ABSTRACT
THOMPSON, CATHERINE E. Tracking Organic Matter from Source to Sink in the Waiapu River Watershed, New Zealand: A Geochemical Perspective. (Under the direction of Neal E. Blair and Elana L. Leithold).
The significant contribution of small mountainous river systems, including the
Waiapu River on New Zealand’s East Cape, to the global fluvial sediment supply motivates investigation into the processes that influence the character and composition of the organic carbon that they carry. Organic matter preserved in continental margin sediments originates from terrestrial sources such as kerogen, fresh and aged soil carbon, as well as marine sources. Due to the reactivity of marine carbon in the seabed, terrestrial sources of carbon are preferentially preserved unaltered. Therefore, identification of specific terrestrial sources of sediment from the watershed preserved on the continental margin can facilitate interpretation of the organic geochemical record and enable reconstruction of the watershed history.
Carbon and nitrogen isotopic analyses and polycyclic aromatic hydrocarbons have been used to apportion terrestrial carbon sources preserved on the shelf, specifically to resolve aged soil contributions. Spatial geochemical patterns on the margin appear to be complicated by structural deformation of the shelf and its influence on marine sediment dynamics. Resolution of riverine sources suggests that gullying, bank failure, and sheetwash are the chronic geomorphic processes delivering most of the organic carbon to the Waiapu sedimentary system. The middle and outer Waiapu continental shelf buries an average of 59 Gg
C/y. Relative to the 200 Gg C/y delivered by the Waiapu River to the ocean, approximately 23% of the riverine carbon is retained on the mid- to outer shelf, matching the sediment inventory. Radiocarbon analysis of DIC indicates that there is no apparent oxidation of kerogen in the seabed. Stable carbon isotopic signatures suggest minimal oxidation of modern terrestrial carbon, signifying that the 77% of the riverine carbon not accounted for has likely been retained and potentially oxidized on the inner shelf or escaped to the slope or beyond the established boundaries of the mid- to outer shelf.
Tracking Organic Matter from Source to Sink in the Waiapu River Watershed, New Zealand: A Geochemical Perspective
by Catherine E. Thompson
A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy
Marine, Earth, and Atmospheric Sciences
Raleigh, North Carolina
May 2009
APPROVED BY:
______Dr. Neal E. Blair Dr. Elana L. Leithold Committee Chair Committee Co-chair
______Dr. Elizabeth G. Nichols Dr. David J. DeMaster
______Dr. J. Paul Liu DEDICATION
For my Mom and Dad, whose love and support has sustained me through life’s
challenges. And for my Aunt Vicki, who has always inspired and encouraged me.
In his heart a man plans his course,
but the Lord determines his steps.
~Proverbs 16:9~
“There are potholes on the road less traveled. Some deep, some not so deep, some
you dig yourself. Most are filled with mud. Many contain rocks. Once in awhile,
however, you’ll be walking along and step in one a bit more accommodating… shabby, green, and pulsing with life. It’ll tickle your feet, like clover.”
Excerpt from Flabbergasted by Ray Blackston
ii BIOGRAPHY
Catherine Elizabeth Thompson graduated from Walter Hines Page High
School in Greensboro, NC. She was a Libby Jones Scholar at the University of
North Carolina at Greensboro, where she majored in chemistry. In her junior and senior years, Catherine was a Glaxo-Wellcome Women in Science Scholar, and conducted organic synthesis research with Dr. James Barborak. After her undergraduate studies, Catherine continued her chemistry research at the University of Michigan under Dr. Edwin Vedejs. She also focused on chemistry education as a
Chemical Sciences at the Interface of Education fellow under Dr. Brian Coppola.
After obtaining Ph.D. candidacy in organic chemistry, Catherine decided that her interests were better suited to environmental chemistry. She took a M.S. in organic chemistry, and joined Dr. Neal Blair and Dr. Lonnie Leithold at North Carolina State
University for a Ph.D. in chemical oceanography, focusing on organic geochemistry.
Her Ph.D. research has focused on the use of isotopic and molecular signatures of organic matter as tracers of sediment generation, transport, deposition, and burial in the Waiapu River watershed in New Zealand.
iii ACKNOWLEDGMENTS
I would like to first thank my advisors, Neal Blair and Lonnie Leithold, without whose guidance, this would not have been possible. Under their tutelage, I have transitioned from an organic chemist to an organic geochemist. They taught me to
“think outside of the beaker”, putting chemistry in an environmental context. I will always be grateful for all they have done. I would also like to thank the other members of my committee, Elizabeth Nichols, Dave DeMaster, and Paul Liu, who have supported my education at NCSU.
Dr. Elizabeth Nichols has provided me immeasurable support over the last five years. In addition to serving on my committee, she has supported my research both with her time and funding. She has always been available to talk about science, school, and life in general. Without her support, my graduate school experience would have been vastly different; there simply are not words to express my gratitude.
Dr. Alan Palmer at Massey University in Palmerston North, New Zealand supported my field research in 2007. He taught me a great deal about soil science and the geology of New Zealand. Alan was also so kind as to open his home to me throughout my tenure in New Zealand. Dr. Chris Osburn, my supervisor in the IRMS
Lab, has provided many insights towards my research. I am very appreciative for his time and efforts on my behalf.
iv I am greatly appreciative of my various funding sources throughout the years.
The departments of Marine, Earth and Atmospheric Sciences and Forest and
Environmental Resources at NCSU supported me through teaching assistantships and as an instrument technician. Grants from the National Science Foundation and the Environmental Protection Agency/North Carolina Department of Environmental and Natural Resources/Division of Water Quality have provided funding for me as a research assistant for Neal Blair, Lonnie Leithold, and Elizabeth Nichols. An EAPSI fellowship through NSF and the Royal Society of New Zealand supported my soil science research in New Zealand in 2007.
Previous and current lab members have contributed technical support.
Specifically, I want to thank Kristen Lloyd and Laurel Childress, the lab monkeys, who have not just been lab mates, but also very good friends. They have been supportive through the good times and bad.
My office mates Anita McCulloch and Linda Waters were with me at NCSU since day one, quickly becoming two of my closest friends and my support system.
We studied together, worked all hours of the night in the office, provided new perspectives on research ideas, and found every possible coffee shop, bookstore, and restaurant in the greater Triangle area with available power outlets and free internet connections. Their contribution to my graduate education and life in general is beyond measure and deserves infinite gratitude and thanks.
Other friends, both at NCSU and at the University of Michigan deserve acknowledgement. Special thanks are extended to Jane Griffiths, Yiyi Wong, Tara
v Conser Hagena, Mark Bielaska, Tim and Beth DeVries, Andrew Callender, Robyn
Gdula, Nancy Santagata, Beth Rueschhoff, Pete Lazaro, Rachel Cook, and Thorne
Gregory. Special thanks also go to Jason Smulik, who pushed and antagonized me to be the best scientist possible.
My father and mother, Rich and Janice Thompson, have been the foundation of my support structure. They have been there for me regardless of the need. Without their love, encouragement, and all of forms of support, I would not have been able to survive the graduate school process. My brother and sister-in- law, Tim and Traci Thompson, have been very supportive of me. My niece Annika and my nephew Dane have been the joy in my life over these last few years. They constantly remind me that life should be approached with curiosity. My aunt, Janet
Register, has supported me as if I were one of her own daughters. My grandparents, Richard and Lois Gainer, have supported me with constant prayer, love, and financial gifts for which I am overwhelmingly thankful. My aunt and uncle,
Vicki and John Winterton have taught me that both work and fun should be approached with same diligence and fervor; that all things should be done to the best degree possible.
Finally, I lost my beloved dog, Gracie, during my tenure at NCSU. Her presence added some normalcy to my life, ensuring that I went home at regular hours and had a life outside of work. She taught me many lessons in patience and devotion. Her love was unconditional; she was always there with a smile and a bark.
vi TABLE OF CONTENTS
List of Tables...... xiii
List of Figures ...... xv
Chapter 1. Introduction ...... 1
1. Introduction...... 2
2. References ...... 12
Chapter 2. Carbon and nitrogen isotopic signatures as tracers of particle associated organic matter in the Waiapu sedimentary system, New Zealand ...... 26
1. Introduction...... 28
2. Site description and methodology...... 33
2.1 Site description...... 33
2.2 Sampling methods...... 37
2.3 Analytical methods ...... 39
2.3.1 Particulate organic carbon ...... 39
2.3.2 Particle size analysis...... 40
2.3.3 Physical separation of particle sizes ...... 41
3. Results...... 42
3.1 Source: Terrestrial organic matter...... 42
3.1.1 Organic content of bedrock...... 42
vii 3.1.2 Organic content of soils...... 46
3.2 Sink: Riverine and marine sediments...... 54
3.2.1 Fluvial transport...... 54
3.2.2 Marine sediment...... 56
4. Discussion ...... 62
4.1 Source identification of preserved particulate organic carbon ...... 62
4.2 Resolution of terrestrial source identification...... 75
4.3 Geologic implications of terrestrial source identification ...... 80
4.4 POC dynamics on the continental margin ...... 82
5. Conclusions...... 88
6. Acknowledgements...... 91
7. References ...... 92
8. Appendices...... 111
8.1 Complete terrestrial sample descriptions...... 112
8.1.1 Bedrock in the Waiapu River watershed...... 112
8.1.2 Bedrock-derived forested soils...... 115
8.1.3 Bedrock-derived pastoral soils...... 118
8.1.4 Tephric soils...... 121
8.1.5 Colluvial soils ...... 126
8.1.6 Alluvial soils...... 131
8.1.7 Thermogenic hydrocarbon sources in the Waiapu watershed ...... 136
8.1.8 Riverine sediment (riverbed and suspended load)...... 137
viii 8.2 Carbon and nitrogen mass balance solutions for marine samples ...... 142
Chapter 3. Geomorphic process identification using background-level polycyclic aromatic hydrocarbons in sediment sources in the Waiapu River watershed, New
Zealand...... 143
1. Introduction...... 145
2. Site description and methodology...... 150
2.1 Site description...... 150
2.2 Sampling methods...... 152
2.3 PAH concentration analyses ...... 156
3. Results...... 158
3.1 TPAH concentrations ...... 158
3.2 PAH distributions...... 163
3.2.1 Classic pyrogenic and petrogenic PAH distributions...... 163
3.2.2 PAH distributions of potential rock carbon sources...... 165
3.2.3 PAH distributions in watershed soils...... 166
3.2.4 PAH distributions of organic carbon delivered to the river and
margin ...... 169
4. Discussion ...... 171
4.1 Detection of contaminant and background PAHs...... 171
4.2 Diagnostic ratios: A quantitative assessment of PAH distributions...... 174
4.2.1 Weathering...... 174
ix 4.2.2 Source apportionment using diagnostic ratios ...... 177
4.3 Source apportionment by principal component analysis ...... 181
4.4 Implications for geomorphic processes ...... 188
5. Conclusions...... 193
6. Acknowledgements...... 196
7. References ...... 197
8. Appendices...... 214
8.1 PAH structures of interest...... 215
8.2 Full details of quality assurance/quality control ...... 216
8.2.1 Precision/analysis replication...... 217
8.2.2 Separation efficiency...... 218
8.2.3 Recovery...... 219
8.3 PAH concentration data...... 221
8.4 Complete PAH histograms for all samples...... 228
8.4.1 PAH histograms of bedrock formation samples ...... 228
8.4.2 A complete record of soil profile PAH histograms ...... 232
8.4.3 PAH histograms of thermogenic hydrocarbon sources within the
Waiapu watershed...... 242
8.4.4 PAH histograms of sediment taken directly from the riverbed...... 243
8.4.5 PAH histograms of marine sediment down core and with increasing
distance from the river mouth ...... 244
x Chapter 4. The fate of terrestrial organic carbon from a small mountainous river on the adjacent continental shelf: Waiapu River sedimentary system, New
Zealand...... 249
1. Introduction...... 251
2. Site description and methodology...... 256
2.1 Site description...... 256
2.2 Sampling methods...... 259
2.3 Analytical methods ...... 260
2.3.1 Particulate organic carbon ...... 260
2.3.2 Dissolved inorganic carbon...... 261
3. Results...... 262
3.1 Particulate organic carbon...... 262
3.2 Dissolved inorganic carbon ...... 268
4. Discussion ...... 271
4.1 Source identification of preserved particulate organic carbon ...... 271
4.2 Source identification of remineralized organic carbon ...... 275
4.3 The Waiapu margin carbon budget ...... 281
4.3.1 Burial rate of particulate organic carbon...... 282
4.3.2 Burial rate of dissolved inorganic carbon ...... 285
4.3.3 Carbon outputs from the seabed: DIC diffusive flux rates...... 286
4.3.4 Retention of riverine carbon on the Waiapu shelf...... 289
xi 4.3.5 Burial efficiencies of kerogen and terrestrial, non-rock organic
carbon ...... 294
5. Conclusions...... 297
6. Acknowledgements...... 299
7. References ...... 300
Chapter 5. Overall Conclusions ...... 315
1. Conclusions...... 316
2. References ...... 324
xii LIST OF TABLES
Chapter 2. Carbon and nitrogen isotopic signatures as tracers of particle associated organic matter in the Waiapu Sedimentary system, New Zealand...... 26
Table 1. Location and geochemistry of potential terrestrial organic matter
sources...... 43
Table 2. Location, geochemistry, and grain size analysis of marine
sediment...... 57
Table 3. Particulate organic carbon characteristics of surficial continental shelf
sediment samples...... 61
Table 4. Isotopic mass balance sensitivity test: Terrestrial carbon treated as
entirely modern vs. incorporating an aged soil component...... 65
Table 5. Averages of the source fractions from mass balance solutions using
entirely modern and non-rock (aged soil inclusive) estimates ...... 70
Table A1. Two end-member marine carbon and nitrogen mass balance
solutions ...... 142
Chapter 3. Geomorphic process identification using background-level polycyclic aromatic hydrocarbons in sediment sources in the Waiapu River watershed, New
Zealand...... 143
Table 1. Sample locations by type...... 154
Table 2. Inventory of 42 Alkylated and Non-Alkylated PAHs of Interest ...... 158
xiii Table 3. PAH concentrations and selected diagnostic ratios...... 160
Table 4. Mean TPAH concentrations in urban waterways and the remote
Waiapu...... 172
Table A1. Breakdown of samples run for 42 polycyclic aromatic
hydrocarbons...... 216
Table A2. Paired t-test results of replicated sample analysis...... 218
Table A3. Percent recovery of chrysene-d12...... 220
Table A4. Concentrations of individual PAH analytes...... 222
Chapter 4. The fate of terrestrial organic carbon from a small mountainous river on the adjacent continental shelf: Waiapu River sedimentary system, New
Zealand...... 249
Table 1. POC and DIC concentrations and isotopic compositions for selected
depths of representative profiles along the northern transect...... 268
Table 2. Application of mass balance calculations to partition POC burial rates by
carbon fraction...... 275
Table 3. Organic carbon delivered to the Waiapu Margin...... 284
Table 4. Sediment and POC Yields from New Zealand Rivers...... 289
Table 5. Carbon retention on the mid to outer Waiapu continental shelf ...... 293
xiv LIST OF FIGURES
Chapter 2. Carbon and nitrogen isotopic signatures as tracers of particle associated organic matter in the Waiapu Sedimentary system, New Zealand...... 26
Figure 1. Map of the Waiapu River Watershed, New Zealand...... 33
13 15 Figure 2. Organic geochemistry of the ten soil profiles: δ C, δ N, C/N atm ratio,
and %C org ...... 47
Figure 3. Stable carbon isotopic analysis ( δ13 C) of the Waiapu River sedimentary
system ...... 51
Figure 4. Nitrogen isotopic analysis ( δ15 N) of the Waiapu River sedimentary
system ...... 52
Figure 5. C/N atm ratios for the Waiapu River sedimentary system ...... 53
Figure 6. Median diameter of surface sediment (0-2 cm) on the Waiapu
margin...... 58
Figure 7. Mass balance partitioning of particulate organic carbon in surficial
sediment along the northern transect ...... 66
Figure 8. Organic matter partitioned by fractions of riverine and marine carbon
across the Waiapu margin ...... 72
Figure 9. Organic matter partitioned by fractions of riverine and marine nitrogen
across the Waiapu margin ...... 74
Figure 10. Principal component analysis of the Waiapu sedimentary system
source to sink ...... 79
xv Figure 11. Principal component analysis of surficial marine sediment...... 86
Chapter 3. Geomorphic process identification using background-level polycyclic aromatic hydrocarbons in sediment sources in the Waiapu River watershed, New
Zealand...... 143
Figure 1. Map of the Waiapu River Watershed, New Zealand...... 150
Figure 2. Map of terrestrial and marine samples collected from the Waiapu River
sedimentary system for PAH analysis ...... 155
Figure 3. Pyrogenic and petrogenic PAH histogram patterns in soil and rock,
respectively...... 164
Figure 4. Diagnostic ratio cross plots used to assess the degree of PAH
weathering in environmental samples...... 176
Figure 5. Diagnostic ratio cross plots quickly illustrate relationships between
various environmental samples ...... 178
Figure 6. Principal component analysis of 42 individual PAH concentrations
normalized to TPAH...... 185
Figure A1. Examples of PAH parent structures ...... 215
Figure A2. PAH histograms for Tertiary bedrock samples...... 229
Figure A3. PAH histograms for Cretaceous bedrock samples...... 230
Figure A4. PAH histograms for bedrock samples taken from soil profiles ...... 231
Figure A5. Barton’s Mouth profiles: A forested, Tertiary bedrock-derived soil
profile...... 232
xvi Figure A6. Forest Soil profiles: A forested, Cretaceous bedrock-derived soil
profile...... 233
Figure A7. Burdett’s Waiapu Bridge Overlook: A pastoral, bedrock-derived soil
profile taken from an area prone to sheetwash...... 234
Figure A8. Landslide Waiapu: A pastoral, bedrock-derived soil profile taken from
a landslide scar...... 235
Figure A9. W1 Terrace: A stable, pastoral, tephric soil...... 236
Figure A10. Mata Slip: A pastoral, tephric soil undergoing sheetwash, landsliding,
and tunnel gullying...... 237
Figure A11. Manuel Terrace: A stable, organic farm on a colluvial terrace...... 238
Figure A12. Tutumatai Station: A stable, pastoral colluvial terrace...... 239
Figure A13. Tutumatai Station Lowest Holocene Terrace: A pastoral, rapidly
failing alluvial soil along the riverbank ...... 240
Figure A14. Youngest Holocene Terrace: A rough pastoral alluvial soil
terrace...... 241
Figure A15. Te Puia Springs and Williams Gas Seep both represent natural
sources of hydrocarbons to the watershed...... 242
Figure A16. Bedload mud taken directly from the Waiapu River and its tributaries
to track organic carbon through the watershed...... 243
Figure A17. The PAH histograms from the sediment core taken at 61 m water
depth...... 244
xvii Figure A18. The PAH histograms from the sediment core taken at 83 m water
depth...... 245
Figure A19. The PAH histograms from the sediment core taken at 108 m water
depth...... 246
Figure A20. The PAH histograms from the sediment core taken at 128 m water
depth...... 247
Figure A21. The PAH histograms from the sediment core taken at 615 m water
depth...... 248
Chapter 4. The fate of terrestrial organic carbon from a small mountainous river on the adjacent continental shelf: Waiapu River sedimentary system, New
Zealand...... 249
Figure 1. Map of the Waiapu River Watershed, New Zealand ...... 256
Figure 2. Organic matter partitioned by fractions of riverine and marine carbon
across the Waiapu margin ...... 265
Figure 3. Depth profiles of POC content and stable carbon isotopic signatures for
sediment...... 266
Figure 4. Particulate organic and dissolved inorganic carbon isotopic analysis of a
seaward transect from the river mouth ...... 267
Figure 5. DIC concentration and stable carbon isotopic signatures of porewater with
depth in cores ...... 270
xviii Figure 6. Mass balance partitioning of particulate organic carbon in surficial
sediment...... 273
Figure 7. Mass balance calculations of DIC...... 277
Figure 8. Source apportionment of remineralized carbon in porewater by mass
balance calculations ...... 279
xix CHAPTER 1
Introduction
1 1. Introduction
The study of riverine systems and adjacent coastal environments has profound societal relevance as greater than 50% of the world’s population lives within 200 km of the coast and 2/3 of the global population is within 400 km
(Hinrichsen, 1998). These fluvial and coastal environments provide water and energy for these societies. An understanding of the natural behaviors of these systems must be attained in order to determine the impact of anthropogenic activity.
Increased erosion results from practices including deforestation, agriculture, and commercial and private land development (e.g. Milliman and Syvitski, 1992; Eden and Page, 1998; Page et al., 2001; Scott et al., 2006; Parkner et al., 2007). The resulting increased sediment load causes a decline in water quality and river flow
(Hinrichsen, 1998).
Land management can be improved by understanding the coupled watershed-continental margin system; therefore, research that spans the entire coupled system is of great societal relevance. In order to gain predictive capability of geomorphic response to natural and human-induced perturbations, it is critical to first understand how different environmental forcings (i.e. tectonic uplift, earthquakes, landslides, storms, floods) affect various parts of the watershed and how the effects of these forcings are propagated through the system (NSF, 2003; i.e. which geomorphic processes are responsible for sediment transport in a given system).
Transformations of intrinsic geochemical signatures of particle-associated organic
2 carbon due to transport and burial processes present the opportunity to use particulate organic matter as a tracer of sediment movement to examine these geomorphic perturbations.
Perceptions concerning the fate of particulate organic carbon (POC) on river- dominated continental margins are largely shaped by studies of large to moderate- sized systems, such as those of the Amazon (Hedges et al., 1986; Aller et al., 1996;
Keil et al., 1997; Mayorga et al., 2005; Aller and Blair, 2006; Aufdenkampe et al.,
2007), Mississippi (Gordon et al., 2001; Gordon and Goñi, 2004; Wang et al., 2004;
Dagg et al., 2005; Goñi et al., 2006a; Wysocki et al., 2006; Bianchi et al., 2007) and
Columbia (Small et al., 2004; Walsh et al., 2008). However, the nature of the POC transported and ultimately buried at sea varies as a function of river characteristics.
The Earth’s major rivers, such as the Amazon, are predominantly on passive margins where organic carbon must pass through multiple bioactive reservoirs on its way to the ocean (Blair et al., 2004; Aller and Blair, 2006). In these reservoirs, the sediment-associated POC is exposed to a variety of biogeochemical processes and aged, altering the chemical composition of the POC (Blair et al., 2004; Aller and Blair,
2006; Alin et al., 2008). Once delivered to a wide, energetic margin, the chemical composition of the POC is further altered by exposure to O 2, marine carbon, and metal oxidants (Aller et al., 1996; Aller and Blair, 2004). This process is reflected by the variable nature of the age and reactivity of the now complex mixture of organics that are buried in the seabed (Blair et al., 2004; Aller and Blair, 2006; Alin et al.,
2008).
3 In contrast to these major rivers, which have historically received the bulk of research attention, small mountainous rivers (SMRs) are in far greater abundance.
Despite individually small sediment loads, these types of rivers collectively transport more than 40% of the global sediment flux (Milliman and Syvitski, 1992). Between
17 and 35% of the global riverine POC flux is estimated to come from Oceania alone
(Milliman and Syvitski, 1992; Lyons et al., 2002; Alin et al., 2008). Much of the sediment derives from modern terrigenous organic matter or erosion of the uplifted sedimentary rocks of which the terrain is composed (Leithold et al., 2006). SMRs on active margins have little space for storage, so the sediment is transported to the margin more rapidly, leaving the POC relatively unaltered (Blair et al., 2003; Blair et al., 2004; Goñi et al., 2005; Leithold et al., 2005; Leithold et al., 2006; Goñi et al.,
2006b; Goñi et al., 2008). The contribution of aged soils in these systems is not yet understood (Blair et al., 2003; Drenzek et al., 2009). Limited storage reservoirs restrict aging on land, suggesting bimodal delivery of modern and ancient carbon
(Blair et al., 2003); however, biomarker evidence indicates aged carbon contributions may be underestimated (Drenzek et al., 2009). The significant contribution of sediment and associated POC by SMRs to the global ocean provides an excellent opportunity to explore organic matter transformations from its terrigenous source through a watershed to the continental margin (the marine sink).
Until recently, the understanding of the source-to-sink system has been limited by research conducted piecemeal by geologists in watersheds and by oceanographers on margins around the world. In order to integrate this research,
4 the National Science Foundation developed a Source-to-Sink Initiative as part of their Margins Program which provided a community forum with a collaborative focus to study sediment production, transport, and burial in selected environments (NSF,
2003).
One goal of the Source-to-Sink effort is to advance the predictive capability of sediment fluxes across the Earth’s continental margins, clarifying the role of geochemical cycles in ecosystem changes (NSF, 2003). For example, enhanced knowledge of particle-associated carbon transport throughout a system can provide information on the global carbon cycle as it relates to climate change. At present, we cannot forecast how perturbations in one part of a coupled watershed-continental margin system will propagate, which is concerning as increased anthropogenic activity is leading to increased erosion in these systems. This program aims to provide quantitative explanations for and predictive capability concerning the effects of perturbations in watersheds on the geologic record preserved in the stratigraphic record on the continental margin (NSF, 2003).
The Source-to-Sink initiative focuses on the Fly River of Papua New Guinea and the Waipaoa River of New Zealand (NSF, 2003). Both rivers are on tectonically active margins and deliver high sediment yields to the adjacent sedimentary basin.
These rivers also have significant differences including different climate regimes, hydrography, sediment sources, oceanographic forcings, and stratigraphy (NSF,
2003). The ability to compare and contrast these systems provides the opportunity to differentiate the impacts of various environmental forcings and potentially improve
5 predictions of tsunami and landslide activity, which will benefit the inhabitants of these regions.
Although not one of the Source-to-Sink sites, the opportunity arose to examine the Waiapu River sedimentary system of New Zealand, adjacent to the
Waipaoa River watershed, using the source-to-sink method. The Waiapu River is also a small mountainous river with a sediment yield larger than that of the Waipaoa
(Mazengarb and Speden, 2000). A high sediment discharge river system ensures that the river is the primary source of particulates to the continental margin, and also creates a detailed stratigraphic record. Direct transport of terrestrial carbon to the continental margin limits the opportunity for carbon alteration from biogeochemical processes in intermediate sediment reservoirs, potentially conserving the terrestrial sediment source signature in the seabed and providing a geochemical tracer for sediment origin and transport mechanisms (Leithold and Hope, 1999; Blair et al.,
2003; Blair et al., 2004; Leithold et al., 2006).
While not part of the NSF Margins Initiative, much is already known about this system (e.g. Mazengarb and Speden, 2000; Page et al., 2001; Parkner et al., 2006;
Wright et al., 2006; Addington et al., 2007; Ma et al., 2008; Wadman and McNinch,
2008; Kniskern et al., 2009). Sediment transport and burial processes are well defined and a well-resolved event record exists in the seabed (Wright et al., 2006;
Addington et al., 2007; Ma et al., 2008; Wadman and McNinch, 2008; Kniskern et al.,
2009). The Waiapu River watershed is also more remote than the Waipaoa and relatively undeveloped. This minimizes contamination of the system by
6 anthropogenic sources of organic carbon, which is particularly advantageous in the molecular characterization of the watershed.
Herein, we have addressed the three main objectives of the Source-to-Sink agenda for the Waiapu River system (NSF, 2003). Consideration has been given to how forcing parameters such as tectonic activity and severe weather events impact sediment generation, transport, and eventual burial and how these forcings regulate geomorphic processes responsible for transporting sediment. Specifically, we have examined the contributions of rock and soil sources of organic matter to the margin delivered by tectonically driven gullying and storm-induced landslides, respectively.
Finally, we have developed a molecular profile as a tracer for organic carbon that can be extended to examine variations of sediment sources throughout the stratigraphic record.
In Chapter 2, carbon and nitrogen isotopic signatures are utilized as tracers of particle-associated organic matter to examine sediment origin, fluvial transport, deposition and accumulation on the continental margin. Rock and soil samples were acquired to establish isotopic (δ13 C and δ15 N) and elemental (%C and %N)
signatures of potential sediment sources. Riverine sediment is analyzed to track the
incorporation of this terrestrial material into the fluvial network. Once the organic
matter is transported from the river to the continental margin, bulk sediment
properties (e.g. chemical composition; particle size) across the margin are explained
by physical sorting processes caused by the known physical oceanographic regime
7 and the addition of marine carbon (Wright et al., 2006; Addington et al., 2007;
Wadman and McNinch, 2008; Ma et al., 2008; Kniskern et al., 2009).
Multivariate statistical modeling, specifically principal component analysis
(PCA), is utilized to identify the chemical similarity between potential sources of
terrestrial organic matter and the river and marine sediments, thereby suggesting the
origin of the sediment preserved on the shelf and slope (Shaw, 2003; Giles et al.,
2007). Mass balance equations incorporating the carbon isotopic signatures of
riverine and marine organic matter end-members are then applied to quantify the
percent of terrestrial and marine carbon buried in the sediment across the margin
(Blair et al., 2003; Blair et al., 2004; Leithold et al., 2005; Leithold et al., 2006).
In Chapter 3, the identification of terrestrial organic matter sources was
extended to include molecular fingerprints. A new application for the use of
polycyclic aromatic hydrocarbons (PAHs) as sediment tracers has been developed.
Because the Waiapu River is so remote, PAH contamination customarily detected by
these compounds is limited (Venkatesan, 1988; Page et al., 1996; Burns et al., 1997;
Short et al., 1999; Stout et al., 2001a; Stout et al., 2001b; Stout et al., 2004).
Therefore, intrinsic, background levels of these compounds can be used to
fingerprint terrestrial organic matter sources. While bedrock can be characterized by
petrogenic PAHs, the slash and burn deforestation practices add pyrogenic PAHs to
the soils (Lima et al., 2005). The tephric nature of some soils in the watershed
(Landcare Research, 2009) minimizes the petrogenic PAH signature from the
8 bedrock, thereby increasing the detection of this pyrogenic input signature and facilitating the use of PAH profiles as sediment tracers.
While mass balance models using isotopic analysis allows for the quantification of terrestrial organic matter as a category, principal component analysis allows identification of specific terrestrial sources that are actively delivering sediment to the river. PAH concentration profiles serve as a more complex fingerprint for each potential source to be used in PCA. Score plots of these principal components reveal the chemical similarity between each terrestrial source and the riverine and marine sinks, signifying terrestrial sources that are likely contributing to the sediment and further resolving the terrestrial origin of sediment and organic matter in the watershed (Mudge et al., 2002; Shaw, 2003).
Geomorphic processes active in the watershed access different organic matter sources: gullying delivers rock carbon (Derose et al., 1998; Page et al., 2001;
Gomez et al., 2003a; Kasai et al., 2005; Marden et al., 2005a; Parkner et al., 2006); sheetwash delivers topsoil, down to the top 10 cm of soil (Page et al., 2004; Leithold et al., 2006; Marden et al., 2006); landsliding delivers a larger portion of the soil profile, down to about 1 m (Page and Trustrum, 1997; Eden and Page, 1998;
Marutani et al., 1999; Page et al., 1999; Gomez et al., 2002; Gomez et al., 2003b;
Reid and Page, 2003; Page et al., 2004; Kasai et al., 2005; Liebault et al., 2005;
Marden et al., 2006); and bank failure delivers riverbank soils (Marden et al., 2005b;
Phillips et al., 2007). Because these processes access unique organic matter pools,
9 the principal component source apportionment model of the potential terrestrial sources is applied to assess active geomorphic activity in the watershed.
In Chapter 4, post-depositional processing of organic matter is considered by examining the sediment cores obtained from the continental margin. Down-core patterns of each carbon fraction in the particulate organic carbon down to 50 cm depth were relatively conserved, confirming no major change or variation in organic matter source over recent history represented by the sediment core in the sampled horizons. Stable carbon isotopic analysis of the dissolved inorganic carbon (DIC) in the porewater was used to determine the origin of the organic carbon that is oxidized in the seabed (marine or terrestrial) (Aller and Blair, 2004; Aller and Blair, 2006; Aller et al., 2008). Radiocarbon analysis of porewater DIC indicates the age and corresponding potential source of the oxidized carbon (Aller and Blair, 2004;
Mayorga et al., 2005; Aller and Blair, 2006; Aller et al., 2008). Any presence of ancient carbon remineralization has the potential to clarify the poorly understood role of kerogen oxidation in the global carbon cycle (Hedges, 1992; Blair et al., 2003;
Komada et al.; 2004; Alin et al., 2008).
The carbon preserved or remineralized at each site is dictated by the physical oceanographic dynamics on the shelf. This also controls burial rates of POC and
DIC, as well as diffusive rates of DIC. The methods used to collect DIC samples prohibit the accurate estimate of DIC diffusion, thereby eliminating the possibility of quantifying the marine carbon contribution and the corresponding total carbon delivery rate to the continental shelf. However, the fraction of buried terrestrial POC
10 can be compared with the estimated riverine input to the shelf in order to determine how much riverine carbon is retained on the mid- to outer shelf.
Our ability to interpret the stratigraphic record is limited by our understanding of processes that affect sediment generation, transport, deposition, and burial.
Understanding the modern depositional environment off the Waiapu is key to interpreting the stratigraphic record throughout geologic time. This research contributes to the goals of both the Source-to-Sink Initiative and New Zealand’s
Kyoto Protocol goals by associating isotopic and molecular terrestrial source identification with geomorphic processes in the watershed as well as burial processes in the seabed to determine the fate of sediment and associated carbon in a coupled watershed-continental margin system. Furthermore, mass balance partitioning and quantification of the terrestrial and marine carbon fractions preserved in the seabed were used to both determine the retention of riverine carbon and assess its reactivity (i.e. oxidation) on the mid- to outer Waiapu shelf.
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25 CHAPTER 2
Carbon and nitrogen isotopic signatures as tracers of particle-associated organic
matter in the Waiapu sedimentary system, New Zealand
26 Abstract
Terrestrial organic carbon delivered by the Waiapu River to the continental margin appear to be unaltered during early marine diagenesis and can by resolved into fraction of various origin using carbon and nitrogen isotopes. Modern terrestrial organic matter in the Waiapu system, such as plant litter, is depleted in both 13 C and
15 N relative to 12 C and 14 N respectively ( δ13 C range is approximately -30 to -27.5‰;
δ15 N range is approximately -4 to 2.5‰). The bedrock is enriched in the heavier
isotopes of both carbon and nitrogen (typical δ13 C ranges from -26.5 to -23‰; δ15 N
ranges from 2 to 5.5‰). Soil organic matter has intermediate isotopic values
(ranges: -27 to -25.5‰ and 4 to 6.5‰; δ13 C and δ15 N respectively). Once sediment is delivered to the continental margin, stable carbon and nitrogen isotopic signatures of marine sediment typically become more enriched in the heavier isotope with increasing distance from the river mouth (from -25 to -22‰ for δ13 C; from 3.5 to
5.5‰ for δ15 N).
Mass balance models indicate terrestrial carbon accounts for >90% of carbon on the inner shelf, between 70 and 85% on the mid-shelf, and 60-70% of carbon on the slope. Of this terrestrial carbon, kerogen appears to account for between 65 and
80%. Mass balance equations also indicate that marine carbon is generally added to relatively recalcitrant terrestrial carbon, except in areas of sediment reworking such as bathymetric highs. Principal component analysis (PCA) has been used to distinguish specific terrestrial sources of sediment and associated particulate organic
27 carbon in the Waiapu system. Though not well resolved, river and marine sediments are most chemically similar to Cretaceous Whangai Fm. mudstones, riverbank alluvial soils, Tertiary Wanstead Fm. sandstones, and some horizons of colluvial, tephric, and bedrock-derived soils. Both mass balance and PCA indicate that gully erosion of ancient rock carbon is the dominant geomorphic process delivering sediment to the Waiapu margin.
Spatial geochemical patterns on the margin appear to be complicated by
structural deformation of the shelf and its influence on marine sediment dynamics.
The inner shelf and the region of highest sediment accumulation rate on the middle
shelf are both dominated by terrestrial organic matter, whereas the remainder of the
mid- to outer shelf is dominated by marine carbon. Fine sediment is primarily found
on the outer shelf due to fining processes and in the highest sediment accumulation
zone where rapid burial precludes sorting and offshore transport.
1. Introduction
Three percent of the global land area produces 17-35% of the global
sediment flux; with tectonically active, small islands contributing a disproportionate
1% relative to the 0.1% of the land they comprise (Milliman and Syvitski, 1992;
Lyons et al., 2002; Scott et al., 2004). Rivers are the main conduit for sediment and
its associated particulate organic carbon (POC) from terrestrial to marine
28 environments (Hedges and Keil, 1995). Though individually, small mountainous rivers (SMRs) deliver small sediment loads, collectively these rivers are responsible for the transport of greater than 40% of the global sediment flux (Milliman and
Syvitski, 1992). Therefore, SMRs in these high yield sedimentary systems are a critical component in the global carbon cycle.
High sediment yields in connection with rapid POC burial promote the preservation of terrestrial carbon across the margin, thereby preserving the source signatures of the POC transported through the watershed (Blair et al., 2003; Blair et al., 2004; Goñi et al., 2005; Leithold et al., 2005; Goñi et al., 2006a; Goñi et al., 2006b;
Leithold et al, 2006; Goñi et al., 2008). Mass balance equations using stable and radiocarbon isotopic analyses have accordingly been utilized to identify and quantify terrestrial sources preserved in the seabed (Blair et al., 2003). Studies of SMR sedimentary systems associated with these tectonically active watersheds indicate that the organic carbon buried in marine depositional sites has a bimodal character that reflects two dominant sources of carbon: modern (terrestrial and marine) and ancient (kerogen) carbon (Leithold et al., 2001; Blair et al., 2003; Blair et al., 2004;
Leithold et al., 2005; Leithold et al., 2006). Estimates are now being refined to account for less dominant fractions such as aged soil contributing to the POC
(Drenzek et al., 2009; Blair et al., 2009).
To address the significance of aged soil contributions in the Waiapu River and adjacent margin, stable carbon and nitrogen isotopic signatures have been used herein to characterize a suite of potential rock and soil sources of organic matter
29 from the Waiapu watershed to be compared with riverine and marine sediment. The isotopic signatures of sedimentary rocks are the product of the paleo-environment and post-depositional diagenesis and catagenesis (Sadofsky and Bebout, 2004), whereas isotopic compositions of soils have integrated more recent surficial terrestrial transformations. Carbon is gained by plant and microbe inputs as well as animal waste product inputs, and is lost by diagenesis. Nitrogen is gained by N 2
fixation, nitrification, plant litter and animal waste inputs as well as fertilizer
applications, and is lost by denitrification (Ehleringer, 2000, Kao and Liu, 2000;
Galloway et al., 2004; Holloway and Smith, 2005). Relationships between the δ15 N of soils and/or vegetation with precipitation, temperature, soil age, and topography have also been observed and may be derived in part from isotopically selective loss mechanisms (Handley et al., 1999; Amundson et al., 2003; Bedard-Haughn et al.,
2003).
In marine environments, sedimentary POC is gained through the addition of marine carbon to terrestrial carbon across the margin (Blair et al., 2003). POC can be lost by oxidation of marine or terrestrial carbon, or other diagenetic processes
(Aller et al., 1996; Aller and Blair, 2004; Aller et al., 2008). Nitrogen is gained through remobilization of particulate organic matter and lost by denitrification on the seabed (Galloway et al., 2004). Aside from biogeochemical transformations, the composition of buried POC on the shelf can also be influenced by hydrodynamic sorting during transport (Vannote et al., 1980; Goñi et al., 1997; Goñi et al., 1998;
Leithold and Hope, 1999). Globally, as much as 90% of riverine sediments are
30 deposited on the continental margin; therefore, this zone plays a key role in linking the global terrestrial and oceanic carbon cycles (Thomas et al. 2004). Because particle-associated organic matter that reaches the continental shelf reflects its complex origins in the watershed, the terrestrial source can be identified in the seabed using the isotopic signatures that characterize soil and rock particulate organic matter.
As various geomorphic processes access distinct carbon pools and terrestrial carbon is preserved in the seabed, organic matter attributed to these carbon sources in the seabed suggest active geomorphic processes within the watershed. Gully erosion, shallow landsliding, sheetwash, and bank failure are dominant mechanisms of sediment delivery to the channel network in small mountainous watersheds. In the East Cape region of the North Island of New Zealand, gully erosion is a chronic source of sediment driven by high frequency, low magnitude storms (Derose et al.,
1998; Hicks et al., 2000; Gomez et al., 2003; Hicks et al., 2004; Kasai et al., 2005;
Marden et al., 2005; Parkner et al., 2006; Parkner et al., 2007). Deep incision into bedrock by gullying delivers rock debris along with its associated kerogen (ancient
OC) to the river. Conversely, landsliding is typically triggered by intense episodic storm events, and delivers entire soil profiles (of 1 m average thickness) along with the mostly modern and aged plant-derived carbon concentrated in the topsoil (Page et al., 1994; Page and Trustrum, 1997; Eden and Page, 1998; Page et al., 1999;
Trustrum et al., 1999; Page et al., 2001; Gomez et al., 2002; Page et al., 2004;
Marden et al., 2006). Similarly, sheetwash delivers modern soil organic matter, but
31 typically only down to about 10 cm in a soil profile (Page et al., 2004; Marden et al.,
2006). Additionally, bank failure of riverbank soils delivers modern, aged, and ancient POC to the river (Marden et al., 2005; Phillips et al., 2007).
Mass balance models using carbon isotopes have previously been used to link geomorphic processes to the sedimentary record by identifying terrestrial sources preserved in the seabed (Leithold et al., 2005). In systems with lower sediment yields, the dominant portion of the POC delivered to the margin was modern plant material delivered by sheetwash and shallow landsliding processes
(Leithold et al., 2005). However, in systems with increasingly larger sediment yields, gullying of ancient rock carbon contributes more to the POC (Leithold et al., 2005).
The addition of nitrogen isotopes can further resolve terrestrial sources, allowing more insight into the geomorphic processes contributing aged carbon in addition to modern and ancient carbon.
In this study, the isotopic signatures of sedimentary rocks and soils in the
Waiapu watershed, bedload and suspended sediment in the river, and marine sediment from the continental margin have been used to: a) investigate possible sources of particulate organic matter to the margin, b) consider potential processes that transport sediment to the river, and c) examine possible physical parameters that may influence sediment deposition and accumulation on the adjacent continental margin. Stable carbon and nitrogen isotopic signatures used for source apportionment improve our ability to link the organic geochemical record to both present and paleo-environmental geomorphic processes.
32
2. Site description and methodology
2.1 Site description
The Waiapu River watershed is located on the East Cape of New Zealand’s North Island
(Figure 1). The river flows 130 km eastward to the Pacific Ocean from the Raukumara Range, and is oriented within the Hikurangi subduction margin (Moore and Mazengarb, 1992). Most of the rocks in the watershed are part of the East
Coast Allochthon, primarily Cretaceous and
Tertiary sandstones and mudstones which have been displaced in a series of thrust sheets. Figure 1. Map of the Waiapu Figure 1. Map of the Areas underlain by the Allochthon are Waiapu River Watershed, New Zealand characterized by high erosion rates because of extensive crushing of the rocks during faulting (Mazengarb and Speden, 2000; Page et al., 2001; Parkner et al., 2006). The Whangai Fm. within the East Coast
Allochthon is composed of Late Cretaceous to Paleocene mudstones and underlies the largest part of the catchment (Parkner et al., 2006).
33 The vegetation in the Waiapu watershed has changed drastically during the
Holocene Epoch. Maori settlers burned the native podocarp and hardwood forests
(600-700 years B.P.) and Europeans cleared the forests for pasture (from 1890-
1920) (Wilmshurst et al., 1999). Today, the area is sparsely populated and the land is primarily used for sheep and beef farming (Mazengarb and Speden, 2000).
Soils in the watershed can be categorized as bedrock-weathered, tephric,
colluvial, and riverbank alluvial. When bedrock-weathered soils are formed, the soil
organic matter reflects both the overlying plant litter and weathered underlying
bedrock (Marden et al., 2008). Through much of the upper Waiapu watershed, soils
are not built on bedrock but rather are tephric soils formed on a mantle of volcanic
ash dating back to 55,000 years B.P., with deposits of the most recent eruption, 665
years B.P., being the most widespread (Eden et al., 2001). Erosion rates of tephric
soils are generally low except on steep slopes or exposed sites (Landcare
Research, 2009). Colluvial and alluvial soils are both mixtures of other soil types.
Colluvial soils are those occupying depositional areas within the watershed that
incorporate bedrock-derived and tephric soils delivered by mass wasting processes
(e.g. landslide debris tails) (Marden et al., 2008). Alluvial soils are mixtures of soil
types deposited from water suspension including floodplains, alluvial terraces, and
riverbank soils, the latter of which was sampled herein (Phillips et al., 2007).
At 20,520 T km -2 yr -1, the Waiapu’s sediment yield is one of the highest in the
world (Page et al., 2001; Parkner et al., 2006), with a sediment discharge of
approximately 36 x 10 6 T/yr from a drainage area of only 1734 km 2 (Hicks et al.,
34 2000; Page et al., 2001; Parkner et al., 2006). High intensity storm events, high rates of uplift and steep slopes, and unstable lithologies contribute to the highly degraded nature of the catchment, with extreme mass wasting and high sediment yields
(Marutani et al., 1999; Page et al., 1999; Hicks et al., 2000). The small drainage basin limits the storage capacity for the eroding sediment within the watershed caused in part by a yearly rainfall in excess of 2.4 m (Page et al., 2001). Gully erosion into sedimentary bedrock is estimated to account for more than half of the high sediment yield of the Waiapu River catchment (Page et al., 2001; Parkner et al.,
2006). To a first approximation, the suspended POC in the Waiapu has been shown to be a bimodal mixture containing roughly 15-25% modern plant-derived and 75-
85% rock (kerogen) OC, with surface erosion and gullying as the primary processes generating the sediment from the soil and bedrock (Leithold et al., 2006; this study).
Modern sediment transport on the Waiapu margin is controlled by currents, waves, and gravity flows under high concentration conditions (Wright et al., 2006;
Addington et al., 2007; Ma et al, 2008; Wadman and McNinch, 2008; Kniskern et al.,
2009). Two currents affect the Waiapu continental margin: the East Cape Current
(ECC) flowing southward along the shelf break and upper continental shelf and the
Wairarapa Coastal Current which flows northward inshore of the ECC (Chiswell,
2000; Wadman and McNinch, 2008; Kniskern et al., 2009). The seabed up to 60 m water depth is regularly agitated by wave action (Wadman and McNinch, 2008).
Sediment transport and burial processes on the margin have been studied to resolve the event record in the seabed (Addington et al., 2007; Ma et al., 2008;
35 Wadman and McNinch, 2008; Kniskern et al., 2009). Boundary layer instrumented tripods off the Waiapu mouth at 40 and 60 m water depths have shown dense, near- bottom hyperpycnal flows during moderate river floods (Wright et al., 2006). The deposits of episodic flood events are revealed in the down-core geochemical records by 210 Pb-poor sediment layers on the mid-shelf. The high sediment concentrations during such events overwhelm the local supply of 210 Pb and do not provide enough time to permit scavenging of oceanic 210 Pb (Sommerfield and Nittrouer, 1999;
Sommerfield et al., 1999; Kniskern et al., 2009). During non-flood conditions, energetic waves and currents rework sediment deposited in the shallow shelf, resulting in higher 210 Pb inventories due to greater exposure time to the water
column (Sommerfield and Nittrouer, 1999; Sommerfield et al., 1999; Kniskern et al.,
2009).
210 Pb inventories have also been used to determine sediment accumulation rates on the shelf that range from 0.2 to 3.5 cm/y (Kniskern, 2007; Kniskern et al.,
2009). The 200 m isobath defined the eastern boundary of the Waiapu shelf, while shelf deposit thickness, particle size, and other trends were used to define the north and south boundaries (Lewis et al., 2004; Kniskern, 2007; Kniskern et al., 2009).
The shelf was divided into four regions of accumulation: the inner shelf and three regions of 0.7, 1.4, and 2.7 cm/y accumulation on the mid- to outer-shelf representing estimated areas of approximately 217, 330, and 67 km 2 each,
respectively (map in Kniskern et al., 2009). Beryllium-7 indicates that the inner shelf
(<60 m) behaves as a bypass zone or temporary storage, and sediment is then
36 transported to deeper water (Addington et al., 2007; Kniskern, 2007; Wadman and
McNinch, 2008; Kniskern et al., 2009). The highest sediment accumulation rates occur on the mid to outer shelf (1.4-2.7 cm/y), and then decrease (0.7 cm/y) towards the shelf break (Kniskern, 2007; Kniskern et al., 2009).
2.2 Sampling methods
Terrestrial samples were collected to describe organic carbon sources throughout the Waiapu River watershed. This includes rock, soil, and riverine sediment, complete descriptions of which can be found in Appendix 1.
In summary, representative samples of the major bedrock formations were
collected in place from two gullies, one incised into Cretaceous rocks and one
incised into Tertiary lithologies (Table 1). The Cretaceous Gully sampled is
underlain by rocks from the Whangai Formation, the Mokoiwi Formation, the Tolaga
Group, and the Weber Formation. The Tertiary gully sampled is underlain by rocks
of the Wanstead and Waipawa Formations. The Tikihore Fm. was sampled in place
at the base of a soil profile in the watershed. A loose sample tentatively determined
to be the Whakai Fm. was obtained from the base of a soil profile collected on the
W1 alluvial terrace. Mudstones comprise the Whangai and Waipawa Formation
samples, while sandstones comprise all other rock samples (Appendix 1).
Soil profiles throughout the watershed were sampled to represent the soil
carbon contribution to the margin (Table 1). Two soil profiles were collected from
37 each of the following types of soils: forested bedrock-derived soils, pastoral soils developed from bedrock weathering, tephric soils, colluvial soils and riverbank alluvial soils (Table 1). Variations in land stability (stable terrain vs. areas prone to sheetwash, landsliding, and bank failure) and land usage (e.g. pasture and forest) were accounted for in each type of soil (Appendix 1). In addition, two sites that were likely to be contaminated by natural thermogenic hydrocarbon sources (hot springs and a natural gas seep) were sampled (Table 1). Riverine sediment was collected to characterize the organic matter being transported by the river (Table 1).
Sediment was taken directly from the riverbed; suspended sediments were also obtained by centrifugation or filtration of surface river water.
In order to trace terrestrial carbon through its deposition and burial in the marine environment, sediment samples were collected on the continental margin adjacent to the Waiapu River mouth from the R/V Kilo Moana in May 2004 (Table 2).
Box cores (up to 50 cm in length) were obtained, subsampled, and immediately sectioned on ship. Approximately seven intervals of sediment 2 cm thick were collected from each core; the depths of these sections were chosen randomly based on the length of the core to best represent the entire profile with seven evenly spaced samples. The sediment was transferred to a plastic bag and frozen.
Samples were shipped frozen and archived as such until time of analysis.
38 2.3 Analytical methods
2.3.1 Particulate organic carbon
Bulk sediment samples for carbon analysis were freeze-dried and acidified
(4N HCl, 4 days) prior to analysis to remove carbonates. Dried subsamples were
placed in tin boats and analyzed for %OC, %N, and δ13 C with a continuous flow elemental analyzer (Flash CE 1112)-IRMS (Thermo Electron Delta V) (±2% (RSD) and 0.1‰, respectively). Selected samples were subsequently analyzed for 14 C by
cryogenically trapping the CO 2 produced by the oxidation of POC via the elemental analyzer. These samples were sent to the National Ocean Sciences Accelerator
Mass Spectrometry (NOSAMS) facility at Woods Hole Oceanographic Institution.
14 The CO 2 was converted to graphite and analyzed for C content, which is reported as fraction modern relative to the National Bureau of Standards and Technology
Oxalic Acid I Standard normalized to a δ13 C of -19‰ (Olsson, 1970), where modern
is defined as 95% of the radiocarbon concentration in AD 1950. Further corrections
normalize the fraction modern to a δ13 C VPDB of -25‰ to account for natural
isotopic fractionations. The average error of Fm reported by NOSAMS on POC
samples was 0.0023. This error is the larger of the internal error (± √n, where n equals the total number of counts for a given target) or external error (reproducibility of counts for multiple exposures for a given target) calculated.
39 It has been shown that the δ15 N is significantly altered in acidified samples, and should be analyzed on untreated dried samples, making separate EA-IRMS runs necessary for the two isotope measurements (Ryba and Burgess, 2002;
Kennedy et al., 2005). With two runs for each sample, %N is obtained for both the acidified and untreated samples. The significance of the effect of acidification on
%N has been debated, and appears to vary somewhat with sample type (Ryba and
Burgess, 2002; Kennedy et al., 2005). In this study, the %N in acidified samples is systematically lower than that in untreated samples for all sample types (rocks, soils, and sediments). However, the difference in the %N is statistically insignificant
(p=0.106, df=183) within a 95% confidence interval. Therefore, reported δ15 N data
13 are reported from untreated samples, whereas %C org , %N, C/N ratios, and δ C data
are reported from acidified samples.
2.3.2 Particle size analysis
Grain size analysis was conducted by laser diffraction. Bulk samples were
thawed and approximately 1 g of wet sediment was placed in a clean
beaker. Deionized water (approximately 10 to15 mL) was added to the sample
which was suspended in the water using a magnetic stir bar. Sodium
metaphosphate (approximately 1 mL of 2 g/L solution) was added to the sample as a
dispersant to prevent flocculation. Disposable polyethelene pipettes were used to
transfer the sample to the water column of the Beckman Coulter LS 13 320,
40 Universal Liquid Module. Samples were diluted in the instrument to obtain obscuration levels between 9-12%. The water column was then sonicated for 15 seconds and analyzed for 60 seconds. Duplicates were run on all samples; coarse grained samples typically had a higher standard deviation than fine grained samples for the mean particle size.
2.3.3 Physical separation of particle sizes
Between 3 and 5 g of dry sediment was washed through a 25 µm sieve into a
1L graduated cylinder using DI water. DI water was used to transfer the >25 µm fraction from the sieve to a beaker, frozen and freeze-dried to isolate that fraction for acidification and isotopic analysis. Sodium metaphosphate (5 mL) was added to the cylinder containing the <25 µm fraction to prevent flocculation. The total volume of
DI water in the cylinder was brought up to 600 mL. Rubber tubing affixed to glass tubing was run through a peristaltic pump. The glass tubing was placed exactly 10 cm below the surface of the water in the cylinder. The sample was stirred thoroughly, and the cylinder was left to settle undisturbed for two hours. After 2 hours, the top 10 cm of fluid was pumped out of the cylinder and centrifuged to isolate the <4 µm fraction which was then dried, acidified, and analyzed by EA-
IRMS.
41 3. Results
3.1 Source: Terrestrial organic matter
3.1.1 Organic content of bedrock
Pale grey-green calcareous mudstones of the Tertiary Wanstead Formation
(Mazengarb and Speden, 2000), are depleted in 13 C (-26.4 and -27.6‰, respectively), but somewhat enriched in 15 N (5.3 and 5.4‰, respectively). These
rocks have approximately 0.3%C org and 0.03%N. Intercalated glauconitic sandstones from this formation contain considerably less organic matter (0.01%C org
and 0.01%N). These samples are also more enriched in 13 C (-24.2 and -22.9‰, respectively) and more depleted in 15 N content (5.1 and 3.8‰, respectively). The
Tertiary Waipawa Fm., a poorly bedded, grey to black, non-calcareous mudstone
(Mazengarb and Spenden, 2000), has high %C org (2.5 and 10.3%, respectively) and
is very enriched in 13 C relative to all other samples (-20.5 and -16.5‰, respectively).
Samples from the Waipawa Fm. also have correspondingly high %N (0.1 and
0.37%), and are the most enriched samples in 15 N collected (6.4 and 6.7‰,
respectively).
42 Table 1. Location and geochemistry of potential terrestrial organic matter sources 13 15 Sample Detail Latitude Longitude Elevation %C %N C/N atm δ C δ N Rocks Barton's Gully 1 Jar Wanstead -37.8408 178.2182 197 0.33 0.03 13.59 -26.41 5.25 Barton's Gully 2 Jar Wanstead -37.8408 178.2182 197 0.27 0.03 9.86 -27.62 5.44 Barton's Gully 2 Jar Wanstead -37.8408 178.2182 197 0.27 0.03 9.86 -27.62 5.44 Barton's Gully Rock 1 Waipawa -37.8408 178.2182 197 2.48 0.11 27.16 -20.45 6.42 Bartons Gully Rock 2 Wanstead -37.8408 178.2182 197 0.01 0.01 0.89 -24.23 5.07 Bartons Gully Rock 3A Wanstead -37.8408 178.2182 197 0.02 0.01 2.11 -22.93 3.82 Bartons Gully Rock 3B Waipawa -37.8408 178.2182 197 10.31 0.37 32.32 -16.51 6.67 Barton's Gully Stream Bed 1 Wanstead -37.8408 178.2182 197 0.05 0.02 3.57 -23.85 2.63 Barton's Gully Stream Bed 1 Wanstead -37.8408 178.2182 197 0.05 0.02 3.57 -23.85 2.63 Cretaceous Gully 1 Jar Mokoiwi -37.8768 178.2136 101 0.06 0.02 3.28 -25.06 3.14 Cretaceous Gully 2 Jar Mokoiwi -37.8768 178.2136 101 0.46 0.05 11.70 -25.50 2.11 Cretaceous Gully 3 Jar Tolaga Group -37.8768 178.2136 101 0.11 0.02 6.35 -25.31 2.30 Cretaceous Gully 4 Jar Weber -37.8768 178.2136 101 0.06 0.02 3.29 -25.16 1.42 Cretaceous Gully 5 Jar Whangai -37.8768 178.2136 101 0.51 0.05 12.01 -25.51 2.20 Cretaceous Gully Rock 1 Whangai -37.8768 178.2136 101 0.41 0.04 11.23 -25.48 2.04 Cretaceous Gully Rock 1 Whangai -37.8768 178.2136 101 0.41 0.04 11.23 -25.48 2.04 LW Br Whangai -37.8079 178.3742 118 0.11 0.02 6.13 -25.44 4.43 Mata Slip Br Tikihore -37.9541 178.1990 157 0.08 0.02 4.65 -23.36 1.99 Mata Slip Br Tikihore -37.9541 178.1990 157 0.08 0.02 4.65 -23.36 1.99 W1 rock Whakai -37.8698 178.3173 125 0.05 0.02 3.39 -24.02 2.94 Soils Bartons Mouth 0-5cm forested B-D -37.8400 178.2120 167 28.18 1.62 20.25 -28.38 1.11 Bartons Mouth 5-10cm forested B-D -37.8400 178.2120 167 5.35 0.40 15.45 -25.33 4.60 BWBO 00-5cm pastoral B-D -37.8959 178.2914 95 6.56 0.66 11.67 -27.24 6.57 BWBO 05-10cm pastoral B-D -37.8959 178.2914 95 2.96 0.28 12.23 -27.35 6.12 BWBO 05-10cm pastoral B-D -37.8959 178.2914 95 2.96 0.28 12.23 -27.35 6.12 BWBO 20-25cm pastoral B-D -37.8959 178.2914 95 0.95 0.08 14.12 -27.19 4.11 BWBO 45-55cm pastoral B-D -37.8959 178.2914 95 0.96 0.07 15.19 -27.45 4.32 Forest Soil 0-4cm forested B-D -37.8754 178.2066 140 13.01 0.74 20.50 -28.72 -3.21 Forest Soil 0-4cm forested B-D -37.8754 178.2066 140 13.01 0.74 20.50 -28.72 -3.21 Forest Soil 0-4cm forested B-D -37.8754 178.2066 140 13.01 0.74 20.50 -28.72 -3.21 Forest Soil 10-15cm forested B-D -37.8754 178.2066 140 3.39 0.30 13.29 -26.28 1.01 LW 00-8cm pastoral B-D -37.8079 178.3742 118 6.44 0.54 13.83 -27.66 1.91 LW 08-19cm pastoral B-D -37.8079 178.3742 118 2.25 0.20 13.10 -25.98 4.22 LW 30-40cm pastoral B-D -37.8079 178.3742 118 0.87 0.09 11.23 -25.82 5.32 LW 60-70cm pastoral B-D -37.8079 178.3742 118 0.59 0.07 10.24 -25.53 5.56 LW 81-92cm pastoral B-D -37.8079 178.3742 118 0.64 0.08 9.87 -25.55 5.21 Mata Slip 0-5cm tephric -37.9541 178.1990 157 2.80 0.26 12.67 -27.40 3.53 Mata Slip 10-15cm tephric -37.9541 178.1990 157 2.85 0.24 14.01 -26.26 4.26 Mata Slip 20-25cm tephric -37.9541 178.1990 157 1.68 0.17 11.82 -26.05 5.21 Mata Slip 40-50cm tephric -37.9541 178.1990 157 0.96 0.11 9.93 -25.81 5.75 Mata Slip 52-62cm tephric -37.9541 178.1990 157 0.63 0.07 10.16 -25.67 5.09 Mata Slip 80-90cm tephric -37.9541 178.1990 157 0.21 0.03 7.58 -25.59 4.26 MT 0-5cm colluvial -37.8029 178.3863 34 5.83 0.56 12.07 -25.99 4.30 MT 13-18cm colluvial -37.8029 178.3863 34 3.32 0.36 10.69 -26.19 6.44 MT 20-25cm colluvial -37.8029 178.3863 34 2.62 0.29 10.40 -26.22 6.87 MT 30-35cm colluvial -37.8029 178.3863 34 0.94 0.11 9.59 -25.61 7.25 MT 45-50cm colluvial -37.8029 178.3863 34 0.60 0.08 8.79 -25.73 6.77 MT 70-75cm colluvial -37.8029 178.3863 34 0.28 0.04 8.55 -25.49 5.93 MT 85-90cm colluvial -37.8029 178.3863 34 0.36 0.05 8.35 -25.91 5.51
43 Table 1. Location and geochemistry of potential terrestrial organic matter sources (cont'd) 13 15 Sample Detail Latitude Longitude Elevation %C %N C/N atm δ C δ N TS 00-5cm colluvial -37.8462 178.3015 88 5.82 0.53 12.82 -27.35 3.60 TS 010-15cm colluvial -37.8462 178.3015 88 4.40 0.40 12.93 -26.50 4.47 TS 030-40cm colluvial -37.8462 178.3015 88 0.53 0.05 11.29 -25.27 5.33 TS 055-65cm colluvial -37.8462 178.3015 88 0.37 0.04 9.80 -25.21 4.30 TS 080-90cm colluvial -37.8462 178.3015 88 0.32 0.04 10.50 -24.78 4.44 TS 110-120cm colluvial -37.8462 178.3015 88 0.18 0.03 8.07 -24.98 4.38 TSLHT 00-5cm recent alluvial -37.8433 178.3022 76 4.05 0.35 13.37 -23.95 2.39 TSLHT 015-20cm recent alluvial -37.8433 178.3022 76 2.13 0.18 13.48 -23.41 3.77 TSLHT 015-20cm recent alluvial -37.8433 178.3022 76 2.13 0.18 13.48 -23.41 3.77 TSLHT 030-40cm recent alluvial -37.8433 178.3022 76 0.53 0.06 10.94 -22.02 3.94 TSLHT 110-120cm recent alluvial -37.8433 178.3022 76 0.17 0.02 8.59 -24.03 3.68 W1 00-5cm tephric -37.8698 178.3173 125 12.47 1.11 13.14 -27.04 3.75 W1 05-10cm tephric -37.8698 178.3173 125 10.35 0.93 13.03 -25.68 4.17 W1 15-20cm tephric -37.8698 178.3173 125 8.74 0.79 12.86 -25.37 3.99 W1 15-20cm tephric -37.8698 178.3173 125 8.74 0.79 12.86 -25.37 3.99 W1 15-20cm tephric -37.8698 178.3173 125 8.74 0.79 12.86 -25.37 3.99 W1 30-35cm tephric -37.8698 178.3173 125 5.36 0.46 13.53 -24.94 5.89 W1 45-50cm tephric -37.8698 178.3173 125 2.64 0.22 14.29 -25.57 6.87 W1 75-80cm tephric -37.8698 178.3173 125 0.93 0.10 11.11 -24.93 7.51 W1 75-80cm tephric -37.8698 178.3173 125 0.93 0.10 11.11 -24.93 7.51 W1 75-80cm tephric -37.8698 178.3173 125 0.93 0.10 11.11 -24.93 7.51 W1 95-100cm tephric -37.8698 178.3173 125 0.47 0.05 10.05 -24.80 5.77 W1 95-100cm tephric -37.8698 178.3173 125 0.47 0.05 10.05 -24.80 5.77 YHT 010-15cm raw alluvial -37.8636 178.1861 0 0.52 0.05 12.60 -26.57 1.81 YHT 010-15cm raw alluvial -37.8636 178.1861 0 0.52 0.05 12.60 -26.57 1.81 YHT 040-45cm raw alluvial -37.8636 178.1861 0 0.45 0.04 14.62 -25.75 2.19 YHT 040-45cm raw alluvial -37.8636 178.1861 0 0.45 0.04 14.62 -25.75 2.19 YHT 080-90cm raw alluvial -37.8636 178.1861 0 0.41 0.04 12.04 -25.52 2.39 YHT 110-115A raw alluvial -37.8636 178.1861 0 3.56 0.33 12.51 -25.68 2.18 Thermogenic Hydrocarbons Te Puia Springs 1 hc -38.0594 178.3035 81 1.49 0.11 15.12 -29.79 -1.51 Te Puia Springs 2 hc -38.0588 178.3036 81 0.91 0.09 11.55 -27.54 -7.39 Williams Gas Seep 1 hc -38.0602 178.2555 123 6.45 0.50 14.96 -25.09 -0.01 Williams Gas Seep 2 hc -38.0602 178.2555 123 13.20 1.12 13.79 -28.39 1.66 Riverbed Sediment BG Mud mud -37.8408 178.2182 197 0.22 0.02 11.74 -25.02 4.23 Makarika Stream Mud mud -37.9552 178.2288 106 0.35 0.03 13.08 -24.66 3.04 Mangaoporo Mud mud -37.8371 178.2926 0 0.27 0.02 15.01 -25.28 3.07 Mata River Mud mud -37.9491 178.2122 97 0.53 0.05 12.68 -26.08 2.51 Tikitiki Mud mud -37.7999 178.4129 0 0.22 0.02 11.99 -25.44 2.96 YHT Mud mud -37.8636 178.1861 0 0.30 0.03 11.60 -25.24 2.48 Riverine Suspended Sediments Makarika Stream Susp Sed susp sed -37.9552 178.2288 106 0.79 0.07 13.05 -25.67 2.90 Mata River Susp Sed susp sed -37.9491 178.2122 97 0.71 0.07 12.09 -26.26 2.70 WU 030520 susp sed -37.8949 178.2956 ~50 0.50 0.05 12.70 -25.22 WU 030521 susp sed -37.8949 178.2956 ~50 0.54 0.05 13.13 -25.37 WU 030826 susp sed -37.8949 178.2956 ~50 0.55 0.06 11.62 -25.37 WU 030827 susp sed -37.8949 178.2956 ~50 0.59 0.06 12.00 -25.57 WU 030917 susp sed -37.8949 178.2956 ~50 0.60 0.06 12.39 -25.38
44
Rocks collected from the Cretaceous gully included the Whangai Fm., composed of a non-calcareous and calcareous, micaceous, and siliceous shale and mudstone, the Mokoiwi Fm., an alternating centimeter to decimeter bedded, fine to medium grain sandstone, the Tolaga Group, a minor component of the gully, with massive bedded, slightly calcareous, sand and mudstones, and the Weber Fm., a very minor Oligocene contribution to this gully, which is comprised of poorly bedded, pale grey calcareous mudstone with thin glauconitic sandstone beds (Mazengarb and Speden, 2000). All of these rocks have approximately the same δ13 C signature
(-25.3 ± 0.2‰), though the Whangai Fm. mudstones have considerably higher %C org than the sandstones (0.5 vs. 0.1%C org ). The nitrogen signature of the Whangai Fm. is 2.1 ± 0.2‰, with 0.45%N. The nitrogen signature for the Mokoiwi Fm. is 2.6 ±
0.5‰ (n=2), 2.3‰ (0.02%N) for the Tolaga Group; and 1.42‰ (0.02% N) for the
Weber Formation.
The Whangai Fm. sampled from bedrock associated with a soil profile (rather than from a gully) has considerably lower organic matter content (0.11%C org and
0.02%N). Its δ13 C signature is comparable to other Whangai Fm. rocks (-25.4‰);
however, its δ15 N is enriched (4.4‰). A sandstone from the Tikihore Fm., a
Cretaceous unit composed of alternating fine-grained sandstone and mudstone
(Mazengarb and Speden, 2000) underlying a soil profile, is relatively depleted in
13 15 both C (-23.4‰) and N (2.0‰), with 0.08%C org and 0.02%N. A sandstone tentatively identified as the Whakai Fm., composed of alternating sandstone and
45 mudstone beds (Mazengarb and Speden, 2000), has 0.05%C org and 0.02%N with isotopic signatures of -24.0‰ and 2.9‰, δ13 C and δ15 N, respectively.
3.1.2 Organic content of soils
The stable carbon and nitrogen isotopic signatures of soils tend to become
more positive (enriched in the heavier isotope) with depth (Figure 2). This has been
hypothesized to be a result of a change in source over time, mixing between modern
surface and rock organic matter, or diagenetic processes (Ehleringer et al., 2000;
Blair et al., 2003; Bostrom et al., 2007). In the Waiapu River watershed, modern
terrestrial organic matter, such as plant litter, is isotopically light ( δ13 C range is approximately -30 to -27.5‰; δ15 N range is approximately -4 to 2.5‰) (Table 1).
The bedrock is isotopically heavier (typical δ13 C ranges from -26.5 to -23‰; δ15 N
ranges from 2 to 5.5‰) (Table 1; Figures 3 and 4). Therefore, soil organic matter
that forms from the mixing of these two end-members would result in a positive trend
in isotope signature. Diagenetic processes result in lighter isotopes being
preferentially used in microbial reactions because of lower energetic requirements,
leaving the heavier isotope behind to enrich the soil, showing the same trend as in
soil mixing processes (Ehleringer et al., 2000).
Bedrock-derived soils, both forested and pastoral, are enriched in the heavier
13 15 isotope ( δ C and δ N) and %C org and %N decreased precipitously from the plant
litter horizon to subsoil, being concentrated in the topsoil (Figure 2; Table 1), as
46 δ13 C δ15 N -28 -26 -24 -22 -2 0 2 4 6 8 0 20 40 Bartons Mouth Bartons Mouth 60 BWBO BWBO Forest Soil Forest Soil 80 LW LW Depth (cm) Depth Mata Slip Mata Slip MT MT 100 TS TS TSLHT TSLHT 120 W1 W1 YHT YHT C/N %C org atm 0 5 10 15 20 25 30 5 10 15 20 25 0 20
Bartons Mouth Bartons Mouth 40 BWBO BWBO Forest Soil Forest Soil 60 LW LW Mata Slip Mata Slip 80 MT MT Depth (cm) Depth TS TS 100 TSLHT TSLHT W1 W1 YHT YHT 120
Figure 2. Organic geochemistry of the ten soil profiles (data can also be 13 15 found in Table 1). Clockwise from the top left is δδδ C, δδδ N, C/N atm ratio, and %C org . Typically, topsoils are isotopically depleted in the heavier isotope and are enriched downward. Topsoils also have the highest %C org , which decreases exponentially with depth and higher C/N ratios which also decrease with depth. expected for a typical soil profile (Ehleringer et al., 2000). An exception to this occurs on the bedrock-derived, pastoral soil at Burdett’s Waiapu Bridge Overlook
(BWBO; Figure 2; Table 1). Like other profiles, the organic matter is concentrated in the topsoil, and %C org and %N decrease with depth; however, the stable carbon
47 isotope signature is fairly constant downward in the profile (-27.3±0.1‰). The soil at this site appears to dry out during the summer, so marks of dung or topsoil in subsoil samples occurs by a natural process that causes cracks to fill with topsoil (Palmer,
2007). Organic matter is therefore mixed downward and averaged over the profile.
In addition to this process, the nitrogen isotopes are affected strongly by the input of animal waste, as the land is heavily trafficked by cattle and sheep. The δ15 N
becomes depleted with depth; the topsoil is very enriched (~6‰) and becomes
lighter with depth.
Similar to the bedrock-derived soils, the %C org and %N both decrease with
depth in the tephric soils, as organic matter is more concentrated in the topsoil
(Figure 2; Table 1). The δ15 N signature becomes more positive up to the allophanic horizon (pedology by Palmer, 2007), but then becomes more depleted again below that. The tephric sites are differentiated by their stable carbon isotopic composition
(Figure 2; Table 1). At the W1 Terrace site, a pasture with no obvious evidence of erosion, the topsoil is isotopically depleted in 13 C from plant litter, but the remainder
of profile is evenly mixed (-25.3 ± 0.4‰). In the Mata Slip profile, collected from an
actively eroding hillslope (with evidence of sheetwash, landsliding, and tunnel
gullying) used for pasture, the stable carbon isotopic signature behaves typically,
becoming more positive with increasing depth in the profile.
The %C org and %N decrease precipitously with depth in both colluvial soil
profiles (Figure 2; Table 1). These soils are influenced by various inputs through
time, rather than being completely formed in place (Marden et al., 2008) and this
48 variability is reflected in the isotopic signatures downward in the profile. Manuel’s
Terrace has a fairly constant δ13 C throughout (-25.85 ± 0.35‰), while the core at
Tutumatai Station behaves as typically expected, becoming enriched downward in
the profile. The nitrogen isotopic signature for both profiles becomes more enriched
in 15 N to a certain depth, and then becomes more depleted (MT enriches from 4.3 to
7.3‰ at 40 cm, then down to 5.5‰; TS enriches from 3.6 to 5.3‰, then down to
4.4‰) (Figure 2; Table 1). This is proposed to be a shift in source through time
feeding the intermediate storage reservoirs.
Upstream sources of organic matter are incorporated in riverbank alluvial
soils that serve as sediment storage along the river. In the recent riverbank alluvial
soil (which has had time to develop a topsoil (Landcare, 2009)), %C org and %N
decrease downward in the profile. Both δ13 C and δ15 N become more positive
(enriched in the heavier isotope) with increasing depth, until the horizon in direct
contact with the river, which is more negative than the overlying horizon. The raw
riverbank alluvial soil (no developed topsoil) is most depleted in the heavier isotope
at the surface because of the input from surficial plants (-26.6‰ and 1.8‰, δ13 C and
δ15 N respectively). The remainder of the profile is more enriched than the surficial
soil, but is relatively uniform throughout (-25.63 ± 0.12‰ ( δ13 C) and 2.2 ± 0.2‰
15 (δ N)). The %C org and %N decrease down to the horizon in contact with the river
where it increases considerably (7 times the surface horizon) (Figure 2; Table 1).
Additionally, in the Waiapu River watershed, there are sites with natural
thermogenic hydrocarbons present (Table 1). At Te Puia Springs, sediment from
49 two hot springs has very depleted isotopic signatures and high %C org and %N (-29.8
13 15 and -27.5‰ ( δ C), 1.5 and 0.9%C org , -1.5 and -7.4‰ ( δ N), and 0.11 and 0.09%N.
At Williams Gas Seep, soil surrounding a natural gas containment structure is very
13 15 isotopically depleted (-28.4‰ ( δ C); 13.2%C org ; 1.7‰ ( δ N); 1.12%N. Three
meters away, soil exhibits an isotopic and chemical signature similar to the local
13 15 bedrock (-25.1‰ ( δ C); 6.5%C org ; -0.01‰ ( δ N ); 0.5%N).
Each soil profile and rock sample has been shown to possess a characteristic
isotopic signature (Figure 2; Table 1). The contribution of each of these samples to
the fluvial system is better understood when examined spatially. A map has been
developed (ArcGIS) to spatially assess the stable carbon isotopic signature (Figure
3), the nitrogen isotopic signature (Figure 4), and the C/N ratio (Figure 5) of the
potential terrestrial sources in the watershed. Soil parameters shown on the map
are the weighted average (relative to %C org of each horizon) of the soil profile.
Sources are either depleted (green) or enriched (red) in the heavy (13 C, 15 N) relative
to the lighter ( 12 C, 14 N) isotope.
The isotopic fingerprints of soil and rock sources that actively provide sediment to the river are integrated into the suspended and bedload sediments in the river. The rocks ( ) and soils ( ) are incorporated into the rivers channel
bottom ( ) and suspended ( ) sediments. This results in an averaged signal of both δ13 C and δ15 N revealed in the sediment nearest the river mouth at Tikitiki, which accounts for the averaging of extreme isotopic signatures of the soils that contribute to the sediment yield, and also closely resembles the signature of the Cretaceous
50 Figure 3. Stable carbon isotopic analysis ( δδδ13 C) of the Waiapu River sedimentary system. The mixing of rock ( ) and soil ( ■) sources of terrestrial organic matter may explain intermediate riverine isotopic values ( for riverbed, for suspended sediment). The river sediment is dispersed across the continental margin ( ), becoming more enriched in 13 C with distance from the river mouth. (Soil values represent the weighted average for the entire soil profile.)
51 Figure 4. Nitrogen isotopic analysis ( δδδ15 N) of the Waiapu River sedimentary system. Contributing sources of terrestrial organic matter from the varied potential rock ( ) and soil ( ■) origins in the watershed are homogenized in the river ( for riverbed sediment, for suspended sediment), explaining intermediate isotopic values. The river sediment is dispersed across the continental margin, becoming more enriched in 15 N with distance from the river mouth ( ). (Soil values presented are the weighted average for the entire soil profile.)
52 Figure 5. C/N atm ratios for the Waiapu River sedimentary system. The integration of terrestrial organic matter from varied potential rock ( ) and soil ( ■) origins in the watershed results in a mixed C/N ratio in the river ( for riverbed sediment, for suspended sediment). The C/N ratio decreases as river sediment is dispersed across the continental margin ( ). (Soil values presented are the weighted average for the entire soil profile.)
53 rock carbon which is thought to dominate the sediment yield (Page et al., 2001;
Parkner et al., 2006).
As the maps show, only the lower half of the watershed was sampled. This was due to the inability to collect samples on inaccessible and/or private land.
However, care was taken to ensure that the major bedrock formations and soil types in the watershed were all sampled. Riverbed (and in some cases, suspended) sediments were collected on each tributary to determine the contributions of organic matter to the sediment upriver of soil and rock sampling sites as well.
3.2 Sink: Riverine and marine sediments
3.2.1 Fluvial transport
The incorporation of sediment produced from the rocks and soil described above into the river system was examined using isotopic signatures of riverine sediment collected once within five days of a typical low magnitude storm event
(Table 1; Figures 3, 4, and 5). Farthest upriver, sediment from the riverbed of the
Mata River is the most depleted riverine sediment sampled (-26.08‰ ( δ13 C), 2.5‰
15 (δ N)) and also the highest %C org and %N (0.5% and 0.05%, respectively). The
suspended sediment at the Mata River site is the same isotopically, but has higher
%C org and %N (0.7% and 0.07%, respectively). The Makarika Stream that feeds into the Mata river is the most enriched riverbed sediment in 13 C (-24.7‰), but exhibits
54 an average δ15 N signature (3.04‰). The suspended sediment in the Makarika
Stream is more depleted in 13 C (-25.7‰), however, the nitrogen isotopic signature is
the same as the riverbed sediment.
The stable carbon isotopic signatures of the riverbed sediment at Barton’s
Gully, the Mangaoporo River, and the YHT riverbank alluvial soil site all have stable carbon isotopic signatures indistinguishable from the bedrock end-member (-25.15 ±
0.15‰). For the Mangaoporo and YHT sites, the underlying Cretaceous bedrock is approximately -25‰. Barton’s Gully bedrock (Tertiary) has a much wider range of stable carbon isotopic signatures (Table 1); however, they could potentially average out to -25‰ by mixing together in certain proportions (Table 1). Barton’s Gully is also enriched in 15 N (4.23‰) relative to the riverbed sediment at the Mangaoporo
and the YHT sites (2.75 ± 0.25‰). This corresponds to the 15 N-enriched rocks found throughout Barton’s Gully relative to the Cretaceous rocks (Table 1).
Tikitiki is the farthest site sampled downriver before the Waiapu empties into the ocean (Table 1). At this site, the isotopic signature of the riverbed sediment is a mixture of all sources feeding the margin. This value is equivalent to the suspended sediment at the Piggery Rd. bridge (-25.4‰ ( δ13 C)), the site farthest downriver
where suspended sediment was acquired multiple times over the course of a year to
determine an average isotopic signature being delivered to the continental margin.
The isotopic fingerprints of riverine sediment also confirms that, despite the
presence of some C4 plants in the watershed, the modern terrestrial plant matter
55 appears to be dominated by C3 plants (Smith and Epstein, 1971; Fry and Sherr,
1984).
3.2.2 Marine sediment
Stable carbon isotopic signatures of bulk sediment become more enriched in
13 C with increasing distance from the river mouth from -25 to -22‰ (Figure 3; Table
2). This trend is consistent with the addition to or replacement of riverine carbon by marine carbon. In bathymetric lows, isotopically depleted organic matter is preserved, likely reflecting the burial of terrestrial organic matter delivered by negatively buoyant plumes prior to addition of marine carbon (Wadman and
McNinch, 2008). Sediment on bathymetric highs shows enrichment in 13 C relative to
12 C in spite of an overall loss of total organic carbon. This trend suggests that these areas of slow sediment accumulation are sites of either a replacement of terrestrial carbon by marine OC or enrichment of kerogen by the loss of more reactive OC phases.
Enrichment in 15 N relative to 14 N with increasing distance from the river mouth and increasing %N is consistent with the addition to or replacement of terrestrial organic matter by marine organic matter as well as the result of denitrification
15 15 (Figure 4; Table 2). N 2 fixation causes a decrease in the δ N signature ( N
depletion) because isotopically light nitrogen is incorporated into the marine organic
matter. However, when nitrogen is lost through denitrification, isotopically light N 2
56 Table 2. Location, geochemistry, and grain size analysis of marine sediment. Depth grain % % 4- % 13 15 Sample Latitude Longitude (m) %OC %N C/N atm δ C δ N size <4 µm 25 µm >25 µm BC22 SS -38.0130 178.3960 34 0.22 0.03 9.16 -23.34 5.02 70.7 7.2 14.1 78.7 BC40 SS -37.8258 178.5200 40 0.24 0.03 11.27 -25.22 3.44 55.4 11.4 19.0 69.6 40m BC SS -37.7988 178.5398 40 0.35 0.03 11.84 -24.97 3.88 14.4 29.8 46.8 23.4 BC63 SS -37.7257 178.5797 42 0.75 0.08 11.08 -25.19 3.96 11.4 31.9 55.1 13.0 BC11 SS -37.7973 178.5480 47 0.58 0.06 11.40 -25.15 3.83 22.9 21.2 41.6 37.2 BC19 SS -37.8890 178.4738 48 0.63 0.07 10.74 -24.38 4.40 13.7 29.7 51.2 19.1 BC21SS -37.9825 178.4315 48 0.39 0.05 8.99 -23.77 4.83 45.3 15.8 27.3 56.9 BC20 SS -37.9388 178.4442 50 0.58 0.07 10.12 -24.20 4.50 16.9 25.4 48.7 26.0 BC18 SS -37.8658 178.5128 54 0.61 0.06 11.57 -25.02 4.04 14.2 27.4 52.8 19.8 BC23 SS -38.0173 178.4510 56 0.56 0.07 8.87 -24.06 4.52 25.4 19.6 41.7 38.7 BC12 SS -37.7582 178.5830 60 0.81 0.08 11.65 -25.87 3.07 8.7 37.6 55.9 6.5 60m BC SS -37.7905 178.5413 61 0.54 0.05 12.01 -25.35 3.39 26.4 19.2 41.1 39.7 BC24 SS -38.0218 178.5210 77 0.89 0.11 9.11 -23.78 5.12 19.6 19.5 50.7 29.9 BC51 SS -37.7058 178.6585 78 0.39 0.04 10.53 -24.72 4.36 39.8 17.9 33.1 49.0 BC7 SS -37.8088 178.5900 78 0.81 0.09 10.84 -24.83 4.14 12.1 31.6 53.4 15.1 BC33 SS -37.9348 178.5317 83 0.89 0.09 11.06 -25.07 3.81 9.6 35.1 55.9 9.0 BC8 SS -37.7710 178.6283 83 0.71 0.08 10.33 -24.72 4.37 10.5 32.5 56.3 11.2 BC5 SS -37.8730 178.5660 88 0.63 0.06 11.59 -24.80 3.76 12.8 26.2 58.9 14.9 BC34 SS -37.9343 178.5845 108 0.89 0.11 9.38 -23.80 4.84 13.5 26.0 56.5 17.5 BC14 SS -37.7603 178.6903 108 0.56 0.06 10.35 -24.17 4.05 12.7 29.3 53.8 16.9 BC25 SS -38.0208 178.5940 110 1.10 0.14 9.00 -23.31 5.19 18.9 20.5 52.4 27.1 BC50 SS -37.7238 178.7092 112 0.51 0.06 10.03 -24.50 4.17 24.0 22.0 41.4 36.5 BC39 SS -37.8841 178.6044 114 0.82 0.09 10.53 -24.31 4.34 10.4 30.2 60.4 9.4 BC13 SS -37.8230 178.6425 121 0.83 0.10 10.05 -24.04 4.41 12.8 29.8 53.2 17.0 BC35 SS -37.9327 178.6222 125 0.99 0.12 9.54 -23.67 4.97 13.6 26.2 56.2 17.5 BC49 SS -37.7363 178.7500 128 0.38 0.04 10.18 -24.42 4.49 19.5 25.0 43.9 31.1 BC48 SS -37.7664 178.7319 128 0.58 0.06 10.75 -24.51 4.15 13.8 28.3 50.8 21.0 BC36 SS -37.9339 178.7054 129 0.31 0.05 7.69 -22.80 5.44 91.0 11.4 21.8 66.7 BC42 SS -37.8562 178.5097 136 0.79 0.09 10.26 -24.41 4.29 11.7 29.9 56.6 13.5 BC38 SS -37.8863 178.6359 137 0.86 0.10 9.70 -24.06 4.67 10.6 31.2 57.9 10.9 BC45 SS -37.8291 178.6841 150 0.86 0.10 9.86 -23.84 4.55 14.0 25.9 55.2 19.0 BC26 SS -38.0257 178.6682 160 1.02 0.13 8.97 -22.94 5.12 15.7 23.0 55.0 22.0 BC37 SS -37.8939 178.6940 169 0.62 0.08 9.06 -22.79 5.29 16.6 21.9 52.8 25.4 BC46 SS -37.8423 178.7247 201 0.53 0.07 9.12 -23.43 4.85 15.2 26.4 51.4 22.2 BC27 SS -38.0263 178.7487 263 0.77 0.10 8.54 -22.53 5.54 13.4 26.7 55.9 17.4 BC28 SS -37.9838 178.7862 444 0.68 0.09 8.54 -22.42 5.45 14.1 25.8 54.1 20.1 BC1 SS -37.7870 178.8345 615 0.83 0.10 9.70 -23.43 4.85 12.9 25.6 58.2 16.2 BC2 SS -37.8247 178.8341 656 0.89 0.11 9.80 -23.56 5.10 12.0 26.1 59.4 14.5 BC67 SS -37.7867 178.8472 695 0.51 0.06 9.63 -23.98 4.51 13.3 30.7 49.8 19.5 BC62 SS -37.9095 178.8355 747 1.03 0.12 10.16 -24.05 4.60 14.9 23.1 56.8 20.1
57 Figure 6. Median diameter of surface sediment (0 -2 cm) on the Waiapu margin. At the top, the average particle diameter of the sediment is shown; below is the partitioning of sediment by grain size: clay (% sediment < 4 µµµm); fine to medium silt (25 < % sediment < 4 µµµm); and coarse silt and sand (% sediment > 25 µµµm). Coarse sediment is predominantly found near the river mouth and on bathymetric highs; fine sediment is associated with regions of high accumulation rates and offshore sediment (a result of offshore fining).
58 and CO 2 is lost from the seabed, leaving behind isotopically enriched particulate organic matter (Saino and Hattori, 1987; Bronk and Gilbert, 1994). Because denitrification outpaces N 2 fixation, nitrogen isotopes are expected to become enriched in 15 N with distance across the margin and increased exposure to the water
column resulting in increased biogeochemical processing as observed on the
Waiapu margin. Sediment in bathymetric lows tends to be isotopically light
suggesting rapid burial of terrestrial organic matter prior to marine organic matter
addition. Conversely, sediment on bathymetric highs experience replacement of and
addition to terrestrial organic matter by isotopically heavier marine organic matter.
Geochemical trends observed across the margin may be influenced by
source variation and diagenetic alteration as discussed; however, hydrodynamic
sorting can also influence observed geochemical trends. Organic matter
composition also varies with grain size; therefore, sedimentary grain size and
geochemical characterization together can clarify sediment dispersal, depositional,
and accumulation processes.
The average particle diameter for surface sediment (0-2 cm) across the
margin suggests sediment deposition patterns across the Waiapu margin. Fining
from larger (green) to smaller (red) particles with distance offshore is typical
winnowing behavior; coarse sediment is found predominantly on the inner shelf
particularly near the river mouth and becomes finer on the mid- to outer shelf (Table
2; Figure 6, top). Structural deformation of the shelf complicates the sediment
dynamics, resulting in localized areas that deviate from these trends. This suggests
59 potential routing of sediment deposition and burial that also influences geochemical composition.
Bathymetric lows identified nearshore (Wadman and McNinch, 2008) serve as regions of accumulation for terrestrial carbon and fine sediments delivered by negatively buoyant plumes with bathymetric control over transport. Rapid sediment accumulation in these depositional basins precludes extensive winnowing, preserving fine sediment nearshore. Bathymetric highs on the shelf are overlain by coarse particles that suggest the effects of winnowing. In one such region near the shelf break, coarse sediment enriched in 13 C suggests this may being a region of low sediment accumulation experiencing either a replacement of terrestrial carbon by marine OC or an enrichment of kerogen by the loss of more reactive OC phases
(Goñi et al., 1997; Goñi et al., 1998; Leithold and Hope, 1999).
The sediment for the 0-2 cm interval for each station was also subdivided into fractions of clay (black), fine to medium silt (yellow), and coarse silt and sand
(purple) (Table 2; Figure 6, bottom). Sand-sized OC can be dominated by plant debris whereas the OC associated with clays is more microbial in nature (Hedges and Oades, 1997; Ransom et al., 1997; Leithold and Hope, 1999). Kerogen C is typically concentrated in the finer fractions in the Eel system (Leithold and Blair,
2001; Blair et al., 2003). However, in the Waipaoa system, the coarse fraction is dominated by sedimentary rock and its associated kerogen (Blair et al., 2009).
Thus, selected samples were physically separated to assess the organic carbon composition associated with each grain size class in the Waiapu (Table 3).
60 Table 3. Particulate organic carbon characteristics of surficial continental shelf sediment samples. Bulk POC >25 µm fraction <4 µm fraction Water δ13 C C/N ∆14 C 14 C δ13 C C/N δ13 C C/N Depth %C Fm %C %C org atomic org atomic org atomic Sample Latitude Longitude (m) (‰) (‰) age (‰) (‰)
Northern Transect: 60m tripod BC 37.7905 178.5413 61 0.54 -25.35 12.0 -583.0 6970 0.420 0.58 -26.18 17.6 0.76 -24.68 9.4 BC8 37.7710 178.6283 83 0.71 -24.72 10.3 -528.7 5990 0.474 0.83 -25.23 12.3 0.83 -24.54 10.7 BC14 37.7603 178.6903 108 0.56 -24.17 10.3 -494.2 5420 0.509 0.34 -24.91 13.1 0.89 -24.10 9.1 BC48 37.7664 178.7319 128 0.58 -24.51 10.8 -516.3 5780 0.487 0.39 -25.04 14.2 0.84 -24.01 11.2 BC1 37.7870 178.8345 615 0.83 -23.43 9.7 -317.8 3020 0.687 0.58 -24.22 11.5 1.18 -23.16 8.7
Southern Transect: 40m tripod BC 37.7988 178.5398 40 0.35 -24.97 11.8 -599.8 7300 0.403 0.24 -25.39 13.6 0.81 -24.65 9.3 BC5 37.8730 178.5660 88 0.63 -24.80 11.6 -618.8 7690 0.384 1.47 -26.23 17.8 0.77 -25.25 10.3 BC39 37.8841 178.6044 114 0.82 -24.31 10.5 -461.1 4910 0.542 1.25 -24.99 12.2 0.83 -24.47 9.6 BC37 37.8939 178.6940 169 0.62 -22.79 9.1 -313.7 2970 0.691 0.24 -23.07 9.3 1.17 -22.93 10.9 BC2 37.8247 178.8341 656 0.89 -23.56 9.8 -381.5 3800 0.623 0.77 -24.14 10.9 1.09 -23.10 8.8
The δ13 C and 14 C of bulk sediment is compared to the stable carbon isotopic
composition of two isolated sediment size fractions, >25 µm (the coarse silt and
sand fraction) and <4 µm (the clay fraction) for two offshore transects (Table 3).
Coarser fractions are consistently more depleted in 13 C relative to the bulk and fine sediment fractions. This indicates that the rock and marine carbon is diluted with non-rock carbon (e.g. plant debris; soil carbon) (Table 3). Organic carbon content of this fraction is high in regions with high accumulation rates and low where accumulation is slow and/or turbulent conditions keep sediment in the surface mixed layer, promoting oxidation of carbon. In all cases, the fine particles are consistently more organic rich than the bulk sediment, as the increased surface area allows for increased organic carbon association (Ransom et al., 1998). These finer fractions are also generally isotopically enriched in 13 C than the coarse fraction and bulk sediment, reflecting the preservation of rock or marine organic matter or microbially- processed modern terrestrial organic matter (Ransom et al., 1997). The bulk sediment typically reflects an average of these size fractions, with moderate %C org
61 and isotopic signature. The radiocarbon age of bulk sediment decreases across the margin, consistent with the addition of modern marine carbon to the terrestrial carbon with distance from the river mouth.
4. Discussion
4.1 Source identification of preserved particulate organic carbon
Delineation of the sources of organic carbon buried on the continental margin offers a potential window into both watershed and seabed processes. On the
Waiapu margin, radiocarbon age decreases while 13 C is typically enriched with distance offshore (Tables 2 and 3; Figure 3), suggesting spatial changes in the proportions of POC from different sources. There are several possible explanations for these cross-shelf isotopic trends including: 1) the addition of modern marine organic matter to riverine organic matter; 2) the loss of aged or ancient carbon through oxidation; and 3) preferential seaward transport of C4-derived organic matter relative to C3 plant-derived organic matter (Gordon and Goñi, 2003). The first two possibilities will be evaluated with the application of mass balance mixing models.
The input of C4 plants is possible in this system; for example, maize and C4 grasses such as kikuyu are present in the watershed (Ross et al., 2000; Ross et al.,
2002; Ulyatt et al., 2002; Majeran and van Wijk, 2009). However, C4 sources were
62 identified only at a few of the sampling sites, none of which coincided with areas of evident severe erosion. Additionally, riverbed mud taken from a site adjacent to a field on which maize was historically farmed showed no significant enrichment relative to other riverbed muds taken from the watershed in areas with no apparent
C4 plant input. The δ13 C of riverine suspended sediment also indicates negligible
C4 plant contribution (refer to the following end-member discussion). While C4 contributions cannot be entirely ruled out, for this analysis, we have assumed that
C4 plant contributions are negligible.
An isotopic mass balance approach based on the stable and radiocarbon isotopic values of particulate organic carbon can resolve the relative contributions of various carbon sources. The prototypical study of source apportionment using carbon isotopic signatures was conducted on the Eel River sedimentary system, which is characterized by a bimodal distribution of ancient kerogen and modern carbon of terrestrial and marine sources on the shelf (Blair et al., 2003; Leithold et al., 2006). In this system, aged soil OC is hypothesized to be a minor source because 1) the OC inventory within a soil profile will be skewed towards the modern as a result of higher concentrations at the surface, 2) surficial material will be preferentially eroded in all but the most extreme events, and 3) material that ages within a watershed does so because it is not naturally prone to erosion.
Using the stable and radiocarbon isotopic signatures, simultaneous equations
(1), (2), and (3) can be used to solve for the fractions of kerogen and modern riverine and marine carbon in the POC:
63 13 Eq. (1) δ C = f mar δ mar + f riv δ riv + f ker δ ker
14 Eq. (2) ∆ C = f mar ∆ mar + f riv ∆ riv + f ker ∆ ker
Eq. (3) 1 = f mar + f riv + f ker where f x is the fraction of either marine, modern riverine, or kerogen C, and δ and ∆
are the isotopic compositions of the respective end-members (Table 4; Figure 7). In
order to use these mass balance equations for source apportionment, isotopic
values for the primary end-member sources of organic carbon on the continental
margin must be identified and assumed to be constant cross-shelf for this model to
hold. Isotopic values for these end-members can be difficult to determine (Weijers
et al., 2009); therefore, uncertainty associated with these values needs to be
accounted for. The stable carbon isotopic end-member values utilized were
empirically determined from samples acquired throughout the watershed. The rock
carbon that is feeding the Waiapu River margin has a δ13 C of -25.3‰ (±0.2‰, n=6).
The average stable carbon isotopic signature for modern terrestrial organic matter sampled from the Waiapu River watershed is -27.8‰ (±0.6‰, n=6). The stable carbon isotopic signature used for the marine carbon end member is -19.2‰
(±1.2‰, n=8). This number was determined by a combination of methods; it is an average of δ13 C values obtained on three local benthic biota as well as the extrapolated isotopic signature for oxidized marine carbon added to the porewater mid-shelf (Chapter 4).
64 Table 4. Isotopic mass balance sensitivity test: Terrestrial carbon treated as entirely modern vs. incorporating an aged soil component. End-members Marine Non-rock terrestrial Kerogen 13 14 Depth (m) %C δδδ C ∆∆∆ C δδδker δδδterr δδδmar ∆∆∆ker ∆∆∆terr ∆∆∆mar fmar %C mar σ %C mar fterr %C terr σ %C terr fker %C ker σ %C ker 0 0.56 -25.51 -724.6 -25.34 -27.8 -19.21 -1000 33 33 0.06 0.03 0.04 0.21 0.12 0.04 0.73 0.41 0.04 61 0.54 -25.35 -583.0 -25.34 -27.8 -19.21 -1000 33 33 0.11 0.06 0.04 0.29 0.16 0.04 0.60 0.32 0.04 Northern 83 0.71 -24.72 -528.7 -25.34 -27.8 -19.21 -1000 33 33 0.20 0.14 0.06 0.25 0.18 0.06 0.54 0.39 0.06 Entirely transect 108 0.56 -24.17 -494.2 -25.34 -27.8 -19.21 -1000 33 33 0.28 0.15 0.05 0.21 0.12 0.05 0.51 0.28 0.05 modern 128 0.58 -24.51 -516.3 -25.34 -27.8 -19.21 -1000 33 33 0.23 0.13 0.05 0.24 0.14 0.05 0.53 0.31 0.05 terrestrial 615 0.83 -23.43 -317.8 -25.34 -27.8 -19.21 -1000 33 33 0.41 0.34 0.11 0.25 0.21 0.11 0.34 0.28 0.11 organic matter 5 0.63 -24.80 -618.8 -25.34 -27.8 -19.21 -1000 33 33 0.17 0.11 0.05 0.20 0.13 0.05 0.63 0.40 0.05 Southern 39 0.82 -24.31 -461.1 -25.34 -27.8 -19.21 -1000 33 33 0.27 0.22 0.08 0.25 0.21 0.08 0.48 0.39 0.08 transect 37 0.62 -22.79 -313.7 -25.34 -27.8 -19.21 -1000 33 33 0.49 0.30 0.10 0.18 0.11 0.10 0.34 0.21 0.10
0 0.56 -25.51 -724.6 -25.34 -26.7 -19.21 -1000 -136 33 0.03 0.02 0.05 0.28 0.16 0.05 0.69 0.39 0.05 61 0.54 -25.35 -583.0 -25.34 -26.7 -19.21 -1000 -136 33 0.08 0.05 0.05 0.38 0.21 0.05 0.53 0.29 0.05 Non-rock Northern 83 0.71 -24.72 -528.7 -25.34 -26.7 -19.21 -1000 -136 33 0.18 0.12 0.07 0.34 0.24 0.07 0.49 0.35 0.07 terrestrial transect 108 0.56 -24.17 -494.2 -25.34 -26.7 -19.21 -1000 -136 33 0.25 0.14 0.07 0.28 0.16 0.07 0.46 0.26 0.07 organic 128 0.58 -24.51 -516.3 -25.34 -26.7 -19.21 -1000 -136 33 0.21 0.12 0.06 0.31 0.18 0.06 0.48 0.28 0.06 matter (include 615 0.83 -23.43 -317.8 -25.34 -26.7 -19.21 -1000 -136 33 0.39 0.32 0.14 0.33 0.27 0.14 0.29 0.24 0.14 aged soil 5 0.63 -24.80 -618.8 -25.34 -26.7 -19.21 -1000 -136 33 0.15 0.09 0.06 0.26 0.17 0.06 0.59 0.37 0.06 estimate) Southern 39 0.82 -24.31 -461.1 -25.34 -26.7 -19.21 -1000 -136 33 0.24 0.20 0.10 0.33 0.27 0.10 0.42 0.35 0.10 transect 37 0.62 -22.79 -313.7 -25.34 -26.7 -19.21 -1000 -136 33 0.47 0.29 0.12 0.24 0.15 0.12 0.30 0.18 0.12
The kerogen fraction is assigned a ∆14 C of -1000‰ as fossil carbon is radiocarbon dead. The radiocarbon signature of the modern marine fraction is approximated at +33‰, using a benthic feeding animal as a proxy for modern marine carbon that reaches the seabed (Fm = 1.04). The modern terrestrial carbon is estimated at +33‰ as well, taken from hand-picked, non-blackened wood fragments (Fm = 1.00 and 1.07) isolated from the adjacent Waipaoa river system
(Blair et al., 2009). Previous studies have assigned modern terrestrial and marine fractions a ∆14 C end member value of +100‰, assuming completely modern contributions from these fractions inherited from the contemporary atmosphere (Blair et al., 2003). Within the margin of error inherent in these calculations, this assumption provides the same relative source apportionment.
Preliminary estimates of the relative contributions of these end-member sources have been determined for the Waiapu margin (Figure 7B; Table 4; Appendix
2). The uncertainty in end-member values affords a wide range of possible source
65 0.90 A %C marine B %C marine 0.80 %C riverine %C terrestrial 0.70 %C kerogen 0.60
org 0.50
%C 0.40 0.30 0.20 0.10 0.00 0 61 83 108 128 615 0 61 83 108 128 615 Water depth (m) Water depth (m)
Figure 7. Mass balance partitioning of particulate organic carbon in surficial sediment along the northern transect (0 m depth refers to the riverine end-member sample). A) The two end-member mass balance calculation apportions marine from riverine carbon using the stable carbon isotopic signatures. B) The three end-member mass balance calculation includes radiocarbon dating to apportion ancient from modern terrestrial carbon and marine carbon. Within the errors of the end-members, these calculations reinforce one another, and indicate that riverine carbon accounts for >90% of the carbon on the inner shelf, between 75 and 90% on the mid-shelf, and 60 to 70% of carbon on the slope. Of that riverine carbon, kerogen accounts for between 65 and 80%. contributions; however, general patterns can be determined for the various fractions.
The mass balance calculations indicate that the terrestrial carbon behaves conservatively across the shelf along the northern transect (Figure 7B). Terrestrial organic matter is more refractory than the relatively labile marine organic matter and thus expected to be buried preferentially to the marine carbon (Weijers et al., 2009).
Little loss of the rock organic carbon via oxidation is expected because exposure to
O2 is short and the ancient OC is recalcitrant by nature, having already survived a cycle of diagenesis and catagenesis (Aller and Blair, 2004; Aller et al., 2008).
Increasing contributions of marine carbon are added on top of the terrestrial carbon
66 transported from the river with increasing distance across the margin. The addition of marine carbon enriched in 13 C across the margin causes the bulk stable carbon isotope data to become more positive (Figure 4). The radiocarbon age also supports this interpretation, becoming younger as fresh marine carbon is added to the preserved terrestrial carbon across the margin (Table 3).
Within the errors of the end-members, these calculations indicate that riverine carbon accounts for approximately ~90% of the carbon on the inner shelf, between
70 and 85% on the mid-shelf, and approximately 60% of carbon on the slope (Figure
7B; Table 4; Appendix 2). The addition of marine carbon across the shelf decreases the percentage of terrestrial carbon relative to the total (Table 4; Appendix 2).
Kerogen accounts for between 65 and 80% of the riverine carbon preserved on the margin, with an average of 72% (Table 4). Kerogen ranges from 0.3 to 0.45%C org depending on transect and depositional environment (Figure 7B; Table 4). Modern terrestrial carbon ranges from 0.11 to 0.20%C org (Figure 7). The primary exception to this occurs on the bathymetric high (on the southern transect; Table 4), where the
POC shows a reduction in rock carbon (0.21%C ker ) accompanied by a significant increase in marine carbon (0.30%C mar ). It is not known whether this reduced riverine carbon content is due to physical sorting, oxidation, or both. Physical sorting processes occurring on the shelf may account for the small deviations observed in the preserved terrestrial carbon concentrations and added marine carbon as was seen on the Waipaoa shelf (Brackley et al., 2009).
67 While serving as a first approximation of the reactivity of terrestrial source contributions (Kao et al., 2008), this assessment oversimplifies the Waiapu system, resulting in an overestimation of contributions from both modern and ancient terrestrial organic matter. It has been shown that in several small mountainous river watersheds such as the Fly (Alin et al., 2008) the Eel (Drenzek et al., 2009) and the
Waipaoa (Blair et al., 2009), aged soil organic carbon is delivered to the continental shelf. Sediment budgets suggest that while aged soils are not a major source of particulates to the Waiapu River, contributions are evidenced during large storm events that cause shallow landsliding (Page et al., 1994; Page et al., 1999; Page et al., 2001). Though the record of such storms have not been identified in the sampled horizons herein, flood layers that effectively bury organic matter to a sufficient depth to prevent rapid turnover have been identified by others on the
Waiapu shelf (Kniskern et al., 2009). Thus, some consideration is due to its contributions.
In systems such as the Fly River where aged soil is a dominant source, three
end-member mass balance equations (1), (2), and (3) can be solved twice,
substituting SOM for kerogen (as in Alin et al., 2008). In this river, these sequential
mixing models indicated that SOM does contribute to the system; however, kerogen
is required as a source to account for all of the carbon (Alin et al., 2008). The
considerable kerogen content of sediment in the Waiapu precludes the effective use
of this model.
68 Therefore, a rock and non-rock terrestrial source model has been developed
(Blair et al., 2009) to account for the unknown aged soil contribution to the river.
While the rock carbon end-member has been empirically determined, the non-rock riverine carbon must be estimated using isotopic mass balance equations as follows:
Eq. (4) F riv*c riv = F ker *c ker + F terr *c terr
Eq. (5) c riv = c ker + c terr
Eq. (6) F riv * criv = F terr * criv + c ker (F ker – F terr ) where F is the fraction modern and c is the %C for the riverine suspended sediment
(riv), the kerogen fraction (ker, rock), and the terrestrial, non-rock fraction (terr).
When F riv * c riv is plotted as a function of c riv , the linear relationship represents the
%C in the river that is added to the rock carbon and the slope of this line is the fraction modern of the riverine, non-rock carbon (Fterr ) (Blair et al., 2009).
Radiocarbon data is limited for the Waiapu; however, the radiocarbon signature of non-rock carbon for the adjacent Waipaoa has been determined to be -
136 ±109‰ (Blair et al., 2009). This represents a mixture of modern plant material with an aged soil organic matter signature of -355‰ ( ±100‰) at 83 cm deep in a soil profile (a depth near the extent of the reach of shallow landsliding, providing an upper limit of the age of soil delivered in this manner) (Blair et al., 2009). The stable carbon isotopic signature of the riverine, non-rock carbon can also be determined
13 this way, substituting δ C for F m. For the Waiapu, the resultant isotopic signature of the non-rock organic matter is -26.7‰ (r 2=0.998; n=5), reflecting a dominance of C3
69 plant input. This is comparable to the weighted average of all soil organic matter collected in the watershed (-26.9‰ ( ±1.3‰, n=58)).
Using these rock and non-rock end-members for the terrestrial carbon input to the seabed, the mass balance equations (1), (2), and (3) are again solved to determine the sensitivity of the mass balance solutions to the contributions of aged soil organic matter in this system (Table 4). Substituting non-rock riverine values for the entirely modern riverine values results in consistently lower mass balance solutions for the modern marine and rock carbon fractions. However, the solutions to the mass balance equations in the two models are within the range of uncertainty.
Although a 4% difference in rock carbon is not a large difference relative to the overall concentration (Table 4), the 7% difference in non-rock, riverine carbon does result in a 33% increase in this fraction relative to the entirely modern fraction. Thus, the source fractions for each core are determined by taking an average of the Table 5. Averages of the source fractions from mass balance solutions mass balance solutions from each using entirely modern and non-rock (aged soil inclusive) estimates. model (Table 5).
Depth (m) fmar fterr fker Radiocarbon analyses are cost 0 0.05 0.24 0.71 61 0.10 0.34 0.56 prohibitive. While we can only study 83 0.19 0.29 0.52 Northern 108 0.26 0.25 0.49 transect two transects thoroughly (Table 4; 128 0.22 0.28 0.51 615 0.40 0.29 0.31 Figure 7), if a single tracer can be used 5 0.16 0.23 0.61 Southern 39 0.26 0.29 0.45 transect to estimate the general behavior of 37 0.48 0.21 0.32
70 organic matter in the system, a more detailed view of carbon behavior across the
Waiapu continental shelf can be observed. Using only the stable carbon isotopic signatures, simultaneous equations (7) and (8) can be used to solve for the fractions of riverine and marine carbon in the POC:
13 Eq. (7) δ C = f mar δ mar + f riv δ riv
Eq. (8) 1 = f mar + f riv
13 where f x is the fraction of marine (mar) or riverine (riv) carbon and δ is the δ C end- member for each of these fractions. The averaged riverine signal incorporating ancient, aged, and modern carbon that is exported to the margin as determined by suspended and riverbed sediments acquired from the mouth of the river is -25.3‰
(±0.35‰, n=11). The 2 end-member mass balance models are less informative than
3 end-member models, not differentiating modern from ancient organic carbon sources. However, within the errors of the end-members, these calculations corroborate one another, indicating that the total amount of riverine carbon on the margin accounts for approximately 90% of the carbon on the inner shelf, between 70 and 85% on the mid-shelf, and 60 to 70% of carbon on the slope (Figure 7;
Appendix 2).
Therefore, samples across the entire margin have been partitioned into fractions of riverine and marine carbon (Figure 8). The rapid burial of sediment in the Waiapu margin (as much as 2.7 cm/yr; Addington et al., 2007; Kniskern, 2007;
Kuehl, 2007; Kniskern et al., 2009) promotes the conservation of terrestrial organic carbon across most of the margin, thereby preserving the source signatures of the
71 Figure 8. Organic matter partitioned by fractions of riverin e and marine carbon across the Waiapu margin. Terrestrial carbon dominates the composition of the shelf. Marine carbon is typically added to the terrestrial carbon with distance from the river mouth. Minimal marine carbon is added to the sediment on the inner shelf or to that buried in the region of highest accumulation (in pink). Accumulation rates for the color-coded regions are 2.7 cm/y (pink); 1.4 cm/y (green); and 0.7 cm/y (blue) as determined by Kniskern (2007). The size of the bar refers to the %C, for example, the bar in the legend corresponds to 0.44% C .
72 organic carbon transported through the watershed. Marine carbon is then added to it, as was observed in the 3 end-member model (Figure 7). The primary exception to this trend is on the bathymetric highs where hydrodynamic sorting and reworking is likely responsible for the overall loss of carbon (Figure 8).
Aside from the inner shelf, terrestrial organic carbon is more dominant with
increasing accumulation rate, indicating that riverine carbon is buried too rapidly to
acquire much marine carbon (Figure 8; Appendix 2). The most riverine carbon
preserved on the mid to outer shelf is located where sediment accumulates at 2.7
cm/y (pink region, Figure 8; Kniskern, 2007). The fraction of riverine carbon is
decreased in regions where accumulation rates are lower (e.g. green, 1.4 cm/y;
blue, 0.7 cm/y; Kniskern, 2007).
Nitrogen isotopes serve as an additional tracer to improve understanding of
sediment dynamics on the shelf and can be partitioned into fraction riverine and
fraction marine as done with stable carbon isotopes (Figure 9; Table 2). The riverine
N end-member, determined empirically from suspended and riverbed sediment, is
2.81 ± 0.24‰ (n=7). The marine N end-member is 5.34 ± 0.24‰ (r 2=0.93), determined by mass balance equations to equal the slope of δ15 N*%N as a function of %N (adapted from Aller et al., 2008). Like carbon, nitrogen isotopic signatures of sediment typically become more enriched in the heavier isotope from 3.5 to 5.5‰ with increasing distance from the river mouth (Figure 4). Enrichment in 15 N relative to 14 N is consistent with the addition to or replacement of terrestrial organic matter by marine organic matter as well as the result of denitrification. Nitrogen is also
73 Figure 9. Organic matter partitioned by fractions of riverine and marine nitrogen across the Waiapu margin. Marine nitrogen dominates the composition of the shelf; however, terrestrial nitrogen is observed nearshore and in the region of highest accumulation rates (in pink). Accumulation rates for the color-coded regions are 2.7 cm/y (pink); 1.4 cm/y (green); and 0.7 cm/y (blue) as determined by Kniskern (2007). The size of the bar refers to the %N, for example, the bar in the legend corresponds to 0.067% N.
impacted N 2 fixation and the large variations of isotopic signature based on land use
(forest vs. pasture, Table 1) (Bronk and Gilbert, 1994; Galloway et al., 2004).
Unlike the carbon apportionment, the organic matter on the shelf is predominantly marine, as the terrestrial nitrogen is rapidly turned over. However, the relationship between accumulation rate and fraction of riverine carbon holds for
74 nitrogen. The inner shelf reflects the input of terrestrial nitrogen from the river mouth. Terrestrial nitrogen is more dominant with increasing accumulation rate, indicating that riverine organic matter is buried rapidly enough to preserve the terrestrial signal rather than be replaced by marine nitrogen (Figure 9; Appendix 2).
The mid- to outer shelf preserves the most terrestrial nitrogen in the region where sediment accumulates at 2.7 cm/y (Figure 9, pink; Kniskern, 2007), while the fraction of terrestrial nitrogen is decreased in regions where accumulation rates are lower
(e.g. green, 1.4 cm/y; blue, 0.7 cm/y; Kniskern, 2007).
4.2 Resolution of terrestrial source identification
While the mass balance mixing models can partition organic matter by categories (i.e. riverine vs. marine; kerogen vs. non-rock terrestrial vs. marine), they do not resolve specific terrestrial sources (i.e. soil by type, rock formations). In contrast, multivariate statistics can be used as a qualitative assessment of individual identifiable organic matter sources (e.g. Giles et al., 2007). This enables the identification of sources actively eroding and contributing sediment to the river and margin.
Principal component analysis (PCA) is a multivariate statistical analysis that
can be used to resolve sources of organic matter that may be contributing to a
sample by identifying common trends or distributions in data based on statistical
associations (variance and covariance) (Shaw, 2003; Giles et al., 2007). PCA
75 enables classification of data according to the chemical similarity rather than pre- classification according to the nature of the source (Stout et al., 2003). The advantage of PCA is the ability to incorporate many variables (in this case isotopic and elemental compositions) that together create a complex and unique fingerprint of each source; whereas mixing models are limited to fewer variables that may not account for the variance of each source. Thus, possible sources of organic carbon in riverine and marine sediments can be identified with better resolution than cross
13 15 plots of individual analytes such as %C org vs. %N or δ C vs. δ N or mass balance models (e.g. Burns et al., 1997; Blair et al., 2003; Giles, 2007).
PCA generates new independent variables that are linear combinations of the original interrelated input variables (Stout et al., 2001; Shaw, 2003). Eigenvector decomposition mathematically reduces the dimensionality of a data set to a few important, uncorrelated principal components (PCs) that best describe variations in data (Stout et al., 2001; Shaw, 2003). Each PC accounts for a progressively smaller percentage of the variance within a data set (Shaw, 2003). If all of the variability between samples can be accounted for by a small number of PCs, then the relationships between multivariate samples can be assessed by simple inspection of
2 and 3 dimensional graphs.
The PC score plot is a graphical representation of the principal components determined by the PCA. When interpreting PC score plots, spatial relationships between samples are representative of their chemical relationships (Shaw, 2003;
Giles et al., 2007). Samples that plot close to one another tend to be chemically
76 similar; the further samples plot from one another, the more chemically distinct they tend to be (Stout et al., 2001). Samples that spread out along a continuum in a score plot indicate a chemical relationship with non-identical distributions (Stout et al., 2001; Shaw, 2003). For example, trends connecting end-member sources can indicate mixing (Stout et al., 2001). The underlying chemical cause for trends in a
PC score plot can be ascertained by examination of the factor loadings calculated for each input variable used in the PCA (Stout et al., 2001). Factor loadings can be visualized with a cross plot, revealing which variables are responsible for “pulling” particular samples in a given direction on a PC score plot (Stout et al., 2001).
The variation in scale for some parameters (‰ for isotopic values or % for carbon and nitrogen composition) requires normalization to compare data. These data have been normalized using a z-factor,
Eq. (1): z = x i – µ / s, where x i is the individual observation (a specific PAH concentration or isotopic
value), µ is the mean of all values for that data category, and s is the standard
deviation for that category. Without z-score normalization, results are biased to
variables with the largest numerical values. Z-scores leave the shape of the data
distribution unchanged, but re-scales each component axis to overall equivalent
units (Shaw, 2003).
A PCA model has been developed in the JMP 7.0 software by SAS using
complex chemical fingerprints of the isotopic and elemental compositions of rocks,
soils, and riverine and marine sediments from the Waiapu River sedimentary
77 system. The PCA score plot (Figure 10) reveals the chemical similarity between potential terrestrial organic matter sources throughout the watershed and those of riverine and marine sediments. Because the terrestrial organic matter has been shown to be relatively preserved across the shelf by mass balance equations, the particle-associated terrigenous organic matter delivered to and preserved in the marine environment will have similar chemical signatures and therefore be in close spatial proximity to the riverine and marine sediments in the PCA score plot.
Marine and riverine sediments are all in close spatial proximity of one
another, likely a consequence of homogenization of the potential sources during
delivery to the shelf. On the score plot, the sediments also appear in the middle of
many possible terrestrial sources, including the Cretaceous Whangai Fm.
mudstones, the riverbank alluvial soils, the Tertiary sandstones of the Wanstead
Fm., and some horizons of colluvial, tephric, and bedrock-derived soils (Figure 10).
Aside from discharge by gully stream directly into the river, rock is incorporated into
soils by weathering; thus, similar chemical composition of the rocks, sediments, and
most of the soils in the watershed is possible. The score plot suggests that the
contribution of rock carbon permeates through the Waiapu sedimentary system,
consistent with the results of mass balance calculations.
The factor loading plot is employed to understand the chemical composition
driving the distributions observed on the PC score plot. Detailed analysis of the
loading plot for the first two PCs for the concentration normalized PCA reveals that
%C org and %N pull principal components to the lower left (low organic matter
78 Rocks: Tertiary mudstone 4 Tertiary sandstone enriched δ13 C 15 high %C & %N Other rocks enriched δ N Cretaceous sandstone 2 Cretaceous mudstone
Soils: 0
■ Colluvial Prin2 ■ Tephric ■ TSLHT (alluvial) ■ YHT (alluvial) -2 ■ Bedrock weathered δ13 low %C & %N depleted C ■ Plant influenced topsoils depleted δ 15 N Thermogenic hydrocarbons -4 l River sediment -2 0 2 4 l Marine sediment Prin1
Figure 10. Principal component analysis of the Waiapu sedimentary system source to sink. Because terrestrial organic carbon is preserved mostly unaltered on the margin, sources of this POC can be resolved by chemical similarity between potential sources and riverine and marine sediment. Using %C, %N, δδδ13 C, and δδδ15 N, almost 85% of the total variance in the source to sink data is accounted for in the first two PCs (59% in PC1; 25.7% in PC2). The sources that are most chemically similar to the riverine and marine sediments are the Cretaceous Whangai Fm. mudstones, the riverbank alluvial soils, the Tertiary sandstones of the Wanstead Fm., and some horizons of colluvial, tephric, and bedrock -derived soils. content) and the upper right (high organic matter content) of the score plot. For example, the topsoils which are high in organic carbon and nitrogen are distinguished from other samples in the system (Figure 10). Isotopic composition pulls principal components to the opposite quadrants (samples enriched in 13 C and
15 N are found in the upper left quadrant; those depleted are found in the lower right quadrant). The skewed pattern observed in the riverine and marine sediments is a
79 result of isotopic enrichment with increasing distance from the river mouth concurrent with the addition of marine organic matter.
Because of the influence of rock carbon throughout the system, terrestrial sources are not well-resolved using only isotopic and elemental compositions.
Therefore, additional chemical parameters are desired to take full advantage of the capabilities of the principal component statistical analyses to incorporate multiple variables in order to develop complex organic matter fingerprints. Molecular level analyses, for example, could provide many additional chemical variables for each potential source of organic matter in the watershed to be compared with the riverine and marine sediment. However, this analysis does differentiate a few of the potential sources from the riverine and marine sediment. The soils distinguished along PC1 in Figure 10 are mostly either topsoils or tephric soils, which are likely to show negligible bedrock influence. The rocks of the Tertiary Waipawa Fm. are more isotopically enriched and the thermogenic hydrocarbons are more isotopically depleted in 13 C and 15 N than the other rocks, soils, and sediments. The chemical
distinction between these sources and the marine sediment suggests they are less
likely to contribute to the sediment load of the river.
4.3 Geologic implications of terrestrial source identification
Organic matter from individual rock and soil sources is delivered to the river in
unequal proportions because of varying intensity of geomorphic processes
80 throughout the watershed responsible for their transport (Gomez et al., 2003). The deforestation by Maori and European settlers has destabilized the land, increasing gullying and shallow landsliding in the basin (Hicks et al., 2000; Page et al., 2001).
The tendency of each profile to erode into the river and the mechanism by which the sediment is transported determines the degree to which it contributes to the sediment load and its isotopic signature.
For example, elevated erosion rates are known to occur under non-forested land in steep hill country on soft or fractured rock types (Page et al., 2004; Parkner et al., 2006). Tephric soils are relatively stable, unless on exposed areas already prone to erosion (Landcare, 2009). Intense episodic storm events can initiate widespread shallow landsliding and deliver soil-derived material to the continental shelf (Page et al., 1994; Page et al., 1999; Page et al., 2001). A profile that is susceptible to landsliding can deliver modern and aged soil/regolith that has an isotopic signature reflecting the organic matter from each horizon delivered, down to approximately 1 m depth (Page et al., 2004). Sheetwash would deliver the modern carbon associated with the topsoil of a profile preferentially, where the bulk of organic matter is concentrated, and is typically isotopically light. Therefore, variations of land use, topography, and stability (e.g. forested vs. pastoral, stable vs. actively eroding, terrace vs. hillside) were sought for the various soil types to determine how these factors may impact the geochemistry of the soil profiles.
Though terrestrial sources of carbon feeding the margin remain poorly resolved, PCA does reveal the relative similarity between the sediment and several
81 rock and soil sources. PCA does resolve some non-contributing organic matter sources, including the chemically distinct Tertiary mudstones and thermogenic hydrocarbons. Both PCA and mass balance source apportionment suggest that gully erosion of ancient rock carbon is the dominant geomorphic process delivering sediment to the Waiapu sedimentary system. The rocks are more consistently chemically similar to the sediment than the soils according to the PCA (Figure 10); mass balance solutions indicate that rock carbon accounts for greater than 50% of the organic carbon preserved on the shelf (Figure 7). This is in agreement with geomorphologic research (Page et al., 2001; Parkner et al., 2006) that gullies are the primary source of sediment in the watershed. About 900 active gullies cover more than 20% of the watershed (Page et al., 2001; Parkner et al., 2006). Similarly, in the neighboring Waipaoa River watershed, deep gully incision of rock carbon in excess of 3 m/yr (DeRose et al., 1998) vastly exceeds soil contributions from sheetwash and landslide processes (Gomez et al., 2003). Gully complexes produce sediment during normal flow conditions, regulating the suspended sediment and
POC yields, and larger storms capable of initiating landslides and earthflows simultaneously increase sediment output from gullies.
4.4 Particulate organic carbon dynamics on the continental margin
The primary control on particulates to the margin is the local riverine sediment supply (Wadman and McNinch, 2008). River plumes are most often hypopycnal;
82 that is, the freshwater from the river is less dense than the saline water into which it empties. The direction of hypopycnal river plumes and associated sediment deposition is primarily driven by winds. Depending on prevailing winds, buoyant river plumes typically hug the coast and flow either north or south (Wadman and
McNinch, 2008). Fine grained sediments are temporarily deposited near the river mouth and rapidly resuspended by energetic waves and currents, and then transported offshore (Harris and Wiberg, 2002; Wright et al., 2006; Addington et al.,
2007; Ma et al, 2008; Wadman and McNinch, 2008; Kniskern et al., 2009). This winnowing process coarsens the inner shelf, transporting the entrained fine sediment away from river mouth (Harris and Wiberg, 2002). The resultant enrichment in 13 C and 15 N cross shelf is likely due to the prolonged exposure of the sediment and associated riverine carbon to the water column and marine carbon as it moves across the shelf (e.g. Blair et al., 2003). The winnowing process also results in a loss of terrestrial carbon observed in the little organic carbon that is preserved with coarse sediment sequestered on bathymetric highs.
When the suspended sediment load exceeds the threshold for hyperpycnal flow (36 kg/m 3), a negatively buoyant plume may be initiated. The Waiapu River
watershed experiences flooding events several times a year that cause this to occur
(Hicks et al., 2000; Hicks et al., 2004; Wright et al., 2006; Addington, et al., 2007;
Wadman and McNinch, 2008; Kniskern et al., 2009). Sediment transported by
negatively buoyant flows is expected to follow bathymetry independently of the
waves and currents (Wadman and McNinch, 2008).
83 Seismic profiles identified the spatial variation of stratigraphy and antecedent geology on the Waiapu shelf (Addington et al., 2007; Wadman and McNinch, 2008).
The inner shelf is most commonly thought as an area of fine sediment bypassing and sediment segregation. However, it has been shown that on the Waiapu shelf, a paleobathymetric low extends offshore to the south of the river mouth following a paleochannel. There have been previous reports of relict geology influencing modern day coastal sedimentation (Browder and McNinch, 2006), which is observed on the Waiapu, where a modern day bathymetric low follows the paleochannel
(Wadman and McNinch, 2008). The fate of high concentration density driven flows is controlled via bathymetric steering and nearshore accumulation zones for terrestrial carbon and fine sediment that inhibit particle sorting (Addington et al.,
2007; Wadman and McNinch, 2008). In these bathymetric lows, mass balance models indicate that there is little contribution from marine carbon and nitrogen, as terrestrial organic matter is likely buried rapidly and preserved there from the negatively buoyant river plume (Figures 8 and 9; Wadman and McNinch, 2008).
As with source apportionment, principal component analysis was utilized to simultaneously examine the chemical and physical similarity of the marine sediment
13 15 composition using multiple parameters (%C org , %N, δ C, δ N, % clay, % fine to
moderate silt, and % coarse silt and sand; Figure 11). More than 90% of the total
variance in the dataset is accounted for in the first two principal components. PC1
accounts for 56.7% of the total variance of the data; PC2 accounts for 33.9% of the
variance. Factor loading plots reveal that the isotopic composition of the organic
84 matter pulls the principal components towards the upper (enriched in the heavier isotopes) and lower (depleted in the heavier isotope) halves of the score plot.
Sediment particle size pulls the principal components to the upper left (coarse silt to sand) or the lower right (clay to medium silt) quadrants.
Samples are color coded by 210 Pb sediment accumulation rates (Figure 11) to visualize the chemical similarity of sediment within these depositional regions
(Kniskern, 2007; Kuehl, 2007; Kniskern et al., 2009). The inner shelf ( ) is primarily
dominated by organic matter that is isotopically depleted in 13 C and 15 N feeding in
from the river mouth. The average particle size varies on the inner shelf. Coarse
sediment in this region is a consequence of hydrodynamic sorting after sediment is
temporarily deposited, resuspended, and transported offshore (Harris and Wiberg,
2002; Addington et al., 2007; Wadman and McNinch, 2008; Kniskern et al., 2009).
The fine sediment on the inner shelf occurs along the bathymetric low, where fine
sediment is trapped by rapid delivery and burial from hyperpycnal river plumes
(Wright et al., 2006; Wadman and McNinch, 2008).
The region of highest accumulation (2.7 cm/y) is the smallest of the
depositional areas, accounting for only 67 km 2 of the shelf (Kniskern, 2007).
Sediments in this region ( +) are dominated by fine sediment that is depleted in 13 C
and 15 N, indicative of terrestrial carbon (Figures 8, 9, and 11). As seen with the fine sediment preserved on the inner shelf, the physical and chemical composition of the sediment in this region is determined by rapid delivery and burial of riverine
85 3 enriched in 13 C and 15 N
2 coarse silt and sand Sediment 1 Accumulation
Rates (cm/y): 0 0 ■ 0.7 Prin2 fine to -1 □ 1.4 medium silt + 2.7 -2
-3 depleted in 13 C and 15 N clay
-5 -4 -3 -2 -1 0 1 2 Prin1
Figure 11. Principal componen t analysis of surficial marine sediment only using %C, %N, δδδ13 C, δδδ15 N, and sediment size fractions. More than 90% of the total variance in the data is accounted for in the first two PCs (56.7 and 33.9% for PC1 and 2, respectively). The sediment within each depositional region has a distinct particle size distribution and chemical signature.
sediment with minimal exposure to marine carbon (Figure 8; Wadman and McNinch,
2008).
Moving radially outward from the region of highest accumulation, the next
depositional region of 1.4 cm/y is largest region of the shelf (330 km 2) (Kniskern,
2007). Sediment in this region ( □) is predominantly fine sediments transported by
hydrodynamic sorting of inner shelf sediment (Harris and Wiberg, 2002; Wadman
and McNinch, 2008). The associated organic matter is a mixture of isotopic
compositions, suggesting the addition of marine to riverine carbon (Figure 8, 9, and
11).
86 The fourth region on the Waiapu shelf, the most remote from the river mouth to the north, south, and east, accumulates sediment at a rate of 0.7 cm/y and accounts for 217 km 2 (Figure 3, 4, 8, and 9; Kniskern, 2007). The sediment in this region ( ■) is the most isotopically enriched in 13 C and 15 N, indicating the addition of marine carbon to the organic matter which has had the most exposure to the water column (Figure 8, 9, and 11). Sediment is both fine and coarse grained in this region (Figure 6 and 11). Fine sediment is the consequence of hydrodynamic sorting of sediment from the inner shelf that is transported offshore and deposited here (Harris and Wiberg, 2002; Wadman and McNinch, 2008). The coarse sediment occurs on a bathymetric high ( ), where the sediment is mixed and resuspended,
causing the winnowing away of the fine particulates and leaving behind the coarse
sediment (Figure 6). The samples in this region have obvious carbon losses,
indicating that the marine carbon is not only added to but also replaces the terrestrial
carbon (Figure 8).
Minimal bioturbation is observed, especially in the high accumulation zones
indicating that physical processes, such as hydrodynamic sorting and rapid burial by
negatively buoyant plumes, dominate shelf behavior (Kniskern, 2007). Increased
frequency of bioturbation and decreased frequency of lamination radially away from
the river mouth indicates that riverine input rather than wave energy controls
sediment mixing patterns (Kniskern, 2007). The knowledge of the physical
oceanographic domain of the Waiapu margin facilitates the interpretation of the
chemical composition of organic matter preserved on the shelf. In turn,
87 understanding the chemical compositions of sediment in terms of the geophysical parameters that dictate shelf behavior can improve interpretations of paleo- environmental processes using the sedimentary organic geochemical record.
5. Conclusions
The large contribution of small mountainous rivers to the global sediment supply has motivated the investigation of sediment transport and burial processes in these systems. A stable carbon and nitrogen isotopic and elemental analysis of potential terrestrial organic carbon sources in the Waiapu River watershed has provided unique chemical fingerprints to be used in source apportionment of sediment in the adjacent continental margin. The extensive range of potential sources of terrestrial organic matter have heterogeneous isotopic signatures ( δ13 C,
∆14 C, and δ15 N) that are incorporated in riverine sediment and transported through
the watershed to the ocean. Once sediment is delivered to the continental margin,
stable carbon and nitrogen isotopic signatures of marine sediment typically become
more enriched in the heavier isotope with increasing distance from the river mouth
(from -25 to -22‰ for δ13 C; from 3.5 to 5.5‰ for δ15 N).
Mixing models indicate that the Waiapu River delivers terrestrial carbon that is
typically conserved; therefore, the isotopic shift towards more enriched carbon and
nitrogen signatures is likely the result of an addition of rather than replacement by
marine carbon, except in areas of sediment reworking such as bathymetric highs.
88 Because terrestrial carbon is preserved relatively unaltered on the margin, these sources of organic matter to the marine environment can be identified and quantified using carbon and nitrogen isotopic signatures. Terrestrial carbon accounts for >90% of carbon on the inner shelf, between 70 and 85% on the mid-shelf, and 60-70% of carbon on the slope. Of this terrestrial carbon, kerogen appears to account for between 65 and 80%. These models are unable to account for the contribution of aged soil material or to identify specific soil or rock types.
Multivariate statistics provide another means of source apportionment, comparing the complete elemental and isotopic composition of each potential terrestrial source with that of the riverine and marine sediment. Because the terrestrial carbon is conserved, specific sources of this carbon are determined by chemical similarity to the sediment. Principal component analysis has been used to characterize the specific terrestrial sources of sediment and associated particulate organic carbon in the Waiapu system further than the capability of mass balance equations. Though still not well-resolved, river and marine sediments are most chemically similar to Cretaceous Whangai Fm. mudstones, riverbank alluvial soils,
Tertiary Wanstead Fm. sandstones, and some horizons of colluvial, tephric, and bedrock-derived soils. The narrow range of values for each isotopic tracer used to distinguish terrestrial sources complicates the apportionment, even with multivariate statistics. Because PCA can accommodate multiple chemical fingerprints for each source, the development of additional tracers such as molecular level biomarkers will further the investigation of organic carbon from source to sink.
89 PCA is a qualitative assessment and cannot be used to quantify unique source contributions. Thus, the combined use of mass balance mixing models with
PCA best describes the terrestrial organic matter delivered to the continental margin.
Both mass balance and PCA indicate that gully erosion of ancient rock carbon is the dominant geomorphic process delivering sediment to the Waiapu sedimentary system.
Spatial geochemical patterns on the margin appear to be complicated by structural deformation of the shelf and its influence on marine sediment dynamics.
Bathymetric features appear to direct sedimentation across the margin, affecting particle size and geochemistry. Sediment on bathymetric highs tends to be coarser and poorer in organic carbon, suggesting the effects of winnowing by currents and waves. Sediment in bathymetric lows, on the other hand, are enriched in terrestrial carbon, perhaps the result of more rapid rates of accumulation and reduced exposure to the water column. PCA of marine sediment elemental, isotopic and size data indicates that the chemical composition of marine sediment corresponds to the regions defined by sediment accumulation rates. Organic matter of terrestrial origin dominates sediment on the inner shelf near the river mouth and in the region of highest sediment accumulation rate. The remainder of the mid- to outer shelf is dominated by marine carbon. Fine sediment is primarily found on the outer shelf due to fining processes and in the highest sediment accumulation zone where burial occurs too rapidly to be sorted and transported offshore. The spatial distributions of sediment particle size and chemical composition are better understood in context of
90 the known physical forcings on the shelf, and consequently, can be used to improve interpretations of paleo-marine environments.
6. Acknowledgements
The NSF project “Age Distribution of Particulate Organic Carbon (POC)
Discharged from Small Mountainous Rivers- the Influence of Sediment Yield and
Soil Residence Time” (EAR- 0222584, Leithold and Blair) provided the primary support for this study. Marine samples were obtained in 2004 on the R/V Kilo
Moana, with special thanks due to Steve Kuehl and Jesse McNinch who allowed us to participate in their research cruise. The field collection of rock and soil samples in
2007 was sponsored by the National Science Foundation East Asia and Pacific Rim
Summer Institute program which was co-sponsored by the Royal Society of New
Zealand and Massey University, Institute of Natural Resources, Fertilizer and Lime
Research Centre in Palmerston North, New Zealand. The National Science
Foundation project “Source to Sink generation of biogeochemical stratigraphic signals across the Waipaoa margin, New Zealand” (OCE-0646159, Leithold and
Blair) also supplemented this work.
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110 8. Appendices
111 8.1 Complete terrestrial sample descriptions
8.1.1 Bedrock in the Waiapu River watershed
Tertiary Bedrock
The Tertiary gully sampled is locally referred to as Barton’s Gully (BG).
Barton’s Gully is along the Tapuaeroa River, approximately 3 km from the road along a true left tributary. The elevation is 197 m at the base of the gully.
Vegetation around the gully includes kanuka, toitoi, tutu, kahitatea, cabbage tree, sedges, rough grasses, and weeds. Mudballs that accumulate and encase pebbles are located all around the floor of the gully.
In this gully, seven samples were obtained, two mudstones and five sandstones, corresponding to the three bedrock formations present (Mazengarb and
Speden, 2000). BG Jars 1 & 2 and BG Rock 3A are samples of the primary rock being eroded from the gully, the Wanstead Formation, which is a pale grey-green calcareous and non-calcareous smectitic mudstone with intercalated beds of glauconitic and lithic sandstones. BG Rock 2 is a green glauconitic sandstone with authigenic mica formed on the seafloor from the intercalated beds of the Wanstead.
These samples comprise the group of “Tertiary sandstones” that will be used in statistical modeling. The Waipawa Fm., a poorly bedded, grey to black, non- calcareous mudstone, is the other major bedrock formation in the Tertiary gully,
112 corresponding to samples BG Jar 1 and BG Rock 3B, which were both dark black, shiny mudstones. These samples comprise the “Tertiary mudstone” category that will be referred to in statistical analysis.
Cretaceous Bedrock
The Cretaceous Gully is part of the Cretaceous Allochthon. The stratigraphy
is broken up because it is sheered. There are some volcanic rocks present on the
gully floor, including tischnite and chert coming from the headwaters. Vegetation in
the area includes toetoe, tusik grass, paspalun, kanuka, cock’s foot, and various
weeds.
In the Cretaceous Gully (CG, known locally as the Blue Gully), six samples
were obtained from the four bedrock formations present (Mazengarb and Speden,
2000). The mudstone samples (CG Jar 5 and Rock 1, “Cretaceous mudstones”) are
both argillites from the Whangai Fm., which is comprised of a non-calcareous and
calcareous, micaceous, and siliceous shale and mudstone, and are the primary
rocks eroding into the river.
The sandstones are from three bedrock formations: the Mokoiwi, Tolaga, and
Weber. CG Jars 1 and 2 are both samples of the Mokoiwi Fm., an alternating
centimeter to decimeter bedded, fine to medium grain sandstone. Both samples are
rippled sandstones, with centimeter to decimeter bedded fine sandstone and argillite
laid down during weak turbidity flows (underwater landslides). CG Jar 3 contains the
113 Tolaga Group, a minor component of the gully, with massive bedded, slightly calcareous, sand and mudstones. This thick bedded, massive sandstone sample is much more competent (harder) than samples from the Mokoiwi Fm. CG Jar 4 is the
Weber Fm., a minor Oligocene contribution to this gully, which is comprised of poorly bedded, pale grey calcareous mudstone with thin glauconitic sandstone beds. This sample is a part of the mélange; the glauconitic sandstone is a minor component but does contribute to the gully sediment load.
Other (non-gully) bedrock samples
A few other bedrock samples from the watershed were obtained that were directly associated with specific soil profiles (Mazengarb and Speden, 2000). LW Br, the bedrock associated with the Landslide Waiapu soil profile, is part of the Whangai
Fm. previously described. MS Br, the bedrock associated with the Mata Slip soil profile, is from the Tikihore Fm., a Cretaceous, fossiliferous centimeter to meter bedded alternating fine grained sandstone and mudstone. W1 Rock, the rock associated with the soil profile taken from the W1 terrace, is the Whakai Fm., a well- bedded alternating sandstone and mudstone. Because this tephric soil profile was obtained atop an alluvial terrace base, the rock was not collected in place, therefore, this is a tentative assignment based on rock characteristics.
114 8.1.2 Bedrock-derived forested soils
Forested sites are comprised of organic soils, which are formed in the partly decomposed remains of forest litter (Landcare, 2009). The soil is dominated by organic matter. These types of soils account for only about 1% of New Zealand
(Landcare, 2009). These soils have high cation exchange capacities, are typically strongly acidic, and nutrient deficient. High C/N ratios represent slow decomposition rates (Landcare, 2009; Chapter 2, Table 1 and Figure 5).
Forest Soil (FS; forested, bedrock-derived soil)
A forest-covered bedrock-derived soil profile was sampled just upstream of the Cretaceous Gully. At 140 m elevation and approximately 50 m higher than the floor of the riverbed, the sampled site is on the shoulder of a spur at a slope of 27 degrees with a direction of 038 (NE). The forest is about 50% bare ground, with dominant kanuka commonly between 20 and 30 cm in diameter and approximately
200 to 300 years old. This soil has been stable for the life of the trees, but probably not much more than that given the absence of older vegetation. Other vegetation includes mapu, mahooe, myrcenae divericata, kawakawa, sedge, moss, and ferns on the forest floor.
Leaf litter was noted from 1 to 0 cm. The A h horizon descends from 0 to 4 cm. There was common leaf litter and many fine roots in the dark brown and black,
115 humus-rich silty loam. This soil is very friable, non-sticky, and non-plastic with strong coarse crumb structure and a distinct boundary. The A h2 horizon extends to
18 cm depth. This friable, slightly sticky, slightly plastic, very dark brown silty clay loam has moderate fine block structure, common fine roots, and few medium roots with a distinct boundary. The B g layer, from 18 to 36 cm, is dark grey, slightly firm,
sticky, and plastic. This silty clay loam has weak fine block structure, common fine
roots, few medium roots, few organic coatings on roots, and few fragments of
sandstone <1 cm in size and ranging from soft to hard. A gradational boundary
separates the B g from the C g horizon. The grey, gritty, silty clay loam in the C g horizon is firm, sticky, and slightly plastic with weak blocky structure and few medium roots. Increasing quantities of dark grey sandstone and argillite with depth were very angular, soft to very hard, and mostly <5 cm. This profile is built upon the same bedrock being weathered from the Cretaceous Gully, the Whangai Fm.
Samples were only collected from the A h horizon (0-4 cm) and the A h2 horizon
(10-15 cm).
Barton’s Mouth (BM; forested, bedrock-derived soil)
A soil sample was taken on the true left bank of the stream upstream of the mouth of Barton’s Gully. At an elevation of 167 m, the sample site was 15 to 20 m above the valley floor, with a slope of 38 degrees with an aspect of 224. The soil appears to have been stable for some time. The sample was taken uphill in a
116 heavily forested area of pine trees estimated to be about 50 to 100 years old. This area was also a patch of native forest with titoki, kawakawa, mire, tawa (estimated at
>100 yr), myrsine divaricata, ferns, weakly scattered grasses, mahoe, and kanuka.
There is topsoil from 1 to 0 cm containing leaf litter. The A h horizon,
extending from 0 to 5 cm is a black, non-plastic, non-sticky, very friable, gritty sandy
loam with moderate medium crumb structure, many fine roots, and common leaf
fragments. A gradational boundary separates this layer from the B w horizon. From 5 to 13 cm deep, a dark brown, non-sticky, non-plastic, friable, gritty silt loam has weak fine nut and strong coarse crumb structure. There are common fragments of rusty weathered bedrock mostly <1 cm present in this horizon. A gradational boundary lies between the B w horizon and the B r horizon, of which, 50% is the soil as described in the B w horizon, and 50% is bedrock. This bedrock is a dark grey
siltstone weathering to reddish-brown and brown. Below 25 cm is moderately hard
bedrock which can be sheared by a knife with some difficulty.
Samples were taken of the A h horizon (0-5 cm) and the B w horizon (5-10 cm).
The bedrock is the same primary bedrock in Barton’s Gully, the Wanstead Fm.
117 8.1.3 Bedrock-derived pastoral soils
Burdett’s Waiapu Bridge Overlook (BWBO, bedrock weathering)
A soil profile was taken at 95 m elevation from the Burdett’s Farm overlooking the bridge at Piggery Rd. crossing the Waiapu River. The soil profile was taken from the hillside with slope of 29 degrees at 345 degrees. The land is used for cattle and sheep pasture, therefore abundant dung and plant litter are present all over the hillside. Vegetation in the area includes paspalun, brown top, chickweed, a bit of clover, yarro, and moss. The ground is approximately 40% bare with severe sheet erosion of the slope exacerbated by wind. The dark grey siltstone bedrock is exposed in places. The soil dries out during the summer, so marks of dung or topsoil in subsoil samples is not contamination, but rather a natural process that causes cracks to fill with topsoil (Palmer, 2007).
The A p horizon extends from 0 to 5 cm. The dark greyish brown, gritty silt loam is friable, non-sticky, and slightly plastic with moderately developed coarse crumb structure and many fine roots. It is made up of approximately 20% angular fragments of unweathered siltstone mostly <1 cm. A sharp wavy boundary separates the A p from the B w horizon, extending to 11 cm. The dark yellowish brown gritty silt loam is friable, non-sticky, and non-plastic with weak fine nut and moderate coarse crumb structure and many fine roots. This layer contains 30% fine unweathered siltstone fragments which are very angular and mostly <2 cm in size.
118 There are a few strong brown mottles and staining around the siltstone fragments. A gradational boundary divides the B w from the B r horizon that reaches to 29 cm. The
Br horizon is a 50/50 mixture of unweathered siltstone fragments and a gritty, sandy
loam matrix. The matrix is a 50/50 mixture of strong brown and yellowish brown,
coarsely mottled, weathered bedrock with fragments of the unweathered siltstone.
The horizon is loose, non-sticky, and non-plastic with weakly developed crumb
structure. There are many fine roots and a gradational boundary. The R horizon is
dark grey to dark greyish brown weathered bedrock with strong brown iron staining
along closely spaced joints and a few fine roots. The rock is easy to dislodge with a
knife. The rocks break into very angular fragments up to 5 cm long but mostly very
fine.
One sample was taken from each horizon: the A p horizon (0-5cm), B w (5-
10cm), B r (11-29cm), and R (45-55cm).
Landslide Waiapu (LW, weathered bedrock soil)
Halfway between Tikitiki and Ruatoria, a soil profile was collected on the
Waitoa farms. The land is particularly prone to landsliding, so the sampling was done at the top of a steep slope (38 degrees, 118 m elevation) at the headwall of a landslide on the shoulder rolling over to the backslope of the hill. The landslide sampled was approximately 50 m across. The land is used as pasture, and there was abundant sheep dung. Abundant areas of bare ground showing between
119 grasses across the pasture gave evidence to sheet erosion. Vegetation in the area includes brown top, indicative of a low producing pasture, and kikuyu, paspallun, subterranean clover, yarro, and scattered kanuka.
The soil is a brown soil which is dominant in the hill country. The A p horizon descends to 8 cm. The very dark greyish brown silty clay loam is friable, slightly sticky, and plastic with a moderately developed fine nut and coarse crumb structure.
There are abundant fine roots and a diffuse wavy boundary. The AB w horizon, which extends to 19 cm, is a yellowish brown silty clay loam with 20% very dark greyish brown worm mottles. This horizon is a friable, slightly sticky, and plastic with moderately developed fine nut and coarse crumb structure. There are abundant fine roots, and 5% subangular siltstone fragments mostly <2 cm with a rusty coating and a grey interior. A diffuse boundary separates the AB w from the B w1 horizon,
extending to 55 cm. This brownish-yellow silty clay loam is friable, sticky, plastic,
and moderately developed, with a fine and medium nut and coarse crumb structure,
and 10% siltstone fragments as described in the AB w horizon. A diffuse boundary
separates the B w1 from the B w2 horizon, which reaches to 81 cm. This yellow silty
clay loam is friable, sticky, and plastic, with weak fine blocky structure and 20%
siltstone fragments as above. A sharp wavy boundary separates the B w2 from the
2B w horizon, from 81-92 cm. This layer is composed of 70% subangular siltstone
clasts with a brown exterior and a grey interior. They are commonly 5-10 cm across
the long axis, and are not in place, but rather from upslope. A yellow gritty silty clay
loam (as in the B w2 horizon) matrix encases these rocks. A sharp wavy boundary
120 separates this layer from the weathered bedrock (B r). The bedrock is a grey
siltstone with a yellow matrix.
One sample was taken from each horizon: the A p (0-8 cm), the AB w (8-19
cm), the B w1 (30-40 cm), the B w2 (60-70 cm), the 2B w (81-92 cm), and the B r layer
(bedrock).
8.1.4 Tephric soils
Tephric soils are composed in part of allophane, imogolite, and ferrihydrite minerals from volcanic tephra and weathered sandstones (Landcare, 2009). These minerals coat the sand and silt grains, giving the soils a distinctly greasy feel when moistened and rubbed firmly between the fingers (Landcare, 2009). They occur predominantly in the North Island volcanic ash, and in the weathering products of other volcanic rocks, covering approximately 5% of New Zealand (Landcare, 2009).
Some gleying occurs in these soils, where waterlogging caused by the affinity of allophane for water chemically reduces the soils. This is recognized by light grey subsoils, usually with reddish brown or brown mottles (Landcare, 2009).
W1 Terrace (W1; airfall tephra; stable pasture)
The W1 terrace sample was taken along the Mangaoporo River at an
elevation of approximately 125 m. Approximately 200 m west of the road, the
121 sample site is greater than a kilometer from the river, sloping 4 degrees at 195 degress altitude. The sampled soil is allophanic (andesol), which bonds well to organic matter (Palmer, 2007; Landcare, 2009). It is a well drained soil, but the allophane present holds a lot of water. Vegetation covering the land includes brown top, white clover, Yorkshire fog, plantain, yarro, and rushes.
This terrace represents erosion surfaces cut in bedrock by marine processes, overlain by younger tephras, and subsequently raised by tectonic uplift (Mazengarb and Speden, 2000). The terrace was constructed by the Mangaoporo River during the height of the last glacial maximum, between 30 and 18 ka before present (Eden et al., 2001). At the end of this period, the climate warmed substantially, but during the terrace formation, the temperature was 5 to 7 degrees cooler than present, with a lot of frost (no glaciers). There were strong westerly winds moving north, and very unstable conditions. There was no vegetative cover, massive erosion and massive aggradation (Eden et al., 2001). By clearing the forests and converting the land to pasture, these glacial conditions are being recreated without the colder temperature
(Palmer, 2007).
The W1 terrace is one of the most stable sites in the current landscape. If
cultivated as on this site, wind erosion may be a problem, like in the W3 terrace
which is the same type of soil with more exposure (Palmer, 2007). The terrace
slopes to the south. The sample was taken from a rough pasture with low fertility.
Due to the neglected upkeep, the land which could be high producing is not because
of the way it is being farmed with very low to no rates of fertilizer application. The
122 upper meter of the land is airfall tephra supplied from Rotorua (rhyolite), Taupo
(rhyolite), and Ruapeho (andesitic) (Eden et al., 2001). The land was cleared by several cycles of Maori deforestation and also burnt scrub from Europeans. The sampling site is about 200 m upwind of the road, thus vehicle emissions are blown away from the sampling site. In the other direction, only open pastoral land can be seen.
The A p horizon, developed for pasture stocking (grass cover and ploughed), spanned the top 24 cm. This horizon is a non-sticky, non-plastic, very friable black sandy loam with moderate structure, medium nut and strong coarse crumb. There were also many fine roots, abundant near the surface. A gradual boundary separates the A p horizon from the AB w transitional horizon, which is 50% black and
50% brown in color with normal weathering evident. It is a non-sticky, non-plastic, very friable sandy loam with a moderate fine nut and strong coarse crumb ranging from 24 to 35 cm deep. There are many fine roots throughout and a gradual boundary. The B w horizon extends from 35 to 60 cm. This brown to strong brown
sandy loam has a greasy feel typical for allophane. It is non-sticky, slightly plastic,
and very friable with strong medium crumb structure, few fine roots, and a gradual
boundary. Black mottling extends down the root channels in this horizon. The B w2 horizon from 60 to 96 cm has slightly different coloring, but is still greasy, non-sticky, slightly plastic, and friable, with weak fine blocky structure breaking to moderate medium crumb. The last horizon sampled extends down from 96 cm. This 2C horizon is made up of a different soil material, which is pale brown to light yellowish
123 brown in color. This silty clay loam is sticky, plastic, soft, and massive, with no roots but a few root channels, and weeps water.
Three samples (0-5, 5-10, and 15-20 cm) were taken from the A p horizon.
One sample each was taken from the AB w (30-35 cm), B w (45-50 cm), B w2 (75-80 cm), and 2C (95-100 cm) horizons. A rock sample was also acquired just over the edge of the paddock at an open cliff face has been tentatively described as the
Whakai Formation (Mazengarb and Speden, 2000).
Mata Slip (MS; tephric; prone to landslips)
A soil profile was taken from the Mata Slip on Horehore Rd along the Mata
River at 157 m elevation. The soil profile was taken from the hillside with slope of 38 degrees at 105 degrees. The hillside is behind a middle to early Holocene terrace.
The terrace has a tephric cover, principally the Waimahia tephra. Therefore, the hillside was eroding when the river was forming the terrace sometime prior to the
Waimahia tephra. The Waimahia accumulated in places on the hillside and soil has formed, probably with bush cover. Now that the bush has been removed and land farmed, soil slips are steadily removing the soil mantle. There is a lot of evidence of past slips and a few recent ones. The soil was sampled at the headwall of a recent small slip approximately 5 m across on a mid-slope position.
Blocks of bedded sandstone are seen all over the hillslope, exposed by landslips. The land is used for cattle and sheep pasture with abundant dung over
124 the hillside. Also on the hillside are several tunnel gullies in the footslope colluvium
(tunnel gullies cannot form into bedrock) (Lynn and Eyles, 1984). Livestock on the hillside fall into these gullies and die because they are unable to get back out, and were likely hurt when they fell in. Vegetation in the area includes brown top,
Yorkshire fog, paspalun, white clover, buttercup, scattered sweet briar, rushes, and kanuka.
The A p horizon extends from 0 to 19 cm. The very dark grey, silt loam is
friable, non-sticky, and non-plastic with moderately developed fine nut and coarse
crumb structure and many fine roots. It is made up of approximately 5% rusty brown
to pale yellow, angular weathered sandstone clasts. A gradational boundary
separates the A p from the AB w horizon, extending to 27 cm. This transitional horizon is a 50% very dark grey and 50% dark yellowish brown, fine sandy loam, which is slightly firm, non-sticky, and slightly plastic, with weak fine nut and block structure and many fine roots. There are also a few weathered sandstone fragments up to 10 cm across, as in the A p horizon. There is a gradational boundary between AB w and
Bw. The B w horizon extends from 27 to 52 cm. The yellowish brown sandy loam is tephric, slightly firm, non-sticky, and slightly plastic with weak fine nut and medium crumb structure. There are a few weathered sandstone clasts as above, with a few fine roots, and a gradational wavy boundary. The 2B w horizon is the Waimahia tephra, from 52 to 62 cm. This layer is 3400 14 C-years in age. It is a yellowish brown gritty sandy loam. The grit is pumice lapilli (2 to 64 µm in size), and the <2
µm fraction is ash. This layer has a weak medium crumb structure with a few fine
125 roots and a sharp boundary. The 3C horizon extends to 113 cm. The brown to light olive brown sandy clay loam is firm, slightly sticky, and plastic with no structure and no roots. There is approximately 20% large, slightly to moderately weathered, fine sandstone clasts up to boulder size, but angular with olive grey interiors. The matrix has a few light grey and strong brown coarse mottles. There is a sharp contact to bedrock. The R horizon, which extends below 113 cm, is a decimeter bedded, fine sandstone bedrock.
Two samples were taken from the A p horizon (0-5 and 10-15 cm). One sample was taken from each of the other horizons: the AB w horizon (20-25 cm), B w
(40-50 cm), 2B w (52-62 cm), 3C (80-90 cm), and R (bedrock).
8.1.5 Colluvial soils
Colluvial soils are made up of sediment that has been eroded from hillslopes and accumulates at the bottom of a steep or shallow slope or a barrier on a slope
(Marden et al., 2008). Gravity transports soils disturbed by sheetwash, landsliding, or other means of erosion to these depositional areas, often in colluvial fan deposits
(Marden et al., 2008). These soils are comprised of mixtures of soils on the slopes surrounding them: tephric or bedrock-derived, forested or pastoral, transported by sheetwash or landslides, etc.
126 Manuel Terrace (colluvial mixture of tephric and bedrock soils; pastoral)
Taken from the oldest Holocene terrace, this tephric soil profile was obtained from the all-organic Manuel Farm, which has received no fertilizers. This is a Brown soil with some gleying with imperfect drainage. The sample site is located approximately 300 m west of State Highway 35, and approximately 3 km southwest of Tikitiki. It sits on a high flat Holocene terrace 40 m west of the terrace riser down to the next Holocene terrace on which the road sits. There is also a stream 40 m east of the sample site. There is maize upstream from the sample site.
The younger Holocene terrace next to the road is covered with tawa and titoki. Rushes lie west of the sample site extending about 1 km, at which point, kohikitia appears at the base of the W1 terrace and an area that appears to be the
W3 terrace. An area of Ngati Porou forest of kanuka and pine tress separates these regions. The W3 terrace shows evidence of sheet erosion and possible landslips.
The Ngati Porou forest appears to be covering a naturally healed gully system.
The soil profile is an aggrading profile, still upbuilding. Each layer would have
been topsoil at one time. The carbon has been oxidized and buried. In addition to
tephric airfall, particularly the Waimahia tephra, some sediment would have come
from the W1 terrace and forested gully above. Vegetation on the sampled terrace
includes paspalun, plantain, brown top, rats tail, yarro, minor clover, and a few
rushes. The grasses show signs of phosphate deficiency. Manure is all around as
127 the area is used for lax cattle grazing, however, it is poor pasture due to the lack of fertilization. There is no discernible slope.
The tephric A p horizon descends to 18 cm. The brown fine sandy loam is
friable, non-sticky, and slightly plastic with a moderate medium nut breaking to
medium coarse crumb. There are abundant fine roots and scattered charcoal
especially towards the base. Charcoal gets deep in the soil profile because burned
shrubs get ploughed under. A diffuse boundary separates this layer from the AB w
horizon, which extends to 26 cm. This horizon is 50% dark greyish brown and 50%
yellowish brown. There is distinct worm mottling in this tephric, fine sandy loam. It
is friable, slightly sticky, and slightly plastic with moderate medium nut breaking to
medium coarse crumb and common fine roots. A diffuse boundary leads to the B w horizon which reaches 40 cm in the profile. The slightly firm, sticky, very plastic silty clay loam is brown to yellowish-brown. It has moderate fine blocky structure with few fine roots and few fine faint grey and orange mottles towards the base. A gradual bound separates the B w horizon from the B g horizon. The B g horizon is 50%
light yellowish brown, 30% light grey, and 20% brownish yellow. There are medium
distinct mottles in this firm, sticky, plastic silty clay loam with strong fine blocky
structure and few fine roots. There are also pale yellowish brown clay coatings on
peds and root channels. A gradual boundary at 56 cm separates the B g from B g2 horizon. Extending to 82 cm depth, this soft, slightly sticky, plastic sandy clay loam is 60% light grey and 40% reddish-yellow with prominent medium mottles. The B g2 layer has a weak medium block structure, few fine roots, and few root channels lined
128 with dark reddish-brown coatings. A sharp, undulating boundary due to groundwater effects separates the B g2 from the B g3 horizon. The B g3 horizon is the deepest horizon sampled in this profile, extending past the 100 cm depth cleared for the profile. The firm, very sticky, very plastic clay is 70% light grey and 30% reddish- yellow with medium prominent mottles, weak medium blocky structure, a few root channels, and a few fine roots.
Two samples were taken from the A p horizon (0-5cm and 13-18 cm), and one sample was taken from each of the other horizons: AB w (20-25 cm), B w (30-35 cm),
Bg (45-50 cm), B g2 (70-75 cm), and B g3 (85-90 cm).
Tutumatai Station (TS; colluvial pasture)
A soil profile was taken from the Tutumatai Station at 88 m elevation. The soil site is approximately 150-200 m from the road and ~0.5 km from the Mangaoporo
River. The soil profile was taken from an exposed side of a flat terrace in the pasture, cleared for sampling.
The land is on a colluvial terrace fan, where part of the material present comes from the hills, part is locally formed from bedrock weathering, and part came from the Mangaoporo River. It is an early Holocene terrace, which is reasonably fertile, and has likely seen fertilizer. The land is used for cattle and sheep pasture with abundant dung all over the terrace. Vegetation in the area includes rye grass,
129 paspalun, brown top, white clover, plantain, and a few rushes. Nearby the pasture are cypress, willow, and poplar trees.
The A p horizon extends from 0 to 19 cm. The very dark greyish brown silty loam is friable, slightly sticky, and slightly plastic with moderately developed fine to medium nut and granule structure breaking to crumb. There are abundant fine roots and a gradational boundary. The AB w horizon reaches to 27 cm, and is a 50/50
mixture of the above very dark greyish brown silty loam and the yellowish brown B w
horizon described below with medium distinct worm mottles. It is friable, slightly
sticky, and plastic with moderately developed fine to medium nut and crumb
structure, many fine roots, and a gradational wavy boundary. The B w horizon (27-55 cm) is a firm, yellowish brown, slightly sticky, plastic, silty clay loam with moderate medium and coarse blocky structure and many fine roots. There are also a few light grey and strong brown fine distinct mottles above a sharp wavy boundary with a thin discontinuous iron pan that is <2 mm. The 2C horizon reaches 72 cm. There is not much structure to this layer, as it is gravel rather than silt. It is a fine gravel with a dark greyish brown sandy matrix. The gravels are mostly <1 cm, subangular to subrounded, and weakly weathered with a few iron coatings. A distinct wavy boundary separates the 2C horizon from the 3C g horizon from 72 to 107 cm. The
3C g horizon is a blend of 60% yellowish brown, 30% light yellowish brown, and 10%
yellow sandy clay loam. There are coarse faint mottles becoming more distinct
towards the base. It is firm, slightly sticky, and plastic with a few pebbles particularly
in the middle of the horizon and a few roots. It is mostly structureless, but vertically
130 cracks into coarse columns running through it to a sharp boundary. The 4C horizon extending beyond 107 cm is a dark greyish brown sandy gravel with few fine roots and unweathered pebbles, many having rusty iron coatings.
Two samples were taken from the A p horizon (0-5 and 10-15 cm), and one sample was taken from each of the following horizons: B w (30-40 cm), 2C (55-65
cm), 3C g (80-90 cm), and 4C (110-120 cm).
8.1.6 Alluvial soils
As a river flows from its headwaters, particulates being transported can be picked up and re-deposited elsewhere (Phillips et al., 2007; Marden et al., 2008).
Alluvial soils are mixtures of these upriver soil and rock particulates deposited from water suspension, including floodplains, alluvial terraces, and riverbank soils, the latter of which was sampled herein (Phillips et al., 2007; Marden et al., 2008). The sampled riverbank alluvial soils are categorized as either raw with no topsoil development or recent having been in place long enough to develop a topsoil
(Landcare, 2009).
Raw soils are very young soils that occur where active erosion and deposition processes prevent the development of a topsoil (Landcare, 2009). These soils cover about 3% of New Zealand in association with mountains and associated rivers, such as the Waiapu. Fertility is limited, due to nitrogen deficiency and the lack of organic matter accumulating along the riverbank (Landcare, 2009).
131 Recent soils are slightly older, weakly developed, but show signs of soil- forming processes (Landcare, 2009). A distinct topsoil is present, but B horizons are either absent or only beginning to develop. These soils occur on young surfaces primarily less than 1000 to 2000 years old, and cover about 6% of New Zealand
(Landcare, 2009). They have high natural fertility due to the water at their base, and therefore are typically covered with vascular plants (Landcare, 2009). Because only riverbank alluvial soils were acquired, this was not observed in the described profiles.
Tutumatai Station Late Holocene Terrace (TSLHT; upstream alluvial soil)
In order to obtain a recent soil, a soil profile was obtained across the road due
north of the Tutumatai Station along the Mangaoporo River at 76 m elevation. An
electric fence runs 2 m from the riverbank, which drops off 3-4 m into the river. The
paddock was recently fenced to keep livestock out of the river. Manure on the
ground indicates that the low fertility pasture was recently active, though was not
currently being grazed by cattle or sheep. Because of the ~4 m drop into the river,
the flat, late Holocene terrace is unlikely to flood (no indications of regular flooding)
except in extreme weather events. Vegetation includes paspalun, rye grass, brown
top, plantain, yarro, hemlock, moss, and minor clover.
The A p horizon extends to 20 cm. The very dark grey fine sandy loam has a
few large boulders to the surface. It is friable, non-sticky, and non-plastic with weak
132 fine nut structure breaking to strong coarse crumb. The A p horizon has a few small pebbles, many fine roots, and a distinct wavy boundary. The BC horizon (20-46 cm) is a greyish brown very stony loamy sand with stones of all sizes up to 30 cm. There are unweathered sandstones, few siltstones, and dark brown shale, the latter two of which are weathered and soft. This horizon is loose, non-sticky, and non-plastic with common fine roots, weak crumb around the roots, and a gradational boundary. The
C horizon extending beyond 46 cm is composed of sandy river gravels with sandstone and siltstone lithologies comparable to the BC horizon. Stones are subrounded to rounded and constitute 70% of the horizon. The remaining 30% is a grey sandy matrix.
Two samples were taken from the A p horizon (0-5 and 15-20 cm) and one sample was taken from the BC horizon (30-40 cm) and the C horizon (110-120 cm).
Youngest Holocene Terrace (YHT; alluvial riverbank)
The Youngest Holocene Terrace sample was taken on the banks of the aggrading Tapuaeroa River, 70 m south of Tapuaeroa Valley Road at the Hukanui
Ford. It is a very young, raw soil which has not had enough time to form a topsoil.
This site serves actively as both a source and temporary sink of sediment in the river by streambank erosion and flood deposition, respectively. River floods will submerge the soil profile completely. A major source of sediment to the profile is storm-induced debris flow from the gullies upriver.
133 The land around the sample site is an unfenced rough grazing area that has not been fertilized for around 50 years, however, across the river, superphosphate is used as a fertilizer for grazing land. While the road is nearby, it is not heavily traveled but it does include logging. The wind from the west takes vehicle emissions and fertilizers away from the site rather than towards. There are lots of worms in the soil profile. There is a high natural fertility in the soil due to abundant white clover.
Other vegetation includes Yorkshire fog, rye grass, plantain, and brown top.
A piece of wood was taken from the base of a debris flow at 1.07 m below the surface of the river bank. A debris flow present contained angular rocks in a silty matrix unlike what is seen in the river flow, which contains more worn rocks with a sandy matrix, which was also sampled.
The soil profile sampled extends 2 m above the water level of the river. The top 3 cm is a dark greyish brown loamy fine sand. This C horizon is non-sticky and non-plastic with loose, weak fine crumb structure around many fine roots and a sharp contact. The C2 horizon extends from 3 to 16 cm. The non-sticky, non- plastic, olive loamy fine sand is massive with no structure and an undulating sharp boundary. The C3 horizon is a non-sticky, non-plastic laminated to finely bedded fine sand and silt with flood layers present from 16 to 56 cm. The sand is dark greyish brown; the silt is grey. There is about 5% fine, distinct, strong brown iron mottling, particularly within the silt layer. The C4 horizon, from 56 to 70 cm, is a dark grey, non-sticky, non-plastic, loamy fine sand with common fine roots. There is a sharp contact between this horizon and the 2C debris flow horizon extending from
134 70 to 107 cm. The debris flow is composed of greywacke stones in a silt loam to fine sandy loam grey matrix. The stones are clast-supported rather than matrix supported and angular to subrounded (primarily angular to subangular). The average size of the unweathered rocks is 2 cm, with a maximum of 6cm.
A sharp contact separates the debris flow from the 3A b horizon, extending from 107 to 115 cm. This firm buried paleosol soil is a dark grey, slightly sticky, slightly plastic, silty loam. The weak medium blocky structure has common fine root channels and a few fine pebbles. A gradational boundary separates this horizon from the 3AB horizon. This horizon reaches from 115 to 125 cm. It is a dark grey, slightly sticky, non-plastic soil with weak fine blocky structure and common fine root channels. There are 10% strong brown fine distinct mottles and matching 1-2 cm coatings on the 20% unweathered, subangular to subrounded greywacke stones. A gradational boundary separates this from the 3C horizon, a very dark grey, non- sticky, non-plastic coarse sand grit and 10% subrounded unweathered greywacke stones from 125 to 140+cm.
Samples include soils from the C2 (10-15 cm), C3 (40-45 cm), 2C (80-90 cm), and 3 Ab (110-115cm) horizons, as well as a wood fragment from the bottom of the debris flow and mud taken from the submerged riverbed.
135 8.1.7 Thermogenic hydrocarbon sources in the Waiapu watershed
Williams Gas Seep (natural gas seep)
The Williams family originally settled this land and farmed it for five generations prior to selling to the current owners. This farm is located over a natural gas seep. The family used to pipe the natural gas up to the farm and store it in order to heat the home. The gas seep is known to migrate, and, was an unreliable source of energy for the farm. Following an earthquake, it can no longer be piped. To find this gas seep, the pipeline was followed across the rolling terrain across sheep and cattle pasture down to the stream, about 2 km from the house. Because the stream was high and rapid, the smell of gas could not be detected, though an old storage container was located in the ground. Thus, soil around the tank was sampled to see if hydrocarbons may have seeped into the surrounding soil. Pipes covering the lid of the collection structure indicate that at one time, there was immense pressure built up. Ten meters from the tank, a crush zone was evident in hillside rocks. Rocks are more competent both upstream and downstream of the crush zone, therefore, it is possible that gas is seeping up a fault line to the general area. There is a grey pug with black material along the cracks in the rocks, which appears to be seeping up, rather than forming in the soil. This material, which smelled slightly of hydrocarbons, was sampled as well.
136 Te Puia Springs (Sulfur springs on border of watershed)
Te Puia Springs is a small town with a hotel at some hot springs with an
overwhelming smell of hydrogen sulfide and hydrocarbons. Pipes are used to pump
water from the springs to the hot tubs at the hotel, so the pipes were followed to their
source to obtain samples. Smoky, foggy mist covered the ground and water
saturated the ground on the hike up the hillside all the way to the spring source.
Non-native trees surrounding the area have fallen into the springs, adding an
anthropogenic contribution to the carbon pool. Sediment was taken from the bottom
of the pools of water at two of the openings of the springs. Sediment from the first
spring opening at 81 m elevation is greyish, while the second spring opening to the
northeast of the first at 149 m elevation has much more leaf litter and is much
darker, including parts which are black and oil-like in appearance. Vegetation
around the springs include Bracken ferns, kiprosma, non-native large palms,
macrocarpa (large cypress), eucalyps, various grasses, and Pinus radiata.
8.1.8 Riverine sediment (riverbed and suspended load)
Makarika Stream (suspended sediment and riverbed)
Sediment and water were collected from the Makarika Stream, which
branches off the Mata River, a primary tributary of the Waiapu. The water level was
137 quite high and receding due to recent rains, and the riverbed sediment sampled was under standing water that had been undisturbed. Mostly Cretaceous rocks mixed with few Tertiary rocks were present in the riverbed. There was a high suspended load, so two cubitainers of water were obtained and centrifuged to collect suspended sediment. Vegetation surrounding the area includes sedge and cock’s foot grasses, rough grasses, crack willow, kanuka, and tutu. Alongside the stream is a cow pasture, which was once a maize field that would have seen high nitrogen fertilizer input. There is very low traffic across the Makarika Rd. bridge (~1-2 cars/hour).
Mata River (suspended sediment and riverbed)
Sediment and water were collected from the Mata River at the bridge crossing on Makarika Rd. The water level was quite high and receding due to recent rains resulting in a high suspended load. Two cubitainers of water were obtained and centrifuged to collect suspended sediment, and riverbed sediment was acquired from the riverbed. The rocks are mostly Cretaceous in the river. Upriver, there are both landslide terrains and rock gullies. Vegetation surrounding the area includes crack willow, kanuka, rough grasses including Cock’s foot, clover, tutu, silver poplar, and fennel. Alongside the river is a well-developed and intensively used cow and sheep pasture with rye grass clover. Thus, there is considerable dung and possible fertilizers used. However, the land is naturally quite fertile, therefore the recent
138 sediments are unlikely to add much fertilizer unless there is ongoing cropping.
There is very low traffic across the Makarika Rd. bridge (~1-2 cars/hour).
Mangaoporo Mud (riverbed mud sample)
A fresh mud sample was taken from the riverbed of the Mangaoporo River
which was flooded up to the road five days before sampling. The riverbed is
dominantly sand with a thin coat of mud from the storm. The sample was taken
approximately 100 m west of the bridge and about 60 m south of the road paralleling
the river. There is a possibility of anthropogenic hydrocarbon contamination in the
water here due to logging trucks fording across the river.
The area is not pasture, but is used for rough grazing. Fertilizer exposure is
not likely, other than manure. The terrace is very young; there is no sign of tephric
coverage. The primary source of local sediment is bank failure, but upriver sources
of riverbed sediment are also dominant, particularly due to the recent flood. Local
vegetation includes rye grass as the primary pasture grass, rough grasses, kikuyu,
plantain, yarro, buttercup, white clover, rushes, sedges, and kanuka. Both Tertiary
and Cretaceous rocks are found in the riverbed, however, it appears to be
dominantly Cretaceous.
139 Barton’s Gully Mud
Riverbed sediment was taken from Barton’s Gully, the Tertiary rock gully along the Tapuaeroa approximately 3 km from the road along a true left tributary, far removed from likely anthropogenic influence. The elevation is 197 m at the base of the gully. The mud was taken from the stream cutting through the gully. Vegetation around includes kanuka, toitoi, tutu, kahitatea, cabbage tree, sedges, rough grasses, and weeds. Mudballs that accumulate and encase pebbles can be found all over the gully floor.
YHT mud
In addition to the soil profile obtained at of the Youngest Holocene Terrace, riverbed mud was also collected next to banks of the aggrading Tapuaeroa River, 70
m south of Tapuaeroa Valley Road at the Hukanui Ford. This site serves actively as
both a source and sink of sediment in the river by streambank erosion and flood
deposition, respectively. The land around the sample site is an unfenced rough
grazing area that has not been fertilized for around 50 years, however, across the
river, superphosphate is used as a fertilizer for grazing land. There is a high natural
fertility in the soil due to abundant white clover. Other vegetation includes Yorkshire
fog, rye grass, plantain, and brown top. While the road is nearby, it is not heavily
140 traveled but it does include logging traffic. The wind from the west takes vehicle emissions and fertilizers away from the site rather than towards.
Tikitiki (Riverbed mud sample)
The sample collected the farthest downriver was riverbed mud taken from the
Waiapu riverbed directly at the town of Tikitiki at 10 m elevation and approximately
0.5 km from the road. The area was flooded five days prior to sampling and was receding since, so the sediment in the river was likely an integration of upstream sources. Vegetation surrounding the area includes gorse, golden willow, broom, rough grasses, kikuyu, rye grass, and plantain. The lack of clover gives evidence to too many cattle and horses having used the surrounding land as pasture. A maize paddock is located along the road upstream from Tikitiki. Suspended sediment was taken here as well, but due to the low sediment concentration, insufficient sample was collected for analysis. Samples here should show evidence the incorporation of contributions from all upriver sources including fertilizers, rocks, and soils.
141 8.2. Carbon and nitrogen mass balance solutions for marine samples
Table A1. Two end-member marine carbon and nitrogen mass balance solutions Carbon Nitrogen Depth %C %C %N %N 13 15 Sample (m) %C δ C F riv F mar riv mar %N δ N F riv F mar riv mar BC22 SS 34 0.22 -23.34 0.68 0.32 0.15 0.07 0.03 5.02 0.13 0.87 0.00 0.02 BC40 SS 40 0.24 -25.22 0.98 0.02 0.24 0.00 0.03 3.44 0.75 0.25 0.02 0.01 40m BC SS 40 0.35 -24.97 0.94 0.06 0.33 0.02 0.03 3.88 0.58 0.42 0.02 0.01 BC63 SS 42 0.75 -25.19 0.98 0.02 0.74 0.02 0.08 3.96 0.54 0.46 0.04 0.04 BC11 SS 47 0.58 -25.15 0.97 0.03 0.56 0.02 0.06 3.83 0.60 0.40 0.04 0.02 BC19 SS 48 0.63 -24.38 0.85 0.15 0.54 0.10 0.07 4.40 0.37 0.63 0.03 0.04 BC21SS 48 0.39 -23.77 0.75 0.25 0.29 0.10 0.05 4.83 0.20 0.80 0.01 0.04 BC20 SS 50 0.58 -24.20 0.82 0.18 0.47 0.11 0.07 4.50 0.33 0.67 0.02 0.04 BC18 SS 54 0.61 -25.02 0.95 0.05 0.58 0.03 0.06 4.04 0.51 0.49 0.03 0.03 BC23 SS 56 0.56 -24.06 0.79 0.21 0.44 0.12 0.07 4.52 0.33 0.67 0.02 0.05 BC12 SS 60 0.81 -25.87 1.09 -0.09 0.88 -0.07 0.08 3.07 0.90 0.10 0.07 0.01 60m BC SS 61 0.54 -25.35 1.00 0.00 0.55 0.00 0.05 3.39 0.77 0.23 0.04 0.01 BC24 SS 77 0.89 -23.78 0.75 0.25 0.66 0.22 0.11 5.12 0.09 0.91 0.01 0.10 BC51 SS 78 0.39 -24.72 0.90 0.10 0.35 0.04 0.04 4.36 0.39 0.61 0.02 0.03 BC7 SS 78 0.81 -24.83 0.92 0.08 0.75 0.07 0.09 4.14 0.47 0.53 0.04 0.05 BC33 SS 83 0.89 -25.07 0.96 0.04 0.86 0.04 0.09 3.81 0.60 0.40 0.06 0.04 BC8 SS 83 0.71 -24.72 0.90 0.10 0.64 0.07 0.08 4.37 0.38 0.62 0.03 0.05 BC5 SS 88 0.63 -24.80 0.91 0.09 0.57 0.05 0.06 3.76 0.62 0.38 0.04 0.02 BC34 SS 108 0.89 -23.80 0.75 0.25 0.67 0.22 0.11 4.84 0.20 0.80 0.02 0.09 BC14 SS 108 0.56 -24.17 0.81 0.19 0.45 0.10 0.06 4.05 0.51 0.49 0.03 0.03 BC25 SS 110 1.10 -23.31 0.67 0.33 0.73 0.36 0.14 5.19 0.06 0.94 0.01 0.13 BC50 SS 112 0.51 -24.50 0.86 0.14 0.44 0.07 0.06 4.17 0.46 0.54 0.03 0.03 BC39 SS 114 0.82 -24.31 0.83 0.17 0.68 0.14 0.09 4.34 0.39 0.61 0.04 0.06 BC13 SS 121 0.83 -24.04 0.79 0.21 0.66 0.18 0.10 4.41 0.37 0.63 0.04 0.06 BC35 SS 125 0.99 -23.67 0.73 0.27 0.72 0.27 0.12 4.97 0.15 0.85 0.02 0.10 BC49 SS 128 0.38 -24.42 0.85 0.15 0.33 0.06 0.04 4.49 0.34 0.66 0.01 0.03 BC48 SS 128 0.58 -24.51 0.87 0.13 0.50 0.08 0.06 4.15 0.47 0.53 0.03 0.03 BC36 SS 129 0.31 -22.80 0.59 0.41 0.18 0.13 0.05 5.44 -0.04 1.04 0.00 0.05 BC42 SS 136 0.79 -24.41 0.85 0.15 0.67 0.12 0.09 4.29 0.41 0.59 0.04 0.05 BC38 SS 137 0.86 -24.06 0.79 0.21 0.69 0.18 0.10 4.67 0.26 0.74 0.03 0.08 BC45 SS 150 0.86 -23.84 0.76 0.24 0.65 0.21 0.10 4.55 0.31 0.69 0.03 0.07 BC26 SS 160 1.02 -22.94 0.61 0.39 0.62 0.40 0.13 5.12 0.09 0.91 0.01 0.12 BC37 SS 169 0.62 -22.79 0.59 0.41 0.36 0.26 0.08 5.29 0.02 0.98 0.00 0.08 BC46 SS 201 0.53 -23.43 0.69 0.31 0.36 0.16 0.07 4.85 0.20 0.80 0.01 0.05 BC27 SS 263 0.77 -22.53 0.54 0.46 0.42 0.35 0.10 5.54 -0.08 1.08 -0.01 0.11 BC28 SS 444 0.68 -22.42 0.53 0.47 0.36 0.32 0.09 5.45 -0.04 1.04 0.00 0.10 BC1 SS 615 0.83 -23.43 0.69 0.31 0.57 0.26 0.10 4.85 0.19 0.81 0.02 0.08 BC2 SS 656 0.89 -23.56 0.71 0.29 0.64 0.26 0.11 5.10 0.09 0.91 0.01 0.10 BC67 SS 695 0.51 -23.98 0.78 0.22 0.40 0.11 0.06 4.51 0.33 0.67 0.02 0.04 BC62 SS 747 1.03 -24.05 0.79 0.21 0.82 0.22 0.12 4.60 0.29 0.71 0.03 0.08
142 CHAPTER 3
Geomorphic Process Identification Using Background Level Polycyclic Aromatic
Hydrocarbons in Sediment Sources in the Waiapu River Watershed, New Zealand
143 Abstract
The significant contribution of small mountainous river systems, including the
Waiapu River on New Zealand’s East Cape, to the global fluvial sediment supply motivates investigation into the processes that influence the character and composition of the organic carbon that they carry. Organic matter preserved in continental margin sediments originates from terrestrial sources such as kerogen, fresh and aged soil carbon, as well as marine sources. Due to the reactivity of marine carbon in the seabed, terrestrial sources of carbon are preferentially preserved. Therefore, identification of specific terrestrial sources of sediment from the watershed preserved on the continental margin can facilitate interpretation of the organic geochemical record and enable reconstruction of the watershed history.
Carbon and nitrogen isotopic analyses have been used to apportion terrestrial carbon sources preserved on the shelf (Chapter 2); however, these methods have not been able to resolve aged soil contributions. Therefore, polycyclic aromatic hydrocarbons (PAHs) associated with organic matter have been used herein to further clarify terrestrial sources of sediment transported to and preserved on the continental margin. Examination of a suite of 42 alkylated and non-alkylated PAHs establishes concentration profiles for potential terrestrial sediment sources, including various rocks and soil types. Likely sources of organic matter feeding the river and margin are determined by the chemical similarity between PAH profiles of possible terrestrial sources and riverine and marine sediments, as determined by principal
144 component analysis (PCA). The PCA loading plot reveals that the major influences on chemical similarity are the degree of weathering and the relative inputs of petrogenic and pyrogenic PAHs. The score plot suggests that the primary sources of terrestrial organic matter buried over recent history on the continental shelf are
Cretaceous bedrock, raw riverbank soils, and topsoils. Resolution of these sources suggests that gullying, bank failure, and sheetwash are the geomorphic process delivering most of the organic carbon to the Waiapu sedimentary system.
1. Introduction
Rivers are the main conduit for sediment and particulate organic carbon
(POC) from terrestrial to marine environments (Hedges and Keil, 1995). The composition of the POC transported and ultimately buried at sea varies as a function of river characteristics. Though individually, short mountainous rivers deliver small sediment loads, collectively these rivers are responsible for the transport of greater than 40% of the global sediment flux (Milliman and Syvitski, 1992). Therefore, these rivers are a critical component in the global carbon cycle.
Recent discoveries (Blair et al., 2003, 2004; Leithold et al., 2001, 2005, 2006) indicate that the organic carbon buried in depositional sites associated with small mountainous river sedimentary systems, such as the Eel River in California, have a bimodal character reflecting two dominant sources of carbon: modern (terrestrial and
145 marine) and ancient (kerogen). High sediment yields in connection with rapid POC burial promote the preservation of terrestrial carbon across the margin, thereby preserving the source signatures of the POC transported through the watershed
(Blair et al., 2003, 2004; Goñi et al., 2005, 2006a, 2006b, 2008; Leithold et al., 2005,
2006). Mass balance equations using stable and radiocarbon isotopic analyses have then been utilized to link geomorphic processes to the sedimentary record by identifying and quantifying terrestrial sources preserved in the seabed (Leithold et al., 2006). These models indicated that in systems with lower sediment yields, the dominant portion of the POC delivered to the margin was modern plant material delivered by sheetwash and shallow landsliding processes (Leithold et al., 2006).
However, in systems with increasingly larger sediment yields, gullying of ancient rock carbon contributes more to the POC (Leithold et al., 2006).
These calculations assume a bimodal POC distribution; however, aged soil contributions that were assumed negligible may potentially have been underestimated (Blair et al., 2003; Drenzek et al., 2009; Blair et al., 2009). To address the significance of aged soil contributions in the Waiapu River and adjacent margin, stable carbon and nitrogen isotopic signatures have been used to further characterize a suite of potential rock and soil sources of organic matter from the
Waiapu watershed (Chapter 2). This isotopic analysis indicates that the gullying of
Cretaceous mudstones of the Whangai Fm. does constitute the dominant source of organic carbon to the continental margin; however, the contribution of aged soil carbon components is not resolved (Chapter 2; Alin et al., 2008). Molecular level
146 analyses are required to further refine terrestrial source contributions of organic carbon from the watershed on the continental margin.
Polycyclic aromatic hydrocarbons (PAHs) have been selected as a tracer for sediment generation, transport, and burial in the Waiapu River watershed and continental margin. PAHs are particularly robust, being thermally stable, having low water solubility, and surviving over hundreds of millions (or even billions) of years
(Mahajan et al., 2001). As PAHs are hydrophobic, they tend to associate with particles (e.g. soils and sediments) rather than partition into the dissolved phase
(Lima et al., 2005; Planas et al., 2006). The presence of carbonaceous geosorbents such as black carbon, unburned coal, and kerogen can cause strong sorption of
PAHs (Cornelissen et al., 2006). Particle bound species are not readily bioavailable, which greatly slows degradation, rendering them excellent tracers of POC through geologic time due to their persistence in the environment (Lima et al., 2005).
The PAHs of interest are categorized as petrogenic or pyrogenic (Table 2).
Petrogenic PAHs are produced by the slow thermal degradation of sedimentary organic carbon over geologic time (as with fossil fuels); pyrogenic PAHs are derived from incomplete combustion of modern biomass (e.g. wood) and fossil fuels (e.g.
Lima et al., 2005; Planas et al., 2006; Stout and Emsbo-Mattingly, 2008).
Petrogenic PAHs are abundant in many sedimentary rocks and the specific PAH compounds present reflect the time and temperature of formation. Thus, petrogenic
PAH compositions may provide a signature for not only a rock source in general, but for specific formations as well. The pyrolytic composition of pyrogenic PAHs is
147 controlled by factors such as type of fuel (e.g. biomass source), amount of available oxygen for combustion, and temperature of formation (e.g. Laflamme and Hites,
1978; Lima et al., 2005; Stout and Emsbo-Mattingly, 2008). Additionally, biogenic
PAHs, while few, can appear in environmental samples in the presence of biologic processes and early diagenesis (e.g. Laflamme and Hites, 1978; Wakeham et al.,
1980; Stout et al., 2001a; Lima et al., 2005; Planas et al., 2006).
Because the molecular compositions of petrogenic and pyrogenic PAHs differ, they have been used to distinguish sources in the environment, particularly in sediments (Lima et al., 2005; Burns et al., 1997; Stout et al., 2004). Concentration distributions in PAH groups (i.e. naphthalenes, anthracenes, fluorenes, and chrysenes) and individual PAH analytes provide a discrete signature for each potential organic matter source (Stout et al., 2001a; 2001b; Stout and Emsbo-
Mattingly, 2008). Over the last 30 years, research in the environmental sciences and petroleum industry has developed methods for source identification and allocation of PAHs in sediments (e.g. Burns et al., 1997; Stout et al., 2004).
Because of their relative stability and resistance to degradation, PAH fingerprinting and source-specific diagnostic ratios can differentiate between biological formation of PAHs (Venkatesan, 1988), combustion products (pyrogenic), oil seeps
(petrogenic), and spill oil or diesel oil (petrogenic) (Burns et al., 1997). For example,
PAH fingerprinting was used to determine the origin of hydrocarbons in Prince
William Sound, where PAH signatures derived from erosion of coal deposits, oil
148 seeps, and eroding source rocks from rivers (Bence et al., 1996; Page et al., 1996;
Short et al., 1999; Mudge, 2002).
PAHs are generally used to distinguish contaminant sources to a given environment, while the pre-existing background concentrations are considered negligible (e.g. Wakeham et al., 1980; Page et al., 1996; Benlahcen et al., 1997;
Burns et al., 1997; Zeng and Vista, 1997; Short et al., 1999; Headley et al., 2002;
Stout et al., 2004; Oren et al., 2006; Liu et al., 2008). However, in a system isolated from anthropogenic interferences, these background PAH signatures can be exploited to identify specific organic matter sources from the watershed to the continental margin. Despite pervasive applications in soil and petroleum research, to our knowledge, PAHs have not before been used as a tracer to associate terrestrial sediment production processes with the organic geochemical record of a coupled watershed-continental shelf system.
PAH profiles are used herein as chemical tracers to resolve specific terrestrial sources of organic matter delivered from the Waiapu River watershed to its adjacent continental shelf (Stout et al., 2003). The Waiapu sedimentary system provides an excellent test site for PAHs as a natural sediment tracer because its remote location minimizes anthropogenic influences. The molecular profiles of potential terrestrial sources have been compared with those in the continental margin sediments (Stout et al., 2003). Chemical similarity between specific terrigenous sources and riverine and marine sediments are used to interpret sediment contributions from various geomorphic processes which deliver POC with unique PAH compositions.
149 Ultimately, this source apportionment method can be applied to watershed history reconstruction, given its capability to distinguish geomorphic processes responsible for sediment delivery in the stratigraphic record.
2. Site description and methodology
2.1 Site description
The Waiapu River watershed is located on the East Cape of New Zealand’s North Island
(Figure 1). The river flows 130 km eastward to the
Pacific Ocean from the Raukumara Range, and is oriented within the Hikurangi subduction margin
(Moore and Mazengarb, 1992). Active tectonic deformation of the area results in a highly friable terrain of primarily Cretaceous and Tertiary Figure 1. Map of the Waiapu sandstones and mudstones (Mazengarb and Figure 1. Map of the Waiapu River Watershed, Speden, 2000). New Zealand
Soils are categorized as bedrock-weathered, tephric, colluvial, and riverbank alluvial. Bedrock-weathered soil organic matter reflects the overlying plant litter and weathered underlying bedrock of which it is comprised (Marden et al., 2008).
Tephric soils occur in areas of volcanic ash deposition, which date back to 55,000
150 years B.P., with deposits of the most recent eruption, 665 years B.P., being the most widespread (Eden et al., 2001). Erosion rates of tephric soils are generally low except on steep slopes or exposed sites (Landcare Research, 2009). Colluvial and alluvial soils are both mixtures of other soil types. Colluvial soils are those occupying depositional areas within the watershed that incorporate bedrock-derived and tephric soils delivered by mass wasting processes (e.g. landslide debris tails)
(Marden et al., 2008). Alluvial soils are mixtures of soil types deposited from water suspension including floodplains, alluvial terraces, and riverbank soils, the latter of which was sampled herein (Phillips et al., 2007).
At 20,520 T km -2 yr -1, the Waiapu’s sediment yield is one of the highest in the world (Page et al., 2001), with a sediment discharge of approximately 36 x 10 6 T/yr from a drainage area of only 1734 km 2 (Page et al., 2001). High intensity storm events, high rates of uplift and steep slopes, and unstable lithologies contribute to the highly degraded nature of the catchment, with extreme mass wasting and high sediment yields (Marutani et al., 1999; Page et al., 1999a; Hicks et al., 2000). The small drainage basin limits the storage capacity for the eroding sediment within the watershed caused in part by a yearly rainfall in excess of 2.4 m (Page et al., 2001).
Modern sediment transport on the Waiapu margin is controlled by currents, waves, and gravity flows under high concentration conditions (Wright et al., 2006;
Addington et al., 2007; Ma et al, 2008; Wadman and McNinch, 2008; Kniskern et al.,
2009). Two currents affect the Waiapu continental margin: the East Cape Current
(ECC) flowing southward along the shelf break and upper continental shelf and the
151 Wairarapa Coastal Current which flows northward inshore of the ECC (Chiswell,
2000; Wadman and McNinch, 2008; Kniskern et al., 2009). The deposits of episodic flood events are revealed in the down-core geochemical records by 210 Pb-poor sediment layers on the mid-shelf. These 210 Pb inventories have also been used to determine sediment accumulation rates on the shelf that range from 0.2 to 3.5 cm/y
(Kniskern, 2007; Kniskern et al., 2009). Increased frequency of bioturbation and decreased frequency of lamination radially away from the river mouth indicates that riverine input rather than wave energy controls sediment mixing patterns (Kniskern,
2007). POC composition across the margin reflects terrestrial and marine sources as well as hydrodynamic sorting (Chapter 2; Kniskern, 2007; Kniskern et al., 2009).
2.2 Sampling methods
Detailed sampling information has been previously described (Chapter 2). In summary, representative bedrock samples were collected from the major
Cretaceous and Tertiary formations exposed in active gullies throughout the watershed. Soil profiles were collected from stable terrain and areas with sheetwash and landslide exposures. Most soil profiles were collected from pastoral land; however, two forested soil profiles were also obtained to compare land usage effects on molecular fingerprints of carbon sources.
Riverbank alluvial soils, riverbed sediment, and suspended sediments obtained by centrifugation or filtration of surface river water were collected to
152 observe the transport of rock and soil organic matter down the river. Marine samples were collected from the continental margin adjacent to the Waiapu River mouth from the R/V Kilo Moana in May 2004. Box cores (up to 50 cm in length) were obtained and immediately sectioned on ship. To avoid PAH contamination, terrestrial and marine samples were stored in organic carbon-free glass jars and frozen until time of analysis. Specific sample locations in the Waiapu sedimentary system are detailed in Table 1 and Figure 2.
153
Table 1. Sample locations by type
Sample site Sample type Latitude Longitude
Bartons Gully gully rock 37°50.45' 178°13.09' Cretaceous Gully gully rock 37°52.61' 178°12.81'
Bartons Mouth forested b-d soil 1 37°50.40' 178°12.72' Forest Soil forested b-d soil 1 37°52.52' 178°12.40'
BWBO pastoral b-d soil 1 37°53.75' 178°17.48' Landslide Waiapu pastoral b-d soil 1 37°48.48' 178°22.45'
W1 terrace tephric soil 37°52.19' 178°19.04' Mata Slip tephric soil 37°57.24' 178°11.94'
Manuel Terrace colluvial soil 37°48.17' 178°23.18' Tutumatai Stn. colluvial soil 37°50.77' 178°18.09'
Te Puia Springs thermogenic hc mud 2 38°03.56' 178°18.21' Williams Gas Seep thermogenic hc soil 2 38°03.61' 178°15.33'
Tutumatai Stn. Lowest alluvial (recent) soil 37°50.60' 178°18.13' Holocene Terrace Youngest Holocene Terrace alluvial (raw) soil 37°51.81' 178°11.16'
Tikitiki riverine sediment 37°47.99' 178°24.77' Mangaoporo riverine sediment 37°50.22' 178°17.55' Bartons Gully riverine sediment 37°50.45' 178°13.09' Makarika Stream riverine sediment 37°57.31' 178°13.73' Mata River riverine sediment 37°56.95' 178°12.73' Youngest Holocene Terrace riverine sediment 37°51.81' 178°11.16'
Water Depth (m) 61 marine sediment 37°47.43' 178°32.48' 83 marine sediment 37°46.26' 178°37.70' 108 marine sediment 37°45.62' 178°41.42' 128 marine sediment 37°45.99' 178°43.92' 615 marine sediment 37°47.22' 178°50.07' 1b-d denotes bedrock-derived soils; 2hc denotes hydrocarbon
154
Figure 2. Map of terrestrial and marine samples collected from the Waiapu River sedimentary system for PAH analysis. To compensate for the lack of sampling in the headwaters due to private property restrictions, river sediment was collected to examine the carbon being transported down the river from those regions.
155 2.3 PAH concentration analyses
Samples were freeze-dried and ground. For each sample, two Teflon centrifuge tubes containing approximately 15 g of dried sample each were spiked with 100 µL of 20.04 ng/ µL deuterated chrysene (C0-d12 ) in dichloromethane (DCM) as a recovery standard prior to solvent extraction. DCM (30 mL) was added to each tube and shaken for 24 hours. After shaking, samples were centrifuged at 2000 rpm for 10 minutes and the supernatant was decanted into amber EPA vials. An additional 20 mL DCM was added to each sample and the extraction procedure was repeated for another 24 hours. A third extraction sequence also included an hour of sonication.
Solvent extracts were combined and concentrated by evaporation with nitrogen gas, with care not to evaporate the samples to dryness. The solvent was exchanged to hexane and concentrated with N 2 gas to 5 mL extract for each sample.
An activated neutral alumina column was used to clean up 1 mL of each sample prior to analysis by GC/MS. Hexane (15 mL) was first run through the column to separate the aliphatic fraction. Then, DCM (80 mL) was used to isolate the aromatic fraction. The aromatic fraction was concentrated to less than 1 mL and brought up to exactly 1 mL, 100 µL of which was transferred to a vial for analysis on GC-MS.
Extracts were spiked with deuterated phenanthrene (d 10 , 500 ng/mL) and benzo[a]pyrene (d 8, 500 ng/mL) as internal standards (e.g. Planas et al., 2006).
156 Extracts were analyzed for 42 PAHs (Table 2; Appendix 1) using a modified method of EPA 8270 (US EPA, 1986). GC/MS select ion monitoring (SIM) mode analyses was conducted on a HP5890 Series II GC equipped with electronic pressure control connected to an HP5970 or HP5972 Mass Selective Detector
(MSD) using a Restek 30m x 0.25 mm Rtx-5 (film thickness 0.25 µm) MS with
Integra-Guard column. The method is similar to that described elsewhere (Stout et al., 2001a; Stout et al., 2001b; Brenner et al., 2002; Luellen and Shea, 2002; Stout et al., 2002).
Data quality control indicators were utilized to ensure precision and accuracy of PAH extraction and analysis procedures. The recovery averages for the chrysene-d12 standard spike ranged from 68 to 85% for the five different sample types, with an overall average percent recovery of 77%. The extraction and analysis methods resulted in a narrow range of percent recovery across all sample types with an average relative standard deviation of less than 20% (Stout et al., 2004; Planas et al., 2006). The aliphatic fraction of one sample was applied both as a procedural blank and a proof of aliphatic and aromatic fraction separation during column chromatography cleanup. The concentrations of PAH analytes in this fraction were negligible, establishing effective separation and minimal influence of procedural or analytical artifacts. Complete details of the quality assurance procedures can be reviewed in Appendix 2.
157 Table 2. Inventory of 42 Alkylated and Non-Alkylated PAHs of Interest
Analyte Abbrev Rings Type Analyte Abbrev Rings Type biphenyl BP 2 petro fluoranthene FL 4 pyro acenaphthylene ACL 3 petro pyrene PY 4 pyro acenaphthene ACE 3 petro C1-fluoranthenes/pyrenes FP1 4 pyro dibenzofuran Dfu 3 petro retene Re 4 bio naphthalene N0 2 petro benz[a]anthracene BaA 4 pyro C1-naphthalenes N1 2 petro chrysene C0 4 pyro C2-naphthalenes N2 2 petro C1-chrysenes C1 4 pyro C3-naphthalenes N3 2 petro C2-chrysenes C2 4 petro C4-naphthalenes N4 2 petro C3-chrysenes C3 4 petro fluorene F0 3 petro C4-chrysenes C4 4 petro C1-fluorenes F1 3 petro benzo[b]fluoranthene BbF 5 pyro C2-fluorenes F2 3 petro benzo[k]fluoranthene BkF 5 pyro C3-fluorenes F3 3 petro benzo[e]pyrene BeP 5 pyro dibenzothiophene D0 3 petro benzo[a]pyrene BaP 5 pyro C1-dibenzothiophene D1 3 petro perylene Pryl 5 bio C2-dibenzothiophene D2 3 petro indeno[1,2,3-cd]pyrene ID 6 pyro C3-dibenzothiophene D3 3 petro dibenz[a,h]anthracene DA 5 pyro anthracene AN 3 pyro benzo[g,h,i]perylene BgP 6 pyro phenanthrene (PHEN) P0 3 mixed coronene Co 6 pyro C1-PHEN/anthracenes P1 3 mixed C2-PHEN/anthracenes P2 3 petro C3-PHEN/anthracenes P3 3 petro C4-PHEN/anthracenes P4 3 petro petro = petrogenic; pyro = pyrogenic; bio = biogenic
3. Results
3.1 TPAH concentrations
The total concentration of PAHs in each sample is summarized in Table 3
(TPAH). Concentrations of the 42 individual PAH analytes detected in each sample
158 are provided in Appendix 3. The total concentration of extractable PAHs, ranged from 0.013 to 26.85 µg PAH/g sediment for the 42 PAHs quantified, including sixteen
US EPA priority PAH pollutants (naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene, and benzo[g,h,i]perylene) (Table
3). For the priority pollutants alone, the PAH concentrations extended from 0.005 to
2.88 µg PAH/g sediment. The median PAH concentrations were 0.46 and 0.057 µg
PAH/g sediment for the total and priority pollutant PAHs respectively.
The concentration of total PAHs in rock samples ranged from 0.017 to 26.85
µg PAH/g sediment. The highest concentration for an individual PAH extracted from any sample was 4.77 µg PAH/g solid of C2-naphthalenes in the Tertiary Waipawa rock formation. This rock formation also had the highest concentration of a priority pollutant with 1.75 µg PAH/g solid of naphthalene. The soil samples had the lowest
PAH concentrations, ranging from 0.013 to 5.44 µg PAH/g soil. The heterogeneous multitude of rocks and soils that are potential sources of organic carbon to the river drive the large ranges of extractable PAHs from samples in these categories. The riverbed mud samples that incorporate this carbon had a much smaller range from only 1.18 to 3.38 µg PAH/g sediment due to homogenization during transport. As the localized sink for terrestrial sediment, these samples include both the
159 Table 3. PAH concentrations ( µg PAH/g) and selected diagnostic ratios TPAH/ %2 %3 %4-6 Sample detail %OC TPAH %OC D2/P2 P0/P2 D0/P0 P2/P1 FL/PY BaA/C0 ring ring ring Rocks Barton's Gully Jar 1 Wanstead Fm. 0.33 0.87 261.0 0.32 0.81 0.10 0.97 0.05 0.91 62.9 22.0 15.1 Barton's Gully Jar 2 Wanstead Fm. 0.27 0.62 229.2 0.21 0.38 0.09 1.32 0.03 1.07 49.1 27.3 23.6 Barton's Gully Jar 2 Wanstead Fm. 0.27 0.59 221.0 0.24 0.36 0.09 1.29 0.03 1.02 47.1 27.8 25.1 Bartons Gully Rock 1 Waipawa Fm. 2.48 9.80 395.2 0.95 0.39 0.36 1.31 0.07 1.00 66.8 25.1 8.1 Barton's Gully Rock 2 Wanstead Fm. 0.01 0.03 286.7 0.42 11.50 0.04 0.42 1.48 0.00 35.8 59.5 4.7 Barton's Gully Rock 3 Wanstead Fm. 0.02 0.13 744.2 0.30 5.70 0.07 0.46 0.80 0.20 89.9 8.7 1.3 Bartons Gully Rock 4 Waipawa Fm. 10.31 26.85 260.4 2.20 0.26 1.23 1.37 0.16 0.61 60.0 31.6 8.4 BG Stream Bed Wanstead Fm. 0.05 0.03 54.2 0.00 17.13 0.00 0.32 23.67 0.01 1.8 79.8 18.4 BG Stream Bed Wanstead Fm. 0.05 0.02 44.4 0.00 14.16 0.00 0.33 16.89 0.01 0.2 75.7 24.1 Cretaceous Gully 1 Mokoiwi Fm. 0.06 0.47 805.5 0.15 1.38 0.03 0.68 1.34 0.03 80.9 16.6 2.5 Cretaceous Gully 2 Mokoiwi Fm. 0.46 12.85 2774.6 0.08 0.67 0.09 0.80 0.25 0.12 67.4 27.0 5.6 Cretaceous Gully 3 Tolaga Group 0.11 2.10 1844.6 0.08 0.81 0.09 0.75 0.38 0.12 68.1 27.0 4.9 Cretaceous Gully 4 Weber Fm. 0.06 0.44 803.7 0.07 0.76 0.09 0.86 0.32 0.07 62.0 24.4 13.6 Cretaceous Gully 5 Whangai Fm. 0.51 9.18 1803.8 0.05 0.67 0.06 0.79 0.30 0.09 64.7 28.4 7.0 Cretaceous Gully 6 Whangai Fm. 0.41 3.46 844.0 0.07 0.95 0.10 0.65 0.30 0.07 37.2 50.9 11.9 Cretaceous Gully 6 Whangai Fm. 0.41 2.16 527.6 0.07 0.97 0.10 0.65 0.28 0.07 26.1 60.0 13.9 LW Br Whangai Fm. 0.11 0.02 14.7 0.61 13.11 0.08 0.34 0.26 1.21 35.8 51.8 12.4 Mata Slip Br Tikihore Fm. 0.08 0.03 33.8 0.00 10.24 0.03 0.44 1.44 0.14 72.9 24.5 2.6 Mata Slip Br Tikihore Fm. 0.08 0.02 29.4 0.00 11.06 0.03 0.45 1.20 0.17 74.9 22.2 2.9 W1 Rock Whakai Fm. 0.05 0.09 179.8 1.10 5.80 0.06 0.42 2.63 0.00 90.9 8.2 0.9 Soils Bartons Mouth 0-5cm BW Forest 28.18 0.13 0.5 0.00 1.03 0.07 0.96 0.49 0.37 32.6 43.5 23.9 Bartons Mouth 5-10cm BW Forest 5.35 0.36 6.8 0.83 1.58 0.06 1.04 0.34 0.21 80.4 14.1 5.5 BWBO 0-5cm BW Pastoral 6.56 0.12 1.9 0.19 1.14 0.08 0.73 0.46 0.16 29.2 40.4 30.4 BWBO 5-10cm BW Pastoral 2.96 0.07 2.3 0.20 1.96 0.04 0.70 1.79 0.13 55.8 31.4 12.8 BWBO 5-10cm BW Pastoral 2.96 0.18 6.1 0.36 1.57 0.06 0.76 1.04 0.16 75.4 18.1 6.5 BWBO 20-25cm BW Pastoral 0.95 0.54 57.0 0.48 1.34 0.06 0.86 11.33 0.02 80.4 14.3 5.3 BWBO 45-55cm BW Pastoral 0.96 0.04 4.3 0.26 3.27 0.08 0.43 1.88 0.00 34.3 45.6 20.1 Forest Soil 0-4cm BW Forest 13.01 0.59 4.6 0.08 1.13 0.04 0.66 0.67 0.15 57.2 30.6 12.3 Forest Soil 0-4cm BW Forest 13.01 0.78 6.0 0.09 1.31 0.03 0.60 0.68 0.14 53.0 35.3 11.7 Forest Soil 0-4cm BW Forest 13.01 0.50 3.9 0.08 1.16 0.03 0.73 0.67 0.16 47.1 37.5 15.5 Forest Soil 10-15cm BW Forest 3.39 0.88 25.9 0.06 1.59 0.02 0.64 0.62 0.15 66.8 21.0 12.2 LW 0-8cm BW Pastoral 6.44 0.07 1.1 0.00 1.57 0.10 0.69 0.22 0.24 47.2 18.0 34.8 LW 8-19cm BW Pastoral 2.25 0.06 2.8 0.00 4.86 0.10 0.46 0.09 0.17 64.3 19.8 15.9 LW 30-40cm BW Pastoral 0.87 0.07 8.0 0.00 4.75 0.09 0.49 0.07 0.25 68.8 18.9 12.3 LW 60-70cm BW Pastoral 0.59 0.16 27.7 0.00 7.16 0.10 0.39 0.11 0.25 87.1 9.0 3.9 LW 81-92cm BW Pastoral 0.64 0.60 93.8 0.00 10.89 0.08 0.29 0.53 0.33 92.4 7.0 0.7 Mata Slip 0-5cm tephric 2.80 0.16 5.9 0.69 5.13 0.05 0.45 1.55 0.32 88.1 10.2 1.8 Mata Slip 10-15cm tephric 2.85 0.17 6.0 0.70 5.37 0.05 0.48 1.85 0.15 89.5 8.7 1.8 Mata Slip 20-25cm tephric 1.68 0.16 9.5 0.00 3.34 0.06 0.75 2.18 0.38 88.6 7.0 4.3 Mata Slip 40-50cm tephric 0.96 0.10 10.7 1.08 6.28 0.05 0.46 2.00 0.25 90.6 7.7 1.7 Mata Slip 52-62cm tephric 0.63 0.14 22.2 0.00 6.19 0.06 0.41 1.31 0.33 92.9 6.4 0.7 Mata Slip 80-90cm tephric 0.21 0.16 75.9 1.08 5.77 0.06 0.47 1.70 0.25 93.1 6.2 0.7 MT 0-5cm colluvial 5.83 0.19 3.2 0.52 3.01 0.06 0.55 1.12 0.09 85.4 10.6 4.0 MT 13-18cm colluvial 3.32 0.27 8.1 0.61 3.11 0.06 0.60 2.83 0.14 92.2 6.8 1.0 MT 20-25cm colluvial 2.62 0.18 6.8 0.44 1.32 0.06 1.22 4.25 0.00 85.9 13.0 1.1 MT 30-35cm colluvial 0.94 0.13 13.8 0.98 4.62 0.06 0.53 2.29 0.00 93.4 6.1 0.6 MT 45-50cm colluvial 0.60 0.09 14.7 1.42 6.42 0.06 0.37 2.50 0.00 91.7 7.7 0.6 MT 70-75cm colluvial 0.28 0.10 37.3 0.00 8.18 0.06 0.35 1.55 0.00 92.0 7.3 0.7 MT 85-90cm colluvial 0.36 0.11 31.8 1.69 8.86 0.07 0.24 2.67 0.50 89.5 9.8 0.7 TS 0-5cm colluvial 5.82 0.30 5.2 0.00 5.82 0.05 0.38 1.40 0.34 89.3 7.3 3.3 TS 10-15cm colluvial 4.40 0.20 4.6 0.53 5.01 0.04 0.42 1.49 0.32 86.3 10.2 3.6 TS 30-40cm colluvial 0.53 0.17 32.7 0.00 8.17 0.06 0.33 1.04 0.18 89.5 8.6 1.9 TS 55-65cm colluvial 0.37 0.19 49.5 0.00 7.87 0.07 0.36 0.97 0.04 89.7 8.5 1.8 TS 80-90cm colluvial 0.32 0.15 45.2 0.38 5.61 0.03 0.40 10.00 0.03 87.7 10.4 1.8 TS 110-120cm colluvial 0.18 0.24 133.7 0.00 7.84 0.05 0.35 5.29 0.06 91.2 7.6 1.2
160 Table 3. PAH concentrations ( µg/g) and selected diagnostic ratios (cont'd) TPAH/ %2 %3 %4-6 Sample detail %OC TPAH %OC D2/P2 P0/P2 D0/P0 P2/P1 FL/PY BaA/C0 ring ring ring TSLHT 0-5cm alluvial 4.05 0.07 1.6 0.11 2.51 0.02 0.56 0.79 0.23 31.2 40.3 28.5 TSLHT 15-20cm alluvial 2.13 0.07 3.1 0.08 2.44 0.02 0.55 2.39 0.17 30.7 53.8 15.6 TSLHT 15-20cm alluvial 2.13 0.21 10.1 0.28 2.50 0.04 0.56 2.44 0.13 68.1 25.6 6.3 TSLHT 30-40cm alluvial 0.53 0.16 30.1 0.07 2.45 0.05 0.58 0.68 0.03 45.2 40.8 13.9 TSLHT 110-120cm alluvial 0.17 0.13 76.6 0.12 2.36 0.05 0.58 1.27 0.00 28.8 36.3 34.9 W1 0-5cm tephric 12.47 0.04 0.3 0.09 2.08 0.04 0.84 1.04 0.31 47.2 32.1 20.7 W1 5-10cm tephric 10.35 0.03 0.3 0.00 1.78 0.03 0.98 1.18 0.32 44.5 32.8 22.7 W1 15-20cm tephric 8.74 0.03 0.4 0.00 1.80 0.03 0.96 1.12 0.30 44.4 35.6 20.0 W1 15-20cm tephric 8.74 0.06 0.7 0.13 2.07 0.03 0.84 1.18 0.32 41.0 38.6 20.4 W1 15-20cm tephric 8.74 0.05 0.5 0.08 2.13 0.03 0.88 1.10 0.32 40.8 38.1 21.1 W1 30-35cm tephric 5.36 0.02 0.4 0.00 2.60 0.03 1.02 1.30 0.16 12.9 61.9 25.3 W1 45-50cm tephric 2.64 0.02 0.6 0.00 9.33 0.03 0.45 1.22 0.14 59.5 31.7 8.8 W1 75-80cm tephric 0.93 0.03 2.8 0.00 8.44 0.03 0.69 1.00 0.18 56.5 23.8 19.7 W1 75-80cm tephric 0.93 0.04 4.6 0.41 4.22 0.03 0.84 1.26 0.51 39.6 26.1 34.4 W1 75-80cm tephric 0.93 0.05 4.8 0.50 8.15 0.03 0.52 1.24 0.32 63.6 27.5 8.9 W1 95-100cm tephric 0.47 0.02 4.4 0.00 7.09 0.04 0.66 1.38 0.17 67.2 27.3 5.4 W1 95-100cm tephric 0.47 0.01 2.8 0.00 4.40 0.03 0.75 1.67 0.09 34.6 52.9 12.5 YHT 10-15cm alluvial 0.52 1.54 299.0 0.07 0.83 0.07 0.68 3.06 0.08 40.9 48.7 10.3 YHT 10-15cm alluvial 0.52 2.04 396.0 0.09 0.80 0.07 0.70 11.27 0.03 42.6 46.8 10.6 YHT 40-45cm alluvial 0.45 1.63 362.4 0.07 1.19 0.05 0.60 2.64 0.18 43.5 43.5 13.0 YHT 40-45cm alluvial 0.45 1.55 345.5 0.06 1.00 0.05 0.64 3.57 0.13 32.4 53.4 14.2 YHT 80-90cm alluvial 0.41 2.47 601.9 0.08 0.79 0.08 0.73 0.56 0.11 45.2 38.1 16.7 YHT 110-115cm alluvial 3.56 1.03 29.0 0.06 0.90 0.06 0.70 0.62 0.13 30.8 47.6 21.6 Te Puia Springs 1 thermogenic 1.49 5.44 366.0 0.18 2.89 0.09 0.29 0.43 0.56 29.3 63.7 7.0 Te Puia Springs 2 thermogenic 0.91 2.69 296.7 0.17 0.93 0.04 0.27 0.49 0.40 37.2 55.8 7.0 Williams Gas Seep 1 thermogenic 6.45 0.36 5.6 0.38 1.88 0.05 0.59 1.13 0.15 81.1 15.7 3.2 Williams Gas Seep 2 thermogenic 13.20 0.66 5.0 0.00 1.88 0.05 0.67 0.78 0.33 76.7 14.5 8.9 Sediment Barton's Gully Mud riverbed 0.22 1.18 529.1 0.39 1.46 0.11 0.65 0.43 0.82 64.7 22.8 12.5 Makarika Stream Mud riverbed 0.35 2.81 798.6 0.16 0.80 0.11 0.70 0.47 0.21 54.6 32.9 12.5 Mangaoporo mud riverbed 0.27 1.35 504.0 0.18 1.51 0.06 0.55 0.77 0.61 53.6 30.3 16.1 Mata River mud riverbed 0.53 2.02 381.3 0.12 0.85 0.07 0.76 0.54 0.48 58.0 26.1 15.9 Tikitiki mud riverbed 0.22 1.33 616.8 0.13 0.79 0.07 0.72 0.58 0.21 49.0 38.5 12.5 YHT mud riverbed 0.30 3.38 1124.0 0.10 0.74 0.08 0.72 0.55 0.11 50.5 39.4 10.1 60m BC 0-2cm marine 0.54 2.49 457.2 0.11 0.85 0.07 0.72 0.60 0.26 51.3 33.3 15.4 60m BC 14-16cm marine 0.62 2.18 350.0 0.11 0.73 0.07 0.75 0.60 0.30 47.3 35.6 17.1 60m BC 27-29cm marine 0.48 1.55 323.7 0.12 0.79 0.07 0.74 0.64 0.31 45.8 36.5 17.7 60m BC 39-41cm marine 0.50 1.76 351.9 0.12 0.76 0.07 0.71 0.64 0.29 45.6 35.9 18.5 BC8 0-2cm marine 0.71 2.07 291.0 0.11 0.92 0.07 0.67 0.62 0.28 43.2 37.4 19.4 BC8 7-9cm marine 0.68 1.55 227.7 0.10 0.76 0.07 0.76 0.70 0.22 42.5 37.9 19.6 BC8 14-16cm marine 0.66 2.04 307.7 0.10 0.76 0.07 0.76 0.67 0.23 40.6 39.5 19.9 BC8 14-16cm marine 0.66 1.75 263.8 0.10 0.91 0.06 0.68 0.65 0.26 42.9 36.7 20.5 BC8 29-31cm marine 0.67 1.40 207.9 0.10 0.83 0.07 0.72 0.65 0.24 42.0 37.1 20.9 BC8 29-31cm marine 0.67 1.43 213.7 0.11 0.77 0.07 0.75 0.67 0.20 40.4 39.4 20.1 BC8 43-45cm marine 0.62 1.01 161.5 0.12 0.78 0.07 0.78 0.65 0.15 42.1 39.6 18.3 BC8 43-45cm marine 0.62 1.03 165.3 0.11 0.86 0.07 0.70 0.63 0.24 41.3 37.5 21.2 BC14 0-2cm marine 0.56 1.42 255.8 0.10 0.81 0.06 0.73 0.74 0.27 36.6 39.2 24.2 BC14 7-9cm marine 0.62 1.53 245.0 0.11 0.79 0.07 0.78 0.63 0.28 38.7 37.9 23.4 BC14 28-30cm marine 0.52 1.15 222.0 0.11 0.79 0.07 0.75 0.57 0.28 36.8 38.3 24.9 BC14 43-45cm marine 0.57 1.15 200.0 0.09 0.86 0.08 0.70 0.57 0.23 49.8 34.7 15.4 BC48 0-2cm marine 0.58 1.37 236.5 0.11 0.94 0.07 0.60 1.09 0.52 33.3 31.8 34.9 BC48 13-15cm marine 0.65 1.41 215.5 0.09 0.82 0.07 0.71 0.57 0.24 42.3 38.5 19.2 BC48 27-29cm marine 0.57 1.20 212.3 0.10 0.81 0.07 0.65 0.60 0.23 42.3 38.3 19.5 BC48 40-42cm marine 0.55 1.24 225.6 0.10 0.75 0.07 0.70 0.63 0.28 44.7 36.5 18.8 BC1 0-2cm marine 0.83 1.68 203.3 0.10 0.80 0.06 0.44 0.66 0.16 39.9 42.1 18.1 BC1 8-10cm marine 0.78 1.48 188.8 0.11 0.84 0.06 0.50 0.64 0.18 44.2 39.0 16.8 BC1 25-27cm marine 0.78 1.36 174.0 0.43 1.30 0.08 0.50 1.21 0.00 40.2 49.4 10.4 BC1 42-44cm marine 0.72 1.22 170.0 0.09 0.77 0.07 0.76 0.66 0.22 44.0 36.9 19.2 *abbreviations for ratio compounds can be found in Table 2
161 highest median concentration at 1.69 µg PAH/g sediment and the highest minimum total PAH concentration of the four major sample types: rock (0.017), soil (0.013), riverine (1.18), and marine (1.01 µg PAH/g sediment). The sediment transported from the river to the continental margin, being even further homogenized, has an even smaller range of PAH concentrations (1.01 to 2.49 µg PAH/g sediment).
The total concentration of PAHs in each sample is dependent on its %C org as well as the PAH distributions themselves and was therefore normalized to %C org
(Stout and Emsbo-Mattingly, 2008). After normalization, the concentration of extractable PAHs ranges from 0.3 to 2775 µg PAH/g C for all 42 analytes or from 0.1 to 400 µg PAH/g C for the priority pollutants alone (Table 3). The patterns of PAH concentration distribution between the sample groups remain the same. Rock and soil samples have a larger span of PAH concentrations due to the heterogeneity of the potential sources (14.7 to 2774 and 0.3 to 602 µg PAH/g C, respectively).
Riverine and marine samples exhibit smaller ranges of PAH concentrations (381 to
1124 and 161 to 457 µg PAH/g C, respectively) from the incorporation of carbon sources actually delivered to the river and margin. The highest concentration of
PAHs in the watershed is still found in a rock; however, this occurs in a different sample (the Cretaceous sandstone from the Mokoiwi Fm.) than the raw concentrations (the Tertiary mudstone from the Waipawa Fm.). Normalization by
%C org adjusts for the relative contribution of PAHs to other organic compounds in
162 sediment, allowing for comparison of the PAH composition between samples with varying %C org (Stout and Emsbo-Mattingly, 2008).
3.2 PAH distributions
3.2.1 Classic pyrogenic and petrogenic PAH distributions
Aside from differences between the total PAH concentrations of individual rock and soil sources, the distributions of individual and groups of PAHs also vary and can therefore be used to identify contributing source composition in riverine and marine sediments. PAH fingerprints of pyrogenic PAHs are characterized by discrete peaks of non-alkylated PAHs and the relative absence of alkylated homologs due to a higher temperature of formation (Figure 3, top; Stout et al.,
2001a; Stout et al., 2001b; Stout et al., 2003), resulting in a pattern of decreasing concentrations of PAHs with increasing substitution (Laflamme and Hites, 1978).
Enrichment in high molecular weight (4-6 ring) PAHs is also characteristic of pyrogenic PAHs (Stout et al., 2003). Natural background sediments may also display a few, discrete non-alkylated PAH peaks (fluoranthenes, pyrene, benzo(a)pyrene) (Stout et al, 2001a; Stout et al., 2001b) from pyrogenic PAH sources such as natural fires (Burns et al., 1997). Petrogenic PAH profiles, concentrated in rock samples, contain more alkylated PAHs than soil samples, with bell shaped distribution patterns (Stout et al., 2003; Figure 3, bottom). PAH
163 distribution patterns for potential terrestrial organic matter sources of sediment to riverine and seabed sediments have been determined.
0.005 W1 0-5 cm 0.004 pyrogenic patterns 0.003 0.002 0.001 0.000 2.5 Cretaceous Gully 2 2.0 petrogenic patterns
g PAH g /gPAH sample 1.5 µ 1.0 0.5 0.0 l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y f l l l l y y t a o r r r h t y r e t t y y y p p p p a c c c c a y s s s s r a r o p h o t h h a a a a o o o r n y r r n n p p p r i t u P y y y y r z t l o o o o a a a a R ] ] e ] e n r r r r p h h h h u u u i i i i h r r r r a h h a a h h P o B n h t t t t l l l t t e a d t p F a r / h h h h r r P a h h h h h h h h [ [ ] p C i e p h h h h F F F t t t t n t t t t s n c n C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e B t t t t u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure 3. Pyrogenic and petrogenic PAH histogram patterns in soil and rock, respectively. At the top, the classical pyrogenic PAH pattern is detected in this tephric topsoil; the concentration decreases with increasing alkyl substitution. At the bottom, the PAH distribution profile displays the classic bell-shaped alkyl substitution pattern seen for petrogenic PAHs as in this sandstone from the Mokoiwi Fm.
164 3.2.2 PAH distributions of potential rock carbon sources
PAH distributions in bedrock (Appendix 3 and 4.1) are primarily petrogenic in origin due to catagenesis, although paleo-forest fires may contribute minor quantities of pyrogenic PAHs to rock composition. The Tertiary Wanstead Formation, a pale grey-green calcareous and smectitic mudstone with intercalated beds of glauconitic and lithic sandstones, exhibits classic petrogenic bell-shaped curves for alkyl- substituted PAH compound groups. The PAHs in the glauconitic sandstone beds from the Wanstead Fm. are composed only of highly weathered naphthalenes and phenanthrene.
The Mokoiwi Fm., an alternating centimeter to decimeter bedded, fine to medium grained Cretaceous sandstone, contains only bell-shaped napthalenes and phenanthrene. The Cretaceous Tolaga Group, with massive bedded, slightly calcareous, sandstone and mudstones and the Oligocene Weber Fm., composed of poorly bedded, pale grey calcareous mudstone with thin glauconitic sandstone beds, both have classic, bell-shaped, petrogenic PAH distributions. The Tikihore Fm. (a
Cretaceous, fossiliferous, centimeter to meter bedded, alternating fine-grained sandstone and mudstone) and the Cretaceous Whakai Fm. (a well-bedded alternating sandstone and mudstone) samples obtained have low concentrations of
PAHs (<0.08 µg PAH/g sample), petrogenic PAH distributions, no trace of high molecular weight PAHs, and relatively high concentrations of phenanthrenes.
165 Mudstones tend to have higher concentrations of PAHs than sandstones.
Both the Tertiary Waipawa Fm. (a poorly bedded, grey to black, non-calcareous mudstone) and the Cretaceous Whangai Fm. (a non-calcareous and calcareous, micaceous, and siliceous shale and mudstone) had very high concentrations of petrogenic PAHs with classic bell-shaped distribution profiles of alkylated PAHs.
Minimal traces of high molecular weight PAHs in the baseline suggest possible contributions of pyrogenic PAHs preserved from ancient forest fires.
3.2.3 PAH distributions in watershed soils
Depending on the type of soil, the PAH signature may contain a mixture of pyrogenic PAHs from biomass burning (from wildfires and slash and burn deforestation), petrogenic PAHs from underlying bedrock from which the soil is derived, and biogenic PAHs from overlying organic matter incorporated into the soil
(Appendix 3 and 4.2). On the forested hillslopes, soils have low TPAH concentrations with bell-shaped distributions, indicating petrogenic PAHs from the underlying bedrock. These soils also contain biogenic PAHs from the degrading organic matter.
On the bedrock-derived hillslope soils that have been cleared for pasture, the biogenic PAHs are no longer evident, but pyrogenic PAHs have been added to the soil by slash and burn deforestation. The topsoil is influenced by high molecular weight pyrogenic PAHs and pyrogenic distribution patterns in higher molecular
166 weight alkylated compounds (i.e. phenanthrene/anthracenes). With increasing depth in the core, the profile looks more petrogenic as the influence of the underlying bedrock begins to be more dominant, and the soil reflects the bedrock rather than the topsoil. The high concentration of the petrogenic PAHs overwhelms the pyrogenic compounds in all but the uppermost portion of the soil profile. However, since the landscape actively experiences sheetwash, the PAH components of the topsoil will be transported through the watershed.
Tephric soils are built on volcanic ash, therefore the less concentrated pyrogenic PAHs deposited from biomass burning are more evident. On a stable pasture which was cleared by burning, high molecular weight pyrogenic compounds and weathered pyrogenic PAH distributions (refer to Discussion section 4.2) are present throughout the upper portion of the profile, likely from tillage mixing the top layer down. The pyrogenic PAHs decrease from the topsoil down, except in the allophanic horizon where they are the most concentrated. On an actively eroding pastoral hillslope with preserved tephra layers, the PAH distribution patterns reflect weathered naphthalenes and a pyrogenic distribution of the phenanthrene/ anthracene compounds are the only PAHs present downcore. This terrain is actively being eroded by sheetwash, landsliding, and tunnel gullying. (Tunnel gullying frequently occurs in the presence of other erosional processes; water moves through subsurface cracks and eluviates subsurface material, often resulting in surface collapse (Lynn and Eyles, 1984; Page et al., 1994).) The combination of these
167 erosional processes is likely responsible for the relatively uniform PAH profiles observed downcore.
Both colluvial soils have similar profiles, as they both incorporate multiple types of soils (i.e. tephric, forested bedrock-derived, pastoral bedrock-derived) from the watershed. The only PAHs present in both colluvial profiles are weathered naphthalenes and pyrogenic distributions of phenanthrene/anthracene alkylated compounds. These PAH distributions seem consistent with the aggradational nature of the soils where exposure causes weathering and slash and burn deforestation adds pyrogenic PAHs. These profiles are both stable and so are not likely to be sediment sources to the river.
The different horizons of alluvial soils record episodic river flood events, so systematic vertical trends are not typical. Rather, organic matter from upstream sources is incorporated in alluvial soils that serve as storage along the river. It is likely that soils would be represented in these profiles along with rock carbon contributed from gully erosion that is transported constantly down the river. Most horizons have bell-shaped distribution profiles in the low molecular weight PAH compounds. The presence of high molecular weight pyrogenic PAH compounds suggest pyrogenic PAH input. In the recent riverbank alluvial soil (which has a developed topsoil), pyrogenic distribution patterns in the higher molecular weight alkylated PAHs indicate that these layers were most likely formed during floods that moved terrestrial soil carbon in addition to the rock carbon typically transported during lower river stages. In the raw riverbank alluvial soil (no topsoil developed),
168 while high molecular weight pyrogenic PAHs are present, all alkylated PAH patterns are bell-shaped, suggesting that the bedrock overwhelms pyrogenic soil contributions. Thus, the pyrogenic signal is only detected in the high molecular weight pyrogenic compounds.
Natural thermogenic hydrocarbons produced in the watershed “contaminate” the sediment (hot springs) and soil (natural gas) in a few places. Both show petrogenic PAH distributions and a complete absence of high molecular weight pyrogenic PAHs (Appendix 3 and 4.3). The naphthalene compounds at both sites are weathered, likely due to natural evaporative processes.
3.2.4 PAH distributions of organic carbon delivered to the river and margin
Samples of sediment were obtained from the riverbed at six locations throughout the watershed (Appendix 3 and 4.4) to examine contributions of terrestrial carbon at different points along the river. All of these samples were dominated by petrogenic PAH distribution patterns with trace amounts of high molecular weight pyrogenic PAHs. This is indicative of rock carbon being the dominant fraction delivered to the river. However, this does not rule out the incorporation of soil carbon (modern or aged). The presence of high molecular weight PAHs could also indicate pyrogenic PAHs from soil sources that are overwhelmed by the petrogenic patterns observed in each alkylated PAH series.
The similarity of the riverbed muds at all locations analyzed could suggest that the
169 primary terrestrial source of organic carbon to the river is found throughout the watershed or that the dominant source of sediment is at the headwaters of the river and overwhelms downstream contributions.
Sediment along a transect on the continental shelf and slope off the river mouth has the same PAH distribution patterns for each core and each interval within the cores (Appendix 3 and 4.5), suggesting no major change in source over recent history (approximately the last 70 years; Kuehl, 2007; Kniskern, 2007; Kniskern et al., 2009). Like the riverine sediment, all alkyl-substituted patterns in the PAH distributions of the marine sediments are bell-shaped, indicating that petrogenic
PAHs dominate the continental margin. Trace amounts of high molecular weight (4-
6 ring) pyrogenic PAHs are also present. Thus, the sediment does potentially reflect contributions from a variety of terrestrial organic carbon sources; however, the low concentrations indicate that these contributions would be minor compared to the petrogenic source. In addition, biogenic perylene is added to the sediment in the marine environment (Venkatesan, 1988); however, biogenic PAHs do not appear to be a major influence on PAH profiles relative to the terrestrial input. The similarity of marine and riverine PAH distributions coupled with the lack of variation in PAH distribution across the margin underscores the dominance of riverine organic carbon across the entire margin.
The similarities between PAH distributions for various terrestrial sources makes the process of determining specific sources in riverine and marine sediment based on the descriptive histograms subjective. This qualitative method of source
170 apportionment can be improved upon using other methods of data and statistical analysis.
4. Discussion
4.1 Detection of contaminant and background PAHs
PAH signatures have historically been used to discern contaminant point sources such as oil seeps, oil spills, eroding source rocks, and coal deposits. This source apportionment can be used to assess responsibility in environmental disasters such as in the case of the Exxon-Valdez oil spill (Bence et al., 1996; Page et al., 1996; Page et al., 1999b; Short et al., 1999; Mudge, 2002). PAH source apportionment was also used to differentiate creosote from urban runoff and natural background PAHs at a wood treatment facility, determining the effectiveness of a sediment cap intended to prevent dispersion of creosote that seeped into the harbor
(Brenner et al., 2002).
In addition to contaminant point sources, there are also non-point sources.
Such sources include stormwater runoff, surface runoff (e.g. roadways and bridges), direct atmospheric deposition (e.g. particles from biomass, coal and petroleum combustion), and other small but persistant discharges (Zeng and Vista, 1997;
Brenner et al., 2002; Stout et al., 2004). This “urban background” has been established for several major urban waterways on the east and west coast of the
171 United States, ranging from 1.5 to 25.5 µg PAH/g sediment with an average of 12
µg PAH/g sediment (Stout et al., 2004; Table 4). Urban waterways in Washington were found to contain as much as 50 µg PAH/g sediment, with an average of 37.5
µg PAH/g sediment (Stout et al., 2003). Each environment exhibits its own distinct distribution of PAHs, indicating that while urban background sources are common, each setting is unique, and a representative background distribution cannot be resolved (Stout et al., 2004).
Table 4. Mean TPAH concentrations in urban waterways 1 & the remote Waiapu TPAH Waterway (µg PAH/g sediment) San Francisco 1.5 Portland Harbor 4.7 Alameda Pt., CA 4.4 Thea Foss, WA 19.9 Eagle Harbor, WA 17.5 Boston Harbor 14.9 Elizabeth River 25.5 All urban sites 12.0 Waiapu 2 2.0 3 1unless otherwise denoted, all data comes from Stout et al., 2004; 2this study; 3mean value of 6 river sites sampled throughout the watershed.
Relative to these sites, the Waiapu River watershed is removed from urban non-point source contamination as well as point source contaminants, with minimal anthropogenic influence aside from agriculture (Mazengarb and Speden, 2000). The mean TPAH concentration of 2.0 µg PAH/g sediment found in the Waiapu River is six times lower than the average of the urban backgrounds (Table 4; Stout et al.,
172 2004). In the Waiapu, the most concentrated terrigenous sample is more than 15 times less concentrated than the minimum common cleanup standard (common soil and sediment total petroleum clean up criteria range from 500 to 2000 µg/g total extractable hydrocarbons) (Table 3; Stout and Emsbo-Mattingly, 2008). The lowest concentration cleanup standard for the priority pollutants is for the carcinogenic benzo[a]pyrene at approximately 660 ng/g (Stout and Emsbo-Mattingly, 2008). This is two orders of magnitude greater than the highest BaP concentration found in the
Waiapu watershed (6.62 ng BaP/g solid for the Waipawa Tertiary rock formation which is not found to be a major source of POC to the river (refer to section 4.3)).
In the absence of industrial and urban contamination, low concentrations of background PAHs derived from natural and anthropogenic fires, natural oil seeps, eroded rocks, and early diagenetic processes are proposed to distinguish organic carbon sources in the watershed that can be traced to the adjacent continental margin. Deforestation by burning appears to be the only major anthropogenic source of PAHs in this region, and this addition of pyrogenic PAHs to known areas of affected land aids in the discrimination of background PAHs sources.
On the continental shelf and slope, relative PAH concentrations decrease downward in each of the cores and across the margin (Appendix 3 and 4.5). This may be the result of dilution, volatility, and biodegradation processes. Dilution of terrigenous petrogenic PAHs by addition of marine organic matter has been observed in other studies (Stout et al., 2003; Fang et al., 2009), decreasing the concentration of TPAH with distance from the source. Volatility and biodegradation
173 processes involve low molecular weight PAH loss from the sample. Naphthalenes are the most concentrated PAHs in these sediment cores, and therefore have the largest impact on the decrease in PAH concentration with depth and distance across the margin. Resuspension and transport of sediment may initiate volatilization of the susceptible, lower molecular weight naphthalenes and reduce their concentration
(Murphy and Morrison, 2007). Additionally, naphthalene-degrading bacteria have been previously isolated from continental margin sediments (Lima et al., 2005; Nair et al., 2008). These bacteria can efficiently utilize particle-associated naphthalene, causing total PAH concentrations to decrease downcore. It is important to note that despite the volatility and degradability of low molecular weight PAHs, in general, higher molecular weight PAHs (3-ring structures and larger) are very persistent in the environment (Lima et al., 2005). This persistence supports their use as tracers of organic matter through a small mountainous river watershed and margin.
4.2 Diagnostic ratios: A quantitative assessment of PAH distributions
4.2.1 Weathering
In addition to visual inspection (histograms, Appendix 4), the concentrations of specific PAHs can be used for quantitative evaluation of PAH distributions. This is particularly useful as the distinction between petrogenic and pyrogenic patterns becomes more difficult with weathering (Stout et al., 2003). Weathering processes
174 (physical, chemical, and biological) can impact the shape of PAH distribution patterns, complicating the interpretations of the histograms (Douglass et al., 1996;
Stout et al., 2001b; Stout et al., 2002; Stout et al., 2004; Gregory and Nichols, 2008).
PAHs are more resistant to weathering with increased alkyl substitution; therefore, weathering preferentially removes the parent compounds over their alkylated equivalents (Stout et al., 2001a; Stout et al., 2003). Thus, in soils, when pyrogenic PAHs are weathered, the classic pattern of decreasing concentrations in
PAHs expected with increasing alkyl substitution are skewed, decreasing concentrations in the parent and mono- (and possibly di-) substituted compounds
(C1 and C2), such that a new bell-shaped distribution resembles that of unweathered petrogenic PAHs. The presence of high molecular weight pyrogenic
PAHs (e.g. fluoranthenes, pyrenes, coronene) aid interpretation of weathered pyrogenic PAH histograms. The weathering of petrogenic PAHs causes the bell- shaped distribution of alkylated PAH homologs to be skewed towards more highly substituted homologs which are more resistant (Stout et al., 2001a; Stout et al.,
2001b). This results in a pattern of increasing petrogenic PAH concentration with increased substitution.
Diagnostic ratios such as C3-naphthalenes/C2-phenanthrenes (N3/P2) can be useful in determining the degree of recent weathering in samples of petrogenic source (Figure 4; Douglas et al., 1996; Page et al., 1996; Gregory and Nichols,
2008). Because petrogenic PAHs are dominant in this watershed, when N3/P2 is plotted against diagnostic source ratios such as C3-dibenzothiophene/C3-
175 15 10 l Rocks A 12 l Soils 8 B l Riverine Mud 9 l Marine Sediment 6
N3/P2 6 N3/P2 4
3 2
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 D3/P3 D3/C3
Figure 4. Diagnostic ratio cross plots used to assess the degree of PAH weathering in environmental samples. Samples closer to the origin are more highly weathered. Rock and soil samples both show variation in weathering patterns. Mixed soils (colluvial and alluvial), pastoral topsoils, and exposed sandstones show the most weathering.
phenanthrenes (D3/P3; Figure 4A; Douglas et al., 1996; Page et al., 1996; Gregory and Nichols, 2008), relative weathering of potential sources can be visualized.
Decreasing ratios approaching zero are increasingly weathered (Page et al., 1999b).
Selective evaporation of low molecular weight PAHs is a possible artifact of sample preparation; therefore the interpretation of weathering in low molecular weight PAH patterns such as naphthalenes must be cautiously rendered.
Weathering preferentially reduces the proportion of low molecular weight PAHs to high molecular weight PAHs (Stout et al., 2001a). Thus, chrysene homologs are not as susceptible to weathering as lower molecular weight PAHs (Stout et al., 2003).
Substituting C3-chrysene for C3-phenanthrene in a weathering diagnostic plot provides a simplified picture of longer-term weathering influences, significantly
176 distinguishing the relative degree of weathering among the potential rock carbon sources (Figure 4B).
Weathering impacts many of the Waiapu samples herein (Figure 4). The histograms of samples including pastoral topsoils, buried soils that were once topsoils, and colluvial soils that have formed from soils mixing during mass wasting processes all show evidence of weathering to some degree (Figure 4; Appendix 4).
However, the resulting reduced concentrations of certain PAHs from weathering are part of each sample’s molecular profile, incorporating the weathering influence of a specific source in its PAH signature used for apportionment.
4.2.2 Source apportionment using diagnostic ratios
In addition to determining the degree of weathering in environmental samples, the concentrations of specific PAHs can also be used to calculate source-specific diagnostic ratios (e.g. Stout et al., 2001a; Yunker et al., 2002; Bucheli et al., 2004;
Stout and Emsbo-Mattingly, 2008). In order to use diagnostic ratios for source apportionment, the relative thermodynamic stability of different parent PAHs, the characteristics of PAH sources, and changes in PAH composition between source and sink must all be considered (Yunker et al., 2002; Lima et al., 2005). For example, combustion processes produce a larger proportion of the less stable PAH compound, thus the ratio of unstable to stable compounds provides an estimate of the organic matter source (Lima et al., 2005).
177 1.4 40 l Rocks 1.2 l Soils A B 1.0 l Riverine Mud 30 l Marine Sediment 0.8 20 0.6 BaA/C0 0.4 10 0.2 %4-6 ring PAHs 0.0 0 0 1 2 3 15 0.01 0.1 1 10 100 FL/PY TPAH
Figure 5. Diagnostic ratio cross plots illustrate relationships between various environmental samples. The marine and riverine sediment samples are in close proximity with intermediate values for both ratios relative to soils and rocks, suggesting a mixture of both sources. A) FL/PY>1 and BaA/C0>0.9 indicates pyrogenic PAHs. With few exceptions, rocks have low diagnostic ratios relative to the soils. B) Rocks typically show a low percentage of high molecular weight pyrogenic PAHs relative to soils. Circles drawn are not statistically significant, but rather demonstrate proximity only.
Characteristic relationships between certain compounds have been defined in the literature to differentiate pyrogenic and petrogenic sources of PAHs (e.g. Stout et al., 2001a; Table 2). For example, the PAH source ratio allocation model used the diagnostic source ratios of C 2-dibenzothiophene/C 2-phenanthrene (D2/P2) to distinguish Exxon Valdez crude oil from natural background oil seeps, the latter of which was nearly identical to the offshore sediment diagnostic PAH signatures
(Page et al., 1996). Selected diagnostic ratios are shown in Table 3.
These source specific diagnostic ratios normalize the data to account for concentration variability between samples (Christensen et al., 2004), thereby
178 facilitating the comparison of organic matter in potential sources and sediments.
Distinct relationships, determined by similar diagnostic ratios, are evident between possible carbon terrestrial sources (soil and rocks) and marine sediment samples
(Table 3). Because there are so many possible sources in the watershed, direct comparison of these ratios with sediment ratios is challenging. Therefore, cross plots of diagnostic ratios are used to graphically analyze chemical similarity between terrestrial and marine samples to determine potential specific sources of terrigenous carbon delivered to the margin.
In Figure 5, two such cross plots of diagnostic ratios are shown. Chemical similarity is determined by spatial proximity of potential sources with the sediment being apportioned. The cross plot diagram of fluoranthene/pyrene (FL/PY) ratios against benz[a]anthracene/ chrysene (BaA/C0) indicates an apparent mixing of rock and soil sources (Figure 5A). A ratio of FL/PY>1 is indicative of pyrogenic sources, thus, the ratio increases as the proportion of pyrolytic input in a sample increases
(Benlahcen et al., 1997; Readman et al., 2002; Yunker et al., 2002; Lima et al.,
2005). In the Waiapu, this is seen as increased FL/PY ratios correspond to soils that have been cleared by deforestation, adding pyrogenic PAHs to the soils. This is similar to that seen in urban backgrounds where increased combustion-derived particulate matter adds pyrogenic PAHs (Stout et al., 2004). The rocks have predominantly low FL/PY ratios as pyrogenic PAHs are due to combustion from natural fires prior to lithification rather than recent anthropogenic combustion processes.
179 As with FL/PY, BaA and C0 are abundant in combustion-derived samples, thus low levels of these indicate predominantly petrogenic origin (Zeng and Vista,
1997). A BaA/C0 ratio greater than 0.9 is considered pyrolytic in origin, whereas a value less than 0.4 indicates petrogenic origin (Readman et al., 2002). As the contribution of combustion products increases, this ratio increases. The riverbed mud links the rock and soil organic matter sources to the river with the marine sediment, where the terrestrial carbon is ultimately buried. The general clustering of riverine and marine sediment in the center of the continuum of potential sources suggests a mixing of sources, but does not resolve specific sources (Stout et al.,
2001a; Stout et al., 2003).
An increase in the percent of high molecular weight PAHs relative to the total of PAHs (TPAH) is typically indicative of more pyrogenic inputs (Figure 5B; Stout et al., 2003). The soils tend to have a higher percentage of pyrogenic PAHs despite lower concentrations overall. Rock samples have a propensity towards lower percentages of high molecular weight PAHs regardless of total PAH concentrations with few exceptions likely influenced by paleo-forest fires or extensive weathering.
Again, the clustering of riverine and marine sediment suggests a mixing of rock and soil sources, but these are not well resolved (Stout et al., 2001a; Stout et al., 2003).
Diagnostic cross plots are useful for quickly visualizing the relationships between many samples in the watershed-continental margin system to establish chemical similarity between specific sources (soils and rocks) and buried organic matter in the river and margin. The convenience of these cross plots is the rapid
180 assessment of similarity between ratios that have pre-defined relationships based on origin and PAH type. However, diagnostic ratio cross plots are subject to interpretation and must be used with care in context of a specific environment (Lima et al., 2005). The accurate use of diagnostic ratios requires source signatures to be distinctive (Lima et al., 2005). Ratios can be helpful in distinguishing petrogenic from pyrogenic, but sources may not be well resolved. Unfortunately, these plots are limited to the influence of only four PAH compounds, as each axis takes into account the relationship between only two parameters, therefore restricting comparisons to a small subset of data.
4.3 Source apportionment by principal component analysis
In order to overcome the limitations of diagnostic ratio cross plots, a quantitative chemometric analysis has been applied (Stout et al., 2004). Principle component analysis (PCA) is a multivariate statistical analysis that can be used to resolve sources of organic matter that may be contributing to a sample by identifying common trends or distributions in data based on statistical associations (variance and covariance) (Shaw, 2003; Stout et al., 2003). PCA enables classification of data according to the chemical similarity rather than pre-classification according to the nature of the source (Stout et al., 2003). Molecular distributions can be combined in
PCA to develop complex fingerprints of PAH sources in lieu of simple ratios between selected compounds (Mudge, 2002). Thus, possible sources of PAHs in sediments
181 can be identified with better resolution using all or selected PAH analytes in PCA
(Burns et al., 1997; Christensen et al., 2004).
PCA generates new independent variables that are linear combinations of the original interrelated input variables (Stout et al., 2001a; Stout et al., 2001b; Shaw,
2003; Stout et al., 2004). Eigenvector decomposition is a simple mathematical procedure that reduces the dimensionality of a data set to a few important, uncorrelated principal components (PCs) that best describe variations in data (Stout et al., 2001a; Stout et al., 2003). Each PC accounts for a progressively smaller percentage of the variance within a data set. If all of the variability between samples can be accounted for by a small number of PCs, then the relationships between multivariate samples can be assessed by simple inspection of 2 and 3 dimensional graphs.
The PC score plot is a graphical representation of the principal components determined by the PCA. When interpreting PC score plots, spatial relationships between samples are representative of their chemical relationships (Stout et al.,
2001a; Giles et al., 2007). Samples that plot close to one another tend to be chemically similar; the further samples plot from one another, the more chemically distinct they tend to be (Stout et al., 2001a; Stout et al., 2003). Samples that spread out along a continuum in a score plot indicate a chemical relationship with non- identical distributions (Stout et al., 2003). For example, trends connecting end member sources can indicate mixing and/or weathering (degradation, dissolution,
182 evaporation) (Stout et al., 2001a; Stout et al., 2003). This is similar to the continuum seen in the diagnostic ratio source plots (Figure 4 and 5; Stout et al., 2003).
The underlying chemical cause for trends in a PC score plot can be ascertained by examination of the factor loadings calculated for each input variable used in the PCA (Stout et al., 2001a). Factor loadings can be visualized with a cross plot, revealing which variable, in this case PAH analyte, is responsible for “pulling” particular samples in a given direction on a PC score plot (Stout et al., 2001a).
Common spatial distributions of individual factor loadings can indicate relationships between certain analytes (e.g. categories such as petrogenic and pyrogenic PAHs among the 42 individual analytes), suggesting the potential basis of the trends observed in a score plot.
Because PCA can compare large numbers of variables simultaneously, the
PAH concentration data for all 42 PAHs identified can be analyzed to give each potential source a unique PAH fingerprint to compare with the riverine and marine sediment. However, the variation in scale for some parameters requires normalization to compare data. Often, these data are normalized using a z-factor,
Eq. (1): z = x i – µ / s, where x i is the individual observation (a specific PAH concentration or isotopic value), µ is the mean of all values for that data category, and s is the standard deviation for that category. Without z-score normalization, results are biased to variables with the largest numerical values. Z-scores leave the shape of the data
183 distribution unchanged, but re-scales each component axis to overall equivalent units (Shaw, 2003).
An initial PCA model was developed in the JMP 7.0 software by SAS using quantitative chemical fingerprints of the entire suite of 42 PAH analytes to compare each potential terrestrial organic matter source analyzed in the Waiapu River watershed with the riverine and marine sediments on its adjacent margin (Chapter 2;
Burns et al., 1997; Stout et al., 2001a; Mudge, 2002). Using z-factors to scale the concentrations, giving a standard deviation of 1 to each variable, this model produced a linear pattern determined to be a “flying v” (Johnson, 2009). All of the soils were grouped together at the point of the v, riverine and marine sediments were at one of the terminal ends, and rock samples were spread throughout.
Though this analysis accounts for a large fraction of the sample variance (72%; 55% in PC1 and 18% in PC2), this pattern is known to indicate that the data is concentration driven (Johnson, 2009). Because the PAH concentrations in the soils are so low relative to the sediment and most of the rock samples, chemical similarities within distribution profiles were negligible as compared to the concentration difference. Therefore, it was determined that the data must be normalized by concentration rather than by z-score.
PAH data were normalized to the total PAH concentration to remove the effect of widely varying concentrations between samples and between individual analytes, giving each equal weight and assessing chemical similarity based on PAH distributions (Stout et al., 2001a; Stout et al., 2003). This model (Figure 6) accounts
184 10 Rocks: Tertiary mudstone 8 HMW pyrogenic PAHs Tertiary sandstone Other rocks 6 Cretaceous sandstone Cretaceous mudstone 4
Soils: 2
■ Colluvial Prin2 ■ Tephric 0 ■ TSLHT (recent alluvial) ■ YHT (raw alluvial) ■ Bedrock weathered -2 ■ Plant influenced LMW non-alkylated topsoils -4 PAHs / highly alkylated weathered petrogenic PAHs l River sediment -6 samples l Marine sediment
-6 -4 -2 0 2 4 6 Prin1
Figure 6. Principal component analysis of 42 individual PAH concentrations normalized to TPAH. The first two PCs account for 46.5% of the variance in the data (PC1 for 31.5%; PC2 for 15.0%). PC1 is influenced by the degree of weathering and degree of alkylation, while PC2 is influenced by petrogenic, pyrogenic, and biogenic PAHs. Non-alkylated low molecular weight petrogenic PAHs dominate the left side of the score plot, while the upper right quadrant represents the dominance of pyrogenic PAHs and the lower right quadrant is populated by samples dominated by alkylated petrogenic PAHs and biogenic PAHs. for 46.5% of the data variance in the first two principal components (31.5 and 15%, respectively) and 56.4% if the third PC is considered. (PCA analysis of data normalized by concentration and z-score afforded the same statistical results as concentration normalization alone.)
The PCA score plot (Figure 6) reveals the chemical similarity between PAH distributions of terrestrial organic matter from different sources throughout the
185 watershed and those of riverine and marine sediments. Marine sediments are in close spatial proximity of one another, indicative of similar PAH distributions likely resultant from their homogenization during delivery to the shelf. Riverine samples are relatively close to the marine samples, indicative of their role as the transport mechanism between watershed sources and the marine environment. They are somewhat less tightly grouped than the marine samples, as the six river sites are located throughout the watershed, suggesting slightly different distributions based on local terrigenous sources.
Sources of sediment and particle-associated terrigenous organic matter delivered to and preserved in the river and marine environments will have similar chemical signatures (Chapter 2). PCA offers higher resolution of these sources, as the chemical signature is based on 42 parameters for each sample rather than one or two. The terrestrial sources in closest proximity to the riverine and marine sediments in the PC score plot are the Cretaceous Whangai Fm. mudstones, the raw riverbank alluvial soils, plant-influenced topsoils, and the Tertiary sandstones of the Wanstead Fm. (Figure 6). These sources of organic matter are therefore the most chemically similar to the riverine and marine sediments, suggesting their active role in sediment contribution to the margin.
The factor loading plot is employed to understand the chemical composition driving the distributions observed on the PC score plot. Detailed analysis of the loading plot for the first two PCs for the concentration normalized PCA revealed that the left half of the PC score plot is influenced by low molecular weight, non alkylated
186 compounds (such as naphthalene, dibenzofuran, anthracene, phenanthrene to the upper left and biphenyl and acenaphthene to the lower left). Principal components are pulled to the upper right quadrant in the presence of high molecular weight pyrogenics PAHs, such as fluoranthenes, benzo[g,h,i]perylene, benzo[e]pyrene, benz[a]anthracene, indeno[1,2,3-cd]pyrene, chrysene, and C1/C2 chrysenes and phenanthrene/anthracenes. Compounds that appear to pull PCs back to the origin of the loading plot include acenaphthylene, benzo[a]pyrene, fluorene, dibenzothiophene, coronene, dibenz[a,h]anthracene, and pyrene. The lower right quandrant is dominated by alkylated petrogenic PAHs and biogenic PAHs (i.e. alkylated naphthalenes, fluorenes, dibenzothiophenes, fluoranthene/pyrene; C3/C4 chrysenes and phenanthrene/anthracenes; and perylene and retene.
PC1 is influenced primarily by the degree of weathering and corresponding molecular weight and alkylation constraints. PC2 separates the pyrogenic PAHs from the petrogenic and biogenic PAHs. PC3 appears to be most heavily influenced by molecular weight. Residual variance after PC3 is not noise in the analysis. To adequately describe the system variance, 14 PCs are required. However, after the first four, corresponding to petrogenic, weathered petrogenic, petrogenic/pyrogenic mixture, and petrogenic/biogenic mixture end-members, PCs begin to describe variations within sample types which are expected for background level environmental samples.
Because biogenic and petrogenic PAHs appear to pull samples in the same direction on the loading plot where the riverine and marine samples are located,
187 source apportionment between marine samples and terrestrial sources in this quadrant is ambiguous. Therefore, biogenic PAHs were removed from the analysis, as the margin is dominated by riverine input rather than in situ marine productivity.
The resultant 40 variable PCA returned the same distribution of samples, indicating that the biogenic PAHs indeed appear to have little influence in the variance of the data. Marine samples appear to be influenced primarily by the petrogenic PAHs.
4.4 Implications for geomorphic processes
Gully erosion, shallow landsliding, sheetwash erosion, and bank erosion are dominant mechanisms for sediment generation in small mountainous river watersheds and consequently are hypothesized to control the composition of the terrigenous particulate organic carbon (POC) preserved on the margin. These processes erode different components of terrestrial organic matter (e.g. Page et al.,
2004; Marden et al., 2005a; 2005b; Leithold et al., 2006; Parkner et al., 2006).
Erosion from an active gully is a continual process driven by high frequency, low magnitude storms that erode the watershed slowly over time delivering rock debris along with its associated ancient carbon (kerogen). Gully development is a threshold event, and occurs in given terrains when a combination of factors including bedrock susceptibility, mountainous topography, recent forest clearance, and extreme precipitation events act together (Zhang et al., 1993; Parkner et al., 2007).
The threshold conditions for gully initiation are less extreme under pasture than
188 forest cover; therefore deforestation for agricultural land use increases gully development (Zhang et al., 1993; Derose et al., 1998; Gomez et al., 2003a; Kasai et al., 2005; Marden et al., 2005a; Parkner et al., 2006).
Shallow landsliding and sheetwash each contribute a component of terrestrial plant and soil organic matter to the river and adjacent continental margin. Shallow landsliding is a threshold event, typically triggered by intense episodic storm events causing high sediment yields, preferentially delivering modern carbon associated with topsoil (Page et al., 1994; Page and Trustrum, 1997; Eden and Page, 1998;
Marutani et al., 1999; Page et al., 2001; Gomez et al., 2002; Gomez et al., 2003b;
Reid and Page, 2003; Page et al., 2004; Kasai et al., 2005; Liebault et al., 2005;
Marden et al., 2006). This process can also deliver aged soil organic matter as landslides typically have failure planes at the soil-bedrock interface, on the order of
1 m depth on the East Cape of New Zealand (Page et al., 2004; Blair et al., 2009).
Sheet erosion contributes topsoil typically to no deeper than a few centimeters; therefore this process is responsible for transporting primarily modern terrestrial organic matter to the river (Page et al., 2004; Leithold et al., 2006; Marden et al., 2006). This process does not move large quantities of sediment; however, unlike landsliding, it can supply sediment under normal precipitation conditions rather than primarily during severe weather events. Therefore, it is a regionally important process, particularly in areas that have been destabilized due to deforestation, pasture conversion, and landsliding (Marden et al., 2006).
189 Bank failure contributes alluvial soil and sediment stored along the margin of the river channel (Marden et al., 2005b; Phillips et al., 2007). This POC is a composite of upstream sediment sources, and should reflect those terrigenous organic matter origins. This includes rock carbon from gully erosion, modern plant and soil carbon from sheetwash and landsliding, and aged soil carbon from landslides. These sources accumulate on the riverbank and form alluvial soils particularly during flood conditions that overwash the existing riverbank (Marden et al., 2005b; Phillips et al., 2007).
In the Waiapu River watershed, deforestation by Maori tribes beginning approximately 700 years B.P. (Hicks et al., 2000) and European settlers beginning about 100 years B.P. (Page et al., 2001) destabilized the land and increased erosion beyond natural levels. Because gully erosion, landsliding, sheetwash, and bank failure each contribute POC of different rock and soil composition to marine sediments over time (Kasai et al., 2005), source apportionment should be able to distinguish active geomorphic processes contributing terrestrial POC to the margin based on the carbon preserved.
The PCA model suggests Cretaceous mudstones, Tertiary sandstones, topsoils, and raw riverbank soils are the primary sources of organic carbon to the margin over recent history, being the most chemically similar to riverine and marine sediment sampled. Aerial photography and digital elevation models have previously identified gullying of the highly erodible Cretaceous Whangai Fm. to be the primary source of sediment to the Waiapu River during normal flow conditions (Page et al.,
190 2001; Parkner et al., 2006). This dominance is due to the destabilization of the large area of the highly erodible Whangai Fm. underlying and exposed in the mountainous watershed in conjunction with extreme precipitation events (Derose et al., 1998;
Marden et al., 2005a; Kasai et al., 2005; Parkner et al., 2006). Tertiary sandstones are also chemically similar to the sediment after normalization by concentration; however, this is likely a minor source since these rocks underlie less of the watershed and also tend to contain lower concentrations of organic carbon and
PAHs than the Cretaceous mudstones (Derose et al., 1998). After the formation of a gully, a continuous supply of sediment is generated by low magnitude, high frequency rainfall events. At present, there are over 900 active gullies in the Waiapu
River watershed (Parkner et al., 2006). Twenty-two percent of the catchment is prone to gullying, accounting for significantly more than 50% of the sediment yield
(Page et al., 2001; Parkner et al., 2006). Incised sedimentary rock is therefore a primary component of the particulate load (Page et al., 2001).
Like gullying, sheetwash only requires low magnitude, high frequency storms to deliver topsoil, particularly from hillslope areas destabilized by deforestation such as are abundant in this watershed. Thus, while it does not transport a large quantity of soil organic matter, sheetwash appears to be an important process in the watershed, providing a continuous supply of topsoil to the river. While landsliding is evident throughout the watershed, it does not emerge as a primary source of sediment to the river except during major episodic storm events, which were not identified in the sampled marine horizons herein (Page and Trustrum, 1997; Eden
191 and Page, 1998; Marutani et al., 1999; Page et al., 2001; Reid and Page, 2003;
Page et al., 2004; Kasai et al., 2005; Marden et al., 2006).
The contribution of riverbank soils indicates that bank failure, particularly of raw riverbank alluvial soils (those that have not developed a topsoil), may be a major process contributing sediment to the margin. The degree to which bank failure contributes soil to the margin is difficult to establish because of the function of riverbank alluvial soils as storage sites for upriver sediment sources. The riverbank soil may simply be a sediment sink reflecting upriver sources in the same manner as the riverine and marine sediments. Due to the similarity of raw riverbank soils to riverbed and marine sediments and the dual function of these soils as both sediment source and sink, the distinction of riverbank alluvial soils as a significant source or sink of sediment in this river system has not been resolved with this method.
Relative changes in these processes throughout geologic time should be recorded in the organic geochemical record preserved in the continental margin sediment. For example, in many small mountainous watersheds, land use changes such as clearing for timber harvesting and agricultural purposes have a clear signature in the offshore record (Leithold et al., 2005). Discerning the origin of organic carbon transported by these rivers and preserved on the adjacent margins elucidates the processes involved in present-day sediment production and carbon deposition and burial. This approach can clarify interpretations of the organic geochemical record, in turn improving reconstructions of watershed history and carbon cycling.
192
5. Conclusions
Isotopic analyses are limited in their usefulness for resolving organic carbon sources in the Waiapu sedimentary system, particularly the aged soil component
(Chapter 2 and 4). Therefore, polycyclic aromatic hydrocarbons have been investigated as molecular tracers to monitor organic carbon transport from source to sink in a small mountainous river sedimentary system. PAHs make excellent tracers of sediment transport due to their resistance to degradation, a result of the stability that aromaticity provides.
The unique conditions of formation and intrinsic recalcitrance of PAHs provide information about environment-specific disturbances that result in sediment generation and transport within the Waiapu River watershed. Petrogenic PAHs form in the bedrock during catagenesis and lithification. Bedrock formations have unique compositions, but exhibit classic petrogenic histogram shapes. Soils are composed of PAH mixtures, including petrogenic PAHs from the weathering of the bedrock from which they are derived. Forested soils are also influenced by biogenic PAHs from the overlying plant matter. The practice of deforestation by burning landcover adds pyrogenic PAHs to the pastoral soils. Tephra layers dilute soil organic matter with volcanic ash, so tephric pastoral soils have a more pronounced pyrogenic component as compared to bedrock-derived soils without tephric material. The degree of weathering a soil or rock is exposed to also impacts the PAH composition.
193 PAH signatures differentiate the various bedrock formations and soil types in this watershed, providing a valuable tool for resolving POC sources and the geomorphic processes that deliver this carbon to the river and continental margin.
Gully erosion has been previously estimated to account for more than half of the high sediment yield of the Waiapu River catchment, signifying that incised sedimentary rock is the primary component of the particulate load (Page et al.,
2001). Principal component analysis of PAHs provides geochemical evidence to support the aerial photography and digital elevation models (Parkner et al., 2006) that determined gullying of the Cretaceous Whangai Fm. to be the primary source of organic carbon to the margin during non-flood conditions. Additionally, topsoils and raw riverbank alluvial soils were also shown by PCA to be chemically similar to the riverine and marine sediment. Topsoils appear to be a continuous source of modern soil carbon to the river and margin by sheet erosion. Riverbank soils are mixtures of modern, aged, and ancient carbon from rock and soil deposits; the dual nature of riverbank soils as both sediment source and sink prevent the resolution of this source component.
Shallow landsliding requires initiation by intense periodic storm events in order to supply massive modern and aged soil transport downriver to be buried on the continental shelf and is therefore not a continuous process like gullying, sheetwash, and bank failure (Page et al., 1994; Eden and Page, 1998; Marutani et al., 1999; Page et al., 1999a; Page et al., 2001; Gomez et al., 2002; Gomez et al.,
2003; Reid and Page, 2003; Page et al., 2004; Kasai et al., 2005). PAH analysis of
194 storm layers in the stratigraphic record from storms like Cyclone Bola against watershed sources is proposed to indicate contributions of more aged soils in addition to bedrock and modern SOM, signifying landsliding activity in the watershed
(Evans et al., 1990; Marutani et al., 1999; Kasai et al., 2005). Unfortunately, no major weather events were identified in the sediment intervals analyzed for PAH composition. The analysis of a system that is not prone to gullying, but rather only landsliding such as Lake Tutira on New Zealand’s North Island would also be an excellent test site of soil delivery by landsliding (Page and Trustrum, 1997; Gomez et al., 2002; Page et al., 2004).
The use of PAHs as tracers advances our understanding of organic carbon/sediment transport dynamics through further resolution of carbon sources within the watershed, allowing for assessment of active geomorphic processes. Like other methods used for source apportionment, PAHs can be used downcore to look for changes in source throughout history (Wakeham et al., 1980; Stout et al., 2001a).
Their inherent stability may enable more lucid interpretation of the stratigraphic record in future studies, thereby improving our ability to link the organic geochemical record to both present and paleo-environmental geomorphic processes. Ultimately, this can be used to enhance the reconstruction of watershed history, providing a better understanding of the organic carbon cycle.
195 6. Acknowledgements
The field collection of rock and soil samples in 2007 was sponsored by the
National Science Foundation East Asia and Pacific Rim Summer Institute program which was co-sponsored by the Royal Society of New Zealand and Massey
University, Institute of Natural Resources, Fertilizer and Lime Research Centre in
Palmerston North, New Zealand. The National Science Foundation project “Source to Sink generation of biogeochemical stratigraphic signals across the Waipaoa margin, New Zealand” (OCE-0646159, Leithold and Blair) also supplemented this work. Marine samples were obtained on the R/V Kilo Moana, sponsored by the NSF project “Age Distribution of Particulate Organic Carbon (POC) Discharged from
Small Mountainous Rivers- the Influence of Sediment Yield and Soil Residence
Time” (EAR- 0222584, Leithold and Blair). PAH analyses were supported by the
Environmental Protection Agency/North Carolina Department of Environmental and
Natural Resources/Division of Water Quality 319 Program (EW6028, Nichols).
Special thanks go to Pete Lazaro of the NCSU Department of Toxicology who assisted with the analysis of PAH samples.
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213 8. Appendices
214 8.1 Polycyclic aromatic hydrocarbon structures of interest
O
Biphenyl Dibenzofuran Napthalene Fluorene
S
Dibenzothiophene Anthracene Phenanthrene Pyrene
CH3
CH3
CH3 Retene Chrysene Benzo[e]pyrene
Benzo[a]pyrene Perylene Coronene
Figure A1. Examples of PAH parent structures.
215 8.2 Full details of quality assurance/quality control
Of the 113 samples run on the GC-MS, there are 94 separate samples
including 5 different sample types (Table A1). For quality assurance, duplicates (12
samples) and triplicates (3 samples) of various sample types were run to ensure
replication of extraction and isolation methods (Table A1 and A2). Accuracy is
assured by running samples in four discrete sets, with replicate samples run in
different sets to ensure the equipment was operating properly over time. Internal
standards of deuterated PAHs (benzo[a]pyrene-d12 and phenanthrene-d10) in
known concentrations are used to quantitate the spectra. The aliphatic fraction of
one of the samples run in duplicate was also run to ensure good separation between
aliphatic and aromatic fractions. Furthermore, chrysene-d12 was added to 72% of
the samples analyzed to determine percent recovery of high molecular weight PAHs
(Table A3).
Table A1. Breakdown of samples run for 42 polycyclic aromatic hydrocarbons.
Samples Rocks Soils HC Mud drape Sediment Total Individual Samples 16 47 4 6 21 94 Duplicate Run 4 8 -- -- 3 15 Triplicate Run -- 3 ------3 Aliphatic Fraction ------1 1 % QA/QC Samples 25 23 -- -- 19 20
216 8.2.1 Precision/analysis replication
Replicated samples were compared using a paired t-test analysis to determine the significance or lack thereof of the difference between runs. The degrees of freedom for each analysis were 42, given the 43 variables compared in the analysis including the TPAH concentration and the concentration of the 42 individual PAHs measured.
Of the duplicate runs (Table A2), only one sample pair compared was significantly different at the 95% confidence level (YHT 10-15cm). The differences between the remaining 11 pairs analyzed were insignificant (p>>0.05). The average probability of all 12 pairs was significantly above 0.05, indicating that the concentrations of PAHs in the samples replicated were indeed statistically similar.
Therefore, using duplicate analysis, the precision of analysis is shown to be sufficient. The demonstration of precision in technique was extended to a few triplicate analyses (Table A2). Again, the average probability of all analyses was greater than 0.05, therefore, the difference between replicated samples are deemed insignificant. Finally, the average of all probabilities calculated for replicate analysis resulted in p>0.05, even at the extent of the average standard deviation.
217 Table A2. Paired t-test results of replicated sample analysis.
Sample p1,2 p2,3 p1,3 Duplicate Runs Barton’s Gully Jar 2 0.1506 Barton’s Gully Stream Bed 1 0.3053 Cretaceous Gully Rock 1 0.0561 Mata Slip Br 0.3113 BWBO 5-10cm 0.2426 W1 95-100cm 0.2467 YHT 10-15cm 0.0219 YHT 40-45cm 0.2724 TSLHT 15-20cm 0.1467 BC8 14-16cm 0.0584 BC8 29-31cm 0.1828 BC8 43-45cm 0.2778 Average 0.1894 Standard Deviation 0.1025
Triplicate Runs W1 15-20cm 0.3017 0.3039 0.2964 W1 75-80cm 0.3075 0.6637 0.3053 Forest Soil 0-4cm 0.1725 0.0501 0.5622 Triplicate Total Average 0.3292 Triplicate Total Std Deviation 0.1844
Overall Probabilities Average 0.2493 Standard Deviation 0.1562
8.2.2 Separation efficiency
To ensure that the method sufficiently separated aliphatic from aromatic fractions, three replicates of BC8 43-45cm were run. Two samples were independent analyses of the aromatic fraction. The third analysis was the aliphatic fraction separated by column cleanup from one of the above extractions. If the
218 separation technique is effective, the aliphatic fraction should contain insignificant trace amounts of PAHs, if any. According to a paired t-test, the difference between the two aromatic fractions was shown to be statistically insignificant (p=0.2778). The differences between the aliphatic fraction and each aromatic fraction, however, were shown to be statistically significant within the 90% confidence interval (p=0.0772 and
0.0940, respectively). Thus, the separation of the aromatic and aliphatic fractions was effective.
8.2.3 Recovery
Chrysene-d12 was used as a recovery standard. This standard was added to
81 samples prior to initial extraction. The efficiency of the entire sample preparation and analysis process including extraction, cleanup, concentration, chromatography, and mass spectrometry is assessed by comparing the amount of chrysene-d12 recovered in the mass spectrum to the known quantity added. The average overall recovery was 76.65% with a relative standard deviation of 0.18. This can be further broken down by sample type, as seen in Table A3. Because it is easier to extract low molecular weight PAHs relative to high molecular weight PAHs, it is not surprising that the percent recovery of chrysene-d12, a high molecular weight PAH, is lower than 100%. However, a percent recovery >75% for deuterated chrysene is considered to be effective extraction (Stout et al., 2004). The concentrations for
PAHs were not corrected to the percent recovery because of the varying difficulty of
219 extraction between low and high molecular weight PAHs. Therefore, the reported concentrations of high molecular weight PAHs discussed herein are minimal concentrations present, however the relative comparisons between samples is not affected as this would be a systematic error throughout.
Table A3. Percent recovery of chrysene-d12.
Sample Type n % recovery RSD Rocks 14 73.53 0.15 Soils 44 68.41 0.19 Hydrocarbon 4 79.37 0.32 Mud drape 6 84.92 0.08 Sediment 13 77.02 0.16 Total (Average) 81 (76.65) (0.18)
220 8.3 PAH concentration data
Table A4 provides the concentrations of individual PAH analytes for all samples in
µg PAH / g sediment.
221 Table A4. Concentrations of individual PAH analytes (ug PAH/g sediment) Sample detail BP ACLACE Dfu N0 N1 N2 N3 N4 F0 F1 F2 F3 Rocks Barton's Gully Jar 1 Wanstead 0.002 0.000 0.009 0.003 0.037 0.103 0.153 0.151 0.088 0.008 0.017 0.025 0.028 Barton's Gully Jar 2 Wanstead 0.004 0.000 0.006 0.002 0.020 0.049 0.083 0.083 0.057 0.004 0.012 0.025 0.030 Barton's Gully Jar 2 Wanstead 0.003 0.000 0.006 0.002 0.019 0.045 0.077 0.078 0.050 0.004 0.011 0.023 0.030 Bartons Gully Rock 1 Waipawa 0.107 0.000 0.102 0.028 0.876 1.252 1.686 1.623 0.870 0.069 0.228 0.396 0.367 Barton's Gully Rock 2 Wanstead 0.001 0.000 0.000 0.002 0.001 0.001 0.002 0.002 0.001 0.001 0.001 0.001 0.001 Bartons Gully Rock 3A Wanstead 0.084 0.000 0.000 0.003 0.009 0.003 0.004 0.008 0.002 0.001 0.001 0.000 0.000 Bartons Gully Rock 3B Waipawa 0.140 0.000 0.325 0.070 1.752 2.709 4.768 4.215 2.128 0.238 0.807 1.103 0.935 Barton's Gully Stream Bed 1 Wanstead 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 Cretaceous Gully 1 Mokoiwi 0.131 0.000 0.000 0.015 0.034 0.073 0.069 0.046 0.011 0.005 0.007 0.008 0.006 Cretaceous Gully 2 Mokoiwi 0.846 0.000 0.007 0.275 1.049 2.447 2.380 1.285 0.377 0.261 0.503 0.490 0.283 Cretaceous Gully 3 Tolaga Group 0.234 0.000 0.001 0.049 0.104 0.290 0.394 0.266 0.091 0.021 0.056 0.080 0.047 Cretaceous Gully 4 Weber 0.023 0.000 0.000 0.011 0.024 0.068 0.080 0.052 0.018 0.005 0.008 0.013 0.012 Cretaceous Gully 5 Whangai 0.454 0.000 0.003 0.217 0.410 1.495 1.885 1.115 0.356 0.040 0.080 0.399 0.375 Cretaceous Gully Rock 1 Whangai 0.054 0.000 0.002 0.110 0.000 0.011 0.359 0.551 0.201 0.135 0.271 0.215 0.137 Cretaceous Gully Rock 1 Whangai 0.004 0.000 0.001 0.050 0.000 0.000 0.070 0.296 0.143 0.086 0.205 0.160 0.094 LW Br Whangai 0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.001 0.000 0.001 0.000 0.001 0.001 Mata Slip Br Tikihore 0.004 0.000 0.000 0.001 0.006 0.002 0.002 0.003 0.001 0.001 0.000 0.000 0.000 Mata Slip Br Tikihore 0.003 0.000 0.000 0.001 0.006 0.002 0.002 0.003 0.000 0.001 0.000 0.000 0.000 W1 Rock Whakai 0.072 0.000 0.000 0.001 0.002 0.002 0.002 0.005 0.001 0.000 0.000 0.000 0.000 Soils Barton's Mouth 0-5cm B-W forested 0.001 0.000 0.000 0.002 0.003 0.006 0.010 0.010 0.008 0.002 0.004 0.006 0.007 Barton's Mouth 5-10cm B-W forested 0.210 0.000 0.001 0.005 0.021 0.007 0.010 0.025 0.013 0.001 0.002 0.000 0.000 BWBO 0-5cm B-W pastoral 0.001 0.000 0.000 0.002 0.003 0.004 0.008 0.009 0.008 0.002 0.002 0.006 0.007 BWBO 5-10cm B-W pastoral 0.005 0.000 0.000 0.002 0.006 0.005 0.007 0.005 0.008 0.001 0.001 0.001 0.001 BWBO 5-10cm B-W pastoral 0.103 0.000 0.000 0.002 0.002 0.003 0.005 0.010 0.008 0.001 0.001 0.003 0.004 BWBO 20-25cm B-W pastoral 0.312 0.000 0.001 0.007 0.024 0.013 0.024 0.030 0.024 0.002 0.002 0.000 0.000 BWBO 45-55cm B-W pastoral 0.000 0.000 0.000 0.001 0.002 0.003 0.004 0.002 0.002 0.001 0.001 0.002 0.004 Forest Soil 0-4cm B-W forested 0.027 0.007 0.000 0.015 0.022 0.056 0.103 0.057 0.052 0.003 0.006 0.017 0.013 Forest Soil 0-4cm B-W forested 0.032 0.008 0.000 0.018 0.028 0.071 0.127 0.066 0.060 0.003 0.009 0.038 0.022 Forest Soil 0-4cm B-W forested 0.015 0.007 0.000 0.010 0.009 0.026 0.070 0.049 0.049 0.003 0.009 0.020 0.016 Forest Soil 10-15cm B-W forested 0.177 0.000 0.001 0.034 0.054 0.107 0.115 0.070 0.028 0.004 0.004 0.000 0.000 LW 0-8cm B-W pastoral 0.006 0.000 0.001 0.002 0.000 0.003 0.005 0.010 0.006 0.001 0.000 0.000 0.000 LW 8-19cm B-W pastoral 0.007 0.000 0.001 0.002 0.012 0.004 0.004 0.011 0.000 0.001 0.002 0.000 0.000 LW 30-40cm B-W pastoral 0.008 0.000 0.001 0.002 0.015 0.004 0.004 0.013 0.000 0.002 0.002 0.000 0.000 LW 60-70cm B-W pastoral 0.085 0.000 0.001 0.004 0.021 0.006 0.007 0.018 0.000 0.002 0.001 0.000 0.000 LW 81-92cm B-W pastoral 0.413 0.000 0.003 0.013 0.045 0.018 0.016 0.036 0.010 0.005 0.004 0.000 0.000 Mata Slip 0-5cm tephric 0.113 0.000 0.000 0.003 0.010 0.004 0.004 0.008 0.002 0.001 0.001 0.002 0.002 Mata Slip 10-15cm tephric 0.115 0.000 0.001 0.004 0.012 0.006 0.005 0.009 0.003 0.001 0.001 0.000 0.000 Mata Slip 20-25cm tephric 0.112 0.000 0.000 0.003 0.010 0.003 0.003 0.008 0.002 0.001 0.001 0.000 0.000 Mata Slip 40-50cm tephric 0.071 0.000 0.000 0.002 0.007 0.002 0.002 0.006 0.002 0.001 0.000 0.000 0.000 Mata Slip 52-62cm tephric 0.101 0.000 0.000 0.002 0.010 0.003 0.003 0.008 0.002 0.001 0.001 0.000 0.000 Mata Slip 80-90cm tephric 0.117 0.000 0.000 0.002 0.011 0.004 0.003 0.009 0.002 0.001 0.001 0.000 0.000 MT 0-5cm colluvial 0.116 0.000 0.000 0.003 0.011 0.004 0.006 0.011 0.007 0.001 0.001 0.000 0.000 MT 13-18cm colluvial 0.207 0.000 0.000 0.003 0.010 0.004 0.005 0.009 0.009 0.001 0.001 0.000 0.000 MT 20-25cm colluvial 0.121 0.000 0.000 0.002 0.008 0.003 0.004 0.008 0.008 0.001 0.001 0.002 0.003 MT 30-35cm colluvial 0.102 0.000 0.000 0.002 0.002 0.002 0.003 0.007 0.003 0.000 0.001 0.000 0.000 MT 45-50cm colluvial 0.068 0.000 0.000 0.001 0.002 0.001 0.002 0.006 0.001 0.000 0.000 0.000 0.000 MT 70-75cm colluvial 0.081 0.000 0.000 0.002 0.003 0.002 0.002 0.004 0.001 0.001 0.001 0.000 0.000 MT 85-90cm colluvial 0.085 0.000 0.000 0.002 0.002 0.002 0.002 0.007 0.001 0.001 0.001 0.000 0.000 TS 0-5cm colluvial 0.205 0.000 0.001 0.005 0.021 0.009 0.008 0.018 0.004 0.001 0.001 0.000 0.000 TS 10-15cm colluvial 0.129 0.000 0.001 0.004 0.014 0.006 0.006 0.012 0.003 0.001 0.001 0.000 0.002 TS 30-40cm colluvial 0.106 0.000 0.001 0.004 0.014 0.007 0.007 0.012 0.003 0.002 0.001 0.000 0.000 TS 55-65cm colluvial 0.113 0.000 0.001 0.005 0.017 0.008 0.007 0.012 0.003 0.002 0.001 0.000 0.000 TS 80-90cm colluvial 0.094 0.000 0.000 0.003 0.011 0.005 0.005 0.007 0.002 0.001 0.001 0.000 0.000 TS 110-120cm colluvial 0.168 0.000 0.001 0.004 0.017 0.007 0.006 0.012 0.003 0.001 0.001 0.000 0.000 TSLHT 0-5cm alluvial 0.003 0.000 0.000 0.001 0.003 0.003 0.005 0.003 0.002 0.001 0.001 0.002 0.002 TSLHT 15-20cm alluvial 0.003 0.000 0.000 0.002 0.004 0.003 0.004 0.004 0.000 0.001 0.002 0.002 0.002 TSLHT 15-20cm alluvial 0.110 0.000 0.000 0.004 0.004 0.006 0.008 0.011 0.003 0.001 0.002 0.005 0.005 TSLHT 30-40cm alluvial 0.011 0.000 0.002 0.006 0.015 0.012 0.013 0.008 0.004 0.003 0.003 0.007 0.006 TSLHT 110-120cm alluvial 0.005 0.000 0.001 0.003 0.006 0.007 0.007 0.006 0.002 0.002 0.002 0.006 0.005
222 Table A4. Concentrations of individual PAH analytes (ug PAH/g sediment) cont'd Sample detail BP ACLACE Dfu N0 N1 N2 N3 N4 F0 F1 F2 F3 W1 0-5cm tephric 0.003 0.000 0.000 0.001 0.003 0.003 0.003 0.003 0.002 0.001 0.001 0.001 0.000 W1 5-10cm tephric 0.003 0.000 0.000 0.001 0.003 0.002 0.003 0.002 0.001 0.000 0.000 0.001 0.000 W1 15-20cm tephric 0.003 0.000 0.000 0.001 0.003 0.002 0.003 0.002 0.001 0.001 0.000 0.001 0.001 W1 15-20cm tephric 0.006 0.000 0.000 0.002 0.006 0.003 0.003 0.004 0.001 0.001 0.001 0.002 0.002 W1 15-20cm tephric 0.004 0.000 0.000 0.002 0.004 0.003 0.003 0.002 0.001 0.001 0.001 0.001 0.001 W1 30-35cm tephric 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.001 W1 45-50cm tephric 0.002 0.000 0.000 0.001 0.002 0.001 0.001 0.001 0.000 0.000 0.000 0.000 0.000 W1 75-80cm tephric 0.003 0.000 0.000 0.001 0.004 0.002 0.001 0.002 0.001 0.001 0.000 0.000 0.000 W1 75-80cm tephric 0.003 0.000 0.000 0.001 0.005 0.002 0.002 0.002 0.001 0.001 0.000 0.001 0.001 W1 75-80cm tephric 0.005 0.000 0.000 0.002 0.008 0.003 0.004 0.005 0.001 0.001 0.001 0.001 0.001 W1 95-100cm tephric 0.003 0.000 0.000 0.001 0.004 0.002 0.001 0.002 0.000 0.001 0.000 0.000 0.000 W1 95-100cm tephric 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.003 0.001 0.000 0.000 0.000 0.001 YHT 10-15cm alluvial 0.038 0.000 0.001 0.020 0.031 0.113 0.216 0.161 0.050 0.022 0.054 0.063 0.043 YHT 10-15cm alluvial 0.140 0.000 0.001 0.028 0.048 0.160 0.273 0.175 0.045 0.025 0.053 0.082 0.056 YHT 40-45cm alluvial 0.030 0.000 0.002 0.028 0.060 0.152 0.232 0.157 0.047 0.019 0.056 0.047 0.038 YHT 40-45cm alluvial 0.023 0.000 0.001 0.026 0.001 0.054 0.199 0.157 0.042 0.023 0.051 0.063 0.051 YHT 80-90cm alluvial 0.065 0.000 0.003 0.038 0.056 0.217 0.363 0.275 0.098 0.021 0.047 0.113 0.095 YHT 110-115cm alluvial 0.024 0.000 0.002 0.014 0.027 0.067 0.093 0.065 0.027 0.007 0.015 0.058 0.052 Te Puia Springs 1 thermogenic 0.102 0.001 0.007 0.044 0.152 0.245 0.420 0.413 0.210 0.158 0.356 0.223 0.098 Te Puia Springs 2 thermogenic 0.196 0.000 0.009 0.005 0.018 0.028 0.319 0.297 0.131 0.019 0.241 0.170 0.061 Williams Gas Seep 1 thermogenic 0.159 0.000 0.001 0.008 0.029 0.027 0.040 0.030 0.000 0.002 0.003 0.000 0.000 Williams Gas Seep 2 thermogenic 0.322 0.000 0.001 0.014 0.027 0.029 0.042 0.047 0.025 0.002 0.004 0.000 0.000 Sediment Barton's Gully Mud riverine 0.163 0.000 0.005 0.017 0.108 0.142 0.152 0.119 0.059 0.013 0.024 0.028 0.026 Makarika Stream Mud riverine 0.176 0.000 0.008 0.031 0.111 0.318 0.447 0.314 0.130 0.041 0.079 0.102 0.069 Mangaoporo mud riverine 0.172 0.000 0.009 0.018 0.061 0.113 0.162 0.134 0.057 0.023 0.038 0.042 0.033 Mata River mud riverine 0.135 0.000 0.006 0.018 0.105 0.240 0.317 0.238 0.113 0.021 0.048 0.061 0.045 Tikitiki mud riverine 0.127 0.000 0.002 0.019 0.022 0.067 0.177 0.171 0.067 0.015 0.035 0.049 0.040 YHT mud riverine 0.277 0.000 0.004 0.044 0.081 0.290 0.512 0.372 0.129 0.042 0.100 0.138 0.093 60m tripod BC 0-2cm marine 0.207 0.000 0.005 0.032 0.082 0.225 0.343 0.271 0.112 0.031 0.065 0.087 0.061 60m tripod BC 14-16cm marine 0.123 0.000 0.004 0.026 0.062 0.184 0.290 0.237 0.105 0.028 0.057 0.084 0.062 60m tripod BC 27-29cm marine 0.092 0.000 0.003 0.018 0.045 0.121 0.194 0.163 0.073 0.019 0.041 0.063 0.049 60m tripod BC 39-41cm marine 0.121 0.000 0.004 0.019 0.048 0.132 0.215 0.181 0.081 0.021 0.046 0.071 0.045 BC8 0-2cm marine 0.051 0.000 0.004 0.028 0.079 0.214 0.317 0.243 0.097 0.028 0.060 0.074 0.060 BC8 7-9cm marine 0.031 0.000 0.003 0.018 0.050 0.138 0.208 0.165 0.072 0.021 0.045 0.058 0.052 BC8 14-16cm marine 0.033 0.000 0.003 0.018 0.052 0.142 0.208 0.157 0.065 0.022 0.050 0.080 0.057 BC8 14-16cm marine 0.034 0.000 0.003 0.020 0.049 0.146 0.230 0.186 0.071 0.025 0.048 0.083 0.057 BC8 29-31cm marine 0.027 0.000 0.003 0.015 0.042 0.114 0.185 0.144 0.060 0.018 0.041 0.077 0.044 BC8 29-31cm marine 0.031 0.000 0.003 0.017 0.047 0.121 0.202 0.158 0.063 0.018 0.035 0.063 0.040 BC8 43-45cm marine 0.018 0.000 0.002 0.010 0.028 0.078 0.124 0.100 0.041 0.012 0.027 0.031 0.037 BC8 43-45cm marine 0.024 0.000 0.002 0.012 0.034 0.088 0.139 0.112 0.044 0.013 0.027 0.042 0.031 BC14 0-2cm marine 0.027 0.000 0.003 0.017 0.035 0.114 0.185 0.153 0.065 0.017 0.037 0.063 0.052 BC14 7-9cm marine 0.027 0.000 0.003 0.018 0.037 0.122 0.194 0.160 0.070 0.019 0.042 0.067 0.053 BC14 28-30cm marine 0.019 0.000 0.002 0.013 0.025 0.085 0.144 0.122 0.055 0.014 0.033 0.054 0.046 BC14 43-45cm marine 0.021 0.000 0.002 0.013 0.026 0.087 0.148 0.128 0.059 0.014 0.033 0.062 0.046 BC48 0-2cm marine 0.032 0.000 0.003 0.020 0.042 0.121 0.192 0.157 0.066 0.017 0.032 0.054 0.049 BC48 13-15cm marine 0.022 0.000 0.003 0.015 0.027 0.093 0.160 0.136 0.060 0.020 0.041 0.063 0.053 BC48 27-29cm marine 0.021 0.000 0.002 0.013 0.027 0.085 0.140 0.121 0.056 0.013 0.031 0.067 0.045 BC48 40-42cm marine 0.021 0.000 0.002 0.012 0.025 0.081 0.139 0.120 0.055 0.015 0.035 0.061 0.046 BC1 0-2cm marine 0.033 0.000 0.003 0.020 0.050 0.143 0.221 0.181 0.074 0.021 0.045 0.074 0.060 BC1 8-10cm marine 0.028 0.000 0.003 0.018 0.039 0.126 0.198 0.156 0.064 0.018 0.038 0.062 0.051 BC1 25-27cm marine 0.024 0.000 0.003 0.015 0.033 0.110 0.178 0.147 0.067 0.016 0.038 0.061 0.053 BC1 42-44cm marine 0.023 0.000 0.002 0.013 0.027 0.093 0.153 0.124 0.058 0.016 0.035 0.067 0.048 *abbreviations for PAHs can be found in Table 2
223 Table A4. Concentrations of individual PAH analytes (ug PAH/g sediment) cont'd Sample D0 D1 D2 D3 AN P0 P1 P2 P3 P4 FL PY FP1 Re BaA Rocks Barton's Gully Jar 1 0.002 0.008 0.007 0.007 0.000 0.018 0.023 0.023 0.016 0.007 0.003 0.057 0.032 0.002 0.002 Barton's Gully Jar 2 0.001 0.004 0.005 0.005 0.000 0.009 0.019 0.025 0.019 0.009 0.002 0.067 0.046 0.005 0.002 Barton's Gully Jar 2 0.001 0.004 0.006 0.006 0.000 0.009 0.019 0.024 0.020 0.009 0.002 0.065 0.045 0.004 0.002 Bartons Gully Rock 1 0.029 0.133 0.198 0.320 0.000 0.080 0.159 0.209 0.197 0.077 0.015 0.225 0.148 0.024 0.006 Barton's Gully Rock 2 0.000 0.000 0.000 0.000 0.000 0.008 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Bartons Gully Rock 3A 0.000 0.001 0.000 0.000 0.000 0.005 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Bartons Gully Rock 3B 0.193 0.790 1.307 1.102 0.012 0.157 0.433 0.593 0.544 0.269 0.047 0.285 0.355 0.067 0.017 Barton's Gully Stream Bed 1 0.000 0.000 0.000 0.000 0.000 0.017 0.003 0.001 0.000 0.000 0.002 0.000 0.000 0.001 0.000 Cretaceous Gully 1 0.000 0.001 0.002 0.003 0.000 0.014 0.015 0.010 0.006 0.002 0.001 0.000 0.002 0.003 0.000 Cretaceous Gully 2 0.029 0.047 0.039 0.024 0.001 0.319 0.596 0.478 0.302 0.095 0.008 0.033 0.144 0.191 0.004 Cretaceous Gully 3 0.006 0.010 0.007 0.004 0.000 0.069 0.115 0.086 0.049 0.015 0.002 0.006 0.020 0.024 0.001 Cretaceous Gully 4 0.001 0.002 0.001 0.001 0.000 0.013 0.020 0.018 0.011 0.003 0.001 0.002 0.003 0.006 0.001 Cretaceous Gully 5 0.019 0.032 0.022 0.017 0.001 0.295 0.564 0.443 0.237 0.078 0.010 0.035 0.100 0.138 0.005 Cretaceous Gully Rock 1 0.021 0.027 0.016 0.007 0.000 0.217 0.350 0.229 0.108 0.028 0.007 0.025 0.085 0.112 0.002 Cretaceous Gully Rock 1 0.016 0.020 0.012 0.005 0.000 0.166 0.260 0.170 0.085 0.019 0.005 0.019 0.060 0.083 0.001 LW Br 0.000 0.000 0.000 0.000 0.000 0.004 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 Mata Slip Br 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mata Slip Br 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 W1 Rock 0.000 0.000 0.001 0.000 0.000 0.003 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Soils Barton's Mouth 0-5cm 0.001 0.000 0.000 0.000 0.000 0.010 0.010 0.009 0.006 0.000 0.002 0.004 0.007 0.003 0.001 Barton's Mouth 5-10cm 0.001 0.002 0.006 0.005 0.000 0.011 0.007 0.007 0.007 0.003 0.001 0.004 0.003 0.006 0.000 BWBO 0-5cm 0.001 0.001 0.001 0.001 0.000 0.008 0.010 0.007 0.004 0.001 0.001 0.003 0.002 0.003 0.001 BWBO 5-10cm 0.000 0.001 0.001 0.001 0.000 0.006 0.004 0.003 0.002 0.001 0.001 0.001 0.001 0.002 0.000 BWBO 5-10cm 0.000 0.001 0.002 0.001 0.000 0.007 0.006 0.004 0.003 0.001 0.001 0.001 0.001 0.002 0.000 BWBO 20-25cm 0.001 0.003 0.007 0.005 0.000 0.018 0.016 0.014 0.007 0.002 0.004 0.000 0.004 0.012 0.000 BWBO 45-55cm 0.000 0.001 0.000 0.000 0.000 0.005 0.003 0.001 0.001 0.000 0.001 0.001 0.000 0.002 0.000 Forest Soil 0-4cm 0.001 0.003 0.002 0.001 0.000 0.037 0.049 0.033 0.015 0.000 0.003 0.005 0.013 0.012 0.001 Forest Soil 0-4cm 0.001 0.003 0.003 0.002 0.000 0.047 0.060 0.036 0.025 0.024 0.004 0.006 0.017 0.015 0.001 Forest Soil 0-4cm 0.001 0.002 0.002 0.002 0.000 0.038 0.044 0.032 0.017 0.000 0.003 0.005 0.013 0.013 0.001 Forest Soil 10-15cm 0.001 0.002 0.002 0.001 0.000 0.056 0.055 0.035 0.016 0.005 0.005 0.009 0.012 0.026 0.002 LW 0-8cm 0.000 0.000 0.000 0.000 0.000 0.004 0.003 0.002 0.002 0.000 0.001 0.004 0.001 0.000 0.000 LW 8-19cm 0.000 0.000 0.000 0.000 0.000 0.004 0.002 0.001 0.001 0.000 0.001 0.006 0.000 0.000 0.000 LW 30-40cm 0.000 0.000 0.000 0.000 0.000 0.005 0.002 0.001 0.000 0.000 0.001 0.007 0.000 0.000 0.000 LW 60-70cm 0.001 0.001 0.000 0.000 0.000 0.006 0.002 0.001 0.000 0.000 0.001 0.005 0.000 0.000 0.000 LW 81-92cm 0.002 0.002 0.000 0.000 0.001 0.020 0.006 0.002 0.000 0.000 0.001 0.002 0.000 0.000 0.000 Mata Slip 0-5cm 0.000 0.001 0.001 0.000 0.000 0.006 0.002 0.001 0.000 0.000 0.001 0.000 0.000 0.000 0.000 Mata Slip 10-15cm 0.000 0.001 0.001 0.000 0.000 0.007 0.003 0.001 0.001 0.000 0.001 0.001 0.000 0.000 0.000 Mata Slip 20-25cm 0.000 0.000 0.000 0.000 0.000 0.005 0.002 0.001 0.001 0.000 0.001 0.000 0.000 0.000 0.000 Mata Slip 40-50cm 0.000 0.000 0.001 0.000 0.000 0.004 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mata Slip 52-62cm 0.000 0.000 0.000 0.000 0.000 0.004 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mata Slip 80-90cm 0.000 0.000 0.001 0.000 0.000 0.004 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MT 0-5cm 0.000 0.001 0.001 0.001 0.000 0.007 0.004 0.002 0.002 0.000 0.001 0.001 0.001 0.000 0.000 MT 13-18cm 0.000 0.001 0.001 0.001 0.000 0.007 0.003 0.002 0.001 0.000 0.001 0.000 0.000 0.000 0.000 MT 20-25cm 0.000 0.001 0.001 0.001 0.000 0.004 0.003 0.003 0.003 0.000 0.001 0.000 0.000 0.001 0.000 MT 30-35cm 0.000 0.000 0.001 0.000 0.000 0.003 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MT 45-50cm 0.000 0.000 0.001 0.000 0.000 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MT 70-75cm 0.000 0.000 0.000 0.000 0.000 0.004 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 MT 85-90cm 0.000 0.001 0.001 0.000 0.000 0.005 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 TS 0-5cm 0.001 0.001 0.000 0.000 0.000 0.011 0.005 0.002 0.001 0.000 0.002 0.001 0.001 0.000 0.000 TS 10-15cm 0.000 0.001 0.001 0.000 0.000 0.008 0.004 0.002 0.001 0.000 0.002 0.001 0.001 0.000 0.000 TS 30-40cm 0.000 0.001 0.000 0.000 0.000 0.007 0.003 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000 TS 55-65cm 0.001 0.001 0.000 0.000 0.000 0.008 0.003 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000 TS 80-90cm 0.000 0.000 0.000 0.000 0.000 0.007 0.003 0.001 0.000 0.000 0.001 0.000 0.000 0.000 0.000 TS 110-120cm 0.000 0.001 0.000 0.000 0.000 0.009 0.003 0.001 0.000 0.000 0.001 0.000 0.000 0.000 0.000 TSLHT 0-5cm 0.000 0.000 0.000 0.000 0.000 0.009 0.006 0.004 0.001 0.000 0.001 0.002 0.002 0.001 0.000 TSLHT 15-20cm 0.000 0.000 0.000 0.000 0.000 0.012 0.009 0.005 0.002 0.000 0.001 0.001 0.001 0.001 0.000 TSLHT 15-20cm 0.001 0.001 0.002 0.001 0.000 0.016 0.011 0.006 0.003 0.001 0.002 0.001 0.002 0.001 0.000 TSLHT 30-40cm 0.001 0.001 0.001 0.000 0.000 0.018 0.013 0.007 0.003 0.001 0.002 0.003 0.002 0.001 0.000 TSLHT 110-120cm 0.001 0.001 0.001 0.001 0.000 0.012 0.009 0.005 0.002 0.001 0.002 0.001 0.001 0.001 0.000
224 Table A4. Concentrations of individual PAH analytes (ug PAH/g sediment) cont'd Sample D0 D1 D2 D3 AN P0 P1 P2 P3 P4 FL PY FP1 Re BaA W1 0-5cm 0.000 0.000 0.000 0.000 0.000 0.004 0.002 0.002 0.001 0.000 0.001 0.001 0.001 0.000 0.000 W1 5-10cm 0.000 0.000 0.000 0.000 0.000 0.003 0.002 0.002 0.001 0.000 0.001 0.001 0.001 0.000 0.000 W1 15-20cm 0.000 0.000 0.000 0.000 0.000 0.004 0.002 0.002 0.001 0.001 0.001 0.001 0.001 0.000 0.000 W1 15-20cm 0.000 0.000 0.000 0.001 0.000 0.008 0.004 0.004 0.002 0.001 0.003 0.002 0.001 0.001 0.000 W1 15-20cm 0.000 0.000 0.000 0.000 0.000 0.006 0.003 0.003 0.001 0.000 0.001 0.001 0.001 0.000 0.000 W1 30-35cm 0.000 0.000 0.000 0.000 0.000 0.005 0.002 0.002 0.001 0.001 0.001 0.001 0.000 0.000 0.000 W1 45-50cm 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 W1 75-80cm 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 W1 75-80cm 0.000 0.000 0.000 0.000 0.000 0.004 0.001 0.001 0.001 0.000 0.001 0.001 0.001 0.000 0.001 W1 75-80cm 0.000 0.000 0.000 0.000 0.000 0.005 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000 W1 95-100cm 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 W1 95-100cm 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 YHT 10-15cm 0.008 0.013 0.010 0.006 0.000 0.114 0.201 0.137 0.062 0.018 0.009 0.003 0.032 0.017 0.002 YHT 10-15cm 0.010 0.016 0.016 0.010 0.000 0.146 0.260 0.183 0.080 0.018 0.014 0.001 0.035 0.021 0.001 YHT 40-45cm 0.007 0.010 0.008 0.005 0.000 0.140 0.194 0.117 0.053 0.015 0.022 0.008 0.044 0.029 0.005 YHT 40-45cm 0.008 0.012 0.010 0.006 0.000 0.150 0.233 0.149 0.062 0.014 0.022 0.006 0.045 0.031 0.005 YHT 80-90cm 0.010 0.015 0.013 0.008 0.000 0.125 0.215 0.157 0.090 0.032 0.012 0.021 0.056 0.042 0.005 YHT 110-115cm 0.004 0.007 0.005 0.003 0.000 0.077 0.121 0.085 0.042 0.015 0.009 0.015 0.031 0.014 0.004 Te Puia Springs 1 0.079 0.093 0.055 0.025 0.006 0.880 1.055 0.305 0.101 0.028 0.038 0.087 0.118 0.015 0.012 Te Puia Springs 2 0.006 0.049 0.026 0.009 0.005 0.143 0.563 0.154 0.045 0.010 0.026 0.053 0.054 0.006 0.005 Williams Gas Seep 1 0.001 0.002 0.003 0.003 0.000 0.015 0.013 0.008 0.006 0.002 0.002 0.001 0.002 0.004 0.000 Williams Gas Seep 2 0.002 0.000 0.000 0.000 0.001 0.031 0.025 0.017 0.010 0.004 0.005 0.006 0.005 0.013 0.001 Sediment Barton's Gully Mud 0.005 0.009 0.011 0.008 0.001 0.043 0.045 0.029 0.019 0.008 0.009 0.021 0.035 0.012 0.005 Makarika Stream Mud 0.012 0.024 0.023 0.014 0.001 0.113 0.201 0.140 0.080 0.027 0.010 0.021 0.073 0.069 0.005 Mangaoporo mud 0.004 0.008 0.009 0.006 0.003 0.073 0.089 0.049 0.025 0.010 0.017 0.022 0.058 0.033 0.008 Mata River mud 0.004 0.011 0.009 0.007 0.001 0.069 0.106 0.081 0.047 0.018 0.011 0.020 0.051 0.040 0.008 Tikitiki mud 0.005 0.009 0.011 0.007 0.001 0.068 0.120 0.086 0.048 0.017 0.008 0.014 0.039 0.028 0.003 YHT mud 0.013 0.025 0.023 0.015 0.001 0.167 0.316 0.227 0.125 0.047 0.012 0.021 0.078 0.038 0.003 60m tripod BC 0-2cm 0.008 0.016 0.015 0.010 0.002 0.113 0.184 0.133 0.076 0.027 0.015 0.025 0.073 0.049 0.007 60m tripod BC 14-16cm 0.007 0.014 0.014 0.010 0.002 0.095 0.173 0.130 0.073 0.027 0.015 0.025 0.067 0.050 0.008 60m tripod BC 27-29cm 0.005 0.010 0.011 0.008 0.002 0.072 0.125 0.092 0.053 0.015 0.014 0.022 0.052 0.034 0.006 60m tripod BC 39-41cm 0.005 0.012 0.012 0.007 0.002 0.079 0.146 0.104 0.060 0.021 0.016 0.025 0.059 0.039 0.007 BC8 0-2cm 0.008 0.012 0.011 0.007 0.002 0.099 0.164 0.115 0.061 0.018 0.012 0.022 0.058 0.048 0.005 BC8 7-9cm 0.005 0.010 0.009 0.006 0.001 0.076 0.129 0.099 0.048 0.015 0.012 0.019 0.052 0.036 0.005 BC8 14-16cm 0.006 0.010 0.010 0.006 0.002 0.086 0.154 0.092 0.051 0.020 0.043 0.040 0.072 0.039 0.035 BC8 14-16cm 0.006 0.010 0.010 0.006 0.002 0.089 0.153 0.108 0.057 0.019 0.013 0.023 0.056 0.038 0.006 BC8 29-31cm 0.004 0.009 0.008 0.005 0.001 0.065 0.124 0.081 0.043 0.015 0.012 0.019 0.047 0.035 0.004 BC8 29-31cm 0.004 0.008 0.009 0.005 0.001 0.065 0.124 0.087 0.046 0.017 0.012 0.019 0.048 0.034 0.005 BC8 43-45cm 0.003 0.007 0.006 0.004 0.001 0.048 0.139 0.061 0.037 0.012 0.010 0.014 0.036 0.025 0.002 BC8 43-45cm 0.003 0.006 0.006 0.005 0.001 0.049 0.117 0.058 0.033 0.011 0.009 0.014 0.034 0.024 0.002 BC14 0-2cm 0.005 0.009 0.009 0.006 0.001 0.069 0.117 0.084 0.045 0.014 0.012 0.019 0.044 0.032 0.007 BC14 7-9cm 0.005 0.009 0.010 0.007 0.001 0.077 0.129 0.090 0.049 0.017 0.013 0.020 0.049 0.034 0.008 BC14 28-30cm 0.004 0.008 0.008 0.005 0.001 0.057 0.099 0.074 0.039 0.012 0.012 0.018 0.040 0.028 0.004 BC14 43-45cm 0.004 0.008 0.009 0.005 0.001 0.056 0.093 0.072 0.040 0.014 0.011 0.018 0.039 0.029 0.003 BC48 0-2cm 0.005 0.009 0.008 0.006 0.000 0.079 0.115 0.079 0.040 0.011 0.012 0.019 0.038 0.030 0.005 BC48 13-15cm 0.005 0.009 0.009 0.007 0.001 0.072 0.122 0.089 0.047 0.015 0.017 0.024 0.056 0.032 0.008 BC48 27-29cm 0.004 0.007 0.008 0.005 0.001 0.057 0.092 0.072 0.039 0.013 0.012 0.019 0.040 0.028 0.007 BC48 40-42cm 0.004 0.008 0.008 0.006 0.001 0.060 0.101 0.075 0.042 0.013 0.011 0.019 0.041 0.028 0.009 BC1 0-2cm 0.006 0.010 0.010 0.006 0.001 0.087 0.142 0.095 0.052 0.018 0.014 0.022 0.056 0.037 0.006 BC1 8-10cm 0.005 0.009 0.008 0.005 0.001 0.076 0.123 0.084 0.045 0.015 0.013 0.020 0.048 0.033 0.007 BC1 25-27cm 0.004 0.008 0.009 0.005 0.001 0.065 0.112 0.085 0.045 0.014 0.013 0.019 0.045 0.033 0.005 BC1 42-44cm 0.004 0.008 0.007 0.005 0.001 0.058 0.100 0.076 0.042 0.013 0.012 0.017 0.042 0.028 0.005 *abbreviations for PAHs can be found in Table 2
225 Table A4. Concentrations of individual PAH analytes (ug PAH/g sediment) cont'd Sample C0 C1 C2 C3 C4 BbF BkF BeP BaP Pyrl ID DA BgP Co TPAH Rocks Barton's Gully Jar 1 0.002 0.008 0.005 0.002 0.000 0.001 0.001 0.003 0.001 0.001 0.001 0.000 0.007 0.003 0.870 Barton's Gully Jar 2 0.002 0.007 0.000 0.000 0.000 0.001 0.001 0.003 0.001 0.002 0.000 0.000 0.006 0.002 0.616 Barton's Gully Jar 2 0.001 0.007 0.008 0.000 0.000 0.001 0.001 0.003 0.002 0.002 0.000 0.000 0.006 0.002 0.594 Bartons Gully Rock 1 0.006 0.084 0.072 0.064 0.036 0.003 0.002 0.004 0.007 0.085 0.002 0.000 0.009 0.001 9.798 Barton's Gully Rock 2 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.027 Bartons Gully Rock 3A 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.125 Bartons Gully Rock 3B 0.028 0.654 0.115 0.112 0.092 0.005 0.004 0.005 0.000 0.468 0.004 0.000 0.008 0.001 26.853 Barton's Gully Stream Bed 1 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.029 Cretaceous Gully 1 0.001 0.001 0.001 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.469 Cretaceous Gully 2 0.037 0.037 0.030 0.023 0.013 0.042 0.008 0.043 0.004 0.003 0.013 0.005 0.054 0.027 12.852 Cretaceous Gully 3 0.006 0.006 0.005 0.003 0.003 0.005 0.001 0.006 0.001 0.001 0.002 0.001 0.007 0.004 2.098 Cretaceous Gully 4 0.008 0.008 0.006 0.002 0.000 0.006 0.001 0.008 0.000 0.000 0.001 0.001 0.005 0.003 0.444 Cretaceous Gully 5 0.051 0.043 0.029 0.018 0.009 0.048 0.008 0.047 0.003 0.006 0.012 0.005 0.051 0.025 9.180 Cretaceous Gully Rock 1 0.025 0.023 0.019 0.012 0.000 0.025 0.004 0.022 0.002 0.002 0.005 0.003 0.028 0.012 3.462 Cretaceous Gully Rock 1 0.019 0.017 0.012 0.011 0.000 0.018 0.003 0.016 0.002 0.001 0.005 0.001 0.020 0.009 2.164 LW Br 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.017 Mata Slip Br 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.026 Mata Slip Br 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.022 W1 Rock 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.094 Soils Barton's Mouth 0-5cm 0.001 0.002 0.002 0.001 0.000 0.001 0.000 0.001 0.000 0.003 0.000 0.002 0.001 0.000 0.127 Barton's Mouth 5-10cm 0.002 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.362 BWBO 0-5cm 0.005 0.004 0.003 0.002 0.000 0.003 0.001 0.003 0.000 0.000 0.001 0.005 0.001 0.001 0.123 BWBO 5-10cm 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.067 BWBO 5-10cm 0.002 0.001 0.001 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.179 BWBO 20-25cm 0.003 0.001 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.541 BWBO 45-55cm 0.003 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.042 Forest Soil 0-4cm 0.007 0.007 0.004 0.000 0.000 0.008 0.001 0.005 0.000 0.001 0.001 0.000 0.003 0.002 0.592 Forest Soil 0-4cm 0.009 0.008 0.005 0.000 0.000 0.011 0.002 0.006 0.000 0.000 0.001 0.001 0.004 0.002 0.777 Forest Soil 0-4cm 0.007 0.006 0.006 0.005 0.000 0.008 0.000 0.005 0.000 0.000 0.001 0.000 0.003 0.001 0.502 Forest Soil 10-15cm 0.012 0.008 0.005 0.000 0.000 0.010 0.001 0.008 0.000 0.000 0.002 0.001 0.004 0.002 0.877 LW 0-8cm 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.004 0.000 0.000 0.004 0.000 0.007 0.070 LW 8-19cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.000 0.000 0.062 LW 30-40cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.070 LW 60-70cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.164 LW 81-92cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.601 Mata Slip 0-5cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.165 Mata Slip 10-15cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.172 Mata Slip 20-25cm 0.001 0.000 0.000 0.000 0.000 0.001 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.160 Mata Slip 40-50cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.102 Mata Slip 52-62cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.140 Mata Slip 80-90cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.159 MT 0-5cm 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.186 MT 13-18cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.268 MT 20-25cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.178 MT 30-35cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.129 MT 45-50cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.088 MT 70-75cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.104 MT 85-90cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.114 TS 0-5cm 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.304 TS 10-15cm 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.203 TS 30-40cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.173 TS 55-65cm 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.185 TS 80-90cm 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.146 TS 110-120cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.238 TSLHT 0-5cm 0.002 0.001 0.000 0.000 0.000 0.003 0.001 0.002 0.000 0.000 0.001 0.000 0.002 0.001 0.066 TSLHT 15-20cm 0.002 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.066 TSLHT 15-20cm 0.002 0.001 0.001 0.000 0.000 0.002 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.214 TSLHT 30-40cm 0.004 0.003 0.001 0.000 0.000 0.004 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.158 TSLHT 110-120cm 0.011 0.007 0.005 0.000 0.000 0.011 0.001 0.003 0.000 0.000 0.000 0.000 0.000 0.001 0.127
226 Table A4. Concentrations of individual PAH analytes (ug PAH/g sediment) cont'd Sample C0 C1 C2 C3 C4 BbF BkF BeP BaP Pyrl ID DA BgP Co TPAH W1 0-5cm 0.001 0.001 0.000 0.000 0.000 0.002 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.040 W1 5-10cm 0.001 0.001 0.000 0.000 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.032 W1 15-20cm 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.035 W1 15-20cm 0.001 0.001 0.001 0.000 0.000 0.002 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.065 W1 15-20cm 0.001 0.001 0.000 0.000 0.000 0.001 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.045 W1 30-35cm 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.021 W1 45-50cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.016 W1 75-80cm 0.000 0.000 0.000 0.000 0.000 0.001 0.001 0.001 0.000 0.000 0.001 0.000 0.001 0.000 0.027 W1 75-80cm 0.001 0.001 0.000 0.000 0.000 0.002 0.002 0.001 0.000 0.000 0.001 0.000 0.001 0.000 0.043 W1 75-80cm 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.045 W1 95-100cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.021 W1 95-100cm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.013 YHT 10-15cm 0.024 0.022 0.012 0.004 0.003 0.015 0.002 0.010 0.000 0.000 0.001 0.002 0.000 0.002 1.541 YHT 10-15cm 0.044 0.035 0.016 0.007 0.002 0.024 0.003 0.009 0.000 0.000 0.000 0.002 0.000 0.001 2.041 YHT 40-45cm 0.028 0.021 0.010 0.004 0.003 0.018 0.005 0.012 0.000 0.000 0.001 0.002 0.001 0.002 1.629 YHT 40-45cm 0.034 0.025 0.012 0.000 0.000 0.022 0.003 0.010 0.000 0.000 0.001 0.002 0.000 0.002 1.553 YHT 80-90cm 0.047 0.045 0.032 0.015 0.010 0.033 0.005 0.035 0.000 0.001 0.008 0.004 0.028 0.013 2.466 YHT 110-115cm 0.032 0.027 0.017 0.008 0.006 0.020 0.003 0.017 0.000 0.000 0.003 0.003 0.010 0.005 1.033 Te Puia Springs 1 0.022 0.020 0.016 0.005 0.004 0.007 0.007 0.007 0.005 0.003 0.005 0.001 0.007 0.002 5.437 Te Puia Springs 2 0.012 0.007 0.004 0.001 0.000 0.003 0.003 0.003 0.001 0.003 0.002 0.000 0.003 0.002 2.693 Williams Gas Seep 1 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.362 Williams Gas Seep 2 0.004 0.003 0.000 0.000 0.000 0.004 0.002 0.003 0.000 0.005 0.002 0.000 0.003 0.002 0.663 Sediment Barton's Gully Mud 0.006 0.007 0.006 0.003 0.002 0.005 0.003 0.006 0.003 0.009 0.003 0.001 0.009 0.005 1.184 Makarika Stream Mud 0.023 0.028 0.020 0.010 0.007 0.017 0.004 0.015 0.002 0.017 0.004 0.002 0.015 0.008 2.810 Mangaoporo mud 0.013 0.012 0.007 0.004 0.003 0.010 0.004 0.008 0.001 0.000 0.004 0.001 0.009 0.004 1.354 Mata River mud 0.016 0.020 0.014 0.007 0.005 0.013 0.004 0.011 0.002 0.073 0.004 0.002 0.014 0.008 2.021 Tikitiki mud 0.013 0.014 0.010 0.003 0.002 0.009 0.002 0.009 0.000 0.000 0.002 0.001 0.006 0.003 1.332 YHT mud 0.033 0.035 0.028 0.013 0.010 0.021 0.003 0.022 0.000 0.000 0.004 0.002 0.014 0.006 3.384 60m tripod BC 0-2cm 0.027 0.028 0.018 0.011 0.008 0.019 0.004 0.019 0.005 0.041 0.005 0.003 0.017 0.010 2.487 60m tripod BC 14-16cm 0.025 0.027 0.017 0.009 0.006 0.018 0.004 0.015 0.004 0.050 0.003 0.002 0.017 0.010 2.178 60m tripod BC 27-29cm 0.019 0.019 0.011 0.007 0.004 0.013 0.003 0.010 0.003 0.037 0.003 0.001 0.011 0.006 1.550 60m tripod BC 39-41cm 0.024 0.023 0.013 0.008 0.005 0.016 0.004 0.015 0.003 0.041 0.003 0.001 0.015 0.009 1.758 BC8 0-2cm 0.022 0.023 0.016 0.008 0.007 0.017 0.003 0.016 0.003 0.036 0.002 0.002 0.014 0.006 2.072 BC8 7-9cm 0.023 0.024 0.015 0.007 0.006 0.018 0.003 0.016 0.002 0.035 0.003 0.002 0.014 0.007 1.555 BC8 14-16cm 0.067 0.036 0.019 0.008 0.007 0.069 0.053 0.056 0.033 0.038 0.029 0.009 0.046 0.013 2.037 BC8 14-16cm 0.026 0.025 0.017 0.010 0.008 0.022 0.004 0.018 0.003 0.036 0.005 0.002 0.016 0.008 1.746 BC8 29-31cm 0.019 0.019 0.012 0.007 0.006 0.016 0.003 0.013 0.001 0.037 0.003 0.001 0.012 0.006 1.396 BC8 29-31cm 0.018 0.018 0.011 0.006 0.005 0.015 0.003 0.013 0.001 0.042 0.003 0.001 0.011 0.005 1.435 BC8 43-45cm 0.015 0.014 0.009 0.005 0.004 0.013 0.004 0.011 0.000 0.006 0.002 0.001 0.008 0.004 1.008 BC8 43-45cm 0.013 0.012 0.008 0.003 0.003 0.011 0.003 0.010 0.000 0.012 0.002 0.001 0.007 0.003 1.031 BC14 0-2cm 0.028 0.026 0.015 0.007 0.005 0.021 0.005 0.019 0.001 0.021 0.005 0.002 0.018 0.010 1.421 BC14 7-9cm 0.031 0.029 0.020 0.009 0.005 0.023 0.006 0.022 0.001 0.014 0.006 0.002 0.021 0.011 1.531 BC14 28-30cm 0.021 0.018 0.011 0.006 0.004 0.015 0.004 0.014 0.000 0.011 0.003 0.001 0.012 0.008 1.151 BC14 43-45cm 0.020 0.017 0.011 0.005 0.003 0.014 0.004 0.013 0.000 0.002 0.003 0.001 0.011 0.007 1.150 BC48 0-2cm 0.022 0.019 0.011 0.005 0.004 0.015 0.004 0.015 0.000 0.017 0.004 0.001 0.013 0.007 1.374 BC48 13-15cm 0.029 0.024 0.014 0.007 0.004 0.021 0.005 0.018 0.002 0.048 0.005 0.002 0.016 0.009 1.407 BC48 27-29cm 0.025 0.024 0.014 0.006 0.004 0.020 0.006 0.019 0.001 0.022 0.006 0.002 0.018 0.009 1.201 BC48 40-42cm 0.031 0.030 0.018 0.008 0.005 0.025 0.008 0.023 0.001 0.011 0.006 0.002 0.022 0.011 1.240 BC1 0-2cm 0.023 0.023 0.014 0.007 0.004 0.017 0.004 0.017 0.002 0.047 0.005 0.002 0.017 0.010 1.681 BC1 8-10cm 0.026 0.025 0.014 0.008 0.004 0.020 0.005 0.019 0.001 0.022 0.005 0.002 0.018 0.012 1.476 BC1 25-27cm 0.024 0.021 0.012 0.006 0.004 0.018 0.005 0.016 0.001 0.013 0.004 0.002 0.015 0.010 1.359 BC1 42-44cm 0.021 0.020 0.013 0.006 0.004 0.016 0.004 0.015 0.001 0.011 0.005 0.002 0.014 0.009 1.216 *abbreviations for PAHs can be found in Table 2
227 8.4 Complete PAH histograms for all samples
8.4.1 PAH histograms of bedrock formation samples
Figures A2 through A4 give the complete record of PAH histograms for analyzed for the bedrock sampled throughout the watershed.
228 0.18 Barton's Gully Jar 1 0.12 0.06
0.09 Barton's Gully Jar 2 0.06 0.03 2.0 1.5 Barton's Gully Rock 1 1.0 0.5 0.009 Barton's Gully Rock 2 0.006 0.003 0.08
g PAH /g sample Barton's Gully Rock 3A
µ 0.02 0.01
4.00 Barton's Gully Rock 3B
2.00
0.020 0.015 Barton's Gully Stream Bed 1 0.010 0.005
l y e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e n n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n e e r e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h y h f e e e e e e e h h h h c e e e e h e c s e e e e h h y c y n t a l l l l o r r r h c c c c t y r e y s s s s t t y y y p h o a a a a p p p p a t y a r r a r o i t h th u o o o r a a a a n P R r y y y y n n p p e p r e r z h h h h l io io io io n r r r r h r r r r ] ] ] B h p h t t t t lu lu lu h a P h a a e a d h p o a n F h h h h t a h h h h r / t h h h h r r [ [ P t ] p e p h h h h F F F t t t t n n t t t t o s n C o o c n i C a n C C C C o o - , b a p p p p - - - o o o o A e n n n n u e a u u a h n e i a a a a z z z z l ] - - - - l l z z 3 ] , c N h A A A A n f f n n , , e D 1 2 3 n n n n / / / / F a ] ] 2 g N N N N P e [ 1 2 3 4 k e e , h [ c A C C C e e e e s s s s h z [b [ , - - - - e e e e t C C C C B B 1 a o A ib ib ib ib n o o [ [ z 1 2 3 4 n n n n n e z z o z D D D D e e e e a n C C C C r r r r r B n n n n e - - - o e e e e th th th th B 1 2 3 u B B d ib n n n n l n C C C F I a a a a D n n n n - e e e e h h h h 1 P P P P C
- - - - 1 2 3 4 C C C C Figure A2. PAH histograms for Tertiary bedrock samples.
229 0.15 0.12 Cretaceous Gully 1 0.09 0.06 0.03
2.0 Cretaceous Gully 2 1.5 1.0 0.5
0.3 Cretaceous Gully 3 0.2 0.1
0.08 Cretaceous Gully 4 0.06 0.04 g PAH/gg sample
µ 0.02
1.5 Cretaceous Gully 5 1.0 0.5 0.0 Cretaceous Gully Rock 1 0.4
0.2
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h a a a a y r i h u o o o r n P r y y y y n n p p p r r t z t l o o o o a a a a R ] ] e ] e h h h h n r r r r p u u u i i i i h r r r r a P h h a a h o h t t t t p B n h F l l l t a r / t r r e a P d t h h h h h h h h ] a h h h h C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z o z n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C Figure A3. PAH histograms for Cretaceous bedrock samples.
230 0.006 LW Br 0.004
0.002
0.008 Mata Slip Br
0.002
0.060 g PAHg/g sample W1 Rock
µ 0.008 0.006 0.004 0.002
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A4. PAH histograms for bedrock samples taken from soil profiles.
231 8.4.2 A complete record of soil profile PAH histograms
Figures A5 through A14 are the complete PAH histograms for the ten soil profiles obtained.
0.012 Barton's Mouth 0-5cm 0.008
0.004
0.20 0.04 Barton's Mouth 5-10cm
0.02
0.09
g PAH /g sample /g PAH g Barton's Gully bedrock (Jar 2) µ 0.06
0.03
l s s s s s s s s s s s s s s s s y e e n e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e n n n a n n n n n n n n n n n n n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l e e e e e e e c e e e e e c s e e e e c h y h f h h h h h h h y y n t a l l l l o r r r h t y r e t t y y y p p p p a c c c c a y s s s s r a r o p h o h a a a a t y r i t h u o o o r n P r y y y y n n p p p r r z t l o o o o n a a a a R ] ] e ] e p h h h h u u u i i i i h r r r r a P h h r r r r a a h o B h n h t t t t F l l l t a r / t r r e a P d t p a h h h h h h h h h h h h [ [ ] p p h h h h t t t t t t t t C c i C e F F F n n o s n o o n , a n C C C C o o - b a p p p p - - - o o o o e n n n n u e a u u a e i A l l l z z 3 ] h n a a a a z z z z ] - - - - , N h A A A A n f f , , c D 1 2 3 / / / / F a ] ] n n e N N N N n n n n e [ 2 h g P 1 2 3 4 b k e e , [ c A e e e e s s s s , - - - - C C C h z [ [ e e e e t C C C C 1 a o A b b b b B B i i i i n o o [ [ z n n n n n 1 2 3 4 e z z o z D D D D a n e e e e n C C C C r r r r r B n n n e - - - h h h h o e e e e t t t t B 1 2 3 u B B d b n n n n l i n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4 C C C C
Figure A5. Barton’s Mouth profiles: A forested, Tertiary bedrock-derived soil profile.
232
0.15 0.12 Forest Soil 0-4cm 0.09 0.06 0.03
0.16 Forest Soil 10-15cm 0.12 0.08 0.04
0.3 Cretaceous Gully 3 0.2
g PAH /g sample 0.1 µ
Cretaceous Gully Rock 1 0.4
0.2
l e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e y e e n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A6. Forest Soil profiles: A forested, Cretaceous bedrock-derived soil profile.
233 0.009 BWBO 0-5cm 0.006 0.003
0.006 BWBO 5-10cm
0.003
0.30 BWBO 20-25cm 0.04
g PAH sample /g g 0.02 µ
0.004 BWBO 45-55cm
0.002
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z o z n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A7. Burdett’s Waiapu Bridge Overlook: A pastoral, bedrock-derived soil
profile taken from an area prone to sheetwash.
234 0.009 LW 0-8cm 0.006 0.003
0.015 LW 8-19cm 0.010
0.005
0.015 LW 30-40cm 0.010
0.005
0.08 LW 60-70cm 0.02 gPAH /g sample
µ 0.01
0.35 LW 81-92cm 0.04 0.02
0.004 LW Br
0.002
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y f l l l l y y t a r r r h t y r e t t y y y o c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p a h h h h h h h h h h h h [ [ ] p h h h h C i e p F F F t t t t n n t t t t o s n o o c n C n o o - , a C C C C b a p p p p o o o o n n n n e a a - - - A e u u u h e i l ] l l z z 3 ] n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e B t t t t u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C Figure A8. Landslide Waiapu: A pastoral, bedrock-derived soil profile taken from a
landslide scar.
235 0.004 W1 0-5cm
0.002
0.004 W1 5-10cm 0.002
0.006 W1 15-20cm
0.003
0.004 W1 30-35cm
0.002
0.003 W1 45-50cm 0.002
0.001 g PAH /g sample /g PAH g µ
0.004 W1 75-80cm
0.002
0.004 W1 95-100cm 0.002
0.060 W1 Rock 0.005
l s s s s e e n e s s s s e s s s e e e e e e e e s e e e s s s s e e e e e e e e e y e e e e n n a n e e e e n e e e n n n n n n n n e n n n e e e e n n n n n n n n n n r n n n n e e e n n n n e n n n e e e e e e e e n e e e n n n n e e e e e e e e e e l u l r t r r l r l r e e e e r e c c h f e e e e e e e h h h h c h s e e e e h h n h y a l l l l r t t y y y y y t o r r r h c c c c t y e p p p p a a y s s s s r a r o h o t p h h a a a a o o o r n y r r n n p p p r i t u a a a a P y y y y e r z t l o o o o n R ] ] e ] h h h h i i i i r r r r r r r r p u u u h a P h h a a h o h t t t t l l l p B n h F t a r / t r r e a P d t h h h h h h h h h h h h [ [ ] a C i p e p h h h h F F F t t t t t t t t s n c n C n n o o o , n C C C C o o - a p p p p o o o n n n n b a - - - o e u e a u u a i A z z h e l ] - - - - l l 3 ] n a a a a z z z z n f f , , , N h A A A A c / / / / a ] ] n n e D 1 2 3 n n n n F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A9. W1 Terrace: A stable, pastoral, tephric soil.
236 0.120 Mata Slip 0-5cm 0.008
0.120 Mata Slip 10-15cm
0.008
0.12 Mata Slip 20-25cm 0.01
0.060 Mata Slip 40-50cm 0.008
0.100
g PAH /g sample /g PAH g Mata Slip 52-62cm
µ 0.008
0.120 Mata Slip 80-90cm 0.008
0.008 Mata Slip Br 0.003
l s s s s s s s e e e e s s s s s e s s s s e e e e e y e e n e e e e e e e e e e e e n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e le u le r t r r l r le e r e e e h h h c e e e e r e c s e e e e h h c h y h f l le le le h h y y n t a o r r r h c c c c t y r e t t y y y o p p p p a t a y s s s s r a r o p h h h a a a a o o o r n y r r n n p p p r i t t u a a a a P R y y y y e ] e r z h h h h l n r r r r r r r r ] ] p u io io io io h a P h h a a h o B h n h t t t t l lu lu t / t r e a d t p F h h h h a h h h h r h h h h r P p a h h h h t t t t C [ [ i] e p F F F t t t t n n o s n o o c n C n o o - , a p p p p o o o o n n n n e C C C C b a - - - A e a u a h e i lu ] l lu z z 3 ] n a a a a z z z z A A A A n - - - - f f , , , N h n n c D 1 2 3 n n n / / / / F a ] ] g e N N N N n e [ 2 h P s s s s 1 2 3 4 b k e e , , [ c A e e e e - - - - C C C h z [ [ e e e e t C C C C B B 1 a o A b b b b n o o [ [ i i i i n n n n n z 1 2 3 4 e z z o z D D D e e e e a n D n C C C C r r r r r B n n n e - - - h h h h o e e e e t t t t B 1 2 3 B B d n n n n lu ib n C C C a a a a F I D
n n n n -
e e e e 1 h h h h P P P P C
- - - -
1 2 3 4 C C C C Figure A10. Mata Slip: A pastoral, tephric soil undergoing sheetwash, landsliding,
and tunnel gullying.
237 0.12 Manuel Terrace 0-5cm 0.01
0.20 Manuel Terrace 13-18cm 0.01
0.120 Manuel Terrace 20-25cm 0.006
0.100 Manuel Terrace 30-35cm
0.006
0.060
g PAH /g sample /g PAH g Manuel Terrace 45-50cm µ 0.004
0.080 Manuel Terrace 70-75cm
0.003
0.080 Manuel Terrace 85-90cm
0.005
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l l l u r r r t r r l r h e e e e e e e h h h h c e e e e h e c s e e e e h h c f y n h y a l l l l y r t t y y y y t o r r r h c c c c t e p p p p a a y s s s s r a r o p h o a a a a t h h o o o r n y r r n n p p p r i t u a a a a P y y y y r z t l o o o o R ] ] e ] e n r r r r p h h h h u u u i i i i h r r r r a P h h a a h h t t t t o B n h l l l t / t e a d t p F a r h h h h r r P a h h h h h h h h [ [ ] p C i e p h h h h F F F t t t t t t t t s n c n C n n o o o , n C C C C o o - a p p p p o o o o n n n n b a - - - e u e a u u a i A z z 3 h e l ] - - - - l l ] n a a a a z z z z n f f , , , N h A A A A c / / / / a ] ] n n e D 1 2 3 n n n n F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ - - - - [ e e e e t C C C C 1 a o B B A b b b b n o [ i i i i o [ z n n n n n 1 2 3 4 e z z o z n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C Figure A11. Manuel Terrace: A stable, organic farm on a colluvial terrace.
238 0.20 TS 0-5cm 0.02
0.01
0.14 TS 10-15cm 0.01
0.12 TS 30-40cm 0.01
0.12 0.02 TS 55-65cm
0.01 g PAH /g sample
µ 0.10 TS 80-90cm 0.01
0.18 0.02 TS 110-120cm
0.01
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y f l l l l y y t a o r r r h t y r e t t y y y p a c c c c a y s s s s r a r o p h o p p p t h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A12. Tutumatai Station: A stable, pastoral colluvial terrace.
239 0.009 TSLHT 0-5cm 0.006 0.003
0.012 TSLHT 15-20cm 0.008
0.004
0.015 TSLHT 30-40cm 0.010
g PAH /gsample 0.005 µ
0.012 TSLHT 110-120cm 0.009 0.006 0.003
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n a n e e e e e e e n n n e e e e n n e n n n e e e e n n n n n n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l e e e e e e e h c e e e e h e c s e e e e h h c n h y h f h h h t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A13. Tutumatai Station Lowest Holocene Terrace: A pastoral, rapidly failing
alluvial soil along the riverbank.
240 0.2 YHT 10-15cm
0.1
0.2 YHT 40-45cm
0.1
0.3 YHT 80-90cm 0.2 0.1 g PAH /g sample /g PAH g µ 0.12 0.09 YHT 110-115cm 0.06 0.03
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e n n n n n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A14. Youngest Holocene Terrace: A rough pastoral alluvial soil terrace.
241 8.4.3 PAH histograms of thermogenic hydrocarbon sources within the Waiapu watershed (Figure A15).
1.2 Te Puia Springs 1 0.8 0.4
0.6 Te Puia Springs 2 0.4
0.2
0.10 Williams Gas Seep 1 0.04
0.02 g g PAH /gsample
µ 0.30 0.06 Williams Gas Seep 2 0.04 0.02
l y e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e n n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n e e r e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l o r r r h t y r e t t y y y y y o p p p p a t c c c c a y s s s s r a r o ip h h h a a a a u o o o r n P y r r n n p p p r r t z t l o o o o n a a a a R y y y y ] ] e ] e p h h h h u u u i i i i h r r r r a P h h r r r r a a h o B h n h t t t t F l l l t a r / t r r e a P d t p a h h h h h h h h C h h h h [ [ ] p e p h h h h F F F t t t t n n t t t t o s n o o c n i C n C C C C o o - , a b a p p p p - - - o o o o A e n n n n u e a u u a e i l ] - - - - l l z z 3 ] h n N a a a a z z z z h A A A A n f f , , , c 1 2 3 / / / / F a ] ] n n e D N N N N n n n n e [ 2 h g P s s s s 1 2 3 4 b k e e , , [ c A C C C e e e e h z [ [ - - - - e e e e t C C C C B B 1 a o A b b b b n o o [ [ i i i i n n n n n z 1 2 3 4 e z z o z D D D D e e e e a n C C C C r r r r r B n n n n e - - - e e e h h h h o e B t t t t B d b 1 2 3 lu B i n n n n n C C C a a a a F I D
n n n n -
e e e e 1 h h h h P P P P C
- - - -
1 2 3 4 C C C C
Figure A15. Te Puia Springs and Williams Gas Seep both represent natural sources
of hydrocarbons to the watershed.
242 8.4.4 PAH histograms of sediment taken directly from the riverbed (Figure A16).
0.20 0.16 Barton's Gully Mud 0.12 0.08 0.04
0.4 Makarika Stream Mud 0.2
0.15 Mangaoporo Mud 0.10 0.05
0.3 Mata River Mud 0.2 0.1
g PAH /g sample /g PAH g YHT Mud
µ 0.4 0.2
0.15 Tikitiki Mud 0.10 0.05
l s s s s e e n e s s s s e s s s e e e e e e e e s e e e s s s s e e e e e e e e e y e e e e n n a n e e e e n e e e n n n n n n n n e n n n e e e e n n n n n n n n n n r e e e n n n n n n n e e e e e e n n n n e n e e n n n n e e e e e e e e l l e e e le r l e u e e e e r e e e h h h h c r e e e e r e t c e e e e h h r r c h y h f l l l l h r s y y y n t a o r r r h c c c c t y e s s s s t t y y a r h o a a a a p p p p a t y a y r o p h h u o o o r a a a a n r r y y y y n n p p p r r i t z t l o o o o n r r r r P R ] ] e ] e p h h h h u u u i i i i h a P h h r r r r a a h o B h n h t t t t F l l l t a h h h h r / t r r e a P d t p a h h h h t t t t h h h h [ [ i] p e p h h h h F F F t t t t n n o s n C o o c n C n C C C C o o - , a b a p p p p o o o o n n n n e a u u a e i - - - A e u ] l l z z 3 ] h n a a a a z z z z A A A A l n - - - - f f , , , c N h / / / / a ] ] n n e D 1 2 3 n n n n F e [ 2 h g N N N N P s s s s 1 2 3 4 b k e e , , [ c A C C C e e e e h z [ [ - - - - e e e e t 1 a o A b b b b n C C C C o B B [ i i i i n n n n n o [ z 1 2 3 4 e z z o z D D D D e e e e a n C C C C r r r r r B n n n n e - - - h h h h o e e e e t t t t B u d b 1 2 3 n n n n l B B i n C C C a a a a F I D n n n n - e e e e h h h h 1 P P P P C
- - - -
1 2 3 4 C C C C
Figure A16. Bedload mud taken directly from the Waiapu River and its tributaries to
track organic carbon through the watershed.
243 8.4.5 PAH histograms of marine sediment down core and with increasing distance from the river mouth (Figure A17 – A21).
0.3 60m tripod BC 0-2cm 0.2
0.1
0.3 60m tripod BC 14-16cm
0.2
0.1
0.3 60m tripod BC 27-29cm 0.2
0.1 g PAH/g sample µ
0.3 60m tripod BC 39-41cm 0.2
0.1
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h ] a h h h h C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z o z n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A17. The PAH histograms from the sediment core taken at 61 m water
depth.
244 0.3 BC8 0-2cm 0.2 0.1
0.3 BC8 7-9cm 0.2 0.1
0.3 BC8 14-16cm 0.2 0.1
0.3 BC8 29-31cm g PAH sample /g g
µ 0.2 0.1
0.3 BC8 43-45cm 0.2 0.1
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h ] a h h h h C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 e e [ c A s s s s b k , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n C C C C r r r r r B n n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A18. The PAH histograms from the sediment core taken at 83 m water
depth.
245 0.15 BC14 0-2cm 0.10 0.05
0.15 BC14 7-9cm 0.10 0.05
0.15 BC14 28-30cm 0.10 g PAH /gg sample
µ 0.05
0.15 BC14 43-45cm 0.10 0.05
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y f l l l l y y t a o r r r h t y r e t t y y y p a c c c c a y s s s s r a r o h o p p p t p h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h ] a h h h h C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 e e [ c A s s s s b k , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A19. The PAH histograms from the sediment core taken at 108 m water
depth.
246 0.15 BC48 0-2cm 0.10 0.05
0.15 BC48 13-15cm 0.10 0.05
0.15 BC48 27-29cm 0.10 g PAH /g sample /g PAH g µ 0.05
0.15 BC48 40-42cm 0.10 0.05
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h a a a a y r i h u o o o r n P r y y y y n n p p p r r t z t l o o o o a a a a R ] ] e ] e h h h h n r r r r p u u u i i i i h r r r r a P h h a a h o h t t t t p B n h F l l l t a r / t r r e a P d t h h h h h h h h ] a h h h h C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z z o n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B B d b 1 2 3 u B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A20. The PAH histograms from the sediment core taken at 128 m water
depth.
247 0.20 BC1 0-2cm 0.15 0.10 0.05
0.20 BC1 8-10cm 0.15 0.10 0.05
0.20 BC1 25-27cm 0.15 0.10
g PAH g /gsample 0.05 µ 0.20 BC1 42-44cm 0.15 0.10 0.05
l e e n e s s s s e s s s e e e e e e s s s s e e s e e e s s s s e e e e e e e e e y n n a n e e e e n e e e n n n n n n e e e e n n e n n n e e e e n n n n n n n n n n r e e e n n n n e n n n e e e e e e n n n n e e n e e e n n n n e e e e e e e e e e l u l r r r t r r l r l h f e e e e e e e h h h h c e e e e h e c s e e e e h h c n h y t a l l l l r r r t y r e t t y y y y y o h c c c c y s s s s o p p p p a t a r a r o p h h h a a a a o o o r n y r r n n p p p r i t u a a a a P R y y y y r z t l o o o o n ] ] e ] e p h h h h u u u i i i i h r r r r a h h r r r r a a h h t t t t P o B n h F l l l t a r / t r r e a P d t p h h h h h h h h h h h h ] a C [ [ p e p h h h h F F F t t t t n t t t t s n c n i C n o o o , n C C C C o o - a a p p p p o o o o n n n n b - - - A e u e a u u a e i l ] l l z z 3 ] h n a a a a z z z z - - - - , N h A A A A n f f , , c a ] ] n n e D 1 2 3 n n n n / / / / F g N N N N e [ 2 h P 1 2 3 4 k e e [ c A s s s s b , , C C C e e e e h z [ [ - - - - o e e e e t C C C C B B 1 a A b b b b n o i i i i o [ [ z n n n n n 1 2 3 4 e z z o z n D D D D e e e e a n n n C C C C r r r r r B n e - - - e h h h h o e e e t t t t B u B d b 1 2 3 B i n n n n l n C C C a a a a F I D
n n n n -
e e e e 1 h h h h C P P P P
- - - -
1 2 3 4
C C C C
Figure A21. The PAH histograms from the sediment core taken at 615 m water
depth.
248 CHAPTER 4
The fate of terrestrial organic carbon from a small mountainous river on the adjacent
continental shelf: Waiapu River sedimentary system, New Zealand
249 Abstract
Characterization of organic matter buried on the middle and outer continental shelf offshore from the Waiapu River in New Zealand points to the extensive preservation of riverine carbon, including rock-derived carbon. While marine carbon is deposited along with recent and ancient terrestrial carbon at those sites, analyses of dissolved inorganic carbon in the porewater indicates its rapid diagenetic processing in the seabed. On the shallow, energetic innermost portion of the shelf, however, apparent physical reworking of sediment leads to oxidation of both modern terrestrial and marine carbon.
The middle and outer Waiapu continental shelf can be partitioned into three distinct regions where average POC burial rates are 40, 107, and 222 Mg C/km 2/y.
When normalized to shelf area, these same regions bury 9, 35, and 15 Gg C/y, totaling an average of 59 Gg C/y. Relative to the 200 Gg C/y delivered by the
Waiapu River to the ocean, approximately 23% of the riverine carbon is retained on the mid- to outer shelf, matching the sediment inventory. Radiocarbon analysis of
DIC indicates that there is no apparent oxidation of kerogen in the seabed. Stable carbon isotopic signatures suggest minimal oxidation of modern terrestrial carbon, signifying that the 77% of the riverine carbon not accounted for has likely been retained and potentially oxidized on the inner shelf or escaped to the slope or beyond the established boundaries of the mid- to outer shelf.
250 1. Introduction
The biogeochemical transformations experienced by particulate organic
matter as it moves through sedimentary systems depend in part on tectonic setting.
Organic carbon must pass through multiple bioactive reservoirs on its way to the
ocean in rivers on large, predominantly passive margins. Extended time in bioactive
reservoirs allows for alteration of the initial terrestrial carbon source to the shelf (Blair
et al., 2004; Aller and Blair, 2006; Alin et al., 2008). Thus, riverine sediment
delivered to the shelf by rivers such as the Ganges-Brahmaputra, which sequesters
30-40% of its sediment flux in its floodplain (Goodbred and Kuehl, 1998), is distinct
from its terrestrial origin. Once delivered to a wide, energetic margin, resuspension
processes enhance exposure to higher order oxidants (e.g. O2, metal oxides) and
reactive marine organic matter is proposed to prime oxidation of the relatively
recalcitrant terrestrial OC (Aller et al., 1996; Aller, 1998; Aller and Blair, 2004). On
the Amazon shelf, the %OC decreases due to a loss of terrestrial carbon relative to
the constant background of marine organic matter in the fluid muds (Aller et al.,
1996; Aller, 1998; Aller and Blair, 2006). Marine carbon does not accumulate due to
the high rate of oxidation relative to the rate of addition in the energetic shelf. This
process is reflected by the variable nature of the age and reactivity of the now
complex mixture of organics that are buried in the seabed.
In contrast, many small mountainous river (SMR) systems such as the
Waiapu River system in New Zealand exhibit low to moderate degrees of
251 intermediate storage coupled with high sediment discharge, favoring the preservation of terrestrial organic matter across the continental margin (Lyons et al.,
2002; Blair et al., 2003). Organic carbon buried on the continental margins associated with SMR systems have a predominantly bimodal character reflecting two distinct sources of carbon: modern (terrestrial and marine) and ancient
(kerogen) (Blair et al., 2003; Blair et al., 2004; Leithold et al., 2001; Leithold et al.,
2005; Leithold et al., 2006). The bimodal particulate organic carbon (POC) composition on SMR margins has been hypothesized to be due to episodic, short- lived flood events that rapidly transport and bury particles prior to biogeochemical alteration by higher order oxidants (Blair et al., 2003; Blair et al., 2004; Leithold et al., 2006). On the continental shelf offshore from the Eel River in California for example, the organic carbon content increases, indicative of marine carbon being added to the sediment across the shelf without observably impacting the terrestrial carbon fraction, which is buried too rapidly to be altered (Blair et al., 2003).
The implications of the bimodal delivery of ancient and fresh terrestrial carbon on SMR margins are not yet fully appreciated. There is potential to improve the understanding of geomorphic processes involved in present-day sediment transport based on the resolution of distinct carbon pools delivered to the continental margin
(e.g. gullying delivers ancient rock carbon; sheetwash delivers modern soil organic carbon). Additionally, source apportionment of carbon in the seabed can enhance understanding of the processes associated with deposition and burial, thereby improving interpretations of the organic geochemical record. Finally, the kerogen
252 cycle is poorly understood, and further study of the fate of ancient carbon transported to the margin can clarify kerogen oxidation processes, leading to a better understanding of the global carbon cycle.
In this study, stable carbon and nitrogen isotopic signatures as well as polycyclic aromatic hydrocarbon (PAH) molecular distributions of potential terrestrial organic matter sources of sediment to the Waiapu margin in New Zealand were compared to riverine and marine sediment (Chapter 2 and 3). If a bimodal source of
OC to the riverine sediments is assumed, non-zero 14 C-contents indicate the input of
modern carbon from terrestrial plants and marine organic matter as well as kerogen.
However, in contrast to the Eel, the Waiapu system has the additional complexity of
aged soil organic matter inputs (Chapter 2 and 3). This is concurrent with the
adjacent Waipaoa watershed, where alluvial deposits are composed of upriver
terrestrial sources including rock and soil organic matter (Blair et al., 2009).
Streambank erosion produces sediment during low intensity, high frequency storms,
providing a continuous source of both aged and ancient carbon (Blair et al., 2009).
While most studies of seabed carbon dynamics look only at the POC, dissolved inorganic carbon (DIC) in the porewater has been established to be an integral part of the system as well (e.g. Aller and Blair, 2004; Aller and Blair, 2006;
Aller et al., 2008). During early diagenesis in the seabed, the most energetically favorable fraction of buried organic carbon will be preferentially remineralized
(Lehmann et al., 2002). The oxidation of this organic carbon buried in the sediment adds CO 2 to the porewater. The stable carbon isotopic signature of the buried DIC
253 reflects the source (terrestrial or marine) of the reactive carbon (Aller and Blair,
2004; Aller and Blair, 2006; Aller et al., 2008). While the carbon preserved in the
POC at each site is a function of the depositional environment (chapter 2), the fate of organic carbon across the margin can be further clarified by examining the composition of the reactive fraction of organic matter being oxidized (Aller and Blair,
2006; Aller et al., 2008; Alin et al., 2008).
The predominance of rock carbon delivered to and buried on the Waiapu continental margin also presents the unique opportunity to investigate the reactivity of the insoluble kerogen fraction associated with the ancient organic carbon. The completion of the global carbon cycle requires the oxidation of kerogen over geologic time scales; however this process is poorly understood. Kerogen is relatively recalcitrant, as it has already survived at least one cycle of burial, preservation, and uplift (Pilson, 1998). While microbial activity facilitates the oxidation of kerogen when exposed on the Earth’s surface (Leythaeuser, 1973;
Clayton and Swetland, 1978; Petsch et al., 2000; Petsch et al., 2001), much can escape to the ocean by river transport in a highly erodible terrain, such as a short mountainous river on an active margin prior to oxidation (Blair et al., 2003). Due to increased mass wasting caused by deforestation, more kerogen is thought to escape oxidation on land in recent history (Blair et al., 2003). Once delivered to the continental margin, the fate of kerogen in the ocean is largely unknown. It is not known, for example, to what extent kerogen might be oxidized in the seabed.
Because of its recalcitrant nature, we anticipate that the ancient sedimentary OC will
254 experience little if any detectable loss from particles during transport from the bedrock source to the continental margin. This input will be explored because the fate of increased kerogen delivery in a modern sedimentary environment has implications for the control of atmospheric chemistry as part of the global carbon cycle.
Mass balance mixing models of organic carbon delivered to the Waiapu margin have been employed herein to assess the impact of depositional environment on the fate of sedimentary carbon. Distinct depositional environments have been identified (Kniskern, 2007; Kniskern et al., 2009) and examined using stable and radiocarbon isotopic signatures of POC and DIC to characterize the preserved and oxidized fractions of carbon in the seabed. Additionally, radiocarbon analysis has been used to examine kerogen dynamics, thereby potentially enhancing the understanding of the impact of small mountainous river sedimentary systems on the overall global carbon cycle.
Furthermore, research initiatives to determine the carbon budget for New
Zealand are necessary as the country endeavors to be compliant with the Kyoto
Protocol (Scott et al., 2006). Local carbon budgets are particularly necessary in areas such as the East Cape where high carbon mobilization is induced by erosion
(Scott et al., 2006). While previous terrestrial work has been done to model and estimate the carbon being eroded from regions across New Zealand including the
East Cape (e.g. Gomez et al., 2003; Scott et al., 2006), the fate of this OC in the marine system has not previously been considered. A C-budget has been estimated
255 herein so as to determine the fate of the riverine POC on the continental shelf adjacent to the Waiapu River.
2. Site description and methodology
2.1 Site description
The Waiapu River watershed is located on the East Cape of New Zealand’s North Island
(Figure 1). The river flows 130 km eastward to the
Pacific Ocean from the Raukumara Range along the Hikurangi subduction margin (Moore and Figure 1. Map of the Waiapu Figure 1. Map of the Mazengarb, 1992). Active tectonic deformation of Waiapu River Watershed, New Zealand the area results in a highly friable terrain of primarily Cretaceous and Tertiary sandstones and mudstones (Mazengarb and
Speden, 2000). Upland soils are built on volcanic ash layers dating back to 55,000 years B.P., with deposits of the most recent eruption, 665 years B.P., being the most widespread (Eden et al., 2001). The vegetation in the Waiapu watershed has changed drastically during the late Holocene; Maori settlers burned the native podocarp and hardwood forests (600-700 years B.P.) and Europeans cleared the forests for pasture (from 1890-1920) (Wilmshurst et al., 1999).
256 Gully erosion into sedimentary bedrock is estimated to account for more than half of the high sediment yield of the Waiapu River catchment (Page et al., 2001;
Parkner et al., 2006). Intense episodic storm events can also initiate widespread shallow landsliding and sediment discharge to the continental shelf (Page et al.,
1994; Page et al., 1999; Page et al., 2001). The deforestation by Maori and
European settlers has increased gullying and shallow landsliding in the basin (Hicks et al., 2000; Page et al., 2001). To a first approximation, the suspended POC in the
Waiapu has been shown to be a bimodal mixture containing roughly 15-25% modern plant-derived and 75-85% rock (kerogen) OC, with surface erosion and gullying as the primary processes generating the sediment from the soil and bedrock (Leithold et al., 2006; this study).
At 20,520 T km -2 yr -1, the Waiapu’s sediment yield is one of the highest in the
world (Page et al., 2001), with a sediment discharge of approximately 36 x 10 6 T/yr from a drainage area of only 1734 km 2 (Page et al., 2001). High intensity storm
events, high rates of uplift and steep slopes, and unstable lithologies contribute to
the highly degraded nature of the catchment, with extreme mass wasting and high
sediment yields (Hicks et al., 2000; Marutani et al., 1999; Page et al., 1999). The
small drainage basin limits the storage capacity for the eroding sediment within the
watershed caused in part by a yearly rainfall in excess of 2.4 m (Page et al., 2001).
Sediment transport on the Waiapu margin is dictated by waves, currents, and
sediment-gravity (hyperpycnal) flows (Wright et al., 2006; Addington et al., 2007;
Wadman and McNinch, 2008; Kniskern et al., 2009). Two currents affect the
257 Waiapu continental margin: the East Cape Current (ECC) flowing southward along the shelf break and upper continental shelf and the Wairarapa Coastal Current which flows northward inshore of the ECC (Chiswell, 2000; Wadman and McNinch, 2008;
Kniskern et al., 2009). Boundary layer instrumented tripods off the Waiapu mouth at
40 and 60 m water depths have shown dense, near-bottom hyperpycnal flows during moderate river floods (Wright et al., 2006). The deposits of episodic flood events are revealed in the down-core geochemical records by 210 Pb-poor sediment layers on the mid-shelf. The high sediment concentrations during such events overwhelm the local supply of 210 Pb and do not provide enough time to permit scavenging of
oceanic 210 Pb (Sommerfield and Nittrouer, 1999; Sommerfield et al., 1999; Kniskern
et al., 2009). During non-flood conditions, energetic waves and currents rework
sediment deposited in the shallow shelf, resulting in higher 210 Pb inventories due to greater exposure time to the water column (Sommerfield and Nittrouer, 1999;
Sommerfield et al., 1999; Kniskern et al., 2009).
210 Pb profiles have been used to determine sediment accumulation rates on
the shelf that range from 0.2 to 3.5 cm/y (Kniskern, 2007; Kniskern et al., 2009).
The 200 m isobath defined the eastern boundary of the Waiapu shelf, while shelf
deposit thickness, particle size, and other trends were used to define the north and
south boundaries (Lewis et al., 2004; Kniskern, 2007; Kniskern et al., 2009). The
shelf was divided into four regions of accumulation: the inner shelf and three regions
of 0.7, 1.4, and 2.7 cm/y accumulation on the mid- to outer-shelf representing
estimated areas of approximately 217, 330, and 67 km 2 each, respectively (map in
258 Kniskern et al., 2009). Beryllium-7 indicates that the inner shelf (<60 m) behaves as a bypass zone or temporary storage (Addington et al., 2007; Kniskern, 2007;
Wadman and McNinch, 2008; Kniskern et al., 2009) The highest sediment accumulation rates occur on the mid to outer shelf (1.4-2.7 cm/y), and then decrease
(0.7 cm/y) towards the shelf break (Kniskern, 2007; Kniskern et al., 2009). Minimal bioturbation is observed, especially in the high accumulation zones indicating that physical processes dominate shelf behavior (Kniskern, 2007). Increased frequency of bioturbation and decreased frequency of lamination radially away from the river mouth indicates that riverine input rather than wave energy controls sediment mixing patterns (Kniskern, 2007).
2.2 Sampling methods
Sediment samples were collected on the continental margin adjacent to the
Waiapu River mouth aboard the R/V Kilo Moana in May 2004. Box cores (up to 50 cm in length) and Kasten cores (approximately 2 m) were obtained and immediately subsampled and sectioned on ship. Seven evenly spaced intervals of sediment 2 cm thick were collected from each core. Pore water was extracted from the sediment by pressure squeezing with nitrogen (Reeburgh, 1967) through a combusted glass fiber filter and transferred into 5 mL glass ampoules under nitrogen atmosphere, flame sealed, and frozen. The sediment was transferred to a plastic bag and stored frozen until time of analysis.
259 2.3 Analytical methods
2.3.1 Particulate organic carbon
Sediment samples were freeze-dried and acidified (4N HCl, 4 days) prior to analysis to remove carbonates. Dried subsamples were then placed in tin boats and analyzed for %OC and δ13 C with a continuous flow elemental analyzer (Flash CE
1112)-IRMS (Thermo Electron Delta V) interfaced with a Thermo Conflo III. The errors for EA and the IRMS are ± 2% (relative standard deviation) and ± 0.1‰, respectively. Selected samples were subsequently analyzed for 14 C by cryogenically trapping the CO 2 produced by the oxidation of POC via the elemental analyzer.
These samples were sent to the National Ocean Sciences Accelerator Mass
Spectrometry (NOSAMS) facility at Woods Hole Oceanographic Institution. The CO 2 was converted to graphite and analyzed for 14 C content, which is reported as fraction
modern relative to the National Bureau of Standards and Technology Oxalic Acid I
Standard normalized to a δ13 C of -19‰ (Olsson, 1970), where modern is defined as
95% of the radiocarbon concentration in AD 1950. Further corrections normalize the fraction modern to a δ13 C VPDB of -25‰ to account for natural isotopic
fractionations. The average error of fraction modern (Fm) reported by NOSAMS on
POC samples was 0.0023. This error is the larger of the internal error (± √n, where n equals the total number of counts for a given target) or external error (reproducibility of counts for multiple exposures for a given target) calculated.
260
2.3.2 Dissolved inorganic carbon
Pore water samples were thawed and transferred from ampoules to a syringe to minimize atmospheric CO 2 dissolution during measurement. Between 0.05 and
0.4 mL of porewater was then added to 2 mL Wheaton glass bottles containing 0.1 mL of 1M H 3PO 4/0.1M CuSO 4 solution that had been sealed with a crimped red rubber stopper and flushed with helium. The resulting CO 2 from the acid/sample solution was stripped by helium flow, dried via passage through magnesium perchlorate and Nafion® tubing, and analyzed for concentration (± 0.15 mM) and
δ13 C (± 0.1‰) via continuous flow isotope ratio mass spectrometry using a Conflo III
interfaced with a Thermo Electron Delta V IRMS.
Selected samples were subsequently analyzed for 14 C content. Between 2 and 5 mL of the porewater sample was added to 40 mL Wheaton glass bottles containing 2 mL of 1M H 3PO 4/0.1M CuSO 4 solution that had been sealed with a
crimped red rubber stopper and flushed with helium. The resulting CO 2 from the
acid/sample solution was stripped by helium flow, dried via passage through
magnesium perchlorate and Nafion tubing, and cryogenically trapped. These
samples were sent to the National Ocean Sciences Accelerator Mass Spectrometry
(NOSAMS) facility at Woods Hole Oceanographic Institution for analysis as above.
Due to the small sample size (<10 µmol C) of CO 2 trapped from the DIC samples, the average error of Fm is higher at 0.0086, calculated in the same manner as in
261 POC 14 C samples, where the error reported by NOSAMS is the larger of the internal
and external errors.
3. Results
3.1 Particulate organic carbon
POC and DIC were characterized downcore in sediment profiles from two
transects, one north and one south of the mouth of the Waiapu River (shown
encircled in Figure 2). POC contents ( ) of sediment from these two transects range from 0.5 to 0.9% and are relatively uniform downcore, changing on average only between 0.1 and 0.2% with depth (Figure 3). The %C org tends to be lowest in areas of active resuspension (Figure 3.A and H), such as on the inner shelf
(Wadman and McNinch, 2008) and on certain bathymetric features such as the bathymetric high that influences the core at 169 m on the southern transect
(Addington et al., 2007). Active sorting processes in these areas tend to preserve coarser sediment which tends to preserve less organic carbon (Keil et al., 1997;
Leithold and Hope, 1999; Gordon and Goñi, 2004; Bianchi et al., 2007). Variations in %C could indicate the addition of marine carbon across the shelf or increased terrestrial carbon due to a flood within a specific horizon of a core. Variations could also be due to different sediment delivery mechanisms, including gravity-driven flow,
262 water column resuspension, and dilute suspensions/hypopycnal river plumes, over time (Kniskern, 2007).
The δ13 C of the POC ( ) becomes slightly more positive as the POC is
enriched in 13 C down core likely reflecting early diagenesis; however, this change is rarely more than 0.5‰ throughout the cores (Figure 3 and Table 1). As with stable carbon isotopic signatures, only minimal change was observed in the radiocarbon age of selected depth intervals in the top 50 cm of the sediment in individual cores on the northern transect (Table 1).
Across the Waiapu margin, particulate organic carbon generally becomes enriched in 13 C and 14 C relative to 12 C with increasing distance from the river mouth
and increasing water depth (Figure 3 and 4; Table 1; Chapter 2). Exceptions to this
trend are typically the result of bathymetric features on the seabed impacting
sedimentation patterns and particle sorting processes (Figures 2 and 4; Chapter 2;
Addington et al., 2007; Wadman and McNinch, 2008). Along the northern transect,
the stable carbon isotopic signatures of the POC range from -25.3 to -23.4‰ (Figure
3 and 4; Table 1), mostly becoming more positive with distance. The radiocarbon
age of surface sediment particulate organic carbon ranges from about 7000 years
BP to 3000 years BP, getting younger with distance from the river mouth (Figure 4;
Table 1). These data suggest either a change in the source of carbon delivered or
alteration of carbon preserved cross-shelf (e.g. Alin et al., 2008; Kuzyk et al., 2008;
Tesi et al., 2008; etc).
263 The variation within a core is generally less than that between the cores across the shelf (Figure 3). This suggests that there has been minimal variation in organic matter source to the margin during this period of accumulation (between 18 and 70 years based on sediment accumulation rates (Kuehl, 2007)) and that depositional environment impacts the organic carbon preserved across the margin.
Though specific horizons sampled in this study have not been identified as flood layers, such layers have been identified by others using 210 Pb, and have more terrestrial isotopic signatures than non-event layers, indicating the preservation of riverine carbon due to rapid burial (Kniskern, 2007; Kniskern et al., 2009).
264 Figure 2. Organic matter partitioned by fractions of riverine and marine carbon across the Waiapu margin. Terrestrial carbon dominates the composition of the shelf. Marine carbon is typically added to the terrestrial carbon with distance from the river mouth. Minimal marine carbon is added to the sediment on the inner shelf or to that buried in the region of highest accumulation (in pink). Accumulation rates for the color-coded regions are 2.7 cm/y (pink); 1.4 cm/y (green); and 0.7 cm/y (blue) as determined by Kniskern (2007). The size of the bar refers to the %C, for example, the bar in the legend corresponds to 0.44% C. Pink circles denote the stations along the northern and southern transects.
265 %C
0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0 A B C D E 10
20
30 depth(cm) 40
61 m 83 m 108 m 128 m 615 m 50 -25 -24 -23 -25 -24 -23 -25 -24 -23 -25 -24 -23 -25 -24 -23
0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0 F G H 10 13 20 δ C %C org 30 depth(cm) 40
88 m 114 m 169 m 50 -25 -24 -23 -25 -24 -23 -25 -24 -23 13 δ C
Figure 3. Depth profiles of POC content and stable carbon isotopic signatures for sediment in the cores along the northern (A-E) and southern (F-H) transects. Downcore trends are relatively uniform, while cross shelf patterns reveal isotopic enrichment in 13 C with increasing distance from the river mouth and lower %C org in cores that experience agitation and mixing of the seabed. Sampling intervals (2 cm thick) are shown as vertical error bars.
266 -18 -300 δ13 C POC ∆14 C POC 13 -400 -20 δ C DIC -500 -22 C (‰) C (‰) 14
13 -600 δ -24 ∆ -700 -26 0 50 100 150 600 Bathymetric Depth (m)
Figure 4. Particulate organic and dissolved inorganic carbon isotopic analysis of a seaward transect from the river mouth. In pink, stable () and radiocarbon ( ) isotopic signatures of surficial sediment (0-2 cm) POC with relation to bathymetric depth. The 0 m water depth represents the terrestrial end-member taken from riverine suspended sediments (n=5). Sediment typically becomes isotopically heavier and younger across the shelf, consistent with the addition of marine carbon. In blue, the stable carbon isotopic signatures of dissolved inorganic carbon added to the porewater in these cores indicates the origin of carbon being oxidized in the seabed. Nearshore, terrestrial organic carbon is oxidized, while on the middle shelf, marine carbon is preferentially oxidized.
267
Table 1. POC and DIC concentrations and isotopic compositions for selected depths of representative profiles along the northern transect. POC DIC 13 14 14 13 14 14 14 Depth %C org δ C ∆ C C age Fm [DIC] δ C ∆ C C age Σ CO 2 Sample (cm) (‰) (‰) (mM) (‰) (‰) Fm
0-2 0.54 -25.35 -583.0 6970 0.420 2.85 -4.95 ND ND ND 61 m 8-10 0.60 -25.38 -521.1 5860 0.482 4.46 -12.72 36.01 >Mod 1.043 21-23 0.62 -25.24 -575.6 6830 0.427 5.81 -16.09 28.76 >Mod 1.036
0-2 0.56 -24.17 -494.2 5420 0.509 ND ND ND ND ND 9-11 0.58 -24.11 -478.5 5180 0.525 4.00 -7.57 4.32 >Mod 1.011 108 m 30-32 0.50 -24.21 -542.9 6230 0.460 3.42 -8.35 2.03 >Mod 1.009 45-47 0.58 -24.25 -495.2 5440 0.508 4.07 -10.45 18.63 >Mod 1.025
0-2 0.58 -24.51 -516.3 5780 0.487 ND ND ND ND ND 9-11 0.62 -24.68 -534.1 6080 0.469 2.33 -4.14 35.51 >Mod 1.042 128 m 22-24 0.58 -23.96 -482.7 5240 0.521 3.33 -6.79 8.39 >Mod 1.015 29-31 0.62 -23.91 -497.8 5480 0.506 3.54 -8.53 7.80 >Mod 1.014 42-44 0.53 -24.08 -555.4 6460 0.448 4.14 -10.84 -25.39 155 0.981 *ND means not determined
3.2 Dissolved inorganic carbon
The concentration of DIC ( ) typically increases with depth in most profiles from approximately 2 to 2.5 mM just below the sediment-water interface to concentrations ranging from 4 to 8 mM in the upper 50 cm of the sediment columns
(Figure 5, Table 1). The main exception to this pattern of increasing concentration with depth was found at 169 m depth along the southern profile. The bathymetric high near this site likely induced mixing of the porewater while the sediment was reworked in that profile; therefore, the concentration profile of DIC was uniform
268 throughout and close to that of the overlying seawater around 2.5 to 3 mM (Figure
5.G). The shape of the concentration profile can also contribute to the understanding of DIC behavior in the seabed. Simple upward diffusion of the DIC produced is expected to produce a linear gradient, while diffusion coupled with production creates a concave profile (Reeburgh, 1976; Martens and Berner, 1977;
Thomas et al., 2002). Patterns observed on the Waiapu shelf fall between these two expected patterns.
The δ13 C of DIC ( ) generally became progressively depleted (more negative) in 13 C, from approximately -2 to -4‰ near the sediment-water interface to
approaching from -9 to -16‰ (Figure 5; Table 1). This trend is most likely the result
of OC oxidation in the seabed (e.g. Thomas et al., 2002). The addition of 13 C- depleted DIC from the oxidation of POC to seawater DIC (~0‰) results in a systematic depletion in 13 C with depth in the seabed concurrent with the increase in
CO 2 concentration (Figure 5). Again, because of sediment disturbance at 169 m water depth on the bathymetric high, the isotopic trend at this site is subdued relative to the other sites. While still becoming increasingly depleted in 13 C, the isotopic change is only 3‰ downcore to 40 cm depth, from approximately -2.5 to -5.5‰.
Radiocarbon analyses on all but one sample of porewater indicate that the DIC is entirely modern (Table 1). The sole exception was a sample at the deepest interval of the 128 m core that had an uncorrected 14 C-age of 155 years B.P.
269 [DIC] (mM) 2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8 0 A B C D 10
20
30 depth (cm) depth 40 61 m 83 m 108 m 128 m m = 4.98 m = 12.8 m = 6.78 m = 20.94 r2 = 1 r2 = 0.77 2 2 50 r = 1 r = 0.95
-16 -12 -8 -4 -16 -12 -8 -4 -16 -12 -8 -4 -16 -12 -8 -4
2 4 6 8 2 4 6 8 2 4 6 8 0 E F G 10 13 20 δ C [DIC] 30 depth (cm) depth 40 88 m 114 m 169 m m = 2.95 m = 5.99 m = n/a r2 = 1 r2 = 0.96 r2 = n/a 50 -16 -12 -8 -4 -16 -12 -8 -4 -16 -12 -8 -4 13 δ C
Figure 5. DIC concentration () and stable carbon isotopic signatures (o) of porewater with depth in cores along the northern (A-D) and southern (E-G) transects. DIC concentration typically increases with depth, while the stable carbon isotopic signature 13 becomes depleted in C as POC is oxidized to CO 2 and added to the porewater. Sampling intervals (2 cm thick) are shown as vertical error bars. Dotted black lines indicate the approximate concentration gradient that can be determined at this resolution.
270 4. Discussion
4.1 Source identification of preserved particulate organic carbon
In order to develop a carbon inventory for the kerogen, terrestrial non-rock,
and marine fractions on the Waiapu shelf, the sedimentary carbon has first been
partitioned into these contributing sources (Chapter 2). An isotopic mass balance
approach based on the stable and radiocarbon isotopic values of particulate organic
carbon resolves the relative contributions of various carbon sources. The
prototypical study of source apportionment using carbon isotopic signatures was
conducted in the Eel River sedimentary system, characterized by a bimodal
distribution of ancient kerogen and modern carbon of terrestrial origin in addition to
the marine carbon sources (Blair et al., 2003). While these are the dominant
sources of carbon (Chapter 2 and 3), the aged soil carbon contribution to the
sediment delivered to the shelf by the Waiapu River should not be overlooked.
Therefore, the riverine carbon is characterized by two mass balance models
(an ancient vs. modern and a rock and non-rock riverine contribution (Chapter 2)),
and is quantified as the average of the two methods (Table 2; Chapter 2). Using the
stable and radiocarbon isotopic signatures, simultaneous equations (1), (2), and (3)
can be used to solve for the fractions of modern marine, modern terrestrial
plant/non-rock riverine, and kerogen fractions. The δ13 C and ∆14 C values define
these fractions by the following equations:
271 13 Eq. (1) δ C = f mar δ mar + f terr δ terr + f ker δ ker
14 Eq. (2) ∆ C = f mar ∆ mar + f terr ∆terr + f ker ∆ ker
Eq. (3) 1 = f mar + f terr + f ker where f x is the fraction of either marine, modern terrestrial/non-rock riverine or kerogen C, and δ and ∆ are the isotopic compositions of the respective end-
members. The stable carbon isotopic end-member values utilized were empirically
determined from samples acquired throughout the watershed. The rock carbon that
is feeding the Waiapu River margin has a δ13 C of -25.3‰ (±0.2‰, n=6); the average stable carbon isotopic signature for modern terrestrial organic matter -27.8‰
(±0.6‰, n=6) (Chapter 2). The non-rock, riverine isotopic signature (determined by mass balance) is -26.7‰ (r 2=0.998; n=5), reflecting a dominance of C3 plant input
and incorporation into the river sediments (Blair et al., 2009). The averaged riverine
isotopic signature incorporating ancient, aged, and modern carbon -25.3‰ (±0.35‰,
n=11); the stable carbon isotopic signature for the marine carbon end-member is -
19.2‰ (±1.2‰, n=8) (Chapter 2). Radiocarbon end-members are -1000‰ for
kerogen, as fossil carbon is radiocarbon dead; +33‰ for the modern marine organic
matter; +33‰ for the modern terrestrial organic matter; and -136 ± 109‰ for the
non-rock, riverine carbon (Chapter 2; Blair et al., 2009).
The mass balance calculations indicate that the terrestrial carbon behaves
conservatively across the shelf along the northern transect with increasing
contributions of marine carbon added with distance from the river mouth (Figures 4
and 6; Chapter 2). The radiocarbon age supports this interpretation, becoming
272 younger as modern marine carbon is added to the preserved terrestrial carbon across the margin (Figure 4; Table 2). Within the errors of the end-members, these calculations indicate that terrestrial carbon accounts for approximately >90% of the carbon on the inner shelf, between 70 and 85% on the mid-shelf, and 60 to 70% of carbon on the slope (Figure 6 and Table 2). Kerogen accounts for between 65 and
80% of the riverine carbon on the margin, with an average of 72% (Table 2).
Kerogen ranges from 0.3 to 0.45%C depending on transect and depositional environment. Modern terrestrial carbon ranges from 0.11 to 0.20%C (Table 2). The
0.90 %C marine %C marine 0.80 A %C riverine B %C terrestrial 0.70 %C kerogen 0.60
org 0.50
%C 0.40 0.30 0.20 0.10 0.00 0 61 83 108 128 615 0 61 83 108 128 615 Bathymetric depth (m) Bathymetric depth (m)
Figure 6. Mass balance partitioning of particulate organic carbon in surficial sediment along the northern transect (0 m bathymetric depth refers to the riverine end-member sample). A) The two end-member mass balance calculation apportions marine from terrestrial carbon using the stable carbon isotopic signatures. B) The three end-member mass balance calculation includes radiocarbon dating to apportion ancient from modern terrestrial carbon and marine carbon. Within the errors of the end-members, these calculations reinforce one another, and indicate that terrestrial carbon accounts for >90% of the carbon on the inner shelf, between 75 and 90% on the mid-shelf, and 60 to 70% of carbon on the slope. Of that riverine carbon, kerogen accounts for between 65 and 80%.
273 primary exception to this occurs on the bathymetric high (Table 2), where the POC shows a loss of riverine carbon (0.16%C ker and 0.06%C terr ) accompanied by a
significant increase in marine carbon (0.20%C mar ) (Table 2). It is not known whether
this loss of terrestrial carbon is due to physical sorting, oxidation, or both (Brackley
et al., 2009).
Similar three end-member mass balance calculations based on the down-
core patterns of each carbon fraction in the particulate organic carbon down to 50
cm depth were relatively conserved. This supports the conjecture that no major
change or variation in organic matter source over recent history is represented by
the sediment core in the sampled horizons, as was suggested by the relatively
13 14 uniform %C org , δ C and ∆ C data (Table 1). Some variation is due to weather events such as Cyclone Bola (Marutani et al., 1999; Page et al., 1999; Kasai et al.,
2005) throughout the time analyzed that have discharged more modern and aged terrestrial material along with the rock carbon; however, these pulses of organic matter variance were not identified in the sampled horizons herein.
274
Table 2. Application of mass balance calculations to partition POC burial rates by carbon fraction e
Water 2 13 14 %C %C %C POC burial rate (g C/m /y) Depth δ C ∆ C %C a b c org mar terr ker a b c (m) Total Mar Terr Ker
River d 0 -25.5 -724.6 0.56 0.03 0.11 0.42 NA NA NA NA
61 -25.4 -583.0 0.50 0.05 0.14 0.31 NA NA NA NA 83 -24.7 -528.7 0.55 0.11 0.13 0.31 81.6 15.9 19.1 46.7 Northern 108 -24.2 -494.2 0.58 0.16 0.11 0.31 86.1 23.0 16.6 46.5 Transect 128 -24.5 -516.3 0.53 0.12 0.11 0.30 39.3 8.8 8.5 22.0 615 -23.4 -317.8 0.71 0.28 0.16 0.27 52.7 21.1 11.6 20.0
88 -24.8 -618.8 0.67 0.11 0.12 0.44 191.8 31.2 35.3 125.3 Southern 114 -24.3 -461.1 0.89 0.23 0.20 0.45 254.7 66.4 58.4 129.9 Transect 169 -22.8 -313.7 0.42 0.20 0.06 0.16 31.2 14.8 4.6 11.7 aMarine; bTerrestrial; cKerogen; d suspended sediment collected at the Piggery Rd bridge; eelemental analysis and burial rates are shown for the lowest horizon sampled in each core
4.2 Source identification of remineralized organic carbon
Unlike POC, which represents the organic carbon being preserved in the sediment, dissolved inorganic carbon concentrations and isotopic signatures of porewater indicate the source of organic carbon that is oxidized in the seabed
(Ogrinc et al., 2003; Aller and Blair, 2004; Aller and Blair, 2006; Aller et al., 2008).
Herein, this fraction of carbon is termed diagenetically active carbon, as this is the organic carbon that is preferentially remineralized in the top 50 cm of the sediment column, adding CO 2 to the porewater. The source of this carbon indicates the
275 relative reactivity of the various carbon pools (Schillawski and Petsch, 2008). Stable carbon isotopic analysis of the porewater is used to determine the origin of the remineralized carbon (marine or terrestrial), while radiocarbon analysis indicates the age of the oxidized carbon (ancient or modern) (Thomas et al., 2002). Mass balance equations express the relationship between the concentration and stable carbon isotopic signature of DIC in seabed porewater as follows:
Eq. (4) [DIC] pw = [DIC] sw + [DIC] a
13 13 13 Eq. (5) δ Cpw *[DIC]pw = δ Csw *[DIC] sw + δ Ca*[DIC] a, where the δ13 C and concentration of DIC are described for the porewater (pw) in terms of those for seawater (sw) and that which is added (a) to the seawater by oxidation. These two equations can be combined and rearranged to afford
13 13 13 13 Eq. (6) δ Cpw *[DIC] pw = δ Ca*[DIC] pw + ( δ Csw -δ Cpw )*[DIC] sw .
13 The slope of the linear function of [DIC] pw *δ Cpw versus [DIC] pw represents the
stable isotopic signature of the particulate organic matter being oxidized (Blair et al.,
2003). The isotopic compositions of the remineralized carbon were established for
both the northern and southern transects for comparison of the DIC behavior under
different regions of accumulation (Figure 7).
In the northern transect, the δ13 C of the remineralized carbon is considerably more positive for the mid-shelf stations (-19 + 1‰) relative to the inner shelf station
(-25‰), consistent with a shift in POC source from terrestrial to marine carbon with
increased distance from the river mouth and a change in depositional environment
(Figure 7.A-D and Figure 4) (e.g. Thomas et al., 2002; Blair et al., 2003). On the
276 0 A B C D -20 C 13
δ -40
-60
[DIC] x[DIC] -80 61 m 83 m 108 m 128 m m = -24.8 m = -18.8 m = -18.0 m = -20.55 2 2 2 2 -100 r = 0.95 r = 0.96 r = 0.99 r = 0.97
2 3 4 5 6 2 3 4 5 6 2 3 4 5 6 2 3 4 5 6
0 E F G -20 C 13
δ -40
-60
[DIC] x x [DIC] -80 88 m 114 m 169 m m = -19.9 m = -20.1 m = -2.1 2 2 2 -100 r = 0.91 r = 0.98 r = 0.01 2 3 4 5 6 2 3 4 5 6 2 3 4 5 6 [DIC] (mM)
Figure 7. Mass balance calculations of DIC for the northern (A-D) and southern (E-G) transects indicate the isotopic signature of the carbon being added to the porewater during oxidation, thereby revealing the source of the organic carbon driving early diagenesis in the seabed. inner shelf, seabed agitation and reworking likely causes the oxidation of both marine and terrestrial carbon, leaving behind a more depleted 13 C isotopic signature for the buried DIC reflecting the more recalcitrant terrestrial organic matter. The isotopic signatures of the carbon oxidized in the mid- to outer-shelf cores reflect a close similarity to marine carbon end-member within its error tolerance (-19 + 1‰;
Figure 7.B-D and Figure 8). While it is possible there may be minimal oxidation of terrestrial carbon at these sites, the isotopic signatures suggest that marine carbon drives early diagenesis, overwhelming any contributions from terrestrial sources
(Aller et al., 1996).
277 For the southern transect (Figure 7.E-G), a nearshore core was not obtained; however, it is anticipated that it would behave similarly to the 61 m core in the northern transect, oxidizing a mixture of terrestrial and marine carbon on the energetic inner shelf. Two of the mid-shelf cores along the southern transect analyzed for DIC at 88 m and 114 m (Figure 7.E and F) indicate that marine carbon
(-20‰) is preferentially remineralized as seen on the northern transect (Lehmann et al., 2002; Aller et al., 2008). Conversely, the bathymetric high is prominent enough at the most offshore site (169 m) that the profile has been physically reworked and mixed, as evidenced by 13 C-enriched POC accompanied by a coarse average
particle size and little to no variation in the DIC concentration or isotopic signature
down core (Figure 7.G). Thus, the down core isotopic signature describing the
dominant source of diagenetically active carbon at this site cannot be determined
(Figure 7.G). The carbon remineralized at each site is dictated by the overall carbon
preserved and available for oxidation (Alin et al., 2008). The POC shows loss of
terrestrial carbon at this site (Figure 2; Table 2); however, the mixed profile can not
distinguish whether this loss of terrestrial carbon is due to physical sorting, oxidation,
or both. Enrichment in 13 C in the POC relative to landward cores in the same
transect also implies the addition of marine carbon (Figure 3) (e.g. Leithold and
Hope, 1999; Blair et al., 2003).
As with POC, the stable carbon isotopic signature calculated from the mass
balance model to describe the source of remineralized carbon can be further
partitioned into contributions of marine and terrestrial carbon (Figure 8).
278 Radiocarbon analyses of porewater samples from the northern transect were almost exclusively greater than modern in age (Table 1). This suggests rapid remineralization of modern terrestrial and marine carbon (<100 years). Oxidation of the more recalcitrant, radiocarbon-dead kerogen would be evidenced by depleted
14 C content, resulting in an older radiocarbon age for the overall porewater sample
(Alin et al., 2008). Thus, to account for the lack of kerogen oxidation, a two end- member source apportionment was employed, distinguishing marine from non-rock carbon. While the marine end-member value remains at -19 + 1‰, the terrestrial end-member for DIC reflects only the modern carbon (-27.8 + 0.6‰) (similar to those in Aller and Blair, 2004 and Aller et al., 2008; however, empirically derived for the
Waiapu watershed). The three mid-shelf cores reflect little to no oxidation of
1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 Fractioncarbon 0.20 0.10 0.00 f marine 60 83 108 128 f riverine Water depth (m)
Figure 8. Source apportionment of remineralized carbon in porewater by mass balance calculations . On the inner shelf, oxidation of both fresh terrestrial and marine carbon occurs; however, the oxidation of marine carbon appears to dominate early diagenesis for the remainder of the shelf.
279 terrestrial carbon in lieu of marine (Figure 8). However, on the inner shelf, physical reworking promotes remineralization and efficient recycling of 13 C-depleted terrestrial
POC along with 13 C-enriched marine carbon (Figure 8; Aller et al., 1996; Aller and
Blair, 2004; Wadman and McNinch, 2008).
The preferable reactivity of marine carbon has been observed in previous studies of dissolved inorganic carbon as well. Marine OC was found to be the dominant source of porewater DIC on the North Carolina shelf and slope (Boehme et al., 1996; Thomas et al., 2002), the Amazon shelf (Blair and Aller, 1995; Blair et al.,
2004), and the Fly River delta offshore of Papua New Guinea (Aller and Blair, 2004).
14 C-measurements indicated that the DIC was typically younger ( 14 C-enriched) than
the co-existing POC (Table 1). The Fly and Amazon shelves also show evidence of
co-oxidation of aged terrestrial OC in addition to the marine carbon (Blair and Aller,
1995; Blair et al., 2004; Aller and Blair, 2004, 2006; Alin et al., 2008).
Like the Waiapu River, a significant portion of terrestrial organic carbon from
the Fly River is delivered during flood conditions, the rapid burial of which prevents
alteration, preserving the source signature (Alin et al., 2008; Goñi et al., 2008). The
receiving Gulf of Papua (GoP) has a broad shelf compared to the Waiapu; however,
both have similar sediment accumulation rates (1-4 cm/y for GoP, 0.7 to 2.7 cm/y for
the Waiapu) (Aller and Blair, 2004; Kuehl, 2007). The GoP also experiences rapid
and efficient remineralization of both terrestrial and marine organic carbon (Aller and
Blair, 2004). As seen in the Waiapu, the DIC in the GoP reveals the oxidation of
young terrestrial carbon inshore with increasing remineralization of marine carbon
280 offshore (Aller et al., 2008). Frequent physical reworking and exposure cause the oxidation of terrestrial deposits. Like the Fly, the organic carbon along the northern transect of the Waiapu can be described older, less reactive terrestrial organic matter to which young, rapidly recycled marine carbon is added (Figures 4, 6, and 8;
Aller and Blair, 2004). The terrestrial particulate organic matter is typically conserved while marine carbon is added to it, increasing with distance from shore, enriching the sediment in 13 C (Figure 6) except in regions experiencing high levels of resuspension and reworking (Figure 3.A and H; Aller et al., 2008).
4.3 The Waiapu margin carbon budget
After apportioning the carbon delivered to the seabed into its riverine and marine constituents, a carbon inventory of the shelf can be determined. Accounting for the fraction of each source in the cores, POC burial rates of kerogen and modern terrestrial carbon can be compared with the riverine carbon delivered to the margin to determine how much of the riverine carbon delivered to the ocean is retained on the shelf and in which depositional environment. Burial efficiency of riverine carbon fractions can also be estimated by examining burial rates along with DIC analysis.
281 4.3.1. Burial rate of particulate organic carbon
2 The burial rate of carbon (J B, g C/m /y) on the continental margin can be
calculated as follows:
Eq. (7) JB = SR * ρb * %C org ,
3 where SR is the sediment accumulation rate (m/y), ρb is the dry bulk density (g/m ), and %C org is the weight percent of organic carbon in the sediment (Thomas et al.,
2002; Alperin et al., 2002). The accumulation rate for various parts of the Waiapu margin has been established using 210 Pb techniques (Table 3; Kuehl, 2007;
Kniskern, 2007; Kniskern et al., 2009). The bulk density is calculated using the relationship:
Eq. (8) ρb = p s (1 - φ), where ρs is the density of the solid and φ is the porosity. The density of the solid is
approximated with the density of quartz (2.65 g/cm 3) (Schulz, 2000). The uniform average seabed porosity ( φ), the porewater (pw) volume, for the Waiapu shelf is
estimated at 0.6 cm 3 pw/cm 3 sediment (Kniskern, 2007; Wadman, 2008; Kniskern et
3 al., 2009). This results in a ρb of 1.06 g/cm , which is comparable to a previously used value of 1.16 g/cm 3 for bulk density of sediment in the East Cape watershed
(Page et al., 2004). The organic carbon content in the upper 50 cm of each sediment core analyzed is relatively uniform (Figure 3), therefore the burial rate would only be mildly altered by the depth chosen to represent the organic carbon content of the profile (Alperin et al., 2002). Thus, the organic carbon content of the
282 lowest depth interval sampled for each core was used for burial rate calculations
(Table 2).
As the sediment accumulation rate in the energetic inner shelf up to 60 m water depth (Wadman and McNinch, 2008) is reported as zero, the calculated POC burial rate is consequentially zero (Table 2). Sediment accumulation on the remainder of the shelf ranges from 0.7 to 2.7 cm/y, resulting in POC burial rates ranging from 31 to 255 g C/m 2/y (Table 2 and 3). The burial of POC is highest along
the southern transect, concurrent with the highest sediment accumulation rates
(Kniskern et al., 2009), where a paleobathymetric channel directs sedimentation
during hyperpycnal flow conditions that can occur during major storm events
(Addington et al., 2007; Wadman and McNinch, 2008). Event layers preserved from
these conditions are too thick to be completely reworked, resulting in rapid burial of
sediment with increased organic carbon content (Addington et al., 2007; Wadman
and McNinch, 2008). POC burial decreases with distance from shore on the mid-
and outer shelf, as accumulation rates slow and organic carbon content is
diminished on the continental shelf (Table 2). Once on the slope, the increased
carbon content due to marine organic matter accumulation in addition to the
terrestrial organic matter results in a slight elevation of burial rate relative to the
outer shelf (Table 2 and 3; Figure 3.E).
Worldwide shelf accumulation rates typically average from 0.1 to 1 cm/y
(Wheatcroft and Drake, 2003). Higher accumulation rates are seen on large river-
dominated shelves like the Amazon (up to an estimated 10 cm/y) (Kuehl et al.,
283 1996). The accumulation rates on the Waiapu margin are three times higher than usually observed on a continental shelf (Kniskern, 2007). Accordingly, the POC burial rates are higher. Comparatively, the shelf off Peru only approaches the lower end of the organic carbon burial seen on the Waiapu margin at 40-70 g C/m 2/y
(Henrichs and Farrington, 1984; Thomas et al., 2002). These burial rates have been
attributed to upwelling conditions on the Peru margin (Thomas et al., 2002). The
Amazon shelf buries 58 g OC/m 2/y (Aller et al, 1996). This is roughly on par with the
burial of carbon on the Waiapu shelf except in the area of highest accumulation.
The high burial rate of carbon in that region likely reflects the impeded oxidation
during rapid burial of organic matter with high organic carbon content delivered by
episodic flood events (France-Lanord and Derry, 1997; Blair et al., 2003; Leithold et
al., 2006; Alin et al., 2008).
Table 3. Organic carbon delivered to the Waiapu Margin Water Sed Rate DIC Diffusive DIC DIC burial rate POC burial rate Depth %C 2 1 2 2 (cm/y) (mM) org flux (gC/m /y) (gC/m /y) (gC/m /y) (m)
61 0.0 5.8 0.50 1.9 0.0 0.0 83 1.4 5.2 0.55 0.9 0.5 81.6 Northern Transect 108 1.4 4.4 0.58 0.9 0.4 86.1 128 0.7 4.1 0.53 0.6 0.2 39.3 615 0.7 ND 2 0.71 ND 2 ND 2 52.7
88 2.7 6.3 0.67 2.5 1.2 191.8 Southern Transect 114 2.7 9.0 0.89 1.9 1.7 254.7 169 0.7 2.8 0.42 ND 2 0.1 31.2 1DIC diffusive flux is a gross underestimate due to sample resolution; 2ND is not determined
284
4.3.2 Burial rate of dissolved inorganic carbon
Dissolved inorganic carbon produced from the oxidation of POC in the seabed either accumulates in the porewater, diffuses out of the seabed, or precipitates as carbonate. Burial rates (J B) of dissolved species are determined using the relationship:
Eq. (9) JB = φ * SR * [DIC], where φ is the porosity of the sediment, SR is the sediment accumulation rate (m/y), and [DIC] is the concentration (mM) of CO 2 where the profile becomes asymptotic.
As described for POC burial rates, the uniform core porosity of the sediment on the
Waiapu margin is estimated at 0.6 (Kniskern, 2007; Wadman, 2008), and the
accumulation rates were previously determined with 210 Pb (Table 3; Kuehl, 2007;
Kniskern, 2007; Kniskern et al., 2009). Burial terms are typically calculated at a
depth where concentrations become asymptotic, however, the cores analyzed did
not reach this point. Therefore, the burial rates of DIC in this study were calculated
using the concentration of CO 2 at the deepest interval in each sediment column
where the concentration was the highest (Thomas et al., 2002). The resultant burial
rates are consequently minimum estimates as concentrations may continue to
increase downcore (Thomas et al., 2002).
The DIC burial rates range from 0.1 to 1.7 g C/m 2/y along the Waiapu shelf
(Table 2). On the northern transect, rates range from 0.2 to 0.5 g C/m 2/y,
285 decreasing seaward with increasing water depth and decreasing accumulation rates and %C org associated with the sediment. On the southern transect, there are two distinct regions of DIC burial in the analyzed cores. The landward profiles have DIC burial rates of 1.2 to 1.7 g C/m 2/y, reflecting the high sediment accumulation rates
and POC burial (Kniskern, 2007). At the most seaward site along the southern
transect, the DIC burial rate is only 0.1 g C/m 2/y. As seen in the DIC profile (Figure
5.G), the concentration of DIC is uniform throughout the sampled core. In the POC profile (Figure 5.H), the organic carbon content is relatively uniform and has the most enriched stable carbon isotopic signature. The carbon composition suggests mixing and reworking of the sediment throughout this core which is located on a bathymetric high on the shelf. These processes minimize burial of DIC produced in this core which is also impeded by a lower sediment accumulation rate. In comparison to the Waiapu, the DIC burial on the Cape Hatteras slope is low but on the same order of magnitude, ranging from 0.012 to 0.6 g C/m 2/y in an environment
with a lower sediment accumulation rate and correspondingly lower POC burial rates
(Thomas et al., 2002).
4.3.3 Carbon outputs from the seabed: DIC diffusive flux rates
The DIC formed in the seabed that diffuses back out into the overlying water
can be quantified using Fick’s first law of diffusion as it applies to sediments. The
diffusive flux (J D) of CO 2 out of the sediment is directly proportional to the
286 concentration gradient (dC/dx) assuming steady state conditions (Schulz, 2000).
This relationship is described by:
Eq. (10) J D = - φ * D sed * dC/dx
where φ is porosity, D sed is the diffusion coefficient for CO 2 from the sediment, and
dC/dx is the concentration gradient of CO 2 in the porewater through the sediment
-10 2 column (Schulz, 2000). Dsed (5.099 x 10 m /s) for the Waiapu shelf is calculated
from:
2 Eq. (11) D sed = D sw / θ ,
-9 2 where D sw (1.03 x 10 m /s) is the diffusion coefficient for CO 2 in free solution for
seawater at 5 °C and θ is tortuosity ( θ2 = 2.02). Tortuosity accounts for the deviations required for diffusion in the porewater around sedimentary particles and is directly proportional to porosity ( φ) (Schulz, H.D. 2000). Direct measurement of electrical resistivity of the sediment/porewater system and the porewater alone is used to quantify tortuosity through the calculation of a formation factor which is multiplied by porosity (Schulz, 2000). However, when these measurements are not possible, Boudreau’s law (Schulz, 2000) approximates tortuosity as follows:
Eq. (12) θ2 = 1 – ln( φ2), through its empirical relationship with porosity. The concentration gradient (dC/dx) was determined by linear regression of concentration with respect to depth of each profile (Figure 5, dotted black lines), utilizing the whole core if linear or the top two sampling intervals if exponential.
287 Estimates of interface gradients generally decrease as the distance of extrapolation increases (Reimers and Smith, 1986). Thus, near-surface porewater gradients such as those obtained for this study cannot be used to accurately estimate benthic fluxes because gradient transport relationships are not correctly parameterized (Jahnke et al., 2005). Oxygen concentrations decrease exponentially over the top 1.5 cm, suggesting that most of the particulate organic carbon that reaches the seabed is oxidized most rapidly within the upper few millimeters of the sediment column (Reimers and Smith, 1986). This is supported as the highest organic carbon, bacterial abundance, and porosity are found in the top 2 mm of the seabed (Reimers and Smith, 1986). In order to obtain meaningful diffusive flux measurements, flux chambers (Jahnke et al., 2005) or mm-scale microelectrode measurements (Jahnke, 1996) should be used to accurately measure fluxes in and out of porewater directly. Sampling and analytical protocols employed herein afford large uncertainty in benthic flux rates, providing non-useful constraints due to poor vertical resolution (Jahnke, 1996).
In this study, the diffusive flux on the Waiapu shelf is calculated to be less than 2.5 g C/m 2/y for the two transects analyzed for porewater carbon chemistry
(Table 3). This calculated DIC flux for the Waiapu shelf is estimated to be between
10 and 100 times too low. The gross underestimation of diffusive flux from the
Waiapu can be observed by comparing this calculated flux with other regions of high sediment accumulation rates such as the shelf of Peru (0.6 to 20 g C/m 2/y; Henrichs
and Farrington, 1984) and the Amazon shelf (>220 g C/m 2/y; Aller et al., 1996).
288
4.3.4 Retention of riverine carbon on the Waiapu shelf
It is estimated that 209 ± 21 Tg/y of suspended sediment are delivered to the ocean from the New Zealand landscape (Hicks et al., 2002; Page et al., 2004).
Deforestation of the watershed has also enhanced the erosion rates (Mazengarb and Speden, 2000), increasing sedimentation and accordingly, the OC burial rates
(France-Lanord and Derry, 1997). Associated with these high sediment yields, New
Zealand’s rivers are estimated to deliver some of the greatest carbon yields globally
(100-300 g C/m 2/y) (Table 4; Lyons et al., 2002; Page et al., 2004). Upper limits on carbon yields are observed in small, wet, mountainous watersheds that experience rapid erosion similar to the Waiapu (Stallard, 1998; Page et al., 2004). Cumulatively, the rivers of New Zealand are estimated to export 10 ± 3 Mg C/km 2/y of POC to the ocean (Scott et al., 2006), which is 6 times the global average (Ludwig et al., 1996;
Stallard, 1998). This difference is attributed to the tectonically active environment in
New Zealand (Scott et al., 2006).
Table 4. Sediment and POC Yields from New Zealand Rivers Mean OC Drainage area Sed yield POC yield River conc (%) (km 2) (tons/km 2/y) (Mg C/km 2/y) Hokitika a 0.25 352 17,000 43 Cropp a 0.41 29 30,000 52 Haast a 0.56 1020 12,700 168 Hikuwai a 1.6 307 13,890 222 Waipaoa b 1.1 1580 6750 55 Waiapu c 0.56 1734 20,520 115 aLyons et al., 2002; bGomez et al., 2003; cthis paper; estimated shelf (%C x sediment yield)
289
Areas of the highest sediment and POC fluxes occur in regions where rapid uplift of soft or highly fractured rocks are exposed to frequent storm events (Scott et al., 2006). Similar to global “hot spots” of riverine OC export (Lyons et al., 2002), localized hot spots of riverine sediment and associated OC occur within New
Zealand (Scott et al., 2006). Approximately 9% of New Zealand is responsible for
65% of its total POC exported to the ocean, including regions of the East Cape, including the Waiapu watershed that exceed 40 Mg C/km 2/y (Scott et al., 2006;
Table 4). Like the adjacent Waipaoa watershed, the Waiapu hot spot region is part
of the 1.7% of New Zealand’s land area composed of steep terrain typically
underlain by soft mudstone and crushed sedimentary rock (Page et al., 2004).
These environments are estimated to yield approximately 110 Mg C/km 2/y (Page et al., 2004; Scott et al., 2006), and are responsible for around 20% of the POC delivered to the ocean from New Zealand (Scott et al., 2006).
The total organic carbon delivered to the continental shelf includes this riverine carbon with kerogen, modern terrestrial, and SOM components and marine carbon. The total organic carbon delivered to the Waiapu margin would be estimated by the summation of burial fluxes of both POC and DIC as well as the diffusive flux of DIC out of the seabed (Rullkotter, 2000; Thomas et al., 2002):
Eq. (13) TOC Del = J D (DIC) + J B (DIC) + J B (POC), where TOC Del is the total organic carbon delivered, J D (DIC) is the diffusive flux of
DIC out of the sediment, and J B is the burial flux of DIC and POC, respectively. This
290 accounts for the three primary fates of organic carbon on this margin: burial as particulate-associated organic matter or remineralization to CO 2 which is either
buried in the porewater or released back to the overlying seawater after diffusing out
of the sediment. Because the diffusive flux estimate is grossly underestimated, we
can not quantify the marine carbon flux on the Waiapu continental shelf. We can,
however, assess the fate of the buried carbon on the shelf.
Average POC burial rates of 40, 107, and 222 Mg C/km 2/y were calculated for the 0.7, 1.4, and 2.7 cm/y accumulation rate zones, respectively, described for the
Waiapu mid- to outer continental shelf (Kniskern, 2007; Kniskern et al., 2009).
When normalized to shelf area, these same regions bury 9, 35, and 15 Gg C/y, totaling an average of 59 Gg C/y (ranging from 37 to 76 Gg C/y) (Table 5). Utilizing the mass balance solutions partitioning buried POC into its modern marine, modern terrestrial, and ancient kerogen source contributions, the burial rate of carbon associated with individual fractions can be determined (Table 2, 3 and 5; Alperin et al., 2002; Blair et al., 2003). It is acknowledged that some fraction of the terrestrial material attributed to kerogen and modern terrestrial carbon is in fact SOM.
However, as the sampled horizons in the cores sampled were not identified as flood layers and the SOM fraction remains poorly constrained, the SOM fraction is assumed to be a negligible fraction in the individual component burial rate calculations herein. The rate of kerogen burial ranges from 20 to 41 Gg C/y with an average of 32.5 Gg C/y on the mid to outer shelf. Approximately 12 Gg C/y (8 to 16
291 Gg C/y) of modern terrestrial carbon is retained. Marine carbon is buried at a rate of
14.5 Gg C/y (37 to 76 Gg C/y).
These carbon burial rates can be compared with the river flux of 200 Gg C/y to determine how much of the riverine carbon is retained on the shelf and in which depositional areas. Burial retention (RB) indicates the fraction of the riverine POC that can be accounted for on the shelf and is calculated by:
Eq. (14) R B = (POC burial flux / total POC delivered) x 100.
Using calculated burial rates within each depositional region, an inventory of the carbon retained on the shelf can be determined. An average of 23% (ranging from
14 to 28%) of the riverine carbon delivered from the Waiapu River is buried across the three accumulation zones on the mid and outer Waiapu shelf. Of that, only slightly more (26%) of the terrestrial fraction is retained, while 22% of the kerogen fraction is buried on the shelf (Table 5). Accounting for greater than half of the area of the mid- to outer shelf, the 1.4 cm/y accumulation zone accounts for more than half of the carbon retained on the shelf (Table 5).
292
Table 5. Carbon retention on the mid to outer Waiapu continental shelf POC Burial %C Trapped on 8 Rate Area Gg C/ y Shelf 2 2 (MgC/km /y) (km ) Tot 1 Mar 2 Terr 3 Ker 4 Riv 5 Terr 3 Ker 4
Riverine Output NZ lower limit 6 100 1734 173.4 43 130 NZ higher limit 6 300 1734 520.2 130 390 Estimate 7 115 1734 199.4 50 150
Mid to Outer Shelf Accum. Zone (cm/y) 0.7 42 217 9.2 3.2 1.7 4.3 3.0 3.4 2.9 Average %C 9 1.4 107 330 35.3 8.1 7.5 19.6 13.6 15.1 13.1 per zone 2.7 222 67 14.8 3.1 3.1 8.6 5.9 6.2 5.8 Total Margin Average 371 614 59.3 14.5 12.3 32.5 22.5 25.6 21.8
0.7 73 217 15.9 5.6 2.9 7.4 5.2 5.8 5.0 Maximum %C 9 1.4 133 330 43.8 10.1 9.3 24.3 16.9 18.7 16.3 per zone 2.7 238 67 15.9 3.4 3.3 9.3 6.3 6.6 6.2 Total Margin Maximum 444 614 75.6 19.0 15.5 41.0 28.4 31.1 27.4
0.7 28 217 6.1 2.1 1.1 2.9 2.0 2.3 1.9 Minimum %C 9 1.4 58 330 19.0 4.4 4.1 10.6 7.3 8.1 7.1 per zone 2.7 180 67 12.1 2.6 2.5 7.0 4.8 5.0 4.7 Total Margin Minimum 266 614 37.3 9.1 7.7 20.5 14.1 15.4 13.7 1Total; 2Marine; 3Terrestrial; 4Kerogen; 5Riverine; 6Lyons et al., 2002; Page et al., 2004; 7estimated by multiplying %C by sediment flux; 8 %C retention based on estimated riverine output; 9avg, max, and min based on %C of all samples in each accumulation zone (n=9 for 0.7 cm/y; n=10 for 1.4 cm/y; n=4 for 2.7 cm/y)
The carbon budget tracks that of the inventory of sediment retained on the
Waiapu continental shelf, calculated by Kniskern (2007) using sediment accumulation rates. Of the 36 Mg/y of sediment delivered from the Waiapu River
(Page et al., 2001), only approximately 23% (8.1 Mg/y) (ranging from 17 to 38%) is retained on the mid- to outer shelf over the last 80 to 100 years (Kniskern, 2007;
Kniskern et al., 2009). An additional 8-15% is accounted for by the inner shelf
293 (Kniskern, 2007). The remainder escapes beyond the boundaries of the shelf to the slope or beyond the northern boundary (Kniskern, 2007). Errors of the estimates include spatial resolution of sampling, averaged unrealistic uniform porosity, and no variability of annual river input (Kniskern, 2007).
4.3.5 Burial efficiencies of kerogen and terrestrial, non-rock organic carbon
Approximately 100 Tg of kerogen carbon may be oxidized globally each year based on the assumption that burial rates must be balanced by kerogen oxidation to maintain atmospheric O 2 levels (Berner and Canfield, 1989; Hedges, 1992).
However, little is known about the process of kerogen oxidation back to CO 2.
Terrestrial exposure of kerogen results in the loss of organic carbon from
sedimentary rocks (Leythaeuser, 1973; Clayton and Swetland, 1978; Petsch et al.,
2000). After being exposed on the Earth’s surface, the lifetime of kerogen is
estimated to be up to 10 4 yr (Keller and Bacon, 1998; Petsch et al., 2000). Due to the recalcitrance of kerogen, this process requires microbial mediation (Petsch et al.,
2001). Rock characteristics including permeability and resistance to erosion influence kerogen oxidation by controlling the exposure time to O 2 and water. Thus, sandstones are likely to experience more oxidation than shales (Petsch et al., 2000).
Small mountainous rivers such as the Waiapu that flow through highly erodible landscapes may transport greater than 40 Tg kerogen C to the ocean (Blair et al., 2003). Oxidation of kerogen on land potentially suggests a similar fate in the
294 seabed. The presence of 14 C-depleted DIC in groundwater suggests subsurface
oxidation of ancient sedimentary POC (Keller and Bacon, 1998). In the marine
environment, kerogen oxidation is proposed to be primed by either 1) the exposure
of previously protected surfaces by disaggregation of sedimentary rock, 2)
+3 +4 resuspension rates that increase contact with higher order oxidants (O 2, Fe , Mn ) or 3) the co-metabolism of kerogen with reactive marine carbon (Aller et al., 1996;
Aller, 1998; Aller and Blair, 2004; Alin et al., 2008). Oxidation of older terrestrial organic carbon has been identified on the Fly River (Alin et al., 2008). The addition of relatively small amounts of 14 C-depleted kerogen should be readily apparent in the dissolved inorganic carbon.
Resuspension events near shore and on bathymetric highs are capable of keeping sediment in the surface mixed layer (Wadman and McNinch, 2008).
Kerogen oxidation in the seabed would most likely be expected to occur under these aerobic conditions. According to the mass balance solutions, as little as a 9% contribution by oxidized ancient sedimentary organic carbon to the DIC would be detectable based on the precision of the radiocarbon measurements. As the radiocarbon ages of dissolved inorganic carbon samples were almost exclusively greater than modern across the entire margin, <9% of the oxidized carbon would be attributable to ancient carbon sources on the Waiapu River continental shelf. This suggests quantitative burial efficiency of kerogen.
Except for the core on the innermost shelf, all DIC profiles on the Waiapu shelf have stable isotopic signatures enriched in 13 C, indicating the oxidation of
295 marine organic carbon rather than terrestrial. Where terrestrial organic matter is remineralized, particularly on the inner shelf, greater than modern radiocarbon signatures indicate that this is the result of modern terrestrial carbon oxidation.
Because the diffusive flux cannot be quantified, the modern terrestrial carbon oxidized cannot be quantified, thus a burial efficiency cannot be determined.
However, in the largest region of the shelf (>50% of the area has a sediment accumulation of 1.4 cm/y), there is no indication of terrestrial carbon oxidation in the
DIC (Figure 8). This suggests quantitative burial efficiency of terrestrial carbon in that region. Quantitative burial efficiency of terrestrial organic carbon has been reported for the Bay of Bengal as well (Galy et al., 2007). The terrestrial carbon loading of sediment is approximately equivalent to that exported from the Ganges-
Brahmaputra; this extreme burial efficiency is attributed to the high erosion rates from the Himalayas which afford negligible oxidative loss of terrestrial carbon (Galy et al., 2007). Similarly, as New Zealand has some of the highest sediment and carbon yields globally, it follows that the rapid burial would prevent oxidation, causing the burial efficiency of terrestrial carbon to be near quantitative as seen in the Bengal Fan (Lyons et al., 2002; Page et al., 2004; Galy et al., 2007).
In the region where sediment accumulates at 2.7 cm/y, 8-10% of the buried
DIC comes from the oxidation of terrestrial matter; whereas, approximately 16% of the buried DIC is attributed to modern terrestrial carbon in the 0.7 cm/y region.
While the quantity of oxidized modern terrestrial carbon cannot be determined, these low percentages of terrestrial carbon oxidation do indicate that marine carbon is
296 more diagenetically favorable than modern terrestrial carbon on the mid- to outer shelf. The inner shelf shows such a high percentage of dissolved terrestrial carbon buried (~66%), likely because the high energy conditions of the inner shelf do not promote marine carbon burial (Figure 6 and 8). A high burial efficiency is more likely in regions of high accumulation rates, where the sediment is buried rapidly, minimizing exposure to higher order oxidants. Conversely, lower burial efficiency reflects increased exposure time prior to burial (Aller et al., 1996; Aller, 1998; Aller and Blair, 2004; Alongi et al., 2005). For example, environments such as energetic inner shelves where resuspension keeps sediment and associated organic carbon entrained in the surface mixed layer facilitate remineralization. Rapid burial on continental margins such as the Waiapu protects riverine POC from oxidation (Table
2 and 5; France-Lanord and Derry, 1997; Sommerfield et al., 1999).
5. Conclusions
As sediment is transported offshore from the Waiapu River across the continental margin, the riverine POC behaves relatively conservatively, and increasing marine carbon is added to the POC pool with increasing distance from the river mouth. This behavior is consistent with previous work offshore from small mountainous river systems (e.g. Blair et al., 2003). Short residence times in biogeochemically active reservoirs such as soils and the surface mixed layer of the seabed are proposed to limit alteration of terrestrial carbon prior to burial. This
297 pattern deviates for sediment deposited on bathymetric highs, where reworking of the sediment allows for loss of terrestrial carbon.
The addition of radiocarbon analysis distinguishes the kerogen from the
modern carbon in the terrestrial fraction; the ancient kerogen fraction is estimated to
account for greater than 50% of the terrestrial carbon in the POC. These
calculations assume that aged soils are a minor source of organic carbon to the
sedimentary system. Dissolved inorganic carbon analysis indicates that the marine
carbon is the diagenentically active carbon pool remineralized in the seabed.
Oxidation of the terrestrial POC is limited to a short-term depositional region near the
river mouth. The source of organic matter being oxidized near the bathymetric high
was indeterminate because of physical sorting and reworking processes active on
this part of the shelf which have mixed this porewater profile. In addition to the
marine rather than terrestrial source determined for the majority of carbon oxidized
in the seabed by stable carbon isotopic analysis, kerogen oxidation was also refuted
by the modern radiocarbon age of the porewater. This suggests a quantitative burial
efficiency for kerogen across the inner and outer shelf.
The carbon budget for the Waiapu shelf was determined. Three distinct
regions of sediment accumulation have been previously described for the Waiapu
mid- to outer continental shelf (Kniskern, 2007; Kniskern et al., 2009). These
regions had sediment accumulation rates of 0.7, 1.4, and 2.7 cm/y and average
POC burial rates of 40, 107, and 222 Mg C/km 2/y, respectively. This is equivalent to a total of 59 Gg C/y (9, 35, and 15 Gg C/y for the three regions in order of increasing
298 accumulation rate). Because of the resolution of porewater concentration measurements, the DIC diffusive flux cannot be calculated. Therefore, the total carbon delivered to the seabed and thus, the marine carbon budget, could not be quantified.
When apportioned by mass balance calculations into its respective
components (kerogen, modern terrestrial, and marine), POC burial rates can be
used to examine the fate of the riverine carbon. Relative to the 200 Gg C/y
delivered by the Waiapu River to the ocean, approximately 23% of the riverine
carbon is retained on the mid to outer slope, matching the sediment inventory
established by Kniskern (2007). Stable carbon isotopic signatures suggest minimal
oxidation of modern terrestrial carbon, signifying that the 77% of the riverine carbon
not accounted for has likely been retained and potentially oxidized on the inner shelf
or escaped to the slope or beyond the established boundaries of the mid to outer
shelf.
6. Acknowledgements
This project was funded by NSF projects “Age Distribution of Particulate
Organic Carbon (POC) Discharged from Small Mountainous Rivers- the Influence of
Sediment Yield and Soil Residence Time” (EAR-0222584, Leithold and Blair) and
”Source to Sink Generation of Biogeochemical Stratigraphic Signals across the
Waipaoa Margin, New Zealand” (OCE-0646159, Leithold and Blair). Special thanks
299 are extended to the Kuehl and McNinch groups from VIMS and the crew of the R/V
Kilo Moana for assistance in obtaining marine sediment samples.
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314 CHAPTER 5
Overall Conclusions
315 1. Overall Conclusions
The large contribution of short mountainous rivers to the global sediment supply has motivated the investigation of sediment generation, transport, and burial processes in these systems (Blair et al., 2003; Leithold et al., 2006). Therefore, the
Waiapu River sedimentary system has been examined in an integrated source to sink fashion, similar to that being utilized for the adjacent Waipaoa River watershed as part of the Source-to-Sink Margins Program through the National Science
Foundation. The Waiapu River has one of the highest sediment yields in the world
(Hicks et al., 2004; Leithold et al., 2006), creating a detailed stratigraphic record.
There are also fewer possible sources of organic matter in the Waiapu watershed
(Mazengarb and Speden, 2000; Landcare Research, 2009) to be eroded into the river, making sediment generation, transport, and burial more straightforward to trace than the Waipaoa system. In particular, extensive gullying throughout the watershed delivers more kerogen to the margin than is seen in other small mountainous river systems (Mazengarb and Speden, 2000; Page et al., 2001;
Leithold et al., 2006).
Potential sources of terrestrial organic matter to the river and continental shelf were characterized using stable carbon and nitrogen isotopic signatures. The elemental (%C and %N) and isotopic (δ13 C and δ15 N) data were not distinct enough
to resolve any soil contributions to the riverbed sediment because the underlying
bedrock permeated the isotopic signature of the soils and sediment of the Waiapu
316 system. This analysis provides geochemical data to support conclusions based on aerial photography and digital elevation models that the highly erodible Whangai Fm. that underlies a large portion of the watershed is the primary source of particulates to the river and margin (Page et al., 2001; Parkner et al., 2006).
Once on the continental margin, bulk particulate organic carbon signatures were consistent with physical sorting processes caused by the known physical oceanographic regime (Wright et al., 2006; Addington et al., 2007; Wadman and
McNinch, 2008; Ma et al., 2008). The isotopic signatures of organic matter associated with the isolated clay and coarse silt/sand fractions also correspond to these sorting processes (Hedges and Oades, 1997; Leithold and Hope, 1999;
Ransom et al., 1998; Blair et al., 2003). Sediment on bathymetric highs on the shelf was characterized by larger particle sizes with lower %C, suggesting that winnowing processes sort this sediment. Sediment on bathymetric lows near the river mouth preserved fine sediment with a terrestrial composition.
Because ancient rock carbon was determined to be a primary source of terrestrial carbon to the margin, mass balance equations incorporating the stable carbon isotopic signatures of riverine and marine organic matter end members were applied to quantify the percent of terrestrial and marine carbon buried in the sediment across the margin (Blair et al., 2003; Blair et al., 2004; Leithold et al., 2005;
Leithold et al., 2006). The quantity of terrestrial and marine carbon preserved at various points across the margin was consistent with expectations based on particle sorting processes caused by waves, currents, and other physical oceanographic
317 parameters of the shelf. In general, marine carbon was added onto relatively conserved terrestrial organic carbon fractions. However, on the bathymetric highs, for example, less organic carbon overall was preserved with the coarser fraction of sediment, and what was preserved was almost half marine carbon, as particle reworking processes caused the replacement of terrestrial with marine carbon.
Contributions of non-rock terrestrial material was distinguished from ancient rock carbon by incorporating radiocarbon data with the mass balance equations
(Blair et al., 2003; Gomez et al., 2003; Blair et al., 2004; Blair et al., 2009). The magnitude of ancient carbon that escapes oxidation in the watershed and is reburied on the margin after fluvial transport is an unconstrained component of the carbon cycle (Hedges, 1992; Blair et al., 2003). In the transect with available radiocarbon data, rock carbon was determined to be relatively conserved across the margin. The modern terrestrial carbon also experienced minimal alteration, whereas marine carbon was added onto the terrestrial carbon with distance from the shore.
Radiocarbon analysis of a transect influenced by a bathymetric high would be expected to show different behavior due to the reworking of the terrestrial carbon, and would be an excellent addition to this study if funding permits in the future.
Molecular level analyses were utilized to further resolve the terrestrial sources for source apportionment. Polycyclic aromatic hydrocarbons are commonly used to track environmental contamination (Venkatesan, 1988; Page et al., 1996; Burns et al., 1997; Short et al., 1999; Stout et al., 2004). While the Waipaoa River empties into Poverty Bay at Gisborne, the Waiapu River watershed is more remote,
318 eliminating potential contaminant sources of PAHS. Therefore, background PAH signatures have been exploited herein to identify sources of eroding organic matter in order to explain active geomorphic processes in a watershed is a novel application of this technology.
Petrogenic PAHs are associated with rock samples; deforestation by burning extant vegetation adds pyrogenic PAHs to the soil (Lima et al., 2005). While bedrock-derived soils have both petrogenic and pyrogenic PAHs, the Waiapu watershed also has tephric soils which dilute petrogenic inputs. PAH concentration histogram shapes of Waiapu samples confirm that rock sources have primarily petrogenic PAH signatures, whereas soil profiles have pyrogenic in addition to petrogenic contributions (Stout et al, 2001a; 2001b; Stout et al., 2004).
As with isotopic signatures, PAH diagnostic ratios commonly applied for
source apportionment do not resolve the organic carbon sources within the
watershed well (Page et al., 1996; Burns et al., 1997; Christensen et al., 2004; Stout
et al., 2004). The 42 individual PAH concentrations determined for each
environmental sample are used to develop a more comprehensive fingerprint of
possible riverine sources to be compared with marine sediments using principal
component analysis. PCA provides a more resolved analysis of potential organic
carbon sources to the Waiapu shelf, indicating that Whangai Fm., raw riverbank
alluvial soils, and topsoils are the most chemically similar to the riverine and marine
sediments. This suggests gullying, bank failure, and sheetwash are the primary,
chronic geomorphic processes active in the watershed. However, because raw and
319 recent alluvial soils serve as storage for organic matter being transported down the river as well as a sediment source via bank failure, this method cannot resolve the relationship between the raw riverbank alluvial soil and riverine sediment.
Once the sources of organic carbon to the margin were identified, the fate of organic carbon on the continental shelf can be examined in the context of early diagenetic processing. In three cores along a seaward transect from the river mouth, the fractions of POC (kerogen, modern terrestrial, and modern marine) within each core were all relatively conserved down to 50 cm depth. Stable nitrogen isotopic signatures downcore also showed only minimal (<1 ‰) variations throughout recent history. This suggests that no major change or variation in organic matter source occurred over recent history (approximately 70 years) in the sampled intervals in each core.
Unlike POC which represents the organic carbon being preserved in the sediment, dissolved inorganic carbon concentrations and stable and radiocarbon isotopic signatures of porewater indicate the origin and age of organic carbon that is oxidized in the seabed (Aller and Blair, 2004; Aller and Blair, 2006; Aller et al., 2008).
The carbon remineralized at each site is dictated by the overall carbon preserved and available for oxidation. In the particulate fraction, marine carbon is added with distance from shore, enriching the sediment in 13 C. In all but the shallow, heavily
reworked sites (i.e. the inner shelf and bathymetric highs) where fresh terrestrial
carbon is oxidized, this enriched marine carbon is preferentially remineralized.
320 Radiocarbon signatures were obtained for some horizons in the northern transect; all but one DIC sample were greater than modern in radiocarbon age. This is expected because ancient carbon buried on the margin is likely to be recalcitrant, having already undergone a cycle of diagenesis, lithification, and uplift (Hedges,
1992; Hedges et al., 2000; Blair et al., 2003). The stable and radiocarbon signatures of the DIC is consistent with the oxidation of modern marine carbon rather than kerogen oxidation.
The middle and outer Waiapu continental shelf buries an average of 59 Gg
C/y. Relative to the 200 Gg C/y delivered by the Waiapu River to the ocean, approximately 23% of the riverine carbon is retained on the mid- to outer shelf, matching the sediment inventory. Because stable carbon isotopic signatures suggest minimal oxidation of modern terrestrial carbon in this region, the 77% of the riverine carbon not accounted for has likely been retained and potentially oxidized on the inner shelf or escaped to the slope or beyond the established boundaries of the mid- to outer shelf.
In addition to using carbon and nitrogen isotopic analysis of downcore profiles,
molecular fingerprinting was also applied. The PAH signatures change only in
concentration, not distribution. The decreased concentration may be caused by
dilution, volatility, or biodegradation. Marine carbon is added onto the terrestrial
signature, diluting the terrestrial biomarker signature from the watershed, but not
actually changing the PAHs preserved (Fang et al., 2009). The changes in
concentration are primarily seen in the low molecular weight naphthalenes which
321 have also been shown to undergo volatilization (Murphy and Morrison, 2007) and biodegradation (Nair et al., 2008). Because of possible changes in naphthalenes over time, more emphasis should be placed on the higher molecular weight PAHs
(3+ rings).
As a novel application of background PAH fingerprints has been developed for source identification of eroding sediment in a gully-dominated watershed, follow up research could include a similar site that is only impacted by landsliding of soil organic matter. For example, sediment taken from nearby Lake Tutira (New
Zealand) should show PAH signatures dominated by landslide-driven sedimentation
(Page and Trustrum, 1997; Gomez et al., 2002; Page et al., 2004). Additionally, a temporary change in source to the Waiapu margin could be evidenced by analyzing a defined storm layer, such as Cyclone Bola, that represents a pulse of fresh plant and aged soil carbon delivered to the shelf by storm-induced landslides (Eden and
Page, 1998; Marutani et al., 1999; Hicks et al., 2000; Gomez et al., 2003; Kasai et al., 2005).
This method could be applied to piston cores to examine sedimentation over geologic time, potentially allowing the identification of shifts in terrestrial organic matter sources feeding the continental margin. Finally, the application of isotopic analysis to the aliphatic and aromatic fractions (Pancost et al., 2001; Younes 2001a
& b; Younes, 2003; Sun et al., 2005) and compound specific stable isotope analysis of individual PAHs (Freeman et al., 1994; Lichtfouse et al., 1997; Smirnov et al.,
1998; McRae et al., 2000; Gray et al., 2002; Reddy et al., 2002; Shouakar-Stash et
322 al., 2003; Dawson et al., 2005; Lima et al., 2005; Boyd et al., 2006) should allow even more precise chemical fingerprinting to improve source resolution further.
In addition to using stable carbon and nitrogen isotopic signatures, we have developed a new application for PAH distribution analysis to identify potential sources of organic matter in a small mountainous river watershed that may contribute to the carbon buried on the adjacent continental margin using a comprehensive geochemical fingerprint. Geomorphic processes actively eroding the watershed have been suggested based on the primary sources of organic matter being transferred to the continental shelf. Organic matter delivered to the shelf has been quantified according to age and origin, and a source-specific carbon budget for the Waiapu shelf has been determined. Finally, the fate of organic carbon has been explored to determine what fraction of the organic carbon initially buried in the seabed survives early diagenetic processing to be preserved in the stratigraphic record. The identification and quantification of organic matter buried in this small mountainous river-adjacent continental margin provides insight into the behavior of
POC within these globally important systems (Milliman and Syvitski, 1992). This study complements previous studies of other small mountainous river systems (e.g.
Blair et al., 2003; Gomez et al., 2003; Leithold et al., 2006), distinguishing these from rivers on large passive margins and reinforcing the need for further study.
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