View of Silicate Weathering Cycle……………………………………

PHYSICAL AND CHEMICAL WEATHERING PROCESSES AND ASSOCIATED CO2 CONSUMPTION FROM SMALL MOUNTAINOUS RIVERS ON HIGH-STANDING ISLANDS

DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the Graduate

School of The Ohio State University

Steven Todd Goldsmith, M.S.

Geological Sciences Graduate Program

The Ohio State University 2009

Dissertation Committee:

Professor Anne E. Carey, Advisor

Professor W. Berry Lyons

Professor Michael Barton

Professor Wendy Panero

Steven Todd Goldsmith

2009

ABSTRACT

Recent studies of chemical weathering of on high standing islands (HSIs) have shown these terrains have some of the highest observed rates of chemical weathering and associated CO2 consumption yet reported. However, much remains unknown about controlling process. To determine the role these islands play on climate the following were evaluated: 1. dissolved, particulate and organic carbon fluxes delivered to the ocean from a small-mountainous river on an HSI during an intense storm event (i.e., typhoon);

2. relationship between physical and chemical weathering rates on an HSI characterized by ranges of uplift rates and lithology; 3. water and sediment geochemical fluxes and

CO2 consumption rates on HSIs with andesitic-dacitic volcanism; and 4. the overall chemical weathering fluxes and CO2 consumption rates from andesitic-dacitic terrains on

HSIs of the Pacific and the East and Southeast Asia region.

Sampling of the Choshui River in Taiwan during Typhoon Mindulle in 2004 revealed a particulate organic carbon (POC) flux of 5.00x105 tons associated with a sediment flux of 61 million tons during a 96 hour period. The linkage of high amounts of

POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the first known evidence for the rapid delivery and burial of

POC from the terrestrial system. These fluxes, when combined with storm derived CO2

consumption of 1.65x108 moles from silicate weathering, elucidate the important role of these tropical cyclone events on small mountainous rivers as a global sink of CO2.

Geochemical sampling of Taiwan rivers during spring 2004 and summer 2005 revealed carbonate weathering supplies a significant portion of the total cation yields

(44–93%) while silicate weathering plays a lesser role. However, absolute silicate weathering rates are so high (5.8–149 tons km-2a-1) that they fall at the upper end of those previously determined from sedimentary and metamorphic terrains of HSIs.

Comparisons of chemical weathering yields to potential controlling parameters revealed slightly positive correlations with basin average mean annual rainfall and average basin runoff as well as between silicate weathering rates with annual suspended sediment yields. However, the high p-value suggests more data are necessary to obtain an accurate determination. Silicate and carbonate weathering yields also had different relationships with post-uplift age of the landscape. H2SO4 weathering, originating from the dissolution of pyrite, accounts for 13–33 % of the total chemical weathering in these systems. After correction for H2SO4, calculated CO2 consumption from silicate weathering ranges from

236 to 2640 x 103 moles km-2 a-1 and is highly elevated over world average values. Such

CO2 consumption likely represents the upper limit for a non-volcanic active margin setting.

Sampling and stream gauging of Dominica rivers in July 2006 and March 2008 revealed distinct wet and dry season solute concentrations. A cluster analysis of the stream geochemical data shows the importance of parent material age on the overall delivery of solutes. Observed Ca:Na, HCO3:Na and Mg:Na ratios suggest crystallinity of

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the parent material may also play an important role in determining weathering fluxes.

Observed chemical weathering yields (6–105 t km-2 a-1) were similar to those previously

-2 -1 determined for basalt terrains. Silicate yields (3.1–58.4 t km a ) and associated CO2 consumption (143–2040 x 103 mol km-2 a-1) are amongst the highest determined to date.

These chemical yields confirm the weathering potential of andesitic-dacitic terrains.

Chemical weathering yields from two additional andesite terrains, Mt. Pinatubo in the Philippines and Volcán Barú in western Panama are combined with existing datasets in an attempt to calculate a regional CO2 drawdown value for this material. Annual chemical and silicate yields from both regions were some of the highest recorded to date

3 -2 -1 and corresponding CO2 consumption values (1532–2882 x10 moles km a ) are so high they fall in-line with those previously determined for basaltic terrains. A compilation of the new and existing datasets shows rivers draining andesite material are characterized by relatively high Na-normalized molar ratios and low Ca:Mg molar ratios compared to those draining continental silicates. No discernible relationship with material age was observed while runoff and temperature were shown to be the dominant controls on solute fluxes. From these relationships and a new highly-detailed lithology map, CO2 consumption from andesite weathering on HSIs of 0.49 x 1012 moles a-1 and for East and

Southeast Asia of about 0.59 x 1012 moles a-1 were determined. These values represent between 5.7 and 6.8% of the annual CO2 consumption previously calculated from continental silicate weathering and between 16 and 19% of the value previously calculated from basaltic terrains worldwide thereby confirming the importance of

andesite weathering as a CO2 sink. These terrains thus may play an important role on climate evolution over geologic time.

This study is the first comprehensive evaluation of weathering processes on HSIs and provides valuable insights on the relationship of silicate weathering and global CO2 drawdown on various timescales. Weathering fluxes observed from the sedimentary and metamorphic terrains of Taiwan may represent the upper end of what may have occurred during the early stage collision of the Himalayas. Evaluation of CO2 drawdown from andesite terrains on HSIs shows the importance of this material as a weathering substrate and lends further support that andesite terrain be considered separately when calculating global CO2 sequestration from silicate weathering. Additional evaluation of andesite terrains worldwide is warranted in order to accurately delineate this annual value.

To Mom and Dad

ACKNOWLEDGEMENTS

I wish to thank my advisor, Dr. Anne Carey, for her help throughout the whole course of my Ph.D. and agreeing to take a chance on an environmental consultant in dire need of a career change. Her help was invaluable. Conversations with Dr. W. Berry

Lyons regarding my data were both exciting and rewarding. Discussions on andesite volcanics with Dr. Michael Barton were extremely helpful as well. I also would like to thank Dr. Wendy Panero for agreeing to serve on my committee. I am indebted to the following members of the Carey and Lyons research groups for their valuable help at various intervals during the process: Sue Welch, Kathy Welch, Brent Johnson, Gregg

McElwee, and Annette Trierweiler. I also wish to thank Dr. Shuh-Ji Kao of the

Academia Sinica, Research Center for Environmental Changes, Taipei, Taiwan for serving as my host during my two summers (2004 and 2005) in the National Science

Foundation East Asia and Pacific Summer Institute.

A number of acknowledgements are required for specific help I received with this work: T.-Y. Lee, A.-J. Song, Jill Chien, Jack Hu, and S.-Y. Chou aided with sample collection of the Choshui River during Typhoon Mindulle (Chapter 2); Jean Chen and

Jeff Owen for helping me to adjust to life in Taiwan while conducting my research

(Chapters 2 and 3); Anne Carey, Jill Chien and T.-Y. Lee for their aide in sample collection of Taiwan rivers during the spring and summer of 2005; Jean Chen for providing help with field logistics and data analysis; T.-Y. Lee for accessing long-term

vii

gauging records of the Taiwan Water Resources Agency; Anthony Lutton for assistance with sample analysis on the ICP-OES (Chapter 3); Cecil Shillingsworth of the Office of

Disaster Management for his help with field logistics and the people of Dominica for their kind access to streams for gauging and sampling; Anne Carey and Brent Johnson for their assistance with sampling and gauging of Dominica streams; Brent Johnson for aiding with watershed area determination (Chapter 4); Dr. Russell Harmon of the U.S.

Army Research Office and Eric Nicoliasen and Alonzo Iglesias of TRAX-IAESA for their assistance with logistics and sample collection in Panama; Dr. Maria Luisa Tejada,

Mark Lapus, Raymond Rodolfo, and Allan Salas of the National Institute of Geological

Sciences, University of the Philippines – Diliman for their assistance with logistics and sample collection in the Philippines; Sue Welch for help with sample analysis on the ion chromatograph (Chapter 5). This work was also supported by the East Asia and Pacific

Summer Institutes (OISE 0413475) and Hydrologic Sciences (EAR 0309564) programs of US National Science Foundation; the Geological Society of America Graduate Student

Research Grant (2006), Friends of Orton Hall Fund; The Ohio State University Office of

International Affairs Graduate Student Dissertation Research Travel Grant (2007); The

Ohio State University Alumni Grants for Graduate Research and Scholarship (2007); and

The Ohio State University Presidential Fellowship (2009).

I am also grateful to Anne Carey for allowing me the opportunity to serve as a research mentor during my tenure at OSU. Assisting in the research projects of the following undergraduate students was extremely rewarding, kept me humble and

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confirmed my desire to teach: Christopher Gordon, Brent Johnson, Matthew Dugan,

Justin Von Bargen, Lindsay Hannah, and Claire Mondro.

I would also like to thank The Flying Pizza for providing the closest semblance to

NY style pizza for this at times homesick New Yorker. I am also indebted to Mom, John and Irene for their support during this process. Finally, I would like to thank Sarah,

Lauren, and Nicholas Goldsmith for their inquisitiveness into the workings of the world that aids my own.

VITA

October 10, 1973……………………. Born – Yonkers, New York

May 1995……………………………. B.A. Geology, B.A. Environmental Studies, Ohio Wesleyan University

September 1995- April 2001………... Associate – Senior Environmental Geologist, AquaTerra Environmental Services, Inc., New York, NY

April 2001 – June 2002……………... Regional Manager, Certified Environments, Inc., White Plains, N.Y.

June 2002 - September 2003………... Project Professional, Delta Environmental, Inc., Armonk, N.Y.

September 2003-Present Graduate Teaching and Research Associate, The Ohio State University Columbus, Ohio, United States

PUBLICATIONS Research Publications

1. Goldsmith S.T., Carey A.E., Johnson B.M., Welch S.A., Lyons W.B. and McDowell, W.H. (2009) Stream geochemistry, chemical weathering and CO2 consumption potential of andesitic terrains, Dominica, Lesser Antilles. Geochim. Cosmochim. Acta, in press.

2. Goldsmith S.T., Kao S-.J., Carey A.E. (2008) Geochemical fluxes from the ChoShui River during Typhoon Mindulle, July 2004. Geology. 36, 483-486.

3. Goldsmith S.T., Carey A.E., Lyons W.B., Hicks, M. (2008) Geochemical fluxes and weathering on high standing islands: Taranaki and Manawatu-Wanganui Regions, New Zealand. Geochim. Cosmochim. Acta, 72, 2248-2267.

4. Carey A.E., Gardner C.B., Goldsmith S.T., Lyons W.B., and Hicks D.M. (2005) Organic carbon yields from small, mountainous rivers, New Zealand. Geophys. Res. Lett. 32, L15404, doi:10.1029/2005GLO23159.

FIELDS OF STUDY

Major Field: Geological Sciences Area of Emphasis: Geochemistry x

TABLE OF CONTENTS

Page

Abstract…………………………………………………………………………. ii

Dedication………..……………………………………………………………... vi

Acknowledgements……………………………………………………………... vii

Vita……………………………………………………………………………… x

List of Tables…………………………………………………………………… xv

List of Figures…………………………………………………………………... xvii

Chapters:

1. Introduction……..………………………………………………………. 1

2. Extreme Storm Events, Landscape, Denudation, and Carbon Sequestration: Typhoon Mindulle, Choshui River, Taiwan 2.1. Abstract….…………………………………………………. 6 2.2 Introduction………………………………………………… 6 2.3. Study Area Background and Typhoon Mindulle…………... 8 2.4. Methods……………………………………………………. 9 2.4.1 Sample Collection and Analysis………………… 9 2.4.2. Storm Flux Calculations………………………… 10 2.5. Results……………………………………………………… 11 2.6. Discussion and Conclusions……………………………….. 13 2.7. Tables………………………………………………………. 17 2.8. Figures……………………………………………………… 18

3. Chemical Weathering on High-Standing Islands: Central and Coastal Ranges, Taiwan 3.1. Abstract….…………………………………………………. 20 3.2. Introduction………………………………………………… 21 3.3. Study Area Background ……………………….………….. 24

3.3.1. Tectonics and Geology………..………………… 24 3.3.2. Climate and Topography………………………... 26 3.4. Sampling and Analytical Methods………………………… 27 3.4.1. Sample Methodology…………………………… 27 3.4.2. Water Analysis………………………………..... 27 3.4.3. Data Interpretation……………………………… 28 3.5. Results and Discussion……………………………………. 30 3.5.1. Solute Geochemistry………………………...... 30 3.5.2. Relative Input of Cations from Weathering……. 34 3.5.3. Intensity of Silicate Weathering………………... 36 3.5.4. Importance of Pyrite Weathering………………. 37 3.6. Flux Determination and Potential CO2 Consumption…….. 39 3.6.1. Chemical Weathering Rates……………………. 39 3.6.2. Silicate Weathering Rates……………….……… 41 3.6.3. Carbonate Weathering Rates…………………… 42 3.6.4. H2SO4 Input to Weathering…………………….. 43 3.6.5. CO2 Consumption Rates………………………... 44 3.7. Conclusions…………………………………………….. 47 3.8. Tables…………………………………………………... 48 3.9. Figures………………………………………………….. 55

4. Stream Geochemistry, Chemical Weathering and CO2 Consumption Potential of Andestic Terrains, Dominica, Lesser Antilles 4.1. Abstract….…………………………………………………. 61 4.2. Introduction………………………………………………… 62 4.3. Study Area Background ……………………….…………... 65 4.3.1. Geology………..………………………………... 65 4.3.2. Climate and Hydrology…………………………. 67 4.3.3. Topography and Soils…………………………… 69 4.4. Sampling and Analytical Methods………………………… 71 4.4.1. Sample Methodology…………………………… 71 4.4.2. Stream Gauging and Annual Discharge Determination...... 71 4.4.3. Water Analysis………………………………..... 72 4.4.4. Data Interpretation……………………………… 73 4.5. Results and Discussion……………………………………. 75 4.5.1. Representativeness of the Data………………… 75 4.5.2. Concentration of Major Elements and Statistical Analysis………………………………………… 76 4.5.3. Source of Solutes………………………………. 79 4.5.4. Cluster Analysis………………………………… 82 4.6. Dominica River Chemistry Compared to Silicate Terrains Worldwide………………………………………………… 83

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4.7. Chemical Weathering Rates and Potential CO2 Consumption………………………………………………. 85 4.7.1. Chemical Weathering Rates……………………. 85 4.7.2. Silicate Weathering and CO2 Consumption……. 87 4.8. Conclusions…………………………………………….. 92 4.9. Tables…………………………………………………… 94 4.10. Figures………………………………………………….. 100

5. Andesitic-Dacitic Terrains of HSIs and their Role on Global Carbon Cycle 5.1. Abstract….………………………………………………… 106 5.2. Introduction……………………………………………….. 107 5.3. Study Area Background ……………………….…………. 111 5.3.1. Geology………..……………………………….. 111 5.3.1.1. Volcán Barú, Panama………………. 111 5.3.1.2 Mt. Pinatubo, Philippines…… ……. 113 5.3.2. Climate and Hydrology………………………… 114 5.3.2.1. Volcán Barú, Panama………………. 114 5.3.2.2 Mt. Pinatubo, Philippines…… ……. 115 5.4. Sampling, Analytical Methods, and Data Compilation Methodology …………………………………………. 115 5.4.1. Sample Methodology…………………………... 115 5.4.2. Stream Gauging and Annual Discharge Determination...... 116 5.4.3. Water Analysis………………………………..... 117 5.4.4. Data Interpretation……………………………… 118 5.5. Results and Discussion……………………………………. 119 5.5.1. Correction for Geothermal Input………………. 119 5.5.2. Concentration of Major Elements and Statistical Analysis…………………………….. 121 5.5.2.1. Mt. Pinatubo, Philippines………….. 121 5.5.2.2. Volcán Barú, Panama …… ……….. 123 5.5.3. Chemical Weathering Rates…………………… 125 5.6. Worldwide Summary of Andesitic Terrains and Search for Controls ……………………….……………………... 126 5.6.1. Summary of Datasets and Subsequent Corrections……………………………………… 126 5.6.2. Elemental Ratios……………………………….. 128 5.6.2.1. Ca:Mg…………………………….... 128 5.6.2.2. Na Normalized Ratios…… ……….. 129 5.6.3. Evaluation of Potential Controls on Andesitic Weathering……………………………………… 130 5.6.3.1. Average SiO2 Content of Parent Material……………….. …………… 130

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5.6.3.2. Parent Material Age…. ……………. 132 5.6.3.3. Runoff and Temperature…………… 133 5.7. Present-day CO2 Drawdown from Andesitic Weathering on HSIs and East and Southeast Asia……………………. 135 5.8. Conclusions……………………………………………….. 138 5.9. Tables…………………………………………………….. 140 5.10. Figures……………………………………………………. 147

6.0 Conclusions and Future Research…..………………………………….. 156 6.1. Conclusions……………………………………………….. 156 6.2. Future Directions………………………………………….. 158

Bibliography……………………………………………………..…………….. 161

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LIST OF TABLES

Table Page

2.1 Silicate, chemical yields and CO2 consumption based on silicate weathering…………………………………………………………….… 17

2.2 Particulate organic carbon (POC) yields……………………...………… 17

3.1 Sample location summary (Taiwan watersheds)……………………….. 48

3.2 Dissolved load of Taiwan rivers (Spring 2005)………………………… 49

3.2 Dissolved load of Taiwan rivers (Summer 2004 & 2005)……………… 50

3.3 Taiwan and worldwide Ca:Mg and Na normalized molar ratios in stream water…………………..……………………………………… 51

3.4 Taiwan Si:(Na+K-Cl) ratios normalized ratios in stream water……….. 52

3.5 Chemical, silicate, and carbonate weathering yields and CO2 consumption in Taiwan watersheds………………………………...... 53

3.6 Chemical, silicate, and carbonate weathering yields and CO2 consumption in other locales …………………………………………… 54

4.1 Sample location summary (Dominica watersheds)……………………... 94

4.2 Dissolved load of Dominica rivers (Summer 2006)……………………… 95

4.2 Dissolved load of Dominica rivers (Spring 2008)……………………… 96

4.3 Dominica Fw values in stream water and seasonal averages…………… 97

4.4 Dominica and worldwide Ca:Mg ratios in stream water……………….. 98

4.5 Dominica, continental silicate end member, and basalt molar ratios……………………………………………………………………. 98

4.6 Chemical, and silicate weathering yields and CO2 consumption in Dominica watersheds and other locales ………………. 99

5.1 Sample location and watershed area summary ………………………… 140

5.2 Dissolved load of Panama Rivers (Summer 2006 and Winter 2007).….. 141

5.2 Dissolved load of Philippines Rivers (Winter 2008).………………….. 142

5.3 Chemical, Silicate, and carbonate weathering yields and CO2 consumption in Panama and Philippines.………………………………. 143

5.4 Mean solute concentration, climatic parameter and rates for andesitic watersheds ……………………………………………………. 143

5.5 Andesite, continental silicate end member, and basalt molar ratios……. 144

5.6 Summary of geochemical data for andesite terrains in this study ……… 145

5.7 Age of volcanic included in study ……………………………………… 145

5.8 Lithology breakdown, runoff, annual temperature and calculated CO2 drawdown for each andesitic province ……………………………. 146

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LIST OF FIGURES

Figure Page

1.1 View of silicate weathering cycle……………………………………..... 1

2.1 Shaded relief map of Taiwan….………………………………………... 18

2.2 Rainfall and discharge for the Typhoon Mindulle 96 h storm Hydrograph……………….. ….……………………………………….. 19

3.1 Map of Taiwan showing stream water sampling locations……………. 55

3.2 Plot of total cation input in stream water from rainwater, carbonate, and silicate weathering………………………………………………… 56

3.3 Ternary diagram of HCO3-Si-(Cl-SO4) in μeq/l in stream water…… 57

3.3 Ternary diagram of lot of Ca-Mg-(Na+K) in μeq/l in stream water….... 57

+2 +2 - 3.4 Plot of Ca + Mg vs. HCO3 in μeq/l in stream water (corrected for precipitation).………………………………………………………. 58

+2 +2 - -2 3.4 Plot of Ca + Mg vs. HCO3 and SO4 in μeq/l in stream water (corrected for precipitation).…………………………………………… 58

3.5 Plot of silicate weathering rates plotted vs. latitude and altitude of watershed divide and uplift rate…………………………………….. 59

3.5 Plot of carbonate weathering rates plotted vs. latitude and altitude of watershed divide and uplift rate……………………………………. 60

4.1 Map of Dominica stream water sample locations and sample locations with respect to lithology………………….………………….. 100

4.2 Plot of fraction of solutes derived from weathering reactions (Fw) vs. Si in μmol/l in stream water corrected for precipitation………. 101

4.3 Plot of total cations (TZ+) in μmol/l vs. total dissolved solids (TDS) in μg/kg in stream water corrected for precipitation ………….... 101

xvii

4.4 Results of a cluster analysis of the dry season Dominica stream water samples………………………………………………………….. 102

4.5 Mg/Na vs. Ca/Na and HCO3/Na vs. Ca/Na of Dominica rivers corrected for atmospheric inputs ……………………….…………….... 103

4.6 Chemical weathering in Dominica watersheds as function of watershed average annual precipitation …………..…………….…...... 104

4.7 Plot of HCO3_ concentrations vs. 1/T (calculated using mean annual temperature) for rivers draining both basaltic and andestic terrains…………………………...…...………………...... 105

5.1 Map of Mt. Pinatubo in Luzon Philippines showing stream water sampling locations.…………………………………………………….. 147

5.1 Map of Volcán Barú in western Panama showing stream water sampling locations …………………………………………………….. 148

5.2. Cl-SO4-HCO3 ternary diagram for Pinatubo springs and wells and stream water samples………………………………………………… 149

5.3 Mg/Na vs. Ca/Na and of rivers draining andesitic terrain corrected for atmospheric inputs ……………………………...………………….. 150

5.3 HCO3/Na vs. Ca/Na of rivers draining andesitic terrain corrected for atmospheric inputs ……….…..……………………………………. 150

5.4 Silica vs. total alkalis variation diagram for the published andesite values.……………….…………………………………………………. 151

5.5 Plots of mean bicarbonate concentrations, TDScat, and TDSw Average SiO2 content of the parent material ………………………….. 152

5.6. Plots of mean bicarbonate concentrations, TDScat, and TDSw versus age of most recent eruption/eruptive period ……….………….. 153

5.7 Plots of mean bicarbonate concentrations, TDScat, and TDSw versus mean annual runoff …………………………………………….. 154

5.8. Plots of mean bicarbonate concentrations, TDScat, and TDSw versus mean annual temperature……………………………………….. 155

xviii

CHAPTER 1

INTRODUCTION

Global climate change has brought urgency to the determination of an accurate annual global carbon budget. Over geologic time, the cumulative impact of CO2 removal from the atmosphere is believed to be controlled by the rate of CO2 input from volcanic outgassing and metamorphism and the rate of removal by silicate weathering (Raymo and

Ruddiman, 1992) (Figure 1.1). The generally accepted relationship between temperature and atmospheric CO2 concentration established for the last 140 k.y. from the Vostok ice- core records (Barnola et al., 1987) lends further importance to net CO2 fluxes in the atmosphere over geologic time.

Fig 1.1 View of the silicate weathering cycle (Berner, 1999).

High-standing islands (HSIs) are islands whose streams’ headwaters lie at elevations greater than 1,000 meters above sea level. Because of their geographical proximity to the oceans, HSIs experience precipitation events at high frequency and intensity. More importantly, the islands are characterized by short rivers with steep gradients, providing rapid transport of sediments to the oceans. As a result of these characteristics, HSIs have been estimated to contribute up to 33% of the total sediment entering the world’s oceans annually (Lyons et al., 2002).

Previous studies of chemical weathering rates (a measure of the chemical dissolution rate of rock) on HSIs have shown some of the highest yet observed rates of chemical weathering and CO2 consumption (Jacobson and Blum, 2003; Lyons et al.,

2005). Studies point to the mass of sediment being transported and not simply climate as the ultimate factor determining chemical weathering rates at active margins (Lyons et al.,

2002). Chemical weathering in these environments also depends on basin geology, which has also been found to exert control on both the physical and chemical weathering yields for HSIs (Hicks et al., 1996). Tectonics (rock uplift rate) and lithology have also been argued to exert control on annual CO2 consumption rates as a result of high silicate weathering rates (Edmund and Huh, 1997; Huh and Edmond, 1999).

However, while these initial studies have shed light on the magnitude of fluxes originating from HSIs, much remains unknown about potential controlling processes.

For example, if the hydrology of HSIs is heavily influenced, if not dominated, by intense aperiodic storm events, what role do these events play on the annual delivery of fluxes

(solute, sediment, and particulate organic carbon) from these systems? Furthermore, do the characteristically high physical erosion rates of HSIs play a controlling role on the

delivery of fluxes from these locales? Finally, if a lithology of particular weathering importance composes a significant portion of the terrain on HSIs, could the resulting weathering rates and associated CO2 consumption be high enough to play a major role on the annual global carbon cycle?

This study focused on the determination of weathering fluxes from HSIs and the collective role these islands play on earth’s climate. My central hypotheses are that chemical weathering rates and CO2 consumption on HSIs are dependent on such factors as physical weathering rates, lithology and uplift rates, and that the majority of these dissolved and particulate fluxes are delivered during intense aperiodic precipitation events (i.e., hurricanes, typhoons, monsoons). The following specific objectives were pursued to achieve the central hypothesis tested by pursuing:

Objective 1: Determine particulate, particulate organic carbon, and dissolved fluxes

delivered to the ocean from a small mountainous river on an HSI during a typhoon.

Objective 2: Determine the relationship between physical and chemical weathering rates

on an HSI characterized by varying uplift rates and lithology.

Objective 3: Determine water and sediment geochemical fluxes and CO2 consumption

rates on HSIs characterized by andesitic-dacitic volcanism.

Objective 4: Calculate chemical weathering and CO2 consumption rates from andesitic-

dacitic terrains on HSIs of the Pacific.

The first objective was addressed through monitoring the sediment and solute delivered from Taiwan during Typhoon Mindulle in June of 2004. Dissolved Si, suspended sediment, and sediment POC were multiplied by published Taiwan Water

Resources Agency (WRA) hourly discharge rates in order to calculate storm fluxes. A 3

subsequent comparison of these fluxes to annual yields from other HSIs was performed to determine whether the majority of these fluxes are delivered during a storm event.

Data collected from the storm monitoring also improved the understanding of the role of extreme storm events at these locales on organic carbon burial. For example, if carbon fluxes were delivered with river sediment concentrations capable of producing a negatively buoyant plume upon reaching the ocean, then the organic carbon could be subject to rapid burial.

The second objective was addressed through the monitoring of the solute chemistry of Taiwan rivers during the spring and summer of 2005. Precipitation- corrected water chemistry was further corrected for relative input from carbonate and silicate weathering in order to delineate the geographic input of each throughout the island. Chemical erosion fluxes were determined by multiplying the precipitation- corrected TDS concentrations for the wet and dry season data by published Taiwan WRA stream discharge data for the respective season and summing the values. Silicate weathering fluxes and CO2 consumption values were determined in a similar fashion by summing the cation concentrations in µeq/l and cation concentrations in uquev/l, respectively, and multiplying by the seasonal discharge values. These fluxes were then compared to parameters such as basin average annual rainfall, basin average annual runoff, annual suspended sediment yields, elevation of uplift divide and annual uplift rate in an effort to determine controls on chemical weathering.

The third objective was addressed through the monitoring of solute chemistry and manual gauging of stream flow of Dominica rivers during the spring of 2007 and summer of 2008. Precipitation-corrected water chemistry was utilized in a cluster analysis to

determine the effect of age of parent material on observed solute chemistry. Chemical and silicate erosion yields, and CO2 consumption values were calculated using the methodology outlined above except using wet and dry season discharge data obtained from manual gauging of stream flow in the rivers. The resultant geochemical fluxes were subsequently compared to parameters such as watershed gradient, basin average annual rainfall, and age of parent material in an effort to determine controls on chemical weathering.

The final objective was addressed through the collection of new geochemical data from the andesite terrains of Volcán Barú in western Panama and Mt. Pinatubo in the

Philippines (November 2008). These new datasets were then combined with existing andesite weathering datasets from Costa Rica; Dominica (Chapter 4); Guadeloupe;

Martinique; Mt. Taranaki, New Zealand; and Western Oregon in order to identify controls on andesite weathering. The dataset was compared to parameters such as age and average SiO2 content of the parent material, along with mean annual runoff and mean annual temperature in an effort to identify overall controls on andesite weathering. An equation taking into account the role of both runoff and temperature was then utilized with a new highly detailed lithology map of East and Southeast Asia to determine annual

CO2 drawdown from andesite weathering on HSIs.

This work utilizes applied geochemical techniques in order to evaluate the extent and potential controls on chemical weathering on HSIs. To understand the controls on chemical erosion at these locales will determine the potential role HSIs play in annual

CO2 consumption, and therefore their possible control of climate over geologic time.

CHAPTER 2

EXTREME STORM EVENTS, LANDSCAPE DENUDATION, AND CARBON SEQUESTRATION: TYPHOON MINDULLE, CHOSHUI RIVER, TAIWAN

2.1. Abstract

This study represents the first known semi-continuous monitoring of particulate organic carbon (POC) fluxes and dissolved Si concentrations delivered to the ocean during a typhoon. Sampling of the Choshui River in Taiwan during Typhoon Mindulle in

2004 revealed a POC flux of 5.00x105 tons associated with a sediment flux of 61 million tons during a 96 h period. The linkage of high amounts of POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the first known evidence for the rapid delivery and burial of POC from the terrestrial system. These fluxes, when combined with storm derived CO2 consumption of

1.65x108 moles from silicate weathering, elucidate the important role of these tropical cyclone events on small mountainous rivers as a global sink of CO2.

2.2. Introduction

Recent work on small mountainous rivers (SMRs) has demonstrated that they are major sources of sediment (Milliman and Syvitski, 1992; Milliman et al., 2007), particulate organic carbon (POC) (Kao and Liu, 1996; Lyons et al., 2002, Carey et al.,

2005), and dissolved fluxes to the ocean (Lyons et al., 2005). Because of their

geographic locations, many SMRs are affected by periodic to aperiodic intense precipitation events such as monsoonal activity, ENSO variation and tropical cyclonic activity. Infrequent aperiodic events can determine their long-term water discharge and sediment delivery and strongly affect the rate of landscape denudation (Filippelli, 1997;

Gomez et al., 2004). Although many studies have focused on the rapid denudation and/or geochemical delivery associated with these regimes (Milliman and Syvitski, 1992; Kao and Liu, 1996; Lyons et al., 2002, Carey et al., 2005; Lyons et al., 2005), little information is available to quantify the overall importance of these aperiodic events, in part because they are difficult to sample and study.

The island of Taiwan, with its rapid uplift of ~5–10 mm yr-1 (Shin and Teng,

2001), erosion rates up to 2–8 mm yr-1 (Fuller et al, 2003) and approximate typhoon frequency of four per year (Wu and Kuo, 1999), provides an ideal example of a denudation and weathering regime characterized by sediment-laden SMRs affected by intense aperiodic events. Recent studies have shown the importance of human activities such as agriculture and road building on the control of sediment delivery to the coastal ocean globally (Wilkinson and McElroy, 2007; Syvitski and Milliman, 2007). In Taiwan, anthropogenic activities (Kao and Liu, 2002), natural variability in drainage basins

(Dadson et al., 2003), and friable lithology (Kao and Liu, 2002) have been linked to short-term localized effects on sediment transport, but cumulative seismic activity and temporal variability in runoff (particularly during typhoon-related floods) have been shown statistically to control modern decadal scale physical erosion (Dadson et al.,

2003). Coseismic weakening of the substrate material allows for its initial downslope mobilization, which can then be remobilized and ultimately delivered to the river channel

during a typhoon related landslide (Dadson et al., 2004). No known studies have focused on the POC content of the sediment fluxes or the dissolved fluxes from silicate weathering delivered during one of these events. Therefore, this study investigates the quantity of POC matter transported and CO2 consumption via silicate weathering during a typhoon event.

2.3. Study Area Background and Typhoon Mindulle

The Choshui River watershed (drainage area of 3150 km2), located in the Western

Foothills fold and thrust belt of the Central Mountain Range in southwestern Taiwan

(Figure 1.1), is characterized by friable Miocene and Pliocene sedimentary, meta- sedimentary, and low-grade metamorphic rocks and Quaternary alluvium deposits (Ho,

1988). Recent studies of the Choshui and other rivers draining Taiwan indicate that the majority of the sediment load is delivered to the oceans as hyperpycnal flows during typhoon events (for the case of Taiwan, sediment concentrations >40 g L-1) (Dadson et al., 2003; Milliman and Kao, 2005; Dadson et al., 2005). These studies have focused on the measurement and estimation of total sediment yields from Taiwan rivers during a typhoon while relying on sediment rating curves based on few data points.

Typhoon Mindulle originated as a tropical depression in the western Pacific near

Guam on 23 June 2004. The storm encountered the northern tip of Luzon, where it weakened before making landfall in southeast Taiwan on 1 July 2004. Rainfall total for the 96 h period, averaged from five stations throughout the watershed, was 650 mm with peak intensities reached on 3 July 2004 at 0200 h (28 mm hr-1) and on 4 July 2004 at

0900 h (76 mm hr-1) (Water Resources Agency, 2004). The staggered rainfall pattern

resulted in a double-pulsed storm hydrograph with a peak river discharge of 6600 m3 sec-1 at 1100 h on 4 July 2004 (Figure 1.2A) (Water Resources Agency, 2004).

2.4. Methods

2.4.1. Sample Collection and Analysis

Water and suspended sediment samples were obtained at approximately three hour intervals over a 96 h period from 1 July 2004 through 5 July 2004 from a downstream location of the Choshui River during Typhoon Mindulle. Samples were collected from the Jangyung Bridge (23° 47.264’ N; 120° 38.184’ E) located ~50 km (81 m in elevation) upstream of the river mouth and well above the influence of the tidal zone and agriculture.

Four one-liter water samples were collected at each time interval in new, low- density polyethylene (LDPE) deionized water (18 MΩ) soaked bottles. The bottles were manually lowered into the stream in a weighted Teflon holder. The commonly-used

USGS DH-48/76 sampler was not used because of its difficulty in sinking in high flow velocities (~5 m s-1). Total suspended sediment (TSS) concentrations were determined gravimetrically using pre-combusted 0.7 m pore-size Whatman GF/F filters. The mean of the blank, calculated from 10 replicates is 0.05 ± 0.01mg L-1. This blank value is well below the weight of sediment on the filter (generally >50 g L-1). The filtered sediment was isolated and analyzed for organic-matter concentration using a modified methodology of Lyons et al. (2002), whereby the POC concentration was determined as

33% of the weight loss of dried samples after ignition at 550°C for 4 hours. The relative standard deviations (RSDs) of multiple analyses were <7%, with the majority <3%.

Water samples were filtered through 47 mm diameter Nuclepore® (nominal pore size of 0.4 m) polycarbonate filters into an HNO3 washed 60 ml LDPE bottle for Si analysis. Filter blanks were created by filtering 18 MΩ water into clean LDPE bottles using the same methods used for samples. Trip blanks were created prior to sample collection with 18 MΩ water in clean HDPE bottles. Upon return from the field, each filtered sample for Si analysis was acidified to pH 2–3 with Fisherbrand® trace metal

® grade HNO3. Dissolved Si concentrations were determined using a Perkin-Elmer Sciex

ELAN 6000® inductively coupled plasma optical emission spectrometer (ICP-OES).

Internal standards were used and check standards were run every 3–4 samples to account for instrument drift. Triplicate analyses did not reveal any RSDs greater than ± 5%.

2.4.2. Storm Flux Calculations

Dissolved Si fluxes for the 96 h storm event were calculated by multiplying published hourly Taiwan Water Resources Agency (2004) stream discharge data (Q) and the laboratory determined Si values for each of the sample intervals. The Si fluxes for each time interval were summed to provide the total storm flux. Total sediment fluxes for the storm were calculated by multiplying water discharge and TSS values for each of the sample intervals. This value was subsequently multiplied by the POC concentration for the suspended sediment sample from the associated time interval in order to obtain the

POC flux. During the last two hours of the hydrograph, no samples for POC or dissolved

Si were collected. In calculation of total hydrograph flux, the minimum observed values for Si and POC were used for the last integration interval.

This sampling location at the Jangyung Bridge and the WRA gauge are located

~15 km downstream of the Chi-Chi water supply impoundment. However, the storm hydrograph at another gauging station upstream of the impoundment near the Yuifong

Bridge, had an average water discharge (3230 m3) over the falling limb of the hydrograph

~4% larger than the average discharge (3100 m3) recorded at the sampling location and gauging station downstream of the impoundment. Therefore, the flux estimates for the falling limb of the hydrograph are unlikely to have been affected by any drainage from the reservoir.

2.5. Results

The analysis of dissolved Si (DR Table 1) revealed a 96 h storm flux of 2.32x103 tons (T), and a watershed normalized Si yield of 0.74 T km-2 (Figure 1.2(b) with the methodology of Edmond and Huh (1997), whereby CO2 consumption is calculated as twice the dissolved Si yield from silicate weathering alone, I determined CO2 consumption as twice the dissolved Si yield from silicate weathering, yielding calculated

3 -2 CO2 consumption for the storm of 52.5x10 moles km . That the Choshui storm value represents up to 31% of the annual CO2 consumption values previously determined for the high silicate yielding SMR catchments of the North and South Islands of New

Zealand (Lyons et al., 2005) (Table 2.1) supports the notion that the majority of the annual dissolved solute weathering fluxes for these regimes is delivered during storm events. While this sampling methodology cannot readily distinguish relative input of Si from overland flow and groundwater, the Typhoon Mindulle value represents the overall weathering of silicate sedimentary rock and the subsequent delivery of dissolved Si from

one of these events. Furthermore, this Si flux may be higher for SMRs elsewhere which are covered by greater areas of volcanic terrain.

Total sediment fluxes delivered during the 96 h period of Typhoon Mindulle is

61.4 MT. Although this value is ~15% lower than the 72 MT estimated by Milliman et al. (2007), their methodology employed a sediment rating curve from measurements collected ~40 km downstream of Jangyung Bridge and below the confluences of additional tributaries. While the Mindulle sediment fluxes are also far lower than the 130

MT recorded during Super-Typhoon Herb in 1996 and the 175 MT recorded during the post Chi-Chi earthquake Typhoon Toraji in 2001, those two earlier storms had peak discharge of 14,800 m3 s-1 and 28,000 m3 s-1, respectively, far in excess of the Typhoon

Mindulle peak of 6600 m3 s-1 (Dadson et al., 2005). The Mindulle sediment fluxes are an order of magnitude greater than composite values determined for the mountainous watersheds of the island of Puerto Rico (2.4 MT) during Hurricane Georges (peak discharges ranging from 22 to 2,140 m3 s-1), the only previously known hurricane/typhoon-magnitude storm event sediment flux estimate for another geographic location (Warne et al., 2005). Hyperpycnal sediment concentrations (>40 g L-1) were recorded for Mindulle during ~80% of the 96 h monitored, including 64 of the last 72 h of the hydrograph (Figure 2.2(b)).

POC analysis of the suspended particles revealed a total storm POC flux of

5.00x105 T, and a watershed area normalized POC yield of 159 T km-2. This single storm normalized POC yield is greater than previously calculated total annual fluxes for many of the high POC-yielding rivers of New Zealand (Lyons et al., 2002; Carey et al., 2005) and Taiwan (Kao and Liu, 1996) and is between 72% and 95% of the annual POC flux

for the highest yielding world rivers (Table 2.1). The annual POC yields for New

Zealand and Taiwan rivers are an order of magnitude larger than any other worldwide value (Lyons et al., 2002). Approximately 4.66x105 T, or 93%, of the Mindulle POC storm flux was delivered when the Choshui was experiencing hyperpycnal sediment concentrations (Figure 1.2D). These fluxes likely represent a minimum given that bedload is believed to compose ~30% of the total sediment load for Taiwan (Dadson et al., 2003), input from woody debris was not taken into account, and the conservative methodology used for TSS flux calculations.

2.6. Discussion and Conclusions

Recent evaluation of hyperpycnal plumes points to the importance of SMRs as having the flashy hydrographs and sediment abundance necessary to produce these negatively buoyant fluvial discharges (Mulder and Syvitski, 1995). Their offshore movement can result in either rapid deposition as margin deposits or erosion of previous deposits and channeling of sediments to more remote basin environments (Milliman and

Kao, 2005; Dadson et al., 2005; Mulder and Syvitski, 1995). Previous sampling of an offshore mud-dominated sediment deposit discharged from the Choshui immediately following Typhoon Mindulle revealed C/N and 13C ratios indicative of land-derived organic matter (Milliman et al., 2007). This correlation further reinforces the idea that these plumes can result in rapid burial and possible preservation of terrestrial organic matter.

Hyperpycnal plumes originating from the Choshui and other rivers draining northwest Taiwan are believed to be entrained in the northward flowing North Taiwan

Current during the summer months (Lee and Chao, 2003). Sediments deposited in the

Taiwan Strait from hyperpycnal events appear to be subsequently remobilized and deposited as discontinuous sediment bands along the Taiwan Straight (Milliman et al.,

2007) and may ultimately be transported around the north tip of Taiwan into the Okinawa

Trough (Milliman and Kao, 2005). Although the ultimate fate of sediments from this portion of Taiwan is not yet fully understood, the fact that rivers from the northeast, east, and southwest portion of Taiwan discharge into deep offshore basins (Okinawa Trough,

Hautung Basin, South China Sea Basin, respectively) supports the notion of rapid transport and subsequent burial of sediment. A review of Taiwan WRA long-term sediment gauge records (1970–1998) revealed nine other rivers, all of which have either previously approached or exceeded hyperpycnal sediment concentrations during storm events prior to the Chi-Chi earthquake (Dadson et al., 2005). These records strongly suggest that the production and potential rapid burial mechanisms of POC exhibited by the Choshui River may extend to other systems.

In an effort to determine the importance of the Typhoon Mindulle storm fluxes on a global scale, a comparison of these fluxes to previously established annual worldwide values was conducted. Mindulle-induced CO2 consumption from silicate weathering represents ~0.003% of a previously established worldwide estimate of 0.058 Gt C yr-1

(Gaillardet et al., 1999). Although this value may be minor on a global scale, it still represents ample input of Ca and Mg from immature siliciclastic sedimentary rocks dominant in the Choshui watershed (Ho, 1988). However, the Typhoon Mindulle POC storm flux represents ~1% of present (prehuman) annual burial fluxes globally for deltaic-shelf sediments (Berner, 1982). Although this comparison requires all of the POC

delivered from the Choshui River to be buried, given that Taiwan experiences on average four typhoons per year (Wu and Kuo, 1999), this value may not be far off base. Again, while subsequent remobilization of this sediment in the Taiwan Strait may not represent ideal conditions for carbon preservation during burial, these fluxes only represent one

Taiwan river during one storm event and do not take into account input from northeast, east, and southwest Taiwan. Fluxes from these locales would likely bypass the nearshore environment during a hyperpycnal event and have an estimated annual sediment delivery of 222 MT yr-1 (Dadson et al., 2003).

The large POC fluxes reported herein, coupled with hyperpycnal suspended sediment concentrations generated from a typhoon-magnitude storm event, may provide the necessary conditions for the removal and potential rapid burial of substantial quantities of terrestrially-derived organic carbon. These POC fluxes, when combined with storm derived CO2 consumption from silicate weathering, demonstrate the important role that short-term aperiodic storm events on SMRs, particularly those on high standing islands (HSIs) of Oceania and other tectonically active areas, have on modifying Earth’s climate over geologic time (Lyons et al., 2002; Lyons et al., 2005). As recent studies have suggested that cyclone intensity and monsoonal frequency (Goswami et al., 2006) may be increasing owing to global warming (Emanuel, 2005) even larger POC fluxes, greater silicate weathering-driven CO2 consumption, and more rapid organic carbon burial may be associated with HSIs of Oceania in the future. Furthermore, it is reasonable to expect that other geographically similar regions such as central Asia, eastern Asia (Wu et al., 2005) or the Caribbean and Central America may also be affected by more intense and more frequent aperiodic precipitation events (i.e., hurricane/typhoon

and/or monsoonal activity). The discharge, dissolved silicon, POC, and suspended sediment fluxes reported herein for the Choshui River’s response to Typhoon Mindulle strongly suggest that increased hurricane/typhoon activity may enhance silicate weathering rates and exacerbate current sediment delivery loads, thereby increasing POC delivery and burial.

2.7. Tables

H4SiO4 CO2 flux, Interval/ Location Yield x103 moles tons km-2 km-2 Mindulle Storm Fluxes 96-hour 0.73 52.5 Other Regions (Annual Yields) North Island, New Zealand (East Cape 2.4 - 15.1 170 - 1074 Region)* South Island, New Zealand (Southern 4.2 - 13.3 296 - 946 Alps)* Andes† --- 220 - 1000 Himalayas† --- 100 - 320 *Data from Lyons et al. (2005) †Data from Edmund & Huh (1997)

Table 2.1 Silicate, chemical yields and CO2 consumption based on silicate weathering.

POC Yield, Interval/ Location x106 g km-2 Mindulle Storm Fluxes 96-hour 159 Other Regions (Annual Yields) North Island, New Zealand (Taranaki and East Cape 1.3 - 222 Regions)*,† South Island, New Zealand 1.4 - 168 (Southern Alps)*,† Lanyang Hsi, Taiwan‡ 53 *Data from Lyons et al. (2002) †Data from Carey et al. (2005) ‡Data from Kao & Liu (1996)

Table 2.2 Particulate organic carbon (POC) yields.

2.8. Figures

Figure 2.1 Shaded relief map of Taiwan showing the outline of the Choshui River watershed. Locations of the Jangyung Bridge sampling location/gauging site (1) and the

Yuifong Bridge gauging site (2) are shown. Total watershed area is 3150 km2.

Figure 2.2 (a) Rainfall and discharge for the Typhoon Mindulle 96 h storm hydrograph.

Discharge data (solid circles) were measured hourly by the Taiwan Water Resources

Agency (WRA) at a gauging station adjacent to the sampling location. Hourly rainfall data, represented by the vertical bars, are the average of five stations throughout the

Choshui River watershed, (b) Calculated Si flux for the 96 h storm hydrograph, (c)

Suspended sediment concentrations for the 96 h storm hydrograph. Horizontal line denotes suspended sediment concentration (40 g L-1) for hyperpycnal flows, (d)

Calculated POC flux for the 96 h storm hydrograph.

CHAPTER 3

CHEMICAL WEATHERING ON HIGH STANDING ISLANDS: CENTRAL AND COASTAL RANGES, TAIWAN

3.1. Abstract

HSIs such as Taiwan have garnered much recent attention for their inproportionately high annual sediment fluxes to the ocean. While recent geochemical investigations from these locales have shown highly elevated absolute values of silicate weathering yields and CO2 consumption compared to world averages, much remains unknown about controlling process. Initial estimates have shown a strong link between physical and chemical erosion rates as well as lithologic effects. Recent sampling of watersheds throughout the subtropical island of Taiwan was performed in an effort to characterize the water and fluxes from an HSI which is experiencing rapid uplift and erosion and is underlain by sedimentary and metamorphic terrains. Carbonate weathering supplies a significant portion of the total cation yields (44–93%) while silicate weathering plays a lesser role. However, absolute silicate weathering rates are so high (5.8–149 tons km-2a-1) that they fall at the upper end of those previously determined from sedimentary and metamorphic terrains of HSIs. Comparisons of chemical weathering yields to potential controlling parameters revealed slightly positive correlations with basin average

mean annual rainfall and average basin runoff as well as between silicate weathering rates with annual suspended sediment yields. However, high p-value suggests more data are necessary to obtain an accurate determination. Silicate and carbonate weathering yields also had varying relationships with post-uplift age of the landscape. H2SO4 weathering, originating from the dissolution of pyrite, accounts for 13–33 % of the total chemical weathering in these systems. After correction for H2SO4, calculated CO2

3 -2 -1 consumption from silicate weathering ranges from 236 to 2640 x 10 moles km a and is highly elevated over world average values. While these CO2 fluxes presented herein should be taken with some caution as they do not account for weathering originating from metamorphic CO2, they still likely represent the upper limit for a non-volcanic active margin setting.

3.2. Introduction

The uplift weathering hypothesis of Raymo and Ruddiman (1992) suggests that the uplifting Himalaya and Qinghai-Tibet Plateau was responsible for large scale silicate weathering and subsequent drawdown of atmospheric CO2, and global climate cooling during the Cenozoic. Several subsequent geochemical studies from both major rivers and headwater streams in the Himalayas and Tibetan Plateau have shown stream water chemistry is dominated by carbonate weathering originating from carbonate exposures and disseminated calcite, or has a strong metamorphic CO2 component (Galy and France-

Lanord 1999; Gaillardet et al., 1999; Jacobson and Blum, 2000; Dalai et al., 2002;

Jacobson et al., 2002; West et al., 2002; Wu et al., 2005; Hren et al., 2007; Wolff-

Boenisch et al., 2009). This is important as the weathering of carbonate results in no net

CO2 drawdown from the atmosphere over time scales longer than a million years (Berner,

1983). Dominance of carbonate weathering has led to the subsequent proposal of increased silicate weathering occurring further downstream where sediments have accumulated and climate conditions are more favorable (West et al., 2002; France-Lanord et al., 2003; Wolff-Boenisch et al., 2009). Regardless, Himalayan silicate weathering rates fall far below those determined for volcanic terrains, both basaltic and andesitic

(Gaillardet et al., 1999; Dessert et al., 2003, Rad et al. 2006; Goldsmith et al., 2008a), and has partly directed the search for the connection between physical and chemical erosion

(and subsequent CO2 drawdown) to other high sediment yielding locales.

HSIs, those whose streams’ headwaters lie at greater than 1000 m above sea level), have garnered much attention since the late 1970s as studies from Taiwan (Li,

1976), New Zealand (Adams, 1980; Griffiths, 1981), and Papua New Guinea (Pickup,

1980) revealed annual sediment yields exceeding 2,000 t km-2 yr-1 and in the case of

Taiwan as high as 20,000 t km-2 yr-1. These studies were followed by Milliman and

Syvitski’s (1992) summation of suspended sediment yields for the HSIs of Oceania and

New Guinea, which showed that while these islands represent only a small portion of the continental land mass they deliver up to 25% of the sediment to the world’s ocean annually. These high sediment yields were later attributed to correspondingly high uplift rates, to other factors such as runoff and, to a lesser extent, lithology (Dadson et al.,

2003). Initial investigations into the link between physical and chemical erosion rates from HSIs has shown that while the ratio of physical to chemical weathering rates is low, the absolute chemical weathering rates are some of the highest recorded to date (Carey et al., 2002; Jacobson and Blum, 2003; Lyons et al., 2005). Interestingly enough, some of these studies focusing on weathering of non-volcanic terrains have also shown the

importance of carbonate weathering in these systems (Jacobson and Blum, 2003; Lyons et al., 2005).

The island of Taiwan, with its rapid uplift of ~5–10 mm yr-1 (Shin and Teng,

2001), erosion rates of 2–8 mm yr-1 (Fuller et al, 2003) and typhoon frequency of approximately four per year (Wu and Kuo, 1999), provides an ideal opportunity to explore the relationship of physical and chemical weathering rates. Previous studies interpreting long-term suspended sediment transport data have shown cumulative seismic activity and temporal variability in runoff (particularly during typhoon-related floods) to play a statistically significant control on modern decadal scale physical erosion (Dadson et al., 2003) while anthropogenic activities (Kao and Liu, 2002), natural variability in drainage basins (Dadson et al., 2003), and friable lithology (Kao and Liu, 2002) have been linked to short-term localized effects. Sediment movement is initiated by a coseismic weakening of the substrate material, which allows for its initial downslope mobilization and subsequent remobilization and delivery to the river channel during a typhoon related landslide (Dadson et al., 2003).

While preliminary studies on chemical weathering in Taiwan have revealed a potential relationship with these high physical erosion rates (Carey et al., 2002; Carey et al., 2005; Selvaraj and Chen, 2006; Goldsmith et al., 2008b) the studies either rely upon a limited dataset or focus solely on fluxes delivered during a storm event. This study is the first to determine chemical weathering yields and CO2 drawdown throughout Taiwan and evaluate the link between physical and chemical erosion as well as other potential controls on these rates.

3.3. Study Area Description

3.3.1. Tectonics and Geology

The island of Taiwan is located along the convergence of the Luzon Arc on the

Philippine Sea plate and the Asian continental margin (Figure 3.1). The oblique collision between the plates is manifested in the N-S linear mountain belt (Central Range) with more than 25 peaks over 3,000 m in elevation (Ho, 1988). Rates of uplift have been found to vary between 5–10 mm yr-1 with the highest rates observed in the Central Range itself (Shin and Teng, 2001). Uplift rates are matched by correspondingly high erosion rates averaging 3–7 mm yr-1 and locally up to 60 mm yr-1 (Dadson et al., 2003).

Previous studies examining long-term sediment discharge rates have shown cumulative seismic activity and temporal variability in runoff (particularly during typhoon-related floods) to play a statistically significant physical control on modern decadal scale physical erosion rates (Dadson et al., 2003). However, a recent examination of a similar dataset showed that controls on physical erosion throughout the island are highly variable and can also be locally affected by lithology, gradient, and anthropogenic activities (Kao and Milliman, 2009). Regardless, these factors have contributed to annual island-wide suspended sediment flux of approximately 222 Mt

(Dadson et al., 2003) of which significant quantities are delivered in hyperpycnal conditions during typhoons (Dadson et al., 2003; Milliman et al., 2008; Goldsmith et al.,

2008b). Fluvial bedrock incision (Hartshorn et al., 2002), landsliding (Hovius et al.,

2000), and debris flows (Lin et al., 2004) have been exposed as the main driving mechanisms in the ultimate delivery of this material to stream channels.

The southward propagation of uplift over the last 5 Myr has also played a major role on the surficial geology over time as the island has slowly shed its soft sedimentary cover. The highly metamorphosed core in the northeast consists of localized pegmatite, migmatite and marble of late Paleozoic to Mesozoic age, and grades downward to regional outcrops of black- and greenschist grade formations of similar age (Ho, 1988).

From west to east across central Taiwan, the metamorphic grade increases from poorly consolidated Upper Tertiary sediments in the Western Foothills thrust belt, through slates in the Hsuehshan and western Central ranges, to greenschist grade pre-Tertiary meta- sediments in the eastern Central Range (Ho, 1988; Hovius et al., 2000). As one moves south along the Central Range exposures change from the metamorphic core towards

Eocene slates and weakly cleaved sedimentary rocks in southernmost Taiwan (Willet et al., 2003). Although the island links the Ryuku and Manila subduction systems, there is no active volcanism and exposures of igneous rocks are limited to the extinct andesitic

Tatun Volcano Group located at the northern extremity of the island and the highly limited outcrops of accreted mafic volcanics in the Coastal Range (Ho, 1988). Rivers draining these volcanic areas were not evaluated as part of this study.

Watersheds included in this study traverse a wide range of lithologies. The

Jhuoshuei watershed on average exhibits the youngest lithology, which is largely a result of its location in the Western Foothills thrust belt. Its poorly consolidated sediments have played a large role in its highly elevated annual suspended yields of ~40 Mt yr-1 (Kao et al., 2005). At the opposite end of the spectrum, the Hualien watershed is located in the heart of the metamorphic core and is largely underlain by Late Paleozoic to Mesozoic aged metamorphosed marbles and schist. The Beinan, Siouguluan, and other eastern

watersheds are underlain by lithologies encompassing a wider age scale and consist of argillites, sandstones, and shales, which have undergone varying degrees of metamorphism.

3.3.2. Climate and Topography

The island exhibits a subtropical to tropical climate with long-term basin- integrated rainfall ranging from 1600 to 3200 mm yr-1 and up to 4000 mm yr-1 in the mountains. The precipitation is highly seasonal with July-October rainfall accounting for up to 75% of the annual rainfall in the south and east (Kao and Milliman, 2009). This summer precipitation is enhanced by Taiwan’s location in typhoon alley, which results in a storm frequency of approximately four per year (Wu and Kuo, 1999). This large seasonal difference in rainfall has an immediate impact on stream discharge with most rivers generally averaging less than 10–50 m3 s-1 during low-rainfall months to as high as

1000–10,000 m3 s-1 in the summer. During typhoons discharge has been known to exceed

3 -1 20,000 m s in some large rivers (Kao and Milliman, 2009).

Taiwan’s topography has also played an important role in the geomorphology of these river systems. Eastern rivers tend to be more mountainous, with greater rainfall and runoff, which can approach or exceed 2000 mm yr-1. These factors have resulted in regularly spaced transverse rivers draining the eastern Central Range (Willemin and

Knuepfer 1994; Hovius 1996), which crosscut its structural grain and ultimately merge into three larger rivers: the Hualien, Siouguluan, and Beinan (Hovius et al., 2000). The largest watershed that drains to the west, the Jhuoshuei, exhibits a lower topographic gradient and lower basin wide runoff (~1200 mm yr-1).

3.4. Sampling and Analytical Methods

3.4.1. Sample Methodology

Water samples were collected from four locations (C-1, J-1, R-1, S-1) in July

2004, twenty-seven (27) locations (T05-10 through T05-31 and T05-33 through T05-39) in March 2005, and twenty-three (24) locations (T05-43 through T05-56; T05-60 through

T05-64; and T05-66 through T05-71) in July 2005 (Figure 3.1 and Table 3.1). Together the samples constitute a wet and dry season composite dataset for 27 sites. Several of the sites consist of mainstem and tributary samples associated with a particular river system:

Jhoshuei (4 sites), Beinan river system (8 sites), Siouguluan (6 sites), and the Hualien (3 sites). Six of the remaining seven sites are from rivers located at the southern end of the

Central Range and will be hereafter referred to as “other eastern rivers.” The remaining river, the Liwu is located at the northern end of the sampling transect. Samples were collected by hand within 1 m of the riverbank, and generally from river mouth locations well above the influence of the tidal zone (where applicable).

3.4.2. Water Analysis

Water samples were collected in new, deionized water (18 MΩ) soaked low- density polyethylene (LDPE) bottles, which were rinsed in river water three times prior to sample collection. A small aliquot of sample was used for in-field measurement of pH and temperature. Samples were stored in the dark at room temperature prior to filtration within 36 hours of collection. Samples were filtered through a 47 mm diameter (nominal pore size of 0.4μm) polycarbonate filter directly into an acid washed 60 ml LDPE bottle for cation analysis and a deionized water soaked 60 ml LDPE bottle for anion analysis.

Filter blanks were created by filtering 18 MΩ water into clean LDPE bottles using the same methods used for samples. Trip blanks were created prior to sample collection with

18 MΩ water in clean LDPE bottles. The samples, trip blanks, and filter blanks were subsequently shipped to Ohio State and chilled at ~4˚C until analysis. Upon return, each filtered sample for cation analysis was acidified to pH 2–3 with trace metal grade HNO3

+ + + + 2+ 2+ for preservation. Major ion concentrations for cations (Li , Na , NH4 , K , Ca , Mg )

- - - - - 3- 2- and anions (F , Cl , NO2 , Br , NO3 , PO4 , SO4 ), were determined via ion chromatography (IC) using a Dionex-120® ion chromatograph using the methods of

Welch et al. (1996). Precision was determined using five replicate check standards per run with relative standard deviations (RSDs) all better than ±5% and usually better than

±2%. Dissolved Si and Sr2+ concentrations were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES). External standards were used and check standards were run every 3–4 samples to account for instrument drift.

Triplicate analyses did not reveal RSDs greater than ±5%. Major and trace elemental concentrations measured in the stream water samples are provided in Tables 3.2a & b.

3.4.3. Data Interpretation

- Calculations were performed to determine alkalinity (HCO3 ) in these streams

- from atmospheric input and rock weathering. Sample HCO3 was determined via the method of Lyons et al. (1992) as the difference between Σcations in microequivalants and

Σanions in microequivalants. This method assumes little contribution to alkalinity from

- dissolved species other than HCO3 . Previous work has demonstrated that alkalinity

- values derived from this technique provide excellent estimates of HCO3 in river and stream waters (Lyons et al, 1992).

Precipitation contribution to stream water was determined via a modified method of Stallard and Edmond (1981). Based on the absence of evaporite rocks shown on

- existing geological maps for the study area, all Cl in stream water samples was determined to be from precipitation. With the lowest Cl- concentration in stream water from each season assumed to represent concentrations closest to actual precipitation, 44.6

- + + µmol/l for spring (T05-28) and 19.9 µmol/l for summer (T05-55), ratios of Cl to Na , K ,

2+ 2+ 2- Mg , Ca , and SO4 were determined using seasonal precipitation data from the Fu- shan Experimental Forest in northeastern Taiwan (E 121° 34'; N 24° 34') (Lin et al.,

+ 2+ - - 2000). Ratios of Li , Sr , F , and Br were determined from sea salt data of Bruland

(1983). Any remaining Cl- in the streamwater samples was assumed to be from the dissolution of halite (NaCl), therefore, the complementary concentration of Na+ was removed from each of the samples. A review of Taiwan geologic maps did not identify the presence of any evaporites in the watersheds, so any halite in the watersheds was assumed to be strictly from evapoconcentration. It was assumed that all the Si in the streams originates from chemical weathering (Stallard and Edmond, 1981).

Total cation contribution from silicate and carbonate weathering was determined using the average molar ratios of Ca/Na (0.24) and Mg/Na (0.45) previously determined for bulk sedimentary rock (argillite, metapelite, phyllite, and sandstone) by Lan et al.

(2002). This low Ca/Na value for Taiwan rocks is a direct result of the low Ca content in the majority of the sedimentary rocks themselves (< 2 wt %) with the exception of argillite (2.53 wt %). The high Mg ratios are likely the result of a higher content of Mg

rich minerals in the sedimentary and low-grade metamorphic rocks such as biotite and chlorite (Selveraj and Chen, 2006). Any Ca2+ and Mg2+ in excess of these respective molar ratios was attributed to the dissolution of carbonate present in the watershed system. The “corrected” Ca2+ and Mg2+ values were summed with the precipitation- corrected Mg2+, Na+, and K+ concentrations to calculate cation weathering input from silicate weathering only. The remaining “excess” Ca2+ and Mg2+ concentrations were summed to determine the cation weathering input from carbonate weathering.

Chemical erosion fluxes were determined by multiplying the precipitation- corrected TDS concentrations for the wet and dry season data by the calculated stream discharge data for the respective season and summing the values. Chemical erosion yields were determined by multiplying the precipitation corrected TDS concentrations by the mean annual stream discharge data and dividing by the watershed area. Mean annual discharge data from long-term (20+ years) river gauge records were provided by Taiwan

Water Resources Agency (WRA). Watershed areas in the calculations represent the area of the drainage basin above the respective river gauge. Silicate weathering yields were determined in similar fashion with multiplying the dissolved Si concentrations for the wet and dry season data by the calculated stream discharge data for the respective season, summing the values, and dividing the result by the watershed area.

3.5. Results and Discussion

3.5.1. Solute Geochemistry

Seasonal contrast in the dataset was apparent with mean pH values ranging between 7.82 for the spring (dry season) to the slightly more alkaline 8.40 for the summer

(wet season) (Table 3.2a and 3.2b). Water temperature also varied sharply seasonally, 30

17.2–28.6 ⁰C measured for the spring and 22.2–30.3 ⁰C measured for the summer. Two- tailed tests rejected the null hypothesis of no difference between the mean values for both pH (α = 0.05; p =7.59E-11) and temperature (α = 0.05; p =1.54E-06).

Mean discharge for the days sampled ranged from 0.29 m3/s (Dawu River) to

121.9 m3/s (Jhuoshuei River mainstem) for the dry season and 1.78 m3/s (Lijia) to 133.2 m3/s (Jhuoshuei River mainstem) for the wet season. Of the 22 sample sites with government provided discharge data, 18 sites experienced seasonal differences of discharge on the day sampled ranging from 1.1x for the Maan and Jhuoshuei rivers to 20x for the Dawu River. Of note, four of the measured streams (Chingshuei, Beinan, Wanli, and Mugua) actually experienced higher discharge values during the spring sampling period which may be result of above normal spring rainfall in 2005. A two-tailed t-test of

TDS concentrations (not corrected for precipitation) for the 18 sites which experienced an increase in seasonal discharge accepted the null hypothesis of no difference between the mean values (α = 0.05; p =0.16). This statistically insignificant difference in TDS may reflect the importance of carbonates in these systems as their weathering rates tend to increase with runoff unlike silicates which can undergo dilution above some threshold runoff value (Stallard and Edmund, 1987; Berner and Berner, 1996). While no samples included in this dataset were collected during a typhoon, a previous detailed sampling of the Jhuoshuei during Typhoon Mindulle in 2004 did not find a significant dilution of Si concentration with increasing discharge (Goldsmith et al., 2008a).

The geochemistry of the streams, after correction for precipitation input, revealed little seasonality. The precipitation-corrected total cation chemistry (TZ+) for the streams

2+ in the wet season demonstrates that Ca is the major cation from weathering and it

constitutes approximately 68% of the total cation charge, followed by Mg (21%), Na

(10%), and K (1%). All but one of the stream water samples show the importance of carbonate weathering with Ca dominating the cation abundance (in moles) with 17 samples exhibiting Ca>Mg>Na>K and 9 sites exhibiting Ca>Na>Mg>K and the one anomalous site (Chingshuei River (C-1)) exhibiting Na>Ca>Mg>K. All but one of the

- wet season samples (tributary of Jhuoshuei (S-1) exhibit HCO3 as the dominant anion

2 - 2- over SO4 , with HCO3 to SO4 ratios ranging from ~1.01 (Jhuoshuei River mainstem (J-

1)) to ~10.8x (Siouguluan River mainstem (T05-60)). The dry season samples exhibited

2+ remarkable similarity with respect to the total cation charge with Ca constituting approximately 69% of the total followed by Mg (20%), Na (9%), and K (1%). All of the stream water samples show Ca dominating the cation abundance (in moles) with 16 samples exhibiting Ca>Mg>Na>K and 11 sites exhibiting Ca>Na>Mg>K. All but two of the dry season samples (Wulu (T05-27) and unnamed tributary of the Beinan (T05-27))

- 2- - exhibit HCO3 as the dominant anion over SO4 , with the HCO3 concentration ranging

2- from ~1.73 (Lu Ye River (T05-21)) to ~9.2x (Dawu River (T05-19)) the respective SO4 concentration.

Precipitation-corrected total dissolved solids (TDS) concentrations ranged between 137 to 824 mg/kg measured during the wet season and 180 to 502 mg/kg measured during the dry season. Median TDS concentrations for both the wet and dry seasons was 317 mg/kg. The lack of difference between the seasons further stresses the relatively negligible dilution effect exhibited between the seasons and points towards the impact of carbonate weathering on these systems. The mainstem of the Siouguluan River exhibited the highest TDS concentration for the wet season (824 mg/kg) while the Wulu

River, a tributary of the Beinan, exhibited the highest TDS concentration for the dry season (501 mg/kg), exceeding those of rivers many times its size.

A statistical analysis of the precipitation-corrected dissolved load data was performed in order to determine the existence of significantly different average concentrations between the wet and dry seasons. A two-tailed t-test with a null hypothesis of no difference in mean elemental values (α = 0.05) between the seasons was conducted for all analyzed species and calculated alkalinities. The null hypothesis of no

- difference between mean ionic concentrations was rejected only for HCO3 (p = 0.08) with significantly higher concentrations observed during the wet season.

Ca:Mg ratios (after correction for precipitation) ranged from 1.4 to 8.9 for the wet season and 1.9 to 10.1 for the dry season (Table 3.3). The Beinan watershed (n = 7) exhibited the highest average Ca:Mg ratios for both the wet and dry season (6.1 and 5.6, respectively) while the Jhuoshuei watershed (n = 4) exhibited the lowest for both seasons

(1.5 and 2.0, respectively). The seasonal Ca:Mg ratios for the Siouguluan (4.0 and 3.9, respectively), Hualien (3.3, both seasons) and other eastern watersheds (3.5 and 3.7, respectively) exhibited more intermediate values compared to the these two end members. The lower values in the Jhuoshuei watershed may indicate the importance of the weathering of Mg-rich minerals in parental lithology for this watershed, such as biotite and chlorite. With the exception of the Jhuoshuei watershed, these values are well in excess of the world average of 2.4 (Harmon et al., 2009) and likely indicate the importance of calcite dissolution in these watersheds. The Ca:Mg values determined for this study are similar to those previously published for other Southeast Asian rivers (after correction for precipitation), such as Hong River (3.8) by Moon et al. (2007) and the

upper and lower Yellow River (1.7 and 1.4, respectively) by Wu et al. (2005) and Hu et al. (1982), respectively. Of note, the Yellow River has been previously proposed to be a source terrain for Taiwan’s sedimentary rocks (Lan et al., 1995).

Molar ratios of Ca/Na and Mg/Na were calculated with the precipitation-corrected water chemistry (after Gaillardet et al., 1999) in order to compare the Taiwan samples to previously established silicate and carbonate end member ratios for world’s rivers and to ratios determined for rock and riverbed sediments for Taiwan and southeast Asia (Table

3.3). All molar ratio averages and ranges were greater for the dry season than the wet season. The highest Ca/Na, Mg/Na, and HCO3/Na ratios for the wet season were observed for the Maan River, a tributary of the Hualien. Average basin scale molar ratios generally decreased from north to south, which may reflect lower carbonate exposures in these systems. All molar ranges and averages are greater than the continental silicate end member ratios determined by Gaillardet et al. (1999) and were in line with those previously determined for the Taroko Gorge by Yoshimura et al. (2001) and other southeast Asian rivers draining marine sedimentary rock of similar nature such as the

Hong (Red) River basin by Moon et al. (2007) and the Upper Huang He (Yellow River) by Wu et al. (2005). In addition all of the Ca/Na ratios were well in excess of those previously identified for silicate bedrock in Taiwan (0.23) (Selveraj and Chen, 2006) further indicating the importance of carbonate weathering in these systems.

3.5.2. Relative Input of Cations from Weathering

In an effort to determine the relative input from each component into the streamwater samples a plot of cation input from rainwater, evaoporite, carbonate and silicate weathering was constructed (Figure 3.2). The plot revealed average cation inputs 34

of 3% rain, 3% evaporite, 73% carbonate and 22% silicate for the wet season and 1% rain, 2% evaporite, 63% carbonate, and 34% silicate for the dry season. Precipitation input to the total cation flux revealed no regional pattern in the total percentage. The dominance of carbonate weathering in the river systems was apparent with cations from carbonate weathering ranging from 45% to 83% of the cation total in the wet season upwards to 44% to 93% of the cation total in the dry season. These ranges are in agreement with other mixed silicate-carbonate lithologies in southeast Asia such as the

~30% calculated for the Han River basin by Ryu et al. (2008) and the 4 to 95% determined for the Hong (Red) River basin by Moon et al. (2007). The Beinan watershed exhibited the highest basin average carbonate cation input for the wet season (60%) while the Hualien watershed exhibited the highest basin average value for the dry season

(80%). The Jhuoshuei watershed exhibited the lowest seasonal average basin carbonate inputs of 47 and 65%, respectively. While a review of Taiwan Ministry of Economic

Affairs (1974) 1:250,000 scale geology maps did not identify the presence of carbonate lithology in the watershed, it is likely that carbonate is being released in the form of disseminated trace calcite, which has been previously observed to contribute substantially to cation dissolved load in active margin locales, such as the Himalayas and New Zealand

(Jacobson and Blum, 2000; Jacobson and Blum, 2003). Calcareous schist has also been proposed as a source for carbonate weathering input previously observed in the Liwu watershed in Taiwan by Yoshimura et al. (2001).

In an effort to delineate the importance of carbonate weathering in these streams, calcite saturation indices were calculated for the both the wet and dry seasons using the software program PHREEQC (Parkhurst and Appelo, 1999). The calculations revealed

24 of the 27 dry season samples (89%) and 15 of the 27 wet season samples (56%) were saturated with respect to the calcite. Interestingly enough, watersheds with no known carbonate exposures such as the Jhuoshuei and five of the six “other eastern watersheds” showed saturation with respect to the calcite in several of the samples for both the west and dry season further indicating the importance of disseminated calcite in these systems.

A marked increase in the silicate weathering portion of the total cation input was observed between the spring and summer sampling periods in all but one of the sampling sites (Jinlung River (T05-17/ T05-45). Total seasonal increases ranged from 8% (other eastern rivers) to 20% (Jhuoshuei) of the total cation input. These results are in-line with moderate chemical weathering previously observed in Taiwan river, lake, and off-shore sediments (Selveraj and Chen, 2006).

3.5.3. Intensity of Silicate Weathering

The Si:(Na+K-Cl) ratio was used (after Stallard and Edmond, 1987 and Edmond et al., 1995) to determine the extent and/or intensity of silicate weathering occurring in the Taiwan watersheds. Watersheds with lower ratios are indicative of less intensive silicate weathering while higher Si:(Na+K-Cl) are presumably indicative of an increased regional weathering rate (Table 3.4). The overall Si:(Na+K-Cl) values for the Taiwan rivers were indicative of low intensity of silicate weathering with the Hualien watershed exhibiting the highest Si:(Na+K-Cl) for both the wet and dry seasons (0.7 and 0.8, respectively) and the Jhuoshuei exhibiting the lowest pattern for each season (0.3, both seasons).

The watershed averages for both the wet and dry season increased from south to north. The relatively low Si:(Na+K-Cl) concentrations for the samples from the southern 36

portions of the Central Range are suggestive of mechanical weathering of soils in the upland areas of these watersheds exposing new surfaces from which easily soluble Na+ and K+ could leach. This may result from a combination of higher uplift rates for this area, particularly for the other eastern watersheds and a decrease in relative rock hardness and post uplift age from north to south along the sampling transect (Willet et al., 2003).

Furthermore, numerous landslides recorded in the Jhuoshuei watershed as a result of the

M 7.9 1999 Chi-Chi Earthquake have resulted in single-storm sediment fluxes well in excess of the pre-earthquake annual sediment delivery value even five years later

(Milliman et al., 2007). This resulting exposure of fresh surfaces likely supplies an abundance of new surfaces which can be readily weathered and is probably responsible for the highly elevated solute fluxes previously observed for this watershed (Goldsmith et al., 2009a). Alternatively the higher Si:(Na+K-Cl) ratios observed for the northern watersheds could be the result of a greater water-rock interaction time as has been proposed from a deep groundwater component for the Liwu watershed (Camels et al.,

2009) or from the fact that underlying lithology in the younger rocks is composed largely of sediments that have already experienced previous cycle(s) of weathering and are not fresh bedrock. This latter idea, along with an average shale to kaolinite Si:(Na+K-Cl) value of 1.0 (Huh et al., 1998), may be responsible for the low values of this ratio observed in a marine sedimentary draining headwater tributary of the Yangtze by Yoon et al. (2008).

3.5.4. Importance of pyrite weathering

Pyrite oxidation has been found to contribute significantly to the dissolved fluxes of other southeast Asian rivers underlain by varying quantities of marine sedimentary 37

rocks (Moon et al., 2007; Ryu et al., 2008). The process is important as pyrite supplies not only sulfate into water but also H+ ions via the following simplified reaction:

2- + FeS2 + 15/4 O2 +7/2 H2O  Fe(OH)3 + 2SO4 + 4H (Eq. 3.1)

Therefore, the weathering of one mole pyrite will result in the creation of two moles of sulfuric acid (Stumm and Morgan, 1996). Furthermore, this resultant creation of H2SO4 in the watersheds will lead to chemical weathering not derived from atmospheric CO2 and if not properly accounted for can result in an overestimate of CO2 drawdown. The presence of pyrite has been previously documented in black schists of the metamorphic basement rocks of eastern Central Range by Juang (1984) and Ho (1988) and the marine sedimentary rocks of the Western Foothills Fold and Thrust Belt by Kao et al. (2004).

Pyrite oxidation and resultant carbonate weathering via H2SO4 has been proposed for the waters draining the Taroko Gorge in eastern central Taiwan (Yoshimura et al., 2001).

2- Finally, it is assumed that no SO4 in these systems originates from gypsum dissolution and there is little atmospheric component from pollution.

A series of plots was constructed in order to determine the influence of pyrite dissolution on these systems. A ternary diagram of HCO3-Si-(Cl-SO4) in charge equivalent units shows all the data falling along the alkalinity and Cl-SO4 axis indicating the importance of pyrite dissolution along with carbonate weathering (Figure 3.3a). The fact that most data are off the line itself indicates that silicate weathering is taking place in these systems as well. Furthermore, the fact that none of the points plots near the Cl-

SO4 apex indicates the lack of gypsum dissolution in these systems. A complementary ternary diagram of Ca-Mg-(Na+K) in equivalents shows the importance of Ca in these

systems and to a lesser extent Na +K (Figure 3.3). Subsequent plots of Ca2+ and Mg2+

- versus HCO3 in μeq/l in stream water corrected for precipitation represents the upper limit of bicarbonate input from the weathering of carbonate rocks (Figure 3.4a). All sites plotted at varying enrichments on the Ca2+ and Mg2+ side of the 1:1 line, indicating the

2- 2+ importance of SO4 in balancing out the cation charge in these systems. A plot of Ca

2+ - 2- and Mg versus HCO3 and SO4 in μeq/l in stream water corrected for precipitation

2- confirmed the role of SO4 , as almost all sites plotted on or in close proximity of the 1:1 line which suggests the importance of pyrite oxidation in these systems (Figure 3.4b).

2- Many of the data points exhibited enrichment on the HCO3 + SO4 side of the 1:1line indicating the importance of Na+ + K+ or other cations from silicate weathering in balancing out the anion charge. Similar charge equivalent relationships between

2+ 2+ - 2- concentrations of Ca and Mg versus HCO3 and SO4 were previously observed for stream waters draining the Taroko Gorge by Yoshimura et al. (2001).

3.6. Flux Determination and Potential CO2 Consumption

3.6.1. Chemical Weathering Rates

Chemical weathering yields, based on a compilation of TDS and calculated

-2 -1 discharge values for each season, ranged from 331 to 10,393 tons km a (Table 3.5).

The highest value was identified with the Wulu River, a tributary of the Beinan River.

Field reconnaissance identified hydrothermal alteration of outcrops within the watershed, however, no direct spring input was observed to sample. Regardless, it is likely this sample is heavily influenced by hydrothermal activity. After the anomalously high value for the Wulu, the next highest observed rate was observed for the mainstem of the Beinan

River (1825 tons km-2a-1). These rates are high and no doubt heavily influenced by input 39

from carbonate weathering. Average basin scale watershed chemical weathering yields

-2 -1 3 -2 -1 ranged as follows: Beinan (2602 tons km a ), Siouguluan (671 x 10 tons km a ), other

3 -2 -1 3 -2 -1 eastern rivers (618 x 10 tons km a ), Jhuoshuei (562 x 10 tons km a ), and the

3 -2 -1 Hualien (513 x 10 tons km a ). Two tailed t-tests between watersheds (ten possible scenarios) revealed no significant differences of average basin-scale chemical weathering yields between the watersheds. Furthermore, the calculated chemical weathering yields for these watersheds are remarkably similar to an average chemical weathering rate previously determined by Li (1976) for rivers draining the Central Range of 650 tons

-2 -1 km a .

In an attempt to evaluate potential controls on chemical weathering in these watersheds, the calculated chemical weathering yields (excluding those for the Wulu) were compared with the following parameters: basin average mean annual rainfall, basin average runoff, and annual suspended sediment discharge rates. A slight positive correlation was observed between chemical weathering yield and basin average mean annual rainfall (r2 = 0.25, p = 0.01) and basin average runoff (r2 = 0.34, p = 0.004). The paucity of Taiwan WRA suspended sediment data collection centers for the sampled watersheds meant that the comparison with annual suspended sediment yields was limited to the mainstem sites of the Jhuoshuei, Beinan, and Siouguluan Rivers. While no correlation was observed between the chemical weathering yields and annual suspended sediment yields (r2 = 0.0.01, p = 0.7) the high p-value suggest more data are necessary to obtain an accurate determination. The observed chemical weathering yield for the

Jhuoshuei fell off the trend of increasing silicate weathering rates with suspended sediment yields. The extraordinary high suspended sediment yield for this river (40 Mt

and approximately twice that for the next highest watershed) may point to some critical level where silicate weathering no longer increases with corresponding physical erosion rates.

3.6.2. Silicate Weathering Yields

Determination of silicate weathering yields for each of the watersheds was based on a compilation of summed cation concentrations (corrected for carbonate weathering) and calculated discharge values for each season. Silicate weathering yields ranged from

5.8 tons km-2a-1 for the Mugua River (a tributary of Hualien River) to 149 tons km-2a-1 for a tributary of the Jhuoshuei River (Table 3.5). Average basin scale silicate weathering

-2 -1 -2 -1 yields were as follows: Beinan (71 tons km a ), Jhuoshuei (67 tons km a ), other

-2 -1 -2 -1 eastern rivers (75 tons km a ), the Siouguluan (27 tons km a ), and the Hualien (18

-2 -1 tons km a ). Silicate weathering rates surpass those previously determined for sedimentary and metamorphic terrains of New Zealand, 2.4–15.1 tons km-2 yr-1 (Jacobson and Blum, 2003; Lyons et al., 2005; Goldsmith et al., 2008b), other southeast Asian rivers such as the Hong (red) River in China, 0.7–10.6 tons km-2 yr-1(Moon et al., 2008), and the Han River in China, 0.1–34.2 tons km-2 yr-1 (Ryu et al., 2008) (Table 3.6).

No systematic relationship was observed between silicate weathering yield and basin average mean annual rainfall (r2 = 0.002, p = 0.9) and basin average runoff (r2 =

0.03, p = 0.4) while a slightly positive correlation was observed between the silicate weathering yields and annual suspended sediment yields (r2 = 0.26, p = 0.7). However, the high p-values suggest more data are necessary to obtain an accurate determination.

Given the limited suspended sediment data, the silicate weathering yields were subsequently compared to the continuous elevation of divide for the catchment, uplift rate 41

(mm/yr) from Dadson et al. (2003), and the relative post-uplift age of the landscape from

Willet et al. (2003) in order to delineate a the effects of physical erosion on these systems

(Figure 3.5A). The plot did not show any correlation between silicate weathering yields and elevation of watershed divide (r2 = 0.06, p = 0.8) nor observable trend with uplift rates. Of note, the three highest silicate weathering yields were located in the section exhibiting the middle range of post uplift age, which may point to some ideal relationship among relief, age of rocks exposed, or their mineralogy.

3.6.3. Carbonate Weathering Yields

Carbonate weathering yields for each of the watersheds was based on a compilation of summed excess Ca2+ and Mg2+ concentrations (corrected for silicate weathering) and calculated discharge values for each season. Carbonate weathering yields ranged from 48.7 tons km-2a-1 for the Jinlung River to 1840 tons km-2a-1 for a tributary of the Wulu River (Table 3.6). Average basin-scale carbonate weathering yields for the watersheds showed little variation from silicate weathering yields and ranged as

-2 -1 -2 -1 follows: Beinan (429 tons km a ), Jhuoshuei (118 tons km a ), the Siouguluan (99

-2 -1 -2 -1 -2 -1 tons km a ), other eastern rivers (86 tons km a ), and the Hualien (81 tons km a ).

These yields are at the upper end of those previously observed for other rapidly uplifting regions underlain by sedimentary and metamorphic terrains such as the Southern Alps of

New Zealand, southeast Asia, and headwater watersheds in the Himalayas (Table 3.6).

No systematic relationship was observed between carbonate weathering yields and basin average mean annual rainfall (r2 = 0.03, p = 0.3), basin average runoff (r2 =

0.04, p = 0.7), or annual suspended sediment yields (r2 = 0.06, p = 0.8). Plots of silicate weathering yields (Figure 3.5b) revealed no correlation of carbonate weathering yields 42

with elevation of watershed divide (r2 = 0.06, p = 0.9) nor observable trend with uplift rates. However, a correlation was observed between carbonate weathering yields and the middle level of post-uplift age with a t-tailed t-test rejecting the null hypothesis of no difference of mean carbonate weathering yields between the two landscape ages (α =

0.05; p = 3.88E-08).

3.6.4. H2SO4 Input to Weathering

In order to determine atmospheric CO2 drawdown were from silicate weathering in these systems, the calculated cation yields needed to be corrected for weathering input

2- originating from sulfuric acid. All the SO4 in these systems (after correction for precipitation) was assumed to derive from pyrite oxidation rather than from gypsum

2- dissolution. A correction coefficient for SO4 was determined for each streamwater sample after the method of Galy and France-Lanord (1999) using the following formula:

2- - XSO4 = [SO4]sulfide/([SO4 ]tot + [HCO3 ]tot) (Eq. 3.2)

where [SO4]tot represents the total input of sulfide in each sample from both rainfall and rock weathering, [SO4]sulfide represents the remaining sulfide in each sample after

- - correction for precipitation, and [HCO3 ]tot represent all inputs of HCO3 in each sample.

The method assumes sulfuric and carbonic acids alter carbonate and silicate without any selectivity. While higher XSO4 values would be attributed to a greater proportion of pyrite weathering in the respective watershed, no distinguishable patterns of average basin XSO4 values for the four watersheds were observed. Average basin scale XSO4 values showed that H2SO4 weathering accounted for 13–33% of the solute flux for the wet season and

21–41% for the dry season. The XSO4correction coefficient was subsequently multiplied 43

by the precipitation corrected concentration for Na+, K+, Ca2+, and Mg2+ in order to delineate the weathering contribution derived solely from sulfuric acid. The remaining cation concentrations in each streamwater sample are assumed to derive solely from weathering via carbonic acid.

3.6.5. CO2 Consumption Yields

Average basin-scale CO2 consumption yields from silicate weathering (фCO2sil) for each of the watersheds was determined by summing the cation concentrations from silicate weathering in equivalents after correction for H2SO4weathering. The highest

фCO2sil value was identified with the Wulu River, a tributary of the Beinan River (2738 x

3 -2 -1 10 moles km a ) (Table 3.5). As previously discussed it is likely this watershed is affected by a hydrothermal component and some of the weathering is likely derived from

CO2 originating from within the Earth. Therefore, the sample is not considered to be representative of CO2 drawdown solely from atmospheric CO2. The next highest values

3 -2 -1 were recorded for the mainstem of the Beinan River (2640 x 10 moles km a ) and a

3 -2 -1 tributary of the Beinan, the DaLuen (2244 x 10 moles km a ), respectively. Average basin scale watershed values for the watersheds revealed values decreased in the

3 -2 -1 3 -2 - following order: Beinan (1793 x 10 moles km a ), Jhuoshuei (1252 x 10 moles km a

1 3 -2 -1 3 -2 ), other eastern rivers (1050 x 10 moles km a ), the Siouguluan (845 x 10 moles km

-1 3 -2 -1 a ), and the Hualien (509 x 10 moles km a ). Of note, the order of the basin scale averages did not deviate from those from silicate weathering despite the corrections for

H2SO4 suggesting all watersheds were similarly affected by pyrite dissolution. Two tailed t-tests between the watersheds (ten possible scenarios) revealed significant differences of average basin-scale фCO2sil values between the Beinan and Hualien ( = 44

0.05, P = 0.01) and the Beinan and Siouguluan ( = 0.03, P = 0.01) watersheds only. The higher average basin-scale фCO2sil averages for the Beinan watershed are likely reflective of the relative lack of carbonate lithology in its basin compared to the abundance of carbonate rocks in the Hualien.

Average basin-scale CO2 consumption yields from carbonate weathering

2+ 2+ (фCO2carb) for each of the watersheds was determined by summing the Ca and Mg concentrations from carbonate weathering in equivalents after correction from

3 -2 -1 H2SO4weathering. The фCO2carb values ranged from 2686 x 10 moles km a

5 -2 -1 (Chingshuei River a tributary of the Jhuoshuei) to 121 x 10 moles km a (Wulu River).

The фCO2carb values represented anywhere between 1.4x to 18.3x the respective фCO2sil value and clearly show the dominance of carbonate weathering in these systems. With the exception of the Beinan, the average basin scale фCO2carb averages for the watersheds showed the impact of H2SO4 on carbonate weathering by the different order of carbonate

3 -2 -1 weathering yields: Beinan (excluding the Wulu) (6496 x 10 moles km a ), the

3 -2 -1 3 -2 -1 Siouguluan (5284 x 10 moles km a ), other eastern rivers (4083 x 10 moles km a ),

3 -2 -1 3 -2 -1 the Hualien (3735 x 10 moles km a ), and the Jhuoshuei (5284 x 10 moles km a ).

Regardless of the low фCO2sil/фCO2carb ratios for the majority of Taiwan rivers sampled, the фCO2sil values for Taiwan still remain highly elevated over the average

3 -2 -1 value for the 60 largest rivers of 63 x 10 moles km a determined by Gaillardet et al.

(1999) and fall amongst the highest yet reported for sedimentary and metamorphic terrains on other HSIs by Lyons et al. (2005) and Jacobson and Blum (2003) (Table 3.6).

Furthermore, the фCO2sil values are in excess of what is being observed today for both

3 headwater regions and large catchment draining the Himalayas (1.5 – 641 x 10 moles

-2 -1 km a ) (Gaillardet et al., 1999; Wu et al., 2005; Wolff-Boenisch et al., 2009). However, the фCO2sil values presented herein should be viewed with caution as these rates do not account for carbonic acid weathering driven solely from out-gassing of CO2 derived from metamorphic processes. Previous studies of spring and stream waters in the Liwu watershed in eastern Taiwan indicate that deep water interaction with metamorphic derived CO2 can account for substantial portions of the total alkalinity (Yoshimura et al.

2001) and anywhere between 20 to 40% of the total chemical weathering flux for this system (Calmels et al., 2009). While this input was attributed to water movement through deep-seated faulting it is currently unclear whether deep CO2 cycling is a local or regional process. Furthermore, even with a 40% reduction, the фCO2sil rates presented herein would still be at the upper end of those determined for non-volcanic active margin

3 -2 -1 settings (53–1584 x 10 moles km a ).

However, while the Taiwan silicate weathering and CO2 drawdown values are elevated over world averages, they likely represent the upper end of what could be expected from a non-volcanic active margin setting. As the island slowly sheds its soft sedimentary cover to expose a metamorphic complex, the physical erosion rates would slow or at least level off along with a concomitant decrease in chemical weathering rates.

If the early stage collision of the Himalayas occurred in a setting such as Taiwan rather than in the Sumatra type subduction complex, then uplift of the Himalaya would likely not produce the supposed silicate weathering rates and associated CO2 drawdown attributed to the early growth of the mountain chain. Therefore, this study further points to the importance of the weathering of volcanic terrains as potential tipping points for climate over geologic time.

3.7. Conclusions

This study investigated geochemical fluxes from the subtropical HSI of Taiwan.

Seasonal sampling of streamwater for both major and trace elements show the strong influence of carbonate weathering on these systems. The limited exposure of carbonate units in some of these watersheds points to the importance of weathering of disseminated calcite. Comparisons of silicate weathering yields to potential controlling parameters revealed slight positive correlations between chemical weathering rates and basin average mean annual rainfall and average basin runoff and a slight positive correlation of silicate weathering rates with annual suspended sediment yields. Silicate and carbonate weathering yields also revealed varying relationships with post-uplift age of the landscape, potentially pointing to some ideal relationship among relief, age of rocks exposed, their mineralogy, and weatherability. However, H2SO4 weathering, originating from the dissolution of pyrite, accounts for up to a third of the total chemical weathering in these systems. After correction for H2SO4 weathering, calculated CO2 consumption values from silicate weathering and carbonate still remain elevated compared to world average. These CO2 fluxes presented herein should be viewed with some caution as they do not account for weathering originating from metamorphic CO2. Regardless, the silicate weathering rates and associated CO2 drawdown fluxes likely represent the upper limit for a non-volcanic active margin setting. If Taiwan represents an analogy for early stage Himalayan uplift, then early stage Himalayan weathering fluxes would be similar to present day values for Taiwan and only slightly elevated with average present day value for the world’s major rivers.

3.8. Tables

Table 3.4 Taiwan Si:(Na+K-Cl) ratios normalized ratios in stream water.

3.9. Figures

Figure 3.1 Map of Taiwan showing stream water sampling locations.

Figure 3.3(a) Ternary diagram of Ca-Mg-(Na+K) in μeq/l in stream water.

Figure 3.3(b) Ternary diagram of HCO3-Si-(Cl-SO4) in μeq/l in stream water.

+2 +2 - Figure 3.4(a) Plot of Ca + Mg vs. HCO3 in μeq/l in stream water (corrected for precipitation).

+2 +2 - -2 Figure 3.4(b) Plot of Ca + Mg vs. HCO3 and SO4 in μeq/l in stream water (corrected for precipitation).

Figure 3.5(a) Plot of silicate weathering rates plotted vs. latitude and altitude of watershed divide (top) and uplift rate (bottom). Uplift rates are taken from Dadson et al. (2003). Relative post uplift age of the landscape is divided into three sections with age decreasing from gray to white (from Willet et al., 2003).

Figure 3.5(b) Plot of carbonate weathering rates plotted vs. latitude and altitude of watershed divide (top) and uplift rate (bottom). Uplift rates are taken from Dadson et al. (2003). Relative post uplift age of the landscape is divided into three sections with age decreasing from gray to white (from Willet et al., 2003).

CHAPTER 4

STREAM GEOCHEMISTRY, CHEMICAL WEATHERING AND CO2 CONSUMPTION POTENTIAL OF ANDESITIC TERRAINS, DOMINICA, LESSER ANTILLES

4.1. Abstract

Recent studies of chemical weathering of andesitic-dacitic material on high standing islands (HSIs) have shown these terrains have some of the highest observed rates of chemical weathering and associated CO2 consumption yet reported. However, the paucity of stream gauge data in many of these terrains has limited determination of chemical weathering product fluxes. In July 2006 and March 2008, stream water samples were collected and manual stream gauging was performed in watersheds throughout the volcanic island of Dominica in the Lesser Antilles. Distinct wet and dry season solute concentrations reveals the importance of seasonal variations on the weathering signal. A cluster analysis of the stream geochemical data shows the importance of parent material age on the overall delivery of solutes. Observed Ca:Na, HCO3:Na and Mg:Na ratios suggest crystallinity of the parent material may also play an important role in determining weathering fluxes. From total dissolved solids concentrations and mean annual discharge calculations chemical weathering yields of (6–106 t km-2 a-1) were calulated, which are similar to those previously determined for basalt terrains. Silicate fluxes (3.1–55.4 t km-2

-1 3 -2 -1 a ) and associated CO2 consumption (190–1575 x 10 mol km a ) determined from this

study are amongst the highest determined to date. The calculated chemical fluxes from this study confirm the weathering potential of andesitic-dacitic terrains and that additional studies of these terrains are warranted.

4.2. Introduction

Over geologic time scales the Earth’s climate has been controlled by the balance of CO2 input into the atmosphere from volcanism and contact metamorphism and removal from silicate weathering plus burial of terrestrially derived organic carbon

(Berner, 1983; Raymo and Ruddiman, 1992). In order to determine controls on silicate weathering, much attention has focused on the determination of an accurate present day annual global carbon budget. A recent attempt to quantify CO2 drawdown via silicate weathering from the world’s 60 largest rivers by Gaillardet et al. (1999) has demonstrated the effect of high physical weathering rates on correspondingly high chemical weathering rates. The results show small mountainous rivers at active margins may play an important role in the silicate weathering cycle and has drawn much attention to the weathering of high standing islands (those whose headwaters lie greater than 1,000 m above sea level), which have some of the highest physical erosion rates observed to date (Milliman and

Syvitski, 1992; Hicks et al., 1996, Dadson et al., 2003; Milliman et al., 2007). Initial chemical weathering studies from sedimentary and metamorphic terrains at these settings have revealed some of the highest silicate weathering rates recorded (Carey et al., 2002;

Jacobson and Blum, 2003; Lyons et al., 2005) and rival only those previously determined for volcanic terrains (Dessert et al., 2001; Louvat and Allègre, 1997).

The role of volcanic terrains on weathering rates has been investigated in studies of the flood basalts of the Deccan traps in west-central India (Dessert et al., 2001) and the

basaltic island of Réunion in the Indian Ocean (Louvat and Allègre, 1997), where the easily erodible lithology has yielded some of the highest silicate weathering and CO2 consumption rates recorded (580–2540 x 103 mol km-2 a-1 and 1300–4400 x 103 mol km-2 a-1, respectively). In fact, silicate weathering rates for basaltic terrains are so high that they are estimated separately from granitic rocks when determining global CO2 consumption rates (Gaillardet et al., 1999; Dupré et al., 2004; Dessert et al., 2003). A recent study of the andesitic-dacitic volcanic terrain of the Taranaki region in New

3 Zealand determined silicate weathering rates and CO2 consumption (217–2926 x 10 mol km-2 a-1) in the range previously determined for basaltic terrains (Goldsmith et al., 2008).

Another study recently conducted on the volcanic islands of Guadeloupe and Martinique confirms the weathering potential of andesitic volcanic rock (Rad et al., 2006). Those results suggest the weathering potential of other HSIs (e.g., Dominica, Philippines, and

Indonesia), which are covered by significant quantities of andesitic-dacitic material, should be investigated as their importance in global chemical weathering, and hence CO2 consumption, greatly exceeds their percentage of global extent.

However, these high weathering rates observed for andesitic terrains present a conundrum. Previous compilation of laboratory weathering rates for igneous rock has shown andesites weather an order of magnitude slower than basalts (Wolf-Boenisch et al., 2006). The laboratory results were attributed to the corresponding increase of interlinked Si-O bonds as silicates become more evolved. However, as shown by

Goldsmith et al. (2008) the effects of the crystallinity of tephra in andesitic terrains may play an important role in observed field weathering rates. Effects of crystallinity on chemical weathering rates for basalt on Iceland has been previously documented by

Stefánsson and Gíslason (2001) where the presence of basaltic glass was found to enhance fluxes of Na, Si, Ca, F, and S by a factor of 2 to 5 at a constant runoff factor and vegetative cover. These studies suggest that in areas with an abundance of this andesite material, high mean annual rainfall, and means of removal significant fluxes and CO2 consumption could be observed.

The volcanic island of Dominica in the Lesser Antilles, with an annual rainfall up to >10,000 mm yr-1 (Reading, 1991) along with one of the highest river densities on Earth

(Walsh, 1985), provides an ideal field setting for the chemical weathering of andesitic material. The island has experienced an evolution of volcanic activity beginning with

Miocene age submarine to aerial basaltic flows (Wills, 1974; Bellon, 1988) laying the foundation for the abundant andesitic-dacitic volcanic activity since the Pliocene (Wills,

1974; Bellon, 1988; Sigurdsson and Carey, 1991; Lindsay et al., 2005). The latter explosive activity has culminated with three major periods of Plinian activity within the last 100 k.y. producing tens of km3 of pyroclastic material (Smith et al., 2003). This subsequent production of pyroclastic deposits with distinct ages in several of the island’s watersheds provides an ideal opportunity to determine the impact of material residence time on weathering yields. Previous studies of riverine nutrient export and availability chemistry in Dominica have consisted solely of preliminary studies (Hart and Hart, 1969;

McDowell et al., 1990) which have suggested the abundance of this readily weatherable volcanic material as the source for the high quantities of solutes observed (McDowell et al., 1995).

The purpose of this investigation is to determine chemical weathering fluxes and

CO2 consumption in watersheds hosted solely in andesitic deposits on the island of

Dominica. In order to do so, manual gauging of streams was conducted so that wet and dry season fluxes could be determined and summed to estimate the annual weathering fluxes. Delineation of seasonal hydrology and solute chemistry signatures provides insight to the predominant weathering processes. Geochemical analyses of the water samples were combined with the manually obtained hydrological data to estimate chemical fluxes and CO2 consumption via silicate weathering. Furthermore, determination of the effects of material age on weathering fluxes will aid in the estimation of chemical weathering rates and CO2 consumption for other locales underlain by andesitic deposits. The resulting fluxes were compared to existing data for basaltic terrains in order to determine whether a separate quantification of fluxes for andesitic terrains is also warranted when calculating global CO2 consumption from silicate weathering.

4.3. Study Area Background

4.3.1. Geology

The island of Dominica (15° 25' 00" N 61° 20' 00" W) is located in the center of the 850 km long Lesser Antilles volcanic chain, which extends from the island of Saba in the north to the island of Grenada in the south. The chain marks the westward subduction of the North American plate beneath the Caribbean plate and has been previously described as the agglomeration of three well-defined segments, each with a distinct angle and rate of subduction, and subsequent form of magmatism (Smith et al., 1980).

Dominica is located in the central segment, which is characterized by a high angle of subduction (45º to 50º) at a convergence rate of 2cm yr-1 for the last 30 Ma (Rosencrantz and Sclater, 1986). This relatively high angle of subduction combined with Dominica’s 65

extensive crustal building activity through volcanism has been hypothesized to allow for the emplacement of large shallow-level batholith in the arc crust. This shallow level chamber may be responsible for the characteristic voluminous andesitic dacitic volcanism observed throughout Dominica since the Pleistocene (Sigurdsson and Carey, 1981).

Volcanism on Dominica (based on K-Ar and radiocarbon dates) can be divided into four units: Miocene, Pliocene, Older Pleistocene, and Younger Pleistocene-Recent

(Lindsay et al., 2005). The oldest surfical volcanic rocks on Dominica date to ~7 Ma and consist of deeply incised low-K basalt and breccias limited to the eastern portion of the island (Wills, 1974; Bellon, 1988). During the Pliocene the magmas began to evolve, and by ~3.7 Ma, large basaltic to basaltic andesitic stratovolcanoes and their associated pyroclastic deposits dominated the volcanism (Wills, 1974; Bellon, 1988; Sigurdsson and

Carey, 1991). These now extinct, deeply dissected centers and their associated deposits created the base for the major Pleistocene volcanic centers throughout the island. Older

Pleistocene rocks consisting of Pelean domes and associated aprons of block and ash flow deposits are limited to the north part of the island, with activity centering around

Morne aux Diables and Morne Diablotins (Lindsay et al., 2005).

At approximately 1 Ma, a major shift in explosive activity occurred with volcanism moving to the southern portion of the island culminating with three major periods of Plinian activity within the last 100 k.y. These eruptions are estimated to have

3 produced tens of km of pyroclastic material associated with Morne Diablotins, and the calderas of Morne Trois Pitons and Wotten Waven/Micotrin (Smith et al., 2003). The ~28

Ka eruption at the Trois Pitons Micotrin volcanic center was particularly significant

(largest in the Antilles in the last 200 k.y.) as it produced ignimbrite and welded tuff

deposits, which are presently exposed in the Roseau, Layou, and Rosalie river valleys

(Sigurdsson, 1972; Sigurdsson and Carey, 1991). In fact, the eruption was so voluminous it resulted in submarine pyroclastic flow deposition over 250 km into the Grenada back- arc basin to the west and ash fall deposits throughout the North Atlantic to the east of the arc (Carey and Sigurdsson, 1980). Other notable Quaternary pyroclastic deposits include the >22 Ka to >40 Ka Grand Savanne Ignimbrite from Morne Diablotins (Sparks et al.,

1980) and the 39 Ka Grand Bay Ignimbrite from the Trois Pitons-Micotrin volcanic center (Lindsay et al., 2003). Deposits of the Grand Savanne Ignimbrite extend outward in five radial tongues from Diablotin (Sparks et al., 1980) while outcrops of the Grand

Bay ignimbrite can be found in the Geneva river valley (Lindsay et al., 2003). At present, there are seven active centers located on the southern end of the island, and nearly all of these areas have been active in the last 10 Ka (Lindsay et al., 2005).

Sedimentary formations in Dominica are limited to raised Pleistocene age volcanic conglomerates and coral reef limestone on discontinuous portions of the west coast, which were exposed due to volcanism-related uplift (Sigurdsson, 1972; Sigurdsson and Carey, 1991).

4.3.2. Climate and Hydrology

Dominica can be classified as having a humid tropical climate, however, microclimates can range from highly seasonal on the leeward coast and weakly seasonal on the windward coast to perennial wet in the mountainous interior (Walsh, 1980).

Prevailing trade winds from the east are responsible for this orographic precipitation effect with rainfalls varying from 2,000 mm yr-1on the east coast to 1,000 mm yr-1on the west coast (Reading, 1991). Annual values in the interior mountain rainforest have been 67

-1 found to exceed 10,000 mm yr (Reading, 1991). A review of long-term monthly average rainfall data for the Melville Hall Airport (20 yr record) recorded by NOAA, confirmed a wet season from June to December and a dry season from January to May. Mean monthly temperature ranges from 24.8ºC in February to 27.4ºC in July and August

(http://www.climate-charts.com).

The island experiences a hurricane frequency of once every 15 yrs (Neumann et al., 1978) and can also be affected by systems that pass in the nearby vicinity that result in abundant rainfall. The last direct hit from a major tropical event was Hurricane David in 1980, which traversed the south-central portion of the island. Wind gusts from the storm topped 200 mph and caused considerable tree fall ranging 30–60% in the center and 60–100% in the south of the island (Rouse et. al., 1986). This defoliation, combined with rainfalls of 185–200 mm from Hurricane David along with an additional 300 mm from Hurricane Frederic, which passed just to the north of the island approximately one week later, saturated the landscape and resulted in numerous landslides throughout the island (Walsh, 1982). Despite the low frequency of hurricanes, Hurricane Dean passed to the south of the island during August, 2007.

Dominica’s abundant rainfall, combined with its mountainous topography, has resulted in one of the largest river densities on Earth (Lindsay et al., 2005). Rivers tend to form in an outward radial pattern from volcanic centers. The slopes of these cones can be exceptionally steep (> 40º), resulting in fast flowing streams with steep gradients capable of incising bedrock (Reading, 1991). Type and age of volcanic material has also been found to play a role in river development. A previous study on drainage densities in

Dominica by Walsh (1985) revealed slower rates of drainage network evolution in

younger materials (late Pleistocene to present), such as dome deposits and pyroclastics, as a result of their higher permeability. With the exception of limited United States

Geological Survey (USGS, 2007) gauging records of the Layou River between 1984–

1990, no stream flow records were available for this study

4.3.3. Topography and Soils

2 Dominica covers 751km of extremely mountainous terrain and has the roughest terrain of any island in the Lesser Antilles, with no contiguous area greater than 1km2 being flat (Lindsay et al. 2005). An unbroken chain of young volcanic centers, nine of which are greater than 1000 m, forms the backbone of the island. The volcanoes include the second highest peak in the Lesser Antilles and the highest peak in Dominica, Morne

Diablotin (1421m) (Sigurdsson and Carey, 1991). Valley slopes, especially in the south, frequently exceed 40º, and directly result from the youthful volcanic relief associated with mid-Pleistocene uplift (Wills, 1974) and the characteristically high drainage density

2 for Dominica covers 751km of mountainous terrain with no contiguous area greater than

1km2 being flat (Lindsay et al. 2005). An unbroken chain of young volcanic centers, nine of which are greater than 1000 m, forms the backbone of the island. The volcanoes include the second highest peak in the Lesser Antilles and the highest peak in Dominica,

Morne Diablotin (1421m) (Sigurdsson and Carey, 1991). Valley slopes, especially in the south, commonly exceed 40º, and directly result from the youthful volcanic relief associated with mid-Pleistocene uplift (Wills, 1974) and the characteristically high drainage density for the island (Walsh, 1985). Older topographic features such as the large numbers of collapsed domes also contribute to the ruggedness of the island.

The relatively young age of both the parent lithology (Figure 4.1B) and the landscape (as a result of landslides) has allowed only a limited time for soil development

(Reading, 1991). The relatively uniform and recent andesitic-dacitic eruptive history has also led to variations in rainfall as being the dominant determinant for soil types. The four main types of surficial soils and their associated climatic regimes are: 1) montmorillonite/ smectoid soils in the western coastlands which have a marked dry season, 2) kaolin/kandoid clays in the rain forest areas with a weak dry season, 3) allophone latosols in very wet mountainous interior, and 4) allophone podzolics in areas with greater than

7000 mm annual rainfall (Walsh, 1980; Rouse et al., 1986). Although topsoil permeability is extremely high, subsurface soil hydrology varies strikingly with soil type and has been shown to control runoff processes. Saturated surface overland flow is characteristic of the weakly permeable smectoids while deep throughflow is characteristic of the kandoids and allophone latosoils where storm runoff accounts for less than 20% of total runoff (Rouse et al., 1986).

Landslides are the dominant erosional process and are a product of sheer stress being overcome at the soil/bedrock interface due to subsoil saturation, particularly in allophone latosols and kandoids on slopes (Rouse et al., 1986). However, due to high horizontal and vertical permeability in these soils, subsoil saturation is usually only achieved after a series of high intensity storms (Rouse et al., 1986). Surface suction has also been proposed as a process that keeps these soils together in-place until critical pore water content has been achieved (Rao, 1996). Initiating mechanisms for mass movement appear to be shallow rotational or transitional slides which eventually led to a flowslide

(Rouse, 1990).

4.4. Sampling and Analytical Methods

4.4.1. Sample Methodology

Twenty-eight (28) stream water samples (D06-1 through D06-13 and D06-15-

D06-29) were collected from twenty-five (25) locations throughout the island in July

2006 (Figure 4.1 and Table 4.1). In 2006, three locations were sampled twice, during both baseflow and a high-flow event. In addition, twenty-one (21) water samples (D08-1 through D08-21) were collected from twenty-one (21) locations throughout the island in

March 2008 (Figure 4.1 and Table 4.1). Samples were collected by hand within 1 m of the riverbank, and generally from river mouth locations above the influence of the tidal zone and in the case of those watersheds draining the western portion of the island, above the influence of any previously identified carbonate lithology (Roobal and Smith, 2004).

In addition, a review of available literature confirmed all sampling locations were outside the influence of any previously identified hydrothermal activity (Smith and Kirkley,

2004). Additional samples were collected from upstream locations where conditions allowed.

A total of eight (8) tuff/ignimbrite samples were collected from various watersheds throughout Dominica during March 2008 (Figure 4.1 and Table 4.1). Of note, the tuff/ignimbrite samples appeared to have undergone various degrees of weathering.

4.4.2. Stream Gauging and Annual Discharge Determination

Manual gauging of river flow was conducted at thirteen (13) of the sites using

United Stated Geological Survey methodology for measurement and computation of streamflow (Rantz et al., 1992). A depth profile of the channel was determined at each location prior to using a General Oceanic Inc.™ flow meter to obtain flow readings at 71

20% and 80% above the base of the channel at 11 locations. Instantaneous discharges were calculated by integrating over the river cross-sectional area. A previous statistical evaluation of two-tenths and eight tenths method methodology conducted by Carter and

Anderson (1963) showed multiple measurements obtained from sample locations with measurement stations > 11 would yield a standard deviation of percentage errors of < 5%.

Annual discharge values for each of the gauged streams were estimated from the discharge measurements for the wet and dry season for the annual hydrograph (Harmon et al., 2009). As previously discussed, Dominica experiences a wet season from June to

December and a dry season from January to May (http://www.climate-charts.com).

Instantaneous flows for the wet and dry seasons were subsequently extrapolated out to monthly flows and summed for eight months for the wet season and four months for the dry season, respectively. This methodology will probably result in an under representation of annual flows from rivers draining the eastern portion of the island. In the case of the Layou River, a previously estimated discharge value of 8.21 m3/s for the wet season (Diaz et al., 1985) and a 0.3 m3/s baseflow value for the dry season

(McDowell et al., 1995) were utilized for flux determination.

4.4.3. Water Analysis

Water samples were collected in new, deionized water (18 M ) soaked low density polyethylene (LDPE) Nalgene™ bottles, which were rinsed in river water three times prior to sample collection. A small aliquot of sample was used for in-field measurement of pH. Samples were stored in the dark at room temperature prior to filtration (usually within 36 hours of collection). Samples were filtered through a 47 mm

diameter (nominal pore size of 0.4μm) polycarbonate filter directly into an acid washed

60 ml LDPE bottle for cation analysis and a deionized water soaked 60 ml LDPE bottle for anion analysis. Filter blanks were created by filtering 18 M water into clean LDPE bottles using the same methods used for samples. Trip blanks were created prior to sample collection with 18 M water in clean LDPE bottles. The samples, trip blanks, and field blanks were subsequently shipped to the Ohio State University and chilled at ~4°C until analysis. Upon return, each filtered sample for cation analysis was acidified to pH

+ + 2–3 with Ultrex™ HNO3 for preservation. Major ion concentrations of cations (Li , Na ,

+ 2+ 2+ - - - - - 3- 2- NH4 , K+, Ca , Mg ) and anions (F , Cl , NO2 , Br , NO3 , PO4 , SO4 ), were

® determined via ion chromatography (IC) using a DX-120 ion chromatograph using the methods of Welch et al. (1996). Precision was determined using five replicate check standards per run with relative standard deviations (RSDs) usually better than ±1% and never greater than ±5%. Dissolved Si, and Ba2+ and concentrations were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES). External standards were used and check standards were run every 3–4 samples to account for instrument drift. Triplicate analyses did not reveal RSDs greater than ±5%.

4.4.4. Data Interpretation

Prior to the comparison of water sample results for the three regions, calculations

- - were performed to determine alkalinity (HCO3 ) in these streams. Sample HCO3 was determined via the method of Lyons et al. (1992) as the difference between Σcations in microequivalants and Σanions in microequivalants. This method assumes little

- contribution to alkalinity from dissolved species other than HCO3 . Previous work has

demonstrated that alkalinity values derived from this technique provide excellent

- estimates of HCO3 in river and stream waters (Lyons et al., 1992).

A review of available literature was conducted in order to bias the sampling locations to areas generally outside the influence of any previously identified hydrothermal activity (Smith and Kirkley, 2004). However, both hydrothermal and precipitation contributions to stream water were determined via a modified method of

Louvat at al. (1993) which was previously adapted from Minister [1977]. Atmospheric, hydrothermal, and low-temperature alteration inputs were corrected using mixing

2+ 2+ + + - 2- equations of X/Na (X being Ca , Mg , Na , K , Cl , SO4 , and SiO2). End member ratios for atmospheric input were determined using precipitation data from from El Verde in the Luquillo Experimental Forest, Puerto Rico (McDowell et al., 1990) while end member ratios for hydrothermal input were determined using the average elemental ratios for 14 hot springs on Dominica (McCarthy et al., 2005). End member ratios for andesite weathering were determined from studies of geochemical studies of rock and tephra throughout Dominica (Deurling, 2008; Lindsay et al. 2005; Sigurdsson and Carey, 1991).

For each dissolved X species of a river, the respective concentrations were determined using the following mixing equation:

(Eq. 4.1)

where α#(Na) is the Na proportion derived from # = rain, andesite weathering, or thermal springs with α rain(Na) + α andesite weathering(Na) + α thermal springs (Na) = 1. Using 74

eq. 1 seven equations were obtained with twenty-eight parameters from which all three

α#(Na) values are the unknowns. Given the number of equations is greater than the number of unknowns there is an over-constrained system and an inverse method using a least square fit was used to obtain a solution.

Chemical erosion yields were determined by multiplying the precipitation- corrected TDS concentrations for the wet and dry season data by the calculated stream discharge data for the respective season and summing the values. The yields were subsequently divided by the watershed area in order to compare between watersheds.

Silicate weathering yields were determined in similar fashion with multiplying the dissolved Si concentrations for the wet and dry season data by the calculated stream discharge data for the respective season, summing the values, and dividing the result by the watershed area.

4.5. Results and Discussion

4.5.1. Representativeness of the Data

Measured instantaneous discharge readings ranged from 0.0002 m3/s (St. Joseph’s

River) to 1.9 m3/s (Rosalie River) for the dry season and 0.02 m3/s (Geneva River

(upstream)) to 3.4 m3/s (Rosalie River) for the wet season. Previous analysis of Layou

River (the largest watershed in Dominica) discharge data determined an estimated average discharge value of 8.21 m3/s for the wet season (Diaz et al., 1985) and 0.3 m3/s baseflow value for the dry season (McDowell et al., 1995). This 27x difference between wet and dry season flows does not appear to be uncommon in Dominica streams as the seasonal discharge data showed differences ranging from 1.7x for the Dublanc River to

232x for the Hampstead River (n = 13). Of note, this range does not include the 75

anomalous 0.2x value for the upstream portion of the Geneva River, which might be attributed to unexpected rainfall in the headwaters region of this watershed during the dry season sampling. The characteristic response of discharge to seasonal variations in rainfall for small mountainous rivers located in the tropics is common and has been previously observed in other tropical settings (McDowell et al. 1995; Newbold et al.,

1995; Townsend-Small et al., 2008).

In order to further evaluate the representativeness of the discharge sampling protocol, a detailed analysis of the four year-year USGS dataset for the Layou River was conducted. Analysis of the of the mean daily flow record revealed for both March and

July showed a rapid return to baseflow following precipitation events. Furthermore the size of the Layou represents anywhere between 2.3 to 84x the remaining watersheds for which flux calculations were determined. Therefore, it is expected that each of these watersheds would have exhibit a much faster return to baseflow levels after a storm event. In fact, this is what was observed in the field with high flows from an event returning to pre-storm baseflow levels no later than the following day. Given the field sampling was biased towards days with no storms both the solute concentrations and discharge measurements obtained are representative of the baseflow values for the respective seasons.

4.5.2. Concentration of Major Elements and Statistical Analysis

Rainfall-corrected solute concentrations were subsequently used to determine the fraction of solutes derived from weathering reactions (Fw). The Fw is calculated using the

TDS and TDSc (Eq.1), where TDSc is the total dissolved solids due to cyclic salts.

(Eq.4.2)

During the dry season, the stream water chemistry is dominated by the weathering signal, which accounts for approximately 70% of the solutes in the rivers (Fw = 0.42 – 0.91; 0.68 average). The data suggest a large baseflow component to the solute flux in these rivers.

This is in agreement with a previous study of subsurface weathering rates on Martinique and Guadeloupe, which indicated weathering rates in the subsurface could be 2 to 5 times higher than those for surface waters (Rad et al., 2007). However, during the wet season, a strong positive correlation with rainfall was observed (Fw = 0.38 – 0.84; 0.60 average)

(Table 4.3). The highest values of Fw were associated with the Upstream Geneva River

(0.91) and Geneva site (0.48) site and the Rosalie River (0.84). The high Fw values for these rivers may be attributed to their watersheds being underlain by young Pleistocene age pyroclastic deposits. Geography was also found to play a minor role. Samples from the leeward coast (n=10) exhibited higher average Fw values than their windward coast

(n=13) counterparts for both the wet and dry seasons (0.65 and 0.75 versus 0.58 and 0.63, respectively). A plot of Fw vs. Si (Figure 4.2) showed higher concentrations of Fw in accordance with higher Si concentrations for the dry season data set. Furthermore, a two- tailed t-test of the seasonal Fw:Si ratios revealed significantly different values for the data sets (a = 0.05, P = 0.0004). The results of the graphical and statistical analysis suggests both parameters serve as good indicators of the extent of chemical weathering occurring in the system. While the two Geneva River and the one Cairo River sites for the wet season were notable exceptions to this pattern, these sites are again influenced by the young Pleistocene age pyroclastic deposits.

The geochemistry of the streams, after correction for precipitation input, revealed distinct chemical differences between the wet and dry seasons. The precipitation- corrected total cation chemistry (TZ+) for the streams in the wet season demonstrates that

+ Na is the major cation from weathering and it constitutes approximately 43% of the total cation charge, followed by Ca (25%), Mg (20%), and K (4%). Twenty-one of the 28 stream water samples exhibit a cation abundance of Na>Ca>Mg>K while the remaining

- seven samples exhibit Ca>Na>Mg>K. All samples exhibit HCO3 as the dominant anion

2- - over SO4 , with the HCO3 concentration ranging from ~2.6 (North Fork of the Pagua

2- River (D06-22)) to ~36x (Geneva River (upstream) (D06-3)) the respective SO4 concentration. The dry season samples exhibited similarities with respect to the total

2+ 2+ + + cation charge albeit with higher proportions of Ca and Mg with respect to Na and K .

2+ Na constituted approximately 38% of the total cation charge and was followed by Ca

(37%), Na (23%), and K (3%). The stream water samples were evenly divided with respect to the order of cation abundance with with 11 samples exhibiting Ca>Na>Mg>K and 11 sites exhibiting Na>Ca>Mg>K. Anion chemistry was similar to that for the wet

- 2- season with all samples exhibiting HCO3 as the dominant anion over SO4 , with the

- HCO3 concentration ranging from ~2.4 (North Fork of the Pagua River (D08-18)) to 36x

2- (Cairo River, D08-6) the respective SO4 concentration.

This distinct solute chemistry for each season resulted in a factor of three difference in precipitation-corrected total dissolved solids (TDS) concentrations with a range of 8.7 to 70.4 mg/kg measured during the wet season and a range of 33.7 to 84.2 mg/kg measured during the dry season. Median TDS concentrations for the wet and dry seasons consisted of 24.9 mg/kg and 53.8 mg/kg, respectively. The Geneva River

exhibited the highest TDS concentration for the wet season (70.4 mg/kg) while the Cairo

River exhibited the highest TDS concentration for dry (84.2 mg/kg). The Geneva and

Cairo rivers both drain younger Pleistocene volcanic material. Compare this to the Pagua

(North Fork) which solely drain Miocene and Pliocene volcanics, and has the lowest TDS of the rivers sampled. Solute concentrations in the dry season ranged from approximately

1.4 to 5.1 times greater during the respective wet season value. While this may in part reflect dilution of weathering products by rainfall and differences in stream flow, it is also worth noting that the composition of the weathering products are different in the wet and dry seasons.

A statistical analysis of the precipitation-corrected dissolved load data was performed in order to determine the existence of significantly different average concentrations among the wet and dry seasons. A two-tailed t-test with a null hypothesis of no difference in mean elemental values (α = 0.05) between the seasons was conducted for all analyzed species and calculated alkalinities. The null hypothesis of no difference

+ + between mean ionic concentrations was rejected for Na (p = 5.16E-05), K (p = 0.009),

2+ 2+ - - Mg (p = 2.12E-05), Ca (p = 9.59E-05), Si (p = 1.99E-05), NO3 (p = 0.05), and HCO3

- 2- (p = 0.0002). The null hypothesis could not be rejected for, F (p = 0.5) and SO4 (p =

0.08).

4.5.3. Source of Solutes

Ca:Mg ratios for the Dominica samples ranged from 1.0 to 2.3 for the wet season and 0.9 to 2.3 for the dry season (Table 4.4). These ratios are similar to those previously published for rivers throughout Dominica of 0.6–4.2 (Hart and Hart, 1969) and for the

Layou River of 2.1 (McDowell et al., 1995). The upper range of the Hart and Hart (1969) data set is almost twice that for this study, but after removal of their two highest values, none of their remaining 26 samples exhibited Ca:Mg ratios greater than 3.2. Furthermore, their study did not exclude settings potentially affected by geothermal activity.

Almost all of the samples for this study exhibited Ca:Mg ratios much lower than the world average of 2.4 (Harmon et al., 2009) but are similar to those previously established for the andesitic terrain of the Taranaki region of New Zealand (1.8 to 2.9) by

Goldsmith et al. (2008), the basaltic/andesitic western coast of Costa Rica (0.4 to 0.9) by

Pringle et al., (1990), the basaltic/andesitic islands of Martinique and Guadeloupe (0.9 to

2.6) by Rad et al. (2007), and Puerto Rico (0.86-0.95) by McDowell and Asbury (1994).

These low Ca:Mg ratios for Dominica rivers indicate the importance of the weathering of

Mg-rich minerals in these watersheds. Previous studies on Dominica have shown that easily weathered Mg-rich minerals such as pyroxenes (ortho- and clino-) and hornblende compose significant portions of both the phenocrysts and groundmass in the volcanic rock (Wills, 1974; Lindsay et al., 2003; Lindsay et al., 2005) and tephra/ignimbrite

(Wills, 1974; Sigurdsson, 1972; Carey and Sigurdsson, 1980).

These results are consistent with the mineralogy of the rock samples determined

2+ as part of this study (Section 4.3). The contribution to stream water of Ca from both secondary and pedogenic calcite has been shown to play a role in basaltic settings

(Raiswell and Thomas, 1984; Stefánsson and Gíslason, 2001). However, the relatively low Ca:Mg ratios of Dominica rivers are more consistent with those previously determined for basaltic terrains (0.9–3) by Dessert et al. (2003) than with the higher

Ca:Mg values characteristic of carbonates. Furthermore, a literature review did not

identify the presence of calcite in any previous soil (Rouse et al., 1986; Walsh et al.,

1988; Reading, 1991), bedrock (Wills, 1974; Sigurdsson and Carey, 1991; Lindsay et al.,

2003; Smith et al., 2003; Lindsay et al., 2005), or tephra/ignimbrite (Wills, 1974;

Sigurdsson, 1972; Carey and Sigurdsson, 1980; Sparks et al., 1980)

2+ geochemical/mineralogical studies of the island. While the contribution of Ca from disseminated calcite cannot be ruled out in these systems, there are currently no data to support its occurrence.

Chemical weathering in watersheds has been previously calculated as the sum of cations in stream water (Ca + Mg + Na + K = TZ+) after correction for atmospheric input

+ (Edmond and Huh, 1997). A strong positive correlation was observed between TZ and

2 2 TDS for both the wet (r = 0.98) and dry season data (r = 0.92) (Figure 4.3). The

Downstream Geneva River exhibited the highest TZ+ for the wet season while the Cairo

River exhibited the highest TZ+ for the dry season.

The surficial soils throughout much of the mountainous interior portion of the island are dominated by allophone latosols whose high permeability results in most storm runoff (>80%) occurring as soil through flow (Rouse et al., 1986). Water transport in these settings has been hypothesized to consist of either slowly moving soil water at the deep soil/ rock interface or as groundwater (Rouse et al., 1986). Either process would allow rainwater to interact continually with new mineral surfaces at depth.

Smectite surficial soils have been previously identified on the strongly seasonal western portion of the island while kandoids have been found in rain forest areas with a weak dry season (Rouse et al., 1986). The presence of kandoids and smectites in other regions lends support to their potential existence at the deep soil/bedrock interface in

Dominica. A previous summary of stream water chemistry from basaltic settings (Bluth and Kump, 1994) showed that cation exchange reactions, particularly from 2:1 clays such as smectite, played a strong control on the observed stream water chemistry. A similar survey of andesite/quartz diorite terrain drained by Rio Tanama yielded similar results

(Drever, 1988). The abundance of Ca and Mg-rich primary minerals, such as calcic- plagioclase, pyroxenes (ortho-, clino-, and hypersthene) and hornblende in Dominica parent material minerals could supply the Ca and Mg for these 2:1 clays.

4.5.4. Cluster Analysis

A cluster analysis of the dry season data was conducted after Harmon et al. (2009) using SPSS© software in an effort to determine statistical similarities amongst the solute data. The dry season data (corrected for precipitation) were used because sample analysis showed the fraction of weathering (Fw) dominated the stream water chemistry during this time of year and therefore effects of material age on ion contribution to stream water could be more easily differentiated among sample locations. A Q-mode cluster analysis of the samples themselves was performed using the furthest neighbor method with measures based on squared Euclidean distance. All transform values were standardized by z-scores. Eight chemical parameters for each of the samples were used and included

+ 2+ 2+ + - - 2- Si, Na , Ca , Mg , K , HCO3 , Cl , and SO4 .

The cluster analysis resulted in two distinct clusters with age of lithology appearing to play a major role (Figure 4.4). Cluster B is composed of six samples, four of whose watersheds predominantly drain Miocene and/or Pliocene age deposits (Pagua,

Pagua North Fork, Castle Bruce, and Richmond). The remaining two rivers (Cairo and

Malabuka) drain watersheds underlain by Younger Pleistocene age (<1.8 Ma) pyroclastic 82

deposits. However, both of these watersheds are small in size (1.8 and 2.8 km2,

- respectively) and were found to contain extremely high concentrations of Cl (>890

μmol/kg). This may have resulted in an underestimate of the chemical weathering input from these streams given the conservative methodology for precipitation correction.

Cluster A comprises 16 samples, 11 of which are underlain by Pleistocene age pyroclastic deposits (Rosalie, Layou (upstream), Layou (downstream), Tributary of Layou, Bateli,

Geneva (downstream), Geneva (upstream), La Ronde, Tweed, St. Josephs, and Dublanc), one with these deposits in its headwater regions (Hampstead), and one underlain by older

Pleistocene age (~2.01 and 1.8 Ma) deposits. The remaining two rivers in Cluster A

(Mahaut and Douce) are underlain by older Miocene age deposits. The fact these rivers plot with those which drain Pleistocene age deposits may represents some common source (i.e. ash fall deposits, regional soil through flow/groundwater) which may be contributing to the dissolved load of these rivers. This is supported by the fact these rivers fall geographically within the range of the other Cluster A, as shown in Figure 4.1. The statistical separation of rivers with regards to age and/or abundance of pyroclastic material lends support to the idea of these factors playing a major role on delivery of the dissolved load.

4.6. Dominica River Chemistry Compared to Silicate Terrains Worldwide

Molar ratios of Ca/Na, HCO3/Na, and Mg/Na ratios were calculated for the precipitation corrected water chemistry (after Gaillardet et al., 1999) in order to compare the Dominica samples to previously established silicate end member ratios for world’s rivers and to ratios determined for basaltic terrains (Dessert et al., 2003). These elemental ratios were chosen to avoid dilution and evaporation effects when comparing different 83

weathering regimes worldwide (Gaillardet et al., 1999). A lack of Na data by Hart and

Hart (1969) and for HCO3 by McDowell et al. (1995), precludes using those datasets in this exercise. The Dominica samples exhibited distinctive values for both the wet and dry seasons (Table 4.5). All molar ratio averages and ranges were greater for the dry season than the wet season. The highest Ca/Na, Mg/Na, and HCO3/Na ratios for the wet season were observed for the Geneva River. Highs among the dry season ratios were more variable between the rivers with the highest Ca/Na ratio observed for the North Fork of the Pagua River, the highest Mg/Na ratio for the Malabuka River, and the highest

HCO3/Na ratio for the Geneva River. The Ca/Na molar ranges and averages for both seasons and the Mg/Na averages and ranges for the dry season are greater than the continental silicate end member ratios determined by Gaillardet et al. (1999) and are similar to those for basaltic terrains (Dessert et al., 2003) (Table 4.5).

Plots of Na-normalized molar data (Figure 4.5 A&B) prepared after Gaillardet et al. (1999) show that approximately half of the Dominica samples fall outside the range determined for average continental silicate (granitic) rock and fall on the linear trend between the silicate and carbonate end members as observed for basaltic terrains by

Dessert et al. (2003). Dessert et al. (2003) explained this pattern either as the result of mixing between the silicate end member and the dissolution of calcite disseminated in basalt, or as the result of the weathering of Ca- and Mg-rich volcanic rock. Calcite has not been identified in any soil or bedrock studies nor in the tuff/ignimbrite samples collected as part of this study. Furthermore, the relatively low dissolved Ca/Mg ratios of

Dominica rivers are more consistent with those previously determined for basaltic and

andesitic rocks than to the higher Ca/Mg values expected for carbonate rocks and points to the weathering of andesitic rock/tephra as the source.

The fact that half of the Dominica molar ratios fall within the range previously identified for basaltic terrains worldwide confirms the chemical weathering potential of andesitic material and suggests that some mechanisms (e.g., ideal soil thickness, relative abundance of tephra/pryoclastic material, crystallinity of parent material, and/or presence of conduits to soil/bedrock interface) can readily supply these constituents to stream water. Furthermore, high concentrations of Ca and Mg being delivered in stream water in

Dominica suggest that andesitic terrains should not be combined with bulk silicate weathering in determining global CO2 consumption values. This separate treatment from granitic terrains has previously been proposed for basaltic terrains by Gaillardet et al.

(1999) and Dessert et al. (2003).

4.7. Chemical Weathering Rates and Potential CO2 Consumption

4.7.1. Chemical Weathering Rates

Chemical weathering yields, based on a compilation of TDS and calculated

-2 -1 discharge values for each season, ranged from 6 to 106 tons km a (Table 4.6). Six of the seven highest yielding rivers (Layou, Rosalie, Geneva, Blenheim, Dublanc, and

Hampstead) are all found in watersheds underlain by mid-Pleistocene to recent age

-2 -1 pyroclastic deposits. In particular, the Geneva (83 tons km a ) is underlain by the Grand

-2 -1 -2 -1 Bay Ignimbrite (~39 Ka) while the Layou (106 tons km a ), Rosalie (104 tons km a ),

-2 -1 and Dublanc (64 tons km a ) are underlain by pyroclastic deposits originating from the

~28 Ka eruption from the Trois Pitons-Micotrin volcanic center. All but three of the watersheds had chemical weathering rates well in excess of the global mean value of 24 85

-2 -1 tons km a (Gaillardet et al., 1999). Furthermore, these values are similar to those

-2 -1 previously established for the andesitic terrains of New Zealand (15–310 tons km a ) by

Goldsmith et al. (2008) and Guadeloupe and Martinique (100–120 tons km-2 a-1) by Rad

-2 -1 et al. (2006), and for the basaltic terrains of the Deccan Traps (21-63 tons km a ) by

-2 -1 Dessert et al. (2001) and Reunion Island (63–170 tons km a ) by Louvat and Allègre

(1997).

Comparison of chemical weathering rates to stream gradient and watershed average annual rainfall showed no systematic relationship between the chemical erosion yield and the stream gradient (r2 = 0.12). In fact, the Layou River exhibited the highest chemical erosion rates for the study area despite having the lowest gradients. A slightly

2 positive correlation (r = 0.53, P = 0.5) with annual watershed average annual rainfall was observed (Figure 4.6). While a two tailed t-test (a = 0.05) did not reveal significantly higher basin-scale chemical weathering rates for those rivers draining Pleistocene age

-2 -1 volcanics (62 tons km a ; n = 9) compared to those draining the Miocene and Pliocene

-2 -1 aged volcanics (28 tons km a ; n = 2) a high p-value associated with the analysis (p =

0.13) suggests more data is necessary in order to reach a more definitive conclusion. A comparison of chemical weathering rates to physical erosion rates was not possible given the lack of suspended sediment gauging records for the island. Previous work by

Goldsmith et al. (2008) in the andestic terrains of New Zealand did not demonstrate a positive correlation between chemical and physical weathering rates, however, those authors acknowledge a lack of physical erosion for the region may have contributed to the lack of correlation.

4.7.2. Silicate Weathering and CO2 Consumption

On geologic time scales the weathering of silicate rocks acts like a global thermostat (Walker et al. 1981). As part of this silicate weathering process, the dissolution of one mole of a calcium-bearing aluminosilicate mineral will result in the consumption of two moles of CO2 (Eq. 2), one of which one is precipitated as CaCO3 upon delivery to the ocean. This reaction results in the net loss of one mole of CO2 from the atmosphere (Eq. 3). This differs from the weathering of carbonate minerals (Eq. 4), which result in the consumption of one mole of CO2, which is ultimately returned to the atmosphere. The cumulative impact of this removal of CO2 from the atmosphere via silicate weathering becomes important when looking at periods of time one million years or longer (Berner, 1983; Raymo and Ruddiman, 1992).

2+ - 2CO2 + 3H2O + CaSiO3  Ca + 2HCO3 + H4SiO4 (Eq. 4.3)

2+ - Ca + 2HCO3  CaCO3 +CO2 + H2O (Eq. 4.4)

2+ - CO2 + H2O + CaCO3  Ca + 2HCO3 (Eq. 4.5)

While weathering of silicate rocks from the world’s major rivers has been shown by Gaillardet et al. (1999) to play a strong role in global consumption of CO2, that study did not include data from rivers draining large igneous provinces or those located on

HSIs. A recent compilation of CO2 consumption studies in basaltic terrains (Dessert et al.,

2003) showed these previously neglected settings may be responsible for a CO2 drawdown equivalent of up to a one-third of the previously determined worldwide value from silicate weathering. Lyons et al. (2005) speculated that active margins associated with HSIs may play a disproportionately large role in global CO2 consumption via

chemical weathering. Furthermore, recent studies of CO2 consumption for the weathering of andesitic terrains have shown values from these neglected terrains can rival those for basalt (Rad et al., 2006; Goldsmith et al., 2008). The data suggest that in global CO2 consumption determination, neglect of active margins, including HSIs, which are covered by significant quantities of andesitic materials, may result in a dramatic underestimate of this worldwide flux.

Previous world-wide lithology compilations have shown that andesite composes a significant percentage of land coverage with estimates ranging from 2.4% to 3% of the total land surface (Amiotte-Suchet et al. 2003; Meybeck, 1987, respectively). This compares well with previous estimates for worldwide cover estimates of 4.15% and 6% for basalt terrain (Meybeck, 1987, Amiotte-Suchet et al. 2003, respectively). In addition, current day locations of andesite terrains such as the HSIs of the Southwest Pacific, the

Lesser Antilles, Central America and portions South America are often located in tropical and subtropical areas as opposed to basalts, where flood basalt provinces are often found outside these climatic zones (i.e. Siberia Traps, Columbia River). For example, a recent lithology compilation for East and Southeast Asia by Peucker-Ehrenbrink and Miller

(2004) shows intermediate volcanic rocks compose 3.46 % of the land surface compared to 2.13% for mafic volcanic rocks. Furthermore, andesite terrains are often associated with active margin locales where their proximity to the ocean subjects them to aperiodic intense precipitation events (i.e., typhoons).

Silicate weathering yields determined for this study, based on a compilation of dissolved Si concentrations and calculated discharge values for the wet and dry seasons,

-2 -1 ranged 3.1–55.4 tons km a (Table 4.6). Eight of the nine highest yielding rivers were

-2 -1 recorded for locations draining Pleistocene aged volcanics (3.1–55.4 tons km a ).

-2 -1 Miocene and Pliocene aged volcanics exhibited far lower yields (7.9–10.1 tons km a ).

A two tailed t-test (a = 0.05; p = 0.01) revealed significantly higher basin-scale silicate

3 weathering averages for those rivers draining Pleistocene age volcanics (29.5 x 10 tons

-2 -1 km a ; n = 9) compared to those draining the Miocene and Pliocene aged volcanics (9.0

3 -2 -1 -2 -1 x 10 tons km a ; n = 2). Furthermore, the silicate flux of 55.4 tons km a determined

-2 -1 for the Layou River is consistent with a previous estimate of 80 tons km a (McDowell et al., 1995). The slightly lower value for this study may be the result of the conservative precipitation correction methodology.

Average basin-scale CO2 consumption yields by silicate weathering were calculated as TZ+ (Ca + Mg + Na + K) in equivalents (corrected for sea salts), multiplied by discharge and divided by watershed area. This precipitation methodology is based on

- the assumption that all Cl in stream is attributed to rainfall, and so the CO2 consumption values presented should be taken as minima. As with the chemical and silicate weathering yields the values presented are a summation of both wet and dry season data. The three

3 highest CO2 consumption yields were determined for the Layou River (1575 x 10 mol

-2 -1 3 -2 -1 3 km a ), Rosalie River (1523 x 10 mol km a ), and Blenheim River (1410 x 10 mol

-2 -1 km a ) (Table 4.6). While a two tailed t-test (a = 0.05) did not reveal significantly higher basin-scale CO2 averages for those rivers draining Pleistocene age volcanics (1359

3 -2 -1 x 10 mol km a ; n = 9) compared to those draining the Miocene and Pliocene aged

3 -2 -1 volcanics (449 x 10 mol km a ; n = 2) a high p-value associated with the analysis (p =

0.1) suggests more data is necessary in order to reach a more definitive conclusion.

However, the relatively higher mean concentrations reflect the fact that the younger

terrain produces higher amounts of dissolved Si resulting in these higher CO2 consumption rates and implies the relative age of lithology should be taken into account when developing models of potential fluxes from andesitic terrains. Silicate weathering fluxes and associated CO2 consumption for Dominica exceed all previously established values for high-standing islands and all andesitic/dacitic terrains worldwide (Table 4.6).

Three of Dominica’s rivers, the Geneva, Layou and Rosalie, have CO2 consumption

3 similar to the maximum value for the Taranaki Region of New Zealand (2926 x 10 mol

-2 -1 km a ) determined by Goldsmith et al. (2008). In fact, the Dominica CO2 consumption rates are so high they only fall within the range of values previously determined for the

3 -2 -1 basaltic terrains of the Deccan Traps (580–2450 x 10 mol km a ) by Dessert et al.

3 -2 -1 (2001), Réunion Island (1300–4400 x 10 mol km a ) by Louvat and Allègre (1997),

3 -2 -1 and Iceland (141–1764 x 10 mol km a ) by Stefánsson and Gíslason (2001), and the

3 -2 basaltic/andesitic volcanic islands of Guadeloupe and Martinique (1.1–1.4 x 10 mol km a-1) (Rad et al., 2006).

Interestingly, laboratory experiments suggest that Dominica’s lithology of andesite/dacite should weather slower than basaltic rocks (Wolff-Boernish et al. 2006), however this is clearly not the case. In their summary, Wolff-Boernish et al. (2006) identified both the abundance of polymerized (interconnected) Si-O bonds and the crystallinity of parent material as controlling factor on weathering rates. Andesite’s greater number of these interlinked Si-O bonds than basalt suggests crystallinity of parent material must play a role for these extremely high yields observed on Dominica. These data support the hypothesis that silicate weathering fluxes and associated CO2 consumption are highest in rivers that flow through fresh Pleistocene tephra material

(Table 4.6, Figure 1). While recent work has tried to link the physical and chemical erosion rates on HSIs (Carey et al. 2005; Lyons et al. 2005), it is now clear that lithology must also be taken into account. HSIs, such as Dominica, provide an excellent field laboratory for the interplay of these effects as the characteristically high slope gradients on the island cause instability resulting in landslides (Reading, 1991), particularly during prolonged precipitation events. When a landslide does occur, it limits relative maximum soil thickness and generates fresh surface area for this highly erosive material to weather.

This repeatedly happens and leads to the high silicate weathering flux and CO2 consumption observed for Dominica.

Dissolution of basaltic rock as a function of temperature has been previously modeled through the use of Arrhenius’ law by Dessert et al. (2001). Using their plot of

- atmospheric CO2 consumption via silicate weathering (HCO3 concentration) vs. 1000/T

(K-1), the Dominca dry and wet season values plot outside the general trend of the negative correlation (Fig. 7). However, the effects of seasonal weathering processes on

CO2 consumption potential are clearly shown. The fact the dry season Dominica values plot higher than those observed by Rad et al. (2006, 2007) for the basaltic-andesite islands of Guadeloupe and Martinique may point to some ideal soil thickness as key to this weathering process for andesitic terrains. Alternatively, sampling protocol from those earlier studies may not have captured the importance of those seasonal variations. While the values previously determined for the Taranaki region of New Zealand fit Dessert’s

Arrhenius equation for basaltic terrains, it is clear that more data will be necessary to determine whether their equation would fit andesitic terrains or whether a separate worldwide compilation is warranted. Furthermore, the observed trend of younger material

age with higher chemical yields for Dominica may also play a role in the determination of weathering rates for andesitic settings. This study clearly points to the need for additional field and laboratory experiments on the weathering rates of andesitic tephra.

The observed CO2 consumption rates for Dominica become even more impressive when viewed in the light of a recent study which indicated subsurface delivery of solutes to the ocean from these locales may underestimate total annual chemical weathering rates by 30–60% (Rad et al., 2007). An absence of groundwater wells prevented a comparative study for Dominica. In addition, calculated weathering yields may be further underestimated because the role of solute leaching from suspended material delivered to the ocean has not yet been determined (Gíslason et al., 2006). Crystallinity associated with tephra material on Dominica should further enhance this effect. These factors all point to the CO2 drawdown potential by weathering of volcanics at active margins and lead to the possibility of their potential control or at the very least moderating factor through post-Phanerzoic geologic time.

4.8. Conclusions

Our study investigated geochemical fluxes in watersheds on the andesitic-dacitic

HSI of Dominica in the Lesser Antilles. A combination of seasonal sampling and manual gauging of streams was conducted to provide a glimpse into yields from andesitic terrains not normally covered by such investigations. Water samples analyzed for major and trace elements showed distinct seasonal signatures. Crystallinity and age of parent material also exhibited importance in the delivery of high concentrations of solutes in stream water. Annual chemical weathering yields for the watersheds produced from both wet and dry season yields were found to be similar to those previously determined for 92

andesitic and basaltic terrains. Silicate weathering yields and subsequent CO2 consumption are amongst the highest known and fall only within the upper range of those previously determined for the basaltic terrains of the Deccan Traps and Reunion Island.

These fluxes point to the importance of crystallinity of andesitic parent material to produce fluxes capable of rivaling those previously determined from the weathering of basalt. The results clearly show the importance of weathering of andesitic terrains at active margins, particularly those on HSIs, and that these terrains should be evaluated as a whole when considering global geochemical fluxes. In addition, age of parent material is an important factor in controlling solute weathering fluxes.

4.9. Tables

Table 4.1 Sample Location Summary.

Table 4.3 Dominica Fw values in stream water and seasonal averages.

Table 4.4 Dominica and worldwide Ca:Mg ratios in stream water.

Table 4.5 Dominica, continental silicate end member, and basalt molar ratios

Table 4.6 Chemical, and Silicate Weathering Yields and CO2 Consumption in Dominica watersheds and other locales.

4.10. Figures

100

Figure 4.2 Plot of fraction of solutes derived from weathering reactions (Fw) vs. Si in μmol/l in stream water corrected for precipitation.

Figure 4.3 Plot of total cations (TZ+) in μmol/l vs. total dissolved solids (TDS) in μg/kg in stream water corrected for precipitation.

101

Figure 4.4 Results of a cluster analysis of the dry season Dominica stream water samples. Samples divide into two distinct clusters (A & B).

102

Figure 4.5 Mg/Na vs. Ca/Na and HCO3/Na vs. Ca/Na of Dominica rivers corrected for atmospheric inputs (after Gaillardet et al., 1999). Basaltic compositions are taken from Dessert et al (2003). End member compositions for granitic silicates and carbonates are those determined by Négrel et al. (1993) and by Gaillardet et al. (1999).

103

Figure 4.6 Chemical weathering in Dominica watersheds as function of watershed average annual precipitation. Chemical weathering yields were calculated using geochemical analysis of samples from locations indicated in Figure 1. Watershed average annual precipitation was determined using average annual rainfall maps from Rouse et al. (1986) analyzed for watershed area using ArcGIS.

104

Figure 4.7 Plot of HCO3_ concentrations vs. 1/T (calculated using mean annual temperature) for rivers draining both basaltic and andestic terrains (after Rad et al., 2006 and Goldsmith et al., 2008). Both the dry and wet season value falls within below the general trend for basaltic terrains previously defined by Dessert et al. (2001) and more in line with those previously established for Martinique and Guadeloupe by Rad et al. (2006).

105

CHAPTER 5

ANDESITIC-DACITIC TERRAINS OF HSIs AND THEIR ROLE IN THE GLOBAL CARBON CYCLE

5.1. Abstract

Recent studies of andesite-dacite weathering at active margins show the importance of this material when calculating annual CO2 drawdown fluxes from silicate weathering. While these fluxes have been previously linked to the both the age and abundance of tephra material, efforts to calculate current regional and global CO2 drawdown is limited by paucity of datasets for these regions, particularly for younger deposits. Herein, chemical weathering yields from two andesite-dacite terrains are presented, Mt. Pinatubo in the Philippines and Volcán Barú in western Panama and combine the results with existing datasets to calculate CO2 drawdown. These two new datasets allow evaluation of the impact of abundant fresh tephra material from historical eruptions on silicate weathering rates. Dry season (Mt. Pinatubo) and annual (Volcán

Barú) chemical and silicate yields are among the highest recorded to date. Corresponding

3 -2 -1 CO2 consumption values (1532–2882 x10 moles km a ) are so high they are rivaled only with those previously determined for basaltic terrains. Compilation of the new and existing datasets shows rivers draining andesite-dacite material are characterized by relatively high Na-normalized molar ratios and low Ca:Mg molar ratios compared to

106

those draining continental silicates. Comparison of the dataset to potential controlling parameters revealed the primary importance of runoff and temperature. No discernible relationship was observed between average aqueous HCO3, total dissolved cations, or total dissolved solids with SiO2 content or age of the parent material while runoff and temperature were shown to be the dominant controls on solute fluxes. From these relationships and a new highly-detailed lithology map I calculate a CO2 consumption rate from andesite-dacite weathering for HSIs of 0.49 x 1012 moles a-1 and for East and

Southeast Asia of about 0.59 x 1012 moles a-1. These values represent between 5.7 and

6.8% of the annual CO2 consumption values previously calculated from continental silicate weathering and between 16% and 19% of the value previously calculated from basaltic terrains worldwide. This result confirms the importance of andesite-dacite weathering as a CO2 sink. These terrains may thus play an important role in climate evolution over geologic time.

5.2 Introduction

Global climate change has brought urgency to the determination of an accurate annual global carbon budget. This endeavor has renewed attention towards understanding the modern day behavior of two processes believed to control the balance of atmospheric

CO2 over geologic time; silicate weathering and burial of terrestrially derived organic carbon (Berner, 1983; Raymo and Ruddiman, 1992). From the pioneering work of

Garrels and McKenzie (1971) and Berner et al. (1983) onward many attempts have been made to identify potential controls of silicate weathering rates and CO2 drawdown on a local, regional, and global scale. A recent endeavor to quantify CO2 drawdown via silicate weathering from the world’s 60 largest rivers by Gaillardet et al. (1999) has 107

demonstrated the effect of high physical weathering rates on producing correspondingly high chemical weathering rates. This linkage of physical and chemical weathering rates is further reinforced from several studies at active margins (Carey et al., 2002; Louvat and

Allègre, 1997; Millot et al., 2002; Jacobson and Blum, 2003; and Lyons et al., 2005) where the majority of the physical erosion on Earth’s surface is occurring today. Several studies have demonstrated the importance of high-standing islands (HSIs), whose headwaters lie greater than 1,000 m above sea level and have some of the highest physical erosion rates observed to date (Milliman and Syvitski, 1992; Hicks et al., 1996,

Dadson et al., 2003; Milliman et al., 2007), as providing abundant mineral surfaces for chemical weathering reactions to occur.

The role of lithology, particularly basalt, on weathering rates has also been investigated by several authors who have shown the readily erodible nature of this parent material contributes to high observed solute fluxes in both the laboratory (Wollf-

Boenisch et al., 2006; Gislason et al., 1996) and in the field (Meybeck, 1987; Amiotte-

Suchet and Probst, 1993). More recent investigations concerning silicate weathering and potential CO2 drawdown from these terrains have been approached both at a regional scale such as the Parana and Deccan Traps (Benedetti et al., 1994; Dessert et al., 2001) and at a local scale on volcanic islands such as Iceland (Gíslason et al., 1996), Réunion

(Louvat and Allègre, 1997), and Java (Louvat, 1997), the last of which has the highest values determined to date (6400 x 103 mol km-2 a-1). These efforts led to several compilation studies which identified CO2 drawdown values rates as high as 30-35% of those for granitic terrains despite their limited aerial extent (Gaillardet et al., 1999;

Dessert et al., 2003; Dupré et al., 2004). Further exploration into controls of worldwide

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basalt weathering by Dessert et al. (2003) showed the importance of temperature and to a lesser extent runoff. However, given that many active margins currently characterized by basalt terrain eventually evolve to volcanics of more intermediate composition, investigations of andesitic lithologies appeared warranted.

Recent silicate weathering and CO2 consumption studies from Martinique and

Guadeloupe (Rad et al., 2006; 2007), the Taranaki Region of New Zealand (Goldsmith et al. 2008), and the island of Dominica (Goldsmith et al. 2009) have shown weathering of andesitic material can result in solute fluxes similar to those determined for basaltic terrains. These field observations were realized by Goldsmith et al. (2008a) to present a conundrum as a previous compilation of laboratory weathering rates for igneous rock showed andesites weather at an order of magnitude slower than basalts (Wolf-Boenisch et al., 2006). Given the laboratory results were attributed to the corresponding increase of interlinked Si-O bonds as silicates become more evolved, the author’s hypothesized that the abundance of tephra in andesitic terrains may play an import role in controlling field weathering rates. This effect of crystallinity on chemical weathering rates has been previously shown for basalt on Iceland where the presence of basaltic glass was found to enhance fluxes of Na, Si, Ca, F, and S by a factor of 2 to 5 at a constant runoff factor and vegetative cover (Stefánsson and Gíslason, 2001). Regardless, these initial weathering studies for andesitic terrains suggested the weathering potential of other active margins

(including HSIs), which are covered by significant quantities of andesitic-dacitic material, should be investigated.

The volcanics of Volcán Barú in western Panama and Mt. Pinatubo on the island of Luzon in the Philippines present an ideal opportunity to evaluate the impact of

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abundant “fresh” tephra material from historical eruptions on silicate weathering rates.

Large-scale plinian style eruptions from each location occurred ~500 yrs BP (Sherrod et al., 2008) and 19 years BP, respectively. Historical eruptive activities at Volcán Barú have resulted in two large scale land features, extensive debris avalanche deposits and a lahar plain (Siebert et al., 2006; Sherrod et al., 2008) the 1991 eruption of Mt. Pinatubo resulted in the deposition of ~5-6 km3 of pyroclastic material in the immediate vicinity of the cone (Scott et al., 1996). Both sites have exhibited evidence of repeated large-scale extensive volcanic activity over the last 50 k.y. (Sherrod et al., 2008; Newhall et al.,

1996). Finally, both study sites are located well within the tropics and experience distinctive wet and dry seasons, but the Philippines experience a typhoon frequency of approximately four per year (Wu et al., 2005). Therefore, it is possible one or both of these locales may provide a high rate end-member value for chemical weathering from andesitic terrains.

The purpose of this paper is two-fold. First, new stream water geochemical data from Volcán Barú in western Panama and from Mt. Pinatubo on the island of Luzon in the Philippines will be presented. Geochemical analyses of the water samples will be combined with governmental and manually obtained hydrological data, respectively, to estimate chemical fluxes and CO2 consumption via silicate weathering. Second, the new data will be combined with existing datasets from Dominica (Goldsmith et al. 2009);

Martinique and Guadeloupe (Rad et al., 2006; Rad et al., 2007); Northwestern Costa Rica

(Newbold et al., 1995); Mt. Taranaki, New Zealand (Goldsmith et al., 2008); and

Western Oregon (Martin and Harr, 1988) to evaluate potential controls on silicate weathering and CO2 consumption from andesitic terrains. Factors such as parent material

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chemistry and age, runoff, temperature, and physical weathering rates (where available) will be evaluated. Finally, weathering equations derived from the dataset analysis will be used in conjunction with a new highly-detailed lithology map for East and Southeast Asia in an effort to calculate CO2 drawdown from the chemical weathering of andestic terrain for this region.

5.3. Study Area Background

5.3.1. Geology

5.3.1.1. Volcán Barú, Panama

Active volcanism in western Panama is related to the northwestward subduction of the Nazca Plate along with the Cocos Ridge beneath southern Central America (Figure

5.1a). Volcanic centers in the region are part of a Quaternary age chain of stratovolcanoes that form the backbone of Panama. Previous studies on these volcanic centers revealed ages from 17.5 Ma to 300 yr B.P. with the majority of dates concentrated between 12–7 Ma and around 1.3±1 Ma. Volcanism appears to have been significantly reduced between 6 and 2 Ma, while 14C ages have provided evidence for recent activity

(≤500 yr B.P.) at Volcán Barú (elev. 3447 m) and La Yeguada (elev. 1297 m) (deBoer,

1991). Basalts are generally absent from the volcanic centers near the Costa Rica border

(Volcán Barú and Tisinigal) where basaltic andesites and andesites predominating.

Volcanic rocks of Barú range from 52 to 65% SiO2 (basaltic andesite to low silica dacite), contain moderate Na2O and K2O (Le Bas and Streckeisen, 1991), and exhibit fractionated rare-earth-element geochemistry (high La:Rb ratio), high Sr content, and no Eu anomoly

(deBoer et al., 1988). The trace and rare earth element signature is characteristic of what

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might be expected at the ends of a volcanic arc where a diminished magmatic component is commensurate with a lower rate of subduction (Sherrod et al., 2008).

Volcanism at Volcán Barú commenced approximately 0.5 Ma (Universidad

Tecnológica de Panamá, 1992) and is younger than that of Volcán Tsinigal located adjacent to the west. Radiocarbon data from wood and bulk soil charcoal in paleosols interspersed with volcanic deposits point to at least four major eruptive periods in the past

1,600 years with the youngest occurring sometime approximately 400 14C calibrated years before present (A.D. 1550). Each of these eruptive periods is believed to have had consisted of prolonged dome growth, explosive eruptions, and the spalling of numerous block-and-ash flows followed by a period of quiescence (Sherrod et al., 2008). This historical explosive activity has produced a composite cone formed predominantly of numerous overlapping pyroclastic flows, lahars and lava flows.

The short but often violent history of Volcán Barú has resulted in two large scale land features that compose the bulk of the surficial deposits and corresponding landforms characteristic of the study area. First, an extensive debris avalanche southwest of the cone covering an area ~30 km long by ~20 km wide (25-30 km3 total volume) has been identified in the field as an area of hummocky topography with scattered volcanic blocks

(Siebert et al., 2004; 2006). The age of the debris avalanche is unknown but believed older than 50,000 years B.P. due to a radiocarbon depleted tree sample from a deposit located directly beneath (Sherrod et al., 2008). The debris avalanche deposits are the largest yet discovered in Latin America and approximately tenfold larger than those associated with the 1980 eruption of Mt. Saint Helens. Secondly, a slightly dissected volcaniclastic fan comprising largely lahars and pyroclastic flows grading to an

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assortment of lahars and alluvial deposits further downslope extends to the southeast of the cone. The lahar plain covers approximately 630 km2 and encompases a volume in excess of 60 km3 and has been proposed to encompass an additional 30km3 further south of lat 8⁰ 30′ (Sherrod et al., 2008).

5.3.1.2. Mt. Pinatubo, Luzon, Philippines

The arc front volcanoes of the central portion of Luzon Island in the Philippines are associated with andesite/dacite eruptive activity beginning approximately 7 Ma

(Defant et al., 1989). Mt. Pinatubo (15.13°N 120.35°E ; elev. 1486 m), an andesite-dacite dome complex and stratovolcano surrounded by an extensive apron of pyroclastic flow and lahar deposits, provides a perfect example of recent explosive activity. The eruptive history of Pinatubo is divided into two parts: ancestral (~1 Ma to an unknown time before

35 ka) and modern (35 ka to the present) (Newhall et al., 1996). No evidence of large scale eruptions prior to 35 ka have been identified, while at least six and up to twelve major eruptive periods have occurred since. These explosive periods are largely associated with pyroclastic flows and lahars which possibly diminished in volume over time, with the eruption occurring ~35 ka believed to be 5x as large as the eruption of

1991 (Newhall et al., 1996).

The 1991 eruption, Pinatubo’s second largest in the 20th century, resulted in a summit elevation loss of ~260 m and created a 2.5 km wide caldera (Jones and Newhall,

1996). While the eruption is largely noteworthy for its injection of 17–20 Mt of SO2 into the atmosphere (Bluth et al., 1992) which resulted in a global decrease in temperature of

~0.5 °C (Hansen et al., 1996) it also injected ~1.3–1.8 1016 g of tephra into the

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atmosphere (Openheimer, 2004) partly resulting in the deposition of ~5–6 km3 of pyroclastic flow deposits in the immediate vicinity of the cone (Scott et al., 1996). As with past activity, the eruption was probably triggered by the injection of a more mafic magma into a highly evolved dacitic magma chamber. Evidence of this trigger mechanism is believed to be in the composition of tephra material itself, which changed in composition from andesitic to dacitic during the main eruption period from June 12-15,

1991 (Hoblitt et al., 1996). Although the volume of tephra material extruded during the

1991 eruption pales in comparison to previous events, it resulted in widespread lahars which are still readily mobilized during typhoons.

5.3.2. Climate and Hydrology

5.3.2.1. Volcán Barú, Panama

Panama is characterized by a humid tropical climate with annual rainfall strongly influenced by the location in the Intertropical Convergence Zone (ITCZ). A northern shift of the ITCZ during boreal summer results in 90% of the annual precipitation derived from local thermally driven convection generated storms occurring between May and

December. A marked dry season occurs from January to April (Harmon et al., 2009).

The orographic effect of the Cordillera de Talamanaca, of which Volcán Barú is a part, also enhances precipitation in the region which can reach an annual value of 4280 mm.

(Espinosa et al. 1997). Long-term stream gauging records maintained by Empresa de

Transmisión Eléctrica S.A. (ETESA) confirm this seasonal effect of rainfall on discharge with average monthly flows for the wet season three to four times greater than in the dry season.

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5.3.2.2. Mt. Pinatubo, Luzon, Philippines

West-central Luzon is characterized by a humid tropical climate which is governed by seasonal changes in positions of dominant air masses and attendant wind shifts. The region experiences a mean annual rainfall of ~3800 mm with a distinct wet season from June to October followed by a marked dry season from November to May

(Rodolfo et al., 1996). However, unlike western Panama, west-central Luzon also comes under the influence of tropical cyclones due to its more northerly location (15⁰ N to 8⁰ N, respectively). The region annually experiences approximately four typhoons, which tend to originate just to the east over the Pacific Ocean (Wu et al., 2005). The high annual rainfall along with the large-scale pyroclastic deposits from the 1991 eruption led to at least two years of extensive lahars on several of the main rivers systems draining the peak itself (Scobia-Bamban, Gumain, O’Donnell, Pasig and Porac to the east and Marella,

Santo Thomas, Maloma, and Bucao River to the west) (Major et al., 1996; Rodolfo et al.,

1996). Several of these lahars resulted in the alteration and infilling of the channels themselves, which in some cases are still in the process of being incised. Field reconnaissance in November 2008 identified this material in abundant supply in several of these channels.

5.4. Sampling, Analytical Methods, and Data Compilation Methodology

5.4.1. Sample Methodology

A total of eight (8) stream water samples were collected from four (4) watersheds in the vicinity of Volcán Barú in western Panama in July 2006 and in February 2007

(Figure 5.1(a) and Table 5.1). In addition, sixteen (16) stream water samples from eleven

(11) watersheds in the vicinity of Mt. Pinatubo in the Philippines in November 2008 115

(Figure 5.1(b) and Table 5.1). Samples were collected by hand within 1 m of shore, and generally from river mouth locations well above the influence of the tidal zone (where applicable). Additional samples were collected from upstream locations where conditions allowed. Three (3) hot spring/spring water samples were also collected in the

Philippines in November 2008. No fresh water springs were encountered during the

Panama field reconnaissance.

5.4.2. Stream Gauging and Annual Discharge Determination

Mean annual discharge data from long-term river gauge records were provided by

ETESA. The stream gauge records vary in length between 30 years and greater than 40 years. Annual discharge values for each of the gauged streams were estimated from the discharge measurements for the wet and dry season for the annual hydrograph.

Manual stream gauging of river flow was conducted at eight (8) of the Philippines sites using United Stated Geological Survey methodology for measurement and computation of streamflow (Rantz et al., 1992). A depth profile of the channel was determined at each location prior to using a General Oceanic Inc.™ flow meter to obtain flow readings at 20% and 80% above the base of the channel at minimum of three locations for small streams and 11 locations for large streams. Instantaneous flows were calculated by integrating over the river cross-sectional area. Instantaneous flows for the dry season were extrapolated out to monthly flows and summed for seven months for the dry season.

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5.4.3. Water Analysis

Water samples were collected in new, deionized water (18 MΩ) soaked low density polyethylene (LDPE) Nalgene™ bottles, which were rinsed in river water three times prior to sample collection. A small aliquot of sample was used for in-field measurement of pH. Samples were stored in the dark at room temperature prior to filtration. Samples were filtered through a 47 mm diameter (nominal pore size of 0.4μm) polycarbonate filter directly into an acid washed 60 ml LDPE bottle for cation analysis and a deionized water soaked 60 ml LDPE bottle for anion analysis. Filter blanks were created by filtering 18 MΩ water into clean LDPE bottles using the same methods used for samples. Trip blanks were created prior to sample collection with 18 MΩ water in clean LDPE bottles. The samples, trip blanks, and field blanks were subsequently shipped to the Ohio State University and chilled at ~4°C until analysis. Upon return, each filtered sample for cation analysis was acidified to pH 2–3 with Ultrex™ HNO3 for

+ + + 2+ 2+ 2+ preservation. Major ion concentrations of cations (Li , Na , NH4 , K+, Ca , Mg , Sr )

- - - - - 3- 2- and anions (F , Cl , NO2 , Br , NO3 , PO4 , SO4 ), were determined via ion

® chromatography (IC) using a DX-120 ion chromatograph using the methods of Welch et al. (1996). Precision was determined using five replicate check standards per run with relative standard deviations (RSDs) usually better than ±1% and never greater than ±5%.

Dissolved Si concentrations for 11 of the 16 Panama samples were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES). External standards were used and check standards were run every 3-4 samples to account for instrument drift. Triplicate analyses did not reveal RSDs greater than ±5%. Reactive silica for the five remaining Panama samples and all of the Philippines samples were

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determined by colorimetrically with a Shimadzu UV-mini 1240 UV-Vis

Spectrophotometer using a 1 cm quartz cell based on a modified method of Mullin and

Riley (1955).

5.4.4. Data Interpretation

For both the Panama and Philippines water samples, calculations were performed

- - to determine alkalinity (HCO3 ) in these streams. Sample HCO3 was determined via the method of Lyons et al. (1992) as the difference between Σcations in microequivalants and

Σanions in microequivalants. This method assumes little contribution to alkalinity from

- dissolved species other than HCO3 . This previous work has demonstrated that alkalinity

- values derived from this technique provide excellent estimates of HCO3 in river and stream waters (Lyons et al., 1992).

Precipitation contribution to stream water dissolved chemistry was determined via a modified method of Stallard and Edmond (1981). Based on the absence of evaporite

- rocks shown on existing geological maps for the study areas, all Cl in stream water

- + + samples was assumed to be from precipitation. For Panama, ratios of Cl to Na , K ,

2+ 2+ 2- Mg , Ca , and SO4 were determined using seasonal precipitation data from a nearby location in Costa Rica (Eklund et al., 1997) while for the Philippines the same elemental ratios to Cl- were determined using precipitation chemistry for Northern Luzon (Baguio)

+ 2+ - - (Cole et al., 2004). Ratios of Li , Sr , F , and Br were determined from sea salt data of

Bruland (1983). This is a conservative approach to account for marine input into rainwater and may result in an underestimation of TDS from chemical erosion, since

- some Cl present in river water may result from chemical weathering or volcanic activity.

Major and trace elemental concentrations measured in the stream water samples are 118

provided in Table 5.2. It is assumed that all the Si in the streams originates from chemical weathering (Stallard and Edmond, 1981).

Chemical erosion yields were determined by multiplying the precipitation corrected TDS concentrations by the seasonal stream discharge data and dividing by the watershed area.

Watershed areas used in the calculations are for the area of the drainage basin above the respective river gauge or sampling location. Watershed areas for Panama were provided by ETESA while those for the Philippines were determined using Google™ Earth Pro

5.0. The yields were subsequently divided by the watershed area in order to compare between watersheds. Silicate weathering yields were determined in similar fashion by multiplying the dissolved Si concentrations for the wet and dry season data by the calculated stream discharge data for the respective season, summing the values (where applicable), and dividing the result by the watershed area.

5.5. Results and Discussion

5.5.1. Correction for Geothermal Input

Prior to the 1991 eruptive activities at Mt. Pinatubo, springs extended up to 10 km outwards along a north-northwest trend on both sides of the cone (Delfin et al., 1992,

1996). Geochemical analysis of the pre-eruptive springs showed their composition varied with distance from the cone with Cl-HCO3 springs founds at low elevations and SO4-

HCO3 and Cl-SO4 type springs on the summit itself, all with near neutral pHs. The highest elevation features were low pH Cl-SO4 boiling pools (Delfin et al., 1992, 1996). 119

A compilation of spring, stream, and deep well water samples collected before and after the 1991 eruption by Stimac et al. (2004) shows that Pinatubo’s modern day system is dominated by meteoric water and influx from Cl-SO4 and Cl-HCO3 water with localized

2- 2+ peaks in SO4 and Ca concentrations resulting from the leaching of anhydrite and aerosol-laden tephra.

In order to correct for the geothermal input in the Pinatubo stream water samples, a three component end member was constructed using data from the previous compilation data from Stimac et al. (2004) and samples collected from this study. A Cl-HCO3-SO4 ternary diagram clearly shows these three end members as well as their influence on the stream water samples (Figure 5.2). Subsequent correction for geothermal input (using the precipitation corrected dataset) was based on a modified method of Dessert et al., (2009).

Using the Cl content of rainwater previously collected in Northern Luzon (37.5 µmol/l) by Cole et al. (2004) all remaining Cl was attributed to geothermal input. The amount of each solute in stream water originating from high temperature weathering was calculated as follows:

[X] geoth = [Cl] geoth * (X/Cl)hot-springs

Whereas [Cl]geoth represents the Cl in each stream water sample in excess of 37.5 µmol/l and (X/Cl)hot-springs represents the average chlorine normalized geothermal end member ratios for each solute. While Dessert et al. (2009) used an average of their end member geothermal ratios, this best fit model quantifies relative input of the three end member waters. The premise of the model is that no sample can show greater geothermal input for any one element in excess of its stream water concentration and that relative input of

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the end members should be in agreement with the previous geographical distribution of geothermal water types determined by Stimac et al. (2004). Correction of HCO3 concentrations for geothermal input was also conducted in a similar fashion in order to avoid over estimating HCO3 originating solely from the atmosphere.

5.5.2. Concentration of Major Elements and Statistical Analysis

5.5.2.1. Mount Pinatubo

The correction methodology showed samples from the upstream O’Donnell watershed are distinctly influenced by the SO4 end member while samples further away from the cone showed stronger influence from both the HCO3 and Cl end members. Of note, samples collected from O’Donnell River, Sapang Bato River, freshwater tributary of Sapang Bato River, Sapang Bato River (confluence), and the Santo Thomas River did not show a non-geothermal component with this model. However, field reconnaissance revealed two major geothermal sources to the stream water in close proximity to the sampling sites for the O’Donnell and Sapang Bato, and the crater lake of Mt. Pinatubo forms the headwaters of the Santo Thomas River of Mt. Pinatubo. These samples were subsequently removed from discussion of the dataset. The findings of end member dominance in the remaining samples are in geographical agreement with the current hydrothermal model proposed by Stimac et al. (2004) (i.e. sulphate dominance close to the cone and HCO3 and Cl dominance further away) suggesting that the hydrothermal input into the stream water samples has been adequately estimated.

The precipitation and hydrothermal total cation chemistry for the remaining

2+ stream water samples demonstrate that Mg is the major cation from weathering and it constitutes approximately 55.8% of the total cation charge, followed by Ca (25%), Na 121

(24%), and K (0.2%). However, this overall value is skewed by four of the values from rivers draining the western flank of the volcano which are dominated by Mg abundance

(Baquilan River, Bucao River (mouth), Maloma River, and Gorangoro River), which may reflect the fact these rivers drained terrain also underlain by the Zambales Ophiolite

Complex, an easterly-dipping slab of Eocene ocean crust uplifted during the late

Oligocene (Villones, 1980). Three of the remaining seven stream water samples exhibited a cation abundance of Ca>Mg>Na>K while the remaining four sites exhibiting an order of Ca>Na>Mg>K. All but two of the samples (Bamban River and Pasig River)

- 2- - had HCO3 as the dominant anion over SO4 , with the HCO3 concentration ranging from

~3 (Tributary of the Porac River (PH08-12)) to ~50x (Gorangoro River (PH08-17)) the

2- respective SO4 concentration.

The precipitation-corrected total dissolved solids (TDS) concentrations had a range of 36.3 to 436 mg/kg with a median TDS concentration of 171 mg/kg. The Bamban

River exhibited the highest TDS concentration (436 mg/kg) exceeding those of rivers

2+ 2- many times its size. Its high concentrations of Ca and SO4 may reflect a large input from the weathering of anhydrite. Anhydrite has been found in abundance as a primary phenocryst in tephra deposits from the 1991 eruption (Bernard et al., 1991; Bernard et al.,

1996; Jakubowski et al., 2002) and has been linked to vapor phase deposition from a magma with a high sulfur content (Jakubowski et al., 2002). Furthermore, the rapid breakdown of anhydrite in natural and geothermal waters has been previously regarded as

2+ 2- a major source of both elevated Ca and SO4 in stream water and spring samples in the upstream O’Donnell watershed (Stimac et al., 2004). To delineate the importance of anhydrite weathering in these streams, anhydrite saturation indices were calculated for

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the uncorrected stream water chemistry using the software program PHREEQC

(Parkhurst and Appelo, 1999). The calculations revealed six of the sixteen dry season stream water samples (38%) were saturated with respect to the anhydrite.

In order to delineate the influence of anhydrite weathering in the samples previously corrected for precipitation and hydrothermal activity, Ca2+ was removed at a

2- 1:1 ratio with the remaining SO4 . Correction for anhydrite lowered to the total TDS in the samples from a range of 83 to 436 mg/kg to a range of 40 to 193 mg/kg. The anhydrite corrected amounts are between 38 and 78% of the previously corrected TDS.

All remaining Ca2+ in the stream water samples is attributed solely to silicate weathering.

5.5.2.2. Volcán Barú, Panama

The geochemistry of the streams, after correction for precipitation input had distinct chemical differences between the wet and dry seasons. The precipitation- corrected total cation chemistry (TZ+) for the streams in the wet season demonstrates that

2+ Ca is the major cation from weathering and it constitutes approximately 48% of the total cation charge, followed by Mg (27%), Na (20%), and K (5%). The stream water samples

- all exhibited a cation abundance of Ca>Mg>Na>K. All samples exhibit HCO3 as the

2- - dominant anion over SO4 , with the HCO3 concentration ranging from ~26 (low order tributary of Rio Chiriqui Viejo (060820-3)) to ~7080x (Rio Caisen (060820-4))) the

2- respective SO4 concentration. The dry season samples had similiar total cation charge

2+ 2+ albeit with higher proportions of Mg . Ca constituted approximately 38% of the total cation charge and was followed by Mg (35%), Na (21%), and K (6%). The stream water samples showed more variability with respect to the order of cation abundance with three samples exhibiting Ca>Mg>Na>K, two sites exhibiting Mg>Ca>Na>K, two sites 123

exhibiting Ca>Na>Mg>K and one site exhibiting Na>Mg>Ca>K. Anion chemistry was similar to that for the wet season, albeit with SO4 showing an increasing relative

- 2- importance, with all samples exhibiting HCO3 as the dominant anion over SO4 and

- HCO3 concentrations ranging from ~5.3 (Rio Chiriqui Viejo at Nueva Suiza (060820-1))

2- to ~6200x (Rio Platanal (060819-9)) the respective SO4 concentration.

This distinctive solute chemistry for each season resulted in a factor of three difference in precipitation-corrected total dissolved solids (TDS) concentrations with a range of 42.7 to 90.4 mg/kg during the wet season and a range of 47.3 to 157 mg/kg during the dry season. Median TDS concentrations for the wet and dry seasons were 59.3 mg/kg and 106 mg/kg, respectively. Solute concentrations in the dry season ranged from approximately 1.8 to 2.4 times greater during the respective wet season value. While this may in part reflect dilution of weathering products by rainfall and differences in stream flow, it is also worth noting that the composition of the weathering products are different in the wet and dry seasons.

A statistical analysis of the precipitation-corrected dissolved load data was performed in order to determine the existence of significantly different average concentrations among the wet and dry seasons. A two-tailed t-test with a null hypothesis of no difference in mean elemental values (α = 0.05) between the seasons conducted for all analyzed species and calculated alkalinities rejected the null hypothesis of no

2+ - difference between mean ionic concentrations for Mg (p = 0.006) and F (p = 0.05), The

+ + 2+ null hypothesis could not be rejected for Na (p = 0.13), K (p = 0.13), Ca (p = 0.34), Si

- 2- - (p = 0.34), NO3 (p = 0.069), SO4 (p = 0.12), and HCO3 (p = 0.1). However, the

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correspondingly high p-values associated with some of these analyses indicates more data are necessary to reach a definitive conclusion.

5.5.3. Chemical Weathering Rates

Chemical weathering yields, based on a compilation of TDS and calculated

-2 -1 discharge values for each season, ranged from 143 to 260 tons km a for the rivers

-2 -1 draining Volcán Barú and a dry season yield of 5 to 44 tons km a for those draining Mt.

Pinatubo (Table 5.3). While the dry season values for the Pinatubo may not appear formidable, it is worth noting that 84% of the annual precipitation for the region occurs during the wet season from June to October (NOAA, 2009). Two of the rivers draining

Mt. Baru are located in areas underlain by the extensive debris avalanche deposits (Rio

Chiriqui Viejo and Rio Chico) while the remaining river is located within the volcaniclastic fan consisting largely of lahars and pyroclastic flows (Rio Chiriqui) thereby confirming the importance both of these substrates on producing high chemical erosion yields. The Pasig, Porac, and Gumain rivers all drain the eastern flank of Mt.

Pinatubo and served as depocenters of pyroclastic flow deposits from the 1991 eruption

(Major et al., 1996; Rodolfo et al., 1996). A comparison of chemical weathering rates to physical erosion rates was not possible given the lack of suspended sediment gauging records for the regions.

Silicate weathering yields determined for this study, based on a compilation of dissolved Si concentrations and calculated discharge values for the wet and dry seasons,

-2 -1 -2 ranged from 23–42 tons km a for Volcán Barú and a dry season flux of 1–8 tons km

-1 a for Mt. Pinatubo (Table 5.3). Average basin-scale CO2 consumption yields by silicate weathering were calculated as TZ+ (Ca + Mg + Na + K) in equivalents (corrected for sea 125

salts), multiplied by discharge and divided by watershed area. The corresponding CO2

3 -2 -1 consumption values for western Panama ranged from 1532–2882 x10 moles km a

(Table 5.3). The CO2 consumption values for the western Panama rivers are similar to the maximum values for other andesite-dacite terrains such as the islands of Guadeloupe

3 -2 -1 and Martinique (1.1–1.4 x 10 mol km a ) (Rad et al., 2006), the Taranaki Region of

3 -2 -1 New Zealand (2926 x 10 mol km a ) (Goldsmith et al., 2008), and the island of

3 -2 -1 Dominica (580–2450 x 10 mol km a ) (Goldsmith et al., 2009). In fact, the western

Panama CO2 consumption rates are so high they only fall within the range of values

3 previously determined for the basaltic terrains of the Deccan Traps (580–2450 x 10 mol

-2 -1 3 -2 -1 km a ) by Dessert et al. (2001), Réunion Island (1300–4400 x 10 mol km a ) by

3 -2 -1 Louvat and Allègre (1997), and Iceland (141–1764 x 10 mol km a ) by Stefánsson and

Gíslason (2001). Therefore, these high CO2 consumption values for the rivers draining

Volcán Barú and Mt. Pinatubo confirm the weathering potential of andesite-dacite lithology on a global basis.

5.6. Worldwide Summary of Andesitic Terrains and Search for Controls

5.6.1. Summary of Datasets and Subsequent Corrections

The Mt. Pinatubo and Volcán Barú datasets were combined with the following published datasets in order to evaluate potential controls on andesite-dacite weathering:

Dominica (Goldsmith et al. 2009); Martinique and Guadeloupe (Rad et al., 2006; Rad et al., 2007); Northwestern Costa Rica (Newbold et al., 1995), Mt. Taranaki, New Zealand

(Goldsmith et al., 2008); and Western Oregon (Martin and Harr, 1988). The datasets consisted of either seasonal or annual averages.

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Where applicable, data were corrected for precipitation using the method detailed by the authors. The data for Costa Rica were corrected for precipitation using ratios of

- + + 2+ 2+ 2- Cl to Na , K , Mg , Ca , and SO4 determined using seasonal precipitation data from a nearby location in Costa Rica (Eklund et al., 1997). Precipitation correction for the

Western Oregon data was conducted in a similar fashion using precipitation chemistry from the main meteorological station at the H.J. Andrews Experimental Forest (Martin and Harr, 1988). Corrections for geothermal input were also conducted where applicable.

For example, Ramos-Escobedo and Vazquez (2001) identified geothermal signatures in the Manantiales River and therefore this stream was removed from the dataset. In the case of the Taranaki dataset, Ca and Mg concentrations were corrected using silicate end member values for andesitic terrains of 0.5 as determined by Gaillardet et al. (1999) in order to account for input from sedimentary terrains in the lower reaches of the watersheds.

In the same manner as Dessert et al.’s evaluation of basaltic terrains, this study focuses on the following indicators of chemical weathering composed from the precipitation corrected datasets: mean bicarbonate concentration, mean cationic concentration (Ca, Mg, Na, and K) (TDScat), and mean total dissolved solids ( SiO2, Ca,

Mg, Na, K, SO4) (TDSw). Average concentration for each of these parameters is presented in Table 5.4.

Average riverine bicarbonate concentrations ranged between 353 and 1550 µmol/l and reflect atmospheric CO2 consumption from the weathering of andesitic volcanics.

CO2 consumption rates for the respective datasets were calculated using runoff values from the respective sampling period (where available) or using known average annual

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6 -2 values (Table 5.4). CO2 consumption values ranged from 0.5 to 4.3 x 10 moles km yr-1, are higher than most of the world’s major rivers (Gaillardet et al., 1999) and are similar to those previously determined for basaltic terrains (Dessert et al., 2003). The highest value of 4.3 x 106 moles km-2 yr-1 for the Philippines is exceeded only in the

6 -2 -1 basaltic dataset by Java (6.43 x 10 moles km yr ). TDScat concentrations ranged from

7 to 34 mg/l. Cationic weathering fluxes were calculated in a similar fashion using

-2 -1 TDScat and runoff values, and ranged from 9 t km yr for western Oregon to 93 tons km-2 yr-1 for Pinatubo. TDSw concentrations ranged from 20 to 77 mg/l and corresponding chemical weathering rates ranged from 27 to 207 tons km-2 yr-1.

5.6.2. Elemental Ratios

5.6.2.1. Ca:Mg

Average Ca:Mg molar ratios (corrected for atmospheric input) for all the sites range between 1.5 - 2.7 (Table 5.5). Except for the Bamban River (Ca:Mg ratio of 11.8), from the area draining Mt. Pinatubo in the Philippines, the range of Ca:Mg ratios also showed remarkable consistency (0.1 - 3.8). Almost all of the samples for this study exhibited Ca:Mg ratios much lower than the world average of 2.4 (Harmon et al., 2009) and are consistent with those previously determined for basaltic terrains (0.7 - 1.8) by

Dessert et al. (2003). These low averages indicate the importance of the weathering of

Mg-rich minerals such as pyroxenes (ortho-, clino-, and hypersthene) and hornblende which has been found to compose significant portions of both the phenocrysts and groundmass in both the volcanic rock and tephra/ignimbrite samples for these settings

(Goldsmith et al., 2008; Goldsmith et al., 2009).

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5.6.2.2. Na Normalized Concentrations

Molar ratios of Ca/Na, HCO3/Na, and Mg/Na ratios were calculated for the precipitation corrected water chemistry (Table 5.5). These elemental ratios were chosen to avoid dilution and evaporation effects when comparing different weathering regimes worldwide (Gaillardet et al., 1999). The majority of the HCO3/Na molar ratios varied between 0.5 and 8 with a mean value around 3.8; Ca/Na ratios ranged between 0.2 and 10 with a mean value around 1.6; and Mg/Na ratios vary between 0.4 and 2.6 with a mean value close to 0.9. In general, the ratios were in excess of silicate end member values determined for the world’s major rivers by Gaillardet et al. (1999) and well within the range previously determined for basaltic terrains by Dessert et al. (2003). The Mt.

Pinatubo rivers exhibited the highest Na normalized ratios of the dataset, which may reflect the young age of deposits and their ability to be readily weathered.

With the exception of some of the Martinique and Guadeloupe streamwater samples, a general mixing trend between the silicate and carbonate end members was observed within the dataset (Figure 5.3). However, the low Ca:Mg ratios of the dataset is more indicative of the weathering of intermediate volcanic rocks than the input from carbonate or the possible presence of disseminated calcite in the parent material.

Regardless, the fact that the majority of the molar ranges and averages for andesitic terrains are greater than the continental silicate end member ratios and fall within the range previously identified for basaltic terrains worldwide (Table 5.5; Figure 5.3) confirms the chemical weathering potential of andesitic material. These findings suggest that some mechanisms (e.g., ideal soil thickness, relative abundance of tephra/pryoclastic material, crystallinity of parent material, and/or presence of conduits to soil/bedrock

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interface) can readily supply these constituents to stream water. Furthermore, the high concentrations of Ca and Mg being delivered in stream water confirm that andesitic terrains should not be combined with bulk silicate weathering in determining global CO2 consumption values.

5.6.3. Evaluation of Potential Controls on Andesitic Weathering

Previous attempts to calculate global CO2 consumption from silicate weathering have focused on the effects of runoff and temperature. Herein I expand the evaluation to also include the average SiO2 content as well as the average age of the parent material to determine the role these parameters play, if any, on observed fluxes. The relative importance of factors such as soil thickness and physical erosion rates could not be addressed with the available data.

5.6.3.1. Average SiO2 Content of Parent Material

One of the primary difficulties in summarizing fluxes from volcanic terrains of intermediate composition at active margins is establishing a common geochemical control for lithology as major petrologic differences can result from factors such as slab collision rate, angle of subduction, thickness of mantle wedge, slab melt vs. sediment fluid input, and relatively maturity of the arc itself. In an effort to determine whether these geochemical difference in lithology can play a role on fluxes data from both volcanic edifice/lava flow and pyroclastic samples were compiled for the study area sites

(Dominica (n = 62), Taranaki New Zealand (n = 29), Guadeloupe (n = 62), Martinique (n

= 168), Panama (n = 72), and the Philippines (n = 54)). Geochemical information for the

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inactive volcanoes Orosí and Cerro Cacao in Northwestern Costa Rica and the H.J.

Andrews Experimental Forest in Western Oregon was not available.

A plot of total alkalis (in weight %) vs. SiO2 (in weight %) based on the classification scheme of Le Bas and Streckeisen (1991) revealed a wide range of compositions with SiO2 concentrations of surficial units from 44.50 wt % (basalt) to

75.00 wt% (dacite) (Table 5.6; Figure 5.4). In general, the Mt. Pinatubo, Dominica, and

Martinique samples were the most evolved while the Mt. Taranaki and Guadeloupe samples were the least evolved. This range in values reflects the difficulties in assessing the lithologic impact on fluxes as a previous compilation of laboratory weathering rates for igneous rock has shown a decrease in chemical weathering rates associated with a corresponding increase in Si composition. The previous results were attributed to an increase in interlinked Si-O bonds as silicates become more evolved (Wolf-Boenisch et al., 2006).

A plot of average HCO3 vs. average SiO2 of the dataset had a strong positive correlation (r2 = 0.83, p = 0.04). This correlation could possibly be explained as an indication of the explosive nature of the eruption itself and the subsequent availability of tephra ability which can be readily weathered, but plots of average TDScat and TDSw showed only slight positive correlations (r2 = 0.27, p = 0.5; and r2 = 0.11, p = 0.8 ; respectively). The result could also be explained as an artifact of a slight positive

2 correlation of SiO2 with runoff (r = 0.45, p = 0.45). The high p- value associated with this calculation confirms the need for additional data points.

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5.6.3.2. Parent Material Age

One of the primary difficulties in analyzing fluxes from volcanic terrains of intermediate composition at active margins is the ability to determine the effect of parent material age on observed fluxes. As shown in Chapter 4, weathering yields in streams draining relatively younger parent material exhibit higher silicate weathering fluxes and

CO2 consumption. However, it remains unknown whether these preferential effects could be sustained over longer time periods.

A summary of parent material ages for the study was compiled to determine these effects (Table 5.7). Age of the last major eruption/or eruptive period is provided because the most recent eruption is the source of the primary pyroclastics and ash to large-scale areas of the respective basins which could be readily weathered. Specific ages for the volcanism in Northwestern Costa Rica and Western Oregon were not available. Ages ranged from approximately 19 yrs before present for Mt. Pinatubo (Newhall et al., 1996) to ~30,000 yrs for Dominica (Sigurdsson (1972); Sigurdsson and Carey (1991)). The range of surficial ages of parent material at each setting is also provided in order to highlight the complications in delineating the average age of parent material for each setting. Comparisons of the last major eruption/or eruptive period to the geochemical

- 2 weathering parameters did not reveal any correlation with average HCO3 (r = 0.02; p =

0.02), TDScat (r2 = 0.07; p = 0.003), or TDSw (r2 = 0.007; p = 0.009) (Figure 5.5).

Furthermore, removal of the most recent dataset (Mt. Pinatubo) would only further decrease the r2 values. While clearly more datasets are needed in order to determine these sustained effects over time, this lack of correlation points to some mechanism in these terrains which sustains high rates of solute delivery over time. Does a process such

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as physical erosion provide new surfaces the exposure of earlier previously unaltered deposits which can be readily weathered and allow weathering rates to be sustained over time? Such a process might be aided by the nature of active margins themselves which would allow for periodic deposition of andesitic-dacitic material over time.

5.6.3.3 Runoff and Temperature

Previous attempts to calculate annual global CO2 drawdown from silicate weathering have focused on the relationship between runoff and weathering rate

(Meybeck, 1987; Bluth and Kump, 1994; Amiotte-Suchet et al. 2003; Ludwig et al.,

1988). This approach was duplicated by plotting CO2 consumption rates, cationic weathering rates and silicate weathering rates vs. runoff (Figure 5.7). Of note, neither specific rates nor runoff is an independent parameter since they are calculated from:

f = Rf x C (Eq. 5.1)

6 -2 -1 -2 -1 where f is the specific weathering flux in either 10 mol km a or t km a , Rf is runoff and C is either the average bicarbonate concentration in µmol/l or the TDScat /TDSw concentration in mg/l. Runoff rates for the dataset were either obtained directly from the literature (Guadeloupe, Martinique, Northwestern Costa Rica and Western Oregon) or calculated as the average value of annual discharge divided by watershed area for all the streams in each the study areas (Dominica, Panama, Philippines, Taranaki New Zealand).

The origination of the runoff value for each setting is given in Table 5.4.

The runoff plot (Figure 5.8) also shows that for any particular runoff value, differences in vertical position of the data points can be attributed to an increase in the mean annual temperature. Correlations of the weathering parameters to temperature 133

- (Figure 5.8) revealed correlations for average HCO3 (0.71 to 0.48), TDSw (0.76 to 0.69),

TDScat (0.68 to 0.76) similar to those determined for basaltic terrains by Dessert et al.

(2003). However, respective p-values fro the respective regression analyses where high

(0.8, 0.9, and 0.8, respectively) and indicate future datasets from andesitic terrains are warranted to confirm the strength of this link.

The scatter at the upper end of the temperature range with the chosen weathering parameters could be the result of a number of factors. The highest point from the trend line on all three plots is the one for Mt. Pinatubo. The streams draining Mt. Pinatubo have a known geothermal component and may represent an under compensation for this high-temperature weathering contribution in the correction methodology. In contrast, thicker soil profiles in Guadeloupe and Martinique have been previously suggested as an explanation for the lower chemical weathering yields for those islands (Rad et al., 2006).

Lack of physical erosion data for all these locales prevents further exploration of this potential correlation. Regardless, the data clearly show both temperature and runoff play a role in the weathering in these locales. These exponential equations of best fit between the weathering parameters and temperature were combined with Eq. (5.1) to produce the following three equations:

fCO2 = Rf x 259.75 exp(0.0429 T) (Eq. 5.2)

fcat = Rf x 5.29 exp(0.0534 T) (Eq. 5.3)

fw = Rf x 14.15 exp(0.0502 T) (Eq. 5.4)

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This approach allows for each locality and an exponential equation was used to account for the scatter in the temperature data. This combined approach was previously shown to better account for yield determination from basaltic terrains by Dessert et al. (2003).

5.7. Present-day CO2 Drawdown from Andesitic Weathering on HSIs and East and Southeast Asia

The solute fluxes presented herein clearly show the chemical weathering of andesitic terrain needs to be taken into account when calculating global silicate weathering rates and associated CO2 consumption. To date there has been no attempt to calculate this significance even at a regional scale. As clearly shown for basaltic terrains by Dessert et al. (2003), world river average compositions are insufficient as they do not delineate the proportions of elements coming from granite, basalt, or (in this case of this study) andesite-dacite weathering. Adding to this complication is andesite’s association with active margin settings where small mountainous rivers are numerous and geochemical data are rare. Furthermore, geology maps for such setting settings are often limited by poor spatial resolution or with respect to geochemistry of the volcanic themselves. These problems are highlighted by previous attempts at calculating average annual CO2 drawdown from silicate weathering globally by Bluth and Kump (1991) and

Amiotte-Suchet et al. (2003), which utilized geologic maps with a 2⁰ by 2⁰ and 1⁰ by 1⁰ spatial resolution, respectively. In addition, the UNESCO (1976) geology map and those introduced by Amiotte-Suchet et al. (2003) and Gibbs and Kump (1994) often simply divide volcanic terrains into acidic and basic subsets, thereby under compensating for intermediate terrain. This categorization methodology also lumps dacitic rocks as acid volcanics, thereby underestimating their weathering and CO2 drawdown potential.

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After Dessert et al. (2003), CO2 drawdown from andesitic terrain on HSIs and the regional province East and Southeast Asia andesitic province was calculated using

(Eq.5.2) and a new high resolution quantitative analysis of area-age distribution geology for the region by Peucker-Ehrenbrink and Miller (2004) and a 1:250,000 Geologic Map of New Zealand-North Island (NZGS, 1947). Spatial resolution of the Peucker-

Ehrenbrink and Miller (2004) dataset varies from 44 km2 per polygon in Japan to 1659 km2 per polygon in Taiwan, with an average of 490 km-2. This dataset is at the highest resolution to date and an order of magnitude better than the most recent global compilations of bedrock lithology. Lithology classifications are divided differently than that of UNESCO and for the sake of this study will consist of the following: mafic- intermediate, intermediate, and intermediate-felsic. As shown in Section 5.3.1, these classifications correspond to the intermediate litholgies with the greatest representation at active margin terrains (basaltic andesite, andesite, and dacite). Countries summarized for this compilation include Brunei, Cambodia, China, East Timor, Indonesia, Japan, Laos,

Malaysia, North Korea, Papua New Guinea, Philippines, South Korea, Taiwan, and

Thailand. This methodology can be extended to other regions when more detailed lithology maps become available. The New Zealand geology map also describes lithology as intermediate to acidic. Average annual runoff and mean annual temperature values utilized in the equations and their respective sources are detailed in Table 5.8.

The methodology resulted in a total area of andesite-dacite terrains on HSIs in the

Pacific Ocean of approximately 0.41 x106 km2 and for East and Southeast Asia of 0.92 x106 km2 (Table 5.8). Using these areas and the equation detailed in Section 3.1, a regional CO2 drawdown value for andesitic-dacitic terrains on HSIs in the Pacific Ocean

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of 0.49 x 1012 moles yr-1 and for East and Southeast Asia of 0.59 x 1012 moles yr-1 were calculated (Table 5.8). These values represent between 5.7 and 6.8% of the CO2 drawdown previously calculated for continental silicates using data for world’s major rivers by Gaillardet et al. (1999). More importantly, the values represent between 16% and 19% of that annual CO2 drawdown value for the weathering of basalts. The total area of andesite-dacite material included in this study only represents between 5.9%

(HSIs) and 13% (East and Southeast Asia) of the total global area of basalts determined by Dessert et al. (2003).

The relatively higher CO2 drawdown value per given area for andesite-dacite is likely reflective of the large quantity of this material found in tropical and subtropical areas, unlike basalts, where flood basalt provinces are often found outside these climatic zones (i.e. Siberia Traps, Columbia River). Future compilations of andesite-dacite materials for areas such as Central and South America as well as the Lesser Antilles will likely further increase. The higher CO2 drawdown resulting from andesitic-dacitic rock weathering. Furthermore, the disproportionately high CO2 drawdown per unit area determined for andesite-dacite terrains on HSIs compared to basaltic terrains is likely reflective of their proximity of these islands to the ocean, where they are subject to aperiodic intense precipitation events (i.e., typhoons).

The CO2 consumption associated with the chemical weathering of andesite-dacite for HSIs and East and Southeast Asia clearly show a worldwide calculation is warranted.

However, limitations in the availability of both geochemical datasets and detailed digital maps preclude further compilation at this time. For example, even though lithology summaries have been completed for andesitic terrains in other geographic provinces such

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as North America the dataset is currently limited at the lower end of the temperature spectrum (7.5⁰C; Western Oregon). Future mapping summaries for tropical/subtropical settings with abundant intermediate lithology including Central America and the Lesser

Antilles will likely contribute significantly to the CO2 drawdown values.

The results presented within are readily acknowledged as an initial attempt to accurately quantify the fluxes and CO2 drawdown in andesitic terrains. Future endeavors which effectively quantify hydrothermal input and thus adequately address volcanic driven CO2 and SO4 weathering will be necessary in order to further constrain the values with regards to atmospheric CO2 consumption. Additional inquiries into the effects of age and whether fluxes can be sustained over time also appear warranted. Finally, it is becoming increasingly clear that focusing solely on surface and near surface weathering at these locales underestimates the role of these terrains as a solute delivery source to the world’s ocean. As shown by Rad et al. (2007), subsurface fluxes to the ocean from volcanic HSIs could be as much as two to five-fold higher than surface fluxes.

Therefore, it is likely future comprehensive flux evaluations from active margin locales will alter our understanding of elemental cycling on both short and long-term time scales.

5.8. Conclusions

Data for rivers draining the andesitic regions of Mt. Pinatubo in the Philippines and Volcán Barú in western Panama revealed some of the highest chemical and silicate yields recorded to date. Corresponding CO2 consumption values are as high as or higher than those previously determined for basaltic terrains. A subsequent compilation of the new and existing datasets shows rivers draining andesite-dacite material are characterized

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by relatively high Na-normalized molar ratios and low Ca:Mg molar ratios compared to those draining continental silicates. Comparison of the dataset to potential controlling parameters revealed the primary importance of runoff and an additional correlation between average HCO3 concentration and the average Si content of the parent material.

No discernible relationship with material age was observed but runoff and temperature were shown to be the dominant controls on solute fluxes. From these relationships and a new highly-detailed lithology map calculations of CO2 consumption value from andesite- dacite weathering on HSIs of 0.49 x 1012 moles a-1 and for East and Southeast Asia of about 0.59 x 1012 moles a-1 were made. These values represent significant portions of annual CO2 consumption values previously calculated for continental silicate weathering and basaltic terrains worldwide and confirm the importance of andesite-dacite weathering as a CO2 sink. These terrains may play an important role in climate evolution over geologic time. This study also points to the need for additional datasets from andesitic terrains to further delineate controls on andesitic weathering.

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5.9 Tables

Table 5.1 Sampling location and watershed area summary.

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Table 5.3 Chemical, silicate, and carbonate weathering yields and CO2 consumption in Panama and the Philippines.

Table 5.4 Mean solute concentration, climatic parameters and rates for andesitic watersheds.

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Table 5.6 Summary of geochemical data for andesite terrains in this study.

Table 5.7 Age of volcanic material included in study (data compiled in 2009).

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Table 5.8 Lithology, runoff, annual temperature and calculated CO2 drawdown for each andesitic province.

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5.10. Figures

Figure 5.1(a) Map of Mt. Pinatubo in Luzon Philippines showing stream water sampling locations. Only watersheds sampled and their respective tributaries are shown.

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Figure 5.1(b) Map of Volcan Baru in Luzon western Panama showing stream water sampling locations. Only watersheds sampled and their respective tributaries are shown. RCV = Rio Chiriqui Viejo.

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Figure 5.2 Cl-SO4-HCO3 ternary diagram for Pinatubo springs and wells and stream water samples.

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Figure 5.3 (a) Mg/Na vs. Ca/Na and (b) HCO3/Na vs. Ca/Na of rivers draining andesitic terrain corrected for atmospheric inputs (after Gaillardet et al., 1999). Basaltic compositions are taken from Dessert et al (2003). End member compositions for granitic silicates and carbonates are those determined by Négrel et al. (1993) and by Gaillardet et al. (1999).

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Figure 5.4 Silica vs. total alkalis variation diagram for the published andesite values utilized in this study. Classification grid from LeBas and Streckeisen (1991). Sources of data points are provided in Table 5.6.

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Figure 5.5 Plots of mean bicarbonate concentrations, TDScat, and TDSw versus average SiO2 content of the parent material. While a strong positive correlation was observed with mean bicarbonate no relationships were observed with TDScat and TDSw (exponential equation).

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Figure 5.6 Plots of (a) mean bicarbonate concentrations, (b) TDScat, and (c) TDSw versus age of most recent eruption/eruptive period. No relationship was observed for any of the weathering parameters with age (exponential equation).

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Figure 5.7 Plots of (a) mean bicarbonate concentrations, (b) TDScat, and (c) TDSw - versus mean annual runoff. Parallel lines represent constant values of HCO3 , TDscat and TDSw. Sources of runoff data are provided in Table 5.4.

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Figure 5.8 Plots of (a) mean bicarbonate concentrations, (b) TDScat, and (c) TDSw versus mean annual temperature.

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CHAPTER 6

CONCLUSIONS AND FUTURE DIRECTIONS

6.1. Conclusions

This study, the first comprehensive evaluation of weathering processes on HSIs, provides valuable insights into the relationship of silicate weathering and global CO2 drawdown on various timescales. Semi-continuous sampling of the Choshui River of central-western Taiwan during Typhoon Mindulle was performed over a 96 hour period from 1 July 2004 through 5 July 2004. Comparison of sediment and sediment POC data with published Taiwan WRA discharge records revealed a POC flux of 5.00x105 tons associated with a sediment flux of 61 million tons during a 96 h period. The linkage of high amounts of POC with sediment concentrations capable of generating a hyperpycnal plume upon reaching the ocean provides the first known evidence for the rapid delivery and burial of POC from the terrestrial system. These fluxes, when combined with storm

8 derived CO2 consumption of 1.65x10 moles from silicate weathering, shed light on the important role of tropical cyclone events on small mountainous rivers as a global sink of

CO2.

Seasonal sampling of Taiwan stream water for both major and trace elements shows the strong influence of carbonate weathering on these systems. The limited exposure of carbonate units in some of these watersheds suggests the importance of 156

weathering of disseminated calcite. Comparisons of silicate weathering yields to potential controlling parameters revealed slight positive correlations between chemical weathering rates and basin average mean annual rainfall and average basin runoff and a slight positive correlation of silicate weathering rates with annual suspended sediment yields. Silicate and carbonate weathering yields also revealed varying relationships with post-uplift age of the landscape, potentially pointing to some ideal relationship among relief, age of rocks exposed, their resulting mineralogy, and weatherability. However,

H2SO4 weathering, originating from the dissolution of pyrite, accounts for up to a one- third of the total chemical weathering in these systems. After correction for H2SO4 weathering, calculated CO2 consumption values from silicate weathering and carbonate still remain elevated compared to world average and likely represent the upper limit for a non-volcanic active margin setting.

A combination of seasonal sampling and manual gauging of Dominica streams revealed the importance of crystallinity and age of parent material on the delivery of high concentrations of solutes in stream water. Annual chemical weathering yields for the watersheds produced from both wet and dry season sampling were found to be similar to those previously determined for andesitic and basaltic terrains. Silicate weathering yields and subsequent CO2 consumption are amongst the highest known and fall only within the upper range of those previously determined for the basaltic terrains of the Deccan Traps and Réunion Island. The fluxes pointed to the importance of crystallinity of andesitic parent material in producing fluxes capable of rivaling those previously determined from the weathering of basalt. The results clearly showed the importance of weathering of

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andesitic terrains at active margins, particularly those on HSIs, when considering global geochemical fluxes.

Data obtained from sampling rivers draining the andesitic-dacitic lithology of

Volcán Barú in Panama and Mt. Pinatubo in the Philippines revealed some of the highest annual silicate weathering yields and CO2 consumption values known and confirmed the importance of this material as a weathering substrate. Compilation of the new and existing datasets shows rivers draining andesite material are characterized by relatively high Na-normalized molar ratios and low Ca:Mg molar concentrations, similar to those for regions draining basalt. No discernible relationship with material age was observed with average SiO2 content or age of the parent material. Runoff and temperature were seen to be the dominant controls on solute fluxes. From these relationships and a new highly-detailed lithology map calculations of CO2 consumption rate from andesite weathering on HSIs of 0.49 x 1012 moles a-1 and for East and Southeast Asia of 0.59 x

12 -1 10 moles a were made. These values represent significant portions of annual CO2 consumption values previously determined for continental silicate weathering and basaltic terrains worldwide and confirm the importance of andesite weathering as a CO2 sink. These terrains may thus play an important role in climate evolution over geologic time. The studies also point to the need of additional datasets from andesitic terrains to further delineate controls on andesitic weathering.

6.2. Future Directions

The data presented herein clearly provide valuable insights on weathering processes on HSIs. However, the investigation also sheds light on additional processes that require future examination. The geochemical investigation of stream waters draining

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the Mt. Pinatbuo volcanics showed the importance of anhydrite and apatite weathering in andesitic terrains. Anhydrite weathering provides an additional supply of Ca2+ to the oceans which could be assimilated into calcite and thereby effect climate. Whether this effect remains significant over geologic time would depend on the abundance of parent material and whether the magma reservoir can maintain high sulfur content. Apatite weathering can also play a crucial role in CO2 drawdown as it can enhance these locales as a biological sinks of CO2 as well. Each mole of apatite that weathers provides to the aquatic system three moles of phosphate into the aquatic system, which can then be utilized by biota. Of note, sampling of Pinatubo streams in November 2008 showed the majority of the rivers were saturated with respect to fluoroapatite. This may be the result of the biologic system not having completely evolved since the 1991 eruption to take up all the available phosphorous in the system. Studies linking phosphorous availability to age of volcanics and chemical erosion rates may show that chemical weathering plays a controlling role on ecosystem development at these volcanic locales. This type of study could be achieved by exploring the link of DOC and chemical weathering rates, along with phosphorous availability.

The Mt. Pinatubo volcanics play an interesting role in their potential to sequester

CO2 as a mitigation strategy. The rapid ongoing buildup of atmospheric CO2 from fossil fuel combustion, and its potential impact on climate have led to the IPCC’s endorsement of carbon capture and storage as a viable remediation option. Of the three types of carbon capture and storage proposed or practiced today (geological, ocean, and mineral), only mineral storage results in the transformation of CO2 into a solid byproduct.

Therefore, it is currently the only permanent, testable, and enforceable carbon

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sequestration technique. Large scale andesite tephra deposits which, due to high surface area and lack of crystallinity, present an ideal weathering substrate. While the data presented herein show andesitic material does present a potentially ideal substrate for carbon storage and capture techniques, little is known about the sequestration potential or reaction kinetics for this lithology. Therfore, future laboratory investigations focusing on andesite reaction rate kinetics are necessary to determine whether this is a viable atmospheric CO2 mitigation strategy.

Finally, the study also shows that the in-situ weathering processes themselves require further evaluation. This may be aided by the determination of residence time of water in the subsurface of these respective environments, which could be accomplished through detailed sampling of river systems throughout the year with a bias towards storm sampling during the rainy season. Storm sampling of O and H isotopes along with traditional weathering solutes would determine how quickly water moves through the subsurface of these respective environments and could shed light on whether weathering processes are occurring during the dry season or early in the wet season. Furthermore, it is becoming increasingly clear that a focus solely on surface and near surface weathering at these locales drastically underestimates the role of these terrains as an effective solute delivery source to the world’s ocean. As shown by Rad et al. (2007), subsurface fluxes to the ocean could be up to two to five fold higher than those observed from the surface.

Therefore, it is likely future comprehensive flux evaluations from active margin locales will alter our understanding of elemental cycling on both short and long-term time scales.

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BIBLIOGRAPHY

Adams J. (1980) High sediment yields from major rivers of the western Southern Alps, New Zealand. Nature. 287, 88–89.

Amiotte-Suchet P., Probst J.-L. and Ludwig W. (2003) Worldwide distribution of continental rock lithology: Implications for the atmospheric/soil CO2 uptake by continental weathering and alkalinity river transport to the oceans. Glob. Biogeochem Cycl. 17, 1038, doi:10.1029/2002GB001891.

Amiotte-Suchet P. and Probst J.L. (1993) Modelling of atmospheric CO2 consumption by chemical weathering of rocks: application to the Garonne, Congo and Amazon basins. Chem. Geol. 107, 205–210.

Barnola J.M., Raynaud D., Korotevich Y.S. and Lorius C. (1987) Vostok ice core provides 160,000-year record of atmospheric CO2. Nature. 329, 408–414.

Bellon H. (1988) Reconnaissance chronologique des deux premieres phases d’activité volcanique en Dominique (Petites Antilles). CR. Acad. Sci. Paris. 306, 1487– 1492.

Benedetti M.F., Menard O., Noack Y., Carvalho A. and Nahon D. (1994) Water rock interactions in tropical catchments: field rates of weathering and biomass impact. Chem. Geol. 118, 203– 220.

Bernard A., Demaiffe D. and Mattielli N. (1991) Anhydrite-bearing pumices from Mount Pinatubo: further evidence for the existence of sulphur-rich silicic magmas. Nature. 354, 139–140.

Bernard A., Knittel U., Weber B., Weis D., Albrecht A., Hattori K., Kleian J. and Oles D. (1996) Petrology and geochemistry of the 1991 eruption products of Mount Pinatubo. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 767–798.

Berner R.A. (1999) A new look at the Long-term Carbon Cycle. GSA Today. 9, 1-6.

Berner R.A., Lasaga A.C. and Garrels R.M. (1983) The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Am. J. Sci. 284, 641–683.

Berner R.A. (1982) Burial of organic-carbon and pyrite sulphur in the modern ocean: its geochemical and environmental significance. Am. J. Sci. 282, 451–473.

161

Bluth G.J.S. and Kump L.R. (1994) Lithologic and climatologic controls of river chemistry. Geochim. Cosmochim. Acta. 58, 2341–2359.

Bluth G.J., Doiron S.D., Schnetzler C.C., Kruger A.J. and Walter L.S. (1992) Global tracking of the SO2 clouds from the June 1991 Mount Pinatubo eruptions. Geophys. Res. Lett. 19, 151–154.

Bluth G.J.S. and Kump R.L. (1991) Phanerozoic paleogeography. Am J. Sci. 291, 281- 308.

Boudon G., Le Friant A., Villemant B. and Viode J.-P. (2005) Martinique. In Volcanic Hazard Atlas of the Lesser Antilles (Eds. J.M. Lindsay, R.E.A. Robertson, J.B. Shepherd and S. Ali). The University of the West Indies, Trinidad and Tobago, W.I., pp. 127–146.

Bruland K.W. (1983) Trace Elements in seawater. In Chemical Oceanography (Eds. J.P. Riley and R. Chester), Vol. 8, Academic Press, London, pp. 157–220.

Calmels D., Galy A., Bickle M.J, Hovius, N., Chen M.-C. and Chapman, H. (2009) Deep chemical weathering in a rapidly eroding mountain belt, Taiwan, Geophys. Res. Abstrs. 11. EGU2009-12631-1.

Carey A.E., Gardner C.B., Goldsmith S.T., Lyons W.B. and Hicks D.M. (2005) Organic carbon yields from small, mountainous rivers, New Zealand. Geophys. Res. Lett. 32, doi: 10:.1029/2005GL023159.

Carey A.E., Kao S.-J., Hicks D.M., Nezat C.A. and Lyons W.B. (2005) The geochemistry of rivers in tectonically active areas of Taiwan and New Zealand. In Tectonics, Climate and Landscape Evolution (Eds. S.D. Willett, N. Hovius, M.T. Brandon and D.M. Fisher). Special GSA Paper 398, pp. 339–351.

Carey A.E., Nezat C.A., Lyons W.B., Kao S-J. and Owen J.S. (2002) Trace metal fluxes to the ocean: The importance of high-standing islands. Geophys. Res. Lett. 29 (23), 2099, doi:10.1029/2002GL015690, 2002.

Carey S. and Sigurdsson H. (1980) The Roseau ash: Deep-sea tephra deposits from a major eruption on Dominica, Lesser Antilles arc. J. Volcanol Geoth. Res. 7 (1-2), 67–86.

Cogley J.C. (1988) GGHYDRO. Global Hydrographic Data. Release 2.2 Trent Climate Note 98-1. Trent University, Ontario.

Cole I.S., Ganther W.D., Sinclair J.D, Lau D. and Paterson D.A. (2004) A study of the wetting of metal surfaces in order to understand the processes controlling atmospheric corrosion. J. Electrochem. Soc. 151, B627–B635.

162

Dadson S., Hovius N., Pegg S., Dade W.B., Horng M.J. and Chen, H. (2005) Hyperpycnal river flows from an active mountain belt. J. Geophys. Res. 110, doi: 10.1029/2004JF000244.

Dadson S.J., Hovius N., Chen H.G., Dade W.B., Hsieh M.-L., Willett S.D., Hu J.-C., Horng M.-J., Chen M.-C., Stark C.P., Lague D. and Lin J.-C. (2003) Links between erosion, runoff variability and seismicity in the Taiwan orogen. Nature. 426, 648–651.

Dalai T.K., Krishnaswami S. and Sarin M.M. (2002) Major ion chemistry in the headwaters of the Yamuna river system: Chemical weathering, its temperature dependence and CO2 consumption in the Himalaya. Geochim Cosmochim. Acta. 66, 3397–3416. deBoer J.Z., Defant M.J., Stewart R.H. and Bellon H. (1991) Evidence for active subduction below western Panama. Geology. 19, 649–652. deBoer J.Z., Defant M.J., Stewart R.H., Restrepo J.F., Clark L.F. and Ramirez A.H. (1988) Quaternary calc-alkaline volcanism in western Panama: Regional variation and implication for the plate tectonic framework: J. S. Am. Earth Sci. 1, 275–293.

Defant M.J., Jacques D., Maury R.C., deBoer J. and Joron J.-L. (1989) Geochemistry and tectonic setting of the Luzon arc, Philippines. Geol. Soc. Amer. Bull. 101, 663– 672.

Delfin F.G. Jr., Sussman D., Ruaya D. and Reyes A.G. (1992) Hazard assessment of the Pinatubo volcanic-geothermal system: clues prior to the June 15, 1991 eruption. Trans. Geotherm. Res. Council. 16, 519–527.

Delfin F.G. Jr., Villarosa H.G., Layugan D.B., Clemente V., Candelaria M.R. and Ruaya J.R. (1996) Geothermal exploration of the pre-1991 Mount Pinatubo hydrothermal system. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp 197–212.

Dessert C., Gaillardet J., Dupre B., Schott J. and Pokrovsky O.S. (2009) Fluxes of high- versus low-temperature water-rock interactions in aerial volcanic areas: Example from the Kamachatka Peninsula, Russia. Geochim Cosmochim Acta. 73, 148–169.

Dessert C, Dupré B., Gaillardet J., Francois L. and Allègre C.J. (2003) Weathering laws and the impact of basalt weathering on the global carbon cycle. Chem. Geol. 202, 257–273.

163

Dessert C.B., Dupré L.M., Francois J., Schott J., Gaillardet J., Chakrapani G. and Bajapi S. (2001) Erosion of Deccan Traps determined by river geochemistry: Impact on the global climate and the 87Sr/86Sr ratio of seawater. Earth Planet. Sci. Lett. 188, 459–474.

Deurling K. (2008) A stratographic, geochemical, and petrologic investigation of the most recent eruption of Morne Diablotins, Dominica, Lesser Antilles. University of Florida, undergraduate thesis, 50 p.

Díaz P.L., Lugo A.E. and McDowell W.H. (1985) General hydrology and water quality of Layou River in Dominica, Buccament River in St. Vincent, and Troumasse River in St. Lucia, British West Indies. In Proceedings of the Second Caribbean Islands Water Resources Congress, May 6- 8, 1985, San Juan, Puerto Rico (Eds. F. Quinones and A.V. Sanchez). American Water Resources Association, Bethesda, pp. 46–49.

Drever J.I. (1988) The Geochemistry of Natural Waters, 2nd ed., Prentice Hall, Englewood Cliffs, 437 pp.

Dupré B., Dessert C., Oliva P., Goddéris Y., Viers J., Francois L., Millot R. and Gaillardet J. (2004) Rivers, Chemical weathering and Earth’s climate. CR Geosci. 335 (16), 1141–1160.

Edmond J.M. and Huh Y. (1997) Chemical weathering yields from basement and orogenic terrains in hot and cold climates. In Tectonic Uplift and Climate Change (Ed. W.F. Ruddiman). Springer, New York. pp. 329–351.

Eklund T.J., McDowell W.H. and Pringle C.M. (1997) Seasonal variation of tropical precipitation chemistry: La Selva, Costa Rica. Atmos. Environ. 31, 3903–3910.

Emanuel K. (2005) Increasing destructiveness of tropical of tropical cyclones over the past 30 years. Nature. 436, 686–688, doi: 10.1038/nature03906.

Filippelli G.M. (1997) Intensification of the Asian monsoon and a chemical weathering event in the late Miocene early Pliocene: Implications for late Neogene climate change. Geology. 25, 27–30.

Fuller C.W., Willet S.D., Hovius N. and Slingerland R. (2003) Erosion rates for Taiwan mountain basins: New determinations from suspended sediment records and a stochastic model of their temporal variation. J. Geol. 111, 71–87.

Gaillardet J., Dupré B., Louvat P. and Allègre C.J. (1999) Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chem. Geol. 159 (1), 3–30.

164

Galy A. and France-Lanord C. (2001) Higher erosion rates in the Himalaya: geochemical constraints on riverine fluxes. Geology. 29 (1), 23–26.

Garrels, R.M. and Mackenzie F. (1971) Evolution of Sedimentary Rocks. Norton, New York.

Gibbs M.T. and Kump L.R. (1994) Global chemical erosion during the last glacial maximum and the present: Sensitivity to changes in lithology and hydrology. Paleoceanography. 9, 529–543.

Gíslason S.R., Oelkers E.H. and Snorrason A. (2006) Role of river-suspended material in the global carbon cycle. Geology. 34 (1), 49–52.

Gislason S.R., Arnorsson S. and Armannsson H. (1996) Chemical weathering of basalt as deduced from the composition of precipitation, rivers and rocks in SW Iceland. Am. J. Sci. 296, 837– 907.

Goldsmith S.T., Carey A.E., Johnson B.M., Welch S.A. and Lyons W.B. (2009) Stream Geochemistry, Chemical Weathering, and CO2 consumption potential of andesitic terrains, Dominica, Lesser Antilles. Geochim. Cosmochim. Acta., in press.

Goldsmith S.T., Carey A.E., Lyons W.B. and Hicks M. (2008a) Geochemical fluxes and weathering on high standing islands: Taranaki and Manawatu-Wanganui Regions, New Zealand. Geochim. Cosmochim. Acta. 72 (9), 2248–2267.

Goldsmith S.T., Carey A.E, Lyons W.B., Kao S.-J., Lee T.-Y. and Chen J. (2008b) Extreme storm events, landscape denudation, and carbon sequestration: Typhoon Mindulle, Choshui River, Taiwan. Geology. 36, 483–486.

Gomez B., Carter L., Trustrum N.A., Palmer A.S. and Roberts A.P. (2004) El Nino Southern Oscillation signal associated with middle Holocene climate change in intercorrelated terrestrial and marine sediment cores, North Island, New Zealand. Geology. 32, 653–656.

Goswani B.N., Venugopal V., Sengupta D., Madhusoodanan M.S. and Xavier P.K. (2006) Increasing trend of extreme rain events over India in a warming environment. Science. 314, 1442–1445.

Griffiths G.A. (1981) Some suspended sediment yields from South Island catchments, New Zealand. Water Res. Bull. 17, 662–671.

Gunn B.G., Roobol M.J. and Smith A.L. (1984) Petrochemistry of the Pelean-Type Volcanoes of Martinique. Geol Soc Am. Bull. 85, 1023-1030.

165

Gunn B.M., Roobol M.J. and Smith A.L. (1980) Geochemistry of the volcanoes of Basse- Terre, Guadeloupe – An example of intra-island variation. Bull. Volcanol. 43, 403-411.

Hansen J., Ruedy R., Sato M. and Reynolds R. (1996) Global surface air temperature in 1995: Return to pre-Pinatubo level. Geophys. Res. Lett. 23, 1665-1668, doi:10.1029/96GL01040.

Harmon R.S., Lyons W.B., Long D.T., Wörner G., Mitasova H., Ogden F.L., Gardner C.B., Welch K.A. and Witherow R.A. (2009) Geochemistry of Four Tropical Montane Watersheds, Central Panama. Appl. Geochem. 24, 624–640.

Harr D.R., Levno A. and Mersereau R. (1982) Streamflow changes after logging 130- Year-Old Douglas Fir in two small watersheds. Water Res. Res. 18, 637-644.

Hart C. and Hart D.G. (1969) Bredin-Archibold Smithsonian Biological Survey of Dominica – A Contribution to the Limnology of Dominica, Lesser Antilles. P. Acad. Nat. Sci. Phil. 121, 109–126.

Hartshorn K., Hovius N., Dade W.B. and Slingerland R.L. (2002) Climate-driven bedrock incision in an active mountain belt. Science. 297, 2036–2038.

Hicks D.M., Hill J. and Shankar U. (1996) Variation of suspended sediment yields around New Zealand: the relative importance of rainfall and geology. In Erosion and Sediment Yield: Global and Regional Perspectives (Eds. D.E. Walling and B. Webb). Proceedings of the Exeter Symposium, July 1996, Wallingford, IAHS Publication 236, pp. 149–156.

Hoblitt R.P., Wolfe E.W., Scott W.E., Couchman M.R., Pallister J.S. and Javier D. (1996) The preclimactic eruptions if Mount Pinatubo. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 545–570.

Ho C.S. (1988) An introduction to the geology of Taiwan, Ministry of Economic Affairs, Taipei, 192 p.

Hovius N., Stark C.P., Chu H.T. and Lin J.C. (2000) Supply and removal of sediment in a landslide dominated mountain belt: Central Range, Taiwan. J. Geol. 108, 73–89.

Hovius N. (1996) Regular spacing of drainage outlets from linear mountain belts. Basin Res. 8, 29–44.

Hren M.T., Chamberlain C.P., Hiley G.E., Blisniuk P.M. and Bookhagen B. (2007) Major ion chemistry of the Yarlung Tsangpo-Brahmaputra river: Chemical

166

weathering, erosion, and CO2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya. Geochim Cosmochim Acta. 71, 2907–2935.

Huh Y., Panteleyev G., Babich D., Zaitsev A. and Edmond J.M. (1998) The fluvial geochemistry of the rivers of Eastern Siberia: II. Tributaries of the Lena, Omoloy, Yana, Indigirka, Kolyma, and Anadyr draining the collisional/accretionary zone of the Verkhoyansk and Cherskiy ranges. Geochim. Cosmochim. Acta. 62, 2053– 2075.

Jacobson A.D. and Blum J.D. (2003) Relationship between mechanical erosion and atmospheric CO2 consumption in the New Zealand Southern Alps. Geology. 31, 865–868.

Jacobson A.D., Blum J.D. and Walter L.M. (2002) Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters. Geochim. Cosmochim. Acta 66, 3417–3429.

Jacobson A.D. and Blum J.D. (2000) Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Naga Parbat: influence on river-water chemistry. Geology. 28, 463–466.

Jakubowski R.T., Fournelle J., Welch S., Swope R.J. and Camus P. (2002) Evidence for magmatic vapor deposition of anhydrite prior to the 1991 climactic eruption of Mt. Pinatubo. Am. Mineral. 87, 1029–1045.

Jones J.W. and Newhall C.G. (1996) Preeruption and Posteruption Digital-Terrain Models of Mount Pinatubo. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 571–582.

Jose A.M., Sosa L.M. and Cruz N.A. (1996) Vulnerability assessment of Angat water reservoir to climate change. Water Air Soil Poll. 92, 191-201.

Juang W-S. (1984) The origin of the Tungshan Cupriferous Pyrite Deposit of Ilan-Hsien, Eastern Taiwan-A geochemical approach. Sp. Pub. Centr. Geol. Surv. 3, 171–194.

Kao S.-J. and Milliman J.D. (2009) Water and sediment discharge from small mountainous rivers, Taiwan: The roles of lithology, epsisodic events, and human activities. J. Geol. 116, 431–448.

Kao S.J., Lee T.Y. and Milliman J.D. (2005) Calculating highly fluctuated suspended sediment fluxes from mountainous rivers in Taiwan. Terres. Atmos. Ocean. Sci. 16(3), 653–675.

167

Kao S.-J., Horng C.-S., Roberts A.P. and Liu K.-K. (2004) Carbon –sulfur-iron relationships in sedimentary rocks from southwest Taiwan: influence of geochemical environment on greigite and pyrrhotite formation. Chem. Geol. 203, 153–168.

Kao S.-J. and Liu K.-K. (2002) Exacerbation of erosion induced by human perturbation in a typical Oceania watershed: Insight from 45 years of hydrological records from the Lanyang-Hsi River, northeastern Taiwan. Global. Biogeochem. Cy. 16, 1016, doi: 10.1029/2000GB001334.

Kao S.-J. and Liu, K.-K. (1996) Particulate organic carbon export from a subtropical mountainous river (Lanyang Hsi) in Taiwan. Limnol. Oceanogr. 41, 1749–1757.

Komorowski J.-C., Boudon, G., Semet M., Beauducel F., Antenor-Habazac C., Bazin S. and Hammouya G. (2005) Guadeloupe, In Volcanic Hazard Atlas of the Lesser Antilles (eds. J.M. Lindsay, R.E.A. Robertson, J.B. Shepherd and S. Ali). The University of the West Indies, Trinidad and Tobago, W.I. pp. 65–102.

Korozoun, V.I., Sokolov A.A., Budyko M.I., Voskresensky K.P., Kailinin, G.P., Konoplyanstev A.A., Korotkevich E.S. and Lvovich M.I. (1977) Atlas of World Water Balance. The UNSESCO Press, Paris.

Le Bas M.J. and Streckeisen A.L. (1991) The IUGS systematics of igneous rocks. J. Geol. Soc. London. 148, 825–833.

Lee H.-J. and Chao S.-Y. (2003) A climatological description in and around the East China Sea. Deep-Sea Res. Pt. II. 50, 1065–1084.

Li Y.-H. (1976) Denudation of Taiwan Island since the Pleistocene Epoch. Geology. 4, 105–107.

Lin C.W., Shieh C.L., Yuan B.D., Shieh Y.C., Liu S.H. and Lee S.Y. (2004) Impact of the Chi-Chi earthquake on the occurrence of landslides and debris flows: Example from the Chenyulan River watershed. Eng. Geol. 71, 49–61.

Lin T.-C., Hamburg S.P., King H.-B. and Hsia Y.-J. (2000) Through fall patterns in a subtropical rain forest of northeastern Taiwan. J. Env. Qual. 29, 1186–1193.

Lindsay J.M., Smith A.L., Roobol M.J. and Stasiuk M.V. (2005) Dominica. In Volcanic Hazard Atlas of the Lesser Antilles (Eds. J.M. Lindsay, R.E.A. Robertson, J.B. Shepherd and S. Ali). The University of the West Indies, Trinidad and Tobago, W.I., pp. 1–48.

168

Lindsay J.M., Stasiuk M.V. and Shepherd J.B. (2003) Geological history and potential hazards of the late Pleistocene to Recent Plat Pays volcanic complex, Dominica, Lesser Antilles. B. Volcanol. 65 (2-3), 201–220.

Louvat P. (1997) Etude ge´ochimique de l’e´rosion fluviale d’iles volcaniques a` l’aide des bilans d’e´le´ments majeurs et traces. PhD Thesis. Universite´ Paris-7 Paris. 322 pp.

Louvat P. and Allègre C.J. (1997) Present day denudation rates on the island of Réunion determined by river geochemistry: basalt weathering and mass budget between chemical and mechanical erosions. Geochim. Cosmochim. Acta. 61(17), 3645– 3669.

Ludwig W., Amiotte Suchet P., Munhoven G., and Probst J.L. (1998) Atmospheric CO2 consumption by continental erosion: Present-day control and implications for the Last Glacial Maximum. Global Planet. Change. 16–17, 107–120.

Lyons W.B., Carey A.E., Hicks D.M. and Nezat C.A. (2005) Chemical weathering in high sediment yielding watersheds, New Zealand. J. Geophys. Res. 110 (FO1008), doi:10.1029/2003JF000088.

Lyons W.B., Nezat C.A., Carey A.E. and Hicks D.M. (2002) Organic carbon fluxes to the ocean from high-standing islands. Geology. 30, 443–446.

Lyons W.B., Lent R.M., Djukic N., Maletin S., Pujin V. and Carey A.E. (1992) Geochemistry of surface waters of Vojvodina, Yugoslavia. J. Hydrol. 137, 33–55.

Major J.J., Janda R.J. and Daag A.S. (1996) Watershed disturbance and lahars on the east side of Mount Pinatubo during the mid-June 1991 eruptions. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 895–919.

Martin G.W. and Harr R.D. (1988) Precipitation and Streamwater Chemistry from undisturbed watersheds in the Cascade Mountains of Oregon. Water. Air. Soil. Poll. 42, 203–219.

McCarthy K.T., Pichler T. and Price R.E. (2005) Geochemistry of Champagne Hot Springs shallow hydrothermal vent field and associated sediments, Dominica, Lesser Antilles. Chem. Geol., 224, 55–68.

McDowell W.H., Sánchez C.G., Asbury C.E. and Pérez C.R.R. (1990) Influence of sea salt aerosols and long range transport on precipitation chemistry at El Verde, Puerto Rico. Atmos. Environ. 24 (11), 2813–2821.

169

McDowell W.H. and Asbury C.E. (1994) Export of carbon, nitrogen, and major ions from three tropical montane watersheds. Limnol. Oceanogr. 39, 111–125.

McDowell W.H., Lugo A.E. and James A. (1995) Export of nutrients and major ions from Caribbean catchments. J. N. Am. Benthol. Soc. 14 (1), 12–20.

Meybeck M. (1987) Global chemical weathering of surficial rocks estimated from river dissolved loads. Am. J. Sci. 287, 401–428.

Milliman J.D., Lin S., Kao S.J., Liu J.P., Liu C.S., Chiu J.K. and Lin Y.C. (2007) Short term changes in seafloor character due to flood-derived hyperpycnal discharge: Typhoon Mindulle, Taiwan, July 2004. Geology. 35, 779–782.

Milliman J.D. and Kao S.-J. (2005) Hyperpycnal discharge of fluvial sediment to the ocean: Impact of Super-Typhoon Herb (1996) on Taiwanese rivers. J. Geol. 113, 503–516.

Milliman J.D. and Syvitski J.P.M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: The importance of small mountainous rivers. J. Geol. 100, 525–544.

Millot R., Gaillardet J., Dupré B. and Allègre C.J. (2002) The global control of silicate weathering rates and the coupling with physical erosion: New insights from rivers of the Canadian Shield. Earth Planet. Sci. Lett. 196, 83– 98.

Ministry of Economic Affairs (1974) Geology Map of Taiwan, 1:250,000, Taipei, Sheets 1–4.

Moon S., Huh Y., Qin J., and Pho N.V. (2007) Chemical weathering in the Hong (Red) River basin: Rates of silicate weathering and their controlling factors. Geochim Cosmochim Acta. 71, 1411–1430.

Mulder T. and Syvitski J.P.M. (1995) Turbidity currents generated at river mouths during exceptional discharges to the world oceans. J. Geol. 103, 285–299.

Mullin J.B. and Riley J.P. (1955) The colorimetric determination of silicate with reference to sea and natural waters. Anal. Chim. Acta, 12, 162–176

NASA (2009) Goddard Institute for Space Studies Surface Temperature Analysis http://data.giss.nasa.gov/gistemp/station_data, accessed July (2009).

Négrel P., Allègre C.J., Dupré B. and Lewin E. (1993) Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: the Congo Basin case. Earth Planet. Sci. Lett. 120 (1-2), 59–76.

170

Newbold J.D., Sweeney B.W, Jackson J.K. and Kaplan L.A. (1995) Concentrations and Export of Solutes from Six Mountain Streams in Northwestern Costa Rica. J. N. Am. Benthol. Soc. 14 (1), 21–37.

Newhall C.G., Daag A.S., Deflin Jr. F.G., Hoblitt R.P., McGeehin J., Pallister J.S., Regalado M.T.M., Rubin M., Tubianosa B.S., Tamoyo Jr. R.A. and Umbal J.V. (1996) Eruptive history of Mount Pinatubo. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 165–195.

New Zealand Geologic Survey (1947) Geologic Map of New Zealand-North Island. 1:250,000.

NOAA (2009) Climate Global Warming, and Daylight Charts and Data, http://www.climate-charts.com, accessed July (2009).

Norton D. (1974) Chemical mass transfer in the Rio Tanama system, west-central Puerto Rico. Geochim. Cosmochim. Acta. 38, 267–277.

Neumann C.J.G., Cry G.W., Caso E.L. and Jarvian, B.R. (1978). Tropical Cyclones of the North Atlantic Ocean, 1871-1977. National Climatic Center, Asheville, N.C., 48 pp.

Parkhurst D.L. and Appelo, C.A.J. (1999) User's guide to PHREEQC (version 2) – a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, U.S. Geological Survey Water-Resources Investigations Report 99-4259, 312 pp.

Peucker-Ehrenbrink B. and Miller M.W. (2004) Quantitative bedrock geology of east and Southeast Asia (Brunei, Cambodia, eastern and southeastern China, East Timor, Indonesia, Japan, Laos, Malaysia, Myanmar, North Korea, Papua New Guinea, Philippines, far-eastern Russia, Singapore, South Korea, Taiwan, Thailand, Vietnam). Geochem. Geophys. Geosys. 5, Q01B06, doi:10.1029/2003GC000619.

Pichavant M., Martel C., Bourdier J.-L. and Scaillet B. (2002) Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles Arc). J. Geophys Res. 107, B5, 2093, doi:10.1029/2001JB00315.

Pickup, G. (1980) Hydrologic and sediment modeling studies in the environmental- impact assessment of a major tropical dam project. Earth Surf. Proc. Landf. 5, 61–75.

Price R.C., Smith I.E.M., Stewart R.B. and Woodhead J.D. (1999) Petrogenesis of high- K arc magmas: evidence from Egmont Volcano, North Island, New Zealand. J. Petrol. 40(1), 167–197.

171

Pringle C.M., Triska F.J. and Browder G. (1990) Spatial variation in basic chemistry of streams draining a volcanic landscape on Costa Rica's Caribbean slope. Hydrobiologia. 206, 73–85.

Rad S.D., Allègre C.J. and Louvat P. (2007) Hidden erosion on volcanic islands. Earth Planet. Sci. Lett. 262 (1-2), 109–124.

Rad S.D., Louvat P., Gorge C., Gaillardet J. and Allègre C.J. (2006) River dissolved and solid loads in the Lesser Antilles: New insight into basalt weathering processes. J. Geochem. Explor. 88 (1-3), 308–312.

Raiswell R. and Thomas A.G. (1984) Solute acquisition in glacial melt waters. I. Fjallsjökull (South-East Iceland): Bulk melt waters with closed-system characteristics. J. Glaciol. 30, 35–43.

Rantz S.E. and others. (1992) Measurement and Computation of Streamflow Discharge. U.S. Geological Survey, Water Supply Paper 2175, 284 pp.

Rao S.M. (1996) Role of apparent cohesion in the stability of Dominican allophone soil slopes. Eng. Geol. 43(4), 265–279.

Rauch S. (2007) Geochemical signatures in subduction zone magmatism at Volcán Barú, Panama. University of Göttingen, Master’s Thesis, 76 p.

Raymo M.E. and Ruddiman W.F. (1992) Tectonic forcing Cenozoic climate change. Nature. 359 (6391), 117–122.

Reading A.J. (1991) Stability of tropical residual soils from Dominica, West Indies. Eng. Geol. 31, 27–44.

Restrepo J.F. (1987) A geochemical investigation of Pleistocene to recent calc-alkaline volcanism in western Panama: Tampa, University of South Florida, Master’s Thesis, 103 pp.

Rodolfo K.S., Umbal J.V., Alonso R.A., Remotigue C.T., Paladio-Melosantos M.L., Salvador J.H.G., Evangelista D. and Miller Y. (1996) Two Years of Lahars on the western flank of Mount Pinatubo: Initiation, flow processes, deposits and attendant geomorphic and hydraulic changes. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 989–1013.

Roobal M.J. and Smith A.L. (2004) Geologic Map of Dominica, West Indies. Geology Department, University of Puerto Rico at Mayaguez, 1:100,000.

172

Rosencrantz E. and Sclater J.G. (1986) Depth of age in the Cayman Trough. Earth Planet. Sci. Lett. 79 (1-2), 133–144.

Rouse W.C. and Reading A.J., Walsh R.P.D. (1986) Volcanic soil properties in Dominica, West Indies. Eng. Geol. 23 (1), 1–28.

Rouse C. (1990) The mechanics of small tropical flowslides in Dominica, West Indies. Eng. Geol. 29 (33), 227–239.

Ruddiman W.F. (2000) Earth’s climate: past and future. New York: W.H. Freeman, 465pp.

Ryu J.-S., Lee K.-S., Chang H.-W. and Shin H.S. (2008) Chemical weathering of carbonates and silicates in the Han River basin, South Korea. Chem. Geol. 247, 66–80.

Samper A., Quidelleur X., Mollex D. and Lahitte P. (2007) Timing of effusive volcanism and collapse events within an oceanic arc island: Basse-Terre, Guadeloupe archipelago (Lesser Antilles Arc). Earth Planet Sci Lett. 258, 175–191.

Scott W.E., Hoblitt R.P., Torres R.C, Self S., Martinez M. and Nillos T. Jr. (1996) Pyroclastic flows of the June 15, 1991 climactic eruption of Mount Pinatubo. In: Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines (eds. C.G. Newhall and R.S. Punongbayan). University of Washington Press, Seattle. pp. 545–570.

Selvaraj K. and Chen C.-T.A. (2006) Moderate chemical weathering of subtropical Taiwan: constraints from solid-phase geochemistry of sediments and sedimentary rocks. J. Geol. 114, 101–116.

Sherrod D.R., Vallance J.W., Espinosa A.K. and McGeehin J.P. (2008) Volcán Barú- Eruptive History and Volcanon-Hazards. UGSS. U.S. Department of Interior, Open-File Report 2007-1401, 33p.

Shin T.-C. and Teng T.-L. (2001) An overview of the 1999 Chi-Chi, Taiwan, earthquake. B. Seismol. Soc. Am. 91, 895–913.

Siebert L., Alvarado G.E., Vallance J.W. and van Wyk de Vries B. (2006) Large-volume volcanic edifice failures in Central America and associated hazards. In: Natural hazards in El Salvador (eds.W.I. Rose, J.J. Bommer, D.L. López, M.J. Carr, and J.J. Major). Geol. Soc. Am. S. 412, 1–26.

Siebert L., Kimberly P. and Pullinger C.R. (2004) The voluminous Acajutla debris avalanche from Santa Ana volcano, western El Salvador, and comparison with other Central American edifice- failure events. In: Natural hazards in El Salvador

173

(eds.W.I. Rose, J.J. Bommer, D.L. López, M.J. Carr, and J.J. Major). Geol. Soc. Am. S. 375, 5–23.

Sigurdsson H. and Carey S.N. (1991) Caribbean Volcanoes: a Field Guide to Martinique, Dominica and St. Vincent. Geological Association of Canada, Field Trip Guidebooks, Toronto, pp. 1–101.

Sigurdsson H. and Carey S.N. (1981) Marine tephrochronology and Quaternary explosive volcanism in the Lesser Antilles arc. In Tephra Studies (Eds. S Self and R.S.J. Sparks). NATO Series, Reidel, Holland, pp. 255–280.

Sigurdsson H. (1972) Partly-welded pyroclastic flow deposits in Dominica, Lesser Antilles. B. Volcanologique. 36 (1), 148–163.

Smith A.L. and Kirkley J. (2004) Geothermal Anomalies of Dominica, W. I., http://www.caribbeanvolcanoes.com/dominica/geothermaldata.htm, Department of Geological Sciences, California State University, accessed May (2007).

Smith A.L., Roobol M.J. and Gunn, B.M. (1980) The Lesser Antilles - A discussion of the island arc magmatism. B. Volcanol. 43 (2), 287–302.

Smith A.L., Roobol M.J., Rheubottom A., Kirkley J., Freeman Z., Karnes K., Hinojosa I., Stultz H., Harris S. and Pacheco O. (2003) The Geology of Dominica, Lesser Antilles. Geol. Soc. Am. Abstr. Prog. 35, 323.

Sparks R.S.J., Sigurdsson H. and Carey S.N. (1980) The entrance of pyroclastic flows into the sea, I. Oceanographic and geologic evidence from Dominica, Lesser Antilles. J. Volcanol. Geoth. Res. 7 (1-2), 87–96.

Stallard R.F. and Edmund J.M. (1987) Geochemistry of the Amazon. 3. Weathering chemistry and limits to dissolved inputs. J. Geophys. Res. 92(C8), 8293–8302.

Stallard R.F. and Edmond J.M. (1981) Geochemistry of the Amazon: 1. Precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge. J. Geophys. Res. 86 (C10), 9844–9858.

Stefánsson A. and Gíslason S.R. (2001) Chemical weathering of basalts, southwest Iceland: effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean. Am. J. Sci. 301 (6), 513–556.

Stimac J.A., Goff F., Counce D., Larocque A.C.L., Hilton D.R.and Morgenstern U. (2004) The crater lake and hydrothermal system of Mount Pinatubo, Philippines: evolution in the decade after eruption. Bull. Volcanol. 66, 149–167.

174

Stum W. and Morgan J.J. (1996) Aquatic Chemistry, 3rd ed., Wiley Interscience, New York, 1040 pp.

Syvitski J.P.M. and Milliman J.D. (2007) Geology, geography, and humans battle for dominance over the delivery of fluvial sediment to the coastal ocean. J. Geol. 115, 1–19, doi: 10.1086/509246.

Townsend-Small A., McClain M.E., Hall B., Noguera J.L., Llerena C.A. and Brandes J.A. (2008) Suspended sediments and organic matter in mountain headwaters of the Amazon River: Results from a 1-year time series study in the central Peruvian Andes. Geochim. Cosmochim. Acta. 72 (3), 732–740.

Turner M.B., Cronin S.J., Bebbington M.S. and Platz T. (2008) Developing probabilistic eruption forecasts for dormant volcanoes; a case study from Mt. Taranaki, New Zealand. Bull. Volcanol. 70, 507–515.

Universidad Tecnológica de Panama (1992) Evaluación de la amenaza, estimación de la vulnerabilidad y del factor costo del riesgo del Volcán Barú, Republica de Panamá: Departamento de Geotécnica Facultad de Ingenieria Civil, Universidad Tecnológica de Panamá, con apoyo del Centro para la Prevención de Desastres Naturales en América Central (CEPREDENAC), 129 p., 2 pls., scala 1:100,000.

UNESCO (1976) Geological World Atlas. UNESCO Press, Paris.

USGS, Layou River at Layou Valley, Dominica, http://waterdata.usgs.gov/nwis/inventory/?site_no=152450061234400&, USGS Puerto Rico Water Science Center, accessed May (2007).

Villones (1980) The Aksitero Formation: Its implications and relationship with respect to the Zambales ophiolite. Philippine Bureau of Mines and Geosciences. Technical Information Series. 16-80, 21 p

Walker J.C.G., Hays P.B. and Kasting J.F. (1981) A negative feedback mechanism for the long-term stabilization of Earth’s surface temperature. J. Geophys. Res. 86 (C10), 9776–9782.

Walsh R.P.D. and Howells K.A. (1988) Soil pipes and their role in runoff generation and chemical denudation in a humid tropical catchment. Earth Surf. Proc. Land. 13, 9–17.

Walsh R.P.D. (1985) The influence of climate, lithology and time on drainage density and relief development in the tropical volcanic terrain of the Windward Islands. In Environmental Change and Tropical Geomorphology (Eds. I. Douglas and T. Spencer). George Allen & Unwin, London. pp. 93–122.

175

Walsh R.P.D. (1982) A provisional survey of the effects of Hurricane David and Hurricane Frederic in 1979 on the terrestrial environment of Dominica, West Indies. Swansea Geogr. 19, 28–35.

Walsh R.P.D. (1980) Runoff processes and models in the humid tropics. Z. Geomorphol. 36, 176–202.

Warne A.G., Webb R.M.T and Larsen M.C. (2005) Water, sediment, and nutrient discharge characteristics of rivers in Puerto Rico, and their potential influence on coral reefs. U.S. Geological Survey Scientific Investigations Report 2005–5206, 58p.

Water Resources Agency (2005) Hydrological year book of Taiwan: Republic of China, 2005: Ministry of Economic Affairs.

Water Resources Agency (2004) Hydrological year book of Taiwan: Republic of China, 2004: Ministry of Economic Affairs.

Welch K.A., Lyons W.B., Graham E., Neumann J., Thomas J.M. and Mikesell D. (1996) Determination of major element chemistry in terrestrial waters form Antarctica by ion chromatography. J. Chromatogra A. 739, 256–263.

West A.J., Bickle M.J., Collins R. and Brasington J. (2002) Small-catchment perspective on Himalayan weathering fluxes. Geology. 30, 355–358.

Wilkinson B.H. and McElroy B.J. (2007) The impact of humans on continental erosion and sedimentation. Geol Soc. Amer. Bull. 119, 140–156, doi: 10.1130/B25899.1.

Willemin J. H. and Knuepfer P.L.K. (1994) Kinematics of arc-continent collision in the eastern Central Range of Taiwan inferred from geomorphic analysis. J. Geophys. Res. 99, 20267–20280.

Willet S.D., Fisher D., Fuller C., En-Chao Y. and Cia-Yu L. (2003) Erosion rates and orogenic-wedge kinematics in Taiwan inferred from fission-track thermochronology. Geology. 31, 945–948.

Wills K. (1974) The Geological History of Southern Dominica and Plutonic Nodules from the Lesser Antilles, Ph.D. Thesis, University of Durham.

Wolff-Boenisch D.W., Gabet E.J., Burbank D.W., Lagner H. and Putkonen, J. (2009) Spatial variations in chemical weathering and CO2 consumption in Nepalese High Himalayan catchments during the monsoon season. Geochim. Cosmochim. Acta. 73, 3148–3170.

176

Wolff-Boenisch D., Gislason S.R. and Oelkers E.H. (2006) The effect of crystallinity on dissolution and CO2 consumption capacity of silicates. Geochim. Cosmochim. Acta. 70 (4), 858–870.

Wu C.-C. and Kuo Y.-H. (1999) Typhoons affecting Taiwan: Current understanding and future challenges. B. Am. Meteorol. Soc. 80, 67–80.

Wu L., Wang B. and Geng S. (2005) Growing typhoon influence on east Asia. Geophys. Res. Lett. 32, doi: 10.1029/2005GL022937.

Wu L., Huh Y., Qin J., Du G., and van Der Lee S. (2005) Chemical weathering in the Upper Huang He (Yellow River) draining the eastern Tibetan Plateau. Geochim. Cosmochim. Acta. 69, 5279–5294.

Wu W., Xu S., Yang J. and Yin H. (2008) Silicate weathering and CO2 consumption deduced from the seven Chinese rivers originating in the Qinghai-Tibet Plateau. Chem. Geol., 249, 307–320.

Yoon J., Huh Y., Lee I., Moon S., Noh H. and Qin J. (2008) Weathering processes in the 87 86 34 18 Min Jiang: major elements, Sr/ Sr, SSO4, and OSO4. Aquat. Geochem. 14, 147–170.

Yoshimura K., Nakao S., Noto M., Inokura Y., Urata K., Chen M. and Lin P.-W. (2001) Geochemical and stable isotope studies on natural water in the Taroko Gorge karst area, Taiwan- chemical weathering of carbonate rocks by deep source CO2 and sulfuric acid. Chem. Geol. 177, 415–430.

177