Molecular level studies on the distribution and fate of organic
matter at continental margins
Dissertation
zur Erlangung des Doktorgrades
der Naturwissenschaften
Dr. rer. nat.
Am Fachbereich Geowissenschaften
der Universität Bremen
vorgelegt von
Frauke Schmidt
Bremen
Mai 2009
1. Gutachter: Prof. Dr. Kai Uwe Hinrichs
2. Gutachter: Prof. Dr. Boris Koch
Preface
Preface
The studies presented in this thesis were funded by the Deutsche Forschungsgemeinschaft through the DFG Research Center / Excellence Cluster “The Ocean in the Earth System” and the Bremen International Graduate School for Marine Sciences (GLOMAR). The thesis was prepared in the time from March 2006 to May 2009 in the Organic Geochemistry group of
Prof. Dr. Kai Uwe Hinrichs at the University of Bremen.
This thesis contributes to a better understanding of the processes and mechanisms behind the distribution and early diagenesis of organic matter in marine sediments. In the first chapter general aspects of organic matter cycling in the ocean and in marine sediments are examined to provide a base for the studies in Chapters 2 – 4. Both study sites, the NW
Iberian margin and the northern Black Sea, are briefly introduced and the chapter concludes with a general description of the methodological approach. Chapter 2 (submitted to Marine
Chemistry) investigates the sources, transport and, distribution of organic matter at the NW
Iberian margin. Chapters 3 and 4 focus on early diagenetic processes in the sedimentary dissolved organic matter pool at the NW Iberian margin (Chapter 3, published in Geochimica et Cosmochimica Acta) and in the northern Black Sea (Chapter 4, draft in preparation for submission). The data from both chapters were generated in collaboration with Bruker
Daltonik GmbH, Bremen, and the complete data set is provided in the electronic appendix.
The thesis closes with the major findings of the studies and perspectives for future investigations.
Table of contents
TABLE OF CONTENTS
Abstract 4
Zusammenfassung 6
1 Introduction 11 1.1 Organic matter in the contemporary ocean 12 1.1.1 Composition of organic matter 14 1.1.2 Alteration of OM in the water column 16 1.1.3 Early diagenesis of OM in marine sediments 17 1.1.4 Production and significance of DOM in sediment pore waters 18 1.2 OM at continental margins 19 1.2.1 Study area 1: The Galicia Minho Shelf 20 1.2.2 Study area 2: The northern Black Sea 21 1.3 Main objectives and thesis outline 23 1.4 Methodological approach 25 References 30
2 Sources, transport, and partitioning of organic matter at a highly dynamic continental margin 37 Abstract 38 2.1 Introduction 39 2.2 Study area 40 2.3 Material and methods 42 2.3.1 Sampling 42 2.3.2 Total organic carbon, total carbon and total nitrogen 42 2.3.3 Lipid extraction 42 2.3.4 Bacteriohopanepolyols 43 2.3.5 Branched and isoprenoid tetraether index 43 2.3.6 Lignin extraction 43 2.3.7 Gas chromatography coupled to mass spectrometry or flame ionization detection 44 2.3.8 High performance liquid chromatography 45 2.3.9 Grain size analysis 46 2.3.10 Correlation analysis, cluster analysis, and Ocean Data View 46 2.4 Results 46 2.4.1 Bulk properties of sediments 46 2.4.2 Distributions of lipid biomarkers and lignin phenols 48 2.4.2.1 Biomarker grouping 48 2.4.2.2. Variation of biomarker distributions between samples 48 2.4.2.3 Terrestrial biomarker input 49 2.4.2.4 Marine biomarker input 54 2.4.3 Inventory of lipid biomarkers and lignin phenols in the local rivers 56 2.5 Discussion 58 2.5.1 Sources of organic matter at the Galicia Minho shelf 58 2.5.1.1 The composition of terrestrial organic matter on the shelf 58 2.5.1.2 Marine contributions to sedimentary organic matter 61 2.5.2 Transport and distributional patterns of terrestrial organic matter 64 1 Table of contents
2.5.3 Spatial OM distribution in relation to seasonal plankton ecology 65 2.5.4 OM provinces on the Galicia Minho shelf 66 2.6 Conclusions 69 Acknowledgements 70 References 71 3 Molecular characterization of dissolved organic matter in pore water of continental shelf sediments 79 Abstract 80 3.1 Introduction 81 3.2 Materials and methods 83 3.2.1 Sampling site 83 3.2.2 Sampling 83 3.2.3 Dissolved organic matter extraction 84 3.2.4 Fourier transform ion cyclotron resonance mass spectrometry 85 3.2.5 Dissolved organic carbon and total dissolved nitrogen 86 3.2.6 Total organic carbon, total nitrogen and total sulfur 86 3.2.7 Lipid extraction 87 3.2.8 Lignin extraction 88 3.2.9 Gas chromatography coupled to mass spectrometry or flame ionization detection 88 3.3 Results and discussion 89 3.3.1 Characterization of sedimentary organic matter 89 3.3.2 Spatial distribution of lignin phenols and lipid biomarkers in the particulate organic matter 90 3.3.3 Molecular variations in dissolved organic matter 92 3.3.1 Dissolved organic matter characterization via molar H/C and O/C ratios 92 3.3.3.2 Dissolved organic matter characterization via molecular mass and DBE 96 3.3.3.3 Sources and occurrence of aromatic ring structures 97 3.3.3.3.1 Aromatic compounds with low O/C ( 0.2) 98 3.3.3.3.2 Aromatic compounds with high O/C (> 0.4) 99 3.3.3.4 Spatial variations of nitrogen bearing compounds 100 3.3.3.5 Sulfur compounds as indicators for early diagenesis? 102 3.4 Summary and conclusions 104 Acknowledgements 105 References 106 4 Diagenetic transformation of dissolved organic matter under contrasting sedimentary redox conditions in the Black Sea 111 Abstract 112 4.1 Introduction 113 4.2 Material and methods 114 4.2.1 Sampling 114 4.2.2 Dissolved organic matter extraction 115 4.2.3 Fourier transform ion cyclotron resonance mass spectrometry 115 4.2.4 Dissolved organic carbon and total dissolved nitrogen 116 4.2.5 Total hydrolyzable dissolved amino acids 117 2 Table of contents
4.2.6 Total organic carbon and total nitrogen 117 4.2.7 Stable organic carbon isotopes 118 4.3 Results and discussion 118 4.3.1 Characterization of sedimentary organic matter 118 4.3.2 The effect of early diagenesis on molecule size and molecule saturation 120 4.3.3 Variations in CHO molecules 121 4.3.4 Variations of N bearing compounds in DOM 123 4.3.5 N bearing compounds as products of protein degradation 124 4.3.6 Other sources for N bearing compounds in DOM 127 4.4 Summary and conclusions 128 Acknowledgements 129 References 130
5 Concluding remarks and perspectives 135 References 141
Appendix A1 143
Appendix A2 147
Danksagung 154
Erklärung 157
Electronic Appendix
3 Abstract
ABSTRACT
Continental margins are a major reservoir for organic carbon (OC) burial in the oceans and play an essential role in the global carbon cycle. High primary productivity, supply of terrestrial derived organic matter (OM) and reduced remineralization rates during the shorted sinking times of OM in the water column compared to the open ocean are the main reasons for the high OC accumulation. However, continental margins are often characterized by high water and sediment dynamics promoting OM remineralization. The highly dynamic conditions together with the multiple sources for organic matter complicate an understanding of the processes, responsible for OM preservation and remineralization in continental margin sediments, and at present, the main mechanisms remain elusive.
This thesis provides deeper insights into the molecular composition of OM with the aim to constrain the sources, distribution and early diagenetic processes in the sedimentary OM at continental margins.
The first study examines the effect of hydrodynamic and sedimentological processes on OM at the NW Iberian margin by comparing the distribution of lipid biomarkers and lignin phenols to the grain size distribution as well as to the chemical and isotopic bulk OC composition. The comprehensive lipid biomarker and lignin phenol analyses indicate OM contributions from various terrestrial and marine sources subjected to different transport mechanisms. Terrestrial OM consisting of vascular plant and soil derived material is mainly supplied by local rivers to the shelf. The distribution on the shelf is closely linked to the inorganic sediment fraction which is indicated by the correlation with the grain size distribution in the sediment. The abundance of higher plant wax derived compounds (long chain n alkanes, n alcohols and n fatty acids) in the sediment from all areas of the continental margin, suggests a second input pathway by wind. High water dynamics on the shelf are responsible for an internal sorting of the terrestrial OM according to the hydrodynamic properties, i.e., soft plant tissue (indicated by high ratios of cinnamyl phenols to vanillyl phenols (C/V)) is preferentially transported and deposited offshore, whereas the woody material (represented by plant derived triterpenoids and low C/V ratios) is primarily accumulated close to the source rivers in a local mid shelf mudbelt. Refractory soil derived organic matter was identified via 3,5 dihydroxybenzoic acid (major compound in tannic acid) and is distributed similar to the soft tissue. Algal lipid biomarkers reflect a complex marine plankton community on the shelf and a distribution of the algal derived OM closely
4 Abstract linked to the nutrient conditions, i.e., accumulation of algal derived OM close to the continent and the NW Iberian upwelling cell. The interplay of plankton blooms, hydrodynamic and sedimentological processes results in three different OM provinces on the shelf: (I) the inner shelf region with fresh marine OM, (II) the mid shelf mudbelt as major sink for terrestrial OM, and (III) the outer shelf with a low accumulation of rather refractory terrestrial derived OM. Based on this characterization, the second study elucidates early diagenetic processes in the sediments from the NW Iberian margin. Dissolved organic matter
(DOM) from sediment pore waters was analyzed for the first time by high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) which provided new and important insights into the molecular composition of DOM and thus into early diagenetic processes. More than 2000 molecular formulae were detected in the sediment pore water
DOM from the mudbelt indicating the various OM sources. Compounds with low elemental hydrogen to carbon (H/C) and high oxygen to carbon (O/C) ratios coincided with high concentrations of lignin phenols in the sedimentary OM and are probably released from terrestrial derived particulate OM via hydrolysis. The refractory nature of the OM deposited on the outer shelf results in a lower molecule number in the FT ICR MS data set and smaller variations in their H/C and O/C ratios.
The impact of the redox conditions on early diagenesis were investigated with FT ICR MS in a third study in the northern Black Sea in detail. The bulk sedimentary OM responded to the redox conditions by a high accumulation of refractory OM (total organic carbon (TOC) content of up to 6.2 % with a stable carbon isotopic signature of 13C = 25 ‰) and a chemically reduced DOM pool in the anoxic sediments. In contrast, the different remineralization processes at the oxic sampling site result in a lower TOC content (1.2 %) in the sediment and a higher oxidized DOM pool. A set of nitrogen bearing compounds in the
FT ICR MS data set was interpreted as products from protein degradation via hydrolysis and deamination. The reaction pathway appears to differ depending on the redox conditions; oxidatative deamination predominates under oxic conditions, whereas the anoxic sites contain also molecular formulae that may result from reductive deamination or a coupled reductive and oxidative pathway.
This thesis reveals the processes controlling OM distribution at continental margins and provides new and important insights into molecular reactions during early diagenesis, thus contributing to a better understanding of the fate of OM in marine sediments.
5 Zusammenfassung
ZUSAMMENFASSUNG
Kontinentalränder sind die Hauptablagerungsgebiete für organischen Kohlenstoff im Ozean und nehmen daher eine bedeutende Rolle im globalen Kohlenstoffkreislauf ein. Die wesentlichen Gründe für die verstärkte Akkumulation von organischem Material (OM) sind zum einen die hohe Primärproduktion, und zum anderen der zusätzliche Eintrag von OM vom Kontinent und die geringere Remineralisierung während kürzerer Sinkzeiten durch die
Wassersäule im Vergleich zum offenen Ozean. Andererseits fördern die hohen Wasser und
Sedimentdynamiken die Remineralisierung von OM. Die Kombination von hochdynamischen Bedingungen und von verschiedenartigen Quellen für das OM verhindern ein Verständnis der Prozesse, die zur Erhaltung oder der Remineralisierung von
OM führen. Bis heute sind deren Hauptmechanismen unklar. Diese Dissertation eröffnet tiefere Einblicke in die molekulare Zusammensetzung von OM mit dem Ziel, die Quellen, die Verteilung und früh diagenetische Prozesse im sedimentären OM auf
Kontinentalrändern zu erfassen.
Die erste Studie untersucht die Auswirkung von hydrodynamischen und sedimentologischen Prozessen auf das OM am NW iberischen Kontinentalrand in einem
Vergleich der Verteilung von Lipidbiomarkern und Ligninphenolen mit der
Korngrößenverteilung und der Charakterisierung der Gesamtorganik. Die umfassende
Analyse von Lipidbiomarkern und Ligninphenolen zeigt, dass das OM aus verschiedenen terrestrischen und marinen Quellen stammt und verschiedenen Transportmechanismen unterliegt. Das terrestrische OM stammt zum einen von vaskulären Pflanzen und zum anderen aus Böden und wird hauptsächliche fluviatil eingetragen. Die Verteilung auf dem
Schelf ist eng an die anorganische Sedimentfracht gebunden; dies wird durch die Korrelation mit der Korngrößenverteilung im Sediment angezeigt. Spezifische Verbindungen, die von
Pflanzenwachsen stammen (langkettige n Alkane, n Alkohole und n Fettsäuren), sind in den
Sedimenten des NW iberischen Kontinentalhanges weit verbreitet und weisen damit auf einen weiteren Eintragsweg durch die Atmosphäre hin. Die großen Wasserdynamiken auf dem Schelf sind für eine interne Sortierung des terrestrischen Materials verantwortlich, entsprechend ihren hydrodynamischen Eigenschaften. Weiche Pflanzenmaterialen, wie
Blätter und Gräser (angezeigt in den erhöhten Verhältnissen von Cinnamylphenolen zu
Vanillylphenolen (C/V)) werden bevorzugt in Richtung des äußeren Schelfs transportiert und dort abgelagert, wohingegen hölzerne Materialien (repräsentiert in Pflanzen
6 Zusammenfassung
Triterpenoiden und niedrige C/V Verhältnisse) vor allem nahe der Quellflüsse in einem lokalen Mudbelt auf dem mittleren Schelf abgelagert werden. Refraktäres OM von terrestrischen Böden wurde mittels 3,5 Dihydroxylbenzoesäure identifiziert (einem
Hauptbestandteil in Tanninsäure) und wird ähnlich wie die weichen Pflanzenmaterialien verteilt. Lipidbiomarker für Algen spiegeln eine komplexe marine
Planktonvergesellschaftung auf dem Schelf wider. Die Verteilung der Algenbiomasse scheint eng an die Nährstoffbedingungen in der Wassersäule geknüpft zu sein, d.h. Akkumulation nahe dem Kontinent und der NW iberischen Upwelling Zelle. Das Zusammenspiel von
Planktonblüten, hydrodynamischen und sedimentologischen Prozessen führt zu der
Ausbildung von drei verschiedenen Provinzen für OM auf dem Schelf: (I) der innere Schelf mit frischem marinen OM, (II) der Mudbelt im Bereich des mittleren Schelfes, der das
Hauptablagerungsgebiet für terrestrisches OM darstellt und (III) der äußere Schelf mit geringer Akkumulation von eher refraktärem OM. Basierend auf dieser Charakterisierung werden in der zweiten Studie frühdiagenetische Prozesse in den Sedimenten des NW iberischen Kontinentalhanges untersucht. Gelöstes organisches Material (DOM, von engl. dissolved organic matter), das aus sedimentären Porenwässern stammt wurde erstmalig mittels hochauflösender Fourier Transform Ion Cyclotron Resonance Massenspektrometrie
(FT ICR MS) untersucht, was neue und tiefere Einblicke in die molekulare
Zusammensetzung von DOM und die darin abgebildeten frühdiagenetischen Prozesse gewährte. Über 2000 molekulare Formeln konnten im sedimentären Porenwasser DOM aus dem Mudbelt ermittelt werden, die die mannigfaltigen Quellen für OM abbilden.
Molekulare Formeln mit niedrigen elementaren Wasserstoff zu Kohlenstoff (H/C)
Verhältnissen und hohen Sauerstoff zu Kohlenstoff (O/C) Verhältnissen fallen mit erhöhten
Ligninphenol Konzentrationen im sedimentären OM zusammen und werden vermutlich durch Hydrolyse aus terrestrischem partikulären Material freigesetzt. Die refraktäre Natur des OM auf dem äußeren Schelf zeigt sich in der niedrigeren Anzahl an identifizierten molekularen Formeln und geringeren Variationen in den H/C und O/C Verhältnissen dieser
Moleküle.
Die Bedeutung der Redoxbedingungen während der Frühdiagenese wurde detailliert mittels
FT ICR MS in einer dritten Studie im Schwarzen Meer untersucht. Die sedimentäre
Gesamtorganik reagiert auf die Redoxbedingungen mit der Akkumulation von refraktärem
OM (Gesamtorganischer Kohlenstoffgehalt (TOC, von engl.: total organic carbon) bis zu 6,2%
7 Zusammenfassung mit einer stabilen Kohlenstoffisotopie ( 13C) von 25‰) und einem chemisch reduzierten
DOM Pool in den anoxischen Sedimenten. Im Gegensatz dazu führen die unterschiedlichen
Remineralisierungsprozesse an der oxischen Probenahmestelle zu einem geringeren TOC
Gehalt von 1,2% und einem höher oxidierten DOM Pool. Eine Reihe von stickstoffführenden
Verbindungen in den FT ICR MS Daten konnte als Produkte von Proteinabbau via
Hydrolyse und Deaminierung interpretiert werden. Die Reaktionswege scheinen von den
Redoxbedingungen abzuhängen: oxidative Deaminierung dominiert unter oxischen
Bedingungen, wohingegen die anoxische Probenahmestelle ebenso molekulare Formeln enthält, die aus reduktiver Deaminierung oder einer gekoppelten reduktiven oxidativen
Reaktion hervorgehen können.
Die Dissertation zeigt die Prozesses auf, die hinter der Verteilung von OM an
Kontinentalrändern stehen und gibt neue und bedeutende Einblicke in molekulare
Reaktionen während der Frühdiagenese. Damit trägt sie wesentlich zu einem besseren
Verständnis hinsichtlich des Verhaltens von OM in marinen Sedimenten bei.
8 9 10
Chapter 1
Introduction Chapter 1
1.1 ORGANIC MATTER IN THE CONTEMPORARY OCEAN
The organic carbon (OC)1 reservoir of the present oceans is divided into four compartments, the living marine biota (3 Gt C) , particulate organic matter (POM, 20 Gt C) and dissolved organic matter (DOM, 680 Gt C) in the water column, and organic matter (OM) deposited in surface sediments (100 Gt C) (Hedges, 1992). The oceanic surface water is in direct exchange with the atmosphere having a balancing effect on the atmospheric carbon dioxide budget via equilibration (Stuiver, 1980). The build up of biomass by photosynthesis extracts carbon dioxide from the surface layer of the oceans. Subsequent burial of this biomass in marine sediments leads to a net removal of carbon dioxide from the atmosphere and to an oxygen input to the atmosphere (Eq. 1.1).
6 CO2 + 12 H2O (CH2O)6 + 6 H2O + 6 O2 (1.1)
Accordingly, OM preserved in marine sediments plays an essential role in the global carbon cycle and in controlling atmospheric carbon dioxide and oxygen on long time scales (Berner,
1989; Hedges, 2002). Remineralization processes in the ocean waters and surface sediments release carbon dioxide and recently deposited OM represents an important link between the active biological carbon cycle in the ocean and the inactive carbon pool in deeper sediments, which is preserved over geological timescales and thus decoupled from the global cycle.
Most of the OM sequestered in marine sediments is derived from marine organisms inhabiting the euphotic zone of the oceans (Wakeham and Lee, 1993), although marine sediments also act as sink for terrestrial OM supplied by rivers (0.4 Gt year 1; Schlesinger and
Melack, 1981; Siegenthaler and Sarmiento, 1993) or wind (0.1 Gt C year 1; Romankevich,
1984). Within the group of marine producers, phytoplankton represents 95% of the global primary production (Killops and Killops, 2005). The production rates of phytoplankton are coupled to the nutrient availability and magnitude of insolation. Accordingly, the highest primary productivity is observed in coastal oceans due to nutrient supply from nearby continents and in upwelling regions where cold deep ocean waters ascend (Wollast, 1991).
After the death of the organisms in the upper layers of the oceans the biomass sinks as POM incorporated in fecal pellets and aggregates to the sea floor (Alldredge et al., 1993). However, a large part of the planktonic biomass is efficiently remineralized on the way through the
1 The term organic matter refers in this thesis to natural organic matter, i.e., products from living organisms comprising carbon, oxygen, hydrogen and nitrogen, whereas the term organic carbon refers to organic matter normalized to carbon.
12 Introduction water column and assimilated by heterotrophic organisms remaining in the active pool (Fig.
1.1).
Figure 1.1. OM cycling in the contemporary oceans (adapted from Wakeham and Lee, 1993).
Approximately 1% of the OM originally produced in the surface waters survives the transport to the sea floor of 4000 m depth (Suess, 1980) and subsequent to the losses in OM through remineralization in the water column, approximately 90% of the OM deposited on the sea floor is degraded. Thus, only 0.1% of the OM, originally synthesized in the upper ocean escapes remineralization and is ultimately preserved (Berger et al., 1989). On a global base, the burial efficiency exceeds 0.1% because a significant amount of OM is deposited under more favorable preservation conditions (see Table 1.1 and Berner, 1989). Today, 6% of all OC being deposited in marine sediments is accumulated under regions of high productivity with pronounced oxygen minimum zones and about 90% is buried at lower concentrations in sediments of continental margins (Berner, 1989; Hedges and Keil, 1995).
The mechanisms that finally determine whether a marine setting acts as a sink for OC are complex and vary considerably depending on the depositional circumstances. Therefore, detailed information about the OM composition in respect to its sources and quality are essential for a comprehensive understanding of remineralization or preservation processes
13 Chapter 1 which in return is prerequisite for evaluating the potential of marine sediments for atmospheric carbon dioxide removal.
Table 1.1. Approximate organic carbon burial in the contemporary ocean (calculated from the yearly OC burial rate after Berner, 1982). Environment Organic carbon burial (%) Delta 44 Shelf and upper slope 42 Shallow water carbonate 4 High productivity slope 4 High productivity pelagic 2 Low productivity pelagic 3 Anoxic basin <1
1.1.1 COMPOSITION OF ORGANIC MATTER
The molecular composition of OM is an important factor controlling the extent of OM remineralization. Organic tissue consists of a variety of molecules having different properties and reactivities. On a compound class level, all organisms are composed of the same basic chemical classes, of which geochemical most important are proteins, carbohydrates and lipids (Fig.1.2). OM in marine sediments can be simply classified as being derived either from marine or terrestrial sources. Marine OM is generally considered as phytoplankton detritus and is composed of 25 – 50% proteins, 5 – 25% lipids and up to 40% carbohydrates
(Raymont, 1983). Terrestrial OM consists of living biomass, plant litter and soil OM. The latter has been subjected to degradation and consists therefore of pre aged compounds that might be refractory with a high potential for preservation while the labile compounds are lost. Vascular plants contain ~5% proteins, 30 – 50% carbohydrates (mainly cellulose) and very little amounts of lipids which are mostly concentrated in the fruiting bodies and leaf cuticles (Killops and Killops, 2005). Instead, lignins are major chemical components and occur exclusively in vascular plants representing an important tracer for terrestrial OM
(Hedges and Mann, 1979b). Lignins consist of non repeating units which are linked together in a random network by ether and carbon carbon bonds (Fig. 1.2d). This combination increases the chemical stability of the macromolecules and enhances the preservation potential by limiting the number of micro organisms that are able to consume lignins
(Sarkanen and Ludwig, 1971). Proteins on the other hand are rather labile compounds,
14 Introduction because the peptide bond linking the amino acids subunits are more susceptible to hydrolysis and various organisms synthesize specific enzymes (proteases) enabling protein consumption (e.g., Hollibaugh and Azam, 1983). The same applies for glycosidic linkages in carbohydrates (Fig. 1.2). However, under certain conditions, even labile molecules are preserved in marine sediments, e.g., protected in frustules of marine organisms (Ingalls et al.,
2003). Moreover, it is assumed that proteins and carbohydrates are incorporated into macromolecular OM and therefore shielded against further hydrolysis (Tissot and Welte,
1978; Hedges et al., 2001). This macromolecular OM is widely referred to as molecularly uncharacterized OM (MU OM) due to the incapability to resolve the composition of this residual OM on a molecular level by conventional analytical techniques (Wakeham et al.,
1997; Hedges et al., 2000; Hedges et al., 2001). MU OM sometimes accounts for up to 80% of the OM deposited in marine sediments (Wakeham et al., 1997) possibly formed during OM degradation by abiotic recombination of reactive intermediates, such as monomeric sugars or amino acids (Tissot and Welte, 1978; Hedges, 1988; Killops and Killops, 2005).
Figure 1.2. Structures of exemplary molecules representing the most important OM compound classes in marine sediments: a) carnosine (peptides, i.e., protein building block), b) chitin (carbohydrates), c) lecithin (lipids), d) lignins (possible structure, adapted from Glazer and Nikaido, 1995), e) perylene (black carbon), f) kerogen, g) murein (peptidoglycan).
15 Chapter 1
Other OM components in the oceans are black carbon, products of incomplete biomass burning, and recycled kerogen, both having a high preservation potential due to low reactivities. Bacterial derived OM, i.e., re assimilated and re packed OC, might be important in marine sediments due to the recalcitrance of the cell wall component peptidoglycan (Keil et al., 2000; Pedersen and Thomsen, 2001; Lomstein et al., 2006), particularly in anoxic environments in the absence of grazers (Lee, 1992).
1.1.2 ALTERATION OF OM IN THE WATER COLUMN
Prior to deposition in the marine sediments, the composition of OM is substantially transformed during sinking in the marine water column (see Fig. 1.1). In general, more labile, reactive compounds are selectively lost and more refractory components become concentrated. The extent by which OM is remineralized in the water column is variable and closely linked to the environmental conditions. The most important parameters are:
1) The amount and composition of the OM, such as the extent of the primary
productivity in the euphotic zone (Suess, 1980; Pace et al., 1987) and/or the
proportion of refractory terrestrial OM (Westrich and Berner, 1984).
2) The transport mechanisms controlling the residence time of the OM particles.
Aggregation, adsorption and incorporation in fecal pellets act protectively and
increase the sinking velocity which depends on particle size and shape (McCave,
1975; Armstrong et al., 2002).
3) The biological communities mediating the transformation reactions (Arnosti et al.,
2005).
4) The physical properties of the water column, e.g., depth (Suess, 1980), redox state
(Wakeham, 1990), etc.
The effects of these parameters vary strongly between the different OM components. A preferential loss of pigments, amino acids and lipids was observed from plankton to particulate OM in the water column and to the surface sediment in the central equatorial pacific. At the same time the fraction of MU OM increases from <20% to >80% of the total
OM (Wakeham et al., 1997). However, within the single compound classes selected components show a consistent pattern in their relative abundance with depth, still indicating the source material.
16 Introduction
1.1.3 EARLY DIAGENESIS OF OM IN MARINE SEDIMENTS
Once deposited in the marine sediment, OM undergoes several transformation processes resulting either in the remineralization or the preservation (Fig. 1.3). Generally during diagenesis, the elemental composition of the initial OM molecule shifts in direction to pure carbon, primarily by the release of oxygen and finally by the loss of hydrogen. The first step in OM remineralization is the chemically and/or enzymatically hydrolysis of biopolymers, producing smaller water soluble molecules. The hydrolysis products may either be converted to the inorganic end products in microbial mediated oxidation and/or fermentation reactions or they may be shielded from further remineralization by adsorption onto mineral surfaces and/or condensation reaction resulting in their preservation.
Figure 1.3. Schematic of early diagenesis in marine sediments (adapted from Henrichs, 1992).
The exact processes and mechanisms controlling OM preservation versus remineralization remain widely elusive, partly due to the complexity of compounds and processes involved, and partly due to analytical limitations. However, the following factors promote evidentially
OM burial in marine sediments:
Reduced oxygen exposure times due to high sedimentation rates/fast burial and little
bioturbation and/or little physical sediment reworking (Aller, 1994; Cowie et al.,
1995; Hartnett et al., 1998).
17 Chapter 1
The availability and physical association with mineral phases (Keil and Hedges,
1993; Keil et al., 1994; Mayer, 1994).
The OM provenance (Hedges et al., 1988b; Prahl et al., 1997; Hulthe et al., 1998).
Naturally, the relative contributions of these factors to OM preservation vary considerably, depending upon the specific depositional circumstances. Variations among individual compound classes during early diagenesis are indicated by the relative reactivities which are in the order: total hydrolysable amino acids (THAA) total carbohydrates (TCHO) > total organic carbon (TOC) > total nitrogen (TN) > lipids (Wang et al., 1998).
1.1.4 PRODUCTION AND SIGNIFICANCE OF DOM IN SEDIMENT PORE WATERS
Remineralization of POM in marine sediments produces large amounts of DOM having various molecule sizes and reactivities (Henrichs, 1992; Alperin et al., 1994). The net production of DOM in the surface sediments is reflected in elevated DOC concentrations in the sediment pore water compared to the marine bottom water. In consequence of the concentration gradient pore water DOM is released via diffusion into the bottom water contributing to the oceanic DOM pool (Williams and Druffel, 1987; Mopper et al., 1991;
Hedges, 1992; Burdige and Homstead, 1994) and probably to the nutrient pool for benthic organism communities (Burdige and Zheng, 1998). However, since the DOM flux accounts for less than 10% of the sediment carbon oxidation rate (Burdige, 2002), sediments appear to be efficient in retaining DOM. The processes producing and consuming DOM intermediates in the sediment are tightly coupled to the overall remineralization of sediment OM.
Accordingly, DOM passes through similar reactions as POM (Fig. 1.3). Evidence for the preservation of DOM on geological time scales comes from fluorescence data of pore water
DOM (Burdige, 2001; Burdige et al., 2004) and the decrease in POM remineralization rates and reactivities (Middelburg, 1989; Burdige, 1991) implying a decrease in the bulk reactivity of the pore water DOM subsequently allowing an accumulation with depth. The refractory
DOM is possibly a crucial factor in preservation of OM through condensation reactions as suggested in the classical geopolymerization model (Tissot and Welte, 1978) or in the mesopore protection model (Mayer, 1994; Hedges and Keil, 1995). The latter proposes DOM sorption in small mesopores on mineral surfaces which physically protect DOM from microbial enzyme attack and catalyzes abiotic condensation reactions.
18 Introduction
1.2 OM AT CONTINENTAL MARGINS
Chapter 1.1 emphasizes the importance of continental margins as sinks for OC. Reasons for the high OC burial rates particularly on continental shelves are the elevated marine productivity which is fueled by nutrient input from the nearby continent and/or coastal upwelling (Wollast, 1991), the high supply of terrestrial OM, and the shallow water depths (<
200 m) implying less OM remineralization in the water column. 25 – 50% of the OM produced in the euphotic zone of shelves reaches the seafloor (Suess, 1980), and 5 – 25% of this fraction overcomes further remineralization and is subsequently buried in the sediment
(Henrichs and Reeburgh, 1987).
The high input of terrestrial OM which comprises a large amount of soil derived and highly refractory OM (Meybeck, 1982; Hedges et al., 1994) suggests an elevated proportion of terrestrial OM in the shelf sediments. However, several studies indicated that the OM deposited in shelf sediments derives predominantly from marine sources (Gough et al., 1993;
Hedges and Keil, 1995; Hedges et al., 1997). One reason is possibly that continental shelves are usually sites of high sediment dynamics and the buried OM is subjected to oscillating redox conditions which result in similar remineralization rates for reactive marine OM and refractory terrestrial OM (Cowie et al., 1995). However, given that the calculated terrestrial
OM supply exceeds three times the total OC burial rate in marine sediments (Berner, 1989;
Hedges and Keil, 1995), similar remineralization rates are not sufficient to explain the predominance of marine OM in continental shelf sediments. Comprehensive studies regarding the molecular composition of OM and the variations in shelf sediments can help to elucidate the responsible mechanisms. It is necessary to cover a wide spectrum of continental margins, since environmental conditions, and thus the mechanisms, vary considerably between different sites influenced by local factors, including marine conditions (e.g., oceanographic current system, the marine productivity) as well as the continental setting
(e.g., geology, morphology and climate controlling the vegetation, the river run off and aeolian flux).
The studies in this thesis were carried out in two different continental margin settings in order to cover a wide range of environmental conditions that influence the mechanisms behind OM distribution and early diagenesis. The Galicia Minho shelf at the NW Iberian margin is an example for a highly dynamic continental shelf with complex hydrodynamic and sedimentological processes, whereas the northern Black Sea shelf provides a suited
19 Chapter 1 environment for investigations on early diagenesis under contrasting redox conditions. Both settings are briefly introduced and characterized in the following.
1.2.1 Study area 1: The Galicia Minho Shelf
The Galicia Minho shelf is located between 41.5 – 43.0 °N and 9.0 – 9.8 °W in the temperate zone of South Europe with a mild maritime climate (Fig. 1.4a). Due to seasonal upwelling in summer, the Galicia Minho shelf ranks among the areas with the highest primary productivity in the world (Antoine et al., 1996) and the high shear wave velocities in winter, particularly during frequent storm events, result in extensive sediment reworking (Vitorino et al., 2002).
Figure 1.4. a) Satellite image of the Iberian Peninsula (NASA visible earth) with the Galicia Minho shelf (yellow square) and b) conceptual model of the sedimentological processes on the Galicia Minho shelf adapted from Dias et al. (2002).
The present day shelf system was the subject of several detailed studies regarding the oceanography (e.g., Coelho et al., 2002; Mc Cave and Hall, 2002; Peliz et al., 2002), marine productivity (e.g., Estrada, 1984; Bao et al., 1997; Castro et al., 1997; Bode et al., 2002), and sedimentation processes (e.g., Jouanneau et al., 2002; Schmidt et al., 2002; Van Weering et al.,
2002; Vitorino et al., 2002). The results of these studies were summarized by Dias et al. (2002) in a conceptual model (Fig. 1.4b) reflecting the complex sedimentological processes. The
20 Introduction contemporary sediment distribution is dominated by the winter dynamics: sediment supplied by rivers (Douro and Minho in Fig. 1.4b) is deposited initially at the inner shelf close to the source, resuspended during winter storms and downwelling conditions and subsequently carried offshore. This results in sediment sorting, i.e., fine sediment is successively transported offshore leaving the coarse sediment fraction behind. The fine sediment is partly incorporated in a local mudbelt, situated in the mid shelf area at favorable hydrodynamic conditions. Sediments in deeper water depth (>100 m) are only reworked and resuspended during large storm events which can favor sediment export across the shelf break. The sources and particularly the fate of OM in this highly dynamic system remain elusive in this model and are examined in Chapter 2.
1.2.2 Study area 2: The northern Black Sea
The Black Sea is the world’s largest anoxic basin with permanent water column stratification caused by a large salinity difference between dense saline deep waters (22.3‰) and the surface layer, with a salinity of 18‰ (Murray et al., 1991), receiving large amounts of fresh water volumes by rivers (e.g., Danube, Dnieper, etc.). Slow renewal times for deep water
(Lee et al., 2002) in combination with oxygen consuming remineralization of POM during sinking from the surface to the lower layers (Sorokin, 1983) leads to permanent anoxic conditions below a water depth of 140 – 160 m.
Figure 1.5. a) Satellite image of the Black Sea (NASA earth observatory) indicating sampling locations (red stars) on the northern shelf and in the central basin; b) cross section showing the dominating processes in water column and sediment depending on the redox conditions.
21 Chapter 1
The oxic anoxic transition of the stratified water column separates the shelf in an oxic and an anoxic zone providing a suitable environment for investigations regarding the effect of redox conditions on OM composition and remineralization (Fig. 1.5b). The oxic shelf sediments are characterized by organic matter oxidation mediated by micro and macro organism activity and effective sediment reworking, whereas the anoxic shelf sediments are not bioturbated and dominated by bacterial sulfate reduction (e.g., Albert et al., 1995; Thamdrup et al., 2000;
Jørgensen et al., 2001).
22 Introduction
1.3 MAIN OBJECTIVES AND THESIS OUTLINE
The thesis was prepared in the frame of the project “Sediment partitioning and transformation on the shelf” in the research area “Sedimentation processes” of the DFG
Research Center / Excellence Cluster “The Ocean in the Earth System”. A comprehensive understanding of the processes controlling the distribution and partitioning of sediments, and particularly the organic sediment compartment, is important due to the prominent position of continental margins as transition between coastal areas/continents and the open ocean. A comprehensive knowledge on the fate of the OM at continental margins is the key to an understanding of OM remineralization and preservation.
The overarching goal of this thesis was to obtain deeper insights into the sources, distribution, and early diagenetic changes in the sedimentary OM pool in relation to sedimentation processes and environmental conditions. The two study sites in this thesis represent two distinct different continental margin settings, i.e., the NW Iberian margin with the Galicia Minho shelf is a high energy system with complex hydrodynamic and sedimentological processes, whereas the northern Black Sea is characterized by contrasting redox conditions. The methodological approach (Chapter 1.4) combines established geochemical tools for the analysis of particulate organic matter (lipid biomarker, lignin phenol, and amino acid analyses) with analyses of pore water DOM by high resolution
Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS), sediment grain size analyses, and chemical and isotopic characterization of bulk OM. The approach aims at a detailed molecular characterization of the sedimentary OM and DOM pool with the intent to:
(1) identify the sources, transport mechanisms, and distribution of the OM at continental
margins.
(2) determine the impact of hydrodynamic and sedimentological processes on the
distribution and composition of OM and, more specifically, to determine whether
different types of OM (e.g., algal OM, vascular plant derived OM, soil OM, etc.)
response differently to these processes?
(3) examine the biogeochemical transformation of OM, and particularly of the
sedimentary DOM pool, during early diagenesis in relation to:
a) hydrodynamic and sedimentological processes
b) prevailing redox conditions in the sediment and water column
23 Chapter 1
Chapter 2 investigates the sources, transport, and distribution of OM at the NW Iberian margin in a multi biomarker approach combined with grain size analyses and the bulk OC characterization. Various terrestrial and marine OM sources were identified and the relationship between OM deposition and sedimentological processes was examined. As a result of hydrodynamic sorting, sedimentological processes, and seasonal and spatial variations in the marine productivity, the continental margin could be subcategorized into three different depositional areas with distinct OM provenance.
Chapter 3 examines the variations in the molecular composition of the pore water DOM from the NW Iberian margin aiming specifically at early diagenetic processes. For the first time, DOM extracted from marine sediment pore waters was analyzed by high resolution
FT ICR MS providing detailed molecular information on the DOM composition. Since FT
ICR MS provides no structural information, the lipid biomarker and lignin phenol inventory
(Chapter 2) helped to explain the detected variations in the molecular composition. The study provides evidence for the close relationship between the sedimentary POM and DOM pool; the OM provinces, identified in Chapter 2, were also reflected in the molecular composition of DOM and resulted in distinct early diagenetic signals.
Chapter 4 investigates early diagenetic processes under contrasting redox conditions in the
Black Sea. FT ICR MS analyses were performed aiming at different molecular patterns in the
DOM pool which are related to oxic and anoxic remineralization reactions. Dissolved organic nitrogen compounds were the special focus of this study; amino acid analyses gave evidence for the presence of protein degradation products in the DOM extracts and the FT ICR MS data provided insights into possible degradation pathways.
The complete data set of all studies is provided in the appendix. The printed appendix contains an abstract of a manuscript (A1) in which I contributed through my lignin expertise, and the correlation matrix from Chapter 2 (A2). The electronic appendix comprises the biomarker and grain size data from Chapter 2 (A3 – A5) and the complete FT ICR MS data set from Chapter 3 (A6 – A13) and from Chapter 4 (A14 – A20).
24 Introduction
1.4 METHODOLOGICAL APPROACH
In the following, selected methods applied in the studies are briefly introduced, some more in detail than others. The application of gas chromatography coupled to mass spectrometry or a flame ionization detector (GC MS, GC FID) and high performance liquid chromatography (HPLC) in combination with MS for lipid biomarker, lignin phenol, and amino acid analyses in the scope of this thesis, are standard procedures in Organic
Geochemistry, and therefore will not be further discussed. On contrary, FT ICR MS is a relatively new approach for OM analyses and will be examined more closely. More detailed discussions of their application can be found in the corresponding studies in chapter 2 – 4.
Fig. 1.6 shows the methodological approach and an overview for the indications of the derived parameters.
Figure 1.6. Overview of the analytical approach applied in the single studies (study titles in italic, numbers refer to the Chapter). * 13C was not included in Chapter 3.
The analytical window. Bulk parameters are useful for an overall characterization of the total OM pool; however, they do not allow deeper insights into the OM composition. At present, the characterization of OM on molecular level suffers from the analytical limitations to resolve complex organic mixtures (Chapter 1.2). Therefore, direct source information is restricted to the molecularly characterizable fraction, which comprises in this study, solvent extractable compounds (lipid biomarkers and lignin phenols) and water soluble compounds in a mass window of 200 – 600 Da.
25 Chapter 1
Bulk parameters. Source information for the total OM pool can be derived from the elemental and isotopic composition of the bulk OM, directly measured in the sediment by an
Elemental Analyzer. Variations in the elemental carbon and nitrogen composition can be used to determine terrestrial contributions to the sedimentary OM (Meyers, 1994). Lignins and cellulose, major components of higher land plants, contain only little nitrogen in their structures which is reflected in total organic carbon to total nitrogen ratios (TOC/TN) > 20
(Hedges et al., 1986). On contrary, fresh algal OM consists of a large part of proteins and therefore displays TOC/TN ratios of 5 – 7 (Redfield, 1958). The carbon isotopic composition
( 13CTOC) is controlled by the carbon assimilation pathway of the organism and the 13C of the carbon source (Hayes, 1993). The predominating plant type in the study areas are C3 plants which have 13CTOC values of 25‰ to 27‰ (Meyers, 1990; Talbot and Johannessen, 1992), whereas 13CTOC values of marine algae range between 22‰ and 20‰ (Prahl et al., 1980;
Gearing et al., 1984).
Lipid biomarkers. Biomarkers are, in general, organic compounds that are specific for one organism or one organism group (Hedges and Prahl, 1993). Further requirements are given by their application, e.g., biomarkers as indicators for OM sources should be resistant to chemical and biological degradation, whereas indicators for transformation processes have to be sensitive. Lipids are particularly suited as biomarkers, due to their high degree of structural complexity, i.e., they comprise specific compounds for both applications. Specific compounds produced by marine organisms such as dinosterol (from dinoflagellates; Boon et al., 1979), highly branched isoprenoids (from diatoms; Belt et al., 2000; Massé et al., 2004) and alkenones (from coccolithophoridae; Conte et al., 1994) can be used as indicators for marine
OM and marine productivity, whereas specific pentacyclic triterpenoids (e.g., amyrin, taraxerol; Ten Haven et al., 1992) and long chained n alkanes with an odd over even predominance in carbon number indicate contributions from terrestrial higher plants
(Eglinton and Hamilton, 1967) (see Chapter 2). More labile compounds such as polyunsaturated fatty acids, highly abundant in fresh algal biomass, decrease strongly during early diagenesis (Wakeham et al., 1997), and therefore they are indicative of the OM quality of the marine fraction.
Lignin phenols. Lignins are valuable biomarkers in marine sediments due to their high resistance to chemical and biological degradation and their specific occurrence in higher land plants. The analytical approach via chemical cleavage through a copper oxide oxidation
26 Introduction releases lignin phenols from the marginal positions in the macromolecule (Fig. 1.7). The lignin phenols can be subdivided into four different groups (vanillyl, syringyl, cinnamyl and p hydroxyl group; Fig. 1.7) that form the base of several parameters regarding the plant type and degradation state. All higher land plants contain vanillyl, whereas only angiosperm tissue contains syringyl phenols and cinnamyl phenols are indicative of non woody plant tissue such as leaves, needles and grasses (Hedges and Mann, 1979a). Alteration of higher plant tissue is displayed in elevated concentrations of the acidic lignin phenols, i.e., vanillic acid and syringic acid (Hedges et al., 1988a) and/or in the loss of syringyl and vanillyl phenols relative to p hydroxyl phenols (Opsahl and Benner, 1995) depending on the environmental conditions and the responsible organism group.
Figure 1.7. a) Possible lignin macromolecule structure (adapted from Glazer and Nikaido, 1995) and b) lignin oxidation products.
Amino acids. The amino acid pool in sediment pore waters consists of free and combined amino acids, the latter including amino acids which are bound in proteins, peptides (Lee and
Bada, 1977) and other macromolecules such as glycoproteins and humic substances (Lytle and Perdue, 1981). Due to different structural associations, individual amino acids can serve as diagnostic markers for sources and diagenesis; e.g., D alanine and D glutamic acid are highly abundant in the bacterial cell wall component peptidoglycan and therefore indicate bacterial derived biomass (Madigan et al., 2000; Pedersen and Thomsen, 2001). Amino acids
27 Chapter 1 associated with structural components (e.g., cell walls) such as glycine, serine and threonine are more stable during diagenesis and are consequently enriched, whereas phenylalanine, glutamic acid, tyrosine, deriving from cell plasma, are more easily degraded (Dauwe and
Middelburg, 1998; Dauwe et al., 1999).
Molecular characterization of DOM. FT ICR MS is a relatively novel tool in Organic
Geochemistry and is further introduced in the following. FT ICR MS was established in 1974
(Comisarow and Marshall, 1974) and first performances on DOM in combination with electrospray ionization (ESI) are dated to the early 2000’s (e.g., Kujawinski et al., 2002;
Stenson et al., 2002). The high resolving power (Fig. 1.8) enables the analyses of complex, water soluble, organic mixtures.
Figure 1.8. FT ICR mass spectrum at m/z 178 for isobaric species of neoxaxiton (structure below) showing the high mass resolving power in a comparison to quadrupole time of flight collision induced dissociation spectrum (QqTOF (CID)) displayed as dashed line (taken from Marshall and Hendrickson, 2008).
The application as well as the data interpretation is rapidly evolving and this thesis contributes to the development of FT ICR MS through the first analyses of DOM from marine sediment pore waters. FT ICR MS analyses provide exact mass information of single
OM components (Fig. 1.8) which can be translated into molecular formulae. Different source materials or transformation reactions within the OM are all reflected in the molecular
28 Introduction composition and can be extracted from the data set. Several successful applications proved the potential of this method for the characterization of DOM in limnic and marine environments (e.g., Kim et al., 2003; Koch et al., 2005; Hertkorn et al., 2006). The extension of the application of FT ICR MS performed on DOM extracted from sediment pore water opened new perspectives and insights which are discussed in the corresponding Chapters 3 and 4.
29 Chapter 1
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32 Introduction
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33 Chapter 1
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34 Introduction
Siegenthaler, U. and Sarmiento, J. L., 1993. Atmospheric carbon dioxide and the ocean. Nature 365, 119 125. Sorokin, Y. I., 1983. The Black Sea. In: Ketchum, P. H. (Ed.), Ecosystems of the World. Elsevier, Amsterdam. Stenson, A. C., Landing, W. M., Marshall, A. G., and Cooper, W. T., 2002. Ionization and Fragmentation of Humic Substances in Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Analytical Chemistry 74, 4397 4409. Stuiver, M., 1980. 14C distribution in the Atlantic Ocean. Journal of Geophysical Research 85, 2711 2718. Suess, E., 1980. Particulate organic carbon flux in the oceans—surface productivity and oxygen utilization. Nature 288, 260 263. Talbot, M. R. and Johannessen, T., 1992. A high resolution palaeoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter. Earth and Planetary Science Letters 110, 23 37. Ten Haven, H. L., Peakman, T. M., and Rullkötter, J., 1992. Early diagenetic transformation of higher plant triterpenoids in deep sea sediments from Baffin Bay. Geochimica et Cosmochimica Acta 56, 2001 2024. Thamdrup, B., Rossello Mora, R., and Amann, R., 2000. Microbial Manganese and Sulfate Reduction in Black Sea Shelf Sediments. Applied Environmental Microbiology 66, 2888 2897. Tissot, B. P. and Welte, D. H., 1978. Petroleum formation and occurrence: a new approach to oil and gas exploration. Springer Verlag, Berlin. van Weering, T. C. E., de Stigter, H. C., Boer, W., and de Haas, H., 2002. Recent sediment transport and accumulation on the NW Iberian margin. Progress in Oceanography 52, 349 371. Vitorino, J., Oliveira, A., Jouanneau, J. M., and Drago, T., 2002. Winter dynamics on the northern Portuguese shelf. Part 2: bottom boundary layers and sediment dispersal. Progress In Oceanography 52, 155 170. Wakeham, S. G., 1990. Algal and bacterial hydrocarbons in particulate matter and interfacial sediment of the Cariaco Trench. Geochimica et Cosmochimica Acta 54, 1325 1336. Wakeham, S. G. and Lee, C., 1993. Production, transport, and alteration of particulate organic matter in the marine water column. In: Engel, M. H. and Macko, S. A. Eds.), Organic Geochemistry: principles and applications. Plenum Press, New York. Wakeham, S. G., Lee, C., Hedges, J. I., Hernes, P. J., and Peterson, M. J., 1997. Molecular indicators of diagenetic status in marine organic matter. Geochimica et Cosmochimica Acta 61, 5363 5369. Wang, X. C., Druffel, E. R. M., Griffin, S., Lee, C., and Kashgarian, M., 1998. Radiocarbon studies of organic compound classes in plankton and sediment of the northeastern Pacific Ocean. Geochimica et Cosmochimica Acta 62, 1365 1378. Westrich, J. T. and Berner, R. A., 1984. The role of sedimentary organic matter in bacterial sulfate reduction: The G model tested. Limnology and Oceanography 29, 236 249. Williams, P. M. and Druffel, E. R. M., 1987. Radiocarbon in dissolved organic matter in the central North Pacific Ocean. Nature 330, 246 248. Wollast, R., 1991. The coastal organic carbon cycle: fluxes, sources and sinks. In: Mantoura, R. F. C., Martin, J. M., and Wollast, R. Eds.), Ocean Margin Processes in Global Change. Wiley, New York.
35 36
Chapter 2
Sources, transport, and partitioning of organic matter at a highly
dynamic continental margin
Frauke Schmidt, Kai Uwe Hinrichs, Marcus Elvert
Submitted to Marine Chemistry in May 2009
MARUM – Center for Marine Environmental Sciences, Leobener Straße, D 28359 Bremen,
Germany
Chapter 2
ABSTRACT
Continental shelves play a major role as transition zone during transport of multiply sourced organic matter into the deep sea. In order to obtain a comprehensive understanding of the origin and fractionation processes of organic matter at the NW Iberian margin, 40 surface sediment samples were analyzed for a structurally diverse range of lipid biomarkers, lignin phenols, grain size distribution, organic carbon content (TOC), its stable carbon isotopic composition ( 13CTOC), and the organic carbon to nitrogen ratio (TOC/TN). The biomarker inventory reflected a heterogeneous mixture of organic matter from various marine and terrestrial sources. Soil and vascular plant derived continental organic matter, indicated by lignin phenols and plant derived triterpenoids, was primarily associated with the silt fraction and transported by river run off. The spatial distribution patterns of higher plant waxes derived long chain n alkanes, n alcohols, and n fatty acids suggested distinct different transport mechanisms and/or sources. The branched tetraether index, a molecular proxy expressing the relative abundance of branched dialkyl tetraethers versus crenarchaeol and considered to signal soil derived organic matter, was not as sensitive as other molecular indicators in detecting continental organic matter. Hydrodynamic sorting processes on the shelf resulted in a separation of different types of terrestrial organic matter; grass and leaf fragments and soil organic matter were preferentially transported offshore and deposited in areas of lower hydrodynamic energy. Algal lipid biomarker distributions indicated a complex community of marine plankton contributing to organic matter. Spatial and seasonal patterns of phytoplankton growth primarily controlled the distribution of algal organic matter components. The interplay of all of these processes control production, distribution, and deposition of organic matter and result in three distinct provinces at the Galicia Minho shelf: (I) fresh marine organic matter dominated the inner shelf region; (II) high inputs of terrestrial organic matter and high TOC content characterized the mid shelf deposited mudbelt; (III) lower concentrations of relatively degraded organic matter with increased proportions of refractory terrestrial components dominated the outer shelf and continental slope.
KEY WORDS: lipid biomarkers, lignin phenols, BIT index, bacteriohopanepolyols, degradation ratios, Galicia Minho shelf
38 Sources, transport, and partitioning of organic matter
2.1 INTRODUCTION
90% of the global organic carbon stored in modern marine sediments is deposited at continental margins (Hedges and Keil, 1995). Therefore, they are key locations for organic matter (OM) burial and play an essential role in the global carbon cycle. Nevertheless, due to varying OM composition, sedimentological regimes, and geochemical processes OM accumulation, preservation, and remineralization are not well understood. Common to all continental shelves is the diversity of OM sources. Terrestrial OM from plants and soils accumulate together with marine, estuarine or even riverine OM derived from autotrophic and heterotrophic organisms. Accumulation of OM from these various sources in sediments is further influenced by transport and sinking behaviors of carrier particles and differing chemical reactivity (Wang et al., 1998). Sediment remobilization and resuspension enhances the susceptibility of the OM to degradation due to extended oxygen exposure (Cowie et al.,
1995; Sun et al., 2002; Moodley et al., 2005). Within the different OM fractions distinct transport behaviors and preservation potentials are observed under similar environmental conditions and consequently affect the ultimately preserved OM content and quality. For example, pre aged terrestrial OM from plants and soils is more recalcitrant than fresh algal
OM (e.g., Prahl et al., 1997).
The existing studies on OM burial in continental shelf sediments (e.g., Prahl, 1985; Prahl et al., 1994; Goñi et al., 2000; Goñi et al., 2005; Xu et al., 2006; Tesi et al., 2007; Kuzyk et al., 2008;
Ramaswamy et al., 2008; Volkman et al., 2008; Yoshinaga et al., 2008; Schmidt et al., 2009) indicate complex mechanisms behind the OM distribution and strong variations in consequence of varying environmental conditions at each setting. Studies with a comprehensive set of geochemical indicators are needed to obtain a deeper understanding of the diverse processes that control OM distribution and preservation in shelf sediments. Bulk properties of sedimentary OM such as its stable carbon isotopic composition ( 13CTOC) and total organic carbon to total nitrogen ratio (TOC/TN) were frequently used to evaluate the sources of OM (e.g., Hedges and Parker, 1976; Peters et al., 1978; Alt Epping et al., 2007;
Perdue and Koprivnjak, 2007; Ramaswamy et al., 2008). However, TOC/TN is seriously affected by the preferential remineralization of nitrogen in marine sediments or nitrogen sorption onto clay minerals (Schubert and Calvert, 2001) and 13CTOC values of a mixture of
C3 and C4 plants could mimic marine algae (e.g., Goñi et al., 1998). Furthermore, both indices cannot provide detailed information about specific OM sources. Lipid biomarkers, on
39 Chapter 2 the other hand, can provide such information and, to a certain extent, they allow a determination of the degradation state of OM (e.g., Eglinton and Hamilton, 1963; Poynter and Eglinton, 1990; Ten Haven et al., 1992; Wakeham et al., 1997).
In order to overcome the obstacles associated with the interpretation of single OM portions in sediments of continental shelves we analyzed a broad range of lipid biomarkers and lignin phenols in surface sediments from the NW Iberian margin. Our aim was to reveal distributional and early diagenetic patterns of OM from various sources. The NW Iberian margin with the Galicia Minho shelf is a high energy shelf system characterized by high primary productivity in summer due to seasonal upwelling and sediment remobilization during severe winter storms. The material is deposited in the mid shelf area in a local mudbelt which is primarily fed by material from the Douro River and Minho River (see Fig.
4.1 and Araújo et al., 2002; Jouanneau et al., 2002). This manuscript links the spatial variations of source specific biomarkers to bulk sediment properties in order to constrain the relationship between OM deposition and sedimentological processes.
2.2 STUDY AREA
The study area is located between 41.33 °N and 42.49 °N latitude and 8.58 °W and 9.45 °W longitude at the NW Iberian margin (Fig. 2.1). The shelf is almost 50 km wide and bordered by sediment free rocky outcrops in the coastal areas and along the shelf edge in the southern parts, the latter acting as sediment traps. Terrestrial sediment is mainly delivered by the
Douro and the Minho River to the shelf, although the supply has been suppressed in recent years due to several dam constructions in the rivers (Jouanneau et al., 2002; Oliveira et al.,
2002; Vitorino et al., 2002). The smaller rivers along the Galicia Minho shelf (e.g., Cavado
River, Ulla River) contribute only minor amounts of sediment (Araújo et al., 2002), whereas the Rías Baixas located in the north, with islands off the coast, retain most of the sediment supplied by the rivers (Rey Salgado, 1993).
The oceanographic regime of the NW Iberian margin is controlled by two current systems; the warm subtropical Eastern North Atlantic Central Waters (ENACW) from beneath 40 °N is in exchange with cold less saline subpolar ENACW (>45 °N) by a poleward slope undercurrent and on the surface by the Portugal Coastal Counter Current (PCCC). The
PCCC is controlled by the predominating wind system: winds from north to northeast in
40 Sources, transport, and partitioning of organic matter summer direct the PCCC to the south and favor upwelling of cold nutrient rich subpolar
ENACW at the N and NW Iberian margin (Frouin et al., 1990). The movement of these water masses southward in a subsurface front results in enhanced primary productivity of multiple algal groups at the Galicia Minho shelf (Bode et al., 2002). During autumn and winter the
PCCC shifts northwards in response to the southerly winds, creating a downwelling front
(Frouin et al., 1990) that results in the deposition of sediment on the shelf. During the frequent winter storm events with increased bottom wave velocities, sediment is remobilized and transported northward by the bottom currents (Vitorino et al., 2002) and eventually deposited in the mid shelf mudbelt (Fig. 2.1). Hence, the upper 10 to 15 cm of sediment on the inner shelf (above 100 m water depth) represent a mixing layer which shows a decrease in mixing frequency and depth offshore (Jouanneau et al., 2002). Although the PCCC hinders material export from the shelf in general, a transport of fine material occurs during storm events from the mid shelf area that is subsequently eroded due to the northward slope current at the outer shelf (Vitorino et al., 2002).
Figure 2.1. Sampling locations at the NW Iberian margin (Sample names: GeoB 110..) and the local rivers. Color code refers to TOC content in the analyzed sediment samples. The mid shelf mudbelt (red dashed lines), mesozoic and cenozoic rocky outcrops (bricks), plutonic and metamorphic bedrock (crosses) are displayed according to Dias et al. (2002a).
41 Chapter 2
2.3 MATERIAL AND METHODS
2.3.1 Sampling
Sampling was performed with a giant box corer in August 2006 during the GALIOMAR expedition (P342) with the German RV Poseidon at the Galicia Minho shelf (Hanebuth and participants, 2007). Surface sediments were sampled from a depth interval of 0 2 cm for lipid biomarker, lignin phenol and bulk analyses. Additional sediments were collected from the river bank of three local rivers (Minho, Cavado and Ulla Rivers) and from the tidal flat of the
Ría de Vigo. All samples were stored in pre combusted brown glass bottles at 20 °C to avoid
OM degradation until further preparation in the home laboratory.
2.3.2 Total organic carbon, total carbon and total nitrogen
Total organic carbon (TOC), total carbon (TC) and total nitrogen (TN) concentrations were analyzed from the freeze dried homogenized sediment. TOC was measured with a Leco CS
200. Prior to TOC measurements, 50 mg of each sample were treated with 12.5% hydrochloric acid to remove carbonates. For TN and TC analyses, 25 mg of each sample was packed in tin boats and measured on a Vario EL III Elemental Analyzer. Bulk organic carbon stable isotope analyses were carried out using an Heraeus elemental analyzer connected to a
Finnigan MAT Delta Plus isotope ratio mass spectrometer. Prior to analysis, the freeze dried samples were decalcified with 12.5% hydrochloric acid, homogenized and packed into tin boats. Standard deviation was below 0.1‰ as determined from repeated analysis of a reference sample. Values are quoted in the 13C notation in per mil relative to the Vienna Pee
Dee Belemnite (V PDB) standard.
2.3.3 Lipid extraction
30 g of wet sediment were spiked with a mixture of internal standards consisting of 5 cholestane, nonadecan 2 one, 1 nonadecanol and 2 methyloctadecanoic acid. Lipid biomarkers were extracted three times with a solvent mixture of dichloromethane:methanol
(DCM:MeOH, 2:1, v/v) in a microwave extraction system (MARS X, CEM) at 80 °C. The combined extracts were washed with 0.05 M potassium chloride and residual water was removed from the organic phase with sodium sulfate. The remaining solvent was evaporated under a stream of nitrogen. The total lipid extract was separated in a hexane soluble
(maltene) and insoluble (asphaltene) fraction. The maltene fraction was further separated on
42 Sources, transport, and partitioning of organic matter
SPE cartridges (Supelco LC NH2, 500 mg sorbent) into four fractions (hydrocarbons, esters and ketones, alcohols, and free fatty acids) according to the protocol by Hinrichs et al. (2000).
Prior to analyses, alcohols were reacted with bis (trimethylsilyl)trifluoroacetamide (BSTFA,
Merck) in pyridine to form trimethylsilyl(TMS) derivatives. Fatty acids (FA) were derivatized with 12% boron trifluoride in methanol (Merck) and analyzed as fatty acid methylesters. Both reactions were conducted at 70 °C for 1 h followed by the removal of solvent and reagent under a stream of nitrogen. All four fractions were stored at –20 °C in the dark until further analysis.
2.3.4 Bacteriohopanepolyols
An aliquot of the asphaltene fraction from each sample was prepared for bacteriohopanepolyol (BHP) analysis according to the protocol by Talbot et al. (2001). Briefly, the fraction was acetylated with acetic anhydride in pyridine (1:1, v/v; Roth) for 1 h at 50 °C and left at room temperature overnight. Afterwards, the reagents were removed under a stream of nitrogen and stored at –20 °C in the dark until analysis. Prior to analysis, each sample fraction was dissolved in MeOH:isopropanol (3:1, v/v) and spiked with an injection standard (5 cholestane) for component quantification.
2.3.5 Branched and isoprenoid tetraether index
For the branched and isoprenoid tetraether index (BIT) measurements, the apolar fraction was isolated from the total lipid extract after the protocol by Hopmans et al. (2004). Aliquots of the total lipid extract were separated over an Al2O3 column (Merck, active basic 70 – 230 mesh) into an apolar and a polar fraction using hexane:DCM (9:1, v/v) and DCM:MeOH (1:1, v/v), respectively. The solvent was removed from the apolar fraction, containing the isoprenoidal tetraethers, under a stream of nitrogen and the fraction redissolved in a defined volume of hexane:isopropanol (99:1, v/v), corresponding to an amount of 2 mg/ml. Prior to analysis, the fraction was filtered through a PTFE filter (pore size 0.45 m, Roth) in order to remove small particles.
2.3.6 Lignin extraction
The protocol used for lignin extraction was adapted from Hedges and Ertel (1982). Briefly, 1
2 g of solvent extracted sediment was filled into a microwave vessel followed by addition of
43 Chapter 2
100 mg ammonium iron(II) sulfate hexahydrate (Fe2(NH4)2(SO4) * 6 H2O, p.a. Fluka), 1 g
DCM extracted CuO powder, and 8 mL of nitrogen purged sodium hydroxide (8% p.a. grade, Merck) under oxygen free conditions. The oxidation was conducted under nitrogen atmosphere in a sealed extraction vessel in MARS X at 150 °C for 3 h. After the reaction was completed, an internal standard mixture consisting of ethylvanillin and t cinnamic acid was added to the sediment slurry. The sediment was sonicated three times with 15 ml of 16% sodium hydroxide and each liquid phase was separated from the sediment by centrifugation.
The combined extracts were acidified with hydrochloric acid to pH 1 and lignin derived phenols were extracted from the aqueous solution three times with distilled diethylether, which was treated with Fe2(NH4)2(SO4) * 6 H2O in aqueous solution a priori. The combined extracts were dried with sodium sulfate and the solvent was removed under a stream of nitrogen. Prior to analysis by gas chromatography (GC) either coupled to mass spectrometry
(MS) or a flame ionization detector (FID), the phenols were reacted with BSTFA in pyridine to form TMS derivatives.
In order to check for ester bond phenols deriving from cutin and suberin, two random pre extracted sediment samples were subjected to base hydrolysis to release ester bound phenols. 2 – 3 g of sediment were heated with 25 ml of 1 M methanolic potassium hydroxide solution at 80 °C for 3 h. After cooling, 5 ml of 22% hydrochloric acid was added and the mixture was sonicated three times with 30 ml of DCM/MeOH (2:1, v/v) for 15 min. Each extract was separated from the sediment by centrifugation and 50 ml of MilliQ were added to the combined extract. Phenols were extracted from the aqueous solution according to the lignin extraction protocol reported above. The extracted, saponified sediment was subjected to CuO oxidation and the phenol composition was compared to the corresponding extracted sediment.
2.3.7 Gas chromatography coupled to mass spectrometry or flame ionization detection
All four lipid fractions and the phenol extract were analyzed by GC MS and GC FID. The GC
(Thermo Electron Trace GC) was equipped with a 30 m Rtx 5MS fused silica capillary column (0.25 mm i.d., 0.25 m film thickness) and helium was used as carrier gas (flow rate
1.0 ml min 1). The lipid fractions were injected in hexane using an injection temperature of
60°C, a temperature ramp of 10 °C min 1 up to 150 °C followed by a temperature ramp of 4
°C min 1 up to 320 °C (hold time: 30 min). The lignin phenol extract was injected in
44 Sources, transport, and partitioning of organic matter pyridine:BSTFA (4:1 v/v) in order to keep all carboxylic groups as trimethylsilyl derivatives.
The initial temperature of 100 °C was followed by a temperature ramp of 4 °C min 1 up to 310
C (hold time: 12 min). BHPs were analyzed on a 30 m DB XLB fused silica capillary column
(0.25 mm i.d., 0.1 m film thickness). Helium was used as carrier gas with a flow rate of 1.0 ml min 1 and samples were injected in methanol:isopropanol (3:1, v/v) at an initial temperature of 100 °C. A temperature ramp of 10 °C min 1 up to 300 °C was followed by a temperature ramp of 4 °C min 1 up to 360 °C (hold time: 24 min).
All components were identified via their mass spectra and concentrations were calculated from their peak area in the FID chromatogram relative to the internal standard with the exception of BHPs which were quantified via an injection standard.
Blanks for lipid and lignin extraction were prepared in parallel in the same manner as sediments to check for contaminations during sample preparation. No target components or other substances with similar retention times were detected in the blanks. Three random samples were analyzed in duplicate for lignin phenols in order to detect variations during sample preparation and analysis. Relative standard deviation was below 15% which is in the previously reported range for lignin analysis (Hedges and Ertel, 1982; Houel et al., 2006). The relative standard deviation between lignin phenols obtained from pre extracted sediments with and without basic hydrolysis was below 13% indicating that ester bound phenols can be neglected as an important source at the Galicia Minho shelf.
2.3.8 High performance liquid chromatography
Non isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) were analyzed on an Agilent
1100 series high performance liquid chromatograph (HPLC) coupled to an Agilent 1200 MSD equipped with automatic injector and HP Chemstation software. A sample volume of 20 l was injected on an Alltech Prevail Cyano column (150 mm x 2.1 mm; 3 m) with hexane:isopropanol (99:1, v/v) as eluent at a flow rate of 0.2 ml min 1. After 5 min of isocratic elution, a linear gradient to hexane:isopropanol 98.2:1.8 (v/v) within 45 min was applied followed by a linear gradient within in 2 min to hexane:isopropanol 95:5 (v/v) that was kept for 8 min.
The compounds were ionized in positive APCI mode and expressed as relative abundances of non isoprenoidal GDGTs compared to crenarcheol by integrating the peak areas of the
[M+H]+ ions. The BIT index was calculated according to Hopmans et al. (2004). All samples
45 Chapter 2 were analyzed in duplicate and the maximum deviation of individual values for the BIT index was 0.01 units.
2.3.9 Grain size analysis
The grain size distribution was determined using a laser particle analyzer (Coulter LS 200).
Prior to analysis, 0.5 g of pre extracted sediment was mixed with aqueous sodium pyrophosphate solution (3 mg in 50 ml of water) and heated up to 100 °C in order to avoid the formation of aggregates. Analyses were performed in the range of grain sizes between
0.04 and 1000 m in triplicate. The silt percentage was calculated based on the granulometric interval of 62.5 – 3.9 m.
2.3.10 Correlation analysis, cluster analysis, and Ocean Data View
Correlation and cluster analysis was performed using the software PAST (version 1.75;
Hammer, 2001). Correlation analysis was performed using Spearman’s rank correlation coefficient and for 40 samples a correlation was significant if r > 0.418 for a level of confidence = 0.01 and r > 0.325 for = 0.05. Distribution maps were constructed in the software Ocean Data View (ODV; Schlitzer, 2002) using the diva gridding algorithm.
2.4 RESULTS
2.4.1 Bulk properties of sediments
Table 2.1 lists the bulk properties of surface sediments from the NW Iberian margin.
Generally, the silt content varied between 11 and 83% corresponding to a mean grain size distribution between 187 m and 21 m, respectively. Exceptions from this were the continental slope sample GeoB 11033 (92% silt, 9.4 m mean grain size) and the mid shelf sample GeoB 11011 (7.7% silt, 368 m mean grain size). The small grain sizes at the continental slope sample indicate an accumulation of very fine grained, clay rich sediment in this region, whereas the large grain sizes at station GeoB 11011 suggest a preferential accumulation of carbonate debris which is corroborated by a high total inorganic carbon
(TIC) content of 7.3%. Apart from this, the highest silt contents (> 50%) were observed in the inner to mid shelf area (Fig. 2.2a) confirming the previously reported extensions of the
Galicia mudbelt (Dias et al., 2002a). A decrease in the silt content was observed offshore at
46 Sources, transport, and partitioning of organic matter
Table 2.1. Location and bulk properties of samples from the Galician margin and adjacent rivers. Mudbelt samples are indicated by a silt content >50%, whereas the remaining zonation corresponds to water depth. 13 Sample Location Latitude (N) Longitude Water TOC [%] TIC [%] CTOC (‰) TOC/TN Silt [%] Mean grain (GeoB) (W) depth (m) size [μm]
11001 mudbelt 42°25 9°08 136 0.92 0.76 -23.5 9.55 69.2 28.8 11002 mudbelt 42°17 8°99 112 0.91 0.49 -23.6 9.61 63.7 34.6 11003 mudbelt 42°17 9°04 129 1.43 0.66 -23.7 10.51 83.0 21.4 11004 outer shelf 42°17 9°10 141 0.49 0.18 -22.8 8.93 48.7 59.4 11005 outer shelf 42°17 9°18 161 0.20 0.26 -21.7 5.91 17.7 122.8 11006 outer shelf 42°17 9°33 235 0.26 1.58 -22.0 8.89 22.5 77.6 11007 outer shelf 42°17 9°23 184 0.30 1.31 -21.9 8.06 22.7 73.7 11008 outer shelf 42°36 9°22 157 0.22 0.11 -22.1 7.77 17.6 145.1 11009 outer shelf 42°43 9°27 166 0.14 0.19 -21.4 4.96 10.6 187.3 11010 mudbelt 42°42 9°11 119 1.06 1.08 -23.2 8.99 69.6 27.6 11011 mid-shelf 42°53 9°20 100 0.16 7.30 -21.0 4.53 7.7 367.9 11012 mudbelt 42°71 9°27 119 0.73 1.37 -22.6 8.14 53.2 41.3 11013 mid-shelf 42°71 9°35 130 0.49 0.97 -22.6 8.07 46.1 53.5 11014 outer shelf 42°71 9°46 153 0.36 1.33 -22.8 8.17 33.7 73.3 11015 outer shelf 42°77 9°47 158 0.39 0.81 -22.8 8.78 40.5 55.8 11016 mid-shelf 42°82 9°35 132 0.28 0.92 -22.3 7.20 22.7 84.0 11017 mudbelt 42°52 9°24 120 0.81 2.23 -22.9 8.29 61.5 48.0 11018 mid-shelf 42°52 9°27 124 0.29 0.56 -22.2 7.64 31.6 83.5 11019 outer shelf 42°51 9°31 147 0.32 0.22 -22.4 8.05 32.4 88.6 11020 outer shelf 42°51 9°32 154 0.27 0.31 -22.2 7.04 28.7 104.2 11021 cont. slope 42°49 9°43 484 0.62 0.69 -22.5 12.45 45.2 44.7 11022 outer shelf 42°58 9°42 289 0.43 0.92 -22.8 8.83 35.0 77.6 11023 cont. slope 42°71 9°56 405 0.37 1.20 -22.5 8.33 30.1 66.8 11024 cont. slope 42°70 9°76 1823 0.44 2.16 -23.1 7.63 41.1 47.4 11025 mid-shelf 42°63 9°36 131 0.50 0.89 -22.7 8.20 44.2 53.5 11027 outer shelf 41°97 9°18 137 0.27 2.34 -22.2 7.25 21.1 103.2 11028 mudbelt 41°97 9°09 127 1.28 1.01 -24.1 11.06 76.4 26.0 11029 mudbelt 41°97 9°05 115 1.32 0.61 -24.5 12.62 69.9 32.9 11030 inner shelf 41°97 8°99 94 0.54 0.28 -23.7 8.86 37.4 58.4 11031 outer shelf 42°08 9°16 149 0.21 0.85 -21.8 7.66 17.7 90.6 11032 outer shelf 42°09 9°25 166 0.25 1.50 -22.0 7.41 25.2 72.0 11033 cont. slope 42°17 9°56 1873 1.10 3.18 -22.6 8.41 92.0 9.4 11037 inner shelf 41°72 8°98 80 0.53 0.63 -22.6 8.31 44.0 55.9 11038 inner shelf 41°64 8°89 78 0.35 0.98 -22.9 7.97 27.2 77.9 11039 mudbelt 41°55 9°08 99 2.21 1.98 -25.1 15.55 57.9 45.4 11040 mid-shelf 41°64 9°07 99 0.28 1.73 -22.8 7.73 20.5 120.7 11041 inner shelf 41°80 9°02 93 0.51 0.68 -23.7 11.02 31.1 76.3 11042 mudbelt 41°72 9°02 95 0.75 0.78 -23.7 10.08 52.8 47.6 11043 inner shelf 41°55 9°00 84 0.41 0.80 -23.2 9.94 30.3 70.7 11044 mid-shelf 41°89 9°17 126 0.36 4.17 -22.2 8.36 30.2 92.6 MR1 River 42°05 8°52 - 0.87 0.00 -27.1 17.27 - - MR2 River 41°93 8°78 - 0.24 0.06 -25.4 6.47 - - CR River 41°52 8°74 - 2.80 0.53 -26.5 9.76 - - UR River 42°52 8°71 - 1.42 0.13 -25.9 11.33 - - RdV Tidal flat 42°35 8°71 - 4.16 0.40 -26.3 12.81 - - the southern inner shelf and close to rocky outcrops in the northern shelf area, where the sediment is mainly composed of fine to middle sand. The TOC content varied from 0.14 –
47 Chapter 2
2.21% with the lowest values found at the outer shelf (Fig. 2.1) and was significantly correlated with the silt distribution (r = 0.95) as observed in several studies before (Keil et al.,
1994; Mayer, 1994). The mudbelt itself had TOC contents between 0.73 and 2.21%, which is contrasting to all other inner to mid shelf stations that ranged between 0.28 and 0.54%.
Slightly lower values were observed in the sediments from the outer shelf region (0.14 –
0.49%). The TOC/TN ratio was significantly correlated with TOC and silt content (r = 0.82 and 0.72, respectively), whereas 13C of TOC was inversely correlated (r = 0.84 and 0.74, respectively).
Figure 2.2. a) Silt content (grain size interval of 62.5 – 3.9 m), b) 13CTOC, and c) TOC/TN ratio at the NW Iberian margin. Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, 100 m).
2.4.2 Distributions of lipid biomarkers and lignin phenols
2.4.2.1 Biomarker grouping
A correlation analysis was performed on all quantified biomarkers and bulk property data to reveal internal relationships. In order to reduce the data set and to facilitate data interpretation, the most abundant biomarkers were combined into groups according to their sources (Table 2.2) and correlation coefficients (Table A2, Appendix A2). For example, all
5,22 and 5 sterols in the range of C27 to C29 were correlated (r > 0.80) and therefore combined as indicator of fresh marine biomass (sterols). Due to specific source assignments, sterol biomarkers such as dinosterol, 24 methylenecholestadienol, the C37:2 alkene or the triterpenoid friedelin were considered individually. All bulk parameters indicating terrestrial input ( 13CTOC, TOC/TN, TOC content, silt content) generally agree well with the spatial
48 Sources, transport, and partitioning of organic matter
distribution of lignins, plant derived triterpenonoids, BIT index, long chain C25 to C35 n alkanes with high odd over even carbon number predominance (LCalk), even numbered long chain n alcohols with a chain length of 22 to 32 carbon atoms (LCOH), C16:0 mono O alkyl glycerol ether (C16:0 MAGE) and to a lesser extent the amount of plant derived triterpenols, p hydroxyl phenols and even numbered long chain n fatty acids with a chain length of 22 to 32 carbon atoms (LCFA). Sterols, short chain n alcohols (SCOH including
C16:0 and C16:1 alcohol) and the C18 FA are inversely correlated with 13CTOC. Several specific terrestrial biomarkers (LCalk, LCOH, LCFA, plant derived triterpenoids, lignin) and C16:0
MAGE, SCOH and C18 FA correlated with the TOC content in the sediment. In contrast, the ratio of 3,5 dihydroxybenzoic acid to vanillyl phenols (3,5 Bd/V), the acid to aldehyde ratio of vanillyl ((Ad/Al)V) and the ratio of p hydroxyl phenols to syringyl and vanillyl phenols
(P/(S+V)) increased with decreasing TOC. Marine lipid biomarkers were not related to TOC in the sediment. Similar patterns were observed in the relationship to the grain size distribution, i.e., most terrestrial biomarkers increased with increasing silt content, whereas marine biomarkers did not correlate with the silt content. LCalk, lignin, and the BIT index were correlated with TOC/TN ratio, while lignin degradation ratios ((Ad/Al)V, P/(S+V)) were inversely correlated with TOC/TN ratios.
2.4.2.2. Variation of biomarker distributions between samples
Biomarker concentrations were generally highest in the mudbelt samples and decreased with increasing distance from the source rivers along the inner and mid shelf area to the outer shelf and the continental slope (Table 2.3). Fig. 2.3 shows the biomarker distribution of a typical mudbelt sample (GeoB 11002). Highest concentrations of up to 190 g/g dry weight
(dw) were observed for lignin phenols in the southern parts of the mudbelt at station GeoB
11039 (Table 2.3). In general, concentrations of fatty acids and sterols, both absolute and relative to other biomarkers, were highest in the southern regions, with maximum values of
180 g/g dw and 33 g/g dw for short chain fatty acids (SCFA) and sterols, respectively. Both compound groups showed the highest concentration at location GeoB 11042, indicating a strong input of algal OM. LCFA and LCOH yielded concentrations of 1.7 to 11 g/g dw and
3.1 to 12 g/g dw in the mudbelt. LCalk were in the same concentration range, maximizing at n C31 (Fig. 2.2a).
49 Chapter 2
Table 2.2. Biomarker grouping, internal correlation, and main sources of single biomarkers at the Galician margin. n. c. – no correlation. Correlation indices of S and V refer to the phenolic and the acidic oxidation product.
Group/biomarker components correlation main source (reference)
LCalk Odd numbered C25 – C35 n-alkanes r > 0.72 Epicuticular plant waxes (Eglinton and Hamilton, 1967)
C37:2 alkene Coccolithophoridae, e.g., Emiliana huxleyi (Volkman et al., 1980)
HBI Monocyclic C25-triene, C25-triene 1, r > 0.57 Diatoms from the genus Rhizosolenia, Haslea, Navicula, Pleurosigma (Belt et al., C25-triene 2 2000; Belt et al., 2001; Massé et al., 2004)
Alkenone C37:2/37:3 alkenone r = 0.98 Coccolithophoridae, e.g., Emiliana huxleyi (Conte et al., 1994)
LCOH Even-numbered n-alcohols with a r > 0.62 Epicuticular plant waxes (Eglinton and Hamilton, 1967) chain length of C22-C32
Phytol Ester-linked side-chain of chlorophyll-a, degradation of chlorophyll (Johns et al., 1980; Sun et al., 1998)
Sterols 27 5,22, 27 5, 28 5,22, 28 5, 29 5,22, r > 0.80 Phytoplankton (Review by Volkman et al., 1998; Volkman, 2003) 29 5
Dinosterol Dinoflagellates (Boon et al., 1979)
24-Methylene- 28 5,24(28) Abundant in diatoms (Volkman, 1986 and references therein) cholestadienol
Alkyl-diols 1,15-C30:1-, 1,14-C30:0-, 1,15-C30:0- r > 0.75 Eustigmatophytes (Volkman et al., 1999) alkyl-diols & 12-hydroxy-C28 -fatty acid Plant-derived Taraxerol, -amyrin, lupeol r > 0.87 Angiosperm resin triterpenols
Plant-derived Taraxerone, -amyrone, lupenone r > 0.88 Oxidation of triterpenols (Medeiros and Simoneit, 2008) triterpenones
Friedelin Angiosperms LCFA Even numbered n-FA with a chain r > 0.70 Epicuticular pant waxes, minor source: microalgae, bacteria (Eglinton and length of C20-C30 Hamilton, 1967)
Sat.SCFA Saturated C14 – C16-fatty acid r > 0.83 Unspecific marine (Cranwell, 1982)
MUFA C16:1 5-, C16:1 7-, C18:1 7-, C18:1 9- r > 0.79 Alteration (Cranwell et al., 1987) fatty acid
PUFA Polyunsaturated fatty acids r = 0.81 Fresh phytoplankton biomass, mainly from diatoms (Shaw and Johns, 1985; including C20:4FA, C20:5FA Canuel and Martens, 1993)
C18:1 9-FA Zooplankton (Sargent, 1976)
C18:1 7-FA Bacteria (Cranwell, 1982)
BrFA i-+ai-C15, i-+ai-C17, 10-Me-C16-fatty r > 0.88 Bacteria (sulfate reducers) (Taylor and Parkes, 1983; Edlund et al., 1985; Kaneda, acid 1991)
C16:0-MAGE C16:0-Mono-O-alkyl glycerol ether Bacteria (Rütters et al., 2001)
Diploptene a) Soil (Prahl, 1985; Prahl et al., 1992) b) Marine OM (Venkatesan, 1988; Elvert et al., 2001)
C32-HOH C30-Hopanol, C32-bishomohopanol r = 0.80 Degradation product of highly functionalized biohopanoids (Innes et al., 1997; Winkler et al., 2001)
C32- -HA C32- -Hopanoic acid Degradation product of highly functionalized biohopanoids (Ourisson and Rohmer, 1992; Innes et al., 1997)
BHPs 32, 34-Anhydrobacterio- r > 0.81 Bacteria (Rohmer et al., 1980; Ourisson and Rohmer, 1982) hopanetetrol, bacteriahopanetetrol
3,5-Bd/V 3,5-Dihydroxybenzoic acid to vanillyl Soil (Prahl et al., 1994; Gordon and Goñi, 2003)
Lignin Vanillyl (V), syringyl (S), cinnamyl r > 0.50 Vascular plants (C), p-benzoic phenols (P) r > 0.70 (for S, V, P)
Vanillyl phenols Acetovanillone, vanillin, vanillic acid r = 0.87 Woody gymnosperms (Hedges and Mann, 1979)
Syringyl phenols Acetosyringone, syringealdehyde, r = 0.57 Woody angiosperms (Hedges and Mann, 1979) syringic acid
Cinnamyl phenols p-Coumaric acid, ferulic acid r = 0.52 Non-woody material (Hedges and Mann, 1979) p-Hydroxyl phenols p-Benzophenone, p-benzaldehyde, n.c. Lignin (Hedges et al., 1988) p-benzoic acid
50 Sources, transport, and partitioning of organic matter
51 Chapter 2
52 Sources, transport, and partitioning of organic matter
Plant derived triterpenoids such as amyrin, lupeol, taraxerol, their corresponding ketone derivatives, and friedelin were present in even lower concentrations than the plant wax components (LCalk, LCOH, LCFA), whereas bacteriohopanetetrol (BHT) and anhydrobacteriohopanetetrol (anhydro BHT) varied strongly in a concentration range from
2.1 to 17 g/g dw.
Figure 2.3. Gas chromatograms (FID) of lipid biomarker fractions and CuO oxidation products from the mudbelt sample GeoB 11002. a) hydrocarbon, b) ester and ketone fraction, c) alcohol and sterol fraction, d) fatty acid fraction, and f) lignin fraction. 53 Chapter 2
2.4.2.3 Terrestrial biomarker input
Within the group of plant wax components, LCFA diverged slightly in the southern parts of the inner and mid shelf from the distribution of LCalk and LCOH, i.e., showing a relatively balanced distribution (Fig. 2.4, Table 2.3), whereas LCalk and LCOH were primarily accumulated at stations GeoB 11039 and 11028. Plant wax constituents were increased at the continental slope with contents of up to 15 g/g dw. At the northern outer shelf, LCOH showed high values comparable to the nearby mid shelf area. Long chain n alkanes with a chain length of 36 to 40 carbon atoms (n C36 40) but no carbon number predominance (e.g.,
Fig. 2.3a) were observed in all samples. Their spatial distribution diverged only slightly from the plant wax pattern and thus these compounds may be derived from the same sources and/or transported by similar mechanisms. p hydroxyl phenol concentrations ranged between 2.4 and 37 g/g dw with the highest amounts found at the northern shelf and were significantly correlated with vanillyl and syringyl phenols (r = 0.831), suggesting an origin of the p hydroxyl phenols from lignin.
Possible marine precursors of p hydroxyl phenols such as proteins, amino sugars, and amino acids (Goñi and Hedges, 1995) were removed from the sediment samples by the lipid biomarker extraction procedure prior to the CuO oxidation treatment.
Figure 2.4. Distribution of terrestrial organic matter on the NW Iberian margin indicated by a) Lignin phenols, and b) plant wax components (n C25 35, LCOH and LCFA). Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, 100 m).
The sum of all lignin phenols reached the highest concentration in the southernmost sample
GeoB 11039 (190 g/g dw), followed by station GeoB 11025 (150 g/g dw) at the northern shelf and samples from the southern mudbelt with concentrations between 110 and 140 g/g
54 Sources, transport, and partitioning of organic matter
(GeoB 11042, 11028 and 11029). The individual phenol groups of vanillyl, syringyl, p hydroxyl and cinnamyl phenols generally confirmed the partitioning of total lignin phenols, except for the vanillyl phenols at station GeoB 11025. The relative contributions of single lignin phenol groups reflect different types of plant tissue (Table 4.2; Hedges and Mann,
1979). The ratio of syringyl to vanillyl phenols (S/V) and cinnamyl to vanillyl phenols (C/V) reached rather low values of 0.31 to 0.81 and 0.02 to 0.35 in the sediments of the southern to central inner and mid shelf (Table 2.4). Both ratios displayed a general trend to higher values at the outer shelf and in the northern part of the inner and mid shelf area, with a particular enrichment in the central continental slope sample GeoB 11033. The ratio of 3,5 Bd/V, a lignin based indicator for soil input (Prahl et al., 1994; Gordon and Goñi, 2003), yielded low values in the mudbelt and increased at the outer shelf (GeoB 11020, 11022, 11032, 11007) and the northern shelf region (e.g., GeoB 11025, 11016; Fig. 2.5a). By contrast, the BIT index, proposed as relative marker for soil input (Hopmans et al., 2004), showed only small variations at the Galicia Minho shelf ranging between 0.04 in the northern outcrop and mudbelt area and 0.10 in the central and southern mudbelt (Fig. 2.5b, Table 2.4).
Figure 2.5. Distribution of soil derived OM on the NW Iberian margin indicated by a) 3,5 Bd/V ratio, b) BIT index, and c) (Ad/Al)V ratio. Color code below refers to typically observed values and was adapted from a) Houel et al. (2006), b) Hopmans et al. (2004), and c) Goñi et al. (1993). Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, 100 m).
Plant derived triterpenoids varied strongly in the inner shelf and mudbelt samples (0.54 to
6.6 g/g dw) with enhanced concentrations at station GeoB 11039 and 11037 in the south and at station GeoB 11010 in the central mudbelt. Friedelin was less abundant and showed an
55 Chapter 2 accumulation at the mudbelt stations GeoB 11039 and 11003. Generally, these terrestrial biomarkers showed a decrease at the outer shelf and the continental slope.
2.4.2.4 Marine biomarker input
Fig. 2.6 shows the distribution of lipid biomarkers from marine algae. Dinosterol is widely distributed in the sediments of the mudbelt and inner/mid shelf area with the highest concentration of 15 g/g dw at the southern inner shelf station GeoB 11037. Similar distributional patterns were observed for other algal markers such as the alkyl diols, the sterol group, and 24 methylenecholestadienol. Marine biomarkers decreased offshore with the exception of the central outer shelf station GeoB 11032. Highly branched isoprenoids
(HBIs) showed the highest concentration of up to 1.4 g/g dw in the northern parts of the shelf (GeoB 11016), regardless of grain size distribution or TOC content in the sediment (Fig.
2.6a). Areas of elevated accumulation of alkenones and the C37:2 alkene appeared to be randomly spread over the whole shelf and exhibited the highest concentrations of 1.7 g/g dw at the central mudbelt station GeoB 11003.
Figure 2.6. Marine organic matter production at the NW Iberian margin indicated by a) HBIs (diatoms), b) dinosterol (dinoflagellates) and c) C37 alkenones (coccolithophorides). Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, 100 m).
SCOH were rather accumulated in the central mudbelt (1.2 to 2.4 g/g dw) and showed only low values in samples close to outcrops. Polyunsaturated fatty acids (PUFA) and monounsaturated short chain fatty acids (MUFA) showed in general elevated values in the inner/mid shelf area and decreased offshore. Remarkably high concentrations of bacterial biomarkers (up to 33 g/g dw) were found for the branched fatty acids (BrFA including i /ai
56 Sources, transport, and partitioning of organic matter
C15 and C17 FAs and 10 methyl C16 FA), C16:0 MAGE, and C18:1 7 FA at the southern mudbelt station GeoB 11042 and the northern inner shelf station GeoB 11011. In contrast, BHPs as another indicator of bacterial input showed very high concentrations (up to 38 g/g dw) at other locations such as the inner/mid shelf stations GeoB 11016 and 11030 and the central outer shelf at station GeoB 11008 (Table 2.3). Diploptene and other hopanoids are elevated in several mudbelt samples (GeoB11012, 11028, and 11039) and decline at the outer shelf and continental slope.
The general distribution of terrestrial OM vs. aquatic OM is shown in the ratio of
SCFA/LCFA (Table 2.4); the highest values of 15 to 20 were observed at the southern shelf at stations GeoB 11041, 11042, and 11043. In the central outer shelf SCFA were elevated, whereas the northern offshore regions were rather dominated by LCFA.
2.4.3 Inventory of lipid biomarkers and lignin phenols in the local rivers
River bank sediments from three local rivers (Minho, Cavado, and Ulla River) and sediment samples from the tidal flat of the Ría de Vigo (RdV) were analyzed as a reference for river and land derived OM input. TOC ranged between 0.24% in the Minho River (MR2) and
4.16% in the RdV sample (Table 2.1). TOC/TN and 13CTOC reflected terrestrial values of 9.8 to
17.3 and 27.1 to 25.4‰, respectively. The most abundant lipid biomarkers in the river bank sediments were saturated SCFA with highest concentrations of 57 and 76 g/g dw in MR1 and the Cavado River (CR), respectively. Sterols comprised the second most abundant compound group and were dominated by sitosterol. Dinosterol was present in all three river samples in concentrations up to 14 g/g dw, indicating a migration of dinoflagellates into the Galician rivers, probably in conjunction with red tides (e.g., Figueiras and Ríos,
1993). HBIs and several other algal biomarkers such as alkenones and C37:2 alkenes were either observed at low concentrations or below the detection limit.
The BIT index was in the range of 0.72 at MR1 and 0.96 at the Ulla River (UR) indicating a strong impact of soil OM (Table 2.4). Lignin concentrations in the rivers of Galicia varied from 16 g/g dw at station MR2 to 110 g/g dw at the CR. An even higher amount of up to
420 g/g dw was obtained from the tidal flat sediments of the RdV. 3,5 Bd varied strongly between the samples from 1.8 g/g dw (MR2) to 13 g/g dw (UR). Consequently, the 3,5
Bd/V ratio was as low as 0.25 and therefore below typical shelf values (Table 2.4).
Degradation products of BHPs were only present in minor amounts, with C32 hopanoic
57 Chapter 2
acid (C32 HA) reaching the highest concentrations of 1.7 g/g dw at station MR2, whereas total BHPs varied strongly between the individual sampling sites from 1.0 g/g dw at station
MR1 to 32 g/g dw in the RdV.
2.5 DISCUSSION
2.5.1 Sources of organic matter at the Galicia Minho shelf
2.5.1.1 The composition of terrestrial organic matter on the shelf
The broad range of values in TOC/TN (4.5 to 15.6) and 13CTOC ( 21.0 to 25.1‰) in sediments of the Galicia Minho shelf is consistent with various degrees of OM mixing from continental and marine sources (see also Schmidt et al., 2009). Elevated TOC/TN values between 8.3 and
15.6 and depleted 13CTOC values in the central mudbelt and the southern inner shelf may indicate substantial contributions of OM from terrestrial sources (Hedges et al., 1986) delivered by the Douro and Minho Rivers. A predominant terrestrial OM signal would show
13CTOC values in the range of 33 to 25‰ (Peters et al., 1978; Middelburg and Nieuwenhuize,
1998).
Particularly in the mudbelt, vascular plant material plays an important role as suggested by the high lignin content. S/V values of 0.32 to 1.87 (Table 2.4) reflect low contributions from gymnosperms and a predominance of angiosperm material (Goñi and Hedges, 1992; see also
Fig. 2.7). This is supported by a range of plant derived triterpenoids in the sediment that reflect the vegetation of the hinterland. The catchment area of the Minho and Douro Rivers is predominated by angiosperm vegetation with oak trees and garigue in semi arid central
Spain and deciduous and mixed woodland as well as farmland in the humid north.
However, due to recent massive dam constructions in the river course, the plant OM in the shelf sediments is probably mainly derived from the coastal region. The dominating angiosperm vegetation is also mirrored in the plant wax components. The maximum in the
LCalk distribution at n C31, closely followed by n C29, displayed in the average chain length
(ACL) of n alkanes in Table 2.4, and the maximum of LCOH at n C26 is indicative for a mixture of angiosperm grasses and trees (Collister et al., 1994a; Rommerskirchen et al., 2006).
Due to the rich, tree dominated vegetation at the river sampling sites, the maximum for
LCalk is shifted to n C29, a typical indication for angiosperm trees. This trend is evident in
C/V values below 0.22 (Table 2.4 and Fig. 2.7) for woody material (Goñi and Hedges, 1992).
58 Sources, transport, and partitioning of organic matter
Table 2.4. Vegetation and soil ratios in all analyzed surface sediments from the NW Iberian margin and source rivers. ACL – n alkane average chain length which is the weighted average number of C atoms in the
range from C25 to C33 (Poynter and Eglinton, 1990).
Sample (GeoB) Location ACL SCFA/LCFA C/V S/V 3,5-Bd/V BIT 11001 Mudbelt 29.8 8.98 0.11 0.31 0.01 0.09 11002 Mudbelt 29.7 6.24 0.06 0.44 0.08 0.09 11003 Mudbelt 29.9 2.09 0.07 0.80 0.46 0.06 11010 Mudbelt 30.0 4.93 0.30 0.81 0.07 0.05 11012 Mudbelt 29.8 4.93 0.06 0.55 0.22 0.08 11017 Mudbelt 29.9 6.10 0.30 0.62 0.08 0.05 11028 Mudbelt 29.9 6.23 0.12 0.42 0.03 0.08 11029 Mudbelt 29.8 3.70 0.27 0.80 0.05 0.10 11039 Mudbelt 29.7 4.13 0.19 0.65 0.04 0.08 11042 Mudbelt 29.6 19.20 0.30 0.44 0.04 0.08 11011 Mid-shelf 29.6 10.63 0.47 0.93 0.87 0.04 11013 Mid-shelf 29.9 5.39 0.75 0.55 0.05 0.05 11016 Mid-shelf 30.0 6.25 0.32 0.68 0.67 0.04 11018 Mid-shelf 29.9 5.54 0.31 0.61 0.45 0.06 11025 Mid-shelf 30.0 10.71 0.64 1.87 1.60 0.09 11030 inner shelf 29.7 9.51 0.10 0.44 0.02 0.07 11037 Inner shelf 29.8 11.27 0.02 0.50 0.22 0.06 11038 Inner shelf 29.6 7.59 0.26 0.44 0.33 0.09 11040 Mid-shelf 29.7 7.49 0.08 1.03 0.26 0.05 11041 Inner shelf 29.3 15.81 0.35 0.61 0.07 0.08 11043 Inner shelf 29.5 15.02 0.12 0.50 0.14 0.07 11044 Mid-shelf 29.9 4.07 0.27 0.39 0.32 0.05 11004 Outer shelf 29.8 3.77 0.33 0.69 0.17 0.07 11005 Outer shelf 29.8 3.37 0.21 0.54 0.23 0.05 11006 Outer shelf 30.4 5.19 0.42 0.85 0.13 0.05 11007 Outer shelf 30.2 4.36 0.31 0.89 0.91 0.04 11008 Outer shelf 30.1 6.16 0.46 0.73 0.43 0.04 11009 Outer shelf 29.9 4.72 0.39 0.93 0.63 0.04 11014 Outer shelf 30.2 1.82 0.06 0.60 0.38 0.07 11015 Outer shelf 29.8 8.97 0.09 0.64 0.57 0.08 11019 Outer shelf 30.1 2.78 0.30 0.76 0.47 0.06 11020 Outer shelf 30.0 8.12 0.27 0.84 1.84 0.06 11022 Outer shelf 30.0 6.27 0.31 0.79 1.40 0.07 11027 Outer shelf 29.9 6.87 0.51 0.76 0.32 0.05 11031 Outer shelf 29.9 8.17 0.28 0.71 0.57 0.06 11032 Outer shelf 29.9 15.10 0.21 1.53 1.20 0.06 11021 Cont. slope 30.2 3.39 0.44 1.00 0.43 0.06 11023 Cont. slope 29.9 2.70 0.19 0.75 0.18 0.09 11024 Cont. slope 29.8 1.48 0.33 0.78 0.05 0.05 11033 Cont. slope 30.2 5.60 0.77 1.74 0.25 0.05 MR1 River 28.8 2.97 0.15 0.32 0.19 0.72 MR2 River 28.7 16.26 0.22 0.93 0.25 0.86 CR River 29.4 3.58 0.04 0.45 0.10 0.82 UR River 29.2 1.94 0.01 0.49 0.23 0.96 RdV Tidal flat 30.0 0.82 0.07 0.46 0.05 0.82
59 Chapter 2
In the shelf sediments, plant waxes account for one quarter of the plant wax concentrations detected in the riverine sediments. LCOH were always most abundant, closely followed by
LCFA and LCalk, indicating rather fresh OM at the shelf (Cranwell, 1981). Moreover, the carbon preference index (CPI) for n alkanes (Table 2.5) showed relatively high values that are in the range of fresh plant tissue (4.3 to 40.3; Collister et al., 1994b).
Figure 2.7. C/V ratio vs. S/V ratio for all marine (Sample names: GeoB 110..) and riverine samples (MR1, MR2, UR, CR, RdV)analyzed in this study. Squares indicate typical ranges for woody and non woody tissue of angio and gymnosperms, respectively. Symbols refer to sampling location: black circles – mudbelt, grey circles – inner and mid shelf, yellow squares – outer shelf, red rhomb – continental slope, green cross rivers.
The soil derived fraction of OM is identified by the BIT index and the 3,5 Bd/V ratio in our data set. Based on BIT, the contribution of soil OM to the sediment OM off Iberia appeared to be small. High BIT indices between 0.72 and 0.96 testified to soil OM in riverbank sediments but BIT values in shelf sediments were low and ranged from 0.04 to 0.10. These low BIT values are not unusual for shelf sediments as shown in previous studies (Hopmans et al.,
2004; Herfort et al., 2006; Kim et al., 2006). However, they are in stark contrast to the soil index 3,5 Bd/V (Fig. 2.5). 3,5 Bd/V appeared to be a more sensitive indicator of soil OM deposition at the NW Iberian margin and yielded values of 0.01 to 1.84 with generally higher values found offshore. These values are comparable to those reported in studies of other shelf sediments (e.g., Prahl et al., 1994; Goñi et al., 2000). The disparity of the two soil indices suggests that they are associated with different soil fractions. 3,5 Bd was detected in high
60 Sources, transport, and partitioning of organic matter concentrations in wood and tannic acids (Dickens et al., 2007) and was particularly enriched in the mineral horizon of soils (Houel et al., 2006), whereas non isoprenoidal GDGTs used for calculation of the BIT index are presumed to be produced by anaerobic soil bacteria
(Weijers et al., 2006). Generally, the application of the BIT index is critical in regions with a high primary productivity and consequently high autochthonous crenarchaeol input, which will lower the BIT values and thus mask the allochthonous signal. Variations in primary productivity strongly affect the BIT index and may misleadingly appear to indicate changes in soil input and river run off. Inspection of the raw data suggests that the variation of the
BIT index is largely governed by the spatial variation of crenarchaeol rather than the branched tetraethers. 3,5 Bd/V represents a more robust, absolute measure of the soil input and is therefore more suitable for detecting soil OM in the studied system.
2.5.1.2 Marine contributions to sedimentary organic matter
Particularly in the northern shelf areas and offshore, elevated 13CTOC values and TOC/TN ratios in the range of the Redfield ratio of ~6.7 (Redfield, 1958) suggested a predominance of marine algal OM (Fig. 2.2b, c). The high abundance of sterols and SCFA indicated the accumulation of relatively fresh algal OM, probably as consequence of high marine productivity in areas of increased upwelling. Low stanol to stenol ratios ( 0/ 5 < 0.6, Table
2.5) in all areas reflected fresh unaltered material (Nishimura, 1982; Wakeham, 1989). The distributional pattern of sterols in river and marine sediments suggested that sterol contributions from higher land plants to the Galician shelf are only of minor importance.
Land plant derived sitosterol dominated by far in the river sediments, whereas the shelf sediments showed the following sterol abundance: cholesterol > sitosterol (29 5) > brassicasterol (28 5) > dinosterol > cholesta 5,22 dienol (27 5,22) 24 methylenecholestadienol
(28 5,24(28)) stigmasterol (29 5,22). In addition, sitosterol significantly correlated with the other algal sterols (r > 0.77) and algal compounds (e.g., alkyl diols, 12 OH C28 FA), suggesting diatoms or haptophyceae as main sources (see Volkman, 2003, and references therein).
More specific markers are HBIs which are produced only by a restricted range of diatom species (Table 2.2). Other characteristic biomarkers such as dinosterol indicate important contributions from dinoflagellates, whereas alkenones and C37:2 alkene originate from specific haptophyceae algae such as Emiliania huxleyi.
61 Chapter 2
Table 2.5. Biomarker degradation ratios at the NW Iberian margin and source rivers. CPI – carbon preference index for n alkanes calculated over the range C24 to C34 (Eglinton and Hamilton, 1963); HPA – higher plant alkanes index based on the concentrations of C24,26,28 n alcohols to C27,29,31 n alkanes (Poynter and Eglinton, 1990). P/(V+S) indicates subaquatic degradation (Dittmar and Lara, 2001).
0 5 Sample (GeoB) Location CPI HPA / Alkene/ Alkenone (Ad/Al)V P/(S+V) 11001 Mudbelt 4.21 0.54 0.20 0.88 0.47 0.29 11002 Mudbelt 3.80 0.61 0.16 1.20 0.08 0.52 11003 Mudbelt 5.62 0.64 0.41 0.09 0.31 0.58 11010 Mudbelt 4.17 0.65 0.43 0.73 0.39 0.31 11012 Mudbelt 3.57 0.65 0.21 0.45 0.41 0.91 11017 Mudbelt 4.05 0.71 0.44 0.92 0.40 0.41 11028 Mudbelt 6.00 0.61 0.33 0.70 0.29 0.39 11029 Mudbelt 5.43 0.63 0.33 1.20 0.40 0.21 11039 Mudbelt 6.11 0.62 0.23 0.75 0.34 0.20 11042 Mudbelt 4.26 0.66 0.10 0.75 0.78 0.35 11011 Mid-shelf 2.69 0.72 0.27 0.48 11.56 1.72 11013 Mid-shelf 3.53 0.69 0.21 0.54 2.11 0.81 11016 Mid-shelf 5.08 0.62 0.34 0.80 0.98 1.30 11018 Mid-shelf 5.12 0.54 0.36 0.72 1.26 0.72 11025 Mid-shelf 6.07 0.74 0.33 0.37 1.81 0.39 11030 inner shelf 4.15 0.70 0.25 0.56 0.48 0.47 11037 Inner shelf 4.08 0.60 0.55 0.91 0.09 1.05 11038 Inner shelf 4.23 0.72 0.30 0.61 0.43 0.85 11040 Mid-shelf 4.30 0.73 0.47 1.74 0.55 1.01 11041 Inner shelf 3.41 0.63 0.15 0.60 0.88 0.60 11043 Inner shelf 4.37 0.70 0.13 0.58 0.14 0.87 11044 Mid-shelf 5.82 0.66 0.40 0.40 0.67 0.85 11004 Outer shelf 4.21 0.70 0.16 0.51 0.47 0.29 11005 Outer shelf 4.12 0.64 0.19 0.57 0.92 0.54 11006 Outer shelf 4.40 0.13 0.38 0.57 0.62 0.53 11007 Outer shelf 4.98 0.73 0.43 0.46 5.73 0.65 11008 Outer shelf 6.89 0.69 0.38 0.52 2.92 0.60 11009 Outer shelf 4.98 0.68 0.31 0.54 14.88 1.33 11014 Outer shelf 4.42 0.76 0.35 0.90 0.24 0.53 11015 Outer shelf 3.38 0.56 0.29 0.59 0.44 1.11 11019 Outer shelf 5.37 0.72 0.45 0.79 1.15 0.66 11020 Outer shelf 4.85 0.61 0.87 0.72 10.83 0.88 11022 Outer shelf 3.47 0.76 0.25 0.37 4.86 0.83 11027 Outer shelf 3.28 0.66 0.45 0.97 1.44 0.96 11031 Outer shelf 4.01 0.69 0.51 0.35 0.92 1.14 11032 Outer shelf 3.61 0.79 0.41 1.05 0.76 0.95 11021 Cont. slope 5.40 0.59 0.34 1.29 1.41 0.74 11023 Cont. slope 4.38 0.67 0.25 0.38 0.47 0.35 11024 Cont. slope 3.51 0.65 0.12 0.52 1.56 1.04 11033 Cont. slope 1.80 0.64 0.22 0.73 0.93 0.73 MR1 River 6.86 0.60 0.26 - 0.53 0.19 MR2 River 5.36 0.63 0.19 - 0.66 0.42 CR River 2.82 0.43 0.27 - 0.03 0.23 UR River 9.61 0.80 0.43 - 0.11 0.45 RdV Tidal flat 5.14 0.70 0.31 - 0.74 0.41
62 Sources, transport, and partitioning of organic matter
These three algal groups were previously reported as main primary producers on the
Galicia Minho shelf (e.g., Estrada, 1984; Bode et al., 2002). A more recent study found evidence that cyanobacteria account for 55 to 65% of the primary production at the NW
Iberian shelf (Lorenzo et al., 2005). However, the low concentrations of hopanes and the lack of specific cyanobacterial markers such as mid chain methylheptadecanes (Shiea et al., 1990;
Kenig et al., 1995) in this study suggest that cyanobacteria are not major contributors to the sedimentary OM.
The organisms producing the abundant 1,15 C30:1 , 1,14 C30, and 1,15 C30 alkyl diols and the
12 OH C28 FA are not fully constrained. C32 alkyl diols with a predominance of the 1,15 C32:0 and the 1,15 C32:1 isomer were detected in eustigmatophyceae species (Volkman et al., 1992), and diatoms of the genus Proboscia abundantly produce saturated and monounsaturated C28
1,14 diols and 12 OH C28 FA (Rampen et al., 2007). However, the sources for C30 diols, which usually are the most abundant diols in marine sediments remain unclear (see also review by
Versteegh et al., 1997). In late Pleistocene sediments underlying highly productive surface waters in the Southeast Atlantic, the abundance of alkyl diols was strongly correlated with other major algal biomarkers and TOC, suggesting that the producing algae were major contributors to paleoproductivity (Hinrichs et al., 1999). At the Galicia Minho shelf, the distributional pattern of alkyl diols and 12 OH C28 FA resembled that of sterols but differs from HBIs (Tables 2.3, Figs. 2.6a and b), suggesting that the HBIs and alkyl diols are produced by different groups of organisms.
Low TOC/TN values in the range of 5.5 to 7.7 as found at the northern outcrop station GeoB
11011 could indicate elevated contributions of bacterial biomass to the sedimentary OM pool
(Fukuda et al., 1998). However, bacterial biomarkers were not correlated with TOC/TN and the origin of several of the bacterial biomarkers, i.e., in situ produced biomass vs. input from the water column or from terrestrial sources, were hard to assign. For example, diploptene correlated with marine derived HBIs and the C37:2 alkene, which could indicate a dominant marine origin (Venkatesan, 1988; Elvert et al., 2001). On the other hand, it was also significantly correlated with LCalk and was present in the river samples, which would point to a terrestrial source, e.g., from soil OM (Prahl, 1985; Prahl and Hayes, 1992). Likewise, the exact sources of BHPs in the shelf sediments could not be unambiguously identified.
Amongst others, BHT was identified in several environments, such as soils, marine sediments and cyanobacterial mats (see Talbot et al., 2008 and references therein). It
63 Chapter 2 correlated significantly with anhydro BHT, a possible degradation product (Schaeffer et al.,
2008), but showed a distinct distributional pattern from all other biomarkers. Furthermore, both compounds showed no correlation with degradation products of BHPs, i.e., hopanes, hopanols and hopanoic acids (Table 2.2).
2.5.2 Transport and distributional patterns of terrestrial organic matter
The major fraction of the terrestrial OM is transported to the shelf in winter and spring during periods of maximum rainfall in the Iberian hinterland and occasional river floods
(Dias et al., 2002b). The terrestrial matter is primarily deposited close to the source rivers and redistributed during dynamic winter conditions (Vitorino et al., 2002). High shear wave velocities in winter, particularly during storm events, induce sediment remobilization, northward transport with the bottom currents, and probably also export of a sediment fraction across the shelf break (Dias et al., 2002b; Vitorino et al., 2002). These processes influence the lateral distribution of OM (e.g., Figs. 2.4 and 2.7). Apparently, plant debris was hydrodynamic sorted and deposited in different areas of the shelf. High C/V ratios at the outer shelf and continental slope indicated an accumulation of grass and leaf material, i.e., soft plant tissues. Woody material with low C/V ratios was deposited closer to the continent and the source rivers in the mudbelt sediments, thus showing a higher total lignin phenol concentration, whereas the soft plant tissue was effectively transported offshore, probably due to its hydrodynamic properties. Plant derived triterpenoids resembled a partitioning similar to the woody lignin fraction (Table 2.3) due to an enrichment of resins in wood (e.g.,
Bull et al., 2000). Due to the high abundance of LCalk, LCOH and LCFA in epicuticular plant waxes (Eglinton and Hamilton, 1967), which are primarily associated with soft tissues, one would expect a spatial distribution of plant wax components comparable to C/V. However, the high concentrations of plant wax components indicated a general abundance of plant waxes in sediments off NW Iberia (Fig. 2.4b). Due to their position on the plant surface, wax components are more susceptible to weathering than lignins which are part of the cell walls.
Therefore, beside remobilization by water leaching, run off, and riverine transport, plant waxes are also transported via the atmosphere (e.g., Schefuss et al., 2003; Rommerskirchen et al., 2006). The frequent wild fires in Galicia and northern Portugal during the hot and dry summer months could promote a high abundance of plant wax components in the aerosols and the offshore directed winds from north and northeast in summer favor an aeolian input.
64 Sources, transport, and partitioning of organic matter
Soil OM was mainly transported offshore as indicated by elevated 3,5 Bd/V ratios (Fig. 2.6a).
Coinciding with the increase in 3,5 Bd/V is an increase in (Ad/Al)V offshore (Table 2.5), likewise observed in soils (Houel et al., 2006), river estuaries (Louchouarn et al., 1997) and on shelves (Prahl et al., 1994). (Ad/Al)V indicates aerobic degradation of lignin. As a consequence of propyl side chain oxidation in lignin macromolecules by white rot fungi
(Goñi et al., 1993), the acidic CuO oxidation products increase and result in (Ad/Al)V ratios >
0.5. Low (Ad/Al)V values in the sediments from the inner/mid shelf area suggested a deposition of fresh plant tissue, whereas values higher than 5 in the outer shelf and the northern outcrop region indicated highly degraded, pre aged material. This fraction probably derived from degraded plant material that was accumulated in the soils, leached, and transported to the continental margin by water and river run off as part of soil OM.
2.5.3 Spatial OM distribution in relation to seasonal plankton ecology
Particle transport on the Galicia Minho shelf occurs preferentially in the surface nepheloid layer (SNL) and the bottom nepheloid layer (BNL), the former enriched in large organic particles and the latter in small inorganic material (Oliveira et al., 2002). The association of terrestrial OM with the silt fraction, as discussed above, implies a distribution of terrestrial
OM that is controlled mainly by the BNL, whereas the non correlation of marine biomarkers with silt suggests a transport and partitioning that is distinct from the terrestrial OM and occurred most likely in the SNL. However, the distribution of marine biomarkers is not only the result of a transport behavior that is distinct from the terrestrial OM; it also reflects the seasonal and spatial succession of plankton assemblages. The seasonal phytoplankton succession starts in spring with the onset of upwelling resulting in a diatom bloom, followed by a predominance of heterotrophic organisms in summer and a dinoflagellate bloom in autumn, the latter induced by stratified water conditions after the wind relaxation (Figueiras and Ríos, 1993).
The high concentrations of HBIs in the northern parts of the shelf (> 42.25 °N, Fig. 2.6a) suggested an accumulation of diatom derived OM close to the NW Iberian upwelling cell further up in the North of the study area. Diatoms dwell under eutrophic conditions that are caused by the nutrient rich upwelling waters which are dominantly located in the northern parts of the Galicia Minho shelf during the diatom bloom period in spring (e.g., Bode et al.,
2002). The preferential occurrence of HBIs right in front of the Rías Baixas could alternatively
65 Chapter 2 indicate a biomass export out of the Rías (e.g., Tilstone et al., 2000). The primary productivity in the Rías Baixas exceeds the shelf production by far due to their favorable growth conditions, i.e., high nutrient input from land and particularly from the upwelling cell into a semi closed protected environment (Prego, 1993; Alvarez Salgado et al., 1996; Alvarez
Salgado et al., 1999). Nevertheless, such an export of biomass should be visible in the dinosterol distribution, since dinoflagellates have been reported to account for 13 to 30% of the primary productivity in the Ría de Vigo (Lorenzo et al., 2005). The dinosterol concentration maxima are, however, more or less restricted to the inner/mid shelf (Fig. 2.6b), which correspond to the favored habitat of dinoflagellates in autumn. The high dinosterol concentrations at stations GeoB 11010 (northern mudbelt) and GeoB 11020 (outer shelf) could indicate remobilization and redistribution of dinoflagellate derived OM during the dynamic winter months. The former station particularly showed also increased concentrations of most of marine and terrestrial biomarkers as well as TOC which is most likely caused by the proximity to rocky outcrops that act as sediment traps (Fig. 2.1).
Haptophytes do not successfully compete with other phytoplankton groups such as diatoms or dinoflagellates and therefore are more adapted to oligotrophic waters (Winter et al., 1994).
This explains the fairly high alkenone concentrations farther offshore. In winter, the downwelling front separates two contrasting environments (Castro et al., 1997); the coastal areas exhibit mesotrophic conditions due to the nutrient supply from land and provide an environment for diatoms and larger dinoflagellates (Alvarez Salgado et al., 2003). The surface mixed layer of the PCCC with its oligotrophic conditions advances towards the continent together with the downwelling front, which shifts the habitat of haptophyceae algae and small flagellates to the mid shelf region. The elevated alkenone concentrations in the mid shelf area (Fig. 2.6c) may reflect the winter conditions and the onshore movement of the oligotrophic plankton assemblage during that time.
2.5.4 OM provinces on the Galicia Minho shelf
Representative biomarkers were subjected to a cluster analysis (Fig. 2.8) which revealed a partitioning of the NW Iberian margin into three main regions with distinct OM provenances
(Fig. 2.9): the inner shelf, the mud dominated, and the offshore region. The inner shelf region
(<130 m water depth) is characterized by a high OM input from marine sources as indicated by the highest values of SCFA/LCFA. The primary productivity in this region is fuelled by
66 Sources, transport, and partitioning of organic matter nutrient supply from the continent and upwelling, resulting in high concentrations of algal biomarkers such as sterols, alkyl diols, and SCFA. The comparably high abundance of PUFA and MUFA signals fresh OM. Lignin concentrations vary strongly in the inner shelf region, consistent with by passing of terrestrial derived OM to the mudbelt region and eventually farther offshore.
Figure 2.8. Cluster analysis of selected parameters and biomarkers for all sediment samples (Sample names: GeoB 110..) from the NW Iberian margin (variables: water depth, TOC, silt, TOC/TN, 13CTOC, BIT, n C25 35, n C36 40, HBIs, alkenones, SCOH, LCOH, sterols, dinosterol, 24 methylenecholesterol, diols, higher plant derived triterpenoids, friedelin, LCFA, sat. SCFA, MUFA, PUFA, C18 FA, C18:1 9, C18:1 7, BrFA, C16:0 MAGE, diploptene, 3,5 Bd, V, S, C, P).
The terrestrial mud dominated region extends across the mid shelf area from 41.8° to 42.8°
N, comprising most of the mudbelt. Here, concentrations of most terrestrial lipid biomarkers and lignin are particularly high. Compared to the marine dominated inner shelf region, the absolute concentrations of marine biomarkers decrease only slightly (e.g., dinosterol, alkyl diols and 12 OH C28 FA) or are invariable (e.g., alkenones, phytol, sterols, SCOH, HBIs).
High accumulation of terrestrial OM in the mid shelf region generally dilutes the regular background sedimentation. Terrestrial OM from the Minho and Douro Rivers is redistributed and accumulated with the sedimentary material in this region, yielding the highest silt and TOC contents. The southernmost branch of this region (station GeoB 11039) is very likely directly fed by the Douro River including sediment remobilization from the
67 Chapter 2
Douro mud patch located further in the south (Araújo et al., 1994; Drago et al., 1998). The extension of the terrestrial mud dominated region to the outer shelf in the North (GeoB
11020, 11022 and 11025) is mainly attributed to the accumulation of lignin and soil OM in these sediments. The transport of terrestrial OM not only northward but also offshore could indicate an export pathway for particulate matter in the northern shelf area as suggested by
Dias et al. (2002). A second export pathway runs possibly across the outer shelf, in the north of the rocky outcrops bordering the southern shelf break (Fig. 2.9) as implied by a biomarker pattern at station GeoB 11032 that is similar to the marine dominated inner shelf region.
Figure 2.9. Organic matter provinces at the NW Iberian margin identified by biomarkers and bulk parameters from the cluster analysis (Fig. 2.8). Blue regions: marine dominated inner shelf; green region: terrestrial mud dominated mid shelfbelt; red regions: outer shelf and continental slope, yellow regions: starved outer shelf and continental slope. Pie plots show main OM fractions indicated by selected biomarkers. Bacteria include BrFA, C18:2 FA, C18:1 7, UFA – unsaturated fatty acids, i.e., MUFA and PUFA, MPP – main primary producers, i.e., dinosterol, alkenones and HBIs.
Sediment remobilization, resuspension, and ultimately deposition are the main controlling factors in the terrestrial mud dominated mid shelf region. Consequently, increased degradation due to prolonged exposure to oxygen is expected. However, the fairly high
68 Sources, transport, and partitioning of organic matter values of the higher plant alkane index (HPA, Table 2.5) in the inner/mid shelf areas and the low abundance of triterpenones in the sediment (less than 10% of the total plant derived triterpenoids), suggest low oxidative degradation in the surficial sediments. Moreover, low
P/(S+V) values in the terrestrial mud dominated region also indicate rather fresh lignin relative to the other OM provinces further corroborating the finding of reduced in situ degradation in this region.
The offshore region finally comprises the outer shelf and the continental slope and is dominated by low sedimentation, resulting in a low silt and TOC content of the sediment.
The oligotrophic conditions in the PCCC result in a low marine productivity, which is reflected in the low concentrations of sterols and fatty acids. Terrestrial biomarker concentrations in this region were lower on an absolute level but elevated relative to marine biomarkers, a trend that is especially pronounced in the starved offshore regions (Stations
GeoB 11006, 11023 and 11024; Fig. 2.9). This observation can be explained by the lower productivity in combination with the higher water depth that enhances remineralization of marine OM, while pre aged and chemically more resistant terrestrial OM is more likely to be preserved.
2.6 CONCLUSIONS
The multi biomarker approach applied in this study enabled the identification of the major sources in the heterogeneous sedimentary OM pool at the NW Iberian margin. Accordingly, the continental shelf was classified into three distinct OM regions:
The inner shelf region is predominated by marine OM as indicated by the high
abundance of marine lipid biomarkers in sediments with intermediate contents of silt
and TOC as well as by TOC/TN and 13CTOC values consistent with a predominant
OM input by marine algae.
The terrestrial dominated mid shelf region is the main depositional area on the shelf
and revealed the highest contents of silt and TOC and the lowest 13CTOC values,
corresponding to the Galicia Minho mudbelt (Dias et al., 2002b and references
therein). The sediments comprise high concentrations of marine and terrestrial lipid
biomarkers, and strikingly high lignin phenol contributions at a low degradation
state.
69 Chapter 2
The offshore region, i.e., the outer shelf and the continental slope receives only little
sediment and OM input, resulting in characteristically low concentrations of silt,
TOC, and lipid biomarkers. Furthermore, lipid biomarkers provide evidence that
starved regions are indicated by an advanced degradation state of marine and
terrestrial OM and a relative enrichment of terrestrial lipid biomarkers. This
predominance of terrestrial components is, however, not reflected in TOC/TN and
13CTOC.
The distribution of OM is controlled by a complex and seasonally varying interplay of ocean currents, primary productivity, atmospheric conditions, and river run off. Controls differ between terrestrial and marine OM pools with hydrodynamic sorting governing the partitioning of terrestrial OM. The transport and deposition of soft tissue is favored offshore, whereas woody material is accumulated in the mid shelf mudbelt after shorter transport times. Soil OM behaves in a similar manner to the soft tissue and is preferentially accumulated in the northern parts of the study area and on the outer shelf. On the contrary, the distribution of marine OM reflects the seasonal and spatial succession of plankton assemblages and is directly controlled by the oceanographic conditions.
Acknowledgements
We thank the crew and shipboard scientific party of RV Poseidon cruise P342 GALIOMAR, particularly the chief scientist Till Hanebuth. We thank Brit Kockisch for assistance with sample collection and for TOC measurements. We are grateful to Gesine Mollenhauer for providing access to her HPLC MS system for BIT analysis and to Jan Berend Stuut for assistance in grain size analysis. We thank Hella Buschoff for TN and TC analysis, Monika
Segl for 13CTOC measurements, Xavier Prieto and Kevin Becker for support in the lab, and
Daniel Birgel and Arne Leider for fruitful discussions and comments on data and the manuscript. Funding was provided by the “Deutsche Forschungsgemeinschaft” through
DFG Research Center / Excellence Cluster “The Ocean in the Earth System” and the Bremen
International Graduate School for Marine Sciences (GLOMAR).
70 Sources, transport, and partitioning of organic matter
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78
Chapter 3
Molecular characterization of dissolved organic matter in pore water of
continental shelf sediments
Frauke Schmidta, Marcus Elverta, Boris P. Kochb, Matthias Wittc, Kai Uwe Hinrichsa
Published in Geochimica et Cosmochimica Acta
vol 73, issue 11, page 3337 3358 doi:10.1016/j.gca.2009.03.008
© 2009 Elsevier Ltd.
Appendix from publication has been moved to the electronic appendix
aMARUM – Center for Marine Environmental Sciences, Leobener Straße, D 28359 Bremen,
Germany bAlfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D 27570
Bremerhaven, Germany cBruker Daltonik GmbH, Fahrenheitstraße 4, 28359 Bremen, Germany
Chapter 3
ABSTRACT
Dissolved organic matter (DOM) in sediment pore water is a complex molecular mixture reflecting various sources and biogeochemical processes. In order to constrain those sources and processes, molecular variations of pore water DOM in surface sediments from the NW
Iberian shelf were analyzed by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) and compared to river and marine water column
DOM. Weighted average molecular element ratios of oxygen to carbon ((O/C)wa) and hydrogen to carbon ((H/C)wa) provided general information about DOM sources. DOM in local rivers was more oxygenated ((O/C)wa 0.52) and contained less hydrogen ((H/C)wa 1.15) than marine pore water DOM (mean (O/C)wa 0.50, mean (H/C)wa 1.26). The relative abundance of specific compound groups, such as highly oxygenated aromatic compounds or nitrogen bearing compounds with low H/C ratios, correspond to a high concentration of lignin phenols (160 g/g sediment dry weight) and a high TOC/TN ratio (13.3) in the sedimentary organic matter and were therefore assigned to terrestrial sources. The lower degree of unsaturation and a higher relative abundance of nitrogen bearing compounds in the pore water DOM reflected microbial activity within the sediment. One sampling site on the shelf with a high sediment accumulation, and a humic rich river sample showed a wide range of sulfur compounds in the DOM, accompanied by a higher abundance of lipid biomarkers for sulfate reducing bacteria, probably indicating early diagenetic sulfurization of organic matter.
80 Characterization of DOM in sediment pore water
3.1 INTRODUCTION
Continental margins are the dominant reservoir of organic matter (OM) burial in the marine environment. In the modern ocean, approximately 90% of the OM is buried along continental margins (Hedges and Keil, 1995). In particular, continental shelves receive a high input of
OM derived from different sources. 0.21 Gt C in form of dissolved organic matter (DOM) and 0.17 Gt C in form of particulate organic matter (POM) are annually transported by rivers to the ocean (Ludwig et al., 1996). Besides, the high riverine input of inorganic nutrients fuels the marine primary production on continental shelves. High OM production and short sinking times in shallow water depths are the reason that less OM is remineralized on the shelf and the upper slope. Therefore, around 68% of the marine produced organic carbon is buried in these regions (Hedges and Keil, 1995).
The variety of sources and processes such as sediment remobilization and resuspension due to currents and storm induced bottom waves complicate studying OM preservation in continental shelf sediments. Transport and subsequent deposition cause a mixing of old and fresh OM from various sources and enable increased chemical or biological transformation of
OM. Selective adsorption of OM onto mineral surfaces, a process sensitive to changes in chemical conditions (such as salinity and pH, e.g., during the transition river – ocean), can also bias the composition of DOM and POM in the sediment (White et al., 2007; Mead and
Goñi, 2008). Both OM pools share common sources and are closely linked by dynamic processes, e.g. dissolution of POM by microbial exoenzymes contributes to DOM (Burdige and Gardner, 1998), whereas DOM flocculates in the course of polymerization (Amon and
Benner, 1996; Verdugo et al., 2004). Similar stable carbon isotope ratios of pore water DOM and sedimentary POM emphasize the interlinkage of those processes in marine sediments
(Bauer et al., 1995).
DOM in sediment pore waters plays an important role for at least two reasons. On the one hand, it is assumed that pore water DOM is involved in sedimentary preservation of OM
(Hedges et al., 1992; Henrichs, 1992; Hedges and Keil, 1995), and on the other hand, it is an important source of nutrients (e.g. Burdige and Zheng, 1998). The integrated dissolved organic carbon (DOC) flux of 0.19 GT C year 1 from coastal and continental margin sediments
(Burdige et al., 1999) is comparable to the assumed carbon burial rate of 0.16 GT C year 1 of all marine sediments (Hedges and Keil, 1995). These fluxes suggest that refractory pore water DOM plays an important role in the global oceanic DOM cycle on long terms. Since the
81 Chapter 3 nature of the benthic DOM flux (refractory or reactive) is still under debate (Burdige et al.,
2002), the identification of specific molecular compounds that serve as markers for sedimentary pore water DOM and their detection in oceanic DOM could be the key to this problem.
One reason for the gap in the knowledge of OM composition and transformation is the analytical challenge to resolve complex molecular mixtures. Thus, 80% of the OM accumulated in sediments cannot be characterized on the molecular level (Hedges et al.,
2000). The limitations of DOM analysis are attributed to its high complexity, low concentration and high polarity. Hence, DOM is not amenable to analysis by gas chromatography coupled to mass spectrometry (GC MS). Other approaches such as pyrolysis or thermochemolysis are invasive methods and can form new products during analysis. NMR spectroscopy, on the other hand, provides an insight into the functionalities of DOM, but due to the complexity of the sample matrix cannot provide in depth molecular details.
In recent years, the application of Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) has been applied to the molecular characterization of DOM from different natural environments, e.g. from the marine water column (Koch et al., 2005;
Hertkorn et al., 2006; Sleighter and Hatcher, 2008), rivers (Kim et al., 2003), mangrove estuaries (Koch et al., 2005; Tremblay et al., 2007), groundwater (Einsiedl et al., 2007) and soils (Kujawinski et al., 2002a; Kramer et al., 2004; Hockaday et al., 2006). FT ICR MS is capable of resolving complex molecular mixtures and provides information about the exact elemental composition of individual compounds. Although analytical challenges such as chemical selectivity during ionization do not allow quantitation, the analytical method of FT
ICR MS and its data interpretation has been improved over the last years providing a valuable tool for the molecular characterization of low molecular weight (LMW) DOM
(<1000 Da).
Here, we analyzed for the first time small volumes (50 ml) of marine sediment pore water
DOM with FT ICR MS from five distinct locations at the shelf and continental slope of the
NW Iberian margin. The FT ICR MS data were compared to DOM from local rivers which are the major sources of terrestrial material to the shelf (Dias et al., 2002). The associated sedimentary OM was additionally characterized via lipid biomarkers and lignin phenols, which are both routinely used for the identification of OM sources, degradation state and
82 Characterization of DOM in sediment pore water environmental conditions (e.g. Canuel and Martens, 1996; Goñi et al., 2000; Birgel et al.,
2004). By this two pronged approach we specifically aim to identify molecules and compound groups in the sediment pore water DOM that can serve as diagnostic markers for either different OM sources or transformation processes during early diagenesis.
3.2 MATERIALS AND METHODS
3.2.1 Sampling site
All samples derive from the Galicia Minho shelf, the adjacent continental slope and local rivers. The shelf is a storm dominated, highly energetic environment, located at the NW
Iberian margin (Fig. 3.1). The modern oceanography of Galicia was part of several studies
(e.g., Coelho et al., 2002; Oliveira et al., 2002; Vitorino et al., 2002a; Vitorino et al., 2002b;
Alvarez Salgado et al., 2003). In summer, the northward flow of the Eastern North Atlantic
Central Waters (ENACW) is controlled by trade winds and shifted offshore resulting in upwelling of cold nutrient rich water masses (Frouin et al., 1990) and an enhanced primary production from March to October. In winter, SW storms produce downwelling conditions resulting in sediment resuspension, remobilization and eventually sediment deposition on the shelf (Jouanneau et al., 2002; Vitorino et al., 2002b) or sediment transport across the shelf break (Dias et al., 2002). Most of the modern sediment accumulates in the mid shelf mudbelt
(Dias et al., 2002). However, sedimentation rates at the Galicia Minho shelf are difficult to assess due to a sediment mixing layer that comprises the upper 10 cm of sediment at the inner shelf (Jouanneau et al., 2002). The major fraction of sediment at the shelf is supplied by the Douro and Minho River during storms and river floods. Subsequent to the primary sedimentation close to the source, the terrestrial material is remobilized and transported northwards with the predominant currents (Oliveira et al., 2002; Vitorino et al., 2002b).
Due to their importance as sedimentary OM suppliers, Douro and Minho River were sampled to obtain a terrestrial DOM fingerprint. An additional DOM sample was taken from the small Ave River (Fig. 3.1), which was enriched in humic substances, and serves as a reference site for degraded DOM.
3.2.2 Sampling
Sampling was performed with a giant box corer in August 2006 during the GALIOMAR expedition (P342) with the German research vessel RV Poseidon at the Galicia Minho shelf.
83 Chapter 3
50 ml of pore water was extracted with rhizons (Eijkelkamp, pore size 0.1 m) from each surface sediment sample (depth interval of 0 2 cm). Afterwards, sediments, including additional pore waters, of the same depth horizon were sampled for sedimentary OM analysis (lipids, lignin phenols) and bulk measurements. 50 ml of water from three local rivers were collected and filtered through GF/F filter (Whatman, pore size 0.7 m). River sampling sites at the local rivers were located at: Douro River 41°06’45 N, 8°32’18 W; Ave
River 41°21’06 N, 8°40’54 W; Minho River 42°03’06 N, 8°40’54 W. All water samples were stored in pre combusted glass bottles without headspace at +4°C in the dark until further preparation in the home laboratory. Sediment samples were stored in pre combusted brown glass bottles at 20°C to avoid OM degradation.
Figure 3.1. Sampling sites (GeoB, open circles) at the Galicia Minho shelf and associated local rivers (distribution of the mid shelf mudbelt adapted from Dias et al., 2002). Latitude (°N), longitude (°W) and water depth at the sampling sites are listed in Table 3.1.
3.2.3 Dissolved organic matter extraction
Water samples (50 ml) were concentrated by solid phase extraction on SPE cartridges (PPL bond elut, 200 mg sorbent, suitable for highly polar to nonpolar substances, Varian). Prior to extraction, samples were acidified to pH 2 with hydrochloric acid (HCl, p.a. grade, Merck).
After DOM adsorption, the cartridges were rinsed with 6 ml 0.1 M HCl solution in order to
84 Characterization of DOM in sediment pore water remove any salt from the cartridges, which is a prerequisite for subsequent electrospray ionization (ESI) in FT ICR MS analysis. DOM was then eluted with 1 ml methanol
(LiChrosolv, Merck) into pre combusted glass ampoules and stored under nitrogen atmosphere at 18°C in the dark until analysis.
3.2.4 Fourier transform ion cyclotron resonance mass spectrometry
Analyses were performed with an Apex Qe mass spectrometer (Bruker Daltonics Inc.
Billerica, USA) equipped with a 9.4 T superconducting magnet (Bruker Biospin,
Wissembourg, France) and an Apollo II electrospray source. DOM extracts were analysed in a methanol:water solution (50:50 v/v) with ESI in negative ion mode (capillary voltage: +4 kV) at an infusion flow rate of 2 L min 1. Spectra were calibrated with arginine clusters and
400 to 500 scans were added to one spectrum. All ions were singly charged and mass accuracy was below 0.4 ppm. The latter was obtained by internal calibration with compounds, which were repeatedly identified in marine DOM samples (e.g. Koch et al.,
2007). For the calculation of molecular formulas, a mass range of 200 600 m/z was selected for peaks with a signal to noise ratio (S/N) of >3. Formula assignment included the elements
1H0 , 12C0 , 16O0 , 32S0 2, 14N0 10 and 13C0 2. A formula tolerance of ± 0.5 ppm was considered as a match. The data set was restricted to a molecular element ratio of O/C 1.2 and to integer double bond equivalent (DBE) values. DBE defines the number of double bonds and/or rings in a molecule and it is calculated from the number of atoms (Ni) and the valence (Vi) of each element (i) after the following equation:
i max B N i (Vi 2) DBE =1 i (3.1) 2
For the final dataset we focused on ions with a relative abundance > 2% (corresponding to a
S/N ratio > 20 in the spectra with the highest intensities (GeoB 11002 and 11006), the most abundant sample compound was set to 100%), using following restrictions with respect to the molecular composition: 1H0 , 12C0 , 16O0 , 32S0 2, 14N0 2. With this approach, one unequivocal molecular formula was assigned for each peak in the mass range of 200 500 m/z.
Multiple formula matches in the mass range 500 600 m/z were excluded by the homologues series/building block approach (see Koch et al., 2007 for a summary).
85 Chapter 3
Weighted average double bond equivalents (DBEwa), molecular element ratios (e.g. (O/C)wa,
(H/C)wa, (C/N)wa) and molecular weight ((m/z)wa) were calculated for each sample from the intensity (Int) of each assigned peak (x) using equation 3.2: B(O / C) x Int x ) (O / C) (3.2) wa B Int
Aromatic compounds were identified using the modified aromaticity index AImod (Koch and
Dittmar, 2006) after equation 3.3: 1 C 0.5O S 0.5H AI (3.3) mod C 0.5O S N
3.2.5 Dissolved organic carbon and total dissolved nitrogen
To determine the DOC concentration (excluding MeOH) and total dissolved nitrogen (TDN) in the solid phase extract, an aliquot of the extract was dried with N2 at room temperature, redissolved in ultrapure water and measured with high temperature catalytic oxidation using a Shimadzu TOC/TN analyzer equipped with an infrared and a chemiluminescence detector (gas flow oxygen: 0.6 l min 1). In the autosampler, 6 ml of sample volume in pre combusted vials were acidified with 0.12 ml HCl (2 M) and sparged with oxygen to remove inorganic carbon. 50 l sample volume was directly injected on the catalyst (heated to
680°C). Final DOC concentrations were average values of triplicate measurements. Detection limit (5 of the blank) was 7 M C with an accuracy of ±2 M C determined with low carbon water and seawater reference material (DOC CRM, Hansell Research Lab, University of
Miami, US). Due to the limited availability of porewater samples the extraction efficiency could only be exemplary determined for one sample which was available in a larger volume
(sample GeoB 11033). The DOC concentration in the redissolved methanol extract was divided by the concentration factor (50) and the DOC concentration in the porewater sample.
With this approach we determined an extraction yield of 53% of the total DOC (305 M
DOC) in that sample. Generally, the described method extracts 43 65% of the pelagic DOC
(Dittmar et al., 2008).
3.2.6 Total organic carbon, total nitrogen and total sulfur
Total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) concentrations were analyzed from the freeze dried homogenized sediment. TS and TOC was measured with a
86 Characterization of DOM in sediment pore water
Leco CS 200. Prior to TOC measurement, the samples were treated with 12.5% hydrochloric acid to remove carbonates. For the TN and TC analysis, 25 mg of the sample was packed in tin boats and measured on a Vario EL III Elemental Analyzer.
Figure 3.2. Scheme of analytical methods applied in this study. Bulk parameters comprise dissolved organic matter (DOM) and particulate organic matter (POM). At each sampling location, DOM was separated by solid phase extraction (SPE) from pore or river water; lipid biomarkers and lignin phenols were analyzed in the sedimentary OM.
3.2.7 Lipid extraction
Lipid biomarkers were extracted three times from 30 g of freeze dried homogenized sediment with a solvent mixture of DCM:methanol (2:1 v/v) with a microwave extraction system (MARS X, CEM) at 80°C. Prior to extraction, an internal standard mix consisting of
5 cholestane, behenic acid methylester, 1 nonadecanol and 2 methyloctadecanoic acid was added. The combined extracts were washed with 0.05 M potassium chloride. Water was removed from the organic phase with sodium sulfate and the solvent was evaporated under a stream of nitrogen. Subsequently, the total lipid extract was separated into a hexane soluble (maltene) and insoluble (asphaltene) fraction. The maltene fraction was further separated on SPE cartridges (Supelco LC NH2, 500 mg sorbent) into four fractions
(hydrocarbons, esters and ketones, alcohols, and free fatty acids, Fig. 3.2) according to the protocol by Hinrichs et al. (2000). Prior to analysis, alcohols were derivatized with bis
(trimethylsilyl)trifluoroacetamide (BSTFA, Merck) in pyridine and analyzed as trimethylsilyl (TMS) derivatives; fatty acids were reacted with 12% boron trifluoride in
87 Chapter 3 methanol (Merck) yielding fatty acid methylesters. Both reactions were conducted at 70°C for
1 h. Afterwards, solvent and reagent were removed under a stream of nitrogen and the fractions were stored until analysis at 20°C in the dark.
3.2.8 Lignin extraction
The extraction protocol was adapted from (Hedges and Ertel, 1982). Briefly, 1 2 g of already extracted sediment was oxidized in a microwave vessel under oxygen free conditions using
CuO. Prior to the reaction, 100 mg ammonium iron(II) sulfate hexahydrate (Fe2(NH4)2(SO4) *
H2O, p.a. Fluka), 1 g DCM extracted CuO powder, and 8 ml of nitrogen purged sodium hydroxide (p.a. grade, Merck) were added to the sediment under N atmosphere. The reaction was conducted in a sealed extraction vessel in MARS X at 150 °C for 3 h. After the reaction was completed, an internal standard mixture (ethylvanillin, t cinnamic acid) was added and the sediment was sonicated three times with 15 ml 1 M sodium hydroxide. The solid and the liquid phase were separated by centrifugation and the combined extracts were acidified with hydrochloric acid to pH 1. Phenols were extracted from the aqueous solution three times with distilled diethylether, which was treated with Fe2(NH4)2(SO4) * H2O in aqueous solution a priori. The extract was dried with sodium sulfate and the solvent was removed under a stream of nitrogen. Phenols were reacted prior to analysis to TMS derivatives using BSTFA in pyridine.
3.2.9 Gas chromatography coupled to mass spectrometry or flame ionization detection
All four lipid fractions and the phenol extract were analyzed by GC either coupled to a MS or connected to a flame ionization detector (FID). The GC (Thermo Electron Trace GC) was equipped with a 30 m Rtx 5MS fused silica capillary column (0.25 mm i.d., 0.25 m film thickness) and helium was used as carrier gas (flow rate 1 ml min 1). The lipid extracts were injected in hexane using an injection temperature of 60 °C, a temperature ramp of 10 °C min 1 until 150 °C, a temperature ramp of 4 °C min 1 and a final temperature of 320 °C (hold time:
30 min). The lignin phenol extract was injected in pyridine:BSTFA (4:1 v/v) in order to keep all carboxylic groups as trimethylsilyl derivatives. The initial temperature of 100 °C was followed by a temperature ramp of 4 °C min 1 and a final temperature of 310 °C (hold 12 min). All components were identified via their mass spectrum and quantified by their peak area in the FID chromatogram.
88 Characterization of DOM in sediment pore water
3.3 RESULTS AND DISCUSSION
3.3.1 Characterization of sedimentary organic matter
The TOC concentrations in the sediment indicated an OM accumulation in the mudbelt and at the continental slope, whereas the outer shelf sediment received only minor amounts of
TOC, indicating a by passing of material in this area (Table 3.1). Within the mudbelt, the southernmost sediment sample contained the highest amount of TOC and a high TOC/TN ratio of 13.3. TOC/TN is widely used to determine the proportion of OM from terrestrial and marine sources. The elemental composition of marine phytoplankton is characterized by the
Redfield ratio (TOC:N:P 106:16:1, Redfield, 1934; Redfield, 1958) with a TOC/TN value of 6.7, whereas TOC/TN in vascular plants ranges between 20 and 400 (Hedges et al., 1986). An elevated TOC/TN ratio of 13.3 in the southern mudbelt implied that these sediments received the highest input of terrestrial plant material and emphasized the importance of the Douro
River, which has been identified as the major terrestrial OM source at the Galicia Minho shelf (Dias et al., 2002). The TOC/TN ratio decreased in our study area in northern direction and offshore (Table 3.1), corresponding to a loss of terrestrial material along the main transport pathways at the shelf. TOC/TN ratios of ca. 7.5 suggested a TOC composition dominated by marine algae and sedimentary microbes. As a fraction of the bulk sediment data, DOCSPE and TDNSPE were measured solely in the extracts of the sediment pore waters.
The DOCSPE/TDNSPE ratio reflects similar trends as the TOC/TN data, but in comparison to the bulk sediment the pore water was always enriched in carbon. Similar results were observed in DOM and POM of the marine water column (Hopkinson et al., 1998) and can be explained by a preferential remineralization of easily accessible, fresh DON by microorganisms.
Table 3.1. Location and water depth of all marine sampling sites in this study. Content of total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) in %, and their ratios (TOC/TN, TOC/TS) analyzed in the bulk sediment from the Galicia Minho shelf. Station GeoB Depth Longitude (°N) Latitude (°W) TOC TN TOC/TN TS TOC/TS 11039 99 41°33’04 9°04’40 2.21 0.17 13.3 0.28 7.89 (mudbelt) 11002 112 42°09’60 8°59’24 0.91 0.11 8.2 0.14 6.50 (mudbelt) 11012 119 42°42’31 9°15’58 0.73 0.11 7.0 0.16 4.56 (mudbelt) 11006 235 42°10’00 9°20’01 0.26 0.03 7.6 0.09 2.89 (outer shelf) 11033 1873 42°10’11 9°33’50 1.10 0.15 7.2 0.20 5.50 (continental slope)
89 Chapter 3
3.3.2 Spatial distribution of lignin phenols and lipid biomarkers in the particulate organic matter
Lignin is a major compound in vascular land plants and its phenol moieties can be used to trace riverine input of terrestrial material in marine systems (e.g., Ertel and Hedges, 1984;
Prahl et al., 1994; Goñi and Hedges, 1995; Goñi et al., 2000). The sum of all detected lignin phenols (vanillin, acetovanillone, vanillic acid, syringealdehyde, acetosyringone, syringic acid, p coumaric acid, ferulic acid) in the sedimentary OM is denoted as Lig (Table 3.2) and resembled the TOC/TN ratio. Accordingly, high concentrations of lignin phenols in the southern mudbelt (Fig. 3.3b) indicated the accumulation of terrestrial OM. The lignin phenol concentration of 160 g/g sediment dry weight (dw) is 40 times higher than in the sediment of the continental slope, reflecting a loss of vascular plant material along the transport pathways to the north and offshore.
Table 3.2. Total lignin abundance ( Lig) and representative lipid biomarkers (in g/g dw) in the sediment. The acid to aldehyde ratio of the vanillyl phenols ((Ad/Al)V) was calculated from the total concentration of vanillic acid to vanillin. The terr/ ster ratio indicates the relative contributions of terrestrial biomarkers ( terr including n alkanes (odd numbered C25 C35), n alcohols (even numbered C22 C30) and n fatty acids (even numbered C22 C30)) and marine sterol markers ( ster including cholesterol, brassicasterol, and dinosterol) in the sediment. C14 to C19 mono O alkyl glycerol ethers ( MAGEs) as well as iso and anteiso C15 fatty acids (i + ai C15FA) derive from autochthonous sulfate reducing bacteria.
Station GeoB Lig (Ad/Al)V n- n- n-fatty terr Chole- Brassica- Dino- ster terr/ MAGEs i- + ai- alkanes alcohols acids sterol sterol sterol ster C15FA 11039 159.9 0.34 2.6 4.8 3.9 11.3 2.5 2.7 2 7.2 1.6 3.4 0.6 (mudbelt)
11002 49.6 0.08 1.7 2.7 2.1 6.5 2.4 1.7 1.3 5.4 1.2 3.1 0.5 (mudbelt) 11012 33.6 0.41 1.8 3.5 2.4 7.7 2.4 1.8 1.3 5.5 1.4 3 0.3 (mudbelt) 11006 9.9 0.62 0.9 1.9 2.5 5.3 0.9 0.8 0.7 2.4 2.2 0.3 0.3 (outer shelf) 11033 4.2 0.93 1.4 1.9 1.4 4.7 0.8 0.6 0.4 1.9 2.5 0.4 0.3 (continental slope)
The relative proportion of specific lignin phenols in the sediment can provide information on the degradation state of lignin (Ertel and Hedges, 1985). Side chain oxidation, demethylation and aromatic ring cleavage are the major processes during biodegradation of lignin (Tien and Kirk, 1983; Hedges et al., 1988). Propyl side chain oxidation by white rot fungi in terrestrial systems results in an increase in acidic CuO reaction products, whereas subaqueous microbial degradation is mainly reflected in a decrease of syringyl and vanillyl phenols due to demethylation reactions (Opsahl and Benner, 1995). The acid to aldehyde ratio of vanillyl (i.e., vanillic acid to vanillin, (Ad/Al)V) indicates terrestrial lignin 90 Characterization of DOM in sediment pore water
degradation, if (Ad/Al)V values are greater than 0.4 (Goñi et al., 1993). At the Galicia Minho shelf, the (Ad/Al)V ratio increased from values below 0.4 in the mudbelt sediments to 0.62 and 0.93 in the sediments of the outer shelf and the continental slope, respectively. This increase coincided with the decrease of the total lignin phenols in the sediments (Fig. 3.3d) along the main transport trajectory, indicating enhanced subaqueous lignin alteration and at the same time a preferential transport or preservation of pre degraded terrestrial OM.
Figure 3.3. Gas chromatograms (FID) of the maltene (a, c) and lignin fractions (b, d) of samples from the southern mudbelt (GeoB 11039; a, b) and the continental slope (GeoB 11033; c, d). FA – unsaturated and saturated fatty acids, MAGE – mono O alkyl glycerol ethers (C14 – C19), circle – n alkanes (C25 – C35), triangle – n alcohols (C22 – C30), black star – cholesterol, cross – brassicasterol, rhomb – dinosterol. IS – internal standard, 1 – vanillin, 2 – acetovanillone, 3 – vanillic acid, 4 – syringaldehyd, 5 – acetosyringone, 6 – syringic acid, 7 – p coumaric acid.
Lipid biomarkers can also be used to identify sources of OM in the sediment (e.g. Mccallister et al., 2006; Medeiros and Simoneit, 2008; Volkman et al., 2008; Waterson and Canuel, 2008;
Yoshinaga et al., 2008). The lipid biomarker distribution, however, showed a slightly different picture than the lignin phenol and TOC/TN data at the Galicia Minho shelf. Table
91 Chapter 3
3.2 lists the absolute abundance of representative lipids at the Galicia Minho shelf. Long chain n alkanes with an odd over even carbon number predominance and even numbered long chain n alcohols and n fatty acids are commonly used as terrestrial biomarkers due to their high abundance in leaf waxes of vascular plants (Eglinton and Hamilton, 1967). For all of these biomarkers, enhanced concentrations were observed in the mudbelt sediments, in particular in the southern mudbelt, which is consistent with the lignin phenol and TOC/TN data. Concentrations of terrestrial markers decreased in the outer shelf and continental slope sediments, but their relative proportions to marine phytoplankton derived biomarkers such as brassicasterol or dinosterol increased. Together with the low amounts of lignin found here, the lipid biomarker pattern indicates a different transport behavior of single OM fractions, i.e., transport of lignin containing terrestrial material seems to be restricted in
South North direction, whereas n alkanes, n alcohols and n fatty acids bypass the outer shelf and are also deposited offshore.
3.3.3 Molecular variations in dissolved organic matter
3.3.1 Dissolved organic matter characterization via molar H/C and O/C ratios
A representative FT ICR mass spectrum of marine sediment pore water DOM is shown in
Fig. 3.4a. All spectra showed a similar pattern with a slight shift to higher m/z in the river water spectra (Table 3.3). We observed typical mass spacing patterns such as 14.0156 Da for
CH2 groups and an increase of 36.4 mDa for the replacement of O by CH4, as well as a strong predominance of odd over even mass ions as described elsewhere for various other humic rich DOM samples (Kujawinski et al., 2002a; Kujawinski et al., 2002b; Stenson et al., 2003;
Kramer et al., 2004; Koch et al., 2005). An exception was observed for the Ave River spectrum, which was very distinct from all other samples (Fig. 3.4c). The spectrum was less intense and the peak intensities were more evenly distributed in the range between 230 and
580 Da, whereas the peaks in the mass spectra of the pore water and other river waters were normally distributed with a maximum in the range of 420 to 450 Da. An expanded section at the nominal mass 407 of the Ave River DOM spectrum revealed a trend to higher relative intensity of lower masses at each nominal mass (relative intensity maximum: C19H29O10, Fig.
3.4d) compared to the pore water DOM (relative intensity maximum: C20H24O9; Fig 3.4b).
Douro and Minho River spectra showed even higher peak intensities at lower masses. This shift is produced by the higher mass defect of 1H (1.0078) than of 16O (15.9949) and therefore,
92 Characterization of DOM in sediment pore water it reflects a higher relative O content and/or a lower relative H content in the river DOM
(Koch et al., 2008). In total, we identified between 2146 and 2693 unequivocal molecular formulas per sample in the three river DOM samples (Table 3.3). In the marine pore waters the number of identified molecular formulas was significantly lower and showed the highest number of 2020 molecular formulas in DOM from the southern mudbelt, the sample most strongly influenced by terrestrial input. The decrease in molecular complexity during the coastal export is consistent with other studies (Koch et al., 2005).
Figure 3.4. ESI negative FT ICR mass spectra and expanded sections of mass spectra at nominal mass 407 of: a) and b) northern mudbelt derived sediment pore water DOM (GeoB 11012) and c) and d) Ave River DOM. Open rhomb – C13H28O10S2, open square – C26H19O3N2; following symbols refer to series that are characterized by the replacement of O by CH4: filled circles – CHO series from C17H12O12 to C23H36O6, cross – nitrogen bearing series from C17H16O10N2 to C19H24O8N2, open triangle – sulfur bearing series from C18H16O9S to C21H28O6S. (O/C)wa and (H/C)wa refer to CHO compounds in the presented mass range and reflect a lower relative O content and a higher relative H content in the pore water DOM.
A widely used tool for the visualization of compositional variations from FT ICR MS data is the van Krevelen diagram (e.g., Kim et al., 2003; Kim et al., 2004; Kujawinski et al., 2004; Wu et al., 2004; Koch et al., 2005; Kim et al., 2006) in which individual molecular formulas are plotted on the basis of their molar H/C and O/C ratios. Thus, each dot in the diagram represents one or more molecular formulas with a specific O/C and H/C ratio. The pattern in the van Krevelen diagram can reflect the sources of DOM but also reaction pathways(Kim et al., 2003), e.g., enzymatic hydroxylation by microbes or photodegradation. Fig. 3.5 compares changes of DOM along the main transport route from the Douro River across the shelf
93 Chapter 3
system (central mudbelt – outer shelf – continental slope). Distinct features were revealed
among the different samples such as specific molecular formulas at low O/C and high H/C
values in the marine sediment pore waters as well as molecular formulas at low H/C and
intermediate O/C values, which were only present in the river DOM. In general, the
composition of sediment pore water DOM was gradually shifted in the van Krevelen
diagram to higher H/C and lower O/C ratios along the transport pathway and (O/C)wa and
(H/C)wa ratios of pore water DOM were similar to those of DOM from the marine water
column (Atlantic deep water as marine reference sample in Table 3.3). Similar trends were
observed in earlier studies by comparing marine DOM samples with river DOM (Sleighter
and Hatcher, 2008) or mangrove pore water DOM (Koch et al., 2005). The variations in the
oxygen and hydrogen content of DOM appear to be linked to changes in OM sources. River
DOM is more oxygenated and contains less hydrogen, which points to a higher aromaticity
in the terrestrial OM due to high contributions of lignins or tannins. The contribution of
aliphatic compound types from algal detritus and/or microbial biomass in marine sediment
is reflected in lower (O/C)wa and higher (H/C)wa ratios.
Table 3.3. Characteristic parameters of pore water and river DOM as well as the contents (in M) of SPE extracted dissolved organic carbon (DOCSPE) and total dissolved nitrogen (TDNSPE). Displayed are carbon to nitrogen ratios in the extract (DOCSPE/TDNSPE), numbers of identified molecular formulas (nIon), the intensity weighted average values of the molecular weight ((m/z)wa), double bond equivalents (DBEwa), molar oxygen to carbon ((O/C)wa) and hydrogen to carbon ratios ((H/C)wa), number of carbon (Cwa), nitrogen (Nwa) and sulfur atoms (Swa).
Station GeoB DOCSPE TDNSPE DOCSPE/ nIon (m/z)wa DBEwa (O/C)wa (H/C)wa Cwa Nwa Swa TDNSPE 11039 153 10 15 2020 421.08 8.61 0.52 1.26 19.48 0.32 0.09 (mudbelt) 11002 94 4* 26* 1980 411.99 8.40 0.50 1.26 19.24 0.37 0.02 (mudbelt) 11012 189 15 13 1418 430.44 8.91 0.51 1.24 20.15 0.34 0.03 (mudbelt) 11006 270 12 23 1780 426.92 8.95 0.50 1.24 20.09 0.35 0.02 (outer shelf) 11033 189 16 12 1858 422.63 8.63 0.49 1.29 19.85 0.61 0.01 (continental slope) Douro River 150 8* 20* 2146 444.31 9.55 0.52 1.18 20.66 0.20 0.03 Minho River 107 6* 17* 2211 449.57 10.18 0.52 1.13 21.02 0.16 0.02 Ave River 209 22 11 2694 401.88 8.92 0.49 1.19 18.81 0.16 0.30 Atlantic deep - - - 624 453.03 9.14 0.50 1.24 21.34 0.11 - water ) * TDNSPE close to the limit of quantitation
A change in (H/C)wa and (O/C)wa ratios may also be explained by biodegradation processes,
which have been shown to selectively remove oxygen rich molecules from DOM (Kim et al.,
2006). This is probably the case for Ave River DOM with a typical terrestrial (H/C)wa ratio of
94 Characterization of DOM in sediment pore water
1.19 and a low (O/C)wa ratio of 0.49, i.e., the latter being similar to marine Atlantic deep water
DOM.
Figure 3.5. Van Krevelen diagram of CHO formulas in DOM from the Galicia Minho shelf. A shift in direction of the arrow is observed to higher H/C and lower O/C ratios from the Douro River (green rhombs) to the central mudbelt (GeoB 11002, light blue triangles) down to the outer shelf (GeoB 11006, blue circles) and the continental slope (GeoB 11033, black dashes). Black circles correspond to general compound classes (adapted from Kim et al., 2003; Sleighter and Hatcher, 2008) and are indicated for reference, not necessarily implying the presence of these compounds in the DOM samples.
Referring to the peak heights in the spectra, the most abundant compounds in all samples were located in the center of the van Krevelen diagram (O/C 0.5 0.8, H/C 1.0 1.5), correlating with O/C and H/C ratios of lignins and tannins (Fig. 3.5). However, in a recent study by Lam and coworkers (2007) no lignin methoxy groups were detected by NMR spectroscopy in
Lake Ontario DOM. 8% of the carbon in Lake Ontario DOM was associated with aromatics,
17% with carbohydrates and the remaining 75% with terpenoid derived material (linear and alicyclic terpenoids). One particular compound group belonging to the latter fraction comprises carboxylic rich alicyclic molecules (CRAM, molecules with a cyclic backbone and a high degree of carboxylation), previously identified as major compounds in deep ocean
DOM (Hertkorn et al., 2006). Several molecular formulas that are assigned to CRAM were present in all DOM samples from Galicia (e.g. C28H32O13, corresponding to a mass of 576.546
Da). However, the samples contained also molecular formulas of degraded lignin (e.g.,
C27H28O12, see Stenson et al., 2003) and especially in river DOM and the pore water DOM from the southern mudbelt lignin contributions are very likely, since high lignin phenol concentrations were detected in the sedimentary OM.
95 Chapter 3
Fig. 3.6 shows the molecular differences between the samples in a cluster diagram. The cluster analysis was carried out on the basis of all identified molecular formulas and the relative intensity of the referring ion in the spectrum. By this, weighted average element ratios, molecular weight distribution, etc. are directly reflected in the degree of similarity between the samples, e.g., the exceptional position of the Ave River was supported in the lowest degree of similarity compared to all other samples. Douro and Minho River grouped together (S = 84), well separated from the pore water samples. Within the marine samples the pore water DOM from the continental slope differed with S 0.72.
Figure 3.6. Cluster analysis (Bray Curtis Similarity) based on all identified molecular formulas having a relative intensity 2%. Cluster analyses visualize similarities/dissimilarities between samples based on a multidimensional data set. The x axis represents the degree of similarity; the vertical lines of the branches indicate the similarity value. A similarity value of S = 100 would mean that all molecular formulas in the samples are identical.
3.3.3.2 Dissolved organic matter characterization via molecular mass and DBE
DOM from the three environments, i.e., sediment pore water, river, and Atlantic deep water, differed significantly regarding their (m/z)wa and DBEwa (Table 3.3). The (m/z)wa in marine pore water DOM ranged from 411.99 to 430.34, which is much lower compared to the
Atlantic deep water ((m/z)wa 453.03) and the river DOM ((m/z)wa 444.27 and 449.57 for Douro and Minho River, respectively).
In the sediment pore water, lower (m/z)wa was correlated with lower DBEwa. The loss of aromatic molecules and molecules with high molecular weight (low DBEwa and (m/z)wa, respectively) was observed previously by Tremblay and coworkers (2007) who compared mangrove pore water and water column DOM in a Brazilian estuary. These authors
96 Characterization of DOM in sediment pore water explained this loss with photodegradation that took place during transport of DOM in the water column. At the Galicia Minho shelf, a shift to lower DBEwa and (m/z)wa from river water to sediment pore water suggested that in situ transformation processes are taking place. Sediment pore water DOM is a heterogeneous mixture of organic compounds including large molecules, e.g., proteins, humic substances, and smaller molecules, e.g., amino acids and short chain organic acids (e.g. Henrichs, 1992). The DOC concentration in marine pore water is usually up to a magnitude higher than in the surrounding bottom water, implying net production of DOM in the sediment as a result of remineralization
(Burdige et al., 2002). Intermediates during remineralization processes such as hydrolysis and fermentation produce smaller molecules and LMW DOM, thus reducing (m/z)wa, which is consistent with the trends observed in our study. Besides, the highly degraded DOM in the
Ave River (see also 3.3.5) had by far the lowest (m/z)wa and a DBEwa value closely similar to sediment pore water DOM. Furthermore, recent studies with FT ICR MS reported a shift to lower m/z in stream DOM (Kim et al., 2006) and fulvic acids (Einsiedl et al., 2007), which were both exposed to microbial activity.
3.3.3.3 Sources and occurrence of aromatic ring structures
In order to separate aromatic compounds from the bulk FT ICR MS dataset, we applied the modified aromaticity index (AImod, Koch and Dittmar, 2006). This ratio emphasizes the proportion of double bonds in a molecule compared to the total number of carbon atoms in consideration of the presence of heteroatoms such as sulfur and nitrogen. Based on the assumption that the carboxyl group is the dominant oxygen containing functional group in
DOM (Hertkorn et al., 2006; Sleighter and Hatcher, 2008), AImod considers that 50% of the oxygen is bound with bonds as carbonyl oxygen, which would artificially increase the number of DBE in a molecule. In our samples, we distinguished two groups of aromatic structures (Table 3.4), one with high O/C ratios (O/C > 0.4, DBE O from 2 to 11) and one with low O/C ratios (O/C < 0.2, DBE O from 15 to 32). Both groups comprised CHO compounds as well as compounds containing heteroatoms (S, N) and will be separately discussed in the following.
97 Chapter 3
Table 3.4. Distributional pattern and source assignments of identified compound classes in river and sediment pore water DOM.
Compound class Distribution Potential sources Previous Reference CHO-compounds Sediment pore Aliphatic molecules from Koch et al. (2005) O/C < 0.6 waters algal and microbial biomass Sleighter and Hatcher (2008) H/C > 1.7
CHO-compounds Rivers, S’mudbelt, Terrestrial OM, e.g., fulvic Kim et al. (2003) O/C > 0.5 central mudbelt acids Stenson et al. (2003) H/C < 0.8 Koch et al. (2005) Sleighter and Hatcher (2008)
CHO-compounds Non specific CRAM, lignins, tannins Kim et al. (2003) O/C 0.5 – 0.8 Hertkorn et al. (2006) H/C 1.0 – 1.5 Tremblay et al. (2007) Sleighter and Hatcher (2008)
Aromatic compounds Non specific Black carbon from soils and Kim et al. (2003) O/C 0.2 wild fires Kramer et al. (2004)
Aromatic compounds Rivers, S’mudbelt Humic acids from soils or Kim et al. (2004) O/C > 0.4 modified BC Kramer et al. (2004) Hockaday et al. (2006) Dittmar and Koch (2006)
Nitrogen-bearing compounds Continental slope Degraded proteins from Kujawinski et al. (2004) H/C > 1.5 fresh microbial and algal Sleighter and Hatcher (2008) biomass
Nitrogen-bearing compounds Rivers, S’mudbelt Terrestrial compounds O/C 0.4 – 0.8 H/C 0.6 – 0.8
Sulfur-bearing compounds Ave River, Early diagenetic sulfurization S’mudbelt of OM
3.3.3.3.1 Aromatic compounds with low O/C ( 0.2)
In recent studies, aromatic compounds with low O/C values in DOM samples were assigned to black carbon (BC) (Kim et al., 2004; Kramer et al., 2004). BC, a group of incomplete combustion products of fossil fuels and biomass, is very heterogeneous with respect to its size, age, and chemical composition (Schmidt and Noack, 2000). At the Galicia Minho shelf,
BC was present in all samples and showed no obvious trend in its distribution. The presence of BC in river water DOM can be best explained by the frequent occurrence of wildfires in northern Spain and Portugal. During the time of sampling in August 2006, large parts of
North West Iberia were affected by those forest fires, which provide a possible source of BC in the three rivers. Marine sediments can receive BC on several pathways. One is air transport and dry fall out of combustion products from wildfires or anthropogenic production, another one is river transport which is promoted by the fairly high resistance to biodegradation (Kim et al., 2004). However, the absence of typical combustion products from
98 Characterization of DOM in sediment pore water wild fires such as 3 methoxyfriedelane, n nonacosan 10 ol or methoxyphenols (see Simoneit,
2002) in the lipid fraction of our samples pointed to a different origin, potentially hydrocarbons from ship fuels. Alternatively, BC like compounds in the pore water could also have an older age and derive from wild fires in former times. Compared to solvent extractable combustion products in the sedimentary OM, it is most likely that BC compounds survive during transport from the soil to the sediments due to their very high resistance against degradation.
3.3.3.3.2 Aromatic compounds with high O/C (> 0.4)
We detected a high number of aromatic structures with high O/C values in the river DOM as well as in the sediment pore water DOM from the southern mudbelt (Fig. 3.7). These aromatic compounds were highly abundant in number in the river DOM and decreased gradually in the mudbelt northward along the main transport route and were not detected in the pore water DOM from the northern mudbelt, outer shelf and continental slope. The significant correlation with lignin and n alkanes in the sedimentary OM (r = 0.9, for a confidence level = 0.05) suggests a terrestrial origin of those compounds. Aromatic compounds with high O/C values could be derived from microbially mediated oxidation of
BC (Dittmar and Koch, 2006), which takes place in soils and sediments. Several microbes are able to metabolize BC via enzymatic reactions and accordingly increase the number of carboxylic, hydroxylated and methoxylated aliphatic groups, which then increases the O/C ratio and generally the water solubility (Rudolphi et al., 1991; Willmann and Fakoussa, 1997;
Decesari et al., 2002). In recent studies, carboxylic aromatic ring structures with high oxygen content were detected as major constituents in humic acid leachates of soil charcoal
(Hockaday et al., 2006), in volcanic ash soil humic acids (Kramer et al., 2004) and in bog and black water DOM (Kim et al., 2004). Therefore, likely sources of these compounds in river
DOM are humic acids, washed out from soil or modified BC compounds. For the occurrence of those compounds in the pore water, two major pathways are conceivable: BC is oxidized in soils, subsequently dissolved, transported via rivers to the ocean, and incorporated into the oceanic DOM pool and ultimately into sediment pore water due to exchange processes with the bottom water. Alternatively, particulate BC, which is transported in the POM fraction via rivers or as aerosols via the atmosphere, can be oxidized by microbes in marine sediments, where the products dissolve and accumulate in the pore water DOM.
99 Chapter 3
Figure 3.7. Detailed van Krevelen diagram of aromatic compounds (modified aromaticity index AImod 0.67), including N and S for: a) the Douro River, b) the southern (GeoB 11039) and c) the central mudbelt (GeoB 11002). The non modified aromaticity index 0.67 (red line) assigns all condensed aromatics with low oxygen content. Circles mark highly oxygenated aromatics of terrestrial origin.
3.3.3.4 Spatial variations of nitrogen bearing compounds
Compounds with molecular formulas including one or two nitrogen atoms were present in all DOM samples in varying relative abundance. The FT ICR mass spectra revealed a change from low numbers of nitrogen bearing compounds in the river DOM (Nwa = 0.16 to 0.20) to higher amounts in northward direction along the mudbelt (Nwa = 0.32 to 0.37) and offshore at the continental slope (Nwa = 0.61). Similar results of an increase in nitrogen bearing compounds offshore were detected in a recent study along a river to ocean transect of the
Chesapeake Bay (Sleighter and Hatcher, 2008).
100 Characterization of DOM in sediment pore water
The van Krevelen diagram in Fig. 3.8 reflects the high variety in nitrogen bearing compounds in the pore water DOM from the continental slope in comparison to Douro River
DOM.
Figure 3.8. Van Krevelen diagram of nitrogen bearing molecular formulas in DOM of: a) the Douro River (rectangle marks compounds of terrestrial origin) and b) the continental slope (GeoB 11033). Circles correspond to O/C and H/C ratios of proteins and amino sugars (Kim et al. 2003).
Striking features are the contributions from compounds with two N atoms, plotting mainly in regions of the van Krevelen diagram where proteins, oligopeptides, amino sugars and highly aromatic compounds are expected. These compounds were exclusively present in the continental slope DOM. A high relative abundance of nitrogen bearing compounds, which were concentrated in the protein area of the van Krevelen diagram, was also detected by
Kujawinski and coworkers (2004) who analyzed DOM produced by bacteria and protozoan cultures. Although proteins are not in our analytical window (200 < m/z < 600), the occurrence of degradation products and intermediates with lower molecular weight are expected constituents in pore water DOM. The lack of those molecules in the other pore water samples may be related to variations in the exchange between pore water and bottom water at the different sampling locations. Sediment deposits at the Galicia Minho shelf are subjected to sediment mixing due to strong currents and storm induced bottom waves in winter (Jouanneau et al., 2002; Vitorino et al., 2002b) and therefore, pore water and bottom water are regularly exchanged on a seasonal basis. In contrast, calm sedimentation conditions at the continental slope result in a continuously accumulation of clay rich sediment. Additionally, diffusion rates of pore water are reduced in clay rich densely packed
101 Chapter 3 sediments (e.g., Tryon et al., 2001; Janssen et al., 2005), enhancing the preservation of nitrogen compounds in the pore water DOM at the continental slope. Alternatively, this distinct signal in the continental slope pore water DOM may also reflect a different OM quality, e.g., similarly shown in Amazon River POM (e.g., Hedges et al., 1986).
A specific compound group with O/C ratios of 0.4 to 0.7 and relatively low H/C ratios of 0.6 and 0.8 was observed in the river DOM (rectangle in Fig. 3.8a). Molecular formulas related to these ratios (e.g. C19H15O11N) were also present in the southern mudbelt but missing in all other pore water samples, thus pointing to a terrestrial source of this compound group.
Nevertheless, at this point, the knowledge of the occurrence of distinct nitrogen compounds in DOM is still limited and therefore needs further research to understand their distribution and structural diversity.
3.3.3.5 Sulfur compounds as indicators for early diagenesis?
All DOM samples contained several compounds with one or two sulfur atoms in the formulas. Compounds with higher sulfur (and also nitrogen) content have been omitted, since it was necessary to restrict the number and type of isotopes for the molecular formula calculation in an a priori assumption (Koch et al., 2007).
In general, the relative abundance of Swa in the FT ICR MS data was around 0.02. Intriguing was the high number of sulfur bearing compounds in the Ave river (Swa 0.30) and in the sediment pore water DOM from the southern mudbelt (Swa 0.09). Exemplary FT ICR mass spectra at the mass 353 are shown in Fig. 3.9a, c, e (outer shelf, southern mudbelt and Ave
River). The outer shelf represents samples with a Swa value of 0.02 and exhibited no sulfur compounds at the nominal mass 353, whereas the southern mudbelt and the Ave River showed several sulfur formulas, e.g. C16H34O6S. In the spectrum of the Ave River, the sulfur bearing compounds reached intensities of up to 2/3 relative to CHO compounds, emphasizing the importance of dissolved organic sulfur compounds in this sample. The van
Krevelen diagram of all sulfur formulas in the DOM at the outer shelf revealed two specific compound groups. One group located at 0.4 to 0.8 O/C and 1.1 to 1.6 H/C values and the other at 0.0 to 0.2 O/C and 0.8 to 1.0 H/C values (Fig. 3.9b). By contrast, the Ave River and the southern mudbelt sample showed a widespread occurrence of sulfur bearing compounds in the van Krevelen diagram (Fig. 3.9d and f). Since none of the observed compounds (e.g.
C15H26O6S, C22H34O9S2) contained nitrogen, a relationship to degradation products of proteins
102 Characterization of DOM in sediment pore water
Figure 3.9. FT ICR mass spectra at 353 m/z (for: a) the outer shelf (GeoB 11006), c) the southern mudbelt (GeoB 11039) and e) the Ave River) and van Krevelen diagram of all characterized S bearing compounds (for: b) the outer shelf (11006), d) the southern mudbelt (GeoB 11039) and f) the Ave River). Molecular formulas and exact masses refer to filled circles (a), to open circles (b) and to open triangles (c). Cross – nitrogen series, where CH4 replaces O from C14H14O9N2 to C16H22O7N2, filled squares – C12H18O12, C13H22O11 is unlikely. We therefore suggest that early diagenetic sulfurization of organic matter is the primary source of the detected sulfur compounds in the Ave River and the southern mudbelt
DOM. Although total sulfur concentrations in the sediment from the Galicia Minho shelf were low and the TOC/TS values of 2.89 to 7.89 exceeded the commonly assumed average values for modern shelves of 2.8 (Berner, 1982), the southern mudbelt showed an 3 fold higher TS concentration than the outer shelf. Sulfurization reactions are known to proceed
103 Chapter 3 during the early stages of sedimentary diagenesis and have been detected even at the sediment water interface (Sinninghe Damste and De Leeuw, 1990; Wakeham et al., 1995). The southern mudbelt probably provides at least seasonally (summer) anoxic conditions, which are prerequisite for the sulfurization reactions. Furthermore, a range of mono O alkyl glycerol ethers (C14 to C19) and iso/anteiso C15:0 fatty acids, both groups indicative of sulfate reducing bacteria (Taylor and Parkes, 1983; Rütters et al., 2001), were detected in the sedimentary OM samples of the mudbelt with the highest amounts found at the southernmost sampling site (Table 3.2 and Fig. 3.3a). The activity of sulfate reducing bacteria in these sediments provides hydrogen sulfide and other reduced sulfur species, which could subsequently react with the OM.
The Ave River DOM, however, represents probably older, highly degraded OM, enriched in humic substances. OM at this site had repeatedly passed anoxic conditions in soils or anoxic bottom waters, in which sulfurization processes took place, thus explaining the occurrence and diversity of sulfur bearing compounds. Further research is needed to elucidate the origin and significance of sulfur containing DOM.
3.4 SUMMARY AND CONCLUSIONS
Although FT ICR MS delivers extensive molecular data for humic material it must be considered that this method so far is not quantitative. The ionization efficiency might vary strongly between aromatic and more apolar aliphatic compounds. The isolation of single compounds by separation methods or in the ICR cell as well as structure elucidation in fragmentation experiments will be the prerequisite for any quantitative approach and is one of the goals of our future research. Moreover, since the molecular structures of the compounds in this study are not exactly determined, tests with artificial standards cannot be applied so far. The conclusions which were drawn in this study are based on relative peak abundances and allowed a fingerprinting approach for organic matter sources. The observed variations of the molecular DOM composition on the shelf are linked to both source variations and biogeochemical processes in the sediment. Compositional variations in DOM are also reflected in distributions of compounds in the sedimentary OM, i.e., lignin phenols, selected lipid biomarkers, and in elemental compositions of bulk OM. Table 3.4 summarizes the source assignments of major compound groups detected in pore water DOM and the resulting interpretations. The major observations are:
104 Characterization of DOM in sediment pore water
1. Highly oxygenated aromatic compounds and nitrogen bearing compounds
with H/C 0.6 0.8 are probably derived from terrestrial sources as suggested by
their distributional pattern similar to the lignin phenol abundance and
TOC/TN ratios in concurrently collected sediment samples.
2. Another nitrogen bearing compound group with high H/C ratios was
dominantly found in continental slope DOM and appears to be indicative for
microbial activity in the marine sediment, which was also reflected in the
lower number of DBEs.
3. Remarkably high concentrations of sulfur compounds in the southern
mudbelt and Ave River DOM probably result from early diagenetic alteration
of OM and have not been documented in previous FT ICR MS studies.
Acknowledgements
We thank the crew of RV Poseidon, Till Hanebuth and the scientific shipboard party of the
GALIOMAR cruise. Further thanks go to Brit Kockisch for support during sampling and the performance of TOC measurements. We also like to acknowledge Hella Buschoff for TN analysis and Xavier Prieto for support in the lab. Funding was provided by the “Deutsche
Forschungsgemeinschaft” through DFG Research Center / Excellence Cluster “The Ocean in the Earth System” and the Bremen International Graduate School for Marine Sciences
(GLOMAR).
105 Chapter 3
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107 Chapter 3
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108 Characterization of DOM in sediment pore water
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109 Chapter 3
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110
Chapter 4
Diagenetic transformation of dissolved organic matter under contrasting
sedimentary redox conditions in the Black Sea
Frauke Schmidta, Boris P. Kochb, Marcus Elverta, Matthias Wittc, Kai Uwe Hinrichsa
Draft, in preparation for submission to Environmental Science and Technology
aMARUM – Center for Marine Environmental Sciences, Leobener Straße, D 28359 Bremen,
Germany bAlfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D 27570
Bremerhaven, Germany cBruker Daltonik GmbH, Fahrenheitstraße 4, 28359 Bremen, Germany Chapter 4
ABSTRACT
Dissolved organic matter (DOM) in sedimentary environments is produced during remineralization of particulate organic matter. For a comprehensive understanding of early diagenesis of DOM, samples from sediment pore waters and bottom water from contrasting redox conditions in the northern Black Sea were analyzed by high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) and compared to the associated bulk sedimentary organic matter and the total hydrolysable dissolved amino acids
(THDAA). The diversity and number of molecules in DOM in bottom and pore water were strongly elevated compared to the water column. The intensity weighted average molecular weight ((m/z)wa) and double bond equivalents (DBEwa) showed the lowest values in the bottom water and were probably the result of increased DOM production due to the elevated microbial activity at the sediment water interface. At the oxic sampling site, DOM was mainly derived from the oxidation of algal organic matter and characterized by relatively high elemental oxygen to carbon ratios (O/C). In contrast, bulk sedimentary OM at the anoxic site consisted of refractory organic matter with higher contributions from bacterial biomass which was reflected in the exclusive presence of several compounds with low O/C ratios. Distinct compounds with one or two nitrogen atoms were identified in all samples and were more abundant in pore waters than in bottom waters coinciding with higher concentrations of THDAA. These compounds were possibly products from protein and peptide degradation via hydrolysis and deamination and from early diagenetic reactions between pore water ammonium and functional groups of DOM. A set of N bearing compounds exclusively present in the oxic bottom water was interpreted to result from oxidative deamination of peptides, whereas several compounds in the anoxic DOM samples were attributed to reductive deamination or a coupled oxidative and reductive deamination reaction.
112 Diagenetic transformation of DOM
4.1 INTRODUCTION
Dissolved organic matter (DOM) in marine sediment pore waters is a heterogeneous mixture of organic compounds with wide spectra of both reactivities and molecular sizes (Burdige and Martens, 1988; Arnosti and Holmer, 2003). It derives primarily from the dissolution of particulate organic matter (POM) in the sediment through hydrolysis, oxidative cleavage, leaching, cell autolysis, and remineralization by sedimentary organisms (Henrichs, 1992).
The widely accepted model for DOM production in sediment pore waters starts with an initial production of high molecular weight (HMW) DOM through depolymerization of
POM, followed by a subsequent remineralization of the HMW DOM to monomeric low molecular weight (LMW) DOM (Amon and Benner, 1996; Burdige and Gardner, 1998;
Komada et al., 2004). A fraction of the HMW DOM is converted into polymeric, apparently recalcitrant LMW DOM, which survives further transformation processes (Santschi et al.,
1995; Amon and Benner, 1996). The decrease in reactivity and remineralization rates of POM with sediment depth (Middelburg, 1989; Burdige, 1991), coinciding with an increase of the dissolved organic carbon (DOC) content (e.g., Shaw and McIntosh, 1990; Bauer et al., 1995;
Holcombe et al., 2001), suggests accumulation and increasing recalcitrance of pore water
DOM with time and advancing POM remineralization. DOM in sediment pore waters is important as nutrient pool for benthic organisms through the diffusional exchange with bottom water (e.g., Burdige and Zheng, 1998) and furthermore, it is probably a controlling factor for organic matter preservation in marine sediments (Henrichs, 1992; Hedges and Keil,
1995). However, up to now most of the pore water studies focused on concentrations and cycling of LMW DOM compounds or on the HMW DOM pool and only few studies investigated the molecular composition of pore water DOM on a compound class level (see
Burdige, 2002, and references therein) leaving a large fraction of the pore water DOM uncharacterized. The lack of compositional data on a molecular level is mainly due to analytical limitations in resolving the complex DOM mixture which hinders a comprehensive understanding of the factors controlling preservation and remineralization of organic matter in marine sediments.
In order to obtain insights into early diagenetic processes on a molecular level, we analyzed
DOM in bottom and pore waters from the Black Sea by high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS). FT ICR MS resolves complex organic mixtures by providing the exact masses of single molecules. This technique has been
113 Chapter 4 successfully applied to characterize a fraction of the marine DOM pool (e.g., Koch et al., 2005;
Hertkorn et al., 2006; Koch et al., 2008; Reemtsma et al., 2008; Sleighter and Hatcher, 2008).
Although it has not been possible to assign molecular structures to each of the thousands of compounds detected as molecular formula in a typical marine sample, FT ICR MS provides an substantial amount of new information on the molecular composition of DOM. In a previous study, we have for the first time applied FT ICR MS to marine sedimentary pore water and identified sources and early diagenetic processes in the polymeric LMW DOM in surface sediments (Schmidt et al., 2009). Now, we aim at constraining the initial steps of organic matter remineralization and its dependence of contrasting redox conditions in a set of DOM samples from the water column, bottom water, and sediment pore water of the
Black Sea. The modern Black Sea is permanently stratified with anoxic conditions below 150 to 200 m water depth (Glenn and Arthur, 1985) and is therefore suited for systematic comparison of organic matter remineralization under contrasting redox conditions. Two of the sampling sites are located on the northern continental shelf where the effect of different redox conditions on the DOM pool was investigated (oxic versus anoxic remineralization).
The third sampling site at 2000 m water depth in the central basin is influenced by the older age of the overlying water masses (Lee et al., 2002), whereas the pore water sample derives from a well preserved turbidite layer (20 MUC 15 cm, Table 4.2).
In this study, we focused particularly on N bearing compounds in DOM in order to understand the factors influencing remineralization and remobilization of nutrient N during early diagenesis. The abundance of nitrogen in labile biomolecules such as proteins, amino acids, RNA, DNA, etc. links the nitrogen cycle closely to early diagenetic transformations of organic matter at the sediment water interface (e.g., Val Klump and Martens, 1983; Boynton and Kemp, 1985; Klump and Martens, 1987; Kemp et al., 1990; Mayer and Rice, 1992;
Dell anno and Corinaldesi, 2004).
4.2 MATERIAL AND METHODS
4.2.1 Sampling
The anoxic water column of the northern Black Sea was sampled using a Pump CTD (Table
4.2) during a cruise with RV Meteor in May 2007. Bottom waters and sediment pore waters were taken from multicorers (MUC). Bottom water in a volume of 50 ml was sampled from the sediment water interface of each MUC and filtered through GF/F filters (Whatman,
114 Diagenetic transformation of DOM
0.7 m pore size). Sediment pore water (50 ml) was obtained by rhizon sampling
(Eijkelkamp, pore size 0.1 m) in 15 – 17 cm sediment depth. All water samples were acidified to pH 2 with hydrochloric acid (HCl, p.a. grade, Merck) and stored in sealed, pre combusted vials at +4 °C in the dark until further preparation. Additionally to the bottom and pore water, the associated sediment (0 – 1 cm and 15 – 17 cm) was sampled for analysis of total organic carbon (TOC), total nitrogen (TN) and stable carbon isotopic composition
( 13CTOC) of bulk organic matter. Sediment samples were stored in pre combusted glass vials at 20 °C in the dark.
4.2.2 Dissolved organic matter extraction
Water samples (50 ml) were concentrated by a factor of 50 with solid phase extraction on pre cleaned SPE cartridges (PPL bond elut, 200 mg sorbent, Varian; extraction efficiency amounts to 43 – 65% of the pelagic DOC (Dittmar et al., 2008)). After DOM adsorption onto the PPL cartridges salt was removed from the samples by rinsing with 6 ml 0.1 M HCl solution. After elution with 1 ml methanol (LiChrosolv, Merck) into pre combusted glass ampoules the
DOM extracts were stored under nitrogen atmosphere at 20 °C in the dark until analysis.
4.2.3 Fourier transform ion cyclotron resonance mass spectrometry
Prior to FT ICR MS analyses DOM extracts were diluted in a ratio of methanol:water 50:50
(v/v). All samples were analyzed from a volume of 400 l with electrospray ionization (ESI,
Apollo II electrospray source) in negative ion mode (capillary voltage: +4 kV) at an infusion flow rate of 2 l min 1 on an Apex Qe mass spectrometer (Bruker Daltonics Inc. Billerica,
USA) equipped with a 9.4 T superconducting magnet (Bruker Biospin, Wissembourg,
France). Spectra were calibrated with arginine clusters and 300 scans were added to one spectrum. Internal calibration with compounds, which were repeatedly identified in marine
DOM samples (m/z: 339.10854, 369.11911, 411.12967, 469.13515, 541.15628, 595.20323) yielded a mass accuracy below 0.3 ppm. Molecular formulae were calculated in a mass range of 200 –
600 m/z for peaks with a signal to noise ratio (S/N) of >3 and were restricted to a molecular element ratio of O/C 1.2 and to integer double bond equivalent (DBE) values. DBE was calculated from the number of atoms (Ni) and the valence (Vi) of each element (i) after Eq.
(4.1) and defines the number of double bonds and/or rings in a molecule.
115 Chapter 4
i max B N i (Vi 2) DBE =1 i (4.1) 2
A formula tolerance of ± 0.5 ppm was considered as valid formula and for the final dataset we focused on ions with a relative abundance I 4% (the most abundant sample compound in each spectrum was set to 100%), using following restrictions with respect to the molecular composition: 1H0 120, 12C0 50, 16O0 35, 14N0 2. This approach provided unequivocal molecular formulae in the mass range 200 – 500 m/z and multiple formulae in the mass range 500 – 600 m/z were filtered with the homologous series/building block approach (e.g., Koch et al.,
2007).
Due to the lack of standard substances and possible variations in the ionization efficiency of different substance classes during ESI ionization, FT ICR MS is not a quantitative method for
DOM. However, the relative intensity approach permits semi quantitative statements for samples analyzed under similar conditions. Weighted average values were calculated for several parameters (m/z, DBE, N and molecular element ratios such as O/C, H/C and C/N) in each sample from the intensity (Int) of the assigned peak (x) in the spectrum using Eq. (4.2): B(O / C) x Int x ) (O / C) (4.2) wa B Int
4.2.4 Dissolved organic carbon and total dissolved nitrogen
DOC and total dissolved nitrogen (TDN) concentration in the SPE extract were analyzed by high temperature catalytic oxidation using a Shimadzu TOC/TN analyzer equipped with an infrared and a chemiluminescence detector (gas flow oxygen: 0.6 l min 1). Prior to analysis an aliquot of the solid phase extract was dried with nitrogen, gentle heated (40 °C) and re dissolved in ultrapure water. 6 ml of sample volume were acidified with 0.12 ml HCl (2 M) in the autosampler and purged with oxygen to remove inorganic carbon. Afterwards, 50 l of the sample was directly injected on the catalyst (heated to 680 °C). Detection limit (5 of the blank) was 7 M C with an accuracy of ±2 M C and final DOC concentrations were average values of triplicate measurements.
116 Diagenetic transformation of DOM
4.2.5 Total hydrolyzable dissolved amino acids
Total hydrolysable dissolved amino acids (THDAA) were analyzed using a modified method after Fitznar et al. (1999). Aliquots of the DOM methanol extract (100 l) were hydrolyzed with 2 ml of 32% HCl in glass ampoules. The ampoules were flushed with nitrogen, melted down and kept for 24 h at 110 °C. The solution was neutralized with 1.780 ml borate buffer, and the pH was adjusted to 8.5 with 32% sodium hydroxide. Samples were analyzed on a high performance liquid chromatography (HPLC) system equipped with a C18 column
(Phenomenex Hyperclone 5 , BDS C18; 250 x 4 mm; pre column: 2 x 4 mm) and a fluorescence detector. A binary gradient of sodium acetate buffer (eluent A; 25 mmol sodium acetate in ultrapure water + 20 ml acetonitrile adjusted to pH 6.00 with diluted HCl and filled up to 1 l with ultrapure water) and acetonitril (eluent B, LiChroSolv, MERCK) were used for the injection volume of 20 l. Compared to Fitznar et al. (1999) the gradient was changed according to Table 4.1.
Table 4.1. Binary gradient (eluent A: sodium acetate buffer, eluent B: acetonitrile)for HPLC separation for TDHAA.
T (min) Solvent B (%) Flow: (ml/min) 0 3 0.850 3 3 0.850 80 19 0.850 90 21 0.850 100 35 0.850 105 100 0.850 110 100 0.850 115 3 0.850
D/L ratio was calculated from alanine, arginine, aspartic acid, asparagine, glutamic acid, glutamine, histidine, methionine, serine, threonine, tryptophan, tyrosine, and valine.
THDAA include additionally glycine, L isoleucine, L leucine, D phenylalanine, L and D aminobutyric acid, and aminobutyric acid.
4.2.6 Total organic carbon and total nitrogen
The TOC content in the freeze dried homogenized sediment was analyzed using a Leco CS
200. Prior to analyses, 50 mg of each sample were treated with 12.5% HCl to remove carbonates. For TN measurements, 25 mg of the freeze dried homogenized sediment were weighted into tin boats and analyses were carried out using a Vario EL III Elemental
Analyzer.
117 Chapter 4
4.2.7 Stable organic carbon isotopes
13CTOC was measured on an Heraeus Elemental analyzer connected to a Finnigan MAT Delta
Plus. Prior to analysis, the freeze dried samples were decalcified with 12.5% HCl, homogenized and packed into tin boats. Standard deviation was determined from routine and replicate measurements of a reference sample and always below 0.1‰. Values are quoted in the 13C notation in per mil relative to the Vienna Pee Dee Belemnite (V PDB) standard.
4.3 RESULTS AND DISCUSSION
4.3.1 Characterization of sedimentary organic matter
The three sediment sampling sites differed considerably regarding the sediment type and
TOC content (Table 4.2). The upper 2 cm of sediment at the oxic sampling site (16 MUC) consisted of green to grey shelly mud with a TOC content of 1.19%. The TOC decreased with depth and the sediment consisted of pure green to grey mud below 2 cm. The low TOC/TN and enriched 13CTOC signatures in the surface sediment (16 MUC 0 cm) indicated a marine origin of the organic matter (Redfield, 1958). The sediments of the anoxic shelf site (14 MUC) and the anoxic central basin site (20 MUC) represent typical anoxic Black Sea sediments, i.e., dark grey to black mud with a fluffy organic rich layer on top and lamination formed from diatom ooze below 5 cm. The TOC content was much higher at both sites with a maximum of
2.60% (14 MUC 0 cm) and 6.16% (20 MUC 0 cm) in the surface sediment. The comparably high TOC/TN ratios of 8.50 and 8.89 as well as the low 13CTOC values of 24.1 and 25.0 in sediments from the anoxic shelf and the anoxic central basin, respectively, may indicate the preferential remineralization of labile marine organic matter and hence the accumulation of more recalcitrant, possibly terrestrial derived organic matter. The THDAA pool at the anoxic sites contains a higher proportion of amino acids from bacterial peptidoglycan (bacterial AA in Table 4.3), i.e., D alanine, D asparagine, D aspartic acid, D serine, D glutamine, and D glutamic acid (e.g., Pedersen and Thomsen, 2001; Lomstein et al., 2006) which is more recalcitrant (McCarthy et al., 1998; Grutters et al., 2002). Such recalcitrant organic matter is preferentially preserved in the anoxic sediments, whereas under oxic conditions of sampling site 16 MUC 0 cm degradation is less selective (Cowie et al., 1995; Kristensen et al., 1995;
Hulthe et al., 1998). Low L histidine (L His) and elevated glycine (Gly) concentrations in the pore waters from the oxic (16 MUC 15 cm) and the anoxic (14 MUC 15 cm) shelf reflect
118 Diagenetic transformation of DOM increased diagenesis compared to the bottom water. In the central basin of the Black Sea (20
MUC), this trend was reversed, i.e., L His was relative enriched and Gly relative depleted in the pore water compared to the bottom water, indicating a better preservation of the
THDAA pool in the turbidite layer (20 MUC 15 cm), possibly due to the rapid burial of organic matter.
Table 4.2. Water depths, sediment and organic matter properties (TOC and TN in %, 13CTOC in ‰) of the sampling locations in the northern Black Sea.
13 Sampling site Longitude (°N) Latitude (°E) Depth (m) Sediment TOC TN TOC/TN CTOC 16-MUC 0 cm 44°42'98 36°24'86 71 Green to grey shelly mud 1.19 0.21 6.46 -22.9 (oxic) 16-MUC 15 cm Grey mud 0.89 0.13 8.04 -23.5 14-MUC 0 cm Dark grey to black mud, 44°35'80 36°21'28 268 2.60 0.31 9.92 -24.1 (anoxic) fluffy layer Laminated dark grey to 14-MUC 15 cm 1.68 0.20 10.01 -24.3 black mud 20-MUC 0 cm Dark grey to black mud, 43°57'26 35°36'46 2046 6.16 0.69 10.37 -25.0 (anoxic) fluffy layer 20-MUC 15 cm Dark grey to black mud 4.72 0.46 11.85 -25.0 13-PCTD 44°33'01 36°20'00 150 (Water sample, 2l) - - - -
Table 4.3. Characteristic parameters of water column, bottom water and pore water DOM analyzed from the solid phase extract including DOCSPE and TDNSPE (in M/l), total hydrolysable dissolved amino acid (THDAA in nM/l) and their D/L ratio. Bacterial AA (in mol % THDAA) comprise D alanine, D asparagine, D aspartic acid, D glutamine, D glutamic acid and D serine. Glycine (Gly in mol % THDAA) and L histidine (L His in mol % THDAA) reflect diagenesis. Gly L-His Sampling site DOC TDN THDAA D/L bacterial AA SPE SPE THDAA (%THDAA) (%THDAA) 16-MUC 0 cm 151 102.7 298 0.21 9.2 13.5 24.7 16-MUC 15 cm 380 19.5 1541 0.26 11.4 14.7 8.1 14-MUC 0 cm 70 6.5* 156 0.31 12.3 15.9 26.3 14-MUC 15 cm 278 14.3 794 0.26 11.0 16.6 13.7 20-MUC 0 cm 122 2.4* 146 0.36 11.3 19.4 12.7 20-MUC 15 cm 263 5.3* 775 0.26 11.4 14.9 19.3 13-PCTD 68 3.0 98 0.33 11.8 19.0 14.0
) * TDNSPE close to the limit of quantitation
While TOC decreased with sediment depth, the pore water DOC concentration was a factor of 2.5 4 higher than in the bottom water (Table 4.3) reflecting remineralization of POM and accumulation of degradation products and intermediates in the pore water. The DOC and the relative TDN content in the water column sample (13 PCTD) were lower than in deep bottom water. The TDN depletion is most likely caused by the high consumption of labile and semi labile DOM (i.e., DOM with low DOC/TDN ratios) above the suboxic anoxic interface (Ducklow et al., 2007) and the subsequent relative enrichment of nitrogen depleted
DOM in the anoxic water layer below. The sediments on the other hand store larger amounts
119 Chapter 4 of organic matter providing therefore an extensive organic matter pool for the initial diagenesis, as it is also reflected in the increased number of total identified molecules (nIon,
Table 4.4).
Table 4.4. Characteristic parameters of water column, bottom water and pore water DOM derived from FT ICR MS analysis. Displayed are numbers of identified molecular formulae (nIon), the intensity weighted average values of the molecular weight ((m/z)wa), double bond equivalents (DBEwa), molar oxygen to carbon ((O/C)wa) and hydrogen to carbon ratios ((H/C)wa), the number of nitrogen atoms per molecule (Nwa) and the ratio of the total intensity of CHO formulas versus CHNO formulas((CHO/CHNO)wa).
Sampling site nion (m/z)wa DBEwa (O/C)wa (H/C)wa (N)wa (CHO/CHNO)wa 16-MUC 0 cm* 1081 379.77 7.63 0.54 1.25 0.19 6.2 16-MUC 15 cm* 1147 384.13 7.68 0.51 1.27 0.22 5.14 14-MUC 0 cm 995 368.35 7.35 0.51 1.27 0.19 6.7 14-MUC 15 cm 1346 380.71 7.87 0.51 1.24 0.25 4.7 20-MUC 0 cm 942 385.25 7.93 0.53 1.23 0.18 7.6 20-MUC 15 cm 1137 388.00 7.99 0.53 1.23 0.21 5.6 13-PCTD 564 385.52 7.73 0.51 1.26 0.06 26.97
4.3.2 The effect of early diagenesis on molecule size and molecule saturation
Up to 1346 molecular formulae were assigned in each DOM sample (Table 4.4). In general, the number of compounds increased from the bottom water to the pore water in 15 cm sediment depth corresponding to the observed DOC increase. The intensity weighted average molecular weight (m/z)wa and DBEwa decreased both from the anoxic water column in 150 m water depth (13 PCTD) to the bottom water. Similar variations were previously observed in DOM from river water compared to DOM from surficial sediment pore waters at the NW Iberian margin (Schmidt et al., 2009). The accumulation of smaller and more saturated molecules in the sediment pore water was interpreted as increased DOM production due to a higher microbial activity in the surface sediments. In the Black Sea, the bottom waters had the lowest (m/z)wa and DBEwa and both values were again relatively increased in pore waters, especially at the anoxic sampling sites. This suggests that (m/z)wa and DBEwa are linked to the microbial activity in the sediment. The impact of microbial activity on the DOM pool is expected to be the highest at the sediment water interface. The concentrations of amino acids from bacterial biomass showed indeed a decrease with depth at the anoxic shelf site (14 MUC, Table 4.3). Consistent with this trend, the D/L ratio at both anoxic sampling sites showed much higher values in the bottom water compared to the pore water (Table 4.3). High D/L ratios can be a product of high contributions from bacterial
120 Diagenetic transformation of DOM biomass or of the conversion of L amino acids to D amino acids during diagenesis (Bada et al., 1970; Kvenvolden et al., 1970). The high D/L ratio for 13 PCTD may indicate contributions from bacteria to the DOM pool which graze on sinking POM in the anoxic water column of the Black Sea (e.g., Wakeham, 1995). The larger molecule sizes in the pore water could indicate an accumulation of refractory molecules or, alternatively, they could be a product of specific reactions in the pore water such as molecule rearrangements and condensation in the course of diagenesis.
4.3.3 Variations in CHO molecules
Fig. 4.1 shows the composition of the DOM samples in van Krevelen diagrams. Each plot compares compositional variations between two samples. Van Krevelen diagrams are convenient for an overall comparison between different samples regarding their elemental
CHO composition. Molecular formulae of all pure CHO compounds are arranged in Fig. 4.1 according to their elemental oxygen to carbon (O/C) and hydrogen to carbon (H/C) ratios.
Figure 4.1. Van Krevelen diagram of CHO molecules in a) water column sample (13 PCTD) and the bottom water from the anoxic shelf (14 MUC 0 cm), b) oxic bottom water (16 MUC 0 cm) and anoxic bottom water from the shelf (14 MUC 0 cm), and c) oxic bottom water (16 MUC 0 cm) and pore water from the same location (16 MUC 15 cm). Grey dashes indicate CHO formulas present in each of the two compared samples.
The first plot (Fig. 4.1a) is a comparison of the water column and bottom water of the anoxic shelf. The number of identified compounds is much larger in the bottom water (see also
Table 4.4) and the compounds are distributed over a wider O/C and H/C range. The water column DOM lacks compounds with either high or low H/C and O/C ratios, but showed several exclusive compounds with moderate O/C and H/C ratios at high molecule masses
(m/z = 526 – 598) and low relative intensities (I = 4 – 7%). In comparison, the oxic and the
121 Chapter 4 anoxic bottom water samples share more compounds with similar molecular formulae (Fig.
4.1b). However, the oxic bottom water is characterized by exclusive formulae with high O/C ratios, most of them in the O/C range of 0.35 to 0.90, but with low relative intensities (I = 4 –
9%), whereas the majority of exclusive formulae in the anoxic bottom water DOM range at
O/C ratios of 0.10 to 0.60 with a wide range of relative intensities up to 99%. This pattern reflects most likely variations in the degradation mechanisms and in the source material under the contrasting redox conditions. The higher oxidation rate for POM in the oxic surface sediment (e.g., Hulthe et al., 1998; Komada et al., 2004) produces more strongly oxidized pore water DOM with low relative intensities due to a rapid DOM turnover. Under the oxygen limited conditions of sampling site 14 MUC, the sedimentary organic matter consists of more recalcitrant compounds with lower oxygen content as also indicated by
13CTOC, TOC/TN and bacterial amino acids which is reflected in the DOM. The increase in the number of molecular formulae in the pore water resulted probably from the accumulation of refractory DOM with sediment depth and progressive diagenesis and the comparably lower O/C ratios of these compounds in the pore water reflect then the increasing recalcitrance of the source material.
A cluster analysis revealed a close relation of the bottom water from the central basin (20
MUC 0 cm) and from the oxic shelf (16 MUC 0 cm) (Fig. 4.2a). The similarity based on a high number of common CHO compounds in both samples (see also (O/C)wa and (H/C)wa in Table
4.4). This is surprising, since the predominating degradation reactions and the source material at both sites differ considerably (Table 4.2).
Figure 4.2. Cluster analysis (Bray Curtis Similarity) based on a) all CHO molecules and b) all nitrogen bearing molecules. 122 Diagenetic transformation of DOM
4.3.4 Variations of N bearing compounds in DOM
We identified up to 352 and 75 compounds with one two nitrogen atoms, respectively (14
MUC 15 cm). Compounds with three or four nitrogen atoms were not detected in the FT
ICR MS data set, probably due to their low abundance (see also Koch et al., 2007). The N bearing compounds were distributed over a mass range from 250 to 550 Da and were higher abundant in the pore waters at all sampling sites corresponding to higher values of nIon,
DOC, THDAA and their weighted average molecular weight (N(m/z)wa). N bearing compounds were least abundant in the water column (13 PCTD) with only 60 compounds with N = 1 and none detected with N = 2 (Fig. 4.3). Generally, N bearing compounds with high relative intensities (I = 10 – 24%) were present in all bottom and pore water samples regardless of the prevailing redox conditions. Interestingly, the molecule with the highest relative intensity is in all samples, except for the water column, C15H19N1O9.
Figure 4.3. Molecular mass distribution of nitrogen bearing compounds in all samples. Black bar indicates compounds with N = 1, grey bar refers to compounds with N = 2, and red dashed arrows indicate the maximum molecule size of di and tripeptides, respectively.
Compositional differences between individual samples are evident among molecules with relative intensities below 10%. The comparison of the water column DOM (13 PCTD) and the bottom water DOM from the anoxic shelf (14 MUC 0 cm) (Fig. 4.4a) reflect not only the strong increase in the number of molecules but also a wider H/C and O/C range in the 123 Chapter 4 bottom water DOM as reported above for the pure CHO compounds. Most of the N bearing molecular formulae in the anoxic bottom water ranged between O/C = 0.35 – 0.80 and H/C =
0.90 – 1.70, whereas the water column DOM is restricted to O/C = 0.44 – 0.69 and H/C = 1.06 –
1.53. The bottom water DOM from the anoxic and the oxic shelf (Fig. 4.4b) as well as the bottom water and the pore water at each sampling site (Fig. 4.4c) share a high number of molecular formulae in the range of O/C = 0.40 – 0.90 and H/C = 0.39 – 1.65. Similar to the pure CHO compounds, the oxic bottom water contained exclusively a set of N bearing compounds with molecular formulae at higher O/C ratios (O/C = 0.40 – 0.90), whereas a set of molecular formulae with lower O/C ratios (O/C = 0.15 – 0.75) was exclusively present in anoxic bottom water and pore water with the highest diversity in the latter.
Figure 4.4. Sample comparison of nitrogen bearing compounds in van Krevelen diagrams for a) water column sample (13 PCTD) and the bottom water from the anoxic shelf (14 MUC 0 cm), b) oxic bottom water (16 MUC 0 cm) and anoxic bottom water from the shelf (14 MUC 0 cm), and c) oxic bottom water (16 MUC 0 cm) and pore water from the same location (16 MUC 15 cm). Grey dashes indicate compounds with one nitrogen common in both samples, black dashes assign compounds with two nitrogen common in both samples. Filled symbols/cross refer to compounds with one nitrogen in individual samples, open symbols indicate compounds with two nitrogen atoms in individual samples.
4.3.5 N bearing compounds as products of protein degradation
Most of the nitrogen in biomass is bound in proteins, whereas amino sugars (e.g., in chitin), nucleic acids in DNA and RNA and tetrapyrroles in pigments form a minor fraction
(Stankiewicz and Van Bergen, 1998). DON in the water column predominantly occurs in the amide form (McCarthy et al., 1997) with a strong relative increase in protein derived amides from the surface to the deep water (Aluwihare et al., 2005). Although the contribution of amide N to the DOM pool in sediment pore waters has not yet been exactly determined, there are clear indications that a fraction of the DOM derives from protein degradation.
124 Diagenetic transformation of DOM
Lomstein and co workers (1998), for example, were able to identify 40% of the DON
(corresponding to 10% of the DOC) in coastal sediment pore waters as dissolved free and combined amino acids, probably products of protein degradation. In the DOM extracts from the Black Sea, the increase in THDAA from the bottom to the pore water corresponded to the increase in N bearing compounds in the FT ICR MS data.
Theoretically, the N = 1 compounds of up to 343 Da and the N = 2 compounds of up to 507
Da could represent protein derived di and tripeptides (the masses correspond to peptides formed from two and three tyrosine molecules, respectively). Protein degradation starts rapidly after cell death, initially and most importantly by hydrolysis of the peptide bonds, which successively breaks down the macromolecular protein structure into smaller oligopeptides, dipeptides and amino acids (e.g., Hollibaugh and Azam, 1983; Mayer and
Rice, 1992; Pantoja et al., 1997). In addition to abiotic mechanisms, protein hydrolysis proceeds via enzymatic reaction, performed intracellularly by macroorganisms or extracellularly by bacteria that are not able to transport molecules larger than 600 Da across their cell membranes (Nikaido and Vaara, 1985). The extracellular hydrolysis results in an accumulation of dipeptides in DOM due to slower hydrolysis compared to peptides with more than two amino acids, possibly resulting from a lack of specific exoenzymes (Pantoja and Lee, 1999). The accumulation of dipeptides in DOM could be an explanation why only molecules with one or two nitrogen atoms in the mass window of 200 – 600 Da were detected. In the van Krevelen diagram, proteins, peptides, and amino acids would plot in the upper left corner (Fig. 4.4b). Molecular formulae in the O/C and H/C range of proteins were previously detected by FT ICR MS in DOM from protozoa cultures (Kujawinski et al., 2004) and in pore water in continental slope sediment (Schmidt et al., 2009); in both studies proteins have been suggested as sources for such compounds. However, the samples from the Black Sea showed only a small number of molecular formulae with H/C and O/C ratios similar to proteins (Fig 4.4).
Assuming that the N bearing compounds detected by FT ICR MS are actually protein degradation products, we would need to conclude that processes in addition to hydrolysis alter the proteins structurally and in their elemental composition. Additional processes related to protein, peptide, and amino acid degradation involve deamination and decarboxylation (Gale, 1940; Morris and Fillingame, 1974). The identified molecular formulae in our samples could derive from peptide deamination, which would also explain the lack of
125 Chapter 4 molecular formulae with more than two nitrogen atoms. Under the oxic conditions of sampling site 16 MUC, the predominating deamination process is probably microbially catalyzed oxidative deamination. During this reaction, the terminal amino group is oxidized to form a carbonyl group as illustrated in Eq. (4.3) for glutamic acid.
COOH CHNH2 (CH2)2 COOH + NADP+ + H2O
COOH CHO (CH2)2 COOH + NADPH + H+ + NH3 (4.3)
This reaction results in a shift to higher O/C and lower H/C ratios (Fig. 4.4b). Exclusive N bearing compounds in the bottom water from the oxic shelf site could be formed via the reaction in Fig. 4.5a. Two amino groups are substituted by oxygen in the peptide formed by valine, glutamic acid and asparagine, yielding the molecular formula C14H18N2O9. Oxidative deamination also is not restricted to oxic environments; it has been reported for methanogens, sulfate reducers and acetogens (Stams and Hansen, 1984; Örlygsson et al.,
1994). The presence of oxidative deamination could explain the relatively high similarity of
N bearing compounds from oxic and anoxic samples in the cluster analysis (Fig. 4.2b).
Reductive deamination, on the other hand, is probably more important under the anoxic conditions of the sampling locations 14 MUC and 20 MUC. This process lowers the H/C ratios of the altered peptide (Fig. 4.4b) due to the loss of two hydrogen atoms. Several molecular formulae with rather low O/C ratios were exclusively found in the anoxic samples; we suggest that these compounds are the products of reductive deamination of peptides as it is demonstrated in Fig. 4.5b.
Another possible deamination reaction performed by anaerobic bacteria (e.g., Clostridia) is a coupled oxidation and reduction of amino acids in the Stickland reaction (Seto, 1980). During this reaction, the electron is intramolecularly transferred from the oxidized to the reduced amino group site. Fig. 4.5c shows an example how the molecular formula C18H22N2O8, exclusively found in the anoxic samples, could be formed via Stickland type reaction by reductive deamination of tyrosine and oxidative deamination of glutamine.
126 Diagenetic transformation of DOM
Figure 4.5. Possible precursor peptides and deamination reactions for exemplary sum formulas from the FT ICR MS data set: a) oxidative deamination of a peptide built from valine (Val), glutamic acid (Glu) and asparagine (Asn) result in the substitution of two ammonium groups by oxygen (yellow circle) and in the sum formula C14H18N2O9 which was exclusively found in the oxic bottom water (16 MUC 0 cm), b) reductive deamination substitutes the ammonium group by hydrogen in the peptide built from tyrosine (Tyr) and threonine (Thr), and c) in the Stickland reaction of the peptide build from Tyr, glutamine (Gln) and Thr, Tyr is deaminated reductively, whereas Gln is deaminated oxidatively. Sum formulae of the products from reaction b) and c) were only detected in the anoxic bottom waters and the pore water samples.
4.3.6 Other sources for N bearing compounds in DOM
The fact that a significant fraction of the compounds with a single N atom exceed the mass of potential dipeptides (Fig. 4.3; maximum molecular weight of dipeptide Tyr Tyr is 343 Da) indicates that alternative sources and processes need to be considered. A possibility is the formation through reactions of ammonium with reactive functional groups of N free DOM in the pore water. Such reactions followed by cyclyzation were proposed to start already during early diagenesis at the sediment water interface and could explain the change In structure and functionality from the amide N predominating in living biomass and water column DON to heterocyclic nitrogen preserved in coal and petroleum (Patience et al., 1992;
Vairavamurthy and Wang, 2002). The exact mechanisms of the rearrangement reactions
127 Chapter 4 remain elusive, however, Burdige (2002) suggested that the reactions occur via DON intermediates. Generally, the molecular formulae in all samples had relatively high oxygen contents ((O/C)wa, Table 4.4). A fraction of this oxygen must be bound in carboxyl and hydroxyl groups which eventually react with abundant ammonium, typically observed in anoxic sediments (e.g., Klump and Martens, 1987). The new appearance of a high number of
N bearing compounds with low O/C ratios in the sediment pore water (Fig. 4.4c) could intermediates of the rearrangement reactions which shift the molecular composition to higher H/C and lower O/C ratios through the exchange of oxygen by two hydrogen and one nitrogen atom (Eq. (4.4)).
C21H31O11 COOH + NH3 C22H31O11 CONH2 + H2O (4.4)
4.4 SUMMARY AND CONCLUSIONS
The molecular composition of polymeric LMW DOM from contrasting redox conditions in the water column, bottom waters and sediment pore waters of the Black Sea were analyzed with FT ICR MS. Variations in the molecular composition reflected different remineralization processes associated with the prevailing environmental conditions:
DOM in the anoxic water column of the Black Sea is recalcitrant, has a low nitrogen
content and a less diverse molecular composition compared to bottom water and
sediment pore water; NIon and the number of exclusive molecular formulae increase
successively in the order water column – bottom water – pore water.
At the oxic shelf, TOC is lower than in anoxic sediments and TOC/TN and 13CTOC
indicate marine algae as main organic matter source. Exclusive molecular formulae
with high O/C ratios reflect the increased oxidation of POM at the sediment water
interface. N bearing compounds with one or two nitrogen atoms are present in all
samples and increase in number with sediment depth corresponding to the increase
in THDAA. The majority of N bearing compounds in the oxic bottom water probably
derive from protein degradation via oxidative deamination of peptides.
The anoxic conditions at sampling site 14 MUC on the shelf and 20 MUC in the
central basin of the Black Sea resulted in high TOC concentrations in the sediment.
TOC/TN and 13CTOC suggest preferential degradation of labile algal material under
anoxic conditions and an accumulation and preservation of more refractory organic
matter, i.e., terrestrial organic matter and peptidoglycan from bacterial cell walls, the
128 Diagenetic transformation of DOM
latter is indicated by elevated D/L ratio and bacterial derived amino acids. The anoxic
conditions are reflected in a set of exclusively present molecular formulae at low O/C
ratios. Such compounds are either formed by hydrolysis of relatively reduced POM
or by reduction of high O/C DOM. The diversity of N bearing compounds suggests
that protein and peptide degradation is controlled by oxidative, reductive, and
coupled oxidative and reductive deamination by anaerobic microbes.
Furthermore, the average molecule size (m/z)wa and DBEwa appear to be closely linked to
microbial activity at the sediment water interface mediating the DOM release from the
sediment POM. Both values are low in the bottom water DOM at all sampling sites
corresponding to increased concentrations of bacterial amino acids in the DOM extracts.
In general, elevated numbers of compounds in the pore water DOM correspond to higher
DOC contents and larger numbers of exclusive N bearing and CHO compounds with
low O/C ratios. Both compound groups are suggestive of the accumulation of recalcitrant
DOM. Furthermore, a fraction of the N bearing compounds is possibly a product of
reactions from ammonium with functional groups in the DOM.
Acknowledgements
We thank the crew, Christian Borowski and the scientific shipboard party of the RV Meteor cruise M72 5. We are grateful to Brit Kockisch for TOC measurements, Hella Buschoff for TN and TC analysis, Monika Segl for 13CTOC, and Kai Uwe Ludwichowski for amino acid measurements. Furthermore, we thank Xavier Prieto and Kevin Becker for support in the lab.
Funding was provided by the “Deutsche Forschungsgemeinschaft” through DFG Research
Center / Excellence Cluster “The Ocean in the Earth System” and the Bremen International
Graduate School for Marine Sciences (GLOMAR).
129 Chapter 4
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133 134
Chapter 5
Concluding remarks and perspectives
Chapter 5
Mechanisms of OM preservation and remineralization are highly complex, particularly at continental margins, where OM sources and sedimentation processes vary significantly. The performed studies in this thesis provided molecular insights into the composition of OM that aimed for a better understanding of OM distribution and early diagenetic changes at continental margins. The complex interplay of hydrodynamic, sedimentological and biogeochemical processes required an extended scope of investigation which was realized by the combination of well established geochemical analyses (bulk TOC characterization, lipid biomarkers, lignin phenols, amino acids) with the novel technique of FT ICR MS. The latter targeted the complex molecular information encoded in the dissolved pool of OM.
In the following sections I will summarize the major findings of this thesis and elucidate their implications for future investigations on OM composition, distribution and diagenesis.
Lipid biomarkers and lignin phenols as indicators for sources, transport and distribution of OM at continental margins
OM in the sediments at the Galicia Minho shelf derives from various terrestrial and marine sources which are reflected in a rich lipid biomarker inventory. The comprehensive biomarker data set (see Table A3 for the complete data set and Table A4 for the internal relationship of the biomarkers in the electronic appendix) provides important insights into the applicability of lipid biomarkers and lignin phenols as indicators for OM sources, transport, and deposition. The multiple sources and transport mechanisms at the NW Iberian margin needed an analytical approach that considered each individual factor. The application of single lipid biomarkers or a bulk TOC characterization is not sufficient for constraining all processes in a complex continental margin setting. The approach in this thesis enabled a classification of the NW Iberian margin into three provinces with different
OM types and input pathways. This result is the key for the understanding of the effects of hydrodynamic and sedimentological processes on OM preservation. Moreover, the study in
Chapter 2 provides a solid base for reconstructing the paleoenvironmental conditions of the
NW Iberian margin.
Multiple algal blooms, fuelled by the intrusion of waters from the NW Iberian upwelling cell in the North and nutrient supply from the Iberian Peninsula, contribute to the marine OM and are reflected in high concentrations of sterols and SCFAs. The distributional pattern of most sterols and fatty acids is similar and contain no specific information on algal groups or
136 Concluding remarks and perspectives
environmental conditions. In contrast, HBIs, dinosterol and C37 alkenones provide important insights into the seasonal and spatial variations in the algal community of the three major marine producers (diatoms, dinoflagellates and haptophyte algae). The distribution of their biomass in the sediment is closely related to the nutrient conditions on the shelf. By transferring the spatial pattern of the algal derived OM to the past, valuable insights might be gained regarding the development of the NW Iberian upwelling cell and the oceanographic current system.
The modern continental shelf system evolved after the last glacial maximum due to the global sea level rise (Dias, 2000) implying major changes in the currents, climate, and vegetation that are most likely preserved in the composition and distribution of OM.
Terrestrial lipid biomarkers and lignin phenols in the sedimentary OM preserve information on the source material and the transport mechanism. Lignin phenols were in general very sensitive for the identification of terrestrial OM. Due to the association of individual lignin phenol groups with different types of plant tissue they can ideally serve to differentiate between angiosperm (high S/V ratios) and gymnosperm vegetation (low S/V ratios), and between woody (low C/V ratios) and non woody plant tissue (high C/V ratios) (Hedges and
Mann, 1979). For paleoenvironmental reconstructions, these ratios may reveal insights in the post glacial vegetation changes of the Iberian Peninsula. Furthermore, variations in the distribution and concentration of terrestrial OM may reflect changes in the response of river run off due to climate warming and post glacial sea level rise.
The comparison of sedimentary lipid biomarker and lignin phenol distribution with sediment grain sizes revealed different transport mechanisms within the terrestrial OM fraction; i.e., long chain n alkanes are supplied by rivers and wind, whereas lignin phenols and plant derived triterpenoids are very well suited to identify riverine input (Chapter 2).
The river supplied terrestrial OM is associated with the inorganic river load and distributed with the fine inorganic sediment fraction on the shelf (see Table A2 in the appendix).
Therefore, the conceptual shelf model by Dias et al. (2002) (Fig. 1.4.b, Chapter 1) can be adopted to explain the fate of this OM fraction at the NW Iberian margin.
Hydrodynamic sorting is in general an important process controlling sediment partitioning on continental shelves (e.g., Keil et al., 1994; Gordon and Goñi, 2004; Waterson and Canuel,
2008) and is responsible for the internal separation of the terrestrial OM at the NW Iberian margin. Lignin phenol analyses proved to be very suited for tracing sorting processes due to
137 Chapter 5 the association of individual lignin phenols with different types of plant tissue. Leaf and grass fragments (elevated C/V ratios) are preferentially transported and deposited offshore, whereas woody material (low C/V ratios and plant derived triterpenoids) is accumulated in the Galician mudbelt.
Soil derived OM accounts for an important proportion of the terrestrial OM delivered to the oceans (Hedges et al., 1986; Prahl et al., 1994; Goñi et al., 1998). However, studies on OM distribution in marine sediments often underestimate soil OM due to the lack of appropriate soil indicators resulting from a strong decrease in recognizable compounds during degradation processes and humification on the continents. Previously, the ratio 3,5 Bd/V has been applied for tracing soil derived OM in marine sediments (Prahl et al., 1994; Louchouarn et al., 1999; Goñi et al., 2000). At the NW Iberian margin, the 3,5 Bd/V ratio indicates a deposition of soil derived OM offshore, in areas with marine bulk TOC signatures. Such masking of soil derived OM on a global base can be a reason why a large part of the terrestrial OM in the global river load remains undetected in marine sediments (see Chapter
1.6 and Gordon and Goñi, 2003).
Molecular characterization of DOM by FT ICR MS and its implications for an understanding of early diagenesis on a molecular compound level
The production of DOM in sediments is closely coupled to the overall remineralization of sedimentary OM and pore water DOM reflects the molecular signature of the biogeochemical processes during early diagenesis (Henrichs, 1992; Alperin et al., 1994). At present, pore water DOM is not well characterized on a molecular level due to the analytical challenge of resolving such complex organic mixtures. The application of ESI FT ICR MS for the characterization of DOM provides information on the composition of thousands of individual compounds in the polar fraction of DOM and their reactivity within biogeochemical processes (Kim et al., 2003; Stenson et al., 2003). FT ICR MS has been successfully applied for the characterization of DOM from limnic, marine, and coastal environments (e.g., Kim et al., 2004; Koch et al., 2005; Tremblay et al., 2007; Sleighter and
Hatcher, 2008). Variations in the detected masses between different samples can be used to identify the molecular level impact of different biogeochemical processes, e.g., microbial modifications (Kujawinski et al., 2004; Kim et al., 2006) or photochemical transformations of
DOM (Kujawinski et al., 2004; Tremblay et al., 2007).
138 Concluding remarks and perspectives
In this thesis, FT ICR MS was for the first time applied for the analysis of marine pore water
DOM (Chapter 3). The approach gave new and important insights into the relationship between the particulate and dissolved fraction of OM in continental margin sediments, thus specifically investigating early diagenetic processes. Since no structural information and source indications are provided by DOM analyses using FT ICR MS the comparison with data from the associated particulate OM fraction (e.g., bulk TOC, lipid biomarkers, and lignin phenols) greatly helped to explain the observed patterns of molecular formulae and provided evidence for the close relationship between both pools. For example, the source material for DOM is mirrored in the elemental composition; sediment OM containing large amounts of terrestrial OM produces DOM with high elemental oxygen to carbon (O/C) and low hydrogen to carbon (H/C) ratios, i.e., higher oxidized molecules with a higher aromaticity due to contributions from lignins, cellulose etc.. In contrast, hydrolysis of aliphatic algal derived OM is reflected in molecular formulae with lower O/C and higher
H/C ratios. The lower reactivity of refractory OM (e.g., OM deposited on the outer shelf,
Chapter 2) results in lower numbers of identified DOM molecules and a much smaller variation in their elemental ratios (O/C and H/C) compared to the fresh OM deposited in the
Galician mudbelt. Sulfur bearing compounds in the pore water DOM, if coinciding with elevated concentrations of specific sedimentary biomarkers for sulfate reducing bacteria (i.e., i and ai C15 and C17 fatty acids, and C14 to C19 monoalkyl glycerolethers), are very likely related to early diagenetic sulfurization of OM.
The redox conditions of the water column and sediment are a key factor during early diagenesis (Wakeham, 1990; Cowie et al., 1995). In order to evaluate this effect on the DOM pool a dedicated study was performed under varying oxygen contents in the Black Sea
(Chapter 4). The overall molecular DOM composition directly corresponded to the redox conditions with a more oxidized DOM pool (high number of molecular formulae with elevated O/C ratios) at the oxic sampling site and a chemically more reduced DOM pool in the anoxic environment (molecular formulae exclusively at low O/C ratios). A specific set of nitrogen bearing DOM compounds was observed and analyses of the THDAA pool from the same extract gave indications for the presence of protein degradation products in the pore water. The variations of the molecular formulae which possibly result from protein degradation depended on the oxygen content of the samples suggesting distinct reaction pathways. Oxidative deamination appears to predominate under oxic conditions, whereas
139 Chapter 5 this process is partly substituted by reductive deamination as sole reaction or coupled with the oxidative pathway in form of the Stickland reaction (Seto, 1980) at lower oxygen contents. However, an alternative origin of the nitrogen bearing DOM compounds has to be considered since the reaction between pore water ammonium and functional groups in the
DOM pool (e.g., carboxyl and hydroxyl groups) can lead to a similar set of compounds. It has been proposed that such compounds act as intermediates in the transformation reaction from biological nitrogen bearing molecules, e.g., proteins, to heterocyclic nitrogen preserved in coals and petroleum via ammonification and cyclyzation reactions (e.g., Patience et al.,
1992). However, it is still an open question whether this diagenetic pathway exists in nature which needs to be considered in future research. Investigations are envisioned combining structural analyses either by isolation of fractions or even single compounds as well as fractionation experiments in the ICR cell. Structural analyses can be also strengthened by focussing on functional groups via nuclear magnetic resonance (NMR) spectroscopy. Such combinations will be the ultimate step for a successful data interpretation and will guide to a full explanation of the observed patterns in the FT ICR MS data.
The studies in this thesis provided new and detailed insights on molecular reactions during early diagenesis which were not detected on that level before due to analytical limitations.
The technique of ESI FT ICR MS opens new perspectives for investigations on OM remineralization in various environments (soils, lake sediments, seeps, deep biosphere, etc.).
Controlled studies under defined conditions and defined microbial cultures would offer further insights into the diagenetic processes and may provide molecular fingerprints for specific organisms or reactions.
140 Concluding remarks and perspectives
REFERENCES (Dias, 2000)
Alperin, M. J., Albert, D. B., and Martens, C. S., 1994. Seasonal variations in production and consumption rates of dissolved organic carbon in an organic rich coastal sediment. Geochimica et Cosmochimica Acta 58, 4909 4930. Cowie, G. L., Hedges, J. I., Prahl, F. G., and de Lance, G. J., 1995. Elemental and major biochemical changes across an oxidation front in a relict turbidite: An oxygen effect. Geochimica et Cosmochimica Acta 59, 33 46. Dias, J. A., Boski, T., Rodrigues, A., Magalhaes, F., 2000. Coast line evolution in Portugal since the Last Glacial Maximum until present a synthesis. Marine Geology 170, 177 186. Goñi, M. A., Ruttenberg, K. C., and Eglinton, T. I., 1998. A reassessment of the sources and importance of land derived organic matter in surface sediments from the Gulf of Mexico. Geochimica et Cosmochimica Acta 62, 3055 3075. Goñi, M. A., Yunker, M. B., Macdonald, R. W., and Eglinton, T. I., 2000. Distribution and sources of organic biomarkers in arctic sediments from the Mackenzie River and Beaufort Shelf. Marine Chemistry 71, 23 51. Gordon, E. S. and Goñi, M. A., 2003. Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico. Geochimica et Cosmochimica Acta 67, 2359 2375. Gordon, E. S. and Goñi, M. A., 2004. Controls on the distribution and accumulation of terrigenous organic matter in sediments from the Mississippi and Atchafalaya river margin. Marine Chemistry 92, 331 352. Hedges, J. I., Clark, W. A., Quay, P. D., Richey, J. E., and Devol, A. H., 1986. Compositions and fluxes of particulate organic material in the Amazon River. Limnology and Oceanography 31, 717 738. Hedges, J. I. and Mann, D. C., 1979. The characterization of plant tissues by their lignin oxidation products. Geochimica et Cosmochimica Acta 43, 1803 1807. Henrichs, S. M., 1992. Early diagenesis of organic matter in marine sediments: progress and perplexity. Marine Chemistry 39, 119 149. Keil, R. G., Tsamakis, E., Fuh, C. B., Giddings, J. C., and Hedges, J. I., 1994. Mineralogical and textural controls on the organic composition of coastal marine sediments: Hydrodynamic separation using SPLITT fractionation. Geochimica et Cosmochimica Acta 58, 879 893. Kim, S., Kaplan, L. A., Benner, R., and Hatcher, P. G., 2004. Hydrogen deficient molecules in natural riverine water samples evidence for the existence of black carbon in DOM. Marine Chemistry 92, 225 234. Kim, S., Kaplan, L. A., and Hatcher, P. G., 2006. Biodegradable dissolved organic matter in a temperate and a tropical stream determined from ultra high resolution mass spectrometry. Limnology and Oceanography 51, 1054 1063. Kim, S., Kramer, R. W., and Hatcher, P. G., 2003. Graphical Method for Analysis of Ultrahigh Resolution Broadband Mass Spectra of Natural Organic Matter, the Van Krevelen Diagram. Analytical Chemistry 75, 5336 5344. Koch, B. P., Harder, J., Lara, R. J., and Kattner, G., 2005. The effect of selective microbial degradation on the composition of mangrove derived pentacyclic triterpenols in surface sediments. Organic Geochemistry 36, 273 285. Kujawinski, E. B., Del Vecchio, R., Blough, N. V., Klein, G. C., and Marshall, A. G., 2004. Probing molecular level transformations of dissolved organic matter: insights on
141 Chapter 5
photochemical degradation and protozoan modification of DOM from electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Marine Chemistry 92, 23 37. Louchouarn, P., Lucotte, M., and Farella, N., 1999. Historical and geographical variations of sources and transport of terrigenous organic matter within a large scale coastal environment. Organic Geochemistry 30, 675 699. Patience, R. L., Baxby, M., Bartle, K. D., Perry, D. L., Rees, A. G. W., and Rowland, S. J., 1992. The functionality of organic nitrogen in some recent sediments from the Peru upwelling region. Organic Geochemistry 18, 161 169. Prahl, F. G., Ertel, J. R., Goni, M. A., Sparrow, M. A., and Eversmeyer, B., 1994. Terrestrial organic carbon contributions to sediments on the Washington margin. Geochimica et Cosmochimica Acta 58, 3035 3048. Seto, B., 1980. The stickland reaction. In: Knowles, C. J. (Ed.), Diversity of bacterial respiratory systems. Sleighter, R. L. and Hatcher, P. G., 2008. Molecular characterization of dissolved organic matter (DOM) along a river to ocean transect of the lower Chesapeake Bay by ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Marine Chemistry 110, 140 152. Stenson, A. C., Marshall, A. G., and Cooper, W. T., 2003. Exact Masses and Chemical Formulas of Individual Suwannee River Fulvic Acids from Ultrahigh Resolution Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectra. Analytical Chemistry 75, 1275 1284. Tremblay, L. B., Dittmar, T., Marshall, A. G., Cooper, W. J., and Cooper, W. T., 2007. Molecular characterization of dissolved organic matter in a North Brazilian mangrove porewater and mangrove fringed estuaries by ultrahigh resolution Fourier Transform Ion Cyclotron Resonance mass spectrometry and excitation/emission spectroscopy. Marine Chemistry 105, 15 29. Wakeham, S. G., 1990. Algal and bacterial hydrocarbons in particulate matter and interfacial sediment of the Cariaco Trench. Geochimica et Cosmochimica Acta 54, 1325 1336. Waterson, E. J. and Canuel, E. A., 2008. Sources of sedimentary organic matter in the Mississippi River and adjacent Gulf of Mexico as revealed by lipid biomarker and 13CTOC analyses. Organic Geochemistry 39, 422 439.
142
Appendix A1
Molecular level studies on the distribution and fate of organic matter at continental margins. Dissertation am Fachbereich Geowissenschaften der Universität Bremen, Mai 2009 Frauke Schmidt Appendix A1
ACETOGENESIS IN DEEP SUBSEAFLOOR SEDIMENTS OF THE
JUAN DE FUCA RIDGE FLANK: A SYNTHESIS OF GEOCHEMICAL,
THERMODYNAMIC, AND GENE BASED EVIDENCE.
Mark A. Lever*, Verena Heuer‡, Yuki Morono§, Noriaki Masui§, Frauke Schmidt‡, Marc J.
Alperin*, Fumio Inagaki§, Kai Uwe Hinrichs‡¶, and Andreas Teske*
Accepted for publication pending minor revisions in Geomicrobiology Journal
*Department of Marine Sciences, 340 Chapman Hall CB #3300, University of North Carolina at
Chapel Hill, Chapel Hill, NC 27599, USA
‡Organic Geochemistry Group, Department of Geosciences & MARUM Center for Marine
Environmental Sciences, University of Bremen, D 28334 Bremen, Germany
§Kochi Institute for Core Sample Research, Japan Agency for Marine Earth Science and
Technology, Kochi, Japan
144 Appendix A1
ABSTRACT
In deep subsurface sediments of the Juan de Fuca Ridge Flank, porewater acetate that is depleted in 13C relative to sedimentary organic matter indicates an acetogenic component to total acetate production. Thermodynamic calculations indicate common fermentation products or lignin monomers as potential substrates for acetogenesis. Autotrophic acetogenesis may contribute as well provided that dihydrogen (H2) concentrations are not drawn down to the thermodynamic thresholds of the competing processes of sulfate reduction and methanogenesis.
A high diversity of novel formyl tetrahydrofolate synthetase (fhs) genes throughout the upper half of the sediment column indicates the genetic potential for acetogenesis. Our results suggest that a substantial fraction of the acetate produced in marine sediment porewaters may derive from acetogenesis, in addition to the conventionally invoked sources fermentation and sulfate reduction.
145 146
Appendix A2
Molecular level studies on the distribution and fate of organic matter at continental margins. Dissertation am Fachbereich Geowissenschaften der Universität Bremen, Mai 2009 Frauke Schmidt Appendix A2
Table A2. Correlation analysis of sample parameters, individual or groups of biomarker and biomarker ratios (for abbreviation see Chapter 2). Significant correlation in grey for level of confidence = 0.01. 13 Latitude Longitude Water TOC [%] TIC [%] CTOC TOC/TN Silt Mean grain n-C25-35 n-C36-40 C37:2 (N) (W) depth (m) (‰) [%] size [μm] alkene Latitude (N)
Longitude (W) -0,809 Water depth (m) 0,522 -0,802 TOC [%] -0,145 0,263 -0,322 TIC [%] 0,040 -0,255 0,057 0,029 13C (‰) TOC 0,312 -0,455 0,506 -0,835 0,077 TOC/TN -0,347 0,348 -0,272 0,817 -0,094 -0,757 Silt [%] 0,016 0,111 -0,187 0,953 -0,009 -0,743 0,717 Mean grain size [μm] -0,007 -0,077 0,096 -0,918 -0,093 0,691 -0,719 -0,935 LCalk -0,172 0,289 -0,395 0,732 -0,065 -0,568 0,569 0,716 -0,625 n-C36-40 0,281 -0,283 0,120 0,397 0,055 -0,085 0,091 0,515 -0,446 0,683 C alkene 37:2 -0,308 0,267 -0,335 0,076 -0,025 -0,089 0,072 0,028 -0,026 0,523 0,323 HBIs 0,166 0,157 -0,407 0,348 -0,055 -0,268 0,149 0,420 -0,286 0,656 0,523 0,378 Alkenone -0,156 0,189 -0,205 0,097 -0,060 -0,031 0,092 0,051 -0,060 0,419 0,327 0,591 SCOH -0,141 0,334 -0,349 0,658 -0,269 -0,461 0,418 0,694 -0,599 0,610 0,403 0,172 LCOH -0,267 0,397 -0,490 0,565 -0,037 -0,514 0,384 0,513 -0,426 0,728 0,498 0,623 Phytol 0,185 -0,356 0,213 0,149 0,092 -0,018 0,062 0,187 -0,268 0,266 0,443 0,287 Sterols -0,419 0,567 -0,650 0,392 -0,105 -0,432 0,294 0,322 -0,253 0,582 0,212 0,627 Dinosterol -0,298 0,418 -0,489 0,337 -0,109 -0,365 0,169 0,304 -0,153 0,506 0,313 0,547 24-Methylene- cholestadienol -0,289 0,475 -0,492 0,347 -0,099 -0,345 0,136 0,320 -0,245 0,393 0,200 0,377 Diols -0,314 0,513 -0,573 0,223 -0,136 -0,295 0,014 0,203 -0,105 0,418 0,209 0,547 Plant-derived Triterpenoids -0,367 0,504 -0,561 0,453 -0,161 -0,487 0,277 0,406 -0,278 0,607 0,346 0,572 Friedelin -0,301 0,479 -0,503 0,514 -0,208 -0,627 0,461 0,474 -0,404 0,494 0,059 0,302 LCFA -0,349 0,496 -0,557 0,421 -0,105 -0,434 0,312 0,332 -0,254 0,548 0,208 0,473 SCFA -0,524 0,654 -0,747 0,108 -0,095 -0,239 0,099 -0,013 0,035 0,223 -0,079 0,437 sat. SCFA -0,497 0,636 -0,693 0,066 -0,193 -0,196 0,078 -0,039 0,068 0,174 -0,096 0,386 MUFA -0,515 0,657 -0,768 0,160 0,010 -0,267 0,113 0,035 -0,028 0,244 -0,056 0,435 PUFA -0,449 0,575 -0,620 0,293 -0,017 -0,315 0,219 0,196 -0,115 0,451 0,184 0,507 C -FA 18 -0,277 0,419 -0,574 0,480 0,107 -0,475 0,342 0,397 -0,396 0,395 0,064 0,204 C -FA 18:1 9 -0,446 0,604 -0,740 0,155 0,076 -0,274 0,124 0,030 -0,057 0,227 -0,064 0,419 C -FA 18:1 7 -0,481 0,626 -0,749 0,181 0,071 -0,244 0,135 0,047 -0,059 0,317 0,008 0,492 BrFA -0,463 0,575 -0,606 -0,066 -0,148 -0,070 -0,039 -0,165 0,180 0,086 -0,121 0,402 C -MAGE 16:0 -0,397 0,599 -0,631 0,460 -0,104 -0,534 0,342 0,387 -0,353 0,615 0,183 0,441 Diploptene -0,117 0,105 -0,170 0,265 0,025 -0,273 0,365 0,225 -0,232 0,571 0,274 0,505 C -HOH 32 -0,163 0,246 -0,354 0,352 0,054 -0,283 0,145 0,342 -0,215 0,517 0,425 0,543 C - -HA 32 -0,331 0,464 -0,457 0,200 0,006 -0,176 0,165 0,136 -0,082 0,269 0,031 0,420 BHP -0,053 0,293 -0,285 0,238 -0,268 -0,245 0,219 0,153 -0,129 0,452 0,248 0,393 3.5-Bd 0,259 -0,130 0,038 -0,074 -0,091 0,156 -0,093 -0,027 0,094 0,045 0,178 0,184 Lignin -0,050 0,273 -0,330 0,576 -0,082 -0,570 0,499 0,521 -0,464 0,370 0,106 0,071 Vanillyl phenols -0,120 0,361 -0,386 0,628 -0,159 -0,633 0,567 0,566 -0,481 0,422 0,066 0,070 Syringyl phenols -0,038 0,229 -0,257 0,573 -0,094 -0,537 0,490 0,535 -0,478 0,369 0,151 0,098 Cinnamyl phenols -0,034 0,097 -0,099 0,384 -0,068 -0,358 0,406 0,337 -0,243 0,230 0,095 -0,090 p-Hydroxyl phenols 0,037 0,175 -0,328 0,406 0,046 -0,426 0,276 0,371 -0,317 0,325 0,163 0,166 ACL -0,506 0,320 -0,339 0,194 0,155 -0,259 0,219 0,075 -0,143 -0,056 -0,242 -0,065 SCFA/LCFA -0,308 0,373 -0,441 -0,095 -0,022 -0,064 -0,034 -0,159 0,129 -0,071 -0,228 0,105 C/V 0,228 -0,427 0,374 -0,271 0,109 0,395 -0,246 -0,231 0,240 -0,237 0,161 -0,309 S/V 0,241 -0,473 0,504 -0,259 0,182 0,374 -0,263 -0,219 0,136 -0,311 0,126 -0,168 3,5-Bd/V 0,357 -0,418 0,395 -0,612 -0,036 0,654 -0,578 -0,529 0,543 -0,398 0,041 -0,022 BIT -0,163 0,283 -0,321 0,531 -0,215 -0,638 0,584 0,492 -0,481 0,327 -0,138 0,056 CPI -0,133 0,073 -0,030 0,065 -0,282 -0,070 0,207 0,025 0,018 0,207 0,141 0,292 HPA 0,023 -0,046 0,002 -0,305 0,199 0,154 -0,331 -0,318 0,331 -0,487 -0,316 -0,079 0/ 5 -0,027 -0,052 0,088 -0,317 0,094 0,391 -0,326 -0,284 0,318 -0,001 0,265 0,280 alkene/ alkenone -0,234 0,183 -0,213 0,188 0,013 -0,220 0,128 0,198 -0,165 0,346 0,220 0,501
(Ad/Al)V 0,344 -0,503 0,494 -0,579 -0,035 0,625 -0,592 -0,531 0,548 -0,451 -0,010 -0,233 P/(S+V) 0,167 -0,222 0,125 -0,601 0,177 0,557 -0,656 -0,570 0,523 -0,315 0,009 0,049
148 Appendix A2
Table A2. Continued. HBIs Alkenone SCOH LCOH Phytol Sterols Dinosterol 24-Methylene- Diols Plant-derived Friedelin cholestadienol Triterpenoids Latitude (N)
Longitude (W)
Water depth (m)
TOC [%]
TIC [%]
13C (‰) TOC TOC/TN
Silt [%]
Mean grain size [μm]
LCalk n-C 36-40 C alkene 37:2 HBIs
Alkenone 0,283 SCOH 0,438 0,105 LCOH 0,512 0,531 0,552 Phytol 0,020 0,229 0,104 0,211 Sterols 0,447 0,421 0,546 0,755 0,131 Dinosterol 0,464 0,309 0,451 0,762 -0,019 0,774 24-Methylene- cholestadienol 0,434 0,255 0,551 0,665 -0,032 0,754 0,833 Diols 0,457 0,401 0,537 0,725 0,032 0,824 0,832 0,860 Plant-derived Triterpenoids 0,488 0,291 0,523 0,815 0,041 0,801 0,959 0,830 0,836 Friedelin 0,494 0,345 0,527 0,531 0,054 0,446 0,304 0,353 0,384 0,412 LCFA 0,302 0,349 0,399 0,610 0,015 0,532 0,382 0,284 0,506 0,479 0,415 SCFA 0,173 0,215 0,226 0,407 0,063 0,671 0,486 0,545 0,618 0,559 0,284 sat. SCFA 0,117 0,188 0,225 0,365 0,076 0,645 0,451 0,479 0,586 0,516 0,221 MUFA 0,208 0,261 0,248 0,454 0,041 0,709 0,532 0,631 0,674 0,589 0,278 PUFA 0,321 0,356 0,332 0,590 -0,051 0,615 0,531 0,475 0,646 0,599 0,251 C -FA 18 0,243 -0,047 0,348 0,274 -0,027 0,417 0,210 0,336 0,273 0,309 0,227 C -FA 18:1 9 0,247 0,264 0,165 0,412 0,014 0,656 0,440 0,575 0,602 0,508 0,254 C -FA 18:1 7 0,267 0,348 0,205 0,489 0,044 0,729 0,505 0,607 0,644 0,567 0,253 BrFA 0,120 0,273 0,104 0,339 0,097 0,574 0,449 0,485 0,562 0,490 0,230 C -MAGE 16:0 0,427 0,405 0,537 0,580 0,179 0,761 0,495 0,617 0,622 0,592 0,595 Diploptene 0,532 0,336 0,143 0,412 0,172 0,322 0,218 0,202 0,202 0,274 0,374 C -HOH 32 0,524 0,303 0,425 0,761 0,049 0,703 0,936 0,812 0,747 0,898 0,253 C - -HA 32 0,341 0,130 0,325 0,344 -0,153 0,437 0,344 0,377 0,432 0,385 0,288 BHP 0,386 0,375 0,142 0,480 0,022 0,366 0,393 0,337 0,319 0,427 0,249 3.5-Bd 0,229 0,350 0,051 0,293 -0,088 0,135 0,262 0,269 0,255 0,213 0,080 Lignin 0,326 0,147 0,349 0,521 -0,104 0,387 0,483 0,536 0,402 0,549 0,422 Vanillyl phenols 0,350 0,151 0,419 0,527 -0,152 0,407 0,496 0,518 0,393 0,566 0,514 Syringyl phenols 0,299 0,121 0,389 0,561 -0,087 0,396 0,521 0,564 0,444 0,582 0,383 Cinnamyl phenols 0,109 0,029 0,170 0,295 -0,023 0,188 0,361 0,335 0,177 0,398 0,215 p-Hydroxyl phenols 0,350 0,217 0,229 0,503 -0,054 0,409 0,480 0,538 0,441 0,505 0,304 ACL -0,283 0,065 0,145 0,051 0,159 0,168 -0,033 0,097 0,133 0,001 0,126 SCFA/LCFA 0,061 0,028 0,000 0,015 0,013 0,406 0,303 0,474 0,361 0,288 0,050 C/V -0,287 -0,137 -0,215 -0,286 0,253 -0,294 -0,270 -0,251 -0,317 -0,305 -0,374 S/V -0,352 -0,151 -0,226 -0,168 0,179 -0,281 -0,179 -0,128 -0,152 -0,200 -0,431 3,5-Bd/V -0,134 0,170 -0,339 -0,192 0,036 -0,223 -0,101 -0,092 -0,070 -0,216 -0,348 BIT 0,229 -0,018 0,399 0,291 0,048 0,373 0,259 0,319 0,220 0,346 0,530 CPI 0,057 0,352 0,049 0,272 0,043 0,058 0,173 -0,122 0,063 0,238 0,215 HPA -0,299 0,046 -0,160 0,150 -0,075 0,057 0,101 0,138 0,183 -0,013 -0,016 0/ 5 0,237 0,173 -0,189 0,170 -0,075 -0,064 0,296 0,116 0,181 0,219 -0,215 alkene/ alkenone 0,272 -0,305 0,305 0,279 0,078 0,370 0,420 0,314 0,309 0,471 0,085
(Ad/Al)V -0,351 -0,075 -0,400 -0,400 0,135 -0,355 -0,241 -0,235 -0,266 -0,332 -0,468 P/(S+V) -0,141 0,040 -0,426 -0,303 0,062 -0,145 -0,242 -0,198 -0,115 -0,325 -0,490
149 Appendix A2
Table A2. Continued. LCFA SCFA sat. SCFA MUFA PUFA C18-FA C18:1 9-FA C18:1 7-FA BrFA C16:0- Diploptene MAGE Latitude (N)
Longitude (W)
Water depth (m)
TOC [%]
TIC [%]
13C (‰) TOC TOC/TN
Silt [%]
Mean grain size [μm]
LCalk
n-C 36-40 C alkene 37:2 HBIs
Alkenone
SCOH
LCOH
Phytol
Sterols
Dinosterol
24-Methylene- cholestadienol Diols
Plant-derived Triterpenoids Friedelin
LCFA
SCFA 0,608 sat. SCFA 0,611 0,973 MUFA 0,563 0,966 0,910 PUFA 0,815 0,679 0,664 0,706 C -FA 18 0,343 0,455 0,386 0,535 0,441 C -FA 18:1 9 0,435 0,848 0,776 0,921 0,657 0,675 C -FA 18:1 7 0,548 0,911 0,846 0,962 0,705 0,601 0,956 BrFA 0,505 0,931 0,942 0,876 0,548 0,236 0,732 0,808 C -MAGE 16:0 0,379 0,544 0,489 0,586 0,485 0,523 0,593 0,628 0,418 Diploptene 0,219 0,040 -0,048 0,102 0,166 0,161 0,184 0,198 -0,002 0,253 C -HOH 32 0,283 0,372 0,312 0,452 0,462 0,238 0,399 0,462 0,341 0,455 0,295 C - -HA 32 0,546 0,536 0,479 0,569 0,663 0,496 0,550 0,552 0,426 0,302 0,280 BHP 0,343 0,284 0,266 0,296 0,339 0,057 0,257 0,370 0,225 0,422 0,328 3.5-Bd 0,127 0,075 0,016 0,110 0,178 -0,189 0,142 0,139 0,091 -0,029 0,113 Lignin 0,346 0,277 0,200 0,346 0,344 0,289 0,349 0,347 0,178 0,336 0,225 Vanillyl phenols 0,383 0,268 0,203 0,314 0,342 0,300 0,288 0,305 0,170 0,386 0,213 Syringyl phenols 0,382 0,262 0,197 0,321 0,375 0,207 0,283 0,304 0,169 0,299 0,191 Cinnamyl phenols 0,153 0,175 0,134 0,171 0,120 -0,021 0,061 0,164 0,168 0,215 0,096 p-Hydroxyl phenols 0,340 0,318 0,235 0,401 0,377 0,295 0,436 0,413 0,230 0,321 0,255 ACL 0,116 0,292 0,264 0,367 0,157 0,316 0,332 0,316 0,186 0,146 0,031 SCFA/LCFA -0,171 0,578 0,545 0,631 0,075 0,384 0,692 0,616 0,577 0,405 0,057 C/V -0,282 -0,100 -0,057 -0,162 -0,312 -0,337 -0,252 -0,159 0,002 -0,239 -0,240 S/V -0,153 -0,159 -0,134 -0,167 -0,083 -0,356 -0,171 -0,177 -0,099 -0,340 -0,313 3,5-Bd/V -0,290 -0,129 -0,101 -0,148 -0,158 -0,474 -0,101 -0,124 0,024 -0,292 -0,230 BIT 0,070 0,110 0,061 0,140 -0,015 0,205 0,155 0,125 0,007 0,402 0,310 CPI 0,305 0,019 0,065 -0,060 0,279 -0,214 -0,101 -0,020 0,045 0,007 0,117 HPA -0,108 0,038 0,055 0,086 -0,048 -0,201 0,059 0,040 0,147 -0,095 -0,342 0/ 5 0,016 0,001 0,012 0,004 0,191 -0,253 0,017 0,035 0,134 -0,235 0,020 alkene/ alkenone 0,257 0,304 0,287 0,253 0,287 0,307 0,227 0,232 0,183 0,191 0,273
(Ad/Al)V -0,411 -0,205 -0,155 -0,250 -0,388 -0,427 -0,286 -0,238 -0,021 -0,306 -0,266 P/(S+V) -0,226 -0,010 0,005 0,019 -0,069 -0,036 0,142 0,069 0,054 -0,181 -0,109
150 Appendix A2
Table A2. Continued. C32-HOH C32- -HA BHP 3.5-Bd Lignin Vanillyl Syringyl Cinnamyl p-Hydroxy-benzene phenols phenols phenols phenols ACL Latitude (N)
Longitude (W)
Water depth (m)
TOC [%]
TIC [%]
13C (‰) TOC TOC/TN
Silt [%]
Mean grain size [μm]
LCalk
n-C 36-40 C alkene 37:2 HBIs
Alkenone
SCOH
LCOH
Phytol
Sterols
Dinosterol
24-Methylene- cholestadienol Diols
Plant-derived Triterpenoids Friedelin
LCFA
SCFA
sat. SCFA
MUFA
PUFA
C -FA 18 C -FA 18:1 9 C -FA 18:1 7 BrFA
C -MAGE 16:0 Diploptene
C -HOH 32 C - -HA 32 0,382 BHP 0,393 0,202 3.5-Bd 0,304 0,093 0,147 Lignin 0,520 0,275 0,270 0,514 Vanillyl phenols 0,505 0,300 0,288 0,417 0,971 Syringyl phenols 0,553 0,261 0,277 0,530 0,972 0,937 Cinnamyl phenols 0,396 0,097 0,278 0,216 0,681 0,695 0,693 p-Hydroxyl phenols 0,531 0,240 0,209 0,631 0,907 0,826 0,862 0,497 ACL -0,093 0,118 -0,158 -0,359 -0,022 -0,014 -0,058 -0,076 -0,068 SCFA/LCFA 0,256 0,126 0,047 -0,016 0,120 0,057 0,037 -0,005 0,218 0,293 C/V -0,208 -0,264 -0,079 -0,188 -0,363 -0,416 -0,312 0,278 -0,385 -0,002 S/V -0,121 -0,344 -0,173 0,250 -0,103 -0,257 0,034 0,035 -0,027 -0,147 3,5-Bd/V -0,067 -0,291 -0,045 0,631 -0,210 -0,319 -0,176 -0,223 0,018 -0,386 BIT 0,229 0,028 0,202 0,085 0,493 0,562 0,459 0,234 0,331 0,170 CPI 0,121 -0,033 0,250 0,270 0,207 0,259 0,232 0,277 0,092 -0,079 HPA 0,108 -0,106 -0,130 0,197 -0,035 -0,084 0,021 -0,023 0,034 0,142 0/ 5 0,316 -0,070 0,129 0,299 -0,115 -0,177 -0,053 -0,167 -0,019 -0,302 alkene/ alkenone 0,388 0,351 0,148 -0,142 -0,005 -0,004 0,060 -0,073 0,013 -0,134
(Ad/Al)V -0,179 -0,345 -0,170 0,041 -0,351 -0,427 -0,327 0,124 -0,237 -0,190 P/(S+V) -0,209 -0,133 -0,173 0,052 -0,519 -0,635 -0,577 -0,634 -0,166 -0,086
151 Appendix A2
Table A2. Continued. alkene/ 0 5 SCFA/LCFA C/V S/V 3,5-Bd/V BIT CPI HPA / alkenone (Ad/Al)V Latitude (N)
Longitude (W)
Water depth (m)
TOC [%]
TIC [%]
13C (‰) TOC TOC/TN
Silt [%]
Mean grain size [μm]
LCalk
n-C 36-40 C alkene 37:2 HBIs
Alkenone
SCOH
LCOH
Phytol
Sterols
Dinosterol
24- Methylenecholestadienol Diols
Plant-derived Triterpenoids Friedelin
LCFA
SCFA
sat. SCFA
MUFA
PUFA
C -FA 18 C -FA 18:1 9 C -FA 18:1 7 BrFA
C -MAGE 16:0 Diploptene
C -HOH 32 C - -HA 32 BHP
3.5-Bd
Lignin
Vanillyl phenols
Syringyl phenols
Cinnamyl phenols
p-Hydroxyl phenols
ACL SCFA/LCFA C/V -0,022 S/V -0,146 0,486 3,5-Bd/V 0,061 0,217 0,548 BIT 0,178 -0,475 -0,386 -0,326 CPI -0,341 -0,073 0,026 0,081 0,065 HPA 0,064 0,113 0,237 0,333 -0,141 -0,091 0/ 5 -0,034 -0,038 0,361 0,482 -0,388 0,282 0,099 alkene/ alkenone 0,132 -0,185 -0,040 -0,242 0,085 -0,015 -0,244 0,181
(Ad/Al)V 0,035 0,753 0,496 0,531 -0,529 -0,066 0,217 0,090 -0,303 P/(S+V) 0,246 0,107 0,210 0,565 -0,482 -0,365 0,111 0,255 -0,092 0,401
152 153 Danksagung
DANKSAGUNG
Ich danke Prof. Dr. Kai Uwe Hinrichs, dass er mir die Möglichkeit gegeben hat, meine
Dissertation unter seiner hervorragenden wissenschaftlichen Betreuung durchzuführen, für seine Förderung und das mir entgegengebrachte Vertrauen während der letzten Jahre.
Für die Übernahme des Zweitgutachtens und für die ausgezeichnete wissenschaftliche
Zusammenarbeit während der letzten drei Jahre danke ich Prof. Dr. Boris Koch. Besonders möchte ich mich für die geduldige Einführung in FT ICR MS und die daraus resultierende
Datenflut bedanken.
Dr. Marcus Elvert danke ich besonders für die Einführung in die Welt der Organischen
Geochemie, für seine exzellente wissenschaftliche Betreuung, seine uneingeschränkte
Unterstützung und das Vertrauen, das er mir entgegengebracht hat.
Das Weiteren danke ich den folgenden Personen, die auf dem einen oder anderem Weg zu dem Entstehen dieser Dissertation beigetragen haben:
Dr. Daniel Birgel für seine Hilfe bei wissenschaftlichen Problemen und Fragen
Xavier Prieto Mollar dafür, dass er den Laborbetrieb und die Geräte am Laufen hält
Birgit Schmincke für ihre Hilfe in allen administrativen Sachen
Enno Schefuß für seine Ideen und Anregungen während der Treffen des Thesis
Committees
Arne Leider, Julius Lipp, Lars Hoffmann, Stefanie Tille, Verena Heuer, Marcos
Yoshinaga, Julio Sepulveda, für wertvolle Kommentare zur Laboranalytik und
Datenbewertung und Manuskripten
Laura Garcia Pardo und Kevin Becker für die Unterstützung bei Laborarbeiten
Den Kollegen aus C6 und Projektleiter Till Hanebuth für anregende wissenschaftliche
Diskussionen
Bruker Daltonik GmbH und Matthias Witt für die Messung meine Proben auf ihrem
FT ICR Massenspektrometer
154 Danksagung
dem wissenschaftlichen Team der P342 Galiomar Expedition und der Besatzung der
FS Poseidon; Till Hanebuth für die hervorragende Fahrtleitung; insbesonder danke
ich Michelle und Brit
den Fahrtteilnehmern und der Schiffsbesatzung der Meteor Ausfahrt M72/5 im
Schwarzen Meer; Christian Borowski für seine humorvolle Fahrtleitung; Olga, Olaf,
Bert, Barnie und Helge für den Spaß bei der Arbeit und in der Schiffsbar
allen aktuellen und ehemaligen Mitgliedern der AG Organische Geochemie, Prof. Dr.
Jörn Peckmann und der AG Geobiologie für die wunderbare Arbeitsatmosphäre,
entspannte Kaffeerunden, Grillparties… Ganz besonders möchte ich mich bei meinen
lieben Office Mates Arne, Julio und Simone bedanken.
der deutschen Forschungsgemeinschaft für die Finanzierung meiner Arbeit im
Rahmen des DFG Excellence Cluster “Der Ozean im System Erde”
Bremen International Graduate School for Marine Sciences (GLOMAR) für die
Finanzierung von Konferenzteilnahmen
Meinen Freunden im Norden: Kate (im Kampf gegen jede Widrigkeit), Benny (im
Kampf gegen d.Z.d.Z.), Esther, Francois, Michelle, Brit, Steph, Lars, Robyn und
meinen Mädels in Tübingen: Silke, Steffi, Ulli, Nelli und Claudi. Für Graptas
Hammer und vieles mehr danke ich Ellen und Henning
meinem Bruder Gunnar, meiner Schwester Anke und ihrer Familie und ganz
besonders meinen Eltern für ihre fortwährende Unterstützung und den Rückhalt, den
sie mir geben.
155 156 N a m e : ...... Datum ......
Anschrift : ......
E r k l ä r u n g
Hiermit versichere ich, dass ich
1. die Arbeit ohne unerlaubte fremde Hilfe angefertigt habe,
2. keine anderen als die von mir angegebenen Quellen und Hilfsmittel benutzt habe und
3. die den benutzten Werken wörtlich oder inhaltlich entnommenen Stellen als solche kenntlich gemacht habe.
______, den ......
------(Unterschrift)
157