Molecular and Isotopic Characterization of Terrestrial Organic Carbon Released to (Sub-)Arctic Coastal Waters

Molecular and isotopic characterization of terrestrial organic carbon released to (sub-)Arctic coastal waters

Jorien Elisabeth Vonk

Doctoral Thesis Department of Applied Environmental Science Stockholm University Stockholm, 2010

Printed in Sweden by US-AB, Stockholm 2010 Distributor: Department of Applied Environmental Science (ITM)

Cover: Kolyma coast, East Siberian Sea, September 2008 (photo by Johan Gelting) “... half past 9 in the evening [shiptime], the sun has just risen. A desolate coastline appears. Brown- orange in colour, friendly roling hills above an eroding shore. Unbelievable to be here, once in a lifetime?”

Een beetje avontuur verhoogt je weerstand

Loesje

ABSTRACT Arctic soils store half of the global soil organic carbon (OC) pool and twice as much C as is currently present in the atmosphere. A considerable part of these carbon pools are stored in permafrost. Amplified climate warming in the Arctic will thaw permafrost and remobilize some of these substantial carbon stocks into the active carbon cycle, potentially causing positive feedback to global warming. Despite the global importance of this mechanism, our understanding of the fate of these thawing organic carbon (OC) pools is still poor, particularly regarding its degradation potential. This makes good estimates on greenhouse gas emissions versus coastal reburial impossible. This doctoral thesis aims to improve our understanding on the fate of high-latitude terrestrial OC during fluvial and coastal transport. In two study regions, the Bothnian Bay and the East Siberian Sea, we apply a wide range of bulk, molecular and isotopic geochemical analyses to reveal information on sources, age, degradation and transport routes. Our results show that both study regions receive and store large amounts of terrestrial OC, largely derived from peatlands (paper I, II and IV). This terrestrial matter undergoes extensive degradation in both the water column and surface sediments (paper I, III and IV). Surface sediments in the East Siberian Sea show a offshore-decreasing input of riverine OC and a considerable and constant input of OC from coastal erosion. The strong imprint of rapidly settling coastal OC far out on the shelf may be explained by a strong benthic boundary layer transport in combination with offshore ice-transport and selective preservation of erosion OC compared to riverine OC (paper IV). Molecular radiocarbon data allowed us to distinguish between two (sub-)Arctic soil OC pools that show a remarkably different susceptibility to degradation upon arrival in the coastal system; a young and easily degradable pool originating in surface peatlands, and an old and recalcitrant pool originating in deep mineral soils and coastal mineral Pleistocene deposits (paper III and IV). Our first estimates suggest that, in the Bothnian Bay coastal system, mineral soil OC is at least 20 times less susceptible to degradation than peatland OC (paper III). Hence, a considerable part of the thaw-released mineral OC pool may simply be relocated to coastal sediments instead of being emitted to the atmosphere.

NEDERLANDSE SAMENVATTING Arctische bodems bevatten de helft van het totale organisch koolstof (OC) reservoir op aarde en twee keer zoveel C als in de atmosfeer. Het grootste deel van deze reservoirs wordt geborgen in permafrost. Versterkte klimaatverandering in Arctische gebieden ontdooit de permafrost en maakt deze aanzienlijke voorraden beschikbaar voor actieve recycling in de C cyclus. Dit kan leiden tot een versterkt broeikaseffect. Ondanks het globale belang van deze kwestie, is ons inzicht in de dynamiek van ontdooiende OC reservoirs nog steeds minimaal, met name wat betreft de afbraak gevoeligheid. Goede schattingen met betrekking tot broeikasgas emissies versus heropslag in zeebodems zijn daardoor niet mogelijk. Dit proefschrift heeft als doel meer inzicht te verkrijgen in de processen die terrestrisch OC beïnvloeden tijdens fluviaal en kust transport. Met behulp van geochemische analyses op bulk, moleculair en isotoop niveau in twee studiegebieden, de Botnische Golf en de Oost-Siberische Zee, verkrijgen we informatie over de oorsprong, leeftijd, afbraak en transport routes van terrestrisch materiaal. Onze resultaten laten zien dat beide studie locaties grote hoeveelheden terrestrisch OC ontvangen en herbergen, grotendeels afkomstig van veengebieden (artikel I, II en IV). Dit terrestrisch OC ondergaat verregaande afbraak zowel in het water als in het sediment (artikel I, III en IV). Oppervlakte sediment in the Oost-Siberische Zee bevat een fluviaal OC signaal dat afneemt met toenemende afstand tot de kust, en een constant en aanzienlijk signaal afkomstig van kusterosie OC. De sterke invloed van snel zinkend kusterosie OC op grote afstanden van de kust kan verklaard worden door overheersende transport routes over de zeebodem en in zee ijs, en door selectieve preservatie van kusterosie OC ten opzichte van fluviaal OC (artikel IV). C- 14 analyse op moleculair niveau maakte het mogelijk onderscheid te maken tussen twee (sub- )Arctische OC bodem reservoirs die een verbazingwekkend verschillende gevoeligheid voor afbraak vertonen na aankomst in de zee; een jong en makkelijk afbreekbaar reservoir afkomstig uit veen, en een oud, refractief reservoir afkomstig uit minerale bodems en kustafzettingen (artikel III en IV). Onze eerste schattingen zeggen dat mineraal OC minstens 20 keer minder gevoelig is voor afbraak als veen OC (artikel III). Dit heeft als gevolg dat een aanzienlijk deel van het ontdooiende minerale OC reservoir simpelweg heropgeslagen wordt in sedimenten op de zeebodem in plaats van te worden uitgestoten als broeikasgas in de atmosfeer.

ABBREVIATIONS

CPI Carbon preference index DOC Dissolved organic carbon ESS East Siberian Sea GC Gas chromatography GHG Greenhouse gases GRARs Great Russian Arctic Rivers HMW High-molecular weight IQR Interquartile range LMW Low-molecular weight MS Mass spectrometry OC Organic carbon OM Organic matter pcGC preparative capillary gas chromatography POC Particulate organic carbon POM Particulate organic matter SOC Soil organic carbon (paper III) / Sedimentary organic carbon (paper I and IV) SPM Suspended particulate matter TerrOM Terrestrial organic matter TN Total nitrogen Tg Terragram, 1012 g TLE Total lipid extract TOC Total organic carbon

TABLE OF CONTENTS

ABSTRACT ...... 2 NEDERLANDSE SAMENVATTING ...... 3 ABBREVIATIONS ...... 4 TABLE OF CONTENTS...... 5 LIST OF PAPERS ...... 6 STATEMENT ...... 7 1. OBJECTIVES OF THESIS ...... 8 2. INTRODUCTION ...... 11 2.1 Arctic carbon pools under climate pressure ...... 11 2.2 Our approach in contributing towards solving this puzzle ...... 14 2.3 The International Siberian Shelf Study 2008 (ISSS-08) ...... 18 3. METHODS ...... 20 3.1 Field sampling ...... 20 3.2 Bulk geochemical methods and their usage ...... 21 3.3 Molecular analyses of soil OC biomarkers ...... 22 3.4 Distribution patterns and relative abundance of soil OC indicators ...... 22 3.5 Bulk and molecular radiocarbon analyses ...... 25 4. SUMMARY AND DISCUSSION OF MAIN RESULTS ...... 27 4.1 Sources of terrestrial OC (Paper I - IV) ...... 27 4.2 A proxy for (sub-)Arctic Sphagnum (Paper II) ...... 27 4.3 Origin and age of soil OC (Paper III and IV) ...... 28 4.4 Relative input of coastal erosionOC and fluvial OC (Paper IV) ...... 30 4.5 Terrestrial OC degradation in (sub-)Arctic coastal waters (Paper I, III and IV) ...... 30 4.6 Terrestrial OC burial (Paper III and IV) ...... 31 5. CONCLUSIONS ...... 33 6. FUTURE PERSPECTIVES ...... 35 ACKNOWLEDGEMENTS – DANKWOORD – TACK! ...... 38 REFERENCES ...... 41

LIST OF PAPERS

Paper I Lipid biomarker investigation of the origin and diagenetic state of sub-arctic terrestrial organic matter presently exported into the northern Bothnian Bay. Vonk, J. E., B. E. van Dongen, and Ö. Gustafsson (2008), Marine Chemistry, 112, 1-10.

Paper II Calibrating n-alkane Sphagnum proxies in sub-Arctic Scandinavia. Vonk, J. E., and Ö. Gustafsson (2009), Organic Geochemistry, 40, 1085-1090.

Paper III Selective preservation of old organic carbon fluvially released from sub-arctic soils. Vonk, J. E., B. E. van Dongen, and Ö. Gustafsson (2010), accepted in Geophysical Research Letters.

Paper IV Molecular and radiocarbon constraints on sources and degradation of terrestrial organic carbon along the Kolyma paleoriver transect, East Siberian Sea. Vonk, J. E., L. Sánchez-García, I. Semiletov, O. Dudarev, T. Eglinton, and Ö. Gustafsson, manuscript.

Papers are reprinted with permission from Elsevier (paper I and II) and the American Geophysical Union (paper III).

STATEMENT

I, Jorien Elisabeth Vonk, contributed to the papers as follows:

Paper I The sampling campaign in 2005 was planned and performed by others. I carried out all laboratory analyses, and made interpretations in collaboration with others. I did the main part in writing the paper.

Paper II I sampled and/or coordinated all Sphagnum collection. I had an important role in planning and sampling in the 2007 field campaign where we collected most of the other samples. I carried out all laboratory analyses, and took the lead role writing the paper.

Paper III The sampling campaign in 2005 was planned and performed by others. I carried out all laboratory analyses on the surface sediment samples, Bart van Dongen was responsible for all laboratory work regarding the suspended particulate matter. Data interpretation was done in close collaboration with others, and I had the main role in writing the paper.

Paper IV I was responsible for the sediment sampling and co-responsible for the suspended particulate matter collection during the 2008 sampling campaign. I carried out all laboratory analyses and took the lead role writing the paper.

1. OBJECTIVES OF THESIS

The overall objective of this thesis was to improve our understanding on the fate of terrestrial OC being released from sub-Arctic and Arctic land masses to coastal waters. By performing a wide range of bulk, molecular and isotopic geochemical analyses on surface seawater particulate matter and underlying surface sediments, we obtain information on sources, degradation, reservoir ages and transport mechanisms.

The specific objectives, also shown as text boxes 1-6 in Figure 1, are:

1. To obtain information on the dominant sources of terrestrial OC delivered to the northern Baltic Sea (paper I - IV) and the East Siberian Arctic (paper IV).

2. To establish a molecular proxy for (sub-)Arctic Sphagnum that can be used to identify and quantify peatland-derived terrestrial OC at northern latitudes (paper II).

3. To trace the origin and age of soil OC, coastally-exported from northern Scandinavia (paper III) and East Siberia (paper IV) by measuring radiocarbon contents on a bulk and molecular level.

4. To estimate the relative input of coastal erosion OC and fluvial OC in the East Siberian Arctic (paper IV).

5. To reveal information on terrestrial OC degradation in coastal waters, by studying both surface water suspended particulate matter and underlying bottom sediments (paper I, III and IV).

6. To obtain a qualitative and quantitative picture of terrestrial OC burial in coastal and ocean sediments (paper I, III and IV).

Figure 1 Conceptual model of a (sub-)Arctic region with rivers draining into the ocean through different vegetation zones, underlain by discontinuous to continuous permafrost. The three cross sections represent different phases of land-to-ocean terrestrial organic carbon (OC) transport: (A) soils, (B) coasts and (C) estuaries/oceans. The text boxes contain topics that the papers in this thesis refer to.

2. INTRODUCTION

2.1 Arctic carbon pools under climate pressure

The Arctic is a remote and relatively undisturbed region that has attracted adventurous explorers and scientists for centuries, and still does so. Nowadays, the Arctic also happens to be a suspected “hot spot” for the initiation of positive feedback processes that can amplify global warming trends. One main reason for these concerns is the massive amounts of carbon (C) stored in the permanently frozen ground. Approximately half of the global soil OC pool resides in the circum-Arctic (Tarnocai et al., 2009; Figure 2A), mainly in the top few meters. This is more than twice as much as the current atmospheric C pool (Kuhry et al., 2009). The largest part of this pool is represented by mineral soils and peatlands, the two soil types with highest mean soil OC contents. In addition, a large fraction is stored in so-called yedoma deposits that accumulated during glacial ages in the Pleistocene (Schirrmeister et al., 2002; Zimov et al., 2006) and are widespread in large parts of northern Russia.

Figure 2 (A) Distribution of soil OC contents in the northern circum-Arctic permafrost region (from Tarnocai et al., 2009). (B) Drainage basins of the six largest Arctic rivers and the extent of permafrost currently found within them (continuous 90–100% areal coverage, discontinuous 50–90%, sporadic 10–50%, isolated patches 0–10%; from Brown et al., 1998 and Frey and McClelland, 2009).

These significant soil OC pools are currently to a large extent freeze-locked into permafrost: ground that remains at or below 0°C for more than two years (van Everdingen, 1998). Permafrost is widespread in the circum-Arctic (Figure 1B) and has existed for millenia. Organic matter stored in permafrost has often experienced little decay before its change into a frozen state. This is particularly valid for peatlands and yedoma (e.g. Khvorostyanov et al., 2008; Kuhry et al., 2009). The implication is that these soil OC pools may be labile and would respire quickly upon thawing, releasing greenhouse gases (Dutta et al., 2006; Dorrepaal et al., 2009; Schuur et al., 2009).

Figure 3 (A) Ongoing climate warming in the Arctic as illustrated by observed spring (March- April-May) temperature anomalies (°C) for 2000-2005 compared to a 1968-1996 base period (from Richter-Menge et al., 2006). (B) The estimated extent of permafrost in the Northern Hemisphere in 2090-2100 (light purple); pink areas indicate the loss of permafrost compared to 1990-2000 (from Euskirchen et al., 2006).

Temperature increases associated with climate warming are predicted to be highest in Arctic regions, and are already being observed on larger and larger scales (e.g. Serreze et al., 2000; ACIA, 2004; Richter-Menge et al., 2006; Romanovsky et al., 2007; Figure 3A). This will result in a wide range of local and large-scale impacts on high latitude ecosystems, for instance a reduction in sea ice cover, northward movement of the tree line, increasing contaminant transport or higher forest fire frequencies (IPCC, 2001; ACIA, 2004). One of the most influential changes is the expected widespread permafrost thaw (Figure 3B). This could release substantial

12 amounts of the currently freeze-locked soil OC, most likely creating positive feedback to global warming (Schuur et al., 2008). Frozen terrestrial C pools subject to increasing air temperatures will experience permafrost thaw. This will occur through different mechanisms (Schuur et al., 2008). (1) The surface ground layer that thaws in summer (active layer) will gradually deepen, and (2) year- round unfrozen areas (talik) will slowly develop when the summer-thawed layer does not refreeze anymore completely in winter. (3) Furthermore, coastlines will be exposed to thermal degradation in combination with higher sea levels and increased storm and wave fetch due to reduced coastal sea-ice extent (IPCC, 2001; Stein and Macdonald, 2004). Estimates of average coastal retreat rates are scarce but considerable and are between 3-13 m/year with maxima up to ca. 30 m/year (Semiletov et al., 1999; Rachold et al., 2000; Dudarev et al., 2006; Overduin et al., 2007). Finally, (4) slumping events and ground subsidence as a result of thawing of ice-rich grounds (thermokarst) will increase. The speed at which these different mechanisms occur is difficult to predict. Note that permafrost degradation and development is not only linked to air temperature, but also depends on precipitation, vegetation, soil type and soil moisture content (Johansson et al., 2006). In any case, these thaw processes will either directly expose the unfrozen permafrost OC to microbial degradation, or will increase the interaction of the OC pool with the hydrological cycle, allowing for further degradation and transport. Active layer deepening and talik formation will initiate the development of shallow groundwater flow paths (Bense et al., 2009; Frey and McClelland, 2009; Lyon et al., 2009). Together with other factors such as enhanced precipitation, this has already increased river discharge to the Arctic Ocean (e.g. Savelieva et al., 2000; Peterson et al., 2002) by 7% from 1936 to 1999. The increase in hydrological fluxes combined with intensification of coastal erosion will most likely increase the transport of thaw- remobilized soil OC to Arctic coastal regions.

Overall, one can say that the high latitude permafrost C pools are under serious climate pressure. However, our understanding of the fate of thaw-remobilized soil OC pools is still poor (e.g. Gruber et al., 2004; Stein and Macdonald, 2004; Schuur et al., 2008; Kuhry et al., 2009).

Will it be converted to greenhouse gases (CO2, CH4) or will it simply be reburied in ocean sediments with no net effect on the atmospheric C pool? This thesis follows high-latitude soil OC during its transport through estuaries and coastal oceans. These coastal reactors (Figure 1C) are actively remineralizing terrestrial OC (e.g. Algesten et al., 2006; Semiletov et al., 2007; van Dongen et al., 2008b; Anderson et al., 2009). By applying a variety of organic geochemical tools, we gain insight in the origin of the various soil OC pools, their relative susceptibilities to degradation and preferential transport mechanisms. This allows for better estimates on the fate and reactivity of thaw-remobilized soil OC being released from (sub-)Arctic regions and, consequently, a better assessment of feedback mechanisms to climate warming.

2.2 Our approach in contributing towards solving this puzzle The Arctic drainage basin is an enormously heterogeneous landmass and consists of a wide variety of vegetation, soil types, hydrology and permafrost regimes. Harsh climatic conditions and logistical challenges provide only limited opportunities to access these regions. It is therefore a challenge to study the fate of thaw-remobilized terrestrial OC, particularly on larger spatial scales. Yet, the need is high for integrated studies (McGuire et al., 2009) to be able to estimate the future role of permafrost OC in a changing climate. This thesis studies various components of the natural system and uses them as tools to obtain an integrated picture of terrestrial matter release to (sub-)Arctic coastal waters. In the following sections (2.2.1 – 2.2.6) we will describe these components, and the information they provide on our study region.

2.2.1 Rivers provide information on their watersheds (paper I - IV) Small mountainous creeks, peatland brooks and larger downstream tributaries; all the water in a catchment that does not evaporate will eventually end up at the point where the river meets the ocean. Fluvially-transported organic matter thus reflects the characteristics of the surrounding landmass and changes in the watershed will sooner or later impact the river signal. Seasonal changes in the watershed will for example impact the river signal, by changing the lability of dissolved OC (DOC) (Neff et al., 2006; Holmes et al., 2008). In late summer, active

14 layer deepening allows for enhanced processing of DOC making it less labile upon arrival in the coastal ocean. In contrast, spring flood DOC is quickly flushed from a mostly frozen system, inhibiting microbial processing on its way to the ocean. Furthermore, vegetation cover and soil types such as peat strongly impact the river chemistry at high-latitudes (Humborg et al., 2004). Loads of total OC (TOC; equals DOC plus POC) are generally higher with extensive peatland coverage in the catchment (e.g. Arvola et al., 2004). This thesis utilizes the signal that is recorded in river-transported terrestrial POC in coastal environments to deduce information about processes ongoing in the watershed.

2.2.2 Different OC matrices provide information on different fractions of terrestrial OC (paper I, III and IV) Different fractions of fluvially transported OC and their relative abundance are indicative of different ongoing processes in the catchment. Dissolved compounds commonly are young and originate from modern biomass, and fluctuations in DOC will thus predominantly reflect changes in plant ecology (e.g. Benner et al., 2004; Guo et al., 2007; Raymond et al., 2007). In contrast, particulate matter is generally older and reflects processes connected to permafrost degradation (e.g. Guo and Macdonald, 2006; Guo et al., 2007) such as river bank and coastal erosion, thermokarst development and active layer deepening. Whereas POC and DOC in the water column are snapshots of ongoing fluvial transport, the OC fraction in coastal surface sediments provides an integrated picture over annual or longer time scales. The terrestrial OC in sediments also represents the fraction that is relatively recalcitrant, since it is not remineralized during fluvial and coastal transport. This thesis concentrates on both surface water POC and surface sediments to trace terrestrial C release and its susceptibility to degradation.

2.2.4 Terrestrial biomarkers provide information on higher plant input (paper I - IV) By looking at sediments and surface water POC on a molecular level, we can distinguish in more detail between the different sources. In the field of organic geochemistry, molecular biomarkers are often used as a tool to reveal information on the environment. A biomarker is a

15 molecule that is specific for an organism or a group of organisms. If you detect this biomarker, you know that this organism lives here, has been living here or has been transported here. For example, asterosterol is a molecule that can be found in the northern Pacific sea star Asterias amurensis (Kobayashi et al., 1973). If you detect this molecule in surface sediments in the East Siberian Sea, you know that these animals are living in this region or transported here. Terrestrial input in marine environments can be traced in a similar way, by looking at terrestrial biomarkers such as long-chain n-alkanes, n-alkanoic acids and n-alkanols. These compounds originate from waxes that cover leaves of higher plants (e.g. Eglinton and Hamilton, 1963) and are not formed by marine organisms. By focusing on molecules with a pure terrestrial source, one can obtain a “clean” terrestrial signal, without interference from other sources of OC (such as freshwater and marine plankton). Particularly if biomarkers are resistant to degradation, which is the case for these leaf wax compounds (e.g. Brown et al., 1972; Prahl et al., 1997), they are valuable as markers of recalcitrant soil OC. Even if the total amounts of these biomarkers generally are not more than a few milligram per gram OC, their stability and source-specific character makes them valuable in terrestrial organic carbon cycling studies. In general, terrestrial OC is less labile than marine OC (Hedges et al., 1997; Hedges and Oades, 1997) due to its structure and protection by mineral associations (Keil et al., 1994; Huguet et al., 2008). Higher plants also produce different kind of sterols such as β-sitosterol and campesterol (Patterson and Nes, 1991; Goad and Akihisa, 1997) that could be used as terrestrial biomarkers. However, these products are also synthesized by freshwater phytoplankton (e.g. Volkman, 1986) and are more susceptible to degradation than straight-chain leaf wax compounds.

2.2.5 Radiocarbon content provides information on source and fate of remobilized OC (paper III and IV) Bulk 14C contents reveal relative inputs of different fractions with different reservoir ages and have been applied fairly often in studies on biogeochemical cycling (e.g. Guo et al., 2004; McNichol and Aluwihare, 2007 and references therein; van Dongen et al., 2008a). Terrestrial OC that is delivered to coastal environments consists of fresh material (e.g. recent

16 plant litter, surface soils), sources with intermediate ages (deeper soils) and old pools (e.g. Pleistocene yedoma). Simply put, if you know the approximate ages of these different pools, you can estimate their relative contributions to the TOC fraction in your samples (paper IV). Radiocarbon analyses on a molecular level can reveal even more detailed information on specific inputs and their reservoir ages. If one analyzes terrestrial molecules in marine environments, there is no interference from younger marine OC sources. Although this type of analyses is powerful, studies are still relatively scarce (Eglinton et al., 1997; Pearson et al., 2001; Ohkouchi et al., 2003; Smittenberg et al., 2006; Drenzek et al., 2007; Mollenhauer and Eglinton, 2007). We perform molecular radiocarbon analyses on both POC and sediments, and this allows us to distinguish between different soils in the catchment with different reservoir ages and the fate of different soil OC pools in the coastal system (paper III).

2.2.6 Model system Kalix River/Bothnian Bay provides information on Ob and Yenisey Rivers/ western Russian Arctic (paper I, II and III) The Kalix River in northern Sweden is one of the largest unregulated rivers in northern Europe. It drains a catchment rich in peatland and its upper reaches are located in the discontinuous permafrost zone. The Russian Arctic rivers Ob and Yenisey drain one of the largest peatland systems in the world (Figure 2A, pink areas; Kremenetski et al., 2003). The northern parts of their watersheds are also underlain by discontinuous and continuous permafrost (Figure 2B). The annual mean isotherm of 0°C crosses all three drainage basins and makes these ecosystems particularly sensitive to a warming climate. Permafrost thaw in the Swedish mountains is already ongoing (e.g. Christensen et al., 2004) and climate warming reported in the Ob and Yenisey basins (Frey and Smith, 2003) is expected to amplify C release (Frey and Smith, 2005). Several studies have shown similarities in hydrogeochemical properties of these three rivers Kalix, Ob and Yenisey (Gustafsson et al., 2000; Guo et al., 2004; Ingri et al., 2005; van Dongen et al., 2008a). The easily accessible Kalix River and the receiving Bothnian Bay, a well- studied system, can therefore act as a valuable model system for the much less accessible Russian-Arctic rivers.

2.3 The International Siberian Shelf Study 2008 (ISSS-08) From time to time, logistical challenges and climatic conditions do not withhold people from planning and performing extensive field campaigns in the remote (Russian) Arctic. In July 1878, Adolf Erik Nordenskiöld and his crew started what was to become their most famous journey onboard the Vega (Nordenskiöld, 1880). They sailed from southern Sweden all the way to the Bering Strait where they arrived a bit more than a year later, with a 10-month break when they were frozen into the ice. They were the first to succeed in sailing through the complete Northeast Passage. Nordenskiöld claimed that previous expeditions through the Northeast Passage had failed because they encountered much ice on their route far away from the coast. Instead, the Vega made use of the large and relatively warm freshwater inflow of the large Siberian Rivers, and navigated through open waters close to the shore.

Figure 4 (left) Adolf Erik Nordenskiöld with his ship Vega on the background (painted by Georg von Rosen (Wikipedia)), (right) the Nordenskiöld archipelago: a group of 90 windswept and desolate islands west of the Vilkitskiy Strait. We sailed past these islands with our ship the Yakob Smirnitskiy on 21 August 2008 (picture by Johan Gelting).

In the footsteps of Nordenskiöld, an international team of 30 scientists departed in August 2008 from Kirkenes in northern Norway to follow the same route almost exactly 130

18 years later. This International Siberian Shelf Study 2008 (ISSS-08) performed extensive sampling along the complete Northeast Passage with the main focus on the understudied Laptev and East Siberian Sea. In addition to a wide variety of sampling while underway, the biogeochemistry team onboard collected more than 250 surface sediments, around 100 sediment cores, and filtered more than 50,000 liters of sea water. Paper IV reports on some of the first results from the southern East Siberian Sea.

To be able to join such an expedition is a once-in-a-lifetime experience that, in my opinion, one should never say no to. The tremendous size of the preparations, continuously changing working schedules, cooperating in harsh conditions; it sure was not a piece of cake, but one of the most unforgettable and valuable learning experiences in my (PhD) life.

3. METHODS

This section describes the analytical methods that are used in this thesis, and the way in which the results can be used and interpreted to reveal information on the environment.

3.1 Field sampling Sample collection in this research area often takes place during ship-based expeditions. Surface sediments can be obtained with grab samplers such as the Ekman or van Veen. To be able to collect the most recent sediments, one should use a spoon or spatula to carefully take out the top layer, transfer to a clean container and store frozen (at least -18°C).

During fieldwork in this thesis, sediment cores have been collected with a GEMAX (or “Gemini”) corer (Figure 5). This dual gravity corer (from Oy Kart AB, Finland; modified at Stockholm University) retrieves two parallel cores of 9 cm i.d. and maximally 80 cm long. The original stainless steel cutting part of the corer was adjusted so that the plexiglass tubing could be taken out after every cast without taking out the liners. Lead weights can be added (4 x 5 kg) to maximize sediment penetration. After retrieving the GEMAX, the plexiglass core tubes filled with sediment and overlying water should be sealed and labelled. It is

Figure 5 GEMAX dual gravity corer.

20 possible to sample overlying water and the turbid bottom (nepheloid) layer for analyes. The cores should be sliced and frozen as soon as possible to minimize degradation. Core sectioning into slices of 0.5 or 1.0 cm is done with an extruder (also from Oy Kart AB, Finland). Suspended particulate matter is collected by pumping water through a glass fibre filter (GF/F) system. We have used a submersible pump (model AN19, Debe Pumpar AB, Sweden) that is connected with reinforced silicon and PVC tubing and stainless steel connections to the filtration set-up. The GF/F system is built up of a stainless steel filter holder (293 mm or 142 mm) that contains filters with a pore size of 0.7 µm (Whatman). A pressure meter is attached before the filter, and an in-line electronic flow meter after the filter to monitor back pressures and water volumes, respectively. Back pressures should be kept below 1 bar to avoid cell breakage and clogging/cracking of the filter. Before transfer into a clean aluminum envelope and frozen storage, excess (sea)water should be drained. In the study of molecular compounds and natural isotopes in the environment we cannot permit to encounter any (traces of) isotope spiking or oil contamination. It is therefore extremely important to have clean working routines both during sampling and laboratory analyses. We are doing this by for example (1) pre-combusting (12h, 450°C) the materials we use such as glass and aluminum foil, (2) avoiding the use of any kind of plastic material, (3) thorougly pre-rinsing all the sampling equipment and laboratory glass, and (4) performing swab tests to check for radiocarbon contamination in all new field or laboratory spaces.

3.2 Bulk geochemical methods and their usage After acidification of the sediments and surface water suspended particulate matter samples to remove inorganic C, the TOC, total nitrogen (TN) and stable C isotopes (δ13C) were analyzed at the Davis Stable Isotope Facility (USA) (paper I and IV) and the Department of Geological Sciences of Stockholm University (Sweden) (paper IV). A detailed description can be found in the papers. The TOC/TN content in a sample can be used to distinguish between different sources. Generally, marine algae have TOC/TN mass ratios between 4 and 10, whereas higher plants have values >20 (Hedges et al., 1986; Meyers, 1994). However, the ratios can be misleading due

21 to a contribution of inorganic N. This either occurs in samples with very low C contents, or in regions with a high contribution of clay minerals (i.e. illite). These minerals bind ammonium to soil-derived organic matter and thus lower the TOC/TN ratios. The inorganic N fraction can be as high as 34-63% of the TN in the central Arctic Ocean (Schubert & Calvert, 2001). Stable C ratios are also indicative of different OC sources. Marine sources typically show values around -21‰ while terrestrial plants (using the C3 fixation pathway) commonly are around -27‰ (Fry and Sherr, 1984; Meyers, 1997). By applying two-endmember mixing models (e.g. Semiletov et al., 2005) one can derive the relative fractions of marine and terrestrial OC in Arctic samples.

3.3 Molecular analyses of soil OC biomarkers Freeze dried and homogenized samples were extracted and fractionated as schematically described in Figure 6. Further details can be found in paper I-IV. Paper II analyzed the molecular composition of different Sphagnum species from northern Scandinavia. These samples were collected in aluminum envelopes and air-dried in paper bags before being extracted and fractionated according to Figure 6.

3.4 Distribution patterns and relative abundance of soil OC indicators The presence and relative concentrations of terrestrial biomarkers can also be used to reveal different OC sources. Long-chain n-alkanes, n-alkanoic acids and n-alkanols (C numbers >20) from higher plant leaf waxes (Eglinton and Hamilton, 1967) are characterized by an odd-over- even (n-alkanes) and even-over-odd predominance (n-alkanoic acids and n-alkanols) among the high-molecular weight (HMW) homologues (Bray and Evans, 1961; Figure 7). This is caused by the structure of their building blocks. This predominance is expressed by means of the carbon preference index (CPI), typically >5 for extant plants (Rieley et al., 1991). A high degree of degradation causes CPIs to approach 1 due to cleavage of alkyl chains that produces n-alkanes without odd-over-even predominance (Killops and Killops, 2005).

Figure 6 Schematic representation of extraction, fractionation and purification of wax lipids from sediment and particulate matter samples.

Distribution patterns in high-molecular weight n-alkanes can be indicative for specific groups of plants. Ficken et al. (2000) introduced the n-alkane proxy Paq (C23+C25)/ (C23+C25+C29+C31) for submerged/floating water plants. The relative abundance of mid-chain length n-alkanes (C23 and C25) in these species resulted in Paq values of around 0.69 compared to values around 0.09 for higher plants (abundant in C29 and C31). Later, more specific studies on Sphagnum revealed that C23 was the most abundant n-alkane (Nott et al., 2000; Pancost et al., 2002), and the ratio

C23/(C23+C29) became more commonly used, also because the C31 n-alkane is abundant in some Sphagnum species growing in drier habitats (Nichols et al., 2006). However, all these studies have analyzed Sphagnum species from mid-latitudes (e.g. UK, Ireland, The Netherlands). In paper II, we analyzed multiple Sphagnum species from six different sites throughout northern

Scandinavia and showed that the C25/(C25+C29) n-alkane ratio is most suitable to trace (sub- )Arctic Sphagnum-derived soil OC. The loss of functional groups is indicative of degradation. Long-chain n-alkanoic acids will thus degrade faster than long-chain n-alkanes and one can use the ratio of long-chain n- alkanoic acids over n-alkanes as a comparative measure of degradation status in sediment and POC samples. Additionally, one can compare the relative abundance of stanols, degradation products of sterols, to their “mother” sterols. Killops and Killops (2005) suggested to use the ratio of the sum of cholestanol, campestanol and stigmastanol divided by the sum of cholesterol, campesterol and β-sitosterol. A high ratio will then reflect a high degree of degradation. Marine influence on organic matter can also be detected on a molecular level. Aquatic phytoplankton produces low-molecular weight n-alkanes (C17+C19), n-alkanols (C16+C18) and n- alkanoic acids (C14+C16+C18) (e.g. Fahl and Stein, 1997 and references therein). The ratios of these low-molecular weight (LMW) compounds against their HMW counterparts from higher plants can thus be used as an indicator of relative marine/terrestrial input. Certain sterols (e.g. dinosterol, Boon et al., 1979; asterosterol, Kobayashi et al., 1973; cis-22-dehydrocholesterol, Idler and Wiseman, 1971) can also be used as biomarkers for marine organisms.

Figure 7 Structure and characteristic distribution patterns of terrestrial biomarkers originating from leaf waxes in higher plants.

3.5 Bulk and molecular radiocarbon analyses Isolation of recalcitrant soil OC markers was performed using preparative capillary Gas Chromatography (pcGC) as was first described by Eglinton et al. (1996). After extensive extraction and purification, isolation of individual compounds takes place in trap capillaries in the preparative fraction collector unit (Gerstel, Germany) that receives approximately 99% of the flow from the pcGC megabore column (Figure 8). The purities are determined by running the harvested fractions in scan mode on GC-MS. Laboratory swab blank tests (following Buchholz et al., 2000) excluded background contamination in our labs. Details are described in the supplementary information of paper III. The time-intensive isolation of individual target molecules resulted in a minimal amount of ca. 40 µg C, required for Accelerator Mass Spectrometry (AMS). Bulk and molecular radiocarbon analyses were performed at the US National Ocean Sciences AMS (NOSAMS) facility of the Woods Hole Oceanographic Institution. Isotope fractionation as a result of partial collection of the harvested compounds can be

25 excluded (Zencak et al., 2007). Radiocarbon results for n-alkanoic methyl esters were corrected for the methyl group of the derivatization agent. Compound-specific radiocarbon analysis on terrestrial biomarkers can, among other things, be used to determine the relative input of fossil or ancient sedimentary kerogen and contemporary sources (Eglinton et al., 1997; Drenzek et al., 2007) and examine soil OC sequestration and build up (Smittenberg et al., 2006; Drenzek et al., 2009).

Figure 8 Isolation of target compounds from the nonpolar and derivatized acid fraction (Figure 5) by means of preparative capillary Gas Chromatography (pcGC).

4. SUMMARY AND DISCUSSION OF MAIN RESULTS

This section follows the division of the previously described “Specific objectives” as illustrated by the six text boxes in Figure 1.

4.1 Sources of terrestrial OC (Paper I - IV) Both study locations in this thesis (Bothnian Bay and southern East Siberian Sea) show a dominant input of terrestrial organic matter from vascular plants as indicated by (1) the abundant presence of long-chain n-alkanes, n-alkanoic acids and n-alkanols, (2) high CPIs, (3) relatively high HMW/LMW ratios, (4) the presence of higher plant sterols such as β-sitosterol, campesterol and cholesterol, (5) terrestrial δ13C signatures and (6) terrestrial TOC/TN ratios (in Bothnian Bay) (paper I and IV). A significant fraction of the particulate terrestrial organic matter transported from northern Scandinavia is derived from peatlands, as illustrated by the abundant presence of mid- chain n-alkanes (C23 and C25) diagnostic for Sphagnum species (paper I and II). The imprint of peatland-derived material is stronger in the open Bothnian Bay, influenced by Finnish rivers with relatively high peatland coverage in their watersheds, than in the sheltered Kalix River estuary. The large unregulated Kalix River extends large-scale east-west trends across the Eurasian continent in transported terrestrial organic matter characteristics, and resembles the western Russian Arctic river Ob draining a similar peatland-rich watershed (paper I). The impact of peatland/wetland-derived particulate terrestrial organic matter is less, but still substantial, in the southern East Siberian Sea (paper IV).

4.2 A proxy for (sub-)Arctic Sphagnum (Paper II) Moss-covered northern peatlands contain massive amounts of OC. The relative contribution of these Sphagnum-rich peatlands can be inferred from unique distribution patterns among their n-alkanes. Analyses of eight Sphagnum species from six different locations throughout northern Scandinavia showed that the C25 homologue was dominant in nearly all samples. The C25/(C25+C29) ratio showed lowest variations between species and

27 between different latitudinal regimes. In addition, this ratio demonstrates a larger dynamic range between Sphagnum and higher plant endmember values than other n-alkane proxies (i.e.

Paq and C23/(C23+C29)), and hence makes two-source mixing model calculations more reliable. We therefore propose that this proxy should be used for future studies on fluxes of Sphagnum/peatland-derived terrestrial OC from (sub-)Arctic regions (paper II). The first application of this ratio in the Bothnian Bay suggested a Sphagnum/peatland contribution of 82-88% to surface sediments and 68-100% to surface water suspended particulate matter. Peatland coverage in the catchments of the largest rivers draining into the Bothnian Bay lies roughly between 20 and 40% (Arvola et al., 2004; HELCOM, 2004) but the contribution of peatland-OC to the total flux of fluvial OC is certainly much larger than 20-40% because of high soil OC contents in peat. Arvola et al. (2004) showed that annual TOC loads are significantly higher from peatland basins than from those dominated by fields. Additionally, previous studies in northern Scandinavia have inferred that peatland is the dominant landscape compartment for TOC delivery, both during spring flood and storm events (Pettersson et al., 1997; Ingri et al., 2005). A lower contribution of peatland-derived particulate terrestrial matter in the East

Siberian Sea, C25/(C25+C29) ratios are ~0.45 instead of ~0.65 in northern Scandinavia, likely represent the lower areal extent of wetlands/peatlands in its drainage basin (paper IV). However, a drier climate and/or a different species composition could also cause a lower abundance of mid-chain n-alkanes in the fluvial molecular fingerprint.

4.3 Origin and age of soil OC (Paper III and IV) Bulk terrestrial OC transported into the Bothnian Bay consists mostly of contemporary sources, as illustrated by high 14C contents (around -50‰) in the surface sediments (paper III). On the contrary, organic matter delivered to the East Siberian Sea is much older (around -500‰ in surface sediments; paper IV). The west-east Eurasian trend towards older OC sources as suggested by Guo et al. (2004) and van Dongen et al. (2008a) is thus corroborated by this thesis. Molecular radiocarbon measurements of lipid biomarkers in the coastal reactor (paper III) allowed us to distinguish between (sub-)Arctic soil OC pools with remarkably different

28 susceptibility to degradation. While settling processes in the Bothnian Bay coastal system are in the order of days to weeks, we detected a fractionation of ca. 420‰ (or ~6000 14C years) in HMW n-alkanoic acids that settled from surface waters to the underlying sediments. This suggests the delivery of two different pools of soil OC to the coastal environment; a young and easily degradable surface peat pool, and an old and refractory (deep) mineral soil pool (Table 1). The young pool most likely originates from upper soil horizons and surface peatlands. The low density of such humic aggregates makes this pool susceptible to extensive mineralization during fluvial and coastal transport (paper III and IV). The old soil pool is likely bound to minerals; this matrix-association provides protection against substantial degradation, and serves as ballast for selective settling upon release into coastal waters (paper III and IV). Paper IV confirmed the release of these two dominant soil OC pools by showing that surface POC was highly degraded but young ( 14C ca. -60‰ corresponding to ca. 500 14C years), whereas the underlying sediments were less degraded but old ( 14C ca. -500‰ corresponding to ca. 5500 14C years). The old mineral soil OC that is fluvially transported into the Bothnian Bay probably originates from deeper mineral soils formed in the early Holocene (MacDonald et al., 2006) or from relict OC formed in pre-glacial times (Fabel et al., 2002) (Paper III). The old fraction of OC in the East Siberian Sea sediments is presumably delivered through two different transport mechanisms; the release of Pleistocene yedoma from eroding coasts, and the fluvial release of thawing inland (deep) mineral soils (paper IV) (Table 1).

Table 1 (Sub-)Arctic soil types rich in OC with key characteristics for degradation potential, major transport routes and their dominant fate after (thaw-)remobilization.

4.4 Relative input of coastal erosionOC and fluvial OC (Paper IV) A three-endmember mixing model, using radiocarbon and stable C isotope signatures, revealed an offshore-decreasing fraction of fluvial OC (~50% to ~10%) and a constant fraction of coastal erosion OC (~50%) in the surface sediments of the southern East Siberian Sea. Although erosion OC tends to settle rapidly close to shore, we still measure a contribution of ~50% on mid-shelf, ca. 500 km from the coast. We believe that the distribution of old and recalcitrant OC far out on the East Siberian Sea shelf can be attributed to predominant transport mechanisms in the benthic boundary layer and through incorporation in sea-ice. Additionally, considerable proportions of land-derived OC may be transported offshore through incorporation in sea ice during ice formation (Ogodorov, 2003; Stein and Macdonald, 2004). Furthermore, preferential degradation of riverine OC relative to erosion OC during bottom transport might play a role. This could explain the decrease in the riverine OC fraction relative to the erosion OC fraction in the surface sediments along the transect.

4.5 Terrestrial OC degradation in (sub-)Arctic coastal waters (Paper I, III and IV) Fluvially delivered particulate terrestrial OC undergoes active degradation in the water column and surface sediments in the northern Baltic Sea and the southern East Siberian Sea (paper I and IV). This terrestrial OC can be subdivided in various fractions that have different propensities to degradation. For example, transformation of most of the higher plant sterols to their degradation products stanols occurs at a rate that is slower than watershed flushing and settling rates (paper I). Furthermore, OC-normalized long-chain n-alkane concentrations increase off-shore, despite the addition of plankton-derived OC to the TOC pool, and thus indicate a higher recalcitrancy of these compounds compared to the rest of the OC (paper I). These tracers of terrestrial OC are recalcitrant because of their structure and their physical protection. Molecular radiocarbon analyses on soil OC tracers revealed two soil compartments with a substantial difference in remineralization potential; a fresh surface peat pool that undergoes rapid degradation, and a deep, mineral soil OC pool that is preferentially relocated to coastal sediments without undergoing substantial degradation (paper III).

The degradation extent of terrestrial OC in coastal surface sediments, as illustrated by the relative abundance of HMW n-alkanoic acids over HMW n-alkanes, clearly increases offshore in both our study regions. However, the Bothnian Bay sediments seem more degraded than terrestrial OC in the southern East Siberian Sea (HMW n-alkanoic acid to HMW n-alkane ratios of 0.5 - 1.5 and 1.0 - 6.0 respectively). The low level of degradation in the eastern Russian Arctic surface sediments might reflect the recalcitrant nature of the yedoma OC fraction. Yedoma formation occurs after rapid freeze-up of top organic matter layers when aeolian or river-borne materials cover surface soils. The lack of in situ degradation in combination with a physical protection due to mineral-associations is probably manifested in the low extent of degradation we detect in the surface sediments (paper IV). Particulate matter degradation shows opposite trends; here the Bothnian Bay material is less degraded (HMW n-alkanoic acid to HMW n-alkane ratios between 1.0 and 3.0) than fluvially released OC in the eastern Russian Arctic (values around 1.0). These trends are probably partly related to seasonal sampling effects. In the Bothnian Bay, sampling took place during the spring flood when fresh riverine material is being delivered to the coastal system. In contrast, the sampling campaign in East Siberia was in late summer, when maximal permafrost thaw remobilizes degraded OC and spring flood delivered material had been subject to weeks-month long aging in the water column (paper I and IV).

4.6 Terrestrial OC burial (Paper III and IV) Thaw-remobilization of the enormous permafrost OC pool is generally assumed to be directly linked to considerable contributions of greenhouse gases to the atmosphere (IPCC, 2007; Khvorostyanov et al., 2008; Schuur et al., 2008). However, paper III suggests that a substantial part of this remobilized pool is simply fluvially relocated to coastal sediments without undergoing significant degradation during transport. Paper IV confirms and details these findings. Preferential burial is attributed to mineral matrix associations that protect the OC from degradation and ballasts it for rapid settling. Molecular radiocarbon analyses on tracers for recalcitrant soil OC in river water and surface sediments allowed us to estimate the difference in preservation of the recalcitrant mineral soil OC pool compared to the rapidly

31 degrading surface peat soil OC. The recalcitrant pool appeared to be at least 20 times less susceptible to degradation than the younger peatland pool. This implies that the assumption of a causal relationship between thawing permafrost OC and greenhouse gas emissions needs a revision.

5. CONCLUSIONS

Coastal environments in the Bothnian Bay and East Siberian Sea show a dominant input of terrestrial organic matter from higher plants. In the Bothnian Bay, this mostly consists of peatland-derived material. In the East Siberian Sea, the contribution of peatland/wetland organic matter is less, but still substantial.

The n-alkane proxy C25/(C25+C29) is most suitable for tracing peatland-derived particulate terrestrial OC in (sub-)Arctic regions. Application of this proxy in the Bothnian Bay suggests that peatlands supply the major fraction of OC to the total fluvial POC flux.

In general, the bulk terrestrial OC released into the Bothnian Bay is young and degraded, whereas terrestrial OC released into the East Siberian Sea is older and more recalcitrant. More specifically, the total (particulate) terrestrial OC pool can be subdivided in two dominant pools that exhibit different susceptibilities to degradation; a surface (peat) OC pool and a mineral soil OC pool. The young surface peat OC is easily degradable because it is fluvially delivered in a light and humic suspension to the coastal system. The old mineral soil pool behaves mostly recalcitrant due to mineral-OC bonds that protect the OC from degradation and ballast it for rapid settling. We suggest this pool to be fluvially released from deep mineral soils in the catchments, and additionally through coastal erosion of yedoma in the East Siberian Arctic.

East Siberian Sea surface sediments are strongly affected by the input of recalcitrant yedoma OC from coastal erosion. This imprint is equally strong in coastal regions as in areas further out on the shelves while the riverine input decreases offshore. This suggests dominant transport in the benthic boundary layer and in sea-ice and preferential degradation of riverine OC compared to erosion OC.

Fluvially delivered particulate terrestrial OC undergoes active degradation in the water column and in surface sediments in the Bothnian Bay and the southern East Siberian Sea. Differences in degradation extent can be attributed to chemical structures, seasonal (sampling) effects, protection due to associations with minerals, and selective settling.

Our first estimates suggest that, in the coastal system, mineral soil OC is at least 20 times less susceptible to degradation than peatland OC. Hence, a considerable part of the mineral OC pool is resistant to degradation and selectively buried in coastal sediments. This implies that different (sub-)Arctic soil OC pools demonstrate different potentials for positive feedback to climate warming.

6. FUTURE PERSPECTIVES

We are starting to unravel more and more of the remote and inaccessible Arctic puzzle, but there still remain huge gaps in our understanding of northern biogeochemical processes and their interactions with the hydrological cycle. This thesis has helped to reveal information that can act as a starting point for further research.

The development of a Sphagnum-proxy for northern Scandinavia (paper II) is certainly an improvement in circum-Arctic studies on peatland-derived terrestrial organic matter, compared to proxies developed in lower latitude regions. However, climatological and hydrological differences among (sub-)Arctic wetlands can cause very heterogeneous Sphagnum species distributions and/or slightly different n-alkane patterns. Furthermore, the relative occurrence of Sphagnum in northern peatlands varies within the Arctic; the wetter regions in the West-Siberian Lowlands contain huge amounts of these mosses, whereas the drier bogs in Eastern Siberia also consist of many other species. For these reasons, the proxy should be applied with caution. Ideally, one should sample different species representative for the different vegetation covers in a catchment when source-apportioning terrestrial OC in coastal regions. Radiocarbon and geochemical analyses on a molecular level revealed different soil OC pools with significantly different susceptibility to degradation. Paper III hypothesizes two dominant pools; a surface peat component that rapidly degrades upon arrival in the coastal system, and a deeper soil component that is mineral-bound and ballasted for rapid settling. Obviously, the division is not that clear-cut and there must exist many more levels of recalcitrancy within the soil OC pool. Similar to earlier studies (e.g. Keil et al., 1994; Huguet et al., 2008) we attribute the relative recalcitrancy of different soil OC components to the degree in which OC is associated with mineral soil components. In addition, other factors play a role such as types of remobilization (e.g. sudden thermokarst, slow active layer thaw, surface peat run-off), seasonality (e.g. spring flood versus winter base flow, light ice-free conditions in summer versus dark and ice-covered winters) and availability of nutrients and micro-organisms to breakdown the terrestrial OM. We need to understand the relative degradability of different

35 fractions of the soil OC pool to be able to estimate its potential to be converted into greenhouse gases. This is crucial for global estimates of linkages between thawing permafrost OC and positive feedback to a warming climate. For this reason, future studies could divide soil OM into different fractions based on size, density or chemical properties (Trumbore and Zheng, 1996) to distinguish between potentially labile and refractory components. One could then determine the OC contents of these fractions and measure their susceptibilities to degradation or respiration (e.g. by molecular analyses or incubations). Dutta et al. (2006) followed a similar approach with thawing yedoma in the Kolyma delta. They measured respiration fluxes (CO2) over time and the relative contributions to these fluxes by different fractions of the yedoma (mineral, humus, organic fragments and DOC). Since an important part of remineralization occurs during fluvial and coastal transport, future studies could additionally follow different soil OC fractions and their degradation along the hydrological route from the moment of thaw to deposition in the ocean. These type of studies should ideally be performed on different soils on a wide spatial scale within several vegetation zones. More studies confirm earlier approximate estimates on the large impact of coastal erosion OC in the Arctic shelf seas. This thesis also supports this by showing an equally strong imprint of coastal yedoma OC and fluvial OC in near-coast sediments (paper IV). Further offshore, the erosion signal stays constant whereas the riverine fraction decreases. This implies a greater recalcitrancy of the erosion OC compared to fluvial OC and/or preferential transport mechanisms in the benthic boundary layer and sea-ice. We need to find tracers or proxies to distinguish between these two dominant terrestrial OC sources (i.e. erosion and fluvial), with different vulnerabilities towards degradation and thus potential for positive feedback. The coupled isotope approach in this thesis (Δ14C and δ13C) is a good but expensive method to differentiate between these two pools of different ages (i.e. Pleistocene versus Holocene). First indications of bulk coastal erosion tracers such as high POC/DOC ratios or high ε280 (Sánchez- García et al., in preparation) should be expanded and corroborated by other bulk or molecular proxies.

One interesting issue here is the heterogenic character of Yedoma. This material consists of mineral material with large amounts of organic matter (e.g. peat fragments, plant macrofossils, roots, animal bones) intercepted by massive ice wedges (Schirrmeister et al., 2002). Respiration rates will vary among the different fractions and most likely increase with soil OC content (Dutta et al., 2006). The fractions we trace in coastal regions are probably the most recalcitrant parts that will settle fast due to their mineral ballast. We need to take this into account when developing and interpreting yedoma erosion tracers in the marine environment. Throughout the year, different landscape compartments are interacting with the hydrological cycle. This results in seasonal differences in type, quantity and lability of released organic matter (e.g. Neff et al., 2006; Holmes et al., 2008). There are regional differences, but generally one can say that hydrological flow paths will change in a warmer climate; permafrost thaw will increase groundwater flow (Bense et al., 2009, Lyon et al., 2009), winter base flows will be higher (Andréasson et al., 2004) and fluvial discharge will increase (Peterson et al., 2002). We need more seasonal studies to improve our understanding of seasonal hydrological- biogeochemical couplings and its relation to climate warming, to reveal periods when the most labile and recalcitrant OC pools are remobilized and to trace their origin. By analyzing particulate matter and surface sediments in coastal regions, we obtain an impression of current (i.e. seasonal or annual) release of terrestrial OM. It would be valuable to place these results in an historical context, especially in Arctic regions that now experience climate warming. If we can compare the current state of terrestrial OM release with earlier trends in pre-industrial periods, this enables us to detect and quantify recent and future changes in terrestrial OC export from thawing Arctic permafrost. This historical context can be provided by collecting coastal sediment cores with consistent age-depth relationships. This is a major challenge since sediments on the shallow Arctic shelves almost always are disturbed by ice-scouring and storms, and reliable chronologies are therefore rare. Undisturbed sediment cores would be invaluable since they integrate fluvial terrestrial OC release over time. Alternatively, sediment cores from lakes in Arctic deltas can also be used to trace historical trends in OC release and place the ongoing climate-warming induced signal in perspective.

ACKNOWLEDGEMENTS – DANKWOORD – TACK!

Stockholm, Sweden. Five unforgettable years. How I love the archipelago, the strong seasons, biking through the summer nights, Söder, the nature in the city, the Swedes and their dedication to traditions and decency in traffic. I learnt to dance, to like sea food (fermented or not), to hate boiled potatoes, to talk Swedish, to love research even more. And I found out that I am more Dutch than I realized.

Most of all, I want to thank Örjan, my supervisor. I have truly enjoyed working with you, and could not have wished myself a better supervisor. Strict but righteous, inspiring and motivating. I respect and appreciate you very much. Even in harsh times (believe me; lack of sleep, together with stress and sea sickness is not the best combination..) we managed very well, knowing that some verbal collisions are only natural, and wouldn’t harm our connection. The amount of energy and drive you have for research is amazing and contagious. I trust we can keep on collaborating! A solid Alma mater. And of course, Bart, the talkative Dutch, I want to thank you too! For supply of appeltaart, kip- saté, pepernoten and frequent Dutch news flashes (the death of Swiebertje, the arrival of Sinterklaas, the Dutch medals at the Olympics, you name it!). But of course mostly for all the things you taught me within the field of organic geochemistry, your unstoppable flow of ideas and the inspiring discussions.

There are many other people at ITM I want to thank; Karin, for your help with an endless list of administrative stuff, but much more for your enthusiasm, support, and the twinkle in your eyes. Yngve for great dancing, Kerstin for many weather and holiday updates, Marsha (I loved our Åland islands and Joensuu trips!), Margot (I’ll never forget your birthday cake with candles in Hellasgården!), Merle (you really are my favorite German!) and plenty of others for party company and Swedish mountain holidays (Jenny, Ullis, Åsa, Maria L., Robin, Niklas, Anna- Karin, ....).

Our “little” MPG group (including the alumni) has always been a solid and pleasant base for me. Tack! to Hanna for being the analytical-lab-routine endmember (versus Bart the chaotic organic geochemist) and for being a “steady rock in the waves” in rough times. Zdenek for gossiping like a woman, helping out with the pcGC in the weekends, and joining in on many activities. Rebecca: Texas professor! Finnhamn, Argo dinners, a lot of talking, we had an ultimate summer. Marie: sediment core queen with the bright laughter! thanks for cosy times at work, in Oslo and on Ornö. And also thanks to the rest of the crew: Zofia, mr. Recumbant Henry, Maria U., Pelle, dr. Anderson (the a-typical Swede with an even louder laugh) and the Sect members alias “the swamp” (i.e. Charline, Axel, Emma, Elena, Christoph, Daniel, Laura, Brett, Michael Jr., Michael Sr., and all the others that feel they belong here!) for adding some spice to the office life and a never-ending flow of activities (particularly during our honeymoon in höst- 08/vår-09); stormy kayak weekends, Russian Disco in Berlin, dancing, drinking, BBQ’s and camping trips, etc. etc. All the best to you, whereever you may end up! And no more accidents please.

But there are a few that really deserve more than a few words in these acknowledgements (supplementary info?). Vanja, where to start? We share so much. Thanks for many cosy evenings, Stockholm life, carbon cycle discussions, feeding me chocolate cake when I was in “grönsaks” mode on the ship, great times in Oslo...(tack also Vidar!) Baby, just stay online in Skype! Martin: we were the dreamteam onboard the Smirnitsky! Working with you in the GFF container on the silent pre-breakfast shifts, zipping coffee, hearing the ship crush the ice and talking about life... some of the most special moments of the expedition! You are the one that keeps me updated on the latest food and health trends, supplies advice on various topics and dad of my favorite twins. And last but not least... James: yes, to the flow! My buddy in Belgian Beer drinking, drum ‘n bass, climbing Kebne, exploring the dodgy streets of Barcelona, biking along Amsterdam canals and many, many creative cooking experiments. We have spent so much time together. I really enjoyed it. Life in Stockholm is not the same now you’re gone!

I also want to thank Fredrik and Johan (the “Lule-killar”) for cozy early morning coffee- moments in the electrically charged wet lab. Igor for your devotion, humor, energy and neverending compliments on the ISSS08 trip, Oleg for always being able to master the winch monster, and the others in the Russian crowd onboard the Smirnitskiy (Vova, Sveta, Irina). Per for your amazing love for surströmming, I’ll do my best to spread this tradition! I also enjoyed hanging out with some “Geo-folks” like Stina, Arvid and Daniela (for inspiring IPY activities!) and Annika, Gustaf, Steffen and Britta.

Wow, this list is neverending! A good sign I suppose :-) Of course there are many people outside work life that have been (and are!) special to me: in and outside Stockholm, over the phone/sms/e-mail, during wine tastings, hiking trips and many reunions and travels; Fiona, Tanya, Jennifer, Yvonne, Garrelt, Stan and Célia, Sonja & Jouni, Dave, Pekka and other Svalbardians. And all my friends in the Netherlands, despite the distance you have always shown support and interest in what I am doing here. Jullie betekenen veel voor me! (Winita, Erny, Lissy, Anke, Sanne, Ilse, Rishi, Alex, Remco, Mara, Yvette). Finally, I want to thank my family; bro Bart and Marjolein and of course my “producers”: mum Els and dad Albert-Jan. For letting me grow up close to nature; climbing trees, boat rides, playing with mud, rabbits, cable cars and kayaks. This has made me into who I am now! I love you!

Yes: a proper dream changes reality. I am very happy I moved here. What will be next?

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