Elsevier

Organic Geochemistry

Volume 91, January 2016, Pages 16-30
Organic Geochemistry

Historical records of organic matter supply and degradation status in the East Siberian Sea

https://doi.org/10.1016/j.orggeochem.2015.10.008Get rights and content

Highlights

  • We present the first centennial record of OC deposition on the Siberian Arctic Shelf.

  • Spatial differences (inner vs outer shelf) were larger than down-core changes.

  • Terrigenous OC was more recalcitrant after burial than during cross-shelf transport.

  • Marine OC readily degraded in situ with a half-life of about 20–30 years.

Abstract

Destabilization and degradation of permafrost carbon in the Arctic regions could constitute a positive feedback to climate change. A better understanding of its fate upon discharge to the Arctic shelf is therefore needed. In this study, bulk carbon isotopes as well as terrigenous and marine biomarkers were used to construct two centennial records in the East Siberian Sea. Differences in topsoil and Pleistocene Ice Complex Deposit permafrost concentrations, modeled using δ13C and Δ14C, were larger between inner and outer shelf than the changes over time. Similarly, lignin-derived phenol and cutin acid concentrations differed by a factor of ten between the two stations, but did not change significantly over time, consistent with the dual-carbon isotope model. High molecular weight (HMW) n-alkane and n-alkanoic acid concentrations displayed a smaller difference between the two stations (factor of 3–6). By contrast, the fraction for marine OC drastically decreased during burial with a half-life of 19–27 years. Vegetation and degradation proxies suggested supply of highly degraded gymnosperm wood tissues. Lipid Carbon Preference Index (CPI) values indicated more extensively degraded HMW n-alkanes on the outer shelf with no change over time, whereas n-alkanoic acids appeared to be less degraded toward the core top with no large differences between the stations. Taken together, our results show larger across-shelf changes than down-core trends. Further investigation is required to establish whether the observed spatial differences are due to different sources for the two depositional settings or, alternatively, a consequence of hydrodynamic sorting combined with selective degradation during cross-shelf transport.

Introduction

Amplified global warming in the Arctic may affect regional and global carbon cycles. A putative carbon-climate coupling process in the Arctic is the accelerated destabilization and remobilization of carbon currently held in permafrost. Remobilization of this vulnerable carbon pool can occur as a result of several climate-driven mechanisms including deepening of the permafrost active layer, thermokarst processes, enhanced river discharge as well as thermal destabilization and erosion of coastal permafrost (Peterson et al., 2006, Wagner et al., 2011, Vonk and Gustafsson, 2013). Considering the vast amount of carbon stored in permafrost soil (1307 Pg; Hugelius et al., 2014), the translocation and ultimate fate of the terrigenous organic carbon (TerrOC) in the coastal ocean is insufficiently understood (Vonk and Gustafsson, 2013).

Furthermore, marine ecosystems are also undergoing rapid changes with rising temperatures (Walther et al., 2002, Hoegh-Guldberg and Bruno, 2010). Shrinking ice-cover and a warmer Arctic Ocean have the potential to highly increase marine primary production (Slagstad and Wassmann, 1996, Arrigo et al., 2008). Organic matter dynamics in the Arctic shelf seas, however, remain understudied, due to the complex nature of the system with heterogeneous pools and pathways from different sources, in addition to the logistical challenges for field campaigns.

The East Siberian Arctic Shelf (ESAS), the world’s largest continental shelf, receives TerrOC that is delivered by the Great Russian Arctic Rivers that drain extensive areas of permafrost (e.g., Stein and Macdonald, 2004, van Dongen et al., 2008, Gustafsson et al., 2011, Holmes et al., 2012, Semiletov et al., 2012). An additional important source of particulate organic carbon (POC) derives from the continuous erosion of the vast coastline, that includes extensive outcrops of Yedoma (Pleistocene Ice-Complex Deposits, ICD) (e.g., Stein and Macdonald, 2004, Schirrmeister et al., 2011, Vonk et al., 2012, Sánchez-García et al., 2014). Insights into the sources and degradation status of TerrOC on the ESAS have been gained from applying isotope-based source apportionment models to the surface sediments (e.g., Semiletov et al., 2005, Vonk et al., 2010, Vonk et al., 2012, Vonk et al., 2014) and biomarkers for both surface sediments (e.g., Stein and Macdonald, 2004, Feng et al., 2013, Tesi et al., 2014) and POC in the water column (e.g., Charkin et al., 2011, Karlsson et al., 2011). Yet, despite increased awareness of potential changes in the Arctic carbon cycle, knowledge is still lacking on dispersal and accumulation of both TerrOC and marine organic carbon on the Arctic shelves, and whether any changes have occurred over the past century. Most studies have focused on surface sediments, and do not address this temporal dimension.

This study explores how sources and degradation status of organic matter have evolved over the last century and assesses whether recent changes in the Arctic carbon cycle are evident in the shelf sediment record. Our analysis focused on two sediment cores from the East Siberia Sea (ESS) that record up to the last 120 years of sediment accumulation. The cores were collected in different depositional regimes: one nearshore located outside the Kolyma estuary and the other one from the outer shelf about 600 km from the coastline (Fig. 1). To reconstruct a historical record of the terrigenous input to the ESAS and to compare temporal to spatial changes we developed a 210Pb-based age model and measured down-core profiles of a suite of well-known terrigenous and marine biomarkers (solvent extractable high-molecular weight wax lipids and CuO oxidation products) as well as bulk carbon isotopes (δ13C, Δ14C) at these two locations.

Section snippets

Study area

The East Siberian Sea (ESS) is the largest marginal sea in the Arctic Ocean (Stein and Macdonald, 2004, and citations therein), spanning an area of 987,000 km2 that is bordered by the Laptev Sea along the western edge of the New Siberian Islands and the Chukchi Sea along Wrangel Island to the east. With a mean depth of 58 m and low bathymetric gradients, it also belongs amongst the shallowest shelf seas in the Arctic. The Indigirka and Kolyma are the main rivers discharging into the ESS (54 and

Geochronology and sediment surface area

In order to establish historical sediment records of organic matter input and degradation status an age model for dating the sediment cores is necessary. The radionuclide 210Pb with a half-life of 22.2 years has been widely used for marine sediment dating on a timescale of 100 years (e.g., Nittrouer et al., 1979, Strobl et al., 1998). In undisturbed sediment cores with constant sedimentation rates the relationship between the natural logarithm of the unsupported fraction of 210Pb, i.e. the

Conclusions

This study comprises the first down-core investigation of changes in the East Siberian Sea carbon cycling over the last century. A suite of terrigenous and marine biomarkers as well as source-diagnostic bulk carbon isotopes were analyzed in two cores from different depositional regimes in this Arctic shelf sea: one close to the Kolyma estuary and the other one from the outer shelf.

The sediment mass accumulation rates were relatively uniform over the past century at both stations and did not

Acknowledgments

We thank crew and personnel of the R/V Yakob Smirnitskyi and the sub-expedition on TB0012. The International Siberian Shelf Study 2008 (ISSS-08) expedition was supported by the Knut and Alice Wallenberg Foundation, Headquarters of the Far Eastern Branch of the Russian Academy of Sciences, the Swedish Research Council (VR Contract No. 621-2004-4039 and 621-2007-4631), the US National Oceanic and Atmospheric Administration (OAR Climate Program Office, NA08OAR4600758/Siberian Shelf Study), the

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