Sedimentary Carbon Flux and Source Apportionment in the Laptev and East Siberian Sea: Strong Shelf Imprint of Old Organic Carbon from Coastal Erosion

The Arctic Ocean has unusually large and shallow continental shelves, covering more than 50% of its total area. Large amounts of riverine transported terrestrial organic carbon (OC) are delivered to the Siberian Arctic Shelves, in addition to input of coastally eroded material that, based on very limited data, is estimated to be equally large. While the East-Siberian Arctic landmass undergoes the strongest climate warming on earth, the fate of these large-scale releases of terrestrial OC into the shelf seas is still poorly understood. Improving our understanding of terrestrial OC processing on the Eurasian Arctic shelves was one of the main objectives of the International Siberian Shelf Study, a 50-day research expedition in August/September 2008. We combined 2008 bulk elemental, 13C and 14C isotopic results with data from previous campaigns (2003, 2004 and 2007) to facilitate a spatially comprehensive investigation (200 locations) of the sediment composition on the Siberian Arctic shelfs. Our results show that coastal regions in the Laptev and East Siberian Sea generally exhibit higher OC contents, show stronger terrestrial influence and are older (up to 10 ky) than outer shelf regions. To determine the relative input of marine, erosion and riverine OC to the sedimentary OC fraction in the Laptev and East Siberian Sea surface sediments, we applied a dual-carbon isotope, three-end-member mixing model. The contributions of the three sources to the surface OC show a distinct spatial pattern; riverine OC contributes with 20-30% in the coastal regions and decreases to ca. 5% on the outer shelf. Coastal erosion OC makes up at least 50% of the surface OC in the outer shelf regions and up to 75% on the shallowest part of the shelves (<40m). The marine OC imprint is between 20-30% in Laptev Sea, and in the East Siberian Sea between 5-50%. The sedimentary imprint of old coastal erosion OC on the entire Laptev and East Siberian shelf is enormous. We hypothesize this may be explained by mineral protection of this OC, in contrast to the more bioavailable and buoyant riverine and marine OC. Bottom transport and ice-rafting are likely responsible for its widespread dispersal over the entire shelf, after fast sedimentation near the coast. Annual sediment OC accumulation rates for the East Siberian Sea, based on radiochronological calculations, will be presented and evaluated within a broader terrestrial OC budget for this climate-sensitive region.