Organic geochemical investigation of sources, transport and fate of terrestrial organic matter in the southeast Laptev Sea

Abstract: Permafrost carbon stores have been suggested to react to warming trends with increased terrestrial loading to its coastal waters. Presently, the warming has been seen to be especially high in the East Siberian Arctic and the fate of the major release of terrestrial matter to these coasts is yet to be detailed.Our work is focused on the East Siberian Shelf (ESS) – which is the largest continental shelf in the world. It receives substantial inputs of terrestrial organic matter both from the large Russian rivers and from eroding coastlines. The largest of its rivers, the Lena, discharges in Buor-Khaya Bay, which is also a hot spot for coastal erosion.Previous studies of molecular and ?¹3C and ?¹?C composition of terrestrial organic matter received by Arctic coastal waters have suggested a different propensity of different terrestrial source materials towards bacterial degradation. This detailed isotopic and molecular marker survey which is the basis for this thesis reveals clearly distinguished source patterns both between surface water (POC) and sea floor (SOC) as well as with distance away from the sources. The heavy terrestrial dominance over marine/planktonic sources are clearly detected in gradients of high POC and SOC levels, as well as depleted ?13C -OC and high HMW/LMW n-alkane ratios. Furthermore, data suggests that terr-OC was substantially older yet less degraded in the surface sediment than in the surface waters. This unusual vertical degradation trend was only recently found also for the coastal and central East Siberian Sea. It seems that the riverine terr-OC component comprising mainly annual thaw layer surface soil dominates the buoyant surface water POC and is readily degraded. This is in contrast to the coastline-erosion terr-OC which is associated with minerals and therefore ballasted to the sediments where it makes up the key OC component and seems relatively protected from degradation.The study area of this work is a region with strong terrestrial influence hosting many of the important carbon cycling processes, and data reveal two important OC contributors of different origin, mineral associated coastal erosion matter and riverine borne surface soil matter. These two components may well represent different propensities to contribute to a positive feedback to climate warming by converting OC from coastal and inland permafrost into CO?.