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. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Accepted. Paper 4: Manuscript.

The Arctic Region is currently experiencing an amplified warming if compared to the rest of the world. The soils in this region store approximately half of the global soil organic carbon (OC), mainly locked in the permanently-frozen ground (permafrost). This carbon sink is sensitive to global warming meaning that the predicted warming will likely increase the thaw-release of this ‘old’ carbon. However, what happens to this remobilized OC once it is transported to the Arctic Ocean, including the potential conversion to greenhouse gasses causing a positive feedback to climate warming, remains unclear. In this work, we further investigate the fate of terrestrial derived OC (terrOC) in the Eurasian Arctic Region. The key findings of this work are: • Glycerol dialkyl glycerol tetraethers (GDGTs) and bacteriohopanepolyols (BHPs) are present in marine sediments of the Eurasian Arctic Region and the associated Branched and Isoprenoidal tetraether (BIT) and Rsoil indices can be used to trace terrOC in marine realm. However, a slight modification in the Rsoil index is suggested (R’soil). • Analyses indicate that the behaviour of BIT is largely controlled by a marine GDGT contribution while the R’soil index is mainly controlled by the removal of soil marker BHPs. Although both indices suggest a non-conservative behavior for the terrOC, this leads to differences in the estimations for the percentage terrOC present. A multi-proxy approach is essential since the use of a single-proxy approach can lead to over/under estimation.• Comparison of BIT and 13Csoc indices across the East Siberian Shelf indicates that the BIT index is possibly reflecting a predominantly fluvial input while 13Csoc represents a mixed fluvial and coastal erosion input.• The macromolecular terrOC composition varies along a west-east Eurasian Arctic climosequence and is mainly controlled by the river runoff of surface derived terrOC and wetland coverage (sphagnum vs. higher plants) but is not affected by the presence/absence of continuous permafrost. • The phenols/(phenols+pyridines) ratio was suggested as a proxy to trace terrOC at the macromolecular level along the Kolyma River-East Siberian Sea transect. The results indicate a non-conservative behavior of the macromolecular terrOC comparable to the bulk of the terrOC.All molecular analyses/based proxies used showed that the remobilized terrOC in the Eurasian Arctic region behaves non-conservatively potentially causing a positive feedback to global climate change.