Selective Sorting and Degradation of Permafrost Organic Matter in the Nearshore Zone of Herschel Island (Yukon, Canada)

Erosion of permafrost coasts due to climate warming releases large quantities of organic carbon (OC) into the Arctic Ocean. While burial of permafrost OC in marine sediments potentially limits degradation, resuspension of sediments in the nearshore zone potentially enhances degradation and greenhouse gas production, adding to the "permafrost carbon feedback." Recent studies, focusing on bulk sediments, suggest that permafrost OC derived from coastal erosion is predominantly deposited close to shore. However, bulk approaches disregard sorting processes in the coastal zone, which strongly influence the OC distribution and fate. We studied soils and sediments along a transect from the fast-eroding shoreline of Herschel Island-Qikiqtaruk (Yukon, Canada) to a depositional basin offshore. Sample material was fractionated by density (1.8 g cm-3) and size (63 mu m), separating loose OC from mineral-associated OC. Each fraction was analyzed for element content (TOC, TN), carbon isotopes (delta 13C, Delta 14C), molecular biomarkers (n-alkanes, n-alkanoic acids, lignin phenols, cutin acids), and mineral surface area. The OC partitioning between fractions changes considerably along the transect, highlighting the importance of hydrodynamic sorting in the nearshore zone. Additionally, OC and biomarker loadings decrease along the land-ocean transect, indicating significant loss of OC during transport. However, molecular proxies for degradation show contrasting trends, suggesting that OC losses are not always well reflected in its degradation state. This study, using fraction partitioning that crosses land-ocean boundaries in a way not done before, aids to disentangle sorting processes from degradation patterns, and provides quantitative insight into losses of thawed and eroded permafrost OC. The Arctic coastline is vulnerable for erosion, since it mostly consist of permafrost (permanently frozen ground), which undergoes rapid warming due to climate change. Thaw and erosion of permafrost along the coast releases organic carbon (OC), that has been stored for millennia, into the ocean. There, in the ocean, this carbon can be degraded back into CO2, potentially enforcing climate warming, or it can settle to the sediments, and potentially be stored again for millennia. The permafrost and the sediment consist of particles, for example, sand, silt, and clay, and loose fragments of vegetation. In this study, we separated the OC that is associated with fine particles (silt and clay) from the loose fragments of organic matter. We observe that these two fractions of sediment have a very different composition and source, and end up somewhere else in the marine system. This study further shows that a large part of the OC associated with minerals (silt and clay) is lost during transport, but also that indicators for degradation show contrasting results. Overall, we highlight the importance of looking at sediment fractions separately, and also emphasize studying the transitional zone between land to ocean for the permafrost carbon cycle. Size and density fractionation of sediment organic carbon (OC) reveals substantial contrasts between different fractions and environmentsOC- and biomarker loading shows that a significant amount of terrestrial OC is lost along a land-ocean transectCombining methods in dynamic environments such as the nearshore zone is key in studying the fate of permafrost OC