Organic Carbon, Mercury, and Sediment Characteristics along a land – shore transect in Arctic Alaska

Abstract. Climate warming in the Arctic results in thawing permafrost and associated processes like thermokarst, especially in ice-rich permafrost regions. Since permafrost soils are one of the largest organic carbon reservoirs of the world, their thawing could lead to the release of greenhouse gases, further exacerbating climate warming. To enhance predictions of potential future impacts of permafrost thaw, we studied how soil characteristics change in response to permafrost landscapes affected by thermokarst processes in an Arctic coastal lowland. We analysed six sediment cores from the Arctic Coastal Plain of northern Alaska, each representing a different landscape feature along a gradient from upland to thermokarst lake and drained basin to thermokarst lagoons in various development stages. For the analysis, a multiproxy approach was used including sedimentological (grain size, bulk density, ice content), biogeochemical (total organic carbon (TOC), TOC density (TOCvol), total nitrogen (TN), stable carbon isotopes (δ¹³C), TOC/TN ratio, mercury (Hg)), and lipid biomarker (n-alkanes, n-alkanols, average chain length (ACL), 𝑃_𝑎𝑞, 𝑃_𝑤𝑎𝑥, carbon preference index (CPI), higher plant alcohol index (HPA)) parameters. The results showed highest TOC contents in samples of the thermokarst lake and the drained thermokarst lake basin. Lowest TOC contents were measured in the samples of the semi-drained thermokarst lagoon. The comparison of unfrozen and frozen deposits showed significantly higher TOCvol and TN in the unfrozen deposits. Indicated by the ACL, δ¹³C and the 𝑃_𝑎𝑞, 𝑃_𝑤𝑎𝑥 we found a stronger influence of aquatic organic matter (OM) in the OM composition in the soils covered by water, compared to those not covered by water. Moreover, it was indicated by the results of the δ¹³C, TOC/TN ratio, and the CPI that the saline deposits contain stronger degraded OM than the deposits not influenced by saltwater. Additionally, we found positive correlations between the TOC and TOCvol and the Hg content in the deposits. The results indicate that thermokarst-influenced deposits tend to accumulate Hg during thawed periods and thus contain more Hg than the upland permafrost deposits that have not been impacted by lake formation. Our findings offer valuable insights into the dynamics of carbon storage and vulnerability to decomposition in coastal permafrost landscapes, reflecting the interplay of environmental factors, landform characteristics, and climate change impacts on Arctic permafrost environments.