Sedimentary organic carbon dynamics in a glaciated Arctic fjord: tracing contributions of terrestrial and marine sources in the context of Atlantification over recent centuries

Abstract. In this study, we investigated sedimentary organic carbon (OC) dynamics in Kongsfjorden, Svalbard, using three multicores collected during the HH22 and HH23 cruises aboard the RV Helmer Hanssen in 2022 and 2023. We assessed the relative contributions of petrogenic, soil-derived, plant-derived, and marine OC by applying a four-source apportionment approach based on Δ14Corg, δ13Corg, and lignin parameters, including the (Ad / Al)v ratio and lignin phenol concentrations, with Monte Carlo (MC) analysis. Age–depth models based on 210Pb and 226Ra data were used to evaluate temporal variations in the accumulation rates (ARs) of sedimentary OC. Our results revealed a marked increase in marine OC ARs over the past decades, closely linked to enhanced Atlantic Water (AW) inflow. The increasing AW influence in Arctic fjords observed in our record points to the potential for continued Atlantification under Arctic warming, as retreating sea ice and glaciers lower barriers to AW intrusion, enabling deeper and more persistent inflow. By putting this recent AW trend into historical context, our study provides valuable insights into the biogeochemical consequences of ongoing and future climate change in Arctic fjord systems.