A centennial record of fluvial organic matter input from the discontinuous permafrost catchment of Lake Tornetrask

High-latitude regions are underlain by the most organic carbon (OC)-rich soils on earth and currently subject to intense climate warming, potentially increasing remobilization and mineralization of soil OC. Sub-Arctic Scandinavia is located on the 0 degrees C mean annual isotherm and is therefore particularly vulnerable to climate change. This study aimed to establish a baseline for soil OC release over the past century into Lake Tornetrask, the largest lake in sub-Arctic Scandinavia, through bulk geochemical and molecular radiocarbon analyses in chronologically constrained sediment cores. Our results suggest a dominance of peat-derived terrestrial OC inflow. We show that the annual terrestrial OC inflow to the lake is similar to 12 times higher than the in-lake produced particulate OC, and consists for a large part (ca. 60%) of old OC from deep reservoirs in the catchment. The sedimentary record shows signs of increasing inflow of more degraded terrestrial matter since similar to 1975, as indicated by increasing % TOC concentrations, a lower delta C-13 value and lower TOC: TN ratios. Based on simultaneous changes in local climate and reported signs of permafrost degradation (e.g., active layer deepening, mire/peat erosion), the observed changes in the sedimentary record of Scandinavia's largest mountain lake likely reflect a climate warming-induced change in terrestrial OC inflow.