Riverine transport of terrestrial organic carbon in permafrost-dominated floodplains

Arctic permafrost soils represent a major organic carbon (OC) pool, storing twice the amount of carbon than atmosphere. The permafrost carbon pool is sensitive to climate change. While warming climate may promote primary production and OC storage, warming also accelerates OC loss through temperature-modulated erosion and oxidation. The pace and magnitude of warming-induced OC loss remain poorly constrained, obscuring our understanding of how climate change affects the net permafrost carbon stock. Here we measure OC carried by the Koyukuk River in Alaska to constrain OC cycling in permafrost-dominated floodplains. The Koyukuk is a major tributary of the Yukon River that flows through discontinuous permafrost. We collected depth profiles along the river to capture OC across sediment grain sizes and hydraulic conditions. We measured the radiocarbon composition of OC to separate carbon sourced from modern biomass, aged permafrost soils, and bedrock. Combining carbon, grain size, and hydraulic data, we calculated OC fluxes loaded on different size classes. Comparing the characteristics of riverine OC along the river course and to OC in bank deposits, we estimated the loss of OC when mobilized across floodplains. Overall, our study provides new constraints on the flux and fate of permafrost OC, shedding light on OC cycling in high-latitude areas under a warming climate.