Erosional and hydrological controls on the age and thermochemical stability of particulate organic carbon in an Arctic river

Understanding the mechanisms that drive the mobilization and fate of organic carbon (OC) in Arctic landscapes is important for modeling the feedbacks among permafrost thaw, carbon cycling, and climate change. While significant progress has been made toward measuring in situ OC decomposition in permafrost soils and bulk particulate organic carbon (POC) export from Arctic rivers, few studies have distinguished the source and lability of POC across Arctic landscapes, limiting our ability to predict whether mobilized POC will be oxidized to CO₂ and CH₄ or buried in downstream depositional environments. This study uses ramped pyrolysis/oxidation radiocarbon (RPO-¹⁴C) analyses to investigate spatial and temporal variations in the thermochemical stability and radiocarbon content of fluvial POC during downstream transport from mountains to the coast in the Canning River (North Slope, Alaska). Fluvial POC in the headwaters is predominantly comprised of high activation energy, thermally recalcitrant petrogenic OC (OC_petro) derived from shale bedrock. Moving into the foothills and low-relief coastal plains, river bank erosion primarily drives mobilization of labile, low activation energy, soil-derived OC (OC_soil). Fluvial POC in mountainous upstream reaches consisted of ∼70% OC_petro and just ∼30% OC_soil, while POC in the downstream coastal plain reaches comprised ∼85% OC_soil and ∼15% OC_petro. The high relative lability of POC exported to the coast indicates high susceptibility to oxidation and microbial decomposition, which could enhance CO₂ release as the Arctic hydrologic cycle intensifies. However, the persistence of refractory OC_petro in the suspended load indicates the potential for long-term burial of rock organic carbon in marine sediments.