Permafrost Degradation in the Koyukuk River System and the Timescale Dependence of Sulfur-Carbon-Climate Feedbacks

Permafrost degradation due to anthropogenic climate change is altering biogeochemical processes throughout Arctic landscapes. In turn, these changes feedback on the concentration of atmospheric carbon dioxide (CO2) over a range of temporal and spatial scales by altering the fluxes of dissolved inorganic carbon (DIC) and alkalinity (ALK) associated with organic matter transformations, sulfide oxidation, and silicate and carbonate weathering. However, understanding the timescale-dependent impact of these reactions on atmospheric CO2 remains a major knowledge gap for both predicting the response of the carbon cycle to permafrost thaw and for interpreting evidence of changing fluvial solute fluxes in permafrost rivers. Here we report new observations of chemical weathering dynamics from the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the town of Huslia in summer 2018. Sulfate (SO42-) sulfur (34S/32S) and oxygen (18O/16O) isotope ratios in mainstem river water samples indicated that approximately 2/3 of SO42- derived from sulfide oxidation and the remainder from evaporite dissolution. The relative 34S/32S ratios, major ion abundances, DIC 13C/12C ratios, and sulfur x-ray absorption spectra of pore fluid, mainstem, and secondary channel river water and sediment samples indicated sulfur cycling within the floodplain. The MEANDIR inversion model was used to quantify the relative importance of weathering processes and their impact on pCO­2. Inversion results suggested that weathering fluxes of ALK and DIC were comparable in both mainstem and slough water samples over short timescales of floodplain carbon storage while weathering resulted in higher pCO2 values over geologic timescales. Furthermore, comparison with previous observations revealed increases in the absolute and relative concentration of SO42- in the Koyukuk River over the preceding decades. We interpreted this increase as enhanced sulfur oxidation, consistent with changes in hydrology due to permafrost thaw. Overall, our findings implicate sulfur oxidation as a substantial component of permafrost degradation and suggest that the sulfur cycle responds to permafrost thaw through a positive feedback on warming.