Climate sensitive methane release from sediment-laden channels in Arctic rivers 

Arctic rivers can act as a route for methane (CH4) to enter the atmosphere from landscapes impacted by ongoing permafrost thaw and climate warming. Thermokarst erosion and thaw induced mass-wasting are underway across the Arctic, increasing organic matter supply to river systems that could act as substrates for methanogenesis. In addition, warming of air and water temperatures could increase methanogenesis in analogy with responses seen in other aquatic, non-fluvial settings. Despite this recognition, the source, drivers and sensitivity of Arctic river CH4 emissions to geomorphic and climate change remain obscured.Here, we apply novel sampling methods and use radiocarbon and a multi-stable isotope approach to quantify CH4 emissions, age and source in Arctic rivers of the Mackenzie River Basin across two field campaigns in winter 2023 and summer 2024. Despite evidence for CH4 oxidation, we find that sediment-laden Arctic Rivers are hotspots of CH4 release, both downstream of sites of increased thaw-driven mass wasting and within large channels of the river delta. We find that river CH4 emissions increase by three times in the summer season compared to the winter, sustained by an aged, but higher quality organic matter substrate. A detailed reach-scale CH4 budget reveals a high apparent temperature sensitivity of river CH4 emissions that has not been recognized before, suggesting that ongoing warming, permafrost thaw and increased erosion will increase river CH4 emissions in the Arctic.