Isotopic seasonality of fluvial-derived greenhouse gases implies active layer deepening

Abstract

Climate change in the northern circumpolar regions is rapidly thawing organic-rich permafrost soils, leading to the substantial release of dissolved CO _2 and CH _4 into river systems. This mobilization impacts local ecosystems and regional climate feedback loops, playing a crucial role in the Arctic carbon cycle. Here, we analyze the stable carbon ( δ ^13 C) and radiocarbon (F ^14 C) isotopic compositions of dissolved CO _2 and CH _4 in the Sagavanirktok and Kuparuk River watersheds on the North Slope, Alaska. By examining spatial and seasonal variations in these isotopic signatures, we identify patterns of carbon release and transport across the river continuum. We find consistent CO _2 isotopic values along the geomorphological gradient, reflecting a mixture of geogenic and biogenic sources integrated throughout the watershed. Bayesian mixing models further demonstrate a systematic depletion in ^13 C and ^14 C signatures of dissolved CO _2 sources from spring to fall, indicating increasing contributions of aged carbon as the active layer deepens. This seasonal deepening allows percolating groundwater to access deeper, older soil horizons, transporting CO _2 produced by aerobic and anaerobic soil respiration to streams and rivers. In contrast, we observe no clear relationships between the ^13 C and ^14 C compositions of dissolved CH _4 and landscape properties. Given the reduced solubility of CH _4 , which facilitates outgassing and limits its transport in aquatic systems, the isotopic signatures are likely indicative of localized contributions from streambeds, adjacent water saturated soils, and lake outflows. Our study illustrates that dissolved greenhouse gases are sensitive indicators of old carbon release from thawing permafrost and serve as early warning signals for permafrost carbon feedbacks. It establishes a crucial baseline for understanding the role of CO _2 and CH _4 in regional carbon cycling and Arctic environmental change.

Keywords

• carbon dioxide
• methane
• radiocarbon
• permafrost
• fluvial systems
• Arctic