Extreme hydrological events amplify weathering-derived inorganic carbon fluxes in the Arctic permafrost active layer

Chemical weathering plays a key role in regulating long-term atmospheric CO₂, yet the balance between CO₂-consuming and CO₂-releasing weathering pathways in Arctic catchments remains poorly constrained. Here, we present the first high-resolution, multi-decadal (1997–2022) assessment of net carbon fluxes from a heavily monitored small High Arctic river catchment in NE Greenland. Net CO₂ release is dominated by sulfuric acid weathering of carbonates driven by sulfide oxidation from pyrite minerals, as supported by δ34SSO4, δ18OSO4, and δ18OH2O isotopic tracers that together trace sulfide from other sulfur sources in river waters. River pH has increased by >1.5 units over the last 20+ years, consistent with progressive acid neutralisation by carbonate dissolution and coincident with progressive deepening of the active layer. Our results reveal intensifying sulfuric acid-carbonate weathering in response to Arctic warming and a strengthening hydrological cycle, highlighting the sensitivity of Arctic carbon fluxes to deepening of active layers, evolving flowpaths, enhanced water-rock interactions, and geomorphic disturbance. Further, we show that extreme erosional events exert contrasting controls on catchment-scale carbon fluxes: glacial lake outburst floods reduce or temporarily reverse net CO₂ release. Erosion therefore does not exert a unidirectional control on weathering-driven carbon fluxes in Arctic systems.These findings challenge the assumption that enhanced Arctic weathering will necessarily promote long-term CO₂ sequestration and underscore the need to account for lithology-specific and process-driven controls when assessing cryosphere–carbon feedbacks under future climate change.