Permafrost thaw in the Arctic enables the biogeochemical transformation of vast stores of organic carbon into carbon dioxide (CO₂). This CO₂ release has significant implications for climate feedbacks, yet the potential counterbalance from CO₂ fixation via chemical weathering of minerals exposed by thawing permafrost is entirely unstudied. We show that thermokarst in the western Canadian Arctic can enable rapid weathering of carbonate tills, driven by sulfuric acid from sulfide oxidation. Unlike carbonic acid‐driven weathering, this caused significant and previously undocumented CO₂ production and outgassing in headwater streams. Increasing riverine solute fluxes correspond with long‐term intensification of thermokarst and reflect the regional predominance of sulfuric acid‐driven carbonate weathering. We conclude that thermokarst‐enhanced mineral weathering has potential to profoundly disrupt Arctic freshwater carbon cycling. While thermokarst and sulfuric acid‐driven carbonate weathering in the western Canadian Arctic amplify CO₂ release, regional variation in sulfide oxidation will moderate the effects on the permafrost carbon‐climate feedback.

Climate change is increasing the frequency and intensity of thermokarst, and accelerating the delivery of terrestrial organic material from previously sequestered sources to aquatic systems, where it is subject to further biochemical alteration. Rapid climate change in the glacially conditioned ice-rich and ice-marginal terrain of the Peel Plateau, western Canada, is accelerating thaw-driven mass wasting in the form of retrogressive thaw slumps, which are rapidly increasing in area, volume and thickness of permafrost thawed. Despite major perturbation of downstream sedimentary and geochemical fluxes, few studies have examined changes in flux and composition of particulate organic carbon (POC) in streams and rivers as a result of permafrost thaw. Here we show that the orders of magnitude increase in total organic carbon, nitrogen, and phosphorus mobilized to streams from thaw slumps on the Peel Plateau is almost entirely due to POC and associated particulate nitrogen and phosphorus release. Slump-mobilized POC is compositionally distinct from its dissolved counterpart and appears to contain relatively greater amounts of degraded organic matter, as inferred from base-extracted fluorescence of particulate organic matter. Thus, slump-mobilized POC is potentially more recalcitrant than POC present in non-slump affected stream networks. Furthermore a substantial portion of POC mobilized from thaw slumps will be constrained within primary sediment stores in valley bottoms, where net accumulation is currently exceeding net erosion, resulting in century to millennial scale sequestration of thermokarst-mobilized POC. This study highlights the pressing need for better knowledge of sedimentary cascades, mobilization, and storage reservoirs in slump-affected streams, and baseline assessments of the biodegradability of POC and cycling of particulate nutrients within a sedimentary cascade framework. Explicit incorporation of POC dynamics into our understanding of land-water carbon mobilization in the face of permafrost thaw is critical for understanding implications of thermokarst for regional carbon cycling and fluvial ecosystems.

Thermokarst can rapidly mobilize vast amounts of sediment, solutes, and organic carbon previously maintained in frozen soils to inland waters. Streams provide a critical pathway for transforming these materials into carbon dioxide (CO₂) and methane (CH₄), yet the direct effects of thermokarst on fluvial C gas efflux from streams to the atmosphere are largely unknown. Working on the Peel Plateau in the western Canadian Arctic, we show that CO₂efflux in rill runoff thaw streams (runoff) within retrogressive thaw slumps (RTSs) was four times greater than in adjacent streams and contributed modestly but disproportionately to efflux at the landscape scale. In contrast, CH₄efflux was generally greater in adjacent streams than in RTS runoff and, overall, was within the range of values reported for other northern streams. While RTS occurrence was a primary driver of CO₂efflux, CH₄efflux was more strongly associated with conditions reflective of biological activity. Transects downstream of two RTSs revealed that CH₄consistently and rapidly degassed to the atmosphere, while elevated CO₂was sustained downstream of one RTS feature. At the watershed scale, streams adjacent to RTSs rather than runoff streams within RTSs dominated fluvial CO₂and CH₄efflux. Intensifying thermokarst activity in the western Canadian Arctic will likely amplify contributions from runoff streams in RTSs to watershed‐scale fluvial C gas efflux.

Significance: Russian rivers are the predominant source of riverine mercury to the Arctic Ocean, where methylmercury biomagnifies to high levels in food webs. Pollution controls are thought to have decreased late–20th-century mercury loading to Arctic watersheds, but there are no published long-term observations on mercury in Russian rivers. Here, we present a unique hydrochemistry dataset to determine trends in Russian river particulate mercury concentrations and fluxes in recent decades. Using hydrologic and mercury deposition modeling together with multivariate time series analysis, we determine that 70 to 90% declines in particulate mercury fluxes were driven by pollution reductions and sedimentation in reservoirs. Results suggest that Russian rivers likely dominated over all other sources of mercury to the Arctic Ocean until recently.