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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 CO2 efflux 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, CH4 efflux 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 CO2 efflux, CH4 efflux was more strongly associated with conditions reflective of biological activity. Transects downstream of two RTSs revealed that CH4 consistently and rapidly degassed to the atmosphere, while elevated CO2 was sustained downstream of one RTS feature. At the watershed scale, streams adjacent to RTSs rather than runoff streams within RTSs dominated fluvial CO2 and CH4 efflux. Intensifying thermokarst activity in the western Canadian Arctic will likely amplify contributions from runoff streams in RTSs to watershed-scale fluvial C gas efflux.

Plain Language Summary Rapid Arctic warming is thawing perennially frozen ground (permafrost) and driving terrain subsidence in ice-rich areas (thermokarst). In glaciated permafrost landscapes, thermokarst features called retrogressive thaw slumps, which are caused by thawing of ice-rich permafrost, release sediment, solutes, and organic materials that have been preserved in permafrost for millennia. Following thaw, permafrost carbon can rapidly transform into the greenhouse gas carbon dioxide and methane in streams. Yet the effects of thermokarst on fluvial gas flux are largely unknown. Working on the Peel Plateau in northwest Canada, we found that carbon dioxide efflux to the atmosphere from runoff draining from thaw slumps was four times greater than in streams adjacent to thaw slumps. In contrast, methane efflux in adjacent streams was twice that in runoff streams and both were similar to streams in other northern regions. While thaw slump occurrence was a primary driver of carbon dioxide efflux, methane efflux was more strongly associated with conditions reflective of biological activity. Generally, both carbon dioxide and methane were rapidly degassed downstream. While efflux of carbon dioxide and methane in thaw slumps were a small component of watershed-scale fluvial C gas efflux, these fluxes will likely increase as regional thermokarst activity accelerates.