Thermokarst amplifies fluvial inorganic carbon cycling and export across watershed scales on the Peel Plateau, Canada

As climate warming and precipitation increase at high latitudes, permafrost terrains across the circumpolar north are poised for intensified geomorphic activity and sediment mobilization that are expected to persist for millennia. In previously glaciated permafrost terrain, ice-rich deposits are associated with large stores of reactive mineral substrate. Over geological timescales, chemical weathering moderates atmospheric inline-formulaCO₂ levels, raising the prospect that mass wasting driven by terrain consolidation following thaw (thermokarst) may enhance weathering of permafrost sediments and thus climate feedbacks. The nature of these feedbacks depends upon the mineral composition of sediments (weathering sources) and the balance between atmospheric exchange of inline-formulaCO₂ vs. fluvial export of carbonate alkalinity (inline-formulaΣ[inline-formula $M4inlinescrollmathmlchem{\mathrm{normal\; HCO}}_{\mathrm{normal\; 3}}^{-}$ 33pt16ptsvg-formulamathimg2fbeb9b29341401f243f2ced851d27df bg-17-5163-2020-ie00001.svg33pt16ptbg-17-5163-2020-ie00001.png , inline-formula $M5inlinescrollmathmlchem{\mathrm{normal\; CO}}_{\mathrm{normal\; 3}}^{\mathrm{normal\; 2}-}$ 30pt17ptsvg-formulamathimgc7035c2c75c9854db2b1eb7c42f957db bg-17-5163-2020-ie00002.svg30pt17ptbg-17-5163-2020-ie00002.png ]). Working in the fluvially incised, ice-rich glacial deposits of the Peel Plateau in northwestern Canada, we determine the effects of slope thermokarst in the form of retrogressive thaw slump (RTS) activity on mineral weathering sources, inline-formulaCO₂ dynamics, and carbonate alkalinity export and how these effects integrate across watershed scales (inline-formula∼ 2 to 1000 inline-formulakm²). We worked along three transects in nested watersheds with varying connectivity to RTS activity: a 550 inline-formulam transect along a first-order thaw stream within a large RTS, a 14 inline-formulakm transect along a stream which directly received inputs from several RTSs, and a 70 inline-formulakm transect along a larger stream with headwaters that lay outside of RTS influence. In undisturbed headwaters, stream chemistry reflected inline-formulaCO₂ from soil respiration processes and atmospheric exchange. Within the RTS, rapid sulfuric acid carbonate weathering, prompted by the exposure of sulfide- and carbonate-bearing tills, appeared to increase fluvial inline-formulaCO₂ efflux to the atmosphere and propagate carbonate alkalinity across watershed scales. Despite covering less than 1 % of the landscape, RTS activity drove carbonate alkalinity to increase by 2 orders of magnitude along the largest transect. Amplified export of carbonate alkalinity together with isotopic signals of shifting DIC and inline-formulaCO₂ sources along the downstream transects highlights the dynamic nature of carbon cycling that may typify glaciated permafrost watersheds subject to intensification of hillslope thermokarst. The balance between inline-formulaCO₂ drawdown in regions where carbonic acid weathering predominates and inline-formulaCO₂ release in regions where sulfides are more prevalent will determine the biogeochemical legacy of thermokarst and enhanced weathering in northern permafrost terrains. Effects of RTSs on carbon cycling can be expected to persist for millennia, indicating a need for their integration into predictions of weathering–carbon–climate feedbacks among thermokarst terrains.