Abstract
Abstract. 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 CO2 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 CO2 vs. fluvial export of carbonate alkalinity (Σ[HCO3-, CO32-]). 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, CO2 dynamics, and carbonate alkalinity export and how these effects integrate across watershed scales (∼ 2 to 1000 km2). We worked along three transects in nested watersheds with varying connectivity to RTS activity: a 550 m transect along a first-order thaw stream within a large RTS, a 14 km transect along a stream which directly received inputs from several RTSs, and a 70 km transect along a larger stream with headwaters that lay outside of RTS influence. In undisturbed headwaters, stream chemistry reflected CO2 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 CO2 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 CO2 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 CO2 drawdown in regions where carbonic acid weathering predominates and CO2 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.
Highlights
Riverine export of carbonate alkalinity ( [HCO−3, CO23−]), generated by the chemical weathering of silicate and carbonate minerals, is a key component of the global carbon cycle and Earth’s long-term climate (Berner, 1999; Gaillardet et al, 1999; Hilton and West, 2020; Torres et al, 2017)
Water discharge (Q) in Stony Creek tributaries was estimated from a hydraulic-geometry model (Gordon et al, 2004) that we developed using flow measurements made in Peel Plateau streams during 2015–2017, and width (W ) was estimated from on-site measurements or photos from 2017 with a known scale
4.3 Integration of retrogressive thaw slump (RTS) effects on carbon cycling across watershed scales. These findings enable us to develop a conceptual model of catchment chemical characteristics and how the effects of RTS activity on carbon cycling integrate across watershed scales on the Peel Plateau (Fig. 7)
Summary
Riverine export of carbonate alkalinity ( [HCO−3 , CO23−]), generated by the chemical weathering of silicate and carbonate minerals, is a key component of the global carbon cycle and Earth’s long-term climate (Berner, 1999; Gaillardet et al, 1999; Hilton and West, 2020; Torres et al, 2017). Rapid warming at northern latitudes (Serreze and Barry, 2011) is thawing permafrost (Biskaborn et al, 2019), increasing vegetation productivity (Bjorkman et al, 2018), intensifying hydrologic cycles (Rawlins et al, 2010), and strengthening land–fresh water linkages (Vonk et al, 2019; Walvoord and Kurylyk, 2016). These processes are activating large amounts of mineral substrate into biogeochemical cycles, with significant implications for DIC cycling (Lacelle et al, 2019; Wadham et al, 2019). Glaciated permafrost terrain hosting ice-rich deposits of reactive sediments are thought to be distributed across the northern permafrost zone, raising the prospect that terrain consolidation following thaw (thermokarst) and associated carbonate alkalinity production and export may have stronger influence on climate feedbacks in such regions (Zolkos et al, 2018)
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