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Permafrost thaw in the Arctic enables the biogeochemical transformation of vast stores of organic carbon into carbon dioxide (CO 2 ). This CO 2 release has significant implications for climate feedbacks, yet the potential counterbalance from CO 2 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 CO 2 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 CO 2 release, regional variation in sulfide oxidation will moderate the effects on the permafrost carbon-climate feedback.Plain Language Summary In the Arctic, perennially frozen ground (permafrost) in previously glaciated regions stores abundant minerals and is often ice-rich. Therefore, this permafrost can rapidly thaw and collapse, resulting in thermokarst and exposing minerals to breakdown by chemical weathering. Mineral weathering by carbonic acid fixes CO 2 , making it less likely to enter the atmosphere. However, the effect of thermokarst on mineral weathering, carbon cycling, and rising atmospheric CO 2 levels is unknown. We show thermokarst enhances weathering in streams in the western Canadian Arctic can rapidly produce significant and previously undocumented CO 2 because carbonate weathering in this region is driven by sulfuric acid (from weathering of sulfide minerals) instead of carbonic acid. Long-term river chemistry reveals that this weathering is intensifying as thermokarst accelerates. Across the Arctic, increasing thermokarst will profoundly impact freshwater carbon cycling, yet the influence of weathering on climate feedbacks will depend on regional variation in the mineral composition of permafrost soils. À

Abstract. The intensification of thaw-driven mass wasting is transforming glacially conditioned permafrost terrain, coupling slopes with aquatic systems, and triggering a cascade of downstream effects. Within the context of recent, rapidly evolving climate controls on the geomorphology of permafrost terrain, we (A) quantify three-dimensional retrogressive thaw slump enlargement and describe the processes and thresholds coupling slopes to downstream systems, (B) investigate catchment-scale patterns of slope thermokarst impacts and the geomorphic implications, and (C) map the propagation of effects through hydrological networks draining permafrost terrain of northwestern Canada. Power-law relationships between retrogressive thaw slump area and volume (R2=0.90), as well as the thickness of permafrost thawed (R2=0.63), combined with the multi-decadal (1986–2018) increase in the areal extent of thaw slump disturbance, show a 2 order of magnitude increase in catchment-scale geomorphic activity and the coupling of slope and hydrological systems. Predominant effects are to first- and second-order streams where sediment delivery, often indicated by formation of recent debris tongue deposits, commonly exceeds the transport capacity of headwater streams by orders of magnitude, signaling centennial- to millennial-scale perturbation of downstream systems. Assessment of hydrological networks indicates that thaw-driven mass wasting directly affects over 5538 km of stream segments, 889 km of coastline, and 1379 lakes in the 994 860 km2 study area. Downstream propagation of slope thermokarst indicates a potential increase in the number of affected lakes by at least a factor of 4 (n>5692) and impacted stream length by a factor of 8 (>44 343 km), and it defines several major impact zones on lakes, deltas, and coastal areas. Prince of Wales Strait is the receiving marine environment for greatly increased sediment and geochemical fluxes from numerous slump-impacted hydrological networks draining Banks Island and Victoria Island. The Peel and Mackenzie rivers are globally significant conveyors of the slope thermokarst cascade, delivering effects to North America's largest Arctic delta and the Beaufort Sea. Climate-driven erosion of ice-rich slopes in permafrost-preserved glaciated terrain has triggered a time-transient cascade of downstream effects that signal the rejuvenation of post-glacial landscape evolution. Glacial legacy, ground-ice conditions, and continental drainage patterns dictate that terrestrial, freshwater, coastal, and marine environments of western Arctic Canada will be an interconnected hotspot of thaw-driven change through the coming millennia.

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.

Retrogressive thaw slumps (RTSs) are thermokarst features created by the rapid thaw of ice-rich permafrost, and can mobilize vast quantities of sediments and solutes downstream. However, the effect of slumping on downstream concentrations and yields of total mercury (THg) and methylmercury (MeHg) is unknown. Fluvial concentrations of THg and MeHg downstream of RTSs on the Peel Plateau (Northwest Territories, Canada) were up to 2 orders of magnitude higher than upstream, reaching concentrations of 1,270 ng L–1 and 7 ng L–1, respectively, the highest ever measured in uncontaminated sites in Canada. MeHg concentrations were particularly elevated at sites downstream of RTSs where debris tongues dammed streams to form reservoirs where microbial Hg methylation was likely enhanced. However, > 95% of the Hg downstream was typically particle-bound and potentially not readily bioavailable. Mean open-water season yields of THg (610 mg km–2 d–1) and MeHg (2.61 mg km–2 d–1) downstream of RTSs were up to an order of magnitude higher than those for the nearby large Yukon, Mackenzie and Peel rivers. We estimate that ∼5% of the Hg stored for centuries or millennia in northern permafrost soils (88 Gg) is susceptible to release into modern-day Hg biogeochemical cycling from further climate changes and thermokarst formation.

Land−ocean linkages are strong across the circumpolar north, where the Arctic Ocean accounts for 1% of the global ocean volume and receives more than 10% of the global river discharge. Yet estimates of Arctic riverine mercury (Hg) export constrained from direct Hg measurements remain sparse. Here, we report results from a coordinated, year-round sampling program that focused on the six major Arctic rivers to establish a contemporary (2012−2017) benchmark of riverine Hg export. We determine that the six major Arctic rivers exported an average of 20 000 kg y −1 of total Hg (THg, all forms of Hg). Upscaled to the pan-Arctic, we estimate THg flux of 37 000 kg y −1 . More than 90% of THg flux occurred during peak river discharge in spring and summer. Normalizing fluxes to watershed area (yield) reveals higher THg yields in regions where greater denudation likely enhances Hg mobilization. River discharge, suspended sediment, and dissolved organic carbon predicted THg concentration with moderate fidelity, while suspended sediment and water yields predicted THg yield with high fidelity. These findings establish a benchmark in the face of rapid Arctic warming and an intensifying hydrologic cycle, which will likely accelerate Hg cycling in tandem with changing inputs from thawing permafrost and industrial activity.

Permafrost degradation is delivering bioavailable dissolved organic matter (DOM) and inorganic nutrients to surface water networks. While these permafrost subsidies represent a small portion of total fluvial DOM and nutrient fluxes, they could influence food webs and net ecosystem carbon balance via priming or nutrient effects that destabilize background DOM. We investigated how addition of biolabile carbon (acetate) and inorganic nutrients (nitrogen and phosphorus) affected DOM decomposition with 28‐day incubations. We incubated late‐summer stream water from 23 locations nested in seven northern or high‐altitude regions in Asia, Europe, and North America. DOM loss ranged from 3% to 52%, showing a variety of longitudinal patterns within stream networks. DOM optical properties varied widely, but DOM showed compositional similarity based on Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) analysis. Addition of acetate and nutrients decreased bulk DOM mineralization (i.e., negative priming), with more negative effects on biodegradable DOM but neutral or positive effects on stable DOM. Unexpectedly, acetate and nutrients triggered breakdown of colored DOM (CDOM), with median decreases of 1.6% in the control and 22% in the amended treatment. Additionally, the uptake of added acetate was strongly limited by nutrient availability across sites. These findings suggest that biolabile DOM and nutrients released from degrading permafrost may decrease background DOM mineralization but alter stoichiometry and light conditions in receiving waterbodies. We conclude that priming and nutrient effects are coupled in northern aquatic ecosystems and that quantifying two‐way interactions between DOM properties and environmental conditions could resolve conflicting observations about the drivers of DOM in permafrost zone waterways.

Recent advances in remote sensing and ecological modeling warrant a timely and robust investigation of the ecological variables that underlie large-scale patterns of breeding bird species richness, particularly in the context of intensifying land use and climate change. Our objective was to address this need using an array of bioclimatic and remotely sensed data sets representing vegetation properties and structure, and other aspects of the physical environment. We first build models of bird species richness across breeding bird survey (BBS) routes, and then spatially predict richness across the coterminous US at moderately high spatial resolution (1 km). Predictor variables were derived from various sources and maps of species richness were generated for four groups (guilds) of birds with different breeding habitat affiliation (forest, grassland, open woodland, scrub/shrub), as well as all guilds combined. Predictions of forest bird distributions were strong (R 2 = 0.85), followed by grassland (0.76), scrub/shrub (0.63) and open woodland (0.60) species. Vegetation properties were generally the strongest determinants of species richness, whereas bioclimatic and lidar-derived vertical structure metrics were of variable importance and dependent upon the guild type. Environmental variables (climate and the physical environment) were also frequently selected predictors, but canopy structure variables were not as important as expected based on more local to regional scale studies. Relatively sparse sampling of canopy structure metrics from the satellite lidar sensor may have reduced their importance relative to other predictor variables across the study domain. We discuss these results in the context of the ecological drivers of species richness patterns, the spatial scale of bird diversity analyses, and the potential of next generation space-borne lidar systems relevant to vegetation and ecosystem studies. This study strengthens current understanding of bird species-climate-vegetation relationships, which could be further advanced with improved canopy structure information across spatial scales.