Fluvial inorganic carbon cycling across divergently evolving permafrost landscapes (Yukon and Northwest Territories, Canada)

Across the circumpolar north, rapid warming and intensifying hydrologic cycles are accelerating permafrost thaw and strengthening land-freshwater linkages. Among the most significant implications of this change is the mobilization of large amounts of previously sequestered organic and inorganic substrate from thawing permafrost into modern aquatic biogeochemical cycles. While microbial oxidation of permafrost organic carbon can produce carbon dioxide (CO2) and methane (CH4) upon thaw, current understanding about the sources and transformation (cycling) of carbon – and thus potential climate feedbacks – in permafrost thaw-affected freshwaters is driven by research in relatively organic-rich terrains. Much less is known about carbon cycling in permafrost terrains containing a relatively greater proportion of inorganic substrate, where the chemical weathering of minerals may shift the direction and magnitude of permafrost carbon-climate feedbacks. This work investigates the summertime (2014–2017) magnitude, drivers, and regional variability in inorganic carbon cycling in a series of northern watersheds in diverse permafrost terrains across the Yukon and Northwest Territories (Canada). Much of this work occurred on the Peel Plateau (Northwest Territories), where terrain subsidence following permafrost thaw (thermokarst) releases tills into fluvial networks. Measurements of dissolved inorganic carbon (DIC, Σ[CO2, HCO3-, CO32-]) showed that the thaw and chemical weathering of carbonate- and sulfide-bearing tills on the Peel Plateau reshaped fluvial inorganic carbon cycling across watershed scales. The substantial thermokarst-driven increase in weathering was reflected by a 100-fold increase in bicarbonate (HCO3–) concentrations between non thermokarst-affected headwaters and the Stony Creek outlet into the Peel River. Ions and stable isotopes of DIC (13CDIC) and CO2 (13CCO2) indicated that carbonate weathering was primarily driven by sulfuric acid from sulfide oxidation, which contributed to CO2 supersaturation within thermokarst features. This CO2 was rapidly effluxed to the atmosphere in headwaters, while the downstream evolution of HCO3– and other ions, 13CDIC, and 13CCO2 along transects spanning nested watersheds reflected inputs from RTS-affected tributaries and revealed a modest increase in biotic CO2 production. Our finding that H2SO4 carbonate weathering is traceable across watershed scales aligns with our hydrochemical modeling results, which show a multi-decadal increase in H2SO4 carbonate weathering products within the Peel River from ca. 1975–2015, in conjunction with accelerating thermokarst activity. While our estimates of CO2 and CH4 efflux from within thermokarst features indicate these fluxes are currently modest contributions to the atmosphere, results from our abiotic mineral weathering experiment are consistent with observations that intensifying thermokarst – which exposes deeper, previously unthawed tills – will amplify H2SO4 driven carbonate weathering and HCO3– export and increase atmospheric CO2 over geological timescales via HCO3– transformation within marine precipitation reactions. Trends in fluvial carbon cycling in nearby permafrost terrains revealed that inorganic carbon cycling and export (as HCO3–) dominated in mountainous terrain with carbonate bedrock, and dissolved organic carbon (DOC) export and CO2 and CH4 efflux dominated in low-relief terrains with a relatively greater proportion of organic matter. Fluvial carbon cycling was intermediate on the Peel Plateau, except where thermokarst enhanced carbonate weathering and sediment release by unearthing mineral-rich tills. From an ecosystem carbon balance perspective, fluvial C export was equivalent to 8% of CO2 uptake by terrestrial vegetation, except in the watersheds affected by thermokarst (~1% by area), where fluvial C export was equivalent to roughly 40% of CO2 uptake by vegetation. Consistent with observations elsewhere, our results show that the magnitude and drivers of fluvial carbon cycling can vary considerably across proximal yet diverse landscapes. This research is among the first to document the effects of thermokarst on fluvial carbon cycling in a relatively inorganic-rich permafrost terrain. We conclude that the regional variability in the mineral composition of permafrost, and thus the degree to which carbonate weathering is coupled with sulfide oxidation, will moderate CO2 consumption or release via chemical weathering of thawed minerals. Although we show that carbonate- and sulfide-bearing thermokarst terrains span the circumpolar north, carbonate and silicate weathering by carbonic acid (H2CO3*, including CO2) will consume CO2 and be carbon neutral or a sink over geological timescales. Determining the effects of thermokarst on fluvial inorganic carbon cycling in northern permafrost terrains is a top research priority for refining models of the Arctic carbon cycle and constraining permafrost carbon-climate feedbacks.

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