Permafrost Landscape History Shapes Fluvial Chemistry, Ecosystem Carbon Balance, and Potential Trajectories of Future Change
Suzanne E. Tank,
Steven V. Kokelj,
Robert G. Striegl,
William L. Quinton,
Edward A. G. Schuur,
Peter M. Lafleur,
Ryan C. Sullivan,
David P. Billesbach,
Global Biogeochemical Cycles, Volume 36, Issue 9
Abstract Intensifying permafrost thaw alters carbon cycling by mobilizing large amounts of terrestrial substrate into aquatic ecosystems. Yet, few studies have measured aquatic carbon fluxes and constrained drivers of ecosystem carbon balance across heterogeneous Arctic landscapes. Here, we characterized hydrochemical and landscape controls on fluvial carbon cycling, quantified fluvial carbon fluxes, and estimated fluvial contributions to ecosystem carbon balance across 33 watersheds in four ecoregions in the continuous permafrost zone of the western Canadian Arctic: unglaciated uplands, ice‐rich moraine, and organic‐rich lowlands and till plains. Major ions, stable isotopes, and carbon speciation and fluxes revealed patterns in carbon cycling across ecoregions defined by terrain relief and accumulation of organics. In previously unglaciated mountainous watersheds, bicarbonate dominated carbon export (70% of total) due to chemical weathering of bedrock. In lowland watersheds, where soil organic carbon stores were largest, lateral transport of dissolved organic carbon (50%) and efflux of biotic CO 2 (25%) dominated. In watersheds affected by thaw‐induced mass wasting, erosion of ice‐rich tills enhanced chemical weathering and increased particulate carbon fluxes by two orders of magnitude. From an ecosystem carbon balance perspective, fluvial carbon export in watersheds not affected by thaw‐induced wasting was, on average, equivalent to 6%–16% of estimated net ecosystem exchange (NEE). In watersheds affected by thaw‐induced wasting, fluvial carbon export approached 60% of NEE. Because future intensification of thermokarst activity will amplify fluvial carbon export, determining the fate of carbon across diverse northern landscapes is a priority for constraining trajectories of permafrost region ecosystem carbon balance.
Permafrost thaw in the Arctic enables the biogeochemical transformation of vast stores of organic carbon into carbon dioxide (CO2). This CO2 release has significant implications for climate feedbacks, yet the potential counterbalance from CO2 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 CO2 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 CO2 release, regional variation in sulfide oxidation will moderate the effects on the permafrost carbon‐climate feedback.