Iron Oxidation-Reduction Processes in Warming Permafrost Soils and Surface Waters Expose a Seasonally Rusting Arctic Watershed

Abstract

Landscape-scale changes in the Arctic as a result of climate change affect the soil thermal regime and impact the depth to permafrost in vulnerable tundra watersheds. When top-down thaw of permafrost occurs, oxygen and porewaters infiltrate deeper in the soil column exposing fresh, previously frozen material and altering redox conditions that govern the mobility of geochemical constituents. Redox conditions play a critical role in the carbon cycle processes that link permafrost carbon stocks with potential feedbacks to climate warming. As such, there remains a gap in knowledge understanding how redox stratifications in thawing permafrost impact the geochemistry of watersheds in response to climate change and how investigations into redox may be scaled by coupling extensive geophysical mapping techniques. In this study, we collected soils and soil porewaters from three soil pits and surface water samples from an Arctic watershed on the North Slope of Alaska and analyzed for trace metals iron (Fe) and manganese (Mn) and Fe oxidation state using bulk and microscale techniques, including X-ray synchrotron spectroscopy. We also used geophysical mapping and soil thermistors to measure active layer depths across the watershed to relate accelerating permafrost thaw to watershed geochemistry. We found that Fe(II) and Fe(III) co-occur in the soils, porewaters, and surface waters of Imnavait Creek watershed with Fe(II) comprising up to 37% of the total Fe concentrations in the 40-60 cm soil depth and up to 17% in the 60-80 cm soil depth. In comparison to the surface (0-20 cm) and deeper in the permafrost (80-100 cm), Fe(II) was found to be enriched in the soils at the permafrost-active layer transition zone in two of the three soil pits and that translated to mobilization of Fe(II) to porewaters upon thaw at 40-60 cm, contributing up to 72% of the total Fe. Further, Fe(II) was found to be mobilized in all porewater samples from 60 to 100 cm depth and comprised 56-70% of the total Fe. In the surface water, Fe and Mn concentrations were linked to seasonality with higher concentrations coinciding with the deepest yearly extent of the active layer thaw progression. Overall, we found evidence that Fe and Mn could be useful as geochemical indicators of permafrost thaw and release of Fe(II) from thawing permafrost and further oxidation to Fe(III) could translate to a higher degree of seasonal rusting coinciding with the warming and thawing of near surface-permafrost.