Permafrost soils contain substantial amounts of organic carbon (OC), often associated with iron(III) (oxyhydr)oxide minerals. Thawing permafrost releases aqueous iron (Fe2+aq) and associated OC under reducing conditions. This released OC and Fe, in both dissolved and particulate forms, are transported to small thaw ponds experiencing redox fluctuations. Seasonal changes in runoff and extensive waterlogging shift redox conditions from oxic to anoxic, affecting the extent of Fe(II) oxidation and Fe(III) reduction. The impact of these redox fluctuations on mineral-organic interactions, and consequently on Fe and OC mobilization and greenhouse gas emissions, remains unclear. In this study, we first characterized the Fe-OC interactions in thaw ponds in partially and fully thawed soils (“bog” and “fen” thaw ponds, respectively) in a permafrost peatland complex in Abisko, Sweden. We collected pond samples and characterized them using size fractionation (large particulate fraction (LPF), small particulate fraction (SPF), and dissolved fraction (DF)), analyzing their elemental composition, Fe oxidation state, and particle size using both bulk and nanoscale techniques. Also, the pond samples were subjected to laboratory incubation under varying redox conditions where we also monitored the dynamics of dissolved organic carbon, along with CO2 and CH4 emissions. Our findings indicate thaw ponds have high concentrations of dissolved organic carbon and redox-active iron (Fe). The Fe speciation varies depending on the stage of thawing as well as the redox fluctuations in these ponds. Bulk Fe and OC in ponds showed similar dynamics, suggesting their association in dissolved and particulate phases. These particulates increase in size and stability with more redox cycles. Ultimately, our results imply that released Fe and OC in thaw ponds may re-associate, which could protect OC from microbial decomposition and stabilize the redox state of Fe. However, the redox fluctuations within these ponds may cause carbon mobilization and the release of greenhouse gases.
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