| Summary: | Mineral organic carbon (OC) interactions in soils and ice-rich sediments are key to stabilize OC and mitigate greenhouse gas emissions upon permafrost thaw. However, changes in soil water pathways upon permafrost thaw are likely to affect the stability of mineral OC interactions. Dissolution of mineral surfaces can release associated OC which contributes to carbon loss, whereas precipitation of new mineral surfaces can promote loci for OC interactions limiting carbon loss. Here we use radiogenic Sr isotopes to locate in situ dissolution-precipitation processes of mineral OC interactions along a gradient of permafrost thaw in soils and ice-rich sediments. In these materials, about 20% of OC is stabilized as organo-mineral associations (association between ferrihydrite and OC) and organo-metallic complexes (associations between Fe, Mn, Al, Ca polyvalent cations and organic acids). We target Sr adsorbed or occluded to metal oxides or participating in organo-metallic complexes. We hypothesize that a change in the Sr isotopic signature of such mineral OC interactions upon permafrost thaw indicates a destabilization of the binding between mineral surfaces and Sr, and hence OC. We show that it is in saturated layers that mineral OC interactions have remained undissociated and preserved since their formation. At the redox interface, the data highlight processes of dissolution and precipitation of the mineral OC interactions, supporting loci for the loss and gain in OC stabilization potential. Given the importance of stabilizing surfaces and cations involving Fe for the stabilization of OC in permafrost environments, we propose an approach to estimate at larger scale in the Arctic the proportion of this “reactive iron†in permafrost soils.
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