| Summary: | Mineral organic carbon (OC) interactions in soils are key to stabilize carbon 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 by inducing processes of dissolution and precipitation of these interactions: dissolution can release OC and thereby contribute to carbon loss, whereas precipitation can promote loci for carbon gain or limited carbon loss. Here we tested a new approach using radiogenic Sr isotopes to locate in situ dissolution-precipitation processes of mineral OC interactions along a gradient of permafrost thaw at Eight Mile Lake, AK, USA. In these soils, we find that between 6% and 59% of total OC (mean 20%) 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 involved in these mineral OC interactions either adsorbed or occluded to metal oxides or participating in organo-metallic complexes. We argue 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. Our data show that in saturated soil layers, between 4% and 64% of mineral OC interactions have remained undissociated since their formation, supporting the preservation of OC associated under saturated soil conditions. At the redox interface, the data highlight processes of dissolution and precipitation of the mineral OC interactions, thereby supporting loci for C loss and gain in fluctuating redox conditions depending on drainage and lateral OC transfer. Finally, the results demonstrate that primary mineral weathering resulting from permafrost thaw releases mineral elements contributing to OC stabilization mechanisms, thereby limiting OC loss.
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