Iron (hydr)oxide formation in Andosols under extreme climate conditions

Abstract

Redox-driven biogeochemical cycling of iron plays an integral role in the complex process network of terrestrial ecosystems, such as carbon and phosphorous cycling, the fate of nutrients and greenhouse gas emissions. We investigate Fe-(hydr)oxide (trans)formation pathways from volcanic glass and titanomagnetite phenocrysts in acidic topsoils of South Patagonian Andosols, combining bulk geochemical analyses with micrometer-scale-measurements on individual aggregates. Our data indicate that Fe-(hydr)oxide (trans)formation is closely linked to tephra weathering and organic matter turnover, regulated by frequent hydrological perturbations related to the hyper-humid climate. We document abiotic and microbially-mediated pathways, including Fe-(hydr)oxide nucleation after glass dissolution and complexation with organic ligands, maghemitization and dissolution-(re)crystallization processes from metastable precursors. Ultimately, hematite represents the sole thermodynamically stable Fe-(hydr)oxide formed under these conditions. The insights into iron cycling in pristine Andosols have implications for a better understanding of the colloidal delivery of bio-available iron and carbon sequestration in the unique Patagonian fjord ecosystem.