Interplay of Fe and S biogeochemistry shapes in situ iron mineral transformations in contrasting intertidal sediments

The transformation and stability of iron (Fe) minerals in coastal sediments are closely linked to the sulfur (S) cycle, influencing the fate of nutrients, carbon, and contaminants. However, in situ studies of these interactions in coastal sediments remain limited. We investigated the transformation of lepidocrocite, goethite, and mackinawite in three intertidal field plots with contrasting Fe and S biogeochemistry. Fe minerals were enriched with ⁵⁷Fe and mixed with the sediment, allowing close contact with the other inorganic and organic components of the sediment. After 8 weeks, transformation products were assessed using ⁵⁷Fe Mössbauer spectroscopy. Regular porewater analysis complemented solid-phase analyses, supporting the understanding of transformation pathways and extents. Under low-sulfide, Fe-reducing conditions, lepidocrocite did not transform to more crystalline Fe-oxides such as goethite or magnetite. Instead, ∼20% of the lepidocrocite transformed, mostly into a disordered Fe-phase, due to reductive dissolution and a small extent of sulfidation. Goethite, in contrast, remained apparently unchanged under the same conditions. These results indicate that both Fe-oxides may persist during extended anoxic periods under Fe-reducing conditions in coastal sediments and thus may influence elemental cycles. However, in sulfidic environments, lepidocrocite and goethite transformed into amorphous, nonstoichiometric Fe-sulfide and greigite. We hypothesize that amorphous Fe-sulfide precipitated first, later transforming into greigite; a potential precursor of pyrite formation. This is further supported by the transformation of synthetic mackinawite into greigite under high sulfide conditions, suggesting a sulfidation pathway that may eventually lead to pyrite formation in coastal sediments.