Metastable Iron (Mono)sulfides in the Shallow-Sea Hydrothermal Sediments of Milos, Greece

Metastable iron sulfides are involved in a series of biotic and abiotic processes in the marine environment, including the mineralization of organic matter. However, naturally occurring metastable iron (mono)sulfide minerals are rarely reported in marine sediments, and current information about their formation and characteristics comes from synthetic sulfides. Here, we studied sulfur speciation and mineralogy in a submarine surface core (0-22 cm depth) from an active, shallow-sea hydrothermal system (Milos, Greece) that is dominated by sulfur-metabolizing microorganisms. Geochemical analysis results showed S-Fe-As enrichment in the bottom layers of the core, which were further characterized using a suite of techniques. Powder X-ray diffraction and Synchrotron-based μ-X-ray diffraction did not show crystalline Fe-S compounds whereas scanning electron microscopy and Synchrotron-based X-ray fluorescence mapping indicated the presence of Fe-S(-As) phases and sulfur particles. Sulfur microspeciation by X-ray absorption near-edge structure spectroscopy showed a mixture of oxidation states, including organic sulfur species, indicative of active sulfur biogeochemical cycling. Ultimately, transmission electron microscopy was used for the identification of the Fe-S mineral assemblage in the samples that included arsenic-bearing pyrite and the metastable mackinawite, monoclinic pyrrhotite and greigite, alongside elemental sulfur nanoparticles. Previous studies on the mineralogy of Milos hydrothermal sediments omitted the presence of metastable iron sulfides, that were up to now known to form in marine sediments from estuaries and anoxic/euxinic basins. Our results highlight that the use of standard microscopic, spectroscopic and diffraction techniques may overlook the presence of metastable iron sulfides in natural samples. Considering that metastable iron sulfides are implicated in critical biogeochemical processes for the marine ecosystems, their role in sulfur, iron, and carbon cycling in modern and ancient marine sediments might be underrated.