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

Metastable iron sulfides are involved in a series of biotic and abioticprocesses in the marine environment, including the mineralization of organicmatter. However, naturally occurring metastable iron (mono)sulfide minerals arerarely reported in marine sediments, and current information about theirformation and characteristics comes from synthetic sulfides. Here, we studiedsulfur speciation and mineralogy in a submarine surface core (0-22 cm depth)from an active, shallow-sea hydrothermal system (Milos, Greece) that isdominated by sulfur-metabolizing microorganisms. Geochemical analysis resultsshowed S-Fe-As enrichment in the bottom layers of the core, which were furthercharacterized using a suite of techniques. Powder X-ray diffraction andSynchrotron-based mu-X-ray diffraction did not show crystalline Fe-S compoundswhereas scanning electron microscopy and Synchrotron-based X-rayfluorescencemapping indicated the presence of Fe-S(-As) phases and sulfur particles. Sulfurmicrospeciation by X-ray absorption near-edge structure spectroscopy showed amixture of oxidation states, including organic sulfur species, indicative of activesulfur biogeochemical cycling. Ultimately, transmission electron microscopy wasused for the identification of the Fe-S mineral assemblage in the samples thatincluded arsenic-bearing pyrite and the metastable mackinawite, monoclinic pyrrhotite and greigite, alongside elemental sulfurnanoparticles. 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 useof standard microscopic, spectroscopic and diffraction techniques may overlook the presence of metastable iron sulfides in naturalsamples. 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.