Pathways to Pyrite: Direct Evidence for Metastable Precursors in Marine Sediments

The formation of pyrite is a critical process in sedimentary environments, central to the global iron and sulfur cycles and redox-sensitive biogeochemical processes. Yet the pathways and mechanisms driving pyrite formation, particularly the roles of metastable iron sulfide (FeSx) phases and polysulfides (Sn2-), remain poorly understood under natural conditions. Here we provide the first direct evidence for polysulfide-mediated pyrite formation in marine sediments, using Mössbauer spectroscopy and X-ray photoelectron spectroscopy, on cores from Saanich Inlet. We identify two distinct nanoparticulate FeS1+x phases: a mackinawite-like and a greigite-like phase. We find that under highly reducing conditions the greigite-like phase can compete with pyrite as the prevailing Fe-S mineral. Our results also reveal limitations of sequential chemical extractions, particularly in underestimating highly reactive mineral phases. These findings highlight the advantage of high-resolution deterministic spectroscopic tools for the identification of nanoparticulate complexes. While the polysulfide pathway has been widely inferred, direct detection of polysulfides and intermediate FeSx phases in marine sediments has remained elusive. Our study confirms the polysulfide pathway, bridging a long-standing gap between experimental models and environmental observations. These findings refine our understanding of early-diagenetic Fe-S transformations and how sulfide minerals form and persist in Earth’s dynamic sedimentary environment.