UNCOVERING THE INTERMEDIATES IN PYRITE FORMATION UNDER FERRUGINOUS CONDITIONS

The chemical pathways leading to the formation of sedimentary pyrite, one of Earth’s most abundant minerals, are diverse and often debated. In low temperature settings, pyrite formation is initiated by microbial sulfate reduction to form sulfide (S2-), which is then oxidized to the average S1- state that we find in the mineral pyrite (FeS2). However, the mechanisms that lead to the oxidation of sulfur in pyrite are poorly constrained, with little knowledge about what intermediates may be present during the transformation of pyrite. Lab-based experiments show that pyrite can form via the reaction of FeS with elemental sulfur (S0) or polysulfides (S1- and S0), or from sulfidation of Fe(III) (oxyhydr)oxides. However, we are still lacking in observations of these pathways during pyrite formation in anoxic sediments.

To investigate sulfur transformation pathways in anoxic sediments, we collected water-column particulates and sediments from Brownie Lake in Minneapolis, MN. Brownie Lake is an meromictic lake with ferruginous (anoxic and iron-rich) bottom water with dissolved iron concentrations of >1000 μM, and maximum sulfate concentrations ranging between 80-100 μM. The distribution and abundance of sulfur species was determined using bulk and microscale techniques on both solids and (pore)water. Bulk X-Ray fluorescence (XRF) of Brownie sediments shows 0.5 wt.% S, and synchrotron-based X-Ray Absorption Spectroscopy (XAS) document pyrite formation within sediments. Mossbauer spectroscopy of water column particulates shows a highly reactive particulate FeS phase, while voltammetry documented an aqueous FeS phase. Microscale synchrotron-based X-Ray Absorption Spectroscopy (XAS) indicates the presence of S intermediates, such as thiosulfate and sulfite, that may be involved in the transformation of FeS to pyrite under predominantly ferruginous conditions.