Characterizing sulfur redox state and geochemical implications in deep-time using mineral chemistry network analysis

Abstract

Sulfur (S) is a central element in global biogeochemical cycling and Earth’s redox evolution. Minerals that contain S are an important record of local environmental conditions at the time of their formation based on chemical speciation and redox. However, the oxidation state of S for hundreds of different S-containing minerals and thousands of S-containing mineral localities is unknown, largely sulfides and sulfosalts, and the redox state alone does not fully capture mineral chemistry diversity, thus limiting understanding of S redox evolution. Here, we use mineral chemistry network analysis and the weighted Mineral Element Electronegativity Coefficient of Variation (wMEECV) metric to investigate the element interactions and localities of S-containing minerals from the Mineral Evolution Database (MED) to infer the redox state of S in minerals where the redox state is unknown (SU). Louvain community detection of the S mineral chemistry redox network reveals that there are three main network communities that are separated by redox state. The S6+ community includes minerals that contain the S6+ redox state and a small number of S4+ and S2+ minerals, the S2− community includes S2−-containing minerals, and the SU community includes minerals in which the redox state of S is unknown. The wMEECV values of the SU community closely overlap with the wMEECV values of the S2− community, and do not overlap with the wMEECV values of the S6+ community, indicating the SU community minerals contain predominately reduced S. Assuming that SU community minerals contain reduced S, as supported by their network chemical associations and wMEECV values, then reduced S-containing minerals make up approximately 81 % of S-containing mineral localities in the S mineral chemistry network, even though the majority of all mineral localities (S-containing and non-S-containing) are oxygen (O)-containing minerals. Additionally, reduced S-containing minerals make up the majority (∼75 %) of all non-O containing mineral localities in the MED, representing the importance of reduced S as an electron source and substrate in the evolution of microbial metabolic networks. The range wMEECV values of S6+ community minerals expands through time due primarily to formation of chemically diverse sulfate minerals, coinciding with crustal oxidation from the late Proterozoic to Phanerozoic and the expansion of the marine sulfate reservoir. The intersection of shared constituent elements among reduced and oxidized S in the mineral chemistry network represents redox convergence of weathered S in the geosphere that was crucial in the formation of natural resource deposits and the evolution of biogeochemical cycles.