Bulk iron-sulfur ratio (Fe/S) conditional proxy for sulfurized, TOC-rich sediments: insights and comparison of compositional model with real data utilizing the Pyrite-TOC-Fe-S compositional space

Matthew Wehner

Bulk iron-sulfur (Fe/S) ratio can be a proxy for sulfurized organic matter in organic-rich shales, particularly those prone to be siliceous or carbonate. First part uses a theoretical model of a compositional system consisting solely of iron, sulfur, TOC, and pyrite to demonstrate how the Fe/S ratio can indicate the presence of sulfurized organic matter using the model’s assumption of sulfur being only in either in pyrite as sulfide or in TOC as organic sulfur. The second part identifies 25 studies where iron, sulfur, TOC, and pyrite content measurements (or have elemental data for organic matter) are made for organic-rich shales in 11 countries and 16 formations spanning Devonian to Plio-Pleistocene geologic time scales. These datasets clearly shows that Fe/S ratios less than 0.87 correspond to sulfurized TOC in organic-rich sedimentary rocks. Also a checklist is provided to assist researchers assess the validity of the Py-TOC-Fe-S model to their datasets and if so how to interpret the Fe/S proxy. In a way, the Fe/S ratio could be thought of complementary to degree of pyritization (DOP) since DOP is valid fore Fe/S > 0.871 while Fe/S proxy is valid for Fe/S < 0.871