A GEOCHEMICAL FRAMEWORK FOR THE APPLICATION OF STABLE SULFUR ISOTOPESTO FOSSIL PYRITIZATION

Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts ei ther as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of por ewater chemistry and saturation stale with respect to carbonates and i ron sulphides. A review of modern marine sediments suggests that near- surface porewaters are always over-saturated, or close to saturation, with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide. However, significant variations in carbonate sa turation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversatu rated until the later stages of methanogenesis. Here undersaturation m ay return if there is a substantial input of carbon dioxide, and if li ttle alkalinity has arisen from iron reduction. Organic matter pyritiz ation requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the sur rounding porewaters. The modem sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction, where there are iron-rich porewaters which are saturated with iron sulphides. Pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occu r either during the earliest stages of sulphate reduction, or when met hanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morp hologies can precipitate on carbonate shells. Sulphur isotope data pro vide support for these associations of porewater chemistry and style o f fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hunsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on sh ell pyritization with the range of isotopic compositions indicating fo rmation through all stages of diagenesis.