Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron 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 saturation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversaturated until the later stages of methanogenesis. Here undersaturation may return if there is a substantial input of carbon dioxide, and if little alkalinity has arisen from iron reduction.
Organic matter pyritization requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the surrounding porewaters. The modern 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 occur either during the earliest stages of sulphate reduction, or when methanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells. Sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hünsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis.