A model for iron deposition to euxinic Black Sea sediments

Sulfur isotope and C-S-Fe elemental data from the Lower Devonian (Siegenian/Emsian) Hunsruck Slate of Germany allow the conditions that led to the pyritization of soft-bodied fossils to be determined. The slates preserving pyritized soft-tissues generally contain low concentrations of organic matter (0.3-0.4 percent C) and pyrite sulfur (0-0.2 percent S) respectively but are unusually rich in total and HCl-extractable iron (concentrations of iron are significantly lower in samples of the Hunsruck Slate from areas where fossils lack pyritized soft-tissues). The fossil pyrite is very enriched in S-34 compared to fine-grained, disseminated pyrite In the adjacent slates (Delta(fossil-sediment) = 10 to 50 permil), suggesting that pyritization of the fossils persisted into later stages of authigenic mineralization. Pyrite formation in the sediments was limited by the low concentrations of metabolisable organic matter, leaving residual sulfate and iron available for continued fossil pyritization in organic-rich microenvironments, A diffusion-with-precipitation model indicates that a critical control on the initial preservation of readily metabolisable soft tissue in this way is the presence of high concentrations of sediment iron capable of being solubilized during shallow burial diagenesis in association with low concentrations of metabolisable organic matter in the matrix. Pore waters rich in dissolved iron allow the sulfide generated by the decay of readily metabolisable soft tissue to be trapped efficiently within the carcass.