CONTROLS ON THE PYRITIZATION OF EXCEPTIONALLY PRESERVED FOSSILS - AN ANALYSIS OF THE LOWER DEVONIAN HUNSRUCK SLATE OF GERMANY

Sulfur isotope and C-S-Fe elemental data from the Lower Devonian (Sieg enian/Emsian) Hunsruck Slate of Germany allow the conditions that led to the pyritization of soft-bodied fossils to be determined. The slate s preserving pyritized soft-tissues generally contain low concentratio ns of organic matter (0.3-0.4 percent C) and pyrite sulfur (0-0.2 perc ent S) respectively but are unusually rich in total and HCl-extractabl e iron (concentrations of iron are significantly lower in samples of t he 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 persist ed 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 contin ued fossil pyritization in organic-rich microenvironments, A diffusion -with-precipitation model indicates that a critical control on the ini tial 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 c oncentrations of metabolisable organic matter in the matrix. Pore wate rs rich in dissolved iron allow the sulfide generated by the decay of readily metabolisable soft tissue to be trapped efficiently within the carcass.