Paleoredox and pyritization of soft-bodied fossils in the Ordovician Frankfort Shale of New York

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Multiple beds in the Frankfort Shale (Upper Ordovician, New York State), including the original “Beecher9s Trilobite Bed,” yield fossils with pyritized soft-tissues. A bed-by-bed geochemical and sedimentological analysis was carried out to test previous models of soft-tissue pyritization by investigating environmental, depositional and diagenetic conditions in beds with and without soft-tissue preservation. Highly-reactive iron (FeHR), total iron (FeT), δ34S, organic carbon and redox-sensitive trace elements were measured. In particular, the partitioning of highly-reactive iron between iron-carbonates (Fe-carb), iron-oxides (Fe-ox), magnetite (Fe-mag), and pyrite (FeP) was examined. Overall, the multi-proxy sedimentary geochemical data suggest that the succession containing pyritized trilobite beds was deposited under a dysoxic water-column, in agreement with the paleontological data. The data do not exclude brief episodes of water-column anoxia characterized by a ferruginous rather than an euxinic state. However, the highest FeHR/FeT values and redox-sensitive trace element enrichments occur in siltstone portions of turbidite beds and in concretions, suggesting that subsequent diagenesis had a significant effect on the distribution of redox-sensitive elements in this succession. Moderately high FeHR/FeT and FeP/FeHR, low organic carbon, enriched δ34S, and the frequent presence of iron-rich carbonate concretions in beds with soft tissue preservation confirm that pyritization was favored where pore-waters were iron-dominated in sediments relatively poor in organic carbon.