Farrell, Una C.3; Briggs, Derek E. G.3; Hammarlund, Emma U.6; Sperling, Erik A.4; Gaines, Robert R.5
1 Department of Biology, Faculty of Science, SDU2 Nordic Center for Earth Evolution (NordCEE), Department of Biology, Faculty of Science, SDU3 Yale University4 Harvard Univ, Dept Earth & Planetary Sci, Cambridge, MA 02138 USA5 Pomona Coll, Dept Geol, Claremont, CA 91711 USA6 Nordic Center for Earth Evolution (NordCEE), Department of Biology, Faculty of Science, SDU
Multiple beds in the Frankfort Shale (Upper Ordovician, New York State), including the original "Beecher's 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 (Fe-HR), total iron (FeT), delta S-34, 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 Fe-HR/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 Fe-HR/FeT and FeP/Fe-HR, low organic carbon, enriched delta S-34, 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.
American Journal of Science, 2013, Vol 313, Issue 5, p. 452-489