期刊
AMERICAN JOURNAL OF SCIENCE
卷 313, 期 5, 页码 452-489出版社
AMER JOURNAL SCIENCE
DOI: 10.2475/05.2013.02
关键词
Dysoxia; iron paleoredox proxy; Konservat Lagerstatte; taphonomy; trace elements; sulfur isotopes; distal turbidite; paleoenvironment
资金
- Yale Institute of Biospheric Studies
- Geological Society of America
- Paleontological Society
- International School of Aquatic Sciences, Denmark
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.
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