Pyritization and Paleoecology of Olenid Trilobite Faunas from the Ordovician of Upstate NY

Beecher's Trilobite Bed (BTB) (Ordovician, Frankfort Formation, NY) is known for the preservation of trilobite appendages in pyrite, in particular those of the olenid trilobite Triarthrus. A new locality that also yields exceptionally preserved olenids has been discovered in the Ordovician Whetstone Gulf Formation, NY (Martin Quarry, MQ). Olenid trilobites have been interpreted as having a symbiotic relationship with sulfur bacteria allowing them to live in low-oxygen, borderline sulfidic conditions.

Field-work has revealed that conditions suitable for pyritization were not confined to one bed but are found in multiple horizons. Pyritization in these horizons is variable in extent and quality.

This study includes geochemical analyses of the highly reactive iron (Fe_HR) and d³⁴S in the sediment. The ratio of Fe_HR to total iron (FeT) provides a method of distinguishing anoxic from oxic bottom waters. The proportion of pyrite iron (Fe_py) provides an indication of the possible presence of sulfide in the water. Results show fluctuating values of Fe_HR/FeT and Fe_HR/Fe_py suggesting that bottom waters were occasionally anoxic but rarely sulfidic. However, the signature is not simply correlated to paleoecological data, although beds with pyritized fossils have consistently high ratios of Fe_HR/FeT and high d³⁴S.

Paleontological analyses at BTB show a low-diversity and sparse fauna consisting of graptolites, cephalopods, brachiopods, bivalves, and fragments of trilobites. Beds with whole trilobites (almost all olenids) often have shallow trace fossils at the top. Overall diversity at MQ is much higher, with the addition of crinoids, bryozoans, gastropods and other trilobites. The pyritized olenid trilobites are nevertheless found in otherwise unfossiliferous beds.

In conclusion, bottom-water conditions fluctuated and were occasionally inhospitable. Chemoautotrophic symbiosis could have been a way to take advantage of such an environment, although adaptation to low-oxygen may have been sufficient. Pyritization of soft-tissues occurred in distal turbidites with distinctive geochemical signatures.