SETTING A BACKGROUND FOR THE CHEMICAL CONDITIONS OF MICROBIALITE FORMATION: A CASE STUDY OF REDOX PROXIES FROM THE FILLMORE FORMATION, UTAH

The Early Ordovician was a unique time in the history of life representing the last period of microbialite abundance as well as experiencing the increased radiation of life during the Great Ordovician Biodiversification Event (GOBE). This combination of microbialite abundance and metazoan diversification during the Early Ordovician is particularly striking due to the fact that it has been suggested that the lack of microbialites in the Phanerozoic is correlated to the increase of grazing and burrowing metazoans. Here we begin to explore the chemical conditions for microbialite formation through the analysis of global and local redox proxies during the Early Ordovician from carbonate successions including horizons of microbialite patch reefs. Sulfur isotopic compositions of carbonate associated sulfate (CAS) act as a global redox proxy. Abundances of total organic carbon (TOC) and total sulfur (TS) act as redox proxies for local depositional environments.

We present sulfur isotopic compositions of CAS (δ³⁴S_CAS) and abundances of TOC and TS in order to constrain the background redox conditions of Early Ordovician microbialites from Lower Ordovician carbonates of the Fillmore Formation, Utah (C Section of Hintze & Davis, 2003). δ³⁴S_CASanalyses were completed at the stable isotopes laboratory at UC Riverside. TOC and TS abundances were measured on an ELTRA CS2000 carbon and sulfur determinator.

Our δ³⁴S_CAS values average 35.0‰ VCDT and range from 27.9-49.4‰ VCDT. The two microbialitic horizons from 50-55m and from 103-120m average 31.9‰ VCDT and 30.5‰ VCDT, respectively. There are no unique trends represented in the microbialitic horizons. Abundances of TOC and TS are uniformly low with respective averages of 0.2 wt.% and 0.02 wt.%. The uniformly low abundances of TOC and TS suggest that the local depositional environment of the C Section microbialites was not anoxic. However, our relatively heavy δ³⁴S_CAS values suggest that the Early Ordovician deep oceans were anoxic, as has been previously suggested (Thompson and Kah, 2012). This paradox between oxygenated local environments and anoxic deep oceans sets an interesting environmental background for the C Section microbialites. Our ongoing research will work to better constrain the chemical environment of these Early Ordovician microbialites.