HOW DID DEEP OCEAN EUXINIA AFFECT THE GOBE? A CASE STUDY FROM THE UPPER KANOSH AND LEHMAN FORMATIONS, UTAH

The Great Ordovician Biodiversification Event (GOBE) was a pronounced radiation within the Paleozoic and Modern Evolutionary Faunas. It was an interval of about 25 million years (Servais et al., 2009) during which there was a significant increase in marine invertebrate fossil diversity. A number of factors are thought to have contributed to the GOBE including increased continental shelf area as well as climatic and ecological factors. Although life in shallow marine environments was thriving, recent geochemical research suggests that the deep ocean was euxinic (e.g., Thompson and Kah, 2012). However, it is still unclear how this deep ocean euxinia may have affected the GOBE.

One way to investigate how deep ocean euxinia affected shallow marine life is to study redox proxies in continental shelf rocks formed during the GOBE. For this study, carbonates and shales were collected from the Middle Ordovician Kanosh and Lehman Formations of Millard County, Utah (the Camp section of Hintze and Davis, 2003). Total organic carbon (TOC) and total sulfur (TS) abundances were measured as redox proxies using an Eltra Carbon and Sulfur Determinator. Overall the TOC and TS abundances for the shales and limestones of the Camp section were low. The shale TOC abundance averaged 0.3 wt. %., with a maximum result of 0.8 wt. %. The shale TS abundance averaged 0.03 wt. % with a maximum of 0.05 wt. %. Similarly, limestone TOC abundance averaged 0.3 wt. % with a maximum of 0.7 wt. %, while limestone TS abundance averaged 0.03 wt. % with a maximum of 0.05 wt. %. These data indicate that this basin was well oxygenated without significant intervals of anoxia. Ongoing research will help constrain how deep ocean euxinia may have affected the GOBE.