Contrasting long-term global and short-term local redox proxies during the Great Ordovician Biodiversification Event: A case study from Fossil Mountain, Utah, USA

Abstract

The Great Ordovician Biodiversification Event (GOBE) was a time of pronounced radiation within the Paleozoic and Modern evolutionary faunas. Despite increases in metazoan diversity during both the Cambrian and the Ordovician, recent geochemical evidence suggests that the Early Paleozoic may have experienced recurring deep ocean euxinia. The dynamic connections between deep ocean redox and fossil-rich shelf environments remain unclear, but one way to elucidate these relationships is to study rocks formed during the GOBE using both long-term global and short-term local redox proxies. Here we report geochemical results from the Middle Ordovician Juab Formation and Kanosh Formation at Fossil Mountain, Utah. The Kanosh Formation has been interpreted by different workers either as an anoxic restricted basin or as a well-oxygenated normal marine basin based on sedimentological and paleontological characteristics.Here we employ the sulfur isotopic composition of carbonate associated sulfate (CAS) as a proxy for long-term, whole-ocean redox changes during the Middle Ordovician. We use the abundances of organic carbon and total sulfur as proxies for short-term, localized changes in redox during the deposition of the Fossil Mountain sediments. Organic carbon abundances are low overall (average=0.7wt.%, maximum=1.2wt.% using the most conservative estimate), although some units may have had considerably higher organic content before burial. Total sulfur is uniformly low, with an average value of 0.02wt.% and a maximum value of 0.07wt.%. The total sulfur and organic carbon abundance results suggest that anoxia was not prevalent in the depositional environment of the Juab and Kanosh Formations. Similarly, the sulfur isotopic results correlate well with those reported from time-equivalent strata in Newfoundland, suggesting that the Fossil Mountain sediments were not deposited in a restricted basin. Even though the total sulfur and organic carbon data suggest that the local environment was not anoxic, our sulfur isotopic results support the hypothesis that the global ocean was likely euxinic during the Middle Ordovician.