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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. [Copyright &y& Elsevier]

The Cambrian and Ordovician radiations were the most important biodiversification events in the history of animal life, yet they were separated by intervals of recurrent anoxic sedimentation and regional biomere extinction events in the Late Cambrian and Early Ordovician. Recent work linked biomere extinction events to positive carbon isotopic excursions interpreted to represent enhanced organic matter burial under anoxic conditions. This led to the hypothesis that an increase in oceanic oxygenation following the last of the biomere extinctions paved the way for the rapid diversification of the Ordovician Radiation. Here we test the hypothesis of increasing ocean oxygenation using Th/U preserved in carbonate rocks from the Lower–Middle Ordovician succession near Ibex, Utah, USA. The global seawater uranium inventory is expected to vary inversely with the amount of deep-water anoxic deposition due to uranium sequestration in black shales. Therefore in shallow water carbonates higher Th/U ratios may reflect increased global ocean anoxia. Our data show that the highest Th/U values are coincident with the heaviest carbon isotopic values reported from the base of this succession, which is latest Tremadocian in age. Th/U then decreases and remains low through the Floian up through the contact with the Kanosh Formation, a shale-dominated unit with interspersed limestone units. The geochemistry of the carbonates within the Kanosh Formation likely represents local rather than global conditions in the marine environment. We interpret the overall decrease in Th/U ratios throughout the Tremadocian to Floian interval to represent an increase in global seawater uranium concentration. These data are consistent with the hypothesis that an increase in ocean oxygenation occurred prior to the major pulses of diversification associated with the Ordovician Radiation. [ABSTRACT FROM AUTHOR]