A NEGATIVE SULFUR ISOTOPIC EXCURSION DURING THE HEIGHT OF THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT: A BRIEF INTERRUPTION IN LONG-TERM DEEP-OCEAN ANOXIA?

The Great Ordovician Biodiversification Event (GOBE) was a pronounced radiation within the Paleozoic and Modern evolutionary faunas during the Early and Middle Ordovician (e.g., Droser & Finnegan, 2003). A number of factors likely contributed to the GOBE, one of which was the availability of shallow, warm continental shelf environments resulting from the breakup of Rodinia as well as the high sea level resulting from warm climate during the Ordovician (e.g., Servais et al., 2009). Despite the biological radiation occurring during this time, recent geochemical evidence suggests that the deep ocean was likely anoxic during the Early Paleozoic (e.g., Gill et al., 2011; Thompson & Kah, 2012; Marenco et al., 2013). However, warm, shallow continental shelf settings may have been decoupled from the deep-ocean anoxia, which would have allowed for the GOBE to progress largely independently of redox conditions in the deep.

Here we present organic carbon and total sulfur abundances from the Lower and Middle Ordovician of the Ibex area of Utah, which when combined with fossil data suggest an absence of anoxic deposition for much of the succession. We also present sulfur isotopic compositions of carbonate associated sulfate (CAS) that are relatively elevated (between +30 and +40 permil) for the majority of the succession, suggesting a long period of deep oceanic anoxia, consistent with data from other regions (Thompson & Kah, 2012). However, in Middle Ordovician strata, during the height of the GOBE, the sulfur isotopic composition of CAS shows a pronounced decrease to +20 permil and a return to +30 permil over an interval of approximately 100m. This excursion may represent a temporary increase in global oxygen during the most intense part of the GOBE.