OXYGENATION AS A DRIVER OF THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT (Invited Presentation)

The largest radiation of Phanerozoic marine animal life, the Great Ordovician Biodiversification Event (GOBE), quadrupled genus-level diversity by the Late Ordovician. The causes of this event are not well understood but a leading hypothesis is that cooling of the Ordovician climate lowered sea surface temperatures into the thermal tolerance window of some modern animal groups (e.g. brachiopods and corals). An essential role for oxygenation of subsurface environments has been inferred based on increasing abundance of skeletal carbonate, but strong evidence for rising atmospheric oxygen (O₂) levels has yet to be discovered. Most estimates for atmospheric O₂ come from carbon (δ¹³C) and sulfur (δ³⁴S) isotope mass balance or iron-speciation data, but O₂-dependent fractionation of carbon isotopes by photosynthesis represents an independent method that is largely unexplored.

Here we use high-resolution δ¹³C isotope records to estimate atmospheric O₂ during the GOBE using O₂-dependent carbon isotope fractionation. We compare this O₂-dependent approach with the traditional carbon-sulfur isotope mass balance approach using the GEOCARBSULF model and new high-resolution isotope data (δ¹³C, δ³⁴S, and ⁸⁷Sr/⁸⁶Sr). We identify a strong link between rising O₂ and marine animal biodiversification during the Darriwilian Stage in both approaches that show atmospheric O₂ increased from ~14% to 23–25% during this interval. Most of the GOBE is composed of the Paleozoic Evolutionary Fauna (e.g. articulated brachiopods, bryozoans, crinoids), suggesting that these taxa thrived in oxygenated seas, which prior to the GOBE were periodically anoxic and likely stressful to benthic life. Oxygenated environments might have worked in concert with other factors to facilitate diversification, such as cooling temperatures, ecological forces (e.g. niche competition and predation), and CaCO₃-saturated shallow seas that favored taxa with biomineralized CaCO₃ skeletons. The high-resolution O₂-dependent approach also records a previously unrecognized ~10% O₂ decrease (from 25% to 15%) that coincides with the first phase of the end-Ordovician mass extinction. The possibility of an oxygen crisis during this extinction event could explain why metabolically active or heavily skeletonized taxa suffered disproportionately.