URANIUM ISOTOPE EVIDENCE FOR WELL-OXYGENATED OCEANS IMMEDIATELY PRIOR TO THE END-CRYOGENIAN MARINOAN GLACIATION (Invited Presentation)

Emerging paleo-redox data suggest highly dynamic Ediacaran ocean redox conditions after the ‘Snowball Earth’ episodes, with intervals of intense ocean anoxia punctuated by transient oxygenation events often associated with negative excursions in the δ¹³C composition of marine carbonates. Comparatively few paleo-redox constraints exist, however, for the Cryogenian oceans between the two ‘Snowball Earth’ (Sturtian and Marinoan) glaciations. In particular, uranium isotope (δ²³⁸U) data suggest an interval of temporary ocean oxygenation after the Sturtian glaciation, followed by a return to strongly anoxic oceans for the remainder of the interglacial period. In this study, we present new carbonate δ²³⁸U data from a profound negative δ¹³C event that immediately underlies Marinoan glacial deposits in south Australia—the Trezona anomaly. Previous calcium isotope (δ⁴⁴Ca) evidence suggested the potential for fluid-buffered diagenetic alteration in the declining and climbing limbs leading into and out of the δ¹³C excursion, but indicated that the nadir of δ¹³C values was characterized by sediment-buffered conditions that preserve the isotopic composition of primary seawater on the shallow carbonate platform. In the well-preserved nadir of δ¹³C values within the Trezona anomaly, our existing δ²³⁸U record exhibits high δ²³⁸U values close to modern Bahamian carbonates. If these δ²³⁸U values are representative of global seawater, they indicate an episode of well-oxygenated global oceans immediately preceding the Marinoan glaciation. Importantly, this evidence for ocean oxygenation in the Trezona Formation is found in the same section as recent discoveries of articulated sponge-grade animal fossils. The co-occurrence of negative δ¹³C excursions, positive δ²³⁸U shifts indicating transient ocean oxygenation, and fossil evidence for biological innovation is a recurring feature of the Neoproterozoic geological record, despite background conditions of widespread ocean anoxia that may have persisted well into the Paleozoic.