RE-OS GEOCHRONOLOGY AND OS-SR ISOTOPE DATA FOR THE BITTER SPRINGS EXCURSION, WYNNIATT FORMATION, SHALER SUPERGROUP, CANADA (Invited Presentation)

The Tonian Period (ca. 1000-717 Ma) represents a pivotal point in the history of eukaryotic life punctuated by supercontinent amalgamation, carbon isotope excursions (CIEs), and novel eukaryotic evolutionary innovation, including early records of biomineralization. One of the most prominent geochemical records from this time is the Bitter Springs Excursion (BSE), a ≥5‰ negative CIE that has been documented from sedimentary successions globally. The broad coincidence of the BSE with redox fluctuations and the emergence of eukaryotic biomineralization highlights its importance for investigating geobiological change during the Tonian. However, the cause of the BSE is unresolved, and global radioisotopic age constraints are few. These roadblocks have limited efforts to 1) understand the mechanism(s) responsible for the BSE, and 2) produce a robust global age model to correlate stratigraphic, biogeochemical and paleontological data associated with the BSE.

Here, we report a new Re-Os age from a syn-BSE interval of organic-rich (3-10 wt. % TOC) calcilutite from the upper carbonate member of the Wynniatt Formation, Shaler Supergroup, NWT, Canada. These data provide a valuable updated age constraint for the Wynniatt Formation, and represent the only reported syn-BSE radioisotopic age. By propagating this age into existing age models, we can now constrain the duration of the BSE to ≥7.4 Myrs. We also provide seawater Os isotope ratios (Osi) and ⁸⁷Sr/⁸⁶Sr chemostratigraphy bracketing the BSE onset in the Wynniatt Formation to test a chemical weathering-related trigger. Together, these geochemical data constrain fluctuations in chemical weathering intensity and basin restriction, which have geobiological implications for changing nutrient flux and basin water chemistry leading into the BSE.