Defliese, William F.1
1School of Earth and Environmental Sciences, The University of Queensland, St Lucia, QLD, Australia
It has been long recognized that glacial episodes can affect the δ18O value of ocean water, where preferential storage of 16O in ice changes the 18O/16O ratio of the ocean. However, these effects are generally thought of as transient, as Cenozoic glaciation has had neither the magnitude nor duration to cause long-term change with ocean water buffered to values close to 0±2 ‰ VSMOW by tectonic processes. The Snowball Earth glaciations of the Cryogenian have the potential to cause much larger changes in ocean water δ18O values due to their increased ice volume and long duration relative to Cenozoic glaciation, but these effects have not been previously investigated.
Here, I use a numerical box model to investigate ocean water δ18O values over the Proterozoic and Phanerozoic. The model simulates various temperature and tectonics dependant fluxes of 18O, while also incorporating a zero-dimensional climate model and ice volume component to model glacial cycles. Monte Carlo simulations of the Sturtian and Marinoan glaciations reveal that these had the potential to alter ocean water δ18O values for hundreds of millions of years after the termination of glaciation, providing a mechanism for secular change in the δ18O value of ocean water. This occurs as a very large volume of ice (presumably, but not necessarily 18O depleted) is sequestered from the ocean, causing the ocean to become enriched enough in 18O for exchange at mid-ocean ridges to remove 18O from the ocean and slowly change the overall ocean water δ18O value. If Snowball Earth ice volumes were as large as proposed (~28-32% of ocean volume), present day values of ice δ18O would cause significant secular change in ocean water δ18O extending into the Phanerozoic. An additional finding of this work is that the duration of the Sturtian glaciation required a very low CO2 degassing rate on the order of ~2 Tmol/year, significantly less than that estimated from most other mass balance approaches for the Phanerozoic.
Dr. William Defliese is Lecturer in Geochemistry in the School of Earth and Environmental Sciences at UQ. His research interests include stable isotope geochemistry, clumped isotope geochemistry, carbonate sediments, basin analysis, and paleoclimatology. He joined UQ in 2019 after postdocs at UCLA and Texas A&M University.