The impact of Snowball Earth glaciation on ocean water δ18O values

It has been long recognized that glacial episodes can affect the delta O-18 value of ocean water, where , preferential storage of O-16 in ice changes the O-18/O-16 ratio of the ocean. However, these effects are generally thought of as transient, as Cenozoic glaciation has had neither the magnitude or duration to cause long-term change with ocean water buffered to values close to 0 +/- 2%o VSMOW by tectonic processes. The Snowball Earth glaciations of the Cryogenian have the potential to cause much larger changes in ocean water delta O-18 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 delta O-18 values over the Proterozoic and Phanerozoic. The model simulates various temperature and tectonics dependant fluxes of O-18, 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 delta O-18 values for hundreds of millions of years after the termination of glaciation, providing a mechanism for secular change in the delta O-18 value of ocean water. This occurs as a very large volume of ice (presumably, but not necessarily O-18 depleted) is sequestered from the ocean, causing the ocean to become enriched enough in O-18 for exchange at mid-ocean ridges to remove O-18 from the ocean and slowly change the overall ocean water delta O-18 value. If Snowball Earth ice volumes were as large as proposed (similar to 28-32% of ocean volume), present day values of ice delta O-18 would cause significant secular change in ocean water delta O-18 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 similar to 2 Tmol/year, significantly less than that estimated from most other mass balance approaches for the Phanerozoic. Crown Copyright (C) 2020 Published by Elsevier B.V. All rights reserved.

Automated summary

Glacial episodes can affect the delta O-18 value of ocean water, with Cenozoic glaciation not causing long-term change due to ocean water buffering. In contrast, Snowball Earth glaciations in the Cryogenian have the potential to significantly alter ocean water delta O-18 values. Monte Carlo simulations show that glaciations could change delta O-18 values for hundreds of millions of years, with Sturtian glaciation requiring a low CO2 degassing rate.