LAUSR

Abstract It has been long recognized that glacial episodes can affect the δ 18 O 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 or 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 δ 18 O 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 δ 18 O 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 δ 18 O values for hundreds of millions of years after the termination of glaciation, providing a mechanism for secular change in the δ 18 O 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 δ 18 O value. If Snowball Earth ice volumes were as large as proposed (∼28-32% of ocean volume), present day values of ice δ 18 O would cause significant secular change in ocean water δ 18 O 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.