Modelling sulfate concentrations in the global ocean through Phanerozoic time

Understanding the long-term variance of seawater sulfate concentrations ([SO42-]sw) is critical to understanding the dynamic relationship between the sulfur, carbon, calcium, and oxygen cycles, and their influence on Earth's habitability. Here, we explore how [SO42-]sw has changed throughout the Phanerozoic, and its impact on other elemental cycles. We do this by utilising the biogeochemical box model GEOCARBSULFOR. The model suggests that [SO42-]sw rose throughout the Paleozoic, declined during the Mesozoic, and then rose once more in the Cenozoic, generally matching geochemical proxies. Atmospheric oxygen mirrors [SO42-]sw changes during the Paleozoic and Mesozoic, but intriguingly, decouples during the Cenozoic. We further explored controls on [SO42-]sw by modifying the modelled gypsum fluxes via the incorporation of evaporite data from the geological record. We find that forcing gypsum burial with the observed evaporite deposition data causes the model to better match proxy records at some times, but worsens predictions at others. Finally, we investigate model reliance on a prescribed record of marine calcium concentrations, finding that it is a dominant control on modelled Phanerozoic [SO42-]sw, and that removing this control seriously degrades model predictions. We conclude that no model can yet simulate a reasonable evolution of both the calcium and sulfur cycles.