Modelling sulfate concentrations in the global ocean through Phanerozoic time

Abstract

Understanding the long-term variations in seawater sulfate concentrations ([SO2−4 ]sw) is crucial to our understanding of the dynamic relationships between the sulfur, carbon, calcium and oxygen cycles, and their influence on the habitability of the Earth. Here, we explore how [SO2−4 ]sw has changed throughout the Phanerozoic and its impact on other elemental cycles. We do this by utilizing the biogeochemical box model GEOCARBSULFOR. The model suggests that [SO2−4 ]sw increased throughout the Paleozoic, decreased during the Mesozoic and then increased once more in the Cenozoic, generally matching geochemical proxies. Atmospheric oxygen mirrors [SO2−4 ]sw changes during the Paleozoic and Mesozoic, but, intriguingly, decouples during the Cenozoic. We further explored the controls on [SO2−4 ]sw by modifying the modelled gypsum fluxes via the incorporation of evaporite data from the geological record. We found 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. We also investigated the reliance of the model on a prescribed record of marine calcium concentrations, finding that it is a dominant control on modelled Phanerozoic [SO2−4 ]sw and that removing this control seriously degrades the model predictions. We conclude that no model can yet simulate a reasonable evolution of both the calcium and sulfur cycles.