Understanding the long-term variations in seawater sulfate concentrations ([SO ₄ ²⁻ ] _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 [SO ₄ ²⁻ ] _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 [SO ₄ ²⁻ ] _sw increased throughout the Paleozoic, decreased during the Mesozoic and then increased once more in the Cenozoic, generally matching geochemical proxies. Atmospheric oxygen mirrors [SO ₄ ²⁻ ] _sw changes during the Paleozoic and Mesozoic, but, intriguingly, decouples during the Cenozoic. We further explored the controls on [SO ₄ ²⁻ ] _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 [SO ₄ ²⁻ ] _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. Supplementary material: Figures S1–S5 are available at https://doi.org/10.6084/m9.figshare.c.7164928 Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/topic/collections/sulfur-in-the-earth-system