Iron speciation in ancient sedimentary rocks is widely used to reconstruct oceanic redox conditions over geological time, specifically to assess the extent of oxic, euxinic (anoxic containing sulfide), and ferruginous (anoxic containing iron) conditions. We explore how post-depositional sedimentary processes can skew particular geochemical signals in the rock record. One such process is when aqueous sulfide—including that produced in the sediment column—reacts with sedimentary iron, converting non-sulfide, highly reactive iron minerals to iron-sulfide minerals; this can lead to increased preservation of iron as pyrite and an overestimation of seafloor euxinia. We show that sedimentary rocks with higher (>5 wt%) total iron content are more buffered to this effect and thus are a more reliable indicator of true water-column euxinia. When considering this effect in the geological past, we estimate that true euxinia in the mid-Proterozoic may have been as much as fourfold less than previously thought—more in line with other recent paleoredox proxies not based on iron minerals. Marine iron and sulfate concentrations were more equivalent in Proterozoic–Neoproterozoic oceans, suggesting this time period was particularly susceptible to this post-depositional alteration, explaining the extent of euxinia suggested for this geological interval.