The ocean is hypothesized to have been anoxic throughout the Marinoan “Snowball Earth” event, from ca. 649 to 635 Ma, with potentially catastrophic implications for the survival of eukaryotic life. However, the precise nature of ocean redox chemistry across this critical interval, and hence the factors that governed the persistence of eukaryotes, remains unknown. We report records of pyrite iron and sulfur isotopes, combined with Fe speciation, for glaciogenic diamictites from the Nantuo Formation of South China. These data provide constraints on seawater redox state across the Marinoan glaciation, and they reveal that the redox state of the ocean fluctuated in concert with waxing and waning extents of glaciation, to include intervals of expanded oxygenation. The input of meltwater-derived oxygen provides a potential explanation for the persistence of eukaryotes through the Cryogenian “Snowball Earth” events, which ultimately paved the way for subsequent intervals of rapid biological innovation.