At 28 mM seawater sulphate is one of the largest oxidant pools at Earth’s surface and its concentration in the oceans is generally assumed to have remained above 5 mM since the early Phanerozoic (400 Ma). Intermittent and potentially global oceanic anoxic events (OAEs) are accompanied by changes in seawater sulphate concentrations and signal perturbations in the Earth system associated with major climatic anomalies and biological crises. Ferruginous (Fe-rich) ocean conditions developed transiently during multiple OAEs, implying strong variability in seawater chemistry and global biogeochemical cycles. The precise evolution of seawater sulphate concentrations during OAEs remains uncertain and thus models that aim to mechanistically link ocean anoxia to broad-scale disruptions in the Earth system remain largely equivocal. Here we show that during OAE1a, 125 Ma, the oceans were anoxic and ferruginous for more than 1 million years. Development of ferruginous conditions at this time requires low seawater sulphate concentrations, which likely dropped to < 70 μM or more than a hundred times lower than modern. This collapse in the seawater sulphate pool over just a few hundred thousand years or less, reveals previously unrecognized dynamics in Phanerozoic Earth surface redox budgets with potential to dramatically alter global biogeochemical cycles, marine biology, and climate on remarkably short time-scales.