Ferruginous oceans during OAE1a and collapse of the marine sulfate pool

Seawater sulfate is one of the largest oxidant pools at Earth's surface today and its concentration in the oceans is generally assumed to have varied between 5 and 28 mM since the early Phanerozoic Eon. Intermittent and potentially global Oceanic Anoxic Events (OAEs) are accompanied by changes in seawater sulfate 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 sulfate concentrations during OAEs, however, is uncertain and thus models that aim to mechanistically link oceanic anoxia to broad-scale disruptions in the Earth system remain incomplete. We use analyses of Fe-speciation and redox sensitive trace metals in sediments deposited in the Tethys and Pacific oceans to constrain seawater sulfate concentrations and underlying dynamics in marine chemistry during OAE1a, ∼120 Ma. We find that large parts of the global oceans were anoxic and ferruginous for more than 1 million years. Calculations show that the development of ferruginous conditions requires that seawater sulfate concentrations drop below 600 μM and possibly below 100 μM, which is an order of magnitude lower than previous minimum estimates. Such a collapse of the seawater sulfate pool in less than one-hundred thousand years is a key and previously unrecognized feature of Phanerozoic Earth surface redox budgets. This sensitivity of the Earth system to changes in seawater sulfate concentrations illustrates potential for dramatically altered global biogeochemical cycles with corresponding climate impacts on remarkably short timescales.