Multiple sulfur isotope evidence for massive oceanic sulfate depletion in the aftermath of Snowball Earth

The terminal Neoproterozoic Era (850-542 Ma) is characterized by the most pronounced positive sulfur isotope ((34)S/(32)S) excursions in Earth's history, with strong variability and maximum values averaging δ(34)S∼+38‰. These excursions have been mostly interpreted in the framework of steady-state models, in which ocean sulfate concentrations do not fluctuate (that is, sulfate input equals sulfate output). Such models imply a large pyrite burial increase together with a dramatic fluctuation in the isotope composition of marine sulfate inputs, and/or a change in microbial sulfur metabolisms. Here, using multiple sulfur isotopes ((33)S/(32)S, (34)S/(32)S and (36)S/(32)S ratios) of carbonate-associated sulfate, we demonstrate that the steady-state assumption does not hold in the aftermath of the Marinoan Snowball Earth glaciation. The data attest instead to the most impressive event of oceanic sulfate drawdown in Earth's history, driven by an increased pyrite burial, which may have contributed to the Neoproterozoic oxygenation of the oceans and atmosphere.