A Paleoproterozoic collapse in seawater sulfate and its influence on the global methane cycle

The initial accumulation of atmospheric oxygen is referred to as the Great Oxidation Event (GOE) and is fairly well-constrained to between 2,450 and 2,320 Ma. However, the magnitude and duration of the GOE are subject to debate and it is not clear how early Paleoproterozoic oxidation relates to the long-lived intermediate redox state of the mid-Proterozoic. In order to investigate Paleoproterozoic surface oxidation, we used a combination of pyrite multiple-sulfur (32S, 33S and 34S) and organic carbon isotopes from a suite of early Paleoproteorzoic marine black shales. We analyzed the 2,320 Ma Rooihoogte and Timeball Hill Formations, from which the GOE is dated; the 2,200 to 2,100 Ma Sengoma Argillite Formation, deposited during the peak of the Lomagundi carbon isotope excursion, and the Upper Zaonega Formation of the Ludikovian Series, Russian Karelia, deposited in the immediate aftermath of the Lomagundi carbon isotope excursion. Our results suggest that a large marine sulfate reservoir was an immediate result of the GOE, however, sulfate concentrations collapsed rapidly following the Lomagundi carbon isotope excursion resulting in invigoration of the methane cycle as recorded in the organic C isotope record.