Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion:: evidence from sulphate evaporites in the ∼2.2-2.1 Gyr shallow-marine Lucknow Formation, South Africa

Past oceanic sulphate concentration is important for understanding how the oceans' redox state responded to atmospheric oxygen levels. The absence of extensive marine sulphate evaporites before similar to 1.2 Gyr probably reflects low seawater sulphate and/or higher carbonate concentrations. Sulphate evaporites formed locally during the 2.22-2.06 Gyr Lomagundi positive delta C-13 excursion. However, the similar to 2.2-2.1 Gyr Lucknow Formation, South Africa, provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input. These marginal marine deposits contain evidence for evaporite molds, pseudomorphs after selenite gypsum, and solid inclusions of Ca-sulphate in quartz. Carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines. The apparent precipitation of gypsum before halite requires >= 2.5 mM L-1 sulphate concentration, higher than current estimates for the Paleoproterozoic. During the Lomagundi event, which postdates the 2.32 Gyr initial rise in atmospheric oxygen, seawater sulphate concentration rose from Archean values of <= 200 mu M L-1, but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state.