RISE IN SEAWATER SULFATE CONTENT DURING THE LOMAGUNDI (CA. 2.22-2.1 GA) CARBON ISOTOPE EXCURSION

Precipitation of anhydrite or gypsum in open-marine evaporite settings constrains seawater sulfate content above that provided by photochemical SO₂ disproportionation in the Archean atmosphere and requires riverine sulfate flux provided by oxidative continental weathering. It has been argued that either high carbonate alkalinity or low seawater sulfate content inhabited calcium sulfate precipitation in Paleoproterozoic open-marine evaporite settings until ca. 1.7 Ga. Evidence for precipitation of calcium sulfate beds and layers in pre-1.7 Ga sedimentary successions are indeed rare or absent. However, pseudomorphs after displacive calcium sulfate crystals and nodules formed during early diagenesis are common in the Paleoproterozoic carbonate and siliciclastic successions deposited during the ca. 2.22-2.1 Ga carbon isotope excursion worldwide (e.g. Melezhik et al., 2005; Bekker et al., 2006). These successions are not associated with or underlain by volcanics and were deposited in shallow, open-marine settings based on associated tidal signatures. Sulfur isotope values of preserved sulfate in these successions range from +11.4 to +34.2 permil, CDT.

Appearance of sulfate evaporites at the early stage of the Lomagundi carbon isotope excursion is most likely genetically related to the rise of atmospheric oxygen associated with the excessive organic carbon burial and initiation of oxidative continental weathering. Detrital pyrite grains are indeed lacking in mature siliciclastic successions deposited after the rise of atmospheric oxygen. Absence of evaporite beds and layers older than 1.7 Ga likely reflects lower redox state of the deep ocean where biological sulfate reduction and pyrite formation served as a major sink for seawater sulfate. Consequently, lower seawater sulfate content rather than high carbonate alkalinity inhibited calcium sulfate precipitation from seawater in open-marine evaporite setting during the Paleoproterozoic Era.