Primitive Os and 2316 Ma age for marine shale: implications for Paleoproterozoic glacial events and the rise of atmospheric oxygen

Re–Os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the Rooihoogte and Timeball Hill formations, Transvaal Supergroup, South Africa, provides a precise isochron age of 2316±7 Ma (±4 statistical uncertainty if error on decay constant is excluded) and a chondritic initial 187Os/188Os ratio of 0.1121±0.0012. These units were deposited between what are interpreted as the second and third of three Paleoproterozoic global glacial events, and thus provide minimum and maximum ages, respectively, for these events. The Rooihoogte Formation is correlative with the Duitschland Formation, which records previously undated carbon isotope excursions. Because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative δ34S values, the rise of atmospheric oxygen most likely began prior to 2.32 Ga. The chondritic initial 187Os/188Os ratio requires that primitive hydrothermal sources of Os dominated the Os budget of seawater at 2.32 Ga. Os is mobile only under oxidizing conditions. Therefore, the chondritic initial 187Os/188Os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic Os to offset the primitive hydrothermal/magmatic flux. Even if atmospheric oxygen levels had increased significantly by 2.32 Ga, anoxic conditions throughout the Archean most likely limited Os enrichment in Archean marine shales, so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic Os to Paleoproterozoic seawater.