Evidence for a gradual rise of oxygen between 2.6 and 2.5 Ga from Mo isotopes and Re-PGE signatures in shales

Abstract

Concerning the question of how and when free oxygen started to accumulate in the Earth’s atmosphere and hydrosphere, we report Mo concentrations and isotopic compositions as well as Platin group element and Rhenium (PGE-Re) data from black shales of the Transvaal Supergroup (Ghaap Group, 2.64–2.5 Ga, and Pretoria Group, 2.45–2.15 Ga). For comparison we also include new data from the 3.23 Ga Fig Tree Group, Barberton Greenstone Belt. This time range covers the period between the first robust evidence for cyanobacteria at 2.7 Ga and the disappearance of mass independent sulfur isotope fractionation (MIF) at 2.32 Ga. Due to the redox dependent solubility of Mo, Re and Os, such data are important proxies for changes in oxygen levels of the early atmosphere and oceans. In particular, Mo isotope fractionation can only occur when Mo is in solution as oxyanions, requiring free oxygen. In the Fig Tree Group samples, Mo is not fractionated relative to the continental crust, and PGE-Re abundance patterns reflect those of komatiites, indicating a purely detrital input of these element, a sign of no free oxygen. A general increase in Mo concentration and isotope fractionation, as well as an enrichment in Re, compared to the continental crust and Fig Tree Group, can be seen within the Ghaap Group indicating a gradual rise of oxygen between 2.64 and 2.5 Ga. However, Mo concentrations and δ 98/95Mo values vary strongly in this period. Local conditions of sedimentation (changes in redox and input conditions) and/or global oxic/anoxic fluctuations could have caused these Mo variations. Samples of the overlying Pretoria Group (2.45–2.15 Ga) show Mo concentrations and isotopic compositions somewhat below the continental input, while PGE-Re abundance patterns are quite similar to those for the Ghaap. The apparent contradiction between the decoupled Mo and PGE-Re values can be resolved assuming a stratified ocean with almost total scavenging of Mo. Such an increased scavenging can result from enhanced biological sedimentation between and especially after glacial events. This period coincides with the Lomagundi-Jatulian carbon isotope excursion, which indeed indicates very enhanced organic carbon burial.