Sulfur isotope mass-independent fractionation (S-MIF) is a unique geologic record of Archean atmospheric chemistry that provides important constraints on the evolution of the early Earth’s atmosphere and its impact on early life. In this contribution, we report multiple-sulfur (33S/32S, 34S/32S, and 36S/32S) isotope ratios of sulfide minerals and carbon (13C/12C) isotope ratios of organic carbon for shale in the ~2.96 to ~2.84 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. The δ13C of organic carbon shows two populations: one with δ13C of ~ −26 ‰ and another with δ13C of −32 ‰. The Δ33S values from nine samples ranges from −0.49 to +0.36 ‰, which is considerably smaller than what was measured for the sulfide and sulfate minerals from other Archean intervals but outside the range of Δ33S values measured for post-2.0 Ga sulfide and sulfate minerals. Moreover, some samples from the Mozaan Group yield Δ36S/Δ33S ratios that are different from Phanerozoic sulfides, suggesting sulfide sulfur from the Mozaan Group carries mass-independent isotope fractionation originated from atmospheric photochemistry.
The relatively small Δ33S values for the Mozaan Group may suggest that the atmosphere became slightly oxidized at ~2.9 Ga with oxygen level above 10−5 but below 10−2 times present atmospheric level. This intermediate oxygen level would allow production of S-MIF in atmospheric chemistry but prohibit preservation of large S-MIF signatures in surface deposits. Our hypothesis implies the evolution of oxygenic photosynthesis as early as ~2.9 Ga. Such an ephemeral oxidation event could have triggered the Mozaan-Witwatersrand glaciation by destabilizing an existing methane-rich Archean atmosphere.