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Early evolution of atmospheric oxygen from multiple-sulfur and carbon isotope records of the 2.9 Ga Mozaan Group of the Pongola Supergroup, Southern Africa

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015 e-mail: s.ono{at}gl.ciw.edu

Nicolas J. Beukes

Department of Geology, Auckland Park Campus, University of Johannesburg, 2006 Auckland Park, South Africa e-mail: njb{at}na.rau.ac.za

Douglas Rumble

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015 e-mail: rumble{at}gl.ciw.edu

Marilyn L. Fogel

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015 e-mail: m.fogel{at}gl.ciw.edu

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 ¹³C of organic carbon shows two populations: one with ¹³C of ~ –26 and another with ¹³C of –32 . The ³³S 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 ³³S values measured for post-2.0 Ga sulfide and sulfate minerals. Moreover, some samples from the Mozaan Group yield ³⁶S/³³S 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 ³³S 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.