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

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 (S-33/S-32, S-34/S-32, and S-36/S-32) isotope ratios of sulfide minerals and carbon (C-13/C-12) isotope ratios of organic carbon for shale in the similar to 2.96 to similar to 2.84 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. The delta C-13 of organic carbon shows two populations: one with delta C-13 of similar to -26 parts per thousand and another with delta C-13 of -32 parts per thousand. The Delta S-35 values from nine samples ranges from -0.49 to +0.36 parts per thousand, which is considerably smaller than what was measured for the sulfide and sulfate minerals from other Archean intervals but outside the range of Delta S-33 values measured for post-2.0 Ga sulfide and sulfate minerals. Moreover, some samples from the Mozaan Group yield Delta S-36/Delta S-33 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 Delta S-33 values for the Mozaan Group may suggest that the atmosphere became slightly oxidized at similar to 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 similar to 2.9 Ga. Such an ephemeral oxidation event could have triggered the Mozaan-Witwatersrand glaciation by destabilizing an existing methane-rich Archean atmosphere.