Atmospheric sulfur rearrangement 2.7 billion years ago: Evidence for oxygenic photosynthesis

Mass-independently fractionated sulfur isotopes (MIF-S) provide strong evidence for an anoxic atmosphere during the Archean. Moreover, the temporal evolution of MIF-S shows increasing magnitudes between 2.7 and 2.5 Ga until the start of the Great Oxidation Event (G.O.E.) at around 2.4 Ga. The conclusion of a completely anoxic atmosphere up to the G.O.E. is in contrast to recent studies on redox-sensitive elements, which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the G.O.E. In order to investigate this apparent inconsistency, we present multiple sulfur isotopes for 2.71 Ga pyritic black shales derived from the Kidd Creek area, Ontario, Canada. These samples display high positive Delta S-33 values up to 3.8% and the typical late Archean slope in Delta S-36/Delta S-33 of 0.9. In contrast, the time period before (32-2.73 Ga) is characterized by greatly attenuated MIF-S magnitudes and a slope in Delta S-36/Delta S-33 of 1.5. We attribute the increase in Delta S-33 magnitude as well as the contemporaneous change in the slope of Delta S-36/Delta S-33 to changes in the relative reaction rate of different MIF-S source reactions and changes in atmospheric sulfur exit channels. Both of these are dependent on atmospheric CH4:CO2 and O-2 mixing ratios. We propose a distinct change in atmospheric composition at 2.7 Ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed Neoarchean MIF-S record. Our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2.7 billion years ago. (C) 2013 Elsevier B.V. All rights reserved.