Photochemistry of sulfur dioxide yields anomalous sulfur isotope fractionation under anoxic atmosphere. The isotope anomaly can be transported into sedimentary sulfide and sulfate, thus is useful to trace atmospheric chemistry before the Great Oxidation Event (GOE) at about 2.3 billion years ago. Based on the isotopic analysis of Archean sedimentary rocks, the δ36S/δ33S trend seems to have changed through time, though the factors controlling the δ36S/δ33S signal are still uncertain. Our new laboratory experiments have shown that the δ36S/δ33S trend are mainly controlled by pSO2 and redox state of the atmosphere even without molecular oxygen. Thus, the observed change of the sulfur isotopic signal may reflect volcanic SO2 flux as well as redox fluctuation before GOE. Furthermore, our results indicate that the Archean atmosphere is not the pure CO2, but should be more reducing to explaining the observed δ36S/δ33S trend in the Archean rocks.
