Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative δ³³S anomaly on the early Earth

Results of multiple sulfur isotope analyses from five Archean paleosols show the widespread presence of mass-independently fractionated sulfur in the regolith developed on the pre-2.5Ga Earth. Analysis of sulfur from a small set of diamictite samples gave similar results. All values of δ³³S are negative, indicating that the Archean surface environments preferentially retained atmospheric S from the SO₄^2- pathway, which carried a negative δ³³S signal, whereas a portion of the S from the S₈ pathway, with a positive δ³³S, was transferred to the oceans. The soil SO₄^2- was then converted to sulfide by bacterial sulfate reduction with terrestrial organic matter in the weathering horizon acting as a reductant. Some S from the S₈ pathway also included, which resulted in a net soil δ³³S anomaly from atmospheric S in the range -0.3 to -0.6‰, similar to values for pyrites from floodplain sediments. Excess S₈-derived S was carried to the oceans, resulting in the negative/positive dichotomy between terrestrial and marine systems. Pyrite that formed in paleosols and pyrite grains that formed in flood-plain deposits, both carrying the terrestrial δ³³S signal, were then recycled into detrital pyrite grains now found in sandstones and conglomerates deposited before the rise of atmospheric oxygen. Therefore the Earth's early regolith constituted a reservoir of S with negative δ³³S values that could be important for balancing the predominantly positive δ³³S signature found in marine sediments.