Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative Δ33S anomaly on the early Earth

- We measured multiple S isotopes from Archean paleosols and diamictites.
- Results show that the continental surface received atmospheric S as sulfate with a negative Δ33S.
- This S was trapped as pyrite whereas elemental S with positive Δ33S was lost to the oceans.
- This partitioning created a terrestrial reservoir of negative Δ33S.
- Excess elemental S in the oceans created a positive marine reservoir.

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-2.5 Ga 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 SO42 − 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 SO42 − 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.