Neoarchean carbonate-associated sulfate records positive Δ³³S anomalies.

Mass-independent fractionation of sulfur isotopes (reported as Δ(33)S) recorded in Archean sedimentary rocks helps to constrain the composition of Earth's early atmosphere and the timing of the rise of oxygen ~2.4 billion years ago. Although current hypotheses predict uniformly negative Δ(33)S for Archean seawater sulfate, this remains untested through the vast majority of Archean time. We applied x-ray absorption spectroscopy to investigate the low sulfate content of particularly well-preserved Neoarchean carbonates and mass spectrometry to measure their Δ(33)S signatures. We report unexpected, large, widespread positive Δ(33)S values from stratigraphic sections capturing over 70 million years and diverse depositional environments. Combined with the pyrite record, these results show that sulfate does not carry the expected negative Δ(33)S from sulfur mass-independent fractionation in the Neoarchean atmosphere.