Bulk and grain-scale minor sulfur isotope data reveal complexities in the dynamics of Earth's oxygenation

The disappearance of mass-independent sulfur isotope fractionation (S-MIF) within the c. 2.3-billion-year-old (Ga) Rooihoogte Formation has been heralded as a chemostratigraphic marker of permanent atmospheric oxygenation. Reports of younger S-MIF, however, question this narrative, leaving significant uncertainties surrounding the timing, tempo, and trajectory of Earth's oxygenation. Leveraging a new bulk quadruple S-isotope record, we return to the South African Transvaal Basin in search of support for supposed oscillations in atmospheric oxygen beyond 2.3 Ga. Here, as expected, within the Rooihoogte Formation, our data capture a collapse in Delta(3x) S values and a shift from Archean-like Delta S-36/Delta S-33 slopes to their mass-dependent counterparts. Importantly, the interrogation of a Delta S-33-exotic grain reveals extreme spatial variability, whereby atypically large Delta S-33 values are separated from more typical Paleoproterozoic values by a subtle grain-housed siderophile-enriched band. This isotopic juxtaposition signals the coexistence of two sulfur pools that were able to escape diagenetic homogenization. These large Delta S-33 values require an active photochemical sulfur source, finger-printing atmospheric S-MIF production after its documented cessation elsewhere at similar to 2.4 Ga. By contrast, the Delta S-33 monotony observed in overlying Timeball Hill Formation, with muted Delta S-33 values (<0.3) and predominantly mass-dependent Delta S-36/Delta S-33 systematics, remains in stark contrast to recent reports of pronounced S-MIF within proximal formational equivalents. If reflective of atmospheric processes, these observed kilometer-scale discrepancies disclose heterogenous S-MIF delivery to the Transvaal Basin and/or poorly resolved fleeting returns to S-MIF production. Rigorous bulk and grain-scale analytical campaigns remain paramount to refine our understanding of Earth's oxygenation and substantiate claims of post-2.3 Ga oscillations in atmospheric oxygen.

Automated summary

This study uses a new bulk quadruple S-isotope record to investigate support for atmospheric oxygen oscillations in the Rooihoogte Formation in the South African Transvaal Basin. The study finds coexistence of different sulfur pools and extreme spatial variability.