![]() Here, in the absence of photon-shielding O 2/O 3, photolysis and/or photo(de)excitation reactions are understood to yield reduced (i.e., S 0 or S 8) and oxidised (i.e., H 2SO 4) sulfur phases that carry positive and negative ∆ 33S values, respectively 12, 13, 16, 17, 18. 1 and 2) 12, 14, 15, the generation and geological preservation of S-MIF is linked to low atmospheric oxygen concentrations in several ways: Starting with its production, anoxic SO 2 photochemistry remains the only experimentally demonstrated means of creating S-MIF that remotely resembles the geological record 13 (Fig. Manifest as non-zero ∆ 3X-values (“Methods” Eqs. Kazput Formation, BQ Beasley River Quartzite, CB Cheela Springs Basalt. Reitfontain Member, WR Woongarra Rhyolite, BF Boolgeeda Iron Formation, MB1, 2 the diamictites within the Meteorite Bore Member, KF Koolbye Formation, Kaz. Contrastingly, the persistence of low-magnitude non-zero ∆ 33S values throughout cores T2–T3 has been ascribed to the crustal memory effect, with atmospheric oxygenation occurring much earlier (blue horizontal bar, e, Supplementary Information). ![]() The subsequent blue horizontal bars in panel c mark isolated instances of S-MIF within the Timeball Hill Formation (TBH) that have been interpreted to represent returns to an anoxic atmospheric state ( Supplementary Information). The lowermost horizontal blue band in c, d corresponds to the apparently sustained presence of mass-independent sulfur isotope fractionation (S-MIF) within the Rooihoogte Formation (Ro(oi)) seen throughout the Carletonville area 22, 25, 30, 37. Each of the chronologically constrained lithostratigraphic columns follow those presented with the ∆ 33S data 22, 25, 30, 32, 33, with the superimposed vertical grey bars illustrating the ☐.3‰ threshold for identifying S-MIF by ∆ 33S alone 25. Stratigraphic distributions of ∆ 33S data from the South African Kaapvaal ( c core EBA-2 d core KEA-4) and Western Australian Pilbara ( e cores T1–T3) cratons, whose discrepant interpretations have led to the emergence of conflicting models of atmospheric oxygenation. Triangular and circular points represent spot and bulk ∆ 33S analyses respectively. The grey box locates the interval over which atmospheric oxygen is thought to have first accumulated, while the horizontal bars illustrate more traditional redox indicators, whose red and blue colouration discriminates between those disclosing oxic and anoxic conditions, respectively 4, 5. b The secular evolution of ∆ 33S values compiled herein (Supplementary Data File 1). Importantly, these apparent atmospheric relapses were fundamentally different from older putative oxygenation episodes, implicating an intermediate, and potentially uniquely feedback-sensitive, Earth system state in the wake of the Great Oxidation Event.Ī A schematic representation of competing ideas surrounding the oxygenation of Earth’s atmosphere 1 contextualised within a framework of biological innovations. Accepting others’ primary photochemical interpretation, our approach demands that these implied atmospheric dynamics were ephemeral, operating on sub-hundred-thousand-year timescales. Next, utilising statistical approaches, supported by new MSI data, we show that the reconciliation of adjacent, yet seemingly discrepant, South African MSI records requires that the rare instances of post-2.3-billion-year-old S-MIF are stratigraphically restricted. Herein, however, after accounting for unrecognised temporal and spatial biases within the Archaean–early-Palaeoproterozoic MSI record, we demonstrate that the global expression of the CME is barely resolvable thereby validating S-MIF as a tracer of contemporaneous atmospheric chemistry during Earth’s incipient oxygenation. ![]() Nevertheless, S-MIF recycling through oxidative sulfide weathering-commonly termed the crustal memory effect (CME)-potentially decouples the multiple sulfur isotope (MSI) record from coeval atmospheric chemistry. Given its origin in oxygen-free photochemistry, mass-independent sulfur isotope fractionation (S-MIF) is widely accepted as a geochemical fingerprint of an anoxic atmosphere. ![]() Understanding the timing and trajectory of atmospheric oxygenation remains fundamental to deciphering its causes and consequences.
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