Dartmouth Events

Understanding the timing and trajectory of atmospheric oxygenation remains fundamental to deciphering its causes and consequences. 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. However, although it is generally agreed that oxygen began to accumulate within the atmosphere between 2.5–2.3 Ga, different readings of the S-MIF record have been used to portray dramatically different oxygenation trajectories—with some workers arguing for a unidirectional geologically rapid rise in O₂, while others envisage an oscillatory trajectory whereby pO₂ repeatedly crossed the threshold necessary to resume S-MIF genesis. Here, we take a deeper look at the S-MIF record in an attempt to reconcile these conflicting narratives. Our statistical and modeling analyses, as well as new sulfur isotope data, implicate an intermediate, and potentially uniquely feedback-sensitive, Earth system state in the wake of the Great Oxidation Event, rather than a sweeping step-function(s) between bi-stable endmembers.

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