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

Abstract

Significance The permanent disappearance of mass-independent sulfur isotope fractionation (S-MIF) from the sedimentary record has become a widely accepted proxy for atmospheric oxygenation. This framework, however, neglects inheritance from oxidative weathering of pre-existing S-MIF–bearing sedimentary sulfide minerals (i.e., crustal memory), which has recently been invoked to explain apparent discrepancies within the sulfur isotope record. Herein, we demonstrate that such a crustal memory effect does not confound the Carletonville S-isotope record; rather, the pronounced Δ 33 S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed 33 S-enrichments within the succeeding Timeball Hill Formation, however, contrasts with our record, revealing kilometer-scale heterogeneities that highlight significant uncertainties in our understanding of the dynamics of Earth’s oxygenation.

Rapid timescale for an oxic transition during the Great Oxidation Event and the instability of low atmospheric O 2

“…Occasionally, big perturbations to the atmosphere, such as an LIP, might have lowered O 2 concentrations enough for sulfur isotope MIF to reoccur. Note that the above explanation for the cause of O 2 oscillations prior to 2.2 Ga is complicated by S-MIF data presented in Izon et al ( 11 ), which do not suggest the same O 2 variability found by Poulton et al ( 9 ).…”

“…Here, we use our time-dependent photochemical model to address the controversy of the reversibility of the oxic transition ( 9 , 11 ). Fig.…”

“…Overall, geochemical evidence tentatively suggests that O 2 concentrations and climate were unstable for 200 million years until 2.2 Ga, which marks the most recent estimated timing of the permanent oxygenation of the atmosphere ( 9 ). However, interpretations of oscillating O 2 have been questioned ( 11 ). While the geologic evidence for the O 2 oscillations remains equivocal, the data have raised significant questions regarding the feasibility and timescales for Earth’s great oxidation.…”

Reconciling discrepant minor sulfur isotope records of the Great Oxidation Event

“…Stratigraphic distributions of Δ 33 S 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. Each of the chronologically constrained lithostratigraphic columns follow those presented with the Δ 33 S data 22,25,30,32,33 , with the superimposed vertical grey bars illustrating the ±0.3‰ threshold for identifying S-MIF by Δ 33 S alone 25 . The lowermost horizontal blue band in c, d corresponds to the apparently sustained presence of massindependent sulfur isotope fractionation (S-MIF) within the Rooihoogte Formation (Ro(oi)) seen throughout the Carletonville area 22,25,30,37 , while others envisage an oscillatory trajectory whereby pO 2 repeatedly crossed the threshold necessary to resume S-MIF genesis [24][25][26] (Fig.…”

“…1a-d). While inadequate age constraints and ambiguous stratigraphic correlations [22][23][24][26][27][28][29][30] currently prevent discrimination between these models (Supplementary Information), both interpretations are reliant on the assumption that sedimentary S-MIF chronicles contemporaneous atmospheric conditions. Again, this premise is not universally accepted and some contend that the operation of a so-called crustal memory effect (CME) decouples the MSI record from coeval atmospheric chemistry 16,[31][32][33] .…”