Rapid oxygenation of Earth's atmosphere 2.33 billion years ago

Abstract

Molecular oxygen (O2) is, and has been, a primary driver of biological evolution and shapes the contemporary landscape of Earth's biogeochemical cycles. Although "whiffs" of oxygen have been documented in the Archean atmosphere, substantial O2 did not accumulate irreversibly until the Early Paleoproterozoic, during what has been termed the Great Oxygenation Event (GOE). The timing of the GOE and the rate at which this oxygenation took place have been poorly constrained until now. We report the transition (that is, from being mass-independent to becoming mass-dependent) in multiple sulfur isotope signals of diagenetic pyrite in a continuous sedimentary sequence in three coeval drill cores in the Transvaal Supergroup, South Africa. These data precisely constrain the GOE to 2.33 billion years ago. The new data suggest that the oxygenation occurred rapidly-within 1 to 10 million years-and was followed by a slower rise in the ocean sulfate inventory. Our data indicate that a climate perturbation predated the GOE, whereas the relationships among GOE, "Snowball Earth" glaciation, and biogeochemical cycling will require further stratigraphic correlation supported with precise chronologies and paleolatitude reconstructions.

Keywords: GOE; Great Oxidation Event; Oxygen; Paleoproterozoic; atmosphere; sulfur isotopes.

Fig. 1. Geologic sequence in South Africa between 2.5 and 2.2 Ga.

Stratigraphic correlation of the Rooihoogte and Duitschland Formation between the Carltonville and Duitschland area in the Transvaal Basin, South Africa. Note that the diagram is not drawn according to the real stratal thicknesses. The coarse green wavy line represents the sequence boundary in the middle Rooihoogte/Duitschland Formation that was previously proposed as the time interval in which the GOE occurred (34). Previously, the transition from S-MIF to S-MDF was obscured by an apparent hiatus in the Duitschland Formation. The pink interval represents the GOE identified in this study. The dashed vertical lines in the S-MIF box represent small Δ³³S values (<2‰), whereas the solid lines represent the presence of large Δ³³S values (>2‰). *From Hannah et al. (40). †From Rasmussen et al. (41).

Fig. 2. The δ³⁴S and Δ³³S profiles of pyrite in the three cores.

All isotope data are reported relative to VCDT (Vienna Cañon Diablo Troilite). The dashed lines represent three-point moving averages, whereas the shaded regions represent 1 SEM on the three points. The transitional interval highlighted in pink is defined on the basis of variation of Δ³³S records. The stars adjacent to the KEA-4 lithologic column correspond to samples for which thin-section photomicrographs are shown in fig. S2. The Re-Os date of 2316 ± 7 Ma on syngenetic pyrite from the boundary between the Timeball Hill and Rooihoogte formations in the EBA-2 core is from Hannah et al. (40).

Fig. 3. Triple sulfur isotope cross-plots of the three intervals of the study sequence.

(A) δ³⁴S versus δ³³S. (B) Δ³⁶S versus Δ³³S. The green dashed lines in (A) represent the MDF array. The blue and purple dashed lines in (B) represent the mass-independent and the theoretical mass-dependent fractionation slope, respectively (1, 44). The red dashed lines in both panels represent the regression line of samples in the transitional interval.

Fig. 4. Model results showing the increase in pO₂ and seawater sulfate level across the GOE.

The red line represents the atmospheric pO₂. The blue solid and dashed lines represent seawater sulfate levels modeled with the pyrite burial flux coupled or decoupled with pO₂, respectively.