Lithofacies control on multiple-sulfur isotope records and Neoarchean sulfur cycles

Triple-sulfur isotope ratios (³²S/³³S/³⁴S) were measured for 141 bulk rock samples from two Agouron scientific drill cores (GKP01 and GKF01) that recovered Neoarchean successions of the Transvaal Supergroup, South Africa. These two deep-time cores are correlated to each other with 14 tie lines using volcanic and impact spherule layers in a sequence stratigraphic framework, allowing us to evaluate both lithofacies and temporal controls over multiple-sulfur isotope systematics. The ca. 2.5 Ga (giga-annum before present) basinal Klein Naute Formation and the ca. 2.6 Ga peritidal Boomplaas and Vryburg Formations yield an array of data characterized by δ³³S ≈ 1.4 × δ³⁴S. These linear trends are found in both shallow water and deepwater facies but are characteristic to rocks with high-iron content suggesting these may reflect isotopic compositions of aqueous sulfide-elemental sulfur reservoirs in the Neoarchean oceans. Data that deviate from this linear array are interpreted as resulting from additional inputs of sulfide from microbial sulfate reduction with or without contribution from sulfur disproportionation. Sulfate-derived sulfur evolved to be either enriched or depleted in ³⁴S depending on the local depositional environment. For example, the Reivilo Formation in core GKF01 is characterized by abundant microbialite textures and shows an isotopic signature of closed system sulfate reduction. Rapid cementation of these carbonate fabrics may have attenuated the supply of sulfate to pore waters resulting in the progressive ³⁴S enrichments during bacterial sulfate reduction below the sediment-water interface. In contrast, signatures of open-system sulfate reduction are associated with slope facies, dominated by granular dolostones, preserved in the upper Nauga Formations in GKF01 and GKP01. The two different sulfur isotope patterns, interpreted to reflect closed versus open-system sulfate reduction are both found in the lower Reivilo Formation in both cores. This lateral variation of isotope signals documents that observed ³⁴S shifts can be controlled locally, and may not have temporal significance. This study demonstrates critical importance of recovering sulfur isotope data from stratigraphically correlated drill cores to evaluate both geographical and temporal shifts of sulfur cycles, and their links to the great oxidation event at the end of the Archean Eon.