Microbial sulphur-cycling and atmospheric signatures in the 2.52 Ga Gamohaan Formation, South Africa

Abstract

The Late Archean atmosphere is thought to have been largely reducing as suggested by the preservation of large Mass-Independent fractionations of S isotopes (MIF-S) in sedimentary sulphides and sulphates. However, there is a growing body of evidence for transient oxygen accumulation in the Neoarchean oceans, especially from shallow marine environments. Here ∼2.52 Ga carbonates in the upper Campbellrand Group of South Africa are investigated from the shallow marine carbonate shelf Kogelbeen Formation to the deeper water basinal Gamohaan Formation. Previous studies of these rocks have found large positive Δ33S signatures, but with differing interpretations of the controls on, and pathways for, the preservation of Δ33S. Here bulk S isotope measurements from sedimentary sulphides and carbonate associated sulphate (CAS) are reported, combined with in-situ S isotope measurements by Secondary Ion Mass Spectrometry (SIMS) from well-preserved pyritised microbial mat microtextures of the Gamohaan Formation.Large positive Δ33S signatures are observed in sulphide grains of the Gamohaan and Kogelbeen formations (max Δ33SSIMS=12.6‰) confirming the isotopic effects of photolytic reactions in an anoxic atmosphere. Small, early pyrites in fenestrate microbialites record an atmospheric signature (average δ34SSIMS=7.6‰, Δ33SSIMS=8.5‰, Δ36SSIMS=−7.6‰) reflecting the preservation of atmospherically derived Sn particles in the microbial carbonates. Contorted microbial mat layers, also from the carbonate slope, exhibit large mass-dependent fractionations with δ34SSIMS of up to 30.6‰ plotting off the Archean reference array, suggesting oxygenated seawater on the shelf-slope. Pyrite concretions in organic rich carbonates from deeper water facies show S isotope values (average δ34SSIMS=3.1‰, Δ33SSIMS=−0.2‰, Δ36SSIMS=2.5‰) indicative of microbial sulphate reduction (MSR).The CAS data show positive Δ33S with values of up to 8.49‰ and positive δ34S values that are more similar to sulphide data measured from each sample than estimated late-Archean seawater sulphate. This is interpreted to record the early oxidation of sulphides with positive Δ33S by shallow oxygenated waters in the upper diagenetic zone that moved through the sediment and was then incorporated into the platform carbonates. In this way, we hypothesise that positive Δ33S can be inherited by the Neoarchean diagenetic sulphate reservoir and preserved in carbonates.