Sulphur cycling in a Neoarchaean microbial mat

Multiple sulphur (S) isotope ratios are powerful proxies to understand the complexity of S biogeochemical cycling through Deep Time. The disappearance of a sulphur mass‐independent fractionation (S‐MIF) signal in rocks <~2.4 Ga has been used to date a dramatic rise in atmospheric oxygen levels. However, intricacies of the S‐cycle before the Great Oxidation Event remain poorly understood. For example, the isotope composition of coeval atmospherically derived sulphur species is still debated. Furthermore, variation in Archaean pyrite δ(34)S values has been widely attributed to microbial sulphate reduction (MSR). While petrographic evidence for Archaean early‐diagenetic pyrite formation is common, textural evidence for the presence and distribution of MSR remains enigmatic. We combined detailed petrographic and in situ, high‐resolution multiple S‐isotope studies (δ(34)S and Δ(33)S) using secondary ion mass spectrometry (SIMS) to document the S‐isotope signatures of exceptionally well‐preserved, pyritised microbialites in shales from the ~2.65‐Ga Lokammona Formation, Ghaap Group, South Africa. The presence of MSR in this Neoarchaean microbial mat is supported by typical biogenic textures including wavy crinkled laminae, and early‐diagenetic pyrite containing <26‰ μm‐scale variations in δ(34)S and Δ(33)S = −0.21 ± 0.65‰ (±1σ). These large variations in δ(34)S values suggest Rayleigh distillation of a limited sulphate pool during high rates of MSR. Furthermore, we identified a second, morphologically distinct pyrite phase that precipitated after lithification, with δ(34)S = 8.36 ± 1.16‰ and Δ(33)S = 5.54 ± 1.53‰ (±1σ). We propose that the S‐MIF signature of this secondary pyrite does not reflect contemporaneous atmospheric processes at the time of deposition; instead, it formed by the influx of later‐stage sulphur‐bearing fluids containing an inherited atmospheric S‐MIF signal and/or from magnetic isotope effects during thermochemical sulphate reduction. These insights highlight the complementary nature of petrography and SIMS studies to resolve multigenerational pyrite formation pathways in the geological record.