Abstract
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 δ34S 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 (δ34S and Δ33S) 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 δ34S and Δ33S = −0.21 ± 0.65‰ (±1σ). These large variations in δ34S 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 δ34S = 8.36 ± 1.16‰ and Δ33S = 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.
Highlights
The sulphur isotope record has played an integral role in shaping our understanding of key events in Earth’s geological and biological history
Archaean sulphur mass-independent fractionation (MIF) has been widely attributed to atmospheric photochemical reactions involving SO2; these reactions would have been blocked in the Palaeoproterozoic due to the UV-shielding effects caused by increased atmospheric O2 and O3 concentrations (e.g., Farquhar, Bao, & Thiemens, 2000)
We suggest two scenarios whereby the circulation of S-rich fluids from the above sources could have contributed to the formation of type 2 pyrite and its associated sulphur mass- independent fractionation (S-MIF) signal: one via thermochemical sulphate reduction of S-bearing fluids from a more recent sulphur source, and one via migration of S-rich fluids from stratigraphically older sediments carrying an inherited Archaean S-MIF signal
Summary
The sulphur isotope record has played an integral role in shaping our understanding of key events in Earth’s geological and biological history. S-isotope ratios that largely follow a linear trend in δ34S versus Δ33S along an Archaean reference array Combining these data with photochemical models, Ono et al (2003) suggested that the Δ33S sign was positive for elemental sulphur and negative for sulphate. Additional studies in the Griqualand West Basin are required to test whether these textural and geochemical interpretations can be applied to pyrite from other palaeoenvironments and depositional ages To address these gaps in our current understanding of Archaean sulphur cycling, we measured in situ, μm-scale, multiple sulphur isotope ratios (δ34S and Δ33S) in exceptionally well-preserved pyritised microbialites in shales (sample-3184) from the ~2.65-G a Lokammona Formation, Ghaap Group, South Africa. By petrographically correlating pyrite phases with sulphur isotope fingerprints, we can determine the MDF and MIF processes that contributed to the formation of multiple pyrite generations in these sediments
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