Abstract
The sulfur isotope composition of pyrite in marine sedimentary rocks is often difficult to interpret due to a lack of precise isotopic constraints for coeval sulfate. This study examines pyrite and barite in the Late Devonian Canol Formation (Selwyn Basin, Canada), which provides an archive of δ34S and δ18O values during diagenesis. Scanning electron microscopy (SEM) has been combined with microscale secondary ion mass spectrometry (SIMS) analysis (n = 1,032) of pyrite (δ34S) and barite (δ34S and δ18O) on samples collected from nine stratigraphic sections of the Canol Formation. Two paragenetic stages of pyrite and barite formation have been distinguished, both replaced by barium carbonate and feldspar. The δ34Sbarite and δ18Obarite values from all sections overlap, between +37.1‰ and +67.9‰ (median = +45.7‰) and +8.8‰ and +23.9‰ (median = +20.0‰), respectively. Barite morphologies and isotopic values are consistent with precipitation from diagenetically modified porewater sulfate (sulfate resupply << sulfate depletion) during early diagenesis. The two pyrite generations (Py-1 and Py-2) preserve distinct textures and end-member isotopic records. There is a large offset from coeval Late Devonian seawater sulfate in the δ34Spyrite values of framboidal pyrite (-29.4‰ to -9.3‰), consistent with dissimilatory microbial sulfate reduction (MSR) during early diagenesis. The Py-2 is in textural equilibrium with barite generation 2 (Brt-2) and records a broad range of more positive δ34SPy-2 values (+9.4‰ to + 44.5‰). The distinctive highly positive δ34Spyrite values developed from sulfate limited conditions around the sulfate methane transition zone (SMTZ). We propose that a combination of factors, including low sulfate concentrations, MSR, and sulfate reduction coupled to anaerobic oxidation of methane (SR-AOM), led to the formation of highly positive δ34Spyrite and δ34Sbarite values in the Canol Formation. The presence of highly positive δ34Spyrite values in other Late Devonian sedimentary units indicate that diagenetic pyrite formation at the SMTZ may be a more general feature of other Lower Paleozoic basins.
Highlights
Methane is a powerful greenhouse gas that is produced during the final stage of organic matter fermentation (Knittel and Boetius, 2009)
Two distinct stages of pyrite formation formed via microbial sulfate reduction (MSR) under contrasting levels of sulfate availability
The initial stage of pyrite formation developed during early diagenesis under relatively opensystem conditions, which resulted in the precipitation of the framboidal pyrite (Py-1) and preservation of negative δ34S values
Summary
Methane is a powerful greenhouse gas that is produced during the final stage of organic matter fermentation (Knittel and Boetius, 2009). Changes in the flux of methane (from sediment to oceans) have been linked with major climatic impacts at particular stages of earth history (e.g., Dickens et al, 1995). Authigenic pyrite (FeS2) and barite (BaSO4) can both form as by-products of SR-AOM, meaning these phases provide a potential archive of methane oxidation (e.g., Borowski et al, 2013; Wood et al, 2021). Pyrite and barite provide an important archive for sulfur isotopes in marine environments, which can be used to reconstruct biogeochemical processes that link the sulfur, carbon, and iron cycles (Bottrell and Newton, 2006; Fike et al, 2015). Stratigraphic variability in δ34S values have been used to infer regional to global-scale changes in the sulfur cycle that reflect enhanced pyrite burial (e.g., Goodfellow and Jonasson, 1984) and the size of the marine sulfate reservoir (e.g., Kah et al, 2004)
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