Refining the extraction methodology of carbonate associated sulfate: Evidence from synthetic and natural carbonate samples

Abstract

Sulfur and oxygen isotope analyses of trace and whole mineral sulfate are valuable in investigating diagenetic processes and the microbial communities that produced them, seawater anoxia, and paleoclimate. Oxygen isotopes are particularly useful in that they may also record alterations to the original isotope ratio, be it from post-depositional processes or oxidation of sulfide minerals during the chemical extraction procedure. Here we rigorously test several common methodological procedures of extracting carbonate associated sulfate (CAS) for sulfur and oxygen isotope analyses in order to generate a method that will extract only the CAS, while preserving associated organic reduced sulfur and sulfides for analysis. The results of these experiments on synthetically generated carbonates demonstrate that our proposed protocol sufficiently removes all non-CAS sulfate and does not result in oxidation of included sulfides. Analytical reproducibility (standard deviation) of synthetic carbonates is 0.1‰ (1σ) for δ34S and 0.3‰ (1σ) for δ18O. Extractions of low pyrite, high organic matter geologic samples from the Monterey Formation across a range of facies types demonstrate a reproducibility (1σ) of 0.4‰–0.7‰ for δ34S and 0.8‰–1.3‰ for δ18O, resulting from sample heterogeneity. δ34S and δ18O from Monterey Formation samples do not demonstrate oxidation of organic matter, suggesting our proposed protocol will preserve organic sulfur. A high pyrite-concentration Jet Rock concretion demonstrates that additional sulfate can be produced during the non-CAS leaching processes from oxidation of pyrite. We show that pyrite from the Jet Rock concretion ceases to oxidize when the sample is leached under an anoxic environment.