ISOTOPE FINGERPRINTS OF CARBONATE FORMATION MEDIATED BY SULFATE REDUCTION

Oxidation of organic matter coupled to microbial or thermogenic sulfate reduction can be a powerful driver for the formation of authigenic carbonates. Assessing if an organic source contributed to the carbonate formation is typically straightforward: commonly, isotopically light carbon points to the addition of carbon from organic matter or hydrocarbons. Pinning down the role of sulfate reduction, on the other hand, turns out to be a challenge. Biomarkers for sulfate reduction can demonstrate the potential presence of sulfate reducing organisms, whereas authigenic minerals typical for sulfidic conditions (i.e. pyrite) can establish that sulfide was present at some time during the formation of the rock. However, these markers for sulfate reduction are not always present in the rock record, and it can be difficult to unambiguously show that there is a link between authigenic carbonate formation and actual sulfate reduction.

The sulfur and oxygen isotope composition of sulfate that is associated with authigenic carbonates (referred to as carbonate associated sulfate CAS) may offer a complimentary and robust approach to assess the role of sulfate reduction in the genesis of authigenic carbonates. In case of a dominant role of this process, sulfate that is trapped in authigenic carbonates will bear the sulfur and oxygen isotope fingerprint of sulfate reduction. Interestingly, the isotope composition of CAS also acts as a recorder of the openness or closed-ness of the system with respect to sulfate, providing valuable insights into the conditions under which the carbonates formed. To assess the usefulness of this tool we measured the isotope composition of CAS from authigenic calcitic and dolomitic carbonates that were presumably formed under conditions where sulfate reduction prevailed, such as carbonate caprocks on salt domes and sulfur-rich lacustrine environments.