Tracing past microbial communities with isotopes of traces of ancient sea water
Fossil carbonate shells contain trace amounts of sulfate from the seawater in which the organisms lived. Geochemists can extract the “carbonate associated sulfate” to investigate changes in global geochemical cycles in the past. However, the extraction process can oxidize the very small amounts of sulfide minerals in the carbonate adding extra, artifact sulfate. Our aim is not to look at changes in seawater sulfate but to examine carbonates precipitated during burial diagenesis to investigate microbial processes that oxidize organic matter and in particular sulfate-reduction which imparts a very distinctive sulfur isotope value. The problem we have overcome relates to the fact that buried sediments also accumulate iron sulfides in relatively large amounts, which too can be oxidized. We have tested a new method with synthetic mixtures of chemicals and natural samples and have already produced some very exciting new results on paleo-microbial ecology.
Theiling BP and Coleman M (2015) Refining the extraction methodology of carbonate associated sulfate: Evidence from synthetic and natural carbonate samples. Chem. Geol. 411 36–48. A PDF copy of the paper can be found here.