Peter K. Swart

Although several studies have shown the possibility of kinetic isotopic effects during the precipitation of carbonates using the dual clumped isotope proxy (Δ47 and Δ48), this approach has not yet been applied to the study of dolomite. We present Δ47 and Δ48 values of two types of Cenozoic Bahamian dolomites formed by distinctly different mechanisms. One dolomite, present in a core drilled on the island of San Salvador, is believed to have formed from normal seawater in the open system, while the other, taken from the Clino core drilled on the margin of the Great Bahama Bank, was precipitated associated with microbial sulfate reduction in the closed system. The Δ48-derived temperatures of San Salvador dolomites are consistent with their Δ47-temperatures, which are in turn close to the expected value of normal seawater. In contrast, dolomites from Clino show slightly elevated Δ47-derived temperatures (~15 °C higher than the expected) and significantly increased Δ48-temperatures (~145 °C) that are positively correlated with the sulfur isotopes of carbonate associated sulfate, in excess of +30‰. Such an association suggests a non-equilibrium process driven by microbial sulfate reduction, which elevates the alkalinity and depresses the pH of dolomitizing fluids. Notably, the greater amplitude of disequilibrium in Δ48 relative to Δ47 values demonstrates the sensitivity of the Δ48 proxy to kinetic isotope effects in dolomite. Conclusively, this study shows the potential of the dual clumped isotope proxy to identify microbial-mediated dolomitization in modern and ancient records.