Identification of microbial influences on carbonate precipitation using dual clumped isotopes

The pioneering work of Judith McKenzie and her colleague Kerry Kelts identified the presence of dolomites associated with the oxidation of organic material in sediments obtained from the Deep Sea Drilling Project.  This discovery led to the wide spread recognition that dolomites form in association with microbial sulfate reduction.   Later Judy’s work proposed that the microbes were more than agents of creating a suitable geochemical environments, but actually were instrumental in precipitating dolomite.  This work stimulated the research of many, but the questions have always arisen as to what degree microbial processes are responsible for dolomite formation and other carbonate minerals in the ancient record.  Some have proposed different geochemical indices such as carbon or magnesium isotopes or the concentration of certain elements, maybe diagnostic of microbial processes.  However, these tools frequently provide equivocal evidence and therefore are not definitive. In this presentation we provide several examples of the use of a new geochemical tool, the dual clumped isotope proxy (Δ₄₇ and Δ₄₈).  Deviations from equilibrium, particular of Δ₄₈ values, provide strong evidence of the influence of different mechanisms of the precipitation of dolomite and calcite.  In this presentation we present several examples in which the dual clumped isotope proxy has been employed.  These include not only dolomites, but also meteoric LMC calcite cements, previously believed to have form without the influence of microbial processes.  While the dual clumped proxy may also yield equivocal results, it will be a welcome addition to the tools used to understand the roles of microbes during carbonate precipitation.