Microbial Autotrophy Recorded by Carbonate Dual Clumped Isotope Disequilibrium

The proliferation of microbial carbon fixation is a key control on the evolution of the biosphere and global carbon cycle. Most records of these metabolisms in ancient rocks come from organic matter or fossils, which are not always preserved. Here, we present a potential proxy for microbial carbon fixation (autotrophy) based on the isotopic composition of carbonate minerals. Autotrophs influence carbonate chemistry in the cellular microenvironment by decreasing CO2 concentration and increasing the carbonate saturation state. This can induce rapid precipitation of carbonate minerals that are out of isotopic equilibrium with their environment. Recent work has identified disequilibrated dual clumped isotope compositions (triangle 47 and triangle 48) in the skeletal fossils of marine calcifying organisms. Here we test whether the same is true of non-skeletal carbonate fabrics associated with microbial autotrophs in modern and Eocene lakes. We found that microbial carbonate formed via autotrophic metabolism recorded lower triangle 47 and higher triangle 48 values (-triangle 47/+triangle 48) than predicted for thermodynamic equilibrium mineral formation. Our findings are supported by models of dual clumped isotope kinetics in the DIC system, and disequilibrium in the oxygen isotope system. We hypothesize that the inverse trajectory away from the equilibrium line (+triangle 47/-triangle 48) should be recorded by carbonates formed in association with alkalinizing heterotrophs, such as sulfate reducers. If so, carbonate dual clumped isotopes could be a powerful tool to identify the proliferation and rate of heterotrophic and autotrophic metabolisms in the carbonate rock record on Earth and (perhaps) other planets. Primary producers have fueled the biosphere since the origin of life on Earth. The production of organic molecules by autotrophs is essential for the evolution of heterotrophic bacteria, and eventually fungi and animals. However, identifying where and when specific metabolisms were active in Earth history is challenging because we lack reliable biosignatures of these processes. Identifying the presence of ancient metabolic activity can provide context to how microorganisms steered global biogeochemical cycles and the carbon budget in ancient oceans, atmosphere, and on land. In this study, we present a potential fingerprint of microbial autotrophy (e.g., photosynthesis, methanogenesis) preserved in carbonate minerals in modern and ancient settings. Microbial autotrophy imparts a dual clumped isotope and oxygen isotope disequilibrium signature on carbonate minerals This disequilibrium biosignature can be preserved in the rock record post-burial Dual clumped isotope signatures of microbially associated carbonates are differentiable from abiotic carbonates that formed from the same water and experienced the same burial history