Abstract
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 CO2concentration and increasing
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 (∆47 and ∆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 ∆47 and higher ∆48 values
(-∆47/+∆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 (+∆47/-∆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.