ISOTOPIC FINGERPRINTS OF MICROBIAL RESPIRATION IN ARAGONITE FROM MODERN BAHAMIAN STROMATOLITES

In honor of Prof. Ginsburg and the numerous insightful conversations we have had with him, we present new results on processes causing lithification in modern Bahamian stromatolites. As Ginsburg has argued for longer than we have been scientists, understanding how modern stromatolites or “strombolites” form and lithify is critical to properly interpreting the origins of ancient stromatolites. Microbial roles in stromatolite lithification can be traced in carbonates because microbial communities produce carbon isotopic shifts when they cycle local DIC (dissolved inorganic carbon). Thus, the isotopic composition of carbonates can record ancient microbial CO₂ cycling, providing insights into the processes of stromatolite growth and microbial influences on carbonate chemistry.

In shallow subtidal modern stromatolites from Highborne Cay, Bahamas, authigenic aragonite preserves a carbon isotopic record of heterotrophic microbial influences on DIC; authigenic aragonite is >1 per mil depleted in ¹³C relative to aragonite that precipitated in equilibrium with local seawater. Even though cyanobacteria raise pH during peak photosynthesis, more aragonite precipitates when and where respiration influences local DIC. These results are consistent with 1) sulfate reduction promoting carbonate precipitation and 2) calcium release during decay of exopolymeric substances as previously reported. Thus, heterotrophs play a more important role than phototrophs in Bahamian stromatolite lithification on a local scale. However, organic matter produced by cyanobacteria supports the heterotrophic community. Thus, the absence of an autotrophic isotopic signature in the rock record cannot imply the absence of photosynthetic activity.

Heterotrophic signatures may also be difficult to observe in ancient stromatolites. With continued lithification, the 1-2‰ δ¹³C shifts will be diluted by carbonate precipitation during early diagenesis and burial. Thus, microbial isotopic signatures in shallow subtidal stromatolites are only likely to be identified in exceptional circumstances. Identifying ancient microbial signatures will require careful sampling to separate carbonate components with different isotopic signatures and carbonates that precipitated on the spatial scales influenced by microbial communities.