The temporal evolution of the carbon isotope composition of calcite in the presence of cyanobacteria

Quantifying the link between cyanobacterial activity and the carbon isotope signature of precipitated carbonate minerals is crucial for reconstructing the environmental conditions present at the time of carbonate mineral formation. In this study, calcite was precipitated in the presence and absence of Synechococcus sp. cyanobacteria in batch reactors. The temporal evolution of the carbon isotope composition of calcite (δ¹³C_Calcite) and dissolved inorganic carbon (δ¹³C_DIC) was monitored to evaluate the rate and degree to which the carbon isotope compositions in calcite are modified during and after its precipitation. The presence of cyanobacteria promoted calcium carbonate formation by increasing fluid pH and the CaCO₃ saturation state. It also changed significantly the carbon isotope composition of dissolved inorganic carbon due to the preferential incorporation of ¹²C into the biomass. This generated an isotope disequilibrium between the calcite and the aqueous fluid phase over time after the calcite precipitated. The carbon isotope composition of the calcite evolved continuously towards mineral-fluid isotope equilibrium after its precipitation, at geometric surface area normalized rates ranging from 1.75 × 10⁻¹⁴ to 1.71 × 10⁻¹³ mol ¹³C/m²/s. These rates are sufficiently fast such that the δ¹³C_DIC value of aqueous fluids in calcite-rich rocks would be buffered by the δ¹³C_Calcite value of the co-existing calcite. Mass balance calculations suggest that the carbon isotope composition of calcite could change noticeably when the calcite is in isotope disequilibrium with its co-existing fluid, for example through the presence of a local ¹²C sink such as photosynthetic microorganisms or ¹²C source such as decomposition of organic material.