Carbon isotope fractionation during calcium carbonate precipitation induced by urease‐catalysed hydrolysis of urea

Stable carbon isotopic fractionation during calcium carbonate precipitation induced by urease‐catalysed hydrolysis of urea was experimentally investigated in artificial water at a constant temperature of 30 °C. Carbon isotope fractionation during urea hydrolysis follows a Rayleigh distillation trend characterized by a 13C‐enrichment factor of − 20 to − 22‰. CaCO3 precipitate is up to 17.9‰ 13C‐depleted relative to the urea substrate (− 48.9 ± 0.07‰). Initial CaCO3 precipitate forms close to isotopic equilibrium with dissolved inorganic carbon. Subsequent precipitation occurs at − 2 to − 3‰ offset from isotopic equilibrium, suggesting that the initial δ13C value of CaCO3 is reset through dissolution followed by reprecipitation with urease molecules playing a role in offsetting the δ13C value of CaCO3 from isotopic equilibrium. Potentially, this isotopic systematics may provide a tool for the diagnosis of ureolytically‐formed carbonate cements used as sealing agent. Moreover, it may serve as a basis to develop a carbon isotope tool for the quantification of ureolytically‐induced CO2 sequestration. Finally, it suggests carbon isotope disequilibrium as a hallmark of past enzymatic activity in ancient microbial carbonate formation.

► A Rayleigh‐mode fractionation of 13C occurs during urease‐catalysed ureolysis.
► δ13C values of CaCO3 formed during ureolysis show isotopic disequilibrium with DIC.
► CaCO3 formed during ureolysis is 13C‐depleted relative to the urea substrate.