The continuous re-equilibration of carbon isotope compositions of hydrous Mg carbonates in the presence of cyanobacteria

The hydrous magnesiumcarbonate minerals dypingite (Mg-5(CO3)(4)(OH)(2)center dot 5H(2)O) and nesquehonite (MgCO3 center dot 3H(2)O) were precipitated in the presence and absence of Gloeocapsa sp. and Synechococcus sp. cyanobacteria in batch reactors. The Delta C-13(mineral-DIC) is similar in both biotic and abiotic systems. Stable carbon isotope analyses of the precipitated minerals and co-existing fluids indicated that the delta C-13 values of the precipitated carbonates co-vary over time with the delta C-13(DIC) of the fluid phase, even after the precipitation of the carbonate is complete. This observation indicates the continuous isotopic exchange between the carbonates and the fluid, a process that efficiently resets the original delta C-13 values of the solids. Therefore although cyanobacterial-induced Mg carbonates are 10 +/- 5% enriched in C-13 compared to inorganic carbonates, the delta C-13 values of natural hydrousMg carbonates may reflect post precipitation processes and may not be reliable for paleo-environmental reconstructions. (C) 2015 Elsevier B.V. All rights reserved.