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

• Published in: Chemical geology 2015-05, Vol.404, p.41-51
• Main Authors: Mavromatis, Vasileios, Bundeleva, Irina A., Shirokova, Liudmila S., Millo, Christian, Pokrovsky, Oleg S., Bénézeth, Pascale, Ader, Magali, Oelkers, Eric H.
• Format: Article
• Language: English
• Subjects: Carbon isotopes; Carbonates; Cyanobacteria; Fluid dynamics; Fluid flow; Fluids; Gloeocapsa; Hydrous magnesium carbonates; Isotope re-equilibration; Minerals; Precipitation; Sciences of the Universe; Synechococcus
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2015.03.016

The hydrous magnesium carbonate minerals dypingite (Mg5(CO3)4(OH)2·5H2O) and nesquehonite (MgCO3·3H2O) were precipitated in the presence and absence of Gloeocapsa sp. and Synechococcus sp. cyanobacteria in batch reactors. The Δ13Cmineral–DIC is similar in both biotic and abiotic systems. Stable carbon isotope analyses of the precipitated minerals and co-existing fluids indicated that the δ13C values of the precipitated carbonates co-vary over time with the δ13CDIC 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 δ13C values of the solids. Therefore although cyanobacterial-induced Mg carbonates are 10±5‰ enriched in 13C compared to inorganic carbonates, the δ13C values of natural hydrous Mg carbonates may reflect post precipitation processes and may not be reliable for paleo-environmental reconstructions. •Hydrous Mg-carbonates formed in the presence and absence of cyanobacteria.•The carbon isotope composition is affected by photosynthetic activity.•The carbon isotope composition exhibits continuous re-equilibration.