Magnesium isotope fractionation during hydrous magnesiumcarbonate precipitation with and without cyanobacteria

Mavromatis, Vasileios; Pearce, Christopher R.; Shirokova, Liudmila S.; Bundeleva, Irina A.; Pokrovsky, Oleg S.; Benezeth, Pascale and Oelkers, Eric H. (2012). Magnesium isotope fractionation during hydrous magnesiumcarbonate precipitation with and without cyanobacteria. Geochimica et Cosmochimica Acta, 76 pp. 161–174.

DOI: https://doi.org/10.1016/j.gca.2011.10.019

Abstract

The hydrous magnesium carbonates, nesquehonite (MgCO₃.3H₂O) and dypingite (Mg₅(CO₃)₄(OH)₂.5(H₂O)), were precipitated at 25 oC in batch reactors from aqueous solutions containing 0.05 M NaHCO₃ and 0.025 M MgCl₂ and in the presence and absence of live photosynthesizing Gloeocapsa sp. cyanobacteria. Experiments were performed under a variety of conditions; the reactive fluid/bacteria/mineral suspensions were continuously stirred, and/or air bubbled in most experiments, and exposed to various durations of light exposure. Bulk precipitation rates are not affected by the presence of bacteria although the solution pH and the degree of fluid supersaturation with respect to magnesium carbonates increase due to photosynthesis. Lighter Mg isotopes are preferentially incorporated into the precipitated solids in all experiments. Mg isotope fractionation between the mineral and fluid in the abiotic experiments is identical, within uncertainty, to that measured in cyanobacteria bearing experiments; measured δ²⁶ ranges from -1.54‰ to -1.16‰ in all experiments. Mg isotope fractionation is also found to be independent of reactive solution pH and Mg, CO₃^2-, and biomass concentrations. Taken together, these observations suggest that Gloeocapsa sp. cyanobacterium does not appreciably affect magnesium isotope fractionation between aqueous fluid and hydrous magnesium carbonates.