Magnesium isotope fractionation during hydrous magnesium carbonate precipitation with and without cyanobacteria

The hydrous magnesium carbonates, nesquehonite (MgCO3 center dot 3H(2)O) and dypingite (Mg-5(CO3)(4)(OH)(2)center dot 5(H2O)), were precipitated at 25 degrees C 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 delta Mg-26 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, CO32-, 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. (C) 2011 Elsevier Ltd. All rights reserved.