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

Abstract

How to cite:Mavromatis, Vasileios; Pearce, Christopher R.; Shirokova, Liudmila S.; Bundeleva, Irina A.; Pokrovsky, Oleg S.; Benezeth, Pascale and Oelkers, Eric H. (2012) AbstractThe hydrous magnesium carbonates, nesquehonite (MgCO 3 Á3H 2 O) and dypingite (Mg 5 (CO 3 ) 4 (OH) 2 Á5(H 2 O)), were precipitated at 25°C in batch reactors from aqueous solutions containing 0.05 M NaHCO 3 and 0.025 M MgCl 2 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 cyanobacteriabearing experiments; measured d 26 Mg ranges from À1.54& to À1.16& in all experiments. Mg isotope fractionation is also found to be independent of reactive solution pH and Mg, CO 3 2À , 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.

The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling

“…The reproducibility of δ 26 Mg measurements for the liquid Mg standards DSM3, CAM1, and OUMg and the dolomite carbonate standard JDo-1 used during this study was better than 0.02‰, 0.08‰, 0.09‰ and 0.10‰, respectively. This precision is similar to that reported earlier from our laboratory Beinlich et al, 2014;Mavromatis et al, 2012Mavromatis et al, , 2013Mavromatis et al, , 2014aMavromatis et al, , 2014bShirokova et al, 2013), and fall within the previously reported values for these standards reported by other groups (Li et al, 2012(Li et al, , 2014Tipper et al, 2006;Bolou-Bi et al, 2009;Geske et al, 2015;Immenhauser et al, 2010;Pogge von Strandmann, 2008;Wombacher et al, 2009Wombacher et al, , 2011.…”

“…cyanobacteria. As emphasized by Mavromatis et al (2012), Gloeocapsa sp. does not affect the extent of Mg isotope fractionation between hydrous Mg carbonates and the aqueous solution.…”

“…Thus, Mg in minerals and plants grown from organic-rich solutions can be enriched in either heavy or light Mg isotopes depending on the identity of the aqueous species incorporated in the inorganic and organic solid phases. Table 3 lists the data obtained by Mavromatis et al (2012) for Mg isotope fractionation during nesquehonite and dypingyte precipitation in batch reactors at ambient temperatures in the presence and absence of Gloeocapsa sp. cyanobacteria.…”

“…The importance of Mg in global and past geochemical processes has also motivated experimental studies aimed at quantifying Mg isotope fractionation during the formation of calcite (Immenhauser et al, 2010;Li et al, 2012;Mavromatis et al, 2013), magnesite , dolomite (Li et al, 2015), nesquehonite/ dypingite (Mavromatis et al, 2012;Shirokova et al, 2013), brucite (Wimpenny et al, 2014;Li et al, 2014), and epsomite (Li et al, 2011). These studies indicate that 24 Mg is preferentially incorporated into all precipitated carbonate minerals, such that the minerals have a lower 26 Mg/ 24 Mg ratio than in the fluid phase.…”

The efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at Earth surface conditions

“…Black et al, 2008;Bolou-Bi et al, 2010;Li et al, 2010;Wimpenny et al, 2010;Opfergelt et al, 2012Opfergelt et al, , 2014Pogge von Strandmann et al, 2008;Shirokova et al, 2013;Teng et al, 2010;Tipper et al, 2010;Hwang et al, 2012;Ilina et al, 2103;Mavromatis et al, 2012Mavromatis et al, , 2014. Such processes could lead to global riverine fluxes that are isotopically lighter in Mg compared to the homogenous chondritic composition of the Earth's mantle (Tipper et al, 2006).…”