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Experimentally determined biomediated Sr partition coefficient for dolomite: Significance and implication for natural dolomite |
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| Authors: | Mónica Sánchez-Román Judith A McKenzie Christopher S Romanek Antonio Sánchez-Navas |
| Institution: | a ETH-Zürich, Geological Institute, 8092 Zürich, Switzerland b NASA Astrobiology Institute and Department of Geology, University of Georgia, Drawer E, Aiken, SC 29808, USA c Centro de Astrobiología, INTA-CSIC, Ctra Ajalvir Km 4, Torrejón de Ardoz, 28850 Madrid, Spain d Department of Chemistry, PUC, 22453-900 Rio de Janeiro, Brazil e Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506, USA f Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain |
| Abstract: | Two strains of moderately halophilic bacteria were grown in aerobic culture experiments containing gel medium to determine the Sr partition coefficient between dolomite and the medium from which it precipitates at 15 to 45 °C. The results demonstrate that Sr incorporation in dolomite does occur not by the substitution of Ca, but rather by Mg. They also suggest that Sr partitioning between the culture medium and the minerals is better described by the Nernst equation (DSrdol = Srdol/Srbmi), instead of the Henderson and Kracek equation (DSrdol = (Sr/Ca)dol/(Sr/Ca)solution. The maximum value for DSrdol occurs at 15 °C in cultures with and without sulfate, while the minimum values occur at 35 °C, where the bacteria exhibit optimal growth. For experiments at 25, 35 and 45 °C, we observed that DSrdol values are greater in cultures with sulfate than in cultures without sulfate, whereas DSrdol values are smaller in cultures with sulfate than in cultures without sulfate at 15 °C.Together, our observations suggest that DSrdol is apparently related to microbial activity, temperature and sulfate concentration, regardless of the convention used to assess the DSrdol. These results have implications for the interpretation of depositional environments of ancient dolomite. The results of our culture experiments show that higher Sr concentrations in ancient dolomite could reflect microbial mediated primary precipitation. In contrast, previous interpretations concluded that high Sr concentrations in ancient dolomites are an indication of secondary replacement of aragonite, which incorporates high Sr concentrations in its crystal lattice, reflecting a diagenetic process. |
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