Experimentally determined biomediated Sr partition coefficient for dolomite: Significance and implication for natural dolomite

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 degrees 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 (D-Sr(dol) = Sr-dol/Sr-bmi), instead of the Henderson and Kracek equation (D-Sr(dol) = (Sr/Ca)(dol)/(Sr/Ca)(solution). The maximum value for D-Sr(dol) occurs at 15 degrees C in cultures with and without sulfate, while the minimum values occur at 35 degrees C, where the bacteria exhibit optimal growth. For experiments at 25, 35 and 45 degrees C, we observed that D-Sr(dol) values are greater in cultures with sulfate than in cultures without sulfate, whereas D-Sr(dol) values are smaller in cultures with sulfate than in cultures without sulfate at 15 degrees C. Together, our observations suggest that D-Sr(dol) is apparently related to microbial activity, temperature and sulfate concentration, regardless of the convention used to assess the D-Sr(dol). 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. (C) 2010 Elsevier Ltd. All rights reserved.