PRECIPITATION OF CARBONATE MINERALS BY MICROORGANISMS - IMPLICATIONS FOR SILICATE WEATHERING AND THE GLOBAL CARBON-DIOXIDE BUDGET

Direct light and electron microscopic studies show that cyanobacterial cells serve as nucleation sites for carbonate mineral precipitation i n a variety of fresh to saline-alkaline lakes on the Cariboo Plateau i n central British Columbia, Canada, and in mineralized crusts on weath ered basalt in Iceland. The carbonate minerals found in association wi th the cyanobacteria were extremely fine-grained, and invariably occur red on the external surfaces of the cells. Carbonate mineralogy was va riable, ranging from calcite to magnesite, depending on differences in lake and groundwater chemistry (i.e., saturation state of the water w ith respect to individual carbonate minerals). In microcosm experiment s, phototrophic cyanobacterial growth increased alkalinity and the deg ree of oversaturation with respect to calcite. Calculated values for t he saturation state of calcite and magnesite in Cariboo Plateau natura l waters exhibited two distinct trends, with (1) high magnesite satura tion values in areas where the weathering of magnesium olivine-rich ba salt bedrock determines water chemistry and (2) high calcite saturatio n values where bedrock is a mix of basic lava flows, limestone argilli te, and chert. Similar calculations for Iceland show that cold surface waters are generally oversaturated with respect to calcite, as expect ed for the weathering of calcium plagioclase-rich lava. These observat ions of microbial carbonate precipitation in the Caribou Plateau regio n of British Columbia, Canada, and on the Budarhaun lava plain in Icel and suggest that weathering of silicate minerals in bedrock is biogeoc hemically coupled to the deposition. of carbonate minerals by microorg anisms. This process may provide a sink for atmospheric carbon dioxide in terrestrial environments.