INVESTIGATION ON MECHANISM FOR DOLOMITE PRECIPITATION IN DEEP SPRINGS LAKE, CALIFORNIA

The formation of dolomite has two aspects: failure to precipitate dolomite experimentally at Earth surface temperature conditions due to kinetically inhibition; and the lack of modern analogues to explain the abundance in the geological record. Modern dolomite is observed to form in hypersaline and alkaline marine environments. Such modern dolomite precipitation is commonly attributed to microbial mediation. It was reported that there are primary dolomite precipitation in Deep Springs Lake in California, a hypersaline playa. (Jones, 1961) The biomass in this lake is dominated by micro-organisms of halophilic archeabacteria and green alga Dunaliella salina. In order to understand the roles of microbes in mediating dolomite precipitation, we use the biomass collected from Deep Springs Lake in California in dolomite precipitation systems.

The experimental results prove that disordered dolomite can be synthesized at low temperature using the biomass collected from Deep Springs Lake. Based on the glycosyl composition analysis of the biomass, about ten monosaccharides were detected with glucose, ribose and xylose as the most abundant. These bacterially derived extracellular polymeric substances (EPS) might play a crucial role in dolomite precipitation. It is proposed that polysaccharides can be strongly adsorbed on Ca-Mg carbonate surfaces through hydrogen bonding, and weaken the chemical bonding between Mg and water molecules, which can enhance Mg incorporation into carbonate, therefore contribute to the growth of disordered dolomite.

XRD patterns of synthesized HMC (high-magnesium calcite) and disordered dolomite demonstrate that there is a positive relationship between biomass concentration and mole percent of MgCO₃ in the precipitates. No dolomite precipitates in solutions without biomass. The Mg²⁺/Ca²⁺ ratio from initial solution and temperature also have strong effects on synthesized dolomite based on our experiments. EDS results demonstrate Mg:Ca ratio of precipitates could be as high as 45% under room temperature and 60% under 40°C. SEM images indicate synthesized HMC and dolomite grew on the surface of calcite seed crystals. TEM images reveal that synthesized dolomites are nano-crystals with similar crystallographic orientations.