The Role of Sulphate-Reducing Bacteria and Unconsolidated Organic Matter in the Formation of Primary Dolomite

Laboratory experiments succeeded in producing primary dolomite at Earth-surface conditions using components collected from Christmas Bay near Brazosport, Texas and the Bahamas. Dolomite and calcite precipitates were observed within an anoxic sludge layer of siliciclastic sediment intermingled with decaying particulate organic vegetable matter, e.g., plant stems and roots. After experimentation lasting from 3 to 5 weeks, dolomite was identified by its elemental spectrum via energy-dispersive x-ray spectroscopy compared to known standards. Communities of sulphate-reducing bacteria in anoxic conditions gathered on the decomposing organic matter within the sludge layer. The bacteria on the decaying organic matter changed the microenvironment around them so that dolomite became supersaturated within their immediate vicinity. Although slight changes were made to the solution included in the experiment to decrease the kinetic barriers of the dolomitization reaction, it is concluded that sulphate-reducing bacteria were primarily responsible for the generation of the dolomite precipitates because (a) dolomite was only observed as being directly associated with the decaying organic matter where sulphate-reducing bacteria are thought to have preferentially accumulated, and (b) controlled changes of the chemistry of the solution appeared to have little effect on the precipitation of dolomite. Bacterial precipitates of dolomite and calcite were only found directly on or embedded within organic matter and were preferentially concentrated on organic matter devoid of siliciclastic detritus. Furthermore, dolomite was only synthesized in the presence of ooids which increased the speed and likelihood of dolomitization by contributing Ca2+ and CO32- ions through aragonite dissolution. It is suggested that the reason why primary dolomite is not found naturally within the Christmas Bay sediments is because the process of primary dolomite precipitation requires the presence of carbonate minerals.