GEOCHEMISTRY OF MICROBIALITES AND WATER IN STORR'S LAKE, SAN SALVADOR, BAHAMAS

Several morphological types of modern microbialites have been identified in Storr’s Lake, San Salvador Island, Bahamas. This hypersaline lake provides a natural laboratory to study the formation and early diagenesis of microbialites. In previous studies, the precipitation of carbonate minerals in microbialites was considered to be induced by the metabolic activities of microbes, through extracellular polymeric substance (EPS) production and degradation and by favorably modifying pH and alkalinity. Variations in the mineralogical compositions (calcite and aragonite) were ascribed to early diagenetic processes and/or changes in lake water chemistry. In this research, detailed geochemical studies of microbialites and water samples from Storr’s Lake will be presented to gain insight into the environmental conditions and/or microbial activity during the formation and early diagenesis of the microbialites.

Major and trace element composition as well as in situ water chemistry data of water samples were analyzed along with the lake water saturation state that could highlight any spatial and temporal variations within the lake system. Results indicated that Storr’s Lake water samples are most similar to seawater and differ from well and conduit water samples, suggesting that the source of lake water is primarily seawater. X-ray diffraction (XRD) analyses have been applied in conjunction with thin section observation to characterize the mineralogical compositions of sublayers within the microbialites. The results suggest that pinnacle mound and plateau-mushroom microbialites have a considerable amount of aragonite, while other types of microbialites only contain high-Mg calcite. We also observed that the ratio of aragonite to high-Mg calcite varies within the sublayers of a microbialite sample. Ca/Mg and Ca/Sr molar ratios will be coupled with aragonite/calcite ratios to discuss the controls on the mineralogical compositions. Oxygen and carbon isotopic analyses will be combined with thin section observations to identify the extent of early diagenesis and variations in microbial activities in these sublayers as well.