THROMBOLITES DEVELOPING IN HARD WATER, MARINE, AND HYPERSALINE ENVIRONMENTS: COMPARISON OF FABRIC AND BIOGEOCHEMICAL CONDITIONS

Thrombolites are clotted microbialites that are likely formed by the lithification of benthic microbial communities (e.g., through mineral precipitation). This precipitation, also referred to as microbially-induced organomineralization, is controlled by the so-called “alkalinity engine”, which is the result of the combined metabolic reactions of the entire microbial community. In addition, physicochemical changes in the macroenvironment may influence thrombolite formation, either directly (e.g., when the microbialites are exposed to environmental stress) or indirectly (e.g., by influencing microbial physiology).

By comparing physical and chemical data from contemporary thrombolite-forming environments including Highborne Cay, Bahamas; Green Lake, New York; and Shark Bay, Western Australia, it may be possible to gain insight into the environmental and microbial processes affecting thrombolite formation.

We hypothesize that the pronounced environmental differences that exist between these sites (e.g., in salinity, temperature, alkalinity) may affect the fabric of the microbialites. If this is the case, it may aid in the interpretation of thrombolites in the rock record.

Here we present data from samples collected in the field and our experiments conducted in the laboratory. These results highlight the importance of hydrochemical and physical factors such as water chemistry, temperature, light, and sediment supply as well the importance of microbial activity to the formation of thrombolites. Thin section and SEM images detail sediment-trapping and organomineralization that create the thrombolitic fabric. Initial analyses suggest differences in thrombolite microstructure not only based on geographical location, but also on seasonal variations. Though macrostructure similarities exist between the thrombolites from the various sites, differences are apparent when analyzing samples at a microscopic scale. Variations in environmental factors and water chemistry appear to alter the degree of mineralization, its location and morphology, highlighting the influence of the environment on the formation of thrombolitic fabric.