Geomicrobial Investigation of Thrombolites in Green Lake, New York and Highborne Cay, Bahamas

Thrombolites are non-laminated, clotted microbialites that form by the lithification of microbial communities through the trapping and binding of sediment and mineral precipitation. This precipitation (microbially-induced organomineralization) is controlled by both intrinsic (microbial; e.g. microbial metabolisms) and extrinsic (environmental; e.g. temperature) factors, which dictate the carbonate saturation index and ultimately the precipitation of carbonate minerals within the microbialite structure (i.e. EPS matrix). Thrombolites date back in the rock record to the early Paleoproterozoic (ca. 1.9 Ga) and are currently forming around the world in various climates and under a range of environmental conditions. This study compares two markedly different modern thrombolite environments: a hard water, meromictic lake (Green Lake, New York) and an open marine, intertidal environment (Highborne Cay, Bahamas). In this study we use geochemical and physical data to characterize these extant thrombolite-forming environments, analyze the fabric of the thrombolitic structure, and study some of the processes of microbe-mineral interactions that lead to the formation of a microbialite. To understand the extrinsic factors affecting microbialite growth, data such as chemical composition of the water, temperature, salinity, light, and sediment supply are important and were collected seasonally. Microbialite samples were analyzed using petrographic thin section, dissection, and scanning electron microscopy (SEM) to provide a context for mineral formation and thrombolite fabric. Between Green Lake and Highborne Cay, differences in exposure, temperature, water composition, and sediment supply impact the degree and location of precipitation within the surface microbial mat and the resulting thrombolitic fabric. The emerging geomicrobiological model for thrombolite formation in Green Lake shows mat thickness and precipitation as a factor of seasonality (i.e. temperature, sediment supply, light). In contrast, the model for Highborne Cay, Bahamas shows that the growth of and precipitation within the microbialite is relatively continuous and dependent on sediment supply. This research provides insight into the role the macroenvironment and microorganisms play in mineral precipitation, lithification, and overall formation of the thrombolite structures. Additional research may help providing a link to the fossil record, using the knowledge of these structures and their formation to understand microbialite formation in ancient environments