Characteristics and controls on the distribution of sublittoral microbial bioherms in Great Salt Lake, Utah : Implications for understanding microbialite development

Side-scan sonar and Compressed High Intensity Radar Pulse mapping of Great Salt Lake, Utah, linked to reprocessing of acoustic data from bathymetric surveys, has enabled the distribution of microbial bioherms to be assessed. Bioherms occupy an estimated area >700 km2 in the south arm and >300 km2 in the north arm. Distributions vary from statistically dispersed to clustered, and in this latter case, are predominantly located on metre-scale, fault-controlled topographic highs, with sediment infilling intervening lows between adjacent offsets. Individual bioherms are circular to oblate and range from centimetres to over 2 m in diameter. In some areas, bioherm heights were measured at more than 1.5 m above adjacent substrate. Sublittoral bioherms are made of aragonite, calcite and minor dolomite precipitated due to physico-chemical, biologically induced and influenced carbonate mineralization processes in association with microbial mats. Bioherm fabrics vary at the millimetre to centimetre-scale and consist of leiolitic and clotted peloidal micrite-grade carbonate, sinuous threads of spherulitic fibrous aragonite crystals, laminated micrite boundstone and internal carbonate mud sediment with peloids and ooids. The identification of factors that influence microbial bioherm occurrence and spatial distribution in Great Salt Lake is limited to a set of collinear physical, chemical and biological variables that are confined to a localised closed system, such as salinity, water depth, wave energy, stable substrate and sediment accumulation. Anthropogenic modifications to Great Salt Lake resulting in increased salinity have exceeded the salinity range in which bioherm-mediating microbial communities can survive, effectively defining an upper limit of salinity for bioherm microbial community viability. The better understanding of the distribution of microbial bioherms has significant implications for managing and protecting the lake ecosystem and may provide insights into the physical and chemical controls that existed during the formation of fossil microbialites in deep time.