--> Distributions and Environmental Controls of Microbialites in Great Salt Lake, Utah

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Distributions and Environmental Controls of Microbialites in Great Salt Lake, Utah

Abstract

Although the presence of microbialites in Great Salt Lake (GSL) has been known for over 70 years, new research designed to characterize microbialite distributions in GSL has revealed both the world's largest known extent of modern microbial bioherms and evidence of environmental controls affecting their occurrence and distribution. Integration of multiple acoustic technologies and analytic techniques provides details on the shapes, sizes, and densities of microbial bioherms in Great Salt Lake and confirms that meter-scale microbial bioherms occupy an estimated area of more than 700 km2 in the south part of the lake and 300 km2 in the north. Bioherms are observed throughout shallow areas of the lake and vary from statistically dispersed individuals to clustered, laterally-connected forms. Individuals typically are circular to oblate and range in size from centimeters to over 2 meters in diameter with measured heights of more than 1.5 meters above adjacent substrate. Sub-decimeter resolution digital CHIRP profiles, collected simultaneously with side scan sonar data, reveal the intimate relationship between microbialite occurrence and subtle tectonically-controlled, micro-topographical changes on the shallow lake floor. Even though recent tectonic activity has produced only meter-scale offsets in lake-floor topography, this displacement controls the distribution of modern microbial bioherms: bioherm development preferentially occurs on the highs of the footwalls and results in abrupt facies changes across faults. Based on the observed distribution of microbial bioherms, it appears that interrelated environmental conditions found at the topographic highs (light availability, substrate type, and/or wave-base energy) play a critical role in microbial bioherm occurrence. In addition, anthropogenically-induced increases in salinity, related to a rock-filled causeway that subdivides GSL, have resulted in a widespread die-off of bioherm-forming microbial communities in the hypersaline environment of the north part of the lake, effectively defining an upper limit for bioherm viability in hypersaline waters. Great Salt Lake provides an ideal scenario for studying environmental controls on the occurrence and spatial distributions of microbial bioherms within a mid-latitude saline shallow lacustrine environment. It also provides a unique analogue for assessing the possible range of environmental conditions associated with microbial life during early Earth history.