| Description |
Microbial bioherms constitute some of the earliest forms of life on Earth. Although the primary form of life on earth during the Proterozoic (Hoffman, 1994), modern occurrences that spread over an area greater than a few square kilometers are rare. Microbial bioherms in Great Salt Lake were reported more than 75 years ago (Eardley, 1938); however, no additional studies have been done on the extent of these structures nor on how environmental factors may influence their distribution. This dissertation documents the development of a methodology to examine the occurrence and spatial distribution of microbial bioherms in Great Salt Lake, Utah, using multiple lines of evidence including marine acoustic technologies, various spatial and aspatial analyses, Geographic Information Systems (GIS) technologies, and in situ verification of bioherm occurrence. Microbial bioherms occupy an estimated area of more than 700 km2 in the south part of the lake and more than 300 km2 along the margins of the north part of Great Salt Lake. Distributions vary from statistically dispersed to clustered and are closely associated with structurally controlled, positive, microtopographical changes in the benthos. Individuals typically are circular to oblate and range in size from centimeters to over 2 meters in diameter. In some areas of the lake, bioherm heights were measured at more than 1.5 meters above adjacent substrate. Observations, videography, and samples of microbial bioherms from the south part of Great Salt Lake, in modern conditions of salinity averaging about 15 percent, are dark green in color, the result of an abundant surface-based phototrophic community that populates the highly rugose surface of the bioherms. Exposed shallow-water bioherms in the north part of the lake are white and not populated by either surface-based phototrophic communities or macroinvertebrate grazers. The environmental conditions in the north part of Great Salt Lake, averaging about 27 percent salinity, have exceeded the ability of bioherm-forming microbial communities to survive. Great Salt Lake provides a glimpse into what Earth was like during the first 3 billion years of its existence and before the emergence of more complex life forms. This investigation has identified how tectonics and environmental conditions influence the initiation and growth of microbially induced carbonates in saline environments and places important constraints on the conditions for microbial life during the present and during early Earth history. |
