UNRAVELING RADIAL OOID FORMATION IN GREAT SALT LAKE (UTAH): INSIGHTS FROM RADIOCARBON CHRONOLOGY AND MOLECULAR BIOLOGY

Ooids (laminated coated grains, formed as layers precipitate upon a nucleus) are common in the geologic record in lacustrine and marine systems. Traditionally interpreted as abiogenic precipitates, recent work suggests that microbial metabolism/byproducts may enhance the calcium carbonate precipitation of some ooids. Thus, the processes that govern ooid formation remain enigmatic, making it difficult to assess their significance as biosigntatures and environmental indicators in modern/ancient environments.

The Great Salt Lake (GSL), Utah, represents a unique environment to assess growth rate of aragonitic ooids and the microbial communities associated with them. Ooids collected near Antelope Island were sieved to separate size fractions. The microbial communities associated with each size fraction were compared to those in the lake water using small subunit rRNA gene sequencing. 50% of the ooids studied contain peloidal nuclei hypothesized to be fecal pellets from the Great Salt Lake Artemia (brine shrimp); thus, Artemia pellets were also collected and sequenced, for comparison. The microbial communities associated with ooids closely resembled those of the Artemia fecal pellets, which were markedly different than those of the lake water. Alphaproteobacteria, Deltaproteobacteria, Planctomycetes, and Bacteriodetes were the most abundant bacterial phyla present within ooid samples, whereas, the lake water microbial community was largely composed of Halobacteria (Euryarchaeota).

To investigate age, 355-500 µm ooids were sequentially leached and ¹⁴C of the evolved gas samples was analyzed. The oldest inorganic carbon (C) of this size fraction has an apparent ¹⁴C age of ~6600 yr BP, with subsequent growth spanning over 6,000 years. Organic carbon from the nuclei has a ¹⁴C age that is nearly contemporaneous with the oldest carbonate. We conclude that 1) ¹⁴C-dating using sequential leaching methods indicates 355-500 µm ooids began forming over 6,000 yr BP and growth has continued to the present; and 2) relict DNA (and by extension, other organic molecules) in ooid nuclei may skew the results when investigating the role of microbes in ooid formation.