Great Salt Lake ooids: insights into rate of formation, potential as paleoenvironmental archives, and biogenicity

Ooids are laminated coated grains that are most commonly composed of calcium carbonate which precipitates around a nucleus. As accretionary structures, they have the potential to record a time series of hydrologic and geochemical change in marine and lacustrine environments. Ooids are common in carbonate environments throughout Earth’s history, but the mechanism by which they form remains unclear. In particular, the rate of ooid growth remains elusive in all but a few modern marine environments. Furthermore, modern marine ooids display tangentially-oriented aragonite crystals within their cortices, whereas many ancient ooids have radially-oriented cortices. Ooids are traditionally viewed as abiogenic precipitates, however the role of biology in ooid formation has recently been suggested because certain microbial metabolisms may either foster carbonate precipitation or dissolution by manipulating dissolved inorganic carbon and alkalinity. The following dissertation uses ooids from Great Salt Lake (GSL), Utah, a well-known site of primary radial ooids, to investigate ooid growth rate and age, the potential of ooids to resolve lacustrine hydrologic change, and the biogenicity of lacustrine ooids. ❧ We used ¹⁴C to establish a sequential chronology for ooids from GSL. Ooids from the coarse size fraction were sequentially dissolved and ¹⁴C ages were obtained for each dissolution step to create a time series of ooid growth. The results of the sequential dating indicate that the formation of coarse Great Salt Lake ooids began between 5800-6600 ± 60 ¹⁴C yr BP while their outer cortices are nearly modern. Sequentially dated ooids from Antelope Island (southern part of Lake) record a nearly linear growth history with linear growth rates of ∼0.01 – 0.015 µm/yr, whereas ooids from Spiral Jetty (NE part of Lake) record somewhat faster growth between ∼6000 and 4000 years ago (0.03 – 0.06 µm/yr) followed by a slower growth history for the remainder of their lifespan (0.003 – 0.008 µm/yr). The lifespan of Great Salt Lake radial aragonitic ooids ranges is two and six times longer than those from modern marine environments, and thus provides a unique end member for understanding the mechanisms behind radial ooid formation. The antiquity of the ooids would suggest that geochemical parameters measured from bulk ooid dissolution would integrate over ∼6000 years and thus do not represent geochemical snapshots in time as some previous studies suggest. ❧ Lake level indicators are sparse in GSL between ∼10,000 ybp and the onset of historical times, so the Holocene lake variation is somewhat poorly resolved. We evaluate the ¹⁴C age and stable carbon isotope composition of ooids that grew in the lake over the last 7,000 years. Since δ¹³C and δ¹⁸O covary in closed basin lakes, changes in the δ¹³C through time can be interpreted to represent changes in lake volume if other assumptions we make hold true. Ooids were sequentially dissolved to create a paired radiocarbon chronology and δ¹³C profile with depth in the ooid. Petrographic analysis reveals that, for the majority of ooids, the initial precipitation upon the nucleus is represented by a comparatively large aragonite crystal ray texture, followed by alternations of finely crystalline radial aragonite and thin radial-concentric layers. Net growth rates derived from radiocarbon ages indicate the large crystal ray fabric corresponds to 7498 to 2073 cal yr BP with δ¹³C between 4.4±0.15 and 4.8±0.15 ‰, likely indicating a relatively stable lake level. The alternating fabric precipitated between 2073 cal yr BP to present, and the δ¹³C values of ooids trending toward lighter values (from ∼4.5‰ to 3.7‰), suggests a largely transgressive phase in GSL. The youngest inorganic carbon sample from north arm ooids falls off this trend as it has a significantly heavier δ¹³C of 5.6±0.15‰. Modern dissolved inorganic carbon from north arm lake water indicates aragonite precipitating today would have δ¹³C values between 9.1±0.55‰ to 9.8±0.75‰, therefore the young heavy inorganic carbon of north arm ooids likely suggests that ooid formation has occurred after the construction of the railroad causeway in 1959, which isolated the north arm from the riverine-influenced southern portion and thus left the reservoir to generate an enriched δ¹³C pool, representing the first robust evidence that ooid formation has occurred in modern times. Furthermore, the entire chronology of GSL ooids from both sample sites record over 2‰ variability in δ¹³C over several thousand years, which calls into question the utility of bulk geochemical analyses from accretionary structures such as ooids in dynamic systems such as closed-basin lakes. ❧ To investigate the biogenicity of ooids in GSL, we evaluated the microbial community of ooids from the southern arm of GSL and the brine shrimp (Artemia) pellets upon which they commonly form. The small subunit rRNA analysis revealed ooids and Artemia pellets have similar microbial assemblages dominated in Bacteroidetes, Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria. The striking similarity between the microbial communities of GSL ooids and brine shrimp gut microbiota suggests relict DNA residing in oolitic nuclei may be an important source of DNA in microbial community analysis of ooids and other accretionary structures. The calcium carbonate saturation state of GSL is supersaturated with respect to aragonite, though it ranges from 100% to 250% saturation seasonally. Calcification rates of GSL ooids are several orders of magnitude lower than experimental precipitation of aragonite, suggesting aragonite may be precipitating in GSL just above saturation, conditions that do not necessitate the geochemical manipulation of microbiota to achieve in this system.