Variations in Stable Isotope Composition of Great Salt Lake Carbonate: is There a Microbial Impact on the Sedimentary Record?

Carbon (d¹³C_c) and oxygen (d¹⁸O_c) stable isotopes from carbonate minerals formed in ancient lakes are often used to infer about past environmental conditions. To improve the use of lacustrine carbonate in paleoclimatic and paleoenvironmental reconstructions, we test the necessary assumption that carbonate that precipitates at approximately the same time across a single lake basin will record a consistent isotopic signature which can be paired to a particular environmental parameter. We study Great Salt Lake (UT, USA; GSL) as a proxy for the alkaline, hypersaline lakes that were more common in the past to better understand the effects of early diagenesis on the stable isotopes of carbonate precipitated from this unique lake chemistry. We hypothesize that microbial activity mediates the carbonate chemistry in the water column and below the sediment-water interface, thus affecting d¹³C_c and d¹⁸O_c. We measured d¹³C_c and d¹⁸O_c of microbialites collected at the sediment-water interface and five cores of various depths across the north-south extent of GSL and compared isotopic compositions at the sediment-water interface between sites to see if coeval sediments record comparable isotopic compositions. δ¹⁸O_carb from four of the five cores shows down-core and between-site variability of 3‰, suggesting that cores with similar carbonate facies are robust paleoenvironmental proxies. However, δ¹³C_c varies by 5‰ down-core within and between coring sites, perhaps indicating that microbial activity (e.g. respiration) affects the early diagenesis of lacustrine carbonate in GSL. Finally, although we expect a closed basin hydrological system like GSL to have a positive δ¹³C_c-δ¹⁸O_c correlation, we find no significant correlation. We believe this is because δ¹⁸O_c is mainly controlled by hydrology; this is not appreciably changed on the timescale of our cores' recorded sedimentation. Therefore, any perturbations from the expected positive correlation result from early diagenesis of δ¹³C_c. As such, we suggest that the main force controlling the down-core isotopic variability in GSL is a microbial process that mediates carbon cycling between the inorganic and organic carbon pools, changing the pH and alkalinity of the lake/porewater and altering the dissolved inorganic carbon pool from which carbonate precipitates.