GREAT SALT LAKE (UTAH) MICROBIALITE CHRONOLOGY AND ISOTOPE GEOCHEMISTRY: IMPLICATIONS FOR PALEOLAKE EXTENT AND BIOGEOCHEMICAL EVOLUTION

GSL hosts extensive lacustrine microbialites that range in morphology and are thought to cover >20% of the oxic benthic area. Microbialites host cyanobacteria, diatoms, and green algae that contribute significantly to lake primary production, and provide substrate/food for brine fly larvae and brine shrimp that serve as a food source for large, GSL-dependent, bird populations. Despite their importance to GSL biogeochemistry and bird habitat, few studies have focused on the microbialites, in part because they have historically been submerged. To date, research on the GSL microbialites has focused on their surface microbial communities, their distribution, morphology and facies association, and limited chronology. We have taken advantage of historically low GSL levels and exposures of microbialites, and report new microscopic, stable isotopic, and age data of microbialites with implications for lake evolution through the Holocene.

Microbialite C (δ¹³C_PDB) and O (δ¹⁸O_PDB) stable isotope values covary significantly (R²=0.8) and range from 1.0 to 6.6 and -4.3 to -1.2 ‰, respectively. Aragonite ooids interpreted as modern have an average δ¹³C_PDB = 3.9 and δ¹⁸O_PDB = -3.9 ‰. Lake water collected at the time of microbialite sampling had a δ¹³C_PDB of 2.0 ‰, and δ²H_SMOW and δ¹⁸O_SMOW of -59.5 and -3.4 ‰, respectively. Calculated aragonite values in equilibrium with lake water are similar to the ooid composition indicating that carbonate is likely forming in equilibrium with the lake water, and this is important for subsequent interpretation of the microbialite values. δ¹³C_PDB and δ¹⁵N_AIR values of organic matter vary from -18.4 to -13.6 ‰ and 6.3 to 10.1 ‰, respectively. Uncalibrated radiocarbon ages from extracted organic matter from core to rim in one microbialite are 9359, 9502, 10728, and 6747 yr BP. Associated mean calendar ages are 8640, 8920, 10733, and 5667 yr BCE. Currently, the magnitude of the reservoir effect on these ages is unknown. For comparison, the reservoir effect based on carbonate radiocarbon ages of modern GSL gastropods has been reported at 1 - 4.5 kyr, and this likely far exceeds the effect on organic matter ages. We suggest that C and O stable isotope data trends are tied to periods of lake level rise and fall with implication for resolving Holocene variations in the lake hydrograph and biogeochemistry.