RADIOCARBON AND U-TH DOUBLE-DATING AND ISOTOPE GEOCHEMISTRY OF GREAT SALT LAKE MICROBIALITES: IMPLICATIONS FOR THE 14C RESERVOIR AND PALEOLAKE BIOGEOCHEMICAL EVOLUTION
The oldest radiocarbon and U-Th dates are from microbialites exposed at Lakeside (~18 – 15 ka) and cannot be interpreted due to inconsistencies with the GSL hydrograph and large Th corrections. However, microbialite U-Th and radiocarbon dates from Antelope Island and the North Arm are strongly positively correlated (r2= 0.98) and internally consistent from 11.4 to 1.7 ka. Assuming the U-Th dates are the true value (which may not be accurate), linear regression suggests a radiocarbon reservoir effect of ~1560 years.
Positive correlations between carbonate δ18O and δ13C in some microbialites are consistent with a holomictic (mixes at least once per year), hydrologically closed-basin lake with fluctuations in volume, chemistry, and associated changes in lake primary production. However, inverse δ18O and δ13C correlations present in a number of microbialites are enigmatic, but may imply periods of higher salinity and stable lake stratification (meromixis) similar to modern GSL conditions. The new and revised geochronology and isotope geochemistry in this study suggest two periods of meromixis between ~11.4 and 8 ka, and 3.8 and 1.7 ka, separated by periods dominated by holomictic conditions in GSL.