RADIOCARBON AND U-TH DOUBLE-DATING AND ISOTOPE GEOCHEMISTRY OF GREAT SALT LAKE MICROBIALITES: IMPLICATIONS FOR THE 14C RESERVOIR AND PALEOLAKE BIOGEOCHEMICAL EVOLUTION

Extensive lacustrine microbialite deposits exposed along the shores of Great Salt Lake (GSL), Utah potentially preserve a rich continental paleoenvironmental record; however, geochronology on these deposits is limited. We report coupled radiocarbon and U-Th dates from microbialite carbonate in concert with carbon and oxygen stable isotope ratios. This double-dating effort attempts to inform the radiocarbon reservoir effect and identify open-system or other challenges with the U-Th system in GSL microbialites. Previously reported uncalibrated ¹⁴C dates range from 14,747 ± 50 to 3362 ± 26 yr B.P. These calibrate to mean probability dates from 17,945 to 3606 cal yr BP, assuming no ¹⁴C reservoir effect. New U-Th dates were acquired from splits of the radiocarbon sample locations in these microbialites. Uncorrected U-Th dates range from 47,074 ± 89 yr to 4,975 ± 19 yr. Applying a crustal Th correction, the date range is 18,300 ±26 yr to 1,691 ± 16 yr. In both cases the reported uncertainties are solely based on uncertainties in the decay constants and measured ratios of ²³⁰Th/²³⁸U, ²³⁴U/²³⁸U and ²³⁰Th/²³²Th.

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.