RHIZOLITH STABLE ISOTOPE PATTERNS FROM PILOT VALLEY PLAYA, UTAH

Carbonates often accompany lake and lake-margin deposits in both modern and ancient geologic settings. If precipitated in equilibrium with the local waters, their stable isotope records reflect the prevailing climate (arid vs. wet) and regional ecological parameters. Carbonate rhizoliths and water samples were collected from a playa lake in western Utah. Pilot Valley (~43°N) is a closed-basin, remnant playa from the Quaternary desiccation of paleo-Lake Bonneville.  Freshwater springs dot the playa margin at the base of an alluvial fan. The regional climate is one of low precipitation (<150mm/year), high summer evaporation, and extreme temperatures with moderately high wind speeds during the spring. Values from rhizolith samples range from 0.6‰ to 9.7‰ and –14.7‰ to –6.7‰ for d¹³C and d¹⁸O respectively (V-PDB).  The average d¹³C value is 3.0‰, and the average d¹⁸O value is –11.2‰ (1-s S.D.=1.2‰ and 1.8‰ respectively).  Published d_water values vary from –16‰ (V-SMOW) in the springs to –3 ‰ in the playa proper.  Covariance between carbon and oxygen isotopes within individual rhizoliths is not apparent.  The oxygen isotope signal reflects primarily the highly evaporative conditions on the playa flat where the rhizoliths form.  If there was no evaporative increase in the d_water, the rhizolith d¹⁸O values would require formation temperatures colder than 12°C.  Assuming a more realistic calcification temperature of 30°C, the rhizolith d¹⁸O values would indicate precipitation in waters with d_water values of –12 to –4‰ (V-SMOW), consistent with the observed values.  Certainly both variables (temperature and d_water) change.  However the potential for CaCO₃ precipitation is enhanced with increased alkalinity and evaporation.  We interpret the high d¹³C values as reflecting gas exchange between the playa waters and the atmosphere.  At 30°C, atmospheric d¹³C values of ~-8‰ exchange with water DIC and the resulting calcite precipitate should record ~ 2‰ d¹³C, which is similar to the measured average rhizolith d¹³C values.  The lack of covariance is due to high carbonate alkalinity, which weakens the d¹³C response to changes in freshwater input.  The range in each isotope creates a shallow trend to the data that is unique to semiarid climates.