URANIUM-SERIES DATING OF PEDOGENIC CARBONATE FROM THE PROVO SHORELINE, LAKE BONNEVILLE, UTAH, USA

The timing and rate of climate-induced regression of Lake Bonneville, from the Provo stillstand to the pre-Holocene lowstand, has been the focus of a number of climate studies that have endeavored to correlate regional and global records. Recent published analyses of radiocarbon data from shoreline facies suggests that Lake Bonneville maintained prolonged, high frequency oscillations at or near the Provo level before undergoing rapid regression to near modern lake levels by 11.5 ¹⁴C ka (Godsey et al. 2005. Quat. Res. 63, 212-223). Although the relative timing of Lake Bonneville pluvial events is well constrained, direct correlation between published radiocarbon ages and regional or global climate records is not straightforward due to uncertainties inherent in the dating technique such as the reservoir effect.

Uranium series dating of a calcic soil profile, developed within a Provo gravel shore zone deposit near Hyrum, provides a vertical chronosequence of some complexity. Preliminary uranium-series ages, determined from inner pedogenic carbonate rinds, display a broad age distribution between c.30-10 ka, with the majority of the samples in the range c.13.5-10 ka with 2σ errors between 5-10%. Although the bulk of the older sub-set may be discounted due to high initial ²³⁰Th derived from detrital contaminants, a minority reflect an older age distribution that may be genuine and should be considered in models of Lake Bonneville evolution. The suggested regression of the Provo shoreline by c.11.5 ¹⁴C ka (c.13.5 cal ka [Fairbanks1204 calibration: Fairbanks et al. 2005. Quat. Sci. Rev. 24, 1781-1796]) broadly correlates with the onset of calcic soil development constrained by the younger sub-set (c.13.5-10 ka), although a number of older outliers should not be discounted. Analysis of the dataset suggests that pedogenic carbonate sampled from basal gravel units are more likely to be free of significant detritus, resulting in inner rind ages of improved precision. Further work will better constrain the potential correlation between radiocarbon and uranium-series ages and may help determine both the extent of the reservoir effect in Lake Bonneville and the correlation between regional and global climate records.