Amino acid paleothermometry of Quaternary ostracodes from the Bonneville Basin, Utah

Unlike many other proxy climate recorders, temperature-dependent chemical reactions that take place in fossils are independent of paleoprecipitation changes, and therefore allow paleotemperatures to be calculated uniquely. In this study, I use the extent of amino acid racemization in fossil ostracode shells to reconstruct the temperature history of the Bonneville Basin, Utah. The enantiomeric composition (D/L ratios) of 64 ostracode samples, each comprising an average of 18 subsamples, from 25 sites were analyzed by reverse phase HPLC. The effective diagenetic temperature (EDT) for five intervals of the Quaternary, including the last full-glacial period, was calculated for two amino acids (aspartic and glutamic acids). Errors associated with the EDT calculations were derived using a Monte Carlo procedure. Age control was provided by both new and previously published AMS ¹⁴C, luminescence, tephra, and amino acid geochronology. The EDT for the southern Bonneville Basin during the middle Pleistocene interval from 620-150ka (oxygen-isotope stages (OIS) 16-6) was 5.7 ± 1.5°C. This compares with an EDT of 10.9 ± 1.3°C for the late Holocene (last 5.8 cal ka), and a current mean annual temperature of about 9.4°C at nearby sites (40°N latitude). The EDT then decreased to 3.3 ± 1.3°C for the interval from OIS 6-2 (150-12 cal ka). During the last full-glacial period (24-12 cal ka = OIS 2), the EDT was 1.1 ± 2.5°C, or about 10 ± 3°C lower than during the late Holocene. Paleotemperatures this low suggest that reduced evaporation rather than increased precipitation could have accounted for the increased effective moisture and the growth of Lake Bonneville.