Amino acid racemization in lacustrine ostracodes, part II: Paleothermometry in Pleistocene sediments at Summer Lake, Oregon

The extent of amino acid racemization in fossil ostracodes of known age has served as a valuable proxy indicator of Pleistocene temperatures in the Great Basin of the western U.S. Here we assess the utility of this technique for estimating paleotemperatures in the Summer Lake basin of south-central Oregon. The enantiomeric composition (D/L values) of aspartic acid and glutamic acid was measured in Candona and Limnocythere ostracode valves collected from 32 sedimentary horizons within a ∼16-m section of lacustrine sediments associated with pluvial Lake Chewaucan and exposed along the Ana River. Independent geochronologic control spanning the past 250,000 years is provided by a recently revised age-depth model for the Ana River section. Measured D/L values in the fossil ostracodes generally increase down-section, but the trend is not monotonic and contains stratigraphic reversals. The D/L values from the Ana River section are significantly lower than those measured in ostracodes of comparable age from Lake Bonneville sediments, implying that the Summer Lake basin was colder than the Bonneville basin during the middle and late Quaternary. Although a higher north-south temperature gradient during the Pleistocene is expected considering the proximity of the Ana River site to former ice sheets, published amino acid paleothermometry equations for Candona yield unreasonably cold Pleistocene temperatures at Summer Lake. Measurements of fossil Candona valves from deposits across the western U.S. and northern Sonora reveal abnormally high amino acid concentrations in the subset of Ana River ostracodes analyzed, possibly related to unusual preservation and environmental factors at this site. This suggests that ostracode preservation has an influence on rates of racemization which complicates the application of amino acid paleothermometry at the Ana River site.