A PRELIMINARY LATE-LAST-GLACIAL-PERIOD PALEOTEMPERATURE RECORD FOR SUMMER LAKE, OR FROM CLUMPED ISOTOPE THERMOMETRY

The growth of pluvial lake systems across the internally-drained Great Basin, USA during the Late Pleistocene is a primary indicator of regional hydroclimate change during and following the last glacial period; however, the mechanisms for individual pluvial lake expansions (increased precipitation vs. reduced evaporative water loss as a result of cooler mean annual temperatures) remain poorly resolved due to limited independent paleotemperature constraints on pluvial lake proxy records. We present a preliminary paleotemperature record derived from clumped isotope study of micrites preserved in the Summer Lake subbasin of pluvial Lake Chewaucan in central Oregon. Summer Lake micrites with measured radiocarbon ages of 27-17 cal ka BP exhibit variability in δ¹³C (–4.7 to +4.1‰) and δ¹⁸O (–11.4 to –2.6‰) that reflects closed-basin hydrologic conditions in pluvial Lake Chewaucan throughout the late last glacial period. Clumped isotope formation temperatures (T(Δ₄₇)) for 27-17 cal ka BP micrites average 2 ± 3ºC which is significantly colder than modern mean annual lake temperature of 13ºC. These new micrite T(D₄₇) values overlap with the cold end of the range of published Summer Lake paleotemperature estimates derived from ostracod Mg/Ca ratios (~4 to 30ºC; Cohen et al., 2000) and are similar to average T(Δ₄₇) for ~ 18 ka tufa samples (6 ± 2ºC) from neighboring Lake Abert, OR published in Hudson et al., 2017. These preliminary findings are encouraging and highlight the value of Summer Lake sediments as a proxy archive for Late Pleistocene paleoenvironmental change.