SPATIAL TRENDS IN OPTICAL AGES AND CHEMISTRY FOR DUNE SANDS AND LOESS, NEBRASKA SAND HILLS

Optical ages of eolian sand and loess from in and adjacent to the Nebraska Sand Hills show several periods of eolian activity beginning at approximately 16 ka, with the most recent regional activity between approximately 950 and 650 a, during the Medieval Climate Anomaly (MCA). These optical ages were determined over several years by different researchers using both Risø and Daybreak readers. Samples from the MCA period were re-analyzed by the first author using a single Risø reader in order to eliminate any small systematic errors. The data show a distinct spatial trend in OSL ages, from ~ 750 – 900 a in the northwest to ~ 550 – 750 a along the southeastern margin of the Sand Hills. OSL ages from upland dunes are consistent with previously determined radiocarbon ages from peat layers bounding sand sheets in interdune peatlands. OSL ages for loess samples collected southeast of the Sand Hills are consistent with ages for nearby dune sands. The trend in optical ages may represent either two distinct periods of drought, or a single episode differentially preserved as a result of earlier stabilization of the dunes near wet interdunes found in the northwest core of the Sand Hills. Chemical data show a spatial trend perpendicular to sand-drift wind direction, consistent with a dual sand source, with sand in the southern Sand Hills derived from near the Platte River, and sand in the northern Sand Hills from near the Niobrara River.