DECIPHERING LATE HOLOCENE EOLIAN SAND DEPOSITIONAL RECORDS FROM WESTERN NEBRASKA: LANDSCAPE RESPONSE TO EXTREME DROUGHT

Stabilized dune fields are common throughout the Great Plains of North America. These landforms attest to periods of pronounced aridity during the Holocene that exceeded drought conditions in the 20th century. Our field research has focused on deciphering the activity of parabolic and barchanoid-ridge paleodunes in Dundy and Cherry counties, western Nebraska in the past 2000 years. The internal stratigraphy of dunes is well exposed in sections adjacent to lakes, with many of the lakes formed with dune damming of drainages. These sections reveal intercalated weak soils (A/C or A/Bw) within eolian sand sequences that indicate repeated dune reactivation during the late Holocene. Quartz grains from these eolian sediments are directly dated using recent advances in optically stimulated luminescence, single aliquot regeneration (OSL-SAR) protocols providing decadal to century precision for the past 2 ka. Stratigraphic analyses combined with OSL-SAR ages identify eolian sand depositional events separated by paleosols at 60 ± 20, 150 ± 20, 460 ± 30, and 1310 ± 100 cal. yr BP which are coincident with the limited 14C age control and with droughts inferred from the tree-ring or paleolimnologic record. One of the thickest (>4 m) eolian sand deposits in western Nebraska, exhibiting clear cross-stratification indicative of dune movement is dated by OSL-SAR to the late 16th century which is coincident with a well recognized and widespread North American “megadrought” in the tree-ring record. SAR-OSL ages also place deposition of a cover sand in the mid-19th century associated with a persistent drought identified in dendroclimatic records from the Rocky Mountains and western Great Plains. There is also compelling geomorphic and stratigraphic evidence for accumulation of a local cover sand during the 1930’s drought, and is confirmed by aerial photography from 1939 that shows abundant blows, incipient parabolic dunes and dried lakes and wetlands.