Reconstruction of paleoclimatic conditions and timing of the last dune reactivation in the Nebraska Sand Hills

Dune fields across the Great Plains are currently stabilized, but have a history of recurring mobility throughout the past few thousand years. A detailed understanding of the conditions and timing of the most recent dune reactivation (during the Medieval Climatic Anomaly, from 900 to 1300 AD) is important given the possibility for future reactivations. Wind patterns, loss and return of vegetative cover to the dunes, and timing of the last reactivation were all examined. Paleowinds from dune crest orientations and internal structures were compared with modern meteorological data. Strong northwesterly winds are recognized from both periods. A major shift in secondary winds is recognized with dry southwesterly winds present during the last dune reactivation and moist south to southeasterly winds present today. Pocket gopher burrows were studied across the Nebraska Sand Hills to determine the pattern of vegetative loss during the Medieval Climatic Anomaly. Most of these burrows occur in areas of low-angle cross-bedded strata, suggesting vegetative loss occurred first on the crests of the newly formed dunes, while vegetation persisted in the interdunes. Pocket gophers likely survived initially in areas of remaining vegetation and subsequently on roots in the recently buried soil. Ultimately, the gophers were unable to reach their food source and they were forced to abandon the uplands. The timing of the last dune reactivation in the Nebraska Sand Hills was studied using optically stimulated luminescence dating. A spatial trend occurs across the area with older ages (∼850 to 1,000 years) in the northwestern Sand Hills, and younger ages (∼500 to 600 years) in the southeast. These dates indicate the spread of vegetation back onto the dunes at the end of the last major drought. An additional spatial trend in ages is recognized in linear dunes across the Sand Hills with linear dunes in the northwest originating during the Medieval Climatic Anomaly (∼800 years) and those in the southeast built up around older cores thousands of years older