LATE PLEISTOCENE DUNE ACTIVITY IN THE CENTRAL GREAT PLAINS

Multiple episodes of Holocene dune activity in the Nebraska Sand Hills and other central Great Plains dunefields are now well-documented. Published evidence for Late Pleistocene dune mobility in this region remains limited, however, despite early interpretations of the Sand Hills as Pleistocene relicts. Assuming that preservation of Pleistocene sands would be best within large, slowly migrating dunes, we collected drill cores from that setting across the Sand Hills and nearby dunefields. All but one of these cores contained at least some Late Pleistocene sand, with optically stimulated luminescence (OSL) ages between 19 and 14 ka. Several of the dunes we sampled clearly contain a substantial volume of Late Pleistocene sand, despite extensive Holocene reactivation. Eolian sands interbedded with Pleistocene loess south of the Sand Hills yield ages of 20-25 ka. Thus, dune activity was extensive during and shortly after the last glacial maximum (LGM) on the central Great Plains, at about the same time that extraordinarily rapid Peoria Loess deposition also occurred in this area. The large barchanoid ridges and megabarchans of the Sand Hills may have been initially constructed by northerly to westerly winds during the late Pleistocene, though with substantial modification and development of superimposed dune forms in the Holocene. Although much attention has been focused on the possibility of easterly surface winds associated with anticyclonic circulation over the Laurentide Ice Sheet, many numerical models of LGM and late-glacial climates actually indicate prevalence of westerly to northerly winter and spring winds on the Great Plains south of the ice sheet margin, consistent with large Sand Hills dune forms and loess transport directions inferred from thickness and grain size trends in the Peoria Loess. These climate simulations also indicate reduced precipitation in the central Plains under glacial conditions. This effect is offset by reduced evaporation in much cooler summers, however, and reduced effective moisture alone is unlikely to provide an adequate explanation for the magnitude of Late Pleistocene dune activity and dust production. Effects of shorter and colder growing seasons and enhanced sediment supply must also be considered as controls on Late Pleistocene eolian activity.