9709742 Loope The Nebraska Sand Hills, constituting the largest sand sea in the western hemisphere, are presently stabilized by prairie vegetation. There are several lines of evidence that the dune field has been active many times during the Holocene, but it has been difficult to differentiate climatic from non-climatic "triggers", and a high-resolution record of dune activity has not been previously located. In the central Sand Hills, peat-accumulating wetlands fed almost exclusively by ground-water are present between elongate dunes that reach over 100 m in height. The basal peat, which typically lies at a depth of 5 to 6 m and contains abundant spruce pollen, is about 12,000 years old. Peat apparently began to accumulate after major eolian activity caused dunes to block pre-existing fluvial systems. Structureless sand beds about 10 to 60 cm thick are interbedded with the peat. Because the region is blanketed in dune sand, rivers do not flood. For this and several other reasons a fluvial origin for the sand beds can be ruled out. Our observations strongly favor an eolian origin for the sand beds. One of our lines of evidence involves the connection of the sand beds to convex-up sand ridges that are present near the center of some of the interdune valleys. The ridges lie parallel to the axes of the valleys and some are not yet completely buried by peat. We interpret the sand beds as eolian sand sheets and the sand ridges as sand drifts produced along the upwind margins of shrunken wetlands by the baffling action of dense vegetation. Saltating sand cannot penetrate dense vegetation, therefore delivery of eolian sand across the surfaces of interdune peatlands would require a major ecological change, presumably the result of a significant decline in the regional water table. Preliminary work indicates that the thin sand beds in the upper 2 m of the peat are laterally extensive and less than 2000 years old. These shallow sands are present in at least 27 interdune valleys. The a bsence of Dust Bowl sand sheets on the surface of these wetlands indicates that droughts of greater persistence and severity than any in recorded history would be required to drop water table levels for emplacement of the late Holocene sand sheets. Paleoclimatological studies of Minnesota lakes have recently revealed strong evidence for a series of droughts ending just prior to 1200 AD. These drought intervals have been hypothesized to correlate with western US droughts that were revealed by studies of tree rings and lake fluctuations. Synchronous drought across such an area would require a persistent upper-level blocking ridge of tremendous size. Because of the central location of the Sand Hills and the abundance of datable material, the peatlands described here provide an excellent opportunity to test this idea. We are eager to compare the paleoclimatic record of the peatlands with the record based on buried paleosols exposed in blowouts high in the adjacent dunes. With the limited data we have from the buried upland soils, we are unsure whether they record regional climatic fluctuations or more local environmental changes.
