LOESS SECTIONS DOCUMENT DRY CLIMATE AND EOLIAN ACTIVITY IN THE EARLY TO MID-HOLOCENE, CENTRAL GREAT PLAINS

It has been supposed that the climate in the central Great Plains during the early to mid-Holocene was characterized by a dry climate associated with extensive eolian activity. Little evidence has been available to support this hypothesis, however, because mid-Holocene eolian deposits have seldom been identified within dune fields of the central Plains, and other paleoclimatic records are rare. Here we report new data from Holocene Bignell Loess sections immediately downwind from dune fields in western Nebraska, which document the history of eolian activity throughout the Holocene. Optical stimulated luminescence (OSL) dating, especially the newly developed SAR technique, produces reliable ages from Nebraska loess. At the Wauneta and Logan Road Cut exposures, Bignell loess is as thick as 3-6 meters. The basal Bignell Loess is a transition zone immediately above the prominent Brady Soil. Above the transition zone, light colored loess represents rapid and continuous eolian deposition, with a preliminary age estimate of 9,000 yr B.P. to 6,000 yr B.P. from OSL dating. This probably represents the driest part of the Holocene in the central Great Plains. Previous studies of paleolimnological records in the northern Great Plains are in good agreement with these eolian records from the central Plains. For example, at Moon Lake in North Dakota, salinity, indicated by diatoms, was the highest during early to mid-Holocene, suggesting the low effective moisture at that time.

At the same times OSL ages from these sections, together with stratigraphic and soil geomorphic evidence, demonstrate eolian sand activity and loess deposition in the late Holocene, including significant loess accumulation after 2500 yr BP. The late Holocene experienced rapid variation of climate, because the upper part of the sections contains multiple depositional increments separated by weak buried soils. In addition, OSL results indicate that eolian deposition was over twice as rapid during the early to mid-Holocene as during late Holocene.