BIMODAL GRAIN SIZE OF LOESS IN THE CENTRAL GREAT PLAINS AND THE ENVIRONMENT OF THE LAST GLACIAL-INTERGLACIAL TRANSITION

The Peoria Loess and Bignell Loess of the central Great Plains display bimodality in particle size distributions, with a coarse peak of medium silt to very fine sand and a shoulder or secondary peak of fine silt. In most of the Peoria Loess and more rapidly deposited parts of the Bignell Loess, the coarse mode is dominant and the fine mode is minor. The fine mode becomes much more prominent in the uppermost Peoria Loess (~15-14 ka) and particularly in the upper part of the Brady Soil, which formed ~14-10 ka, and it also increases to a lesser extent in Holocene paleosols. Interpreting these changes in bimodality as a response to environmental change is an important but challenging problem. In the semi-arid western Great Plains, development of the fine mode through mineral weathering is unlikely. Mason et al. (2003, Catena 53:377-397) documented evidence for sedimentary aggregates in Peoria Loess, accounting for at least part of the fine mode. On the other hand, the lamination found in most thick Peoria Loess of western Nebraska appears to record alternating sedimentation of relatively coarse and much finer grains, at a frequency that could be annual, providing evidence that grain-size bimodality originates at least partially because of bimodality in transport conditions. Physical models of dust transport suggest such large grain-size variations are more likely to reflect changing transport distance than changes in wind strength alone. Therefore, a viable explanation for the increased fine dust deposition during the glacial-interglacial transition is that the coarse mode represents dust last entrained in active dune fields, 10s of km upwind of the thickest Peoria Loess and Bignell Loess, and those dune fields were increasingly stabilized after 15 ka. As dust emission from the local source area decreased—but probably never ceased altogether—the influx of finer dust continued, from drier, colder, less vegetated, and possibly windier environments 100s of km farther upwind. The relative prominence of the Brady Soil may in part reflect this ongoing influx of dust, dominated by fine silt, which favored organic matter accumulation and preservation. After Brady Soil formation, reactivation of proximal dune fields and possibly wind erosion of exposed Peoria made the coarse mode dominant in the Bignell Loess.