LOESS STRATIGRAPHY OF NEBRASKA IN A GLOBAL CONTEXT

Loess sequences in northern China, Central Asia, and Central Europe are routinely correlated with global climatic records extracted from cores of marine sediment and glacier ice. In contrast, the loess stratigraphy of the North American Midcontinent has traditionally been interpreted in a more local context, often in relation to fluctuations of the Laurentide Ice Sheet. A substantial portion of the Midcontinent loess succession is now known to be nonglaciogenic, and its depositional and pedogenic patterns may well record continental- or global-scale climatic change. Our work on loess in Nebraska, however, demonstrates that correlation of loess stratigraphy with global climate-proxy records is not straightforward, even with the incorporation of a great deal of new stratigraphic information and age control. Intensive subsurface work on the Middle and Late Pleistocene loesses in eastern Nebraska clearly suggests an upward increase in the rate of loess accumulation (averaged over glacial-interglacial cycles), culminating in the extraordinarily thick and rapidly deposited Peoria Loess. No other major loess system shows such a large Late Quaternary increase in dust production and transport. In western Nebraska, new age control on the Late Pleistocene to Early Holocene loess succession requires a reappraisal of the paleoclimatic significance of the Brady Soil. This prominent buried soil formed during a regional interval of limited eolian activity that clearly includes both the Younger Dryas chronozone and subsequent rapid global warming. We know of no directly correlative pedostratigraphic unit in other loess systems.