MASS ACCUMULATION RATES AND COMPETING LOESS SOURCES DURING THE LAST GLACIAL PERIOD IN THE MISSOURI RIVER BASIN, NORTH AMERICAN MIDCONTINENT

Previous geochemical, sedimentological and isotopic studies of last-glacial loess (Peoria Loess) in the North American Midcontinent demonstrate that loess was derived from both glacial (valley train) and nonglacial sources during the last glacial period. A high-resolution chronology for Peoria Loess of the Missouri River Basin, constructed by applying optically stimulated luminescence (OSL) dating to 35-50ìm quartz, yields insights into the timing and magnitude of dust flux from these two sources. Glacial-source dust from the Missouri River Valley, which drained the southwestern margin of the Laurentide Ice Sheet, began to accumulate approximately 23,500 yr B.P. Dust originating from nonglacial sources in the northwestern part of the Missouri River Basin began to accumulate at a later date between 20,000 and 19,000 yr B.P., and dominated over glacial-source dust throughout the basin until last-glacial loess accumulation ended shortly after 14,000 yr B.P. Mass accumulation rates (MARs) for Missouri River Basin last-glacial loess calculated using the OSL ages are extremely high from 18,000 to 14,000 yr B.P. – surpassed worldwide only by those from localities located to the west that are proximal to the nonglacial loess source areas. This period of extreme MARs coincides with a late-glacial period when paleoenvironmental proxies indicate significantly cooler conditions than predicted by paleoclimate simulations from atmospheric global circulation models. We suggest that radiative forcing resulting from the high atmospheric dust loading implied by these MARs may explain the mismatch between paleoenvironmental proxies and modeled paleoclimate during the late-glacial period in central North America.