MIS 3 VIEWED THROUGH PEDOGENESIS ON THE LOESS-MANTLED UPLANDS OF THE CENTRAL GREAT PLAINS

In the central Great Plains, MIS 3 is represented by the Gilman Canyon Formation, a loess unit heavily overprinted by pedogenesis. The formation has been recognized from Nebraska to Oklahoma and from Colorado through Kansas, with temporal equivalents to the east. Thickness is typically about 1 m, but can range up to several meters adjacent to major river valleys (e.g., Platte, Missouri). Over 250 numerical ages derived from about 115 localities provide relatively good chronological control for the formation, particularly the soil-dominated interval. Following termination of the Sangamon Soil (Last Interglacial), Gilman Canyon loess began to accumulate as early as 55 ka, at an average mass accumulation rate of about 2500 g/mm/yr. Pedogenesis began between ~42 ka and 38 ka and is expressed typically as two to three welded soils. At locations where loess accumulation was extraordinarily high or where pedogenesis was occurring on a slope, two or more of these episodes are separated by loess intervals. Terminal pedogenesis dates to about 25 ka, and shortly thereafter the soil was inundated by the Late Wisconsinan Peoria Loess. Existing age data suggest that the history of the formation is temporally (but not necessarily causally) correlated to global and North Atlantic events, in that initial loess accumulation began around the start of MIS 3 (~59 ka) as an upward extension of the Sangamon Soil (no erosional hiatus). Further, the lower and dominant soil forming interval ended about 31 ka, around Heinrich Event 3; the second and final intervals of soil formation occurred 28-26 ka, between Heinrich Events 3 and 2. The lower soil also coincides with the 30^oN insolation peak. Stable C isotope analyses of samples from multiple sites, including the type locality (southwestern NE), indicate a rapid shift from C₃ dominance in the loess to C₄ dominance in the lower soil, but gradually shifts back to C₃ dominance in subsequent soils to a level characteristic of the Last Glacial Maximum Peoria Loess; phytolith and other biosilicates data corroborate the isotopic signal.