LATE PLEISTOCENE (≥MIS 3) ENVIRONMENTAL CHANGE AND PEDOGENESIS IN THE LOESS MANTLE OF THE CENTRAL GREAT PLAINS

Due to recurrent intervals of landscape stability, loess sequences of the Central Great Plains exhibit an array of intercalated paleosols. In recent years, research has focused largely on the younger part of this sequence (MIS 1 & 2). In this study, loess-paleosol sequences coeval with MIS 3 and earlier have been examined using stable isotope, rock magnetic and other parameters. Five sites were investigated: Buzzard's Roost, Eustis ash pit, and Hueftle borrow pit in southwestern Nebraska, Elba canal cut in central Nebraska, and Barton County landfill in central Kansas. Pedostratigraphic units include the soil complex of the Middle Wisconsinan Gilman Canyon Formation (MIS 3), Late Illinoian Sangamon soil complex (MIS 5), three or more paleosols within the lower Illinoian loess (Loveland Formation), and pre-Illinoian paleosols. At two localities, the sequence is underlain by Lava Creek B tephra (640ka). The Gilman Canyon Formation typically exhibits two to three welded soils, with the lowermost soil being best developed. At some localities, A horizons of the Gilman Canyon soil complex differentiate, such as at the Hueftle borrow pit. The Sangamon soil is usually expressed as one and sometimes two well-weathered Bt horizons; this window of soil development spanned a long period (c. 130-75ka) and likely reflects the complexity of MIS 5. What is believed to be the lower Loveland Formation contains a series of paleosols, four of which occur regionally as Bt and Btk horizons. A pronounced shift in magnetic susceptibility signals what appears to be the base of the Loveland Formation. Pre-Loveland pedogenesis is best exposed at the Elba canal cut (e.g., Walnut Creek soil?). In most instances, stable carbon isotope data indicate periods of soil formation are characterized by marked increases in C4 grasses. Opal phytolith data confirm the increase in C4 grass contribution during periods of pedogenesis.