LUMINESCENCE AND RADIOCARBON CHRONOLOGY OF LOESS-PALEOSOL SEQUENCES IN THE LOWER OHIO RIVER VALLEY

Seven samples from a 16-meter-thick section of loess and associated paleosols at the Bon Harbor Hills in western Kentucky were dated using conventional radiocarbon and luminescence methods. The Peoria Loess, which is nearly 10 m thick, contains banding that represents at least nine incipient soil (A) horizons; several discernable, weakly developed soils; prismatic calcareous concretions; and fossil terrestrial gastropods. Optically stimulated luminescence ages for samples collected at depths of 2.2 m, 2.6 m, 5.3 m, and 9.3 m in the Peoria Loess were 11.5 ± 0.7 ka, 12.2 ± 3.2 ka, 17.8 ± 3.4 ka, and 23.5 ± 1.3 ka, respectively. The Roxana Silt underlies the Peoria Loess and is approximately 0.75 m thick. Although there is no apparent soil separating the Peoria Loess from the Roxana Silt, soil organic matter from an organic-rich zone between them was dated at 25,800 ±140 yr BP (30.8 ± .3 ka), which corresponds to the Farmdale paleosol. The Sangamon paleosol, a well-developed interglacial soil more than 2 m thick, underlies the Roxana Silt and is developed in the underlying Loveland Loess. Samples collected from the base of the Loveland Loess were dated at 104 ± 11.4 ka using OSL, and 116 ±8.3 ka using thermoluminescence. These ages are too young for the Loveland Loess, which is approximately 160 ka. Since the Sangamon paleosol completely penetrates the Loveland Loess, however, these samples were slightly pedogenically altered, and the ages likely reflect the timing of Sangamon soil development. The Loveland Loess is underlain by thin, 0.3-m-thick colluvial gravel that was deposited on a pre-Loveland paleosol. The pre-Loveland paleosol is developed through a silt-rich deposit approximately 1 m thick and into the underlying regolith. This basal paleosol-silt sequence may represent the Yarmouth paleosol and Crowleys Ridge Loess, respectively.

Loess-paleosol stratigraphy and radiometric dating indicate that the Bon Harbor Hills loess section contains a continuous terrestrial record of environmental changes for the past 160 ka (oxygen isotope stages 1-6), and possibly extends back to 260 ka (oxygen isotope stage 8), making this section particularly useful as a proxy for middle to late Pleistocene paleoclimatic changes within the Ohio River Valley.