PALEOSOLS WITHIN THE PRE-ILLINOIAN TILL SEQUENCE IN NORTHERN MISSOURI RECORD CHANGES IN PLEISTOCENE CLIMATE

Exposures in northern Missouri preserve the Sangamon Geosol developed in Loveland Silt above the Yarmouth Geosol in till. Reasons for the distinctive red coloration within these two paleosols (temperature vs. time) have been debated for many years, but previous workers did not have a sequence of older paleosols for comparison. Here, four pre-Yarmouth paleosols are also preserved locally within the Pre-Illinoian till sequence, and cosmogenic isotope dates for these tills also provide the age of the soils along with the duration of soil development. Generally, the older paleosols formed over a much longer time than the younger. For example, the oldest till (Atlanta Formation) was deposited at ~2.4 Ma and was buried by the overlying Moberly Formation at ~1.3 Ma. Thus, the Atlanta paleosol developed over ~1.1 myr, compared to no more than ~0.1 myr for the Sangamon.

The oldest paleosols rarely developed hues redder than 10YR, but rubification increases within the younger sola, despite shorter durations of weathering, culminating in bright red hues near 5YR within the Yarmouth and Sangamon Geosols. Thus, these paleosols record increasing interglacial temperatures, beginning in the Middle Pleistocene.

Caliche is nearly always present within the older weathering profiles, but is absent within the modern and Sangamon soils in this area. Thus, the older paleosols developed under drier conditions than the more recent climates. The depth to the top of caliche also increases systematically with younger age, recording increases in mean annual precipitation during the Early and Middle Pleistocene. In summary the paleosol sequence in northern Missouri records a trend toward warmer and moister climates during the Pleistocene.