Late Quaternary climate change, loess sedimentation, and soil profile development in the central Great Plains: A pedosedimentary model

Abstract

Research on soil genesis often assumes a “top-down” model, in which the soil profi le develops downward from a stable land surface. This model is inapplicable to upland landscapes affected by frequent dust deposition, where soils grow upward as they develop. On the central Great Plains, late Quaternary loess sections proximal to immediate source areas contain the Brady Soil, a prominent marker separating late Pleistocene Peoria Loess from Holocene Bignell Loess. Farther from immediate dust source areas, the Brady Soil and Bignell Loess are not recognizable in the fi eld. On loess tablelands in these distal regions, surface soils typically contain a prominent, clay-rich B horizon below a thick silty A horizon. Assuming top-down pedogenesis, this could be interpreted as a postglacial soil profi le formed in Peoria Loess, with the B horizon produced by weathering and clay illuviation. We propose a strikingly different interpretation, in which the upper B horizon at distal sites is the Brady Soil A horizon that has been transformed by burial, organic matter loss, and modern subsoil structure formation processes. The overlying modern A horizon represents Bignell Loess. Properties of the Brady Soil at proximal sites (a distinctive burrowed zone, high clay content and TiO 2 /ZrO 2 , and low volcanic glass content) can be traced to the B horizon in distal soils. A decrease in smectite abundance above the Brady Soil at proximal sites is identifi able at the top of the clay-rich B horizon in distal soils. The spatial variation of clay content in loess and soil horizons is best explained by eolian sedimentation patterns. The higher clay content in the Brady Soil and distal B horizons defi nes a fi ne-grained zone that represents a late phase of Peoria Loess accumulation. Evidence of chemical weathering is minimal, and illuvial clay is rare to absent in the clay-rich B horizons. Illuvial clay does often occur deep in the solum and is related to the depth below the top of the Brady Soil. The depth of occurrence of illuvial clay is not related to modern climate parameters, although depth to secondary carbonate appears to be in equilibrium with modern climate. Upland soils in the central Great Plains are composite soils; their properties are the result of a pedosedimentary history linked to regional climate change that has infl uenced sedimentation and pedogenesis since the late Pleistocene.