AGGRADATIONAL PEDOGENESIS: THE UPLAND LANDSCAPE RESPONSE TO CLIMATE CHANGE

Research on soil genesis often assumes a “top-down” model, in which the soil profile 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, source-proximal late Quaternary loess sections contain the Brady Soil, a prominent marker separating late Pleistocene Peoria Loess from Holocene Bignell Loess. Farther from dust sources, the Brady Soil and Bignell Loess are not recognizable in the field. 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 monogenetic profile 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, transformed by burial and organic matter loss. The overlying modern A horizon represents Bignell Loess. Properties of the Brady Soil at proximal sites (high clay content and Ti/Zr, low volcanic glass content, and a distinctive burrowed zone) can be traced to the B horizon in distal soils. An abrupt upward decrease in smectite abundance above the Brady Soil at proximal sites is identifiable at the top of the clay-rich B horizon in distal soils. In this case, the Brady Soil is the key to interpretation of late Quaternary loess stratigraphy, but the loess stratigraphy is also the key to interpreting the soil landscape.