Aggradational Pedogenesis: A New Model to Describe Soil Profile Development in the Central Great Plains of North America.
Peter Jacobs, University of Wisconsin - Whitewater, Department of Geography and Geology, 800 W. Main St., Whitewater, WI 53190-1790 and Joseph A. Mason, University of Wisconsin - Madison, Department of Geography, 550 North Park St., Madison, WI 53706.
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, these distal profiles 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, organic matter loss, and overprinting with prismatic structure. The overlying modern A horizon represents Bignell Loess. This model is supported by stratigraphic, chemical, and mineralogical evidence. Properties of the Brady Soil at proximal sites, including a distinctive burrowed zone in the AB horizon, relatively high clay content, high Ti/Zr ratio, and low volcanic glass content can be traced to the B horizon in distal soils. Furthermore, the depth to these indicators varies systematically across the study area, consistent with downwind thinning of an overlying loess deposit. Within the Brady Soil and distal soil B horizons, clay content, glass content, and Ti/Zr ratios also vary systematically from the Brady Soil to B horizons, suggesting a sedimentary origin. A 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. Bignell Loess at proximal sites is several meters thick and contains multiple buried soils that at distances within 10 km become compressed into a pedocomplex of multiple A (and sometimes AB or Bw) horizons above the Brady Soil, and then at distances >10 km the soil profiles appear monogenetic with thick dark A (and AB) horizons over a clay-rich “Bt” horizon. Many authors have noted little to no illuvial clay in the clay-rich Bt horizons. Illuvial clay does occur in these soils, but excepting the easternmost sites, all illuvial clay occurs beneath secondary pedogenic carbonate and is stratigraphically associated with the lower Brady Soil (lower clayey Bt horizons), indicating that clay illuviation is not an active pedogenic process over much of this region. Secondary calcium carbonate was emplaced during the Holocene in the Brady Soil and overlying horizons formed in Bignell Loess, and its depth is statistically predictable with modern climate data. We propose that the upward transition from largely unaltered C horizon loess to a clay-rich B horizon, overlain by a thick A horizon with lower clay content, is primarily a reflection of changes in the accumulation rate and grain size of atmospheric dust over the late Pleistocene and Holocene. Post- and syndepositional pedogenesis does have a significant effect on soil morphology in this setting, but the overall soil horizon sequence is more a stratigraphic record of climatically driven dust influx, than a product of pedogenesis as traditionally defined. We conclude that upland soils in the central Great Plains are composite soils with properties that are the result of a pedosedimentary history linked to regional climate change since the late Pleistocene. We propose the term aggradational pedogenesis to emphasize the active roles played by both sedimentation and pedogenic processes in the upward growth of soil profiles in aggrading landscapes. Soils formed by aggradational pedogenesis form part of a continuum, from sedimentation too rapid for any significant pedogenesis, to topdown pedogenesis beneath a completely stable land surface.