Landscapes across the Midwestern United States are at least partially mantled by loess, wind-blown dust deposited during the Late Pleistocene and then extensively redistributed by hillslope erosion. Many observations suggest that the presence of a thin loess mantle can increase the rates of soil-forming processes and influence ecologically important properties such as nutrient or water retention. This research project will investigate the relative importance of a loess mantle in the evolution of soil catenas, which are predictable patterns of soil variation down hillslopes. At one extreme in the range of possible interpretations, the local pattern of loess thickness may have been determined in the Late Pleistocene, as dust was deposited and rapidly redistributed downslope. Since that time, the loess mantle has acted as a critically important but largely static control on soil formation. On the other hand, biological processes, such as mixing and sorting of soil materials by burrowing animals, may augment or even replace the role played by a loess mantle in determining the present spatial pattern of soil properties. This interpretation implies a much more dynamic history of catena development, as biomechanical processes progressively move soil downslope and generate a fine-grained surface mantle that is conducive to rapid soil development. In this project, the investigators will conduct analyses of particle size and mineralogy to estimate the actual amount of loess present in soil catenas of study areas in Minnesota and Wisconsin. Where loess is present, the timing of downslope redistribution will be interpreted from the sediments in natural "traps" at the foot of slopes. Several methods, including optically stimulated luminescence dating, will be used to characterize the extent of soil mixing. Finally, the relative influence of loess and biomechanical processes on ecologically important soil properties will be investigated in catenas of both study areas.
The results of this project will have broad implications for long- and short-term change in the soil landscapes of the Midwest and many other regions of the world where a loess mantle may influence catena evolution. If local loess thickness is a largely static control over soil-forming processes, then accelerated erosion that removes loess may irreversibly change the rates or directions of soil development, which in turn can alter plant productivity and suitability for many human land uses. This would be true of prehistoric erosion accelerated by climatic change or rapid erosion triggered by recent land use change and would clearly need to be taken into account in estimating tolerable rates of soil erosion. On the other hand, if biomechanical processes play a major role in enhancing soil formation, then the soil landscape can ultimately be renewed over the long run, even after severe erosion. These implications of this research will be presented to the scientific community and through public outreach in the study areas, where there is a keen and growing interest in sustainable land use. Both graduate and undergraduate students will participate in the project, learning many of the most important techniques of research on soil genesis and geomorphology.
