GEOMORPHOLOGY OF THE UPWIND BOUNDARY OF THICK LOESS, CENTRAL GREAT PLAINS, AND IMPLICATIONS FOR LONG-TERM SEDIMENT AND SOC STORAGE

In the Central Great Plains, USA, there are abrupt and often dramatic boundaries between dune fields or surfaces of aeolian transport with minimal loess cover and very thick loess deposits immediately downwind, often forming distinctive tablelands (a similar abrupt transition occurs at the northern edge of the Chinese Loess Plateau). While these spatial transitions between minimal and thick loess accumulation often appear to be persistent features, there is also abundant stratigraphic and geomorphic evidence for transitions over time between net accumulation and net erosion of loess—by wind or water—at given locations. We use examples from the Central Great Plains to illustrate the nature, controls, and significance of these temporal transitions, and implications for long-term storage of sediment in loess deposits as well as the accumulation and long-term storage of soil organic carbon (SOC).

In the Central Great Plains, there is evidence that greater moisture availability and vegetation density along stream valleys produced barriers to aeolian sand transport that allowed thick loess accumulation downwind. These must have been leaky barriers, as aeolian sand clearly migrated into thick loess areas at times, associated with truncation and/or burial of the loess and altering local soil hydrology and SOC storage where surficial sand now covers loess. While these sand incursions may have been initiated by climate change, their likelihood would have decreased as thick loess accumulated, forming windward facing scarps at the edge of high tablelands that were themselves barriers to sand transport. This developing topography also favored “clifftop” accumulation of coarse loess, especially in the Holocene, leading to significant SOC burial and preservation. While the timing of loess accumulation in this setting is closely linked to Holocene climatic change, a specific geomorphic context was necessary for it to occur. Localized deflation at the surface of thick loess produced closed basins and wind-aligned troughs. These deflation features then influenced local patterns of soil development and SOC storage. Landscape evolution modeling demonstrates how internal drainage associated with closed basins is essential to preservation of loess tablelands and the sediment and SOC stored there.