AEOLIAN PROCESSES LINKING DUNEFIELDS AND LOESS IN THE CENTRAL GREAT PLAINS, USA

The central Great Plains contain North America’s largest dunefield, Late Pleistocene loess deposited at extraordinary rates, and a detailed stratigraphic record of episodic Holocene loess accumulation. Major regional episodes of dune activity and loess accumulation—identified by OSL age clusters, bounding paleosols, and distinctive dune forms—were approximately synchronous. This in part reflects the fact that both systems responded to similar climatic changes. However, I have proposed a conceptual model in which dune activity and loess accumulation are also linked by aeolian processes, implying that dune activity may directly influence downwind loess accumulation. The dunefields are immediately upwind of the loess, and when active they acted as surfaces of transport across which dust from more distant sources—indicated by provenance studies—was conveyed by repeated resuspension, facilitated by saltation. The sand-loess transition coincides with potential barriers to dune migration, such as stream valleys. In the absence of sand transport beyond those barriers, thick loess accumulated. The highly irregular form of the abrupt sand-loess boundary, jumping northwest or southeast between stream valleys, is compelling evidence for the model. In this talk I will summarize other geomorphic evidence, describe new data that support the model (though with modifications), and consider alternatives such as loess formation through abrasion of dune sand. Portable In-Situ Wind Erosion Laboratory (PI-SWERL) experiments show that when dune sand is mobilized, significant dust emission occurs quickly (Sweeney and Mason, in press, JGR: Earth Surf.). While the fraction of dust-sized material in the sand is quite small, it is much more readily mobilized than other reservoirs of fine material such as crusted loess or lake sediment. Thus, it appears that active dunefields can act as effective downwind conveyers of loess, quickly remobilizing dust deposited on the dunes. The loess is not inherently stable in the absence of saltating sand, however; without a crust or vegetation cover it is susceptible to rapid wind erosion, and large-scale wind erosion contributed to the irregularity of the sand-loess border. Both the rapid rates of loess accumulation and provenance data cast doubt on abrasion as major source of the loess.