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Even Longer Records

Changes in Great Plains Landcover
Forman, S.L. and Oglesby, R.
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Summary:
The semi-arid Great Plains is covered by >100,000 km2 of eolian (wind blown) deposits which contain evidence for episodes of dune formation, or reactivation of old dunes, over the past 10,000 years. The region of multiple sand dunes and eolian deposits across the central United States (figure on this page) is currently stabilized by vegetation. However, analyses of sediments within sand dunes (stratigraphic records) and the shapes of the dunes (geomorphic records) provide information about multiple past droughts, which resulted in the removal of vegetation, and the movement and accumulation of eolian sand. Accumulations of eolian sediment and formation of dunes occurs when there is adequate sediment supply, when winds exceed the speed needed to move particles of sand, and when there is a lack of stabilizing vegetation or landforms. On the Great Plains, there is adequate sediment supply. Wind strength is already sufficient in much of the Great Plains to move silt and sand, and create widespread active dunes. When a decrease in moisture results in a reduction of vegetation cover below a threshold (~30% decrease in rainfall), dunes can once again become active.

The combination of integrated geomorphic and stratigraphic studies and advances in dating techniques has lead to an enhanced understanding of the timing and location of Holocene (the last 10,000 years) eolian activity on the Great Plains. For larger viewing image, please click image. The data indicate that relatively moist conditions, leading to landscape stability and soil development, were repeatedly interrupted by eolian deposition during periods of drought. Despite the uncertainties associated with the preservation of soil horizons (i.e., the mixing of sediments within a dune) or in correlating dune deposits across the Great Plains, there is regionally consistent evidence for activation of dune systems between 8,000 and 5,000 years ago. Although not as spatially coherent, evidence also indicates the occurrence of a number of drought-related dune formation events in the past 1500 years.

The figure to the right (click here or on the figure for a larger version) , from the work of Forman and Munyikwa, shows the spatial distribution and extent of stabilized dune fields (in brown) in the North American Great Plains. Superimposed on this map are 15 time series for the last 10,000 years with evidence for episodes of dry and moist conditions (LS denotes landscape stability, DR denotes dune reactivation. HW denotes high water level, LL denotes low water level).

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