EOLIAN RECORDS OF LATE QUATERNARY ENVIRONMENTAL CHANGE IN NORTHERN NORTH AMERICA

Eolian deposits of known age indicate local and regional surface air circulation patterns and how these change through time. They also indicate periods of erosion and transport caused by changes in wind regimes, vegetation cover or sediment input related to climatic, biotic or geologic effects. Approximately 1300 radiocarbon, optical and tephra ages on dunes and loess plotted on published biome-maps spanning last glacial maximum to modern times, show how circulation changed in response to changing ice sheet extent and how former climates affected eolian activity. Within glaciated areas, most postglacial dune fields overlie or extend downwind from proglacial deposits. In these areas, the former positions of ice sheets, proglacial lakes and rivers define maximum limiting ages of dune fields. The small size of these dune fields relates to short-lived episodes of glacio-fluvial source-sediment deposition, and brief postglacial periods of eolian activity. During full glacial conditions, northern North American dunes and loess formed beyond the ice sheets, under a dominant west-to-east wind regime south of the Laurentide Ice Sheet (LIS). Within glaciated regions, dunes formed soon after deglaciation, predominantly under ice-proximal (tundra) settings. A glacial anticylone developed between full glacial conditions and 8.0 ka BP (9.0 cal ka), but was possibly partially suppressed by localized winds along the margins of the Cordilleran and Laurentide ice sheets prior to 12 k BP (14.1 cal ka). Anticylonic winds were well-expressed thereafter, with the Cordilleran and Laurentide ice sheets well-separated and the LIS residing east of the influence of the Pacific air mass. Dune activity continued in northwestern Canada under parkland settings during the Younger Dryas interval, when dunes were stable and the Brady Soil formed in the US Great Plains. Most dunes in northwestern Canada stabilized, thereafter, from reduced wind strength and increased vegetation cover. The LIS suppressed early Holocene warming and dune re-activation in western Canada, when dune and loess activity, including deposition of Bignell Loess, occurred in the grasslands of central United States. Late Holocene dune activity may be closely synchronized with droughts across the Great Plains, particularly at 6.6, 2.8, 1.5 and 0.8 cal ka.