REGIONAL ARIDITY IN NORTH-AMERICA DURING THE MIDDLE HOLOCENE

Increased aridity throughout the Great Plains and Rocky Mountain regio n during the middle Holocene has been documented from pollen records, aeolian proxy variables in lake cores, and active sand dune migration. Varve calibration provided by a continuously varved record of the Hol ocene from a core from Elk Lake, northwestern Minnesota, shows that th e influx of aeolian elastic material increased beginning about 8 ka an d ended about 3.8 ka, with peak aeolian activity at about 6 ka. if aeo lian influx to Elk Lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in Minnesota dune fields, then the varve calibration in Elk Lake provides precise time calibration of pe riods of peak aeolian activity in Minnesota. Palaeowind studies from t he Minnesota dune fields show that the dominant wind direction when th e dunes were active was from the northwest, the same as the dominant w ind direction in dune fields throughout the Great Plains and Rocky Mou ntains. if the mid-Holocene aeolian activity in Minnesota was driven b y an increase in westerly zonal winds, then the varve calibration can be extended to more precisely determine the timing of activity of dune s over a much broader area. We suggest that an increase in the westerl y zonal wind field might have a solar-geomagnetic cause.