PATTERNS OF HOLOCENE CLIMATIC-CHANGE IN THE MIDWESTERN UNITED-STATES

The Holocene pollen sequence in the Minnesota area is "asymmetric" around the so-called prairie period: the early Holocene is dominated by elm (with pine in the north) and the late Holocene by oak. The elm zone is interpreted as a manifestation of summer monsoonal rains enhanced by the Milankovitch insolation maximum, and the pine in the north is interpreted as a result of summer cooling near the retreating ice sheet. As the summer insolation waned during the Holocene, its associated monsoonal rains from the Caribbean moisture source lasted longer in the south (northeastern Iowa and southern Wisconsin), where the inferred mesic elm forest changed to prairie as late as 5000 yr B.P., compared to 8000 yr. B.P. in Minnesota (and 9000 yr B.P. in the Dakotas). © 1992.