STABLE ISOTOPES AND SEDIMENTS FROM PICKEREL LAKE, SOUTH-DAKOTA, USA -A 12KY RECORD OF ENVIRONMENTAL-CHANGES

Sedimentological parameters and stable O- and C-isotopic composition o f marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest(ca. 12ky) paleoenvironmental records from the north ern Great Plains. The late Glacial to early Holocene climate in the no rthern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic chan ges were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that t he cold and dry phase may correspond to the Younger Dryas event. A sal inity maximum was reached between 10.3 and 9.5 ka, after which Pickere l Lake shifted from a system controlled by atmospheric changes to a sy stem controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was es tablished at approximately 8.7 ka, and lasted until about 2.2 ka. Duri ng this mid-Holocene prairie period, drier conditions than today preva iled, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than d ry and warm. The last 2.2 ka are characterized by higher climatic vari ability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age. Although the signal of changing atmospheric ci rculation is overprinted by fluctuations in the positions of the ice s heet and glacial Lake Agassiz during the late Glacial-Holocene transit ion, a combination of strong zonal circulation and strong monsoons ind uced by the presence of the ice sheet and high insolation may have pro vided mechanisms for increased precipitation. Zonal flow introducing d ry Pacific air became more important during the prairie period but see ms to have been interrupted by short periods of stronger meridional ci rculation with intrusions of moist air from the Gulf of Mexico. More f requent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the l ast 2.2 ka.