HR: 1340h
AN: PP33B-1576    [Abstracts]
TI: Early Holocene Change in Atmospheric Circulation in the North-Central USA
AU: * Dean, W E
EM: dean@usgs.gov
AF: USGS, MS980 Federal Center, Denver, CO 80225
AB: Numerous proxies in cores from Elk Lake, northwestern Minnesota, have provided a record of climatic and environmental change with annual resolution for the last 10,000 years. The proxies that allow reconstruction of the lake's physical and chemical paleolimnology (diatoms, redox-sensitive trace metals, and 18O values) show that that prior to about 8.2 cal ka the lake was a stable, dimictic lake that was strongly stratified. The same proxies show that after 8.2 cal. ka the lake was turbulent, well-mixed and shallower. The proxies that are related to climate factors external to the lake (dust as % Al and % Si, varve thickness, and pollen) show that prior to 8.2 cal. ka the lake was receiving relatively little dust, implying little wind activity. After 8.2 cal ka, there was a marked increase in the influx of dust indicating an increase in westerly winds. Lastly, the ostracode faunal assemblages, which provide information about the limnology and watershed characteristics, indicate that, for 1000 years prior to 8.2 cal. ka, the lake was stable and dilute with characteristics typical of lakes in boreal forests. At 8.2 cal. ka, the ostracode assemblage abruptly shifted to an assemblage typical of Canadian prairie lakes that exhibit large seasonal variability in physical characteristics. This marks the northward displacement of the polar front and beginning of westerlies. The Elk Lake record further shows that the so-called 8.2 cal. yr cold event, recognized in ice-core and other records from the circum-North Atlantic, and thought by some to be caused by catastrophic drainage of freshwater from proglacial lakes Agassiz and Ojibway, was but a brief manifestation of a more fundamental and lasting change in the climate of North America. This fundamental climate change was the result of changes in atmospheric circulation in response to marked changes in the relative proportions of land, water, and, especially, glacial ice in North America during the early Holocene, the beginning of the altithermal or prairie period in Minnesota. Substantial changes in salinity also occurred at many lacustrine sites in the Northern Great Plains around 8.2 cal. ka so that such changes are not unique to Elk Lake, and thus the driver of these changes must be regional or global in extent. Bear Lake, Utah and Idaho, is a mesosaline-alkaline lake that historically was bipassed by the Bear River to the east of the lake. However, during the last glacial interval the lake and river were connected until about 16 cal. ka. As the salinity of the lake increased without Bear River influx, carbonate began to precipitate, first as calcite and then as aragonite, the dominant carbonate mineral deposited during most of the Holocene. C- O- and Sr-isotope data indicate that at about 9.5 cal ka the lake abruptly freshened suggesting that Bear River was again reconnected to the lake. The cause of the reconnection of Bear River with Bear Lake may have been tectonic, geomorphic, or climatic. However, the dominance of a boreal ostracode assemblage prior to 8.2 cal ka in Elk Lake, indicating that the polar front was far south of its present position, may have permitted the temporary establishment of a polar low over Utah, Idaho, and Wyoming in winter, increasing the snow pack in the Uinta Mountains and the Bear River Range to the west of the lake thereby increasing the surface- and ground-water flow to the lake. By 8.5 cal ka the salinity of the lake had decreased sufficiently to trigger another pulse of calcite precipitation that lasted about 1000 years. At 8.2 cal ka, when the Elk Lake ostracode record indicates that the polar front had retreated to the north, the Bear River was disconnected from the lake, and by 7.5 cal ka aragonite was again forming.
DE: 0458 Limnology (1845, 4239, 4942)
DE: 1030 Geochemical cycles (0330)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1620 Climate dynamics (0429, 3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005