2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 10
Presentation Time: 1:30 PM-5:30 PM


TIAN, Jian, Geology, Univ of Illinois, 265 Morrill Hall, Plant Biology, University of Illinois, 505 S. Goodwin Ave, Urbana, IL 61801, NELSON, David, Program in Ecology and Evolutionary Biology, Univ of Illinois, 265 Morrill Hall, Plant Biology, University of Illinois, 505 S. Goodwin Ave, Urbana, IL 61801, HU, Feng Sheng, Univ Illinois - Urbana-Champaign, 505 S Goodwin Ave, Urbana, IL 61801-3707, BROWN, Thomas A., Ctr for Accelerator Mass Spectrometry, LLNL L-397, PO BOX 808, 7000 East Avenue, Livermore, CA 94551, STEFANOVA, Ivanka, Geology & Geophysics, Univ of Minnisota, 1426 Hythe St, St Paul, MN 55108 and WRIGHT, Herbert E., Jr, Univ Minnesota - Twin Cities, 310 Pillsbury Dr SE, Minneapolis, MN 55455-0219, jiantian@uiuc.edu

We analyzed sediment cores from Steel Lake (SL, 46º 58’ N, 94º 41’ W) and Lake Olaf (LO, 46º 37’ N, 96º 11’ W), located in the prairie-forest ecotone of Minnesota, for d18O and d13C of bulk carbonate, d13C of charcoal, mineral composition, charcoal abundance, and pollen assemblages. These data provide new information on Holocene climatic changes in the mid-continent of North America. The SL chronology is based on 26 AMS 14C dates of terrestrial plant macrofossils, and the LO chronology is established with 8 AMS 14C dates of charcoal flakes. An abrupt transition to warm/dry climatic conditions of the middle Holocene occurred at 7.8 cal ka, as indicated by increases in d18O (-9.8‰ to -7.3‰, SL), quartz content (10% to 43%, SL), aragonite/calcite ratio (0.7 to 1.4, LO), and non-arboreal pollen abundance (34% to 56%, SL). This transition probably resulted from a major reorganization in global atmospheric circulation, but it appeared to postdate the 8.2 ka event. Within the middle Holocene, our isotopic, mineralogical, and biological data exhibit two distinct climatic periods with warmer/drier conditions between 7.8 and 5.8 cal ka than between 5.8 and 3.5 cal ka. Superimposed on this pattern are millennial-scale shifts in drought intensity and prairie vegetation, as evidenced by oscillations in aragonite/calcite ratio, % Poaceae pollen, charcoal abundance, and C4-plant abundance. For example, the aragonite/calcite ratio of the LO sediments reaches peak values at 7.5 cal ka, 6.2 cal ka, and 5.0 cal ka, suggesting driest climatic conditions at these times, and minimal values at 6.8 cal ka and 5.5 cal ka, suggesting wetter phases of the middle Holocene. These oscillations have a ~1300-year pacing but do not appear to coincide with the ice-rafted-debris events in the North Atlantic.