Paleorecords from Minnesota and adjacent areas have often been used to evaluate large-scale climatic processes in the mid-continent of North America. However, most of these records are compromised by chronological flaws, making problematic any comparisons with climatic interpretations based on other records (e.g., GISP2 in Greenland). We report here a high-resolution pollen record with a secure chronology constrained by 26 14C dates on terrestrial macrofossils from Steel Lake, central Minnesota. About 11.2ka cal BP (11,000 calibrated yr before present) the late-glacial Picea forest near Steel Lake was succeeded abruptly by Pinus banksiana and/or resinosa. The Pinus forest began to open 9.4ka cal BP with the expansion of prairie taxa, and a pine parkland or savanna prevailed until about 8ka cal BP, when Quercus replaced Pinus to become the dominant tree in the prairie areas for 4500 yr. The close chronological control permits the correlation of key vegetational changes with those at other reliably dated sites in the eastern Dakotas and in Minnesota, suggesting that the abrupt decline of the spruce forest was time-transgressive from southwest to northeast during 2000 yr, and that the development of prairie was time-transgressive in the same direction over 2600 yr. Correlation of key pollen horizons at Steel Lake with those in the high-resolution pollen profiles of Elk Lake, ca. 50km northwest of Steel Lake, suggests that the well-known Elk Lake varve chronology for the early Holocene is about 1000 yr too young.
Bibliographical noteFunding Information:
Palynological advice and use of reference slides were kindly provided by E.J. Cushing and B.C.S. Hansen. Helpful manuscript reviews were afforded by E.C. Grimm, P.J. Bartlein, and D.M. Nelson. Financial support came from the Limnological Research Center and from a National Science Foundation grant (EAR 99–05327 to F.S. Hu and H.E. Wright). This work was performed, in part, under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.