VEGETATION OF THE NORTH AMERICAN MIDCONTINENT DURING THE LAST (SANGAMON) INTERGLACIAL PERIOD

Climate models (atmospheric general circulation models, or AGCMs) indicate that the last interglacial period (LIG) of ~125-115 ka could have been somewhat drier than present in midcontinental North America (Harrison et al., 1995). This would have resulted in a warm grassland at ~125 ka and a cool grassland at ~115 ka, with a prairie-forest border to the east of, or in the same position as at present (Fig. 1). This model was tested with studies of LIG (Sangamon) soils in a 1200-km long transect (east to west: Zanesville, OH; Mt. Vernon, IN; Morrison, IL; Loveland, IA; Bellevue, NE; Lincoln, NE) using soil morphology and carbon isotopes. Carbon isotopes in organic matter of modern soils along this transect reflect the presettlement vegetation distribution (Fig. 1), with lighter values in areas of forest (dominated by C3 plants) and heavier values in areas of warm-season grassland (dominated by C4 plants). Carbon isotopes in modern soils were then compared with those in LIG soils at several localities where there is good stratigraphic control and LIG soils are well preserved, such as at Morrison, IL (Fig. 2). Figure 2 also shows the range of carbon isotope values for C3 and C4-dominated plants.

Figure 1

Figure 2

LIG soils at all localities have thick, well developed Bt horizons that are more likely to have formed under forest than prairie. Even in the Great Plains at Lincoln, NE, the LIG soil has an argillic Bt horizon with 7.5YR hues and well-expressed clay cutans on ped faces. At Mt. Vernon and Morrison, LIG soils also have E horizons in addition to well expressed argillic Bt horizons. Under prairie, E horizons are rare, whereas they are ubiquitous under forest. Except for Lincoln, LIG soils at all localities (Fig. 3) have carbon isotope values that range from -22 to -27 o/oo, indicating a dominance of C3 plants (trees or cool-season grasses). At Lincoln, all horizons of the LIG soil have carbon isotope values ranging from -18 to -20 o/oo, indicating a dominance of C4, warm-season grasses.

Figure 3

Figure 4

The results suggest that during at least parts of the LIG, forest extended as far west as the Missouri River, and the prairie-forest ecotone may have been in eastern Nebraska. This represents a movement of the forest border ~500 km to the west of its present western limit (Fig. 4). Given the possible long duration of the LIG (up to ~60 ka), it is likely that more than one vegetation regime dominated the region, and the carbon isotope data presented here may represent only the most recent vegetation type. Soils data suggest at least slightly wetter climates during the LIG, not drier climates as suggested by the AGCM simulations.

REFERENCES
(*references in bold are project products)

Bettis, E.A., III, Muhs, D.R., Rutledge, E.M., and Johnson, W.C., 1998, Soil and carbon isotopic evidence of North American midcontinent vegetation during the last interglacial period: Geological Society of America Abstracts with Progams, v. 30, no. 7, p. A-261.

Harrison, S.P., Kutzbach, J.E., Prentice, I.C., Behling, P.J., and Sykes, M.T., 1995, The response of northern hemisphere extratropical climate and vegetation to orbitally induced changes in insolation during the last interglaciation: Quaternary Research, v. 43, p. 174-184.

Muhs, D.R. and Bettis, E.A., III, 1998, Climate of the last interglacial in the midcontinent of the U.S.: evidence from soil morphology and carbon isotopes: American Quaternary Association Program and Abstracts of the 15th Biennial Meeting, p. 136.