Full-glacial summer temperatures in eastern North America as inferred from Wisconsinan vegetational zonation

Abstract

A variety of types of fossil evidence — pollen, macrofossils, and snails — document the occurrence of boreal forest species at low latitudes of eastern North America duringthe last full glacial (18,000 yr B.P.). Both the northern and southern boundaries of boreal coniferous forest are determined by limiting values of summer warmth. Knowledge of these limiting factors permits construction of a gradient of Wisconsinan summer temperatures. The reconstructed temperature departures are 5–12°C for the eastern part of the continent south of the ice, with temperature departures of no less than 5–6°C along the Gulf coast. Information regarding occurrence of tundra and depression of treeline in the Appalachian mountains provides another estimate of summer temperature depression that is in agreement with these results. Comparison with proxy data from other parts of the world shows that the degree of temperature depression is of about the same degree as that postulated for the tropics (Rind and Peteet, 1985; Lui and Colinvaux, 1985) on the basis of the elevational shifts in vegetational zones and snowline. Quantitative estimates of past temperatures (or temperature departures) of the type produced here are valuable because they permit comparison with model simulations of the full-glacial climate. The depression in summer temperatures indicated is lower than most recent modeling results using CLIMAP boundary conditions and adds to suggestions that the reconstructed sea-surface temperatures used as boundary conditions in simulations of glacial and Holocene climates may be in error.