The Late Wisconsin Glaciation and deglaciation of the Laurentide Ice Sheet

Abstract

During the Late Wisconsin Glaciation, the Laurentide Ice Sheet covered an area approximately equivalent to that of the present Antarctic Ice Sheet, about 12.6 × 10⁶ km². The ice sheet extended between 75° and ca. 45° N and between 64° and 120°W. It existed across major climatic gradients and was an important modifier of climate (e.g., Kutzbach and Wright, 1985; Manabee and Broccoli, 1985). In this chapter the deglacial history of this massive Northern Hemisphere ice mass is examined via four topics that constrain its area/volume relation and that indicate boundaries against which glaciologic, climatologic, and oceanographic reconstructions of the late-glacial world must be tested and verified. These topics are (1) the evidence of glacial extent during the middle Wisconsin interstade, (2) the data on the timing of the last glacial maximum and the extent of this glaciation, (3) the field evidence for ice-marginal profiles and hence the thickness of the ice sheet, and (4) the chronology, rates, and mechanisms of deglaciation. This chapter is concerned with the glacial-geological evidence for ice-sheet extent and thickness. Additional evidence on the complex problem of reconstructing the Laurentide Ice Sheet is contained in the chapters by Hughes and by Peltier, who examine glaciological and glacial-isostatic aspects of the problem. Chapters by Ruddiman and by Mix also concern deglacial history; specifically, the influence and interaction between the ice sheets and different oceanographic parameters. The chronology and mechanisms of deglaciation for the southern and southwestern margins are also dealt with by Teller (this volume). This chapter draws heavily on recent syntheses for the whole or parts of the Laurentide Ice Sheet, including reports of the I.G.C.P. Project #24 (Fulton, 1984), Dyke and others, (1987, and other chapters therein), Michelson and others (1983), and chapters in Teller and Clayton (1983). Most Quaternary sediments in North America north of 45°N post-date the last deglaciation. This volume looks at those extensive deposits from the standpoints of timing, cause, and mechanism of the wastage of North American ice during the last deglaciation and the accompanying environmental changes in the nonglaciated and deglaciated areas. It particularly examines the mechanisms by which a mass of ice equivalent to 100 m of global sea-level was returned to the ocean within about 8,000 years. A truly comprehensive synthesis of marine and terrestrial information in 22 chapters grouped into five sections: Chronology of Disintegration of the North American Ice Sheets, Ice Core and Other Glaciological Data, the Nonglacial Physical Record on the Continent, Biological Record on the Continent, and Analysis and Summary. Includes two oversize color plates showing time-series maps of pollen densities and vegetation changes since 18 ka.