Ice dynamics and deglaciation models when ice sheets collapsed

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Abstract
The Quaternary Period is remarkable because of its dramatic environmental fluctuations, recorded most profoundly in the geological evidence for a succession of worldwide glaciations accompanying global climatic oscillations, especially in mid and high northern latitudes. North America, which spans these latitudes, has been a major stage for this continuing drama. Broecker and van Donk (1970), on the basis of oxygen-isotope records from nine core sites in the Atlantic and Caribbean basins, discovered a sawtooth pattern for Brunhes glaciation cycles (the last 730,000 yr): glaciation was prolonged and unsteady, the deglaciation was swift and sure. They used the word “terminations” to identify times of rapid deglaciation. The last termination took place from about 17 to 7 ka. It was a time when ice sheets collapsed. But how did the ice sheets collapse, and from what size? These are the questions addressed in this chapter. Research aimed at understanding cycles of glaciation can be guided by two competing philosophical viewpoints. On the one hand, the succession of major glaciations and interglaciations may be the most extreme perturbations of a global climatic machine in stable equilibrium, and analogous to a ball in a hollow: if it is rolled and released, it rolls back to its starting position. For stable equilibrium, climate dynamics is understood best by focusing research on conditions during the extremes of climate, when the slope of a climate-versus-time curve is zero. On the other hand, the succession of major glaciations and interglaciations may be the manifestation of a global climatic machine in unstable equilibrium, and analogous to a ball on a hilltop: if it is rolled and released, it continues to roll forward. For unstable equilibrium, climate dynamics is understood by focusing research on conditions during the most rapid climatic change, when the slope of a climate-versus-time curve is maximum. These conditions exist when ice sheets collapse. By examining mechanisms during the last glacial and interglacial maxima, CLIMAP (Climate: Long-range Investigation, Mapping, and Prediction; a project of the International Decade of Ocean Exploration, 1970–1980) addressed the possibility of stable climatic equilibrium. This chapter examines deglaciation mechanisms that promote unstable climatic equilibrium.