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Broecker used changes in oxygen isotopes in deep-sea cores, analyzed by Emliani during 1955-1966, to point out the sawtooth nature of glacial cycles—a slow cooling followed by rapid warming. (Shown here are four different cores, with variation of oxygen-18 toward the right representing higher temperature. Time runs downward in thousands of years Before Present.)

From Broecker & van Donk, Reviews of Geophysics and Space Physics (1970) p. 171,
reproduced by permission of American Geophysical Union.