Since the abundance of the O18 isotope in calcium carbonate varies with the temperature at which it is deposited from water, the variation in abundance can be used as a thermometer. This paper discusses the following problems: (1) the magnitude of the effect expected, (2) the mass spectrometer of high sensitivity, (3) the preservation of the record during geological time, (4) the constancy of the isotopic composition of the ocean, (5) the impossibility of using skeletons of air breathing animals, and (6) the temperature of marine animals relative to their surroundings.

A Jurassic belemnite is used to show that the record has been retained since Jurassic times, and belemnites of the Upper Cretaceous of the United States, England, and Denmark are used to determine the temperature of this time at these localities. The temperatures are about 15°–16°C and indicate nearly uniform temperature over this latitudinal belt. Because of the possible variation in O18 content of the oceans and the limited number of samples, these temperatures are regarded as preliminary.

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