Oxygen-18/Oxygen-16 ratios of marine limestones and calcareous organisms have been widely applied to the determination of water temperatures of ancient oceans after the technique was developed by Urey et al. The use of the carbonate paleogeothermometer has not been extended much past the Cretaceous because of uncertainties in the oxygen isotopic composition of pre-Cretaceous seawater and because of the possibility of isotopic exchange between marine carbonates and isotopically lighter intrastratal fluids.

Freshwater limestones and fossils have not been used for paleotemperature determinations because of the great variation in the O¹⁸O¹⁶ ratio of the water in which these carbonates were precipitated. Such variations arise from differences in latitutde and altitude at which meteoric precipitation of water took place, the origin and history of the air mass, and the extent of evaporation of stream and lake water. Duplicate isotopic analyses of 157 proven fresh-water limestones of Devonian to Quaternary age which were formed in large bodies of water, excluding saline lakes, and which show no signs of isotopic alteration, suggest that the variation is random and that a large number of samples may provide a mean ^dgrO¹⁸ value which is temperature dependent. Me n ^dgrO¹⁸ values are: Devonian (-8.57), Pennsylvanian (-5.25), Permian (-4.36), Triassic (-5.12), Jurassic (-8.52), Cretaceous (-10.22), Tertiary (-9.65), Quaternary (-8.15), in permil, relative to the Chicago PDB standard. The results form a regular sinusoidal curve with O¹⁸/O¹⁶ maxima in the colder periods of Permo-Caboniferous and Quaternary glaciation, and a minimum for the Cretaceous. Large bodies of fresh water, because of a greater response to worldwide temperature fluctuations than the oceans, form limestones whose isotopic composition may yield significant paleoclimatic information in the form of relative climatic temperatures.

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