Oxygen-isotope paleotemperature studies of marine microfossils, primarily from DSDP cores at several laboratories, have in the past few years yielded a great deal of information about the three-dimensional temperature structure of the oceans and its evolution during the past 100 m.y. As expected, because of problems of sample preservation, climatic information is progressively less detailed for successively older times.

The general trend of ocean temperatures since middle Cretaceous time has been downward. However, the downward trend has not been uniform, but has been punctuated by frequent intervals of rising temperatures. Temperatures fell and then rose again near the time of transition from Tertiary to Cretaceous, but the magnitude of the temperature decline was much smaller than other temperature declines observed in the marine isotopic record of the past 100 m.y. Two striking features

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of the record are (1) a rapid drop of both high- and low-latitude temperatures at approximately Eocene-Oligocene time, and (2) a drop in high-latitude temperature correlated with a warming in low latitudes in early middle Miocene time. The latter probably corresponds to the onset of major Antarctic icecap development.

The steepness of the thermocline in low latitudes has varied. The vertical thermal structure of Oligocene oceans resembled that of the modern ocean most closely, and the thermal structures of Paleocene and Late Cretaceous times differed most from the modern structure.

Causes of the observed climatic variations are not known, but the variations may be related at least in part to changes in oceanic circulation resulting from changing continental positions, and to changes in eustatic sea level.

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