The temperature decrease of surface sea-water in high latitudes and of abyssal-hadal water in open oceanic basins during the past 75 million years

Oxygen isotopic analysis of Upper Cretaceous belemnites from Alaska and Siberia gave temperatures of about 14°C. Similar measurements on shells of calcareous benthonic Foraminifera from sections of Globigerina-ooze sediments of Tertiary age from the eastern equatorial Pacific gave a temperature of 10·4°C for the Middle Oligocene, decreasing to 2·2°C in the Late Pliocene. Thus, a temperature decrease of both surface water at high latitudes and abyssal-hadal water in open oceanic basins, amounting to about 12°C, appears to have occurred during the past 75 million years. This temperature decrease was probably not linear with time and, although very slow, may have had an important effect on the abyssal and hadal fauna. By comparison, temperatures similar to the present ones may have obtained throughout geologic time in at least some portions of the equatorial thermosphere. If constant temperature is more important for the survival of archaic forms than other factors, the equatorial pelagic fauna should show more archaic affinities than other marine faunas.