The oxygen isotopic composition of deep-sea microfossils reveals two trends for the past 100 m.y.: a long term (10⁸ years) increase in the ¹⁸O content of deep-sea benthic foraminiferal carbonate which suggests a progressive cooling of polar regions related to changes in ocean basin-continent geometry, and the poleward shift of land area since the Cretaceous; and 10⁶-year steplike fluctuations in the ¹⁸O content of planktonic and benthic microfossils related to changes in the area of shelf seas, relative and eustatic sea level, and polar glaciation.

Benthic isotopic results, after correction for probable ice volume effects in the Oligocene and post-middle Miocene, correspond closely to sea level fluctuation. This correlation appers to be the result of climatic (largely temperature) effects caused by changes in global albedo patterns. During the sea level highstands in the Cretaeous and early Tertiary, shallow seas covered more than 50 × 10⁶ sq km which maximized heat storage in the ocean. The planetary thermal gradient was low, with polar regions producing warm bottom waters (10 to 15°C). In this regime, sea level fluctuations controlled climate. The cause of the sea level fluctuations is unclear. After the middle Eocene, falling eustatic sea level, the reduction of shelf seas to less than about 30 × 106 sq km and the initiation of glaciation in Antarctica produced a rapid cooling of ocean bottom waters and a change in the global heat flux. In this regime, which became fully established with the closure of Tethys and the expansion of southern hemisphere glaciation in the Miocene (ca 15 m.y.), there has been a progressive cooling of deep waters in the ocean and an increase in the vertical thermal gradient. Eustatic and relative sea level fluctuations have been controlled by climatic events in polar regions.

End_of_Article - Last_Page 920------------