that time, unlikely as it may seem, southeastern England was cloaked by tropical jungles like those of modern Malaysia. Fossils of deep-sea ostracodes, minute crustaceans that are distant cousins of crabs and lobsters, reveal that a major change took place in the deep-sea about 40-million-years ago. The types of ostracodes that occupy the oceanic abyssal plain today, having adapted to frigid conditions, made their first appearance at that time. Oxygen isotopes in foraminifera confirm the observation of this trend toward frigidity.

Before that cooling began, the deep-sea may have reached temperatures as warm as 15°C. It did not cool to its modern temperature immediately, of course, but gradually lost heat until 30-million-years ago when the cold basal currents of the modern oceans became firmly entrenched.

Plate tectonics offers a possible explanation for cooling in the Southern Hemisphere 40-million-years ago. In the vicinity of Antarctica, microfossils preserved in deep-sea sediments testify to a drastic change in thermal conditions. Millions of years earlier the supercontinent of Pangea had rifted apart to form many of the fragments that constitute the continents of the modern world. South America and Australia remained attached to what is now Antarctica, which was positioned on the South Pole close to its present location. While these connections remained, cool water was deflected equatorward and warm water poleward along the coasts. Microfossil and other data indicate that about 40-million-years ago South America began to drift away from Antarctica, allowing a continuous current to flow around Antarctica—the Circumantarctic Current. This current isolated the continent thermally, and the change in circulation marked the origin of the refrigeration system for the deep-sea that operates in this region today, trapping water and allowing it to cool and sink. About the same time this refrigeration system was supplemented by another. The Arctic Ocean became connected to the Atlantic over the Iceland sill, allowing cold Arctic surface waters to descend into the deep-sea.

Throughout earth history a cold basal layer of ocean water must have formed each time at least one of the poles became frigid. Fossil data verify the occurrence of such an event 450-million-years ago, when the supercontinent of Gondwana encroached on the South Pole and accumulated massive ice sheets that left extensive glacial deposits in what is now the Sahara Desert in Africa. Careful stratigraphic research into the period has shown that brachiopods and other creatures that had colonized the seafloor at cooler high latitudes progressively shifted into deep-water habitats at all latitudes, apparently tracking the movement of cool waters into the deep-sea.

Even after the origin of the modern cold basal layer and before the start of the modern ice age, the oceans experienced major thermal changes. Substantial alterations occurred between about 22 million and 5-million-years ago. Important clues have come from carbon isotopes in fossil foraminifera. Gradients of 13C/12C ratio, detected by the study of fossil foraminifera, reveal that prior to about 14-million-years ago water flowed from the Mediterranean Sea into the Indian Ocean and southward toward Antarctica. It traveled at intermediate depths, apparently having sunk below the surface because it was more saline than normal seawater. The intense salinity resulted from a high evaporation rate in the Mediterranean region. This water and others that it entrained apparently joined the Circumantarctic Current at depth. The outflow of this saline plume ended about 14-million-years ago, probably when collision of Arabia with Asia closed the eastern end of the Mediterranean. The cutoff of warm-water flow toward Antarctica may have resulted in the buildup of the West Antarctic ice cap, which has been documented to have occurred at this time period on the basis of other geological evidence. Apparently the tectonic movements that pinched off the flow from the Mediterranean had profound climatic repercussions in regions thousands of kilometers away.

Problems in distinguishing among the effects of glacial expansion, temperature change, and variation in salinity frustrate detailed investigations of paleoclimate that use the oxygen isotopes preserved within fossil skeletons. A partial remedy is now in sight, and it comes from an unexpected source. A particular family of calcareous phytoplankton includes several living species that produce lipids called alkenones. The degree of hydrogen saturation in these fatty compounds varies markedly with the temperature at the time of production. They retain their original chemical composition over millions of years, even after bacterial decay releases them and they end up in deep-sea sediment. Their changing patterns of chemical composition, as displayed in ice age cores, correlate closely with those of oxygen isotope ratios, but the alkenone composition can be scaled to approximate absolute temperature.

The analysis of fossil alkenones promises a large volume of ocean temperature data extending back tens of millions of years. One important controversy inviting resolution relates to the warm interval that preceded the origin of the modern cold basal



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement