gases, thereby significantly lowering surface temperatures. As the continents
rose higher, the ocean basins dropped lower.The change in shape of the ocean
basins greatly affected the course of ocean currents (Table 7), which, in turn,
had a profound effect on the climate.
All known episodes of glaciation occurred at times when sea levels
should have been low, although not all mass extinctions were associated with
lowered sea levels. The recession of the seas made continental margins less
extensive and narrower, confining marine habitats to nearshore areas. Such an
occurrence might have had a major influence on the great extinction at the
end of the Paleozoic era. During this time, land once covered with great coal
sw amps completely dried out as the climate grew colder, culminating in the
deaths of large numbers of species.
Ocean temperatures also remained cool for a considerable time follow-
ing the late Permian ice age. Episodes of climatic cooling are detrimental to
species that do not adapt to the new, colder conditions or migrate to warmer
refuge. Marine invertebrates that escaped extinction lived in a narrow margin
near the equator.Corals,which live only in warm, shallow water, were partic-
ularly hard hit as evidenced by the lack of coral reefs in the early Mesozoic.
When the great glaciers melted and the seas began to warm to their preglacialHistorical Geology
TABLE 7 HISTORY OF THE DEEP CIRCULATION
OF THE OCEAN
Age (millions
of years ago) Event3 An Ice Age overwhelms the Northern Hemisphere.
3–5 Arctic glaciation begins.
15 The Drake Passage is open; the circum-Antarctic current is formed. Major sea
ice forms around Antarctica, which is glaciated, making it the first major glaciation of the
modern Ice Age. The Antarctic bottom water forms. The snow limit rises.
25 The Drake Passage between South America and Antarctica begins to open.
25–35 A stable situation exists with possible partial circulation around Antarctica. The
equatorial circulation is interrupted between the Mediterranean Sea and the Far East.
35–40 The equatorial seaway begins to close. There is a sharp cooling of the surface
and of the deep water in the south. The Antarctic glaciers reach the sea with glacial
debris in the sea. The seaway between Australia and Antarctica opens. Cooler bottom
water flows north and flushes the ocean. The snow limit drops sharply.
> 50 The ocean could flow freely around the world at the equator. Rather uniform
climate and warm ocean occur even near the poles. Deep water in the ocean is much
warmer than it is today. Only alpine glaciers exist on Antarctica.