Ordovician Ice Age

ORDOVICIAN ICE AGE Species that evolved rapidly during the early Cambrian advanced significantly in the warm Ordovician seas. The warming was largely because the atmosphere held as much as 16 times its current carbon dioxide content, enough to heat the climate to tropical levels even though the Sun was 4 percent dimmer than it is today. Global temperatures averaged about 18 degrees Celsius, about 8 degrees hotter than today. Corals, needing warm water, began to build extensive carbonate reefs. In addition, the first fish appeared in the ocean. The presence of freshwater jawless fish on the continent suggests that lakes and rivers are inhabited by red and green algae. Plants began invading the soil and expanded to all parts of the world during the late Ordovician about 450 million years ago. Early land plants absorbed large amounts of atmospheric carbon dioxide. Rapid burial under anaerobic conditions deposits organic carbon into geological columns, where it is converted into coal. Plants also help the weathering process by leaching minerals from rocks. Carbonate rocks, such as limestone deposited by shelled organisms from the Cambrian onwards (Fig. 67), lock in large amounts of carbon dioxide. The withdrawal of large amounts of carbon dioxide from the atmosphere weakens the greenhouse effect. The resulting climate cooling largely began by plant invasions gave birth to a great ice age at the end of the Ordovician about 440 million years ago. At the time, the southern edge of Gondwana was just above the South Pole, where the ice sheet grew to about 80 percent the size of today's Antarctica. Middle and late Carboniferous glacial glaciations about 330 million and 290 million years ago may have been affected by reductions in atmospheric carbon dioxide to roughly a quarter of its current value. Atmospheric scientists have collected information about global geochemical cycles to ascertain the causes of radical changes in atmospheric carbon dioxide content. Data from the deep ocean core suggest that variations in carbon dioxide preceded changes in the rate of more recent ice ages, suggesting that earlier glacial epochs may have also been affected. Variations in carbon dioxide levels may not be the only cause of glaciation. However, when combined with other processes, such as variations in Earth's orbital motion or decreased solar radiation, they can be a powerful influence. Continental movements may also have been responsible for the glaciation of the late Ordovician. The magnetic orientation of rocks from different parts of the world indicates the position of the continents relative to the magnetic poles at various times in Earth's history. Paleomagnetic studies in Africa revealed very strange findings. They placed North Africa directly above the South Pole during the Ordovician, which led to glaciations around the world. Additional evidence for such widespread glaciation comes from another surprising location—the middle of the Sahara Desert. Geologists exploring petroleum in the region discovered a series of giant grooves cut into the underlying strata by glaciers. Rocks embedded in the base of glaciers roam the landscape as ice sheets move back and forth. Other collaborative evidence suggests that thick ice sheets blanket the Sahara and include large, erratic boulders placed by moving ice and ice, which are sinuous sand deposits from glacial outwash flows. An episode of major mountain buildings from the Cambrian to the Ordovician deformed areas between all continents comprising the supercontinent south of Gondwana, showing their collision during this interval. The middle Paleozoic fern Glossopteris (Figure 68), named after a Greek word meaning "feather-like," and whose fossil leaf tracks actually look like feathers, is found in coal seams in the southern subcontinent and India. However, the plant is suspiciously absent in the northern continent. This indicates the existence of two large continents, one located in the southern hemisphere and another in the northern hemisphere, separated by a large open ocean. Matches between the mountains of Canada, Scotland, and Norway show their assembly to the supercontinent north of Laurasia during this time. At the end of the Ordovician, glaciation reached its peak. About 430 million years ago, the ice sheet largely disappeared. As Gondwana continues to drift south, the farther south, the smaller the ice sheet. As the center of the continent approaches the South Pole, winters in the interior become colder. However, the soil is warm enough during the summer to melt the ice. Meanwhile, the southern glacial edge of Gondwana moved north to the warmer sea, and the glaciers soon left.