Carbon Glaciation Course Materials

CARBON GLACIATION Terrestrial flora first appeared about 450 million years ago and was abundant and varied during the Carboniferous. True forests of seed ferns and trees quickly spread throughout Gondwana and Laurasia at the beginning of the Carboniferous. Primitive amphibians inhabit swampy forests. These forests are buzzing with hundreds of insect species, including huge cockroaches and giant dragonflies. As the climate grew colder and widespread glaciation enveloped the southern continent by the end of the period, the first reptiles appeared. They began to displace amphibians as the dominant terrestrial vertebrates. During the latter part of the Carboniferous about 290 million years ago, Gondwana was in the south polar regions. The glacial center extends across the southern continent, as evidenced by glacial deposits of tillites along with striations in ancient rocks. Rocks strongly notched by advancing glaciers show lines of ice flow away from the equator and toward the poles. This would not have been possible if the continents were located where they are today. Furthermore, ice would have to flow from the sea to land in many areas, which is highly unlikely. Instead, the southern continent floated en masse above the South Pole, and a huge ice sheet crossed the current continental boundary. Glacial deposits are interspersed with marine sediments. Large rock deposits distort the finer sediments in which they are located. This suggests they fell into the seafloor mud from an ice raft. Apparently, floating ice sheets extended out of land as they do today on the huge ice shelf of Antarctica (Fig. 113). When the iceberg moves away from the ice sheet and melts, the debris embedded in the ice falls into sediments on the seafloor over a large area. The so-called erratic strange and out-ofplace rocks, which consist of rock types found nowhere else on one continent, match those on the opposite continent. Glacial deposits are overwritten by thick sequences of terrestrial sediments. In turn, this is covered by a large outpouring of basalt lava flows. On top of this volcanic rock there is a layer of coal containing similar fossil plant material. Glacial deposits in equatorial regions today suggest that in the past these areas were much colder. Fossilized coral reefs and coal deposits in the Arctic region suggest a tropical climate was there at one time. In addition, salt deposits in the Arctic region indicate an ancient desert climate. Either the climate in the past changed dramatically or the continents shifted position with respect to the equator. During the early part of the glacial epoch, the maximum glacial effect was in South America and South Africa. Later, as the continent drifted south, the major glacial centers moved to Australia and Antarctica, clearly showing that the southern continent consisting of Gondwana hovered above the South Pole. Continents that are near the poles are often the cause of long periods of glaciation. This is because soils at higher latitudes have high albedo (reflective qualities) and low heat capacity, encouraging ice accumulation. The ice sheet covers most of east-central South America, South Africa, India, Australia, and Antarctica. In Australia, marine sediments are interspersed with glacial deposits. Tillites are separated by coal seams. This suggests that periods of glaciation were interspersed with warm interglacial spells, when vast forests grew. In South Africa, the Karroo Series, consisting of a sequence of late Paleozoic tillites and coal seams, spanned an area of several thousand square miles and reached a total thickness of 20,000 feet. Among the coal seams, the best in Africa, are fossilized leaves of Glossopteris. Their existence on the southern continent is one of the best evidence for the theory of continental drift. That glaciation may have been caused by the loss of atmospheric carbon dioxide. Burial of carbon dioxide in the Earth's crust may be the key to the onset of all the great ice ages since life evolved on Earth. Substantial carbon dioxide repositories during the latter part of the Paleozoic were large forests scattered throughout the country. Plants invaded the land and expanded to all parts of the world starting about 450 million years ago. The dense forests that grow during the Carboniferous store large amounts of carbon in their timber tissue. Burial under a layer of sediment compacts the vegetative material and turns it into a thick coal seam (Fig. 114). The reduction in carbon dioxide content in the atmosphere greatly weakens the greenhouse effect, causing the climate to cool. The continental margins become less wide and narrow, limiting marine habitats to nearshore areas. This may have influenced a major extinction at the end of the Paleozoic. However, no major extinction events occurred during widespread Carboniferous glaciation about 330 million years ago. The relatively low extinction rate may be due to the limited number of species vulnerable to extinction after the late Devonian extinction. As glaciers left, the first reptiles appeared to replace amphibians as the dominant terrestrial vertebrates. The climate of the tropics became more arid, and the swamps began to disappear. The land once covered with large coal swamps began to dry up as the climate got colder.