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The Karoo Ice Age from 360–260 million years ago (Mya) was the second major ice age of the Phanerozoic Eon. It is named after the tillite (Dwyka Group) found in the Karoo region of South Africa (and adjacent areas), where evidence for this ice age was first clearly identified in the 19th century. The tectonic assembly of the continents of Euramerica (later with the Uralian orogeny, into Laurasia) and Gondwana into Pangaea, in the Hercynian-Alleghany Orogeny, made a major continental land mass within the Antarctic region, and the closure of the Rheic Ocean and Iapetus Ocean saw disruption of warm-water currents in the Panthalassa Ocean and Paleotethys Sea, which led to progressive cooling of summers, and the snowfields accumulating in winters, causing mountainous alpine glaciers to grow, and then spread out of highland areas, making continental glaciers which spread to cover much of Gondwana. At least two major periods of glaciation have been discovered: *The first glacial period was associated with the Mississippian Period (359.2–318.1 Mya): ice sheets expanded from a core in southern Africa and South America. *The second glacial period was associated with the Pennsylvanian Period (318.1–299 Mya); ice sheets expanded from a core in Australia and India. The extent of ancient glaciations in Antarctica is not well known, because the present ice sheet hides the evidence. ==Causes of the Karoo Ice Age== The evolution of land plants with the onset of the Devonian Period, began a long-term increase in planetary oxygen levels. Large tree ferns, growing to 20 m high, were secondarily dominant to the large arborescent lycopods (30–40 m high) of the Carboniferous coal forests that flourished in equatorial swamps stretching from Appalachia to Poland, and later on the flanks of the Urals. Oxygen levels reached up to 35%, and global carbon dioxide got below the 300 parts per million level, which is today associated with glacial periods. This reduction in the greenhouse effect was coupled with lignin and cellulose (as tree trunks and other vegetation debris) accumulating and being buried in the great Carboniferous Coal Measures. The reduction of carbon dioxide levels in the atmosphere would be enough to begin the process of changing polar climates, leading to cooler summers which could not melt the previous winter's snow accumulations. The growth in snowfields to 6 m deep would create sufficient pressure to convert the lower levels to ice. Earth's increased planetary albedo produced by the expanding ice sheets would lead to positive feedback loops, spreading the ice sheets still further, until the process hit limit. Falling global temperatures would eventually limit plant growth, and the rising levels of oxygen would increase the frequency of fire-storms because damp plant matter could burn. Both these effects return carbon dioxide to the atmosphere, reversing the "snowball" effect and forcing greenhouse warming, with CO2 levels rising to 300 ppm in the following Permian period. Over a longer period the evolution of termites, whose stomachs provided an anoxic environment for methanogenic lignin-digesting bacteria, prevented further burial of carbon, returning carbon to the air as the greenhouse gas methane. Once these factors brought a halt and a small reversal in the spread of ice sheets, the lower planetary albedo resulting from the fall in size of the glaciated areas would have been enough for warmer summers and winters and thus limit the depth of snowfields in areas from which the glaciers expanded. Rising sea levels produced by global warming drowned the large areas of flatland where previously anoxic swamps assisted in burial and removal of carbon (as coal). With a smaller area for deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet. By 250 Mya, planet Earth had returned to a percentage of oxygen similar to that found today. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Karoo Ice Age」の詳細全文を読む スポンサード リンク
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