Karoo Ice Age 3988315 188006903 2008-01-30T21:40:14Z SieBot 4005189 robot Adding: [[nl:Karoo-ijstijd]] The Karoo [[Ice Age]] from 300–400 [[Annum|Ma]] (million years ago) was the second major period of [[glaciation]] of the [[Phanerozoic]] [[Eon]]. It is named after the glacial [[till]]s found in the [[Karoo]] region of [[South Africa]] where evidence for this ice age was first clearly identified. The [[tectonic plates|tectonic]] assemble of the continents of [[Euramerica]] (later with the [[Uralian orogeny]], into [[Laurasia]]) and [[Gondwanaland]] into [[Pangaea]], in the [[Hercynian]]-[[Alleghany]] [[Orogeny]], made a major continental landmass 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 [[Tethys Ocean|Paleotethys Sea]], which led to progressive cooling of summers, and the snowfields accumulating in winters, causing mountainous alpine [[glacier]]s to grow, and then spread out of highland areas, making [[continental glacier]]s which spread to cover much of Gondwanaland. At least two major periods of glaciation have been discovered:- *The first glacial period was associated with the [[Mississippian]] era (359.2 myr-318.1 myr): ice sheets expanded from a core in southern Africa and South America. *The second glacial period was associated with the [[Pennsylvanian]] era (318.1 myr-299 myr); ice sheets expanded from a core in [[Australia]] and [[India]]. The extent of glaciation in [[Antarctica]] is not exactly known, due to its present ice sheet. It is thought that the shift in glacial expansion cores is due to polar wandering and tectonic movements of Pangaea. ==Causes of the Karoo Ice Age== It is thought that 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 meters high, were secondary dominant to the large arborescent [[Lycopods]] (30-40 metres high) of the [[Carboniferous]] [[coal forest]]s that flourished in equatorial [[swamp]]s stretching from [[Appalachia]] to [[Poland]], and later on the flanks of the [[Ural Mountains|Urals]]. [[Oxygen]] levels reached anything 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 metres deep would create sufficient pressure to convert the lower levels to ice. Further pressure would melt the bottom layer, lubricating and letting the snowfield begin moving downslope as a [[glacier]]. 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 CO<sub>2</sub> levels rising to 300 parts per million in the following Permian period. Over a longer period the evolution of [[termites]], whose stomachs provided an [[anoxia|anoxic environment]] for [[methanogenic bacteria|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 higher planetary albedo resulting from the fall in size of the glaciated areas, would have been enough to 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 million years ago, planet Earth had returned to a percentage of oxygen similar to that found today. ==The effects of the Karoo Ice Age== The rising levels of oxygen in the Karoo Ice Age had major effects upon [[evolution]] of plants and animals. Higher oxygen concentration (and accompanying higher atmospheric pressure) enabled energetic metabolic processes which encouraged evolution of large land-dwelling vertebrates and flight, with the dragonfly-like [[Meganeura]], an aerial predator, with a wingspan of 60 to 75 cm. The inoffensive stocky-bodied and armoured millipede-like [[Arthropleura]] was 1.8 meters long, and the semi-terrestrial [http://www.palaeos.com/Invertebrates/Arthropods/Eurypterida/Hibbertopterina.html Hibbertopterid] [[eurypterid]]s were perhaps as large, and some [[scorpion]]s reached 50 or 70cm. The rising levels of oxygen also led to the evolution of greater fire resistance in vegetation and ultimately to the evolution of flowering plants. Genetic studies have shown that this was when [[Angiosperms]] separated from [[Cycads]] and [[Gymnosperms]]. ==See also== *[[Quaternary glaciation]] – the current ice age, proceeding the Karoo Ice Age. ==Resources== * Berner, Robert A. (1999) "Atmospheric oxygen over Phanerozoic time" http://www.pnas.org/cgi/content/full/96/20/10955#F2 * Beerling, D. J. and Berner, R. A. "Impact of a Permo-Carboniferous high O2 event on the terrestrial carbon cycle" http://www.pnas.org/cgi/content/full/97/23/12428 [[Category:Glaciology]] [[Category:Ice ages]] [[Category:Carboniferous]] [[Category:Permian]] [[nl:Karoo-ijstijd]]