- Karoo Ice Age
Ice Agefrom 300–400 Ma (million years ago) was the second major period of glaciationof the Phanerozoic Eon. It is named after the glacial tills found in the Karooregion of South Africawhere evidence for this ice age was first clearly identified.
The tectonic assemble of the continents of
Euramerica(later with the Uralian orogeny, into Laurasia) and Gondwanalandinto Pangaea, in the Hercynian- Alleghany Orogeny, made a major continental landmass within the Antarctic region, and the closure of the Rheic Oceanand Iapetus Oceansaw disruption of warm water currents in the PanthalassaOcean 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 Gondwanaland.
At least two major periods of glaciation have been discovered:-
*The first glacial period was associated with the
Mississippianera (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
Pennsylvanianera (318.1 myr-299 myr); ice sheets expanded from a core in Australiaand India.
The extent of glaciation in
Antarcticais 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 oxygenlevels. Large tree ferns, growing to 20 meters high, were secondary dominant to the large arborescent Lycopods(30-40 metres high) of the Carboniferous coal forests that flourished in equatorial swamps stretching from Appalachiato Poland, and later on the flanks of the Urals. Oxygenlevels reached anything up to 35%, and global carbon dioxidegot below the 300 parts per million level which is today associated with glacial periods. This reduction in the greenhouse effectwas coupled with ligninand 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
albedoproduced by the expanding ice sheets would lead to positive feedbackloops, 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 parts per million 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 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
evolutionof 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 Arthropleurawas 1.8 meters long, and the semi-terrestrial [http://www.palaeos.com/Invertebrates/Arthropods/Eurypterida/Hibbertopterina.html Hibbertopterid] eurypterids were perhaps as large, and some scorpions reached 50 or 70 cm. 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 Angiospermsseparated from Cycadsand Gymnosperms.
Quaternary glaciation– the current ice age, proceeding the Karoo Ice Age.
* Beerling, D. J. and Berner, R. A. (2000) " [http://www.pnas.org/cgi/content/full/97/23/12428 Impact of a Permo-Carboniferous high O2 event on the terrestrial carbon cycle] ", "Proc. Nat. Acad. Sci.", 97 (23), p. 12428-12432
* Berner, R.A. (1999) " [http://www.pnas.org/cgi/content/full/96/20/10955#F2 Atmospheric oxygen over Phanerozoic time] ", "Proc. Nat. Acad. Sci.", 96 (20), p. 10955-10957
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