Late Paleozoic icehouse

The Karoo Ice Age from 350-250 million years ago was the second major period of Glaciation of the Pharerozoic Era. It is named after the glacial tills found in the Karoo region of South Africa where evidence for the Ice Age was first clearly identified.

The tectonic assembly of the continents of Euramerica (later with the Uralian orogeny, into Laurasia) and Gondwanaland into Pangaea, in the Hercynian-Alleghany Orogeny, resulted in a major continental landmass within the Antarctic region, and the closure of the Rheic and Iapetus Seas saw disruption of warm water currents in the Panthallasic Ocean and Paleotethys Sea, which led to the progressive cooling of summers, and the accumulation of snowfields in winters, causing mountain glaciers to grow. Upon leaving highland areas, continental glaciers 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), saw Ice Sheets expanding from a core in Southern Africa and South America

• the second glacial period was associated with the Pensylvanian Era (318.1 myr-299 myr), saw Ice Sheets expanding from a core in Australia and India.

The extent of glaciation in Antarctica (due to its present Ice Sheets) is not exactly known.

It is thought that the shift in glacial expansion cores is due to polar wandering and tectonic rotationary movements of Pangaea itself.

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 a height of 20 meters were secondary dominant to the large arborescent Lycopods (30-40 metres) of the Carboniferous forests that flourished in Equatorial Swamps stretching from Apalachia to Poland, and later on the flanks of the Urals. Oxygen levels rose as high as 35%, and a reduction in global carbon dioxide below the 300 parts per million level, today associated with glacial periods. This reduction in the Greenhouse Effect was coupled with the accumulation of lignin, the form of cellulose associated with tree trunks, and its burial 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 the cooler summers, incapable of melting last winter's snow accumulations. The growth in snowfields to a depth of 6 metres would create sufficient pressure to convert the lower levels to Ice. Further pressure would melt the bottom layer, creating a lubrication and allowing the snowfield to begin moving down-slope as a glacier.

The reduced planetary albedo, produced by the expanding Ice Sheets would lead to positive feedback loops, spreading the Ice Sheets still further, until limits were achieved. Falling global temperatures would eventually limit plant growth, and the rising levels of oxygen would increase the frequency of fire-storms, in which even wet plant matter can burn. Both these effects return carbon dioxide to the atmosphere, reversing the "snowball" effect and forcing greenhouse warming. Over a longer period the evolution of termites, whose stomachs provided an anoxic environment for methanogenic lignin digesting bacteria, prevented the 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 the burial and removal of carbon (in the form of coal). With a smaller area for the deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet. By 250 million years ago, the planet had returned to a percentage of oxygen not dissimilar to that found today.

The rising levels of oxygen in the Karoo Ice Age had major effects upon evolution of both plants and animals. Higher oxygen concentration (and the higher atmospheric pressure) enabled energetic metabolic processes encouraging the 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 Hibbertopterid eurypterids were perhaps as large, while some scorpions reached 50 or 70cm. The rising levels of oxygen also led to the evolution of greater fire resistence in vegetation and ultimately to the evolution of flowering plants. Genetic studies have shown that this was the period in which Angiosperms separated from Cycads and Gymnosperms.

• 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