Why glaciation occurs.

As well as global cooling due to changes in the proportion of gases in the atmosphere mentioned in the section on volcanism, there have been other suggestions as to why glaciation occurs and can be cyclical.

• The Milankovich cycle.
  The Milankovich cycle could also explain the fluctuations in glaciation.
  The cycle uses variations in the Earth's rotational axis (obliquity) every 42,000 years, changes in orbit patterns (eccentricity) every 100,000 years and 'wobbles' of the axis (precession) which vary ever 26,000 year, to give reasons for the timing of past glacial events.
  An example of this is that if the Earth's eccentricity is at a maximum, the obliquity at a minimum and the precession furthers from the sun in the summer, it cools the northern hemisphere. This cooling could lead to glaciation.
  If any of the factors were to change, there would be a temperature change and glaciation would cease. If they were altered regularly this would cause variations in temperature and lead to fluctuations in the appearance of ice sheets, and between there presence in the northern and southern hemispheres.

  However, the Milankovich cycle would only account for fluctuations in the size of the sheet as opposed to actual ice sheet formation as the temperature changes may not be great enough to create them.
  For details of the Milankovich cycle this is a good site to look at.

• Land over the poles.

  Having land over the poles promotes glaciation. The has almost always been ice at the poles. If there was ice in the sea, the salinity of the sea would limit the extent of the sheet. On land this does not melt the fresh water ice and the sheet can spread until it reaches too warm latitudes. Ice at the poles has a knock on effect in that because the ice is not floating in the water, it has actually taken water out of the oceans lowering sea level. See effects of glaciation.

Evidence for glaciation.

Evidence for glaciation can be seen in abundant glacial deposits of Permian age found in Australia, Siberia and in the North sea.
Sea level change due to ice sheets also lead to layers of Shale, Siltstone, Limestone, Sandstone, Marl and Dolomite indicating a regressive period.

Effects of Glaciation.

• From geochemical evidence, these ice sheets caused a global drop in temperature . This had the affect of forcing species from normally low latitude habitats to move towards the tropics and the equator, and tropical and equatorial species to die out as they had nowhere to migrate to. Glaciation does not necessarily cause climate cooling in the lower latitudes, but has been linked with it.

• Glaciation affects global sea level. As ice takes up large quantities of water in its formation, during glaciation the sea level would have dropped. As glaciation was fluctuating while Pangea moved between poles, this gave rise to sea level fluctuations.
  The low lying nature of the Pangean topography meant that quite small fluctuations in sea level would have repeatedly flooded and exposed large areas of coast line. Regression would have drained inland water bodies and river estuaries leading to creatures having to migrate and adapt to new conditions, or die. The repeated changes in type and size of coastal habitats would have lead to species living there being repeatedly disturbed. A link has been shown between the size of habitat available, and the number of creatures that can live there, so if reduction of habitat occurred the number of species that could live there would be reduced.

• During prolonged periods of glaciation, along with the limited coast line of Pangea, the low water level would lead to a reduction in shallow seas. As shallow seas are the most productive regions, over all marine productivity would be reduced leading to a strain on the marine environment. The strain could have been due to less space to live or lack of resources, and lead to extinction. As the shallow shelves are the main areas for primary production there would have been repercussive affects further up the food chain.

• Sea level fluctuations are linked to another method of extinction. Changing depths of water change currents and bring up anoxic water from the deep oceans. This de-oxygenates the upper levels of the water column causing death. There is only convincing evidence for this at the end Devonian and end Ordovician periods so it may not be a viable hypothesis for the Permian Mass Extinction.

Opposing views.

It has been suggested that the anoxia mentioned in the section above, occurred too late to have caused the Permian extinction, and is also insignificant compared to other factors.

There is some speculation that there was an unglaciated time at the end of the Palaeozoic, due to Pangea moving off the south pole. Glaciation was apparent in the southern hemisphere from the mid Devonian, into the middle Permian because of the landmass over the south pole, but most of the extinctions occurred at the end of the Permian and into the Triassic when Pangea had moved. This would suggest that glaciation was not a contributing factor to the end Permian extinction.
An explanation for why these different opinions occur is because Pangea was so large and stretched from far up in the northern hemisphere, to low down in the southern hemisphere.
As the supercontinent moved off the south pole it became unglaciated, but the continent encroached on the north pole and ice sheets were formed in the north. This meant that throughout much of the Permian there was fluctuating glaciation between the south and north.

In a paper by Hallam and Wignall it is suggested that regression, which is often linked with glaciation, did not occur.
Although sea level drop is considered one of the main causes of the extinctions they suggest that evidence from biostratigraphy shows that there was not a regression at all. Although taxa became extinct in end Permian Salt Range sediments of North Pakistan, there is no discontinuity in deposition. This suggests that regression did not happen at the time of extinction.
Other evidence supporting their hypothesis comes from the Dolomites of northern Italy where no change in deposition can be seen at the end of the Permian.

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All these contradictions are proof that the subject is still a hotly debated area and that as evidence is gained from new studies, different hypotheses are formed.

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