Glaciation

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.

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: because the ice is not floating in the water, it has actually taken water out of the oceans lowering sea level.

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.