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Pleistocene Ice Age Mammals

An ice age is a period of time with extensive glaciation over a significant area.  Several ice ages have occurred through geological time, the most well known is the most recent: the Pleistocene ice age.

 

Conditions are not static during an ice age but vary between glacials, where the ice sheet is growing and spread over a large area and interglacials, where the ice retreats and a warmer climate is present for a time.  There are also intermediate periods called interstadials when the ice sheets retreat but the climate is not as warm and the retreat does not last as long as during an interglacial.

 

Glacial and interglacials can be recognised in the rock and fossil record by examining oxygen isotope ratios.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The glacials and interglacials also correspond to changes in sea level.  As the volume of water stored as ice changes, the ocean volume changes accordingly.  Therefore sea level is related to climatic change by way of temperature and ice volume.  The extensive area of the oceans means that for sea level to drop by 1m, 400,000 km3 of ice must be formed.

 

At the beginning of a glacial age continental ice sheets develop from specific centres. These centres are usually of high elevation as the snow and ice covering is thickest at these points. As the ice thickens it engulfs the local landscape and spreads out radially from the area with the thickest ice.

 

The landscape today still bears evidence of the ice ages of the past.

· ‘U’ shaped valleys carved out by the movement of glaciers are present in much of the Northern hemisphere and USA. The base of the valley is far wider than the ‘V’ shaped valleys formed by rivers.

· The strength of the glaciers can be seen from the large boulders that have been displaced from their place of origin, these are known as “erratics”.

· Large lakes “kettles” are present where softer sediment was scoured out by the rocks at the base of the glacier.

· Hanging valleys with waterfalls into the main valley where smaller side glaciers eroded down but not as far as the main larger glacier.

· Fjords are ‘U’ shaped valleys flooded with sea water as seen along much of the Norwegian coast.

· Isostatic rebound is still occurring in many areas.  When ice was present the added weight resulted in subsidence of the crust. Then the weight of the ice was removed and the crust slowly started recovering or ‘rebounding’. Evidence of this rebound can be seen as raised beaches.

· The weight of 100m of ice loading depresses the continental crust by 27 m

· Isostatic rebound in Scotland is currently occuring at a rate of 1-3 mm yr-1.

 

 

1 Press, Siever, Grotzinger, Jordan. (2004) Understanding Earth fourth edition. W.H. Freeman and Company, New York.

Ice Ages

Text Box: Oxygen Isotopes: isotopes are measured from foraminifera fossils, the carbonate shells of which have variable ratios of 16O:18O according to the temperature of the water at the time.  Therefore the ratio of oxygen isotopes in a fossil reveals the temperature and examining these fossils from a period of time shows the changes in temperature, as seen in the graph below1.













During periods of glaciation the ratio is higher in 18O isotopes because the lighter 16O is preferentially evaporated from the oceans and trapped into the glacial ice.

By Laura Wilkinson and Lynne Toland
Level 2 Palaeontology and Evolution MSci students (May 2005)