In Deep Time
(the last 2 billion years)

Ice Ages and Hot Spells

In the bedrocks of Canada's Arctic there is evidence of ancient glaciers that, two billion years ago, carried boulders and dropped them into soft sediments. By contrast, about 70 million years ago, dinosaurs roamed through the leafy forests of the Arctic, while sharks and giant reptiles swam in warm, polar seas. Clearly, the earth's climate has varied widely throughout time. In fact, these fluctuations have likely had a profound influence in shaping life's diversity. Furthermore, the relationship between life and climate is a reciprocal one – organisms themselves have an impact on the climate of our planet. As the current greenhouse gas debate attests, the levels of carbon dioxide in the atmosphere and oceans have a key influence in the regulation of climate. Plants, which trap carbon dioxide and expel oxygen, play an important role in the balance of these two gases.

In fact, it is thought that it was the removal of carbon dioxide from the atmosphere by primitive plants that plunged the earth into its very first ice age, 2 billion years ago. There is evidence for other glaciations about 800–640 million years ago and again about 350–250 million years ago. Minor glaciations also occurred from 460–430 million years ago. Although the causes of these ice ages remain uncertain, there are plenty of theories, some of which are discussed in the What Causes the Ice Ages section.

 

Snowball Earth

About 700 million years ago, our planet was in the grip of the most extreme ice age in its history. The ice sheets extended close to the equator, as did permafrost. Evidence of glaciers from this period exists on every continent, usually in the form of extensive deposits of material left behind by a retreating ice sheet. This layer of debris is 182-metres thick north of Lake Superior and over 3.5-kilometres thick in northern Utah! Some of these glaciers occurred near sea level, even in the Tropics, unlike the Pleistocene ice ages, in which glaciers remained above 3000 m in equatorial regions! During the coldest periods, the ice-covered planet might have resembled a snowball to an observer in outer space – a very cold place indeed! In fact, the Precambrian period, from about 800–540 million years ago was characterized by episodes of this extreme cold, with estimated global temperatures as low as -50°C. Aside from the cold, the ice advances were very persistent, lasting for 10 million years at a time.

It is now thought that this period was actually punctuated by several episodes of severe heat! One explanation for these flip-flops in temperature is volcanic activity. Although it may be difficult to imagine smoking volcanoes poking out from ice-strewn fields, Mount Erebus in Antarctica is a modern example of such an active volcano. By spewing carbon dioxide into the atmosphere, which eventually accumulated, the Precambrian volcanoes resulted in a drastic rise in global temperature.

It is also thought that the only plants which lived on the ice fields of "snowball earth" were blue-green algae and that only a few, simple organisms survived in the sea. Researchers interested in these Precambrian lifeforms are studying the present day communities that exist within the sea ice of the Arctic. These communities include primitive, single-celled algae, bacteria, and some protozoans.

When the series of Precambrian ice ages finally ended, about 575–525 million years ago, life on earth exploded! A wide range of multicellular organisms evolved – the likes of which has never occurred before or has since. Termed the "Cambrian Explosion", this brief period led to the evolution of the ancestors of many modern-day organisms.

For more information on what the earth's climate has looked like through the ages, click here http://www.scotese.com/climate.htm