Environmentally Speaking....

        THE ICE AGES

       

                

The term "ICE AGE” or "GLACIAL AGE” is a geological period of long-term reduction in the temperature of the Earth’s surface and atmosphere, resulting in the expansion of the continental ice sheets, polar ice sheets and the alpine glaciers.  Ice ages are a natural system.  Within a long-term ice age, individual pulses of extra cold climate are termed "GLACIAL PERIODS” or also known as "GLACIALS” or "GLACIATIONS”, and intermittent warm periods called "INTERGLACIALS”.  An ice age implies the presence of extensive ice sheets in the northern and southern hemispheres, we are still in the ice age that began at the start of the Pleistocene (reason being is that Greenland and Antarctic ice sheets still exist).

 

The "THE ICE AGE” refers to the most recent colder period that peaked at the Last Glacial Maximum approximately 20,000 years ago, in which extensive ice sheets lay over large parts of the North American and Eurasian continents. 

 

The causes of ice ages are not fully understood for both the large-scale ice age periods and the smaller ebb and flow of glacial-interglacial periods within an ice age.  The consensus is that several factors are important:  atmospheric composition (the concentrations of carbon dioxide, methane); changes in the Earth’s orbit around the Sun known as Milankovitch cycles (and possibly the sun’s orbit around the galaxy); the motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on the earth’s surface, which affect wind and ocean currents; variations in solar output; the orbital dynamics of the Earth-Moon system; and the impact of relatively large meteorites, and volcanism including eruptions of super volcanoes.

 

Some of these factors influence each other.  For example, changes in earth’s atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO₂).   

 

 Major Ice Ages

There have been at least five major ice ages in the earth’s history.  Rocks from the earliest well established ice age, called the Huronian, formed around 2.4 to 2.1 GA (billion) years ago during the early Proterozoic Eon. Probably the thickest ice (approximately 3,300m) occurred over the Hudson Bay.

 

The next well-documented ice age, and probably the most severe of the last billion years, occurred from 850 to 630 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which glacial ice sheets reached the equator, possibly being ended by the accumulation of greenhouse gases such as CO₂ produced by volcanoes.  The presence of ice on the continents and                                             

pack ice on the oceans would inhibit both silicate weathering and photosynthesis, which are the two major sinks for CO₂ at present.

 

A minor ice age, the Andean-Saharan, occurred from 460 to 430 million years ago, during the Late Ordovician and the Silurian period.  There were extensive polar ice caps a intervals from 350 to 260 million years ago in South Africa during the Carboniferous and early Permian Periods, associated with the Karoo Ice Age. 

 

The Karoo Ice Age occurred from 360 to 260 million years ago.  It is thought that this ice age was largely the cause of the evolution of land plants with the onset of the Devonian period.  The Earth during this time was covered with an immense degree of vegetation compared to earlier times, and this caused a long term increase in planetary oxygen levels and reduction of CO₂  levels that resulted in this ice age.

 

An ice sheet on Antarctica began to grow some 20 million years ago.  The current ice age, the Pliocene Quaternary glaciation, started about 2.58 million years ago during the late Pliocene when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of  glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called glacials (glacial advance) and interglacials ( glacial retreat).  The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago.  All that remains of the continental ice sheets are the Greenland, Antarctica ice sheets and smaller glaciers such as on Baffin Island.      

 

 

Huronian Glaciation

 

The Huronian glaciation extended from 2400 Ma to 2100 Ma, during the Siderian and Rhyacian periods of the Paleoproterozoic era, following the oxygen catastrophe.  It was one of the most severe ice ages in geologic history and some geologists believe that it was very similar to the Snowball earth ice age that happened in the Neoproterozoic era.

 

 

 

Cryogenian Glaciation

 

The Cryogenian is a geologic period that lasted from 850 to 630 Ma.  The Cryogenian forms the second geologic period of the Neoproterozoic era, preceded by the Tonian Period and followed by the Ediacaran.  The Sturtian and Marinoan glaciations, which are the greatest ice ages known to have occurred on Earth and may have covered the entire planet, occurred during this period.  These so-called "snowball earth” events are the subject of much scientific controversy.  The main debate involves whether these glaciations were truly global or merely localised events.

 

Note:  The period has not received the international ratification that all geological time periods undergo.  The period is defined only on the ages of the rocks and not on any observable and documented global event.  This is problematic as estimates of rock ages are variable and are subject to laboratory error.  Example:  The Cambrian Period is marked not by rock younger than a given age (542 Ma), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblage.  This means that rocks can be recognised as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date.  As yet, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, and its base is only loosely set to 850 Ma.   

 

Climate  

 

Glacial deposits indicate that Earth suffered the most severe ice ages in its history during this period.  Glaciers extended and contracted in a series of rhythmic pulses, possible reaching as far as the equator.  It is generally considered to be divisible into at least two major worldwide glaciations.  The Sturtian glaciation persisted from 750 million years ago to 700 Ma, and the Marinoan/Varanger glaciation terminated at circa 635 Ma.  The deposits of glacial tillites also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply-frozen planetary oceans called "Snowball Earth”.

 

Paleogeography

 

During the Cryogenian, the supercontinent Rodinia broke up, and the supercontinent Pannotia began to form. 

 

Cryogenian biota and fossils

 

Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period.  During the Cryogenian period, the oldest known fossils of sponges make an appearance. 

 

 

 

Andean-Saharan Glaciation

 

The Andean-Saharan glaciation was from 460 Ma to 430 Ma, during most of the Silurian period and the beginning of the Devonian period.

 

 

 

Karoo Ice Age  

 

The Karoo Ice Age from 360-260 Ma was the second major ice age of the Phanerozoic Eon.  It is named after the glacial tills found in the Karoo regions of South Africa where evidence for this ice age was first clearly identified.  

 

At least two major periods of glaciation have been discovered:

 

·         The first glacial period was associated with the Mississippian era (359.2-318.1 Ma):  ice sheets expanded from a core in southern Africa and South America.

·         The second glacial period was associated with the Pennsylvanian era (318.1-299 Ma); ice sheets expanded from a core in Australia and India. 

 

The extent of glaciation in Antarctica is not exactly known, due to its present ice sheet.

 

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

 

Causes of the Karoo Ice Age

 

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.  Oxygen levels reached anything up to 35%, and global carbon dioxide got below the 300 parts per million level which is today associated with glacial periods.  This reduction in the greenhouse effect was coupled with lignin and cellulose (as tree trunks and other vegetation debris) accumulating and being buried 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 cooler summers which could not melt the previous winter’s snow accumulations.  The growth in snowfields to 6 metres deep would create sufficient pressure to convert the lower levels to ice.  Further pressure would melt the bottom layer, lubricating and letting the snowfield begin moving downslope as a glacier.  Earth’s increased planetary albedo produced by the expanding ice sheets would lead to positive feedback loops, spreading the ice sheets still further, until the process hit limit.  Falling global temperatures would eventually limit plant growth, and the rising levels of oxygen would increase the frequency of firestorms because damp plant matter could burn.  Both these effects return carbon dioxide to the atmosphere, reversing the "snowball” effect and forcing greenhouse warming, with CO₂ levels rising to 300 parts per million in the following Permian period.  Over a longer period the evolution of termites, whose stomachs provided an anoxic environment for methanogenic lignin digesting bacteria, prevented 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 lower planetary albedo resulting from the fall in size of the glaciated areas would have been enough for 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 burial and removal of carbon (as coal).  With a smaller area for deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet.  By 250 million years ago, planet Earth had returned to a percentage of oxygen similar to that found today. 

 

The effects of the Karoo Ice Age  

 

The rising levels of oxygen in the Karoo Ice Age had major effects upon evolution of plants and animals.  Higher oxygen concentration (and accompanying higher atmospheric pressure) enabled energetic metabolic processes which encouraged 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, and some scorpions reached 50 or 70 cm.  The rising levels of oxygen also led to the evolution of greater fire resistance in vegetation and ultimately to the evolution of flowering plants. 

 

In addition, the Karoo Ice Age has unique sedimentary sequences called cyclothems.  These were produced by the repeated alterations of marine and nonmarine environments.