Glaciations have occurred all over the world and throughout the earth’s history. The relationships between glaciers, ice sheets, and climate are exceptionally complex.  This hypertext is designed to further explain and correlate these  complex interactions with  their causes and effects. 
 
 
 
 
 

     Precambrian
       When the topic of glacial periods arise people think of the ice age we currently live in. There is evidence of glaciations several times through the geologic past.  The first evidence of glaciation occurs in the Precambrian, with two separate glacial periods.  The first or oldest glaciation occurred in the early Proterozoic about 2.5-2.0 billion years ago. In the Gowganda formation, which is 12,000 meters thick and located north of Lake Huron, there are deposits known as tillites.  Tillites are believed to be lithified versions of poorly sorted glacial sediments that represent subaqueous varves. Debris with in the tillites also show glacial features such aspolish and striations.  The Gowganda formation represents the first known glacial period known as the Huronian glaciation.
       The second Precambrian glaciation occurred in the late Proterozoic 800-570 million years ago. Again the evidence for this glaciation are tillite deposits which are found on every continent except Antartica.
       The probable cause for the first glaciations to occur is a positive feedback loop triggered by the evolution of chlorophyll bearing plants or blue-green algae over 3.0 billion years ago. These algae could now perform photosynthesis, which produces free oxygen from carbon dioxide (CO2). This free oxygen could now be available to form Ozone(O3).  Eventually a thin layer of ozone formed in the atmosphere, this layer reflects incoming short wave radiation which heat up the earth’s surface.  As the amount of incoming radiation decreases, so does the earth’s climate.  This cooling was significant enough to form ice caps.  The main evidence of this explanation is found in the rock record as an increase in late Precambrian stromatolites.
 
 

    Paleozoic 
       Three separate glaciations occurred in the Paleozoic era. The first of these took place in the Ordovician period ( 500 million years ago).  At this time the African continent was located approximately  at the south pole and held an ice sheet  possibly six square kilometers larger than the current Antarctic ice sheet.  The evidence of the African ice sheet are tillites found on Africa, South America, Scotland, and North America. This wide spread evidence suggests that the ice sheet was not restricted to the South pole region.
        There is fragmentary evidence for a Devonian age glaciation found in South America and South Africa. However little is known about this glacial period.
        The last glaciation to occur prior to the Quaternary ice ages took place from the Carbiniferous to the Permian period (350-280 million years ago).  The Gondwanalandice sheet was centered on the continent of Antarctica  and south Africa, with a maximum extent that could have been twice as large as the current Antarctic ice sheet.  The Gondwanaland ice sheet was believed to have had radial flow over South America, southern Australia, and southern India. The evidence for this  is in the Dwyka series found in Africa. The Dwyka series is a 600 meter thick series of tillites, Underneath these tillites the bedrock contains grooves, striations and polish which indicate paleo flow directions. The Dwyka series is also evidence for the breakup of Pangea, for the fact that no evidence of glaciation corresponding to this time period is found in the northern continents.

        In contrast to older parts of the Earth’s history, the significant changes within the Quaternary are not changes in faunal composition, rather changes in climate.  Research shows that cooling of climate and glacial expansion began about 2.5 million years ago.  Mid-latitudinal glaciers have been present since then and fluctuated greatly.  During the  last 900,000 years the fluctuations have been amplified due to the presence of the largest mid-latitudinal ice sheets forming.  It is these fluctuations, some relatively rapid, which make the later Quaternary so interesting. For a look at the climate fluctuations of the last 900,000 years, click here.
       The cause for these fluctuations is not well understood.  There are many hypotheses, yet none explain the seemingly regular patterns except for the Milankovitch cycles.  It is thought that the Milankovitch cycles serve as triggers which start the stages of glacial and interglacial periods.  There is evidence from oxygen-isotope analyses of  as many as 21 glacial cycles that have occurred during the Quaternary period.  Yet, any terrestrial evidence is restricted to at most 4 events:

-Wisconsin glaciation (began about 80,000 years ago until holocene)
-Sangamon interglaciation
-Illinoian glaciation 
-Yarmouth interglaciation
-Kansan Glaciation
-Aftonian interglaciation
-Nebraskan glaciation

        As already mentioned, a history of  Pleistocene glaciation can be constructed with better resolution using oxygen isotope analyses.   The ratio of the stable isotopes 18O and 16O in both the oceans and atmosphere are a function of temperature.  This is due to the fact that water molecules containing 16O are more readily evaporated than molecules with 18O (because they are lighter.)  If those water molecules are trapped as snow or ice, the oceans become depleted in 16O relative to 18O, producing  a larger oxygen isotope ratio in the oceans.  Conversely, a smaller ratio is found in trapped in the ice.  Thus, ratios found in ice cores reflect the temperatures of the atmosphere across time and can be used to infer glacial periods. 
 

Click here to see a diagram of Oxygen isotope ratios with respect to glacial and interglacial periods.

      This method of reconstructing paleoclimates  has produced a more complex stratigraphic sequence than any terrestrial evidence has inferred.  As a result, there has been somewhat of an abandonment of at least the later terminology describing glacial periods above, notably Nebraskan and Kansan. Glacial and interglacial stages are now commonly referred to by their isotopic stage. 

       For a look at more oxygen isotope

       Glacial Cycles with Isotope Ratios

        The reasons or causes for these fluctuations has focused much attention to the Milankovitch cycles.  Milankovitch cycles are cycles in the Earth’s orbit that influence the amount of solar radiation strking different parts of the Earth at different times of the year.  To explain this we are interested in three types of variations in Earth’s orbital patterns.  These are: the eccentricity of the orbit, obliquity (axial tilt), and precession.  For explanation and description of these processes click below.

        The effects of the three parameters has been mathematically combined to produce a curve showing the relation of  varying insolation and time.  These curves have been highly promising in correlating the Milankovitch signals with curves of oxygen isotope records.  Thus, there is direct evidence of the Milankovitch cycles as being, at least trigger cause of glacial periods.  For a closer look at the results of this concept, visit Click here.
 

          More links to other Milankovitch concerns:

     Liberty Australia's Global Warming Page

    NOAA-Paleoclimatology Program

    U.S. Navy Academy