Environmentally Speaking....

       EVIDENCE FOR ICE AGES

There are three main types of evidence for ice ages:  geological, chemical, and paleontological.

 

Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics.  Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret.  Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials.  The advent of sediment and ice cores revealed the true situation:  glacials are long, interglacials short.  It took some time for the current theory to be worked out.

 

                       

 

       Glaciers scrape and scour

 

 

 The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks and ocean sediment cores.  For the most recent glacial periods ice cores provide climate proxies from their ice, and atmospheric samples from included bubbles of air.  Because water containing heavier isotopes has a higher heat of evaporation, its proportion decreases with colder conditions.  This allows a temperature record to be constructed.  However, this evidence can be confounded by other factors recorded by isotope ratios.

 

 

 

                                          GISP2 ice core at 1837 meters depth with clearly visible annual layers

 

 

 The paleontological evidence consists of changes in the geographical distribution of fossils.  During a glacial period cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes.  This evidence is also difficult to interpret because it requires;

 

• sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated
• ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed
• the finding of the relevant fossils, which requires a lot of luck.

 

                   

                                        

 

The palaeontological site of Miguasha National Park, in south-eastern Quebec on the southern coast of the Gaspé Peninsula, is considered to be the world's most outstanding illustration of the Devonian Period known as the 'Age of Fishes'. Dating from 370 million years ago, the Upper Devonian Escuminac Formation represented here contains five of the six fossil fish groups associated with this period. Its significance stems from the discovery there of the highest number and best-preserved fossil specimens of the lobe-finned fishes that gave rise to the first four-legged, air-breathing terrestrial vertebrates – the tetrapods.

 

 

Despite the difficulties, analyses of ice core and ocean sediment cores have shown periods of glacials and interglacials over the past few million years.  These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics.  Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are founding layers created much earlier than the time range for which ice cores and ocean sediment cores are available.