Evidence of Glacial Cycles
There are three main types of evidence of past ice ages.
1. Debris
2. Amount of CO2 (carbon dioxide) in the atmosphere
3. Amount of heavy oxygen in single cell organisms
Debris
Glaciers scour the earth below them as they slide towards lower grounds. Deposits of debris moved by glaciers can be seen here in Central NewYork. Drumlins are tear-drop deposites left by ice masses during the last ice age which peaked about 14,000 years ago. Other signs of glacier movements are the Finger Lakes, deep valleys produced by the motion of ice masses. Cayuga Lake is particularly deep (- 400 meters) and its bottom is actually below see level. To see some pictures, click here (from the Hartwick Geology Department).
Amount of CO2 in the atmosphere
Bubbles of air are found to be trapped in today's glaciers. By taking a sample as a function of depth, the composition of air bubbles which were trapped thousands of years ago can be analyzed. It is found that the content of CO2 (carbon dioxide) changed over the years in concert with the thickness of deposited ice. High level of carbon dioxide is associated with high ambient temperatures, or retreating glaciers. To see the data, click here (from the Hartwick Geology Department).
This discovery has other consequences. Most of the carbon dioxide is actually stored in the oceans, and there is a continuous exchange of carbon dioxide between air, land, and the oceans. Thus it seems plausible that the oceans should be considered in the study of dramatic global climatic changes, as they occured during the ice ages.
Amount of heavy oxygen in single cell organisms
The most abundant isotope of oxygen is Oxygen-16, where the number 16 stands for the sum of protons (8) nad neutrons (8) in the nucleus of an oxygen atom. There is also a much less abundant isotope of oxygen, Oxygen-18, with 8 protons and 10 neutrons. This latter type, when combined with hydrogen to form water (H2O), falls slightly more readily to the ground than water containing the lighter variety of oxygen. Cold air holds less moisture than warm air. Thus, during the ice ages, there would have been less Oxygen-18 in the air and more in the oceans.
Foraminifera are simple organisms which inhabit shells. When they die and fall to the ocean bottoms, their debris retain the signature of the composition of seawater at that time. By extracting and analyzing samples of seafloor, researchers found that the change in the content of Oxygen-16 vs. Oxygen-18 tracked the change of glacial cover.
This page was produced by Gianfranco Vidali.