In the early 20th century, it was theorized that ice ages resulted from variations in the Earth's orbit. No one could attempt to prove this theory, however, because there was not enough trustworthy data about when ice ages occurred and how long they lasted. Recently, however, a group of scientists discovered how to determine this chronology by using two oxygen isotopes, 16 and 18, that are found in ocean residue. Almost all oxygen in ocean water is oxygen 16, but the forming of ice sheets results in less water evaporating from the ocean, and an increasing ratio of oxygen 18 molecules. The greater the ratio of these heavier molecules found in ocean sediments, the more land ice was present. The author of this passage is primarily interested in