How Many Ice Ages?—An Ocean of Clues
So, we have an immense amount of evidence that ice ages really occurred. But, on land, one advancing glacier might erode the record of an older one. A pile of four tills deposited by glaciers and separated by soils from warmer times may record four advances, or forty, with most of the record having been eroded away and only a few deposits remaining. To learn how many ice ages there were, we go to places in the deep oceans, where sediment has been piling up without erosion for millions of years. We can identify glaciations and warmer times using characteristics of the shells in those sediments, as we discuss next, and we can learn their ages.
Water in the oceans is not all the same—roughly one molecule in 500 has an extra neutron or two in one or more of the oxygen or hydrogen atoms. Such “heavy” water is still water but weighs a little extra. (If you don’t remember isotopes, go back and look at the introduction to chemistry near the start of the course.) Not surprisingly, light molecules evaporate more easily than heavy molecules. Water vapor, rain, and snow thus are slightly “lighter” than the ocean; that is, the ratio of light water molecules to heavy ones is larger in vapor, rain, and snow than in the ocean from which the vapor, rain, and snow came.
During an ice age, roughly 300 feet of this slightly light water evaporates from the ocean and piles up on land as gigantic ice sheets, leaving the oceans a bit heavier isotopically. When the ice age ends and the ice melts, that light water from the ice sheets is returned to the ocean, making the ocean isotopically lighter.
These changes are small - the ocean is water at all times! Rounding just a little, in the modern ocean 1 of each 500 water molecules is heavy, which is the same as saying that 1000 of each 500,000 water molecules are heavy. When the ice sheets were big, roughly 1001 of each 500,000 water molecules were heavy. This difference does not affect the water, but it is easy to measure with modern instruments.
Many plants and animals that grow in the ocean build shells of calcium carbonate (the stuff of limestone) or silica, both of which contain oxygen obtained from the water, and record the isotopic composition of the water. During times with more ice, the shells that grow and then fall to the sea floor have slightly heavier oxygen isotopes. As the shells pile up, they record a history of the ice volume on Earth with the youngest layers on top. With enough care, knowledge, and instrumentation, dedicated workers can obtain consistent, reproducible data that tell a wonderful, clear story. (There are a few additional details, but the main story is this simple.)
Amazingly, the story was predicted correctly decades before scientists gained the ability to test it, as we’ll see in the next section! The story is that, over the most recent 800,000 years, ice has generally grown for about 90,000 years, shrunk for 10,000 years, grown for 90,000 years, shrunk for 10,000 years, etc. Superimposed on this are smaller wiggles, with a spacing of about 19,000 years and 41,000 years. (As described in the Enrichment, the ice was growing and shrinking more than 800,000 years ago, and there have been times in Earth’s history with no ice on Earth, and other times when the Earth was completely ice-covered. We will revisit some of these issues later.)