How can the oxygen-isotope ratio in the calcium carbonate shell remains of organisms found in ocean sediment provide information about past sequences of glacier advances and retreats?

Answer 1

Because certain organisms that once lived near the surface of water can only live within a narrow range of temperatures, the distribution and type of organisms within the sediment indicate the temperature of the surface water. Most of the oxygen in seawater is composed of 8 protons and 8 neutrons in its nucleus, giving it an atomic weight of 16. However, about one out of every thousand oxygen atoms contains an extra 2 neutrons, giving it an atomic weight of 18. When ocean water evaporates, the oxygen-18 tends to be left behind. Consequently, during periods of glacier advance, the oceans, which contain less water, have a higher concentration of oxygen-18. The shells of marine organisms are constructed from the oxygen atoms exiting in ocean water, so determining the ratio of oxygen-18 to oxygen-16 within these shells yield information about how the climate may have varied in the past. A higher ratio of oxygen-18 to oxygen-16 in the sediment suggests a colder climate, whereas a lower ratio suggests a warmer climate.