Water molecules from ice cores and shells taken from sediments have another story to tell. They contain oxygen of two different types and two different types of hydrogen.
Most chemical elements exist as two or more isotopes. Different isotopes of an element are identical chemically, but they have different atomic masses. Oxygen has three isotopes, two of which are important: 16O and
18O. Seawater contains 99.76 percent H216O and 0.2 percent H218O. It
Carbonate from the seabed
also contains 0.03 percent HDO; the remainder contains the third oxygen isotope, H217O. HDO is deuterium oxide or "heavy water," in which a deuterium (D) atom substitutes for one of the hydrogen atoms. The difference is that a hydrogen nucleus consists of a single proton and a deuterium nucleus consists of one proton and one neutron, so it is heavier by the weight of the neutron.
When water evaporates, more H216O enters the air than H218O or HDO, because both of these molecules are heavier than H216O. A high rate of evaporation therefore depletes seawater of H216O. Being heavier, cloud drops of H218O and HDO fall as precipitation sooner than drops of H216O. Most rain and snow soon melt and the water returns to the ocean, but snow that falls onto the polar ice sheets remains there. Consequently, if the polar ice lasts for a very long time, the oceans become steadily more depleted of H216O. When the ice melts, its accumulated H216O returns to the oceans and the original balance is restored.
Ice obtained from ice cores reveals the proportions of H216O, H218O, and HDO present in the water. This tells scientists more than simply whether or not the ice sheets were extensive at a particular time. The rate of seawater evaporation depends on the air temperature. When the temperature is high, more H218O and HDO evaporate. If these molecules become more abundant in the ice, it means the weather was warm in middle latitudes, which is where most of the water in polar ice caps originates. If H216O becomes more abundant, it means conditions were cooler in middle latitudes.
Was this article helpful?