The water cycle is a key part of understanding Earth’s climate. One process in the water cycle is evaporation. Evaporation is where molecules of H2O leave the liquid state becoming molecules of H2O vapor or gas in the air above. In the gaseous state, H2O can then move through the atmosphere.
Water, of course, contains both hydrogen and oxygen. Oxygen is one of the most significant keys to deciphering past climates. Oxygen comes in heavy and light varieties, or isotopes, which are useful for paleoclimate research. Like all elements, oxygen is made up of a nucleus of protons and neutrons, surrounded by a cloud of electrons. All oxygen atoms have 8 protons, but the nucleus might contain 8, 9, or 10 neutrons. “Light” oxygen-16, with 8 protons and 8 neutrons, is the most common isotope found in nature, followed by much lesser amounts of “heavy” oxygen-18 with 8 protons and 10 neutrons. Water, therefore, will contain hydrogen as well as either oxygen-16 or oxygen-18 (shown as H216O and H218O respectively).
So not all molecules of water are created equal – that’s an important point. Some molecules have more mass than others.
The demonstration your teacher will do illustrates the difference in the evaporation of the two types of water molecules (H216O and H218O ) in the water cycle. Transport of these molecules out of the ocean (plexiglass) shows movement in the water cycle. But why were there two types of particles to represent H2O molecules? You should have seen that they were raised above the fan at different heights (H216O higher and H218O lower). The diagram to the right shows H2O molecules A and B. Look at how they differ. The difference isn’t big, but it’s important in the water cycle.
Molecule A has an oxygen with a mass = 16 coming from 8 protons and 8 neutrons. With two hydrogens, the total mass of H216O is 18. Molecule B has an oxygen with a mass = 18 from 8 protons and 10 neutrons. Adding two hydrogens, the mass of H218O is 20. These two molecules have different masses because 16O and 18O are two isotopes of oxygen. (Remember: an isotope is an element that has the same number of protons, but a different number of neutrons). Because of their different masses, H216O and H218O behave differently in the water cycle.
As water condenses and precipitates out of the atmosphere it can be incorporated into existing ice sheets. Currently, the major ice sheets exist in the polar regions. At times in the past, ice sheets have also existed across the northern hemisphere. They covered much of the United States, all of Canada, and most of northern Europe. Existing ice sheets provide a continuous record of atmospheric conditions dating back as far as 1.5 million years. The water that comprises the frozen ice sheets contains a mixture of light and heavy isotopes. As water evaporates from the oceans, more of this water tends to be the low mass molecule of water (H216O). Over time, the precipitation that falls over the poles becomes part of the ice sheets. Thus, the ice sheets will have relatively more H216O than H218O.
Condensation and Precipitation
As air rises in the atmosphere and cools, the water vapor condenses and falls as precipitation. On a global scale, warm air rises at the equator and circulates towards the poles. Because it is heavier, H218O will tend to condense first and the ratio of H218O to H216O will be greater in rain that falls at lower latitudes. As the moist air mass continues to circulate towards the poles, the precipitation that falls will become increasingly depleted of H218O molecules. However, changes in global temperatures impact how far H218O can travel from the equator before precipitating out as moisture.
Let’s focus for a moment on the precipation of the different isotopes of water in the atmosphere. During periods of warmer global temperatures there is more heat energy in the atmsophere. This allows heavy water (H218O) to stay in the gaseous phase longer and travel farther from the equator before precipitating out. Thus, during times of higher global temperatures, the ratio of heavy water to light water (H216O) found in precipitation that falls at the poles changes with a higher percentage of heavy water being present. Scientists have an indicator for the relative amount of H218O in water. It is δ18O, and is pronounced “delta-18-O”. The delta, δ, stands for “change,” and the 18O represents the oxygen-18 in H218O. So, in ice core samples taken at the poles, a higher ratio of heavy water to light water indicates higher average global temperatures. The following figure illustrates this concept:
The concentration of 18O in precipitation decreases with temperature. This graph shows the difference in 18O concentration in annual precipitation compared to the average annual temperature at each site. The coldest sites, in locations such as Antartica and Greenland, have about 5 percent less 18O than ocean water. (Graph from http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/, adapted from Jouzel et. al., 1994)
Scientists have performed global atmospheric measurement that allow them to correlate temperature to the ratio of heavy water compared to light water. This laboratory data is then used as a standard to infer past temperatures based on the ratio of isotopes found in ice cores.