isotopes of oxygen-16

Oxygen is one of the most important 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 can 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 smaller amounts of "heavy" oxygen 18, with 8 protons and 10 neutrons.

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The ratio (relative amount) of these two types of oxygen in water changes with the weather. By determining how the ratio of heavy and light oxygen in marine sediments, ice cores, or fossils is different from a universally accepted standard, scientists can learn something about climate changes that have occurred in the past. The standard scientists use for comparison is based on the ratio of oxygen isotopes in ocean water at a depth of 200 to 500 meters.

What climatic factors influence the proportion of oxygen isotopes in ocean water?
Evaporation and condensation are the two processes that most influence the ratio of heavy oxygen to light oxygen in the oceans. Water molecules are made up of two hydrogen atoms and one oxygen atom. Water molecules that contain light oxygen evaporate slightly more easily than water molecules that contain a heavy oxygen atom. At the same time, the water vapor molecules that contain the wide variety of oxygen condense more easily.

The Oxygen-18 isotope has two additional neutrons, for a total of 10 neutrons and 8 protons, compared to the 8 neutrons and 8 protons in a normal oxygen atom. The slightly greater mass of 18O (12.5 percent more than 16O) results in the differentiation of isotopes in Earth's atmosphere and hydrosphere. Scientists measure differences in oxygen isotope concentrations to reveal past climates. [Mouse over cores to animate.] (Illustration by Robert Simmon, NASA GSFC)

Graph of oxygen depletion 18 as a function of temperature
As air cools rising into the atmosphere or moving toward the poles, moisture begins to condense and fall as precipitation. At first, rain contains a higher proportion of water made of heavy oxygen, since those molecules condense more easily than water vapor that contains light oxygen. The remaining moisture in the air is depleted into heavy oxygen as air continues to move poleward to colder regions. As moisture reaches higher latitudes, falling rain or snow is made up of more and more water molecules that contain light oxygen.


Heavy oxygen-rich ocean waters: During ice ages, colder temperatures extend toward the equator, so heavy oxygen-containing water vapor leaves the atmosphere at even lower latitudes than in milder conditions. Light oxygen-containing water vapor moves toward the poles, eventually condensing, and falling onto the ice sheets where it remains. The remaining water in the ocean develops an increasing concentration of heavy oxygen compared to the universal standard, and the ice develops a higher concentration of light oxygen. Therefore, the high concentrations of heavy oxygen in the ocean tell scientists that the light oxygen was trapped in the ice sheets. Exact oxygen ratios can show how much ice the Earth covered.

Light Oxygen Rich Ocean Waters - Conversely, as temperatures rise, ice sheets melt and fresh water runs into the ocean. The melting returns light oxygen to the water and reduces the salinity of the world's oceans. Higher-than-standard global light oxygen concentrations in ocean water indicate that global temperatures have warmed, resulting in less global ice cover and less saline waters. Because oxygen-containing water vapor condenses and falls as rain before light oxygen-containing water vapor, higher-than-standard local concentrations of light oxygen indicate that basins that flow into the sea at that The region experienced heavy rains, which produced more diluted waters. Therefore, scientists associate lower levels of heavy oxygen (again, compared