How does oxygen isotope dating work

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Thus, glaciers are relatively enhanced in 16 O, while the oceans are relatively enriched in 18 O. This imbalance is more marked for colder climates than for warmer climates. In fact, it has been shown that a decrease of one part per million 18 O in ice reflects a 1.

While there are complexities with the analysis, a simple measurement of the isotopic ratio of 18 O in ice cores can be directly related to climate. Ice cores from Greenland or Antarctica are often layered, and the layers can be counted to determine age.

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The heavy oxygen ratio can then be used as a thermometer of ancient climates. The oxygen in the H 2 O is enriched in the lighter 16 O. This H 2 O condenses in clouds, falling on land as precipitation. Thus, H 2 O that is part of the terrestrial water cycle is enriched in the light 16 O isotope and sea water is enriched in the heavier 18 O isotope.

Glacial ice is therefore made up primarily of water with the light 16 O isotope. The same isotopic analyses can be made in ocean sediment cores on the shells of dead marine organisms Figure 6.

Carbon & Oxygen Isotopes

Some of these organisms are made up of calcium carbonate CaCO 3 , and the oxygen in the carbonate reflects the isotopic abundance in the shallow waters where the creatures lived. Thus if we can find and date ever more ancient sediments made up of old sea shells, we can determine the isotopic ratio of oxygen and infer the sea surface temperature at that time.

The more 18 O found in the sediment, the colder the climate inverse relationship to that of glacier ice.

The calculated ratio of the masses of each present in the sample is then compared to a standard, which can yield information about the temperature at which the sample was formed - see Proxy climate for details. The addition of more energy is required to vaporize H 2 18 O than H 2 16 O, and H 2 18 O liberates more energy when it condenses.

In addition, H 2 16 O tends to diffuse more rapidly. Because H 2 16 O requires less energy to vaporize, and is more likely to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H 2 16 O, and the residual liquid is enriched in H 2 18 O.


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When water vapor condenses into liquid, H 2 18 O preferentially enters the liquid, while H 2 16 O is concentrated in the remaining vapor. As an air mass moves from a warm region to a cold region, water vapor condenses and is removed as precipitation.

Oxygen isotope ratio cycle - Wikipedia

The precipitation removes H 2 18 O, leaving progressively more H 2 16 O-rich water vapor. Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures. The subsequent uptake of hurricane rainfall in trees, creates a record of the passing of hurricanes that can be used to create a historical record in the absence of human records. As colder temperatures spread toward the equator, water vapor rich in 18 O preferentially rains out at lower latitudes.

The remaining water vapor that condenses over higher latitudes is subsequently rich in 16 O. Since large amounts of 16 O water are being stored as glacial ice, the 18 O content of oceanic water is high.


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  • Ice core dating using stable isotope data – University of Copenhagen.
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A plot of ancient water temperature over time indicates that climate has varied cyclically, with large cycles and harmonics , or smaller cycles, superimposed on the large ones. This technique has been especially valuable for identifying glacial maxima and minima in the Pleistocene.

Oxygen isotope ratio cycle

Limestone is deposited from the calcite shells of microorganisms. Calcite, or calcium carbonate , chemical formula CaCO 3 , is formed from water , H 2 O, and carbon dioxide , CO 2 , dissolved in the water. The carbon dioxide provides two of the oxygen atoms in the calcite. The calcium must rob the third from the water. The isotope ratio in the calcite is therefore the same, after compensation, as the ratio in the water from which the microorganisms of a given layer extracted the material of the shell.

A higher abundance of 18 O in calcite is indicative of colder water temperatures, since the lighter isotopes are all stored in the glacial ice.