Oxygen isotope ratio cycle

Isotopes of oxygen

An limestone deposits in the ocean floor.

The calculated ratio of the masses of each present in the sample is then compared to a standard ratio representing a standard temperature. The ancient sea water in which the limestone was deposited is then either hotter or cooler by a quantitative amount. The method becomes statistical when many samples are considered.

Connection between calcite and water

Limestone is deposited from the calcite shells of microorganisms. Calcite, or 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.

Connection between isotopes and temperature

O-18 is two neutrons heavier than O-16 and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is therefore required to vaporize it than for O-16, and the molecule must lose less energy to condense.

Energy adds to or takes from the vibrational motion of the molecule, expressed as dew point, the molecules adhere into droplets and fall out of the atmosphere as rain or snow. Below the boiling point, the equilibrium between the number of molecules that fly out and the number that return is a function of water temperature.

A warmer water temperature means that the molecules require less energy to vaporize, as they already have more energy. A cooler water temperature means that the water requires more energy to vaporize. As a heavier, O-18 water molecule requires more energy than an O-16 water molecule to depart from the liquid state, cooler water releases vapor that is higher in O-16 content. Cooler air precipitates more O-18 than warmer air. Cooler water therefore collects more O-18 relative to O-16 than does warmer water.

Connection between temperature and climate

The O-18/O-16 ratio provides an accurate record of ancient water temperature. Water 10 to 15 degrees Fahrenheit) cooler than present represents glaciation. Precipitation and therefore glacial ice contain water with a low O-18 content. Since large amounts of O-16 water are being stored as glacial ice, the O-18 content of oceanic water is high. Water up to 5 degrees Celsius (9 °F) warmer than today represents an interglacial, when the O-18 content is lower. 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.

See also

• Geologic temperature record
• Ice age
• Marine isotopic stage
• Temperature record
• Environmental isotopes

References

• Encyclopedia Britannica under Climate and Weather, Pleistocene Climatic Change
• Harmon Craig, 1961, "Isotopic variations in meteoric waters", Science 133, pp.1702-03