Oxygen isotope ratio cycle

Oxygen isotope ratio cycles are cyclical variations in the ratio of the mass of oxygen with an atomic weight of 18 to the mass of oxygen with an atomic weight of 16 present in calcite of the oceanic floor as determined by core samples. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates. Cycles in the ratio mirror climate changes in geologic history.  

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Isotopes of oxygen

An oxygen molecule (chemical symbol O) has three naturally occurring isotopes: O-16, O-17 and O-18, where the 16, 17 and 18 refer to the atomic weights. The most abundant is O-16, with a small percentage of O-18 and an even smaller percentage of O-17. Oxygen isotope analysis considers only the ratio of O-18 to O-16 present in a core sample taken from 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 carbonate, chemical formula CaCO₃, is formed from water, H₂O, and carbon dioxide, CO₂, 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.

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 temperature. At the boiling point, the vibration is sufficiently high to overcome the adhesion between water molecules and they fly into the space of the container or the atmosphere. At the 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 Celsius (18 to 27 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