Oxygen isotope ratio cycle

Oxygen (chemical symbol O) has three naturally occurring isotopes: ¹⁶O, ¹⁷O, and ¹⁸O, where the 16, 17 and 18 refer to the atomic mass. The most abundant is ¹⁶O, with a small percentage of ¹⁸O and an even smaller percentage of ¹⁷O. Oxygen isotope analysis considers only the ratio of ¹⁸O to ¹⁶O present in a sample.

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.

¹⁸O is two neutrons heavier than ¹⁶O and causes the water molecule in which it occurs to be heavier by that amount. The additional mass changes the hydrogen bonds so that more energy is required to vaporize H₂¹⁸O than H₂¹⁶O, and H₂¹⁸O liberates more energy when it condenses. In addition, H₂¹⁶O tends to diffuse more rapidly.

Because H₂¹⁶O requires less energy to vaporize, and is more likely to diffuse to the liquid phase, the first water vapor formed during evaporation of liquid water is enriched in H₂¹⁶O, and the residual liquid is enriched in H₂¹⁸O. When water vapor condenses into liquid, H₂¹⁸O preferentially enters the liquid, while H₂¹⁶O 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. The precipitation removes H₂¹⁸O, leaving progressively more H₂¹⁶O-rich water vapor. This distillation process causes precipitation to have lower ¹⁸O/¹⁶O as the temperature decreases. Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures.

Due to the intense precipitation that occurs in hurricanes, the H₂¹⁸O is exhausted relative to the H₂¹⁶O, resulting in relatively low ¹⁸O/¹⁶O ratios. 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.[1]

In laboratories, the temperature, humidity, ventilation and so on affect the accuracy of oxygen isotope measurements.[2] Solid samples (organic and inorganic) for oxygen isotope measurements are usually stored in silver cups and measured with pyrolysis and mass spectrometry. Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.[2]

The ¹⁸O/¹⁶O ratio provides a record of ancient water temperature. Water 10 to 15 °C (18 to 27 °F) cooler than present represents glaciation. As colder temperatures spread toward the equator, water vapor rich in ¹⁸O preferentially rains out at lower latitudes. The remaining water vapor that condenses over higher latitudes is subsequently rich in ¹⁶O.[3] Precipitation and therefore glacial ice contain water with a low ¹⁸O content. Since large amounts of ¹⁶O water are being stored as glacial ice, the ¹⁸O content of oceanic water is high. Water up to 5 °C (9 °F) warmer than today represents an interglacial, when the ¹⁸O content of oceanic water 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.

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. A higher abundance of ¹⁸O in calcite is indicative of colder water temperatures, since the lighter isotopes are all stored in the glacial ice. The microorganism most frequently referenced for identifying marine isotope stages is foraminifera.[4]

Earth's dynamic oxygenation evolution is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. Responsible for these fluctuations in oxygenation were likely driven by the Lomagundi carbon isotope excursion.[5]

• δ18O
• Isotope fractionation

• Encyclopædia Britannica under Climate and Weather, Pleistocene Climatic Change
• Craig Harmon (1961). "Isotopic variations in meteoric waters". Science. 133 (3465): 1702–1703. Bibcode:1961Sci...133.1702C. doi:10.1126/science.133.3465.1702. PMID 17814749. S2CID 34373069.
• Epstein S.; Mayeda T. (1953). "Variation of O18 content of waters from natural sources". Geochimica et Cosmochimica Acta. 4 (5): 213–224. Bibcode:1953GeCoA...4..213E. doi:10.1016/0016-7037(53)90051-9.
• Veizer Ján; Godderis Yves; François Louis M (2000). "Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon" (PDF). Nature. 408 (6813): 698–701. Bibcode:2000Natur.408..698V. doi:10.1038/35047044. PMID 11130067. S2CID 4372892. Archived from the original (PDF) on 2013-10-06. Retrieved 2014-10-23.

• NASA Earth Observatory: The Oxygen Balance
• Scripps O2 Global Oxygen Measurements