Oxygen isotopes - (n.)
Among the most commonly studied stable isotopes in climate research are those of oxygen, especially the ratio between oxygen-16 (the common atom) and oxygen-18 (the rarer form). Water containing oxygen-18 (or deuterium) enters the vapor phase less readily than normal water with an oxygen-16 atom and two hydrogen atoms. Also, the water with a heavier isotope condenses more easily from vapor during precipitation as rain or snow. Thus, the distribution of these isotopes in natural waters (including the ocean) reflects evaporation and precipitation processes (and, in the oceans, the motion and mixing of different water masses). The precipitation history of a parcel of air with its associated vapor is largely controlled by temperature changes. Thus, the ratio between heavy and light isotopes of hydrogen and of oxygen in polar ice can be used to reconstruct the temperature of the precipitation of snow on a given glacier. In the reconstruction of ice age cycles, the content of oxygen-18 within the shells of foraminifers (small one-celled organisms) has proved crucial. The ratio of 18O to 16O (expressed as the deviation δ from the ratio in a standard: δ18O = [sample ratio]/[standard ratio]-1) changes in marine carbonate shells as a function of both the temperature of the water in which the shells (made of CaCO3) are precipitated, and the amount of water that has been extracted from the ocean and locked up in polar ice. The polar ice is enriched in oxygen-16 relative to the ocean, and thus the ocean is enriched in oxygen-18 whenever ice shields are large. Maximum polar ice buildup during the last several hundred thousand years changes the ocean's delta value by about 1.2 ‰ (or 0.12 percent), corresponding to a change of 0.1 ‰ for each 10 m of sea level change. The portion of a change in isotope ratio that is due to temperature change follows the rule that 0.2 ‰ of change in the delta value corresponds to a change in temperature of 1°C. (‰ is the symbol for permil, in contrast to %, the symbol for percent.)