Foraminifera shells are composed of calcium carbonate (CaCO) and are found in many common geological environments. The ratio of O to O in the shell is used to indirectly determine the temperature of the surrounding water at the time the shell was formed. The ratio varies slightly depending on the temperature of the surrounding water, as well as other factors such as the water's salinity, and the volume of water locked up in ice sheets.
 also reflects local evaporation and freshwater input, as rainwater is O-enriched—a result of the preferential evaporation of the lighter O from seawater. Consequently, the surface ocean contains greater proportions of O around the subtropics and tropics where there is more evaporation, and lesser proportions of O in the mid-latitudes where it rains more.
Similarly, when water vapor condenses, heavier water molecules holding O atoms tend to condense and precipitate first. The water vapor gradient heading from the tropics to the poles gradually becomes more and more depleted of O. Snow falling in Canada has much less HO than rain in Florida; similarly, snow falling in the center of ice sheets has a lighter  signature than that at its margins, since heavier O precipitates first.
Changes in climate that alter global patterns of evaporation and precipitation therefore change the background  ratio.
Solid samples (organic and inorganic) for oxygen isotope analysis 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.