18O

Oxygen-18 (18O) is a stable, non-radioactive isotope of oxygen with mass number 18. It consists of eight protons and ten neutrons. Along with 16O and 17O, it is one of the three naturally occurring isotopes of oxygen.

Natural abundance and distribution are modest: 18O makes up about 0.204% of Earth's oxygen atoms, with 16O comprising about 99.76% and 17O about 0.037%. The 18O/16O ratio varies among materials and environments (for example in water and carbonate minerals) and is commonly expressed as delta-18O, a measure used in paleoclimatology and hydrology to infer temperature and hydrological cycles.

Nucleosynthesis and occurrence in nature: 18O is produced in stellar interiors during helium burning and through related nuclear reactions. It is stabilized in nature and accumulates in Earth’s crust and oceans, where its relative abundance is influenced by geological and atmospheric processes.

Applications: The 18O/16O ratio is a key proxy in geochemistry, paleoclimatology, and hydrology. It helps reconstruct past temperatures, precipitation patterns, and water cycle changes by analyzing samples from ice cores, minerals, and waters. In isotopic labeling, 18O is used in tracer studies and in spectroscopic analyses to study molecular vibrations and reaction mechanisms.

Measurement and methods: 18O content is typically measured by isotope ratio mass spectrometry (IRMS) or by high-precision spectroscopic techniques. Substituting 18O for 16O also shifts vibrational frequencies in molecules, aiding analytical and structural studies.