Delta18O

Delta18O, often written as δ18O, is a stable isotope ratio notation used in geochemistry, hydrology, and paleoclimatology. It expresses the relative abundance of the heavy oxygen isotope 18O to the most common isotope 16O in a sample, compared with the same ratio in a defined standard. The value is written on a per mil (‰) scale: δ18O = [ (18O/16O)sample / (18O/16O)standard − 1 ] × 1000. The reference standard is Vienna Standard Mean Ocean Water (VSMOW); older literature may use SMOW. Positive δ18O indicates enrichment of 18O relative to the standard, while negative values indicate depletion.

Measurement is typically performed by isotope ratio mass spectrometry or laser-based spectrometry. Values for natural materials range widely, from strongly negative in some meteoric waters to strongly positive in certain carbonates, with units in per mil.

Interpretation: δ18O reflects fractionation of oxygen isotopes during phase changes and chemical processes. In meteoric water, δ18O generally decreases with increasing latitude, altitude, and distance from the ocean, due to Rayleigh-type fractionation during evaporation and re-evaporation. In paleoclimate records, δ18O in ice cores, speleothems, and carbonate minerals serves as a proxy for past temperatures and hydrological conditions, subject to regional controls such as the amount effect in the tropics and changes in moisture source.

Applications: tracing water sources, studying the water cycle, reconstructing past climates, and calibrating temperature proxies.

Limitations: local processes, evaporation effects, post-depositional alteration, and the need for appropriate calibrations and multiple proxies. See also: global meteoric water line, isotope ratio mass spectrometry, stable isotopes.