Oxygen has three naturally-occuring stable isotopes: 16O, 17O, 18O, with 16O being the most abundant (99.762%).
Two international reference standards are used to report variations in oxygen isotope standards: PDB and SMOW. The use of the PDB standard in reporting oxygen isotope composition is restricted to carbonates of low-temperature origins (e.g., oceanic, lacustrine ). The conversion between SMOW and PDB scales is given by:
δ18O may be measured on: