Noun
English Wikipedia has an article on:oxygen-18Wikipedia
oxygen-18 (uncountable)
(physics) A minor stable isotope of oxygen, 8O, having eight protons and ten neutrons; it amounts to about 0.2% of the element in nature
Full replacement with heavy atom isotopes can be accomplished in higher organisms with other non-radioactive heavy isotopes (such as carbon-13, nitrogen-15, and oxygen-18), but this cannot be done for the stable heavy isotope of hydrogen. Source: Internet
Marine organisms then incorporate more oxygen-18 into their skeletons and shells than they would in a warmer climate. Source: Internet