ODM2 Units Type Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of the Unit or the more general quantity that the Unit represents. Stable isotope delta For stable isotopes, isotope ratios are typically reported using the delta (?) notation where ? represents the ratio of heavy isotope to light isotope in the sample over the same ratio of a standard reference material, reported using units of in "per mil" (‰, parts per thousand) and reported relative to the specific standard reference material. Chemistry Wikipedia https://en.wikipedia.org/wiki/Isotope_geochemistry#Stable_isotope_geochemistry