Triple oxygen isotopes in the water cycle

The past decade has seen a remarkable expansion of studies that use mass-dependent variations of triple oxygen isotopes (16O, 17O, 18O) in isotope hydrology and isotope geochemistry. Recent technological and analytical advances demonstrate that small deviations of δ′18O and δ′17O from a mass-dependent reference relationship are systematic and are explained by well-known equilibrium and kinetic fractionations. Measurements of δ′18O and δ′17O complement traditional metrics like deuterium-excess, constrain isotope effects of kinetic fractionation that are impossible to discern with δ18O alone, and help reconstruct past environmental conditions from geologic records. In this review, we synthesize published meteoric (derived from precipitation) water triple oxygen isotope data with a new, near-global surface water dataset of δ′18O, δ′17O, δ2H, deuterium-excess, and ∆′17O, where ∆′17O is defined as δ′17O – λref δ′18O, δ′ notation is a logarithmic definition of the common δ value (δ′=ln(δ + 1), and λref is equal to 0.528. The expanded dataset shows that meteoric water δ′18O and δ′17O fit multiple regression lines and indicates that one global meteoric water line does not adequately describe all triple oxygen isotope data. Instead, this isotope system may be sensitive to processes such as moisture transport, rainout, and evaporation that do not affect the water cycle equally across the globe. This review provides a practical guide to understand ∆′17O variation in waters, explains the utility of this isotope system in hydrologic and paleoclimate studies, and outlines directions of future work that will expand the use of ∆′17O.