Equilibrium fractionation of triple-oxygen and hydrogen isotopes between ice and water

Abstract

The triple-oxygen isotopic fractionation associated with freezing is a fundamental property of water, knowledge of which is essential for reconstructions of the hydrological cycle from the triple-oxygen isotopic composition of natural materials. We constrained this isotopic fractionation, in freshwater and seawater, in a series of freezing experiments over a range of temperatures and freezing rates. The freshwater freezing experiments with the lowest freezing rates, which we consider closest to isotopic equilibrium, yield 18O/16O, 17O/16O and 2H/1H fractionations of 2.82±0.12‰, 1.49 ± 0.07‰ and 20.05 ± 0.72‰, respectively. The slowest-freezing seawater experiments yield 18O/16O, 17O/16O and 2H/1H fractionations of 2.92 ± 0.08‰, 1.55 ± 0.03‰ and 21.18 ± 1.85‰, respectively. The 18O/16O and 2H/1H fractionation estimates in freshwater and seawater are within error of each other and in broad agreement with past estimates. Our newly determined 17O/16O fractionations constrain the triple-oxygen mass dependence of water freezing to be ≈0.528, but with large uncertainty. If this mass dependence is accurate, then ice formation and melting processes in the hydrological cycle are expected to generate variability that is on the Global Meteoric Water Line.