Triple Oxygen Isotopic Variation in Continental Waters and Potential Applications to Paleoclimate Research

¹⁸O/¹⁶O ratios are widely used in paleoclimate studies as proxies for temperature, precipitation amount and hydrologic change, but interpretations of these records are often challenged by the multiple factors that can influence them. Variation in ¹⁷O/¹⁶O ratios of Earth materials have long been assumed to covary with ¹⁸O/¹⁶O ratios in predictable and uniform ways such that they were not considered useful in studies of Phanerozoic climate. However, recent advances in the ability to measure small differences in ¹⁷O-excess, the deviation from an expected relationship between ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios, in both waters and low-temperature minerals and rocks (e.g., carbonates, bioapatites, silicates, oxides) present the opportunity to use triple oxygen isotope measurements in hydrological and paleoclimate studies. In particular, the sensitivity of ¹⁷O-excess to kinetic fractionation means that it can be used to constrain the influence of kinetic effects on variations in δ¹⁸O.

Here we review recently generated datasets on the triple oxygen isotope composition of the hydrosphere and show that there is considerably more variation in ¹⁷O-excess of continental waters than initially proposed. A compilation of ¹⁷O-excess data from precipitation, which includes snow from polar regions, tropical storms and weekly precipitation collections from mid-latitudes, shows that the ¹⁷O-excess of precipitation can range from -0.06 to +0.07‰. A continent-wide survey of tap waters from the U.S. mirrors the variation observed in precipitation. Among leaf waters, ¹⁷O-excess values range from -0.28 to +0.04‰ and can vary by as much as 0.16‰ in a plant within a single day. The mass-dependent effects associated with kinetic fractionation are likely responsible for the majority of the observed variation in waters, either during re-evaporation of rainfall at warmer temperatures, snow formation at very cold temperatures, or evapotranspiration within leaf waters.

In summary, the combination of the observed variation of ¹⁷O-excess in continental waters and the emerging techniques for measuring ¹⁷O-excess in a wide range of geologic materials means that it is possible to use ¹⁷O-excess to monitor the effects of kinetic fractionation on meteoric waters and provide additional constraints on variation of δ¹⁸O in sedimentary archives.