Kinetic 17O effects in the hydrologic cycle:: Indirect evidence and implications

The abundances of O-18 and deuterium in the present and past hydrologic cycle have proven to be an important tool in Earth systems science. In contrast, the abundance of O-17 in precipitation has thus far been assumed to carry no additional information to that of O-18. Here, we demonstrate, using known constraints on oxygen isotope abundances from the O-2 cycle and existing data about the natural abundance of O-17 in water, that the relationship between the discrimination against O-17 and O-18 in water may vary. This relationship, presented here as theta = In ((17)alpha)/In ((18)alpha), is found to be 0.511 +/- 0.005 for kinetic transport effects and 0.526 +/- 0.001 for equilibrium effects, with very low temperature sensitivity. As a result, the (17)Delta of precipitation is controlled primarily by kinetic effects during evaporation of the initial vapor and, in contrast to the deuterium excess, is independent of the temperature at the evaporation (and condensation) site. This makes (17)Delta a unique tracer that complements O-18 and deuterium, and may allow for a decoupling of changes in the temperature of the ocean, that serves as the vapor source, from changes in the relative humidity above it. In addition, the (17)Delta of ice caps is influenced by the kinetic effects in ice formation, and therefore measurement of ice (17)Delta can be used as an additional constraint for better understanding and parameterization of these effects. Copyright (C) 2004 Elsevier Ltd