Kinetic and equilibrium fractionation of O2 isotopologues during air-water gas transfer and implications for tracing oxygen cycling in the ocean

Abstract

Oxygen isotopologues are useful tools for understanding biogeochemical processes and chemical budgets in the ocean. For example, the triple‑oxygen isotope composition of dissolved oxygen in the ocean mixed layer (i.e., its δ17O and δ18O values) is widely used to estimate gross oxygen productivity (GOP), a quantity closely related to gross primary productivity. While recent work has demonstrated the importance of upwelling and horizontal transport to these estimates, the isotopic effects of gas exchange when the mixed layer is out of solubility equilibrium have only been measured for 18O/16O. Oxygen is rarely at 100% saturation in the surface ocean, so most regions experience net ingassing or outgassing; kinetic fractionation across the air-water boundary is therefore expected to be important. Here, we present the results of air-water gas transfer experiments designed to obtain the kinetic and equilibrium fractionation factors for the four rare O2 isotopologues 16O17O, 16O18O, 17O18O, and 18O18O relative to 16O16O. Furthermore, we examine their potential effects on isotopologue-based GOP estimates and connect the observed air-water kinetic fractionation factors to dissolved-phase diffusive isotopic fractionation. These kinetic fractionation effects may provide additional constraints on O2 cycling at the surface and in the deep ocean.