Boda Li, Laurence Y. Yeung, Huanting Hu, and Jeanine L. Ash

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 possible 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 in the surface and deep ocean.

Mar. Chem. (2019)  10.1016/j.marchem.2019.02.006