The δ18O of dissolved O2 in seawater: A unique tracer of circulation and respiration in the deep sea

The isotopic composition of dissolved O 2 in seawater, expressed as the δ 18 O of O 2 , is unique among the bioactive tracers of the aphotic zone in that it is not linearly related to oxygen utilization via the stoichiometry of organic matter decomposition. In fact, δ 18 O of O 2 depends on the history of water mixing and O 2 consumption in the sample studied (Craig and Kroopnick, 1970; Kroopnick and Craig, 1976). For this reason, the variation of δ 18 O of O 2 with O 2 concentration depends on regional circulation patterns and oxygen utilization rates. The δ 18 O of O 2 can be used to chartacterize these processes by decoupling their effects. As an example of this assertion, we interpret the covariation between the concentration of O 2 and its isotopic composition in the Pacific Ocean as reported by Kroopnick (1987), using four simple representations of seawater mixing and respiration. Kroopnick's data are in general accord with an elementary model of isopycnal mixing represented by diffusive exchange and oxygen utilization in the ocean's interior, coupled with atmospheric equilibrium at the point where the isopycnals outcrop at the sea surface. This specific result illustrates the general point that δ 18 O of O 2 in seawater can serve as an important constraint on more extensive and sophisticated physical models used to estimate rates of oxygen utilization in the deep sea.