On the use of dissolved oxygen isotopologues as biogeochemical tracers in the Pacific Ocean

Abstract

The isotopic composition of dissolved oxygen offers a family of potentially unique tracers of respiration and transport in the subsurface ocean. Uncertainties in transport parameters and isotopic fractionation factors, however, have limited the strength of the constraints offered by ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios in dissolved oxygen. In particular, puzzlingly low ¹⁷O/¹⁶O ratios observed for some low-oxygen samples have been difficult to explain. To improve our understanding of oxygen cycling in the ocean’s interior, we investigated the systematics of oxygen isotopologues in the subsurface Pacific using new data and a 2-D isotopologue-enabled isopycnal reaction-transport model. We measured ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios, as well as the “clumped” ¹⁸O¹⁸O isotopologue in the northeast Pacific, and compared the results to previously published data. We find that transport and respiration rates constrained by O₂ concentrations in the oligotrophic Pacific yield good measurement-model agreement across all O₂ isotopologues only when using a recently reported set of respiratory isotopologue fractionation factors that differ from those most often used for oxygen cycling in the ocean. These fractionation factors imply that an elevated proportion of ¹⁷O compared to ¹⁸O in dissolved oxygen―i.e., its triple-oxygen isotope composition―does not uniquely reflect gross primary productivity and mixing. For all oxygen isotopologues, transport, respiration, and photosynthesis comprise important parts of their respective budgets. Mechanisms of oxygen removal in the subsurface ocean are discussed.