At depth, O2 depleted because of organic matter remineralization is generally estimated based on apparent oxygen utilization (AOU). However, AOU is an imperfect measure of oxygen utilization because of O2 air-sea disequilibrium at the site of deep-water formation. Recent methodological and instrumental advances have paved the way to further deconvolve processes driving the O2 signature. Using numerical model simulations of the global ocean, we show that measurements of the dissolved O2/Ar ratio, which so far have been confined to the ocean surface, can provide improved estimates of oxygen utilization, especially in regions where the disequilibrium at the site of deep-water formation is associated with physical processes. We discuss applications of this new approach and implications for current tracers relying on O2 such as remineralization ratios, respiratory quotients and preformed nutrients. Finally, we propose a new composite geochemical tracer, [O2]*bio combining dissolved O2/Ar and phosphate concentration. Being insensitive to photosynthesis and respiration, the change in this new tracer reflects gas exchange at the air-sea interface at the sites of deep-water formation.

Plain Language Summary

Oxygen utilization at depth offers insight into organic matter remineralization and the strength of the biological carbon pump. However, the oxygen concentration in the ocean interior is also impacted by additional biotic and abiotic processes occurring at the sites of deep-water formation and during transit in the ocean interior. In this study, we summarize, formalize, and model these processes to explore how decomposing the O2 signal at depth with new tools can provide new insight and more refined budgets of the broad-scale oceanic biogeochemical cycling of oxygen and nutrients. This is particularly important in light of the recent evidence that the role of physical processes in regions of convective deep-water formation may currently be underestimated.