A case for a pragmatic oxygen-based approach to quantifying the biological contribution to the marine carbon sink

Abstract. AOU, the ‘apparent oxygen utilization’, is a widely used concept in ocean biogeochemistry to assess and interpret ocean deoxygenation and changes of the marine carbon cycle. It provides an estimate of oxygen (O₂) used and dissolved inorganic carbon (DIC) released during organic matter degradation in the ocean interior. AOU is calculated from observations of temperature, salinity and O₂. This calculation relies on the assumption that surface water O₂ is in equilibrium with the atmosphere when water masses form and sink into the interior ocean. Using specifically designed idealized model tracers, we here provide an evaluation of the reliability and uncertainty of this approach. Global AOU reliably estimates total oxygen debt, defined as the sum of ‘true’ oxygen utilization (TOU) since last contact with the atmosphere and biotic oxygen disequilibrium (O₂^dis,bio). AOU and TOU + O₂^dis,bio agree to within about 10 %, both for the preindustrial state estimate and the transient climate change situations explored. Taking differences of the biotic components of the O₂ and DIC disequilibrium (DIC^dis,bio) into account, we find that, for the pre-industrial state estimate, the carbon equivalent of AOU is an about 15 % underestimate of the sum of DIC^remin and DIC^dis,bio. For the climate change transient we find that 𝛥AOU underestimates our estimates of the disequilibrium corrected change in C^soft (DIC^remin + DIC^{dis,bio,COU*-BGC*}) by 25 % . In summary, we suggest that AOU in particular can be used to assess causes of ocean deoxygenation, and is a very useful proxy for changes of storage of biologically processed carbon under various climate change scenarios.