Technical Note: The effect of vertical turbulent mixing on gross O₂ production assessments by the triple isotopic composition of dissolved O₂

Abstract. The ¹⁷O excess (¹⁷Δ) of dissolved O₂ has been used, for over a decade, to estimate gross O₂ production (G¹⁷OP) rates in the mixed layer (ML) in many regions of the ocean. This estimate relies on a steady-state balance of O₂ fluxes, which include air–sea gas exchange, photosynthesis and respiration but notably, not turbulent mixing with O₂ from the thermocline. In light of recent publications, which showed that neglecting the turbulent flux of O₂ from the thermocline may lead to inaccurate G¹⁷OP estimations, we present a simple correction for the effect of this flux on ML G¹⁷OP. The correction is based on a turbulent-flux term between the thermocline and the ML, and use the difference between the ML ¹⁷Δ and that of a single data-point below the ML base. Using a numerical model and measured data we compared turbulence-corrected G¹⁷OP rates to those calculated without it, and tested the sensitivity of the GOP correction for turbulent flux of O₂ from the thermocline to several parameters. The main source of uncertainty on the correction is the eddy-diffusivity coefficient, which induces an uncertainty of ∼50%. The corrected G¹⁷OP rates were 10–90% lower than the previously published uncorrected rates, which implies that a large fraction of the photosynthetic O₂ in the ML is actually produced in the thermocline.