Technical Note: The effect of vertical turbulent mixing on gross O<sub>2</sub> production assessments by the triple isotopic composition of dissolved O<sub>2</sub> Academic Article uri icon

abstract

  • <p><strong>Abstract.</strong> The <sup>17</sup>O excess (<sup>17</sup>&amp;Delta;) of dissolved O<sub>2</sub> has been used, for over a decade, to estimate gross O<sub>2</sub> production (G<sup>17</sup>OP) rates in the mixed layer (ML) in many regions of the ocean. This estimate relies on a steady-state balance of O<sub>2</sub> fluxes, which include air–sea gas exchange, photosynthesis and respiration but notably, not turbulent mixing with O<sub>2</sub> from the thermocline. In light of recent publications, which showed that neglecting the turbulent flux of O<sub>2</sub> from the thermocline may lead to inaccurate G<sup>17</sup>OP estimations, we present a simple correction for the effect of this flux on ML G<sup>17</sup>OP. The correction is based on a turbulent-flux term between the thermocline and the ML, and use the difference between the ML <sup>17</sup>&amp;Delta; and that of a single data-point below the ML base. Using a numerical model and measured data we compared turbulence-corrected G<sup>17</sup>OP rates to those calculated without it, and tested the sensitivity of the GOP correction for turbulent flux of O<sub>2</sub> from the thermocline to several parameters. The main source of uncertainty on the correction is the eddy-diffusivity coefficient, which induces an uncertainty of &amp;sim;50%. The corrected G<sup>17</sup>OP rates were 10–90% lower than the previously published uncorrected rates, which implies that a large fraction of the photosynthetic O<sub>2</sub> in the ML is actually produced in the thermocline.</p>