Estimates of vertical turbulent mixing used to determine a vertical gradient in net and gross oxygen production in the oligotrophic South Pacific Gyre

Mixed layer (ML) gross (GOP) and net (NOP) oxygen production rates based on in situ mass balances of triple oxygen isotopes (TOI) and O2/Ar are influenced by vertical transport from below, a term traditionally difficult to constrain. Here we present a new approach to estimate vertical eddy diffusivity (Kz) based on density gradients in the upper thermocline and wind speed-based rates of turbulent shear at the ML depth. As an example, we use this Kz, verified by an independent Be-based estimate, in an O2/TOI budget at a site in the oligotrophic South Pacific Gyre. NOP equaled 0.31 ± 0.16mmolm 2 d 1 in the ML (~55–65m depth) and 1.2 ± 0.4mmolm 2 d 1 (80%) beneath the ML, while GOP equaled 74± 27mmolm 2 d 1 (86%) in the ML and 12± 4mmolm 2 d 1 (14%) below, revealing a vertical gradient in production rates unquantifiable without the Kz estimate.