Apparent oxygen utilization rates based on tritium-helium dating in the South China Sea: Implications for export production

We present tritium (³H) and helium isotope (δ³He) data from the South China Sea (SCS), to estimate apparent oxygen utilization rates (AOURs). The observed δ³He values are close to the theoretical solubility equilibrium value of -1.7% in the upper mixed layer, followed by an increase with depth down to ~1500 m. Below 1500 m depth, δ³He is homogenously distributed with a value of 20.9% ± 1.0%. The distribution of δ³He reveals that a significant fraction of ³He throughout the water column over the entire SCS basin is allochthonous, derived from mantle sources originated from the Pacific Ocean. By using the salinity-normalized “potential” alkalinity (NPA) as a conservative mixing tracer, ³He produced by tritium decay (tritiogenic) was separated from mantle ³He, and the apparent ³H–³He ages of the SCS water masses were subsequently calculated to be 11 ± 5 years at 100 m depth and 50 ± 4 years at 1000 m. Together with the observed dissolved oxygen concentrations, we estimated the mean AOURs and obtained values of 4.15 ± 0.27 μmol kg⁻¹·yr⁻¹ for the depth range of 100–500 m, and 0.81 ± 0.23 μmol kg⁻¹·yr⁻¹ for that between 800 and 1000 m depth. The depth-integrated AOURs between 100 and 1000 m depth, yield a spatially and temporally averaged export flux of organic carbon of 1.96 ± 0.16 mol-C·m⁻²·yr⁻¹, comparable with previous estimates based on either ²³⁸U/²³⁴Th disequilibrium or nutrient and oxygen mass balance calculations in the SCS.