Quantifying vertical nutrient fluxes along GEOTRACES sections using helium observations

The GEOTRACES programme has expanded the global ocean datasets for a suite of trace metals and also noble gases. However, estimates of the vertical fluxes that support primary production in the upper ocean remain unclear. Helium is biologically and chemically inert, and therefore is an ideal tracer of major physical processes. The subduction, advection and radioactive decay of tritium has generated an inventory of helium-3 in the ocean interior. Helium time-series measurements have previously been exploited to estimate the vertical entrainment of nitrate into the surface mixed layer.

Here, we utilise GEOTRACES helium observations to quantify vertical exchanges between the seasonal mixed layer and main thermocline in the subtropical North Atlantic and southern equatorial Pacific. We apply multiple air-sea exchange parameterizations to provide a range of helium outgassing estimates. The helium flux through the base of the mixed layer is inferred from the outgassing flux by accounting for additional helium sources (in situ tritium decay and surface bubble injection). The corresponding nitrate flux is quantified via the established correlation between helium and nitrate. Extending our analysis to other key nutrients such as iron permits multiple estimates of in situ nutrient fluxes to be quantified for both GEOTRACES zonal sections.

We compare our physical flux estimates to corresponding shipboard measurements of aeolian deposition. We show how the prevalence of each flux term varies (1) across the subtropical North Atlantic, (2) across the southern equatorial Pacific and (3) between nutrients, thereby constraining the role of two major processes which drive upper ocean resource availability.