Articles | Volume 12, issue 24
Research article
21 Dec 2015
Research article |  | 21 Dec 2015

Coastal upwelling off Peru and Mauritania inferred from helium isotope disequilibrium

R. Steinfeldt, J. Sültenfuß, M. Dengler, T. Fischer, and M. Rhein

Abstract. Upwelling is an important process, bringing gases and nutrients into the ocean mixed layer. The upwelling velocities, however, are too small to be measured directly. Here we use the surface disequilibrium of the 3He / 4He ratio measured in two coastal upwelling regions off Peru in the Pacific ocean and off Mauritania in the Atlantic ocean to calculate the regional distribution of vertical velocities. To also account for the fluxes by diapycnal mixing, microstructure-based observations of the vertical diffusivity have been performed on all four cruises analysed in this study. The upwelling velocities in the coastal regions vary between 1.1 ± 0.3 × 10−5 and 2.8 ± 1.5 × 10−5 m s−1 for all cruises. Vertical velocities are also inferred from the divergence of the wind-driven Ekman transport. In the coastal regimes, both methods agree within the error range. Further offshore, the helium-derived vertical velocity still reaches 1 × 10−5 m s−1, whereas the wind-driven upwelling from Ekman suction is smaller by up to 1 order of magnitude. One reason for this difference is ascribed to eddy-induced upwelling. Both advective and diffusive nutrient fluxes into the mixed layer are calculated based on the helium-derived vertical velocities and the vertical diffusivities. The advective part of these fluxes makes up at about 50 % of the total. The nutrient flux into the mixed layer in the coastal upwelling regimes is equivalent to a net community production (NCP) of 1.3 ± 0.3 g C m2 d−1 off Peru and 1.6–2.1 ± 0.5 g C m2 d−1 off Mauritania.

Short summary
The coastal upwelling systems, e.g. off Peru and Mauritania, are key regions for the emissions of climate relevant trace gases from the ocean into the atmosphere. Here, gases and nutrients are transported into the ocean mixed layer from below. The upwelling velocities, however, are too small to be measured directly. We use the enhancement of helium-3 in upwelled waters to quantify the vertical velocity, which varies between 1.0 and 2.5 metres per day in the coastal regions.
Final-revised paper