Effects of 238U variability and physical transport on water column 234Th downward fluxes in the coastal upwelling system off Peru

Ruifang Xie (1) , Frédéric A.C. Le Moigne (2) , Insa Rapp (1) , Jan Lüdke (1) , Beat Gasser (3) , Marcus Dengler (1) , Volker Liebetrau (1) , Eric Achterberg (1)

1 GEOMAR - Helmholtz Centre for Ocean Research [Kiel]
2 MIO - Institut méditerranéen d'océanologie
3 IAEA - Environment Laboratories

Abstract. The eastern boundary region of the southeast Pacific Ocean hosts one of the world's most dynamic and productive upwelling systems with an associated oxygen minimum zone (OMZ). The variability in downward export fluxes in this region, with strongly varying surface productivity, upwelling intensities and water column oxygen content, is however poorly understood. Thorium-234 (234Th) is a powerful tracer to study the dynamics of export fluxes of carbon and other elements, yet intense advection and diffusion in nearshore environments impact the assessment of depth-integrated 234Th fluxes when not properly evaluated. Here we use ADCP current velocities, satellite wind stress and in situ microstructure measurements to determine the magnitude of advective and diffusive fluxes over the entire 234Th flux budget at 25 stations from 11° S to 16° S in the Peruvian OMZ. Contrary to findings along the GEOTRACES P16 eastern section, our results showed that weak surface wind stress during our cruises induced low upwelling rates and minimal upwelling 234Th fluxes, whereas vertical diffusive 234Th fluxes were important only at a few shallow shelf stations. Similarly, horizontal advective and diffusive 234Th fluxes were negligible due to small alongshore 234Th gradients. Our data indicated a poor correlation between seawater 238U activity and salinity. Assuming a linear relationship between the two would lead to significant underestimations of the total 234Th flux by up to 40 % in our study. Proper evaluation of both physical transport and variability in 238U activity is thus crucial in coastal 234Th flux studies. Finally, we showed large temporal variations on 234Th residence times across the Peruvian upwelling zone, and cautioned future carbon export studies to take these temporal changes into consideration while evaluating carbon export efficiency.

Domaines: Océan, Atmosphère

Ruifang Xie, Frédéric A.C. Le Moigne, Insa Rapp, Jan Lüdke, Beat Gasser, et al.. Effects of 238U variability and physical transport on water column 234Th downward fluxes in the coastal upwelling system off Peru. Biogeosciences, 2020, ⟨10.5194/bg-2020-57⟩. ⟨hal-02960677⟩