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

[摘要] The eastern boundary region of the southeastern 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 ( 234 Th ) 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 234 Th fluxes when not properly evaluated. Here we use vessel-mounted acoustic Doppler current profiler (VmADCP) current velocities, satellite wind speed and in situ microstructure measurements to determine the magnitude of advective and diffusive fluxes over the entire 234 Th flux budget at 25 stations from 11 to 16 ∘  S in the Peruvian OMZ. Contrary to findings along the GEOTRACES P16 eastern section, our results showed that weak surface wind speed during our cruises induced low upwelling rates and minimal upwelled 234 Th fluxes, whereas vertical diffusive 234 Th fluxes were important only at a few shallow shelf stations. Horizontal advective and diffusive 234 Th fluxes were negligible because of small alongshore 234 Th gradients. Our data indicated a poor correlation between seawater 238 U activity and salinity. Assuming a linear relationship between the two would lead to significant underestimations of the total 234 Th flux by up to 40 % in our study. Proper evaluation of both physical transport and variability in 238 U activity is thus crucial in coastal 234 Th flux studies. Finally, we showed large temporal variations on 234 Th residence times across the Peruvian upwelling zone and cautioned future carbon export studies to take these temporal variabilities into consideration while evaluating carbon export efficiency.

[学科分类] 大气科学