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. The eastern boundary region of the southeastern Pacific Ocean hosts one ofthe world's most dynamic and productive upwelling systems with an associatedoxygen minimum zone (OMZ). The variability in downward export fluxes in thisregion, with strongly varying surface productivity, upwelling intensitiesand water column oxygen content, is however poorly understood. Thorium-234( 234Th ) is a powerful tracer to study the dynamics of export fluxes ofcarbon and other elements, yet intense advection and diffusion in nearshoreenvironments impact the assessment of depth-integrated 234Th fluxeswhen not properly evaluated. Here we use vessel-mounted acoustic Doppler currentprofiler (VmADCP) current velocities,satellite wind speed and in situ microstructure measurements to determine themagnitude of advective and diffusive fluxes over the entire 234Th fluxbudget at 25 stations from 11 to 16 ∘  S in thePeruvian OMZ. Contrary to findings along the GEOTRACES P16 eastern section,our results showed that weak surface wind speed during our cruises inducedlow upwelling rates and minimal upwelled 234Th fluxes, whereas verticaldiffusive 234Th fluxes were important only at a few shallow shelfstations. Horizontal advective and diffusive 234Th fluxes werenegligible because of small alongshore 234Th gradients. Our dataindicated a poor correlation between seawater 238U activity andsalinity. Assuming a linear relationship between the two would lead tosignificant underestimations of the total 234Th flux by up to 40 % inour 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 timesacross the Peruvian upwelling zone and cautioned future carbon exportstudies to take these temporal variabilities into consideration whileevaluating carbon export efficiency.

biogeosciences

Effects of &amp;lt;sup&amp;gt;238&amp;lt;/sup&amp;gt;U variability and physical transport on water column &amp;lt;sup&amp;gt;234&amp;lt;/sup&amp;gt;Th downward fluxes in the coastal upwelling system off Peru