Description: 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 (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 VmADCP current velocities, satellite wind speed 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 speed during our cruises induced low upwelling rates and minimal upwelled 234Th fluxes, whereas vertical diffusive 234Th fluxes were important only at a few shallow shelf stations. Horizontal advective and diffusive 234Th fluxes were negligible because of 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 variabilities into consideration while evaluating carbon export efficiency.

Description: A sensor package for measuring and autonomously logging oceanographical parameters was installed on the ROV-Squid during the METEOR research cruise M183. The suite of sensors included an RBR Concerto logger for measurements of pressure, temperature, dissolved O₂ concentration and turbidity, a Pyroscience AquapHOx-LX for measurements of pH and a Trios OPUS nitrate sensor. The duration and frequency of the sampling were defined on every dive in order to have the best sensors performance/battery usage combination. Generally RBR Concerto data were recorded every 10 seconds, pH data every 30 seconds while nitrate data every 5 minutes. Following sensor post-calibration, a maximum pH offset of -0.05 was applied. Nitrate data were processed according to Nehir et al., 2021. While the RBR Concerto logger recorded during the entire dives, data for both the pH and nitrate sensors include bottom time and ROV ascent only.