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

Xie, Ruifang C , Le Moigne, Frederic A. C. , Rapp, Insa, Lüdke, Jan , Gasser, Beat, Dengler, Marcus , Liebetrau, Volker and Achterberg, Eric P. (2020) Effects of 238U variability and physical transport on water column 234Th downward fluxes in the coastal upwelling system off Peru. Open Access Biogeosciences (BG), 17 . pp. 4919-4936. DOI 10.5194/bg-17-4919-2020.

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Abstract

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 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 234Th 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 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.

Document Type: Article
Keywords: Eastern tropical South Pacific, 234Th tracer, uranium-salinity correlation, physical processes, residence time
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
OceanRep > The Future Ocean - Cluster of Excellence
OceanRep > SFB 754
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BI Biological Oceanography
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/bg-17-4919-2020
ISSN: 1726-4189
Projects: SFB754, Future Ocean
Expeditions/Models/Experiments:
Date Deposited: 09 Mar 2020 09:09
Last Modified: 08 Feb 2021 07:30
URI: http://oceanrep.geomar.de/id/eprint/49192

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