Uranium-238 data and physical transports off the Peruvian coast are questioning the thorium-uranium export model

Thorium-234 (234Th) is a widely used tracer for element export fluxes from the ocean surface. However, its application requires a relatively quiet ocean dynamic and constant distribution of the conservative uranium-238 (238U): both conditions are not optimal off the Peruvian coast, to say the least. Using vessel-mount acoustic current Doppler profiler instrument, satellite wind speed and in situ microstructure measurements, Xie and co-workers (2020, see reference below) determined insignificant advective and diffusive 234Th fluxes at 25 stations from 11 to 16ºS in the Peruvian oxygen minimum zone. They also revealed that in this area, 238U is not anymore correlated to salinity, in other words conservative. Despite these obstacles, they provide 234Th- export fluxes at 100 m ranging from 850 to 3400 dmp.m-2.d-1, although warn the community of the limits of the Th-U model in such coastal area!

Figure: Study area and 234Th flux results from Xie et al. (2020).
Map on the top panel shows locations of stations from M136 (circles) and M138 (white) off the coast of Peru. Color boxes schematically divide the four shelf-offshore transects. The bar chart on bottom panel shows the total 234Th flux (white circles) as a consequence of radioactive production and decay (blue), upwelling (pink) and vertical diffusion (grey) for the depths at 5-20 m below the ML (top) and 100 m below sea surface (bottom).


Reference:

Xie, R. C., Le Moigne, F. A. C., Rapp, I., Lüdke, J., Gasser, B., Dengler, M., Liebetrau, V., Achterberg, E. P. (2020). Effects of 238U variability and physical transport on water column 234Th downward fluxes in the coastal upwelling system off Peru. Biogeosciences, 17(19), 4919–4936. DOI: https://doi.org/10.5194/bg-17-4919-2020

Latest highlights

Science Highlights

Deep sea lithogenic weathering a source of iron colloids for the ocean

Homoky and co-workers determined the isotope composition of dissolved iron profiles in shallow surface sediments of the South Atlantic Uruguayan margin…

28.03.2021

Science Highlights

Adding external sources allow a better simulation of the oceanic rare earth elements cycles

Oka and colleagues demonstrate that the global distribution of REE can be reproduced by considering the internal cycle associated with reversible scavenging and external REEs inputs around continental regions.

26.03.2021

Science Highlights

First direct measurements of luxury iron uptake in natural phytoplankton communities: surprising results!

This study demonstrates the importance of biology and ecology to understanding iron biogeochemistry.

19.03.2021

Science Highlights

Air-sea gas disequilibrium drove deoxygenation of the deep ice-age ocean

This study provides one of the first mechanistic explanations for Last Glacial Maximum deep ocean deoxygenation.

18.03.2021

Rechercher