Radium-224 and radium-223: powerful chronometers to estimate the residence time of Amazon waters on the Brazilian continental shelf

The Amazon River is the largest source of fresh water into the ocean, carrying enormous amounts of dissolved and particulate matter that have a major influence on the chemical composition and biogeochemical cycles of the Atlantic Ocean. A consortium of scientists1 from LEGOS and HSM (France), the University of Kiel (Germany), the University of South Carolina (USA) and the Instituto de Pesquisas Energéticas e Nucleares (Brazil) (2022, see references below) has gathered a unique dataset acquired in different hydrological contexts during i) American (AmasSeds in 1991), ii) French (AMANDES² / GEOTRACES process study GApr01 in 2007–2008) and iii) German (M147 in 2018) cruises. In this study, radium, a natural radioactive isotope, is used as a tracer for the waters of the Amazon that mix with those of the Atlantic Ocean. In particular, the radioactive decay of radium isotopes (radium-224 and radium-223) along the Amazon plume allows to measure the transit time of these waters on the Brazilian continental shelf up to more than 500 km from the mouth.

The study shows that it takes between 9 and 14 days for the waters of the Amazon to reach the northwest limit of the Brazilian continental shelf (off French Guiana) and between 12 and 21 days to reach the northeastern limit of the Brazilian continental shelf opposite the mouth. The average speed of transport to the northwest along the Brazilian coasts is thus estimated at 30 cm.s-1. The residence times obtained from radium isotopes are consistent with those estimated from high-resolution numerical simulations based on the NEMO model. The results obtained also suggest that the high seasonal variability in river flow has little influence on the residence time of the waters on the continental shelf. Since there are very few current observations on the Brazilian continental shelf, these estimates derived from geochemical tracers are therefore very valuable.

Figure: Map of A) the location of the samples collected in the AMANDES cruises and B) the ages (in days) of waters determined using NEMO model for AMANDES 4 (July 2008). The water parcels are released in the model along the green line, but the age counter begins when the particles are in waters with a salinity below 1. The colored circles show radium apparent ages (in days).

Reference:

Léon, M., van Beek, P., Scholten, J., Moore, W. S., Souhaut, M., De Oliveira, J., Jeandel, C., Seyler, P., & Jouanno, J. (2022). Use of 223 Ra and 224Ra as chronometers to estimate the residence time of Amazon waters on the Brazilian continental shelf. Limnology and Oceanography. doi:10.1002/lno.12010

1 The consortium brings together the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS, Université de Toulouse/ CNRS/ CNES/ IRD/ Université Toulouse III Paul Sabatier), the Montpellier Hydrosciences Laboratory (HSM, IRD/CNES/ Université de Montpellier), the University of Kiel (Germany), the University of South Carolina (USA) and the Instituto de Pesquisas Energéticas e Nucleares (IPEN), Sao Paulo (Brazil).

2 AMANDES project supported by ANR (Agence Nationale de la Recherche; ANR-BLA-2005, NT05-3_43160; PIs: Catherine Jeandel and Patrick Seyler), IRD (Institut de Recherche pour le Développement) and CNRS/INSU (Centre National de la Recherche Scientifique/Institut National des Sciences de l’Univers).

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