Rare earth elements in the North Atlantic, part II: Partition coefficients

• Published in: Chemical Geology
• Main Authors: Lagarde, Marion, Pham Viet, Quoc, Lemaitre, Nolwenn, Belhadj, Moustafa, Jeandel, Catherine
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2024
• Subjects: Rare earth elements; dissolved particulate exchanges; partition coefficients; suspended particle mass; manganese oxides; iron hydroxides; GEOTRACES; GEOVIDE; North Atlantic
• Online Access: https://archimer.ifremer.fr/doc/00902/101363/112078.pdf; https://archimer.ifremer.fr/doc/00902/101363/112079.docx; https://archimer.ifremer.fr/doc/00902/101363/112080.xlsx; https://doi.org/10.1016/j.chemgeo.2024.122298; https://archimer.ifremer.fr/doc/00902/101363/

This study presents the first basin scale section of the partition coefficients (Kd) of 14 rare earth elements (REE) along the GEOVIDE (GEOTRACES GA01 cruise) transect, in the subpolar North Atlantic (May–June 2014, R/V Pourquoi Pas?). Although Kd data are very scarce in the literature, it is a key parameter of element cycle modeling, controlling the scavenging efficiency by particles. Because of this lack of data, Kd are generally adjusted in the models to obtain the best agreement between modeled and observed concentrations. This shortcoming likely contributes to make the modeling of the oceanic REE cycles challenging. This is well illustrated by the difficulty to satisfactorily simulate both Nd concentrations and isotopic compositions. Here, we determined Kd(REE) at 10 stations, calculated from previously published dissolved and particulate REE concentrations, and from suspended particulate matter (SPM) concentrations. The data required for the calculation of SPM concentrations were obtained in the framework of the GEOVIDE cruise. Kd profiles displayed minimum values at the surface, variations in the upper 400 m, and an increase with depth below 400 m. The expression of the ratio of particulate to dissolved REE concentrations as a function of SPM concentrations showed that manganese oxides (MnO2) were the main driver of REE scavenging, followed by the lithogenic phase and iron hydroxides (Fe(OH)3). Our study also highlighted that REE scavenging preferentially depended on the relative proportion of these three phases rather than on their absolute concentrations. These conclusions were evaluated by a Pearson correlation test, with correlation coefficients of 0.83, 0.84, and 0.75 between Kd(Nd) and the MnO2, lithogenic, and Fe(OH)3 fractions, respectively. Finally, we proposed two Kd(Nd) parameterizations as a function of the particulate phase fractions. The first parameterization included MnO2 and Fe(OH)3, phases that are currently not represented in Nd cycle models, and showed a good agreement between observed ...