The 226Ra-Ba relationship in the North Atlantic during GEOTRACES-GA01

• Main Authors: Le Roy, Emilie, Sanial, Virginie, Charette, Matthew A., Van Beek, Pieter, Lacan, François, Jacquet, Stéphanie H. M., Henderson, Paul B., Souhaut, Marc, García-ibáñez, Maribel I., Jeandel, Catherine, Pérez, Fiz, Sarthou, Géraldine
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2018
• Subjects: Canada; Greenland; West European Basin; Newfoundland; North Atlantic; North East Atlantic
• Online Access: https://archimer.ifremer.fr/doc/00412/52305/53073.pdf; https://archimer.ifremer.fr/doc/00412/52305/53074.pdf; https://archimer.ifremer.fr/doc/00412/52305/56812.pdf; https://archimer.ifremer.fr/doc/00412/52305/56813.pdf; https://doi.org/10.5194/bg-2017-478; https://archimer.ifremer.fr/doc/00412/52305/

We report detailed sections of radium-226 (226Ra, T1/2 = 1602 y) activities and barium (Ba) concentrations determined in the North Atlantic (Portugal-Greenland-Canada) in the framework of the international GEOTRACES program (GA01 section – GEOVIDE project, May–July 2014). Dissolved 226Ra and Ba are strongly correlated along the GA01 section, a pattern that reflects their similar chemical behavior. Since 226Ra and Ba have been widely used as tracers of water masses and ocean mixing, we investigated more thoroughly their behavior in this crucial region for thermohaline circulation taking advantage of the contrasting biogeochemical patterns existing along the GA01 section. We used an Optimum Multiparameter (OMP) analysis to distinguish the relative importance of physical transport (water mass mixing) from non-conservative processes (sedimentary, river, or hydrothermal inputs; uptake by particles, and dissolved-particulate dynamics) on the 226Ra and Ba distributions in the North Atlantic. Results show that 72 % of the 226Ra and 68 % of the Ba can be explained by conservative mixing along the section and therefore, they can be considered as conservative tracers of water mass transport in the ocean interior. However, regions where 226Ra and Ba displayed non-conservative behavior were also identified, mostly at the ocean boundaries (seafloor, continental margins, and surface waters). Elevated 226Ra and Ba concentrations found in deep waters of the West European Basin reflect that lower North East Atlantic Deep Water (NEADWl) accumulates excess 226Ra and Ba from sediment diffusion during transport. In the upper 1500 m, deficiencies in 226Ra and Ba are likely explained by their incorporation in planktonic siliceous shells, or in barite (BaSO4) (Bishop, 1988). Finally, since Ba and 226Ra display different source terms (mostly deep-sea sediments for 226Ra and rivers for Ba), strong decoupling between 226Ra and Ba were observed at the land-ocean boundaries. This is especially true in the shallow stations near the coasts of Greenland and Newfoundland where high 226Ra / Ba ratios at depth reflect the diffusion of 226Ra from sediment and low 226Ra / Ba ratios in the upper water column reflect the input of Ba associated with meteoric waters.