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

We report detailed sections of radium-226 (Ra-226, T-1/2 = 1602 years) activities and barium (Ba) concentrations determined in the North Atlantic (Portugal-Greenland-Canada) in the framework of the international GEO-TRACES program (GA01 section - GEOVIDE project, May-July 2014). Dissolved Ra-226 and Ba are strongly correlated along the section, a pattern that may reflect their similar chemical behavior. Because Ra-226 and Ba have been widely used as tracers of water masses and ocean mixing, we investigated their behavior more thoroughly 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 nonconservative processes (sedimentary, river or hydrothermal inputs, uptake by particles and dissolved-particulate dynamics) on the Ra-226 and Ba distributions in the North Atlantic. Results show that the measured Ra-226 and Ba concentrations can be explained by conservative mixing for 58 and 65% of the samples, respectively, notably at intermediate depth, away from the ocean interfaces. Ra-226 and Ba can thus be considered conservative tracers of water mass transport in the ocean interior on the space scales considered here, namely, on the order of a few thousand kilometers. However, regions in which Ra-226 and Ba displayed nonconservative behavior and in some cases decoupled behaviors were also identified, mostly at the ocean boundaries (seafloor, continental margins and surface waters). Elevated Ra-226 and Ba concentrations found in deep-water in the West European Basin suggest that lower Northeast Atlantic DeepWater (NEADWl) accumulates Ra-226 and Ba from sediment diffusion and/or particle dissolution during transport. In the upper 1500m of the West European Basin, deficiencies in Ra-226 and Ba are likely explained by their incorporation in planktonic calcareous and siliceous shells, or in barite (BaSO4) by substitution or adsorption mechanisms. Finally, because Ba and Ra-226 display different source terms (mostly deep-sea sediments for Ra-226 and rivers for Ba), strong decoupling between Ra-226 and Ba were observed at the land-ocean boundaries. This is especially true in theshallow stations near the coasts of Greenland and Newfound-land where high Ra-226/Ba ratios at depth reflect the diffusion of Ra-226 from sediment and low Ra-226/Ba ratios in the upper water column reflect the input of Ba associated with meteoric waters.

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Le Roy Emilie, Sanial Virginie, Charette Matthew A., Van Beek Pieter, Lacan Francois, Jacquet Stephanie H. M., Henderson Paul B., Souhaut Marc, Garcia-Ibanez Maribel I., Jeandel Catherine, Perez Fiz F, Sarthou Geraldine (2018). The Ra-226-Ba relationship in the North Atlantic during GEOTRACES-GA01. Biogeosciences. 15 (9). 3027-3048.,

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