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

Abstract. We report detailed sections of radium-226 (²²⁶Ra, T_1∕2 =  1602 years) 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 section, a pattern that may reflect their similar chemical behavior. Because ²²⁶Ra 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 ²²⁶Ra and Ba distributions in the North Atlantic. Results show that the measured ²²⁶Ra 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. ²²⁶Ra 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 ²²⁶Ra 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 ²²⁶Ra and Ba concentrations found in deepwater in the West European Basin suggest that lower Northeast Atlantic Deep Water (NEADWl) accumulates ²²⁶Ra and Ba from sediment diffusion and/or particle dissolution during transport. In the upper 1500 m of the West European Basin, deficiencies in ²²⁶Ra 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 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.