Volume 15, issue 9 | Copyright

Special issue: GEOVIDE, an international GEOTRACES study along the OVIDE...

Biogeosciences, 15, 3027-3048, 2018
https://doi.org/10.5194/bg-15-3027-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 17 May 2018

Research article | 17 May 2018

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

Emilie Le Roy1, Virginie Sanial1,2,a, Matthew A. Charette2, Pieter van Beek1, François Lacan1, Stéphanie H. M. Jacquet3, Paul B. Henderson2, Marc Souhaut1, Maribel I. García-Ibáñez4,b, Catherine Jeandel1, Fiz F. Pérez4, and Géraldine Sarthou5 Emilie Le Roy et al.
  • 1LEGOS, Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (Université de Toulouse, CNRS/CNES/IRD/UPS), Observatoire Midi Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France
  • 2Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
  • 3Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO), UM110, 13288 Marseille, France
  • 4Instituto de Investigaciones Marinas (IIM, CSIC), Eduardo Cabello 6, 36208 Vigo, Spain
  • 5Laboratoire des Sciences de l'Environnement Marin (LEMAR), UMR 6539, IUEM, Technopôle Brest Iroise, 29280 Plouzané, France
  • anow at: Department of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA
  • bnow at: Uni Research Climate, Bjerknes Centre for Climate Research, Bergen 5008, Norway

Abstract. We report detailed sections of radium-226 (226Ra, T1∕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 1500m 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 226RaBa ratios at depth reflect the diffusion of 226Ra from sediment and low 226RaBa ratios in the upper water column reflect the input of Ba associated with meteoric waters.

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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 section, which may reflect their similar chemical behavior.
We report detailed sections of radium-226 (226Ra, T1/2 = 1602 y) activities and barium (Ba)...
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