228Ra/226Ra and 226Ra/Ba ratios in the Western Mediterranean Sea : barite formation and transport in the water column

P. van Beek (1) , E. Sternberg (1) , J. L. Reyss (2, 3) , M. Souhaut (1) , Eric Robin (2) , C. Jeandel (1)

1 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
2 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
3 OCEANIS - Océan et Interfaces

226Ra, 228Ra and Ba distributions as well as 228Ra/226Ra and 226Ra/Ba ratios were measured in seawater, suspended and sinking particles at the DYFAMED station in the Western Mediterranean Sea at different seasons of year 2003 in order to track the build-up and fate of barite through time. The study of the 228Raex/226Raex ratios (Raex = Ra activities corrected for the lithogenic Ra) of suspended particles suggests that Baex (Baex = Ba concentrations corrected for the lithogenic Ba, mostly barite) formation takes place not only in the upper 500 m of the water column but also deeper (i.e. throughout the mesopelagic layer). Temporal changes in the 228Raex/226Raex ratios of sinking particles collected at 1000 m depth likely reflect changes in the relative proportion of barite originating from the upper water column (with a high 228Ra/226Ra ratio) and formed in the mesopelagic layer (with a low 228Ra/226Ra ratio). 228Raex/226Raex ratios measured in sinking particles collected in the 1000 m-trap in April and May suggest that barite predominantly formed in the upper water column during that period, while barite found outside the phytoplankton bloom period (February and June) appears to form deeper in the water column. Combining ratios of both the suspended and sinking particles provides information on aggregation/disaggregation processes. High 226Raex/Baex ratios were also found in suspended particles collected in the upper 500 m of the water column. Because celestite is expected to be enriched in Ra [Bernstein R. E., Byrne R. H. and Schijf J. (1998) Acantharians: a missing link in the oceanic biogeochemistry of barium. Deep-Sea Res. II 45, 491–505], acantharian skeletons may contribute to these high ratios in shallow waters. The formation of both acantharian skeletons and barite enriched in 226Ra may thus contribute to the decrease in the dissolved 226Ra activity and 226Ra/Ba ratios of surface waters observed between February and June 2003 at the DYFAMED station.

Mots clés: Mediterranean Sea; barite; radium isotopes; acantharian; DYFAMED

Domaines: Environnement et Société; Radiochimie; Océan, Atmosphère

P. van Beek, E. Sternberg, J. L. Reyss, M. Souhaut, Eric Robin, et al.. 228Ra/226Ra and 226Ra/Ba ratios in the Western Mediterranean Sea : barite formation and transport in the water column. Geochimica et Cosmochimica Acta, 2009, 73, pp.4720-4737. ⟨10.1016/j.gca.2009.05.063⟩. ⟨hal-00404768⟩