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

Ra-226, Ra-228 and Ba distributions as well as Ra-228/Ra-226 and Ra-226/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 Ra-228(ex)/Ra-226(ex) ratios (Ra-ex = Ra activities corrected for the lithogenic Ra) of suspended particles suggests that Ba-ex (Ba-ex = 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 Ra-228(ex)/Ra-226(ex) 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 Ra-228/Ra-226 ratio) and formed in the mesopelagic layer (with a low Ra-228/Ra-226 ratio). Ra-228(ex)/Ra-226(ex) 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 Ra-226(ex)/Ba-ex 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. H 45, 491-505], acantharian skeletons may contribute to these high ratios in shallow waters. The formation of both acantharian skeletons and barite enriched in Ra-226 may thus contribute to the decrease in the dissolved Ra-226 activity and Ra-226/Ba ratios of surface waters observed between February and June 2003 at the DYFAMED station. (C) 2009 Elsevier Ltd. All rights reserved.