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

Ra, Ra and Ba distributions as well as Ra/Ra and Ra/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 Raex/ Raex 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 (ie., throughout the mesopelagic layer). Temporal changes in the Raex/ Raex 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/Ra ratio) and formed in the mesopelagic layer (with a low Ra/Ra ratio). Raex/ Raex 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