Cycling of radium and barium in the Black Sea

We report the first data for the concentrations of Ba and 226Ra in rivers draining into the Black Sea. For Ba, concentrations are in the range 215–405 nM L-1; for 226Ra, concentrations are in the range 0.4–4 Bq m-3. These are dissolved concentrations and do not represent additional inputs due to desorption from fluvial particles. We also report new data for the concentrations of Ba and 226Ra in Black Sea surface waters from 1991 to 1995. Earlier studies documented significant reductions in the concentrations of these elements in Black Sea surface waters for the periods 1957–1988 (Ra) and 1975–1988 (Ba). During the period 1988–89 to 1995, Ba concentrations followed the earlier trend and decreased by an additional 35%. On the other hand, 226Ra concentrations reversed the trend and increased by about 60% from 1988 to 1995. The reason for the recent divergence in behavior of these two elements is unknown.