Depletion of barium and radium-226 in Black Sea surface waters over the past thirty years

Abstract

THE nearly landlocked waters of the Black Sea support a valuable fishery, but are also particularly vulnerable to anthropogenic disturbance. Here we use dissolved barium and radium-226 as tracers, to investigate the biogeochemical health of the sea. Both elements are brought to surface waters by vertical mixing of deeper, enriched waters^1,2, and by rivers^3–8; these inputs should ordinarily be balanced by outflow of surface waters at the Bosporus, and by biologically mediated removal of ²²⁶Ra-bearing barite. We show, however, that surface-water inventories have been substantially depleted over the past few decades: recent (1988–89) barium concentrations were 1.6 times lower than in 1975⁹; and recent ²²⁶Ra concentrations were 3.5 times lower than in 1958¹⁰ and 1967¹¹. These observations suggest that steady-state cycling of these elements has been perturbed by increased primary productivity, presumably fuelled by nutrients from industry and agricultural runoff, and to a lesser extent by decreased fluvial sediment loads owing to extensive impoundment of rivers in the region.