137Cs water profiles in the South Indian Ocean – An evidence for accumulation of pollutants in the subtropical gyre

Abstract

The 137Cs seawater transect along the 20°S latitude in the Indian Ocean, sampled during the round the globe BEAGLE2003 (Blue Ocean Global Expedition) clearly shows a presence of two cores of higher 137Cs concentrations at 100°E and 55°E in water column depths between 100 and 200 m. The subsurface maximum of 137Cs activity concentration (2.1 mBq/L) observed near 100°E is the highest level found in southern hemisphere seawater, which is comparable to the maximum 137Cs concentration observed in North Pacific surface water. This transect represents the first high density sight of 137Cs distribution in South Indian Ocean waters. The estimated 137Cs water column inventories (up to 1240 Bq/m 2), normalized for the 1000 m water depth along the 20°S latitude are by a factor of three higher than would be expected from global fallout in the South Indian Ocean. The estimated residence time of 137Cs in surface waters of the South Indian Ocean (∼30 years) is by about a factor of two higher than for the North Pacific. The 137Cs data confirm a transport of surface waters from the North Pacific via Indonesian seas to the South Indian Ocean, and to the subtropical gyre. The subtropical gyre is an important reservoir of anthropogenic pollutants that have been transported from the North Pacific and Indian Oceans to the south on a time-scale of several decades.