210Pb in the western Indian Ocean: distribution, disequilibrium, and partitioning between dissolved and particulate phases

Abstract

Dissolved 210Pb profiles are presented for 13 GEOSECS stations in the western Indian Ocean. In surface water away from high southern latitudes, 210Pb is in excess over 226Ra due to the atmospheric fallout from decay of 222Rn. Except in the Circumpolar region, the dissolved 210Pb profiles display a gentle mid-depth maximum similar to the corresponding 226Ra profiles. The 210Pb/ 226Ra activity ratio ranges from 1.6 in the surface water east of Madagascar to 0.4 or less in the bottom water of all the basins. The lowest ratio observed was 0.1 in the Gulf of Aden very close to the continental land mass. A ratio of 0.6 divides the western Indian Ocean horizontally into two portions, with the contour at shallower depth in the north than in the south. The deep water disequilibrium is thus more extensive north of Madagascar than south of it. It appears that locality and bottom topography play a strong role in controlling the distributions of 210Pb and 226Ra as well as their extent of disequilibrium in the water column. The mean residence time for Pb with respect to particulate and boundary scavenging in the deep water ranges from about 15 to 75 years.