<SUP>210</SUP>Po and <SUP>210</SUP>Pb distributions in the central and eastern Indian Ocean

Disequilibrium between 210Po and 210Pb and between 210Pb and 226Ra has been mapped in the eastern and central Indian Ocean based on stations from Legs 3 and 4 of the GEOSECS Indian Ocean expedition. 210Po/210Pb activity ratios are less than 1.0 in the surface mixed layer and indicate a residence time for Po of 0.6 years. 210Po and 210Pb are generally in radioactive equilibrium elsewhere in the water column except at depths of 100-500 m, where Po may be returned to solution after removal from the surface water, and in samples taken near the bottom at a few stations. 210Pb excesses relative to 226Ra are observed in the surface water but these excesses are not as pronounced as in the North Pacific and North Atlantic. The difference is attributable to a lower flux of 210Pb from the atmosphere to the Indian Ocean. Below the main thermocline, 210Pb activities increase with depth to a broad maximum before decreasing to lower values near the bottom. Departures from this pattern are especially evident at stations taken in the Bay of Bengal (where 210Pb/226Ra activity ratios as low as 0.16 are observed) and near the Mid-Indian Ridge. The data suggest that removal of 210Pb at oceanic boundaries, coupled with eddy diffusion along isopycnals, can explain gradients in 210Pb near the boundary. Application of a simple model including isopycnal diffusion, chemical removal, production and radioactive decay produces fits the observed 210Pb/226Ra gradients for eddy diffusion coeffients of ~ 107 cm2/s. High productivity in surface waters of the Bay of Bengal makes this region a sink for reactive nuclides in the northern Indian Ocean