²¹⁰Po and ²¹⁰Pb distributions in the central and eastern Indian Ocean

Abstract

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.