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

• Main Authors: Cochran, J. K., Bacon, M. P., Krishnaswami, S., Turekian, K. K.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier Science 1983
• Subjects: Indian; Pacific; North Atlantic
• Online Access: http://repository.ias.ac.in/16747/; http://linkinghub.elsevier.com/retrieve/pii/0012821X83901802

Disequilibrium between 210 Po and 210 Pb and between 210 Pb and 226 Ra has been mapped in the eastern and central Indian Ocean based on stations from Legs 3 and 4 of the GEOSECS Indian Ocean expedition. 210 Po/ 210 Pb activity ratios are less than 1.0 in the surface mixed layer and indicate a residence time for Po of 0.6 years. 210 Po and 210 Pb 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. 210 Pb excesses relative to 226 Ra 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 210 Pb from the atmosphere to the Indian Ocean. Below the main thermocline, 210 Pb 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 210 Pb/ 226 Ra activity ratios as low as 0.16 are observed) and near the Mid-Indian Ridge. The data suggest that removal of 210 Pb at oceanic boundaries, coupled with eddy diffusion along isopycnals, can explain gradients in 210 Pb near the boundary. Application of a simple model including isopycnal diffusion, chemical removal, production and radioactive decay produces fits the observed 210 Pb/ 226 Ra gradients for eddy diffusion coeffients of ~ 10 7 cm 2 /s. High productivity in surface waters of the Bay of Bengal makes this region a sink for reactive nuclides in the northern Indian Ocean.