U-238 DECAY SERIES NUCLIDES IN THE NORTHEASTERN ARABIAN SEA - SCAVENGING RATES AND CYCLING PROCESSES

The extent of radioactive disequilibrium between Th-234-U-238, Po-210- Pb-210 and Pb-210-Ra-226 has been measured in four vertical profiles f rom the northeastern Arabian Sea, a region characterized by oxygen def icient and denitrification layers at intermediate depths. The most com mon feature of the data is that the dissolved concentrations of the da ughter nuclides are considerably less than those of their parents, exc eptions being the near equilibrium or excess concentrations of Po-210 (relative to Pb-210) at similar to 100 m and that of Pb-210 relative t o Ra-226 in surface waters. These results suggest that scavenging of d issolved Th-234, Po-210 and Pb-210 occurs throughout the water column in the northeastern Arabian Sea. In the mixed layer, the scavenging re sidence time of Th is similar to 1-2 months and that of Po and Pb is s imilar to 4-8 months. At depths greater than similar to 300 m, the mea n dissolved [Po-210/Pb-210] and [Pb-210/Ra-226] are similar to 0.75 an d similar to 0.4, respectively, representing some of the most pronounc ed disequilibria observed in the deep sea. The particle reactivity of the measured nuclides increases as Th > Po > Pb. The U-238 concentrati on and the extent of Th-234-U-238 disequilibrium are similar in the ox ic surface layers and suboxic intermediate waters, indicating that U a nd Th distributions are not significantly affected by oxygen depletion and denitrification processes. On the contrary, both Po-210-Pb-210 an d Pb-210-Ra-226 disequilibria show enhanced scavenging of 210po and Pb -210 at stations that are characterized by intense denitrification. Mo delling of dissolved Po-210 profile in the surface similar to 100 m sh ows its potential to yield eddy diffusion coefficients in surface wate rs and to place constraints on the new production rates. Results obtai ned in this study, although encouraging, stress the need for further w ork to confirm some of the assumptions of the model.