<SUP>238</SUP>U decay series nuclides in the northeastern Arabian Sea: scavenging rates and cycling processes

The extent of radioactive disequilibrium between 234Th-238U, 210Po-210Pb and 210Pb-226Ra has been measured in four vertical profiles from the northeastern Arabian Sea, a region characterized by oxygen deficient and denitrification layers at intermediate depths. The most common feature of the data is that the dissolved concentrations of the daughter nuclides are considerably less than those of their parents, exceptions being the near equilibrium or excess concentrations of 210Po (relative to 210Pb) at ~100m and that of 210Pb relative to 226Ra in surface waters. These results suggest that scavenging of dissolved 234Th, 210Po and 210Pb occurs throughout the water column in the northeastern Arabian Sea. In the mixed layer, the scavenging residence time of Th is ~1-2months and that of Po and Pb is ~4-8months. At depths greater than ~300m, the mean dissolved [210Po/210Pb] and [210Pb/226Ra] are ~0.75 and ~0.4, respectively, representing some of the most pronounced disequilibria observed in the deep sea. The particle reactivity of the measured nuclides increases as Th &gt; Po &gt; Pb. The 238U concentration and the extent of 234Th-238U disequilibrium are similar in the oxic surface layers and suic intermediate waters, indicating that U and Th distributions are not significantly affected by oxygen depletion and denitrification processes. On the contrary, both 210Po-210Pb and 210Pb-226Ra disequilibria show enhanced scavenging of 210Po and 210Pb at stations that are characterized by intense denitrification. Modelling of dissolved 210Po profile in the surface ~100m shows its potential to yield eddy diffusion coefficients in surface waters and to place constraints on the new production rates. Results obtained in this study, although encouraging, stress the need for further work to confirm some of the assumptions of the model