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

Abstract

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 > Po > 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.