Nd isotopic composition and REE pattern in the surface waters of the eastern Indian Ocean and its adjacent seas

The Nd isotopic composition and dissolved rare earth elements (REEs) have been measured in the surface waters along the 1996/97 R.V. Hakuho-Maru Expedition route from Tokyo to the Southern Ocean, southwest of Australia, through the Philippine and Indonesian Archipelago, the eastern Indian Ocean, the Bay of Bengal and the South China Sea. The radiogenic epsilon(Nd) values of -1.3 and -1.4 were found in the Sulu Sea and near the Lombok Strait, indicating the strong influence of surrounding volcanic islands, whereas non-radiogenic epsilon(Nd) values of less than -10 were found in the Southern Ocean and the Bay of Bengal suggesting Nd of continental origin. The dissolved Nd concentrations also showed a wide range of variation from 2.8 to 19.6 pmol/kg and the trivalent REE patterns exhibited characteristic features that can be grouped into each different oceanic province. The geographical distribution of dissolved Nd is different from that of atmospherically derived Pb-210, but generally resembles that of coastally derived Ra-228. This strongly suggests that fluvial and coastal input predominates over eolian input for dissolved Nd in the surface ocean. However, the riverine dissolved Nd flux appears to be relatively minor, and remobilization of Nd from coastal and shelf sediments may play an important role in the total Nd input to the ocean. By modeling the distributions of the isotopic composition and concentration of Nd together with the activity ratio of Ra-228/Ra-226 in the southeastern Indian Ocean, we estimate a mean residence time of Nd in the surface mixed layer to be 1.5-2.6 years. The short mean residence time is comparable with, or slightly longer than that of Pb-210 suggesting similar chemical reactivity. Copyright (C) 2000 Elsevier Science Ltd.