Concentrations and isotopic compositions of neodymium in the eastern Indian Ocean and Indonesian straits

Four profiles of Nd concentration and isotopic composition were determined at two stations in the eastern Indian Ocean along a north/south section between Ball and Port-Hedland and at two others in the Timer and Sumba straits. Neodymium concentrations increase with depth, between 7.2 pmol/L at the surface to 41.7 pmol/L close to the bottom. The epsilon(Nd) of the different water masses along the section are -7.2 +/- 0.2 for the Indian Bottom Waters and -6.1 +/- 0.2 for the Indian Deep Waters. The intermediate and thermocline waters are less radiogenic at st-10 than at st-20 (-5.3 +/- 0.3 and -3.6 +/- 0.2, respectively). In the Timer Passage and Sumba Strait, epsilon(Nd) of the Indonesian waters is -4.1 +/- 0.2 and that of the North Indian Intermediate Waters is -2.6 +/- 0.3. These distinct isotopic signals constrain the origins of the different water masses sampled in the eastern Indian Ocean. They fix the limit of the nonradiogenic Antarctic and Indian contributions to the southern part of the section whereas the northern part is influenced by radiogenic Indonesian flows. In addition, the neodymium isotopic composition suggests that in the north, deep waters are influenced by a radiogenic component originating from the Sunda Arch Slope flowing deeper than 1200 m, which was not documented previously. Mixing calculations assess the conservativity of epsilon(Nd) on the scale of an oceanic basin. The origin of the surprising radiogenic signal of the NIIW is discussed and could result from a remobilization of Nd sediment-hosted on the Java shelf, requiring important dissolved/particulate exchange processes. Such processes, occurring in specific areas, could play an important role in the world ocean Nd budget. Copyright (C) 1998 Elsevier Science Ltd.