Spatial distribution of dissolved neodymium and ε_Nd in the Bay of Bengal: role of particulate matter and mixing of water masses

Abstract

The concentration and isotope composition of dissolved Nd have been measured in the water column along an 87°E transect (GIO1 section of International GEOTRACES Program) in the Bay of Bengal (BoB) to investigate the effect of water mass mixing and Nd release from particulate matter in determining these properties. The concentration of Nd in surface waters of the BoB shows a North–South decreasing non-linear trend (∼46 to ∼22 pmol/kg) with salinity, whereas its depth profiles typically show a high value in surface waters, a minimum (∼15 to ∼23 pmol/kg) in the shallow subsurface (∼50–200 m), followed by a gradual increase with depth. The Nd concentration of BoB waters is generally higher than that in nearby oceanic basins. On the other hand, the ε_Nd values in the BoB are less radiogenic compared to those reported for other regions of the global oceans (except Baffin Bay, the North Atlantic Subpolar Gyre and the Niger delta margin), and show a greater variation in the upper water column. Surface waters of the southernmost profile (∼6°N) show a more radiogenic εNd value ∼−8, which decreases to −15 in the northernmost profile (∼20°N), close to the values for dissolved and particulate phases of the Ganga–Brahmaputra (G–B) Rivers. This latitudinal trend is most likely a result of variations in mixing proportion between the more radiogenic Indonesian Throughflow surface waters (IW) and unradiogenic BoB low salinity water; the former’s signature being more clearly discernible in surface waters of the two southernmost profiles (∼6°N and ∼8.5°N). Attempts to balance the Nd budget in the water column based on an inversion model, suggest that in addition to water masses other source(s) is required, the strength of which is estimated to vary from 1% to 65% of the measured Nd concentration. The calculations also show that the εNd of this additional source(s) has to be in the range of ∼−16 ± 2, typical of G–B river sediments. These observations, coupled with the North–South distribution of dissolved Nd and εNd, indicate that this additional source is release from particulate phases supplied by the G–B river system. The calculations also bring out the presence of “hot-spots” of Nd release (excess Nd) near the sediment–water interface along the northern slope of the bay, indicating supply of Nd from continental margin sediments. This study underscores the significant role of dissolved/particulate Nd from the Ganga–Brahmaputra river system in contributing to the dissolved Nd budget of the global oceans.