Seasonal variations in dissolved neodymium isotope composition in the Bay of Bengal

Constraining the dissolved neodymium (Nd) cycle in the ocean is paramount for using Nd isotopic composition (εNd) as a tracer to reconstruct deep-sea paleocirculations or continental weathering on different time scales. Dissolved εNd has been measured in seawater samples from six hydrological stations collected along ∼89°E North-South transect in the Bay of Bengal (BoB) in order to assess the impact of seasonal freshwater and sediment discharges from the continental river systems. Seawater samples collected in this study during June 2012 reveal more radiogenic εNd (a difference of ∼2 Epsilon units for the upper 2000 m, and ∼0.5 Epsilon unit below 2000 m) and ∼3-8 pmol/kg lower Nd concentrations than the reported values of nearby seawater samples collected in November 2008. These observations are most plausibly explained by a seasonal variations in dissolved Nd concentrations and εNd in the BoB, induced by seasonal variations in the freshwater and sediment discharges from the Ganges-Brahmaputra (G-B) river system. However, we cannot entirely exclude the possibility of spatial differences given that the water stations collected in this study are not exactly the same positions collected in November 2008. A two-box model suggests, (1) the particulate Nd inputs from the G-B river system mainly control the seasonal shift of εNd observed in the BoB seawater, and (2) a very rapid Nd exchange exists between lithogenic particles and seawater (at least on the scale of a few months). Seasonal changes in seawater εNd may also occur in other marginal seas and in the outflows of major rivers, and these need to be taken into account when using the εNd proxy in the ocean.