The distribution of neodymium isotopes and concentrations in the Eastern Equatorial Pacific: Water mass advection versus particle exchange

The radiogenic isotope composition of the Rare Earth Element (REE) neodymium (Nd) is a powerful water mass proxy for present and past ocean circulation. The processes controlling the Nd budget of the global ocean are not quantitatively understood and in particular source and sink mechanisms are still under debate. In this study we present the first full water column data set of dissolved Nd isotope compositions and Nd concentrations for the Eastern Equatorial Pacific (EEP), where one of the globally largest Oxygen Minimum Zones (OMZ) is located. This region is of particular interest for understanding the biogeochemical cycling of REEs because anoxic conditions may lead to release of REEs from the shelf, whereas high particle densities and fluxes potentially remove the REEs from the water column. Data were obtained between 1 degrees 40'N and 16 degrees S along a nearshore and an offshore transect. Near surface zonal current bands, such as the Equatorial Undercurrent (EUC) and the Subsurface Countercurrent (SSCC), which are supplying oxygen-rich water to the OMZ are characterized by radiogenic Nd isotope signatures (epsilon(Nd) = -2). Surface waters in the northernmost part of the study area are even more radiogenic (epsilon(Nd) = +3), most likely due to release of Nd from volcanogenic material. Deep and bottom waters at the southernmost offshore station (14 degrees S) are clearly controlled by advection of water masses with less radiogenic signatures (epsilon(Nd) = -7) originating from the Southern Ocean. Towards the equator, however, the deep waters show a clear trend towards more radiogenic values of up to epsilon(Nd) = -2. The northernmost station located in the Panama basin shows highly radiogenic Nd isotope signatures in the entire water column, which indicates that particle scavenging, downward transport and release processes play an important role. This is supported by relatively low Nd concentrations in deep waters (3000-6000 m) in the EEP (20 pmol/kg) compared to locations in the Northern and Central Pacific (40-60 pmol/kg), which suggests enhanced removal of Nd in the EEP. (C) 2012 Elsevier B.V. All rights reserved.