Neodymium isotopes and concentrations in Caribbean seawater: Tracing water mass mixing and continental input in a semi-enclosed ocean basin

We present the first full water column Nd isotope ( ε Nd ) and concentration data for Caribbean seawater, as well as for stations close to the Orinoco River mouth and in the Florida Straits. The surface inflow into the southeastern Caribbean via the Guyana Current is characterized by an ε Nd signature of −10.9, which is a consequence of the mixing of relatively unradiogenic ε Nd signatures (−13.6) supplied by the Orinoco River with contributions from the Amazon River (∼−10). Despite the proximity to land, sub-surface and intermediate waters within the Caribbean largely retain the ε Nd signatures of their source water masses in the Atlantic. In contrast, the deep waters of the Caribbean show ε Nd signatures at least 3 ε Nd units more radiogenic than the inflowing Upper North Atlantic Deep Water (UNADW). A ε Nd shift from −13 to −9.7 can be explained by addition of radiogenic Nd to the deep Caribbean through weathering inputs from land. However, in order to balance such large shifts in ε Nd with at the same time modest increases in Nd concentrations, Nd must also be removed from seawater within the basin. It is suggested that the long residence time of deep waters in the Caribbean allows significant interaction of seawater with sinking particles and seafloor sediments resulting in more radiogenic values. These findings have implications for the use of ε Nd as a proxy for paleocirculation in restricted basins, in which the residence times of the deep waters are long.