Exchange of neodymium and its isotopes between seawater and small and large particles in the Sargasso Sea

Abstract

The concentrations and isotopic ratios of Nd were measured in small (1–53 μm) and large (> 53 μm) in situ filtered particles, sediment trap material, sediment, and aerosol samples, collected during oligotrophic and productive conditions in the Sargasso Sea. Atmospheric input is the main source of exogeneous Nd into the Sargasso Sea. Based on Nd Al ratios in trapped material, we suggest that up to 50% of atmospheric Nd could dissolve at the air/ sea interface. The concentration and isotopic composition of Nd vary with both depth and the particle size. Neodymium concentrations of small and large particles increase with depth (from 2.9–12 μg/g and 5.8–11.8 μg/g, respectively). At 40 m, the Nd signature of large particles is significantly more negative ( ϵ Nd(0) = -13.1 ± 0.2) than that of the small ones ( ϵ Nd(0) = -11.7 ± 0.3 and the particles filtered deeper. Below 237 m, the ϵ Nd(0) values for small particles match the seawater, whereas the ϵ Nd(0) values of large particles remain less radiogenic. These differences are related to the different behaviors of the particulate fractions. Small particles apparently exchange Nd with seawater and then sink rapidly to depth as biologically formed aggregates (large particles). These aggregates isolate the small particles, slowing the exchange with the water to produce different signatures between large particles and seawater. Since these exchange processes modify the particulate Nd signature, the shallow radiogenic ϵ Nd(0) signature observed in the surface and subsurface waters in the NW Atlantic is not due to external contributions and is likely of advective origin. At depth below 800 m, the magnitude of the aggregation/ disaggregation process varies significantly with season; the fraction of small particles repackaged into large aggregates can vary from 0–8% to 80–100% in productive and oligotrophic conditions, respectively. This seasonal variation may correspond to sinking particles of different zooplanktonic origin. Vertical fluxes of Nd into and out of the Sargasso Sea, as well as exchange fluxes in the water column are examined. About 50% of the particulate flux from the surface is remineralized at depth; a net scavenging flux of 0.8 10 −4 g/m2/y is required. This value corresponds to a scavenging Nd residence time of ∼ 1900 y, considerably longer than previous estimates of global oceanic residence time of Nd on the order of 400 y.