EXCHANGE OF NEODYMIUM AND ITS ISOTOPES BETWEEN SEAWATER AND SMALL ANDLARGE PARTICLES IN THE SARGASSO SEA

The concentrations and isotopic ratios of Nd were measured in small (1 -53 mu m) and large (> 53 mu m) in situ filtered particles, sediment t rap material, sediment, and aerosol samples, collected during oligotro phic 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 atmospher ic Nd could dissolve at the air/sea interface. The concentration and i sotopic composition of Nd vary with both depth and the particle size. Neodymium concentrations of small and large particles increase with de pth (from 2.9-12 mu g/g and 5.8-11.8 mu g/g, respectively). At 40 m, t he Nd signature of large particles is significantly more negative (eps ilon(Nd)(0) = -13.1 +/- 0.2) than that of the small ones (epsilon(Nd)( 0) = -11.7 +/- 0.3) and the particles filtered deeper. Below 237 m, th e epsilon(Nd)(0) values for small particles match the seawater, wherea s the epsilon(Nd)(0) values of large particles remain less radiogenic. These differences are related to the different behaviors of the parti culate fractions. Small particles apparently exchange Nd with seawater and then sink rapidly to depth as biologically formed aggregates (lar ge particles). These aggre gates isolate the small particles, slowing the exchange with the water to produce different signatures between la rge particles and seawater. Since these exchange processes modify the particulate Nd signature, the shallow radiogenic epsilon(Nd)(0) signat ure observed in the surface and subsurface waters in the NW Atlantic i s 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 parti cles repackaged into large aggregates can vary from 0-8% to 80-100% in productive and oligotrophic conditions, respectively. This seasonal v ariation may correspond to sinking particles of different zooplanktoni c origin. Vertical fluxes of Nd into and out of the Sargasso Sea, as w ell 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/m(2)/y is required. This value corres ponds to a scavenging Nd residence time of similar to 1900 y, consider ably longer than previous estimates of global oceanic residence time o f Nd on the order of 400 y.