Fluxes and balance of 210Pb in the tropical northeast Atlantic

Activities of the natural radionuclide 210Pb in particulate matter are reported from sediment traps deployed in the tropical northeast Atlantic at two sites, oligotrophic and mesotrophic, of the French EUMELI programme (EUtrophic, MEsotrophic and oLIgotrophic). Whereas 210Pb specific activities in settling particles do not vary with time at the oligotrophic site, they exhibit marked temporal variations at the mesotrophic site. Relationships between activities of 210Pbxs (i.e. that scavenged from seawater) and major constituents of trapped particles are examined in order to determine the influence of particle composition on 210Pb partitioning between solution and particles. In this oceanic area, 210Pb scavenging is directly related to the composition of particulate matter, and micro-particles of aluminosilicate are shown to be the primary phases carrying this radionuclide from the water column to the sea floor. Particulate organic carbon (POC) fluxes also appear to play an important role. 210Pbxs fluxes are linearly related to POC fluxes, as observed elsewhere, up to 10 mg m−2 day−1, but are limited to near-constant values when POC fluxes are higher than 10–20 mg m−2 day−1. This limit is ascribed to the depletion of the suspended aluminosilicate particle standing crop in the water column. To assess the relative importance of vertical and horizontal transport on scavenging processes, an oceanic 210Pb mass balance is drawn up for the tropical northeast Atlantic. This budget clearly demonstrates that this area of the tropical Atlantic eastern margin constitutes a significant sink for 210Pb.