Fluxes and balance of Pb-210 in the tropical northeast Atlantic

Activities of the natural radionuclide Pb-210 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 Pb-210 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 Pb-210(xs) (i.e. that scavenged from seawater) and major constituents of trapped particles are examined in order to determine the influence of particle composition on Pb-210 partitioning between solution and particles. In this oceanic area, Pb-210 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. Pb-210(xs) 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 Pb-210 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 Pb-210. Copyright (C) 1996 Elsevier Science Ltd