BE-10 AND BA CONCENTRATIONS IN WEST-AFRICAN SEDIMENTS TRACE PRODUCTIVITY IN THE PAST

Particle reactive elements are scavenged to a higher degree at ocean m argins than in the open ocean due to higher fluxes of biogenic and ter rigenous particles. In order to determine the influence of these proce sses on the depositional fluxes of Be-10 and barium we have performed high-resolution measurements on sediment core GeoB 1008-3 from the Con go Fan. Because the core is dominated by terrigenous matter supplied b y the Congo River, it has a high average mass accumulation rate of 6.5 cm/kyr. Biogenic Be-10 and Ba concentrations by subtracting the terri genous components of Be-10 and Ba, which are assumed to be proportiona l to the flux of Al2O3. The mean Ba/Al weight ratio of the terrigenous component was determined to be 0.0045. The unusualy high terrigenous Be-10 concentrations of 9.1 x 10(9) atoms/g Al2O3 are either due to in put of particles with high Be-10 content by the Congo River or due to scavenging ofoceanic Be-10 by riverine particles. The maxima of biogen ic Be-10 and Ba concentrations coincide with maxima of the paleoproduc tivity rates. Time series analysis of the Be-10 and of Ba concentratio n profiles reveals a strong dominance of the precessional period of 24 kyr, which also controls the rates of paleoproductivity in this core. During the maxima of productivity the flux of biogenic Ba is enhanced to a larger extent than that of biogenic Be-10. Applying a model for coastal scavenging, we ascribe the observed higher sensitivity of Ba t o biogenic particle fluxes to the fact that the ocean residence time o f Ba is approximately 10 times longer than that of Be-10.