PARTICLE MIXING RATES IN SEDIMENTS OF THE NORTHEAST TROPICAL ATLANTIC - EVIDENCE FROM PB-210(XS), CS-137, TH-228(XS) AND TH-234(XS) DOWNCORE DISTRIBUTIONS

Particle mixing rates were estimated from distributions of Cs-137 and excess activities of (210)pb, Th-228 and Th-234 measured in nine sediment cores taken from the northeast tropical Atlantic, i.e. at the three sites of the French EUMELI (EUtrophic, MEsotrophic and oLIgotrophic) programme. At the oligotrophic site, activities of Cs-137 and excess Pb-210 decrease smoothly with depth. On the other hand, at the mesotrophic and eutrophic sites, Pb-210(xs) and Cs-137 profiles suggest a mixing process with three different regimes, with subsurface sediments being mixed more rapidly than those at the surface. Bioturbation rates were found to be slowest at the oligotrophic site, intermediate at the mesotrophic site, and fastest at the eutrophic site. These mixing intensities appear closely linked to benthic fauna abundance, organic carbon flux to the sediments, and primary production in overlying surface waters. The use of a multitracer approach for studying sediment particle reworking has allowed bioturbation rates to be better constrained and an assessment of whether radionuclide distributions in surface sediments are in steady state. For only one core from the mesotrophic site were surface sediments not in steady state, perhaps because of the occurrence of a benthic animal (i.e., a xenophyophore) at the surface of this core.