Upwelling intensity and ocean productivity changes off Cape Blanc (northwest Africa) during the last 70,000 years: geochemical and micropalaeontological evidence

The accumulation of biogeochemical (organic carbon, calcium carbonates, mol ybdenum and iodine), micropalaeontological (benthic foraminifera) and terri genous markers (grain-size, Si/Al, Zr/Al, K/Al) over the last 70,000 years in one core (Sedorqua-11K) on the northwest African margin off Cap Blanc ha s been used to reconstruct past variations of local upwelling intensity and oceanic productivity. The study demonstrates that productivity in this are a increased during Stage 3, particularly between 40,000 and 50,000 yr B.P., and during the last deglaciation between 6000 and 15,000 yr B.P. During mo st of isotopic Stage 2, and particularly during the last glacial maximum, p roductivity was much lower. These variations can be attributed to changes i n local wind stress and seasonality that are related to variability in mons oon pressure intensity. Because of the establishment of upwelling cells ove r the shelf due to high sea-level, the conditions of sedimentation during t he last 5-6 thousand years on the upper slope (site 11K) are largely domina ted by advection from the shelf, leading to strong sorting prior to deposit ion. Advection seems to have been minor during the other periods of enhance d productivity. A conceptual model is proposed to link the productivity var iations to atmospheric circulation, in particular to the wind stress, direc tion and seasonality. (C) 1999 Elsevier Science B.V. All rights reserved.