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, molybdenum and iodine), micropalaeontological (benthic foraminifera) and terrigenous 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 has been used to reconstruct past variations of local upwelling intensity and oceanic productivity. The study demonstrates that productivity in this area 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 most of isotopic Stage 2, and particularly during the last glacial maximum, productivity was much lower. These variations can be attributed to changes in local wind stress and seasonality that are related to variability in monsoon pressure intensity. Because of the establishment of upwelling cells over the shelf due to high sea-level, the conditions of sedimentation during the last 5-6 thousand years on the upper slope (site 11K) are largely dominated by advection from the shelf, leading to strong sorting prior to deposition. Advection seems to have been minor during the other periods of enhanced productivity. A conceptual model is proposed to link the productivity variations to atmospheric circulation, in particular to the wind stress, direction and seasonality. (C) 1999 Elsevier Science B.V. All rights reserved.