Atmospheric and Oceanic Circulation Patterns off Northwest Africa During the Past 25 Million Years

Abstract

The sediments of the eastern Atlantic contain excellent historical records of the patterns of oceanic and atmospheric circulation in the subtropics. This is particularly the case at the low relief northwest African continental margin which favors unrestrained interaction of the land- sea climatic system and which forms a schematic, vertical cross section through the ocean from the equator to Mediterranean latitudes. Our synthesis of the Neogene and Quaternary evolution of paleoenvironments along this margin tries to show that oceans and land respond in a complementary way to global climatic events and mechanisms.

Composition and fabrics of surface sediments produce a detailed record on (1) the stratification of modern deep-water circulation (Antarctic Bottom Water, North Atlantic Deep Water, various currents along the slope such as the South Atlantic Central Water, oxygen-minimum layer), (2) the geometric patterns of ecologic surface-water masses (for example, the North Atlantic eastern boundary current, north equatorial water, up-welling water, freshwater discharge), (3) their temperature distribution in general, and (4) the major structures of climate and wind circulation (continental wetness and aridity, zonal and meridional wind systems, and the respective wind speeds, summer position of the Intertropic Convergence Zone). This record basically contributes to the understanding of the Quaternary and Neogene history which, however, shows a number of additional environmental patterns which have no modern counterparts.

Estimates from the planktonic foraminiferal record suggest that during the last interglacial climatic optimum at 6000 years B.P., surface-water temperatures of the Canary Current were higher during summer and only slightly higher during winter as compared to today. This increase of seasonality can signal a summer breakdown of coastal upwelling and of meridional Trade Winds which correlates with the reduced supply of northwest Saharan dust. The Harmattan dust outbreaks and the ITCZ occupied approximately the same summer position as today.

Continental wetness spread over most parts of the Sahara and resulted in an extended discharge of fluvial muds.

During the last glacial mode at 18,000 years B.P., the estimates from planktonic foraminiferal counts show that the North Atlantic polar front had shifted to lat. 42°–45°N and bordered almost directly upon the subtropical gyre, whose geometry and temperatures differed only slightly from today. The intensified Canary Current cooled down by 6°–8°C and expanded some 500 km further south. The productivity of coastal upwelling was high as a result of both a subsurface expansion of nutrient-rich SACW and strenghtened and perennial Trade Winds.

Distribution and composition of aeolomarine dust and the upwelling record suggest that during glacial stages the ITCZ was essentially in its interglacial position with seasonal migrations comparable to that of today. Meridional winds intensified and zonal winds decreased by about 50%. As a result, continental aridity and dune fields spread over large parts of north Africa between lat. 100 and 28°N, river load disappeared, even in front of the Niger delta.

In the deep-water circulation, the record of benthic foraminifers suggests a replacement of the well- oxygenated North Atlantic Deep Water by less oxygenated water masses. The last (short) transition from glacial to interglacial modes shows time transgressive pulses in the occurrence of planktonic groups advancing from south to north, and a slightly delayed onset of continental wetness near 12,000 years B.P.

The Neogene oxygen-isotopic record of the Deep-Sea Drilling Sites 366 and 397 suggests six major glacial- style intervals (24–20 m.y., 18–14 m.y., 13–?9.5 m.y., 7.5–5.3 m.y., 3.2–1.9 m.y., and since 0.73 m.y., marking a stepwise general climatic deterioration leading to the Present. Each interval correlated with enhanced oceanic circulation and deepsea erosion, glacial-style meridional winds, and aridity spreading over north Africa. The intervening phases amounted to an equal range of time (20–18 m.y., 14–13 m.y., ?9.5–7.5 m.y., 6.8–6.0 m.y., 5.3–3.2 m.y., and 1.9–0.73 m.y., and led to climatic stability or amelioration. They correlated with high sea level, calm oceanic and prevailing zonal wind circulation, and with continental wetness. During two time intervals, during the Early Miocene until 14 m. y. B.P. and during the last 3 m.y., sediments record enhanced fertility at the continental margin which was induced by upwelling as well as by intense river discharge. The Neogene northward drift of Africa resulted in a gradual southward shift of the position of wind systems and the correlated climatic belts measured relative to the African plate.