Palaeoecology of Africa and the Surrounding Islands - Volume 26

Heine Klaus

• Publisher: Routledge
• Year: 2018
• Print ISBN: 9789054104766
• eBook ISBN: 9781351426015
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

This volume offers comprehensive and up-to-date information on research in many different disciplines which give an overall insight into the environmental history of Africa.

Quaternary and recent palynology

Marine-terrestrial interaction of climate changes in West Equatorial Africa of the last 190,000 years

LYDIE DUPONT & RALPH SCHNEIDER
Geosciences, University of Bremen, Germany
ANNEKE SCHMÜSER
Institute of Palynology and Quaternary Sciences, Goettingen, Germany
SUSANNE JAHNS
Institute of Palynology and Quaternary Sciences, Goettingen, Germany (Present adress: Deutsches Archaeologisches Institut, Berlin, Germany)

ABSTRACT

Palynological investigation of a marine piston core GeoB 1008-3 (6°35′S/10°19′E) from the highly productive area off the Congo fan provides a dinoflagellate cyst record reflecting marine surface water conditions during the last 190 ka and a pollen record of the vegetation changes in Central Africa induced by climatic fluctuations for the same period. The freshwater discharge of the Congo river is related to the intensity of the West African monsoon. During warmer and more humid phases, dinoflagellate cyst flux decreased in relation to lower productivity and increased freshwater input in the eastern South Atlantic. At the same time on the continent, the rain forest expanded. During the cooler interstadials of Stage 5, the Afromontane forest represented by Podocarpus pollen expanded to lower altitudes and occupied former areas of rain forest during periods showing intermediate levels for sea surface temperatures and river discharge. During the colder and more arid phases of glacial Stages 6 and 2, when freshwater run-off into the Gulf of Guinea decreased, dinoflagellate cysts were much more abundant. However, Stage 4 shows high levels of river discharge and a moderate dinoflagellate cyst flux coupled to low sea-surface temperatures of the eastern South Atlantic, while Stage 3 shows moderately high pollen percentages of the rain forest during phases with fluctuating fresh-water discharge and very low sea-surface temperatures. High sea levels at the beginning of the Last Interglacial and the Holocene favoured the expansion of mangroves.

INTRODUCTION

Trade winds and monsoon circulation control the climate of West Africa. The surface wind field over the Angola Basin and the Gulf of Guinea is dominated by the SE-trade winds, that change their direction from SE to SW thereby becoming the SW-monsoon that penetrates far into the continent during boreal summer. In boreal winter, atmospheric circulation reverses over the continent when NE trade-winds blow over Africa and the adjacent subtropical Atlantic and reach the Gulf of Guinea (Leroux, 1983; Hastenrath, 1992).

Monsoon circulation results from the differing heat capacities of land and water: Sensible heating warms land surfaces much more rapidly man the ocean mixed layer. Over West Africa, summer heating over central North Africa during boreal summer drives the inflow of moisture-laden air from the adjacent eastern equatorial Atlantic. Strong trade winds of the Southern Hemisphere cross the equator and penetrate into central Africa. The monsoon delivers precipitation deep into central subtropical Africa. Variations of the monsoon, however, are strongly coupled to sea surface temperature fluctuations of the Indian Ocean (Kutzbach & Street-Perrot, 1985; Kutzbach, 1987; Prell & Kutzbach, 1987).

Additional influence on the West African monsoon is from the North Atlantic Ocean. General circulation model experiments of DeMenocal & Rind (1993) indicate that cooler North Atlantic sea surface temperatures (SST’s) reduce the monsoonal inflow into North Africa and decrease surface air temperature by 2 to 4°C and precipitation up to 1 mm/day. Summer southwest African monsoon winds are reduced by the same high-pressure cell over the North Atlantic strengthening the winter NE trade-wind circulation over northwestern Africa. The precipitation decreases over Africa can be related to increased surface air outflow from the region owing to the thermal effects of the cooler North Atlantic SST’s (DeMenocal & Rind, 1993).

Because of the influence of different oceanic regions on the strength of the monsoon, its response to climatic variation is highly complicated. On the one hand, there is the influence of the SST’s of the North Atlantic in response to glacial-interglacial variation and obliquity, while, on the other hand, there is the direct response of continental heating to insolation and the precessional controlled SST’s of the eastern equatorial Atlantic (McIntyre et al., 1989; Schneider et al., 1995; Mix & Morey, 1996; Schneider et al., 1996) and of the Indian Ocean. Glacial-interglacial high latitude variation intermingles with the response to low-latitude precessional insolation (DeMenocal, 1995).

By comparing terrestrial and oceanographic proxy data from a core off the mouth of the Congo river, we try to identify the direct influence of the SST of the eastern equatorial Atlantic on the climate of west equatorial Africa (Congo basin and surroundings) for the last 190 ka. To this purpose, a new dinoflagellate cyst diagram is presented from the Congo fan (core station GeoB 1008) and compared with a pollen diagram from the same locality (Jahns, 1996). The present study continues and extends investigations about the vegetation changes in western equatorial Africa to the south. Changes in palaeoprecipitation can be inferred from shifts in vegetation zones. The dinoflagellate cyst record holds information about marine surface production and salinity variation as a result of river induced upwelling and oceanic/coastal upwelling as well as oscillations of the Congo freshwater discharge. A firm land-sea correlation is established by the combined records of dinoflagellate cysts and terrestrial palynomorphs from the same samples. Additional information is provided by alkenone-derived palaeo-SST’s (Schneider et al., 1995) and by Al/K-ratios indicating chemical wheathering conditions on the continent (Schneider et al., 1997).

Laboratory cultures of the coccolithorid species Emiliania huxleyi as well as studies of modern plankton, sediment-trap material, and surface sediments established a relationship between the saturation index of C37-alkenones produced by coccolithophorids and the temperature of the water in which the coc-colithophorids were growing (Prahl et al., 1988; Brassell, 1993). Biological studies show that coccolithophorids bloom in offshore surface waters off SW Africa above a well developed thermocline (Giraudeau et al., 1993). Sediment-trap experiments on the Walvis Ridge revealed that alkenones were exported from the surface waters throughout the years, with two prominent maxima during austral spring and fall. Alkenone-derived palaeo-SST’s, therefore, most likely reflect annual average temperatures of the mixed layer (Schneider et al., 1995).

Fluctuations in the ratios of Al/K and kaolinite/feldspar are attributed to changes in the composition of the Congo river particulate sediment load, assuming the river outfall to be the major source for kaolinite in the Angola Basin sediments (Eisma et al., 1978; Sholkovitz et al., 1978). Because kaolinite and feldspar strongly differ in their aluminium content, most of the variation in the Al/K-ratio reflects variation in the kaolinite/feldspar-ratio with which it generally covaries. Changes in the Al/K-ratio in Congo fan sediments can be considered in terms of weathering of the central African hinterland, whereby high Al/K-ratios correspond to intensified chemical weathering (Schneider et al., 1997).

PRESENT OCEANOGRAPHIC SETTING

The surface waters off the Congo mourn are dominated by southward flowing warm surface and sub-surface currents: Angola current and South Equatorial Counter-Current. The Angola Dome and the cyclonic-gyre circulation of the SE Atlantic, which transports warm equatorial waters southward through the Angola Current, influence the region. A small amount of northward flowing cold sub-surface waters from the Benguela Coastal Current also reaches the core station. However, the main front between the warm surface waters of the Angola Current from the north and the cold surface waters of the Benguela Coastal Current from the south — called the Angola-Benguela Front — is situated farther south, between 12 and 14°S (Fig. 1). The area is charaterized by high productivity caused by oceanic, river-induced, and coastal seasonal upwelling, (van Bennekom & Berger, 1984; Berger et al., 1989; Stramma & Peterson, 1989; Peterson & Stramma, 1991).

Figure 1. Ocean currents (AC = Angola Current, BOC = Benguela Ocean Current, BCC = ...

Table of contents

• Cover Page
• Halftitle Page
• Title Page
• Copyright Page
• Contents
• Forword
• Preface
• Obituary
• Palaeopalynology
• Quaternary and recent Palynology
• Pollen morphology
• Book reviews
• List of authors