Land–sea linkages during deglaciation: High-resolution records from the eastern Atlantic off the coast of Namibia and Angola (ODP site 1078)

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Abstract

The distribution of pollen in marine sediments is used to record vegetation change on the continent. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in the marine surface sediments and the occurrence of the source plants on the adjacent continent. To investigate land–sea interactions during deglaciation, we compare proxies for continental (pollen assemblages) and marine conditions (alkenone-derived sea surface temperatures) of two high-resolution, radiocarbon-dated sedimentary records from the tropical southeast Atlantic. The southern site is located West of the Cunene River mouth; the northern site is located West of the Angolan Huambe Mountains. It is inferred that the vegetation in Angola developed from Afroalpine and open savannah during the last Glacial maximum (LGM) via Afromontane Podocarpus forest during Heinrich Event 1 (H1), to an early increase of lowland forest after 14.5 ka. The vegetation record indicates dry and cold conditions during the LGM, cool and wet conditions during H1 and a gradual rise in temperature starting well before the Younger Dryas (YD) period. Terrestrial and oceanic climate developments seem largely running parallel, in contrast to the situation ca. 5° further South, where marine and terrestrial developments diverge during the YD. The cool and wet conditions in tropical West Africa, South of the equator, during H1 suggest that low-latitude insolation variation is more important than the slowdown of the thermohaline circulation for the climate in tropical Africa.

Introduction

The thermohaline circulation (THC) is an important distributor of heat (Broecker, 1994); particularly in the Atlantic, the THC transports heat from the tropical ocean to the North Atlantic, whereby the South Atlantic loses heat (Crowley, 1992). Consequently, a slowdown of the THC during the Younger Dryas (YD, 12.5–11.8 ka) period and Heinrich Event 1 (H1, 18–15 ka) warms the tropical Atlantic surface and intermediate waters (Arz et al., 1999; Rühlemann et al., 1999; DeMenocal et al., 2000; Hüls and Zahn, 2000; Rühlemann et al., 2004). Thus, the effects of THC slow-down are different at low and at high latitudes, which is in contrast to the direct effects of greenhouse warming. However, greenhouse warming and subsequent ice melting and fresh-water discharge in the Arctic Ocean may initiate a slow-down of the THC.

Numerical modelling indicates that changing sea surface temperatures (SSTs) should affect the tropics of Africa in numerous ways (DeMenocal and Rind, 1993; Kutzbach and Liu, 1997; Claussen et al., 1999; Nicholson, 2000), because: (1) the trade winds may become stronger during an increased latitudinal temperature gradient in the North Atlantic; (2) the average summer position of the Intertropical Convergence Zone (ITCZ) is affected, which has consequences for the spatial distribution of precipitation in the tropics of Africa; (3) changes of the SSTs in the tropics would affect the humidity gradient between ocean and continent and thus the water carrying capacity of the monsoon.

In NW Africa, an arid interval corresponding to the YD period interrupted the humid conditions associated with a ‘green’ Sahara between 14.8 and 5.5 ka (DeMenocal et al., 2000; Gasse, 2000). Also, in the Sudanian savannah (ca. 10°N), a distinct dry event occurred during the YD period (Salzmann et al., 2002). However, a clear YD oscillation has not been recorded in the lake sediments of Barombi Mbo (Giresse et al., 1991; Maley and Brenac, 1998). Comparison of the deglacial pattern in SST changes off the Congo River mouth, with West African records indicating that the rapid changes recorded in African lake levels are related to changes in the tropical SST and slow-down of the THC (Mulitza and Rühlemann, 2000). Feedback mechanisms between precipitation and vegetation cover increase the amplitude of the environmental changes (Kutzbach et al., 1996; Texier et al., 1997; Claussen et al., 1999). Hence, we want to investigate the terrestrial climate change in tropical SW Africa during periods of slow-down of the thermohaline circulation, and subsequent warming of tropical Atlantic surface and subsurface waters.

The monsoon is the major climate driver in tropical Africa. Its variability depends largely on the variation in low latitude insolation, which is precession-dependent and places the African monsoon in phase with the maximum summer insolation in the northern hemisphere (Kutzbach, 1981; Schneider et al., 1995; Dupont et al., 1999). The period of YD took place during the rise in northern hemisphere summer insolation following the last Glacial maximum (LGM). During YD, the THC collapsed in a period of increasing insolation, while during H1, the THC was low at a minimum in the northern hemisphere summer insolation. Records from the Cariaco basin (Haug et al., 2001) and the subtropical NE Atlantic (DeMenocal et al., 2000) show the effects of YD as an interruption of the deglaciation process in South America and northwest Africa, respectively. They illustrate that the influence of insolation in the tropics can be countered by the effects of reduced THC overturning. The question arises as to the sensitivity of the climate in the southern tropics of Africa. The importance and the actuality of this question are underlined by the emergence of a new view of the tropics controlling global climate change (e.g. Visser et al., 2003). Here, we present a study of the vegetation development of tropical SW Africa (southern part of the Congo Basin and the highlands of Angola) from the LGM (ca. 22 ka) to the Holocene optimum (ca. 9 ka), and compare marine and terrestrial records from the tropical SE Atlantic.

Section snippets

Material and methods

The material originates from sedimentary cores of two locations—ODP 1078C at 11°55′S 13°24′E in 426 m water depth and GeoB 1023-5 at 17°09′S 11°01′ E in 1978 m water depth (Fig. 1; Wefer et al., 1988, Wefer et al., 1998). ODP 1078C and GeoB 1023 are dated with 18 and 9 AMS-radiocarbon dates, respectively (Shi et al., 2000; Kim et al., 2003). All ages are given in 1000 years calibrated before present (ka cal BP). Sea surface temperature records of both cores have been published by Kim et al., 2002

Modern vegetation

The vegetation of Africa, South of the equator, ranges from rain forest to desert, and from lowland to Afroalpine vegetation (Fig. 1A). Phytogeographical regions are the Congolean, Zambezian, Kalahari–Highveld and Karoo–Namib desert/semi-desert phytochoria; the Cape Flora and the Afromontane vegetation (White, 1983).

South of the Congo basin, with its swamp forests and rain forests, the transition to the Zambezian dry evergreen forest is nowadays formed by a mosaic of lowland rain forest and

Vegetation development in tropical SW Africa during deglaciation

We studied the terrestrial environmental changes in tropical SW Africa during the deglaciation using the relative abundance of various pollen and spores in sediments of ODP site 1078. The source areas of the pollen and spores lie mainly in Angola. Predominantly, during austral fall and winter, easterly and southeasterly winds transport pollen and spores from the Angolan highlands westward to the Angolan Basin in the southeast Atlantic (Dupont and Wyputta, 2003). Few major rivers have their

Discussion and conclusions

The relative grass pollen abundance at 17°S and at 12°S is anti-correlated between 20 and 14 ka, which we interpret as a southward shift of the open savannah (Fig. 3). Comparable latitudinal shifts of open grass-rich vegetation zones have been reported to occur repeatedly over glacial–interglacial transitions, both in SW and in NW Africa (Dupont and Hooghiemstra, 1989; Dupont and Wyputta, 2003).

The pollen record indicates an expansion of the Afromontane Podocarpus forest during H1. The

Acknowledgments

This paper is a contribution to the PAGES/START/INQUA Africa workshop organized by Daniel Olago in Nairobi. We thank Daniel Olago, Norm Catto and Henry Lamb for their constructive comments and suggestions. Financial support was given by the Deutsche Forschungsgemeinschaft (DFG). Data are available in PANGAEA.

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