Land–sea linkages during deglaciation: High-resolution records from the eastern Atlantic off the coast of Namibia and Angola (ODP site 1078)
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.
References (43)
- et al.
The deglacial history of the western tropical Atlantic as inferred from high resolution stable isotope records off northeastern Brazil
Earth and Planetary Science Letters
(1999) - et al.
Abrupt onset and termination of the African humid period: rapid climate responses to gradual insolation forcing
Quaternary Science Reviews
(2000) - et al.
Reconstructing pathways of aeolian pollen transport to the marine sediments along the coastline of SW Africa
Quaternary Science Reviews
(2003) - et al.
Southwest African climate independent of Atlantic sea surface temperatures during the Younger Dryas
Quaternary Research
(2004) - et al.
Pollen evidence of late Quaternary vegetation and inferred climate changes in Congo
Palaeogeography, Palaeoclimatology, Palaeoecology
(1994) Hydrological changes in the African tropics since the Last Glacial Maximum
Quaternary Science Reviews
(2000)- et al.
Sedimentation and palaeoenvironment in crater lake Barombi Mbo, Cameroon, during the last 25,000 years
Sedimentary Geology
(1991) - et al.
Interhemispheric comparison of deglacial sea-surface temperature patterns in Atlantic eastern boundary currents
Earth and Planetary Science Letters
(2002) - et al.
Vegetation dynamics, palaeoenvironments and climatic changes in the forests of western Cameroun during the last 28,000 years B.P
Review of Palaeobotany and Palynology
(1998) - et al.
African monsoonal precipitation modulated by interhemispheric temperature gradients
Quaternary Research
(2000)
The nature of rainfall variability over Africa on time scales of decades to millenia
Global and Planetary Change
Late Quaternary climate and vegetation of the Sudanian zone of northeast Nigeria
Quaternary Research
Correlation between vegetation in southwestern Africa and ocean upwelling in the past 21,000 years
Quaternary Research
Massive iceberg discharge as triggers for global climate change
Nature
Simulation of an abrupt change in Saharan vegetation in the mid-Holocene
Geophysical Research Letters
Phytogeography, flora and endemism
Fynbos
North Atlantic Deep Water cools the southern hemisphere
Paleoceanography
Sensitivity of Asian and African climate to variations in seasonal insolation, glacial ice cover, sea surface temperature, Asian orography
Journal of Geophysical Research
The Saharan–Sahelian boundary during the Brunhes chron
Acta Botanica Neerlandica
Marine–terrestrial interaction of climate changes in West equatorial Africa of the last 190,000 years
Palaeoecology of Africa
Cited by (27)
A 30,000yr record of land-ocean interaction in the eastern Gulf of Guinea
2013, Quaternary Research (United States)Citation Excerpt :The pollen record off the Sanaga shows that savannah, swamp and Afromontane vegetation were significantly present in the Sanaga catchment region. In this respect, these data concur with other marine pollen records from farther south (e.g., Dupont and Behling, 2006) and west (Lezine and Cazet, 2005) (see Fig. 1 for site location). Charred grass cuticles are also particularly abundant during this period and may be related to higher incidence of savannah fires.
A petrographic and geochemical record of climate change over the last 4600years from a northern Namibia stalagmite, with evidence of abruptly wetter climate at the beginning of southern Africa's Iron Age
2013, Palaeogeography, Palaeoclimatology, PalaeoecologyAfrica
2013, Encyclopedia of Quaternary Science: Second EditionEffect of aridity and rainfall seasonality on vegetation in the southern tropics of East Africa during the Pleistocene/Holocene transition
2012, Quaternary ResearchCitation Excerpt :In much of North Africa, an abrupt return of aridity is observed resulting from a southward ITCZ displacement, and vegetation cover was greatly reduced (Hooghiemstra, 1988; Zhao et al., 2000). In contrast, rainforests remained significant until the early Holocene in coastal southwestern Africa (Dupont and Behling, 2006). Multi-proxy analysis of sediments covering the YD chronozone (13–11.8 ka) from Lake Malawi (9–14°S) have suggested a return to drier, cooler conditions and intense northeasterly (NE) tradewinds (Filippi and Talbot, 2005; Powers et al., 2005; Barker et al., 2007; Brown et al., 2007; Castañeda et al., 2007; Woltering et al., 2011).
Millennial-scale changes in vegetation records from tropical Africa and South America during the last glacial
2010, Quaternary Science ReviewsCitation Excerpt :However, marked vegetation changes occur during periods that may be associated with HS 1 and HS 2. The marine core from ODP Site 1078 C (Dupont and Behling, 2006; Dupont et al., 2008) provides a record of vegetation changes in tropical southwest Africa from 31.7 ka onwards (Fig. 3, Tables 1 and 2). During glacial times the vegetation in the source area (Angola and the southern Congo basin) was savannah and xerophytic scrubland, and grassland and dry shrubland (including open montane vegetation).
Late Quaternary hydrological dynamics in the Middle Kalahari: Forcing and feedbacks
2009, Earth-Science Reviews