Land-Ocean Interactions: Climate Variability off West Africa for the Last 21, 000 years
by
Marret, Fabienne
School of Ocean Sciences, University of Wales (Bangor), Menai Bridge, LL 59 5AB, U.K.
Coauthors: James Scourse

The growing concern for understanding the abrupt changes that the present-day climate system is currently undergoing, has led to explore high-resolution palaeoenvironmental archives within a multidisciplinary approach. Within this perspective, study of marine cores for their terrestrial and marine proxy content enables to reconstruct together oceanic, continental and atmospheric conditions that are closely linked. In comparison to temperate regions that are well studied as the access to marine and many continental sites is relatively easy, the number of African high-resolution records remains relatively low, although many palaeoenvironmental research have been conducted. The sensitivity of African ecosystems to any abrupt climate change has been well evident as, for instance, the impact of drought on the population and their environment over the last century. Desertification and regression of the tropical rainforest are also one of the main concerns, and knowledge of the natural variation of these ecosystem is crucial.

This paper will present a review of marine studies of subtropical and tropical vegetation and oceanic changes for the period covering from the Last Glacial Maximum (LGM) to the late Holocene. Three selected sites, two from NW Africa and one in the east equatorial Atlantic illustrate the evolution of the phytozones in relation to oceanic and atmospheric conditions (Figure 1).

In NW Africa, off Morocco, core KS78007 (Figure 2) provide a continuous record since the LGM, showing a twos-step deglaciation (Marret and Turon, 1994). Xeric conditions on the adjacent land, marked by the increase of open vegetation taxa (Artemisia, Chenopodiaceae/Amaranthaceae, Ephedra) and the development of the montane taxon, Cedrus atlantica, are observed before the Termination IA (TIA) and during the Younger Dryas. These intervals are also marked by increase of Pinus pollen grains, with indicates a strengthening of the NE trade winds. Proxies for oceanic surface conditions indicate that SSTs (sea-surface temperatures) were cooler during the TIA and the Younger Dryas, probably as a result of intensified coastal upwelling in relation to the strengthening of the atmospheric surface circulation. Oak forests, which are an important part of the present-day vegetation of the Maghreb region, began to developed around ca 8.5 ka BP.

Further south, off Western Sahara, core GIK 12379-4 (Marret, unpub. data) (Figure 3) covers the last 30,000 years and illustrate the variations of Saharan (Artemisia, Ephedra, Caryophyllaceae), Saharan-Sahelo (Chenopodiaceae/Amaranthaceae, Poaceae), Sahelo-Soudanian (e.g. Celtis, Rhus) vegetations and long-distance transport with Soudanian elements (Combretaceae) and Afromontane taxon (Podocarpus). Before 15 ka BP, Saharan elements are relatively important, in association with strong upwelling conditions as illustrated by the relatively high occurrence of the dinocyst Trinovantedinium applanatum (upwelling indicator) as well of Pinus, a trade-wind indicator. From 15 ka BP, increase of Cyperaceae as well as of Poaceae suggest wetter conditions with expansion of grassland, only interrupted at around 12-11 ka BP, with peaks of Ephedra and Caryophyllaceae indicating a return to drier conditions. The onset of "the Green Sahara" may be observed at around 6 ka BP with the strong decrease of Artemisia and Ephedra, whereas, sea-surface conditions record less prominent upwelling conditions.

These two relatively high-resolution records integrating marine and terrestrial proxies clearly illustrate the relationship between the occurrence and strength variation of the coastal upwelling and the adjacent vegetation.

In the equatorial belt of Africa, palaeoenvironmental studies undertaken and covering the last climatic cycle, in particular for the Last Glacial Maximum and the Holocene period, enable to reconstruct abrupt climate shifts in terms of dry/wet conditions. The review of terrestrial vegetation records from the equatorial central Africa document the dynamics of the lowland rainforest and savannahs in relation to climate changes during the Holocene. For the period before the Holocene, continental records are scarce in this vast region and/or only allow to reconstruct local vegetation. The investigation of marine sediment sequences from the Congo River deep-sea fan enables to reconstruct tropical palaeoenvironmental changes for the last 14,000 years based on integrated oceanic and terrestrial proxies (Marret et al., 2001, subm.). This record highlights the significance of abrupt changes in Congo River palaeodischarge, vegetation composition of the Congo drainage basin and regional hydrography set within precessional forcing of the monsoon. Terrestrial proxies (pollen, spores, charred grass cuticles) signal changes in the expansion/regression of the lowland rainforest in relation to the migration of the Intertropical Convergence Zone (ITCZ). Periods of dry climate, highlighted by dominant open-vegetation indicators also recorded increased savannah fires as illustrated by higher fluxes of burned grass cuticles (Figure 4).

The expansion of the lowland rainforest and dry forest that started around 13 cal ka BP was at its greatest around between 9 cal ka BP and 4.5 cal ka BP. The late Holocene period is marked by abrupt and possibly rapid climatic changes as reflected in the grass and afromontane vegetation signals. These major change in the vegetation show a rapid increase of herbaceous vegetation all over the Atlantic central Africa. This general regression of the forest is probably linked to deterioration of climate rather than anthropogenic influence. However, this opening of the vegetation may have initiate human population movements. The study of this integrated record of marine and terrestrial proxies illustrates the complexity of interactions between land-ocean and atmosphere systems and emphasize the need of high-resolutions records for enabling satisfactory explanations of our palaeodata reconstructions.

References Marret F. and Turon J.L., 1994. Paleohydrology and paleoclimatology off northwest Africa during the last glacial-interglacial transition and the Holocene: Palynological evidences. Marine Geology, 118: 107-117 Marret F., Scourse J., Versteegh G., Jansen J.H.F., and Schneider R., 2001. Integrated marine and terrestrial evidence for abrupt Congo River palaeodischarge fluctuations during the last deglaciation. Journal of Quaternary Science 16(8): 761-766.

Marret, F., Maley, J. and Scourse, J. Climatic instability in west equatorial Africa during the Mid- and Late Holocene. Submitted to Special Publication of Geological Society of London.

Date received: November 28, 2003

Copyright © 2003 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # camu-16.