Amaral, P.G.C.

Buchet, G.

Deschamps, Pierre

Guiot, J.

Sylvestre, Florence

Vincens, A.

Horizon / Pleins textes

Geophysical Research Abstracts
Vol. 13, EGU2011-12005, 2011
EGU General Assembly 2011
© Author(s) 2011
Vegetation reconstruction to the mid-Holocene transition in Sahel and
Sahara: first palynological results of a lacustrine record from the
Mega-Lake Chad
Paula G. C. do Amaral, Annie Vincens, Joel Guiot, Guillaume Buchet, Pierre Deschamps, and Florence Sylvestre
CEREGE, UMR 6635 - CNRS, IRD, Collège de France, Univ. Aix-Marseille - Europôle Méditerranéen de l’Arbois, BP 80,
13545 Aix-en-Provence Cedex 4
One of the most prominent environmental changes over the past 10,000 years is the abrupt change observed
during the mid-Holocene, ca. 5500 years ago, in northern Africa with the transition from the “green Sahara” to the
present hyperarid desert. Sedimentary records show that the current area of the Sahel and part of the Sahara was
moister during the Late Pleistocene to middle Holocene (between ∼14 to 5.5 kyr cal BP), period known as the
African Humid Period (AHP) [1]. The end of the AHP and the transition to the present arid climate was triggered
by Earth orbital changes and the decrease of the African monsoon influence in response to a weaker insolation of
the Northern Hemisphere during summer. However, many questions related to this climate change are unresolved
and the timing, duration, mode and spatial pattern of the AHP termination are still matter of debate [2, 3].
Lake Chad represents a key site for investigating these questions, with a drainage basin that extends from
5◦ to 25◦N and 8◦ to 28◦E. Lake Chad is ideally located to document paleoclimatic and paleovegetational changes
that occurred at a regional scale. Furthermore, during the past humid periods, numerous sedimentological and
paleontological evidences testify the existence of giant water bodies. The last lacustrine episode occurred during
the AHP, where the paleolake, called the Mega-Lake Chad, had a surface area of more than 350,000 km2 [4].
Here we present the first palynological results obtained from a core collected in Lake Chad that encompasses the
mid-Holocene transition.
Pollen-based biome reconstructions were made using the biomization method, where the pollen data are
assigned to plant functional types (PFT) and biomes. This method was also applied to modern pollen rain
data [5] around Lake Chad in order to (i) establish a modern calibration, and (ii) to compare fossil pollen as-
semblages and the reconstructed potential biomes within the core with the modern pollen vegetation representation.
When compared with the local vegetation, the biomes from the modern dataset are correctly reconstructed
for most of modern samples, except for those collected in desert area. Pollen types from southern and moister
biomes are better represented in the sample with direct influence of fluvial flood supply (Chari river delta station).
For all fossil samples the dominant potential biome reconstructed is savanna. However, both pollen assemblages
and biome scores affinity indicate an important change evidencing a higher contribution of southern moister
biomes before 6 kyr cal BP. These results could indicate a modification in the fluvial transport likely associated
with a decrease in rainfall in the southern part of Lake Chad basin, reducing the transport of pollen types of distant
biomes after 6 ka. From 6 kyr cal BP onwards, fossil samples present very similar characteristics (pollen types
and percentages, and biome scores affinities) with the modern sample collected in the Chari river delta station
suggesting that comparable conditions to modern ones probably established in the Chad basin at this time.
These first results show the highly promising potential of lacustrine records retrieved from Lake Chad and
highlight the need to develop a higher resolution dataset spanning the entire Holocene. A more detailed and
thorough study involving mutliproxy quantification and ecosystem modelling will allow us to better understand
the mid-Holocene climatic transition and will provide an original contribution to the active debate about the AHP
termination.
[1] DeMenocal et al. 2000, Quaternary Science Reviews 19, 347.
[2] Kropelin et al. 2008, Science 320, 765.
[3] Brovkin and Claussen 2008, Science 322, 1326.
[4] Leblanc et al.2006,Palaeogeography Palaeoclimatology Palaeoecology 239, 16.
[5] Maley 1972, Pollen et Spores XIV, 263.


Vegetation reconstruction to the mid-Holocene transition in Sahel and Sahara : first palynological results of a lacustrine record from the Mega-Lake Chad

Vegetation reconstruction to the mid-Holocene transition in Sahel and Sahara : first palynological results of a lacustrine record from the Mega-Lake Chad

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