Palynological evidence for gradual vegetation and climate changes during the African Humid Period termination at 13°N from a Mega-Lake Chad sedimentary sequence 1CEREGE, Aix-Marseille Université, CNRS, IRD, Collège de France, Europôle Méditerranéen de l'Arbois, BP 80, 13545 Aix-en-Provence cedex 4, France
29 Jan 2013
2Département des Sciences de la Terre, Université de N'Djamena (UNDT) BP 1027 N'Djamena, Chad
Received: 16 May 2012 – Published in Clim. Past Discuss.: 18 Jun 2012 Abstract. Located at the transition between the Saharan and Sahelian zones, at the
center of one of the largest endorheic basins, Lake Chad is ideally
located to record regional environmental changes that occurred in the past.
However, until now, no Holocene archive was directly cored in this lake. In
this paper, we present pollen data from the first sedimentary sequence
collected in Lake Chad (13° N; 14° E; Sahel region).
Dated between ca. 6700 and ca. 5000 cal yr BP, this record is continuous and
encompasses part of the termination of the African Humid Period (AHP).
Vegetation reconstructions are based on standard analyses of pollen diagrams
and are strengthened by quantitative approaches. Potential biomes are
reconstructed using the biomization method and mean annual precipitation
(Pann) is estimated using the modern analogues technique.
Revised: 18 Dec 2012 – Accepted: 19 Dec 2012 – Published: 29 Jan 2013
Results show that, between ca. 6700 and ca. 6050 cal yr BP, a vegetation
close to humid woodland or humid savanna, including elements currently found
further southward, thrived in the vicinity of the Mega-Lake Chad in place of
the modern dry woodland, steppe and desert vegetation. At the same time,
montane forest populations extended further southward on the Adamawa
Plateau. The high abundance of lowland humid pollen taxa, particularly of
Uapaca, is interpreted as the result of a northward migration of the corresponding
plants during the AHP. This preferential zonal occurrence of these taxa in
Lake Chad Basin (LCB) (rather than extrazonal) is driven by more humid
local and regional climate conditions at this latitude, as shown by mean
Pann estimated values of ca. 800 (−400/+700) mm during this period.
However, we cannot rule out that an increase of the Chari–Logone inputs into
the Mega-Lake Chad might have also contributed to control the abundance of
these taxa. Changes in the structure and floristic composition of the
vegetation towards more open and drier formations occurred after ca.
6050 cal yr BP, following a decrease in mean Pann estimates to
approximately 600 (−230/+600) mm. But, the constant significant
presence of lowland humid taxa until ca. 5000 cal yr BP, contemporaneous
with a slight increase in steppic taxa, demonstrates that at this date, the
modern vegetation was not yet established in the vicinity of Lake Chad. Our
data indicate that between ca. 6700 and ca. 5000 cal yr BP vegetation and
climate changes must have occurred progressively, but that century-scale
climate variability was superimposed on this long-term mid-Holocene drying
trend as observed around ca. 6300 cal yr BP, where pollen data indicate more
Citation: Amaral, P. G. C., Vincens, A., Guiot, J., Buchet, G., Deschamps, P., Doumnang, J.-C., and Sylvestre, F.: Palynological evidence for gradual vegetation and climate changes during the African Humid Period termination at 13°N from a Mega-Lake Chad sedimentary sequence, Clim. Past, 9, 223-241, doi:10.5194/cp-9-223-2013, 2013.