Palynological evidence for gradual vegetation and climate changes during the African Humid Period termination at 13°N from a Mega-Lake Chad sedimentary sequence

Abstract

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. 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 humid conditions.

Out of the African Humid Period

“…Tierney and deMenocal have studied a sediment core from the Gulf of Aden that provides detailed insights into the beginning and end of the African Humid Period. The work complements previous studies of sediment cores taken off the coasts of Mauritania ( 2, 3) and Somalia ( 11) and from Lake Yoa ( 5,12) and Lake Chad ( 14). The African Humid Period and its termination affected human civilizations, as illustrated by a rock painting from the Tassili n'Ajjer, Algeria, and the pyramid of pharaoh Djoser in Saqqara, Egypt.…”

“…Of particular importance is the Holocene history of Lake Chad, which was at least 10 times as large during the African Humid Period than it is today ( 13). Pollen data from Lake Chad ( 14) indicate that vegetation changes occurred progressively over about two millennia, but that century-scale variability was superimposed on the mid-Holocene dry-ing trend. Additional proxies remain to be measured and longer continuous cores to be collected and studied from Lake Chad to advance understanding of the African Humid Period.…”

Humans as Agents in the Termination of the African Humid Period

“…In a study of vegetation response to AHP termination from Lake Mbalang in northern Cameroon, the impacts of steadily decreasing precipitation on the floristic composition of the site were gradual, which is attributed to the pre-event "stability of vegetation" that preceded climate change (Vincens et al, 2010). Similar landscape dynamics have been documented from pollen records recovered from Lake Tilla (Salzmann et al, 2002), Lake Yoa (Lézine, 2009), Lake Mega-Chad (Amaral et al, 2013), portions of the eastern Sahara (Neumann, 1989), and from an offshore sediment core near Senegal (Niedermeyer et al, 2010;Schefuß et al, 2015) where the vegetation was slow to respond to changing rainfall patterns until clear human impacts are documented in the proxy record.…”

“…Further south, a sediment core from Lake Chad provides evidence for a gradual increase in non-arboreal pollen (NAP) and shrub vegetation at the expense of AP and non-wetland grasses between 6700 and 5000 years BP (Amaral et al, 2013) with an abrupt switch to arid conditions after 5000 years BP ( Figure 3F; Armitage et al, 2015). This time period has been posited, though not yet archeologically proven, to be when domesticated plants and animals arrived in the region (MacEachern, 2012).…”

Using a novel climate–water conflict vulnerability index to capture double exposures in Lake Chad

“…They are considered representative of farming, fishing and pastoral villages made up of livelihood groups that are generally and historically exposed to disruptive climate extremes and conflicts in the region (Table 1). Average annual rainfall is approximately 200-500 mm with maximum rainfall observed during July-September, while average temperature is approximately 27°C, ranging from 21 to 36°C throughout the year (Amaral et al 2013). Since the 1970 s, intense droughts have impacted water supplies and, in turn, have intensified aggression and conflicts around the Lake for which several hundreds of lives have been lost (Onuoha 2009).…”