Holocene landscape history and palaeohydrology evidenced by stable carbon isotope ( δ13C) analysis of alluvial sediments in the Mbari valley (5°N/23°E), Central African Republic

Abstract

Pleistocene to Holocene as well as recent trends in climate have an influence on the composition of savanna–forest vegetation fringes in Africa, dominated mainly by savanna (C 4) and mainly forest (C 3) groups of plants. The modified vegetation cover plays an important role on the runoff processes and on the discharge of the draining river systems. Because the majority of forest–savanna borders in Central Africa is situated on geologically old planation surfaces, the main sources of palaeoenvironmental information are alluvial sediments of rivers. Therefore, this study focuses on the examination of alluvial soils and the determination of stable carbon isotopes ( δ 13C) of organic sediments on the Mbomou plateau and in the Mbari valley in the southeast of the Central African Republic (CAR). It has been shown that there is some evidence for an ongoing increase in C 3-dominated forest plants, reducing the recent extension of savannas in the study area. The most important reasons for this trend are sufficient amount of annual rainfall (>1500 mm), decrease in bushfire frequency, and negative migration processes of the rural population due to the economic crisis in Central Africa. δ 13C values in fossil soil horizons show that a greater extent of forest on the Mbomou plateau occurred around 7–7.5 ka and between 2.5 and 3 ka. Drier, savanna-dominated vegetation patterns were found at 5 ka and from 1 ka to the present. The more humid and arid climate periods during the Holocene partly correspond with high and low lake levels of Lake Chad. The findings also seem to be confirmed by other studies neighbouring Central African regions as Cameroon, Gabon and Congo–Brazzaville (Batéké Plateau), which indicate a more general validity of the findings from the Mbomou plateau, especially for the period since 3 ka.