Past environmental and climatic changes during the last 7200 cal yrs BP in Adamawa Plateau (Northern-Cameroun) based on fossil diatoms and sedimentary 13C isotopic records from Lake Mbalang
1University of Dschang, Department of Plant Biology, P.O. Box, 67 Dschang, Cameroon
2Institut des Sciences de l'Evolution (UMR CNRS 5554), Université Montpellier II – pl. E. Bataillon, case courrier 061, 34095 Montpellier cedex 05, France
3IRD Bondy, 32, Avenue Henri Varagnat, 93143 Bondy, France
Abstract. Past limnological conditions of Lake Mbalang (7°19´ N, 13°44´ E, alt: 1130 m) and vegetation type were reconstructed from diatoms and sedimentary stable carbon isotope records (δ13C) since 7200 cal yrs BP. The data showed that before 3600 yrs cal BP the water column was preferentially cold and stable except around 5000–5300 cal yrs BP where diatom evidenced mixed upper water layer, δ13C data suggest more forested vegetation in the landscape. These stable conditions can be explained by a strong monsoonal flux and correlatively northern position of the ITCZ that entailed high/low rainfall well distributed over the year to allow the development mountainous forest taxa. The decreasing trend of the monsoonal flux towards mid-Holocene was however affected by several centennial to millennial time scale abrupt weakening at 6700, 5800–6000, 5000–5300, 4500 and 3600 cal yrs BP although their impact on vegetation is not visible probably because rainfall distribution was favourable to forest maintenance or extension. After 3600 cal yrs BP, water column became very mixed as a result of more intense NE trade winds (Harmattan) that led at ~3000 cal yrs BP to the instalment of savana in the vegetation landscape. At that time, rainfall was probably reduced following the southwards shift of the ITCZ and the distribution of yearly rainfall was no more favourable to forest development. Thus a strong seasonality with a well marked dry season was established, conditions that maintained the savana vegetation till today. Diatom data suggest the lake did not dried during the last 7200 cal yrs BP, however, a low lake level observed at 2400–2100 cal yrs BP is contemporaneous to a climatic event evidenced in several areas of tropical Africa and could correspond to the southernmost position of the ITCZ. Other low lake levels are observed at 1800 and 1400 cal yrs BP, after which lake rose to its present level.