Past environmental and climatic changes during the last 7200 cal yr BP in Adamawa plateau (Northern-Cameroun) based on fossil diatoms and sedimentary carbon isotopic records from Lake Mbalang

摘要

Past limnological conditions of Lake Mbalang (7°19'N, 13°44'E, altitude: 1130 m) and vegetation type were reconstructed from diatoms and sedimentary stable carbon isotope records (δ~(13)C) since 7200calyr BP. The data showed that before 3600 cal yr BP, the water column was dominantly stable except around 5000-5300 cal yr BP where diatoms evidenced a mixed upper water layer and δ~(13)C data suggest more forested vegetation in the landscape. These stable conditions can be explained by a strong monsoon flux and relatively northern position of the ITCZ that entailed high or low rainfall well distributed over the year, allowing the development of mountainous forest taxa. The decreasing trend of the monsoon flux towards the mid-Holocene was affected by several abrupt centennial to millennial-scale weakening at 6700, 5800-6000, 5000-5300, 4500 and 3600 cal yr BP. However, their impact on the vegetation is not visible, probably because rainfall distribution was favourable to forest maintenance or extension. After 3600 cal yr BP, the water column became very mixed as a result of more intense NE trade winds (Harmattan) that led at -3000 cal yr BP to the establishment of savannah in the vegetation landscape. At that time, rainfall was probably reduced following the southward shift of the ITCZ, and the distribution of yearly rainfall was not favourable anymore to forest development. A strong seasonality with a marked dry season was established, conditions that maintained the savannah vegetation until today. Diatom data suggest the lake did not dry up during the last 7200 cal yr BP; however, a low lake level observed at 2400-2100 cal yr 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 1400calyr BP, after which the lake rose to its present level.