LATE QUATERNARY PALEOENVIRONMENTS IN THE LAKE BAROMBI MBO (WEST CAMEROON) DEDUCED FROM POLLEN AND CARBON ISOTOPES OF ORGANIC-MATTER

A sediment core, 23.5 m long, was recovered from a water depth of ca. 110 m in Lake Barombi Mbo, a maar crater of the Cameroon volcanic chain. This paper presents a carbon isotopic curve of organic matter linked to the main results of the pollen analysis. Interpretation of this carbon isotopic curve is simplified (1) because of the weak role played by the phytoplankton in the balance of the organic particle flux, and (2) because of the presumed absence of diagenetic alteration of the isotopic composition. The carbon isotopic curve exhibits an almost linear correlation with that of total grass pollen which form the main part of the C4 biomass. The major fluctuations of isotopic and pollen curves allow four main phases to be distinguished: from ca. 25,000 to 20,000 yr B.P., deltaC-13 values of -25 to -30 parts per thousand (PDB) are related to a forest environment associated with a montane floral element; from 20,000 to 13,000 yr B.P., the deltaC-13 values are between -23 and -28 parts per thousand and are linked to openings of landscape with a mosaic of forest and savanna and a fall of lake level with colonization of the shores by Cyperaceae and other aquatic plants; from 13,000 to 10,000 yr B.P., the forest extended again reaching a maximum density during the period from 9500 until 3000 yr B.P. and with an average deltaC-13 value of -32 parts per thousand; a new dry phase occurred at around 2500-2000 yr B.P., with temporary openings in the forest, marked by a deltaC-13 excursion above -30 parts per thousand, but during the last 2 millennia, the forest developed again with deltaC-13 values of -32 parts per thousand.