Petrographic (palynofacies) and geochemical (Rock Eva1 pyrolysis) studies of sedimentary organic matter of a core from a crater lake in Madagascar show a peaty sedimentation before 36 ky and between ca 28 and 15 ky, and between 6 and O ky. Between 36 and 28 ky, the organic sedimentation is characterized by an alternance between phytoplanktonic sedimentation and migrations of vegetation from the basin slope to the lake itself. Contributions of oxidized allochthonous organic matter prove also the presence of vegetation on the surrounding basin and soil leaching. The variation of the ecosystems and their development reveal the hydroclimatic changes during the last 36 kyrs, in terms of arid and humid periods.
