THE LACUSTRINE ORGANIC SEDIMENTATION IN TROPICAL HUMID ENVIRONMENT (CARAJAS, EASTERN AMAZONIA, BRAZIL) - RELATIONSHIP WITH CLIMATIC CHANGESDURING THE LAST 60,000 YEARS BP

The petrographic and geochemical studies of the organic material from a sediment core recovered from the central parr of a marshland basin s ituated on a narrow plateau surrounded by the Amazonian rainforest has allowed characterization and evaluation of the relative percentages o f the different organic fractions. The petrographical approach allowed confirmation and a better understanding of the geochemical results. I n order to prove the potential of organic palaeoenvironmement markers, the results of this study were compared with those of the palynology obtained from the same samples. This comparison is consistent and prov ides a new framework of interpretation of palaeoclimatic changes. Inde ed, during full development of the rain forest surrounding the plateau , the resistent organic material which one observes in the organic lev els of lacustrine sediments, is principally composed of algal material and translucid ligno-cellulosic debris which have a weak vitrinite re flectance and a grain size ranging from 60 mu m-100 mu m. Elsewhere th ese levels are characterized by high fluxes of total organic carbon (T OC) and nitrogen (N). On the contrary, during regeneration of the fore st (humid conditions) following the opening of the rain forest (dry co nditions), the organic material, situated in siderite levels, is compo sed of amorphous organic matter (autochthonous organic fraction) and o paque ligno-cellulosic debris (allochthonous fraction) with small grai n size. The distribution of vitrinite reflectance in these levels is m ultimodal with average and high values. The increase of fluxes of TOC and N and the low C/N ratios shows improvement in the quality and quan tity of organic matter. In the organic levels, emplaced during the Hol ocene forest regression, the maximum fluxes of TOC coincide with the p resence of carbonized wood debris, proving the existence of frequent f orest fires.