THE LACUSTRINE ORGANIC SEDIMENTATION IN TROPICAL HUMID ENVIRONMENT (CARAJAS, EASTERN AMAZONIA, BRAZIL) - RELATIONSHIP WITH CLIMATIC CHANGES DURING 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 situated on a narrow plateau surrounded by the Amazonian rainforest has allowed characterization and evaluation of the relative percentages of the different organic fractions. The petrographical approach allowed confirmation and a better understanding of the geochemical results. In 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 provides a new framework of interpretation of palaeoclimatic changes. Indeed, during full development of the rain forest surrounding the plateau, the resistent organic material which one observes in the organic levels of lacustrine sediments, is principally composed of algal material and translucid ligno-cellulosic debris which have a weak vitrinite reflectance and a grain size ranging from 60 mu m-100 mu m. Elsewhere these levels are characterized by high fluxes of total organic carbon (TOC) and nitrogen (N). On the contrary, during regeneration of the forest (humid conditions) following the opening of the rain forest (dry conditions), the organic material, situated in siderite levels, is composed of amorphous organic matter (autochthonous organic fraction) and opaque ligno-cellulosic debris (allochthonous fraction) with small grain size. The distribution of vitrinite reflectance in these levels is multimodal 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 quantity of organic matter. In the organic levels, emplaced during the Holocene forest regression, the maximum fluxes of TOC coincide with the presence of carbonized wood debris, proving the existence of frequent forest fires.