The late quaternary vegetation and climate history of the southern Amazonian rainforest-savanna boundary, eastern Bolivia

Pollen and charcoal analyses have been undertaken on a 43 000 year old sediment core from Laguna Chaplin, in Noel Kempff Mercado National Park, at the southern Amazon margin. The aims of this research were to 1. determine when the present day rainforest communities became established in the park, 2. determine the composition of these former savannas and/or forests, 3. determine the broad-scale significance of rainforest dynamics in NKMNP in the context of previously published palaeoclimatic studies in other parts of Amazonia and the Bolivian Andes and 4. reconstruct the late Quaternary movements of the ITCZ. Vegetation reconstructions have revealed that savanna and dry forest communities surrounded the lake from ca.43 000 yr BP until ca.2000 yr BP. A high abundance of charcoal particles throughout this time period supports the pollen interpretation. Although grasses (Ppaceae) dominate the pollen assemblages throughout this time period, the contribution of other pollen types is variable. Pleistocene communities are dominated by the presence of Machaerium/Byrsonima, Paullinia/Roupala and Erythroxylum whereas during the Holocene Curatella americana, Acacia and members of the Anacardiaceae family are important. From ca.2000 yr BP tropical rainforest communities (dominated by Moraceae pollen) expanded in the vicinity of the lake at the expense of the savannas and dry forests. This rainforest expansion coincides with decreasing charcoal abundance, as one would expect in localities of moist forest vegetation. Comparison of this pollen sequence to that compiled from another lake in the park has established that the vegetation signal from Laguna Chaplin is indeed a regional vegetation signal. The expansion of rainforest communities into the park after ca.3000 yr BP has been correlated to evidence for expanding rainforest and/or increasing lake levels from other parts of southern Amazonia and the Bolivian Altiplano. This apparent increase in regional moisture availability has been attributed to enhanced southerly migration of the ITCZ.