Registros palinológicos em sedimentos deltaicos do Lago Cabaliana na planície aluvial do Rio Solimões

Abstract

A sediment core (PD-67) was collected in the alluvial plain of the Solimões river located at the varzea in Central Amazonia. A profile of 160 cm was raised from Lago Cabaliana delta and samples were prepared using standard pollen analytical techniques and acetolysis. Three sediment samples were dated by AMS-radiocarbon dates. The dates from the lower part of the sediment samples of Lago Cabaliana indicate an age of 2570 ± 40 14C yr BP, followed by an age of 1570 ± 40 14C yr BP and 830 ± 40 14C yr BP in the uppermost part At about 2600 14C yr BP Alchornea, Cecropia and Symmeria were predominant likewise the herbaceous elements: grasses, Asteraceae and Alternanthera. The presence of Arrabidaea, Rubiaceae, Tabebuia, Sapotaceae, Rutaceae, Protium and Symphonia implies a well-drained upland forest and adjacent wetlands, indicated by the presence of Sagittaria pollen and fern spores. At this period, the sedimentation was characterized by compact clay, and it indicates that the flood was probably much lower during this time interval. Cecropia has remained relatively constant since 2700 14C yr BP and later, around 2200 14C yr BP, showed an increase. Grasses and other herbaceous pollen show a slight fall, occurred around 2200 14C yr BP, and together with Byrsonima and other plants from late successional stages in the floodplain - Sapium and Myrtaceae – we can infer a decrease in height water column. At 700 14C yr BP, the climax vegetation of varzea becomes predominant, despite the frequency of late secondary succession elements like Pseudobombax and Laetia. Pollen of Cassia, Vismia, Neoxythece and upland táxons: Doliocarpus, Simaba and Scleronema suggest the proximity or the mixture of elements from forest upland and varzea. There were two drier periods, represented by the effective decrease in precipitation around 2700 14C years BP and 1200 to 700 14C years BP. The other phases (2400 14C yr BP and from 1500 to 1200 14C yr BP) are indicative of less pronounced dry periods, i.e., small variations in the levels of flood in the basin. The succession dynamics occurring in the lake Cabaliana indicate that the varzea was recently established and is composed of a mosaic of different successional stages of vegetation, mainly controlled by the flood pulse. Alternating wet and dry phases caused by ENSO in the late Holocene were detected, increasing seasonality of precipitation. The drainage pattern of the Solimões River was affected, which caused fluctuations in the flood pulse and therefore variations in floristic composition of vegetation surrounding Lake Cabaliana and changes in depositional processes. Thus, it is possible to propose that the recent climate history of Central Amazon is a reflection of changes in precipitation patterns in the basin.