O aumento do nível do mar pós-glacial afetou a dinâmica das florestas de várzea da Amazônia durante os últimos 5000 anos?

Abstract

Four sediment cores, between three and ten meters depth, were sampled from fluvial plains and abandoned channels along the Rio Branco-Roraima. The stratigraphic analyses were based on grain size, structures and pollen content. In addition, the age of events were determined by seven radiocarbon dating. The data show cycles of channel abandonment, resulting in formation of lakes and, latter, its reactivation. The lake phases permitted the accumulation of mud sediments in different time intervals, suitable for pollen preservation. Pollen analyze indicated a combination of at least seventy taxa, the most representative are: Moraceae, Euphorbiaceae, Caesalpiniaceae, Fabaceae, Rubiaceae, Melastomataceae, Combretaceae, Sapindaceae, Poaceae, Cyperaceae, Aizoaceae and Apiaceae. They characterize arboreal and herbaceous vegetation, similar to modern vegetation of the study, since the middle Holocene. Considering the dry period occurred in the Amazon region during the early and middle Holocene, followed by a wet period in the late Holocene, the data presented in this work do not suggest significant changes in vegetation as a consequence of that climatic changes. Therefore, climatic and hydrologic conditions have favored the stability of vegetation in the study site for at least the last 5500 years. Three hypotheses are presented to justify this stability: 1) the early and middle Holocene dry period did not affect vegetation pattern; 2) a dry climatic phase was never present; or 3) the stabilization of the relative sea level about 6000 cal yr BP along the northern Brazilian littoral may have influenced the water table, and favored the establishment and maintenance of Amazonian lowland forest during the mid and late Holocene. In addition, this process may have attenuated the impact of that dry period in areas under most fluvial influence.