Dinâmica da vegetação da região de Humaitá - AM durante o Pleistoceno tardio e o Holoceno

Abstract

Vegetation dynamics of Western Amazonia during the past 42000 cal yr BP was studied by pollen, sedimentary facies, 14C dating δ13C and C/N. Two sediment cores were sampled from Humaitá in southern Amazonas state, and Porto Velho, northern Rondônia. These sites, located in western Brazilian Amazonia, are covered by grassland and tropical rainforest vegetation, respectively. The sedimentary deposits are predominantly composed of massive sand, heterolithic mud/sand, and laminated or massive mud representative of sediment accumulation in floodplain, active channel, abandoned channel, and oxbow lake sedimentary environments. In these settings, reducing and low energy subaqueous conditions were developed, locally favoring preservation of a pollen community of herbaceous vegetation and glacial forest represented mainly by Alnus, Drymis, Hedyosmum, Podocarpus, and Weinmannia trees at least between > 42,033 – 43,168 cal yr BP and <34,804 – 35,584 cal yr BP. The herbaceous and arboreal vegetation ecotone persisted from the early to mid-Holocene, whilst the cold pollen assemblage became extinct. In this work, we record a significant plant population, at present restricted to Andean areas located at altitudes higher than 2000 - 3000 m, in areas of the Amazonia lowland toward the onset of the Last Glacial Maximum. This suggests that this time interval might have seen temperatures lower than previously proposed for this region.