Late quaternary vegetation and climate dynamics in western amazonia

Abstract

The Amazon is a shelter for a huge variety of fauna and flora with the largest fluvial system and the highest diversity in the world. The biotic and abiotic factors of the floodplains serve to control the development and evolution of the ecosystems of the region. In the Amazon lowlands, little is known about past vegetation and ecosystem dynamics and to an even lesser extent human disturbances. Therefore, this project presents three palaeoecological multi-proxy studies from the western part of the Amazon region in Brazil. The aim is to reconstruct the vegetation and climate changes as well as possible human impact during the late Quaternary. A palaeoecological record from the floodplain areas of the Acre River, southwestern Brazil revealed the development and vegetation dynamics of a young oxbow lake (Lago Amapá) since the 19th century. The Lago Amapá record showed two main periods of changes. First the lake was influenced by fluvial input and afterwards the lake had a complete lacustrine phase. From ca. 1900 AD (Anno Domini) the main factors of transformation in vegetation were the fluvial dynamics, reflected by the input of sediments and pollen, transported by the river originating from Peruvian headwaters. From ca. 1950 AD the lake was rather isolated and only filled up when exceptional severe flood events occurred. Due to its close proximity to the City of Rio Branco at this time, it is assumed that the vegetation dynamics were altered by human activities. The increase of pioneer taxa, changes in sediment source and a comparison from Landsat images from the last 30 years, which showed a vast transformation in the vegetation cover close to the lake, displayed an extensive human impact. Towards the northwestern part of the Amazon region, a mid-Holocene record from a uniquely isolated plateau, the Serra do Tepequém, reflected a regional change from dry to wetter conditions, and an early expansion of the Mauritia flexuosa palm. A dry period was detected by the presence of grassland taxa, creeping pteridophytes and a high regional signal of fires between the period 7570 to 6190 cal BP (calibrated years Before Present). Subsequently, a wetter environment was detected by the changes in forest composition, reduction of grasslands and an increase on Mauritia flexuosa palm from 6190 to 4900 cal BP. In addition, the study showed that the early increase in M. flexuosa developed differently to the increase in fire and savanna expansion found in the savanna region in Venezuela The third study, a multi-proxy record from the remote region of the middle-upper Rio Negro showed the role of climate and river dynamics on the vegetation assemblages during the early and late Holocene. The Lake Acarabixi sediment core revealed that the forest was resilient to changes in the amount of precipitation and the filling dynamics of the valley of Rio Negro, which ended around 8000 cal BP in the Lake Acarabixi. In the early Holocene (10,840 to 8240 cal BP), herbs and open forest occurred, reflecting reduced precipitation with a seasonal climate. After 8240 to 1600 cal BP a large hiatus in sedimentation occurred. In the late Holocene (1600 to 650 cal BP) the lake reflected more lacustrine phases, and forest were closed with more flooding tolerant species.