Late Pleistocene glacial forest of Humaitá—Western Amazonia

Abstract

Glacial-aged vegetation dynamics of the Humaitá—Western Brazilian Amazonia were studied by pollen, sedimentary facies, 14C dating, δ13Corg and C/Nmolar. Two sediment cores were taken to a depth of 10 and 8m from areas covered by grassland and dense/open forest, respectively. The deposits represent a succession of sediment accumulation in active channel (>42,600cal yr B.P.), abandoned channel/floodplain (>42,600 to ~39,000cal yr B.P.), and oxbow lake sedimentary environments (~39,000cal yr B.P. to modern). The predominance of mud sediments, depletion of δ13Corg and decrease in C/Nmolar values identify the lake establishment. In these settings, low energy subaqueous conditions were developed, locally favoring preservation of a pollen assemblage representing herbaceous vegetation, some modern taxa from Amazonia and cold-adapted plants from the Andes represented by Alnus (2–11%), Hedyosmum (2–17%), Weinmannia (0–18%), Podocarpus (0–4%), Ilex (0–4%) and Drymis (0–1%), at least between >42,600 and <35,200cal yr B.P. The herbs and modern taxa from Amazonia persisted through the Holocene, while the cold pollen assemblage became absent. The co-occurrence of Alnus with other cold adapted plants from the Andes during the late Pleistocene indicates that Alnus probably penetrated the Western Amazonia lowland or was growing closer to the study site due to cooler temperatures during glacial times. The present study presents the first report of a glacial age forest containing Alnus in areas of the Brazilian Amazonian lowlands. In addition to its palaeogeographical importance, this work demonstrates the effectiveness of using a combination of proxies for reconstructing sedimentary environments associated with vegetation.