Abstract
The main goal of this study was to reconstruct the Late Pleistocene-Holocene floristic composition in an area of the northern Brazilian Amazonia, comparing the results with other Amazonian localities in order to discuss the factors that have influenced phytophysiognomic changes over this time period. The work in eastern Marajó Island at the mouth of the Amazonas River was approached based on analysis of 98 pollen and diatom samples from core data distributed along a proximal to distal transect of a paleoestuarine system. The results indicated high concentration of Rhizophora, associated with arboreal pollen grains typical of the modern Amazonian rainforest during the last 40,000 cal yrs BP. Pollen composition also included wetland herbs. Diatoms were dominated by marine and fresh water taxa. Wetland forest, mangrove and, subordinately herbs remained constant during most of the latest Pleistocene-early/middle Holocene. At 5,000 cal yrs BP, there was a distinguished change from forest and mangrove to wet grassland savanna due to sea level fluctuation. As marine influence decreased, the estuary gave rise to fresh water lacustrine and swamp environments, with establishment of herbaceous campos. A main conclusion from this study is that solely the occurrence of herbaceous savanna can not be used as a definitive indicator of past dry climates in Amazonian areas.
Highlights
Interpreting late Quaternary climate fluctuations in Amazonia based on vegetation changes is not yet an issue of overall consensus
These records were not sensitive to detect any noticeable change in vegetation during the Late Pleistocene and early/middle Holocene, when the landscape was dominated by mosaics of wetland forest and mangrove
A drastic change occurred in the middle to late Holocene, with a remarkable increase in herb frequency, mostly Poaceae
Summary
Interpreting late Quaternary climate fluctuations in Amazonia based on vegetation changes is not yet an issue of overall consensus. Sedimentological, stratigraphic, isotopic (13C/12C and 15N/14N) and 14C data from eastern Marajó Island recorded Late Pleistocene and Holocene (i.e., 42,580 14C yr BP to 3,340 14C cal yr BP) fine- to coarse-grained, parallel-laminated or cross stratified sands, massive or laminated muds and heterolithic deposits (Castro et al 2010). According to these authors, these proxies indicate that the organic matter in the sediments derived mostly from marine and fresh water phytoplankton sources. The latter fall led to the progressive establishment of continental conditions as the coast prograded around 45 km northward, culminating with replacement of the estuarine system into the modern Lake Arari
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