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.
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