The integration of palynological, chemical, isotopic and stratigraphic data with radiocarbon
dating, obtained from two cores, allowed the identification of two periods during the last
millennium in the coastal plain of the Soure in Marajó Island. The first period, from 1275-
1165 cal yr BP to 760-480 cal years BP, indicates the expansion of herbaceous vegetation,
characterized by the presence of the Cyperaceae, Poaceae and Fabaceae. During this period it
was recorded the largest continental influence through the relationship δ
13C (-30 and -25 ‰)
and C/N ratio (20-40). During the last 500 years the mangrove vegetation has expanded, being
represented by the Rhizophora and Avicennia pollens. The C/N ratio decreased to values <20,
indicating an increase in the contribution of aquatic organic matter (phytoplankton). This
aquatic influence was observed on the lithology and sediment structure, indicated by the
predominance of silt-clay sediments with parallel-laminated and some millimetrics lens of
clay, which may have their origin associated to tidal flat. This trend during the last centuries
on the Marajó Island may be associated to the eustatic sea-level rise, changes in the Amazon
rivers discharge due to climatic changes or tectonic effects.
|
