VEGETATION AND MORPHOLOGY CHANGES IN THE MOUTH OF THE AMAZON AND DOCE RIVER DURING THE LATE QUATERNARY

This work compares the vegetation and morphological changes occurred along the littoral of the Marajó Island, Amazonian littoral, and the coastal plain of the Doce river, southeastern Brazil, during the late Quaternary, focused specifically on the response of the mangroves to the sea-level fluctuations and the climate change, which have been identified in several studies along the Brazilian coast. This integrated approach combined radiocarbon dating, description of sedimentary features, pollen data, and organic geochemical indicators (δ¹³C, δ¹⁵N and C/N). On the coastal plain of the Doce River between ~47,500 and ~29,400 cal yr BP, a deltaic system was developed in response mainly to sea-level fall. The post-glacial sea-level rise caused a marine incursion with invasion of embayed coast and broad valleys, and it favored the evolution of a lagoonal/estuary system with wide tidal mud flats occupied by mangroves between at least ~7400 and ~5100 cal yr BP. Considering the Marajó Island during the early and middle Holocene (~7500 and ~3200 cal yr BP) mangrove area increased over tidal mud flats with accumulation of estuarine/marine organic matter. It was a consequence of the marine incursion caused by post-glacial sea-level rise, further driven by tectonic subsidence. Dry conditions in the Amazon region during this time led to a rise is tidal water salinity and contributed to mangrove expansion. Therefore the effect of relative sea-level (RSL) rise was determinant to the mangrove establishment in the southeastern and northern region. During the late Holocene (~3050 – 1880 cal yr BP) the mangroves in both regions were retracted to a small area, with some areas replaced by freshwater vegetation. That was caused by the increase in river discharge associated to a wet period recorded in the Amazon region, and considering the coastal plain of the Doce River (southeastern Brazil), the mangroves shrank in response to an increase in fluvial sediment input associated to a sea-level fall.