Studies on Holocene mangrove ecosystem dynamics of the Braganca Peninsula in north-eastern Para, Brazil

Three sediment cores from the Braganca Peninsula located in the coastal region in the north-eastern portion of Part State have been studied by pollen analysis to reconstruct Holocene environmental changes and dynamics of the mangrove ecosystem. The cores were taken from an Avicennia forest (Bosque de Avicennia (BDA)), a salt marsh area (Campo Salgado (CS)) and a Rhizophora dominated area. (Furo do Chato). Pollen traps were installed in five different areas of the peninsula to study modem pollen deposition. Nine accelerator mass spectrometry radiocarbon dates provide time control and show that sediment deposits accumulated relatively undisturbed. Mangrove vegetation started to develop at different times at the three sites: at 5120 C-14 yr BP at the CS site, at 2170 C-14 yr BP at the BDA site and at 1440 C-14 yr BP at the FDC site. Since mid Holocene times, the mangroves covered even the most. elevated area on the peninsula, which is today a salt marsh, suggesting somewhat higher relative sea-levels. The pollen concentration in relatively undisturbed deposits seems to be an indicator for the frequency of inundation. The tidal inundation frequency decreased, probably related to lower sea-levels, during the late Holocene around 1770 C-14 yr BP at BDA, around 910 C-14 yr BP at FDC and around 750 C-14 yr BP at CS. The change from a mangrove ecosystem to a salt marsh on the higher elevation, around 420 C-14 yr BP is probably natural and not due to an anthropogenic impact. Modern pollen rain from different mangrove types show different ratios between Rhizophora and Avicennia pollen, which can be used to reconstruct past composition of the mangrove. In spite of bioturbation and especially tidal inundation, which change the local pollen deposition within the mangrove zone, past mangrove dynamics can be reconstructed. The pollen record for BDA indicates a mixed Rhizophora/Avicennia mangrove vegetation between 2170 and 1770 C-14 yr BP. Later Rhizophora trees became more frequent and since ca. 200 C-14 yr BP Avicennia dominated in the forest. (C) 2001 Elsevier Science B.V. All rights reserved.