Morphological and vegetation changes on tidal flats of the Amazon Coast during the last 5000 cal. yr BP

Abstract

The analysis of morphology, sedimentary facies, palynology, carbon and nitrogen isotopes, C/N ratio and radiocarbon data from tidal flats sediments influenced by the Amazon River indicates that vegetation development during the last 5000 cal. yr BP was controlled by morphological changes associated with marine and fluvial process. The proximal portion of the tidal flat corresponds to abandoned channels surrounded by terrestrial vegetation since 5280–5160 cal. yr BP. During the last 2840–2750 cal. yr BP, autochthonous organic material became prevalent as well as the increase in contribution of terrestrial organic matter mainly from ‘várzea’ vegetation. Herbaceous plants already colonized the tidal flat before 3170–2970 cal. yr BP, and after 2630–2310 cal. yr BP and 1350–1290 cal. yr BP they were replaced by mangrove vegetation following the development of secondary channels connected to the sea with brackish water influence. Furthermore, the erosion of the herbaceous field to form chenier ridges/tidal mixed flat after 3170–2970 cal. yr BP and before 220–140 cal. yr BP in the distal portion of the tidal flat suggests an overall interpretation of a retrogradation cycle. This process was driven by a relative sea-level rise influenced by the increase of Amazon discharge during the late Holocene.