Lowermost Amazon River: evidence of late Quaternary sea-level fluctuations in a complex hydrodynamic system

Abstract

Abstract Sedimentological, geochemical, and high-resolution seismic studies on the lowermost Amazon River (Xingu Mouth–Macapa–Estreitos de Breves) allow reconstruction of the sediment dynamics of this large river system in response to late Quaternary sea-level changes. Areas of sediment transport and deposition on an older river bottom are delineated by seismic reflectors and are related to inundation and incision caused by late Quaternary rising and falling of sea level. Older substrates have been identified as the Alter do Chao Formation (Cretaceous) and the Barreiras Formation (Tertiary). The initial AMS-radiocarbon dates yielded late Pleistocene to Holocene ages, and indicate a high sedimentation rate (0.6 cm/yr) for the last 800 yr. Three stages in the late Quaternary evolution of the Amazon mouth system can be distinguished: (i) relative sea-level fall, probably related to the Last Glacial Maximum with channel incision; (ii) deposition and consolidation of mud found near Afua during relative sea-level rise between late Pleistocene and middle Holocene, corresponding to the “freshwater lake” or the evolution of large floodplain areas in Amazonia; and (iii) stable sea-level conditions since 6000 yr BP resulting in the modern pattern of deposition and erosion, controlled by complex interactions of fluvial and oceanic processes.