Numerical study of circulation on the inner Amazon Shelf

Author Posting. © Springer, 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Ocean Dynamics 58 (2008): 187-198, doi:10.1007/s10236-008-0139-4.We studied the circulation on the coastal domain of the Amazon Shelf by applying the hydrodynamic module of the Estuarine and Coastal Ocean Model and Sediment Transport - ECOMSED. The first barotropic experiment aimed to explain the major bathymetric effects on tides and those generated by anisotropy in sediment distribution. We analyzed the continental shelf response of barotropic tides under realistic bottom stress parametrization (Cd), considering sediment granulometry obtained from a faciologic map, where river mud deposits and reworked sediments areas are well distinguished, among others classes of sediments. Very low Cd values were set in the fluid mud regions off the Amapa coast (1.0 10-4 ), in contrast to values around 3:5 10-3 for coarser sediment regions off the Para coast. Three-dimensional experiments represented the Amazon River discharge and trade winds, combined to barotropic tide influences and induced vertical mixing. The quasi-resonant response of the Amazon Shelf to the M2 tide act on the local hydrodynamics by increasing tidal admittance, along with tidal forcing at the shelf break and extensive fluid mud regions. Harmonic analysis of modeled currents agreed well with analysis of the AMASSEDS observational data set. Tidal-induced vertical shear provided strong homogenization of threshold waters, which are subject to a kind of hydraulic control due to the topographic steepness. Ahead of the hydraulic jump, the low-salinity plume is disconnected from the bottom and acquires negative vorticity, turning southeastward. Tides act as a generator mechanism and topography, via hydraulic control, as a maintainer mechanism for the low-salinity frontal zone positioning. Tidally induced southeastward plume fate is overwhelmed by northwestward trade winds so that, along with background circulation, probably play the most important role on the plume fate and variability over the Amazon Shelf

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

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 'varzea' 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.CNPq [562398/2008-2, 43518/2008-9, 302943/2008-0