SEASONAL SEDIMENT STORAGE ON MUDFLATS ADJACENT TO THE AMAZON RIVER

Pb-210 and Th-234 activity profiles in sediment cores from underconsolidated mudflats 300 km downdrift of the Amazon river mouth record an ephemeral surface layer of fine-grained sediment up to 1.5 m thick. This layer contains about 1.5 x 10(8) tons of Amazon sediment deposited rapidly (similar to 1 cm/d) from a fluid-mud suspension (10-400 g/l) during the months between January and June. Virtually the entire layer is remobilized in July-December and the sediment is advected alongshore to the northwest. Seasonal variations in trade-wind strength and in supply of Amazon shelf sediment are thought to control emplacement, and removal of this ephemeral deposit. Solitary surface gravity waves characteristic of this setting generate a net landward sediment flux, which, with shore-normal tidal currents, controls spatial geometry of the surface layer. The resultant lens-shaped deposit dissipates incident wave energy and provides a substrate above mean high water for mangrove colonization and irregular shoreline progradation of meters per year. Macroscale (sand/silt laminations) and microscale (plasmic fabric) sedimentary structures in the ephemeral layer record diverse temporal variations (e.g., tidal and wave-induced) in bottom shear stress and sediment supply. Ephemeral deposition of 10(8) tons is inferred to be common in coastal areas associated with large and energetic river dispersal systems.