SEASONAL SEDIMENT STORAGE ON MUDFLATS ADJACENT TO THE AMAZON RIVER

Pb-210 and Th-234 activity profiles in sediment cores from underconsol idated mudflats 300 km downdrift of the Amazon river mouth record an e phemeral surface layer of fine-grained sediment up to 1.5 m thick. Thi s layer contains about 1.5 x 10(8) tons of Amazon sediment deposited r apidly (similar to 1 cm/d) from a fluid-mud suspension (10-400 g/l) du ring the months between January and June. Virtually the entire layer i s remobilized in July-December and the sediment is advected alongshore to the northwest. Seasonal variations in trade-wind strength and in s upply of Amazon shelf sediment are thought to control emplacement, and removal of this ephemeral deposit. Solitary surface gravity waves cha racteristic of this setting generate a net landward sediment flux, whi ch, 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 mangrov e colonization and irregular shoreline progradation of meters per year . Macroscale (sand/silt laminations) and microscale (plasmic fabric) s edimentary structures in the ephemeral layer record diverse temporal v ariations (e.g., tidal and wave-induced) in bottom shear stress and se diment supply. Ephemeral deposition of 10(8) tons is inferred to be co mmon in coastal areas associated with large and energetic river disper sal systems.