FLUID-MUD PROCESSES ON THE AMAZON CONTINENTAL-SHELF

A sediment transport study conducted on the Amazon continental shelf a s part of AmasSeds (A Multi-disciplinary Amazon Shelf SEDiment Study) revealed extensive regions of dense nearbed suspensions of sediment, k nown as fluid mud (suspended-sediment concentration >10 g l(-1)). Flui d mud was found near the river mouth on the inner- and middle-shelf, i n the region of the bottom salinity front, and was most extensive duri ng periods of rising and high river discharge. Fluid mud, up to 7.25 m thick, but generally 1-2 m thick, appears to form by processes simila r to those occurring at an estuarine turbidity maximum, i.e. enhanced settling and lateral convergence of near-bottom flows. A modeling stud y showed that vertical mixing was controlled by the suppression of tur bulence, due to the stratification induced by suspended sediment, and established an upper bound for the total amount of suspended sediment that may be carried in suspension. Sediment leaving the Amazon River a ppears to go through cycles of trapping and resuspension at the river mouth, before being partially advected seaward and alongshelf, where i t is largely incorporated into fluid mud along the bottom salinity fro nt. The fluid muds have far reaching effects on the Amazon shelf syste m by reducing boundary shear stresses, affecting water-column seabed e xchange, and serving as the agent of outward growth of the subaqueous delta through episodic offshore transport.