Fluid-mud processes on the Amazon continental shelf

A sediment transport study conducted on the Amazon continental shelf as part of AmasSeds (A Multi-disciplinary Amazon Shelf SEDiment Study) revealed extensive regions of dense nearbed suspensions of sediment, known as fluid mud (suspended-sediment concentration >10 g l(-1)). Fluid mud was found near the river mouth on the inner- and middle-shelf, in the region of the bottom salinity front, and was most extensive during 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 similar to those occurring at an estuarine turbidity maximum, i.e. enhanced settling and lateral convergence of near-bottom flows. A modeling study showed that vertical mixing was controlled by the suppression of turbulence, 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 appears to go through cycles of trapping and resuspension at the river mouth, before being partially advected seaward and alongshelf, where it is largely incorporated into fluid mud along the bottom salinity front. The fluid muds have far reaching effects on the Amazon shelf system by reducing boundary shear stresses, affecting water-column seabed exchange, and serving as the agent of outward growth of the subaqueous delta through episodic offshore transport.