Dispersal of Zaire river suspended matter in the estuary and the Angola Basin

Abstract

During two cruises, in November 1976 and May 1978, the suspended matter distribution in the Zaire estuary, on the shelf and in the adjacent ocean was studied. The concentrations decreased from ∼30 g·m −3 in the river to <0.05 g·m −3 in the ocean, with higher turbidity in the surface water and near to the bottom. A strong decrease in concentration was found in the estuary where the surface water looses about half its suspended matter between the head of the canyon and the river mouth in spite of current velocities of 1.5 to 2.5 m·s −1. In the surface water the turbidity is determined by particle supply through the river plume, mixing and particle settling, and, probably, particle aggregation in the surface water and disaggregation at greater depth down to the oxygen minimum. The turbidity in the bottom water is determined by particle settling, disaggregation at the head of the canyon, bottom currents keeping fine particles in suspension, stirring up of material slumped downward from the canyon walls, and probably, transport up-canyon of suspended particles. Downward transport through the canyon is by episodic turbidity currents, and probably also by more regular density flow. The distribution of turbidity in the oceanic bottom water decreases towards the centre of the Angola Basin, which may be related to the bottom current velocity distribution in the basin or to downslope density flow. The particle size distribution of the suspended material shows a gradual decrease in size from the river and the estuary towards the ocean, reflecting settling out of the large sized particles (predominantly > 16 to 18 μ). But also fine particles of 2 to 4 μ settle out, which is only conceivable by aggregation in the surface water and disaggregation near to the bottom, chiefly at the head of the canyon, where POC is being transformed into DOC. Salt flocculation, if occuring, affects only a small part of the total amount of suspended material present in the water. The suspended matter supplied by the Zaire river is partly deposited on the shelf north of the river mouth, and partly on the continental slope and on the ocean floor (where a large submarine fan has been formed) or is supplied to the ocean surface water through the river plume, which extends as far as 800 km from the river mouth. Estimates made of the supply of suspended matter to the deep ocean floor and deposition on the submarine fan differ by a factor of 2 to 3 but the uncertainties in the various estimates are large, not allowing more precise conclusions. The suspended matter supplied to the ocean surface water through the river plume, even when a low estimate is used, is sufficient to supply all the inorganic detrial particles in a surface layer of 100 m thickness over an area of 10 7 km 2. The oceanic suspensions show a log-normal size distribution with a median and mode and between 2 to 4 μ, irrespective of the composition and the origin of the particles. Turbulent mixing of the particulate suspended matter is therefore considered to be far more rapid than supply or settling of the particles.