Abstract

The concentration of suspended matter in surface waters off northeastern Brazil is generally very low (less than 0.5 mg/l) even near the coast. The main exception is off the Sao Francisco River where high concentrations (9-10 mg/l) occur within 10 km of the river mouth. Both organic and inorganic suspensates decrease away from the coast, indicating control by terrigenous supplies of mineral grains and nutrients. Only small quantities of terrigenous mineral grains reach the sea, and more than half of the nearshore suspensates, even off the Sao Francisco, are usually organic, as is nearly all of the suspensate further offshore. A substantial proportion of the organic fraction consists of diatoms, which are particularly concentrated off the Sao Francisco in response to the abundant nutrients supplied by that river. The water discharged by the Sao Francisco becomes brackish and is forced south along the coast by the Brazil Current, nearly as far as Salvador. This brackish water carries only a tiny part of the river's suspended load, most of which appears to be deposited, mainly by flocculation, in the lower reaches of the river or just offshore: calculations suggest that only about 1% of the suspended load escapes as far as 10 km from the river mouth. From the available data, and by analogy with the Atlantic shelf of the United States, it appears that there is little or no movement of sediment in surface or bottom waters from the coast out over the continental shelf, except on the Sao Francisco delta within about 5 km of the shore. This virtual absence of terrigenous sedimentation helps explain the widespread growth of various calcareous algae and the development of the almost pure carbonate sands which characterize the continental shelf of northeastern Brazil.

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