The Influence of Upwelling on Suspended Matter and Shelf Sediments off Southeastern Brazil

Abstract

ABSTRACT Surface waters off southeastern Brazil contain very little terrigenous material in suspension, even off the mouths of major rivers. As a result of the low rate of supply of terrigenous sediment, calcarenites and live algal reefs are widespread on the middle and outer continental shelf. Upwelling near the coast and over offshore banks, is associated with elevated amounts of suspensates, which are mainly planktonic and non-skeletal, and reach concentrations of 0.5 to 1.0 mg/l. Beneath the biologically productive upwelled surface waters there is remarkably little sedimentation of this organic matter. Also, although phosphorite deposits are usually associated with upwelling centers, there are no phosphatic sediments off southeastern Brazil. The absence of both organogenic sediment and pho phorite differentiates this coast from the areas of upwelling in the eastern Atlantic, and probably results from the high degree of oxygenation of upwelled water, which contains up to 6 ml/l of dissolved oxygen (more than three times as much as found in upwelling centers off northwest Africa, for example). Phosphorite and organically enriched sediment would be absent from paleo-upwelling centers of similar type, but these centers might be recognized through local enhancements of organic matter in the silt and clay fraction of marine sediments.