Sandstream on the northeast Brazilian shelf

Abstract

A preliminary characterization of seabed morphology and mapping of algal patches within a sandstream on the northeast Brazilian continental shelf off Cape Calcanhar (where the South American coast turns sharply to the west) is presented. The study area (30 km × 30 km) is swept by unidirectional, wind-driven, tidally rectified currents, under the influence of the North Brazil Current. The study was made by use of Landsat Thematic Mapper (TM) data of the seafloor down to 40 m depth, digital bathymetric data from smooth sheets furnished by the Brazilian Navy, echo-sounder profiling and underwater surveying by SCUBA diving. A mesoscale shore-parallel natural zonation of the sandstream was observable on the TM-images: (a) an inner zone dominated by sediment resuspension caused by wave-driven turbulence and tidal currents; (b) an intermediate zone dominated by sand ribbons and other longitudinal bedforms; (c) an outer zone dominated by large-scale sand waves (underwater sand dunes) limited offshore by shore-parallel sandbanks. Image-assisted in situ work, carried out on the latter zone, gave the following results: (1) Morphology and sand composition—the asymmetric sand waves are made of medium quartz sand, with almost straight crests of lengths up to 4 km, heights between 3 and 7 m, and avalanche lee slopes of 30°; on the stoss sides we recorded the presence of ripples and absence of megaripples and benthic ecosystems; on interdune areas, benthic ecosystem patches dominated by foliaceous and calcareous algae were found. (2) Sediment transport—active bedload transport was observable only during the winter season, when the strongest wind-forced currents induce flow separation at the crests and avalanche streams on the lee slope. Within the interdune areas, under the influence of the lee vortices, benthic community data confirms no appreciable abrasion due to sand transport. (3) Biology—species composition and biomass estimates from the algal patches in the first upstream trough are presented, and species composition of bryozoa observed as epibionts on live gastropod shells are analysed. This sandstream shows some similarities to one described from the southeast African continental shelf, which is also under the influence of unidirectional currents.