The Abrolhos Reefs of Brazil

Abstract

The Abrolhos reefs represent the most extensive and richest area of coral reefs in Brazil and in the Southwestern Atlantic. Among the first scientific reports on Abrolhos coral reefs are those from the Darwin visit to the Abrolhos Archipelago; however, it was the Thayer Expedition led by Louis Agassiz that produced the seminal work with a first detailed description of coral zonation on the reefs (Hartt 1870). The Abrolhos reef ecosystem, which occupies approximately 6,000 km² of the northern Abrolhos Bank, is comprised of two reef arcs (Coastal and Outer Arcs) almost parallel to the coast, as well as of volcanic islands, sand shoals and channels. The Abrolhos Bank is an enlargement (up to 200 km off Caravelas) of the southern part of the generally narrow (average width 50 km) Eastern Brazilian continental shelf (Fig. 6.1A). The shelf around the Abrolhos reefs is shallow (<30m) with a slope of 0°08′ and the shelf edge at about 70 m. The topography of the inner shelf is smooth and flat while narrow channels and sandbanks are common on the middle and outer shelves. A shallow (Caravelas Channel 15 m) and a deeper (Abrolhos Channel 20–30 m) channel separate the coast from the coastal reef arc and the outer reef arc, respectively (Fig. 6.1A). Topographic features of the shelf evolved during the last Pleistocene regression when the shelf was subaerially exposed and the surface deeply eroded by the fluvial drainage system discharging into the Abrolhos Depression. Depositions of terrigenous sediments were later replaced by marine biogenic carbonate depositions (Vicalvi et al. 1978) which mainly occur on the middle and outer shelves but extend onto the inner shelf around the reefs, while terrigenous sediments are confined to the inner shelf (Leão 1982; Leão et al. 1988). Volcanic accretions to the shelf formed five islands (Abrolhos Archipelago), which consist of sedimentary Cretaceous rocks, and basalt sills and dykes, originated from Tertiary intrusive and volcanic activity. Until today the Abrolhos reef ecosystem has an unchallenged scientific importance because it differs in many aspects from other well-established coral reef models.