6 The Abrolhos Reefs of Brazil
Z.M.A.N. LEAO and R.K.P. KIKUCHI
6.1 Introduction
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 km 2 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 Channel15 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 (Leao 1982; Leao 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.
Ecological Studies, Vol. 144
U. Seeliger and B. Kjerfve (eds.)
Coastal Marine Ecosystems of Latin America
© Springer-Verlag Berlin Heidelberg 2001
Z.M.A.N. LEAO and R.K.P. KIKUCHI
6.1 Introduction
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 km 2 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 Channel15 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 (Leao 1982; Leao 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.
Ecological Studies, Vol. 144
U. Seeliger and B. Kjerfve (eds.)
Coastal Marine Ecosystems of Latin America
© Springer-Verlag Berlin Heidelberg 2001