Abstract
The Abrolhos reefs, off the coast of Brazil, are the southernmost coral reefs in the Atlantic and are significantly different from the well known reefs in the Caribbean. These differences are in morphology, surrounding sediments, reef-building organisms and Holocene history. The reefs form two arcs which occupy a total area of approximately 6,000Km('2). The basic element of most of the reefs is the "chapeiroes", mushroom-shaped pinnacles, 5 to 25m high and 5 to 50m in diameter. The flared top of the "chapeirao" that is a growth form, resembles the top of algal reefs of the North Atlantic. In the Coastal arc, the top of the adjacent "chapeiroes" coalesce to form bank or platform reefs, 1 to 20km long, with varied shapes. These bank reefs do not display the well marked zones of Caribbean reefs, but there are well developed algal rims on the windward edges of the reefs, like those in Pacific reefs. The outer arc has fringing reefs surrounding volcanic islands and "chapeiroes" that do not coalesce. Corals, millepores and coralline algae are the major framebuilders of the reefs. The number of coral species is less than half that of the Caribbean; they are dominantly archaic, endemic species that are the combined result of isolation of a late Tertiaty community and the stress of periodically high turbidity. Millepores and coralline algae contribute more to these reefs than in the Caribbean, and there is a substantial amount of cemented internal sediment. In contrast with the predominance of carbonate sediments surrounding most reefs in the Caribbean and in the Pacific, the coastal reefs of Abrolhos are surrounded by muddy sediments which contain 40 to 70% quartz sand and clay minerals. Despite the abundance of this terrigenous sediment, the Holocene sequence of one coastal reef has an average growth rate of 3.7m/1,000 years, as revealed by a core boring, which is comparable to the rate of accumulation of reefs, in clear water.