Late Holocene reef growth and relative sea-level changes in Atol das Rocas, equatorial South Atlantic

Abstract

Shallow drilling provided the first detailed record of vertical reef accretion rates for the last 4,000 years from the oceanic atoll Atol das Rocas. Six cores up to1-m long from windward, leeward, and intertidal hardground environments were radiocarbon dated. Frameworks are dominated by the coralline alga Porolithon cf. pachydermum with minor contributions of Lithophyllum sp. Coralline bindstone and framestone facies were identified. Vertical accretion rates (VAR) form three groups: group A frameworks were formed between 3,490±45 years BP and 2,770±45 years BP, and VAR are 0.85, 1.4, and 1.6 mm/year; group B frameworks were formed between 2,510±45 year BP and 490±45 year BP, and VAR are 0.25, 0.46, and 0.42 mm/year; group C frameworks were formed between 900±50 year BP and 655±45 year BP, and VAR are 3.2, 9.75, and 18.4 mm/year. Results indicate that coralline-algal reefs may display a catch-down response to a falling sea level similar to the way corals respond to a rising sea level. In this case, present day reef topography may be the result of late Holocene SW Atlantic sea-level changes. The calculated VAR of 18.4 mm/year is the highest rate known to date for a coralline-algal reef and close to the maximum rates recorded for corals.