Buracas: Novel and unusual sinkhole-like features in the Abrolhos Bank

Abstract

Novel and unusual sinkhole-like features are described for the Abrolhos Bank continental shelf, eastern Brazil, based on geophysical and geological assessments. These unusual structures are large cup-shaped depressions similar to sink/blueholes (10–75m in diameter, 8–39m in height), occurring in a consolidated carbonate substrate in the mid and outer shelf. A total of 36 such features, locally named buracas, were found between 41 and 161km off the coast and between 24 and 65m depth (the bottom of these features may be as deep as 93m). The buracas’ walls are mainly composed of encrusting coralline algae. Radiocarbon dating has provided ages of 5400±90yr Cal BP, 8630±90yr Cal BP and 39,200±400yr BP. Besides providing a comprehensive description of these novel structures within the mesophotic zone, the potential mechanisms by which these sinkhole-like structures originated are discussed here, as well as the possible active mechanisms impeding their filling with sediments and biogenic material. We hypothesize that their origin could be related to either typical sinkhole formation during subaerial exposure or to a partially Holocene growth pattern influenced by antecedent morphology and gas/fluid escaping due to organic matter decomposition. The buracas are relevant not only because they comprise outstanding novel features, but also because they are relevant targets for marine conservation, as they enhance productivity and aggregate biomass in a region under growing fishing pressure.