The geomorphic evolution of slopes and sediment chutes on forereefs

Frequent slope failures of the forereef dropoff occur in living coral reefs on the island of Bonaire in the southern Caribbean. Topographic profiles o f ten sites were taken perpendicular to shore, followed by an estimation of coral ages (M. annularis) along the topographic profiles. The coral ages w ere estimated from published rates of growth and sizes of the sampled coral s. Comparisons of coral age and adjacent slope angle indicated that the ste eper slopes are older and that mass failure of the slope reduces the steepn ess of the forereef front and destroys the coral community along the distur bed slope. Slopes grow until reaching a critical steepness, after which und erwater sliding of the over-steepened forereef front results. In observed s lides, the mechanical weakness in the reef structure that facilitates a sli de failure was the result of a weak mud and coral rubble layer deposited 2 m below the living coral cover. The remaining slide scarp evolves into a se diment chute that conveys the resulting flow of biogenic sands from the sha llow forereef terrace down the forereef edge. Because of the constant flow of sand down these channels, no corals can recolonize the sediment chutes a nd the chutes become stable topographic features. In addition, these indent ations in the forereef front also serve as channels for the vertical densit y mixing of water of varying salinities and temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.