Abstract

Observations and samples taken during submersible operations on the deep fore-reef and island slope (-55 m to -305 m) off Discovery Bay, north Jamaica, revealed a surprising lack of reef-derived sediments on the deeper parts of the island slope as a result of the barrier effect of sill reefs at the top of the deep fore-reef. In those areas where sill reefs are absent such as the area immediately above the Discovery Bay submarine canyon, compositional and textural data, together with bulk carbon-14 age dates that range from 900 to 3,530 years BP, indicate occasional catastrophic movement of discrete sediment masses down the axis of the canyon, separated by long periods of suspended sediment addition and biologic mixing. During the present sedimentologic regimen, the coarse reef-derived debris found in the canyon does not continue down-slope into the adjacent deep basin (Cayman-Bartlett trench), but seems to be restricted to the island slope above 800 m. The green alga, Halimeda , is a major reef-sediment source, particularly in the deep fore-reef where it was found alive and growing in significant quantities to depths as great as 100 m. The various species of Halimeda can be useful sediment tracers, with H. goreauii and H. opuntia being characteristic of fore-reef slope environments and H. cryptica essentially restricted to the deep fore-reef. The present study confirms that boring sponges particularly Cliona sp. generate significant quantities of easily recognizable silt-sized detritus as a result of their boring activity. More than 5% of the total volume of the island slope sediment samples can be attributed to the boring activity of the clionid sponges with a mean of 24% of the silt fraction being composed of clionid debris. Where sponge chips contain microscopic algal borings from the original substrate, clionid debris may serve as a powerful sediment tracer from shallow to deep water. The submersible NEKTON Gamma made possible direct observation and sampling of a hitherto unknown, highly complex environment. These observations, and the data generated from these samples, have significantly reshaped our understanding of fore-reef sediment transport and deposition.

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