BATHYAL SEDIMENTS AND ASSOCIATED UNIQUE DEEP-WATER ECOSYSTEM FROM THE NORTHWESTERN SLOPE OF THE GREAT BAHAMA BANK–POTENTIAL MODERN ANALOG FOR DEEP SEA SKELETAL CARBONATES

Dredging surveys at 400 to 700 m along the northwestern edge of the Great Bahama Bank, from Victory Cay northward to South Bimini, revealed an existence of an unusual benthic assemblage residing on a large coarse-grained underwater talus slope. This offshore zone is influenced by meanders of the Florida Current (Gulf Stream). The Gulf Stream also comes close to shore along this stretch of the coast, and brings perpetual tropical waters to the shelf area. Because of this, large coral patch reefs grow along the edge of the shelf, and their dead fragments tumble down to form a series of coral talus slopes.

Along the northern part of the Bimini Chain, off the South Bimini Island, the rubble talus slope is wider and the Bimini Wall is sloping gradually. This wide gently dipping slope is made of coarse skeletal gravels. Farther south, along the Cat Cays, Victory Cay, and Castle Rock, the Bimini Wall and the resulting talus slope are much steeper and the slope is made of finer sediment, predominantly shell hash. Skeletal gravel being the most coarse-grained sediment composed predominantly of coral and molluscan fragments originated in shallow water (80%) with minor amounts of benthic and planktonic foraminifera, and pteropods (5%). The accumulated pavement of dead coral and mollusk hard parts provides hard substrate for a diverse assemblage of mollusks and attachment of a number of cnidarians and poriferans. The lithoherms of deep sea hydrocoral Stylaster laevigata and the branching scleractinian Oculina sp. occur on the terraces of the talus slope contributing up to 15% of the skeletal material mainly in the coarse fraction (above 5 mm).

These unusually coarse sediments exist within a narrowest portion of the Florida Straits and result from a funneling effect of the Gulf Stream Current. The presence of the diverse and unique benthic community with distinct tropical shallow water affinities is attributed to a current and temperature asymmetry between the western and eastern slopes of the Straits of Florida. This current asymmetry forces warm, surface-derived, water down to bathyal depths on the eastern slopes of the Straits of Florida. Our data suggests that these unique sediments and associated benthic community does not exist within similar depth range, to the South of Victory Cay and North of South Bimini Island.