FAULT-ASSOCIATED SEABED MOUNDS (CARBONATE KNOLLS) OFF WESTERN IRELAND AND NORTH-WEST AUSTRALIA

Groups of seabed mounds, suspected to be modern carbonate knolls or bioherms, have been located on reflection seismic data in the Porcupine Basin off western Ireland and in the Vulcan Sub-basin off north-west Australia. The size of these mounds is impressive, often being over 1 km across and over 100 m high. In the Porcupine Basin they occur in water depths of 650-1000 m, whereas in the Vulcan Sub-basin they occur in water depths of 200-350 m. Shallow seabed coring in the Porcupine Basin has provided data on the age, lithological composition and hydrocarbon concentrations in the upper 3 m layer of sediment both on and off the mounds. In both basins the mounds occur close to faults which may provide conduits for upward migrating hydrocarbons. The seepage of hydrocarbons is suspected to have caused local eutrophication or 'fertilization' by providing nutrients to bacteria, which in turn are part of the food chain for higher organisms. The dominant organism in the Porcupine Basin examples appears to be the deep-water coral Lophelia sp., whereas in the Vulcan Sub-basin the mounds appear to be Halimeda-dominated.