Authors: Beaman, R.J., Webster, J.M.
Publication: H. Neil and D. Tracey (Editors), 4th International Symposium on Deep-Sea Corals, 1-5 December 2008. National Institute of Water and Atmospheric Research, Wellington, New Zealand.
Coldwater coral habitats are usually associated with large geological features, such as deepwater seamounts and guyots, or found as smaller mounds on relatively steep continental slope and basin areas. Although seamounts are typically of volcanic origin, which explains their complex topography and hard substrates, the genesis and initial control of the smaller mound settings on soft sediment is currently a point of debate. For example various seabed features, such as hydrocarbon seeps, slumps and mud volcanoes, may provide the initial topographic relief, then is followed by phases of mound development due to favourable oceanographic conditions and the successive build-up of coldwater coral communities. Here we present new high-resolution multibeam bathymetry datasets from the Great Barrier Reef (GBR) margin. Combined with sub-bottom profiles and rock dredge samples, these data provides a fresh insight into the origin and spatial distribution of a new coldwater coral habitat discovered in the adjacent Queensland Trough. These findings suggest that large blocks may have broken off the GBR margin as catastrophic landslides, moving down the lower slope and coming to rest in the basin where they now form a preferential habitat for a deep, coldwater coral community.
The multibeam and sub-bottom data reveal a spectacular network of submarine canyons, slump scars and landslide deposits on the continental slope and upper basin. The canyons often terminate in the Queensland Trough as slide scarps and debris fields where progressive upslope erosion has reduced the stability of the parent margin sediments. Lying downslope of the canyons and slump scars, the multibeam maps show a cluster of eight knolls up to two km long and over 100 m high in depths of about 1100 m. Moats or scours are prominent around the larger knolls and are indicative of impinging bottom currents. Sub-bottom profiles across the knolls show they are discrete, seismically-opaque blocks capped by a ~15 m of soft sediment. The blocks protrude through multiple, parallel sub-surface reflectors from the adjacent basin sediments. A rock dredge taken across the top of the largest knoll recovered evidence of a coldwater coral community, including live gorgonian and dead scleractinian corals, barnacle plates, gastropods, serpulid worms, and manganese-covered concretions within a matrix of carbonate mud. We named these fascinating features the Gloria Knolls, and they represent the first documented case of a coldwater coral habitat adjacent to the GBR World Heritage Area (GBRWHA). Significantly, the multibeam maps show the potential for other geomorphic features with similar origins to be used as proxies for deep seabed biodiversity distribution along the GBR margin.