Gloria Knolls: A new coldwater coral habitat on theGreat Barrier Reef marginRobin J. Beaman1 and Jody M. Webster2

IntroductionColdwater coral habitats are usually associated with large geo-logical features, such as deepwater seamounts and guyots, or found as smaller mounds on relatively steep continental slope and basin areas1. The genesis and initial control of the smaller mound settings on soft sediment is currently a point of debate2,3.

For example various seabed features, such as hydrocarbon seeps, slumps and mud volcanoes, may provide the initial topo-graphic relief, then is followed by phases of mound development due to favourable oceanographic conditions and the successive build-up of coldwater coral communities2.

Voyage 07-2007We present new high-resolution multibeam bathymetry datasets from the Great Barrier Reef (GBR) margin4. Combined with seismic profiles and rock dredge samples, these data provides a fresh in-sight into the origin and spatial distribution of a new coldwater coral habitat discovered in the 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 pref-erential habitat for a deep, coldwater coral community. We have named these fascinating features the Gloria Knolls and they repre-sent the first documented case of such a habitat adjacent to the GBR World Heritage Area (GBRWHA).

ObjectivesTo find geophysical evidence that links the Gloria Knolls to a submarine landslide origin.To understand the role of the landscape in defining the cold-water coral habitat boundaries using multibeam and seismic profile analyses. To study the origin and biodiversity of the coldwater coral com-munity using 14C AMS dates, taxonomic and sediment analy-ses (to be completed in 2009).

MethodsThe project utilised ArcGIS and Fledermaus visualisation soft-ware to analyse the geophysical spatial datasets. Destructive techniques and geochemical analyses were conducted on sediment. Taxonomic specialists provided information on the biogenic samples obtained in the dredge.

1. Setting The survey of the GBR margin about 80 km east of Cairns revealed 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.

2. Multibeam, oceanography 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 1100-1300 m. Moats or scours are prominent on the north-side of the larger knolls and are indicative of impinging bottom currents. A CTD profile in the vicinity confirmed the near-seabed temp. is ~4°C and a northerly-flowing Subantarctic Mode Water in the Queensland Trough.

3. Seismics Profiles reveal large, steep-sided blocks embedded within a matrix of low-relief Trough sediments ~30 m thick with 15-20 parallel sub-bottom reflectors. Moats lie north of the larger blocks up to 40 m deeper than the adjacent Trough seafloor. The largest blocks are opaque with caps of 6-14 m thick semi-opaque parallel reflectors. Smaller buried opaque blocks are covered by Trough sediments that preserves a higher seabed relief above the blocks.

1School of Earth and Environmental Sciences, James Cook University, Cairns QLD 4870, Australia. Email: robin.beaman@jcu.edu.au2School of Geosciences, University of Sydney, Sydney, NSW, 2006, Australia

4. Surface difference A huge landslide scarp lies 20 km upslope from the Gloria Knolls with dimensions about 20 km long, 5 km wide and 300-1000 m in depth. Small canyons have downcut into the scarp face. Fledermaus software was used to calculate the erosion volume of the scarp (32.6 km3) relative to a pre-slide surface. The total Gloria Knolls volume is only 0.4 km3 above the surrounding flat seabed. Most of the landslide debris now appears to be buried.

5. Landscapes The Gloria Knolls multibeam data were analysed in ArcGIS for slope and Benthic Terrain Modeler5 basic zones to highlight the physical landscape boundaries. In addition to the eight larger knolls, the results show numerous smaller-sized (less than 1 km long, 10-20 m high) features with east-side depressions, suggesting initial debris transport from the GBR margin.

6. Dredge sample A rock dredge taken from the crest of the largest knoll in 1170 m re-covered evidence of a coldwater coral community, including live gorgonians, dead scleractinian and bamboo corals, barnacle plates, gastropods and serpulid worms. The sediment matrix comprised a carbonate mud showing progressive calcification into hard nodules, which also provide hard attach-ment surfaces for the sessile fauna. Taxonomic analysis is preliminary but highlights similarities with Australian tropical province scleractinia and New Zealand Subantarctic cirripedia.

7. Conclusions The Gloria Knolls are the remnants of a large debris flow transported into the Queensland Trough due to a catastrophic landslide on the GBR margin. Hemi-pelagic sediments have subsequently buried most of the debris blocks except for the largest knolls. Multiple sub-bottom reflectors from the surrounding Trough sediments and the small canyons downcut into the scarp, points to the landslide being an ancient event.The knolls deflect northerly-flowing currents and provide a favourable habitat for a coldwater coral community on the crests. Numerous smaller knolls may provide additional positive relief habitats for corals. The limited taxonomic analysis shows similarities with other western Pacific coldwater fauna.Significantly, submarine landslides are common on the margin and there is potential that similar debris flow features can be used as proxies for coldwater coral distribution in the GBRWHA.

AcknowledgementsWe thank the Captain and crew of the RV Southern Surveyor for their assistance during the survey. We also acknowledge the support of the staff at Australia’s Marine National Facility (http://www.marine.csiro.au/nationalfacility/). This project was funded by the National Geographic Society and the European Consortium for Ocean Research Drilling. The authors would like to thank the Australian Institute of Nuclear Science andEngineering for providing financial assistance (Award No. AINGRA09006) to enable 14C AMS work on the dredge samples to be conducted.

References1. Freiwald A., Fossa J. H., Grehan A., Koslow T., and Roberts J. M. (2004). Cold-water coral reefs: Out of sight – no longer out of mind. UNEP-WCMC Report, Cambridge, U.K.2. Davies A. J., Wisshak M., Orr J. C., and Roberts J. M. (2008). Predicting suitable habitat for the cold-water coral Lophelia pertusa (Scleractinia). Deep-Sea Research I 55, 1048-1062. doi:10.1016/j.dsr.2008.04.0103. De Mol B., Van Rensbergen P., Pillen S., Van Herreweghe K., Van Rooij D., McDonnell A., Huvenne V., Ivanov M., Swennen R., and Henriet J. P. (2002). Large deep-water coral banks in the Porcupine Basin, southwest of Ireland. Marine Geology 188, 193-231.4. Webster J. M., Beaman R. J., Bridge T., Davies P. J., Byrne M., Williams S., Manning P., Pizarro O., Thornborough K., Woolsey K., Thomas A., and Tudhope S. (2008). From corals to canyons: The Great Barrier Reef margin. EOS 89(24), 217-218.5. Wright, D.J., Lundblad, E.R., Larkin, E.M., Rinehart, R.W., Murphy, J., Cary-Kothera, L., Draganov, K., 2005. ArcGIS Benthic Terrain Modeler. Oregon State University, Davey Jones Locker Seafloor Mapping/Marine GIS Laboratory and NOAA Coastal Services Center, Corvalis, U.S.A.

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