AIMS-GA survey 2011

In 2008/09, AIMS and Geoscience Australia combined their complementary strengths under an MOU of co-operation to survey deep water habitats within the Ningaloo Marine Park in Western Australia. In 2010-11, the two agencies completed the second of two voyages of discovery to characterise deep water habitats in the Joseph Bonaparte Gulf west of Darwin. The surveys included intensive studies of four areas along a cross-shelf transect on the eastern side of the Sahul Shelf, which is a large drowned segment of the continental shelf that was once dry land dissected on its seaward margins by multiple river valleys that are today submarine canyons. This area is known as the Van Diemen Rise. Behind it are deeper areas that are today submerged sedimentary basins, some full of mud, but that would once have been lakes or flood plains when sea levels were lower in the past.

The survey obtained detailed geological (sedimentological, geochemical, geophysical) and biological data from the submerged banks, channels and plains to establish the late-Quaternary evolution of the region and to investigate modern relationships between the physical environment and ecology for biodiversity prediction over a broader section of the Shelf. At the four locations, the bathymetry was mapped accurately with multibeam sonar data on both visits to follow the stability of sand banks and channels not formed from hard materials. In the basins, sub-bottom acoustic profilers were used to estimate the depth of sediment deposited since the last sea level rise and to reveal the underlying hard structures.

More than 1,154 km2 of multibeam sonar data and 340 km of sub-bottom profiles were collected. The data showed that the seabed on the Van Dieman Rise is complex with numerous ridges, banks, terraces, channels, and deep holes. This includes a channel on the middle shelf, which is the second deepest known channel on the northern Australian shelf. The banks and ridges are characterised by diverse sponge and soft coral gardens in contrast to the Halimeda(a type of calcareous algae) and hard coral constructions found elsewhere on the northern Australian shelf. At the innermost coastal location, significant pockmarks in the soft bottom had acoustic signatures consistent with fluid and gas seepage, as detected elsewhere in the Arafura Sea.

Information and data collected on the survey will be used to support the work programs of the Department of Resources, Energy and Tourism and the Department of Sustainability, Environment, Water, Population and Communities. The new surveys, when combined with existing data, will provide a regional picture of seabed environments in the eastern Joseph Bonaparte Gulf that can be used to support resource development by the oil and gas industry in this highly prospective area as well as characterise proposed marine protected areas.