from hub partner Geoscience Australia 

New high resolution multibeam sonar mapping of the seabed along the Carnarvon continental shelf in Western Australia reveals in unprecedented detail a complex submarine terrain of coral reefs and sandy seabed. 

These new data collected by the Marine Biodiversity Hub show the area seaward of Ningaloo Reef World Heritage Area where the inner shelf is covered by hundreds of small but often interconnected reefs (bommies) up to 5 m high and ridges up to 20 m high.

Underwater photography and seabed sampling of this area shows that these patchy reefs and ridges are covered in dense coral and sponge communities, whereas the surrounding seafloor is mostly flat and sandy with few benthic organisms.

Some of these reefs look similar in size and shape to the dunes on the adjacent mainland, and corals appear to have grown on dunes that were cemented into hard structures and then drowned as sea level rose. Importantly, these drowned ancient dunes are now alive with diverse and colourful corals and other tropical marine animals. The outer edge of the inner shelf, in 60 m water depth, is marked by well-defined ridges that extend parallel to the shoreline for more than 15 km, likely formed by the drowning of a shoreline barrier. These ancient coastal sand deposits were likewise cemented to form limestone prior to sea level rising to its present position and drowning the old coastline.

These new data and images clearly demonstrate the value of multibeam sonar mapping to resolve and describe critical biological habitats and as a tool for mapping these habitats over large areas of the seafloor.

Map of the inner shelf at Point Cloates, showing mounds and ridges that cover a combined area of 80 km2 and provide habitat for dense coral and sponge communities (top right), in contrast to the bare sand flats of the surrounding seafloor (bottom right).