The Deep Osprey Reef mapping project is a collaboration between James Cook University (JCU) and the scuba diving vessel MV Undersea Explorer, to map the deep seabed of the remote Osprey Reef in the Coral Sea. Because the reef is located about 130 km from the northern Great Barrier Reef shelf edge, it is frequently visited by adventure diving vessels to experience spectacular diving with manta rays, sharks, and the nautilus shells found here.
Osprey Reef lies on the north-western Queensland Plateau which is a huge submerged platform that was once part of mainland Australia, then separated and subsided during the break-up of Gondwana millions of years ago (Map below). Dotted over the Queensland Plateau are coral atolls which have kept pace with platform subsidence and have continued growing up to present day sea-levels. Osprey Reef is one such atoll and the steep walls of this oceanic reef drop more than one km into the Queensland Trough which divides the Australian mainland from the plateau.
Figure 1. 3D view of shallow Osprey Reef
In 2006, we obtained Royal Australian Navy (RAN) lidar, or laser, depth data from Osprey Reef which had mapped the shallow parts of the reef to 40 m depth (Figure 1 & Movie below). The main feature of the reef is a lagoon 25 km long by 8 km wide with a depth of about 40 m, rimmed by an outer reef flat 500-700 m wide. A deep outlet channel called The Entrance drains the lagoon water to the outside sea, then continues along the West Wall to terminate at distinct point called North Horn.
A study of the Navy lidar data raised questions about what are the underwater landscape, or seascape, details of the slope below 40 m depth? Were there any large canyons cut into the upper levels of the reef? Do the walls show any signs of lower sea-level erosion, similar to those found on the adjacent Great Barrier Reef margin where terraces record sea-levels about 110 m below present sea-levels? Could the 3D seascape reveal details about the capture depths and habitat preferences for the live nautilus shells that are part of a tag and release study by marine researchers?
The objectives of the Deep Osprey Reef mapping project were:
To map the deeper seabed of Osprey Reef;
To examine the 3D seascape for clues relating to lower sea-levels or for evidence of canyons; and
To describe the shape, gradient and seabed profiles of the live nautilus habitat depths.
Figure 2. 3D map of deep NW Osprey Reef
To map the reef, we developed a computer program called bedLogger, which automatically records the singlebeam echosounder depth data as the MV Undersea Explorer as she traveled between dive sites. Global Positioning System (GPS) location and time were also recorded to the computer every two seconds. The Raymarine DSM300 echosounder collected depth data to about 800 m. After each trip, the computer data was downloaded and then merged with other depth data to build up a 3D map of the slope.
After several years of singlebeam echosounder data collection, the 3D map of north-western Osprey Reef emerged. Fledermaus 3D visualization software (IVS3D) helped reveal the seascape of Osprey Reef down to about 800 m depth (Figure 2). We used rainbow colours ranging from pink to represent the shallow top of the reef, through to violet which represents the deepest mapped seabed at 800 m.
Figure 3. Sea-level for the past 500,000 years
Below 40 m, the outer slope drops nearly vertically to about 100 m depth where a narrow terrace interrupts the steep gradient of the wall. The terrace possibly indicates an erosion surface caused when sea-levels were lower during previous glacial periods. About 20,000 years ago during the last ice age, global sea-levels had dropped about 110 m below present day sea-levels (Figure 3). This ice-age sea-level closely coincides with the depth of the narrow terrace found along the wall.
The 3D map shows the reef wall continues its near-vertical drop below the 100 m terrace until a dramatic change in the slope gradient at 250-300 m depth. Here, the seabed angles away from the vertical wall and a number of canyons appear, with their heads starting around 250-300 m and continuing down to the limit of the map at about 800 m depth.
The canyon heads are divided by a pinnacle in the centre of the mapped area, which lies at about 250 m depth. The axes of the canyons then drain away from this central pinnacle. The canyons are about 100-200 m wide and incise about 60 m into the slope. The canyons likely funnel eroded sediments draining from the shallow reef down into the deep abyss surrounding Osprey Reef.
The Deep Osprey Reef mapping project will continue to build up the 3D map of Osprey Reef so as to provide more detail about the underwater landscape of this fascinating atoll.
Osprey Reef, Australia [Fledermaus sd]