Osprey Reef lies on the northern Queensland Plateau, which is a huge submerged carbonate platform that was once part of mainland Australia, then separated and subsided during the break-up of Gondwana millions of years ago. Scattered over the Queensland Plateau are emergent coral reefs that have kept pace with platform subsidence and continued growing level to present day sea-levels. Osprey Reef is one such reef and the steep walls drop more than 1.5 km into the surrounding Queensland Plateau.
In 2006, we obtained RAN lidar bathymetry (depth) data from Osprey Reef which mapped the shallower parts of the reef to about 40 m depth. 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.
The Deep Osprey Reef mapping project was developed to explore the underwater landscape of the slope below about 40 m as collaboration between James Cook University, adventure diving operators and other institution researchers. The objectives of the project were to map the deeper seabed looking for clues relating to lower sea-levels or evidence of canyons, and to describe the seabed habitats and marine life of the mesophotic (twilight) and aphotic (dark) zones.
To map the deeper parts of the reef, we automatically recorded the singlebeam echosounder depth data as the MV Undersea Explorer traveled between dive sites. GPS location and time were also recorded to the computer every two seconds. A Raymarine DSM300 echosounder collected depth data to about 932 m. After each trip, the computer data were downloaded and then merged with other depth data to build up a 3D map.
Below 30 m, the slope drops nearly vertically to about 130 m depth. This near-vertical cliff is pockmarked with caves and overhangs betwen 110 and 130 m, possibly indicating 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 120 m. This ice-age sea-level closely coincides with the depth of the caves and overhangs.
Deeper than 130 m, the wall is still very steep with occasional pinnacles and a narrow ledge around 250 to 300 m. Between about 250 and 300 m down to 450 m is a rough zone of steep walls interspersed with gullies. Then around 450 m, the seabed gradient angles away from the rough zone above and a number of canyons appear within this apron zone, continuing down to the limit of the map at 932 m. The canyons likely funnel eroded sediments draining from the shallow reef into the deep platform and basin surrounding Osprey Reef.
Since the project started we have been fortunate to collaborate with Japanese and German scientists using remotely operated vehicles to provide more detail about the deep marine life of Osprey Reef (see Additional media below).
- Related blog post - Deep Down Under expedition to the Coral Sea
- Related blog post - PICASSO-1 studies Osprey Reef
- Conference paper - ROV transect reveals the mesopelagic Osprey Reef
- Peer-reviewed literature - In situ observation of Denise’s pygmy seahorse Hippocampus denise associated with a gorgonian coral Annella reticulata
- Peer-reviewed literature - The untethered remotely operated vehicle PICASSO-1 and its deployment from chartered dive vessels for deep sea surveys
- Peer-reviewed literature - Spatial patterns in the distribution of benthic assemblages across a large depth gradient in the Coral Sea, Australia
- Related blog post - Living fossils and rare corals revealed in the deep Coral Sea