Drowned shelf edge reefs in the Great Barrier Reef
Drowned (or submerged) reefs are tropical coral reefs that grew during previous lower sea-levels, and they now lie in depths greater than those typically associated with the vigorous growth of modern coral reefs. Studies around the world have found that drowned reefs are very important as records of global sea-level rise and climate change, as the corals that grew during these earlier periods are now preserved in the limestone rock of the drowned reefs.
Figure 1. Map of Grafton Pass
On Australia's Great Barrier Reef, an extensive line of drowned reefs are found along the outer-continental shelf in depths of about 40 to 70 metres (Figure 1). These drowned reefs lie in deeper waters compared to the live coral reefs that most people see when they visit the GBR. Scientific studies in the 1980s and 1990s looked for drowned reefs on the outer-shelf between Cooktown and Mackay, and found they were narrow in width - just a few hundreds of metres wide - and run parallel with the edge of the shelf break, which is where continental shelf meets the higher-gradient continental slope.
Then several years ago, we were looking at new 3D seabed depth data collected by the Royal Australian Navy (RAN) and could see the drowned reefs in far more detail than ever viewed before (Figure 2). This led to many questions, such as how extensive are these features, what are their range of depths, what could they tell us about previous climate and sea-level history of the GBR, and importantly, what marine animals and plants are now living on the drowned reefs?
Figure 2. 3D view of Escape Pass
We applied for ship-time on Australia's largest blue-water research vessel, the RV Southern Surveyor, and were granted about three weeks at sea in 2007 to conduct the 'Evolution of drowned shelf edge reefs in the GBR: Implications for understanding abrupt climate change, coral reef response, and modern deep water benthic habitats'. We also received a National Geographic Committee for Research and Exploration (CRE) grant to help pay for the expedition.
The goals of the drowned shelf edge reefs expedition were:
To define the ages of the shelf edge reefs;
To describe the spatial (geographical) extent of the reefs along the GBR shelf;
To understand their relationship with past sea-levels, such as how they grew and died; and
To map the living biological communities which now use the dead reefs as a habitat.
Scientists and students with an interest in drowned reefs were invited from James Cook University (JCU), University of Sydney (USyd), University of Oxford (UOxford), and the University of Edinburgh (UEdinburgh). The ship departed from Cairns in north Queensland and traveled along edge of the GBR outer-shelf and in the Coral Sea for about 700 km before finishing at Mackay in central Queensland (Map below).
Figure 3. Autonomous underwater vehicle
We used advanced underwater technologies mounted on the ship to locate the drowned shelf edge reefs. These were a multibeam echosounder and a sub-bottom profiler which uses various sound frequencies to explore the seafloor and provide us with a detailed 3D view of the underwater landscape or seascape. Computer visualization software gave us the ability to ‘glide' through the seascape so we could closely examine the shape of the seabed.
Once the shelf edge reefs were located, we sent down an underwater robot, or autonomous underwater vehicle (AUV), owned by the Australian Centre for Field Robotics (ACFR) (Figure 3 & Movie below). The AUV robot was not tethered by a cable to the RV Southern Surveyor and could operate by itself after being programmed with instructions for where to find the reefs. Its job was to use its propellers to swim over the reefs, which were too deep for scuba diving, and to take underwater photos of the marine life living on the drowned reefs.
Figure 4. Soft corals at 70 metres depth
The AUV could also take measurements of the temperature, salinity and other water records to help describe the environment at the reefs. The robot worked for many hours each day at the sites we visited along the GBR shelf, and came back with hundreds of high-quality images showing the diversity of marine life living on the deeper GBR shelf (Figure 4). Because the images were taken in stereo (twin cameras side-by-side) we could see the detailed marine life attached to the drowned reefs.
We also used rock dredges, or large steel mesh bags, to precisely trawl over the drowned reefs and bring back samples of the limestone rock that makes up the drowned reefs. This sampling method worked very well as we knew exactly where to lower the dredge by using computer-generated 3D views of the seabed. These samples of limestone rock were once living coral colonies, so we will study these pieces in a laboratory to understand what the coral chemistry and internal structure can tell us about the ocean climate when they were living corals.
The drowned shelf edge reefs in the GBR expedition will give scientists valuable records of the past GBR response to climate and sea-level change. The detailed 3D seabed maps and high-resolution images will help us to understand how these reefs provide a habitat for the living marine life attached to their surface. This new information will also help GBR Marine Park managers to protect and preserve the these interesting deep reef environments.
