Vast undersea canyons are giving scientists insight into the ancient history of the Great Barrier Reef, and how it has responded to past climate change. “Beyond the familiar coral reefs of the Great Barrier Reef Marine Park are deep basins and troughs, as deep as around three kilometres, bordering the north-east Australian continental shelf,” Associate Professor Jody Webster from the University of Sydney said.
“We have been studying the submarine canyons that connect the shelf to the deep basins. These are massive canyons that cut into the seafloor and play a major role as pathways in delivering sediment to the deep ocean.”
Along the northern Great Barrier Reef, over 100 individual canyons have so far been mapped with multibeam sonar.
“Sediment transport through the canyons is delivered as pulses of coarse sand, or turbidites, which can be triggered by gravity, earthquakes and surface storms,” said Dr Angel Puga-Bernabeu, formerly of the University of Sydney and now at the University of Granada in Spain.
Studying sediment cores for the interplay of quartz sediments (from land) and carbonate sediments (from coral reefs) found within the turbidites reveal the responses of the Great Barrier Reef to climate change.
A major environmental study has now reconstructed the composition, timing and frequency of these coarse sand turbidites along a huge section of the Great Barrier Reef, offshore from Cooktown to Townsville, over a distance of 400 km.
“This gives us information going back to more than 60 thousand years ago, past the last ice age,” Dr Puga-Bernabeu said.
The research, published in the latest edition of the journal Marine and Petroleum Geology, was undertaken Dr Puga-Bernabeu, Associate Professor Webster and Dr Robin Beaman from James Cook University.
“Submarine canyons are some of the largest undersea landscape features on Earth. This is the first study of its kind in the submarine canyons bordering the Great Barrier Reef margin,” said Dr Webster, from the School of Geosciences at the University of Sydney.
There are major spatial (geographic) and temporal (timing) patterns of canyon activity, with very active canyons found in the northern study area and less active canyons found along the southern Great Barrier Reef margin.
The presence of barrier reefs, such as the extensive Ribbon Reefs in the north, control how active canyons are compared to areas where coral reefs are more dispersed on the shelf.
More active canyons were also found to be connected directly to the shelf and possibly ancient river channels, once exposed at lower sea levels during the last ice age, but are now drowned and found preserved on the shelf.
These river-canyon connections clearly show high quantities of quartz originated from the continent and then delivered as river sediment to the edge of the shelf at lower sea levels.
In contrast, the many canyons bordering the coral reefs found on the more open parts of the shelf deliver large quantities of shallow coral sediment into the deep basin, with coral fragments found in sediment cores to depths of about 2 km.
These pulses of shallow corals were interrupted at the peak of the ice age, about 20 thousand years ago, when the ‘carbonate factory’ was turned off.
Another finding was that the further south along the Great Barrier Reef, the canyons are replaced by broad landslides, also delivering vast amounts of finer sediments to the deep basins, but with fewer coarse sand turbidites.
“For the first time we now know which canyons are more active than others, with the implication that those canyons with more frequent disturbance by pulses of sediment will affect the seabed marine life in those areas, compared to areas that are not as disturbed,” said Dr Beaman.
The team has plans to extend the project to study the marine life within these canyons and provide important geological and biological baseline data for the Great Barrier Reef Marine Park.