Underwater dunes towering more than five stories tall have been seen on continental slopes for the first time, likely created by extremely powerful waves in the South China Sea.
The South China Sea is home to the largest so-called "internal waves" seen in the oceans — ones that oscillate within the sea rather than on its surface. These waves, which are generated by tidal forces, can sometimes reach more than 575 feet (175 meters) "high" underwater.
As scientists were mapping these internal waves, they unexpectedly detected undulations on the continental slope of the South China Sea — the continental slopes being the farthest edges of those parts of the Earth's crust that support the continents. These undulations, seen 525 feet to 1,970 feet (160 to 600 meters) underwater, turned out to be huge dunes more than 50 feet (16 meters) high and 1,150 feet (350 meters) long — the first ever seen on continental slopes.
The dunes can't be directly imaged because they are so far under the ocean — and so large — that it would be nearly impossible to image them. Instead, sonar readings show the existence of the dunes.
These new dunes are in the top 5 to 10 percent in size when it comes to underwater dunes. The researchers suggest they are the result of the internal waves traveling across the basin of the South China Sea, scattering extremely large amounts of energy on the continental slope, reshaping the sand there.
This is a newfound way to form underwater dunes, different from how tides, rivers and winds typically form shallow-water dunes and how seafloor bottom currents generate deep-water dunes, the researchers said.
"It's exciting just to be part of such an unexpected discovery," ocean acoustician Benjamin Reeder at the Naval Postgraduate School in Monterey, Calif., told OurAmazingPlanet.
There remain many open questions about these dunes, Reeder said. For instance, how do the dunes influence the seafloor habitats in the South China Sea? Do the waves help the mixing of nutrients and thus enhance life, and if so, on what time scale? How widespread are the dunes, and how fast do they migrate? How do they affect the propagation of sound waves underwater?
Reeder and his colleagues want to map the rest of the South China Sea continental slope to chart the extent of these sand dunes.
"Sand dunes on the continental slope in other locations are possible since internal waves are ubiquitous in the ocean's margins, but I suspect they would be much smaller than the South China Sea dunes," he said.
The scientists detailed their findings in the Jan. 15 issue of the journal Marine Geology.