Far out of sight at the bottom of the Mediterranean Sea, the Red Sea, and the Gulf of Mexico lurk rare features known as brine pools. These basins of extremely salty, nearly oxygen free water can be a death trap to unsuspecting animals like eels, crabs, and mussels that wander into these “underwater lakes.”
Now researchers studying a brine pool at the bottom of Orca Basin in the Gulf of Mexico have shown that submarine landslides can generate massive waves of brine within pools. These tsunami-like events can send deadly brine spilling out into adjoining basins.
Half a Million Tons per Year
A team of scientists led by Derek Sawyer, a marine geologist at The Ohio State University in Columbus, has spent the past year studying Orca Basin, a feature about 350 kilometers southwest of New Orleans, La. They focused on the brine pool at its bottom, which is 123 square kilometers in area—about twice the size of Manhattan—and one of the world’s largest.
At about 8,000 years old, this brine pool is fed by an outcropping of salt dating to the Jurassic period. As seawater dissolves this salt, the resulting brine flows downhill because of its high density and pools in the lowest reaches of Orca Basin. This process is ongoing: Researchers have estimated that 500,000 metric tons of salt dissolve per year.
The brine pool is already about 8 times saltier than normal seawater, and it’ll only grow larger and saltier over time, Sawyer explained. His team studies brine pools because of their parallels to conditions in other times and places: Modern-day brine pools are probably good analogs for conditions on early Earth, and pools might also provide a toehold for life on other planets.
Sawyer and his colleagues examined Orca Basin’s brine pool, 2,200 meters below sea level, using detailed maps of the seafloor and subsurface imaging. These data were collected, in part, by energy companies looking for oil and gas deposits in the Gulf of Mexico.
The researchers found evidence that multiple underwater landslides had hit the brine pool in the past: Scarps dot the edge of Orca Basin, debris like large boulders litters the bottom of the brine pool, and a sediment core drilled in 1983 revealed a 16-meter-thick landslide deposit.
These landslides were likely caused by “salt tectonics,” Sawyer and his team proposed. Bodies of salt under the seafloor can move and flow over time, eventually pushing up on the seafloor, Sawyer said. “It’s like stepping on a tube of toothpaste.” Over time, this motion can build up the angle of the seafloor and create steep slopes susceptible to landslides. In Orca Basin, slopes as steep as 22° have been recorded—that’s about 6 times steeper than the maximum permissible road grade on U.S. highways.
Laboratory experiments have shown that fast-moving sediments striking a dense fluid such as a brine can trigger large waves. But just how large?
Using estimates of landslide speed based on the timing of telegraph cable breaks after an underwater landslide in 1929, the density difference between seawater and brine water, and other parameters, Sawyer and his team calculate that wave heights ranging from 90 to 360 meters could slosh through Orca Basin’s brine pool following a landslide.
These amplitudes are far larger than normal waves and rival only the largest historical tsunami, Sawyer noted.
Landslide-induced waves in Orca Basin are large enough to potentially overspill the basin, Sawyer and his colleagues concluded: The lowest spill point of Orca Basin is only 139 meters above the surface of the brine pool.
“It’s not unreasonable” that brine might make it over this spill point and escape into adjoining basins, said Sawyer. That’d certainly be bad news for sea creatures living nearby. “If the brine gets sloshed on them, it’s not going to be a nice day for them,” said Sawyer. These results were published this month in Scientific Reports.
These results are “very interesting,” said Erik Cordes, a biologist at Temple University in Philadelphia, Pa., not involved in the research. “Brine would likely cause a significant die-off of the animals that it came in contact with further downslope.”
Age dating the landslides that have occurred in Orca Basin would help pin down how often brine gets tossed into surrounding ecosystems, said Sawyer. “I’d love to get some more cores.”