The mystery of the circular slopes off the coast of California has been partially solved

Round depressions about 150 meters (500 feet) wide off the coast of Central California are quite ancient and owe their longevity to sediment flows, new research reveals. However, their original cause has not yet been discovered.

The Sur Pockmark Field has puzzled oceanographers since its discovery in 1998. More than 5,000 shallow circular depressions are clustered in an area the size of Los Angeles about 40 kilometers (26 miles) off the coast of Big Sur, Central California.

The formations are only about 5 meters (16 ft) deep, although some can be nearly 10 times that. Similar formations elsewhere around the world have been attributed to pockets of methane leaking through sediments, such as the craters that have recently begun to appear in Siberia.

The release of all this methane is a serious problem for the planet, given the gas’s global warming potential if it reaches the surface, but there’s also a more local problem. The area off Big Sur is zoned for offshore wind farms, and the presence of bubbling methane where the floating towers are anchored would be a problem.

This prompted the Monterey Bay Aquarium Research Institute (MBARI) to investigate the topic further, and they found another explanation. The researchers concluded that the depressions were caused by gravity flows of sediments, sort of like an underwater avalanche of mud and sand.

“There are many unanswered questions about the seafloor and its processes,” MBARI’s Eva Lundsten said in a statement. “This research provides important seafloor data for resource managers and others considering potential offshore sites for subsea infrastructure to guide decision-making.”

Lundsten and co-authors began by deploying underwater robots to map more than 300 spots and their surroundings, providing a surface-based resolution sonar can’t match. They also took more than 500 sediment samples in and around five spots, none of which showed any signs of methane.

The key to the reason lies in something not obvious from the cards; the field is located on the inclined continental margin, between the shelf and the deep ocean. This tilt created the conditions for massive gravity flows, and the team found that the sediment included layers of sandy deposits called turbidites left by gravity flows over the past 280,000 years. The last flow was about 14,000 years ago, at the end of the last ice age.

The team concluded that gravity flows erode sediments that accumulate in the spots under more normal conditions, maintaining the depressions, but the initial formation process remains a mystery.

“We collected a huge amount of data that allowed us to make a surprising connection between patchiness and sediment gravity flows. We have not been able to determine exactly how these marks formed in the first place, but with MBARI’s advanced underwater technology, we have gained new insight into how and why these features have persisted on the seafloor for hundreds of thousands of years,” said Lundsten.

Adding to the mystery of the formation of the spots is that they are relatively evenly spaced, a common feature of scar fields elsewhere. They range from 10 to 700 meters (33 to 2,297 ft), with the larger ones in deeper water. The team also found evidence that individual patches can sometimes migrate along the ocean floor.

Wind farm developers may not like the sound of massive avalanche-like activity in their area, but the timescales between these events suggest they are less of a threat than methane releases.

The work makes the Sur Pockmark field one of the most well-explored offshore regions in North America, and therefore possibly in the world.

The research is open access in the Journal of Geophysical Research Earth Surface.

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