Thousands of strange circular holes discovered in seabed off California coast leave scientists baffled

The holes measure 11 meters wide and one meter deep, on average, according to scientists who mapped the seabed. The survey was done to evaluate the feasibility of building a wind energy farm on the site


                            Thousands of strange circular holes discovered in seabed off California coast leave scientists baffled

Thousands of holes or pits have mysteriously appeared in the deep seafloor off Big Sur in California. These "micro-depressions" measure 11 meters wide and one meter deep, on average, according to scientists from Monterey Bay Aquarium Research Institute (MBARI), who mapped the seabed recently.

The seabed also plays host to larger micro-depressions — which are nearly circular and fairly evenly spaced. Called pockmarks, they average 175 meters across and five meters deep, some of which were discovered in 1999, during a seafloor survey.  

No one knows how these mystery holes appeared, but they likely made their first appearance 400,000 years ago, according to research presented at an annual meeting of the American Geophysical Union (AGU) by Charles Paull, a marine geologist and senior scientist with the MBARI.

This exercise is a part of the survey done to evaluate the feasibility of building a wind energy farm on the site. So a team of scientists from MBARI surveyed the seafloor extensively, using sonar — a technique that uses sound to detect objects deep in the ocean.

Seafloor map showing pockmark and micro-depressions in the seafloor off Big Sur. (2019 MBARI)

"The pockmarks and micro-depressions in this area are both holes in the seafloor that occur in softer sediments, but they are morphologically distinct,"  says Eve Lundsten, one of the study authors. But, she adds, the micro-depressions are recently formed features.

This is not the first time scientists have discovered pockmarks. In fact, in the late 1960s, scientists found one off the coast of Nova Scotia in Canada. Triggering their development, scientists believe, is either methane gas or other fluids.

"Such methane releases could potentially cause the seafloor to be unstable, which could pose risks for structures such as offshore oil platforms or wind turbines," the statement read. 

In the next few years, MBARI and other organizations carried out additional surveys, where scientists found 5,200 pockmarks spreading over 1,300 square kilometers (500 square miles), making this area the largest known pockmark field in North America, says the statement.

The wind farm proposal meant that scientists had to perform exhaustive surveys using sonar mounted on autonomous underwater vehicles before anything gets built.

In addition to identifying 15,000 holes, scientists also detected objects homing in on them, including rocks, animal bones and bits of kelp, and marine trash such as fishing gear and plastic debris. Marine organisms find refuge in these holes.

"The presence of these objects provides microhabitats for fish, that were commonly observed in ROV [remotely operated vehicle] dives stirring up the fine-grained sediment, which is then carried away by sea-bottom currents, further contributing to carving out the eroded hole(s) left behind," the authors wrote in their abstract.

A close-up view of the seafloor inside a micro-depression, showing trash, rocks, seafloor animals, and fish (2019 MBAR)

Next, scientists wanted to find out whether methane was creating the holes. However, MBARI researchers found no evidence of methane in the sediment or seawater in this region. In fact, sonar data showing layers of seafloor sediments suggest that these pockmarks have been inactive for the last 50,000 years.

"The cause and persistence of the pockmarks still remains a mystery, but we find no evidence they were created from gas or fluid in the seafloor in the recent past," explains Lundsten.

According to Reidulv Bøe, a marine geologist at the Geological Survey of Norway, who was not involved in the study, seafloor surveys like the one the MBARI team conducted can shed light on how humans are affecting the ocean environment. "In a way, you can map garbage by counting micro-depressions," Bøe told Science magazine.

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