5,251 Underwater Holes Mysteriously Line California’s Coast—and We Finally Know Why

  • First surveyed in 1998, the Sur Pockmark Fields stretch along California’s central coastline and lie in an area ripe for future wind energy installations.

  • Using a fleet of underwater robots to gather mapping data and sediment core samples, scientists discovered that these mysterious pockmarks are caused by a geological phenomenon known as gravity flows rather than by bubbling methane, as was previously believed.

  • While the research team isn’t sure how these pockmarks first formed, they’ve convincingly mapped how these 5,251 depressions formed over hundreds of thousands of years.

Off the coast of California, some 1,600 to 5,200 feet below sea level, there lies a strange geological phenomenon known as the Sur Pockmark Field. Roughly the size of Los Angeles, this field contains approximately 5,251 shallow holes, each measuring around 500 feet across and roughly 16 feet deep. It’s the largest such pockmark field in North America, and its existence has puzzled oceanographers ever since its discovery during a Monterey Bay Aquarium Research Institute (MBARI) survey in 1998.

Previous research in other parts of the world suggested that these depressions were possibly caused by methane bubbling up through underwater sediment, which brings up potential stability concerns for any future wind farm applications across this stretch of LA-adjacent ocean. To figure out the cause of these depressions, researchers from MBARI—along with scientists from the US Geological Survey (USGS) and Stanford University—used a suite of impressive underwater robots to create maps, gather bathymetric data, and collect sediment core samples of these pockmarks.

What they discovered is that these formations are caused by sediment gravity flows—kind of like an avalanche, but with sand, water, and mud instead of snow—that have occurred for an estimated 280,000 years. The results of the study were published Tuesday in the Journal of Geophysical Research: Earth Surface.

“We collected a massive amount of data, allowing us to make a surprising link between pockmarks and sediment gravity flows,” Eve Lundsten, MBARI Senior Research Technician and lead author of the study, said in a press statement. “We were unable to determine exactly how these pockmarks were initially formed, but with MBARI’s advanced underwater technology, we’ve gained new insight into how and why these features have persisted on the seafloor for hundreds of thousands of years.”

First, the team sent its torpedo-shaped autonomous underwater vehicle (AUV) that’s purpose-built for seafloor mapping to take a look. Diving to preprogrammed waypoints, the AUV traveled roughly 50 meters above the ocean floor and used multibeam sonar and a Chirp (Compressed High-Intensity Radar Pulse) sub-bottom profiler to peer some 40 meters beneath the surface layer. Then, the researchers employed two remotely-operated vehicles (ROVs)—the ROV Doc Ricketts and miniROV—to capture 185 hours of footage of 21 separate pockmarks. Finally, Doc Ricketts collected 107 five-foot-long sediment samples known as “vibracores,” as well as 433 10-inch samples, or “pushcores.”

All of this data showed no signs of a methane gas, but the subsurface information revealed alternating layers of fine and coarse sediment, likely caused by intermittent (often thousands of years apart) gravity flows bringing coarser material to an otherwise fine-grained underwater sediment field. These flows eroded the center of pockmarks, and while scientists aren’t sure why (yet), they theorize that the area’s “unique seafloor morphology” may be the cause.

Being so close to a major population center like Los Angeles, US Bureau of Ocean Energy Management marked this general area as a possible candidate for building wind farms in 2018. Now, thanks to this study, future green energy projects can rest easy knowing that these shifting sands are not a methane-induced disaster waiting to happen.

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

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