On the seabed off the coast of central California, there is a vast field of mysterious pockmarks. The field extends over approximately 1,300 square kilometers and there are approximately 5,200 of these strange pits, which measure about 175 meters on average and 5 meters deep.
No one knows how these mysterious holes appeared, but they probably first appeared 400,000 years ago, according to research presented at the December 9th Annual Meeting of the American Geophysical Union (AGU) by Charles Paul, Marine Geologist and Senior Scientist at the Institute for Underwater Research. Monterey Bay (MBARI).
When Paul's team sent underwater robots into the depths of the ocean to map the location of these mysterious holes, they made an unexpected discovery. On the seabed, around the large circular holes, thousands of much smaller holes, or microdepressions, were scattered about only 11 m wide and 1 m deep. Microdepressions have outnumbered holes by about three times, which means that the hole field contains about 15,000 of these small objects that have not been previously detected.
Until about three years ago, this part of the seabed near Big Sur was a "research backwater" with very little data showing what it looked like, Poll said. But marine researchers began to explore the region more thoroughly after the area was considered for the construction of an offshore wind farm.
The unusual dimples were first mapped by scientists from the National Oceanic and Atmospheric Administration (NOAA), MBARI and other agencies using sonar, but MBARI's autonomous underwater vehicles (AUV) diving robots were required to study them at high resolution.
A modern NOAA instrument can obtain a resolution of about 10 meters. A diving robot can do this 10 times better, reaching 1 meter resolution.
At this high resolution, the indentations were smooth and perfectly round. The exceptional detailing of these new images also revealed microdepressions for the first time. They had steeper sides than large pits and had "tails" that extended in one direction.
"From what we know about the rate of deposition in this area, which comes with C-14 [carbon-14] dating, this indicates that the layers deposited at this level were deposited 400,000 years ago, which suggests that these features have been preserved over time, "said Poll. Moreover, the larger pockmarks have not changed for over 50,000 years.
The usual explanation for such seabed pits is that they are formed by flows of underground fluids or methane gases; this could create unstable seabed conditions that would be unsuitable for a wind farm.
But scientists have not seen any visual signs of methane release in the pits. And when they removed the sedimentary cores from the internal pockmarks and microdepressions and analyzed the chemistry of the water, they found no chemical trace indicating the presence of methane.
“Both the pockmarks and the microdepressions that we found do not indicate that there is a methane release. Thus, the general hypothesis of their formation in this area has not been confirmed, and more research will be required to determine what formed these pits and potholes, Paul said.
In the meantime, the riddle remains unsolved.