Circular 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 root cause has yet to be discovered.
The Sur Pockmark field has puzzled oceanographers since its discovery in 1998. More than 5,000 shallow circular depressions are packed into 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 feet) deep on average, although some can be nearly 10 times that. Similar formations elsewhere in the world have been attributed to pockets of methane escaping through sediments, such as the craters that have recently begun to appear in Siberia.
The release of all that methane is a serious problem for the planet, given the global warming potential of the gas if it reaches the surface, but there is also a more local problem. The area near Big Sur has been designated for offshore wind farms and the formation of methane bubbles where the floating towers are anchored would be a concern.
This prompted the Monterey Bay Aquarium Research Institute (MBARI) to look deeper into this topic and they found another explanation. The researchers concluded that the depressions were caused by gravity flows of sediment, a kind of underwater avalanche of silt and sand.
“There are many unanswered questions about the seafloor and its processes,” MBARI’s Eve Lundsten said in a statement. “This research provides important seafloor data for resource managers and others considering potential offshore sites for underwater infrastructure to guide decision-making.”
Lundsten and co-authors began by deploying underwater robots to map more than 300 holes and their surroundings, providing a resolution unmatched by surface sonars. They also took more than 500 sediment samples in about five holes, none of which showed any sign of methane.
The clue to the cause lies in something that is not visible from the maps; the field lies on the sloping edge of the continent, between the shelf and the deep ocean. That tilt creates the conditions for massive gravity flows, and the team found that the sediment includes layers of sandy deposits called turbidites left behind by gravity flows over the past 280,000 years. The most recent flow was about 14,000 years ago, at the end of the last ice age.
The team’s conclusion is that gravity flows erode sediments that accumulate in the wounds under more normal conditions, retaining the depressions, but the initial formation process remains a mystery.
“We collected a huge amount of data, which allowed us to make a surprising connection between smallpox and sediment gravity flows. “We haven’t been able to determine exactly how these pore formations originally formed, but with MBARI’s advanced underwater technology, we have gained new insight into how and why these features have existed on the sea floor for hundreds of thousands of years,” said Lundsten.
Adding to the mystery of the pox formation is that they are fairly evenly spaced, a common feature of pox fields elsewhere. They vary in diameter from 10 to 700 meters (33 to 2,297 feet), and are larger in deeper water. The team also found evidence that individual smallpox 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 time lags between these events suggest they are less of a threat than methane releases.
The work makes the Sur Pockmark Field one of the best-studied offshore regions in North America, and thus probably in the world.
The study is available in the Journal of Geophysical Research Earth Surface.