Circular depressions around 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 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 around 40 kilometers (26 miles) off the coast of Big Sur, Central California.
The formations are only around 5 meters (16 feet) deep on average, although some can be almost 10 times that. Similar formations elsewhere in the world have been attributed to pockets of methane escaping through sediments, like the craters that have recently started appearing in Siberia.
The release of all that methane is a serious problem for the planet, given the gas’s global warming potential if it reaches the surface, but there is a more local issue as well. The area off Big Sur is marked for offshore wind farms, and having methane bubbling up where the floating towers are anchored would be a concern.
This spurred the Monterey Bay Aquarium Research Institute (MBARI) to investigate the topic more deeply, and they found another explanation. The researchers concluded the depressions are caused by sediment gravity flows, a sort of underwater avalanche of mud and sand.
“There are many unanswered questions about the seafloor and its processes,” said MBARI’s Eve Lundsten in a 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.”
Lundsten and co-authors started by deploying underwater robots to map more than 300 pockmarks and their surrounds, providing resolution surface-based sonar could not match. They also took more than 500 sediment samples in and around five pockmarks, none of which showed any sign of methane.
A clue to the cause lies in something not obvious from maps; the field lies on the sloping continental margin, between the shelf and the deep ocean. That slope creates the conditions for enormous gravity flows, and the team found the sediment to include layers of sandy deposits called turbidites left behind by gravity flows over the last 280,000 years. The most recent flow was around 14,000 years ago, at the end of the last Ice Age.
The team’s conclusion is that the gravity flows erode sediments that build up in the pockmarks under more normal conditions, maintaining the indentations, but the initial formation process remains a mystery.
“We collected a massive amount of data, allowing us to make a surprising link between pockmarks and sediment gravity flows. 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,” Lundsten said.
Adding to the mystery of the pockmarks’ formation is that they are fairly evenly spaced, a common feature of pockmark fields elsewhere. They vary from 10 to 700 meters (33 to 2,297 feet) across, with the larger ones in deeper waters. The team also found evidence individual pockmarks can sometimes migrate along the ocean floor.
Windfarm developers may not like the sound of enormous avalanche-like activities in their field, but the timescales between these events suggest they’re 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 therefore probably the world.
The study is open access in the Journal of Geophysical Research Earth Surface.
Source Link: The Mystery Of Circular Depressions Off California’s Coast Has Been Partially Solved
