Mystery deepens over origin of thousands of massive holes on the ocean floor

A team of researchers has shed new light on the mysterious pockmarks that dot the seafloor off the coast of Central California. The findings reveal that powerful sediment flows, not methane gas eruptions, may be responsible for maintaining these prehistoric formations.

The research was conducted by a collaborative team from the Monterey Bay Aquarium Research Institute (MBARI), the United States Geological Survey (USGS), and Stanford University.

Their work provides crucial information to guide decision-making about responsible use and management of the seafloor off California, including site assessments for the development of offshore wind farms.

Solving the mystery of the Sur Pockmark Field

The Sur Pockmark Field, located off the coast of Big Sur, California, spans an area comparable to the size of Los Angeles and contains over 5,200 circular depressions.

Each of these pockmarks measures approximately 200 meters across and five meters deep. Resource managers were particularly interested in understanding the role of methane in these formations due to planned offshore wind farms in Central California.

However, data collected by the experts found no evidence of methane gas in the pockmarks. Instead, they discovered that sediment gravity flows, akin to underwater avalanches of mud, sand, and water, have been maintaining these formations for hundreds of thousands of years.

“There are many unanswered questions about the seafloor and its processes,” said MBARI Senior Research Technician Eve Lundsten, who led the study. “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.”

Sending in the robots: Autonomous Underwater Vehicles (AUVs)

The research team utilized MBARI’s advanced underwater robots to study the Sur Pockmark Field. Autonomous underwater vehicles (AUVs) mapped the region with greater detail than previous sonar surveys.

These maps guided the sampling process conducted by MBARI’s remotely operated vehicle (ROV) Doc Ricketts, which collected sediment samples to reconstruct the history of individual pockmarks.

The scientists discovered multiple layers of sandy deposits, known as turbidites, within the pockmarks, indicating that large sediment gravity flows have occurred intermittently for at least the past 280,000 years. These flows cause erosion at the center of each pockmark, maintaining their distinctive shape over time.

“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,” said Lundsten.

Implications for offshore wind energy development

Pockmarks found in other parts of the world have been associated with the release of methane gas, which could destabilize the seafloor and pose risks to structures like offshore wind turbines.

The findings from the Sur Pockmark Field are crucial for ensuring the stability and safety of proposed offshore wind energy projects in Central California.

MBARI’s research, driven by the need for renewable energy development, has provided critical insights into the stability of the seafloor in this region.

“Expanding renewable energy is critical to achieving the dramatic cuts in carbon dioxide emissions needed to prevent further irreversible climate change. However, there are still many unanswered questions about the possible environmental impacts of offshore wind energy development,” said MBARI President and CEO Chris Scholin.

“This research is one of many ways that MBARI researchers are answering fundamental questions about our ocean to help inform decisions about how we use marine resources,” Scholin continued.

The Sur Pockmark Field is now one of the best-studied seafloor areas on the west coast of North America, thanks to the efforts of MBARI and other institutions.

However, questions remain about the initial formation of these pockmarks. Further research will be necessary to fully understand the processes that shaped these unique underwater features.

Next Steps: Computer modeling and further research

In summary, by deploying advanced underwater technology and fostering collaborative research efforts, scientists have uncovered the surprising link between sediment gravity flows and the maintenance of the prehistoric formations in the Sur Pockmark Field.

This discovery enhances our knowledge of the deep sea ecosystem and provides crucial information to guide decision-making for responsible use and management of the seafloor, particularly in the context of offshore wind energy development.

As we continue to explore the mysteries of the deep, studies like this underscore the importance of ongoing research and collaboration in advancing our understanding of the complex and dynamic processes that shape our oceans.

The study is published in the Journal of Geophysical Research Earth Surface.

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