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New MBARI research has shown that sediment flows, not methane gas eruptions, maintain the prehistoric formations of pockmarks — large, circular depressions on the seafloor offshore of Central California. The Sur Pockmark Field, an area about the size of Los Angeles, contains over 5,200 circular depressions. Previous research suggested that methane gas was responsible for these formations, but the recent research found no evidence of methane at this site, proposing instead that sediment gravity flows have been maintaining these seafloor formations for hundreds of thousands of years.

The research team deployed MBARI’s advanced underwater robots to study the Sur Pockmark Field. Autonomous underwater vehicles (AUVs) mapped the region, and the data collected guided sampling with MBARI’s remotely operated vehicle (ROV) Doc Ricketts. The team found multiple layers of sandy deposits in the sediment samples taken from the pockmarks, indicating that large sediment gravity flows have been occurring intermittently in the region for at least the last 280,000 years, causing erosion in the center of each pockmark.

The research team was able to make a surprising link between pockmarks and sediment gravity flows, providing new insights into how and why these features have persisted on the seafloor for hundreds of thousands of years. While the exact formation of these pockmarks remains unclear, the data collected by MBARI’s advanced underwater technology has shed new light on the processes responsible for maintaining these unique underwater morphologic features over time.

The Sur Pockmark Field is one of the best-studied areas of seafloor on the west coast of North America, thanks to the extensive efforts of MBARI, USGS, BOEM, and NOAA. Still, there are many unanswered questions about these pockmarks, including how they were initially formed. The research has significant implications for the responsible use and management of the seafloor off California, particularly in guiding potential offshore sites for underwater infrastructure like offshore wind farms.

Funded by the David and Lucile Packard Foundation, BOEM, and USGS, this research is part of MBARI’s active research program that uses advanced robots to study and map the seafloor offshore of Central California. The Sur Pockmark Field, located offshore of Big Sur, has historically been one of the least-studied regions of the continental margin off the west coast of North America. For the past six years, MBARI’s Continental Margin Processes Team has been working to understand the origins of the pockmark formations.

The research team used mapping AUVs to visualize the Sur Pockmark Field in greater detail, capturing data that revealed the smooth, gradually-sloped sides of the pockmarks. These surveys allowed for targeted sampling of pockmarks within the field, and subsequent analysis found no evidence of methane gas in any of the samples collected. Instead, the sediment profiles and layers of alternating fine and coarse sediment indicated that sediment gravity flows have been eroding the pockmark centers and maintaining these features over time. Further research is needed to understand the flow patterns over the seabed and confirm the mechanism behind the erosion and maintenance of the pockmarks.

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