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A recent study from researchers at Monterey Bay Aquarium Research Institute, Stanford University, and USGS debunks the longstanding theory that methane gas is responsible for the formation of thousands of pockmarks—large, circular depressions—on the seafloor off Central California. Instead, the research team attributes these features to ancient underwater sediment flows.

Map of study area showing the Sur Pockmark Field offshore of central California
Map of study area showing the Sur Pockmark Field.

The study, conducted in an area slated for the development of a wind farm offshore of Central California, mapped more than 5,000 pockmarks at depths ranging from 500 to 1,500 meters. Researchers used an array of tools, including high-resolution seafloor maps, video footage, sediment cores, and subsurface images, to investigate these curious formations.

Contrary to the prevalent belief that many pockmarks are created and maintained by methane seepage, the collected data revealed no significant signs of methane gas. Instead, the team found that the pockmarks contain alternating layers of fine sediments and sandy deposits. The sandy layers, identified as turbidites, are the result of large sediment flows that periodically scour the pockmark centers. 

These turbid sediment flows not only carve out the pockmarks but also cause them to shift positions, sometimes over distances of tens of kilometers. This phenomenon has been occurring intermittently over the past 280,000 years, continuously reshaping the seafloor.

Interestingly, the study found that the pockmarks are not randomly scattered but are distributed in a regular, equally spaced pattern. This regular spacing further supports the hypothesis that the pockmarks are maintained by periodic sediment flows rather than by random methane fluxes.

The findings have significant implications for the proposed wind farm project. Understanding the provenance of these pockmarks allows development to proceed with a clearer picture of the seafloor's stability and history, minimizing potential risks to infrastructure.

Read a related press release from MBARI.

Read the study, Pockmarks Offshore Big Sur, California Provide Evidence for Recurrent, Regional, and Unconfined Sediment Gravity Flows, in the Journal of Geophysical Research: Earth Surface.


Diagram showing patterns of erosion and deposition that produce seafloor pockmarks
A diagram illustrating how the patterns of erosion and deposition associated with three sediment gravity flows could produce the stacked pockmarks and lateral migration observed in AUV-collected Chirp profiles. Long, uneventful episodes of hemipelagic drape (a, d, g, j) are intermittently interrupted by sediment gravity flows that either: 1b. Erode through the accumulated hemipelagic drape, deepening and potentially shifting the pockmark centers before 1c. Leaving a regional turbidite with thicker deposits in pockmark centers. 2e. Pass by without noticeable erosion, and 2f. Deposit a regional turbidite with thicker deposits in pockmark centers or 3h. Cause significant erosion in pockmark centers, eroding through hemipelagic drape as well as buried turbidites, and 3i. Leave a regional turbidite with a thick flat-top sandy deposit in pockmark centers.

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