Documenting the dynamic processes that shape the seafloor

The ocean covers 70 percent of Earth’s surface. Beneath the water lies a seafloor with a landscape as complex as dry land. The seafloor plays an important ecological and societal role. It provides vital habitat for marine life and supports our modern infrastructure. The landscape of the ocean floor is diverse and dynamic, yet many fundamental questions remain about the geological processes that shape this environment. A better understanding of seafloor geology helps us understand how human activities impact seafloor ecosystems and how geohazards like landslides, earthquakes, and tsunamis may affect coastal communities and underwater infrastructure.

Members of MBARI’s Continental Margin Processes Team examine sediment cores collected from the Sur Pockmark Field — MBARI’s Continental Margin Processes Team collaborated with researchers from USGS and Stanford University to better understand prehistoric pockmark formations on the seafloor offshore of Central California. Image courtesy of Nora Nieminski/USGS

MBARI scientists and engineers work together to design and build innovative tools that give us unprecedented access to the deep seafloor. Our researchers use a variety of technologies to study the seafloor from our backyard of Central California to the Arctic Ocean.

The seafloor offshore of this remote stretch of the Central California coastline 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 unusual pockmark formations—large, circular depressions—on the seafloor.

The Sur Pockmark Field—an area about the size of the city of Los Angeles located off the coast of Big Sur, California—contains more than 5,200 circular depressions. These formations are approximately 200 meters (656 feet) across, roughly the distance of two football fields, and five meters (16 feet) deep. Spread out over 1,300 square kilometers (500 square miles), this area is the largest known pockmark field in North America.

Past research in other parts of the world has suggested that similar large seafloor depressions were formed and maintained by methane gas bubbling up through the sediments. As the construction of wind farms is being considered offshore Central California, resource managers were concerned about how the presence of methane gas might impact the stability of the seafloor in this region. MBARI researchers, along with our collaborators from the United States Geological Survey (USGS) and Stanford University, have been working to establish whether these formations are geologically active and determine if they are areas of special biological significance.

A map of the seafloor offshore of Central California shows the location of the Sur Pockmark Field. The California coast is on the right in solid, flat brown, with the seafloor extending to the left in a textured gradient of white to orange to yellow to green to blue. Red circles represent pockmark formations. Three areas of the map with a high-density of pockmarks are outlined in teal and labeled with the words Sur Pockmark Field. Location references are labeled with the words Lucia Chica Channel, San Simeon Channel, and Santa Lucia Bank. At the bottom is a white scale bar labeled 20 km. In the bottom left is a key labeled Depth (m) with a box with a color gradient from white to orange to yellow to green to blue to purple that is labeled with 0 at the top with the white color and 2350 at the bottom with the dark purple color. In the top right is a compass alongside a white illustration of the state of California with a red box calling out the location of the map. On the edges are notches for latitude and longitude.

The data collected by MBARI researchers and our collaborators found no evidence of methane at this site. Instead, the research team has proposed that sediment gravity flows—similar to an avalanche of mud, sand, and water moving along the seafloor—have occurred in this region intermittently for hundreds of thousands of years, maintaining these seafloor formations.

MBARI’s advanced underwater robots were integral to this research. First, autonomous underwater vehicles (AUVs)—torpedo-shaped, self-guided robots—mapped the region. Previous maps of the seafloor were collected by sonar mounted on ships, but the distance between the ocean surface and the seafloor resulted in low-resolution data. AUVs can travel closer to the seafloor to visualize the terrain below in much greater detail. MBARI’s seafloor mapping AUVs also carried technology to profile the sub-bottom layers of sediment below the seafloor. These maps then guided sampling with MBARI’s remotely operated vehicle (ROV) Doc Ricketts. Operated by the research team in the control room aboard an MBARI research vessel, the ROV Doc Ricketts collected sediment samples to reconstruct the history of individual pockmarks.

Visualizing layers of sediment below the seafloor surface with MBARI’s seafloor mapping autonomous underwater vehicle (AUV) — MBARI’s mapping AUV collected bathymetry of the present-day seafloor of the Sur Pockmark Field. Acoustic technology on the AUV also allowed researchers to visualize the sediment layers underneath that were deposited over thousands of years. Illustration: Eve Lundsten © 2024 MBARI

These pockmarks are located on the continental margin, a dynamic section of the seafloor that connects the relatively shallow continental shelf to the deep sea. Sediment gravity flows can move massive amounts of material through this region intermittently. The data and samples collected by MBARI technology helped the research team piece together the history of sediment movements over this part of the seafloor.

The team found multiple layers of sandy deposits, called turbidites, in the sediment samples taken from the pockmarks and the sub-bottom images of the pockmark field. These deposits indicated that large sediment gravity flows in the region have occurred intermittently for at least the last 280,000 years. These sediment gravity flows appear to cause erosion in the center of each pockmark, maintaining these unique underwater morphologic features over time.

While researchers were unable to determine exactly how these pockmarks were initially formed, MBARI’s advanced underwater technology gave them new insight into how and why these features have persisted on the seafloor for hundreds of thousands of years.

Because of the extensive efforts of MBARI, USGS, BOEM, and NOAA as part of the interagency Expanding Pacific Research and Exploration of Submerged Systems (EXPRESS) cooperative research campaign, the Sur Pockmark Field is now one of the better-studied areas of seafloor on the west coast of North America.

A sample of multi-colored seafloor bathymetry shows circular depressions called pockmarks on the seafloor. The image color represents a depth gradient with shallower depths in orange on the right, transitioning to yellow, green, and blue on the left to represent deeper depths. The pockmark formations are circular in shape and spaced evenly apart. There are 18 large pockmark formations in the frame, as well as four smaller pockmark formations and a handful of other small, dark pits on the seafloor.

MBARI research on a field of pockmarks offshore of the Central California coast has revealed that powerful sediment flows, not explosions of methane gas, maintain these large, circular depressions on the seafloor. Image: © 2019 MBARI

Expanding renewable energy is critical to achieving the dramatic cuts in carbon dioxide emissions needed to prevent further irreversible climate change. However, many unanswered questions remain about the possible environmental impacts of offshore wind energy development. 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.

Funding for this work was provided by the David and Lucile Packard Foundation, the U.S. Bureau of Ocean Energy Management (BOEM), and USGS.

"There are many unanswered questions about the processes that sculpt the seafloor. MBARI research is providing important data to guide decision-making about this environment.”
—Senior Research Technician Eve Lundsten

MBARI researchers are investigating how the morphology of the continental margin—where the continental shelf transitions to the abyssal plain—is sculpted and changed over time. This work extends far beyond Monterey Bay, with an active research program in the Canadian Beaufort Sea.

MBARI’s Mapper 2 seafloor mapping autonomous underwater vehicle (AUV) preparing to descend underwater for a mapping survey — MBARI’s seafloor mapping AUVs are autonomous robots that can visualize the bathymetry of the seafloor down to a resolution of a one-meter square (11-square feet) grid, or roughly the size of a dinner table. Image: Dave Caress © 2022 MBARI

Since 2003, MBARI has been part of an international collaboration to study the seafloor at the edge of the Canadian Arctic shelf. This remote area only recently became accessible to scientists as warmer temperatures caused sea ice to retreat.

A mapping survey by Canadian researchers in 2010 first uncovered the region’s distinctively rugged seafloor terrain. In 2013, MBARI researchers and their collaborators conducted the region’s first high-resolution mapping surveys. Using an MBARI AUV, the research team documented the seafloor terrain in detail.

Five mapping surveys—two conducted from Canadian research ships and three with MBARI’s advanced underwater technology—in this area over a 12-year period revealed 65 newly-formed craters on the seafloor. The largest crater was the size of a city block of six-story buildings.

In 2022, the team returned to the Arctic aboard KOPRI’s icebreaker research vessel Araon. They first used MBARI’s two seafloor mapping AUVs to identify recently formed craters. Then, they conducted visual surveys within those specific craters with MBARI’s MiniROV. This portable, remotely operated vehicle developed by MBARI engineers can be configured for various science missions. Equipped with cameras and sampling equipment, it has been integral to studying the Arctic seafloor.

A map created by MBARI’s advanced underwater robots reveals the unique seafloor terrain in a remote region of the Arctic Ocean. The right-hand side of the map has several wide, flat orange mounds and yellowish-green pits, representing mounds and sinkholes. The left-hand side of the map is yellowish-green with several small, pointy green mounds. A color key in the top right reads Water Depth (m) with a color gradient from white to orange to yellow to green to light blue to dark blue. The label on the right under white reads 120 for 120 meters water depth. The label on the left under dark blue reads 205 for 205 meters water depth. At the bottom left is a black scale bar labeled ~300 m to represent 300 meters of distance in the map. Above is a compass arrow pointed diagonally at approximately 45 degrees to the middle left and labeled N in a black script font for North.

Repeated surveys with MBARI’s seafloor mapping autonomous underwater vehicles have revealed the unique terrain at the edge of the continental shelf in the Canadian Beaufort Sea, including the rapid formation of massive craters and large mounds. Image: Eve Lundsten © 2022 MBARI

While exploring the seafloor with the MiniROV, MBARI researchers and our collaborators from the Korea Polar Research Institute (KOPRI), the Korea Institute of Geoscience and Mineral Resources, the Geological Survey of Canada, and the U.S. Naval Research Laboratory observed exposed layers of ice inside the recently formed seafloor craters.

These layers of ice are not the same as the ancient permafrost formed during the last ice age, but rather were created under present-day conditions above the submerged permafrost. This recently discovered ice is sourced from deeper layers of ancient submarine permafrost. Isotopic analysis of samples from these formations and the surrounding seafloor confirmed that the ice found in newly formed craters came from brackish groundwater that began on land. This ice was created partly by the melted ancient permafrost rising up through the sediment, and refreezing as it approaches the seafloor, where the ambient temperature is colder than the deeper sediments below.

Marine operations crew members launch a robotic submersible over the side of a research ship into the water. The crew members are on the left side of the frame. The submersible has a yellow float, black frame, and a red mechanical arm, and is equipped with various scientific instruments. A black tether extends offscreen at the top of the frame to a crane on the research ship. Marine operations crew members are holding two yellow tethers, one to carry video from the submersible’s cameras and one to stabilize the submersible during deployment. The background is gray-blue ocean and overcast gray sky.

MBARI’s MiniROV is equipped with tools for studying the seafloor. Surveys with this nimble robotic submersible revealed new permafrost ice formations inside massive sinkholes on the Arctic seafloor. Image: Dave Caress © 2022 MBARI

The complex morphology of the seafloor in this region of the Arctic tells a story that involves both the melting of ancient submerged permafrost and the disfiguration of the modern seafloor that occurs when released water refreezes and melts again.

After the last ice age, sea levels rose and covered the ancient permafrost on the Arctic shelf. The base of this body of ancient permafrost is slowly warming and thawing because of heat flowing up from the Earth’s core. When this melted groundwater migrates up to the colder seafloor, it freezes. Freezing ice pushes up ridges and mounds on the seafloor. When seawater seeps into the blistered seafloor surface, the change in salinity melts the ice layers and leaves behind the massive sinkholes discovered by MBARI scientists. The dynamic interplay between large changes in salinity and small changes in temperature near the seafloor drives this dynamic process.

MBARI researchers and collaborators watching a live video feed from MBARI’s MiniROV aboard the IBRV Araon — MBARI Senior Scientist Charlie Paull (second from left) led an international team of researchers that has uncovered the dynamic processes that create massive craters and new submarine permafrost ice on the seafloor in a remote part of the Arctic Ocean. Image: Dave Caress © 2022 MBARI

Minor temperature and salinity variations cause shifts between freezing of ascending brackish groundwater and melting of near-seafloor ice layers. These ongoing processes work in tandem to create a dramatic submarine landscape composed of numerous depressions and ice-filled mounds of varying ages. This discovery of near seafloor ice above ancient submerged permafrost expands our understanding of submarine permafrost in the continental shelves of the Arctic Ocean.

This research revealed that permafrost ice is both actively forming and decomposing near the seafloor over widespread areas, creating a dynamic underwater landscape with massive sinkholes and large mounds of ice covered in sediment. These dramatic and ongoing seafloor changes have huge implications for policymakers who are making decisions about underwater infrastructure in the Arctic.

This work was funded by the David and Lucile Packard Foundation, the Korean Ministry of Ocean and Fisheries (KIMST grant No. 20210632), the Geological Survey of Canada, and the U.S. Naval Research Laboratory.

Research Publications:

Lundsten, E., C.K. Paull, R. Gwiazda, S. Dobbs, D.W. Caress, L.A. Kuhnz, M. Walton, N. Nieminski, M. McGann, T. Lorenson, G. Cochrane, and J. Addison. 2024. Pockmarks offshore Big Sur, California provide evidence for recurrent, regional, and unconfined sediment gravity flows. JGR Earth Surface. 129, e2023JF007374. https://doi.org/10.1029/2023JF007374

Paull, C.K., J.K. Hong, D.W. Caress, R. Gwiazda, J.-H. Kim, E. Lundsten, J.B. Paduan, Y.K. Jin, M.J. Duchesne, T.S. Rhee, V. Brake, J. Obelcz, and M.A.L. Walton. 2024. Massive ice outcrops and thermokarst along the Arctic shelf edge: by-products of ongoing groundwater freezing and thawing in the sub-surfaces. JGR Earth Surface, 129: e2024JF007719 https://doi.org/10.1029/2024JF007719

Documenting the dynamic processes that shape the seafloor

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