The seafloor of Penobscot Bay has been in the news quite a bit lately due to controversy around a proposed dredging project in Searsport. A little-discussed aspect of the dredge proposal is that the Army Corp of Engineers is proposing to deposit the dredge spoils into an expansive cavern on the sea floor in western Penobscot Bay, called a “pockmark.”
The existence of these giant pockmarks is not widely known outside the scientific and fishing communities, but has been the subject of study for three decades by our leading marine geologists in Maine. It turns out that Penobscot Bay hosts the largest known concentration of these pockmarks anywhere in the world, each one formed by the release of naturally occurring methane gas.
The University of Maine’s Dan Belknap and Joe Kelley started studying these features in 1984. On their first cruise, they partnered with a local lobsterman because fishermen have known about these depressions for decades. At the time they weren’t on the charts, but fishermen observed that lobsters congregate in the depressions, and traps set in them came up carrying sticky blue clay.
The blue clay was the first clue indicating glacial sediments and methane.
“We have had incredible cooperation from fishermen over the years,” Belknap said. “They have provided us with very good clues about what is actually going on. It would have taken us a lot longer to figure this out on our own.”
Three decades later, after ship-based surveys with side-scan and multi-beam sonar, ROV (remotely-operated vehicle) surveys, and even manned submersible visits, Belknap and his colleagues now know a lot more about these formations. But they still have unanswered questions.
In a sonar image the pockmarks appear as a field of cone-shaped depressions crowded together in groups or strung out in chains “as if connected like a string of pearls,” explained Belknap.
There are thousands of pockmarks in northwestern Penobscot Bay, but they also occur in smaller clumps or chains in muddy seafloor areas up and down the coast of Maine. A medium sized pockmark is about 60 feet wide and 20 feet deep. Many are smaller. A few are as large as the Rose Bowl.
Sonar and sediment core data pointed to naturally-formed methane gas as the force creating these pockmarks. The methane seems to have been formed by microbes acting on rich organic material that was deposited when these areas were wetlands at the end of the ice age, over 11,000 years ago. Over time, as the methane bubbles have escaped through the surrounding sediments and up through the water, the seafloor has slumped, forming the cone-shaped features.
While the pocks are ubiquitous in the bay, the total amount of methane is small, and capturing it commercially would not be economically feasible.
But questions about the pockmarks still persist: Do they form in rapid events or through slow outgassing? Are they still being formed? Are they stable?
There are anecdotal reports of fishermen seeing bubbles come to the surface—one lobsterman described it as looking like a submarine was surfacing—but this outgassing has not been observed or recorded during scientific surveys.
The most recent scientific evidence indicates the pockmarks are not likely actively forming, Belknap noted. Nonetheless, uncertainly persists about whether methane is being released and what the contribution could be to local ocean chemistry, including acidification.
What the scientists do know is that the pockmark walls contain what appear to be vents, and that these walls are not stable.
“I was in a submersible one time too close to the edge of a pockmark and we started an underwater landslide,” Belknap recalled. “The captain of the submersible said ‘We are not doing that again!'”
While we do not know everything we would like to about these features, they serve as a reminder that the seafloor off our own coast can be a fascinating and ever-changing environment—not the silent deep many of us think of as sitting below the waves.
Dr. Heather Deese is an oceanographer and VP of Strategic Development at Island Institute. Dr. Susie Arnold is a marine ecologist and Marine Scientist at Island Institute.