Pockmarks beneath sea mystify scientists

ABOARD THE J.W. POWELL — After completing a sixth dive to the bottom of Belfast Bay, the pilot lifted the hatch of the tiny submersible craft and stepped onto the rain-washed deck of the 200-foot tender ship.

“That’s it,” Bob Wicklund said, shaking his head disgustedly as a scientist wriggled out of the sub behind him. “I’m not going down there again. It’s the pits.”

Wicklund, who has piloted these two-man Delta research subs for almost six years, spent much of the weekend bringing scientists to the bottom of northwestern Penobscot Bay to examine a vast field of unusual “pockmarks” that dot the muddy ocean floor.

At 70 feet and more below the surface, visibility had been poor throughout the morning on Friday. As a heavy rain fell in the afternoon, however, and lightning appeared in the distance over Searsport, the visible distance was reduced further to a dangerous few feet of floodlit mud gliding by the portholes.

“I can’t even see the end of the sub anymore down there,” Wicklund complained.

After several trips to the murky bottom earlier in the day, the 10 marine scientists from the University of Maine and the Maine Geological Survey still were not certain how the pockmarks were formed. But with a vacuum device called a “slurp gun,” and a mechanical claw protruding from the sub, they had managed to collect enough core samples of mud and organisms to keep them busy at their microscopes for the next several weeks.

Researchers said they are confident of one thing already: Natural gas or methane is seeping from the bottom of the bay, blowing off the mud and leaving behind saucer-shaped craters lined with clay. But the evidence to prove that theory — gas emanating from the pockmarks — has been elusive.

“We thought we would find bubbles of gas when we went down, but we didn’t,” said Dr. Daniel F. Belknap of the University of Maine. “The bottom line is that we just don’t have all the answers yet.”

Eventually, though, the answers from the laboratory could help lobstermen understand more about their local fishing grounds. Belknap said that lobstermen routinely place their traps on the pockmarks, which may contain organic material preferred as food by lobsters.

“But the pockmarks could move the gear and the lobstermen could be losing gear without knowing why,” Belknap said.

Since methane traps heat in the atmopshere 20 times more effectively than carbon dioxide, Belknap said, the pockmarks might be releasing a gas that plays a significant role in the greenhouse effect.

“People have been making large models of the greenhouse effect, but they have yet to plug methane into the equation,” Belknap said as he tracked the movement of the tiny sub on a screen in the ship’s pilothouse.

The pockmarks — 1,300 in all — range in diameter from 10 to 125 meters, or up to 410 feet across. The deepest craters reach 115 feet below the seafloor, with soft mud sides that slope sharply into the black depths.

Unlike the pockmarks found in the North Sea and the Nova Scotia Shelf, researchers concluded, the Penobscot Bay formations were not caused by the powerful release of petroleum gases from deep inside the earth.

“These pockmarks are formed by gases from organic decay, not from petroleum deposits,” Belknap said. “We believe there is methane gas seeping out. We’ve held matches over the core samples and saw the mud flare.”

Joseph Kelley, a marine geologist, said the gases might have been caused by the decomposition of sawdust that accumulated on the bay floor during the lumbering boom of 100 years ago.

“There were millions of board feet of wood cut every year on the Penobscot River, and the sawdust used to go into the river,” Kelley said, shortly after returning from a half-hour dive in the sub. “The river could have carried all that sawdust down and deposited it on the bottom.”

Kelley said there is also a possibility also that the pockmarks were formed as earthquakes shook the sea floor, causing the gas bubbles in the decaying sediment to consolidate and push through the soft bottom.

“The difficulty with that hypothesis,” he said, “is that there haven’t been any earthquakes recorded here recently, so why do the pockmarks still look so fresh?”

Steve Dickson of the Maine Geological Survey wrestled with that question, too, as he stood on the deck of ship after a dive of 80 feet.

“What really bothers us is that we thought the pockmarks were old,” Dickson said, rubbing a glistening sample of mud between his fingers. “But there are 30-degree slopes on the sides, and this soft mud doesn’t hold that kind of shape. One winter season and the mud would slough off and fill in the pockmarks. So if there is no gas bubbling out, what is keeping the holes clear?”

Theories are plentiful about the origin of pockmarks elsewhere around the world. Soon after oil platforms were planted in the North Sea, scientists began to speculate that the strange pockmarks beneath them had been formed by everything from meteors to bombs to falling chunks of icebergs. After dismissing those theories, scientists were left to conclude that fluid or gas was being belched from deep within the earth.

By early this week, as the Delta sub and the 200-foot tender steamed toward another research project in Alaska, the Maine scientists were back in their labs to analyze what they had brought up from the bottom of Penobscot Bay. By checking the sediment for traces of methane, and by looking for organisms that might synthesize the gas, they hope to answer a few of the questions raised by the mysterious field of pockmarks.

“Then we all meet to sling the bull and throw around our ideas,” Belknap said. “Weeks from now, if all goes well, we hope to be able to see the bigger picture of what’s going on down there.”