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Posted April 28, 2000

Natural Gas May Explain Pockmarks in Gulf of Maine

Accumulations of natural gas may be the cause of a distinctive sea floor feature of some Maine bays. According to a theory proposed by University of Maine geologists Joseph Kelley, Daniel Belknap and students Jeff Rogers of Old Town and Allen Gontz of Orono, gas bubbling from decomposing vegetation may be trapped by mud and other sediments until it is released by natural events or human activities.

Earthquakes, sea floor dragging and even anchors dropped from boats may cause bubbles to be released in areas where gas has accumulated in adequate concentrations.

While these gases do not appear to be economically recoverable, they may lead to the thousands of small craters or pockmarks that dot parts of the seafloor in Penobscot Bay, Belfast Bay, Casco Bay and other locations. With funding from the Maine/New Hampshire Sea Grant Program, Kelley and his colleagues have been using sonar devices to map the distribution and sizes of pockmarks.

Under most circumstances, sonar can detect bedrock well below the sediments. “Sonar waves can see through the mud, but bubbles of natural gas are impenetrable. They reflect the sonar waves, and where we see pockmarks, we also see those reflections,” says Kelley.

Such “acoustically impenetrable” layers have been documented in the Gulf of Maine and off the coast of Nova Scotia since the early 1970s by other researchers. Several theories have been proposed to explain their existence. It has been suggested that whales or flowing groundwater may have been a cause, or even bombs dropped from the surface. However, no evidence has been found to support those suggestions.

The pockmarks found by the UMaine team vary in width and depth, from less than 15 feet wide to more than 325 feet wide and up to 100 feet deep. “We've observed thousands of these on the sea floor. In some places there can be more than 650 pockmarks per square mile,” says Kelley.

While natural gas is known to bubble up from the sea floor in other bays around the world, pockmarks appear to be unique to muddy, formerly glaciated areas, he adds. Thick glacial sediments may play an important role by trapping gas that would escape more rapidly in other settings.

Kelley and Belknap are continuing to study the possibility that decomposing vegetation associated with ancient lakes and wetlands may be responsible for the gas and the resulting pockmarks. They are mapping shorelines that have long ago been drowned by the sea and have since been covered by sediments.

It is also possible, Kelley suggests, that the escaping gas may affect sea floor ecosystems by reducing oxygen concentrations in the water.

For more information, see the Department of Geological Sciences' Web page.

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