Methane Bubbles Up From Seafloor Near Seattle

Seattle

SEATTLE, Wash., January 30, 2022 (ENS) – At least 349 plumes of methane gas are bubbling up from the seafloor in Puget Sound, a bay in the Pacific Ocean, a University of Washington research team has discovered. Methane is rising just outside the city of Seattle and along the ferry routes that serve the greater Seattle area and surrounding islands.

“These gas emission sites are co-located over traces of three major fault zones that breach the entire forearm crust of the Cascadia subduction zone,” the UW scientists write in their new study in the January issue of the journal Geochemistry, Geophysics, Geosystems.

“Multi-beam and single-beam sonar data from cruises conducted in 2011, 2018, 2019, 2020 and 2021 identified the acoustic signature of 349 individual bubble plumes. Dissolved CH4 [methane] Gas from the plumes combine to increase methane concentrations in seawater throughout the Puget Sound estuary,” writes the UW team.

Methane is a primary contributor to the formation of ground-level ozone, a dangerous air pollutant and greenhouse gas. Human exposure to methane at ground level causes a million premature deaths every year, says the United Nations Environment Program (UNEP).

Methane is also a powerful greenhouse gas. Over a 20-year period, UNEP said in a report last August, it is 80 times more warming than carbon dioxide, the most common greenhouse gas.

While the columns of methane bubbles could be seen along the inner shore of Puget Sound, they are most numerous in two places – off the coast of Alki Point and off the coast of the Kingston Ferry Terminal.

Methane bubbles over faults

The two persistent bubble cloud fields occur over geological faults, the scientists learned. The Alki Bubbles are over a branch of the Seattle Fault and the Kingston Bubbles are over the South Whidbey Fault.

“It’s likely that the bubbles are related to the underlying geology,” said Dr. Paul Johnson, the UW professor of oceanography who led the research team.

This map of Puget Sound shows the locations of methane plumes as yellow and white circles. Black lines show the South Whidbey Island fault zone, the Seattle fault zone, and the Tacoma fault zone. 2022 (Johnson et al./University of Washington map)

Alki, at the westernmost point of the country in the West Seattle Borough, was the original 1850s settlement that would become the city of Seattle. The Duwamish tribe had inhabited this area for many centuries before white settlers arrived in the 1850s.

Today, two ferries, to Bainbridge Island and to Bremerton, depart and arrive a few miles north of downtown Seattle.

The other methane plume hotspot was found 23 miles north of Seattle, near the terminal in the small town of Kingston, where a ferry takes passengers to and from Edmonds on the other bank.

“There are methane clouds all over Puget Sound,” said Dr. Johnson, the lead author of the study. “Individual feathers are everywhere, but the big clusters of feathers are at Kingston and at Alki Point.”

Previous UW research found methane gushing from the Pacific Ocean coasts of Washington and Oregon. The bubbles in Puget Sound were first discovered by surprise in 2011 when the UW’s global research vessel, the RV Thomas G. Thompson, kept its sonar beams on as it returned to its homeport on the UW campus.

Underwater images generated by the sound waves showed clear, persistent plumes of bubbles as the ship rounded Kingston Ferry Terminal.

Since then, the team has analyzed sonar data collected during 18 cruises on the UW’s smaller research vessel, the RV Rachel Carson.

Methane plumes have been seen from Hood Canal to the Everett coast to south of the Tacoma Narrows. At Alki, the bubbles rise 200 meters (656 feet), about as high as the Space Needle, to reach the ocean’s surface.

“Off Alki, there’s a crisp, sharp hole in the sea floor about every three feet, about 3 to 5 inches in diameter,” said Dr. Johnson. “There are holes everywhere, but bubbles or liquid aren’t coming out everywhere. Occasionally there is a burst of bubbles and then another 50 feet away that has a new burst of bubbles.”

The UW study is an early step in investigating the release of methane from estuaries or places where salt and fresh water meet, a topic that is more widely studied in Europe.

Although only a small amount of natural methane is released compared to human sources, with climate change it is becoming increasingly important to understand how greenhouse gases cycle through ecosystems.

Johnson said, “To understand methane in the atmosphere and control the human sources, we need to know the natural sources.”

Puzzling about what the gas emits

Questions remain about the origins of the bubbles. The idea that the bubbles could have originated from the Cascadia subduction zone was not supported. The gas bubbles do not show the same characteristic chemistry as nearby hot springs and deep wells connected to the subduction zone deep underground.

Man does not seem to be responsible. While Puget Sound has historically been a dumping ground for waste or sediment, violent tides wash that material into the open ocean, Johnson said. Sewage drains, gas lines, and freshwater storm drains are also inconsistent with plume locations.

Instead, a biological methane source beneath the sea floor seems likely, Johnson observed.

“The source may be in the dense clay sediments deposited after the last Ice Age, when glaciers first eroded the Puget Sound basin,” he said. “The methane appears to be biological in origin, and the bubbles also support mats of methane-eating bacteria in the surrounding water.”

In follow-up work this fall, scientists used underwater microphones to eavesdrop on the bubbles. The team also hopes to return to Alki Point with a remote-controlled vehicle that could place instruments in a vent hole to fully analyze the leaking liquid and gas.

Susan Merle of the National Oceanic and Atmospheric Administration was a member of the research team. The research was funded by the National Science Foundation.

Click here to access the study in Geochemistry Geophysics Geosystems, “Methane Plume Emissions Associated With Puget Sound Faults in the Cascadia Forearc.”

Featured Image: Naval Engineer Sonia Brugger, right, and Naval Engineer Tor Bjorklund aboard RV Rachel Carson collecting data near the Alki Point methane gas spill field. Alki Point can be seen in the distance. Puget Sound, Washington, December 2020 (Photo courtesy University of Washington)