Mysterious bubbles in Puget Sound: UW researchers track hundreds of seeping gas plumes

Tor Bjorklund, an oceanographer from the University of Washington, stood surrounded by blinking monitors on the research vessel Rachel Carson as it drifted through windswept waters during a routine trip on Lake Washington.

“This is a pretty small plume,” he said, pointing to a screen displaying measurements taken in December by a hydrophone, an aquatic device used to detect sound and movement in the ocean.

The clunky but effective contraption was resting just outside on the ship’s main deck.

For years, Bjorklund and his colleagues have used sonar transducers — like the one attached to the hydrophone — mounted on the Carson’s underside to detect and locate nearly 350 plumes of methane gas along the bottom of Puget Sound. Their findings were published in December.

Natural sources of methane gas in aquatic estuaries are neither rare nor do they account for the largest source of methane emission, but never before had they been discovered in Puget Sound — and especially not in such great numbers.

Scientists are investigating a possible link between the abundance and distribution of these methane plumes and seismic fault lines located directly beneath them. Now they seek more funding for research to better understand the plumes’ biological and chemical composition.

The question, Bjorklund said, is whether methane plumes serve as an indicator of seismic activity.

“There’s a lot left to learn,” he said.

The discovery itself was an accident.

Researchers stumbled upon the plumes in 2011 when oceanographers aboard the Thomas G. Thompson, one of the UW’s global research vessels, forgot to turn off the ship’s sonar beams as it returned to port. The data it collected later revealed bubbles rising from plumes near the Kingston ferry terminal.

After 19 trips spanning nearly a decade, scientists aboard the Carson were able to identify 349 methane plumes from Hood Canal to the Tacoma Narrows.

But it seems they’ve only begun to scratch the surface.

The hydrophone used on the Carson covers an area roughly 25 meters wide.

If a proposal for more funding from the National Science Foundation is approved, Bjorklund said they would install a multi-beam system on the ship’s hull to widen its underwater sonar vision.

“I do think that 350 plumes is conservative,” Bjorklund said. “I think it’s a vast underestimation of how much methane is coming out.”

The plumes are scattered across Puget Sound but most concentrated near Kingston and Alki Point, according to the scientists’ December report.

Paul Johnson, a UW oceanographer and lead author of the report, said the biological and tectonic implications are significant, but little is known about the plumes and their chemical composition.

“The difference in Puget Sound is that the methane is coming in from the seafloor, from the big faults out there,” he said. “It tells you that there’s an enormous amount of methane dissolved in Puget Sound seawater that gets emitted to the atmosphere … and it tells you something about which of the branches (on fault lines) are allowing gas and fluid to pass upward through them.”

On the Washington coast, methane vents were first discovered in 2009.

Ten years later, a comprehensive study by researchers from the UW and Oregon State University found more than 1,700 plumes about 30 miles from shore.

Previously, UW scientists believed rising ocean temperatures could be releasing frozen methane, but more recent research strongly suggests methane along the Pacific coast is emerging from old sites connected to shifts in tectonic fault lines, not warming temperatures.

Methane is a powerful heat absorber that, according to 2019 data from the Environmental Protection Agency, accounted for 10% of all greenhouse gas emissions in the United States.

“Any methane that’s released, wherever its released, contributes to methane in the atmosphere, and because methane is a potent greenhouse gas, that contributes to global warming,” said Brian Lamb, a professor at Washington State University whose research focuses on regional air quality and atmospheric pollutants.

The biggest sources of methane emission are agriculture — namely livestock and manure — natural gas or petroleum leaks and waste from homes, businesses and landfills.

While its life span in the atmosphere is much shorter than carbon dioxide, methane is 25 times more efficient over a 100-year period at trapping radiation.

Atmospheric methane concentration reached 1,900 parts per billion in 2021, according to research published in January by the National Oceanic and Atmospheric Administration, which is 2.6 times more than that which was seen during the pre-industrial era, according to the World Meteorological Organization.

Johnson estimated the 349 plumes in Puget Sound were collectively releasing about 50,000 metric tons of methane every year. If scientists are correct in assuming many plumes in Puget Sound remain uncounted, further research could provide a better understanding of how methane is impacting one of the state’s most precious estuaries.

“Any time you find a new seep like this, it’s certainly worthwhile to go back and collect new measurements,” Lamb said. “It just adds another data point to what we know about methane.”