Scientists look for answers in methane bubbles rising from bottom of Puget Sound

In 2011, sonar operators aboard the ocean-going Research Vessel Thomas G. Thompson inadvertently recorded a surprising natural phenomenon, as the 274-foot ship traversed through Puget Sound while returning to port at the University of Washington.

At the time, researchers on board were focused on a host of other projects. They might not have known that the ship’s multi-beam sonar was even turned on. They certainly didn’t realize that the sonar was picking up images that would later be interpreted as multiple plumes of methane bubbles rising from the bottom of Puget Sound.

“Nobody looked at the data until about three years ago, when a former student of mine was working on a project looking at bubble plumes out on the Washington (Coast) margins,” said Paul Johnson, a UW professor of oceanography. “What she found was astonishing.”

The initial discovery of the methane plumes, by Susan Merle of Oregon State University, would lead to further discoveries of methane bubbles throughout most of Puget Sound. The findings have raised many interesting questions while providing implications related to the Puget Sound food web, studies of earthquake faults and even worldwide climate-change research. Johnson, Merle and other collaborators just published their first report on Puget Sound’s methane bubbles in the journal “Geochemistry, Geophysics, Geosystems.”

Nobody was even looking for plumes of bubbles in Puget Sound when Merle, a senior research assistant at OSU’s Cooperative Institute for Marine Resources Studies, began looking at eight-year-old archived sonar data from the RV Thompson. Following the ship’s tracklines, she observed the data as the sonar picked up images of methane bubble plumes along the coast. The sonar was still on when the ship entered Puget Sound. Merle kept following the data, not realizing that the surprising bubble plumes being revealed by the recorded sonar were all the way into Central Puget Sound, off Kingston on the Kitsap Peninsula.

“Nobody knew that there were methane bubble plumes there,” Johnson said after confirming her findings. “I said, ‘This is incredible. I wonder if there are other data out there to verify this.’”

The UW’s smaller 72-foot Research Vessel Rachel Carson operates with a less sophisticated single-beam sonar, but the ship travels all over Puget Sound, carrying student as well as professional researchers, generally on short trips. Like the RV Thompson, the RV Carson records sonar soundings wherever it goes, and those data records are kept on file.

Johnson retrieved the data from 35 cruises and found much more evidence of bubble plumes.

“There were these bubble plumes all over the place,” Johnson said, “so I said, ‘Let me have a day with the Carson,’ and we went up to Kingston in 2019.”

An instrument package was dropped to the bottom to pick up samples of water and gas around the plumes. “Sure enough, it was methane,” Johnson noted.

Thanks to a grant from the National Science Foundation for “speculative” research that might lead to breakthroughs, Johnson and his colleagues began to map bubble plumes throughout Puget Sound. They found bubbles from the Tacoma Narrows to Everett and also in Hood Canal, some 350 plumes in all.

Besides Kingston, the deep water off Seattle’s Alki Point contained a surprising number of the plumes, which are described as clusters of holes in the sea bed through which the bubbles pass. Johnson said one can get a general idea of the effect by turning a kitchen colander upside down and submerging it in a sink full of water to see bubbles emerging through the holes.

By using remotely operated vehicles, the researchers can record video of the bubbles emerging out of sharp, well-defined holes, 3 to 5 inches in diameter and roughly 3 feet apart. More than a few holes appeared to be abandoned, not producing any bubbles. Others intermittently released a series of bubbles that rose to the surface.

“You can tell which are active because of bacteria mats,” Johnson said, explaining that the bubble plumes can be a rich feeding ground for methane-loving bacteria, which grow around the holes.

In mapping the bubble plumes, it became clear that large numbers were aligned along geologic fault zones, primarily the ones running east and west, known as the Seattle, Tacoma and South Whidbey faults. Others lined up with smaller north-south faults, but the greatest number of bubble plumes occurred where the faults intersected, such as off Alki Point in West Seattle.

Much of this phenomenon has yet to be explained, Johnson said. One idea is that the methane gas is largely confined beneath a layer of clay and compressed sediments laid down during the last glacial period. If so, the methane may be rising up through cracks in the confining layer, cracks created through tectonic activity.

Methane gas is produced naturally during the breakdown of organic compounds found in all living things. Biogenic methane is produced during digestion by certain types of bacteria. Thermogenic methane occurs at higher temperatures, especially under pressure. (See discussion in Science Direct.)

Because of the lower temperatures in Puget Sound, Johnson said he suspects that the methane is from biological processes. Off the Washington and Oregon coasts, both biogenic and thermogenic methane are being released from thousands of bubble plumes, with pronounced clusters in a north-south band some 30 miles off the coast. This region is along the tectonic boundary where the Juan de Fuca oceanic plate collides with the North American continental plate.

High temperatures and pressures in this subduction zone leads to the release of fluids and methane gas. The vast majority of plumes are seen on the seaward side of the continental shelf in waters about 500 feet deep. Faults in this region, created by powerful subduction earthquakes, appear to be the routes for methane gas and fluids to escape to the surface.

An early hypothesis suggested that the bubbles in Puget Sound might be coming up from this underlying subduction zone, but that has not panned out. The chemical signature of the methane in Puget Sound, as revealed through isotope analysis, does not match that from sources deep underground, where samples can be obtained from terrestrial hot springs and water wells.

Because the methane feeds bacteria at the base of the food web, bubble plumes off the coast have been found to flourish with biological activity, including large populations of krill and fish, Johnson said.

“Fishermen know where these areas are, because they are biological hotspots,” he said.

How this methane may affect the Puget Sound ecosystem is yet to be studied in detail, Johnson said. The answer may depend on the location and specific physical and chemical conditions. While the methane is likely to increase biological productivity, it may also play a role in the low-oxygen conditions that can affect sea life and create other problems.

Because the bubble plumes seem to be coming up through faults underlying Puget Sound, seismologists might be able to use them to locate unknown geological features, identify changes over time, or determine which faults are active.

These findings also are relevant to climate change, as scientists search to find other natural sources of methane. Since methane is a powerful greenhouse gas, climatologists are challenged to identify all natural as well as human-caused sources in order to predict the effects of reduced emissions. (See “Methane Budget,” Global Carbon Project.)

Globally, between 35 and 50 percent of methane emissions are believed to come from natural sources, including wetlands, according to the Environmental Protection Agency. 

Methane’s lifetime in the atmosphere is much shorter than carbon dioxide, but methane is more efficient at trapping radiation. That’s why this gas raises major concerns. Pound for pound, the impact of methane is 25 times greater than carbon dioxide over a 100-year period, according to a report from the Intergovernmental Panel on Climate Change. In 2019, methane was said to account for about 10 percent of all U.S. greenhouse gas emissions from human activities.

The total amount of methane released from Puget Sound is relatively small when considering the total methane from many natural and human sources — including natural-gas leaks, raising livestock and garbage dumps. Still, Johnson hopes to launch a project that would estimate the total atmospheric emissions from the bubble plumes, while continuing to examine what is venting from all these holes. These new findings also point to ways to search for other natural methane sources around the world.

Related work by Shima Abadi, an associate professor at UW Bothell, involves analyzing the sound that the bubbles make and determining how that might relate to the amount of gas being released and other factors.

Other authors of the new paper are Tor Bjorklund, an engineer in UW oceanography; Chenyu (Fiona) Wang, a former UW undergraduate; Susan Hautala, a UW associate professor of oceanography; Jerry (Junzhe) Liu, a senior in oceanography; Tamara Baumberger, assistant professor at OSU; Nicholas D. Ward, affiliate assistant professor in UW Oceanography; and Sharon L. Walker of NOAA’s Pacific Marine Environmental Laboratory.