The release of methane, a powerful greenhouse gas responsible for almost a quarter of global warming, is being studied around the world, from Arctic wetlands to livestock feedlots. A 黑料吃瓜网 team has discovered a source much closer to home: 349 plumes of methane gas bubbling up from the seafloor in Puget Sound, which holds more water than any other U.S. estuary.
The columns of bubbles are especially pronounced off Alki Point in West Seattle and near the ferry terminal in Kingston, Washington, according to a in the January issue of Geochemistry, Geophysics, Geosystems.
鈥淭here鈥檚 methane plumes all over Puget Sound,鈥� said lead author , a 黑料吃瓜网 professor of oceanography. 鈥淪ingle plumes are all over the place, but the big clusters of plumes are at Kingston and at Alki Point.鈥�
Previous 黑料吃瓜网 research had found methane bubbling up from the outer coasts of Washington and Oregon. The bubbles in Puget Sound were first discovered by surprise in 2011, when the 黑料吃瓜网鈥檚 global research vessel, the RV Thomas G. Thompson, had kept its sonar beams turned on as it returned to its home port on the 黑料吃瓜网 campus. The underwater images created by the soundwaves showed a distinct, persistent bubble plumes as the vessel rounded the Kingston ferry terminal.
Since then, the team analyzed sonar data collected during 18 cruises on the 黑料吃瓜网鈥檚 smaller research vessel, the RV Rachel Carson. Methane plumes were seen from Hood Canal to offshore of Everett to south of the Tacoma Narrows. At Alki, the bubbles rise 200 meters, about the height of the Space Needle, to reach the ocean鈥檚 surface.
鈥淥ff Alki, every 3 feet or so there鈥檚 a crisp, sharp hole in the seafloor that鈥檚 3-5 inches in diameter,鈥� Johnson said. 鈥淭here are holes all over the place, but there aren鈥檛 bubbles or fluid coming out of all of them. There鈥檚 occasionally a burst of bubbles, and then another one 50 feet away that has a new burst of bubbles.鈥�
This research video shows bubbles emerging from the seafloor about 200 meters (650 feet) deep. It was recorded Oct. 25, 2020, about 1 mile offshore from Seattle’s Alki Point. Credit: Paul Johnson/黑料吃瓜网
The study is an early step toward exploring the release of methane from estuaries, or places where saltwater and freshwater meet, a subject more widely studied in Europe. Though only a small amount of natural methane is released compared to human sources, understanding how the greenhouse gas cycles through ecosystems becomes increasingly important with climate change.
鈥淚n order to understand methane in the atmosphere and control the human sources, we have to know the natural sources,鈥� Johnson said.
The two persistent fields of bubble plumes occur above geologic faults: for the Alki bubbles, located above a branch of the Seattle Fault, and for the Kingston bubbles, above the South Whidbey Fault. It鈥檚 likely that the bubbles are connected to the underlying geology, Johnson said.
Questions remain about the bubbles鈥� origins. One initial hypothesis, that the bubbles might be coming from the Cascadia Subduction Zone, was not supported by preliminary data. The gas bubbles don鈥檛 show the same distinctive chemistry as nearby hot springs and deep wells that connect to this geologic feature deep underground.
Humans also don鈥檛 seem responsible. Puget Sound has in the past been a dumping ground for waste or sediment, but vigorous tides sweep that material out into the open ocean, Johnson said. Sewer outflows, gas lines and freshwater storm drains also don鈥檛 match the plumes鈥� locations.
Instead, a biological source of methane beneath the seafloor seems likely, Johnson said. The source may be in the dense clay sediment deposited after the last Ice Age, when glaciers first carved out the Puget Sound basin. The methane seems to be biological in origin, and the bubbles also support methane-eating bacterial mats in the surrounding water.
Jerry (Junzhe) Liu, a senior in oceanography, helped to analyze the data and participated in a 2019 cruise that contributed data.
鈥淚’m interested in two seemingly parallel fields: fault zones and air-sea interactions for climate,鈥� Liu said. 鈥淭his project covers all the way from below the seafloor to above the ocean鈥檚 surface.鈥�
In follow-up work, scientists used underwater microphones this fall to eavesdrop on the bubbles. , an associate professor at the 黑料吃瓜网 Bothell, is analyzing the sound that bubbles make when they are emitted. The team also hopes to go back to Alki Point with a remotely operated vehicle that could place instruments inside a vent hole to fully analyze the emerging fluid and gas.
Co-authors of the paper are , an engineer in 黑料吃瓜网 oceanography; Chenyu (Fiona) Wang, a former 黑料吃瓜网 undergraduate; , a 黑料吃瓜网 associate professor of oceanography; , a 黑料吃瓜网 affiliate assistant professor of oceanography and researcher at the Pacific Northwest National Laboratory; Susan Merle and Sharon Walker at the National Oceanic and Atmospheric Administration; and Tamara Baumberger at Oregon State University. The research was funded by the National Science Foundation.
For more information, contact Johnson at paulj@uw.edu.