Bubbles of methane rising from seafloor in Puget Sound

Date:
January 19, 2022
Source:
University of Washington
Summary:
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 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.



FULL STORY ==========================================================================
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 University of Washington 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 study in the January issue of Geochemistry, Geophysics, Geosystems.

"There's methane plumes all over Puget Sound," said lead author Paul
Johnson, a UW professor of oceanography. "Single plumes are all over the
place, but the big clusters of plumes are at Kingston and at Alki Point." Previous UW 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 UW's global research vessel,
the RV Thomas G.

Thompson, had kept its sonar beams turned on as it returned to its home
port on the UW 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 UW's 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's surface.

"Off Alki, every 3 feet or so there's a crisp, sharp hole in the
seafloor that's 3-5 inches in diameter," Johnson said. "There are holes
all over the place, but there aren't bubbles or fluid coming out of all
of them. There's occasionally a burst of bubbles, and then another one
50 feet away that has a new burst of bubbles." 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.



==========================================================================
"In order to understand methane in the atmosphere and control the human sources, we have to know the natural sources," 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's likely
that the bubbles are connected to the underlying geology, Johnson said.

Questions remain about the bubbles' 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't show the
same distinctive chemistry as nearby hot springs and deep wells that
connect to this geologic feature deep underground.

Humans also don't 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't match the plumes' 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.

"I'm interested in two seemingly parallel fields: fault zones and air-sea interactions for climate," Liu said. "This project covers all the way
from below the seafloor to above the ocean's surface." In follow-up
work, scientists used underwater microphones this fall to eavesdrop
on the bubbles. Shima Abadi, an associate professor at the University
of Washington 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 Tor Bjorklund, an engineer in UW oceanography; Chenyu (Fiona) Wang, a former UW undergraduate; Susan Hautala, a UW
associate professor of oceanography; and Susan Merle at the National
Oceanic and Atmospheric Administration. The research was funded by the
National Science Foundation.

========================================================================== Story Source: Materials provided by University_of_Washington. Original
written by Hannah Hickey. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. H. Paul Johnson, Susan G. Merle, Tor A. Bjorklund, Susan L. Hautala,
Tamara Baumberger, Sharon L. Walker, Junzhe Liu, Nicholas D. Ward,
Chenyu Wang. Methane Plume Emissions Associated With Puget
Sound Faults in the Cascadia Forearc. Geochemistry, Geophysics,
Geosystems, 2021; 23 (1) DOI: 10.1029/2021GC010211 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220119135047.htm

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