Bubble
plumes off Washington, Oregon suggest warmer ocean may be releasing
frozen methane
Sonar image of bubbles rising from the seafloor off the Washington coast. The base of the column is 1/3 of a mile (515 meters) deep and the top of the plume is at 1/10 of a mile (180 meters) depth. Credit: Brendan Philip/University of Washington
14
October, 2015
Warming
ocean temperatures a third of a mile below the surface, in a dark
ocean in areas with little marine life, might attract scant
attention. But this is precisely the depth where frozen pockets of
methane 'ice' transition from a dormant solid to a powerful
greenhouse gas.
New
University of Washington research suggests that subsurface warming
could be causing more methane
gas to bubble up off the Washington and Oregon coast.
The
study, to appear in the journal Geochemistry,
Geophysics, Geosystems,
shows that of 168 bubble plumes observed within the past decade, a
disproportionate number were seen at a critical depth for the
stability of methane
hydrates.
"We
see an unusually high number of bubble plumes at the depth where
methane hydrate would decompose if seawater has warmed," said
lead author H. Paul Johnson, a UW professor of oceanography. "So
it is not likely to be just emitted from the sediments; this appears
to be coming from the decomposition of methane that has been frozen
for thousands of years."
Methane
has contributed to sudden swings in Earth's climate in the past. It
is unknown what role it might contribute to contemporary climate
change, although recent studies have reported warming-related methane
emissions in Arctic permafrost and off the Atlantic coast.
Of
the 168 methane plumes in the new study, some 14 were located at the
transition depth - more plumes per unit area than on surrounding
parts of the Washington and Oregon seafloor.
If
methane bubbles rise all the way to the surface, they enter the
atmosphere and act as a powerful greenhouse gas. But most of the
deep-sea methane seems to get consumed during the journey up. Marine
microbes convert the methane into carbon dioxide, producing
lower-oxygen, more-acidic conditions in the deeper offshore water,
which eventually wells up along the coast and surges into coastal
waterways.
Map
showing locations of the 168 bubble plumes used in the study. Credit:
University of Washington
"Current
environmental changes in Washington and Oregon are already impacting
local biology and fisheries, and these changes would be amplified by
the further release of methane," Johnson said.
Another
potential consequence, he said, is the destabilization of seafloor
slopes where frozen
methane
acts as the glue that holds the steep sediment slopes in place.
Methane
deposits are abundant on the continental margin of the Pacific
Northwest coast. A 2014 study from the UW documented that the ocean
in the region is warming at a depth of 500 meters (0.3 miles), by
water that formed decades ago in a global warming hotspot off Siberia
and then traveled with ocean currents east across the Pacific Ocean.
That previous paper calculated that warming at this depth would
theoretically destabilize methane deposits on the Cascadia subduction
zone, which runs from northern California to Vancouver Island.
At
the cold temperatures and high pressures present on the continental
margin, methane gas in seafloor sediments forms a crystal lattice
structure with water. The resulting icelike solid, called methane
hydrate, is unstable and sensitive to changes in temperature. When
the ocean warms, the hydrate crystals dissociate and methane gas
leaks into the sediment. Some of that gas escapes from the sediment
pores as a gas.
The
2014 study calculated that with present ocean warming, such hydrate
decomposition could release roughly 0.1 million metric tons of
methane per year into the sediments off the Washington coast, about
the same amount of methane from the 2010 Deepwater Horizon blowout.
The
new study looks for evidence of bubble plumes off the coast,
including observations by UW research cruises, earlier scientific
studies and local fishermen's reports. The authors included bubble
plumes that rose at least 150 meters (490 feet) tall that clearly
originate from the seafloor. The dataset included 45 plumes
originally detected by fishing boats, whose modern sonars can detect
the bubbles while looking for schools of fish, with their
observations later confirmed during UW research cruises.
Results
show that methane
gas
is slowly released at almost all depths along the Washington and
Oregon coastal margin. But the plumes are significantly more common
at the critical depth of 500 meters, where hydrate would decompose
due to seawater warming.
"What
we're seeing is possible confirmation of what we predicted from the
water temperatures: Methane hydrate appears to be decomposing and
releasing a lot of gas," Johnson said. "If you look
systematically, the location on the margin where you're getting the
largest number of methane plumes per square meter, it is right at
that critical depth of 500 meters."
Still
unknown, however, is whether these plumes are really from the
dissociation of frozen methane deposits.
"The
results are consistent with the hypothesis that modern bottom-water
warming is causing the limit of methane hydrate stability to move
downslope, but it's not proof that the hydrate is dissociating,"
said co-author Evan Solomon, a UW associate professor of
oceanography.
Solomon
is now analyzing the chemical composition of samples from bubble
plumes emitted by sediments along the Washington coast at about 500
meters deep. Results will confirm whether the gas originates from
methane hydrates rather than from some other source, such as the
passive migration of methane from deeper reservoirs to the seafloor,
which causes most of the other bubble plumes on the continental
margin.
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