Quakes may help release methane

Major earthquakes can topple buildings, cause landslides and spawn
tsunamis. Now scientists say they can do something else: set off the
release of methane gas from the seabed.
In a study published in thejournal Nature Geoscience, European
researchersreportthat an underwater quake off Pakistan nearly 70 years
ago likely fractured seafloor sediments and created pathwaysfor
methane, a potent greenhouse gas, to bubble up from below. The
researchers say the phenomenon may be widespread enough that climate
scientists should takeit into account when estimating the amounts of
heat-trapping gases in the atmosphere.
"We suggest there is a new source that they might want to consider in
the future," said David Fischer, a postdoctoral researcher at the
University of Bremen in Germany and the lead author of the study.
Methane, which is formed by the decomposition of organic material,
seeps from reservoirs under the seafloor in many places. But under
certain conditions it mixes with seawater to form icy compounds,called
gas hydrates, in the top layers ofsediments. The hydrates act almost
like cement, creating a barrier that prevents more free methane from
coming up from below.
Fischer and his colleagues analyzed sediment cores taken in 2007 from
two locations in the northern Arabian Sea where hydrates were present
and seepage was occurring. They found chemical signatures in the cores
suggesting that the methane flow greatly increased sometime in the
mid-20th century. Looking through seismic records, Fischer found that
a magnitude 8.1 quake occurred in the area in 1945. The quake
wascentered less than 15 miles from where the cores were taken, and a
resulting tsunami killed up to 4,000 people.
The conclusion was inescapable, Fischer said: "The quake broke open
gas-hydrate sediments and the free gas underneath migrated to the
surface."
The hydrates themselves did not dissolve.
"They remain there," he said.
Fischer said the researchers chose the core locations in the Arabian
Sea because they wanted to get a better understanding of how methane
seepage was related to tectonics, and the area is in an active zone
where one of the Earth's tectonic plates slides beneath another. But
they were not thinking about the effect of individual earthquakes, and
his discovery of the 1945 quake in the records"was probably a moment
I'll never forget," he said.
The upward flow of methane is continuing, and the researchers do not
know when it might stop. All told, they estimate that nearly 10
million cubic yards of methane have been released from the core sites.
But that is a conservative figure, Fischer said, because immediately
after the quake the flow would have been much higher.
Joel E. Johnson, a geology professor at the University of New
Hampshire who was not involved in the study, said the conclusion that
quakes can set off methane releases "does seem solid." Apart from
monitoring gas release in real time during an earthquake — a practical
impossibility — "this is about as close as you're going to get" to
proving that it occurs, he said.
-New York Times Service