Quakes may help release methane
In a study published online recently in the journal Nature Geoscience, European researchers report that an underwater quake off Pakistan nearly 70 years ago likely fractured seafloor sediments and created pathways for methane, a potent greenhouse gas, to bubble up from below. The researchers say the phenomenon may be widespread enough that climate scientists should take it 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 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 of sediments. The hydrates act almost like cement, creating a barrier that prevents more free methane from coming up from below. Fischer and his team analysed 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 was centered 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.
Scientists have studied how factors like increasing water temperatures could affect the flow of methane from the seabed, Johnson said, but those kinds of changes occur over very long periods of time. “A big part of this is the question of whether the methane gets to the atmosphere,” he said. Unless the rate of flow is fast enough, methane that bubbles from the seafloor usually dissolves in the seawater before reaching the surface, he said.