Study on Methane Gas Emission from Deep-Sea Sediments Released
Earthquakes may play a larger role in triggering the release of greenhouse gases, like for instance methane, than previously believed. In a study, which was published online in Nature Geoscience on July 28th, an international team of scientists investigated the aftermath of a magnitude 8.1 earthquake that took place in the Northern Arabian Sea in 1945.
They postulated that this event caused the release of about 7.4 million cubic meters methane, into the ocean. In 2007, during a research cruise off the coast of Pakistan, the scientists obtained several sediment cores. One of these cores contained methane hydrates, a solid ice-like structure of methane and water, just 1.6 meters below the sea floor. Investigations of these cores enabled the scientists to relate the 1945 earthquake to the concomitant release of methane.
A team of scientists from the MARUM Institute at the University of Bremen, the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Bremerhaven, and the ETH Zürich investigated hydrocarbon cold seeps at the Pakistani continental margin. During their expedition with the research vessel METEOR the researchers extracted sediment core samples, which they closely investigated in the lab.
“First, we examined the pore water, which is the water between sediment grains in the core”, explains first author Dr. David Fischer from MARUM. “At two separate coring sites, one with hydrates the other without, we found unusual pore water sulfate profiles indicating a substantial increase in upward methane flux in the recent past.”
The presence of methane at shallower sediment depths was further corroborated by enrichments of the mineral barite at these depths. These barite enrichments were calculated to have started to form between 1916 and 1962. “Both the sulfate and the barite analyses indicated that in the recent past, something must have amplified the methane flux from below. We started going through the literature and found that a major earthquake had occurred close-by in 1945,” explains Fischer. “Based on several indicators, we postulated that the earthquake led to a fracturing of the sediments, releasing the gas that had been trapped below the hydrates into the ocean.”
The conservative estimate of the methane released since the earthquake, not taking into account how much was discharged directly after the quake, is equivalent to roughly 7.4 million cubic meters of methane gas at standard conditions at the earth’s surface, which equals ten large gas tankers. “There are probably even more sites in the area that had been affected by the earthquake”, says the MARUM scientist.
This study sheds light on yet another natural trigger of greenhouse gas release from marine sediments. In order to formulate more accurate estimates of the global carbon budget, all sources need to be accounted for. “The Intergovernmental Panel on Climate Change (IPCC) calculates the planet’s carbon budget by estimating all possible natural sources of carbon to the atmosphere, and we now provide a new mechanism of carbon export that had not been considered before,” explains Fischer.
Press Release, July 29, 2013