Methane Found Bubbling Up from Arctic Sea

Greenhouse gas leaking from Arctic Ocean floor

Environmental Science and Technology, Sept. 16, 2009

Scientists have reported the presence of previously unknown sources of methane -- a greenhouse gas some 25 times more powerful than CO₂ at trapping heat -- bubbling up from the Arctic Ocean seafloor north of Norway. Gradual warming of a regional current has caused temperature-sensitive methane hydrate below the seabed to break down and discharge the gas, the researchers say.

For years, scientists have debated whether the planet's rising temperatures would turn methane deposits in permafrost regions into a -- ticking bomb -- that, once detonated, could liberate vast quantities of methane to the atmosphere, possibly triggering disastrous climate-feedback effects. Some paleoclimate studies have argued that such scenarios have occurred in the past, and that the processes of hydrate formation and disintegration have been a primary driver of glacial cycles.

Over the past couple of decades, as the tools for oceanographic exploration have grown more sophisticated, researchers have documented about 90 oceanic locations of methane hydrate, estimated to contain as much as 63,000 gigatons or more of carbon. Previously, International Polar Year (2007) surveys of the East Siberian Arctic shelf uncovered abundant methane seeps and measured record-breaking summertime concentrations of the gas in northern polar waters.

Hydrate usually forms in sediment beneath the seabed and is stable at depths below 300-500 meters (m), depending upon temperature, pressure, salinity, and the types of gases present, according to Graham Westbrook of the University of Birmingham (U.K.). However, on a research cruise in 2008, Westbrook and colleagues collected sonar images of more than 250 plumes of methane gas rising from the seafloor at depths ranging between 150 and 400 m. They found these plumes along a section of continental margin washed by the West Spitsbergen Current (WSC), an arm of the Gulf Stream that delivers Atlantic seawater to the Arctic. As the WSC has warmed by 1 °C over the past 30 years, the depth at which hydrate in the area is stable has fallen from 360 to 396 m, liberating methane, Westbrook says.

The plumes averaged several meters in diameter, with the largest reaching within 50 m of the sea surface. The researchers have not yet calculated the actual volume of gas being released. However, on the basis of previous studies of hydrate concentrations in the area, they estimate that the 30-kilometer-long zone of plume occurrence could be losing about 27 kilotons per year from dissociating hydrate.

"If this process [of hydrate dissociation due to rising ocean temperatures] becomes widespread along Arctic continental margins, tens of megatons of methane per year -- equivalent to 5 to 10% of the total amount released globally by natural sources -- could be released into the ocean," Westbrook warns. Although most, if not all, of the methane is dissolving in the seawater, a tiny fraction of it transfers to the atmosphere by equilibration. Furthermore, the dissolved methane lowers oxygen levels and contributes to ocean acidification, the researchers note.

An assessment by Matthew T. Reagan and others of Lawrence Berkeley National Laboratory concluded that rapid discharges of methane are possible for shallow-lying hydrates in both warm and cold regions. If Arctic hydrates prove as widespread as some evidence indicates, the assessment suggests, this could pose a particular threat to regional or even global ecology. "The [Westbrook] study is noteworthy in its documentation of significant methane releases occurring at locations corresponding to the limit of gas hydrate stability for a system that has seen documented temperature changes at the seafloor," Reagan says. "This is a good example of what methane release due to climate change might look like," he contends.

Veteran hydrate researcher Peter Brewer of the Monterey Bay Aquarium Research Institute says, "The depths at which the plumes have been detected [in the study] are consistent with hydrate dissolution, but it's not proof that climate change has caused this. Modeling shows that it takes a very long time for heat to penetrate deep into ocean sediments. So this is not yet a smoking gun."

However, Brewer points out, if widespread hydrate dissolution does happen, another ominous consequence could follow: if ocean floor sediments become increasingly gas-saturated, they will likely turn highly unstable, so that earthquakes are more likely to unleash tsunami-like landslides.

Copyright © 2009 American Chemical Society