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April 23rd, 2009

Type of methane found in ancient ice is good news for planet

Vasilii Petrenko (INSTAAR postdoc) led a large international team in developing and applying a new technique for analyzing the carbon-14 content of methane in ancient Greenland ice. Their analyses suggest that a spike in the greenhouse gas methane about 11,600 years ago originated from wetlands rather than the ocean floor or from permafrost, a finding that is good news from a global warming standpoint.

The research team spent five field seasons extracting several tons of ancient ice from the western margin of the Greenland ice sheet, the largest ice samples ever recovered for a climate change study. The researchers cut the ice into blocks with electric chain saws, dumped 17 cubic feet at a time into a vacuum melting tank heated by powerful propane torches, and transferred ancient air released from bubbles in the ice into cylinders for subsequent laboratory analysis.

Carbon 14 of methane was measured in those cylinders as a "tracer" to date and distinguish wetland methane from methane clathrates, which are bound in ocean sediments and permafrost. As Earth emerged from the last ice age, temperatures in some places in the Northern Hemisphere shot up about 18 degrees Fahrenheit in just 20 years. Scientists have been concerned that such abrupt warming events could trigger huge oceanic methane "burps" caused by the dissociation of seafloor clathrates, providing a positive climate feedback mechanism that could drive up Earth's temperatures still further.

"If we found that clathrates release a lot of methane to the atmosphere during abrupt episodes of warming, that could signal big trouble for the planet, " said Petrenko. "But even though wetlands appear be the primary source, it's still something to be concerned about."

The team's research was published in Science on April 24.

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