Ice cores suggest methane spike may be caused by plants, not hydrates

A dramatic increase about 12,000 years ago in levels of atmospheric methane, a potent greenhouse gas, was most likely caused by higher emissions from tropical wetlands or from plant production, rather than a release from seafloor methane deposits, a new study concludes.

This research, to be published Friday in the journal Science, contradicts some suggestions that the sudden release of massive amounts of methane frozen in seafloor deposits may have been responsible – or at least added to - some past periods of rapid global warming, including one at the end of the last ice age.

The findings were made with analysis of carbon isotopes from methane frozen in Greenland ice core samples, by researchers from Oregon State University, the University of Victoria, University of Colorado, and the Scripps Institution of Oceanography at the University of California-San Diego.

For climate researchers, an understanding of methane behavior is of some significance because it is the second most important "greenhouse gas" after carbon dioxide. Its atmospheric concentration has increased about 250 percent in the last 250 years, and it continues to rise about 1 percent a year.

...researchers studied two stable isotopes of carbon found in methane, that can provide a better idea of where the methane came from during a period thousands of years ago when Earth was emerging from its most recent ice age, and entering the interglacial period that it is still in. At that time, methane concentration went up 50 percent in less than 200 years.

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"There have been estimates that releasing even 1 percent of the methane hydrates in the seafloor could double the atmospheric concentration of methane," said Ed Brook, an associate professor of geosciences at OSU and co-author on the study. "So we looked to the past to see if that may have happened during previous periods of rapid global warming."

Based on their isotopic analysis of the methane from the Greenland ice cores, the researchers concluded that it did not come from seafloor hydrate deposits or "gas bursts" of methane associated with them. The most likely candidates, they said, were higher emissions from tropical wetlands or larger amounts of plants, or some other combination of sources.

If the rise in methane had come from seafloor hydrate deposits, the study found, the atmospheric levels of methane would have had a different isotopic "signature" than they actually did.

...The current understanding of methane sources and sinks does not completely explain the isotopic signature of methane now found in the atmosphere. This indicates that estimates of methane emissions, including the human-made contribution, may have to be revised.

There are also concerns, they said, about methane trapped in permafrost across wide areas of the Earth's Arctic regions. There are significant amounts of methane found in this permafrost that could be released if it melted, and also organic material associated with melting permafrost that could cause further increases in methane. This might cause "a fairly significant rise in the total level of atmospheric methane of around 20 percent," Schaefer said.

Hinrich Schaefer, Michael J. Whiticar, Edward J. Brook, Vasilii V. Petrenko, Dominic F. Ferretti, Jeffrey P. Severinghaus 2006 Ice Record of 13C for Atmospheric CH4 Across the Younger Dryas-Preboreal Transition, Science 25 August 2006: Vol. 313. no. 5790, pp. 1109 - 1112

We report atmospheric methane carbon isotope ratios (13CH4) from the Western Greenland ice margin spanning the Younger Dryas–to–Preboreal (YD-PB) transition. Over the recorded 800 years, 13CH4 was around –46 per mil (); that is, 1 higher than in the modern atmosphere and 5.5 higher than would be expected from budgets without 13C-rich anthropogenic emissions. This requires higher natural 13C-rich emissions or stronger sink fractionation than conventionally assumed. Constant 13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.

Here we address these problems with measurements of isotopes of nitrogen and argon gas trapped in air bubbles in the GISP2 (Greenland Ice Sheet Project 2) ice core. Bubble 15N/14N and 40Ar/36Ar record a signal of rapid temperature change at the surface of the ice sheet

Seeing this abstract (anybody seen the complete study?) it does not appear that they corrected for this gravitational and molecular fractionation of the CH4 gas, did they?