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March 15th, 2011

‘Horizontal ice core’ shows history of atmospheric methane

Photo by Robb Kulin.

INSTAAR postdoctoral researcher Vasilii Petrenko was interviewed by Public Radio International (PRI) while helping drill shallow ice cores on the Taylor Glacier in Antarctica.  The interview opens with a bit of fun, listening to the eerie pinging sounds created by dropping leftover ice blocks back into a borehole 20 m (70 ft) deep.

Petrenko was part of a team that drilled numerous and unusually large ice cores (9.5 in / 24 cm diameter) and melted them in the field to collect ancient air trapped inside the ice. This air contains a record of past atmospheric composition, including methane, a potent greenhouse gas. The ancient air was shipped to the United States for laboratory analyses that will include the carbon-14 content of methane. Petrenko's collaborators in this project include Jeff Severinghaus (UC San Diego) and Ed Brook (Oregon State).

Photo by Robb Kulin.

The team collected ancient air from boreholes spread along the ice surface in Taylor Glacier’s unusually long ablation zone –where ice is being removed from the surface either by sublimation or melting.  Ice in this zone formed long ago and far up the glacier, and is very old by the time it flows downhill and becomes exposed at the surface. The team surveyed more than 20 km (12.5 miles) of the ablation zone during their first field season (2009/2010).  They found the age of the ice surface changes with distance, spanning a time interval from 8,000 to 70,000 years ago. In this way, Taylor Glacier presents an opportunity to collect a “horizontal ice core”.

In a regular, vertical ice core (such as the WAIS Divide core that INSTAAR scientists like Jim White are also working on), ancient ice is laboriously recovered from a glacier’s accumulation zone – where snow is being added to the surface and slowly compressed into ice. Drilling downward as deep as 2 miles below the surface of the accumulation zone takes great effort. In contrast, the ablation zone of Taylor Glacier exposes ice of many of the same ages right at the surface. What’s more, it exposes essentially unlimited amounts of it (as compared to relatively small amounts recovered from vertical ice cores). This allows the team to analyze gases present in ultra-trace concentrations, such as carbon-14 of methane.

Petrenko participated in a similar project in Greenland from 2001-2005 that used chain saws to collect ice samples of sufficient size. Subsequent analysis of carbon-14 of methane in the Greenland ancient air samples suggested that a spike in the greenhouse gas methane about 11,600 years ago originated from wetlands rather than the ocean floor or from permafrost.

Petrenko and his colleagues want to verify their Greenland-based results at Taylor Glacier, which has the added advantage of cleaner ice samples, with less organic matter.  The Antarctic team is interested in not only the abrupt spike in methane from 11,600 years ago but an earlier spike from about 15,000 years ago. Their analyses of carbon-14 of methane will provide a more definitive test of whether melting methane hydrates from the seafloor were chiefly responsible for these spikes in methane as the last Ice Age waned and the world warmed.

The team’s results could also tell us something about what might happen in the future.  As Petrenko pointed out to the PRI radio crew, the ancient methane spikes provide insight as to whether future warming will destabilize methane hydrates and release methane into the atmosphere.   And that’s particularly important because methane is itself a potent greenhouse gas. More methane in the atmosphere leads to more warming, which could in turn lead to the release of more methane.

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