Latest ice core may solve mystery of ancient volcanic eruptions

A team of Ohio State University researchers has returned from an expedition in southeastern Alaska with the longest ice core ever drilled from a mountainous glacier.

The core measures 460 meters (1,509 feet) and is 150 meters (492 feet) longer than the previous longest core – a record of ice from the Guliya ice cap in western China that eventually relinquished a climate record stretching back 760,000 years – the oldest such record retrieved to date.

Until the new core is analyzed in detail, scientists won’t know if it exceeds the historic record provided by the Guliya core, but visible evidence in the new core itself suggests that the ice might reach back through several ice ages.

In April, Lonnie Thompson, professor of geological sciences at Ohio State, led this expedition – his 44th – to an ice cap that straddled the col, or saddle, between Mount Churchill and Mount Bona in the Wrangell-St. Elias National Park on the U.S.-Canadian border.

Mount Churchill is a 15,638-foot (4,766-meter) stratovolcano while Mount Bona is its 16,420-foot (5,005-meter) neighbor. The drill site on the col stood at an altitude of 14,500 feet (4,300 meters).

The core also revealed evidence that suggests geologists may have to rethink their understanding of the volcanic history of this region.

For years, scientists have believed that Mount Churchill was the source of a massive deposit of tephra, or volcanic ash, called the White River Ash that rose from at least two eruptions within the last 2000 years. That blast released between 25 and 30 cubic kilometers of ash across at least 340,000 square kilometers (131,275 square miles) in northwestern Canada and adjoining eastern Alaska.

Thompson’s team expected to find a layer of White River Ash several meters thick in the core since Mount Churchill, sitting adjacent to the drill site, is the believed source. They were shocked to discover that the ice core contained no such ash layer, making it unlikely for Churchill to have been the cause of the deposit.

“There simply was no ash there,” Thompson said. “To me, that argues that this mountain couldn’t have been the source and that instead, Churchill may have served as a barrier to the ash that was coming from another source.”

As if the absent ash layer wasn’t enough evidence, Thompson’s team found more obvious clues at their feet early in the expedition. On the ground surrounding the crater rim at the top of Mount Churchill, they found boulders and rocks composed of granodiorite, an igneous rock that is not produced by volcanic eruptions.

“These granodiorite pieces at the crater rim are sculpted by ice. These aren’t fragments — they are smooth boulders. This kind of rock shouldn’t be present at the crater rim,” Thompson said.
“If there had been an explosion at Mount Churchill large enough to have produced the White River Ash, it would have either blown the rocks away or buried them deeply.”

The discovery of the granodiorite evidence at the crater rim may have been serendipitous. That area is normally covered under deep layers of snow most of the year and clears only for a few weeks at the end of winter. The Ohio State team just happened to reach the site when the ground was clear and the rocks were visible.

“A few weeks later and all of them would have been covered, he said. “That’s probably why no one has noticed this evidence before.”

Even more surprising for expedition members was the discovery of several layers of pebbles of this same granodiorite in the bottom portion of the core. At a depth of 432 meters (1,417 feet), the team hit two layers of pebbles in the ice, separated by about seven centimeters (2.75 inches) of clean ice. They also encountered numerous pebble layers in the last two meters (6.5 feet) of the core.

While a precise explanation for these pebble layers must await a detailed analysis of the core, Thompson suggests that they may be evidence that massive ice sheets had overrun the mountain in the past.

During the last ice age, the eastern United States was covered by the Laurentide Ice Sheet while the American west was blanketed by the 4,000-foot-thick Cordilleran Ice Sheet. As these massive sheets moved, they scraped off stones from the underlying rock and transported them for thousands of miles.

Thompson believes both the stones at the crater rim and the pebble layers are remnants of those ice sheets. And the fact that they found multiple layers in the core suggests the area had been overrun by ice several times. Once the ice in the core is dated, they should have a better idea of when the layers were formed.

The Ohio State team returned with six cores altogether, all packed in 105 insulated boxes each containing six meters (19.6 feet) of ice. Along with the longest 460-meter core, they drilled a 116-meter (380.5-foot) core that will provide a complementary record to confirm the upper part of the longer core, and together both cores are expected to provide a climate history for this region reaching back thousands of years. They also drilled four 12-meter (39.3-foot) cores to help determine the snow accumulation rate at the site.

The Office of Polar Programs in the National Science Foundation supported the project.

Contact: Lonnie Thompson, (614) 292-6652; or thompson.3@osu.edu.
Written by Earle Holland, (614) 292-8384; Holland.8@osu.edu.