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Ice cores disagree on origin of White River ash deposit

15.12.2004


One anticipated component missing from an ice core drilled through a high-mountain, Alaskan ice field may force researchers to rethink the geologic history of that region.



Ohio State University scientists had expected to find a thick layer of volcanic tephra - evidence of a massive historic eruption - near the bottom of core they drilled between Mount Bona and Mount Churchill, both ancient volcanoes, in southeast Alaska’s St. Elias Mountain Range. That tephra layer would provide new evidence that Mount Churchill had been the source of an eruption that blanketed hundreds of thousands of square miles in the Pacific Northwest, creating a deposit known as the White River Ash.

The problem is that the ice core contains no ash layer. “Our drill site was so close to the crater of Mount Churchill that if it had erupted in 803 A.D., then ash would have been preserved somewhere in our record in the core,” explained Tracy Mashiotta, a research associate with the Byrd Polar Research Center. “We drilled all the way through the glacier to the bedrock below and didn’t find any ash.” “Without a visible ash layer in the core, we don’t believe that Mount Churchill could have been the source for that deposit.” explained Lonnie Thompson, professor of geological sciences and researcher with the Byrd Polar Research Center.


They reported their findings today at the meeting of the American Geophysical Union in San Francisco. The researcher’s conviction lies with the analysis of six cores they recovered in 2003 from an ice field in a saddle between the two peaks. Along with five shallow cores ranging from 10.5 meters (34.4 feet) to 114 meters (374 feet), the researchers drilled a complete 460-meter (1,509-foot) core through the ice to bedrock, capturing a climatological record preserved in the ice.

The five shallow cores contain a record ranging from eight to 64 years. The long core - itself the longest ever drilled through a mountainous glacier – dates back nearly 1,500 years. And that presents a problem for the current interpretation of geology in the region. Researchers have dated the two White River Ash deposits to two eruptions in the last two millennia - one in approximately 803 A.D. and the other in around 63 A.D – that were thought to come from the same volcano. And many scientists have suspected Mount Churchill was the site of those eruptions. If so, those blasts would have deposited layers of volcanic ash at the top of the mountain, including on the ice present at the time. And over time, that layer would have been buried deep in the ice.

Expecting to find it, the OSU team had engineered a new drill bit for their drill capable of piercing the expected one-meter-thick (3.2-foot) ash layer. But it wasn’t there. “Both ash deposits have been thought to come from the same volcano,” Mashiotta said. “If we didn’t see evidence from the ice core pointing to an eruption in 803 A.D., it’s unlikely that Mount Churchill was the source of the earlier deposit from 63 A.D.

Besides the missing ash layer, the core offered more evidence that the White River Ash wasn’t born on Mount Churchill. Towards the bottom of the long core, they discovered layers of pebbles trapped in the ice. The pebbles were made of granodiorite, a non-volcanic rock. In fact, granodiorite is widely seen as a marker that shows the passing of the massive ice sheets that once blanketed the Western Cordillera of North America. Thompson said the bottom two meters (6.5 feet) of the core contained numerous layers of these pebbles as well.

The research team serendipitously arrived at a time when the surface at the summit of Mount Churchill - near the crater - was free of snow cover. There they found large granodiorite stones and boulders. Had the mountain erupted since they were deposited, Thompson said, they would have been covered by deep ash. 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.

Another clue was missing from the core that might have pointed to Mount Churchill as the source of the deposit. They found distinct markers of high sulfate content at various points in the core, traceable to several famous eruptions but nothing traceable to Churchill. “We found sulfate from the eruptions of Katmai in Alaska in 1912, from Tambora in Indonesia in 1815 and from Laki in Iceland in 1783,” Mashiotta said. “If in 803, there was a big eruption of Mount Churchill 300 meters from our drill site, we would have seen sulfate there. And we didn’t.”

The core contains other “gems” as well as clues to the White River Ash. The climate record includes the period of the “Little Ice Age,” a period from 1450 A.D. to 1850 A.D., when the climate dramatically cooled, as well as evidence of “Medieval Warm Period,” from 1000 A.D. to 1400 A.D. The core should even provide a long-term history of dust blown from north-central China across the Pacific Ocean to fall on the Alaskan mountain.

Along with Mashiotta and Thompson, Mary Davis, a research associate with the Byrd Polar Research Center, is working on this project. The National Science Foundation supported this research.

Lonnie Thompson | EurekAlert!
Further information:
http://www.osu.edu

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