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Wednesday, December 14, 2005

Antarctica

Antarctic Ice core samples reveal 2,000 years of climate history

PRESS RELEASE
Date Released: Monday, December 12, 2005
Source: The Antarctic Sun


By Emily Stone, Antarctic Sun staff

The five tents sitting on the white expanse of the Clark Glacier look more like a winter camping site than a scientific field camp. Skis stick out of the snow. A pot on the propane stove holds melted snow to make tea. An MP3 player pipes music into the cozy dining tent.

The camp's real purpose becomes clear inside the largest of the tents, where an ice-coring drill hums away, taking a 160-meter sample from deep inside the glacier.

Principal investigator Karl Kreutz will use the ice core to study the climate of the McMurdo Dry Valleys over the last 2,000 years. He'll compare that to climate data from different spots around the world during the same time period. This will help scientists better understand how changes in one region affect the climate in another, with the ultimate goal being to use information about the past to help predict climate changes in the future.

"You can really start to study how these regional climate patterns evolve," said Kreutz, of the University of Maine. "It would tell you something about how these different regions are connected."

Kreutz, two students and two drillers took the Clark Glacier core and a 140-meter core from the Upper Victoria Glacier this season. Last year, they took a core from the Commonwealth Glacier, as well as another core from the Clark Glacier. They will combine the information they retrieve from the cores to put together a picture of how the climate has changed there in the past two millennia, a time period known as the late Holocene era. He can date the different layers in the cores to within at least five years.

The ice cores will actually go back four or five thousand years, but that shorter time period is the focus for two reasons, Kreutz explained. The major factors that affect climate on a large scale - like the Earth's orbit around the sun or the number of volcanoes on the planet - have stayed constant over the last 2,000 years, which rules out blaming them for climate change.

There's also a lot of climate data from around the world for the late Holocene era from tree rings, lake sediment cores, farm records and diaries. This means that although there are still holes in the history of the Southern Hemisphere, regional climate data is fairly complete for the Northern Hemisphere. For example, it's known that there was a "Little Ice Age" in Europe between 1300 and 1850, Kreutz said. But it's unclear if there was a similar change in Antarctica. Determining this will shed light on whether the climates in Antarctica and Europe are connected and whether they influence each other.

The data will also help verify the computer models that scientists use to try to predict future changes. They test these models by trying to project backward in time and seeing if their models can recreate what happened in the past. The data from the cores helps them know if their models are accurate.

Kreutz will analyze two components of the core to determine what the temperature and atmospheric circulation patterns were in the Dry Valleys. The group looks at "proxy records" for climate, meaning they measure things that are influenced by temperature and not the temperature itself. One test will look at the dust in the core to figure out which way the winds were blowing at a given point. The other looks at isotopes in the ice, which indicate what the relative temperature was at the time.

The chemical composition of the dust indicates where the dominant winds were coming from. If the winds were coming from the east off the Ross Sea, they would have deposited a noticeable amount of sea salt in the snow. Westerly winds would have carried dust from the valley floors that has a different chemistry. Changes in the wind patterns affect temperatures in the Dry Valleys by bringing air from different regions.

The isotope analysis looks at the oxygen molecules in the ice. There are eight protons and eight neutrons in 99.9 percent of oxygen. But every now and then a couple extra neutrons will show up in a molecule. Kreutz will measure the ratio of the standard oxygen molecules to the ones with 10 neutrons.

The extra neutrons make the molecules heavier, which makes them harder to transport in the air. When it's cold, there isn't as much energy in the air to support the heavy molecules. The warmer it is, Kreutz said, "the more of the heavy little guys make it down here."

Kreutz's group has been testing these techniques - chemical analysis and dust composition measurements - in snow pits. They've dug pits at each of their coring sites, as well as on other glaciers in the area. They dig a two-meter-deep pit with another hole just behind it so that sun can shine in through one of the walls. Inside the pit, that wall is lit up so it's easy to tell one year from another.

Kreutz demonstrates the "finger test" against the wall. Winter snow is more dense, making it difficult to push through. The summer snow is more airy and his finger easily pokes into it. One solid layer plus one lighter layer equals one full year.

"It ends up being like a tree ring," he said. "You can just count back in time."

A weather station atop the Clark Glacier pit has been recording the weather and snow accumulation there for the past year. By cross referencing the weather data, the information about exactly when the snow fell, and the chemistry analysis of the dust and isotopes in that snow, the group has determined that their tests do show relative warming and cooling.

Kreutz will analyze the cores from this season when he returns to Maine.

Inside the big, orange tent, drillers Terry Gacke and Mike Waszkiewicz retrieve the core about half a meter at a time. The three-meter-long drill is lowered into the hole on a long cable. Gacke stops it at the right level, then starts the drill spinning. The drill's outer tube uses springs to anchor into the wall of the hole so it doesn't rotate. The inner tube spins to create the core. The men then pull the drill back up. Once it's clear of the hole, they rotate the tube 90 degrees so that it's parallel to the ground.

They then carry the inner tube outside where Kreutz waits at a table and they empty out its contents. Kreutz slips each section into a clear, plastic bag, labels the top and marks where it fits in the sequence. He staples the bag closed and stores the cores in a nearby pit until they can be shipped by helicopter back to McMurdo Station. He is often helped by students Bruce Williamson and Toby Burdet.

Kreutz holds up his latest core, which is about half a meter long. There are countless air bubbles inside, which are thousands of years old. Every now and then you can see bits of brown dust trapped in there, and can make out the slightly different colors of the summer and winter layers as well.

"You're looking for one smooth chunk of ice," Gacke said, referring to the core's exterior.

"The Holy Grail," Kreutz added.

NSF-funded research in this story: Karl Kreutz, University of Maine; http://climatechange.umaine.edu

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