UCI device probes ancient wine, climate

Fri, 02/11/2011

John Southon in the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory

Picture Credit: 
Pat Brennan, OC Register
Pat Brennan, OC Register
The Orange County Register, OC Science

Deep in an Armenian cave, copper-age vintners pressed and fermented their grapes for what were likely solemn occasions: Their production room was part of a large cemetery complex.

Four or five centuries later, someone lost a leather shoe near the entrance to the same group of caves shortly before they were completely abandoned.

Each became famous as the oldest item of their kind ever found. And UC Irvine’s Keck accelerator played an important role in confirming both discoveries.

The accelerator — a large, ring-shaped machine in a basement laboratory — takes samples of a variety of materials that have been converted into chunks of graphite, then blasts them to atoms.

By tracking the amount of a special atom, carbon 14, in each, the accelerator allows scientists to put dates on the samples — in the case of the Armenian wine press, from 6,100 years to 5,900 years ago.

The dates yielded by the UCI machine were checked against those from a similar device at Oxford University.

But archaeology is part-time work for the accelerator. Its main job is to track carbon in its various forms through land, ocean and atmosphere — unraveling secrets of Earth’s long-vanished climates, and sharpening understanding of present-day climate, including the potential effects of global warming.

“Most of what we do is related to climate change and the carbon cycle, and the interactions between them,” said John Southon, a UC Irvine Earth System Science researcher and one of the leaders of the Keck lab.

Southon and the scientists he collaborates with trace carbon through oceans, corals, wood, even the skeletons of microscopic animals that fall to the sea floor over millennia.

Carbon 14 is the key. Created when high-energy cosmic rays hit nitrogen particles in the atmosphere, it forms radioactive carbon 14.

Tracking carbon around the globe

That mixes into just about everything — air, water, plants, animals, human bodies.

The radioactive atoms decay at a known rate; by measuring how much carbon 14 is left in a sample, the scientists can use it as a kind of clock, revealing the sample’s age.

It can be used on samples as old as about 55,000 years.

Since firing up in 2002, the UCI accelerator has been put to fascinating use.

The ancient wine-press made headlines recently because of so much of it was preserved intact, complete with dessicated grapes and grapevines.

“What we really have is totally new, this absolutely excellent condition of preservation of organic material,” said Gregory Areshian, assistant director of the Cotsen Institute of Archaeology at the University of California, Los Angeles.

“This was a very large, copper-age cemetery 6,000 years ago, in which we have this wine production,” said Areshian, who helped investigate the site. “We have a very special use of wine — a ritual burial ceremony.”

The oldest known leather shoe, found earlier near the entrance to the cave complex that contained the wine press, is younger by 400 to 500 years, he said; the UCI accelerator helped pin down that age as well.

“The shoe immediately predates the abandonment of the cave, caused by a very major seismic event,” he said.

The UCI accelerator also weighs in on the sizzling topic of climate change.

When scientists were arguing about whether trees in the Amazon could reliably capture and store carbon in their tissues, perhaps helping curb global warming, the accelerator threw some cold water on the idea.

A 2005 study showed that many Amazonian trees, which were difficult to age because they had no rings, were improbably ancient — some as old as 1,400 years.

Some researchers had assumed that the ringless trees grew fast and died young. That would be good for carbon storage. But old, slow-growing trees would not.

The accelerator also played a role in demonstrating that simple corn husks could be used to measure how much fossil fuel is being burned in different parts of the planet.

Regions with a lot of burning of fossil fuels — oil, petroleum and natural gas — have relatively less carbon 14 in the air. The fuels have been buried so long that all its carbon 14 has decayed away.

That means corn husks from those regions have less carbon 14 in their tissues than corn husks from areas with little fossil-fuel burning.

It might sound trivial, but corn husks are much cheaper than automated air-sampling stations.

Lately, the scientists who work with the accelerator have been trying to solve a vexing problem that is keeping climate models from accurately portraying the distant past.

“If models can’t reproduce the past, it gives you a queasy feeling about their ability to predict the future,” Southon said.

The problem is a steep rise in carbon dioxide after the last ice age, some 18,000 years ago, revealed by analyzing ice cores. The levels have gone up and down over many glacial cycles.

At times of high levels of carbon dioxide, the planet gets warmer — as a large majority of climate scientists agree is happening now because of human emission of carbon dioxide and other greenhouse gases.

So where did the carbon go during the ice ages?

“The best explanation people could come up with was a bunch of carbon was sequestered in the deep ocean,” Southon said. “So now people have been looking for this deep old carbon reservoir.”

A popular idea was that the carbon was mostly sequestered in the southern ocean and the Pacific. That is, until Southon’s colleague, Ricardo De Pol-Holz, came along.

He used the accelerator to study sediment cores from the Pacific basin near Chile. If the idea was right, he should have found an abundance of ancient carbon.

“Unfortunately for what was actually a beautiful theory, or hypothesis, in the intermediate waters off Chile it was absolutely not present, any of that carbon,” De Pol-Holz said.

While De Pol-Holz said the finding does not jeopardize climate science of the present day, it frustrates scientists who wish to create models of climate in the distant past.

But then, those kinds of problems are why scientists like Southon got into the science business in the first place.

“The whole picture is still pretty murky,” Southon said. “But that’s what’s kind of fun. We’re in the middle of figuring this out.”