READING ROCKS: Chemical clues about global warming processes can be found in the remnants of Ice Age glaciers.
ISTOCKPHOTO/CASCOLY

Aptly named for its location behind a ball field in New York City's Central Park, Umpire Rock may offer a useful vantage point for calling balls and strikes. For scientists, however, it has served as a speed gun for calculating the trajectory and timing of an ancient glacier that once played an active role in global climate change.

"The Laurentide coughs and the climate will change," says Joerg Schaefer, a geochemist at Columbia University's Lamont–Doherty Earth Observatory.

Schaefer refers to the Laurentide Ice Sheet that covered the island of Manhattan, along with the northern third of the U.S. and most of Canada, until about 18,000 years ago when the great glacier finally retreated. It had spent more than 70,000 years affecting and reflecting the world's weather through periods of melting and growth. Today, only carved terrain and rocky remnants remain, including the popular leftover that lies a short walk east of West 62nd Street.

Umpire Rock is just one of many enormous boulders—from Antarctica to New Zealand—created under the weight and movement of glacial ice. With increasingly sophisticated techniques, Schaefer and other scientists are more closely studying the chemical footprints on these rocks, thereby gaining valuable insights into climate change.

"Glaciers are great climate indicators," says Richard Alley, a glaciologist at The Pennsylvania State University, in an e-mail. "Although they respond to many things (such as increased snowfall, for example), they are controlled primarily by changes in summer temperatures. Warming melts ice—and it is almost that simple."

A glacier's response to climate can be very sensitive to even the slightest temperature fluctuations: "Just one degree [Celsius] has forced glaciers to move back and forth," Schaefer says. As researchers determine precisely when and where glaciers have advanced and retreated, they can add to a global map of summer temperatures during the Holocene epoch, which spans from 10,000 years ago, after the last ice age, to today. Schaefer thinks a better understanding of variations in this era could fill in missing key predictors for Earth's future climate. "We can only evaluate how dramatic changes will be if we know this natural variability," Schaefer says. "Everything humans do to the climate now is on top of this."

Shedding new light on the climate
Schaefer and other scientists are using a combination of old stonemason tools and cutting-edge machinery, along with some basic geochemistry, to decipher and date this distribution of ancient temperatures.

When a glacier starts its retreat, it exposes the surface it had entombed to daylight. This accumulation of debris, known as a moraine, is often identified when geologists see large boulders that were formed as a result of sediment compacted over thousands of years under a glacier's weight. Cosmic rays zipping through Earth's atmosphere begin bombarding these previously protected rocks at the moment they are coughed out from under the receding ice sheet. The collisions trigger chemical reactions that form a unique radioactive element, beryllium 10. Scientists can then count these newborn nuclides, or isotopes (an element whose atoms have a different number of neutrons, giving it slightly different properties from the common variety), using a technique known as cosmogenic dating, to determine exactly when the glacier pulled back and the bombardment began. "The glory of cosmogenic dating is that you can ask a rock how long it has been sitting there being zapped by cosmic rays," Alley notes.

To get an answer from the rock, the nuclide is isolated from a chiseled sample—about the top centimeter on its surface—and then analyzed via mass spectrometry. With chemists' ability to isolate pure beryllium 10 and spectrometric sophistication growing over the last decade, Schaefer says, "tiny, tiny amounts of these nuclides can now be measured with very high precision."

The total count is then compared with the background rate at which the beryllium 10 is naturally produced in the atmosphere, and a precise age is derived. In the best cases, Schaefer says, a date can now be stamped with an error of plus or minus 10 years. (Only one mass spectrometer accelerator, located at the Lawrence Livermore National Laboratory in California, is currently powerful enough to provide these kinds of estimates.)

Cosmogenic dating has been around since the 1980s, but this improved sensitivity is allowing the possibility of dating much younger rocks that have had less time to accumulate beryllium 10. "The isotope records now overlap our historic record," Schaefer says. "This opens up a climate record that we couldn't read out before."

Tree rings, ice cores and marine life also offer glimpses into climate's past, yet they are not always available, nor always precise. "You could get lucky and run into a tree stump, but most glaciers leave deposits that are always traceable by this technique," says John Stone, a geochemist at the University of Washington in Seattle who uses cosmogenic methods for glaciers in Antarctica. "This allows a wide variety of glacial records to be obtained."

Global data set in stone
Schaefer wants to see the improved dating method taken around the world, and has been chiseling rocks himself from New York to New Zealand. He is the lead author of a study published in the May 7, 2009, issue of Science that investigated the ebbs and flows of glaciers in New Zealand's Southern Alps.

Unlike the majority of the world's glaciers, some in this region are advancing, rather than retreating. But hope that this glacial behavior means a less dire global climate predicament is arrested by Schaefer's findings: Glaciers in the Northern Hemisphere seem to behave independently from those in the south, and these "growing glaciers" have been expanding at an increasingly slower rate over the last several thousand years.