By Thomas Guengerich
Right: Nelia Dunbar explains the finer details of her electron microprobe laboratory at New Mexico Tech.
SOCORRO, N.M., Aug. 21, 2008 – New Mexico Tech researcher Nelia Dunbar is uncovering ancient secrets about global climate by studying the chemical composition of volcanic ash in the West Antarctic Ice Sheet.
Dunbar, a geochemist and lab director at New Mexico Tech, presented her findings at an Antarctic Earth Science meeting in Santa Barbara in August 2007, and again at a Geochemical Society meeting in Vancouver in early July 2008. Along with fellow researchers at New Mexico Tech and the University of Maine, Dunbar is using her laboratory sleuthing skills to find the record of volcanic eruptions in ice that preserves a record of fluctuations in global temperatures as far back as 100,000 years ago.
From November 2007 to January 2008 – that’s summer in Antarctica –an army of scientists, engineers, technicians and students extracted a cylinder of ice from the 11,000-foot-thick ice sheet. The ice core is like a living record of precipitation, volcanic eruptions, greenhouse gases and other naturally-occurring atmospheric particles.
Left: Nelia Dunbar points out the graphic interface software that makes the electron microprobe very user-friendly.
Dunbar’s specialty is volcanology. She is using her knowledge of volcanic activity and processes and her laboratory acumen to help the nation’s community of geologists and geophysicists deduce a broad picture of Earth’s climatic movements over the eons.
“We’re looking at specific layers of dust from specific volcanic eruptions,” Dunbar said. “This ice core contains a frozen record of the earth’s atmospheric history.”
The ice core is expected to be the first section of an 11,000-foot column of ice detailing 100,000 years of Earth’s climate history. So far, the first core of 1,800 feet has been drilled and transported to the National Ice Core facility in Denver.
The top section of ice can be visually broken down to year-by-year layers going back about 40,000 years. Below the first few hundred feet, however, the ice is too compressed to visually count the layers. At that point, other methods must be used to determine the age of the ice. One of the methods is to identify volcanic ash layers using an electron microprobe to identify the chemical profile of ash.
One main reason for drilling the ice core is to examine the history of atmospheric carbon-dioxide content and global temperatures over the last 100,000 years.
“The ice core contains a beautiful, detailed climate record,” Dunbar said. “But we need to know when changes in the climate happened. The volcanic record helps us to understand the chronology of the core.”
Right: Tech researchers – and husband-and-wife team – Nelia Dunbar and Bill McIntosh huddle in an ice cave on Antarctica.
New Mexico Tech researchers Matt Heizler and Bill McIntosh have built a state-of-the-art Argon Geochronology Lab, and one of the specialties of the lab is to date volcanic rocks,– by vaporizing the rocks with a laser beam.
When volcanic rocks are formed, many contain crystals that are made, in part, of the element potassium. Over time, the potassium decays to an isotope of argon at a known rate. By vaporizing the crystals and releasing the argon, McIntosh can analyze the argon to determine a rock’s age. From a similar sample, Dunbar can determine the chemical composition of rocks associated with specific eruptions because each volcanic eruption has its own chemical signature.
The new WAIS ice core contains ash particles that are far too small and sparse to be dated in the Argon Lab. However, the chemical composition of the ash particles can be measured, and can be matched to ashes sampled found near the source volcanoes, where the deposits are thick and can be dated in the New Mexico Tech Argon Lab. Only Dunbar’s lab sleuthing can truly determine the chemical composition – and, hence, the age – of the deeply buried layers of volcanic ash in the ice. From ash pieces as small as 10 microns (1/100th of a millimeter) found in the ice core, she can determine the chemical composition of the ash, then associate that composition with a known, dated, volcanic eruption in West Antarctica.
“It’s a puzzle and that’s why a lot of science is intriguing,” Dunbar said. “We gather information, put it together and figure out something that you can’t observe directly. By putting together the pieces of the puzzle that we find in this ice core, we can learn about ancient occurrences.”
Nelia Dunbar talks on the radio while surveying a blue ice sheet on Antarctica.
Dunbar said the West Antarctic Ice Sheet Divide is one of the best spots on the planet to recover ancient ice containing trapped air bubbles – samples of the Earth's atmosphere as old as 100,000 years.
Finding good places to sample volcanic ash near a source volcano in West Antarctica can be challenging, because of the thick ice cover over most of the area. However, at certain locations on the ice sheet, ancient deposits of volcanic ash have been lifted to the surface by natural flows. In the summit crater of an extinct West Antarctic volcano, Mount Moulton, Dunbar and colleagues located and sampled a section of such ice.
“It’s the Rosetta Stone of volcanic ash,” she said. “We found a blue ice field on Mount Moulton where the ice has captured evidence of 40 separate and distinct eruptions of a nearby active volcano, Mount Berlin.”
Using the New Mexico Tech argon lab, McIntosh has dated a number of well-preserved crystals.
Scientists at the University of Maine are slowly, painstakingly dismantling and analyzing the ice core. When they find a layer of ash, they preserve a piece for Dunbar.
“This ice is very precious,” Dunbar said. “They can’t give me a huge section. They take a little portion of ice from that horizon of the core, melt it, filter it and send me the filter paper which has trapped the volcanic ash.”
Dunbar can then remove the ash and place it in an epoxy disk about 1-inch across. She then polishes the disk, which can be placed in the electron microprobe.
That device works like a microscope, but uses electrons instead of light. A microprobe can determine the chemical composition of a 1 micron spot on a sample – a feat no other instrument can do. The electron microprobe also can show spatial resolution of chemical variability on a sample surface.
In 1996 New Mexico Tech became the first university in the nation to install a Cameca SX-100 electron microprobe. The $400,000 instrument – which costs about $750,000 today – is now a staple of research universities. The device functions as both a microscope and a mass spectrometer, providing an image of tiny objects and determining the objects’ chemical composition.
Dunbar needs at least six ash fragments from one layer to get a good chemical fingerprint. She prefers to analyze up to 30 fragments to minimize the possibility of natural contamination.
“We want a good, homogeneous, population of ash to look at the chemical composition,” Dunbar said. “If you find a single, random ash shard from South America, you would get the wrong impression. This shard might have been blown in by wind, rather than being brought by the volcanic eruption. Some eruptions have a very simple chemical composition, while others have a range. We can look at a chemical analysis and say it’s homogenous or we might find a sample that's chemically variable."
Dunbar is using her laboratory wizardry to help the scientific community expand its understanding of the past.
Scientists from more than 30 universities and government agencies around the nation are studying different aspects of the WAIS ice core – natural dust, biological materials, carbon-dioxide levels, methane levels and many other aspects.
“Each investigator offers information on a different aspect of the core,” Dunbar said. “You put it all together and you get a very complete picture of what Earth’s climate was doing. … No one person can do all that. That’s why it’s so multi-faceted.”
Together, they are putting together a large puzzle. In October, they will all convene in Denver to share their preliminary findings at the West Antarctic Ice Sheet Divide Ice Project.
“Scientists get together and share papers. There’s a huge amount of discussion. It’s exciting because we’re presenting data that is hot off the presses,” she said.
While other ice cores from Greenland have been used to develop longer records of Earth’s atmosphere, the record from Dunbar’s expedition will allow a more detailed study of the interaction of previous increases in greenhouse gases and climate change. Eventually, this information will improve computer models that are used to predict how the current high levels of greenhouse gases in the atmosphere caused by human activity might influence future climate, she said.
– NMT –