| Dr. Nelia Dunbar in her electron microprobe lab at the Bureau of Geolog on the campus of New Mexico Tech.
| Dunbar in Antarctica during a previous expedition.
| Dunbar processing an ice core from a previous project.
Logan Mitchell, from Oregon State University, saws a section of an ice core destined for gas measurements. Photo by Peter Rejcek/National Science Foundation
Each silver tube on these shelves contains a one-meter long section of the WAIS Divide ice core. This repository is at the National Ice Core Laboratory in Lakewood, Colo. The NICL freezer is kept at -36 degrees C, to preserve the integrity of the ice. Photo by Peter Rejcek/National Science Foundation
Dunbar’s latest project is part of her ongoing “tephrochronology” research work, in which she dates and characterizes the ash layers – or tephra – that is found in the 3,300-foot-thick ice of West Antarctica.
The field of Antarctic tephrochronology has been progressing steadily, and is on the cusp of having a fully integrated tephra framework for large parts of the continent, Dunbar said.
“The tephra framework is mature in the western U.S., meaning that, through detailed work by many scientists, we know the source volcanoes, eruptive age, and geochemistry for most of the major volcanic ashes. ” Dunbar said. “We are finally getting to that point in Antarctica, but it has taken many years of work by many researchers. We’re moving toward really understanding the big important ash layers, where they came from and how old they are.”
On a previous NSF grant, Dunbar examined the tephra records in the top 600 meters of the ice core, which represents about 2,000 years of ice. With the new grant, she will examine the remainder – another 2,700 feet of ice, which represents about 60,000 to 70,000 years of Earth history.
The new grant will permit Dunbar to hire a doctoral student to assist in the study. Nels Iverson, a master’s student in geochemistry, will help with the lab work and analysis. Iverson recently returned from Antarctica, where he collected samples of volcanic rock and ice for his master’s thesis.
“This is a very interesting project,” Iverson said. “The WAIS Divide Ice Core is deep and has lots of different tephra layers. We’ll be doing different chemical analyses to figure out where they’re from.”
Iverson has completed two tours of duty in Antarctica and has worked in the Argon Lab. He’s looking forward to being on the first project to date ice-core tephra samples in the Argon Lab. Most of the tephra layers contain too little material to be dated; however, one particularly large deposit is one-centimeter thick and estimated to be about 20,000 years old.
“The age of the oldest ice is one of the questions that needs to be answered,” Dunbar said. “That’s not completely known. They thought it would be about 120,000 years old based on accumulation rates, but current thinking is that it may be much younger.”
New Mexico Tech researchers – including Dunbar, Dr. Bill McIntosh and Dr. Phil Kyle – were among the first people to explore the geology of western Antarctica’s volcanos.
“For some of the West Antarctic volcanoes, some of which are almost as large at Mt. Ranier, we are the only people who have ever been there – the first people to put feet on the ground on those volcanoes” Dunbar said.
Assembling the tephra records – and the volcanic history – of Antarctica is akin to putting together a jigsaw puzzle.
Other geologic investigations have created a reasonably complete record of the history of volcanic eruptions, including ages and the chemical fingerprints, largely via work at New Mexico Tech’s Argon Laboratory by McIntosh and Matt Heizler.
From the ice cores that Dunbar is now examining, she will not be able to gather enough ash material to date the layers precisely. However, she will be able to look at the unique chemical fingerprint and associate that layer to known eruptions identified in the Argon Lab.
Sounds reasonable enough? Not so fast. The science is complicated, but the logistics are equally as tricky. Drilling a 3,300-foot, 4-inch diameter ice core certainly is no easy task. Transporting that ice core from Antarctica to Denver presents even more challenges.
“A few years back, a shipping container lost its cooling system while on the cargo ship, and part of an ice core was melted,” Dunbar said. “Part of the invaluable ice sampled during the coring process ended up, literally, going down the drain.”
Once extracted, the ice core is transported to a freezer in McMurdo Station on the coast of Antarctica, where the ice sits for a year “relaxing.” Researchers found that brittle sections of ice tend to explode, once relieved of the pressure of thousands of feet of ice. Once the ice has stabilized structurally, it can be shipped to California in containers refrigerated to -15C. The cores are then loaded into refrigerated trucks and driven to the National Ice Core Lab in Denver.
The core is then cut into pieces for each scientist who has submitted a successful proposal for various ice core tests.
“It’s a really complicated cut pattern so each investigator gets a piece,” Dunbar said. “It’s fully allocated and I get a little piece of core. I go to Denver to work in the ice core lab, where it’s really cold – colder than Antarctica.”
After cutting sections, Dunbar will thaw the ice, filter the ash and return to Socorro. At the Bureau of Geology, she uses her electron microprobe to analyze the chemical composition of the tephra.
Another element of the grant will allow Dunbar to work with a colleague in Wales who is refining the methods of trace element analysis for small ash samples.
“I look at major elements,” Dunbar said. “He’s pushing the limits of beam size to detect trace elements. Nels and I will go to his lab and learn his techniques.”
– NMT –
By Thomas Guengerich/New Mexico Tech
Tephrochronology is a geochronological technique that uses discrete layers of tephra – volcanic ash from a single eruption – to create a chronological framework in which paleoenvironmental or archaeological records can be placed. Such an established event provides a “tephra horizon.” The premise of the technique is that each volcanic event produces ash with a unique chemical “fingerprint” that allows the deposit to be identified across the area affected by fallout. Thus, once the volcanic event has been independently dated, the tephra horizon will act as time marker.
The main advantages of the technique are that the volcanic ash layers can be relatively easily identified in many sediments (including ice) and that the tephra layers are deposited in a geologically instantaneous event over a wide spatial area. Therefore, tephra layers provide accurate temporal markers that can be used to verify or corroborate other dating techniques, linking sequences widely separated by location into a unified chronology that correlates climatic sequences and events.