Author: Melina Devoney
Carrying bear spray and 70-pound backpacks, geology professor Ann Blythe, Thomas Warfel (senior), David Phillips (senior) and four other geology professors and students trudged through the shrubs and permafrost of the eastern Denali range. The 18 hours of sunlight of the Alaskan summer created a slushy terrain that led to soaking wet feet but also ideal conditions for scooping up soggy tephra samples from the fault’s sediment layers.
In this week’s blog, we return to Alaska to once again to unearth the wonders hidden in the inelegant form of rocks and dirt.
Actually, calling tephra a “rock” or “dirt” is a vile misnomer that would elicit a cringe from a real geologist. It is a conglomerate of minerals that carries more grandiose meaning than something a neighborhood rascal throws at your window or your dog rolls around in.
Tephra is essentially any solid material ejected during a volcanic eruption. Glass fragments that form in the layers of tephra sediment are key components of a geological timepiece called fission-track tephrochronology.
Blythe set out to develop this relatively underutilized dating method.
“The technique is to date volcanic ash layers that nobody can date with other methods,” Blythe said.
According to Blythe, the sediments are roughly between two and five million years old, but tephrochronology will allow them to assign a more precise age range.
Tephra is abundant in sediments layers around the Denali Fault, so Blythe flew to Fairbanks, Alaska along with Warfel and Phillips, who are both researching tephrochronology for their senior comprehensive projects (comps).
In Fairbanks, the Occidental team met up with professor Paul Fitzgerald from Syracuse University, professor Jeff Benowitz from University of Alaska Fairbanks and professor Ken Ridgway and his graduate student from Purdue University.
The crew, carrying out various projects centered in Denali, hiked about eight hours southeast of Fairbanks and set up tent in makeshift campsites for six days. They were out of contact with any other humans for the entirety of the trip.
Blyth and Phillips’ research relied solely on collecting tephra samples, while Warfel had more freedom to assist the other professors because he focused his comps on the tephra layers from which early hominids were excavated in Ethiopia.
Geologists have been unable to date the Ethiopian fossils, but tephrochronology could provide an age of the sediment layers and therefore a time range of when those hominids roamed the earth, or rather, when they ceased to do so and became fossils.
Erin DiMaggio, a geology professor at Occidental from 2013 to 2014, sent Warfel some Ethiopian tephra samples from her current research position at Pennsylvania State University. She had unsuccessfully attempted argon-argon dating — which utilizes the decay rates of argon isotopes — and hoped that Warfel could get satisfactory results through tephrochronology.
Warfel started studying up on tephrochronology last spring. He practiced preparing the samples and tracking fission zones in lab samples until he was ready for hands-on work with the Alaskan tephra this summer.
Warfel said that he had less of an incentive than Blythe and Phillips to find tephra and more motivation to learn as much as he could about working in the field and helping with the simultaneous projects. He spent one day searching for appetite in granite for Fitzgerald’s alternative fission tracking method and another helping Benowitz utilize the position and ages of the tephra layers to analyze how the Denali fault has been slipping.
According to Blythe, their team easily found tephra samples because the rock layers along the fault were tilted due to tectonic activity or eroded by rivers. Both processes expose the underlying tephra layers.
Warfel said that they developed tricks to find tephra. One major clue was coal layers; since volcanic eruptions smother and kill sessile organisms within range of their volcanic dust, ancient plants and animals decayed into coal layer underneath the tephra. That tephra acted as fertilizer for the next generation of organisms, which grew abundantly but inevitably died and turned into a thick coal layer just like their ancestors.
Removing the tephra was also easier than usual due to the climate.
“Most of the tephras we collect here — in the Mojave desert and nearby — look pretty different than they do in Alaska where it rains a lot more,” Warfel said.
The main difference is that tephra erodes slower in dry climates and therefore protrudes out of its eroded bedrock, whereas it erodes faster in Alaska’s wet climate.
“Tephra turns into glue, essentially,” Blythe said. “It weathers into clay, so it was loose and we didn’t even need a rock hammer.”
In fact, they were able to dig some samples out with a big spoon-like instrument or their bare hands.
“It was pretty fruitful,” Warfel said. “We were making good progress the whole time.”
Once they filled eleven sandwich bags with the clay, the crew returned to Los Angeles to analyze the samples.
Back in Occidental’s geology lab, Warfel and Phillips began counting the fission tracks in the tephra. They abstracted the volcanic glass pieces that are diffused throughout tephra, grinding up the pieces that were too large, and fixed them to a microscope slide to look for the fission tracks.
Fission tracks are essentially damage zones etched into the glass when uranium 238 (U-238) — the most common isotope of the element found in tephra glass — splits into two nuclei that repel each other and fly off in opposite directions. The students set a measured area of the glass and counted the number of damage zones in those parameters.
Mastering the counting of fission events is an uncommon and tedious skill, according to Warfel.
The tracks are only 15 microns (or 0.015 millimeters) across, so researchers use hydrofluoric acid to preferentially etch the damage zones, leaving troughs in the glass that are visible under a microscope.
A gridded microscope eyepiece is used to scan across the sample to count the tracks. A caveat is making sure that every fission track, and only fission tracks, are counted.
“Anything else that could make a pothole in the glass could also look like a track,” Warfel said.
The last piece of the puzzle, measuring the amount of uranium in them, will allow them to determine the ages of the tephra. By counting the number of fission tracks and determining the amount of uranium that made those tracks, the Occidental team can use the known rate of uranium fission to date all of the tephra.
The fission rate is based on uranium radioisotopes. Isotopes are forms of an atom with the same number of protons but a different number of neutrons. They are considered radioisotopes when the nuclei are unstable due to this variation and undergo radioactive decay—converting back to stable forms and emitting radiation in the process. This radiation can be used to “tag” the atom because it emits a relatively large amount of energy. Scientists have experimentally determined the fission rate of uranium.
The amount of uranium in each glass sample is measured in a nuclear reactor.
Blythe explained that, once she sends the samples to the Oregon State University’s TRIGA (Training, Research, Isotopes, General Atomic) Reactor, they will be bombarded with thermal neutrons (those that are not bound within an atomic nucleus and have an average kinetic energy corresponding to the average energy of the materials in the reactor) in order to create fission in uranium 235 (U-235). Since U-235 and U-238 occur in a constant ratio, the number of fission tracks created in the nuclear reactor by fission of U-235 can be used to extrapolate how much U-238 is present in each sample.
After all that hot nuclear activity, the samples must stay in the reactor for about a month until they are cool enough to handle. Blythe is hopeful that they will receive dates this November and present their findings in conferences in San Francisco and Pomona this Spring.
In the meantime, Warfel is looking into writing a grant proposal to investigate the homogeneity of uranium concentration in their samples. He hopes to validate their assumption that uranium distributes evenly throughout the entire sample.
“It would be interesting to see how the uranium concentration varies across the sample,” Warfel said. “I hope to find that it doesn’t at all, but we’ll see.”
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