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Down and dirty, yet cosmic
“No gold, no oro!” wasn’t convincing the police at the Panama City airport. Here was a young guy flying home to the US with boxes of dirt as checked luggage—what was he smuggling? By the time a translator could say “sediment specimens,” officers were carefully sifting the whole lot. Finding no contraband ores, they finally let the passenger, and his dirt, onto the plane.
The American was Kyle Nichols, now on Skidmore’s geosciences faculty, and the dirt was a treasure of sorts, containing traces of an element precious to geology research. Nichols studies sedimentation and erosion; his work in Panama, funded by the US Army Research Office and recently by the National Science Foundation, focuses on three river basins near the Panama Canal. His calculations rely on a high-tech method for measuring minuscule amounts of a chemical formed when cosmic rays strike the earth’s surface. The more of the chemical found, the longer time the soil was exposed to cosmic rays, and therefore the slower the erosion rate has been.
The key vehicle in Nichols’s research is quartz, a component of sandy soils. He explains, “Quartz is composed of silicon and oxygen. When a certain kind of radiation hits an oxygen atom, it can spall, or break open. Neutrons and protons are released to redistribute, and among the new atoms formed are beryllium-10.” Beryllium is a lightweight metal; Be-10 is a variant, an isotope with one extra neutron. Such atomic fission happens very rarely—about four Be-10 atoms get formed in a gram of Panamanian quartz in a year. That’s in quartz at the surface; even fewer are formed deeper in the soil, and none about six feet under. (Other cosmogenic isotopes are less rare—like helium-3 in olivine rocks, or calcium-41 in carbonate rocks—but the amounts are still vanishingly small.)
Nichols collects anywhere from a cup to a gallon of sediment per sample. Rinsing, acid baths, and density separation leave him with 20–40 grams of pure quartz granules. These go to a specially equipped lab at the University of Vermont (where Nichols did his PhD work), which extracts the Be-10 and forms it into bullet-sized packets. These in turn go to Lawrence Livermore National Laboratory in California, home to one of the world’s few accelerator mass spectrometers, which can analyze elements in the tiniest quantities. Through a few dizzying computations, the lab’s count of Be-10 atoms helps Nichols to determine how long the soil was on the surface of the landscape.
“In this kind of research,” he says, “your evidence, by definition, is not there. It’s been eroded away.” So he figures out how long it used to be there before it ended up as river silt or landslide rubble. He’s found that Panama river basins average a “fairly high” erosion rate of 100 millimeters per 1,000 years. Not enough to clog the canal anytime soon … unless climate change or deforestation speeds up the process.
With his new NSF grant, Nichols and colleagues are now analyzing several existing data sets from various terrains to evaluate the impact of grain size (gravel vs. sand) and sediment delivery on their calculations. Does Be-10 sampling on a steep slope dotted with landslides give a skewed erosion rate compared to sampling flatter land? Let the number-crunching begin.
From atom-by-atom isotope extraction to massive melding of worldwide data banks, the science is painstaking. Good thing Nichols is a big fan of line graphs, scatter charts, and bell curves. “The data-crunching can be very exciting,” he says. “But I also love fieldwork. I love landscapes and discussing their origins and evolutions.” He adds, “I was always a history buff, ever since my dad used to bring home arrowheads he found in a cornfield by our house. If I hadn’t pursued earth science, I might have become an archeologist. I need to dig in the dirt and figure out what happened there.” —SR