Ellerbroek studies ancient climate via stalagmites
Rebecca Ellerbroek began her senior thesis research by spending the summer of 2008 at the University of New Mexico’s Radiogenic Isotope Laboratory with Professor Rhawn Denniston. Rebecca’s project centers on a stalagmite from the Kimberley region of northwestern Australia that was collected by Australian collaborators and shipped to Denniston that spring.
Her goal is to reconstruct variations in Australian monsoon rainfall over the last 10,000 years in order better understand what conditions have pushed huge areas of Australia into and out of drought. The work at the University of New Mexico is step one in a two-step process.
“Stalagmites are made of calcium carbonate but contain trace amounts of uranium and thorium,” Ellerbroek notes, “and we’re measuring the uranium and thorium isotopes to determine the age of our stalagmite.”
The work involves chemical procedures to separate the uranium and thorium from the bulk of the stalagmite material, and then analyze the isotopic ratios of these elements on a mass spectrometer. As Denniston points out, this can be far from easy.
“The instrument at UNM is truly state-of-the-art. The uranium concentrations in Becca’s stalagmite are so low that this work simply wouldn’t have been possible on older instruments.”
In the fall, Becca will perform step two of her project by analyzing the isotopes of oxygen in this stalagmite at the University of Michigan.
“Oxygen isotopes in stalagmites can tell us air temperature or the amount of rainfall in the past,” Ellerbroek says. “In northern Australia, the temperature remains more or less constant year round, but the monsoon varies a great deal through time. During times of heavier rainfall, 18-O is less abundant and 16-O more abundant in the precipitation. The rainwater percolates through the ground and traps this monsoon signal in a stalagmite.”
Professor Yemane Asmerom, who directs the UNM lab, enjoys having Cornell College students working with his group. “One of my great pleasures is seeing undergraduate students doing world-class research,” he says.