November 20, 2013
Moraine Research May Alter Thinking On Climate’s Effect On Glaciers
redOrbit Staff & Wire Reports - Your Universe Online
A Dartmouth College-led study published in the latest edition of the journal Quaternary Geochronology describes a newly discovered, more accurate method to date the soil and rock debris left behind by high altitude tropical glaciers.The research could alter pre-existing interpretations of how climate change affected equatorial ice masses, the study authors explained in a statement Tuesday. Currently, scientists use a variety of different dating methods to determine the ages of these glacial moraines, which are sedimentary deposits marking the past extents of glaciers.
The timing of glacial fluctuations marked by these moraines can help scientists better understand past variations in the climate because the ice masses are sensitive to weather changes, especially at high altitudes and latitudes, the researchers said. Furthermore, the research could help experts predict how glaciers will respond to future changes.
“In the tropics, glacial scientists commonly use beryllium-10 surface exposure dating. Beryllium-10 is an isotope of beryllium produced when cosmic rays strike bedrock that is exposed to air,” the New Hampshire-based Ivy League school said. “Predictable rates of decay tell scientists how long ago the isotope was generated and suggest that the rock was covered in ice before then.”
While the production rate of beryllium-10 is often influenced by factors such as elevation and latitude, scientists have typically used rates obtained from calibration sites around the world rather than rates determined locally at the sites being studied. For their study, the Dartmouth-led research team looked at beryllium-10 concentrations in moraine boulders deposited by the Quelccaya Ice Cap.
Quelccaya is the largest ice mass in the tropics and is located 18,000 feet above sea level in the Peruvian Andres, the study authors said. Over the past few decades, it has retreated significantly, and the study authors determined a new locally-calibrated production rate that is between 11 and 15 percent lower than the traditional global rate.
“The use of our locally calibrated beryllium-10 production rate will change the surface exposure ages reported in previously published studies at low latitude, high altitude sites and may alter prior paleoclimate interpretations,” explained lead author Meredith Kelly, an assistant professor and glacial geomorphologist at Dartmouth College.
“The new production rate yields beryllium-10 ages that are older than previously reported, which means the boulders were exposed for longer than previously estimated,” the school added. “Prior studies suggested glaciers in the Peruvian Andes advanced during early Holocene time 8,000 -10,000 years ago, a period thought to have been warm but perhaps wet in the Andes. But the new production rate pushes back the beryllium-10 ages to 11,000 -12,000 years ago when the tropics were cooler and drier.”
In addition, glaciers expanded in the northern hemisphere during this time, indicating a relationship between the mechanisms that caused cooling in both that region and the southern tropics. The discoveries suggest that using the new production rate could more precisely date the ages of moraines in low-latitude, high-altitude regions such as the tropical Andes. This would allow scientists to accurately reconstruct past glacial and climatic variations, Kelly said.