“The technique allows us to collect samples from the present surface and, based on observations, infer how they cooled through 80 degrees Celsius to 20 degrees Celsius (176 to 68 Fahrenheit) over the last few million years, and thus, how deep they were when they cooled,” Shuster said.
At the moment, the technique works only with crystals of apatite, a calcium phosphate mineral found mainly in plutonic rocks, such as granite, that solidify from magma deep underground. The apatite crystals contain uranium and thorium, which over millions of years decay radioactively, producing helium-4. The helium gradually leaks out of the crystal into the surrounding rock, but the rate of leakage decreases as the crystal cools.
Using special equipment at the BGC, the geologists were able to date the cooling of the minerals by measuring the amount of uranium and thorium in each crystal as well as the total amount of helium-4. The new technique involves irradiating the crystal with a proton beam to create helium-3, then measuring the outgassing of both helium isotopes to obtain a cross section of the helium-4 concentration in the crystal. They then calculated the crystal’s cooling history based on the helium diffusion rate.
The samples, all of them younger than 2.5 million years, showed a large range of temperature, and thus depth, histories. Cuffey and Shuster used a computer model to test various scenarios and concluded that only one fit the data: Glaciers initially scoured the U-shaped valleys on the flanks of the mountain range, and only later began eating away at their headwater regions, including cirques and drainage divides.
“… this morphology resembles modern analogs in Norway and Antarctica, where steep valley ramps descend to level floors,” the authors wrote.
The common thread is that the rock erodes faster where the ice sits on a steep slope, they said. Thus, the erosion rate of a glacier is greatest where the flowing river of ice descends steeply downstream.
“This scenario is consistent with a subglacial erosion rate dependent on ice sliding velocity, but not ice discharge,” Shuster said.
Cuffey, coauthor with W. S. B. Paterson of the fourth edition of the book “The Physics of Glaciers” (2010), hopes to use the new information to improve computer models of glacial sculpting of landscapes. Meanwhile, Shuster and Cuffey plan to apply thermochronometry to chart the history of Yosemite National Park before and after the arrival of Pleistocene glaciers, going back as far as 40 million years into the Cenozoic era.
The research was supported by the National Science Foundation. The work of the BGC was supported by the Ann and Gordon Getty Foundation.