BERKELEY—The beautiful and distinctive U-shaped glacial valleys typical of alpine areas from Alaska to New Zealand have fascinated and frustrated geologists for centuries.
While it seems obvious that glaciers scoured the bedrock for millions of years, what the landscape looked like before glaciers appeared, and how the glaciers changed that landscape over time, have remained a mystery. The glaciers erased all the evidence.
Now, University of California, Berkeley, and Berkeley Geochronology Center (BGC) scientists have employed a clever technique to reconstruct the landform history of a 300-square-mile area of Fiordland in New Zealand, from the early Pleistocene some 2.5 million years ago, when the world cooled and glaciers formed, through today’s warmer interglacial period.
“The first question we asked was, how much of the current landscape and relief is a result of glacial erosion?” said David Shuster, who developed the novel technique, called helium-4/helium-3 thermochronometry. “The answer is, all of it.”
Shuster is an associate adjunct professor of earth and planetary science at UC Berkeley and a geochemist at the Berkeley Geochronology Center.
“Geologists have wondered, what did the landscape look like 200,000 years ago, or 400,000 years ago, or back before the Pleistocene glaciations began?” said glaciologist Kurt Cuffey, professor and chair of geography and a professor of earth and planetary science at UC Berkeley. “Did the valleys start out as V-shaped canyons submerged in ice, and the glacier just widened and deepened them? Or perhaps the relief was sculpted by glaciation, and it didn’t matter what the rock landscape looked like before.”
“David’s work opens up a whole new world of investigation to tell us how the alpine landscape progressed, with implications for how glaciers today act on the landscape,” he said.
Shuster, Cuffey, UC Berkeley graduate student Johnny Sanders and BGC researcher Greg Balco report their conclusions in the April 1 issue of the journal Science.
Glaciers carved their mouths first, then their heads
The team found that in the Fiordland, a well-known tourist destination in the Southern Alps of New Zealand, the rock currently on the surface was about 1.5 miles (2 kilometers) underground when the glaciers began forming about 2.5 million years ago. Since then, the mountains rose as a result of tectonic activity, while the glaciers flowed downhill, scouring the landscape and gouging U-shaped valleys on their way to the sea.
What surprised the geologists was that most of the valley-making occurred at the downstream mouths of glaciers for the first million years, essentially stopping about 1.5 million years ago. For the next million years, until about 500,000 years ago, erosion took place primarily at the heads of glaciers, which steadily ate into their headwalls, characterized by steep, amphitheater-like cirques. As a result, the deep valleys advanced up their drainage basins toward the range divide, producing razorback ridges in the process.
“Apparently, the heads of glaciers would be directly opposite one another on either side of a high ridge, and faster erosion at the headwalls caused the glaciers to eat their way inward to the spine of the mountain range, farther from the glacier’s outlet,” Cuffey said.
Major changes to the mountain topography essentially stopped about half a million years ago. The current interglacial period started about 12,000 years ago, after warming temperatures caused the glaciers to melt and recede. The fact that these Fiordland valleys are now ice-free allowed the researchers to collect surface rock samples from 33 sites in four glacial valleys over six days with the assistance of a helicopter. The valleys end in Milford Sound or Lake Te Anau.
Temperature as a proxy for depth
Shuster developed helium-4/helium-3 thermochronometry while a graduate student at Caltech, from which he obtained his Ph.D. in 2005. The technique makes it possible to determine the temperature of a mineral as it cooled over geological time. Because temperature increases with depth, the temperature history of the mineral tells how deeply it was buried over a period of millions of years.
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