Kurt Cuffey overlooking the glacier-carved Bowen River drainage (middle), Mount Tutoko (far right) and Milford Sound (left) in Fiordland National Park of New Zealand. (Photo by Johnny Sanders)
beautiful and distinctive U-shaped glacial valleys typical of alpine
areas from Alaska to New Zealand have fascinated and frustrated
geologists for centuries.
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.
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.
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.”
is an associate adjunct professor of earth and planetary science at UC
Berkeley and a geochemist at the Berkeley Geochronology Center.
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
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.
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
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
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.
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.
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
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.
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
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.
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.
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.
One of the study sites, the North Clinton drainage in Fiordland National Park, New Zealand. (Kurt Cuffee, UC Berkeley)
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.
scenario is consistent with a subglacial erosion rate dependent on ice
sliding velocity, but not ice discharge,” Shuster said.
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.
research was supported by the National Science Foundation. The work of
the BGC was supported by the Ann and Gordon Getty Foundation.