Down the hole
Special instrument provides complementary information to ice core records
Posted August 28, 2009
Ice cores from Antarctica, Greenland and elsewhere in the world serve as a way for scientists to travel back in time to understand past climate. They analyze such things as the trapped bubbles of gas, chemicals, insoluble dust and trace metals found in the ice to reconstruct the cycles of glacial advance and retreat, the waxing and waning of temperature, the sudden appearance of droughts and volcanic eruptions.
But those long, skinny cylinders of ice can only tell so much of the story, according to Ryan Bay , a research physicist at the University of California Berkley . Bay and colleagues use an instrument they developed called an optical dust logger to measure the dust and particulates, or bits of matter, not captured by the original ice core.
Graphic Credit: Ryan Bay
Plots compare the results of the optical dust logger at IceCube (top) with ice-core records at Dome C and Dronning Maud Land (middle). The bottom graph shows how the comparison helps date the South Pole ice sheet.
“We use the borehole as an access point. We don’t measure the borehole itself or the wall. We’re trying to look outside the borehole into the surrounding ice,” Bay explained. “We can see things that other people can’t see in the core. We can tell other people where to look in the core for interesting things, particularly very brief fallout layers like volcanic ash.”
Bay and team member Delia Tosi will take the latest and greatest version of their optical dust logger to Antarctica this coming field season. They’ll send the instrument down the deepest hole ever drilled in the ice sheet, where European scientists recovered the oldest ice to date from a high-altitude spot on the polar plateau called Dome C in East Antarctica.
The 3,270-meter-long ice core drilled by the European Project for Ice Coring in Antarctica (EPICA) reaches back at least 800,000 years. Bay said he is particularly interested in the last 100,000 years of climate history and finding signatures of volcanic eruptions and impacts from comets or asteroids that may have caused abrupt climate changes.
“One of the things we’re interested in is what caused these climate changes,” Bay said. “Some of the best examples of abrupt climate change occurred on the millennial time scale in which you had temperature changes of 10 or 20 degrees [centigrade] within a few decades.”
For instance, a major volcanic eruption can send a great amount of sulfate particles into the atmosphere. The sulfate aerosols reflect sunlight, effectively shielding the lower atmosphere of the planet and cooling it near the surface. Bay said this abrupt cooling event could then affect the succeeding climate for millennia.
“There’s a lot of tantalizing information in those particulates,” he said.
IceCube is dusty
Bay and colleagues have used the optical dust logger since 2000, analyzing dust concentration as a proxy for climate change, as well as a way to date and correlate different ice cores in Antarctica and Greenland. Most recently, they’ve sent it down six of the 59 holes drilled so far at the South Pole for the IceCube Neutrino Observatory .
A “telescope” buried within a cubic kilometer of ice between 1,450 and 2,450 meters near the geographic South Pole, IceCube seeks to track down cosmic neutrinos. These high-energy particles are produced from violent events in the distant corners of the universe, such as exploding stars or by the formation of black holes.
Physicists believe neutrinos carry unadulterated information about such intergalactic events thanks to their ability to zip through anything in the universe without changing course. IceCube can help scientists trace the neutrinos back to their place of origin by capturing the incredibly brief interaction of the particles with other atoms as they speed through ice.
The subatomic traffic accident produces another particle called a muon, which leaves a trail of blue light in its wake. It’s that flash of light that IceCube captures using strings of digital optical modules frozen in the ice sheet.
The optical dust logger has dated the bottom of the IceCube array at between 90,000 and 100,000 years old. It also revealed quite a bit of distortion and tilting of the dust layers, a feature that can allow the scientists to unravel atmospheric conditions in the past.
“We can do things like try to map the wind speed by looking for the tiny distortions from one dust record to another dust record a few hundred meters away,” Bay explained. “You can sort of tease out what the surface of the snow looked like when the dust was laid down.”
That information helps map out the height of the sastrugi, the irregular patterns of the snow at the surface, which leads to how fast the wind was blowing tens of thousands of years ago.
More importantly, for the purpose of IceCube, mapping the dust layers provides information that could affect the instrument. For example, a dust layer sits more than 50 meters thick about 2,000 meters down in the middle of the array, cutting out the light in the ice.
“It has a pretty profound impact on the performance of the telescope,” Bay said.
Open for business
The logger uses a very bright, highly focused laser to measure the dust particles. At shallow depths, the light hits bubbles in the ice as well as dust. Deeper in the hole, it only encounters dust, which absorbs and scatters the light. A fraction of the scattered light goes back into the hole, detected by a phototube on the optical dust logger.
The amount of light tells researchers about the dust concentrations within several square meters of the ice adjoining the hole. The ice-core holes like the EPICA one at Dome C are kept open using antifreeze liquid for just these sorts of experiments, according to Bay.
“The borehole gives you another perspective on the ice sheet,” he said. “Borehole logging allows you to do things you can’t do any other way. It’s very important to the community that these boreholes stay open for business.”
NSF-funded research in this story: Ryan Bay, University of California Berkley, Award No. 0739743 .