During the Pleistocene Epoch, which lasted from about 2.6 million to 11,700 years ago, great ice sheets covered much of the northern hemisphere. Since the end of that period, both the Laurentide Ice Sheet, which covered much of North America, and the Greenland Ice Sheet, have melted considerably. However, there is evidence that the ice sheet edges have pulsed, growing and shrinking in response to subsequent climate changes.
Nicolás Young at Columbia University’s Lamont-Doherty Earth Observatory (LDEO) and colleagues Joerg Schaefer (LDEO), Jason Briner (SUNY Buffalo) and Gifford Miller (University of Colorado Boulder) want to know just how sensitive those ice margins are to climate perturbations. The team has launched a three-year study to investigate the two ice sheets’ responses to two short-term cooling events (several decades to a few centuries in duration) that occurred 9.3 and 8.2 thousand years ago.
During the study, Young, a post-doctoral fellow at LEOD, and his team will visit Baffin Island, Arctic Canada, and west Greenland to collect samples for beryllium-10 exposure dating. This method will help them decipher the timing and magnitude of glacial change during the last seven to ten thousand years, a time period with a climate similar to today.
“We are interested in the Laurentide and Greenland ice sheets’ sensitivity to short periods of climate change like the cooling events that occurred at 8.2 and 9.3 thousand years ago. These cold snaps only lasted a couple of centuries. Did they affect the ice sheets?” says Young. “Glaciers are sensitive to temperature and precipitation so it’s intuitive to think that when the climate is warm, the ice sheet melts, and when it’s cold, the ice sheet grows. But there's a lag time between changes in the climate and changes in the ice sheet. We are interested in knowing if the Laurentide and Greenland ice sheets were sensitive enough to actually respond to these very short-lived cold snaps. We know the ice sheet has been bigger and smaller in the past, but how much did the ice sheets change and why?”
Land-terminating versus Outlet Glaciers
The project builds on the group’s similar studies of Jakobshavn Isbrae, one of Greenland’s largest, and more famous, outlet glaciers. Outlet glaciers, which carry ice from the ice sheet to the oceans, are probably more sensitive to climate change than glaciers that terminate on land. Previous work showed that Jakobshavn did respond to the 8.2 and 9.3 thousand-year-old climate cooling events by reversing its course of retreat and re-advancing across the landscape. However, Jakobsahvn Isbræ, which is a high ice velocity outlet glacier, is not fully representative of the broader Greenland Ice Sheet.
“So, for this project we’ll be looking at the more subtle parts of the ice sheet, the land-terminating part of the ice sheet that flows at much slower velocities, to compare and contrast. These areas are generally more sluggish so we don’t know if they could respond to these abrupt cooling events. We will then be able to compare and contrast these data to outlet glaciers like Jakobshavn,” Young says.
Unlocking the Data
As a glacier moves down a valley, it picks up rocks that become entrained in the ice. That entrained material is eventually deposited as piles of rock debris, called moraines. Cosmic rays bombard the surface of the earth, interacting with and changing atoms found in rocks - unless the rocks are covered by ice. By measuring the build up of the element beryllium-10 in the surfaces of Greenland’s exposed rocks, Young will be able to pinpoint when in time the rock first became ice-free as the ice sheet retreated and also determine the rate at which the ice margin pulled back during the early Holocene.
Most samples come from the crests of moraines because the rocks at the top have probably been exposed to the atmosphere the longest, which means they will provide the most accurate age for the deposition of the moraine. For the chemical methods Young’s team uses, they need rocks that contain the mineral quartz, which is made from silicon and oxygen. Luckily, almost all of the rocks in the field area contain quartz. Young says the larger the boulder, the better.
“We look for larger boulders because these boulders have likely remained stable for the duration of their exposure to the atmosphere and thus gives us a more accurate age. Once we identify a sample, we get out hammers, chisels and rock saw. Our bulk rock sample averages 1-2 kilograms, which will provide us with about 100 grams of quartz. This is a good amount for our initial measurement and having enough archive material. The longer a rock is exposed, the more beryllium-10 it will have.”
The project begins in the office where the team will spend much of the spring creating preliminary maps of the study area’s topographic landforms, using aerial photographs. This allows them to target potential sampling locations. Once in the field, Young’s group will then focus on sampling for beryllium-10 dating.
This summer, the scientists will fly to Baffin Island where they will then be supported by helicopter out of their study area on the Cumberland Peninsula, and will focus on the history of the Laurentide Ice Sheet – the North American counterpart to the Greenland Ice Sheet. They plan to spend 2 to 3 weeks working out of the King Harvest Valley, an area of alpine glaciated valleys.
In the lab, the team isolates quartz minerals, which are then dissolved. Everything is removed until only beryllium remains.
“At this point, the samples will be only beryllium, but there will be two kinds: beryllium-9 and beryllium-10. We will send the samples to a particle accelerator at Lawrence Livermore National Laboratory to measure the 10Be/9Be ratio. We add a known amount of beryllium-9 to our samples in the laboratory, so once we have that measured ratio we can calculate the total amount of beryllium-10 in our samples. In order to turn the total amount of beryllium-10 into years of exposure, we need to know the rate at which beryllium-10 is produced in rock surfaces. Luckily, we have a pretty good handle on the rate at which beryllium-10 is produced; we divide the total amount of beryllium-10 by the rate at which it is produced to get the years of exposure.”
Young says the study will allow them to track the history of Laurentide and Greenlandic ice margins at unprecedented detail. Data from these slow velocity areas of west Greenland and Baffin Island will complement the existing data from ocean-terminating glaciers. Ultimately, these high-resolution studies will help improve the accuracy of ice-sheet models that attempt to accurately predict ice sheet response in a changing climate. —Marcy Davis