Scientists would like to know how quickly ice sheets in Antarctica and Greenland will melt as a result of climate change, and how much that will contribute to a rise in sea levels. And one way they’re figuring that out is by looking at how glaciers melted right here in New England, thousands of years ago.
For geologists, every hike is something of a treasure hunt. When you understand geological history, every twist in a trail reveals a fascinating new relic from another era. A team hiking Massachusetts’ tallest mountain, Mt. Greylock in the Berkshires, recently was looking for something specific.
“We are here searching for boulders,” said Jeremy Shakun of Boston College. “Glacially dropped boulders that have been sitting around for probably fifteen thousand years or so, since last ice age ended.”
And just how that ice age ended – and how that increased sea levels - is what they’d like to figure out. Back then, North America was covered it the biggest ice sheet in the world.
“I mean, picture Antarctica in your mind today," Shakun said. "You just see a huge, featureless white landscape of ice covering everything. That’s what this would have been like.”
The end of the last ice age was caused by a tilt in the earth’s axis. Today, scientists agree the climate is changing because of an increase in greenhouse gases in the atmosphere. “So this time the cause is different, but the way it plays out -- warming planet, melting ice, rising sea levels -- it’s the same bottom line," he said. "Just a different trigger.”
Shakun explained it’s glaciers on land, like the one that used to be here in New England and that now exist in Greenland and Antarctica, that cause sea level rise when they melt. Melting sea ice, he said, is like ice cubes in a glass. When they melt, the water level in the glass doesn’t go up. The biggest problem is ice sitting up on the land. “As it melts, it’s pouring more and more water into the ocean," Shakun said. "Adding more water to the glass. That’s the stuff that raises sea level. So that’s the stuff we’re really concerned about.”
And that’s what’s so important about understanding how quickly the last ice age ended. The secret is hidden in the rocks.
Off the trail, behind a mess of vines and brush, Shakun and a geologist from nearby Williams College found what they were looking for: a boulder with milky looking veins of quartz running through it.
Shakun called over BC masters student Chris Halstead.
“Oh yeah, that’s pretty good," Halstead said when he saw it. "Not a bad surface to sample on.” They took some measurements, pulled out a hammer and a chisel, and started chipping away at the quartz.
All around us, Shakun explained, there are cosmic rays – radiation from space, that you can’t see or feel, but they’re there. And when those cosmic rays come in contact with quartz, they form a rare isotope called Beryllium 10. And by measuring how much Beryllium 10 is in the rocks, they can tell how long it’s been since a glacier melted, exposing the rocks to the radiation.
“What we would imagine here is that as the ice age was ending, the top of the mountain was the first with the first part to get exposed as the ice surface melted down into the valley," he said. "So we would expect the rocks up here at the tip to have more Beryllium 10 than the rocks do at the bottom that didn't thaw out of the ice until later.”
If there’s about the same amount of Beryllium 10 in rocks at the top of the mountain as there is in the rocks at the bottom, the ice melted quickly. “If there’s a big difference," he said, "then the ice must have thinned slowly, and it was much more gradual melt out of the big glaciers.”
They took turns chipping at the rock, because it was pretty tiring. Also, the black flies were brutal. And doing this work can be a bit dangerous -- it's easy to hit a hand with the hammer. After 15 minutes or so of chipping away, they had enough for a good sample. And they headed back out on the trail to get more samples at different elevations.
They’ll be at it for months, getting rocks at mountains all over New England, which they’ll crush, extract the quartz, and measure how much Beryllium 10 is in each. Ultimately, Shakun said they’ll have a 3D computer model illustrating how the ice sheets here collapsed thousands of years ago. And that’ll provide just a glimpse into what we may be in for now with a changing climate.
“We can use the past to help guide the future," Shakun said. "But to some extent, we’re doing a really wild experiment with the climate that doesn't have precedent.”