The melting of ice caps and glaciers in the Canadian Arctic has exposed plants that died tens of thousands of years ago. Radiocarbon dating of the plants and surrounding rocks has revealed when the ice first crept over the plants and preserved the landscape beneath.
“Glaciers are in some ways purely reactionary to climate: It warms, they shrink. It cools, they grow,” Simon Pendleton told Eos. “The plant ages and the radiocarbon in the rocks tell us that they’ve likely had continuous ice cover for the last at least 40,000 years.” Pendleton is a geoscientist at the Institute of Arctic and Alpine Research at the University of Colorado Boulder and the lead author on the study that published these results in Nature Communications on 25 January.
In addition, the researchers found that the plants and landscapes are exposed today because “the most recent century of warming that we have experienced was greater than [that of] any century in the past 115,000 years,” Pendleton said.
Delicate Dead Plants
The researchers traveled to Baffin Island in Nunavut, Canada, in 2013, 2014, and 2015 to study dead plants and rocks at the edges of its ice caps and glaciers. Glaciers on high-elevation, low-relief terrain like Baffin Island are useful for paleoclimate studies because they adhere to the rock rather than sliding and eroding the surface.
“They actually preserve the landscape underneath to a really high degree,” Pendleton said. When the ice cap retreats, “those plants come out from underneath the ice in the exact same position that they went under the ice thousands of years ago.” The team collected dead plants—mostly fragile mosses and lichen—from the edges of 30 ice caps and glaciers on Baffin Island.
The team determined the approximate year the plants died using radiocarbon measurements. “Radiocarbon dating in organic material is based on the idea that living organisms—plants, you, me, anything that breathes—exchange CO2 [carbon dioxide] with the atmosphere,” Pendleton explained. “That CO2 is a combination of 12C and [radioactive] 14C.”
The amount of radioactive carbon left in dead organic material directly relates to when it died and stopped replenishing its levels of 14C. The team found that most of the plants died at least 40,000–50,000 years ago, marking the time span that those plants were covered by ice.
At nine of the sample sites, the team also measured the 14C concentration in surface rocks. The radioactive carbon in surface rocks is caused by energetic particles from space striking the surface. Ice cover shields the rocks from these particles. “Basically, it’s an on-off signal,” Pendleton explained. “When the landscape is exposed, you’re producing 14C; when it’s covered by ice, you’re not.”
The researchers compared 14C measurements to models of radiocarbon concentration that depend on the ice cover thickness. This can reveal whether a rock was exposed after the glacier first expanded. They found that in eight of the areas, the ice that originally killed the plants 40,000 years ago never melted away—until now.
Undoing Consistent Ice Cover
“We know the Arctic is changing,” Pendleton said. “We see it in shrinking sea ice and retreating glaciers and changing ecosystems. What we don’t know is, When was the last time the Arctic was actually as warm as it is today?”
Ages from radioactive carbon can reach back only so far, Pendleton explained. The researchers wanted to know when Baffin Island last experienced warming similar to today’s and thus when their field sites may have been ice free.
For this, they looked to ice cores collected in Greenland by other teams. The Greenland ice cores showed that the most recent time before the plants died that the Arctic had warming similar to today’s was 115,000 years ago, during the Last Interglaciation.
“You can have a year here and a year there where the temperatures may be pretty warm or very cold,” Pendleton said. “The climate system is naturally chaotic. But now what we’re seeing is consistent warmth undoing this continuous ice coverage of the past 115,000 years.”
Running Out of Time
“This is an exciting paper that documents the magnitude of recent warming in the Arctic,” Meredith Kelly, a glacial geologist at Dartmouth College in Hanover, N.H., told Eos. Kelly, who was not involved with this research, said that the study “is amazing, but alarming, since it shows that the recent melt is unprecedented in the Holocene. The small ice caps on Baffin Island are highly sensitive to temperature, and their retreat is emblematic of the Arctic cryosphere.”
Kelly also said that using radiocarbon measurements from the plants and rocks, combined with ice core ages, is a promising research technique for paleoclimate and cryosphere research. “It’s a relatively new methodology that could and should be applied in the Arctic and elsewhere to determine a more detailed record of past ice extents,” she said.
Pendleton said that the team will return to Baffin Island in the near future and collect more samples from the ice caps. “So many of these small ice caps you’re seeing are going to be gone in the next 10 years,” he said. With time running out, scientists must “try and preserve that record, or at least access that record, before it’s gone.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer