By Bryce Gray
Unassuming muck often derided as a messy nuisance.
But there’s more to it than that.
It’s an amalgamation of plant and animal parts, sediment, and detritus of all kinds, deposited in a continuous record and entombed together in a preservative matrix. (Getting excited yet?)
Raiders of the Lost Muck
Last summer, climate scientist Yarrow Axford and two of her colleagues descended on coastal lakes in southwest Greenland hunting elusive quarry: 12,000-year-old mud.
Although climate research in Greenland is most closely associated with ice cores, Axford and others in the field of paleolimnology (“the study of ancient lakes”) turn their attention to lake cores–tubes of sediment from Arctic lake bottoms. Axford, a professor at Northwestern University, has spent years digging into such mud to answer questions about Greenland’s response to changes in climate since the last Ice Age.
“Ice cores do a good job recording what’s happening with the atmosphere,” says Axford, 41. “That leaves out what’s happening with glaciers themselves, and what’s happening with vegetation.” Mud covers both of those, since it can contain the ground-up soils created and moved by the glaciers, as well as preserved bits of plants and animals that lived nearby.
No Place Like Greenland
These questions about ancient landscapes are especially pertinent in southwest Greenland, where there are hints that the area has behaved differently than other Arctic regions throughout the Holocene–the nearly 12,000-year period since the last Ice Age.
In a warming world, Axford says it’s important to understand climate change as more than an abstract increase in temperature. That’s where lake cores can provide answers.
Clues that contextualize the terrestrial response to changes in climate can be preserved by lakes, which Axford hails as “passive collectors of a huge range of things that tell us about the environment.”
“If parts of Greenland experience something very different, I think it matters to the rest of the world because it affects the future of the ice sheet and global sea level,” Axford says.
Listen to the Lake
“There are so many things in a lake core: bug heads, minerals, elements. They can all tell you something,” says Everett Lasher, a 27-year-old Ph.D. candidate at Northwestern who has joined Axford in Greenland for multiple research expeditions.
The bug heads belong to chironomids–midges that are often the first species to repopulate deglaciated landscapes. Their chitinous head capsules preserve well, making them a coveted “paleothermometer” for researchers aiming to reconstruct past climate trends.
In the field, however, Axford and Lasher are looking for starker indications of the area’s glacial past. Ideally a quick glance at a core will reveal clear divisions of gray, glacially deposited sediment, and dark, organic, non-glacial sediment. Those layers correspond with area glacial activity, essentially time-stamping local shifts related to climate.
“That’s what you look for: glacier, no glacier. It’s almost black and white,” says Axford.
A Case in Point
Axford was hoping to find that clear distinction last summer when she set her sights on an ice cap perched in the mountains squeezed between the polar ice sheet and Greenland’s coast.
She hypothesized that the ice cap did not exist in the early Holocene–a period for which there is strong evidence that the Arctic was experiencing a warmer climate than today. If her hypothesis were correct, the team would be likely to find organic sediment older than known glacial activity in the watershed.
But at the first lake where Axford and her team went to work, cores did not reveal any of the sought-after organic material that could predate the ice cap perched at the lake’s headwaters.
At a second lake-coring site, however, the team may have had a breakthrough.
While capping a freshly retrieved core, Lasher noticed shells in the mud at the bottom of the tube.
The team took the shells as a sign that the core had likely reached marine sediments deposited when Greenland’s coastline was pushed below sea level by the overhead ice sheet during the last ice age.
After the ice receded, the earth began a rebound process known as isostatic uplift–a phenomenon that is still occurring in landscapes where glaciers are retreating today.
Axford believes the shells indicate the team captured a record of the entire Holocene at the site. She says the finding reinforces the dramatic change that the landscape’s lakes have undergone.
“They go from being under an ice sheet, to being part of the ocean to being a lake,” Axford says. “It’s pretty radical change during a time when, overall, climate is usually described as being fairly stable.”
Back on Northwestern’s campus, Axford and Lasher have begun their analysis of the cores to see what has changed in the mud’s composition over time.
“There’s a lot more going on inside the core than you can see from the outside,” says Lasher.
The Future’s on Ice
There’s plenty of lab analysis ahead, but with funding secured for three future field seasons in southwest Greenland, Axford is looking forward to gathering more cores.
She is open to challenging her initial hypothesis, but wants additional data for a more complete picture of the region’s past. “One thing I really want to know is, do we have evidence that the ice cap was present through the whole Holocene?” she asks. “To me, that would be surprising.”
For now, that answer remains locked in the mud.