
Scientists describe carbon cycle in a subglacial freshwater lake in Antarctica for first time
Subglacial lakes that never see the light of day are among science’s most inaccessible frontiers, overflowing with more untold stories than even our solar system’s planets.
One thing is certain: where there is water, there is life—even if that water is at the bottom of a cold lake, in complete darkness, beneath the West Antarctic Ice Sheet.
Scientists analyzed the chemical fingerprint of the ocean and microbes retrieved from sediments and water at the bottom of a subglacial lake called Mercer Lake to describe, for the first time, where the feisty microbes get carbon, the energy source du jour, and move it through this fiercely desolate system, as reported in AGU Advances.
They inferred the geologic history of this location using data from sediment, microorganisms, and the carbon cycle, and the conclusions startled them.
While scientists previously believed that the ice over Mercer Lake had been stable for hundreds of millennia, this new research shows that the lake was connected to the ocean around 6,000 years ago, and the West Antarctic Ice Sheet was smaller than it is today.
This was a time when the climate was stable in comparison to the end of the last ice age and even today’s human climate change.
“This is the first time we have unequivocal geologic proof that the grounding line of the West Antarctic Ice Sheet, which is like its shoreline where the ice meets the ocean, was at least 250 kilometers further inland than it is today—possibly more,” said Ryan Venturelli, lead author of the study and assistant professor at the Colorado School of Mines. Venturelli performed the work with her former Ph.D. advisor, marine geologist Brad Rosenheim at the University of South Florida College of Marine Science.
In other words, just a few thousand years ago, the West Antarctic Ice Sheet receded around 155 miles—roughly the distance between New York and Boston—before re-growing to its current size.
“These are real numbers from water and sediment samples that can now be used to validate ice sheet models,” Venturelli said.
Venturelli and Rosenheim collected the samples from Mercer Lake, a few hundred miles from the South Pole, as part of a 25-person team on the Subglacial Antarctic Lake Scientific Access, or SALSA, mission. From December 2018 to January 2019, they were on-site.
The researchers used a clean-access, bespoke hot water drill to extract the longest subglacial lake core to date—about seven feet long. They had to dig through more than a half mile of ice, racing against the time as the water-filled hole refroze.
It was only the second time in history that scientists have retrieved a sediment core from a subglacial lake. (The first was retrieved from Lake Whillans in 2013.)
“This work is a big step forward for us,” Rosenheim added. “We thought the glacier would retreat back to where it is now, but it went much further back, indicating that the ice is much more dynamic than we realized.
” Now we must incorporate this new knowledge into models in order to better predict what will happen in the future as the world heats.”
To find out how carbon cycles through the system, the researchers used geochemical procedures such as isotope analysis and radiocarbon dating. They paired this with known estimates of microbial metabolism to statistically confirm when the West Antarctic Ice Sheet’s grounding line receded.
“Prior to this study we had not yet confirmed the maximum extent of the last deglaciation,” Venturelli said.
6,000-year-old carbon tastes just fine
“It turns out the bacteria living in this environment are hardy little guys that make do with what they have down there,” Venturelli said. “This project truly confirms that where there is water, life can persist.”
The lake’s bacteria feed on 6,000-year-old carbon introduced when this region was still connected to the ocean.
As a reminder, the sun does not power life in most lakes on Earth through photosynthesis. “This is not a lake like we know it,” said research co-author and University of Florida microorganism expert Brent Christner.
“In Mercer Lake, in addition to that legacy carbon from 6,000 years ago, microbes can use chemical energy from physical processes associated with the ice sheet itself,” Christner explained.
The rock beneath the ice is broken into minute particles that are mobilized in the water, and microbes—mostly bacteria and Archaea—access those minerals for energy during a process known as chemosynthesis.
Archaea are microorganisms that are unique from bacteria seen in other severe conditions, such as hot springs on land and deep sea hydrothermal vents.
The carbon pool in the silt at the bottom of Mercer Lake is at least 100 times larger than any other carbon pool in the cycle, according to Venturelli, and the bacteria use it efficiently.
They also utilise carbon brought into the system by upstream water bodies. Subglacial lakes can be transitory, resembling a braided subglacial river system rather than a closed-off lake system.
“Think of these lakes not as separate ecosystems but as a network of communities that are connected by the transport of water and sediment,” Christner said.
Painting the bigger picture
The fate of the Antarctic ice sheet has enormous ramifications for global sea level rise. Some estimates predict that melting the entire West Antarctic Ice Sheet would contribute more than nine feet to coasts around the world.
“There is a lot of hopeless doom and gloom out there in climate science,” Venturelli says. “I find hope in the fact that this work highlights that the ice sheets are much more dynamic than we previously appreciated, and we need to probe this idea of reversibility—what were the forcing mechanisms that caused the ice sheet to re-advance to where it is today?—so we can better predict future scenarios.”
Scientists estimate that there may be more than 650 subglacial lakes in Antarctica, thus they have only scratched the surface of their mysteries with two key data points between this core investigation and the first in 2013.
But one thing is certain, according to Venturelli: looking at the glacier’s base, particularly the water and sediment in these subglacial lake systems, is worthwhile.
“We couldn’t have learned these things by poking at marine sediments from the outside from a ship,” she said. “Sometimes it takes looking at an old problem in a new way to really unearth exciting findings.”