Scientists describe for the first time the carbon cycle in a subglacial freshwater lake in Antarctica

Subglacial lakes, which never see the light of day, are among the most inaccessible frontiers of science and are more untold stories than even the planets of our solar system. One thing seems certain: where there is water, there is life – even if the water is at the bottom of a cold lake, in total darkness, under more than half a mile of the West Antarctic Ice Sheet.

As reported in AGU progress, scientists analyzed the chemical fingerprint of the ocean and microbes extracted from sediments and water at the bottom of a subglacial lake called Mercer Lake to describe for the first time where the vibrant microbes harvest and move carbon, the day’s energy source extremely desolate system. They used the data from the sediment, microbes and carbon cycle to infer the geological history of this region, and the results surprised them.

While they previously thought the ice over Mercer Lake may have been stable up to hundreds of millennia, this new work confirms that the lake was connected to the ocean about 6,000 years ago and the West Antarctic ice sheet was smaller than it is today. This was a period when the climate was stable relative to the end of the last Ice Age and even to present-day anthropogenic climate change.

“This is the first time we have unequivocal geological evidence that the baseline 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 an assistant professor at the Colorado School of Mines. Venturelli led 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 retreated about 155 miles — a few dozen miles less than the distance from New York to Boston — before growing back to its modern configuration.

“These are real numbers from water and sediment samples that can now be used to validate ice sheet models,” Venturelli said.

Venturelli and Rosenheim retrieved the samples from Mercer Lake, a few hundred miles from the South Pole, as part of a 25-person team on the historic expedition called Subglacial Antarctic Lake Scientific Access, or SALSA. They were there from December 2018 to January 2019. The team used a clean-access custom hot-water drill to extract the longest core to date from a subglacial lake — about two meters long. They drilled through more than half a mile of ice to get it, working against the clock while the water-filled hole froze over again.

It was only the second time in history that scientists have extracted a sediment core from a subglacial lake. (The first was recovered from Lake Whillans in 2013.)

“This work is a big step forward for us,” said Rosenheim. “We thought the glacier would retreat to where it is now, but it went well beyond that, suggesting the ice is much more dynamic than we thought. Now we need to incorporate this new understanding into models so we can better predict what may happen in the future as the planet warms.”

The team used geochemistry tools, including isotopic analysis and radiocarbon dating, to figure out how carbon circulates through the system. They combined this with known estimates of microbial metabolism to mathematically confirm when the West Antarctic Ice Sheet groundline retreated.

“Prior to this study, we had not yet confirmed the maximum extent of the last deglaciation,” Venturelli said.

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6,000 year old carbon simply tastes good

“It turns out that the bacteria that live in that environment are tough little guys that can get by with what they have down there,” Venturelli said. “This project really confirms that where there is water, life can exist.”

The microbes in the lake feed on 6,000-year-old carbon introduced when this region was still connected to the ocean.

As a reminder, there is no sun down there fueling life through photosynthesis like there is in most lakes on Earth. “This is not a lake as we know it,” said study co-author and microbe expert Brent Christner of the University of Florida. “In Mercer Lake, in addition to this carbon deposit from 6,000 years ago, the microbes can harness chemical energy from physical processes associated with the ice sheet itself,” Christner said.

As the ice moves, the underlying rock is pulverized into small particles that are mobilized in the water, and the microbes – mainly bacteria and archaea – access these minerals for energy during a process called chemosynthesis. Archaea are microorganisms distinct from bacteria found in other extreme environments such as B. hot springs on land and hydrothermal vents in the deep sea.

The carbon stock in the sediment at the bottom of Mercer Lake is at least 100 times greater than any other carbon stock in the cycle — and the microbes are using it efficiently, Venturelli said. They also use carbon injected into the system from upstream water bodies. Subglacial lakes can be short-lived and resemble a braided subglacial river system rather than a closed lake system.

“Think of these lakes not as separate ecosystems, but as a network of communities connected by the transport of water and sediment,” Christner said.

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Paint the whole picture

The fate of the Antarctic ice sheet is having extraordinary implications for global sea level rise. If the entire West Antarctic Ice Sheet were to melt, it is estimated that the coastline around the globe would be lengthened by more than 2.6 meters.

“There’s a lot of doom and gloom in climate science,” Venturelli said. “I find hope in the fact that this work highlights that the ice sheets are much more dynamic than we previously thought, and we need to explore this idea of ​​reversibility – what were the coercive mechanisms that caused the ice sheet to move again.” where has it moved to today? – so that we can better predict future scenarios.”

Scientists estimate there may be more than 650 subglacial lakes in Antarctica — so between this core study and the first in 2013, they’ve literally only just begun to scratch the surface of their mysteries with two main data points. But one thing is for sure, Venturelli says: Looking at the glacier base, particularly the water and sediments in these subglacial lake systems, is worth the extra effort.

“We couldn’t have learned these things by poking around in ocean sediments from the outside of a ship,” she said. “Sometimes it takes a fresh look at an old problem to unearth really exciting insights.”