An international team of researchers has discovered groundwater that is more than a billion years old deep below Earth’s surface - only the second time such a discovery has been made.
The water, which is 1.2 billion years old, was recovered from a goldand uranium-producing mine in Moab Khotsong, South Africa, confirming that groundwater of such a vintage is more abundant than previously thought.
The find sheds new light on how life is sustained below Earth’s surface and how it may thrive on other planets.
"Ten years ago, we discovered billion-year-old groundwater from below the Canadian Shield - this was just the beginning, it seems," says University Professor Barbara Sherwood Lollar of the department of Earth sciences in the University of Toronto’s Faculty of Arts & Science and co-author of a study published in Nature Communications .
"Now, 2.9 kilometres below the Earth’s surface in Moab Khotsong, we have found that the extreme outposts of the world’s water cycle are more widespread than once thought."
What’s different compared to the 2013 discovery at Kidd Creek Mine near Timmins, Ontario is that high local uranium levels made the find more of a challenge, as the mineral was obscuring the age of the water deep inside the subsurface rock.
Uranium and other radioactive elements naturally occur in the surrounding host rock that contain mineral and ore deposits. Understanding the role of these elements has revealed novel ways of thinking about groundwater’s role as a source of energy for rock-eating micro-organisms previously discovered in Earth’s deep subsurface. The micro-organisms draw chemical energy from the rock to flourish in the absence of sunlight.
When elements like uranium, thorium and potassium decay in the subsurface, the resulting alpha, beta, and gamma radiation has ripple effects, triggering radiogenic reactions in the surrounding rocks and fluids. The radiation also breaks apart water molecules in a process called radiolysis, producing large concentrations of hydrogen - an essential energy source for subsurface microbial communities that are unable to access energy from the sun for photosynthesis.
Researcher Oliver Warr collects samples 2.9 kilometers beneath the Earth’s surface
In the groundwater samples recovered from Moab Khotsong, the researchers found large amounts of radiogenic helium, neon, argon and xenon, and an unprecedented discovery of an isotope of krypton - a never-before-seen tracer of this powerful reaction history.
While the almost impermeable nature of the rocks where these waters are found means the groundwaters themselves are largely isolated and rarely mix - accounting for their 1.2-billion-year age - diffusion of hydrogen, helium and neon among other gases can still take place.
"Solid materials such as plastic, stainless steel and even solid rock are eventually penetrated by diffusing helium, much like the deflation of a helium-filled balloon," says Oliver Warr , a research associate in University of Toronto’s department of Earth sciences and lead author of the study. "Our results show that diffusion has provided a way for 75 to 82 per cent of the helium and neon originally produced by the radiogenic reactions to be transported through the overlying crust and captured for industrial applications."
The researchers stress that the study’s new insights on how much helium diffuses up from deep inside Earth is a critical step forward as global helium reserves run out and the transition to more sustainable resources gains traction.
"For the first time, we have insight into how energy stored deep in Earth’s subsurface can be released and distributed more broadly through its crust over time," says Warr. "Think of it as a Pandora’s box of helium-and-hydrogen-producing power, one that we can learn how to harness for the benefit of the deep biosphere on a global scale.
"Humans are not the only life-forms relying on the energy resources of Earth’s deep subsurface. Since the radiogenic reactions produce both helium and hydrogen, we can not only learn about helium reservoirs and transport, but we can also calculate the variability of hydrogen energy that can sustain subsurface microbes on a global scale."
Warr notes that such calculations are vital for understanding how subsurface life is sustained on Earth, and what energy might be available from radiogenic-driven power on other planets and moons in the solar system and beyond - informing upcoming missions to Mars, as well as to Saturn’s moons Titan, Enceladus and Jupitor’s moon Europa. The findings hint at the possibility that subsurface water may persist on long timescales despite surface conditions that no longer provide a habitable zone.
The paper’s other co-authors include C.J. Ballentine from the University of Oxford, researchers from Princeton University and the New Mexico Institute of Mining and Technology.