An international team of researchers has discovered groundwater more than a billion years old deep below Earth’s surface for the second time, and outside of Canada for the first time.
The 1.2-billion-year-old water recovered from a gold- and uranium-producing mine in Moab Khotsong, South Africa confirms that groundwater of such a vintage is more abundant than previously thought, and shedding 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 at the University of Toronto 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 discovery more of a challenge, as the mineral was cloaking or obscuring the true residence times of the water deep inside the subsurface rock.
Ten years ago, we discovered billion-year-old groundwater from below the Canadian Shield — this was just the beginning, it seems.
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 the groundwater’s role as a source of energy for rock-eating micro-organisms, previously discovered in Earth’s deep subsurface that 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 what are called 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.
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.
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. 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.
“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, research associate in the Department of Earth Sciences at the University of Toronto 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,” Warr continues. “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 these 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, Titan, Enceladus and 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.
Additional co-authors of the paper include C.J. Ballentine from the University of Oxford, and researchers from Princeton University and the New Mexico Institute of Mining and Technology.
Funding for the study was provided by the Natural Sciences and Engineering Research Council of Canada, the Nuclear Waste Management Organization of Canada, the University of Oxford and the Earth 4D: Subsurface Science and Exploration Program of the Canadian Institute for Advanced Research. The National Science Foundation and the International Continental Scientific Drilling Program funded the drilling and installation of sampling equipment.