An international team of researchers has discovered groundwater more than a billion years old deep below Earth’s surface – only the second time such a discovery has been made. The 1.2 billion years old water was discovered in a gold and uranium-producing mine in Moab Khotsong, South Africa. It confirms that such groundwater is more abundant than previously thought.
The find sheds new light on how life is sustained below Earth’s surface. It also provides an indicator of 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,” said University Professor Barbara Sherwood Lollar of the department of Earth sciences at 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, we have found that the extreme outposts of the world’s water cycle are more widespread than once thought.”
What’s different from the Kidd Creek Mine is that high local uranium levels made the find more of a challenge. The mineral obscured the age of the water deep inside the subsurface rock.
Radioactive elements found in groundwater
Uranium and other radioactive elements naturally occur in the surrounding host rock that contains 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 without sunlight.
When elements like uranium, thorium and potassium decay in the subsurface, the resulting radiation has ripple effects. It triggers radiogenic reactions in the surrounding rocks and fluids. The radiation also breaks apart water molecules in a process called radiolysis. It produces large concentrations of hydrogen. Hydrogen is an essential energy source for subsurface microbial communities that cannot 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. There was an unprecedented discovery of an isotope of krypton – a never-before-seen tracer of this powerful reaction history.
The almost impermeable nature of the rocks means the groundwaters themselves are largely isolated and rarely mix. Their 1.2-billion-year age allowed for the diffusion of hydrogen, helium and neon, among other gases.
“Solid materials such as plastic, stainless steel and even solid rock are eventually penetrated by diffusing helium,” said Oliver Warr, research associate and lead author. “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. They can be transported through the overlying crust and captured for industrial applications.”
Helium diffusion proof of age in groundwater find
The researchers stress that the insights on how much helium diffuses from deep inside Earth are a critical step forward as global helium reserves run out. The transition to more sustainable resources will gain 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,” said 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 learn about helium reservoirs and transport. We can 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. It also teaches scientists what energy might be available on other planets and moons in the solar system and beyond. Future missions to Mars, as well as to Titan, Enceladus and Europa will benefit from this information. The findings suggest 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. The research was supported by the Natural Sciences and Engineering Research Council of Canada, the Nuclear Waste Management Organization of Canada, the University of Oxford and 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.
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