Geologists discovered 1.2 billion-year-old groundwater about 3 km below the surface in Moab Khotsong, a South African gold and uranium mine. This ancient groundwater contains the highest concentrations of radiogenic products ever found in fluid.
|Dr. Oliver Warr gathers data in Moab Khotsong, South Africa. credit: Oliver Warr.
The discovery has implications beyond Earth, as subsurface water on rocky planets like Mars may persist over long timescales despite surface conditions that no longer provide a habitable zone.
Uranium and other radioactive elements are found naturally in the host rock that contains mineral and ore deposits.
These elements reveal new details about groundwater's role as a power source for previously discovered chemolithotrophic, or rock-eating, groups of co-habiting microorganisms in the Earth's deep subsurface.
When elements such as uranium, thorium, and potassium decay in the subsurface, the resulting alpha, beta, and gamma radiation cause ripple effects in the surrounding rocks and fluids, triggering radiogenic reactions.
University of Toronto researcher Oliver Warr and colleagues discovered large amounts of radiogenic helium, neon, argon, and xenon, as well as an unprecedented discovery of krypton-86 — a never-before-seen tracer of this powerful reaction history — at Moab Khotsong, a gold and uranium mine located in the Witwatersrand Basin, within the Kaapvaal Craton, South Africa.
The radiation also breaks apart water molecules in a process known as radiolysis, resulting in high concentrations of hydrogen, an essential energy source for deep-sea microbial communities that cannot access solar energy for photosynthesis.
Helium and neon are particularly useful for identifying and quantifying transport potential due to their extremely small masses.
While the extremely low porosity of the crystalline basement rocks in which these waters are found means that the groundwaters are largely isolated and rarely mix, explaining their 1.2-billion-year age, diffusion can still occur.
Dr. Warr explained that "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."
"Our findings show that diffusion has allowed 75-82 percent of the helium and neon produced by radiogenic reactions to be transported through the overlying crust."
The authors emphasize that understanding how much helium diffuses up from deep Earth is a critical step forward as global helium reserves deplete and the transition to more sustainable resources gains traction.
"Humans are not the only life forms that rely on the deep subsurface's energy resources," Dr. Warr explained.
"Because radiogenic reactions produce both helium and hydrogen, we can learn about helium reservoirs and transport, as well as calculate hydrogen energy flux from the deep Earth that can sustain subsurface microbes on a global scale."
"These calculations are critical for understanding how subsurface life on Earth is sustained, as well as what energy may be available from radiogenic-driven power on other planets and moons in the Solar System and beyond, informing future missions to Mars, Titan, Enceladus, and Europa."
Reference : doi: 10.1038/s41467-022-31412-2