Geologists have discovered 1.2-billion-year-old groundwater about 3 km below surface in Moab Khotsong, a gold- and uranium-producing mine in South Africa. This ancient groundwater is enriched in the highest concentrations of radiogenic products yet discovered in fluid. The discovery has implications beyond Earth, where on rocky planets such as Mars, subsurface water may persist on long timescales despite surface conditions that no longer provide a habitable zone.
Uranium and other radioactive elements naturally occur in the surrounding host rock that contains mineral and ore deposits.
These elements hold new information about the groundwater’s role as a power generator for chemolithotrophic, or rock-eating, groups of co-habitating microorganisms previously discovered in the Earth’s deep subsurface.
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.
At Moab Khotsong, a gold and uranium mine located in the Witwatersrand Basin, within the Kaapvaal Craton, South Africa, University of Toronto researcher Oliver Warr and colleagues found large amounts of radiogenic helium, neon, argon and xenon, and an unprecedented discovery of krypton -86 — a never-before-seen tracer of this powerful reaction history.
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 deep in the Earth that are unable to access energy from the sun for photosynthesis.
Due to their extremely small masses, helium and neon are uniquely valuable for identifying and quantifying transport potential.
While the extremely low porosity of crystalline basement rocks in which these waters are found means the groundwaters themselves are largely isolated and rarely mix, accounting for their 1.2-billion-year age, diffusion 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,” Dr. Warr said.
“Our results show that diffusion has provided a way for 75-82% of the helium and neon originally produced by the radiogenic reactions to be transported through the overlying crust.”
The authors stress that the insights on how much helium diffuses up from the deep Earth is a critical step forward, as global helium reserves run out, and the transition to more sustainable resources gains traction.
“Humans are not the only life forms relying on the energy resources of the Earth’s deep subsurface,” Dr. Warr said.
“Since the radiogenic reactions produce both helium and hydrogen, we can not only learn about helium reservoirs and transport, but also calculate hydrogen energy flux from the deep Earth that can sustain subsurface microbes on a global scale.”
“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 discovery is described in a paper published in the journal NatureCommunications.
O.Warr et al. 2022. 86Kr excess and other noble gases identify a billion-year-old radiogenicly-enriched groundwater system. Nat Common 13, 3768; doi: 10.1038/s41467-022-31412-2