Who isn’t surprised that water can be hundreds of years old? That’s what happens to water that is retained underground in crystalline rocks with very low porosity, when it remains isolated and rarely mixes. An international team of researchers from the Universities of Toronto in Canada, Oxford University in the UK, Princeton University and the New Mexico Institute of Mining and Technology in the US have found water with these characteristics, 2.9 km deep in the Moab Khotsong mine in South Africa.
The scientists have compiled their findings in a paper entitled “86Kr excess and other noble gases identify a billion-year-old radiogenically-enriched groundwater system”, published on 30th June in the journal Nature Communications. The paper describes their discovery of 1.2 billion-year-old groundwater deposits, some of the oldest water on Earth.
In addition to gold, minerals such as uranium, thorium and potassium can be found in the Moab Khotsong mine. When these elements disintegrate naturally, they emit alpha, beta and gamma radiation, whose waves trigger what scientists call “radiogenic reactions” in the surrounding rocks and fluids. The product of these reactions, and therefore of the decay of these rocks and fluids, are the noble gases that geologists have found at Moab Khotsong in unprecedented quantities.
According to the scientists in their paper, time scales are essential to understanding these crustal processes. However, these scales are beyond the reach of conventional short-lived tracers such as tritium or carbon-14. To reach the profound timescales in which such geological processes take place, it is necessary to resort to the measurement and interpretation of radiogenic noble gas isotope accumulation. Helium, neon, argon and xenon are among the gases accumulated, in the highest concentration yet discovered in a fluid, in the ancient subsurface waters the researchers found at Moab Khotsong. On this occasion, they also detected an isotope never before seen in the context of such reactive processes: krypton-86.
But radiation also breaks down water molecules and produces, in a process called radiolysis, large concentrations of hydrogen. Hydrogen is precisely the energy source essential for the life of microbial communities in the deep Earth, which do not have access to the sun’s energy for photosynthesis. According to declarations made to Science News by Dr Oliver Warr, a research associate in the Department of Earth Sciences at the University of Toronto and lead author of the study, “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”. After all, this shows that, on rocky planets like Mars, groundwater present over long timescales could sustain life forms, even if conditions do not allow it on the surface.
