In 2016, the world’s oldest water was found 3 km deep at the bottom of a Canadian mine. Since the previous record of finding ancient waters had been set three years earlier at a higher level than the mine itself, there seemed to be something special about this site. Today, the same team found water at a similar depth in the Moab Khotsong Gold and Uranium Mine in South Africa, which is at least 1.2 billion years old. This water contains elements that allow life to survive without any access to energy from the sun, such as Canadian waters.
Many life forms survive without the need for direct sunlight, as we find in caves and on the ocean floor, to name a few. But most organisms still depend on the sun as their primary source of energy. For example, species that live on the sea floor depend on foods that come from the ocean surface, with the exceptions being the life forms that live around hydrothermal vents on the sea floor, and the microbes that live in the ocean. It lives on hydrogen deep in the earth.
We have not yet determined the depth limits for hydrogen-eating life forms, but a new paper from Nature Communications provides compelling evidence for the availability of very deep ancient habitat sites. The waters in the depths of the Moab Khotsong Mine contain higher concentrations of radioactive decay elements than had been seen before, some of which provide opportunities for life.
Dr. Oliver Warr, a researcher at the University of Toronto, and co-authors found water within precambrian crystalline rocks 2.9 kilometers below the surface. They pointed out that these rocks cover an estimated 72% of the Earth’s continental crust by surface area, and may represent up to 30% of the planet’s groundwater.
The interactions between water and certain types of rocks produce hydrogen gas in this place. Although production is slow at any given interface, over such a large area can produce a huge volume of gas over time, providing a major source of energy for microbes, or perhaps humans if we can make use of it. Some of the hydrogen reacts with carbon to produce methane and more complex hydrocarbons, expanding the range of microorganisms it might support.
Meanwhile, radioactive decay of unstable isotopes results in alpha particles, which become helium through electron capture, providing arguably the most finite resource. Uranium, thorium, and potassium decompose in the surrounding rock to produce lighter elements, including the noble gases (helium, neon, and argon) whose concentrations build up over time and provide a measure of the age of the water in which they are trapped.
“Think of it as Pandora’s box of helium and hydrogen energy, a fund we can learn to harness for the benefit of the deep biosphere on a global scale,” Warr said in a statement.
Notably, the paper does not explore to what extent life has benefited from what the Moab Khotsong Mine has to offer. However, future studies may not only reveal an extremely exotic ecosystem, but may provide insight into the possibility of life in the depths of other worlds, where water is abundant without sunlight.
This is only a second example of groundwater that is more than a billion years old, but there is one important difference from the previous example. But the lighter noble gases seeped in by surging through the rocks, leading to differences in concentration between the different elements.
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