Some of the most ancient rocks on Earth have been found to host water that is rich in hydrogen. The chemistry is similar to that around deep ocean vents, candidates for the birthplace of life, and the discovery could open up new opportunities to explore what is known as the deep carbon cycle.
Professor Barbara Sherwood Lollar of the University of Toronto has explored deep Precambrian shield rocks in Canada, South Africa and Scandinavia, and reports evidence for hydrogen-rich waters, sometimes containing 30% hydrogen by volume. In Nature, she explains the hydrogen's presence is not primarily from radioactivity as previously thought, but as the product of chemical reactions.
The presence of hydrogen in these environments is important because, as previous studies have shown, it can provide a source of energy that allows microbial communities to thrive—whether at hydrothermal vents or in rocks reached by South Africa's deepest gold mines.
"This represents a quantum change in our understanding of the total volume of Earth's crust that may be habitable," Sherwood Lollar says, calling the vast areas of Precambrian rocks a sleeping giant with the potential to transform our thoughts on the origin and quantity of life on Earth.
“Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres,” the paper notes.
Previous attempts to estimate the amount of hydrogen gas released by Precambrian crustal rocks found it to be very low. However, Sherwood Lollar produces figures that range from 40 to 350 times higher, “comparable to estimates from marine systems.”
The higher-than-anticipated levels of hydrogen could be the product of chemical reactions known as “serpentinization,” a name the paper says is “an umbrella term encompassing a suite of reactions all of which produce H2 as a by-product of hydration of minerals” rich in magnesium and iron. Older sections of the Earth's crust contain more rocks rich in these metals.
While the paper is likely to lead to further explorations in search of deep-delving microorganisms, the most significant implications may lie in the other direction. "If the ancient rocks of Earth are producing this much hydrogen, it may be that similar processes are taking place on Mars," says Sherwood Lollar. The same may go for moons in the outer solar system.
Sherwood Lollar et al. Precambrian rocks make up much of the Earth's crustal material and may be rich in deeply buried life.