Tiny Bubbles in the Crust Add Up to a Big Reservoir of Abiotic Methane

Once thought to be hard to find, methane that forms in the absence of life may be incredibly widespread on Earth and elsewhere in the solar system. It exists in microscopic bubbles that form from water-rock reactions in the oceanic crust and mantle.

Scientists have detected abiotic methane, which forms from geochemical reactions without any input from organic matter or life, at a few unusual locations worldwide. On land, abiotic methane leaks from alkaline springs and ophiolites, which are chunks of ocean crust and upper mantle thrust up onto a continent, while on the seafloor, it seeps from hydrothermal vents. A new study, however, finds that abiotic methane may be much more common than we had expected, available primarily in tiny pockets of fluids within the mineral olivine. 

DCO Deep Energy Community members Frieder Klein and Jeffrey Seewald (Woods Hole Oceanographic Institution, USA) with former graduate student, Niya Grozeva (now at the French Alternative Energies and Atomic Energy Commission, France) discovered tiny trails of “fluid inclusions” full of abiotic methane in more than 100 samples from locations around the world. They propose that the methane comes from chemical reactions that occur when fluids infiltrate and become trapped inside olivine. This process has the potential to occur across much of Earth’s oceanic crust and upper mantle, and may also occur on other rocky planetary bodies that have or once had liquid water. The researchers report their findings in a new paper [1] in Proceedings of the National Academy of Sciences.

“Any rock that contains olivine can form these inclusions when exposed to water at high temperatures. If the trapped water contains dissolved carbon, methane can form through reactions with the olivine host when the rock cools below a certain temperature. This means that abiotic methane formation may be a lot more widespread than we previously thought,” said Klein.

The researchers used multiple analytic techniques to examine 160 rock samples collected from past ocean drilling, dredging, and submersible expeditions from Klein’s own collection, and from samples contributed by colleagues. They saw microscopic trails of inclusions containing abiotic methane and hydrogen in the majority of the sampled rocks with olivine. “I had no clue at the time how widespread the methane actually would be,” said Klein. “Basically everywhere you looked, it would be like ‘yeah, it’s there too.” Olivine is the most abundant mineral in Earth’s upper mantle and a major mineral in the lower oceanic crust.

Using Raman spectroscopy, a laser-based technique that can identify fluids and minerals in thin slices of rock, the researchers identified characteristic minerals inside the inclusions that form when the temperature of the rocks dropped below 400 degrees Celsius and the trapped fluids and olivine reacted in a process called serpentinization. That process releases hydrogen, which can combine with any dissolved inorganic carbon in the fluids to make abiotic methane.

methane bubbles
Methane forms inside the mineral olivine when fluids become trapped within the rocks. This process can occur in any rocks containing olivine, under the correct temperature range, both on land and in the seafloor. Credit: Klein et al., Proceedings of the National Academy of Sciences

Each bubble of methane and hydrogen is only about the size of a microbe, so this is not a source of gas that could be mined for energy. The bubbles, however, likely occur across wide areas of Earth’s oceanic crust and upper mantle. The researchers estimate that the lower oceanic crust alone may contain roughly 4.8 billion tons of carbon in the form of abiotic methane inclusions. 

“The methane that we found in these inclusions is probably one of the largest reservoirs of abiotic methane, which means that it is definitely playing a role in the global carbon cycle in deep Earth,” said Klein. While some of this methane reaches the surface when the surrounding rocks dissolve or crack, much of it likely stays buried underground, where it might be an energy source for deep microbes, or get recycled back into the mantle. 

The new findings potentially explain the earlier, puzzling observation that methane from hydrothermal vents coming out of serpentinite rocks, which form from serpentinization, appears to have the same isotopic compositions as methane from vents that occur in basalt rocks. “People have scratched their heads and wondered why the carbon isotopes look the same but couldn’t come up with a solution for a common source or reaction pathway,” said Klein. He suspects that both types of vents may be supplied with abiotic methane from olivine-bearing rocks, specifically gabbro and peridotite.

Earth may not be the only planet generating abiotic methane in fluid inclusions within olivine. “Other planetary bodies in our solar system have the ingredients and that process may be going on today,” said Klein. Since these bubbles can remain trapped for millions or billions of years, even planets that once had water, but lost all or most it, like Mars, could have ancient stores of this gas. Additionally, icy moons, where melted ice water interacts with olivine in the rocky core, such as Europa and Enceladus, could also host abiotic methane. 

 

Main image: Methane- and hydrogen-rich fluid inclusions show up as trails of black dots in olivine rock. Credit: Frieder Klein

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