Deep Squeeze: Experiments point to methane in Earth’s mantle

Although today’s fossil fuel reserves reside in Earth’s crust, a new study suggests that hydrocarbon fuel might also nestle in the mantle, at depths of 100 kilometers or more. Determining whether fossil fuels can form in these extreme environments could not only point to new energy sources but also open a new avenue for origin-of-life researchers.

EXTREME CHEMISTRY. During a simulation of the conditions in Earth’s mantle, this bubble of methane formed when researchers mixed iron oxide, calcite, and water at high temperature and pressure. PNAS

Commercial oil and gas reserves originate strictly from the breakdown of plants and animals, according to most scientists. Recently, however, geologists have shown that hydrocarbons such as methane—the main component of natural gas—can form in Earth’s crust abiogenically, or in the absence of once-living matter. These scientists have speculated that this same abiogenic process might give rise to methane in the mantle.

Henry Scott of Indiana University South Bend and his colleagues decided to test the theory by simulating the extreme heat and pressure of Earth’s upper mantle. Using a small anvil with diamond jaws, the researchers squeezed a mixture of iron oxide, calcite, and water inside the tool’s microscopic chamber. After increasing the pressure to 5 gigapascals—50,000 times the air pressure at sea level—the researchers heated the material to temperatures between 500° and 1,500°C.

“Sure enough, methane had formed,” says Scott. Spectroscopic and X-ray analyses confirmed the presence of methane in the chamber, the group reports in the Sept. 28 Proceedings of the National Academy of Sciences.

Although excited by the findings, Scott is quick to point out that scientists have no way yet to say how much methane, if any, is present in the mantle.

Still, he says, the idea that hydrocarbon fuels could form in the mantle and rise to Earth’s surface is compelling. “If you think about what volume of the planet is represented by the crust, it’s not that much,” says Scott. However, when one considers the size of the mantle in terms of storing fossil fuels, “that’s a significant fraction of the planet’s volume,” he says.

Barbara Sherwood Lollar, a geologist at the University of Toronto, says the new study provides “excellent scientific evidence” of the formation of abiogenic hydrocarbons in the mantle. However, she’s skeptical that these hydrocarbons could accumulate in the mantle in volumes that would be economically important. She also points out that abiogenic hydrocarbons in Earth’s crust aren’t a significant component of the world’s usable oil and gas reserves.

On the other hand, the new diamond-anvil study could have huge implications for understanding the origin of life on Earth and, possibly, other planets, says Sherwood Lollar. Given that some bacteria feed off of methane, its formation below Earth’s surface could help explain how microorganisms subsist in extreme environments. As scientists look for evidence of life on Mars, a search for methane below that planet’s surface could offer some valuable clues.

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