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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Non-fossil fuels

Abiogenic methane made in the mantle from carbonate?

My Mind and Matter column in the Wall Street Journal is on abiogenic methane

Coal, oil and gas are "fossil" fuels, right? They are derived from ancient life-forms and are nonrenewable, stored energy, extracted from prehistoric sunlight. In the case of coal and most oil, this is obviously true: You can find fossil tree trunks and leaves in coal seams and chemicals in oil that come from plankton.

But there's increasing doubt about whether all natural gas (which is 90% methane) comes from fermented fossil microbes. Some of it may be made by chemical processes deep within the earth. If so, the implications could be profound for the climate and energy debates.

When the Earth was forming, the meteorites that crashed into its surface were about 3% carbon by weight. Some carbon was in the form of simple hydrocarbons. Gases like methane would have bubbled out of the rocks as the Earth's core heated up. This "outgassing" still happens on planets. It probably explains why there are lakes of liquid methane and ethane on the surface of Titan, a moon of Saturn.

When geologists started drilling for oil and gas, they began to speculate about where it came from. Broadly speaking, the Americans backed the fossil theory, while the outgassing heresy was championed by some Russians, led by the great chemist Dmitri Mendeleev, and later revived by the astrophysicist Tommy Gold. To date, the fossil theory has worked well in predicting that oil and gas would be found where ancient marine plankton had been trapped in the shallow crust.

By contrast, a couple of wells drilled to more than 20,000 feet in Sweden in the 1980s in a place where a meteorite had cracked the granite crust, which might have let methane seep up from the molten mantle far beneath, proved a costly failure. The Russians have kept Mendeleev's flame alive, however, and at a recent conference in Kazan, in Russia, the idea that some gas fields are chemical, not fossil, gained some support.

The meeting also discussed another idea: that a lot of natural gas is made chemically but that life plays a part in gathering one of the ingredients. The ocean floor accumulates not just the soft bodies of plankton but also their shells and skeletons, made in effect from dissolved carbon dioxide, which build up to thick layers of carbonate rocks (such as the white cliffs of Dover in England).

When the ocean floor is driven down deep into the molten mantle, in the so-called subduction zones where continents are barging their way over the oceanic crust, this carbonate gets heated and pressurized. In 2004, Henry Scott of Indiana University and his colleagues discovered that ideal conditions exist for this carbonate to lose its oxygen and gain hydrogen instead, making methane on a massive scale.

In effect, this would recycle the Earth's carbon dioxide by turning it back into the fuel from which it was made when burned or breathed. Maybe this explains why so much methane bubbles up through hydrothermal vents on the ocean floor. Moreover, a new paper by Vladimir Kutcherov of the Royal Institute of Technology in Stockholm argues that this might also explain why vast quantities of hydrated methane (known as fire-ice) have been found under the seabed near the continental margins: Perhaps it has come up from the mantle. Recently the Japanese announced a successful pilot project to extract some of this methane as a source of energy.

Dr. Kutcherov thinks the evidence "confirms the presence of enormous, inexhaustible resources of hydrocarbons in our planet." If he is right—and America's new Deep Carbon Observatory aims to resolve the question in the next few years—natural gas may effectively never run out.