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MARGINALIA

Old Gas, New Gas

Methane—made and taken apart by microbes, in the Earth, by people

Roald Hoffmann

Origins

In a hurry to get to methane hydrates, I began by writing, "Along with petroleum and coal, methane is a fossil fuel, of plant origin…"—at which point I got corrected by the president of Sigma Xi. When I changed what I wrote, geologists gave me more trouble. I had stumbled right into a nest of controversies. Or, an area of current research.

It appears that methane on Earth has not one source, but many. Most (but not all) of the commercial methane in natural gas is thermogenic—thought to derive from petroleum (originally from plants) that is heated and processed deep underground. It's old.

A great deal of additional methane, however, is sequestered in sediments, at sea bottom and in permafrost, in a remarkable set of structures I will soon describe. And its origins are controversial. Much (some think all) is made by archaeans—the neither-bacterial-nor-eukaryotic microorganisms that were only distinguished in recent decades.

But there is an abiogenic source of sequestered methane too. Mantle rocks that contain the mineral olivine (which describes a range of minerals from Mg2SiO4 to Fe2SiO4) are often altered to serpentine ((Mg,Fe)3Si2O5(OH)4), a change that also produces brucite ((Mg,Fe)(OH)2) and magnetite (Fe3O4). The chemistry of this "serpentinization" reaction is roughly this (to balance the equation, we'd have to specify the olivine):

Click to Enlarge Image

The important thing about this reaction is that the olivine is a source of electrons that convert the protons in water to H2. Combining it with CO2 results in methane, courtesy of the so-called Fischer-Tropsch reaction:

Click to Enlarge Image

This reaction proceeds in geological strata at accessible temperatures and pressures in the presence of the necessary catalysts. Some geologists think that most methane is created this way.

But the reaction also runs (catalyzed by enzymes now) in microbes—methanogens—at temperatures hundreds of degrees lower. To return to the very different setting of thermogenic methane, geologist and chemist John M. Hayes of Woods Hole Oceanographic Institution suggests that even there microbes could have catalyzed thermogenic CH4 formation.

I am staying tuned. But let's return to that underwater methane, wherever it comes from.








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