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):
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:
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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