Geology Reference
In-Depth Information
mission. Geochemist Ed Young and his colleague Edwin Schauble, both at the University
of California in Los Angeles, think isotopes will be the key to determining whether a seep
of methane on the ocean floor was produced by a rock or a microbe. But their theoretical
calculations can't be tested with any ordinary measurement of heavy versus light isotopes.
Ed Young wants to measure “isotopologs.”
Isotopologsarechemicallyidenticalmoleculesthatdifferinthearrangementoftheiriso-
topes. Methane, with one carbon atom and four hydrogen atoms, comes in a variety of iso-
topologs. About 99.8 percent of all carbon atoms are the lighter carbon-12 variety, while
one in every five hundred atoms is the heavier carbon-13 isotope. By the same token, hy-
drogen comes in a lighter version (technically “hydrogen-1,” but always referred to simply
as hydrogen) as well as the heavier hydrogen-2 isotope, which is always called deuterium.
On Earth, the typical hydrogen-to-deuterium ratio is about a thousand to one. These ratios
mean that about one in every five hundred methane molecules holds a carbon-13 isotope,
while about four in every thousand methane molecules contain a deuterium.
Trace amounts of either of these two heavy isotopes are hard enough to measure, but
that's not what Ed Young and his colleagues are after. They want to measure the elusive
doubly substituted methane isotopologs—the roughly one-in-a-million molecule of meth-
ane that holds both a carbon-13
and
a deuterium (denoted
13
CH
3
D) or else two deuteriums
(
12
CH
2
D
2
). According to Edwin Schauble's calculations, the ratio of those two rare isoto-
pologs in any given sample of methane should provide a sensitive indicator of the temper-
ature at which the methane formed. Temperature is the key: if a batch of methane formed
at temperatures below 200 degrees, then it must be microbial; if it formed at temperatures
above 1,000 degrees, then it is most likely abiotic.
The idea looks great on paper. The trouble is, there's not an instrument in the world that
can tease out the ratio of
13
CH
3
D to
12
CH
2
D
2
. Conventional isotope analysis is based on
mass spectrometry, the process of separating molecules according to their masses. These
two isotopologs differ by less than a hundredth of 1 percent in mass, posing significant
problems in resolving one type from the other. What's more, the isotopologs are present at
extremely low concentrations that challenge conventional analysis. Ed Young and his col-
leagues need a new instrument that enhances both mass resolution
and
molecular sensitiv-
ity. That's why one of the first actions of the Deep Carbon Observatory was to help fund
a $2 million prototype instrument designed specifically to measure the isotopolog ratios of
methane. (Inasatisfying displayofcooperation, theU.S.National Science Foundation,the
U.S. Department of Energy, Shell Oil Corporation, and the Carnegie Institution of Wash-
ington are also supporting the effort.) It's a risky endeavor. It will take years to build the
instrument, years more before we know if it works. But a definitive answer to the question
of the sources of deep methane, and the resulting insights into a methane-driven feedback
loop that may drastically alter Earth's climate, is well worth the chance.
