The release of massive amounts of carbon from methane hydrate frozen under
the seafloor 56 million years ago has been linked to the greatest change in
global climate since a dinosaur-killing asteroid presumably hit Earth 9 million
years earlier. New calculations by researchers at Rice University
show that this long-controversial scenario is quite possible.
Nobody knows for sure what started the incident, but there’s no doubt
Earth’s temperature rose by as much as 6 C. That affected the planet for up to
150,000 years, until excess carbon in the oceans and atmosphere was reabsorbed
Earth’s ecosystem changed and many species went extinct during the Paleocene-Eocene
Thermal Maximum (PETM) 56 million years ago, when at least 2,500 gigatonnes of
carbon, eventually in the form of carbon dioxide, were released into the ocean
A new report by Rice scientists in Nature
Geoscience suggests that at the time, even though methane-containing gas
hydrates—the “ice that burns”—occupied only a small zone of sediment
under the seabed before the PETM, there could have been as much stored then as
there is now.
This is a concern to those who believe the continued burning of fossil fuels
by humans could someday trigger another feedback loop that disturbs the
stability of methane hydrate under the ocean and in permafrost; this change
could warm the atmosphere and prompt the release of large amounts of methane, a
more powerful greenhouse gas than carbon dioxide.
Some who study the PETM blame the worldwide burning of peat, volcanic
activity or a massive asteroid strike as the source of the carbon, “but
there’s no crater, or any soot or evidence of the burning of peat,” says
Gerald Dickens, a Rice professor of Earth science and an author of the study,
who thinks the new paper bolsters the argument for hydrates.
The lead author is graduate student Guangsheng Gu; co-authors are Walter
Chapman, the William W. Akers Professor in Chemical Engineering; George
Hirasaki, the A.J. Hartsook Professor in Chemical Engineering; and alumnus
Gaurav Bhatnagar, all of Rice; and Frederick Colwell, a professor of ocean
ecology and biogeochemistry at Oregon State University.
In the ocean, organisms die, sink into the sediment, and decompose into
methane. Under high pressure and low temperatures, methane molecules are
trapped by water, which freezes into a slushy substance known as gas hydrate
that stabilizes in a narrow band under the seafloor.
Warmer oceans before the PETM would have made the stability zone for gas
hydrate thinner than today, and some scientists have argued this would allow
for much less hydrate than exists under the seafloor now. “If the volume—the
size of the box—was less than today, how could it have released so much
carbon?” Dickens asks. “Gu’s solution is that the box contains a
greater fraction of hydrate.”
“The critics said, ‘No, this can’t be. It’s warmer; there couldn’t have
been more methane hydrate,'” Hirasaki says. “But we applied the
numerical model and found that if the oceans were warmer, they would contain
less dissolved oxygen and the kinetics for methane formation would have been
With less oxygen to consume organic matter on the way down, more sank to the
ocean floor, Gu says, and there, with seafloor temperatures higher than they
are today, microbes that turn organic matter into methane work faster.
“Heat speeds things up,” Dickens says. “It’s true for almost all
microbial reactions. That’s why we have refrigerators.”
The result is that a stability zone smaller than what exists now may have
held a similar amount of methane hydrate. “You’re increasing the
feedstock, processing it faster and packing it in over what could have been millions
of years,” Dickens says.
While the event that began the carbon-discharge cycle remains a mystery, the
implications are clear, Dickens says. “I’ve always thought of (the hydrate
layer) as being like a capacitor in a circuit. It charges slowly and can
release fast—and warming is the trigger. It’s possible that’s happening right
That makes it important to understand what occurred in the PETM, he says.
“The amount of carbon released then is on the magnitude of what humans
will add to the cycle by the end of, say, 2500. Compared to the geological
timescale, that’s almost instant.”
“We run the risk of reproducing that big carbon-discharge event, but
faster, by burning fossil fuel, and it may be severe if hydrate dissociation is
triggered again,” Gu says, adding that methane hydrate also offers the
potential to become a valuable source of clean energy, as burning methane emits
much less carbon dioxide than other fossil fuels.
The calculations should encourage geologists who discounted hydrates’ impact
during the PETM to keep an open mind, Dickens says. “Instead of saying,
‘No, this cannot be,’ we’re saying, ‘Yes, it’s certainly possible.'”