Melting methane: New thermometer for ancient ocean?

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Melting methane: New thermometer for ancient ocean?

A staggering amount of natural gas is trapped under the edge of the continental shelf, frozen in a substance called methane hydrate. When this solid material warms, it breaks down and releases methane, a powerful greenhouse gas.

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Side-view diagram showing seafloor slope and depth of gas hydrate stability zone

From original diagram by United States Geological Survey
Methane stabilizes in hydrate where pressure is high and temperature is low, often under the seafloor. Oceanic methane hydrates could hold 500 to 2,500 billion tons of carbon — up to half as much as the estimated 5,000 billion tons held in other fossil fuel reserves.

The ongoing warming of the ocean could release this methane into the atmosphere, starting a nightmare feedback: warming releases methane, which begets more warming, which releases more methane.

But the hydrates are trapped under remote ocean bottoms. They are so difficult to study that basic data is scanty, even on “simple” matters like temperature. A study published this week, however, shows that methane hydrates themselves can tell us about ocean temperatures long ago.

Because Earth gets warmer with depth, methane hydrates can only exist in a sliver of the seafloor where the pressure is high and temperature is low. Below that, the rocks are so warm that methane can only exist as a gas.

It is this relation among temperature, pressure and the physical state of the methane that was used as a retrospective thermometer in the new Nature study.

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Deep sandy seabed artificially lit with many small bubbles rising

Methane bubbles up from seafloor off the coast of British Columbia, as scientists monitored changes in hydrate distribution, depth, structure, properties and venting, due to earthquakes, slope failures and plate motions.

To begin, Benjamin Phrampus and his thesis adviser, Matthew Hornbach, an associate professor of earth sciences at Southern Methodist University, used seismic data — the reflections of sound waves that reveal Earth’s inner structure. Sound waves change speed at the bottom of the hydrate layer, where rocks holding solid gas hydrate transition into warmer, deeper rocks that contain gaseous methane.

Going down

As the warm Gulf Stream passes over the site, Phrampus says, its heat slowly penetrates downward through sediment, rocks and methane hydrate, eventually reaching the bottom of the hydrate layer. But when the scientists used today’s water temperature to calculate where that lower boundary should be, it was not at that depth.

The reason, Hornbach and Phrampus concluded, was warming water, likely due to a change in the location of the Gulf Stream. “The essence of our argument is that the water we are seeing today is warmer than what we would expect, based on the stability regime we see on the continental margin,” Phrampus told us.

The calculated warming came to about 8°C, and started very roughly 1,000 years ago. The warming signal is likely to continue for another 4,000 years, Phrampus says. Over that 50-century period, 2.5 billion tons of methane could be released from hydrates in the 10,000 square kilometers of a Carolina tract.

Although that is equivalent to about 23 percent of the carbon released in one year through fossil fuel combustion, methane hydrates in other places are probably adding to the global release of methane.

Methane molecule diagram centered in pentagonal water molecule cage.

Methane molecules in methane hydrate, AKA methane clathrate, are trapped in a cage made of water molecules.

Contemplating collapse

The Carolina Ridge was the site of a massive landslide — roughly 14,000 to 30,000 years ago — and a future collapse could feed on itself as hydrate breaks down, destabilizing the seabed and releasing more methane from hydrate (and perhaps from methane gas stored below the hydrate zone). As Phamprus and Hornbach note, “If continuing hydrate destabilization triggers slope failure at this site, the amount of methane released could be an order of magnitude greater.”

Methane hydrates are one of the big unknowns in geoscience, and they are inherently difficult to study, according to Arne Biastoch, who has studied the interplay between ocean conditions and Arctic hydrates at the GEOMAR Helmholtz Centre for Ocean Research, in Kiel, Germany. “The temporal evolution of the temperatures in the deep ocean remains highly uncertain; most studies, therefore, work with vague estimates, which naturally result in highly uncertain figures for the affected gas hydrate.”

Biastoch says better numbers are needed to evaluate the importance of stored methane in affecting the atmosphere and ocean. “Especially in the light of increasing temperatures under global warming, solid estimations are needed to study potential threats arising by out-gassing (enhancing climate warming), slope instability (triggering tsunamis) and ocean acidification (affecting the oceanic biosphere).”

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Mechanical arm inserts column into white methane mound on seafloor to take sample

The study of gas hydrates requires rugged research rigs. This machine is sampling 865 meters below the ocean’s surface, a depth greater than the height of nine Statues of Liberty.

A need-to-know basis

Although some of the released methane will be eaten by ocean bacteria and never reach the atmosphere, that could feed a different problem: acidification of the ocean. Why? Because the critters that eat the methane-eating bacteria will decay, releasing carbon dioxide dissolves in water, making the ocean more acidic and weakening the shells of clams and other crustaceans.

However, the biggest fear is that massive releases of methane could trigger a warming episode like the Paleocene–Eocene thermal maximum. About 55 million years ago, the global temperature rose 6°C after thousands of billions of tons of carbon entered the atmosphere.

Although the Carolina Rise alone would only add 0.2 percent of the carbon needed to rerun that epoch, “This only represents a fraction of the global estimated inventory of gas hydrates,” Biastoch notes. And it’s hardly the only source of carbon dioxide entering the atmosphere …

– David J. Tenenbaum

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Terry Devitt, editor; Emily Eggleston, project assistant; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Recent changes to the Gulf Stream causing widespread gas hydrate destabilization, Benjamin J. Phrampus & Matthew J. Hornbach, Nature 25 October 2012.
  2. What else is down in the deep sea?
  3. Gas hydrate climate crisis? Probably not.
  4. The potential of methan hydrates for energy
  5. Gas hydrates 101