Published online 31 January 2008 | Nature | doi:10.1038/news.2008.542

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Deep-ocean vents are a source of oil and gas

Hydrocarbons bubble up from the mid-Atlantic's Lost City.

The Lost CityDeep-sea vents could offer a non-biological source of oil and gas.D. KELLEY & M. ELEND, UNIV. WASHINGTON INST. FOR EXPLORATION/URI-IAO/NOAA/THE LOST CITY SCIENCE TEAM

Undersea thermal vents can yield unexpected bounty: natural gas and the building blocks of oil products. In a new analysis of Lost City, a hydrothermal field in the mid-Atlantic, researchers have found that these organic molecules are being created through inorganic processes, rather than the more typical decomposition of once-living material.

Most of the planet's oil and natural gas deposits were created when decomposing biological matter is 'cooked' in high temperatures underground. But non-biological hydrocarbons have also been found deep inside the Earth, where chemical processes create the molecules from inorganic sources such as rock.

Although researchers have seen some evidence for inorganic production of hydrogen in the ocean, Lost City “is the first really clear example of a marine, deep-sea world where hydrocarbons are being synthesized abiotically,” says Giora Proskurowski of the Woods Hole Oceanographic Institution in Massachusetts, one of the researchers who made the discovery.

The Lost City hydrothermal vents, some of which are 60 metres tall, sit above magnesium- and iron-rich deposits called 'ultramafic' rock. The minerals contained in the rocks interact with water to produce an environment with plentiful hydrogen, making it chemically favourable for the creation of the hydrocarbon molecules that make up oil and gas.

In 2003, Proskurowski and his team descended 800 metres under the waves to collect the liquid bubbling from these vents. The team returned in 2005 with a remotely operated submarine to collect more samples. By analysing carbon isotopes in the hydrocarbons they brought back, the team found that the carbon and hydrogen atoms in the molecules seemed to come from Earth’s mantle and not from biological matter that had settled on the ocean floor. The results of the study are published this week in the journal Science1.

Bubbling crude

Sampling fluids at this depth under the sea can be tricky; collecting just 150 millilitres of fluid requires a container surrounded by more than 9 kilograms of titanium to prevent depressurization, says Proskurowski.

To rule out the possibility that the hydrocarbons collected from the vents were created from biological material, the team analysed several different isotopes.

Among other measurements, the team analysed the amount of carbon-13 in methane, which contains one carbon atom, and in hydrocarbons containing two, three, and four carbon atoms. As the number of carbon atoms rose, the concentration of carbon-13 fell — the opposite trend to that seen in biologically derived hydrocarbons.

Instead, the pattern of isotopes suggest that a chemical process called the Fischer-Tropsch process is at work in Lost City, creating bigger and bigger hydrocarbons in the hydrogen-rich environment. Although the concentrations were too low to detect without a filter, small amounts of larger hydrocarbons such as kerosene and octane may also be produced.

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The team also found that the methane in Lost City contained no carbon-14, suggesting the carbon source for the hydrocarbons comes from within the mantle, far away from organisms that might have had contact with the global carbon cycle at the surface.

“There was always some nagging doubt that there could be some biological contribution such as decomposing organic matter,” says Tom McCollom of the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder. “This really provides a means of clearing up that uncertainty.”

At this point, McCollom adds, no one knows how many hydrocarbon sources there are in the deep sea, but the types of rocks found in Lost City are widespread in other ocean regions, suggesting that it may be a common phenomenon. 

  • References

    1. Proskurowski, G. et al. Science 319, 604-607 (2008).
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