Methane Deep In Ocean Crust Could Feed Chemical-Hungry Microorganisms

December 15, 1996

Evidence is surfacing that searing temperatures and crushing pressures are creating a storehouse of nutrients needed by microorganisms living at the seafloor and, possibly, deep within the earth's crust.

Microorganisms in the ocean depths thrive where there is no light and dine on chemicals toxic to other life. Deborah Kelley, a University of Washington oceanographer, presents a poster at this week's American Geological Society meeting in San Francisco saying that a significant reservoir of methane and hydrocarbons may be found in rock beneath the seafloor. Such a large food source bolsters speculation about how pervasive these life forms might be.

Kelley has been studying the source of these nutrients in what is called "layer three" of the oceanic crust. Located at a depth of about two and a half miles, this layer consists of rock that was once part of molten magma chambers. Although the basalts found nearly everywhere on the seafloor come from the same submarine chambers, Kelley says that the fluids in layer-three rock are very different from those in basalts because of the environment in which they cooled and evolved.

"In the past everyone commonly assumed that the composition of fluid in basalts on the seafloor is what could be expected deeper in the crust," Kelley says. Starting about 10 years ago, scientists started rethinking this. Kelley's latest work shows that layer-three rock has concentrations of methane that are 50 times greater than that measured in the gases from seafloor volcanoes, the places where lava has been able to reach the surface of the seafloor rather than remaining locked below.

In addition, Kelley and Gretchen Fruh-Green, a colleague from Zurich, Switzerland, have done the first-ever isotopic work on rocks from layer three and the methane does not appear to come from the breakdown of sedimentary material.

Kelley thinks methane forms in two ways in magma chambers below the seafloor:

The first involves fluids -- mainly water and fluids rich in carbon dioxide -- that are present when magma chambers are filled with liquid rock. As the rock and tiny bubbles of these fluids cool to 500 C to 600 C, the fluids evolve into very methane-rich compositions.

The second happens as the rock cools to temperatures of 400 C and lower. As the rock solidifies it fractures and cracks, seawater rushes in and the rock "heals" around tiny amounts of the liquid. The high temperatures cause the seawater to react with the surrounding rock and fluids to evolve into methane-rich compositions. The process can take thousands of years.

Kelley says that the second process could be pervasive throughout all of layer three, which makes up 60 percent of the oceanic crust. Even though the rock of layer three may contain only minute bubbles of methane, the amount becomes significant when considered over such a vast area.

This considerable reservoir of methane is unlikely to be exploited by humans. Most of the methane below the seafloor is trapped in rock as bubbles so tiny that they can be seen clearly only with a microscope.

It is more likely that the methane may be used by deep-sea microorganisms. As the rock ages it continues to fracture and admit seawater. It could be that the seawater carries microorganisms down into the rock where they mine the methane or it could be that seawater leeches methane out of the rocks and carries it into the oceans, Kelley says. There is evidence that this happens at hydrothermal vent fields where the seafloor has fractured and emits hot water and dissolved minerals.

With funding from the National Science Foundation, Kelley studies minuscule amounts of seawater for clues about what could be a major source of methane and hydrocarbons. She has access to samples of a drill core from the Southwest Indian Ridge near Madagascar. It's there that a portion of layer three has been thrust up to where humans can drill it for the first time and get samples that have not been affected or weathered by seawater or the atmosphere.

For her analysis she creates paper-thin slices of rock and mounts them on a stage that heats them to 700 C or cools them to -200 C as she watches the minute bubbles of fluids through her microscope. By noting such things as the melting temperature of the ice formed, she can determine the composition of the fluid, the generations of fluids and even calculate the temperature at which those fluids were once circulating through the crust.

University of Washington

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