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Role of gas hydrates in carbon cycling and environmental change noted

Science has long oversimplified the processes of carbon cycling on Earth

Rice University

As mankind pumps more and more carbon into the atmosphere, those studying global climate change are becoming increasingly interested in the fate and consequences of all that extra carbon. At the same time, new findings about poorly understood compounds called clathrate hydrates -- ice-like substances formed by water, methane and other gases located just beneath the ocean floor -- show that science has long oversimplified the processes of carbon cycling on Earth.

"Somewhere between 2 trillion and 20 trillion tons of methane in gas hydrates lie on the outer edges of the continents, just beneath the ocean floor," said Gerald Dickens, associate professor of earth science at Rice University. "This enormous pool of methane might contain more carbon than all the world's oil, coal and natural gas reserves combined, but the real surprise is that carbon that goes there doesn't get locked away for eons. In fact, significant volumes of carbon are always entering and leaving these hydrate reservoirs via several processes, each of which is poorly quantified."

Dickens is presenting an analysis of the role that gas hydrates may play in carbon cycling and environmental change at today's annual meeting of the American Association for the Advancement of Science in Seattle. Gas hydrates, also called clathrate hydrates, are formed via natural biological and geological processes occurring in deep-sea sediments. Organic matter produced in surface water or on land accumulates on the sea floor. Bacteria infiltrate these muddy sediments, digest the organic matter and release methane gas. Under the right set of conditions -- a narrow range of cold temperatures, moderate pressures and high concentrations of gas -- the methane molecules become trapped inside an ice-like lattice of water crystals. Hydrates can become unstable. They dissolve when exposed to sea water without gas, and they can rapidly melt, releasing their methane gas, when temperature increases or pressure decreases.

There is growing evidence that significant amounts of carbon are added to and removed from gas hydrates all the time. These findings necessitate a change in the way scientists have historically thought about the carbon cycle, the process by which carbon moves across Earth's surface and through the environment. Traditionally, the carbon cycle has been viewed as comprising the ocean, atmosphere and biosphere. Carbon slowly enters the cycle via volcanic activity and erosion, and is passed between the three realms before returning to the earth in sediments.

"In this context, once carbon enters the earth, even if it's a few feet below the surface, it's gone from the picture completely and no longer figures in the cycle," Dickens said. "But now we're finding that the hydrate reservoir isn't a stable system. Instead, it acts like an enormous capacitor, charging and discharging variable amounts of methane constantly, in response to factors that we don't yet understand."

The fragility of gas hydrates has led to Dickens and others to theorize that even minor changes in ocean temperatures and currents could trigger a massive release of methane -- a major greenhouse gas -- from the world's oceans. Evidence for this comes from the fossil record. Due to the chemical processes involved in hydrate formation, the carbon that is deposited in hydrates contains a disproportionately low amount of the isotope carbon-13.

In prior work, Dickens and colleagues studied the geologic record and found carbon samples that were notably lacking in carbon-13 during a 10,000 year period in the late Palaeocene, about 55 million years ago. The period was marked by a worldwide temperature increase of 4-8 degrees Celsius, leading Dickens to conclude that this and other global carbon isotope anomalies may document times of greatly enhanced methane output from marine sediments.

Climatologists and earth scientists are just beginning to appreciate the causes and consequences of methane fluxes to and from the seafloor. In his presentation, Dickens will outline several of the most pressing questions in the field.


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