Gas hydrates, El Niņo, and global change:
A case study

December 8, 1997

SAN FRANCISCO, CA —On the seafloor off Northern California researchers have found evidence that deposits of methane hydrate—the ice-like, solid form of the greenhouse gas, methane—are poised in a delicate balance that could shift with even small increases in ocean temperatures, possibly unleashing a cascade of environmental effects.

Peter Brewer and colleagues from MBARI, the U.S. Geological Survey, the National Oceanic and Atmospheric Administration/Pacific Marine Environmental Lab, and Stanford University used MBARI’s remotely operated vehicle (ROV) last August to investigate a site about 25 km (15 miles) off shore, where other scientists had earlier documented the presence of gas hydrate. The MBARI researchers found the site, located at a depth of 521 meters (1,700 feet) in the Eel River Basin, populated with vesicomyid clams and bacterial mats—all of which depend on methane and hydrogen sulfide vented from beneath the seafloor. They also saw extensive slabs of calcium carbonate, formed by bacterial action on methane. However, while the team observed, via an underwater video camera, a methane seep pumping out about 200 liters of gas per minute (STP), they found no solid gas hydrate either at the seafloor or in cores of sediments the ROV extracted at the site.

Temperature readings and other measurements made in the water surrounding the site indicated that conditions had changed there since the methane hydrate was discovered in 1987. Many gases react with water and convert to solid hydrates, but only at precise combinations of pressure and temperature. "This year," Brewer said, "with the northward transport of enormous volumes of warm water due to El Niņo, the water temperature at the depths where hydrates would occur is about a degree warmer than it was in 1987." The temperature increase depresses the threshold at which hydrate converts to gas and vice versa; thus Brewer and his team found bubbling gas at 521 meters, instead of solid hydrate.

If a short-term increase in water temperature such as an El Niņo episode can lower the gas-solid boundary and destabilize hydrate deposits at greater depths, triggering the release of gas, what effects would long-term ocean warming have? "It’s an open question," Brewer said. "We don’t yet know the quantities of hydrates at the seafloor. But we do see the chemical signatures of the twentieth century—chlorofluorocarbons, radioactive compounds from nuclear explosions, and such—mixed down to the ocean depths. So we would expect to see the corresponding increases in global temperatures expressed there as well."

Based on the concentration of chlorofluorocarbons (CFCs) absorbed by the seawater from the atmosphere, Brewer and his cohorts calculated the age of the water bathing the gas seep at the research site to be about 30 years. This "ventilation age," or number of years since water at that depth was last at the surface, indicates the time lag between changes in the atmosphere and their manifestation in the deep sea.

Worldwide, methane hydrate deposits are estimated to contain about 20 quadrillion cubic meters of methane gas, and some fraction of this material is exposed in seafloor sediments. (Most hydrate reserves lie at the edges of continents, but some are contained in the arctic permafrost.) A sustained rise in global temperatures could trigger large-scale melting of hydrates. While the resulting changes would be site-specific, in the ocean such a breakdown would likely generate a cascade of effects, says Brewer. "We know that hydrate decomposition has occurred in the past, and it certainly could occur in the future. Probably some of the methane would rise up to the atmosphere and contribute to greenhouse warming, but much would be oxidized by bacteria, lowering oxygen levels in the ocean. The increased carbon-dioxide levels from bacterial oxidation would lower the ocean’s capacity to absorb CO2 from the atmosphere."

While the significance of gas hydrate decomposition and its potential to accelerate global climate change is largely unknown, the results of this work strongly suggest that short-term climate variability can affect hydrate stability. "We can see that where gas hydrates are exposed at the seafloor they are poised at their thermodynamic boundary and thus sensitive to small perturbations," he concluded. The question remains whether these findings presage more serious, global effects.

Contact: Debbie Meyer, 831-775-1807