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 hydratethe ice-like, solid form of the greenhouse
gas, methaneare 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 MBARIs 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 matsall 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? "Its an open
question," Brewer said. "We dont yet know the quantities of hydrates at
the seafloor. But we do see the chemical signatures of the twentieth
centurychlorofluorocarbons, radioactive compounds from nuclear explosions, and
suchmixed 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 oceans 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