|Enlarge ImageEvidence is steadily growing that methane seeping and bubbling from the seafloor is a widespread, but previously overlooked, natural phenomenon. (High-definition image copyright Woods Hole Oceanographic Institution and the BBC Natural History Unit, courtesy of the WHOI Advanced Imaging and Visualization Laboratory and Johnson-Sea-Link submersible, Harbor Branch Oceanographic Institution.)
|Enlarge ImageMethane, the same natural gas that we use as fuel, solidifies in the cold, pressurized depths. It is encapsulated by frozen water to form an ice-like substance called methane hydrates, which could prove to be an abundant source of energy in the future. (High-definition image copyright Woods Hole Oceanographic Institution and the BBC Natural History Unit, courtesy of the WHOI Advanced Imaging and Visualization Laboratory and Johnson-Sea-Link submersible, Harbor Branch Oceanographic Institution.)
|Enlarge ImageMethane seeping from the seafloor sustains microbes that serve as the base of the food chain for communities of animals, like these tubeworms, which thrive in the sunless depths. (High-definition images copyright Woods Hole Oceanographic Institution and the BBC Natural History Unit, courtesy of the WHOI Advanced Imaging and Visualization Laboratory and Johnson-Sea-Link submersible, Harbor Branch Oceanographic Institution.)
|Enlarge ImageMethane seeping from the seafloor sustains microbes that serve as the base of the food chain for communities of animals, like these tubeworms, which thrive in the sunless depths. (High-definition images copyright Woods Hole Oceanographic Institution and the BBC Natural History Unit, courtesy of the WHOI Advanced Imaging and Visualization Laboratory and Johnson-Sea-Link submersible, Harbor Branch Oceanographic Institution.
Seeping methane gas
Scientists are discovering that abundant quantities of methane gas are continually seeping from the seafloor throughout the oceans. This widespread but overlooked natural phenomenon has potentially dramatic implications on world energy supplies, life in the oceans, and Earth's climate.
Illustration by Jayne Doucette
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Jean K. Whelan, Senior Research Specialist
Marine Chemistry and Geochemistry Dept.
Woods Hole Oceanographic Institution
Far more natural gas is sequestered on the seaflooror leaking from
itthan can be drilled from all the existing wells on Earth. The ocean
floor is teeming with methane, the same gas that fuels our homes and
In more and more locations throughout the world’s
oceans, scientists are finding methane percolating through the
seafloor, bubbling into the water column, collecting in pockets beneath
seafloor sediments, or solidifying in a peculiar icelike substance,
called methane hydrate, in the cold, pressurized depths of the ocean.
Massive deposits of methane hydrates could prove to be abundant
reservoirs of fuel. But in the past, these massive storehouses of
methane also may have “thawed” suddenly and catastrophically, releasing
great quantities of climate-altering greenhouse gas back into the
In some places, seeping methane sustains thriving
communities of exotic organisms that harness the gas as an energy
source in their sunless environment. Below the seafloor, an unknown but
potentially vast biosphere of microbes may be making the methane that
percolates upward. (See Is Life Thriving Deep Beneath the Seafloor?)
Other places on the seafloor show evidence that pockets of gas trapped
beneath sediments have exploded to form “mud volcanoes,” or may have
triggered seafloor avalanches and tsunami waves.
An underestimated phenomenon
Until recently, scientists have largely overlooked seafloor methane and
its potentially dramatic impacts. The problem is that methane commonly
vents out of isolated cracks in the seafloorsome so small that they
are easily missed by oceanic surveillance systems. Once out into the
ocean, the methane usually is diluted rapidly by seawater, or it
dissolves in seawater and is consumed by microorganisms that convert it
metabolically into carbon dioxide. Unless you happen to be looking in
the right place at the right time, you’ll miss the show.
evidence has steadily accumulated that natural seepage of methane from
the seafloor is a large, continuous, and ubiquitous phenomenon. When
oceanographers happen upon these vents (often called “cold seeps”), the
scene is often spectacular.
Several researchers have
documented large craters or pockmarks on the seafloor, while others
have described huge carbonate mounds (formed by organisms that ingest
methane and produce carbonate). Both are often relics of past seafloor
gas venting. Sometimes gas simply seeps from the ocean floor and
sustains communities of unusual tubeworms, mussels, and other creatures
like those found at hydrothermal vents. (See The Evolutionary Puzzle of Seafloor Life.)
Gas frozen solid at the seafloor
The deep ocean floor around gas seep sites is often covered by methane
hydrates. These are solid crystals of methane encapsulated in ice,
which form under the low temperatures and high pressures typical of
ocean depths greater than about 1,500 feet.
look like seafloor carbonate, but when chunks are broken off, methane
hydrates float upward (carbonates sink). As those hydrates rise into
higher temperatures and lower pressures, they decompose, releasing
methane gas into the oceana process akin to releasing the pressure on
a bottle of soda.
Energy companies have been eyeing methane
hydrates as a potentially tremendous new source of natural gas. Since
the 1930s, the use of natural gas has increased fivefold to account for
more than 25 percent of the world’s energy consumption. With existing
technology, the world gas supply is estimated to be 5,300 trillion
cubic feet (tcf), Robert Kleinberg of Schlumberger and Peter Brewer of
Monterey Bay Aquarium Research Institute reported in American Scientist. At the current rate of global consumption (about 85 tcf per year), a 60-year supply remains.
But the amount of gas at various locations around the world varies
widely. Russia and the Persian Gulf each have about 1,700 tcf, while
the total for North America is about 260 tcf. Japan and Europe import
nearly all of their natural gas, while India and China have very small
A potential new energy resource
The untapped well of methane hydrates holds the promise of energy
independence for nations close to oceans or permafrost regions (where
conditions and consistently cold temperatures also create methane
hydrates). Offshore methane hydrates would provide the U.S. alone an
estimated potential natural gas reserve of 300,000 tcf. Projections of
hydrate gas reserves in the ocean south of Japan are 2,000 times that
country’s very small existing natural gas reserves, according to
Kleinberg and Brewer.
Most of the world’s gas hydrates are
sequestered in the deep ocean, presenting great challenges for
potential commercial production. Hydrates dissolve quickly when removed
from the unique conditions on the ocean bottom, so researchers must
figure out how to either stabilize them or produce and transfer fuel
directly from the seafloor.
Many known deep-water deposits,
such as the Blake-Bahamas Plateau off the Carolinas, are very diluted
or spread across relatively thin layers over wide areas, making them
very difficult to “mine” economically. And deep-sea hydrates are often
associated with complex biological communities that would be disrupted
or destroyed by gas extraction and production.
Recharged oil wells
Recent work by a number of laboratories suggests that free gas
streaming through the seafloor or seafloor hydrate deposits may
constitute yet another large oceanic methane source. On the northern
continental slope of the Gulf of Mexico, for instance, a process known
as “gas washing” fills subsurface petroleum reservoirs with natural gas
that flows upward from even deeper reservoirs in the Earth’s crust.
It has been estimated that less than 2 percent of generated oil and gas
ever makes its way into commercial reservoirs. Of the residual oil,
about half remains dispersed in the source rock and sediments.
The residual oil and organic matter in deeper sediments is subjected to
more heating and natural processing and is broken down into natural
gas. The gas streams upward, washing out clogged pore spaces and
recharging many fuel reservoirs. Evidence comes from oil wells in the
northern Gulf of Mexico, where we have observed significant changes in
oil compositions over time scales as short as 10 years. The wells
continue to produce long after their expected lifetimes.
other half of the residual oil leaks upward and out of the sediments
into ocean bottom waters. Remarkable satellite photographs of the Gulf
of Mexico and other regions reveal slicks extending for miles in areas
where no oil production is occurring. Similar photographs are now being
used to locate new oil and gas accumulations.
Beyond the geological “cooking and squeezing” processes that produce
petroleum and gas, large quantities of gas also are being produced
biologically. Many gas hydrate accumulations and ocean-floor gas seeps
consist of methane largely derived from microorganisms.
Bacteria living in oxygen-poor areas beneath deep-sea sediments on the
seafloor produce methane as a major product of their metabolism. Some
models suggest that bacteria in sediments may account for 10 percent of
the living biomass on Earth. In addition, microbial communities beneath
the seafloor, whose numbers are entirely unknown, may also be producing
vast amounts of methane.
Global warming and tsunamis
The pervasive and ongoing movement of methane gasfrom seeps,
decomposing hydrates, gas washing, and microbial sourcesleads to some
fascinating phenomena and important questions.
Methane is a
greenhouse gas that traps heat about 20 times more effectively than
carbon dioxide. If methane deposits and seeps prove to be ubiquitous in
the oceans, they are a potentially significant contributor to global
Relatively modest changes in global ocean
temperatures or sea level could trigger a massive release of oceanic
methane. If a change in ocean bottom pressure or a rise in water
temperatures passes a certain threshold, sizable methane hydrate
deposits could decompose rapidly and release a large quantity of
heat-trapping gas back into the atmosphere. This scenario has been
proposed as a possible cause for some past episodes of rapid global
Evidence from the past suggests that upward-seeping
methane may pose another threat: underwater avalanches. Landslides at
the edge of the continental slope just off the East Coast of the United
States may have been triggered by pockets of methane gas that had built
up pressure under a lid of overlying sediments and exploded. Similar
landslides today might generate tsunamis that would hit the U.S. coast.
An offshore oil-drilling platform that accidentally hit such a gas
pocket would also be endangered.
A wide-open new field
Many challenges remain ahead for researchers. Methane seeps are widely
distributed around the world’s oceans, yet their discovery remains
mainly serendipitous. The volume of oil and gas seeping to the floor
throughout the world’s oceans is also unknown, as most of the seafloor
Even in the cases of known seepsespecially
those found in and around known oil and gas fieldsdata on the rates of
seepage are scarce. Yet evidence suggests that gas seeps and methane
hydrate deposits may be even more pervasive than their known extent
today and may play a fundamental role in regulating ocean chemistry,
sustaining marine life, and shaping seafloor geology.
Posted: February 13, 2004