Methane Mimosas On Ice

Scientists Explore A Reservoir Of Frozen Methane On The Seafloor Off The Northwest Coast

By Jennifer Glass

Fifty miles west of Vancouver Island, the shallow platform of the North American continent plummets to the great depths of the Pacific Ocean.

The slopes of this undersea plateau are, by and large, composed of the assemblage of sediment you might expect for this setting: a mixture of sand, clay, bits of gravel carried by storms and landslides, sea stars and deep sea mollusks.

But every now and then, as one might find on a steep city sidewalk on a late January afternoon, a small snowdrift manages to stay frozen on an isolated patch of Vancouver's slope.

Snow on the seafloor? Not snow exactly, but cages of ice crystals encasing gas molecules.

The ice cages, most often filled with methane gas, are sometimes called "clathrates,” from the Latin clathratus, "to encage.”

Last fall, an international team of scientists probed these frozen gas deposits from the deck of a giant drill ship. Expedition 311 of the Integrated Ocean Drilling Program bored one thousand feet down into the sediments off Vancouver's western slope to determine the distribution and concentration of gas hydrates.

"We had a certain model of how gas hydrates form,” says Expedition 311 co-chief scientist Michael Riedel, formerly a research scientist at Natural Resources Canada in Sidney, B.C., and now a professor at McGill University in Montreal. "We found out that it doesn't work that way. Gas hydrates are more opportunistic. They form wherever they find the right conditions.”

Gas hydrates can remain frozen at temperatures warmer than the melting point of ice, but require high pressures to stay in solid form. Letting chlorine gas effervesce through water at elevated pressures, the great British chemist Sir Humphry Davy was first to discover the mechanism for gas hydrate creation in 1810.

Gas hydrates were considered only a chemical oddity until they were rediscovered in the 1930s, clogging oil pipelines in cold climates.

More and more deposits of gas hydrates were found throughout the twentieth century, the bulk of them were located beneath the seafloor on continental margins, where cool temperatures and moderate pressure keep the hydrates stable.

"Water ice is very sensitive to temperature, and not pressure. Gas hydrates are sensitive to both,” says Anne Tréhu, a professor of marine geology and geophysics at Oregon State University (OSU) in Corvallis and scientist on Expedition 311.

When gas hydrates were brought to the ship deck in drill cores on Expedition 311, they immediately began to decompose into gas and water. "You take a chip of it in your hand, and it starts scooting around like a hydroplane and popping,” says Tréhu. "You have to work fast.”

Gas hydrates can be kept solid for future experiments if they are stored in liquid nitrogen or under pressure.

Measurements of the chemical composition of these gas hydrates and the consortia of microbial communities living on them are revealing that the Pacific Northwest continental shelf has hosted a treasure trove of unique icy deep-sea ecosystems for millions of years.

While methane seeps are biological oases in the middle of vast plains of barren clay and sand, they also pose the hazard of releasing their hoards of caged methane to the ocean and atmosphere, heating up a world already threatened by global climate change.

Melting Methane Ice Caps

The first evidence for gas hydrates on the slopes of Vancouver Island came in the late 1980s. Geophysicists from the University of Victoria (UVic) and Natural Resources Canada had been beaming sound at the seafloor to image sediment layers. They found a suspicious reflection in the seismic records, marking the boundary between the region where methane hydrates were stable and the underlying sediments.

Seismic imaging helped scientists locate pockets of gas hydrates in various locations offshore of Vancouver Island and Oregon in the years to follow.

In the mid-1990s, a site of focused methane venting was discovered off Vancouver Island. Concentric rings of seismic reflections revealed that hydrates formed within a network of fractures penetrating up through the sediment to the seafloor.

Riedel, a Ph.D. student at UVic at the time, and his advisor George Spence named the site "Bullseye Vent” after the unusual seismic image.

The new millennium brought discoveries of massive seafloor gas hydrate deposits on Vancouver's slopes.

In November 2000, while trawling an undersea canyon thirty miles southeast of the Bullseye Vent, fishermen on the commercial fishing ship Ocean Selector were surprised to find their trawl nets bobbing to the surface, filled with over 2,000 pounds of gas hydrate frothing and hissing "like Alka-Seltzer.”

Two years later, UVic researchers explored of the canyon where the fishing boat discovery had been made, using the Canadian remote-controlled deep-sea submersible ROPOS. Gas hydrate mounds large enough to snag a trawl net fifty feet wide were discovered.

On Expedition 311 in September and October of 2005, Bullseye Vent was found to be capped by a mushroom-shaped deposit the size of the largest radio telescope, composed of alternating layers of mud and methane hydrate.

But a massive gas hydrate off of Vancouver is the exception, not the rule. Methane hydrate in this region generally does not form massive deposits, but instead occupies the pore spaces in between sand grains.

The discovery of a predominance of small hydrate bodies is good news from the perspective of the threat of massive methane hydrate release during a subduction zone earthquake in the Pacific Northwest. Less than thirty miles west of Bullseye Vent sits the trench where the eastward-moving Juan de Fuca tectonic plate dips under the North American continent.

If it manages to escape the jaws of the trench, sediment layered on the Juan de Fuca plate's basaltic crust is shoved up onto the slopes of Vancouver Island, like food items piling up at the end of the conveyor belt at the grocery store.

These sediments contain the tissue of phytoplankton that once lived on the sea surface. This organic matter is converted to methane by microorganisms. It is this methane that gets locked up in frozen cages under high pressure and low temperature.

In the case of a subduction zone earthquake, like the one that devastated South Asia with the December 2004 tsunami, giant submarine landslides can rip up blocks of massive deep-sea methane hydrate locked in sediments.

These giant methane-filled ice cubes then shoot to the sea surface and melt, releasing to the atmosphere massive quantities of methane, a greenhouse gas many times more potent than carbon dioxide. Such a massive methane hydrate release event may have been triggered by a giant landslide off of Norway 8,000 years ago.

Methane hydrate blocks the size of refrigerators grow at another Pacific Northwest gas hydrate location, Hydrate Ridge, fifty miles west of Newport, Ore. Some say the joined hillocks comprising Hydrate Ridge resemble a giant peanut, fifteen miles long and ten miles wide.

The trench where the tectonic plates scrape against each other is only six miles west of Hydrate Ridge. If Hydrate Ridge's peanut shell cracked open during a Pacific Northwest subduction zone earthquake, a refrigerator factory's inventory of methane-filled ice chests could float to the sea surface, expelling massive quantities of methane gas and warming the already-feverish atmosphere.

Perhaps more worrisome than earthquake-triggered methane release is warming of ocean bottom water temperatures as a result of global climate change. Higher temperatures of deep water may destabilize small pods of gas hydrate distributed through the sediments, like on the slopes of Vancouver Island, and pump the ocean full of methane, which will eventually cycle up to the atmosphere.

Massive releases of methane from continental slopes are thought to have helped thaw the world at the end of the last ice age 15,000 years ago and may have caused major global warming during an event geologists refer to as the Paleocene-Eocene Thermal Maximum, 55 million years ago.

She Studies Sea Shells by the Seashore

Ruth Martin, a graduate student in the Department of Earth and Space Sciences at the University of Washington (UW) in Seattle, recently discovered that methane was already venting along the continental slope of Washington State five million years ago.

Martin drives out to the Olympic Peninsula every summer, when the lowest low tides occur along the Pacific Northwest coast. This is the only time of year that the sea cliffs on the Quinault Indian Reservation are exposed. With permission from the Quinault tribal authorities, she scours the beaches, where close inspection reveals the same sedimentary assemblage as is being deposited on the slopes of Vancouver Island today.

The west side of the Olympic Mountains is a classic example of an "accretionary wedge,” named for a characteristic isosceles triangular shape. This wedge was created when the assortment of seafloor debris was thrust off the overflowing conveyor belt and uplifted onto land over the course of millions of years.

"The sedimentary structures are magnificent,” Martin says of the Quinault Formation, the portion of the accretionary wedge exposed along the reservation's coastal bluffs. "There are sediments preserved that were deposited in all marine environments, from estuarine to outer shelf. There are siltstones, mudstones, storm deposits, and flood deposits that cut channels in the sediments.”

Most exciting to Martin is evidence of methane seepage through the ancient sediments of the Quinault Formation. Climbing the sea cliffs with her advisor, Burke Museum curator of invertebrate paleontology Elizabeth Nesbitt, and scientific colleague Kathy Campbell of the University of Auckland in New Zealand, Martin identified fossils of Acharax, a genus of bivalve clams that only inhabit sites with high concentrations of hydrogen sulfide, such as methane vents.

Martin collected sediment samples from the Quinault Formation and took them back to the UW's Burke Museum, where she laboriously sieved for fossil species that had made their calcium carbonate shells using carbon in the pore water in between sediment grains.

Species of sediment-dwelling foraminifera, single-celled marine protists with intricately-carved shells, turned out to be perfect for the job. Martin's foraminifers had lived several inches down in the sediment their entire lives, using carbon in the pore water to make their shells.

Measuring carbon isotopes of the shells, Martin found that the shells were very rich in carbon-12, the light stable isotope of carbon. Methane is simply four hydrogen atoms surrounding a central carbon atom. When it is found to have abundant carbon-12, it is evidence of a methane source fueled by microbiological decomposition of phytoplankton in deep sea sediments.

The carbon isotopes of Martin's foraminifera show that millions of years before the last ice age, methane-vent ecosystems were thriving off the Pacific Northwest coast, as they still are today.

Bubbling Against The Tide

The lunar pull that exposes the Quinault sea cliffs during the summer months is also felt two thousand feet below the sea surface at Hydrate Ridge, off Oregon.

At various sites along Hydrate Ridge, methane bubbles manage to escape their icy confines and rise through fractures and faults in the sediment to vent into the ocean water.

Using the submersible ROPOS, Marta Torres, associate professor of chemical oceanography at OSU, found a correlation between the times of highest methane gas flow on Hydrate Ridge and the times of low tides during the summer of 1998.

"Because the gas in fractures beneath the seafloor is more compressible than water, the change in pressure generated by the tides is enough to generate the signal of the changing flux of methane gas,” says Torres.

During low tides, less ocean water sits on top of the ridge, allowing up to 80 gallons of methane gas to escape from the sediments every hour.

At the southern summit of Hydrate Ridge, Torres and colleagues discovered mysterious hummocks created an undulating seafloor topography. Depressions between the mounds were hypothesized to have formed when slabs of gas hydrate broke free from the sediment, possibly during a subduction zone earthquake, and bobbed to the surface.

Perhaps most importantly, Torres' research confirmed that that peanut-shaped Hydrate Ridge does indeed have a shell. Crusts of calcium carbonate, the same material out of which foraminifera make their shells, paved the northern reaches of Hydrate Ridge.

German researchers have studied the microorganisms living deep in the sediment underneath the carbonate pavements.

They discovered that in these ecosystems, devoid of oxygen from seawater, microbes work together to strip oxygen from sulfur compounds in order to get energy from the methane. By-products of this microbial reaction are hydrogen sulfide, and bicarbonate, which combines with calcium in seawater to form the calcium carbonate peanut shell.

Hydrogen sulfide, the chemical compound that gives volcanoes their rotten egg odor, is produced in such abundance by microorganisms living within Hydrate Ridge that it is the only location yet discovered where gas hydrates house not only methane gas molecules, but also "guest molecules” of hydrogen sulfide gas.

Microbes living on the surface of Hydrate Ridge use this waste product as a food source. Thick mats of bacteria called Beggiatoa, which combine hydrogen sulfide with oxygen for energy, stretch over areas the size of football fields.

Similar bacteria live inside the gills of clams, providing energy to their hosts. Fields of clams on Hydrate Ridge include living specimens of Acharax, the same species fossilized in the Quinault sea cliffs, as well as others with equally exotic scientific names, like Calyptogena pacifica.

While Hydrate Ridge remains the most thoroughly studied gas hydrate location along the Pacific Northwest continental slope, further south along the Oregon continental shelf a modern-day equivalent of the fossilized methane vents exposed along the beaches of the Olympic Peninsula can be found, at depths too shallow for gas hydrate formation.

A Sea of Greenies and a Yellow Submarine

"Greenie Spot,” the nickname given by fishermen in the 1980s, is a pitted pockmark cemented with calcium carbonate crusts, once home to schools of yellowtail rockfish, known as "greenies.”

In the late 1980s, a scientist sitting at a bar stool in southern Oregon happened to overhear a horde of inebriated fishermen roaring about their favorite rockfish catch site. It was the weirdest place, they said, easily recognizable by the permanent ring of gas bubbles, which rose five hundred feet through the water column, bursting into sea spray at the sea surface.

News of Greenie Spot, thirty miles southwest of Coos Bay, Ore., spread through the scientific community. In the early 1990s, two Pacific Northwest oceanographers dove in the deep-sea submersible DELTA (a genuine yellow submarine) to collect samples of bubbling gas from the pit. The bubbles, sure enough, were filled with methane gas.

From the submersible's porthole, Robert Collier, now a professor of marine geochemistry at OSU, and Marvin Lilley, professor of chemical oceanography at the UW, observed thick salmon-pink bacterial mats coating the edges of the cream-colored carbonate crater. Mats of Beggiatoa formed smaller circles around the pit's rim.

In what could have been a scene straight out of the Disney cartoon The Little Mermaid, Collier and Lilley observed crabs frolicking among methane bubbles, apparently trying to capture the bubbles in their claws.

Unlike the crabs, the scientists were successful in collecting methane gas samples with a funnel and syringe mounted to DELTA. They measured the carbon isotopes of the bubbles and found them to be heavier than the carbon in Martin's foraminifera shells.

Methane bubbling up from Greenie Spot had experienced a more complicated history than the methane caged in frozen clathrates at Hydrate Ridge, or the methane venting five million years ago along Washington's ancient coast.

Deep within the two-mile-thick sediment pile plastered against Oregon, "thermogenic” methane is created. Instead of a microbial source, this methane is cooked up by high pressures and temperatures along the western side of the North American continent.

"Take offshore organic matter,” says Collier, referring to the sunken decomposing phytoplankton corpses, "and smack it up against a continent. The deeply buried sediments heat up, slowly converting the marine carbon into natural gas.”

The Fool's Gold Rush

Who cares about methane, once you strike gold?

As a Master's student at UVic, Ivana Novosel thought she had discovered a source of supplemental graduate student salary her first day on the job of sediment core analysis training with geologist Tark Hamilton at Natural Resources Canada.

In sediment cores taken within Bullseye Vent, Novosel, now a geochemist at Shell Oil Company, found foraminifera fossils coated with gold. Unfortunately, it was only fool's gold, or pyrite.

Running a magnetic coil along the sediment cores, Novosel noted very low magnetism. That seemed odd, because these sediments usually contained abundant grains of magnetite, a light, flaky, highly-magnetic mineral weathered out of continental rocks and transported long distances before settling out of the water column.

Work Novosel had done as an undergraduate at McMaster University, studying the breakdown of magnetite under high methane concentrations in the soils of southern Ontario, helped her form a hypothesis to explain the coincidence of both low magnetism and pyritized foraminifera at Bullseye Vent.

Lining the sides of the sediment cores, Novosel had observed pink-orange slime mats of the type found at Greenie Spot. Members of this microbial mimosa were busily eating methane and emitting hydrogen sulfide until they were captured and brought to the surface by Novosel and her colleagues.

"When magnetite is exposed to hydrogen sulfide, it starts to dissociate,” says Novosel. "The highly magnetic mineral magnetite is reduced to a more stable mineral pyrite, which is non-magnetic.”

Novosel imagines that the magnetic anomaly resulting from high pyrite content near methane vents could be used in the future to search for more methane hydrate deposits beneath the seafloor. Researchers at Natural Resources Canada have already started using magnetic techniques to search for gas hydrate in an even chillier region than the deep sea, the Canadian Arctic.

Frosty Methane: Fuel of the Future?

Microorganisms aren't the only ones who know how to use methane gas as an energy source.

In the early 1900s, residents of the small frontier town of Charleston, Ore., cradled in between the present-day town of Coos Bay and the Pacific Ocean, tried to fuel their streetlights with methane gas from local vents. One nearby methane source, Fat Elk Well, must have also been frequented by portly wildlife.

One industrious Oregon resident still heats his house with methane from a well in his backyard.

Now, countries like Japan, India, Korea and the U.S. are investing in research and development to determine the energy potential of hydrates in their own backyards. With a huge estimated global resource of methane, clathrates may be the energy source of the future.

Scott Dallimore, Riedel's colleague at Natural Resources Canada, headed an international research program called Mallik 2002 Production Research Well Program. This program drilled through the permafrost in the Mackenzie River Delta in Canada's Northwest Territories to collect gas hydrate samples and test small-scale gas production.

Methane hydrates locked up under hundreds of feet of permafrost in the Arctic are likely more viable for drilling for natural gas than deep sea hydrates, due to their higher concentration and location near operational oil fields.

"Mallik is one of the most concentrated deposits known in the world,” says Dallimore of the gas hydrates under the Mackenzie Delta. "To date I am not aware of a well-documented site in a marine setting that is as extensive and concentrated.”

Drilling for gas hydrates under the permafrost presents the same challenge of preservation as drilling beneath the seafloor. In order to minimize hydrate dissociation, the samples are immediately cooled when they reach the surface, even at Mallik, far north of the Arctic Circle.

Dallimore and colleagues at Natural Resources Canada are currently working with Japanese oil and gas companies to return to the Mackenzie Delta and undertake longer-term production testing.

Meanwhile, Japan has started its own gas hydrate test well program, MH21, which will run for the next ten years. MH21 will explore the gas hydrates of offshore Japan, using new drilling technology that allows coring to be carried out at temperatures and pressures similar to those found in methane hydrate deposits.

U.S. oil companies like Chevron, ConocoPhillips, and Halliburton are turning one eye from the deserts of the Middle East to the icy seafloor methane mounds in the Gulf of Mexico, where exploratory wells will be drilled in coming years in order to inventory accessible U.S. methane hydrate deposits.

Neptune's New Trident: Fiber-Optic Cables on the Seafloor

The future is bright for the new field of methane hydrate research. The drill holes cored on the slopes of Vancouver Island during Expedition 311, as well as others drilled on Hydrate Ridge off the Oregon coast on a previous Ocean Drilling Program expedition, may soon be filled with sensors that will transmit data to shore in real time using fiber optic cables on the seafloor.

The Canadian NEPTUNE ("North-East Pacific Time-series Undersea Networked Experiments”) cabled observatory is currently being installed. NEPTUNE's U.S. counterpart, ORION ("Ocean Research Interactive Observatory Networks”) is being developed.

These two projects together will result in 2,000 miles of fiber-optic cable encircling the entire Juan de Fuca plate to collect real-time scientific information about deep-sea processes happening off the Pacific Northwest coastline.

"We need real-time observatories to get the time dimension of gas hydrate research,” says Tréhu. "We now conduct short-term experiments, but with fiber-optic cables, we can learn how gas hydrate systems evolve with time.”

The methane ice buried under the sediment off the Pacific Northwest coast has remained frozen for millions of years. Only time will tell whether the scattered snowdrifts can survive a combination of changing ocean temperatures, tectonic shaking and the insatiable human appetite for Mother Nature's filleted prehistoric phytoplankton.

Jennifer Glass is a 2006 University of Washington graduate holding double degrees in Earth and Space Sciences and Oceanography.

Images

Top: A mound of gas hydrate exposed on the seafloor, thirty miles off of Vancouver Island. White spots around the hydrate mound are clams.

Photo: Ross Chapman/University of Victoria

Middle: Burning the methane that is released as a gas hydrate sample decomposes. Hands belong to Robert Collier (OSU). Photo: Marta Torres/OSU

Bottom: Gas hydrate chunks fizz in the water after being netted by the commercial fishing ship Ocean Selector in 2000. Image: Chris Cleary/Archipelago Marine Research