The thing about the mud that marine geologist and geochemist Erwin Suess hoisted from the bottom of the Pacific in 1996, half a mile down and 60 miles off Newport, Oregon, was that it was seething—cold, but seething. On the TV monitor aboard the research vessel Sonne, he could see the mud as it was brought to the surface. A few minutes later, on deck, “the whole cubic meter of stuff was bubbling,” he says. “It was just one smelly olive green mess of sediment. People were hesitating, standing back, because it was just an awful smell, the hydrogen sulfide. So I rolled up my sleeves and reached in that mess and stood up to my ankles in it, and then touched this hard stuff—it was cold, ice cold. I threw it on the deck to get rid of the sediment, because it was all covered, and it broke open. It was pure white.”
Graphic courtesy of NOAA National Geophysical Data Center
FROZEN: Scientists excavating the ocean floor have found huge chunks of frozen methane along Hydrate Ridge, about 60 miles off the coast of Oregon. The ridge is accretionary and lies only about 6 miles east of the deformation front where the Pacific tectonic plate plunges under North America.
A lot of photos were snapped on the Sonne
that day and on later cruises to the same spot. The photos show rubber-gloved hands holding slimy hunks with the color contrast of a coconut: brownish on the outside, white on the inside. In some of the pictures you can see little craters in the white stuff, the hemispheric outlines of vanished bubbles. In others you see people boring into it with power drills, slicing it with knives, plunging it for safekeeping into basins of liquid nitrogen. You see young scientists grinning ear to ear as they hold pieces of the white stuff in their hands—showing the camera how it can be ignited, how it burns brightly with an orange flame.
The white stuff is methane hydrate, and it is weird. Researchers call it icelike. It is frozen solid, but it is not exactly ice. It is a form of clathrate, a term derived from the Latin word for cage. Methane hydrates consist of molecules of methane trapped in cages of H20. Typically, six water molecules surround a single methane molecule, and many of those cages linked together form a crystal. Only at high pressures can methane insinuate itself into water this way. But if the pressure is more than 30 times the normal atmospheric pressure—easily exceeded under a thousand feet or more of water—hydrates can form at temperatures above 32 degrees Fahrenheit. This usually happens just beyond the continental shelf, where it slopes down into the abyss.
There is a precise curve of temperatures and pressures that define the depth at which methane hydrates exist. The bottom of that zone, deep in the seafloor mud, is where the temperature gets too high, toward Earth’s hot interior; the top of the zone is where the pressure gets too low, moving toward the surface. Leave the zone in either direction and the partnership of methane and water dissolves. When you bring a chunk of methane hydrate up to the surface, the water melts and drips through your fingers; the methane gas wafts into the air.
Hydrates have been a curiosity for nearly 200 years. Natural methane hydrates were first discovered by Russian scientists in the late 1960s in Siberian permafrost—where the ground is so cold that hydrates can form at shallower depths and at lower pressures than under the sea—and then, in the 1970s, at the bottom of the Black Sea. (So much methane comes out of the Black Sea that sailors have reported seeing lightning igniting it at the surface.) It was not until the 1980s that researchers drilling into the ocean floor first began to understand that this stuff is everywhere. Usually, though, the frozen hydrates they brought up in cores were small, like pencil erasers.
The chunk that Suess brought up off Oregon was a foot and a half across. There was plenty of it to study. Over the years, as researchers have returned to the place they named Hydrate Ridge, they have learned a lot about how methane hydrate is created there. Hydrate Ridge is one of several accretionary ridges off Oregon, long layers of mud that get scraped off the Pacific tectonic plate as it plunges under the North American continent. Methane gets squeezed out of the deepest layers of sediments like water from a sponge and migrates up toward the seafloor. The southern summit of Hydrate Ridge, about 2,500 feet below the sea surface, is a field of mounds and depressions 10 to 20 feet across. It was from one of those mounds that Suess’s team grabbed chunks of thick, pure methane hydrate.
There is a lot of methane under Hydrate Ridge. Under most frozen hydrate deposits is a layer of free methane gas occupying the pore spaces in the sediment. Typically, that’s how the deposits are discovered because the boundary reflects sound well enough for a survey ship to detect it. Under some parts of Hydrate Ridge there is so much methane gas, says German geologist Gerhard Bohrman, that it is constantly bubbling up into the hydrate zone. There is so much methane that, as it freezes instantaneously to form hydrate, it draws all the water out of the seafloor ooze and dries it out completely—and often there is methane left over, trapped as large bubbles in the porous hydrate. Bohrman proved that by bringing a sediment core up in an autoclave. He kept it under pressure while he had it CT-scanned in a clinic in Palo Alto. Before that he and his shipmates had seen how buoyant the hydrate was: As they worked off the coast, large blocks of it sometimes bobbed to the surface near the ship.
There is so much methane rising up under the southern summit of Hydrate Ridge that some of it bubbles all the way to the seafloor. “You build up too much free gas, and then you have an overpressured column,” says Gerald Dickens, a marine geochemist at Rice University who went to Hydrate Ridge on a drill ship in 2002. “And the gas just cracks the sediment and migrates right up to the seafloor.” Seafloor gas chimneys have been turning up in many places, Dickens says, now that researchers know how to recognize them on seismic readouts. And places where methane bubbles into the seawater, as it does at Hydrate Ridge, are commonplace around the world.
Ultimately, that methane is derived from bigger and more complex organic compounds in the buried sediment. That’s why hydrates, like oil—and like fish—tend to be found along the world’s coastlines, where the waters are rich in nutrients and plankton corpses fall like thick snow to the seafloor. It used to be thought that the methane in hydrates was made the way oil is—that Earth’s internal heat makes methane, the smallest hydrocarbon, by cracking bigger hydrocarbons at depths of more than a mile below the seafloor. But then researchers started looking at the carbon isotopes in hydrates. They found that most hydrates, compared with the sediments around them, are enriched in the isotope carbon-12 and depleted of the heavier carbon-13. Heat would not be choosy that way about the molecules it cracks. Life, however, is choosy: All living things selectively take up carbon-12 and reject carbon-13.
The carbon-isotope ratio of seafloor hydrates indicates that the methane was made by microbes. “These microbes are forming enormous amounts of gas,” says Dickens. “But it’s not like the hydrates are just building up over time, because we’re also losing methane out of these systems.” The puzzling thing is that methane isn’t bubbling up everywhere. But that puzzle was solved at Hydrate Ridge.