Hydrate Ridge
off the Oregon Coast
Hydrate Ridge is an area of ocean floor about 60 miles from the Oregon coast. Scientists have found massive rock of frozen methane along Hydrate Ridge. The ridge is located about 6 miles east of the deformation front where the Pacific tectonic plate slides under North America.
Scientists call the material ice-like. It is frozen solid but does not feel the same as ice. It is actually a form of clathrate – this word comes from the Latin word for `cage`. In its frozen form the gas is called methane hydrates. These are molecules of methane trapped in `cages` of H20. Six water molecules envelope a single methane molecule then the cages are connected together form a crystal. Enormous pressure is required to release methane into the water. However, if the pressure is more than 30 times the normal atmospheric pressure water hydrates can form at temperatures above 32 degrees Fahrenheit. It is common just off the continental shelf where it drops into the dark cold water,
Methane hydrates react to the temperatures and pressures that form above and define how deep the methane hydrates exist. Deep below the ocean floor is where the temperature gets too high, toward the Earth’s hot core. The top of the zone is where the pressure is too low and the hydrates rise to the surface. One either side of this zone, methane and water molecules separate. A piece of frozen methane hydrate brought to the surface melts away from the heat and the methane gas is released into the air.
In the 1960s, Russian scientists found natural methane hydrates in Siberian permafrost. The permafrost layer is cold enough to allow hydrates to form at shallower waters and at lower pressure forces than those under the sea. In the 1970s, methane hydrates were found to be abundant at the bottom of the Black Sea. It is said that there is so much methane that seeps onto the water surface that it has caused lightening to shoot upwards into the sky.
Conventional wisdom said that lightening could only come down from storm clouds. But sailors knew about this phenomenon long before scientists had it figured out. It was a decade before researchers drilled into the ocean and pulled up small cores of frozen hydrates. This is where the new science was developed for studying this process and the realization that frozen methane hydrates were massive.
Hydrate Ridge is one of several ridges off Oregon the Oregon coast. As the Pacific tectonic plate slides underneath the North American continent the movement stirs up layers of muddy sediment. Methane is so pressurized that it seeps out of the deepest layers of sediments and travels upward. At the summit of the southern region of Hydrate Ridge at approximately 2,500 feet below the sea surface is a range of small peaks and valleys some 10 to 20 feet across. It was here that scientists found thick, pure methane hydrate.
Underneath Hydrate Ridge, there is a huge reservoir of methane. There is a layer of frozen hydrate deposits and on top of that a layer of free methane gas filling up pores and cracks in the sediment. This is how deposits are usually found since the dividing line of frozen and free methane can be detected by a survey ship’s sonar. Under some parts of Hydrate Ridge there is so much methane gas, German geologist Gerhard Bohrman, says that it is constantly percolating up into the hydrate zone. The vast amounts of methane freeze instantly to form hydrate and in doing so suck up the water from the seafloor, leaving behind methane trapped like bubbles in the porous hydrate. To prove this theory, Bohrman raised up a sediment core up in an autoclave. While keeping it under pressure, he ran a CT scan in a clinic in Palo Alto. It was obvious that the hydrate was buoyant. They had witnessed this early in the research when large chunks popped up to the surface close to the ship.
The methane rising up under the southern summit of Hydrate Ridge is reaching the seafloor. Gerald Dickens a marine geochemist at Rice University went to Hydrate Ridge in 2002 that saw bubbles at that depth. Dickens explained: “You build up too much free gas, and then you have an over pressured column. And the gas just cracks the sediment and migrates right up to the seafloor. Seafloor gas chimneys have been turning up in many places now that researchers know how to recognize them on seismic readouts.” There are many regions around the world where methane seeps into the seawater like it does at Hydrate Ridge.
Hydrates consumed along the food chain end up in larger fish. Along coastlines, methane is in abundant where the waters are rich in nutrients and dead plankton fall to the sea floor. At one time it was believed that the methane in hydrates was produced the same way oil is from the earth’s internal heat. The theory was that intense heat produced methane, with the smallest hydrocarbon, by opening cracks in depths of over a mile below the seafloor.
However, when scientists examined the carbon isotopes in hydrates more closely they were surprised at their findings. Compared with the sediments located in proximity, they found that most hydrates are enriched in the isotope carbon-12 and in heavier carbon-13. Heat simply cracks all the molecules and all living things specifically take up carbon-12 and reject carbon-13.
According to Dickens: “The carbon-isotope ratio of seafloor hydrates indicates that the methane was made by microbes. These microbes are forming enormous amounts of gas. But it’s not like the hydrates are just building up over time, because we’re also losing methane out of these systems.” But what surprised scientists is that methane is not escaping evenly around the globe. But a closer look at Hydrate Ridge revealed the answers.
In a research expedition off the cost of Oregon in 1999, Antje Boetius, bio geologist for Marine Microbiology in Bremen, collaborated closely with Seuss’s group. She was working on her own research and wanted to understand the fundamental techniques of molecular ecology related to her studies in the Indian Ocean. Boetius’ research was not related to hydrates but she made a discovery at Hydrate Ridge that changed the focus of her research.
Boetius noticed clusters of organisms growing where methane seeps from the ocean floor. These clumps of organisms were first found by Suess and are called a ‘cold-seep community’ In earlier research, colleagues explored a seafloor hot spring near the Galápagos Islands, bringing up specimens of giant white clams collected with the submersible Alvin. In 1984 Suess went down in Alvin about 5,000 miles northwest of the Galápagos and just a few miles west of Hydrate Ridge. Although there were no hot springs Suess found “...there were the same damned critters.” he says. The variety of organisms living there were not only clams but also tube worms and thick mats of bacteria, white or bright orange. Many years later Suess would locate the same mats of bacteria all over Hydrate Ridge.
Boetius’s discovery came while she was examining sediment slices with her microscope. She used probes to detect 20 different species of sulfate-reducing bacteria until she found one that would produce glowing dots on the screen. The probe from DeLong and Hinrichshad worked right away: the Hydrate Ridge sediments were abundant with their methane eater. It turns out that the bacteria was not actually bacteria but a species of Archaea, an ancient group of microbes that evolved from bacteria billions of years ago but are genetically distinct from them now.
Boetius was counting the glowing dots showing on her computer screen. She wanted to determine how many of each were contained in her sediment samples. Counting these tiny microbes was a tedious undertaking until Boetius noticed something. “I see these stupid clusters of archaea, and now I see these stupid clusters of sulfate reducers. And they had a very funny shape. The archaea looked like real clumps—lots and lots of cells sitting together. The sulfate reducers were like shells, a circle of sulfate reducers with nothing in the middle. And, really, I sat there for two hours before it finally popped into my head.”
The sulfate reducers were attached to the archaea, forming a shell around them. Tori Hoehler’s idea of a microbial consortium seemed closer to solving the mystery. In earlier research, scientists had repeatedly injected methane into the sediment. In this experiment though the methane vanished and sulfide appeared instead. Each clump was less than one-thousandth of an inch across and contained hundreds of cells. It is estimated that there are in the vicinity of 900 million clumps in every ounce of sediment at Hydrate Ridge.
The archaea in the Boetius clumps were close relatives of other archaea, those that produce the methane to begin with. The archaea methane makers use the gas from hydrogen and carbon dioxide; the methane eaters are similar but do not give off hydrogen. They pass energy onto the sulfate reducers that surround them. “There’s some kind of delicate interaction that we do not understand, says Widdel. He has an experiment underway to grow a consortium in a laboratory. Past attempts had failed but with the understanding that microbes grow very slowly Widdell says: “We know it will take time. We might need two or three Ph.D. theses.”
What is so important about this kind of research? Wherever the researchers have looked they have found the consortium in over 20 or so other places around the world: in a mud volcano in the Arctic Ocean and at cold seeps and hydrate mounds in the Gulf of Mexico. Recently, in a crater off the Democratic Republic of the Congo at 10,000 feet down, a team of French researchers led by Myriam Sibuet of the French Research Institute for Ocean Exploitation, found an amazing cold seep with a large tracks of clams and mussels, blue shrimp, purple sea cucumbers, and six-foot-long tube worms growing in bushes next to mounds of gas hydrate. Boetius’s microbes were also in the mud and she thinks her consortium provides sulfide at cold seeps everywhere. This is the basis for the food chain for a world that exists on the ocean floor and beyond.
