The crystal fuel

Natural History, May, 1997 by Kevin Krajick

In 1976 scientists on an international drilling expedition off the coast of Guatemala brought up a core of gritty mud from the deep-sea floor. In it were several softball-size nodes of a whitish, icelike material no one on board had ever seen before. The nodes popped, hissed, and snapped like frying bacon, sputtering out gas and throwing off droplets of liquid. Within minutes the only thing left was a puddle of water. "We measured and photographed it every which way," says Bill Harrison, a geologist at the Idaho National Environmental Engineering Laboratory, who was on board. "We knew what we had."

What they had were bits of gas hydrates--chemical compounds that are increasingly being studied for their potential as huge reservoirs of energy, as possible causes of sea floor instability, and even as significant contributors to global warming. Scientists had long speculated about them and even made them in the laboratory, but until now only a few had been seen in nature.

Hydrates are a peculiar combination of two common substances: water and natural gas, usually methane. If these meet in sediments where pressure is high and temperature low--a combination produced under oceans and the permanency frozen subsoil (permafrost) of polar regions--they join together in an extraordinarily compact form found nowhere else. But retrieving a chunk is extremely difficult. Unlike diamonds or oil, hydrates quickly decompose into their two components when they are pulled up from the depths toward warmth and low pressure--thus the rush of gas and water that the scientists saw on deck. They were lucky to witness even that; decomposition usually happens long before hydrates hit the surface.

Since the 1970s, scattered studies have spotted about fifty areas where significant deposits may exist, says Keith Kvenvolden, a U.S. Geological Survey scientist in Menlo Park, California. There is a rough consensus that 20 quadrillion cubic meters of methane are locked up in hydrates--enough to blanket the earth's surface with gas 130 feet thick. Measured another way, hydrates may hold 10 trillion tons of carbon, twice as much as the earth's coal, oil, and conventional gas reserves combined. The estimate is based on just a few dozen recovered hydrate samples, as well as on seismic profiles of the sea floor, chemical analyses of drill cores, and extrapolations about where conditions exist for hydrate formation. "There's still a lot of speculation," notes Kvenvolden. "God knows what the reality is."

Studies show that most of the gas for hydrates is made when anaerobic bacteria break down organic matter under the sea floor, producing methane and an array of gaseous byproducts, including carbon dioxide, hydrogen sulfide, and small amounts of ethane and propane, all of which can be incorporated into hydrates; methane, however, predominates. Various thermal reactions can also produce methane for gas hydrates. As gases rise, they are dissolved in whatever water is in the sediment. At the right pressure-temperature combination--the "stability zone"--hydrates will form. In theory, most ocean bottoms provide stability zones, but in practice, hydrates seem confined to the edges of continents, where nutrient-rich waters send a rain of organic detritus into the muds for bacteria to turn into methane and where the water is at least 1,000 feet deep. Although gas hydrates can be found at the sea floor, their usual range is 325 to 3,600 feet beneath it. In permafrost, they can occur at shallower depths because of colder temperatures.

The basic hydrate unit is a hollow crystal of dozens of water molecules; inside, a single gas molecule floats freely. The crystals fit together in an extremely tight latticework. To the naked eye, hydrates (also known as clathrates, from the Greek and Latin words for "cage-work") look like ice. But hydrate!crystals are cubic rather than hexagonal, and hydrates usually form above the freezing point of water. Also, unlike ice, hydrates can be set on fire because they are amazingly efficient packages for flammable methane. The crystal structure effectively contains the compressed gas; in a cubic foot of hydrates, gas will expand in volume to about 164 cubic feet at standard temperature and pressure.

Depending on local conditions, hydrates may be found in nodules, veins, solid layers, or finely dispersed particles between sediment pores. But they usually can't be seen. Only in a few places--the Gulf of Mexico is one--do visible lobes crop out on the sea floor. Hydrates have also been found under sediment off the Oregon coast, in 10,000-foot-deep water. In 1995 geologists drilling along the peat-rich Arctic coast of Canada's Northwest Territories discovered hydrates for the first time in permafrost itself, 400 feet down. Russian and Japanese scientists have found hydrates deep in the Antarctic ice sheet, formed from ancient bubbles of atmospheric nitrogen and other gases.

Since hydrates are nearly impossible to observe directly, researchers have relied on a surrogate: making them in the laboratory. This involves shaking, heating, cooling, or otherwise manipulating gas and water or ice particles for hours or days--a process that probably little resembles what actually happens in nature.