Tsunamis, megafarts, and mountains of the deep
It was Earth's biggest fart ever. Fifty-five million years ago, more than a trillion tons of methane burst from the ocean, sending temperatures soaring by up to i8°F extinguishing two thirds of the species in the ocean depths, and causing a major evolutionary shock at the surface. The story, while from long ago, is a reminder that methane lurks in prodigious quantities in many parts of the planet—not just in frozen bogs—and that one day it could be liberated in catastrophic quantities.
The first whiff of this prehistoric megafart was unearthed in 1991, from a hole drilled about a mile into a submarine ridge just off Antarctica. Examining the different layers of the ancient sediment removed from the hole, the geologists James Kennett, of the University of California at Santa Barbara, and Lowell Stott, of the University of Southern California in Los Angeles, found evidence of a sudden mass extinction of organisms living on the sea floor 55 million years ago. They had apparently disappeared from the ocean within a few hundred years—perhaps less. Kennett and Stott soon discovered that other researchers had detected evidence of similar extinctions from the same era, in Caribbean and European marine sediments. This was clearly a global event—one of the largest extinctions in the history of the planet.
What happened? Looking at the chemistry of fossils in the drilled sediment, the two geologists found some intriguing clues. There was, for instance, a sudden change in the ratio of two oxygen isotopes, known as oxygen-18 and oxygen-16. The ratio in the natural environment is very sensitive to temperature, and this isotopic "signature" in sediments and ice cores is a widely used indicator of past temperatures. Kennett and Stott concluded that after rising gradually for several million years, ocean temperatures had soared much more dramatically about 5 5 million years ago. The change happened at the same time as the extinctions.
The sediments also revealed a second isotopic shift, this time between isotopes of carbon. Earth's organic matter suddenly contained a lot more carbon-12. From somewhere, trillions of tons of the stuff had been released into the environment. Clearly a greenhouse gas, either carbon dioxide or methane, had caused both changes. The problem was finding a likely source with sufficient capacity to do the job.
Jerry Dickens, a biochemist at James Cook University, in Townsville, Australia, set himself the task of working out where this carbon-12 might have come from. The first suggestion was carbon dioxide in volcanic eruptions, which are a rich source of carbon-12 in the modern atmosphere. But, says Dickens, that would have required volcanic eruptions at an annual rate a hundred times the average over the past billion years. Fossil fuels like coal, oil, and natural gas were possible sources. But they are mostly buried out of harm's way, sealed in rocks. Given that there were no creatures digging them up and burning them at the time, that, too, seemed implausible. The same was true for methane from swamps and wetlands like those found today in Borneo and Siberia. About three times as many of them existed then, but even so, they could not have delivered the amount of carbon-12 required. Only one last source—big enough and accessible enough to unleash a climatic eruption—was left. That, Dickens suggested, had to be the vast stores of methane that geologists have recently been discovering frozen in sediment beneath the oceans: methane clathrates.
Methane clathrates are an enigma. They have until recently escaped the attention of oil and gas prospectors, because they don't turn up in the kind of deep and confined geological formations where prospectors traditionally look for fossil fuels. Nor are they the product of current ecosystems, such as tropical and Arctic bogs. They are generally close to the surface of the ocean floor but frozen—confined not by physical barriers but by high pressures and low temperatures, in a lattice of ice crystals rather like a honeycomb. Scientists still debate exactly how and when they were formed, but they seem to arise when cold ocean water meets methane created by microbes living beneath the seabed. Seismic surveys have revealed these structures in the top few hundred yards of sediments beneath thousands of square miles of ocean. They exist unseen, usually just beyond the edge of continental shelves. Many of these frozen clathrate structures trap even larger stores of gaseous methane beneath, where heat from Earth's core keeps them from freezing.
Dickens estimates that between i and 10 trillion tons of methane is tied up today in or beneath clathrates. But its confinement may not be permanent. Release the pressure or raise the temperature, and the lattices will shatter, pouring methane up through the sediment into the ocean and finally into the atmosphere. It seems that some such event must have happened 55 million years ago. Moreover, if this was the source of the great release of carbon-12, it would also explain why the extinctions appeared to be most serious in the ocean depths, where extensive acidification would have been almost certain. "Right now, most everybody seems to accept that the release of methane clathrates is the only plausible explanation for what happened 55 million years ago," says Dickens.
His chronology goes like this. For several million years, the world was warming, probably because of extraterrestrial influences such as the sun. The warming gradually heated sediments on the seabed until the clathrates started to shatter and release methane. Perhaps it happened in stages, with warming releasing methane that caused further global warming that released more methane. But at any rate, over a few centuries, or at most a few thousand years, trillions of tons of methane were eventually released into the atmosphere—enough to cause the observed global shift in carbon isotopes and a large and long-lasting hike in temperatures.
"The world just went into chaos," as Dickens puts it. Life on Earth was transformed almost as much as by the asteroid hit 10 million years before that wiped out the dinosaurs. Once the methane releases had ended, the planet's ecosystems gradually absorbed the remainder of the great fart, the climate recovered its equilibrium, and the oceans settled down again. But the evolutionary consequences of that long-ago event have lasted to this day. By the time the climate had recovered, many land and ocean species had become extinct, while others evolved and flourished.
"At the same time as the great warming, there was a major evolution and dispersal of new kinds of mammals," says Chris Beard, a paleontologist at the Carnegie Museum of Natural History, in Pittsburgh. It was "the dawn of the age of mammals." Among those on the evolutionary move were all kinds of ungulates—including the ancestors of horses, zebras, rhinos, camels, and cattle—and primates. And among the new primates evolving in the balmy conditions were the omomyids, the ancestors of simians, who in turn spawned humans.
Could such a cataclysm happen again? Maybe in the twenty-first century? Certainly there is still enough methane buried beneath the oceans. But could current global warming provide the trigger for its release? Some say that is unlikely; modern seawater is still much colder than it was 55 million years ago. But Deborah Thomas, of the University of North Carolina, who has analyzed the event in detail, is not so sanguine. The oceans may still be cooler, but they are also warming faster than they were 5 5 million years ago. And the pace of change may be as dangerous as the extent. If so, she says, "the trigger on the clathrate gun will be a lot touchier than it was 55 million years ago."
Apparently seaworthy ships can disappear from the ocean without warning for many reasons. They can be hit by giant waves, upturned by submarines, punctured by icebergs, or dashed onto rocks in storms. Could huge slugs of methane bursting from the ocean depths be another cause? Some say so. Take the strange case of Alan Judd and Witch Hole.
Judd is a British marine geologist at the University of Newcastle with a long interest in methane clathrates. In the late 1990s, he persuaded a French oil company to fund work in the North Sea to map giant pockmarks on the seabed. Geologists regard these otherwise inexplicable pockmarks as the aftermath of past methane eruptions from clathrates deep in the sediment. One day, about 90 miles off Aberdeen, Judd's remote-controlled probe was exploring a particularly large crater, about a hundred yards across and known to mariners as Witch Hole, when it crashed into something. Something large, metal, and unidentified, which destroyed the probe.
In the summer of 2000, Judd returned to try to find out what his probe had struck. This time he had money from a television company and a tiny remote-controlled submarine equipped with a video camera. He found the culprit. It was the steel hull of an 8o-foot trawler dating, judging by its design, from the early twentieth century. The ship sat upright on the seabed, in the middle of the crater, apparently unholed. "The boat didn't go in either end first; it went down flat," Judd said later. "It looks as though it was just swamped." The ship could have gone down in a storm, but "for the boat to have randomly landed within Witch Hole would be an amazing coincidence," he said. "It is tempting to suggest that it is evidence of a catastrophic gas escape."
Efforts to identify the ship and find contemporary reports of why it went to the bottom have so far yielded nothing. And funds for another survey have failed to materialize. But the story remains an intriguing mystery to set beside other stories of ships that apparently disappeared in calm waters. Some say methane emissions from the depths could explain the mysterious loss of ships in the area of the Atlantic known as the Bermuda Triangle, for instance. Certainly, methane clathrates have been found in the area. So, while there is much mythology and misinformation about the Triangle, it may contain some truth. "When the gas bubbles to the surface, it lowers the density of the water and therefore its buoyancy," says Judd. "Any ship caught above would sink as if it were in a lift shaft." Any people jumping overboard to save themselves would sink, too. No trace would remain—at the surface.
Meanwhile, pockmarks are turning up on the seabed almost everywhere that clathrates are found: from the tropics to the poles, from the Atlantic, the Pacific, and the Arctic to the Indian and Southern Oceans. Evidence of when methane was released from the ocean floor remains sketchy, but the signs are of major releases. At Blake Ridge, off the eastern U.S., marine geologists have found pockmarks 700 yards wide and up to 30 yards deep, like huge moon craters. And drilling studies suggest that the ridge may still have around 15 billion tons of frozen methane hidden beneath the craters, with at least as much again trapped as free gas in warmer sediments beneath the frozen zone. European researchers have found pockmarks just as big in the Barents Sea southeast of Svalbard. The widely quoted estimate that 1 to 10 trillion tons of methane is trapped down there remains a bit of a stab in the dark, but the scale sounds right.
The lattice structures that hold methane clathrates survive only at low temperatures and high pressures, so sightings are rare. Occasionally they survive briefly at the ocean surface. Fishing nets bring lumps to the surface from time to time. They fizz away on the ship's deck, releasing their methane. Alarmed fishermen usually throw them back fast. Researchers have found white clathrate chunks "the size of radishes" sitting in the mud on the bottom of the Barents Sea; sometimes they track small plumes of methane rising from the seabed to the surface. Russian researchers have reported clathrates bursting out of the Caspian Sea and igniting "like a huge blowtorch, producing flames that rise several hundred metres high." But these events are mild curiosities compared with the evidence being pieced together of major catastrophic events caused by methane releases from beneath the ocean—including events that occurred much more recently than 55 million years ago.
On the east coast of Scotland, cliff faces often show a mysterious layer of gray silt about 4 inches thick sandwiched between layers of peat. The silt seems unremarkable, except that it extends right up the coast for hundreds of miles, and is full of the remains of tiny marine organisms that are normally found only on the ocean floor. The silt was deposited about 8,000 years ago by a tsunami that surged across the North Sea after the collapse of an underwater cliff on the edge of the continental shelf west of Norway. This was a huge event. The 250-mile-long cliff slumped more than 1.5 miles vertically down the slope onto the floor of the deep ocean, taking with it a staggering 1 billion acre-feet of sediment. It spread across an area of seabed almost the size of Scotland.
The scars left by this huge submarine slide were first spotted in 1979 by Norman Cherkis, of the Naval Research Laboratory in Washington, D.C. Cherkis was using sonar equipment to scour the ocean bed for hiding places for military submarines. He assumed at first that the slide had been caused by an underwater earthquake, though there was little seismological evidence for this. That presumption was shaken by a Norwegian marine geologist, Juergen Mienert, of the University of Tromso, who saw that the area of seabed that had slumped, known as Storegga, also contained large numbers of pockmarks associated with bursts of clathrates.
Mienert suggested that the slide coincided with a rise of 1 1°F in ocean temperatures off Norway as currents carrying the warm tropical waters of the Gulf Stream became much stronger in the aftermath of the last ice age. The strong wash of warm water over a previously cold seabed would have been enough, he said, to melt clathrates. Since just i oo cubic feet of clathrate contains enough methane to produce 16,000 cubic feet of gas at normal atmospheric pressure, the releases would have had explosive force, stirring up the seabed sediments over a huge area, and creating more releases and a cataclysmic slide.
Mienert estimated that this undersea eruption released between 4 and 8 billion tons of methane—enough to heat the global atmosphere by several degrees. His theory gained dramatic support when analysis of Greenland ice cores showed a big rise in methane concentrations in the air at that time. Some argue that the methane surge came from tropical wetlands that grew as the world warmed and became wetter. Mienert disagrees, but the argument has yet to be resolved.
The tsunami certainly had a huge impact. A 40-foot wave crashed into the Norwegian coast and deposited silt 20 feet above the shoreline in Scotland. The Shetland Islands took the brunt, receiving at least two waves that left a slimy trace 65 feet above what was then sea level. In the hours after Storegga slipped, many Stone Age people must have died on the shores of Europe. And it wasn't an isolated event. There appear to have been several earlier slips at Storegga. The fear must be that it could happen again here. "There is still a lot of methane on the north side of the slide," Mienert says.
Since the discovery of the Storegga slip, the remains of a number of other, similar slips have been discovered in areas of the ocean known to harbor methane clathrates. They have turned up off British Columbia, off both the East and West Coasts of the U.S., and at the mouths of great rivers like the Amazon and the Congo, where huge offshore fans of sediment contain methane generated by rotting vegetation from the rainforests upstream. Exactly when these slips occurred is not yet certain, but Mienert believes that the thermal shock caused by Storegga may have had a domino effect, releasing other clathrates stocks already made vulnerable by the warming postglacial oceans.
Some researchers postulate a "clathrate gun" theory of climate change, in which, at the end of the ice ages and perhaps at other times, successive releases of methane instigated a worldwide warming. They see the catastrophic event 55 million years ago as just the biggest in a whole family of methane-related climate disasters.
When I met Juergen Mienert in his lab, on a hill overlooking a fjord on the edge of Tromso (suitably raised, we joked, in case of a tsunami), he was planning a major new European research project to find more remains of slides and clathrate blowouts. The Euro margins project, which he chairs, "will be targeting areas where there are both pockmarks, indicating past clathrate releases, and warm ocean currents, indicating a risk of destabilization," he said.
He is already on the trail of an ancient slip high in the Arctic, off the north coast of Spitzbergen. This area is currently warming fast and is bathed periodically in warm waters from the Gulf Stream that break through the Fram Strait into the Arctic. "Some of the world's richest methane deposits lie right below that current," he said. He showed me new survey images of the seabed there, taken on a cruise two months before, in an area known as Malene Bay. They reveal another huge event. "Look at this," he enthused. "Look at the height of the cliff that fell. It was 1,500 yards high.
The prognosis, Mienert says, is worrying. Current conditions are disturbingly similar to those in which the great methane releases of the past happened—fast-rising sea temperatures penetrating the sediment and defrosting the frozen methane. Global warming, he believes, "will cause more blowouts and more craters and more releases." The risk of a giant tsunami blasting into Europe, the most densely populated continent on Earth, at the same time that a huge outburst of methane pushes climate change into overdrive is disturbing, to say the least.
Some argue that such concerns are exaggerated. It would take decades or even centuries for a warming pulse from the ocean to penetrate sediment to the zones where methane clathrates generally cluster. But Mienert counters that clathrates are being found ever closer to the surface, particularly in the Arctic. In any event, there is a second and much faster route downward for the heat. The U.S. naval researcher Warren Wood has discovered that seabed sediments often contain cracks that extend into the frozen clathrate zone. Warm water takes no time to penetrate the cracks and can quickly unleash the methane. As Richard Alley said of the crevasses inside ice caps, "Cracks change everything."
Methane is only the third most important greenhouse gas, after water vapor and carbon dioxide. But, says Euan Nisbet, "arguably it is the most likely to cause catastrophic change." This is "because the amount needed to change climate is smaller than for carbon dioxide, and because the amount of the gas available, in soils and especially methane clathrates, is so large." Methane has clearly had catastrophic effects in the past. In the dangerous world of sudden and unstoppable climate change, methane is the gunslinger.
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