Andreia Plaza Faverola/CAGE
Massive craters pockmark the bottom of the Barents Sea, north of Norway. They point to past explosions there, a new study finds. As domes of frozen methane destabilized within this seabed some 12,000 years ago, they blasted open the seafloor. Triggered by the waning of the last Ice Age, these events are truly ancient history. But they also point to the power of Arctic warming — underway once again — to reshape the polar oceans.
The craters are huge and extensive. What’s more, they formed violently, the new study concludes. Karin Andreassen is a marine geologist at the Arctic University of Norway in Tromsø. She notes that the craters were first discovered in the early 1990s. Until now, however, no one realized how big most were or how extensively they pitted the seafloor.
Andreassen’s team found more than 100 giant craters within an area covering 440 square kilometers (170 square miles) — half the size of Dallas, Texas. Some craters span a kilometer (0.6 mile) across. The team also discovered previously unknown, 20-meter (65-foot) tall domes near some of the craters.
Andreassen and her team described their findings June 2 in Science.
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Rich deposits of methane hydrate underlie much of the Arctic seafloor. This material is basically ice made from methane gas. It occurs in seabed layers at sites (such as the Barents Sea) that are rich in methane. Cold temperatures, combined with the weight of water and rock there, crush the gas into a frozen, super-concentrated form.
Some 16,000 years ago, the Barents Sea was covered by an ice sheet two kilometers (1.2 miles) thick. That provided all the pressure and cold needed to keep the methane hydrate stable, Andreassen explains. But then, this massive sheet of ice began to melt. The zone where methane hydrate remained stable now shrank. In time, the icy hydrate began collecting in bulging domes atop the seafloor.
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When the ice thinned and temperatures warmed enough, the methane hydrate disintegrated. It literally exploded back into a gas like ginormous burps. That gas burst out of the domes, leaving giant craters. What makes this really impressive, Andreassen notes, is that “the craters are carved into bedrock, not just sediment. That means the force must have been quite enormous when they blew.”
The researchers also discovered plumes of methane that are still streaming out of these craters, 12,000 years later.
To collect their data, Andreassen and her team spent weeks on a research ship on the stormy Barents Sea. (This meant that much of the time they were fighting seasickness, she says.) The researchers used a range of tools to map the seafloor’s topography — changes in its highs and lows. For instance, they broadcast acoustic signals, creating pressure waves at the seafloor. By listening for the return signal — a technique known as echo sounding — they were able to see the craters and domes in high resolution, three-dimensional detail. They also used seismic waves to explore structures below the seabed. Finally, they tracked methane plumes by monitoring the sound waves caused by rising gas bubbles.
Later, back in Tromsø, the researchers ran computer models. These attempted to determine how ice thicknesses, temperatures and gas-hydrate stability likely changed over tens of thousands of years.
This multipronged approach makes the study especially useful, says geologist Carolyn Ruppel. She heads the gas hydrates program at a U.S. Geological Survey field center in Woods Hole, Mass., and was not involved in the new work. “The data is really complete,” says Ruppel. “They have the data below the seafloor, the seafloor data — and to top it off they have the water column with the gas plumes.”
The domes look strikingly similar to dry-land Arctic features called pingos. Indeed, she says, the new study provides an “elegant” explanation for how pingo-like structures might form underwater.
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Domes of doom?
Today, Earth is experiencing a rapid global warming. And no place is warming faster than the Arctic. The new findings point to what might occur if Earth’s fever continues to rise.
Methane is a powerful greenhouse gas, for example. Normal methane plumes only bubble up about 200 meters into the water, notes Andreassen. Violent dome explosions, however, may be a different story. “I think when the craters formed, methane definitely had the potential to reach the surface,” she says.
Might future exploding methane domes, triggered by melting ice, release extra methane into the atmosphere? Or, will valuable reservoirs of this natural gas be blown away in a blast of bubbles? (Research centers for years have been looking at these hydrates as a fossil-fuel source that might be mined to provide energy.) Answers to these questions are unknown, says Andreassen.
Yet people may glean some answers soon. Days after this study was published, a second, related study came out. It was authored by a team of scientists who were led by Andreassen’s graduate student Pavel Serov. Its new report appeared June 5 in the Proceedings of the National Academy of Sciences.
Its discovery? More big domes on the bottom of the Barents Sea, not far from Andreassen’s ancient craters. But the newly reported domes aren’t old. They are fresh, and they are leaking methane like crazy. Any year now, the domes could blow, Serov’s group now predicts. All it will take is a bit more warming.
acoustic Having to do with sound or hearing.
Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice.
bedrock The thick, solid rock layer than underlies the soil and other broken, rocky materials on Earth’s surface.
crater A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.
echo sounding A type of sonar, it’s a technique to measure the depth of water to an object or the seafloor. Pressure waves are broadcast into the water. The time it takes for the return wave — its echo — offers a clear gauge of the distance.
elevation The height or altitude at which something exists.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
geologic An adjective that refers to things that are related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.
global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.
graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
ice age Earth has experienced at least five major ice ages, which are prolonged periods of unusually cold weather experienced by much of the planet. During that time, which can last hundreds to thousands of years, glaciers and ice sheets expand in size and depth. The most recent ice age peaked 21,500 years ago, but continued until about 13,000 years ago.
ice sheet A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.
literally A term that means precisely what it says. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.
marine Having to do with the ocean world or environment.
methane A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.
methane hydrate Molecules of methane gas trapped — and compressed under pressure — within a lattice of water ice. If warmed or the pressure drops, this material will again become regular water and methane (the principle component of natural gas). As it melts, one cubic meter of gas hydrate will release 164 cubic meters of natural gas. Deposits of this may be quite thick meters thick and tend to develop under permanently frozen Arctic soils or beneath the polar seafloor.
natural gas A mix of gases that developed underground over millions of years (often in association with crude oil). Most natural gas starts out as 50 to 90 percent methane, along with small amounts of heavier hydrocarbons, such as propane and butane.
pingo Hills that form across the Arctic and subarctic landscape as water collects underground, freezes and then expands to push the soil upward and out.
plume (in geology) Fluids (air, water or magma typically) that move, largely intact, in a feather-like shape over long distances.
pressure Force applied uniformly over a surface, measured as force per unit of area.
reservoir A large store of something. Lakes are reservoirs that hold water. People who study infections refer to the environment in which germs can survive safely (such as the bodies of birds or pigs) as living reservoirs.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sediment Material (such as stones and sand) deposited by water, wind or glaciers.
seismic wave An energy wave traveling through the ground produced by an earthquake or some other means.
sound wave A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.
Texas The second largest state in the United States, located along the southern border with Mexico. It is about 1,270 kilometers (790 miles) long and covers an area of 696,000 square kilometers (268,581 square miles).
U.S. Geological Survey (or USGS) This is the largest nonmilitary U.S. agency charged with mapping water, Earth and biological resources. It collects information to help monitor the health of ecosystems, natural resources and natural hazards. It also studies the impacts of climate and land-use changes. A part of the U.S. Department of the Interior, USGS is headquartered in Reston, Va.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
Journal: P. Serov et.al. Postglacial response of Arctic Ocean gas hydrates to climatic amelioration. Proceedings of the National Academy of Sciences. Posted online June 5, 2017. doi: 10.1073/pnas.1619288114
Journal: K. Andreassen et al. Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor. Science. Vol. 356, June 2, 2017, p. 948. doi: 10.1126/science.aal4500.