Listening for gas bubbles

What if we could cheaply and efficiently detect a potent new energy source, while also monitoring for environmental safety? Olivier Carrière, a physicist in the Marine Physical Laboratory at the Scripps Institution of Oceanography, and other researchers are using the symphony of sound produced in the ocean to do just that.

When natural gas is released from the seafloor, it produces bubbles; similarly, gas leaking from a pipeline also produces bubbles. Instead of traditional acoustic methods that use active surveys of the ocean floor with sonar or seismic techniques, researchers are developing a revolutionary method that listens for these bubbles passively. If successful, this new advancement could change the way we survey the oceans.

The new passive acoustic techniques allow researchers to listen to the bubbles to identify both gas hydrate deposits – which could be an energy source or a potential hazard – and to keep watch over subsea natural gas pipelines. Read more about this online at http://www.earthmagazine.org/article/listening-gas-bubbles or order the full March issue at EARTH online (http://www.earthmagazine.org).

Make sure to check out the other stories in this month’s issue of EARTH: discover whether the Japanese tsunami made their people more vulnerable, find out what mystifying metal was discovered in the Earth’s mantle, and see if you have the skills to crack this month’s “Where on Earth?”

The physics of earthquake forecasting

One year on from the magnitude-9.0 earthquake that unleashed a devastating tsunami and caused a partial meltdown of the Fukushima Daiichi nuclear plant, this month’s special issue of Physics World, on the theme of “Physics and the Earth”, includes an investigation by journalist Edwin Cartlidge into the latest advances in earthquake forecasting.

In addition to the special issue, physicsworld.com hosts an exclusive video documentary reviewing the fundamental science behind earthquakes and assessing the current efforts that are being made around the world to forecast these events. From Monday 27 February the video can be viewed here http://physicsworld.com/cws/article/multimedia/48784.

The huge responsibility that comes with assessing the likelihood of earthquakes was never more evident than in March 2009, when a group of 11 Italian scientists met to discuss the risk of a powerful earthquake striking the town of L’Aquila, after a swarm of small quakes had hit the area continuously for four months.

After concluding that there were no grounds for alarm, a devastating magnitude-6.3 earthquake struck the town on 6 April that year, leaving 308 people dead. Now, seven of those 11 scientists are on trial for manslaughter.

Thomas Jordan, chairman of the International Commission on Earthquake Forecasting (ICEF), argues in Physics World that the tragedy at L’Aquila highlights how vital it is for us to understand which are the most reliable types of forecasting, so that we have the best possible information at our fingertips.

Finding specific, natural events that may flag up an impending earthquake has been given a lot of thought; for example, a long-standing idea is that animals flee a specific area after somehow sensing an upcoming quake.

These precursors are unconvincing, however; and while we are unlikely to ever be able to predict precisely when, where and with what magnitude particular earthquakes will strike, much can be gained from short-term “probabilistic” forecasting, which can give the odds that an earthquake above a certain size will occur within a given area and time.

Still, these short-term “probabilistic” methods have their limitations, as was demonstrated a year ago this month when even the most up-to-date models did not predict the Japanese earthquake.

“This approach is tricky because no-one can quite agree on which are the best models. So, we have uncertainty on uncertainty. But can we ignore the information that they give
us? The earthquakes in L’Aquila and New Zealand taught us we don’t have that luxury,” says Jordan.

The special issue of Physics World can be downloaded as a PDF free of charge from Thursday 8 March at physicsworld.com.

Also in this issue

  • Lessons from Fukushima — Mike Weightman, UK’s chief inspector of nuclear installations — discusses what we can learn from last year’s nuclear incident

  • Physics and fracking — journalist Jon Cartwright examines how physicists can help assess the controversial process of releasing gas from shale by pumping sand and chemicals in at high pressures

  • The Earth from afar — a set of stunning images of our planet produced using a range of visualization techniques

  • Prospecting with geoneutrinos — how tiny almost massless neutrinos, generated from radioactive decay deep within the Earth, could shed light on the interior of our planet

  • When north moves south — could the movement of tectonic plates explain the variation in the rate of reversal of the Earth’s magnetic field?

  • A pressing matter — studies of the conditions deep inside our planet suggest that its core may contain immense crystals of iron up to 10 km long

When continents collide: A new twist to a 50 million-year-old tale

Fifty million years ago, India slammed into Eurasia, a collision that gave rise to the tallest landforms on the planet, the Himalaya Mountains and the Tibetan Plateau.

India and Eurasia continue to converge today, though at an ever-slowing pace. University of Michigan geomorphologist and geophysicist Marin Clark wanted to know when this motion will end and why. She conducted a study that led to surprising findings that could add a new wrinkle to the well-established theory of plate tectonics – the dominant, unifying theory of geology.

“The exciting thing here is that it’s not easy to make progress in a field (plate tectonics) that’s 50 years old and is the major tenet that we operate under,” said Clark, an assistant professor in the Department of Earth and Environmental Sciences in the College of Literature, Science, and the Arts.

“The Himalaya and Tibet are the highest mountains today on Earth, and we think they’re probably the highest mountains in the last 500 million years,” she said. “And my paper is about how this is going to end and what’s slowing down the Indian plate.”

Clark’s paper is scheduled for online publication Feb. 29 in the journal Nature.

In it, she suggests that the strength of the underlying mantle, not the height of the mountains, is the critical factor that will determine when the Himalayan-Tibetan mountain-building episode ends. The Earth’s mantle is the thick shell of rock that separates the crust above from the core below.

According to the theory of plate tectonics, the outer part of the Earth is broken into several large plates, like pieces of cracked shell on a boiled egg. The continents ride on the plates, which move relative to one another and occasionally collide. The tectonic plates move about as fast as your fingernails grow, and intense geological activity – volcanoes, earthquakes and mountain-building, for example – occurs at the plate boundaries.

The rate at which the Indian sub-continent creeps toward Eurasia is slowing exponentially, according to Clark, who reviewed published positions of northern India over the last 67 million years to evaluate convergence rates. The convergence will halt – putting an end to one of the longest periods of mountain-building in recent geological history – in about 20 million years, she estimates.

And what will cause it to stop?

Until now, conventional wisdom among geologists has been that the slowing of convergence at mountainous plate boundaries was related to changes in the height of the mountains. As the mountains grew taller, they exerted an increasing amount of force on the plate boundary, which slowed the convergence.

But in her Nature paper, Clark posits that a different model, one based on the strength of the uppermost mantle directly beneath the mountains, best explains the observed post-collisional motions of the Indian plate.

By “strength” Clark means the uppermost mantle’s ability to withstand deformation, a property called viscous resistance. Clark suggests that the relatively strong mantle directly beneath Tibet and the Himalayas acts as a brake that slows – and will eventually halt – the convergence of the two continents.

“My paper is arguing that it’s not the height of the mountains, it’s the strength of the mantle that’s controlling this slowing,” Clark said. “This is something that hasn’t been considered before and basically grew out of field observations in northern Tibet.”

But viscous resistance doesn’t tell the whole story. Other factors may also contribute to the slowing of the Indian plate, Clark said.

“For me, critical field observations showed that the northern edge of the Tibetan Plateau hasn’t moved since the collision 50 million years ago,” she said. “Therefore, the Tibetan Plateau is getting smaller in width. It’s like squeezing a box and making it narrower while squeezing it up.”

The rate at which the box is being squeezed is the average rate of mountain-building, and it provides important clues about the factors controlling plate motion. Clark analyzed how the convergence is slowing as compared to the shrinking of the plateau.

“If the height of the mountains were important in slowing India’s convergence, then the rate of mountain-building should also slow down as the Himalaya and Tibet grew to high elevation,” Clark said. “But when I analyzed how the mountain-building rate changed over the past 50 million years, I was surprised to find that it didn’t change at all.

“From this I conclude that the strength of the uppermost mantle is keeping this mountain- building constant. But as the box is shrinking, the plate motion must slow down to keep the shrinking rate the same,” she said.

3 scientific expeditions seek treasure under the ice in the Frozen Continent

In a modern iteration of the great age of Antarctic exploration of the 19th and 20th centuries, three teams of scientists are rushing to reach not the South Pole like Roald Amundsen, Robert Falcon Scott and Ernest Shackleton, but lakes deep below the surface of the Frozen Continent believed to hold scientific treasures. That quest by Russian, British and American scientific teams for water samples is the topic of an article in the current edition of Chemical & Engineering News, the weekly newsmagazine of the American Chemical Society (ACS), the world’s largest scientific society.

C&EN European Correspondent Sarah Everts explains that the Russian mission to Lake Vostok captured global headlines recently when the team bored 2.5 miles through Antarctic ice to reach the lake’s ancient water, undisturbed for 15 million years. They want to analyze the lake for signs of life and clues about how life might survive in Earth’s most inhospitable places – or on other planets. But that step must wait until late in 2012 when the Antarctic winter ends, allowing travel into the Frozen Continent.

But the Russians are only one team of several trying to understand what kind of life can survive in water beneath the Antarctic ice sheet and how these organisms might do it. The other two may yield even greater scientific treasures. One is an American team that plans to drill with hot water – rather than mechanically, as the Russians did – into a river of ice one half mile below the surface that carries water from several underground lakes to the ocean.

Using the same method, British scientists will try to reach Lake Ellsworth, almost two miles below the surface, which may have been isolated for a million years. They hope to make a complete survey of life and nutrient sources in Ellsworth, which is not yet possible for the deeper, colder and more ancient Lake Vostok.

Contamination of La Selva geothermal system in Girona, Spain

The system works by refilling meteoric waters that penetrate the earth in high areas, move underground and reach an unknown thermal hot spot, where they heat up and acquire CO2, and probably metals as well. Afterwards the water leaches out (dissolve) the host rocks and flows out from the upwellings. -  SINC/A. Navarro et al.
The system works by refilling meteoric waters that penetrate the earth in high areas, move underground and reach an unknown thermal hot spot, where they heat up and acquire CO2, and probably metals as well. Afterwards the water leaches out (dissolve) the host rocks and flows out from the upwellings. – SINC/A. Navarro et al.

Monitoring the construction of wells, avoid over-exploiting cold groundwater close to hot groundwater, and controlling mineral water extraction. These are the recommendations from the Polytechnic University of Catalonia and the University of Barcelona, after analyzing the contamination of La Selva geothermal system, above all by arsenic pollution. In this region, which is known for its spa resorts and bottling plants, as well as in other Catalan coastal mountain ranges, uranium levels higher than what is recommended by the WHO have been detected.

The groundwater in La Selva (Girona, Spain) area show high levels of arsenic, antimony and other polluting elements. The area’s geothermal system, where hot and cold groundwater flow naturally, are the cause of this situation, according to a study that researchers from the Polytechnic University of Catalonia (UPC) and the University of Barcelona (UB) have published in the journal Geothermics.

“The system works by refilling meteoric waters that penetrate the earth in high areas, move underground and reach an unknown thermal hot spot, where they heat up and acquire CO2, and probably metals as well “Andrés Navarro, a lecturer at UPC and co-author of the project explained to SINC. “Afterwards the water leaches out (dissolve) the host rocks and flows out from the upwellings”.

In this process the waters naturally pick up pollutants. Consequently, the researchers have found high volumes of arsenic, silver, lead, antimony, zinc and other metals in the hydrothermal deposits, especially in the area of Caldes de Malavella (Girona), an area famous for its spa resorts and mineral water bottling companies.

The results show that groundwater in some areas has arsenic levels of up to 0.069 mg/l, when the legal limit in Spain and the rest of the European Union was 0.01 mg/l in water for human consumption.

“Fortunately, a few years ago a legislation for this was created, and since then bottled mineral has been controlled, although before then this did not occur” the researcher says. Furthermore, a plant to remove arsenic from public water supplies has been built in Caldes de Malavella.

In any case, the study recommends controlling the extraction of water for bottling plants, as well as over-exploiting cold groundwater close to hot water springs. This way the mixing of waters is avoided and so are the pollutants.

For the same reason it is not advisable to build wells close to geothermal upwellings, especially illegal ones for private supplies or irrigation. Studying geothermal liquid outlets in areas of diffused discharge, such as some humid areas, is also proposed.

The problem of uranium in waters

“Making a model for managing the whole aquifer to rationalize water extraction and consumption would be the solution” Navarro says. He also highlights the need to take action regarding the natural water pollutants: uranium.

The analysis in La Selva shows “relatively high” levels (37.7 microg/l) of this element, especially in samples from sources and wells not directly linked to thermal activity.

The mobility of uranium is linked to granite rocks and is frequently found in Catalan coastal mountain ranges, where researchers have carried out specific studies on the topic. The samples have been taken from wells and drilling at a depth of up to 100 metres.

The results published in the journal Tecnología del agua and they show “significant concentrations” of uranium in groundwater used for public supply and bottling. Specifically, in some parts of the Montseny-Guilleries mountain range, these levels are more than 140 microg/l.

There are no legal limits for uranium concentrations in water in the European Union, but the analysis carried out in Catalan coastal mountain ranges shows that these levels far exceed the recommendations of the World Health Organisation (WHO), or for example, the standard established by the Environmental Protection Agency (EPA) in the USA.

Both the EPA and the WHO establish a maximum uranium level of 30 microg/l. The European Food Safety Authority (EFSA) is considering establishing a guideline level for this element, but at the moment the legal loophole remains.

The toxicity of uranium is linked to the solubility of the compound: the more soluble it is, the more toxic it is. The experiments carried out on animals and people show the most affected organ is the kidney, and alterations in reproduction and development are seen when this element exists in high concentrations.

Salty soil can suck water out of atmosphere: Could it happen on Mars?

The frigid McMurdo Dry Valleys in Antarctica are a cold, polar desert, yet the sandy soils there are frequently dotted with moist patches in the spring despite a lack of snowmelt and no possibility of rain.

A new study, led by an Oregon State University geologist, has found that that the salty soils in the region actually suck moisture out of the atmosphere, raising the possibility that such a process could take place on Mars or on other planets.

The study, which was supported by the National Science Foundation, has been published online this week in the journal Geophysical Research Letters, and will appear in a forthcoming printed edition.

Joseph Levy, a post-doctoral researcher in OSU’s College of Earth, Ocean, and Atmospheric Sciences, said it takes a combination of the right kinds of salts and sufficient humidity to make the process work. But those ingredients are present on Mars and, in fact, in many desert areas on Earth, he pointed out.

“The soils in the area have a fair amount of salt from sea spray and from ancient fjords that flooded the region,” said Levy, who earned his doctorate at Brown University. “Salts from snowflakes also settle into the valleys and can form areas of very salty soil. With the right kinds of salts, and enough humidity, those salty soils suck the water right out of the air.

“If you have sodium chloride, or table salt, you may need a day with 75 percent humidity to make it work,” he added. “But if you have calcium chloride, even on a frigid day, you only need a humidity level above 35 percent to trigger the response.”

Once a brine forms by sucking water vapor out of the air, Levy said, the brine will keep collecting water vapor until it equalizes with the atmosphere.

“It’s kind of like a siphon made from salt.”

Levy and his colleagues, from Portland State University and Ohio State University, found that the wet soils created by this phenomenon were 3-5 times more water-rich than surrounding soils – and they were also full of organic matter, including microbes, enhancing the potential for life on Mars. The elevated salt content also depresses the freezing temperature of the groundwater, which continues to draw moisture out of the air when other wet areas in the valleys begin to freeze in the winter.

Though Mars, in general, has lower humidity than most places on Earth, studies have shown that it is sufficient to reach the thresholds that Levy and his colleagues have documented. The salty soils also are present on the Red Planet, which makes the upcoming landing of the Mars Science Laboratory this summer even more tantalizing.

Levy said the science team discovered the process as part of “walking around geology” – a result of observing the mysterious patches of wet soil in Antarctica, and then exploring their causes. Through soil excavations and other studies, they eliminated the possibility of groundwater, snow melt, and glacial runoff. Then they began investigating the salty properties of the soil, and discovered that the McMurdo Dry Valleys weather stations had reported several days of high humidity earlier in the spring, leading them to their discovery of the vapor transfer.

“It seems kind of odd, but it really works,” Levy said. “Before one of our trips, I put a bowl of the dried, salty soil and a jar of water into a sealed Tupperware container and left it on my shelf. When I came back, the water had transferred from the jar to the salt and created brine.

“I knew it would work,” he added with a laugh, “but somehow it still surprised me that it did.”

Evidence of the salty nature of the McMurdo Dry Valleys is everywhere, Levy said. Salts are found in the soils, along seasonal streams, and even under glaciers. Don Juan Pond, the saltiest body of water on Earth, is found in Wright Valley, the valley adjacent to the wet patch study area.

“The conditions for creating this new water source into the permafrost are perfect,” Levy said, “but this isn’t the only place where this could or does happen. It takes an arid region to create the salty soils, and enough humidity to make the transference work, but the rest of it is just physics and chemistry.”

Volcanoes deliver 2 flavors of water

Seawater circulation pumps hydrogen and boron into the oceanic plates that make up the seafloor, and some of this seawater remains trapped as the plates descend into the mantle at areas called subduction zones. By analyzing samples of submarine volcanic glass near one of these areas, scientists found unexpected changes in isotopes of hydrogen and boron from the deep mantle. They expected to see the isotope “fingerprint” of seawater. But in volcanoes from the Manus Basin they also discovered evidence of seawater distilled long ago from a more ancient plate descent event, preserved for as long as 1 billion years. The data indicate that these ancient oceanic “slabs” can return to the upper mantle in some areas, and that rates of hydrogen exchange in the deep Earth may not conform to experiments. The research is published in the February 26, 2012, advanced on line publication of Nature Geoscience.

As Carnegie coauthor Erik Hauri explained, “Hydrogen and boron have both light and heavy isotopes. Isotopes are atoms of the same element with different numbers of neutrons. The volcanoes in the Manus Basin are delivering a mixture of heavy and light isotopes that have been observed nowhere else. The mantle under the Manus Basin appears to contain a highly distilled ancient water that is mixing with modern seawater.”

When seawater-soaked oceanic plates descend into the mantle, heavy isotopes of hydrogen and boron are preferentially distilled away from the slab, leaving behind the light isotopes, but also leaving it dry and depleted of these elements, making the “isotope fingerprint” of the distillation process difficult to identify. But this process appears to have been preserved in at least one area: submarine volcanoes in the Manus Basin of Papua New Guinea, which erupted under more than a mile of seawater (2,000 meters). Those pressures trap water from the deep mantle within the volcanic glass.

Lead author Alison Shaw and coauthor Mark Behn, both former Carnegie postdoctoral researchers, recognized another unique feature of the data. Lab experiments have shown very high diffusion rates for hydrogen isotopes, which move through the mantle as tiny protons. This diffusion should have long-ago erased the hydrogen isotope differences observed in the Manus Basin volcanoes.

“That is what we typically see at mid-ocean ridges,” remarked Hauri. “But that is not what we found at Manus Basin. Instead we found a huge range in isotope abundances that indicates hydrogen diffusion in the deep Earth may not be analogous to what is observed in the lab.”

The team’s * finding means is that surface water can be carried into the deep Earth by oceanic plates and be preserved for as long as 1 billion years. They also indicate that the hydrogen diffusion rates in the deep Earth appear to be much slower than experiments show. It further suggests that these ancient slabs may not only return to the upper mantle in areas like the Manus Basin, they may also come back up in hotspot volcanoes like Hawaii that are produced by mantle plumes.

The results are important to understanding how water is transferred and preserved in the mantle and how it and other chemicals are recycled to the surface.

New research points to erosional origin of linear dunes

Linear dunes, widespread on Earth and Saturn’s moon, Titan, are generally considered to have been formed by deposits of windblown sand. It has been speculated for some time that some linear dunes may have formed by “wind-rift” erosion, but this model has commonly been rejected due to lack of sufficient evidence. Now, new research supported by China’s NSF and published this week in GSA BULLETIN indicates that erosional origin models should not be ruled out.

The linear dunes in China’s Qaidam Basin have been proposed to have formed as self-extending lee dunes under a unidirectional wind regime owing to a high level of total silt, clay, and salt content or cohesiveness of sediments, and they have undergone southward lateral migration at rates of up to 3 m/yr.

New GSA BULLETIN research examines the sediments, internal structures, and optically stimulated luminescence ages of the linear dunes in the central Qaidam Basin approximately 80 km north of the city Golmud. The study’s findings suggest that the linear dunes are most likely of erosional origin similar to yardangs with orientations controlled by strikes of joints.

According to the study’s lead author, Jianxun Zhou of the China University of Petroleum’s State Key Laboratory of Petroleum Resource & Prospecting, “If the control of tectonic structures on the orientation of wind-eroded ridges is taken into account, morphodynamic interpretations for the wind-rift model may become much simpler. No one has considered the possibility of erosional origin for the linear dunes on Titan. Nearly all researchers consider the linear dunes on Titan to be of depositional origin, but their morphodynamic interpretations are complicated and their relationships to wind directions are in dispute. If an erosional origin is considered, the morphodynamic interpretations of the linear dunes on Titan can also be greatly simplified.”