Corn syrup model splits Yellowstone’s mantle plume in 2

A corn syrup mantle plume rises near the subducting plate, which induces fluid flow that distorts and deforms the plume as it rises toward the surface. -  Kelsey Druken
A corn syrup mantle plume rises near the subducting plate, which induces fluid flow that distorts and deforms the plume as it rises toward the surface. – Kelsey Druken

One of the greatest controversies in science is what’s underneath the Yellowstone supervolcano. The controversy surrounds a unique relationship between a mantle plume (like the one that powers Hawaiian volcanoes) and the subduction zone off the Washington-Oregon coast. Cutting-edge research using a common kitchen ingredient is explored in the latest issue of EARTH Magazine.

Recently published research explores this problem in 3-D, using a model created with corn syrup, fiberglass and a series of hydraulic pistons. What the scientists saw was a plume sliced in half by the subducting plate. Before this research, different scientific teams had only investigated the subducting tectonic plate or the mantle plume, but not both at the same time.

The resulting model of a bifurcated mantle plume potentially answers key questions about the Yellowstone supervolcano. Read about how these results impact volcano research in Washington, Oregon, Montana, Wyoming and the South Pacific in the July issue of EARTH Magazine: http://bit.ly/153lVat. For complete access – including to see the corn syrup apparatus subscribe to Earth Magazine at: http://www.earthmagazine.org/digital.

Don’t miss the other great articles in the July issue of EARTH Magazine. Uncover ancient earthquake damage in a Roman Mausoleum, Arctic ozone depletion, and plankton growth caused by Icelandic volcanoes, all in this month’s issue of EARTH, now available on the digital newsstand at http://www.earthmagazine.org/digital.

Scientists solve a 14,000-year-old ocean mystery

At the end of the last Ice Age, as the world began to warm, a swath of the North Pacific Ocean came to life. During a brief pulse of biological productivity 14,000 years ago, this stretch of the sea teemed with phytoplankton, amoeba-like foraminifera and other tiny creatures, who thrived in large numbers until the productivity ended-as mysteriously as it began-just a few hundred years later.

Researchers have hypothesized that iron sparked this surge of ocean life, but a new study led by Woods Hole Oceanographic Institution (WHOI) scientists and colleagues at the University of Bristol (UK), the University of Bergen (Norway), Williams College and the Lamont Doherty Earth Observatory of Columbia University suggests iron may not have played an important role after all, at least in some settings. The study, published in the journal Nature Geoscience, determines that a different mechanism-a transient “perfect storm” of nutrients and light-spurred life in the post-Ice Age Pacific. Its findings resolve conflicting ideas about the relationship between iron and biological productivity during this time period in the North Pacific-with potential implications for geo-engineering efforts to curb climate change by seeding the ocean with iron.

“A lot of people have put a lot of faith into iron-and, in fact, as a modern ocean chemist, I’ve built my career on the importance of iron-but it may not always have been as important as we think,” says WHOI Associate Scientist Phoebe Lam, a co-author of the study.

Because iron is known to cause blooms of biological activity in today’s North Pacific Ocean, researchers have assumed it played a key role in the past as well. They have hypothesized that as Ice Age glaciers began to melt and sea levels rose, they submerged the surrounding continental shelf, washing iron into the rising sea and setting off a burst of life.

Past studies using sediment cores-long cylinders drilled into the ocean floor that offer scientists a look back through time at what has accumulated there-have repeatedly found evidence of this burst, in the form of a layer of increased opal and calcium carbonate, the materials that made up phytoplankton and foraminifera shells. But no one had searched the fossil record specifically for signs that iron from the continental shelf played a part in the bloom.

Lam and an international team of colleagues revisited the sediment core data to directly test this hypothesis. They sampled GGC-37, a core taken from a site near Russia’s Kamchatka Peninsula, about every 5 centimeters, moving back through time to before the biological bloom began. Then they analyzed the chemical composition of their samples, measuring the relative abundance of the isotopes of the elements neodymium and strontium in the sample, which indicates which variant of iron was present. The isotope abundance ratios were a particularly important clue, because they could reveal where the iron came from-one variant pointed to iron from the ancient Loess Plateau of northern China, a frequent source of iron-rich dust in the northwest Pacific, while another suggested the younger, more volcanic continental shelf was the iron source.

What the researchers found surprised them.

“We saw the flux of iron was really high during glacial times, and that it dropped during deglaciation,” Lam says. “We didn’t see any evidence of a pulse of iron right before this productivity peak.”

The iron the researchers did find during glacial times appeared to be supplemented by a third source, possibly in the Bering Sea area, but it didn’t have a significant effect on the productivity peak. Instead, the data suggest that iron levels were declining when the peak began.

Based on the sediment record, the researchers propose a different cause for the peak: a chain of events that created ideal conditions for sea life to briefly flourish. The changing climate triggered deep mixing in the North Pacific ocean, which stirred nutrients that the tiny plankton depend on up into the sea’s surface layers, but in doing so also mixed the plankton into deep, dark waters, where light for photosynthesis was too scarce for them to thrive. Then a pulse of freshwater from melting glaciers-evidenced by a change in the amount of a certain oxygen isotope in the foraminifera shells found in the core-stopped the mixing, trapping the phytoplankton and other small creatures in a thin, bright, nutrient-rich top layer of ocean. With greater exposure to light and nutrients, and iron levels that were still relatively high, the creatures flourished.

“We think that ultimately this is what caused the productivity peak-that all these things happened all at once,” Lam says. “And it was a transient thing, because the iron continued to drop and eventually the nutrients ran out.”

The study’s findings disprove that iron caused this ancient bloom, but they also raise questions about a very modern idea. Some scientists have proposed seeding the world’s oceans with iron to trigger phytoplankton blooms that could trap some of the atmosphere’s carbon dioxide and help stall climate change. This idea, sometimes referred to as the “Iron Hypothesis,” has met with considerable controversy, but scientific evidence of its potential effectiveness to sequester carbon and its impact on ocean life has been mixed.

“This study shows how there are multiple controls on ocean phytoplankton blooms, not just iron,” says Ken Buesseler, a WHOI marine chemist who led a workshop in 2007 to discuss modern iron fertilization. “Certainly before we think about adding iron to the ocean to sequester carbon as a geoengineering tool, we should encourage studies like this of natural systems where the conditions of adding iron, or not, on longer and larger time scales have already been done for us and we can study the consequences.”

Some volcanoes ‘scream’ at ever-higher pitches until they blow their tops

It is not unusual for swarms of small earthquakes to precede a volcanic eruption. They can reach a point of such rapid succession that they create a signal called harmonic tremor that resembles sound made by various types of musical instruments, though at frequencies much lower than humans can hear.

A new analysis of an eruption sequence at Alaska’s Redoubt Volcano in March 2009 shows that the harmonic tremor glided to substantially higher frequencies and then stopped abruptly just before six of the eruptions, five of them coming in succession.

“The frequency of this tremor is unusually high for a volcano, and it’s not easily explained by many of the accepted theories,” said Alicia Hotovec-Ellis, a University of Washington doctoral student in Earth and space sciences.

Documenting the activity gives clues to a volcano’s pressurization right before an explosion. That could help refine models and allow scientists to better understand what happens during eruptive cycles in volcanoes like Redoubt, she said.

The source of the earthquakes and harmonic tremor isn’t known precisely. Some volcanoes emit sound when magma – a mixture of molten rock, suspended solids and gas bubbles – resonates as it pushes up through thin cracks in the Earth’s crust.

But Hotovec-Ellis believes in this case the earthquakes and harmonic tremor happen as magma is forced through a narrow conduit under great pressure into the heart of the mountain. The thick magma sticks to the rock surface inside the conduit until the pressure is enough to move it higher, where it sticks until the pressure moves it again.

Each of these sudden movements results in a small earthquake, ranging in magnitude from about 0.5 to 1.5, she said. As the pressure builds, the quakes get smaller and happen in such rapid succession that they blend into a continuous harmonic tremor.

“Because there’s less time between each earthquake, there’s not enough time to build up enough pressure for a bigger one,” Hotovec-Ellis said. “After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes.”

She is the lead author of a forthcoming paper in the Journal of Volcanology and Geothermal Research that describes the research. Co-authors are John Vidale of the UW and Stephanie Prejean and Joan Gomberg of the U.S. Geological Survey.

Hotovec-Ellis is a co-author of a second paper, published online July 14 in Nature Geoscience, that introduces a new “frictional faulting” model as a tool to evaluate the tremor mechanism observed at Redoubt in 2009. The lead author of that paper is Ksenia Dmitrieva of Stanford University, and other co-authors are Prejean and Eric Dunham of Stanford.

The pause in the harmonic tremor frequency increase just before the volcanic explosion is the main focus of the Nature Geoscience paper. “We think the pause is when even the earthquakes can’t keep up anymore and the two sides of the fault slide smoothly against each other,” Hotovec-Ellis said.

She documented the rising tremor frequency, starting at about 1 hertz (or cycle per second) and gliding upward to about 30 hertz. In humans, the audible frequency range starts at about 20 hertz, but a person lying on the ground directly above the magma conduit might be able to hear the harmonic tremor when it reaches its highest point (it is not an activity she would advise, since the tremor is closely followed by an explosion).

Scientists at the USGS Alaska Volcano Observatory have dubbed the highest-frequency harmonic tremor at Redoubt Volcano “the screams” because they reach such high pitch compared with a 1-to-5 hertz starting point. Hotovec-Ellis created two recordings of the seismic activity. A 10-second recording covers about 10 minutes of seismic sound and harmonic tremor, sped up 60 times. A one-minute recording condenses about an hour of activity that includes more than 1,600 small earthquakes that preceded the first explosion with harmonic tremor.

Upward-gliding tremor immediately before a volcanic explosion also has been documented at the Arenal Volcano in Costa Rica and Soufrière Hills volcano on the Caribbean island of Montserrat.

“Redoubt is unique in that it is much clearer that that is what’s going on,” Hotovec-Ellis said. “I think the next step is understanding why the stresses are so high.”

Continuous satellite monitoring of ice sheets needed to better predict sea-level rise

The findings, published in Nature Geoscience, underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise.

The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth’s glacier ice which would raise global sea level by some 63m if it were to melt completely. The ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behaviour. They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored. One common approach is to extrapolate observed changes to estimate their contribution to sea level in the future.

Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth’s gravity field resulting from changes in mass distribution, including movement of ice into the oceans. Using these changes in gravity, the state of the ice sheets can be monitored at monthly intervals.

Dr Bert Wouters, currently a visiting researcher at the University of Colorado, said: “In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and that the rate at which these losses occurs is increasing. Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the atmosphere (for example, shifting pressure systems in the North Atlantic, or El Niño and La Niña events) and slow changes in ocean currents.

“So, if observations span only a few years, such ‘ice sheet weather’ may show up as an apparent speed-up of ice loss which would cancel out once more observations become available,” Dr Wouters said.

The team of researchers compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. They found that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record.

At the moment, the ice loss detected by the GRACE satellites is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not.

The study suggests that although there may be almost enough satellite data to detect a speed-up in mass loss of the Antarctic ice sheet with a reasonable level of confidence, another ten years of satellite observations is needed to do so for Greenland. As a result, extrapolation of the current contribution to sea-level rise of the ice sheets to 2100 may be too high or low by as much as 35 cm. The study, therefore, urges caution in extrapolating current measurements to predict future sea-level rise.

Research reveals Earth’s core affects length of day

Research at the University of Liverpool has found that variations in the length of day over periods of between one and 10 years are caused by processes in the Earth’s core.

The Earth rotates once per day, but the length of this day varies. A yeas, 300million years ago, lasted about 450 days and a day would last about 21 hours. As a result of the slowing down of the Earth’s rotation the length of day has increased.

The rotation of the earth on its axis, however, is affected by a number of other factors – for example, the force of the wind against mountain ranges changes the length of the day by plus or minus a millisecond over a period of a year.

Professor Richard Holme, from the School of Environmental Sciences, studied the variations and fluctuations in the length of day over a one to 10 year period between 1962 and 2012. The study took account of the effects on the Earth’s rotation of atmospheric and oceanic processes to produce a model of the variations in the length of day on time scales longer than a year.

Professor Holme said: “The model shows well-known variations on decadal time scales, but importantly resolves changes over periods between one and 10 years. Previously these changes were poorly characterised; the study shows they can be explained by just two key signals, a steady 5.9 year oscillation and episodic jumps which occur at the same time as abrupt changes in the Earth’s magnetic field, generated in the Earth’s core.

He added: “This study changes fundamentally our understanding of short-period dynamics of the Earth’s fluid core. It leads us to conclude that the Earth’s lower mantle, which sits above the Earth’s outer core, is a poor conductor of electricity giving us new insight into the chemistry and mineralogy of the Earth’s deep interior.”

Scientists cast doubt on theory of what triggered Antarctic glaciation

This is a physiographic map of the present-day Scotia Sea, Drake Passage and adjacent land masses. The white arrows show the present path of the several branches of the deep Antarctic Circumpolar Current (ACC) centered on its core. The area of study in the central Scotia Sea (CSS) is shown by the black box to the south of South Georgia island (SG). The volcano symbols mark the active South Sandwich volcanic arc (SSA). (WSS = western Scotia Sea; ESS = eastern Scotia Sea) -  University of Texas at Austin
This is a physiographic map of the present-day Scotia Sea, Drake Passage and adjacent land masses. The white arrows show the present path of the several branches of the deep Antarctic Circumpolar Current (ACC) centered on its core. The area of study in the central Scotia Sea (CSS) is shown by the black box to the south of South Georgia island (SG). The volcano symbols mark the active South Sandwich volcanic arc (SSA). (WSS = western Scotia Sea; ESS = eastern Scotia Sea) – University of Texas at Austin

A team of U.S. and U.K. scientists has found geologic evidence that casts doubt on one of the conventional explanations for how Antarctica’s ice sheet began forming. Ian Dalziel, research professor at The University of Texas at Austin’s Institute for Geophysics and professor in the Jackson School of Geosciences, and his colleagues report the findings today in an online edition of the journal Geology.

The Antarctic Circumpolar Current (ACC), an ocean current flowing clockwise around the entire continent, insulates Antarctica from warmer ocean water to the north, helping maintain the ice sheet. For several decades, scientists have surmised that the onset of a complete ACC played a critical role in the initial glaciation of the continent about 34 million years ago.

Now, rock samples from the central Scotia Sea near Antarctica reveal the remnants of a now-submerged volcanic arc that formed sometime before 28 million years ago and might have blocked the formation of the ACC until less than 12 million years ago. Hence, the onset of the ACC may not be related to the initial glaciation of Antarctica, but rather to the subsequent well-documented descent of the planet into a much colder “icehouse” glacial state.

“If you had sailed into the Scotia Sea 25 million years ago, you would have seen a scattering of volcanoes rising above the water,” says Dalziel. “They would have looked similar to the modern volcanic arc to the east, the South Sandwich Islands.”

Using multibeam sonar to map seafloor bathymetry, which is analogous to mapping the topography of the land surface, the team identified seafloor rises in the central Scotia Sea. They dredged the seafloor at various points on the rises and discovered volcanic rocks and sediments created from the weathering of volcanic rocks. These samples are distinct from normal ocean floor lavas and geochemically identical to the presently active South Sandwich Islands volcanic arc to the east of the Scotia Sea that today forms a barrier to the ACC, diverting it northward.

Using a technique known as argon isotopic dating, the researchers found that the samples range in age from about 28 million years to about 12 million years. The team interpreted these results as evidence that an ancient volcanic arc, referred to as the ancestral South Sandwich arc (ASSA), was active in the region during that time and probably much earlier. Because the samples were taken from the current seafloor surface and volcanic material accumulates from the bottom up, the researchers infer that much older volcanic rock lies beneath.

Combined with models of how the seafloor sinks vertically with the passage of time, the team posits that the ASSA originally rose above sea level and would have blocked deep ocean currents such as the ACC.

Two other lines of evidence support the notion that the ACC didn’t begin until less than 12 million years ago. First, the northern Antarctic Peninsula and southern Patagonia didn’t become glaciated until less than approximately 12 million years ago. And second, certain species of microscopic creatures called dinoflagellates that thrive in cold polar water began appearing in sediments off southwestern Africa around 11.1 million years ago, suggesting colder water began reaching that part of the Atlantic Ocean.

Geothermal power facility induces earthquakes, study finds

An analysis of earthquakes in the area around the Salton Sea Geothermal Field in southern California has found a strong correlation between seismic activity and operations for production of geothermal power, which involve pumping water into and out of an underground reservoir.

“We show that the earthquake rate in the Salton Sea tracks a combination of the volume of fluid removed from the ground for power generation and the volume of wastewater injected,” said Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and lead author of the study, published online in Science on July 11.

“The findings show that we might be able to predict the earthquakes generated by human activities. To do this, we need to take a large view of the system and consider both the water coming in and out of the ground,” said Brodsky, a professor of Earth and planetary sciences at UCSC.

Brodsky and coauthor Lia Lajoie, who worked on the project as a UCSC graduate student, studied earthquake records for the region from 1981 through 2012. They compared earthquake activity with production data for the geothermal power plant, including records of fluid injection and extraction. The power plant is a “flash-steam facility” which pulls hot water out of the ground, flashes it to steam to run turbines, and recaptures as much water as possible for injection back into the ground. Due to evaporative losses, less water is pumped back in than is pulled out, so the net effect is fluid extraction.

During the period of relatively low-level geothermal operations before 1986, the rate of earthquakes in the region was also low. Seismicity increased as the operations expanded. After 2001, both geothermal operations and seismicity climbed steadily.

The researchers tracked the variation in net extraction over time and compared it to seismic activity. The relationship is complicated because earthquakes are naturally clustered due to local aftershocks, and it can be difficult to separate secondary triggering (aftershocks) from the direct influence of human activities. The researchers developed a statistical method to separate out the aftershocks, allowing them to measure the “background rate” of primary earthquakes over time.

“We found a good correlation between seismicity and net extraction,” Brodsky said. “The correlation was even better when we used a combination of all the information we had on fluid injection and net extraction. The seismicity is clearly tracking the changes in fluid volume in the ground.”

The vast majority of the induced earthquakes are small, and the same is true of earthquakes in general. The key question is what is the biggest earthquake that could occur in the area, Brodsky said. The largest earthquake in the region of the Salton Sea Geothermal Field during the 30-year study period was a magnitude 5.1 earthquake.

The nearby San Andreas fault, however, is capable of unleashing extremely destructive earthquakes of at least magnitude 8, Brodsky said. The location of the geothermal field at the southern end of the San Andreas fault is cause for concern due to the possibility of inducing a damaging earthquake.

“It’s hard to draw a direct line from the geothermal field to effects on the San Andreas fault, but it seems plausible that they could interact,” Brodsky said.

At its southern end, the San Andreas fault runs into the Salton Sea, and it’s not clear what faults there might be beneath the water. A seismically active region known as the Brawley Seismic Zone extends from the southern end of the San Andreas fault to the northern end of the Imperial fault. The Salton Sea Geothermal Field, located on the southeastern edge of the Salton Sea, is one of four operating geothermal fields in the area.

Distant quakes trigger tremors at US waste-injection sites, says study

Large earthquakes from distant parts of the globe are setting off tremors around waste-fluid injection wells in the central United States, says a new study. Furthermore, such triggering of minor quakes by distant events could be precursors to larger events at sites where pressure from waste injection has pushed faults close to failure, say researchers.

Among the sites covered: a set of injection wells near Prague, Okla., where the study says a huge earthquake in Chile on Feb. 27, 2010 triggered a mid-size quake less than a day later, followed by months of smaller tremors. This culminated in probably the largest quake yet associated with waste injection, a magnitude 5.7 event which shook Prague on Nov. 6, 2011. Earthquakes off Japan in 2011, and Sumatra in 2012, similarly set off mid-size tremors around injection wells in western Texas and southern Colorado, says the study. The paper appears this week in the leading journal Science, along with a series of other articles on how humans may be influencing earthquakes.

“The fluids are driving the faults to their tipping point,” said lead author Nicholas van der Elst, a postdoctoral researcher at Columba University’s Lamont-Doherty Earth Observatory. “The remote triggering by big earthquakes is an indication the area is critically stressed.”

Tremors triggered by distant large earthquakes have been identified before, especially in places like Yellowstone National Park and some volcanically active subduction zones offshore, where subsurface water superheated by magma can weaken faults, making them highly vulnerable to seismic waves passing by from somewhere else. The study in Science adds a new twist by linking this natural phenomenon to faults that have been weakened by human activity.

A surge in U.S. energy production in the last decade or so has sparked what appears to be a rise in small to mid-sized earthquakes in the United States. Large amounts of water are used both to crack open rocks to release natural gas through hydrofracking, and to coax oil and gas from underground wells using conventional techniques. After the gas and oil have been extracted, the brine and chemical-laced water must be disposed of, and is often pumped back underground elsewhere, sometimes causing earthquakes.

From a catalog of past earthquake recordings, van der Elst and his colleagues found that faults near wastewater-injection sites in and around Prague, Snyder, Tex., and Trinidad, Colo., were approaching a critical state when big earthquakes far away triggered a rise in local earthquakes. Injection at the three sites had been ongoing for years, and the researchers hypothesize that passing surface waves from the big events caused small pressure changes on faults, triggering smaller earthquakes.

“These passing seismic waves are like a stress test,” said study coauthor Heather Savage, a geophysicist at Lamont-Doherty. “If the number of small earthquakes increases, it could indicate that faults are becoming critically stressed and might soon host a larger earthquake.”

The 2010 magnitude 8.8 Chile quake, which killed more than 500 people, sent surface waves rippling across the planet, triggering a magnitude 4.1 quake near Prague 16 hours later, the study says. The activity near Prague continued until the magnitude 5.7 quake on Nov. 6, 2011 that destroyed 14 homes and injured two people. A study earlier this year led by seismologist Katie Keranen, also a coauthor of the new study, now at Cornell University, found that the first rupture occurred less than 650 feet away from active injection wells. In April 2012, a magnitude 8.6 earthquake off Sumatra triggered another swarm of earthquakes in the same place. The pumping of fluid into the field continues to this day, along with a pattern of small quakes.

The 2010 Chile quake also set off a swarm of earthquakes on the Colorado-New Mexico border, in Trinidad, near wells where wastewater used to extract methane from coal beds had been injected, the study says. The swarm was followed more than a year later, on Aug. 22 2011, by a magnitude 5.3 quake that damaged dozens of buildings. A steady series of earthquakes had already struck Trinidad in the past, including a magnitude 4.6 quake in 2001 that the U.S. Geological Survey (USGS) has investigated for links to wastewater injection.

The new study found also that Japan’s devastating magnitude 9.0 earthquake on March 11, 2011 triggered a swarm of earthquakes in the west Texas town of Snyder, where injection of fluid to extract oil from the nearby Cogdell fields has been setting off earthquakes for years, according to a 1989 study in the Bulletin of the Seismological Society of America. About six months after the Japan quake, a magnitude 4.5 quake struck Snyder.

The idea that seismic activity can be triggered by separate earthquakes taking place faraway was once controversial. One of the first cases to be documented was the magnitude 7.3 earthquake that shook California’s Mojave Desert in 1992, near the town of Landers, setting off a series of distant events in regions with active hot springs, geysers and volcanic vents. The largest was a magnitude 5.6 quake beneath Little Skull Mountain in southern Nevada, 150 miles away; the farthest, a series of tiny earthquakes north of Yellowstone caldera, according to a 1993 study in Science led by USGS geophysicist David Hill.

In 2002, the magnitude 7.9 Denali earthquake in Alaska triggered a series of earthquakes at Yellowstone, nearly 2,000 miles away, throwing off the schedules of some of its most predictable geysers, according to a 2004 study in Geology led by Stephan Husen, a seismologist at the Swiss Federal Institute of Technology in Zürich. The Denali quake also triggered bursts of slow tremors in and around California’s San Andreas, San Jacinto and Calaveras faults, according to a 2008 study in Science led by USGS geophysicist Joan Gomberg.

“We’ve known for at least 20 years that shaking from large, distant earthquakes can trigger seismicity in places with naturally high fluid pressure, like hydrothermal fields,” said study coauthor Geoffrey Abers, a seismologist at Lamont-Doherty. “We’re now seeing earthquakes in places where humans are raising pore pressure.”

The new study may be the first to find evidence of triggered earthquakes on faults critically stressed by waste injection. If it can be replicated and extended to other sites at risk of manmade earthquakes it could “help us understand where the stresses are,” said William Ellsworth, an expert on human-induced earthquakes with the USGS who was not involved in the study.

In the same issue of Science, Ellsworth reviews the recent upswing in earthquakes in the central United States. The region averaged 21 small to mid-sized earthquakes each year from the late 1960s through 2000. But in 2001, that number began to climb, reaching a high of 188 earthquakes in 2011, he writes. The risk of setting off earthquakes by injecting fluid underground has been known since at least the 1960s, when injection at the Rocky Mountain Arsenal near Denver was suspended after a magnitude 4.8 quake or greater struck nearby-the largest tied to wastewater disposal until the one near Prague, Okla. In a report last year, the National Academy of Sciences called for further research to “understand, limit and respond [to]” seismic events induced by human activity.

The contribution of the Greenland ice sheet to sea-level rise will continue to increase

New research has shown surface ice melt will be the dominant process controlling ice-loss from Greenland. As outlet glaciers retreat inland the other process, iceberg production, remains important but will not grow as rapidly.

The Greenland ice sheet is often considered an important potential contributor to future global sea-level rise over the next century or longer. In total, it contains an amount of ice that would lead to a rise of global sea level by more than seven metres, if completely melted.

Changes in its total mass are governed by two main processes – fluctuations in melting and snowfall on its surface, and changes to the number of icebergs released from a large number of outlet glaciers into the ocean.

The ice loss from the ice sheet has been increasing over the last decade, with half of it attributed to changes in surface conditions with the remainder due to increased iceberg calving – the process by which ice detaches from the glacier to become an iceberg.

Researchers from the Vrije Universiteit Brussel, funded by ice2sea, a European Union project, tackled the question of how both processes will evolve and interact in the future. This was done with a computer model, which projects the future ice sheet evolution with high accuracy using the latest available techniques and input data.

They devised a method to generalize projections made in earlier research which concerned just four of Greenland’s outlet glaciers. By doing so they could apply the earlier findings to all calving glaciers around the Greenland ice sheet. Their results indicate a total sea-level contribution from the Greenland ice sheet for an average warming scenario after 100 and 200 years of 7 and 21 cm, respectively.

The balance between the two processes by which ice is lost is, however, changing considerably in the future so that iceberg calving may only account for between 6 % and 18 % of the sea-level contribution after 200 years. This is important, because variations in outlet glacier dynamics have often been suspected to have the potential for very large sea-level contributions.

Lead author Dr Heiko Goelzer, of the Vrije Universiteit Brussel, says,

“Our research has shown that the balance between the two most important mass loss processes will change considerably in the future so that changes in iceberg calving only account for a small percentage of the sea-level contribution after 200 years with the large remainder due to changes in surface conditions.”

The limited importance of outlet glacier dynamics in the future is the result of their retreat back onto land and of strongly increasing surface melting under global warming, which removes ice before it can reach the marine margin.

Ice2sea coordinator Professor David Vaughan, of the British Antarctic Survey says,

“This scenario is no reason to be complacent. The reason the significance of calving glaciers reduces compared to surface melting is, so much ice will be lost in coming decades that many glaciers currently sitting in fjords will retreat inland to where they are no longer affected by warming seas around Greenland.”

Study shows how early Earth kept warm enough to support life

This is an artist's conception of the Earth during the late Archean, 2.8 billion years ago. Weak solar radiation requires the Earth have increased greenhouse gas amounts to remain warm. CU-Boulder doctoral student Eric Wolf Wolf and CU-Boulder Professor Brian Toon use a three-dimensional climate model to show that the late Archean may have maintained large areas of liquid surface water despite a relatively weak greenhouse. With carbon dioxide levels within constraints deduced from ancient soils, the late Archean may have had large polar ice caps but lower latitudes would have remained temperate and thus hospitable to life. The addition of methane allows the late Archean to warmed to present day mean surface temperatures. -  Charlie Meeks
This is an artist’s conception of the Earth during the late Archean, 2.8 billion years ago. Weak solar radiation requires the Earth have increased greenhouse gas amounts to remain warm. CU-Boulder doctoral student Eric Wolf Wolf and CU-Boulder Professor Brian Toon use a three-dimensional climate model to show that the late Archean may have maintained large areas of liquid surface water despite a relatively weak greenhouse. With carbon dioxide levels within constraints deduced from ancient soils, the late Archean may have had large polar ice caps but lower latitudes would have remained temperate and thus hospitable to life. The addition of methane allows the late Archean to warmed to present day mean surface temperatures. – Charlie Meeks

Solving the “faint young sun paradox” — explaining how early Earth was warm and habitable for life beginning more than 3 billion years ago even though the sun was 20 percent dimmer than today — may not be as difficult as believed, says a new University of Colorado Boulder study.

In fact, two CU-Boulder researchers say all that may have been required to sustain liquid water and primitive life on Earth during the Archean eon 2.8 billion years ago were reasonable atmospheric carbon dioxide amounts believed to be present at the time and perhaps a dash of methane. The key to the solution was the use of sophisticated three-dimensional climate models that were run for thousands of hours on CU’s Janus supercomputer, rather than crude, one-dimensional models used by almost all scientists attempting to solve the paradox, said doctoral student Eric Wolf, lead study author.

“It’s really not that hard in a three-dimensional climate model to get average surface temperatures during the Archean that are in fact moderate,” said Wolf, a doctoral student in CU-Boulder’s atmospheric and oceanic sciences department. “Our models indicate the Archean climate may have been similar to our present climate, perhaps a little cooler. Even if Earth was sliding in and out of glacial periods back then, there still would have been a large amount of liquid water in equatorial regions, just like today.”

Evolutionary biologists believe life arose on Earth as simple cells roughly 3.5 billion years ago, about a billion years after the planet is thought to have formed. Scientists have speculated the first life may have evolved in shallow tide pools, freshwater ponds, freshwater or deep-sea hydrothermal vents, or even arrived on objects from space.

A cover article by Wolf and Professor Brian Toon on the topic appears in the July issue of Astrobiology. The study was funded by two NASA grants and by the National Science Foundation, which supports CU-Boulder’s Janus supercomputer used for the study.

Scientists have been trying to solve the faint young sun paradox since 1972, when Cornell University scientist Carl Sagan — Toon’s doctoral adviser at the time — and colleague George Mullen broached the subject. Since then there have been many studies using 1-D climate models to try to solve the faint young sun paradox — with results ranging from a hot, tropical Earth to a “snowball Earth” with runaway glaciation — none of which have conclusively resolved the problem.

“In our opinion, the one-dimensional models of early Earth created by scientists to solve this paradox are too simple — they are essentially taking the early Earth and reducing it to a single column atmospheric profile,” said Toon. “One-dimensional models are simply too crude to give an accurate picture.”

Wolf and Toon used a general circulation model known as the Community Atmospheric Model version 3.0 developed by the National Center for Atmospheric Research in Boulder and which contains 3-D atmosphere, ocean, land, cloud and sea ice components. The two researchers also “tuned up” the model with a sophisticated radiative transfer component that allowed for the absorption, emission and scattering of solar energy and an accurate calculation of the greenhouse effect for the unusual atmosphere of early Earth, where there was no oxygen and no ozone, but lots of CO2 and possibly methane.

The simplest solution to the faint sun paradox, which duplicates Earth’s present climate, involves maintaining roughly 20,000 parts per million of the greenhouse gas CO2 and 1,000 ppm of methane in the ancient atmosphere some 2.8 billion years ago, said Wolf. While that may seem like a lot compared to today’s 400 ppm of CO2 in the atmosphere, geological studies of ancient soil samples support the idea that CO2 likely could have been that high during that time period. Methane is considered to be at least 20 times more powerful as a greenhouse gas than CO2 and could have played a significant role in warming the early Earth as well, said the CU researchers.

There are other reasons to believe that CO2 was much higher in the Archean, said Toon, who along with Wolf is associated with CU’s Laboratory for Atmospheric and Space Physics. The continental area of Earth was smaller back then so there was less weathering of the land and a lower release of minerals to the oceans. As a result there was a smaller conversion of CO2 to limestone in the ocean. Likewise, there were no “rooted” land plants in the Archean, which could have accelerated the weathering of the soils and indirectly lowered the atmospheric abundance of CO2, Toon said.

Another solution to achieving a habitable but slightly cooler climate under the faint sun conditions is for the Archean atmosphere to have contained roughly 15,000 to 20,000 ppm of CO2 and no methane, said Wolf. “Our results indicate that a weak version of the faint young sun paradox, requiring only that some portion of the planet’s surface maintain liquid water, may be resolved with moderate greenhouse gas inventories,” the authors wrote in Astrobiology.

“Even if half of Earth’s surface was below freezing back in the Archean and half was above freezing, it still would have constituted a habitable planet since at least 50 percent of the ocean would have remained open,” said Wolf. “Most scientists have not considered that there might have been a middle ground for the climate of the Archean.

“The leap from one-dimensional to three-dimensional models is an important step,” said Wolf. “Clouds and sea ice are critical factors in determining climate, but the one-dimensional models completely ignore them.”

Has the faint young sun paradox finally been solved? “I don’t want to be presumptuous here,” said Wolf. “But we show that the paradox is definitely not as challenging as was believed over the past 40 years. While we can’t say definitively what the atmosphere looked like back then without more geological evidence, it is certainly not a stretch at all with our model to get a warm early Earth that would have been hospitable to life.”

“The Janus supercomputer has been a tremendous addition to the campus, and this early Earth climate modeling project would have impossible without it,” said Toon. The researchers estimated the project required roughly 6,000 hours of supercomputer computation time, an effort equal to about 10 years on a home computer.