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Cascading Down the Mountain

Mon, 06/24/2019 - 13:57

The collision of tectonic plates that forms the tallest and steepest mountains on Earth produces large and destructive earthquakes. After the immediate shaking of the ground, a chain of events can be initiated that can affect the built environment and human communities in the short term and modify the entire landscape over a much longer geologic time scale. A recent paper in Reviews of Geophysics explores earthquake-induced chains of geological hazards in mountainous regions over days, years, and millennia. Here, the authors of the paper answer some questions about these hazards and suggest where further research is needed.

What is the greatest geological hazard caused by earthquakes that happen in mountainous regions?

Landslides: thousands of them! While most earthquake-triggered landslides are small in size and travel only short distances, their total mass is huge. However, some of the largest landslides have enough momentum to reach major rivers, damming their course and forming “quake lakes”. A dam may hold for hours or months until the water overtops or breaches, producing a sudden discharge of millions of tons of water, and flooding the valley below, maybe for tens of kilometers. Some dams can hold for millennia, acting as a barrier to the fluvial export of sediments, with their lakes altering the local ecosystem and microclimate profoundly.

Chains of geological hazards triggered by a strong continental earthquake: different types of coseismic landslides (red), post-seismic cascade of hazards in days to years later (blue), and the long-term impact of an earthquake, from years to decades later, and perhaps longer (yellow). Credit: Fan et al. [2019], Figure 1How long after an earthquake might landslides occur?

An initial landslide caused by the earth shaking is one thing, but earthquake aftershocks and heavy rains can mobilize the debris again and again. Up to years later, masses of water-saturated debris can generate fearsome debris flows that can flood valleys and swallow entire towns at frightening speeds. Studies have demonstrated that, in the first years after a strong earthquake, landslides can be triggered by rainstorms 5 to 10 times weaker than those that would have caused a landslide before the earthquake.

Beichuan town sits astride the Yingxiu-Beichuan fault that generated the great 2008 Wenchuan earthquake. The town was severely affected by the coseismic and post-seismic chain of geological hazards following the magnitude 7.9 quake. Two large coseismic landslides buried nearly half of the town, including a middle school, killing near 2500 people. Many landslides dammed the Jian River, forming several large landslide dams both upstream and downstream of the town. Subsequently, frequent debris flows induced by heavy rainfalls flooded the town with both water and sediment. The town has now been rebuilt on another site, leaving the old town as an open-air museum. Credit: Xuanmei Fan

What was significant about the earthquake-induced landslides in Taiwan (1999) and Sichuan (2008) and what did scientists learn from these events?

The 1999 Chi-Chi earthquake (Taiwan, China, magnitude 7.6), and the 2008 Wenchuan earthquake (Sichuan, China, magnitude 7.9) prompted intense research into earthquake-induced landsliding. They have presented an opportunity to make direct observations of the ongoing geological impacts of recent large quakes.

Researchers have used these two events to investigate the controlling factors of coseismic landslides, the mechanism and changing rainfall thresholds of post-seismic debris flows, patterns of sediment transport, and much more.

The Chi-Chi earthquake was possibly the first time that high-resolution satellite observations were used to track, even on a monthly basis, the activity of the earthquake-induced landslides and the post-earthquake sediment dynamics.

Meanwhile, records of sediment concentration in river waters collected routinely at river gauging stations in Taiwan and Sichuan made it possible to quantify the magnitude and timing of the earthquake perturbation.

Multi-year suspended-sediment and event-based landslide records from the Chi-Chi earthquake in Taiwan (top) and the Wenchuan earthquake in Sichuan (bottom). In both cases, the notable seismic enhancement of sediment fluxes gradually decays to baseline levels over about 5 years, similar to the timescale over which post-seismic landslide activity also wanes. Credit: Fan et al. [2019], Figure 18The most significant increases in sediments in rivers were associated with intense rainfall events that occurred in subsequent years, which demonstrated the importance of fluvial transport capacity in regulating the pace of landslide-sediment evacuation.

What is the overall balance between seismic uplift and landscape erosion?

It’s very intriguing to consider whether large earthquakes are more important for the generation of new topography through seismic uplift or for the erosion of topography through landslides. They certainly counterbalance one another, but the frequency and spread of earthquakes compared to the rate at which sediment is removed can substantially influence the calculation of this mass balance. Reconciling this has not yet received formal consideration in the scientific literature. One fascinating hypothesis is that landslide-driven seismic erosion can focus near active faults. The resulting unloading of the crust could, in turn, trigger more earthquakes and generate possible feedbacks between earthquakes and erosion.

Besides topographic changes, what are the other effects of earthquakes and their associated landslides?

One notable impact of landsliding is that it can alter the carbon budget. Carbon stored in rock, soil and vegetation that is mobilized by a landslide may be retained in the debris pile; it may be removed by a river and either transported away from the area or buried in sedimentary deposits; or it may be oxidized either in landslide deposits or during fluvial transit releasing carbon back to the atmosphere as carbon dioxide. Data from the Wenchuan region showed that the flux doubled after the earthquake.

Summary of possible effects of earthquake-triggered landslides on the carbon cycle. Credit: Fan et al. [2019], Figure 22Moreover, landslide deposits can act as weathering reactors.

Silicate mineral weathering produces alkalinity that consumes carbon dioxide through precipitation of carbonates.

In contrast, sulfide oxidation can release carbon dioxide into the atmosphere.

The net effect of coseismic landslide-induced chemical weathering on the carbon cycle could thus depend on the balance between these two processes.

Altogether, earthquakes and associated landslides could play a meaningful role in linking tectonic activity and global climate. Understanding this role remains challenging, yet extremely fascinating.

What are some of the other unresolved questions where additional research, data or modeling is needed?

In modeling sediment dynamics associated with landslide, two important issues remain particularly challenging. The first is what determines how much landslide sediment from a given event is delivered to the main river channels, as opposed to being isolated on the slopes. The second is what determines the ability of earthquake-derived sediment to move through a fluvial network, in particular how coarse landslide material can be mobilized and transported downstream. Addressing these questions is key for obtaining a systematic understanding of this sedimentary chain, and thus setting the foundations for predictive modeling.

We also advocate for more multidisciplinary collaboration to study earthquake-induced hazards.We also advocate for more multidisciplinary collaboration to study earthquake-induced hazards. This field requires the understanding of landscape morphology, landslide mechanisms, erosion and sediment transport, landslide hazard and risk, and much more. Quantifying and predicting a chain of geologic hazards that may develop over short and long timescales requires a joint effort.

—Xuanmei Fan (email: fanxuanmei1@gmail.com), Gianvito Scaringi, Qiang Xu, and Runqiu Huang, Chengdu University of Technology, China

Microbes Spotted in “Polyextreme” Hot Springs

Mon, 06/24/2019 - 13:56

There’s nothing relaxing about soaking in Ethiopia’s Dallol hot springs—they’re as acidic as battery acid and nearly boiling. But fieldwork in this extreme world suggests that the hot springs are home to tiny, spherical organisms whose relatives might have once thrived on Mars, new research shows.

“It’s a really interesting, extreme environment.” It also looks “like hell.”Felipe Gómez, an astrobiologist at the Astrobiology Center—National Institute of Aerospace Technology in Madrid, and his colleagues traveled to the Danakil Depression in northern Ethiopia in January 2017. They were there, in one of the lowest spots on Earth, to study an otherworldly landscape: a salt plain near the Dallol volcano.

Magma just below the surface heats the groundwater, dissolving salts and producing bright yellow, orange, and red mineral formations known as “chimneys” that spew briny, nearly boiling water. A sulfurous, yellow-tinged fog occasionally envelops the area, forcing anyone nearby to evacuate.

“It’s a really interesting, extreme environment,” said Gómez. It also looks “like hell.”

A Surprise in the Salt

The researchers focused on a chimney roughly 1 meter tall and scraped salt precipitates from its walls into sterile glass vials. Back in the lab, they extracted DNA from the samples and found a close match with the class Nanohaloarchaea, tiny, salt-loving organisms similar to those spotted in hypersaline ponds in Spain.

Using imaging techniques such as scanning electron microscopy and transmission electron microscopy, Gómez and his team recorded spherical structures approximately 50–500 nanometers in size. That’s smaller than most archaea, said Gómez. On the basis of the structures’ high carbon content, the researchers concluded they were biological.

Finding these microorganisms was a surprise, said Gómez, because Nanohaloarchaea haven’t been spotted in acidic, high-temperature environments before.

Implications for Early Mars

This discovery has implications for what forms of life might have thrived on Mars long ago, Gómez and his team propose.

“The presence of life in the Dallol hot springs expands our understanding of the limits of habitability on Earth and beyond.”Early Mars was volcanically active, and some regions, such as Nili Patera Caldera, might have resembled the geology of the Dallol hot springs, the researchers stated in their paper, published in Scientific Reports in May. “The presence of life in the Dallol hot springs expands our understanding of the limits of habitability on Earth and beyond,” they wrote.

Although these results are promising, follow-on work is necessary, said Kenneth Stedman, a virologist at Portland State University in Oregon who was not involved in the research.

An important next step will be to show that these microorganisms are metabolically active, said Stedman, to rule out that they were blown into the salt and preserved there. It’s also critical to study samples taken directly from the brines, said Stedman. “I’d like to see what’s in the water.”

—Katherine Kornei (@katherinekornei), Freelance Science Journalist

Ultraprecise Clock Will Facilitate Space Exploration

Fri, 06/21/2019 - 12:13

Atomic clocks—the world’s most precise timepieces—are the basis of modern-day navigation and telecommunication. This month, an ultraprecise, toaster-sized atomic clock is slated to leave our planet’s surface and enter low-Earth orbit. It’ll pave the way for placing atomic clocks on spacecraft, an advance that will allow interplanetary spacecraft to more quickly and accurately determine their location in space. That’s important as humans prepare to travel to other planets like Mars.

Light Is Fast, But Space Is Big

Here on Earth, we pretty much know where we are at all times: The signals that zip between our smartphones and orbiting GPS satellites do so in far less than a second. That’s thanks to the relative proximity of GPS satellites (only about 20,000 kilometers away). But things are different for spacecraft, which don’t currently have GPS capabilities.

To determine where a spacecraft is, Earth-based researchers send it a radio signal, which it returns. Atomic clocks on Earth time the journey of those radio waves, and a bit of math reveals the spacecraft’s distance from Earth. Measuring location is therefore “fundamentally a problem of very accurately measuring time,” said Jill Seubert, the deputy principal investigator of the Deep Space Atomic Clock Technology Demonstration Mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Alerting a spacecraft of its location requires a third radio signal, however, and all of that light travel time adds up. For a spacecraft near Mars, the delay can top 40 minutes, said Seubert.

An Atomic Clock of Their Own

Much more efficient navigation would be possible, researchers realized, if spacecraft were equipped with their own atomic clocks. They’d then be able to receive a radio signal from Earth (typically from one of NASA’s Deep Space Network antennae located in California, Spain, and Australia) and very accurately measure the signal transit time to calculate their distance. Rather than sending radio signals on three trips, only one would be needed. That’s the idea behind the Deep Space Atomic Clock.

Outfitting spacecraft with their own atomic clocks is a major step toward making them autonomous, said Seubert. “If you design the computer and the radio properly, you can have the spacecraft do its own navigation.”

The Deep Space Atomic Clock is substantially smaller and lighter than most earthbound atomic clocks, roughly the size of a four-slice toaster and just 16 kilograms.

It also runs extremely consistently. Its stability derives from a small cloud of mercury ions that “steer” the frequency of a quartz oscillator. The principle hinges on the mercury ions changing energy state when they absorb a particular frequency of microwave radiation (40,507,347,997 hertz). By continuously correcting the frequency of the quartz oscillator, scientists ensure the steady “heartbeat” of the Deep Space Atomic Clock.

“It can go about 10 million years before it would drift off by 1 second,” said Seubert. “It’ll be the most stable clock in space.”

Can’t Call a Postdoc

Deep Space Atomic Clock will launch aboard a SpaceX Falcon Heavy rocket from NASA’s Kennedy Space Center in Florida as early as 24 June. It will enter low-Earth orbit at an altitude of roughly 720 kilometers and remain there for about 1 year while scientists analyze its performance.

Turning it on for the first time—after its jarring rocket flight—will be an achievement in and of itself, said Seubert. “If one piece breaks, I can’t send a postdoc.”

Scientists aren’t sure yet which spacecraft will be the first to be outfitted with a version of the Deep Space Atomic Clock. Given the large number of spacecraft and rovers imaging and exploring Mars’s surface, however, something near the Red Planet may win out, said Seubert.

“While there are no specific plans yet, the next Mars orbiter would be a great candidate to fly this technology as it could demonstrate autonomous navigation, support radio science experiments, and perhaps one day serve as a node in a future Mars network,” she said.

A GPS-Like Network Around Another World

“Once we have this technology demonstrated…we could build GPS-like networks at other planets and moons.”Seubert sees a lot of uses for atomic clock technology in space. For instance, she envisions building constellations of Deep Space Atomic Clocks orbiting other worlds to provide inhabitants with location information.

“Once we have this technology demonstrated…we could build GPS-like networks at other planets and moons,” Seubert said. “We use GPS every dang day here on the Earth. I think we take it for granted. If we’re sending humans to Mars, how are they going to find their way around?”

For Seubert, who has been working on the Deep Space Atomic Clock since she joined the Jet Propulsion Laboratory in 2011, the upcoming launch is a big deal. She’ll be flying to Florida to watch it.

“I will be there,” said Seubert. “I’ve been waiting a long time for this.”

—Katherine Kornei (@katherinekornei), Freelance Science Journalist

Planetary Low Tide May Force Regular Sunspot Sync Ups

Fri, 06/21/2019 - 12:11

For more than 1,000 years, the number of sunspots hit a minimum within a few years of a major planetary alignment. A recent study showed that tides created by this alignment every 11 years are strong enough to tug on material near the Sun’s surface and synchronize localized changes in its magnetic field.

“We noticed from historical data that there is an astonishing degree of regularity” in the sunspot cycle, Frank Stefani, lead author of the study, told Eos. Stefani is a fluid dynamics research fellow at Helmholtz-Zentrum Dresden-Rossendorf in Dresden, Germany. “We definitely have a clocked process,” he said. “But then the question was, What is the clock?”

The study expands upon the commonly accepted model for the solar dynamo and supports a long-held theory that planetary configurations are responsible for the sunspot cycle and magnetic solar cycle.

Wound, Twisted, and Unstable A simplified schematic of a single magnetic field line as it wraps around the Sun (omega effect) and then twists upon itself (alpha effect). The arrows indicate the direction that solar material moves as it drags the field line with it. Credit: NASA/MSFC

As a giant spinning ball of plasma, the Sun’s magnetic field is extremely complicated. Its magnetic field lines start as parallel lines running from the north to the south pole. But because the Sun rotates faster at its equator than at its poles, those pole-to-pole magnetic field lines slowly wind and wrap around the Sun, stretching like taffy from the middle of the line to become horizontal.

On top of the rotational motion of solar plasma, convection moves material from the equator to the poles and back again. That twists the field lines around each other into loops and spirals.

The winding and twisting of the Sun’s magnetic field lines are described by the alpha-omega dynamo model. In that model, alpha represents the twisting, and omega represents the wrapping. Tangled field lines can create instabilities in the local magnetic field and cause sunspots, flares, or mass ejections.

This model is the commonly accepted explanation for the behavior of the Sun’s magnetic field, but it’s not perfect, Stefani explained. It predicts that the instabilities’ twistedness will oscillate randomly every few years. But the model can’t explain why the number of sunspots waxes and wanes on a roughly 11-year cycle or why the Sun’s magnetic field flips polarity every 22 years.

Low Tide, Low Activity

Another solar system phenomenon happens every 11 years: Venus, Earth, and Jupiter align in their orbits. These three planets have the strongest tidal effect on the Sun, the first two because of their proximity to the Sun and the third because of its mass. Past observational studies have shown that minima in the sunspot cycle have occurred within a few years of this alignment for the past 1,000 years or so.

“If you look at the trend, it has an amazing parallelism,” Stefani said.

The researchers wanted to test whether the planetary alignment could influence the Sun’s alpha effect and force an interplanetary low tide at regular intervals. The team started with a standard alpha-omega dynamo model and added a small tidal tug to the alpha effect every 11 years to simulate the alignment.This synchronization could also suppress (or generate) sunspots across the Sun at roughly the same time.

“Our dynamo model is not a completely new one,” Stefani explained. “We’re really building on the old-fashioned, or conventional, alpha-omega dynamo.”

The simulation showed that even a weak tidal tug of 1 meter per second every 11 years forced unstable magnetic twists to pulse with that same period. The simulated dynamo’s polarity oscillated with a 22-year period just like the real solar dynamo.

“With a little bit of this periodic alpha,” Stefani said, “we can indeed synchronize the dynamo period to 22 years [with] planetary forcing.”

Because those magnetic instabilities are connected with solar activity, the researchers argue, this synchronization could also suppress (or generate) sunspots across the Sun at roughly the same time—in other words, the sunspot cycle. The team published these results in Solar Physics in late May.

A Counterintuitive Result?

“This is an intriguing paper,” said Steve Tobias, a solar dynamo researcher at the University of Leeds in the United Kingdom who was not involved with this research. Tobias argued that the combined planetary tides are too weak to directly set the length of the solar cycle—plasma dynamics deep within the Sun are the more likely cause, he told Eos.Other planetary systems might have tidally dominant planets that resonate with their suns, but it’s not likely that we’ll be able to prove it.

Nevertheless, this study “seems to show that even a tiny amount of forcing from tidal processes can resonantly synchronize the cycle,” Tobias said. “This counterintuitive result should be explored further by investigating the behavior of proxies for solar activity such as the production rates of isotopes of beryllium deposited in ice cores.”

It’s possible that other planetary systems might have tidally dominant planets that resonate with their suns like ours do, Stefani said, but it’s not likely that we’ll be able to prove it.

For most stars, “we have observations going back about 40 years,” he said. “And people are happy if they can identify two or three or four periods. Only for our Sun do we have all the historical observations. We have beryllium data. We can go back for thousands of years.”

“Our Sun is quite an ordinary star, but it is quite special in that sense.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

House Science Committee Approves Bill to Stop Sexual Harassment

Thu, 06/20/2019 - 19:24

The House Committee on Science, Space, and Technology unanimously approved bipartisan legislation today, 20 June, to stop sexual harassment in the sciences. The bill follows a 2018 report by the National Academies of Sciences, Engineering, and Medicine that found that 58% of individuals in the academic workplace experience sexual harassment.

The Combating Sexual Harassment in Science Act of 2019 “is an important step in tackling the serious issue of sexual harassment in the sciences,” committee chair Rep. Eddie Bernice Johnson (D-Texas) said at a committee markup that advanced the bill for consideration by the full House.

Johnson and committee ranking member Rep. Frank Lucas (R-Okla.) jointly introduced the legislation on the first day of the 116th Congress.

The bill calls for the National Science Foundation to expand research efforts to better understand the causes and consequences of sexual harassment and gender harassment in the science, technology, engineering, and mathematics (STEM) workforce and to examine interventions to reduce harassment. The bill also calls for the White House Office of Science and Technology Policy to establish an interagency working group to coordinate federal science agency efforts to reduce sexual and gender harassment.

“Unfortunately, too many women have been driven out of STEM careers due to a culture of harassment and abuse.”The bill states that “sexual harassment undermines career advancement of women” and that many women have reported leaving their jobs at institutions of higher education because of sexual harassment.

“I hope [the bill] can play an important role in focusing federal efforts to stamp out sexual harassment in the sciences,” Johnson said at the hearing.

“Unfortunately, too many women have been driven out of STEM careers due to a culture of harassment and abuse,” Lucas stated. “H.R. 26 takes the first steps to addressing the problem.”

He said that engaging more women in STEM studies and careers “is essential to American competitiveness. Women make up half of the workforce but account for less than 25% of America’s STEM workforce.”

Bipartisan Committee Effort

In an interview with Eos, Lucas said that the legislation is an example of bipartisanship on the committee.

“My chairman indicated early on in the year after we’d gone through all the investigations and all the background work last year that there was a major problem about sexual harassment,” he told Eos. “I agreed with it. It was something that we could address working within the jurisdiction of the committee, and it seemed like the appropriate way to send the signal by filing this [bill] as one of the first pieces of legislation at the beginning of the session that we were going to do something. And you’ve seen this committee now become a very product oriented group. Progress.”

Lucas, however, would not venture a prediction about how the legislation might fare in the Senate. “I’ve been around long enough to know that you can’t guess the other body,” he said. “We can only do our work and do it in an efficient and effective way, and lay it out before them and encourage them to be responsive. They’ll step up when and how they see fit, but we’ve laid great groundwork here today.”

—Randy Showstack (@RandyShowstack), Staff Writer

Senator Urges Ending Dark Money’s Stifling of Climate Action

Thu, 06/20/2019 - 18:29

Anonymous dark money political funding that has poisoned the U.S. political process and throttled action on climate change needs to be unmasked and stopped, Sen. Sheldon Whitehouse (D-R.I.) said at a 19 June forum in Washington, D.C.

“The fossil fuel industry’s dark money has polluted our politics as badly as its carbon emissions have polluted our atmosphere and oceans.”“The fossil fuel industry’s dark money has polluted our politics as badly as its carbon emissions have polluted our atmosphere and oceans,” Whitehouse said at the forum hosted by the Washington, D.C.–based League of Conservation Voters (LCV) and the End Citizens United Action Fund (ECU), a group advocating to counter the impacts of the Supreme Court’s 2010 Citizens United decision that led to unprecedented political spending.

Whitehouse called for continued pressure in Congress and the courts, among other measures, on the fossil fuel industry to curtail and expose the influence of dark money, which is loosely defined as anonymous political spending by organizations such as political action committees (PACs). The For the People Act of 2019 (H.R. 1), which the House of Representatives passed in March, addresses campaign spending. Companion legislation, awaiting action in the Senate, includes the DISCLOSE Act that Whitehouse introduced to increase transparency surrounding special interest campaign contributions.

Since the Citizens United decision, entities within the energy and natural resources sector have provided more than $668 million in campaign contributions, including more than $185 million in corporate PAC contributions, according to data provided by LCV and ECU that cite information from the Center for Responsive Politics, a group that tracks money in U.S. politics. Three quarters of those contributions have gone to Republican candidates, according to the information.

“Our failure in Congress to address climate change is directly connected to the secret empire of dark money that Citizens United launched,” Whitehouse said. “Climate denial and dark money are two sides of the same coin.”

EPA’s Replacement of the Clean Power Plan

Other speakers at the forum also called for an end to dark money. “If we want to protect our air and our water and reduce the impact of climate change, we have to sever the ties between special interest money and candidates for office,” said ECU president Tiffany Muller.

Tiernan Sittenfeld, LCV’s senior vice president for government affairs, said that the 2020 elections and potential victories by environmental champions “cannot come soon enough.”

“I say that on the day when the Trump administration has just pulled back the Clean Power Plan, perhaps the single biggest thing our nation has ever done to combat the climate crisis,” she added. The Environmental Protection Agency (EPA) announced on 19 June its final Affordable Clean Energy rule, which would replace the plan put in place by former president Barack Obama.

At the forum, Whitehouse said that the Trump administration’s favoring of the fossil fuel industry goes beyond dark money.

“Time is not our friend with climate change.”“With the Trump administration, there’s something that’s so kind of shameless about their operation that they really don’t need to do a whole lot in terms of dark money. They just pick a fossil fuel stooge right out of the fossil fuel industry and put them in as a decision-maker,” he said.

EPA director Andrew Wheeler is a former coal lobbyist with the Faegre Baker Daniels consulting firm.

Whitehouse said that although environmental groups may challenge Trump administration rulings and win in court, time is not on the side of the environment. He said that if all the administration is doing is stopping action that needs to be taken to move forward on climate change efforts, “then the advantage is theirs.”

“With all the delays and all the fuss on the Clean Power Plan, and the very shabby plan that was announced today, time is not our friend with climate change,” Whitehouse said. “So even if they do a complete hash of a job through the administrative procedures act process and then losing court, we’re still in the situation where we’ve lost another 3 or 4 years where EPA could have done something useful [about climate change] if they tried.”

Snuffing Out Bipartisan Climate Efforts

Whitehouse said that when he got to the Senate in 2007 and for a few years afterward, there was bipartisan progress on climate change. Sen. John McCain had a strong climate position when he ran for president in 2008, and there were hopes for bipartisan legislation.

However, that bipartisan effort on climate change “all fell apart” following the Supreme Court ruling that “opened the spigots to the fossil fuel industry to flood unlimited money into our politics,” he said. “With its Citizens United weaponry, ruthlessly, through dark money attacks and threats, the fossil fuel industry snuffed out Senate bipartisanship on climate change. Weaponization of that new unlimited dark money power by the fossil fuel industry cost us a decade of climate progress.”

“I think we can disable their dark money power structure, shame corporate America into stepping up in a way that they to this point have not, and keep enormous pressure on the fossil fuel industry.”Whitehouse told Eos that he tells congressional colleagues who publicly supported climate action prior to Citizens United that time is running out for meaningful action on climate change.

“You will be on the right side of history” if you support climate action, Whitehouse said he tells colleagues. “When these fossil fuel pirates blow up, which they will—the whole thing is a big charade; it’s phony as a $3 bill—you don’t want to go down with them.”

At the forum, Whitehouse said he is hopeful that the influence of dark money on climate change efforts can be turned around. He said that corporations are vulnerable to public opinion, and he pointed to polls showing that Americans are concerned about corruption in government and about the impacts of climate change.

“I think we can disable their dark money power structure, shame corporate America into stepping up in a way that they to this point have not, and keep enormous pressure on the fossil fuel industry,” he said.

He remains hopeful, too, about the upcoming 2020 elections.

“The dark money problem and the fraudulent climate denial, that whole mess, is something we can attack, and that is a real vulnerability for Republicans,” he told Eos. “I think that if we had a Democratic president and Democratic majority leader even without a filibuster-proof Senate, and a Democratic speaker, we absolutely could find a way to get a very meaningful bill passed.”

—Randy Showstack (@RandyShowstack), Staff Writer

New Perspectives on 2,000 Years of North Atlantic Climate Change

Thu, 06/20/2019 - 11:58

Historical and natural clues suggest that Earth’s climate underwent small changes over the past 2,000 years, and variations in North Atlantic ocean circulation may have been a key driver. In a new paper, Moffa-Sánchez et al. compile recent advancements in analytical tools used to probe this period of ocean circulation, presenting a comprehensive overview of current knowledge.

The past 2 millennia have seen several centuries-long climate shifts, such as the Medieval Warm Period, followed by the Little Ice Age, which were particularly recorded around the North Atlantic. The North Atlantic is an important climatological region  because of the strong interactions between ocean, atmosphere, and sea ice, as well as the overturning circulation between surface and deep-ocean currents. Scientists have traditionally proposed that changes in this Atlantic Meridional Overturning Circulation played a key role in the observed climate shifts, but this view remains under debate because of the lack of clear evidence.

The new review synthesizes 20 years’ worth of rapid progress toward understanding the role of ocean circulation in historical climate change. The authors highlight advancements in two major fields: observational data that provide proxy clues to past oceanic and climate conditions, such as sediment cores and the remains of shelled organisms, and models that simulate the past climate.

The authors unite a number of previously published proxy data sets to paint a picture of North Atlantic ocean circulation over the past 2,000 years. This compilation reveals details of past conditions in various regions. For example, the past 2,000 years around Greenland show progressively cooler and icier surface waters reaching the coldest conditions during the Little Ice Age. The compilation also underscores a need for more comprehensive data sets on deep North Atlantic waters for this period.

To gain further insight, the authors selected and compared three recently developed climate models that are particularly well suited for studying the North Atlantic’s past climate over the past millennium. They identified key areas of uncertainty and disagreement between the models that could be addressed in future research. They also demonstrated agreement between model and proxy data estimates of sea surface temperatures for some historical periods and regions but disagreement for others.

These findings highlight important advancements toward understanding how changes in ocean circulation may have affected historical climate change while emphasizing areas for improvement. The authors note the promise of using models to fill gaps in observational data but also call for continued collection and improvement of proxy data sets. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003508, 2019)

—Sarah Stanley, Freelance Writer

Spy Satellite Reveals Accelerated Pace of Himalayan Glacier Melt

Wed, 06/19/2019 - 19:34

Declassified images taken during the Cold War show that the thickness of Himalayan glaciers has been declining twice as fast since 2000.

A new study released on 19 June 2019 in Science Advances compares the thickness of 650 glaciers in the Central Himalaya over a 40-year period. The results relied on modern methods to digitize declassified film photographs taken by U.S. spy satellites between 1973 and 1976. The analysis revealed that even over large swaths of the Himalaya, which have a range of local climates and pollution levels, scientists found a detectable link between diminishing glacial ice and warming air temperature.

“We see the clearest picture yet of how Himalayan glaciers have responded to climate change.”“We see the clearest picture yet of how Himalayan glaciers have responded to climate change,” first author and doctoral student at Columbia University Josh Maurer told Eos. “As temperatures continue to rise, ice loss will continue to accelerate.” He warned of drier days to come for those downstream as water stores melt away.

Tracking glacier melt in the Himalaya can be a tricky business. Unlike some glaciers that recede as they melt, like Exit Glacier in Alaska, Himalayan glaciers often keep their spatial extent but simply become thin. The glacier loses mass, dwindling in height, but the change is difficult to assess from top-down snapshots, like those available in the 20th century when air temperatures began to ramp up due to global warming.

Starting in the 1950s, however, the United States designed sophisticated cameras to spy on the former Soviet Union and allied European and Asian countries. The KH-9 Hexagon spy satellite, first launched in 1971, snapped images from hundreds of kilometers above at such fine resolution that U.S. officials could count the number of launchpads at Soviet missile sites. Images from Hexagon and other spy satellites were declassified in the 2000s, giving scientists a new trove of historical data.

An artist’s illustration outlines the KH-9 Hexagon satellite and its camera components. The satellite used nearly 100 kilometers (60 miles) of film per deployment. Credit: National Reconnaissance Office

The declassified Hexagon images present researchers with a new angle that traditional satellite images couldn’t: The spy satellite took photos overlapping by more than 50%, so that U.S. intelligence officials back in Washington could create three-dimensional images. Having the overlapping images allowed Maurer to extract not only the extent of the glaciers but also their volume over time.

“That third dimension is really important,” Maurer explained. He created a digital elevation model for the Himalayan region using the old black-and-white film and compared it with three-dimensional images taken today.

A Landscape Melting Away

The latest study shows the quickening pace of the Himalayan glacial melt. According to the research, the glaciers shrank by an average of a quarter of a meter between 1975 and 2000. Since 2000, however, the glaciers lost twice that amount over the same length of time.

The glaciers now have just under three quarters of their 1975 ice mass.All told, Himalayan glaciers now lose billions of tons of ice per year, Maurer said, enough to fill 3.2 million Olympic-sized swimming pools annually. The glaciers now have just under three quarters of their 1975 ice mass.

The effect wasn’t isolated to just one part of the Central Himalaya. “We see a rather homogenous pattern of ice loss across a large and climatically complex region,” Maurer explained.

Using measurements from weather stations in the area, the study points to global warming as the underlying cause. “The correlation we observed between rising air temperatures and acceleration of glacier melts over the past 4 decades really highlights how vulnerable these glaciers are to climate change,” Maurer noted.

The 650 glaciers considered in the study contain only about half the glacial mass in the Central Himalaya. But Maurer said that the study is representative of the region, because their analysis included the largest glaciers, which have the most to lose, and spans a wide area.

Glaciologist Etienne Berthier, from the French National Centre for Scientific Research, called the paper’s doubling pace of ice loss “very convincing” but also said that scientists should wait until further study to attribute ice melt to warming temperatures. “This work paves the way toward more thorough attribution studies,” he told Eos.

Maurer plans to apply this method to other parts of High Mountain Asia, such as the Hindu Kush mountain range at the Afghan and Pakistani border. He said that the Hexagon program didn’t cover just U.S. adversaries but has images worldwide.

“They were taking images wherever they could, all over the globe,” Maurer said. “There are lots of images that are just sitting there in an archive waiting to be used.”

—Jenessa Duncombe (@jrdscience), News Writing and Production Fellow

The Tides They Are a-Changing

Wed, 06/19/2019 - 12:24

When is it high tide?

Do we have a spring tide?

These questions are asked by tourists getting ready for a day at the beach and by sailors arriving at port. The tide makes a difference to their day. But tides may have an influence on far larger timescales, too.

At the recent General Assembly of the European Geosciences Union in Vienna, Austria, two presentations suggested that tides may have changed the history of our planet on more than one occasion.

Snowball Earth

Tides may have helped initiate a “snowball Earth” phase by becoming exceptionally weak, according to Mattias Green of the School of Ocean Sciences at Bangor University in the United Kingdom, along with researchers from the University of Lisbon in Portugal, Monash University in Australia, and Northwestern University in the United States. They reached this conclusion from model calculations of tides the world over, taking into account the location of continents, ocean depth, and ice cover.

Originally, Green told Eos, he and his colleagues were looking for arguments against the snowball Earth hypothesis, which posits that during the Cryogenian, the geologic period from 720 million to 635 million years ago, two extreme glaciations covered the planet with ice, except possibly for a band along the equator.Tidal weakness supported the stable cold phase of the planet by starving ice sheets of any warm water that could have kept them in check.

There is no consensus, however, that glaciation really was that extreme. Green thought that if tides turned out to be strong during the Cryogenian, it might not be possible for so much ice to exist.

“We know that the tides on Earth today are important around the Antarctic and in Greenland,” Green said. “They lift the ice, which sets up stress fractures. But most importantly, the tide brings warm water in, which melts the ice on the underside. The cold, fresh meltwater would then insulate the ice, but the tide gets rid of that. There are studies that suggest that if you don’t have tides in Antarctica, the melt rate would be a quarter of what it is now.”

After the calculations were complete, Green had to conclude that the opposite was true in the Cryogenian: Tides were exceptionally weak, about 10% of their present amplitude.

“They were doing nothing,” Green said.

But that doesn’t mean that tides played no role during snowball Earth. On the contrary, their weakness supported the stable cold phase of the planet by starving ice sheets of any warm water that could have kept them in check.

“All the other processes that the tides are contributing to, like vertical transport, ocean circulation, [they aren’t] there. So it’s possible that along with a lot of other things, this is one potential mechanism that helped bring along the snowball stage,” said Green.

One initial reason for weak tides during the Cryogenian was that the familiar continents of Earth were gathered into one supercontinent, surrounded by one ocean. According to an earlier study by Green, when there are more continents and smaller oceans, the resonance of tidal flows is more likely to be amplified, just like water will slosh higher and higher in a bathtub if you move your body at just the right beat. Right now, this is the reason tides in the Atlantic are particularly high.

Once Earth entered the snowball stage, for reasons that are still debated, conditions on the planet further dampened the tides, according to Green. Water turning into ice means shallower oceans and thus less water for the Moon and the Sun to pull around. Friction on the underside of the ice means tidal currents are weakened.

The feedback would work in the other direction once the supercontinent started breaking up and the ice lost its grip on Earth and its tides. The Cryogenian ended.

Tidal Change in Evolution

In a warmer world, life flourished in the sea and eventually on land. And in that transition, too, tides may have played a pivotal role, according to another study by Green, together with Steven Balbus of the University of Oxford in the United Kingdom and Per Ahlberg and Hannah Byrne of Uppsala University in Sweden.

Researchers argue that during the Devonian, tides forced certain lobe-finned fish to make the move onto land. These organisms were the ancestors of modern tetrapods.The researchers argue that during the Devonian, 419 million to 359 million years ago, tides forced certain lobe-finned fish to make the move onto land. These organisms were the ancestors of modern tetrapods, or four-limbed vertebrates, including humans.

An explanation for that move onto land is needed, Balbus told Eos.

“That’s not something that naturally occurs. Fish don’t come out of the ocean regularly and make themselves happy on land. The other way is very common. Lots of species love to go back to the ocean; they evolve, and they become very fishlike: whales, porpoises, and before them ichthyosaurs. They can swim, and if they need oxygen, they go up. If you’re a fish on the land and you need to have water over your gills, what are you going to do? So it requires some kind of special impetus for that to happen.”

That impetus, he proposed in a 2014 paper in Proceedings of the Royal Society A, could have come from the phenomenon of spring tides and their opposite, neap tides. These tidal variances occur because the gravitational pulls of the Sun and the Moon, comparable in size, sometimes reinforce and sometimes work against each other.

“For the spring tides, the ocean comes up very far onto the land. The next time there’s a high tide, it doesn’t come as far,” Balbus explained. “That means there’s an isolated tidal pool. If you’re a fish, it’s not good.”

That’s because with Earth tides being what they are, a fish will have to wait roughly half a month for a splash of water and a chance to escape the pool—unless it can call on some extra capabilities.

“If the fish can somehow flail out, and wouldn’t die immediately, there would be another place reasonably nearby to get to, that was replenished a little more often, because it’s closer to the sea,” Balbus said.

In other words, on a coast with high tides, even rudimentary locomotion would be advantageous for a fish, making the development of fleshy, lobed fins into weight-bearing limbs a good evolutionary adaptation.

Balbus and his colleagues investigated tidal patterns and evolution by analyzing the geography of 400 million years ago. “The idea is to take reconstructions of what the continents during the Devonian looked like, work out what the Moon’s orbit was back then, and ask: What tidal patterns were there?”

His team was the perfect multidisciplinary group, Balbus said: he himself is an astrophysicist, Green is an oceanographer, and Ahlberg and Byrne are paleontologists. They did the calculations and at the Vienna conference presented two regions in what are now southern China and the Baltics that seem particularly promising to establish a correlation between tidal range and the emergence of early tetrapods.“We are a long way from proving [our theory]. People argue about the reasons why these fish developed limbs; you can think of all kinds of reasons why that would be good.”

“There are bays with very high tidal responses,” Balbus said. “And, in fact, they are associated with very interesting fossil records that tell us that a large radiation of key transitional fish species occurred. And the early tetrapods were located there.”

An article on the findings has been submitted to the Proceedings of the National Academy of Sciences of the United States of America, but Balbus admits that “we are a long way from proving it. People argue about the reasons why these fish developed limbs; you can think of all kinds of reasons why that would be good.”

One way to clinch the debate would be for the group to point to locations that had very high tides in the Devonian and then find transitional or early tetrapod fossils there. “That would be a very important vindication, if we can calculate where to look for fossils,” said Balbus.

Tides on Earth are nowadays a bit stronger than they were in the Devonian, and all over the world, tidal pools are a common feature of the coastal landscape. So the same evolutionary pressure for fish to adapt to live on land still exists, Balbus agrees.

Modern fish, however, have a problem that the lobe-finned fish of the Devonian did not: “God help the fish that can’t do very well on land. There are a lot more creatures now that will have them for breakfast than when this first happened. But there are fish that live in these intertidal regions, and you can see what kind of biological adaptations they’ve developed. Air breathing capacity and also some sort of mobility. In Asia, perches actually climb trees, and they inhabit areas that I guess are similar to the sort of areas we are talking about here, extensive swamps.”

Such behavior shows these areas to be, he said, the most likely places for vertebrates to have come on shore for the first time. “And it seems to have happened.”

—Bas den Hond (bas@stellarstories.com), Science Writer

Banned CFC Emissions Tracked to Eastern China

Wed, 06/19/2019 - 12:16

In 2010, a chlorofluorocarbon known as CFC-11 was phased out of production because of its detrimental impact on Earth’s protective ozone layer. But since 2013, a slowdown has been observed in the rate at which the banned substance has been declining in the atmosphere, suggesting that the foaming agent is once again being manufactured somewhere on Earth. A new study is helping to narrow down the source of the easily dispersed gas to two industrial areas in eastern China, an important step in curbing its production.

In 1987, the Montreal Protocol on Substances That Deplete the Ozone Layer was enacted in an effort to halt the production of chemicals that break down the ozone layer, which protects the planet from harmful ultraviolet rays. To date, it is the only United Nations treaty to be ratified by every country on Earth.

The sheer magnitude of the emissions strongly suggests that the source must be new production, not the breakdown of existing materials.Last year, a study published in Nature showed an unexpected and persistent increase in CFC-11 emissions, starting in 2013. The paper’s “findings suggested that somebody, somewhere, has started to emit CFC-11 in quite substantial quantities, on the order of 10,000 tons per year,” says Matthew Rigby, an atmospheric chemist at the University of Bristol in the United Kingdom and lead author of the new study, also published in Nature.

But narrowing down a point source of an easily dispersed gas is not easy, Rigby says. “The atmosphere is very diffusive. The further you get from the source, the more diffuse the signal becomes.”

To help pinpoint the source of the emissions, Rigby and his colleagues turned to two atmospheric monitoring stations in South Korea and Japan run by the Advanced Global Atmospheric Gases Experiment and the Japanese National Institute for Environmental Studies.

By combining the monitoring data with models that work backward to trace how CFC-11 is dispersed through the atmosphere, the team was able to gauge that starting around 2013, an extra 7,000 tons of CFC-11 annually was coming from eastern China, particularly from in or around the provinces of Shandong and Hebei.

Atmospheric observations show an increase in CFC-11 emissions from eastern China between 2008–2012 and 2014–2017. The emissions rise is primarily from Shandong, Hebei, and surrounding provinces. Credit: Matt Rigby and Luke Western, University of Bristol

“We can’t yet say if it’s being emitted from one place or several in those areas,” Rigby says. But the sheer magnitude of the emissions strongly suggests that the source must be new production, not the breakdown of existing materials such as through the demolition of old buildings containing CFC-11 materials, he says.

Stopping the production will likely fall to Chinese authorities, says Neil Harris, an atmospheric chemist at Cranfield University in the United Kingdom who was not involved in the new study. “I think that the attention these studies is getting in the scientific community and in the media will be enough to motivate the Chinese government to regulate their factories.”

Detecting the other fraction of the rogue global CFC-11 emissions will take a more comprehensive monitoring network, Rigby says. Currently, the United States, parts of Europe, and Japan have reasonably good coverage with existing networks, but “there’s a huge swath of the planet we don’t monitor at all,” including India, South America, and Africa.

“If there’s one thing these two studies have shown, [it] is how absolutely crucial atmospheric monitoring measures are for the continued success of the Montreal Protocol,” Rigby says. “If it [weren’t] for these records, we would have no idea that new production was occurring.”

—Mary Caperton Morton (@theblondecoyote), Science Writer

Extending the Envelope for Known Safe Locations in Space

Wed, 06/19/2019 - 11:30

Most operational spacecraft orbit Earth within 7 Earth radii. Over recent decades, a few research missions have sampled the plasma and energetic particle environment out to 20 Earth radii. From the limited (but growing) number of observations beyond geosynchronous orbit, Denton et al. [2019] have created an empirical model from the CLUSTER mission, a quartet of orbiting spacecraft with identical instruments.

This model supports prediction of electron flux in the energy range ~45 eV to ~325 keV, as a function of local-time and radial distance from the Earth near the equatorial plane. This is a key energy band associated with spacecraft charging. The model is parameterized by a geomagnetic activity index (Kp index) that ranges from 0 to 9. This new model supports international efforts in benchmarking extreme particles fluxes that may damage spacecraft or individual space-based instruments.

Citation: Denton, M. H., Taylor, M. G. G. T., Rodriguez, J. V., & Henderson, M. G. [2019]. Extension of an empirical electron flux model from 6 to 20 Earth radii using Cluster/RAPID observations. Space Weather, 17. https://doi.org/10.1029/2018SW002121

—Delores J. Knipp, Editor in Chief, Space Weather

Demystifying Sea Level Changes Along the New England Coast

Tue, 06/18/2019 - 11:13

The Atlantic Meridional Overturning Circulation (AMOC), a system of currents in the Atlantic Ocean that transports warm surface waters northward and cooler, deeper waters southward, is a crucial component of Earth’s climate system. Because the AMOC is part of a conveyor belt of oceanic circulation that redistributes heat around the globe, variability in its strength can have significant climate consequences.



Previous modeling studies have concluded that the AMOC’s strength is negatively correlated to sea level along the New England coastline, such that a weakening of the North Atlantic Current or the Gulf Stream leads to a rise in sea level at the coast. This relationship, however, has been difficult to detect in observational records.

Now Piecuch et al. are challenging the conventional wisdom that a direct causal connection exists between the AMOC and coastal sea level in this region. After obtaining monthly RAPID monitoring program observations of the overturning circulation at 26°N and monthly sea level records collected between 2004 and 2017 at eight New England tide gauges, the team examined the physical relationships between the two records.

The authors concluded that widespread atmospheric teleconnections can simultaneously trigger changes in both the AMOC at 26°N and coastal New England sea level. Although these phenomena are temporally correlated with each other, the team concludes that they are not causally linked. The researchers argue that the local atmospheric forcing mechanisms driving coastal New England sea level change are instead related to the North Atlantic Oscillation and other surface atmospheric variations.

Although this study represents a valuable contribution to improving our understanding of how coastal sea level is related to oceanic circulation, the authors caution that their results apply to only the time period studied and that the negative correlation between coastal sea level and overturning at 26°N should not be considered representative of the AMOC at other latitudes. The researchers suggest that future studies could shed new light on the processes occurring at higher latitudes, where new AMOC monitoring arrays have recently been established. (Geophysical Research Letters, https://doi.org/10.1029/2019GL083073, 2019)

—Terri Cook, Freelance Writer

Ancient Water Underlies Arid Egypt

Tue, 06/18/2019 - 11:12

Aside from the Nile River’s green corridor, much of northeastern Africa is desert. But the arid landscape hides a secret: Vast quantities of groundwater fill an underground aquifer that spans four countries.

In the future, as Egypt’s population and agriculture expand, groundwater will become a more important resource.A new study using chloride isotopes to date the groundwater under Egypt’s Eastern Desert has found that the water in smaller, shallower aquifers is refilled by the larger, deeper aquifer, where vast quantities of groundwater date to the last ice age.

Most Egyptians live along the Nile and get their drinking and irrigation water from the river. Currently, just 7% of the country’s water usage is supplied by groundwater, but that number is expected to rise, said Mahmoud Sherif, a hydrogeochemist at the University of Delaware and lead author of the new study, published in Earth and Planetary Science Letters. “In the future, as Egypt’s population and agriculture expand, groundwater will become a more important resource.”

The research team set out to date the age of the groundwater under the Eastern Desert to determine the aquifers’ responses to climate conditions and the recharge rates of shallower formations called alluvial aquifers. “These aquifers are closer to the surface and easier to access than the deeper Nubian aquifer,” Sherif told Eos.

“We expected the water in the shallow aquifers to be less than 100 years old,” Sherif said, with the clock starting when the rainwater falls from the atmosphere onto Earth. Instead, some of the water samples were more than a thousand times that age.

Mahmoud Sherif, lead author of a new study on aquifers in Egypt’s Eastern Desert, collects a water sample from a groundwater pipe. Credit: Mahmoud Sherif

The researchers used the radioactive isotope chlorine-36, which has a half-life of 300,000 years, to date groundwater samples collected from 29 wells scattered around the Eastern Desert. They found that the oldest samples in the alluvial aquifers were more than 200,000 years old. “This was really surprising,” Sherif said.

Mixing with the Nubian aquifer helps explain the age: “This region is tectonically active and has a lot of deep-seated faults. Groundwater from the Nubian aquifer is making its way up along these faults and recharging the alluvial aquifers,” Sherif said.

The Nubian Sandstone Aquifer System is the largest known fossil water aquifer in the world. Spanning more than 2 million square kilometers across Sudan, Chad, Libya, and Egypt, it contains more than 150,000 cubic kilometers of groundwater—more water than the Nile River discharges in 500 years.

The water in the Nubian aquifer dates back to the Pleistocene epoch, when Earth weathered periodic deep freezes.

“During the Ice Age, what is now desert was a lot greener and wetter,” said Cliff Voss, a hydrogeologist with the U.S. Geological Survey in Menlo Park, Calif., who was not involved in the new study. Today, the Nubian aquifer may receive a little water during flash flood season, but the recharge rate is “effectively zero,” Voss said.

The new study offers new data on the little-studied Eastern Desert, and the findings match up well with previous studies of the Western Desert and the rest of the region underlain by the Nubian aquifer, said Voss.

“In 2014, we mapped out the Nubian aquifer in the hopes that the four countries wouldn’t have to compete over their share of the water,” Voss said of his 2014 study published in the Hydrogeology Journal. “Fortunately, all four countries essentially have water forever, especially Egypt and Libya.”

To date, Libya is the only country to tap into the vast water reserves of the Nubian aquifer on a large scale. The pipelines known as the Great Man-Made River carry water from 1,300 wells more than 2,800 kilometers inland across the desert to the coastal cities of Tripoli, Benghazi, and Sirte.

The connection between the alluvial aquifers and the Nubian aquifer means that communities and agricultural operations in the Eastern Desert have an almost unlimited supply of groundwater without having to drill very deep.

“It’s a gorgeous supply of water, clean, not salty,” Voss says. “You can drink it without any filtration or treatment.”

—Mary Caperton Morton (@theblondecoyote), Science Writer

Monitoring Volcanic Craters with Infrasound “Music”

Mon, 06/17/2019 - 11:30

Chile’s Villarrica volcano erupted suddenly on 3 March 2015, disgorging a lava fountain more than 2 kilometers high. The eruption—Villarrica’s first in 30 years—was unexpected in terms of its rapid onset and its violence. It was also remarkably short-lived. Within an hour, the explosive activity had ended. Within about a month, the volcano had returned to its usual state, which featured a roiling lava lake situated deep within the steep-walled summit crater.

We now recognize that Villarrica’s changing sounds provided a warning that lava was rising within the crater.Forecasting such violent eruptions is the holy grail for applied volcano science. Toward this objective, volcanologists deploy seismometers to detect tremors, tiltmeters and GPS to identify swelling, and multispectral detectors to monitor gas and heat output. Infrasound sensors, which record the low-frequency sounds produced by volcanoes, are an increasingly important component of this diverse tool kit.

Volcanologists traditionally have used infrasound surveillance to both count explosions and track eruption intensity, important capabilities when the view of the volcano is obscured [Fee and Matoza, 2013; Johnson and Ripepe, 2011]. Recent studies have demonstrated that infrasound monitoring can also be used to identify important eruption precursors [e.g., Ripepe et al., 2018]. Villarrica gave indications of its unrest through the changing character of its infrasound. We now recognize that Villarrica’s changing sounds provided a warning that lava was rising within the crater [Johnson et al., 2018a].

These observations were made serendipitously as part of a National Science Foundation–sponsored research project, Volcano Acoustics: From Vent to Receiver, that studied the long-distance propagation of the infrasound produced at Villarrica. During the 2015 field expedition, we installed sensors on the summit and flanks of the volcano. Although the 3 March eruption destroyed the summit deployment, sensors outside the damage zone collected data that yielded a full chronology of the volcano’s increasing unrest.

Volcanoes as Giant Musical Instruments

Volcanoes generate infrasound, low-frequency sounds below the threshold of human perception. Despite varied eruptive behaviors, many volcanoes radiate their most intense sounds within a few octaves of 1 hertz, corresponding to sound wavelengths of hundreds of meters. It is no coincidence that this dimension is similar to the dimension of volcanic craters, which play a critical role in modulating the radiated sound [e.g., Kim et al., 2015].

As with a musical horn, a volcano’s timbre and pitch are particular to a crater’s shape.In many ways, a volcano is like a giant musical instrument. As with volcanoes, the size of a musical horn controls the pitch of the sound it makes: Bigger horns make lower-pitched sounds. Musical sounds tend to be pleasing because of the horn’s resonance; air pressure waves sloshing back and forth within a length of brass tube project sonorously from the horn’s bell. The shape of the bell’s flare is important and controls whether a note is sharp and short or rich and reverberating. This quality, which is independent of a note’s frequency or loudness, is referred to broadly as its timbre.

As with a musical horn, a volcano’s timbre and pitch are particular to a crater’s shape. Volcanoes with deep craters have a tendency to produce low-frequency sounds, whereas shallow craters radiate higher-frequency sounds [Spina et al., 2014; Richardson et al., 2014]. Narrow conduits often resonate for extended periods, but broad, dishlike craters might not reverberate at all. Although volcanic sound sources can be varied, vents at the bottom of a crater acting as mouthpieces often generate infrasound. The violent expulsion of gas from vents or from a lava lake surface can induce the crater to resonate.

Volcanic Unrest and Changing Sound Quality Fig. 1. During the few days leading up to Villarrica’s 3 March 2015 explosion, the volcano’s characteristic explosion infrasound changed (top and bottom). Colored disks represent the spatial equivalents of the respective infrasound time series, which were recorded 4 kilometers from the vent; oscillations are mostly absent on 2 March. Waveforms on 27 February had well-defined oscillations that were mostly absent by 2 March (middle). Draped topography was created by the authors from the Shuttle Radar Topography Mission digital elevation model using an image from NASA Earth Observatory. VID and VIC are the stations that recorded the waveform data.

Volcano infrasound merits particular attention when it changes over time. This can happen when volcanoes change their shape as crater walls slump, floors collapse, or a lava lake rises and falls. Villarrica’s lava lake dynamism, for instance, is considered to be responsible for changing infrasound leading up to the violent eruption in 2015. Frequency fluctuations had previously been attributed to oscillating lava lake stages [Richardson et al., 2014], but in 2015, scientists noted a systematic variation that led up to the violent eruption on 3 March. A study by Johnson et al. [2018a] reported two primary observations: The frequency content of the sounds increased around 1 March (from 0.7 to 0.95 hertz), and the timbre changed (Figure 1). Prior to 1 March, reverberations were evident, but afterward, the sound became like a thunk. In other words, the crater’s acoustic source had dampened.

Villarrica’s crater resembles a funnel, with a conical upper section and a narrow conduit beneath. The absence of resonance in early March is important because according to numerical models, it signifies a high stand of the lava lake situated near the flaring section of the crater. During Villarrica’s typical background state, the surface of the lava lake is deeper—and often hidden—within the vertical-walled shaft. By 2 March, the infrasound signals suggest that the lava lake was approaching the crater rim; the horn had become a loudspeaker, as illustrated in the video below.



The trigger for the dramatic 3 March lava fountain, which started at 3:00 a.m. local time, remains enigmatic, but the end result was a violent paroxysm that caused property damage, forced thousands of people to evacuate the area, and made worldwide headlines. Infrasound observations told us that the surface of the lava lake had reached a high level several days before the eruption. These insights may help us to anticipate future eruptions at open-vent volcanoes.

Volcano Resonance on Steroids

Every volcano has a unique infrasound signature. Compared with Volcán Villarrica, whose resonance evolved during a few days from noticeable to absent, infrasound from Ecuador’s Cotopaxi volcano was notable because it rang consistently in 2016 (Figure 2). Villarrica’s infrasound oscillations lasted cumulatively for a few seconds, but a single oscillation at Cotopaxi lasted for 5 seconds. As many as 16 oscillations were detected in some of the infrasound signals, which, incredibly, lasted more than a minute (Figure 3).

Fig. 2. Cotopaxi and Villarrica volcano photos and satellite imagery from NASA Earth Observatory show the relative size of their summit craters, which produce discrete infrasound signals. Yellow squares in both satellite images are 1 square kilometer. Credit: NASA International Space Station photo archive (Cotopaxi satellite photo), NASA Earth Observatory Fig. 3. The infrasound signal time series illustrates the nature of the resonance at Villarrica and Cotopaxi (top left). Each waveform is a composite stack of 50 events, which occurred during 1 day at Villarrica and during 6 months at Cotopaxi. A detail of the first 10 seconds from this time series shows the contrast in sound signatures from the two volcanoes (top right). Frequency spectra peak at 0.2 hertz for Cotopaxi and 0.75 hertz for Villarrica; damping factors α indicate the time constant for characteristic decay in reciprocal seconds (bottom)..

. . . .

A study of the Cotopaxi events recorded in 2016 refers to these beautiful signals as infrasound tornillos, the Spanish word for screws, because the pressure recording resembles a screw’s profile [Johnson et al., 2018b]. Such waveforms attest to an exceptionally low damping and thus a high quality factor of the crater acoustic source. (Sources with higher quality factors have less damping, and they ring or vibrate longer.)

If Villarrica is like a large trombone, with a leadpipe length that changes over time, then Cotopaxi is like a giant tuba, with relatively unchanging dimensions during much of 2015 and 2016. After explosions in August 2015 opened up Cotopaxi’s crater, the visible conduit extended steeply downward from its 5,900-meter summit. Throughout the first half of 2016, the crater bottom was not visible to aircraft flying over the summit. Aerial observations showed a vertical-walled crater at least 200 meters deep, a dimension corroborated by the modeled infrasound, which suggested a 350-meter shaft.

Sources of Crater Resonance

Infrasound’s journey from volcano source to receiver can be understood only by considering the dramatic modulating effects produced by crater topography [Kim et al., 2015]. It is most plausible that both Cotopaxi’s impressive tornillos and Villarrica’s subdued oscillations are induced by short-duration impulses occurring at the bottom of their craters. An abrupt explosion, or an impulse, contains a broad spectrum of frequencies; however, only those that excite the crater in resonance are well sustained.

Typically, volcano scientists who analyze remote infrasound recordings are generally less interested in the oscillatory “breathing” of the crater outlet (i.e., its infrasound resonance) than in extracting important information about the explosion’s source, such as its duration or mass flux. It is this information that contributes to our growing understanding of how gas accumulates and separates from magma and how it powers volcanic explosions.

However, with recent developments in the understanding of crater acoustic effects, we are better poised to recover important parameters related to the sources of explosions. Cotopaxi and Villarrica represent just two of the dozens of volcanoes active worldwide where infrasound is contributing to our fundamental understanding of eruption dynamics and to our ability to forecast future paroxysms.

Acknowledgments

This work was funded in part by National Science Foundation grants EAR-0838562 and EAR-1830976 and by the Fulbright Scholar Program.

Linking Regional Weather and Climate to Remote Events

Mon, 06/17/2019 - 11:30

Regional weather is affected by remote events. Jenney et al. [2019] present an index called STRIPES (Sensitivities To the Remote Influence of Periodic EventS) to quantify the remote impacts of quasi-period events on regional weather and climate. The index combines both spatial and temporal lag of the sensitivities. The authors applied this index to examine the variations of surface temperature, geopotential height, and precipitation in North America in relation to the Madden-Julian Oscillation, a tropical intraseasonal oscillation. It clearly identified what areas and what aspects of the regional weather climate are sensitive to the Madden-Julian Oscillation. Better insights into the spatial-temporal characteristics of remote impact are valuable for process understanding and prediction.

Citation: Jenney, A. M., Randall, D. A., & Barnes, E. A. [2019]. Quantifying regional sensitivities to periodic events: Application to the MJO. Journal of Geophysical Research: Atmospheres, 124, 3671– 3683. https://doi.org/10.1029/2018JD029457

—Minghua Zhang, Editor in Chief, JGR: Atmospheres

Ceramics Trace a 14th Century Indonesian Tsunami

Mon, 06/17/2019 - 11:29

The first waves rolled ashore in Banda Aceh, on the northern tip of Sumatra, barely 20 minutes after a magnitude 9.1 megathrust earthquake struck the Indian Ocean on 26 December 2004. After the last of a series of tsunamis slammed the community, the death toll in Indonesia exceeded 150,000.

Following the so-called Boxing Day tsunamis, survivors opted to rebuild their lives within the inundation zone rather than abandon the exposed coastline. Now, scientists have used archaeological evidence from the coast of Sumatra dating back to the 14th century to show that returning to a tsunami-devastated region has a long historical precedent. Their results were published last month in the Proceedings of the National Academy of Sciences of the United States of America.

No Shortage of Tsunamis

Patrick Daly, an archaeologist at Nanyang Technological University in Singapore and lead author of the new study, works just across the Strait of Malacca from the Indonesian island of Sumatra. He remembers visiting Banda Aceh, the capital of Indonesia’s Aceh Province and the site of some of the worst destruction from the 2004 Indian Ocean tsunami, in 2006.

Seeing the extensive wreckage firsthand got Daly thinking about how frequently tsunamis strike the region. “Have there been other societies that have dealt with similar things?” he remembers wondering.

The answer, Daly and his colleagues soon realized, was a definitive yes. In a study published in 2017, a team of researchers, including Daly, found evidence that 11 tsunamis had struck near Banda Aceh between 7,400 and 2,900 years ago. Other research teams analyzing sand deposits and growth patterns of corals suggested that tsunamis also struck the area more recently, in about 1394 and 1450. Daly and his collaborators set about looking for archaeological evidence from the 14th and 15th centuries to determine the impacts of these more recent tsunamis.Working with a team of over 60 Acehnese individuals recruited through the International Centre for Aceh and Indian Ocean Studies, the research team collected more than 30,000 pieces of broken ceramic pottery (sherds).

Sherds, Sherds Everywhere

The scientists examined a 40-kilometer section of the Sumatran coastline near Banda Aceh.

Working with a team of over 60 Acehnese individuals recruited through the International Centre for Aceh and Indian Ocean Studies, they collected more than 30,000 pieces of broken ceramic pottery (sherds). The team found the roughly 5- to 15-centimeter fragments, which had been exposed by erosion and the 2004 tsunamis, both on the ground and in beach-facing cliff faces. These pieces were trade ceramics, the researchers concluded, and were originally made in places as far-flung as China, India, Syria, Thailand, and Vietnam. They were “the type of stuff you’d find in museums of Asian art,” said Daly.

On the basis of the style and design of the pottery, the team grouped the sherds into five time periods ranging from pre-1400 to 1650–1800.

The researchers found that the ceramics tended to be clustered in sites, implying geographically distinct settlements engaged in trade. These settlements, 10 in total, likely corresponded to modest-sized fishing villages, the researchers concluded. Nine were located within the inundation zone of the 2004 tsunami, and the 10th was situated on a promontory roughly 60 meters above sea level.

Trading from a Hill

Mining through their extensive database of age-dated, geographically tagged sherds, Daly and his collaborators noted a curious result: The settlements contained over 3,800 sherds confidently dated to before 1400 but only 70 sherds confidently dated to 1400–1450.

This fiftyfold decrease was consistent with the occurrence of a tsunami in 1394 that temporarily wiped out trading, the team reasoned.

“A powerful tsunami in the middle ages around 1394, analogous with the 2004 event, does indeed give the best fit with the detailed archaeological data set, ” Hendrik J. Bruins, a geoarchaeologist at Ben-Gurion University of the Negev, Israel, who was not involved in the research, told Eos.

Further support for this hypothesis soon emerged: 56 of the 70 sherds dated to 1400–1450 were found at the 10th settlement, the one atop the promontory and therefore presumably above the reach of tsunami waves. “Cluster 10 clearly retained connections to its international trading partners over this period,” the researchers wrote.

Daly and his colleagues went on to find that the hilltop settlement was abandoned by roughly 1550. Around the same time, trade started to increase at the low-lying villages, sites that would have likely been destroyed by the 1394 tsunami.  Researchers don’t know what caused the shift in trading patterns, but Daly and his team hypothesize that outsiders may have been moving into the low-lying settlements.

“You’re getting new groups of people taking advantage of the depopulation to set up a new trading infrastructure,” said Daly.

The area around Banda Aceh is an ideal meeting place for traders, he said, because it’s situated near the Bay of Bengal and therefore readily accessible from India, China, and Southeast Asia.There’s a long history of people moving back into tsunami-prone regions after a disaster, in part because there are “massive social and economic consequences to relocating people.”

Accepting Risk

The results in the new paper show that there’s a long history of people moving back into tsunami-prone regions after a disaster, said Daly. That’s partially because there are “massive social and economic consequences to relocating people.” Humans are also remarkably good at accepting a certain degree of risk, particularly for rare events like tsunamis, Daly said.

The Acehnese coast will surely be hit by another tsunami in the future, said Daly, who notes that only time will tell if people will once again rebuild in such a disaster-prone area. He and his team are continuing to piece together the tsunami record in the area, particularly focusing on the last 2,000 years, he said. “We’re telling this big archaeological, environmental story.”

—Katherine Kornei (@katherinekornei), Freelance Science Journalist

Report Examines New Tools to Protect Coral Reefs

Fri, 06/14/2019 - 18:38

With coral reefs under threat worldwide, a new report examines and provides a framework to assess novel intervention options that could provide a way forward to protect them.

In the face of threats including habitat destruction, pollution, and climate change, the aim of these interventions is “to increase the ability of these coral reefs to persist in these rapidly degrading environmental conditions,” according to the report, A Decision Framework for Interventions to Increase the Persistence and Resilience of Coral Reefs, which was released on 12 June by the National Academies of Sciences, Engineering, and Medicine (NASEM).

The tools themselves, which also were detailed by NASEM in 2018, include genetic and reproductive interventions such as managed selection and breeding; physiological interventions including pre-exposure of corals to increase their tolerance to stress factors; environmental interventions such as marine and atmospheric shading; and managed relocations of coral populations.

“These new tools are needed because established approaches for managing coral reefs are neither sufficient, nor designed, to preserve corals in a changing climate.”Seven of the 23 examined tools already have been field-tested in specific locations, and all of the interventions have potential risks that need to be carefully weighed against perceived benefits, according to the report, which was requested and funded by the National Oceanic and Atmospheric Administration (NOAA), with additional support provided by the Paul G. Allen Family Foundation. For instance, different types of managed selection present the risk of a decrease in genetic variation. Genetic manipulation could alter the wrong genes and result in unknown risks. Another tool, shading, would alter light regimes.

“These new tools are needed because established approaches for managing coral reefs are neither sufficient, nor designed, to preserve corals in a changing climate,” the report states. “Coral interventions that address the impacts of ocean warming and ocean acidification are part of a three-pronged approach for coral reef management that crucially also includes the mitigation of greenhouse gas emissions and the alleviation of local stressors.”

Managers and decision makers “are faced with the task of evaluating the benefits and risks of a growing number of interventions, separately and in combination,” the report continues. “The interventions have different risks, benefits, and feasibilities in different regions.”

Because there is “no single generalizable approach” for coral reef interventions, the report recommends a structured and adaptive management framework that engages a wide range of stakeholders and that is tailored to local environmental and ecological settings, management objectives, and preferred intervention options.

A Bridge to the Future

“Mitigating [greenhouse gas] emissions is the only way that corals are going to be able to thrive into the far, far future,” Stephen Palumbi, chair of the NASEM committee that produced the report, said at a 12 June briefing. Palumbi, a coral scientist, is a professor of marine sciences and a senior fellow with the Woods Institute for the Environment at Stanford University. “But in this century, when we are hopefully getting a handle on mitigation of the emissions, and things will eventually be getting better by the century, it will take coral interventions now in order for those coral systems to bridge between now and the end of the century.”

In an interview with Eos, Palumbi summed up the report: “Coral reefs are in trouble, there are some things we can do about them, and we now have the tools to begin to be able to make that work in the future.”

“Corals are not just pretty things that we’d like to have around,” Palumbi said. “They support hundreds of millions of people.”

Shallow-water coral reefs, which cover less than 1% of the Earth’s surface, conservatively provide an estimated $172 billion per year in benefits to people in the form of food production, property protection, and tourism, according to NOAA’s Coral Reef Conservation Program Strategic Plan.

Moment of Opportunity

There is “a moment of opportunity” to help protect coral reefs if these interventions are managed properly and are ready for deployment when needed.Marissa Baskett, a member of the NASEM committee that produced the report, told Eos that there is “a moment of opportunity” to help protect coral reefs if these interventions are managed properly and are ready for deployment when needed.

“We have a variety of potential interventions that can increase coral persistence in the future,” said Baskett, an associate professor of environmental science and policy at the University of California, Davis. “All come with uncertainties and risks. But if we leverage that uncertainty, we can learn from the process to mitigate risk, maximize learning, and improve the future.”

Baskett said that there needs to be a stakeholder-driven and scientifically driven process for understanding the potential risks and benefits of these interventions. “We have an extraordinary opportunity right now to be ready to deploy them when they are necessary” and to be proactive rather than reactive, she said.

Committee members also have briefed Congress, the White House, and NOAA about the report. NOAA, which received the report about a week ago, currently is developing its response, according to Tali Vardi, a coral scientist with ECS, a federal contracting company, who is NOAA’s point person to the NASEM study.“How do we stop just cataloging those declines and start putting our energy into doing things? That’s what I want this report to be.”

“There is a lot of work to do” to protect coral reefs, she told Eos. “Reefs are disappearing while we sit here and chat.”

Palumbi told Eos that he wants this report to make a real difference for the future of coral reefs.

“Everybody I know who has worked on reefs for the last 20 or 30 years knows places that were fabulous and are virtually dead now. How do we turn that around? How do we stop just cataloging those declines and start putting our energy into doing things?” Palumbi said. “That’s what I want this report to be. I want it to be the foundation on which people say, there are things to do, there’s energy to do it. There’s a goal. We have to wrap it into climate mitigation. We have to wrap it into the local stressors thing. But there is a way forward. Let’s take it, because what are we going to do if we don’t take it?”

—Randy Showstack (@RandyShowstack), Staff Writer

Shallow Low Frequency Tremors in Japan Trench

Fri, 06/14/2019 - 12:12

Low frequency tremor is a newly discovered type of seismic activity indicative of slow slip of a fault, rather than the typical “fast” slip that occurs in regular earthquakes. While such activity has been discovered in many global subduction zones, it has yet to be identified in northern Japan.

Tanaka et al. [2019] report the discovery of shallow low-frequency tremors near the Japan Trench by using the newly installed seafloor seismic observation network (S-Net). They show that tremor activity in this region, co-located with low-frequency earthquakes, is distributed in two main clusters, separated by a gap where large earthquakes have nucleated and where aftershock activity of the 1994 Sanriku-Oki earthquake was also located. This indicates fairly rapid along-strike variations in the frictional properties of the shallow plate interface and helps us to better understand the driving mechanisms for both slow and regular earthquakes.

Citation: Tanaka, S., Matsuzawa, T., & Asano, Y. [2019]. Shallow low‐frequency tremor in the northern Japan Trench subduction zone. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL082817

—Gavin P. Hayes, Editor, Geophysical Research Letters

Arctic Glacial Retreat Alters Downstream Fjord Currents

Fri, 06/14/2019 - 12:10

As climate change progresses, glaciers continue to retreat worldwide. In the Arctic, glacial meltwater delivers sediments and nutrients to fjords and, ultimately, to the ocean. New research by Normandeau et al. reveals how glacial retreat impacts downstream sediment delivery and associated currents in Arctic fjords.

Heavier flows resulting from glacial retreat can boost the amount of sediment delivered to the mouths of rivers that empty into fjords, rapidly increasing the size of the river deltas. However, the links between glacial retreat and underwater delta dynamics are complex and have been poorly understood, limiting predictions of how, exactly, glacier retreat will reshape Arctic nearshore fjords.

To get a better picture of this system, the authors of the new study mapped the underwater features of 31 river mouths in fjords along the eastern coast of Canada’s Baffin Island. The maps incorporated high-resolution bathymetric data collected over several years from aboard the R/V Nuliajuk and the CCGS Amundsen as part of the ArcticNet program.

These mapping efforts revealed which deltas contained sediment waves, large-scale patterns in deposited sediment that are formed by the fast, downhill flow of sediment-laden water. These fast flows are known as turbidity currents, and their presence or absence depends on upstream glacial and watershed dynamics.

Statistical analysis of links between the mapping data and watershed data compiled for each river mouth showed that the presence of turbidity currents depends on the presence and size of upstream glaciers, which erode material that becomes transported as sediment. However, if lakes form upstream from fjords during glacial retreat, they may trap sediment, keeping it from flowing downstream and halting turbidity currents.

The researchers used these findings to create a model of evolving delta dynamics over the course of upstream glacial retreat. The model accounts for the formation of lakes that halt downstream turbidity currents, as well as reactivation of turbidity currents that may occur if lakes later fill up with sediment. The scientists applied their model to 644 rivers emptying into fjords along Baffin Island, predicting which are likely to contain turbidity currents.

This work could help improve predictions of future coastal changes worldwide, including effects on marine ecosystems that rely on nutrients transported in sediments. It could also help refine understanding of past glacial retreat. (Journal of Geophysical Research: Earth Surface, https://doi.org/10.1029/2018JF004970, 2019)

—Sarah Stanley, Freelance Writer

Oldest Meteorite Collection Found in World’s Oldest Desert

Fri, 06/14/2019 - 12:08

Each year, millions of meteors intersect with Earth. Most of these burn up on entering our atmosphere, but some larger space rocks survive the journey and land on Earth’s surface.

A new study looking at a sampling of more than 300 meteorites collected in Chile’s Atacama Desert is shedding some light on the rate and variety of meteor strikes over the past 2 million years.

Meteorites can land anywhere on Earth, but those that fall in deserts and on ice sheets are more likely to be preserved and recovered, says Alexis Drouard, an astrophysicist at Aix-Marseille University in France and lead author of the new study, published in Geology.

But both locations have drawbacks: Most deserts on Earth are only a few thousand years old, and meteorites that land on ice sheets are often transported and concentrated by glacial processes, making it difficult to determine how many meteors might have fallen in a given time period, a statistic known as the meteorite flux.

“This confirms the long-term, multi-million-year stability of the Atacama Desert surfaces and offers a unique opportunity to study the meteorite flux to Earth.”“We wanted to see how the meteorite flux to Earth changed over longer timescales, over millions of years,” says Drouard.

To find evidence of older meteorites in a stable environment, Drouard and his colleagues turned to a collection of over 300 meteorites found in Chile’s Atacama Desert. “The Atacama is the oldest desert on Earth,” Drouard says. “The Sahara was green 5,000 years ago, but the Atacama has been arid for at least 7 million years and maybe as long as 20 million years.”

The team subjected a sample of 54 rocky meteorites to cosmogenic age dating using the chlorine-36 isotope and found that the oldest samples fell to Earth between 1 and 2 million years ago, with a mean age of 710,000 years, making this the oldest meteorite collection found to date on Earth’s surface.

“This confirms the long-term, multi-million-year stability of the Atacama Desert surfaces and offers a unique opportunity to study the meteorite flux to Earth and meteorite weathering over the million-year time scale,” the team wrote in Geology.

Being able to study the meteorite flux sheds some light on cosmic processes and events, such as collisions, that may produce more meteorites or change the type of debris. The team found that the flux of meteorites remained constant over a 2-million-year time span with 222 meteorites more massive than 10 grams falling per square kilometer every million years.

One of the larger chondrites found in the Atacama Desert sits among smaller, lighter rocks and a rock hammer for scale. Credit: Jérôme Gattacceca (CEREGE)

“It’s extremely very rare to find a record like this that spans such a long, continuous chunk of time,” says Philipp Heck, a meteorist at the Field Museum in Chicago who was not involved in the new study.

The team also found that the type of meteorite that fell on the Atacama changed over the time period studied. All 54 meteorites studied were ordinary chondrites, the most common type of rocky meteorite, but the collection falls into three groups: high iron (H type; 25 meteorites), low iron (L type; 26 meteorites), and low iron, low metal (LL type; 3 meteorites). The team detected a sharp increase in the proportion of H chondrites over L chondrites between 1 and 0.5 million years ago.

“It’s an interesting and important result that they found an overabundance of H chondrites between 1 and 0.5 million years ago,” Heck says. “When one type of meteorite dominates, it’s most likely related to an event such as a collision that released those objects from the parent body.”

For a follow-up study, Drouard’s team could use cosmic ray exposure dating to determine how long the meteors traveled through space before entering Earth’s atmosphere, Heck says. “This can tell us something about where they came from and the trajectory they were traveling before they intersected with the Earth.”

—Mary Caperton Morton (@theblondecoyote) Science Writer

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