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November Meeting

Join us Tuesday, November 27, 2018 at the Fernbank Museum of Natural

History, 760 Clifton Road NE, Atlanta GA. The social dinner starts at 6:30 pm and the meeting starts approximately

7:00 p.m.

This month’s presentation is “Walking in Hutton’s Footsteps” presented by Ben Bentkowski P.G.

Please find more information about the presentation and Mr. Bentkowski

bio on the next page.

Please come out, enjoy a bite to eat, the camaraderie, informative presentation and perhaps some discussion on the importance of

accurate mineral characterization.

www.atlantageologicalsociety.org

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Atlanta Geological Society Newsletter

ODDS AND ENDS Dear AGS members, And just like that, another year is nearly done. Because we don’t have a December meeting, the November AGS meeting is the first end of the year occurrence on my calendar. With elections this next Tuesday, I would encourage anyone to become more involved by tossing their hat into the ring. I think we could improve the mechanics of our membership rolls. The spreading of AGS information and the newsletter are all tied to having a membership that you have confidence in. There is a membership position, but it hasn’t been very active or effective. This is not an elected position but rather a volunteer position. We sure could use someone to head up those activities going into 2019. We will be looking for someone to serve as the newsletter editor. Jim Ferreira has done an admirable job the last couple of years, but he would like to move on. I must say that I really enjoyed my time as Editor. It gave me a reason to read all those geology news stories. Just 10 issues a year, please consider volunteering. Another transition as Kaden Borseth has moved to the Washington DC area. Amanda Gore will be our primary Fernbank contact going forward. With change comes the opportunity for others to grow in new directions. If you see Amanda, please tell her thanks for stepping up. And now, back to my slide preparation… Ben Bentkowski, President

AGS November 2018

This Month’s Atlanta Geological Society Speaker

“Walking in Hutton’s Footsteps” Speaker Mr. Ben Bentkowski’s Bio:

Ben Bentkowski is a senior hydrogeologist at the Atlanta office of the EPA where he works on solving contaminated groundwater problems for the Superfund Division, plus other duties as assigned. He is a registered geologist in Georgia and serves on the P.G. Licensing board. He has two geology degreed; Bachelor’s from Florida Atlantic University and Master’s from Oklahoma State University. He is the current president of the Atlanta Geological Society. He is a firm believer in always taking time for a little (or a lot) of geology when he travels for work or pleasure.

Abstract:

This month’s presentation is titled Walking in Hutton’s Footsteps. On a recent trip to Scotland Ben made a point to visit some of the outcrops that convinced James Hutton and his colleagues of the concepts of geologic time, uniformitarianism and plutonism. An exploration of the Isle of Skye was also part of this trip but that will be reserved for another Tuesday evening.

“The past history of our globe must be explained by what can be seen to be happening now. No powers are to be employed that are not natural to the globe, no action to be admitted except those of which we know the principle.” James Hutton

Hutton’s Unconformity at Jedburgh, Scotland – one of the most important locations in the development of geological science. Hutton said this location had at times been under the sea. The bottom section of rocks in the image is the oldest. Here rock layers formed over an immense amount of time were tilted to be almost vertical when they were uplifted as they rose out of the sea. They were then eroded, forming the layer immediately above them. The land then sank below the sea again and more horizontal layers were laid down over another very long period of time. The rocks then emerged again from the sea.

Hutton’s Rock Cycle

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A Large Asteroid Struck Greenland in the Time of Humans. How did it Affect the Planet?

On a bright July day 2 years ago, Kurt Kjær was in a helicopter flying over northwest Greenland—an expanse of ice, sheer white and sparkling. Soon, his target came into view: Hiawatha Glacier, a slow-moving sheet of ice more than a kilometer thick. It advances on the Arctic Ocean not in a straight wall, but in a conspicuous semicircle, as though spilling out of a basin. Kjær, a geologist at the Natural History Museum of Denmark in Copenhagen, suspected the glacier was hiding an explosive secret. The helicopter landed near the surging river that drains the glacier, sweeping out rocks from beneath it. Kjær had 18 hours to find the mineral crystals that would confirm his suspicions.

What he brought home clinched the case for a grand discovery. Hidden beneath Hiawatha is a 31-kilometer-wide impact crater, big enough to swallow Washington, D.C., Kjær and 21 co-authors report this week in a paper in Science Advances. The crater was left when an iron asteroid 1.5 kilometers across slammed into Earth, possibly within the past 100,000 years.

Though not as cataclysmic as the dinosaur-killing Chicxulub impact, which carved out a 200-kilometer-wide crater in Mexico about 66 million years ago, the Hiawatha impactor, too, may have left an imprint on the planet's history. The timing is still up for debate, but some researchers on the discovery team believe the asteroid struck at a crucial moment: roughly 13,000 years ago, just as the world was thawing from the last ice age. That would mean it crashed into Earth when mammoths and other megafauna were in decline and people were spreading across North America.

The impact would have been a spectacle for anyone within 500 kilometers. A white fireball four times larger and three times brighter than the sun would have streaked across the sky. If the object struck an ice sheet, it would have tunneled through to the bedrock, vaporizing water and stone alike in a flash. The resulting explosion packed the energy of 700 1-megaton nuclear bombs, and even an observer hundreds of kilometers away would have experienced a buffeting shock wave, a monstrous thunderclap, and hurricane-force winds. Later, rock debris might have rained down on North America and Europe, and the released steam, a greenhouse gas, could have locally warmed Greenland, melting even more ice.

The news of the impact discovery has reawakened an old debate among scientists who study ancient climate. A massive impact on the ice sheet would have sent meltwater pouring into the Atlantic Ocean—potentially disrupting the conveyor belt of ocean currents and causing temperatures to plunge, especially in the Northern Hemisphere. “What would it mean for species or life at the time? It's a huge open question,” says Jennifer Marlon, a paleoclimatologist at Yale University.

A decade ago, a small group of scientists proposed a similar scenario. They were trying to explain a cooling event, more than 1000 years long, called the Younger Dryas, which began 12,800 years ago, as the last ice age was ending. Their controversial solution was to invoke an extraterrestrial agent: the impact of one or more comets. The researchers proposed that besides changing the plumbing of the North Atlantic, the impact also ignited wildfires across two continents that led to the extinction of large mammals and the disappearance of the mammoth-hunting Clovis people of North America. The research group marshaled suggestive but inconclusive evidence, and few other scientists were convinced. But the idea caught the public's imagination despite an obvious limitation: No one could find an impact crater.

Proponents of a Younger Dryas impact now feel vindicated. “I'd unequivocally predict that this crater is the same age as the Younger Dryas,” says James Kennett, a marine geologist at the University of California, Santa Barbara, one of the idea's original boosters.

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A Large Asteroid Struck Greenland in the Time of Humans. How did it Affect the Planet? (Continued)

But Jay Melosh, an impact crater expert at Purdue University in West Lafayette, Indiana, doubts the strike was so recent. Statistically, impacts the size of Hiawatha occur only every few million years, he says, and so the chance of one just 13,000 years ago is small. No matter who is right, the discovery will give ammunition to Younger Dryas impact theorists—and will turn the Hiawatha impactor into another type of projectile. “This is a hot potato,” Melosh tells Science. “You're aware you're going to set off a firestorm?”

IT STARTED WITH a hole. In 2015, Kjær and a colleague were studying a new map of the hidden contours under Greenland's ice. Based on variations in the ice's depth and surface flow patterns, the map offered a coarse suggestion of the bedrock topography—including the hint of a hole under Hiawatha.

Kjær recalled a massive iron meteorite in his museum's courtyard, near where he parks his bicycle. Called Agpalilik, Inuit for “the Man,” the 20-ton rock is a fragment of an even larger meteorite, the Cape York, found in pieces on northwest Greenland by Western explorers but long used by Inuit people as a source of iron for harpoon tips and tools. Kjær wondered whether the meteorite might be a remnant of an impactor that dug the circular feature under Hiawatha. But he still wasn't confident that it was an impact crater. He needed to see it more clearly with radar, which can penetrate ice and reflect off bedrock.

Kjær's team began to work with Joseph MacGregor, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who dug up archival radar data. MacGregor found that NASA aircraft often flew over the site on their way to survey Arctic sea ice, and the instruments were sometimes turned on, in test mode, on the way out. “That was pretty glorious,” MacGregor says.

The radar pictures more clearly showed what looked like the rim of a crater, but they were still too fuzzy in the middle. Many features on Earth's surface, such as volcanic calderas, can masquerade as circles. But only impact craters contain central peaks and peak rings, which form at the center of a newborn crater when—like the splash of a stone in a pond—molten rock rebounds just after a strike. To look for those features, the researchers needed a dedicated radar mission.

Coincidentally, the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, had just purchased a next-generation ice-penetrating radar to mount across the wings and body of their Basler aircraft, a twin-propeller retrofitted DC-3 that's a workhorse of Arctic science. But they also needed financing and a base close to Hiawatha.

Kjær took care of the money. Traditional funding agencies would be too slow, or prone to leaking their idea, he thought. So he petitioned Copenhagen's Carlsberg Foundation, which uses profits from its global beer sales to finance science. MacGregor, for his part, enlisted NASA colleagues to persuade the U.S. military to let them work out of Thule Air Base, a Cold War outpost on northern Greenland, where German members of the team had been trying to get permission to work for 20 years. “I had retired, very serious German scientists sending me happy-face emojis,” MacGregor says.

Three flights, in May 2016, added 1600 kilometers of fresh data from dozens of transits across the ice and evidence that Kjær, MacGregor, and their team were onto something. The radar revealed five prominent bumps in the crater's center, indicating a central peak rising some 50 meters high. And in a sign of a recent impact, the crater bottom is exceptionally jagged. If the asteroid had struck earlier than 100,000 years ago, when the area



was ice free, erosion from melting ice farther inland would have scoured the crater smooth, MacGregor says. The radar signals also showed that the deep layers of ice were jumbled up—another sign of a recent impact. The oddly disturbed patterns, MacGregor says, suggest “the ice sheet hasn't equilibrated with the presence of this impact crater.”

But the team wanted direct evidence to overcome the skepticism they knew would greet a claim for a massive young crater, one that seemed to defy the odds of how often large impacts happen. And that's why Kjær found himself, on that bright July day in 2016, frenetically sampling rocks all along the crescent of terrain encircling Hiawatha's face. His most crucial stop was in the middle of the semicircle, near the river, where he collected sediments that appeared to have come from the glacier's interior. It was hectic, he says “one of those days when you just check your samples, fall on the bed, and don't rise for some time.”

In that outwash, Kjær's team closed its case. Sifting through the sand, Adam Garde, a geologist at the Geological Survey of Denmark and Greenland in Copenhagen, found glass grains forged at temperatures higher than a volcanic eruption can generate. More important, he discovered shocked crystals of quartz. The crystals contained a distinctive banded pattern that can be formed only in the intense pressures of extraterrestrial impacts or nuclear weapons. The quartz makes the case, Melosh says. “It looks pretty good. All the evidence is pretty compelling.”

Now, the team needs to figure out exactly when the collision occurred and how it affected the planet.

THE YOUNGER DRYAS, named after a small white and yellow arctic flower that flourished during the cold snap, has long fascinated scientists. Until human-driven global warming set in, that period reigned as one of the sharpest recent swings in temperature on Earth. As the last ice age waned, about 12,800 years ago, temperatures in parts of the Northern Hemisphere plunged by as much as 8°C, all the way back to ice age readings.

The hidden crater Under a lobe of ice on northwest Greenland, airborne radar and ground sampling have uncovered a giant and remarkably fresh impact crater. Though not as large as the dinosaur-killing Chicxulub impact, Hiawatha crater may have formed as recently as the end of the last ice age, as humans were spreading across North America. Meltwater from the impact could have triggered a thousand-year chill in the Northern Hemisphere by disrupting currents in the Atlantic Ocean.

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They stayed that way for more than 1000 years, turning advancing forest back into tundra.

The trigger could have been a disruption in the conveyor belt of ocean currents, including the Gulf Stream that carries heat northward from the tropics. In a 1989 paper in Nature, Kennett, along with Wallace Broecker, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, and others, laid out how meltwater from retreating ice sheets could have shut down the conveyor. As warm water from the tropics travels north at the surface, it cools while evaporation makes it saltier. Both factors boost the water's density until it sinks into the abyss, helping to drive the conveyor. Adding a pulse of less-dense freshwater could hit the brakes. Paleoclimate researchers have largely endorsed the idea, although evidence for such a flood has been lacking until recently.

Then, in 2007, Kennett suggested a new trigger. He teamed up with scientists led by Richard Firestone, a physicist at Lawrence Berkeley National Laboratory in California, who proposed a comet strike at the key moment. Exploding over the ice sheet covering North America, the comet or comets would have tossed light-blocking dust into the sky, cooling the region. Farther south, fiery projectiles would have set forests alight, producing soot that deepened the gloom and the cooling. The impact also could have destabilized ice and unleashed meltwater that would have disrupted the Atlantic circulation.

The climate chaos, the team suggested, could explain why the Clovis settlements emptied and the megafauna vanished soon afterward. But the evidence was scanty. Firestone and his colleagues flagged thin sediment layers at dozens of archaeological sites in North America. Those sediments seemed to contain geochemical traces of an extraterrestrial impact, such as a peak in iridium, the exotic element that helped cement the case for a Chicxulub impact. The layers also yielded tiny beads of glass and iron—possible meteoritic debris—and heavy loads of soot and charcoal, indicating fires.

The team met immediate criticism. The decline of mammoths, giant sloths, and other species had started well before the Younger Dryas. In addition, no sign existed of a human die-off in North America, archaeologists said. The nomadic Clovis people wouldn't have stayed long in any site. The distinctive spear points that marked their presence probably vanished not because the people died out, but rather because those weapons were no longer useful once the mammoths waned, says Vance Holliday, an archaeologist at The University of Arizona in Tucson. The impact hypothesis was trying to solve problems that didn't need solving.

The geochemical evidence also began to erode. Outside scientists could not detect the iridium spike in the group's samples. The beads were real, but they were abundant across many geological times, and soot and charcoal did not seem to spike at the time of the Younger Dryas. “They listed all these things that aren't quite sufficient,” says Stein Jacobsen, a geochemist at Harvard University who studies craters.

Yet the impact hypothesis never quite died. Its proponents continued to study the putative debris layer at other sites in Europe and the Middle East. They also reported finding microscopic diamonds at different sites that, they say, could have been formed only by an impact. (Outside researchers question the claims of diamonds.)

Now, with the discovery of Hiawatha crater, “I think we have the smoking gun,” says Wendy Wolbach, a geochemist at DePaul University in Chicago, Illinois, who has done work on fires during the era. The impact would have melted 1500 gigatons of ice, the team estimates—about as much ice as Antarctica has lost


Leery of the earlier controversy, Kjær won't endorse that scenario. “I'm not putting myself in front of that bandwagon,” he says. But in drafts of the paper, he admits, the team explicitly called out a possible connection between the Hiawatha impact and the Younger Dryas.

THE EVIDENCE STARTS with the ice. In the radar images, grit from distant volcanic eruptions makes some of the boundaries between seasonal layers stand out as bright reflections. Those bright layers can be matched to the same layers of grit in cataloged, dated ice cores from other parts of Greenland. Using that technique, Kjær's team found that most ice in Hiawatha is perfectly layered through the past 11,700 years. But in the older, disturbed ice below, the bright reflections disappear. Tracing the deep layers, the team matched the jumble with debris-rich surface ice on Hiawatha's edge that was previously dated to 12,800 years ago. “It was pretty self-consistent that the ice flow was heavily disturbed at or prior to the Younger Dryas,” MacGregor says.

Other lines of evidence also suggest Hiawatha could be the Younger Dryas impact. In 2013, Jacobsen examined an ice core from the center of Greenland, 1000 kilometers away. He was expecting to put the Younger Dryas impact theory to rest by showing that, 12,800 years ago, levels of metals that asteroid impacts tend to spread did not spike. Instead, he found a peak in platinum, similar to ones measured in samples from the crater site. “That suggests a connection to the Younger Dryas right there,” Jacobsen says.

For Broecker, the coincidences add up. He had first been intrigued by the Firestone paper, but quickly joined the ranks of naysayers. Advocates of the Younger Dryas impact pinned too much on it, he says: the fires, the extinction of the megafauna, the abandonment of the Clovis sites. “They put a bad shine on it.” But the platinum peak Jacobsen found, followed by the discovery of Hiawatha, has made him believe again. “It's got to be the same thing,” he says.


because of global warming in the past decade. The local greenhouse effect from the released steam and the residual heat in the crater rock would have added more melt. Much of that freshwater could have ended up in the nearby Labrador Sea, a primary site pumping the Atlantic Ocean's overturning circulation. “That potentially could perturb the circulation,” says Sophia Hines, a marine paleoclimatologist at Lamont-Doherty.

NASA and German aircraft used radar to see the contours of an impact crater beneath the ice of Hiawatha Glacier.

In 2016, Kurt Kjær looked for evidence of an impact in sand washed out from underneath Hiawatha Glacier. He would find glassy beads and shocked crystals of quartz.

AGS November 2018


Yet no one can be sure of the timing. The disturbed layers could reflect nothing more than normal stresses deep in the ice sheet. “We know all too well that older ice can be lost by shearing or melting at the base,” says Jeff Severinghaus, a paleoclimatologist at the Scripps Institution of Oceanography in San Diego, California. Richard Alley, a glaciologist at Pennsylvania State University in University Park, believes the impact is much older than 100,000 years and that a subglacial lake can explain the odd textures near the base of the ice. “The ice flow over growing and shrinking lakes interacting with rough topography might have produced fairly complex structures,” Alley says.

A recent impact should also have left its mark in the half-dozen deep ice cores drilled at other sites on Greenland, which document the 100,000 years of the current ice sheet's history. Yet none exhibits the thin layer of rubble that a Hiawatha-size strike should have kicked up. “You really ought to see something,” Severinghaus says.

Brandon Johnson, a planetary scientist at Brown University, isn't so sure. After seeing a draft of the study, Johnson, who models impacts on icy moons such as Europa and Enceladus, used his code to recreate an asteroid impact on a thick ice sheet. An impact digs a crater with a central peak like the one seen at Hiawatha, he found, but the ice suppresses the spread of rocky debris. “Initial results are that it goes a lot less far,” Johnson says.

EVEN IF THE ASTEROID struck at the right moment, it might not have unleashed all the disasters envisioned by proponents of the Younger Dryas impact. “It's too small and too far away to kill off the Pleistocene mammals in the continental United States,” Melosh says. And how a strike could spark flames in such a cold, barren region is hard to see. “I can't imagine how something like this impact in this location could have caused massive fires in North America,” Marlon says.

It might not even have triggered the Younger Dryas. Ocean sediment cores show no trace of a surge of freshwater into the Labrador Sea from Greenland, says Lloyd Keigwin, a paleoclimatologist at the Woods Hole Oceanographic Institution in Massachusetts. The best recent evidence, he adds, suggests a flood into the Arctic Ocean through western Canada instead.

Banded patterns in the mineral quartz are diagnostic of shock waves from an extraterrestrial impact.



An external trigger may be unnecessary in any case, Alley says. During the last ice age, the North Atlantic saw 25 other cooling spells, probably triggered by disruptions to the Atlantic's overturning circulation. None of those spells, known as Dansgaard-Oeschger (D-O) events, was as severe as the Younger Dryas, but their frequency suggests an internal cycle played a role in the Younger Dryas, too. Even Broecker agrees that the impact was not the ultimate cause of the cooling. If D-O events represent abrupt transitions between two regular states of the ocean, he says, “you could say the ocean was approaching instability and somehow this event knocked it over.”

Still, Hiawatha's full story will come down to its age. Even an exposed impact crater can be a challenge for dating, which requires capturing the moment when the impact altered existing rocks—not the original age of the impactor or its target. Kjær's team has been trying. They fired lasers at the glassy spherules to release argon for dating, but the samples were too contaminated. The researchers are inspecting a blue crystal of the mineral apatite for lines left by the decay of uranium, but it's a long shot. The team also found traces of carbon in other samples, which might someday yield a date, Kjær says. But the ultimate answer may require drilling through the ice to the crater floor, to rock that melted in the impact, resetting its radioactive clock. With large enough samples, researchers should be able to pin down Hiawatha's age.

Given the remote location, a drilling expedition to the hole at the top of the world would be costly. But an understanding of recent climate history—and what a giant impact can do to the planet—is at stake. “Somebody's got to go drill in there,” Keigwin says. “That's all there is to it.” Read more about this article at: http://science.sciencemag.org/content/362/6416/738

Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe INTRODUCTION The latest projections of soil organic carbon (SOC) changes from different Earth system models (ESMs) show a very wide range of values, adding uncertainty on land-atmospheric carbon (C) feedback under climate change. Because the soil is the largest terrestrial C pool, different representations of SOC turnover may amplify the magnitude and even the sign of the C balance. Recent studies suggest that the general soil C sink simulated by 2100 is too optimistic, as ESMs underestimate the mean age of SOC. Moreover, the level of confidence of current projections and the possibility of improving process descriptions of SOC dynamics may be further limited by the exclusion of relevant processes, such as soil erosion. This erosional process not only redistributes SOC across the landscape but also generates different feedbacks. In eroding landscape positions, the C that is laterally displaced can be partially replaced with new photosynthates stabilized by the less C-saturated subsoil exposed by erosion, generating a continuous C sink (that is, dynamic replacement). The displaced SOC can be mineralized during transport and preserved in depositional sites with different degrees of efficiency. Depending on the strength of these processes, which remain the topic of scientific debate, erosion has been claimed to induce both a net source and net sink of C. A recent study, for instance, estimates that erosion has offset one-third of C emissions from land cover change since the onset of agriculture. This global-scale analysis highlights how agricultural perturbation of more stable land use (grassland and forest) could generate a C sink over millennia. In these approaches, the net effect of soil erosion on soil-atmosphere C exchange is inferred from the measurement of SOC stock change against a reference soil profile, which may integrate the effect of other drivers in addition to the induced erosion effect on the C cycle (the so-called black box model). For instance, primary productivity and soil respiration can be affected by anthropogenic interventions in eroding/depositional areas, leading to a SOC change not entirely driven by the “erosion effect.” Therefore, inventory-based approaches are useful for tracking past changes but are unlikely to be useful for predicting future change in which the impacts of projected climate on both vertical C

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Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) fluxes (photosynthesis and respiration) and, indirectly, C redistribution by rainfall erosivity variation are important drivers. As erosion is a widespread phenomenon, especially on current agricultural land use, any approach oriented toward projecting future changes requires a framework that can dynamically assess variation of drivers at the same time, integrating current erosion rates as a baseline condition.

The erosion-induced C feedbacks have been studied at the scale of small hillslopes and watersheds, but the lack of large-scale data, in conjunction with gaps in process understanding, has prevented the coupling of erosion/distribution processes in large-scale modeling. For explaining past changes, several large-scale data-driven approaches or integrated small-scale modeling studies have been published, but no study to date has attempted to project future changes, which is the subject of this paper.

Biogeochemistry-erosion model framework Here, we use a consistent biogeochemistry-erosion model framework to quantify the impact of future climate on the C cycle, taking into account accelerated water erosion through variation in rainfall erosivity. The model couples the process-based biogeochemistry model CENTURY and the RUSLE2015 erosion model, the latter based on the Revised Universal Soil Loss Equation (RUSLE). The strength of this approach is that (i) any SOC change related to land use or management activity, including the effect of climate on plant productivity and soil respiration, propagates as a different amount (and type) of displaced SOC, potentially inducing a dynamic feedback, and (ii) land use and management directly affect the soil erosion rates, giving a bidirectional feedback on C balance.

This integrated model has already been used to quantify the current impact of erosion in the agricultural soil of the European Union (EU) at a resolution of 1 km2 (fig. S1), showing a high level of accuracy in simulating dynamic replacement in comparison with previous studies and the overall SOC budget (section S2: figs. S2 to S5). Some of the previous assumptions on sediment distribution that are required for running the model at continental scale were further tested using a delivery/sedimentary model, as detailed in section S3 (figs. S6 and S7 and table S1). The model was run for the Representative Concentration Pathway (RCP) 4.5 climate scenario as simulated by the HadGEM2 climate model, with current erosion (CE) and accelerated erosion (AE), the latter induced by the rainfall erosivity change as calculated recently by Panagos et al. When the model incorporates erosion, the C balance accounts for C displacement components (see Materials and Methods) and does not generate CO2 fluxes until the C is mineralized. As we were interested in the net C exchange between soil and atmosphere, the net soil C flux was calculated

(1)

where CI and CH are the so-called vertical fluxes representing the atmospheric CO2 fixed in litter inputs and released by soil heterotrophic respiration, respectively, while CTm and CBm are the C mineralized during transport and in the burial deposits. There is limited consensus in the literature about C mineralization during transport and its preservation at depositional sites. To provide a range of model uncertainty, we ran the model with two configurations according to previous model sensitivity analysis: the first enhancing the erosion-induced C sink effect (e) and the second minimizing it (r) (see Materials and Methods). Last, we calculated the contribution of accelerated erosion (ΔCAE) on the net C exchange as the difference between the scenario with accelerated and current erosion (both under e and r uncertainty configurations)

(2)


Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) RESULTS AND DISCUSSION The effect of climate change on the terrestrial C cycle is predominantly explained by productivity-driven changes in ESMs, with potential significant biases when omitting the land management effect. Our model framework, which includes a detailed description of land use and agricultural management, also showed an increase in C input by 2100 (Fig. 1A), driven by the higher productivity under the HadGEM2 projection observable at mid-high latitude. This increase was almost offset by enhanced soil respiration, which showed a similar continental response to C input fluxes (Fig. 1B). By the end of the century, the model also predicted an overall increase of C displaced by erosion, with an opposite trend in western countries (Fig. 1C) and some large areas such as Andalusia and central France. This pattern partially overlapped with the change in rainfall erosivity (fig. S8), as eroded C is dependent not only on soil erosion but also on the evolution of SOC stock due to trends in productivity and soil respiration. These complex feedbacks emerged in the net soil flux figure (Fig. 1D), where no clear continental patterns of land C sink and source were predicted over the period 2016–2100. Looking at the continental C budget in the AE_e scenario (Fig. 2), we estimated a small C sink from agricultural land equal to 0.09 Tg C year−1 (CI − CH). Despite the fact that eroded C was more than one order of magnitude lower than the vertical fluxes, its contribution was important; the additional losses generated by C displacement (CTm and CBm) offset the vertical C sink, leading to a C source from land equal to 0.92 Tg C year−1 over the period 2016–2100. When we ran the model without erosion, the C source from land was even higher and equal to 1.8 Tg C year−1 (table S2). This result suggests that climate change was the overarching driver on net CO2 losses, which were partially offset by soil erosion through dynamic replacement (fig. S5), under this conservative model configuration. In the AE_r scenario, the halving of the enrichment factor caused a direct reduction of eroded C, which amounted to 63% than the AE_e (Fig. 2). Despite the lower SOC loss from the soil profile by erosion, the concomitant lower dynamic replacement led to a positive feedback on the vertical fluxes (CI < CH with a value of −0.56 Tg C year−1). In this model configuration, we estimated a net soil C flux of −10.1 Tg C year−1, largely

Fig. 1 Response of C fluxes (Mg C km−2 year−1) under the RCP 4.5 (implemented in HadGEM2) climate scenario for the accelerated soil erosion simulation (AE_e).

(A to C) Average difference in C input, soil heterotrophic respiration, and eroded C, respectively, between the 2090–2100 and 2000–2010 time frames. (D) Cumulative change in net soil C flux over the period 2016–2100 (Mg C km−2) in EU agricultural soils. In (D), negative values represent a C source to the atmosphere, while positive values are a C sink.

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Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) driven by the lower burial efficiency of buried SOC in the model setup (that is, 0.2). Although this value represents a quite extreme configuration of our model framework, a recent study indicated burial efficiencies of 0.22 and 0.14 in colluvium and alluvium stores of temperate zones, respectively. In this scenario, the C feedbacks generated by erosion resulted in a net soil C flux five times higher than that driven by climate change alone (−10.1 versus −1.8 Tg C year−1).

Fig. 2 Cumulative C budget (Tg C year−1) over the period 2016–2100 at the EU level in the accelerated (AE) and current (CE) soil erosion scenarios.

The numbers in brackets [e; r] are the outcomes of the two model configurations: enhanced erosion-induced C sink (e), with the mineralization during transport set to 2%, the burial efficiency to 95%, and the enrichment factor to 2, and reduced erosion-induced C sink (r), where the same parameters were set to 10%, 20%, and 1, respectively. Dark arrows represent C displacements, while blue arrows represent C fluxes as C exchanges with the atmosphere (CO2-C). For the net soil flux, negative values represent a C source to the atmosphere, while positive values represent a C sink (see Materials and Methods for the full C balance component details). The agricultural area simulated (arable crops, grassland, and permanent crops) covers 1.88 Mkm2.

The C displacement between eroding and depositional areas also induced different C feedbacks mediated by the associated nitrogen (N) (that is, in the organic matter), in turn affecting crop productivity at different landscape positions (see fig. S9 and related discussion). At the EU level, we calculated a CI loss of 0.05 Tg C year−1 per Tg of C eroded, which suggests relatively small reductions in productivity over the period 2016–2100, as supported by other studies. Conversely, the effect may be much higher locally and the magnitude of our estimate may be conservative if the increase in rainfall erosivity will be associated with a higher frequency of extreme events. In CENTURY, the unique feedback on plant productivity due to erosion is related to the displacement of N (associated with SOC) and its turnover, while the model does not simulate mechanistically physicochemical soil changes (for example, in structure, texture, and loss of all nutrients) due to rain-driven intensive erosion events or the direct impact on the vegetation. Direct and indirect effects of erosion on plant productivity may then be very important. Despite the fact that the direct plant response to erosion is only modulated by N in our model, this is the first attempt to dynamically include lateral C fluxes in a process-based model framework at continental scale. Soil erosion is a widespread process that currently affects EU agricultural soils; therefore, the evaluation of future C feedbacks driven by its change requires a baseline reflecting the current conditions. To quantify the potential C feedback of accelerated erosion in the future, we ran the model, keeping erosion rates constant at current levels (current erosion: CE scenarios). With this run, we estimated that the future variation in rainfall erosivity led to 35% more C displaced by erosion in both configurations (e and r; Fig. 2). Overall, the climate-induced erosion feedback (ΔCnetAE) was marginal in the e scenario but


Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) was more pronounced in r, with a loss of 2.34 Tg C year−1 (Fig. 2 and table S2). Considering the contrasting ranges of parameterization in the two configurations, it is likely that we are catching the magnitude of this feedback, although there are no data against which to compare the model.

Despite the fact that accelerated erosion induced a negligible C source continentally under the AE_e scenario (0.01 Tg C year−1), we observed a marked regional variability (Fig. 3A). In 35% of the cells (that is, 66 Mha), we estimated an induced C sink by the accelerated erosion, completely offset by sources for the remaining agricultural areas. In the AE_r scenario, the erosion-induced C sink was present in only 13% of the agricultural soils (Fig. 3B), while lower values than those in AE_e were estimated all over the continent.

Projections of land C fluxes in current Earth system simulations under climate change are particularly affected by C turnover in soils and are still missing the erosion feedbacks over decadal scales. This study may be a preliminary step toward coupling landscape dynamics in the biogeochemistry component of land schemes. Our results highlight how the interaction among soil variability, local management, climate, and geophysical processes leads to a variable C response, both in space and in sign of emissions. Upscaling of inventory-based approaches to large areas should therefore be done with care so that processes that have inherent variability are not oversimplified. Furthermore, our results challenge the suggestion that erosion will lead to a C sink in the near future, but they also claim for new researches investigating poorly known landscape C processes.

CONCLUSIONS Our modeling framework has some limitations. Soil erosion by water is a selective process that redistributes different size particles across the landscape, in turn potentially changing the environmental conditions affecting plant growth and SOC decomposition. In addition, the displaced C is associated to different mineral/particle fractions (for example, C bound to mineral particles, C in aggregates, and particulate C), and the turnover of these different fractions is not yet implemented into the large-scale models, which are still currently based on the compartment approach of “conceptual” C pools. We estimate that the development of full sediment delivery models coupled with new generations of biogeochemical models based on measurable C fractions will take another decade. To do this, we will need more information and increased process understanding to transfer microscale processes, such as aggregate breakdown, to millions of hectares. Meanwhile, we could account for

Fig. 3 Climate-induced erosion feedback on net soil C flux (Mg C km−2 year−1).

The values represent the difference in net soil C flux (ΔCnetAE) between the accelerated erosion (AE) and current erosion (CE) scenarios (Eq. 2) over the period 2016–2100. (A and B) The e and r model configurations. Negative values indicate an erosion-induced source of C to the atmosphere driven by a variation in rainfall erosivity, while positive values show an erosion-induced C sink.

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Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) lateral C fluxes with feasible approaches, as we have described here. Future land C projections rely on complex Earth system and biogeochemical models that do not incorporate the erosion process, lowering the level of confidence of our predictions. We have attempted to include the main lateral and vertical C fluxes as driven by a climate change projection to increase the confidence of future projections of soil C erosion on the C cycle.

MATERIALS AND METHODS Model framework and configuration The model framework was based on coupling the process-based biogeochemistry model CENTURY to the RUSLE2015 erosion model. This model integration has been presented in previous studies, and a summary flowchart is given as fig. S1. We summarized the model setup and main assumptions as follows.

1) The CENTURY model was ran at a resolution of 1 km2 for the agricultural soil of the EU, using the soil erosion from RUSLE2015 model as input for CENTURY. Starting from 1900, the erosion process was implemented, keeping the climate, soil, and topographic factors (R, K, and LS, respectively) constant. While we considered K and LS factors quite invariable on a centennial scale, the C factor associated with the crop type was dynamically varied with crop rotations and land use changes. The simulated land use was based on the CORINE Land Cover 1990, 2000, and 2006, supplemented with Eurostat (Statistical Office of the European Communities)/FAO (Food and Agriculture Organization of the United Nations) statistics to build up crop rotations and implement consistent agronomic inputs (fertilization, irrigation, etc.). Before 1990, we assumed the same land use but with different agricultural techniques characterized by lower productivity crops, lower rates of mineral N, and different rotation schemes.

2) Originally, we assumed that each 1-km2 grid cell is composed of an eroding area and a depositional area, the latter retaining a proportion of eroded C. The partition was based on the study of Van Oost et al., who found that 53 to 95% of eroded SOC were retained in the catchment and redeposited in a limited area (14 to 35%) within the same catchment. Taking the central values, we assumed that 25% of our grid cells were depositional areas, which received 70% of eroded soil. The remaining 30% was accounted for as leaving the grid cell as potential sediments and C discharged to riverine systems. These assumptions on sediment distributions, necessary to work at continental scale, were further tested using a delivery/sedimentary model in regional simulations, as detailed in section S3. After the intercomparison between the original and the sedimentary model–driven configuration, we set a new sediment delivery ratio of 0.11, as most of the sediments were predicted to be retained in land under the delivery/sedimentary model runs. 3) The replacement of eroded soil in the fixed profile comes by “recruitment” from the subsoil layers (SSLs), characterized by a SOC composition defined quantitatively and qualitatively as a partition among the three CENTURY SOC pools (active, slow, and passive). These pools are functionally defined on the basis of mean residence time, as opposed to measured fractions, and thus cannot be constrained by measurements applicable at the EU scale. The most rational approach was to adopt the calibration of Harden et al. (7), which implicitly related the subsoil composition to the topsoil composition; accordingly, the subsoil SOC pools at time t = 0 (that is, 1900) were assumed to be composed as a fraction of the top soil pools, as follows

The lower horizon pools were decremented at each time step using the following relationships for each of the three SOC pools (i)


Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) where t is the time and FLOST is the fraction of topsoil lost to erosion [for the sake of clarity, SSLi(t− 1) × FLOST(t) is the C moving up from subsoil annually].

Although CENTURY does not explicitly simulate depositional processes, they can be mimicked while ensuring conservation of mass (soil and C) within the system boundaries. First, we calculated the amount of soil transported from the eroding to the depositional fraction of each grid cell. Second, we simulated the burial effect of the deposition event as a “negative” erosion event, which moves a SOC fraction below the fixed soil profile (C burial flux), proportional to the amount of soil deposited on the surface. Last, the associated amount of deposited C was controlled by setting the incoming SOC pool in the model. This procedure required an iterative process, running the model year by year, first in the eroding area and then in the depositional area of each pixel. A check of the C balance closure was done at the end of simulations.

Future rainfall erosivity and erosion The methodology for the estimation of the future rainfall erosivity was presented in a very recent study. Summarizing, this made use of the REDES (Rainfall Erosivity Database at European Scale) and a statistical approach (Gaussian process regression), used to spatially interpolate rainfall erosivity data with climatic scenarios. Using the HadGEM2 general circulation model climate projections (for RCP 4.5), available at the WorldClim repository, the rainfall erosivity was calculated for two time periods, namely, 2050 and 2070. Therefore, the soil erosion was recalculated, keeping the current factors of RUSLE, except the rainfall erosivity. The ratio between the future and the current soil erosion by water is reported in fig. S8.

Climate change scenarios with and without accelerated erosion The model was run with the current climate up to 2015 and with the same HadGEM2-ES_rcp45 scenario for the 2016–2100 time frame for consistency. Monthly precipitation and maximum and minimum air temperature were downloaded from the World Climate Research Programme–Coordinated Regional Downscaling Experiment (WCR-CORDEX) portal (https://esgf-node.ipsl.upmc.fr/search/cordex-ipsl/). The present land use and agricultural management remained the same to isolate the climate effect on both productivity/turnover and erosion C feedbacks.

In the accelerated erosion (AE) scenario, we linearly interpolated the soil erosion rates since 2016, considering the two soil erosion projections at 2050 and 2070. The name “accelerated erosion” refers to the average effect of rainfall erosivity change on increasing soil erosion. Higher and lower rainfall erosivity patterns are present at the EU level, as depicted in fig. S8. In the current erosion (CE) scenario, we kept the actual soil erosion rates.

Given the fact that there is limited consensus on C preservation/decomposition upon its displacement, we ran the model with two configurations, enhancing (e model setup) and minimizing (r model setup) the erosion as an induced C sink (or source). In particular, in the e configuration, the enrichment factor (that is, the C concentration in eroded sediments with respect to the bulk soils) was set to 2, the C mineralization during sediment transport to 2% yearly, and the burial efficiency (that is, the amount of C preserved when moved downward into the soil profile) to 95%. In the r configuration, the three parameters were set to 1, 10, and 20%, respectively.

According to previous sensitivity analyses, a higher enrichment factor promotes a higher dynamic replacement of C (inducing a C sink in eroding position), while lower mineralization and higher burial efficiency preserve the displaced C. Considering the ranges of parameters from different studies, our configuration should reflect the outer boundary of the erosion effect in inducing a sink/source of C.

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Soil Erosion is Unlikely to Drive a Future Carbon Sink in Europe (Continued) C Budget The CENTURY model was parameterized to simulate the soil C and N turnover in a fixed profile of 0 to 30 cm. When the model incorporates soil erosion, an upward C flux from a subsoil pool follows the erosion event (as described in point 3 of the “Model framework and configuration” section). Moreover, the C balance accounts for C displacement components, some of them representing C moving outside the land domain (such as C to rivers, lakes, and ocean). For the purpose of the study, we set our boundary conditions to the land and, despite estimating a lump term accounting for C delivered to the riverine system, we did not calculate its decomposition (to CO2) related to the turnover outside the land. For the eroding area, the C balance is

(3) where ΔSOC is the SOC stock change in the fixed profile, CI is the C input through the remaining net primary productivity (NPP; after C exportation by harvest but including roots and manure), CH is the heterotrophic respiration, CS is the incoming SOC from a deeper layer, CE is the lateral C flux by sediment transport, and DOC is the C exported as dissolved organic C. For the depositional area, the balance is

(4) where CD is the C deposited coming from eroding areas and CB is the C that is moved out (that is, buried) of the simulated profile as a consequence of soil deposition. However, the net changes in SOC storage do not directly equate to the net CO2 exchange from soil to the atmosphere due to the combination of vertical C fluxes (CI − CH) of eroding and depositional areas and the mineralized fraction of SOC upon displacement. Therefore, the net soil C flux (as CO2) in the grid cell is given by

(5) where [CI − CH] is the net vertical C flux, CTm represents the fraction of eroded SOC mineralized during the transport, and CBm is the fraction of buried C that is mineralized in depositional area. Because the uncertainty in simulating C turnover in deeper layers increases with depth (due to cumulative sediment deposition), the cumulative CB fluxes, over the period 2016–2100, were assumed to be preserved from decomposition with two contrasting burial efficiency rates over a 100-year horizon (0.95 and 0.2). The corresponding CO2 flux to the atmosphere was then calculated as CB × (1 − burial efficiency).

Read more about this article at: http://advances.sciencemag.org/content/4/11/eaau3523

Do Slow Slip Events Trigger Large and Great Megathrust Earthquakes?

INTRODUCTION Since their discovery more than two decades ago, it has been suggested that slow slip events (SSEs) may trigger subduction zone earthquakes, perhaps by stress loading of adjacent sections of the fault. Alternatively, SSEs reduce the probability of large earthquakes by relieving strain and reducing the magnitude of coseismic slip. It is also possible that both are true: SSEs limit rupture area, reducing the long-term risk from earthquakes, but elevate the short-term probability of a seismic event through perturbations of the stress state. Unfortunately, it has been difficult to test these hypotheses as, in subduction zones, the critical region occurs offshore, where geodetic networks have limited sensitivity. Offshore geodetic techniques exist, but their deployment has been limited because of high cost and lower precision.


Do Slow Slip Events Trigger Large and Great Megathrust Earthquakes? (Continued)

In Japan, an SSE may have triggered the giant earthquake of 2011. While Global Positioning System (GPS) did not record the offshore SSE, eight offshore pressure sensors recorded deformation of the seafloor associated with an SSE. However, the signal was close to the noise level of the technique. Seismic activity associated with SSEs was observed to propagate toward the epicenter. Similar observations were made, leading up to the moment magnitude (Mw) 8.1 Iquique, Chile earthquake in 2014, with indicators preceding the earthquake by 8 months. An SSE was also observed several days before the Mw 6.9 Valparaiso, Chile earthquake in 2017. Table S1 summarizes the current database for earthquakes associated with SSEs. In most cases, slow slip is postulated on the basis of migrating foreshocks or a few geodetic stations. Hence, it has been difficult to investigate the physical mechanism linking slow slip to earthquakes. In many subduction zones, SSE repeat times are short (one to several years) compared with earthquake recurrence intervals (30 to 500 years or longer), making it possible that their correlations are coincidental. In particular, it has been difficult to show that SSEs migrate in the vicinity of the earthquake nucleation point based on geodetic measurements.

In late February 2012, an SSE began in northern Costa Rica, 6 months before the 5 September 2012 Mw 7.6 earthquake. Both the SSE and earthquake were well recorded due in part to a peninsular region that allows instrumentation immediately above the seismogenic zone. Preliminary analysis of nine GPS stations noted that the change in Mohr-Coulomb failure stress (MCS) associated with the SSE at the nucleation site of the earthquake was small. Since that time, data from 11 additional stations have become available (figs. S1 to S6), allowing considerable refinement of our knowledge about this episode, including the relationship with foreshocks, migration pattern, and a more accurate estimate of MCS.

The Nicoya Peninsula lies along the Middle America trench where the Cocos plate is subducting beneath the Caribbean plate at a rate of ~9 cm/year. The peninsula extends toward the trench, with the plate surface of ~15 km beneath the coastline. SSEs have been identified both updip and downdip of the peninsula, with recurrence times of about 22 months. On 5 September 2012, a Mw 7.6 earthquake took place within a region that had been previously identified as a locked patch (Fig. 1). Using newly available GPS data, a large geodetic signal consistent with an SSE (southwest motion) is visible mid-February, persisting until the day of the Mw 7.6 earthquake (Fig. 2). The signal appears first on stations located southeast of the peninsula in February (CIQU, JACO, PUNT, and RIDC). The initial phase lasts for 4 months, with interseismic-like GPS velocities reappearing in July, 1.5 months before the Mw 7.6 earthquake. A second pulse of motion is observed in early August and continued until the earthquake. Coastal stations (GRZA, EPZA, and SAJU) show a final increased south-westward movement 2 weeks before the mainshock.

Fig. 1 Map of the study area in Nicoya Peninsula, Costa Rica.

Light pink areas are regions with interseismic SSEs. Red arrow represents Cocos-Caribbean convergence direction. Dashed line marks the transition between oceanic crust from Cocos-Nazca spreading (CNS) center and East Pacific Rise (EPR). Blue contours mark the slab depth. Yellow triangles mark the GPS stations. Mainshock focal mechanism is indicated by red beach ball. Red star marks the epicenter of the 2012 El Salvador earthquake, 450 km to the northwest of the Nicoya Peninsula.

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Fig. 2 Average slip rate and GPS time series.

Average SSE slip rate for different time periods (left), color coded to match individual site displacement time series (right). Black lines showing modeled fit due to fault slip, and scatter showing the horizontal displacements. Red triangle (left) marks the epicenter of the 2012 earthquake.

RESULTS We invert the GPS displacements for the time-dependent slip on the megathrust. We include the vertical GPS displacements in the datasets, weighting them ~3 times less than the horizontal position estimated. Incorporating the vertical GPS displacements is critical for estimating the downdip limit of slip, and tests removing the vertical GPS time series were unstable. Model results indicate that the initial transient started southeast of the peninsula, under Herradura (Fig. 2 and movie S1). Slip rates were highest during this initial transient, reaching 5 mm/day (fig. S6). This transient is similar to other deep Nicoya SSEs, with slip magnitudes peaking at ~6 cm (Mw 6.5). The transient migrates to the northwest where it slowly decays beneath the locked zone. About 3 weeks before the earthquake, a second shallow slip pulse appears in the vicinity of the locked zone, which is the rupture area of the 5 September earthquake. Kinematic modeling indicates that the earthquake nucleated near or immediately downdip of the region of shallow SSEs. The migration of slip toward the seismogenic zone is constrained by the displacement of coastal stations, particularly stations SAJU and GRZA. Peak slip rate occurs on this shallow section, immediately before the 2012 El Salvador earthquake, which occurred 10 days before and 450 km to the northwest of the Nicoya earthquake. Slip rates decay following this event despite a brief increase in seismicity associated with the El Salvador event. In the month preceding the Nicoya earthquake, there is a good correlation between foreshock productivity (defined as any earthquakes in the 30 days before the mainshock) and slip rate, with foreshocks clustering updip of the region of the largest aseismic slip (Fig. 3). Earthquake productivity in the Nicoya region during the interseismic period, before the events mentioned here, is 25 (±5) earthquakes per day.

Fig. 3 Slip rate and foreshock activity before the Nicoya earthquake.

Gray bars are daily earthquake counts (17). Blue line is the modeled slip rate. Purple vertical line is the 2012 El Salvador earthquake. Red vertical line is the 2012 Nicoya earthquake.



DISCUSSION To explore possible triggering by the SSE, we used Coulomb failure stress (CFS) analysis. CFS represents the relative contributions of normal stress change and shear stress change, resolved onto the fault in the direction of fault slip. Increased CFS implies that a fault is closer to failure. We find that, while positive in the time-dependent case, CFS due to the SSE is less than 0.1 bar at the nucleation point (Fig. 4), well below the threshold typically found in static earthquake–triggering cases [~1 bar] but above the level for modulating SSEs. While CFS is affected by the slip gradient, which is dependent on regularization, it is unlikely to have been increased by an order of magnitude. We find that reduction of regularization constraints requires more slips in the coseismic region, leading to further decrease in the Coulomb stress near the earthquake nucleation point. To explore the effects of this regularization, we also modeled the SSE using a static approach (Fig. 5). We used the cumulative offset estimated through fitting a cubic spline through the GPS time series. The time period spans the same period analyzed in the time-dependent modeling. This approach gives qualitatively similar results to the time-dependent inversion mentioned earlier in the region of the peninsula and requires slip within the cosesimic region regardless of choice of regularization (figs. S7 and S8). Static modeling requires slips to the south of the peninsula, in a region of poor geodetic resolution. We attribute this slip to differences in network geometry for the static inversion, where discontinuous time series cannot be included. Some signal at station JACO is identified as a bench mark motion in the Network Inversion Filter (NIF) modeling because of its early onset, and its displacement might be overestimated in the static modeling. However, slip is required in this region even when JACO is excluded.

Perhaps a more dynamic process is responsible for triggering. The presence of slow slip, both updip and downdip of the seismogenic zone during most Nicoya interseismic SSEs, suggests that slow slip is able to cross the frictional barrier between seismic slip and aseismic slip. The magnitude of shallow slip is above our uncertainty levels, although resolution decreases offshore (fig. S9). At 60-km depth, where the SSE initiated, the presence of fluid is thought to promote aseismic slip, as evidenced by the large signal inland. Perhaps the SSE allows fluid to migrate updip toward the seismogenic zone at 15- to 25-km depth. While conditions within the seismogenic zone are unfavorable for aseismic slip, fluids could weaken this region through dilatancy hardening or other mechanism. Further updip, conditions once again favor slow slip and do so persistently. If the seismogenic zone is close to failure, as was the case for 2012 earthquake when the Nicoya segment was more than 20% past its average 50-year

Fig. 4 Cumulative slip and Coulomb stress.

Left: Cumulative slip for the SSE, with chartreuse line marking the 5-mm contour. Right: CFS on the megathrust fault associated with the cumulative slip. White and cyan contours mark the 1, 2, and 3 m of coseismic slip, and red focal mechanism marks the 2012 earthquake epicenter. Gray circles are foreshocks in the 30 days before the earthquake, with denser concentrations appearing black.

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characteristic recurrence time, then additional fluids driven by the SSE were sufficient to trigger the earthquake.

Migrating seismicity before large earthquakes has been reported. It has been hypothesized that this migration is indicative of aseismic slip behavior. Our results provide strong evidence that foreshocks can be temporally associated with the slip rate of SSEs. Precursor microseismicity rates may therefore be a reasonable proxy for aseismic slip behavior, albeit at lower slip rates than previously reported. Our data also allow us to rule out at least one model for earthquake triggering by SSE, whereby rupture is initiated through power law acceleration of slip. In this case, both slip rates and foreshock rates were higher in the weeks before the rupture.

While there seems to be a strong case for temporal correlation between foreshocks (seismic behavior) and slow slip (aseismic behavior), spatiotemporal patterns of seismicity do not necessarily track SSE behavior. Notably, foreshocks cluster near the megathrust rupture but predominantly outside the SSE region (Fig. 4).

The use of foreshocks and SSEs for earthquake forecasting remains challenging, as most subduction zones lack the necessary monitoring. In particular, identifying foreshock sequences that culminate in a large earthquake in real time has not been possible. When the precursor SSE in Nicoya initiated, it was indistinguishable from interseismic SSEs, which often begin with a high slip under the Gulf of Nicoya. Both the timing of the SSE [22-month recurrence] and earthquake [50-year recurrence] were consistent with historical records. This suggests that near-term hazard forecasts should incorporate information about the timing of SSEs, particularly as a fault enters the later stage of the earthquake cycle. We note that SSEs are not required for a nucleation of a megathrust earthquake. Better measurements of the offshore region of subduction zones will be required to separate precursor activity from normal interseismic behavior.

MATERIALS AND METHODS GPS processing GPS data were processed using GIPSY-OASIS 6.4 software, with orbits and clock estimated provided by the NASA Jet Propulsion Laboratory. Daily solutions were calculated using the precise point positioning method. Phase ambiguity resolution was performed, and ocean loading was removed using FES2004. Tropospheric delay was estimated using the VMF1 (Vienna mapping function 1) mapping function, and ionospheric delay was estimated using the Ionex model. Stations were processed in the IGb08 reference frame.

Seasonal signal removal Seasonal signals were removed from the time series via a least-squares fit of a function that included annual, semiannual, a linear term, and H function (offset). Periods in which SSE was occurring were masked, and a Heaviside function was used to remove the offset. Parameters were estimated using the Levenberg-Marquardt algorithm, as implemented in the Python package lmfit (http://lmfit.github.io/lmfit-py/). The seasonal terms were then subtracted from the time series, and the estimated trend was included as an a priori trend during the subsequent time-dependent inversion for fault slip.

Fault slip inversion using the NIF We used a modified version of the NIF. We generated Green’s function using a high-resolution model of the subducting interface of the Cocos plate discretized into triangular elements, with approximate spatial dimensions of 20 km2. The modeled times series were a function of the slip rate on the plate interface, network error, random



walk error, and common-mode error. Regularization was imposed on both the temporal and spatial smoothness and was enforced via maximum likelihood. The slip rake direction is constrained to be parallel to the relative motion between the Cocos and Caribbean plates (29° east of North). Modeled displacement estimates were compared to observed position time series in fig. S1.

Read more about this article at: http://advances.sciencemag.org/content/4/10/eaat8472

Gravitational Collapse of Mount Etna’s Southeastern Flank

INTRODUCTION Volcanic flanks can slide in response to various internal and external forces. For example, the unbalanced weight distribution of a volcanic edifice and horizontal “pushing” due to magmatic intrusions can trigger flank spreading. Unstable flanks can fail catastrophically and result in giant landslides, such as those at the submarine slopes off Hawaii. Catastrophic collapses of ocean island volcanoes or those built at the shoreline pose the largest threat as the sudden displacement of large amounts of material in water can trigger tsunamis with extreme effects. Assessing the hazard potential of catastrophic collapse requires a profound understanding of the mechanisms that cause flank movement, which is also crucial for the design of appropriate monitoring strategies.

Numerous hypotheses have been proposed to explain flank sliding at Mount Etna, including increases in magma pressure, eruptive activity, repeated dyke intrusions, basement uplift, gravitational spreading, gravitational reorganization, gravity-driven instability accelerated by inflation and/or lateral intrusions, or combined magmatic inflation and continental margin instability. All hypotheses are derivatives of two basic processes capable of triggering flank instability: horizontal pushing of ascending magmatic intrusions or gravitational pull. These end-member mechanisms have fundamentally different hazard implications: While magma dynamics can trigger slope failures near the magma pathways, gradual deep-seated gravitational deformation can induce catastrophic collapse as in the cases of Mombacho, Kilauea, other Hawaiian volcanoes, and Ritter Island, Papua New Guinea. The overall consensus for Etna has been that it is mainly the magnetic plumbing system that drives movement of the unstable southeastern flank, rather than gravitational or tectonic forces.

Fig. 5 Static inversion modeling of GPS offsets associated with SSE. Slip exists within the co-seismic region.

Left: Static inversion of GPS time series with preferred regularization weighting. Right: Coulomb stress associated with the static inversion modeling results.

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Gravitational Collapse of Mount Etna’s Southeastern Flank (Continued) Uncertainties regarding the causes of flank sliding originate from the lack of information on the dynamics of the submarine part of Etna volcano. Onshore geodetic measurements have documented large-scale continuous seaward motion at an average rate of 3 to 5 cm per year since the early 1980s, immediately evidencing the highest rates at the coast. However, no information on the movement of the submarine part of the flank existed before this study. Here, we document rapid deformation of Etna’s offshore flank and combine the offshore measurements with onshore ground deformation. Our combined onshore-offshore data define the dynamics of the entire volcanic flank.

Seafloor displacement measurements at Etna’s submerged flank Established satellite-based geodetic tools are not adaptable for use in the marine environment due to the opacity of seawater to electromagnetic waves. Underwater, distances can be estimated with the sound speed of water and travel time measurements between transponders on the seafloor. Periodic back-and-forth acoustic interrogations between several transponders equipped with absolute pressure sensors and arranged in a network allow continuous determination of seafloor displacement in horizontal and vertical directions within the network. A network of five such transponders was placed on both sides of the submerged southern boundary of Etna’s unstable flank at a water depth of ~1200 m. Changes in distance between transponders across the fault and increases in pressure at transponders to the north of the fault indicate movement of the presumed unstable flank relative to the stable surrounding. Our seafloor network is the first to monitor an offshore strike-slip event in subcentimeter resolution, therewith proving the feasibility of the emerging acoustic direct-path ranging method to monitor volcanic flank instability.

On land, the spatial outline of the unstable flank is well defined by geodetic, geophysical, and geological methods (Fig. 1): Along the northern boundary of the unstable flank, deformation focuses along the left-lateral Pernicana fault. To the south, the right-lateral Tremestieri and Acitrezza (ATF) fault systems accommodate most of the flank movement. Off the coast, the Riposto Ridge forms the prolongation of the northern boundary. In distal direction, two anticlines observed in seismic reflection data mark the seaward termination of the unstable volcanic flank (Fig. 1). To the south, a right-lateral transpressive fault north of Catania Canyon, interpreted as the offshore prolongation of onshore fault systems, represents the southern boundary of the unstable flank.

Fig. 1 Morphologic map of Mount Etna including tectonic features of the southeastern flank.

Onshore topography in gray and offshore bathymetry in green to blue colors. Contour line interval is 300 m. Main features are shown as dashed and solid black lines. The thick gray line delineates the coastline. The orange rectangle marks the location of the seafloor geodetic network.


Gravitational Collapse of Mount Etna’s Southeastern Flank (Continued) This fault is a pronounced west-east striking feature in the bathymetry (Figs. 1 and 2C). Seismic data indicate distinct reflection characteristics on either side of the fault (fig. S1). On the basis of these observations, we deployed transponders 1 and 4 south of the fault and transponders 2, 3, and 5 north of the fault (Fig. 2C). All transponders were in line of sight of each other, resulting in 10 baselines with distances between 144 and 1254 m. All transponders monitored distances to all other transponders and pressures every 90 min. Periodic data upload via an acoustic link provided a continuous time series from April 2016 to July 2017.

RESULTS For most parts of the observation period, acoustic distances between transponders remained stable within approximately 0.5 cm (Fig. 2 and fig. S2). However, a significant change in distances occurred between 12 and 20 May 2017. Only baselines across the fault recorded the 8-day-long aseismic fault motion that stands out from the background noise (Fig. 2 and fig. S2). Relative distance changes during the May 2017 event ranged between 0.6 and −3.9 cm for different transponder pairs (Table 1, Fig. 2, and fig. S2). As expected for a dextral strike-slip fault, length changes are dependent on the angle of the baseline to the fault (fig. S3). This angle can be used to determine true fault slip. The main uncertainty in slip results from the lack of knowledge of the exact fault trace on the seafloor. The ranging data confirm that the fault trace must run in the very narrow corridor between transponders 1 and 3 (Fig. 2C) within a range of 5°. Taking into account all fault crossing baselines, the true slip is between 3.87 and 4.23 cm (Table 1). We also observe that transponders on the north side of the fault showed a downward vertical displacement of 1 cm relative to those on the south side during the May 2017 event (Fig. 2 and fig. S4).

Overall, no significant changes in distances or depths occurred between transponders that were located on the same fault side (Fig. 2 and figs. S2 to S4). We exclude the possibility of a local landslide coherently moving these transponders based on the lack of evidence for soft sediments in seismic and sediment echosounder data, as well as in seafloor samples. The observed distance changes are in all aspects consistent with right-lateral strike-slip movement separating transponders 2, 3, and 5 from transponders 1 and 4 (Fig. 2C).

Fig. 2 Seafloor deformation across the fault that marks the offshore southern boundary of Mount Etna’s unstable flank, as recorded by the network of five autonomous monitoring transponders.

(A and B) Relative changes in distances between transponder pairs (blue and green colors indicate active interrogation and passive response of acoustic signals, respectively) and relative vertical displacement between transponder pairs (gray line, 3-day moving average). Time series for all other transponder pairs are shown in figs. S2 and S3. (C) Map view of relative distance changes within the array during the observation period plotted on gray-shaded bathymetry (see Fig. 1for location). Black numbers indicate transponder numbers.

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Gravitational Collapse of Mount Etna’s Southeastern Flank (Continued) Notably, the observed length change in the network of ~4 cm provides a minimum estimate of the true slip along the fault during the May 2017 event. The gross motion of the unstable flank might not have been fully captured, leading to a potential underestimation of slip. The southern boundary fault splits into several branches toward the seafloor, as imaged in seismic data (fig. S1). The network of transponders, however, does not span over all fault branches. Branches out of the reach of our network may have also accommodated flank movement during the investigated time period.

A slip of 4 cm corresponds to a moment magnitude release equivalent to a Mw of 4.3 to 5.3 earthquake. Since the initiation of instrumental seismic recording at Etna in the 1980s, no earthquake with a magnitude larger than 4 has been observed in the area. Hence, the main style of deformation of the offshore volcanic flank is episodic and aseismic sliding rather than seismic rupture.

Overall flank dynamics Our offshore observations show that the submarine part of Mount Etna’s southeastern flank moves in east and downward direction with a minimum aseismic fault slip of at least 4 and 1 cm relative subsidence, respectively (Fig. 2). The total slip may be even larger as not all fault branches could be captured by the seafloor network (fig. S1). Onshore, the seaward flank motion at Etna in the observation period April 2016 to July 2017 manifested in continuous deformation (fig. S8) rather than in episodic slip, as observed offshore. Cumulative displacements were highest along the coast (Fig. 3). SISTEM (simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements) integration of GPS (fig. S5) and DInSAR (Differential Interferometry Synthetic Aperture Radar) (fig. S6) data shows that flank movement mainly occurred across the ATF and San Leonardello fault (Fig. 3 and fig. S7) with a maximum slip of ~2 cm along each fault. The offshore flank movement was thus in the same order of magnitude as the sum of onshore fault slips for identical periods of time. Therefore, the offshore fault probably cumulated the slip of both the ATF and San Leonardello fault.

Gross onshore and offshore movements are kinematically consistent (Fig. 4) and, therefore, are expressions of the same underlying process related to flank instability. The observed differences in fault slip mode during the observation period, i.e., continuous creep onshore and slow slip offshore, can result from variations in fault properties, such as temperature, fluid pressure, or fault gouge material, while still representing the same overall dynamics. Nevertheless, onshore deformation at Etna’s unstable flank also manifests in slow slip events along the coastline, as monitored by continuous GPS.

DISCUSSION Reasons for instability of Mount Etna’s southeastern flank have been related either to the volcano’s magmatic plumbing system or to gravitational forces. Displacement induced by magma injection strongly decays with distance to the dyke. Inflation of the volcanic edifice caused by uprising magma is expected to cause the highest displacements near the volcanic center, which is inconsistent with our data. In contrast, our geodetic measurements demonstrated that flank movement increases away from the summit toward the coast and into the Ionian Sea, while no increase in magma activity was noticed simultaneous to the May 2017 offshore event, implying that magma dynamics cannot be solely responsible for the observed deformation pattern. The comparison of onshore and offshore fault slip further suggests that offshore deformation focuses along one fault north of Catania Canyon and that strain is partitioned near the coast into two fault systems (Fig. 4). The observations of (i) largest deformation away from and (ii) strain partitioning toward the summit indicate that the


Gravitational Collapse of Mount Etna’s Southeastern Flank (Continued) basal shear zone accommodating flank movement began offshore and has developed retrogressively landward. Therefore, the forcing mechanism that controls the bulk of Mount Etna’s flank movement must have its origin seaward and is separated from the volcanic edifice. Gravitational pull of the subsiding continental margin is a potential tectonic trigger.

Yet, magmatic activity also influences flank movement as episodic accelerations of onshore flank movement have been related to dyke intrusions and magma ascent repeatedly. Analyses of onshore seismic and ground deformation data show a clear decoupling of the shallow and deep strain regimes beneath the eastern flank at a depth of 2 km during an inflation period. Inflation and dyke intrusions can thus favor episodic accelerations of flank movement in addition to large-scale continuous gravitational sliding. Both processes may well interact with and influence each other, as demonstrated by analog models.

Marine geological records off the Canary Islands document that large-scale submarine flank failures occurred in multiple stages, all preceding explosive eruptions. A similar pattern is recorded in sediment cores at Etna’s submerged flank, where ash layers overlie landslide deposits. These observations further support a close interaction of flank movement and magmatic activity. However, eruptions do not trigger catastrophic flank collapses, implying that gravitational sliding is the governing process.

Our results show that only the combination of onshore and offshore ground deformation data gives a clear picture of overall volcano flank dynamics, from which the hazard of catastrophic flank collapse can be assessed. In the case of Mount Etna, our shoreline-crossing deformation analysis implies a greater hazard for flank collapse than previously assumed, as deep-seated gravitational sliding can potentially lead to catastrophic collapse. Onshore ground deformation analyses reveal signs of ongoing flank instability at numerous coastal and ocean island volcanoes today. Volcanoes, including those in Hawaii, the Canary Islands, and La Réunion, are potentially liable to collapse, but shoreline-crossing ground deformation analyses are needed to obtain a comprehensive view of the dynamics and constrain the hazard. Our results demonstrate both that seafloor geodetic investigations are capable of characterizing the dynamics of submerged volcanic flanks and that such investigations provide deformation data at a resolution comparable to GPS.

MATERIALS AND METHODS Bathymetry Bathymetric data were acquired during research vessel (RV) Meteor expedition M86/2 in 2012 with hull-mounted Kongsberg Simrad EM122 and EM710 multibeam sounders. Standard data processing with MB-System produced a grid with a cell size of 30 m by 30 m. Coastal bathymetry was acquired in the framework of the MaGIC (Marine Geohazards along the Italian Coast) project.

Seafloor geodesy The direct-path acoustic ranging method provides relative positioning by using high-precision acoustic transponders [Sonardyne Autonomous Monitoring Transponders (AMT)]. Multiple transponders installed at the seafloor measure the time of flight of acoustic signals between them with a microsecond resolution and water sound speed, temperature, and absolute pressure. Travel time observations were converted into distances with millimetric precision. Pressure measurements provided information on vertical displacement. Dual-axis inclinometers detected changes in instrument tilt. Repeated interrogations over months to years allowed the determination of displacements and, hence, deformation of the seafloor inside the network for extended periods, depending on battery capacity.

Here, we used five transponders from GEOMAR’s GeoSEA array. The transponders communicated with 8-ms

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Gravitational Collapse of Mount Etna’s Southeastern Flank (Continued) calculated by cross-correlation of the interrogation and receiving signals. The AMTs logged pressure, temperature, tilt, and sound speed. The log period for each transponder was set to 90 min. We noted instability in the sound speed measurement and recalculated the sound speed using the high-resolution temperature and pressure measurements at each transponder and assuming a constant salinity of 34 practical salinity units. We removed the tide signals from the pressure data using the data provided by the Istituto Superiore per la Protezione e la Ricerca Ambientale tide gauge in the port of Catania (www.mareografico.it). Pressure was converted to depth with the seawater density of 1024 kg/m3. For better comparison to the relative distance measurements obtained by acoustic telemetry, and because we are mostly interested in the relative movement of the unstable sector compared to the stable sector, we only showed relative vertical displacement between transponder pairs. These were obtained by subtracting the time series recorded by one transponder from that of another transponder.

The autonomous monitoring transponders were located at the outcrop of a fault at the seafloor. Locations for individual transponders were chosen on the basis of a closely spaced high-resolution two-dimensional (2D) seismic survey and swath bathymetric data. The network design ensures that at least two AMTs sit at each side of the fault and are in acoustic sight of each other. The AMTs were mounted on anchored buoyancy bodies. The deployed trapeze-shaped setup results in 10 monitored baselines. Besides transponder 1, all baselines were recorded in two directions (forward and backward measurements), resulting in six bidirectional baselines and four unidirectional baselines. Distances for forward (for example, measuring the travel time from AMT 1 to 2 and return) and backward measurements (measuring from AMT 2 to 1 and return) closely agree for all transponder pairs.

We deployed the transponders in April 2016 during RV Poseidon expedition POS496 at meter precision using ultrashort baseline acoustic positioning in water depths of 950 to 1180 m. Data stored in each station were uploaded from the seafloor to the surface with an acoustic modem.

Onshore geodesy We processed and integrated the onshore data covering the same period as the offshore data acquisitions to compare the results and extend the information about the deformation measured by the seafloor network. GPS data collected during the first week of April 2016 and the last week of July 2017 were proc

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