Cratonic lithosphere with a high-density root undergoes delamination when perturbed by mantle plumes from beneath. The removed cratonic root then thermally grows back, with its rock fabrics preserving recent mantle deformation. Credit: Lijun Liu
A University of Illinois-led team has identified unexpected geophysical signals underneath tectonically stable interiors of South America and Africa. The data suggest that geologic activity within stable portions of Earth’s uppermost layer may have occurred more recently than previously believed. The findings, published in Nature Geoscience, challenge some of today’s leading theories regarding plate tectonics.
The most ancient rocks on Earth are located within continental interiors, far from active tectonic boundaries where rocks recycle back into the planet’s interior. These strong, buoyant and deeply rooted blocks of Earth, called cratons, have been drifting on the surface for billions of years, seemingly undisturbed. They occasionally join and break apart along their edges in a dance called the supercontinent cycle.
“We usually think of cratons as being cold, stable and low-elevation,” said professor of geology and study co-author Lijun Liu. “Cold because the rocks are far above the hot mantle layers, stable because their crusts have not been disturbed significantly by faulting or deformation, and their low elevation is because they have been sitting there, eroding down for billions of years.”
However, there are places where cratons don’t follow these rules.
“For example, there are regions of high topography within the cratons of South America and Africa,” said graduate student and lead author Jiashun Hu.
The researchers processed geophysical data with the Blue Waters supercomputer at the National Center for Supercomputing Applications at Illinois hoping to better understand these high-elevation regions. The thick roots of cratons have been thought to be buoyant due to their low-density mineral content, allowing them to float on top of the hot underlying mantle. However, the new data indicate that the cold mantle that lies below these regions in South America and Africa – once joined as part of the supercontinent Pangea – has a layered structure and that the lower layer was more dense in the past than it is today, Liu said.
This density difference could be the result of a process called mantle delamination. During delamination, the denser lower mantle layer peels away from the buoyant upper layer under the crust of the craton after interacting with hot magma from mantle plumes, the researchers said.
“From several types of seismic imaging data, we can see what we think are delaminated mantle slabs sinking into the hot, viscous deep mantle,” Liu said.
“The material that subsequently grows back at the roots of the cratons after delamination, due to cooling from above, is probably compositionally much less dense than what was there before,” said geology professor Craig Lundstrom. “That adds buoyancy, and that force from buoyancy could be what forms the anomalously high topography.”
This multidisciplinary study is beginning to give the team a very logical – albeit complicated – update on the story of Earth’s tectonic history, the researchers said.
“The high topography of Africa and South America is only part of the story,” Hu said. “There are many geologic phenomena such as the location of hotspot trajectories, continental volcanism, surface uplift and erosion, as well as seismically imaged deformation within the craton roots that all seem to correlate well with the proposed delamination event, implying a potential causal relationship.”
There is also evidence to support other locations of craton-plume interaction during other times in Earth’s history.
“The rock record shows that uplift and erosion events have taken place during previous supercontinent cycles,” said geology professor and School of Earth, Society and Environment director Stephen Marshak. “A related study discusses what might be a similar event, namely continental uplift possibly related to delamination of cratonic lithosphere that caused the period of global erosion resulting in the Great Unconformity, which is the contact between Precambrian basement rock and Paleozoic sedimentary strata.”
For now, it is not clear if and how craton-plume interaction may affect modern-day earthquake activity and volcanism in areas thought of as geologically inactive. However, the study marks new thinking in how geologists may understand the so-called stable cratons.
Reference:
Jiashun Hu et al. Modification of the Western Gondwana craton by plume–lithosphere interaction, Nature Geoscience (2018). DOI: 10.1038/s41561-018-0064-1
Rhynia gwynne-vaughanii — 400 million-year-old fossil plant stem from Aberdeenshire, Scotland. Credit: The Natural History Museum, London.
For the first four billion years of Earth’s history, our planet’s continents would have been devoid of all life except microbes.
All of this changed with the origin of land plants from their pond scum relatives, greening the continents and creating habitats that animals would later invade.
The timing of this episode has previously relied on the oldest fossil plants which are about 420 million years old.
New research, published today in the journal Proceedings of the National Academy of Sciences, indicates that these events actually occurred a hundred million years earlier, changing perceptions of the evolution of the Earth’s biosphere.
Plants are major contributors to the chemical weathering of continental rocks, a key process in the carbon cycle that regulates Earth’s atmosphere and climate over millions of years.
The team used ‘molecular clock’ methodology, which combined evidence on the genetic differences between living species and fossil constraints on the age of their shared ancestors, to establish an evolutionary timescale that sees through the gaps in the fossil record.
Dr Jennifer Morris, from the University of Bristol’s School of Earth Sciences and co-lead author on the study, explained: “The global spread of plants and their adaptations to life on land, led to an increase in continental weathering rates that ultimately resulted in a dramatic decrease the levels of the ‘greenhouse gas’ carbon dioxide in the atmosphere and global cooling.
“Previous attempts to model these changes in the atmosphere have accepted the plant fossil record at face value — our research shows that these fossil ages underestimate the origins of land plants, and so these models need to be revised.”
Co-lead author Mark Puttick described the team’s approach to produce the timescale. He said: “The fossil record is too sparse and incomplete to be a reliable guide to date the origin of land plants. Instead of relying on the fossil record alone, we used a ‘molecular clock’ approach to compare differences in the make-up of genes of living species — these relative genetic differences were then converted into ages by using the fossil ages as a loose framework.
“Our results show the ancestor of land plants was alive in the middle Cambrian Period, which was similar to the age for the first known terrestrial animals.”
One difficulty in the study is that the relationships between the earliest land plants are not known. Therefore the team, which also includes members from Cardiff University and the Natural History Museum, London, explored if different relationships changed the estimated origin time for land plants.
Leaders of the overall study, Professor Philip Donoghue and Harald Schneider added: “We used different assumptions on the relationships between land plants and found this did not impact the age of the earliest land plants.
“Any future attempts to model atmospheric changes in deep-time must incorporate the full range of uncertainties we have used here.”
Reference:
JL Morris, MN Puttick, J Clark, D Edwards, P Kenrick, S Pressel, CH Wellman, Z Yang, H Schneider and PCJ Donoghue. Timescale of early land plant evolution. Proceedings of the National Academy of Sciences, 2018. DOI: 10.1073/pnas.1719588115
Wastewater created during oil and gas production and disposed of by deep injection into underlying rock layers is the probable cause for a surge in earthquakes in southern Kansas since 2013, a new report in the Bulletin of the Seismological Society of America concludes.
Until 2013, earthquakes were nearly unheard of in Harper and Sumner counties, the site of recent intensive oil and gas production. But between 2013 and 2016, 127 earthquakes of magnitude 3 or greater occurred in Kansas, with 115 of them taking place in Harper and Sumner counties. Prior to 1973, there were no felt earthquakes reported in the area, and only one magnitude 2.0 earthquake between 1973 and 2012.
Using data collected by a network of seismic stations installed by the U.S. Geological Survey, lead researcher Justin Rubinstein and his colleagues analyzed 6,845 earthquakes that occurred in the counties between March 2014 and December 2016.
They found that the dramatic uptick in seismicity correlated in time and location with increases in wastewater disposal that began in 2012 — and that decreases in seismicity during that time also corresponded to decreases in disposal rates.
Between 1974 and 2012, there were no magnitude 4 or greater earthquakes in the study area. Between 2012 and 2016, six such earthquakes occurred. “The probability of this rate change occurring randomly is approximately 0.16%,” Rubinstein and colleagues write in the BSSA study.
Kansas had the second-highest statewide earthquake rate in the central United States between 2013 and 2016, coming in behind Oklahoma, where a similar dramatic increase in seismicity also has been linked to wastewater injection.
In the southern Kansas study area, wastewater injection decreased significantly in 2015, falling from an average of 5 million barrels a month from July to December 2014 to 3.8 million barrels per month in March 2015. This decrease was likely due in part to a drop in oil and gas production as the prices for those commodities dropped, the researchers said, noting that the price of a barrel of oil fell by half between August 2014 and January 2015.
During the same time, the Kansas Corporation Commission developed rules to limit wastewater injection in the study area, with the rules going into full effect in July 2015. Since then, wastewater injection in the study area has dropped by almost 50 percent.
There is a corresponding decrease in seismicity in 2015, but Rubinstein said it is difficult to tell how much economic shifts or new regulation contributed to that trend. “We can’t fully disentangle it. But there’s no question that economics plays a role here because injection started to drop off before the new rules went into place. ”
“It certainly seems probable that regulations have an effect,” he said, “but we would need to speak to individual [oil and gas] operators to determine the extent of that effect.”
The researchers say that fluids injected into the crystalline rock basement below the Kansas oil and gas sites increased pressures in the rock pores and reduced friction along faults to trigger these induced earthquakes. Not all wastewater injection disposal sites have earthquakes associated with them, Rubinstein noted. In some cases, the fluid pressures may not be able to get to depths where earthquakes occur, or there may be some places in the rock basement that are more susceptible than others to fluid effects.
It’s also difficult to know how long these stresses might continue to produce earthquakes, he added. Seismicity related to an injection well in Youngstown, Ohio in 2013 lasted only a few weeks, while fluid injection at Rocky Mountain Arsenal in Colorado stopped in 1966 but significant seismicity continued in the area until the 1980s.
“It’s hard to say how long it’s going to last given that what we’re looking at in Kansas is a much higher rate of injection than in the places where seismicity slowed quickly, and many, many more wells,” said Rubinstein. “If they shut off all the injection, the decay could still take years, just because there’s been such a dramatic change in the regional pressure field.”
Rubinstein will continue to work in Kansas to learn more about whether seismologists can consistently see foreshocks and earthquake swarms in the seismic record. “We have an incredible network there, and one of the best documented cases of induced seismicity with publicly available seismic data,” he said.
Reference:
Justin L. Rubinstein, William L. Ellsworth, Sara L. Dougherty. The 2013–2016 Induced Earthquakes in Harper and Sumner Counties, Southern Kansas. Bulletin of the Seismological Society of America, 2018; DOI: 10.1785/0120170209
This schematic illustration of the 2014 Iquique earthquake off the coast of Chile (magnitude 8.1) shows the locations of foreshocks (blue) and aftershocks (red) relative to the area of large slip on the fault (contour lines). The mainshock involved thrust faulting on the plate boundary between the underthrusting Nazca and the overriding South American plates. Credit: Wetzler et al., Science Advances, Feb-2018
A comprehensive analysis of 101 major earthquakes around the Pacific ring of fire between 1990 and 2016 shows that most of the aftershock activity occurred on the margins of the areas where the faults slipped a lot during the main earthquakes. The findings support the idea that the area of large slip during a major earthquake is unlikely to rupture again for a substantial time.
The idea that earthquakes relieve stress on faults in the Earth’s crust makes intuitive sense and underlies the common assumption that the portion of a fault that has just experienced an earthquake is relatively safe for some time. But not all studies have supported this, according to Thorne Lay, professor of Earth and planetary sciences at UC Santa Cruz.
“This intuition has been challenged by statistical treatments of seismic data that indicate that, based on the clustering of earthquakes in space and time, the area that has just slipped is actually more likely to have another failure,” Lay said. “The truth appears to be more nuanced. Yes, the area that slipped a lot is unlikely to slip again, as the residual stress on the fault has been lowered to well below the failure level, but the surrounding areas have been pushed toward failure in many cases, giving rise to aftershocks and the possibility of an adjacent large rupture sooner rather than later.”
In the new study, published February 14 in Science Advances, Lay and other seismologists at UC Santa Cruz and Caltech took advantage of advanced slip-imaging methods applied to recent earthquakes of magnitude 7 or greater. When they examined the locations of aftershocks with respect to the slip during the mainshock, they found that very few aftershocks occur in the regions of a fault that had a large amount of slip, and aftershocks that do occur in the slip zone tend to be weak, with negligible additional slip. Most aftershock activity occurs on the margins of the area that slipped in the mainshock.
“This produces a halo of aftershocks surrounding the rupture and indicates that the large-slip zone is not likely to have immediate rerupture,” Lay said.
These findings indicate that the stress reduction during a major earthquake is large and pervasive over the ruptured surface of the fault. Stress will eventually build up again on that portion of the fault through frictional resistance to the gradual motions of the tectonic plates of Earth’s crust, but that’s a very slow process. Although immediate rerupture of the large-slip zone is unlikely, regional clustering of earthquakes is likely to occur due to the increased stress outside the main slip zone.
The findings also suggest that if unusually intense aftershock activity is observed within the high-slip zone, a larger earthquake in the immediate vicinity of the first event might still be possible. The authors noted that earthquake sequences are highly complex and involve variable amounts of slip and stress reduction.
Reference:
“Systematic deficiency of aftershocks in areas of high coseismic slip for large subduction zone earthquakes” Science Advances (2018). DOI: 10.1126/sciadv.aao3225
The oceanic slow carbon cycle. Credit: Adriana Dutkiewicz
A previously unknown connection between geological atmospheric carbon dioxide cycles and the fluctuating capacity of the ocean crust to store carbon dioxide has been uncovered by two geoscientists from the University of Sydney.
Prof Dietmar Müller and Dr Adriana Dutkiewicz from the Sydney Informatics Hub and the School of Geosciences report their discovery in the journal Science Advances.
Many of us are familiar with the Slow Movement philosophy, which includes slow living, slow cooking, slow fashion, and even slow TV. But most of us would not have heard of the slow carbon cycle, which is about the slow movement of carbon between the solid Earth and the atmosphere.
The slow carbon cycle predates humans and takes place over tens of millions of years, driven by a series of chemical reactions and tectonic activity. The slow carbon cycle is part of Earth’s life insurance, as it has maintained the planet’s habitability throughout a series of hothouse climates punctuated by ice ages.
One idea is that when atmospheric carbon dioxide rises, the weathering of continental rock exposed to the atmosphere increases, eventually drawing down carbon dioxide and cooling the Earth again.
Less well-known is that weathering exists in the deep oceans too. Young, hot, volcanic ocean crust is subject to weathering from the circulation of seawater through cracks and open spaces in the crust. Minerals such as calcite, which capture carbon in their structure, gradually form within the crust from the seawater.
Recent work has shown that the efficiency of this seafloor weathering process depends on the temperature of the water at the bottom of the ocean—the hotter it is, the more carbon dioxide gets stored in the ocean crust.
Prof Müller explains: “To find out how this process contributes to the slow carbon cycle, we reconstructed the average bottom water temperature of the oceans through time, and plugged it into a global computer model for the evolution of the ocean crust over the past 230 million years. This allowed us to compute how much carbon dioxide is stored in any new chunk of crust created by seafloor spreading.”
Dr Dutkiewicz adds: “Our plate tectonic model also allows us to track each package of ocean floor until it eventually reaches its final destination—a subduction zone. At the subduction zone, the crust and its calcite are recycled back into the Earth’s mantle, releasing a portion of the carbon dioxide into the atmosphere through volcanoes.”
The computer model reveals that the capacity of the ocean crust to store carbon dioxide changes through time with a regular periodicity of about 26 million years.
Several geological phenomena including extinctions, volcanism, salt deposits and atmospheric carbon dioxide fluctuations reconstructed independently from the geological record all display 26 million-year cycles.
A previous hypothesis had attributed these fluctuations to cycles of cosmic showers, thought to reflect the Solar System’s oscillation about the plane of the Milky Way Galaxy.
Prof Müller says: “Our model suggests that characteristic 26 million-year periodicity in the slow carbon cycle is instead driven by fluctuations in seafloor spreading rates that in turn alter the capacity of the ocean crust to store carbon dioxide. This raises the next question: what ultimately drives these fluctuations in crustal production?”
Subduction, the sinking of tectonic plates deep into the convecting mantle, is regarded as the dominant plate driving force of plate tectonics. It follows that cyclicities in seafloor spreading rates should be driven by equivalent cycles in subduction.
An analysis of subduction zone behaviour suggests that the driving force in the 26 million-year periodicity originates from an episodicity in subduction zone migration. This component of the slow carbon cycle needs to be built into global carbon cycle models.
Better understanding of the slow carbon cycle will help us predict how the Earth will react to the human-induced rise in atmospheric carbon dioxide. It will help us answer the question: To what extent will the continents, oceans and the ocean crust take up the extra carbon dioxide in the long run?
A new study has shown that monitoring inaudible low frequencies called infrasound produced by a type of active volcano could improve the forecasting of significant, potentially deadly eruptions.
Scientists from Stanford and Boise State University analyzed the infrasound detected by monitoring stations on the slopes of the Villarrica volcano in southern Chile, one of the most active volcanoes in the world. The distinctive sound emanates from the roiling of a lava lake inside a crater at the volcano’s peak and changes depending on the volcano’s activity.
The study demonstrated how changes in this sound signaled a sudden rise in the lake level, along with rapid up-and-down motions of the surging lake near the crater’s rim just ahead of a major eruption in 2015. Tracking infrasound in real time and integrating it with other data, such as seismic readings and gas emission, might help alert nearby residents and tourists that a volcano is about to blow its stack, the researchers said.
“Our results point to how infrasound could aid in forecasting volcanic eruptions,” said study co-author Leighton Watson, a graduate student in the lab of Eric Dunham, an associate professor in the Department of Geophysics of the Stanford School of Earth, Energy & Environmental Sciences and also a co-author. “Infrasound is potentially a key piece of information available to volcanologists to gauge the likelihood of an eruption hours or days ahead.”
The study, published Feb. 14 in the journal Geophysical Research Letters, is led by Jeffrey Johnson, an Associate Professor of Geophysics at Boise State University in Idaho.
Sleeping giant roars awake
Villarrica is a picturesque mountain with an altitude of 9,300 feet. The snowcapped volcano looms over a lake and across from the city of Pucón, which swells to a quarter million people in the summer tourist season. At night, residents of Pucón can often see a scarlet glow from Villarrica’s lava lake, normally hidden well below the volcano’s rim.
The ominous serenity that had held at Villarrica since its last eruption in the mid-1980s ended in the early morning hours on March 3, 2015. An incandescent fountain of lava rocketed from the mountaintop nearly a mile into the sky, spewing ash and debris and triggering bolts of lightning from the thick heat-generated clouds enveloping the summit. Around 4,000 people evacuated the immediate area. The eruption proved short-lived, however, and with risks of mudslides and flooding from melted snow minimal, evacuees soon returned to their homes.
Infrasound monitoring stations established at Villarrica just two months before the 2015 event and maintained by co-author Jose Palma from the University of Concepcion in Chile captured its before-and-after sonic activity. Studying these data, the research team saw that in the build-up to the eruption, the pitch of the infrasound increased, while the duration of the signal decreased. Flyovers in aircraft documented the changes in Villarrica’s lava lake, allowing researchers to explore connections between its height and the sound generation.
Watson offered a music analogy to explain this relationship. Similar to a person blowing into a trombone, explosions from gas bubbles rising and then bursting at the surface of the lava lake create sound waves. Just as the shape of a trombone can change the pitch of the notes it produces, the geometry of the crater that holds the lava lake modulates its sounds. When the lava lake is deep down in the volcano’s crater, the sound registers at a lower pitch or frequency — “just like when a trombone is extended,” said Watson. When the lava lake rises up in the crater, potentially heralding an eruption, the pitch or frequency of the sound increases, “just like when the trombone is retracted,” said Watson.
Warning signs
Future research will seek to tie infrasound generation to other critical variables in volcano monitoring and eruption forecasting, such as seismicity. Ahead of an eruption, seismic activity in the form of small earthquakes and tremors almost always increases. This seismicity emanates from several miles underground as magma moves through the volcano’s “plumbing system” of fractures and conduits that connect the volcano’s opening to magma chambers in our planet’s crust. Volcanologists think that changes in lava lake levels — and their attendant infrasound — result from the injection of new magma through volcanic plumbing, increasing the odds of a violent outburst.
In this way, the collection of infrasound should prove beneficial for forecasting purposes at “open vent” volcanoes like Villarrica, where an exposed lake or channels of lava connect the volcano’s innards to the atmosphere. Closed vent volcanoes, however, where the pooling magma remains trapped under rock until an explosive eruption occurs, do not generate the same kind of infrasound and thus pose additional forecasting challenges. An example of a closed vent volcano is Mount St. Helens in southwestern Washington state, whose eruption in 1980 remains the most lethal and destructive eruption in the history of the United States.
“Volcanoes are complicated and there is currently no universally applicable means of predicting eruptions. In all likelihood, there never will be,” Dunham said. “Instead, we can look to the many indicators of increased volcanic activity, like seismicity, gas emissions, ground deformation, and — as we further demonstrated in this study — infrasound, in order to make robust forecasts of eruptions.”
Reference:
Jeffrey B. Johnson, Leighton M. Watson, Jose L. Palma, Eric M. Dunham, Jacob F. Anderson. Forecasting the eruption of an open-vent volcano using resonant infrasound tones. Geophysical Research Letters, 2018; DOI: 10.1002/2017GL076506
The fossil Waptia from the Burgess Shale, Canada. New Oxford University research suggests that the mineralogy of the surrounding earth is key to conserving soft parts of organisms, and finding more exceptional fossils like the Waptia. Credit: Yale University
Fossils that preserve entire organisms (including both hard and soft body parts) are critical to our understanding of evolution and ancient life on Earth. However, these exceptional deposits are extremely rare. The fossil record is heavily biased towards the preservation of harder parts of organisms, such as shells, teeth and bones, as soft parts such as internal organs, eyes, or even completely soft organisms, like worms, tend to decay before they can be fossilised. Little is known about the environmental conditions which stop this process soon enough for the organism to be fossilised.
New Oxford University research suggests that the mineralogy of the surrounding earth is key to conserving soft parts of organisms, and finding more exceptional fossils. Part-funded by NASA, the work could potentially support the Mars Rover Curiosity in its sample analysis, and speed up the search for traces of life on other planets.
Perhaps the most iconic of all exceptional fossil deposits is the Burgess Shale of Canada, popularised by Stephen J. Gould’s Wonderful Life. Dating to around 500 million years ago, the deposit preserves exceptional fossils from the Cambrian Explosion, an event which saw the rapid diversification of early animal life from simpler single-celled ancestors. Burgess Shale-type fossil localities are now known across the globe and without them roughly 80% of Cambrian organisms (those that have no hard skeleton or shell) would be unknown, distorting our picture of early animal evolution.
Published in Geology, the study, conducted by researchers from Oxford’s Department of Earth Sciences, Yale University, and Pomona College, builds on their previous research which revealed that certain clay minerals are toxic to bacteria that decay marine animals. This time around, the team set out to find geological evidence that rocks composed of the same clay minerals are the hosts of Burgess Shale-type fossils.
The team examined more than 200 Cambrian rock samples using powder X-ray diffraction analysis to determine their mineralogical composition, comparing rocks with Burgess Shale-type fossils with those with only fossilised shells and bones. Nicholas Tosca, Associate Professor of Sedimentary Geology at Oxford, said: ‘The number of samples required for this study was made possible because the diffractometer at Oxford collects mineralogical data 250 times faster than a conventional instrument.’
The findings reveals that soft tissue fossils are generally found in rocks rich in the mineral berthierine, one of the main clay minerals identified by the previous study as being toxic to decay bacteria. Ross Anderson, lead author and fellow at All Souls College, Oxford, explains: ‘Berthierine is an interesting mineral because it forms in tropical settings when the sediments contain elevated concentrations of iron. This means that Burgess Shale-type fossils are likely confined to rocks which were formed at tropical latitudes and which come from locations or time periods that have enhanced iron. This observation is exciting because it means for the first time we can more accurately interpret the geographic and temporal distribution of these iconic fossils, crucial if we want to understand their biology and ecology.’
The study provides a mineralogical signature which can be used to find the more elusive sites that are home to these extraordinary fossils. ‘The mineralogical associations we identified mean that for a given Cambrian sedimentary mudrock we can predict with around 80% accuracy whether it is likely to contain Burgess Shale-type fossils,’ explains Anderson.
Of the project’s wider applications, potentially supporting the search for life beyond our own planet, Anderson adds: ‘For the vast majority of Earth’s history, life has not possessed hard shells or skeletons. This means that if we want to look for fossil evidence of life on other planets like Mars, the chances are we probably need to find fossils of entirely soft organisms, and Burgess Shale-type fossilisation provides a way. NASA’s Curiosity rover has the ability to record mineralogy on the Martian surface, so it could potentially look for the types of rocks which might be most conducive to preserving these fossils.’
To expand their understanding of the exceptional preservation of soft organisms, the team are currently delving further back into Earth history, to investigate the preservation of microbes before macroscopic organisms with skeletons or shells evolved.
Reference:
Ross P. Anderson, Nicholas J. Tosca, Robert R. Gaines, Nicolás Mongiardino Koch, Derek E.G. Briggs. A mineralogical signature for Burgess Shale–type fossilization. Geology, 2018; DOI: 10.1130/G39941.1
Footprints of mammoths, dated to 43,000 years ago, are seen in a portion of a trackway that was uncovered by researchers in 2017 in an ancient dry lake bed in Lake County, Oregon. Credit: Photos by Greg Shine, Bureau of Land Management
A fossilized trackway on public lands in Lake County, Oregon, may reveal clues about the ancient family dynamics of Columbian mammoths.
Recently excavated by a team from the University of Oregon Museum of Natural and Cultural History, the Bureau of Land Management and the University of Louisiana, the trackway includes 117 footprints thought to represent a number of adults as well as juvenile and infant mammoths.
Discovered by Museum of Natural and Cultural History paleontologist Greg Retallack during a 2014 class field trip on fossils at the UO, the Ice Age trackway is the focus of a new study appearing online ahead of print in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
Retallack returned to the site with the study’s coauthors, including UO science librarian Dean Walton, in 2017. The team zeroed in on a 20-footprint track, dating to roughly 43,000 years ago, that exhibited some intriguing features.
“These prints were especially close together, and those on the right were more deeply impressed than those on the left-as if an adult mammoth had been limping,” said Retallack, also a professor in the UO Department of Earth Sciences and the study’s lead author.
But, as the study reveals, the limping animal wasn’t alone: Two sets of smaller footprints appeared to be approaching and retreating from the limper’s trackway.
“These juveniles may have been interacting with an injured adult female, returning to her repeatedly throughout the journey, possibly out of concern for her slow progress,” Retallack said. “Such behavior has been observed with wounded adults in modern, matriarchal herds of African elephants.”
The tracks were made in a layer of volcanic soil at Fossil Lake, a site first excavated by UO science professor Thomas Condon in 1876 and today administered by the Bureau of Land Management.
“America’s public lands are some of the world’s greatest outdoor laboratories. Localities such as this mammoth tracksite are unique parts of America’s heritage and indicate that there are many special sites still to be discovered,” said study co-author Brent Breithaupt, a paleontologist in the Wyoming State Office of the Bureau of Land Management.
Specimens from the 1876 Fossil Lake excavation-along with the rest of Condon’s extensive assemblage of fossils and geologic specimens-were donated to UO in the early 1900s and form the core of the museum’s Condon Fossil Collection, now under Retallack’s direction and boasting upwards of 50,000 fossil specimens.
Last month a new state law went into effect, making the UO museum Oregon’s default repository for fossils found on state lands. The museum is also a designated repository for artifacts and paleontological specimens collected from BLM-administered lands in Oregon, ensuring they are available to future generations for education and research.
As part of the 2017 study, Neffra Matthews of the BLM’s National Operations Center in Denver, helped survey, map and document the trackway using photogrammetry, which helps scientists perform accurate measurements based on land-based or aerial photographs.
“There is a vast storehouse of natural history found on BLM-managed land, and it’s exciting to work with researchers like Professor Retallack in capturing 3D data on fragile paleontological resources,” she said.
Retallack said that trace fossils such as trackways can provide unique insights into natural history.
“Tracks sometimes tell more about ancient creatures than their bones, particularly when it comes to their behavior,” he said. “It’s amazing to see this kind of interaction preserved in the fossil record.”
Elephants once roamed across much of North America. Woolly mammoths (Mammuthus primigenius) were common in Canada and Alaska. Columbian mammoths (Mammuthus columbi) occupied the region that became Washington state to South Dakota and south into Mexico. Most mammoths went extinct about 11,500 years ago, but some isolated Arctic island populations of woolly mammoth persisted until 4,000 years ago.
Reference:
Gregory J. Retallack, James E. Martin, Adrian P. Broz, Brent H. Breithaupt, Neffra A. Matthews, Dean P. Walton. Late Pleistocene mammoth trackway from Fossil Lake, Oregon. Palaeogeography, Palaeoclimatology, Palaeoecology, 2018; DOI: 10.1016/j.palaeo.2018.01.037
Note: The above post is reprinted from materials provided by University of Oregon. Original written by Kristin Strommer.
On a certain level, extinction is all about energy. Animals move over their surroundings like pacmen, chomping up resources to fuel their survival. If they gain a certain energy threshold, they reproduce, essentially earning an extra life. If they encounter too many empty patches, they starve, and by the end of the level it’s game over.
Models for extinction risk are necessarily simple. Most reduce complex ecological systems to a linear relationship between resource density and population growth — something that can be broadly applied to infer how much resource loss a species can survive.
This week in Nature Communications, an interdisciplinary team of scientists proposes a more nuanced model for extinction that also shows why animal species tend to evolve toward larger body sizes. The Nutritional State-structured Model (NSM) by ecologist Justin Yeakel (UC Merced), biologist Chris Kempes (Santa Fe Institute), and physicist Sidney Redner (Santa Fe Institute) incorporates body size and metabolic scaling into an extinction model where ‘hungry’ or ‘full’ animals, great and small, interact and procreate on a landscape with limited resources.
“Unlike many previous forager models, this one accounts for body size and metabolic scaling,” Kempes explains. “It allows for predictions about extinction risk, and also gives us a systematic way of assessing how far populations are from their most stable states.”
In the NSM, hungry animals are susceptible to mortality, and only full animals have the capacity to reproduce. Because animals’ energetic needs change with body size, the researchers based their calculations for replenishment and reproduction on biological scaling laws that relate body size to metabolism.
They found that species of different sizes gravitate toward population states most stable against extinction. The states they derived in the model reproduce two oft-observed patterns in biology. The first, Damuth’s law, is an inverse relationship between body size and population density: the bigger the species, the fewer of individuals cohabitate in a given area. Within the NSM, this fewer/larger more/smaller pattern emerges because large species are most stable against starvation in small numbers, while small species can afford to reach larger population densities.
The second relationship, Cope’s rule, holds that terrestrial mammals tend to evolve toward larger body sizes. This NSM shows that, overall, larger animals with slower metabolisms are the most stable against extinction by starvation. It even predicts an energetically “ideal” mammal, robust in the face of starvation, which would be 2.5 times the size of an African elephant.
“As we incorporated more realism into how quickly organisms gain or lose body fat as they find or don’t find resources, the results of our model began aligning with large-scale ecological and evolutionary relationships. Most surprising was the observation that the NSM accurately predicts the maximum mammalian body size observed in the fossil record,” explains Yeakel. Though the model doesn’t account for predation, it does offer a dynamic and systematic framework for understanding how foragers survive on limited resources.
“The dynamics of foraging and the interaction of body size in foraging and resource availability, these are all rich problems for which there is beautiful phenomenology,” says Redner. “I hope some of this will have relevance in managing resources and ensuring species don’t go extinct.”
Reference:
Justin D. Yeakel, Christopher P. Kempes, Sidney Redner. Dynamics of starvation and recovery predict extinction risk and both Damuth’s law and Cope’s rule. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-02822-y
This is seismic reflection data. Credit: Morelia Urlaub and colleagues, and Geology.
The biggest landslides on Earth aren’t on land, but on the seafloor. These mega-slides can move thousands of cubic kilometers of material, and sometimes trigger tsunamis. Yet, remarkably, they occur on nearly flat slopes of less than three degrees.
Morelia Urlaub, a marine geoscientist at the Geomar Helmholtz Center for Ocean Research in Kiel, Germany, voices the obvious question: “How can you fail on a slope that is so flat?” Now, Urlaub and colleagues may have discovered the answer. The smoking — or in this case, oozing — gun is a layer of siliceous microfossils called diatoms.
The study, published online ahead of print for the Geological Society of America’s journal Geology, is the first to identify the weak layer responsible for a submarine mega-slide. Although the nature of these critical weak layers has been highly debated, studying them has been nearly impossible because they are typically destroyed along with the slides.
Urlaub was compiling ocean drilling data from 1980 when she realized that the core sampled the seafloor just outside the Cap Blanc slide, a 149,000 year-old mega-slide off the coast of northwest Africa. She correlated that data with high resolution seismic reflection data recorded in the same area in 2009. Together, these data revealed diatom-rich layers, up to ten meters thick, that traced directly from the core to the base of slide layers within the mega-slide complex.
What’s more, each diatom layer was topped by a layer of clay-rich sediment. That clay is apparently key. “Diatom layers are very compressible and water rich,” Urlaub says. As pressure builds, she explains, water would be squeezed from the diatom layer into the clay. Ultimately the clay or the interface between the clay and diatoms fails, sending the sediments above sliding.
At the Cap Blanc slide, the seafloor slopes at just 2.8 degrees. Yet when it broke loose, the slide transported over 30 cubic kilometers of material, and extended at least 35 kilometers. Another submarine mega-slide 8,500 years ago off Norway moved a staggering 3,000 cubic kilometers, causing a damaging tsunami. And some scientists speculate that the 2011 Tohoku tsunami in Japan may have been amplified by a submarine mega-slide.
Although such slides don’t occur very often, says Urlaub, their size makes them quite significant. “One-fifth of all tsunamis may be caused by undersea mega-slides,” she says. If diatom layers are a major factor, then understanding where paleoclimate conditions may have favored diatom growth might help reveal potential mega-slide sites.
Reference:
Diatom ooze: Crucial for the generation of submarine megaslides?
M. Urlaub, Jacob Geersen, Sebastian Krastel, and Tilmann Schwenk. Geology: DOI: 10.1130/G39892.1
Carbon levels around 3 million years ago were similar to those of today and temperatures were even warmer. If something so significant is mirrored in the past, what else can we learn about extreme climate changes?
Three million years ago the Earth’s climate was warm enough to permit a forested High Arctic inhabited by large mammals. If the idea of melting icebergs, rising sea levels and 400 parts per million of carbon dioxide in the atmosphere sounds all too familiar – welcome to the Pliocene.
For many researchers, the Pliocene, which lasted from 5.3 million to 2.6 million years ago, is our best reference for today’s warming. It was the last time atmospheric CO2 levels were similar to today’s, trapping heat and raising global temperatures to above the levels Earth is experiencing now. A better understanding of the response of the ice sheets to increasing temperature is needed to make more rigorous projections of how much sea level change could be expected in the future.
We live in uncertain times when it comes to the impact of climate change and global warming, so any insights we can gain from the past is an area of scientific interest. EU support under the PLIOTRANS fellowship is helping to further our understanding of the responses of the ice sheets to a warming climate.
When it comes to ice sheets, one size does not fit all
Recent research by a team of scientists, including PLIOTRANS, has been considering how the planet responded to Pliocene warmth. They have published a new paper presenting, for the first time, the transient nature of ice sheets and sea level during the late Pliocene. They show that the Greenland and Antarctic ice sheets might have responded differently to Pliocene heat, melting at different times.
Their transient ice sheet predictions are forced by multiple climate snapshots derived from a climate model set up with late Pliocene boundary conditions with different orbital forcing scenarios appropriate to two Marine Isotope Stages (MISs): KM5c (from 3.226 to 3.184 million years ago), and K1 (from 3.082 to 3.038 million years ago).
Their findings support previous studies, which have shown model results indicate peak MIS KM5c and K1 interglacial temperatures were not globally synchronous: there are leads and lags in temperature in different regions.
When it comes to modeling, this highlights the potential pitfalls of aligning peaks in proxy-derived temperatures across geographically diverse data sites. A single climate model simulation for an interglacial event is inadequate to capture peak temperature change in all regions.
The team explains, ‘We present a first step toward a fully coupled system of ice volume and climate variability across the late Pliocene (…) The model simulations presented here attempt to capture the transient response of climate and ice volume to orbital variations.’
The shape of the Earth’s orbit, the tilt of its axis and the fact that it wobbles, all have a part to play
The episodic nature of the Earth’s glacial and interglacial periods within the present Ice Age (the last couple of million years) have been caused primarily by cyclical changes in the Earth’s circumnavigation of the Sun. The study found that when the cyclical change known as precession variability is large, caution is advised when directly inferring the behaviour of ice sheets from oxygen isotope records in the Pliocene.
Their simulations indicate that the asynchronous response of ice sheets, combined with their transient modelling, is indeed a key factor in predicting orbital timescale sea level for a climate that is warmer than ours is now.
The PLIOTRANS (PLIOcene TRANSient Climate Modelling: Towards a global consensus between ice volume, temperature and relative sea level for the Late Pliocene) fellowship ended last year. Its goal was to reduce the uncertainties associated with future projections of sea-level change.
Note: The above post is reprinted from materials provided by CORDIS.
More than 100 species of cockroaches were used in the new genomic study. Credit: Cameron Richardson and Nathan Lo
Cockroaches are so hardy, a popular joke goes, that they’ve occupied the Earth long before humans first appeared — and will probably even outlast us long after we have annihilated each other by nuclear war.
But now, researchers have used the latest in genomic data to gain the most detailed information yet of their evolutionary history.
Armed with a vast amount of genomic information, a team of researchers led by Dr. Thomas Bourguignon, now professor at the Okinawa Institute of Science and Technology, has performed the first molecular dating to gain the clearest picture yet of the biogeographical history of cockroaches.
They have traced back the key evolutionary time points of the cockroach — all the way back almost 300 million years ago when the Earth’s mass was organized into the Pangaea supercontinent.
This fossil record of cockroaches suggests that most extant families evolved during the breakup of Pangaea (which began ~200 Ma) and prior to the beginning of continental separation within Gondwana (~135 Ma).
“Our results indicate that extant cockroach families have evolved over periods of up to ~180 million years,” said Bourguignon. “Through reconstructions of the ancestral distribution of cockroaches using the known distributions of extant genera sampled in this study, we found evidence that continental breakup has had important impacts on cockroach biogeography.”
To do so, they estimated divergence times of all living cockroach families, based on the complete mitochondrial genomes of 119 cockroach species (and to help their molecular dating, compared with 13 termites, seven mantis and multiple other outgroups).
Their estimates indicate that the last common ancestor for cockroaches appeared much earlier than fossil evidence, around 235 million years ago. This was about 95 million years before the appearance of the first fossils attributed to modern cockroaches during the Cretaceous period around 140 million years ago, and before Pangaea broke up.
Since cockroaches can’t fly very far, and for the most part would be terrestrially bound, one of the more appealing aspects of the study was to compare the cockroach divergence time with the geological history of the Earth.
The authors speculate that, like riding a raft, cockroaches spread to every part of the globe through the seismic continental drifts that occurred during the transition from Pangaea. This is illustrated by many sister cockroach lineages, which diverged prior Gondwana breakup, and diversified on their respective continental plates. But, in addition, within younger cockroach lineages, they did find evidence of transoceanic dispersal in regions near Australia and Indo-Malaysia.
“We believe that our results point to an important role for vicariance (continental drift) in determining the global distributions of cockroaches,” said Bourguignon. “On a global scale, the fossil record also agrees with our hypothesis.”
The study underscores the importance of continental drift in shaping modern insect distribution, and will provide a new framework for future cockroach biogeographical research.
Reference:
Thomas Bourguignon, Tang Qian, Simon Y W Ho, Frantisek Juna, Zongqing Wang, Daej A Arab, Stephen L Cameron, James Walker, David Rentz, Theodore A Evans, Nathan Lo. Transoceanic dispersal and plate tectonics shaped global cockroach distributions: evidence from mitochondrial phylogenomics. Molecular Biology and Evolution, 2018; DOI: 10.1093/molbev/msy013
Forget rubies, garnets and sapphires. Fluorite may be the world’s most colourful mineral, because of the enormous range of brilliant and even iridescent colours it displays.
The funny thing is, pure fluorite crystals are transparent.
A crystal’s colour is dictated by the way light interacts with the chemicals in it, and by how these are bonded in an orderly structure, or lattice. Any impurities that work their way into fluorite’s lattice can alter its apparent colour. For example, manganese ions turn it orange.
Structural defects within the lattice, known as colour centres, have a similar effect.
Fluorite’s hallmark deep purple hue is the result of a small number of fluoride ions being permanently forced out of their lattice positions by irradiation or heating. When they move, an electron is left behind in each hole. When light hits the crystal, it is absorbed and re-emitted by these electrons, producing the colour we see.
Some fluorite specimens even have bands of different colours.
Fluorite forms in hydrothermal veins in the Earth’s crust and in cavities in sedimentary rocks. Over the centuries, these fissures are constantly opening and closing, sometimes cutting off the fluids needed for fluorite to form. It’s the subtle changes in the chemistry of these fluids that causes colour zoning in the crystals as they grow.
Selenite
Giant selenite crystals in the Cueva de los Cristales (Credit: Javier Trueba/MSF/NPL)
Buried beneath the Sierra de Naica mountain in Chihuahua, northern Mexico, the Cueva de los Cristales (Cave of Crystals) is home to the largest crystals on planet Earth.
Gargantuan, milky white beams of selenite, some as long as 11m and more than 1m wide, criss-cross the underground chamber. “There is no other place on the planet where the mineral world reveals itself in such beauty,” says Juan Manuel García-Ruiz of the University of Granada in Spain, a geologist who studies the crystals.
The crystals were discovered in 2000 by two brothers excavating new tunnels in the Naica mine, in search of fresh reserves of zinc, silver, and lead.
The cavity, which measures about 10m by 30m, had previously been flooded with heated water. Only when the miners started pumping it out were the monumental structures revealed.
In 2007, García-Ruiz and his team figured out how the crystals were able to grow so big.
Around 26 million years ago, volcanic activity beneath the mine filled the cave with hot water rich in the mineral anhydrite. Anhydrite is stable above 58 °C, but as the underlying magma cooled, it dissolved into the surrounding water.
Very slowly, over hundreds of thousands of years, its chemical components reassembled as gypsum, which can take the form of crystals. Large elongate crystals of gypsum are known as selenite.
Within the Cueva de los Cristales, the temperature has consistently hovered around the magic 58 °C mark ever since.
Another crystal cave, discovered closer to the surface in Naica, also contains selenites. They are still spectacular at about 1m in length, but not as large as those of the Cueva de los Cristales, because this cave cooled faster.
Iceland Spar
Iceland spar is a special form of calcite (Credit: Natural History Museum, London/SPL)
The Icelandic sagas of the 10th century record the details of Viking voyages. They describe a mysterious “sunstone”, which Scandinavian seafarers used to locate the Sun in the sky and navigate on cloudy days.
The identity of the stone stumped scholars for centuries, but in 2011 a convincing candidate was put forward: Iceland spar.
This clear variety of calcite is common in Nordic regions. It bends light by two different amounts, producing a double image (see the picture above).
This property is called birefringence. It’s caused by discrepancies in the binding forces that hold the atoms of the crystal together. The forces are stronger in some directions than others.
When light passes through calcite crystals, it is split into two rays. The asymmetry in the crystal’s structure causes the paths of these two beams to be bent by different amounts, resulting in a double image.
How did that help the Vikings? Researchers studied a piece of Iceland spar discovered aboard an Elizabethan ship that sunk in 1592. They found that moving the stone in and out of a person’s field of vision causes them to see a distinctive double dot pattern that lines up with the direction of the hidden Sun.
Quartz
Quartz is one of the most common crystals on Earth (Credit: Sinclair Stammers/SPL)
Quartz also does interesting things because of its structural asymmetries.
If you squeeze a crystal of quartz, it generates a tiny electric current. The pressure on the crystal’s surface forces ions within it to move out of position, upsetting the overall charge balance and turning the crystal into a tiny battery, with oppositely-charged faces.
The phenomenon is known as the piezoelectric effect, and it also works in reverse. Pass an electric current through a quartz crystal, and it will squeeze itself.
Quartz watches use tiny slivers of cut quartz as oscillators to keep precise time. Electricity from the watch battery causes the crystal to oscillate thousands of times per second, and circuits in the watch convert these oscillations to a once-per-second digital beat.
Quartz was also central to our developing understanding of crystals. In 1669, Danish scientist Nicolas Steno noticed that quartz crystals, irrespective of where on Earth they were found, always showed the same angles between similar crystal faces.
By the turn of the 19th century, French crystallographer René Just Haüy had extended this idea. He realised that the same rules underlie the shapes and angles of all crystals.
We now understand that the shapes of crystals are an expression, on a grand scale, of the orderly lattices in which their constituent atoms are arranged.
Galena
Galena is both a source of lead, and the makings of a radio (Credit: Martin Land/SPL)
Galena is the most common lead-rich mineral, and an important ore of both lead and silver. But that’s just its day job.
It’s the crystal’s ability to extract music and voices from radio waves that makes it truly beguiling. It put galena centre stage in the revolutionary crystal radio sets of the early 1900s.
Galena is a semiconductor, meaning that it will conduct electricity under certain circumstances. In metals, free electrons flow as electricity when a voltage is applied. In galena – a non-metal – small crystal impurities or imbalances in its chemical proportions create a situation where, if electrons can be excited enough, they can be ripped from their atoms and made to flow.
In a crystal radio set, a fine metal wire known as a “cat’s whisker” rests delicately on the surface of a galena crystal. This combination allows current to pass easily in one direction but not the other. This converts the oscillating radio waves picked up by an antenna into an electric signal that can be transformed into sound by speakers.
Not every position on the crystal will perform, so fiddling with the cat’s whisker to find a sweet spot takes patience and skill.
Extra-terrestrial carbon crystals
Diamond is the hardest known natural material on Earth. It is the industry standard for grinding, cutting, drilling and polishing jobs.
But two new kinds of ultra-hard carbon crystals, found embedded in a Finnish meteorite in 2010, put the precious stone to shame.
The Haverö meteorite crashed to Earth in 1971. When researchers used diamond paste to polish a slice, they noticed something extraordinary: small pockets of material emerging in relief from the surface. When they analysed the stubborn crystals, they discovered two completely new forms of carbon.
Diamond is so hard because the carbon atoms inside it are arranged in a tetrahedron-shaped lattice that is immensely strong. In Haverö, the researchers found crystalline carbon arranged in a rhombohedral lattice. This type of diamond was predicted to exist decades ago but had never been seen in nature.
The second substance turned out to be a totally new kind of crystalline carbon, which the researchers call “an intermediate between graphite and diamond”.
Graphite, like diamond, is made up entirely of carbon atoms. However, its atoms are arranged in honeycomb-like sheets. The sheets are only weakly attracted to each other, making it soft and slippery.
When the meteorite entered Earth’s atmosphere, the researchers think pressure shocks and intense heat fused sheets of graphite together, much like the way labs make artificial diamonds.
Unfortunately, the crystals are so small that no one has been able to test the limits of their hardness, nor compare them with the artificial ultra-hard diamonds lonsdaleite and boron nitride.
Autunite
Autunite fluoresces under ultraviolet light (Credit: Joel Arem/SPL)
Autunite is a mineral that the big kid in everyone can get excited about. Its tablet-shaped crystals look like lurid yellow-green scales, its uranium content makes it radioactive, and – the icing on the cake of cool – it fluoresces.
When ultraviolet light shines on an autunite crystal, it imparts energy to electrons within the crystal’s uranium atoms. Each excited electron jumps momentarily away from the nucleus of its atom, then falls back.
When the electrons drop back, they release bursts of visible light. The collective effect makes autunite appear to glow green.
Fluorescent minerals stop glowing when the ultraviolet light source is removed. Other minerals are phosphorescent: the electrons remain in an excited state for longer, so phosphorescent minerals continue to glow for a while even after the light is turned off.
Sugar
Give a sugar crystal a hard whack and it glows blue (Credit: Ted Kinsman/SPL)
Want to see a crystal glow, but don’t have access to a mineral library? No problem.
Get yourself some sugar cubes or polo mints, go to a pitch-black room, and use the bottom of a glass to smash them to pieces. You should see a fleeting faint blue glow emanate from the sugary treats. This is called triboluminescence.
Literally meaning “rub light”, it was first noted by 17th Century polymath Francis Bacon. Later, Robert Boyle observed that: “hard sugar being nimbly scraped with a knife would afford a sparkling light”.
Centuries later, quite how sugar can be triboluminescent is still a mystery.
Current theories postulate that when sugar crystals are scraped, fractured or crushed, their structural asymmetry encourages tiny piezoelectric fields to form. This separates positive and negative charges within the crystal, and when these charges recombine, a spark flies. Then, nitrogen molecules trapped within the crystals absorb this energy and luminesce, much as they do during a lightning storm.
If that’s true, triboluminescence is almost literally a storm in a teacup.
Biophotonic crystal
The spines of this sea mouse (Aphrodita sp.) are photonic (Credit: James King-Holmes/SPL)
Photonic crystals are tiny repeating structures, each about a billionth of a metre across. They can control and manipulate how light flows.
Depending on the angles of its faces, a photonic crystal will only allow certain wavelengths of light through, and blocks all the others. This determines its colour.
The blocked wavelengths are called “photonic band gaps”. Wavelengths near these band gaps tend to scatter and interfere with one another. This is what creates the vivid colours and striking iridescence of some insects, particularly butterflies and beetles, whose colours appear to change depending on the angle they’re viewed from.
Humans can make simple photonic crystals from synthetic polymers. We use them to create things like reflective coatings for sunglasses.
If we could only duplicate the most complex photonic structures – like those seen in beetles, butterflies, bees and spiders – we could use them to improve everything from fibre-optic technologies to solar cells.
So far, engineers have struggled to build precisely-organised three-dimensional structures on usable scales. However, new research into the way biophotonic crystals take shape in insects offers some promising pointers.
Volcanic ice crystals
Snow-like ice crystals in the caves of Mount Erebus (Credit: Chadden Hunter/NPL)
Mount Erebus in Antarctica is the southernmost active volcano in the world. Dotted around its summit is a network of ice caves, which harbour fragile ice formations that occur nowhere else on the planet.
The labyrinth of passages is carved into the snowpack by hot gases from the volcano, which seep out through cracks and fissures in the underlying rock. Within the caves, the warm, steamy air from the volcano hits the frigid walls, whereupon the moisture freezes into intricate, feathery shapes, guided by the air currents.
The resulting crystals look like clusters of snowflakes.
Craig Cary of the University of Waikato in New Zealand has spent time in the caves and was struck by the delicacy of the ice formations. “They hang down maybe half a metre from the ice ceiling, and it only takes the wind generated by a slowly passing body underneath to cause them to fall,” he says.
The crystals are an example of hoarfrost, which is formed when moisture condenses and freezes directly onto objects.
When ice grows slowly, as it does in liquid water, it forms solid hexagonal crystals. But if the water vapour is particularly thick, and there is space to grow, the ice will instead grow into the hexagonally symmetrical branching forms seen at Erebus.
Note: The above post is reprinted from materials provided by BBC Earth. The original article was written by Ceri Perkins.
Colored and black points mark the global distribution of mid-ocean ridges, with ages of 66 million years ago created at spreading rates above and below 35 millimeters a year, respectively. Colors indicate the maximum gravity anomaly within 2 degrees. Credit: Courtesy of Joseph Byrnes
The debate goes on: What killed off the dinosaurs?
New University of Oregon research has identified gravity-related fluctuations dating to 66 million years ago along deep ocean ridges that point to a “one-two punch” from the big meteor that struck off Mexico’s Yucatan peninsula, possibly triggering a worldwide release of volcanic magma that could have helped seal the dinosaurs’ fate.
“We found evidence for a previously unknown period of globally heighted volcanic activity during the mass-extinction event,” said former UO doctoral student Joseph Byrnes.
The study by Byrnes and Leif Karlstrom, a professor in the UO’s Department of Earth Sciences, was published Feb. 7 in Science Advances. It details a record of volcanism preserved along the mid-ocean ridges, which mark the oceanic boundaries of tectonic plates. The evidence comes from changes in the strength of gravity above the seafloor.
The findings of the UO’s National Science Foundation-supported study, Karlstrom said, point to a pulse of accelerated worldwide volcanic activity that includes enhanced eruptions at India’s Deccan Traps after the Chicxulub impact. The Deccan Traps, in west-central India, formed during a period of massive eruptions that poured out layers of molten rock thousands of feet deep, creating one of the largest volcanic features on Earth.
The Deccan Traps region has been in and out of the dinosaur debate. Rare volcanic events at such a scale are known to cause catastrophic disturbances to Earth’s climate, and, when they occur, they are often linked to mass extinctions. Huge volcanic events can eject so much ash and gas into the atmosphere that few plants survive, disrupting the food chain and causing animals to go extinct.
Since evidence of the meteor strike near present-day Chicxulub, Mexico, surfaced in the 1980s, scientists have debated whether the meteor or the Deccan Traps eruptions drove the extinction event that killed off all nonavian dinosaurs.
Progressively improving dating methods indicate that the Deccan Traps volcanoes already were active when the meteor struck. Resulting seismic waves moving through the planet from the meteor strike, Karlstrom said, probably fueled an acceleration of those eruptions.
“Our work suggests a connection between these exceedingly rare and catastrophic events, distributed over the entire planet,” Karlstrom said. “The meteorite’s impact may have influenced volcanic eruptions that were already going on, making for a one-two punch.”
That idea gained strength in 2015 when researchers at the University of California, Berkeley, proposed that the two events might be connected. That team, which included Karlstrom, suggested that the meteorite may have modulated distant volcanism by generating powerful seismic waves that produced shaking worldwide.
Similar to the impacts that normal tectonic earthquakes sometimes have on wells and streams, Karlstrom said, the study proposed that seismic shaking liberated magma stored in the mantle beneath the Deccan Traps and caused the largest eruptions there.
The new findings at the UO extend this eruption-triggering in India to ocean basins worldwide.
Byrnes, now a postdoctoral researcher at the University of Minnesota, analyzed publicly available global data sets on free-air gravity, ocean floor topography and tectonic spreading rates.
In his analyses, he divided the seafloor into 1-million-year-old groupings, constructing a record back to 100 million years ago. At about 66 million years, he found evidence for a “short-lived pulse of marine magmatism” along ancient ocean ridges. This pulse is suggested by a spike in the rate of the occurrence of free-air gravity anomalies seen in the data set.
Free-air gravity anomalies, measured in tiny increments call milligals, account for variations in gravitational acceleration, found from satellite measurements of additional seawater collecting where the Earth’s gravity is stronger. Byrnes found changes in free-air gravity anomalies of between five and 20 milligals associated with seafloor created in the first million years after the meteor.
Reference:
Joseph S. Byrnes, Leif Karlstrom. Anomalous K-Pg–aged seafloor attributed to impact-induced mid-ocean ridge magmatism. Science Advances, 2018; 4 (2): eaao2994 DOI: 10.1126/sciadv.aao2994
Note: The above post is reprinted from materials provided by University of Oregon. Original written by Jim Barlow, University Communications.
Underwater terrain revealed by the survey and points surveyed. Credit: Kobe University
Since the Kobe Ocean Bottom Exploration Center (KOBEC) was established in 2015, it has carried out three survey voyages to the Kikai Caldera, south of Japan’s main islands. Based on these voyages, researchers have confirmed that a giant lava dome was created after the caldera-forming supereruption 7300 years ago. The dome is in the world’s largest class of post-caldera volcano, with a volume of over 32 cubic kilometers. The composition of this lava dome is different from the magma that caused the giant caldera to erupt – it shows the same chemical characteristics as the current post-caldera volcano on the nearby Satsuma Iwo-jima Island. It is possible that currently a giant magma buildup may exist under the Kikai Caldera.
These findings were published in the online edition of Scientific Reports on February 9.
There is roughly a 1 percent chance of a giant caldera-forming eruption occurring within the Japanese archipelago during the next 100 years. An eruption like this would see over 40 cubic kilometers of magma released in one burst, causing enormous damage. The mechanism behind this and how to predict this event are urgent questions.
Researchers equipped training ship Fukae Maru, part of the Kobe University Graduate School of Maritime Sciences, with the latest observation equipment to survey the Kikai Caldera. They chose this volcano for two main reasons. Firstly, for land-based volcanoes it is hard to carry out large-scale observations using artificial earthquakes because of the population density, and it is also difficult to detect giant magma buildups with precise visualization because they are often at relatively low depths (roughly 10km). Secondly, the Kikai Caldera caused the most recent giant caldera-forming eruption in the Japanese archipelago (7300 years ago), and there is a high possibility that a large buildup of magma may exist inside it.
During the three survey voyages, KOBEC carried out detailed underwater geological surveys, seismic reflection, observations by underwater robots, samples and analysis of rocks, and observations using underwater seismographs and electromagnetometers.
In their upcoming March 2018 voyage, researchers plan to use seismic reflection and underwater robots to clarify the formation process of the double caldera revealed in previous surveys and the mechanism that causes a giant caldera eruption.
They will also use seismic and electromagnetic methods to determine the existence of a giant magma buildup, and in collaboration with the Japan Agency for Marine-Earth Science and Technology will carry out a large-scale underground survey, attempting to capture high-resolution visualizations of the magma system within the Earth’s crust (at a depth of approximately 30km). Based on results from these surveys, the team plans to continue monitoring and aims to pioneer a method for predicting giant caldera-forming eruptions.
Formation of metallic ore deposits are predicted to accompany the underwater hydrothermal activity, so the team also plan to evaluate these undersea resources.
Reference:
“Giant rhyolite lava dome formation after 7.3 ka supereruption at Kikai caldera, SW Japan” DOI:10.1038/s41598-018-21066-w
Earthquake simulation. Credit: Lawrence Livermore National Laboratory
In the next 30 years, there is a one-in-three chance that the Hayward fault will rupture with a 6.7 magnitude or higher earthquake, according to the United States Geologic Survey (USGS). Such an earthquake will cause widespread damage to structures, transportation and utilities, as well as economic and social disruption in the East Bay.
Lawrence Livermore and Lawrence Berkeley national laboratory scientists have used some of the world’s most powerful supercomputers to model ground shaking for a magnitude (M) 7.0 earthquake on the Hayward fault and show more realistic motions than ever before. The research appears in Geophysical Research Letters.
Past simulations resolved ground motions from low frequencies up to 0.5-1 Hertz (vibrations per second). The new simulations are resolved up to 4-5 Hertz (Hz), representing a four to eight times increase in the resolved frequencies. Motions with these frequencies can be used to evaluate how buildings respond to shaking
The simulations rely on the LLNL-developed SW4 seismic simulation program and the current best representation of the three-dimensional (3D) earth (geology and surface topography from the USGS) to compute seismic wave ground shaking throughout the San Francisco Bay Area. Importantly, the results are, on average, consistent with models based on actual recorded earthquake motions from around the world.
“This study shows that powerful supercomputing can be used to calculate earthquake shaking on a large, regional scale with more realism than we’ve ever been able to produce before,” said Artie Rodgers, LLNL seismologist and lead author of the paper.
The Hayward fault is a major strike-slip fault on the eastern side of the Bay Area. This fault is capable of M 7 earthquakes and presents significant ground motion hazard to the heavily populated East Bay, including the cities of Oakland, Berkeley, Hayward and Fremont. The last major rupture occured in 1868 with an M 6.8-7.0 event. Instrumental observations of this earthquake were not available at the time, however historical reports from the few thousand people who lived in the East Bay at the time indicate major damage to structures.
The recent study reports ground motions simulated for a so-called scenario earthquake, one of many possibilities.
“We’re not expecting to forecast the specifics of shaking from a future M 7 Hayward fault earthquake, but this study demonstrates that fully deterministic 3D simulations with frequencies up to 4 Hz are now possible. We get good agreement with ground motion models derived from actual recordings and we can investigate the impact of source, path and site effects on ground motions,” Rodgers said.
As these simulations become easier with improvements in SW4 and computing power, the team will sample a range of possible ruptures and investigate how motions vary. The team also is working on improvements to SW4 that will enable simulations to 8-10 Hz for even more realistic motions.
For residents of the East Bay, the simulations specifically show stronger ground motions on the eastern side of the fault (Orinda, Moraga) compared to the western side (Berkeley, Oakland). This results from different geologic materials — deep weaker sedimentary rocks that form the East Bay Hills. Evaluation and improvement of the current 3D earth model is the subject of current research, for example using the Jan. 4, 2018 M 4.4 Berkeley earthquake that was widely felt around the northern Hayward fault.
Ground motion simulations of large earthquakes are gaining acceptance as computational methods improve, computing resources become more powerful and representations 3D earth structure and earthquake sources become more realistic.
Rodgers adds: “It’s essential to demonstrate that high-performance computing simulations can generate realistic results and our team will work with engineers to evaluate the computed motions, so they can be used to understand the resulting distribution of risk to infrastructure and ultimately to design safer energy systems, buildlings and other infrastructure.”
Other Livermore authors include seismologist Arben Pitarka, mathematicians Anders Petersson and Bjorn Sjogreen, along with project leader and structural engineer David McCallen of the University of California Office of the President and LBNL.
This work is part of the DOE’s Exascale Computing Project (ECP). The ECP is focused on accelerating the delivery of a capable exascale computing ecosystem that delivers 50 times more computational science and data analytic application power than possible with DOE HPC systems such as Titan (ORNL) and Sequoia (LLNL), with the goal to launch a U.S. exascale ecosystem by 2021. The ECP is a collaborative effort of two Department of Energy organizations — the DOE Office of Science and the National Nuclear Security Administration.
Simulations were performed using a Computing Grand Challenge allocation on the Quartz supercomputer at LLNL and with an Exascale Computing Project allocation on Cori Phase-2 at the National Energy Research Scientific Computing Center (NERSC) at LBNL.
Reference:
Arthur J. Rodgers, Arben Pitarka, N. Anders Petersson, Björn Sjögreen, David B. McCallen. Broadband (0-4 Hz) Ground Motions for a Magnitude 7.0 Hayward Fault Earthquake With Three-Dimensional Structure and Topography. Geophysical Research Letters, 2018; DOI: 10.1002/2017GL076505
An image of a insect fossilized in amber. Credit: University of Otago
The discovery of fossil insects, nematodes and fungi preserved in amber from sites in Otago is shedding new light on New Zealand’s geological and biological history.
University of Otago paleontologists Associate Professor Daphne Lee and Dr. Uwe Kaulfuss, with Professor Alexander Schmidt of the University of Göttingen, co-led a team of international scientists in collecting and analyzing amber deposits from more than 30 sites throughout New Zealand.
The small and fragile fossils are 25 to 15 million years old and include a number of spiders (including web remains with prey), tiny carnivores such as pseudoscorpions, diverse soil-dwelling mites, detritivores such as springtails, biting and gall midges, fungus gnats and chironomids, scale insects, parasitoid wasps, ants, beetles, and bark lice.
“Some of the arthropods and fungi represent the first fossil records of their groups from the entire Southern Hemisphere,” Associate Professor Lee explains.
Hundreds of kilograms of amber were extracted from lignite deposits, largely near Roxburgh, Hyde and Pomahaka, in Otago. Preparation of the commonly opaque, and often brittle and/or fractured amber to expose inclusions for study is challenging. However, new techniques developed in Professor Schmidt’s laboratory in Germany revealed numerous fossils with 3-D preservation.
The amber derives from the ancestors of the kauri, resin-producing conifers belonging to the Araucariaceae family which still live today in northern New Zealand.
“This means that the source of the resin has remained unchanged for at least the past 25 million years. The amber fossils help in understanding the evolution of these long-lasting forest ecosystems on a geologic time scale,” Associate Professor Lee adds.
Amber, fossilized tree resin, preserves life forms, providing access to delicate organisms that are otherwise rare or absent from the fossil record.
Amber deposits are concentrated in the Northern Hemisphere where their inclusions have been studied intensively. Until now, the scarcity of major deposits from the Southern Hemisphere has severely hampered understanding of the global evolutionary history of terrestrial invertebrate and fungal biotas.
Dr. Kaulfuss says the fossils are significant because of what they tell us about the country’s ecological history, as a long-isolated former Gondwanan landmass.
“These fossils are really important for us because they provide a very rare opportunity to look back on what made up New Zealand’s forest and ecosystem 25 million-years-ago.
“We now know what kind of animals and plants were around at that time and what has gone extinct since then.”
Reference:
Alexander R. Schmidt et al. Amber inclusions from New Zealand, Gondwana Research (2017). DOI: 10.1016/j.gr.2017.12.003
Researchers at the University of Birmingham have discovered that the mass extinction seen in plant species caused by the onset of a drier climate 307 million years ago led to extinctions of some groups of tetrapods, the first vertebrates to live on land, but allowed others to expand across the globe. This research is published today (7th February 2018) in the journal Proceedings of the Royal Society B.
The Carboniferous and Permian periods (358 — 272 million years ago) were critical intervals in the evolution of life on land. During the Carboniferous Period North America and Europe lay in a single land mass at the equator which was covered by dense tropical rainforests. These rainforests flourished because of the warm humid climate, providing an ideal habitat for early tetrapods (vertebrates with four limbs), allowing them to diversify into a variety of species.
But towards the end of this period a major global environment change took place — just as the number of tetrapod species began to increase, the rainforests started to disappear. The climate became much drier causing the mass extinction of many species within the dominant plant groups, such as horsetails and club mosses. Despite this being a catastrophic event for plants, it has been unclear how this affected the early tetrapod community.
Previous attempts to estimate the diversity changes during this period have been hindered by the fossil record, which has not been sampled equally in different time intervals or geographic areas. To fill these gaps in the data, the Birmingham researchers compiled a new dataset from the Paleobiology Database and used advanced statistical methods to estimate diversity and biogeographic changes.
The results of the study show that tetrapod diversity decreased after the rainforest collapse and the onset of drier conditions, largely due to the reduction in suitable habitats for amphibians which needed wet environments to survive.
However they also found that after the rainforest collapse surviving tetrapod species began to disperse more freely across the globe, colonising new habitats further from the equator. Many of these survivors were early amniotes, such as early reptiles, whose generally larger size relative to early amphibians allowed them to travel longer distances, and their ability to lay eggs meant they were not confined to watery habitats.
Emma Dunne, from the University of Birmingham’s School of Geography, Earth and Environmental Sciences, said: ‘This is the most comprehensive survey ever undertaken on early tetrapod evolution, and uses many newly developed techniques for estimating diversity patterns of species from fossil records, allowing us greater insights into how early tetrapods responded to the changes in their environment.’
Dunne continued: ‘We now know that the rainiforest collapse was crucial in paving the way for amniotes, the group which ultimately gave rise to modern mammals, reptiles and birds, to become the dominant group of land vertebrates during the Permian period and beyond.’
Reference:
Dunne E, Close R, Button D, Brocklehurst N, Cashmore D, Lloyd G, Butler R. Diversity change during the rise of tetrapods and the impact of the ‘Carboniferous Rainforest Collapse’. Proceedings of the Royal Society B, January 19, 2018 DOI: 10.5061/dryad.n4k45
It is widely accepted that the Earth’s inner core formed about a billion years ago when a solid, super-hot iron nugget spontaneously began to crystallize inside a 4,200-mile-wide ball of liquid metal at the planet’s center.
One problem: That’s not possible-or, at least, has never been easily explained-according to a new paper published in Earth and Planetary Science Letters from a team of scientists at Case Western Reserve University.
The research team-comprised of post-doctoral student Ludovic Huguet; Earth, Environmental, and Planetary Sciences professors James Van Orman and Steven Hauck II; and Materials Science and Engineering Professor Matthew Willard-refer to this enigma as the “inner-core nucleation paradox.”
That paradox goes like this: Scientists have known for more than 80 years that a crystallized inner core exists. But the Case Western Reserve team asserts that this widely accepted idea neglects one critical point-one that, once added, would suggest the inner core shouldn’t exist.
The inner core contradiction
Here’s why: While it is well known that a material must be at or below its freezing temperature to be solid, it turns out that making the first crystal from a liquid takes extra energy. That extra energy-the nucleation barrier-is the ingredient that models of Earth’s deepest interior have not included until now.
To overcome the nucleation barrier and start to solidify, however, the liquid has to be cooled well below its freezing point-what scientists call “supercooling.”
Alternatively, something different has to be added to the liquid metal of the core-at the center of the planet-that substantially reduces the amount of required supercooling.
But the nucleation barrier for metal-at the extraordinary pressures at the center of the Earth-is enormous.
“Everyone, ourselves included, seemed to be missing this big problem-that metals don’t start crystallizing instantly unless something is there that lowers the energy barrier a lot,” Hauck said.
The Case Western Reserve team contends the most obvious solutions are suspect:
” That the inner core was somehow subjected to a massive supercooling of about 1,800 degrees Fahrenheit (1,000 Kelvin)-well beyond the amount of cooling scientists have concluded. If the Earth’s center had reached this temperature, nearly the entire core should be crystallizing rapidly, but the evidence indicates that it is not.
“That something happened to lower the nucleation barrier, allowing crystallization to occur at a higher temperature. Scientists do this in the lab by adding a piece of solid metal to a slightly supercooled liquid metal, causing the now-heterogeneous material to quickly solidify. But it’s difficult to figure on an earth-sized scale how this could have happened, how a nucleation enhancing solid could have found its way to the center of the planet to allow for the hardening (and expansion) of the inner core, Huguet said.
“So, if the core is a pure (homogenous) liquid, the inner core shouldn’t exist at all because it could not have been supercooled to that extent,” Van Orman said. “And if it’s not homogeneous, how did it become so?
“That’s the inner-core nucleation paradox.”
Possible answers
Then how did the solid inner core form?
At the moment, the team’s favored idea is akin to the second solution above: that large bodies of solid metal slowly dropped from the rocky mantle and into the core to lower the nucleation barrier.
But that would require a massive nugget-maybe the size of a large city-to be heavy enough to drop through the mantle and then large enough to make it the core without entirely dissolving.
If that’s the case, “we need to figure out how that could actually happen,” Van Orman said.
“On the other hand,” he said, “is there some ordinary feature of planetary cores that we have not thought of before-something that allows them to overcome that nucleation barrier?
“It’s time for the whole community to think about this problem and how to test it. The inner core exists, and now we have to figure out how it got there.”
This new map of Earth’s stress field in the Permian Basin of West Texas and southeastern New Mexico could help energy companies avoid causing earthquakes associated with oil extraction. Credit: Jens-Erik Lund Snee
Stanford geophysicists have developed a detailed map of the stresses that act in the Earth throughout the Permian Basin in West Texas and southeastern New Mexico, highlighting areas of the oil-rich region that could be at greater risk for future earthquakes induced by production operations.
The new study, published this month in the journal The Leading Edge, provides a color-coded map of the 75,000-square mile region that identifies those potential oil and gas development sites that would be would be most likely to trigger an earthquake associated with fluid injection.
Previous Stanford research has shown that wastewater injected as a step in hydraulic fracturing (fracking) underlies an increase in seismic activity in parts of the central and eastern U.S., particularly in Oklahoma, starting in 2005. While none of these small-to-moderate earthquakes has yet caused significant property damage or injury, they represent an increased probability of larger earthquakes.
Now, Texas is poised to take center stage as the Permian Basin is becoming the country’s most important oil- and gas-producing region. In the 1920s, energy companies began extracting the basin’s bountiful petroleum deposits during a boom that lasted decades. More recently, the advance of hydraulic fracturing techniques has spurred a new development frenzy. Hundreds of thousands of wells could be drilled in the region in the next few decades.
“We want to get out ahead of the problem in Texas,” said study co-author Mark Zoback, the Benjamin M. Page Professor of Geophysics in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth), who led a number of the Stanford studies in Oklahoma. “We want to stop fluid injection from triggering even small earthquakes in Texas so that the probability of larger earthquakes is significantly reduced.”
High-stress environment
To gauge the risk of future quakes, researchers must first understand the direction of the stresses in a region and their approximate magnitude. When the stress field aligns with a pre-existing fault in a certain manner, the fault can slip, potentially producing an earthquake. In regions such as the central and eastern U.S., far from tectonic plate boundaries such as the San Andreas Fault, this slippage occurs as a natural process, but very rarely. But increasing fluid pressure at depth reduces the friction along the fault, sometimes triggering an earthquake.
“Fluid injection can cause a quake on a fault that might not produce a natural earthquake for thousands of years from now,” said study lead author Jens-Erik Lund Snee, a Ph.D. student in the Department of Geophysics at Stanford Earth.
In a previous study, Zoback and postdoctoral scholar Cornelius Langenbruch found that in Oklahoma, fluid injection caused about 6,000 years of natural earthquakes to occur in about five years.
Creating a next-generation stress map
Building on previous efforts to create maps of stress and seismic potential in the Permian Basin, the Stanford researchers added hundreds of new data points from West Texas and southeastern New Mexico, much of the data being provided by the oil and gas industry. Their findings paint a complicated picture of the Permian Basin, which features some relatively consistent horizontal stress areas along with others that show dramatic directional rotations. “We were surprised to see such high variability,” said Lund Snee. “It raises a lot of questions about how you can have rotations like that in the middle of a continental plate, far from a plate boundary.”
“This is the one of the most interesting stress fields I’ve ever seen,” Zoback said. “While the stress field in this region is surprisingly complex, the data is excellent and having documented what it is, we can now take action on this information and try to prevent the Permian Basin from becoming Oklahoma 2.0.”
A tool for safer, more efficient drilling
The Stanford researchers said the new stress map provides oil companies with detailed quantitative data to inform decisions on more effective drilling operations in the Permian Basin. “This is the most complete picture of stress orientation and relative magnitude that they’ve ever had,” Zoback said. “They can use these data every day in deciding the best direction to drill and how to carry out optimal hydraulic fracturing operations.”
Future studies will focus on improving knowledge of fault lines in the region and gaining a better understanding of fluid pressure, specifically how the amount of water injection (both now and in the past) has impacted the geological mechanisms at work in the area.
“There is the potential for a lot of earthquakes in this area,” said Lund Snee. “We want to understand what’s causing them and provide companies with the tools to avoid triggering them.”
Reference:
Jens-Erik Lund Snee et al. State of stress in the Permian Basin, Texas and New Mexico: Implications for induced seismicity, The Leading Edge (2018). DOI: 10.1190/tle37020127.1
