Researchers on the MV Steller are in front of the terminus of Alaska’s LeConte Glacier in August 2016. An over-the-side pole holds the sonar instrument that collects data on the subsurface ice face as the vessel moves slowly through the icy water. Credit: David Sutherland
Researchers have developed a new method to allow for the first direct measurement of the submarine melt rate of a tidewater glacier, and, in doing so, they concluded that current theoretical models may be underestimating glacial melt by up to two orders of magnitude.
In a National Science Foundation-funded project, a team of scientists, led by University of Oregon oceanographer Dave Sutherland, studied the subsurface melting of the LeConte Glacier, which flows into LeConte Bay south of Juneau, Alaska.
The team’s findings, which could lead to improved forecasting of climate-driven sea level rise, were published in the July 26 issue of the journal Science.
Direct melting measurements previously have been made on ice shelves in Antarctica by boring through to the ice-ocean interface beneath. In the case of vertical-face glaciers terminating at the ocean, however, those techniques are not available.
“We don’t have that platform to be able to access the ice in this way,” said Sutherland, a professor in the UO’s Department of Earth Sciences. “Tidewater glaciers are always calving and moving very rapidly, and you don’t want to take a boat up there too closely.”
Most previous research on the underwater melting of glaciers relied on theoretical modeling, measuring conditions near the glaciers and then applying theory to predict melt rates. But this theory had never been directly tested.
“This theory is used widely in our field,” said study co-author Rebecca H. Jackson, an oceanographer at Rutgers University who was a postdoctoral researcher at Oregon State University during the project. “It’s used in glacier models to study questions like: how will the glacier respond if the ocean warms by one or two degrees?”
To test these models in the field, the research team of oceanographers and glaciologists deployed a multibeam sonar to scan the glacier’s ocean-ice interface from a fishing vessel six times in August 2016 and five times in May 2017.
The sonar allowed the team to image and profile large swaths of the underwater ice, where the glacier drains from the Stikine Icefield. Also gathered were data on the temperature, salinity and velocity of the water downstream from the glacier, which allowed the researchers to estimate the meltwater flow.
They then looked for changes in melt patterns that occurred between the August and May measurements.
“We measured both the ocean properties in front of the glacier and the melt rates, and we found that they are not related in the way we expected,” Jackson said. “These two sets of measurements show that melt rates are significantly, sometimes up to a factor of 100, higher than existing theory would predict.”
There are two main categories of glacial melt: discharge-driven and ambient melt. Subglacial discharge occurs when large volumes, or plumes, of buoyant meltwater are released below the glacier. The plume combines with surrounding water to pick up speed and volume as it rises up swiftly against the glacial face. The current steadily eats away from the glacier face, undercutting the glacier before eventually diffusing into the surrounding waters.
Most previous studies of ice-ocean interactions have focused on these discharge plumes. The plumes, however, typically affect only a narrow area of the glacier face, while ambient melt instead covers the rest of the glacier face.
Predictions have estimated ambient melt to be 10-100 times less than the discharge melt, and, as such, it is often disregarded as insignificant, said Sutherland, who heads the UO’s Oceans and Ice Lab.
The research team found that submarine melt rates were high across the glacier’s face over both of the seasons surveyed, and that the melt rate increases from spring to summer.
While the study focused on one marine-terminating glacier, Jackson said, the new approach should be useful to any researchers who are studying melt rates at other glaciers. That would help to improve projections of global sea level rise, she added.
“Future sea level rise is primarily determined by how much ice is stored in these ice sheets,” Sutherland said. “We are focusing on the ocean-ice interfaces, because that’s where the extra melt and ice is coming from that controls how fast ice is lost. To improve the modeling, we have to know more about where melting occurs and the feedbacks involved.”
Reference:
D. A. Sutherland, R. H. Jackson, C. Kienholz, J. M. Amundson, W. P. Dryer, D. Duncan, E. F. Eidam, R. J. Motyka, J. D. Nash. Direct observations of submarine melt and subsurface geometry at a tidewater glacier. Science, 2019 DOI: 10.1126/science.aax3528
Note: The above post is reprinted from materials provided by University of Oregon. Original written by Carolyn Levinn and Jim Barlow, University Communications.
Magma erupting at the Holuhraun lava field in August 2014. Credit: Bob White
The molten rock that feeds volcanoes can be stored in the Earth’s crust for as long as a thousand years, a result which may help with volcanic hazard management and better forecasting of when eruptions might occur.
Researchers from the University of Cambridge used volcanic minerals known as ‘crystal clocks’ to calculate how long magma can be stored in the deepest parts of volcanic systems. This is the first estimate of magma storage times near the boundary of the Earth’s crust and the mantle, called the Moho. The results are reported in the journal Science.
“This is like geological detective work,” said Dr Euan Mutch from Cambridge’s Department of Earth Sciences, and the paper’s first author. “By studying what we see in the rocks to reconstruct what the eruption was like, we can also know what kind of conditions the magma is stored in, but it’s difficult to understand what’s happening in the deeper parts of volcanic systems.”
“Determining how long magma can be stored in the Earth’s crust can help improve models of the processes that trigger volcanic eruptions,” said co-author Dr John Maclennan, also from the Department of Earth Sciences. “The speed of magma rise and storage is tightly linked to the transfer of heat and chemicals in the crust of volcanic regions, which is important for geothermal power and the release of volcanic gases to the atmosphere.”
The researchers studied the Borgarhraun eruption of the Theistareykir volcano in northern Iceland, which occurred roughly 10,000 years ago, and was fed directly from the Moho. This boundary area plays an important role in the processing of melts as they travel from their source regions in the mantle towards the Earth’s surface. To calculate how long the magma was stored at this boundary area, the researchers used a volcanic mineral known as spinel like a tiny stopwatch or crystal clock.
Using the crystal clock method, the researchers were able to model how the composition of the spinel crystals changed over time while the magma was being stored. Specifically, they looked at the rates of diffusion of aluminium and chromium within the crystals and how these elements are ‘zoned’.
“Diffusion of elements works to get the crystal into chemical equilibrium with its surroundings,” said Maclennan. “If we know how fast they diffuse we can figure out how long the minerals were stored in the magma.”
The researchers looked at how aluminium and chromium were zoned in the crystals, and realised that this pattern was telling them something exciting and new about magma storage time. The diffusion rates were estimated using the results of previous lab experiments. The researchers then used a new method, combining finite element modelling and Bayesian nested sampling to estimate the storage timescales.
“We now have really good estimates in terms of where the magma comes from in terms of depth,” said Mutch. “No one’s ever gotten this kind of timescale information from the deeper crust.”
Calculating the magma storage time also helped the researchers determine how magma can be transferred to the surface. Instead of the classical model of a volcano with a large magma chamber beneath, the researchers say that instead, it’s more like a volcanic ‘plumbing system’ extending through the crust with lots of small ‘spouts’ where magma can be quickly transferred to the surface.
A second paper by the same team, recently published in Nature Geoscience, found that that there is a link between the rate of ascent of the magma and the release of CO2, which has implications for volcano monitoring.
The researchers observed that enough CO2 was transferred from the magma into gas over the days before eruption to indicate that CO2 monitoring could be a useful way of spotting the precursors to eruptions in Iceland. Based on the same set of crystals from Borgarhraun, the researchers found that magma can rise from a chamber 20 kilometres deep to the surface in as little as four days.
The research was supported by the Natural Environment Research Council (NERC).
Reference:
Euan J. F. Mutch*, John Maclennan, Tim J. B. Holland, Iris Buisman. Millennial storage of near-Moho magma. Science, 2019, Vol. 365, Issue 6450, pp. 260-264 DOI: http://dx.doi.org/10.1126/science.aax4092
The fossil of Microdocodon gracilis is preserved in two rock slabs, found in a site near the Wuhua village in the Daohugou area of Inner Mongolia, China. Credit: Zhe-Xi Luo
The 165-million-year-old fossil of Microdocodon gracilis, a tiny, shrew-like animal, shows the earliest example of modern hyoid bones in mammal evolution.
The hyoid bones link the back of the mouth, or pharynx, to the openings of the esophagus and the larynx. The hyoids of modern mammals, including humans, are arranged in a “U” shape, similar to the saddle seat of children’s swing, suspended by jointed segments from the skull. It helps us transport and swallow chewed food and liquid — a crucial function on which our livelihood depends.
Mammals as a whole are far more sophisticated than other living vertebrates in chewing up food and swallowing it one small lump at a time, instead of gulping down huge bites or whole prey like an alligator.
“Mammals have become so diverse today through the evolution of diverse ways to chew their food, weather it is insects, worms, meat, or plants. But no matter how differently mammals can chew, they all have to swallow in the same way,” said Zhe-Xi Luo, PhD, a professor of organismal biology and anatomy at the University of Chicago and the senior author of a new study of the fossil, published this week in Science.
“Essentially, the specialized way for mammals to chew and then swallow is all made possible by the agile hyoid bones at the back of the throat,” Luo said.
‘A pristine, beautiful fossil’
This modern hyoid apparatus is mobile and allows the throat muscles to control the intricate functions to transport and swallow chewed food or drink fluids. Other vertebrates also have hyoid bones, but their hyoids are simple and rod-like, without mobile joints between segments. They can only swallow food whole or in large chunks.
When and how this unique hyoid structure first appeared in mammals, however, has long been in question among paleontologists. In 2014, Chang-Fu Zhou, PhD, from the Paleontological Museum of Liaoning in China, the lead author of the new study, found a new fossil of Microdocodon preserved with delicate hyoid bones in the famous Jurassic Daohugou site of northeastern China. Soon afterwards, Luo and Thomas Martin from the University of Bonn, Germany, met up with Zhou in China to study the fossil.
“It is a pristine, beautiful fossil. I was amazed by the exquisite preservation of this tiny fossil at the first sight. We got a sense that it was unusual, but we were puzzled about what was unusual about it,” Luo said. “After taking detailed photographs and examining the fossil under a microscope, it dawned on us that this Jurassic animal has tiny hyoid bones much like those of modern mammals.”
This new insight gave Luo and his colleagues added context on how to study the new fossil. Microdocodon is a docodont, from an extinct lineage of near relatives of mammals from the Mesozoic Era called mammaliaforms. Previously, paleontologists anticipated that hyoids like this had to be there in all of these early mammals, but it was difficult to identify the delicate bones. After finding them in Microdocodon, Luo and his collaborators have since found similar fossilized hyoid structures in other Mesozoic mammals.
“Now we are able for the first time to address how the crucial function for swallowing evolved among early mammals from the fossil record,” Luo said. “The tiny hyoids of Microdocodon are a big milestone for interpreting the evolution of mammalian feeding function.”
New insights on mammal evolution as a whole
Luo also worked with postdoctoral scholar Bhart-Anjan Bhullar, PhD, now on the faculty at Yale University, and April Neander, a scientific artist and expert on CT visualization of fossils at UChicago, to study casts of Microdocodon and reconstruct how it lived.
The jaw and middle ear of modern mammals are developed from (or around) the first pharyngeal arch, structures in a vertebrate embryo that develop into other recognizable bones and tissues. Meanwhile, the hyoids are developed separately from the second and the third pharyngeal arches. Microdocodon has a primitive middle ear still attached to the jaw like that of other early mammals like cynodonts, which is unlike the ear of modern mammals. Yet its hyoids are already like those of modern mammals.
“Hyoids and ear bones are all derivatives of the primordial vertebrate mouth and gill skeleton, with which our earliest fishlike ancestors fed and respired,” Bhullar said. “The jointed, mobile hyoid of Microdocodon coexists with an archaic middle ear — still attached to the lower jaw. Therefore, the building of the modern mammal entailed serial repurposing of a truly ancient system.”
The tiny, shrew-like creature likely weighed only 5 to 9 grams, with a slender body, and an exceptionally long tail. The dimensions of its limb bones match up with those of modern tree-dwellers.
“Its limb bones are as thin as matchsticks, and yet this tiny Mesozoic mammal still lived an active life in trees,” Neander said.
The fossil beds that yielded Microdocodon are dated 164 to 166 million years old. Microdocodon co-existed with other docodonts like the semiaquatic Castorocauda, the subterranean Docofossor, the tree-dwelling Agilodocodon, as well as some mammaliaform gliders.
Reference:
Chang-Fu Zhou, Bhart-Anjan S. Bhullar, April I. Neander, Thomas Martin, Zhe-Xi Luo. New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science, 19 Jul 2019: Vol. 365, Issue 6450, pp. 276-279 DOI: 10.1126/science.aau9345
Researchers searching for fossils kept finding tiny glassy spheres inside ancient clams. After more than a decade, testing suggests they are evidence of one or more undocumented meteorite impacts in Florida’s distant past. Credit: Florida Museum photo by Kristen Grace
Researchers picking through the contents of fossil clams from a Sarasota County quarry found dozens of tiny glass beads, likely the calling cards of an ancient meteorite.
Analysis of the beads suggests they are microtektites, particles that form when the explosive impact of an extraterrestrial object sends molten debris hurtling into the atmosphere where it cools and recrystallizes before falling back to Earth.
They are the first documented microtektites in Florida and possibly the first to be recovered from fossil shells.
Mike Meyer was a University of South Florida undergraduate when he discovered the microtektites during a 2006 summer fieldwork project led by Roger Portell, invertebrate paleontology collections director at the Florida Museum of Natural History.
As part of the project, students systematically collected fossils from the shell-packed walls of a quarry that offered a cross-section of the last few million years of Florida’s geological history. They pried open fossil clams, washing the sediment trapped inside through very fine sieves. Meyer was looking for other tiny objects — the shells of single-celled organisms known as benthic foraminifera — when he noticed the translucent glassy balls, smaller than grains of salt.
“They really stood out,” said Meyer, now an assistant professor of Earth systems science at Harrisburg University in Pennsylvania. “Sand grains are kind of lumpy, potato-shaped things. But I kept finding these tiny, perfect spheres.”
After the fieldwork ended, his curiosity about the spheres persisted. But his emails to various researchers came up short: No one knew what they were. Meyer kept the spheres — 83 in total — in a small box for more than a decade.
“It wasn’t until a couple years ago that I had some free time,” he said. “I was like, ‘Let me just start from scratch.'”
Meyer analyzed the elemental makeup and physical features of the spheres and compared them to microtektites, volcanic rock and byproducts of industrial processes, such as coal ash. His findings pointed to an extraterrestrial origin.
“It did blow my mind,” he said.
He thinks the microtektites are the products of one or more small, previously unknown meteorite impacts, potentially on or near the Florida Platform, the plateau that undergirds the Florida Peninsula.
Initial results from an unpublished test suggest the spheres have traces of exotic metals, further evidence they are microtektites, Meyer said.
Most of them had been sealed inside fossil Mercenaria campechiensis or southern quahogs. Portell said that as clams die, fine sediment and particles wash inside. As more sediment settles on top of the clams over time, they close, becoming excellent long-term storage containers.
“Inside clams like these we can find whole crabs, sometimes fish skeletons,” Portell said. “It’s a nice way of preserving specimens.”
During the 2006 fieldwork, the students recovered microtektites from four different depths in the quarry, which is “a little weird,” Meyer said, since each layer represents a distinct period of time.
“It could be that they’re from a single tektite bed that got washed out over millennia or it could be evidence for numerous impacts out on the Florida Platform that we just don’t know about,” he said.
The researchers plan to date the microtektites, but Portell’s working guess is that they are “somewhere around 2 to 3 million years old.”
One oddity is that they contain high amounts of sodium, a feature that sets them apart from other impact debris. Salt is highly volatile and generally boils off if thrust into the atmosphere at high speed, Meyer said.
“This high sodium content is intriguing because it suggests a very close location for the impact,” Meyer said. “Or at the very least, whatever impact created it likely hit a very large reserve of rock salt or the ocean. A lot of those indicators point to something close to Florida.”
Meyer and Portell suspect there are far more microtektites awaiting discovery in Florida and have asked amateur fossil collectors to keep an eye out for the tiny spheres.
But no one will be recovering microtektites from the original quarry any time soon. It’s now part of a housing development.
“Such is the nature of Florida,” Meyer said.
Peter Harries of North Carolina State University also co-authored the study.
Reference:
Mike Meyer, Peter J. Harries, Roger W. Portell. A first report of microtektites from the shell beds of southwestern Florida. Meteoritics & Planetary Science, 2019; DOI: 10.1111/maps.13299
Groundwater well drilling equipment in California’s Central Valley. Photo Credit: Chad Ress
Groundwater may be out of sight, but for over 100 million Americans who rely on it for their lives and livelihoods it’s anything but out of mind. Unfortunately, wells are going dry and scientists are just beginning to understand the complex landscape of groundwater use.
Now, researchers at UC Santa Barbara have published the first comprehensive account of groundwater wells across the contiguous United States. They analyzed data from nearly 12 million wells throughout the country in records stretching back decades. Their findings appear in the journal Nature Sustainability.
In tackling the work, Debra Perrone and Scott Jasechko had a number of different questions about groundwater usage they wanted to address. First they set out to determine both where in the country wells are located and what purposes they serve — domestic, industrial or agricultural. They also wanted to track the depths of wells in different areas and test to see if wells are being drilled deeper over time.
Focusing on regions known to depend on groundwater, such as California’s Central Valley, the pair collected a wealth of information about different types of wells across the country. Groundwater is generally a matter of state management, so they had to cull their data from a variety of sources. “[That was] one of the biggest hurdles,” said Perrone, an assistant professor in UC Santa Barbara’s environmental studies department.
“It took us about four years to collect and quality-assure all these data sources,” added Jasechko, an assistant professor based in the Bren School of Environmental Science & Management.
Scientists know that groundwater depletion is causing some wells to run dry. Where conditions are right, drilling new and deeper wells can stave off this issue, for those who can afford it. Indeed, Perrone and Jasechko found that new wells are getting deeper between 1.4 and 9.2 times as often as they are being drilled shallower.
What’s more, the researchers found that 79% of areas they looked at showed well-deepening trends across a window spanning 1950 to 2015. Hotspots of this activity include California’s Central Valley, the High Plains of southwestern Kansas, and the Atlantic Coastal Plain, among other regions.
“We were surprised how widespread deeper well drilling is,” Jasechko said. News media had documented the trend in places like the Central Valley, but it is pervasive in many other parts of the country as well. This includes places like Iowa, where groundwater hasn’t been studied as intensively, he noted.
The reasons for drilling deeper are varied, according to Perrone. For instance, people drill deeper to avoid contamination seeping into aquifers from the surface, or to access aquifers that have less stringent withdrawal regulations, she explained. Some people may drill deeper to source more water.
“What we’re finding is that in places where water levels are declining, some people are drilling deeper, maybe to avoid having their primary water supply go dry,” Perrone said. “Regardless of the reasons why Americans are drilling deeper, we suggest that deeper well drilling is an unsustainable stopgap to groundwater depletion.”
Four major factors explain why deeper drilling won’t solve water woes indefinitely. For starters, it costs more, and it requires more energy to pump water from deep underground compared to water closer to the surface.
Geology presents another challenge: Deeper strata are generally less conducive to groundwater extraction. And finally, groundwater tends to get saltier at depth, so at a certain point it becomes unusable if not treated. As a result, in many regions there’s a floor to how deep we can productively drill for water.
This issue hits rural communities particularly hard. “Groundwater is a crucial resource for rural communities,” Perrone said. “Our previous work found that rural groundwater wells are especially vulnerable to going dry.” What’s more, these communities often have less capacity to update their groundwater infrastructure. Groundwater is also important to the agricultural sector, which often relies on it for irrigation, especially during droughts, she added.
Deliberate groundwater governance and management have emerged as active areas in addressing this challenge. The idea is to become more conscientious about how groundwater is used and regulate and monitor the practices more effectively. Additional research is underway on managed aquifer recharge, namely encouraging water to percolate back underground. This could be normal surface water, flood water or treated water. And particularly dry areas are considering whether to increase their use of recycled water.
Researchers, practitioners and policymakers from around the world will discuss the challenges facing groundwater use as well as potential solutions at an upcoming conference on groundwater sustainability. Perrone is one of the lead organizers for the event, which will convene in Valencia, Spain this October.
This new paper provides additional context to one of Perrone and Jasechko’s past studies — completed with professors Grant Ferguson of the University of Saskatchewan, and Jennifer McIntosh at the University of Arizona — where they found that the United States may have less usable groundwater than previously thought. It also ties into Perrone’s work regarding groundwater policy across the U.S. In the future, she plans to look at the legal frameworks surrounding groundwater use. “My goal is to understand what types of laws are being passed in the western 17 states to manage groundwater withdrawals in more sustainable ways,” she said.
The output of the proposed methodology, which is a two-part optimization process that refines a common technique used to image salt bodies. The black area represents the salt region. Credit: Mahesh Kalita
The efficient extraction of oil and gas from within the Earth’s crust requires accurate images of subsurface rock structures. Some materials are hard to capture, so KAUST researchers have developed a computational method for modeling large accumulations of subsurface salt, a challenging material to derive accurately from seismic imaging data.
Seismic imaging involves sending soundwaves into the ground, where they will be reflected at boundaries between rock structures. Scientists analyze the reflected soundwaves to determine subsurface rock types and formations, and to pinpoint fossil fuel reservoirs.
However, in some regions, such as the Gulf of Mexico, the subsurface is peppered with salt bodies, which are huge accumulations of salt formed millions of years ago deep inside the Earth. Salt is a low-density, buoyant substance, meaning that salt bodies gradually rise through the Earth’s crust over time. This causes stress-related complexities between the salt and the surrounding rock layers. Furthermore, the salt’s crystal structure means that soundwaves are reflected at random, and there are no useable low frequencies retained in the seismic data.
“Data from salt zones are presently analyzed by highly trained experts rather than modeled by a computer,” explains Mahesh Kalita, a KAUST Ph.D. student in Tariq Alkhalifah’s group. “This is a time-consuming and expensive process that carries the risk of human error. We’ve developed a robust computational method for interpreting seismic data from salt bodies quickly and more accurately.”
Existing models use a technique called full waveform inversion (FWI) to minimize the disparity between observed and modeled data. However, the lack of low frequencies in soundwave data from salt bodies means that a traditional FWI fails. Kalita and the team developed a two-part optimization process to refine FWI for salt body imaging.
“For the topmost layer of salt, we get a good enough signal to determine where the salt body begins, but then the soundwave energy rapidly disperses,” says Kalita. “Our technique takes the initial data from this top layer and ‘smears’ it across the most likely area that the salt body encompasses. We call this technique ‘flooding’.”
The resulting model is then tested alongside observed data to check that the surrounding rock structures match up and to ensure the model has not been “over-flooded.” Initial trials using a 1990s dataset from the Gulf of Mexico showed promise, with the new technique generating an accurate representation of local salt bodies.
“We will next trial our automated technique on recent, high-quality datasets that incorporate more three-dimensional details,” says Kalita.
Reference:
Mahesh Kalita et al, Regularized full-waveform inversion with automated salt flooding, Geophysics (2019). DOI: 10.1190/geo2018-0146.1
Schematic diagram of water and chlorine transfer by the oceanic crust into the transitional zone of the mantle and the subsequent capture of the resulting material by an Archaean mantle plume. Credit: Evgeny Asafov
“The mechanism which caused the crust that had been altered by seawater to sink into the mantle functioned over 3.3 billion years ago. This means that a global cycle of matter, which underpins modern plate tectonics, was established within the first billion years of the Earth’s existence, and the excess water in the transition zone of the mantle came from the ancient ocean on the planet’s surface,” said project leader and co-author of the article Alexander Sobolev, a member of the Russian Academy of Sciences (RAS) and Doctor of Geological and Mineralogical Sciences who is a Professor at Vernadsky Institute for Geochemistry and Analytical Chemistry under the Russian Academy of Sciences.
The Earth’s crust consists of large continuously moving blocks known as tectonic plates. Mountains are produced when these plates collide and rise up, and the shock of the collisions leads to earthquakes and tsunamis. These plates move very actively under the World Ocean: old oceanic crust, including the minerals that have absorbed seawater, sinks deep into the Earth’s mantle. Some of this water is released again due to the effect of high temperatures and plays a role in volcanic eruptions, such as those that occur in Kamchatka, the Kuril islands and Japan. The water that remains in minerals of the oceanic crust at higher temperatures continues to descend into the deep mantle and accumulates at a depth of 410-660 km in the structure of the minerals wadsleyite and ringwoodite and high-pressure modifications of olivine (magnesium iron silicate), the main mineral of the mantle. Experiments have shown that these minerals can contain significant quantities of water and chlorine. This is how the greatest part of the World Ocean could be “pumped” into the planet’s interior over the billions of years of its existence.
This process is only a part of the global cycle of the Earth’s matter, which is called convection and underpins plate tectonics, a feature that distinguishes our planet from all other bodies in the Solar System. Many scientists study this mechanism, trying to understand at which stage of the Earth’s history it appeared.
In order to study the mantle of our planet and investigate its composition, geochemists (scientists who specialise in the chemical composition of the Earth and the processes of rock formation) use samples of volcanic rocks that consist of solidified magma of the mantle. This is a silicate melt enriched in volatile components, such as water, carbon dioxide, chlorine and sulphur. There are different types of magma: scientists commonly use basaltic lava (with a temperature of approximately 1200°C), but komatiitic magma, which was erupted during the early history of the Earth, is hotter (at 1500-1600°C). It can help to describe the evolution of the Earth’s inner layers, as it matches the composition of the mantle more completely.
Komatiites are a type of volcanic rock that formed from komatiitic magma billions of years ago and whose composition has changed dramatically in the intervening epochs. It no longer provides information about the content of volatile components, such as water and chlorine. But these rocks still contain remnants of the magmatic mineral olivine, which trapped inclusions of solidified magma during the crystallisation process and protected them from subsequent changes. Such inclusions, just tens of microns across, retain detailed information about the composition of komatiitic melts, including the content of water and chlorine and the isotopic composition of hydrogen. In order to extract this information, inclusions of solidified magma must be heated to the natural melting point of over 1500°C and then immediately tempered to produce clear tempered glass that can later be used for chemical analyses.
In 2016, an international group headed by scientists from the Vernadsky Institute for Geochemistry and Analytical Chemistry studied komatiitic magma of the Abitibi greenstone belt in Canada, which is 2.7 billion years old. Greenstone belts are territories consisting of magmatic rocks that contain greenish minerals. This was the first article that the team published in Nature as part of the project supported by the Russian Science Foundation grant. At that time, the scientists collected initial data on the content of water and a variety of labile elements, such as chlorine, lead and barium, in the transition zone between the upper and lower mantle layers at a depth of 410-660 km, which led them to hypothesise that an ancient subterranean water reservoir once existed that was comparable in mass to the present-day World Ocean. The scientists believe that such a quantity of water was accumulated at the early stages of the Earth’s development.
“In the new article, we presented geochemical data indicating that the cycle of global immersion of oceanic crust into the mantle began much earlier than most experts believed, and it could have functioned as early as the first billion years of the Earth’s history,” noted Alexander Sobolev.
In the course of the work, the scientists once again investigated the composition of komatiite magma, but of a different origin: it was collected from the Barberton greenstone belt in South Africa, which is 3.3 billion years old. The magma was heated using a specialised high-temperature apparatus that can withstand temperatures of up to 1700°C. The geochemists found out that the previously discovered deep water-containing reservoir was already present in the Earth’s mantle in the Palaeoarchaean era, 600 million years earlier than established in the previous study.
Reference:
Alexander V. Sobolev et al, Deep hydrous mantle reservoir provides evidence for crustal recycling before 3.3 billion years ago, Nature (2019). DOI: 10.1038/s41586-019-1399-5
Note: The above post is reprinted from materials provided by AKSON Russian Science Communication Association .
The floormen (in yellow jackets) and drillers (in white booths in background) of Expedition 383 retrieve core barrels and sediment cores around the clock. Credit: Jenny Middleton/Lamont-Doherty Earth Observatory
An ocean drilling ship is not an ocean drilling ship without the skilled and experienced personnel that control, execute and overview the drilling operations. The JOIDES Resolution is no exception.
Of the 123 souls currently onboard, about 30 people facilitate the success and safety of all ocean drilling operations—about the same number as scientists onboard. Among them are five drillers, two tool pushers, six floormen and two derrickmen. While drillers control the advance of the drill pipe and the motion of the core barrel within the drill pipe from a control booth, tool pushers oversee the operations and maintain equipment and tools. Floormen and derrickmen perform all actions on the rig floor, from putting the drill pipe together and sending the core barrel down the drill pipe to the sea floor, to eventually pulling a sediment-laden core out of the core barrel. Most of the drill crew have decades of experience.
Drilling ocean sediments for scientific purposes is not simply achieved by forcing a steel rod into the sea floor and hoping for the best recovery. It actually involves building a multi-piece drill bit (called the bottom-hole assembly) that cuts into the seabed, as well as carefully maneuvering and documenting the location of the drill bit near and below the seafloor, and adjusting the drilling tools according to the stiffness and composition of the material. Most importantly, seafloor drilling requires facilitating safe drilling operations, in particular for those right in the center of action on the JOIDES Resolution rig floor, our floormen.
As a sedimentologist onboard, I primarily work in the core laboratory. On the live feed monitors of the core laboratory, we can watch the drill team work on the rig floor. It is winter in the Southern Ocean at the moment, so it is mostly windy or rainy outside, or in fact both. Sometimes this makes me feel a little bit like I am in a golden tower in the core lab, far away from all the danger, grease, and noise of the drill floor. However, this does not diminish my respect and gratitude towards the drill crew and their work, as the quality of the drilled sediments they recover often governs the quality and impact of our scientific results. This particularly struck me when I had the chance to climb up the derrick of the JOIDES Resolution, six weeks into our expedition.
The top of the derrick ascends 65 meters (over 200 feet!) above the sea level, and is used to raise the drill pipe to a vertical position from the hull of the ship, in order to lower it to the sea floor. If you want to be at the highest point of the derrick (called the “crown”), you have to trust your arms, legs and your fall protection device to bring you all the way to the top of the drill rig. I thank Wouter, our Dutch driller, who talked me through the process of how to wear the required six-pound harness, how to use the safety carabiners, and how to change my fall protection system at each of the six wet and greasy ladders.
In order to drill ocean sediments, the drill pipe must first be extended to the seafloor, which requires lowering the pipes through the water column. This is done by adding 30-meter segments of drill pipe (a “stand”) on top of each other using the winch system of the derrick—a process that is called “tripping pipe.” Depending on the water depth of our study site, this usually takes several hours. I, as an impatient person, always have to remind myself that, for the length of a stand of 30m relative to the mean ocean depth of 4,000m (~2.5 miles), a few hours of tripping pipe is actually very fast. For an equivalent scale using the 10cm derrick of our paper model JOIDES Resolution, below, the whole sedimentology team onboard must stand next to each other to mimic the average depth of the ocean and the distance over which we have to trip the pipe.
Tripping pipe only brings us to the bottom of the ocean, but not into the sea floor. Drill tools need to be lowered through the drill pipe in order to recover seabed samples, 4,000 meters below the ship. For marine sediments, this is done through Advanced Piston Coring (APC) or Half-APC, where a piston assembly with an empty core liner is lowered by wireline through the drill pipe. At the chosen depth, the “firing depth,” the driller increases the hydraulic pressure on the core barrel, which allows the piston to suddenly, but in a controlled manner, penetrate 10 meters (APC) or 5 meters (Half-APC) into the sediments below. This firing process fills up the plastic core liner with precious ocean mud samples. This cycle repeats after the core barrel is raised to the rig floor through the wireline, and the outer drill pipe advances 10 meters further into the sea floor.
Ocean drilling is easier said than done. It is fascinating at the same time how technology and the expertise of our drilling and operations team come together to retrieve a mostly undisturbed and continuous climate archive from a far away place and time. I cannot thank them enough for their work as they are a crucial part in the long process of unveiling the inner workings and mechanisms that govern our Earth system.
Relationship between volcanic eruptions and global monsoon hydroclimate. The bottom panel shows the distribution of global monsoon regions. Credit: Zuo Meng/Google images
Volcanic eruptions eject sulfur dioxide gas high into the atmosphere, forming sulfate aerosol chemically, and block the incoming sunlight like an umbrella. This causes surface cooling globally, but the response of global monsoon rainfall is different following volcano eruptions at different latitudes, according to quantitative research by Meng Zuo, a doctoral student from the Institute of Atmospheric Physics, Chinese Academy of Sciences, along with her mentors Prof. Tianjun Zhou and associate Prof. Wenmin Man.
This work is recently published in Journal of Climate.
Monsoon rainfall and volcano eruption
Monsoon rainfall greatly impacts society since it affects over two-thirds of the world’s population. Insufficient monsoon rainfall brings drought and famines to many parts of the world, while too much rainfall causes floods.
Following tropical volcanic eruptions, the monsoon circulation weakens and rainfall decreases consequently. But when we take the latitude of eruptions into consideration, the monsoon rainfall response changes accordingly. This phenomenon has been found in previous studies but the physical mechanism still remains inconclusive. Also the knowledge of the monsoon rainfall response to volcanic eruptions is crucial for adaptation.
The dominant role of atmospheric circulation change
The researchers analyzed large sets of data and the climate model simulations to investigate the different impacts of northern, tropical and southern volcanic eruptions on the global monsoon rainfall. “In contrast to previous work, we used a diagnostic method to quantitatively analyze the factors determining rainfall response, which is helpful to understand the different physical processes of volcanoes at different latitudes affecting the monsoon rainfall,” said Zuo.
The team found that both the proxy and observations show that monsoon rainfall in one hemisphere is reduced by the volcanic eruptions in the same hemisphere, but is enhanced by the volcanic forcing in the other hemisphere. A similar relationship is found in the model simulations.
They further advanced the underlying mechanism by using moisture budget analysis and moisture static energy budget analysis, which can decompose rainfall changes into water vapor change and circulation change. The change in atmospheric circulation is found to play a dominant role in rainfall responses. The dry-wet changes are attributed to weakened monsoon circulation and enhanced hemispheric thermal contrast, respectively. They also found that the response of the extreme rainfall is consistent with that of the mean rainfall, but more sensitive over monsoon regions, which are related to drought and flood hazards.
“This work implies that future volcanic eruptions located in different latitudes will impact the monsoon rainfall differently through circulation changes, which implies that the rainfall response to volcanic eruptions at different hemispheres should be considered in the design of Decadal Climate Prediction Project (DCPP) experiments and the implementation of geoengineering activities,” the corresponding author Tianjun Zhou, highlighted.
Reference:
Meng Zuo et al, Hydroclimate Responses over Global Monsoon Regions Following Volcanic Eruptions at Different Latitudes, Journal of Climate (2019). DOI: 10.1175/JCLI-D-18-0707.1
Many of the opals mined in Ethiopia are hydrophane opals, particularly from the Welo rocks.
Hydrophane Opal is a fresh word designed to define a specific Opal form. Opals have been mined for thousands of years in Europe and more than 165 years in Australia. However, the finding of opals from Ethiopia and Hydrophane has been around for only about 20 years. The business amounts of this product have increased over the past 10 years.
Ethiopian opal is one of today’s most varied and spectacular opal types on the market. Because this type of opal is new, there is no industry-accepted way of grading this type of opal like the black opal. Black opal is in darkness, color brightness and pattern grading.
Likewise, Ethiopian opal is also graded on shape and brightness, but the stone’s body color does not play an enormous part in determining the ultimate quality.
Ethiopian Opals have many distinctive models and the descriptions must be precise on Opal Auctions. A naturally shaped potch row with internet inclusions recognized as honey comb motif is one of the most common motifs in Ethiopian opals. In any other sort of opals, this model is very uncommon. These forms of the honeycomb are also considered to have lighter or distinct colors on occasions than the adjacent opal, which is also a unusual event in nature.
Color And Brightness In Ethiopian Welo Opal
This relates to the opal flame. These Ethiopian opals ‘ transparent inner flame has characteristics that create the flame feel 3D. In most opals, this 3-dimensional color element is uncommon. It appears to have caught flames within the opal.
Where Can You Find Welo Opal?
Ethiopia
The first released Ethiopian gem study with the finding of valuable opal in the district of Menz Gishe, North Shewa Province, was released in 1994. The opal, found mainly in the shape of nodules, was of volcanic origin and was discovered mainly in weathered rhyolite strata.
This opal Shewa Province was mostly colored dark brown and tended to break. These characteristics rendered the jewel trade unpopular. In 2008, a new opal deposit was found about 180 km north of the province of Shewa, near the town of Wegel Tena, in the Wollo province of Ethiopia. Opal from the past Ethiopian opal findings, the Wollo Province was distinct in that it more strongly resembled Australia and Brazil’s sedimentary opals with a light backdrop and often lively play-of-colour. Opal Wollo Province, more frequently referred to as opal “Welo” or “Wello,” has become the dominant Ethiopian opal in jewelry trade.
Are Ethiopian opals valuable?
Opals with a rare or unique pattern are more precious. The Harlequin pattern in opals is the most rare and loved pattern, but it is very rare.
Ethiopian opal is one of today’s market’s most varied and dramatic opal kinds. Ethiopian opal is also graded on shape and luminosity, but the stone’s body tone does not play a major part in determining the ultimate quality.
Aquilarhinus palimentus, or ‘shovel-chinned eagle nose’. Illustration by ICRA Art
The most complete skull of a duck-billed dinosaur from Big Bend National Park, Texas, is revealed in the Journal of Systematic Palaeontology as a new genus and species, Aquilarhinus palimentus. This dinosaur has been named for its aquiline nose and wide lower jaw, shaped like two trowels laid side by side.
In the 1980s, Texas Tech University Professor Tom Lehman (then a Master’s student) was conducting research on rock layers at Rattle Snake Mountain and discovered badly-weathered bones. He and two others from the University of Texas at Austin collected them, but some were stuck together making them impossible to study. Research in the 1990s revealed an arched nasal crest thought to be distinctive of the hadrosaurid Gryposaurus. At the same time, the peculiar lower jaw was recognized. However, the specimen spent additional years waiting for a full description and it was not until recent analysis that the researchers came to realize that the specimen was more primitive than Gryposaurus and the two major groups of duck-billed dinosaurs.
“This new animal is one of the more primitive hadrosaurids known and can therefore help us to understand how and why the ornamentation on their heads evolved, as well as where the group initially evolved and migrated from,” says lead author Dr Albert Prieto-Márquez from the Institut Català de Paleontologia Miquel Crusafont, near Barcelona. “Its existence adds another piece of evidence to the growing hypothesis, still up in the air, that the group began in the southeastern area of the US.”
Duck-billed dinosaurs, also known as hadrosaurids, were the most common herbivorous dinosaurs at the end of the Mesozoic Era, and all had a similar-looking snout. The front of the jaws meet in a U-shape to support a cupped beak used for cropping plants. The beak of some species is broader than in others, but there was no evidence of a significantly different shape and therefore likely also different feeding style in duckbills until Aquilarhinus was discovered. The lower jaws of Aquilarhinus meet in a peculiar W-shape, creating a wide, flattened scoop.
Around 80 million years ago, this particular dinosaur would have been shovelling through loose, wet sediment to scoop loosely-rooted aquatic plants from the tidal marshes of an ancient delta, where today lies the Chihuahuan desert. When the dinosaur died, some of its bones were transported downstream by the tide and became lodged in vegetation. The twice-daily flow of the tide dropped silt that built up the bank of the channel around its body, fossilising the bones in thick ironstone.
The jaw and other characteristics of the specimen show that it does not fit with the main group of duck-billed dinosaurs known as Saurolophidae. It is more primitive than this group, suggesting there might have been a greater number of lineages than previously recognised that evolved before the great radiation that gave rise to the bewildering array of unadorned, solid and hollow-crested forms. Most saurolophids had bony cranial crests of many different shapes and sizes. Aquilarhinus also sported a bony crest, albeit a simple one shaped like a humped nose. The discovery of a solid crest outside the major radiation of hadrosaurids supports the hypothesis that all crests derived from a common ancestor that had a simple humped nose.
The study was funded by the Ramon y Cajal Program, the Ministry of Economy, Industry and Competitivity of Spain, the CERCA Program of the Generalitat de Catalunya, Texas Tech University, and the Sigma Xi National Scientific Honor Society.
The work was conducted under permit from the Science and Resource Management division of Big Bend National Park Service, as well as the Texas Vertebrate Paleontology Collections at The University of Texas at Austin, where the specimen is housed.
Reference:
Albert Prieto-Márquez, Jonathan R. Wagner, Thomas Lehman. An unusual ‘shovel-billed’ dinosaur with trophic specializations from the early Campanian of Trans-Pecos Texas, and the ancestral hadrosaurian crest. Journal of Systematic Palaeontology, 2019; 1 DOI: 10.1080/14772019.2019.1625078
An artist’s rendering of Auroraceratops shows its bipedal posture as well as the beak and frill that characterize it as a member of the horned dinosaurs. Paleontologists from Penn led a team in characterizing this species, discovered in China. (Illustration: Robert Walters)
Many dinosaur species are known from scant remains, with some estimates suggesting 75% are known from five or fewer individuals. Auroraceratops rugosus was typical in this regard when it was named in 2005 based upon a single skull from the Gobi Desert in northwestern China. But that is no longer the case.
In the intervening years, scientists have recovered fossils from more than 80 individual Auroraceratops, bringing this small-bodied plant-eater into the ranks of the most completely known dinosaurs. It is now one of the few very early horned dinosaurs known from complete skeletons. In a collection of articles appearing as Memoir 18 in the Journal of Vertebrate Paleontology this week, researchers from the University of Pennsylvania, Indiana University of Pennsylvania, the Chinese Academy of Sciences, Gansu Agricultural University, and other institutions describe the anatomy, age, preservation, and evolution of this large collection of Auroraceratops.
Their analysis places Auroraceratops, which lived roughly 115 million years ago, as an early member of the group Ceratopsia, or horned dinosaurs, the same group to which Triceratops belongs. In contrast to Triceratops, Auroraceratops is small, approximately 49 inches (1.25 meters) in length and 17 inches (44 cm) tall, weighing on average 34 pounds (15.5 kilograms). While Auroraceratops has a short frill and beak that characterize it as a horned dinosaur, it lacks the “true” horns and extensive cranial ornamentation of Triceratops.
“When I first saw this animal back in 2004, I knew instantly it was a new kind that had never been seen before and was very excited about it,” says paleontologist Peter Dodson, senior author on the work and a professor with appointments in Penn’s School of Veterinary Medicine and School of Arts and Sciences. “This monograph on Auroraceratops is long-awaited.”
In 2005, Dodson and his former students Hai-Lu You and Matthew Lamanna named Auroraceratops (in Latin, “dawn’s horned face”) in honor of Dodson’s wife, Dawn Dodson. You, along with fellow Chinese scientist Da-Qing Li — both authors on the current work — and collaborators followed up on the discovery, identifying more than 80 additional examples of the species, from near-hatchlings to adults.
Eric Morschhauser, lead author who is now on the faculty at Indiana University of Pennsylvania, completed his Ph.D. under Dodson at Penn, focused on characterizing Auroraceratops using this robust dataset.
Auroraceratops represents the only horned dinosaur in the group Neoceratopsia (the lineage leading to and including the large bodied ceratopsians such as Triceratops) from the Early Cretaceous with a complete skeleton. This exclusiveness is significant, the researchers say, because horned dinosaurs transitioned from being bipedal, like their ancestors, to being the large rhinoceros-like quadrupedal animals most people think of as horned dinosaurs during the later parts of the Cretaceous.
“Before this study,” says Morschhauser, “we had to rely on Psittacosaurus, a more distantly related and unusual ceratopsian, for our picture of what the last bipedal ceratopsian looked like.”
Auroraceratops preserves multiple features of the skeleton, like a curved femur and long, thin claws, that are unambiguously associated with walking bipedally in some dinosaurs.
“It can now provide us with a better picture of the starting point for the changes between bipedal and quadrupedal ceratopsians,” adds Morschhauser.
Peter Dodson is a professor of anatomy in the University of Pennsylvania School of Veterinary Medicine’s Department of Biomedical Sciences and a professor of paleontology in the School of Arts and Sciences’ Department of Earth and Environmental Science.
Reference:
Hailu You, Eric M. Morschhauser, Daqing Li, Peter Dodson. Introducing the Mazongshan Dinosaur Fauna. Journal of Vertebrate Paleontology, 2019; 38 (sup1): 1 DOI: 10.1080/02724634.2017.1396995
Tamu Massif made news in 2013, thought to be the world’s largest volcano. New research offers a better look at the volcano’s formation and throws doubt on that claim.
The discovery of Tamu Massif, a gigantic volcano located about 1,000 miles east of Japan, made big news in 2013 when researchers reported it was the largest single volcano documented on earth, roughly the size of New Mexico.
New findings, reported this week in Nature Geoscience, conclude that it is a different breed of volcanic mountain than earlier thought, throwing into doubt the prior claim that it is the world’s largest single volcano.
The study analyzed magnetic field data over Tamu Massif, finding that magnetic anomalies — perturbations to the field caused by magnetic rocks in the Earth’s crust — resemble those formed at mid-ocean ridge plate boundaries.
William Sager, a geophysicist at the University of Houston and senior author for the paper, said the discovery led researchers to conclude that Tamu Massif formed by mid-ocean ridge “spreading,” the geologists’ term for creation of ocean crust at mid-ocean ridge plate boundaries, rather than as a shield volcano, as previously thought. Shield volcanos are formed primarily as stacks of fluid lava flows and are one of the most common types of volcano.
An international group of researchers — from Texas, China and Japan — sought to understand how the massive Tamu Massif volcano formed near the nexus of three spreading ridges. The key, they report, is magnetic anomalies.
Mid-ocean ridges — plate boundaries where oceanic plates move apart — are themselves large volcanoes. These ridges record distinctive linear magnetic anomalies, parallel to the ridge, as they form new crust. This is a result of lava flows and magma being concentrated near the ridge axis where the magnetic minerals in the new crust record reversals of the magnetic field polarity.
A New Understanding of Tamu Massif
Linear magnetic anomalies formed by the three ridges had previously been found around Tamu Massif, but it was unclear where they stopped within the volcano. A paper published in 2013 by Sager and colleagues concluded that Tamu Massif is an enormous shield volcano, formed by far-reaching lava flows emanating from its summit.
The latest study compiled a magnetic anomaly map over Tamu Massif, using 4.6 million magnetic field readings collected over 54 years along 72,000 kilometers of ship tracks. The data set was anchored by a new grid of magnetic profiles, positioned with modern GPS navigation, collected by the study authors using the Schmidt Ocean Institute ship Falkor. The resulting map shows that linear magnetic anomalies around Tamu Massif blend into linear anomalies over the mountain itself — implying that the underwater volcano formed by extraordinary mid-ocean ridge crustal formation.
Sager said the finding is important because it demonstrates that Tamu Massif and other oceanic plateaus are formed by a different process than previously thought. A widely-accepted model suggests a large blob of magma, known as a “mantle plume,” rises through the mantle and creates a massive volcano when it arrives at the surface. This eruption is thought to be analogous to massive eruptions on land, called “continental flood basalts” and it creates a vertical succession of lava flows.
The ocean-ridge-spreading hypothesis suggests the age progression is instead lateral. New material is always added at the center of the ridge as older material drifts laterally away. An implication is that the gradual slopes of Tamu Massif are not caused by lava flow shape but instead by a gradual inflation and then deflation of ridge volcanism as the crust became thicker and then grew thinner.
The new finding also weakens the accepted analogy between eruptions of continental flood basalts and oceanic plateaus because the formation mechanisms are shown to be different, Sager said.
‘Certainly One of the Largest’
With the discovery, Sager said Tamu Massif can no longer be considered the world’s largest shield volcano. That title reverts to Mauna Loa, on the island of Hawaii.
“The largest volcano in the world is really the mid-ocean ridge system, which stretches about 65,000 kilometers around the world, like stitches on a baseball,” Sager said. “This is really a large volcanic system, not a single volcano.”
Researchers now think Tamu Massif formed as part of that mid-ocean ridge system, he said. “Tamu Massif is certainly one of the largest volcanic mountains in the world.”
The 2013 paper was based on what researchers knew at the time, Sager said. “Science is a process and is always changing. There were aspects of that explanation that bugged me, so I proposed a new cruise and went back to collect the new magnetic data set that led to this new result.
“In science, we always have to question what we think we know and to check and double check our assumptions. In the end, it is about getting as close to the truth as possible — no matter where that leads.”
In addition to Sager, corresponding author and professor of geophysics in the UH College of Natural Sciences and Mathematics, other authors on the paper include: co-corresponding author Yanming Huang, Yangtze University; Masako Tominaga and John A. Greene, both of Texas A&M University; co-corresponding author Jinchang Zhang of the Chinese Academy of Sciences; and Masao Nakanishi of Chiba University.
Reference:
William W. Sager, Yanming Huang, Masako Tominaga, John A. Greene, Masao Nakanishi, Jinchang Zhang. Oceanic plateau formation by seafloor spreading implied by Tamu Massif magnetic anomalies. Nature Geoscience, 2019; DOI: 10.1038/s41561-019-0390-y
Along the North Anatolian Fault, Anatolia and the Eurasian Earth Plate push past each other. Image reproduced from the GEBCO world map 2014, www.gebco.net
Collapsed houses, destroyed port facilities and thousands of victims — on 22 May 1766 an earthquake of approximately 7.5 magnitude units and a subsequent water surge triggered a catastrophe in Istanbul. The origin of the quake was located along the North Anatolian fault in the Sea of Marmara. It was the last major earthquake to hit the metropolis on the Bosporus.
Researchers of the GEOMAR Helmholtz Centre for Ocean Research Kiel (Germany), together with colleagues from France and Turkey, have now been able to demonstrate for the first time with direct measurements on the seafloor that considerable tectonic strain has built up again on the North Anatolian fault below the Sea of Marmara. “It would be sufficient to trigger another earthquake with magnitudes between 7.1 to 7.4,” says geophysicist Dr. Dietrich Lange of GEOMAR. He is the lead author of the study published today in the international journal Nature Communications.
The North Anatolian fault zone marks the boundary between the Eurasian and Anatolian plates. “Strong earthquakes occur when the fault zone becomes locked. Then tectonic strain accumulates, and the seismic energy is released in an earthquake,” explains Dr. Lange. The last time this happened was in 1999 at a section of the North Anatolian fault near Izmit, about 90 kilometers east of Istanbul.
Tectonic strain build-up along fault zones on land has been regularly monitored for years using GPS or land surveying methods. This is not possible in seabed fault zones due to the low penetration depth of the GPS satellite signals under water. However, the section of the North Anatolian fault that poses the considerable threat to the Istanbul metropolitan region is located underwater in the Marmara Sea.
Up to now, it has only been possible to extrapolate, for example using land observations, whether the plate boundaries there are moving or locked. However, the methods could not distinguish between a creeping movement and the complete locking of the tectonic plates. The new GeoSEA system developed at GEOMAR measuring acoustic distances on the seabed now enables scientists for the first time to directly measure crustal deformation with mm-precision. Over a period of two and a half years, a total of ten measuring instruments were installed at a water depth of 800 metres on both sides of the fault. During this time, they carried out more than 650,000 distance measurements.
“In order to get measurements accurate within a few millimetres over several hundred of metres, very precise knowledge of the speed of sound underwater is required. Therefore, pressure and temperature fluctuations of the water must also be measured very precisely over the entire period,” explains Prof. Dr. Heidrun Kopp, GeoSEA project manager and co-author of the current study.
“Our measurements show that the fault zone in the Marmara Sea is locked and therefore tectonic strain is building up. This is the first direct proof of the strain build-up on the seabed south of Istanbul,” emphasizes Dr. Lange.
“If the accumulated strain is released during an earthquake, the fault zone would move by more than four metres. This corresponds to an earthquake with a magnitude between 7.1 and 7.4,” adds Professor Kopp. Such an event would very probably have similar far-reaching consequences for nearby Istanbul as the 1999 earthquake for Izmit with over 17,000 casualties.
Reference:
Dietrich Lange, Heidrun Kopp, Jean-Yves Royer, Pierre Henry, Ziyadin Çakir, Florian Petersen, Pierre Sakic, Valerie Ballu, Jörg Bialas, Mehmet Sinan Özeren, Semih Ergintav, Louis Géli. Interseismic strain build-up on the submarine North Anatolian Fault offshore Istanbul. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-11016-z
When carbon emissions pass a critical threshold, it can trigger a spike-like reflex in the carbon cycle, in the form of severe ocean acidification that lasts for 10,000 years, according to a new MIT study.
In the brain, when neurons fire off electrical signals to their neighbors, this happens through an “all-or-none” response. The signal only happens once conditions in the cell breach a certain threshold.
Now an MIT researcher has observed a similar phenomenon in a completely different system: Earth’s carbon cycle.
Daniel Rothman, professor of geophysics and co-director of the Lorenz Center in MIT’s Department of Earth, Atmospheric and Planetary Sciences, has found that when the rate at which carbon dioxide enters the oceans pushes past a certain threshold — whether as the result of a sudden burst or a slow, steady influx — the Earth may respond with a runaway cascade of chemical feedbacks, leading to extreme ocean acidification that dramatically amplifies the effects of the original trigger.
This global reflex causes huge changes in the amount of carbon contained in the Earth’s oceans, and geologists can see evidence of these changes in layers of sediments preserved over hundreds of millions of years.
Rothman looked through these geologic records and observed that over the last 540 million years, the ocean’s store of carbon changed abruptly, then recovered, dozens of times in a fashion similar to the abrupt nature of a neuron spike. This “excitation” of the carbon cycle occurred most dramatically near the time of four of the five great mass extinctions in Earth’s history.
Scientists have attributed various triggers to these events, and they have assumed that the changes in ocean carbon that followed were proportional to the initial trigger — for instance, the smaller the trigger, the smaller the environmental fallout.
But Rothman says that’s not the case. It didn’t matter what initially caused the events; for roughly half the disruptions in his database, once they were set in motion, the rate at which carbon increased was essentially the same. Their characteristic rate is likely a property of the carbon cycle itself — not the triggers, because different triggers would operate at different rates.
What does this all have to do with our modern-day climate? Today’s oceans are absorbing carbon about an order of magnitude faster than the worst case in the geologic record — the end-Permian extinction. But humans have only been pumping carbon dioxide into the atmosphere for hundreds of years, versus the tens of thousands of years or more that it took for volcanic eruptions or other disturbances to trigger the great environmental disruptions of the past. Might the modern increase of carbon be too brief to excite a major disruption?
According to Rothman, today we are “at the precipice of excitation,” and if it occurs, the resulting spike — as evidenced through ocean acidification, species die-offs, and more — is likely to be similar to past global catastrophes.
“Once we’re over the threshold, how we got there may not matter,” says Rothman, who is publishing his results this week in the Proceedings of the National Academy of Sciences. “Once you get over it, you’re dealing with how the Earth works, and it goes on its own ride.”
A carbon feedback
In 2017, Rothman made a dire prediction: By the end of this century, the planet is likely to reach a critical threshold, based on the rapid rate at which humans are adding carbon dioxide to the atmosphere. When we cross that threshold, we are likely to set in motion a freight train of consequences, potentially culminating in the Earth’s sixth mass extinction.
Rothman has since sought to better understand this prediction, and more generally, the way in which the carbon cycle responds once it’s pushed past a critical threshold. In the new paper, he has developed a simple mathematical model to represent the carbon cycle in the Earth’s upper ocean and how it might behave when this threshold is crossed.
Scientists know that when carbon dioxide from the atmosphere dissolves in seawater, it not only makes the oceans more acidic, but it also decreases the concentration of carbonate ions. When the carbonate ion concentration falls below a threshold, shells made of calcium carbonate dissolve. Organisms that make them fare poorly in such harsh conditions.
Shells, in addition to protecting marine life, provide a “ballast effect,” weighing organisms down and enabling them to sink to the ocean floor along with detrital organic carbon, effectively removing carbon dioxide from the upper ocean. But in a world of increasing carbon dioxide, fewer calcifying organisms should mean less carbon dioxide is removed.
“It’s a positive feedback,” Rothman says. “More carbon dioxide leads to more carbon dioxide. The question from a mathematical point of view is, is such a feedback enough to render the system unstable?”
“An inexorable rise”
Rothman captured this positive feedback in his new model, which comprises two differential equations that describe interactions between the various chemical constituents in the upper ocean. He then observed how the model responded as he pumped additional carbon dioxide into the system, at different rates and amounts.
He found that no matter the rate at which he added carbon dioxide to an already stable system, the carbon cycle in the upper ocean remained stable. In response to modest perturbations, the carbon cycle would go temporarily out of whack and experience a brief period of mild ocean acidification, but it would always return to its original state rather than oscillating into a new equilibrium.
When he introduced carbon dioxide at greater rates, he found that once the levels crossed a critical threshold, the carbon cycle reacted with a cascade of positive feedbacks that magnified the original trigger, causing the entire system to spike, in the form of severe ocean acidification. The system did, eventually, return to equilibrium, after tens of thousands of years in today’s oceans — an indication that, despite a violent reaction, the carbon cycle will resume its steady state.
This pattern matches the geological record, Rothman found. The characteristic rate exhibited by half his database results from excitations above, but near, the threshold. Environmental disruptions associated with mass extinction are outliers — they represent excitations well beyond the threshold. At least three of those cases may be related to sustained massive volcanism.
“When you go past a threshold, you get a free kick from the system responding by itself,” Rothman explains. “The system is on an inexorable rise. This is what excitability is, and how a neuron works too.”
Although carbon is entering the oceans today at an unprecedented rate, it is doing so over a geologically brief time. Rothman’s model predicts that the two effects cancel: Faster rates bring us closer to the threshold, but shorter durations move us away. Insofar as the threshold is concerned, the modern world is in roughly the same place it was during longer periods of massive volcanism.
In other words, if today’s human-induced emissions cross the threshold and continue beyond it, as Rothman predicts they soon will, the consequences may be just as severe as what the Earth experienced during its previous mass extinctions.
“It’s difficult to know how things will end up given what’s happening today,” Rothman says. “But we’re probably close to a critical threshold. Any spike would reach its maximum after about 10,000 years. Hopefully that would give us time to find a solution.”
This research was supported, in part, by NASA and the National Science Foundation.
Reference:
Daniel H. Rothman. Characteristic disruptions of an excitable carbon cycle. Proceedings of the National Academy of Sciences, 2019; 201905164 DOI: 10.1073/pnas.1905164116
Polar ice caps on Mars are a combination of water ice and frozen CO2. Like its gaseous form, frozen CO2 allows sunlight to penetrate while trapping heat. In the summer, this solid-state greenhouse effect creates pockets of warming under the ice, seen here as black dots in the ice.
People have long dreamed of re-shaping the Martian climate to make it livable for humans. Carl Sagan was the first outside of the realm of science fiction to propose terraforming. In a 1971 paper, Sagan suggested that vaporizing the northern polar ice caps would “yield ~10 s g cm-2 of atmosphere over the planet, higher global temperatures through the greenhouse effect, and a greatly increased likelihood of liquid water.”
Sagan’s work inspired other researchers and futurists to take seriously the idea of terraforming. The key question was: are there enough greenhouse gases and water on Mars to increase its atmospheric pressure to Earth-like levels?
In 2018, a pair of NASA-funded researchers from the University of Colorado, Boulder and Northern Arizona University found that processing all the sources available on Mars would only increase atmospheric pressure to about 7 percent that of Earth – far short of what is needed to make the planet habitable.
Terraforming Mars, it seemed, was an unfulfillable dream.
Now, researchers from the Harvard University, NASA’s Jet Propulsion Lab, and the University of Edinburgh, have a new idea. Rather than trying to change the whole planet, what if you took a more regional approach?
The researchers suggest that regions of the Martian surface could be made habitable with a material — silica aerogel — that mimics Earth’s atmospheric greenhouse effect. Through modeling and experiments, the researchers show that a two to three-centimeter-thick shield of silica aerogel could transmit enough visible light for photosynthesis, block hazardous ultraviolet radiation, and raise temperatures underneath permanently above the melting point of water, all without the need for any internal heat source.
The paper is published in Nature Astronomy.
“This regional approach to making Mars habitable is much more achievable than global atmospheric modification,” said Robin Wordsworth, Assistant Professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Department of Earth and Planetary Science. “Unlike the previous ideas to make Mars habitable, this is something that can be developed and tested systematically with materials and technology we already have.”
“Mars is the most habitable planet in our Solar System besides Earth,” said Laura Kerber, Research Scientist at NASA’s Jet Propulsion Laboratory. “But it remains a hostile world for many kinds of life. A system for creating small islands of habitability would allow us to transform Mars in a controlled and scalable way.”
The researchers were inspired by a phenomenon that already occurs on Mars.
Unlike Earth’s polar ice caps, which are made of frozen water, polar ice caps on Mars are a combination of water ice and frozen CO2. Like its gaseous form, frozen CO2 allows sunlight to penetrate while trapping heat. In the summer, this solid-state greenhouse effect creates pockets of warming under the ice.
“We started thinking about this solid-state greenhouse effect and how it could be invoked for creating habitable environments on Mars in the future,” said Wordsworth. “We started thinking about what kind of materials could minimize thermal conductivity but still transmit as much light as possible.”
The researchers landed on silica aerogel, one of the most insulating materials ever created.
Silica aerogels are 97 percent porous, meaning light moves through the material but the interconnecting nanolayers of silicon dioxide infrared radiation and greatly slow the conduction of heat. These aerogels are used in several engineering applications today, including NASA’s Mars Exploration Rovers.
“Silica aerogel is a promising material because its effect is passive,” said Kerber. “It wouldn’t require large amounts of energy or maintenance of moving parts to keep an area warm over long periods of time.”
Using modeling and experiments that mimicked the Martian surface, the researchers demonstrated that a thin layer of this material increased average temperatures of mid-latitudes on Mars to Earth-like temperatures.
“Spread across a large enough area, you wouldn’t need any other technology or physics, you would just need a layer of this stuff on the surface and underneath you would have permanent liquid water,” said Wordsworth.
This material could be used to build habitation domes or even self-contained biospheres on Mars.
“There’s a whole host of fascinating engineering questions that emerge from this,” said Wordsworth.
Next, the team aims to test the material in Mars-like climates on Earth, such as the dry valleys of Antarctica or Chile.
Wordsworth points out that any discussion about making Mars habitable for humans and Earth life also raises important philosophical and ethical questions about planetary protection.
“If you’re going to enable life on the Martian surface, are you sure that there’s not life there already? If there is, how do we navigate that,” asked Wordsworth. “The moment we decide to commit to having humans on Mars, these questions are inevitable.”
Reference:
R. Wordsworth, L. Kerber & C. Cockell. Enabling Martian habitability with silica aerogel via the solid-state greenhouse effect. Nature Astronomy, 2019 DOI: 10.1038/s41550-019-0813-0
A fresh surface rupture of the Kekerengu fault, taken 4 days after the Kaikoura earthquake, with lead author Jesse Kearse leaning next to a curved slickenline (the subject of this article). Photo credit – Professor Tim Little (Victoria University of Wellington).
Slip-parallel grooves (striations) on fault surfaces are considered a robust indicator of fault slip direction, yet their potential for recording aspects of earthquake rupture dynamics has received little attention. During the 2016 Kaikōura earthquake (South Island, New Zealand), >10 m of dextral strike-slip on the steeply dipping Kekerengu fault exhumed >200 m2 of fresh fault exposure (free faces) where it crossed bedrock canyons. Inscribed upon these surfaces, we observed individual striae up to 6 m long, all of which had formed during the earthquake. These were typically curved. Using simulations of spontaneous dynamic rupture on a vertical strike slip fault, we reproduce the curved morphology of striae on the Kekerengu fault.
Assuming strike-slip pre-stress, our models demonstrate that vertical tractions induced by slip in the so-called cohesive zone result in transient changes in slip direction. We show that slip-path convexity is sensitive to the direction of rupture propagation. To match the convexity of striae formed in 2016 requires the rupture to have propagated in a northeast direction, a prediction that matches the known rupture direction of the Kaikōura earthquake. Our study highlights the potential for fault striae to record aspects of rupture dynamics, including the rupture direction of paleo strike-slip earthquakes.
The direction of earthquake rupture propagation exerts a profound influence on the distribution of strong ground shaking. As a consequence, damage and loss experienced by a community during an earthquake is dependent on the community’s location relative to the direction of rupture propagation—a relationship especially true for large strike-slip events (Somerville et al., 1997; Gomberg et al., 2001).
This effect has been observed in many large magnitude strike-slip earthquakes in recent decades; e.g., the 1992 Mw 7.3 Landers (California) earthquake (Velasco et al., 1994), the 2002 Mw 7.9 Denali (Alaska) earthquake (Eberhart-Phillips et al., 2003), and the 2018 Mw 7.5 Palu (Indonesia) earthquake (Bao et al., 2019). Based on historical data, it is uncertain whether large magnitude earthquakes systematically repeat their rupture direction; moreover, establishing the direction of past earthquakes using geological data has proved difficult. Previous studies have used structural data such as branching fault intersections (Poliakov et al., 2002; Fliss et al., 2005) and off-fault damage patterns in both ancient fault zones (Di Toro et al., 2005) and recent surface ruptures (Klinger et al., 2018) to infer rupture direction. However, these studies lack a context of seismological data with which to verify the rupture direction during a particular event.
Based on our literature review, out of ~55 surface ruptures observed geologically in the field since the late 1800s (Wesnousky, 2008; Biasi et al., 2013), 20 of the field datasets include measurements of coseismic slickenlines. Of these, 70% are curved, indicating that coseismic changes in slip direction are common (see the GSA Data Repository1 and references therein). Spudich et al. (1998) used the curvature of slickenlines (up to 50°) found on the Nojima fault following the 1995 Kobe (Japan) earthquake to infer the magnitude of shear traction on the fault surface. However, aside from this pioneering work, the potential link between curved slickenlines and the dynamics of earthquake rupture has received little attention. In this study, we propose that striae produced on fault plane surfaces during coseismic slip can reveal aspects of the dynamic rupture process.
We present new slickenline data from fault surfaces that were exposed during the 2016 Mw 7.8 Kaikōura earthquake (South Island, New Zealand)—an event that has a well-established southwest-northeast rupture direction across ~200 km of the Pacific-Australian plate boundary in New Zealand (Hamling et al., 2017; Kaiser et al., 2017; Ando and Kaneko, 2018; Litchfield et al., 2018) (Fig. 1A). We observed curved coseismic striae on multiple fault outcrops along a 10 km section of the Kekerengu fault, the structure that yielded the greatest slip and moment release during this earthquake (Cesca et al., 2017; Holden et al., 2017). The systematic convexity of these striae records temporal changes in the coseismic slip direction on this fault. To uncover the mechanism that was responsible for the observed variation in slip direction, we undertook dynamic rupture modeling of a vertical strike-slip fault, using a simple set of boundary conditions. Here we discuss the potential for curved striae to record the rupture direction of paleo strike-slip earthquakes.
Curved Striae on the Kekerengu Fault
The Kekerengu fault is part of the Marlborough Fault System, a series of mainly dextral faults that transfer slip between the Hikurangi megathrust and the Alpine fault. Dextral offset of Pleistocene terraces and uplift of the Seaward Kaikōura Ranges on its western side show that the Kekerengu fault is primarily strike-slip, with a lesser reverse component (Van Dissen et al., 2016). During the Kaikōura earthquake, many stream canyons were dextrally displaced 8–10 m and uplifted on their northwest sides by 1–2 m across the Kekerengu fault, which dips steeply to the northwest (>65° dip) (Kearse et al., 2018). Offset of these steep canyon walls (slopes of 20–30°) exhumed more than a dozen smooth and near-planar fault surfaces that were ≤7 m long in the slip direction, each coated in a 1–2 cm layer of soft fault gouge. Nine of these free faces hosted coseismic fault striae that were curved. For example, Figure 1D depicts a striation where slip initiated as dextral-reverse motion, and as further displacement accrued, the direction of slip curved to become nearly pure strike-slip.
This convex-up slip path was repeated at six out of the nine sites that contained curved striae, with some sites containing many measurement locations where striae were recorded. Curved striae were found mostly on the upper part of the free face, suggesting they were formed early in the rupture. The angular magnitude of the observed convexity is typically ~15° and is >40° at sites 3 and 7. At two sharp bends in the fault trace, the sense of uplift was opposite to the dominant one, and their striae describe a convex-down slip path. In all cases, the incremental magnitude of fault slip—both vertical and dextral—recorded by the individual fault striae segments (0.2–6 m) amounts to only a fraction of the known coseismic net displacement at these sites, which was 8–10 m.
Post-earthquake erosion of free faces in the days to months following the earthquake undoubtedly curtailed the length of individual straie; however, some striae become increasingly diffuse toward their ‘downstream’ ends , suggesting a different process is responsible for their limited length. Near-fault geodetic and seismological data collected at two stations (the KEKS seismic station, and a high-rate continuous GPS station located 30 km northeast of the study area) on the northwest side of the Kekerengu fault during the earthquake (Fig. 2A; see the Data Repository) imply that the initial ~5 s of rupturing on the Kekerengu fault accumulated up to 4 m of almost pure dextral-slip and <0.3 m of throw, whereas most of the throw (1–2 m) began accumulating 4–7 s after this. The available geological and geophysical data suggest that the curved striae on the Kekerengu fault were inscribed during the early phase of strike-slip–dominated rupturing.
Dynamic Rupture Model
To evaluate possible mechanisms for the curvature of Kekerengu fault striae, we simulated the spontaneous, dynamic rupture of a 50-km-long and 25-km-deep vertical strike-slip fault in layered elastic half space, using a spectral-element code (Ampuero, 2002; Kaneko et al., 2008). A horizontal initial shear traction was applied to the fault surface, with a magnitude that increased linearly with depth. Dynamic rupture was initiated by overstressing a small 3-km-wide patch in the center of the fault at a depth of 12 km, and then the rupture propagated spontaneously across the fault surface, where it was governed by slip-weakening friction laws (for additional details of the model setup, see the Data Repository).
Slip paths for 42 nodes on the fault surface (gray arrows in Fig. 3C) depict cseismic motion of the near-side of the fault relative to the far side. Their lengths depict slip magnitudes, and their shape and curvature illustrates any changes in slip direction. At depths >3 km, slip paths are horizontal and show little change in slip direction with time. For nodes located along mixed-mode rupture directions (Andrews, 1994) at depths <3 km, slip paths are more complex. At depths <3 km, slip follows a curved path during the initial few decimeters; at locations between x = 0 and x = –20 km, curvature is convex-up (Fig. 3A), while on the other side of the hypocenter between x = 0 and x = 20 km (right side of Fig. 3C), curvature is convex-down. As further displacement is accrued, slip directions converge toward a horizontal rake of 180°. These results do not necessarily imply that one side of the fault experiences an opposing sense of net uplift at either end of a rupture (this is only true for cases with pure strike-slip pre-stress). Rather, our results suggest that the convexity of the initial slip path is opposite at either end of the rupture, regardless of the rake of initial shear traction on the fault.
Mechanism for Changes in Slip Direction
The mechanism behind transient changes in slip direction in our dynamic model is consistent with the one proposed by Spudich et al. (1998). Dynamic changes in rake angle are driven by vertical stress changes along mixed-mode rupture directions produced within the so-called cohesive zone (the part of the fault that is slipping but has not yet fully weakened). Seismic waves are continuously generated within this zone, and they carry vertical stresses that induce vertical components of slip on the sliding surface.
At depths below 3 km, confining stresses are large compared to the dynamic loads, a relationship that suppresses the magnitude of these transient vertical motions. Yet slip at these depths retains subtle (<2°) temporal changes in slip direction, and exhibits the same reversal in path convexity on either side of the hypocenter (left versus right sides of Figure 3C are yellow and green, respectively; see the Data Repository).
Near Earth’s surface, where confining stresses are low, the ratio of dynamic to confining stress is larger, promoting a larger magnitude of coseismic vertical slip. As the rupture front propagates along strike, the model predicts opposite senses of slip deflection near the two propagating rupture tips: convex-upward on one side and convex-downward on the other. Figure 4 details changes in shear traction and slip velocity at the rupture tip to highlight the correlation between vertical stress and rotation of the slip direction.
Discussion and Conclusions
We note a correlation between the convexity of curved striae on the Kekerengu fault and results of our dynamic rupture model. As slip initiates, movement of the downstream side of the fault (the side moving in the direction of rupture propagation) follows a convex-up slip path that initially has a rake of ~15° to (and ultimately becomes parallel with) the direction of initial shear stress on the fault (Fig. 3A). Although several of our sites—specifically those at sharp fault bends—showed the opposite sense of slip-path convexity, we suggest that the kinematic differences at these areas induced local stresses that overwhelmed the more general dynamic effects described by our model. We therefore interpret the predominant pattern of slip-path curvature on the Kekerengu fault to have resulted from vertical stress changes in the cohesive zone of the northeast-propagating rupture of the Kaikōura earthquake. In addition, the curvature of striae seen on multiple exposures of the Nojima fault following the 1995 Mw 7.2 Kobe (Japan) earthquake (Otsuki et al., 1997) is consistent with our model, and with the known direction of rupture propagation in this event (Yoshida et al., 1996). These dynamic effects are likely not to be restricted to strike-slip faults, as mixed-mode dynamic rupture on dip-slip faults should also produce transient changes in initial slip direction, which may explain curved striae seen after events such as the 1954 M 7.0 Fairview Peak (Nevada, USA) (Slemmons, 1957) and 2011 Mw 6.6 Fukushima (Japan) earthquakes (Otsubo et al., 2013).
We offer two possible explanations for why striae on the Kekerengu fault capture only an early and small (typically <10% of total) increment of the 2016 slip. One possibility is that, at the onset of slip, the fault plane (in the near surface) was rough compared to the final stages of slip, having only just broken any cohesion gained during the interseismic period. If so, this might promote asperity ploughing and striae production at that early stage (Tesei et al., 2017), thus promoting a temporal bias in the striation record. According to this scenario, the early-formed asperities (i.e., topography in the new fracture surface) were progressively abraded during the accumulating slip, resulting in a smoother fault plane and a termination in striation production. Another possibility (and one that is not mutually exclusive with the former) is that during slip in the near-surface, along-strike translation of large (meter-scale) corrugations in the fault surface (e.g., Candela et al., 2012) promoted transient separation of the fault walls, thereby terminating the engraving process. Both ideas are supported by individual striae that are deeply inset near their inception point but are shallower and more diffuse toward their termination.
Spudich et al. (1998) together with Guatteri and Spudich (1998)—both of which assume depth-independent pre stress—suggest that the fractional stress drop, defined as the ratio of the stress drop to the magnitude of initial shear stress, is ~0.7 on faults that exhibit large (>30°) changes in coseismic slip direction. In our model that features depth-dependent stress, in order to reproduce up to ~20° changes in slip direction on the Kekerengu fault, the fractional stress drop is ~0.2, which corresponds to the ‘high stress’ case of Guatteri and Spudich (1998). Even in the case of ‘low stress’, the pattern of slip-path convexity should still depend on rupture direction.
Striated fault surfaces are routinely measured in neotectonic and paleoseismic studies, including on well preserved Holocene scarps and on excavated fault surfaces in paleoseismic trenches. Crosscutting or overlapping striae seen in such places do not necessarily represent the superposition of multiple earthquakes; and our results provide a simple explanation for transient changes in slip direction during a single slip episode, which can create overlapping striae (e.g., the 2011 Fukushima earthquake; Otsubo et al., 2013). More importantly, our study demonstrates that it may be possible to infer the rupture direction of paleoearthquakes in cases where coseismic striae record temporal changes in slip direction. To do this, we propose that data be collected from structurally simple parts of the fault that are representative of its overall kinematics, and that are also likely to generate and preserve striae. We further suggest that multiple sites be evaluated to establish any systematic patterns in striation convexity.
Temporal changes in slip direction during dynamic rupture are likely to be a widespread characteristic in the near-surface during large-magnitude earthquakes. Geological evidence of this type, when combined with simulations of dynamic rupture, may yield important insight into the near-field dynamics of fault slip, including the rupture direction of paleo strike-slip earthquakes and, by inference, the rupture direction of future strike-slip earthquakes; thereby helping to better constrain the distribution of damaging ground shaking in those events.
Reference:
Jesse Kearse, Yoshihiro Kaneko, Tim Little, Russ Van Dissen; Curved slickenlines preserve direction of rupture propagation. Geology doi: https://doi.org/10.1130/G46563.1
Earthquake in Southern California. Credit: Public Domain
The odds that Southern California will experience another earthquake of magnitude 7 or greater in the next week are now nearly 11%, according to preliminary estimates from seismologists.
And the chances that a quake will surpass the 7.1 temblor that struck near Ridgecrest on Friday night are roughly 8% to 9%, said Caltech seismologist Lucy Jones.
“There’s about a 1 in 10 chance that we could have another 7 in this sequence,” she said.
More likely is that the Owens Valley will experience another temblor of magnitude 6 or greater. The odds of that are slightly greater than 50-50, Jones said. And more quakes of magnitude 5 or greater are a near certainty.
“It would be extremely unusual if we didn’t have another 5” over the coming week, Jones said.
The 7.1 quake on Friday night was about 8 times larger than the 6.4 quake on Thursday morning, said Robert Graves, a seismologist with the U.S. Geological Survey in Pasadena.
Though such quakes would be more likely to occur in the next few days, the shaking could continue for quite some time.
“A magnitude 7 usually has aftershocks that last for years,” Jones said.
Jones said she could not recall a pattern of earthquakes in California where a 6.4 foreshock was followed by a 7.1 event, only to be followed by an even bigger quake.
But that doesn’t mean it can’t happen, she cautioned.
“It is clearly a very energetic sequence, so there’s no reason to think we can’t have more large earthquakes,” she said.
The last time California experienced a quake of this size was in 1999, when the 1999 Hector Mine quake struck the Mojave Desert. There were minor injuries and no deaths.
The years since have been marked by an unusual earthquake drought.
“In California, we expect to have a magnitude 7 once every 10 to 20 years, and the last one was 20 years ago,” Jones said. “Think of this as a return to what California is supposed to be doing.”
But she also said the activity near Ridgecrest is not likely to trigger any activity on the San Andreas fault.
The 7.1 quake occurred on the same fault system as the 6.4 temblor that is now being considered a foreshock. It was further away from Los Angeles, though still in the Owens Valley.
“This happened at the end of the zone that moved previously,” Jones said.
As of 11 p.m., it has already been followed by three quakes above a magnitude 5, 17 above a magnitude 4 and more than 70 that exceeded magnitude 3, Jones said.
She said the fault is now likely to be 25 to 30 miles long.
“The fault is growing,” Jones said.
Thursday’s 6.4 foreshock triggered shaking in Ridgecrest of intensity 8. With Friday’s 7.1 main shock, the shaking intensity reached a 9, Jones said.
“My expectation is that Ridgecrest is having a pretty difficult time tonight,” Jones said.
“We don’t get the 7s very often,” she added.
Note: The above post is reprinted from materials provided by Los Angeles Times . The original article was written by Karen Kaplan .
Turquoise is an opaque, blue-to-green mineral with the chemical formula CuAl6(PO4)4(OH)8·4H2O, a hydrated copper and aluminum phosphate. It is common and useful in higher grades and has been appreciated for thousands of years as a gemstone and ornamental stone due to its distinctive shade. Recently, the implementation of medicines, imitations and synthetics on the market has devalued turquoise, like most other opaque gems.
Turquoise Properties
The finest of turquoise, or slightly more than window glass, reaches a peak Mohs hardness of just under 6. A cryptocrystalline mineral characteristic, turquoise almost never creates single crystals, and all its characteristics are extremely variable. X-ray diffraction analysis proves to be triclinic in its crystal system. With reduced strength produces reduced particular pressure (2.60–2.90) and higher porosity; the grain size depends on these characteristics.
Turquoise luster is typically waxy to subvitreous, and its transparency is generally opaque, but in thin parts it may be semitranslucent. Color is as variable as the other characteristics of the mineral, varying from white to a blue powder to a purple sky, and from a green blue to a green yellow. The blue is ascribed to idiochromatic copper whereas the green may result from either iron impurities (aluminum replacement) or dehydration.
How Turquoise Is Formed?
Turquoise shapes as a secondary mineral by the activity of percolating acidic aqueous solutions during weathering and oxidation of pre-existing minerals. For instance, copper may come from main copper sulfides such as chalcopyrite or secondary carbonates malachite or azurite; aluminum may come from feldspar; and apatite phosphorus.
As turquoise is typically found in arid areas, filling or encrusting cavities and fractures in typically extremely modified volcanic rocks, often associated with limonite and other iron oxides, climatic variables appear to play an significant part. Turquoise is almost invariably combined with copper sulfide deposits weathering products in or around potassium-feldspar-bearing porphyritic intrusives in the southwestern United States.
Potassium aluminum sulfate is a prominent secondary mineral in some cases of alunite. Typically turquoise mineralization is limited to a comparatively shallow depth of less than 20 meters (66 feet), although it occurs in deeper fracture areas where secondary solutions have higher penetration or water table depth is higher.
Turquoise is almost always cryptocrystalline and huge and requires no clear internal form. Crystals are exceedingly uncommon even on a microscopic scale. Typically the shape is in habit of filling vein or fracture, nodular or botryoidal. Forms of stalactitis were recorded. Feldspar, apatite, other minerals, or even fossils can also be pseudomorphically replaced by turquoise. Odontolite is a fossil bone or ivory traditionally believed to have been changed by turquoise or similar minerals of phosphate such as iron phosphate vivianite. Also prevalent is intergrowth with other secondary minerals of copper such as chrysocolla.
Where Turquoise is found?
These are all small-scale activities, often seasonal due to the restricted deposit size and remoteness. Most of them are handmade with little or no mechanization. However, turquoise is often retrieved, particularly in the United States, as a by-product of large-scale copper mining activities.
United States
Arizona, California (San Bernardino, Imperial, Inyo counties), Colorado (Conejos, El Paso, Lake, Saguache counties), New Mexico (Eddy, Grant, Otero, Santa Fe counties) and Nevada (Clark, Elko, Esmeralda counties, Eureka, Lander, Mineral County and Nye counties) are (or were) particularly wealthy. Pre-Columbian Native Americans mined California and New Mexico’s reserves using stone tools, some local and some as far remote as southern Mexico. Cerrillos, New Mexico is believed to be the site of the earliest mines; the state was the biggest producer of the country before the 1920s; it is now more or less depleted. Today, only one mine at Apache Canyon in California works at a business capability.
The turquoise happens in the form of vein or seam fillings and as compact nuggets, most of which are tiny in number. While sometimes quite good fabric is discovered, rivaling Iranian fabric in both color and durability, most American turquoise is of small grade (called “chalk turquoise”); elevated iron levels imply predominance of greens and yellows, and a typically friendly consistency in the untreated state of the turquoise precludes jewelry use.
Currently Arizona is the most important value producer of turquoise. There are several mines in the state, two of which are renowned for their distinctive color and performance and regarded the finest in the sector: in August 2012, the Sleeping Beauty Mine in Globe stopped turquoise mining. Due to the increasing world market price of copper, the mine chose to ship all the ore to the crusher and focus on copper manufacturing. The cost of untreated natural sleeping beauty turquoise has increased dramatically since the closure of the mine. As of 2015, the Kingman Mine is still operating outside the town alongside a copper mine. Other mines include the Blue Bird mine, Castle Dome, and Ithaca Peak, but are largely inactive owing to elevated operating costs and federal laws. Bisbee’s Phelps Dodge Lavender Pit mine came to a halt in 1974 and never had a turquoise contractor. All the turquoise Bisbee was mined with “dinner pail.” It arrived in the dinner pails of miners from the copper ore mine. Morenci and Turkey Peak are either idle or exhausted.
Nevada is the other major producer of the country, with more than 120 mines producing large amounts of turquoise. Unlike elsewhere in the US, most of the Nevada mines were mainly used for their turquoise gem and very little was retrieved as a by-product of other mining activities. Nevada turquoise is discovered as nuggets, fracture fillings and interstices between pieces are discovered in breccias as the cement coating. A majority of the material produced is hard and dense because of the geology of the deposits of Nevada, being of sufficient quality that no treatment or improvement is required. Although almost every state county has yielded some turquoise, the main manufacturers are in the counties of Lander and Esmeralda. Most of Nevada’s turquoise deposits happen along a broad range of tectonic activity coinciding with the thrust fault area of the state. It strikes at a bearing of about 15 ° and stretches from the southern portion of Elko County to the south-west boundary of Tonopah in California. Nevada has created a broad variety of colors and combinations of multiple matrix motifs, with Nevada turquoise coming in distinct colors of blue, blue-green, and green. Some of this unusually colored turquoise may involve important zinc and iron, causing the lovely tones of light green to yellow-green. Some of the colors of blue to green-yellow may effectively be variscite or faustite, which is comparable in shape to turquoise secondary phosphate minerals.
Egypt
Since at least the First Dynasty (3000 BCE) in ancient Egypt, and probably before that, the Egyptians used turquoise and the Sinai Peninsula was mined by them. This area was regarded by the indigenous Monitu as the Turkey Country. The peninsula has six mines, all on its southwest shore, spanning an area of approximately 650 km2 (250 sq mi). From a historical view, the two most significant of these mines are Serabit el-Khadim and Wadi Maghareh, one of the earliest recognized mines. The former mine is about 4 kilometers from an old temple devoted to the Hathor deity.
The turquoise is discovered in basalt-covered sandstone, or was initially overlain. There are works of copper and iron in the region. Large-scale turquoise mining is not lucrative today, but Bedouin populations use homemade weapons to quarry the deposits sporadically. Miners face a danger of flash flooding in the rainy summer months; mortality from the fall of the haphazardly utilized sandstone mine walls is not unheard of even in the dry season. The color of material from Sinai is typically greener than material from Iran, but is believed to be stable and relatively durable. Sinai fabric is typically the most translucent, often referred to as “Egyptian jade,” and its surface structure is found to be peppered with light green disks not seen in fabric from other locations.
Iran
For at least 2,000 years, Iran has been a major source of turquoise. It was originally called “pērōzah” by Iranians meaning “win,” and subsequently it was called “fayrūzah” by the Arabs, which is described as “fīrūzeh” in modern Persian. The black turquoise was used in Iranian architecture to cover the palace towers as its intense red color was also a sign of heaven on earth.
This sample is obviously black and becomes green owing to dehydration when heated. It is limited to a mine-riddled region in Nishapur, Ali-mersai’s 2,012 m (6,601 ft) summit close Mashhad, the capital of Khorasan Province, Iran. A battered and crushed trachyte is the host of the turquoise, discovered both in situ between limonite and sandstone strata and between the scree at the top of the mountain. Together with the Sinai Peninsula, these works are the longest known. Iran also has turquoise mines in the provinces of Semnan and Kerman.
Bulgaria
Turquoise prehistoric artifacts (beads) have been known from sites in the Eastern Rhodope Mountains in Bulgaria since the fifth millennium BCE – the source of the raw material may be related to the nearby Spahievo field of lead – zinc ore.
China
For 3,000 years or more, China has been a small source of turquoise. Gem-quality material in the shape of compact nodules is discovered in Yunxian and Zhushan, Hubei county, broken, silicized calcareous. Marco Polo also revealed turquoise discovered in Sichuan today. Most of the Chinese product is imported, but there are a few jade-like sculptures that have operated. In Tibet, there are allegedly gem-quality deposits in the Derge and Nagari-Khorsum hills in the region’s north and south respectively.
Other sources
Other notable locations include: Afghanistan ; Australia (Victoria and Queensland) ; northern India ; northern Chile (Chuquicamata) ; Cornwall ; Saxony ; Silesia ; and Turkestan.
Volatile elements in magma, primarily water, drive explosive volcanic eruptions, like this eruption of Anak Krakatau in Lampung, Indonesia, in 2018. Experimental geochemists from Washington University in St. Louis have discovered compelling evidence that magmas may be wetter than once thought. Credit: Shutterstock
Volatile elements in magma, primarily water, drive explosive volcanic eruptions. The tricky part is determining just how much volatile content was present before the eruption took place. This is especially difficult when the only evidence scientists have to go on is the end product after all the volatiles have been lost.
New research from Washington University in St. Louis provides compelling evidence that magmas may be wetter than once thought. The work, led by experimental geochemists including Michael J. Krawczynski, assistant professor of earth and planetary sciences in Arts & Sciences, is published in the July 2 issue of the journal American Mineralogist.
The most common method for determining volatile content requires studying melt inclusions, small pieces of magma trapped in crystals erupted in lava. Scientists study these glassy inclusions to determine the amount of hydrogen present—which, through back calculation, can indicate how much water was dissolved in the magma in Earth’s crust before a volcano erupted. This method is generally accepted as an accurate lower limit on volatile content, accounting for some water that might have been lost during the explosion itself.
Krawczynski and Maxim Gavrilenko, a former postdoctoral fellow in Krawczynski’s lab who is now at the University of Nevada, Reno, instead wanted to look at the upper limit—something that had not been investigated experimentally.
“What people have never looked at before and what we’re trying to measure now is, how big is this bucket?” Krawczynski said. “You can imagine if it rained a lot, and your rain gauge was full, then you don’t really know how much it rained. It could have rained more! We just can’t say.”
The same is true of melt inclusions. If a melt inclusion can’t hold all the water, then scientists don’t have the right reading of the upper limit of water content in magma. The bucket is too small.
Creating magma in the lab
Gavrilenko and Krawczynski created synthetic melt inclusions in the lab to figure out how much water a melt possibly could hold. To do this, the researchers reproduced the temperature and pressure conditions that exist 40 kilometers below the Earth’s surface. Next, they melted and quenched (rapidly cooled) the sample, then determined whether their experiment had created a glass. They continued the process, adding more and more water to the sample until the sample could no longer be quenched to form a glass.
“We found out that if you have a lot of water, then eventually you don’t have a glass,” Gavrilenko said. These devitrified (non-glassy) melt inclusions exist in nature, but are preferentially not studied for volatiles—which has led to sample bias in this field of research.
This bias is especially problematic for scientists who are trying to understand how much water gets recycled back to the surface at subduction zones, which are richest in water compared to other tectonic settings. “If the deep magmas in these zones have more than 9 weight-percent of water, then they won’t be correctly measured by the current gold-standard method,” Gavrilenko said. “There is a need to find a new method for how to measure. We need a new, bigger bucket.”
Global water cycle implications
These results align with recent work by Douglas A. Wiens, the Robert S. Brookings Distinguished Professor in earth and planetary sciences. In a paper published in the journal Nature last fall, Wiens concluded that as much as four times more water might subduct into Earth’s mantle than scientists thought. Gavrilenko and Krawczynski’s work points to how the cycle of water in and out of the crust might be rebalanced in the wake of such discoveries.
“If more is going down (into the mantle), more needs to be coming out back into Earth’s crust,” Krawczynski said. “That’s what we’re looking at here. We’ve understood that it’s a cycle that needs to be balanced, but we haven’t had a good grasp of the sizes of the different reservoirs.”
Reference:
Maxim Gavrilenko et al. The quench control of water estimates in convergent margin magmas, American Mineralogist (2019). DOI: 10.2138/am-2019-6735
