Study explains Pacific equatorial cold water region

A new study published this week in the journal Nature reveals for the first time how the mixing of cold, deep waters from below can change sea surface temperatures on seasonal and longer timescales.

Because this occurs in a huge region of the ocean that takes up heat from the atmosphere, these changes can influence global climate patterns, particularly global warming.

Using a new measurement of mixing, Jim Moum and Jonathan Nash of the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University have obtained the first multi-year records of mixing that permit assessment of seasonal changes. This is a significant advance beyond traditional shipboard measurements that are limited to the time that a ship can be away from port. Small instruments fueled by lithium batteries were built to be easily deployed on deep-sea equatorial moorings.

Moum employs a simple demonstration to show how mixing works.

He pours cold, white cream into a clear glass mug full of hot, black coffee, very carefully, using a straw to inject the heavier cream at the bottom of the mug, where it remains.

“Now we can wait until the cream diffuses into the coffee, and we’ll have a nice cuppa joe,” Moum says. “Unfortunately, the coffee will be cold by then. Or, we can introduce some external energy into the system, and mix it.”

A stirring spoon reveals motions in the mug outlined by the black/white contrasts of cream in coffee until the contrast completely disappears, and the color achieves that of café au lait.

“Mixing is obviously important in our normal lives, from the kitchen to the dispersal of pollutants in the atmosphere, reducing them to levels that are barely tolerable,” he said.

The new study shows how mixing, at the same small scales that appear in your morning coffee, is critical to the ocean. It outlines the processes that create the equatorial Pacific cold tongue, a broad expanse of ocean near the equator that is roughly the size of the continental United States, with sea surface temperatures substantially cooler than surrounding areas.

Because this is a huge expanse that takes up heat from the atmosphere, understanding how it does so is critical to seasonal weather patterns, El Nino, and to global climate change.

In temperate latitudes, the atmosphere heats the ocean in summer and cools it in winter. This causes a clear seasonal cycle in sea surface temperature, at least in the middle of the ocean. At low latitudes near the equator, the atmosphere heats the sea surface throughout the year. Yet a strong seasonal cycle in sea surface temperature is present here, as well. This has puzzled oceanographers for decades who have suspected mixing may be the cause but have not been able to prove this.

Moum, Nash and their colleagues began their effort in 2005 to document mixing at various depths on an annual basis, which previously had been a near-impossible task.

“This is a very important area scientifically, but it’s also quite remote,” Moum said. “From a ship it’s impossible to get the kinds of record lengths needed to resolve seasonal cycles, let alone processes with longer-term cycles like El Nino and La Nina. But for the first time in 2005, we were able to deploy instrumentation to measure mixing on a NOAA mooring and monitor the processes on a year-round basis.”

The researchers found clear evidence that mixing alone cools the sea surface in the cold tongue, and that the magnitude of mixing is influenced by equatorial currents that flow from east to west at the surface, and from west to east in deeper waters 100 meters beneath the surface.

“There is a hint – although it is too early to tell – that increased mixing may lead, or have a correlation to the development of La Niña,” Moum said. “Conversely, less mixing may be associated with El Niño. But we only have a six-year record – we’ll need 25 years or more to reach any conclusions on this question.”

Nash said the biggest uncertainty in climate change models is understanding some of the basic processes for the mixing of deep-ocean and surface waters and the impacts on sea surface temperatures. This work should make climate models more accurate in the future.

Mystery of before 370 Ma coral-stromatoporoid reef disappearing from the planet Earth

This shows invading bacteria-algae (yellow arrows) in the Devonian reef-building corals. -  ©Science China Press
This shows invading bacteria-algae (yellow arrows) in the Devonian reef-building corals. – ©Science China Press

The coral-stromatoporoid reef disappearing from the planet earth was one of the most significant and representative phenomena for the Late Devonian F-F transitional mass extinction event. Professor GONG Yiming and his group (Wu Yibu, Feng Qi etc.) from State Key Laboratory of Biogeology and Environmental Geology of China University of Geosciences are trying to tackle this problem. After several years of continuous research, they have discovered that blooming and invading of bacteria and algae played an important role for before 370 Ma (Late Devonian F-F transition) coral-stromatoporoid reef disappearing from the planet earth. And this may be the key that unlock mystery of the modern reef-building coral bleaching. Their work, entitled “blooming of bacteria and algae is a biokiller for mass-extinction of Devonian coral-stromatoporoid reef ecosystems”, was published in SCIENCE CHINA Earth Sciences, 2013, Vol. 56, No.7.

Modern coral reefs are widespread in tropical and subtropical shallow sea, while coral-stromatoporoid reefs were much more widespread and spectacular before 370 Ma. But the former keep reducing because of reef-building corals bleaching; while the latter were extincted before 370 Ma. Why these happened? Are there any relationships between them? Bacteria and algae may be the most potent killer for both of them.

The innovation of this study is that evidences of direct interactions between bacteria and algae and living reef-building coral-stromatoporoids in the Devonian fossil records have been found in terms of the relationships between organisms and environments, as well as interactions between microbes and macro-organisms. These evidences include many kinds of invading bacteria and algae in “living” Devonian reef-builders in thin sections (as shown in the Figure 1). Bacteria and algae prevented growth of coral-stromatoporoids and the latter had the ability of self-healing(as shown in the Figure 2). Hence they suggested that blooming and invading of bacteria and algae was a possible biokiller for before 370 Ma coral-stromatoporoid reef disappearing from the planet earth. It is worth noting that similar phenomena have been widely reported in modern coral reefs.
This study may give the key to unlock mystery of the modern reef-building coral bleaching and also provide a warning and revelation in prevention and treatment of modern coral reef bleaching.

Devastating long-distance impact of earthquakes

In 2006 the island of Java, Indonesia was struck by a devastating earthquake followed by the onset of a mud eruption to the east, flooding villages over several square kilometers and that continues to erupt today. Until now, researchers believed the earthquake was too far from the mud volcano to trigger the eruption. Geophysicists at the University of Bonn, Germany and ETH Zurich, Switzerland use computer-based simulations to show that such triggering is possible over long distances. The results have been published in “Nature Geoscience.”

On May 27, 2006 the ground of the Indonesian island Java was shaking with a magnitude 6.3 earthquake. The epicenter was located 25 km southwest of the city of Yogyakarta and initiated at a depth of 12 km. The earthquake took thousands of lives, injured ten thousand and destroyed buildings and homes. 47 hours later, about 250 km from the earthquake hypocenter, a mud volcano formed that came to be known as “Lusi”, short for “Lumpur Sidoarjo”. Hot mud erupted in the vicinity of an oil drilling-well, shooting mud up to 50 m into the sky and flooding the area. Scientists expect the mud volcano to be active for many more years.

Eruption of mud volcano has natural cause

Was the eruption of the mud triggered by natural events or was it man-made by the nearby exploration-well? Geophysicists at the University of Bonn, Germany and at ETH Zürich, Switzerland investigated this question with numerical wave-propagation experiments. “Many researchers believed that the earthquake epicenter was too far from Lusi to have activated the mud volcano,” says Prof. Dr. Stephen A. Miller from the department of Geodynamics at the University of Bonn. However, using their computer simulations that include the geological features of the Lusi subsurface, the team of Stephen Miller concluded that the earthquake was the trigger, despite the long distance.

The overpressured solid mud layer was trapped between layers with different acoustic properties, and this system was shaken from the earthquake and aftershocks like a bottle of champagne. The key, however, is the reflections provided by the dome-shaped geology underneath Lusi that focused the seismic waves of the earthquakes like the echo inside a cave. Prof. Stephen Miller explains: “Our simulations show that the dome-shaped structure with different properties focused seismic energy into the mud layer and could very well have liquified the mud that then injected into nearby faults.”

Previous studies would have underestimated the energy of the seismic waves, as ground motion was only considered at the surface. However, geophysicists at the University of Bonn suspect that those were much less intense than at depth. The dome-like structure “kept” the seismic waves at depth and damped those that reached the surface. “This was actually a lower estimate of the focussing effect because only one wave cycle was input. This effect increases with each wave cycle because of the reducing acoustic impedance of the pressurizing mud layer”. In response to claims that the reported highest velocity layer used in the modeling is a measurement artifact, Miller says “that does not change our conclusions because this effect will occur whenever a layer of low acoustic impedance is sandwiched between high impedance layers, irrespective of the exact values of the impedances. And the source of the Lusi mud was the inside of the sandwich.”

It has already been proposed that a tectonic fault is connecting Lusi to a 15 km distant volcanic system. Prof. Miller explains “This connection probably supplies the mud volcano with heat and fluids that keep Lusi erupting actively up to today”, explains Miller.

With their publication, scientists from Bonn and Zürich point out, that earthquakes can trigger processes over long distances, and this focusing effect may apply to other hydrothermal and volcanic systems. Stephen Miller concludes: “Being a geological rarity, the mud volcano may contribute to a better understanding of triggering processes and relationships between seismic and volcanic activity.” Miller also adds “maybe this work will settle the long-standing controversy and focus instead on helping those affected.” The island of Java is part of the so called Pacific Ring of Fire, a volcanic belt which surrounds the entire Pacific Ocean. Here, oceanic crust is subducted underneath oceanic and continental tectonic plates, leading to melting of crustal material at depth. The resulting magma uprises and is feeding numerous volcanoes.

Geochemical ‘fingerprints’ leave evidence that megafloods eroded steep gorge

This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. -  U.S. Geological Survey
This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. – U.S. Geological Survey

The Yarlung-Tsangpo River in southern Asia drops rapidly through the Himalaya Mountains on its way to the Bay of Bengal, losing about 7,000 feet of elevation through the precipitously steep Tsangpo Gorge.

For the first time, scientists have direct geochemical evidence that the 150-mile long gorge, possibly the world’s deepest, was the conduit by which megafloods from glacial lakes, perhaps half the volume of Lake Erie, drained suddenly and catastrophically through the Himalayas when their ice dams failed at times during the last 2 million years.

“You would expect that if a three-day long flood occurred, there would be some pretty significant impacts downstream,” said Karl Lang, a University of Washington doctoral candidate in Earth and space sciences.

In this case, the water moved rapidly through bedrock gorge, carving away the base of slopes so steep they already were near the failure threshold. Because the riverbed through the Tsangpo Gorge is essentially bedrock and the slope is so steep and narrow, the deep flood waters could build enormous speed and erosive power.

As the base of the slopes eroded, areas higher on the bedrock hillsides tumbled into the channel, freeing microscopic grains of zircon that were carried out of the gorge by the fast-moving water and deposited downstream.

Uranium-bearing zircon grains carry a sort of geochemical signature for the place where they originated, so grains found downstream can be traced back to the rocks from which they eroded. Lang found that normal annual river flow carries about 40 percent of the grains from the Tsangpo Gorge downstream. But grains from the gorge found in prehistoric megaflood deposits make up as much as 80 percent of the total.

He is the lead author of a paper documenting the work published in the September edition of Geology. Co-authors are Katharine Huntington and David Montgomery, both UW faculty members in Earth and space sciences.

The Yarlung-Tsangpo is the highest major river in the world. It begins on the Tibetan Plateau at about 14,500 feet, or more than 2.5 miles, above sea level. It travels more than 1,700 miles, crossing the plateau and plunging through the Himalayas before reaching India’s Assam Valley, where it becomes the Brahmaputra River. From there it continues its course to the Ganges River delta and the Bay of Benga

At the head of the Tsangpo Gorge, the river makes a sharp bend around Namche Barwa, a 25,500-foot peak that is the eastern anchor of the Himalayas. Evidence indicates that giant lakes were impounded behind glacial dams farther inland from Namche Barwa at various times during the last 2.5 million years ago.

Lang matched zircons in the megaflood deposits far downstream with zircons known to come only from Namche Barwa, and those signature zircons turned up in the flood deposits at a much greater proportion than they would in sediments from normal river flows. Finding the zircons in deposits so far downstream is evidence for the prehistoric megafloods and their role in forming the gorge.

Lang noted that a huge landslide in early 2000 created a giant dam on the Yiggong River, a tributary of the main river just upstream from the Gorge. The dam failed catastrophically in June 2000, triggering a flood that caused numerous fatalities and much property damage downstream.

That provided a vivid, though much smaller, illustration of what likely occurred when large ice dams failed millions of years ago, he said. It also shows the potential danger if humans decide to build dams in that area for hydroelectric generation.

“We are interested in it scientifically, but there is certainly a societal element to it,” Lang said. “This takes us a step beyond speculating what those ancient floods did. There is circumstantial evidence that, yes, they did do a lot of damage.”

The process in the Tsangpo Gorge is similar to what happened with Lake Missoula in Western Montana 12,000 to 15,000 years ago. That lake was more than 10,000 feet lower in elevation than lakes associated with the Tsangpo Gorge, though its water discharge was 10 times greater. Evidence suggests that Lake Missoula’s ice dam failed numerous times, unleashing a torrent equal to half the volume of Lake Michigan across eastern Washington, where it carved the Channeled Scablands before continuing down the Columbia River basin.

“This is a geomorphic process that we know shapes the landscape, and we can look to eastern Washington to see that,” Lang said.

Greening of the Earth pushed way back in time

<IMG SRC="/Images/77184557.jpg" WIDTH="350" HEIGHT="335" BORDER="0" ALT="This is an interpretive view of Diskagma buttonii with exterior view, left, and cross section. The fossils are the size of match heads and were found connected into bunches by threads in the surface of an ancient soil from South Africa. – Courtesy of Gregory Retallack”>
This is an interpretive view of Diskagma buttonii with exterior view, left, and cross section. The fossils are the size of match heads and were found connected into bunches by threads in the surface of an ancient soil from South Africa. – Courtesy of Gregory Retallack

Conventional scientific wisdom has it that plants and other creatures have only lived on land for about 500 million years, and that landscapes of the early Earth were as barren as Mars.

A new study, led by geologist Gregory J. Retallack of the University of Oregon, now has presented evidence for life on land that is four times as old — at 2.2 billion years ago and almost half way back to the inception of the planet.

That evidence, which is detailed in the September issue of the journal Precambrian Research, involves fossils the size of match heads and connected into bunches by threads in the surface of an ancient soil from South Africa. They have been named Diskagma buttonii, meaning “disc-shaped fragments of Andy Button,” but it is unsure what the fossils were, the authors say.

“They certainly were not plants or animals, but something rather more simple,” said Retallack, professor of geological sciences and co-director of paleontological collections at the UO’s Museum of Natural and Cultural History. The fossils, he added, most resemble modern soil organisms called Geosiphon, a fungus with a central cavity filled with symbiotic cyanobacteria.

“There is independent evidence for cyanobacteria, but not fungi, of the same geological age, and these new fossils set a new and earlier benchmark for the greening of the land,” he said. “This gains added significance because fossil soils hosting the fossils have long been taken as evidence for a marked rise in the amount of oxygen in the atmosphere at about 2.4 billion to 2.2 billion years ago, widely called the Great Oxidation Event.”

By modern standards, in which Earth’s air is now 21 percent oxygen, this early rise was modest, to about 5 percent oxygen, but it represented a rise from vanishingly low oxygen levels earlier in geological time.

Demonstrating that Diskagma are fossils, Retallack said, was a technical triumph because they were too big to be completely seen in a standard microscopic slide and within rock that was too dark to see through in slabs. The samples were imaged using powerful X-rays of a cyclotron, a particle accelerator, at the Lawrence Berkeley National Laboratory in California.

The images enabled a three-dimensional restoration of the fossils’ form: odd little hollow urn-shaped structures with a terminal cup and basal attachment tube. “At last we have an idea of what life on land looked like in the Precambrian,” Retallack said. “Perhaps with this search image in mind, we can find more and different kinds of fossils in ancient soils.”

In their conclusion, the researchers noted that their newly named fossil Diskagma is comparable in morphology and size to Thucomyces lichenoides, a fossil dating to 2.8 billion years ago and also found in South Africa, but its composition, including interior structure and trace elements, is significantly different.

Diskagma also holds some similarities to three living organisms, which were illustrated microscopically in the study: the slime mold Leocarpus fragilis as found in Oregon’s Three Sisters Wilderness; the lichen Cladonia ecmocyna gathered near Fishtrap Lake in Montana; and the fungus Geosiphon pyriformis from near Darmstadt, Germany.

The new fossil, the authors concluded, is a promising candidate for the oldest known eukaryote –an organism with cells that contain complex structures, including a nucleus, within membranes.

“Researchers at the UO are collaborating with scientists from around the world to create new knowledge with far-reaching applications,” said Kimberly Andrews Espy, UO vice president for research and innovation, and dean of the graduate school. “This research by Dr. Retallack and his team opens new doors of inquiry about the origins of ancient life on Earth.”

Mapping field camp’s past and present: Exploring a mainstay of geoscience education

Near the site of Wyoming's first oil well, drilled in 1885 in the Dallas Dome, students in the University of Missouri's field camp listen and take notes. -  Copyright Miriam Barquero-Molina
Near the site of Wyoming’s first oil well, drilled in 1885 in the Dallas Dome, students in the University of Missouri’s field camp listen and take notes. – Copyright Miriam Barquero-Molina

Alexandria, VA – In a field like earth science, adventures in the outdoors are commonplace. As this summer’s field season comes to a close and the lanterns are extinguished one last time, EARTH Magazine explores the ritual of field camp as geoscientists’ rite of passage from classroom learner to a workforce-ready scientist.

Earth science is just that, the study of the Earth. Thorough understanding of the surface expression of textbook concepts helps geoscientists provide protection and valuable resources to society. Thousands of U.S. college students participate in field camps each year, staying in accommodations varying from lodges with running water to nomadic tent camps – where they backpack into sites in the middle of nowhere to see, experience and comprehend the landforms people take for granted. No longer are students reading books, rather they’re reading the rocks.

The history of geology field camps dates back to the days of great explorers and naturalists like John Muir and his contemporaries. Now young men and women gear up and pack out to geologically unique locations nationwide. While traditional skill of mapping rock units and structures onto a blank topographic map still represents a foundational skill set, faculty and industry professionals are finding ways to integrate relevant technology to keep students workforce-ready.

Students leave field camp with a varied toolkit that utilizes colored pencils as much as geophones and GPS. Share in the adventures of thousands in this month’s EARTH Magazine cover story at:

Sea level rise: New iceberg theory points to areas at risk of rapid disintegration

In events that could exacerbate sea level rise over the coming decades, stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean, according to new iceberg calving simulations from the University of Michigan.

“If this starts to happen and we’re right, we might be closer to the higher end of sea level rise estimates for the next 100 years,” said Jeremy Bassis, assistant professor of atmospheric, oceanic and space sciences at the U-M College of Engineering, and first author of a paper on the new model published in the current issue of Nature Geoscience.

Iceberg calving, or the formation of icebergs, occurs when ice chunks break off larger shelves or glaciers and float away, eventually melting in warmer waters. Although iceberg calving accounts for roughly half of the mass lost from ice sheets, it isn’t reflected in any models of how climate change affects the ice sheets and could lead to additional sea level rise, Bassis said.

“Fifty percent of the total mass loss from the ice sheets, we just don’t understand. We essentially haven’t been able to predict that, so events such as rapid disintegration aren’t included in those estimates,” Bassis said. “Our new model helps us understand the different parameters, and that gives us hope that we can better predict how things will change in the future.”

The researchers have found the physics at the heart of iceberg calving, and their model is the first that can simulate the different processes that occur on both ends of the Earth. It can show why in northern latitudes-where glaciers rest on solid ground-icebergs tend to form in relatively small, vertical slivers that rotate onto their sides as they dislodge. It can also illustrate why in the southernmost places-where vast ice shelves float in the Antarctic Ocean-icebergs form in larger, more horizontal plank shapes.

The model treats ice sheets-both floating shelves and grounded glaciers-like loosely cemented collections of boulders. Such a description reflects how scientists in the field have described what iceberg calving actually looks like. The model allows those loose bonds to break when the boulders are pulled apart or rub against one another.

The simulations showed that calving is a two-step process driven primarily by the thickness of the ice.

“Essentially, everything is driven by gravity,” Bassis said. “We identified a critical threshold of one kilometer where it seems like everything should break up. You can think of it in terms of a kid building a tower. The taller the tower is, the more unstable it gets.”

Icebergs do have a tendency to form before that threshold though, Bassis suspects, due to cracks that are already there-either formed when capsizing bergs crash into the water and send shockwaves through the surrounding ice, or when melted water on the surface cuts through. The former is believed to have led to the Helheim Glacier collapse in 2003. The glacier had begun to retreat slowly in 2002, but suddenly gave way the following year when the thinner ice had broken away, exposing a thicker ice coast.

The latter-melted water pools-are occurring more frequently due to climate change, and they’re believed to have played a role in the rapid disintegration of the Antarctica’s Larsen B ice shelf, which crumbled over about six weeks in 2002.

When the researchers added random cracks to their model, it could mirror both Helheim and Larsen B.

A third feature is also required for the most dramatic ice collapses to occur. Icebergs can’t float away and make room for more icebergs to break off the main sheet unless the system has access to open water. So areas that border deep, unobstructed ocean rather than fjords or other waterways are at greater risk of rapid ice loss. The researchers point to the Thwaites and Pine Island glaciers in Antarctica and the Jakobshavn Glacier in Greenland, which is already retreating rapidly, as places vulnerable to “catastrophic disintegration” because they have all three components.

“The ice in those places gets thicker as you go back. If our threshold is right, then if these places start to retreat as you expose the thicker calving font, they’re susceptible to catastrophic breakup,” Bassis said.

Retreat of the current ice coasts in these places areas could occur via melting or iceberg calving.

Ancient ice melt unearthed in Antarctic mud

Global warming five million years ago may have caused parts of Antarctica’s large ice sheets to melt and sea levels to rise by approximately 20 metres, scientists report today in the journal Nature Geoscience.

The researchers, from Imperial College London, and their academic partners studied mud samples to learn about ancient melting of the East Antarctic ice sheet. They discovered that melting took place repeatedly between five and three million years ago, during a geological period called Pliocene Epoch, which may have caused sea levels to rise approximately ten metres.

Scientists have previously known that the ice sheets of West Antarctica and Greenland partially melted around the same time. The team say that this may have caused sea levels to rise by a total of 20 metres.

The academics say understanding this glacial melting during the Pliocene Epoch may give us insights into how sea levels could rise as a consequence of current global warming. This is because the Pliocene Epoch had carbon dioxide concentrations similar to now and global temperatures comparable to those predicted for the end of this century.

Dr Tina Van De Flierdt, co-author from the Department of Earth Science and Engineering at Imperial College London, says: “The Pliocene Epoch had temperatures that were two or three degrees higher than today and similar atmospheric carbon dioxide levels to today. Our study underlines that these conditions have led to a large loss of ice and significant rises in global sea level in the past. Scientists predict that global temperatures of a similar level may be reached by the end of this century, so it is very important for us to understand what the possible consequences might be.”

The East Antarctic ice sheet is the largest ice mass on Earth, roughly the size of Australia. The ice sheet has fluctuated in size since its formation 34 million years ago, but scientists have previously assumed that it had stabilised around 14 million years ago.

The team in today’s study were able to determine that the ice sheet had partially melted during this “stable” period by analysing the chemical content of mud in sediments. These were drilled from depths of more than three kilometres below sea level off the coast of Antarctica.

Analysing the mud revealed a chemical fingerprint that enabled the team to trace where it came from on the continent. They discovered that the mud originated from rocks that are currently hidden under the ice sheet. The only way that significant amounts of this mud could have been deposited as sediment in the sea would be if the ice sheet had retreated inland and eroded these rocks, say the team.

The academics suggest that the melting of the ice sheet may have been caused in part by the fact that some of it rests in basins below sea level. This puts the ice in direct contact with seawater and when the ocean warms, as it did during the Pliocene, the ice sheet becomes vulnerable to melting.

Carys Cook, co-author and research postgraduate from the Grantham Institute for Climate Change at Imperial, adds: “Scientists previously considered the East Antarctic ice sheet to be more stable than the much smaller ice sheets in West Antarctica and Greenland, even though very few studies of East Antarctic ice sheet have been carried out. Our work now shows that the East Antarctic ice sheet has been much more sensitive to climate change in the past than previously realised. This finding is important for our understanding of what may happen to the Earth if we do not tackle the effects of climate change.”

The next step will see the team analysing sediment samples to determine how quickly the East Antarctic ice sheet melted during the Pliocene. This information could be useful in the future for predicting how quickly the ice sheet could melt as a result of global warming.

Newly discovered flux in the Earth may solve missing-mantle mystery

It’s widely thought that the Earth arose from violent origins: Some 4.5 billion years ago, a maelstrom of gas and dust circled in a massive disc around the sun, gathering in rocky clumps to form asteroids. These asteroids, gaining momentum, whirled around a fledgling solar system, repeatedly smashing into each other to create larger bodies of rubble – the largest of which eventually cooled to form the planets.

Countless theories, simulations and geologic observations support such a scenario. But there remains one lingering mystery: If the Earth arose from the collision of asteroids, its composition should resemble that of meteoroids, the small particles that break off from asteroids.

But to date, scientists have found that, quite literally, something doesn’t add up: Namely, the Earth’s mantle – the layer between the planet’s crust and core – is missing an amount of lead found in meteorites whose composition has been analyzed following impact with the Earth.

Much of the Earth is composed of rocks with a high ratio of uranium to lead (uranium naturally decays to lead over time). However, according to standard theories of planetary evolution, the Earth should harbor a reservoir of mantle somewhere in its interior that has a low ratio of uranium to lead, to match the composition of meteorites. But such a reservoir has yet to be discovered – a detail that leaves Earth’s origins hazy.

Now researchers in MIT’s Department of Earth, Atmospheric and Planetary Sciences have identified a “hidden flux” of material in the Earth’s mantle that would make the planet’s overall composition much more similar to that of meteorites. This reservoir likely takes the form of extremely dense, lead-laden rocks that crystallize beneath island arcs, strings of volcanoes that rise up at the boundary of tectonic plates.

As two massive plates push against each other, one plate subducts, or slides, under the other, pushing material from the crust down into the mantle. At the same time, molten material from the mantle rises up to the crust, and is ejected via volcanoes onto the Earth’s surface.

According to the MIT researchers’ observations and calculations, however, up to 70 percent of this rising magma crystallizes into dense rock – dropping, leadlike, back into the mantle, where it remains relatively undisturbed. The lead-heavy flux, they say, puts the composition of the Earth’s mantle on a par with that of meteorites.

“Now that we know the composition of this flux, we can calculate that there’s tons of this stuff dropping down from the base of the crust into the mantle, so it is likely an important reservoir,” says Oliver Jagoutz, an assistant professor of geology at MIT. “This has a lot of implications for understanding how the Earth evolved through history.”

Jagoutz and his colleague Max Schmidt, of the Swiss Federal Institute of Technology in Zurich, have detailed their results in a paper published in Earth and Planetary Science Letters.

A mantle exposed

Measuring the composition of material that has dropped into the mantle is a nearly impossible task. Jagoutz estimates that such dense rocks would form at a depth of 40 to 50 kilometers below the surface, beyond the reach of conventional sampling techniques.

There is, however, one place on earth where such a depth of the crust and mantle is exposed: a region of northern Pakistan called the Kohistan arc. Forty million years ago, this island arc was crushed between India and Asia as the two plates collided.

“When India came in, it slammed into the arc, and the arc extended and rotated itself,” Jagoutz says. “Because of that, we now have a cross-section of the mantle-to-crust transition. This is the only place on Earth where this exists.”

On various trips from 2000 to 2007, Jagoutz trekked through the Kohistan arc region, collecting rocks from various parts of the arc’s crust and mantle. Bringing them back to the lab, he analyzed the rocks’ density and composition, discovering that some were “density-unstable” – much denser than the mantle. These denser rocks could potentially sink into the mantle, creating a hidden reservoir.

Adding up to an asteroid origin

The researchers measured the rocks’ composition, and found that the denser rocks contained much more lead than uranium – exactly the ratio predicted for the missing reservoir of material. Jagoutz then performed a mass balance (a simple conservation-of-mass calculation) to determine how much dense rock drops into the mantle, based on the composition of the region’s crust, rocks and mantle: Essentially, the mass of the Kohistan arc, minus whatever material drops into the mantle, should equal the material that comes out of the mantle.

Jagoutz and Schmidt solved the equation for 10 common elements. From their calculations, they found that 70 percent of the magma that rises from the mantle must ultimately drop back down, relatively heavy with lead. Applying this statistic to other island arcs in the world – such as the Andean volcanic belt and the Cascade Range – they found that the amount of material dropped into the mantle globally equals the composition and quantity of the so-called missing reservoir – a finding that suggests that Earth did indeed form from the collision of meteorites.

“If we are right, one of the questions we have is: Why is the Earth capable of hiding something from us? Why is there never a volcano that brings up these rocks?” Jagoutz adds. “You’d think it’d come back up, but it doesn’t. It’s actually interesting.

Researchers shed new light on supraglacial lake drainage

Supraglacial lakes – bodies of water that collect on the surface of the Greenland ice sheet – lubricate the bottom of the sheet when they drain, causing it to flow faster. Differences in how the lakes drain can impact glacial movement’s speed and direction, researchers from The City College of New York (CCNY), University of Cambridge and Los Alamos National Laboratory report in “Environmental Research Letters.”

“Knowledge of the draining mechanisms allows us to improve our understanding of how surface melting can impact sea-level rise, not only through the direct contribution of meltwater from the surface, but also through the indirect contribution on the mass loss through ice dynamics,” says Dr. Marco Tedesco, the principal investigator and lead author.

Dr. Tedesco is an associate professor in CCNY’s Department of Earth and Atmospheric Sciences at CCNY and is currently serving as temporary program director for the National Science Foundation’s Polar Cyberinfrastructure Program. The research described in the paper was funded before Dr. Tedesco accepted the position at NSF.

NSF supported the research along with NASA’s cryosphere program, the Natural Environment Research Council, the U.S. Department of Energy’s earth systems modeling program, St. Catherine’s College (Cambridge), the Scandinavian Studies Fund and the B.B. Roberts Fund.

Over the past decade, surface melting in Greenland has increased considerably.

Previous research already suggested that the water injected from the rapid draining of the supraglacial lakes controlled sliding of ice over the bed beneath it. However, there was no evidence of the impact of the slow draining mechanism, which the paper identified.

Professor Tedesco and colleagues documented that supraglacial lakes have two different drainage mechanisms that cause them to empty rapidly or slowly. The findings are based on analysis of data collected in 2011 from five GPS stations the team installed around two supraglacial lakes in the Paakistoq region of West Greenland.

The smaller of the two lakes, Lake Half Moon, overflowed its banks and drained from the side to reach a moulin. It took approximately 45 hours to empty. The larger lake, Lake Ponting, drained through a crack in the ice beneath it and was voided in around two hours.

“At first, a crack in the ice beneath the lake may be small, but it deepens as water enters it because the pressure of the water overcomes the compressive action of the ice, which is trying to close the crack,” Professor Tedesco explains. “When the crack reaches the bed beneath the glacier, which could be 1,000 meters or more below the surface, the lake empties rapidly, like a bathtub after its plug is pulled.”

Drainage from both lakes accelerated glacial movement. However, water from Lake Ponting caused the glacier to move faster and further. While the slower drainage from Lake Half Moon caused the glacial pace to increase from baseline values of 90 – 100 meters per year to a maximum of around 420 meters a year, glacial movement in the area affected by Lake Ponting reached maximum velocities of 1,500 – 1,600 meters per year, nearly four times greater.

The drainage of the two lakes impacted the glacier’s trajectory differently, as well. The emptying of Lake Half Moon via the moulin did not change the direction of glacial movement. However, when Lake Ponting drained a slight southerly shift in the glacier’s direction was detected.

“Because the different draining mechanisms affect ice velocity, they could also affect the amount of ice lost through calving of glaciers, which results in icebergs,” Professor Tedesco points out. “Because what happens on a glacier’s surface impacts what is going on below, researchers are trying to look at glaciers as a system instead of independent components,” he adds.

“The surface is like the skin of a tissue and the subglacial and englacial channels that develop because of the surface water act like arteries or veins that redistribute this water internally.”