Caltech Geologists Discover Ancient Buried Canyon in South Tibet

A team of researchers from Caltech and the China Earthquake Administration has discovered an ancient, deep canyon buried along the Yarlung Tsangpo River in south Tibet, north of the eastern end of the Himalayas. The geologists say that the ancient canyon—thousands of feet deep in places—effectively rules out a popular model used to explain how the massive and picturesque gorges of the Himalayas became so steep, so fast.

"I was extremely surprised when my colleagues, Jing Liu-Zeng and Dirk Scherler, showed me the evidence for this canyon in southern Tibet," says Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology at Caltech. "When I first saw the data, I said, 'Wow!' It was amazing to see that the river once cut quite deeply into the Tibetan Plateau because it does not today. That was a big discovery, in my opinion." 

Geologists like Avouac and his colleagues, who are interested in tectonics—the study of the earth's surface and the way it changes—can use tools such as GPS and seismology to study crustal deformation that is taking place today. But if they are interested in studying changes that occurred millions of years ago, such tools are not useful because the activity has already happened. In those cases, rivers become a main source of information because they leave behind geomorphic signatures that geologists can interrogate to learn about the way those rivers once interacted with the land—helping them to pin down when the land changed and by how much, for example.

"In tectonics, we are always trying to use rivers to say something about uplift," Avouac says. "In this case, we used a paleocanyon that was carved by a river. It's a nice example where by recovering the geometry of the bottom of the canyon, we were able to say how much the range has moved up and when it started moving."

The team reports its findings in the current issue of Science.

Last year, civil engineers from the China Earthquake Administration collected cores by drilling into the valley floor at five locations along the Yarlung Tsangpo River. Shortly after, former Caltech graduate student Jing Liu-Zeng, who now works for that administration, returned to Caltech as a visiting associate and shared the core data with Avouac and Dirk Scherler, then a postdoc in Avouac's group. Scherler had previously worked in the far western Himalayas, where the Indus River has cut deeply into the Tibetan Plateau, and immediately recognized that the new data suggested the presence of a paleocanyon.

Liu-Zeng and Scherler analyzed the core data and found that at several locations there were sedimentary conglomerates, rounded gravel and larger rocks cemented together, that are associated with flowing rivers, until a depth of 800 meters or so, at which point the record clearly indicated bedrock. This suggested that the river once carved deeply into the plateau.

To establish when the river switched from incising bedrock to depositing sediments, they measured two isotopes, beryllium-10 and aluminum-26, in the lowest sediment layer. The isotopes are produced when rocks and sediment are exposed to cosmic rays at the surface and decay at different rates once buried, and so allowed the geologists to determine that the paleocanyon started to fill with sediment about 2.5 million years ago.

The researchers' reconstruction of the former valley floor showed that the slope of the river once increased gradually from the Gangetic Plain to the Tibetan Plateau, with no sudden changes, or knickpoints. Today, the river, like most others in the area, has a steep knickpoint where it meets the Himalayas, at a place known as the Namche Barwa massif. There, the uplift of the mountains is extremely rapid (on the order of 1 centimeter per year, whereas in other areas 5 millimeters per year is more typical) and the river drops by 2 kilometers in elevation as it flows through the famous Tsangpo Gorge, known by some as the Yarlung Tsangpo Grand Canyon because it is so deep and long.

Combining the depth and age of the paleocanyon with the geometry of the valley, the geologists surmised that the river existed in this location prior to about 3 million years ago, but at that time, it was not affected by the Himalayas. However, as the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging on the river. Suddenly, about 2.5 million years ago, a rapidly uplifting section of the mountain range got in the river's way, damming it, and the canyon subsequently filled with sediment.

"This is the time when the Namche Barwa massif started to rise, and the gorge developed," says Scherler, one of two lead authors on the paper and now at the GFZ German Research Center for Geosciences in Potsdam, Germany.

That picture of the river and the Tibetan Plateau, which involves the river incising deeply into the plateau millions of years ago, differs quite a bit from the typically accepted geologic vision. Typically, geologists believe that when rivers start to incise into a plateau, they eat at the edges, slowly making their way into the plateau over time. However, the rivers flowing across the Himalayas all have strong knickpoints and have not incised much at all into the Tibetan Plateau. Therefore, the thought has been that the rapid uplift of the Himalayas has pushed the rivers back, effectively pinning them, so that they have not been able to make their way into the plateau. But that explanation does not work with the newly discovered paleocanyon.

The team's new hypothesis also rules out a model that has been around for about 15 years, called tectonic aneurysm, which suggests that the rapid uplift seen at the Namche Barwa massif was triggered by intense river incision. In tectonic aneurysm, a river cuts down through the earth's crust so fast that it causes the crust to heat up, making a nearby mountain range weaker and facilitating uplift.

The model is popular among geologists, and indeed Avouac himself published a modeling paper in 1996 that showed the viability of the mechanism. "But now we have discovered that the river was able to cut into the plateau way before the uplift happened," Avouac says, "and this shows that the tectonic aneurysm model was actually not at work here. The rapid uplift is not a response to river incision."

The other lead author on the paper, "Tectonic control of Yarlung Tsangpo Gorge revealed by a buried canyon in Southern Tibet," is Ping Wang of the State Key Laboratory of Earthquake Dynamics, in Beijing, China. Additional authors include Jürgen Mey, of the University of Potsdam, in Germany; and Yunda Zhang and Dingguo Shi of the Chengdu Engineering Corporation, in China. The work was supported by the National Natural Science Foundation of China, the State Key Laboratory for Earthquake Dynamics, and the Alexander von Humboldt Foundation. 

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Photosynthesis: A Planetary Revolution

Watson Lecture Preview

Two and a half billion years ago, single-celled organisms called cyanobacteria harnessed sunlight to split water molecules, producing energy to power their cells and releasing oxygen into an atmosphere that had previously had none. These early environmental engineers are responsible for the life we see around us today, and much more besides.

At 8:00 p.m. on Wednesday, November 19, in Caltech's Beckman Auditorium, Professor of Geobiology Woodward "Woody" Fischer will describe how they transformed the planet. Admission is free.

 

Q: What do you do?

A: I'm a geobiologist of the historical variety. I'm trying to understand both how the earth works, and why it works that way. The whys are hard, because you can't redo this planetary experiment. You have to create clever ways to work backward from what you can observe to answer the question you've posed.

When you boil down the earth's history, there are maybe a half-dozen singularities—fundamental changes in how our planet and the life on it interact. Photosynthetic cyanobacteria reengineered the planet. Photosynthesis led to two more singularities—plants and animals appeared. The remaining singularities are mass extinctions as a result of something happening to the global environment, and photosynthesis likely caused one of those as well. Oxygen can be highly toxic because it's so reactive. It chews up your DNA, and it binds to the metal compounds that cells use to shuttle electrons around. Any microbes that couldn't cope with this new pollutant died off, or were forced to hide in oxygen-depleted environments.

Atmospheric oxygen resulted from a change to a microbe's metabolism that evolved once, at a specific time in the earth's history. We want to know why that happened. What were those bacteria doing beforehand? What forced them to develop this radically new way of making a living?

Bacteria don't leave fossils, per se, but they can leave behind metabolic signatures that sedimentary rocks preserve. They impact the rock's elemental composition, and they alter the ratios between heavier and lighter isotopes of certain elements as well. We can work backward from that information to deduce what the bacteria were doing on the ocean floor and in the seawater above it as those sediments were being laid down.

 

Q: If the earth has had breathable oxygen for billions of years, why should we care where it came from?

A: There are two really good reasons.

One has to do with meeting society's energy demands. There's a tremendous effort at Caltech and elsewhere to develop "solar fuels." Can we do better than green plants? If cyanobacteria did the best they could under tight constraints, maybe not. But if there are a variety of ways to do that chemistry, maybe we can clear the slate and do something entirely different.

The deeper reason is that atmospheric oxygen rewrote life's recipe book. Oxygen-based metabolism provides extra energy that can be invested in cellular specialization. A group of specialized cells can become a tissue, and eventually you have complex creatures with limbs. It's like agriculture—when you start growing crops, you have surplus food. Villages spring up. Craftsmen appear.

It gets to the Big Question—how rare are we? The earth is 4.5 billion years old, and the oldest evidence for life is about 3.5 billion years old. It took another billion years until photosynthesis, and two billion more for animals to develop. Is it possible to evolve advanced creatures under a different set of constraints leading to completely different metabolisms? If we're looking for life on worlds that play by different rules, will we recognize it?

 

Q: How did you get into this line of work?

A: As a small kid, I always loved science. That disappeared somewhere in middle school, so I went to Colorado College in Colorado Springs—a small, liberal-arts school with a really intense curriculum called the block plan. You take one class at a time for a month. You're completely immersed—lecture from nine to twelve, break for lunch, afternoon labs, evening homework. Lather, rinse, repeat. I took a geology class on a whim, because my grandfather had once taught paleontology there. The class vanished into the mountains for a month, and I was hooked.

In graduate school at Harvard, I worked with Andy Knoll, a Precambrian paleontologist who's trying to understand what the world looked like before animals. Andy's primary appointment is actually in the biology department, and I built on my sedimentary-geology background with a lot of biology classes—molecular biology, biochemistry, genomics, comparative biology, evolutionary biology. And then I came here as an Agouron Postdoctoral Scholar in Geobiology in 2007. I was fortunate that they invited me to stay.

 


Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.
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Watson Lecture: "Photosynthesis: A Planetary Revolution"
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Unexpected Findings Change the Picture of Sulfur on the Early Earth

Scientists believe that until about 2.4 billion years ago there was little oxygen in the atmosphere—an idea that has important implications for the evolution of life on Earth. Evidence in support of this hypothesis comes from studies of sulfur isotopes preserved in the rock record. But the sulfur isotope story has been uncertain because of the lack of key information that has now been provided by a new analytical technique developed by a team of Caltech geologists and geochemists. The story that new information reveals, however, is not what most scientists had expected.

"Our new technique is 1,000 times more sensitive for making sulfur isotope measurements," says Jess Adkins, professor of geochemistry and global environmental science at Caltech. "We used it to make measurements of sulfate groups dissolved in carbonate minerals deposited in the ocean more than 2.4 billion years ago, and those measurements show that we have been thinking about this part of the sulfur cycle and sulfur isotopes incorrectly."

The team describes their results in the November 7 issue of the journal Science. The lead author on the paper is Guillaume Paris, an assistant research scientist at Caltech.

Nearly 15 years ago, a team of geochemists led by researchers at UC San Diego discovered there was something peculiar about the sulfur isotope content of rocks from the Archean era, an interval that lasted from 3.8 billion to about 2.4 billion years ago. In those ancient rocks, the geologists were analyzing the abundances of stable isotopes of sulfur.

When sulfur is involved in a reaction—such as microbial sulfate reduction, a way for microbes to eat organic compounds in the absence of oxygen—its isotopes are usually fractionated, or separated, from one another in proportion to their differences in mass. That is, 34S gets fractionated from 32S about twice as much as 33S gets fractionated from 32S. This process is called mass-dependent fractionation, and, scientists have found that it dominates in virtually all sulfur processes operating on Earth's surface for the last 2.4 billion years.

However, in older rocks from the Archean era (i.e., older than 2.4 billion years), the relative abundances of sulfur isotopes do not follow the same mass-related pattern, but instead show relative enrichments or deficiencies of 33S relative to 34S. They are said to be the product of mass-independent fractionation (MIF).

The widely accepted explanation for the occurrence of MIF is as follows. Billions of years ago, volcanism was extremely active on Earth, and all those volcanoes spewed sulfur dioxide high into the atmosphere. At that time, oxygen existed at very low levels in the atmosphere, and therefore ozone, which is produced when ultraviolet radiation strikes oxygen, was also lacking. Today, ozone prevents ultraviolet light from reaching sulfur dioxide with the energy needed to fractionate sulfur, but on the early Earth, that was not the case, and MIF is the result. Researchers have been able to reproduce this effect in the lab by shining lasers onto sulfur dioxide and producing MIF.

Geologists have also measured the sulfur isotopic composition of sedimentary rocks dating to the Archean era, and found that sulfides—sulfur-bearing compounds such as pyrite (FeS2)—include more 33S than would be expected based on normal mass-dependent processes. But if those minerals are enriched in 33S, other minerals must be correspondingly lacking in the isotope. According to the leading hypothesis, those 33S-deficient minerals should be sulfates—oxidized sulfur-bearing compounds—that were deposited in the Archean ocean.

"That idea was put forward on the basis of experiment. To test the hypothesis, you'd need to check the isotope ratios in sulfate salts (minerals such as gypsum), but those don't really exist in the Archean rock record since there was very little oxygen around," explains Woody Fischer, professor of geobiology at Caltech and a coauthor on the new paper. "But there are trace amounts of sulfate that got trapped in carbonate minerals in seawater."

However, because those sulfates are present in such small amounts, no one has been able to measure well their isotopic composition. But using a device known as a multicollector inductively-coupled mass spectrometer to precisely measure multiple sulfur isotopes, Adkins and his colleague Alex Sessions, a professor of geobiology, developed a method that is sensitive enough to measure the isotopic composition of about 10 nanomoles of sulfate in just a few tens of milligrams of carbonate material.

The authors used the method to measure the sulfate content of carbonates from an ancient carbonate platform preserved in present-day South Africa, an ancient version of the depositional environments found in the Bahamas today. Analyzing the samples, which spanned 70 million years and a variety of marine environments, the researchers found exactly the opposite of what had been predicted: the sulfates were actually enriched by 33S rather than lacking in it.

"Now, finally, we're looking at this sulfur cycle and the sulfur isotopes correctly," Adkins says.

What does this mean for the atmospheric conditions of the early Earth? "Our findings underscore that the oxygen concentrations in the early atmosphere could have been incredibly low," Fischer says.

Knowledge of sulfate isotopes changes how we understand the role of biology in the sulfur cycle, he adds. Indeed, the fact that the sulfates from this time period have the same isotopic composition as sulfide minerals suggests that the sulfides may be the product of microbial processes that reduced seawater sulfate to sulfide (which later precipitated in sediments in the form of pyrite). Previously, scientists thought that all of the isotope fractionation could be explained by inorganic processes alone.

In a second paper also in the November 7 issue of Science, Paris, Adkins, Sessions, and colleagues from a number of institutions around the world report on related work in which they measured the sulfates in Indonesia's Lake Matano, a low-sulfate analog of the Archean ocean.

At about 100 meters depth, the bacterial communities in Lake Matano begin consuming sulfate rather than oxygen, as do most microbial communities, yielding sulfide. The researchers measured the sulfur isotopes within the sulfates and sulfides in the lake water and sediments and found that despite the low concentrations of sulfate, a lot of mass-dependent fractionation was taking place. The researchers used the data to build a model of the lake's sulfur cycle that could produce the measured fractionation, and when they applied their model to constrain the range of concentrations of sulfate in the Archean ocean, they found that the concentration was likely less than 2.5 micromolar, 10,000 times lower than the modern ocean.

"At such low concentration, all the isotopic variability starts to fit," says Adkins. "With these two papers, we were able to come at the same problem in two ways—by measuring the rocks dating from the Archean and by looking at a model system today that doesn't have much sulfate—and they point toward the same answer: the sulfate concentration was very low in the Archean ocean."

Samuel M. Webb of the Stanford Synchrotron Radiation Lightsource is also an author on the paper, "Neoarchean carbonate-associated sulfate records positive Δ33S anomalies." The work was supported by funding from the National Science Foundation's Division of Earth Sciences, the Henry and Camille Dreyfus Foundation's Postdoctoral Program in Environmental Chemistry, and the David and Lucile Packard Foundation.

Paris is also a co-lead author on the second paper, "Sulfate was a trace constituent of Archean seawater." Additional authors on that paper are Sean Crowe and CarriAyne Jones of the University of British Columbia and the University of Southern Denmark; Sergei Katsev of the University of Minnesota Duluth; Sang-Tae Kim of McMaster University; Aubrey Zerkle of the University of St. Andrews; Sulung Nomosatryo of the Indonesian Institute of Sciences; David Fowle of the University of Kansas; James Farquhar of the University of Maryland, College Park; and Donald Canfield of the University of Southern Denmark. Funding was provided by an Agouron Institute Geobiology Fellowship and a Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship, as well as by the Danish National Research Foundation and the European Research Council.

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Rock-Dwelling Microbes Remove Methane from Deep Sea

Methane-breathing microbes that inhabit rocky mounds on the seafloor could be preventing large volumes of the potent greenhouse gas from entering the oceans and reaching the atmosphere, according to a new study by Caltech researchers.

The rock-dwelling microbes, which are detailed in the Oct. 14 issue of Nature Communications, represent a previously unrecognized biological sink for methane and as a result could reshape scientists' understanding of where this greenhouse gas is being consumed in subseafloor habitats, says Professor of Geobiology Victoria Orphan, who led the study.

"Methane is a much more powerful greenhouse gas than carbon dioxide, so tracing its flow through the environment is really a priority for climate models and for understanding the carbon cycle," Orphan says.

Orphan's team has been studying methane-breathing marine microorganisms for nearly 20 years. The microbes they focus on survive without oxygen, relying instead on sulfate ions present in seawater for their energy needs. Previous work by Orphan's team helped show that the methane-breathing system is actually made up of two different kinds of microorganisms that work closely with one another. One of the partners, dubbed "ANME" for "ANaerobic MEthanotrophs," belongs to a type of ancient single-celled creatures called the archaea.

Through a mechanism that is still unclear, ANME work closely with bacteria to consume methane using sulfate from seawater. "Without this biological process, much of that methane would enter the water column, and the escape rates into the atmosphere would probably be quite a bit higher," says study first author Jeffrey Marlow, a geobiology graduate student in Orphan's lab.

Until now, however, the activity of ANME and their bacterial partners had been primarily studied in sediments located in cold seeps, areas on the ocean bottom where methane is escaping from subseafloor sources into the water above. The new study marks the first time they have been observed to oxidize methane inside carbonate mounds, huge rocky outcroppings of calcium carbonate that can rise hundreds of feet above the seafloor.

If the microbes are living inside the mounds themselves, then the distribution of methane consumption is significantly different from what was previously thought. "Methane-derived carbonates represent a large volume within many seep systems, and finding active methane-consuming archaea and bacteria in the interior of these carbonate rocks extends the known habitat for methane-consuming microorganisms beyond the relatively thin layer of sediment that may overlay a carbonate mound," Marlow says.

Orphan and her team detected evidence of methane-breathing microbes in carbonate rocks collected from three cold seeps around the world: one at a tectonic plate boundary near Costa Rica; another in the Eel River basin off the coast of northwestern California; and at Hydrate Ridge, off the Oregon coast. The team used manned and robotic submersibles to collect the rock samples from depths ranging from 2,000 feet to nearly half a mile below the surface.

Marlow has vivid memories of being a passenger in the submersible Alvin during one of those rock-retrieval missions. "As you sink down, the water outside your window goes from bright blue surface water to darker turquoise and navy blue and all these shades of blue that you didn't know existed until it gets completely dark," Marlow recalls. "And then you start seeing flashes of light because the vehicle is perturbing the water column and exciting florescent organisms. When you finally get to the seafloor, Alvin's exterior lights turn on, and this crazy alien world is illuminated in front of you."

The carbonate mounds that the subs visited often serve as foundations for coral and sponges, and are home to rockfishes, clams, crabs, and other aquatic life. For their study, the team members gathered rock samples not only from carbonate mounds located within active cold seeps, where methane could be seen escaping from the seafloor into the water, but also from mounds that appeared to be dormant.

Once the carbonate rocks were collected, they were transported back to the surface and rushed into a cold room aboard a research ship. In the cold room, which was maintained at the temperature of the deep sea, the team cracked open the carbonates in order to gather material from their interiors. "We wanted to make sure we weren't just sampling material from the surface of the rocks," Marlow says.

Using a microscope, the team confirmed that ANME and sulfate-reducing bacterial cells were indeed present inside the carbonate rocks, and genetic analysis of their DNA showed that they were related to methanotrophs that had previously been characterized in seafloor sediment. The scientists also used a technique that involved radiolabeled 14C-methane tracer gas to quantify the rates of methane consumption in the carbonate rocks and sediments from both the actively seeping sites and the areas appearing to be inactive. They found that the rock-dwelling methanotrophs consumed methane at a slower rate than their sediment-dwelling cousins.

"The carbonate-based microbes breathed methane at roughly one-third the rate of those gathered from sediments near active seep sites," Marlow says. "However, because there are likely many more microbes living in carbonate mounds than in sediments, their contributions to methane removal from the environment may be more significant."

The rock samples that were harvested near supposedly dormant cold seeps also harbored microbial communities capable of consuming methane. "We were surprised to find that these marine microorganisms are still viable and, if exposed to methane, can continue to oxidize this greenhouse gas long after surface expressions of seepage have vanished." Orphan says.

Along with Orphan and Marlow, additional coauthors on the paper, "Carbonate-hosted methanotrophy represents an unrecognized methane sink in the deep sea," include former Caltech associate research scientist Joshua Steele, now at the Southern California Coastal Water Research Project; Wiebke Ziebis, an associate professor at the University of Southern California; Andrew Thurber, an assistant professor at Oregon State University; and Lisa Levin, a professor at the Scripps Institution of Oceanography. Funding for the study was provided by the National Science Foundation; NASA's Astrobiology Institute; the Gordon and Betty Moore Foundation Marine Microbiology Initiative grant; and the National Research Council of the National Academies. 

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Textbook Theory Behind Volcanoes May Be Wrong

In the typical textbook picture, volcanoes, such as those that are forming the Hawaiian islands, erupt when magma gushes out as narrow jets from deep inside Earth. But that picture is wrong, according to a new study from researchers at Caltech and the University of Miami in Florida.

New seismology data are now confirming that such narrow jets don't actually exist, says Don Anderson, the Eleanor and John R. McMillian Professor of Geophysics, Emeritus, at Caltech. In fact, he adds, basic physics doesn't support the presence of these jets, called mantle plumes, and the new results corroborate those fundamental ideas.

"Mantle plumes have never had a sound physical or logical basis," Anderson says. "They are akin to Rudyard Kipling's 'Just So Stories' about how giraffes got their long necks."

Anderson and James Natland, a professor emeritus of marine geology and geophysics at the University of Miami, describe their analysis online in the September 8 issue of the Proceedings of the National Academy of Sciences.

According to current mantle-plume theory, Anderson explains, heat from Earth's core somehow generates narrow jets of hot magma that gush through the mantle and to the surface. The jets act as pipes that transfer heat from the core, and how exactly they're created isn't clear, he says. But they have been assumed to exist, originating near where the Earth's core meets the mantle, almost 3,000 kilometers underground—nearly halfway to the planet's center. The jets are theorized to be no more than about 300 kilometers wide, and when they reach the surface, they produce hot spots.  

While the top of the mantle is a sort of fluid sludge, the uppermost layer is rigid rock, broken up into plates that float on the magma-bearing layers. Magma from the mantle beneath the plates bursts through the plate to create volcanoes. As the plates drift across the hot spots, a chain of volcanoes forms—such as the island chains of Hawaii and Samoa.

"Much of solid-Earth science for the past 20 years—and large amounts of money—have been spent looking for elusive narrow mantle plumes that wind their way upward through the mantle," Anderson says.

To look for the hypothetical plumes, researchers analyze global seismic activity. Everything from big quakes to tiny tremors sends seismic waves echoing through Earth's interior. The type of material that the waves pass through influences the properties of those waves, such as their speeds. By measuring those waves using hundreds of seismic stations installed on the surface, near places such as Hawaii, Iceland, and Yellowstone National Park, researchers can deduce whether there are narrow mantle plumes or whether volcanoes are simply created from magma that's absorbed in the sponge-like shallower mantle.

No one has been able to detect the predicted narrow plumes, although the evidence has not been conclusive. The jets could have simply been too thin to be seen, Anderson says. Very broad features beneath the surface have been interpreted as plumes or super-plumes, but, still, they're far too wide to be considered narrow jets.

But now, thanks in part to more seismic stations spaced closer together and improved theory, analysis of the planet's seismology is good enough to confirm that there are no narrow mantle plumes, Anderson and Natland say. Instead, data reveal that there are large, slow, upward-moving chunks of mantle a thousand kilometers wide.

In the mantle-plume theory, Anderson explains, the heat that is transferred upward via jets is balanced by the slower downward motion of cooled, broad, uniform chunks of mantle. The behavior is similar to that of a lava lamp, in which blobs of wax are heated from below and then rise before cooling and falling. But a fundamental problem with this picture is that lava lamps require electricity, he says, and that is an outside energy source that an isolated planet like Earth does not have.  

The new measurements suggest that what is really happening is just the opposite: Instead of narrow jets, there are broad upwellings, which are balanced by narrow channels of sinking material called slabs. What is driving this motion is not heat from the core, but cooling at Earth's surface. In fact, Anderson says, the behavior is the regular mantle convection first proposed more than a century ago by Lord Kelvin. When material in the planet's crust cools, it sinks, displacing material deeper in the mantle and forcing it upward.

"What's new is incredibly simple: upwellings in the mantle are thousands of kilometers across," Anderson says. The formation of volcanoes then follows from plate tectonics—the theory of how Earth's plates move and behave. Magma, which is less dense than the surrounding mantle, rises until it reaches the bottom of the plates or fissures that run through them. Stresses in the plates, cracks, and other tectonic forces can squeeze the magma out, like how water is squeezed out of a sponge. That magma then erupts out of the surface as volcanoes. The magma comes from within the upper 200 kilometers of the mantle and not thousands of kilometers deep, as the mantle-plume theory suggests.

"This is a simple demonstration that volcanoes are the result of normal broad-scale convection and plate tectonics," Anderson says. He calls this theory "top-down tectonics," based on Kelvin's initial principles of mantle convection. In this picture, the engine behind Earth's interior processes is not heat from the core but cooling at the planet's surface. This cooling and plate tectonics drives mantle convection, the cooling of the core, and Earth's magnetic field. Volcanoes and cracks in the plate are simply side effects.

The results also have an important consequence for rock compositions—notably the ratios of certain isotopes, Natland says. According to the mantle-plume idea, the measured compositions derive from the mixing of material from reservoirs separated by thousands of kilometers in the upper and lower mantle. But if there are no mantle plumes, then all of that mixing must have happened within the upwellings and nearby mantle in Earth's top 1,000 kilometers.

The paper is titled "Mantle updrafts and mechanisms of oceanic volcanism."

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Looking Forward to 2020 . . . on Mars

A Q&A With Project Scientist Ken Farley

While the Curiosity rover continues to interrogate Gale Crater on Mars, planning is well under way for its successor—another rover that is currently referred to as Mars 2020. The new robotic explorer, scheduled to launch in 2020, will use much of the same technology (even some of the spare parts Curiosity left behind on Earth) to get to the Red Planet. Once there, it will pursue a new set of scientific objectives including the careful collection and storage (referred to as "caching") of compelling samples that might one day be returned to Earth by a future mission. Today, NASA announced the selection of seven scientific instruments that Mars 2020 will carry with it to Mars.

Ken Farley, Caltech's W.M. Keck Foundation Professor of Geochemistry and chair of the Division of Geological and Planetary Sciences, is serving as project scientist for Mars 2020. We recently sat down with him to talk about the mission and his new role.

 

Congratulations on being selected project scientist for this exciting mission. For those of us who do not know exactly what a project scientist does, can you give us a little overview of the job?

Sure. Conveniently, NASA has a definition, which says that the project scientist is responsible for the overall scientific success of the mission. That's a pretty concise explanation, but it encompasses a lot. My main duty thus far has been helping to define the science needs for equipment that we are going to send to Mars. So while we haven't actually done any science yet, we have had to make a lot of design decisions that are related to the science.

The easiest place to illustrate this is in the discussion of what is necessary, from the science point of view, in terms of the samples that we will cache. We have to consider things like how much mass we need to bring back, what kind of magnetic fields and temperatures the samples are going to be exposed to, and how much contamination of different chemical constituents we can allow. Every one of those questions drives a design decision in how you build the drilling system and the caching system. And if you get those wrong, there's nothing you can do. So there's a lot of thought that has to be put into that, and I convey a lot of that information to the engineers.

Now that we have a science team, I will be helping to facilitate all of its investigations and helping the members to work as a team. MSL [the Mars Science Laboratory, Curiosity's mission] is demonstrating how you have to operate when you have a complex tool (a rover) and a bunch of sensors, and every day you have to figure out what you're going to do to further science. The team has to pull together, pool all of its information, and come up with a plan, so an important part of my job will be figuring out how to manage the team dynamics to keep everybody moving forward and not fragmenting.

 

What aspects of the job were particularly appealing to you?

One of the parts of being a division chair that I have really enjoyed is being engaged with something that's bigger than my own research. And there's definitely a lot of that on 2020. It's a huge undertaking. There are not many science projects of this scale to be associated closely with, so this just seemed like a really good opportunity.

The kinds of questions that 2020 is going after—they're really big questions. You could never answer them on your own. The key objective is about life—is there or was there ever life on Mars, and more broadly what does its presence or absence mean about the frequency and evolution of life within the universe? There's no way you could answer these questions on Earth. The simple reason for that is that Mars is covered by rocks that are of the era in which, at least on our planet, we believe life was evolving. There are almost no rocks left of that age on the earth, and the ones that are left have been really badly beaten up. So Mars is a place where you really stand a chance of answering these questions in a way that you probably can't anywhere else.

It's not the kind of science I'm usually associated with, but the mission is trying to address truly profound scientific questions.

 

As you said, space has not been the focus of your research for most of your career. Can you talk a bit about how a terrestrial geochemist like yourself wound up in this role on a Mars mission?

Several years ago, I participated in a workshop about quantifying martian stratigraphy, which was hosted by the Keck Institute for Space Studies [KISS]. One of the topics that was discussed was geochronology—the dating of rocks and other materials—on other planetary bodies, like Mars. This is important for establishing the history of a planet and is particularly challenging because it requires such exacting measurements. After interacting with some people who are now my JPL collaborators at the workshop, it seemed like we might be able to do something special that would help solve this problem. And we got support from KISS to do a follow-on study.

As I was getting deeper and deeper into thinking about how we could do this on Mars, John Grotzinger (the Fletcher Jones Professor of Geology at Caltech and project scientist for MSL) was conducting the landing-site workshops for MSL. He would say things like, "Oh, it would be really great if we could date this." And we'd agree. Then there was a call for participating scientists on MSL. I had no background whatsoever in this, but I knew there was a mass spectrometer on Curiosity. That's one of the analytical instruments we need to make these dating measurements because it allows us to determine the relative abundances of various isotopes in a sample. Since those isotopes are produced at known rates, their abundances tell us something about the age of the sample. So I wrote a proposal basically saying let's see if we can make Curiosity's mass spectrometer work for this purpose. And it did.

 

What do you think led to your selection as project scientist?

Although I don't have a long track record in studying Mars, this mission is possibly the first step in bringing samples back to Earth. In order to do that, you have to answer a lot of questions related to geochemistry, which is my specialty. The geochemistry community is not ordinarily thinking about rocks coming back from Mars. I happen to have enough crossover between what I know about Mars from the work I just described and my background from working in geochemistry labs, especially those working with the type of very small samples we might get back from Mars, to be a good fit.

 

Given Curiosity's success on Mars, why is it important and exciting for us to be sending another rover to the Red Planet?

One thing to realize is that the surface of Mars is more or less equivalent in size to the entire continental surface area of the earth, and we've been to just a few points. It's naturally tempting to look at the few places we have been on Mars and draw grand conclusions from them, but you could imagine if you landed in the middle of the Sahara Desert and studied the earth, you would come up with different answers than if you landed in the Amazon, for example. So that's part of it.

But the big thing that distinguishes Mars 2020 is the fact that we are preparing this cache, which is the first step in a process that will hopefully bring samples back to Earth some day. It's very clear that from the science community's point of view, this is a critical motivation for this mission.

 

How has the experience been working on the mission thus far?

I enjoy it very much. It's extremely different to go from a lab group of two or three people to a project that, at the end of the day, is going to have spent $1.5 billion over the next seven or eight years. It's a completely different scale of operation.

I find it really fascinating to see how everything works. I've spent my entire career among scientists. Suddenly transitioning and working with engineers is interesting because their approach and style is completely different. But they're all extremely good at what they do.

It's a lot of fun to work with these people and to face completely new and unexpected challenges. You never know what new thing is going to pop up.

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Kimm Fesenmaier
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Corals Provide Clues for Climate Change Research

Just as growth rings can offer insight into climate changes occurring during the lifespan of a tree, corals have much to tell about changes in the ocean. At Caltech, climate scientists Jess F. Adkins and Nivedita Thiagarajan use manned submersibles, like Alvin operated by the Woods Hole Oceanographic Institution, to dive thousands of meters below the surface to collect these specimens—and to shed new light on the connection between variance in carbon dioxide (CO2) levels in the deep ocean and historical glacial cycles.

A paper describing the research appears in the July 3 issue of Nature.

It has long been known that ice sheets wax and wane as the concentration of CO2 decreases and increases in the atmosphere. Adkins and his team believe that the deep ocean—which stores 60 times more inorganic sources of carbon than is found in the atmosphere—must play a vital role in this variance.

To investigate this, the researchers analyzed the calcium carbonate skeletons of corals collected from deep in the North Atlantic Ocean. The corals were built up from 11,000–18,000 years ago out of CO2 dissolved in the ocean.

"We used a new technique that has been developed at Caltech, called clumped isotope thermometry, to determine what the temperature of the ocean was in the location where the coral grew," says Thiagarajan, the Dreyfus Postdoctoral Scholar in Geochemistry at Caltech and lead author of the paper. "We also used radiocarbon dating and uranium-series dating to estimate the deep-ocean ventilation rate during this time period." 

The researchers found that the deep ocean started warming before the start of a rapid climate change event about 14,600 years ago in which the last glacial period—or most recent time period when ice sheets covered a large portion of Earth—was in the final stages of transitioning to the current interglacial period.

"We found that a warm-water-under-cold-water scenario developed around 800 years before the largest signal of warming in the Greenland ice cores, called the 'Bølling–Allerød,'" explains Adkins. "CO2 had already been rising in the atmosphere by this time, but we see the deep-ocean reorganization brought on by the potential energy release to be the pivot point for the system to switch from a glacial state, where the deep ocean can hold onto CO2, and an interglacial state, where it lets out CO2."  

"Studying Earth's climate in the past helps us understand how different parts of the climate system interact with each other," says Thiagarajan. "Figuring out these underlying mechanisms will help us predict how climate will change in the future." 

Additional authors on the Nature paper, "Abrupt pre-Bølling–Allerød warming and circulation changes in the deep ocean," are geochemist John M. Eiler and graduate student Adam V. Subhas from Caltech, and John R. Southon from UC Irvine. 

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Katie Neith
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Caltech-Led Team Develops a Geothermometer for Methane Formation

Methane is a simple molecule consisting of just one carbon atom bound to four hydrogen atoms. But that simplicity belies the complex role the molecule plays on Earth—it is an important greenhouse gas, is chemically active in the atmosphere, is used in many ecosystems as a kind of metabolic currency, and is the main component of natural gas, which is an energy source.

Methane also poses a complex scientific challenge: it forms through a number of different biological and nonbiological processes under a wide range of conditions. For example, microbes that live in cows' stomachs make it; it forms by thermal breakdown of buried organic matter; and it is released by hot hydrothermal vents on the sea floor. And, unlike many other, more structurally complex molecules, simply knowing its chemical formula does not necessarily reveal how it formed. Therefore, it can be difficult to know where a sample of methane actually came from.

But now a team of scientists led by Caltech geochemist John M. Eiler has developed a new technique that can, for the first time, determine the temperature at which a natural methane sample formed. Since methane produced biologically in nature forms below about 80°C, and methane created through the thermal breakdown of more complex organic matter forms at higher temperatures (reaching 160°C–200°C, depending on the depth of formation), this determination can aid in figuring out how and where the gas formed.

A paper describing the new technique and its first applications as a geothermometer appears in a special section about natural gas in the current issue of the journal Science. Former Caltech graduate student Daniel A. Stolper (PhD '14) is the lead author on the paper.

"Everyone who looks at methane sees problems, sees questions, and all of these will be answered through basic understanding of its formation, its storage, its chemical pathways," says Eiler, the Robert P. Sharp Professor of Geology and professor of geochemistry at Caltech.

"The issue with many natural gas deposits is that where you find them—where you go into the ground and drill for the methane—is not where the gas was created. Many of the gases we're dealing with have moved," says Stolper. "In making these measurements of temperature, we are able to really, for the first time, say in an independent way, 'We know the temperature, and thus the environment where this methane was formed.'"

Eiler's group determines the sources and formation conditions of materials by looking at the distribution of heavy isotopes—species of atoms that have extra neutrons in their nuclei and therefore have different chemistry. For example, the most abundant form of carbon is carbon-12, which has six protons and six neutrons in its nucleus. However, about 1 percent of all carbon possesses an extra neutron, which makes carbon-13. Chemicals compete for these heavy isotopes because they slow molecular motions, making molecules more stable. But these isotopes are also very rare, so there is a chemical tug-of-war between molecules, which ends up concentrating the isotopes in the molecules that benefit most from their stabilizing effects. Similarly, the heavy isotopes like to bind, or "clump," with each other, meaning that there will be an excess of molecules containing two or more of the isotopes compared to molecules containing just one. This clumping effect is strong at low temperatures and diminishes at higher temperatures. Therefore, determining how many of the molecules in a sample contain heavy isotopes clumped together can tell you something about the temperature at which the sample formed.

Eiler's group has previously used such a "clumped isotope" technique to determine the body temperatures of dinosaurs, ground temperatures in ancient East Africa, and surface temperatures of early Mars. Those analyses looked at the clumping of carbon-13 and oxygen-18 in various minerals. In the new work, Eiler and his colleagues were able to examine the clumping of carbon-13 and deuterium (hydrogen-2).

The key enabling technology was a new mass spectrometer that the team designed in collaboration with Thermo Fisher, mixing and matching existing technologies to piece together a new platform. The prototype spectrometer, the Thermo IRMS 253 Ultra, is equipped to analyze samples in a way that measures the abundances of several rare versions, or isotopologues, of the methane molecule, including two "clumped isotope" species: 13CH3D, which has both a carbon-13 atom and a deuterium atom, and 12CH2D2, which includes two deuterium atoms.

Using the new spectrometer, the researchers first tested gases they made in the laboratory to make sure the method returned the correct formation temperatures.

They then moved on to analyze samples taken from environments where much is known about the conditions under which methane likely formed. For example, sometimes when methane forms in shale, an impermeable rock, it is trapped and stored, so that it cannot migrate from its point of origin. In such cases, detailed knowledge of the temperature history of the rock constrains the possible formation temperature of methane in that rock. Eiler and Stolper analyzed samples of methane from the Haynesville Shale, located in parts of Arkansas, Texas, and Louisiana, where the shale is not thought to have moved much after methane generation. And indeed, the clumped isotope technique returned a range of temperatures (169°C–207°C) that correspond well with current reservoir temperatures (163°C–190°C). The method was also spot-on for methane collected from gas that formed as a product of oil-eating bugs living on top of oil reserves in the Gulf of Mexico. It returned temperatures of 34°C and 48°C plus or minus 8°C for those samples, and the known temperatures of the sampling locations were 42°C and 48°C, respectively.

To validate further the new technique, the researchers next looked at methane from the Marcellus Shale, a formation beneath much of the Appalachian basin, where the gas-trapping rock is known to have formed at high temperature before being uplifted into a cooler environment. The scientists wanted to be sure that the methane did not reset to the colder temperature after formation. Using their clumped isotope technique, the researchers verified this, returning a high formation temperature.

"It must be that once the methane exists and is stable, it's a fossil remnant of what its formation environment was like," Eiler says. "It only remembers where it formed."

An important application of the technique is suggested by the group's measurements of methane from the Antrim Shale in Michigan, where groundwater contains both biologically and thermally produced methane. Clumped isotope temperatures returned for samples from the area clearly revealed the different origins of the gases, hitting about 40°C for a biologically produced sample and about 115°C for a sample involving a mix of biologically and thermally produced methane.

"There are many cases where it is unclear whether methane in a sample of groundwater is the product of subsurface biological communities or has leaked from petroleum-forming systems," says Eiler. "Our results from the Antrim Shale indicate that this clumped isotope technique will be useful for distinguishing between these possible sources."

One final example, from the Potiguar Basin in Brazil, demonstrates another way the new method will serve geologists. In this case the methane was dissolved in oil and had been free to migrate from its original location. The researchers initially thought there was a problem with their analysis because the temperature they returned was much higher than the known temperature of the oil. However, recent evidence from drill core rocks from the region shows that the deepest parts of the system actually got very hot millions of years ago. This has led to a new interpretation suggesting that the methane gas originated deep in the system at high temperatures and then percolated up and mixed into the oil.

"This shows that our new technique is not just a geothermometer for methane formation," says Stolper. "It's also something you can use to think about the geology of the system."

The paper is titled "Formation temperatures of thermogenic and biogenic methane." Along with Eiler and Stolper, additional coauthors are Alex L. Sessions, professor of geobiology at Caltech; Michael Lawson and Cara L. Davis of ExxonMobil Upstream Research Company; Alexandre A. Ferreira and Eugenio V. Santos Neto of Petrobas Research and Development Center; Geoffrey S. Ellis and Michael D. Lewan of the U.S. Geological Survey in Denver; Anna M. Martini of Amherst College; Yongchun Tang of the Power, Environmental, and Energy Research Institute in Covina, California; and Martin Schoell of GasConsult International Inc. in Berkeley, California. The work was supported by the National Science Foundation, Petrobras, and ExxonMobil.

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Kimm Fesenmaier
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Earth-Building Bridgmanite

Our planet's most abundant mineral now has a name

Deep below the earth's surface lies a thick, rocky layer called the mantle, which makes up the majority of our planet's volume. For decades, scientists have known that most of the lower mantle is a silicate mineral with a perovskite structure that is stable under the high-pressure and high-temperature conditions found in this region. Although synthetic examples of this composition have been well studied, no naturally occurring samples had ever been found in a rock on the earth's surface. Thanks to the work of two scientists, naturally occurring silicate perovskite has been found in a meteorite, making it eligible for a formal mineral name.

The mineral, dubbed bridgmanite, is named in honor of Percy Bridgman, a physicist who won the 1946 Nobel Prize in Physics for his fundamental contributions to high-pressure physics.

"The most abundant mineral of the earth now has an official name," says Chi Ma, a mineralogist and director of the Geological and Planetary Sciences division's Analytical Facility at Caltech.

"This finding fills a vexing gap in the taxonomy of minerals," adds Oliver Tschauner, an associate research professor at the University of Nevada-Las Vegas who identified the mineral together with Ma.

High-pressure and temperature experiments, as well as seismic data, strongly suggest that (Mg,Fe)SiO3-perovskite—now simply called bridgmanite—is the dominant material in the lower mantle. But since it is impossible to get to the earth's lower mantle, located some 400 miles deep within the planet, and rocks brought to the earth's surface from the lower mantle are exceedingly rare, naturally occurring examples of this material had never been fully described.

That is until Ma and Tschauner began poking around a sample from the Tenham meteorite, a space rock that fell in Australia in 1879.

Because the 4.5 billion-year-old meteorite had survived high-energy collisions with asteroids in space, parts of it were believed to have experienced the high-pressure conditions we see in the earth's mantle. That, scientists thought, made it a good candidate for containing bridgmanite.

Tschauner used synchrotron X-ray diffraction mapping to find indications of the mineral in the meteorite. Ma then examined the mineral and its surroundings with a high-resolution scanning electron microscope and determined the composition of the tiny bridgmanite crystals using an electron microprobe. Next, Tschauner analyzed the crystal structure by synchrotron diffraction. After five years and multiple experiments, the two were finally able to gather enough data to reveal bridgmanite's chemical composition and crystal structure.

"It is a really cool discovery," says Ma. "Our finding of natural bridgmanite not only provides new information on shock conditions and impact processes on small bodies in the solar system, but the tiny bridgmanite found in a meteorite could also help investigations of phase transformation mechanisms in the deep Earth. "

The mineral and the mineral name were approved on June 2 by the International Mineralogical Association's Commission on New Minerals, Nomenclature and Classification. 

The researchers' findings are published in the November 28 issue of Science, in an article titled "Discovery of Bridgmanite, the Most Abundant Mineral in Earth, In a Shocked Meteorite."

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Katie Neith
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Caltech Faculty Elected to the American Academy of Arts and Sciences

The American Academy of Arts and Sciences has elected three Caltech faculty members as academy fellows. They are John F. Brady, Chevron Professor of Chemical Engineering and Mechanical Engineering and executive officer for chemical engineering; Kenneth A. Farley, W. M. Keck Foundation Professor of Geochemistry and chair of the Division of Geological and Planetary Sciences; and Fiona A. Harrison, Benjamin M. Rosen Professor of Physics.

"It is a privilege to honor these men and women for their extraordinary individual accomplishments," said Don Randel, chair of the academy's board of directors, of the 204 newly elected fellows and 16 foreign honorary members. "The knowledge and expertise of our members gives the academy a unique capacity—and responsibility—to provide practical policy solutions to the pressing challenges of the day. We look forward to engaging our new members in this work."

Brady works in the area of complex fluids and active matter that includes microstructural elements such as suspensions, colloidal dispersions, and self-propelling particles. Understanding these materials led Brady to develop a novel computational method called Stokesian dynamics. He won the 2012 Fluid Dynamics Prize from the American Physical Society and was elected to the National Academy of Engineering in 1999.

Most of Farley's research has focused on terrestrial geochemistry, but he is now increasingly interested in planetary science and especially exploration of the geochemistry, geology, and geomorphology of Mars. In his laboratory on the Caltech campus, Farley and his group measure noble gases such as helium and neon in rock and mineral samples. One major objective of this work is determining the ages and surface exposure history of Earth's geological features. Farley was recently involved in the first-ever experiments of this type carried out on the surface of Mars, via an instrument on board the Mars Science Laboratory's Curiosity rover. He has received the Day Medal of the Geological Society of America and the Macelwane Award of the American Geophysical Union, and was elected to the National Academy of Sciences in 2013.

Harrison specializes in observational and experimental high-energy astrophysics. She is the principal investigator for NASA's NuSTAR Explorer Mission and uses this satellite, along with other satellites and ground-based telescopes, to understand black holes, neutron stars, and supernova remnants. In her labs at Caltech, Harrison's group develops high-energy X-ray detectors and instrumentation for future space missions. She was elected to the American Physical Society in 2012 and won a NASA Outstanding Public Leadership Medal in 2013.

Also named to the academy this year is Katherine T. Faber, the Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University, who will be joining the Caltech faculty on July 1 as the Simon Ramo Professor of Materials Science. Faber's research focuses on understanding fracture and toughening of brittle materials such as those used for high-temperature coatings for power generation applications. She also works on the fabrication of ceramic materials with controlled porosity. She is cofounder and codirector of the Northwestern University-Art Institute of Chicago Center for Scientific Studies in the Arts (NU-ACCESS), which employs advanced materials science techniques for conservation science. Faber is a Distinguished Life Member of the American Ceramic Society (2013), and became a National Science Foundation American Competitiveness and Innovation Fellow in 2010.

The total number of Caltech faculty named to the academy is now 97.

The academy was founded in 1780 by John Adams, James Bowdoin, John Hancock, and other scholar-patriots "to cultivate every art and science which may tend to advance the interest, honor, dignity, and happiness of a free, independent, and virtuous people." The academy has elected as fellows and foreign honorary members the finest minds and most influential leaders from each generation, including George Washington and Ben Franklin in the 18th century, Daniel Webster and Ralph Waldo Emerson in the 19th, and Albert Einstein and Winston Churchill in the 20th. The current membership includes more than 250 Nobel laureates and 60 Pulitzer Prize winners.

A full list of new members is available on the academy website at https://www.amacad.org/content/members/members.aspx.

The academy will welcome this year's new fellows and foreign honorary members at its annual induction ceremony at the academy's headquarters in Cambridge, Massachusetts, on October 11, 2014.

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Cynthia Eller
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