Caltech Researchers Develop High-Performance Bulk Thermoelectrics

PASADENA, Calif.—Roughly 10 billion miles beyond Neptune's orbit, and well past their 30th birthdays, Voyagers 1 and 2 continue their lonely trek into the Milky Way. And they're still functioning—running on power gleaned not from the pinprick sun, but from solid-state devices called thermoelectric generators, which convert heat energy into electricity.

The same technology can be applied here on Earth to recover waste heat when fuel is burned. "Cogeneration," or the production of electricity as a by-product of a heat-generating process, already provides as much as 10 percent of Europe's electrical power. Systems for this purpose typically operate best at very high temperatures, are costly to build and operate, and suffer from substantial inefficiencies. That's why they can be found in spacecraft and power plants but not, say, in cars.

But recently, scientists have concocted a recipe for a thermoelectric material that might be able to operate off nothing more than the heat of a car's exhaust. In a paper published in Nature this month, G. Jeffrey Snyder, faculty associate in applied physics and materials science at the California Institute of Technology (Caltech), and his colleagues reported on a compound that shows high efficiency at less extreme temperatures.

The heart of a thermoelectric generator is a flat array of semiconductor material. In operation, heat from an external source is directed against one side of the array, while the other side is kept cool. Like air molecules in a hot oven, the material within the array flows along the induced temperature gradient: away from the hot side and toward the cool side. But in the crystalline lattice of a semiconductor, there's only one "material" that isn't rigidly fixed: the charge carriers. Consequently, the only things that move in response to the thermal nonequilibrium are these charge carriers and the result is an electrical flow. Build up a circuit by laying out small semiconductor bricks side by side and wiring them together, and you've got a steady electric current.

The lead telluride (PbTe) family of compounds is commonly used in these applications, but regardless of the underlying technology, scientists designing new thermoelectric materials are continually constrained by structural issues at the most microscopic levels. Those moving charge carriers can run afoul of many complex effects, including electrical interactions, heat-induced vibrations (called phonons), and scattering caused by impurities and imperfections within the crystal structure.

The Caltech researchers began with lead telluride and then added a fractional amount of the element selenium, a concoction first proposed by Soviet scientists A. F. Ioffe and A. V. Ioffe in the 1950s. Because any semiconductor's properties are highly sensitive to the exact type and placement of each of its atoms, this small alteration in the formula produces important changes in the crystal's electronic structure.

Specifically, certain regions called "degenerate valleys" arrange themselves in such a way as to provide a more favorable pathway for charge carriers to follow, a trail of equal-energy stepping stones through the material. In addition, adding the selenium creates multiple regions called point defects. "They're like air bubbles trapped in window glass," says Snyder, "and they tend to scatter vibrations. The result is that heat dissipates more slowly through the material."

That dissipation is important, because in order for a material to be efficient, charge carriers should flow much more easily than heat. In other words, electrical resistance should be low, to maximize current, while thermal resistance should be high, to maintain the temperature gradient that causes the charge carriers to flow in the first place. "It's a delicate tradeoff," says Snyder. "Something like trying to blow ice cream through a straw. If the straw's very narrow, the ice cream moves slowly. But if you widen it to help the ice cream move faster, you'll find that you also run out of air faster."

To make sense of these tradeoffs, scientists speak of a quantity known as the "thermoelectric figure of merit," a dimensionless value that can be used to compare the relative efficiency of materials at specific temperatures. The temperature at which peak efficiency is seen depends on the material: each of the Voyager twins, for instance, produces enough juice to power a medium-sized refrigerator, but to do so it must draw heat from decaying radioisotopes. "These new materials are roughly twice as effective as anything seen before, and they work well in a temperature range of around 400 to 900 degrees Kelvin," says Snyder. "Waste heat recovery from a car's engine falls well within that range."

In other words, the heat escaping out your car's tailpipe could be used to help power the vehicle's electrical components—and not just the radio, wipers, and headlights. "You'll see applications wherever there's a solid-state advantage," Snyder predicts. "One example is the charging system. The electricity to keep your car's battery charged is generated by the alternator, a mechanical device driven by a rubber belt powered by the crankshaft. You've got friction, slippage, strain, internal resistance, wear and tear, and weight, in addition to the mechanical energy extracted to make the electricity. Just replacing that one subsystem with a thermoelectric solution could instantly improve a car's fuel efficiency by 10 percent."

As more automotive systems continue their gradual migration from mechanical or hydraulic to electrical—power steering and brakes, for instance, can both be made to run on electricity—the vehicle of the future will sport more than a passing commonality with the spacecraft of the 1970s. "The future of automobiles is electric," says Snyder. "What we're doing now is looking at how to make it all more efficient."

Snyder's coauthors on the paper, "Convergence of electronic bands for high performance bulk thermoelectrics," are Yanzhong Pei, Aaron LaLonde, and Heng Wang of Caltech; and Xiaoya Shi and Lidong Chen of the Shanghai Institute of Ceramics, Chinese Academy of Sciences. The work was supported by NASA-JPL, the DARPA Nano Materials program, and the Chinese Academy of Sciences.

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Dave Zobel

Jorgensen Renovation Kicks into Gear

Those fences you'll soon see sprouting up around Jorgensen Laboratory are among the less classic signs of spring—and yet, they are very much a symbol of growth and rebirth. Now that the interior demolition of the building has been completed, with some 90 percent of the materials removed being reused or recycled, Jorgensen Lab's much-anticipated renovation is about to begin in earnest.

Once completed in spring of 2012, the building will house two of Caltech's key sustainability research efforts: the Resnick Institute and the Joint Center for Artificial Photosynthesis, or JCAP. The lab's structure and infrastructure will embody the sort of innovative work that will be done within its walls, with a low-energy plant design, building energy information systems, and exhibits showcasing Caltech's energy research.

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Lori Oliwenstein
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Working Toward a Greener Future

Celebrate Earth Day with the Caltech community today from 11 a.m. to 2 p.m. along San Pasqual Walk in front of Chandler and the Red Door.

Chevrolet will have its Volt electric car on display, as well as its new fuel-cell car. Visitors will be able to take a look under the hoods and learn more about the cars. Alternative energy exhibitors at the event include Bloom Energy, a company that builds on-site fuel-cell power generators; Suntech and Verengo, manufacturers of solar energy solutions; and Seesmart LED lights, among others.

Student clubs, including the 2011 Solar Decathlon team, will also be exhibiting at Earth Day. Representatives from Engineers for a Sustainable World—an organization dedicated to sustainable development, scientific problem solving, and social entrepreneurship—will have a display at the event, as will members of the Caltech Community Garden Project.

The popular Kogi Truck will be serving up tacos and other Korean BBQ treats from noon until 2 p.m., with Coolhaus providing architectural-themed ice-cream sandwiches for dessert. Arroyo Food Coop and LifeSource Water will also be on hand for lunch options.

For more information about Caltech's year-round sustainability efforts, visit sustainability.caltech.edu.

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Katie Neith
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Solar Decathlon Team Breaks Ground

The joint Solar Decathlon team of Caltech and the Southern California Institute of Architecture (SCI-Arc) will show off their state-of-the-art, energy-efficient house tomorrow in a groundbreaking ceremony at 2 p.m. at the SCI-Arc campus in Los Angeles. After a year of designing and prototyping, the team will start construction on the house, which will travel to the National Mall in Washington, D.C., this fall for the biennial competition. At the ceremony, the team will raise the first wall.

Student team members will also give tours of a full-sized mock-up of the house and talk about its many engineering and design features. At 3 p.m., Eric Owen Moss, the director of SCI-Arc, and Harry Atwater, Caltech's Howard Hughes Professor and professor of applied physics and materials science and director of the Resnick Institute, will also speak. All are welcome and refreshments will be served.

The Solar Decathlon is a competition sponsored by the U.S. Department of Energy (DOE) in which 20 teams from around the world are selected to design and build the most energy-efficient, affordable, and attractive house they can. Taking place on the National Mall, the Solar Decathlon is a high-profile event that's intended to inspire policymakers, industry leaders, and the public to pursue a sustainable future with cutting-edge design and technology.

Somewhat resembling a giant white pillow, the SCI-Arc/Caltech house features a unique shape and a soft, exterior insulation. As the only two-story building in the competition, the house has a spacious interior, despite an area of only about 800 square feet (contest rules limit the area to between 600 and 1,000 square feet). A central computer controls everything from heating to lighting, optimizing energy use. For example, waste heat from the air-conditioning unit is used to provide hot water. Connected to the Internet, the house can even receive weather-forecast data, allowing it to anticipate cloudy skies and conserve the power generated by its solar panels. To learn more about these and other features of the house, you can attend the groundbreaking ceremony at

SCI-Arc Campus
Solar Decathlon Construction Site
350 Merrick Street
Los Angeles, CA 90013

If you would also like to help the team's outreach efforts—one of the 10 challenges on which the team will be judged—you can visit its Facebook page and click "like."

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Marcus Woo
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Warm Water Causes Extra-cold Winters in Northeastern North America and Northeastern Asia

PASADENA, Calif.—If you're sitting on a bench in New York City's Central Park in winter, you're probably freezing. After all, the average temperature in January is 32 degrees Fahrenheit. But if you were just across the pond in Porto, Portugal, which shares New York's latitude, you'd be much warmer—the average temperature is a balmy 48 degrees Fahrenheit.

Throughout northern Europe, average winter temperatures are at least 10 degrees Fahrenheit warmer than similar latitudes on the northeastern coast of the United States and the eastern coast of Canada. The same phenomenon happens over the Pacific, where winters on the northeastern coast of Asia are colder than in the Pacific Northwest.

Researchers at the California Institute of Technology (Caltech) have now found a mechanism that helps explain these chillier winters—and the culprit is warm water off the eastern coasts of these continents.

"These warm ocean waters off the eastern coast actually make it cold in winter—it's counterintuitive," says Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering.

Schneider and Yohai Kaspi, a postdoctoral fellow at Caltech, describe their work in a paper published in the March 31 issue of the journal Nature.

Using computer simulations of the atmosphere, the researchers found that the warm water off an eastern coast will heat the air above it and lead to the formation of atmospheric waves, drawing cold air from the northern polar region. The cold air forms a plume just to the west of the warm water. In the case of the Atlantic Ocean, this means the frigid air ends up right over the northeastern United States and eastern Canada.

For decades, the conventional explanation for the cross-oceanic temperature difference was that the Gulf Stream delivers warm water from the Gulf of Mexico to northern Europe. But in 2002, research showed that ocean currents aren't capable of transporting that much heat, instead contributing only up to 10 percent of the warming.

This image, taken by NASA's Terra satellite in March 2003, shows a much colder North America than Europe--even at equal latitudes. White represents areas with more than 50 percent snow cover. NASA's Aqua satellite also measured water temperatures. Water between 0 and -15 degrees Celsius is in pink, while water between -15 and -28 degrees Celsius is in purple.
Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio; George Riggs (NASA/SSAI)

Kaspi's and Schneider's work reveals a mechanism that helps create a temperature contrast not by warming Europe, but by cooling the eastern United States. Surprisingly, it's the Gulf Stream that causes this cooling.

In the northern hemisphere, the subtropical ocean currents circulate in a clockwise direction, bringing an influx of warm water from low latitudes into the western part of the ocean. These warm waters heat the air above it.

"It's not that the warm Gulf Stream waters substantially heat up Europe," Kaspi says. "But the existence of the Gulf Stream near the U.S. coast is causing the cooling of the northeastern United States."

The researchers' computer model simulates a simplified, ocean-covered Earth with a warm region to mimic the coastal reservoir of warm water in the Gulf Stream. The simulations show that such a warm spot produces so-called Rossby waves.

Generally speaking, Rossby waves are large atmospheric waves—with wavelengths that stretch for more than 1,000 miles. They form when the path of moving air is deflected due to Earth's rotation, a phenomenon known as the Coriolis effect. In a way similar to how gravity is the force that produces water waves on the surface of a pond, the Coriolis force is responsible for Rossby waves.

In the simulations, the warm water produces stationary Rossby waves, in which the peaks and valleys of the waves don't move, but the waves still transfer energy. In the northern hemisphere, the stationary Rossby waves cause air to circulate in a clockwise direction just to the west of the warm region. To the east of the warm region, the air swirls in the counterclockwise direction. These motions draw in cold air from the north, balancing the heating over the warm ocean waters.

To gain insight into the mechanisms that control the atmospheric dynamics, the researchers speed up Earth's rotation in the simulations. In those cases, the plume of cold air gets bigger—which is consistent with it being a stationary Rossby-wave plume. Most other atmospheric features would get smaller if the planet were to spin faster.

Although it's long been known that a heat source could produce Rossby waves, which can then form plumes, this is the first time anyone has shown how the mechanism causes cooling that extends west of the heat source. According to the researchers, the cooling effect could account for 30 to 50 percent of the temperature difference across oceans.

This process also explains why the cold region is just as big for both North America and Asia, despite the continents being so different in topography and size. The Rossby-wave induced cooling depends on heating air over warm ocean water. Since the warm currents along western ocean boundaries in both the Pacific and Atlantic are similar, the resulting cold region to their west would be similar as well.

The next step, Schneider says, is to build simulations that more realistically reflect what happens on Earth. Future simulations would incorporate more complex features like continents and cloud feedbacks.

The research described in the Nature paper, "Winter cold of eastern continental boundaries induced by warm ocean waters," was funded by the NOAA Climate and Global Change Postdoctoral Fellowship, administrated by the University Corporation for Atmospheric Research; a David and Lucille Packard Fellowship; and the National Science Foundation.

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Marcus Woo
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Greening Caltech's Bottom Line

You can see signs of Caltech's commitment to sustainability everywhere you look on campus: there are recycling centers, LEED-certified buildings, and arrays of solar panels, not to mention landscaping that uses drought-tolerant plants, and dining rooms that feature compostable utensils.

But some of the campus's most impressive efforts are also its least visible ones. Caltech's efforts to finance urgently needed energy-efficiency upgrades—and to do so in ways that give back to the Institute—have been recognized by the Sustainable Endowments Institute in its recently released report, Greening the Bottom Line: The Trend toward Green Revolving Funds on Campus.

Green revolving funds are monies set aside to finance needed upgrades that will increase the efficiency of energy use; these projects are funded based on their likelihood to return the capital spent on them—plus some—once the reductions in energy use or operating expenses are realized. And it's that return on capital that allows the funds to revolve—to be used for yet another energy-saving project.

Caltech's own green revolving fund—the Caltech Energy Conservation Investment Program (CECIP)—was initiated in 2009, and it manages $8 million within an existing endowment created to finance capital projects.

"CECIP allows Caltech to effectively deploy capital to realize energy and cost savings from energy-conservation measures that would otherwise be unfunded," says John Onderdonk, Caltech's manager for sustainability programs. "It's a perfect example of Caltech's sustainability vision in that it reduces the campus's environmental footprint while enhancing the Institute's core mission."

The projects' impact has been nothing if not impressive. According to the Sustainability Research Institute, the efficiency measures funded by Caltech's CECIP has allowed an average return on investment of 33 percent, and had—by August 2010—reduced the Institute's energy bills by $1.5 million.

What's perhaps more impressive is that these projects aren't the high-visibility, easy-to-get funded initiatives like rooftop solar panels. "A lot of the work done through CECIP is behind the scenes," Onderdonk says.

Among the projects that CECIP has funded in the past two years are heating, ventilation, and air-conditioning upgrades at the Broad Center; the installation of auto-closing flume hoods in the Schlinger Laboratory; an LED lighting upgrade in the South Wilson Parking Structure; and lighting upgrades at South Mudd, North Mudd, the Beckman Institute, and the Keck Laboratories.

Any member of the Caltech community is welcome to submit a project proposal, says Onderdonk; projects will be approved if they have a 15 percent return on investment or a simple payback period of less than six years.

For more information on the efforts being made to reduce Caltech's environmental impact and promote stewardship within the Caltech community, visit Sustainability at Caltech.

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Lori Oliwenstein
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Solar Decathlon Back on the Mall

Thanks to a campaign led by a joint team of students from Caltech and the Southern California Institute of Architecture (SCI-Arc), the 2011 Solar Decathlon is back on the National Mall in Washington, D.C. The announcement made last week reverses an earlier decision by the U.S. Department of the Interior (DOI) to relocate the event.

The two-week competition, taking place in the fall, will now be held in West Potomac Park, on a swath of land between the Lincoln and Jefferson Memorials. "Everyone's relieved and happy that we're back on the Mall," says Caltech's Fei Yang, a senior majoring in mechanical engineering, and one of the leaders of the SCI-Arc/Caltech team.

The biennial competition, sponsored by the U.S. Department of Energy (DOE), invites 20 teams from around the world and challenges them to build the most energy-efficient, affordable, and attractive house they can. All four previous competitions have taken place on the National Mall—between the Capitol and the Washington Monument—providing a high-profile venue to inspire policymakers, industry leaders, and the public to pursue a sustainable future with state-of-the-art design and technology.

But in January, the DOI, with the approval of the DOE, decided that the event could no longer be held on the Mall, citing the need to protect and preserve the grounds. "It came as a shock," Yang says.

A statement posted January 11 on the DOE Solar Decathlon website noted that the decision to change locations was made "in support of the historic effort underway to protect, improve, and restore the National Mall." The DOI is embarking on a $600 million plan to renovate the grounds and areas around the monuments. But this justification didn't make sense, says Elisabeth Neigert, a recent SCI-Arc graduate and one of the project managers for the SCI-Arc/Caltech team; a clause in each team's contract already makes them financially responsible for any damage.

Furthermore, Neigert says, this sudden change in venue was made three-quarters of the way through the competition, after design, engineering, and logistical decisions had been made based on the expectation that the house would be built on the Mall. And the promise of high-profile exposure was crucial in securing sponsors. "This late in the game, it left us in a really difficult position," she says.

"They tore the heart out of the Solar Decathlon," says Cole Hershkowitz, also a Caltech senior in mechanical engineering and another leader of the team. "One of the reasons why the competition is so prestigious is because the government supported it and it's in arguably the most prominent place in our country, right there between the Capitol and the Washington Monument." There's no monetary prize for the winner, so the prestige and the opportunity to showcase their innovations are the teams' rewards. "If we had known that it would not be on the National Mall, I think only half of us would've applied," Hershkowitz says.  

What followed was a concerted effort led by Neigert and the SCI-Arc/Caltech team to appeal the DOI's decision. Working with the other 19 Solar Decathlon teams, the group of architecture and engineering students suddenly found themselves in the middle of a political campaign. After much lobbying, they received support from organizations such as the National Coalition to Save Our Mall. They also got official backing from more than a dozen senators and other congressional leaders, including California senator Barbara Boxer and Massachusetts representative Edward Markey, ranking member of the Natural Resources Committee.

Then, the Washington Post stepped in to cover the debate, and Neigert and Eric Owen Moss, director of SCI-Arc, penned opinion columns for the Huffington Post. Two days after the op-eds appeared—and after six weeks of relentless campaigning—the DOI reversed its ruling. "I was filled with disbelief, but I was also absolutely elated," Neigert says. "I was pretty speechless, which doesn't happen very often." Even though the event would no longer be right in front of the Capitol steps, the team is satisfied. "It was probably the best outcome we could've gotten, given the circumstances," she says.

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The full-sized model of the SCI-Arc/Caltech house with its soft, insulating exterior.

Now, the team can refocus on building their house, which is computerized to control and monitor energy usage to maximize efficiency. Every house in the competition is required to be net-zero—that is, to use only as much energy as its solar panels can generate. But perhaps what's most striking about the SCI-Arc/Caltech design is its soft, insulating exterior, which gives the building the appearance of a giant pillow. The house is also the only two-story structure in the competition.

With the Solar Decathlon back on the Mall, the team hopes their design will attract plenty of eyes. After all, from historic moments like Martin Luther King's "I Have a Dream" speech to the numerous festivals that attract hundreds of thousands of visitors, the National Mall is a place where things happen and are noticed, Hershkowitz says. "What makes the National Mall so iconic is not how green the grass is, but the quality of events."

The SCI-Arc/Caltech team will hold a groundbreaking event on April 2 at the SCI-Arc campus in Los Angeles.

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Marcus Woo
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Sustaining the Next Generation of Energy Scientists

Training the next generation of scientists capable of creating startling and transformational advances in sustainable energy research is a critical component of the mission of Caltech's Resnick Institute.

And although the institute is not yet two years old—it was created in June of 2009 by a gift from Stewart and Lynda Resnick and began operating the following April—it has already begun to fulfill that mission, thanks to the newly developed Resnick Fellowship program.

The first two Resnick Fellows began their work in the fall of 2010. Last week, the institute put out a call for applications for the next set of two-year awards, which provide tuition plus a $30,000-per-year stipend for graduate students from any discipline who are seeking to explore unusual and creative sustainable-energy research projects on campus.

"The Resnick Fellowships are unique because they provide an opportunity for Caltech students to think outside the box and take a chance on potentially groundbreaking new work in areas that might not otherwise get explored," says Neil Fromer, the institute's executive director. "Caltech has always been a place where creative individuals can have a big impact. With these fellowships, we are similarly enabling students to chart new paths."

As part of his Resnick Fellowship, Matt Smith, a graduate student in bioengineering working in the laboratory of Frances Arnold, is looking for better ways to create second-generation biofuels—biofuels made from cellulose, hemicellulose, or lignin, all of which are components of plant cell walls. Specifically, he's using new protein recombination techniques to try to create active, stable forms of the enzyme beta glucosidase, which is used to cut small-length cellulose chains into individual glucose molecules.

"It's a more unusual, riskier project," Smith says. "When I started talking about it, Frances suggested I apply for a Resnick Fellowship. She felt it was in the spirit of the institute—a project on energy and sustainability that's a bit 'out there.'"

Smith's fellow Fellow, David Abrecht, is working with both Brent Fultz and Theo Agapie on a stationary hydrogen-storage project using ionic liquids. "Hydrogen gas is uneconomical to store," Abrecht explains. But in order to develop hydrogen as a fuel, hydrogen needs to be stored cheaply, at room temperature and pressure, and it needs to be able to release quickly.

The answer, Abrecht says, may lie in liquid storage. "I'd been thinking about liquid-state storage systems for a while," he explains. "Liquid-state storage might allow you to use hydrogen as a buffer to prevent supply spikes in the electrical-generation grid, to fill in the gaps."

Abrecht is looking at what are known as ionic liquids—which are salts in a liquid state—that would be able to bind with hydrogen at more-or-less normal temperatures and pressures. "Trying to find funding for this kind of project from typical sources would have been very difficult," he says. "I would not have been able to do this project without this fellowship."

Smith and Abrecht are looking forward to the next generation of Resnick Fellows. "The more Fellows there are, the more opportunity we'll have for interaction," Smith says. "I'm looking forward to that, to creating a small community of students working toward similar goals."

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Lori Oliwenstein
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Caltech Geobiologists Uncover Links between Ancient Climate Change and Mass Extinction

PASADENA, Calif.—About 450 million years ago, Earth suffered the second-largest mass extinction in its history—the Late Ordovician mass extinction, during which more than 75 percent of marine species died. Exactly what caused this tremendous loss in biodiversity remains a mystery, but now a team led by researchers at the California Institute of Technology (Caltech) has discovered new details supporting the idea that the mass extinction was linked to a cooling climate.

"While it’s been known for a long time that the mass extinction is intimately tied to climate change, the precise mechanism is unclear," says Seth Finnegan, a postdoctoral researcher at Caltech and the first author of the paper published online in Science on January 27. The mass extinction coincided with a glacial period, during which global temperatures cooled and the planet saw a marked increase in glaciers. At this time, North America was on the equator, while most of the other continents formed a supercontinent known as Gondwana that stretched from the equator to the South Pole.

By using a new method to measure ancient temperatures, the researchers have uncovered clues about the timing and magnitude of the glaciation and how it affected ocean temperatures near the equator. "Our observations imply a climate system distinct from anything we know about over the last 100 million years," says Woodward Fischer, assistant professor of geobiology at Caltech and a coauthor.

The fact that the extinction struck during a glacial period, when huge ice sheets covered much of what's now Africa and South America, makes it especially difficult to evaluate the role of climate. "One of the biggest sources of uncertainty in studying the paleoclimate record is that it’s very hard to differentiate between changes in temperature and changes in the size of continental ice sheets," Finnegan says. Both factors could have played a role in causing the mass extinction: with more water frozen in ice sheets, the world’s sea levels would have been lower, reducing the availability of shallow water as a marine habitat. But differentiating between the two effects is a challenge because until now, the best method for measuring ancient temperatures has also been affected by the size of ice sheets.

The conventional method for determining ancient temperature requires measuring the ratios of oxygen isotopes in minerals precipitated from seawater. The ratios depend on both temperature and the concentration of isotopes in the ocean, so the ratios reveal the temperature only if the isotopic concentration of seawater is known. But ice sheets preferentially lock up one isotope, which reduces its concentration in the ocean. Since no one knows how big the ice sheets were, and these ancient oceans are no longer available for scientists to analyze, it's hard to determine this isotopic concentration. As a result of this "ice-volume effect," there hasn’t been a reliable way to know exactly how warm or cold it was during these glacial periods.

Rock strata on Anticosti Island, Quebec, Canada, one of the sites from which the researchers collected fossils.

But by using a new type of paleothermometer developed in the laboratory of John Eiler, Sharp Professor of Geology and professor of geochemistry at Caltech, the researchers have determined the average temperatures during the Late Ordovician—marking the first time scientists have been able to overcome the ice-volume effect for a glacial episode that happened hundreds of millions of years ago. To make their measurements, the researchers analyzed the chemistry of fossilized marine animal shells collected from Quebec, Canada, and from the midwestern United States.

The Eiler lab’s method, which does not rely on the isotopic concentration of the oceans, measures temperature by looking at the "clumpiness" of heavy isotopes found in fossils. Higher temperatures cause the isotopes to bond in a more random fashion, while low temperatures lead to more clumping.

"By providing independent information on ocean temperature, this new method allows us to know the isotopic composition of 450-million-year-old seawater," Finnegan says. "Using that information, we can estimate the size of continental ice sheets through this glaciation." And with a clearer idea of how much ice there was, the researchers can learn more about what Ordovician climate was like—and how it might have stressed marine ecosystems and led to the extinction.

"We have found that elevated rates of climate change coincided with the mass extinction," says Aradhna Tripati, a coauthor from UCLA and visiting researcher in geochemistry at Caltech.

The team discovered that even though tropical ocean temperatures were higher than they are now, moderately sized glaciers still existed near the poles before and after the mass extinction. But during the extinction intervals, glaciation peaked. Tropical surface waters cooled by five degrees, and the ice sheets on Gondwana grew to be as large as 150 million cubic kilometers—bigger than the glaciers that covered Antarctica and most of the Northern Hemisphere during the modern era’s last ice age 20,000 years ago.

"Our study strengthens the case for a direct link between climate change and extinction," Finnegan says. "Although polar glaciers existed for several million years, they only caused cooling of the tropical oceans during the short interval that coincides with the main pulse of mass extinction."

In addition to Finnegan, Eiler, Tripati, and Fischer, the other authors on the Science paper, "The magnitude and duration of Late Ordovician-Early Silurian glaciation magnitude," are Kristin Bergmann, a graduate student at Caltech; David Jones of Amherst College; David Fike of Washington University in St. Louis; Ian Eisenman, a postdoctoral scholar at Caltech and the University of Washington; and Nigel Hughes of the University of California, Riverside.

This research was funded by the Agouron Institute and the National Science Foundation.
 

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Marcus Woo
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Obama Touts Caltech Research

In last night's State of the Union Address, President Obama said, "We're issuing a challenge. We're telling America's scientists and engineers that if they assemble teams of the best minds in their fields, and focus on the hardest problems in clean energy, we'll fund the Apollo projects of our time. At the California Institute of Technology, they're developing a way to turn sunlight and water into fuel for our cars. At Oak Ridge National Laboratory, they're using supercomputers to get a lot more power out of our nuclear facilities. With more research and incentives, we can break our dependence on oil with biofuels, and become the first country to have a million electric vehicles on the road by 2015." Watch video of the speech.

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Allison Benter
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