Science in Progress: The Curious Case of the Shuram Excursion

About 540 million years ago, animal life on Earth suddenly boomed during an event known as the Cambrian explosion. In just tens of millions of years—a mere geological moment—life evolved rapidly, with all of the major groups of animals alive today making their first appearance. While researchers still aren't sure what could have triggered this burst of biodiversity, many suspect it had to do with a sudden rise in oxygen levels in the atmosphere, which would have allowed complex, multicellular organisms—and eventually higher organisms like ourselves—to flourish. Where this flood of oxygen came from is also a mystery, but whatever caused it, scientists know that it must have been a big event.

As one of the most important riddles in geology, this question has long interested Caltech geologists. In particular, professors John Grotzinger and Woody Fischer have been puzzling over a surprising discovery made in the early 1990s—a discovery that provided some clues as to what might have happened all those millions of years ago.

Digging in the oil-rich desert of Oman, geologists had looked at carbon isotopes in rocks. Measuring isotope ratios is a basic part of a geobiologist’s tool kit, allowing these scientists to piece together Earth's environmental history. The researchers found that sedimentary rocks from a time just before the Cambrian explosion—during the so-called Ediacaran period—were curiously short on a carbon isotope called carbon-13. In fact, the ratios of carbon-13 to carbon-12 that these researchers discovered were the lowest that have ever been seen.

The cause of this drop in carbon-13—named the Shuram excursion, after the rocks in which it was found—is a mystery that's still unsolved today. Now, Grotzinger, Fischer, and David Fike of Washington University in St. Louis have offered a new hypothesis in a review paper published in Nature on April 17. They still need to hunt for more data, but their idea is the latest attempt to reconcile seemingly contradictory evidence that has sparked a significant debate in the field.

When geologists first found this depletion in carbon-13, they thought it was just due to the usual set of processes that turn sediment into rock over time—rainwater can react with the rock, for instance, or high temperatures and pressures can induce chemical changes. Called diagenesis, this smorgasbord of reactions alters isotopic ratios. Diagenesis is typically a local process, depending on the environmental and geological conditions of a particular area—in this case, Oman.

In the last decade or so, however, geologists have found similar carbon-13 depletions widely distributed across other parts of the world, including southern China, southern Australia, and just a couple of hours away from Caltech, in Death Valley. In samples from each of these locations, geologists found a huge dip in the carbon isotope ratios at around the same depth—and therefore from around the same time in history. "We've never seen curves like this," Fischer says. "This is rare and unique in the rock record."

The fact that the Shuram excursion appears around the world seems to indicate that it's the chemical fingerprint of some global event that could be related to the rise of oxygen.

One of the most recent analyses came in February, when Caltech graduate student Charlie Verdel (now at the University of Michigan), geology professor Brian Wernicke, and Samuel Bowring at MIT published new data on the sediments in Death Valley. According to Grotzinger, this analysis provides some of the best evidence that the Shuram excursion was indeed a worldwide phenomenon. In another paper that's in press, graduate student Kristin Bergmann, along with Rebecca Zentmyer of Cerritos College and Fischer, have just published more observations of the Death Valley sediments that support this argument.

And yet, the data also support diagenesis. First, the depletion in carbon-13 only exists in inorganic carbon—carbonate minerals—and not in fossilized organic carbon deposited in the same sediment. If the Shuram excursion were caused by some global event, then it should also be recorded in the organic matter deposited in the same rocks.

The other piece of evidence that points to diagenesis is the oxygen-isotope ratios. In the rock samples that showed the Shuram excursion, researchers also found a relative lack of oxygen-18, which could be a sign that the sediment reacted with different kinds of fluids. The ratio of oxygen-18 to oxygen-16 dropped in much the same way as the ratio of carbon-13 to carbon-12 did, suggesting that whatever caused the carbon dip also caused an oxygen dip—a typical sign of diagenesis.

But at the same time, this data could be explained without invoking diagenesis. In 2006, Grotzinger and Fike, who was his graduate student at the time, along with Lisa Pratt at Indiana University and Roger Summons at MIT, published a paper in Nature suggesting that the data could be consistent with a global event related to the worldwide appearance of oxygen.

Field work along the coastline of Oman. Left to right: Lauren Edgar, John Grotzinger, Maggie Osburn, Woody Fischer.

In this scenario, Earth's ocean during the Ediacaran was chock full of organic carbon, which lacks carbon-13. A sudden increase in oxygen would have oxidized this oceanic carbon, producing carbon dioxide that then formed carbonate rock—inorganic carbon that's deficient in carbon-13, which is exactly what characterizes the Shuram excursion.

Another characteristic of the Shuram data is that the organic carbon found in the sediment did not show a similar decrease in carbon-13—but the researchers could explain this, too. There was so much organic carbon in the oceans that its presence would have masked any dip that might be seen in the sediment's organic carbon.

Despite this evidence, some researchers still argue for a purely diagenetic origin of the Shuram excursion, according to Grotzinger. But no matter which side they're on, all scientists in the field are wrestling with this perplexing data.

"We were sort of sitting around the room, thinking of how we could explain this," Grotzinger recalls. He and his colleagues wondered whether it might be possible for the Shuram excursion to have been caused by diagenesis, but—in a departure from the way people have always thought about it—via a unique global version of the process.

The researchers propose that a rise in atmospheric oxygen would have altered the planet's chemistry, helping sediment get better at oxidizing organic carbon by enriching that sediment with such compounds as iron oxide and sulfate-bearing minerals. The sediment could then oxidize any organic carbon that flowed through it as a hydrocarbon-rich fluid, like oil. Oxidizing organic carbon would produce carbon dioxide that would then form carbonate. Again, since the organic carbon in water or oil has low amounts of carbon-13, this would result in similarly low levels in carbonate (inorganic carbon), the prime feature of the Shuram data.

So far, the evidence has seemed to point to both diagenesis and a global event, and previous ideas favored one or the other. "What we're trying to do is open the idea that maybe it's a combination of both," Grotzinger says. Fischer adds, "I'm sure it'll be controversial." The researchers are now starting to gather more data to see if their new explanation is correct. Bergmann and another graduate student, Maggie Osburn, are now collecting more samples in Oman.

How all this links to the Cambrian explosion remains unclear. All researchers know for sure is that the Shuram excursion represents an unprecedented signal in Earth's geological record, and it happened right before the sudden surge in biodiversity. The evidence is circumstantial, but it’s certainly intriguing. "It requires a remarkable mechanism regardless of how you cut it—it's not a business-as-usual scenario," Fischer notes. "I'm totally open to how this shakes out. I can see it both ways now."

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Caltech Researchers Use GPS Data to Model Effects of Tidal Loads on Earth's Surface

PASADENA, Calif.—For many people, Global Positioning System (GPS) satellite technology is little more than a high-tech version of a traditional paper map. Used in automobile navigation systems and smart phones, GPS helps folks find their way around a new neighborhood or locate a nearby restaurant. But GPS is doing much, much more for researchers at the California Institute of Technology (Caltech): it's helping them find their way to a more complete understanding of Earth's interior structure.

Up until now, the best way to explore Earth’s internal structures—to measure geological properties such as density and elasticity—has been through seismology and laboratory experiments. "At its most fundamental level, seismology is sensitive to specific combinations of these properties, which control the speed of seismic waves," says Mark Simons, professor of geophysics at Caltech's Seismological Laboratory, part of the Division of Geological and Planetary Sciences. "However, it is difficult using seismology alone to separate the effects that variations in density have from those associated with variations in elastic properties." 

Now Simons and Takeo Ito, visiting associate at the Seismological Laboratory and assistant professor of earth and planetary dynamics at Nagoya University in Japan, are using data from GPS satellite systems in an entirely new way: to measure the solid earth's response to the movements of ocean tides—which place a large stress on Earth's surface—and to estimate separately the effects of Earth’s density and the properties controlling response when a force is applied to it (known as elastic moduli).

Their work was published in this week’s issue of Science Express.

By using measurements of Earth’s movement taken from high-precision, continuously recording permanent GPS receivers installed across the western United States by the Plate Boundary Observatory (PBO), the researchers were able to observe tide-induced displacements—or movements of Earth's surface—of as little as one millimeter. PBO is a component of EarthScope, a program that seeks to understand the processes controlling earthquakes and volcanoes by exploring the structure and evolution of the North American continent.

The team focused on understanding the properties of the asthenosphere, a layer of weak and viscous upper mantle that lies below Earth's crust, and used those measurements to build one-dimensional models of Earth's response to the diurnal tides in the western United States.

"The asthenosphere plays an important role in plate tectonics, as it lies directly under the plates," explains Ito. "The results of our study give us a better understanding of the asthenosphere, which in turn can help us understand how the plates move."  

The models provided a look at the variations in density from Earth’s surface down to a depth of about 400 kilometers. The researchers found that the density of the asthenosphere under the western United States and the eastern Pacific Ocean is abnormally low relative to the global average.

"Variations in density can either be caused by variations in the chemical makeup of the material, the presence of melt, or due to the effects of thermal expansion, whereby a given material will decrease in density as its temperature increases," explains Simons. "In this study, we interpret the observed density anomaly to be due to the effects of elevated temperatures in the asthenosphere below the western United States and neighboring offshore areas. The required peak temperature anomaly would be about 300 degrees Celsius higher than the global average at those depths."

This type of data provides keys to understanding the chemical and mechanical dynamics of the planet, such as how heat flows through the mantle and how tectonic plates on Earth's surface are evolving.

"It is amazing that by measuring the twice-a-day centimeter-scale cyclic movement of Earth's surface with a GPS receiver, we can infer the variation of density 220 kilometers below the surface," says Simons.

Now that the researchers know it is possible to use GPS to derive measurements of internal Earth structures, they anticipate several new directions for this research.

"We hope to extend the observations to be global in scope, which may require temporary deployments of GPS in important areas that are typically tectonically bland—in other words, devoid of significant earthquakes and volcanoes—and thus do not have existing dense continuous GPS arrays already in place," says Simons. Next steps may also include going beyond the current one-dimensional depth-dependent models to build 3-D models, and combining the GPS approach with more conventional seismic approaches.

"The method we developed for gathering data from GPS devices has significant potential for improving 3-D images of Earth's internal structure," says Ito. 

The findings of the study, "Probing asthenospheric density, temperature and elastic moduli below the Western United States," are available online in Science Express and will be featured in a future issue of the journal Science.

This research was supported by the National Science Foundation; Grants-in-Aid for Scientific Research, part of Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT); JSPS Postdoctoral Fellowships for Research Abroad; and the Gordon and Betty Moore Foundation. 

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New Caltech Research Suggests Strong Indian Crust Thrust Beneath the Tibetan Plateau

PASADENA, Calif.—For many years, most scientists studying Tibet have thought that a very hot and very weak lower and middle crust underlies its plateau, flowing like a fluid. Now, a team of researchers at the California Institute of Technology (Caltech) is questioning this long-held belief and proposing that an entirely different mechanism is at play.

"The idea that Tibet is more or less floating on a layer of partially molten crust is accepted in the research community. Our research proposes the opposite view: that there is actually a really strong lower crust that originates in India," says Jean-Philippe Avouac, professor of geology and director of Caltech’s Tectonics Observatory. 

These insights lead to a better understanding of the processes that have shaped the Himalaya Mountains and Tibet—the most tectonically active continental area in the world.

Alex Copley, a former postdoctoral scholar with Caltech’s Tectonics Observatory, along with Avouac and Brian Wernicke, the Chandler Family Professor of Geology, describe their work in a paper published in the April 7 issue of the journal Nature.

Tibet and the surrounding Himalaya Mountains are among the most dynamic regions on the planet. Avouac points out that underground plate collisions, which cause earthquakes and drive up the Himalaya and Tibet, are common geological processes that have happened repeatedly over the course of Earth's history, but are presently happening with a vigor and energy only found in that area.

Even though the elevation is uniform across the Tibetan Plateau, the type of stress seen within the plateau appears to change along a line that stretches east-west across the plateau—dividing the region into two distinct areas (southern and northern Tibet, for the purposes of this research.)

The researchers propose that a contrast in tectonic style—primarily east-west extension due to normal faulting in southern Tibet and a combination of north-south compression and east-west extension due to strike-slip faulting in northern Tibet—is the result of the Indian crust thrusting strongly underneath the southern portion of the Tibetan Plateau and locking into the upper crust. Strike-slip fault surfaces are usually vertical, and the rocks slide horizontally past each other due to pressure build-up, whereas normal faulting occurs where the crust is being pulled apart. They believe that the locked Indian crust alters the state of stress in the southern Tibetan crust, which can explain the contrast in the type of faulting seen between southern Tibet and northern Tibet.

To test their theory, the team performed a series of numerical experiments, assigning different material properties to the Indian crust. The simulations revealed evidence for a strong Indian lower crust that couples, or locks in, with the upper crust. This suggests that the "channel flow" model proposed by many geophysicists and geologists—in which a low-viscosity magma oozes through weak zones in the middle crust—­is not correct.

"We have been able to create a model that addresses two long-standing debates," says Copley, who is now a research fellow at the University of Cambridge. "We have constrained the mechanical strength of the Indian crust as it plunges beneath the Tibetan Plateau, and by doing so have explained the variations in the types of earthquakes within the plateau. This is interesting because it gives us new insights into what controls the behavior of large mountain ranges, and the earthquakes that occur within them."

According to Wernicke, the results have motivated the team to think of ways to test further the "weak crust" hypothesis, at least as it might apply to the active tectonic system. "One way we might be able to image an extensive interface at depth is through geodetic studies of southern Tibet, which are ongoing in our research group," he says.

The Gordon and Betty Moore Foundation funded the research, described in the Nature paper, "Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet." Pembroke College in the University of Cambridge provided additional funding for Copley.

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Quake Expert Profiled in Los Angeles Magazine

Lucy Jones, a seismologist at the U.S. Geological Survey and visiting associate in geophysics at Caltech, gets personal in this month's Los Angeles magazine, recalling how she first became interested in earthquakes and what it's like to have two seismologists in the house—she's married to Egill Hauksson, senior research associate in Caltech's Seismological Laboratory. In the Q&A column titled "Shock Absorber," Jones also offers quake preparation advice to her fellow Californians: "However much water you've got, store some more. And get a couple of fire extinguishers and make sure you know how to use them." For the full story, click here

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Katie Neith
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Seismologist reflects on his firsthand experience of the Japanese earthquake

Eminent seismologist Hiroo Kanamori, Caltech's Smits Professor of Geophysics, Emeritus, has been studying the movement of the earth his entire career. On March 11 he was in Tokyo, experiencing firsthand the largest earthquake in the country's recorded history.

"It's hard to believe that it was a coincidence that I arrived there on the eve of  'a once in a thousand years' event," says Kanamori.

He says that when the University of Tokyo building he was in squeaked and began to shake—for a long time, with a gradual increase in intensity—he could tell that the earthquake was not centered nearby. Judging from the strength of the shaking, he estimated that the quake might be a magnitude 8 or larger.

Kanamori met up with fellow seismologists from the university when he evacuated the building. With no reliable source of information, they shared initial thoughts and assumptions about the event.

"Early reports said it was a 7.9, which seemed reasonable to me given what I know about the area," he says. "But when I heard that it was actually a 9, I was surprised."

Kanamori and others had previously theorized that where a plate is very young, it tends to produce big earthquakes, such as the event in Chile last year, which happened on a plate that is 10 million years old—a baby when it comes to tectonic plates.

"But where old plates are subducting, we don't normally have giant earthquakes," says Kanamori. "This explanation usually works. However, the plate off the coast of Tohoku where this earthquake occurred is old, about 130 million years. Thus, at first sight, this event was a big exception."

A deep knowledge of earthquakes is not the only expertise that informed his reactions to the event. In October of last year, Kanamori traveled to the coast near Sendai—an area close to the epicenter of the quake and hit hard by the tsunami—with Luis Rivera, a seismologist at the Institut de Physique du Globe de Strasbourg, with whom Kanamori has been working on improving tsunami early-warning systems for the last several years.

"We walked the beach for two days to look at the coastal topography and try to understand how bad it might be when a tsunami came," he recalls. "It was very obvious that it would be devastating—there are no prominent roads or trails that lead to the nearby highlands."

Kanamori emphasizes that the rapid dissemination of information after a big event is extremely important to warning systems for tsunamis and recovery efforts for earthquakes. He says that information needs to be collected on a global scale, since local systems often fail during a natural disaster. This requires good coordination between agencies in different countries.

"In the case of an emergency, they can exchange information and take immediate action," he says.

Kanamori is also quick to point out that the field of seismology is rapidly advancing its methods for data collection and analysis so that we can improve the way we deal with emergencies. But at the end of the day, we have to prepare for the unexpected.  

"In our science, it's very difficult to prove anything because you can't experiment. You have to rely on nature," he says. "Whatever happens in nature, you cannot predict 100 percent, so building robust infrastructures using rapid reliable information to prepare for the unexpected is very important."

For information on supporting the Japanese community at Caltech in their effort to help those in need, click here

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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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Simons Argues Against Hazard Prevention Cuts

Less than two weeks after a 9.0 earthquake and tsunami devastated a large swath of Japan, Caltech geophysicist Mark Simons, in today's Wall Street Journal, calls attention to federal budget proposals that would cut funding for prevention technologies. "Mitigating against future disasters depends on monitoring hazardous regions (earthquake faults, volcanoes, landslides and so on) and preparing to survive and recover once catastrophe strikes," Simons says in the opinion piece. By cutting funding for advanced early-warning and response technologies, our country's long-term security is jeopardized, he argues. For the full story, click here.

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Katie Neith
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Seismic Imaging Provides Bigger Picture for Earthquake Researchers

Caltech scientists and students are among a group of government and university researchers collecting seismic images of the Imperial and Coachella Valleys this week. The pictures—part of the U.S. Geological Survey's Salton Seismic Imaging Project (SSIP)—will help investigate the geometry of the San Andreas Fault, identify hidden faults, and provide more information about the composition of sediments in the area.

"By getting seismic images, we will be able to gain a better understanding of earthquake hazards for all of Southern California," says Caltech seismologist Joann Stock, a collaborating scientist on the project.

By knowing the geometry of the fault, as well as the thickness and shape of the sedimentary basins, she says, researchers can better predict how the ground will shake in future earthquakes.

SSIP uses small underground explosions and underwater bursts of compressed air to produce sound waves. The initial, transmitted sound waves reveal geologic structure by the way they bend or slow; sound waves reflected by rock layers help map the shapes and depths of those rocks and of the fault. These waves are recorded by more than 3,000 seismographs laid out in seven lines crisscrossing the valleys, then analyzed by powerful computers to produce detailed images of the Earth's crust.

There's no reason to worry that generating these kinds of seismic waves will trigger an earthquake, the scientists say. The explosive charges are small and set off 60 feet below the ground in a cased hole identical to a water well. Similar surveys in the past have shown the process to be safe and effective.

"The blasts are the size of quarry blasts, which happen in this region all the time, and they do not cause earthquakes," says Stock. "In fact, while the seismic network that records earthquakes is so sensitive that it often registers quarry blasts, so far, our blasts haven't even shown up on the network."

SSIP is part of the National Earthquake Hazards Reduction Program's ongoing efforts to protect lives and property in the event of an earthquake in Southern California and across the country. Funding for the project is provided by the National Science Foundation and the USGS.  

For more information on the project, visit the SSIP website.

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"Like It or Not, We Are Living on This Planet"

The number of large destructive earthquakes in 2010, plus a flurry of medium magnitude quakes in California, led many people to ask, Are we in a period of heightened temblor activity, and is it likely to continue? It's also raised questions among both scientists and laypeople about whether these events are related—and if so, how. The eruption of an Icelandic volcano, which disrupted air traffic in Europe for weeks, serves as an additional reminder that we live on a volatile planet. What, if anything, does this apparent uptick in geological activity portend, and how does it compare to events in Earth's past history? E&S sat down with Hiroo Kanamori, the Smits Professor of Geophysics, Emeritus, and Joe Kirschvink, the Van Wingen Professor of Geobiology, to hear their thoughts.

Between February and September 2010, earthquakes ranging from magnitude 6.8 to 8.8 occurred in regions as far-flung as Sumatra, China, Chile, New Zealand, and Baja, California. Are we in fact seeing more large quakes than usual?

Hiroo Kanamori: There are a couple of ways to answer this question. If you look at very major earthquakes, we are not seeing as much activity as between 1950 and 1965, when there were three events of magnitude 9 or greater in which an enormous amount of energy was released.

However, if we total up the number of quakes over magnitude 8 that have occurred since the first great Sumatran quake of 2004, we do find that these numbers really have increased. On average about one quake per year is magnitude 8 or larger. Since 2004, on average we have had two quakes of that size or more annually.

Is this statistically significant?

HK: We don't really know! Thanks to a study that's been going on for about the last 18 years, we do know a great deal more than we used to about triggering events in earthquakes. We now know that every large earthquake sends out seismic waves that can travel some distance and potentially activate seismic activity elsewhere. 

How well do scientists understand the physical mechanisms that might touch off a quake cascade like this?

HK: We have several different models and theories. The most straightforward mechanism would be one in which the seismic waves increase stress on other faults that they're passing through. If those faults are already close to rupture, this seismic impact may be enough to push things over the edge.

There are also cases in which this activity is delayed. This appears to be what happened this summer when the magnitude 7.2 quake that had occurred in Baja California in April touched off two moderate quakes in June and July on the San Jacinto fault in Southern California.

Joe, you've made in-depth studies of ancient geological upheavals. Can you put these recent events in perspective for us?

JK: Just to take volcanoes, the Icelandic eruption that we saw this spring was tiny compared to eruptions that have happened previously in Earth's history. In California alone, about three-quarters of a million years ago—which geologically is nothing—the Long Valley Caldera, between Mono Lake and Mammoth, blew its top. The eruption covered the southwestern United States with a blanket of ash that extended all the way to the Mississippi. The sediments that washed off from the Mississippi delta produced deposits that in some places were hundreds of meters thick. That episode was far, far worse than anything in human memory. There was a similar eruption about two million years ago in what is today Yellowstone. 

One question we often hear from both the public and the media is, will we ever be able to predict earthquakes the way we can—more or less—forecast the weather? What are your views?

HK: There are such fundamental differences between weather forecasting and earthquake prediction. With weather, the situation basically changes on an almost daily basis. With quakes, we are dealing with long-term processes in which the timescale for stress buildup and release is very long—100 to 1,000 years or more—while the length of time in which quakes occur is very short.

As I said earlier, we have made major advances in our understanding of how these seismic processes operate over these lengthy timescales. But to be able to say there's a strong likelihood that a magnitude 8 earthquake will occur in some specific area within the next hundred years or so is not necessarily very useful for the average layperson. You simply can't handle it like a weather forecast. If the forecast says, "rain tomorrow," you may take your umbrella, and either it rains or it doesn't. However, in the case of earthquakes, if you say that something big is going to happen tomorrow and nothing happens, that can be a problem. And that's really a key difference between climatology and seismology.

JK: I agree with Hiroo. If you want to see how completely distinct the two areas are, just turn the analogy around. Certainly, meteorology averaged over a very long period of time gives you climate. Or, to put it another way, climate is just long-term weather. But I certainly wouldn't advocate analyzing ancient climates to determine whether you'll have a thunderstorm next Tuesday.

To read the full interview, go to Engineering and Science online.

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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.
 

Writer: 
Marcus Woo
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