Caltech Geologists Discover Ancient Buried Canyon in South Tibet


"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. 

Kimm Fesenmaier
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Photosynthesis: A Planetary Revolution

Watson Lecture Preview




Q: What do you do?







A: There are two really good reasons.





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




Douglas Smith
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Watson Lecture: "Photosynthesis: A Planetary Revolution"
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Robotic Ocean Gliders Aid Study of Melting Polar Ice





Ordinarily when oceanographers like Thompson want to investigate such questions, they use ships to lower instruments through the water or they collect ocean temperature data from above with satellites. These techniques are problematic in the Southern Ocean. "Observationally, it's a very hard place to get to with ships. Also, the warm water is not at the surface, making satellite observations ineffective," he says.




The glider's up and down capability is important for studying ocean stratification, or how water characteristics, such as density, change with depth, Thompson says. "If it was only temperature that determined density, you'd always have warm water at the top and cold water at the bottom. But in the ocean you also have to factor in salinity; the higher the salinity is in the water, the more dense that water is and the more likely it is to sink to the bottom," he says.

In Antarctica the combined effects of temperature and salinity create an interesting situation, in which the warmest water is not on top, but actually sandwiched in the middle layers of the water column. "That's an additional problem in understanding the heat transport in this region," he adds. You can't just take measurements at the surface, he says. "You actually need to be taking a look at that very warm temperature layer, which happens to sit in the middle of the water column. That's the layer that is actually moving toward the ice shelf."


"Eddies are instabilities that are caused by ocean currents, and we often compare their effect on the ocean to putting a spoon in your coffee," Thompson says. "If you pour milk in your coffee and then you stir it with a spoon, the spoon enhances your ability to mix the milk into the coffee and that is what these eddies do. They are very good at mixing heat and other properties."


"Ocean currents are variable, and so if you go just one time, what you measure might not be what the current looks like a day later. It's sort of like the weather—you know it's going to be warm in the summer and cold in the winter, but on a day-to-day basis it could be cold in the summer just because a storm came in," Thompson says. "Eddies do the same thing in the ocean, so unless you understand how the temperature of currents is changing from day to day—information we can actually collect with the gliders—then you can't understand what the long-term heat transport is."

In future work, Thompson plans to couple meteorological data with the data collected from his gliders. In December, the team will use ocean gliders to study a rough patch of ocean between the southern tip of South America and Antarctica, called the Drake Passage, as a surface robot, called a Waveglider, collects information from the surface of the water. "With the Waveglider, we can measure not just the ocean properties, but atmospheric properties as well, such as wind speed and wind direction. So we'll get to actually see what's happening at the air-sea interface."

In the Drake Passage, deep waters from the Southern Ocean are "ventilated"—or emerge at the surface—a phenomenon specific to this region of the ocean. That makes the location important for understanding the exchange of carbon dioxide between the atmosphere and the ocean. "The Southern Ocean is the window through which deep waters can actually come up to 'see' the atmosphere"—and it's also a window for oceanographers to more easily see the deep ocean, he says. "It's a very special place for many reasons."

The work with ocean gliders was published in a paper titled "Eddy transport as a key component of the Antarctic overturning circulation." Other authors on the paper include Karen J. Heywood of the University of East Anglia, Sunke Schmidtko of GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, and Andrew Stewart, a former postdoctoral scholar at Caltech who is now at UCLA. Thompson's glider work was supported by an award from the National Science Foundation and the UK's Natural Environment Research Council; Stewart was supported by the President's and Director's Fund program at Caltech.

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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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Wednesday, October 29, 2014

Meet the Outreach Guys: James & Julius

Oceanographer Andrew Thompson Wins Prestigious Fellowship

Caltech oceanographer Andrew Thompson, who uses autonomous underwater instruments and numerical models to study ocean currents and eddies and their impact on Earth's ecology and climate, has been awarded a Packard Fellowship for Science and Engineering. Packard Fellowships are awarded annually by the David and Lucile Packard Foundation to the nation's "most innovative early-career scientists and engineers" to provide them with "flexible funding and the freedom to take risks and explore new frontiers in their fields," according to the foundation.

Along with this year's 17 other fellows, Thompson, an assistant professor of environmental science and engineering, will receive a grant of $875,000 distributed over five years, to pursue his research.

As is the case for other Packard Fellows, Thompson was surprised by his selection. He recalls being called on a recent Tuesday morning into a meeting with Ken Farley, W. M. Keck Foundation Professor of Geochemistry. Farley had chaired Thompson's division, Geological and Planetary Sciences, up until September 1, when Fletcher Jones Professor of Geology John Grotzinger took over.

"Ken asked to meet with me in the division chair's office. This was already a little odd, because John had already taken over, but I did not think too much of it at the time," Thompson says. "Five minutes into our conversation, the phone rang, and when it was for me, I knew that something was up. Ken had nominated me in the spring so he was the one who delivered the news. I was thrilled. I had to go for a walk immediately after to calm down. It was really an honor to represent Caltech as a nominee."

"This was a great way to complete my term as chair—to have played a part in successfully nominating Andy for this prestigious and valuable award, and to get the chance to see his surprise and happiness when the foundation told him," Farley says.

Although he's only just heard the news, Thompson already has plans for the grant. "Part of the funds will be used to support our work with autonomous ocean instruments—gliders—that allow us to observe remote or dynamic parts of the ocean over long periods of time," he says. "These tools will be used to explore the coupling between ocean circulation, ecosystem dynamics, and biogeochemical cycling in the upper ocean, processes that are difficult to observe using traditional ship-based techniques."

Thompson joins 23 other current Caltech faculty who have been named Packard Fellows since the program's inception in 1988. To date, the Packard Foundation, a private family foundation created in 1964 Hewlett-Packard Company cofounder David Packard and his wife, Lucile Packard, has awarded $346 million to support 523 scientists from 52 national universities. Each year, participating universities are invited to nominate two faculty members for consideration by the 12-member Fellowship Advisory Panel of internationally recognized scientists and engineers, which recommends nominees for approval by the Packard Foundation Board of Trustees.

Kathy Svitil
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Oceanographer Awarded Prestigious Packard Fellowship
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Wednesday, October 29, 2014
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Getting To Know Super-Earths

"If you have a coin and flip it just once, what does that tell you about the odds of heads versus tails?" asks Heather Knutson, assistant professor of planetary science at Caltech. "It tells you almost nothing. It's the same with planetary systems," she says.


That is beginning to change. NASA's Kepler spacecraft, which launched on a planet-hunting mission in 2009, searched one small patch of the sky and identified more than 4,000 candidate exoplanets—worlds orbiting stars other than our own sun. It was the first survey to provide a definitive look at the relative frequency of planets as a function of size. That is, to ask, 'How common are gas giant planets, like Jupiter, compared to planets that look a lot more like Earth?'



"We are left with this situation where super-Earths appear to be the most common kind of exoplanet in the galaxy, but we don't know what they're made of," says Knutson.



In addition to thinking about exoplanets, Knutson and her students use space-based observatories like the Hubble and Spitzer Space Telescopes to learn more about the distant worlds. For example, the researchers analyze the starlight that filters through a planet's atmosphere as it passes in front of its star to learn about the composition of the atmosphere. Molecular species present in the planet's atmosphere absorb light at particular wavelengths. Therefore, by using Hubble and Spitzer to view the planet and its atmosphere at a number of different wavelengths, the researchers can determine which chemical compounds are present.



The first super-Earth that the astronomical community targeted for atmospheric studies was GJ 1214b, in the constellation Ophiuchus. Based on its average density (determined from its mass and radius), it was clear from the start that the planet was not entirely rocky. However, its density could be equally well matched by either a primarily water composition or a Neptune-like composition with a rocky core surrounded by a thick gas envelope. Information about the atmosphere could help astronomers determine which one it was: a mini-Neptune's atmosphere should contain lots of molecular hydrogen, while a water world's atmosphere should be water dominated.




However, again the data came back featureless. One explanation is that HD 97658b is also enveloped in clouds. However, Knutson says, it is also possible that the planet has an atmosphere that is lacking hydrogen. Because such an atmosphere could be very compact, it would make the telltale fingerprints of water vapor and other molecules very small and hard to detect. "Our data are not precise enough to tell whether it's clouds or the absence of hydrogen in the atmosphere that's causing the spectrum to be flat," she says. "This was just a quick first look to give us a rough idea of what the atmosphere looked like. Over the next year, we will use Hubble to observe this planet again in more detail. We hope those observations will provide a clear answer to the current mystery."

It appears that clouds are going to continue to pose a real challenge in studies of super-Earths, so Knutson and other researchers are working to understand the composition of the clouds around these planets and the conditions under which they form. The hope is that they will get to the point where they can predict which worlds will be shrouded in clouds. "If we can then target planets that we think should be cloud-free, that will help us make optimal use of Hubble's time," she says.

Looking to the future, Knutson says there is only one more known super-Earth that can be targeted for atmospheric studies with current telescopes. But new surveys, such as NASA's extended Kepler K2 mission and the Transiting Exoplanet Survey Satellite (TESS), slated for launch in 2017, should identify a large sample of new targets.

Of course, she says, astronomers would love to study exoplanets the size of Earth, but these worlds are just a bit too small and too difficult to observe with Hubble and Spitzer. NASA's James Webb Space Telescope, which is scheduled for launch in 2018, will provide the first opportunity to study more Earth-like worlds. "Super-Earths are at the edge of what we can study right now," Knutson says. "But super-Earths are a good consolation prize—they're interesting in their own right, and they give us a chance to explore new kinds of worlds with no analog in our own solar system."

Kimm Fesenmaier
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Rock-Dwelling Microbes Remove Methane from Deep Sea
















Written by Ker Than

Ker Than
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