Heavy nitrogen molecules reveal planetary-scale tug-of-war

Rice, UCLA, Michigan State, UNM find unusual enrichment in 15N15N molecules

Nature whispers its stories in a faint molecular language, and Rice University scientist Laurence Yeung and colleagues can finally tell one of those stories this week, thanks to a one-of-a-kind instrument that allowed them to hear what the atmosphere is saying with rare nitrogen molecules.

Yeung and colleagues at Rice, UCLA, Michigan State University and the University of New Mexico counted rare molecules in the atmosphere that contain only heavy isotopes of nitrogen and discovered a planetary-scale tug-of-war between life, the deep Earth and the upper atmosphere that is expressed in atmospheric nitrogen.

The research was published online this week in the journal Science Advances.

“We didn’t believe it at first,” said Yeung, the lead author of the study and an assistant professor of Earth, environmental and planetary sciences at Rice. “We spent about a year just convincing ourselves that the measurements were accurate.”

The story revolves around nitrogen, a key element of life that makes up more than three-quarters of Earth’s atmosphere. Compared with other key elements of life like oxygen, hydrogen and carbon, nitrogen is very stable. Two atoms of it form N2 molecules that are estimated to hang around in the atmosphere for about 10 million years before being broken apart and reformed. And the vast majority of nitrogen has an atomic mass of 14. Only about 0.4 percent are nitrogen-15, an isotope that contains one extra neutron. Because nitrogen-15 is already rare, N2 molecules that contain two nitrogen-15s — which chemists refer to as 15N15N — are the rarest of all N2 molecules.

The new study shows that 15N15N is 20 times more enriched in Earth’s atmosphere than can be accounted for by processes happening near Earth’s surface.

Rice Assistant Professor Laurence Yeung. Photo by Jeff Fitlow


“We think the 15N15N enrichment fundamentally comes from chemistry in the upper atmosphere, at altitudes close to the orbit of the International Space Station,” Yeung said. “The tug-of-war comes from life pulling in the other direction, and we can see chemical evidence of that.”

Co-author Edward Young, professor of Earth, planetary and space sciences at UCLA, said, “The enrichment of 15N15N in Earth’s atmosphere reflects a balance between the nitrogen chemistry that occurs in the atmosphere, at the surface due to life and within the planet itself. It’s a signature unique to Earth, but it also gives us a clue about what signatures of other planets might look like, especially if they are capable of supporting life as we know it.”

The chemical processes that produce molecules like N2 can change the odds that “isotope clumps” like 15N15N will be formed. In previous work, Yeung, Young and colleagues used isotope clumps in oxygen to identify tell-tale signatures of photosynthesis in plants and ozone chemistry in the atmosphere. The nitrogen study began four years ago when Yeung, then a postdoctoral researcher at UCLA, learned about a first-of-its-kind mass spectrometer that was being installed in Young’s lab.

The amount of nitrogen molecules in Earth’s atmosphere that contain only heavy isotopes result from a balance between nitrogen chemistry that occurs in the atmosphere, at the surface due to life and within the planet itself. (Photo courtesy of ISS Expedition 7 Crew, EOL, NASA)

“At that time, no one had a way to reliably quantify 15N15N,” said Yeung, who joined Rice’s faculty in 2015. “It has an atomic mass of 30, the same as nitric oxide. The signal from nitric oxide usually overwhelms the signal from 15N15N in mass spectrometers.”

The difference in mass between nitric oxide and 15N15N is about two one-thousandths the mass of a neutron. When Yeung learned that the new machine in Young’s lab could discern this slight difference, he applied for grant funding from the National Science Foundation (NSF) to explore exactly how much 15N15N was in Earth’s atmosphere.

“Biological processes are hundreds to a thousand times faster at cycling nitrogen through the atmosphere than are geologic processes,” Yeung said. “If it’s all business as usual, one would expect that the atmosphere would reflect these biological cycles.”

To find out if this was the case, co-authors Joshua Haslun and Nathaniel Ostrom at Michigan State University conducted experiments on N2-consuming and N2-producing bacteria to determine their 15N15N signatures.

These experiments suggested that one should see a bit more 15N15N in air than random pairings of nitrogen-14 and nitrogen-15 would produce — an enrichment of about 1 part per 1,000, Yeung said.

Researchers from Rice University and UCLA simulated high-energy chemistry in the upper atmosphere to reproduce enriched levels of 15N15N, molecules that contain only heavy isotopes of nitrogen. (Photo by Laurence Yeung/Rice University)

“There was a bit of enrichment in the biological experiments, but not nearly enough to account for what we’d found in the atmosphere,” Yeung said. “In fact, it meant that the process causing the atmospheric 15N15N enrichment has to fight against this biological signature. They are locked in a tug-of-war.”

The team eventually found that zapping mixtures of air with electricity, which simulates the chemistry of the upper atmosphere, could produce enriched levels of 15N15N like they measured in air samples. Mixtures of pure nitrogen gas produced very little enrichment, but mixtures approximating the mix of gases in Earth’s atmosphere could produce a signal even higher than what was observed in air.

“So far we’ve tested natural air samples from ground level and from altitudes of 32 kilometers, as well as dissolved air from shallow ocean water samples,” he said. “We’ve found the same enrichment in all of them. We can see the tug-of-war everywhere.”

Co-authors include Huanting Hu of Rice, Shuning Li, formerly of Rice and UCLA and now with Peking University in Beijing, Issaku Kohl and Edwin Schauble of UCLA and Tobias Fischer of the University of New Mexico.

The research was supported by the NSF, the Deep Carbon Observatory and the Department of Energy’s Great Lakes Bioenergy Research Center.

Reefs near Texas endured punctuated bursts of sea-level rise before drowning

Fossil coral reefs show sea level rose in bursts during last warming


Reefs near Texas endured punctuated bursts of sea-level rise before drowning

Scientists from Rice University and Texas A&M University-Corpus Christi’s Harte Research Institute for Gulf of Mexico Studies have discovered that Earth’s sea level did not rise steadily but rather in sharp, punctuated bursts when the planet’s glaciers melted during the period of global warming at the close of the last ice age. The researchers found fossil evidence in drowned reefs offshore Texas that showed sea level rose in several bursts ranging in length from a few decades to one century.

The findings appear today in Nature Communications.

“What these fossil reefs show is that the last time Earth warmed like it is today, sea level did not rise steadily,” said Rice marine geologist André Droxler, a study co-author. “Instead, sea level rose quite fast, paused, and then shot up again in another burst and so on.

“This has profound implications for the future study of sea-level rise,” he said.

Rice University researchers (from left to right) Pankaj Khanna, André Droxler and Jeffrey Nittrouer. (Photo by Jeff Fitlow/Rice University)

Because scientists did not previously have specific evidence of punctuated decade-scale sea-level rise, they had little choice but to present the risks of sea-level rise in a linear, per-year format, Droxler said. For example, the International Panel on Climate Change, the authoritative scientific source about the impacts of human-induced climate change, “had to simply take the projected rise for a century, divide by 100 and say, ‘We expect sea level to rise this much per year,’” he said.

“Our results offer evidence that sea level may not rise in an orderly, linear fashion,” said Rice coastal geologist and study co-author Jeff Nittrouer.

Given that more than half a billion people live within a few meters of modern sea level, he said punctuated sea-level rise poses a particular risk to those communities that are not prepared for future inundation.

André Droxler (seated) and Pankaj Khanna aboard the research vessel Falkor in 2012. (Photo by Mark Schrope/Schmidt Ocean Institute)

“We have observed sea level rise steadily in contemporary time,” Nittrouer said. “However, our findings show that sea-level rise could be considerably faster than anything yet observed, and because of this situation, coastal communities need to be prepared for potential inundation.”

The study’s evidence came from a 2012 cruise by the Schmidt Ocean Institute‘s research vessel Falkor. During the cruise, Droxler, study lead author and Rice graduate student Pankaj Khanna and Harte Research Institute colleagues John Tunnell Jr. and Thomas Shirley used the Falkor’s multibeam echo sounder to map 10 fossil reef sites offshore Texas. The echo sounder is a state-of-the-art sonar that produces high-resolution 3-D images of the seafloor.

A high-resolution 3-D map of Southern Bank off the South Texas coast clearly reveals terraces, which are a characteristic coral reef response to rising sea level. (Image courtesy of P. Khanna/Rice University)

The fossil reefs lie 30-50 miles offshore Corpus Christi beneath about 195 feet of water. Sunlight does not reach them at that depth, but because corals live in symbiosis with algae, they need sunlight to live and only grow at or very near sea level. Based on previous studies of the Texas coastline during the last ice age as well as the dates of fossils samples collected from the reefs in previous expeditions, the Rice team surmised that the reefs began forming about 19,000 years ago when melting ice caps and glaciers were causing sea level to rise across the globe.

“The coral reefs’ evolution and demise have been preserved,” Khanna said. “Their history is written in their morphology — the shapes and forms in which they grew. And the high-resolution 3-D imaging system on the R/V Falkor allowed us to observe those forms in extraordinary detail for the first time.”

All the sites in the study had reefs with terraces. Khanna said the stair-like terraces are typical of coral reef structures and are signatures of rising seas. For example, as a reef is growing at the ocean’s surface, it can build up only so fast. If sea level rises too fast, it will drown the reef in place, but if the rate is slightly slower, the reef can adopt a strategy called backstepping. When a reef backsteps, the ocean-facing side of the reef breaks up incoming waves just enough to allow the reef to build up a vertical step.

A 3-D representation of Dream Bank, a long-dead reef offshore South Texas. The vertical scale of the image has been increased to clearly illustrate the terrace structures that form due to rising sea levels via a process known as backstepping. (Image courtesy of P. Khanna/Rice University)

“In our case, each of these steps reveals how the reef adapted to a sudden, punctuated burst of sea-level rise,” Khanna said. “The terraces behind each step are the parts of the reef that grew and filled in during the pauses between bursts.”

Some sites had as many as six terraces. The researchers said it’s important to note that even though the sites in the study are as much as 75 miles apart, the depth of the terraces lined up at each site. Droxler and Nittrouer credited the find to Khanna’s determination. Analysis of the data from the mapping mission took more than a year, and the time needed to respond to questions that arose during the publication’s peer-review process was even longer.

“That’s the way science works,” Droxler said. “This is the first evidence ever offered for sea-level rise on a time scale ranging from decades to one century, and our colleagues expected ironclad evidence to back that claim.”

Rice researchers (from left) Caleb McBride, André Droxler and Pankaj Khanna aboard the research vessel Falkor in 2012. (Photo courtesy of A. Droxler/Rice University)

Nittrouer said the scenario of punctuated sea-level rise is one that many scientists had previously suspected.

“Scientists have talked about the possibility that continental ice could recede rapidly,” he said. “The idea is that sudden changes could arise when threshold conditions are met — for example, a tipping point arises whereby a large amount of ice is released suddenly into global oceans. When melted, this adds water volume and raises global sea level.”

Khanna said it’s likely that additional fossil evidence of punctuated sea-level rise will be found in the rock record at sites around the globe.

“Based on what we’ve found, it is possible that sea-level rise over decadal time scales will be a key storyline in future climate predictions,” he said.

The research was supported by Rice University, the Harte Research Institute at Texas A&M University-Corpus Christi and the Schmidt Ocean Institute.


Rice’s Clint Miller ready to study first cores from Gulf of Corinth rift

Rice scientist ready to study first cores from active continental rift

Rice’s Clint Miller ready to study first cores from Gulf of Corinth rift

By Linda Welzenbach
Special to Rice News

Rice geochemist Clint Miller is part of an international team of scientists that is collecting the first sediment samples ever drilled from Greece’s Gulf of Corinth, an active continental rift where Earth’s crust expands and thins.

“The Gulf is somewhat unique because it is an active rift that has oscillated over the last few million years between lake-like conditions and true marine conditions,” said Miller, a postdoctoral research associate in the Department of Earth, Environmental and Planetary Sciences. “Hopefully, we will be able to see this really clearly in the chemistry of the sediments we collect.”

Clint Miller

Miller said scientists are especially interested in such continental rift zones because they are geologically active and often at risk for earthquakes and volcanic eruptions. “They also present a unique opportunity to observe plate tectonics in action,” he said.

Miller and colleagues sailed on the British Geologic Survey drilling vessel Fugro Synergyto collect samples from the Corinth rift in the Mediterranean Sea. The expedition is sponsored by the the International Ocean Discovery Program (IODP) and European Consortium for Ocean Research Drilling.

Gulf of Corinth

Having formed its modern expression over the past 5 million years, the Corinth rift is young for an active continental rift zone. It’s situated across a shallow marine basin about 30 miles west of Athens and has a closed drainage system that Miller said makes it ideal for studying early rift development and the way land forms when it is subject to competing forces from tectonics and climate.Miller and colleagues sailed on the British Geologic Survey drilling vessel Fugro Synergyto collect samples from the Corinth rift in the Mediterranean Sea. The expedition is sponsored by the the International Ocean Discovery Program (IODP) and European Consortium for Ocean Research Drilling.

The rift is spreading at a rate of about 10-15 millimeters per year, and rivers that drain into the Gulf carry sediments that partially fill the rift zone as it spreads. Miller said IOPD Expedition 381 differs from most scientific drilling missions because it aims to drill only three holes and to do so in a way that maximizes what scientists can learn.

He hopes drilling in carefully selected locations will enable them to collect sediment samples from across tens of thousands of years.

“From a paleoceanographic perspective, that is the best scenario for capturing reliable climate cycle information from seafloor sediments,” he said.

One of Miller’s tasks on ship will be to analyze the chemistry of water that is trapped in the pores of sediments that have been buried in the rift for up to 2 million years. Because the water was trapped inside the sediments as they were buried, it can help tell the story of the Gulf’s past.

“Pore water and ocean sediment tell how the climate and biosphere have changed over time,” he said. “From the types of organic carbon and related compounds that are present, we can get the rates of photosynthesis as well as the sediment and water chemistry of the past. That will tell us how life responded to the rift and to changing sea level.”After the cruise, Miller will conduct more in-depth experiments at IODP’s core storage facility in Bremen, Germany.

Ultimately, Miller hopes to help address the hypothesis that environmental fluctuations preserved in the Corinth rift are related to 100,000-year climate cycles.

“We would like to know how the rift environment affects the deposition of carbon during glacial and interglacial time periods,” he said. “This will help us better understand how climate affects Earth’s biosphere, which is increasingly important due to human-induced climate change.”

To track the expedition’s progress or learn more, visit the IODP Expedition 381 blog.

–Linda Welzenbach is a science writer in the Department of Earth, Environmental and Planetary Sciences at Rice.

Rice’s Laurence Yeung named 2017 Packard Fellow

Rice’s Laurence Yeung named 2017 Packard Fellow

Geochemist wins prestigious grant for innovative research into ‘clumped’ isotopes

HOUSTON — (Oct. 16, 2017) — Rice University’s Laurence Yeung has made a career of searching for some of Earth’s rarest molecules and the stories they tell about the planet’s past, present and future. To aid his search, the David and Lucile Packard Foundation has awarded Yeung a 2017 Packard Fellowship for Science and Engineering.

Packard Fellowships, which include a largely unrestricted five-year research grant of $875,000, are among the most prestigious early career awards for U.S. scientists and engineers. Only 18 are awarded each year.

“My first response was excitement: ‘Oh my God. I cannot believe this is happening,’ and then I looked at everyone who’d come before and won one of these, and I thought, ‘Really? How can I be one of these people?’” said Yeung, assistant professor of Earth, environmental and planetary sciences. “The way I think about it is, ‘Congratulations. Now, earn it.’”

Laurence Yeung

Yeung joined Rice in 2015. His research is a mix of physical chemistry, photochemical experimentation, quantum-mechanical theory, atmospheric modeling and more, all aimed at understanding how the atmosphere broadcasts the state of the Earth system in its chemical composition.

“People sometimes call the atmosphere the ‘great communicator’ because it’s the first to respond to any perturbation in Earth systems,” Yeung said. “Altering anything from volcanic emissions to biological productivity can result in changes in the atmosphere’s makeup and its chemistry.”

He said there are many processes scientists still don’t understand about the atmosphere’s basic workings. For example, they cannot confidently predict how much the atmosphere will warm or cool if it has different amounts of greenhouse gases. They also don’t know how the composition of the atmosphere changes as the biosphere responds to global change.

“Understanding these processes and others could help us better understand climate dynamics, the evolution of Earth’s surface and even how to search for life on other planets,” Yeung said.

His core approach typically revolves around counting exact numbers of extraordinarily rare molecules that contain two or more rare isotopes, atoms of the same element that differ only in their mass. Fundamental processes — like photosynthesis in plants or ozone chemistry in the atmosphere — change the odds that “clumped” isotopes will be created. For each process, the odds change in a characteristic way, which means clumped isotopes can serve as calling cards for specific processes.

However, sifting through millions of molecules to find a handful of these ultrarare clumped isotopes is not uncommon. “The measurements we do are incredibly hard,” Yeung said. “It takes big, heavy instruments to measure a small number of samples extremely carefully.”

He said the Packard Fellowship will allow him to take research risks he couldn’t otherwise afford. Some of these risks involve finding new archives for ancient atmospheric properties, while others entail discovering new ways to detect the imprints of life in a variety of environments.

“Basically, every one of these ideas is risky,” he said. “In some cases, you’re not sure about the math or the model that you’ve put together. There could also be questions about your samples. And even if you get all of that right, your instruments might not be good enough to resolve the detail that’s necessary.”

One project he’s considering is attempting to create a compact device that makes isotopic measurements far more accessible.

“It’s unlikely that this device will be able to make measurements with the same precision that we do in the lab today, but if the precision were good enough, and we could go places — a boat, a mountaintop, a drone — and collect tenfold or hundredfold more data, it would be transformative.

“At the end of five years, I’d like to see that some of the risks paid off and some didn’t,” he said. “If they all pay off, then you’re not taking enough chances and pushing to stay right there at the edge of what’s possible.”

Other Rice faculty who have been named Packard Fellows include Cin-Ty Lee and Rajdeep Dasgupta, also of the Department of Earth, Environmental and Planetary Sciences, and Doug Natelson and Tom Killian, both of the Department of Physics and Astronomy.

Since the Packard Fellows program was begun in 1988, the Packard Foundation has awarded $394 million to support 577 scientists and engineers from 54 top universities. Packard Fellows have gone on to receive awards and honors that include the Nobel Prize in Physics, the Fields Medal, the Alan T. Waterman Award, MacArthur Fellowships and election to the National Academies.


A high-resolution IMAGE is available for download at:

CAPTION: Laurence Yeung (Photo by Jeff Fitlow/Rice University)

Related stories from Rice:

Earth scientist Laurence Yeung wins Clarke Award — Feb. 29, 2016

Oxygen atmosphere recipe = tectonics + continents + life — May 16, 2016

Study: Photosynthesis has unique isotopic signature — April 23, 2015

Xiaodong Gao aiming for the six World Marathon Majors

Research scientist aiming for the six World Marathon Majors

When research scientist Xiaodong Gao crossed the finish line at the Berlin Marathon Sept. 24, he reached the halfway mark to his goal of running six of the world’s largest and most-renowned marathons.

Having completed the Chicago Marathon in 2016 and the Boston Marathon this past April, the 40-year-old native of China’s Jiangsu province still needs to run the New York City, Tokyo and London marathons to earn the highly coveted medal awarded to finishers of the six World Marathon Majors.

Xiaodong Gao holds his finisher’s medal from the Berlin Marathon while displaying medals from Houston, Chicago and Boston marathons and the Texas Independence Relay.

That goal wasn’t even remotely on Gao’s radar four years ago when he started running. In October 2013 Gao went for a health checkup and discovered that his cholesterol was “off the charts.”

“I needed to do something to change that,” said Gao, whose weight of 220 pounds was due more to his girth than his height of 6 feet. He began running in his Clear Lake neighborhood early in the morning before going to his lab at Rice, where he studies the chemical signals of biochar.

A year later he was running 50 to 60 miles a week to train for his first 26.2-mile race – the 2015 Houston Marathon, which he finished in 3:25:01. He shaved more than 15 minutes off that time when he ran Houston again in 2016 in 3:09:55 – a time fast enough to qualify for the Boston Marathon.

“Once I qualified for Boston, I thought, ‘Why not try to do all six world marathon majors?’” Gao said. “It would give me the chance to experience different cultures, traditions and food, and you get that nice medal after you finish all six.”

Gao ran Boston in 3:27 and fondly remembers the cheering crowds that lined the race route “like crazy.” He was unable to improve on that time in the Berlin Marathon, which he ran in 3:35, due to rain, heat and 100 percent humidity the day of the race and Houston’s summer temperatures, which are not ideal for marathon training. “But the really excited crowds at the finish line made me very happy,” Gao said. He celebrated by drinking “a lot of beer” in Germany, and he and his wife turned the trip into a mini vacation by visiting several museums in Berlin.

As Gao was upping his mileage to train for marathons, he was having difficulty finding people to run with. So he used social media to co-found the Houston Long Running Club, which now has more than 400 members throughout the Houston area. “Training with a friend makes it easier and more fun,” he said. “We offer our experience to help other people who are learning to run, and we celebrate after a race by drinking and eating together.” One of his favorite post-race foods is mala Sichuan, a spicy and hot Chinese cuisine.

In addition to running marathons, members of the Houston Long Running Club have formed teams to run the Texas Independence Relay, which starts in Gonzales, where the Texas Revolution began, and finishes more than 200 miles away at the San Jacinto Monument, where Texas won its independence, near the Houston Ship Channel. Gao has competed in the two-day relay twice and will be captain of his 12-member team in the 2018 relay.

Gao hopes that 2018 will also be the year his application gets selected in the random drawings for entry in the New York City Marathon. And he plans to run the Houston Marathon again in January with hopes of setting a personal record. “I’d like to break three hours,” he said. “If everything goes right, it will be like winning the lottery.”

Due to the expense and time commitment to travel to London and Tokyo, Gao doesn’t expect to run marathons there until 2019, provided that his name gets picked in the lottery for each race.

Meanwhile, he expects that his total running mileage, which he records daily, will pass 10,000 miles by the end of this year.

“Running is a great way to relieve the pressure from daily life even though it’s difficult to find time to train when you have a full-time job and a family to take care of,” Gao said. “And I want to set a good example for my two kids. You set a goal and work hard for it. If you achieve your goal, that’s great. If not, accept it and work harder next time.”

“Xiaodong is an excellent scientist, and we’re lucky to have him in the department,” said Carrie Masiello, professor of Earth, environmental and planetary sciences, whose research group includes Gao. “He’s a strong analytical chemist and he’s also a thoughtful mentor of students in the lab, transferring his knowledge to others. He’s also especially good at seeing tasks through to completion. He is not deterred by analytical or intellectual challenges, and he likes to close on projects, making sure that his teams’ projects get all the way to publication. I think this drive to finish tasks thoroughly and completely is consistent between his research and marathoning.”

The desire for better health that motivated Gao to start running has paid off: He now weighs 160 pounds. But that presented a problem of its own. When he was visiting his parents in China, he tried to exchange currency at a bank there. The clerk did not think Gao was the person pictured on his Chinese ID because the photo had been taken prior to his dramatic weight loss. “It’s not you,” Gao remembered the clerk saying. “I had to show them some alternate forms of ID to convince them who I am.”

Hot spot at Hawaii? Not so fast

– AUGUST 18, 2017

Hot spot at Hawaii? Not so fast

Rice University scientists’ model shows global mantle plumes don’t move as quickly as thought

HOUSTON – (Aug. 18, 2017) – Through analysis of volcanic tracks, Rice University geophysicists have concluded that hot spots like those that formed the Hawaiian Islands aren’t moving as fast as recently thought.

Hot spots are areas where magma pushes up from deep Earth to form volcanoes. New results from geophysicist Richard Gordon and his team confirm that groups of hot spots around the globe can be used to determine how fast tectonic plates move.

Rice University geophysicists have developed a method that uses the average motion of hot-spot groups by plate to determine that the spots aren't moving as fast as geologists thought. For example, the Juan Fernandez Chain (outlined by the white rectangle) on the Nazca Plate west of Chile was formed by a hot spot now at the western end of the chain as the Nazca moved east-northeast relative to the hotspot forming the chain that includes Alejandro Selkirk and Robinson Crusoe islands. The white arrow shows the direction of motion of the Nazca Plate relative to the hot spot, and it is nearly indistinguishable from the direction predicted from global plate motions relative to all the hot spots on the planet (green arrow). The similarity in direction indicates that very little motion of the Juan Fernandez hot spot relative to other hot spots is needed to explain its trend. Illustration by Chengzu Wang

Gordon, lead author Chengzu Wang and co-author Tuo Zhang developed a method to analyze the relative motion of 56 hot spots grouped by tectonic plates. They concluded that the hot-spot groups move slowly enough to be used as a global reference frame for how plates move relative to the deep mantle. This confirmed the method is useful for viewing not only current plate motion but also plate motion in the geologic past.

The study appears in Geophysical Research Letters.

Hot spots offer a window into the depths of Earth, as they mark the tops of mantle plumes that carry hot, buoyant rock from deep Earth to near the surface and produce volcanoes. These mantle plumes were once thought to be straight and stationary, but recent results suggested they can also shift laterally in the convective mantle over geological time.

The primary evidence of plate movement relative to the deep mantle comes from volcanic activity that forms mountains on land, islands in the ocean or seamounts, mountain-like features on the ocean floor. A volcano forms on a tectonic plate above a mantle plume. As the plate moves, the plume gives birth to a series of volcanoes. One such series is the Hawaiian Islands and the Emperor Seamount Chain; the youngest volcanoes become islands while the older ones submerge. The series stretches for thousands of miles and was formed as the Pacific Plate moved over a mantle plume for 80 million years.

Rice University geophysicists have developed a method that uses the average motion of hot-spot groups by plate to determine that the spots aren’t moving as fast as geologists thought. From left, Chengzu Wang, Richard Gordon and Tuo Zhang. Photo by Jeff Fitlow

The Rice researchers compared the observed hot-spot tracks with their calculated global hot-spot trends and determined the motions of hot spots that would account for the differences they saw. Their method demonstrated that most hot-spot groups appear to be fixed and the remainder appear to move slower than expected.

“Averaging the motions of hot-spot groups for individual plates avoids misfits in data due to noise,” Gordon said. “The results allowed us to say that these hot-spot groups, relative to other hot-spot groups, are moving at about 4 millimeters or less a year.

“We used a method of analysis that’s new for hot-spot tracks,” he said. “Fortunately, we now have a data set of hot-spot tracks that is large enough for us to apply it.”

For seven of the 10 plates they analyzed with the new method, average hot-spot motion measured was essentially zero, which countered findings from other studies that spots move as much as 33 millimeters a year. Top speed for the remaining hot-spot groups — those beneath the Eurasia, Nubia and North America plates — was between 4 and 6 millimeters a year but could be as small as 1 millimeter per year. That’s much slower than most plates move relative to the hot spots. For example, the Pacific Plate moves relative to the hot spots at about 100 millimeters per year.

Gordon said those interested in paleogeography should be able to make use of the model. “If hot spots don’t move much, they can use them to study prehistorical geography. People who are interested in circum-Pacific tectonics, like how western North America was assembled, need to know that history of plate motion.

“Others who will be interested are geodynamicists,” he said. “The motions of hot spots reflect the behavior of mantle. If the hot spots move slowly, it may indicate that the viscosity of mantle is higher than models that predict fast movement.”

“Modelers, especially those who study mantle convection, need to have something on the surface of Earth to constrain their models, or to check if their models are correct,” Wang said. “Then they can use their models to predict something. Hot-spot motion is one of the things that can be used to test their models.”

Gordon is the W.M. Keck Professor of Earth Science. Wang and Zhang are Rice graduate students. The National Science Foundation supported the research.


Read the paper at http://onlinelibrary.wiley.com/doi/10.1002/2017GL073430/full

Data mining finds more than expected beneath Andean Plateau

Seismic data suggests means of producing massive volumes of continental crust

Seismologists investigating how Earth forms new continental crust have compiled more than 20 years of seismic data from a wide swath of South America’s Andean Plateau and determined that processes there have produced far more continental rock than previously believed.

“When crust from an oceanic tectonic plate plunges beneath a continental tectonic plate, as it does beneath the Andean Plateau, it brings water with it and partially melts the mantle, the layer below Earth’s crust,” said Rice University’s Jonathan Delph, co-author of the new study published online this week in Scientific Reports. “The less dense melt rises, and one of two things happens: It either stalls in the crust to crystallize in formations called plutons or reaches the surface through volcanic eruptions.”

Jonathan Delph

Delph, a Wiess Postdoctoral Research Associate in Rice’s Department of Earth, Environmental and Planetary Science, said the findings suggest that mountain-forming regions like the Andean Plateau, which geologists refer to as “orogenic plateaus,” could produce much larger volumes of continental rock in less time than previously believed.

Study lead author Kevin Ward, a postdoctoral researcher at the University of Utah, said, “When we compared the amount of trapped plutonic rock beneath the plateau with the amount of erupted volcanic rock at the surface, we found the ratio was almost 30:1. That means 30 times more melt gets stuck in the crust than is erupted, which is about six times higher than what’s generally believed to be the average. That’s a tremendous amount of new material that has been added to the crust over a relatively short time period.”

A true-color image of the Central Andes and surrounding landscape acquired by the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard NASA’s Terra spacecraft. (Image courtesy of NASA)

The Andean Plateau covers much of Bolivia and parts of Peru, Chile and Argentina. Its average height is more than 12,000 feet, and though it is smaller than Asia’s Tibetan Plateau, different geologic processes created the Andean Plateau. The mountain-building forces at work in the Andean plateau are believed to be similar to those that worked along the western coast of the U.S. some 50 million years ago, and Delph said it’s possible that similar forces were at work along the coastlines of continents throughout Earth’s history.

Most of the rocks that form Earth’s crust initially came from partial melts of the mantle. If the melt erupts quickly, it forms basalt, which makes up the crust beneath the oceans on Earth; but there are still questions about how continental crust, which is more buoyant than oceanic crust, is formed. Delph said he and Ward began their research in 2016 as they were completing their Ph.D.s at the University of Arizona. The pair spent several months combining public datasets from seismic experiments by several U.S. and German institutions. Seismic energy travels through different types of rock at different speeds, and by combining datasets that covered a 500-mile-wide swath of the Andean Plateau, Ward and Delph were able to resolve large plutonic volumes that had previously been seen only in pieces.

West-east cross sections from north (top) to south (bottom) of a 500-mile-wide portion of the Andean Plateau show subsurface features to a depth of 80 kilometers. Colors represent the speed at which seismic waves pass through the Earth; arrows point to plutonic regions of continent-building material in each section. (Image courtesy of J. Delph/Rice University)

Over the past 11 million years, volcanoes have erupted thousands of cubic miles’ worth of material over much of the Andean Plateau. Ward and Delph calculated their plutonic-to-volcanic ratio by comparing the volume of regions where seismic waves travel extremely slowly beneath volcanically active regions, indicating some melt is present, with the volume of rock deposited on the surface by volcanoes.

“Orogenic oceanic-continental subduction zones have been common as long as modern plate tectonics have been active,” Delph said. “Our findings suggest that processes similar to those we observe in the Andes, along with the formation of supercontinents, could have been a significant contributor to the episodic formation of buoyant continental crust.”

Additional co-authors include George Zandt and Susan Beck of the University of Arizona and Mihai Ducea of the University of Arizona and University of Bucharest.

The research was supported by the National Science Foundation and Rice University, and data was obtained by request from the Incorporated Research Institutions for Seismology and the German Research Centre for Geosciences, Potsdam.


Hidden river once flowed beneath Antarctic ice

Antarctic researchers from Rice University have discovered one of nature’s supreme ironies: On Earth’s driest, coldest continent, where surface water rarely exists, flowing liquid water below the ice appears to play a pivotal role in determining the fate of Antarctic ice streams.

Rice University researchers (from left) Lindsay Prothro, Lauren Simkins and John Anderson and colleagues discovered a long-dead river system that once flowed beneath Antarctica’s ice. (Photo by Jeff Fitlow/Rice University)

The finding, which appears online this week in Nature Geoscience, follows a two-year analysis of sediment cores and precise seafloor maps covering 2,700 square miles of the western Ross Sea. As recently as 15,000 years ago, the area was covered by thick ice that later retreated hundreds of miles inland to its current location. The maps, which were created from state-of-the-art sonar data collected by the National Science Foundation research vessel Nathaniel B. Palmer, revealed how the ice retreated during a period of global warming after Earth’s last ice age. In several places, the maps show ancient water courses — not just a river system, but also the subglacial lakes that fed it.

Today, Antarctica is covered by ice that is in some places more than 2 miles thick. Though deep, the ice is not static. Gravity compresses the ice, and it moves under its own weight, creating rivers of ice that flow to the sea. Even with the best modern instruments, the undersides of these massive ice streams are rarely accessible to direct observation.

This schematic depicts a subglacial Antarctic river and overlying ice sheet. Black lines t1, t2 and t3 show where the ice sheet was grounded to the seafloor during pauses in ice retreat. Rice University researchers used such lines from precise maps of the Ross Sea floor to study how liquid water influenced the ice sheet during a period of its retreat starting about 15,000 years ago. (Image courtesy of L. Prothro/Rice University)

“One thing we know from surface observations is that some of these ice streams move at velocities of hundreds of meters per year,” said Rice postdoctoral researcher Lauren Simkins, lead author of the new study. “We also know that ice, by itself, is only capable of flowing at velocities of no more than tens of meters per year. That means the ice is being helped along. It’s sliding on water or mud or both.”

Because of the paucity of information about how water presently flows beneath Antarctic ice, Simkins said the fossilized river system offers a unique picture of how Antarctic water drains from subglacial lakes via rivers to the point where ice meets sea.

“The contemporary observations we have of Antarctic hydrology are recent, spanning maybe a couple decades at best,” Simkins said. “This is the first observation of an extensive, uncovered, water-carved channel that is connected to both subglacial lakes on the upstream end and the ice margin on the downstream end. This gives a novel perspective on channelized drainage beneath Antarctic ice. We can track the drainage system all the way back to its source, these subglacial lakes, and then to its ultimate fate at the grounding line, where freshwater mixed with ocean water.”

An example of seafloor bathymetry data that Rice University oceanographers used to identify a paleo-subglacial channel, grounding line landforms, volcanic seamounts and other features used in their study. (Image courtesy of L. Simkins/Rice University)

Simkins said meltwater builds up in subglacial lakes. First, intense pressures from the weight of ice causes some melting. In addition, Antarctica is home to dozens of volcanoes, which can heat ice from below. Simkins found at least 20 lakes in the fossil river system, along with evidence that water built up and drained from the lakes in episodic bursts rather than a steady stream. She worked with Rice co-author and volcanologist Helge Gonnermann to confirm that nearby volcanoes could have provided the necessary heat to feed the lakes.

Study co-author John Anderson, a Rice oceanographer and veteran of nearly 30 Antarctic research expeditions, said the size and scope of the fossilized river system could be an eye-opener for ice-sheet modelers who seek to simulate Antarctic water flow. For example, the maps show exactly how ice retreated across the channel-lake system. The retreating ice stream in the western Ross Sea made a U-turn to follow the course of an under-ice river. Simkins said that’s notable because “it’s the only documented example on the Antarctic seafloor where a single ice stream completely reversed retreat direction, in this case to the south and then to the west and finally to the north, to follow a subglacial hydrological system.”

Location of study area in the western Ross Sea. (Image courtesy of L. Simkins/Rice University)

Simkins and Anderson said the study may ultimately help hydrologists and modelers better predict how today’s ice streams will behave and how much they’ll contribute to rising sea levels.

“It’s clear from the fossil record that these drainage systems can be large and long-lived,” Anderson said. “They play a very important role in the behavior of the ice sheet, and most numerical models today are not at a state where they can deal with that kind of complexity.”

He said another key finding is that drainage through the river system took place on a time scale measured in tens to several hundreds of years.

“We’re kind of in this complacent mode of thinking right now,” Anderson said. “Some people say, ‘Well, the ice margin seems to be stable.’ Some people may take comfort in that, but I don’t because what this new research is telling us is that there are processes that operate on decadal time scales that influence ice behavior. The probability of us having observed a truly stable condition in the contemporary system, given our limited observation time, is pretty low.”

Additional co-authors are Lindsay Prothro of Rice, Sarah Greenwood of Stockholm University, Anna Ruth Halberstadt and Robert DeConto of the University of Massachusetts-Amherst, Leigh Stearns of the University of Kansas and David Pollard of Pennsylvania State University.

The research was supported by the National Science Foundation and the Swedish Research Council.

New plate adds plot twist to ancient tectonic tale

Rice University scientists say Malpelo microplate helps resolve geological misfit under Pacific Ocean

By Mike Williams

HOUSTON – (Aug. 14, 2017) – A microplate discovered off the west coast of Ecuador adds another piece to Earth’s tectonic puzzle, according to Rice University scientists.

Researchers led by Rice geophysicist Richard Gordon discovered the microplate, which they have named “Malpelo,” while analyzing the junction of three other plates in the eastern Pacific Ocean. 

The Malpelo Plate, named for an island and an underwater ridge it contains, is the 57th plate to be discovered and the first in nearly a decade, they said. They are sure there are more to be found.

Misfit plates in the Pacific led Rice University scientists to the discovery of the Malpelo Plate between the Galapagos Islands and the South American coast. Click on the image for a larger version. Illustration by Tuo Zhang

The research by Gordon, lead author Tuo Zhang and co-authors Jay Mishra and Chengzu Wang, all of Rice, appears in Geophysical Research Letters.

How do geologists discover a plate? In this case, they carefully studied the movements of other plates and their evolving relationships to one another as the plates move at a rate of millimeters to centimeters per year. 

The Pacific lithospheric plate that roughly defines the volcanic Ring of Fire is one of about 10 major rigid tectonic plates that float and move atop Earth’s mantle, which behaves like a fluid over geologic time. Interactions at the edges of the moving plates account for most earthquakes experienced on the planet. There are many small plates that fill the gaps between the big ones, and the Pacific Plate meets two of those smaller plates, the Cocos and Nazcawest of the Galapagos Islands. 

One way to judge how plates move is to study plate-motion circuits, which quantify how the rotation speed of each object in a group (its angular velocity) affects all the others. Rates of seafloor spreading determined from marine magnetic anomalies combined with the angles at which the plates slide by each other over time tells scientists how fast the plates are turning.

“When you add up the angular velocities of these three plates, they ought to sum to zero,” Gordon said. “In this case, the velocity doesn’t sum to zero at all. It sums to 15 millimeters a year, which is huge.”

Rice University researchers have discovered a microplate off the coast of South America. From left, Tuo Zhang, Richard Gordon and Chengzu Wang. Photo by Jeff Fitlow

That made the Pacific-Cocos-Nazca circuit a misfit, which meant at least one other plate in the vicinity had to make up the difference. Misfits are a cause for concern – and a clue.

Knowing the numbers were amiss, the researchers drew upon a Columbia University database of extensive multibeam sonar soundings west of Ecuador and Colombia to identify a previously unknown plate boundary between the Galapagos Islands and the coast.  

Previous researchers had assumed most of the region east of the known Panama transform fault was part of the Nazca plate, but the Rice researchers determined it moves independently. “If this is moving in a different direction, then this is not the Nazca plate,” Gordon said. “We realized this is a different plate and it’s moving relative to the Nazca.”

Evidence for the Malpelo plate came with the researchers’ identification of a diffuse plate boundary that runs from the Panama Transform Fault eastward to where the diffuse plate boundary intersects a deep oceanic trench just offshore of Ecuador and Colombia. 

“A diffuse boundary is best described as a series of many small, hard-to-spot faults rather than a ridge or transform fault that sharply defines the boundary of two plates,” Gordon said. “Because earthquakes along diffuse boundaries tend to be small and less frequent than along transform faults, there was little information in the seismic record to indicate this one’s presence.” 

“With the Malpelo accounted for, the new circuit still doesn’t close to zero and the shrinking Pacific Plate isn’t enough to account for the difference either,” Zhang said. “The nonclosure around this triple junction goes down — not to zero, but only to 10 or 11 millimeters a year. 

“Since we’re trying to understand global deformation, we need to understand where the rest of that velocity is going,” he said. “So we think there’s another plate we’re missing.” 

Plate 58, where are you?

Gordon is the W.M. Keck Professor of Geophysics. Zhang and Wang are Rice graduate students and Mishra is a Rice alumnus. 

The National Science Foundation supported the research.

Read the abstract at http://onlinelibrary.wiley.com/doi/10.1002/2017GL073704/full

This news release can be found online at http://news.rice.edu/2017/08/13/new-plate-adds-plot-twist-to-ancient-tectonic-tale/


Glaciers may have helped warm Earth

Glaciers may have helped warm Earth

Rice professor’s study details effect of glacial versus nonglacial weathering on carbon cycle

It seems counterintuitive, but over the eons, glaciers may have made Earth warmer, according to a Rice University professor.

Weathering of Earth by glaciers may have warmed Earth over eons by aiding the release of carbon dioxide into the atmosphere.
A new study shows the cumulative effect may have created negative feedback that prevented runaway glaciation.
Photo by Paul Quackenbush

Mark Torres, an assistant professor of Earth, environmental and planetary sciences, took a data-driven dive into the mechanics of weathering by glaciation over millions of years to see how glacial cycles affected the oceans and atmosphere and continue to do so.

Mark Torres

Torres, who joined the Rice faculty in July, is lead author of a paper in the Proceedings of the National Academy of Sciences. He wanted to know how and when chemicals released by weathering of the land reached the atmosphere and ocean, and what effect they have had.

The study shows that glaciation, through enhanced erosion, probably increased the rate of carbon dioxide released to the environment. The researchers determined enhanced oxidation of pyrite, an iron sulfide also known as fool’s gold, most likely generated acidity that fed carbon dioxide into the oceans and altered the carbon cycle. The oscillation of glaciers over 10,000 years could have changed atmospheric carbon dioxide by 25 parts per million or more. While this is a significant percentage of the 400 parts per million measured in recent months, present anthropogenic carbon dioxide release is occurring at a much faster rate than it is naturally released by glaciation.

Over long timescales, they found, glaciers’ contribution to the release of carbon dioxide could have acted as a negative feedback loop that may have inhibited runaway glaciation.“The ocean stores a lot of carbon,” Torres said. “If you change the chemistry of the ocean, you can release some of that stored carbon into the atmosphere as carbon dioxide. This release of carbon dioxide affects Earth’s climate, due to the greenhouse effect.”

Glacial runoff appeared to have an outsize effect on carbon dioxide levels compared with that of rivers in warmer climes. Torres, until recently a postdoctoral researcher at the California Institute of Technology, studied glacier-fed rivers and used existing databases to compare their chemical contents with that of thousands of rivers around the world.

The goal was to evaluate the dominant chemical reactions associated with glacial weathering and explore the long-term implications. “Mainly, we’re thinking about the effect of glaciers and glaciation on the way our planet works,” he said. “In particular, we’re looking at rivers that drain areas of land surface that are covered by glaciers, and whether or not there are any differences in the chemical composition of those rivers.”

Mark Torres, an assistant professor of Earth, environmental and planetary sciences,
looked into the mechanics of weathering by glaciation over millions of years to see
how glacial cycles affected the oceans and atmosphere.

The researchers acknowledged that glaciers are equal-opportunity weathering agents, as they also break down silicates in rocks. Silicates release alkalinity that removes carbon from the atmosphere. Still, they believe the net effect of glaciation could be to supply carbon dioxide to the atmosphere rather than to remove it.

The results support a couple of interesting additional theories. One is that billions of years ago in the Archeaneon and Paleoproterozoic era, when the atmosphere contained little oxygen, Earth may indeed have been a “snowball” as oxidative weathering in glaciated regions and the subsequent release of carbon would have been less active. Another is that the growth of a sulfide reservoir in Earth’s crust over time may have helped to stabilize the climate, which is important for maintaining Earth’s habitability over geologic timescales.

The paper’s authors include Nils Moosdorf of the Center for Tropical Marine Ecology in Bremen, Germany, Jens Hartmann of the University of Hamburg, Jess Adkins of the California Institute of Technology and A. Joshua West of the University of Southern California.

Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks PNAS 2017 published ahead of print July 31, 2017doi:10.1073/pnas.1702953114