The CLEVER Planets team and Department of Earth, Environmental and Planetary Sciences at Rice University are seeking applicants for a fully funded postdoctoral scholar position who would explore volcanic-tectonic-climate interactions of rocky planets and how such interactions affect habitability. The contract will be initially for 1 year and is renewable for up to 3 years based on progress and performance. For details on the position and information on how to apply, please visit: https://earthscience.rice.edu/home-page/open-positions/
JADE BOYD – NOVEMBER 19, 2018
Rice U. scientists use Hawaiian hot spot to study movement of Earth’s poles
Earth’s latest ice age may have been caused by changes deep inside the planet. Based on evidence from the Pacific Ocean, including the position of the Hawaiian Islands, Rice University geophysicists have determined Earth shifted relative to its spin axis within the past 12 million years, which caused Greenland to move far enough toward the north pole to kick off the ice age that began about 3.2 million years ago.
Their study in the journal Geophysical Research Letters is based on an analysis of fossil signatures from deep ocean sediments, the magnetic signature of oceanic crust and the position of the mantle “hot spot” that created the Hawaiian Islands. Co-authors Richard Gordon and Daniel Woodworth said the evidence suggests Earth spun steadily for millions of years before shifting relative to its spin axis, an effect geophysicists refer to as “true polar wander.”
“The Hawaiian hot spot was fixed, relative to the spin axis, from about 48 million years ago to about 12 million years ago, but it was fixed at a latitude farther north than we find it today,” said Woodworth, a graduate student in Rice’s Department of Earth, Environmental and Planetary Sciences. “By comparing the Hawaiian hot spot to the rest of the Earth, we can see that that shift in location was reflected in the rest of the Earth and is superimposed on the motion of tectonic plates. That tells us that the entire Earth moved, relative to the spin axis, which we interpret to be true polar wander.”
By volume, Earth is mostly mantle, a thick layer of solid rock that flows under intense pressure and heat. The mantle is covered by an interlocking puzzle of rocky tectonic plates that ride atop it, bumping and slipping against one another at seismically active boundaries. Hot spots, like the one beneath Hawaii, are plumes of hot solid rock that rise from deep within the mantle.
Gordon, the W.M. Keck Professor of Earth, Environmental and Planetary Science, said the new findings build on two 2017 studies: one from his lab that showed how to use hot spots as a global frame of reference for tracking the movement of tectonic plates and another from Harvard University that first tied true polar wander to the onset of the ice age.
“We’re taking these hot spots as marked trackers of plumes that come from the deep mantle, and we’re using that as our reference frame,” he said. “We think the whole global network of hotspots was fixed, relative to the Earth’s spin axis, for at least 36 million years before this shift.”
Like any spinning object, Earth is subject to centrifugal force, which tugs on the planet’s fluid interior. At the equator, where this force is strongest, Earth is more than 26 miles larger in diameter than at the poles. Gordon said true polar wander may occur when dense, highly viscous bumps of mantle build up at latitudes away from the equator.
“Imagine you have really, really cold syrup, and you’re putting it on hot pancakes,” Gordon said. “As you pour it, you temporarily have a little pile in the center, where it doesn’t instantly flatten out because of the viscosity of the cold syrup. We think the dense anomalies in the mantle are like that little temporary pile, only the viscosities are much higher in the lower mantle. Like the syrup, it will eventually deform, but it takes a really, really long time to do so.”
If the mantle anomalies are massive enough, they can unbalance the planet, and the equator will gradually shift to bring the excess mass closer to the equator. The planet still spins once every 24 hours and true polar wander does not affect the tilt of Earth’s spin axis relative to the sun. The redistribution of mass to a new equator does change Earth’s poles, the points on the planet’s surface where the spin axis emerges.
Woodworth said the hot spot data from Hawaii provides some of the best evidence that true polar wander was what caused Earth’s poles to start moving 12 million years ago. Islands chains like the Hawaiians are formed when a tectonic plate moves across a hot spot.
“True polar wander shouldn’t change hot spot tracks because the hot spot track is the record of the motion of the plate relative to the hot spot,” Woodworth said.
Gordon said, “It was only about a 3 degree shift, but it had the effect of taking the mantle under the tropical Pacific and moving it to the south, and at the same time, it was shifting Greenland and parts of Europe and North America to the north. That may have triggered what we call the ice age.”
Earth is still in an ice age that began about 3.2 million years ago. Earth’s poles have been covered with ice throughout the age, and thick ice sheets periodically grow and recede from poles in cycles that have occurred more than 100 times. During these glacial cycles, ice has extended as far south as New York and Yellowstone National Park. Earth today is in an interglacial period in which ice has receded toward the poles.
Gordon said true polar wander is not merely a change in the location of Earth’s magnetic poles. As the planet spins, it’s iron core produces a magnetic field with “north” and “south” poles near the spin axis. The polarity of this field flips several times every million years, and these changes in polarity are recorded in the magnetic signatures of rocks the world over. The paleomagnetic record, which is often used to study the movement of tectonic plates across Earth’s surface, contains many instances of “apparent polar wander,” which tracks the motion of the spin axis and which includes the effects of both plate motion and true polar wander, Gordon said.
He said Earth’s mantle is ever-changing as new material constantly cycles in and out from tectonic plates. The drawing down and recycling of plates via subduction provides a possible explanation for the highly viscous mantle anomalies that probably cause true polar wander.
“In class, I often demonstrate this with lead fishing weights and pliers,” Gordon said. “It’s easy to deform the lead with the pliers, and it’s not brittle. It doesn’t crack or fly apart when it fails. That’s a pretty good analogy for mantle flow because that’s the way silicate rock deforms under intense heat and pressure.”
He and Woodworth are working with colleagues to extend their analysis, both from 12 million years ago to the present as well as further into the past than the 48-million-year start date in the newly published study.
The National Science Foundation supported the research.
Rice scientist’s method helps interpret climate data from lake sediments
– OCTOBER 24, 2018
How cold did Earth get during the last ice age? The truth may lie deep beneath lakes and could help predict how the planet will warm again.
Sediments in lake beds hold chemical records of ages past, among them the concurrent state of the atmosphere above. Scientists led by a Rice University professor and her colleagues have devised a new computational model to interpret what they reveal.
Sylvia Dee, an assistant professor of Earth, environmental and planetary sciences, and her colleagues have created a computational Lake Proxy System Model to translate data from deep beneath lake surface waters in a way that relates more directly to measurable climate model variables.
Scientists who study past climate analyze geochemical signals from archives like corals and ice cores, or encoded in the rings of old trees, but not everyone interprets the data in the same way. Dee’s quest has been to design simple models that help interpret observations of past climate more uniformly with climate models, and in the process make these invaluable archives more relevant to studies of future climate change.
“We have climate model simulations going back thousands of years,” said Dee, who joined Rice this year. “They help us understand the drivers of past temperature and precipitation changes, but we have to use climate data from the past to ground truth the models.
“For example, if a climate model shows strong agreement with the temperature reconstructions we have from lakes, we might conclude this model’s physics are robust and that it can do a better job simulating what will happen under future anthropogenic warming.”
Lake beds store evidence of climate history in layered sediment that can be analyzed and dated by extracting cores. Dee’s study used climate model data to explore and understand the lake archives scientists use to reconstruct atmospheric conditions for a given time.
“Some of the richest temperature and precipitation histories that we have on Earth come from lakes,” Dee said. “People have been measuring indicators in sediments for years, but it isn’t straightforward to compare that data to climate models.
“That’s where I come in,” she said. “I’m part of a group of scientists focused on translating between what climate models tell us about past changes in the climate system and what the data are telling us.”
Dee and her team simulated lake temperatures and climate archives in two lakes in Africa, Malawi and Tanganyika, stretching back to the last glacial maximum about 21,000 years ago, when global temperatures were estimated to be between 3 to 5 degrees Celsius colder than today.
“That’s a clear target for climatologists,” she said. “We have clues from the past that tell us how cold the African continent was. It’s the last time in Earth’s climate history that there was a dramatic shift in mean climate due to carbon dioxide forcing, and we use it as a test bed for climate model performance.”
In the test case, PRYSM’s simulation revealed that lake temperature proxies underestimated air temperature changes. “People generally assume that lake temperature reconstructions from sediments reflect air temperature changes,” Dee said. “We assume they change in tandem. Our model simulations show that the lake is actually damping the temperature signal.
“For example, if modeled air temperatures show a 4 degree Celsius warming since the last glacial maximum in Africa and the lake damps that signal to 3 degrees, we might wrongly conclude that air temperatures were a full degree warmer than they really were. We’re essentially able to quantify how much error one might expect in our interpretation of past temperature change. Those errors are incurred by the lake alone,” she said.
“That’s important to know,” Dee said. “These lake reconstructions are some of the only data available to help us understand how temperatures will evolve on the African continent in a warming world. Our hope is that PRYSM helps pin down these climate changes with higher confidence.”
The open-source PRYSM is version 2.0, designed explicitly to model climate archives in lakes. Dee built the first version to model ice cores, corals, cave deposits and tree-ring cellulose.
“I’m trying to get everyone in the modeling and paleoclimate communities to talk to each other,” Dee said. “PRYSM is an effort to get both communities to understand we cannot compare apples to oranges. We need to compare paleoclimate data and model simulations in a more formal way, and in doing so, we hope to dramatically improve our interpretations of past climate changes.
“The great thing about being able to get snapshots of temperatures in the past is that we can hopefully build a broader understanding of how the planet is going to react to continued anthropogenic climate change,” she said. “We have direct measurements of what’s happened in the last 150 years. But if we look further back in time, we have bigger changes in carbon dioxide, bigger changes in volcanism and larger ice sheets on Earth. Those are heavy hammers. They help us understand how the climate reacts to stronger forcing.”
Co-authors of the paper are James Russell, an associate professor of Earth, environmental and planetary sciences, graduate student Ashling Neary and undergraduate Zihan Chen, all at Brown University, and Carrie Morrill, a research scientist at the Cooperative Institute for Research in Environmental Sciences, a partnership of the National Oceanic and Atmospheric Administration and the University of Colorado Boulder.
The research was supported by the Peter Voss Postdoctoral Fellowship, the Institute at Brown for Environment and Society, the Brown University Department of Earth, Environmental and Planetary Sciences, and the University of Texas at Austin Institute for Geophysics.
– OCTOBER 25, 2018
Rice University scientists uncover relationship between tremors, water at the Cascadia margin
HOUSTON – (Oct. 25, 2018) – The earthquakes are so small and deep that someone standing in Seattle would never feel them. In fact, until the early 2000s, nobody knew they happened at all. Now, scientists at Rice University have unearthed details about the structure of Earth where these tiny tremors occur.
Rice postdoctoral researcher and seismologist Jonathan Delph and Earth scientists Fenglin Niu and Alan Levander make a case for the incursion of fluid related to slippage deep inside the Cascadia margin off the Pacific Northwest’s coast.
Their paper, which appears in the American Geophysical Union journal Geophysical Research Letters, links fluids escaping from deep subduction to the frequent shakes that Delph said happen in relative slow motion when compared to the sudden, violent jolts occasionally felt by Southern Californians at the southern end of the west coast.
“These aren’t large, instantaneous events like a typical earthquake,” Delph said. “They’re seismically small, but there’s a lot of them and they are part of the slow-slip type of earthquake that can last for weeks instead of seconds.”
Delph’s paper is the first to show variations in the scale and extent of fluids that come from dehydrating minerals and how they relate to these low-velocity quakes. “We are finally at the point where we can address the incredible amount of research that’s been done in the Pacific Northwest and try to bring it all together,” he said. “The result is a better understanding of how the seismic velocity structure of the margin relates to other geologic and tectonic observations.”
The North American plate and Juan de Fuca plate, a small remnant of a much larger tectonic plate that used to subduct beneath North America, meet at the Cascadia subduction zone, which extends from the coast of northern California well into Canada. As the Juan de Fuca plate moves to the northeast, it sinks below the North American plate.
Delph said fluids released from minerals as they heat up at depths of 30 to 80 kilometers propagate upward along the boundary of the plates in the northern and southern portions of the margin, and get trapped in sediments that are subducting beneath the Cascadia margin.
“This underthrust sedimentary material is being stuck onto the bottom of the North American plate,” he said. “This can allow fluids to infiltrate. We don’t know why, exactly, but it correlates well with the spatial variations in tremor density we observe. We’re starting to understand the structure of the margin where these tremors are more prevalent.”
Delph’s research is based on extensive seismic records gathered over decades and housed at the National Science Foundation-backed IRIS seismic data repository, an institutional collaboration to make seismic data available to the public.
“We didn’t know these tremors existed until the early 2000s, when they were correlated with small changes in the direction of GPS stations at the surface,” he said. “They’re extremely difficult to spot. Basically, they don’t look like earthquakes. They look like periods of higher noise on seismometers.
“We needed high-accuracy GPS and seismometer measurements to see that these tremors accompany changes in GPS motion,” Delph said. “We know from GPS records that some parts of the Pacific Northwest coast change direction over a period of weeks. That correlates with high-noise ‘tremor’ signals we see in the seismometers. We call these slow-slip events because they slip for much longer than traditional earthquakes, at much slower speeds.”
He said the phenomenon isn’t present in all subduction zones. “This process is pretty constrained to what we call ‘hot subduction zones,’ where the subducting plate is relatively young and therefore warm,” Delph said. “This allows for minerals that carry water to dehydrate at shallower depths.
“In ‘colder’ subduction zones, like central Chile or the Tohoku region of Japan, we don’t see these tremors as much, and we think this is because minerals don’t release their water until they’re at greater depths,” he said. “The Cascadia subduction zone seems to behave quite differently than these colder subduction zones, which generate large earthquakes more frequently than Cascadia. This could be related in some way to these slow-slip earthquakes, which can release as much energy as a magnitude 7 earthquake over their duration. This is an ongoing area of research.”
Levander is the Carey Croneis Professor of Earth, Environmental and Planetary Sciences and Niu is a professor of Earth, environmental and planetary sciences at Rice. The Wiess Postdoctoral Fellowship at Rice supported the research.
Read the abstract at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL079518.
Follow Rice News and Media Relations via Twitter @RiceUNews.
Jonathan Delph website: https://jrdelph.wordpress.com
Incorporated Research Institutions for Seismology (IRIS): https://www.iris.edu/hq/
Global Seismology Group at Rice: http://www.gseis.rice.edu/index.html
– SEPTEMBER 17, 2018
Cross-disciplinary team will track life-essential elements during planets’ early evolution
NASA’s interdisciplinary Nexus for Exoplanet System Science (NExSS) project has awarded Rice University $7.7 million for a multidisciplinary, multi-institutional research program aimed at finding many different recipes nature might follow to produce rocky planets capable of supporting life.
As any cook knows, it takes the right recipe and getting the right ingredients to make a tasty dish, and the same principle applies to habitable rocky planets, said Rice Earth and planetary scientist Rajdeep Dasgupta, the principal investigator on NASA NExSS’s CLEVER Planets research program.“A recipe for life as we know it requires essential elements like carbon, oxygen, nitrogen, hydrogen, phosphorous and sulfur,” said Dasgupta, professor of Earth, environmental and planetary sciences at Rice. “But the first billion years of a rocky planet’s life are turbulent. On Earth, that period was marked by enormous change, not only at the surface but inside the planet as well. For planetary habitability, life-essential elements must survive that period in a bioavailable form.”
“We have the expertise to trace life-essential elements through the first billion-year journey from protoplanetary disks to prebiotic molecules on the surface of young worlds,” Dasgupta said. “Some of the processes that are central to this — the ones happening inside the planet and the feedbacks that link interior processes with those on the surface — are largely unexplored in the context of exoplanets.”
He said the research will be guided by knowledge from the solar system’s rocky planets — Mercury, Venus, Earth and Mars — and other objects, but CLEVER Planets’ goal is to extend knowledge to rocky worlds orbiting distant stars.
“We know more about our own solar system than any other,” Dasgupta said. “That’s very useful for comparative planetology, but the focus of our search is beyond our own backyard. We want to construct and constrain as many possible pathways to rocky planet habitability as we can.”
Co-investigators on the five-year grant include Cin-Ty Lee, professor and chair of the Department of Earth, Environmental and Planetary Science at Rice; Adrian Lenardic, professor of Earth, environmental and planetary sciences at Rice; Laurence Yeung, assistant professor of Earth, environmental and planetary sciences at Rice; Pedram Hassanzadeh, assistant professor of mechanical engineering and of Earth, environmental and planetary sciences at Rice; Andrea Isella, assistant professor of physics and astronomy and of Earth, environmental and planetary sciences at Rice; Tom McCollomof the University of Colorado, Boulder’s Laboratory for Atmospheric and Space Physics; Hilke Schlichting, associate professor of Earth, planetary and space sciences at UCLA; Sarah Stewart, professor of Earth and planetary sciences at the University of California, Davis; Aaron Burton, planetary scientist in the NASA Johnson Space Center’s Astromaterials Research and Exploration Science Division; and Francis McCubbin, astromaterials curator in the NASA Johnson Space Center’s Astromaterials Research and Exploration Science Division. The CLEVER Planets team also includes collaborators Christopher Johns-Krull and David Alexander, both professors of physics and astronomy at Rice.
The Rice-led team was selected from a competition in response to the NASA Astrobiology Institute Cycle 8 Cooperative Agreement, and funding was awarded by NASA’s Science Mission Directorate as part of the agency’s interdisciplinary NExSS Project. NExSS is a NASA research coordination network dedicated to the study of planetary habitability. The goals of NExSS are to investigate the diversity of exoplanets and to learn how their history, geology and climate interact to create the conditions for life.
– AUGUST 29, 2018
Adrienne Correa, assistant professor of biosciences at Rice, and Pedram Hassanzadeh, assistant professor of mechanical engineering and of Earth, environmental and planetary sciences, received 2018 Early-Career Research Fellowships from the Gulf Research Program, part of the National Academies of Sciences, Engineering and Medicine.
The fellowships support emerging scientific leaders as they take risks on untested research ideas, pursue unique collaborations and build a network of colleagues who share their interest in improving not only offshore energy system safety, but also the well-being of coastal communities and ecosystems.
“Because the early years of a researcher’s career are a critical time, the relatively unrestricted funds and mentoring this fellowship provides help recipients navigate this period with independence, flexibility and a built-in support network,” according to the Gulf Research Program.
Correa, a marine biologist, studies how marine microbial communities influence the health and function of marine animals and ecosystems, particularly when human activities alter temperature, nutrient availability and other conditions in coastal environments.
For three years, Correa has helped advise management of coral reefs in the northwest Gulf of Mexico through her research seat on the Flower Garden Banks National Marine Sanctuary Advisory Council. She is currently leading a team of scientists tracking low-salinity water masses, associated microbial communities and measures of coral health in the Gulf of Mexico to develop a predictive framework for assessing whether particular storm events are likely to harm reefs.
Most recently, Correa discovered that depth makes a big difference in the biological erosion that can lead reefs to either grow or shrink. She and her team were able to quantify how barnacles infest stony coral in a variety of conditions and potentially reduce calcium carbonate on reefs.
Hassanzadeh, an expert in environmental fluid dynamics, is interested in large-scale turbulent flows, such as those in the atmosphere. He uses computational, mathematical and statistical modeling to study atmospheric flows related to a broad range of issues, from extreme weather events to wind energy.
His research group is examining why Hurricane Harvey moved so slowly and whether a large-scale weather pattern played a role. The group is also exploring ways to improve the accuracy of NASA’s weather forecast model, searching for a better understanding of atmospheric turbulence and studying data-driven modeling of environmental flows.
Hassanzadeh’s group is also using deep learning to identify and predict extreme weather events. Hassanzadeh will use the fellowship to advance work on determining if more Harvey-style hurricanes are likely in the Gulf region in the future.
The fellowships include a two-year grant of $76,000, which provides funding for research-related expenses such as equipment purchases, professional travel, development courses, trainee support and salary.
The Gulf Research Program is an independent, science-based program founded in 2013 as part of legal settlements with companies involved in the 2010 Deepwater Horizon disaster. Its purpose is to enhance offshore energy system safety and protect human health and the environment. The program funds grants, fellowships and other activities.
Video by Rice Professor Julia Morgan, taken from a helicopter on July 16, shows lava from the ongoing eruption of Kilauea on the Big Island of Hawaii as it moves from the volcano to the sea. Morgan and her colleagues spent a week placing ocean-bottom seismic instruments off the southeastern shore of the island.
Rice researchers help deliver seismometers to analyze Hawaiian volcano, quakes
By Mike Williams
Rice University researchers joined a team of scientists placing seismometers under the ocean off the coast of Hawaii, where the ongoing eruption of Kilauea has already claimed more than 700 homes and added to the island’s landmass. The researchers hope for new insight about the landscape under the ocean floor.
Julia Morgan, a professor of Earth, environmental and planetary sciences, and student David Blank were awarded a National Science Foundation RAPID grant to join a team of researchers and seed the seafloor with a dozen seismic detectors off the southeastern coast of the island in the wake of the 6.9 magnitude earthquake that occurred at the start of the eruption of Kilauea May 4.
The instruments will gather data until September, when they will be retrieved, and are expected to provide an extensive record of earthquakes and aftershocks associated with the eruption of the world’s most active volcano over two months.
“They’re still going on,” said Morgan, who returned to Houston last week after seven days aboard a vessel deployed to place instruments and map the area. “In addition, a bunch of earthquakes occurred in other portions of the (island) flank. That’s what really got my attention.”
Her interest in geologic structures, particularly relating to volcano deformation and faulting, led her to study the ocean bed off the Big Island’s coast for years. In a 2003 paper, Morgan and her colleagues used marine seismic reflection data to look inside Kilauea’s underwater slope for the boundaries of an active landslide, the Hilina Slump, as well as signs of previous avalanches.
The researchers determined that the Hilina Slump is restricted to the upper slopes of the volcano, and the lower slopes consist of a large pile of ancient avalanche debris that was pushed by Kilauea’s sliding, gravity-driven flank into a massive, mile-high bench about 15 miles offshore. This outer bench currently buttresses the Hilina Slump, preventing it from breaking away from the volcano slopes.
“We mapped out the geometry and extent of the slump and tried to develop a history of how it came to be,” she said of the paper.
“Essentially, Kilauea is a bulldozer sliding out on the ocean crust and scraping off packages of strata that have accumulated,” Morgan said. The Hilina Slump rides on top of the sliding flank, she said.
“Remarkably, after this earthquake, all the boundaries of the slump also lit up with small earthquakes. These clearly occurred on a different fault than the main earthquake, suggesting that the slump crept downslope during or after that event,” she said.
Morgan said the bench appears to be stable, presumably supporting the slump — although if it collapsed, the slump would follow and the results could be catastrophic. “If the slump were to fail catastrophically, it would create an amazing tsunami that would hit the West Coast. We have not seen this in historic times,” she said.
“The frequency of these failures is very low and the interval between them is very high,” Morgan said. “We think this happened at Kilauea between 25,000 and 50,000 years ago, and we know it happened on (adjacent volcano) Mauna Loa about 100,000 years ago, and probably more than once before that.”
While the risk of an imminent avalanche is slim, she said, the eruption, earthquake and aftershocks presented an irresistible opportunity to get a better look at the island’s hidden terrain. Every new quake that occurs along Kilauea’s rift zones and around the perimeter of the Hilina Slump and the bench helps the researchers understand the terrain.
Morgan said the United States Geological Survey, which operates the Hawaiian Volcano Observatory, has a host of ground-based seismometers but none in the ocean. She said monitors at sea will reveal quakes under the bench that are too small for land seismometers to sense.
“The (initial) earthquake seems to have caused earthquakes beneath the outer bench,” Morgan said. “If that outer bench is the buttress to the slump, and that bench is beginning to show seismicity, it’s moving. At what point does it collapse?”
The seismometers are deployed around the Hilina Slump, close to shore where lava is entering the ocean and on the outer bench in line with the initial quake. “That way, aftershocks from the earthquake could be picked up and would record characteristics of the fault zone that slipped,” she said.
“If this outer bench is experiencing earthquakes, we want to know what surfaces are experiencing them. Along the base? Within the bench? Some new fault that we didn’t know about? This data will provide us the ability to determine what structures, or faults, are actually slipping.”
While Blank worked days on the ship to help deploy the instruments, Morgan chose the night shift for mapping – and a better view of lava hitting the water. After their duty at sea, both took the unique opportunity to book a helicopter flight over the volcano, following the river of lava to the sea.
“If you’re following the flows, you can look down and watch the lava tear across the countryside,” she said. “Then you go out to the lava ocean entry. You see these littoral explosions as the lava is flowing into the ocean. You might get a big pulse of lava and suddenly it gets cooled and quenched so rapidly that it just explodes up into the air.”
By Mike Williams
HOUSTON – (July 23, 2018) – Scientists have long thought oxygen appeared in Earth’s lower atmosphere 2.7 billion years ago, making life as we know it possible. A Rice University researcher has added evidence to support that number.
The sulfur record held by ancient rock marks the dramatic change in the planet’s atmosphere that gave rise to complex life, but rocks are local indicators. For the big picture, Rice biogeochemist Mark Torres used water that flows over and erodes the rocks as a proxy.
Torres, a Rice assistant professor of Earth, environmental and planetary sciences, and his colleagues report in Nature Geoscience that the balance of sulfur isotope anomalies in Archean rock, a marker of the “great oxygenation event,” can also be recognized and measured in the rivers that erode it.
The researchers sampled water from two of the few places on Earth where Archean rock is exposed in abundance: at the Superior Craton in Canada and in South Africa. They determined that while individual samples of rock may still show an imbalance (the anomalies) of sulfur isotopes, careful analysis of the water that diffuses and transports sulfur from thousands of miles of rock to the ocean shows that the contents are ultimately in alignment with bulk Earth’s sulfur signature.
“Changes in chemistry can tell you something about the environment, and rocks can tell you whether there was oxygen at a particular time,” Torres said. “Early in our history, sulfur isotope anomalies are all over the place. Then, roughly 2.7 billion years ago, they disappear and they never come back.”
Sulfur is a marker because four stable isotopes, known by their molecular masses of 32, 33, 34 and 36, can show different behaviors when present in the atmosphere. “Most sulfur is mass 32, but there are small amounts of the other masses,” Torres said.
Ultraviolet light from the sun reacted with sulfur gas and split it into separate compounds with heavier and lighter isotopes. Eventually, these compounds sunk into and remain in rock that formed at the time.
“But there’s this weird thing: Really old rocks have more 33-sulfur in them than we would expect, based on the relative masses,” Torres said. “Because 33 is one heavier than 32, we should easily be able to predict their relative abundances using physical chemistry. But, we find that 33 is way more abundant than expected. That’s why we call it an anomaly.”
When oxygen appeared, it absorbed ultraviolet light and quenched the sulfur reaction, as seen in the rock. That’s all well and good, Torres said, but the theory doesn’t account for anomalous sulfur that continued to leach from Archean rock into surface water, be carried to the ocean and then condense into new rock that would also have the anomaly.
“This recycling of ancient rock was a way to perpetuate the anomaly even after oxygen had arisen,” he said. The researchers suspected persistence of the anomaly could blur understanding of the timing of oxygen’s rise by as much as 100 million years.
It didn’t, they discovered, but it wasn’t easy. The team included researchers from the California Institute of Technology and the Center for Petrographic and Geochemical Research in Nancy, France. Members collected scores of samples from the Canadian sites to go along with South African samples they already had and checked their sulfur signature after eliminating the effects of contaminants from sulfurous acid rain, ice-melting road salt and dust from local mining operations. But their final calculations showed a robust balance in 33-sulfur collected by river runoff over a wide area.
“Our effort allows us to be confident we’ve got the timing for this great oxidation event, so now we can start to understand the mechanisms,” Torres said. “If you think about the whole scope of Earth’s history, 100 million years is small, but on the evolutionary timeline of organisms, it matters.”
Co-authors of the paper are Guillaume Paris, a former postdoctoral researcher at Caltech and now a researcher at the Center for Petrographic and Geochemical Research; Jess Adkins, the Smits Family Professor of Geochemistry and Global Environmental Science at Caltech; and Woodward Fischer, a professor of geobiology at Caltech.
The National Science Foundation, the David and Lucile Packard Foundation and a Caltech GPS Division Discovery Award supported the research.
Read the abstract at http://dx.doi.org/10.1038/s41561-018-0184-7
Rice University researcher helped find oxygen evidence of atmospheric production
by Mike Williams
HOUSTON – (July 18, 2018) – Sleuthing by a Rice University postdoctoral fellow is part of a new Nature paper that gives credence to theories about Earth’s atmosphere 1.4 billion years ago.
Rice’s Justin Hayles and his colleagues, led by Peter Crockford at McGill University in Montreal, analyzed samples from an ancient Canadian lake bed that turned up anomalous oxygen isotopes embedded in deposits of sulfate. The oxygen provides hints at the extent of life on ancient Earth’s surface.
The researchers found the planet’s gross primary production – a measure of processes like photosynthesis – was a small fraction of modern levels during a stretch of the Proterozoic eon known to researchers as the “Boring Billion” because of the planet’s environmental and evolutionary stability.
“The Boring Billion is called boring because it seemed for a long time that nothing remarkable was occurring on Earth’s surface, but the evolution of Earth and the life on its surface continued,” Hayles said.
Hayles, a National Science Foundation postdoctoral fellow, did the work as a Ph.D. student at Louisiana State University. He joined the Rice lab of Laurence Yeung, an assistant professor of Earth, environmental and planetary sciences, two years ago.
Hayles’ analysis with specialized mass-spectrometry equipment was part of the effort to analyze cores taken from the lake bed. “When the project started, we were just looking to see what sulfates looked like through Earth’s history,” he said. “In the process, we analyzed this one set of samples and found an anomaly.”
That anomaly was an unexpected amount of oxygen-17, one of three stable isotopes of oxygen. “This was shocking because we thought this anomaly could only exist when atmospheric carbon dioxide concentrations are extremely high, such as during a ‘snowball Earth‘ event,” Hayles said. “It turns out that this condition is not needed if concentrations of atmospheric oxygen (O2) and bioproductivity are much lower than today.”
Because oxygen is highly reactive, it easily combined with sulfide in what was then a lake at Ontario’s Sibley Basin. “When you form sulfate from sulfide, you get a little bit of O2 incorporated,” he said. “That is preserved as a capsule of the ancient atmosphere, so it contains oxygen from back in the Proterozoic, 1.4 billion years ago.”
The researchers suggested their discovery is the oldest direct measurement of atmospheric oxygen isotopes by nearly a billion years, taken from a time when microorganisms, including bacteria and algae, were beginning to ramp up production through photosynthesis but had not yet reached the fertile period that triggered a second “oxygenation event.”
“It has been suggested for many decades now that the composition of the atmosphere has significantly varied through time,” said Crockford, now a postdoctoral fellow at Princeton. “We provide unambiguous evidence that it was indeed much different 1.4 billion years ago.”
The researchers said their discovery could help in the search for clues to life on other planets.
“Earth during the Proterozoic was like an alien world compared with the modern Earth,” Hayles said. “The atmosphere had only a small amount of oxygen and the environment was arguably much warmer.
“Knowing how well microbial life thrived tells us what to expect on a hypothetical planet with a similar environment,” he said. “There is potential that if Mars was ever sufficiently Earth-like and the right material found its way to Earth, this technique could provide similar evidence.”
Scientists at McGill University, Louisiana State University, Lakehead University, the Weizmann Institute of Science in Israel, Peking University, Yale University, Princeton University and the University of California, Riverside took part in the study.
The research was supported by the Natural Sciences and Engineering Research Council of Canada, the Fonds de Recherche du Quebec-Nature et Technologies and the University of Colorado Boulder.
Read the abstract at http://dx.doi.org/10.1038/s41586-018-0349-y
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