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

Fossil coral reefs show sea level rose in bursts during last warming
JADE BOYD – OCTOBER 19, 2017
POSTED IN: CURRENT NEWS, FEATURED STORIES

 

https://youtu.be/jv9VA797Veo
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 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:

http://news.rice.edu/files/2017/10/1016_PACKARD-yeung136-lg-1i08jz2.jpg
CAPTION: Laurence Yeung (Photo by Jeff Fitlow/Rice University)

Related stories from Rice:

Earth scientist Laurence Yeung wins Clarke Award — Feb. 29, 2016
http://news.rice.edu/2016/02/29/earth-scientist-laurence-yeung-wins-clarke-award/

Oxygen atmosphere recipe = tectonics + continents + life — May 16, 2016
http://news.rice.edu/2016/05/16/oxygen-atmosphere-recipe-tectonics-continents-life/

Study: Photosynthesis has unique isotopic signature — April 23, 2015
http://news.rice.edu/2015/04/23/study-photosynthesis-has-unique-isotopic-signature/

Xiaodong Gao aiming for the six World Marathon Majors

Research scientist aiming for the six World Marathon Majors
B.J. ALMOND – OCTOBER 6, 2017
POSTED IN: RICE CURRENT NEWS

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.

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

 

Zealandia should hold answers about tectonics and past climate

Zealandia should hold answers about tectonics, past climate

JADE BOYD – JULY 18, 2017

Scientific expedition will explore Tasman Sea for clues about submerged continent

Thirty scientists will sail from Australia July 27 on a two-month ocean drilling expedition to the submerged continent of Zealandia in search of clues about its history, which relates to key questions about plate tectonic processes and Earth’s past greenhouse climate.

Jerry Dickens standing on a map of Zealandia in Rice’s Keith-Wiess Geological Laboratories (Photo by Jeff Fitlow/Rice University)

“We’re really looking at the best place in the world to understand how plate subduction initiates,” said expedition co-chief scientist Gerald Dickens, professor of Earth, environmental and planetary science at Rice University. “This expedition will answer a lot of lingering questions about Zealandia.”

Expedition 371, a cruise sponsored by the National Science Foundation and its international partners in the International Ocean Discovery Program (IODP), will sail from Townsville, Australia, aboard JOIDES Resolution, one of the world’s most sophisticated scientific drill ships. Expedition scientists will join more than 20 scientific crew members in drilling at six Tasman Sea sites at water depths ranging from 1,000 to 5,000 meters. At each site, the crew will drill from 300 to 800 meters into the seafloor to collect cores — complete samples of sediments deposited over millions of years. The cores hold fossil evidence the scientists can use to assemble a detailed record of Zealandia’s past.

“Some 50 million years ago a massive shift in plate movement happened in the Pacific Ocean,” said Jamie Allan, program director in the National Science Foundation’s Division of Ocean Sciences, which supports IODP. “It resulted in the diving of the Pacific Plate under New Zealand, the uplift of New Zealand above the waterline and the development of a new arc of volcanoes. This IODP expedition will look at the timing and causes of these changes, as well as related changes in ocean circulation patterns and ultimately Earth’s climate.”

IODP Expedition 371 map (Image courtesy IODP)

Zealandia is a mass of continental crust about half the size of Australia that surrounds New Zealand. Increasingly detailed seafloor maps have brought Zealandia into focus in recent decades. Unlike other continents, though, more than 90 percent of Zealandia is submerged.

“If you go way back, about 100 million years ago, Antarctica, Australia and Zealandia were all one continent,” Dickens said. “Around 85 million years ago, Zealandia split off on its own, and for a time, the seafloor between it and Australia was spreading on either side of an ocean ridge that separated the two.”

The relative movements of Zealandia and Australia are due to plate tectonics, the constant movement of interlocking sections of Earth’s surface. There are some 25 tectonic plates that fit like puzzle pieces to form Earth’s crust. Plates are in constant motion. They can crash together to form mountain ranges and slide past one another in earthquake zones. Oceanic plates form on either side of ocean ridges and also can slide beneath lighter, more buoyant continental plates in a process known as subduction.

JOIDES Resolution (Photo courtesy IODP)

Expedition 371 will examine a shift that occurred about 50 million years ago in the direction of movement of the enormous Pacific Plate northeast of Zealandia. In its scientific prospectus, the expedition refers to this shift as “the most profound subduction initiation event and global plate-motion change” in the past 80 million years. Prior to the shift, Australia and New Zealand were spreading apart, and after the shift, the area that separated them was under compression for millions of years. Then, in the final stage of the tectonic shift, the Pacific Plate dove beneath Zealandia, forming a new subduction zone. This relieved the compressive forces across the region.

“What we want to understand is why and when the various stages from extension to relaxation occurred,” Dickens said. “The cores will help tell us that. They’ll be analyzed for sediment composition, microfossil components, mineral and water chemistry and physical properties.”

He said the research also could answer many questions about the way Earth’s climate has evolved in the last 60 million years. For example, Zealandia is left out of many climate models, and Dickens said this could be one reason that this region has been among the most difficult parts of Earth to accurately model in greenhouse climates around 50 million years ago.

“When the community does climate modeling for the Eocene (Epoch), this is the area that causes consternation, and we’re not sure why,” he said. “It may be because we had continents that were much shallower than we had thought. Or we could have the continents right but at the wrong latitude. Either way, the cores will help us figure that out.”

IODP is an international research collaboration that coordinates seagoing expeditions to study the history of Earth recorded in sediments and rocks beneath the ocean floor. Scientists apply to participate in IODP expeditions, and participation is open to all scientists from IODP’s member countries. Opportunities exist for researchers, including graduate students, in all shipboard specialties, which include but are not limited to sedimentology, micropaleontology, paleomagnetism, inorganic/organic geochemistry, petrology, petrophysics, microbiology and borehole geophysics. For more information, visit http://iodp.tamu.edu/index.html.

JR in a Minute the Whole Ship!

 

Bounds on Geologically Current Rates of Motion of Groups of Hotspots

 Chengzu Wang, Richard G. Gordon*, and Tuo Zhang

It is widely believed that groups of hotspots in different regions of the world are in relative motion at rates of 10 to 30 mm a–1 or more. Here we present a new method for analyzing geologically current motion between groups of hotspots beneath different plates. In an inversion of 56 globally distributed, equally weighted trends of hotspot tracks, the dispersion is dominated by differences in trend between different plates rather than differences within plates. Nonetheless the rate of hotspot motion perpendicular to the direction of absolute plate motion, vperp, differs significantly from zero for only three of ten plates and then by merely 0.3 to 1.4 mm a–1. The global mean upper bound on |vperp| is 3.2 ±2.7 mm a–1. Therefore, hotspots move slowly and can be used to define a global reference frame for plate motions.

Link: http://onlinelibrary.wiley.com/doi/10.1002/2017GL073430/full

DOI: 10.1002/2017GL073430

The rigid-plate and shrinking-plate hypotheses: Implications for the azimuths of transform faults

Jay Kumar Mishra and Richard G. Gordon*

The rigid-plate hypothesis implies that oceanic lithosphere does not contract horizontally as it cools (hereinafter “rigid plate”). An alternative hypothesis, that vertically averaged tensional thermal stress in the competent lithosphere is fully relieved by horizontal thermal contraction (hereinafter “shrinking plate”), predicts subtly different azimuths for transform faults. The size of the predicted difference is as large as 2.44° with a mean and median of 0.46° and 0.31°, respectively, and changes sign between right-lateral (RL)-slipping and left-lateral (LL)-slipping faults. For the MORVEL transform-fault data set, all six plate pairs with both RL- and LL-slipping faults differ in the predicted sense, with the observed difference averaging 1.4° ± 0.9° (95% confidence limits), which is consistent with the predicted difference of 0.9°. The sum-squared normalized misfit, r, to global transform-fault azimuths is minimized for γ = 0.8 ± 0.4 (95% confidence limits), where γ is the fractional multiple of the predicted difference in azimuth between the shrinking-plate (γ = 1) and rigid-plate (γ = 0) hypotheses. Thus, observed transform azimuths differ significantly between RL-slipping and LL-slipping faults, which is inconsistent with the rigid-plate hypothesis but consistent with the shrinking-plate hypothesis, which indicates horizontal shrinking rates of 2% Ma−1 for newly created lithosphere, 1% Ma−1 for 0.1 Ma old lithosphere, 0.2% Ma−1 for 1 Ma old lithosphere, and 0.02% Ma−1 for 10 Ma old lithosphere, which are orders of magnitude higher than the mean intraplate seismic strain rate of ~10−6 Ma−1 (5 × 10−19 s−1).

 

Link: http://onlinelibrary.wiley.com/doi/10.1002/2015TC003968/full

DOI: 10.1002/2015TC003968