No laughing matter: Nitrous oxide rose at end of last ice age

Researchers measured increases in atmospheric nitrous oxide concentrations about 16,000 to 10,000 years ago using ice from Taylor Glacier in Antarctica. -  Adrian Schilt
Researchers measured increases in atmospheric nitrous oxide concentrations about 16,000 to 10,000 years ago using ice from Taylor Glacier in Antarctica. – Adrian Schilt

Nitrous oxide (N2O) is an important greenhouse gas that doesn’t receive as much notoriety as carbon dioxide or methane, but a new study confirms that atmospheric levels of N2O rose significantly as the Earth came out of the last ice age and addresses the cause.

An international team of scientists analyzed air extracted from bubbles enclosed in ancient polar ice from Taylor Glacier in Antarctica, allowing for the reconstruction of the past atmospheric composition. The analysis documented a 30 percent increase in atmospheric nitrous oxide concentrations from 16,000 years ago to 10,000 years ago. This rise in N2O was caused by changes in environmental conditions in the ocean and on land, scientists say, and contributed to the warming at the end of the ice age and the melting of large ice sheets that then existed.

The findings add an important new element to studies of how Earth may respond to a warming climate in the future. Results of the study, which was funded by the U.S. National Science Foundation and the Swiss National Science Foundation, are being published this week in the journal Nature.

“We found that marine and terrestrial sources contributed about equally to the overall increase of nitrous oxide concentrations and generally evolved in parallel at the end of the last ice age,” said lead author Adrian Schilt, who did much of the work as a post-doctoral researcher at Oregon State University. Schilt then continued to work on the study at the Oeschger Centre for Climate Change Research at the University of Bern in Switzerland.

“The end of the last ice age represents a partial analog to modern warming and allows us to study the response of natural nitrous oxide emissions to changing environmental conditions,” Schilt added. “This will allow us to better understand what might happen in the future.”

Nitrous oxide is perhaps best known as laughing gas, but it is also produced by microbes on land and in the ocean in processes that occur naturally, but can be enhanced by human activity. Marine nitrous oxide production is linked closely to low oxygen conditions in the upper ocean and global warming is predicted to intensify the low-oxygen zones in many of the world’s ocean basins. N2O also destroys ozone in the stratosphere.

“Warming makes terrestrial microbes produce more nitrous oxide,” noted co-author Edward Brook, an Oregon State paleoclimatologist whose research team included Schilt. “Greenhouse gases go up and down over time, and we’d like to know more about why that happens and how it affects climate.”

Nitrous oxide is among the most difficult greenhouse gases to study in attempting to reconstruct the Earth’s climate history through ice core analysis. The specific technique that the Oregon State research team used requires large samples of pristine ice that date back to the desired time of study – in this case, between about 16,000 and 10,000 years ago.

The unusual way in which Taylor Glacier is configured allowed the scientists to extract ice samples from the surface of the glacier instead of drilling deep in the polar ice cap because older ice is transported upward near the glacier margins, said Brook, a professor in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences.

The scientists were able to discern the contributions of marine and terrestrial nitrous oxide through analysis of isotopic ratios, which fingerprint the different sources of N2O in the atmosphere.

“The scientific community knew roughly what the N2O concentration trends were prior to this study,” Brook said, “but these findings confirm that and provide more exact details about changes in sources. As nitrous oxide in the atmosphere continues to increase – along with carbon dioxide and methane – we now will be able to more accurately assess where those contributions are coming from and the rate of the increase.”

Atmospheric N2O was roughly 200 parts per billion at the peak of the ice age about 20,000 years ago then rose to 260 ppb by 10,000 years ago. As of 2014, atmospheric N2Owas measured at about 327 ppb, an increase attributed primarily to agricultural influences.

Although the N2O increase at the end of the last ice age was almost equally attributable to marine and terrestrial sources, the scientists say, there were some differences.

“Our data showed that terrestrial emissions changed faster than marine emissions, which was highlighted by a fast increase of emissions on land that preceded the increase in marine emissions,” Schilt pointed out. “It appears to be a direct response to a rapid temperature change between 15,000 and 14,000 years ago.”

That finding underscores the complexity of analyzing how Earth responds to changing conditions that have to account for marine and terrestrial influences; natural variability; the influence of different greenhouse gases; and a host of other factors, Brook said.

“Natural sources of N2O are predicted to increase in the future and this study will help up test predictions on how the Earth will respond,” Brook said.

Geologists discover ancient buried canyon in South Tibet

This photo shows the Yarlung Tsangpo Valley close to the Tsangpo Gorge, where it is rather narrow and underlain by only about 250 meters of sediments. The mountains in the upper left corner belong to the Namche Barwa massif. Previously, scientists had suspected that the debris deposited by a glacier in the foreground was responsible for the formation of the steep Tsangpo Gorge -- the new discoveries falsify this hypothesis. -  Ping Wang
This photo shows the Yarlung Tsangpo Valley close to the Tsangpo Gorge, where it is rather narrow and underlain by only about 250 meters of sediments. The mountains in the upper left corner belong to the Namche Barwa massif. Previously, scientists had suspected that the debris deposited by a glacier in the foreground was responsible for the formation of the steep Tsangpo Gorge — the new discoveries falsify this hypothesis. – Ping Wang

A team of researchers from Caltech and the China Earthquake Administration has discovered an ancient, deep canyon buried along the Yarlung Tsangpo River in south Tibet, north of the eastern end of the Himalayas. The geologists say that the ancient canyon–thousands of feet deep in places–effectively rules out a popular model used to explain how the massive and picturesque gorges of the Himalayas became so steep, so fast.

“I was extremely surprised when my colleagues, Jing Liu-Zeng and Dirk Scherler, showed me the evidence for this canyon in southern Tibet,” says Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology at Caltech. “When I first saw the data, I said, ‘Wow!’ It was amazing to see that the river once cut quite deeply into the Tibetan Plateau because it does not today. That was a big discovery, in my opinion.”

Geologists like Avouac and his colleagues, who are interested in tectonics–the study of the earth’s surface and the way it changes–can use tools such as GPS and seismology to study crustal deformation that is taking place today. But if they are interested in studying changes that occurred millions of years ago, such tools are not useful because the activity has already happened. In those cases, rivers become a main source of information because they leave behind geomorphic signatures that geologists can interrogate to learn about the way those rivers once interacted with the land–helping them to pin down when the land changed and by how much, for example.

“In tectonics, we are always trying to use rivers to say something about uplift,” Avouac says. “In this case, we used a paleocanyon that was carved by a river. It’s a nice example where by recovering the geometry of the bottom of the canyon, we were able to say how much the range has moved up and when it started moving.”

The team reports its findings in the current issue of Science.

Last year, civil engineers from the China Earthquake Administration collected cores by drilling into the valley floor at five locations along the Yarlung Tsangpo River. Shortly after, former Caltech graduate student Jing Liu-Zeng, who now works for that administration, returned to Caltech as a visiting associate and shared the core data with Avouac and Dirk Scherler, then a postdoc in Avouac’s group. Scherler had previously worked in the far western Himalayas, where the Indus River has cut deeply into the Tibetan Plateau, and immediately recognized that the new data suggested the presence of a paleocanyon.

Liu-Zeng and Scherler analyzed the core data and found that at several locations there were sedimentary conglomerates, rounded gravel and larger rocks cemented together, that are associated with flowing rivers, until a depth of 800 meters or so, at which point the record clearly indicated bedrock. This suggested that the river once carved deeply into the plateau.

To establish when the river switched from incising bedrock to depositing sediments, they measured two isotopes, beryllium-10 and aluminum-26, in the lowest sediment layer. The isotopes are produced when rocks and sediment are exposed to cosmic rays at the surface and decay at different rates once buried, and so allowed the geologists to determine that the paleocanyon started to fill with sediment about 2.5 million years ago.

The researchers’ reconstruction of the former valley floor showed that the slope of the river once increased gradually from the Gangetic Plain to the Tibetan Plateau, with no sudden changes, or knickpoints. Today, the river, like most others in the area, has a steep knickpoint where it meets the Himalayas, at a place known as the Namche Barwa massif. There, the uplift of the mountains is extremely rapid (on the order of 1 centimeter per year, whereas in other areas 5 millimeters per year is more typical) and the river drops by 2 kilometers in elevation as it flows through the famous Tsangpo Gorge, known by some as the Yarlung Tsangpo Grand Canyon because it is so deep and long.

Combining the depth and age of the paleocanyon with the geometry of the valley, the geologists surmised that the river existed in this location prior to about 3 million years ago, but at that time, it was not affected by the Himalayas. However, as the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging on the river. Suddenly, about 2.5 million years ago, a rapidly uplifting section of the mountain range got in the river’s way, damming it, and the canyon subsequently filled with sediment.

“This is the time when the Namche Barwa massif started to rise, and the gorge developed,” says Scherler, one of two lead authors on the paper and now at the GFZ German Research Center for Geosciences in Potsdam, Germany.

That picture of the river and the Tibetan Plateau, which involves the river incising deeply into the plateau millions of years ago, differs quite a bit from the typically accepted geologic vision. Typically, geologists believe that when rivers start to incise into a plateau, they eat at the edges, slowly making their way into the plateau over time. However, the rivers flowing across the Himalayas all have strong knickpoints and have not incised much at all into the Tibetan Plateau. Therefore, the thought has been that the rapid uplift of the Himalayas has pushed the rivers back, effectively pinning them, so that they have not been able to make their way into the plateau. But that explanation does not work with the newly discovered paleocanyon.

The team’s new hypothesis also rules out a model that has been around for about 15 years, called tectonic aneurysm, which suggests that the rapid uplift seen at the Namche Barwa massif was triggered by intense river incision. In tectonic aneurysm, a river cuts down through the earth’s crust so fast that it causes the crust to heat up, making a nearby mountain range weaker and facilitating uplift.

The model is popular among geologists, and indeed Avouac himself published a modeling paper in 1996 that showed the viability of the mechanism. “But now we have discovered that the river was able to cut into the plateau way before the uplift happened,” Avouac says, “and this shows that the tectonic aneurysm model was actually not at work here. The rapid uplift is not a response to river incision.”

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The other lead author on the paper, “Tectonic control of the Yarlung Tsangpo Gorge, revealed by a 2.5 Myr old buried canyon in Southern Tibet,” is Ping Wang of the State Key Laboratory of Earthquake Dynamics, in Beijing, China. Additional authors include Jürgen Mey, of the University of Potsdam, in Germany; and Yunda Zhang and Dingguo Shi of the Chengdu Engineering Corporation, in China. The work was supported by the National Natural Science Foundation of China, the State Key Laboratory for Earthquake Dynamics, and the Alexander von Humboldt Foundation.

Climate capers of the past 600,000 years

The researchers remove samples from a core segment taken from Lake Van at the center for Marine environmental sciences MARUM in Bremen, where all of the cores from the PALEOVAN project are stored. -  Photo: Nadine Pickarski/Uni Bonn
The researchers remove samples from a core segment taken from Lake Van at the center for Marine environmental sciences MARUM in Bremen, where all of the cores from the PALEOVAN project are stored. – Photo: Nadine Pickarski/Uni Bonn

If you want to see into the future, you have to understand the past. An international consortium of researchers under the auspices of the University of Bonn has drilled deposits on the bed of Lake Van (Eastern Turkey) which provide unique insights into the last 600,000 years. The samples reveal that the climate has done its fair share of mischief-making in the past. Furthermore, there have been numerous earthquakes and volcanic eruptions. The results of the drilling project also provide a basis for assessing the risk of how dangerous natural hazards are for today’s population. In a special edition of the highly regarded publication Quaternary Science Reviews, the scientists have now published their findings in a number of journal articles.

In the sediments of Lake Van, the lighter-colored, lime-containing summer layers are clearly distinguishable from the darker, clay-rich winter layers — also called varves. In 2010, from a floating platform an international consortium of researchers drilled a 220 m deep sediment profile from the lake floor at a water depth of 360 m and analyzed the varves. The samples they recovered are a unique scientific treasure because the climate conditions, earthquakes and volcanic eruptions of the past 600,000 years can be read in outstanding quality from the cores.

The team of scientists under the auspices of the University of Bonn has analyzed some 5,000 samples in total. “The results show that the climate over the past hundred thousand years has been a roller coaster. Within just a few decades, the climate could tip from an ice age into a warm period,” says Doctor Thomas Litt of the University of Bonn’s Steinmann Institute and spokesman for the PALEOVAN international consortium of researchers. Unbroken continental climate archives from the ice age which encompass several hundred thousand years are extremely rare on a global scale. “There has never before in all of the Middle East and Central Asia been a continental drilling operation going so far back into the past,” says Doctor Litt. In the northern hemisphere, climate data from ice-cores drilled in Greenland encompass the last 120,000 years. The Lake Van project closes a gap in the scientific climate record.

The sediments reveal six cycles of cold and warm periods


Scientists found evidence for a total of six cycles of warm and cold periods in the sediments of Lake Van. The University of Bonn paleoecologist and his colleagues analyzed the pollen preserved in the sediments. Under a microscope they were able to determine which plants around the eastern Anatolian Lake the pollen came from. “Pollen is amazingly durable and is preserved over very long periods when protected in the sediments,” Doctor Litt explained. Insight into the age of the individual layers was gleaned through radiometric age measurements that use the decay of radioactive elements as a geologic clock. Based on the type of pollen and the age, the scientists were able to determine when oak forests typical of warm periods grew around Lake Van and when ice-age steppe made up of grasses, mugwort and goosefoot surrounded the lake.

Once they determine the composition of the vegetation present and the requirements of the plants, the scientists can reconstruct with a high degree of accuracy the temperature and amount of rainfall during different epochs. These analyses enable the team of researchers to read the varves of Lake Van like thousands of pages of an archive. With these data, the team was able to demonstrate that fluctuations in climate were due in large part to periodic changes in the Earth’s orbit parameters and the commensurate changes in solar insolation levels. However, the influence of North Atlantic currents was also evident. “The analysis of the Lake Van sediments has presented us with an image of how an ecosystem reacts to abrupt changes in climate. This fundamental data will help us to develop potential scenarios of future climate effects,” says Doctor Litt.

Risks of earthquakes and volcanic eruptions in the region of Van

Such risk assessments can also be made for other natural forces. “Deposits of volcanic ash with thicknesses of up to 10 m in the Lake Van sediments show us that approximately 270,000 years ago there was a massive eruption,” the University of Bonn paleoecologist said. The team struck some 300 different volcanic events in its drillings. Statistically, that corresponds to one explosive volcanic eruption in the region every 2000 years. Deformations in the sediment layers show that the area is subject to frequent, strong earthquakes. “The area around Lake Van is very densely populated. The data from the core samples show that volcanic activity and earthquakes present a relatively high risk for the region,” Doctor Litt says. According to media reports, in 2011 a 7.2 magnitude earthquake in the Van province claimed the lives of more than 500 people and injured more than 2,500.

Publication: “Results from the PALEOVAN drilling project: A 600,000 year long continental archive in the Near East”, Quaternary Science Reviews, Volume 104, online publication: (http://dx.doi.org/10.1016/j.quascirev.2014.09.026)

Massive geographic change may have triggered explosion of animal life

A new analysis from The University of Texas at Austin's Institute for Geophysics suggests a deep oceanic gateway, shown in blue, developed between the Pacific and Iapetus oceans immediately before the Cambrian sea level rise and explosion of life in the fossil record, isolating Laurentia from the supercontinent Gondwanaland. -  Ian Dalziel
A new analysis from The University of Texas at Austin’s Institute for Geophysics suggests a deep oceanic gateway, shown in blue, developed between the Pacific and Iapetus oceans immediately before the Cambrian sea level rise and explosion of life in the fossil record, isolating Laurentia from the supercontinent Gondwanaland. – Ian Dalziel

A new analysis of geologic history may help solve the riddle of the “Cambrian explosion,” the rapid diversification of animal life in the fossil record 530 million years ago that has puzzled scientists since the time of Charles Darwin.

A paper by Ian Dalziel of The University of Texas at Austin’s Jackson School of Geosciences, published in the November issue of Geology, a journal of the Geological Society of America, suggests a major tectonic event may have triggered the rise in sea level and other environmental changes that accompanied the apparent burst of life.

The Cambrian explosion is one of the most significant events in Earth’s 4.5-billion-year history. The surge of evolution led to the sudden appearance of almost all modern animal groups. Fossils from the Cambrian explosion document the rapid evolution of life on Earth, but its cause has been a mystery.

The sudden burst of new life is also called “Darwin’s dilemma” because it appears to contradict Charles Darwin’s hypothesis of gradual evolution by natural selection.

“At the boundary between the Precambrian and Cambrian periods, something big happened tectonically that triggered the spreading of shallow ocean water across the continents, which is clearly tied in time and space to the sudden explosion of multicellular, hard-shelled life on the planet,” said Dalziel, a research professor at the Institute for Geophysics and a professor in the Department of Geological Sciences.

Beyond the sea level rise itself, the ancient geologic and geographic changes probably led to a buildup of oxygen in the atmosphere and a change in ocean chemistry, allowing more complex life-forms to evolve, he said.

The paper is the first to integrate geological evidence from five present-day continents — North America, South America, Africa, Australia and Antarctica — in addressing paleogeography at that critical time.

Dalziel proposes that present-day North America was still attached to the southern continents until sometime into the Cambrian period. Current reconstructions of the globe’s geography during the early Cambrian show the ancient continent of Laurentia — the ancestral core of North America — as already having separated from the supercontinent Gondwanaland.

In contrast, Dalziel suggests the development of a deep oceanic gateway between the Pacific and Iapetus (ancestral Atlantic) oceans isolated Laurentia in the early Cambrian, a geographic makeover that immediately preceded the global sea level rise and apparent explosion of life.

“The reason people didn’t make this connection before was because they hadn’t looked at all the rock records on the different present-day continents,” he said.

The rock record in Antarctica, for example, comes from the very remote Ellsworth Mountains.

“People have wondered for a long time what rifted off there, and I think it was probably North America, opening up this deep seaway,” Dalziel said. “It appears ancient North America was initially attached to Antarctica and part of South America, not to Europe and Africa, as has been widely believed.”

Although the new analysis adds to evidence suggesting a massive tectonic shift caused the seas to rise more than half a billion years ago, Dalziel said more research is needed to determine whether this new chain of paleogeographic events can truly explain the sudden rise of multicellular life in the fossil record.

“I’m not claiming this is the ultimate explanation of the Cambrian explosion,” Dalziel said. “But it may help to explain what was happening at that time.”

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To read the paper go to http://geology.gsapubs.org/content/early/2014/09/25/G35886.1.abstract

Rare 2.5-billion-year-old rocks reveal hot spot of sulfur-breathing bacteria

Gold miners prospecting in a mountainous region of Brazil drilled this 590-foot cylinder of bedrock from the Neoarchaean Eon, which provides rare evidence of conditions on Earth 2.5 billion years ago. -  Alan J. Kaufman
Gold miners prospecting in a mountainous region of Brazil drilled this 590-foot cylinder of bedrock from the Neoarchaean Eon, which provides rare evidence of conditions on Earth 2.5 billion years ago. – Alan J. Kaufman

Wriggle your toes in a marsh’s mucky bottom sediment and you’ll probably inhale a rotten egg smell, the distinctive odor of hydrogen sulfide gas. That’s the biochemical signature of sulfur-using bacteria, one of Earth’s most ancient and widespread life forms.

Among scientists who study the early history of our 4.5 billion-year-old planet, there is a vigorous debate about the evolution of sulfur-dependent bacteria. These simple organisms arose at a time when oxygen levels in the atmosphere were less than one-thousandth of what they are now. Living in ocean waters, they respired (or breathed in) sulfate, a form of sulfur, instead of oxygen. But how did that sulfate reach the ocean, and when did it become abundant enough for living things to use it?

New research by University of Maryland geology doctoral student Iadviga Zhelezinskaia offers a surprising answer. Zhelezinskaia is the first researcher to analyze the biochemical signals of sulfur compounds found in 2.5 billion-year-old carbonate rocks from Brazil. The rocks were formed on the ocean floor in a geologic time known as the Neoarchaean Eon. They surfaced when prospectors drilling for gold in Brazil punched a hole into bedrock and pulled out a 590-foot-long core of ancient rocks.

In research published Nov. 7, 2014 in the journal Science, Zhelezinskaia and three co-authors–physicist John Cliff of the University of Western Australia and geologists Alan Kaufman and James Farquhar of UMD–show that bacteria dependent on sulfate were plentiful in some parts of the Neoarchaean ocean, even though sea water typically contained about 1,000 times less sulfate than it does today.

“The samples Iadviga measured carry a very strong signal that sulfur compounds were consumed and altered by living organisms, which was surprising,” says Farquhar. “She also used basic geochemical models to give an idea of how much sulfate was in the oceans, and finds the sulfate concentrations are very low, much lower than previously thought.”

Geologists study sulfur because it is abundant and combines readily with other elements, forming compounds stable enough to be preserved in the geologic record. Sulfur has four naturally occurring stable isotopes–atomic signatures left in the rock record that scientists can use to identify the elements’ different forms. Researchers measuring sulfur isotope ratios in a rock sample can learn whether the sulfur came from the atmosphere, weathering rocks or biological processes. From that information about the sulfur sources, they can deduce important information about the state of the atmosphere, oceans, continents and biosphere when those rocks formed.

Farquhar and other researchers have used sulfur isotope ratios in Neoarchaean rocks to show that soon after this period, Earth’s atmosphere changed. Oxygen levels soared from just a few parts per million to almost their current level, which is around 21 percent of all the gases in the atmosphere. The Brazilian rocks Zhelezinskaia sampled show only trace amounts of oxygen, a sign they were formed before this atmospheric change.

With very little oxygen, the Neoarchaean Earth was a forbidding place for most modern life forms. The continents were probably much drier and dominated by volcanoes that released sulfur dioxide, carbon dioxide, methane and other greenhouse gases. Temperatures probably ranged between 0 and 100 degrees Celsius (32 to 212 degrees Fahrenheit), warm enough for liquid oceans to form and microbes to grow in them.

Rocks 2.5 billion years old or older are extremely rare, so geologists’ understanding of the Neoarchaean are based on a handful of samples from a few small areas, such as Western Australia, South Africa and Brazil. Geologists theorize that Western Australia and South Africa were once part of an ancient supercontinent called Vaalbara. The Brazilian rock samples are comparable in age, but they may not be from the same supercontinent, Zhelezinskaia says.

Most of the Neoarchaean rocks studied are from Western Australia and South Africa and are black shale, which forms when fine dust settles on the sea floor. The Brazilian prospector’s core contains plenty of black shale and a band of carbonate rock, formed below the surface of shallow seas, in a setting that probably resembled today’s Bahama Islands. Black shale usually contains sulfur-bearing pyrite, but carbonate rock typically does not, so geologists have not focused on sulfur signals in Neoarchaean carbonate rocks until now.

Zhelezinskaia “chose to look at a type of rock that others generally avoided, and what she saw was spectacularly different,” said Kaufman. “It really opened our eyes to the implications of this study.”

The Brazilian carbonate rocks’ isotopic ratios showed they formed in ancient seabed containing sulfate from atmospheric sources, not continental rock. And the isotopic ratios also showed that Neoarchaean bacteria were plentiful in the sediment, respiring sulfate and emitted hydrogen sulfide–the same process that goes on today as bacteria recycle decaying organic matter into minerals and gases.

How could the sulfur-dependent bacteria have thrived during a geologic time when sulfur levels were so low? “It seems that they were in shallow water, where evaporation may have been high enough to concentrate the sulfate, and that would make it abundant enough to support the bacteria,” says Zhelezinskaia.

Zhelezinskaia is now analyzing carbonate rocks of the same age from Western Australia and South Africa, to see if the pattern holds true for rocks formed in other shallow water environments. If it does, the results may change scientists’ understanding of one of Earth’s earliest biological processes.

“There is an ongoing debate about when sulfate-reducing bacteria arose and how that fits into the evolution of life on our planet,” says Farquhar. “These rocks are telling us the bacteria were there 2.5 billion years ago, and they were doing something significant enough that we can see them today.”

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This research was supported by the Fulbright Program (Grantee ID 15110620), the NASA Astrobiology Institute (Grant No. NNA09DA81A) and the National Science Foundation Frontiers in Earth-System Dynamics program (Grant No. 432129). The content of this article does not necessarily reflect the views of these organizations.

“Large sulfur isotope fractionations associated with Neoarchaean microbial sulfate reductions,” Iadviga Zhelezinskaia, Alan J. Kaufman, James Farquhar and John Cliff, was published Nov. 7, 2014 in Science. Download the abstract after 2 p.m. U.S. Eastern time, Nov. 6, 2014: http://www.sciencemag.org/lookup/doi/10.1126/science.1256211

James Farquhar home page

http://www.geol.umd.edu/directory.php?id=13

Alan J. Kaufman home page

http://www.geol.umd.edu/directory.php?id=15

Iadviga Zhelezinskaia home page

http://www.geol.umd.edu/directory.php?id=66

Media Relations Contact: Abby Robinson, 301-405-5845, abbyr@umd.edu

Writer: Heather Dewar

New study shows 3 abrupt pulse of CO2 during last deglaciation

A new study shows that the rise of atmospheric carbon dioxide that contributed to the end of the last ice age more than 10,000 years ago did not occur gradually, but was characterized by three “pulses” in which C02 rose abruptly.

Scientists are not sure what caused these abrupt increases, during which C02 levels rose about 10-15 parts per million – or about 5 percent per episode – over a period of 1-2 centuries. It likely was a combination of factors, they say, including ocean circulation, changing wind patterns, and terrestrial processes.

The finding is important, however, because it casts new light on the mechanisms that take the Earth in and out of ice age regimes. Results of the study, which was funded by the National Science Foundation, appear this week in the journal Nature.

“We used to think that naturally occurring changes in carbon dioxide took place relatively slowly over the 10,000 years it took to move out of the last ice age,” said Shaun Marcott, lead author on the article who conducted his study as a post-doctoral researcher at Oregon State University. “This abrupt, centennial-scale variability of CO2 appears to be a fundamental part of the global carbon cycle.”

Some previous research has hinted at the possibility that spikes in atmospheric carbon dioxide may have accelerated the last deglaciation, but that hypothesis had not been resolved, the researchers say. The key to the new finding is the analysis of an ice core from the West Antarctic that provided the scientists with an unprecedented glimpse into the past.

Scientists studying past climate have been hampered by the limitations of previous ice cores. Cores from Greenland, for example, provide unique records of rapid climate events going back 120,000 years – but high concentrations of impurities don’t allow researchers to accurately determine atmospheric carbon dioxide records. Antarctic ice cores have fewer impurities, but generally have had lower “temporal resolution,” providing less detailed information about atmospheric CO2.

However, a new core from West Antarctica, drilled to a depth of 3,405 meters in 2011 and spanning the last 68,000 years, has “extraordinary detail,” said Oregon State paleoclimatologist Edward Brook, a co-author on the Nature study and an internationally recognized ice core expert. Because the area where the core was taken gets high annual snowfall, he said, the new ice core provides one of the most detailed records of atmospheric CO2.

“It is a remarkable ice core and it clearly shows distinct pulses of carbon dioxide increase that can be very reliably dated,” Brook said. “These are some of the fastest natural changes in CO2 we have observed, and were probably big enough on their own to impact the Earth’s climate.

“The abrupt events did not end the ice age by themselves,” Brook added. “That might be jumping the gun a bit. But it is fair to say that the natural carbon cycle can change a lot faster than was previously thought – and we don’t know all of the mechanisms that caused that rapid change.”

The researchers say that the increase in atmospheric CO2 from the peak of the last ice age to complete deglaciation was about 80 parts per million, taking place over 10,000 years. Thus, the finding that 30-45 ppm of the increase happened in just a few centuries was significant.

The overall rise of atmospheric carbon dioxide during the last deglaciation was thought to have been triggered by the release of CO2 from the deep ocean – especially the Southern Ocean. However, the researchers say that no obvious ocean mechanism is known that would trigger rises of 10-15 ppm over a time span as short as one to two centuries.

“The oceans are simply not thought to respond that fast,” Brook said. “Either the cause of these pulses is at least part terrestrial, or there is some mechanism in the ocean system we don’t yet know about.”

One reason the researchers are reluctant to pin the end of the last ice age solely on CO2 increases is that other processes were taking place, according to Marcott, who recently joined the faculty of the University of Wisconsin-Madison.

“At the same time CO2 was increasing, the rate of methane in the atmosphere was also increasing at the same or a slightly higher rate,” Marcott said. “We also know that during at least two of these pulses, the Atlantic Meridional Overturning Circulation changed as well. Changes in the ocean circulation would have affected CO2 – and indirectly methane, by impacting global rainfall patterns.”

“The Earth is a big coupled system,” he added, “and there are many pieces to the puzzle. The discovery of these strong, rapid pulses of CO2 is an important piece.”

Textbook theory behind volcanoes may be wrong

In the typical textbook picture, volcanoes, such as those that are forming the Hawaiian islands, erupt when magma gushes out as narrow jets from deep inside Earth. But that picture is wrong, according to a new study from researchers at Caltech and the University of Miami in Florida.

New seismology data are now confirming that such narrow jets don’t actually exist, says Don Anderson, the Eleanor and John R. McMillian Professor of Geophysics, Emeritus, at Caltech. In fact, he adds, basic physics doesn’t support the presence of these jets, called mantle plumes, and the new results corroborate those fundamental ideas.

“Mantle plumes have never had a sound physical or logical basis,” Anderson says. “They are akin to Rudyard Kipling’s ‘Just So Stories’ about how giraffes got their long necks.”

Anderson and James Natland, a professor emeritus of marine geology and geophysics at the University of Miami, describe their analysis online in the September 8 issue of the Proceedings of the National Academy of Sciences.

According to current mantle-plume theory, Anderson explains, heat from Earth’s core somehow generates narrow jets of hot magma that gush through the mantle and to the surface. The jets act as pipes that transfer heat from the core, and how exactly they’re created isn’t clear, he says. But they have been assumed to exist, originating near where the Earth’s core meets the mantle, almost 3,000 kilometers underground-nearly halfway to the planet’s center. The jets are theorized to be no more than about 300 kilometers wide, and when they reach the surface, they produce hot spots.

While the top of the mantle is a sort of fluid sludge, the uppermost layer is rigid rock, broken up into plates that float on the magma-bearing layers. Magma from the mantle beneath the plates bursts through the plate to create volcanoes. As the plates drift across the hot spots, a chain of volcanoes forms-such as the island chains of Hawaii and Samoa.

“Much of solid-Earth science for the past 20 years-and large amounts of money-have been spent looking for elusive narrow mantle plumes that wind their way upward through the mantle,” Anderson says.

To look for the hypothetical plumes, researchers analyze global seismic activity. Everything from big quakes to tiny tremors sends seismic waves echoing through Earth’s interior. The type of material that the waves pass through influences the properties of those waves, such as their speeds. By measuring those waves using hundreds of seismic stations installed on the surface, near places such as Hawaii, Iceland, and Yellowstone National Park, researchers can deduce whether there are narrow mantle plumes or whether volcanoes are simply created from magma that’s absorbed in the sponge-like shallower mantle.

No one has been able to detect the predicted narrow plumes, although the evidence has not been conclusive. The jets could have simply been too thin to be seen, Anderson says. Very broad features beneath the surface have been interpreted as plumes or super-plumes, but, still, they’re far too wide to be considered narrow jets.

But now, thanks in part to more seismic stations spaced closer together and improved theory, analysis of the planet’s seismology is good enough to confirm that there are no narrow mantle plumes, Anderson and Natland say. Instead, data reveal that there are large, slow, upward-moving chunks of mantle a thousand kilometers wide.

In the mantle-plume theory, Anderson explains, the heat that is transferred upward via jets is balanced by the slower downward motion of cooled, broad, uniform chunks of mantle. The behavior is similar to that of a lava lamp, in which blobs of wax are heated from below and then rise before cooling and falling. But a fundamental problem with this picture is that lava lamps require electricity, he says, and that is an outside energy source that an isolated planet like Earth does not have.

The new measurements suggest that what is really happening is just the opposite: Instead of narrow jets, there are broad upwellings, which are balanced by narrow channels of sinking material called slabs. What is driving this motion is not heat from the core, but cooling at Earth’s surface. In fact, Anderson says, the behavior is the regular mantle convection first proposed more than a century ago by Lord Kelvin. When material in the planet’s crust cools, it sinks, displacing material deeper in the mantle and forcing it upward.

“What’s new is incredibly simple: upwellings in the mantle are thousands of kilometers across,” Anderson says. The formation of volcanoes then follows from plate tectonics-the theory of how Earth’s plates move and behave. Magma, which is less dense than the surrounding mantle, rises until it reaches the bottom of the plates or fissures that run through them. Stresses in the plates, cracks, and other tectonic forces can squeeze the magma out, like how water is squeezed out of a sponge. That magma then erupts out of the surface as volcanoes. The magma comes from within the upper 200 kilometers of the mantle and not thousands of kilometers deep, as the mantle-plume theory suggests.

“This is a simple demonstration that volcanoes are the result of normal broad-scale convection and plate tectonics,” Anderson says. He calls this theory “top-down tectonics,” based on Kelvin’s initial principles of mantle convection. In this picture, the engine behind Earth’s interior processes is not heat from the core but cooling at the planet’s surface. This cooling and plate tectonics drives mantle convection, the cooling of the core, and Earth’s magnetic field. Volcanoes and cracks in the plate are simply side effects.

The results also have an important consequence for rock compositions-notably the ratios of certain isotopes, Natland says. According to the mantle-plume idea, the measured compositions derive from the mixing of material from reservoirs separated by thousands of kilometers in the upper and lower mantle. But if there are no mantle plumes, then all of that mixing must have happened within the upwellings and nearby mantle in Earth’s top 1,000 kilometers.

The paper is titled “Mantle updrafts and mechanisms of oceanic volcanism.”

Mantle plumes crack continents

In some parts of the Earth, material rises upwards like a column from the boundary layer of the Earth’s core and the lower mantel to just below the Earth’s crust hundreds of kilometres above. Halted by the resistance of the hard crust and lithospheric mantle, the flow of material becomes wider, taking on a mushroom-like shape. Specialists call these magma columns “mantle plumes” or simply “plumes”.

Are mantel plumes responsible for the African rift system?


Geologists believe that plumes are not just responsible for creating volcanoes outside of tectonically active areas – they can also break up continents. The scientists offer the Danakil Depression (the lowlands in the Ethiopia-Eritrea-Djibouti triangle) as an example of this. This “triple junction” is extremely tectonically and volcanically active. Geologists believe that the so-called Afar plume is rising up below it and has created a rift system that forks into the Red Sea, the Gulf of Aden and Africa’s Great Rift Valley. However, the sheer length of time required, geologically speaking, for this process to take place, means that nobody is able to confirm or disprove with absolute certainty that the force of a plume causes continental breakup.

Simulations becoming more realistic


Evgueni Burov, a Professor at the University of Paris VI, and Taras Gerya, Professor of Geophysics at ETH Zurich, have now taken a step closer to solving this geological mystery with a new computer model. Their paper has recently been published in the journal Nature. The two researchers conducted numerical experiments to reproduce the Earth’s surface in high-resolution 3D.

These simulations show that the rising flow of material is strong enough to cause continental breakup if the tectonic plate is under (weak) tensile stress. “The force exerted by a plume on a plate is actually too weak to break it up,” says Gerya. In experiments using simple models, the researchers allowed the plumes to hit an unstressed plate, which did not cause it to break, but merely formed a round hump. However, when the geophysicists modelled the same process with a plate under weak tensile stress, it broke apart, forming a crevice and rift system like the ones found around the world.

“The process can be compared to a taut piece of plastic film. Weak, pointed force is enough to tear the film, but if the film is not pulled taut, it is extremely difficult to tear.” This mechanism has already been proposed in the past as a possible model for explaining continental breakup, but had never been outlined in plausible terms before now.

First high-resolution simulations

“We are the first to create such a high-resolution model which demonstrates how a plume interacts with a plate under tensile stress,” says Gerya. Fast and powerful computers and stable algorithms programmed by the scientists themselves were required for the simulations. The researchers benefited from technical advances made and experience accumulated by the ETH professor in this field over the past ten years.

In the model, the deformations are created quickly from a geological point of view. Rift systems several kilometres deep and more than a thousand kilometres long can form after “just” two million years. The processes are therefore up to ten times faster than tectonic processes such as subduction and 50 times faster than the Alpine orogeny, for example.

Disputed idea


The idea of mantel plumes is widely disputed, with some researchers denying that they even exist. “I think it is much more likely that they do exist,” says Gerya. As is often the case in geology, especially when researching the Earth’s interior, such processes and phenomena like the existence of plumes cannot be observed directly. Furthermore, the periods over which geological processes take place are far too long for humans to experience first-hand. “So far, we have only been able to observe the effects that plumes have on the Earth’s surface and on the propagation of seismic waves in the Earth’s interior.”

The scientists are therefore reliant on good, realistic models that show the processes in a geological time lapse. How realistic the calculated simulations are depends on the parameters used. The plume-plate interaction model incorporated physical laws, the characteristics of materials in the Earth’s crust and mantle, and temperature and pressure conditions. “We know the rules, but humans generally lack the intuition to identify how they interact on geological timescales.”

Composition of Earth’s mantle revisited

Research published recently in Science suggested that the makeup of the Earth’s lower mantle, which makes up the largest part of the Earth by volume, is significantly different than previously thought.

Understanding the composition of the mantle is essential to seismology, the study of earthquakes and movement below the Earth’s surface, and should shed light on unexplained seismic phenomena observed there.

Though humans haven’t yet managed to drill further than seven and a half miles into the Earth, we’ve built a comprehensive picture of what’s beneath our feet through calculations and limited observation. We all live atop the crust, the thin outer layer; just beneath is the mantle, outer core and finally inner core. The lower portion of the mantle is the largest layer – stretching from 400 to 1,800 miles below the surface – and gives off the most heat. Until now, the entire lower mantle was thought to be composed of the same mineral throughout: ferromagnesian silicate, arranged in a type of structure called perovskite.

The pressure and heat of the lower mantle is intense – more than 3,500° Fahrenheit. Materials may have very different properties at these conditions; structures may exist there that would collapse at the surface.

To simulate these conditions, researchers use special facilities at the Advanced Photon Source, where they shine high-powered lasers to heat up the sample inside a pressure cell made of a pair of diamonds. Then they aim powerful beams of X-rays at the sample, which hit and scatter in all directions. By gathering the scatter data, scientists can reconstruct how the atoms in the sample were arranged.

The team found that at conditions that exist below about 1,200 miles underground, the ferromagnesian silicate perovskite actually breaks into two separate phases. One contains nearly no iron, while the other is full of iron. The iron-rich phase, called the H-phase, is much more stable under these conditions.

“We still don’t fully understand the chemistry of the H-phase,” said lead author and Carnegie Institution of Washington scientist Li Zhang. “But this finding indicates that all geodynamic models need to be reconsidered to take the H-phase into account. And there could be even more unidentified phases down there in the lower mantle as well, waiting to be identified.”

The facilities at Argonne’s Advanced Photon Source were key to the findings, said Carnegie scientist Yue Meng, also an author on the paper. “Recent technological advances at our beamline allowed us to create the conditions to simulate these intense temperatures and pressures and probe the changes in chemistry and structure of the sample in situ,” she said.

“What distinguished this work was the exceptional attention to detail in every aspect of the research – it demonstrates a new level for high-pressure research,” Meng added.

Rewriting the history of volcanic forcing during the past 2,000 years

Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a  DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI's ultra-trace ice core analytical laboratory. -  M. Sigl
Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI’s ultra-trace ice core analytical laboratory. – M. Sigl

A team of scientists led by Michael Sigl and Joe McConnell of Nevada’s Desert Research Institute (DRI) has completed the most accurate and precise reconstruction to date of historic volcanic sulfate emissions in the Southern Hemisphere.

The new record, described in a manuscript published today in the online edition of Nature Climate Change, is derived from a large number of individual ice cores collected at various locations across Antarctica and is the first annually resolved record extending through the Common Era (the last 2,000 years of human history).

“This record provides the basis for a dramatic improvement in existing reconstructions of volcanic emissions during recent centuries and millennia,” said the report’s lead author Michael Sigl, a postdoctoral fellow and specialist in DRI’s unique ultra-trace ice core analytical laboratory, located on the Institute’s campus in Reno, Nevada.

These reconstructions are critical to accurate model simulations used to assess past natural and anthropogenic climate forcing. Such model simulations underpin environmental policy decisions including those aimed at regulating greenhouse gas and aerosol emissions to mitigate projected global warming.

Powerful volcanic eruptions are one of the most significant causes of climate variability in the past because of the large amounts of sulfur dioxide they emit, leading to formation of microscopic particles known as volcanic sulfate aerosols. These aerosols reflect more of the sun’s radiation back to space, cooling the Earth. Past volcanic events are measured through sulfate deposition records found in ice cores and have been linked to short-term global and regional cooling.

This effort brought together the most extensive array of ice core sulfate data in the world, including the West Antarctic Ice Sheet (WAIS) Divide ice core – arguably the most detailed record of volcanic sulfate in the Southern Hemisphere. In total, the study incorporated 26 precisely synchronized ice core records collected in an array of 19 sites from across Antarctica.

“This work is the culmination of more than a decade of collaborative ice core collection and analysis in our lab here at DRI,” said Joe McConnell, a DRI research professor who developed the continuous-flow analysis system used to analyze the ice cores.

McConnell, a member of several research teams that collected the cores (including the 2007-2009 Norwegian-American Scientific Traverse of East Antarctica and the WAIS Divide project that reached a depth of 3,405 meters in 2011), added, “The new record identifies 116 individual volcanic events during the last 2000 years.”

“Our new record completes the period from years 1 to 500 AD, for which there were no reconstructions previously, and significantly improves the record for years 500 to 1500 AD,” Sigl added. This new record also builds on DRI’s previous work as part of the international Past Global Changes (PAGES) effort to help reconstruct an accurate 2,000-year-long global temperature for individual continents.

This study involved collaborating researchers from the United States, Japan, Germany, Norway, Australia, and Italy. International collaborators contributed ice core samples for analysis at DRI as well as ice core measurements and climate modeling.

According to Yuko Motizuki from RIKEN (Japan’s largest comprehensive research institution), “The collaboration between DRI, National Institute of Polar Research (NIPR), and RIKEN just started in the last year, and we were very happy to be able to use the two newly obtained ice core records taken from Dome Fuji, where the volcanic signals are clearly visible. This is because precipitation on the site mainly contains stratospheric components.” Dr. Motizuki analyzed the samples collected by the Japanese Antarctic Research Expedition.

Simulations of volcanic sulfate transport performed with a coupled aerosol-climate model were compared to the ice core observations and used to investigate spatial patterns of sulfate deposition to Antarctica.

“Both observations and model results show that not all eruptions lead to the same spatial pattern of sulfate deposition,” said Matthew Toohey from the German institute GEOMAR Helmholtz Centre for Ocean Research Kiel. He added, “Spatial variability in sulfate deposition means that the accuracy of volcanic sulfate reconstructions depends strongly on having a sufficient number of ice core records from as many different regions of Antarctica as possible.”

With such an accurately synchronized and robust array, Sigl and his colleagues were able to revise reconstructions of past volcanic aerosol loading that are widely used today in climate model simulations. Most notably, the research found that the two largest volcanic eruptions in recent Earth history (Samalas in 1257 and Kuwae in 1458) deposited 30 to 35 percent less sulfate in Antarctica, suggesting that these events had a weaker cooling effect on global climate than previously thought.