US urban trees store carbon, provide billions in economic value

From New York City’s Central Park to Golden Gate Park in San Francisco, America’s urban forests store an estimated 708 million tons of carbon, an environmental service with an estimated value of $50 billion, according to a recent U.S. Forest Service study.

Annual net carbon uptake by these trees is estimated at 21 million tons and $1.5 billion in economic benefit.

In the study published recently in the journal Environmental Pollution, Dave Nowak, a research forester with the U.S. Forest Service’s Northern Research Station, and his colleagues used urban tree field data from 28 cities and six states and national tree cover data to estimate total carbon storage in the nation’s urban areas.

“With expanding urbanization, city trees and forests are becoming increasingly important to sustain the health and well-being of our environment and our communities,” said U.S. Forest Service Chief Tom Tidwell. “Carbon storage is just one of the many benefits provided by the hardest working trees in America. I hope this study will encourage people to look at their neighborhood trees a little differently, and start thinking about ways they can help care for their own urban forests.”

Tens of thousands of people volunteered to plant and care for trees for Earth Day and Arbor Day this year, but there are opportunities all year long. To learn about volunteer opportunities near your home, visit the Arbor Day Foundation.

The Forest Service partners with organizations like the Arbor Day Foundation and participates in programs like Tree City USA to recognize and inspire cities in their efforts to improve their urban forests. Additionally the Forest Service is active in more than 7,000 communities across the U.S., helping them to better plan and manage their urban forests.

Nationally, carbon storage by trees in forestlands was estimated at 22.3 billion tons in a 2008 Forest Service study; additional carbon storage by urban trees bumps that to an estimated 22.7 billion tons.

Carbon storage and sequestration rates vary among states based on the amount of urban tree cover and growing conditions. States in forested regions typically have the highest percentage of urban tree cover. States with the greatest amount of carbon stored by trees in urban areas are Texas (49.8 million tons), Florida (47.3 million tons), Georgia (42.4 million tons), Massachusetts (39.6 million tons) and North Carolina (37.5 million tons).

The total amount of carbon stored and sequestered in urban areas could increase in the future as urban land expands. Urban areas in the continental U.S. increased from 2.5 percent of land area in 1990 to 3.1 percent in 2000, an increase equivalent to the area of Vermont and New Hampshire combined. If that growth pattern continues, U.S. urban land could expand by an area greater than the state of Montana by 2050.

The study is not the first to estimate carbon storage and sequestration by U.S. urban forests, however it provides more refined statistical analyses for national carbon estimates that can be used to assess the actual and potential role of urban forests in reducing atmospheric carbon dioxide.

More urbanization does not necessarily translate to more urban trees. Last year, Nowak and Eric Greenfield, a forester with the Northern Research Station and another study co-author, found that urban tree cover is declining nationwide at a rate of about 20,000 acres per year, or 4 million trees per year.


State

Carbon Stored (tons)

Texas

49,800,000

Florida

47,300,000

Georgia

42,400,000

Massachusetts

39,600,000

North Carolina

37,500,000

New York

35,400,000

California

34,600,000

Pennsylvania

31,700,000

New Jersey

30,900,000

Connecticut

25,700,000

Ohio

25,300,000

Michigan

25,200,000

Tennessee

20,800,000

Alabama

20,600,000

Illinois

20,600,000

South Carolina

19,100,000

Virginia

18,300,000

Washington

15,200,000

Maryland

13,100,000

Missouri

12,400,000

Louisiana

11,600,000

Indiana

10,700,000

Wisconsin

10,400,000

Minnesota

10,200,000

Oregon

8,900,000

Arkansas

8,500,000

Mississippi

8,200,000

New Hampshire

7,900,000

Kentucky

7,100,000

Arizona

6,000,000

West Virginia

5,700,000

Kansas

5,300,000

Colorado

4,800,000

Oklahoma

4,800,000

Rhode Island

4,600,000

Maine

4,200,000

Iowa

4,100,000

Delaware

2,500,000

Hawaii

2,400,000

Utah

2,300,000

Alaska

2,200,000

New Mexico

2,000,000

Nebraska

1,800,000

Vermont

1,700,000

Nevada

1,400,000

Idaho

1,200,000

South Dakota

800,000

Montana

500,000

North Dakota

500,000

Wyoming

300,000

Total

708,100,000

Landsat thermal sensor lights up from volcano’s heat

An ash plume drifts from Paluweh volcano in Indonesia in this image, taken April 29, 2013, from the Landsat Data Continuity Mission's Operational Land Imager instrument. -  Robert Simmon, NASA's Earth Observatory, using data from USGS and NASA
An ash plume drifts from Paluweh volcano in Indonesia in this image, taken April 29, 2013, from the Landsat Data Continuity Mission’s Operational Land Imager instrument. – Robert Simmon, NASA’s Earth Observatory, using data from USGS and NASA

As the Landsat Data Continuity Mission satellite flew over Indonesia’s Flores Sea April 29, it captured an image of Paluweh volcano spewing ash into the air. The satellite’s Operational Land Imager detected the white cloud of smoke and ash drifting northwest, over the green forests of the island and the blue waters of the tropical sea. The Thermal Infrared Sensor on LDCM picked up even more.

By imaging the heat emanating from the 5-mile-wide volcanic island, TIRS revealed a hot spot at the top of the volcano where lava has been oozing in recent months.

The two LDCM instruments, working together, illustrate a quote from Aristotle: The whole is greater than the sum of its parts, said Betsy Forsbacka, TIRS instrument manager at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“Each instrument by itself is magnificent,” she said. “When you put them together, with the clues that each give you on what you’re seeing on Earth’s surface, it’s greater than either could do by themselves.”

The image of Paluweh also illuminates TIRS’ abilities to capture the boundaries between the hot volcanic activity and the cooler volcanic ash without the signal from the hot spot bleeding over into pixels imaging the cooler surrounding areas. TIRS engineers tested and refined the instrument pre-launch to ensure each pixel correctly represents the heat source it images on Earth’s surface. Otherwise, Forsbacka said, it would be like shining a flashlight in your eyes — the bright light can leave you seeing spots and halos where it should be dark. The same effect can occur with detectors. But the contrast is sharp on the Paluweh image.

“We can image the white, representing the very hot lava, and right next to it we image the gray and black from the cooler surrounding ash,” Forsbacka said. “It’s exciting that we’re imaging such diverse thermal activity so well.”

The TIRS instrument can also pick up subtle shifts of temperatures, within a 10th of a degree Celsius. And, with two different thermal bands instead of the one band on previous Landsat satellites, LDCM is poised to make it easier for scientists to subtract out the effects of the atmosphere on the signal, obtaining a more accurate temperature of Earth’s surface.

Taking Earth’s temperature from space can be difficult because the atmosphere gets in the way and alters the thermal signals, Forsbacka said. Scientists looking to estimate surface temperatures with the single thermal band on previous Landsat instruments needed measurements or assumptions about atmospheric conditions.

TIRS has two thermal bands, however. The atmosphere affects each band slightly differently, resulting in one thermal image that’s a hair darker than the other. By measuring that difference, and plugging it into algorithms, scientists can better address atmospheric effects and create a more accurate temperature record of the Earth’s surface.

The Landsat program is a joint mission of NASA and the U.S. Geological Survey. Once LDCM completes its onboard calibration and check-out phase in late May, the satellite will be handed over to the USGS and renamed Landsat 8. Data from TIRS and OLI will be processed, archived and distributed from the USGS Earth Resources and Observation Science Center in Sioux Falls, S.D., for free over the Internet.

Scientific ocean drilling poised to reveal the secrets of the subseafloor for the next decade

IODP-Management International (IODP-MI) today announced the successful conclusion of the CHIKYU+10 International Workshop held in Tokyo, Japan, from 21-23 April 2013.

The event, convened by the global scientific ocean drilling community and enabled by the Japan Agency for Marine-Earth Science and Technology, focused on Chikyu’s explorations for the next decades in pursuit of new knowledge about Earth’s past and innovative research into today’s global challenges.

“Chikyu will be a keystone of the IODP endeavor for the next decade and beyond, providing the global research community with the capability to reach deep targets inaccessible by any other scientific platform.” says Prof.

Mike Coffin, University of Tasmania, Australia, and Chair of the CHIKYU+10 Steering Committee.

About 400 scientists and engineers from over 20 countries attended the workshop, including senior officials from funding agencies MEXT (Japan’s Ministry of Education, Culture, Sports, Science and Technology) and NSF (U.S. National Science Foundation), representatives of IODP partners, and scientists from 180 prominent universities and other institutions worldwide. The workshop was hosted by CDEX (Center for Deep Earth Exploration of JAMSTEC) assisted by IODP-MI.

“The Workshop was a landmark success. I saw so many new faces from around the world. Workshop participants clearly consider Chikyu as an international scientific asset.” said Kiyoshi Suyehiro, President and C.E.O. of IODP-MI.

Four Co-Chief Scientists of past Chikyu expeditions also made presentations highlighting their teams’ research and describing the transformational capabilities Chikyu offers the Earth, ocean, and life sciences communities. Presentations covered not only the first 10 years of successful Chikyu operations, but also Chikyu’s continued importance to international scientific ocean drilling research in the next decades.

The workshop focused on five thematic areas identified from short white papers accepted in early 2013: Active Faults, Ocean Crust and Earth’s Mantle, Deep Life and Hydrothermal Systems, Continent Formation, and Sediment Secrets.

Thematic discussions highlighted accomplishments of Chikyu’s completed expeditions, proposals to use Chikyu’s deep riser capability, new ideas submitted from the community, and inspiring keynote talks. Deliberations among researchers will help prioritize projects for Chikyu’s next decade of exploration and beyond.

Global climate change, earthquakes, and tsunami generation are some of the most pressing societal research challenges of the 21st century. At the workshop, the scientific community affirmed that Chikyu will continue to play an important role as a key platform for scientific ocean drilling to investigate these phenomena and fundamental questions in Earth, ocean, and life sciences.

Scientists uncover relationship between lavas erupting on sea floor and deep-carbon cycle

Molten magma erupted onto the seafloor freezes to glass that contains clues to its origin in Earth's deep interior and ancient past (field of view ~1 cm). Volcanic glasses like this one may reveal a link between Earth's oxidation state and the deep carbon cycle. -  Glenn Macpherson and Tim Gooding
Molten magma erupted onto the seafloor freezes to glass that contains clues to its origin in Earth’s deep interior and ancient past (field of view ~1 cm). Volcanic glasses like this one may reveal a link between Earth’s oxidation state and the deep carbon cycle. – Glenn Macpherson and Tim Gooding

Scientists from the Smithsonian and the University of Rhode Island have found unsuspected linkages between the oxidation state of iron in volcanic rocks and variations in the chemistry of the deep Earth. Not only do the trends run counter to predictions from recent decades of study, they belie a role for carbon circulating in the deep Earth. The team’s research was published May 2 in Science Express.

Elizabeth Cottrell, lead author and research geologist at the Smithsonian’s National Museum of Natural History, and Katherine Kelley at the University of Rhode Island’s Graduate School of Oceanography measured the oxidation state of iron, which is the amount of iron that has a 3+ versus a 2+ electronic charge, in bits of magma that froze to a glass when they hit the freezing waters and crushing pressures of the sea floor. Due to the high precision afforded by the spectroscopic technique they used, the researchers found very subtle variations in the iron-oxidation state that had been overlooked by previous investigations. The variations correlate with what Cottrell described as the “fingerprints” of the deep Earth rocks that melted to produce the lavas-but not in the way previous researchers had predicted. The erupted lavas that have lower concentrations of 3+ iron also have higher concentrations of elements such as barium, thorium, rubidium and lanthanum, that concentrate in the lavas, rather than staying in their deep Earth home. More importantly, the oxidation state of iron also correlates with elements that became enriched in lavas long ago, and now, after billions of years, show elevated ratios of radiogenic isotopes. Because radiogenic isotopic ratios cannot be modified during rock melting and eruption, Cottrell called this “a dead ringer for the source of the melt itself.”

Carbon is one of the “geochemical goodies” that tends to become enriched in the lava when rocks melt. “Despite is importance to life on this planet, carbon is a really tricky element to get a handle on in melts from the deep Earth,” said Cottrell. “That is because carbon also volatilizes and is lost to the ocean waters such that it can’t easily be quantified in the lavas themselves. As humans we are very focused on what we see up here on the surface. Most people probably don’t recognize that the vast majority of carbon-the backbone of all life-is located in the deep Earth, below the surface-maybe even 90 percent of it.”

The rocks that the team analyzed that were reduced also showed a greater influence of having melted in the presence of carbon than those that were oxidized. “And this makes sense because for every atom of carbon present at depth it has to steal oxygen away from iron as it ascends toward the surface,” said Cottrell. This is because carbon is not associated with oxygen at depth, it exists on its own, like in the mineral diamond. But by the time carbon erupts in lava, it is surrounded by oxygen. In this way, concludes Cottrell, “carbon provides both a mechanism to reduce the iron and also a reasonable explanation for why these reduced lavas are enriched in ways we might expect from melting a carbon-bearing rock.”

Canada’s distinctive tuya volcanoes reveal glacial, palaeo-climate secrets

Kima'Kho tuya forms a high relief structure covering 28 square kilometers rising 1,946 meters above sea level on the Kawdy Plateau near Dease Lake. -  UBC Science
Kima’Kho tuya forms a high relief structure covering 28 square kilometers rising 1,946 meters above sea level on the Kawdy Plateau near Dease Lake. – UBC Science

Deposits left by the eruption of a subglacial volcano, or tuya, 1.8 million years ago could hold the secret to more accurate palaeo-glacial and climate models, according to new research by University of British Columbia geoscientists.

The detailed mapping and sampling of the partially eroded Kima’ Kho tuya in northern British Columbia, Canada shows that the ancient regional ice sheet through which the volcano erupted was twice as thick as previously estimated.

Subglacial eruptions generate distinctive deposits indicating whether they were deposited below or above the waterline of the englacial lakes–much like the rings left on the inside of a bath tub. The transitions from subaqueous from subaerial deposits are called passage zones and define the high stands of englacial lakes. The depth and volume of water in these ephemeral lakes, in turn, gives researchers an accurate measure of the minimum palaeo-ice thicknesses at the time of eruption.

“At Kima’Kho, we were able to map a passage zone in pyroclastic deposits left by the earliest explosive phase of eruption, allowing for more accurate forensic recovery of paleo-lake levels through time and better estimates of paleo-ice thicknesses,” says UBC volcanologist James K Russell, lead author on the paper published this week in Nature Communications.

“Applying the same technique to other subglacial volcanos will provide new constraints on paleoclimate models that consider the extents and timing of planetary glaciations.”

While relatively rare globally, tuyas are common throughout Iceland, British Columbia, Oregon, and beneath the Antarctic ice-sheets. Kima’Kho tuya forms a high relief structure covering 28 square kilometres rising 1,946 metres above sea level on the Kawdy Plateau near Dease Lake. The plateau hosts six other tuyas.

“We hope our discovery encourages more researchers to seek out pyroclastic passage zones,” says Lucy Porritt, a Marie Curie Research Fellow at UBC and University of Bristol. “With more detailed mapping of glaciovolcanic sequences, and the recognition of the importance of these often abrupt changes in depositional environment, our understanding of glaciovolcanic eruptions and the hazards they pose can only be advanced.”

No Redoubt: Volcanic eruption forecasting improved

Forecasting volcanic eruptions with success is heavily dependent on recognizing well-established patterns of pre-eruption unrest in the monitoring data. But in order to develop better monitoring procedures, it is also crucial to understand volcanic eruptions that deviate from these patterns.

New research from a team led by Carnegie’s Diana Roman retrospectively documented and analyzed the period immediately preceding the 2009 eruption of the Redoubt volcano in Alaska, which was characterized by an abnormally long period of pre-eruption seismic activity that’s normally associated with short-term warnings of eruption. Their work is published today by Earth and Planetary Science Letters.

Well-established pre-eruption patterns can include a gradual increase in the rate of seismic activity, a progressive alteration in the type of seismic activity, or a change in ratios of gas released.

“But there are numerous cases of volcanic activity that in some way violated these common patterns of precursory unrest,” Roman said. “That’s why examining the unusual precursor behavior of the Redoubt eruption is so enlightening.”

About six to seven months before the March 2009 eruption, Redoubt began to experience long-period seismic events, as well as shallow volcanic tremors, which intensified into a sustained tremor over the next several months. Immediately following this last development, shallow, short-period earthquakes were observed at an increased rate below the summit. In the 48 hours prior to eruption both deep and shallow earthquakes were recorded.

This behavior was unusual because precursor observations usually involve a transition from short-period to long-period seismic activity, not the other way around. What’s more, seismic tremor is usually seen as a short-term warning, not something that happens months in advance. However, these same precursors were also observed during the 1989-90 Redoubt eruption, thus indicating that the unusual seismic pattern reflects some unique aspect of the volcano’s magma system.

Advanced analysis of the seismic activity taking place under the volcano allowed Roman and her team to understand the changes taking place before, during, and after eruption. Their results show that the eruption was likely preceded by a protracted period of slow magma ascent, followed by a short period of rapidly increasing pressure beneath Redoubt.

Elucidating the magma processes causing these unusual precursor events could help scientists to hone their seismic forecasting, rather than just relying on the same forecasting tools they’re currently using, ones that are not able to detect anomalies.

For example, using current techniques, the forecasts prior to Redoubt’s 2009 eruption wavered over a period of five months, back and forth between eruption being likely within a few weeks to within a few days. If the analytical techniques used by Roman and her team had been taken into consideration, the early risk escalations might not have been issued.

“Our work shows the importance of clarifying the underlying processes driving anomalous volcanic activity. This will allow us to respond to subtle signals and increase confidence in making our forecasts.” Roman said.

Ancient Earth crust stored in deep mantle

Scientists have long believed that lava erupted from certain oceanic volcanoes contains materials from the early Earth’s crust. But decisive evidence for this phenomenon has proven elusive. New research from a team including Carnegie’s Erik Hauri demonstrates that oceanic volcanic rocks contain samples of recycled crust dating back to the Archean era 2.5 billion years ago. Their work is published in Nature.

Oceanic crust sinks into the Earth’s mantle at so-called subduction zones, where two plates come together. Much of what happens to the crust during this journey is unknown. Model-dependent studies for how long subducted material can exist in the mantle are uncertain and evidence of very old crust returning to Earth’s surface via upwellings of magma has not been found until now.

The research team studied volcanic rocks from the island of Mangaia in Polynesia’s Cook Islands that contain iron sulfide inclusions within crystals. In-depth analysis of the chemical makeup of these samples yielded interesting results.

The research focused on isotopes of the element sulfur. (Isotopes are atoms of the same element with different numbers of neutrons.) The measurements, conducted by graduate student Rita Cabral, looked at three of the four naturally occurring isotopes of sulfur–isotopic masses 32, 33, and 34. The sulfur-33 isotopes showed evidence of a chemical interaction with UV radiation that stopped occurring in Earth’s atmosphere about 2.45 billion years ago. It stopped after the Great Oxidation Event, a point in time when the Earth’s atmospheric oxygen levels skyrocketed as a consequence of oxygen-producing photosynthetic microbes. Prior to the Great Oxidation Event, the atmosphere lacked ozone. But once ozone was introduced, it started to absorb UV and shut down the process.

This indicates that the sulfur comes from a deep mantle reservoir containing crustal material subducted before the Great Oxidation Event and preserved for over half the age of the Earth.

“These measurements place the first firm age estimates of recycled material in oceanic hotspots,” Hauri said. “They confirm the cycling of sulfur from the atmosphere and oceans into mantle and ultimately back to the surface,” Hauri said.

Rethinking early atmospheric oxygen

This photo shows researchers doing field sampling of a pyrite-rich black shale outcrop in China. The weathering of such sediments, which contain sulfur originally buried from the ocean, transfers sulfur isotope signals to the ocean to be buried again in marine sediments. -  Chu Research Group, Institute of Geology and Geophysics, Chinese Academy of Sciences.
This photo shows researchers doing field sampling of a pyrite-rich black shale outcrop in China. The weathering of such sediments, which contain sulfur originally buried from the ocean, transfers sulfur isotope signals to the ocean to be buried again in marine sediments. – Chu Research Group, Institute of Geology and Geophysics, Chinese Academy of Sciences.

A research team of biogeochemists at the University of California, Riverside has provided a new view on the relationship between the earliest accumulation of oxygen in the atmosphere, arguably the most important biological event in Earth history, and its relationship to the sulfur cycle.

A general consensus exists that appreciable oxygen first accumulated in Earth’s atmosphere around 2.4 to 2.3 billion years ago. Though this paradigm is built upon a wide range of geological and geochemical observations, the famous “smoking gun” for what has come to be known as the “Great Oxidation Event” (GOE) comes from the disappearance of anomalous fractionations in rare sulfur isotopes.

“These isotope fractionations, often referred to as ‘mass-independent fractionations,’ or ‘MIF’ signals, require both the destruction of sulfur dioxide by ultraviolet energy from the sun in an atmosphere without ozone and very low atmospheric oxygen levels in order to be transported and deposited in marine sediments,” said Christopher T. Reinhard, the lead author of the research paper and a former UC Riverside graduate student. “As a result, their presence in ancient rocks is interpreted to reflect vanishingly low atmospheric oxygen levels continuously for the first ~2 billion years of Earth’s history.”

However, diverse types of data are emerging that point to the presence of atmospheric oxygen, and, by inference, the early emergence of oxygenic photosynthesis hundreds of millions of years before these MIF signals disappear from the rock record. These observations motivated Reinhard and colleagues to explore the possible conditions under which inherited MIF signatures may have persisted in the rock record long after oxygen accumulated in the atmosphere.

Using a simple quantitative model describing how sulfur and its isotopes cycle through the Earth’s crust, the researchers discovered that under certain conditions these MIF signatures can persist within the ocean and marine sediments long after O2 increases in the atmosphere. Simply put, the weathering of rocks on the continents can transfer the MIF signal to the oceans and their sediments long after production of this fingerprint has ceased in an oxygenated atmosphere.

“This lag would blur our ability to date the timing of the GOE and would allow for dynamic rising and falling oxygen levels during a protracted transition from an atmosphere without oxygen to one rich in this life-giving gas,” Reinhard said.

Study results appear in Nature‘s advanced online publication on April 24.

Reinhard explained that once MIF signals formed in an oxygen-poor atmosphere are captured in pyrite and other minerals in sedimentary rocks, they are recycled when those rocks are later uplifted as mountain ranges and the pyrite is oxidized.

“Under certain conditions, this will create a sort of ‘memory effect’ of these MIF signatures, providing a decoupling in time between the burial of MIF in sediments and oxygen accumulation at Earth’s surface,” he said.


According to the researchers, the key here is burying a distinct MIF signal in deep sea sediments, which are then subducted and removed from Earth’s surface.

“This would create a complementary signal in minerals that are weathered and delivered to the oceans, something that we actually see evidence of in the rock record,” said Noah Planavsky, the second author of the research paper and a former UC Riverside graduate student now at Caltech. “This signal can then be perpetuated through time without the need to generate it within the atmosphere contemporaneously.”

Reinhard, now a postdoctoral fellow at Caltech and soon to be an assistant professor at Georgia Institute of Technology, explained that although the researchers’ new model provides a plausible mechanism for reconciling recent conflicting data, this can only occur when certain key conditions are met – and these conditions are likely to have changed through time during Earth’s long early history.

“There is obviously much further work to do, but we hope that our model is one step toward a more integrated view of how Earth’s crust, mantle and atmosphere interact in the global sulfur cycle,” he said.

Timothy W. Lyons, a professor of biogeochemistry at UCR and the principal investigator of the research project noted that this is a fundamentally new and potentially very important way of looking at the sulfur isotope record and its relationship to biospheric oxygenation.

“The message is that sulfur isotope records, when viewed through the filter of sedimentary recycling, may challenge efforts to precisely date the GOE and its relationship to early life, while opening the door to the wonderful unknowns we should expect and embrace,” he said.

The Earth’s center is 1,000 degrees hotter than previously thought

Recreating the Earth's liquid iron core in the laboratory: a speck-sized piece of iron is thermally isolated and placed between the tips of two small conical diamonds. Pressing the two diamonds together produces pressures of 2 million atmospheres and more. As a laser beam heats the sample to temperatures of 3000 to 5000 degrees, a thin beam of synchrotron X-rays is used to detect whether it has started to melt. This will change its crystalline structure, in turn modifying the 'diffraction pattern' of deflected X-rays behind the sample. -  ESRF/Denis Andrault.
Recreating the Earth’s liquid iron core in the laboratory: a speck-sized piece of iron is thermally isolated and placed between the tips of two small conical diamonds. Pressing the two diamonds together produces pressures of 2 million atmospheres and more. As a laser beam heats the sample to temperatures of 3000 to 5000 degrees, a thin beam of synchrotron X-rays is used to detect whether it has started to melt. This will change its crystalline structure, in turn modifying the ‘diffraction pattern’ of deflected X-rays behind the sample. – ESRF/Denis Andrault.

Scientists have determined the temperature near the Earth’s center to be 6000 degrees Celsius, 1000 degrees hotter than in a previous experiment run 20 years ago. These measurements confirm geophysical models that the temperature difference between the solid core and the mantle above, must be at least 1500 degrees to explain why the Earth has a magnetic field. The scientists were even able to establish why the earlier experiment had produced a lower temperature figure. The results are published on 26 April 2013 in Science.

The research team was led by Agnès Dewaele from the French national technological research organization CEA, alongside members of the French National Center for Scientific Research CNRS and the European Synchrotron Radiation Facility ESRF in Grenoble (France).

The Earth’s core consists mainly of a sphere of liquid iron at temperatures above 4000 degrees and pressures of more than 1.3 million atmospheres. Under these conditions, iron is as liquid as the water in the oceans. It is only at the very center of the Earth, where pressure and temperature rise even higher, that the liquid iron solidifies. Analysis of earthquake-triggered seismic waves passing through the Earth, tells us the thickness of the solid and liquid cores, and even how the pressure in the Earth increases with depth. However these waves do not provide information on temperature, which has an important influence on the movement of material within the liquid core and the solid mantle above. Indeed the temperature difference between the mantle and the core is the main driver of large-scale thermal movements, which together with the Earth’s rotation, act like a dynamo generating the Earth’s magnetic field. The temperature profile through the Earth’s interior also underpins geophysical models that explain the creation and intense activity of hot-spot volcanoes like the Hawaiian Islands or La Réunion.

To generate an accurate picture of the temperature profile within the Earth’s centre, scientists can look at the melting point of iron at different pressures in the laboratory, using a diamond anvil cell to compress speck-sized samples to pressures of several million atmospheres, and powerful laser beams to heat them to 4000 or even 5000 degrees Celsius.”In practice, many experimental challenges have to be met”, explains Agnès Dewaele from CEA, “as the iron sample has to be insulated thermally and also must not be allowed to chemically react with its environment. Even if a sample reaches the extreme temperatures and pressures at the centre of the Earth, it will only do so for a matter of seconds. In this short timeframe it is extremely difficult to determine whether it has started to melt or is still solid”.

This is where X-rays come into play. “We have developed a new technique where an intense beam of X-rays from the synchrotron can probe a sample and deduce whether it is solid, liquid or partially molten within as little as a second, using a process known diffraction”, says Mohamed Mezouar from the ESRF, “and this is short enough to keep temperature and pressure constant, and at the same time avoid any chemical reactions”.

The scientists determined experimentally the melting point of iron up to 4800 degrees Celsius and 2.2 million atmospheres pressure, and then used an extrapolation method to determine that at 3.3 million atmospheres, the pressure at the border between liquid and solid core, the temperature would be 6000 +/- 500 degrees. This extrapolated value could slightly change if iron undergoes an unknown phase transition between the measured and the extrapolated values.

When the scientists scanned across the area of pressures and temperatures, they observed why Reinhard Boehler, then at the MPI for Chemistry in Mainz (Germany), had in 1993 published values about 1000 degrees lower. Starting at 2400 degrees, recrystallization effects appear on the surface of the iron samples, leading to dynamic changes of the solid iron’s crystalline structure. The experiment twenty years ago used an optical technique to determine whether the samples were solid or molten, and it is highly probable that the observation of recrystallization at the surface was interpreted as melting.

“We are of course very satisfied that our experiment validated today’s best theories on heat transfer from the Earth’s core and the generation of the Earth’s magnetic field. I am hopeful that in the not-so-distant future, we can reproduce in our laboratories, and investigate with synchrotron X-rays, every state of matter inside the Earth,” concludes Agnès Dewaele.

Unique chemistry reveals eruption of ancient materials once at Earth’s surface

An international team of researchers, including Scripps Institution of Oceanography, UC San Diego, geochemist James Day, has found new evidence that material contained in oceanic lava flows originated in Earth’s ancient Archean crust. These findings support the theory that much of the Earth’s original crust has been recycled by the process of subduction, helping to explain how the Earth has formed and changed over time.

The Archean geologic eon, Earth’s second oldest, dating from 3.8 to 2.5 billion years ago, is the source of the oldest exposed rock formations on the planet’s surface. (Archean rocks are known from Greenland, the Canadian Shield, the Baltic Shield, Scotland, India, Brazil, western Australia, and southern Africa.) Although the first continents were formed during the Archean eon, rock of this age makes up only around seven percent of the world’s current crust.

“Our new results are important because they provide strong evidence not only to tie materials that were once on Earth’s surface to an entire cycle of subduction, storage in the mantle, and return to the surface as lavas, but they also place a firm time constraint on when plate tectonics began; no later than 2.5 billion years ago,” said Day. “This is because mass independent sulfur signatures have only been shown to occur in the atmosphere during periods of low oxygenation prior to the rise of oxygen-exhaling organisms.”

The new study, which will be published in the April 24 issue of the journal Nature, adds further support to the theory that most of the Archean crust was subducted or folded back into the Earth’s mantle, evidence of which is seen in the presence of specific sulfur isotopes found in some oceanic lava flows.

According to the researchers, because terrestrial independently fractionated (MIF) sulfur-isotope isotope signatures were generated exclusively through atmospheric photochemical reactions until about 2.5 billion years ago, material containing such isotopes must have originated in the Archean crust. In the new study, the researchers found MIF sulfur-isotope signatures in olivine-hosted sulfides from relatively young (20-million-year-old) ocean island basalts (OIB) from Mangaia, Cook Islands (Polynesia), providing evidence that the mantle is the only possible source of the ancient Archean materials found in the Mangaia lavas.

“The discovery of MIF-S isotope in these young oceanic lavas suggests that sulfur-likely derived from the hydrothermally-altered oceanic crust-was subducted into the mantle more than 2.5 billion years ago and recycled into the mantle source of the Mangaia lavas,” said Rita Cabral, the study’s primary author and a graduate student in Boston University’s Department of Earth and Environment.

The data also complement evidence for sulfur recycling of ancient sedimentary materials to the subcontinental lithospheric mantle previously identified in diamond inclusions.