Carbon cycle reaches Earth’s lower mantle, Science study reports

<IMG SRC="/Images/347637653.jpg" WIDTH="350" HEIGHT="175" BORDER="0" ALT="Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth's deep interior. The study by Walter et al. in Science reveals mineral inclusions that originated in oceanic crust subducted into the lower mantle. – Image © Science/AAAS”>
Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth’s deep interior. The study by Walter et al. in Science reveals mineral inclusions that originated in oceanic crust subducted into the lower mantle. – Image © Science/AAAS

The carbon cycle, upon which most living things depend, reaches much deeper into the Earth than generally supposed-all the way to the lower mantle, researchers report.

The findings, which are based on the chemistry of an unusual set of Brazilian diamonds, will be published online by the journal Science, at the Science Express Web site, on 15 September. Science is published by AAAS, the non-profit, international science society.

“This study shows the extent of Earth’s carbon cycle on the scale of the entire planet, connecting the chemical and biological processes that occur on the surface and in the oceans to the far depths of Earth’s interior,” said Nick Wigginton, associate editor at Science.

“Results of this kind offer a broader perspective of planet Earth as an integrated, dynamic system,” he said.

The carbon cycle generally refers to the movement of carbon through the atmosphere, oceans, and the crust. Previous observations suggested that the carbon cycle may even extend to the upper mantle, which extends roughly 400 kilometers into the Earth. In this region, plates of ocean crust-bearing a carbon-rich sediment layer-sink beneath other tectonic plates and mix with the molten rock of the mantle.

Seismological and geochemical studies have suggested that oceanic crust can sink all the way to the lower mantle, more than 660 kilometers down. But actual rock samples with this history have been hard to come by.

Michael Walter of the University of Bristol and colleagues in Brazil and the United States analyzed a set of “superdeep” diamonds from the Juina kimberlite field in Brazil. Most diamonds excavated at Earth’s surface originated at depths of less than 200 kilometers. Some parts of the world, however, have produced rare, superdeep diamonds, containing tiny inclusions of other material whose chemistry indicates that the diamonds formed at far greater depths.

The Juina-5 diamonds studied by Walter and colleagues contain inclusions whose bulk compositions span the range of minerals expected to form when basalt melts and crystallizes under the extreme high pressures and temperatures of the lower mantle.

Thus, these inclusions probably originated when diamond-forming fluids incorporated basaltic components from oceanic lithosphere that had descended into the lower mantle, the researchers have concluded.

If this hypothesis is correct, then the carbon from which the diamonds formed may have been deposited originally within ocean crust at the seafloor. A relative abundance of light carbon isotopes in the Juina-5 diamonds supports this idea, since this lighter form of carbon is found at the surface but not generally in the mantle, the authors say.

The diamond inclusions also include separate phases that appear to have “unmixed” from the homogenous pool of material. This unmixing likely happened as the diamonds traveled upward hundreds of kilometers into the upper mantle, the researchers say.

After the diamonds formed in the lower mantle, they may have been launched back near the surface by a rising mantle plume, Walter and colleagues propose.

Diamonds show depth extent of Earth’s carbon cycle

Scientists have speculated for some time that the Earth’s carbon cycle extends deep into the planet’s interior, but until now there has been no direct evidence. The mantle-Earth’s thickest layer -is largely inaccessible. A team of researchers analyzed diamonds that originated from the lower mantle at depths of 435 miles (700 kilometers) or more, and erupted to the surface in volcanic rocks called kimberlites. The diamonds contain what are impurities to the gemologist, but are known as mineral inclusions to the geologist. Analysis shows compositions consistent with the mineralogy of oceanic crust. This finding is the first direct evidence that slabs of oceanic crust sank or subducted into the lower mantle and that material, including carbon, is cycled between Earth’s surface and depths of hundreds of miles. The research is published in the September 15, 2011, online Science Express.

The mantle extends from as little as 5 to 1,800 miles (10-2,900 kilometers) beneath the Earth’s surface. Most diamonds are free from inclusions and come from depths less than 120 miles (200 km). But in a few localities researchers have found super-deep diamonds from the depths of the convecting upper and lower mantle, as well as the transition zone in between. Whereas inclusions in diamonds from the depths of the upper mantle and transition zone have been consistent with a surface-rock origin, none from the lower mantle have borne this signature until now.

The team,* which included Carnegie scientists, was led by former Carnegie postdoctoral fellow Michael Walter, now a professor at the University of Bristol, UK. The scientists analyzed minute (one to two hundredths of a millimeter) mineral grains from six diamonds from the Juina region in Brazil. The analysis showed that diamond inclusions initially crystallized as a single mineral that could form only at depths greater than 435 miles (700 km). But the inclusions recrystallized into multiple minerals as they were carried up to the surface-first probably from a mantle upwelling known as a plume, then as they erupted to the surface in kimberlites

The diamonds were analyzed for carbon at Carnegie. Four of the diamonds contained low amounts of carbon-13, a signature not found in the lower mantle and consistent with an ocean-crust origin at Earth’s surface. “The carbon identified in other super-deep, lower mantle diamonds is chiefly mantle-like in composition,” remarked co-author Steven Shirey * at Carnegie. “We looked at the variations in the isotopes of the carbon atoms in the diamonds. Carbon originating in a rock called basalt, which forms from lava at the surface, is often different from that which originates in the mantle, in containing relatively less carbon-13. These super-deep diamonds contained much less carbon-13, which is most consistent with an origin in the organic component found in altered oceanic crust.”

“I find it astonishing that we can use the tiniest of mineral grains to show some of the motions of the Earth’s mantle at the largest scales,” concluded Shirey.

The cause of Earth’s largest environmental catastrophe

The eruption of giant masses of magma in Siberia 250 million years ago led to the Permo-Triassic mass extinction when more than 90 % of all species became extinct. An international team including geodynamic modelers from the GFZ German Research Centre for Geosciences together with geochemists from the J. Fourier University of Grenoble, the Max Plank Institute in Mainz, and Vernadsky-, Schmidt- and Sobolev-Institutes of the Russian Academy of Sciences report on a new idea with respect to the origin of the Siberian eruptions and their relation to the mass extinction in the recent issue of Nature (15.09.2011, vol. 477, p. 312-316).

Large Igneous Provinces (LIPs) are huge accumulations of volcanic rock at the Earth’s surface. Within short geological time spans of often less than one million years their eruptions cover areas of several hundred thousand square kilometers with up to 4 kilometers thick lava flows. The Siberian Traps are considered the largest continental LI

A widely accepted idea is that LIPs originate through melting within thermal mantle plumes, a term applied to giant mushroom-shaped volumes of plastic mantle material that rise from the base of the mantle to the lithosphere, the Earth’s rigid outer shell. The high buoyancy of purely thermal mantle plumes, however, should cause kilometer-scale uplift of the lithosphere above the plume head, but such uplift is not always present. Moreover, estimates of magmatic degassing from many LIPs are considered insufficient to trigger climatic crises. The team of scientists presents a numerical model and new geochemical data with which unresolved questions can now be answered.

They suggest that the Siberian mantle plume contained a large fraction of about 15 percent of recycled oceanic crust; i.e. the crust that had long before been subducted into the deep mantle and then, through the hot mantle plume, brought back to the Earth’s lithosphere. This recycled oceanic crust was present in the plume as eclogite, a very dense rock which made the hot mantle plume less buoyant. For this reason the impingement of the plume caused negligible uplift of the lithosphere. The recycled crustal material melts at much lower temperatures than the normal mantle material peridotite, and therefore the plume generated exceptionally large amounts of magmas and was able to destroy the thick Siberian lithosphere thermally, chemically and mechanically during a very short period of only a few hundred thousand years. During this process, the recycled crust, being exceptionally rich in volatiles such as CO2 and halogens, degassed and liberated gases that passed through the Earth crust into the atmosphere to trigger the mass extinction. The model predicts that the mass extinction should have occurred before the main magmatic eruptions. Though based on sparse available data, this prediction seems to be valid for many LIPs.

New technology for recovering valuable minerals from waste rock

Researchers report discovery of a completely new technology for more efficiently separating gold, silver, copper, and other valuable materials from rock and ore. Their report on the process, which uses nanoparticles to latch onto those materials and attach them to air bubbles in a flotation machine, appears in the ACS journal Langmuir.

Robert Pelton and colleagues explain that companies use a technique termed froth flotation to process about 450 million tons of minerals each year. The process involves crushing the minerals into small particles, and then floating the particles in water to separate the commercially valuable particles from the waste rock. The water contains “collector” substances that can attach to the valuable particles, causing them to repel water and rise to the bubbling top of the water where they can be easily skimmed off.

The researchers demonstrated an entirely new type of collector technology, consisting of water-repelling nanoparticles. In laboratory experiments using glass beads to simulate actual mineral particles, they showed that the nanoparticles attached so firmly to the beads that flotation produced a recover rate of almost 100 per cent.

Shell partners with UT Austin to pursue new solutions to unlock gas resources

Shell and the University of Texas at Austin (UT) today signed a five-year agreement to invest $7.5 million to address short- and long-term challenges facing the growing worldwide unconventional oil and gas industry.

“This agreement marks an important milestone in Shell’s commitment to continually research and develop innovative technology that will help to meet global demands by bringing more energy resources to market,” said Marvin Odum, president, Shell Oil Company. “We chose to collaborate with UT because it brings together an extraordinary amount of talent from both organizations that will push the technological envelope in the field of developing even the most challenging hydrocarbons safely and responsibly.”

Shale gas is abundant, widely used, and a growing source of energy in the United States. According to the U.S. Energy Information Administration, shale gas, tight gas, and coalbed methane accounted for 50 percent of U.S. production in 2009 and are expected to account for 75 percent of production by 2035.

The new Shell-UT Program on Unconventional Resources will be managed by the university’s Bureau of Economic Geology with participation across the campus, including geoscience, engineering, economics, business, environmental and regulatory affairs. In addition to top-ranking geology and petroleum engineering programs, the university has dedicated centers working in energy law, economics, finance and energy and environmental policy.

“The pursuit of unconventional energy resources is a complex, integrated problem that requires uniting the scientific and engineering efforts below ground with above-ground efforts in water, regulation, and public awareness,” said William Powers, president of The University of Texas at Austin. “As a major research university and leader in energy, we’ve got the integrated expertise to help solve it.”

Scott Tinker, director of the Bureau of Economic Geology and an expert on global energy, notes that unconventional resources such as shale gas could extend natural gas production in the U.S. and globally from 50 to 100 years beyond recent estimates.

“Increased production of shale gas and other unconventional hydrocarbons could significantly enhance U.S. energy security,” said Tinker, “since these are largely available, affordable, and reliable domestic energy sources that contribute directly to the U.S. and global economy.”

In addition to science and engineering, the Shell-UT agreement will try to integrate research to address environmental and public policy issues that could facilitate safer and wider adoption of onshore US energy resources.

The agreement will also support students at The University of Texas at Austin, significantly enhancing the employability of students working on Shell-UT projects in the Jackson School of Geosciences, Department of Petroleum & Geosystems Engineering, and other departments at the university.

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800,000 years of abrupt climate variability

An international team of scientists, led by Dr Stephen Barker of Cardiff University, has produced a prediction of what climate records from Greenland might look like over the last 800,000 years.

Drill cores taken from Greenland’s vast ice sheets provided the first clue that Earth’s climate is capable of very rapid transitions and have led to vigorous scientific investigation into the possible causes of abrupt climate change.

Such evidence comes from the accumulation of layers of ancient snow, which compact to form the ice-sheets we see today. Each layer of ice can reveal past temperatures and even evidence for the timing and magnitude of distant storms or volcanic eruptions. By drilling cores in the ice scientists have reconstructed an incredible record of past climates. Until now such temperature records from Greenland have covered only the last 100,000 years or so.

The team’s reconstruction is based on the much longer ice core temperature record retrieved from Antarctica and uses a mathematical formulation to extend the Greenland record beyond its current limit.

Dr Barker, Cardiff School of Earth and Ocean Sciences said: “Our approach is based on an earlier suggestion that the record of Antarctic temperature variability could be derived from the Greenland record.

“However, we turned this idea on its head to derive a much longer record for Greenland using the available records from Antarctica.”

The research published in the journal Science (8 September) demonstrates that abrupt climate change has been a systemic feature of Earth’s climate for hundreds of thousands of years and may play an active role in longer term climate variability through its influence on ice age terminations.

Dr Barker added: “It is intriguing to get an insight into what abrupt climate variability may have looked like before the Greenland records begin. We now have to wait until longer Greenland records are produced so that we can see how successful our prediction is.”

The new predictions provide an extended testing bed for the climate models that are used to predict future climate variability.

The collaborative research was funded in part by a Leverhulme Trust Philip Leverhulme Prize awarded to Dr Barker at Cardiff University. The prize recognises the achievement and potential of outstanding researchers at an early stage in their careers but who have already acquired an international reputation for their work. The Natural Environment Research Council and National Science Foundation in the United States also funded the research.

Recovering historical weather data

Recovering historical weather data: Meeting in The Netherlands will highlight the value of historic data for understanding the past and projecting future climate.

At the ACRE (Atmospheric Circulation Reconstruction over the Earth) annual workshop in The Netherlands, international climate scientists will present a series of talks about how historic weather data-culled from 100-year-old ship logs and the notebooks of historic weather observers-are critical for understanding Earth’s future climate as well as its past.

WHAT: 4th Annual Atmospheric Circulation Reconstructions over the Earth (ACRE) Workshop

WHERE:Royal Netherlands Meteorological Institute

De Bilt, The Netherlands

WHEN: Sept. 21-23, 2011

Sessions begin at 8:30 a.m. daily

A full agenda is available online

By mathematically stitching together sparse historic observations, climate scientists are creating datasets of information on the state of the atmosphere at various points in time. The work requires weeks of time on some of the world’s most powerful supercomputers. Resulting weather maps – called reanalyses – can be used to address past and future climate variability and change, and to better understand historic events driven by weather or climate patterns.

Gil Compo, from NOAA and its Cooperative Institute for Research in Environmental Sciences (at the University of Colorado at Boulder) is a co-convener of the workshop, with Rob Allan of the UK Met Office Hadley Centre and the ACRE project manager and Albert Klein Tank of the Royal Netherlands Meteorological Institute.

Presentations include:

  • Rescue of Historical U.S. Marine Data in Support of Marine Ecology (Catherine Marzin, NOAA, US)

  • Citizen Science: Old Weather (Philip Brohan, ACRE & Met Office, UK)
  • Storminess (Dave Easterling, NOAA, US)
  • Recovery of Logbooks And International Marine Data (Clive Wilkinson, University of East Anglia, UK)
  • International Comprehensive Ocean-Atmosphere Data Set (Scott Woodruff, NOAA, US)
  • 1894 Thames Flooding Study (Richard Jones, Met Office, UK)
  • Hail Propensity in Queensland (Roger Stone, University of Southern Queensland, Australia)
  • The HMS Plover High School Project (Kevin Wood, NOAA, US)
  • German initiative: Rescue of world-wide historical climate data (Birger Tinz and Gudrun Rosenhagen, Deutscher Wetterdienst, German Weather Service, Germany)
  • AAA project: French historical climate and weather observations rescue (Sylvie Jourdain, Météo-France)
  • Reconstructing Scotland’s weather for the 18th and 19th centuries (Alastair Dawson, University of Aberdeen, UK and Edward Hanna, University of Sheffield, UK)

Where does all the gold come from?

Ultra high precision analyses of some of the oldest rock samples on Earth by researchers at the University of Bristol provides clear evidence that the planet’s accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after the Earth was formed. The research is published today in Nature.

During the formation of the Earth, molten iron sank to its center to make the core. This took with it the vast majority of the planet’s precious metals – such as gold and platinum. In fact, there are enough precious metals in the core to cover the entire surface of the Earth with a four meter thick layer.

The removal of gold to the core should leave the outer portion of the Earth bereft of bling. However, precious metals are tens to thousands of times more abundant in the Earth’s silicate mantle than anticipated. It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit the Earth after the core formed. The full load of meteorite gold was thus added to the mantle alone and not lost to the deep interior.

To test this theory, Dr Matthias Willbold and Professor Tim Elliott of the Bristol Isotope Group in the School of Earth Sciences analysed rocks from Greenland that are nearly four billion years old, collected by Professor Stephen Moorbath of the University of Oxford. These ancient rocks provide a unique window into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment.

The researchers determined the tungsten isotopic composition of these rocks. Tungsten (W) is a very rare element (one gram of rock contains only about one ten-millionth of a gram of tungsten) and, like gold and other precious elements, it should have entered the core when it formed. Like most elements, tungsten is comprised of several isotopes, atoms with the same chemical characteristics but slightly different masses. Isotopes provide robust fingerprints of the origin of material and the addition of meteorites to the Earth would leave a diagnostic mark on its W isotope composition.

Dr Willbold observed a 15 parts per million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks. This small but significant change is in excellent agreement with that required to explain the excess of accessible gold on Earth as the fortunate by-product of meteorite bombardment.

Dr Willbold said: “Extracting tungsten from the rock samples and analysing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks. In fact, we are the first laboratory world-wide that has successfully made such high-quality measurements.”

The impacting meteorites were stirred into the Earth’s mantle by gigantic convection processes. A tantalising target for future work is to study how long this process took. Subsequently, geological processes formed the continents and concentrated the precious metals (and tungsten) in ore deposits which are mined today.

Dr Willbold continued: “Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tonnes of asteroidal material.”

Evidence for a persistently iron-rich ocean changes views on Earth?s early history

Researchers Chris Reinhard (front) and Noah Planavsky dig into a shale exposure in north China. -  Chu Research Group, Institute of Geology and Geophysics, Chinese Academy of Sciences
Researchers Chris Reinhard (front) and Noah Planavsky dig into a shale exposure in north China. – Chu Research Group, Institute of Geology and Geophysics, Chinese Academy of Sciences

Over the last half a billion years, the ocean has mostly been full of oxygen and teeming with animal life. But earlier, before animals had evolved, oxygen was harder to come by. Now a new study led by researchers at the University of California, Riverside reveals that the ancient deep ocean was not only devoid of oxygen but also rich in iron, a key biological nutrient, for nearly a billion years longer than previously thought — right through a key evolutionary interval that culminated in the first rise of animals.

“The implications of our work are far reaching,” said Timothy Lyons, a professor of biogeochemistry and the principal investigator of the study. “We will need to rethink, in a fundamental way, all of our models for how life-essential nutrients were distributed in the ocean through time and space.”

Study results appear in the Sept. 8 issue of Nature.

Previous ocean chemistry models

Most scientists agree that the early Earth, before 2.4 billion years ago, contained only trace quantities of oxygen and that the oceans were dominantly full of dissolved iron. But there is far less agreement among scientists about the chemical composition of the ocean during the middle chapters of Earth’s history in the wake of atmospheric oxygenation-about 2.4 to 0.5 billion years ago-when the diversity of organisms that we know today, including the animals, first got their footing.

Classic models for this time window maintain that the ocean, all depths, became rich in oxygen in parallel with its first accumulation in the atmosphere. This increase in oxygen in seawater has been linked to the disappearance of iron ore deposits known as ‘banded iron formations,’ the source of almost all of the iron used to make steel today. Oxygen, the argument goes, would have ‘rusted’ the oceans, stripping them of dissolved iron.

More than a decade ago, however, another idea gained traction: hydrogen sulfide. Produced by bacteria in the absence of oxygen, hydrogen sulfide, it was argued, might instead have scrubbed the iron out of the ocean during Earth’s middle history, dealing the fatal blow to the iron deposits. In an ocean full of hydrogen sulfide, diverse life-sustaining elements, including iron, can be stripped from the seawater, potentially causing a biotic crisis.

Fresh perspective


“The problem all along was a general lack of physical evidence in the oceans for the amounts of oxygen, iron, and sulfide in the Earth’s middle history, particularly in a critical billion-year window between roughly 1.8 and 0.8 billion years ago,” said Noah Planavsky, a doctoral student in UC Riverside’s Department of Earth Sciences and the lead author of the new study. “Some earlier work supported a return to an iron-rich ocean 0.8 billion years ago. Rather than a return, however, we predicted that iron may have dominated the deep ocean continuously right up to the oxygenation and concomitant rise of animals a mere half-billion years ago.”

Planavsky and his colleagues at UCR and in Canada, Australia, and China sought to remedy the data deficiency. New rock samples they collected from across the globe suggest a previously unknown continuity in ocean chemistry over much of its history. These data, the first of their kind, point towards continuous oxygen-poor, iron-rich conditions for 90 percent of Earth’s history, with oxygen and hydrogen sulfide, when present, limited mostly to the surface layers and along the margins of the oceans, respectively.

The task now is to reconsider whether the purported shortages of nutrients attributed to widespread hydrogen sulfide were indeed real and a throttle on early evolution. “Our new knowledge that the deep ocean was anoxic and iron-rich does not mean life had it easy, though,” Lyons says. “Enough sulfide could have persisted around the edges of the ocean to severely limit other key nutrients. We are still testing this hypothesis.”

Ironing out the details

The researchers’ results also indicate that neither oxygen nor hydrogen sulfide turned off iron deposition around 1.8 billion years ago, when the last major iron ores were seen. They suggest instead that hydrothermal systems on the seafloor are the most important factor controlling the distribution of iron ore.

“These hydrothermal systems are high-temperature vents on the seafloor tied to magmatic activity, and they can pump huge amounts of iron into the ocean,” Planavsky explained. “Previous researchers have suggested that there was a decrease in the amount of iron from hydrothermal systems around 1.8 billion years ago. Our results support this idea with compelling physical evidence, while showing that iron could persist in the ocean at levels below those necessary to form ore deposits.”

“The next step is to better merge this refined chemical perspective with traditional and emerging views of evolving life, recognizing that life and the environment co-evolve in an intimate dance of cause-and-effect relationships,” Lyons added.

Boom in fracking for oil and gas recovery sparks new technology

With a technology called “fracking” sparking energy booms – and controversy – worldwide, Chemical & Engineering News (C&EN) describes advances in the workhorse materials used to produce oil and gas from previously inaccessible deposits deep below Earth’s surface. C&EN is the American Chemical Society’s weekly newsmagazine.

In the article in C&EN’s current edition, Senior Business Editor Melody M. Bomgardner explains that fracking or hydraulic fracturing involves pumping massive amounts of grainy substances, called proppants, down oil or natural gas wells. Proppants enable production from rock formations 10,000 or 20,000 feet below the surface. To access the oil and gas in these deposits, they need to be fractured open with a mixture of fluid and proppants pumped down wells under high pressure. The grains literally prop up the fissures in these rocks so that oil and gas can flow to the surface.

The article describes development of a new genre of proppants to meet the needs of today’s drillers. For wells that reach more than a mile down, drillers may need 10 million to 20 million pounds of proppants to get oil or natural gas flowing. Drilling companies are going after more-difficult-to-access reserves of oil and gas that require tougher proppants. Some of the new materials, for instance, use high-tech ceramics like those used in aerospace and military applications or sand with each particle coated with curable resins.