Issue Date: March 12, 2012
Carbon Goes Deep
The attics of many homes contain dusty mementos of life gone by, boxes of seasonal decorations, and spare furniture. The attic in the Research Building at Carnegie Institution for Science in Washington, D.C., is like that, too. Except that tucked far back in the corner, past the Christmas decorations, the old equipment, and the boxes marked “historic lab items,” is a laboratory.
Inside the small room, Dionysis I. Foustoukos cranks down on a wrench half as big as he is, attempting to unscrew a plug from the end of a large, blue cylindrical device. Although it might not be obvious at first glance, the research scientist points out, the apparatus’ housing is modeled on old cannons from Navy battleships. Such a heavy-duty structure, Foustoukos explains, is necessary for the high-pressure, high-temperature experiments he and others at Carnegie conduct within the device’s walls.
The researchers use the internally heated pressure vessel, as well as other devices, to simulate conditions deep within Earth and to study the response of carbon-based materials to them. It is their hope that these experiments will reveal details about the many forms carbon takes within Earth and how it cycles from inside the planet, to its surface, and back again.
Like the cannon and the lab it sits in, Carnegie Institution’s Washington campus is tucked away in a corner—the northwest one—of the District of Columbia. Few who pass by the wooded campus would suspect the activities within. But it is here that an eclectic group of scientists are doing some of the world’s premier research on carbon and its geochemical cycles.
The original campus in Washington, D.C., is one of four around the country that make up the Carnegie Institution for Science. Established in 1902 to support the research efforts of “extraordinary” individuals, the institution takes its name from steel tycoon and founder Andrew Carnegie.
Mementos of Carnegie’s rich history dot the century-old campus. In the lobby of the institution’s Administration Building, for example, stairs encircle a model ship that faithfully re-creates the seafaring observatory Carnegie, which sailed the oceans from 1909 until 1929 to make detailed measurements of Earth’s magnetic field.
Today’s Carnegie is home not only to a variety of artifacts, but also to a diversity of research efforts. A sizable chunk of the work done in Washington, however, involves the study of carbon in either remote environments or under special conditions: in diamonds, volcanoes, even in outer space.
According to Robert M. Hazen, a senior staff scientist at Carnegie’s Geophysical Laboratory, the biggest carbon mysteries are beneath Earth’s surface, the home of unusual microbes and molten rock. “We’re not sure by a factor of 20 how much total carbon Earth contains,” he says. “We don’t know how much carbon’s in the core or the mantle. We don’t know if that carbon could play a significant role in surface processes,” he adds. “These are things we have to learn.”
Since 2009, Carnegie has been more strongly positioned to answer those questions than ever before. In that year, its Geophysical Laboratory became home of the secretariat, or administrative nexus, for the nascent Deep Carbon Observatory (DCO), an Alfred P. Sloan Foundation-sponsored international initiative with the mission of studying the cycle of carbon deep within Earth. DCO, which Hazen leads as executive director, encompasses nearly 1,000 researchers from a score of countries, all dedicated to understanding carbon’s complexities.
Researchers have grouped the big questions DCO seeks to answer into four areas. For each area, they established an organization called a scientific directorate. The Reservoirs & Fluxes Directorate looks at carbon abundance in Earth’s mantle and core, as well as how carbon might exchange between those locales and the surface. The Deep Life Directorate seeks to understand biological carbon in life-forms that dwell in Earth’s depths. The Energy, Environment & Climate Directorate’s focus is the chemistry and biochemistry of hydrocarbon formation. And the Physics & Chemistry of Carbon Directorate is all about how the element behaves at extremes of temperature and pressure—the kinds of conditions that form a familiar glittering gemstone.
In a lab that’s floors below Foustoukos’ attic space, Russell J. Hemley pulls out a small black jewelers’ box with a flourish. Inside, resting on a soft cushion, sit half-a-dozen gem-quality diamonds—each of them a carat or more in size.
Hemley, the director of Carnegie’s Geophysical Laboratory, proudly displays the dazzlers, which were synthesized right there in the lab, and then lets it drop that he and his research group can do even better.
“We’ve perfected the process” of making diamonds “over the years,” he says. So much so that now “we can make quite large diamonds—on the order of 10-carat diamonds.”
The secret to the researchers’ success, the chemical vapor deposition equipment where the diamonds are synthesized, hums loudly as Hemley explains how the method works. Years ago, he says, scientists realized that large single-crystal diamonds could be grown from small diamond seeds and methane gas (C&EN, Feb. 2, 2004, page 26). After turning the gas into a plasma and then adjusting its pressure and temperature in the deposition chamber, researchers are able to make a wide variety of diamonds—from baubles that are extremely pure to those that are extremely tough.
Much to the chagrin of brides-to-be everywhere, Hemley isn’t making these gems for commercial purposes. He and his group need the diamonds for high-pressure, high-temperature experiments on minerals and other carbon-based substances found inside Earth. Because of its innate strength, diamond is the ideal substance with which to make a test chamber for extreme conditions. But Mother Earth couldn’t provide gems plentiful enough, large enough, or tough enough for Hemley’s purposes. “So we started making diamond ourselves,” he says.
Once the single-crystal diamonds are grown and cut down with tools including lasers, the researchers put two of the gems together, opposing one another, in what is called a diamond anvil cell. It is between these two diamond pieces that the scientists squeeze and heat carbon-based substances with the aim of simulating conditions within the planet.
Using this apparatus, Hemley’s group recently discovered a polymeric form of carbon dioxide that might be stable within Earth as far down as the top of the lower mantle—a distance about 450 miles below the planet’s surface (Earth Planet Sci. Lett., DOI: 10.1016/j.epsl.2011.07.006). In this framework structure, as Hemley calls it, carbon is in fourfold coordination with oxygen, a configuration similar to that of silicon and oxygen atoms in silica. The substance, Hemley says, “could be a reservoir of carbon in our planet.”
Other researchers at Carnegie also use diamond to study the roles of carbon inside Earth. But unlike Hemley, these scientists prefer natural, flawed diamond for their investigations.
Steven B. Shirey, a geochemist in Carnegie’s Department of Terrestrial Magnetism, studies tiny grains, or inclusions, inside diamonds for what they can tell him about how Earth’s continents formed. “The diamonds you wouldn’t really want to make a ring out of—those are the ones important for our research,” Shirey says.
That’s because the inclusions are pieces of Earth’s mantle that got trapped inside the diamonds during the gems’ formation. Diamonds “form in the deep roots of the continents,” Shirey says. “They give us the chance to look at the way carbon has been recycled in the ancient past.”
Diamonds are brought up from Earth’s interior via volcanic eruptions. A greenish rock called kimberlite hosts the precious stones as the materials fracture their way up through the planet’s mantle and crust to the surface. “It’s hard for people to understand the depths these diamonds come from,” Shirey says. Normal diamonds—those used in jewelry—come from about 100 miles beneath Earth’s surface. But there are also diamonds referred to as superdeep diamonds that come up from as far down as 250–500 miles below Earth’s surface.
Once Shirey and his group obtain samples with large enough inclusions for study, they cut the diamonds into thin wafers and study the grains with a stereomicroscope. They measure the carbon and nitrogen isotopic composition of the diamonds to determine where the carbon originally came from. And finally, they dissolve the inclusions and examine the levels of other isotopes, such as rhenium and osmium, to determine the age of the specimens.
Last year, Shirey and colleague Stephen Richardson of the University of Cape Town, in South Africa, compiled their own findings on diamond inclusions as well as data from other papers published in the past two decades. The researchers found that one type of inclusion, a high-pressure equivalent of basalt, appeared in diamonds around 3.2 billion years ago (Science, DOI: 10.1126/science.1206275). Before that, however, the rock was absent. So they took that to mean that a specific cycle of plate tectonics—one in which the continents were breaking up, drifting, and colliding—began at that time.
In addition to studying diamond inclusions, Erik H. Hauri, another staff scientist at Carnegie and cochair of DCO’s Reservoirs & Fluxes Directorate, examines inclusions in volcanic rock. “Volcanoes are the surface expression of the deep-Earth carbon cycle,” Hauri says. They “give us a window into Earth’s interior.”
Using secondary ion mass spectrometry, Hauri and his group examine the composition of volcanic glass trapped within olivine, a mineral typically incorporated into volcanic magma and dragged up to Earth’s surface during eruptions.
“By studying rocks like these from around the planet,” Hauri says, “we can get a better idea of the fluxes of carbon” from Earth’s interior. Scientists still don’t know whether carbon levels in Earth are increasing or decreasing, he adds, but measurements like these will help pin down an answer. The knowledge will help researchers figure out how much deep-Earth’s carbon cycle interacts with the planet’s surface carbon cycle, which influences climate change and weather.
But Hauri doesn’t restrict himself to just studying volcanoes on Earth. He’s recently been comparing Earth’s magma compositions with those of the moon. Last year, Hauri and coworkers examined inclusions from lunar magmas and found that they contain substantial amounts of water (Science, DOI: 10.1126/science.1204626). They even concluded that some regions inside the moon could contain as much water as Earth’s upper mantle.
Other Carnegie researchers’ deep-carbon-based pursuits reach past the moon, to Earth’s nearest planetary neighbor.
In a lab across campus from Hauri’s work space, biochemist Andrew Steele sits in the dark hunched over a lab bench. On the glowing computer screen in front of him is an image of what might appear to be a regular rock. But it is in fact a piece of the most recent Mars meteorite to fall to Earth—the Tissint. Steele, a British-born researcher with a mane worthy of a rock star, is sampling different regions across the surface of the specimen with a laser beam. The whole setup is part of the micro-Raman spectrometer sitting on the lab bench next to him. Steele uses established analytical techniques to hunt for life on Mars and other planets, and he develops new ways to do so as well.
“If you go into a situation or an environment where you think there’s life, it’s very difficult to find if you don’t know what molecules life uses to make itself or to survive,” Steele says. To look for the signals of life amid the background noise of the universe, it’s important to establish a baseline that depicts the chemistry of things that aren’t alive, he explains.
Steele and his collaborators study samples from a variety of environments, including Mars meteorites, lunar rocks, and samples from Earth’s mantle, to set that background level. They catalog ways carbon species react to produce chemistries that might look a little bit like life but are not really alive.
“You just need to know your organic chemistry,” Steele says. “The essence of the deep carbon cycle for me is setting this baseline of organic chemistry, of carbon chemistry, that we can use as a primary life detection strategy.”
Steele, who’s called Steelie by his Carnegie colleagues, says a baseline that aids the search for life on Mars could also answer questions about the origins of life on Earth. He’s set to refine his baseline when the roving analytical chemistry lab Curiosity, designed to study Mars’s habitability, touches down on the surface of that planet this August (C&EN, Nov. 21, 2011, page 32).
Mars isn’t the only heavenly body that’s helping researchers paint a picture of deep carbon on Earth. Work by George D. Cody, a senior staff scientist in the Geophysical Laboratory, suggests that comets provide clues of their own.
While studying organic solids in comets and other solar system bodies, Cody and his collaborators found that these solids represent a class of material called “nongraphitizable carbon” (Earth Planet Sci. Lett., DOI: 10.1016/j.epsl.2008.05.008; Proc. Natl. Acad. Sci.USA, DOI: 10.1073/pnas.1015913108). The substance, also called carbon glass, is especially hardy where heat is concerned: The bonds holding the substance together don’t break at temperatures as high as 1,400 °C. That discovery says a lot about how Earth retained carbon during its earliest days, Cody says.
Every planet forms through a coalescing of small particles, including the types of organic solids found in comets and meteorites, Cody explains. In Earth’s case, the component particles accumulated so much thermal energy that the planet started out as a molten ball. “At one phase in Earth’s history the entire mantle was what we call a magma ocean,” he says. Under those circumstances, it’s not clear how Earth retained volatile species containing carbon. Cody and coworkers propose that carbon species stayed put because the component particles that melted included heat-resistant carbon glass.
Cody also notes that carbon glass was first identified by DNA structure pioneer Rosalind E. Franklin. He was pleased to be able to cite Franklin’s original article in one of his scientific papers, Cody says.
Like Franklin’s fundamental work that led to the structure of DNA, the Carnegie researchers believe their basic research has the potential to pay dividends—in their case, in energy chemistry, materials science, and other areas. “Some people might wonder why we’re studying carbon, which is such an abundant and familiar element,” Hazen says. But it’s clear that carbon holds far more mysteries than its reputation as a familiar element would suggest, he adds. “Carbon is really the most important element in the periodic table.”
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