Calculations by a trio of theoretical physicists at the University of São Paulo (USP) have provided a new clue that could be useful in solving one of the principal enigmas about the composition and mechanics of the Earth’s interior: where are the ultra-deep reservoirs that contain 90% of the world’s carbon? The computational simulations done by Michel Marcondes and Lucy Assali of the USP Physics Institute (IF-USP) and João Francisco Justo Filho of the Polytechnic School (Poli-USP) suggest that some of these carbon deposits could be hidden in underground patches about 1,000 kilometers (km) wide and 100 km thick, through which seismic waves travel relatively slowly.
These patches, called ultra-low velocity zones, or simply ULVZs, are found mainly in the final segment of a layer of Earth known as the lower mantle, between 660 km and 2,890 km deep (see figure). According to a paper published by the three Brazilian authors in the scientific journal Physical Review B in September 2016, the carbon element tends not to mix as an impurity within silicate perovskites, the silicon-based mineral deposits that are prevalent in the lower mantle. Instead of mixing, carbon separates from the silicate perovskites and forms its own deposits of minerals such as magnesite (MgCO3) and calcium carbonate (CaCO3).
The physicists applied the laws of quantum mechanics to run a computer simulation of the atomic structure of several types of minerals under the conditions found in the lower mantle, where temperatures can reach as high as thousands of degrees Celsius and pressures are millions of times greater than on the Earth’s surface. They then calculated how these minerals would behave when traversed by a seismic wave generated by an earthquake. “The presence of carbon minerals considerably reduces the speed at which the waves propagate, as occurs in ULVZs,” Marcondes says. The seismic waves traveled faster in silicon-based mineral deposits. Marcondes used the techniques he learned from Renata Wentzcovitch, a Brazilian physicist at the University of Minnesota, who develops computational methods to study the atomic structure of materials under the extreme temperature and pressure conditions in the Earth’s interior. Such conditions are very difficult to reproduce in a laboratory (see Pesquisa FAPESP Issue nº 198).
“In the last 500 km of the lower mantle, there are large regions in which the speeds of seismic waves are about 5% slower than in the middle,” explains geophysicist Marcelo Assumpção of the Seismology Center at USP’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP). “Before, they were thought to have come from temperature fluctuations in the lower mantle. But recent studies have been showing that variations in the chemical composition of the geologic layers, such as the presence of more or less silica, iron or carbonates, are needed in order to explain the observations.” The work of Marcondes and his colleagues indicates that a larger percentage of carbon-rich minerals, such as magnesites and calcium carbonates, could explain the occurrence of ULVZs. “It remains to be determined how this concentration of carbonates accumulated in the lower mantle, but that’s another story.”
The other carbon cycle
Theories on the origin of the solar system suggest that the total concentration of carbon in the Earth is more or less the same as that found in carbonaceous chondrites, the rocks in a type of meteorite, whose chemical composition has remained practically unchanged since the planet was formed about 4.5 billion years ago. But, when researchers add up the amount of carbon present in all known sources—the atmosphere, the oceans, the surface of the Earth’s crust and several dozen kilometers immediately below it—the total comes to only 10% of the expected amount. “The missing carbon has to be somewhere,” says Marcondes, who focused on the problem in his doctoral dissertation, defended in September 2016. “There is evidence that the remaining 90% is stored in regions deep within the Earth’s interior.”
Carbon, the chemical basis of life, circulates around the Earth’s surface, and researchers have a good understanding of this process. It is a component of the atmosphere, in the form of methane gas (CH4), carbon monoxide (CO) or carbon dioxide (CO2). Through photosynthesis, algae and plants draw CO2 from the air and fix carbon in the form of organic matter, which eventually decomposes and is deposited in fossil fuel reserves in rock layers of the continental and oceanic crust. Plant and animal respiration, along with combustion processes, return carbon into the air.
Recent research studies in the fields of seismology and geochemistry, however, have indicated that this surface carbon cycle is linked to another, deeper and more slowly and occurring, cycle. Over the course of tens of millions of years, the rocks in the mantle behave like a fluid. Their descending currents are believed to drag large pieces of carbon-rich oceanic crust along with them, which then sink down to the lower mantle. The ascending currents in the mantle carry some of the ultra-deep carbon to the surface. As evidence of this process, Marcondes points to what are known as ultra-deep diamonds such as those discovered in mines in the municipality of Juína, in the state of Mato Grosso. They are small pieces of diamond whose chemical impurities indicate that, before being carried to the surface by volcanic eruptions, they originated at depths greater than 670 km. Most diamonds are formed at about 150 km below the surface.
According to a report published by the Deep Carbon Observatory in 2014, a research program led by geophysicist Robert Hazen of the Carnegie Institute, the exact location and extent of the deep carbon reservoirs are still a great unknown. The report concludes, however, that carbon may be a component in a wide variety of materials in addition to ultra-deep diamonds and carbonate minerals. It might be found in the form of microorganisms living deep in the Earth’s crust or in a dozen types of solids and liquids that are mixed in with the Earth’s mantle and core. “It’s a very complicated issue,” Marcondes says. “We still have a long way to go before we find a definitive answer.”
MARCONDES, M. L. et al. Carbonates at high pressures: Possible carriers for deep carbon reservoirs in the Earth’s lower mantle. Physical Review B. V. 94, No. 10. September 2016.