Diamonds that are found deep in the Earth represent massive, subterranean carbon storage, says S.Ananthanarayanan.

While the Earth is faced with rising, but trace CO₂ in the atmosphere, there are worlds where CO₂ is the main component. Life, as we understand it, was able to arise on the Earth thanks to low CO₂ levels. In contrast, on the planet Venus, CO₂ forms 96.5%, by volume, of the atmosphere.

As Venus is nearer the sun, it receives more heat than we do. And then, as the CO₂ in the atmosphere traps and retains the heat, Venus, at 400-500°C, day or night, is one of the hottest places in the Solar System. The atmosphere, because it is all CO₂, is also ninety times heavier than on the Earth, and the pressure at the surface of Venus is like it is 3,000 metres deep in the ocean on Earth.

An important reason the Earth is not like Venus is that chlorophyll in Earth’s green cover reduces CO₂ to release oxygen. But equally important are other ways that are active in sequestering carbon. During a period when the rise of global temperature seemed to have slowed, we were informed that the sea is a great reservoir, of heat, as well as of CO₂. But another important quarter, where carbon has been stored away since ages, is deep within the Earth.

James W.E.Drewitt, Michael J.Walter, HongluoZhang, Sorcha C.McMahon, DavidEdwards, Benedict J.Heinen, Oliver T.Lord, SimoneAnzellini and AnnetteK.Kleppe, from the University of Bristol and the Harwell Science and Innovation Campus, Chilton, in UK, the Carnegie Institution, Washington and the University of Science and Technology of China, write in the journal, Earth and Planetary Science Letters, of the effect that the pressures at great depths below the surface of the Earth have on the behavior of carbon and minerals that contain carbon

The reason the interior of the planet acts as a carbon reservoir on the Earth but not on Venus is that the Earth’s crust, or the outer layer, above the rocky mantle, on Earth, but not on Venus, consists of plates that move, often over one another. The paper in the journal explains that carbon that has settled on the seabed, in the form of carbonates, enters the Earth’s mantle when the plates of the crust move under other plates, and sink because of gravity. While the upper mantle is mostly solid rock, increasing carbon content lowers the melting point, which leads to melting, and carbonaceous material sinks deeper. The Earth’s mantle goes down to a depth of 2,900 kilometers and a release by the University of Bristol says the mantle “represents 75% of the mass of the Earth’s volume, and potentially holds more carbon than all other reservoirs combined.”

Deep within the mantle, the conditions of temperature and pressure may not allow carbon in its mineral forms and carbon may exist mainly in the form of diamond or in combination with iron, as iron carbide, the paper says. Analysis of the carbon content of the Earth’s atmosphere, crust and the upper mantle suggest the possibility of high carbon storage in the deeper mantle. As samples of material that originates at great depths are rare, the current study specifically aimed to determine the processes in the lower mantle and what happens to carbonates and carbon in these regions

The temperature at the start of the mantle, just after the crust, has risen to 1000°C, and goes up to 3,800°C nearer the core. And the pressure can approach half a million times the atmospheric pressure. , As the deepest we have been able to drill is about 12 kilometers, these deep parts of the Earth’s interior are clearly out of reach of any investigation. The effort in the study was hence to simulate these conditions, and see what the behavior of carbonate minerals was at these depths.

Simulation was carried out using an arrangement called a diamond anvil, made with diamond, the hardest material known, which could sustain such pressures. The arrangement had dimensions of less than a tenth of a millimeter and the sample was in a pressed foil a hundredth of a millimeter thick. While pressure approaching a million atmospheres was generated by mechanical means, temperature of 1,000 to 2,200°C was created by lasers. The pressure itself was measured using the Raman Effect, where light beams scattered by the diamond anvil parts change colour, to an extent that indicates the pressure. The structure of the carbonates and resulting forms was studied by the scattering of hard X Rays and again with the help of the Raman effect.

The result of the experiments was that carbonates remain stable till the conditions are as found at about 1,300 km, or half-way down to the core of the earth. At this depth, carbonates react with silica, to form different minerals, mostly a mineral called bridgmanite. This reaction releases carbon, in the form of CO₂, but at the high pressure, in the form of a solid. And with rising temperature, the CO₂ decomposes into carbon, in the form of diamond, and oxygen. The depth at which this happens is also how deep down carbon can be transported

The study represents important insight into another loop in the carbon cycle. The cycle gets completed by the rising of carbon from the depths through volcanic activity. The balance, of carbon capture and carbon release, however, is far from understood. In any case, the cycle also has a geological time scale and may be of little significance in the current crisis, where it is surface carbon, sequestered as oil and coal, that is being released into the atmosphere.

The study also draws attention to the fact that while further reaches of outer space are inaccessible because of the time it would take to reach them, the deeper parts of our planet are also effectively out of reach.

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