The Earth’s mantle is a very complex mixture. Deep under the surface could lie 90% of our planet’s carbon, but new research has suggested that the sheer amount of it could be even larger than scientists first predicted.
A research team of geologists from Royal Holloway, University of London’s Department of Earth Sciences used seismic sensors — devices which measure vibrations to reveal what kinds of substances are deep underground — to map out an area of melting carbon covering 1.8 million square kilometres.
What they found 250 kilometres underneath the Western US was a massive molten lake of carbon, which upturned any previous estimates of how much carbon the Earth contains. Their findings were published in the journal Earth and Planetary Science Letters.
According to Dr Sash Hier-Majumder, who was the lead author of the study, it would be impossible to drill far enough down to physically ‘see’ the Earth’s mantle, so instead they had to rely on it’s vibrations. The upper mantle of Earth is so incredibly hot that solid carbonates melt there, which creates very specific seismic patterns, or vibrations.
By extrapolating their findings to estimate the levels underneath the whole of Earth, the team concluded there could be up to 100 trillion metric tons of carbon dioxide in the upper mantle, which will eventually make its way to the surface through volcanic eruptions.
“100 Trillion metric tons is indeed a lot of carbon,” Hier-Majumder told Business Insider. “This massive reservoir shows how important the role of the deep Earth in the global carbon cycle is.
“We can think of this reservoir as a long term carbon storage, as we estimate the carbon in this reservoir can stay in the mantle for up to a billion years.”
To help put in perspective how massive this amount is, Hier-Majumder said that 1 trillion metric tonnes of carbon dioxide is the equivalent of burning 2.3 trillion barrels of oil.
“While there is no imminent possibility of this carbon making it to the surface, it shows that in absence of such a deep Earth reservoir this carbon would have to be accommodated in the surface, potentially leading to massive global warming,” he said.
So how did the molten carbon get there?
Hier-Majumder said the belief is that carbon gets to the mantle as tectonic plates sink under continents by a process called subduction. There is also carbonate sediment on the ocean floor which is deposited there from the shells of marine animals, and this gets carried into the mantle with the sinking plate, or slab.
“While between 30-40% of this carbon comes back to the surface through volcanoes, the rest get carried into the deeper mantle,” Hier-Majumder said. “The partially molten region we see in the US is clearly adjacent to such an oceanic plate in the mantle.”