Drilling down: scientists update estimates of Earth’s carbon reservoirs

Important as carbon is to the earth’s biosphere – and to its climate via carbon dioxide gas – only a tiny fraction of the Earth’s carbon is on its surface, scientists say.

Instead, reports Celina Suarez, a geologist at the University of Arkansas, Fayetteville, only 0.2% of it can be found in land, air, sea, and life. The remaining 99.8% resides in the subsurface.

That discovery comes from an ambitious international program called the Deep Carbon Observatory, now completing a decade-long quest to understand how subsurface processes affect the Earth’s overall carbon cycle. 

In the process of their studies, scientists on the program’s team have measured volcanic gas emissions, pored over traces of seismic waves propagating through the Earth’s depths, drilled into subduction zones, and studied the composition of diamonds to better understand how much carbon – the fourth most abundant element in the Solar System – lies beneath our feet. 

“It’s almost like a detective story, putting together bits and pieces of evidence,” a researcher said in a telephonic press conference earlier this week.

One important finding, Suarez’s team reported, is that volcanoes and other natural processes emit 300 to 400 million tonnes of carbon per year, either into the air, or for deep-sea volcanoes, into the water. 

That’s a lot, but those natural releases are dwarfed by humanity’s current emissions from the burning of fossil fuels, which emit 40 to 100 times as much carbon per year than all the world’s volcanoes combined, Suarez says.

However, there have been times when the amount of carbon reaching the earth’s atmosphere increased significantly. {%recommended 3735%}

One of these, Suarez says, occurred 200 million years ago, when the ancient supercontinent Pangaea was breaking apart, opening rifts that allowed an area the size of Canada to be inundated by lava – lava that then emitted enormous quantities of carbon dioxide gas into the atmosphere. 

Another such event occurred 100 million years ago, creating what is now known as the Ontong Java large igneous province, an undersea lava bed in the Pacific Ocean northeast of Australia, about the size of Alaska. 

Both of these, she says, created major warming periods, as carbon dioxide built up in the earth’s atmosphere faster than geological processes could remove it.

“The Earth’s interior and its exterior are always trading [carbon] with each other,” says Tobias Fisher of the University of New Mexico, Albuquerque. “It’s almost like we are accountants trying to work out the history of a 4.5-billion-year-old business.” 

The same research can also help planetary scientists piece together the origin of the Earth, says Sami Mikhail of the University of St. Andrews, UK. “The Deep Carbon Observatory’s work isn’t only about establishing how much carbon is in the Earth, but where the carbon came from that made the Earth.”

We’ve long known, he says, that the Earth is made from smaller bits that initially formed at different distances from the Sun – some close in, some farther out. “They all contain carbon in different abundances,” he says. “[So] we play a game: ‘Make Earth with space rocks’: add up their components to make the best match for Earth.” 

Knowing the Earth’s total content of carbon, he says, plays an important role in that, but “[it] has to satisfy every element in the periodic table.”

From that, he says, “We know Earth is not made from one type of space rock. It’s a cocktail of material from [various] parts of our Solar System.”

The research is summarised in a suite of papers in this month’s issue of the journal Elements.

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