The Carbon Cycle Runs Deep

New research on diamonds found deep in the Earth's crust suggests that the planet's carbon cycle reaches far underground.

A diamond from Kankan, Guinea, with mineral impurities.

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Anetta Banas

Charles Q. Choi, Contributor

(Inside Science) -- Diamonds from deep underground are now revealing secrets of how the carbon vital to life on this planet cycles between Earth's interior and its surface, a new study finds.

The carbon that all life on Earth is based on moves through the planet's atmosphere, oceans and crust in a pattern called the carbon cycle. This cycle plays a key role in climate; for instance, carbon dioxide traps heat from the sun to warm the globe.

The most well-understood parts of the carbon cycle occur at or near the Earth's surface, but findings have recently suggested the whole cycle might extend much deeper into the planet's interior than is often thought. For example, ocean sediments or oceanic crust can plunge into the Earth's mantle, the hot rock layer between the crust and core, when one tectonic plate gets forced, or subducted, under another.

"If we want to understand why our planet has evolved into the habitable state it is today and how the surfaces and atmospheres of other planets may be shaped by their interior processes, we need to better understand the carbon cycle," said study lead author Margo Regier, a geochemist at the University of Alberta in Edmonton, Canada.

One way to solve this mystery is to analyze "superdeep" diamonds -- ones from depths of more than 250 kilometers -- to see what carbon from the crust they might contain.

Regier and her colleagues investigated superdeep diamonds bought from miners in Kankan, Guinea, in West Africa. Based on the kinds of mineral impurities found within the diamonds, these gems originated roughly 250 to 700 kilometers deep in the lower mantle. The researchers examined the diamonds and bits of minerals within them, focusing on the different isotopes of carbon, oxygen and nitrogen they possessed. The ratios of the different isotopes vary depending on the origin of the material that ultimately made the diamonds.

The researchers discovered that superdeep diamonds originating above depths of 660 kilometers appear to be mainly derived from volcanic oceanic crust.

In contrast, the isotope ratios from diamonds originating below 660 kilometers revealed they likely formed from carbon from the lower mantle, with little to no carbon from the crust. This suggests that carbon from the crust only cycles so deep before cycling upward. "This is really important for our understanding of the whole carbon cycle," said mantle geochemist Yaakov Weiss at the Hebrew University in Jerusalem, who did not take part in this research.

The scientists noted the deep mantle is rich in metallic iron, and carbon bonds well to it at those depths. However, this iron-carbon alloy breaks down when exposed to water, and the liberated carbon may form diamonds. They suggested this was likely after the breakdown of water-rich minerals in subducting rock.

Future research should investigate superdeep diamonds from across the Earth to get a truly global picture of the deep carbon cycle, said petrologist Michael Walter at the Carnegie Institution for Science in Washington, D.C., who did not participate in this study. Regier and her colleagues also plan to investigate silicon and magnesium isotopes within superdeep diamonds to learn more about how materials from the crust are circulating within the deep mantle, she said.

The scientists detailed their findings online Sept. 9 in the journal Nature.

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Charles Q. Choi is a science reporter who has written for Scientific American, The New York Times, Wired, Science, Nature, and National Geographic News, among others.