Earth’s mantle is a complex and heterogeneous mixture. Understanding the chemical composition of the mantle, which comprises over 80% of Earth’s volume, and how its activities influence our atmosphere, are crucial. In a paper released this week in Science, Katherine Kelley, associate professor at the URI Graduate School of Oceanography, and Elizabeth Cottrell at the Smithsonian’s National Museum of Natural History describe the central role of carbon in controlling mantle chemistry.

Kelley and Cottrell studied 35 mantle-derived mid-ocean ridge basalts, or MORBs, from 13 different geographical locations. The study of such basalts has shown that the upper mantle is composed of enriched and depleted reservoirs. These distinct regions of the mantle have different elemental composition, and therefore play host to different chemical reactions. They have now shown that enriched mantle is in fact more reduced than depleted mantle. This observation led them to question why this counter-intuitive situation should exist, and the answer they came up with was carbon. Deep in the mantle, carbon exists in its reduced, elemental form (graphite or diamond), but as the mantle moves upwards beneath mid-ocean ridges, it oxidizes to carbon dioxide. This requires that reduced carbon steal oxygen from another compound, and in the case of enriched melts the victim of such theft is iron. Thus, in MORBs, the measured oxidation state of iron is skewed by the starting carbon content.

“Despite its importance to life on this planet, carbon is a really tricky element to get a handle on in melts from the deep Earth,” said Cottrell. “That is because carbon also volatilizes and is lost to the ocean waters such that it can’t easily be quantified in the lavas themselves. As humans we are very focused on what we see up here on the surface. Most people probably don’t recognize that the vast majority of carbon—the backbone of all life—is located in the deep Earth, below the surface—maybe even 90 percent of it.”

“It’s important that we do studies like this because it provides us with a way to quantify how much carbon is in the Earth’s mantle,” said Kelley. “Carbon in the mantle is expelled by volcanoes at mid-ocean ridges when they erupt, and that’s ultimately important for understanding our climate, the evolution of the Earth, and the carbon budget.”

Pictured Above
URI Associate Professor Katherine Kelley (right) and Smithsonian scientist Elizabeth Cottrell display samples of rocks from the Earth’s mantle.
Photo courtesy of Katherine Kelley.

For more information, please see the related press release.