Model Describes First-Order Carbon Cycle, Isotope Distributions Within Deep Earth

In a new paper published in PNAS, scientists from Texas Tech University and the Russian Academy of Sciences provide new insights into Earth’s deep carbon cycle.

Relatively little is known about the carbon budget—and the dynamics that shape it—within Earth's interior and core. This uncertainty sits at the root of many of the Deep Carbon Observatory’s decadal goals. In a new paper published in PNAS, scientists from Texas Tech University, USA, and the Russian Academy of Sciences provide new insights into Earth’s deep carbon cycle [1].

The major processes that formed Earth as we know it today, such as accretion, magma-ocean formation, and the violent impact that created the Moon, may have resulted in significant losses of the primordial elements, including carbon, to space and the core. Noting that naturally occurring stable isotopes can serve as useful tracers, Juske Horita and Veniamin Polyakov describe a novel first-order model of the carbon cycle within deep Earth that includes core formation processes and accounts for carbon isotope distributions involving iron-carbon phases.

“A large number of recent experimental studies have described forms of carbon at conditions representative of Earth’s interior, such as graphite, diamond, kimberlitic or carbonatitic melts, carbides, and carbon dioxide degassed from volcanoes. However, these studies were unable to fully trace the initial mantle signature all these forms of carbon,” said Vincenzo Stagno (Earth-Life Sciences Institute, Tokyo Institute of Technology, Japan). “This study links experiments with observations in nature, allowing quantification of the carbon isotope fractionation from deep, primordial sources up to the surface.”

Based on their model, the authors present calculations showing iron and silicon carbides significantly depleted in carbon-13 relative to other carbon-bearing materials at mantle temperatures. Under these conditions, tectonic processes in deep Earth could readily produce diamonds with observed carbon isotopic ratios that currently cannot be explained.

“In order to understand the carbon isotope signatures found in diamonds and other terrestrial and extra-terrestrial materials, it is imperative we understand all the biotic and abiotic mechanisms that can fractionate carbon, even at high temperature,” said Anat Shahar (Carnegie Institution of Washington, USA).  “This paper does an excellent job of predicting that abiotic fractionation could be very important.”

The findings not only impact current theories about the planet’s formation, but also have implications for the carbon isotope bio-signatures of early life on Earth, according to the authors.

Image: Juske Horita. 

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