In This Issue

Relatively little is known about the carbon budget and the dynamics that shape the budget within Earth’s interior and core. The major processes that formed Earth, 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 Earth’s core. Noting that naturally occurring stable isotopes can serve as useful tracers, Juske Horita and Veniamin Polyakov (pp. 31–36) describe a first-order model of the carbon cycle within the deep Earth that includes core formation processes and accounts for carbon isotope distributions involving iron–carbon phases. Based on their model, the authors present calculations that show iron and silicon carbides can be significantly depleted in 13C relative to other carbon-bearing materials at mantle temperatures. The depletion allows tectonic processes in the deep Earth to readily produce diamonds with observed carbon isotopic ratios that currently cannot be explained. The findings have implications for the carbon isotope biosignatures of early life on Earth, according to the authors. — T.J.