A FRAMEWORK FOR UNDERSTANDING THE SOURCE AND FATE OF CARBON ON ROCKY PLANETS

Carbon isotope ratios are used to track large scale geochemical processes where material is exchanged, and possibly fractionated, between planetary reservoirs. Curiously, the long-term ¹³C/¹²C ratios of the Earth’s observable reservoirs have remained remarkably stable for 3.5+ billion years, suggesting the planet’s carbon reservoirs were set in place early in its history, possibly during formation and differentiation. However, when compared to other solar system objects, the bulk silicate Earth has a substantial carbon deficit, and further the planet’s existing carbon has an unexpectedly high ¹³C/¹²C ratio for its accretionary history. If the Earth was formed from ¹³C-depleated carbon reservoirs, then it has either lost a significant amount of light carbon (i.e. through atmospheric escape) or that carbon is hidden somewhere in the solid Earth. An observed density deficit in the core coupled with the solubility of carbon in iron melts at relevant PT conditions, suggests carbon may be present in abundance in the Earth’s core [2].

Here we model the carbon mass balance between the mantle and core during Earth’s early differentiation. We model the delivery of carbon to the core as a Rayleigh fractionation in which carbon is incrementally added from the overlying magma ocean through metal diapirs. Recent estimates of the carbon isotope fractionation accompanying this process [3] are coupled with solar system ¹³C/¹²C ratios (i.e. potential carbon sources) to evaluate the consistency of carbon in the core with various accretionary hypotheses, including the case of carbon addition following core formation (e.g. through a late veneer). Our results indicate that the Earth’s carbon was likely sourced from chondritic like material, and the Earth’s core may contain more than 90% of the planet’s carbon. Further, our modeling indicates the isotopic observations of the planet can still be explained when a late veneer is considered. Our results have important implications for the evolution of carbon reservoirs on rocky and differentiated planets.

[1] Howell, D., et al., 2020, GCA, doi:10.1016/j.gca.2020.02.011.

[2] Bajgain, S.K., et al., 2021, Communications Earth & Environment, doi:10.1038/s43247-021-00222-7.

[3] Horita, J., and Polyakov, V.B., 2015, PNAS, doi:10.1073/pnas.1401782112.