Carbon content of the Earths outer core

Earths outer core comprises primarily of iron with 5-10 % nickel liquid. However, the geophysical observation of density () and sound wave velocity (VP) for the core cannot be fully explained by iron nickel alloy and it is speculated that some light elements are likely to be present in the Earths outer core. Carbon is one such light element and it is crucial to assess whether it is present in the Earths outer core and, also provide constraints on its abundance in the core. This is particularly important since carbon along with oxygen, hydrogen, and nitrogen are elements that are essential for life on the Earth, so the total inventory of these elements in the Solid Earth and how they have been partitioned into the different Earth reservoirs is important. It is also known that carbon is abundant in the cosmos, and it is highly siderophile at temperatures and pressures that were likely during the formation of the Earths core. To estimate the amount of carbon present in the Earths outer core, we examine the effect of carbon on the and VP of liquid iron at conditions relevant to the Earths outer core using first principles molecular dynamics simulations. Our results show that for a Fe-C binary, ~1-4 wt.% carbon can explain the seismologically observed VP. However, this amount of carbon is too little to explain the density deficit in the outer core. We do not find any unique concentration of carbon that can simultaneously explain the and VP of the outer core if carbon is the only light element present in the core. However, we considered the ternary mixtures Fe-C-LE, where LE refers to light elements such as O, S, H, Si, and N and examined these ternaries over a range of plausible core temperatures. We also used cosmochemical constraints from iron meteorites and the density discontinuity at the inner-outer core boundary to refine our estimation on the carbon content of the outer core. We find that 0.3-2.0 wt.% carbon can simultaneously satisfy both the and VP of the outer core. Even at ~0.3 wt.% carbon concentration, Earths outer core would contain 5.5 1024 gram of carbon i.e., the largest reservoir of terrestrial carbon in the Earth. Acknowledgment: This research was supported by an intramural award from the FSU Provosts Office and the OPD. We acknowledge NSF grants (EAR1763215 and EAR1753125) and computing resources from RCC@FSU and XSEDE.