Abstract
Evaluating carbon’s candidacy as a light element in the Earth’s core is critical to constrain the budget and planet-scale distribution of this life-essential element. Here we use first principles molecular dynamics simulations to estimate the density and compressional wave velocity of liquid iron-carbon alloys with ~4-9 wt.% carbon at 0-360 gigapascals and 4000-7000 kelvin. We find that for an iron-carbon binary system, ~1-4 wt.% carbon can explain seismological compressional wave velocities. However, this is incompatible with the ~5-7 wt.% carbon that we find is required to explain the core’s density deficit. When we consider a ternary system including iron, carbon and another light element combined with additional constraints from iron meteorites and the density discontinuity at the inner-core boundary, we find that a carbon content of the outer core of 0.3-2.0 wt.%, is able to satisfy both properties. This could make the outer core the largest reservoir of terrestrial carbon.
Highlights
Evaluating carbon’s candidacy as a light element in the Earth’s core is critical to constrain the budget and planet-scale distribution of this life-essential element
For the core adiabat with TICB = 6000 K, we find that preliminary reference Earth model (PREM)
Density at core–mantle boundary (i.e., TCMB = 4400 K) could be explained by 6.9 ± 0.4 wt.% of carbon dissolved in liquid iron alloy (Fig. 3)
Summary
Evaluating carbon’s candidacy as a light element in the Earth’s core is critical to constrain the budget and planet-scale distribution of this life-essential element. When we consider a ternary system including iron, carbon and another light element combined with additional constraints from iron meteorites and the density discontinuity at the inner-core boundary, we find that a carbon content of the outer core of 0.3-2.0 wt.%, is able to satisfy both properties. Carbon is highly soluble in metallic Fe-rich melt, with solubility reaching ≥5–8 wt.% at the core and core-forming magma ocean conditions[8,11,12]. Experiments that show such a high concentration of carbon in the Ferich melt are generally carbon saturated, whereas the carbon abundance in natural systems is expected to be well below the saturation limit imposed by graphite/diamond.
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