Carbon versus Other Light Elements in Earth's Core jie li, bin chen, mainak mookherjee, and guillaume morard Introduction 2] [3] The core consists of a liquid outer shell ranging from 2971 to 5210 km in depth and a solid inner sphere with a radius of 1220 km. 4 Without direct samples, its iron-dominant composition has been inferred from seismological, geochemical, and cosmochemical observations, together with mineral physics constraints from laboratory measurements and theoretical simulations.

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... oth the outer and inner cores are lighter than iron or iron-nickel alloys at relevant pressure-temperature (P-T) values, indicating the presence of one or more elements with smaller atomic numbers than iron. 5 Candidates for the light alloying elements of the core include hydrogen (H), carbon (C), oxygen (O), silicon (Si), and sulfur (S). Earth's core may be the largest repository for terrestrial carbon. As the fourth most abundant element in the solar photosphere, carbon occurs in carbonaceous chondrites and ordinary chondrites as a major or minor element. 6 The silicate Earth is depleted in carbon with respect to CI chondrite by more than two orders of magnitude, and by fiveto ten-fold after accounting for evaporative loss to outer space during accretion. 7 2] [13] [14] Core sequestration can also explain the 13 C enrichment in silicate Earth relative to Mars, Vesta, and chondrites. 15 Cosmochemical and geochemical considerations suggest that the core may contain as much as 1 wt.% (5 at.%) carbon. 15 lower estimate of 0.2 wt.% carbon in the core is derived by assuming that carbon depletion follows the volatility trend. 7 More details are found in Chapter 2. A core containing 1 wt.% carbon would exceed the combined budget of known carbon in the atmosphere, hydrosphere, biosphere, crust, and mantle by one order of magnitude (Figure 3 .1). Even with the lowest estimate of 0.1 wt.% carbon, the core would still account for more than half of Earth's total carbon budget. Constraining the carbon budget of the core is crucial for identifying Earth's building blocks and reconstructing its accretion history. In this chapter, we review constraints on the carbon content of the core from the phase relation, density, and sound velocities of ironcarbon alloys and compare carbon with other light elements in terms of their ability to 40