Abstract
Earth’s surface environment is largely influenced by its budget of major volatile elements: carbon (C), nitrogen (N), and hydrogen (H). Although the volatiles on Earth are thought to have been delivered by chondritic materials, the elemental composition of the bulk silicate Earth (BSE) shows depletion in the order of N, C, and H. Previous studies have concluded that non-chondritic materials are needed for this depletion pattern. Here, we model the evolution of the volatile abundances in the atmosphere, oceans, crust, mantle, and core through the accretion history by considering elemental partitioning and impact erosion. We show that the BSE depletion pattern can be reproduced from continuous accretion of chondritic bodies by the partitioning of C into the core and H storage in the magma ocean in the main accretion stage and atmospheric erosion of N in the late accretion stage. This scenario requires a relatively oxidized magma ocean (log _{10} f_{{mathrm{O}}_2}gtrsim{mathrm{IW}}-2, where f_{{mathrm{O}}_2} is the oxygen fugacity, mathrm{IW} is log _{10} f_{{mathrm{O}}_2}^{mathrm{IW}}, and f_{{mathrm{O}}_2}^{mathrm{IW}} is f_{{mathrm{O}}_2} at the iron-wüstite buffer), the dominance of small impactors in the late accretion, and the storage of H and C in oceanic water and carbonate rocks in the late accretion stage, all of which are naturally expected from the formation of an Earth-sized planet in the habitable zone.
Highlights
O2 is fO2 at the iron-wüstite buffer), the dominance of small impactors in the late accretion, and the storage of H and C in oceanic water and carbonate rocks in the late accretion stage, all of which are naturally expected from the formation of an Earth-sized planet in the habitable zone
On the growing proto-Earth with planetesimal accretion and several giant impacts, the formation of magma oceans allowed volatiles to be stored within the magma ocean[10]
Core-forming metal could have removed some of the iron-loving elements from the magma ocean during the main accretion stage[11]
Summary
O2 is fO2 at the iron-wüstite buffer), the dominance of small impactors in the late accretion, and the storage of H and C in oceanic water and carbonate rocks in the late accretion stage, all of which are naturally expected from the formation of an Earth-sized planet in the habitable zone. Similar isotopic compositions of volatiles in Earth and chondrites suggests that delivery was made by chondritic materials[4] Their absolute abundances are largely uncertain, the abundances of C–N–H in BSE relative to chondrites are known to have a V-shaped depletion p attern[5,6]. Other works have attributed the discrepancy largely to the accretion of thermally processed or differentiated, non-chondritic b odies[15,16], which are hypothetical and may not satisfy the isotopic constraints[8] We consider another mechanism that can fractionate C/N: the preferential loss of N relative to C and H by impacts during the late accretion stage, where N is partitioned into the atmosphere, while C and H are partitioned into the oceans and carbonate rocks[14]. This work builds on previous studies[5,15,16] in terms of volatile element partitioning, but makes improvements to simulate core formation and atmospheric loss as continuous processes rather than single stage events
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