Fig. 4. Earth’s heterogeneous accretion of nitrogen during its main accretion phase.

a, b The N-content and -isotopic composition of the proto-Earth’s core, mantle, and atmosphere as a function of mass fraction accreted. At the end of Earth’s accretion, the N-content and δ¹⁵N of the proto-Earth’s mantle are ~2.4 ppm and −4.7‰, respectively, consistent with previous estimates for the present-day mantle^5,6,8. The N-content of the proto-Earth’s atmosphere is ~1.7 ppm, consistent with previous estimates for Earth’s early atmosphere⁵² (note that the ppm N in the atmosphere is based on the atmosphere N-mass normalized to the silicate Earth mass). The delivery of oxidized, CI chondrite-like materials plays an important role in enhancing the proto-atmosphere δ¹⁵N from −30‰ to 0‰–+3‰, which are close to Earth’s surface δ¹⁵N (atmosphere + crust; Fig. 1). Earth’s core contains more than 90% of Earth’s bulk N. The error bars at ~99% accretion were based on ±2σ for $D_{\mathrm{N}}^{\mathrm{metal}/\mathrm{silicate}}$ and ∆¹⁵ N^{metal-silicate}. c Illustration showing the delivery of N to the proto-Earth by first reduced, EC-like impactors and then increasingly oxidized impactors. The Earth accreted its first 60% mass from EC-like planetesimals/embryos with δ¹⁵N of −30‰ (Stage-1). After acquiring its 60% mass, the Earth started to accrete from increasingly oxidized impactors which have δ¹⁵N varying from −30‰ to +5‰, and from minimal CI chondrite-like materials which have an average δ¹⁵N of +40‰ (Stages-2 and −3). As shown in Stage-4, inefficient mantle mixing of EC-like impactor materials may explain the negative δ¹⁵N of some deep mantle diamonds²⁵, while a long-term preservation of oxidized impactor materials at Earth’s core–mantle boundary may explain the positive δ¹⁵N of OIB³⁰.