Why might deeper layers of Earth contain more sulfur than the crust?

Deeper layers of the Earth, particularly the core, may contain more sulfur than the crust due to the process of primary geochemical differentiation that occurred during the planet's early history.1

As the Earth formed, massive segregation of elements took place. Because there was insufficient oxygen to combine with all the major metallic elements, the remaining iron formed iron sulfide. While much of the Earth's iron sank to form the core, it carried with it other elements, including sulfur.1

Geophysical evidence supports the presence of sulfur in the core, as it helps account for the core's mean atomic number being lower than that of pure iron. While the crust is enriched in elements that do not readily enter the crystal structures of mantle minerals—such as alkali metals—sulfur's association with the metallic iron phase during the Earth's formation likely resulted in its concentration within the deeper interior rather than the crust.213