Why is Earth’s surface considered a boundary between a reduced core and an oxidizing atmosphere?

Earth’s surface acts as a redox boundary because it separates the planet’s chemically reduced metallic core from its oxidizing atmosphere.1

During the early formation of Earth, massive geochemical differentiation occurred as the planet accreted from solid bodies. Because there was insufficient oxygen to combine with all available metallic elements, iron with a high affinity for the metallic state sank to the center to form the core, leaving the core in a reduced state.23

Conversely, the atmosphere is characterized by oxidizing conditions, largely due to the presence of free oxygen produced by photosynthesis. The Earth's crust is primarily composed of metal oxides, and the oceans consist of water—an oxide of hydrogen—reflecting the tendency of surface materials to be oxidized by the atmosphere. This boundary is dynamic; while atmospheric processes and weathering promote oxidation, life processes continuously renew complex compounds on the surface through the photosynthetic reduction of carbon dioxide.12