Isotopically heavy carbon in the core

Constraining the origin of Earth's carbon requires knowing the isotopic composition of the core-a geochemical signature long presumed to be beyond observation for the planet's largest carbon reservoir. Here, we report high-precision δ¹³C measurements from Hawaiian plume melt inclusions entrapped before significant degassing and sampling distinct mantle sources. Melt inclusions from shield-stage lavas, interpreted to sample the plume's central axis, inferred to be rooted in the core-mantle boundary thermal boundary layer, converge toward a primary δ¹³C signature of -1 ± 2‰. In contrast, melt inclusions from rejuvenated-stage lavas, derived from entrained mantle at the plume's periphery, record a normal mantle source signature of -7 ± 2‰. We interpret the uniquely ¹³C-rich shield-stage signature as the isotopic fingerprint of carbon exchanged from the core into the mantle. Our results point to ¹³C-rich carbonaceous chondrites as the primary source of Earth's carbon.