Nucleosynthetic signatures of primordial origin around supermassive black holes

If primordial black holes (PBHs) seeded the supermassive black holes (SMBHs) at the centers of high-redshift quasars, then the gas surrounding these black holes may reveal nucleosynthetic clues to their primordial origins. We present predictions of altered primordial abundances around PBHs massive enough to seed SMBHs at 𝑧≈6–7.5. We find that if PBHs with initial masses of ∼105  M⊙ are responsible for such SMBHs, they may produce primordial deuterium and Helium fractions enhanced by ≥10%, and lithium abundance depleted by ≥10%, at distances of up to ≈ a comoving kiloparsec away from the black hole after decoupling. We estimate that ∼108  M⊙ of gas is enhanced (or depleted) by at least one percent. Evidence of these modified primordial deuterium, helium, and lithium abundances could still be present if this circum-PBH gas remains unaccreted by the SMBH and in or near the host galaxies of high-redshift quasars. Measuring the abundance anomalies will be challenging, but could offer a novel way to reveal the primordial origin of such SMBH seeds.