A detailed radiocarbon chronology has been established for the deep-sea core CT85-5 from the Tyrrhenian Sea. This chronology, which is based on the analysis of foraminifera shells, shows a set of reversed ¹⁴C ages for sediments deposited during the eruption of the Campanian Ignimbrite (∼40 ka cal BP). The anomalous young ¹⁴C ages coincide with elevated concentrations of ¹⁰Bc measured in the same core. Although reversals in ¹⁴C ages have been previously found in other records at 40 ka cal BP, such extreme changes have not been observed elsewhere. The enhancement in ¹⁴C concentration in CT85-5 sediments associated with the Campanian Ignimbrite is equivalent to an apparent age ∼15 ka younger than the age for the sediments deposited shortly before the eruption. Here, we present consistent results of repeated measurements showing no analytical problems that can explain the observed rapid changes in ¹⁴C of this particular record.