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We report Ar-40-Ar-39 step-heating ages of Paleocene-Eocene (P-E) boundary impact spherules from Atlantic Margin coastal plain and open ocean sites. We test the hypothesis that the P-E spherules are reworked from an earlier event (e.g., K-Pg impact at similar to 66 Ma), which predicts a cooling age discordant from their depositional age of 55.93 +/- 0.05 Ma at the P-E boundary. Isochrons from the step-heating analysis yield Ar-40-Ar-36 intercepts in excess of the modern in most cases, indicating that the spherules have excess radiogenic Ar (Ar-40*), typical of impact glasses incompletely degassed before solidification. The weighted mean of the isochron-corrected plateau age is 54.2 +/- 2.5 Ma (1 sigma), and their isochron age is 55.4 +/- 4.0 Ma, both indistinguishable from their P-E depositional age, not supporting the K-Pg reworking hypothesis. This is consistent with all other stratigraphic and geochemical evidence for an impact at the P-E boundary and ejecta distribution by air fall.

Plain Language Summary We show that the radioisotopic ages of the recently discovered Paleocene-Eocene (P-E) boundary impact melt spherules (54.2 +/- 2.5 Ma) are indistinguishable from their depositional age (55.93 +/- 0.05 Ma). These initial data indicate that the material is unlikely to have been reworked from some earlier event and hence accompany the climate change at the P-E transition. These air-fall ejecta are the most isochronous P-E horizon available. Inherited radiogenic Ar-40 in the spherules is consistent with a P-E impact site at the Marquez Dome crater (eastern Texas); these carbonates overlie petroleum deposits that could have contributed C-12-enriched carbon to the atmosphere upon impact.