Initial 40Ar‐39Ar Ages of the Paleocene‐Eocene Boundary Impact Spherules

We report 40Ar‐39Ar 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 ~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 40Ar‐36Ar intercepts in excess of the modern in most cases, indicating that the spherules have excess radiogenic Ar (40Ar*), typical of impact glasses incompletely degassed before solidification. The weighted mean of the isochron‐corrected plateau age is 54.2 ± 2.5 Ma (1σ), 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 40Ar 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 12C‐enriched carbon to the atmosphere upon impact. Key Points Radiometric age of P‐E boundary impact spherules is 54.2 ± 2.5 Ma, consistent with their depositional age Their origin as reworked K‐Pg ejecta is not supported by these data