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

Morgan F. Schaller, Brent D. Turrin, Megan K. Fung, Miriam E. Katz, Carl C. Swisher

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)9091-9102
Number of pages12
JournalGeophysical Research Letters
Issue number15
StatePublished - Aug 16 2019

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Earth and Planetary Sciences(all)


  • Ar isotopes
  • PETM
  • ejecta
  • extraterrestrial impact
  • radiometric dates


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