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

Abstract

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.