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Abstract We report40Ar‐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 yield40Ar‐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.
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The Ocean's Global 39 Ar Distribution Estimated With an Ocean Circulation Inverse Model
Abstract 39Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of39Ar using a steady ocean circulation inverse model. Our estimates match available39Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that39Ar traces out the world ocean's ventilation pathways and that the39Ar age ΓArand the ideal mean age have broadly similar large‐scale patterns. At the surface,39Ar is close to saturated except at high latitudes. Undersaturation imparts a finite39Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating ΓArby ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of39Ar, thus providing guidance for future measurements.
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- Award ID(s):
- 1658392
- PAR ID:
- 10360180
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 13
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 7491-7499
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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