27 Nov 2023
 | 27 Nov 2023

Short communication: Resolving the discrepancy between U–Pb age estimates for the ‘Likhall’ zircon bed, a key level in the Ordovician timescale 

André Navin Paul, Anders Lindskog, and Urs Schaltegger

Abstract. The ‘Likhall’ zircon bed is a rare case of a single-age zircon population from a carbonate rock, which in this case is contextualised with remarkable biotic and environmental changes as well as meteorite bombardment of Earth after an asteroid breakup in space. Published chemical-abrasion, high-precision isotope-dilution, thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb age estimates disagree at the typical precision of <0.1 % for a 206Pb/238U date, which has led to discrepancies in the interpretation of the timing of events and their possible cause–effect relationships. We evaluate here the relative strengths and weaknesses, and discrepancies in the so far published datasets, propose strategies to overcome them and present a new U-Pb dataset with improved precision and accuracy. Ultimately, we find that domains of residual Pb-loss are a significant source of age-offset between previously published data, amplified by differences in data evaluation strategies. Our new dataset benefits from an improved chemical abrasion protocol resulting in a more complete mitigation of decay-damage induced grain portions, and points to a weighted mean age estimate of 466.37±0.14/0.18/0.53 Ma for the ‘Likhall’ zircon population. This age is intermediate between previous estimates, but outside of analytical uncertainty, and provides a firm tie point for the Ordovician timescale.

André Navin Paul, Anders Lindskog, and Urs Schaltegger

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2597', Anonymous Referee #1, 04 Jan 2024
    • AC1: 'Reply on RC1', André Navin Paul, 28 Feb 2024
  • RC2: 'Comment on egusphere-2023-2597', Anonymous Referee #2, 02 Feb 2024
    • AC2: 'Reply on RC2', André Navin Paul, 28 Feb 2024
  • RC3: 'Comment on egusphere-2023-2597', Anonymous Referee #3, 29 Feb 2024
André Navin Paul, Anders Lindskog, and Urs Schaltegger
André Navin Paul, Anders Lindskog, and Urs Schaltegger


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Short summary
The `Likhall´ zircon bed helps to constrain the timing of increased meteorite bombardment of the Earth, during the Ordovician period. It is important to understand the timing of this meteorite bombardment when attempting to correlate it with biodiversity changes during the Ordovician period. Calibrating a good age for the ´Likhall´ bed is however challenging and benefited in this study from advances in sample preparation.