Iridium concentration and noble gas composition of Cretaceous-Tertiary boundary clay from Stevns Klint, Denmark

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  • OSAWA TAKAHITO
    Neutron Imaging and Activation Analysis Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA)
  • HATSUKAWA YUICHI
    Neutron Imaging and Activation Analysis Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA)
  • NAGAO KEISUKE
    Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo
  • KOIZUMI MITSUO
    Innovative Nuclear Science Research Group, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency (JAEA)
  • OSHIMA MASUMI
    Innovative Nuclear Science Research Group, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency (JAEA)
  • TOH YOSUKE
    Innovative Nuclear Science Research Group, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency (JAEA)
  • KIMURA ATSUSHI
    Innovative Nuclear Science Research Group, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency (JAEA)
  • FURUTAKA KAZUYOSHI
    Innovative Nuclear Science Research Group, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency (JAEA)

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The Cretaceous-Tertiary (K-T) boundary about 65 million years ago records a mass extinction event caused by a bolide impact. K-T boundary clay collected from Stevns Klint, Denmark was investigated in this work. Iridium concentrations of eight clays across the K-T boundary were determined using a multiple gamma-ray analysis system after neutron activation. Anomalously high Ir concentrations were detected in five marl samples, with the highest concentration being 29.9 ppb. Four samples were analyzed for all noble gases. No extraterrestrial Ar, Kr, and Xe were discovered in any of the samples, although most of the 3He which was detected was extraterrestrial. Solar-like Ne was observed only in the sample SK4, which had an Ir concentration of 14.3 ppb, indicating the presence of micrometeorites. The solar-like Ne clearly did not originate from an asteroid/comet associated with the bolide impact, as that asteroid is thought to have been extremely large. Also, because there was no sign of a high accretion rate of micrometeorites at the boundary it could not be ascertained whether the solar-like Ne was related to a catastrophic event that led to the extinction of the dinosaurs.

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