Kun Wang, Brian D. E. Chatterton, Moses Attrep, Jr., Charles J. Orth; Iridium abundance maxima at the latest Ordovician mass extinction horizon, Yangtze Basin, China:Terrestrial or extraterrestrial?. Geology ; 20 (1): 39–42. doi: https://doi.org/10.1130/0091-7613(1992)020<0039:IAMATL>2.3.CO;2
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Neutron activation analyses of the Chinese Ordovician/Silurian (O/S) boundary sections at two distant localities in the Yangtze Basin, spanning the horizon of a major latest Ordovician global extinction event, show the maxima of iridium abundances to be coincident with the extinction horizon at the base of the graptolite Glyptograptus persculptus zone. The 0.23 ppb Ir maximum in the Yichang type section is almost as large as the late Eocene impact Ir anomaly. However, we have observed that the Ir abundances in the Chinese sections are closely correlated with the sedimentation rates, and we therefore have concluded that the Ir maxima do not indicate a cataclysmic extraterrestrial impact at this extinction level.