Ordovician‐Silurian Boundary Extinction and Its Relationship to Iridium and Carbon Isotope Anomalies

The abundance distributions of more than 40 elements in the No. 502 Ordovician-Silurian (O/S) boundary section at Fenxiang, Yichang have been studied by RNAA and INAA. The results show that in the boundary bed, there is a distinctive Ir anomaly because the Ir concentration abruptly increases to 0.64 ppb. Furthermore, the Ir is positively correlated in abundance variation with some siderophile and sulphophile elements. In the same bed of the O/S boundary section at Huanghuachang, Yichang, there is also a δ13C excursion. These geochemical signs support the hypothesis that the terminal Ordovician mass extinction was probably related to extraterrestrial event, and provide new evidence for defining the O / S boundary between the Hirnantia-Kinnella and G. persculptus Zones.