Iridium abundance measurements across bio-event horizons in the geological record

Geochemical studies have been performed on thousands of rock samples collected across bio-event horizons in the fossil record using INAA for about 40 common and trace elements and radiochemical isolation procedures for Os, Ir, Pt, and Au on selected samples. These studies were begun soon after the Alvarez team announced their discovery of the Cretaceous-Tertiary (K-T) Ir anomaly in marine rock sequences in Europe. With their encouragement the Authors searched for the anomaly in nearby continental (freshwater coal swamp) deposits. In collaboration with scientists from the U.S.G.S. in Denver, the anomaly was located and it was observed that a floral crisis occurred at the same stratigraphic position as the Ir spike. Further work in the Raton Basin has turned up numerous well-preserved K-T boundary sections. Although the Authors have continued to study the K-T boundary and provide geochemical measurements for other groups trying to precisely locate it, the primary effort was turned to examining the other bio-events in the Phanerozoic, especially to those that are older than the terminal Cretaceous. A list of horizons that were examined in collaboration with paleontologists and geologists is given. Results are also given and discussed.