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Geochemical measurements have been performed on about 8,000 rock samples collected across bio-event horizons using instrumental neutron activation analysis (INAA) for about 40 major, minor, and trace elements and radiochemical isolation procedures for Ir. On selected samples, Os, Pt, and Au were also radiochemically determined. These studies, some previously reported, have encompassed the time interval from the Precambrian-Cambrian transition to the late Eocene impact (microspherule) horizons. Our early work strengthened the Alvarez impact hypothesis by finding the Ir (platinum group element [PGE]) anomaly at the K/T boundary in continental sedimentary sequences. In collaborations with paleontologists we discovered weak to moderately strong Ir anomalies at the Frasnian-Famennian boundary in Australia, in the Early Mississippian of Oklahoma, at the Mississippian-Pennsylvanian boundary of Oklahoma and Texas, and in the late Cenomanian throughout the western interior of North America and on the south coast of England. We have found no compelling evidence for an impact-related cause for these anomalies, although PGE impact signatures in the two late Cenomanian anomalies could be masked by the strong terrestrial mafic to ultramafic overprint. Thus far, our evidence for extinction events older than the terminal Cretaceous does not support recent hypotheses, which suggest that impacts from cyclic swarms of comets in the inner Solar System were responsible for the periodic mass extinctions identified by Raup and Sepkoski. However, much more work on extinction boundaries needs to be done to resolve the problem, especially tedious searching for microspherules and shocked mineral grains, because comet impacts might provide little or no excess Ir.

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