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.

During the past year we have been measuring trace element abundances and searching for anomalously high iridium (Ir) concentrations in continental sedimentary rocks that span the Cretaceous-Tertiary boundary in the Raton and San Juan Basins of northern New Mexico and southern Colorado. Using neutron activation and radiochemical separations, we have identified anomalous concentrations of Ir in samples from two sites in the Raton Basin: in a drill core at York Canyon, about 50 km west of Raton, New Mexico, and in a road cut near the city of Raton. In both cases the anomaly occurs essentially at the base of thin coal beds, across a thickness span of only a few cm and at the same level at which several species of Cretaceous pollen become extinct and the ratio of angiosperm pollen to fern spores drops sharply. The Ir surface density ranges from 8 to 40 × 10 −9 g cm −2 . In the York Canyon core the Ir concentration reaches a value of 5.6 × 10 −9 g/g of rock over a local background of about 10 −11 g/g; the Pt abundance distribution is similar to that for Ir, while Au reaches its maximum concentration about 10 cm below the Ir peak. Se, V, Cr, Mn, Co, and Zn are about two-fold more abundant at the anomaly zone than in adjacent zones, and mass spectrometric 244 Pu analysis showed the 244 Pu/Ir atom ratio ⩽ 1 × 10 7 . In the San Juan Basin we have located a small Ir spike (55 × 10 −12 g/g over a local background of 8 × 10 −12 g/g) that is accompanied by high concentrations of Co and Mn. It is thought to be due to geochemical enrichment processes.