James S. Gilmore

Disruption of the Terrestrial Plant Ecosystem at the Cretaceous-Tertiary Boundary, Western Interior

The palynologically defined Cretaceous-Tertiary boundary in the western interior of North America occurs at the top of an iridium-rich clay layer. The boundary is characterized by the abrupt disappearance of certain pollen species, immediately followed by a pronounced, geologically brief change in the ratio of fern spores to angiosperm pollen. The occurrence of these changes at two widely separated sites implies continentwide disruption of the terrestrial ecosystem, probably caused by a major catastrophic event at the end of the period.

An Iridium Abundance Anomaly at the Palynological Cretaceous-Tertiary Boundary in Northern New Mexico

An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater swamp conditions in the Raton Basin of northeastern New Mexico. The anomaly occurs at the base of a coal bed, at the same stratigraphic position at which several well-known species of Cretaceous-age pollen became extinct.

Iridium anomaly in the upper devonian of the canning basin, Western australia.

A moderate iridium anomaly, about 20 times the local background, has been found in Upper Devonian rocks in the Canning Basin. It occurs at or near the Frasnian-Famennian boundary, which is known to be associated with a major massextinction event of global extent. The anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions. Its occurrence suggests a causal link with some form of meteoroid impact. Moreover, carbon isotope data indicate that a large reduction in biomass could have occurred at this level. However, the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium Frutexites; iridium, platinum, iron, manganese, cobalt, arsenic, antimony, and cerium are preferentially concentrated in filaments of this organism, with concentrations ranging from two to five times that of the matrix. It is possible that Frutexites extracted these elements directly from seawater, without the need for their derivation from an extraterrestrial source.

Conodont survival and low iridium abundances across the Permian-Triassic boundary in south China

The Permian-Triassic sedimentary sequence of China includes one of the most complete and fossiliferous Paleozoic-Mesozoic boundaries known. Closely spaced sampling across the boundary, which is an important extinction event for most organisms, has produced good conodont faunas that show little diversity change. A drop in conodont abundance is the only apparent response to the extinction event. A low concentration of iridium in the boundary clay (0.002 part per billion �20 percent), as well as in samples immediately below and above, that range from 0.004 to 0.034 part per billion do not support the proposal of an extraterrestrial impact event at this boundary in China.

Late Devonian “Kellwasser Event” mass-extinction horizon in Germany: No geochemical evidence for a large-body impact

The hypothesis that the Late Devonian (Frasnian-Famennian) mass extinction was triggered by an asteroidal impact has received renewed attention with the discovery of a Late Devonian Ir anomaly in Australia. In Europe, the mass-extinction event corresponds stratigraphically to the geographically widespread Kellwasser black-shale and bituminous limestone units, and the biological crisis itself has been alternatively designated the Kellwasser Event. The authors report here the results of an extensive geochemical analysis of the Kellwasser stratigraphic interval in a section with exceptional conodont zonal control in the Federal Republic of Germany. No Ir anomaly was found, neither at the biological crisis horizon recognized in Europe nor at the conodont horizon that corresponds to the Ir anomaly zone reported in Australia. No shock-metamorphosed quartz, sanidine spherules, or siderophile-rich magnetic spherules were found, which might have been indicative of a cometary impact. Oxygen-isotope ratios show little variation across the mass-extinction horizon, though carbon-isotope data suggest a sudden increase in phytoplankton activity. They further note that the Australian Ir anomaly (1) is most likely not associated with a large-body impact because no equivalent Ir signature occurs in Europe and (2) is stratigraphically above the European biological crisis horizon, thus postdating the Kellwasser mass-extinction event.

Terminal Ordovician extinction: Geochemical analysis of the Ordovician/Silurian boundary, Anticosti Island, Quebec

Elemental abundances (including Ir), carbon and oxygen ratios in carbonates, mineral content, and thin sections have been measured in samples collected across the conodont-defined Ordovician/Silurian (O/S) boundary exposed on Anticosti Island, Quebec. The Ir concentrations ranged from 5 to a maximum at the boundary of 58 parts per trillion (ppt). However, there is no evidence, on the basis of these Ir results, for the association of a large-body-Earth impact with the O/S extinction, because the Ir concentrations, like those of most other trace elements, are simply proportional to the clay (Al) content in the carbonate sequence. The /sup 13/C//sup 12/C and /sup 18/O//sup 16/O ratios decrease abruptly at the boundary, then just as abruptly increase to a long period of higher than preboundary ratios. These patterns are probably related to the salinity in the seaway, which was shallowing up to boundary time and then deepened and developed patch-reefs. Fresh-water input from rivers would have been most influential during the shallow-water conditions. 32 references, 4 figures.

Geologic Framework of Nonmarine Cretaceous-Tertiary Boundary Sites, Raton Basin, New Mexico and Colorado

Iridium concentrations are anomalously high at the palynological Cretaceous-Tertiary boundary in fluvial sedimentary rocks of the lower part of the Raton Formation at several localities in the Raton Basin of New Mexico and Colorado. The iridium anomaly is associated with a thin bed of kaolinitic claystone in a discontinuous carbonaceous shale and coal sequence.

Pt-group metal anomalies in the Lower Mississippian of southern Oklahoma

Four iridium (Pt-group elements) abundance anomalies have been found within a stratigraphic span of 3 m in the Lower Mississippian of Oklahoma. In ascending order, the first anomalies occur at the top of the Woodford Shale: Ir = 0.25 ppb, Pt = 48 ppb, Os = 7.5 ppb, and Au = 18 ppb. The anomalies occur just below a redox boundary and we suspect that the enriched elements were precipitated from sea water that contacted the organic- and sulfide-rick black shale. Two more anomalies occur in the Welden Limestone, the lower one weak and the upper one strong (Ir = 0.42 ppb, Pt = 50 ppb, Os = 0.075 ppb, and Au = 0.14 ppb). The excess Ir and Pt (also Co, As, and Ni) might have been enriched from sea water by bacteria at these two horizons. A 70-cm-thick interval of excess heavy siderophiles occurs in the overlying Caney Shale; the interval contains the following peak concentrations: Ir = 0.56 ppb, Pt =150 ppb, Os = 0.51 ppb, Co = 725 ppm, and Ni =1450 ppm. These elements vary in proportion to the Al (clay) content and we suspect that they were carried in with detrital material from erosion of ultramafic source rocks. We found no evidence of microspherules or shocked-mineral grains in any of these anomaly zones.

Iridium Abundances Across the Ordovician-Silurian Stratotype

Chemostratigraphic analyses in the Ordovician-Silurian boundary stratotype section, bracketing a major extinction event in the graptolitic shale section at Dobs Linn, Scotland, show persistently high iridium concentrations of 0.050 to 0.250 parts per billion. There is no iridiumn concentration spike in the boundary interval or elsewhere in the 13 graptolite zones examined encompassing about 20 million years. Iridium correlated with chromium, both elements showing a gradual decrease with time into the middle part of the Lower Silurian. The chromium-iridium ratio averages about 106. Paleogeographic and geologic reconstructions coupled with the occurrence of ophiolites and other deep crustal rocks in the source area suggest that the high iridium and chromium concentrations observed in the shales result from terrestrial erosion of exposed upper mantle ultramafic rocks rather than from a cataclysmic extraterrestrial event.

Stratigraphic occurrences of iridium anomalies at four Cretaceous/Tertiary boundary sites in New Zealand

Three new iridium anomaly sites have been discovered in Cretaceous/Tertiary boundary sequences in New Zealand. These are at Needles Point, Chancet Rocks, and Waipara, where integrated iridium deposition values were 165, 211, and 7 ng/cm2, respectively. In contrast to the previously reported Woodside Creek stratigraphic sequence that had an iridium anomaly of 187 ng/cm2, a ferruginous boundary clay is absent in the three new sites, though the base of the Tertiary is marked by limonite staining. The relatively weak anomaly at the Waipara section is probably due to extensive bioturbation coupled with a high sedimentation rate at the time of deposition. The discovery of these additional boundary rock sequences in New Zealand negates suggestions that the Woodside Creek iridium anomaly was a fortuitous occurrence caused by unusual weathering conditions. The integrated iridium deposition values at these three new sites of somewhat different geology support the previously reported high iridium level for Woodside Creek, which until now was the only iridium anomaly on land in the Southern Hemisphere.