Charles J. Orth

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.

Global fire at the Cretaceous– Tertiary boundary

Cretaceous-Tertiary (K-T) boundary clays from five sites in Europe and New Zealand are 102-104-fold enriched in elemental C (mainly soot), which is isotopically uniform and apparently comes from a single global fire. The soot layer coincides with the Ir layer, suggesting that the fire was triggered by meteorite impact and began before the ejecta had settled.

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.

Palynological and iridium anomalies at Cretaceous-Tertiary boundary, south-central Saskatchewan

The Cretaceous-Tertiary boundary in south-central Saskatchewan is marked by coincident anomalies in abundance of iridium and fern spores at the extinction level of a suite of Cretaceous pollen taxa. Evidence of disruption of the terrestrial flora includes the fern-spore abundance anomaly and local extinction of as much as 30 percent of angiosperm species. The reorganized earliest Tertiary flora is made up largely of surviving species that assumed new roles of dominance. Persistence of climatically sensitive taxa across the boundary indicates that if paleoclimate was altered by the terminal Cretaceous event, it returned quickly to the pre-event condition.

Geochemical evidence for a catastrophic biotic event at the Frasnian/Famennian boundary in south China

A strong {delta}{sup 13}C anomaly coincides with a weak Ir anomaly at the Frasnian/Famennian (F/F) boundary exposed at Xiangtian, Guangxi, south China. The maximum whole-rock Ir abundance is 0.23 ppb (0.35 ppb on a carbonate-free basis) compared with averages of 0.016 and 0.044 ppb above and below the boundary interval. The {delta}{sup 13}C in carbonate abruptly shifts from a late Frasnian level of about +1{per thousand} to {minus}2.49 {per thousand}in the boundary interval and then abruptly returns to pre-boundary levels, suggesting a temporary reduction of surface-water biomass. A widely distributed (over at least several hundred kilometers) limestone breccia unit of possible wave-deposit origin is present directly below the boundary in south China. Enrichments of Al, V, Cr, As, and U, and a dip in the Mn abundance in the boundary interval indicate that reducing conditions might have been associated with the element enrichment. However, an alternative hypothesis is that oceanic impact(s) at the F/F boundary near south China provided the excess Ir and caused the biotic crisis and the formation of breccia deposits observed in the Late Devonian South China Sea.

Elemental abundance anomalies in the late Cenomanian extinction interval: a search for the source(s)

Elemental abundances have been measured by neutron activation methods across the Cenomanian-Turonian (late Cretaceous) extinction interval in samples collected from sixteen sites in the Western Interior Basin of North America and from twelve widely separated locations around the globe, including six ODP/DSDP sites. In most Western Interior Basin sites, in Colombia, and in western Europe (weaker), two closely spaced elemental abundance peaks occur in the upper Cenomanian (∼ 92 m.y.), spanning the ammonite zones ofSciponoceras gracile throughNeocardioceras juddii. Elements with anomalously high concentrations include Sc, Ti, V, Cr, Mn, Co, Ni, Ir, Pt and Au. The lower peak coincides with the disappearance (extinction) of the foraminiferRotalipora cushmani. In North American sectionsR. greenhornensis also disappears at or just below this horizon, but in Europe it disappears considerably earlier thanR. cushmani. A series of molluscan extinction and speciation or migration events also begins near the stratigraphic level of the lower elemental abundance peak. The well-documented positiveδ13C excursion begins just before the extinctions and the elemental anomalies, and continues into the lower Turonian, well above the upper anomaly. This carbon isotope excursion has been observed in East European sections where we find little or no evidence of the elemental anomalies, suggesting that the two phenomena may not be tightly coupled. Elemental abundance ratios in the anomalies closely resemble those of Mid-Atlantic Ridge basalt or Hawaiian lava (tholeiitic), but not those of C1 chondrite, black shale, average crustal rocks, or lamproite and kimberlite of roughly similar age in southeastern Kansas. The excess Ir and other siderophiles hint at possible large-body impact(s) for the source. However, we have not located microspherules (other than biogenic calcispheres) or shocked mineral grains in any of our samples. Furthermore, Sc, Ti, V and Mn are not enriched in differentiated Solar-System bodies. Although the weak geochemical signal from comet impact(s) could be masked by the strong terrestrial-like overprint, these anomalies more likely resulted either from intense seafloor spreading activity or merely from increased circulation of deep, metal-rich water associated with the large late Cenomanian through early Turonian eustatic rise and deep-water opening of the South Atlantic. The flooding of continental seaways and margins also could have contributed to the anomalies by preventing much continental detritus from diluting the normal background marine geochemical component.

The great latest Ordovician extinction on the South China Plate: Chemostratigraphic studies of the Ordovician-Silurian boundary interval on the Yangtze Platform

Continuous samples from four Ordovician-Silurian boundary sections of graptolitic shales and mudstones in the Yangtze Basin (Yangtze Platform) on the South China Plate, spanning the latest Ordovician mass extinction of many unrelated groups of organisms, were chemically analyzed for the abundances of about 40 common and trace elements and iridium. It is observed that the abundances of Ir and other siderophile and chalcophile elements (e.g., Co, Cr, As, Mo, Sb and V) show elevated values in the extinction horizon at the base of the graptolite persculptus Zone, which corresponds to the stratigraphic level for the local systematic boundary in China between the “Ordovician” and “Silurian”. Iridium abundances show maxima at this extinction level in three of the four sections analyzed with varying strength, 230 ppT at Yichang, 92 ppT at Jinxian, and 88 ppT at Youyang (but 50 ppT at Xiushan). These geochemical patterns appear to be of global significance, since similar geochemical anomalies (including weak Ir anomaly) were also recorded at about the same stratigraphic level in many other sections previously studied in Quebec, Yukon, Arctic and Northwest Territories, Canada, and Scotland. Although impact by a comet with low platinum group element abundances could have been responsible for the geochemical anomalies, we find it is most plausible to attribute the geochemical signal to the effects of a lower sedimentation rate and reducing water conditions at this horizon, the beginning of a rapid transgression induced by rapid melting of the ice caps on Gondwana. Carbon isotope excursions in organic material at Yichang, together with other carbon isotope data from central Sweden, northwestern Canada, Latvia, Quebec and Wisconsin, suggest that, after a period of decreasing atmospheric and marine pCO2 in the latest Ordovician (the Hirnantian), a sudden, large pCO2 increase in the ocean and atmosphere may have occurred at the extinction horizon at the base of the persculptus Zone. A greenhouse effect might have existed for a short time. What triggered such a sudden pCO2 increase is uncertain, but a rapid CO2 degassing caused by volcanism, tectonics, and/or extraterrestrial impact could have been responsible.

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.

A new Cretaceous-Tertiary boundary site at Flaxbourne River, New Zealand - Biostratigraphy and geochemistry

Abstract An exceptionally complete rock sequence across the Cretaceous-Tertiary (K-T) boundary has been discovered near the Flaxboume River, Marlborough Province, South Island, New Zealand. The boundary is marked by a large Ir anomaly (21 ng/g on a decalcified basis), with an integrated abundance of 134 ng/ cm2 after correction for background. Above the boundary there is a 30 cm transition zone, in which a few Cretaceous foraminiferal taxa such as Hedbergella monmouthensis and Guembelitria cretacea survived, though with reduced abundance and size (only ca. 1 2 to 1 4 of normal), apparently reflecting environmental stresses. INAA and ICP analyses show that, in addition to Ir, the boundary clay is also enriched in Cr and Ni, mainly from meteoritic material, and As, Co, Cu, Sb, and Zn from terrestrial sources. Volcanic sources, even when scaled to the 10 7 km 3 volume of the Deccan basalts, fail by three orders of magnitude to account for the Ir and As at the K.-T boundary and by even larger factors for Sb, Zn, Cu, etc. Comparison of our data with those from six other K-T boundary sites shows that the Zn Sb , As Sb , and Zn As ratios generally fall between crustal and oceanic values, suggesting contributions from both sources. Mass balance calculations show that As and Sb could be derived from only 300–500 m of ocean water or also from modest amounts (20–36 g/cm 2 ) of average crustal rock. Copper and Zn, on the other hand, can only be derived from crustal or mantle rock (5–15 g/cm 2 ), presumably impact ejecta. Such an amount of ejecta is fairly close to the global fallout of boundary clay (2–5 g/cm 2 ).