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1.
The Permian-Triassic boundary (PTB) event, which occurred about 251.4 million years ago, is marked by the most severe mass extinction in the geologic record. Recent studies of some PTB sites indicate that the extinctions occurred very abruptly, consistent with a catastrophic, possibly extraterrestrial, cause. Fullerenes (C60 to C200) from sediments at the PTB contain trapped helium and argon with isotope ratios similar to the planetary component of carbonaceous chondrites. These data imply that an impact event (asteroidal or cometary) accompanied the extinction, as was the case for the Cretaceous-Tertiary extinction event about 65 million years ago. 相似文献
2.
Tschudy RH Pillmore CL Orth CJ Gilmore JS Knight JD 《Science (New York, N.Y.)》1984,225(4666):1030-1032
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. 相似文献
3.
Ward PD Haggart JW Carter ES Wilbur D Tipper HW Evans T 《Science (New York, N.Y.)》2001,292(5519):1148-1151
The end-Triassic mass extinction is one of the five most catastrophic in Phanerozoic Earth history. Here we report carbon isotope evidence of a pronounced productivity collapse at the boundary, coincident with a sudden extinction among marine plankton, from stratigraphic sections on the Queen Charlotte Islands, British Columbia, Canada. This signal is similar to (though smaller than) the carbon isotope excursions associated with the Permian-Triassic and Cretaceous-Tertiary events. 相似文献
4.
The (40)Ar/(39)Ar ages of a sanidine clast from a melt-matrix breccia of the Manson, Iowa, impact structure (MIS) indicate that the MIS formed 73.8 +/- 0.3 million years ago (Ma) and is not coincident with the Cretaceous-Tertiary boundary (64.43 +/- 0.05 Ma). The MIS sanidine is 9 million years older than (40)Ar/(39)Ar age spectra of MIS shock-metamorphosed microcline and melt-matrix breccia interpreted earlier to be 64 to 65 Ma. Grains of shock-metamorphosed quartz, feldspar, and zircon were found in the Crow Creek Member (upper Campanian) at a biostratigraphic level constrained by radiometric ages in the Pierre Shale of South Dakota that are consistent with the (40)Ar/(39)Ar age of 73.8 +/- 0.3 Ma for MIS reported herein. 相似文献
5.
Alvarez W Kauffman EG Surlyk F Alvarez LW Asaro F Michel HV 《Science (New York, N.Y.)》1984,223(4641):1135-1141
There is much evidence that the Cretaceous-Tertiary boundary was marked by a massive meteorite impact. Theoretical consideration of the consquences of such an impact predicts sharp extinctions in many groups of animals precisely at the boundary. Paleontological data clearly show gradual declines in diversity over the last 1 to 10 million years in various invertebrate groups.Reexamination of data from careful studies of the best sections shows that, in addition to undergoing the decline, four groups (ammonites, cheilostomate beyozoans, brachiopods, and bivalves) were affected by sudden truncations precisely at the iridium anomaly that marks the boundary. The paleontological record thus bears witness to terminal-Cretaceous extinctions on two time scales: a slow decline unrelated to the impact and a sharp truncation synchronous with and probably caused by the impact. 相似文献
6.
Brooks RR Reeves RD Yang XH Ryan DE Holzbecher J Collen JD Neall VE Lee J 《Science (New York, N.Y.)》1984,226(4674):539-542
Iridium and 26 other elements were determined in shale from the Cretaceous-Tertiary boundary at the locus classicus (for iridium anomalies) at Woodside Creek, New Zealand. Iridium, gold, copper, cobalt, chromium, nickel, arsenic, molybdenum, and iron were enriched in the basal 2 millimeters of the 8-millimeter shale parting as compared with the rest of the stratigraphic column. No other shale partings in the column had anomalous concentrations of any element when the data were expressed on a carbonate-free basis. The boundary material showed striking compositional similarities with the Stevns Klint Danish boundary shale. Elemental concentrations were in general much higher in the New Zealand material than in nonboundary shales from elsewhere in the world. The high concentration of iridium (153 nanograms per gram) in the basal layer of the boundary, together with the enrichment of other siderophile elements supports the idea of an extraterrestrial source for much of the material. The iridium/gold ratio of 2.1 is also in accordance with such a source. The iridium content of the basal layer is higher than for any other marine boundary shale obtained on land. The integrated iridium value is 187 nanograms per square centimeter of boundary surface. 相似文献
7.
Continental flood basalt (CFB) volcanism is hypothesized to have played a causative role in global climate change and mass extinctions. Uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of CFB volcanism. Our results document a 25% decline in the marine 187Os/188Os record that predates the Cretaceous-Tertiary boundary (KTB) and coincides with late Maastrichtian warming. We argue that this decline provides a chemostratigraphic marker of Deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of Deccan volcanism was a transient global warming event of 3 degrees to 5 degrees C that is fully resolved from the KTB mass extinction. 相似文献
8.
At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolidewater impact. 相似文献
9.
Analyses of the clay mineralogy of samples from the Cretaceous-Tertiary boundary layer at four localities show that the boundary clay is neither mineralogically exotic nor distinct from locally derived clays above and below the boundary. The significant ejecta component in the clay that is predicted by the asteroid-impact scenario was not detected. 相似文献
10.
Eight recognized or theorized paleoceanographic events during the past 70 million years were tested against changes in the global deep-sea benthic ostracode fauna. Two events, the sudden Cretaceous-Tertiary boundary event at 66 million years ago and the more gradual 40-million-year event (formation of the psychrosphere), show up most dramatically. Before the 40-million-year event, ostracodes freely radiated into the deeper water regions but were provincial. The development of thermal stratification isolated these deep-water taxa, mostly the survivors of the Cretaceous-Tertiary boundary event, which adapted to a new, free-flowing but more frigid ecosystem and spread rapidly throughout the world. 相似文献
11.
Evidence indicates that the Cretaceous-Tertiary boundary is very sharp, and, within the limits of resolution, it is apparently synchronous at the various boundary localities. Arguments to the contrary, particularly those of Officer and Darke, are shown to invalid. 相似文献
12.
The fossil sequences from cores across the Cretaceous-Tertiary boundary show a, range of transition times and transition time intervals depending on the fossil indicators and the location of the site. These variations, together with the pattern of iridium distribution with depth at some sites, differences in total amounts of iridium, variations in noble metal abundances normalized to extraterrestrial concentrations, the depositional effects that might be expected in a reducing environment, and the clay mineralogy of the boundary layer clays, put into question the interpretation that an extraterrestrial event was the cause of the faunal changes and the iridium anomaly in the vicinity of the Cretaceous-Tertiary transition. It seems more likely that an explanation for the changes during the transition will come from continued examination of the great variety of terrestrial events that took place at that time, including extensive volcanism, major regression of the sea from the land, geochemical changes, and paleoclimatic and paleoceanographic changes. 相似文献
13.
Berryman KR Cochran UA Clark KJ Biasi GP Langridge RM Villamor P 《Science (New York, N.Y.)》2012,336(6089):1690-1693
The scarcity of long geological records of major earthquakes, on different types of faults, makes testing hypotheses of regular versus random or clustered earthquake recurrence behavior difficult. We provide a fault-proximal major earthquake record spanning 8000 years on the strike-slip Alpine Fault in New Zealand. Cyclic stratigraphy at Hokuri Creek suggests that the fault ruptured to the surface 24 times, and event ages yield a 0.33 coefficient of variation in recurrence interval. We associate this near-regular earthquake recurrence with a geometrically simple strike-slip fault, with high slip rate, accommodating a high proportion of plate boundary motion that works in isolation from other faults. We propose that it is valid to apply time-dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide. 相似文献
14.
The origin and possible antiquity of faunas at deep-sea hydrothermal vents and seeps have been debated since their discovery. We used the fossil record of seep mollusks to show that the living seep genera have significantly longer geologic ranges than the marine mollusks in general, but have ranges similar to those of deep-sea taxa, suggesting that seep faunas may be shaped by the factors that drive the evolution of life in the deep sea in general. Our data indicate that deep-sea anoxic/dysoxic events did not affect seep faunas, casting doubt on the suggested anoxic nature and/or global extent of these events. 相似文献
15.
(40)Ar/(39)Ar dating of tektites discovered recently in Cretaceous-Tertiary (K-T) boundary marine sedimentary rocks on Haiti indicates that the K-T boundary and impact event are coeval at 64.5 +/- 0.1 million years ago. Sanidine from a bentonite that lies directly above the K-T boundary in continental, coal-bearing, sedimentary rocks of Montana was also dated and has a (40)Ar/(39)Ar age of 64.6 +/- 0.2 million years ago, which is indistinguishable statistically from the age of the tektites. 相似文献
16.
Pillmore CL Tschudy RH Orth CJ Gilmore JS Knight JD 《Science (New York, N.Y.)》1984,223(4641):1180-1183
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. 相似文献
17.
The mineralogy of shocked mineral and lithic grains in the Cretaceous-Tertiary (K-T) boundary claystone worldwide is most consistent with a bolide impact on a continent. Both the concentrations and sizes of these shocked grains are greatest in the western interior of North America. These data suggest that the Manson impact structure in north-central Iowa is a viable candidate for the K-T boundary impact event. Argon-40-argon-39 age spectrum dating of shocked microcline from the crystalline central uplift of the Manson impact structure indicates that there was severe argon-40 loss at 65.7 +/- 1.0 million years ago, an age that is indistinguishable from that of the K-T boundary, within the limits of analytical precision. 相似文献
18.
Heymann D Chibante LP Brooks RR Wolbach WS Smalley RE 《Science (New York, N.Y.)》1994,265(5172):645-647
High-pressure liquid chromatography with ultraviolet-visible spectral analysis of toluene extracts of samples from two Cretaceous-Tertiary (K-T) boundary sites in New Zealand has revealed the presence of C(60) at concentrations of 0.1 to 0.2 parts per million of the associated soot. This technique verified also that fullerenes are produced in similar amounts in the soots of common flames under ambient atmospheric conditions. Therefore, the C(60) in the K-T boundary layer may have originated in the extensive wildfires that were associated with the cataclysmic impact event that terminated the Mezozoic era about 65 million years ago. 相似文献
19.
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. 相似文献
20.
Asher RJ Meng J Wible JR McKenna MC Rougier GW Dashzeveg D Novacek MJ 《Science (New York, N.Y.)》2005,307(5712):1091-1094
We describe several fossils referable to Gomphos elkema from deposits close to the Paleocene-Eocene boundary at Tsagan Khushu, Mongolia. Gomphos shares a suite of cranioskeletal characters with extant rabbits, hares, and pikas but retains a primitive dentition and jaw compared to its modern relatives. Phylogenetic analysis supports the position of Gomphos as a stem lagomorph and excludes Cretaceous taxa from the crown radiation of placental mammals. Our results support the hypothesis that rodents and lagomorphs radiated during the Cenozoic and diverged from other placental mammals close to the Cretaceous-Tertiary boundary. 相似文献