B31E-01 08:00h
The Chicxulub Impactites: What makes a crater lethal ?
So far Chicxulub is the only impact crater clearly linked to a mass extinction event. Its impact-products differ from equivalent lithologies recovered at or around other impact structures, and may indicate that several factors together contributed to the devastation of the biosphere. There is certainly a size and depth effect. The impactites recovered in this 180-km in diameter peak-ring crater contain a high proportion of basement fragments and various types of melt, indicating intense excavation into the deepest part of the Yucatan crust. The carbonates and evaporites present in the upper 3-km of the target rock is another factor. The suevites are rich in carbonates, as clast and within the matrix. Calcite crystals, which morphology indicates a formation by quenching from a carbonate melt also occur. The presence of carbonate melts (~ 300 km3 or more) and the spontaneous recombination with CaO limits the amount of CO2 released by the impact in an already CO2-rich Upper Cretaceous atmosphere. The role of evaporites is critical, but its exact behavior during the cratering process remains poorly understood. Below the impactite at a depth of ~ 895 m, the Yacopoil drill hole encountered a sequence of Upper Cretaceous mega-blocks; based on this core, evaporites formed approximately 30% of the Yucatan target lithology. This more than the proportion (<10%) present in the suevites. Porosity in some suevite units may indicate the presence of now dissolved anhydrite phase, but in rather limited quantity. An explanation might be that that anhydrite was not present in the deepest part of the sedimentary target rock, where from the suevitic material most likely originated. The shallow cores drilled at the crater periphery contain anhydrite / carbonate breccias, deposited around the rim by the ejecta curtain, which probably sampled a more superficial part of the sedimentary target. Perhaps, the more superficial evaporite layers were completely vaporized upon impact. The climate perturbation models based on the injection in the atmosphere of a high abundances of S-bearing gases (~ 300 Gt) should then be favored. However, modeling also shows that beyond a given concentration in S aerosols, the climate forcing reaches a saturation level. Chicxulub appears thus as a rather unique event; it is unclear if similar size impacts on other target or smaller impacts on volatile-rich targets would have the same lethal consequence for the biosphere ?
B31E-02 08:15h
Does He-3 Provide Evidence for Extinction-Causing Impacts?
Elevated He-3 concentrations can provide strong evidence for an extraterrestrial component in many sedimentary rocks, but in themselves are only suggestive of a major impact. Because most extraterrestrial He-3 is attributable to interplanetary dust rather than large impactors, few-Myr episodes of elevated He-3 flux likely signal dust-producing solar system events (comet showers, asteroidal breakups) that may or may not be associated with enhancement of Earth-crossing bodies. Importantly, no post-Permian extinction boundary appears to correspond to such an event. For example, studies of the P/Tr boundary at Meishan, in Austria, and our new data from Opal Creek, Canada reveal no elevated He-3 levels. If impacts played a role in the P/Tr extinction, they were not associated with such solar system events. In contrast neither a few-Myr episode of enhanced He-3 accumulation at ~35 Ma (and two coeval large impact craters) nor a new event we find at ~8 Ma are associated with major extinctions. Thin He-3 rich layers or spikes have been reported in several boundary sediments including the P/Tr. These spikes and their reported association with He-3-bearing fullerenes have been attributed to a major impact event. We failed to detect such spikes at Meishan and in our new study at Opal Creek. Given the large magnitude of the reported spikes, it seems unlikely that our failure arises from differences in analytical sensitivity or procedures. If the previously reported high He-3 concentrations arise from a P/Tr impact, the deposition or preservation of the He-3 must be extremely heterogeneous.
B31E-03 08:30h
Impact vs. Volcanic Causes of the End-Permian Mass Extinction
Although much paleontological, and geochemical evidence is consistent with an extra-terrestrially induced impact at the Permo-Triassic boundary, and suggestive evidence of such an impact has been published, significant dispute exists within the paleontologic community about the viability of this cause. The leading alternative cause involves the effects of massive volcanic eruptions associated with the generation of the Siberian flood basalts over about 1million years. Much of the Siberian flood basalts erupted through a massive coal basin, adding additional atmospheric carbon dioxide to that released by the eruptions, but as Svenson et al recently observed for the Late Eocene, causing thermal generation of methane via oxidation of the coal. . The combination of acid rain from volcanically induced sulfur dioxide, the volcanically released carbon dioxide and the generation of carbon dioxide and methane from the coal beds would cause rapid greenhouse warming and ocean acidification. Such a model is consistent with the evidence for shallow-water anoxia, the rapidity (but not catastrophic nature) of th e extinction and with the geochemical record.
B31E-04 08:45h
Meteorite Impact at the Bedout High, NW Australian Margin, and Seismic Velocities: is There a Connection?
The Bedout High in the Roebuck Basin at the NW Australian Margin (NWAM) appears to be a good candidate for a massive impact structure associated with the global Permian/Triassic extinction event. On a regional and crustal scale, the NWAM is one of the best studied offshore areas of Australia: ocean-bottom seismograph (OBS) survey supplemented by deep reflection seismic studies in the region has enabled co-interpretation of conventional deep seismic reflection data and accurate seismic velocity models on several transects, including one across the Bedout High. The purpose of this research is to investigate if there is any manifestation of the meteorite impact on a crustal scale, and also on a finer scale of seismic velocity variation in the basement. The impact of the suggested magnitude may have significantly modified the crustal structure in the region. Depth conversion of reflection seismic data indicates that depth to basement at the top of the Bedout High is approximately 3.9 km, and that the High stands more than 4 km above the surrounding sedimentary basins. The basement and crust in the Roebuck Basin have a number of features that distinguish it from other basins at the NWAM. Rapid crustal thinning outboard of the Bedout High and the presence of a thick layer of magmatic underplating in the lower crust are among these features. The meteorite impact may have been one of the possible causes to have triggered upper mantle melting and generation of a voluminous layer of underplated material. On a finer scale, OBS-derived seismic velocity variation along the basement is speculatively interpreted to be consistent with impact-related effects. However, existing seismic and potential field data do not allow accurate estimates of the extent of the crust affected by the meteorite impact, and effects that it may have had on the subsequent rifting, thermal, sedimentation and hydrocarbon maturation regimes in the area. Further multidisciplinary research is necessary to answer these questions.
B31E-05 09:00h
Trace Element Geochemistry of Matrix Glass from the Bedout Impact Structure,Canning Basin NW Australia
We report on geochemical and petrographic analysis of separated matrix glass from Lagrange-1 and Bedout-1 drill cores that penetrated the Bedout structure offshore NW Australia. The results support the conclusion that the Bedout structure was produced by a a major ET impact at the end-Permian that generated shock melted glass and impact breccias (Becker et al., Science, v.304, p.1469, 2004) The Bedout structure is a 30 km, circular, 1.5 km uplifted basment high that occurs on the passive margin offshore NW Australia. The isolated feature, covered by 3 km of Triassic to Recent sediments,is not consistent with any typical volcanic province (i.e. arc or hotspot volcanism). This hypothesis is supported by the unique mineralogy and chemistry of the matrix glass. At Lagrange, major elements crudely resemble low-K, Fe-Ti basalts while the trace element patterns have two distinct signatures. The lower 250 m of Lagrange (3260 - 3010 m depth) have essentially flat REE and "spider" patterns that superficially resemble some E-MORB; a signal not typically found in arc, hotspot or continental margin settings. The upper 150 meters (3000 - 2850m) of Lagrange and the entire Bedout core (3030 - 3070m) have similar light REE-enriched patterns but low levels of alkalis, alkaline-earths and high field strength elements. Again, the chemistry is not consistent with an arc or hotspot setting, based on the low Ba and extremely low Sr (30-110 ppm) concentrations. Based on the geophysical, chemical and petrologic evidence, we hypothesize that the Bedout structure formed as the result` of an ET impact with Permian age rift margin basalts and continental sediment. The basalts did not completely melt as evidenced by the abundance of large (1 mm) An$_{50}$ plagioclase,that exist as both crystalline plag and shock melted maskelynite. Plagioclase is the major repository of Sr in basalts and the lack of a plagioclase contribution to the melt glass is reflected in the low Sr abundance. Shock-transformed plagioclase glass that still retains the lath shape and chemistry of An$_{50}$ are one of the major indicators of the impact origin for Bedout.
B31E-06 09:15h
Laser Raman Spectroscopic Confirmation of Maskelynite in the Bedout Impact Breccia, Offshore, NW Australia.
We report the results of a detailed laser Raman spectroscopic study of isotropic plagioclase laths ($\sim$An50) from the 9986 ft. core sample of the Bedout High, interpreted recently by us (Becker et al. Science, 304, p1469, 2004) as the possible remnant of an end-Permian impact crater, offshore of NW Australia. These plagioclase laths are associated with spherulitic glass fragments of nearly pure silica in composition, as well as pure albite and other heterogeneous glassy fragments in a highly brecciated volcanic-like host rock. We performed laser Raman analysis on the same grains shown in Fig 6 of our paper referred above and identified as maskelynite. We also performed a comparative analysis of other known shocked plagioclase grains from the Manicouagan Crater, Canada and from the Lonar Crater, India (to be reported in this meeting, Chakrabarti et al.) Raman scattering measurements were performed using the 514.5 nm line of an argon ion laser at an intensity of 40 kW/cm$^{2}$. An inverted microscope (Nikon TE3000) with 50x objective (NA 0.55) was used for confocal imaging. A holographic notch filter removed residual laser scatter and the Raman scattering was detected by a silicon CCD at -90$^{0}$ C (Princeton Instruments Spec10-400R). Raman spectra were collected from ~250 cm$^{-1}$ through 2000 cm$^{-1}$ in all these samples. An anisotropic plagioclase lath from the same 9986 polished thin section shows prominent peaks at 827 cm$^{-1}$, 1109 cm$^{-1}$ and 1180 cm$^{-1}$ with much smaller peaks at 433 cm$^{-1}$ and 634 cm$^{-1}$. The Raman spectra of the isotropic grains of Fig 6 from our earlier study are conspicuously featureless and show a progressive increase in the luminescent background with higher wave numbers. Our observations are consistent with the Raman spectra of experimentally shocked plagioclase (Heymann and Herz, 1990, Cont. Min. Petr. 17, 38-44, 1990) as well as those of plagioclases from the tektites of the Lonar Impact Crater. Therefore we strongly believe that we had correctly identified shock-induced maskelynite in the Bedout breccia.
B31E-07 09:30h
Stable Isotopic Results Across the Permian /Triassic Boundary in the Karoo of South Africa: Evidence for both Gradual and Abrupt Extinctions
We have sampled for stable isotopes from paleontologically constrained stratigraphic sections in the southern and central Karoo basin of South Africa, which contains the most abundantly fossiliferous succession of late Permian to middle Triassic terrestrial vertebrates yet described. Over a narrow stratigraphic interval at the end of the Permian in this region, we have found a light carbon isotope anomaly (Ccarb) and succession of negative anomalies (Corg) coincident with, and right after an enhanced pulse of extinction among terrestrial vertebrates. While it is clear that a short interval of intense environmental change occurred over our sampled interval, there is no evidence of impact debris or characteristic boundary sedimentology equivalent to that observed in similar terrestrial facies at numerous Cretaceous/Paleogene boundaries. Our isotopic and paleontological data also show marked dissimilarity to similar data from terrestrial K/P sites. The Permian/Triassic succession in South Africa is thus inconsistent with the K/P model of single-impact mass extinction, and to date shows no evidence of impact of any size.
B31E-08 09:45h
Molecular Evidence for Radical Changes in Ocean Chemistry Across the Permian Triassic Boundary at Meishan in South China
Samples from outcrop and from a new core drilled through the Permian Triassic Boundary at the type section at Meishan have been examined for biomarker and isotopic evidence of biotic and associated environmental change. Late Permian sediments from Meishan Beds 22-27 are characterized by indicators of anoxia including low Pr/Ph ratios and abundant aryl isoprenoids and isoreneieratane derived from the precursor carotenoid isorenieratene. The latter compounds are biomarkers for green sulfur bacteria (Chlorobiaceae) and are considered reliable indicators of euxinic water columns where sulfide extends to the photic zone. The peak of Chlorobiaceae biomarker abundance coincides with a rapid and synchronous drop in the O13C and O15N values of kerogen. On passing up into the Early Triassic, the biomarker signal for Chlorobiaceae wanes and is almost absent by Bed 30 where it is replaced by one for cyanobacteria including abundant hopanes and 2-methylhopanes and accompanying methyl and dimethyl alkanes. A very high value for the hopane/sterane ratio from Beds 30-38 indicates continuing dominant cyanobacterial productivity and only minor inputs from an algal plankton. The prevalence of aryl isoprenoids in P-Tr sediments at the Meishan section of South China is also recorded in a recently cored borehole, Hovea-3, of the Perth Basin, Western Australia. This suggests similar paleoenvironmental conditions prevailed across the Tethys Ocean during and immediately after the P-Tr Boundary. In particular, the presence of biomarkers for Chlorobiaceae at two separate locations indicates that water column euxinia was pervasive during the extinction event and suggests that sulfide may have been a key toxic agent.