U24C-01
Stability of the Stratospheric Ozone Shield During the End-Permian Mass Extinction
The end-Permian mass extinction (251 Myr ago) is associated with the eruption of the Siberian Traps, and has also been linked with massive H2S releases from super-anoxic oceans and CH4 degassing from sea-floor clathrates. This presentation will present new calculations using the Cambridge two-dimensional atmospheric chemistry-transport model that assess the potential for each of these scenarios to damage Earth's ozone layer which shields the surface biota from harmful ultraviolet-B radiation. Our calculations include representation of the warm end-Permian greenhouse' climate and its modelled low O2 atmospheric content (15 %). We show that the direct and indirect effects of the Siberian Traps have the greatest potential to exert severe O3 depletion, to an extent that depends on the duration of the main eruptive phase. However, the O3 layer is found to be remarkably resilient to massive oceanic H2S and CH4 releases, both separately, and in combination. The results will be discussed in the context of chemical and transport mechanisms that lead to these different conclusions.
U24C-02
Osmium Isotopic Evidence Against an Impact at the Frasnian-Famennian Boundary
Two sections across the Frasnian-Famennian boundary were analyzed for Re and Os concentrations and 187Os/188Os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction. These sections are from a siltstone and shale sequence at Irish Gulf in New York, US and a calcareous shale and ferromanganese oxide sequence at La Serre in France. The Irish Gulf section, with an initial 187Os/188Os of ~0.49, does not show the characteristic meteoritic Os imprint with a 187Os/188Os value of about 0.13. Both Re and Os are retained in this section, as indicated by the construction of an isochron with an age of 388 ±41 Ma, consistent with independently determined ages for the Frasnian-Famennian boundary. Although the La Serre section, with Os concentrations as high as 33 ppb and Re concentrations ranging from 1.4 to 7.4 ppb, might be expected to show excellent evidence for a meteoritic contribution, the highly radiogenic isotopic composition (187Os/188Os ranges from 2.42-3.61) instead suggests recent massive Re loss or addition of radiogenic Os. This open system behavior prevents the reconstruction of an initial 187Os/188Os value for the boundary at La Serre. Assuming reasonable Re concentrations prior to loss, however, the Os isotopic value is inconsistent with a large meteoritic component. In addition, this study reinforces the need for Os isotopic evidence, not only enriched PGE concentrations, as substantiation for a meteoritic impact.
U24C-03
The Araguainha impact crater at the Permo-Triassic boundary: implications for the carbon isotope excursion and the mass extinction.
The Araguainha crater is a complex crater with a diameter of 40 km exposed on the northern margin of the Parana Basin of central Brazil. This intracontinental basin, correlated to the Karoo Basin of southern Africa, was the locus of marine sedimentation over an area of 5 million km2 throughout the late Paleozoic. Carbonate sedimentation in the early Permian was marked by large accumulations of organic carbon in pyrite-bearing oil shales such as the Irati Fm, considered to be the world's second largest oil shale. Regional borehole data from outside the crater reveals a thickness of 40m for the oil shale horizon, which is partly to completely absent within the crater. Our structural and stratigraphic survey of the Araguainha crater reveal the post-impact rebound of the crater has removed ca. 2-2.5 km of sediments from the ca. 10 km diameter central uplift, with minimal subsequent erosion (<250m). Vaporization of the colliding body and the approximate shadowed target region are assumed, with energy models for impact craters suggesting a body of 2-3 km diameter. Ongoing radiogenic isotope dating of the impact melts and breccias is being undertaken by U-Pb SHRIMP analysis of shocked zircons and 40Ar/39Ar analysis of glassy vein material interpreted as pseudotachylite. Preliminary U-Pb age data yield an impact age of 252.7 +/- 3.8 Ma (2 sigma error), essentially synchronous with the Permo-Triassic boundary. The minimum amount of isotopically light carbon (-17 to -25 per mill PDB) available in the target rocks for release by the impact is estimated at 10 Gigatons, considering only the area of the central uplift. Possible sources of additional, isotopically-light carbon include the remainder of the 20-25 km transient crater, as well as methane clathrates released by impact-induced slope destabilization. We propose that the Araguainha impact could have been responsible for observed shifts shift in global carbon isotopes at the Permo-Triassic boundary. The possible effect of the the Araguainha impact on late Paleozoic biota is probably restricted to ancillary effects of massive carbon release.
U24C-04
Global Warmth and Nutrient Trapping Enhance End-Paleozoic Euxinia in an Earth System Model
The end-Permian mass extinction occurred during an interval characterized by global warmth and falling atmospheric oxygen levels. Although the cause of the extinction remains unresolved, geochemical evidence suggests that the event coincided with widespread anoxia and possible euxinia (anoxic and sulfidic waters). A combination of warm surface ocean temperatures, low atmospheric oxygen content, and high O2 demand in the deep ocean likely induced anoxia. Anaerobic organic matter remineralization by bacterial sulfate reduction then produced euxinic conditions. Phosphate release from surface sediments and decreased phosphate burial under a sulfidic water column further enhanced oceanic euxinia through positive feedbacks to primary productivity. We hypothesize that late Paleozoic climate and geography favored marine euxinia, a potential kill mechanism for the extinction event. Here we use earth system modelling to explore the physical and biogeochemical conditions necessary for the development of intense euxinia during the end-Permian. We use the end-Permian configuration of GENIE (www.genie.ac.uk), an energy-moisture-balance atmospheric model coupled to a 3-D, non-eddy-resolving, frictional geostrophic model to investigate the transition to marine anoxia and euxinia in a greenhouse world. Equilibrium model simulations over a range of oceanic phosphate concentrations relate oceanic nutrient status to the buildup of euxinia and attendant hydrogen sulfide release to the atmosphere. Addition of a marine nitrogen cycle suggests that microbial denitrification reduces, but does not prevent H2S buildup. Deep-ocean hydrogen sulfide appears with a doubling of phosphate, and localized photic zone euxinia develops with a tripling of phosphate. The greatest surface water H2S concentrations are observed in upwelling zones and in the Paleo-Tethys Ocean, where nutrient trapping results in elevated phosphate concentrations. Significant hydrogen sulfide fluxes to the atmosphere result from extremely high nutrient conditions (6-10x modern phosphate). In this scenario, hydrogen sulfide in the ocean-atmosphere system would impose pervasive environmental stress that may promote extinctions both on land and in the ocean. These simulations support the hypothesis that extreme euxinia and episodic H2S eruptions can occur in a nutrient-rich ocean, despite widespread denitrification under anoxic conditions. Spatially resolved predictions from this earth system modelling approach may guide further study of the rock record and help constrain the geochemistry of end-Permian mass extinction.
U24C-05
Biomarker evidence for shallow water marine euxinia through the PTB in the Panthalassic Ocean (Peace River Basin Embayment, Canada)
Protracted euxinic conditions in the late Paleozoic and early Mesozoic oceans may have been an important paleoenvironmental factor in the Permian-Triassic Boundary (PTB) extinction. Release of hydrogen sulfide during upwelling or transgressive events from such an ocean (Kump et al. 2005; Riccardi et al. 2006) may have been a driver of the extinction in both marine and terrestrial environments. Worldwide marine PTB sections show evidence for a stratified water column and the presence of sulfidic deep water, at least episodically (Isozaki 1997; Grice et al. 2005). Taxa that are particularly characteristic of such an environment are the green sulfur bacteria, or Chlorobiaceae. These anoxygenic phototrophic bacteria utilize sulfide as an electron donor for photosynthesis and live in modern stratified water columns where euxinia extends into the photic zone. Indeed, biomarkers derived from these organisms have been identified at a number of the PTB sections. The Peace River embayment in western Canada has been identified as a section that spans the PTB based on conodont biostratigraphy (Henderson 1997). Samples from five drill cores in this section provide new insight into the state of the Panthalassic Ocean during this time of unprecedented turnover in Earth's biota. Using standard biomarker protocols, we identified aromatic hydrocarbons that are diagentic products of the carotenoids isorenieratene and chlorobactene, which are diagnostic for the brown and green strains, respectively, of the Chlorobiaceae. The occurrence of chlorobactane is especially notable since the green-pigmented varieties of the Chlorobiaceae require higher light intensities than the brown-pigmented forms and, in modern environments where they have been found, occur between 13 and 30 m of the surface. This is the first time that chlorobactane has been reported from a PTB section and it suggests a particularly shallow chemocline periodically at this location. The δ13C values for the aryl isoprenoids are 13C-enriched relative to the signatures for n-alkanes and other compounds produced by the dominant phototrophic primary producers in surface waters, a feature distinctive for Chlorobiaceae. These analyses, combined with other biomarker data from these samples also indicate that there was a marked disruption to the community structure of phytoplankton at this location at the end of the Paleozoic. Grice, K., et al. (2005). Science 307: 706-709. Henderson, C. M. (1997). Bulletin of Canadian Petroleum Geology 45: 693-707. Isozaki, Y. (1997). Science 276: 235-238. Kump, L. R., et al. (2005). Geology 33: 397400. Riccardi, A. L., et al. (2006). Geochimica et Cosmochimica Acta 70: 5740-5752.
U24C-06
The Late Guadalupian (Permian) event: when everything geologically unusual started for the Paleozoic-Mesozoic transition
The event across the Paleozoic-Mesozoic transition involves the greatest mass extinction in history and other long-term complicated geologic phenomena in various aspects on the Earth, such as Pangean rifting and superanoxia. The Permo-Triassic mid-oceanic sedimentary records documented the double-phased nature of the extinction and relevant environmental changes at the Middle and Upper Permian or Guadalupian-Lopingian boundary (G-LB) and at the Permo-Triassic boundary (P-TB). As all the coeval ocean floors of Panthalassa were lost by subduction, the dataset from the accreted mid-oceanic rocks is particularly important. The deep-sea chert recorded the double-phased remarkable faunal reorganization in radiolarians (major marine plankton in the Paleozoic), i.e., in the Late Guadalupian and at P-TB. It is noteworthy that the superanoxia (ca. 20 million year long deep-sea anoxia) started in the Late Guadalupian culminated at the P-TB. The mid-oceanic paleo-atoll carbonates primarily deposited on seamounts also recorded double-phased extinction in fusulines (a representative Late Paleozoic shallow marine benthos), double-phased negative shift of stable carbon isotope ratio in the Late Guadalupian and at P-TB, and the Phanerozoic minimum in 87Sr/86Sr isotope ratio in the Late Guadalupian. Mid-oceanic deep- and shallow-water sequences indicate that significant changes have occurred in the Permian superocean in a double-phased manner, and that everything geologically unusual started in the Late Guadalupian. These bio- and chemostratigraphical data are concordant with those from the coeval shallow marine shelf sequences around Pangea. The uniquely concentrated occurrence of rhyo-dacitic tuff beds both around G-L B and P-TB suggests that Panthalassa and eastern Tethys (South China) extensively suffered severe volcanic hazards twice almost at the same time as the G-LB and P-TB extinctions. In the framework of the gPlume Winterh scenario (Isozaki, 2007), the double-phased felsic volcanism, double-phased environmental turmoil in Panthalassa, and double-phased extinction (the end-Permian triple-double) likely correspond to the cause, process, and consequence of the greatest global catastrophe in the Phanerozoic, respectively.