Paul B. Wignall

Large igneous provinces and mass extinctions

Comparing the timing of mass extinctions with the formation age of large igneous provinces reveals a close correspondence in five cases, but previous claims that all such provinces coincide with extinction events are unduly optimistic. The best correlation occurs for four consecutive mid-Phanerozoic examples, namely the end-Guadalupian extinction/Emeishan flood basalts, the end-Permian extinction/Siberian Traps, the end-Triassic extinction/central Atlantic volcanism and the early Toarcian extinction/Karoo Traps. Curiously, the onset of eruptions slightly post-dates the main phase of extinctions in these examples. Of the seven post-Karoo provinces, only the Deccan Traps coincide with a mass extinction, but in this case, the nature of the biotic crisis is best reconciled with the effects of a major bolide impact. Intraoceanic volcanism may also be implicated in a relatively minor end-Cenomanian extinction crisis, although once again the main phase of volcanism occurs after the crisis. The link between large igneous province formation and extinctions remains enigmatic; volume of extrusives and extinction intensity are unrelated and neither is there any apparent relationship with the rapidity of province formation. Violence of eruptions (proportions of pyroclastics) also appears unimportant. Six out of 11 provinces coincide with episodes of global warming and marine anoxia/dysoxia, a relationship that suggests that volcanic CO2 emissions may have an important effect on global climate. Conversely, there is little, if any, geological evidence for cooling associated with continental flood basalt eruptions suggesting little long-term impact of SO2 emissions. Large carbon isotope excursions are associated with some extinction events and intervals of flood basalt eruption but these are too great to be accounted for by the release of volcanic CO2 alone. Thus, voluminous volcanism may in some circumstances trigger calamitous global environmental changes (runaway greenhouses), perhaps by causing the dissociation of gas hydrates. The variable efficiency of global carbon sinks during volcanic episodes may be an important control on environmental effects and may explain why the eruption of some vast igneous provinces, such as the Parana–Etendeka Traps, have little perceptible climatic impact.

Oceanic Anoxia and the End Permian Mass Extinction

Data on rocks from Spitsbergen and the equatorial sections of Italy and Slovenia indicate that the worlds oceans became anoxic at both low and high paleolatitudes in the Late Permian. Such conditions may have been responsible for the mass extinction at this time. This event affected a wide range of shelf depths and extended into shallow water well above the storm wave base.

Lethally Hot Temperatures During the Early Triassic Greenhouse

Too-Hot Times Climate warming has been invoked as a factor contributing to widespread extinction events, acting as a trigger or amplifier for more proximal causes, such as marine anoxia. Sun et al. (p. 366; see the Perspective by Bottjer) present evidence that exceptionally high temperatures themselves may have caused some extinctions during the end-Permian. A rapid temperature rise coincided with a general absence of ichthyofauna in equatorial regions, as well as an absence of many species of marine mammals and calcareous algae, consistent with thermal influences on the marine low latitudes. Sea surface temperatures approached 40°C, which suggests that land temperatures likely fluctuated to even higher values that suppressed terrestrial equatorial plant and animal abundance during most of the Early Triassic. Global warming in the Early Triassic was so severe that equatorial latitudes were uninhabitable for many plants and animals. Global warming is widely regarded to have played a contributing role in numerous past biotic crises. Here, we show that the end-Permian mass extinction coincided with a rapid temperature rise to exceptionally high values in the Early Triassic that were inimical to life in equatorial latitudes and suppressed ecosystem recovery. This was manifested in the loss of calcareous algae, the near-absence of fish in equatorial Tethys, and the dominance of small taxa of invertebrates during the thermal maxima. High temperatures drove most Early Triassic plants and animals out of equatorial terrestrial ecosystems and probably were a major cause of the end-Smithian crisis.

Anoxia as a cause of the Permian/Triassic mass extinction: facies evidence from northern Italy and the western United States

Facies, faunal and geochemical evidence from the Permian/Triassic boundary sediments of the Dolomites and Idaho indicates a major anoxic event in the earliest Triassic. In both regions, the basal beds consist of finely laminated micrites with common syngenetic pyrite. The only fauna consists of occasional bedding plane assemblages of Lingula or Clacaria, a typical lower dysaerobic assemblage. This is a level where previous studies have shown a major negative carbon isotope excursion and a cerium anomaly. In the Dolomites, the pyritic micrite directly overlies strata containing a diverse and typically Permian marine fauna of algae, foraminifers (including fusulinids) and articulate brachiopods, implying an abrupt extinction in contradiction to many previous views. Sequence stratigraphic analysis of the Dolomite boundary sediments reveals a minor sequence boundary in the late Permian followed by extremely rapid transgression leading to the development of the relatively deep water pyritic micrite — a maximum flooding surface at the Permo-Triassic boundary. A further pulsed deepening in the lower Griesbachian, recorded in both the Dolomites and Idaho, lead to the widespread establishment of dysaerobic facies. It is clear that most of the extinctions occurred at the erathem boundary although the subsequent failure of the marine benthos to fill the empty ecospace in the ensuing Griesbachian may have been due to the widespread development of dysaerobic conditions.

Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis

A newly studied Permian-Triassic (P-Tr) boundary section in Jameson Land, East Greenland, contains an abundant and well-preserved marine fauna as well as terrestrial palynomorphs. For the first time it is possible to compare the biotic crises of the marine and terrestrial realms using the same samples from the same section. The sediments record a negative excursion in δ 13 C carb values of 8‰–9‰, and in δ 13 C org values of 10‰–11‰. The presence of the conodont Hindeodus parvus, combined with the δ 13 C carb record, enables correlation with the proposed global stratotype section at Meishan. This shows that the Greenland section is the most expanded P-Tr section known. Collapse of the marine and terrestrial ecosystems took between 10 and 60 k.y. It took a further few hundred thousand years for the final disappearance of Permian floral elements. Collapse of the terrestrial and marine ecosystems began at the same stratigraphic level and preceded the sharp negative excursion in the δ 13 C record.

Interpreting benthic oxygen levels in mudrocks: A new approach

Quantified paleoecology and gamma-ray spectrometry have been applied in the analysis of the Kimmeridge Clay, a highly organic-rich British Jurassic mudrock. Decreasing benthic oxygen trends are reflected in decreasing species richness and dominance-diversity values. Similarly, the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen. The calculation of authigenic uranium values from data gathered by gamma-ray spectrometry shows enrichment in more oxygen-deficient environments. The good correlation between the independently derived paleoecological and authigenic U data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks.

Griesbachian (Earliest Triassic) palaeoenvironmental changes in the Salt Range, Pakistan and southeast China and their bearing on the Permo-Triassic mass extinction

Abstract Facies and faunal analysis from Pakistan and China show that the Permo-Triassic mass extinction of marine invertebrate faunas was associated with a spectacularly rapid Griesbachian transgression which lead to the widespread establishment of deep-water anoxic and dysoxic conditions. The extinction event was thus caused by habitat loss due to the extensive development of inhospitable conditions. The initial Griesbachian transgression in Pakistan produced extensive shallow, normal marine conditions in which Permian holdover taxa were able to survive until the development of dysaerobic facies in the late Griesbachian. The exceptionally complete sections of China show a three-phased deepening and extinction event beginning in the latest Permia. By the late Griesbachian a variety of dysaerobic and anaerobic facies were developed in all the regions studied. Several of these contain evidence for minimal sulphate reducing activity suggesting that marine productivity and thus organic matter flux to the sediments was very low in early Triassic seas.

Large shifts in the isotopic composition of seawater sulphate across the Permo–Triassic boundary in northern Italy

Carbonate-associated sulphate (CAS) extracted from a Permo–Triassic succession at Siusi in northern Italy is shown to preserve a true seawater-sulphate isotope record. Two periods of increasing δ34S and δ18O in CAS provide evidence for increased oceanic anoxia in the Late Permian and the Early Triassic. These two anoxic episodes are separated by an event characterised by the addition of isotopically light sulphur and oxygen to the oceanic sulphate pool. Simple mass balance calculations suggest that this sulphate originates from the reoxidation of bacterially derived H2S during oceanic mixing, rather than a volcanogenic source. A dramatic fall in CAS-δ18O directly above the P–T boundary, not accompanied by a large change in CAS-δ34S, records an oceanic deoxygenation event probably caused by the release of methane from gas hydrates, subsequently recorded in the carbonate-carbon isotope record. The decline of Early Triassic oceanic anoxia is not recorded by a fall in CAS-δ34S, but is preserved by declining CAS-δ18O. This is because of an increase in the flux of reactive iron to the oceans during the Early Triassic anoxic episode, triggered by the demise of land plants. This permanently removes a greater proportion of light sulphur from the oceanic sulphate reservoir as pyrite, and means that the heavy residual sulphate-sulphur isotope signature of Griesbachian anoxic seawater is preserved as a geochemical ‘fossil’ until the beginning of the Middle Triassic.

Pyrite framboid study of marine Permian-Triassic boundary sections: A complex anoxic event and its relationship to contemporaneous mass extinction

Size analysis of pyrite framboids has been undertaken on epicontinental Permian- Triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia. Mid-paleolatitude sections from the margins of the Boreal (Spitsbergen, Greenland) and Neotethyan oceans (Western Australia) reveal intense anoxia throughout the Permian-Triassic boundary interval with euxinic conditions frequently developing, and dysoxia encountered even in relatively shallow-water settings above storm wave base. At equatorial paleolatitudes, weakly oxygenated (dysoxic) conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition, although euxinia was rare. Western and eastern Tethyan locations re- veal a complex and unstable redox history: anoxia in the Hindeodus praeparvus Zone was replaced by oxygenated facies in the Permian- Triassic boundary interval (H. changxingensis to H. parvus zones). Oxygen-poor deposition returned during the succeeding Isarcicella isarcica Zone. The more persistent and intense development of oxygen restriction in cooler water, mid-paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of Permian-Triassic bound- ary anoxia. In higher paleolatitudes the ben- thic invertebrate extinctions occurred during a prolonged phase of oxygen-poor deposition, while in equatorial Tethyan locations benthic losses occurred at the end of the fi rst anoxic phase (in the late H. praeparvus Zone).

Catastrophic soil erosion during the end-Permian biotic crisis

Organic geochemical analyses of sedimentary organic matter from a marine Permian-Triassic transition sequence in northeastern Italy reveal a significant influx of land-derived diagenetic products of polysaccharides. This unique event reflects massive soil erosion resulting from destruction of land vegetation due to volcanogenic disturbance of atmospheric chemistry. The excessive supply of soil materials to the oceans provides a direct link between terrestrial and marine ecological crises, suggesting that ecosystem collapse on land could have contributed to the end-Permian marine extinctions.