Methane Hydrate: Killer cause of Earth's greatest mass extinction
Introduction
The end Permian was the greatest natural catastrophe experienced by life on Earth with its impact recorded in terrestrial and marine rock archives. About 90% of marine species, 70% of terrestrial vertebrate species, 30% of insect orders and an indeterminate percentage of terrestrial and marine plants succumbed during this catastrophe (e.g., Erwin, 1994, Brand et al., 2012). This devastating mass extinction on Earth became the foundation for the rise of Mesozoic marine and terrestrial life. Important considerations of the end Permian mass extinction are documented by a dramatic and global negative carbon isotope excursion (Fig. 1), and (1) the close connection in the demise of the marine and terrestrial fauna and flora (e.g., Schneebeli-Hermann et al., 2013), (2) the rapidity of the onset of this catastrophe (e.g., Brand et al., 2012), and (3) the tight age constraint for the end Permian extinction event (e.g., Burgess et al., 2014). Numerous causes have been proposed for the catastrophic end Permian mass extinction, which may have acted alone or in concert, but with many researchers favoring a tangled web of interacting and related causes and effects such as global warming, anoxia and/or acidification (e.g., Wignall and Hallam, 1992, Cao et al., 2009, Clarkson et al., 2015). This global event is increasingly associated with the Siberian Trap volcanism and emission of the greenhouse gas CO2 leading to global warming (e.g., Svensen et al., 2009, Schneebeli-Hermann et al., 2013). Some suggest that methane from a variety of sources may have played a supporting role (Berner, 2002, Retallack and Jahren, 2008, Brand et al., 2012), while oceanic anoxia was a minor and/or local player (e.g., Brand et al., 2012, Garbelli et al., 2015), and acidification only played a role during the Early Triassic (e.g., Clarkson et al., 2015).
Carbonates may contain gas trapped in minuscule inclusions that are readily released during crushing and subsequently may be analyzed by mass spectrometer for their gas make-up and compositions (e.g., Blamey, 2012). If the gases in these inclusions were trapped during formation/precipitation of the carbonate and remained sealed/vaulted in the inclusions, then they may represent seawater-dissolved gas, and with Henry's Law and mass balance calculations, we may establish a link between the hydrosphere and atmosphere (Brand et al., 2015b). Brachiopods are ideal archives because they precipitate low-Mg calcite shells that are resistant to post-depositional alteration (Brand and Veizer, 1980, Brand, 2004), and they incorporate carbon and oxygen isotopes into shell calcite in or near equilibrium with ambient seawater (cf. Lowenstam, 1961, Brand et al., 2013, Brand et al., 2015a). Furthermore, to reflect ambient environmental oceanographic conditions, biogenic carbonates including brachiopods must incorporate dissolved gases and water into shell calcite inclusions from seawater without modification of the normal atmospheric equilibrium parameters (NAEC; Fig. 2). Inclusions, microstructural defects manifested as pore, in modern brachiopods (Griesshaber et al., 2007, fig. 2; Goetz et al., 2009, fig. 2b) and in the end Permian brachiopod Comelicania sp. indet. (Fig. 3) may be quite small. Even smaller pores (nanostructural defects) are observed in some brachiopod shells (Fig. 4A) and probably quite applicable to the micron-sized micrite in whole rock, are sufficiently ubiquitous (Fig. 4B) to satisfy the volume requirements demanded by the standard operating protocol of the analytical crush fast-scan mass spectrometry (CFS-MS) method for measuring trapped gases in inclusions. Gas and fluids with dissolved gas may be trapped during the depositional growth phase and/or the early diagenetic stage degradation of the organic tissue covering brachiopod fibers and micrite crystallites (Emig, 1990).
Our objectives are, (1) to characterize the gases in modern brachiopods and coral and use them as references and baselines for evaluating gases measured in the end Permian brachiopod shells and whole rock, (2) decipher the atmospheric and oceanographic conditions leading up to the end Permian event, and (3) identify the ultimate source/trigger for the catastrophic global warming and climate change event during the biggest mass extinction in Earth history.
Section snippets
Sample material
Specimens of the modern brachiopod Magellania venosa were collected at the jetty in Huinay, Chile in Comau Fjord, and other modern shallow water brachiopods and corals were collected at Friday Harbor, Washington State and Signy Island, Antarctica (Brand et al., 2013) and from Indonesia (Azmy et al., 2010). The fossil shallow-water brachiopods and whole rock were collected from the uppermost Permian Bulla Member of the Bellerophon Formation and Tesero Member of the Werfen Formation, northern
Diagenetic screening
The physical evidence supports the presence of suitable pores (Fig. 3, Fig. 4) but that is only the first step in the evaluation process. Furthermore, we must establish that the gas contents of brachiopod and other marine archives reflect NAEC of gases in seawater and consequently of the atmosphere. A plot of gas contents of several modern brachiopods and one coral shows that they indeed reflect equilibrium seawater and atmospheric compositions, which confirms their potential as archives of
Gas classification
Further screening was conducted to test for the reliability in retaining NAEC of gases with the passage of time in the carbonates. Gas results of modern brachiopods plot in an area characterizing marine conditions in equilibrium with the atmosphere defined by CO2/CH4 ratios ranging from 10 to 1000, and by N2/Ar ratios ranging from 35 to 130 (Table 1, Fig. 6), on the modified gas classification scheme originally proposed by Norman and Moore (1999) and Moore et al. (2001). Field boundaries have
Carbon Isotope Excursion (CIE)
The negative carbon isotope excursion (nCIE) in marine carbonates has been long recognized as an important marker of the end Permian (PT) event at Meishan, China and global localities, and it may vary by as much as 6.5‰ (Fig. 1; e.g., Jin et al., 2000, Retallack and Krull, 2006, Brand et al., 2012, Garbelli et al., 2015). However, its origin, source and duration remain problematic (Berner, 2002), and consequently, a definitive cause and mechanism for the nCIE and PT event still eludes us. A
End Permian atmospheric conditions
Generally, uncertainties of timing and duration of events limit the interpretations of many important biologic/geologic events, including the end Permian mass extinction (e.g., Rampino et al., 2000, Posenato, 2009, Shen et al., 2010). But this critical issue of time has been resolved with results from recent publications that allow for the improved geochemical characterization of the source(s) for this most dramatic event (Brand et al., 2012, Joachimski et al., 2012, Burgess et al., 2014). The
Methane source
The question arises: is the emission of gas from Siberian Trap volcanism sufficient to cause both the exacerbated warming and negative carbon isotope shift experienced during the end Permian (Fig. 8a)? The answer on both accounts is in the negative (e.g., Berner, 2002, Retallack and Jahren, 2008, Brand et al., 2012), and thus a new question must be posed whether there is sufficient methane stored in permafrost and marine sediment, upon release, to cause a major perturbation of the global carbon
Conclusions
Biogenic and abiogenic carbonates from the end Permian succession in northern Italy carry inclusion gases that formed in normal atmospheric equilibrium between the ambient seawater and atmosphere. Gas measured in inclusions suggests two pulses of methane emission before the end Permian mass extinction event.
Carbon dioxide derived from Siberian Trap volcanism with its δ13C value of about −6‰ would bring about a warming of about 6 °C and a shift in marine carbonate δ13C values by about −2‰. The
Acknowledgements
We thank Dr. Fan-Wei Meng and an anonymous reviewer for their helpful comments on the manuscript, and Mike Lozon (Brock University) for drafting and constructing the figures. We also thank NSERC (No. 7961-15), NSF, Brock, Munich, Memorial, New Hampshire, Ferrara, Milano and Bologna Universities for financial support.
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Present address: State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, Jiangsu 210008, PR China.