Tag Archives: Extinction event

Shock and Er … wait a minute

Chicxulub2

Enhanced gravity map of the Chicxulub crater (credit: Wikipedia)

Michael Rampino has produced a new book (Rampino, M.R. 2017. Cataclysms: A New Geology for the Twenty-First Century. Columbia University Press; New York). As the title subtly hints, Rampino is interested in mass extinctions and impacts; indeed quite a lot more, as he lays out a hypothesis that major terrestrial upheavals may stem from gravitational changes during the Solar System’s progress around the Milky Way galaxy. Astronomers reckon that this 250 Ma orbit involves wobbling through the galactic plane and possibly varying distributions of mass – stars, gas, dust and maybe dark matter – in a 33 Ma cycle. Changing gravitational forces affecting the Solar System may possibly fling small objects such as comets and asteroids towards the Earth on a regular basis. In the 1980s and 90s Rampino and others linked mass extinctions, flood-basalt outpourings and cratering events, with a 27 Ma periodicity. So the books isn’t entirely new, though it reads pretty well.

Such ideas have been around for decades, but it all kicked off in 1980 when Luis and Walter Alvarez and co-workers published their findings of iridium anomalies  at the K-Pg boundary and suggested that this could only have arisen from a major asteroid impact. Since it coincided with the mass extinction of dinosaurs and much else besides at the end of the Cretaceous it could hardly be ignored. Indeed their chance discovery launched quite a bandwagon. The iridium-rich layer also included glass spherules, shocked mineral grains, soot and other carbon molecules –nano-scale diamonds, nanotubes and fullerenes whose structure is akin to a geodesic dome – and other geochemical anomalies. Because the Chicxulub crater off the Yucatán Peninsula of Mexico is exactly the right age and big enough to warrant a role in the K-Pg extinction, these lines of evidence have been widely adopted as the forensic smoking gun for other impacts. In the last 37 years every extinction event horizon has been scrutinized to seek such an extraterrestrial connection, with some success, except for exactly coincident big craters.

The K-Pg event is the only one that shows a clear temporal connection with a small mountain falling out of the sky, most of the others seeming to link with flood basalt events and their roughly cyclical frequency – but hence Rampino’s Shiva hypothesis that impacts may have caused the launch of mantle plumes from the core-mantle boundary. Others have used the ‘smoking gun’ components to link lesser events to a cosmic cause, the most notorious being the 2007 connection to the extinction of the North American Pleistocene megafauna and the start of the Younger Dryas return of glacial conditions. Since 1980 alternative mechanisms for producing most of the impact-connected materials have been demonstrated. It emerged in 2011 that nano-diamonds and fullerenes may form in a candle flame and their global distribution could be due to forest fires. And now it seems that shocked mineral grains can form during a lightning strike (Chen, J. et al. 2017. Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating. Geophysical Research Letters, v. 44, p. 8757-8768; doi:10.1002/2017GL073843). Shocked or not, quartz and feldspar grains are resistant enough to be redistributed into sediments. Although platinum-group metals, such as iridium, are likely to be mainly locked away in Earth’s core, some volcanic exhalations and many flood basalts – especially those with high titanium contents – significantly are enriched in them. So even the Alvarez’s evidence for a K-Pg impact has an alternative explanation. Rampino is to be credited for acknowledging that in his book.

An awful lot of ideas about rare yet dreadful events in the biosphere depend, like many criminal cases, on the ‘weight of evidence’ and defy absolute proof. The evidence generally permits alternatives, such the cunning Verneshot hypothesis for the extinction-flood basalt connection supported by one of the founders of plate tectonics, W. Jason Morgan. As regards The K-Pg extinction, it is certain that a very large mass did fall on Chicxulub at the time of the mass extinction, whereas the Deccan flood basalts span a million years or so either side. But the jury is out on whether either or both did the deed. For other events of this scale and larger ones the money is on internal origins. As for Rampino’s galactic hypothesis, the statistics are decidedly dodgy, but chasing down more forensics is definitely on the cards.

English: From source; an animation showing the...

Animation showing the Chicxulub Crater impact. ( credit: University of Arizona, Space Imagery Center)

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Deccan Trap sprung by bolide?

English: Alvarez and K-T Boundary

Luis and Walter Alvarez at the end-Mesozoic Boundary (credit: Wikipedia)

It was 35 years back that father and son team Luis and Walter Alvarez upset a great many geoscientists by suggesting that a very thin layer of iridium-rich mud that contained glass spherules and shocked mineral grains was evidence for a large meteorite having struck Earth. They especially annoyed palaeontologists because of their claim that it occurred at the very top of the youngest Cretaceous and that the mud was spread far and wide in deep- and shallow-marine stratigraphic sequences and also in those of continental rocks. It marked the boundary between the Mesozoic and Cenozoic Eras and, of course, the demise of the dinosaurs and a great many more, less ‘sexy’ beasts. Luis was a physicist, his son a proper geologist and their co-researchers were chemists. It can hardly be said that they stole anyone’s thunder since the issue of mass extinctions was quiescent, yet their discovery ranks with that of Alfred Wegener; another interloper into the closed-shop geoscientific community. They got the same cold-shoulder treatment, but massive popular acclaim as well, even from a minority of geologists who welcomed their having shaken up their colleagues, 15 years after the last ‘big thing’: plate tectonics. And then the actual site of the impact was found by geophysicists in a sedimentary basin in the Gulf of Mexico off the small town of Chicxulub on the Yucatan peninsula.

Chicxulub impact - artist impression

Chicxulub impact – artist impression (credit: Wikipedia)

As they say, ‘the rest is history’ and a great many geoscientists didn’t just jump but pounced on this potential bandwagon. Central to this activity was the fact that, within error, the ages of the impact, the mass extinction and a vast pile of continental lavas in western India, the Deccan Traps, were more or less the same (around 66 Ma). Flood basalt events are just about as dramatic as mega-impacts because of their sheer scale, of the order of a million cubic kilometres; that they were exuded in a mere million years or so, but in only a few tens of stupendous lava flows; and they are far beyond the direct experience of humans, blurting out only every 30 Ma or so. This periodicity roughly tallies with mass extinctions, great and small, through the Mesozoic. There have been two large bands of enthusiasts engaged in the causality of the end-Mesozoic die-off – the extraterrestrials and the parochialists who favoured a more mundane, albeit cataclysmic snuffing-out. Mass extinctions in general have been repeatedly examined, and in recent years it has become clear that most of those since 250 Ma ago seem to be associated with basalt-flood events and are purely terrestrial in origin. As regards the event that ended the Mesozoic, it has proved difficult to resolve whether to point the finger at the Deccan Traps or the Chicxulub impact. Both might have severely damaged the biosphere in perhaps different ways, so a ‘double whammy’ has become a compromise solution.

The Western Ghat hills at Matheran in Maharash...

Deccan flood basalts forming the Western Ghats in Maharashtra, India (credit: Wikipedia)

Unsurprisingly, a lot of effort from different quarters has gone into charting the progress of the Deccan volcanism. Some dating seemed at one stage to place the bulk of the volcanism significantly before the mass extinction and impact, others had them spot on and there were even signs of an hiatus in eruptions at the critical juncture. The problem was geochronological precision of the argon-argon method of radiometric dating that is most used for rocks of basaltic composition: many labs cannot do better than an uncertainty of 1%, which is ±0.7 Ma for ages around the end of the Mesozoic, not far short of the entire duration of these huge events. Some Deccan samples have now been dated to a standard of ±0.1 Ma by the Ar-Ar lab at the Department of Earth and Planetary Sciences, University of California-Berkeley (Renne, P.R. et al. 2010. State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science, v. 350, p. 76-78). The results, between 65.5 to 66.5 Ma, nicely bracket the K/T (now K/Pg) boundary age of 66.04±0.04 Ma. It looks like the double whammy compromise is the hypothesis of choice. But there is more to mere dating.

Renne and colleagues plot the ages against their position in the volcanic stratigraphy of the Deccan Traps in two ways: against the estimated height from base in the pile and against the estimated volume of the erupted materials as it built up – the extent and thickness of successive flows varies quite a lot. The second plot provided a surprise. After the K/Pg event the mean rate of effusion – the limited number of individual flows capped by well-developed soils shows that the build-up was episodic – doubled from 0.4±0.2 to 0.9±0.3 km3 yr-1. Despite the much larger uncertainty in the extent and volume of individual lava Formations than that of their ages, this is clearly significant. Does it imply that the Chicxulub impact somehow affected the magma production from, the mantle plume beneath the Deccan? It had been suggested early in the debate that the antipodean position of the lava field relative to that of Chicxulub may indicate that the huge seismicity from the impact triggered the Deccan magma production. Few accepted that possibility when it first appeared. However, Renne and co. do think it deserves another look, at least at the possibility of some linked effect on the magmatism. Perhaps the magma chamber was somehow enlarged by increased global seismicity; other chambers could have been added; magma might have been ‘pumped’ out more efficiently, or a combination of such effects. The ‘plumbing’ of flood basalt piles is generally hidden, but huge dyke swarms in Precambrian times have been suggested as feeders to long-eroded flood basalts. Seismicity of the scale produced by asteroid impacts can do a lot of damage. The Chicxulub impactor at around 10 km diameter would have carried energy a million times greater than that of the largest thermonuclear bomb, equivalent to an earthquake of Magnitude 12.4 that would have been a thousand times more powerful than the largest recorded earthquake with tectonic causes. Extensional faulting sourced in this fashion in the Deccan area may have increased the pathways along which magma might blurt out.

Duncan, R. 2015. Deadly combination. Nature, v. 527, p. 172-173.

How the first metazoan mass extinction happened

The end-Ordovician mass extinction was the first of five during the Phanerozoic, andthe first that involved multicelled organisms. It happened in two distinct phases that roughly coincided with an intense but short-lived glaciation at the South Pole, then situated within what is now the African continent. Unlike the other four, this biotic catastrophe seems unlinked to either a major impact structure or to an episode of flood volcanism.

seadiorama ordovician

Artist’s impression of an Ordovician shallow-sea community (credit: drtel)

In 2009 Earth Pages reported the curious occurrence in 470 Ma (Darriwilian Stage) Swedish limestones of a large number of altered chondritic meteorites, possible evidence that there may have been an extraterrestrial influence on extinction rates around that time. In support is evidence that the meteorite swarm coincided with megabreccias or olistostromes at what were then Southern Hemisphere continental margins: possible signs of a series of huge tsunamis. But in fact this odd coincidence occurred at a time when metazoan diversity was truly booming: the only known case of impacts possibly favouring life.

Number One of the Big Five mass extinctions occurred during the late-Ordovician Hirnantian stage (443-445 Ma) and has received much less attention than the later ones. So it is good see the balance being redressed by a review of evidence for it and for possible mechanisms (Harper, D.A.T et al. 2014. End Ordovician extinctions: A coincidence of causes. Gondwana Research, v. 25, p. 1294-1307). The first event of a double-whammy mainly affected free-swimming and planktonic organisms and those of shallow seas; near-surface dwellers such as graptolites and trilobites. The second, about a million years later, hit animals living at all depths in the sea. Between them, the two events removed about 85% of marines species – there were few if any terrestrial animals so this is close to the extinction level that closed the Palaeozoic at around 250 Ma.

No single process can be regarded as the ‘culprit’. However the two events are bracketed by an 80-100 m fall in sea level due to the southern hemisphere glaciation. That may have given rise to changes in ocean oxygen content and in the reduction of sulfur to hydrogen sulfide. Also climate-related may have been changes in the vertical, thermohaline circulation of the oceans, falling temperatures encouraging sinking of surface water to abyssal depths providing more oxygen to support life deep in the water column. Sea-level fall would have reduced the extent of shallow seas too. Those consequences would explain the early demise of shallow water, free swimming animals. Reversal of these trends as glaciation waned may have returned stagnancy and anoxia to deep water, thereby affecting life at all depths. The authors suggest generalized ‘tipping points’ towards which several global processes contributed.

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