Tag Archives: mass extinction

The late-Ordovician mass extinction: volcanic connections

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

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Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

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K-T (K-Pg) boundary impact probed

One of the most eagerly followed ocean-floor drilling projects has just released some results. Its target is 46 km radially away from the centre of the geophysical anomaly associated with the Chixculub impact structure just to the north of Mexico’s Yucatan Peninsula. In the case of large lunar impact craters the centre is often surrounded by a ring of peaks. Modelling suggests such features are produced by the deep penetration of immense seismic shock waves. In the first minute these excavate and fling out debris to leave a cavity penetrating deep into the crust. Within three minutes the cavity walls collapse inwards creating a rebound superficially similar to the drop flung upwards after an object is dropped in liquid. This, in turn, collapses outwards to emplace smashed and partially melted deep crustal material on top of what were once surface materials, creating a crustal inversion beneath a mountainous ring of Himalayan dimensions that surrounds a by-now shallow crater. That is the story modelled from what is known about well-studied, big craters on the Moon and Mercury. Chixculub is different because the impact was into the sea and involved debris-charged tsunamis that finally plastered the actual impact scar with sediments. The drilling was funded for several reasons, some palaeontological others relating to the testing of theories of impact processes and their products. Chixculub is probably the only intact impact crater on Earth, and the first reports of findings are in the second category (Morgan, J.V. and 37 others 2016. The formation of peak rings in large impact craters. Science, v. 354, p. 878-882; doi: 10.1126/science.aah6561).

English: K/T extinction event theory. An artis...

Artist’s depiction of the Chicxulub impact 65 million years ago that many scientists say is the most direct cause of the dinosaurs’ disappearance (credit: Wikipedia)

The drill core, reaching down to about 1.3 km below the sea floor penetrates post-impact Cenozoic sediments into a 100 m thick zone of breccias containing fragments of impact melt rock, probably the infill of the central crater immediately following the first few minutes of impact. Beneath that are coarse grained granites representing the middle continental crust from original depths around 10 km. The granite is intensely fractured and riven by dykes and pods of impact melt, and contains intensely shocked grains that typify impacts that produce a transient pressure of ~60 GPa – around six hundred thousand times atmospheric pressure. From seismic reflection surveys this crustal material overlies as yet un-drilled Mesozoic sedimentary rocks. Its density is significantly less than that of unshocked granite – averaging 2.4 compared with 2.6 g cm3. So it is probably filled with microfractures and sufficiently permeable for water to have penetrated once the impact site had cooled. This poses the question, yet to be addressed in print, of whether or not this near-surface layer became colonised by microorganisms in the aftermath (Barton, P. 2016. Revealing the dynamics of a large impact. Science, v. 354, p. 836-837). That is, was the surrounding ocean sterilised at the time of the K-T (K-Pg) mass extinction?; an issue whose resolution is awaited with bated breath by the palaeobiology audience. OK; so theory about the physical process of cratering has been validated to some extent, but will later results be more interesting, outside the planetary sciences community?

Read more about impacts here and mass extinctions here .

A rational view of the start of human influences on Life and Geology

Regular readers will know that I have strong views on attempts to burden stratigraphy with a new Epoch: the Anthropocene. The central one is that the lead-in to a putsch has as much to do with the creation of a bandwagon, to whose wheels all future geologists will be shackled, as it does to any scientific need for such a novelty. Bound up as it is with the fear that Earth may be experiencing its sixth mass extinction, the mooted Anthropocene will likely become a mere boundary marked by future stratigraphers as a Global Boundary Stratotype Section and Point or GSSP between the existing Holocene Epoch and that sequence of sedimentary strata and their fossil record that will be laid down on top of it. Or not, if humanity becomes extinct should the economically induced, dangerous modifications of our homeworld of the last few decades or centuries not be halted. Either way, it defies the stratigraphic ‘rule book’.

No one can deny that humanity’s activities are now immensely disruptive to surface geological processes. Nor is it possible to rule out such disruptive change to the biosphere in the near-future that a latter-day equivalent of the K/Pg or end-Permian events is on the cards: such confidence does not spring from the interminable succession of grand words and global inaction reiterated in December 2015 by the UN Paris Agreement on economically-induced climate change. Still, it was a bit of a relief to find that palaeontological evidence, or rather statistics derived from the fossil record in North American sedimentary rocks since the Carboniferous, emphasises that there is no need for the adoption of Anthropocene as an acceptable geological adjective.

To ecologists, extinctions are not the be all and end all of disruption of the biosphere. Major shifts in life’s richness are also recorded by the way entire ecosystems become disrupted. A classic, if small-scale, example is that way in which the ecosystem of the US Yellowstone National Park changed since the eradication by 1926 of the few hundred grey wolves that formerly preyed mainly on elk. In the 20 years since wolf reintroduction to the Park in 1995 the hugely complex but fragile Yellowstone ecosystem has showed clear signs of recovery of its pre-extirpation structure and diversity.

A consortium of mainly US ecologists, led by Kathleen Lyons of the National Museum of Natural History at the Smithsonian Institution in Washington DC, has assessed linkages between species of fossil animal and plants since the Carboniferous (S.K. Lyons and 28 others, 2015. Holocene shifts in the assembly of plant and animal communities implicate human impacts. Nature, published on-line 16 December 2015 doi:10.1038/nature16447). They found that of the 350 thousand pairs of species that occurred together at different times throughout the late Palaeozoic to the last Epoch of the Cenozoic, the Holocene, some pairs appeared or clustered together more often than might be expected from random chance. Such non-random association suggests to ecologists that the two members of such a pair somehow shared ecological resources persistently, hinting at relationships that helped stabilise their shared ecosystem. For most of post-300 Ma time an average of 64% of non-random pairs prevailed, but after 11.7 ka ago – the start of the Holocene – that dropped to 37%, suggesting a general destabilisation of many of the ecosystems being considered. This closely correlates with the first human colonisation of the Americas, the last of the habitable continents to which humans migrated. This matches the empirical evidence of early Holocene extinctions of large mammals in the Americas, which itself is analogous to the decimation of large fauna in Australasia during the late Pleistocene following human arrival from about 50 to 60 ka ago. Significant human-induced ecological impact seems to have accompanied their initial appearance everywhere. The ecological effects of animal domestication and agriculture in Eurasia and the Americas mark the Holocene particularly. In fact, in Europe the presence of Mesolithic hunter gatherers is generally inferred, in the face of very rare finds of artefacts and dwellings, from changes in pollen records from Holocene lake and wetland sediments, which show periods of tree clearance that can not be accounted for by climate change.

There is no need for Anthropocene, other than as a political device.

Verneshots (huge volcanic gas blasts) ten years on

One of the most daring hypotheses of modern geosciences: is that of the ‘Verneshot’ reported by Earth Pages in 2004.  Jason Phipps Morgan and colleagues explored the possible consequences of a build-up of volatiles in plume-related magmas at the base of thick continental lithosphere beneath cratons, prior to the eruption of continental flood basalts. They suggested that pressure would eventually result in an explosive release at a lithospheric weak point, followed by collapse above the plume head that would propagate upwards, at hypersonic speeds. Modelling the forces involved, the authors of the novel idea considered that they would be sufficient to fling huge rock masses into orbit.  Verneshots might neatly explain the circumstances around mass extinctions, such as their coincidence with continental flood basalt events; large impact structures, most likely at the antipode of the event; global debris layers containing shocked rock, melt spherules; unusual element suites and compounds (including fullerenes); and enough toxic gas to cause biological devastation.

Ten years on, Verneshots are back, again in the prestigious journal Earth and Planetary Science Letters, and this time among the co-authors are Morgan père et fils (W. Jason a founder of plate tectonics, and Jason P. who launched the idea). This time the yet-to-be –accepted hypothesis comes with evidence of an extremely unusual and fortuitous kind (Vannucchi, P. et al. 2015. Direct evidence of ancient shock metamorphism at the site of the 1908 Tunguska event. Earth and Planetary Science Letters, v. 409, p. 168-174). The origin of the paper lies in an attempt to verify reports of shocked quartz in samples collected close to the centre of the 2000 km2 devastation that resulted from what is now accepted to have been a comet or asteroid air-burst explosion in June 1908 in the Tunguska region of Siberia. Apart from a disputed 300 m crater in the area, the Tunguska Event left no long-lived sign: it ‘merely’ knocked over millions of trees. However, its epicenter lay in a 10 km depression ringed by hills, that has been suggested to be a volcanic centre associated with the end-Permian Siberian Traps.

Trees knocked down and burned over hundreds of square km by the 1908Tunguska Event (credit: Leonid Alekseyevich Kulik deceased)

Trees knocked down and burned over hundreds of square km by the 1908 Tunguska Event (credit: Leonid Alekseyevich Kulik deceased)

The reported shocked quartz locality turned out to associated with an isolated occurrence of quartz-rich sand and rounded clasts of quartzite that contains sedimentary structures. The occurrence is surrounded by basalts of the Siberian Traps, yet is situated topographically above them. The quartzite is thought to be Permian terrestrial sandstone that commonly underlies much of the remaining extent of Siberian Traps.

Quartzite clasts do indeed contain shocked quartz, together with pseudotachylite glass veinlets, quartz and feldspar crystal growth on sedimentary grains and silica-rich glassy spherules. These features are not uniquely diagnostic of shock metamorphism, but are oddly absent from the surrounding Siberian Traps nearby, which suggests that whatever formed them predated the final eruptive stages of the end-Permian large igneous province. Indeed it would be unlikely that airburst of some extraterrestrial bolide in 1908 could produce the metamorphic features of the quartzites without setting ablaze the trees that it felled. A second possibility, that the Tunguska Depression is a Permo-Triassic impact crater and the quartzites being part of an associated central uplift runs into the unlikely coincidence of lying less than 5 km from the 1908 epicentre.

A third hypothesis is that the Tunguska Depression is a massive diatreme associated with a Verneshot. Another odd association lies 8 km to the south of the epicentre, a carbonatite that is one of many, along with smaller pipe-like structures all possibly linked to magmatic gas escape. The Tunguska Event, a mighty puzzle in its own right, may perhaps be eclipsed. Will silence return as it did after the original Verneshot hypothesis was published? Quite possibly, but another quirk about the Siberian Traps was reported by Earth Pages in mid-2014. In a contribution to a link between this massive end-Permian volcanic effusion and the Permian-Triassic mass extinction it was noted that in the Chinese sedimentary repository of evidence for the extinction there is an isolated spike in the abundance of nickel  that is almost certainly of volcanic origin, but only the one when repeated flood basalt events perhaps ought to have led to a series of nickel anomalies. One huge volcanic gas release as the Siberian Traps were building up?

Mass extinctions’ connections with volcanism: more support

Plot the times of peaks in the rates of extinction during the Mesozoic against those of flood basalt outpourings closest in time to the die-offs and a straight line can be plotted through the data. There is sufficiently low deviation between it and the points that any statistician would agree that the degree of fit is very good. Many geoscientists have used this empirical relationship to claim that all Mesozoic mass extinctions, including the three largest (end-Permian, end-Triassic and end-Cretaceous) were caused in some way by massive basaltic volcanism. The fact that the points are almost evenly spaced – roughly every 30 Ma, except for a few gaps – has suggested to some that there is some kind of rhythm connecting the two very different kinds of event.

Major extinctions and flood basalt events during the Mesozoic (credit: S Drury)

Major Mesozoic extinctions and flood basalt events (credit: S Drury)

Leaving aside that beguiling periodicity, the hypothesis of a flood-basalt – extinction link has a major weakness. The only likely intermediary is atmospheric, through its composition and/or climate; flood volcanism was probably not violent. Both probably settle down quickly in geological terms. Moreover, flood basalt volcanism is generally short-lived (a few Ma at most) and seems not to be continuous, unlike that at plate margins which is always going on at one or other place. The great basalt piles of Siberia, around the Central Atlantic margins and in Western India are made up of individual thick and extensive flows separated by fossil soils or boles. This suggests that magma blurted out only occasionally, and was separated by long periods of normality; say between 10 and 100 thousand years. Evidence for the duration of major accelerations, either from stratigraphy and palaeontology or from proxies such as peaks and troughs in the isotopic composition of carbon (e.g. EPN Ni life and mass extinction) is that they too occurred swiftly; in a matter of tens of thousand years. Most of the points on the flood-basalt – extinction plot are too imprecise in the time dimension to satisfy a definite relationship. Opinion has swung behind an instantaneous impact hypothesis for the K-P boundary event rather than one involving the Deccan Traps in India, simply because the best dating of the Deccan suggests extinction seems to have occurred when no flows were being erupted, while the thin impact-related layer in sediments the world over is exactly at the point dividing Cretaceous flora and fauna from those of the succeeding Palaeogene.

Yet no such link to an extraterrestrial factor is known to exist for any other major extinctions, so volcanism seems to be ‘the only game in town’ for the rest. Until basalt dating is universally more precise than it has been up to the present the case is ‘not proven’; but, in the manner of the Scottish criminal law, each is a ‘cold case’ which can be reopened. The previous article  hardens the evidence for a volcanic driver behind the greatest known extinction at the end of the Permian Period. And in short-order, another of the Big Five seems to have been resolved in the same way. A flood basalt province covering a large area of west and north-west Australia (known as the Kalkarindji large igneous province)has long been known to be of roughly Cambrian age but does it tie in with the earliest Phanerozoic mass extinction at the Lower to Middle Cambrian boundary? New age data suggests that it does at the level of a few hundred thousand years (Jourdan, F. et al. 2014. High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early-Middle Cambrian (Stage 4-5) extinction. Geology, v. 42, p. 543-546). The Kalkarindji basalts have high sulfur contents and are also associated with widespread breccias that suggest that some of the volcanism was sufficiently explosive to have blasted sulfur-oxygen gases into the stratosphere; a known means of causing rapid and massive climatic cooling as well as increasing oceanic acidity. The magma also passed through late Precambrian sedimentary basins which contain abundant organic-rich shales that later sourced extensive petroleum fields. Their thermal metamorphism could have vented massive amounts of CO2 and methane to result in climatic warming. It may have been volcanically-driven climatic chaos that resulted in the demise of much of the earliest tangible marine fauna on Earth to create also a sudden fall in the oxygen content of the Cambrian ocean basins.

Nickel, life and the end-Permian extinction

The greatest mass extinction of the Phanerozoic closed the Palaeozoic Era at the end of the Permian, with the loss of perhaps as much as 90% of eukaryote diversity on land and at sea. It was also over very quickly by geological standards, taking a mere 20 thousand years from about 252.18 Ma ago. There is no plausible evidence for an extraterrestrial cause, unlike that for the mass extinction that closed the Mesozoic Era and the age of dinosaurs. Almost all researchers blame one of the largest-ever magmatic events that spilled out the Siberian Traps either through direct means, such as climate change related to CO2, sulfur oxides or atmospheric ash clouds produced by the flood volcanism or indirectly through combustion of coal in strata beneath the thick basalt pile. So far, no proposal has received universal acclaim. The latest proposal relies on two vital and apparently related geochemical observations in rocks around the age of the extinctions (Rothman, D.H. et al. 2014. Methanogenic burst in the end-Permian carbon cycle. Proceedings of the National Academy of the United States, v. 111, p. 5462-5467).

Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)

Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)

In the run-up to the extinction carbon isotopes in marine Permian sediments from Meishan, China suggest a runaway growth in the amount of inorganic carbon (in carbonate) in the oceans. The C-isotope record from Meishan shows episodes of sudden major change (over ~20 ka) in both the inorganic and organic carbon parts of the oceanic carbon cycle. The timing of both ‘excursions’ from the long-term trend immediately follows a ‘spike’ in the concentration of the element nickel in the Meishan sediments. The Ni almost certainly was contributed by the massive outflow of basalt lavas in Siberia. So, what is the connection?

Some modern members of the prokaryote Archaea that decompose organic matter to produce methane have a metabolism that depends on Ni, one genus being Methanosarcina that converts acetate to methane by a process known as acetoclastic methanogenesis. Methanosarcina acquired this highly efficient metabolic pathway probably though a sideways gene transfer from Bacteria of the class Clostridia; a process now acknowledged as playing a major role in the evolution of many aspects of prokaryote biology, including resistance to drugs among pathogens. Molecular-clock studies of the Methanosarcina genome are consistent with this Archaea appearing at about the time of the Late Permian. A burst of nickel ‘fertilisation’ of the oceans may have resulted in huge production of atmospheric methane. Being a greenhouse gas much more powerful than CO2, methane in such volumes would very rapidly have led to global warming. Before the Siberian Traps began to be erupted nickel would only have been sufficiently abundant to support this kind of methanogen around ocean-floor hydrothermal springs. Spread globally by eruption plumes, nickel throughout the oceans would have allowed Methanosarcina or its like to thrive everywhere with disastrous consequences. Other geochemical processes, such as the oxidation of methane in seawater, would have spread the influence of the biosphere-lithosphere ‘conspiracy’. Methane oxidation would have removed oxygen from the oceans to create anoxia that, in turn, would have encouraged other microorganisms that reduce sulfate ions to sulfide and thereby produce toxic hydrogen sulfide. That gas once in the atmosphere would have parlayed an oceanic ‘kill mechanism’’ into one fatal for land animals.

There is one aspect that puzzles me: the Siberian Traps probably involved many huge lava outpourings every 10 to 100 ka while the magma lasted, as did all other flood basalt events. Why then is the nickel from only such eruption preserved in the Meishan sediments, and if others are known from marine sediments is there evidence for other such methanogen ‘blooms’ in the oceans?

Greening the Earth, Devonian forest fires and a mass extinction

Land plants begin to appear in the fossil record as early as the late Ordovician (~450 Ma), show signs of diversification during the Silurian and by the end of the Devonian Period most of the basic features of plants are apparent. During the Carboniferous Period terrestrial biomass became so high as to cause a fall in atmospheric carbon dioxide, triggering the longest period of glaciation of the Phanerozoic, and such a boost to oxygen in the air (to over 30%) that insects, huge by modern standards, were able to thrive and the risk of conflagration was perhaps at its highest in Earth’s history. Yet surprisingly, the first signs of massive forest fires appear in the Devonian when vegetation was nowhere near so widespread and luxuriant as it became in the Carboniferous (Kaiho, K. et al. 2013. A forest fire and soil erosion event during the Late Devonian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 392, p. 272-280). Moreover, Devonian oxygen levels were well below those of the present atmosphere and CO2 was more than 10 times even the post-industrial concentration (387 parts per million in 2013). Such atmospheric chemistry would probably have suppressed burning.

Kunio Kaiho of Tohoku University in Japan and colleagues from Japan, the US and Belgium analysed organic molecules in Belgian marine sediments from the time of the late-Devonian mass extinction (around the Frasnian-Famennian boundary at 372 Ma). A range of compounds produced by hydrocarbon combustion show marked ‘spikes’ at the F-F boundary. The thin bed that marks the extinction boundary also shows sudden increase then decrease in δ13C and total organic carbon, indicative of increase burial of organic material and a likely increase in atmospheric oxygen levels. Another biomarker that is a proxy for soil erosion follows the other biogeochemical markers, perhaps signifying less of a binding effect on soil by plant colonisation: a likely consequence of large widlfires. Unlike the biomarkers, magnetic susceptibility of the boundary sediments is lower than in earlier and later sediments. This is ascribed to a decreased supply of detrital sediment to the Belgian marine Devonian basin, probably as a result of markedly decreased rainfall around the time of the late-Devonian mass extinction. But the magnetic data from 3 metres either side of the boundary also reveal the influence of the 20, 40, 100 and 405 ka Milankovich cycles.

Juan Ricardo Cortes , a placoderm from the Dev...

Dunkleosteus, a giant (10 m long) placoderm fish from the Devonian, which became extinct in the late Devonian along with all other placoderms (credit: Wikipedia)

This set of environmentally-related data encourages the authors to suggest a novel, if not entirely plausible, mechanism for mass extinction related to astronomically modulated dry-moist climate changes that repeatedly killed off vegetation so that dry woody matter could accumulate en masse during the Frasnian while atmospheric oxygen levels were too low for combustion. A mass burial of organic carbon at the end of that Age then boosted oxygen levels above the burning threshold to create widespread conflagration once the wood pile was set ablaze. Makes a change from continental flood basalts and extraterrestrial impacts… Yet it was about this time that vertebrates took it upon themselves to avail themselves of the new ecological niche provided by vegetation to haul themselves onto land.

Late Devonian: mass extinction or mass invasion?

A hand made lookalike for User:Dragons flight'...

Image via Wikipedia

The later part of the Devonian (the Frasnian and Famennian Stages) once marked the second largest marine mass extinction (~375 Ma) of the Phanerozoic Eon: it was one of the ‘Big Five’. For the last decade the drop in marine biodiversity in that interval has come under scrutiny: partly because it may have involved several  events;  no well-supported extinction mechanism has emerged; and extinctions seem have been concentrated on three animal groups – trilobites, brachiopods and reef corals. Something large did happen, as reef-building corals almost disappeared and coral reefs only returned with the rise of modern (scleractinian) corals in the Mesozoic. While the end of the Devonian still figures widely as having experienced a mass extinction event, more detailed palaeontological work at the genus and species level suggests another possibility.

‘Officially’ a mass extinction event must exceed the background extinction rate throughout the Phanerozoic and be above that in immediately preceding and following stages: statistically significant, that is. They always give rise to a marked reduction in biodiversity, but another mechanism can do that without extinctions suddenly increasing. The rate at which new species emerge can fall below that of species extinctions, when the overall number of living species falls. As far as ecosystems are concerned both processes are equally severe, but the causes may be very different.

Hederelloids encrusting a Spiriferida brachiop...

Brachiopod from the Devonian of Ohio, USA. Image via Wikipedia

Reviewing detailed records of Devonian species of two genera of brachiopods and one bivalve genus (50 species in all) in five North American stratigraphic sequences, Alycia Stigall of Ohio University, USA noted apparently significant variations in the local assemblages (Stigall, A. L. 2012. Speciation collapse and invasive species dynamics during the Late Devonian ‘Mass Extinction’. GSA Today, v. 22(1), p. 4-9). Speciation overall fell in the Frasnian and the preceding Givetian, while rate of extinction barely changed. For the three studied genera ,speciation reached low values in the Frasnian and Famennian, but that was accompanied by an equally large fall in extinctions. In this narrow sample we seem to be seeing not an extinction crisis but one of biodiversity. Why?

The Late Devonian saw repeated ups and downs in sea level, which repeatedly connected and disconnected shallow- to moderate-depth marine basins. The fossil record shows repeated cases of species from one basin colonising another, each invasion accompanying rapid marine transgression.. One means whereby species arise is through prolonged isolation of separate populations of the ancestral species through independent genetic drift and mutation. The episodic connection of basins may have prevented such allopatric speciation. Interestingly, the invading species  were dominantly animals with a broad tolerance for environmental conditions.

Whether this mechanism applied to all three main animal groups whose diversity plummeted in Late Devonian times remains to be seen, and it begs the question ‘why didn’t it happen among other animal groups that were less affected by whatever the events were?’ One of the problems associated with decreasing biodiversity in modern marine (and terrestrial) settings is growth in the numbers of invasive species, so the work on 375 Ma fossils might help understand and mitigate current ecological issues. The only difference is that for many of the hyper-successful invader species the means of invasion has been provided by human activities. brachiopod brachioopod

Pristine mantle and basalt floods

The Western Ghat hills at Matheran in Maharash...

Flood basalts of the Deccan Traps in Maharashtra State, India. Image via Wikipedia

Plot the ages of major extinctions against those of flood basalt events and you will get a straight line graph for six co-occurrences since 250 Ma, with very little error. Although the exact mechanism for mass death of species and families is argued over interminably, for those six, flood basalt events have to be deeply implicated. There again, every geologist and their aunties dispute the mechanisms behind monster basalt effusions that bury whole landscapes beneath flow after flow and create very distinctive landforms.  When they are eroded they form regularly stepped mountain sides, hence their formerly popular name trap basalts, after the Swedish word trappa meaning staircase.  There is a hint of cyclicity in their age distribution. But most important of all, no-one has witnessed these vast, pulsating events, the last having mantled the surroundings of the Columbia and Snake River catchments in the US states of Oregon and Washington between 14-17 Ma ago in the Middle Miocene. Some mark episodes of continental break-up, such as those flanking the Central Atlantic at the time of the end-Triassic (~200 Ma) mass extinction, while others are associated with hot spots, such as the Deccan Traps of western India erupted between 60-68 Ma as India drifted over the Reunion hot-spot and those of the Ethiopian highlands (30 Ma) associated with the Afar hot spot.

A common geochemical feature is beginning to emerge concerning the mantle from which the basalts were partially melted. Six sets of flood basalts exhibit the same trace-element and isotopic (Nd, Pb, Hf and He) characteristics, which suggest that their source had been little effected by previous extraction of crust-forming magmas; it is primitive and may be a relic of the original mantle formed at about 4500 Ma shortly after the catastrophic collision between the early Earth and a wandering Mars-sized planet that flung off the Moon (Jackson, M.G. & Carlson, R.W. 2011. An ancient recipe for flood basalt genesis. Nature, online (27 July 2011) doi:10.1038/nature10326). Although undepleted, the chemistry of the mantle source, worked out by back-calculation from that of the flood basalts, is not the same as the once-postulated original accretion of carbonaceous chondrite meteorites: conceivably a result of the chemical reworking when the Moon formed and the remaining Earth was probably molten from top to centre. The important feature is that the recast chemistry is rich in heat-producing elements compared with the source of ‘common-or-garden’ basalts that continually contribute to the ocean floors and island arcs. Wherever the relic mantle is, it is capable of heating itself, over and above the heating from the core and surrounding mantle, and thus likely to generate thermal and material plumes rising through the mantle.

Preceding the work of Jackson and Carlson, another group discovered that when flood basalt events since the Carboniferous are restored to their former geographic positions at the time they were erupted, they cluster above what are now two patches of more ductile mantle close to the cure-mantle boundary (Torsvik, T.H. et al. 2010. Diamonds sampled by plumes from the core–mantle boundary. Nature, v. 466, p. 352–355). If that is the source of basalt flood-forming plumes, then it is still there and, aside from giant impacts with extra-terrestrial projectiles, the most catastrophic upheavals of the Earth system inevitably will continue, perhaps in the next few million years.