Category Archives: Geobiology, palaeontology, and evolution

Mystery of upside-down dinosaurs resolved

Remains of ankylosaurs, a popular family of heavily armoured dinosaurs, occur in sedimentary sequences that range in age from early Jurassic to the close of the Cretaceous. Their defences seem almost impregnable, being constructed of thick, fused scales, often bearing formidable spines, which covered them completely. They bore a crude resemblance to modern armadillos apart from the fact that they were unable to roll-up defensively. In some species the rigid tail bears a large, knob of scale tissue. At up to 3 m long, were the tail to be swung it would have packed bone-cracking momentum. Interestingly, to a poorly sighted predator the club may have been mistaken for the animal’s head at the end of a long neck, so perhaps it lured a potential assailant within range of its devastating power. The largest ankylosaur, from the Cretaceous of western Canada, was the size of a small bus, up to 8m long, 1.5 m wide, standing 1.7 m high and weighing in at around 5 to 8 t. Such dimensions would have made it almost impossible to be bitten, even by the largest predatory dinosaurs, and difficult to turn over. Their teeth show that ankylosaurs were herbivorous, and their somewhat bulbous bodies almost certainly contained a massive digestion system.

Polski: JuraPark Bałtów - Park Dinozaurów - An...

Recfonstruction of Ankylosaurus at the JuraPark Bałtówin Poland (credit: Wikipedia)

The mystery lies in the fact that most ankylosaur fossils are found lying on their backs. Early dinosaur aficionados suggested a tendency for the lumbering beasts to tumble down slopes and become stranded on their backs, so to die miserably. Such clumsiness is hardly a positive characteristic of evolutionary fitness for such a long-lived group, and, besides, the sedimentary formations in which they are found indicate very gentle slopes. So, were they flipped by dextrous predators, as imagined in some early films purporting to look to the distant past? Probably not, for most well preserved fossils show no sign of bites or gnawing. From a study of 36 late-Cretaceous ankylosaurs from Alberta four Canadian and US palaeontologists (Mallon, J.C. et al. 2018. A “bloat-and-float” taphonomic model best explains the upside-down preservation of ankylosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 497, p. 117-127; doi:10.1016/j.palaeo.2018.02.010 support the idea of their carcases or even living animals having been picked up by flood waters when their high centre of gravity would have flipped them upside down. Bloating through decay might then allow them to be transported large distances. Unsurprisingly, their conclusions rest on model simulations.

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When dinosaurs roamed the Western Isles

Cuillin Hills, Isle of Skye, Scotland, UK

Cuillin Hills, Isle of Skye, Scotland, UK (credit: Wikipedia)

The Isle of Skye off the northwest coast of Scotland  is known largely as a prime tourist destination, such as Dunvegan Castle with a real clan chief (The MacLeod of MacLeod) and its Faerie Flag; Britain’s only truly challenging mountains of the Black Cuillin; and, of course, the romantic connection with the Young Pretender, Charles Edward Stuart and his escape, in drag, from the clutches of the Duke ‘Butcher’ Cumberland, hence the Skye Boat Song. Geologists know it best for its flood basalts with classic stepped topography and the exhumed guts of a massive central volcano (the Cuillin), relics of the Palaeocene-Eocene (62 to 54 Ma) North Atlantic Large Igneous Province. The spectacular Loch Coruisk, a glacial corrie drowned by the sea, exposes the deepest part of the main magma chamber. It is also the lair of Scotland’s lesser known Monster, the dread Each Uisge (Water Horse). Yet evidence is emerging for the former presence in the Hebrides of other, more tangible monsters.

Skye’s great volcanic edifice rests on Mesozoic sedimentary rocks including shallow-water muddy limestones of the Great Estuarine Group of Middle Jurassic (Bathonian, 174–164 Ma) age. For dinosaur specialists this is of the time when meat-eating theropods and herbivorous sauropods began growing to colossal sizes. Yet the Bathonian is notable for its global paucity in well exposed terrestrial and near-shore sedimentary sequences. Easily accessible, the Skye Bathonian sequence is much visited and has yielded a rich, though generally fragmentary fauna. A group of recent visiting palaeontologists from the University of Edinburgh, the Chinese Academy of Sciences and Skye’s Staffin Museum have discovered an extensive tract of wave-cut platform on the east shore of the Trotternish Peninsula where lagoonal carbonate muds were trampled by several dinosaurs that left around 50 tracks (dePolo, P.E. et al. 2018. A sauropod-dominated tracksite from Rubha nam Brathairean (Brothers’ Point), Isle of Skye, Scotland. Scottish Journal of Geology, online; doi:10.1144/sjg2017-016).

Dinosaur foot prints from Skye. Left example of a sauropod rear-foot print; right theropod. (credit dePolo, P.E. et al. 2018, modified from Figs 8 and 9a)

Some are of medium-sized sauropods (either Parabrontopodus or Breviparopus – both names for footprints rather than any genus of dinosaur) whose crudely elephant-like footprints are up to 0.5 m across (the largest, from Western Australia, are about 1.7 m across). Although there are fragmentary dinosaur bones from the same strata, assigning the footprint to a known species is not possible. However, foot size can be used to estimate how high the creatures’ hips stood (2 to 2.5 m): hefty beasts but not the true giants of later times A variety of three-toed, clawed, somewhat bird-like, footprints also occur. They are assigned to probably bipedal carnivores or theropods. Variation in foot size suggests a range of hip-height from about 0.9 to 2 metres, so these carnivores would have been pretty formidable.

Hadean potentially fertile for life

The earliest incontrovertible signs of life on Earth are in the 3.48 billion-year-old Dresser Formation in the Pilbara craton of Western Australia, which take the form of carbon-coated, bubble-like structures in fine-grained silica sediments ascribed to a terrestrial hot-spring environment. In the same Formation are stromatolites that are knobbly, finely banded structures made of carbonates. By analogy with similar structures being produced today by bacterial mats in a variety of chemically stressed environments that are inhospitable for multicelled organisms that might know them away, stromatolites are taken to signify thriving, carbonate secreting bacteria. There are also streaks of carbon associated with wave ripples that may have been other types of biofilm. A less certain record of the presence of life are stromatolite-like features in metasediments from the Isua supracrustal belt of West Greenland, dated at around 3.8 Ga, which also contain graphite with carbon-isotopic signs that it formed from biogenic carbon. Purely geochemical evidence that carbonaceous compounds may have formed in living systems are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen.

At present there is little chance of extending life’s record further back in time than four billion years because the Hadean is mainly represented by pre 4 Ga ages of zircon grains found in much younger sedimentary rocks – resistant relics of Hadean crustal erosion. The eastern shore of Hudson Bay does preserve a tiny (20 km2) patch of metamorphosed basaltic igneous rocks, known as the Nuvvuagittuq Greenstone Belt. Dated at 3.77 Ga by one method but 4.28 Ga by another, this could be Hadean. Like the Isua sequence that in Quebec also contains metasediments, including banded ironstones with associated iron-rich hydrothermal deposits. Silica from the vent system shows dramatically lifelike tubules. Yet the ambiguity in dating upsets any claims to genuine Hadean life. There has also been a physical stumbling block to the notion that life may have originated and thrived during the Hadean: the bombardment record.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Nuvvuagittuq supracrustal belt, Canada. Some of these rocks contain quite convincing examples of fossil cells. (credit: Wikipedia)

While oxygen-isotope data from 4.4 Ga zircons hints strongly at subsurface and perhaps surface water on Earth at that time, continued accretion of large planetesimals would have created the hellish conditions associated with the name of the first Eon in Earth’s history. Liquid water is essential for life to have formed, on top of a supply of the essential biological elements C, H, O, N, P and S. The sheer amount of interstellar dust that accompanied the Hadean impact record would have ensured fertile chemical conditions, but would the surface and near-surface of the early Earth have remained continually wet? Judging by the lunar surface and that of other bodies in the solar system, after the cataclysmic events that formed the Moon, many Hadean impacts on Earth were in the range of 100 to 1000 km across, with a Late Heavy Bombardment (LHB)that not only increased the intensity of projectile delivery but witnessed the most energetic single events such as those that created the lunar maria and probably far larger structures on Earth. The thermal energy, accompanied, by incandescent silicate vapour ejected from craters, may have evaporated oceans and even subsurface water with calamitous consequences for early life or prebiotic chemistry. Until 2017 no researchers had been able to model the energetic of the Hadean convincingly.

After assessing the projectile flux up to and through the LHB, and the consequent impact heating Bob Grimm and Simone Marchi of the Southwest Research Institute in Boulder, Colorado modelled the likely thermal evolution of the outer Earth through the Hadean. This allowed them to calculate the likely thermal gradients in the near-surface, the volumes of rock each event would have affected and the times taken for cooling after impacts (Grimm, R.E. & Marchi, S. 2018. Direct thermal effects of the Hadean bombardment did not limit early subsurface habitability. Earth and Planetary Science Letters, v. 485, p. 1-9; doi:10.1016/j.epsl.2017.12.043). They found that subsurface ‘habitability’ would have grown continuously throughout the Hadean, even during the worst events of the LHB. Sterilizing Earth and thus destroying and interrupting any life processes could only have been achieved by ten times more projectiles arriving ten times more frequently over the 600 Ma history of the Hadean and LHB. Although surface water may have been evaporated by impact-flash heating and vaporized silicate ejecta, the subsurface would have been wet at least somewhere on the early Earth. Provided it either originated in or colonised surface sedimentary cover it would have been feasible for life to have survived the Hadean. However, nobody knows how long it would have taken for the necessary accumulation of prebiotic chemicals and to achieve the complex sequence of processes that lead to nucleic acids encapsulated in cells and thus self-replication and life itself.

The rise of the eukaryotes

You and I, and all the living things that we can easily see belong to the most recently evolved of the three great domains of life, the Eukarya. The vast bulk of organisms that we can’t see unaided are prokaryotes, divided into the Bacteria and the Archaea. Their genetic material floats around in their cell’s fluid, while ours resides mainly in the eukaryote cell’s nucleus with a bit in various organelles known as mitochondria and the chloroplasts of plant cells. Unlike the chicken and egg question, that concerning which came first, prokaryotes or eukaryotes, is answered by DNA. Eukaryote DNA contains a lot from prokaryotes, but the converse does not hold. That contrast posed the question of how eukaryotes arose from the two earlier, simpler forms of life, the answer to which Lynn Margulis suggested to be a whole series of symbiotic relationships among various prokaryotes that shared a host cell; her hypothesis of endosymbiosis. Now, the vast majority of eukaryotes depend on free oxygen for their metabolism, so when the first of them arose boils down to the period of geological history following the Great Oxidation Event around 2.4 billion years ago.

Structure of a typical animal cell

Structure of a typical eukaryote (animal) cell (credit: Wikipedia)

Molecular-clock estimates based on the range of variation in the genomes of a wide range of eukaryotes suggest it took place sometime between 1000 and 2000 Ma. A better means of homing in on a date for the Last Eukaryote Common Ancestor (LECA – as opposed to that of the first organism LUCA) would be that of the earliest fossil to show eukaryote affinities. Grypania from 1.85 Ga, a sort of whorl-like fossil, is a good candidate and is widely thought to be the earliest of our kind but lacks signs of actual cells. More convincing fossils – known generically as acritarchs – from times between 1.5 and 1.0 Ga look like primitive fungi, red algae and slime moulds. A comprehensive review of the microfossils of the Palaeoproterozoic (2.5 to 1.6 Ga) includes both prokaryotes and probable early eukaryotes (Javaux, E.J. & Lepot, K. 2017. The Paleoproterozoic fossil record: Implications for the evolution of the biosphere during Earth’s middle-age. Earth Science Reviews, v. 176, p. 68-86; doi: 10.1016/j.earscirev.2017.10.0001). Yet, despite rapidly accumulating evidence, especially from rocks in China, the picture remains one of monotony; for instance Grypania spans the best part of half a billion years. Bacteria and Archaea cannot be distinguished easily in the absence of preserved DNA. Despite evidence for oxygen in the oceans and atmosphere, apart from a few shallow-water oxygenated examples the chemistry of Palaeoproterozoic marine sediments is dominated by mineralogical outcomes of reducing chemistry. Many chemical isotopic environmental proxies ‘flat-line’ to the extent that the early Proterozoic is sometimes referred to as the ‘boring billion’, yet our ultimate precursors were part of the marine ecosystem. That is, unless one accepts the possibility that that fossils labelled ‘eukaryote’ are colonial prokaryotes – evidence for cell nuclei is sparse. Endosymbiosis, although an attractive model for eukaryote origins, is not proven. The reason for lingering scepticism is that there are only a tiny number of modern examples of prokaryote cells ending up inside those of other prokaryotes.

Whatever, chemical biomarkers in sediments older than about 720 Ma indicate that prokaryotes were the only notable primary producers in the oceans until the Neoproterozoic. Microscopic fossils that are inescapably eukaryotes in the form of amoeba suddenly emerge around that time. This development from the lingering marginality of early eukaryotes to thriving ecosystems that they dominated thereafter is a puzzle seeking a plausible explanation. It coincides with the onset of the Snowball Earth glaciations of the Cryogenian Period (850 to 635 Ma) and a rise in atmospheric and presumably oceanic oxygen. Then macroscopic eukaryotes ‘bloomed’ into distinctively different forms in the Ediacaran Period (635 to 541 Ma) and thereafter. Before the Cryogenian we can perhaps regard eukaryan life and the endosymbiosis that may have given rise to it as a series of ecological experiments repeatedly knocked-back by chemical conditions and competition with the vastly more abundant prokaryotes.

 

Banded iron formations (BIFs) reviewed

This image shows a 2.1 billion years old rock ...

2.1 billion years old boulder of banded ironstone. (credit: Wikipedia)

During most of the last hundred years every car body, rebar rod in concrete, ship, bridge and skyscraper frame had its origins in vividly striped red rocks from vast open-pit mines. Comprising mainly iron oxides with some silica, these banded iron formations, or BIFs for short, occur in profitable tonnages on every continent. But commercial reserves are confined mainly to sedimentary sequences dating from about 3 to 2 billion years ago. They are not the only commercial iron formations, but dominate supplies from estimated reserves of around 105 billion tons. From a non-commercial standpoint they are among the most revealing kinds of sediment as regards the Earth system and its evolution. All scientific aspects of BIFs and similar Fe-rich sediments are reviewed in a recent volume of Earth Science Reviews. (Konhauser, K.O. and 12 others 2017. Iron formations: a global record of Neoarchaean to Palaeoproterozoic environmental history. Earth Science Reviews, v. 172, p. 140-177; doi: 10.1016/j.earscirev.2017.06.012).

The chemical, mineral and isotopic compositions of BIFs form a detailed repository of the changing composition of seawater during a crucial period for the evolution of Earth and life – the transition from an anoxic surface environment to one in which water and air contained a persistent proportion of oxygen, known as the Great Oxidation Event (GOE). Paradoxically, BIFs are highly oxidized rocks, the bulk of which formed when other rocks show evidence for vanishingly small amounts of oxygen in the surface environment. The paradox began to be resolved when it was realized that ocean-ridge basaltic volcanism and sea-floor hydrothermal activity would have released vast amounts of soluble, reduced iron-2 into anoxic seawater, in the upper parts of which the first photosynthetic organisms evolved. Evidence for the presence of such cyanobacteria first appears around 3.5 billion years ago, in the form of carbonates whose structure suggests they accumulated from growth of microbial mats. Oxygen generated by photosynthesis in iron-rich water immediately acts to oxidize soluble iron-2 to iron-3 to yield highly insoluble iron oxides and hydroxides and thus deposits of BIFs. While oceans were iron-rich, formation of ironstones consumed ecologically available oxygen completely.

Other biological processes seem to have been involved in ironstone formation, such as photosynthesis by other bacteria that used dissolved iron-2 instead of water as a reductant for CO2, to release iron-3 instead of oxygen. That would immediately combine with OH­ ions in water to precipitate iron hydroxides. Konhauser and colleagues cogently piece together the complex links in chemistry and biology that emerged in the mid- to late Archaean to form a linkage between carbon- and iron cycles, which themselves influenced the evolution of other, less abundant elements in seawater from top to bottom. The GOE is at the centre. The direct evidence for it lies in the sudden appearance of ancient red soils at about 2.4 billion years, along with the disappearance of grains of sulfides and uranium oxides – both readily oxidized to soluble products – from riverine sandstones, which signifies significant oxygen in the atmosphere. Yet chemical changes in Precambrian marine sediments perhaps indicate that oxygen began to rise in ocean water as early as 3 billion years ago. That suggests that for half a billion years biogenic and abiogenic processes in the oceans were scavenging oxygen as fast as it could be produced so that only tiny amounts, if any, escaped into the atmosphere. Among other possible factors, oceanic methane emissions from methanogen bacteria may have consumed any atmospheric oxygen – today methane lasts only for about 9 years before reaction with oxygen forms CO2. If and when methanogens declined free oxygen would have been more likely to survive in the atmosphere.

The theme running through the review is that of changing and linked interactions between life and the inorganic world, mantle, lithosphere, hydrosphere and atmosphere that involved all available chemical elements. The dominant chemical process, as it is today, was the equilibrium between oxidation and reduction – the loss and gain of electrons among possible chemical reactions and in metabolic processes. Ironstones were formed more commonly between 3 to 2 Ga than at any time before or since, and form a substantial part of that periods sedimentary record. Their net product and that of the protracted organic-inorganic balancing act – oxygenation of the hydrosphere and atmosphere – opened the way for eukaryote organisms, their reproduction by way of the splitting and recombination of nuclear DNA and their evolutionary diversification into the animal and plant life that we know today and of which we are a part. It is possible that even a subtly different set of global processes and interactions set in motion during early evolution of a planet apparently like Earth may have led to different and even unimaginable biological outcomes in later times. The optimism of exobiologists should be tempered by this detailed review.

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)

Ancient footprints

To see traces of where our forebears walked, such as the famous Australopithecus afarensis trackway at Laetoli in Tanzania, the footprints of Neanderthal children in 350 ka old Italian volcanic ash (The first volcanologists? Earth Pages March 2003) or even those of Mesolithic families in estuarine mud is about as heart stopping as it gets for a geologist. But imagine the astonishment of members of a multinational team working on Miocene shore-line sediments on Crete when they came upon a bedding surface covered with what are almost certainly the footprints of another bipedal animal from 5.7 Ma ago (Gierliński, G.D. et al. 2017. Possible hominin footprints from the late Miocene (c. 5.7 Ma) of Crete? Proceedings of the Geologists’ Association, online; https://doi.org/10.1016/j.pgeola.2017.07.006). Trackways preserve a few moments in time, however old they are and the chances of their being preserved are very small, yet they can supply information that is lost from even the best preserved fossil, such as gait, weight, speed and so forth.

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Track bearing surface; (b) two footprints in 5.7 Ma old Miocene sediments at Trachilos, Crete (credit: Gierliński, G.D. et al. 2017; Figures 2 and 8)

The tracks clearly indicate that whatever left them was bipedal and lacked claws, and closely resemble those attributed to A. afarensis at Laetoli in a 3.7 Ma old volcanic ash. What they do not resemble closely are those of non-hominin modern primates, such as chimpanzees. They are diminutive compared with adult modern human prints, being about 12.5 cm long (equivalent to a UK child’ shoe size 4 – US size 4.5, EU 20) and about a third to half the size of those at Laetoli. Were they around the age of those at Laetoli or younger there seems little doubt that they would be widely interpreted as being of hominin origin. But being from an island in the Mediterranean as well as far from sites in Africa that have yielded Miocene hominins (Ardipithecus kadabba from Ethiopia, Orrorin from Kenya and Sahelanthropus from Chad),  such an interpretation is bound to create controversy. Somewhat less controversial might be to regard them as having been created by a late-Miocene primate that convergently evolved a hominin-like upright gait and foot. Being preserved in what seem to be coastal marine sediments, there is probably little chance of body fossils being preserved in the exposed horizon. Since foot bones are so fragile, even if a primate fossil is discovered in the late Miocene of Crete the chances of resolving the issue are pretty remote. Yet fossil primate specialists will undoubtedly beat a well-trodden path to the Trachilos site near Kissamos on Crete

Wildfires and climate at the K-Pg boundary

It is now certain that the Cretaceous-Palaeogene boundary 66 Ma ago coincided with the impact of a ~10 km diameter asteroid that produced the infamous Chicxulub crater north of Mexico’s Yucatán peninsula. Whether or not this was the trigger for the mass extinction of marine and terrestrial fauna and flora – the flood basalts of the Deccan Traps are still very much in the frame – the worldwide ejecta layer from Chicxulub coincides exactly with the boundary that separates the Mesozoic and Cenozoic Eras. As well as shocked quartz grains, anomalously high iridium concentrations and glass spherules the boundary layer contains abundant elemental carbon, which has been widely ascribed to soot released by vegetation that went up in flames on a massive scale. Atmospheric oxygen levels in the late Cretaceous were a little lower than those at present, or so recent estimates from carbon isotopes in Mesozoic to Recent ambers suggest (Tappert, R. et al. 2013. Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic. Geochimica et Cosmochimica Acta, v. 121, p. 240-262,) – other estimates put the level substantially above that in modern air. Whatever, global wildfires occurred within the time taken for the Chicxulub ejecta to settle from the atmosphere; probably a few years. It has been estimated that about 700 billion tonnes of soot were laid down, suggesting that most of the Cretaceous terrestrial biomass and even a high proportion of that in soils literally went up in smoke.

Charles Bardeen and colleagues at the University of Colorado, Boulder, have modelled the climatic and chemical effects of this aspect of the catastrophe (Bardeen, C.G. et al. 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. Proceedings of the National Academy of Sciences; doi:10.1073/pnas.1708980114). Despite the associated release of massive amounts of CO2 and water vapour by both the burning and the impact into seawater, giving increased impetus to the greenhouse effect, the study suggests that fine-grained soot would have lingered as an all enveloping pall in the stratosphere. Sunlight would have been blocked for over a year so that no photosynthesis would have been possible on land or in the upper ocean, the temperatures of the continent and ocean surfaces would have dropped by as much as 28 and 11 °C respectively to cause freezing temperatures at mid-latitudes. Moreover, absorption of solar radiation by the stratospheric soot layer would have increased the temperature of the upper atmosphere by several hundred degrees to destroy the ozone layer. Consequently, once the soot cleared the surface would have had a high ultraviolet irradiation for around a year.

The main implication of the modelling is a collapse in both green terrestrial vegetation and oceanic phytoplankton; most of the food chain would have been absent for long enough to wipe out those animals that depended on it entirely. While an enhanced greenhouse effect and increased acidification of the upper ocean through CO2 emissions by the Deccan flood volcanism would have placed gradually increasing and perhaps episodic stresses on the biosphere, the outcome of the Chicxulub impact would have been immediate and terrible.

More on mass extinctions and impacts here and here

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.

Earliest hydrothermal vents and evidence for life

 

That seawater circulates through the axial regions of rifts associated with sea-floor spreading has been known since well before the acceptance of plate tectonics. The idea stems from the discovery in 1949 of brines with a temperature of 60°C on the central floor of the Red Sea, which in the early 60s turned out to be anomalously metal-rich as well. Advanced submersibles that can withstand the high pressures at great depth a decade later produced images of swirling clouds of sediment from large sea-floor springs, first on the Galapagos rift and subsequently on many others. The first shots were of dark, turbulent clouds, prompting the term ‘black smoker’ for such hydrothermal vents and it turns out that others produce light-coloured clouds – ‘white smokers’. Sampling revealed that the sediments in black smokers were in fact fine-grained precipitates of metallic sulfides, whereas those forming white smokers were sulfates, carbonates and oxides of barium calcium and silicon also precipitated from solute-rich brines produced by partial dissolution of ocean floor through which they had passes.

A black smoker known as "the brothers".

A black smoker with associated organism. (credit: Wikipedia)

Excitement grew when hydrothermal vents were shown to have complex animal ecosystems completely new to science. A variety of chemical evidence, most importantly the common presence of proteins and other cell chemicals built around metal sulfide groups in most living organisms, prompted the idea that hydrothermal vents may have hosted the origins of life on Earth. Many fossil vent systems also contain fossils; macrofossils in the Phanerozoic and microbial ones from the Precambrian. But tangible signs of life, in the form of mats ascribed to bacteria or archaea holding together fine-grained sediments, go back no further than 3830 Ma in the Isua area of SW Greenland. Purely geochemical evidence that carbonaceous compounds may have formed in living systems  are ambiguous since quite complex hydrocarbons can be synthesised abiogenically by Fischer-Tropsch reactions between carbon monoxide and hydrogen. Signs of deep sea hydrothermal activity are common in any geological terrain containing basalt lavas with the characteristic pillows indicating extrusion beneath water. So to trace life’s origins all that is needed to trigger the interest of palaeobiologists are the oldest known pillow lavas. Until quite recently, that meant the Isua volcano-sedimentary association, but heating, high pressures and  very strong deformation affected those rocks when they were metamorphosed half a billion years after they were formed; a cause for skepticism by some geoscientists.

The primacy of Isua metavolcanic rocks has been challenged by more extensive metamorphosed basalts in the Nuvvuagittuk area in Quebec on the east side of Hudson Bay, Canada. They contain hydrothermal ironstones associated with pillowed basalts that are cut by more silica-rich intrusive igneous rocks dated between 3750 and 3775 Ma. That might place the age of basalt volcanism and the hydrothermal systems in the same ball park as those of Isua, but intriguingly the basalts’ 146Sm-142Nd systematics suggest a possible age of magma separation from the mantle of 4280 Ma (this age is currently disputed as it clashes with  U-Pb dates for zircon grains extracted from the metabasalts around the same as the age at Isua). Nonetheless, some parts of the Nuvvuagittuk sequence are barely deformed and show only low-grade metamorphism, and they contain iron- and silicon-rich hot spring deposits (Dodd, M.S. et al. 2017. Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature, v. 543, p. 60-64; doi:10.1038/nature21377). As at Isua, the ironstones contain graphite whose carbon isotope proportions have an ambiguous sign of having formed by living or abiotic processes. It is the light deformation and low metamorphism of the rocks that gives them an edge as regards being hosts to tangible signs of life. Extremely delicate rosettes and blades of calcium carbonate and phosphate, likely formed during deposition, remain intact. These signs of stasis are in direct contact with features that are almost identical to minute tubes and filaments formed in modern vents by iron-oxidising bacteria. All that is missing are clear signs of bacterial cells. Ambiguities in the dating of the basalt host rocks do not allow the authors claims that their signs of life are significantly older than those at Isua, but their biotic origins are less open to question. Neither offer definitive proof of life, despite widespread claims by media science correspondents, some of whom tend  metaphorically to ‘run amok ‘ when the phrase ‘ancient life’ appears; in this case attempting to link the paper with life on Mars …

You can find more on early life here