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.

 

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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.

Mega-impacts and tectonics

Because they are fast as well as weighty, destination-Earth asteroids and comets pack quite a punch. That is because their kinetic energy is proportional to the square of their speed (at least 13 km s-1) as well as half their mass. So, even all one half a kilometre across carries an energy a hundred times the solar energy received by Earth in a year, and a great deal more when compared with geothermal heat production. Much of the focus on the effects of impact events has dwelled on the upper crust, the oceans and atmosphere. Yet they also have huge seismic effects, with a proportion of their shock effect being dissipated throughout the entire planet. One obvious consequence would be a thermal anomaly directly beneath the crater as well as some thinning of the lithosphere and body waves affecting the rest of the solid Earth.

Thermal and mechanical processes lie at the core of tectonics, so a big question has been ‘Could impacts create mantle plumes or set new tectonic processes in motion?’ There has been speculation of diverse kinds since impacts became popular following the link between the Chicxulub crater and the end-Cretaceous mass extinction, discovered in 1980. But ‘educated guesses’ have generated more hot air than clear conclusions. Much as most of us are modelling-averse, a mathematical approach is the only option in the welcome absence of any severe extraterrestrial battering to which scientists have borne witness. With refined algorithms that cover most of the nuances of projectiles and targets – conservation of mass, energy and momentum in the context of the solid Earth behaving as a viscous medium –  Craig O’Neill and colleagues at Macquairie University, Australia, and the Southwest Research Centre in Boulder, CO USA, have simulated possible tectonic outcomes during plausible bombardment scenarios during the Hadean (O’Neil, C. et al. 2017. Impact-driven subduction on the Hadean Earth. Nature Geoscience, v. 10, p. 793-797; DOI: 10.1038/NGEO3029).

It appears that truly gargantuan objects – radius >500 km – are required to stimulate sufficient thermal anomalies that would lead to mantle upwellings whose evolution might lead to subduction at their margins. One at the limit posed by lunar cratering history (~1700 km radius) could have resulted in wholesale subduction of the entire lithosphere present at the time about 4 Ma after the impact. In the Hadean, it is likely that the lithosphere would have had a roughly mantle composition, so that the density excess needed for slab descent would have been merely temperature dependent. Note: after the onset of a basalt-capped lithosphere heat flow would have needed to be below the limit at which basalt converts to eclogite at high pressures, and thus to a density greater than that of the mantle, for continuing subduction. The authors’ Hadean scenario is one of episodic subduction dependent on the projectile flux and magnitude; i.e. with an early Hadean with stop-start subduction waning to tectonic stagnation and then a restart during the Late Heavy Bombardment after 4.1 Ga. Evidence for this is clearly scanty, except for Hadean zircons, whose presence indicates differentiation of early magmas with a peak between 4.0 to 4.2 Ga, in which magnetic intensities are preserved that are roughly as predicted by the scenario.

No impacts preserved in Precambrian to Recent times suggest extraterrestrial objects with the power to induce significant changes to global tectonics.

Recycling of continental crust through time

Because continental crust is so light – an average density of 2700 kg m-3 compared with the mantles’ value of 3300 – it has been widely believed that continents cannot be subducted en masse. Yet it is conceivable that sial can be ‘shaved’ from below during subduction and from above by erosion and added to subductable sediment on the ocean floor. Certainly, there is overwhelming evidence for the net growth of continents through time and plenty for periods of increased and dwindling growth in the past. In some ancient orogens there are substantial slabs of continental composition whose mineralogy bears witness to ultra-high pressure metamorphism at depths greater than that of the base of continents. These slabs had been caught-up in subduction but never reached sufficiently high density to be retained by the mantle; they eventually ‘bobbed up’ again. On the other hand, if early continents were less silica rich through incorporation of substantial proportions of rock with basaltic composition parts of them could founder if subjected to high-pressure, low-temperature metamorphism. But not all crustal recycling to the mantle is through subduction. Some abnormally highly elevated parts of the continents that rose quickly in geological terms, such as the Tibetan Plateau, may have formed by lower crustal slabs becoming detached or delaminated from their base. Again modelling can help assess the past magnitude of continental recycling (Chowdhury, P. et al. 2017. Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth. Nature Geoscience, v. 10, p. 698-703; DOI: 10.1038/NGEO3010).

Various lines of evidence suggest that between 65 to 70% of the present continental volume existed by 3 billion years ago, yet that does not manifest itself in the rock record; perhaps a sign that some has returned to the mantle. It is also widely suggested that plate tectonics in the modern style began at about that time. Pryadarshi Chowdhury and colleagues simulate what may happen at depth in continent-continent collision zones – the classic site of orogenies –at different times in the past. Under the hotter conditions in the early Archaean mantle delamination would have been more likely than it has been during the Phanerozoic; i.e. the peeling off and sinking of the denser, more mafic lower crust and the attached upper mantle. The authors show that increased mantle temperature further back in time increases the likelihood and extent of such delamination. It also encourages partial melting of the descending continental material so creating rising bodies of more silicic magma that add to the remaining continent at the surface. Together with the lower crust’s attachment of to a mantle slab, this ensures that the peeled off material is able to descend under its own load. Once below a depth of 250 km felsic rocks are doomed to further descent. Waning of radiogenic mantle heat production encourages descending slabs to fail and break from the connection with lithosphere at higher levels so that a smaller proportion of the lower crust becomes detached and recycled. This evolution suggests that less and less continental crust is recycled with time. This broadly fits with current geochemical ideas based on the record of radiogenic Nd-, Sr- and Pb-isotopes in rocks ranging from early Archaean to Phanerozoic age.

Large earthquakes and the length of the day

Geoscientists have become used to the idea that long-term global climate shifts are cyclical, as predicted by Milutin Milanković. The periods of shifts in the Earth’s orbital and rotational parameters are of the order of tens to hundreds of thousand years. The gravitational reasons why they occur have been known since the 1920s when Milanković came up with his hypothesis, and they were confirmed fifty years later. But there are plenty of other cycles with shorter periods. The last 115 years of worldwide records for earthquakes with magnitudes greater than 7 whose changing annual frequency shows a clear cyclical period of about 32 years. The records show peaks in 1910, 1943, 1970 and 2011 (see Bendick, R. & Bilham, R. 1917. Do weak global stresses synchronize earthquakes? Geophysical Research Letters, v. 44 online; doi/10.1002/2017GL074934). Unlike Milanković cycles, these oscillations were not predicted, but something synchronous with them must be forcing this behavior: a sort of “cross-talk”. Either global seismicity has a tendency for events to trigger others elsewhere on the Earth or some other process is periodically engaging with major brittle deformation to give it a nudge.

Rebecca Bendick, of the University of Montana, Missoula, and Roger Bilham of the University of Colorado, Boulder used a complex statistical method to check for synchronicity between the seismic cycles and other repetitive phenomena. It turns out that there is a close match with historic data for the length of the day which varies by several milliseconds. At first sight this may seem odd, until one realizes that day length is governed by the Earth’s speed of rotation (about 460 m s-1 at the Equator). The correlation is between increases in both major seismicity and the length of the day; i.e. quakes increase as rotation slows.  Day length can vary by a millisecond over a year or so during el Niño, which involves shifts of vast masses of Pacific Ocean water that affect rotation. But what of larger changes on a three-decade cycle? Seismic events and the forces that they release result from buildup of strain in the lithosphere, so the episodic earthquake maxima require some kind of transfer of momentum within the Earth. It does not need to be large, as the Milanković astronomical forcing of climate demonstrates, just a regular pulse.

One possibility is that, as rotation decelerates, decoupling between the liquid outer core and the solid mantle may change the flow of molten iron-nickel alloy.  That may be sufficient to transmit momentum and thus stress through the plastic mantle to the brittle lithosphere so that areas of high elastic strain are pushed beyond the rocks’ strength so that they fail. There are indeed signs that the geomagnetic field also changes with day length on a decadal basis (Voosen, P. 2017. Sloshing of Earth’s core may spike big quakes. Science, v. 358, p. 575; doi:10.1126/science.358.6363.575). Rotational deceleration began in 2011, and if the last century’s trend holds there may be an extra five large earthquakes next year. Could the deadly 7.3 magnitude earthquake at the Iran-Iraq border on 12 November 2017 be the start? If so, will the 32-year connection improve currently unreliable earthquake forecasting? Probably the best we can expect is increased global readiness. The study has nothing to add as regards which areas are at risk: although there is clustering in time there is none with location, even on the regional scale.

Iranians salvage their furniture and household appliances from damaged buildings in the town of Sarpol-e Zahab in Iran’s western Kermanshah province near the border with Iraq, on November 14, 2017

The winter of dinosaurs’ discontent

Under the auspices of the International Ocean Discovery Program (IODP), during April and May 2016 a large team of scientists and engineers sank a 1.3 km deep drill hole into the offshore, central part of the Chicxulub impact crater, which coincided with the K-Pg mass extinction event. Over the last year work has been underway to analyse the core samples aimed at investigating every aspect of the impact and its effects. Most of the data is yet to emerge, but the team has published the results of advanced modelling of the amount of climate-affecting gases and dusts that may have been ejected (Artemieva, N. et al. 2017. Quantifying the release of climate-active gases by large meteorite imp-acts with a case study of Chicxulub. Geophysical Research Letters, v. 44; DOI: 10.1002/2017GL074879).  . From petroleum exploration in the Gulf of Mexico the impact site is known to have been underlain by about 2.5 to 3.5 km of Mesozoic sediments that include substantial amounts of limestones and evaporitic anhydrite (CaSO4) – thicknesses of each are of the order of a kilometre. The impact would inevitably have yielded huge volumes of carbon- and sulfur dioxide gases, as well as water vapour plus solid and molten ejecta. The first, of course, is a critical greenhouse gas, whereas SO2 would form sulfuric acid aerosols if it entered the stratosphere. They are known to block incoming solar radiation. So both warming and cooling influences would have been initiated by the impact. Dust-sized ejecta that lingered in the atmosphere would also have had climatic cooling effects. The questions that the study aimed to answer concerns the relative masses of each gas that would have reached more than 25 km above the Earth to have long-term, global climatic effects and whether the dominant effect on climate was warming or cooling. Both gases would have added the environmental effects of making seawater more acid.

Chicxulub2

3-D simulation of the Chicxulub crater based on gravity data (credit: Wikipedia)

Such estimates depend on a large number of factors beyond the potential mass of carbonate and sulfate source rocks. For instance: how big the asteroid was; how fast it was travelling and the angle at which it struck the Earth’s surface determine the kinetic energy involved and the impact mechanism. How that energy was distributed between atmosphere, seawater and the sedimentary sequence, together with the pressure-temperature conditions for the dissociation of calcite and anhydrite all need to be accounted for by modelling. Moreover, the computation itself becomes extremely long beyond estimates for the first second or so of the impact. Earlier estimates had been limited by computer speeds to only the first few seconds of the impact and could not allow for other than vertical impacts. The new study, by supercomputers and improved algorithms, used a likely 60° angle of impact, new data on mineral decomposition and simulated the first 15 to 30 seconds. The results suggested that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO2 were ejected, compared with earlier estimates of 40-560 Gt of sulfur and 350-3,500 Gt of CO2.  The greater proportion of sulfur release to the stratosphere pushes the model decisively towards global cooling, probably over a lengthy period – perhaps centuries. Taking dusts into account implies that visible sunlight would also have been blocked, devastating the photosynthetic base of the global food chain, in the sunlit parts of oceans as well as on land.

But we have to remember that these are the results of a theoretical model. In the same manner as this study has thrown earlier modeling into doubt, more data – and there will be a great many from the Chicxulub drill core itself – and more sophisticated computations may change the story significantly. Also, the other candidate for the mass extinction event, the flood basalt volcanism of the Deccan Traps, and its geochemical effects on the climate have yet to be factored in. The next few lines of Shakespeare’s soliloquy for  Richard III may well emerge from future work

… Made glorious summer by this sun of York;
And all the clouds that lour’d upon our house
In the deep bosom of the ocean buried …

See also: BBC News comment on 31 October 201

 

Field studies – real or virtual?

Every evening’s TV schedules include either an ad for some kind of ‘virtual reality’ (VR) device or a ‘techie’ programme in which one appears. As well as massively multiplayer online role-playing games, commercial VR offers 3-D encounters with charging rhinoceroses, surfing, wingsuit flying and other ‘experiences’ that are either life threatening or viciously expensive. Second Life, the online virtual world (but not yet compatible with VR goggles), appeared as long ago as 2003 and at present has about a million regular users and many more have passed through its portal, eventually to tire of its cheesiness. Yet, Second Life no longer seems to be a topic of normal conversation; maybe aficionados don’t go out very often. The development software, the speed and resolution of computers, the peripheral technologies and the visual quality of immersive VR seem to be following something like Moore’s law – the observation that the number of transistors in a dense integrated circuit doubles approximately every two years. And VR gaming is clearly very profitable with revenues likely to rise from about US$17 million in 2014 to over US$ 20 billion by 2020.

Douglas McCauley writes in Science (Insights 20 October 2017) about the potential of digital games and simulation for expanding the reach of STEM education, particularly in his own field ecology (McCauley, D.J. 2017. Digital nature: Are field trips a thing of the past? Science, v.  359, p. 298-300; doi: 10.1126/science.aao1919). His view is partly positive, as they match the thirst for armchair experiences and the growing digital expertise of the billion or more gamers and many more whose culture is dominated by electronic media, skewed strongly to the under-24s. For instance, children in the US spend on average 7 hours per day online, but only 4 to 7 minutes of unstructured outdoor play. There are obvious opportunities to familiarise and enthuse young people with the staggering richness of the natural world, which none of us will ever be able to witness first hand. At a time when the UK National Trust reports, for instance, that only a third of British children can recognise a magpie (a distinctive and common European member of the crow family) whereas 9 out of 10 easily recognise a Dalek alien cyborg, there is clearly a need. Sixty years ago David Attenborough’s early monochrome Zoo Quest series on BBC TV definitely drew me into natural science as it did millions of others, and I for one am deeply grateful for his then somewhat awkward efforts. So it would be stupid to condemn the potential of VR and more plain-vanilla gaming methods as they could do much the same and probably a great deal more. But can it really teach the field skills needed by any potential observational scientist rather than just make people more interested?

McCauley is less certain on that front, and so am I. Studies have shown that virtual field trip participants perform no better than their peers who engaged only in conventional illustrated lectures. ‘Immersive’ experiences can simulate some, but not all aspects of real terrain, ecosystems and geological features.  My own geological ventures have involved a ‘virtual’ aspect provided by remote sensing and image interpretation. Those now pretty aged technologies show ‘the big picture’ – with some zoom-in capacity – and provide insights into regional and, with Google Earth, local geological structures and relationships. By capturing imagery outside the humanly visible wavelength range they add a great deal about rock composition that would otherwise require large sample collections, petrographic interpretation of thin sections and some basic geochemistry. A stereoscopic 3-D view and the use of terrain in creating perspective oblique images also permit estimates of dip and strike of strata. But it is all a bit inhuman and alien, much the same as ‘doing’ geology on Mars without the opportunity to behave as a curious being would if actually on the surface. Any field scientist has real experience imprinted for years in much the same way as would her hunter-gatherer forbears, while it has been shown that virtual experiences may persist for a mere few weeks. My view is that often uncomfortable, total immersion in field reality, literally step-by-step and day after day fosters continual reflection during and for a long time after the experience. Much of science in general is about ‘mulling over’ observations at every level of detail; the more detail and the more repetition the deeper the insight and the more profound the breaks through.

As higher education continues along its path of commodification the more supposedly ‘immersive’ virtual experiences are likely to supplant field work, largely for cost reasons – both for students and institutions. In my former institution, to which I am still tenuously attached, a decision was taken 17 years ago to make residential field studies optional, and in 2011 to abandon them almost entirely in favour of ‘virtual’ experiences of one kind or another. The results have been dramatic: enrollment in geoscientific courses has fallen to a third of the pre-2000 level; retention has declined by up to 10% and pass rates have dropped significantly. The bottom line is that what we used to call Earth sciences has become increasingly marginalised as regards the range of courses on offer.

Plate tectonic graveyard

Where do old plates go to die? For the most part, down subduction zones to mix with their original source, the mantle. Earth-Pages has covered evidence for quite a few of the dead plates, which emerges from a geophysical technique known as seismic tomography – analogous to X-ray or magnetic resonance scans of the whole human body. For 20 years geophysicists have been analysing seismograms from many stations across the globe for every digitally recorded earthquake, i.e. virtually all of those since the 1970s. This form of depth sounding goes far beyond early deep-Earth seismometry that discovered the inner and outer core, various transition zones in the mantle and measured the average variation with depth of mantle properties. Tomography relies on complex models of the paths taken by seismic body waves and very powerful computing to assess variations in the speed of P- and S-waves as they travelled through the Earth: the more rigid/cool the mantle is the faster waves travel through it and vice versa. The result is images of deep structure in 2-D slices, but the quality of such sections depends, ironically, on plate tectonics. Most earthquakes occur at plate boundaries. Such linearly distributed, one-dimensional sources inevitably leave the bulk of the mantle as a blur. Around 20 different methodologies have been developed by the many teams working on seismic tomography. So sometimes conflicting images of the deep Earth have been produced.

Results of seismic tomography across Central America showing anomalously fast (in blue) P- (top) and S-wave (bottom) speeds in map view at a fixed mantle depth (1290 km, left) and as vertical sections (right). The blue zones at right are interpreted to show a steeply dipping slab that represents subduction of the eastern Pacific Cocos plate since about 175 Ma ago (credit: van der Meer, D.G et al. ‘Atlas of the Underworld)

The technique has come of age now that superfast computing and use of multiple models have begun to resolve some of tomography’s early problems. The latest outcome is astonishing: ‘The Atlas of the Underworld’ catalogues 94 2-D sections from surface to the core-mantle boundary each of which spans 40° or arc – about a ninth of the Earth’s circumference (see: van der Meer, D.G., van Hinsbergen, D.J.J., and Spakman, W., 2017, Atlas of the Underworld: slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity, Tectonophysics online; doi.org/10.1016/j.tecto.2017.10.004). Specifically, the Atlas locates remnants of relatively cold slabs in the mantle that are suspected to be remnants of former subduction zones, or those that connect to active subduction. The upper parts of active slabs are revealed by the earthquakes generate along them. At deeper levels they are too ductile to have seismicity, so what form they take has long been a mystery. Once subduction stops, so do the telltale earthquakes and the slabs ‘disappear’.

The slabs covered by the ‘Atlas’ only go back as far as the end of the Permian, when the current round of plate tectonics began as Pangaea started to break-up. It takes 250 Ma for slabs to reach the base of the mantle and beyond that time they will have heated up and begun to be mixed into the lower mantle and invisible. Nevertheless, the rich resource allows models of vanished Mesozoic to Recent plates and the tectonics in which they participated, based on geological information, to be evaluated and enriched. Just as important, the project opens up the possibility of finding out how the mantle ‘worked’ since Pangaea broke up, in 3-D; a key to more than plate tectonics, including the mantle’s chemical heterogeneity. Already it has been used to estimate changes in the total length of subduction zones since 250 Ma ago, and thus arc volcanism and CO2 emissions, which correlates with estimates of past atmospheric CO2 levels, climate and even sea levels.

See also:  Voosen, P. 2016. ‘Atlas of the Underworld’ reveals oceans and mountains lost to Earth’s history. Science; doi:10.1126/science.aal0411.

Lee, H. 2017. The Earth’s interior is teeming with dead plates. Ars Technica UK, 18 October 2017.

Human genetic archaeology round-up

English: Vindija cave near Varazdin in Croatia...

Vindija cave near Varazdin in Croatia. (credit: Wikipedia)

Work on Neanderthal genomes continues, as expected. The latest news comes from remains of a Croatian female, whose genome has been determined by a team led by members of the Max Planck Institute for Evolutionary Anthropology in Leipzig (Prüfer, K. and 35 others 2017. A high-coverage Neandertal genome from Vindija Cave in Croatia. Science, online; doi:10.1126/science.aao1887). Her full genome is of higher quality than those previously published for Neanderthals. Dated at around 52 ka, her genetics is likely to be closer to those who mated with the ancestors of modern Eurasians. Kay Prüfer and his colleagues suggest that Neanderthals passed on to modern Eurasians genes associated with plasma levels of LDL cholesterol and vitamin D (mainly produced by skin exposure to sunlight, Vitamin D is essential for healthy bones and supports the immune system), together with risk factors for eating disorders, accumulation of visceral fat, rheumatoid arthritis and schizophrenia. Two other interesting possibilities stem from reconsidering genetic data from other Neanderthals, in the light of the new Croatian analysis. Ancestors of an older Neanderthal (122 ka) from the Altai region of Siberia had interbred with genetically modern humans as long ago as 130 ka. Yet the genomes of the Altai and Croatian Neanderthals are surprisingly similar, suggesting that both lived in isolated small groups around three thousand strong (Gibbons, A. 2017. Neanderthal genome reveals greater legacy in the living. Science, v. 358 p. 21).

Also published on-line by Science is a study of the genomes of 7 individual anatomically modern humans from KwaZulu-Natal in South Africa (Schlebusch, C.M. and 11 others 2017. Southern African ancient genomes estimate modern human divergence to 350,000 to 260,000 years ago. Science online; doi: 10.1126/science.aao6266). They are three 2000 year-old hunter-gatherers (probably San) and four Iron Age farmers, similar to modern Bantu speakers, from 300 to 500 years ago. Although genomes from modern Khoe-San suggest up to 30% admixture from East Africans and Eurasians, comparison between the two ancient groups suggest a very old divergence among African anatomically modern human (AMH) populations, of the order of 350 to 260 ka ago. This is long before the remains from Ethiopia, widely accepted as the oldest known AMH (190 ka), but roughly the same as recently described fossils from Jebel Irhoud in Morocco (325 to 286 ka) reckoned to be early AMH. The Moroccan humans and now the genetic analysis from much more recent South African skeletons point to a pan-African early evolution of modern people rather than some kind of ‘cradle of humanity’.

English: Man in an Upper Paleolithic burial in...

Adult male in an Upper Paleolithic (28-30 ka) burial at Sunghir, Russia. (credit: Wikipedia)

More ancient AMH hunter-gatherers (~28 to 30 ka) occur at Sunghir, about 200 km east of Moscow, a settlement that includes several burials – one of which contained a boy and a girl – many with abundant, ornate grave goods. It seems likely that all the interred individuals were related and so an excellent target for DNA analysis. Four individuals with roughly the same 14C age did yield enough for genome sequencing (Sikora, M. and 26 others 2017. Science, online; doi: 10.1126/science.aao1807). It turned out from their mtDNA that none were more closely related than first-cousins or great-grandchildren. The data suggested a relatively small breeding population (~300) that avoided inbreeding and its often negative consequences, possibly through exogamy (a wide mating network) as practiced by living hunter-gatherers. Remarkably, the data also hinted at relationship with earlier (36 ka) individuals from Kostenki about 300 km to the south-west. DNA from several Siberian Neanderthal individuals do suggest that inbreeding had been an issue. Had it been widespread among Neanderthals – risky to infer from such scanty information – that may account for their lack of competitiveness with AMH and eventual demise.

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)