Earth-logs

Focus on glaciation…and avoid physics envy

Published on February 14, 2016 by zooks7772 Comments

About 1.3 billion years ago two small black holes, each weighing in at about 30 solar masses, ran into one another and fused. At that time Earthly life forms had neither mouths nor anuses, nor even a nervous system, and they were not much bigger than a sand grain. The distant collision involved rapid acceleration of considerable masses. A century ago Albert Einstein predicted that the movement of any matter in the universe should perturb space-time in a wave-like form that travels at the same speed as light. Well, he was right for, at 9:50:45 universal time on 14 September 2015, four exquisitely engineered mirrors deployed in the two set-ups of a Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington states in the US minutely shuddered, first in the Deep South and 0.007 seconds later in the Pacific Northwest. The signal lasted 0.25 seconds and, when rendered as sound, comprised a sort of chirrup starting at 35 Hz and rising to 250 Hz before an abrupt end. Five months later, and silent internationally shared theoretical verification, the story was released to the back slapping, stamping and pawing the air that we have come to expect from clever, ambitious and persuasive people who have spent a great deal of our money and have something to show for it. So now we know that the universe is probably throbbing – albeit very, very, very quietly – with gravitational waves generated by every single motion that has taken place in the whole of ‘recorded’ history since the Big Bang. Indeed, it is claimed, LIGO-like machines may one day detect the big wave itself if, that is, it hasn’t already passed through the solar system. Recall, 13.7 billion years ago the Big Bang didn’t take much longer than this comparatively mundane collision at 1.3 Ga . Physicists are going to have a lot to ponder on now they have a lever to get yet greater funds. To put all this in perspective, the detected chirrup had been traveling for 1.3 Ga, and so too must the actual place in the universe where it took place: I guess we will never know where it is now or what damage or otherwise may have been visited upon planetary systems in its vicinity, if indeed it had even the slightest recognisable geological or ecological consequence.

So, onto the mundane world of glaciology and climate change.

Tibet is the third greatest repository of glacial ice on the surface of the Earth’s continents. It is the focus of one of the planet’s greatest climatic system, the South Asian. While much of the Plateau hasn’t borne glaciers continuously throughout even the last glacial cycle, it is becoming clear that its western margin has remained cold enough to retain ice throughout an even longer period. In the Kunlun mountains is a 200 km² ice cap known as the Guliya. At the start of detailed glacial stratigraphic ventures in 1990s, focused mainly on Greenland and Antarctica, analysis of a core from the Guliya ice cap yielded dates extending back to 130 ka, before the start if the last interglacial. This section lies above ice that at the time could not be dated reliably other than to show that it may be older than about 750 ka. This stemmed from its lack of the radioactive ³⁶Cl formed, similarly to ¹⁴C, by cosmic-ray interactions with stable ³⁵Cl in atmospheric salt aerosols: such cosmogenic chlorine can be used for radiometric dating of ice younger than 750 ka.

A News Feature in the 29 January issue of Science (Qiu, J. 2016. Tibet’s primeval ice. Science, v. 351, p. 436-439) focused on the preliminary results of an expedition, led by Yao Tandong of the Institute of Tibetan Plateau Research, Beijing and Lonnie Thompson of Ohio State University, Columbus, to drill a further five ice cores at Guliya in September 2015, one of which penetrated ove 300 m of glacial ice. It is now possible to date ice layers back to a million years using argon isotopes. Combined with stable isotope and other measurements through the cores, the dating should provide a huge amount of new information on the evolution of the monsoon, which is currently understood only vaguely. Such information would sharpen models of how the monsoon system works and even hint at how it might change during a period of anthropogenic warming. An estimated 1.4 billion people – a fifth of humanity – who live in the Indian subcontinent, China and SE Asia depend for their food-production on the monsoon.

With less humanitarian urgency but equally fascinating is the discovery that, as well as sea-ice, the central Arctic Ocean once hosted vast ice shelves during the last-but-one glacial episode (Jakobsson, M. and 24 others 2016. Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciations. Nature Communications, v. 7, doi:10.1038/ncomms10365. Clues emerged from multibeam sonar bathymetry that created detailed images of topography on the floor of the Arctic Ocean. These revealed sets of parallel ridges on the shallowest parts of the polar basin, thought to have formed when moving ice shelves grounded. The depths of the grooved areas indicate ice thicknesses up to and exceeding 1 km. The grooves look very similar to the large-scale lineaments that formed on the surface of the Canadian Shield when the Laurentide ice sheet ground its way from zones of glacial accumulation. Grounding of an ice shelf would have resulted in its thickening in the upflow direction as a result of plastic deformation of the ice, tending to lock the flow and direct ice escape over the deeper parts of the Arctic basin.

Back-tracking the grooves defines the ice shelf’s source regions in the northern Canadian islands, north Scandinavia and the lowlands of eastern Siberia as well as regional flow patterns and the extent of floating continental ice. The last is a major surprise: at over 4 million km² it was four times larger than all modern Antarctic ice shelves. The ice moved to ‘escape’ to the North Atlantic Ocean through the Fram Strait between East Greenland and Svalbard (Spitzbergen). Dating sediment stratigraphy in the grooved areas using magnetic and fossil data shows that the ice shelves existed between 160 and 140 ka during the penultimate glacial maximum. For such a mass of glacial ice to be expelled into the Arctic Ocean implies that a great deal more snow fell on its fringes then than during the last glacial maximum. Another possibility is that the huge mass of floating ice regulated the salinity and density of the upper Atlantic in a different way from the periodic iceberg ‘armadas’ that characterized the last glacial epoch and help account for a whole number of sudden warming and cooling events.

Domack, E. 2016. A great Arctic ice shelf. Nature, v. 530, p. 163-164.

Mammoth carcass suggests humans inhabited Arctic 10,000 years earlier than thought

So, when did plate tectonics start up?

Published on February 4, 2016 by zooks777Leave a comment

Tiny, 4.4 billion year old zircon grains extracted from much younger sandstones in Western Australia are the oldest known relics of the Earth system. But they don’t say much about early tectonic processes. For that, substantial exposures of rock are needed, of which the undisputedly oldest are the Acasta gneisses 300 km north of Yellowknife in Canada’s North West Territories, which have an age of slightly more than 4 Ga. The ‘world’s oldest rock’ has been something of a grail for geologists and isotope geochemists who have combed the ancient Archaean cratons for 5 decades. But since the discovery of metasediments with an age of 3.8 Ga in West Greenland during the 1970s they haven’t made much headway into the huge time gap between Earth’s accretion at 4.54 Ga and the oldest known rocks (the Hadean Eon).

There have been more vibrant research themes about the Archaean Earth system, specifically the issue of when our planet settled into its modern plate tectonic phase A sprinkling of work on reconstructing the deep structural framework of Archaean relics has convinced some that opposed motion of rigid, brittle plates was responsible for their geological architecture, whereas others have claimed signs of a more plastic and chaotic kind of deformation of the outer Earth. More effort has been devoted to using the geochemistry of all the dominant rocks found in the ancient cratons, seeking similarities with and differences from those of more recent vintage. There can be little doubt that the earliest processes did form crust whose density prevented or delayed it from being absorbed into the mantle. Even the 4.4 Ga zircons probably crystallized from magma that was felsic in composition. Once trapped by buoyancy at the surface and subsequently wrapped around by similarly low density materials continental crust formed as a more or less permanent rider on the Earth’s deeper dynamics. But did it all form by the same kinds of process that we know to be operating today?

Plate tectonics involves the perpetual creation of rigid slabs of basalt-capped oceanic lithosphere at oceanic rift systems and their motions and interactions, including those with continental crust. Ocean floor cools as it ages and becomes hydrated by seawater that enters it. The bulk of it is destined eventually to oppose, head-to-head, the motions of other such plates and to deform in some way. The main driving force for global tectonics begins when an old, cold plate does deform, breaks, bends and drives downwards. Increasing pressure on its cold, wet basaltic top transforms it into a denser form: from a wet basaltic mineralogy (feldspar+pyroxene+amphibole) to one consisting of anhydrous pyroxene and garnet (eclogite) from which watery fluid is expelled upwards. Eclogite’s density exceeds that of mantle peridotite and compels the whole slab of oceanic lithosphere to sink or subduct into the mantle, dragging the younger parts with it. This gravity-induced ‘slab pull’ sustains the sum total of all tectonic motion. The water rising from it induces the wedge of upper mantle above to melt partially, the resulting magma evolves to produce new felsic crust in island arcs whose destiny is to be plastered on to and enlarge older continental masses.

Relics of eclogites and other high-pressure, low-temperature versions of hydrated basalts incorporated into continents bear direct and unchallengeable witness to plate tectonics having operated back to about 800 Ma ago. Before that, evidence for plate tectonics is circumstantial and in need of special pleading. Adversarial to-ing and fro-ing seems to be perpetual, between geoscientists who see no reason to doubt that Earth has always behaved in this general fashion and others who see room for very different scenarios in the distant past. The non-Huttonian tendency suggests an early, more ductile phase when greater radioactive heat production in the mantle produced oceanic crust so fast that when it interacted with other slabs it was hot enough to resist metamorphic densification wherever it was forced down. Faster production of magma by the mantle without slab-pull could have produced a variety of ‘recycling’ turnover mechanisms that were not plate-tectonic.

One thing that geochemists have discovered is that the composition of Archaean continental crust is very different from that produced in later times. In 1985 Ross Taylor and Scott McLennan, then of the Australian National University, hit on the idea of using shales of different ages as proxies for the preceding continental crust from which they had been derived by long erosion. Archaean and younger shales differed in such a way that suggests that after 2.5 Ga (the end of the Archaean) vast amounts of feldspar were extracted from the continent-forming magmas. This left the later Precambrian and Phanerozoic upper crust depleted in the rare-earth element europium, which ended up in a mafic, feldspar-rich lower crust. On the other hand, no such mass fractionation had left such a signature before 2.5 Ga. Another ANU geochemist, now at the University of Maryland, Roberta Rudnick has subsequently carried this approach further, culminating in a recent paper (Tang, M., Chen, K and Rudnick, R.L. 2016. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. Science, v. 351, p. 372-375). This uses nickel, chromium and zinc concentrations in ancient igneous and sedimentary rocks to track the contribution of magnesium (the ‘ma’ in ‘mafic’) to the early continents. The authors found that between 3.0 to 2.5 Ga continental additions shifted from a dominant more mafic composition to one similar to that of later times by the end of the Archaean. Moreover, this accompanied a fivefold increase in the pace of continental growth. Such a spurt has long been suspected and widely suggested to mark to start of true plate tectonics: but an hypothesis bereft of evidence.

A better clue, in my opinion, came 30 years ago from a study of the geochemistry of actual crustal rocks that formed before and after 2.5 Ga (Martin, H. 1986. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology, v. 14, p. 753-756). Martin showed that plutonic Archaean and post-Archaean felsic rocks of the continental crust lie in distinctly different fields on plots of their rare-earth element (REE) abundances. Archaean felsic plutonic rocks show a distinct trend of enrichment in light REE relative to heavy REE as measures of the degree of partial melting decreases, whereas the younger crustal rocks show almost constant, low values of heavy REE/light REE whatever the degree of melting. The conclusion he reached was that while in the post Archaean the source was consistent with modern subduction processes – i.e. partial melting of hydrated peridotite in the mantle wedge above subduction zones – but during the Archaean the source was hydrated, garnet-bearing amphibolite of basaltic composition, in the descending slab of subducted oceanic crust. Together with Taylor and McLennan’s lack of evidence for any fractional crystallization in Archaean continental growth, in contrast to that implicated in Post-Archaean times.

The geochemistry forces geologists to accept that a fundamental change took place in the generation and speed of continental growth at the end of the Archaean, marking a shift from a dominance of melting of oceanic, mafic crust to one where the upper mantle was the main source of felsic, low-density magmas. Yet, no matter how much we might speculate on indirect evidence, whether or not subduction, slab-pull and therefore plate tectonics dominated the Archaean remains an open question.

More on continental growth and plate tectonics

Earthquakes in Nepal

Published on January 19, 2016 by zooks777Leave a comment

The magnitude 7.8 Gorkha earthquake hit much of the Himalayan state of Nepal on 25 April 2015, to be followed by one of magnitude 7.3 150 km to the east 18 days later. As would have happened in any high-relief area both events triggered a huge number of landslides as well as toppling buildings, killing almost 9000 people and leaving 22 000 injured in the capital Kathmandu and about 30 rural administrative districts. Relief and reconstruction remain hindered 9 months on in many of the smaller villages because they are accessible only by footpaths. Nepal had remained free of devastating earthquakes for almost 6 centuries, highlighting the perils of long quiescence in active plate-boundary areas.

Damage in Kathmandu, Nepal, after the Gorkha earthquake in May 2015 (Credit: CNN)

The International Charter: Space and Major Disasters consortium of many national space agencies was activated, resulting in one of the largest ever volumes of satellite images ranging from 30 to 1 m resolution to be captured and made freely available for relief direction, analysis and documentation. This allowed more than 7500 volunteers to engage in ‘crowd mapping’ coordinated by the Humanitarian OpenStreetMap Team (HOT) to provide logistic support to the Nepal government, UN Agencies and other international organizations who were swiftly responding with humanitarian relief. Most important was the location of damaged areas using ‘before-after’ analysis and assessing possible routes to remote areas. The US NASA and British Geological Survey with Durham University coordinated a multinational effort by geoscientists to document the geological, geophysical and geomorphological factors behind the mass movement of debris in landslides etc that was triggered by the earthquakes, results from which have just appeared (Kargel, J.S. and 63 others 2016. Geomorphic and geological controls of geohazards induced by Nepal’s 2015 Gorkha earthquake. Science, v. 351, p. 140 – full text purchase).

The large team mapped 4312 new landslides and inspected almost 500 glacial lakes for damage, only 9 had visible damage but none of them showing signs of outbursts. As any civil engineer might have predicted, landslides were concentrated in areas with slopes exceeding 30° coincided with high ground acceleration due to the shaking effect of earthquakes. Ground acceleration can only be assessed from the actual seismogram records of the earthquakes, though slope angle is easily mapped using existing digital elevation data (e.g. SRTM). It should be possible to model landslide susceptibility to some extent over large areas by simulation of ground shaking based on various combinations of seismic magnitude and epicenter depth modulated by maps of bedrock and colluvium on valley sides as well as from after-the-event surveys. The main control over distribution of landslides seems to have been the actual fault mechanism involved in the earthquake, assessed from satellite radar interferometry, with the greatest number and density being on the downthrow side (up to 0.82 m surface drop): the uplifted area (up to 1.13 m) had barely any debris movements. Damage lies above deep zones where brittle deformation probably takes place leading to sudden discrete faults, but is less widespread above deep zones of plastic deformation.

The geoscientific information gleaned from the Gorkha earthquake’s effects will no doubt help in assessing risky areas elsewhere in the Himalayan region. Yet so too will steady lithological and structural mapping of this still poorly understood and largely remote area. As regards the number of lives saved, one has to bear in mind that few people buried by landslides and collapsed buildings survive longer than a few days. It seems that rapid response by geospatial data analysts to the logistics of relief and escape has more chance of positive humanitarian outcomes.

In the same issue of Science appears another article on Nepalese seismicity, but events of the 12^th to 14^th centuries CE (Schwanghart, W. and 10 others 2016. Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya. Science, v. 351, p. 147-150). As the title suggests, this relates to recent geology beneath a valley floor in which Nepal’s second city Pokhara is located. It lies immediately to the south of the 8000 m Annapurna massif, about 50 km west of the Gorkha epicentre. Sections through the upper valley sediments reveal successive debris accumulations on scales that dwarf those moved in the 2015 landslides. Dating (¹⁴C) of interlayered organic materials match three recorded earthquakes in 1100, 1255 and 1344 CE, each estimated to have been of magnitude 8 or above. The debris is dominated by carbonate rocks that probably came from the Annapurna massif some 60 km distant. They contain evidence of extreme pulverisation and occur in a series of interbeds some fine others dominated by clasts. The likelihood is that these are evidence of mass movement of a more extreme category than landslides and rockfalls: catastrophic debris flows or rock-ice avalanches involving, in total, 4 to 5 km³ of material.

Carbon emissions: It’s an ill wind…

Published on January 15, 2016 by zooks7772 Comments

The original saying emerged in Shakespeare’s Henry IV Part 2 (Act 5, Scene 3) during a jocular exchange when Ancient Pistol brings news from Court to Sir John Falstaff and other old codgers at dinner in Gloucestershire. Falstaff: ‘What wind blew you hither, Pistol?’ Pistol: ‘Not the ill wind which blows no man to good’. In the present context it seems anthropogenic CO₂ emissions have staved off the otherwise inevitable launch of another glacial epoch. Climate-change deniers will no doubt pounce on this in the manner of a leopard seizing a tasty young monkey.

Climatologists at the Institute for Climate Impact Research in Potsdam, Germany, Potsdam University and the Santa Fe Institute in New Mexico, USA set out to develop a means for predicting the onset of ice ages (Ganopolski, A. et al. 2016. Critical insolation-CO₂ relation for diagnosing past and future glacial inception. Nature, v. 529, p. 200-203) Many researchers have concluded from the oxygen isotope data in marine sediments, which tracks changes in the volume of glacial ice on land, that the end of previous interglacial periods by inception of prolonged climatic cooling may be attributed to reduction of solar heating in summer at high northern latitudes. This conclusion stems from Milankovic’s predictions from the Earth’s astronomically controlled orbital parameters and fits most of previous interglacial to glacial transitions. But summer insolation at 65°N is now more or less at one of these minima, with no signs of drastic global cooling; rather the opposite, as part of 7 thousand years of constant global sea level during the Holocene interglacial.

The latest supercomputer model of the Earth System (CLIMBER-2) has successfully ‘predicted’ the last eight ice ages from astronomical and other data derived from a variety of climate proxies. It also forecasts the next to have already begun, if atmospheric CO₂ concentration was 240 parts per million; the level during earlier interglacials most similar to that in which we live. But the pre-industrial level was 280 ppm and the model suggests that would have put off the return of huge ice caps in the Northern Hemisphere for another 50 thousand years – partly because the present insolation minimum is not deep enough to launch a new ice age with that CO₂ concentration – making the Holocene likely to be by far the longest interglacial since ice-age cycles began about 2.5 Ma ago. Based on current, industrially contaminated CO₂ levels and a rapid curtailment of carbon emissions the model suggests no return to full glacial conditions within the next 100 ka and possibly longer; a consequence of the sluggishness of natural processes that draw-down CO₂ from the atmosphere.

English: Ice age Earth at glacial maximum. Bas... — Simulation of the Earth at a glacial maximum. (Photo credit: Wikipedia)

So, does this indicate that unwittingly the Industrial Revolution and subsequent growth in the use of fossil fuels tipped the balance away from global cooling that would eventually have made vast tracts of both hemispheres uninhabitable? At first sight, that’s the way it looks. But the atmospheric carbon content of the 17^th century would have resulted in much the same long drawn out Holocene interglacial; an unprecedented skipping of an ice age in the period covering most of the history of human evolution. This raises a question first posed by Bill Ruddiman in 2003: did human agriculture and associated CO₂ emission begin the destabilisation of the Earth system shortly after Holocene warming and human ingenuity made farming and herding possible since about 10 thousand years ago?

But, consider this, the CLIMBER-2 Earth System model is said to be one of ‘intermediate complexity’ which is shorthand for one that relies on the ages-old scientific method of reductionism or basing each modelled scenario on modifying one parameter at a time. Moreover, for many parameters of the Earth’s climate system – clouds, dust, the cooling effect of increased winter precipitation as snow, and much else – scientists are pretty much in the dark (Crucifix, M. 2016. Earth’s narrow escape from a big freeze. Nature, v. 529, p. 162-163). Indeed it is still not certain whether CO₂levels have a naturally active or passive role in glacial-interglacial cycles, or something more complex than the simple cause-effect paradigm that still dominates much of science.

Further pounding for ideas on the Ediacaran fauna

Published on January 8, 2016 by zooks7773 Comments

About 635 Ma ago fossils of large-bodied organisms first appeared in the geological record: some quilt like, others with a crude bilateral symmetry, more looking like ‘mud-filled bags’ and ribbed discs but none that can easily be distinguished as animals, plants or colonial microorganisms. First found abundantly in the Ediacara Hills of South Australia, hence their sack-name the Ediacaran biota, it now seems that they were distributed globally in the late Neoproterozoic Era. Interpreting their metabolism is risky enough – some are reckoned to be animals that absorbed nutrients through their skin, others said to be dependent on photosynthesis – but a controversy has raged for many years over the kind of environment in which they thrived. In a detailed 2012 study of sedimentary structures petrography in the South Australian sandstones from which they were first described, Gregory Retallack of the University of Oregon inferred that some lived on land and are now found in palaeosols: they include Spriggina, Dickinsonia and Charnia that are among the most favoured candidates for being animals or some kind. Others inhabited shallow water. Anticipating fiery disputes a Nature editorial appeared in same issue in which Retallack published his paper .

Rich fossil assemblage of the Ediacaran Mistaken Point Formation, Newfoundland. (Credit: Alex Liu, Earth Sciences, University of Bristol)

Retallack has now moved on to the even more fossil-rich Ediacaran sediments of Newfoundland (Retallack, G.J 2016. Ediacaran sedimentology and paleoecology of Newfoundland reconsidered. Sedimentary Geology, v. 333, p. 15-31). Eye-wateringly detailed sequence stratigraphy of the now famous Mistaken Point locality and others suggests that the ecosystem there was an intertidal salt marsh. In detail it contains evidence for shallow-water graded bedding, signs of regular storms and perhaps tsunamis together with interbedded palaeosols and subaerial volcanic crystal tuffs whose feldspars survive intact. The palaeosols can be subdivided into several pedogenic types akin to those used to classify modern soils. Unlike the arid setting of the South Australian Ediacaran sediments, whose palaeosols show signs of freezing, the Newfoundland package indicates humid, cool-temperature climes

As in Australia, the palaeosols are rich in Ediacaran fossils, including the best known; the leaf-like Charnia and its discoidal support structure that appears in Retallack’s reconstruction of the environment in an analogous way to salt-tolerant shrubs in modern tidal flats. They occur together with encrusting fossils that bear some resemblance to modern foliose fungi or lichens. Further chuntering in the palaeontological community seems inevitable, but the sedimentological observations alone knock one hypothesis on the head: it has been said that the graded bedding common to both major Ediacaran assemblages constitutes evidence for deep marine origins from turbidity currents. But there is further compost in which controversy may thrive, in that Retallack ascribes the repeated palaeosols to glacially controlled sea-level fluctuations: the Newfoundland sequence contains two diamictites interpreted as tillite, one dated at ~583 Ma the other undated but at the top of the sequence.

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

Published on January 2, 2016 by zooks777Leave a comment

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.

Paris Agreement 2015: Carbon Capture and Storage

Published on December 15, 2015 by zooks7771 Comment

Anyone viewing news that covered the adoption of the Paris Agreement on climate change on 11 December 2015 would have seen clear evidence of the reality of the old saw, ‘There was dancing in the streets’. Not since the premature celebration of the landing of the Philae spacecraft on comet 67P/Churyumov–Gerasimenko 11 months before has there been such public abandonment of normal human restraint. In the case of ‘little Philae’ the object of celebration sputtered out three days after landing, albeit with the collection of some data. Paris 2015 is a great deal more important: the very health of our planet and its biosphere hangs on its successful implementation. At 32 pages long, by UN standards the document agreed to by all 196 UN Member States is pretty succinct considering everything it is supposed to convey to its signatories and the human race at large.

The Bagger 288 bucket wheel reclaimer moves from one lignite mine to another in Germany.

One central and, by most scientific criteria, the most important technology needed as a stopgap before the longed-for adoption of carbon-free energy generation does not figure in the diplomatic screed: carbon capture and storage (CCS) is not mentioned once. Indeed, only 10 Member States have included it in their pledge or ‘intended nationally determined contribution’ (INDC) – Bahrain, Canada, China, Egypt, Iran, Malawi, Norway, Saudi Arabia, South Africa and the United Arab Emirates. Only three of them are notable users of coal-fired power stations for which CCS is most urgent. An article in the January 2016 issue of Scientific American offers an explanation of what seems to be a certain diplomatic timidity about this highly publicized stop-gap measure (Biello, D. 2016. The carbon capture fallacy. Scientific American, v. 314(1) 55-61). David Biello emphasizes the urgency of CCS from more industries than fossil fuel power plants, cement manufacture being a an example. He focuses on the economics and logistics of one of very few CCS facilities that may be on track for commissioning (33 have been shut down or cancelled worldwide since 2010).

The Kemper power station in Mississippi, USA is the most advanced in the US, as it has to be to burn the strip-mined, wet, brown coal or lignite that is its sole fuel. The chemistry it deploys is quite simple but technologically complex and expensive. So Kemper survives only because it aims to sell the captured CO₂ to a petroleum company so that it can be pumped into oil fields to increase dwindling production. However, its extraction costs US$1.50 per tonne, while naturally occurring, underground CO₂ costs US$0.50 to pump out. Moreover, Kemper’s power output at US$11 000 per kW of generating capacity is three times more expensive than that for a typical coal-fired boiler. Mississippi Power is lucky, in that it only needs to pipe the gas 100 km to its ‘partner’ oil field; a pretty small one producing about 5 000 barrels per day. Some coal plants are near oil fields, but the majority are not. To cap it all, only about a third of the CO₂production is likely to remain in long-term underground storage.

Because Kemper has, predictably, hit the financial buffers (almost US$4 billion over budget) to avoid bankruptcy it has raised electricity prices to its customers by 18%. Without the projected revenue from its partnered oil field it would go belly up. Even in the happy event of financial break-even, in carbon terms it would be subsidising the oilfield to produce…CO₂! But the sting in the tail of Biello’s account of this ‘flagship’ project is that the plant is currently neither burning coal nor capturing carbon: it uses natural gas…

Global Tectonics Centenary: Any Inspiring Papers?

Published on December 7, 2015 by zooks77714 Comments

Although Alfred Wegener first began to present his ideas on Continental Drift in 1912 his publication in 1915 of The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane) is generally taken as the global launch of his hypothesis. Apart from support from Alexander du Toit and Arthur Holmes, geoluminaries of the day panned it unmercifully because, in the absence of evidence for a driving mechanism, he speculated that his proposed ‘urkontinent’ (primal continent) Pangaea had been split apart by a centrifugal mechanism connected to the precession of Earth’s rotational axis. This ‘polflucht’ (flight from the poles) is in fact far too weak to have any such influence. Wegener’s masterly assembly of geological evidence for former links between the major continents was ignored by the critics, suggesting that their motive for excoriation of his suggested mechanism was as much spite against an ‘outsider’ as a full consideration of his hypothesis. It must have been hurtful in the extreme, yet Wegener defended himself with a series of revised editions that amassed yet more concrete evidence. What is often overlooked, even now that his ideas have become part of the geoscientific canon, is that in his initial Geologische Rundschau paper in 1912 he mused that the floor of the Atlantic is continuously spreading by tearing apart at the mid-Atlantic Ridge where ‘relatively fluid and hot sima’ rises. Strangely, he dropped that idea in later works. Anyhow, neither 2012 nor 2015 was celebrated in the manner of the centenary-and-a-half of Darwin’s On the Origin of Species: 2009 was marked by palaeobiologists and geneticists metaphorically dancing in the streets, if not foaming at the mouth. There have been a few paragraphs, and some minor symposia about Wegener’s dragging geology out of the 18^th century and into the 20^th, but that’s about it. The best centenary item I have seen is by Marco Romano and Richard Cifelli (Romano, M. & Cifelli, R.L. 2015. 100 years of continental drift. Science, v. 350, p. 916-916).

In the shape of plate tectonics the Earth sciences hosted what was truly a revolution in science, albeit 50 years on from its discoverer’s announcement. It was through the persistent agitation by his tiny band of supporters, that the upheaval was unleashed when the revelations from palaeomagnetism, seismology and many other lines of evidence were resolved as plate tectonics by the discovery of ocean-floor magnetic stripes by Vine, Matthews and Morley in 1963. Despite an explosion of papers that followed, elaborating onthe new theory and showing examples of its influence on ‘big’ geology , counter-revolutionary resistance lasted almost to the first years of the new century. By then so much evidence had emerged from every geological Eon that opponents looked truly stupid. Even so, the skepticism among those sub-disciplines that were ‘left out’ of geodynamic thought continued to blurt out with the emergence of other exciting aspects of the Earth’s history. I remember that, when three of us in the Open University’s Department of Earth Sciences proposed in 1994 that the influence of impacts by extraterrestrial objects ought to figure in a new course on the evolution of Earth and Life we were sneered at as ‘whizz-bang kids’ by those more earth-bound. Trying belatedly in 1996 to introduce students to another revolutionary development – the use of sedimentary and glacial oxygen isotopes in unraveling past climate change – became a huge struggle in the OU’s Faculty of Science. It went to the press eventually and for 2 years our students had the benefit. But the murmuration of dissent ended with a force-majeur re-edit of the course, by someone who had played no role in its development, expunged the lot and changed the ‘offending’ section back to the way it had been a decade before. As they say: ho hum!

Oddly, in the last 15 years or so of trying to follow in Earth-Pages what I considered to be the most exciting developments in the geosciences, it has become increasing difficult to find papers in the top journals that are truly ground-breaking. Of course that may just be ageing and a certain cynicism that often companies it. From being spoiled for choice week after week it has become increasingly difficult from month to month to maintain the standards that I have set for new work. Has Earth science entered the fifth phase of a ‘paradigm shift’ predicted by philosopher Thomas Kuhn in his 1962 book The Structure of Scientific Revolutions? According to him once a science has entered a period when there is little consensus on the theories that might lie at the root of natural processes there is a drift in opinion to a few conceptual frameworks that seem to work, albeit leaving a lot to be desired. Weaknesses at the frontier between theory and empirical knowledge become increasingly burdensome as a result of the steady plod of ‘normal science’ until the science in question reaches a crisis. If existing paradigms fail repeatedly, science is ripe for the metaphorical equivalent of a ‘Big Bang’: maybe an entirely new discovery or hypothesis, or an idea that has been suppressed which new data fits better than any others that have been common currency. Plate Tectonics is the second kind. After the revolution much is reexamined and new lines of work emerge, until in Kuhn’s 5^th phase scientists return to ‘normal science’. That looks like a pretty good story, on paper, but other forces are at work in science; external to scientific objectives. Most of these are a blend of economics, political ideologies and managerial ‘practicalities’. If the Earth sciences have entered the doldrums of novelty, I suspect it is these forces that are bearing some kind of glum fruit.

The old concept of academic freedom has gone by the board. Institutions demand that research is externally funded – the more the better as the institution, at least in the UK, demands a kind of tax (40% of that proposed) supposedly to cover corporate overheads including salaries of support staff. If an academic doesn’t pull in the dosh, she is not much favoured. If the individual doesn’t publish regularly either, there is a weasel sanction: Josephine Soap is declared ‘research inactive’. Consortia of researchers are more and more in vogue: managers and funders like ‘team players’, so individuals who are bright and confident enough to ‘stick their necks out’ cannot do that in a consortium publication and as often as not are ‘left on the bench’. Risk taking is more dangerous now and to stay ‘research active’, and in many cases of non-tenured posts getting a salary, an individual, even a few like-minded colleagues have to publish 2 or 3 papers a year.

It’s worth mentioning that open access publishing is not just all the rage, it has become more or less compulsory. Of course, it has some benefits for scientists in less well-heeled countries, but there is a downside. You have to raise the cash demanded by journals for the privilege or potentially universal access – at least US$1000 a pop, depending on a journals Impact Factor, and that of course is an odiously essential corporate consideration – and having done that woe betide those who do not publish and spend it. Academic publishing is the most profitable sector of the trade, the more so as print is supplanted by electronic delivery – the 50 free reprints is a thing of the past. So there are more and more journals and each of them strives to get out more issues per year, and of course those have to be filled. To me, this all adds up to more and more ‘pot-boiler’ articles and a tendency to maximise the flesh rendered from the body of research work and into the pot. Taken together with the stresses of commodification in higher education and the now vertical corporate structures from which it is constituted, it shouldn’t be a surprise that excitement and inspiration are at a premium in the weekly and monthly output of such a marginal science as that concerned with how the world works.

Some cunning radiometric dating

Published on November 29, 2015 by zooks7771 Comment

At the end of the 1970’s I was invited by the Deputy Director of the Geological Survey of India (Southern Region) to participate in the Great Postal Symposium on the Cuddapah Basin: a sort of harbinger of the Internet and Skype, but using snail-mail. Feeling pretty honoured and most intrigued I accepted; not that I knew the first thing about the subject. A regular stream of foolscap mimeographed contributions kept me nipping out of my office to check my pigeon hole for about 6 months. I learned a lot, but felt unable to comment. Four years on I was taken across the Cuddapahs by my first research student – a budding moto-cross driver with a morbid fear of bullock carts – en route from the Archaean low-grade greenstone-granite terrains of Karnataka for a peek at the fabled charnockites near Chennai (then Madras). A bit of a round-about route but spurred by my memories of the Great Postal Symposium. Sadly, the detour was marred for me by a severe case of sciatica brought on by manic driving, the state of the trans-Cuddapah highway and a misplaced gamma-globulin shot to ward off several varieties of hepatitis: I mainly blamed the nurse who demanded that I drop my drawers and bravely take the huge needle in a buttock – they do these things more humanely these days. Anyhow, apart from seeing many dusty villages build of slates perfect enough to make a full-size snooker table, my mind was elsewhere and I have long regretted that.

Landsat image mosaic showing part of the Cuddapah Basin.

Hosting possibly the world’s only diamondiferous Precambrian conglomerate, the Cuddapah Basin contains a 5 km thickness of diverse sedimentary strata, but no tangible fossils. It rests unconformably on the Archaean greenstone-granite terrain of the Dharwar Craton and so is Proterozoic in age; an Eon that spans 2 billion years. The middle of the lowest sedimentary formations (the Papaghni and Chitravati Groups) contains volcanic rocks dated at ~1.9 Ga; another group is cut by a ~1.5 Ga granite, and hitherto the youngest dateable event is the emplacement of 1.1 Ga kimberlites that sourced the diamonds in the conglomerate. Until recently the stratigraphy has been known in some detail, but how to partition it in Proterozoic time is barely conceivable with just three dates in the middle parts that span 800 Ma. All that can be said about the base of the Cuddapah sediments is that they are younger than the 3.1 to 2.6 Ga Archaean rocks beneath. Since the uppermost beds are truncated by a huge thrust system that shoved deep crustal granulites over them their minimum age is equally vague.

Structurally, the Basin began to form on a stable continent underpinned by the Dharwar Craton, but when that collided with Enderbyland in Antarctica, as part of the accretion of the Gondwana supercontinent, sedimentation may have been in an entirely different setting. Indeed, some of the sediments have been carried over the undisturbed part of the basin by a major thrust system. To explore both sedimentary and tectonic evolution Australian, Indian and Canadian geoscientists combined to sample and radiometrically date the entire pile (Collins, A.S. and 13 others 2015. Detrital mineral age, radiogenic isotopic stratigraphy and tectonic significance of the Cuddapah Basin, India. Gondwana Research, v. 28, p. 1294-1309). By precisely dating detrital micas and zircons from the sediments the team was able to check the source region of sedimentary grains as well as to establish a maximum age for each major stratigraphic unit. This helped establish a 3-part sedimentary and tectonic history. The earliest sediments came from the cratonic area to the west, but there are signs that collisional orogeny between 1590 and 1659 Ma produced a new sedimentary source in metamorphic rocks forming to the east. A return to westward provenance marked the youngest sedimentary setting. This enabled the team to suggest a dual evolution of the Basin, first as an extensional rift opening at the east of what is now the Dharwar craton followed by collisional orogeny that transformed the setting to that of a foreland basin, analogous to the Molasse basin in front of the Alps during Cenozoic times, ending with tectonic inversion when extension changed to compression and thrusting.

But to what extent did the work improve the age subdivision of the Cuddapah Basin? Apparently very little, which may be down to a problem with dating detrital minerals. If magmatic and metamorphic evolution was continuous in the areas from which sediments moved, then the youngest grain is a good guide to the maximum age of the sediment being analysed. The more strata are analysed in this way the better the detail of sedimentary timing. But two tectonic terrains are unlikely to produce zircons time and time again during a period approaching a billion years. The data indicate only 3 or 4 episodes of ‘zirconogenesis’ in the sedimentary hinterlands, between about 900 to 1940 Ma. Apart from helping correlate sedimentary formations that were previously deemed stratigraphically different – which did help in tectonically unravelling this complex major feature – several hundred isotopic analyses of zircons and micas have give much the same timing as was known already in more precise terms from stratigraphy assisted by a few dozen conventional radiometric dates.

Seismic menace of the Sumatra plate boundary

Published on November 19, 2015 by zooks777Leave a comment

More than a decade after the 26 December 2004 Great Aceh Earthquake and the Indian Ocean tsunamis that devastating experience and four more lesser seismic events (> 7.8 Magnitude) have show a stepwise shift in activity to the SE along the Sumatran plate boundary. It seems that stresses along the huge thrust system associated with subduction of the Indo-Australian Plate that had built up over 200 years of little seismicity are becoming unlocked from sector to sector along the Sumatran coast. Areas further to the SE are therefore at risk from both major earthquakes and tsunamis. A seismic warning system now operates in the Indian Ocean, but the effectiveness of communications to potential victims has been questioned since its installation. However, increasing sophistication of geophysical data and modelling allows likely zones at high risk to be assessed.

Recent Great Earthquakes in different segments of the Sumatra plate margin (credit: Tectonics Observatory, California Institute of Technology http://www.tectonics.caltech.edu/outreach/highlights/sumatra/why.html

One segment is known to have experienced giant earthquakes in 1797 and 1833 but none since then. What is known as the Mentawai seismic gap lies between two other segments in which large earthquakes have occurred in the 21^st century: it is feared that gap will eventually be filled by another devastating event. Geophysicists from the Institut de Physique du Globe de Paris and Nanyang Technological University in Singapore have published a high-resolution seismic reflection survey showing the subduction zone beneath the Mentawai seismic gap (Kuncoro, A.K. et al. 2015. Tsunamigenic potential due to frontal rupturing in the Sumatra locked zone. Earth and Planetary Science Letters, v. 432, p. 311-322). It shows that that the upper part of the zone, the accretionary wedge, is laced with small thrust-bounded ‘pop-ups’. The base of the accretionary wedge shows a series of small seaward thrusts above the subduction surface itself forming ‘piggyback’ or duplex structures.

Seismic reflection profile across part of the Sumatra plate boundary, showing structures produced by past seismicity. (credit: Kuncoro et al. 2015, Figure 3b)

The authors model the mechanisms that probably produced these intricate structures. This shows that the inactive parts of the plate margin have probably locked in stresses equivalent to of the order of 10 m of horizontal displacement formed by the average 5 to 6 cm of annual subduction of the Indo-Australian Plate over the two centuries since the last major earthquakes. Reactivation of the local structures by release of this strain would distribute it by horizontal movements of between 5.5 to 9.2 m and related 2 to 6.6 m vertical displacement in the pop-ups. That may suddenly push up the seafloor substantially during a major earthquake, thereby producing tsunamis. Whether or not this is a special feature of the Sumatra plate boundary that makes it unusually prone to tsunami production is not certain: such highly resolving seismic profiles need to be conducted over all major subduction zones to resolve that issue. What does emerge from the study is that a repeat of the 2004 Indian Ocean tsunamis is a distinct possibility, sooner rather than later.

Tsunami and Risk: How Much We Know Now

The core’s influence on geology: how does it do it?

Published on November 16, 2015 by zooks777Leave a comment

Although no one can be sure about the details of processes in the Earth’s core what is accepted by all is that changes in core dynamics cause the geomagnetic field to change in strength and polarity, probably through some kind of physical interaction between core and deep mantle at the core-mantle boundary (CMB). Throughout the last 73 Ma and especially during the Cenozoic Era geomagnetism has been more fickle than at any time since a more or less continuous record began to be preserved in the Jurassic to Recent magnetic ‘stripes’ of the world ocean floor. Moreover, they came in bursts: 5 in a million years at around 72 Ma; 10 in 4 Ma centred on 54 Ma; 17 over 3 Ma around 42 Ma; 13 in 3 Ma at ~24 Ma; 51 over a period of 12 Ma centring on 15 Ma. During the Late Jurassic and Early Cretaceous the core was similarly ‘busy’, the two time spans of frequent reversals being preceded by quiet ‘superchrons’ dominated by the same normal polarity as we have today i.e. magnetic north being roughly around the north geographic pole.

The Cenozoic history of magnetic reversals - black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia) — The Cenozoic history of magnetic reversals – black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

Until recently geomagnetic ‘flips’ between the two superchrons were regarded as random , perhaps suggesting chaotic behaviour at the CMB. But such a view depends on the statistical method used. A novel approach to calculating reversal frequency through time, however, shows peak-trough pairs recurring 5 times through the Cenozoic Era, approximately 13 Ma apart: maybe the chaos is illusory (Chane, J. et al. 2015. The 13 million year Cenozoic pulse of the Earth. Earth and Planetary Science Letters, v. 431, p. 256-263). So, here is a kind of yardstick to see if there may be any connection between core processes and those at the surface, which Chen of the Fujian Normal University, Fushou China and Canadian and Chinese colleagues compared with the very detailed Cenozoic oxygen-isotope (δ¹⁸O) record preserved by foraminifera in ocean-floor sediments, which is a well established proxy for changes in climate. Removing the broad trend of cooling through the Cenozoic resulted in a plot of more intricate climatic shifts that matches the geomagnetism record in both shape and timing of peak-trough pairs. It also turns out, or so the authors claim, that both measures correlate with changes in the rate of Cenozoic subduction of oceanic lithosphere (a measure of plate tectonic activity), albeit negative – peaks in magnetism and climate connecting with slowing in the pace of tectonics.

The analyses involved some complicated maths, but taken at face value the correlations beg the questions why and how? Long-term climate change contains an astronomical signal, encapsulated in the Milankovich hypothesis which has been tested again and again with little room for refutation. So is this all to do with gravitational influences in the Solar System. More exotic still is the possibility of 13 Ma cyclicity linking the Milankovich mechanism with the vaster scale of the Sun’s orbit oscillating through the disc of the Milky Way galaxy and theoretical hints of a mysterious role for dark matter in or near the galaxy. Or, is it a relationship in which climate and the magnetic field are modulated by plate tectonics through varying volcanic emissions of greenhouse gases and the deep effect of subduction on processes at the CMB respectively? To me that seems more plausible, but it is still as exceedingly complex as the maths used to reveal the correlations.

Fascinating glacial feature found on Mars

Published on November 10, 2015 by zooks7771 Comment

Many of the vast wastes of northern Canada and Scandinavia that were ground to a paste by ice sheets during the last glacial cycle show peculiar features that buck the general glacial striation of the Shield rocks. They are round-topped ridges that wind apparently aimlessly across the tundra. In what is now a gigantic morass, the ridges form well-drained migration routes for caribou and became favourite hunting spots for the native hunter gatherers: in Canada they are dotted with crude simulations of the human form, or inugoks, that the Innuit erected to corral game to killing grounds. Where eroded they prove to be made of sand and gravel, which has proved an economic resource in some areas lacking in building aggregate, good but small examples being found in the Scottish Midland Valley that have served development of Glasgow and Edinburgh. They were given the Gaelic name eiscir meaning ‘ridge of gravel’ (now esker) from a few examples in Ireland.

Eskers form from glacial meltwater that makes its way from surface chasms known as moulins to the very bottom of an ice sheet where water flows much in the manner of a river, except in tubes rather than channels. Where the ice base is more or less flat the tubes meander as do normal sluggish rivers, and like them the tubes deposit a proportion of the abundant sediment derived by melting glacial ice. Once the ice sheet melts and ablates away, the sediments lose the support of the tube walls and flop down to form the eponymous low ridges: the reverse of the sediment filled channels of streams that have either dried up or migrated. Eskers are one of the features that shout ‘glacial action’ with little room for prevarication.

The classic form of eskers in the Phlegra Montes of Mars. (credit: Figure 6 in Gallagher and Balme, 2015)

Glacial terrains on Mars have been proposed for some odd looking surfaces, but other processes such as debris flows are equally attractive. To the astonishment of many, Martian eskers have now been spotted during systematic interpretation of the monumental archives of high-resolution orbital images of the planetary surface (Gallagher, C. & Balme, M. 2015. Eskers in a complete, wet-based glacial system in the Phlegra Montes region, Mars. Earth and Planetary Science Letters, v. 431, p. 96-109). The discovery is in a suspected glacial terrain that exhibits signs of something viscous having flowed on low ground around higher topographic features, bombardment stratigraphy suggests a remarkable young age for the terrain or about 150 Ma ago: the Amazonian. Ice and its effects are not too strange to suggest for Mars which today is pretty much frigid, except for a few suggestions of active flow of small watery streams. Eskers demand meltwater in abundance, and Gallagher and Balme attribute some of the other features in the Phlegra Montes to wet conditions. However, the eskers are a one-off, so far as they know. Consequently, rather than appealing to some climatic warm up to explain the evidence for wetness, they suggest that the flowing water tubes resulted from melting deep in the ice as a result of locally high heat flow through the Martian crust, which is a lot more plausible.

Surprising modern-human migrations into China and Africa

Published on October 23, 2015 by zooks777Leave a comment

Caves figure highly in discoveries of hominin remains, fossil riches from those near Johannesburg in South Africa and at Atapuerca in northern Spain having set the world of palaeoanthropology reeling in the last few months. As often as not the caves chosen by hominins for day-to-day living, refuge or ritual, places where carnivores dragged some of our early relatives, or into which they fell accidentally, formed in limestones. There are few places so well endowed with karst features than southern China, a fair number of caves in them having rich deposits of bat guano to which farmers have beaten well-trodden paths to dig it out for fertiliser. One such is Fuyan Cave in Daoxian County, Hunan. Manure mining there had done a great deal of the heavy work faced by archaeologists, having stopped when it reached a hard layer of calcite speleothem or flowstone that underpaves more or less the entire cave floor. Initial trial investigations found three clearly human teeth at the surface, encouraging further work. Digging through the flowstone revealed sediments rich in fossils, mainly teeth which preserve better than other remains in humid conditions. As well as teeth from a variety of mammals, large and small, 47 human teeth emerged. Close study revealed dental features that are irrefutably those of anatomically modern humans (Liu, W. and 13 others 2015. The earliest unequivocally modern humans in southern China. Nature, doi:10.1038/nature15696). Remarkably, many of the teeth are in far better condition than my own, and those of many living people with access to dental expertise.

Some of the Daoxian human teeth. (Credit: Song Xing and Xiu-jie Wu of the 1Key Laboratory of Vertebrate Evolution and Human Origins at the Chinese Academy of Sciences)

The true significance of the excavation emerged only when ²³⁰Th dating revealed the age of the flowstone cap to the old cave sediments. A small stalagmite protruding from its surface yielded a minimum age of ~80 ka: by far the oldest date for anatomically modern human remains outside of Africa and the eastern Mediterranean. The dating produced older ages around 120 ka with equally good precision. Before this discovery the date of migration of Africans to populate Eurasia was thought to be about 60 ka from imprecise dating based on genetics of a range of living Eurasians and Africans – a ‘molecular clock’ – and the earliest sign of humans found in Australia. Consequently, finds in South India of artefacts beneath 74 ka ash from the super-eruption of the Mount Toba caldera have been regarded by many, other than the finders, as having been made by Homo erectus. Dates of 100 ka for modern human occupation of the Levant were thought to represent a failed attempt at migration out of Africa by a northern route. Both these important findings now take on renewed significance. Yet a 30 to 40 ka time gap between the Fuyan people and the previous dates for the earliest signs of migration into China, Borneo and Australia (40-50 ka) begs the question, ‘Did this early group of far-travelled migrants survive to become ancestors of modern Chinese people?’ There are many possible scenarios that only future discoveries might validate: simply goiung extinct; failure to survive the encounter with earlier migrants, such as H. erectus or the Denisovans; assimilation into those older populations.

Mitochondrial DNA-based chart of large human m... — Mitochondrial DNA-based chart of large human migrations: the consensus before these new data. Numbers are millennia before present. ( credit: Wikipedia)

As if to counter this, a multinational group of collaborators have sequenced and analysed the genome from a 4500 year-old male skeleton discovered in the Mota Cave of the Gamo highlands of southern Ethiopia (Llorente, M.G. and 18 others 2015. Ancient Ethiopian genome reveals extensive Eurasian admixture throughout the African continent. Science, DOI: 10.1126/science.aad2879). Comparison with what is now a virtual library of living human genomes showed that this man’s genetic make-up most closely matched that of the Ari, a tribe living in the area today. What was most interesting is that part of the modern Ari genome – between 4 to 7% – is not present in the 4500 year-old sequence. Instead, it matches those of modern Sardinians and a prehistoric German farmer. Yet it occurs in people living not only in Ethiopia, but also in central, western and southern Africa to varying degrees. There seems to have been a ‘backflow’ of people into the whole of Africa from Eurasia, estimated to have occurred some 3500-4000 years ago and probably involving a large influx. By that time farming was already established in Africa, so the migrants may have had some advantage, either culturally or physically, to encourage their wide spread through the continent.

In tropical climates, DNA is likely to break down quickly and little if any fossil DNA has been recovered from prehistoric Africans. In this case, burial in a cave at high elevation may have helped preserve it, but also the target for extraction was the petrous bone from the inner ear whose density seems to allow DNA a better change of long-term survival. With continually improving DNA analysis and sequencing techniques more news is surely going to emerge from past African populations.

Deccan Trap sprung by bolide?

Published on October 15, 2015 by zooks777Leave a comment

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

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

The Western Ghat hills at Matheran in Maharash... — Deccan flood basalts forming the Western Ghats in Maharashtra, India (credit: Wikipedia)

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

Renne and colleagues plot the ages against their position in the volcanic stratigraphy of the Deccan Traps in two ways: against the estimated height from base in the pile and against the estimated volume of the erupted materials as it built up – the extent and thickness of successive flows varies quite a lot. The second plot provided a surprise. After the K/Pg event the mean rate of effusion – the limited number of individual flows capped by well-developed soils shows that the build-up was episodic – doubled from 0.4±0.2 to 0.9±0.3 km³yr^-1. Despite the much larger uncertainty in the extent and volume of individual lava Formations than that of their ages, this is clearly significant. Does it imply that the Chicxulub impact somehow affected the magma production from, the mantle plume beneath the Deccan? It had been suggested early in the debate that the antipodean position of the lava field relative to that of Chicxulub may indicate that the huge seismicity from the impact triggered the Deccan magma production. Few accepted that possibility when it first appeared. However, Renne and co. do think it deserves another look, at least at the possibility of some linked effect on the magmatism. Perhaps the magma chamber was somehow enlarged by increased global seismicity; other chambers could have been added; magma might have been ‘pumped’ out more efficiently, or a combination of such effects. The ‘plumbing’ of flood basalt piles is generally hidden, but huge dyke swarms in Precambrian times have been suggested as feeders to long-eroded flood basalts. Seismicity of the scale produced by asteroid impacts can do a lot of damage. The Chicxulub impactor at around 10 km diameter would have carried energy a million times greater than that of the largest thermonuclear bomb, equivalent to an earthquake of Magnitude 12.4 that would have been a thousand times more powerful than the largest recorded earthquake with tectonic causes. Extensional faulting sourced in this fashion in the Deccan area may have increased the pathways along which magma might blurt out.

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

Evidence that Earth’s first mass extinction was caused by critters not catastrophe

Our ancestors parted from other humans earlier than expected

Published on September 30, 2015 by zooks777Leave a comment

Despite the excitement raised by the discovery of remnants of 15 individuals of Homo naledi in a South African Cave the richest trove of hominin fossils remains that of Sima de los Huesos (‘pit of bones’) in northern Spain. In 2013 bone found in that cave from one of 28 or more individuals of what previous had been regarded as H. heidelbergensis, dated at around 400 ka, yielded mitochondrial DNA. It turned out to have affinities with mtDNA of both Neanderthals and Denisovans, especially the second. The data served to further complicate the issue of our origins, but were insufficient to do more than throw some doubt on the significance of H. heidelbergensis as a distinct species: nuclear DNA would do better, it was hoped by the palaeo-geneticists of the Max Planck Institute for Evolutionary Anthropology in Leipzig. Now a small fragment of those data (about 1 tro 2 million base pairs) have been presented to a London meeting of the European Society for the Study of Human Evolution – though not yet in a peer-reviewed journal. Anne Gibbons summarised the formal presentation in the 18 September 2015 issue of Science (Gibbons, Ann 2015. Humanity’s long, lonely road. Science, v. 349, p. 1270).

The partial nuclear DNA is a great deal more like that of Neanderthals from much more recent times than it is of either Denisovans and modern humans. It seems most likely that the Sima de los Huesos individuals are early Neanderthals, which implies that the Neanderthal-Denisovan split was earlier than 400 ka. That might seem to be just fine, except for one thing: Neanderthal and Denisovan DNA are much more closely related to each other than to that of ourselves. That implies that the last common ancestor of the two archaic human species must have split from the ancestral line leading to modern humans even further back in time: maybe 550 to 765 ka ago and 100 to 400 ka earlier than previously surmised. This opens up several interesting possibilities for our long and separate development. Since Neanderthals and perhaps Denisovans emigrated from Africa to Eurasia several glacial cycles ago, maybe people genetically en route to anatomically modern humans did so too. The Neanderthal and Denisovan genomes suggest that they interbred with each other and that could have been at any time after the genetic split between them. Famously, they also interbred with direct ancestors of living Eurasians, but there is no genetic sign of that among living Africans. The evidence suggests that the insertion of archaic genetic material was into new migrants from Africa around 100 to 60 ka ago at different points along their routes to Europe and East Asia. But, obviously, it is by no means clear cut what passed between all three long-lived groups nor when. It is now just as possible that surviving, earlier Eurasians on the road to modern humans passed on their own inheritance from relationships with Neanderthal and Denisovan to newcomers from Africa. But none of these three genetic groups ever made their way back to Africa, until historic times.

More on Neanderthals, Denisovans and anatomically modern humans

First glimpse inside the Siberian cave that holds the key to man’s origins

Continental hot-spot track in eastern Australia

Published on September 24, 2015 by zooks777Leave a comment

cosgrove volcano track shown on map of australia — The Cosgrove volcanic track on a natural-colour image mosaic of Australia (credit: Drew Whitehouse, NCI National Facility VizLab)

It is sometimes forgotten that not only oceanic lithosphere provides evidence for hot spot tracks, probably because they are so obvious as island and seamount chains on bathymetric maps. They are not so clear on continents, either because of erosion of volcanoes or topography dominated by features that predate volcanism, but they account for about 20% of proposed tracks. Eastern Australia seems well endowed; four of them marked by a variety of volcanic structures that trend parallel to the Indo-Australian Plate’s NNE Cenozoic drift powered by the Southeast Indian Ridge that separates it from the Antarctic Plate. The timing of the volcanism along the proposed tracks is also highly persuasive. The longest of the tracks, extending about 2000 km SSW from Cape Hillsborough on the coast of central Queensland through New South Wales to Cosgrove in Victoria, is marked by sporadic volcanoes whose age decreases from Late Eocene in the north to Late Miocene in Victoria.

Unlike oceanic hot-spot tracks, those on continents are not continuous lines of volcanic occurrences. The Cosgrove track has several volcanic gaps, up to 650 km wide. This kind of patchy feature once encouraged hot-spot sceptics to question the tectonic affinities of what they regarded as fortuitous alignments. Where volcanic age trends consistent with the hypothesis emerged such doubts have faded into the background academic ‘noise’. In the case of the Cosgrove track all but one of the dates of volcanism tally quite well with the Cenozoic absolute motion of the Indo-Australian Plate and their position along the track (Davies, D.R. et al 2015. Lithospheric controls on magma composition along Earth’s longest continental hotspot track. Nature, v. 525, p. 511-514). Yet the objective of the authors, from the Australian National University and the University of Aberdeen in Britain, was not merely to establish the alignment as a hot-spot track, but to suggest what may have resulted in its marked patchiness.

The geochemistry of lavas from the volcanoes turns out to be of two fundamentally different types: ‘common-or-garden’ basalts in the case of Queensland and peculiar potassium-rich basalts containing the K-feldspathoid leucite in New South Wales and Victoria. Why these compositional differences occur where they do emerged very clearly when their positions were plotted on a new map of the thickness variations of the eastern Australian continental lithosphere. The ordinary basalts rest on the thinnest lithosphere (£110 km), whereas the leucitites are underlain by considerably thicker lithosphere (~135 km). This suggests that the rising mantle whose partial melting produced the magmas was halted at different depths, different geochemical ‘signatures’ of basalts depending on the pressure of melting. The most interesting outcome, albeit one based on an absence of evidence, is that the very large volcanic gaps along the track are each above much thicker lithosphere (>150 km). At those depths a rising mantle plume would be much less likely to begin melting.

The ‘star’ hominin of South Africa

Published on September 11, 2015 by zooks777Leave a comment

The week of 7 to 11 September 2015 was one of the most news-rich of the year. To name but two issues: the plight of tens of thousands of refugees fleeing Africa and the Middle East to Europe was made worse by total confusion, little action and downright obstruction by some of the most privileged governments on Earth ; in Britain one of the most exciting political dramas in decades – the leadership elections of the Labour Party – were reaching a climax of press and political skulduggery because of the unexpected direction both had taken. Something else burst onto the media scene that was, if anything, even more out-of-the-blue to the majority of people on Thursday 10 September: the remains of at least 15 individuals of a new hominin species found in a near-inaccessible cave were announced by a multinational team of geologists and anthropologists. The feature that ensured its wide publicity in competition with some pretty serious political and humanitarian developments was the suggestion that the corpses had been ritually laid to rest by beings that lived maybe 2 million years ago. This major scientific stir arose from the publication of two lengthy papers by the open-access, electronic journal eLife (Berger, L. R. and 46 others 2015. Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. eLife DOI: 10.7554/eLife.09560. Dirks, P.H.G.M. and 23 others 2015. Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa. eLife, DOI: 10.7554/eLife.09560).

Artist’s reconstruction of the face of Homo naledi (credit: John Gurche artist, Mark Thiessen photographer, National Geographic)

Homo naledi (naledi means ‘star’ in the Sotho language: the find was in the Rising Star cave system near Johannesburg) is known in more anatomical detail than any early hominin, and most closely resembles H. habilis and H. rudolphensis discovered 3 to 4 thousand miles away in Tanzania and Kenya. The Dinaledi deposit remains undated but likely to come out at around 2 Ma or older. The sheer wealth of anatomical detail, including complete foot- and hand-bone remains from individuals, evidence for a range of ages at death, and plenty of dental and cranial information, actually poses a taxonomic problem of comparison with remains of other early hominins. Most of them are fragmentary, and it seems likely that once a precise date is obtained H. naledi will assume greater importance in comparative anatomy. Comparison with australopithecines is easier because of their abundant remains, and H. naledi is clearly distinct from that clade as regards gait, chewing, overall physiognomy (see reconstruction video) and cranial dimensions, but does have some australopithecine affinities. They were certainly different from their near geographic neighbour Au. sediba, also found in a cave deposit within the great swath of Palaeoproterozoic limestones near Johannesburg, where the Cradle of Humankind UNESCO World Heritage Site is situated. The brain of Homo naledi was on a par with those of australopithecines as regards volume, yet larger than that of H. floresiensis: it does seem that brain size is not necessarily related to the uses to which it is put.

The route into the Dinaledi Chamber where bones of at least 15 individual members of Homo naledi were found (credit: National Geographic magazine http://news.nationalgeographic.com/2015/09/150910-human-evolution-change/)

Interestingly, it is reported that only the most diminutive members of the research team were able to enter the chamber where the remains were found because of the narrowness of the connecting passage. Also, access from the main cave system involved an upward ‘U-bend’, so that although water could – and did from time to time – enter the chamber in the past, it is unlikely that coarse material such as large bones could simply have been washed in, the more so as the chamber is on a minor spur from the main system and its outlet is through small floor drains that could not sustain torrential flow. Nor is there any direct access from the ground surface to this part of the system. Some of the more fragile body parts, such as a hand, are still articulated, which suggests a non-violent movement to the chamber. There are no signs of physical trauma to any of the bones, ruling out action by carnivores or transport by violent floods, nor any indicative of de-fleshing as by cannibalism. However, before fossilisation, many of the bones had been gnawed by beetles and snails. This combination of features leads to the possibility that corpses may have been deliberately placed in the chamber. If they had been, then to get to deepest recess of the cave system and find the Denalidi Chamber required illumination: fire brands. That the chamber was actually a living space is highly unlikely because of its remoteness from the surface. One big question that cannot be answered is whether or not such possible disposal was by ritual or simply for sanitary arrangements. Another possibility, not considered by the authors is seeking refuge from predators and becoming trapped in the desperately constricted space.

The possibility of ritual burial is clearly what has seized headlines. Yet few palaeoanthropologists will accept that: only Neanderthals and anatomically modern humans are definitely considered to have adopted such a practice, in the last hundred thousand years. The association of a bifacial stone tool with 350 ka old H. heidelbergensis remains at Atapuerca in northern Spain has been suggested to be the earliest evidence for ritual burial, but is not widely accepted. There are no reports of artefacts in the Dinaledi Chamber.

Scientists discover new human-like species in South Africa cave

Hotspots and plumes

Published on September 7, 2015 by zooks777Leave a comment

One of the pioneers of plate tectonics, W. Jason Morgan, recognised in the 1970s that chains of volcanic islands and seamounts that rise from the ocean floor may have formed as the movement of lithospheric plates passed over sources of magma that lay in the mantle beneath the plates. He suggested that such hotspots were fixed relative to plate movements at the surface and likened the formation of chains such as that to the west of the volcanically active of the Hawaiian ‘Big Island’ to linear scorching of a sheet of paper moved over a candle flame. If true, it should be possible to use hotspots as a framework for the absolute motion of lithospheric plates rather than the velocities of individual plates relative to the others. But Morgan’s hypothesis has been debated ever since he formulated it. A test would be to see whether or not plumes of rising hot material in the deep part of the mantle can be detected. This became one of the first objectives of seismic tomography when it was devised in the last decade of the 20^th century: a method that uses global earthquakes records to detect parts of the mantle where seismic waves traveled faster or slower than the norm: effectively patches of hot (probably rising) and cold rock. The first such evidence was equally hotly debated, one view being that the magma sources beneath oceanic islands such as Hawaii and Iceland were actually related to plate tectonics and that the hotspot hypothesis had become a kind of belief system.

English: global distribution of 45 identified ... — Global distribution of hotspots ( credit: Wikipedia)

The problem was that mantle plumes supposedly linked to magmatic hotspots in the upper mantle would be so thin that they would be difficult to detect even with seismic tomography. Geophysicists have been trying to sharpen up seismic resolution partly by using supercomputers to analyse more and more seismic records and also by improving the theory about how seismic waves interact with 3-D mantle structure. This has culminated in more believable visualisation of mantle structure (French, S.W. & Romanowicz, B. 2015. Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots). The two researchers from the University of California at Berkeley in fact showed something different, but still robust support for Morgan’s 40-year old ideas. Instead of thin plumes, they have been able to show much broader conduits beneath at least 5 and maybe more active ends of hotspot chains. The zones extend upwards from the core-mantle boundary to about 1000 km below the Earth’s surface, where some bend sideways towards hotspots, perhaps as a result of another kind of upper mantle circulation.

Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz; http://www.nature.com/doifinder/10.1038/nature14876)

The sources of these hot columns at the core-mantle boundary appear to be zones of very low shear-wave velocities; i.e. almost, but not quite molten blobs. French and Romanowicz suggest that the columns are extremely long-lived and may even have a chemical dimension – as in the hypothesis of mantle heterogeneity. Another interesting feature of their results is that the striking vertical linearity of the columns could indicate that the overall motion of the lower mantle is extremely sluggish and punctured by discrete convection.

Thin- or thick-skinned tectonics: a test

Published on August 24, 2015 by zooks777Leave a comment

How the continental lithosphere deforms at convergent plate margins has been a matter of opinion that depends on where observations have been made in ancient orogenic belts. One view is that arc and collisional orogens are dominated by deformation of the upper crust and especially the cover of sedimentary and volcanic rocks above deeper and older basement. This is a ‘thin-skinned’ model in which rocks of the upper crust are detached from those below and thicken more or less independently by thrust faulting, the formation of ductile nappes or a combination of the two. Mountain ranges, in this view, are the product of piling up of thrust slices or nappes, as exemplified by the Alps, Canadian Rockies and the Caledonian thrust belt of NW Scotland. Thick-skinned processes, as the name suggests, see crustal shortening and thickening as being distributed through the crust from top to bottom and even involving the lithospheric mantle. The hinterlands of both the Alps and the Scottish Caledonides show plenty of evidence for entire-crust deformation, deep crustal rocks being found sheared together with deformed rocks of the cover. It stands to reason that orogenic processes on the grand scale must involve a bit of both.

Both hypotheses stem from field work in deeply eroded, structurally complex segments of the ancient crust, and it is rarely if ever possible to say whether both operated together or one followed the other during the often lengthy periods taken by orogeny to reach completion, and the sheer scale of the process. Orogenesis is going on today, to which major seismic activity obviously bears witness. But erosion has not progress from cover through basement so, up to now, only seismicity and geodetic GPS measurements have been available to show that continental crust in general is being shortened and thickened, as well as being moved about. Potentially, a means of assessing active deformation, even in the deep crust, is to see whether or not the speeds of seismic waves at different depths are biased depending on their direction of travel. Such anisotropy would develop if the mineral grains making up rocks were deformed and rotated to preferred directions; a feature typical of metamorphic rocks. But to make such measurements on the scale of active orogens requires a dense network of seismometers and software that can tease directionality and depth out of the earthquake motions detected by it.

A joint Taiwanese-American consortium set up such a network in Taiwan, which is capable of this type of seismic tomography. Taiwan is currently taking up a strain rate of 8.2 cm per year due to motion of the Philippine Plate on whose western flank the island lies: it is part of an island arc currently colliding with the stationary Eurasian Plate and whose crust is shortening. Results of seismic anisotropy (Huang, T.-Y. et al. 2015. Layered deformation in the Taiwan orogen. Science, v. 349, p. 720-723) show that the fast direction of shear (S) waves changes abruptly at about 10 to 15 km deep in the crust. In the upper crust this lines up with the roughly N-S structural ‘grain’ of the orogen. At between 13 to 17 km down there is no discernible anisotropy, below which it changes to parallel the direction of plate motion, ESE-WNW. It seems that thin skinned tectonics is indeed taking place, although probably not above a structural detachment. Simultaneously the deep crust is being deformed but the shearing is ascribed to the descent of lithospheric mantle of the Philippine Plate beneath the Eurasian Plate, while the deep crust remains attached to the upper crust. If it were possible to examine the mineral lineations now forming in both the Taiwanese upper and lower crust where metamorphism is active, then the two directions would be apparent. Although not mentioned by the authors, perhaps the detection of different directionality of aligned metamorphic minerals in low- and high-grade metamorphic rocks might indicate such tectonic processes in the past.

Roman concrete restrains magma

Published on August 17, 2015 by zooks777Leave a comment

Four million people in and around the Italian city of Naples on the shore of the Tyrrhenian Sea have always lived under a double threat of natural disaster. The one that immediately springs to most people’s mind is the huge volcano Vesuvius that looms over its eastern suburbs, for this was the source of the incandescent pyroclastic flow that overwhelmed Pompeii and Herculaneum in 79 CE. Less familiar outside Italy is a cluster of elliptical volcanic features directly to the west of the city: Campi Flegrei or the Phlegraean Fields. In fact the cluster is part of a vast, dormant caldera, half of which lies beneath the sea centred on the ancient Roman port of Puteoli (modern Pozzuoli). This volcanic collapse structure is about 10 km across; about as large as Vesuvius. Campi Flegrei is famous for its sulfur-rich fumaroles including the mythical crater home of Vulcan the god of fire, Solfatara.

The Bay of Naples with Vesuvius to the east of the city and Campi Flegrei to the west. (credit: Google Earth)

Between 1970 and 1984 the ground around Pozzuoli rose more than 2 metres, which may be evidence that the deep seated magma chamber is inflating. Fears that this might presage an eruption in the near future stems from a curious feature affecting archaeological remains, such as upright pillars in the harbour area of Pozzuoli. At many different levels the stonework is pockmarked by curious holes that are the fossil borings of marine molluscs: at some stage the feet of the pillars descended below sea level. Together with historic records since the Roman era these borings help to establish the local ups and downs of the surface over the last two millennia in considerable detail. From a high of 4 m above sea level when the pillars were erected 194 BCE they slowly subsided to reach sea level around 300 CE when Puteoli ceased to be an important harbour and 4 metres below that around 900 CE. For the last millennium they have slowly risen until in 1538 more than 4 metres of inflation took place very rapidly. That was immediately followed by a small eruption of about 0.02 km³ of magma at Mount Nuovo, to the northeast of another recent crater now occupied by a lake: hence the fear surrounding the uplift in 1970-84. Campi Flegrei has a history of eruptions going back 40 thousand years, including two in the ‘super volcano’ category of 200 and 40 km³ that blanketed vast areas in pyroclastic ash.

One tantalising aspect of the ground inflation and deflation is that each cycle lasts of the order of a thousand years. Another seems to be that magma breaks to the surface very rapidly after a long period of inflation, as if whatever was keeping the magma chamber in a metastable state failed in a brittle fashion. Tiziana Vanorio and Waruntorn Kanitpanyacharoen of Stanford and Chulalonkorn universities in the US and Thailand have come up with a possible reason for such gradual crustal warping in volcanic areas and long-delayed eruption, for which Campi Flegrei is a model case (in fact the oscillations there are unsurpassed). Such long-term bending of the crust suggests abnormally strong rock near the surface. The co-workers analysed borehole cores that penetrated to the depth of small shallow earthquakes – in the metamorphic basement of the area – and found that the zone above the seismically active layer is not only stronger than the basement, but closely resembles a construction material to which Roman architecture owes its longevity (Vanorio, T. & Kanitpanyacharoen, W. 2015. Rock physics of fibrous rocks akin to Roman concrete explains uplifts at Campi Flegrei Caldera. Science, v. 349, p. 617-621).

Roman masons discovered that by mixing young, loose volcanic ash with lime mortar (calcium hydroxide) produced a strong concrete when cured. Specifically, the invention of concrete took place at Pozzuoli itself, using volcanic ash from Campi Flegrei and the product was known as pozzolana. Young ash from an explosive volcano is mainly shards of anhydrous silicate glass, which quickly react with water and calcium hydroxide to produce fibres of hydrous calc-silicate minerals, almost as in conventional cement curing, but without the need for heating limestone and clay to very high temperatures. The strength of pozzolano enabled Roman architects to build the great dome of the Pantheon in Rome, still the world’s largest unreinforced concrete dome. Moreover, the speed with which it sets by exothermic reactions enables its use below sea level. Vanorio and Kanitpanyacharoen found that the strong upper zone beneath Campi Flegrei is almost identical to pozzolano, and suggest that it formed as a result of calcium-rich hydrothermal fluids percolating through young pyroclastic rocks. The calcium derives from metamorphic basement rich in calc-silicate layers through which hot groundwater is driven as a result of heat from the underlying magma chamber. It seems the Campi Flegrei caldera has built its own containing dome. But that is perhaps a mixed blessing: the 1970-84 inflation seems now to be deflating and the flexible carapace may make using ground movements as means of predicting eruptions unreliable.

Intérieur du panteon à Rome — Interior view of the dome of the Pantheon in Rome (credit: Wikipedia)

Is Secret to Roman Concrete in Volcanic Rock?

When Earth got its magnetic field

Published on August 13, 2015 by zooks7771 Comment

For a planet to produce life it needs various attributes. Exoplanet hunters tend to focus on the ‘Goldilocks’ Zone’ where solar heating is neither so extreme nor so little that liquid water is unstable on a planet’s surface. It also needs an atmosphere that retains water. Ultraviolet radiation emitted by a planet’s star dissociates water vapour to hydrogen and oxygen and the hydrogen escapes to space. The reason Earth has not lost water in this way is that little water vapour reaches the stratosphere because it is condensed or frozen out of the air as the lower atmosphere becomes cooler with altitude. Given moist conditions survivability to the extent that exists on Earth still needs another planetary parameter: the charged particles emitted as an interplanetary ‘wind ‘by stars must not reach the surface. If they did, their potential to break complex molecules would hinder life’s formation or wipe it out if it ventured onto land. A moving current of electrical charge, which is what a stellar ‘wind’ amounts to, can be deflected by a magnetic field. This is what happens on Earth, whose magnetic field is a good reason why our planet has supported life and its continual evolution since at least about 3.5 billion years ago.

Direct proof of the existence of a geomagnetic field is the presence of aligned particles of magnetic minerals in rocks, for instance in a lava flow, caused by their acquiring magnetisation in a prevailing magnetic field once they cooled sufficiently. The earliest such remanent magnetism was found in igneous rocks from north-eastern South Africa dated at between 3.2 to 3.45 billion years. All older rocks do not show such a feature dating back to their formation because of thermal metamorphism that resets any remanent magnetism to match the geomagnetic field prevailing at the time of reheating. There are, however, materials that formed further back in time and are also known to resist thermal resetting of any alignments of magnetic inclusion. They are zircons (ZrSiO₄), originally crystallised from igneous magmas, which may have locked in minute magnetic inclusions. Zircons are among the most change-resistant materials and they can also be dated with great precision, with the advantage that the U-Pb method used can distinguish between age of formation and that of any later heating. Famously, individual grains of zircon that had accumulated in an early Archaean conglomerate outcropping in the Jack Hills of Western Australia yielded ages going back from 3.2 to 4.4 billion years; far beyond the age of any tangible rock and close to the formation age of the Earth. Quite a target for palaeomagnetic investigations once a suitable technique had been developed.

Western Australia's Jack Hills — Western Australia’s Jack Hills from Landsat (credit NASA Earth Observatory)

John Tarduno and colleagues from the Universities of Rochester and California USA and the Geological Survey of Canada report the magnetic properties of the Jack Hills zircons (Tarduno, J.A. et al. 2015. A Hadean to Paleoarchean geodynamo recorded by single zircon crystals. Science, v. 349, p. 521-524). All of the grains analysed record magnetisation spanning the period 3.2 to 4.2 billion years that indicate geomagnetic field strengths ranging from that found today at the Equator to about an eighth of the modern value. So from 4.2 Ga onwards geomagnetism probably deflected the solar wind: the early Earth was set for living processes from its earliest days. The discovery also supports the likelihood of functioning plate tectonics during the Hadean.

Earth’s magnetic field is at least 4 billion years old: Study

Pleistocene megafaunal extinctions – were humans to blame?

Published on August 4, 2015 by zooks777Leave a comment

Australia and the Americas had an extremely diverse fauna of large beasts (giant wombats and kangeroos in Australia; elephants, bears, big cats, camelids, ground sloths etc in the Americas) until the last glaciation and the warming period that led into the Holocene interglacial. The majority of these megafauna species vanished suddenly during that recent period. To a lesser extent something similar happened in Eurasia, but nothing significant in Africa. Because the last glacial cycle also saw migration of efficient human hunter-gatherers to every other continent except Antarctica, many ecologists, palaeontologists and anthropologists saw a direct link between human predation and the mass extinction (see Earth-Pages of April 2012. Earlier humans had indeed spread far and wide in Eurasia before, and the crude hypothesis that the last arrivals in Australasia and the Americas devoured all the meatiest prey in three continents had some traction as a result: predation in Eurasia and Africa by earlier hominids would have made surviving prey congenitally wary of bipeds with spears. In Australia and the Americas the megafauna species would have been naive and confident in their sheer bulk, numbers, speed and, in some cases, ferocity. Other possibilities emerged, such as the introduction of viruses to which faunas had no immunity or as a result of climate change, but none of the three possibilities has gained incontrovertible proof. But the most popular, human connection has had severe knocks in the last couple of years. A fourth, that the extinctions stemmed from a comet impact proved to have little traction.

Since the amazing success of analysing the bulk DNA debris in sea water – environmental DNA or eDNA – to look at the local diversity of marine animals, the analytical and computing techniques that made it possible have been turned to ancient terrestrial materials: soils, permafrost and glacial ice. One of the first attempts revealed mammoth and pre-Columbian horse DNA surviving in Alaskan permafrost, thanks to the herds’ copious urination and dung spreading. Several articles in the 24 July 2015 issue of Science review ancient DNA advances, including eDNA from soils that chart changes in both fauna and flora over the last glacial cycle (Pennisi, E. 2015. Lost worlds found. Science, v. 349, p. 367-369). Combined with a variety of means of dating the material that yield the ancient eDNA, an interesting picture is emerging. The soil and permafrost samples potentially express ancient ecosystems in far more detail than would fossil animals or pollens, many of which are too similar to look at the species level and in any case are dominated by the most abundant plants rather than showing those critical in the food chain.

The first major success in palaeoecology of this kind came with a 50-author paper using eDNA ‘bar-coding’ of permafrost from 242 sites in Siberia and Alaska IWillerslev, E. and 49 others 2014. Fifty thousand years of Arctic vegetation and megafaunal diet. Nature, v. 506, p. 47-51. doi:10.1038/nature12921). Dividing the samples into 3 time spans – 50-25, 25-15 (last glacial maximum) and younger than 15 ka – the team found these major stages in the last glacial cycle mapped an ecological change from a dry tundra dominated by abundant herbaceous plants (forbs including abundant anemones and forget-me-not), to a markedly depleted Arctic steppe ecosystem then moist tundra with woody plants and grasses dominating. They also analysed the eDNA of dung and gut contents from ice-age megafauna, such as mammoths, bison and woolly rhinos, where these were found, which showed that forbs were the mainstay of their diet. Using bones of large mammals 6 member of the team also established the timing of extinctions in the last 56 ka (Cooper, A. et al. 2015. Abrupt warming events drove Late Pleistocene Holarctic megafaunal turnover. Science, DOI: 10.1126/science.aac4315), showing 31 regional extinction pulses linked to the rapid ups and downs of climate during Dansgaard-Oeschger cycles in the run-up to the last glacial maximum. By the end of the last glacial maximum, the megafauna were highly stressed by purely climatic and ecological factors. Human predation probably finished them off.

Climate Change Doomed the Mammoths

How far has geochemistry led geology?

Published on July 16, 2015 by zooks7774 Comments

In the Solar System the Earth is unique in having a surface split into two distinct categories according to their relative elevation; one covered by water, the other not. More than 60% of its surface – the ocean basins – falls between 2 to 11 km below sea level with a mean around 4 to 5 km deep. A bit less than 40% – land and the continental shelves – stands higher than 1 km below sea level up to almost 9 km above, with a mean around 1 km high. Between 1 and 2 km below sea level is represented by only around 3 % of the surface area. This combined hypsography and wetness is reckoned to have had a massive bearing on the course of climate and biological evolution, as far as allowing our own emergence. The Earth’s bimodal elevation stems from the near-surface rock beneath each division having different densities: continental crust is less dense than its oceanic counterpart, and there is very little crustal rock with an intermediate density. Gravitational equilibrium ensures that continents rise higher than oceans. That continents were underpinned mainly by rocks of granitic composition and density, roughly speaking, was well known by geologists at the close of the 19^th century. What lay beneath the oceans didn’t fully emerge until after the advent of plate tectonics and the notion of simple basaltic magmas pouring out as plates became detached.

In 1915 Canadian geologist Norman Levi Bowen resolved previously acquired knowledge of the field relations, mineralogy and, to a much lesser extent, the chemistry of igneous rocks, predominantly those on the continents in a theory to account for the origin of continents. This involved a process of distillation or fractionation in which the high-temperature crystallisation of mafic (magnesium- and iron-rich) minerals from basaltic magma left a residual melt with lower Mg and Fe, higher amounts of alkalis and alkaline earth elements and especially enriched in SiO₂ (silica). A basalt with ~50% silica could give rise to rocks of roughly granitic composition (~60% SiO₂) – the ‘light’ rocks that buoy-up the continental surface – through Bowen’s hypothetical fractional crystallisation. Later authors in the 1930s, including Bowen’s teacher Reginald Aldworth Daly, came up with the idea that granites may form by basalt magma digesting older SiO₂-rich rocks or by partially melting older crustal rocks as suggested by British geologist Herbert Harold Read. But, of course, this merely shifted the formation of silica-rich crust further back in time

A great deal of field, microscope and, more recently, geochemical lab time has been spent since on to-ing and fro-ing between these hypotheses, as well as on the petrology of basaltic magmas since the arrival of plate theory and the discovery of the predominance of basalt beneath ocean floors. By the 1990s one of the main flaws seen in Bowen’s hypothesis was removed, seemingly at a stroke. Surely, if a basalt magma split into a dense Fe- Mg-rich cumulate in the lower crust and a less dense, SiO₂-rich residual magma in the upper continental crust the bulk density of that crust ought to remain the same as the original basalt. But if the dense part somehow fell back into the mantle what remained would be more able to float proud. Although a neat idea, outside of proxy indications that such delamination had taken place, it could not be proved.

Since the 1960s geochemical analysis has became steadily easier, quicker and cheaper, using predominantly X-ray fluorescence and mass-spectrometric techniques. So geochemical data steadily caught up with traditional analysis of thin sections of rock using petrological microscopes. Beginning in the late 1960s igneous geochemistry became almost a cottage industry and millions of rocks have been analysed. Recently, about 850 thousand multi-element analyses of igneous rocks have been archived with US NSF funding in the EarthChem library. A group from the US universities of Princeton, California – Los Angeles and Wisconsin – Madison extracted 123 thousand plutonic and 172 thousand volcanic igneous rocks of continental affinities from EarthChem to ‘sledgehammer’ the issue of continent formation into a unified theory (Keller, C.B. et al. 2015. Volcanic-plutonic parity and the differentiation of the continental crust. Nature, v. 523, p. 301-307).

In a nutshell, the authors compared the two divisions in this vast data bank; the superficial volcanic with the deep-crustal plutonic kinds of continental igneous rock. The gist of their approach is a means of comparative igneous geochemistry with an even longer pedigree, which was devised in 1909 by British geologist Alfred Harker. The Harker Diagram plots all other elements against the proportionally most variable major component of igneous rocks, SiO₂. If the dominant process involved mixing of basalt magma with or partial melting of older silica-rich rocks such simple plots should approximate straight lines. It turns out – and this is not news to most igneous geochemists with far smaller data sets – that the plots deviate considerably from straight lines. So it seems that old Bowen was right all along, the differing deviations from linearity stemming from subtleties in the process of initial melting of mantle to form basalt and then its fractionation at crustal depths. Keller and colleagues found an unexpected similarity between the plutonic rocks of subduction-related volcanic arcs and those in zones of continental rifting. Both record the influence of water in the process, which lowers the crystallisation temperature of granitic magma so that it freezes before the bulk can migrate to the surface and extrude as lava. Previously. rift-related magmas had been thought to be drier than those formed in arcs so that silica-rich magma should tend to be extruded.

But there is a snag, the EarthChem archive hosts only data from igneous rocks formed in the Phanerozoic, most being less than 100 Ma old. It has long been known that continental crust had formed as far back as 4 billion years ago, and many geologists believe that most of the continental crust was in place by the end of the Precambrian about half a billion years ago. Some even reckon that igneous process may have been fundamentally different before 3 billion years ago(see: Dhuime, B., Wuestefeld, A. & Hawkesworth, C. J. 2015. Emergence of modern continental crust about 3 billion years ago. Nature Geoscience, v. 8, p.552–555). So big-science data mining may flatter to deceive and leave some novel questions unanswered .

Hallucigenia gets a head

Published on July 7, 2015 by zooks777Leave a comment

The Middle Cambrian Burgess Shale of the Canadian Rockies is one of those celebrated sediments that show extraordinary preservation of soft-bodied and easily disarticulated organisms and rich assemblages of fossils. Being one of the earliest known of such lagerstätten, many of the denizens of the ecosystem in which the shale originated were at first regarded as members of hitherto undiscovered and now vanished phyla, the basal branches of the ‘tree of life’. Some certainly looked pretty odd, such as Opabina with a feeding apparatus looking similar to the extension nozzle of a vacuum cleaner; but that is clearly some kind of arthropod. Others turned out to be astonishingly large, once it was realised that parts of their broken bodies had previously been taken to be different organisms, an example being Anomalocaris. But perhaps the oddest, certainly to palaeontologists, was Hallucigenia. However, there are plenty of even more weird and wonderful living creatures, such as the sea pig, although modern creatures are more easily pigeonholed, taxonomically speaking.

The trouble with Hallucigenia was not so much its complexity – it was a fairly simple-looking beast – but that there were two choices as to which way up it lived; a feature that surprisingly led to a great deal of pondering that ended with the scientist who formally described it in 1977 making the wrong choice. That was eventually resolved fourteen years later, but the creature might also have inspired the Pushmi Pullyu in Hugh Lofting’s Dr Doolittle stories for children. Not that it resembled a unicorn-gazelle cross: far from it, for no-one could decide which its front was and which its backside, and even if it may have lain on its side. But Hallucigenia does demonstrate bilateral symmetry beautifully – it must have a front and back, and a top and bottom, even though which was which remained veiled in mystery – and so belongs to the dominant group of animals, imaginatively known as bilaterians.

The Burgess Shale lagerstätte seemingly was heaving with Hallucigenia so would-be taxonomists have had no shortage of specimens to ponder over in the 38 years since Simon Conway Morris made his dreadful mistake: of course, that was not of such enormity as Einstein’s ‘biggest blunder’ in the form of his cosmological constant, and Conway Morris quickly accepted his error when the beast was turned right-way-up in 1991. The problem is, exquisite as they are, Burgess Shale fossils are flattened and all that remains of mainly soft-bodied animals are delicate carbonaceous films, which need electron microscopy to unravel.

The latest reconstruction of Hallucigenia, by palaeontological illustrator Danielle Dufault (http://www.ddufault.com) — The latest reconstruction of Hallucigenia, by palaeontological illustrator Danielle Dufault

In 2015, Hallucigenia’s front end was definitely found and a great deal more besides by Canadian palaeontologists Martin Smith and Jean-Bernard Caron of the Royal Ontario Museum and the University of Toronto (Smith, M.R. & Caron J.-B. 2015. Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. Nature, v. 523, p. 75-78). It has eyes, albeit rudimentary, and a throat, deep within which it has pointy teeth. Hallucigenia was a lobopod, whose living relatives lie within that large and diverse group the Ecdysozoa, which all have throat teeth and include the wondrous water bear (tardigrade) and the velvet- and penis worms (onychophores and priapulids, respectively) as well as lobsters, flies and woodlice. It may indeed have been close to the last common ancestor of all animals who moult their carapaces.

Picture of the month, June 2015

Published on July 1, 2015 by zooks7771 Comment

SpheroidalIMG_4815 — Spheroidally weathered basalt from Turkey. (credit: Francisco Sousa)

Spheroidal weathering of lavas, easily confused with pillows, is also found in other homogeneous igneous rocks. It develops from rectilinear joint sets along which the groundwater responsible for breakdown of silicates initially moves. Hydration reactions begin along the joints but proceed most quickly at corners so that curved surfaces begin to develop. The concentric banding that sometimes culminates in almost spherical relics may involve more than just rotting of anhydrous silicates as the reactions involve volume increases that encourage further rock fracturing. Other factors, such as elastic strain release may also encourage the characteristic concentricity Prolonged, intense chemical weathering leaves isolated, rounded corestones surrounded by saprolite, that can form boulder fields when the softer weathered material has been eroded away.

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