zooks777

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.

Are coral islands doomed by global warming?

Published on June 18, 2015 by zooks777Leave a comment

Among the most voluble and persistent advocates of CO₂ emissions reduction are representatives of islands in the tropics that are built entirely of reef coral. All the habitable land on them reaches only a few metres above high-tide level, so naturally they have more cause to worry about global warming and sea-level rise than most of us. Towns and villages on some atolls do seem to be more regularly inundated than they once were. So a group of scientists from New Zealand and Australia set out to check if there have been losses of land on one Pacific atoll, Funafuti, during the century since tidal observatories first recorded an average 1.7 mm annual rise in global sea level and a faster rate (~3 mm a^-1) since 1993 (Kench, P.S. et al. 2015. Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll. Geology, v. 43, p.515-518).

English: Funafuti (Tuvalu) from space Magyar: ... — Funafuti atoll (Tuvalu) from space (credit: Wikipedia)

Funafuti atoll comprises 32 islands that make up its rim, with a range of sizes, elevations, sediment build-ups and human modifications. The atoll was first accurately surveyed at the end of the 19^th century, has aerial photographic cover from 1943, 1971 and 1984 and high-resolution satellite image coverage from 2005 and 2014, so this is adequate to check whether or not sea-level rise has affected the available area and shape of the habitable zone. It appears that there has been no increase in erosion over the 20^th century and rather than any loss of land there has been a net gain of over 7%. The team concludes that coral reefs and islands derived from their remains and debris are able to adjust their size, shape and position to keep pace with sea level and with the effects of storms.

This is an observation of just one small community in the vastness of the Pacific Ocean, so is unlikely to reassure islanders elsewhere who live very close to sea level and are anxious. It is a finding that bears out longer-term evidence that atolls remained stable during the major sea-level changes of the post-glacial period until about 7 thousand years ago when land glaciers stabilised. Since coral grows at a surprisingly rapid rate, that growth and the local redistribution of debris released by wave action keep pace with sea-level change; at least that taking place at rates up to 3 mm per year. But the study leaves out another threat from global warming. Corals everywhere are starting to show signs of ill thrift, partly resulting from increasing acidity of seawater as more CO₂ dissolved in it and partly from increases in sea-surface temperature, as well a host of other implicated factors. This manifests itself in a phenomenon known as coral bleaching that may presage die-off. Should coral productivity decrease in the Pacific island states then the material balance shifts to land loss and sea level will begin an irresistible threat.

Small atoll islands may grow, not sink, as sea levels rise

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this: