One of the delights of Google Earth is to commit a little Thesigery in the comfort of your front room and traverse the Sahara, the Empty Quarter of Arabia, the Namib or the Gobi. Not only are there dunes on gargantuan scales, but zooming-in from 30 m Landsat to 65 cm Quickbird images on Google Earth reveals a dune hierarchy down to largish ripples. And not all dunes are classic in shape. In the same issue of Nature as a retrospective review of Ralph Bagnold’s classic The Physics of Blown Sand and Desert Dunes, French, Algerian and US workers give a clue to the fundamental controls over dunes systems, that was not available to early researchers (Andreotti, B. et al 2009. Giant aeolian dune size determined by the average depth of the atmospheric boundary layer. Nature, v. 457, p. 1120-1123). They conclude that the general dynamics are analogous to those in flowing water; i.e. like a river, the wind has a capping surface that is the thermal inversion in the atmosphere marked by the tropopause. Flow that is physically bounded involves a series of resonances (as in a flute), which help to explain the tiered nature of dune systems and also their maximum size in a particular area of desert. Together with seasonal shifts in wind direction, fluctuations in the ‘depth’ of the wind combine together to produce the hypnotically addictive disorganised order that makes big sand deserts so attractive, despite their dangers.
Author: zooks777
Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry
Possible effects of mid-Ordovician bombardment
Limestones dated at around 470 Ma in Sweden contain highly altered chondritic meteorites, ranging in mass up to 3.4 kg and up to 20 cm across, along with chromite grains and high iridium. There are so many that investigators have estimated a flux of extraterrestrial debris that was a hundred times greater than at present. The remarkable repository is matched in age by sediments rich in chromites in central China. The Darriwilian Stage (460-470 Ma) of the Ordovician is also notable for evidence of powerful downslope sediment movement in many continental margin sequences. John Parnell of Aberdeen University reviews the many megabreccias or olistostromes of this geologically short time span (Parnell, J. 2009. Global mass wasting at continental margins during Ordovician high meteorite influx. Nature Geoscience, v. 2, p. 57-61). Most seem to be associated with continental margins of the mid-Ordovician Southern Hemisphere. While some occur at what were probably seismically unstable volcanic arcs, most are associated with stable carbonate platforms. Together with the link in time to evidence for enhanced meteorite flux, this association suggests slope failure associated with large impacts. However, the megabreccias are so widespread that they are unlikely to have been formed by a single tsunami resulting from one giant impact. Indeed there is no evidence for a catastrophic event, either as a large crater or evidence for mass extinction: the mid Ordovician was a time of rising faunal diversity (see The Great Ordovician Diversification in September 2008 issue of EPN). Parnell calculates that there may have been as many as 10 Chicxulub-sized impactors per million years during the Darriwilian, but the lack of catastrophic consequences suggests that the megabreccias may have resulted from a great many smaller events, probably of bodies less than 300 m across. That would also explain the lack of global evidence traditional sought to identify impacts, such as iridium, glass spherules and shocked mineral grains. If he is correct, then other olistostromes of different ages in aseismic settings could point to extraterrestrial causes.
Experiments on formation of organic compounds by impacts
Many mechanisms have been speculatively proposed for the origin of complex organic chemicals from which life may have originated on Earth. The best known of these is the 1929 Oparin-Haldane hypothesis that life began with simple organic compounds formed from methane and ammonia in the early atmosphere, followed by more complex compounds formed in the seas through a variety of reactions. This was tested by Miller and Urey in the 1950s, using electrical discharges through a simulation of such a reducing atmosphere, but current views are that the early atmosphere was rich in CO2 and nitrogen rather than reduced methane and ammonia. Another possibility is synthesis of organic compounds as a result of impact energy; very abundant early in Earth’s history. This idea has been tested experimentally using a propellant gun to create high-velocity impacts into a mixture of solid carbon, iron, nickel, water and nitrogen: a highly simplified scenario of ordinary chondrites bombarding atmosphere and ocean (Furukawa,Y. et al. 2009. Biomolecule formation by oceanic impacts on early Earth. Nature Geoscience, v. 2, p. 62-66). The experiments were performed under conditions that excluded possible contamination. Yet they yielded a wealth of organic molecules, including fatty acids, amines and an amino acid (glycene) found in DNA. Scaling up the experimental yields to the mass of meteoritic material accreted to the Earth during the Hadean Eon (of the order of 10 24 g), the authors estimate that at least 1017 g of organic material would have been present in the surface environment by the time life eventually emerged. Furukawa et al. rule out the delivery of ready-made organics by carbonaceous chondrites, in which a great variety has been found. As well as their decomposition by the heat of entry, the lack of metallic iron in carbonaceous chondrites would promote oxidation rather than reduction of organic compounds preformed in early evolution of the Solar System.
Moon-forming impact dated
One of the major discoveries that arose from the lunar samples returned by the Apollo astronauts was that the pale-coloured lunar highlands were made almost entirely of calcium-rich plagioclase feldspar: they are made of anorthosite. In the early 1970s Joe Smith of the University of Chicago realised that the only way vast amounts of such single-mineral igneous rocks could have formed was by massive fractional crystallisation. Low-density feldspar must have floated on top of what had been literally a magma ocean. Although Smith did not put forward the idea that a molten moon had formed through a giant collision between the Earth and a passing Mars-sized planet, it was his concept that pointed strongly in that direction. Inevitably, much of the Earth would also have been melted by such a monstrous catastrophe – material that eventually became the Moon had probably been vaporised before condensing to form our satellite.
The Apollo samples are still objects of research, especially as new analytical methods develop. One such new method is the dating of single, tiny zircons; even of their individual zones. Later impacts on the Moon formed a variety of breccias, samples of which are handy as they include fragments of many rock types in one specimen. One of these has helped zero-in on just when the magma ocean began to crystallise (Nemchin, A. et al. 2009. Timing of crystallization of the lunar magma ocean constrained by the oldest zircon. Nature Geosciences, v. 2, p. 133-136). In fact advanced mass spectrometry dated 41 tiny spots in a single half-millimetre zircon grain, revealing a spectrum of ages between <4.35 Ga and a maximum of 4.417 ± 0.006 Ga. The oldest marks the minimum age for the start of crystallisation of the molten Moon and thus for the impact that formed the Moon. For comparison, the earliest material found on Earth – also a zircon but one transported in sediment to become part of a much younger sandstone – is 4.404 Ga old. The authors suggest that the bulk of the lunar highland crust had solidified within 100 Ma of the collision
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Rainfall-magnetic field link during the Holocene
One of the fuelling factors in the debate about short-term climate change during the Holocene is the suggestion that variations in cosmic-ray bombardment might affect climate through these extra-solar particles’ possibly nucleating low-altitude clouds. It is a complicated idea, because changes in the Sun’s activity – the solar wind – modulate the cosmic ray flux, and short-term changes in solar irradiance at the Earth’s surface have also been suggested as a climate driver. To further obscure matters, any changes in the geomagnetic field would also affect cosmic-ray flux, yet geomagnetism is in turn a measurable proxy for cosmic ray intensities on Earth (Knudsen, M.F. & Riisager, P. 2009. Is there a link between Earth’s magnetic field and low latitude precipitation? Geology, v. 37, p. 71-74). The two Danish scientists have compiled the Holocene record of the geomagnetic dipole moment: effectively a measure of the strength of the magnetic field. In the paper they compare that record with δ18O changes in stalactites from China and the Oman, which are a proxy for changing low-latitude precipitation – in part the signal of the Indian Ocean monsoons. A correlation did emerge from the study, supporting the cosmic ray-climate theory. This further complicates the Earth’s climate system and therefore the models used by climatologists.
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Deeper roots of culture
There has long been a pervasive aroma of eurocentrism in cultural palaeoanthropology, encouraged by the spectacular cave paintings in southern France and northern Spain that are no more than 40 ka in age and the first to be discovered. This undoubted flowering of art as we appreciate it today has been linked to much more than figurative expression. Some have argued that Homo sapiens only became fully human after Europe was colonised. Thankfully, the archaeological record is rapidly being set straight by more and more discoveries of symbolic representation from elsewhere (Balter, M. 2009. On the origin of art and symbolism. Science, v. 323, p. 709-711). Blomberg Cave In South Africa is a repository for 100 ka old inscribed ochre artefacts (Balter, M. 2009. Early start for human art? Ochre may revise timeline. Science, v. 323, p. 569), which represent symbolism of some kind and the imagined uses to which the ochre was put – ritual or cosmetic body painting? But there are tantalising objects that push art back even further. In 1999 a cache of stone tools at Tan-Tan in Morocco was found to include a 6 cm quartzite chunk that looks like a rough version of the ‘Aurignacian Venuses’ of later times, yet the find dates back to 300 to 500 ka. Something similar turned up in the 250 ka site of Berekhat Ram in the Israeli-occupied Golan Heights of Syria. Both predate the evolution of fully modern humans. And what of the tear-drop shaped biface ‘axes’ associated with H. erectus and H. ergaster as far back as 1.6 Ma? These are extremely odd objects, for several reasons: it is hard to visualise their use; many finds are in pristine condition, as if never used; to make one demands a mental model of what potentially lies within a rock; they are more difficult to make than later blade tools that are more utilitarian. Arguably, the ‘Acheulean hand axe’ may be more of a symbol than a tool.
The reason for renewed discussion in print of these matters is, of course, the bicentenary of Charles Darwin’s birth and the 150th anniversary of publication of his Origin of Species. Darwin drew a link between tool making and language in his Descent of Man. He would have been delightedly surprised to learn details of the emergence of new tool-making skills in Africa, from where he insisted we all came (Morgan, L.E. & Renne, P.R. 2009. Diachronous dawn of Africa’s Middle Stone Age: New 40Ar/39Ar ages from the Ethiopian Rift. Geology, v. 36, p. 967-970). Morgan and Renne, of the University of California at Berkeley, discovered that the oldest sites in the Main Ethiopian Rift that contain the novel tools that mark the onset of the Middle Stone Age (MSA) span a much greater interval than assumed hitherto. In one site such tools date to 276 ka, whereas at another such objects appear only at 183 ka. The more delicate work to make MSA points and blades, and a much diversified ‘tool kit’ has been called the Levallois technique, thought to have been associated with a cognitive leap from the Lower Palaeolithic Oldowan and Acheulean techniques. For some it came to signify more: the appearance of fully modern humans. But the new ages do not tally with the fossil record of H. sapiens or with estimates from mitochondrial DNA molecular clocks. All in all, culture, whether art or technology, seems to be characteristic of the genus Homo. Given a push bike, could H. ergaster have ridden it and, more important, had fun? What would a Neanderthal, male or female, have done with a tube of lipstick?
The Neanderthal genome is coming!
Some computer owners take part in the search for extraterrestrial intelligence, allowing SETI to combine their processing power with that of hundreds of others, on the off chance that the meaning of (pi) pops up in a systematic burst of non-static microwaves. Personally I would far rather wait for a message from a relative than from some seriously weird being whose motives we might never guess. A Neanderthal lady – more precisely her leg bone –from Croatia is very close to speaking volumes about our own history. Two teams of DNA sequencers are putting the finishing touches to her genome. That it would ever happen was a fevered dream not so long ago. That it will opens up a revolution in understanding our origins. To keep in touch, read Elizabeth Pennisi’s account of the pending revelations (Pennisi, E. 2009. Tales of a prehistoric human genome. Science, v. 323, p. 866-871). Svante Paabo gave a glimpse of his team’s rough draft of the genome at the AAAS annual meeting in February 2009. When analyses are finished palaeoanthropology will explode onto the news channels, blogs, and among the twittering classes. Should SETI get a result, I would first eat my trousers and then prepare to be eaten myself. As for Darwin, maybe you have noticed his prominent brow ridges…
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Are sheeted dykes significant?
More than abyssal sediments, pillow basalt, differentiated gabbro and depleted peridotite sheeted dyke complexes have long been a primary identifier for oceanic lithosphere preserved in ophiolites. That assumption has recently been questioned (Robinson, P.T. et al. 2008. The significance of sheeted dyke complexes in ophiolites. GSA Today, v. 18 (November 2008), p. 4-10). Ian Gass first discovered units made up solely of dykes that intrude one another with no intervening screens of other host rocks in the Troodos ophiolite of Cyprus in 1968. Sheeted dyke complexes became widely regarded as characteristic of extensional, sea-floor spreading environments connected to basaltic magma chambers, each increment of extension being filled with magma. They have also been imaged in eroded walls of ocean fracture systems and cut through by ocean drill cores, supporting this notion. In fact, many ophiolites are devoid of sheeted complexes, despite having all the other components of mafic-ultramafic lithosphere. Robinson et al. argue that sheeted dykes only form where spreading rates and magma supply are balanced, as expected at true constructive plate margins but far less likely at other extensional zones associated with plate tectonics, such as those in back-arc basins above subduction zones. Even at true spreading centres that generate new ocean floor magma supply may not balance extension, for instance where spreading rates are slow. Moreover, a great many ophiolites show geochemical affinities that are more akin to supra-subduction magmatic processes than those that produce mid-ocean ridge basalt.
Plate tectonics in time and space
Seismic tomography becomes increasingly revealing as the capacity of supercomputers grows. On top of that, more sophisticated software allows present-day mantle cross sections to be reverse modelled with surface plate motions to reconstruct an idea of mantle dynamics back to Mesozoic times. Geophysicists at the California Institute of Technology give a taste of the possibilities from the subduction history of North America (Liu, L. et al. 2008. Reconstructing Farallon plate subduction beneath North America back to the Late Cretaceous. Science, v. 322, p. 934-938). Investigating 3-D evolution is the key to connecting rigid plate tectonics and fluid convection that has long been postulated but remains obscure. However, while reasonable reconstructions of global plate motions are possible using sea-floor magnetic stripes that go back to the Cretaceous, seismic tomography only images the mantle’s present structure. So it might seem that generating a 3-D ‘geomovie’ is more of an expensive illusion than a model of past realities.
The logic behind the modelling is that today’s mantle temperature structure – that is what tomograms show – stems from past plate activity. For instance, a deep cold, slab-like anomaly dipping eastward beneath eastern North America can reasonably be inferred to be a relic of the Farallon Plate, which formerly constituted floor of the eastern Pacific. That plate was subducted beneath the west edge of the continent until around 40 Ma, when the East Pacific Rise that had driven it was subducted. The present thermal structure shown by the tomogram has, in a sense, ‘faded’ as a result of thermal relaxation of the original anomalies by heat diffusion. Choosing geologically reasonable starting conditions for long-term evolution of a mantle segment enables iterative forward modelling to try and achieve the present set-up. While there is an element of circularity in this logic, such a dynamic model has a predictive aspect; i.e. as cold, dense material in the mantle sinks it tends to pull the surface downwards, allowing marine flooding of continental interiors. During the Late Cretaceous this did happen spectacularly in North America, and Liu et al’s model shows this. Yet sea level also rose globally at the time, thereby amplifying the inundation. Although geeomodellers will be excited by Liu et al’s developments, it is modelling and even the simplest of models is acutely sensitive to the chosen starting conditions, as meteorologists with vastly more real data at hand have discovered again and again.
See also: Steinburger, B. 2008. Reconstructing Earth history in three dimensions. Science, v. 322, p. 866-868
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Technology, culture and migration in the Middle Palaeolithic of southern Africa
The period between 300 and 30 ka was critical for the evolution of modern humans. Our mitochondrial DNA indicates that fully modern humans emerged around 200 ka. Projectile weapons that help define the epoch first appeared. Clear signs of self-adornment and symbolism also turn up during the Middle Palaeolithic. All of these developments took place in Africa, and the last two are reflections of the increased efforts by archaeologists in the continent from which we all originated. There is a long way to go to match the density of sites from which later periods in human history have been outlined in Europe, but progress is accelerating. One great hindrance has been dating sites, for the Middle Palaeolithic lies in a time zone where the Ar-Ar and 14C methods are ineffective. A developing chronological ‘workhorse’ for this difficult period depends on the way in which exposure of sand grains to sunlight ‘heals’ the defects in their molecular structure formed when radioactive isotopes in soils emit ionising radiation. Artificial illumination of sand grains containing these defects causes them to luminesce. The degree of luminescence is related to the time over which the defects have built up. Optical dating relies on grains having been exposed at the surface for a time to ‘reset’ the luminescence clock, and then being buried so that new defects can accumulate. Having lots of sunlight and a superabundance of bare sand, Australia has become a hotbed of research into optical dating of events associated with its peopling during the last ice age. Expertise developed there has been applied to many Middle Palaeolithic sites in Southern Africa (Jacobs, Z. et al. 2008. Ages for the Middle Stone Age of Southern Africa: Implications for human behaviour and dispersal. Science, v. 322, p. 733-735).
Archaeological work in South Africa and Namibia has revealed two distinct stone industries in the Middle Palaeolithic, both of which made hafted weapons that would have made hunting more efficient than the whatever weapons were used in earlier times – the most distinctive of the preceding Lower Palaeolithic tools was the bifacial hand axe, whose use is obscure. Both cultures involved the earliest recognisable ornamentation, such as shell beads and materials engraved with symbols, together with indirect evidence for the use of hematite and goethite pigments for body painting (see When and where ‘culture’ began in EPN of November 2007). Genetic evidence famously places modern human origins and their global migration out of Africa within this time frame. So, dating the archaeological sites as accurately as possible is a crucial importance, and a tremendous start has been made by the multinational team lead by Zenobia Jacobs of the University of Woolangong in Australia. Optical ages span 90 to 30 ka, with clusters between 71.9 to 71 ka and 64.8 to 59.5 ka, with a statistically significant gap of about 6.7 thousand years between them. When compared with climatic-change indicators from the Antarctic ice record the developmental episodes do not seem to correlate clearly with any specific warm of cool periods, though the earlier spans the time of the Toba super-eruption in Indonesia and the later one was a period of warming. So any environmental cause for the technological and cultural changes is unclear. However, both fall within the estimated time span of the genetic ‘bottleneck’ between 80 and 60 ka, and the most likely times for the initial ‘Out of Africa’ migrations, probably across the Straits of Bab el Mandab linking Eritrea and Arabia across the Red Sea shallowed by ice-cap linked falls in global sea level.
Childhood and families
Human females are unlikely to break 10 seconds for the 100 metres because of their sashaying gait. It can’t be helped, being due to the evolution of the pelvic girdle of bipedal females to deal with birthing of infants with increasingly large heads. Supposedly, the human female pelvis is now close to the limit that will permit walking on two legs. Such problems do not plague other living primates partly because their young have small heads relative to their bulk, and pelvic anatomy is not constrained by an habitually upright gait. It seems not to have been an ‘issue’ for australopithecines either: they did not possess ‘child-bearing hips’. The intermediate species, Homo erectus, despite having a 1 Ma fossil record (maybe as long as 1.8 Ma for the Asian form) only recently provided substantial pelvic remains (Simpson, S.W. et al. 2008. A female Homo erectus pelvis from Gona, Ethiopia. Science, v. 322, p. 1088-11092). In the words of the authors, this pelvis is ‘obstetrically capacious’ and demonstrates that female skeletal evolution responded to increasing foetal brain size: it would have permitted infants with heads 30 to 50% of the adult size to have been born. Homo erectus has been widely supposed to have had a tall willowy frame analogous to that of fully modern human inhabitants of tropical savannahs, yet the Gona woman was stocky. So, environmental influences seem to have had less of an evolutionary role than the advantages of greater brain development before birth. That places H. erectus even more firmly on the human line; indeed greater in utero brain development seems to have taken place than in modern humans.
The Gona pelvis demands re-evaluation of how foetal and childhood development has progressed over the last two million years (Gibbons, A. 2008. The birth of childhood. Science, v. 322, p. 1040-1043), the unique attributes having appeared during the evolution of our own genus. Among chimpanzees, infants can fend for themselves, with a little help from elders, after 3 years old. Street children from Asia and South America need to be 6 before they can survive without parental care. Growth lines on teeth that appear week by week reveal that previous age estimates for a number of immature australopithecines whose first adult molars had erupted were large overestimates: instead of 6 they point to 4 years old. Another signal feature of human development is the lengthy period to full development (marked by the eruption of the 3rd molar as well as the end of significant growth in stature). The average age when human child bearing begins is around 19, while chimpanzees start at about 11. A fresh examination of the famous Turkana Boy’s skeleton, an H. erectus, that uses tooth microstructure reduces his age at death from 13 to 8, suggesting an earlier onset of independence than in modern children. He grew much more quickly too, and would have reached adulthood somewhat earlier: around 14.5 years old. The picture with Neanderthals is not completely clear, some tooth studies suggest that their children grew significantly more quickly than modern ones, other studies point to the same rates or even longer development if adult brain sizes of Neanderthals are taken into account (larger on average than those of modern humans). Using average life expectancy of gatherer-hunter humans and chimps who survive dependent childhood – 45 and 70 years respectively – along with evidence for child development, suggests that australopithecines could have reached 45 while H. erectus adults could have expected to reach 60 years old.
There are other differences that begin to slot into space with the new data. Both human and chimpanzee females have a similar child-bearing period of around 20-25 years. The difference is that, on average, the natural interval between births is about half as long for human mothers as for chimpanzees. The greater number of human offspring gives a greater chance of the survival of some to reproduce themselves. On the other hand, slower child development places a greater burden on mothers, even after weaning. So there is quite a contradiction between the evolutionary effects, if only child-mother relationships are taken into account. This contradiction was resolved, to some extent, by a seminal paper in the late 20th century by a group of anthropologists from the Universities of Utah and California (see O’Connell, J.F., Hawkes, K. & Blurton Jones, N.G. 1999. Grandmothering and the evolution of Homo erectus. Journal of Human Evolution, v. 36, p. 461-485). They focussed on the potentialities of the early onset of infertility or the menopause among women relative to its appearance among female chimpanzees, which gives, on average, a 30 year non-child-bearing period to older women. This approximately coincides not only with child-rearing periods for their daughters, but for their granddaughters as well. The ‘grandmothering’ hypothesis for human development centres on the great evolutionary advantages of post menopausal women assisting with child rearing. O’Connell et al. suggested that this arose among H. erectus, as far back as 1.8 Ma, and the Gona pelvis together with other new views of H. erectus development add considerable weight to that concept. As well as freeing younger women for food gathering, the cultural significance of older women caring for children adds another dimension that may link to the advantages of delayed post-weaning development that we see today, albeit in many annoying contexts!
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Arsenic risk in the Mekong Delta of Cambodia
Since the awful discovery in the 1980s that millions of people in the delta plains of the northern Indian subcontinent were at risk of chronic arsenic poisoning if they drank water drawn from wells in alluvium, that hazard has been found to exist in other alluvial areas close to sea level. The arsenic is of natural origin and is released when iron hydroxide, the most common sediment colorant and powerful medium for adsorption of many elements including arsenic, breaks down. Iron hydroxide is destabilised in strongly reducing environments, when its component Fe3+ gains an electron to become soluble Fe2+. The most common source of reducing conditions is vegetation buried in alluvial sediments. In Bangladesh and West Bengal, India, the problem is peat layers buried by rapid sedimentation since about 7 thousand years ago that filled channels cut by rivers when sea level was much lower during the ast glacial maximum. The risky areas in the Mekong Delta are more complex (Papacostas, N.C. et al. 2008. Geomorphic controls on groundwater arsenic distribution in the Mekong River Delta, Cambodia. Geology, v. 36, p. 891-894). Areas at risk are strongly focused by recent landforms associated with channel migration, rather than extending across entire flood plains as in Bangladesh. Features such as meander scrolls, point bars and islands that have grown to be incorporated in older floodplains show the highest arsenic concentration in groundwater. These accumulate organic debris in large amounts, whose decay releases arsenic from iron hydroxide veneers on sand grains. Older features of the same kinds show less arsenic contamination in their groundwater, suggesting that eventually either the reductants become exhausted or available arsenic is flushed out. So, careful mapping and dating of fluviatile geomorphology may be a means of screening for arsenic risk in the Mekong and other low-lying delta plains.
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Mantle rock and carbon dioxide sequestration
The peridotite mantle sequence of ophiolites often shows signs of having been altered by processes that form calcite and magnesite (CaCO3 and MgCO3) veins. It is a mundane feature and few geologists have paid it any heed, other than to note the veining. Such theories as there are generally suggest that the veining took place at the time of obduction of the ophiolitic masses onto continental margins, which was generally accompanied by some metamorphism. Nonetheless, the veins must have taken up carbon dioxide from some reservoir, either hydrothermal fluids derived from seawater or groundwater, but ultimately from the atmosphere: there are no primary carbonates in ophiolites. Dating the veins was deemed impossible, but someone had a go at veins in the Oman ophiolite using the 14C method (Keleman, P.B. & Matter, J. 2008. In situ carbonation of peridotite for CO2 storage. Proceedings of The National Academy of Sciences of the USA, v. 105, p. 17295-17300), discovering a great surprise; the veins are very much younger than the Eocene age of ophiolite emplacement. Their ages span 1.6 to 43 ka, about the same as the period over which a surface tufa deposit formed. Calcite and magnesite form by the breakdown of olivine and clinopyroxene in the presence of slightly acid water in which CO2 is dissolved, their young ages suggesting the veins formed during weathering by rainwater, the tufa deposits probably forming through related processes. Keleman and Matter estimated the volume of veins in peridotites exposed in new road cuttings at about 1%. The 15 m thick weathering horizon in the exposed Oman peridotite therefore corresponds to about 1012 kg of CO2, which accumulated at an average rate of around 4 x107 kg of CO2 per year. If this could be increased by 100 thousand times, the Oman peridotite could sequester about 10% of anthropogenic emissions. Is that possible?
Higher temperatures could speed up the carbonation reactions. The reactions are exothermic and sustaining a temperature around 185ºC is feasible by stimulating the reactions through shallow drilling and pumping carbon dioxide and water into shattered rock. Interestingly, the reactions might be capable of limited geothermal power generation. The potential absorption by such a plant in the Oman ophiolite could be up to 1 billion tonnes of CO2, and there are many other ophiolites rich in olivine. But that is not the end of the story: other olivine breakdown reactions involving water generate hydrogen, as discovered by Australian hydrogeologist Gordon Stanger. While conducting his PhD field work in Oman as part of the Open University Oman Ophiolite Project, Stanger discovered natural springs from which hydrogen gas was bubbling (Stanger, G. 1986. The hydrogeology of the Oman mountains. Unpublished PhD thesis, The Open University, Milton Keynes, UK).
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Cycling on Mars
High-resolution remotely sensed data (HiRISE) from the Red Planet is free of charge to registered investigators (it did cost quite a bit to acquire), whereas the Earthly equivalent costing would set you back at least US$25 per square kilometre (for Quickbird. They are wonderfully clear, as Mars’s thin atmosphere causes no haze except during dust storms. They are also in stereo, providing both 3-D views and digital terrain elevation data with a precision of 1 m. HiRISE data have revealed detail equivalent to that from aerial photos of Earth taken from about 5 km above. Not surprisingly, they show a lot of geology, including an area around 500 to 1000 km2 with clear signs of layered sediments (Lewis, K.W. et al. 2008. Quasi-periodic bedding in the sedimentary rock record of Mars. Science, v. 322, p. 1532-1535). Where large craters have exposed sequences in their walls it is possible to measure bedding thickness and count individual strata. In Becquerel crater the layering is very regular, comprising two size ranges around 3.6 and 37 m, the second being made up of several of the first sized layers. The two sets of thickness remain consistent through about 300 m of section, so probably represent cyclical processes on Mars. The most likely driving forces are rotational and orbital, as they are for the Earth’s Milankovich climatic pacing. The 10:1 ratio between the two frequencies of bedding is twice that dominating the Milankovich time series (rotational precession and orbital eccentricity). One possibility for the Martian cycles is the estimated variation of orbital eccentricity on 120 ka, 1.2 Ma and 2.4 Ma timescales, although axial tilt changes through tens of degrees; far more than does that of the Earth’s rotational axis. Thankfully, the authors stick to variations in wind-driven sedimentation to explain the bedding cycles. Changes in insolation on Mars would affect condensation and evaporation of CO2 ice at the poles, and consequently the density of the atmosphere and its ability to move and deposit sediment. Less fortunately, they suggest water must have been involved to lithify the layers. That hardly seems necessary on a planet with low atmospheric pressure, as unconsolidated wind-blown loess in western China maintains the integrity of its layering with little cementation.
Snowball Earth challenged again
Nobody doubts that in the Neoproterozoic there were several massive climate changes that brought frigid conditions to low latitudes. Some demand that the Earth then entered a runaway cooling because the increased albedo cause by continental ice cover would have reflected away a large amount of solar radiation; the Snowball Earth hypothesis is that the entire planet then became icebound. Evidence for the global glacial epochs is in the form of sediments clearly influenced by deposition of debris carried by ice. Later glacial episodes of Late Ordovician and Carboniferous-Permian age left thin tillites – lithified boulder clay – on glaciated land surfaces in northern and southern Africa and other parts of the southern continents, but the main evidence for the much deeper chills of late Precambrian age are thick piles of sediment studded with dropstones from floating ice. These are glaciomarine diamictites as opposed to tillites. Philip Allen and James Etienne of Imperial College, London and Neftex Petroleum Consultants of Abingdon, UK have paid particular attention to the Neoproterozoic diamictites of Oman (Allen, P.A. & Etienne, J.L 2008. Sedimentary challenge to Snowball Earth. Nature Geoscience, v. 1, p. 817-825). These prime candidates for typical products of low-latitude frigidity are over 1 km thick, and therefore require massive supply of precipitation to drive the large ice flows that could transport such large amounts of sediment. Moreover, within the sequence are many sediments that show little sign of glacial influence yet abundant signs of water transport, such as deltaic bedforms. Other strata are marine and contain ripples formed by wave action; a process that would be impossible with total ice cover. Cyclicity is present, as it is in other Neoproterozoic diamictites. That suggests repeated climate change. Snowball Earth aficionados, and others besides, claim just two and possibly three cryogenic episodes in the Neoproterozoic, but Allen and Young point to the wide range of maximum and minimum ages for those diamictites that are amenable to absolute dating. They suggest that, apart from glaciers being able to develop on land at lower latitudes than in subsequent glacial epochs, the late Precambrian was not ‘special, being merely a period of prolonged climate instability akin to those of later times paced by astronomical factors.
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So, when did the core form?
Sometime early in its history the Earth underwent two gigantic redistributions of its chemistry: a gargantuan collision that formed the Moon; separation of a metal plus sulfide core from a silicate remainder. These ‘set the scene’ for all subsequent geological (and perhaps biological) evolution. The current theory about core formation stems from a marked disparity between Hf-W and U-Pb geochronology of the mantle. The first suggests a metal-secreting event about 30 Ma after formation of the Solar System – tungsten is siderophile and would have become depleted in the mantle following segregation of a metallic core. The second points to lead partitioning into a sulfide mass descent to the core around 20-100 Ma later; assuming that lead is chalcophile. The key to explaining the disparity and validating the dual core formation hypothesis lies in establishing just how chalcophile lead is, relative to other metals that are present in the mantle (Lagos, M. et al. 2008. The Earth’s missing lead may not be in the core. Nature, v. 456, p. 89-92). The German and Russian geochemists set up experiments to determine directly the partition coefficients of lead and the other ‘volatile’ elements cadmium, zinc, selenium and tellurium between metal, sulfide and silicate melts at mantle pressures. They found that Pb and Cd are moderately chalcophile and lithophile, but never siderophile; Zn favours silicate melts, and is exclusively lithophile under mantle conditions; Se and Te are both chalcophile and siderophile, so would enter the core in both molten sulfide and metal.
The measured partition coefficients give a basis for comparing the relative proportions of the volatile elements estimated in the mantle with those predicted by the two-event model of core formation. This elegant approach strongly suggests that sulfide or iron-nickel metal segregation from the mantle to the core can explain neither the mantle abundances of the five ‘volatile’ elements nor the lead-isotope ratios in the mantle. It even questions the existence of terrestrial sulfur in the core. The postulated Moon-forming mega-impact alone could have produced the measured geochemical features of the mantle as a result of vaporisation of ‘volatile’ elements.
Mantle heat transfer by radiation
After some early speculation about efficient heat transfer in the mantle by radiation, it became generally accepted that convection and conduction dominate at depth in the Earth. Yet the Stefan-Boltzmann law has the radiant energy flux of a body increasing proportionally to the fourth power of its absolute temperature. So at deep mantle temperatures of up to 4300 K radiation ought to be significant unless mantle minerals become opaque at high pressures. Mantle mineralogy is dominated by iron-magnesium silicates that adopt the perovskite structure. High-pressure experiments with perovskites reveal surprisingly high transparency to visible and near-infrared radiation (Keppler, H. et al. 2008. Optical absorption and radiative thermal conductivity of silicate perovskite to 125 gigapascals. Science, v. 322, p. 1529-1532). It seems that a higher than expected radiative contribution to heat transfer should stabilise large plume structures in the zone above the core-mantle boundary.
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