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.
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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Broody dinosaurs
The most likely ancestors of birds evolved in the Jurassic from a group of nimble and mainly carnivorous theropod dinosaurs known as Deinonychosaurs, which included the now famed Velociraptor. One of the oddest fossils ever found was the skeleton of one of these preserved together with eggs of what were originally thought to have been laid by Protoceratops. This Mongolian animal, seemingly caught in the act, was given the name Oviraptor or ‘egg seizer’. Specimens of Oviraptor and closely related dinosaurs found subsequently show them sitting on eggs; clear evidence of bird-like brooding. If this wasn’t a sufficient surprise, the clutches were enormous: 20 to 30 eggs. Detailed study of the skeletons shows that they are all males (Varricchio, D.J. et al. 2008. Avian parental care had dinosaur origin. Science, v. 322, p. 1826-1828). About 90% of all living bird species involve males in care of chicks, including sharing of incubation (5% of mammals share parental care). However, only among ratites (ostriches and the like) and tinamous do males brood eggs clutches continuously. This behaviour is generally associated with polygamy and large clutches. So the misnamed Oviraptor and its kin were not only progenitors of birds but may well have passed on the peculiarities of avian parenting.
Molecular evidence for the environment of the universal ancestor
If ever there were a ‘holy grail’ for palaeobiologists, it would be the nature and ecology of the original beings from which all life on Earth subsequently evolved. That is, the primitive organism – among perhaps many that were extinguished ‘intestate’ – whose genetic ‘footprint’ alone survived to be common to all three domains of modern life: Archaea, Bacteria and Eucarya. For some time, attention has focused on extant heat-tolerant Archaea and Bacteria species (hyperthermophiles; ³ 80ºC)) found in hot springs, whose genetics seem primitive. This, together with other features such as the adaptation of heat-shock proteins to other functions and the abundance of metals at the cores of other widespread proteins, has led to notions that life originated under high-temperature conditions such as those around sea-floor hydrothermal vents. The ongoing explosion in nucleic acid analysis and software to sift through vast amounts of molecular data from many sources potentially may provide the key to more concrete ideas of the origin of Earth’s life. A recent comparative study of both ribosomal RNA and protein sequences among representatives of all three of life’s domains gives a clue to surprises ahead for palaeobiologists (Boussau, B. et al. 2008. Parallel adaptations to high temperatures in the Archaean eon. Nature, v. 456, p. 942-945). ‘Exobiologists’, who nurture great, but perhaps folorn, hopes of being alive and sentient when extraterrestrial life forms are ‘bagged’ may also find themselves perplexed; such is the fate of hubris without substance.
The team of francophone biochemists claims that their analyses show signs of a two-fold adaptation to changing environments during the earliest period of surviving life. Rather than having emerged from high-temperature conditions, the last common universal ancestor, or LUCA, probably adapted to more temperate conditions (£ 50ºC), the hyperthermophile Bacteria, Archaea and Eucarya evolving from it. Heat tolerance then declined as the later mass of life forms developed. Sadly, the authors do not address the issue of deep ocean-floor origins in their discussion, preferring to speculate about Archaean climate change and rather odd notions about adaptation to high-temperature meteoritic ejection from extraterrestrial sources. It may be that they too are in for surprises when more mature investigations hit the press.
When bacteria became more sturdy
It’s easy for geologists to forget that most of the genetic diversity on Earth is and always has been in organisms that rarely if ever fossilise; those with only a single cell, among the Archaea, Bacteria and Eucarya. All that is known is from those still alive, and they occupy a vast range of environments, most of which are not ‘friendly’ to multi-celled eukaryotes. Unsurprisingly, they don’t look very different from one another; just tiny bags full of water and a tiny amount of complicated biochemistry. They become distinct from their molecular make-up and also from what they do and where they live, some tending to reproduce best within the bodies of eukaryotes, such as ourselves sometimes with no noticeable effect, sometimes beneficially, but most spectacularly when they make us ill. Bacteria and Archaea have long histories, so their genetic material and proteins are easily distinguishable from group to group. This makes them amenable to the use of a ‘molecular clock’ approach in seeking out when and how they evolved. Analysis of these differences among more than 250 species of bacteria in the context of their living in water or under terrestrial conditions has thrown up some surprises (Battistuzzi, F.U. & Hedges, S.B. 2008. A major clade of prokaryotes with ancient adaptations to life on land. Molecular Biology and Evolution, doi:10.1093/molbev/msn247). Two thirds seem to stem from a common ancestor that had colonised the land around 3.2 Ga ago, 800 Ma before preservation of the first undisputed fossils. To live on the continental surface, all have to have evolved or inherited resistance to environmental hazards such as drying out, UV radiation and high salinity. Many pathogenic bacteria belong to the Gram-positive group, whose cell walls are distinctly adapted to terrestrial life. Despite having to live in eukaryote-free world for a billion years or more, their ancestors were especially well-suited to infesting multi-celled life when it emerged, and to being notoriously adaptable when they are threatened with toxicity themselves.
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At last, 4.0 Ga barrier broken
Since the 1960s when Stephen Moorbath of the University of Oxford determined a date of 3.8 Ga for metamorphic rocks in West Greenland discovered by Vic McGregor of the Geological Survey of Greenland, pushing the age of tangible rocks towards that of the Earth itself has been slow. Indeed, geologists found only one geological terrain that pushed the ‘vestige of a beginning’ significantly back in time beyond the famous Isua rocks: the Acasta Gneiss east of Great Slave Lake in northern Canada, dated at 30 Ma more than 4 Ga. In fact in the 30 years between Moorbath’s Greenland date and that for the Acasta Gneiss, stratigraphers seem to have become resigned to a maximum 3.8 Ga age for rocks, and the start of the Archaean was set at that age. All earlier time, some 750 Ma of it, became known as the Hadean – a hellish time from which nothing had survived. Some geochemists perked up with the discovery, sifted from a much younger sandstone in the late 1980s, by Australians Bill Compston and Bob Pidgeon of 17 zircon grains that formed up to 4.4 Ga ago; but they tell us very little about the early world. What had become the lost cause of seeking pre-4 Ga rocks, has suddenly become revitalised with the discovery of a voluminous suite of rocks that are 200 million years closer to Earth’s origin in the eastern part of Arctic Canada (O’Neil, J. et al. 2008. Neodymium-142 evidence for Hadean mafic crust. Science, v. 321, p. 1828-1831).
The rocks are part of a recently mapped greenstone belt on the east shore of Hudson Bay, which contains a variety of mafic igneous rocks along with metasedimentary banded iron formations and cherts. The most dominant of the mafic rocks has yielded a 146Sm-142Nd isochron age of almost 4.3 Ga, and they are intruded by mafic and ultramafic sills dated at around 4.0 Ga. The older meta-igneous rock’s geochemistry suggests that it formed by partial melting of undepleted mantle rocks to produce magmas similar to those forming at modern convergent plate margins. Its major element variability, reflected in very diverse metamorphic mineral assemblages, suggests it to have originally formed as a mafic pyroclastic rock. It would be hard to prove that the BIFs and cherts are the same age in such a structurally complex belt, but that they are as old as the dated material is a distinct possibility. In that case they push back tangible evidence for surface water a great deal more convincingly than the arcane isotopic evidence derived from the oldest known zircons (see Zircon and the quest for life’s origin in the May 2005 issue of EPN). That such a substantial piece of very old crust has turned up a record age owes a great deal to advances in the Sm-Nd dating technique; the use of 146Sm decay to 142Nd (1/2 life of ~108 years), rather than the more readily addressed 147Sm to 143Nd decay (1/2 life of ~1011 years). This proof of concept may unleash a reappraisal of rocks that seem to be the oldest relative to others in Precambrian shields on every continent. It may eventually become possible to show that, apart from its cataclysmic experience that formed the Moon and probably a global magma ocean shortly after accretion, the Earth was by no means a totally hellish period during the ‘Hadean’.
Banding in BIFs
Banded iron formations, or BIFs, from the late Archaean and early Proterozoic are made of interlayered accumulations of iron oxides (and occasionally sulfides) and chert, and are the world’s most important iron ores. The BIFs of the Hammersley Range in Western Australia produce 26 % of the western world’s iron ore, and are hundreds of metres thick. The banding extends down to the scale of a few micrometres, and in some cases seems to record cyclic events. It has been claimed that, sun-spot, tidal, Milankovich and other nature cycles can be discerned. Few dispute that the iron oxides formed by oxidation of dissolved iron(II) ions through the influence of micro-organisms in shallow seawater. A popular candidate is photosynthetic blue-green bacteria, which produce oxygen; abundant reduced iron dissolved in Archaean seawater would have consumed the oxygen to become insoluble iron (III) oxides, delaying the development of an oxygen-bearing environment util about 2.2 Ga. There are other possibilities, such as anoxygenic photosynthesising bacteria, or photoferrotrophs, that could have achieved the Fe(II) to Fe(III) oxidation directly, without the need for free oxygen.. The puzzle is the on-off mechanism needed to produce the banding itself. That may have been resolved by experimental work under simulated Archaean conditions (Posth, N.R. et al. 2008. Alternating Si and Fe deposition caused by temperature fluctuations in Precambrian oceans. Nature Geoscience, v. 1, p. 703-708). The authors based their experiments on primitive, but living photoferrotrophs in conditions that chemically mimic likely Archaean seawater. They discovered that the critical factor in this form of biogenic precipitation of iron is sea-surface temperature: the microbes reproduce fastest to maximise iron-oxide formation at 20-25ºC. Temperatures above or below this range shut down productivity. However, temperatures above 25ºC favour silica remaining in solution, so the alternation of Fe- and Si-rich bands favours cooler sea temperatures for the latter. As well as providing a means of producing the enigmatic BIF banding, the experiments help resolve the controversy over prevailing sea-surface temperatures in the Archaean, which have been suggested by some to be as high as 85ºC. At least for the late Archaean, ocean temperatures seem to have been much the same as at present.
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Two Archaean birds with one stone
There are two major issues concerning the Archaean mantle: was the mantle hotter than it is now; was it in a reduced or oxidised state? The first has implications for Archaean plate tectonics. If loss of the higher radioactive heat produced in the mantle was accomplished by processes similar to those today, i.e. dominantly by mid-ocean volcanism the Archaean geotherm would have been similar to today’s and plate tectonics would have been similar. If this means of heat loss could not cope, then temperatures would increase more rapidly with depth, with implications for the style of plate tectonics, especially subduction. A mantle with reducing conditions would be expected to emit reduced gases, such as methane, as well as carbon dioxide, to produce a reducing atmosphere. If oxidising conditions prevailed, then CO2 would be a dominant emission to the atmosphere. There have been arguments over these two aspects of the Archaean for decade, but now they may have been resolved (Berry, A.J. et al. 2008. Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle. Nature, v. 455, p. 960-963). One factor alone allowed the arguments to damp down: A 2.7 Ga ultramafic lava flow from Zimbabwe preserved a pristine sample of the original magma in the form of small glass blobs trapped in olivine. Measured proportions of Fe(II) and Fe(III) in a melt indicate those in its source, and hence the redox state of the source, mantle peridotite. The Zimbabwe melt inclusions are similar in this respect to those found in modern mid-ocean ridge basalts; they show a high degree of reduction. In turn that suggests that the melting that formed them was almost anhydrous, otherwise dissociation of water would have added oxygen that would have upped the content of Fe(III) in the melt. Experiments show that the degree of anhydrous partial melting of peridotite needed to form ultramafic magma is compatible only with temperatures around 1700ºC, about 400 degrees hotter than those that form modern basalt magma. Significant volumes of the late Archaean mantle, and by extension that of earlier times, had to have been a great deal hotter than it is today.
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Early, microscopic evidence for human control of fire
Which human species first controlled and used fire has been debated for as long as archaeologists began to realise we had a long and complex ancestry. Because sites can easily be contaminated by charcoal from natural fires it has been difficult to present convincing evidence. But there is a way to get believable data. Stone tools and fragments from their manufacture may have fallen in fires set by hominins, and show changes caused by intense heating. One such example comes from a long-occupied site in Israel (Alperson-Afil, N. 2008. Continual fire-making by Hominins at Gesher Benot Ya‘aqov, Israel. Quaternary Science Reviews, v. 27, p. 1733–1739). Nira Alperson-Afil of the Hebrew University of Jerusalem investigated small flint artefacts, probably flaked off during tool making, from eight levels excavated at the site. In all of them some flint shards showed signs of extreme heating, such as discoloration, crazing and tiny bowl-shaped pits (‘potlids’) resulting from exfoliation of hot flint surfaces. The features are reproduced by experimental heating of flint shards, and do not occur in those that have not been heated above 300ºC.
Gesher Benot Ya‘aqov was first occupied around 790 ka, by Homo antecessor, and the excavation levels may span around 100 ka. The site is the earliest to provide convincing evidence not only for the use of fire, but that it was a continuous part of the hominins’ culture: they could make it at will. Alperson-Afil suggests that fire making may have been an integral part of the Acheulean culture, well known for finely crafted biface axes, since its inception around 1.6 Ma ago. Ambiguous evidence for hominin fire use, such as burnt bones and reddened sediments, has been found at several sites in Africa dated between 1 and 1.5 Ma. Alperson-Afil’s meticulous micro-forensics should help African archaeologists and those working at very old sites left by migrating hominins in Georgia and Asia to check whether fire has such a long-lived place in our evolutionary history.
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Evidence for past tsunamis
Since the Indian Ocean disaster of 26 December 2004, coastal areas world wide are increasingly examined for signs of past tsunamis. Much the most common focus is on large boulders on low-relief shorelines never subject to glaciation. On the Bahamas large blocks of coral scattered above sea level suggest past tsunamis perhaps caused by collapse of volcanoes on Atlantic islands such as the Canaries or Azores. Yet, ordinary storm waves, if focused by coastal inlets can literally blast large boulders from well-jointed outcrops and carry them hundreds of metres inland. So peculiar boulders on a coast do not necessarily show that a tsunami once struck, although many around the shores of eastern Britain may well have been dislodged by tsunami triggered by a submarine landslide off western Norway about 7 thousand years ago. In an attempt to get more reliable signs of past tsunamis, the devastated coasts of northern Sumatra and western Thailand have been searched for tangible signs of the 2004 event (Monecke, K. et al. 2008. A 1,000-year sediment record of tsunami recurrence in northern Sumatra. Nature, v. 455, p. 1232-1234. Jankaew, K. et al. 2008. Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand. Nature, v. 455, p. 1228-1231).
Both teams homed in on boggy depressions or swales between fossil beach ridges on broad low-lying shores. There, debris carried by the huge 2004 waves could be trapped and then preserved by regrowth of vegetation. The generally low energy in the swales is also likely to prevent erosion, so that deep superficial sediment can build up that may preserve signs of past tsunamis. This focus paid dividends, in the form of coarse sand just beneath a regrown vegetation mat, with distinctive signs that the sand had been deposited by transport from the seaward side of swales. Coring and trenching then unearthed deeper, older sands with exactly the same structure. The surprise was the antiquity of the tsunami sands: layers carbon-dated around 1300-1400, 780-990 AD and 250 BC. Clearly, more extensive surveys of this kind are necessary wherever coastal conditions permit good preservation. That would give an idea of the periodicity of earthquakes and landslips energetic enough to produce coastal catastrophes around major ocean basins. Yet there is a danger: if, as suggested by the Thai and Indonesia data, several centuries have lapsed between such dreadful events, it presents an excuse not to install costly monitoring devices or permanently shift coastal townships to foretell or prevent future disasters.
See also: Bondevik, S. 2008. The sands of tsunami time. Nature, v. 455, p. 1183-1184.
Chinese PM is a geo
Like me, many EPN readers may have admired the swift, effective and open response of the government of the People’s Republic of China to the Szechuan earthquake disaster in May 2008. They may also be surprised to learn that Wen Jiabao, the prime minister of the PRC, is geologist who worked for 14 years with a provincial geological survey. To read an abbreviated transcript of a dialogue between the editor of Science and Wen Jiabao was refreshing, and quite probably unique in a world where most senior politicians are, to say the least, not science-savvy (Alberts, B. & Jiabao, W. 2008. China’s scientist premier Q&A. Science, v. 322, p. 362-364; full transcript at www.sciencemag.org/cgi/content/full/322/5900/362/DC1).
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Does glaciation preserve the Tibetan plateau?
At first glance this section’s title seems absurd, for glaciation has the highest potential for erosion that there is on Earth. Yet it seems that at the eastern edge of the Tibetan Plateau the long-term potential for river erosion has been impeded by glacial action (Korup, O. & Montgomery, D.R. 2008. Tibetan plateau river incision inhibited by glacial stabilisation of the Tsangpo gorge. Nature, v. 455, p. 786-789). The accepted wisdom is that in the course of powerful rivers, such as the Tsangpo, steep stretches or ‘knick points’ focus erosion that proceeds headwards to drive a wave of dissection towards the sources of the main river and of all its tributaries. The Tsangpo has had the better part of 40-50 Ma since the India-Asia collision to eat away the vast Tibetan Plateau, but it has failed, as have other, lesser river systems. Repeatedly emplaced moraine dams, seem to have locked the knick points associated with the Tsangpo catchment at around 260 separate locations.
See also: Owen, L.A. 2008. How Tibet might keep its edge. Nature, v. 455, p. 748-749.
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