Is weathering due to the weather?

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The name ‘weathering’ has always been taken to indicate a direct relationship between the atmosphere and the breakdown of rocks, i.e. at or very close to the surface. This is so easy to test that it comes as a surprise to find that nobody has really tried until recently (Yokoyama, T. & Matsukura, Y. 2006. Field and laboratory experiments on weather rate of granodiorite: Separation of chemical and physical processes. Geology, v. 34, p. 809-812). Tadashi Yokoyama and Yukinori Matsukura of universities of Osaka and Tsukuba, Japan, placed small cut tablets of identical fresh granodiorite in three position: at the surface, buried above the water table and buried beneath the water table in one small catchment. These samples stayed there for 10 years. The only sample to show much sign of chemical breakdown of minerals was that buried below the water table. Does anyone claim that there is weather in groundwater? Just exposing fresh granodiorite in the laboratory to a constant flow of water chemically similar to the groundwater doesn’t accomplish the weathering (it is 50 times slower than when samples are buried). Chemical weathering needs to involve soaking, when grain boundaries break down so that individual grains can become detached and allow yet more penetration.

Most geoscientists who work on topics that involve chemical weathering, such as the changing release of tracer isotopes of strontium to estimate rates of weathering in the past, assume that it is all done by atmospheric carbon dioxide dissolved in rainwater or released by organisms in soil. It is accomplished by hydrogen ions that can be released by a great deal more processes than the formation of vary weak carbonic acid (e.g. organic acids and breakdown of sulfides). It now seems very clear that chemical weathering is a product of groundwater and burial, so should we call it weathering at all?

Calibrating the deepest ice core

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Although the ice that makes up the upper parts of the Greenland and Antarctic ice sheets is annually layered, for times before about 70 ka the layering disappears because of plastic deformation. Earlier ages have to estimated from models of the deformation, and a second check is to match the data records from ice cores against those from sea floor sediments. Different processes contribute to those records: for instance, the marine record of oxygen isotopes in benthonic forams tracks the changing volume of ice locked on land, while the same record from ice cores depends on the air temperature above the ice cap. The correlation does seem to work, however. But not, it seems, for the very deepest ice recovered from beneath Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) which extends to around 800 ka.

French scientists involved in the EPICA Dome C ice-core project have cunningly discovered a means of checking on the otherwise undateable deep Antarctic ice (Raisbeck, G.M. et al. 2006. 10Be evidence for the Matuyama-Brunhes geomagnetic reversal in the EPICA Dome C ice core. Nature, v. 444, p. 82-84). The core penetrated to an estimated time that should include the most recent magnetic reversal, dated very precisely to 778±2 ka. Although the exact details of how the magnetic field behaved during this reversal, it is known that when its polarity flips the intensity of the field becomes very small. While the field is stable it is sufficiently strong to deflect charged particles, both in the Solar wind and in cosmic rays, so that less pass through the atmosphere. Cosmic rays are so energetic that they can perform isotopic transformations, one product being 10Be. So if the magnetic field decreased so the proportion of 10Be in the atmosphere would go up. Raisbeck and colleagues have examined the 10Be record in the EPICA core in great detail. In a 10 m thick section from a depth of almost 3.2 km the isotope rises to a peak, which they interpret as the signature of the reversal. If correct, this gives a ‘golden spike’ against which the depth to age conversion can be refined.

Balmy shores of the Precambrian

Before the appearance of fossil organisms that could give clues to past climates the only sources of information take the form of proxies. One of the best examples might seem to be the oxygen isotope composition of carbonate rocks that relate to sea-surface temperature. In fact it isn’t useful for the Precambrian because estimates of SST depend on being able to identify the shells of planktonic animals and use their d18O as a proxy. That is a pity, because limestones are common throughout the geological record and various aspects of their geochemistry have been used extensively as proxies for other crucial information, such as the relationship between their strontium isotope composition and the pace of continental weathering. Another palaeothermometer relies on the same temperature dependent fractionation of oxygen isotopes between seawater and the precipitation of dissolved silica to form cherts, whose d18O decreases with temperature. The trouble is that silica is notoriously prone to being remobilised and reprecipitated as pH changes in the fluids within sedimentary rocks. Some results from Precambrian cherts gave such low d18O that seawater temperature would have been tens of degrees higher than they were during the Phanerozoic, but they have been wisely suspected of having been affected by much later alteration by warmer fluids passing through cherty sequences. Now the approach has been given a boost by geochemists at the French National History Museum (Robert, F. & Chaussidon, M. 2006. A paleaotemperature curve for the Precambrian ocean based on silicon isotopes in cherts. Nature, v. 443, p. 969-972).

François Robert and Marc Chaussidon analysed the silicon isotopes in cherts for which oxygen isotope data are available. Since the two isotopic systems would both change, yet would behave differently during hydrothermal or metamorphic alteration, if the results correlate well both should be undisturbed. Except in samples that show the lowest d18O values (i.e. highest temperatures) there is a good correlation. That finding validates many of the O-isotope seawater temperatures, but Si isotopes fractionate during precipitation too, again in relation to temperature. So Robert and Chaussidon take Precambrian ocean temperature data to a new level with estimates based on two methods. Their results are fascinating: as well as confirming a decline from around 70°C 3.4 Ga ago to between 10 to 40°C in the Phanerozoic, the d30Si data show sharp downward ‘spikes’ at about 2.5 Ga and 1.8 Ga. Between about 1.5 Ga to 600 Ma ocean temperature was steady at around 20°C, so there is no sign of continually cold oceans through the period of ‘Snowball Earth’ events – the number of samples cannot yet resolve the individual events, but the ‘Cryogenian’ is an obvious target for more work. The data are also important as they hint at all kinds of possible biological outcomes for such global warmth, and explanations are definitely needed.  Does the record suggest greater geothermal heating, or was it an outcome of the greenhouse effect? Will more details show periods of changing burial of organic carbon? Whatever, the Precambrian has become a stranger world to contemplate.

See also: de la Rocha, C.L. 2006. In hot water. Nature, v. 443, p. 920-921.

Fossil bee: the right place and the right time

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Amber from a mine in Myanmar generates a steady income from sales to palaeoentomologists, each bead of the lithified resin being a possible lagerstätte in its own right. Two scientists at Oregon State University and Cornell were fortunate enough to find a small, Early Cretaceous bee that is so well-preserved as to show even the leg hairs on which bees carry pollen (Poinar, G.O. & Danforth, B.N. 2006. A fossil been from Early Cretaceous Burmese amber. Science, v. 314, p. 614). Indeed, the hairs carry several grains of pollen. This is the oldest known bee by more than 35 Ma, and it coincides with the start of the explosive radiation of flowering plants.

So, farewell planet Pluto…

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One theological mode of discourse is casuistry, best known for disputing the number of angels who can sit on a pinhead. Amongst astronomers, at least those who meet every three years at the General Assembly of the International Astronomical Union (IAU), this form of sophism crops up from time to time.  It does too among geologists, and probably more often, as they have a many things to argue about. At 13.32 GMT on the 24th of August the 26th GA of the IAU in Prague upset a great many people by casting Pluto, formerly known as Planet Pluto, into the indignity of dwarf-planet status. NASA may be well-miffed, as their New Horizon probe has been on its way there since mid-January 2006.

The issue of Pluto’s status popped up after a larger Sun-orbiting object was announced in 2005 (2003 UB313), which, like Pluto is beyond the orbit of Neptune. That new body is the largest known in the dim and distant Kuiper Belt, and Pluto may well be a stray from that region, having a very odd orbit. IAU decided, somewhat late in its existence, to define ‘planet’. Committees were appointed. The primary criterion decided by the final committee to report to IAU was that planets need to orbit the Sun, not another bigger planet. Second, they have to have sufficient mass for their gravitational force to make them nice and round. Sadly, it seems that the committee made quite a gaffe. In order to distinguish trans-Neptunian planets that take more than 200 years to orbit, they suggested the term ‘pluton’ (oh dear). Whatever, that would give the Solar System 12 planets: trans-Neptunian Pluto, Charon (in binary orbit with Pluto) and 2003 UB313; and Ceres, formerly just the largest asteroid known. But the Kuiper Belt might easily have lots of other massive and round objects in it, awaiting discovery. So, has the old Jesuitical mind-expanding exercise been ‘larged-up’? Probably not, in a strictly scientific sense, because additional criterion for planetary status, added by the 26th GA of the IAU, is that one should be massive enough either to have ‘swept’ its orbit clear of minor bodies early on, or to have flung them far away. Since Pluto and Ceres have done neither, they are officially to be considered ‘of diminished stature’. Some worry that traumatised children, fond of Pluto, will be driven from an interest in science. Who knows? But if IAU persists in the name ‘pluton’ as a sop to public opinion, there will be trouble…

Climate moves mountains

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Several times in Earth Pages News the topic of how erosion contributes to uplift has cropped up. That is more than just the iceberg-like bobbing up of the crust as the load on the underlying asthenosphere is eased by surface rock removal. One oddity is that as large valleys are carved the ridges and peaks that they separate can rise higher than the original lands surface from which they developed (see Erosion and plate tectonics in May 2005 issue of EPN). Now it is becoming clear that sideways movement of the crust beneath mountain ranges can also be a response to erosion; thrusts and nappes can respond to erosion as well as to plate tectonic forces. The most likely place where this might be happening is in the Himalaya, which produce a huge contrast in climate and erosion rate between their southern and northern sides by creating the world’s largest rain shadow. The evidence for this possibility is nicely reviewed by Kip Hodges of Arizona Sate University (Hodges, K. 2006. Climate and the evolution of mountains. Scientific American, v. 295 (August 2006 issue), p. 54-61).

The highest erosion rates take place where rainfall during the Indian monsoon is greatest, on the SSW face of the Himalaya, especially in the foothills between about 1000 and 3500 m. The Tibetan Plateau lies in the rain shadow of the Himalaya, and erosion is far less intense. Yet the Tibetan plateau is buoyed up by crust that is double the normal thickness, to an average elevation of around 5 km. In a crude way Tibet can be regarded as having a pressure head ‘dammed’ to the north of the Himalaya. Intense erosion at the foot of the mountain ‘dam’ is likewise akin to one cause of landslides: erosion of the toe of a slope. The gravitational potential of Tibet, combined with continual undermining of the Himalayan front must create a lateral force. Where the crust is able to behave in a plastic fashion, i.e. at depth, and if there are surfaces on which movement is possible — the north-dipping frontal thrusts of the Himalaya — then deep crust should be extruded sideways. In fact there are faults systems just to the north of the Himalaya that have the same dip as the thrusts, but an opposite sense of movement, directed northwards to create extensional detachments. The crustal zone in-between is the most likely to undergo extrusion. GPS measurements there and cosmogenic dating of the surface reveal that indeed this zone is experiencing  anomalously high rates of uplift. It is producing extremely high gradients on both hillsides and valley floors.

Threatening Earth

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The US Geological Survey has recently launched its Natural Hazards Gateway at www.usgs.gov/hazards to give access to data and educational material on volcanoes, landslides, hurricanes, floods, earthquakes, tsunamis and wildfires. The coverage is global, naturally with a great deal on the US. The links within USGS and to other agencies are comprehensive. When USGS sets out its stall, it groans with produce.

The gold bugs defence

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Australia is rightly famous for its gold nuggets and some, such as the ‘Golden Eagle’ found at Coolgardie, were as big as a gap-year’s rucksack. The curious thing about them is that they are generally found in the most featureless parts of the continent, Western Australia being a case in point. What sharpens the paradox is that these flat areas have been peneplains for up to a billion years. A nugget found in a Yukon or Californian stream is easily attributed to high-energy transport in water, and indeed most of those show signs of long transport in water: they are rounded and pitted. The one kilogram and weightier nuggets from Australia could never have been physically moved across the featureless plains, and most of them come from the alluvium deposited by sluggish Cenozoic drainages, now as dry as a bone — the ‘deep leads’ famous for their gold rushes in the past. They are also oddly shaped, the ‘Golden Eagle’ having wing-like flanges, which any physical transport would bend into conformity, for gold is of course very malleable. One long-held hypothesis is that they formed by precipitation from the extremely noxious groundwater that still persists tens of metres beneath the surface, gold being water-transportable in the form of complex ions such as those involving Au and Cl­. But it now seems that the mediator is bacterial in origin (Reith, F. et al. 2006. Biomineralization of gold: biofilms on bacterioform gold. Science, v. 313, p. 233-236).

Frank Reith and his Australian colleagues collected soils that contain small gold grains from goldfields across the continent. A great many have strangely knobbly surfaces and branching structure when scanned under an electron microscope, whereas fine gold grains from primary deposits in hard rock often shows signs of gold’s crystal symmetry, or at least highly angular surfaces. The soil-gold particles do look as though they formed in association with living processes. Using stains that fluoresce when bonded to organic matter the researchers found numerous associations between gold and organisms of some kind. When organic material was leached from separated gold grains it revealed DNA closely similar to a bacterium that is known experimentally to precipitate gold from dissolved Au-Cl  complexes. Ordinary soil grains showed no such genetic tracers. It looks as if Reith et al. have discovered living biofilms coating the gold grains that the constituent bacteria are in the process of growing. Amazingly, they also found gold-plated living bacterial cells. The probable explanation is that the bacteria live in water so rich in gold (by no means a great deal of it, however) that they are defending themselves from gold’s known toxicity — Ralstonia metallidurans, as its Latin name suggests, is a highly metal-tolerant organism. Nuggets may well form as a result of bacterial defence mechanisms.

‘Peace’ (Selam) disturbed

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The Afar Depression of Ethiopia, especially the middle reaches of the Awash River, has become world renowned as the cradle for early humanity. After the revolutionising discovery in 1974 in the Hadar area of the 3.3 Ma old Australopithecus afarensis remains that became known as ‘Lucy’, other finds – Ardepithecus, Orrorin and Sahelanthropus hit the headlines, pushing back the age of possible human ancestors to almost 7 Ma. None of these had Lucy’s degree of preservation, and the vital issue for the origin of humanity – bipedalism – could only be addressed by scanty evidence about the position of attachment of the cranium to the spine. Much else had to be inferred from teeth and facial shape, and odd bits of long limb bones. Lucy and remains of other A. afarensis individuals that rain progressively washes from the badlands of Hadar provide an embarrassment of riches by comparison. There is little doubt that could walk upright, but a question that has lingered is whether or not it also clambered habitually in trees. The other missing information is the vital one of development, for one big difference between apes and us is the grossly extended infancy of modern humans during which the attributes of consciousness, language and much else that is unique arise. To get a grip on developmental issues demands near-complete juvenile remains. The oldest infant fossils that come close are those of a Neanderthal child from 100 ka ago. A dramatic paper (Alemseged, Z. et al. 2006. A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, v. 443, p. 296-301) brings the spotlight back to Middle Awash and to A. afarensis.

The drama has been long in the making. Zeresenay Alemseged, an Ethiopian working in Germany, made the initial find in 2000, collecting more material and painstakingly exposing bones from their sandstone matrix, grain by grain, over the last 5 years. The skull and dentition are complete, and bar the pelvis, lower spine and some limb bones, so is the rest of the skeleton. Morphology points unerringly to A. afarensis, and the stratigraphic position is the same as that entombing ‘Lucy’. Even without the inferences that can be drawn from it, preservation of a complete body is a near-miracle that ranks with that of the ‘Turkana Boy’ (H. ergaster) and ‘Lucy’. The entombing sediments are those of a small stream, which discharged to a large lake that occupied parts of the Middle Awash area during the Pliocene, so that the body was quickly enclosed in fine sands, possibly after the child was washed away in a flash flood. The jaws contain adult teeth waiting to erupt and, by comparison with chimpanzees, they suggest an age at death of about three years, although comparison with human children would probably give an older estimate. The shape of the adult teeth is similar to those of female, so the infant is a ‘she’. Much more work needs to be done on ‘Selam’ (Peace in Amharic), but that reported so far bears strongly on the issue of bipedalism. The shoulder blades and semi-circular canals of the ear, on which balance depends, are ape-like, and a finger bone is curved like that of a chimpanzee. ‘Selam’ was equipped for climbing, but she has leg and foot bones with more human affinities, which would enable upright walking as well. Being a near-complete individual, ‘Selam’ can be compared with whole adult A. afarensis specimens, notably ‘Lucy’, and with modern apes and humans, to assess the crucial issue of development that should throw light on just how close the species was to a transition to the human species that arose about a million years later.

Interestingly, the same issue of Nature includes a mini-biography of the Tunisian-born geologist Maurice Taib. He was the first to work on the terrestrial Pliocene sediments of the middle reaches of the Awash River, thereby opening to road to palaeoanthropolical fame for the likes of Don Johanson, Tim White and two generations of Ethiopian scientists, whom Taib played a major role in training and encouraging (Dalton, R. 2006. The history man. Nature, v. 443, p. 268-269).

See also: Wood, B. 2006. A precious little bundle. Nature, v. 443, p. 278-281. Wynn, J.G. et al. 2006. Geological and palaeontological context of a Pliocene juvenile hominin at Dikika, Ethiopia. Nature, v. 443, p. 332-336.

Drying East Africa

The 7 Ma recorded history of humans and their hominin ancestors was almost exclusively East African, until early members of the genus Homo began to migrate in pulses after about 1.8 Ma. Exodus from Africa on several occasions has been linked with climate change or the opening of routes by falls in sea level during periods of massive ice accumulation at high northern latitudes. Likewise, the evolutionary adoption of a bipedal gait by formerly forest-dwelling apes was probably driven by climate change that saw the spread of more open savannah ecosystems. Records from fossil assemblages in river and lake-bed sediments of East Africa, and from pollen in nearby sea-floor sediments do show a reduction in woodland cover and a spread of grasslands since the Upper Miocene (6 to 8 Ma) – the period of hominin adaptive radiation. Most workers on African climate change in the Neogene attribute the shift to cooling, either through a fall in atmospheric CO2 or the onset of Northern Hemisphere glaciation. Yet East Africa has its own engine for climate and ecosystem change: the formation of the great Rift system and the uplift associated with it. While recognised as a climatic influence tectonics in the region has been downplayed by comparison with global shifts. That is surprising, since in the last 20 Ma, and perhaps more recently, what was an area of low relief has been transformed while rift shoulders rose to more than 3 km, from Eritrea in the north to Malawi 6000 km to the south.

Before rifting began, flood volcanism poured out a basaltic veneer in the late Eocene to mid-Oligocene, to achieve a thickness of more than 2 km in Ethiopia. Rather than creating high ground the flood basalts, being denser than continental crust, probably caused subsidence that roughly maintained low surface elevations. The achieved their present high elevations in the Ethiopian Plateau no earlier than the late Miocene. Large plateaux deflect low altitude winds and seem certain to have influenced climate on a regional scale, as did the Tibetan Plateau. The timing and pace of East African uplift remains poorly constrained, partly because geological evidence shows highly episodic tectonics, with periods of seeming quiescence between episodes of extensive and profound faulting and uplift, and partly because many of the rocks involved are sparsely dated. Yet the present topography and geological infrastructure are sufficiently well known that modelling any morphological influence on climate is possible.  By considering several plausible tectonic scenarios, a team of French palaeoclimatologists have modelled the possibilities (Sepulchre, P. et al. 2006. Tectonic uplift and eastern Africa aridification. Science, v. 313, p. 1419-1423). Their models show that uplift may have shifted atmospheric circulation drastically to establish the strong seasonality that dominates the region nowadays. Applying their results to likely ecosystems results in a pattern of decreased tree-cover.

While convincing, Sepulchre and colleagues’ work demands more precise timing for the establishment of sufficient tectonic topography. Nevertheless, it shows that events, arguably beginning at the core-mantle boundary, that triggered East Africa’s dominant tectonic influence, the Afar plume, probably conditioned our own eventual emergence.

A lot closer in time is an analysis of climate change in the Eastern Sahara desert since the end of the Younger Dryas (<12 ka) that devotees of the ‘English Patient’ will find revealing (Kuper, K. & Kröpelin, S. 2006. Climate-controlled Holocene occupation in the Sahara: motor of Africa’s evolution. Science, v. 313, p. 803-807. Being based on 150 archaeological excavations, the account of sudden humidity after 8.5 ka and then slow aridification since 5.3 ka is persuasive background to the rise of the pharaonic kingdoms of the Nile once nomadic Saharan pastoralism slowly became impossible.

Asian migrations reviewed

Sometime between 100 and 60 ka, fully modern humans found their way from Africa to the Far East and beyond. The timing and the issue of how many migrations were involved are topics in turmoil, now that genetic analyses help trace linkages among modern people. That was semi-popularised by Steven Oppenheimer’s The Peopling of the World (2003, Constable, London), which remains the genetically based ‘straw man’ of human migrations. Like Oppenheimer, Paul Mellars also of the Dept of Archaeology at Cambridge University, argues for single exodus and rapid eastward dispersal, but leaves open the route either via the Straits of Bab el Mandab or through Mesopotamia (Mellars, P. 2006. Going East: new genetic and archaeological perspectives on the modern human colonization of Eurasia. Science, v. 313, p. 796-800). While genetic lines of descent are a most powerful tool, any conclusions need confirmation through ‘hard’ evidence from excavations, and both Arabia and the India subcontinent are irritatingly blank in that regard.  However, there are a few coastal sites that whet the appetite.  As Jonathan Kingdon first suggested, in Self-made Man and His Undoing (1993, Simon and Schuster, London), the most likely routes for migrants would have been along the shoreline. ‘Strandlopers’ would have had easy pickings from littoral food sources, even during periods of aridity related to global cold spells. But there is the problem: with sea levels well below the present ones, most truly ancient sites will now be hidden below the sea. As regards the route taken, much depends on what the Nile valley has to offer archaeologically, for that is the natural way to the eastern Mediterranean and access to the Arab Gulf either across Syria or skirting the mountains of Kurdistan. The route across the Red Sea already has excellent support by the discovery by the Gulf of Zula in Eritrea of abundant evidence for habitation by ‘strandlopers’ around 100 ka.

Detecting and mapping ancient soils

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During the early Cenozoic, and perhaps before that, huge areas of the exposed continental surface were subject to hot humid climatic conditions. That broke down every conceivable rock type to a few simple minerals that were both stable and insoluble. Such intense weathering possibly affected 30% of the land area during those ‘hothouse’ times. Where the surface was flat, the resulting residual soils were preserved to form laterites, strongly layered mineralogically. Since one of the common components is bright-red hematite, and its brown hydrous equivalent goethite, and another is brilliant white kaolinite, laterites are also stunningly layered in colour from white iron-poor clays at their base through an middle mottled yellow, orange, pink and white zone, to brick-red iron-rich ferricrete at the top of the sequence. No-one can fail to see laterites where they are exposed, but few geologists have set out to understand them. A recent paper provides a clear guide to begin that work on a grand scale, and also to chart where their unique properties and socio-economic pros and cons can be developed or avoided respectively (Andrew Deller, M.E. 2006. Facies discrimination in laterites using Landsat Thematic Mapper, ASTER and ALI data—examples from Eritrea and Arabia. International Journal of Remote Sensing, v. 27, p. 2389–2409).

The key to the long and complex chemical and mineralogical evolution of laterites lies in the different layers or facies in these palaeosols. Because they are thin and once present over vast areas of Africa, South America, India and Australia, their presence or absence today is a guide to the history of erosion and intraplate deformation after they formed. Each facies has very different chemical and physical properties, some advantageous, and some decidedly a threat of some kind, recognised and well documented by M.E. Andrews Deller of the British Open University. For instance, the clay zone is a lubricant that can encourage landslides of great thicknesses of overlying rock, yet is a potential resource — it is China Clay. Hard and porous ferricrete, containing both iron minerals and clays, makes it a cheap source of bricks and even road aggregate. But hematite can pose a frightening risk. Its open structure soaks up dissolved ions, including infamously those of arsenic, which lateritisation can set in motion from the rocks on which it develops. Hematite dissolves under reducing conditions, and should those develop on old laterites arsenic might be liberated to groundwater. Another associated compound that laterites can release is magnesium sulfate (Epsom Salts), an natural emetic but also a potential remedy for eclampsia that threatens mothers and their babies throughout laterite-mantled Africa.

Andrews Deller’s paper is a mine of laterite-related information, yet its central theme is the essential first step of mapping them and discriminating their facies. Her starting point is their mineralogical simplicity, and the unique and distinct spectral properties of those constituent minerals. She matches these to the spectral coverage of freely available remote sensing data — Landsat TM, ASTER and ALI — each of which offers nuances to be exploited in uniquely discriminating the zones. Rather than setting out to ‘unveil’ sophisticated new methods of computer analysis (to which few in laterite-encrusted areas would have access), she chose the simplest useful approaches to a previously overlooked challenge: laterite facies have never been discriminated and mapped before. The results in this well-illustrated paper are stunning, and any geologist, and quite likely many lay people can understand what they show, thanks to careful discussion. The result is a paper that combines interest, novelty and usefulness. The last is the best aspect: geologists can learn from the paper how confidently to make highly informative maps cheaply and quickly.

ASTER data and earthquakes

NASA’s Jet Propulsion Laboratory in Pasadena, California is a huge engine of across-the-board innovation. In my field, remotely sensed geology, everyone pounces eagerly on publications by its scientists because they are bound to push techniques and applications forwards, often in surprising contexts, such as archaeology from space. One such nugget is about to be published (probably this month) in the premier geoscience journal EPSL (Avouac, P. et al. 2006. The 2005, Mw 7.6 Kashmir earthquake: Sub-pixel correlation of ASTER images and seismic waveforms analysis. Earth and Planetary Science Letters, in press doi:10.106/j.epsl.2006.06.025) and amply justifies my impatient preview here. It offers great potential for monitoring the effects of natural hazards that involve mass motion using free (for bona fide researchers and, hopefully, humanitarian organizations) satellite image data.

Jean-Phillipe Avouac and colleagues at JPL applied a well-tried approach in remote sensing — comparison of images captured on different dates—in trying to assess the extent and magnitude of ground motion involved in the 8 October 2005 Kashmir earthquake that claimed at least 80 thousand lives. But theirs is a before-and-after study with a revolutionary new slant. ASTER data from the joint US-Japanese Terra satellite resolves the ground with a resolution as sharp as 15 m, in several wavebands of EM radiation. In their own right, these bands contain huge amounts of information about vegetation, rocks and soils, and many other environmental attributes. Particularly with vegetation, comparing data from different years or seasons soon shows up changes and clues as to why they occurred. But ASTER has another potential view to offer. Two of its sensors, one pointing vertically downwards, the other obliquely back along its ground track, constitute a stereopair. They can be viewed together to give dramatic 3-D visualizations of terrain. With the appropriate software, the parallax difference between the location of each point on the ground in the two images produces a map of terrain elevation. The novelty and potential in Avouac et al. is to combine ASTER data from two instants in time to find places that have shifted in position in the meantime. So that they match geographically, they used stereo-derived terrain elevation to remove geometric distortions caused by viewing rugged relief with effectively a wide-angle camera. The key to extracting deformation parameters is applying shape-detection software to images from before and after an event, and then finding the magnitude and direction of the differences between landform shapes to chart movement. The 15 m resolution poses a limit, but the sophistication of the algorithms enables shifts of the order of less than a metre to be detected at a coarse resolution of 150 m. But that is quite sufficient to show what happened in Kashmir along the entire length of fault movement in 2005. Applied to commercially available stereo data (up to 0.65 m resolution) the results would be awesome.

Confused by radiocarbon ages? Hopefully, not anymore

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When we come to the near past, signifying time that has elapsed becomes unclear. Most christians divide the last four thousand years into AD and BC (with some confusion as to whether the division is at 1 or 0 AD), yet muslims place their starting year differently, and so might many other faiths, if they so chose. The adoption of ‘Before the Common Era’ and ‘the Common Era’ (BCE and CE, which are the same as BC and AD) really doesn’t help politically, being based on a now obvious fact: that the dominantly christian US and EU dominate the planet. The only foolproof way to judge elapsed time in years is to have some continual and irrefutably annual events to count. Now, it is not always convenient to use the annual growth rings in a collection of enormous logs of a variety of ages to tell time, and the same goes for snow layers in polar ice caps and layered stalagmites. Using the decay of radioactive 14C in preserved carbon-containing materials revolutionised archaeology and the science of recent climate change. But it has a snag, for 14C, unlike many other geochronometers, is continually being formed, by cosmic ray bombardment of nitrogen in the upper atmosphere. Cosmic ray flux is not constant, so the proportion of 14C to stable carbon was different at any time in the past. Until recently nobody knew how that proportion had varied. Radiocarbon ages have to be calibrated in some way, so that they record events in a truly absolute time-frame. Without calibration, even the most precise age determinations give a warped view of history (see Rationalising radiocarbon dating in the February 2004 issue of EPN). For instance, the date when the Younger Dryas glacial pulse began was a thousand calendar years before its calibrated 14C age. Despite heroic efforts to establish a link between radiocarbon ages and the true passage of years from long annual records in dateable materials, calibration gaps in the ~50 ka period achievable by using the quite short half-life of 14C have caused a problem. Many published and even some new dates are given without calibration, while others are in ‘years before present (BP)’, i.e. before the start of above-ground atomic bomb tests in 1950, which uniformly contaminated all later atmospheric carbon with 14C produced by nuclear transformation. The confusion should soon be resolved as the effort to match productivity of 14C to real time nears completion (Balter, M. 2006. Radiocarbon dating’s final frontier. Science, v. 313, p. 1560-1563). But some workers are impatient to give real ages using calibration curves for difficult periods, which have not yet been verified and are controversial. An interesting case relates to the possible overlap period, roughly around 35 to 30 ka ago, between fully modern humans and Neanderthals in Europe. That awkward era may soon be clarified with the unearthing of monstrous logs from New Zealand swamps, which may contain annual rings back to the 50 ka limit.

Is the idea of Hadean continental crust bunkum?

As these monthly jottings have noted several times, the geological record of the Hadean (before 4 Ga ago) could easily be lost through an ill-timed sneeze: it consists of a few minute zircon grains extracted from common or garden Archaean meta-sandstones in Western Australia. Milligram for milligram, these have become the heaviest punchers in the world of geochemical debate. They undoubtedly crystallized as long ago as 4.4 Ga. More controversially their detailed chemistry has been suggested to indicate that their crystallization was from granitic magma formed by partial melting of materials that interacted with water at around 700°C; materials that were not primarily of mantle composition (see Zircons and early continents no longer to be sneezed at in EPN February 2006 issue). If true, that would suggest low-density crust that found difficulty in being recycled into the mantle only a few tens of Ma after the Earth’s formation. Either that crust was too thin to resist subduction by some kind of tectonic slicing and has gone for ever, or some of it is still out there waiting to be found…by those who become very excited by extremely aged rocks. There is a simple way of putting the early-granite hypothesis to the test — by seeing if zircons in basalts are any different from them (Coogan, L.A. & Hinton, R.W. 2006. Do the trace element compositions of detrital zircons require Hadean continental crust? Geology, v. 34, p. 633-636).

Coogan and Hinton, of the University of Waterloo, Canada and Edinburgh University respectively, show that Hadean zircons cannot be distinguished chemically from those found in gabbros that have differentiated from basaltic magmas at modern mid-ocean ridges. As if that were not sufficiently deflating, they also made crystallization-temperature estimates of the gabbro-derived zircons, using a geothermometer that uses the titanium content of zircon in equilibrium with rutile. Despite the fact that the real temperature of gabbro crystallization is well over 1000°C, these estimates came in at between 700 and 800°C. That is, about the same as those proposed as evidence for the crystallization temperature of Hadean zircons from a granitic magma. Coogan and Hinton were not content, and go on to offer an alternative explanation for the zircon’s oxygen isotopes, used by others as evidence for the influence of water at shallow depths back to 4.4 Ga. The seemingly water-derived 18O excess in the zircons could well have come from carbonates recycled from surface weathering of basalt, to be assimilated by deep basaltic magma chambers.