Models abound for predicting earthquakes from past seismicity and detailed tectonic maps, analogous to those suggested for prediction of volcanic hazards. The Izmit earthquake of 17 August 1999 in Turkey was among the most savage in recent years and killed thousands. It was as powerful (magnitude 7.8) as the celebrated 1906 San Francisco earthquake, and like it stemmed from movement on a continental-scale strike-slip fault. The North Anatolian Fault is almost as well studied as the San Andreas line, and seismicity was known to be heading westwards well before the Izmit catastrophe. Indeed, the Izmit area was predicted to be next on the list, yet no preparation had been made, even by Turkish tectonicians who had been involved in seismic analysis. Chinese geoscientists take a different approach to seismic prediction than those in the west – over the last few centuries, hundreds of thousand Chinese people have perished in earthquakes. They are trying to organise local people to monitor possible precursors to earthquakes, such as rises in water levels in wells and strange behaviour of animals. They have had some notable successes, including preparation for one earthquake in recent years that saved an estimated 80 thousand people in one particularly hazard prone city. The Geological Survey of Israel has been testing a well known correlation between the times of anomalous radon emissions from the ground and earthquakes along the Aqaba Fault that controls the Dead Sea. Over a 7-year period, hourly scintillation-counter readings of radon emissions from springs, wells and especially gravels near known active faults allowed a rigorous test of a possible prediction system, because in that time there were almost 800 minor earthquakes (Steinitz, G. et al. 2003. Statistically significant relation between radon flux and weak earthquakes in the Dead Sea rift valley. Geology, v. 31, p, 505-508). For events beneath the Dead Sea rift, there is a good correlation between the start of radon emission increases and earthquakes, which suggests that about 3-days warning could be given, if the monitoring was widely deployed. The same cannot be said for small tremors with a source outside of the active fault zones. The success may possibly be because sufficient radon to be easily detected is generated by radioactive decay of uranium in a phosphorite bed that underlies the study area. Radon escape to the surface is possibly eased when microfractures begin to open as strains build before an earthquake.
Author: zooks777
Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry
Divine intervention?
Christianity had a hard time in its first four centuries as a faith, especially at the centre of the Roman Empire. Persecution of Christians ended abruptly with the conversion of Emperor Constantine in 312 AD. Legend has it that, while faced with the double problem of northern barbarian hordes at the gates of Rome and dissident Christians within, Constantine saw a vision in the sky while preparing to take on the invaders. Immediately converting to Christianity, he saw off the hordes, albeit temporarily, and the rest, as they say, is history. One version of the legend, from the Sirente region of Central Italy, tells of a new star that came nearer and nearer to disappear behind the mountains, with a blaze of light from horizon to horizon and ground shaking. Unsurprisingly, impact theorists latched onto this because of its similarity to what probably happens when a substantial meteorite strikes the Earth. Geologists from Sweden have discovered a small crater field in the Sirente area, that consists of a 125 m wide, circular lake with a raised and deformed lip, and several lesser craters dotted around it. Preliminary dating gives an age of 412+ 40 years. Although this date is a century later than Constantine’s conversion, contamination with later material might have reduced the actual age. If the link does prove to be substantial, the Sirente impact will rank with other catastrophes that literally made history, such as the filling of the Black Sea which has been argued to be the inspiration for the Biblical Flood and the Epic of Gilgamesh, and the explosive volcanism of Santorini that wiped out Minoan civilisation on Crete and may well be recorded apocryphally in the Old Testament.
Source: Chandler, D.L. 2003. Crater find backs falling star legend. New Scientist, 21 June 2003, p. 13.
Middle Devonian extinction and impactite layer
Around 380 Ma there was a major extinction event (~40% of marine animals) that is recorded world-wide, along with negative shifts in 13C. As with other extinctions since the discovery that the Chixculub crater was exactly the same age as the famous K/T extinction, there has been a quest to link this Middle Devonian event to an extraterrestrial cause. Now there seems to be a positive result (Ellwood, B.B. and 4 others 2003. Impact eject layer from the mid-Devonian: possible connection to global mass extinctions. Science, v. 300, p. 1734-1737). A Devonian section in Morocco contains a thin layer rich in shocked quartz, microspherules of devitrified glass, and metals, that also has low d13C. The carbon-isotope shift could have resulted from either of two possible consequences: collapse of the marine ecosystem; or massive release of methane from gas hydrates destabilised by the impact. Only one crater coincides wit the date of the layer and the extinction, Kaluga in Russia, but it is only 15 km wide, so cannot have had any dramatic biological effect. However, the very presence of a moderate crater at exactly the right age might signify other impacts, because it is becoming increasing clear that impacts come in clusters, perhaps because large, approaching bodies break up before they hit the Earth.
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The gas-hydrate “gun”
The gas-hydrate “gun”
As fears of anthropogenic climate warming have risen, so more geoscientists have looked in detail at the stratigraphic record for signs of past warming, and funds have become more targeted towards palaeoclimatology. One of the most important discoveries was that the end of the Palaeocene, about 55 Ma ago, was a time of sudden global warming during the overall cooling that has characterised the Cenozoic. The first sign that something strange had happened then came from using the oxygen isotope geothermometer on plankton tests from marine drill core that passed through the boundary. There seemed to have been a 7º C jump in surface seawater temperature. An explanation for the thermal spike arose after carbon isotopes revealed a coincident spike in the lighter 12C. Periods of low primary biological production can impose such anomalies, because photosynthesis selectively binds light carbon in carbohydrate. However, some of that light carbon ends up buried in sea-floor sediments, so another explanation for a negative excursion in d13C is that organic carbon has somehow been released from sedimentary storage to the atmosphere. So, either there was a sterile ocean or a massive release of organic carbon at the Palaeocene/Eocene boundary. Some kind of erosion to achieve the second possibility could not have led to such a speedy shift in carbon isotopes. The accepted explanation, suggested in 1995, stemmed from organic carbon that had been metabolised by methanogen bacteria in anaerobic sea-floor sediments to form methane. Given low enough sea-bottom temperatures and sufficient pressure, methane can crystallise with water to form an icy substance, known as gas-hydrate or clathrate, in sea-floor sediments. Being an unstable compound, gas hydrate can break down rapidly if seafloor temperature rises or sea-level falls. And, of course, the methane can rush to the surface as bubbles. Being 4 times more efficient than carbon dioxide at trapping thermal radiation emitted by the Earth’s surface, methane releases are excellent explanations for sudden warmings in the stratigraphic record. And there is a great deal of methane locked as gas hydrate beneath the sea floor, about 2 teratonnes (2 x 1012 t). Quirin Schiermeier reviews the basic concept (Scheiermeier, Q. 2003. Nature, v. 423, p. 681-682), but poses the question of how methane-induced warming is reversed. Methane is quickly oxidised to CO2 in the atmosphere, so lessening its warming effect. So a “spike” that lasts thousands of years has to be fed by continual releases. Since warming drives gas hydrate breakdown, something must intervene to stop the releases before the warming becomes a “runaway greenhouse”. One view, and probably the correct one, is that warmth and more CO2 drives up biological activity so that the increased atmospheric carbon is “pumped” down by living processes, back to sedimentary burial. If sufficient nutrients are available, there is no way of stopping this negative feedback until a balance is restored. Schiermeier reports that new ocean drilling plans to test the hypothesis that the Palaeocene/Eocene warming accelerated continental erosion, which was able to wash the crucial nutrients phosphorus and iron into the oceans. Experiments have shown that increased iron in ocean-surface water far from land – now pretty sterile because it is iron-deficient – sparks up photosynthetic plankton. That is one possible way of artificially drawing down anthropogenic CO2. The problem is, if such a process was involved in cooling the Eocene Earth, it took about 100 thousand years.
Red Sea record links to northern hemisphere climate
In his forthcoming book, Out of Eden: the Peopling of the World (Constable and Robinson, July 2003), Stephen Oppenheimer offers the novel suggestion that fully modern humans left Africa by island hopping on log rafts across the Straits of Bab el Mandab, which connects the Red Sea to the Indian Ocean. The rationale to his suggestion is that sea-level falls during major glaciations would have partially exposed the shelf that lies beneath the Straits, presenting a route to SW Arabia across only 18 km of island-dotted sea. As today, it would have been impossible to trek across the deserts of the Middle East after a northward African migration along the Nile, without chains of wells. His thesis then sees humans migrating along coasts eventually to reach east Asia at about 70 ka. Precisely when the Straits of Bab el Mandab became shallow enough would have been determined by global climatic conditions, for only glacial maxima result in sufficient sea-level falls for such island hopping to be possible.
The shallowing of the shelf across the southern outlet of the Red Sea would have had a profound impact on seawater circulation. Already having restricted connection to the world’s oceans, Red Sea water has elevated 18O levels, because evaporation from it favours loss of lighter 16O. With more restricted circulation, evaporation would have driven this up further. Geoscientists from the Universities of Southampton, Tuebingen and Göttingen, and the Geological Survey of Israel have analysed the variation in oxygen isotopes of foraminifera from a Red Sea core to quantify ups and downs in sea level in more detail than possible from open-ocean cores, which have uncertainties of about ±30m) (Siddall, M. and 6 others 2003. Sea-level fluctuations during the last glacial cycle. Nature, v. 423, p. 853-858). The method that they used models the effects on Red Sea oxygen isotopes of evaporation and changed circulation to estimate how the depth of the Straits of Bab el Mandab changed. They claim a precision of ±12m. Through the period from 70 to 20 ka, leading up to the last glacial maximum, their sea-level record tallies nicely with climate records from both Antarctic and Greenland ice cores, including shifts linked to the short-lived Heinrich and Dansgaard-Oeschger cycles. During the last glacial maximum(18-20 ka), sea-level fell by almost 120 m, so that the Straits of Bab el Mandab were on average only 15 m deep. The first human Exodus out of Africa to populate Eurasia would have been between 120 to 130 ka, as suggested by Oppenheimer, when sea level probably fell a little further. However, at about 65 ka, sea level dropped to about 100 m below modern levels, perhaps presenting another window of opportunity.
Broecker reviews climate triggers
Wallace Broecker, of the Lamont-Doherty Earth Observatory at Columbia University, was the first to quantify in 1975 the 19th century prediction of Svante Arrhenius that increasing atmospheric carbon dioxide would drive up global temperatures. Broecker’s early work lies at the centre of concern about global warming, and his subsequent contributions are enmeshed with the entire study of past climate change. A review by him of current ideas on palaeoclimates of the recent past is therefore compulsory reading, for all geoscientists (Broecker, W.S. 2003. Does the trigger for abrupt climate change reside in the ocean or in the atmosphere? Science, v. 300, p. 1519-1522. As well as the astronomically connected cyclicity that is apparent in all kinds of climate record through the Pleistocene, those records are punctuated by sudden, short-lived phenomena, whose magnitudes and pace are sufficiently dramatic to focus attention on processes that are probably entirely terrestrial. Foremost among these during the last glacial interglacial cycle are the astonishing coolings of Heinrich’s iceberg armada events and the possibly catastrophic (in a human as well as an ecological sense) Younger Dryas, which reversed warming from the last Glacial Maximum, and the equally sudden warmings associated with Dansgaard-Oeschger events. Broecker’s review focuses on the two mechanisms that have been suggested to underlie these overturns. One links such changes to shifts in whole-ocean water circulation, especially the ons and offs of deep-water circulation beneath the North Atlantic, the other to perturbations of the way in which atmosphere and ocean interact in the tropics.
An entirely plausible scenario for climate-driving changes in North Atlantic water circulation is flushes of freshwater from the surrounding continents, so that formation of sea ice leaves residual water that is not saline or dense enough to sink and drag in water from lower latitudes. The problem is that the complete thermohaline cycle, which impacts on global atmospheric circulation, has a period longer than the changes that might be induced by its perturbation in the North Atlantic. Tropical atmosphere-ocean dynamics are the largest elements in global climate, in terms of the energy and mass that are shifted, so they are a natural candidate for a driving mechanism. Tropical climate shifts abruptly today in well-known ways, most important being the El Niño-La Niña cycle. There is no ponderous underlying dynamic that would damp down connections between cause and global effect, and prevent sudden climate change. Yet, some kind of “flywheel” is essential to keep long-term cyclicity going and lock sudden changes into century to millennium-long climate “states”, which should rapidly decay if effect rebounded on cause, as it does in the case of El Niño-La Niña. Broecker covers all the critical evidence that has borne on both hypotheses up to now. His conclusion is interesting. Both hypotheses are very much model led, and in need of as much empirical support as can be had. Yet, and here is the nub, the crucial data are those bearing on correlating times of events that are recognised all over the place. Time resolution is of the greatest importance, since climate transitions are fast; faster in fact than we can presently resolve before historical times. It is entirely likely that suitable resolution of times past may be absolutely impossible. Both hypotheses have a lot of empirical and theoretical support. So, what is the problem of combining them in a cunning way? Partly, that may be because reductionism (controlling a few variables and looking for developments in another simple set) still plagues science. That is odd in climatology, where all motions and energy changes palpably relate to one another, with no control of a rational kind. Reductionism demands ever more staggering computing power and speed, to “keep all the eggs in the air”. There is always the feeling, as Jimmy “Shnozzle” Durante observed in his musical monologue, The Man Who Found The Lost Chord, that if you find a hitherto overlooked connection, then everything goes well; if you can remember it! Broecker suggests that the missing connection must “transmit” from deep ocean water to tropical atmosphere.
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Geochemistry of the vanishingly tiny
The British press has been awash with speculation that the Prince of Wales is worried about nanotechnology and the slim possibility that the next big threat after Osama and SARS might be minute, self-replicating robots that invade our bodily orifices. It stemmed from the Prince of Wales’ having asked experts for a briefing, and that may well have been just HRH’s curiosity about a changing world. There is rarely an issue of the weekly science journals without news of some discovery of phenomena that occur in nanotubes and minuscule cavities; the world at scales less than a micrometre is beginning to seem strange. Rocks are full of pore spaces and inter-grain boundaries with the dimensions on which new wings of the other sciences are emerging. So it is no surprise to learn that there will soon be “nanogeochemistry” (Wang, Y. et al. 2003. Nanogeochemistry: geochemical reactions and mass transfers in nanopores. Geology, v. 31, p. 387-390). The use of natural and artificial zeolites as ionic filters has been around for a long time, so this is a branch with a new name, rather than a fundamental breakthrough. But zeolites are profitable, and only now has “blue-skies” research turned up the magnification.
Typical nanopores and pathways are grain boundaries in crystalline rocks, cleavage planes in phyllosilicates and clay minerals, and pores in fine-grained sediments, such as diatomite and kaolin, and minerals that have been precipitated as amorphous masses rather than discrete crystals, a good example being the iron oxy-hydroxides in soils. To see these structures requires advanced transmission electron microscopy, and even with them the features are somewhat indistinct. Nanopores can make up to 40% of a material’s porosity, and having such minute radii they contribute as much as 90% of the internal surface area that is exposed to chemical reactions. Artificial materials that show nanoporosity have internal surface areas as high as hundreds of square metres per gram. Clearly, such materials in nature must play a major, but largely uncharted role in geochemical change. Among the oddities discovered by Wang and colleagues at the Sandia National Laboratories and the University of New Mexico, are inclusions of native copper in weathered clay minerals and equally small particles of gold along microfractures in mylonites. Their experiments with artificial simulants of natural fine-grained materials focussed on two simple phenomena: the electrical charge on small surfaces in relation to acidty; and their ability to absorb trace elements. The paper is highly technical, but the conclusions are surprising . Nanopores develop unusually high surface-charge densities that should affect their ability to adsorb ions, and also exert controls on reactions that might seem unlikely in macro-scale simulations of geological conditions. Indeed, finely porous materials enrich trace elements by an order of magnitude compared with isolated small particles, and encourage precipitation or solution of different compounds when that would be unexpected in more open systems. As well as bearing on burial of toxic and radioactive wastes, and on mineralising processes, nano-scale processes are probably central to the whole process of weathering. Interestingly, such small scales exclude even the tiniest bacteria, so that the geochemical processes seem unlikely to impinge on life. However, spaces in rocks comprise a nested series of dimensions, and changing conditions may well flush material from one scale to another. In particular, bacteria of various kinds can control pH at the micro-scale, thereby creating the ambient conditions for nano-scale geochemistry.
Potassium in the core
It might seem impossible for planetary cores dominated by iron-nickel alloys to contain any source of heat generation. The main three elements (uranium, thorium and potassium) with long-lived radioactive isotopes and sufficient abundance to produce substantial heat energy are all highly concentrated in the Earth’s crust. That is because they are incompatible with the minerals in mantle rocks, and so readily enter magmas that contribute to continental growth. However, the only natural materials that bear any resemblance to geoscientists’ notions of core materials, metallic meteorites, contain abundant sulphur. Theoretically, potassium can enter sulphide minerals. So, since as long ago as the 1970s there has been debate about whether motion in the core was driven entirely by residual heat from Earth’s accretion and the formation of the core, or that it contained its own heat source in the form of 40K. If the first was true, then the self-exciting dynamo responsible for the Earth’s magnetic field has been running down over geological time, because heat is transferred across the core-mantle boundary, eventually to reach the surface by convection. The existence of a solid inner core might result from such cooling, though its formation would release latent heat of crystallization and prolong inner motion. However, some calculations suggest that core motion and so geomagnetism ought to have vanished long ago, through loss of core heat to the surface. Substantial potassium in the core would demand considerable revision of ideas about the bulk evolution of the Earth, and other rocky planets. Experiments to prove that iron-sulphur alloys can contain abundant potassium have had a chequered history. Research at the University of Minnesota and the Carnegie Institute of Washington has discovered why there were such ambiguous results (Murthy, V.R. et al, 2003. Experimental evidence that potassium is a substantial radioactive heat source in planetary cores. Nature, v. 423, p. 163-165). The problem was in the preparation of samples for analysis. Rama Murthy and colleagues found that the oils used in polishing samples for electron-microprobe analysis actually leach potassium from the sulphides in them, nearly all disappearing in a few days of contact. With great care, they repeated experiments on mixtures of metallic iron, iron sulphide and potassium bearing glass held at high temperature under pressures between 5 and 10 % of those experienced in the core. Their results show that potassium can indeed enter core materials with high sulphur contents. The higher the temperature the more gets in, and their most extreme run saw almost 4 % K in the quenched sulphide. Plan are afoot to discover if uranium and thorium might also be in core materials.
Incidentally, in the week that the film The Matrix: Reloaded was premiered in the USA, a proposal to send a probe to the core-mantle boundary also appeared (Stephenson, D.J. 2003. Mission to Earth’s core – a modest proposal. Nature, v. 423, p. 239). David Stephenson, of the California Institute of Technology, builds on the notion of the “China Syndrome”, in which meltdown of the core of a nuclear reactor would lead to superdense molten uranium melting its way through the mantle. In his proposal, ruggedised instruments in a capsule the size of a grapefruit would make the journey, along with about 10 million tons of molten iron, by propagating a large crack started by a 10 Mt nuclear explosion. Data is to be transmitted by modulated acoustic signals in the kHz range. The article helps to demonstrate the delays in publication, even in a prestigious weekly journal; it should have appeared 6 weeks earlier….
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Long-term prediction of volcanic activity
Unless it is possible to give people who live near dangerous volcanoes sufficient warning that they can escape disaster, eruption prediction might be looked on as a lugubrious topic. Up to now, there have been very few predictions that have been better than a few hours or days. Mexico’s Popocatapetl gave two days warning in late 2001, and that was sufficient for a completely successful evacuation of those threatened. In the case of the eruption of Nyirangongo in eastern Congo, a few months later, warning signs preceded eruption by 5 days, but the people of Goma were not told and 45 people died trying to rescue possessions from the quiet, but relentless movement of a lava stream (see EPN February 2002, Is volcanic eruption predictable?). In both cases it was abnormal seismicity that presaged the events. John Murray, of the British Open University, has analysed the statistics of seismic events and eruptions of possibly the world’s most monitored volcano, Etna on Sicily (Murray, J.B. 2003. Seismicity and time-lagged lava output at Mount Etna: A new method of long-term forecasting at a destructive volcano. Geology, v. 31, p. 443-446). Energy released during 19-year periods by earthquakes beneath the volcano since 1870 shows a inverse relationship with 9-year lava production, which suggests that seismicity and eruption are widely separated in time over long periods. However, by examining the correlation of seismic energy with eruption volume for time differences between the two from 0 to 50 years, Murray has been able to show that Etna increases its productivity roughly 25 years after major releases of seismic energy. Using this as an input to a model that might predict eruption intensity, he has been able to mimic the actual volcanism through the 20th century with fair accuracy. In his opinion, the very high eruption rate since 1950, which reached a peak in the 1990s, is only likely to decline a quarter of a century after large earthquakes (> magnitude 6) return to Sicily. So, Sicilians have a difficult choice. Should they worry about lava flows or earthquake damage? Sadly, data suitable for broadening Murray’s method are available for very few volcanoes, all in quite prosperous countries.
Modelling the duration and extent of mining contaminants
Release of high concentrations of heavy metals and other pollutants to drainages is a natural consequence of geochemical anomalies associated with mineralization. However, these have come to balance with the rest of the environment over periods measured in thousands of years or even longer. The pose perpetual hazards, some of which are known, some not. Environmental disturbance by mining and associated activities scales up releases of pollutants many times over those of natural origin. Even with modern means of waste containment, escapes occur, sometimes of very large magnitude, such as the breaching of tailings dams or landslips in spoil heaps. Of course, these hit the news when they happen, but assessing how long the pollution dwells in downstream areas and how it moves is not easy. It requires some kind of model of the hydrology, erosion and sediment-transport characteristics of the affected drainage basins, that takes into account catchment topography and the size-distribution and density of escaped wastes. Such a modelling tool is now available, having been developed at the University of Wales in Aberystwyth (Coulthard, T.J. & Macklin, M.G. 2003. Modelling long-term contamination in river systems from historical metal mining. Geology, v. 31, p. 451-454). It is complex, because it combines the 3-D shape of basins with water discharge and depth, vegetation cover, depth to bedrock and the properties of released materials. In a simulation of hydrological dynamics. TRACER is able to take account not just of the fate of grains that enter drainages, but how they are deposited in alluvium and then reworked by later changes in hydrology. Coulthard and Macklin apply the model to the base-metal mining district of Swaledale in North Yorkshire, England, where production began in 1700 and ended 200 years later. Swaledale was a minor producer of lead and zinc in modern terms, and the miners paid scant attention to environmental protection. Results suggest that contamination spread downstream to the flat land of the Vale of York in only 10 years after mining started, but the pollution lingers, and seems likely to stay above safe limits until well after the start of the 22nd century. When possible increases in rainfall through global warming are factored in, the simulation remains much the same for 10 to 25 % rises, and only moves towards clean-up with 50 to 100 % increases in precipitation, when clean sediments should dilute the pollutants. As well as predicting the general effects of contaminant releases, TRACER is able to highlight parts of a drainage basin that are particularly at risk due to trapping of sediments. Mining in Swaledale produced, at most, only about 600 thousand cubic metres of metal-rich waste, fine enough to be transported by water. Recent escapes from tailings dams and landslipped spoil heaps, as in Spain and OK Tedi in Papua New Guinea, were orders of magnitude larger.
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Astronauts’ snaps
When directing The Greatest Story Ever Told George Stevens was unimpressed by John Wayne’s delivery of his only line at the very end of the film. Stevens said, “You have to deliver with a little more awe, John”. And so we have one of the great conclusions in cinema history, “Aw, he truly was…..”. Astronauts have had a fair number of those “Aw” moments, and thankfully, most have them have carried cameras. They captured a great many views of odd and awesome phenomena and features of the Earth as they passed over. There are a great many (around 400 thousand), and NASA has compiled the best views of Earth’s surface on its new site at http://eol.jsc.nasa.gov/sseop/EFS/. I put in “faults” to see what came up; there were 184 images from every continent. Downloads are in two sizes, 300 kb and 13 Mb, so the images are of very high quality. The entire archive is searchable at http://eol.jsc.nasa.gov/sseop/.
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Elderly South African Australopithecines
The Sterkfontein Caves near Johannesburg in South Africa have provided some of the best preserved hominid remains, because they are enveloped in chemically precipitated cement. Fossils are also much more plentiful than at other sites, and the caves have yielded about 500 specimens. However, unlike sites in bedded sediments interleaved with volcanic horizons, cave deposits are difficult to date accurately. Up to now, correlation of other fossil animals in the breccias that encase Sterkfontein hominids with those at more amenable sites, together with dating based on palaeomagnetic reversals, have been hotly disputed. A new technique based on the radioactive decay of isotopes that cosmic-ray bombardment induces in quartz grains promises to resolve the paradox of wonderful fossils that cannot be dated. While quartz grains are at the surface, in alluvium or the debris on slopes, cosmic rays produce radioactive aluminium and beryllium isotopes in a fixed proportion. The longer the exposure time, the more radioactive isotopes are produced. But if such irradiated grains are buried, the isotopes decay away, because they are protected by overlying material. Detrital sediments enter cave systems very quickly, so they are near-ideal for the use of cosmogenic dating. Of the two most-used isotopes, 26Al decays quicker than 10Be. So, the 26Al/10Be ratio decreases with time and gives a measure of how long the sediment has been buried. Results from Sterkfontein (Partridge, T.C. et al. 2003. Lower Pliocene hominid remains from Sterkfontein. Science, v. 300, p. 607-612) show that the stratigraphically lowest fossils are much older than previously thought; around 4 Ma.. Previous age estimates suggested that the oldest Sterkfontein hominids lived around the same time as Australopithecus afarensis, of which the famous “Lucy” skeleton was an Ethiopian member. Four million years ago A. anamensis would have been a contemporary, yet the hominids at Sterkfontein seem quite different anatomically. Maybe there were two species in Pliocene Africa, one East African and the other a southern one. In fact, there are hints that perhaps two species of australopithecines, along with a more robust paranthropoid may have been washed into the caves. There are two problems though: cosmogenic dating is notoriously imprecise (the age reported is 4.2±0.3 Ma), and Sterkfontein has such excellent preservation that the number of specimens outweighs those from elsewhere – comparisons are not easy!
Tracking migrations with language
One of the first surprises that arose when genetic relatedness among living people and the estimated time of their separation began to encompass global populations was how well the genetic patterns matched with the distribution of the world’s languages. When populations move they not only carry their genetic heritage but their languages. Probably the greatest migrations in human evolution took place at the end of the last Ice Age, and so it might seem that plotting language distribution ought to chart the paths these wandering people took. Jared Diamond and Peter Bellwood (Diamond, J. & Bellwood, P. 2003. Farmers and their languages: the first expansions. Science, v. 300, p. 597-603) have reviewed just how complex such a task will be. Genes and language can tell only part of the story, because people carry skills and culture too. The two dominant cultures around 11 000 years ago were the age-old ways of the hunter-gatherer and the new agriculture and animal husbandry. There are at least five possibilities involved. Genes, language and lifestyle could mix between both groups when they came into contact. Hunters might take up farming but keep their identity. Hunters were as likely to shift as farmers when climate belts changed. Powerful incomers might impose their language but not their genes. When one group moved, another might take its place. Bearing in mind these caveats, Diamond and Bellwood review the main patterns of linguistic groups, using excellent graphics.
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Plume debates
Jason Morgan’s recognised in the early 1970-s that chains of volcanic islands and seamounts, such as the Hawaii-Emperor Chain, which cross sea-floor magnetic stripes, might have resulted from mantle “hot spots” that are fixed relative to motions of lithospheric plates. He went on to suggest that such magmatic anomalies might reflect narrow thermal upwellings within the deep mantle, and applied the term “plumes” to these notional convective zones. Geochemists have since flocked to active and extinct manifestations of within-plate magmatism, and developed a whole sub-culture of classification and hypotheses concerning their origin and inner workings. By the end of the 1990s over 5000 candidates for underlying plumes had been proposed, some still active and others inferred for past events, such as flood basalt provinces. Processing of seismic signals using supercomputers over the last few years has used them to map variations in P- and S-wave speeds at different depths in the mantle. Speeds below those expected are likely to reflect hot mantle relative to high-speed, colder regions. So seismic tomography potentially charts hot rising mantle and cool, descending parts; seemingly ideal for detecting mantle plumes and how deep they extend. Early results centred on proposed plumes were a mixed bag. Some seemed to have very deep origins, perhaps down to the core-mantle boundary, whereas others appeared to be above hardly anomalous mantle. Most exciting was a zone of hot, probably rising mantle with a source at the top of the core beneath the South Atlantic, yet whose upper parts sloped obliquely upwards towards the Red Sea. It seemed that the Afar plume, believed to have been responsible for continental flood volcanism in Kenya and the Ethiopian Plateau, and perhaps the East African Rift and opening of the Red Sea, still existed. Hot-spot activity is a minor aspect of global tectonics today, so it is not an ideal time to ponder on plumes. If they are real, then periods of massive flood volcanism would have been responses to superplumes, but the last in Ethiopia was 30 Ma ago.
Exciting as seismic tomography is, its resolution is currently too coarse to pick out the most revealing features of the plumes that potentially it could detect. To have sufficient gravitational potential energy to rise through the entire mantle, a very large volume is required, and that is assigned to the “plume head”. Some hotspots are over large volumes of hot mantle, but they lie just beneath the lithosphere, and could have their origin at any level in the mantle. The tracks that they followed, if any, and which might continue to be a conduit for uprising material would be much narrower. Such predicted “plume tails” are too small for resolution by current tomography. A compilation and re-classification of hot spots (Courtillot, V. et al. 2003. Three distinct types of hotspots in the Earth’s mantle. Earth and Planetary Science Letters, v. 205, p. 295-308) has whittled down candidates for mantle plumes to a mere 50 or so, with less than 10 likely to have risen from core depths. Two responses have arisen about this hugely popular topic: that Morgan’s ideas are still basically valid, but need more work (DePaulo, D.J. & Manga, M. 2003. Deep origin of hotspots – the mantle plume model. Sciene, v. 300, p. 920-921); that hotspots might be linked to plate tectonics, and that mantle plumes are nothing more than a “belief system” (Fouger, G.R. & Natland, J.H. 2003. Is “hotspot” volcanism a consequence of plate tectonics? Science, v. 300, p. 921-922). A sensible aim that might resolve matters is to seek materials from the largest magmatic events – flood basalts – that should contain unambiguous geochemical signs that their parent mantle was at some stage exchanging matter with the core, if they had formed after rise of a superplume. But, every line of approach to deep-mantle processes relies on proxy evidence, several steps removed from actual events and properties. That makes David Stephenson’s proposal for a mission to the core (above) so urgently in need of support!
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Extinction at the Precambrian-Cambrian boundary
The very beginning of the Cambrian is associated in every geologist’s mind with the explosive appearance and diversification of animals with hard parts. Why this dramatic introduction to the modern biological world occurred is one of the great questions in evolution. Some connection with the effects of “Snowball Earth” events in the late Neoproterozoic was thrown into doubt by evidence that it had little effect on micro-organisms (see Microbes showed no sign of change following a “Snowball Earth” in May 2003 EPN). Exactly at the boundary there is a marked fall in the abundance of carbon-13, and this negative d13C excursion is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age. One of the places that it occurs is in Oman, reported previously in EPN (A possible fuse for the Cambrian Explosion, January 2003). The paper describing the evidence from Oman that the carbon-isotope excursion relates to a mass extinction is now out (Amthor, J.E. and 6 others 2003. Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman. Geology, v. 31, p. 431-434) The disappearance of the distinctive eukaryote fossils coincides exactly with the carbon anomaly. Luckily, so too does a volcanic ash horizon from which zircons provide a very precise U-Pb age of 542±0.3 Ma. This matches less precise dates for the anomaly from Siberia and Namibia, and seems likely to become accepted as the definitive age for the start of the Phanerozoic.
“Snowball Earth” and evolutionary diversification: Australians speak out
By comparison with the vast amounts of Australian diamictites that span a range of Neoproterozoic ages, the sites elsewhere, from which evidence in support of the “Snowball Earth” hypothesis and possible effects on evolution have been drawn, are puny. Besides that, the Late Precambrian of Australia has the best record of biological change, including the type locality for the Ediacaran fauna that presaged the Cambrian Explosion. Although somewhat less hasty than the flurry of papers on the “Snowball” hypothesis, since 1998, the appearance of published data from the “Red Continent” is sure to push the debate decisively one way or another. Palaeontologists from the Geological Survey of Western Australia, Macquarie University and Mineral Resources Tasmania have just unveiled details of acritarchs from late-Neoproterozoic sediments that overlie the Marinoan (~600 Ma) glaciogenic rocks in South Australia (Grey, K. et al. 2003. Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? Geology, v. 31, p. 459-462). Acritarchs are spore-like fossils, that probably represent encysting algae. Their rapid diversification makes them useful biostratigraphic indicators from the Late Precambrian to the present. Grey et al. Found that the same assemblage of acritarchs occur before the Marinoan glaciogenic strata and after the succeeding “cap” carbonate. They are part of a group that can be traced back to the Mesoproterozoic However, higher in the sequence that they examined there is a distinctive layer of debris that contains evidence of impact-induced shock. This can be correlated with little doubt to the 90 km Acraman structure in South Australia, which formed at 580 Ma with an energy likely to have had a major influence on life. Sure enough, in the strata above this ejecta layer a completely new type of acritarch group appears and diversifies rapidly, while the pre-impact groups simply disappear. Clearly, the Acraman impact is implicated in this sudden biological change; an extinction followed by rapid diversification. Acritarchs are thought to represent the phytoplanktonic base of the Neoproterozoic food chain. Immediately above the strata in which the post-impact acritarchs diversified lie sandstones that contain the famous Ediacara fauna of the first large, soft bodied animals. The Marinoan “Snowball” event seems disconnected from this evolutionary leap.
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Water resources and bullocks
Desalination is often touted as a solution to shortages of clean drinking water, but the most common method, using reverse osmosis, is really a luxury. It relies on electric pumps driving salty water through a membrane, so that salt concentrates on the high-pressure side of the membrane, allowing nearly fresh water through it. This method is widespread among power-rich economies along desert coastlines, but has done nothing to help the less fortunate millions in countries where electricity is unaffordable. Indian scientists, unsurprisingly, have developed a means whereby fresh water might become accessible to most coastal people in the tropics. They have worked out how to gear bullock power to reverse-osmosis pumps, so that a pair can produce up to 3000 litres each day and supply entire villages. If a bullock can do it, then why not donkeys or camels in even more arid coastal areas?
Source: Coghlan, A 2003. All hooves to India’s pumps. New Scientist, 10 May 2003, p. 19.
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