Most of us have grown used to thinking that earthquakes have an epicentre at some fixed point beneath the surface. That is not at all true, as the event that set the Boxing Day 2004 tsunamis in motion as been shown to have been a lengthy rip that propagated from Sumatra NNE to the Nicobar Islands, over a period of about an hour. Even quite small earthquakes are distributed and often migrate along a fault line. Christine arson of the University of Colorado has captured what is effectively a movie of a magnitude 8.3 event off the island of Hokkaido, Japan, which can be viewed at spot.colorado.edu/~kristine/tokachi_rupture.gif. The data that she used comes from a network of a thousand highly sensitive GPS receivers set up throughout Japan. Instead of acceleration, measured by conventional seismometers, GPS records actual position in x, y, z coordinates. That enable the actual motions to be imaged as in the movie.
Author: zooks777
Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry
Interbreeding: louse study leads to head scratching
A challenging question about the origin of fully modern humans is whether or not Homo sapiens interbred with archaic species, such as the Neanderthals or H. erectus. That modern humans occupied the same territory as both, at the same time, is well established for Europe and Asia. The likely time for the first major migration of moderns from Africa is about 70 to 100 thousand years ago, and archaic humans did not become extinct in Eurasia until 30 ka at the earliest. Genetic material from extinct humans is rare and difficult to analyse because of degradation. A couple of mtDNA samples from Neanderthal remains give results that are sufficiently different from ours to rule out retention in modern human populations of the genetic outcome of any interbreeding between ancestral moderns and the population to which the two Neanderthals belonged. Yet it does not rule out such interactions with other archaic groups. We have no idea of the genetic diversity of Neanderthals, whose lineage probably split from that of our own (through that of H. heidelburgensis) as long ago as 700 ka. If they lived in isolated bands of a small population, that diversity could have become substantial over such a long time. So far, no genetic material has been recovered from H. erectus remains. Another approach to the matter has emerged from a genetic study of human head and body lice – Pediculus humanus (Reed DL. et al. 2004. Genetic analysis of lice supports direct contact between modern and archaic humans. Public Library of Science Biology, v. 2, e340 – through www.plos.org). The louse Pediculus humanus is unique to humans, and genetic comparison with that which infests chimpanzees suggests that these two species diverged at about the same time as the split that led to modern humans and chimps, at about 5.6 Ma. That is remarkably similar to molecular timing that uses primate DNA. The interesting feature of the louse genetic analyses by the team from the Universities of Florida, Utah and Glasgow is that there are differences between the lice that leap on us. There are two strains which originated before 1 Ma ago, according to the molecular clock. One has a global distribution, and infests both head and body, whereas the other is exclusively a head louse and only occurs in the Americas.
Around 1 Ma there seems also to have been a major divergence among early humans between a strand of H. erectus, which survived until as recently as 20 ka in Asia, and one that led to European Neanderthals and the modern humans who began to migrate from Africa to Eurasia around 100 ka. The unique occurrence of the head-only louse in the Americas (along with the other strain) suggests that the modern humans who crossed the Bering Straits to colonise the Americas came into direct physical contact with beings who carried that particular strain, en route. The likely candidates would have been Asian H. erectus. Contact had to be direct, because, unlike the flea, the louse cannot leap, and it can only survive on humans. The lack of the New World Pediculus humanus in Eurasia suggests two things: if moderns were “in touch” with archaics, the latter carried the other variant (Neanderthals?); the present Asian population (and that of New Guinea and Australia) possibly did not have close contact with archaics who were alive at the time of colonisation (were there by then very few?). All very interesting, but it does not resolve the question of interbreeding; intimate contact could have been through fighting, trading or interbreeding. There is another, very different human-only louse, Pthirus pubis, which infests pubic hair only, and about which there is very little genetic information, so far…
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Evidence goes against end-Permian impact
In December 2004 EPN commented on what appears to be a serious challenge to claims of geochemical evidence that would support a major impact associated with the largest of all mass extinctions in the Phanerozoic, that at the close of the Permian Period and the Palaeozoic Era, around 251 Ma ago. Newly published analyses from two other well-constrained P-Tr boundary sites found no signs of the elements that would be expected from a major collision with a metal or silicate-rich asteroid (Koeberl, C. et al. 2004. Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component. Geology, v. 32, p. 1053-1056). Koeberl of the University of Vienna and colleagues from the US and UK focussed on platinum-group elements (PGEs), and osmium and helium isotopes. Both sites are stratigraphically similar and dominated by carbonate sediments, with evidence from one site for deepening water that laid down organic-rich marls. Sure enough, there is a “spike” in iridium at the level of these marls, which had been documented at the Austrian site in 1989, and there is another 50 m higher in the sequence. The new work confirmed both, and also found the marl-related “spike” in Italy. But the reason why iridium has been used to suggest extraterrestrial impacts is because, of all the PGEs, it is the easiest to analyse at very low concentrations. That can give rise to “false positives”, for there are purely terrestrial processes that can concentrate PGEs. An unambiguous arbiter between these processes and impacts lies in the isotopic composition of the metal osmium. Rocks of the Earth’s crust have high rhenium (Re) and low osmium (Os) contents, whereas in meteorites the Re/Os ratio is very much smaller. The unstable isotope 187Re decays to produce a daughter 187Os that adds to the common 188Os isotope. Consequently, terrestrial rocks acquire high 187Os/!88Os rapidly after they crystallise from magmas and that “signature” is imparted to the entire surface environment through weathering and solution. On the other hand, meteorites have low 187Os/!88Os ratios, so the two influences on the geochemical record can be distinguished – if you have good enough analytical facilities. The two iridium spikes fail that test, as regards an impact origin. It seems likely that they originated through precipitation of PGEs from sea water under reducing conditions on the deep sea floor. The helium isotope data carry the same negative message; they are typically terrestrial.
Impact-induced extinctions, particularly ones that wipe out a sizeable proportion of all organisms, are likely to be unremittingly sudden – direct effects being felt within hours over the whole planet, and secondary effects such as “nuclear winter” and acid rainfall over a matter of a few years or decades. Radiometric dating is incapable of resolving such short periods, and at the age of the P-Tr boundary probably not even several hundred millennia. Faunal sequences can give a better indication of abruptness. To most intents the marine record at the time does look as if extinction was very sharp, but it does not indicate anything by way of clear evidence for an impact, such as glass spherules, shocked quart grains and other tell-tale signs. The continental record is pretty sparse, so has not figured much in the debate. However, the Karoo basin of South Africa contains thick continental sediments that span the boundary, and is famous for its primitive reptile fauna, some of which became extinct around the time of the P-Tr event. Incidentally, this die-off created the genetic conditions for the adaptive radiation in the Mesozoic that led not only to the dinosaurs but also the mammals and birds. Charting the timing of the Karoo extinctions has proved difficult, although it appears not to have been sudden in a stratigraphic sense. New age data has emerged from studies of palaeomagnetic field reversals in the sediments, together with variations in carbon isotopes, that allow timing to be better assessed through comparison with magnetic and carbon records from other sections (Ward, P.D. et al. 2005. Abrupt and gradual extinction among Late Permian land vertebrates in the Karoo Basin, South Africa. Science [soon to be published, currently available on Sciencexpress at www.sciencemag.org/sciencexpress/recent.shtml]). The signs are that the proto-reptiles died off over tens to hundreds of thousand years due to some protracted crisis, probably connected with the giant continental flood basalt eruptions that formed the Siberian Traps. Those lavas overlap the timing of the P-Tr boundary, and would certainly have added sufficient CO2 to give substantial global warming and also massive emissions of SO2 that would have created chemically hazardous conditions on a global scale.
New predators on the Mesozoic block
Most people have been led to believe that, although the earliest mammals appeared in the Triassic fossil record, throughout the Mesozoic they were tiny and meekly scurried and skulked while the dinosaurs reigned supreme over land, sea and air. They had to wait for the K-T extinction to develop their full ecological potential. That is now a myth, for Chinese strata (yet again) have revealed much larger mammals than ever thought possible, and some of them ate dinosaurs (Hu, Y. et al. 2005. Large Mesozoic mammals fed on young dinosaurs. Nature, v. 433, p. 149-152). One indisputable mammal skeleton contained the bones of young dinosaurs in its body cavity. In fact so many that one wonders if it met its end through greed.
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Mars, planet of 2004
As 2004 was but a few days old, there was much cheering at NASA’s Jet Propulsion Laboratory as the two Mars landers touched down safely and unleashed the two Rovers to deploy their instruments. Celebrations at ESA were not so universal, as the Beagle-2 miniature geochemistry laboratory vanished without trace. Beagle could in principle have proved the existence or otherwise of Martian life, had it survived and landed on suitable ground. Still, ESA’s Mars Express orbiter was safe and promised oodles of highly detailed pictures and other data. What followed was an embarrassment of riches from both the US and EU missions, more or less throughout the year. Then ESA had real cause for partying as 2005 opened, as its Huygens probe landed on the largest and most enigmatic moon in the solar system, Saturn’s Titan, but that is a story that will run this year, and it was carried courtesy of NASA’s Cassini mission. New Scientist featured an excellent summary of the achievements on Mars in its 15th January 2005 issue (Chandler, D.L. 2005. Distant shores. New Scientist 15 January 2005, p. 30-39). Everything has worked better than expected, Rovers Spirit and Discovery having the benefit of sand blasts that cleared the dust off their solar cells. They are still functioning, though not exactly prancing – it has taken a year for them to travel just over 5 km between them. But the treasures they have unfolded have delighted lots of geologists. There is ample evidence at least for the former influence of liquid water at the surface, which has both weathered the Martian surface to produce iron minerals that witness both water and highly acid conditions and also laid down sediments in layer after layer. Some hint at the former existence of a large shallow, salty sea where Discovery landed. Mars Express’s imaging devices have produced high-resolution pictures that confirm the influence of water’s sculpting, seemingly late in its history, and the presence of recent glacial deposits. The orbiter also carries a deeply penetrating radar device (MARSIS) capable of finding water up to a kilometre beneath the surface, though it has yet to be deployed. Perhaps the most intriguing find is that Mars’ atmosphere has more methane in it than seems possible, unless something is continually emitting it. That “something” could be volcanism (2004 also revealed signs of previously unknown, recent eruptions), methane may be leaking from sub-surface gas-hydrates similar to those beneath Earth’s sea floor, it could be emitted by icy material from comet debris, and maybe it signifies some primitive, methanogen life forms that are respiring. The last needs to be tied down very rigorously before scientists get over excited. Even if it matches up with signs of emitted water vapour, which it does, that could still be an abiogenic phenomenon. There can be little doubt that Mars is proving irresistible as a political draw, riding on its kudos to hammer out the old message that “Man Must Go There!”. But consider this: had today’s robotic technology and analytical miniaturisation been possible 35 years ago we would know vastly more than we do about the evolution of our neighbour the Moon. Instead of carrying astronauts and their weighty life support systems, the Apollo missions would have brought back an equivalent mass of lunar rock. The same goes for Mars, surely, on the old basis of getting “more bangs for your buck”. But that is a scientific outlook, and maybe the bucks can only be raised by the romantic notion of some brave souls treading where Edgar Rice Burrough’s John Carter once rode astride his banth. But of course, robotic science can also ride on that “vision”, for what could be more catastrophic to whichever US president succeeds in making George W. Bush’s dream come true to find that it is not safe enough out there, and the astronauts do not come back.
Plotting meteorite falls
Museums host collections of thousands of meteorites donated by collectors over more than a century. Although they are the source of much of our understanding about the timing and processes involved in the origin of the solar system and of the Earth itself, the collections are biased towards those that are most easily spotted on the ground. Metallic meteorites show up much more readily than do those made of silicate minerals, which resemble ordinary terrestrial rocks in colour and density. Only when collectors pore over very uniform, light coloured surfaces, such as ice caps, deserts and bare limestone plateaux, can they be assured of a truly representative selection of types. Also, many meteorite samples are weathered and contaminated with earthly materials, because they have lain around on the ground for a long time. Improved precision and detection limits of the chemical analytical tools that meteorite specialists use demand fresh material, as do researchers interested in organic materials carried from space – the embarrassment of having an announcement of a fossil bacterium in a meteorite and then finding that it is some common bug from soil is career threatening. Most important are trying to overcome the compositional bias and to see from which part of the sky different kinds of meteorite come. Phil Bland of Imperial College, London is trying to solve all problems at a stroke. His idea is to set up a network of wide-angle sky cameras to record meteor trails, so that computer analysis of the film will triangulate the point of impact and also work out the precise orbit of the offending body. The ideal place – easy to get to, safe, flat, dry unvegetated and dominated by pale rock – is the infamous Nullarbor (“No Tree”) Plain of SW Australia, which is one of the most featureless places on Earth. Bland already has one sky camera in place that has sensors that only turn it on if the sky is clear, and an internet connection that e-mails him if something as malfunctioned. In one year it spotted 12 trails bright enough to have resulted in meteorites falling to the surface. With three cameras, he hopes that results will be sufficiently accurate to narrow search areas to a square kilometre. If funded, the extended project will even incorporate e-mail alerts to teams of local collectors, whenever a trail exceeds a certain brightness. They should then be able to pristine recover material in a few days.
Source: Muir, H. 2004. Catch a falling star. New Scientist, 25 December 2004, p. 45-47.
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And was there a mighty wind?
Readers will be familiar with the to-ing and fro-ing that surrounds the idea of Neoproterozoic Snowball Earth episodes from earlier issues of EPN. The leading proponent and sturdy defender of the hypothesis, Paul Hoffman of Harvard University, re-enters the fray as co-author of a paper that builds on the idea that following global glaciation the climate became not only very warm but also violent (Allen, P.A. & Hoffman, P.F. 2005. Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature, v. 433, p. 123-127). They document evidence from “cap carbonates” in northern Canada and Spitzbergen that succeed diamictites of “Marinoan” (~635 Ma) age, in the form of large-scale sedimentary structures. Many of these are submarine ripples with amplitudes up to 40 cm, and forms that suggest they were produced by sea-bed motion due to surface waves, down to 200-400 m, far deeper than modern storm-wave base. Central to their argument is hydrodynamic modelling of wind speeds that might have produced such large ripples, and their specific shapes – steep sided. Being based on experiment and observation of modern sea-bed processes, the theory seems quite rigorous. It retrodicts wave periods that are somewhat longer than those commonly seen in modern ocean storms. From that they derive sustained wind speeds that exceed 70 km per hour across open oceans, extraordinary by modern ocean wind standards.
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Another year passed
Because of the horrific events at the end of 2004, this is not a time to celebrate geoscientific achievements during the year that has passed.
The horror of Boxing Day
Unlike the collapse of Manhattan’s Twin Towers on 11 September 2001 the world’s cameras were unable to focus on the minutiae of terrible events around the rim of the northern Indian Ocean. They did not catch the sudden dawning of fear, but the tsunamis of 26 December 2004 were witnessed by millions of coast dwellers in Indonesia, SW Thailand, Sri Lanka, eastern India and as far away as Somalia, Tanzania and Kenya. At the time of writing the death toll had reached 150 thousand, but it will rise inexorably, and countless people’s lives will be blighted for years to come. The world did change on Boxing Day 2004 in a way that dwarfs the events of “9/11”. The majority of those swept in minutes into a debris-loaded sea were among the poorest of their communities, and the dead are dominated by children and old people who simply did not have the strength to save themselves. News came first from popular tourist resorts dotted on palm-fringed beaches, through cell phones and from hastily shot videos of what must at first have seemed a curiosity. Before great waves appeared, the sea drew back to leave fish flapping on beaches, which local children rushed to gather as an unexpected benefice. Ocean waves driven by great seismic events have immense wavelengths, so previously unseen sea floor lingered for 10 to 20 minutes before devastating surges suddenly rose above the horizon..
Off the western coast of Sumatra, a subduction-zone thrust displaced the sea floor by several metres, into which an unimaginable tonnage of ocean rushed. Its rebound set in motion the most devastating natural phenomena, yet on the open ocean their passage would have been imperceptible because of their broad wavelength. Unlike wind-waves, tsunamis travel extremely fast, around 400 km per hour; they are seismic disturbances affecting the entire water body. The further they travel the greater the volume they affect, so they dissipate with distance. Two days after the initial shock, sea-level rose perceptibly in California, half a world away. When tsunamis meet shallows, the frictional effect causes the wave to slow, rise and steepen. The wave breaks far offshore in shallow water, resulting in a surge that rises inexorably on land. It rips up sea-floor materials, including boulders where they are present. Damage and deaths result mainly from the backwash that can rip debris and victims several kilometres out to sea, until the next tsunami arrives, and in this case there were at least three. We have all seen the aftermath, like nuclear devastation but not sterile. Debris, rotting flesh and sewage breed disease, and as many may die from cholera, insect-borne disease and exposure as perished on Boxing Day morning.
The magnitude of the Sumatran sea-quake was 9.0 on the Richter Scale. That is a logarithmic measure of the ground displacement, so that for every increase of 1.0 in magnitude ground motion increases by 10 times. However, it is the energy released that damages and the corresponding increase is 32 times. The Sumatran sea-quake was the largest recorded since that off Alaska in 1964 (magnitude 9.2) and the fourth largest in a century. Tsunamis generated off Alaska reached a height of over 60 metres close to the epicentre, but they travelled parallel to the coast of the Americas and caused only 130 deaths. Those of 26 December 2004 hit land head on, and there are large, densely populated coastal tracts around the Indian Ocean that are below 10 m above mean sea level. Buildings, particularly for poor people, are fragile and lightweight, so the devastation was almost total, unlike the effects of on-land earthquakes. In their case, single-storey dwellings that are little more than wood and grass structures cause less deaths than in areas with multi-storey dwellings made of stone or concrete, and the effects are localised.
The US National Oceanographic and Atmospheric Administration (NOAA) is responsible for tsunamis warnings for the eastern Pacific (http://wcatwc.gov/), which are issued in the same way as extreme weather warnings. Other organisations maintain a permanent watch and warning service for the entire Pacific basin, which is surrounded by the majority of the world’s large earthquake zones, mainly connected to subduction, and is the most prone to tsunamis. Using bathymetry and landmasses, it is possible to model in detail the wavefronts of tsunamis and their travel times for any circum-Pacific earthquake. So adequate warning is possible for most coastal areas following a major earthquake. The Indian Ocean has only one major, tectonically dangerous plate margin, where the Indian Plate drives beneath Eurasia to form the Sunda Trench off the Indonesian archipelago. Although discussed as recently as mid-2004, no tsunami-warning service is in place for the Indian Ocean. One reason given for this lack of foresight is that the north-western part of the Sunda Arc has had little major seismicity for more than 150 years. Therein lay the danger; subduction was locked and a major earthquake grew more likely the longer the quiescence lasted. All the world’s seismic observatories recorded the massive disturbance and the exact location of the Sumatran event within minutes of its occurrence, but no warnings were issued. The tsunamis arrived in Sri Lanka and India over 4 hours later, though within less than half an hour in Thailand and Sumatra, which were most devastated. For the millions whose lives have ended or are in ruins, the greatest advance in the geosciences, plate tectonics, utterly failed them.
What is to be done?
Many geoscientists take pride (and a fair amount of public funding) in focusing their research on dangerous natural phenomena, supposedly aimed at giving warnings or mitigating their effects. Take any natural calamity, whether it be earthquake or tsunami, volcanic eruption, mudslide, flood or even something so simple as helping provide clean water for the victims of drought or displacement by conflict. Now list the lives saved by the direct efforts of geoscientists against the torrent of their publications and attendances at conferences. Is there any cause for pride in the instrumentation, the theory and the field experience? Or should we reflect on the hubris of scientific endeavour in the aftermath of such awful events? Two days after the disaster struck I put together detailed topographic elevation data (from the Shuttle Radar Topography Mission – SRTM) for the coastlines that surround the Indian Ocean. I had had them for 6 months, but did nothing except make some pretty maps for a conference presentation. I had known what potential they have for predicting areas of flooding, and more important where refuges from inundation might be, but I found “better” things to do. All coastal areas below 15 m elevation are at risk, and a great many correlate with the tsunamis’ worst effects. What we could have done and what we did do generally emerge only in retrospect, and indulging in mea culpa serves no purpose. Individuals have their own agendas, and they are rarely useful in any wider sphere. The organisations that draw scientists together are not in themselves altruistic, but serve largely academic ends. However, human tragedies surely remind us of a wider set of responsibilities, even if only momentarily.
A collective organisation of both knowledge and real needs, which sets aside career and the “advancement of science”, is probably the only means of putting the geosciences to work for fully human benefit. For 40 years such a collective existed in the small form of the Association of Geoscientists for International Development (AGID), which aimed at knowledge transfer from its members to less fortunate areas. For the latter half of its existence, AGID was subsidised by the Canadian International Development Agency. A handful of members met at the 32nd International Geological Congress in Florence during late August 2004, the agenda being wholly about its survival or winding up following CIDA’s withdrawal of financial support. The vote went for continuation. But, despite helping some young geoscientists of the “Third World” make progress, AGID’s small size and limited aims and funding have proved unable to make it a force that matches real needs or the geosciences’ potential for assisting development. Data and theory now present the opportunity to resolve two great challenges: giving every man, woman and child on the planet access to safe drinking water; and predicting and mitigating all natural hazards. Every senior politician in the developed world pays lip service to both, each UN agency convenes to discuss them on a regular basis, and the International Union of Geological Sciences (IUGS) has proposed the International Year of Planet Earth (2005-2007) with those themes at the top of its agenda. IGC-32, the largest ever gathering of geoscientists was dominated by humanitarian themes and the launch of the IUGS initiative. The International Year of Planet Earth was supposed to have been proposed by the Peoples’ Republic of China at the September 2004 UN General Assembly 59. It does not appear on the UN web site, and the supporting web site www.esfs.org gives no news of its adoption. But there have been many “Years of…” and even several “Decades of…” from which we have yet to see any tangible outcomes, and little of the “awareness” that they are supposed to generate, certainly not in those areas of the world towards which they were directed.
One collective of professionals that has had a powerful impact on emergencies since 1971 is Médicins sans Frontières (www.msf.org), founded and administered independently of the world’s “great and good”. Would a geosciences equivalent be feasible and supported? Yet there are measures that even individuals can take. At a UN Office for Outer Space Affairs meeting on the use of satellite data for mitigating disasters (Munich, 18-21 October 2004) www.zki.caf.dlr.de/events/2004/unoosa_workshop/unoosa_programme_en.html – Margaret Andrews Deller of the UK Open University presented her ideas (see link at the above web site) on a simple and low-cost way to reduce the impact of natural disasters. Briefly, she based her suggestions on indigenous people’s deep knowledge of their surroundings. Recognising that, it should be possible to provide communities with graphic images that highlight potential threats, in forms that are low-cost and easily understood by anyone, such as the use of images that incorporate perspective and show features in near-natural colours. Her most important point is that such information would not be just a warning, but a means of showing people their homeland in a way that they can learn from and value. That would bring together those affected with those who come to their aid, should catastrophe strike, and would empower local people to take charge of their lives instead of being victims.
Easily understood information and advice is vital for potential victims of catastrophes, and a quick search of the internet reveals lots on all manner of hazards and how to avoid them. NOAA’s tsunamis website http://wcatwc.gov/ is an excellent example. Such information needs to be recast into forms that people outside the “information society” can easily understand, and to be distributed – not such a massive task. What drew children to south Asian beaches on Boxing Day, the massive withdrawal of the sea, is the first sign of a tsunami. On Pacific islands everyone knows what threat such a weird occurrence signifies, but nobody on the rim of the Indian Ocean did. But isn’t it also essential for geoscientists to donate some of their publicly and industrially funded time to share their expertise directly with those so much less fortunate than ourselves? Without that, our claim to be resolving humanity’s problems is a transparent sham.
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Another large igneous province implicated in mass extinction
At the end of the Triassic Period, around 200 Ma ago, life underwent a major crisis that so far has not been believably connected to either extraterrestrial or geological causes. Previous studies have shown that the mass extinction was accompanied by an decrease in 13C in sediments that suggests a short-lived global warming of between 2-4 °C at the Tr-J boundary. That CO2 levels rose is suggested by a decrease in the density of pores (stomata) on fossil leaves. It has been suspected for some time that the largest known continental igneous event, which accompanied early rifting of the modern Atlantic Ocean basin may have been responsible, but so far the dating of this Central Atlantic magmatic province (CAMP) has not been tied to the boundary conclusively. A large consortium of Italian, French, US, Moroccan and Swiss has addressed the sedimentary and igneous record around Tr-J times in the High Atlas of Morocco (Marzoli, A and 14 others 2004. Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis. Geology, v. 32, p. 973-976). There, one of the few uneroded continental flood basalt sequences of CAMP (most preserved CAMP magmas are in the form of sills and dykes in offshore basins) occurs among Triassic and Jurassic sediments. Their base deforms the underlying sediments, suggesting that eruption was onto unlithified sediments, shortly after their deposition. Fossils from the sediments are of little help in tying down the age of eruption, however, Ar-Ar ages of the lavas are all within error of 200 Ma, and tally with magnetic stratigraphy from the Tr-J boundary elsewhere. Both age and geochemistry of the flows are remarkably similar to those of flood basalts from the other side of the Atlantic. Magmatic duration, like that in other large igneous provinces was of short duration, no more than a couple of million years. So it now seems that three of the “big five” mass extinctions (the others are end-Permian, connected with the Siberian Traps, and the K-T boundary and associated Deccan Traps) have at least a partial cause from CO2 release by massive volcanism.
Iron isotopes enter the Archaean life debate
Some years ago geochemists obtained carbon-isotope data from 3.8 Ga rocks in Greenland that seemed at the time to be persuasive evidence for the emergence of life during or shortly after Earth’s most traumatic period. Up to 3.8 Ga the Moon was bombarded by huge projectiles, and its companion Earth would have received at least 13 times the flux of destruction. The carbon was within sturdy apatite grains from supposed iron-rich metasediments, and may have been preserved from later high-grade metamorphism. Doubt has been cast on that hypothesis, either because of the unlikelihood of any carbon remaining unfractionated by heating, or because some aspects of the rocks’ geochemistry suggested that they we of igneous origin rather than sediments. Readers will have seen in previous years’ EPN that a controversy rages over even tangible signs that suggest cellular material from rocks half a billion years younger. Geochemists from France and the US have taken a different tack with the ancient Greenlandic rocks that ought to at least resolve the igneous versus sedimentary origin of the banded iron-rich rocks (Dauphas, N. et al. 2004. Clues from Fe isotope variations on the origin of Early Archean BIFs from Greenland. Science, v. 306, p. 2077-2080). They found that the heavy iron isotope 57Fe is more enriched in the ironstones than in any igneous rocks, with little chance that the difference was induced by thermal fractionation. They are metasediments. But therein lies a surprise. The heavy-iron signatures are greater than in less aged banded ironstones. One way in which that could have arisen is from biogenic precipitation of soluble reduced Fe-2, perhaps involving anoxygenic photosynthesisers – because of the strong capacity of photosynthesis for setting electrons in motion, all such organic reactions create local oxidising conditions, whether or not oxygen itself is produced.
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Torrid times in the Cretaceous Arctic
Despite its latitude (above the Arctic Circle) the sedimentary depocentre of northern Alaska is becoming famous for its Cretaceous terrestrial flora and fauna. Plant remains indicate luxuriant vegetation cover, and high excitement greeted the discovery of 8 species of dinosaurs (4 herbivores and 4 theropod predators (Fiorillo, A.R. 2004. The dinosaurs of Arctic Alaska. Scientific American, v. 291(6), p. 60-67). How dinosaurs were able to survive the darkness of the Arctic winter is a bit of a mystery, unless the migrated as do modern caribou – Fiorillo cites evidence for small juveniles that would have been unlikely to have migrated far, because compared with adults they were much smaller than young caribou. There would have been sufficient winter biomass for survival during the Cretaceous, but seeing and being active as cold-blooded reptiles pose problems. At least one of the species had unusually large eyes, so one of the conditions for dinosaur’s remaining year-round seems established. New data regarding climatic conditions in the far north have turned up after an most unusual and intrepid programme of drilling through a drifting island of pack ice over the Arctic Ocean’s Alpha Ridge, not far short of the geographic North Pole. An extraordinary feature of the programme is that it took place between 1963-74, the core having only been examined in detail in the last year (Jenkyns, H.C. et al. 2004. High temperatures in the Late Cretaceous Arctic Ocean. Nature, v. 432, p. 888-892). The Late Cretaceous part of the cores is black mud rich in terrestrial vegetation remains and marine diatoms, and totally lacking in evidence for dropstones and other debris from floating ice shelves. Unfortunately, the Arctic sediments lack carbonate-shelled plankton remains, so the now standard method of sea-surface temperature measurement is not possible. However, Jenkyns et al. were able to use a method based on the fatty acids that survive in plankton membranes, results from which match oxygen-isotope palaeo-temperature measurements in Cretaceous cores from lower latitudes. Astonishingly, even at polar latitudes, the Cretaceous Arctic Ocean seems to have been as warm as 15°C. Climate modelling based on lower latitude data and estimates of CO2 concentration in the Late Cretaceous atmosphere falls around 10° short of these levels. The conventional modelling requires 3 to 6 times more “greenhouse” warming than generally accepted, to account for Arctic sea temperatures in which we could swim in moderate comfort. Possibly the modelling is awry. One of the most important features of Late Cretaceous palaeogeography was a major seaway across North America that connected the Arctic with tropical latitudes. It existed because global sea level was far higher than now, probably due to the oceans’ volume having been substantially reduced by huge magmatic outpourings on the floor of the West Pacific basin (the Ontong-Java Plateau), earlier in Cretaceous times, together with higher rates of sea-floor spreading. The seaway would have been shallow, and thereby easily warmed. Had poleward currents been possible in it, their flow would have acted very like the modern Gulf Stream to warm high latitudes. Despite palaeoclimatologists reliance on models of heat circulation, it needs to be remembered that they are based on grossly simplified geographic features. If they get it very wrong indeed for the well-studied Cretaceous, that casts doubts on climate modelling’s predictive powers for the course of current climate evolution.
See also: Poulsen, C.J. 2004. A balmy Arctic. Nature, v. 432, p. 814-815
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An electronic antidote to eclecticism
It is a plain to me as to any reader that EPN is eclectic, and in some cases pretty impressionist; how else to write a monthly weblog about the broad spectrum of geoscientific developments? So it is good to see websites with a much narrower focus, yet that manage to inform entertainingly and provocatively. Such a site is www.mantleplumes.org, organised by Gillian Foulger of Durham University, currently a visiting scientist with the Volcano Hazards Team at USGS, Menlo Park, USA. It covers the whole of “plumeology”; the tectonics, the magmatism, ages and wider features, even ideas about the presence or absence of plume-related features on other planets. It has some powerful contributing essayists, such as Don Anderson and Warren Hamilton, who are not averse to scepticism and critiques, and represent work in progress on a book, Plates, Plumes & Paradigms just submitted to the Geological Society of America – a rare event to see preprints of book chapters. It serves an educational role as well, with well-illustrated and up-to-date reviews of the mechanisms involved in large-igneous provinces., and thumbnails on a continent-by continent basis. Jason Morgan came up with the “hot-spot” idea about 33 years ago and launched a revolutionising force in plate tectonics. It is good to see that there is still a vibrancy about the topic.
Ancient art
The hallmark of modern human’s abilities is the art left behind by our ancestors since about 30-40 thousand years ago. Among the most enigmatic are those by Australian native people, that might date back as far as 50 ka. The first were discovered by Joseph Bradshaw and his brother in the Kimberly Ranges of northern Western Australia in 1891. The Geneva-based Bradshaw Foundation (http://www.bradshawfoundation.com/) is developing a comprehensive archive of rock-art images from across the globe, which will uplift anyone who visits it.
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Bacterial reduction of arsenic contamination
Following the tragic discovery ten years ago that tens of millions of Bangladeshis drink groundwater that is naturally contaminated by arsenic, the lessons learnt there have been applied on a global scale. That has resulted in further cases with similar causes coming to light. Remediation is chemically quite simple, and since the US reduced the maximum permissible arsenic level in public water supplies from 50 to 10 parts per billion in 2001 research into methods of removal have increased rapidly. There are a number of methods that are based on adsorption of arsenic by iron and aluminium hydroxides and are low-cost. But it seems that biological activity in aquifers can be equally effective (Kirk, M.F. et al. 2004. Bacterial sulphate reduction limits natural arsenic contamination in groundwater. Geology, v. 32, p. 953-956). In the anaerobic conditions that favour the dissolution of iron hydroxide, which is often the most important source of arsenic in sediments, the conditions are also suitable for chemotrophic bacteria. Among these are species that obtain metabolic energy from the reduction of sulphate ions to sulphide. Where metal ions are also present, they combine with the sulphur to precipitate sulphide minerals. In turn, sulphides readily accept arsenic from solution, thereby helping decontaminate potentially dangerous groundwater. Arsenic-bearing groundwater is also found to have high methane levels, which suggests that methanogenic bacteria dominate its micro-ecosystem when sulphate ions are at low concentrations. Perhaps it will prove possible to encourage sulphate-reducers to thrive in such waters, by the addition of some sulphate by injection. That would a cheap remedy to what seems to be a growing risk in areas that extract groundwater from aquifers that are full of organic matter that creates the oxygen-free conditions that release arsenic into solution.
Bacteria in groundwater seem to have another benefit. Where landfill contaminates subsurface waters with a cocktail of pollutants, the nutrients encourage bacterial colonisation, often in the form of biofilms in pore spaces. It seems that their metabolism generates electrical currents (Gosline, A. 2004. Bug “batteries” send out pollution alert. New Scientist !8 December 2004, p. 17). These create electrical potentials of several hundred millivolts that are easily detected by passive electrical monitoring. The voltage highs occur at the margins of pollutant plumes in the groundwater, and can therefore be used to monitor spread of contamination and to indicate safe supplies.
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