Earth-logs

Getting to the matter of the root

Published on May 1, 2006 by zooks777Leave a comment

As well as by its own low density, continental crust may be prevented from subduction because of the strength and buoyancy of cold, thick mantle that forms a root beneath the oldest cratonic crust. Geophysics shows that such roots are there, and in the case of African cratons they merge with the deeper mantle without the intermediate, more ductile asthenosphere: in a sense Africa is ‘nailed’ in place and barely moves. Except for xenoliths in some continental volcanic rocks and in kimberlite pipes, samples of the deep continental lithosphere are uncommon. One place where they are abundant at the surface is in the zone of ~400 Ma continent-continent collision in western Norway (Spengler, D. et al. 2006. Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. Nature, v. 440, p.913-917).

These rocks are Archaean (~3.3 Ga) in age, and contain tiny diamonds. Their more common metamorphic minerals indicate that the peridotites stabilised at depths of about 180 to 250 km. Yet they carry trace element and mineralogical evidence that they formed as residues of partial melting from a body of mantle that rose from almost 400 km down. Compositionally, they seem to represent an outcome of high degrees of partial melting, probably to release high-magnesium or komatiitic magmas that are only common in early Archaean greenstone belts. Most likely, this peridotitic root material continued to rise, eventually to underplate Archaean continental crust. Unable to melt any further, being depleted in incompatible elements, the root became a permanent and very rigid fixture once it had formed. Regarding the unending, but probably fruitless quest for crustal materials that predate 4.0 Ga, other than a snuff-pinch of tiny zircons, this well-supported model for cratonisation perhaps offers an explanation. No doubt in the higher heat-producing mantle of Hadean times komatiite magma was the norm for oceanic crust formation, and such depleted, high-pressure peridotite residues formed continually. Unless they rose to adhere to substantial low-density sialic crustal masses, they would be recycled back to deeper levels. Equally, without the support of such rigid underplates, any sialic material at the surface would have been unable to withstand deformation and would become subductible by tectonic mixing with more common, dense, mafic-ultramafic oceanic lithosphere. A great deal of Archaean tectonics suggests that continents then were not fully cratonised – Archaean crustal rocks seem to have been pervasively and repeated deformed, cratons of undeformed old rocks not appearing until the Proterozoic, when modern plate tectonics became established.

Acasta gneiss and another old zircon

Readers may by now be satiated with comment on geriatric zircons. Most of them – and they can be counted – are detrital grains that survived around a billion years of sedimentary processes to end up in an otherwise common-or-garden quartz-rich sandstone in Western Australia. Their number has been added to by one more grain, which might be cause for jollification in some quarters, because its host was a piece of deep continental crust of good provenance (Iizuka, T. et al. 2006. 4.2 Ga zurcon xenocryst in an Acasta gneiss from northwestern Canada: evidence for early continental crust. Geology, v. 34, p. 245-248).

The Acasta gneisses form the western flank of the Slave craton in northern Canada, and are the world’s oldest rocks, having formed at 3.94-4.03 Ga as a series of plutonic rocks of tonalitic to dioritic composition. Archaean geochemists from various Japanese universities, and a lone Briton from Leicester University, understandable wished to confirm and refine the age of the Acasta gneisses as the earliest ‘golden spike’ in the continental crust , and subjected many zircons extracted from gneiss samples to the latest mass spectrometric dating that uses the U-Pb scheme. Indeed they achieved excellent precision to the nearest few tens of Ma. Using an ion microprobe, they were able to date the zoned interiors of the zircons, revealing progressive crystallisation of the grains, mainly as the igneous precursors of the Acasta complex evolved. In a single grain, however, they came upon zircon in its core that was 200 Ma older. That tiny, trapped granule itself had engulfed even smaller particles of apatite, unlike the bulk of the whole grain.

Ion microprobes are wonderful pieces of kit, as they can give extremely precise and revealing trace element abundances in the mineral into which they burn a hole. In the case of the aged zircon core, such analyses revealed clearly that these few micrograms of zirconium silicate had formed from a magma with broadly granitic composition. Their conclusion: pre-4 Ga granitic crust was more widespread than previously thought. No, not the Acasta gneiss, but whatever material its igneous precursors had picked up while they were magma. In the previous comment in this section, I put forward the view that sial may well have formed before tangible continental material had stabilised as a permanent resident at the Earth’s surface. Yet, for reasons that seem to be emerging, such crust would not have resisted subduction and ended up mixed back into the mantle. Since the Acasta gneisses were most certainly not formed before 4.0 Ga, then it is from their mantle source region that their igneous precursors must have picked up this tiny, alien xenocryst. Unless, that is, someone can show me a 2-5 kg lump of gneiss heaving with these blessed grains (preferably with signs of almost as old crustal deformation). There is an obvious prediction to make. Geochemists are fighting in a heap to acquire ion microprobes and inductively-coupled, laser-ablation, plasma-source mass spectrometers, and why ever not? Now they have something to aim for instead of trawling quartz sandstones for relics of Earth’s Hadean past. My prediction is that every single mantle-sourced rock of granitic composition, whatever its age, will contain at least one pre-4.0 Ga zircon granule. Zirconium silicate is sturdy stuff.

Clays and the rise of an oxygenated atmosphere

Published on March 1, 2006 by zooks777Leave a comment

Almost all eukaryote organisms require oxygen to be available in their environment. Therefore the eukaryote cell probably appeared only after oxygen had become a permanent component of the atmosphere and hydrosphere, which itself depended on photosynthetic metabolism outweighing the scavenging of free oxygen by abundant dissolved iron. It also depends on efficient burial of dead organic matter. For the metazoa – multicellular eukaryote animals – the oxygen demand rises with their bulk. The first tangible fossils of metazoans appear in Ediacaran, after the last global glacial episode of the late-Precambrian, around 600 Ma ago. Apart from the evidence for an oxygen bearing atmosphere after about 2.4 Ga, not much is known about actual levels of oxygen and their changes during the Precambrian. The sudden emergence of the soft-bodied but bulky Ediacaran faunas has been ascribed by many to an equally abrupt rise in the availability of oxygen, on which their evolution must have depended. How that might have occurred has been disputed and pretty vague.

The central requirements to boost oxygen levels are increased photosynthesis – difficult if the period preceding the Ediacaran was one where large tracts of ocean were covered with ice – or increased burial of dead organic matter. The second option is also difficult to imagine if ‘snowball’ conditions had reduced living marine biomass to a very low level. What geoscientists have not been able to grasp, is information on the efficiency with which dead organic matter was buried. Mineralogists and geochemists from the Universities of California (Riverside) and Maine have addressed that aspect from the standpoint of the Precambrian history of clay mineral deposition (Kennedy, M. et al. 2006. Late Precambrian oxygenation; inception of the clay mineral factory. Science, v. 311, p. 1446-1449). If organic matter is buried in porous and permeable sea-floor sediments, the chances of its metabolism by bacterial action is high. Research on modern sea floor sediments shows that the bulk of organic debris at continental margins is adsorbed onto clay-mineral particles, thereby increasing its chance of preservation over simple incorporation as particles in silt-sized sediment. Kennedy et al. tested the hypothesis that sedimentation in the late-Precambrian changed from dominance by physically weathered micas and other silicates to one more dominated by products of chemical weathering on the continental surface, i.e. clays.

Around 700 Ma, the record of marine strontium isotopes in limestones began a major change towards higher ⁸⁷Sr/⁸⁶Sr ratios, suggesting an increase in the chemical weathering of ancient continental rocks. Australia provides a continuous sequence, from 850 to 530 Ma, of quietly deposited shelf sediments that span this transition and also contain the Ediacaran. Sure enough, the mudstones in the sequence show a distinct increase in swelling clays and kaolinite, implicated in modern preservation of dead organic matter. Rather than an abrupt step, the increase is linear from about 800 Ma, and is matched by similar data from other Precambrian cratons. What might have started this chemical weathering of the land surface? Possibly it was due to a much earlier colonisation of the land than direct evidence suggests. The DNA-based phylogeny of mosses, fungi, lichens and liverworts – all terrestrial organisms – suggests that they arose between 700 and 600 Ma ago. All would have contributed organic acids to the process of chemical weathering. Kennedy et al. model the rate at which free oxygen would have increased as a result of increased deposition of clays, and conclude that between 730 and 500 Ma retention of oxygen in the environment would have increased six-fold. Thereafter, land-based organisms and further colonisation permanently increased weathering, establishing increasingly efficient marine burial of organic debris, and so creating an environment in which metazoans could evolve and radiate. If confirmed by further analyses, this work establishes yet another non-uniformitarian process in the Earth system.

Google Mars

Published on March 1, 2006 by zooks777Leave a comment

Have you exhausted the possibilities in Google Earth – unlikely – then why not try Google Mars (www.google.com/mars)? Well it’s a bit early, as the site is still under construction, and does not yet include the features that enrich the Google Earth experience or the full planetary surface. Nevertheless the University of Arizona, which produced the data mosaics, has provided a bright, colour-coded elevation map and mosaiced images in visible and infrared wavelengths that show enough detail to easily examine many of the landforms for which the ‘Red Planet’ has become renowned. It is a fine resource for targeting users to find specific kinds of feature – craters, dunes, water-carved valleys and lava flows. Once complete it should satisfy anyone who wants to explore, probably including those with delusions of ‘boldly going…’ before they become too old and infirm….

Breathing life into ‘Snowball Earth’

Paul Hoffman’s hypothesis of episodes, mainly in the late-Precambrian, when Earth was encapsulated in ice from pole to pole has taken repeated knocks since he first proposed it. It seems only natural that he should make the evidence and his ideas more publicly available on the Web – http://www.snowballearth.org. ‘Snowball Earth’ is a live and important aspect of geoscientific debate, for a whole raft of reasons, and it continually evolves. Although Hoffman does use the site as a vehicle for rebuttals to all the objections that further research has raised, it is a great deal more interesting and useful than that: a very well produced resource for anyone interested in a crucial period – the Neoproterozoic – in the evolution of life. Additionally, it helps budding geoscientists come to grips with the intellectual and experimental processes involved in major advances in knowledge and understanding. Besides which, it will save Hoffman a small fortune in air fares to have his say to live audiences!

Gaia: the ultimate frontier

Published on March 1, 2006 by zooks777Leave a comment

That life plays a role in surface geological processes is self-evident. Death and the burial of dead organic matter feed back to climate by removing carbon from the atmosphere and hydrosphere, thereby reducing the ‘greenhouse’ effect and increasing the oxidation potential of the outer Earth – a discovery of the late 20^th century. James Lovelock’s Gaia hypothesis proposes that life’s influence as a means of balancing conditions for its own continuity is a primary factor behind the behaviour of our home world, although a great many geoscientists doubt that bold generalisation. It seems to many that the influence of both deep mantle processes and extraterrestrial forces not only provided the conditions for planetary evolution, both inside and at the surface, but created the conditions for life’s emergence and its survival. Life has been pushed to the brink of complete extinction several times by both truly primary parameters. Yet Gaia is still a persuasive idea, or at least a metaphorical itch that must be scratched from time to time. Perhaps the boldest attempt at pushing Lovelock’s notions to the limit appears in a recent essay (Rosing, M.T. et al. 2006. The rise of continents – An essay on the geologic consequences of photosynthesis. Palaeogeography, Palaeoclimatology, Palaeoecology v. 232, p. 99-113).

Assuming that carbon-isotope evidence from the oldest sediments known (3.8 Ga, West Greenland) that life selectively took up light ¹²C is valid, there seems to be a remarkable coincidence between the origin of life on Earth and the oldest known continental rocks (4.0 Ga, northern Canada). Rosing et al. suggest that this is no coincidence, but the result of the effect of living organisms on magmatism at subduction zones, most particularly on the mineralogy of old oceanic lithosphere that descends there. Their essay starts by emphasizing that modern photosynthesis contributes three times more energy to surface processes than does heat flow from the mantle, and that energy must accomplish a commensurately significant amount of mainly geochemical work, some of which occurs in basalts of the ocean floor as they spread from constructive margins. Continental crust is widely accepted to form as a result of hydrous fluids rising above subduction zones to cause different conditions for melting of the overriding mantle wedge than those for partial melting of mantle rock beneath mid-ocean ridges and oceanic islands. Multistage fractionation processes that operate on basaltic magmas formed by this wedge melting result in separation of residual magmas that are sufficiently enriched in silica and other elements to crystallize as, broadly speaking, granitic rocks. Since they cannot be metamorphosed to a form that exceeds the density of the mantle, such rocks cannot be subducted, unless debris shed from them mixes as sediment with subducting oceanic lithosphere. So continents become more or less permanently growing edifices on the face of the Earth. The central questions that Rosing et al. focus upon are: why did continents not form from the outset of the Earth’s evolution, once tectonics and oceans had stabilized, and why the coincidence? Their answer to both is that life played a fundamental role in increasing the amount of water that ends up in old, cold oceanic crust, thereby helping the peculiarities of wedge melting to become established. Essentially they appeal to life’s ability to transform energy of different sources, for example heat from the mantle and the energy carried by electromagnetic radiation, and transmit it through biogeochemical cycles from its source to the lithosphere. Specifically, they speculate that this life-mediated energy transfer accelerated the conversion of dry minerals in basalt to water-rich clays. In turn, that had its effect on subduction-zone geochemistry.

Rosing et al.’s seems to have a willful flaw: they focus on the incorporation of solar energy into the Earth system by photosynthesis from the time when continental materials first appeared in substantial bulk, between 3.8 and 4.0 Ga. So far there is a mere shred of evidence from ambiguous carbon isotope studies that photosynthesising organisms were around before about 3.4 to 3.5 Ga. There is no trace of such shallow-water organisms as stromatolites until that time. Nor is there any significant sign of where one end product of photosynthesis, oxygen, must have been secreted away by reaction with dissolved iron(II) – banded iron formations only become prominent in the later Archaean. Whatever organic activity might alter ocean-floor basalts, it is hardly likely to have used photosynthesis, unless the early oceans were shallow enough (200-300 m) to pass light to their floor. The key to alteration of anyhydrous minerals in basalt to form clays is the availability of hydrogen ions (products of oxidation) to donate electrons through hydration reactions, and they are available from a great many processes other than living ones. Then, of course, there is the key issue of whether any influence – direct or indirect – by photosynthesis can be seen on modern ocean-floor geochemical processes. Since it doesn’t go on down there, whereas a great many oxidation reactions that produce hydrogen ions do, makes the hypothesis impossible to test. In fact it is not a hypothesis but speculation, and it has a great deal of company from other ideas to explain the missing 600-800 Ma of Earth’s evolution. Most of those centre on the mechanics of slab-pull force, the pace of sea-floor spreading and the angle of likely subduction during geothermally much hotter times. Oddly, the third author, Norman Sleep, introduced a great deal of basic theory behind these other explanations. This is one of two articles from March 2006, whose time of publication – close to 1 April – may give a clue to its weight. It is interesting seasonal reading, and everyone should look forward to further debate. However, like the magnificent Verneshot hypothesis (See Mass extinctions and internal catastrophes in June 2004 issue of EPN), it may die in a deafening silence.

Methane, methanogens and early climate control

Expulsion of methane from gas hydrates in shallow marine sediments has been implicated several times as the likely cause for sudden bouts of global warming, such as that at the end of the Palaeocene 55 Ma ago. The gas, produced by primitive, anaerobic prokaryotes known as methanogens, is more powerful at delaying loss of heat to space than is carbon dioxide. It is a greenhouse gas of enormous potential power, although in an oxygen-rich atmosphere it has a short life before being oxidised to CO₂ and water. Methanogens themselves, which survive only in airless places, evolved very early in the Earth’s history as witnessed by their genetic molecules being very different from those of other members of the Bacteria and Archaea domains. The ambiguities of carbon isotopes in ancient carbonaceous rocks being able to discriminate different metabolic processes, has led to considerable debate about when methanogens first made their appearance. That was probably well before the oceans were able to contain dissolved oxygen, which is highly toxic to anaerobic prokaryotes, i.e. in the Archaean. A good sign that such cells were around would be, in some way, to detect their main metabolic product, methane. The place to look would be in fluid inclusions enclosed in minerals that were definitely produced by seafloor sedimentary processes. The best candidate would be quartz in cherts precipitated from seafloor hydrothermal vents, where such organisms would have both the energy and the fuel to thrive. A group of Japanese geochemists have systematically looked for such fluid inclusions in a variety of Archaean cherts and they found sufficient evidence to at least give a minimum age for the presence of methane-producing bugs (Ueno, Y. et al. 2006. Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era. Nature, v. 516, p. 516-519).

The Dresser Formation (3.45-3.50 Ga) in the early Archaean of Western Australia contains abundant pillow basalts and chemogenic, silica-rich sediments. These cherts seem to have been fed by fissures through which hydrothermal fluids moved, and it is quartz from these syn-sedimentary quartz-rich dykes that revealed abundant fluid inclusions that had clearly formed as the quartz crystals grew. The inclusions contain carbon dioxide with traces of methane. Most important, the carbon in the methane is highly enriched in heavy ¹³C, evidently due to cell processes drawing in the lighter isotope ¹²C; the methane is almost certainly biological in origin. So it is possible to say both that methanogens had evolved before 3.5 Ga, and that they added methane to the Archaean atmosphere. Such a highly reduced gas would become a permanent constituent of the air, because oxygen had yet to be released by other organisms so that methane would be oxidise quickly, as happens today. The discovery by Ueno et al. is important from another standpoint than the appearance of a particular kind of metabolic process.

From the time of its accretion until well into the early Precambrian, the Earth received a great deal less energy from the Sun than it does today. Solar hydrogen fusion had not then achieved the level of efficiency that it has now. Without some means of trapping heat in the atmosphere, the Earths mean surface temperature would have been well below the freezing point of water. Without a ‘greenhouse’ effect, the planet, well endowed with water, would have been inescapably locked inside a thick crust of ice. In some respects it would have resembled a large version of one of the Outer Planet’s icy moons, such as Enceladus (see Yet another weird world later). Life would have found it difficult to emerge, if at all, at such low temperatures. Like Enceladus and other distant moons, some liquid water would have been present due to heating from the mantle and magmas, but the white surface would always have reflected away most of the Sun’s heat – geothermal heat is vastly less than that of solar origin. The most recently proposed means whereby the Earth could have escaped permanent frigidity and sterility from the ‘weak, young Sun’ is that volcanic exhalation of CO₂would eventually have developed ‘greenhouse’ conditions. However, it would have had to reach much higher atmospheric concentrations that now, perhaps greater than some geochemists believe to be theoretically possible. Being a much more powerful ‘greenhouse’ gas, methane helps overcome such theoretical difficulties. It can only be produced in quantity by biological processes, and that poses a conundrum, despite Ueno et al.s discovery. Without an atmosphere containing gases that could trap solar warmth since shortly after planet formation, the cold trap would have taken an icy grip holding back the emergence of life, such as primitive methanogens. Does that therefore imply that such organisms emerged far earlier than the start of tangible geological history?

Puffing up the Moon

Published on March 1, 2006 by zooks777Leave a comment

Since George Bush announced that US manned planetary missions are back on the agenda, albeit in an uncertain future for NASA, barely a month goes by without some kind of scientific justification for a return to the ‘good old days’. The latest as regards future lunar missions was in the 1 April 2006 of New Scientist, as a special report ‘It’s time to go back’. It seems there are unique opportunities that the Moon presents for a range of scientific work (Chandler, D.L. 2006. The ultimate lab. New Scientist,1 April 2006 issue, p. 33-37). The lunar far side, being shielded from radio noise from Earth, is well suited to deploying an array of miniature radio telescopes. Half a dozen 1 m dishes spread over 20 km could simulate an enormous dish. The lack of an atmosphere suggests ideal stable conditions for optical telescopes, although being on a body with a large gravitational attraction would expose instruments to meteor flux. The lunar south pole is said to look good for science. For a start, there is a 5 km peak always lit by the Sun for continuous solar power, as well as data relay back to Earth. Nearby is the deep Shackleton crater that is never lit, and is immensely cold; ideal for an infrared telescope, and maybe harbouring water ice to support a manned lunar base.

The Apollo missions returned sufficient rock and soil samples to whet planetary scientists’ appetites. They answered a lot of questions, and did revolutionise issues of planetary origins, evolution and bombardment history, yet they raised other interesting questions. Answering geological questions from the rocks of other worlds depends a great deal on luck, and the few small sites visited by the Apollo astronauts undoubtedly left out a great deal. What is needed, it seems is a ‘Serendipity Base’. The best one would be a deep crater with steep, rocky sides, and there is one that seems just right. The Aitken basin is 12 km deep and exposes a layered structure in its walls.

Perhaps the greatest attraction is the fact that anything that falls on the Moon remains in its pristine state for all time, provided it is not buried by accumulated meteoritic dust and impact ejecta. The Moon could be a really happy hunting ground for meteorite specialists, although finding interesting ones on the dull, grey surface might pose problems – you can tell a meteorite on Earth, if you search ice sheets, deserts and saline flats, by their contrast with the background. There is a very odd notion, however, that well-preserved ejecta from impacts on the Earth and other planets that found their way to the lunar surface might hold the keys to the origin of life (Ward, P. 2006. House of flying fossils. New Scientist, 1 April 2006 issue, p. 38-41). The reasoning goes like this: like the Moon, all planets in the Solar System have for 4.55 Ga been whacked by impacts, which must have flung debris outside their gravitational attraction. Having a strong gravitational field itself, the Moon must have swept up a sizeable representative sample of all such debris hurtling around the Solar System. Some of the biggest impacts – again as revealed by the lunar surface – were early in planetary evolution. Debris from them would therefore be samples of materials before they had been affected by later geological processes on their parent planets. Analyses of particles in the Apollo samples indicate that perhaps 3 kg of the third of a tonne of material is non-lunar, of which a few grams might be from Earth.

Terrestrial geology effectively stops once we go back to about 4 Ga, besides which very old rocks on Earth have been subject to all manner of chemical, erosive, tectonic and metamorphic influences. That is the reason why incontrovertible fossils and geochemical evidence for life have yet to be found before 3 Ga at the earliest. There are whiffs of earlier life, which people choose to believe or otherwise, but the potential for dispute fuels continual debate. But escaped ejecta from Hadean impacts on the Earth wouldn’t have been altered so much. They could be dated, and thereby tell geoscientists about the earliest crust, now vanished apart from a few minute grains of pre-4 Ga zircons. Most attractive is the possibility that they could harbour well-preserved organic materials that are traces of the very earliest life forms or their complex precursor chemicals. But would they survive the impacts that produced them? Although impacts from objects as small as 100 m could fling debris beyond the Earth’s pull without heating it too much, Hadean impacts would have had awesome energy because the colliders were huge, as witness the mare basins on the Moon that are over 100 km across. Much of the debris from those lunar big hits is in the form of once melted glasses, and the holes that they left filled with magma generated by the huge energies involved. Some meteorites do preserve their original magnetization, which suggests they never reached temperatures above the Curie points of the minerals responsible for it. Ward cites this evidence in support of once living materials being able to survive in ancient terrestrial ejecta that almost certainly will lie on the lunar surface. But he uses it to say that meteorite internal temperatures must have stayed below 100°C: the Curie point for common magnetic minerals is around 600°C. Given the date of publication, might we be reading of a pudding with too much egg? Whatever, the origin, if not the meaning of life exerts more pull on science purse strings than the prospect of gold nuggets hiding in shadowed craters…

Yet another weird world

Saturn is well-endowed with moons: 35 with names and a whole lot of moonlets. The Saturnian System is astonishing in its diversity, and part of the Cassini probe’s mission is to examine in detail as many moons as possible– 20 flown by in the last year. Enceladus is by no means the largest (504 km in diameter), yet it is very odd indeed. One of its singular features is its ability to jet vast amounts of water from warm spots, and the fact that it seems to snow there. The 10 March 2006 issue of Science magazine devotes 40 pages to articles on the oddities of Enceladus. To jet water ice and vapour to more than twice its diameter – in fact to drench much of the planetary system and replenish parts of the famed ring system – there must be a powerful heat source. Just what that is has yet to be worked out: it could be bound up with internal radioactive decay or with vast tidal sources from Saturn itself, and maybe something else entirely. Its south pole is curiously its most active part, with sufficient heat energy beneath to create a major positive anomaly in long-wave infrared images. This is where much of Enceladus’s resurfacing by snow takes place. Saturn’s tidal forces have rucked up the surface to create hilly ridges, perhaps assisted by a kind of icy volcanism. Tidal or internal forces have also opened up great cracks in the surface, which false-colour images that use UV, green and short-wave infrared reveal to be compositionally different from the water-ice bulk of the surface. That may have resulted from hydrocarbon deposits leaking from deeper layers. It is the moon’s interior that causes most excitement. In order for it to spray off watery jets, there must be a deep source of liquid water, either a liquid shell on which an ice ‘lithosphere’ floats or produced as internal plumes by melting at an interface with a rocky core. That there are hydrocarbons suggests that some of the watery solids include gas-hydrates (ices that incorporate both water and gases).

Discoverer of arsenic in Bengal’s water supply speaks out

Published on March 1, 2006 by zooks777Leave a comment

Indian analytical chemist Dipankar Chakraborti of Jadvapur University, Kolkata was born and raised in one of West Bengal’s many small villages on the delta plains of the Ganges. Paying a visit to a friend’s village in 1988, he found people bearing visible symptoms of chronic arsenic poisoning, which had not been diagnosed before. Analysing samples of well water, Chakraborti found extremely high levels of the poisonous element. For years he was reviled by government agencies who paid no heed to his discovery, calling him a ‘panic monger’ – when more recently showing that Bihar and Assam had similar problems he received death threats. Almost single-handed he campaigned for attention to the undoubted problem, until in the mid 1990s it became clear that arsenic in drinking water from recently sunk wells was a plague of biblical proportions across low-lying West Bengal and neighbouring Bangladesh.

Massive funding, both for establishing the extent and distribution of the contamination and for installing means of removing arsenic from well water, flowed form a host of international donors and agencies. To the outside world it has seemed that the tragedy was being remedied by hugely qualified teams of international scientists, and would eventually be held in check. As revealed in a recent interview (Pearce, F & Chakraborti, D. 2006. Drinking at the west’s toxic well. New Scientist, 1 April 2006 issue, p. 48-49), Chakraborti believes that intervention at national and international levels is doing far less than claimed, even exacerbating the problem by pouring in remedial filtration units without teaching villagers to maintain them. Locals’ are encouraged to trust the remedies, yet continue to drink highly contaminated water once the units clog with silts.

Timely review of nuclear waste disposal

The grand old man of biogeochemistry and the Gaia hypothesis, James Lovelock, seems to have lost patience with life’s ability – and that of alternative energy resources – to keep the Earth system in balance. His view that global warming is past the point of no return as regards ‘green’ remedies has been widely publicised in recent months: he has come out in favour of an increase in the contribution of energy by nuclear reactors. He may have fallen out with many environmentalists, but may also have become an ally of politicians who are looking to nuclear power as a way of maintaining ‘business as usual’ yet putting their money where their mouths are, as regards reducing carbon emissions. Nuclear power may yet have a resurgence, but that would pose again the thorny problem of secure disposal of radioactive wastes. Sweden supplies almost 50% of its electricity using eleven nuclear power stations: the highest number per capita anywhere, despite the country’s otherwise ‘green’ outlook. Should nuclear power rise rapidly elsewhere, then Sweden’s approach to waste disposal may well become a model to follow. What that system is summarised in a recent issue of New Scientist (Nielsen, R.H 2006. Final resting place. New Scientist, 4 March 2006, p. 38-41). Sweden has discovered quite a challenge at its experimental nuclear-waste disposal facility, even though most of the country’s rocks are hard and crystalline, and therefore seemingly ideal for disposal sterilised from the outside world. Despite the common view that crystalline basement is totally impermeable, in reality it is not. Water will be present in any rocks used to cache waste, unless they are beneath almost totally arid deserts, of which only the USA among developed countries has one. It is also becoming increasingly clear that even at great depths, extremophile organisms infest the rock. Among the most common are those that use the reduction of sulfate to sulfide ions as a metabolic energy source: they produce sulphuric acid. That seems a considerable risk to the integrity of whatever form the waste is stored in. The response of the Swedish researchers has been to look for lateral solutions that either kill off the bacteria using clay packing, or exploit the potentially preservative effects of others.

Faster recovery after mass extinctions

Published on February 1, 2006 by zooks777Leave a comment

Mass extinctions have been the principal time markers in the Phanerozoic stratigraphic column since 19^th century palaeontologists recognised sudden changeovers in the fossil record. Two close the Palaeozoic and Mesozoic Eras, two more end Periods (Ordovician and Triassic) and others mark Stage boundaries. Greatest focus has been on the magnitudes of each extinction, greatly assisted by the statistics compiled by the late Jack Sepkoski. The adaptive radiations that filled abandoned niches and restored and, in most cases, expanded diversity are equally interesting. Such recoveries from depleted stocks of organisms have been of immense influence over biological evolution. Resulting from chance events, as far as the Earth’s biota are concerned, the families and species that arose would not otherwise have appeared: the most powerful blow to any notion that biological advances are in any way pre-ordained.

Until recently, it seemed that each recovery was an extremely protracted affair. Over 5 to 10 million years seemed to be the case for aftermaths of the largest extinctions. To a marked extent, analysing recoveries from the fossil record is not so easy as tying the great declines in diversity to a time. It is a matter of working out the rate at which new genera arose or originated through speciation, and that is affected by geographic biases in the fossil record. They arise from less collecting in remote areas and variations in the volume of exposed strata in others. Correcting the biases is possible to some extent, but that still leaves the challenge of statistical analysis. From an extraordinary expansion of analytical expertise, which extends to economists’ methods of understanding stock market trends and the flair of physicists, a very different story of restocking seems about to emerge. A technique called vector autoregression applied to faunal diversification corrected for biases suggests that recoveries were very much faster than previously thought, in fact almost immediate by comparison with the time-precision of the stratigraphic column (Lu, P.J. Motohiro Yogo, M and Marshall, C.R., 2006. Phanerozoic marine biodiversity dynamics in light of the incompleteness of the fossil record. Proceedings of the National Academy of Sciences, v. 103, p. 2736-2739).

See also: Kerr, R.A. 2006. Revised numbers quicken the pace of rebound from mass extinctions. Science, v. 311, p. 931.

Is the Cambrian Explosion real evidence for an evolutionary burst?

About 543 Ma ago, remains of organisms that secreted hard parts suddenly appear in the fossil record. Most palaeontology has focussed on such easily fossilised organisms from the Phanerozoic Eon that began at that time. Whether or not the Cambrian Explosion was a truly significant event, bar the appearance of hard parts – that is quite a mystery in itself – is highlighted by the presence of members of almost all modern animal phyla in the Early Cambrian record. Did they all suddenly explode onto the scene at its outset, or were they around well beforehand as almost completely soft-bodied creatures? Comparative molecular biology of living animals, and the concept of molecular ‘clocks’ has for a while suggested that the origination of modern phyla was considerably earlier than the start of the Phanerozoic. Increasing the database on which such ideas can be based helps improve their precision and scope, assisted by novel methods of mathematical analysis. The 23 December 2005 issue of Science contained an analysis of more than 12 thousand amino acids involved in the genomes of members of 9 or 26 extant animal phyla (Rokas, A.. et al. 2005. Animal evolution and the molecular signature of radiations compressed in time. Science, v. 310, p. 1933-1938). Preliminary study suggests that indeed the early history of the metazoans was remarkably compressed in time, probably in the 50 million years after the ~600 Ma Snowball Earth event, and possibly within a few million years of the base of the Cambrian. However, tests of hypotheses based on such indirectly related data are notoriously difficult, and Rokas et al. have taken a bit of stick (Jermiin, L.S. et al. 2005. Is the ‘Big Bang’ in animal evolution real? Science, v. 310, p. 1910-1911). It seems yet more work on molecular biology of the remaining 17 phyla and a great deal of mathematical wrangling is yet to come.

Zircons and early continents no longer to be sneezed at

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Dating of detrital zircon grains found in moderately old Archaean sediments from Western Australia first pushed known geological time beyond the previously impenetrable 4 Ga barrier. The record now goes back to around 4.4 Ga, within 95% of the date when the Earth and the Solar System came into being (4.55 Ga). There has been much written about the oxygen isotopes in this tiny number of resistant minerals regarding whether or not they originated in a crust permeated by liquid water. Because zircon is a mineral most usually associated with rocks of granitic composition, the very presence of extremely old ones seems to suggest that some degree of fractionation of primitive basaltic magmas must have taken place in the Hadean to form highly evolved magmas. But did actual continental material arise so early? Processes in island arcs can generate evolved magmas in which zirconium is moderately enriched. If such a host for the pre-4 Ga zircons was small in volume, it may have been easily recycled back to mantle depths, yet would enough zircons have been eroded from it to yield those preserved in sediments a billion years younger? It is possible to probe the processes involved in zircon formation by using the extremely sluggish radioactive decay of an isotope of the rare-earth element lutetium. The half-life of the ¹⁷⁶Lu to ¹⁷⁶Hf decay scheme (~37 Ga) is far longer than the time since the Big Bang, so detecting changes in the proportion of ¹⁷⁶Hf to other hafnium isotopes is a tough nut to crack, the more so as ¹⁷⁶Lu is very rare indeed.

A consortium of geochemists from Australia, the US, France and the UK have used the famous Jack Hills zircons to test the widely believed hypothesis that substantial continental crust has only emerged since 4 Ga ago (Harrison, T.M. et al. 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science, v. 310, p. 1947-1950). They found that deviations of ¹⁷⁶Hf/¹⁷⁷Hf from those assumed to characterise the bulk Earth (in fact the proxy of chondritic meteorites) show large variations in the zircons. Some of the deviations are negative, which is consistent with the very early formation of continental crust – perhaps from very soon after the Earth formed. On the other hand, some zircons show positive deviations, a sign that the mantle was depleted, also pointing to crust forming events. The authors boldly suggest that such anomalies refer to a very early geochemical upheaval in the Earth, that likely produced continental material. But the 4 Ga barrier for whole rocks seems clearly to suggest that none remains: either it was all subducted away, or was only a tiny fraction from which the Jack Hills zircons miraculously emerged on their long journey to a final resting place.

Commenting on the paper, Yuri Amelin of the Canadian Geological Survey, points out that no one agrees on the true composition of the bulk Earth (Amelin, Y. 2005. A tale of early Earth told in zircons. Science, v. 310, p. 1914-1915). Other isotopic evidence raises the spectre of our planet having accreted from a mixture of geochemically different meteorite types, and has never mixed thoroughly. Moreover, zircons are notorious for being compositionally zoned, as a result of being able to survive engulfment in later magmas from which new layers of zircon grow. The measurement of ¹⁷⁶Hf/¹⁷⁷Hf ratios is so difficult that only whole zircons give useful results, but those data hide the variations among the zones. Finally, he points out that studies of the ¹⁷⁶Hf/¹⁷⁷Hf in post 4 Ga basalts – and therefore the mantle from which they were derived – show that there is a clear divergence from chondritic meteorites that began around 4 Ga, the start of the record of existing continental rocks. In the kindest way, Amelin casts doubt on the sense in studies of such tiny relics of the Earth’s distant past.

When did Tibet rise?

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As plateaux go, that forming Tibet is by far the highest and the largest. Sitting at an average elevation above 5 km and spanning about 3500 x 1500 km, it dwarfs the next in the list, the Andean Altiplano (mean elevation 3.8 km). The position of the Tibetan Plateau, ahead of the Indian subcontinent’s northward collision with Eurasia marks it obviously as being of tectonic origin. Some plateaux are possibly buoyed up by underlying thermal anomalies in the mantle (the Colorado Plateau of North America, underpinned by a subducted spreading centre), while others, such as that of northern Ethiopia, result partly from vast outpourings of flood basalts and partly from thermal effects of active mantle plumes and rebound associated with massive crustal extension.

There are two basic models for Tibet. It may have formed as a result of a near doubling of crustal thickness as Indian crust was driven beneath that of Asia, low density of the thickened continental crust acting to buoy up its vast area. If that is so, then as soon as India collided with Asia, around 40-50 Ma ago, Tibet would have steadily risen and its plateau would have grown in extent. There are however signs of sudden changes in thermal structure, marked by large-scale magmatism of roughly Late Miocene (8-10 Ma) age. That may have been induced by an extraordinary event, the detachment and foundering (delamination) of a large mass of underlying mantle, whose loss resulted in rapid uplift of the whole overlying region. Because Tibet is known to play a central role in the mechanism that drives the South Asian monsoon, assessing the timing of its formation is crucial to understanding the onset of the monsoon and the many phenomena of accelerated weathering and erosion associated with it. Cores from the floor of the Indian Ocean suggest that the monsoon suddenly increased in intensity at around 8 Ma. Both as a sink for carbon dioxide as a result of weathering of the continental crust, and as a means of obstructing and redirecting continental wind patterns, the growth of the Tibetan Plateau and the Himalaya in front of it have been assigned a major role in the decline of global mean temperatures that resulted in northern hemisphere glaciations. So establishing the timing of their formation makes or breaks two major geoscientific hypotheses of recent decades. The key is some form of proxy for past elevations in the area. One such proxy, the stomatal index of plant leaves found in Tibetan sediments of Miocene age, showed that 15 Ma ago the southern Plateau was just as high as today (see When did southern Tibet get so high? in March 2003 EPN). That cast doubt on a later cause of uplift, but remained unconfirmed.

Sediments deposited in lakes that periodically fill Tibet’s many basins form a record that goes back at least 35 Ma. Carbonates in such lacustrine sediments offer a geochemical means of charting changes in elevation (Rowley, D.B. & Currie, B.S. 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, Central Tibet. Nature, v. 439, p. 677-681). That depends on the proportion of ¹⁸O to the lighter ¹⁶O isotope of oxygen (δ¹⁸O) in carbonate, which is believed to be inherited from rainwater that originally drained into the basins. The higher the elevation at which water falls as rain or snow, the less of the heavier oxygen isotope it contains, so δ¹⁸O is a potential means of measuring the evolution of surface elevation. For central Tibet, this shows that the topography was at least 4 km high as early as 35 Ma ago. Results from other basins that span the Tibetan Plateau clearly suggest that 4 km elevation was achieved progressively later from south to north, anging from 40 to 10 Ma ago. So the delamination model for a sudden springing-up of the Plateau seems now to be a less plausible mechanism for the uplift than the simpler model of progressive crustal thickening following the collision of India. That does not entirely rule out an episode of delamination in the Miocene, for which geochemical evidence is fairly convincing. The implication of the new results is that if Tibet has been a major influence over climate, then it was one that developed progressively from the late Eocene.

See also: Mulch, A and Page Chamberlain, C. 2006. The rise and growth of Tibet. Nature, v. 439, p. 670-671. Kerr, R.A. 2006. An early date for aising the roof of the world. Science, v. 311, p. 758.

Climate change and collapse of early civilisations

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About 4200 years ago early civilisations of the Old World underwent decline and collapse. Examples are the Akkadian civilisation in the upper Tigris and Euphrates basins, famed for Hammurabi’s Hanging Gardens of Babylon, the Harappan of the Indus Valley (Mohenjodaro), the phaoronic Old Kingdom and the Minoan of Crete. This period of the Bronze Age has been thought by some to have experienced either massive volcanism – the explosion of Santorini – or even a comet strike. Others have correlated collapses of city states with Biblical events. Whatever happened, its outcome spanned a vast area of western Asia and north-eastern Africa, so another candidate is climatic drying leading to drought and famine. That is perhaps not such a spectacular fate as near-instant environmental upheavals, but probably just as effective for societies dependant on regular agriculture production or, in the case of Crete, on wide-ranging trade.

Detecting climate change is now well established on proxy records of one kind or another, such as those based on isotopes and sedimentation changes from sea-floor sediments and flowstone (speleothem) in caves, and dust records in ice cores. Such time-series from the mid- to late Holocene are increasing in number, with particular interest growing in records from speleothem now that precise age sequences are possible using uranium-series dating. A flowstone record from a cave in northern Italy, has helped link other time series ranging from the North Atlantic floor, in the Middle East and East Africa (Drysdale, R. et al. 2006. Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone. Geology, v. 34, p. 101-104). A team of geochemists ad environmental scientists from Australia, Italy and the UK has shown a remarkable coincidence among these widely different records, centred on 3900-4200 b.p.. From the North Atlantic at high latitudes is an upsurge in fragments deposited by ice rafting, while mean sea-surface temperatures swung downwards. Kilimanjaro ice shows a marked peak in atmospheric dustiness. Carbonate deposition peaked in the Gulf of Oman. Finally, the Italian flowstone shows peaks in d¹⁸O, d¹³C and the magnesium:calcium ratio of its carbonates. The conclusion is a period of climatic cooling and drying that spanned 40 degrees of latitude over a period of several hundred years. This is not the signature likely to have been associated with instantaneous catastrophes. Yet nor is it typical of the episodic climate shifts of the order of a few thousand years, which were now well known features of the last glacial period and the current interglacial. It was certainly sufficiently prolonged and large enough to have wrought havoc on early civilisations, and throughout the Old World it clearly did.

The Digital Earth revolution

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Launched in July 2005, Google Earth (earth.google.com) has become familiar to many Earth scientists. Some, like me, may have needed encouragement to try it out. Whatever, once up and running on a modern PC with Windows 2000 or XP and broadband connection, even the free version of the software that you need to access Google Earth is compelling, even addictive. It takes no more than a few minutes to realise that it revolutionises teaching of many aspects of Earth science, and will be used too as a top-line research tool by anyone interested in spatial data.

Based primarily on natural-colour images that cover the entire Earth, much at Landsat TM 15-30 m resolution but for some areas using other images that resolve to the order of a couple of metres or better, Google Earth also uses global topographic elevation data. This is where it takes on its revolutionising role. It is easy to view the surface of any part of the planet in oblique perspective, when all topographic and a great many geological features show up dramatically. It is the ultimate ‘Swiss Hammer’ – mapping the complex geology of the Alps was only possible by viewing exposures in one massif from the vantage point of another. Choosing appropriate zoom factors connects geological features that are on different scales. Design of the database – it is perfectly seamless, except where resolution changes in mostly urban areas – makes it possible at broadband connection speeds to roam in real time at any scale. This allows you to simulate flight at any altitude and with any downward look angle: ‘grand tours’ to visit all the famous geological sites you have longed for on every continent become simple. The novelty of 3-D simulation also means that there is much to discover.

Sometimes, even in one’s homeland, it is possible to get lost, especially at large scale. By turning on GIS layers for rivers and roads (in many areas populated places, even street names and fast-food outlets show) navigation is made easier. It is the linking of images with other kinds of data that gives Google Earth its potential for research power. Designed as an easy-to-use geographic information system, by purchasing professional versions of some GIS software you can add layers interpreted, almost literally, ‘on the fly’ (Butler, D. 2006. The web-wide world. Nature, v. 439, p. 776-778).

An immediate attraction, both for globe-trotting geoscientists and, more importantly, people engaged in disaster relief, is the way Google Earth makes it easy to become familiar in moderate detail with the terrain that has to be faced. Solving problems of access, assessing where assistance may be most urgently needed is helped enormously by its highly realistic geographic visualisation. Of course, it cuts down the need for very expensive helicopter reconnaissance. Google Earth has already proved invaluable for assessing the aftermath of the October 2005 earthquake in Kashmir. Google facilitates the mosaicing of new images of disaster areas, such as those struck by Hurricane Katrina, and their incorporation into the Google Earth database (Nourbakhsh, I. 2006. Mapping disaster zones. Nature, v. 439, p. 787-788).

A few people get frightened by some of the highest resolution images that are available – even the lines on tennis courts show up – as if their privacy was being invaded. More seriously, some governments worry about security implications of anyone being able to see intimate details of airfields and ports. That is silly – at any time the Quickbird or Ikonos satellites can take a snap of any part of the planet at up to 65 cm resolution for anyone who has the cash to pay for its acquisition; most likely intelligence agencies and military strategists. Privacy, at least from several hundred kilometres above, is a thing of the past. Every geologist would like to get one-metre resolution images of their research areas. If they see something intended to be hidden for one or another reason, they have an obligation to be discrete.

Exactly how does life shape landforms?

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The land’s present topography is not just the frontier between the lithosphere and the atmosphere and hydrosphere, but where plants of many different kinds grow. Whether in the form of cyanobacteria, lichens or luxuriant tropical rain forest, vegetation affects weathering, erosion and the deposition of sediments. Animals – leaving out humans – also have some influence, whether they be subterranean rabbits, moles and worms, or heavy-footed beasts that force soils to move downslope. Inevitably life-land interactions affect landforms, although rock-type and active geological processes tend to dominate. Nonetheless, a planet with life ought to show different styles of surface shapes from one that is organically dead. The central issues for geomorphologists is whether or not it is possible to define absolutely the differences, and then to use them as a means of detecting the likely former influence of life on other worlds.

Central to such a venture (Dietrich, W.E. & Perron, J.t. 2006. The search for a topographic signature of life. Nature, v. 439, p. 411-418) is the ability to map in detail the variation of topographic elevation. Digital topographic elevation data is now available for most of the Earth’s land surface at a resolution of between 90 and 30 m, the second only publicly available for the USA, from the groundbreaking Shuttle Radar Topography Mission of 2000. Aerial photography and high-resolution stereoscopic images from satellite such as Quickbird and Ikonos, allow resolution as sharp as a few metres. Laser scanning from aircraft potentially can even improve that to the scale of a few tens of centimetres, but such high-resolution data are far from global. The planet Mars is now better endowed with elevation data than is our own planet, thanks to photogrammetric instruments carried by ESA’s Mars Express mission, and the shyness of various intelligence agencies to share publicly what they have gleaned from high-altitude aircraft and spy satellites. Nonetheless, it is now possible to analyse elevation data from the entire range of terrestrial biomes to see what signal vegetation has imposed on surface shape. An easy way to visualise that is simple – just use Google Earth (see The Digital Earth revolution above).

Dietrich and Perron review the mathematical approaches to modelling life’s topographic influences, beginning with an equation that relates elevation and time to rates of uplift, erosion and entry of sediment into storage, thereby expressing conservation of mass. All the variables are themselves governed by a variety of processes, theoretically amenable to quantification, summarised in Dietrich and Perron’s review. In each there will be some potential biotic influence. On Earth there are sufficient landscapes devoid of all but a minute veneer of organisms to assess both end-members clearly. Mars and Venus ought to be good tests. But, should such a rigorous quantification of lifeless and lively surfaces at a spectrum of scales be achieved, where would we deploy it?

Yet more on the end-Permian extinction

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Sequences that reveal the Permian-Triassic boundary continue to receive a great deal of attention, spurred by the seemingly cryptic nature of the conditions that caused up to 90% of all living things to die. Globally, the boundary is marked by a sudden and large fall in the proportion of ¹³C in carbonates and sedimentary organic matter. Since the d¹³C anomaly follows the biotic decline, it is less likely to reflect any cause of the extinction, such as a massive methane release from destabilised gas hydrates and global warming, than an effect of whatever went on. Joint research by UK, Dutch and US organic geochemists focused on the P/Tr boundary in northern Italy, where it is dominated by shallow-marine carbonates (Sephton, M.A. et al., 2005. Catastrophic soil erosion during the end-Permian biotic crisis. Geology, v. 33, p. 941-944). They analysed the organic compounds preserved in the section, and found that the extinction zone coincides with a major increase in total organic carbon, which is dominated by large amounts of compounds (polysaccharides) that typify soils and leaf litter. They explain the anomaly as the result of a short period of rapid soil erosion from the terrestrial hinterland of the shallow Late Permian sea. Since virtually all continental crust had stabilised in the Pangaea supercontinent, tens of millions of years beforehand, such erosion was unlikely have been a result of some sudden tectonic uplift. But it might have been triggered by sudden loss of the vegetation that retards soil erosion on the continental surface. The P/Tr extinction affected both marine and terrestrial organisms, and Sephton et al recognise that their discovery of evidence for soil stripping on a grand scale reflects that unified fate. Acid rain from the massive Siberian continental flood volcanism could well have been the trigger for ill thrift of land vegetation, or maybe removal of stratospheric ozone by release of halogen (chlorine and bromine) compounds let in destructive UV radiation.

Culture and human evolution

Published on December 1, 2005 by zooks7771 Comment

Culture in the most general sense that encompasses tools, clothing, habitation and fire has increasingly set humans and their ancestors apart from the rest of the natural world. It might therefore seem that becoming more ‘human’ cushions our line from Darwinian natural selection since we have created our own ‘nature’ and carry it with us. Setting fully modern humans adrift in the environment, without that culture, would undoubtedly result in rapidly extinguishing the species. In that hypothetical context we are far from ‘fit’, in Darwin’s sense. However, the development of humanity’s cultural milieu has itself provided a continually changing, increasingly pervasive artificial set of conditions for natural selection. Culturally, the most dramatic step in human evolution, for which we have tangible evidence, emerged with the explosive appearance of graphic art and a complex ‘toolkit’ around 35 thousand years ago in Europe. That huge advance will undoubtedly be traced back maybe tens of millennia when archaeological finds in Africa and Australia, for instance, are more precisely dated. Evidence from the DNA in male-carried Y chromosomes indicates that a profound genetic shift occurred around 70 ka, perhaps resulting from a decline in global human numbers to a very small population after the climatic disaster wrought by the explosive eruption of the Toba volcano in Indonesia. That too was a time when fully modern humanity distributed itself more thinly by a decisive exodus from Africa. Some specialists have speculated that the cultural explosion stemmed from that evolutionary ‘bottleneck’. There are genetic signs of adaptation to cultural practices and selective pressures that accompanied them after the rise of agriculture and settlement (See Has human evolution stopped?, September 2005 issue of EPN). Recent work on the whole human genome gives an inkling that even more pervasive evolutionary changes took place in the last 50 thousand years (Wang, E.T. et al., 2005. Global landscape of recent inferred Darwinian selection for Homo sapiens. Proceedings of the National Academy of Science, www.pnas.org/cgi/doi/10.1073/pnas.0509691102).

Wang and colleagues from the University of California studied the occurrence of single-letter differences in the genetic code (single-nucleotide polymorphisms – SNPs). Scattered across all human chromosomes are about 1.6 million of these SNPs. They appear not to do anything, but can be linked to nearby genes. When natural selection favours a particular mutated variant of a gene, the associated SNPs can be selected as well. The approach used by Wang et al. is a statistical search for pairs of SNPs that occur together more often than could be possible by chance ‘reshuffling’ that occurs from generation to generation. Their analysis suggests that around 1800 genes, a remarkable 7% of the whole genome, have changed over the last 50 thousand years. Interestingly, that is similar to the degree of genetic change in maize since its domestication from its wild ancestor. As well as genes connected to protein metabolism that could have changed as new diets followed the rise of agriculture, some that are involved in brain function have been selected as well.

Although at an early stage, this kind of research confirms that we are indeed still evolving along Darwinian lines, perhaps unwittingly domesticating ourselves. It is easy to assume that ideas, skills and artistic sensibilities are passed on through language and learning and thereby grow and diversify, but in order for any of these to stimulate the deep feelings that they foster suggests that some aspects have become ‘hard-wired’ in all of us. Everyone unconsciously taps their feet to rhythm, can be moved to a vast range of emotions by music, words and visual stimuli, and can ‘sense’ an environment captured, even in abstraction, by a talented artist. They inspire further development. Until around 50 ka human culture, insofar as we can see evidence for it, remained fixed for more than a million years through several species and subspecies of the genus Homo. Appearing between 1.6 and 1.4 Ma ago the bi-face stone axe endured as humanity’s highest known achievement until those very recent times.

See also: Holmes, R. 2005. Civilisation left its mark on our genes. New Scientist, 24/31 December 2005 issue, p. 8.

Earliest tourism in northern Europe

Some years ago British palaeoanthropologists were in a state of high excitement about finds of stone tools, evidence of prolonged human habitation and fragmentary skeletal remains from a sandpit at Boxgrove on England’s southern coast. They showed the earliest human presence at high latitudes around 400-500 ka. The date of early colonisation has now been pushed back more than half as long before that to 700 ka by finds in a shoreline exposure of riverine sediments on the coast of Suffolk on England’s east coat. The Cromer Forest Bed of Middle Pleistocene age has been know since Victorian times as a rich source of the flora and fauna from one of the earliest interglacials of the current period of 100 ka climate cyclicity. At that time the North Sea had yet to establish a connection that would eventually separate the British Isles from Europe, and the site at Pakefield would have been the estuary of a now-vanished river system draining the Midlands and Wales. So far no human bones have turned up in the excavations, which have to be conducted at low tide. But many flint tools pepper the organic-rich sediments (Parrfitt, S.A. et al., 2005. The earliest record of human activity in northern Europe. Nature, v. 438, p. 1008-1012). As with most terrestrial deposits, establishing the age of human occupation posed the greatest difficulty. A careful documentation of magnetic polarity combined with fossils – including distinct voles – and a new technique that relies on assessing the degree of protein degradation in bivalve shells helped tie-down the age precisely.

Around 800 ka human occupation had begun in Spain and the Pakefield site shows that migration northwards of flora and fauna following a glacial epoch was swift, to establish conditions considerable warmer than in the Holocene. It seems that this Mediterranean climate encouraged such northward penetration by humans, most likely during a short period of particular warmth. Long eyed by archaeologists as a potential source of human remains, patience has paid off in the Cromer Forest Beds. Yet around the world there are many other, equally promising strata or Pleistocene age that have not had such undivided attention for so long, A glance at the distribution of keynote sites for palaeoanthropology shows how narrow the search for human origins and migratory destination has been up to now. Though it is understandable that once finds have been made, funds and scientists cluster where progress is best guaranteed. Very rarely, either a ‘shot in the dark’ pays off or something surprising turns up at a site being excavated for other purposes. Broadening the search may well have high financial and career risks, yet the more discoveries are made at well-trodden sites the greater the likelihood that the full story of human evolution and migration will be revealed by breaking new ground,

See also: Roebroeks, E. 2005. Life on the Costa del Cromer. Nature, v. 438, p.921-922.

Biogeochemical evidence for vegetation change when hominins evolved

A long-held theory that concerns the background to hominin evolution, is that the freeing of hands by bipedalism was triggered by a shift in the ecology of East Africa from forest to more open grassland. That might well have happened as the Neogene uplift associated with development of the East African Rift transformed the regional wind and rainfall patterns to the way they are today, thereby creating the conditions for the modern savannahs and semi-deserts in the area long associated with human origins. The lakes of East Africa are ephemeral in the context of Neogene climate change, and so their sediments are not much use in charting long-term shifts in flora. However, the modern wind systems shift dust and organic particles consistently towards the Gulf of Aden, so sediment cores there potentially provide a continuous record of vegetation change. That is, if they contain ‘biomarkers’ that distinguish the debris of trees from that of grasses. The first biomarker records from the Gulf of Aden seabed powerfully confirm the notion of vegetation change as a possible driver for hominin evolution (Feakins, S.J. et al., 2005. Biomarker records of late Neogene changes in northeast African vegetation. Geology, v. 33, p. 977-980).

Up to about 3.5 Ma the cores contain plant-derived waxes that are characteristic of trees that use C3 metabolic processes, but thereafter evidence for increasing C4 grasses predominates. Coinciding with that broad trend is an increase in ¹³C in soil carbonates on land, which probably reflects increased grassland too. Although records of hominin diversity before about 3 Ma are scanty, later times saw the rise of several bipedal species, grouped as the powerfully jawed parathropoids and the more daintily chewing members of the lineage that led to modern humans. Detail in those sections of marine core that were used – presumably costs prevented continuous measurements – shows that the carbon-isotopic signals in the waxes varied in harmony with evidence for climate change, so the proportions of savannah and woodland probably shifted quite rapidly. However, because cold-dry periods have tended to be longer than those which were warm and more humid, savannah would have had more influence over faunas than ephemeral woodland. Fascinating as this empirical relationship between hominin evolution and vegetation change is, what Africa lacks – as indeed does most of the planet – is data that chart accurately how topography has changed with time. Cosmogenic and U-Th/ He apatite thermochronology, on which so much hope and funding have been invested, has proved spectacularly ineffectual compared with careful work on the likely effects of changing landforms.

Helium and how the Earth convects

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In the last ten years the new technology of seismic tomography that produces ghostly images of high and low density mantle has convinced many geoscientists that two major dynamic features extend to almost to the core mantle boundary (CMB). Dense, high-velocity zones descend from subduction zones, suggesting that the slabs continue to fall through the entire mantle below the ~700 km maximum depth of the earthquakes that Bennioff and Wadati used to define subduction. Some hotspots seem to be above diffuse zones of low seismic velocity that are supposed to signify hot, low density plumes that rise from the CMB. An inkling of a grand theory of mantle convection might then be that the descending slabs ruck up the deepest and hottest mantle layers to set them rising as narrow diapirs. Yet, other tomographic features appear to be restricted to the uppermost mantle, less than the 660 km depth of a major discontinuity long considered to be due to a mineral phase change at high pressure. A whole-mantle theory of convective heat transfer should transfer some geochemical trace of an exchange between core and silicate mantle. Osmium isotopes from plume-related magmatism suggest that there might be an exchange, but those of tungsten do not (see: Mantle and core do not mix, February 2004 issue of EPN). The oldest and perhaps most convincing evidence against whole-mantle convection comes from study of helium in volcanic rocks, neatly reviewed by Francis Albarède (Albarède, F., 2005. Helium feels the heat in Earth’s mantle. Science, v. 310, p. 1777-1778).

Helium is generated by the decay of radioactive uranium and thorium isotopes as alpha particles (⁴He), which generates much of the Earth’s geothermal heat flow. There should be a close correlation between helium and helium, but at mid-ocean ridges the amount of ⁴He is only 5% of that expected from the associated heat flow. One explanation for this is that somewhere in the mantle there is a barrier to upward movement of helium, yet is allows heat to pass through: a thermally conductive layer that bars convective mass transfer. Albarède cites recent work that uses the flow of heat and helium through groundwater in an aquifer (Castro, M.C. et al., 2005. 2-D numerical simulations of groundwater flow, heat transfer and ⁴He transport — implications for the He terrestrial budget and the mantle helium–heat imbalance. Earth and Planetary Science Letters, v. 237, p. 893-910) as analogy of mantle processes. There too helium is less than might be expected, the reason being that the aquifer is recharged by rainwater, low in He. Likewise, ocean-floor basalts are probably affected in the same way by hydrothermal circulation of seawater, thereby diluting the flux of helium from the mantle and perhaps helping to account for anomalously low helium flux. Another widely accepted view that the high ³He/⁴He ratios of hotspot basalts is evidence for their source in primitive mantle – ³He is probably a product of nucleosynthesis and therefore primordial as far as the Earth is concerned – is challenged by a recent paper that shows that helium is dissolved in mantle minerals (Parman, S.W. et al., 2005. Helium solubility in olivine and implications for high ³He/⁴He in ocean island basalts. Nature, v. 437, p. 1140-1143). Parman et al.’s measurements suggest that the high ³He might result from residues of earlier melting in the mantle, rather than coming from parts that have remain in the state they were when the Earth accreted.

Vanished Martian sea or not?

The Mars Rover data from the Opportunity site that showed up masses of sulfate minerals in the large depression that it has roamed for 2 years prompted the notion that they formed as a sizeable body of surface water evaporated. The Rover Opportunity scientists have also speculated on Mars once having had highly acidic ‘weather’, in the form of sulfuric acid rain from SO₂ emitted by volcanoes. The sediments at the Opportunity site also show signs of fluid transport in the form of bedding and cross stratification, ascribed to moving water. Most independent-minded scientists confronted by a united front of vast teams of highly focused scientists sometimes feel that there is more than one way of skinning a cat. Such is the case of Paul Knauth and Donald Burt of Arizona State University and Kenneth Wohletz of the Los Alamos National Laboratory in New Mexico. The visualise the dramatic evidence from Opportunity in an altogether more mundane scenario (Knauth, L.P. et al., 2005. Impact origin of sediments at the Opportunity landing site on Mars. Nature, v. 438, p. 1123-1128). Their main point of departure is quite simple; acidic water full of hydrogen ions is a powerful means of weathering and the production of clay minerals. Clays are very uncommon on Mars, particularly at the Opportunity site, and have only shown up rarely on hyperspectral remote sensing images.

Layered sediments are evidence for fluid deposition, but not only water produces them. As well as wind transport and deposition, they are also formed by gas-rich base surges from explosive volcanism and meteorite impacts – and also during surface nuclear explosions that mimic impacts, hence the Los Alamos connection. Knauth et al. explain the Opportunity deposits as debris originally made of rock, sulphides brines and ice flung from a massive impact. They explain the sulfates as products of interaction between melted ices and sulfides. The extension of the Opportunity team’s hypothesis of evaporating surface water is that it would have been long-lived, perhaps sufficiently so for the emergence of acid-loving organisms, similar to those that infest groundwater in terrestrial massive sulfide deposits. Should the deposit prove to have formed during an extremely rapid event, such as an impact, the idea of it having hosted primitive life forms becomes extremely unlikely. Gleefully, Knauth et al. almost exactly match the Opportunity image mosaic of layered sediments with a photograph of a New Mexico layered, volcanic surge deposit. Surges from large impacts, and Mars was intensely bombarded in its early history, can extend hundreds of kilometres from the crater rim. Many other examples of layered sequences are being revealed by high-resolution orbital images of Mars, and interpreters regularly ascribe them to wind, flowing water or volcanic processes. Ockham’s Razor demands the most likely and simplest explanation for phenomena, and impacts could have formed the lot. The earliest detection of features that only flowing water could have carved – the sinuous canyons on Mars, originally prompted such a simple explanation, that water was released en masse by early massive impacts. Perhaps there is a much wider link between many Martian features and the most common geological agent in the Solar System.

A tragic 2005

Published on December 1, 2005 by zooks777Leave a comment

Readers of EPN do not need reminding that in the last year Earth processes wrought tragedy on a scale rarely witnessed. That scenes from each disaster reached TV screens globally within hours does seem to have been a wake-up call to geoscientists to at least try to make the next event trigger more timely and efficient assistance, hopefully with clearer advance warning. The year has seen increased understanding of seismic processes in general, and the beginnings of greater co-ordination among scientists concerned about natural hazards. Yet we live in a world with more chronic tragedy too: millions dead or whose lives have been shattered by the anarchy in Congo from the scramble for diamonds, gold and even the tantalum used for boom-time cellnet ‘phones; more still across Africa lack water to drink safely; and mineral booty continues to support repressive regimes, that hold back and disrupt most people’s aspirations and talents.

It is not hard to see that geoscientists have a central role that they could play in alleviating such blights, given the will – we certainly have the time as well as the skills to use and share.

Deep-sea mining to realise its promise?

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On paper, metal resources lying on the deep ocean floor look like an economic panacea. Large areas are covered with either a crust or scattered, potato-sized nodules rich in manganese, copper, cobalt, nickel and several other metals. In some ocean basins, one scoop might provide ore grades for all of them, as in the best onshore multi-metal deposits. ‘Black smokers’ and the metal-rich pillars and muds that develop from them seem just as promising for lead, zinc, copper and even gold: such submarine hydrothermal exhalations probably formed many of the rich massive sulphide deposits sought on land. The 1960s and early 70s seemed likely to foster a fundamental shift in metal extraction, but despite rises in metal prices after the 1973 Yom Kippur war and Iranian revolution of 1978, the excitement faded to insignificance. There were a few ironies too. A ship was designed and almost completed by one of Howard Hughes’ many companies, Global Marine, supposedly to harvest ocean-floor manganese nodules. In fact, the venture was to be secretly directed at salvaging a sunken Soviet nuclear submarine, and the code books that it carried, from the floor of the Pacific Ocean. It now seems that ocean-floor mining might be resurrected – assuming that all does not descend into further wrangling over the Law of the Sea and who should benefit from profits (Thwaites, T. 2005. Treasure Ocean. New Scientist, 17 December 2005, p. 40-53). An Australian company called Seacore is soon to drill around New Guinea and New Zealand to evaluate the potential of exhalative deposits. They claim that if thicknesses greater than 15 m, at decent grades for gold, copper, zinc, silver and lead, are found dredging up the ores would be commercially possible. Essentially it would be literally a smash and grab job, unlike the massive logistics of on-shore open-pit and subsurface mining, albeit tempered by problems connected with depths of several kilometres. Understandably, there are environmental concerns about exposing highly anomalous concentrations of metals and associated sulfide minerals, probably in a fine-grained soft state. Ocean ecosystems are fundamentally based on clear water, and mud plumes could wreak havoc far afield. The deposits would have to be sucked to the surface using the air-lift dredge technique pioneered by marine archaeologists, but on a much larger scale. Yet this appears to be more than a means of attracting and siphoning off venture capital, for the groundwork of identifying targets has already been done by Placer Dome, a well-heeled Canadian mining company. Also, the thorny issue of the legality of harvesting the global oceanic ‘commons’ in international waters is being avoided by drilling within national offshore limits, as has long happened with offshore oil development.

Arc-like andesites from the ocean floor

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To most geologists ‘andesite’ spells subduction beneath island arcs and continental margins. Geochemically they share a universal signature: their depletion in the elements niobium and tantalum. Both find the aqueous fluids that rise from subducting slabs repellent and so they stay in the source of arc magmas, almost certainly in amphibole minerals. Negative Nb and Ta anomalies pervade the continental crust, suggesting that it owes its origin to subduction processes of some kind over maybe the whole of recorded geological time. The other dominant means of expelling magmas is through the adiabatic melting of drier upper mantle as it rises along oceanic rift zones. Theoretically and also in innumerable analyses of ocean-floor rocks Nb and Ta behave like other elements that favour melts over the minerals of mantle residues. That there are ocean-floor rocks that show evidence of incompatible behaviour of the two elements comes as quite a surprise. More surprising still is that they are of bulk andesitic to more silica-rich dacitic composition (Haase, K.M. et al., 2005. Nb-depleted andesites from the Pacific-Antarctic Rise as an analogue for early continental crust. Geology, v. 33, p. 921-924). The rocks analysed by the team from the Christian-Albrechts University of Kiel, Germany, occur close to a hotspot in the South Pacific and span about 130 km of the ridge system, along with basalts.

Modelling the geochemistry of the silicic lavas suggests a dominant role for fractional crystallization of magnetite and ilmenite from a basaltic parent magma that itself is enriched in iron and titanium. Yet, associated basalts do not show depleted Nb and Ta, so some other mechanism must be responsible for their occurrence in the andesites. One possibility is production of silicic magma by partial melting of amphibole-rich mafic oceanic crust, and then its mixing with fractionated basalt to form low-density magma that rises. Silicic lavas in Archaean greenstone belts are often associated with basalts that chemical affinities to those in modern oceanic settings. It is therefore possible that a substantial proportion of Archaean continental crust originated in ocean hotspot settings, rather than by some form of subduction process.

Sea level bonanza

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The ups and downs of sea level through geological time constitute a ‘beat’ to which sedimentation responds by inundation of and withdrawal from the land. The ‘big picture’ is one forced by changes in the volume of the ocean basins as plate tectonics waxed and waned, together with long periods when land ice locked sea water away. A closer focus has stemmed from the changes of oxygen isotopes in benthonic (bottom-dwelling) plankton remains that record details about advances and retreats of polar land ice, most spectacularly from the record of the Pliocene and Pleistocene. These ongoing, higher frequency fluctuations in sea level formed the key to verifying Milankovich’s theory of astronomical controls over climate. There are also fluctuations of the order of thousands to tens of thousand years that seem terrestrial in origin, such as the Bond and Dansgaard-Oeschger cycles. Shorter cycles still have had various causes ascribe to them. For inhabitants of near-sea level cities and flat ocean islands, rising sea level is a realistic concern. It is rising just now at about 3 mm per year (in the 1950s the annual rise was half that), mainly because surface sea water is expanding as a result of anthropogenic warning and polar ice is melting.

November 2005 was valuable for geoscientists interested in fluctuating sea level, and most sedimentologists are in that category because the stratigraphic record is primarily governed by this eustatic (world-wide) rhythm. The earliest information on long-term sea-level change came from studies of continental transgressions and regressions that are preserved as onlap and offlap features between strata. That approach was greatly aided by detailed seismic sections gathered by petroleum explorationists, in which such features show up a great deal more readily than they do in limited exposures on land. The results of many different methods of charting eustasy are wonderfully summarised by a large team of US geoscientists (Miller, K.G. and 9 others 2005. The Phanerozoic record of global sea-level change. Science, v. 310, p. 1293-1298). Their review covers the last 543 Ma, and reveals several novel aspects. It has been known for over 30 years that the higher frequency sea-level changes correlate well with oxygen isotope records, because of the preferential evaporation of water that contains light ¹⁷O. When evaporated ocean water ends up in long-term storage as land ice, the proportion of heavier ¹⁸O rises in seawater and in carbonates extracted from it by organisms. The broad view also shows a sea-level – d¹⁸O correlation though, and that probably reflects expansion and contraction of the volume of ocean water as mean global temperature rose and fell on the scale of tens of million years. That the Cretaceous was the period during which sea level reached an all time high during the Phanerozoic has been well known for over a century, and manifested itself in the production of giant ‘carbonate factories’ on shallow shelves of inundated continental lowlands. Famously, that was ascribed to vast production of new oceanic crust, both by accelerated sea-floor spreading and outpouring of huge submarine flood basalts, such as the Ontong Java Plateau of the west Pacific floor. Putting together all the pertinent data, however, suggests that Cretaceous tectonics was not nearly as vigorous as once suspected.

Unsurprisingly, sea level studies are ‘hot’ and researchers have a better than even chance of getting publications into press in the most august of journals, and a readership to boot. There is a great deal of information on past and current sea level fluctuations, and a great deal of thought has gone into acquiring data. Dotted around the world’s coast lines are tide gauges of the most exquisite precision; so precise in fact that the outermost ripples of the Boxing Day tsunamis were detected at the antipode of the earthquake that caused them. Whether or not watching these gauges continuously is a fulfilling task, the long-term records have revealed a surprise (Church, J,A, et al. 2005. Significant decadal-scale impact of volcanic eruptions on sea level and ocean heat content. Nature, v. 438, p. 74-77). Since 1960, global sea level has been up and down like a yo-yo, deviating by ± 2-3 mm from the longer-term mean at a rate measured in decades. This correlates well with five major volcanic eruptions during the last 45 years, such as El Chichon and Pinatubo. The first effect is a rapid fall (6 mm in a year, after Pinatubo erupted), probably resulting from global cooling and reduced rainfall caused by sulfate aerosols injected into the stratosphere, followed by slow recovery. It seems odd that volcanoes have a bigger effect on sea level than overall global warming, yet other records show their profound global effects. The fall in sea level must be dominated by shrinkage of cooled surface water. Interesting, and quite possibly a boost for those in denial over global warming. However, my main concern, living at 250 m above mean sea level, is that my bathroom cistern is always overflowing because of a water level rise of 1 mm in a matter of a few minutes.

Early warning of earthquakes

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Because earthquakes result ultimately from the relative movement of lithospheric plates, and take the form of various kinds of ground motion it is easy to think of them just in mechanical terms. However, such movements affect materials that respond in odd ways to motion and friction. One of the most obvious is the sound near a fault zone during an earthquake, which can range from a rumble to a piercing shriek, depending on the near-surface rocks being dragged past each other. There are other, more subtle effects. For instance, if grains of quartz or dolomite are rubbed against one another they glow – a nice piece of natural magic for the dark days of winter. There have been many reports of so-called ‘Earth lights’ along active fault zones before and during earthquakes, and they might result from this piezoluminescence. Rocks differ in their ability to conduct electricity, but Faraday’s laws of electromagnetism show that if a conductor is moved in a magnetic field, currents flow through it; the principle behind electricity generation. In turn, motion in a magnetic field of a conductor in which electricity flows generates electromagnetic radiation, whose frequency depends on the rate of motion. Electromagnetic effects may also result from build-up of electrical charge derived from minerals in the crust, or from crushing of magnetic minerals. Along with even less well understood phenomena, such as the rise and fall of water levels and various gas discharges in wells, and animal behaviour, physical changes are potential means of earthquake warning, if they can be detected and properly understood, that could supplement and even supersede conventional approaches to early warning.

Minoru Tsutsui of the Kyoto Sangyo University in Japan has concentrated on the EM radiation known to precede earthquakes (Tsutsui, M. 2005. Identification of earthquake epicentre from measurements of electromagnetic pulses in the Earth. Geophys. Res. Lett., 32, L20303, doi:10.1029/2005GL023691). Previously published observations have been limited to noting EM pulses before major seismic events. These showed that in some cases nearby areas experienced increased EM noise up to a few months beforehand, to peak a few hours before events. The radiation is at very low frequencies, i.e. wavelengths are much longer than normal radio waves. Such ultra-low frequency (ULF) radiation passes extremely efficiently through rock, and ULF has been used for secret communications between submarines and their bases, as it passes through the whole Earth. In the context of seismic prediction, detecting ULF changes is not enough: the object is to predict the position of an earthquake’s focus as well as its timing. Tsutsui has developed a means of finding the direction in which ULF radiation moves, which has been calibrated using the ULF from lightning strikes and the position of the thunder clouds found using weather radar systems. A strong ULF EM pulse that accompanied a magnitude 5.5 earthquake, whose epicentre was known from studies of seismograph records, enabled the Kyoto team to try out their method. It succeeded in accurately pinpointing the epicentre, thereby proving that ULF radiation is generated at the site of earth movements. But that is not sufficient to provide a warning system. The equipment and data analysis have to be refined and continually tested to detect and use ULF noise long before events, to see whether or not these preceding signals point to future epicentres.

As Charles Darwin noted in Voyage of the Beagle, following his experience of a major earthquake in Chile, nothing is more frightening than the unexpected movement of the ground on which one stands. Every victim of an earthquake suffers post-traumatic stress disorder, whether or not they are injured or lose people close to them – we all implicitly trust solidity. Yet many survive physically because they instinctively seek some kind of shelter; perhaps one advantage of panic in the face of such a sudden threat. How much warning is needed in order to act according to a learned plan, in the manner of following a fire drill? Would say 20 seconds be enough? With even such a short warning, automated shut-down mechanisms for gas supplies – much damage and fatality is caused by fires in the aftermath of earthquakes – and activation of road and rail warnings would be possible. It would also enable people to escape from small buildings or to seek shelter in larger ones, given an ‘earthquake drill’, and an audible alert, such as a siren.

During research into the way in which faults rupture, based on seismograms of events of all detectable magnitudes, Erik Olson and Richard Allen of the University of Wisconsin, USA, made a potentially useful discovery (Olson, E.L. & Allen, R.M. 2005. The deterministic nature of earthquake rupture. Nature, v. 438, p, 212-215). Previously, the most widely held view was that the magnitude of an earthquake could not be calculated until all its energy had been released. Indeed, the magnitudes of the events that caused the 26 December 2004 Indian Ocean tsunamis and the massive loss of life in Kashmir and northern Pakistan in October 2005 were not calculated until hours afterwards. Olson and Allen found that the energy delivered by the first arrivals of fast seismic P waves correlated closely with the total energy of the full event, i.e. with its magnitude. The key to this finding was their analysis of the frequency of the early P waves, which show sufficiently good correlation with final magnitude for useful prediction of the most damaging events. P waves arrive around 20 to 30 seconds before the most energetic but slower surface waves, and they are rarely noticeable. If frequency analysis of the kind used by the authors were to be systematised at seismic stations, automatic warnings could be generated. They would not be false alarms because they are based on actual seismicity, although imprecision might mean that some alarms were followed by smaller earthquakes than the theory predicts.

See also: Tata, P. 2005. Can Earth’s seismic radio help predict quakes? New Scientist, 19 November 2005, p.28-29.

Yet further back in the Antarctic ice

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The groundbreaking Vostok ice core from Antarctica is the deepest ever to have been drilled. It recorded 440 ka of climate and atmospheric history, but unfortunately the very depth of the ice beneath the drilling station made that the limit in time terms. Thick ice begins to deform and flow, and the lowest parts of the Vostok core were clearly scrambled by that. The European Project for Ice Coring in Antarctica (EPICA) focussed its effort on a region of the East Antarctic ice sheet (Dome Concordia) whose location may always have ensured low accumulation of snow. Hopefully that would ensure that ice thickness was not so much as to result in complex flow at depth and that a fuller record would be preserved. The idea paid off, and the Dome C core penetrates back as far as 740 ka, giving an additional 3 glacial-interglacial cycles during the early part of the 100 ka periodicity; but falling just short of the first of those major cycles that are reflected in the marine oxygen-isotope record.

Results are now starting to emerge from Dome C (Siegenthaler, U and 10 others 2005. Stable carbon cycle-climate relationship during the Late Pleistocene. Science, v. 310, p. 1313-1317. Spahni, R. and 10 others 2005. Atmospheric methane and nitrous oxide of the Late Pleistocene from Antarctic ice cores. Science, v. 310, p. 1317-1321). The results are high-quality, and reveal some new features. The first three cycles conform to the 100 ka signal of the very weak variation in orbital eccentricity, as expected, but show lower amplitude shifts in CO₂ and methane in air trapped in bubbles than do the later four cycles. The two ‘greenhouse’ gases vary in concert, and their earlier low levels match with less extreme shifts in temperature as shown by the changes in deuterium content of the ice itself. This is probably due to the transition from the previous dominance by the 40 ka pace of changing axial tilt. Nitrous oxide values, although patchy down the core, seem to have fluctuated but at much the same amplitude throughout the last 720 00 ka. Dome C has yet to be ‘bottomed out’ so there is a chance that the record may yet reach the 40-100 ka boundary around 900 ka ago. What is striking – and should ring alarm bells – from the results so far is that in each of the previous 7 interglacials atmospheric neither CO₂ nor methane levels came close to those of the last century. Whatever its eventual effects, anthropogenic addition to the ‘greenhouse effect’ is an incontrovertible fact.

See also: Brook, E.J. 2005. Tiny bubbles tell all. Science, v. 210, p. 1285-7

Movies of Mars

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One of the most exciting geoscience websites that you can find is hosted by Arizona State University in Tempe. It centres on the capture of thermally emitted infrared radiation from the Martian surface by the Thermal Emission Imaging System (THEMIS) aboard NASA’s Mars Odyssey orbiter (http://themis.asu.edu). The opening ‘splash’ features thermal images gathered on the fly by THEMIS, as if you were peering down from the spacecraft as it orbits the planet. The movies are not really live, but about 2 weeks old. Nevertheless, they have a hypnotic appeal as one waits to see what is going to turn up – mainly small craters, but sometimes oddities such as the strange terrain of the northern Tharsis Basin that is a tangle of extensional faults that might well be on the floor of the Afar Depression in north-eastern Ethiopia. THEMIS acquires data in several thermal wavelengths, and this is its scientific importance: the multiple channels span the very different emission spectra of silicate minerals.

Using different thermal bands to control the red, green and blue colour guns of a video monitor produces vivid images that are colour-coded for a variety of rock compositions. The great advantage of thermal sensing is that it works at night as well as during the day. So THEMIS images can also tell us a great deal about the way in which rocks heat up and cool, which is another clue to their composition. Having no clouds – there are seasonal dust storms – Mars can be mapped in great geological detail without geologists having to traipse across space and the inhospitable Martian surface. All that a human touch could add would be to bring back some rock samples for geochemists to get their teeth stuck into. What those rock are – basalts, andesites and various sediments – is already becoming known in greater detail than for huge tracts of the Earth’s surface. Fortunately, a sister instrument to THEMIS, called ASTER does orbit the Earth to deploy a similar multispectral thermal imaging system. What is hugely annoying is that the Martian data are 5 times sharper than those of the infinitely more interesting Earth. Yet again, NASA has priorities that that are far from those of most of humanity. One excuse regularly given for better resolution from other planets is that of security issues for Earth images….

Fig leaves over Palaeocene-Eocene boundary

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Methane-induced warming around 55 Ma ago was one of the greatest environmental upheavals of recent geological time. Pretty quickly, all the methane belched out by destabilisation of sea floor gas hydrates would have forced up atmospheric CO₂ concentrations. The estimated climatic effect was astonishing: a global temperature rise of the order of 5-10°C in 10-20 thousand years. The early Eocene world would have become a steamy place, and the changes certainly tally with shifts in a range of faunas, from foraminifera to large mammals. Not many people have reported any coincident changes in plant fossils, even though a moist atmosphere charged with CO₂ would have encouraged growth enormously. A reflection of the changed conditions does come from rapidly changing leaf shapes and sizes, however. One of the key sections that does reveal floral change is in terrestrial sediments preserved in the Bighorn Basin of Wyoming, USA (Wing, S.L. et al. 2005. Transient floral change and rapid global warming at the Paleoene-Eocene boundary. Science, v. 310, p. 993-996). Tied down from a dramatic change in carbon isotopes, the boundary section not only shows the rapid dominance of leaves with extended ‘drip tips’ that allow rainwater to be shed quickly, but an influx of genera unknown from the Palaeocene below. The invasive groups are known from sediments of that age from much further south in the US, and even from Europe at the other side of the opening Atlantic Ocean. So it seems that there was a rapid northward plant colonisation over 4 to 20 degrees of latitude. The section perhaps gives a flavour of floral changes that might occur should modern anthropgenic warming go unchecked.

Dinosaur dung, the Deccan Trap and grass

Yes, it has to come to a pretty pass when geologists will tramp to the very base of the Deccan continental flood basalts, dig up and then finger through dinosaur crap. The temptation of a bed consisting of little other than coprolites deposited by sauropods, especially beneath the very lavas implicated by some in their demise, is huge. It isn’t the first time that coprophilia has struck the vertebrate palaeontological community, for a very good reason: if dinosaurs grew so darned big what did they eat? That it included grass is a surprise for palaeobotanists, but would have been a great treat for the thunder lizard, for there is nothing more toothsome to a herbivore than a hay snack; much better than a monkey puzzle leaf. Indian and Swedish geologists hit the headlines with their discovery (Prasad, V. et al. 2005. Dinosaur coprolites and the early evolution of grasses and grazers. Science, v. 310, p. 1177-1180). The lithified dung contains unmistakable traces of silica-rich phytoliths that occur only in grasses. Some possible grass pollen has been found before in Late Cretaceous sediments, but the crown-group Poaceae, that still thrive today, had been thought to have appeared later than the Early Eocene. It now seems likely that grasses appeared first in Gondwana, being transferred to Eurasia by the collision of its wandering fragment India around 50 Ma ago – India had already begun to move independently at the time of Deccan eruptions. Genetic studies of grasses points to their origin about 80 Ma ago, so it is likely that those in the dung are among the earliest. The Indian titanosaurs that ate them were not grazers, however, because the dung is also full of remains of conifers, palms and other vegetation that would have been abundant in those times. Interestingly, mammals from palaeosols within the Deccan lava sequence have cheek teeth reminiscent of the dominant grazers of later time.

Clay minerals and the origin of life

J.D. Bernal, a former student of J.B.S. Haldane, had as wide a range of interests as his mentor. Though a member of the Communist Party of Great Britain at the height of its loyalty to Stalin, during World War II he was a scientific advisor to Churchill. Among his many contributions was an idea inspired by Haldane’s conviction that life emerged from the inorganic world through simple chemical processes. Bernal thought in terms of a template sufficiently complex to shape early organic molecules, and clay minerals fitted that particular bill because they contain loosely bonded, yet complex passageways between the sheets of linked SiO₄ tetrahedra that form the bulk of their structure. A group of geochemists from Arizona State University have experimented on the organic catalytic potential of clays by simulating conditions around sea-floor vents that may have been the haven in which terrestrial life first formed (Williams, L.B. et al. 2005. Organic molecules formed in a ‘primordial womb’. Geology, v. 33, p. 913-916). Their ‘feedstock’ was dilute methanol and the clays that they chose were montmorillonite, illite and saponite, the last a member of the smectite group with high magnesium that forms by hydrothermal alteration of olivine and pyroxene in basalts. More complex hydrocarbons, with up to 20 carbon atoms per molecule, did indeed form in their experiments. The results suggest that smectite clays protect such unstable hydrocarbons from thermal decay, but no distinct life-forming molecules, such as amino acids, showed up. The products were polycyclic aromatic hydrocarbons, but it is possible that they would have formed a diverse feedstock for other processes once the hydrothermal clays were deposited in cooler conditions.

The geological sources of myths

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Sitting on top of the Kremlin in Red Square is a huge five-pointed red star that is illuminated at night. This is not just a relic of Stalin’s Soviet Union, but has its origins in a common myth that shows up concretely in archaeological digs, particularly in the Middle East, in the form of collections of fossil sea urchins and starfish. They, of course possess the five-fold symmetry unique to the Echinodermata, which also figures in the emblematic pentagram of Denis Wheatley’s awful novels about satanism and on the pointed hats of latter-day wizards and warlocks. I learned of this fascinating link between geology and symbolism at a session on Geology and Mythology at the 32^nd International Geological Congress in Florence (August 2004). This branch of geoscience seems destined to thrive, and Kevin Krajik has helped ensure that it does by reviewing a range of geo-inspired myths (Krajik, K. 2005. Tracking myth to geological reality. Science, v. 310, p. 762-764). His examples range from Pitman and Ryan’s hypothesis linking the flood myth of the Near East, first recorded in the Epic of Gilgamesh, to catastrophic filling of the Black Sea basin as sea level rose and spilled through the Bosporus around 7600 years ago, to the Oracle of Delphi. The most interesting and useful are those myths that incorporate an implicit warning of risk. Among these are pictograms of two headed serpents US which are reputed to shake the ground by native people of the NW who carved them. These a’yahos are found around major active fault zones. Cameroonian taboos include some that relate clearly to exhalation of carbon dioxide from crater lakes, as happened with disastrous effects at Nyos in 1986. The seafaring Moken of western Thailand have a tradition that a rapidly falling tide presages a man-eating wave: no Mokens died during the 26 december 2004 Tsunamis, despite living on the shore that was badly hit.

BIFs and bacteria

Published on November 1, 2005 by zooks777Leave a comment

Banded iron formations (BIFs) are by far the largest repositories of economic iron ore on Earth, and mines in them dwarf all but the largest surface coal mines. They also present one of the most enduring paradoxes in geochemistry. BIFs consist of oxidised iron in the form of iron(III) oxide (mainly hematite, Fe₂O₃), yet formed before about 2 billion years ago, when the Earth’s atmosphere and oceans were devoid of free oxygen. In fact the very formation of BIFs presupposes that iron must have been freely available in seawater as dissolved ions of its reduced form, iron(II). Their formation has been linked to the excretion of oxygen by photosynthesising cyanobacteria in the photoc zone of Archaean and Palaeoproterozoic seas, which would immediately combine with iron(II), thereby buffering environmental oxygen at very low levels. The problem with that hypothesis is BIFs show every sign of having accumulated in extremely quiet conditions: they contain the most exquisitely fine banding that in some cases has been linked to a diurnal cycle. The photic zone would have been one of high wave energy. A more environmentally viable hypothesis has to take account of that and place the environment of BIF deposition in deeper water. Biogeochemists of the California Institute of Technology and the University of Alberta have perhaps helped to resolve all the paradoxes surrounding BIFs (Kappler, A. et al. 2005. Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology, v. 33, p. 865-868). The bacteria that they cite as agents for iron(III) precipitation use the photon energy of ultraviolet radiation to oxidise iron(II) to iron(III), and in doing so use the freed electrons to reduce CO₂ and water to carbohydrate – this is not photosynthesis that uses light energy to increase the energy of electrons so that they perform the life-giving reduction. Solar ultraviolet radiation penetrates to much greater depths than the red light exploited by photosynthesisers, and could therefore fuel BIF formation below storm wave base at depths greater than 200m.

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