Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Exploration for water on the Moon

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There is a grim determination at NASA, and in the current US presidential administration that funds it, to get back to and stay on the Moon. Of course, it would be absurdly costly to ship out all the necessities for survival beyond a few days, the weightiest item being water. Protected by the frigid permanent shadows inside craters near the lunar poles, there may be some very old ice there (see Puffing up the Moon in April 2006 issue of EPN). NASA intends to crash a two-tonne spent rocket stage from a planned pre-landing mission into Shackleton crater, hoping to detect water vapour in the debris plume thrown up by the impact. Once the surveying satellite carried by the mission has done its job, that too is going to be crashed in the hunt for what is clearly more precious than gold for would be lunar colonisers.

Source: News in Brief. Nature, v. 440, p. 858.

Mineral mapping and the history of Mars’ rocks and water

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The principal mineral and rock mapping tool for Mars is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité. OMEGA is every remote sensing geologist’s dream machine, because its coverage of the short-wave end of electromagnetic radiation by 350 narrow bands can match spectra reflected from rocks and soils with those measured under laboratory conditions for several hundred important minerals. For over 18 months it has been steadily building up mineralogical maps of the Martian surface in a series of narrow swathes would round the planet in the manner of wool in a ball (see Mineral maps of Mars in April 2005 issue of EPN for early results). The 90% complete data, combined with dating of surface regions from crater counts and other means of stratigraphic analysis, is beginning to chart the history of the Martian surface in familiar terms of geology and the effects of water (Bibrin, J-P, and a great many others in the OMEGA team 2006. Global mineralogical and aqueous Mars history derived from OMEGA/Mars Express data. Science, v. 312, p. 400-404).

An interesting correlation is emerging. Where Mars’s surface is dominated by large amounts of pyroxene – the stratigraphically older regions of heavily cratered volcanic rocks – there is evidence of hydrated clay minerals (products of non-acid water alteration) and sulfates (formed by acid, hydrous alteration). The younger, brighter regions, which probably formed by surface processes after about 3.5 Ga, are dominated by anhydrous iron(III) oxides that give Mars its overall red colour. Although on Earth this hematite commonly forms by dehydration of iron(III) hydroxide or goethite, there is no sign of relic goethite on Mars. The authors attribute the red-staining hematite to direct oxidation of iron-rich silicates, without the role of water. It seems that in terms of surface processes, water played a role in the very earliest weathering to form clays. For a while conditions became acidic by the oxidative breakdown of igneous sulfides, thereby encouraging the formation of sulfate encrustations and sediments. This ‘wet’ phase may well have involved water vapour emanating from early, huge volcanoes. Once global volcanism became extinguished the supply of water was shut off, and since 3.5 Ga the planet has been hyper-arid. Hydrated minerals above the 5% level are not common on Mars, and if they did in fact encourage some life forms to emerge, the search for them can be finely focused by the OMEGA results.

Getting to the matter of the root

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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

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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

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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

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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 20th 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 CO2 would 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

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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

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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

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Mass extinctions have been the principal time markers in the Phanerozoic stratigraphic column since 19th 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.