Earth-logs

Yet further back in the Antarctic ice

Published on November 1, 2005 by zooks777Leave a comment

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

Published on November 1, 2005 by zooks777Leave a comment

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

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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.

Growing evidence for ‘hobbits’

Published on October 1, 2005 by zooks777Leave a comment

Various shenanigans within the Indonesian palaeoanthropology community have hindered evaluation of all the evidence surrounding the diminutive adult female skeleton found in Liang Bua cave on Flores in 2003. Her skull was damaged after prolonged examination by a leading national figure in the science, and now further excavation in the cave has been blocked indefinitely. Whether she is indeed a member of new species of hominin, Homo floresiensis, or merely an individual modern human dwarfed by some genetic defect, as some claim, seems closer to resolution (Morwood, M.J. and 10 others 2005. Further evidence for small-bodied hominins from the Late Pleistocene of Flores, Indonesia. Nature, v. 437, p. 1012-1017). During the 2004 field season at Liang Bua the Australian-Indonesian team unearthed remains of nine other individuals of similarly diminished stature. They included another jaw bone that is virtually identical to that of the first ‘hobbit’: neither have the chins that unify all fully modern humans. Significantly, the new piece of lower jaw is dated at some 3 ka older than the original, so the chances of both being from physiologically unfortunate modern humans are remote.

The new finds also include stone tools, more advanced than any found in association with one of H. floresiensis’s possible ancestors, H. erectus. Whoever they were, the ‘hobbits’ also butchered prey and cooked meat. There is negative evidence in support of the new species hypothesis too: compared with human sites of the Late Pleistocene, Liang Bua is conspicuously lacking in evidence for any form of art. But the idea is not proven. It would take a definite association between fossils and tools, as for instance in a burial, to show that the implements belonged to ‘hobbits’ rather than having been introduced by a fully human visitor. Moreover, should any evidence for moderns be found in Liang Bua or other caves of interest, the possibility of mixture of cultures and fossils would leave things up in the air.

It is worth noting that Indonesian scientists are not the only ones prone to obstructive tactics as regards hominin sites. They have long been a bone of contention throughout Africa, where both local and visiting scientists have tried to throw spanners in their colleagues’ research ambitions.

See also: Dalton, R. 2005. More evidence for hobbit unearthed as diggers are refused access to cave. Nature, v. 437, p. 934-935; Lieberman, D.E. 2005. Further fossil finds from Flores. Nature, v. 437, p. 957-958.

Congenital disease, human migration and population growth

The way in which genetic features are inherited has become a key feature in distinguishing human populations, the time and route of their migrations as separate groups, and when they merged with other groups. The most familiar outcomes are those based on mitochondrial DNA and lines of female descent that show with little room for manoeuvre, that all of us descend from Africans alive around 150 to 200 ka. Studies of the male Y chromosome help fine tune the record to show short periods when either populations fell so low that human survival passed through only a few small bands (e.g. around 70 ka) or Big Men corralled most women for their own purposes (the now famous case of Ghengis Khan’s genes still dominating the genetics of Central Asian people). Dennis Drayna of the US NIH outlines yet another revealing feature of genetics with historical connotation in the October 2005 issue of Scientific American (Drayna, D. 2005. Founder mutations. Scientific American, v. 293(4), p. 60-67).

Disabling congenital diseases, such as cystic fibrosis and sickle-cell anaemia, together with adverse reaction to alcohol and the ability of adults to tolerate the lactose in milk, are all passed down generations in different ways. Understanding the genetic processes involved obviously stems from medical research on genetic mutations so as to identify groups that are at risk. From it has emerged details on the structure and location of the responsible genes in chromosomal DNA. The feature that unites the four examples above is a special repetition of the same kind of mutant structure. Inherited conditions involve either different mutations in a single gene, or the identical change at a specific location. Of the latter, it seems the most common is an innate tendency in DNA for the same mutation to affect a specific gene – so called ‘hot-spot’ mutation, which occurs in unrelated individuals. More rare is a defect that is embedded in a length of DNA (a haplotype) whose structure is identical in all those who carry the mutation. That common identity suggests that the mutation arose once and has been passed down subsequently; a ‘founder’ mutation.

Since a ‘founder’ mutation arose at some time in the past it can potentially be used to trace population history, and so passes into the realm of palaeoanthropology. The fascinating and most useful feature is that the greater the separation in generations from the individual in whom the mutation occurred, the more restricted becomes the haplotype, in terms of its relative length in DNA. That phenomenon is a consequence of sexual recombination among descendants. In the founding individual, the whole chromosome is the haplotype, and the mutated part becomes increasingly ‘diluted’ with time. Measuring its length today harks back to the time of foundation. What has become clear is that not all founder mutations have any obvious consequence, and instead of being in as few as one millionth of a population, the general case for those causing disability and therefore conferring an adverse effect on natural selection, a few percent of people can carry them. Such abundance indicates either neutral effects or some subtle benefit to fitness. Diseases ascribed to them appear when both parents contribute the mutation: most are recessive.

A good example is a mutation of the HFE gene that confers above normal iron absorption, which is a decided advantage in protection against anaemia from iron-deficient diet. An individual with two copies vastly overcompensates and iron accumulates to deadly levels in their cells. Studies of its incidence in global populations indicate that it arose in Ireland, western Britain and Brittany and then spread south-eastwards. It appears to be a Celtic trait, although not from their original heartland in Central Europe but at the limit of their original migration more than 2000 years ago. Its haplotype is quite long and suggests a founder around 800 AD. There are no records of significant late Celtic migrations, and quite possibly the spread was through wide-ranging Vikings who dominated parts of the western British Islands at that time. A more fascinating case is the founder mutation that prevents people who carry it from tasting bitterness. Most people do experience bitter tastes, and that is very handy for avoiding toxic plants. About 25% do not. Maybe the mutation involved conferred some advantage, but the fact is that the haplotype is exceptionally short, representing a foundation at about 100 ka. It occurs in Africa along with 6 variants of the bitter-taster gene, yet beyond that continent only one taster and the non-taster forms occur commonly. That tallies with the hypothesis of the major movement out of Africa to populate the rest of the world with modern humans, around 75 ka ago. The surveys go intriguingly further: should descendants of those African migrants have bred successfully and regularly with earlier Eurasian hominins (Neanderthals and Erects), then non-African versions of the bitterness detecting gene ought to be present among non-African populations. Not one ‘alien’ haplotype has been detected, and this novel approach seems to have lain to rest that particularly intriguing bit of sociology.

A dialogue concerning world-shattering events

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Scottish Gaelic mythology includes the ‘Dread Coruisk’, the largest of the each uisge, or water horses. “ ‘Tis a thing of which we dinnae care tae speak”, say locals of the Isle of Skye, whose shores it nightly stalks. The same could be said of one of the most daring, and amusing, hypotheses of modern geosciences: that of the ‘Verneshot’ (see Mass extinctions and internal catastrophes in June 2004 issue of EPN). Phipps Morgan, Reston and Ranero explored the possible consequences of a build-up of volatiles in plume-related magmas at the base of thick continental lithosphere beneath cratons, prior to the eruption of continental flood basalts. The suggested that pressure would eventually result in an explosive release at a lithospheric weak point, followed by collapse above the plume head that would propagate upwards, at hypersonic speeds. Modelling the forces involved, the authors of the novel idea considered that they would be sufficient to fling huge rock masses into orbit. The notion neatly might explain the circumstances around mass extinctions: coincidence of CFB events; large impact structures, most likely at the antipode of the event; global debris layers containing shocked rock, melt spherules; unusual element suites and compounds (including fullerenes); and enough toxic gas to cause biological devastation. As with the ‘Dread Coruisk’, little has been said, neither in support nor in dispute over the last year. My comment at the time was, “As with all departures from “accepted wisdom”, the Geomar group’s ideas will come in for a lot of stick, quite possibly from the fans of giant impacts, who not so long ago were themselves dismissed as “whizz-bang kids” by many geoscientists.

It is good to be proved perceptive once in a while. One of the original butts of adverse opinion in the early days of impact hypotheses, Andrew Glikson of the Australian National University, has been the sole commentator (Glikson, A.Y. 2005. Asteroid/comet impact clusters, flood basalts and mass extinctions: Significance of isotopic age overlaps. Earth and Planetary Science Letters, v. 236, p. 933– 937). He points out that Phipps Morgan et al. overlooked 6 overlaps of impact clusters and CFBs, three of which were associated with mass extinctions. Rather than adding grist to their mill, he goes on to say that it is the geochemical blend associated with impactite layers that points unerringly to an extraterrestrial source for the mass involved in creating large impact craters, rather than any known terrestrial rocks. Moreover, the extreme shock-metamorphism that is the hallmark of impactites has never been observed near any gas-rich volcanic structure formed by explosive venting. He returns to the view that impacts of alien origin have sufficient energy to induce large-scale partial melting of the mantle, and thereby generate large igneous provinces.

Unsurprisingly, the original authors were onto Glikson’s comment, in leopard-like manner (Phipps Morgan, J., Reston, T.J & Ranero, C.R. 2005. Reply to A. Glikson’s comment on ‘Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: Are mantle plume-induced lithospheric gas explosions the causal link?’. Earth and Planetary Science Letters, v. 236, p. 938– 941). First they emphasise that their concept of the tremendous power of a ‘Verneshot’ is not based on the explosive release of volatiles, but on the shock pressures associated with the collapse of ~80 km tall pipes due to gas venting, in a very short period of time. As regards the geochemical blend in impactite-related layers, dominated by iridium yet a dearth of other platinum-group metals, they cite evidence that very similar element proportions are released in the carbon- and sulfur-rich gas phases of plume-related volcanoes, as in Hawaii and Reunion. They are not crustal, but of mantle origin, carried by escaping volatiles, and fall in the field normally said to be meteoritic. Phipps Morgan et al. also dispute the likelihood of extraterrestrial-impact induced magmatism from its statistical unlikelihood – the chances of a one in 100 Ma bolide coinciding with 1 in 30 Ma CFB events is, on their count, 1 in 3000 Ma – and from the standpoint of the powers and work involved. They agree that indeed there are extraterrestrial impact structures.

Surely, their well-argued idea is worth bearing in mind and considering as evidence continues to emerge – they do list a plausible set of characteristics that a ‘Verneshot’ would probably produce. There is some essential philosophy that has a good track record in the history of the geosciences, that of plate tectonics for one: the absence of evidence is not evidence of absense.

Irish mineralising fluids

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One of the most revealing field trips that I ever made was to the now-closed Pine Point lead-zinc deposit in Canada’s Northwest Territories. Being in the company of the late Doug Shearman (dcd. 2003) of Imperial College London helped a great deal, but the evidence exposed in and around the mine reawoke my interest in sedimentary processes that lead to economic mineralisation. To cut a long story short, Pine Point developed by the passage of Devonian seawater from a vast evaporating basin through a barrier-reef complex, in which a variety of chemical and biological environments, and products of karst formation encouraged the fluid to deposit the metals that it contained on an awesome scale. Limestone-hosted Pb-Zn ores occur widely in Britain and Ireland in rocks of Carboniferous age, the most familiar to me in the English Pennines being in narrow veins. The biggest in Ireland, and they are world-class, are more pervasive of the carbonate host. How they formed has been conjectural and based on geological relationships in what is a small area by comparison with the vast Late Palaeozoic sedimentary systems of the Canadian Shield. Crucial large-scale evidence is meagre. Studying the chemistry of the ore-bearing solution trapped in Irish fluid inclusions reveals a familiar picture (Wilkinson, J.J. et al. 2005. Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield. Geology, v. 33, p. 805-808).

Multi-element geochemistry plus strontium and sulfur isotope composition of the included fluids in Irish deposits reveals the signature of considerable concentration of the brines by evaporation, together with their having scavenged metals from crustal rocks as they circulated at depth. Returning to the surface along fault-controlled conduits, the metal-rich brine seems to have mixed with another. As at Pine Point, the sulfur needed to precipitate metal ions as insoluble sulfide ore minerals was probably supplied as hydrogen sulfide excreted by anaerobic bacteria that reduce sulfate ions in seawater. Doug demonstrated this phenomenon in 1981 with a linen handkerchief soaked in lead acetate solution, which he dipped into a foetid swamp seething with such ‘bugs’ on the Pine Point muskeg. ‘Instant orebody’, he cried, as the hanky turned black from fine galena particles.

Although the Irish Zn-Pb ores are more related to faults than to limestone reefs, nonetheless local geology demonstrates considerable relief on the floor of the shallow Carboniferous sea. Fully understanding the ‘plumbing’ and the geochemistry requires, as Wilkinson and colleagues suggest, a regional view of Carboniferous tectonics just before Africa collided with Laurussia. Just before that amalgamation, restricted, evaporation-prone basins would have formed. On a continental scale the circulation of their concentrated brines would have followed active faults systems that reached the shallow sea bed: a great deal more complicated than what is plain to see at Pine Point, given the eye of one of post-WW2 Britain’s lions of geology.

State of the art seismic imaging

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For many decades the primary tool of petroleum exploration has been reflection seismic surveying. As oil has become harder to find, industry has hugely improved means of processing seismic waves that return to detectors and expanded data gathering as a means of showing subtle structures and sedimentological detail. From individual seismic sections up to the 1970s, seismic surveys have moved towards multiple lines with ever-decreasing spacing as a means of producing 3-dimensional subsurface maps. Until recently the results of 3-D seismics have been glimpsed only rarely by the academic community, but once their commercial usefulness has been exploited they are increasingly becoming accessible. Richard Davies of the 3DLab at the University of Cardiff, UK and Henry Posamentier of Anadarko Canada provide an exquisite overview of the possibilities for research in the October 2005 issue of GSA Today (Davies, R.J. & Posamentier, H.W. 2005. Geologic processes in sedimentary basins inferred from three-dimensional seismic imaging. GSA Today, v. 15(10), p. 4-9). They show examples of derivatives from 3-D seismics, produced by a variety of image-processing techniques as well as the basic seismic processing, which demonstrate the depth to which these data can be interrogated. Featured are an example of meandering Pleistocene channels beneath the Gulf of Mexico, structures produced by sediment compaction between the Shetland and Faeroe islands in the North Atlantic, and the shapes taken by basaltic sills as they flowed into place. The graphics are wonderful, and would certainly tempt an IT-literate researcher. However, no funding agency could afford to commission such revealing surveys, and the geoscience community will always rely on the activities and generosity of the petroleum industry to enter this awesome world. Some might think of midnight meetings at lonely crossroads or an armful of long-handled spoons. Yet the potential results far transcend the kind of information one might extract from exposed geology.

Multimedia volcanoes

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Virtual field trips made possible by the considerable ingenuity of their authors are excellent means of taking school children and even undergraduates to places well off limits or resources. Most are available only on CD or DVD, but those on the web are especially valuable for all with sufficient connection speed to use them. A Swiss educational organisation hosts the work of Italian volcanologists Roberto Carniel and Marco Fulle with Swiss teacher Jürg Alean. They make it possible to experience volcanological life vividly, by ‘visiting’ the famous Stromboli, Ethiopia’s Erta Ale lava lake, explosive Montserrat in the Caribean and others.

Visit http://www.swisseduc.ch/stromboli

Photosynthesis during a ‘Snowball’ epoch

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In Neoproterozoic sedimentary sequences evidence for low latitude glaciation crops up at two and probably several other times; so-called ‘Snowball Earth’ events. Opinion is divided on several aspects of these events: whether or not they truly coated the Earth in glacial ice; their influence on biological evolution; the processes that started and terminated them. From a biological standpoint, a completely ice-bound surface – both land and oceans – would have stressed organisms to the extreme. Marine life (all that there was in those times) may only have survived in a few refuges from the ice, perhaps around submarine hydrothermal vents or in ephemeral sea-ice leads and polynya. If that were so, then these frigid episodes would have created important evolutionary ‘bottlenecks’, from which sprang several adaptive radiations: ‘Snowball’ epochs may have determined the forms and genetic diversity of all later life, especially among the Eucarya, of which we are a part. Probable deep-ocean anoxia would have been particularly stressful for organisms that depend on oxygen.

The key to establishing whether or not Neoproterozoic frigid episodes did bring eucaryan life to the verge of extinction lies in the diversity of life during those periods. That is not an easy task as all life until just before the Cambrian Explosion was both soft-bodied and minute. One means of assessing diversity is to study biochemical remnants of cell processes preserved in reduced ocean sediments (Olcott, A.N. et al. 2005. Biomarker evidence for photosynthesis during Neoproterozoic glaciation. Science, v. 310, p. 471-474). Olcott and colleagues studied black shales from Brazil whose age is within that of a frigid episode (740-700 Ma), and which contain textural evidence for abundant sea ice and low temperatures. Recovered biochemical compounds indicate considerable diversity, with a mixture of photosynthetic blue-green bacteria and eucaryan algae, with anaerobic bacteria of several types. The results indicate open water to allow photosynthesis – although it is possible for light to penetrate several metres of sea ice – together with deeper anoxic waters. Since the samples span a section almost 100 m thick, it seems this diversity persisted for a long period. However, the most that it can establish with certainty is that thin sea ice or open water did persist at the low palaeolatitude of late-Precambrian Brazil. The Neoproterozoic record has abundant, widespread black shales, and quite possibly there are others associated with evidence for glacial events. The importance of the paper lies in showing that biomarkers can be used as effectively in the Precambrian as in the Phanerozoic, and an expansion of this approach can be expected.

Oxygen and mammalian evolution

So much in the geological history of surface processes depends on either the dearth or the superabundance of oxygen. That is no surprise for a host of reasons, one being that it is the most reactive common element when free of bonds, and another is that the most powerful means of releasing oxygen is the capture of energetic solar photons by the pigments residing at the heart of photosynthesis. To grossly paraphrase James Lovelock, the principal reason for not sending people to Mars to search for life is that the planet’s atmosphere tells us that even if was there, it wouldn’t be very exciting. Oxygen gas is at vanishing low levels on the Red Planet, even if there is lots locked up in its iron-oxide rich surface.

The greatest event in the history of terrestrial life, apart from its emergence, was exploitation of the means of breaking hydrogen-oxygen bonding in water, which is what common photosynthesis is all about. It opened the entire planet to life from the restricted, though diverse habitats of most Bacteria and Archaea in the earlier anoxic world. First, oxygen-excreting cyanobacteria were able to colonise the entire ocean surface, depending on available nutrients. In doing so and generating free oxygen they threatened every other organism that used metabolisms based on other kinds of chemistry: oxygen is highly toxic because of it propensity to grab free electrons. Balanced by its oxidation of iron in early oceans, severe oxygen stress did not emerge until halfway through Earth’s history. Once it did become able to accumulate in air and water, all ecosystems faced havoc. Dominant prokaryotes slunk to rare places of refuge, while others seem to have combined in resisting oxidation. Their creation of the Eucarya that depend completely on available oxygen led, through the emergence of algae and then plants, to an accelerated stoking up of oxygen generation.

Once vegetation began to cloak the land, an extra 30% of the planet’s surface opened new vistas for animals and increased oxygen production and complementary burial of carbon. Indeed, explosive growth of atmospheric oxygen during the Carboniferous resulted in animal expansion to the air, through ominously huge insects. The first clearly traced ancestors of mammals seem to have appeared in the Permian, though their descendants only got the chance to dominate once reptiles, especially dinosaurians, lost their grip as a result of the K-T extinction. At the time of a far greater loss of living diversity, at the end of the Permian, it is now clear that in a relatively short time oxygen levels had fallen from their highest to one of the lowest in the Phanerozoic record (see New twist for end-Permian extinctions in the May 2005 issue of EPN).

Anoxic oceans were a regular feature of the Mesozoic and early Cenozoic. It is their preservation of abundant buried carbon that holds a key to, in an anthropocentric sense, the greatest of evolutionary leaps; the rise of large mammals and ourselves. A large team of US scientists has used the now abundant records of carbon isotopes in both buried organic matter and marine carbonates to reconstruct changes in atmospheric oxygen content (Falkowski, P.G. and 8 others 2005. The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science, v. 309, p. 2202-2204). Their modelling suggests that at the start of the Jurassic, atmospheric oxygen stood at around only 10%. Through that period it rose dramatically to 16%, fell equally abruptly and then rose again to about 18%, thereby creating the conditions for some of the largest sources of petroleum. Cretaceous times saw a slow rise, until around the time of the global warming at the Palaeocene-Eocene boundary (55 Ma). The middle of the Cenozoic was a further period of dramatic increase in oxygen levels, to their highest (~23% in the Oligocene) since the peak during the Carboniferous. Latterly atmospheric oxygen has waned to around 21% today.

Falkowski et al. compare their new atmospheric oxygen curve with evolutionary spurts among mammals, of which the simplest to understand is the parallel rise of mammalian average size. The metabolism of all mammals, like birds, has 3 to 6 times the oxygen demand of reptiles. Not only were Mesozoic mammals challenged in stature by the air they breathed, reptiles were easily able to grow to monstrous proportions because of their less demanding physiological processes. The first signs of the placental nurturing of mammalian foetuses, which requires a high oxygen level, coincides roughly with the Mesozoic maximum (100-65 Ma). The end-Cretaceous extinction of the dominant dinosaurian reptiles removed the main competition against the subtle advantages of placental mammals, and was followed by further increase in oxygen. The Cenozoic permitted terrestrial mammals to reach sizes almost comparable with dinosaurs, and to go beyond them among whales. Moreover, it saw explosive diversification, one branch of which, the primates, leads to ourselves.

Martian methane: a bit of a blow

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In Joseph Heller’s Catch 22, Hungry Joe is noted for ‘…snorting, stamping and pawing the air in salivating lust and grovelling need’. That is a close metaphor for reactions among some scientists (and astronauts) to observations that seem to support the notion that indeed, there is life on Mars. Remember the meteorite ALH84001? In 2004, a spectrometer carried by ESA’s Mars Express probe detected methane in the Martian atmosphere above areas that probably carry sub-surface water ice. Many exobiologists attributed this to exhalations by methanogen bacteria perhaps living in the ice, which seemed plausible. Sadly, it seems that hydrous alteration of the mineral olivine, which is widespread at the Martian surface, to serpentine is even more likely. The reaction can yield hydrogen, which generates methane by reducing carbon dioxide. Exobiologists are keeping their options open…. Meanwhile, it is not implausible that hydrogen from this simple reaction might be used to resolve global warming: olivine is the most abundant mineral in the rocky planets. Incidentally, it is serpentinisation of ultramafic rocks that best explains methane exhalation from the deep ocean floor and from crystalline basement, which Thomas Gold thought had a deep-mantle origin and was responsible for all hydrocarbon deposits.

Source: Schilling, G. Martian methane: rocky birth then gone with the wind? Science, v. 309, p. 1984.

Where do impactors come from?

All the rocky bodies in the Solar System (the Moon, Mars, Mercury, Venus, Earth and moons of the giant planets) preserve to some extent the signs of collisions with errant bodies. One period stands out dramatically: the Late Heavy Bombardment or LHB (4.0-3.8 Ga) that produced the lunar maria, and left its signature in Archaean rocks on Earth (see Tungsten and Archaean heavy bombardment, August 2002 EPN). The planet Venus was entirely resurfaced about 500 Ma ago, and its plains record the later flux of impactors in much smaller more widespread craters, as do the lunar maria, parts of Mars and to a very limited degree the Earth. The LHB stopped abruptly, having appeared equally out of the blue. The influence of astronomical collisions on planetary histories may be an established fact, but is still something of a mystery as regards its pace and intensity. High resolution images of large rocky bodies sustain a thriving cottage industry of measuring, counting and dating craters; the latter from stratigraphic evidence of relative age, such as craters that have been cratered, and ejecta mantles that bear signs of impact themselves.

Hidden inside such statistics are clues to the astronomical processes that lead to impacts (Strom, R.G. et al. 2005. The origin of planetary impactors in the Inner Solar System. Science, v. 309, p. 1847-1850). The crater-size distributions for the early events and those after 3.8 Ga are very different. Those of the later generation show features very like the size distribution of objects whose orbits intersect that of the Earth (near-Earth Objects or NEOs) and largely reflect the element of chance in a more or less stable late Solar System. The LHB pattern extends to craters more than an order of magnitude larger than the younger one, and resemble the size distribution of bodies that now orbit quite happily in the Main Belt of asteroids. It seems that during the period between 4.0 and 3.8 Ga, some main belt asteroids were flung out of their orbits to enter the Inner Solar System in large numbers. The analysis by Strom et al. suggests that the gravitational disturbance during that period might have been due to gradual migration of the giant Outer Planets before they took up their present stable orbits.

Climate change and human evolution

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One clear character of the record of investigations into human evolution is that, rather than becoming clearer as data increase, our origins become more of a puzzle. With every major fossil find the hominin clade or bush of descent acquires what appears to be another branch. With the recent publication of the genome of our closest living relative, the chimpanzee – and its earliest fossil remains – (Nature, v. 437, p. 47-108), it will hardly be surprising if the assumptions about a gene-based time of separation of the two clades (5-7 Ma) comes into question. Studies of the Y-chromosomes of living human males have suggested ‘bottlenecks’ in our recent evolutionary past, interpreted to indicate near-catastrophic declines in numbers to perhaps that of a few scattered bands. One such ‘near-extinction’ seems to have occurred about 70 thousand years ago, which has been linked to the huge explosion of the Toba ‘supervolcano’ in Indonesia in whose ash are poignantly preserved biface axes. Toba would have had a global climatic effect at a time when fully modern humans were migrating rapidly from Africa across Eurasia; thinly spread and easily isolated by disaster. What followed was an explosive development of both material and aesthetic culture, perhaps enabled by some serious selection amongst those who endured Toba’s global blast.

It is always tempting to restrict hypothesizing with the ‘Just gimme the facts’ outlook – as people of my generation will remember from the main detective in the Dragnet TV series. That is, ideas based on hominin remains alone. Yet all evolution takes place within a wider environmental context; for much of our history that of East Africa. Scanty knowledge of tropical climates there and a reliance on distant deep-sea records had led to the widespread belief that this centre of most hominin evolution gradually became drier since the late Miocene. Lake beds in the East African Rift system have held the key to a useful record, and now some of the detail is emerging (Trauth, M.H. et al. 2005. Late Cenozoic moisture history of East Africa. Science, v. 309, p. 2051-2053). Lakes in the Rift are handy for climate study because they span 8 degrees of latitude north and south of the equator, the spread helping to isolate more local effects of volcanism and tectonics on their sedimentary record from those of regional climate change. Many have little outflow and a local supply of water, so their levels depend mainly on the amount of local precipitation compared with evaporation. The actively subsiding basins in which they form have the opportunity to preserve unbroken, thick records of both lake and river sediments.

Trauth et al. compile environmental and chronological information from sediments in seven Rift basins, going back to about 3 Ma. Volcanic events provide plenty of dating opportunities to calibrate and correlate the sedimentary evidence. They show three rift-long episodes of deep lakes spanning broad periods from 2.7-2.5, 1.9-1.7 and 1.1-0.9 Ma. A few sections reveal lake-level fluctuations on Milankovich timescales. The longer episodes link in time to the intensification of Northern Hemisphere glaciation, to a shift in east-west air circulation over Africa and to the switch from the dominant glacial cyclicity of 41 ka to one of 100 ka, respectively. Wisely, they consider the climatic information to be crucial to studies of human evolution, but still too coarse to be used with confidence in relation to details of the fossil record. Long humid periods would have been ‘easy’, whereas the separating drier periods may have experienced ups and downs in humidity on Milankovich timescales. Fluctuating conditions would have been more stressful and likely to witness speciation. One very odd feature is that the 1.9-1.7 Ma period of deep rift lakes is the time when H. erectus became the first tooled-up being to migrate far beyond Africa. Many have regarded migration as a response to environmental stress, but just as likely is an expansion of opportunity.

Climate and the end-Permian extinction

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A time in Earth history (~251 Ma) when life was all but snuffed out and from which the creatures most familiar to us eventually emerged is understandably revisited quite often. Causes ranging from impacts (no convincing evidence as yet), through flood-basalt emissions, catastrophic methane release, low atmospheric oxygen to ocean anoxia have all been proposed. Hesitantly, opinion is converging on a climatic crisis of some kind, and indeed the coincidence of both terrestrial and marine faunal and flora extinctions points to climate being the global transmitter of some cause or a coincidence of causes. After the waning of Southern Hemisphere glaciations, the late Permian was warm, even at high latitudes. Until recently, attempts at modelling the end-Permian climate have not been entirely convincing because of limitations in the models themselves. Jeffrey Kiehl and Christine Shields of the US National Center for Atmospheric Research in Colorado have assembled a model that couples land, atmosphere, oceans, sea-ice and palaeogeography for the period (Kiehl, J.T. & Shields, C.A. 2005. Climate simulation of the latest Permian: Implications for mass extinction. Geology, v. 33, p. 757-760).

The critical test for the model is running it with parameters for the near-present, and it performs well. Several lines of evidence point to a much higher CO₂ level in the Permian atmosphere, so this is the main input parameter. The outcome is a world with a mean surface temperature that is 8° C higher than now. Unlike today, there was no geographic hindrance to poleward heat transport, so the high mean temperature is reflected in the summer warmth and humidity of Permian high-latitude land. The sub–tropics on the other hand were scorching (around an average summer minimum of 51° C, 15° C higher than now); a clear contributor to minimising life there. Sea-surface temperatures at high latitudes are higher in the model outcomes, this warmth extending to depths of 3 km. Surprisingly, low-latitude sea temperature emerges as much the same as now. The model also suggests that seawater was saltier than now, and that results in greater uniformity of density with depth and location: a hindrance to bottomward circulation and mixing. There would probably have been no thermohaline circulation worth speaking of. The model helps confirm the likelihood of an oxygen-free lower ocean and little transfer of nutrients. The oceans too would have been inhospitable. A shutdown of biological productivity and therefore carbon burial would have accelerated warming. So, pushing the biosphere into a mass extinction would have been inevitable. The last straw may have been the additional stress of increasing acidity from sulphur dioxide emissions from the Siberian flood basalts.

Milankovich forcing and Early Jurassic methane

Periods of environmental crisis less severe than those leading to mass extinction appear throughout the fossil record. As well as minor extinction peaks they are often signified by departures of carbon-isotope records from long-lasting norms. Such a crisis appears in the d ¹³C record of the Early Jurassic, and is beautifully preserved in about 15 m of black shales on the North Yorkshire coast of England. Geoscientists from the Open University, UK and the University of Cologne, Germany have produced an extremely high-resolution time series of carbon-isotope data from the section (Kemp, D.B. et al. 2005. Astronomical pacing of methane release in the Early Jurassic period. Nature, v. 437, p. 396-399). The quality is sufficiently good to analyse the time series using Fourier analysis that yields the frequencies that contribute to the observed wave-like patterns in the data. Of course, the time in a stratigraphic time series is measured in metres, unless it is possible to calibrate the section by precise radiometric dating. The Yorkshire Jurassic contains only fossils and no dateable horizons, but the fine stratigraphic division based on ammonites is also widespread and calibration is possible from dates obtained elsewhere. The overwhelmingly dominant frequency in the carbon-isotope curve is 1.23 cycles m^-1, which represents 21 ka after the calibration of depth to time. That is the signal of precession of the equinoxes, part of the astronomical forcing bound up in Miliutin Milankovich’s theory of astronomical forcing of climate.

Astronomical pacing turns up throughout the stratigraphic column, wherever sediments are suitable for time-series analysis (steady, unbroken sedimentation), so a precessional signal is no great surprise. The important feature is the profundity of the d ¹³C excursions; a total of –7‰, largely accomplished by three abrupt shifts of –2 to –3‰. The first two coincide with bursts in extinctions. The most likely phenomenon to have produced these shifts is massive release of methane by destabilization of submarine gas hydrates. Emissions seem to have been blurting out on a regular basis as the Earth’s rotational axis precessed like a gyroscope. So, the complete time period was one in which gas hydrate was unstable, probably due to overall warming. Yet something else must have triggered vast releases three times. The Lower Jurassic extinctions link in time with massive magmatism in Southern Africa and Antarctic (the Karoo-Ferrar large igneous province). Perhaps especially large volcanic events there set the stage for large precessional methane releases. An alternative view is that volcanic emissions of CO₂ gradually produced enough widespread warming for the astronomical trigger to cause breakdown of gas hydrate simultaneously over very wide areas of the ocean floor. Other explanations have been suggested for the Lower Jurassic warming and carbon-isotope excursions, such as wildfires, impacts and connections with petroleum maturation and migration. The clear cyclicity rules them out.

A tsunami’s reach

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The Boxing Day 2004 Indian Ocean tsunamis were recorded by tidal gauges across the planet, both as amplitude and time of arrival. Armed with such calibrating data, detailed ocean-floor bathymetry and means of modelling wave propagation, oceanographers and geophysicists from the US, Canada and Russia have been able to estimate just how the terrible waves travelled the globe (Titov, V. et al. 2005. The global reach of the 26 December 2004 Sumatra tsunami. Science, v. 309, p. 2045-2048). Highlighting their article wonderfully is a colour-coded map that shows offshore amplitude and arrival time for the world’s oceans and shores. Its most fascinating feature is the manner in which the worst of the disturbance was guided by ocean-ridge systems, principally the Ninety-East and Southwest Indian Ridges, but also the mid-Atlantic Ridge. That is of no comfort to the survivors of the disasters around the Bay of Bengal, although the Irriwaddy delta in Myanmar was spared by the influence of the northern part of the Ninety East Ridge. That Madagascar and East Africa, except for northern Somalia, suffered far less than anticipated is thanks to the peculiar effect of the ridge systems.

The fluoride saga

Archaeological work on Icelandic burial grounds of the 18^th century in the early 21^st century exhumed victims of the Laki eruption of XXXX. Many skeletons bore the distinctive signs of bizarre bone growth that characterises massive ingestion of fluoride ions. The victims had endured prolonged and worsening suffering after exposure to hydrogen fluoride-rich gases that seem to characterise Laki’s effusions. It is a now well-documented geotragedy. Equally well recorded are the lives of Iceland’s early inhabitants from the 8^th century onwards, but in the form of epic prose in Old Norse: the Sagas. Being prone to repeated volcanism, an obvious question is, “Did the Viking heroes experience the same problems?”

One of them was huge, both a righter of injustice and a tidy hand with the battleaxe. Egil Skallagrimsson was ‘a man who caught the eye’, reputedly being awesomely ugly and capable of jerking an eyebrow down to his chin line. Such attributes might seem to have been passed on to the legendary centre-half, ‘Skinner’ Normanton, who graced Barnsley football club in the 1950s. The traditions perhaps, but Egil’s visage was probably a result of chronic fluorosis rather than parentage (Weinstein, P. 2005. Palaeopathology by proxy: the case of Egil’s bones. Journal of Archaeological Science, v. 32, p. 1077-1082). His relatives Hallbjorn Half-troll and Grim Hairy-Cheeks seem from the saga to have been equally afflicted, yet successful. As befits a Viking battler, Egil had a thick skull; when exhumed by descendants in the 12^th century, it was found to be ridged like a scallop shell – the attending priest hit it with the back of an axe, to no avail. Some have inferred abnormal bone growth and deformities due to Paget’s disease, but that tends to produce massive but weak growths, following repeated crumbling of bone. Weinstein’s theory may be verifiable, since Egil’s Saga reveals the final resting place of this enigmatic giant.

Source: Pain, S. 2005. Egil the enigmatic. New Scientist, 17 September 2005, p. 48-49

Earth’s biggest ‘bull’s eye’

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Since astronauts and satellite imaging devices first made pictures from orbit, top of the list for oddness is the Richat structure of Mauritania. Sitting out in the Sahara is series of perfectly concentric rings that are almost circular. The structure is at least 40 km across, and even today, many geoscientists use images of Richat as a superb example of a meteorite impact. It is not (Matton, G. et al. 2005. Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex. Geology, v. 33, p. 665-668). Spectacular from space, Richat is not easily accessible. Early field work reported a breccia on a kilometric scale at its high-relief core, which unsurprisingly added to its designation as an impact structure. There are other possibilities: a structural dome, perhaps due to interference between open folds of a couple of generation; the result of upward forces from magmatic activity, such as an underlying plutonic diapir.

The rocks involved are Neoproterozoic to Ordovician sediments of various kinds, which dip radially outwards from Richat’s core, so it is some kind of dome, rather than the sort of circular breach expected of an impact. Two large, basaltic ring dykes, whose centre coincides with that of the dome, cut the sediments. Other igneous materials are: carbonatites (formed from unusual carbonate-rich magmas) in dykes and sills; alkaline silicate-rich intrusions and flows occurring close to the central breccia; kimberlites in the form of plugs and sills. The central breccia is in fact a roughly horizontal lens, about 3 km across, that is made mainly of local sedimentary material, mainly once carbonates, set in a silica-rich matrix. The clasts range from highly angular to rounded, but show abundant evidence of some kind of corrosion and silicification. Matton et al. interpret the breccia as a zone of intense dissolution that caused the original sediments at the structure’s core to collapse as volume was reduced as magmatic gases (supercritical fluids) rushed to the surface. So the Richat structure has all the hallmarks of doming above an alkaline igneous pluton, followed by intense hydrothermal activity that was able to dissolve carbonates and produce features akin to those formed by weathering in areas of karst. Rather than being particularly ancient, the igneous activity dates to the Middle Cretaceous. Richat is still unique. Diatremes (vertical breccia tubes) formed by explosive release of fluids from alkaline magmas are quite common, especially in areas dotted with kimberlites, but nowhere else have they produced doming on such a grand scale and with such a spectacular shape.

Detecting the effects of slab to wedge fluid transfer in subduction zones

A fundamental hypothesis concerning the formation of magmas above subduction zones is that partial melting in the over-riding wedge of mantle is induced by upward transfer of water vapour produced by dehydration of the descending lithospheric slab. Many aspects of the chemistry of igneous rocks in supra-subduction zone settings are explained by such dehydration-hydration. However, such fluid transfer is difficult to demonstrate, other than by its ‘second-hand’ geochemical effects on crustal magmas. It should have another, physical effect: in the presence of water vapour, some of the dominant olivine in mantle rocks should break down to form hydrated minerals of the serpentine family. Since olivine is an iron-magnesium silicate, whereas serpentine contains only magnesium, the hydration reaction should release iron to crystallise in the form of iron oxide; specifically Fe₃O₄ or magnetite. Geophysicists at the US Geological Survey have been able to detect at first hand the effects of this process, thereby allowing zones of hydration in the mantle wedge to be mapped (Blakely, R.J. 2005. Subduction-zone magnetic anomalies and implications for hydrated forearc mantle. Geology, v. 33, p. 445-448). As well as finding substantial magnetic anomalies caused by the release of magnetite by olivine dehydration over the forearc of the Cascadia subduction zone in Oregon, they show gravity anomalies that reflect density variations in the underlying mantle. The other aspect of the olivine-serpentine transformation is a large decrease in density, which should result in a decrease in gravity anomaly should sufficient olivine have been transformed. The coincidence of gravity lows with magnetic highs allowed Blakely et al. to model the location of hydrated mantle wedge in the Cascadia subduction system: probably just above the zone where subducting oceanic crust is transformed to ecologite.

Serpentinite also has a marked effect on the rheology of mantle rocks, because of its ease of ductile deformation. It should allow subduction deformation to proceed in a continuous fashion within the part of the system where it occurs, yet may focus sudden strain in great earthquakes to shallow levels up-dip of its position.

Arsenic removal no cure

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It is now a decade since the enormity of natural arsenic contamination in groundwater below the great plains of northern India and Bangladesh came to light. In 1995 the World Health Organisation announced that this waterborne arsenic was causing the world’s largest case of mass poisoning. Since then other areas at risk have emerged in East and Central Asia and South America. The tragedy is that groundwater generally presents the safest option for drinking water because sediments filter water and encourage biogenic oxidation that remove common pathogens. That tens of million people in West Bengal and Bangladesh face stealthy poisoning results from channels cut in the low-lying plains during the last glacial maximum being filled rapidly with sediment as sea level rose during climatic recovery. Sedimentation buried large amounts of organic debris to form anoxic conditions in the shallower sediments. Reducing conditions encourage breakdown of the common colorant in sediments, iron hydroxide grain coatings that, having adsorbed most arsenic and other ions from water, releases them when it dissolves. That this should occur was unsuspected during a massive programme of well sinking to relieve endemic ill health from waterborne disease, yet early signs that arsenic had replaced pathogens as a hazard was widely ignored, despite a few warning voices who discovered the unmistakable signs of arsenicosis in the 1980s. They include disfiguring pigmented skin spots and horny growths on hands and feet.

By 1995, the rest of the world took notice, pouring in funds to document occurrences and causes, and to remediate a clearly catastrophic situation. There are three main strategies: to remove arsenic from well water using chemical filters; to return to water from surface sources, though with careful processing to remove pathogens; to sink wells below the level known to encourage arsenic release from iron hydroxide dissolution. For two decades affected populations had been bombarded with encouragement to turn to groundwater: against their better judgement – they termed it the Devil’s water. Once using wells they saw that infant mortality plummeted, so they developed a new enthusiasm for water deemed safe. Caught on the horns of a dilemma, when arsenicosis appeared they were reluctant to return to what appeared to be the greater of two evils. In only a few places were wells deepened to safe depths, and the externally sponsored drive for a solution centred on arsenic removal techniques. Even that was not widespread: of millions of risky wells some 2000 were equipped with arsenic extracting devices, at around US$ 1500 each. It now emerges that the technologies chosen are not doing their intended job (Hossain, M.A. (and 10 others) 2005. Ineffectiveness and Poor Reliability of Arsenic Removal Plants in West Bengal, India. Environmental Science & Technology, v. 39, p. 4300-4306). The team, led by Depankar Chakraborti, who first spoke out about arsenicosis in 1983, tested the efficacy of 18 different devices installed in West Bengal. Only two reduced arsenic levels to the maximum of 50 parts per billion accepted by the Indian government, which is itself five times more than that deemed safe by the WHO. The teams view, supported by the agency that did most to encourage the massive well-driving programme since the 1970s (UNICEF), is that the only realistic solution is a return to rainwater harvesting and purification.

See also: Ball, P. 2005. Arsenic-free water still a pipedream. Nature, v. 436, p. 313.

Legendary events at the Gibraltar Straits

Everyone has heard of Atlantis, but few would care either to point to its former position, or to accept its existence without a shed-full of salt. Nevertheless, no lesser an authority than Plato first described the legend of Atlantis in the 4^th century BC, following verbal accounts that originated in pharaonic Egypt. In the last decade a number of legends, if not their religious connotations, have received scientific support. Foremost among these is that of the biblical Flood, which Ryan and Pitman pursued relentlessly, using the Epic of Gilgamesh as a geographic and chronological guide. They discovered that the Black Sea had catastrophically filled through the Bosphorus once global sea level topped the level of its floor, following glacial melting. Their evidence now includes numerous examples of habitations now inundated by the Black Sea.

As with Ryan and Pitman’s work, one key to resolving a real basis for a legend is carefully puzzling out clues in the most detailed accounts of it. In the case of Atlantis, the clues come from Plato himself (Gutscher, M-A. 2005. Destruction of Atlantis by a great earthquake and tsunami? A geological analysis of the Spartle Bank hypothesis. Geology, v. 33, p. 685-688). Marc-André Gutscher and previous workers focused on Plato’s geographic description of Atlantis, as well as its fate. Plato clearly specified an island in the Atlantic beyond the Straits of Gibraltar, and an earthquake and flood that put paid to the Atlanteans in a single day. Indeed, bathymetry does show well-defined shallows (less than 100 m depth) in such a location, but only about 5 km across. This is the Spartel palaeo-island, on which Gutscher turns his focus. Until the final, decisive rise in sea level after around 12 ka, Spartel would have been a low island. Plato’s account is supported by the existence of a proto subduction zone on the Atlantic sea floor off the Straits of Gibratlar, a major earthquake on which devastated Cadiz in 1755, partly because of a 10 m tsunami. Offshore sediments include turbidites that indicate 8 tsunamis since 12 ka, suggesting a 1500- to 2000-year periodicity of large earthquakes at the entrance to the Mediterranean. Plato’s version of the events includes a rough chronology that suggests a time around 11.6 ka before the present. The thickest of the tsunami-driven turbidites is of roughly that age. Unfortunately for the hypothesis that Spartel was Atlantis, at that time only two tiny islets would have stood above the waves. Seismic destruction of coastal regions by tsunamis is something that might easily become legendary, the more so in the distant past. There is one other possibility that might revive the Spartle hypothesis, demonstrated by the great Indian Ocean tsunami of 26 December 2004. Very powerful earthquakes can also result in massive displacement of the crust, or the order of tens of metres. Spartle might have sunk repeatedly since 11.6 ka, as a result of later events.

Documenting the Palaeogene transition from ‘hothouse’ to ‘icehouse’

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It is well-established that the first large ice sheets that presaged descent into the oscillating climate of the Neogene formed about 34 Ma ago (the Eocene-Oligocene boundary) on Antarctica. Some 21 Ma before, at the Palaeocene-Eocene boundary, global temperatures had leaped following what many believe was a massive blurt of methane previously held in cold storage in ocean-floor sediments as gas hydrate. A monstrous ‘greenhouse’ climatic system must sometime in the interim have reverted to the cooling trend begun at the outset of the Cenozoic. Defining that transformation relies on assembling and interpreting newly available, high-resolution records of climatic proxies through the Eocene and Early Oligocene (Tripati, A. et al. 2005. Eocene bipolar glaciation associated with global carbon cycle changes. Nature, v. 436, p. 341-346). Hitherto, the Eocene part of the ocean-floor sedimentary column had been poorly sampled, so that only broad trends showed.

As you might expect, the change was not a simple transition. At about 42 Ma the record of the Pacific Ocean calcite compensation depth (CCD – the depth at which carbonate remains are dissolved in the deep oceans) shows a remarkable perturbation long before the CCD dipped decisively from about 3.5 km to around 5 km at the start of the Oligocene. A close look at the oxygen isotope record of that age in a highly detailed marine sediment core shows an increase in d ¹⁸O that corresponds to either some 6° of cooling or a 120 m fall in sea-level due to build-up somewhere of ice on land. Coinciding with this perturbation are shifts in the carbon-isotope record in carbonates. The authors suggest that the mid-Eocene cooling and continental glaciation that produced falling sea level triggered the weathering of shallow-water carbonates, which together with river transport increased the oceans’ alkalinity. That would have increased deep-water carbonate formation enormously and accelerated the effective ‘burial’ of carbon from the atmosphere

Smithsonian geological timeline

Published on August 1, 2005 by zooks777Leave a comment

A measure of the quality of a science website, apart from its visual appeal, is a mixture of how much it teaches you and what you can snaffle to help teach others. As a point of departure for E-geology, it will be hard to beat the Smithsonian Institutions geotime site (www.nmnh.si.edu/paleo/geotime). That’s because it focuses first on the history, and if you care to you can discover how that was constructed from the geological record. Its central organiser is a slider that can be zoomed, which lays out the geological past – the literal time line divided into stratigraphic Eons, Eras, Periods and Epochs. Each division is clickable, although zooming in several times is needed to see the Cenozoic Epochs. But, hang on, there is no Ediacaran Period, the newest addition, nor the subdivision of the Proterozoic on the timeline. Whatever, clicking on a division opens a thumbnail sketch of each and links to pages that give more detail on the highlights, plus introductions to the founding concepts behind geological time and unravelling Earth and life processes. There is a glossary, which shows the influence of Encarta and Wikipedia. Here is a chance to learn for hours in a most convenient and engaging way, but graphics are few and far between in the various main panes. There are examples of important fossil organisms, but displayed at a size that lacks satisfying detail. What the site needs are maps and explanatory diagrams, which are available elesewhere. So the Smithsonian needs, I think, to liase a bit with other learning resources in the geosciences. It would be good to have a one-stop shop.

Has human evolution stopped?

Published on August 1, 2005 by zooks777Leave a comment

There can be no doubt that the way in which humans consciously build ‘shields’ of many kinds between themselves and their surroundings placed our species, and those leading up to it, in an increasingly different relationship to the environment than those of other organisms. Fire, habitations, tools, weapons and clothing emerged far back in our evolutionary ‘bush’, to be followed more recently by artificial means of feeding ourselves in a vast range of climatic conditions. In the last century these ‘shields’ have been added to by medical protection against pathogens.

Many of the physical traits of the modern human frame would not be ‘fit’ in a purely Darwinian sense for life unprotected by myriads of cultural devices: they arose from genetic potential largely because growing human culture allowed them to be fit for purposes other than survival at its simplest level. The range of basic physiognomies among modern humans does seem to reflect natural selection to suit various climatic regions, such as the differences between cold- and heat adapted peoples. That perhaps began during the great expansion out of Africa some 70 ka ago. But the much greater range of facial characteristics among all populations (a really human characteristic compared with other primates) is probably a result of genetic drift at random, rather than any kind of evolutionary selection. There are also differences that have arisen since the widespread adoption of agriculturally produced foods since about 10 ka ago, as in jaw shapes and those of the skull, probably linked to easier mastication. That can be explained most easily by the manner in which the use of muscle tends to sculpt the bone to which it is attached: it arises during the life of the individual.

With what appears to be the start of a global unification of cultures, and greater security for the more fortunate one third of humanity at least, it might be expected that natural selection is on the wane for humans. A mere 10 thousand years since the rise of agriculture and far less since modern cultures arose, it is perhaps too soon to conclude that we have cut loose from Darwinian processes. Indeed, recent genetic research has come up with several developments that must be recent results of natural selection. One is the split between adults who can metabolise cows’ milk and those who cannot. The first group, a minority, cluster around the Near East (most Europeans) and in a few parts of Africa where cattle domestication arose. A large block of the human genome, about a million base pairs of nucleotides, includes the gene that produces the necessary enzyme lactase, and its persistence in those adults able to digest milk. The large size of the whole haplotype is typical of recent genetic developments, and the researchers are certain that it resulted from selective pressure where dairy farming began at between 5-10 ka.

Genes that confer resistance to infectious diseases that can cut life short before successful reproduction are good candidates for showing the effects of natural selection, especially in those areas where medical care and drugs are not available. For a long while natural resistance among some west Africans to malaria parasites was linked with heritable sickle-cell anaemia, but recent research has shown a more complex reason that involves several genes. Interestingly, ‘dating’ of the associated genetic changes gives recent ages between 3 and 6 ka, perhaps linked to the rise of farming practices. Clearing land and ponding of water on fields would have encouraged the malaria-carrying Anopheles mosquitoes, which are not forest species: a cultural change presaged a genetic one. Similar results have emerged from studies of inherited protection against HIV/AIDS, yet that only appeared in pandemic form very recently (unless misidentified earlier). An explanation may centre on selective pressure on mutation to form the protecting gene as a result of the appearance of previous epidemics, such as plague and smallpox among early Europeans, who seem to have the highest resistance to HIV/AIDS.

So it is hard to say if selective pressures will work in future on the human genome, as culture convergence continues, and (hopefully) equitably shared living standards. Since the limit on human brain size is the skull, and that is limited by the near-maximum pathway through the human female pelvis, it is very difficult to imagine our evolution into big-heads.

Source: Balter, M. 2005. Are humans still evolving? Science, v. 309, p. 234-237.

Modelling the core

Published on August 1, 2005 by zooks777Leave a comment

Judging by the growing procession of research grant proposals aimed at studying the inner workings of the Earth’s core through computer modelling, it would be easy to assume that a major breakthrough was just over the horizon. What you need is some kind of supercomputer to handle the massive complexity of core fluid dynamics and then channel that through one of several concepts of a geodynamo, first towards simulating the present field and then to how the geomagnetic field swirls and occasionally flips. The fourth biggest there is belongs to the Japanese geophysical community; the Earth Simulator, which is certainly well ahead, in terms of power and speed, of facilities available to less endowed scientists. Recently, about 10% of its power was let loose for a 9 month modelling run that focussed on complex motion in the liquid outer core that theory should generate (Takahashi, F. et al. 2005. Simulations of a quasi-Taylor state geomagnetic field including polarity reversals on the Earth Simulator. Science, v. 309, p. 459-461). Hitherto, modelling had produced pictures of varying magnetic intensity that bore some resemblance to the real magnetic field at the Earth’s surface, and did indeed come up with reversals. Yet a variety of models all produced similarly plausible patterns in space and time. The snag was the limit to matching the viscosity of liquid iron with spin rate. Geomagnetists suspect that the Ekman number, which represents that relationship, is very low in the Earth’s core, i.e. there is very low drag in core circulation, and that adds to complexity. Until the Earth Simulator was built, no power on Earth could deal with the high spatial resolution needed to simulate properly motions at low Ekman numbers. Takahashi and colleagues were able to drop the Ekman number 10 times below any previous simulation.

Real-looking features did begin to emerge in the time sequence for the field at the core’s surface. The most interesting was the formation of zones of opposed polarity at high latitudes, soon (in about 1000 years of simulated time) to be followed by a reversal. The zones move progressively polewards to coalesce, when the overall magnetic polarity all but disappears, and then a reversed field becomes established. However, this is not real but a model dependant phenomenon, even though it is possible to see patterns akin to those observed today – many geophysicists believe the Earth is on a magnetic cusp before a reversal. Will it ever be real is an obvious question, in the same way that related climate simulations may flatter to deceive. The problem is not a lack of models, nor conceivably computing power, but a lack of real data. The ocean floor contains masses of information on past reversals, and cunning analyses of palaeomagnetism in lavas that cooled slowly through the Curie point at the time of a reversal show astonishing things that happened. Excellent maps of the modern field are available, but reality in a reversal is a time series of that mapped field. Without such data, and the time to collect it (the modelling simulates evolution over 5200 years) before the next order-of-magnitude jump in computing power (perhaps 10 years off), it is very difficult to see a justification for this kind of modelling, as opposed to that for climate, which does have a more rapid response time.

See also: Kerr, R.A. 2005. Threshold crossed on the way to a geodynamo in a computer. Science, v. 309, p. 364-365.

Did oil and gas fields form during the Precambrian?

Published on August 1, 2005 by zooks777Leave a comment

Since the origin of life it is certain that a proportion of biological materials would have been preserved in sediments after organisms died. As today, such material would have evolved or matured as the host sediments were buried and heated. There is plenty of evidence that such maturation did occur as far back as 3250 Ma ago, but signs that oil-fields formed by migration and trapping have proved elusive. Several lines of evidence, such as carbon-isotope anomalies in Precambrian limestones, point to periods when enormous amounts of organic material were buried, much as happens in the formation of Phanerozoic petroleum source rocks during periods of ocean anoxia. Before about 2400 Ma, when evidence for an oxidising surface environment first appears in the rock record, such conditions would have been pervasive. The first hints of large-scale petroleum formation and migration have been found in the low-grade Pilbara craton (3500-2850 Ma) of Western Australia and 2770-2450 Ma sediments that overlie the older Archaean complex (Rasmussen, B. 2005. Evidence for pervasive petroleum generation and migration in 3.2 and 2.63 Ga shales. Geology, v. 33, p. 497-500). Black shales in the Pilbara contain not only lots of fine-grained carbonaceous matter, but some in forms that clearly suggest that they had been thermally matured (‘cracked’) to low-viscosity fluids that could migrate. There are blobs of bitumen contained within iron sulfide layers that seem to have formed later, to engulf petroleum liquids. Molecules within the bitumens resemble those formed by photosynthesising blue-green bacteria, methanogen and sulfate-reducing bacteria and arguably perhaps primitive eukaryotes. It appears that the bitumens probably formed as residues as lighter and more fluid hydrocarbons migrated out of these substantial source rocks. What has yet to be demonstrated are Archaean and Palaeoproterozoic reservoir rocks where such migrating petroleum accumulated. Another question is whether or not the source rocks, which are extremely widespread and thick, might have retained some potential for sourcing petroleum much later in the geological history of Western Australia and similar cratons elsewhere.

Precise timing of petroleum migration

Published on June 1, 2005 by zooks777Leave a comment

In their slack moments, petroleum geologists ponder on when oil and gas got into a particular reservoir and became trapped. One aspect of the conundrum is easy to answer: after the reservoir rock and trap formed. But timing is not so trivial, for an important consideration in exploration for new oilfields concerns the actual rock that sourced hydrocarbons in known fields, almost always a highly reduced, black mudrock in which unoxidised dead organic matter accumulated and matured. Repeated anoxic events, both regional and global, provide several alternatives in many petroleum provinces.

Hydrocarbons, having formed under highly reducing conditions, contain several metals and other elements well above normal crustal concentrations. Among these are rhenium and osmium, which allow radiometric dating through the decay of ¹⁸⁷Re to ¹⁸⁷Os. In principle, therefore, it is possible to date oil and relate it to a particular source rock. Interestingly, it is easier to date the actual time at which oil has accumulated in a trap. In an analogous way to the equilibration of parent and daughter isotopes in magmas, which is halted by crystallization so that the system evolves and dating can be done, once oil settles in a trap after migration the timing can be dated sing the Re-Os method. David Selby and Robert Creaser of the University of Alberta, Canada applied this approach for the first time, using the vast reservoirs of oil sand in Alberta as a test (Selby, D. & Creaser, R.A. 2005. Direct radiometric dating of hydrocarbon deposits using rhenium-osmium isotopes. Science, v. 308, p. 1293-1295). The oil in the sands were emplaced around 112 ±5 Ma ago, during the Early Cretaceous, not long after the host sandstones had been deposited. Previous work using ideas on oil maturation suggested that migration had taken place during the Early Palaeocene, around 60 Ma ago, when potential source rocks were heated by tectonic burial during the Laramide orogeny. The Re-Os results point to migration from the west while the Cretaceous sedimentary basin was filling. This may explain the high viscosity of the oils as a result of near-surface biodegradation.

Another product of isotopic dating is establishing the initial ¹⁸⁷Os/¹⁸⁸Os ratio of the petroleum system, which relates to that of the original source and its isotopic evolution. In the case of the oil sands this value points to source rocks of earlier Mesozoic and even Palaeozoic age, rather than a Cretaceous source that had been suggested previously.

Britain above convecting mantle?

Published on June 1, 2005 by zooks777Leave a comment

Being able to picture Earth features far beneath the surface is what makes seismic tomography such an exciting tool, even though it is in its infancy. It shows variations in the velocity of P and S waves in 3-D. Regions of fast waves are likely be cooler than those in which wave speeds are relatively slow. The detail depends on the spacing between seismic recorders and the distribution of natural seismic events, whose interactions produce tomographic data. Despite being rarely affected by seismicity themselves, the British Isles have a remarkably dense network of seismic stations that was developed for research. Given arrival times at the different stations by waves from earthquakes that occurred over a wide range of epicentral angles from the British Isles, it becomes possible to probe in detail what lies beneath. Exploiting the potential to the full, a group of British and US geophysicists has shown that the ‘British’ mantle is far from boring (Arrowsmith, S.J. et al. 2005. Seismic imaging of a hot upwelling beneath the British Isles. Geology, v. 33, p. 345-348).

Down to a depth of 600 km, Britain is underlain by a series of significantly slow and fast mantle ‘blobs’. The seismically slow, probably warm mantle zones seem to follow large features last active during Early Palaeogene magmatism that affected the Hebrides and Northern Ireland, and roughly parallel the 60 Ma dyke swarms that radiate from these centres. They also correlate with regions of anomalously high gravity. It seems highly likely that both features are long-lived relics of a spur of the still active Iceland plume that is intimately associated with spreading on the Mid-Atlantic Ridge. The warm zones also underlie those parts of the British Isles that were most affected by uplift and erosion during the Cenozoic: as much as 3 km in the case of the Irish Sea. Such areas also focused extension at the time of the magmatism, and they are still most affected by minor seismicity.

Estimates of the magnitude of the temperature anomaly associated with the slowing of P-waves are as much as 200 °C above ambient mantle temperature; sufficient to be associated with partial melts. That Britain might once more have active volcanoes is highly unlikely, and the anomalies are probable parts of the Iceland plume system that became trapped beneath zones of crustal thinning. Their loss of heat is sufficiently slow for them to have bolstered areas of uplift and erosion for tens of million years. There is even a chance that some form of convection might yet be going on.

New data on starting point for Earth evolution

Published on June 1, 2005 by zooks777Leave a comment

Slowly, geochemists as well as planetary scientists have been taking up the implications of a likely infernal origin for the Earth-Moon system that resulted from a Mars-size planet colliding with the proto-Earth, shortly after planetary accretion. The chemistries of both Earth and Moon have sufficient similarities for a common origin to be almost certain. There is one difference: lunar rocks are more depleted in volatiles than those accessible on the Earth. Terrestrial rocks were at some stage in their evolution purged of some volatile elements. The Moon’s early history seems to be extraordinarily simple. It is recorded in the pale rocks of the lunar highlands that are made dominantly of feldspars. Their low density and abundance suggest that feldspars floated to the top of completely molten rock, in much the same way as similar anorthosites on Earth seem to have formed in large magma chambers. The difference is that lunar anorthosites probably once formed the entire crust of the early Moon, and formed by simple differentiation of a deep, all-encompassing magma ocean. The late Dennis Shaw applied this simple notion to the Earth’s earliest evolution during the 1970s, but his vision was largely ignored by his geochemist peers. A mantle-wide zone of complete melting was resurrected when William Hartmann’s giant impact theory appeared: the energy involved seems to make this an inevitable corollary of his idea.

Indirect analysis of the mantle from the geochemistry of its basaltic products has shown that the mantle is not homogeneous. Some has been partially stripped of basalt-forming elements, and there are other chemical heterogeneities. However, examined from the standpoint of isotopes of neodymium (¹⁴²Nd and ¹⁴⁴Nd) more or less every magmatic rock has been considered to have been ultimately derived from material with the same isotopic composition as chondritic meteorites, and by extension, that of the Galaxy in the vicinity of what became the Solar System. That observation has been a major counter argument to the notion of an early terrestrial magma ocean. Differentiation of such a fundamentally molten Earth would have separated some of the samarium-146 (the source of ^142‑Nd through radioactive decay) from ¹⁴⁴Nd, thereby imparting different growth histories for ¹⁴²Nd/¹⁴⁴Nd ratios to different mantle ‘reservoirs’. The half-life of ¹⁴⁷Sm is about 100 million years, so that radiogenic ¹⁴²Nd would accumulate most in Earth’s early history, thereafter tending towards a constant proportion of neodymium, unlike the ¹⁴³Nd used in radiometric dating that accumulates much more slowly from decay of ¹⁴⁷Sm (half life about 100 billion years).

There was a flaw in this counter argument. The similarity of chondritic and terrestrial Nd isotope patterns might have stemmed from isotopic measurements that were insufficiently precise to detect significant differences. Mass spectrometry has undergone a near-quantum leap in precision. Applied to the chondrite-Earth rock comparison, the neodymium data for chondrites remains as determined earlier, but the ¹⁴²Nd/¹⁴⁴Nd ratios of terrestrial rocks turn out to be 20 parts in a million higher than for chondrites (Boyet, M & Carlson, R.W. 2005. ¹⁴²Nd Evidence for Early (>4.53 Ga) Global Differentiation of the Silicate Earth. Science, Published online June 16 2005; 10.1126/science.1113634). That doesn’t seem very much, but quite sufficient to suggest plausibly that indeed the Earth’s mantle did indeed evolve from a magma ocean. Its upper part was enriched in samarium by its fractionation as a solid that probably crystallised downwards. Whatever was left of the original liquid would be at the base of the protomantle, and in it many other elements that favoured melt over crystals – so-called ‘incompatible’ elements – would have been enriched. Boyet and Carson suggest that such a deep, enriched layer may amount to between 5 to 30% of the current mass of the mantle.

The implications, if the ideas are confirmed, are enormous, because geochemists up to now have taken the bulk of the mantle that supplies basalt magmas – and whose composition is quite well constrained – to represent the whole silicate Earth. That may satisfy geochemical parameters, but worries geophysicists. The ‘standard’ Earth has insufficient radioactive uranium, thorium and potassium to account for the heat that flows to the surface. In fact it generates about a half, leaving the rest to speculation. One school looks to supposed gravitational potential energy locked in the core when it formed by inward collapse of iron-nickel alloy and slowly released thereafter. Another theorises about radioactive potassium-40 combined in sulphides of the core, which also ‘leaks’ out. The possible existence of the last dregs of an early magma ocean, near the core-mantle boundary (CMB), would not only account for 43% of surface heat flow, but might also drive convection in the liquid outer core as a means of generating Earth’s magnetic field. Even more important, it might fuel the rise of plumes from the CMB that are increasingly implicated in periodic repaving of the Earth’s surface by flood-basalt volcanism. Since flood basalts are a popular source for mantle geochemists’ data, why are the signs of such a peculiar source region not clear in their analyses? Either they are not looking with the requisite precision, or the source itself does not move with plumes, merely setting them in motion. Eminent geochemists see a bit of a hectic time ahead…..

See also: Kerr, R.A. 2005. New geochemical benchmark changes everything on Earth. Science, v. 308, p. 1723-1724.

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Dinosaur dung, the Deccan Trap and grass

Clay minerals and the origin of life

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Congenital disease, human migration and population growth

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Oxygen and mammalian evolution

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