Author: zooks777

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

Sea level bonanza

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

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

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

Early warning of earthquakes

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

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

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

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

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

Yet further back in the Antarctic ice

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

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

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

Movies of Mars

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

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

Fig leaves over Palaeocene-Eocene boundary

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

Dinosaur dung, the Deccan Trap and grass

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

Clay minerals and the origin of life

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

The geological sources of myths

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Sitting on top of the Kremlin in Red Square is a huge five-pointed red star that is illuminated at night.  This is not just a relic of Stalin’s Soviet Union, but has its origins in a common myth that shows up concretely in archaeological digs, particularly in the Middle East, in the form of collections of fossil sea urchins and starfish. They, of course possess the five-fold symmetry unique to the Echinodermata, which also figures in the emblematic pentagram of Denis Wheatley’s awful novels about satanism and on the pointed hats of latter-day wizards and warlocks. I learned of this fascinating link between geology and symbolism at a session on Geology and Mythology at the 32nd 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’

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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.