Author: zooks777

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

Mantle avalanches and length of the day?

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One of the most fascinating spin-offs of detailed palaeontology is that the growth layers in corals and the carbonate shells of other organisms can record how many days there once were in a year.  Records of shell growth can even chart variations in the lunar cycle, backed up by subtle features in cyclical sediments.  Such data infer that the speed of the Earth’s rotation has changed (Ravilious, K.  2002.  Wind up.  New Scientist, 23 November 2002, p. 30-33).  As well as the general slowing through the Phanerozoic, from a rate that gave 420, 21-hour days in a Cambrian year, there have also been times when the rate has strangely speeded up again.  Such curious events occurred at 400 Ma and again around 180 Ma.

Planetary spin can be set in motion or changed by very large impacts, in the manner of whipping a spinning top.  But there is little sign for such drama at those times.  Another possibility is a change in the Earth’s moment of inertia by a shift of mass relative to the spin axis, in the manner of a skater speeding up a spin by pulling in her arms.  What could induce such an effect at the scale of our planet?  Cold, dense lithosphere continually sinks at subduction zones, but that is normal behaviour in balance with rotation.  One possible trigger for sudden changes in moment of inertia is the breaking away of a substantial chunk of the mantle that lies above the discontinuity 670 km beneath the surface to sink to deeper levels.  This dramatic suggestion stems from modelling by Philippe Machetel and Emilie Thomassot of the University of Montpellier in France (Machetel , P. & Thomassot , E. 2002.  Cretaceous length of day perturbation by mantle avalanche.  Earth and Planetary Science Letters, v. 202, p. 379-386).  Their model focussed on the transition zone between lower and upper mantle around the 670 km discontinuity, and how it might respond to the fluid dynamics of Earth’s convective heat transfer, particularly that involving heat originating in the core.  The transition, they claim, acts as a “lid” to efficient heat transfer between lower and upper mantle.  Their model suggested that additional deep-mantle heat flow might destabilise the transition’s strength, so that it would no longer support the mass of cooler and more rigid mantle above it.  Failure could then allow a massive slab of upper mantle literally to fall to the core-mantle boundary, spreading out to displace material there upwards as the precursor of a superplume. 

The link to day-length comes from Machetel and Thomassot’s search for evidence that such collapses might have occurred, and they concentrated on the 180 Ma change (Mid Jurassic).  Around 170 Ma the current round of continental drift began in earnest.  In the Early Cretaceous (130 Ma) the geomagnetic field became locked into quiescence, remaining with the same polarity for an unprecedented 40 Ma during which the giant Ontong Java oceanic flood volcanism took place.  Their explanation for both is that upper mantle avalanched, eventually to reach the core-mantle boundary.  When the mass “bottomed out” it cooled the outer core, settling it into regular motion, so that the geomagnetic field became constant.  Coincidence?  I am reminded that when skaters wish to stop their spins, they throw out their arms.  The law of conservation of angular momentum also demands that the Earth behaves in the same way.  In fact it applies to the Earth-Moon system, so that the general slowing of Earth’s rotation has been accompanied by the Moon receding into ever more distant orbit, and gaining momentum.

Review of 2002

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As in previous years, the landmark developments in 2002 chosen by editorial staff of major journals sideline the Earth sciences.  Both Nature and Science consider the discovery of Sahelanthropus tchadensis the only geoscientific advance worthy of a headline (See Bonanza time for Bonzo in Earth Pages News of August 2002).  Scientific misconduct tops Nature’s list, the exposure of monumental fudging by physicists Jan Hendrick Schön and Victor Ninov being something which should concern every scientist.  Molecular biology was, unsurprisingly, the front runner for both august periodicals, with issues related to terrorism, climate change and the soon-forgotten World Summit on Sustainable Development in Johannesburg appearing in both.  Jo’burg received typically trenchant comment from water specialist Fred Pearce in New Scientist, particularly about the weasel phrase “sustainable development” – read “make money”, according to Pearce.  New Scientist’s main look forward to 2003 is Oliver Morton’s perspective on ESA’s Mars Express, which carries the British Beagle 2 miniature life-sniffing lab, and the two NASA Mars rovers scheduled for launch this year.  This is big-budget science, yet carries big risks, judging from the frequency with which giga-dollar missions ended up in flames or the sea recently.  Morton pours scorn on the hype that Mars missions will solve “great mysteries” on which their funding depends – and that of the agencies who launch them.

Anyone who has the brass neck to comment month by month on geoscientific news cannot resist picking developments that most marked the year, so here is my own personal choice.

The most exciting advances were in palaeoanthropology: March (Taking stock of hominid evolution), April (Homo erectus unification?), April (Phyllogeography and “Out of Africa”), August (Bonanza time for Bonzo), November (A considered view) , December (Central Asian Y chromosomes and the source of migrating humans)

Most hammered hypothesis: “Snowball Earth” came in for some stick in February (Meltdown for Snowball Earth?) and December (Snowball Earth hypothesis challenged, again).  Running that a close second was the BLAG hypothesis that subduction metamorphism is a source for CO2 recycling: December (Deep carbon cycling, and gold mineralization)

Biggest technological advances: April (Satellite-based gravitational surveys), October (Microgravity and diamonds); August (Tungsten and Archaean heavy bombardment), September (Very early differentiation of planetary bodies).  The most important technical consolidation was in seismic tomography: May (Mantle motions from seismic tomography), August (Seismic tomography and the African superplume), this issue (Beowulf and mapping the mantle)

Most connective research: November (The lost world of the Galápagos hotspot track), linking plume activity, Pacific and Caribbean tectonics, closure of the Central American climatic “door”, and intercontinental migration of flora and fauna.

The biggest slanging match: April (Doubt cast on earliest bacterial fossils).

The greatest scandal emerged in autumn 2002: October (British Geological Survey sued over arsenic), December (More confusion over Bangladesh arsenic crisis)

March saw hopefully the last word on the influence of extraterrestrial impact on the K-T mass extinction (Extinctions by impacts: smoking artillery) when the fullerenes in the K-T boundary layer were matched with those in carbonaceous chondrites.

Lesser categories: Biggest scam: August (Exploration licence lepton by physicists).  Most amusing discovery: November (Dinosaurs did urinate). Latest frightener: May (Magnetic reversal on the way?)  Most promising palaeontological theory: September (The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic)

A possible fuse for the Cambrian Explosion

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The sudden appearance of shelly fossils between about 544 to 542 Ma is the most astonishing feature of biological evolution, especially as representatives of every modern animal phylum (and some which have vanished) appear at that time.  A means to explain this short-lived blossoming has eluded palaeontologists.  Part of the problem is that the record of the immediately preceding Neoproterozoic Era cannot resolve whether the phyla sprang up at the same time as they developed hard parts, or had been evolving as flaccid forms for much longer.  Another aspect is the difficulty in accounting for the sudden adoption of calcium carbonate and phosphate hard parts.  It seems inescapable that the issue of hard parts, which is really what the “Explosion” is all about, cannot be separated from the chemistry of seawater at the time.

A new insight into what was going on was presented at the October GSA meeting in Denver by John Grotzinger and colleagues at MIT, who have been examining drill cores through the Precambrian-Cambrian boundary beneath the south Omani oil fields.  The Late Neoproterozoic basin in which the deposits began to form was a semi-enclosed basin, dominated by stromatolitic carbonates.  Seawater in it contained excess calcium and carbonate ions.  Periodically, the basin was cut off and evaporites began to form; it became hypersaline.  In the cyclical sequence the very earliest carbonate-shelled organisms (Cloudinia and Namacalathus) left fossil remains.  However, in cycles of earliest Cambrian age they simply disappear, not merely in Oman but world wide.  Moreover, rocks from which they are missing show abnormally light d13C, generally interpreted as a result of mass extinction.  The demise of two organisms, albeit the only ones that could have left any record, may not seem very dramatic.  But Grotzinger and colleagues suggest that a sudden extinction could mark a critical period in evolution that both reduced the population of all organisms and sterilised ecological niches for future adaptive radiation.  Interesting, but still not explaining why hard parts were adopted to become so very necessary in subsequent animal evolution.

Source:  Kerr, R.A. A trigger for the Cambrian Explosion?  Science, v. 298, p. 1547.

The man who found the oldest hominid

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Earth Pages News has a bias towards investigations of human origins, simply because it is that branch of the geosciences with the most immediate bearing on our readers.  Much of the reported material has been technical.  So, it is pleasing to direct readers to a profile of a palaeoanthropologist who is not a self-publicising diva (Gibbons, A. 2002.  One scientist’s quest for the origin of our species.  Science, v. 298, p. 1708-1711).  Michel Brunet, of the University of Poitiers in France has spent his professional life researching Neogene mammals in as many likely sites to which he and his colleagues could gain access.  It has been a risky business, and at least one of his close colleagues died in the field, and Brunet has had many close encounters with acute danger.  Late in his career he hit the bonanza represented by Sahelanthropus tchadensis  (see Bonanza time for Bonzo, August 2002 Earth Pages News).  Not only did the find take his team far beyond the time frame of previous signs of hominid evolution, but completely outside the usual hunting grounds of eastern Africa to Chad.  That hominids were not exclusive to the area of the East African Rifts had already been demonstrated by Brunet and  David Pilbeam of Harvard by their find of 3.5 Ma australopithecine remains there in 1995.  Time will tell if this seemingly quiet academic is turned into yet another diva by the media circus that inevitably scrums around palaeoanthropologists with big finds.  I reckon he will remain as he is.

Mantle recycling

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Somewhere beneath the Americas there is a sizeable volume of what formerly constituted the East Pacific ocean lithosphere.  It represents half the productivity of the East Pacific Rise over more than 100 Ma.  Although there is still considerable uncertainty about where such subducted rugs end up, seismic tomography does suggest that a fair proportion may reach the core-mantle boundary.  That region of the mantle also seems to be the source of at least some mantle plumes.  So it would not be very surprising if lavas formed from some plumes carried a signature from much older lithosphere.  Finding such signs is not so easy, but if one pops out of lava geochemistry it would indicate that mantle convection has not stirred up and chemically blended the mess of subducted material in the lower mantle; a “memory” of bygone tectonics.  At least 3 billion years of plate tectonics has contributed to the geochemistry of the mantle, so finding such a memory has been just a matter of patience, developing a means of teasing it out and luck.

One such signature has emerged from the plume-related islands volcanic islands of the Azores, in the form of an anomalously low 187Os/188Os isotopic ratio (Schaefer, B.F. et al. 2002.  Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume.  Nature, v. 420, p. 304-307).  The study shows that the parent isotope (187Re) was depleted in the Azores source mantle up to 2500 Ma ago, perhaps before.  Rhenium depletion is likely to occur in mantle rocks during partial melting, because it is incompatible, while osmium is compatible with mantle mineral assemblages that constitute the residue of melting.  So the most likely explanation for unusually low 187Os is that oceanic mantle lithosphere, depleted by late-Archaean melting events, has sat around somewhere without being blended with more primitive mantle.  Lead isotopes in modern ocean-floor basalts suggest that recycling on timescales around 2 billion years has occurred, and the Os data from the Azores confirm that.  However, this is the first swallow in what may (or may not) become an osmium-isotope summer for geochemists eager to map the mantle’s evolution.  And there is one big question: from what depth did the Azores plume rise?  There is absolutely no evidence for it having risen from the core-mantle boundary (or anywhere else for that matter).  So all the data really show is that Archaean materials have been incompletely mixed with their mantle surroundings.  They could be products of Archaean subduction, but it requires special pleading to remove the possibility of Archaean lithosphere that resided just beneath the African or American continents before the Atlantic Ocean began to form.

Beowulf and mapping the mantle

Seismic tomography is a child of high-speed computing, of which we could barely dream only 10 years ago, as well as the world-wide network of seismic stations set up to detect nuclear tests.  The grist to its mill is seismographic data supplied near instantaneously by modern broadband data telemetry.  Mathematically it is not an easy subject, so an insight into how it is done is very welcome (Komatitsch, D. et al. 2002.  The spectral-element method, Beowulf computing, and global seismology.  Science, v. 298, p. 1737-1742).  “Beowulf” refers to the use of clusters of ordinary PCs to perform the calculations, rather than single, main-frame supercomputing.  The review outlines the theoretical approach of the spectral-element method (still beyond me!), but is most interesting in assessing the potential of future machines able to operate 100 times faster (petaflop machines) than even the most powerful today.  It begins to look like geophysicists will unveil far more complexity in the mantle than geochemists have been able to sift from their analyses of exposed rocks at the surface.

Water recycling in the mantle

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The cold, dense oceanic lithosphere that descends subduction zones is also rich in water.  These features result from the circulation of seawater through young basaltic crust, the exothermic hydration of originally anhydrous minerals in basalt and efficient convective cooling through hydrothermal processes.  Because of this, it might seem as though subduction is a means of re-introducing water into the mantle, thereby enhancing the ability of rising mantle plumes to melt.  The critical process that destines subducted lithosphere to sink inexorably is the conversion of oceanic crust to eclogite by high-pressure, low-temperature metamorphism in the subduction zone.  Eclogite consists mainly of garnet and the pyroxene omphacite, which confer its higher density than mantle peridotite, and the reactions which form them involve dehydration.  Rise of hydrous fluids from the descending slab is implicated in partial melting of the over-riding wedge of mantle to form the volatile-rich magmas that build volcanic arcs.  The higher gas content of arc magmas, compared with those at constructive margins and above mantle plumes, makes them explosive and able to build volcanoes high above sea level.  Most eclogites found at the Earth’s surface are accompanied by still hydrous metamorphic rocks of basaltic composition – blueschists – and others that clearly formed from the sedimentary veneer of the oceanic crust.  So, it might seem that blueschists and metasediments could carry a substantial amount of water into the mantle.  Eventually, its recycling through the mantle could influence later magmatic processes.

Testing this seemingly reasonable extension of the hydrological cycle depends on assessing the water content of newly erupted magmas.  This is virtually impossible for eruptions at the Earth’s surface, because low pressure results in water escape within the higher parts of the volcanic plumbing system, before lavas can be sampled.  However, eruptions onto the ocean floor deeper than a kilometre experience pressures high enough to keep gases in solution, which is why pillow lavas of true oceanic crust contain no signs of gas bubbles.  Crystallised oceanic basalts soon react with percolating water, and their volatile contents are meaningless.  Only the rapidly chilled margins are likely to retain their original composition, locked into quenched basaltic glass.  Even then, a direct measurement of water content can be misleading.  A cunning approach is to consider H2O as if it behaved like a single element, based on its bulk distribution coefficient between melt and residual solid mantle.  That is close to the values for light rare-earth elements, such as cerium.  So a check for either degassing or contamination of basaltic glass with seawater is the glass’s H2O/Ce ratio (decreased by the first and increased by the second process).  Jacqueline Dixon of the University of Miami, and co-workers from Harvard and the University of Rhode Island have used this method to assess the probable water content of the mantle source for mid-Atlantic Ridge basalts, whose lead and strontium isotopes suggest that their source was contaminated by older, recycled crust (Dixon, J.E. et al. 2002.  Recycled dehydrated lithosphere observed in plume-influence mid-ocean-ridge basalt.  Nature, v. 420, p. 385-389).  The surprising conclusion of their work is that oceanic basalts formed from mantle with a recycled component have considerably less water in them than those formed by melting of pristine mantle.  This suggests that subduction processes are extremely efficient (>92%) at removing volatiles from the subducted slab; lithosphere descending to depth is almost anhydrous.

Incidentally, the paper begins with an excellent explanation of the somewhat arcane distinctions between different mantle sources affected by lithosphere recycling and mixing.

See also: White, W.M. 2002.  Through the wringer.  Nature, v. 420, p. 366-367; and  Tectonics section below

Hair trigger for gas hydrates

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The curious mix of water ice and methane, known as gas hydrate or clathrate, which is stable at ocean depths greater than 300 m, is one of the largest potential components of the active carbon cycle (~1013 t).  Its methane content stems from bacterial breakdown of organic matter buried in anaerobic sea-floor sediments.  As well as being pressure sensitive, gas hydrate also has a narrow stability “window” as regards temperature.  Geothermal heat therefore limits the depth of gas-hydrate accumulations to a few tens to hundreds of metres below the seabed.  Its vast methane content is clearly something on which energy transnationals have an eye.  However, methane is almost four times more powerful as a “greenhouse gas” than CO2 emissions.  Carbon-isotope studies from sedimentary rocks show signs that several times in the distant past methane was released catastrophically to the atmosphere, the timing coinciding with signs of rapid global warming.  The last major event of this kind was around 55 Ma ago, when the end of the Palaeocene Epoch witnessed an 8°C global temperature rise in a matter of a few thousand years (Thomas, D. et al. 2002. Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum.  Geology, v. 30, p.1067-1070).  The warming “spike” eases because methane is quickly oxidised to water and CO2 in the atmosphere, but that still allows abnormally warm conditions to linger.

Sonar surveys of the seabed, including that of the North Sea, reveal pits and funnels that probably mark sites of past methane releases from destabilised gas hydrates.  In theory, two general processes lead to their instability: falling global sea level that reduces the pressure on gas hydrates formed at shallow water depths; a rise in the temperature of ocean-bottom water.  The second could produce more widespread methane release than the first.  Refining these crude prognoses needs detail about the structure of gas-hydrate zones beneath the seabed.  Conventional seismic surveys conducted at the sea surface show the clathrate-rich zones just beneath the sea floor, but no detail.  Towing sources and receivers just above the seabed reveals intricate structures (Wood, W.T. et al. 2002.  Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness.  Nature, v. 420, p. 656-660).  Wood and co-workers from the US Naval Research Laboratory, the University of Victoria and the Pacific Geoscience Centre in British Columbia, Canada surveyed the Pacific floor off Vancouver Island.  Their most striking observation is of many vertical, chimney-like structures that puncture the gas-hydrate zone in the upper sediment layer.  They reckon that these structures are where methane and warm fluids find their way to the seabed; they are probably the expression in cross section of the surface pitting formed by past degassing.  They also may supply gas to the zone where it becomes locked in metastable water ice.  The sheer number of the “chimneys” indicates that the surface area of gas-hydrate stability is many times larger than previously supposed, as a result of their “roughening” effect.  Since the base of the gas-hydrate stability zone is most prone to the effect of warming of sea-bottom water, which shifts the geotherm slightly, an increase in its surface area, together with its closer approach to the seabed around the “chimneys”,  could further increase its sensitivity to small changes.  Up to now, many specialists have suggested that major methane releases resulted from sudden collapses of sea-floor sediments in tectonically unstable areas, such as the Storegga Slide off western Norway.  They may instead have been due to more widespread instability resulting from environmental change.  Since the largest pressure decreases due to sea-level falls accompanied glacial epochs, some clues to whether the “chimney” effect has had an influence may come from a fresh look at methane contents of trapped air bubbles in Antarctic and Greenlandic ice cores.  The extent to which methane releases might effect climate depends on how much is oxidised to CO2 in sea water, before it can enter the atmosphere to enhance the “greenhouse” effect.  Little is know about such processes.

See also:  Pecher, I.A. 2002.  Gas hydrates on the brink.  Nature, v. 420, p, 622-623.

Orphan terranes and tectonic names

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The period from the Early Ordovician to the Late Silurian involved the assembly of much of the continental lithosphere that now surrounds the North Atlantic.  British geologists refer to this as the Caledonian orogeny, a term coined long before the events that welded the bulk of the British Isles were even dreamt of, let alone understood.  They are now in the embarrassing position (although most show few signs of grave discomfiture) of using the same term for at least two completely unrelated tectonic events.  Clinging to the old name, they now refer to mountain-building events around 470 Ma, during which accretion of an arc terrane to Laurentia resulted in the famous “fountain of nappes” of the Dalradian and Moinian Supergroups, as the “Grampian phase of the Caledonian orogeny”.  Now, I am all in favour of retaining a sense of history in nomenclature, but the fact is that northern Scotland is now known to have been part of Laurentia for a good billion years before this event.  Moreover, the offending island arc was first recognised on the eastern seaboard of North America, where it was dubbed the Taconic Arc; hence the Taconic orogeny there.  About 60 to 70 Ma later, the Avalonia terrane (also named first by North American geologists from a peninsula in Newfoundland) collided with this earlier orogenic belt in Laurentia.  North American geologists, for reasons of their own, refer to the deformation and metamorphism that ensued as the Acadian orogeny.  The British Isles experienced exactly the same event, yet it is referred to as the “Acadian phase of the Caledonian orogeny” – not the Cumbrian, as one might expect from the parochial considerations that prefer “Grampian” to Taconic, for the Iapetus suture that divides terranes north and south in Britain probably lies beneath northern Cumbria.  How confusing this is, and how unnecessary!

 The plot thickens in Scandinavia, long renowned for the pandemonium of orogenies dating from Palaeoproterozoic times.  There, tectonic events around 470 Ma are the “Finnmarkian phase of the Caledonian orogeny”, and those which closed the Lower Palaeozoic are the “Scandian phase”.  Norse, Swedish and Finnish geologists can be excused for sticking with their palaeotoponymy, because Scandinavian lithosphere was a separate entity from Laurentia during these times – Baltica.  The comforting isolation of Baltica had been thought to have ended with its accretion to Laurentia when the “Old Red” continent (Laurussia) formed.  Not entirely so.  Norway is now the proud custodian of a bit of the Taconian orogen (Yoshinobu, A.S. et al. 2002.  Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny. Geology, v. 30, p.883-886).  However, that does not make a unification of Baltica’s tectonic nomenclature with Laurentia sensible, because the sliver seems to have travelled a vast distance from its parent.  Hence “orphan”, because it was emplaced as one of the many nappes of western Scandinavia.  British geologists should take no comfort from this, and it is about time that they accepted a common tectonic history for the whole of Laurentia, otherwise their parochially-named orogenies might justifiably be called “bastards”!

Central Asian Y chromosomes and the source of migrating humans

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Assessing relatedness in the male line from Y chromosome samples of large, widespread populations, is becoming an important tool in palaeoanthropology.  It uniquely shows signs of the major migrations by fully modern humans during the last glacial period and the Holocene (see Eve never met Adam December 2000 Earth Pages News and  Multiregionalists nailed by Y chromosome? June 2001 Earth Pages News).  Although the details make difficult reading for non-geneticists, a recent paper by a large multinational team, led by Spencer Wells, Ruslan Ruzibakiev and Nadira Yuldasheva of Oxford University and the Uzbekistan Academy of Science respectively, sheds important light on where these migrants set out from (Wells, R.S. and 25 others 2002.  The Eurasian heartland: A continental perspective on Y-chromosome diversity.  Proceedings of the National Academy of Science, v. 98, p. 10244-10249).  Central Asian men have among the most diverse genetic make up of any living humans.  Genetic markers on Y chromosomes from that population turn up far afield, so that it seems that the great migrations to Europe, to the Indian sub-continent and even North America set out from the region of Afghanistan, Uzbekistan and Pakistan.

Is evolution predisposed to intelligent beings?

Simon Conway Morris of Cambridge University is one of the younger pioneers of palaeobiology, beginning with his doctoral studies of the famous Cambrian creatures of the Burgess Shale.  His discoveries and analyses of them have clearly set him on course for thoughts of a much broader kind, much as did the career of Stephen Jay Gould.  By way of introduction to his forthcoming book (Life’s Solution: Inevitable Humans in a Lonely Universe, Cambridge University Press, scheduled for 2003) a recent article by him (Conway Morris, S. 2002.  We were meant to be….  New Scientist, 16 November 2002, p. 26-29) will cause a stir.  At first sight it smacks of teleology, the predestination of biological processes to create the thinking mind.  It is far from being teleological, because Conway Morris argues from sound evolutionary principles about the role of fitness.  To him, there is evidence of evolutionary convergence towards smart creatures, such as dolphins and even octopuses and social insects; the outcome of gathering and processing information in some kind of integrated mental map.  Unfortunately, detecting signs of such behaviour in the fossil record is not easy, unless advanced intelligence created recognisable artefacts.  Such evidence spans only the last 2.5 Ma, and of course it originated with hominids, and with them alone; we find few signs of the dolphin’s predilection for using snout guards while grubbing in the seabed – a likely tale!.  What he does not address is the difference between intelligence and the consciousness that turns environments into tools for our species, which in turn drive the generation of culture, economy and a free association of individuals.  Much as we might wish to, we cannot converse with a dolphin, an advanced mollusc or an ant.  Which is a shame, because a really smart cookie needs to work on the principle of, “It takes one to know one”!  All manner of living animals use tools of a rudimentary kind, even the song thrush in my back yard, so Conway Morris is mainly restating a truism.  But that is fine as a starting point for speculation, and what I take to be pure fun.  But as a basis for some optimism that when we meet a truly alien intelligence it should be pretty easy to have a good old natter, is being silly.  If he does hold that view, then I can recommend a few hours in the Aztec exhibition in London; as like as not we would be a menu item for any intelligent being which had crossed a thousand light years out of curiosity or for plunder!  Life’s history on Earth has not been simply one of evolution, but of awesome snuffings out, and many other chance combinations of circumstances outwith any kind of biological necessity.  Being ever so clever is little help against a Chixculub or the Siberian Trap.

Britain’s own impact

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While evidence has been accumulating for the influence of asteroid and comet strikes elsewhere, the British geological community has had a disproportionate share of sceptics; those who thought it was all a matter of “whizz-bang” science.  It is welcome news that we now have our own “piece of the action”, for geoscientists from Aberdeen University and the Open University have a discovered a well-preserved impact horizon in Late Triassic terrestrial sediments that contain both devitrified glass spherules and shocked quartz grains (Walkden, G. et al. 2002.  A Late Triassic Impact Ejecta Layer in Southwestern Britain.  Science Express –www.scienceexpress.org, 15 November 2002).  It is not associated with the Triassic-Jurassic boundary, which witnessed on of the “Big Five” mass extinctions, but is dated at 214±2.5 Ma, within error of the major impact at Manicougan (~100 km diameter; Quebec; 214±1 Ma) the lesser Rochechouart structure (~25 km diameter; France; 214±8 Ma).  The Ar-Ar dating did not use spherule glass, but authigenic potassium feldspar that postdates the spherules, but may have formed from potassium released when they became hydrated.  Given its size and position relative to Britain on a Triassic plate reconstruction, Manicougan is a likely culprit.  However, despite its considerable size, there are no signs of significant faunal changes at the time of the Manicougan impact.  The host Triassic rocks in Somerset rest directly on Carboniferous limestones, and primitive mammal remains are known from infillings of a palaeokarst surface in the Mendip Hills.  Now the deposit has come to light, the search is on for similar materials in Late Triassic marine sediments.