Rise of the dinosaurs after the Tr-J event

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Whatever happened at the Triassic-Jurassic boundary (around 200 Ma ago), the palaeontological shifts then coincided with eruptions of flood basalts of the Central Atlantic Province and the start of Atlantic opening (see And now, the Tr-J boundary, Earth Pages May 2002).  Although questioned as a mass extinction event, the boundary contains extremely high proportions of fern spores, that may signify the land being cloaked by rapidly spreading ferns after it had been wiped clean of other vegetation.  New evidence suggesting the influence of an impact at the time emerges from a geochemical study of the fern-rich boundary layer (Olsen, P.E. and 9 others 2002.  Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary.  Science, v. 296, p. 1305-1307), which revealed anomalously high levels of iridium.  High iridium is only one pointer to possible extraterrestrial influences, and the clinching factor of shocked mineral grains has yet to be shown convincingly.

The novel feature of the paper by Paul Olsen of the Lamont-Doherty Earth Observatory and colleagues from the USA, Canada, Italy and Austria is how they used trace fossils to reach a remarkable conclusion.  They combed eastern US terrestrial sediments either side of the boundary for reptilian foot prints.  They tracked time using evidence for climate change paced by Milankovich cycles.  Their records of 10 thousand sets of tracks show a decline in non-dinosaur footprints, and a jump in the proportion left by dinosaurs from 20 to 50% of the total, as the boundary is crossed.  Those of some Triassic reptiles that had survived for 20 Ma end abruptly at the boundary.  It seems that, whatever the boundary event was, early dinosaurs were able to adapt to change better than evolutionarily more primitive reptiles, so that they could speciate rapidly when their Triassic companions bit the dust.  Dinosaur evolution seems to have been similar to that of the mammalian adaptive radiation that followed the K-T extinction event. 

Gigantic claims for “geogenomics”

Fossils and their stratigraphic ages no longer offer the only clues to biological evolution, now that is possible to judge the degree of relatedness between living organisms from sequences of genes and proteins that their cells contain.  The molecularly inferred family trees of modern animals, plants and micro-organisms help scientists to visualize the relative antiquities of the sharing of a common ancestor by different pairs of a living group.  By assuming constant rates for genetic mutation and protein evolution, some palaeobiologists have asserted that they are able to assign absolute ages to evolutionary divergences.  If that were so, then it would be possible to correlate evolutionary milestones with transformations brought on by geological and climatic upheavals, and also with other past changes in the biosphere.  Good examples would be linking fossil and genetic changes in ruminant mammals to the rise of grasses, or the rise and divergence of corals following the end-Permian mass extinction.  The inter-linkage between palaeontology and genomics is in its infancy.  That it promises a great deal by way of insights, as well as possible bloomers, is nicely brought out by a recent review (Benner, S.A. et al. 2002.  Planetary biology – paleontological, geological and molecular histories of life.  Science, v.  296, p. 864-868).  Whether charting the “planetary proteome” will become “a civilization-wide enterprise”, as Steven Benner and his colleagues predict, is something that I would not care to comment on during the 2002 World Cup.  As Bill Shankly once observed, some things are far more important than matters of life and death.

Too much iron, too little phosphorus delayed an oxygen-rich atmosphere

The age of the earliest blue-green bacteria hinges on the imagination of some palaeobiologists and how well they can focus a microscope (Doubt cast on earliest bacterial fossils, Earth Pages April 2002).  Without doubt, it was blue-greens that first began breaking the chemical equilibrium of water to release free oxygen to the environment, yet it was some 2½  billion years after the Earth had formed that atmospheric oxygen had a tangible effect on the Earth’s bare surface.  In rocks around 2.2 to 2.0 Ga old geologists find the first evidence for that in soils that are rich in oxidized Fe-3.  For iron to lose an electron and change from soluble Fe-2 to Fe-3, whose oxides and hydroxides are highly insoluble, demands the abundant presence of an electron acceptor, or oxidizing agent.  The most likely of these in the atmosphere and hydrosphere is oxygen.  However, there are sedimentary rocks that form vast repositories of Fe-3 and oxygen that predate the first well-accepted oxygen-rich atmosphere.  They are known as banded iron formations or BIFs, whose minuscule layering seems to signify that they formed as precipitates from water, when dissolved Fe-2 met a source of oxygen to produce hematite – Fe2O3 – and goethite – Fe(OH)3  BIFs signify deep ocean water devoid of oxygen, to enable soluble Fe-2 to circulate abundantly, yet a sizeable supply of oxygen where they were precipitated. Since only organic photosynthesis is capable of breaking the powerful bond in water, some kind of photosynthetic bacteria are implicated in the formation of BIFs.  Whether or not palaeobiologists and geochemists can demonstrate evidence for the first appearance of such bacteria, BIFs more or less prove their existence, in the absence of any other plausible means of formation.

Until recently, the huge delay in the Earth’s surface environments becoming oxygenated has been ascribed to the mopping up of any biogenic oxygen by its reaction with a vast excess of dissolved Fe-2.  However, once blue-green bacteria evolved photosynthesis, their chemical trick of splitting water molecules to provide hydrogen for processes at the cell level should have meant that they would have spread like wildfire across the ocean surface.  In that respect they are unique among bacteria, most of which exploit very narrow ecological niches.  Oxygen should have quickly come to dominate both oceans and atmosphere.  That is, unless there was some check on the living ocean biomass.  It turns out that BIFs may contain the answer, for they are rich in phosphates, adsorbed onto the surfaces of their iron minerals (Bjerrum, C.J. and Canfield, D.E. 2002.  Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides.  Nature, v. 417, p. 159-162).  Phosphorus is vital in any organism, being an essential component of nucleic acids and phospoholipids.  By working out the partition coefficient between water and iron oxide, and estimating the production rate of BIFs before 1.8 Ga when their production ceased, Bjerrum and Canfield conclude that phosphorus was an order of magnitude less abundant in sea water until then.  Such a deficiency in a vital nutrient would have limited the scope of blue-greens, and the rate at which they produced oxygen. 

Just why the Fe-P checks and balances on oxygen production collapsed around 2.2 to1.8 Ga is something of a mystery.  One possibility is that the iron concentration in sea water fell, perhaps as sea-floor spreading waned from its high early rates; basalt magma provides the main input of iron through ocean-floor hydrothermal activity.  Less production of BIFs would leave more phosphorus in solution, helping greater biological productivity, whose oxygen output would eventually remove soluble iron from sea water.

See also Hayes, J.M. 2002.  A lowdown on oxygen.  Nature, v. 417, p. 127-128.

Glacial floods and climate change

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One of the fundamental discoveries about climate change during the Plio-Pleistocene ice ages is how many climate fluctuations with periods too short to be ascribed to astronomical forcing link to shifts in deep-ocean circulation.  In the case of the North Atlantic Ocean, if high-latitude seas become diluted by fresh water cold dense brines are less able to form.  It is their sinking as a residue from the formation of sea ice that helps drive the “ocean conveyor” and draws warmer water into the Arctic from the tropics.  If they do not form, then the conveyor shuts down and high-latitudes cool.  The most spectacular of these ocean-driven events was the Younger Dryas cooling from about 12.9 to 11.6 ka, and it may well have occurred because of the sudden drainage of a giant lake of glacial meltwater down the St Lawrence Seaway to dilute the North Atlantic.  The waning of every major ice sheet covering North America would have generated vast amounts of freshwater, and because repeated glaciation created basins by erosion and sagging of the low-relief surface, drainage of such lakes would have been characteristic of every transition to interglacial warmth.  Steven Colman of the US Geological Survey reviews recent attempts to model how flooding may have escaped from the ice-sheet margins (Colman, S.M. 2002.  A fresh look at glacial floods.  Science, v. 296, p. 1251-1252).

The Hadean was cool

James Hutton’s observation that the geological history of Scotland had “no vestige of a beginning” applies everywhere, for no-one has dated rocks that are older than about 4.0 billion years (Ga) old, despite a great deal of effort.  It seems that continental crust only became capable of remaining at the surface in large volumes almost 600 Ma after the Earth formed from the Solar nebula.  Indirect isotopic evidence and dating of meteorites do indicate that the Earth accreted from dust and planetesimals about 4.56 Ga ago.  There are terrestrial materials that break the 4 Ga barrier, but they are so few and so tiny that they could be lost with one powerful sneeze.  These are crystals of the highly resistant mineral zircon, found as detrital grains in mid-Archaean sandstones in Western Australia.  The oldest of these is a single grain dated at 4.404 Ga.  All of them formed in igneous rocks produced by partial melting of the mantle, which concentrates zirconium in magma.  Following their liberation to sedimentary processes by weathering, the zircons have probably been through several sedimentary cycles since the formed.  So the pre-Archaean history of our world has left relics, but they are minuscule.  Because of the absence of pre-4Ga crust, that period was probably turbulent, partly through rapid convective turnover of the mantle and higher degrees of melting because of higher heat production, and partly due to far more large impacts that the lunar surface shows during those times.  Dating of lunar cratering and impact glasses suggests that bombardment reached a crescendo around 4.0 to 3.9 Ga.  It is now fairly certain that the Moon formed from incandescent material ejected from the Earth when it collided with a Mars-sized planet around 4.45 Ga.  Earth and its companion would, in that likely scenario, have begun their geological evolution completely molten in the case of the Moon and with a deep magma ocean on Earth.  “Hellish” is a barely adequate adjective for such conditions, and the period before 4 Ga has been termed the Hadean.  A vital question concerns when such extreme conditions waned to become potentially supportive of biochemistry and the origin of life.

Minute as they are, the pre-4.0 Ga zircons provide useful oxygen-isotope data, and their d18O is no different from that of more common zircons throughout the Archaean Aeon.  The explanation for this is that the mantle and the magmas produced from it contained an H2O phase.  Either the mantle has always had a water content – no surprise as it still does – or the magmas from which the zircons crystallized encountered near-surface water vapour, possibly as a result of hydrothermal exchange with a hydrosphere.  Reviewing these data, John Valley and colleagues from the University of Wisconsin USA and Curtin University Australia pursue the second conjecture (Valley, J.W. et al. 2002.  A cool early Earth.  Geology, v. 30, p. 351-354), and argue for a surface temperature below the boiling point of water since 4.4 Ga, only 50 Ma years after geochemical “year zero”.  The crux of their argument is that the high d18O values of four Hadean zircons indicate their equilibration with water vapour at temperatures below water’s critical point (374°C).  If crystallization at depth was below that temperature, then the Earth would have had surface oceans.  But is this such a surprising conclusion?  Loss of heat by radiation being proportional to the fourth power of absolute temperature, an incandescent Earth’s surface at the time of Moon formation would have cooled below 100°C well within 50 Ma, unless it was blanketed by an opaque atmosphere.  Impacts of the size of those which produced the lunar maria around 4.0-3.9 Ga could have boiled away any surface water from time to time, only for the surface to cool quickly once again.  Conditions for bio-geochemistry could well have been present throughout the Hadean.  The significance of that for the origin of life is hard to judge, because large impacts and ocean boiling would have extinguished any progress, so that the process may have had to restart again and again.

The etymology of the Gaia hypothesis

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Amid the desperate search for classical names to lend weight to the study of asteroids palaeotectonic features, and even theories of the Earth system, there has been one particularly unfortunate choice.

Gaia (Earth) emerged from Chaos, the great void of emptiness within the universe.  She gave birth to Uranus (Sky), apparently by some form of parthenogenesis.  Their incestuous coupling produced the 3 Cyclopes, 3 Hecatoncheires and the 12 Titans. Uranus was a bad father and husband. He particularly hated the Hecatoncheires (they had 100 arms and 50 heads each), and stuck them deep within Gaia’s womb causing her to plot against him.  To rid herself of Uranus she begged her children to kill him. All refused apart from the youngest child, Chronos (Time and the father of Zeus).  Gaia made Chronos a flint sickle, which he used to castrate his father and threw his testicles into the sea.  From the spilt blood came the Giants, the Ash Tree Nymphs and the Erinnyes.  When Uranus’ severed genitalia landed in the sea, foam bubbled around them. From this foam sprang Aphrodite (meaning foam-born), Goddess of Love.

This doesn’t quite tally with the eponymous hypothesis, but seems to have a more realistic ring for what we know about Earth history.

Source: http://www.csc.liv.ac.uk/~u9dam/myth/immortals/

Mantle motions from seismic tomography

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Variations in the density and rigidity of the mantle induce changes in the speed at which seismic waves move through it.  Mapping mantle regions with slowed and faster waves in three-dimensions is the basis for assessing temperature anomalies within the deep Earth.  It has been such tomography that has begun to test ideas about the depth from which mantle plumes rise and the fate of subducted slabs of oceanic lithosphere, and an increasingly certain model for mantle motions has evolved with improvements in the resolution of seismic analyses.  However, the P and S waves used in tomography have other properties than simply speed.  These include direction, polarization, signs of conversion of P to S waves, and even interference properties for which the birefringence observed in petrography is an analogue.

Analysing these properties reveals that there are deviations in the structure of the minerals that make up mantle rocks from random arrangement; there are anisotropies (Park, J and Levin, V.  Seismic Anisotropy: tracing plate dynamics in the mantle.  Science, v. 296, p. 485-489).  Deformation lines up minerals in such a way that the bulk rock structure affects the propagation of seismic waves in different directions – again, the way in which crystallographic anisotropy of minerals affects light passing through them is a means of visualizing what happens on vastly larger scales.  In their review, Park and Lewin describe how this novel approach is revealing aspects of convection in the upper mantle, how lithospheric plates have formed and features spatially related to accretionary boundaries in continents.

Field studies of ophiolites have shown that the dominant olivines of mantle peridotite are commonly aligned, probably as convection dragged it at right angles to the axes of lithospheric spreading.  Indeed, seismic anisotropy confirms that view with trends normal to the mid-Atlantic, Pacific and Indian Ocean spreading centres.  Destructive margins show two trends, those parallel to trenches and those in the direction of subduction, but there are complex variations depending on depth.  Once resolved into indicators of past motions, that complexity may tell volcanologists a lot about large-scale variations in magmatism.  The Hawaiian hot spot has associated vertical anisotropy, that is consistent with a disturbance of the overall flow of shallow mantle.  Several ancient orogens in continents, dating back to the Precambrian, show anisotropy in the mantle beneath them, often parallel to the orogenic trends, but occasionally more complex.  Clearly, this use of natural earthquake signals has a lot to contribute, but depends on much more complex computations than “conventional” tomography and awaits the wider distribution of software and powerful hardware.

The latest significant development from tomography based on detection of wave-speed anomalies relates to the Earth’s two major mantle plumes, beneath Africa and the Pacific Ocean (Romanowicz, B. and Gung, Y. 2002.  Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux.  Science, v. 296, p. 513-516).  Both apparently persist through the transition zone of mantle wave speeds at 670 km below the surface, to become deflected laterally beneath the lithosphere.  They may well be supplying heat to the asthenosphere that could find its way to spreading ridge systems.  The lowering of viscosity in the asthenosphere as a result of this heat originally from the core-mantle boundary (some of it may be heat lost by the core) would act as a lubricant for plate motions.  In particular, it could enhance the influence of slab-pull force at subduction zones, such as those around the Pacific, thereby speeding up tectonics.  The mantle beneath the African lithosphere has probably been heated.  The huge topographic and gravitational anomaly generated by massive flood basalt eruptions in Kenya and Ethiopia may more easily have been able to convert the resulting extensional stresses into extensional deformation, thereby driving the East African Rift system above a zone of thermal lubrication.  Far more gravitationally unstable lithosphere beneath young orogens does undergo lateral collapse, but the lack of associated plumes makes it impossible for the entire lithosphere to fail through lack of such lubrication.  And when superplumes eventually wane, as perhaps have those beneath Iceland and western North America, that too would influence both plate tectonics and that on more local scales by increasing viscous drag in the asthenosphere.

A basaltic meteorite, but from where?

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The vast majority of meteorites represent bodies in the Solar System that never became parts of planets; they are fragments of planetesimals.  Of the 20,000 collected meteorites, only about 50 have been suggested from their geochemistry to hail from existing planetary bodies.  They travelled to Earth as fragments that violent impacts on these bodies ejected from their surfaces.  Since most meteorites have been recovered either from glacial ice or the surface of deserts, such suspected planetary fragments arrived recently in geological time, but had probably been travelling for immense periods of time since an impact dislodged them.  Oddly, there are few if any meteorites with Earthly compositions, and only the Moon and Mars seem to be represented in collections.  Suspected planetary meteorites have basaltic compositions, but so too do some likely to have originated from planetesimals.  One of the keys to sorting them is analysis of their oxygen isotopes, as well as conventional element analyses and noble-gas composition.  It was the resemblance of noble gases in the notorious Antarctic meteorite ALH84001, and others like it, to the very imprecise measurements made by the Viking lander in the 1970s that encouraged the view that it was from Mars.  Their odd oxygen-isotope composition has also been said to indicate a Martian origin, mainly because they don’t fit with other specimens most likely to have originated from planetesimals.

In these uncertain times for manned and unmanned space missions, basaltic meteorites are probably as close as planetary scientists will ever get to the objects of their longing, perhaps for several generations. It is hardly surprising that collectors seize on petrogenetically evolved meteorites with glee.  Such a desirable chunk from a desert surface in NW Africa has been analysed comprehensively by scientists from Japan and the USA (Yamaguchi, A. et al. 2002. A new source of basaltic meteorites inferred from Northwest Africa 011.  Science, v.  296, p. 334-336).  Its chemistry fits with no planetesimal or suspected planetary meteorite class, although for the most part it does resemble the eucrites, considered to originate from the large asteroid Vesta.  Rare-earth elements, siderophile metals and oxygen isotopes put it in a class of its own.  Although the authors are content to conclude that it probably evidences a range of planetesimals that underwent differentiation to produce basaltic magmas, some have been tempted to speculate on a planetary origin, perhaps on Mercury (Palme, H. 2002.  A new Solar System basalt.  Science, v, 296, p. 271-273).  I am left wondering why the supposed Martian meteorite class, with all the kudos that such a suggested origin brings, has not been tempered by the likelihood of origin in a large planetesimal; but I am no specialist.

And now, the Tr-J boundary

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Despite the downgrading of the raw data for mass extinctions by their stratigraphic weighting (see The “Big Five” become the “Big Three”? Earth Pages, January 2002), which cast doubt on the magnitude of the Triassic-Jurassic boundary event, the Tr-J draws attention because something out of the ordinary did happen then.  In terms of changes in fauna and flora, the boundary is globally recognisable.  The giant Manicouagan crater in Quebec formed almost at the boundary, and the Central Atlantic Magmatic Province (CAMP) had about the right timing too – 202 to 198 Ma ago.  The Camp was the magmatic expression of the first substatially break up of Pangaea.  Carbon isotopes in sediments reflect changes in the inorganic and organic parts of the C-cycle, and it seems that a significant excursion from the norm characterizes the change from Triassic to Jurassic biota at four important sections in western Canada, Hungary, Britain and East Greenland (Hesselbo, S.P. et al., 2002.  Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: A link to initiation of massive volcanism?  Geology, v. 30, p. 251-254).

Around the palaeontologically defined boundary, each section records a sharp decrease in the proportion of heavy 13C to light 12C (d13C), followed by a protracted period dominated by isotopically light carbon in the earliest Jurassic.  Such a shift must indicate some kind of global change in the C-cycle.  One explanation is an increase in the amount of CO2 expelled from the mantle by major volcanic activity, which tallies with the CAMP.  If that was the reason, then d13C would be a proxy for all manner of other effects of volcanic activity – acid rain, atmospheric dust as well as volcanic enhancement of the “greenhouse effect”.  However, as several long-running sagas have shown, isotopically light carbon recorded in sediments can also be explained by methane escape from deep sediments, and even by a massive reduction in biological activity that would otherwise sequester light carbon through metabolism.  More confusion arises from the carbon that is isotopically analysed.  That in carbonate sediments records the isotopic composition of carbon dissolved in seawater, while that found as carbon in preserved organic matter tells a different story.  The Tr-J data are from bulk organic carbon in reduced sediments, and relates to the reservoir of carbon on which cell metabolism has drawn – CO2 dissolved in seawater, and that in the air for marine and terrestrial life respectively, both of which are in equilibrium.  It looks like the excursion stemmed from an increase in volcanic emissions to the atmosphere, and the CAMP.  However, the study by Stephen Hesselbo and colleagues from the Universities of Oxford and Copenhagen, and the Geological Survey of Denmark and Greenland, reveals a glitch.  The appearance of the ammonite genus Psiloceras has generally been taken to mark the start of the Jurassic.  In Canada, such fossils occur just after the first major shift in carbon isotopes, whereas in Britain Psiloceras appears considerably later.  One takes ones’ choice: either ammonite species appeared synchronously everywhere (assuming that they were pelagic ocean-crossers like modern Nautilus), or signs of change in the C-cycle are truly global.  Because of very rapid mixing of air and water, on geological timescales, the latter is like to be true and faunal zones are not so reliable as stratigraphers believed over the last century.  It seems that as well as tying whatever happened at the Tr-J boundary to massive volcanism, the study marks a turn from traditional palaeontology to geochemical markers as the “golden spikes” in stratigraphy.

There are other means of linking changes in the pace of volcanism and the surface environment, that emerge by careful choice of geochemical proxy data.  A long used, but imprecise approach depends on the much slower rate at which radiogenic 87Sr builds up in the mantle than in the continental crust, because of the much lower rubidium content of the mantle.  Oceanic strontium isotopes, measured for the past by analysing marine carbonates, reflect the derivation of strontium by erosion and weathering of continental crust, and addition of strontium to sea water by its hydrothermal interaction with newly emerged volcanic rocks at constructive margins or on oceanic plateaux (submarine flood basalts).  Since the 87Sr/86Sr ratio of seawater is a commonly used proxy for varying rates of continental weathering, identifying signs of massive increases in submarine volcanism is only possible when carbonates reveal extremely low values of the ratio.  The mantle is enormously richer than continental crust in elements likely to have entered the core preferentially, such as gold and especially the platinum group elements.  Partial melting allows unusually high amounts of such elements to enter basalt magmas, and the lavas and ejecta that enter the surface environment.  Potentially, the environmental abundance of such normally exceeding rare elements is a reliable proxy for major magmatic events.  Osmium isotopes are particularly promising, even in tiny concentrations, because two (187Os and 188Os) are daughters of the decay of unstable rhenium isotopes.  The details of their use is complex.  As well as the exceedingly light carbon isotopes in sediments at and around the Tr-J boundary, marine shales also reveal a sudden increase in osmium abundance, the 187Os/188Os ratio and the abundance of rhenium (Cohen, A.S. and Coe, A.L. 2002.  New geochemical evidence for the onset of volcanism in the Central Atlantic Province and environmental change at the Triassic-Jurassic boundary.  Geology, v. 30, p. 267-270).  The most likely source for globally distributed anomalies of this kind are the volcanic rocks associated with the CAMP, and their reaction with both rainfall and hydrothermal fluids.  However, the data do not rule out an extraterrestrial influence by a major impact.  That the Tr-J boundary is associated with both the CAMP and the Manicougan impact seems likely to vex geochemists hoping to tie things down to any single trigger.

Magnetic reversal on the way?

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Over the last 150 years, the Earth’s dipolar magnetic field has been declining so fast that it will vanish in around a thousand years.  Breakdown of the dipole is known to have characterized past reversals in magnetic polarity, together with a decrease in the field to very low values. That is a worrying prospect, because the strength and polarity of the Earth’s magnetic field serves to deflect the flux of energetic particles from the Sun, which would otherwise bombard the surface with potentially disastrous effects.

The likely source of planetary magnetic fields is turbulent circulation of a liquid iron core.  Movement of such an electrical conductor is bound to generate such a field, in the manner of a self-sustaining dynamo – movement of a conductor in a magnetic field that the motion itself generates results in current flow that sustains the magnetic field. Perturbation of core motion would give rise to continual deviations from a perfect dipole.  Charting such deviations is therefore a means of sensing how the core’s circulation behaves.  There have been two satellites devoted to monitoring the global magnetic field – The US Magsat in 1978 to 1980 and the Danish Oersted launched in 2000.  Comparing results from the two reveals a remarkable patchiness, the largest being one to the south of Africa in which the field points downwards, opposite to the upward-pointing field of the main dipolar field in the southern hemisphere (Hulot, G. et al. 2002.  Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data.  Nature, v. 416, p. 620-623).  The Earth contains “anti-dynamos”, and if they merged and grew, the overall polarity might flip.  Not only that, but for a while at least the poles of the reversed state need not line up with the rotational axis.

Gauthier Hulot of the Institut de Physique du Globe de Paris, with French and Danish colleagues, have modelled the generalized magnetic maps as proxies for core circulation.  The dominant features, other than a slow westward drift, are probably vortices close to the rotational poles, akin to those induced in the atmosphere by large-scale variations in air temperature.  But there are asymmetries, of which that south of Africa is the largest..  They too are probably vortices, perhaps related to convection columns.  Those showing a likely fluid motion linked to the Earth’s rotation cluster beneath the Pacific, whereas counter flows dominate the hemisphere centred on the Atlantic.

See also:  Olson, P. 2002.  The disappearing dipole.  Nature, v. 416, p. 591-594.

Satellite-based gravitational surveys

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Outside of tides, two fundamental processes shift mass in our planet, the convective motion of the mantle and lithosphere, and that of the oceans.  A second-order means of mass transfer is that of water via the atmosphere, from sources of evaporated vapour to sites of precipitation and temporary storage (as soil moisture and in snow and ice).  Any movement of mass should, theoretically, result in changes in the Earth’s gravitational field.  Exploiting that simple notion presents two practical challenges, sufficiently precise measurements of gravity and its continuous monitoring.  Gravimeters used for surveys at the surface are now sensitive enough to give a reading for the mass of a person, provided he or she moves close enough to the instrument (gravity obeys an inverse-square law), but ground-based monitoring is so slow and expensive that continuous monitoring is impossible, except at permanent stations that check micro-gravitational changes near active volcanoes and fault zones.  Variations in the height at which satellites orbit the Earth stem from changes in gravity.  Although the inverse-square law of gravitational attraction smoothes out gravity anomalies at orbital altitudes, such measurements have been used for three decades to assess the shape of the Earth’s surface, were it completely covered with water (the geoid).  However, they are not accurate enough to do much more than that.

A project jointly funded by NASA and the German space agency DLR aims to improve the precision of satellite gravity measurements by more than 100 times that of the best to date (Adams, D.  2002.  Amazing grace.  Nature, v. 416, p. 10-11).  The Gravity Recovery and Climate Experiment (GRACE), launched in March 2002, uses two satellites that follow the same orbit with a spacing of 220 km.  Range finders on each measure their separation distance, and so their ups and downs as gravity varies, with far greater accuracy than any other method.  Every month they will have gathered enough data to assess the global variation of gravity at their orbital height.  That will produce movies of annual and longer term fluctuations, with sufficient detail even to track variations in the Gulf Stream and rises and falls in soil moisture and snow cover, as well as details that relate to deep ocean currents and mantle convection.  Unfortunately, gravity and the drag of Earth’s atmosphere limits GRACE’s lifespan to a mere 5 years.

See: http://www.csr.utexas.edu/grace and http://op.gfz-potsdam.de/grace/index_GRACE.html

Prolonged Cretaceous hothouse

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Hothouse conditions were forced by massive emission of CO2 during the mid-Cretaceous superplume event that created huge submarine basalt plateaux and began the development of many island chains that litter the floor of the central Pacific.  It was at this time that dinosaur-infested forests cloaked high latitudes, almost to both poles.  Terrestrial evidence suggests that conditions cooled somewhat in the later Cretaceous, and sequence stratigraphy indicates cyclic sea-level fluctuations, ascribed by some to the development of Antarctic ice sheets.  Resolving later Cretaceous global mean temperatures, and the ice-sheet question relies on oxygen isotopes from sea-floor sediments.  These are now available with sufficient precision and resolution to show that hothouse conditions lasted a great deal longer than suspected (Huber, B.T. et al. 2002.  Deep-sea paleotemperature record of extreme warmth during the Cretaceous.  Geology, v. 30, p. 123-126).

The Pacific superplume’s maximum activity was over a period of 15 Ma from 125 to 110 Ma (Barremian and Aptian), although it lasted until the early Campanian (80 Ma).  Contrary to the supposed magnitude of CO2 release by volcanism, heating reached a maximum from 94 to 80 Ma.  Even at high southern latitudes, deep-ocean water remained at 14 to 19°C for these 14 Ma.  Until the end of the Cretaceous it rarely fell below 10°C.  The data rule out any circulation of cold, dense brines into the deep ocean basins from the formation of boreal sea ice, and consequently any influence by polar ice sheets.  Sea level reached its highest during this period, almost certainly because the volume of the ocean basins shrank, being floored by young, warm, low-density crust formed by the superplume.  Mid to late-Cretaceous flooding of the continental margins created uniquely favourable conditions for an explosive development of carbonate-secreting organisms of many kinds.  Despite the burial of vast carbonate platforms, as well as thick boreal coal seams, these limestone “factories” seem incapable of having kept pace with greenhouse warming.  Was CO2 the only means then of global warming?

Continental growth and strike-slip tectonics

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Despite the increased precision of radiometric dating and the steady accumulation of ages when segments of continental crust first formed, two nagging oddities refuse to go away.  There seem to have been spurts in continental growth, rather than a steady build up over time.  Odder still, some areas have more crust of a certain age range than seems feasible.  The problem is a fundamental one, because the Earth generates radiogenic heat continually, though the amount has declined as the heat-producing isotopes of uranium, thorium and potassium decay.  Earth scientists assume that most geothermal energy exits to space through the process of sea-floor spreading.  Hot, new oceanic crust is invaded by seawater, thereby losing heat through hydrothermal activity.  The dominantly felsic magmas that build continental crust originate through partial melting processes where old, cold ocean floor descends at subduction zones.  Although some heat escapes through volcanism associated with mantle plumes, most researchers reckon that it is unlikely that this loss has ever come close to the quiet cooling at mid-ocean ridges, except possibly during the Archaean.  Averaged out, subduction and continent formation ought to keep pace with sea-floor spreading, though slowly declining over time.  There are those who focus on massive mantle turnovers in the form of superplumes that build large volcanic plateaux on land and on the sea floor, suggesting that their subduction generates greater volumes of crust than usual.  The main problem is that such plateaux are unlikely to be subducted.

The evidence for periods of accelerated continental growth comes from restricted regions, albeit very large.  Examples are 1900 to 1650 Ma crust in North America, Greenland and Europe, and 800 to 550 Ma crust in NE Africa, whose volumes are equivalent to between 1 and 10 times the present global rate of crust production at volcanic arcs.  Jonathan Patchett and Clement Chase of the University of Arizona offer a solution to the conundrums (Patchett, P.J. and Chase, C.G. 2002.  Role of transform continental margins in major crustal growth episodes.  Geology, v. 30, p. 39-42).  They show that strike-slip movement at modern subduction zones gives a 16% probability of more than 400 km transport of new continental crust parallel to the margins of existing continents.  Such motions are likely to concentrate continental growth where such terranes become docked together.  Such relative plate motions stem from particular configurations of spreading axes and the margins of old continents, and can therefore vary – some periods may have been dominated by head-on subduction, others by a greater amount of oblique relative movements.  By bundling together new continental material generated in magmatic arcs, the second would give the appearance of extraordinary rates of crust formation in some areas.  If the transform faults that channelled such lateral movements became obscure – and early strike-slip motions in ancient terranes are not easy to find or to quantify – the special natures of  terrane dockyards could go unnoticed.  Patchett and Chase note that the seeming pandemonium of 800-550 Ma crustal growth in NE Africa and Arabia has a counterpart in an age gap in the record of the northern continents, and cite several other examples.

While variations in strike-slip motions of terranes helps to resolve the apparent episodicity of continental growth, there is another line of approach.  Not all modern subduction zones generate voluminous magmas, even where plate motions are head to head.  The Andes has two huge segments where active subduction is unaccompanied by volcanism, and the angle of subduction is unusually shallow.  Low-angled subduction is likely where warmer than usual oceanic lithosphere enters a subduction zone, which is what might happen to segments blanketed by young ocean-plateau lavas formed by mantle plumes.  Constant sea-floor spreading need not necessarily result in constant rates of magmagenesis at destructive plate margins.

Homo erectus unification?

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It is difficult to resolve the “multiregional” versus “out-of-Africa” debate about the origin of modern humans on the basis of fossil evidence.  For some time, it has seemed that there were fundamental anatomical differences between earliest members of the genus Homo in Africa and those found in Asia.  The 19th century discovery by Dubois of what he called Pithecanthropus erectus ( now H. erectus) in Indonesia, set the taxonomic framework for recognising that species before early-human remains of similar antiquity (dating from about 1.8 Ma) were found in Africa.  At first regarded as H. erectus, the anatomical peculiarities of the early African remains eventually forced their reclassification as a different, perhaps ancestral species to “true erects” – H. ergaster (“Action Man”).  The fragmentary remains of the earliest Asian hominids do seem to be of this species, as do those dating to 1.6 Ma from Dmanisi in Georgia.  The lack of African fossils from the period up to about 600 ka permitted the view that H. erectus was an exclusively Asian descendant from early migrants; i.e. that there was a species divergence between Africa and Asia.  Two recent finds have cast doubt on that.

The first was of a well-preserved cranium, with associated tools and abundant mammalian remains, from the Danakil area of Eritrea (Abbate et al. 1998.  A one-million-year-old Homo cranium from the Danakil (Afar) Depression of Eritrea.  Nature, v.  393, p. 458-460), which seems to blend features of both H. erectus and H. sapiens.  The latest is claimed to be indisputably an H. erectus, and comes from the highly productive Middle Awash sediments of southern Afar in Ethiopia (Asfaw, B. et al. 2002.  Remains of Homo erectus from Bouri, Middle Awash, Ethiopia.  Nature, v. 416, p. 317-320).  The last also comes from the period around 1 Ma ago.  Such is its resemblance to Asian fossils, that there seems little point in considering any minor differences as being other than the results of the polymorphism which is so characteristic of modern humans (a view long held by the palaeoecologist, Jonathan Kingdon).  The authors also suggest that assigning earlier fossils to H. ergaster is neither necessary nor useful, for the African record now suggests that they are the early members of a lineage towards later “erects”.  The close resemblance between African and Asian “erects” does appear to indicate either repeated migration to Asia or continuous genetic contact between the two populations.

(Note  Acrimony that has no bearing on scientific debate flared up around the potentially revealing Eritrean, middle-Pleistocene sites at the annual meeting of the Palaeoanthropological Society in Denver (March 2002).  One of the members of the  University of Florence team, who discovered the site at Buia in Danakil, reported that on a recent visit local people had begun offering tools and fossils for sale.  Allegedly, the locals said they had been offered money by another team, possibly led by Randall Susman of the State University of New York.  Susman and co-workers strenuously deny offering bounties, yet have had their permit for future work withdrawn by Eritrean authorities (Dalton, R.  2002.  Hints of bone bounties rile fossil hunters.  Nature, v.  416, p. 356).  It seems hardly surprising that perceptive locals, who wrest a meagre living in one of the world’s most inhospitable places, seek to make their lives a little easier by selling what is clearly valuable enough to attract well-heeled scientists to their homeland.  Rather than allow innuendo to fog the scientific issues, it would seem wise to train people who know the area intimately to become skilled fossil hunters, and to pay them a decent wage, much as has happened in Kenya and Tanzania.)

Phyllogeography and “Out of Africa”

While 2001 was becoming the “Year of the Genome”, work continued unnoticed by the press on the growing amount of information about genetic differences between modern people in widely separated parts of the world.  Moreover, computer software developed to give more meaning to that geographic variation; the science of phyllogeography  emerged.  Analysis of genetic data, using sophisticated statistics, potentially reveals the different mutations that have appeared in widely separated populations over time, and also the degree to which genetic information entered such populations as a result of movement into them by people from far-off places.  It is a complex business, but may help resolve or reconcile the two main hypotheses about the origins of modern humans. 

The “out-of-Africa” hypothesis – launched by early work on modern humans’ genetic patterns – starts with the migration of Homo erectus from Africa to colonise Eurasia, perhaps as early as 1.8 Ma ago, thereafter to evolve separately in isolation from early Africans and perhaps one another.  Fully modern humans evolved in Africa and expanded again since about 100 ka to replace and genetically extinguish those older, non-modern populations.  The alternative view of multiregional evolution also accepts an African origin for H. erectus and its early migration outwards, but that it was followed by many genetic contacts of regional populations with Africa through continued migrations over the last 1.8 Ma.  That would allow local populations to differentiate because of the distances between them, yet gene flow between them and Africa would have maintained a single evolutionary lineage.  The many shifts in climate and sea-levels through the Pleistocene would have posed repeated stresses and opportunities for the regular migrations that this multiregional trellis model demands, hence the tenacity with which its supporters hold that view.  However, a notion of modern human populations having evolved in semi-isolation over such a long time carries inevitable connotations that many people find disagreeable.  There are political undertones in the debate that do cloud the scientific issues.

One of the supporters of the multi-regional model, Alan Templeton of Washington University, Missouri USA, has applied new statistical analyses to genetic data from mitochondrial DNA – first claimed as support for the “out-of-Africa” hypothesis – Y-chromosomes and 8 other sources of genetic information (Templeton, A.R. 2002.  Out of Africa again and again.  Nature, v. 416, p. 45-51).  His work confirms the ultimate African origin of all of us, but raises the possibility of at least two expansions out of Africa, at 600 ka and 95 ka.  Now that may seem to bring much needed support to multi-regionalism, but “again and again” is not the same as the many connections required by the hypothesis.  It is a powerful demonstration of how much remains to be done, put in context by one reviewer’s comment that genetic information from 35 individuals on a Pacific island, colonised in only the last 1000 years, is inadequate to say where all the genes came from (Cann, R.L. 2002.  Tangled genetic routes.  Nature, v. 416, p. 32-33).  In the global data used by Templeton to examine more than a million years of evolution, the groupings rely on samples from as few as 35 living individuals.

Extinctions by impacts: smoking artillery

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It’s a measure of the resistance to events controlled by processes outside of Earthly ones that evidence in support of an impact cause for mass extinctions has assumed monumental dimensions.  The iridium anomalies at the K-T boundary, found by Alvarez and Son in 1980, were never enough for a great many palaeontologists.  Nor, for that matter, were the co-occurrences of glass microspherules, shocked quartz grains and soot, discovered by later investigators at 30 to 100 sites worldwide.  Even the remains of the 180 km-wide Chixculub impact crater that formed at the same time as the extinction event, off Yucatan in the Mexican Gulf, was insufficient for the most intransigent sceptics.   That the sooty material contained massive carbon molecules in forms akin to Buckminster Fuller’s geodesic dome, and moreover those fullerenes contained trapped noble gases in proportions that could never have been present on Earth, formed the smoking siege gun for most sensible scientists.  The fullerenes contain helium, neon and argon with isotopic proportions comparable with those in carbonaceous chondrites and interplanetary dust, probably created by processes in a supernova that preceded accretion of the solar nebula.  The hypothesis that such odd materials were delivered to the K-T boundary layer by an extraterrestrial object was amply confirmed by Luann Becker’s discovery that carbonaceous chondrites, never affected by extreme events since they formed, also contain fullerenes (Becker, L. 2002.  Repeated blows.  Scientific American, v. 286(3), p. 62-69).  The latest occurrence of such convincing evidence for impact control of mass extinction comes from Permian-Triassic boundary deposits in China, Japan and Antarctica, that coincide with the most severe disruption of eukaryote life – around 90% of marine and continental families failed to survive it (see Land vertebrates snuffed at the end of the Permian in February 2002 Earth Pages).

It now seems that palaeontologists and a great many others, including creationists who envisage some kind of design within the fossil record, will be compelled to face up to an unearthly influence over the shaping of life on our planet.  There are many impact structures that are candidates for having affecting the biosphere from the Mesoproterozoic onwards, yet no pattern to their timing and energy of formation.  Such is the complexity of gravitational fluctuations that fling asteroids and comets into Earth-crossing orbits, that aside from the inevitability that, given time, they will strike with devastating consequences, they are essentially random events.  Our species is a late development from a vast concatenation of events, both from outside and within the Earth system, that spanned the entire 4.5 billion-year physical evolution of our home world.  No-one has yet turned statistics to estimate the likelihood of such chance occurrences being repeated, with one outcome being conscious beings.  If that were possible, then for the seekers of extraterrestrial intelligence, it might well be as welcome as a Semtex suppository on a wide-bodied jet!

Dinosaur digest

Having suffered vivid nightmares about dinosaurs when a kid – I did not even dare watch Jurassic Park alone as an adult – it comes as a huge relief to learn that the scariest of all monsters, T. rex, was about as agile as I am.  Indeed, it seems highly likely that you or I could outrun one.  The reasoning behind this welcome news (Hutchinson, J.R and Garcia, M. 2002.  Tyrannosaurus was not a fast runner.  Nature, v. 415, p. 1018-1021) stems from scaling up the sprinting powers of chickens (a chicken is surprisingly fast!) to the estimated 6 tonne weight of an adult T. rex.  The analysis involves two factors.  First, muscle proteins have the same capacity for powering movement, and the total power of musculature depends on its cross-sectional area, but while body mass and volume grows, this area and potential power falls behind.  Secondly, the bearing capacity of bone decreases with size too, because this also depends on area rather than volume.  Hutchinson and Garcia’s scaling hens to 6 tonnes, and calculating the necessary mass of leg muscle to propel them in their fearsome dashes to grab a tidbit (you or me), resulted in the absurd vision of a creature with 86% of its body mass in its legs.  Tyrannosaur modelling from their skeletons falls a very long way short of that, and they would be hard pressed to clock much more than 5 ms-1, which I think I could manage quite easily, for a short while.  That they would ever break into more than a fast walk is unlikely, for the second factor poses a limit.  One wrong pounce would be curtains, for they would break a leg.  Two possible life styles seem to emerge from the analysis.  They may have subsisted on carrion.  Alternatively, the far bigger herbivorous dinosaurs would have been even more stately, for the same mechanical reasons, which generates the absurd vision of large carnivorous dinosaurs ambling down their prey.

See also:  Hecht, J. 2002.  T. rex was a lumbering old slow coach.  New Scientist, 2 March 2002, p. 6;  Biewener, A.A. 2002.  Walking with tyrannosaurs.  Nature, v. 415, p. 971-973.

That dinosaurs could survive high-latitude winters, in near total darkness, if not glacial conditions, was first suspected in 1960 when their footprints turned up in Spitzbergen.  Since then, palaeontologists have found fossils of a wide variety of dinosaurs in areas that would have been near-polar during the Jurassic and Cretaceous Periods (Rich, T.H. et al. 2002.  Polar dinosaurs.  Science, v.  295, p. 979-980).  Surely, these dinosaurs must have been warm-blooded, as their containing sediments sometimes show signs of the effects of permafrost.  There are signs in some of the fossils for heightened visual powers too.  In the case of Australian faunas, it seems certain that the abundant dinosaurs there did not migrate to high latitudes in summer, because seaways blocked passage to lower latitudes.

Extremophiles and possibilities for extraterrestrial life

Bacteria can survive extremes of temperature (-10 to 110°C) and chemistry, and the biosphere extends to crustal depths in excess of 2 km, as shown by thriving communities in deep wells.  So far as biologists are aware, temperature forms the limit to life’s range, because of the instability of crucial molecules and of course the boiling point of water.  Since temperature increases with depth in the Earth, due to its self-heating by radioactive decay, the biosphere has a depth limit too, depending on the geothermal gradient.  However, recent experiments on two common bacteria show that life can survive at extremely high pressures (Sharma, A. et al. 2002.  Microbial activity at gigapascal pressures.  Science, v. 295, p. 1514-1516).  By compressing bacterial films on ice in diamond anvil cells, a team from the Carnegie Institute in Washington, DC have shown that simple life can survive pressure as high as 1.6 Gpa, that is equivalent to crustal depths of 50 km or an ocean bed160 km below the surface.  Because subduction takes cold lithosphere downwards, and the associated geothermal gradient is low in such environments, the deepest biosphere may be below volcanic arcs.  However, the most significant implication of the experiments is that probing the icy crusts of Europa, Ganymede or Callisto (and liquid water that might be present at great depths there) and the Martian ice caps, conceivably could reveal living organisms, if life ever evolved on these bodies.  Whereas this possibility encourages various plans for such exploration, what the experiments did not show was replication by the bacteria, and that is central to any living organism.

Taking stock of hominid evolution

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The dearth of fossils along humanity’s early evolutionary path inevitably results in even a single find forcing a rethink of the whole story.  Sometimes it exposes a novel characteristic, or a new date of occurrence, and quite minor deviations in relative durations of different species or minuscule differences in dentition or foot bones assume an importance that would be disproportionate in any other vertebrate group.  The last 2 to 3 years have unearthed evidence for the presence of bipedalism as early as 6 Ma ago, and three new primate divisions that seem on the line to humans rather than other living apes.  The 15 February issue of Science devotes 8 pages of News Focus to reviewing hominid evolution (Balter, M. and Gibbons, A. 2002.  Becoming human.  Science, v. 295, p. 1214-1225).

One picture that emerged more than a decade ago is that the richest pickings occur along the line of the East African Rift system, where continued extension since Miocene times has created room for the deposition of terrestrial sediments and thus chances of preservation.  Moreover, its continual volcanic activity has interleaved sedimentary strata with lava flows and ash beds that present ample opportunities for precise dating.  It is in the Rift that the onset of human-like traits has been pushed further and further back in time.  The discovery of Ardepithecus ramidus (“root Earth-ape) at Aramis in the Afar province of Ethiopia by The Middle Awash Research Team in 1992 (dated at around 4.4 Ma) pushed “Lucy” and the earlier, but fragmentary 4 Ma Australopithecus anamensis out of specialists’ ranking as the first in our line.  Last year Yohannes Haile Selassie published details of an earlier Ardepthicus subspecies from Afar, whose age is between 5.2 to 5.8 Ma.  In both, the central evidence for being hominid rests on foot bones, for the teeth bear a mixture of chimp- and human-like features.  Ardepithecines possibly could walk bipedally, but probably ate soft fruit and leaves in forested hills.  And then there is Orrorin tugenensis (“original man”) from the Tugen Hills in the Kenyan Rift, coming in at 5.72 to 5.88 Ma.  This so-called “Millennium Man”, found by a joint French-Kenyan team.  Its gait has still to rest on what to most of us might seem like flimsy evidence, modelled from three thighbones.  Orrorin’s teeth have mixed human- and chimp-like characters.  Unsurprisingly, Orrorin’s finders claim primacy as well as a nice new name, while those responsible for slightly younger Ardepithecus argue that both are the same genus.  The most important point, assuming that bipedality can be convincingly demonstrated for both, is that neither dwelt in grasslands, but in forests.  Bipedality might not have evolved through pressures that emerged with the spread of African savannah.  Although yet to be published, and dated only by stratigraphic means, an early forest dwelling hominid fossil, found last year in northern Chad by the French-Chadian Palaeoanthropological Mission breaks the stranglehold of the Rift on exploration for early hominids.  Two thousand kilometres from the Rift, the Chadian find implies that hominids roamed over a vast tract of a largely flat continental surface.

As well as a flurry of revisions to the human evolutionary “bush” (and each anthro to their own!), the oldest date comes dangerously close to the 5 to 7 Ma date of last common ancestor between the chimp and the human lines, as estimated from the difference between modern DNA sequences.  One among several possibilities is that the chimp human separation involved acceleration of evolution in our line; something often attributed to a “bottleneck” when numbers of individuals dropped to such a low level that mutations spread rapidly, instead of being “ironed out” in a larger gene pool.  There is one worrying aspect of the hunt for human ancestral fossils – there seems to be little parallel effort to seek early chimp fossils, or at least they are exceedingly rare.  That may be because true tropical rain forest with its highly oxidizing soils destroys the evidence.  Whatever, there is a possibility that among the increasing number of supposedly hominid fossils could be some ancestral chimpanzees!  All that would be required is a reversion to knuckle walking in forest environments.  Bone and tooth enamel cannot resolve that possibility.  The only possible way forward is more finds in a wider geographic diversity of sites, which the finds in Chad suggest is achievable, given Miocene to Pleistocene successions.

The thrust of research shifts from bones to artefacts in the case of Homo species, and how they might be interpreted in terms of cognitive ability.  Most important are signs of  abstraction from the natural world; in a word, art.  There has long been a Eurocentric bias, largely because of the wealth of exquisite objects that explode into the archaeological record there after 40 thousand years ago.  Art is a sure sign of fully human cognitive abilities, no matter how much physical anthropologists might ponder over this or that feature of skulls from the late-Pleistocene, and its role in shaping brain architecture and function.  Sudden European appearance of artistic expression has long spurred the view that its evolution was explosive and unique, probably as a result of some mutation.  That view needed revision as soon as Christopher Hinshilwood of the South African Museum reported his find in January this year of geometrically carved ochre objects close to Cape Town.  They are 77 thousand years old, but are not exactly prancing horses.  More common are tools, and major advances seem to have taken place in Africa, long before they appear in Europe at around the same time as artistic impressions.  Photographs of the engraved ochre objects bear strong resemblance to recent “doodles” by hunter gatherers and even runestones or tally sticks.  It is certainly a case of “Who knows?”, until more finds come to light.

New Japanese tectonics research centre

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The Institute for Frontier Research on Earth Evolution (IFREE) involves 100 Japanese researchers focusing on central aspects of tectonic evolution over the last 200 Ma.  These include Pangaea break-up, mid-Cretaceous global warming and Eocene plate reorganization.  One particularly interesting study begun using initial funding of US$12 million is the Bonin-Mariana subduction zone.  Details at http://www.jamstec.go.jp/jamstec-j/IFREE.

Collapsing islands

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Lots of attention has focused on impacts by Earth-crossing asteroids and comets as potential causes of economic and biological catastrophe, as too on hazards from climate change induced by major volcanic activity.  To these fears can be added the effects of tsunamis, but not those caused by even the largest conceivable eathquake.  Oceanic islands can fall apart by a process that is identical to, though vastly bigger than a landslip, thereby displacing their equivalent volume of seawater.

Britain has experienced the effects of tsunamis driven by collapse of part of the Norwegian continental slope, triggered by massive methane release from gas hydrates in sea-floor sediments.  The last of these was when its shores were colonised by Bronze Age people, and left its mark in the form of high-level sand beds on the flanks of eastern Scotland’s firths.  The north-east part of the Isle of Skye preserves spectacular results of landslips of volcanic rocks, that represent the largest mass movement known in Europe.  However both examples are dwarfed by features off the Hawaiian islands, that sonar has revealed.  There are some 70 debris fields that date back to 20 Ma, some of which contain up to 5 000 cubic kilometres of rock from collapses of the flanks of the growing volcanic islands.  Surveys around other large oceanic islands of volcanic origin suggest that such flank collapses occur around every 10 000 years.  Movement of masses so large involves energy equivalent to the world’s arsenal of nuclear weapons, so flank collapses are comparable in magnitude with moderately sized impacts.  They would generate tsunamis waves as high as 30 metres, that would devastate coastal areas around large ocean basins.

One area on Hawaii is indeed liable to collapse, and in November 2000 it moved, only to stop short of a full collapse.  Geoscientists from the US Geological Survey and Stanford University used GPS receivers to monitor movement on the southern flank of Kilauea, and after a series of barely detectable earthquakes they recorded slips of up to 6 centimetres per day (Cervelli, P. et al. 2002.  Sudden aseismic slip on the south flank of Kilauea volcano.  Nature, v. 415, p. 1014-1018).  Careful analysis of many kinds of motion sensors suggests that the moving block sits on top of a low-angle fault or detachment, that may eventually carry the block seawards to unleash tsunamis.  It is uncertain how much warning there would be of a fully fledged collapse, but is does seem sensible to establish such monitoring on active volcanic islands in the world’s oceans.  Since expansion of humanity following the retreat of the last continental ice sheets would have largely been along coasts, with their easy terrain and abundant food supplies, tsunamis would have been an ever present, but never suspected risk.  Britain’s example is minor in comparison to those that would stem from flank collapses, and perhaps the near-miss of November 2000 may encourage searches for the scars that huge tsunamis generate in relation to maritime archaeological records.

Credit where credit is due?

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A recent book (Crewdson, J.  2002.  Science Fictions: A Scientific Mystery, a Massive Cover-up and the Dark Legacy of Robert Gallo.  Little, Brown; Boston) describes the role of pulling (and enhancing) rank in the history of HIV’s discovery.  In fact there were two histories: the real one in which two post-docs in Gallo’s lab, Bernie Poiesz and Frank Ruscetti, succeeded in isolating human T-cell leukaemia virus – the seminal step on the road to HIV; the “engineered” history, in which credit for the discovery seemed to pass entirely to Robert Gallo.  However that particular revision of reality emerged, building rank through annexation of credit is not uncommon in academic circles.  Peter Lawrence of the Medical Research Council Laboratory of Molecular Biology, Cambridge University has expanded on Crewdson’s careful investigation to produce a useful warning, particularly for beginning and junior researchers in all disciplines (Lawrence, P.A. 2002.  Rank injustice.  Nature, v. 415, p. 835-836).

Lawrence’s thesis is that the scientific community allows experienced researchers to take advantage of the inexperienced, so that credit generally flows up the ladder of rank.  Part of the problem is that graduate students, and even post-docs, nowadays rarely generate projects themselves and increasingly work under the control rather than the guidance of a supervisor, team leader or major grant holder.  It is not always a case of high-ranking scientists mendaciously grasping credit for discoveries made by underlings, for various practices make misplaced credit inevitable.  Lawrence lists a whole number of these.  For me, one is particularly interesting.  It centres on how to stick in one’s peers’ memory.  If the same name appears again and again in publications – it makes little difference where it figures in the list of authors – it is that name that is remembered as an “authority”.  During the 1980s, Gallo managed to figure as an author in up to 90 papers a year, despite mainly travelling back and forth to conferences.

Most people’s view is that whoever does most of the work, discusses its ramifications and draws conclusions should be the first author in a list.  But are they the “senior” author?  In terms of rank that is often not the case, and one need only scan the publications of a large research team to see the same name appearing again and again, often in last position; that of the “owner” of the lab or the funds.  What they have done to appear on the list is rarely clear, but by sheer number of appearances it is their name that is remembered, and more importantly these days, figures in measures of productivity.  As they say, it is a “win-win” scenario.  Any paper, in whose list of authors the “name” appears, that meets peer acclaim serves to boost that “names” citation rating too.  If such a paper turns out to be sloppy or even fraudulent, then someone safe among the “also-rans” can shrug off responsibility.

The same issue’s Editorial (Thoughts on (dis)credits.  Nature, v. 415, p. 819) quotes from a letter submitted by Max Perutz (Peter Lawrence’s former “boss”), shortly before his death on 6 February 2002.  Perutz spent the first 25 years of his career in the Cavendish Laboratory at Cambridge, headed by Ernest Rutherford and then W.L. Bragg, neither of whom put their names on papers to which they had not contributed, despite the fact that a whole number represented epochal breakthroughs inspired by them.  And nor did Perutz.  That generosity damaged none of their careers or reputations, but made them properly respected, admired and fondly remembered.  Will careers based on annexation of credit (an excellent euphemism!) find the same fate?

Review of thermohaline circulation

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The central factor in abrupt climatic shifts during the last glacial period was change in thermohaline circulation (THC), particularly in the Atlantic Ocean.  Two general processes underpin THC: differences in solar heating from low to high latitudes drive polewards flow of surface water; formation beneath sea-ice of dense brine that sinks to form an equatorwards flow of North Atlantic Deep Water (NADW).  Freshwater influx at high latitudes suppresses the formation of NADW, which, together with enhanced low-latitude evaporation, slows polewards surface flow..  Currently, the thermal influence and NADW formation dominates heat transport northwards in the North Atlantic, by carrying about a petaWatt at mid latitudes.  THC is of little consequence in the North Pacific, partly because its fresher surface water hinders dense-brine formation, and partly because any deep water formed beneath sea ice in the Arctic cannot flow through the very shallow Bering Straits.

Clearly THC is a sensitive mechanism, inseparable from other factors in climate forcing.  Having such a vast influence on heat transport, if it changes there are likely to be dramatic outcomes for climate, particularly along the eastern flank of the North Atlantic where much of the transported heat arrives.  Sea-ice formation around Iceland is decreasing, so a review article on THC and rapid climate change is essential reading (Clark, P.U. et al. 2002.  The role of thermohaline circulation in abrupt climate change.  Nature, v. 415, p. 863-869).  It is now known that the last glacial period was punctuated by short-period (~ 1-2 ka) warming-cooling episodes, known as Dansgaard-Oeschger events, one aspect of which was the launching of “armadas” of icebergs to latitudes as far south as Portugal (known as Heinrich events), which left their mark as occasional gravel layers in the otherwise muddy sediments on the deep Atlantic floor.  These episodes involved temperature changes over the Greenland icecap of as much as 15°C.  They began with warming on this scale within a matter of decades followed by slow cooling to minimal temperatures, before the next turn-over.  The deep cooling seems to have accompanied slowing and shut-down of THC.  Current global warming is likely to do three things:  increasing low-latitude evaporation, increasing freshwater influx to high-latitude Atlantic surface water and a decrease in sea-ice formation at the site of NADW formation.  Because all three drive down polewards heat flux, anthropogenic warming may well result in contrary climate shift in Western Europe and Scandinavia – freeze rather than thaw.  If it happens, chances are that it will be upon us with little warning.

Land vertebrates snuffed at the end of the Permian

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Without doubt, the mass extinction at the Permian-Triassic (P-T) boundary was the most important biological event in the history of Phanerozoic evolution.  Around 80-90% of families disappeared, and perhaps more than 50% of species diversity.  But, the evidence stems largely from marine records.  Marine organisms went down at a hasty rate, as evidenced by the superb boundary sequence in China.  However, such is the inconsistency of preservation on land that matching evidence is sparse from the terrestrial realm.  The best chance of examining the response of land animals to whatever wrought such havoc at sea lies in the Karoo sediments of southern Africa.  Roger Smith of the South African Museum and Peter Ward of the University of Washington have combed the mainly fluvial sediments for evidence (Smith, R.M.H. and Ward, P.D.  2001.  Pattern of vertebrate extinctions across an event bed at the Permian-Triassic boundary in the Karoo Basin of South Africa.  Geology, v. 29, p. 1147-1150).  Rather than supporting the general view that terrestrial P-T extinctions took a few million years, they have been able to show that Permian vertebrates disappeared abruptly, to be replaced by a very different fauna equally suddenly in the lowermost Triassic.  Only one genus (Lystrosaurus) spans the boundary, and the boundary itself contains no evidence of life.  Calculations based on estimates of the rate of sedimentation point to around 50 thousand years for the extinction event, about the same as that affecting marine organisms.  Interestingly, the event sharply separates very different sediments, that Smith and Ward interpret as products of perennially wet Permian flood plains and those experiencing ephemeral flow in the Triassic (see End-Permian devastation of land plants in Earth Pages October 2000).  Whatever its cause, the stresses placed on land vertebrates seem to have included the sudden onset of aridity.

Mesozoic fossil hunting in Madagascar

Most papers on palaeontology report the details of years of research on what the fossil hunters have found, with mentioning the months of patient searching.  John Flynn and André Wyss have provided an insight into the tribulations of palaeontological field work in difficult terrains, as well as a broad account of the context of their finds (Flynn, J.S. and Wyss, A.R. 2002.  Madagascar’s Mesozoic secrets.  Scientific American, v. 286, February 2002, p. 42-51).  Madagascar lingered at the heart of the Gondwana supercontinent until it finally began to split into drifting segments during the early-Triassic.  It lay on the eastern flank of an evolving rift basin that filled with mainly terrestrial sediments until the late-Jurassic.  This particular basin remained uninterrupted by volcanism or erosion, and so is a repository for organic remains trapped in a continuous sedimentary sequence.  This period in geological history, particularly the Triassic, spans the emergence and development of both the dinosaurs and primitive mammals.  The wealth of vertebrate fossils that geologists are beginning to unearth suggests that Madagascar may well become the site where the mysterious origins of both are resolved.

The simplest living ecosystem

Hugely complex as life is, at the cell level it has a profound simplicity, at least as regards its fundamental chemistry.  Cell metabolism receives its power from the transfer of electrons from a high to a low energy level.  High-energy electrons stem from chemically active molecules, atoms or ions able to release them; electron donors or reducing agents.  The metabolic path ends in oxidizing agents accepting these electrons.  This process of donating and accepting electrons takes the form, in most cell types, of “pumping” hydrogen ions, or protons back and forth across the cell wall to create an electrochemical gradient that is continually charged and discharged.  Biochemistry reflects this by the ADP-ATP cycle at life’s core, in many different versions.

The simplest provision of electrons is by hydrogen, and arguably a supply of hydrogen gas is a highly likely precondition for the origin of life.  Surprisingly, hydrogen is generated by many geological reactions, although little survives some form of oxidation for long.  In a few places hydrogen gas escapes abundantly, as in the weathering of ultramafic rocks by groundwater.  The essential process is the breakdown of iron and magnesium silicates to various kinds of clay, by the interaction of hot water with fresh igneous rocks.  Geochemists and microbiologists from the USA  analysed such a hydrothermal system 200 m beneath a volcanic area in Idaho, and found a thriving and diverse ecosystem dominated by simple organisms that do depend on hydrogen (Chapelle, F.H. et al. 2002.  A hydrogen-based subsurface microbial community dominated by methanogens.  Nature, v. 415, p. 312-315).  More than 90% of the organisms are methane-producing Archaea, which reduce carbon dioxide to methane, using hydrogen.  No other hot-spring system comes close to this probably highly primitive community.  It is a handy analogue for the kind of ecology that may have developed if life has arisen deep beneath the icy surface of Europa – a target for future NASA missions.

Incidentally, the exploitation of electron and proton transfer that underpins cell metabolism potentially forms a source of electrical power.  Younger readers may have experimented with using fruit as the basis of a simple galvanic battery, thereby exploiting low pH conditions.  Investigation of the potential of bacteria for direct electricity generation recently made a breakthrough (Bond, D.R. et al. 2002.  Electrode-reducing microorganisms that harvest energy from marine sediments.  Science, v. 295, p. 483-485).  A member of the family Geobacteraceae (Desulfuramonas acetoxidans)  has been found to readily produce excess electrons as it metabolises organic material in oxygen-free muds.   Daniel Bond and co-workers from the University of Massachusetts and the US Naval Research Laboratory introduced graphite electrodes into airless fish-tank muds and the upper oxygenated sediments.  Even with such a crude experiment, sufficient current flowed to power a small calculator.  Moreover, D. acetoxidans colonised the electrodes within a matter of days, showing that they were directly involved in the oxidation-reduction system at the root of such a fuel cell.  As well as raising the possibility of powering submarine monitoring devices using bioelectricity, such geobacteria are able to metabolise a range of common organic pollutants.  Marine organic sediments are virtually limitless, so it is not inconceivable that the process may result in yet another renewable power source, albeit difficult to convert to high-power supplies, with the blessing of pollution control as a sideline.

Meltdown for Snowball Earth?

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Following on from their linking carbon-isotope excursions associated with Neoproterozoic diamictite-cap carbonate sequences to methane release (see Methane and Snowball Earth in Earth Pages, December 2001), Martin Kennedy, Nicholas Christie-Blick and Anthony Prave turned to the d13C values in the diamictites for which a glaciogenic interpretation forms the main plank of the Snowball Earth hypothesis (Kennedy, M.J. et al. 2001.  Carbon isotope composition of Neoproterozoic glacial carbonates as a test of paleoceanographic models for snowball Earth phenomena.  Geology, v. 29, p. 1135-1138).  Complete ice cover of the oceans would chemically isolate ocean water from the atmosphere, and would effectively shut down the organic sinks for atmospheric carbon dioxide.  While they operate, the exclusion of 13C relative to lighter carbon by organisms drives up d13C in sea water, to be preserved in carbonate sediments.  The Snowball Earth model predicts negative d13C, approaching the -5‰ of the mantle, in carbonates deposited during all-enveloping glacial epochs.  However, few researchers have made the measurements needed to test that part of the hypothesis.

Kennedy and co-workers show from three such diamictite sequences that the carbonate precipitated as cement in them has consistently positive d13C.  Although that does not disprove the existence of glaciation at tropical latitudes, it is not consistent with the dreadful scenario of totally ice-bound oceans devoid of life.  Nor, for that matter, is there any evidence from strontium isotope variations in carbonate cap rocks for the massive continental weathering that the Snowball Earth devotees propose as a means of escape from the eventual hot-house that build up of volcanic CO2 emissions to release Earth from the mothers of all cold snaps would create.  Expect interesting news in later Earth Pages of how the greatest Earth science debate of the 21st century develops.

American Geophysical Union 2001 Fall Meeting

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The regular AGU Meetings are the largest of all Earth science bun fights, some 8500 souls attending the latest In San Francisco from 10-14 December 2001.  Without attending or browsing the abstracts, the rest of us might wait months if not years for ideas presented at such conferences to emerge as papers, and many presentations never reach the press.  Summaries of conference proceedings shortly after the event are a useful, although rare notice of “what’s up” to the rest of us.  Surely it is in the interest of organisers to arrange postscripts, perhaps through a general website – conferences cost an arm and a leg to attend, let alone organise, and paying someone to write them up would have a minuscule cost. But there has always been a cachet to being one of the “in crowd”, and no one satirised that better than David Lodge, in his 1980 novel Small World (Penguin Press).  In the case of the AGU and GSA Meetings the crowd is around that at a Nationwide League Division 1 match at 3 pm on a winter Saturday, but is somewhat less focussed.

So a summary of AGU 2001 Fall was a welcome sight in Science (Kerr, R.  2002.  Of ocean weather and volcanoes.  Science, v. 295, p. 260-261), even though it covered only some of the themes in the poster sessions.  A sizeable number of AGU attendees do sea time, and they must have been delighted to learn that the US Naval Research Laboratory  now provides 30-day “sea forecasts” on-line (www.73320.nrlssc.navy.mil/global_nlom/).  An outcome of work from the Ocean Drilling Program along the Hawaii-Emperor sea-mount chain is clear palaeomagnetic evidence that the Hawaiian hot spot has not always been fixed relative to moving lithospheric plates.  From late-Cretaceous to early-Oligocene times it was shifting southwards relative to the north magnetic pole at a rate comparable with that of sea-floor spreading, thereby helping to explain the 60° bend in the chain, and perhaps that less well seen in other Pacific hot-spot tracks.  As well as demanding an explanation for lateral dynamics in the deep mantle, hot spots that move challenge some basics of plate tectonics.  The current mood among plate tectonicians seems somewhat similar to that of a zoologist colleague in the 1960s.  He had ended up on a psychiatrist’s couch when he stoutly maintained that a red blemish on his torso moved, but only when it was he who was examining it.  That turned out to be a giraffe parasite, picked up during field work in the Serengeti.  More immediately worrying for the US public was news in spring 2001 that a sizeable bulge was growing in Oregon, close to dormant volcanoes.  Pushing 30 mm a year, the carbuncle’s growth rate suggested that 0.02 cubic kilometres of magma were on the move – that is a lot of potential volcano.  USGS volcanologists have been on a bulge watch for 2 decades, and were the first to announce a far larger manifestation beneath a northern Californian caldera.  At the AGU, worries were damped down by reassurances that neither seemed likely to amount to a can of beans (but see Is volcanic eruption predictable? below).

Interstellar carbonates and “fossils” from Mars

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Part of the argument used to support the notion that life may have arisen on Mars early in its history depends on the presence of carbonates in the notorious meteorite ALH84001 found in Antarctica.  Supposedly having been ejected by an impact on the Martian surface (based on its oxygen isotope composition and the blend of noble gases trapped within it), ALH84001 also contains the minute structures that were prematurely announced in a blaze of publicity as fossilized alien life forms by US and British meteorite specialists in 1996.  The discoverers claimed that carbonate minerals within it clearly evidenced the rotting of silicates by liquid water containing dissolved CO2; so they do in terrestrial rocks. However, carbonates also occur in meteorites that by no shred of the imagination can have formed within sizeable planets.  Many probably accreted in a near vacuum from dusts that occur in clouds within our galaxy, while the solar system was forming.

Using infrared spectra to assess the mineral composition of dust clouds surrounding stars, a team of European and American cosmochemists has found that in two cases such dust contains calcite and perhaps dolomite (Kemper, F. et al. 2002.  Detection of carbonates in dust shells around evolved stars.  Nature, v. 415, p. 295-297).  Because liquid water cannot exist in a near vacuum, production of these carbonates cannot have taken place by the familiar silicate-rotting process.  More likely, they formed on the surfaces of silicate dust or ice grains by reactions between calcium and magnesium ions and those in which carbon and oxygen were combined.

Hydrocarbon source rocks and ocean anoxia events

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Much of the world’s oil resources formed by maturation and migration of hydrocarbons from organic-rich, marine mudrocks, which seem to have formed episodically during Earth history.  A widely accepted view is that such source rocks’ content of organic matter fell to the ocean floor as the remains of tiny organism.  That they were not oxidized by bacterial action seems to suggest that the periods when source rocks accumulated were characterized by low oxygen levels in bottom waters.  Each major source rock has been linked to such ocean-anoxia events, and to periods when deep-ocean circulation effectively stopped, so cutting off oxygen supplies to deep levels.  However, studies of modern deposition of organic matter in marine sediments at continental margins reveals that discrete particles of organic matter are far outweighed by biological molecules that coat the surfaces of minerals, particularly those of clay minerals.  The amount of organic carbon in a modern sediment depends largely on its content of clay minerals derived from intense chemical weathering of continental rocks.  Such coatings are protected from normal processes of decay, so that the adsorbed organic carbon compounds can be buried, more or less intact

It should be possible to check whether ancient source rocks are similar to modern carbon-rich sediments by seeking a strong correlation between clay content and organic content – mudrocks also contain fine silt particles made of non absorbent quartz. It seems that in at least one Cretaceous source rock in the US Mid-West such a correlation is clear (Kennedy, M.J., Pevear, D.R. and Hill, R.J. 2002.  Mineral surface control of organic carbon in black shale.  Science, v. 295, p. 657-660).  This suggests that oil-shale deposition is as much related to the intensity of continental weathering of silicates as it is to ocean-water chemistry.  Since clays, especially the sponge-like smectites, adsorb organic molecules from solution in seawater, they draw on a vast pool of material, so that enhanced biological productivity need not be linked to oil-shale formation either.  The fact that most organic material in such rocks is structureless kerogen, rather than identifiable particles, also supports this alternative hypothesis.

Both petroleum geologists and palaeoclimatologists have assumed a source rock – ocean anoxia connection in both exploration strategies and assessment of past climate shifts.  So Martin Kennedy et al.’s painstaking findings are sure to cause a major stir.  However, what cannot be avoided is that increased chemical weathering of the continents is likely to accompany globally warm conditions, and they in turn sponsor growth in planktonic productivity.  Likewise, global warmth does not favour the formation of dense, cold and therefore oxygenated sea-surface water, which sinks to aerate deep oceans when the planet is cool.

Measuring erosion rates.

So many landscapes show evidence of changes in the rate of erosion, such as terraces, waterfalls and signs of changing rates of sediment deposition, that a means of accurately measuring rates opens up an important new phase in geomorphological research.  Precise dating of modern surfaces is not possible using stratigraphic or radio-carbon methods, and this has hidden much of landform history.  Once a surface is exhumed, it becomes exposed to cosmic ray bombardment.  These particles travel at near-relativistic speeds, and so have sufficient energy to transmute common element nuclei to unstable isotopes.  The longer the exposure of a surface material, the more radioactive it becomes, albeit very weakly so  Since erosion and sedimentary processes move and quickly bury particles dislodged from a surface, material has a finite time during which it can be irradiated.  The particles themselves carry the isotopic signature of their surface residence time, the slower erosion is the more radioactive are particles derived from the surface.

Cosmogenic dating uses sedimentary grains from sands deposited in a drainage basin, particularly those of quartz that are common and stable.  Oxygen and silicon in silica can become 10Be and 26Al when struck by cosmic rays.  Although sampling is fraught with pitfalls, essentially it amounts to scooping up a handful of sand that represents the past erosion of the entire catchment above a sample point.  Measuring the minute concentrations of new isotopes  costs of the order of $1000 per sample, using a high-energy accelerator mass spectrometer.  Since dozens of samples provide sufficient data for meaningful interpretation, this is not a method that will spread widely to places that come anywhere near fully reflecting the intricacies of erosional shifts over the large age range that cosmogenic dating can address.  Nonetheless, its early results are astonishing.  Work in Idaho suggests that through the period of the last glacial maximum into the early Holocene the average rate of erosion was 17 times faster than it is at present.  That possibly signifies either continual high erosion, that has petered out, or, more likely, that erosion has had episodic, catastrophic pulses.  As might be expected, anthropogenic disturbance of the surface enhances erosion rates, but a cosmogenic study of river sediments in Sri Lanka indicates that 200 years of intensive farming in rugged highland areas have resulted in a 20- to 100-fold acceleration.  Most awkward of all, another study of long-term erosion in California’s Sierra Nevada showed no relation between weathering and erosion rates and climate change.  Geochemists contributing to the debate over climate controls by weathering take note.  It seems that the primary control of erosion rates in western California was purely tectonic, which could tally with the notion that newly rising mountains have a major influence over sequestering of CO2 by silicate breakdown. 

The obvious next step is blending cosmogenic sediment dating with that of crustal exhumation from Ar-Ar and U-Th/He dating of cooling due to uplift and erosion.

Source:  Greensfelder, L. 2002.  Subtleties of sand reveal how mountains crumble.  Science, v. 295, p. 256-258.

Vertical tectonics and formation of Archaean crust

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Since Pentti Eskola’s recognition in 1949 that many Precambrian granitic rocks form domes surrounded by cusp-like synclines of supracrustal rocks, such mantled gneiss domes have been found in most cratons.  Probably the best example characterizes the 3.5 Ga Pilbara province of the West Australian Shield.  How they formed has long been a vexed topic, the most popular views being as a result of low-density basement rising through denser cover that contains abundant volcanic rocks, or as a result of regional-scale fold interference.  Precise dating of the Pilbara granitic rocks and greenstones shows a common age range, with some older greenstones,  The age data suggest that the dome and cusp structure is a product of the co-evolution of both, probably from a primary oceanic-like crust of mafic composition (Zegers, T.E. and van Keken, P.E. 2001.  Middle Archean continent formation by crustal delamination.  Geology, v. 29, p. 1083-1086).

Archaean rocks of broadly granitic composition (dominantly tonalites, trondhjemites and granodiorites, or TTG) have geochemical features setting them apart from post-Archaean varieties.  Rather than signifying their origin by supra-subduction melting of the mantle wedge with fractional crystallization and crustal assimilation in the lower crust (the dominant crust-forming process in post-Archaean times), all Archaean TTG seem to have formed by partial melting of a garnet-rich mafic source.  One of several possibilities is that their source was eclogite.  Based on the peculiar regional structure of the Pilbara and its dominance of the whole crust, as shown by maps of gravitational potential and magnetic field strength, Zegers and van Keken revisit earlier ideas of dominantly vertical tectonics that underlay early crust formation.  They suggest that efficient cooling by hydrothermal circulation allowed thick mafic crust (similar in some respects to that formed in the Mesozoic beneath ocean plateaux) to enter the field of eclogite stability at its base to form a layer denser than ultramafic mantle.  Once sufficiently thick, this layer would begin to founder, or delaminate, to be replaced by hot mantle.  Rebound of the remaining crust would set in motion rapid crustal uplift and extension, together with decompression melting of rising mantle (to form high-magnesium basalts high in the crustal sequence)and melting induced in the remaining mafic crust (to generate TTG magmas).  Indeed, the kimberlites that puncture other Archaean cratons carry abundant eclogite xenoliths from mantle depths.  Seemingly well-documented, this tectonic model does not explain all Archaean crust formation, for other cratons, such as that of west Greenland, are more readily accounted for by seemingly familiar subduction-zone processes.

Is volcanic eruption predictable?

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Inhabitants of the eastern Congolese town of Goma have suffered three disasters in 8 years – the aftermath of the Rwanda massacres of 1994, the episodic war centred on control of Congo’s immense physical resources since 1995, and now the devastating eruption of the Nyiragongo volcano that threatens half a million people.  The last is a grim reminder of the difficulty in predicting geological disasters, and follows closely on claims that spotting impending volcanic eruptions is now “sorted” (Marshall, T. 2002.  There she blows.  New Scientist 12th January 2002, p. 29-31; Horizon, BBC2 17th January 2002, Volcano Hell).  There are four phenomena that have been investigated as signifying threats of  eruption.  Most obvious are increases in temperature at existing vents that can easily be measured using infrared images from daily orbits of meteorological and environmental satellites.  A remote sensing approach is so cheap that it ought to be applicable world-wide, yet most devastating eruptions emerge with insufficient  time following thermal signs for emergency evacuations to begin.  Fundamentally the clearest evidence that magma beneath a volcano is rising is that the edifice swells.  Interferometric radar can detect millimetre-scale changes in surface topography, and such pre-eruption inflation is detectable (see Interferometric radar and faults of the Mojave Desert in Earth Pages, December 2001).  However, the lengthy periods between overpasses by radar imaging satellites (two images are a minimum for radar interferometry), and the need for immensely powerful computer processing has rendered this approach one of retrospection rather than early warning.  Individual volcanoes’ ground motions, and the minute changes in their gravitational potential that also relate to magma movements can be monitored at permanent ground stations, but apart from a select few on which volcanologists conduct long-term research, some thousands of dangerous volcanoes go unwatched.

The central theme of both the Horizon programme and the New Scientist article was a method based on monitoring low-energy seismicity emanating from magmatic movements.  The observation of low-frequency, long-period seismicity  by US Geological Survey volcanologists while Mount St Helen’s was active in 1980 is probably connected to a natural resonance of each volcano as magma begins to move.  Follow-up work at a small number of volcanoes has fine tuned such signals to the timing of eruptions, with sufficient confidence levels that believable warnings are possible.  Believability is essential, for a mass evacuation followed by no threat to life could deter future responses by endangered people, on the “crying Wolf” principle.  Mexican volcanologists were able to give two day’s warning of the immense eruption of Popocatapetl on 18th December 2001, and evacuation prevented any loss of life.  However, none would have been threatened, as it happened, for the eruption on the vast massif was far from habitations.  Yet so spectacular were the fire fountains, that the exercise served to habituate locals to take such warnings very seriously indeed.

Nyiragongo volcano and its companions in the western African Rift regularly erupt low-viscosity lavas that flow quietly over long distances.  They pose less violent threat to life than explosive volcanoes, such as those around the Pacific rim, but chance may channel such flows through inhabited areas disrupting communications and destroying buildings.  Many of the 45 confirmed deaths in Goma arose when people tried to rescue belongings from their engulfed homes.  The current Goma disaster is not one primarily of volcanic origin, but of poverty, poor communications and fragile provision of basic necessities, such as unpolluted water and emergency food supplies.  After the 1994 humanitarian tragedy, and threats from Nyirangongo to the 800 thousand Rwandan refugees camped around Goma, the US Geological Survey and Japanese volcanologists set up seismometers to monitor the volcano’s internal activity.  Five days before the eruption, only two remained functional, yet transmitted signs of abnormal seismic activity (Clarke, T. 2002.  Seismic rumbling foretold Congo eruption.  Nature. v. 415, p, 353).  Despite that, warning did not get through to Goma in time for local people to flee, or any assistance to arrive. There was nowhere for the victims to go and relief followed only days and weeks after the event, when the damage was done.  The same fate hangs over millions of people living in volcanic areas in poor countries – they favour such risky areas to live because of the richness of soils and the encouragement of rainfall by high mountains..  As things stand, communities in volcanic areas of  North America, New Zealand, Japan and a few of the richer 3rd World countries stand a good chance of escaping magmatic events because of believable warnings and efficient communication.  For the majority, survival is a matter of luck alone.

Popper refuted

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In mid-Victorian times, Lord Kelvin peered down his nose at Charles Lyell’s estimation of sedimentation rate from the historic silting of the port of King’s Lynn, as a means to judge the vast time span represented by the stratigraphic column.  His words were not kind; “…when you cannot measure [what you are speaking about], when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind”.  Geologists cringed, particularly when Kelvin went on to reckon an age of 20 to 40 Ma for the Earth based on its cooling from a molten mass, using the physical laws of conduction and radiation.  He was fundamentally wrong on most counts, partly because he knew nothing of radioactive heat generation nor convective heat transfer.  Sadly his corpse could not be revived to eat his mean-spirited words.  Nonetheless, the gibe of Earth scientists’ being “unscientific” has stuck.  We rarely stick to the “scientific method”, reputedly stemming from the Elizabethan philosopher, Francis Bacon and his rationalization of the inductive method of reductionist experimentation.  There are few universal “truths” in Earth history, and the interweaving of limitless processes with a vast spectrum of rates, scales and magnitudes renders reductionism absurd.  Even more prone to reductio ad absurdem is the chemist Karl Popper’s supposedly logical insight that “proper” science rigorously subjects hypotheses to a “risky test”; an experiment that should yield evidence of refutation if the notion is unsound.  Popper’s method of falsification consigns to the dustbin of research any hypothesis which fails the test, with the corollary that in is not “best practice” to seek confirmation for a hypothesis.

Carol Cleland of the University of Colorado (Cleland, c.e. 2001.  Historical science, experimental science, and the scientific method.  Geology, v. 29, p. 987-990) demolishes the “recipe-book” approach to science, which has laid a dead hand on not only the Earth sciences, from the standpoint of philosophy and reality.  She starts from the position of Thomas Kuhn, by pointing out that, for Popper, the whole of Newtonian celestial mechanics should have bitten the dust when 19th century astronomers discovered that the orbit of Uranus deviated from Newtonian prediction.  A sustained search for reasons why concluded that there must be gravitational forces from planets beyond Uranus, and sure enough astronomers discovered Neptune.

There is an air of bullying about the “scientific method”, which has warped investigations and dulled imagination and curiosity for centuries.  It provides ammunition for those who carp and pontificate from the sidelines, and in many cases from positions of considerable power.  Cleland does us all a service by discussing philosophical matters of science in the context of the realities that confront us all, in an accessible way.  Her analogy is Holmesian detection (Sherlock was a deductionist, by the way, proceeding from the general to the particular), which discovers events and proceeds to trace their circumstances – the search, to my mind, for the artillery rather than a single “smoking gun” is far richer than the events themselves, because that deepens our sense of context for particular events, however dramatic they might seem to be.