Author: zooks777

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

Forensic geochemistry to foil “fencing” of conflict diamonds

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The longest and most devastating wars in history have centred rather more on economic interests than nationalism or chivalrous defence of principles, and in some case a specific commodity created an issue that annexation served to resolve.  For instance, the 1914-18 war was not unconnected with the vast iron ore reserves of Alsace-Lorraine.  Similarly, the Nigerian civil war of the late 1960s was bound up with the oil reserves of the Niger delta, and that of Congo centred on base-metal resources of the Copper Belt, particularly the fact that vast strategic reserves of cobalt occur in its Congolese sector.  The running sores of present conflicts in Africa – Angola, Congo, Sierra Leone, Liberia – are about and financed by gems that adorn the rich, the self-regarding and the lazy.  These diamond wars are a direct concern of geologists, for who else finds the elusive kimberlites and traces the natural dispersion of the diamonds that they contain?

More than 30 years on from the start of gem-related carnage in Africa, in which dealers and giant mining corporations have been implicated up to their collective eyebrows, local people have been drawn into “illicit” diamond mining when their livelihoods have been destroyed by perpetual danger and insecurity.  Preyed on by many so-called “rebel” groups, even kids as young as 8 or 9 have been armed and set upon one another and the inhabitants of regions blighted by the presence of what is no more than an allotrope of carbon.  Eugenie Samuel writes on a possible means of defining the source of diamonds “fenced” by the gem trade from on-going conflict zones (Samuel, E. 2002.  Diamond wars.  New Scientist, 25 May 2002, p. 6-7).  It seems that ultra-thin coatings on rough diamonds carry a geochemical signature from the chemically diverse kimberlites and other unusual mafic rocks that carry them from the mantle.  Given research on rough stones from every kimberlite province it should be possible for this forensic approach to help stamp out what is the world’s largest blood trade. 

The problems are many.  For a start, trade in “conflict diamonds” is now illegal, so it is unlikely that rough stones used to calibrate the technique would be given a bona fide provenance by dealers. It would be a courageous geochemist who went sampling in interior Congo, Angola, Liberia or Sierra Leone.  The method clearly requires funds, yet the obvious source, diamond mining and trading companies, are engaged in their own tagging schemes that use using ion beams to bar-code their products on a minute scale.  In fact this tagging method was developed under great secrecy to distinguish from the “real” thing perfect artificial diamond gems synthesized by Russian geochemists.  Finding diamonds requires considerable exploration, which involves systematic sampling of sediments along streams that drain likely kimberlite-bearing ground.  Although high-quality rough found by geologists would be sold, there must be small diamonds archived from such sampling by mining companies and geological surveys.  They could be supplied to forensic geochemists to calibrate the method.  In Sierra Leone, for instance, the diamond fields were located in the early 1950s by geologists of the then Overseas Geological Survey – part of what became the modern British Geological Survey.  Belgian and Portuguese equivalents may well have archival material from Congo and Angola.

Nut-cracking chimps provide clues to the origin of tools

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Ethoarchaeology attempts to use observation or experimental approaches to animal behaviour to shed light on  features of fossil occurrences that relate to human origin.  One example is examining the gnaw marks on bones in the dens of predators to check if they match similar signs on the bones of early hominids.  Another is knapping flints to see if the flakes or debris produced match finds of broken fragments at sites with no clear sign of early-human involvement.  Chimps use lumps of stone to break nuts on wooden anvils, and so provide natural subjects to probe what early hominids may have been up to.  Anthropologists from George Washington University in the USA and the Max Planck Institute for Evolutionary Anthropology in Germany have painstakingly excavated the debris from a nut-cracking site beneath a large tree “traditionally” used as a source of nut protein by Ivory Coast chimps (Mercader, J. et al. 2002.  Excavation of a chimpanzee stone tool site in the African rainforest.  Science, v. 296, p. 1542-1455).

Broken fragments inadvertently created by the chimpanzee troupe do resemble the earliest Oldowan tools, which appear in the fossil record at around 2.5 Ma.  The chimps can be shown to have brought hammer stones from several rock outcrops.  However, any old rock serves their purpose and there is no sign of deliberate selection, unlike the makers of Oldowan tools, who clearly selected rocks that break to give sharp edges from outcrops up to several kilometres from the fossil sites.  The first Oldowan tools demonstrate that they are the end product of what was probably a progression from accidental stone breakage.  The way in which broken fragments from patterns around chimps favourite anvils for nut cracking should help identify earlier assemblages in the steps towards proper tool making.  With luck, they may relate to fossils of the actual beings who were involved.  The 2.5 Ma Oldowan tools from Ethiopia have yet to be linked to a hominid species.  The earliest direct link between tools and their makers is the association of Oldowan artefacts with remains of Homo habilis about 2 Ma ago.

Serpentine: the Vaseline of subduction

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Although they are seismically precarious, the major coastal cities of the Americas and East Asia that lie close to destructive plate margins probably owe their survival to a greasy assemblage of hydrated ultramafic minerals – serpentine, talc and magnesium hydroxide (brucite).  Detailed tomographic images using the records of natural earthquakes along the subduction zone beneath western North America show a zone of exceptionally reduced S-wave speeds at the “corner” formed by the subducted slab and the base of the crust (Bostock, M.G. et al. 2002.  Inverted continental Moho and serpentinization of the forearc mantle.  Nature, v. 417, p. 536-538).  This low-speed zone coincides with the fore-arc region of the destructive margin, roughly along the coast.  Normally the Moho marks a sudden increase in wave speed in the mantle underlying the crust, but here the situation is reversed (inverted).  The best explanation is that S-wave speed slows because of an abundance of weak rock, between 35 and 60 km down.  The likely candidate is mantle peridotite that has become hydrated by fluids seeping upwards from cold, wet oceanic lithosphere as it begins to be subducted.  Low-temperature, high-pressure metamorphism of hydrothermally altered basaltic crust begins to transform it to anhydrous eclogite, so releasing masses of water vapour.  It is this fluid release that is implicated in the generation of magmas beneath volcanic arcs, because it reduces the beginning-of-melting temperature in the overriding mantle wedge.  However, such partial melting is possible only when temperature is high.  In the cooler, shallow regions of the fore arc rising watery fluids serve to convert peridotite to hydrous minerals, especially serpentine.  One outcome is the creation of anomalously low-density mantle, which bulges upwards to create fore-arc ridges at some destructive margins, even squirting serpentinite upwards in bizarre mud volcanoes.  Yet all hydrated, ultramafic minerals are natural lubricants, and would act to ease sudden rupture along the subduction zone, thereby preventing extremely high-magnitude earthquakes whose surface effects would be devastating.

See also: Zandt, G. 2002.  The slippery slope.  Nature, v. 417, p. 497-498

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.

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.

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.