Author: zooks777

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

High- and low-latitude climate changes almost match

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Ten years ago the records of climate proxies from the Greenland ice sheet set new benchmarks for understanding how climate has varied over the last 100 thousand years – annual ice layers allowed division of that data to as fine as decades.  Variations in the ice cores helped explain many of the variations found in more blurred data from sea-floor sediment cores in the Northern Hemisphere.  Variations could be correlated with changes in the formation of North Atlantic deep water at high latitudes and the destabilisation of North American and Scandinavian glaciers.  The whole hemisphere behaved in concert, through long-distance connections in climatic processes, but high-latitude processes seemed to dominate.  Development of 234U/230Th dating extended high precision to carbonates that have been precipitated from groundwater to form stalagmites or speleothem.  The latest results from speleothem, collected on the Indian Ocean island of Socotra, cover 14 thousand years between 56 and 42 ka, and resolve down to only 8 year intervals (Burns, S.J.  et al. 2003.  Indian Ocean climate and an absolute chronology over Dansgaard/Oeschger events 9 to 13.  Science, v. 301, p. 1365-1367).  They show variations in rainfall on the island, though the d18O proxy, and thus changes in the strength of the Indian Ocean monsoon.  In terms of shape, the stalagmite record closely resembles d18O changes in the Greenland ice cores, although the two have opposite senses, because the Greenland proxy is for air temperature above the ice cap.  During the frigid Heinrich events that saw massive southward waves of icebergs, rainfall over Socotra was low.  It became higher as high-latitude conditions warmed in Dansgaard-Oeschger events.  The fine speleothem resolution shows a dramatic change-over that took only 25 years or so.  The explanation is that warmer conditions increased equatorial evaporation from the oceans.  But water vapour is the dominant “greenhouse” gas, and a wetter atmosphere would become warmer.  So the question of whether low- or high latitudes drove the changes is still an open one.  If North Atlantic events were the driver, then the tropical processes would greatly amplify their effects.  One big problem emerges from the joint research by US, Swiss and Yemeni scientists.  The highly reliable U/Th dating gives ages for each event that are about 3000 years older than those interpreted from the ice cores.  The authors are convinced that the ice-core ages need revision, yet there are discrepancies with the event-ages from other similarly dated speleothems.  Commenting on the paper, Frank Sirocko of Johannes Gutenberg University of Mainz in Germany (Sirocko, F. 2003.  What drove past teleconnections.  Science, v. 301, p. 1336-1337) makes the point that maybe the quality and age of ice core records lie behind the widely accepted view that high-latitude process drive climate.  He presents an excellent global image of modern sea-surface temperatures that show the main oceanic shifts of energy – the leakage of cold circum-Antarctic waters northwards, the westward movement of equatorial warm waters to which the El Niño – Southern Oscillation (ENSO) is due, and the unique movement of warm water to Arctic regions in the North Atlantic that is connected to deep water formation.  To that he adds the major effect of continental winter snow cover in central Eurasia, that affects albedo and the size of the winter high-pressure zone there.  Is there a teleconnection between that and events in the North Atlantic?  Nobody knows, because there are no data to compare, yet.  Another uncharted but likely linkage is between the ENSO and processes in the circum-Antarctic current.  Using currently accepted dating of ice cores, records from those in the Antarctic show air temperature changes that precede those from Greenland by several thousand years.  In that respect, the Socotra record possibly has a link with the South Polar climate.  Until the issue of dating is sorted out, it will always be difficult to make concrete statements about global climate change.

Interestingly, in the same issue of Science, sea-floor data (between 9 and 16 ka) from the Cariaco Basin off Venezuela, at about the same latitude as Socotra, mimic the Greenland records to within 30 to 90 years (Lea, D.W. et al, 2003.  Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination.  Science, v.  301, p. 1361-1364).

“Greenhouse” controls challenged

There’s data gathering and there’s theorising.  In palaeoclimate studies the two come into conflict.  Theory suggests that CO2 is likely to be the principal driver for climatic ups and downs, probably on all time scales.  Atmospheric CO­2 estimates from the past are based on proxies of different kind, and the various models that they support do not tally vary well.  Worst of all they do not fit climate records through the Phanerozoic at all well, except in the crudest possible way.  Only the long-lived Carboniferous to Permian “icehouse” and Tertiary cooling tally, and then only in Berner’s GeocarbIII model.  One of the best records of major climate shifts, aside from continental tillites, are marine sediments that contain ice-rafted debris, in particular the palaeolatitudes to which they extend.  They record four major cooling episodes: Late Ordovician; Devonian to Late Permian; Late Jurassic to Mid Cretaceous; and those since about 35 Ma ago.  The oxygen isotope record from Phanerozoic fossils, partly correlated with ocean temperatures also suggest 4 global coolings in the last 545 Ma.  Either the CO2 modelling needs more detail, or the whole issue of the “greenhouse” effect is under question.  That is the conclusion of a study by Nir Shaviv of the Hebrew University of Jerusalem, and Ján Veiser of the Ruhr University and The University of Ottawa (Shaviv, N.J. & Veiser, J.  2003.  Celestial driver of Phanerozoic climate?  GSA Today, Huly 2003, p. 4-10).  Veiser has been analysing the chemistry of carbonates, especially their oxygen isotopes, for his 30 year career, and has amassed more data than any other geochemist on carbonate-related issues.  The two have worked together because their interests fit together extremely well.  Shaviv has reconstructed the variation of cosmic ray flux from studies of the exposure of iron meteorites to them, blended with analysis of how the Solar System moves through the spiral arms of our galaxy.  Cosmic rays are known to affect the Earth’s cloudiness and therefore albedo.  Greater cosmic ray flux should increase the amount of solar energy reflected away by the Earth, thereby causing global cooling.  The degree of fit between the cosmic ray flux and palaeoclimatic records is so good that up to 2/3 of climate variation may be connected with the Earth’s celestial position.  That is, as it passes through the star-rich spiral arms cosmic rays intensities go up.  This happens every 140 Ma or so, which fits very well with the 4 icehouse periods during the Phanerozoic.  They even suggest that the climate-CO2 relationship may be the opposite of that generally agreed; climate might drive carbon dioxide levels.  A secondary role for “greenhouse” gases wreaks havoc on attempts at modelling climate change feared to result from increasing anthropogenic releases.  Shaviv and Veiser’s work comes at a particularly awkward time for climate modellers, who have just initiated a programme for  running huge simulations by corralling the combined computing power of millions of home PC users, similar to the approach pioneered by the SETI Institute (Allen, M.R.  Possible or probable.  Nature, v. 425, p. 242).  Perhaps the view of Phillip Stott, that climate modelling is a complete waste of time (Stott, P. 2003.  You can’t control the climate.  New Scientist, 20 September 2003, p. 25) might sink in as a result of the possible link between cosmic ray flux and climates of the past.  Stott believes that acting on the output of such models might perhaps even be dangerous, since we clearly do not understand short-term climate change well enough.

Precambrian CO2 levels

Whether or not fluctuations in the “greenhouse” effect drive climate change, the fact remains that CO2, methane and water vapour all act to retain solar heat in the Earth system.  Were it nor for their presence in the atmosphere, the Earth would be about 33 degrees colder than it is.  It would be covered by ice.  Theoretical modelling of how stars evolve suggests that the Sun had progressive less energy output going back in Earth’s history.  Only gaseous heat retention could have prevented a sterile, frigid planet.  Yet periods of cooling sufficient to hold large amounts of water in surface ice have occurred only a few times, 4 in the Phanerozoic, a flurry of so-called “Snowball” epochs in the Neoproterozoic and the earliest known glaciation around 2200 Ma ago.  The earliest coincided with the first evidence for free oxygen in the atmosphere, and may have been caused by that.  Methane, a more powerful “greenhouse” gas than water or carbon dioxide and abundantly produced by anaerobic decay, is easily oxidised.  In later time, it has been ephemeral in the atmosphere, unless continuously released, for instance by destabilisation of gas hydrate in sea-floor sediments.  Warming by CO2 has undoubtedly kept total frigidity at bay since then.  The problem is charting just how much was in the air, because most estimates have been based on studies of palaeosols that give odd and very imprecise results for the early Palaeozoic (see Shaviv and Veiser, 2003; previous item).

Photosynthetic organisms derived their carbon from CO2, either in the air or dissolved in water through equilibration with the atmosphere.  The extraction favours lighter 12C, so biological activity results in their products being depleted in the heavier 13C by about 25 parts per thousand (‰) relative to carbon in air and water.  If organic carbon becomes buried, the remaining carbon in the surface environment gets richer in 13C, and that signature becomes fixed in contemporaneous carbonates, both organic and inorganic.  It is therefore possible to use the two carbon-isotope signatures to estimate the reservoir of CO2; its proportion in contemporary air. However, the degree of fractionation depends on the specific carbon metabolism of different organisms, yet most organic carbon in sediments is a mixed product of widely differing life styles.  That severely blurs estimates of atmospheric carbon dioxide content.  What is needed are data from a single source with known metabolism.  Acritarchs are fossil remains of single-celled marine eukaryotes that were, and still are, marine photosynthesisers.  They are made of degraded hydrocarbons.  Advanced ion-microprobe resolution is now sufficient to produce carbon-isotope measurements of individual fossils (about 200 micrometres across).  Sediments from northern China, roughly 1400 Ma old, contain abundant little-altered acritarchs and carbon isotope data from them give good estimates of atmospheric CO2 levels, that are independent of other methods (Kauffman, A.J. & Xiao, S. 2003.  High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils.  Nature, v.  425, p. 279-282).  The estimates suggest between 10 to 200 times higher contents than today, but just about sufficient to keep the Earth above the limit of glacial temperatures when solar luminosity was about 88% of the present.  Acritarchs are present throughout the Neoproterozoic, and it should prove possible to examine the critical periods of “Snowball” conditions using this method.

Another K-T row

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Since the discovery of the buried Chicxulub impact crater off the Yucatán Peninsula, Mexico, many geologists have regarded it as the “smoking gun” for the end-Cretaceous mass extinction.  Such is the heft of K-T studies that money has been raised to drill into the crater and its overlying sediments.  That began in late 2001 at an onshore site on the flank of the structure, and results are starting to emerge.  However, research has been slow in getting underway on the crucial part of the core that goes through the boundary itself.  That section was taken from the project’s headquarters in Mexico City to the Free University of Amsterdam, by Jan Smit, one of the pioneers of K-T boundary studies.  Samples began to reach other researchers in December 2002, 6 months after the boundary section arrived in Amsterdam.  For many, this was a little too slow and suspicions have been raised.  Everyone wanted to get abstracts into the AGU/EGS/EUG bun fight in Nice in April 2003, where a conference session on Chicxulub had been scheduled.  One report presented there seems set to stun the pro-impact school.  Gerta Keller of Princeton University studied foraminifera in the samples immediately above the impact breccia – there were plenty.  She claimed that they represented a period of about 300 thousand years of sedimentation that followed the impact.  Moreover, they occurred below the level of  a thin glauconite-rich horizon, which seems to represent the K-T extinction event itself.  Not surprisingly, Keller concluded that the impact could not have caused the extinction.  Smit dismisses the allegation of “hogging” the core samples, and also suggests that the foram-rich layers represent sediment that was washed back into the crater soon after it formed.  It has always struck me as odd that whenever something startling emerges from scientific research, a sort of preciousness overwhelms supposed scientific “objectivity”.  Counter claims and new variants of ideas rapidly evolve on the periphery of the discovery.  There are reputations to be built, and defended, and of course “sexy” themes attract cash.  The initial work that led to the recognition of a global layer of mass destruction, carried out by the Alvarez father and son team in the late 1970s, was a purer form of science – driven by curiosity and little else.

Sources:  Dalton, R. 2003.  Hot tempers, hard core.  Nature, v. 425, p. 13-14.  McKie, R. 2003.  I’ve got a bone to pick with you, say feuding dinosaur experts.  The Observer, 7 September 2003, p. 22.

Gamma-ray bursts and mass extinctions

There is a Gaelic saying, which roughly translated goes: There are more ways of killing a cat than drowning it in butter.  It seems to apply to mass extinctions.  A team of astrophysicists and palaeontologists from the University of Kansas and NASA, headed by Adrian Melott of the University of Kansas, has found peculiarities in the trilobite record after the Late Ordovician mass extinction (443 Ma) that are difficult to explain by the usual culprits.  Planktonic trilobites were decimated, but those living in deeper water largely came through the extinction.  Graptolites too incurred major changes, only the monograptids surviving until the Silurian.  Many palaeontologists link the end-Ordovician extinctions to global cooling, evidenced by glacial rocks mainly in Africa.  Melott and colleagues suggest that a realistic reason for a depth-related extinction pattern could be due to intense gamma rays emitted by the collapse of a nearby giant star into a black hole.  Although most would be blocked by the Earth’s atmosphere, that would be at the expense of nitrogen oxides being created in large volumes from oxygen and nitrogen molecules.  Nitrogen dioxide, the yellow colorant in photochemical smog would prevent solar radiation reaching the surface and trigger cooling.  Also acid rain would lower the pH of surface water.  Such a process could also explain the Late Ordovician glaciation of Africa.

Source  Hecht, J. 2003.  Did a gamma-ray burst devastate life on Earth?  New Scientist, 27 September 2003, p. 17

Fossil oddities – a golfing trilobite and the ox-sized rodent

Gamblers and golfers do not like distractions, and many wear eye shades of some design or other.  So it is intriguing to learn that a Devonian trilobite, Erbenochile,  found in Morocco evolved a similar device.  Richard Fortey and Brian Chatterton, of the British Museum of Natural History and the University of Alberta, respectively, analysed the peculiar eyes of this phacopid trilobite, and found that their tops had a sort of rim.  Light shining down on the beast put the compound facets in shadow (Fortey, R. & Chatterton, B.  2003.  A Devonian trilobite with an eyeshade.  Science, v. 301, p. 1689).  Not only would this arthropod have been undistracted from its activities by goings on above, but it could also see over its back.

Not since the discovery of the Late Miocene Bullockornis in Australia (see The Ducks of Death in EPN June 2000) have Neogene palaeontologists come up with a record beater.  But now they have (Sanches-Villagra, M.R. et al. 2003.  The anatomy of the world’s largest extinct rodent.  Science, v. 301, p. 1708-1710).  The Late Miocene of Venezuela has yielded a rodent (Phoberomys), whose bones suggest that it weighed in at about 0.7 tonnes.  It is related to modern guinea pigs, and probably had much the same herbivorous habits.  Its teeth suggest that it was grazer too, and like the modern capybara (one tenth the size of Phoberomys) it lived in swamps.  Rodents now rank as the mammalian order with the greatest range of sizes.  Because the digestive systems of mammals cannot efficiently break down the high cellulose content of grasses without the aid of internal bacteria, the bigger their gut, the more efficient they are as herbivores.  So giant rodents make sense as regards their metabolism.  However, they are not as well known for galloping as many other grazers, which is why smaller rodents prefer to escape predation by diving into burrows or among boulders.  That would be difficult for a creature as big as an ox.  Swamp dwellers, like the capybara and Phoberomys, can get away with not being fleet of foot, but would not do well on open grassland.

The compiler of EPN welcomes news of odd and awesome fossils, and hopes soon to learn of mighty hamsters and their adaptation to natural treadmills.

See also:  Alexander, R.M. 2003.  A rodent as big as a buffalo.  Science, v. 301, p. 1678-1679).

Wetting oceanic lithosphere

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Loss of watery fluids from downgoing subduction zones and their rise into the over-riding mantle wedge is the main reason why arc magmas form there by partial melting under high pH2O conditions.  It is usually assumed that all oceanic crust becomes thoroughly hydrated by circulation of seawater shortly after it forms at constructive plate margins.  However, many oceanic basalts from ophiolites or dredged from the ocean floor are very fresh.  It also seems that to explain the depth of fluid-influenced melting in some volcanic arcs, large amounts of water must be coming from the mantle part of the subducted slab.  That is more difficult to hydrate by sea-floor hydrothermal processes.  German and US geophysicists have found abundant evidence for faults oceanwards of where the Cocos Plate bends to descend below the Middle America Trench (Ranero, C.R. et al. 2003.  Bending-related faulting and mantle serpentinization at the Middle America Trench.  Nature, v. 425, p. 367-373).  The faults show up clearly on detailed bathymetric images as wrinkles on the ocean floor off Nicaragua, and high-resolution seismic reflection profiles show that they penetrate deep into the mantle part of the Cocos Plate.  Water can easily make its way down to form serpentinite from mantle peridotites just before the slab plunges down the subduction zone.

Archaean sea-floor hydrothermal fluids

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The circulation of ocean water through new oceanic crust not only cools oceanic lithosphere sufficiently for it to droop and help drive sea-floor spreading.  It also re-emerges as hot submarine springs that today host curious ecosystems, which depend entirely on energy and chemicals that spew out of these “smokers”.  The chemistry of life molecules, particularly the metals in them, reveals a blend that is surprisingly similar to that of hydrothermal fluids.  This, along with other matters, such as the highly primitive genetics of thermophilic bacteria, make sea-floor hydrothermal vents or the crust beneath them excellent candidates for the cradle of life’s origin.  So getting samples of the very earliest such fluids has to be among the most exciting discoveries relevant to palaeobiology.  Jacques Touret of the Free University of Amsterdam, one of the pioneers of fluid inclusion studies, believes that he has found some (Touret , J.L.R. 2003. Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland.  Precambrian Research, v. 126, p. 219-233).  The host rock is an undeformed, but metamorphosed breccia made of basaltic pillows from the famous Isua greenstone belt of West Greenland, which formed about 2.8 billion years ago.  Quartz crystals in amygdales and veins that cement the breccia together contain minute fluid inclusions.  There is little of interest in that fact alone, for most igneous or metamorphic minerals trap samples of the fluids involved in the origin of the host rocks.  What is intriguing abut the Isua fluids is their high content of methane and brine; just as expected from low temperature hydrothermal fluids.  Their chemistry compares well with that of inclusions in altered basalts from modern oceanic crust, in which bacterial activity is implicated.  Metamorphism generally results in carbon dioxide as the main carbon-containing gas in fluid inclusions.  Formation of methane in sea-floor environments can be biologically controlled, but the hydration of deeper ultramafic rocks to serpentine can also generate enough hydrogen to reduce CO2 to methane abiogenically.  The full association at Isua suggests carbon-dominated hydrothermal activity, which today precipitates carbonates at vents, forming so-called “white smokers”.  [“Black smokers” are sulphur dominated, and take their name from the massive precipitation of metal sulphides when the fluids emerge at the seabed.]  These create alkaline conditions that are well suited to bacterial growth.  Touret does not claim that the inclusions indicate living processes, merely that the right conditions were around in the earliest Archaean for life to thrive.  It would be an immense feat if he subsequently discovers bacterial fossils in the inclusions, but that is highly unlikely.  However, the brines might provide proxy evidence, because living cells uniquely accumulate bromine from sea water.  Anomalous ratios of chlorine to bromine might point strongly towards life having been around during Isua times.

See also:  Hecht, J.  2003.  Droplets may reveal life’s oceanic beginnings.  New Scientist, 13 September 2003, p. 25.

Iron and nickel in life’s origins

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The crucial step in assembling amino acids into the proteins that are central to living organisms is the formation of peptide bonds.  Amino acids are found even in meteorites and seem to form abiogenically with some ease.  Peptide bonds link simple amino acids into long chains that are the essence of complex proteins, but this does not happen spontaneously.  The bonds form in the presence of carbon monoxide, but require some kind of catalysis.  Researchers at the University of Munich, Germany have discovered that very fine-grained precipitates of iron and nickel sulphides readily perform such catalytic functions (Huber, C. et al. 2003.  A possible primordial peptide cycle.  Science, v.  301, p. 938-940).  This tallies nicely with one of the co-workers’ (Günter Wächtershäuser) hypothesis for the chemoautotrophic origin of life near sea-floor hydrothermal vents, where Fe, Ni and S are abundant, as is CO in the hot water that emanates from them.

Dinosaurs galore

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They are all at www.dinodata.net, seriously!  Dutch enthusiast, Fred Bervoets puts a vast resource and copious links at anyone’s disposal, even including a forum and a chat rooms.  Technical drawings and artistic impressions of many species are there, together with guides to where specimens can be seen in museums, and major fossil sites.  Skin, eggs, diet, controversies, companion species and sources for replicas……

Setting up subduction

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Although they have roughly the same size and overall density, and probably very similar bulk compositions, Earth and Venus behave in very different ways.  The Earth has plate tectonics, whereas radar images how that Venus has no such phenomenon.  For the most part, Earth loses its internal heat production steadily and plate movements are intimately bound up with that generalised convective heat transfer.  The surface of Venus has seen no significant deformation in half a billion years.  In fact, that surface was probably formed by a massive blurt of magma around late Cambrian times.  In some respects that is similar to the roughly 30 Ma appearance of flood-basalt volcanism on Earth, but on a scale that dwarfs large igneous provinces such as the Deccan and Siberian Traps.  Quite probably, Venus builds up thermal energy in its mantle, until its release by massive partial melting.  The key to Earth’s behaviour seems to be the fact that its oceanic lithosphere is able to break and descend into the mantle.  The gravitational force down a subduction zone is sufficient to keep plate tectonics going.  But why does it start?  Oceanic lithosphere is as strong as that beneath continents, and the other main force involved in plate tectonics, due to the gravitational effect of deepening sea floor as it cools away from constructive margins, is so low that it is unlikely to result in lithospheric failure.  This vital, but often overlooked topic is nicely reviewed by Stephen Battersby, a consultant to New Scientist (Battersby, S. 2003.  Eat your crusts.  New Scientist, 30 August 2003, p. 30-33).

A possible explanation lies in the way in which the strength of the main mantle mineral, olivine, varies with the presence of water.  Even minute amounts of water allow hydrogen ions to enter the olivine molecular lattice, thereby creating defects that can migrate and result in softening of the mineral.  Experimental deformation under mantle conditions, carried out at the University of Minnesota, show ten-fold decrease in olivine’s strength with as little as 20 parts per million of available water.  Subduction at continental margins might therefore be set in motion by the weight of sediments accumulating on the ocean floor, and with time that weight increases as the continents are eroded.  The other factor, perhaps bearing on the start of intra-oceanic subduction that forms island arcs, is the effect of transform faults and fracture zones that separate segments of different age and therefore density.  Maybe that sets up forces that stress the oceanic lithosphere.  The big problem is that the bulk of the oceanic lithosphere, is mantle rock, and when it has been left as a residue by the basalt melting at constructive margins, it is well-nigh anhydrous.  To soften it demands a source of water that permeates the peridotite.  An obvious source is seawater penetration, but at the depths involved any pathways seal up tightly.  Possibly there are wet masses in the deeper mantle, either as a result of earlier subduction or dating back to Earth’s origin.  Slow convection in the deep mantle could bring these into contact with the base of the oceanic lithosphere, where their water could permeate and weaken it to the point of failure.  Just an idea, maybe.  However, seismic tomography, so effective at charting the distribution of hot and cold (low- and high-velocity) mantle rocks, is also able to suggest places where damp, weak rock occurs in the deep mantle.  One such low-velocity blob occurs beneath the eastern seaboard of North America (maybe a relic of the Palaeozoic Iapetus subduction zone that runs parallel to the present margin), where there is, as yet, no sign of subduction.  But there is little sign that the blob is abnormally hot, and in all probability it is damp.  The history of tectonics suggests that no ocean remains with passive margins forever, and inevitably subduction ends up devouring it, in 200 Ma at most (the greatest age of today’s ocean floor).  Given time the eastern USA  may rank with the Andes!

So why does Venus behave so differently?  Although we cannot yet analyse any Venus rock (there are no accredited Venusian meteorites!) there is a plausible scenario.  Venus is the greenhouse planet.  It is highly unlikely that it ever harboured life, particularly of a photosynthetic kind which could have produced free oxygen.  In the Earth’s atmosphere, it is the presence of ozone in the stratosphere that gives the atmosphere its peculiar thermal structure, especially the tropopause.  That marks a sudden cooling that limits the height to which water vapour can rise before freezing out.  In the stratosphere temperature warms up with height, due to the minor “greenhouse” effect of ozone.  Venus probably never has a tropopause, so that clouds of water vapour could rise to the outer limits of the atmosphere warmed by high CO­2 levels.  In contact with ultraviolet light, water dissociates to hydrogen and oxygen, and at high levels the hydrogen leaks away to space.  Any oxygen is quickly drawn down by oxidation of iron at its surface.  So Venus has progressively lost all its water and as a result is a tough nut to crack, as regards forces in its interior.  Earth on the other hand is a bit like a fondant chocolate…

Wandering hot spots

It was once an axiom of plate tectonics that volcanic-island and seamount chains provided robust evidence for sea-floor spreading.  Jason Morgan in 1971 developed the notion, based on a pre-plate tectonic idea by John Tuzo Wilson, that within-plate oceanic volcanic islands derived their magma from upward moving plumes in the mantle below the lithosphere.  Many of them in the Pacific have extinct volcanic islands and seamounts arranged in straight chains that parallel the direction of sea-floor spreading shown by magnetic stripes.  He likened their formation to the burn mark on a sheet of paper passed slowly over a candle flame.  The Hawaii-Emperor chain bucks this hypothesis, by being profoundly bent from a WNW trend in its youngest part to north for ages greater than about 50 Ma.  The problem is that neither leg is at right angles to the magnetic stripes, which does rather suggest that hot spots move.  Hot spots have long been used as a frame of reference for absolute plate motions, but if one has moved then so might all the rest, and how they have moved would probably be independently of one another.  Absolute motions then are hard to judge.  The key to checking on the suspected hot-spot drift is to look at the palaeolatitude of differently aged volcanic rock samples along a chain.  This has been achieved using palaeomagnetic measurements from the S-N Emperor chain (Tarduno, J.A. et al. 2003.  The Emperor seamounts: southward motion of the Hawaiian hotspot plume in Earth’s mantle.  Science, v. 301, p. 1064-1069).  The test proved positive; the hotspot itself moved southwards between 81 to 47 Ma, while the Pacific plate was itself moving.  Other tests suggest that hotspots in the Indian and Atlantic Oceans were indeed fixed for long periods, but the Pacific ones seem to have had a tendency to wander.  Why that has happened is possibly connected to deep mantle flow, which might bend the plumes to which the hot spots owe their magmatic activity.  Maybe their source region in the mantle shifts for entirely different reasons.  Seismic tomography of the mantle has had some success in tracking the shapes of plumes, but not for relatively small ones because of its present poor resolution.  One large plume that has an enormous tilt in the vertical dimension starts near the core-mantle boundary beneath the South Atlantic and hits the lithosphere in the Red Sea.  No-one knows why, but its magmatic expression in the volcanic rocks of east Africa suggest that it too has moved from beneath Kenya about 50 Ma ago, across Ethiopia to its present position that fuels active volcanoes in the Afar Depression of NE Ethiopia, Djibouti and Eritrea.

See also: Stock, J. 2003.  Hotspots come unstuck.  Science, v. 301, p. 1059-1060.

Arsenic threat widens

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The threat of arsenic poisoning from the use of groundwater (see October and December 2002 issues of EPN) is wider that the well-publicised delta of the Ganges-Brahmaputra rivers in Bangladesh (Pearce, F. 2003.  Arsenic’s fatal legacy grows.  New Scientist, 9 August 2003, p. 4-5).  Although springs from rocks that contain arsenic-bearing sulphides, particularly mine drainages, were once the main hazard, increasing use of water from tube wells into alluvium have greatly increased the incidence of arsenic-induced ailments.  This is sadly ironic, because massive investment in well boring since the 1960s aimed at reducing the endemic gastro-intestinal infections and parasites from polluted surface water in many third-world countries.  Arsenic is a cumulative poison, building up to dangerous levels over several years.  So ill-health, including fatal liver cancer, does not immediately appear in populations that are at risk.  Areas in which metals are mined are obvious places where caution is needed in groundwater development, particularly where the ores are sulphides – arsenopyrite is a common waste mineral in gold mining.  However, mines produce relatively small zones of risk.  The alluvium derived from large mountain ranges, in which sulphides occur commonly in sediments and igneous rocks, pose the widest hazards.  That is the case in Bangladesh.  However, reports are emerging of similar problems in the Ganges flood plain in Bihar, India and Nepal, the Mekong Delta in Vietnam, lowland China and the Argentine Pampas, each affecting more than half a million people, together with lesser cases in 11 other countries, including the USA.  Over a billion people world-wide have no access to clean drinking water, and a favoured solution is to develop local groundwater.  The arsenic tragedy is not going to stop that necessary improvement in people’s lives, but rigorous testing for chemical contaminants is now a must.  Also, there are means of cheaply removing arsenic from contaminated water – it is almost totally adsorbed by the iron hydroxides that form rust when conditions are oxidising.  In fact, if wells are driven into zones of oxygen-rich groundwater, dissolved arsenic is rarely apparent – part of the problem in Bangladesh is extraction from levels where groundwater has reducing chemistry.

Senile dementia and copper

The chemical constituents of drinking water vary a lot, according to where you live, and some like arsenic are widely feared.  Having a well drilled into pure silica sand fed with rainwater is not the answer.  Humans get a sizeable proportion of essential elements from the water that they drink, and pure water would result in deficiencies of many elements.  Upper limits for many potentially harmful elements are set legally in some countries, and the World Health Organisation offers useful advice (see http://www.who.int/water_sanitation_health/GDWQ/Summary_tables/Tab2a.htm).  However, little is known about the geochemistry of human health, when it lies within advised limits.  Recent biomedical research reveals a possible link between copper in drinking water and Alzheimer’s Disease (Sparks, D.L. & Schreurs, B.G. 2003.  Trace amounts of copper in water induce {beta}-amyloid plaques and learning deficits in a rabbit model of Alzheimer’s disease. Proceedings of the National Academy of Sciences, 14 August 2003 – online publication).  Two experiments investigating the effects of high-cholesterol intake on rabbits both suggested that beta-amyloid plaques, implicated in human senile dementia, build up with cholesterol intake.  Nothing too surprising in that.  However, the results differed significantly between the two laboratories, one in the USA, the other in New Zealand.  Trying to work out why two labs should get such different results, Larry Sparks of the Sun Health Institute in Arizona discovered that the New Zealand rabbits drank tap water, whereas his were given distilled water.  The US rabbits had significantly less plaque build-up than those studied in New Zealand, so perhaps water chemistry had an input.  Sparks and his colleague varied the copper content of their rabbits’ water, and found that even with one-tenth the maximum safe concentration advised by the WHO, plaque built up 50% faster in the hapless animals.  However, it is early days in this research.  Cells possibly contain numerous mechanisms that fight off accumulation of potentially harmful elements, and perhaps the plaques implicated in Alzheimer’s play such a role.  One line of investigation is to check records of the incidence of Alzheimer’s against local water chemistry, but both kinds of record, even in well-heeled countries like the USA and Britain, are rudimentary to say the least.  If there is a risk, it is likely to be highest among people who use local well water in metal mining areas, or where bedrock includes sediments that contain high copper concentrations, sulphidic shales being a widespread example.

Source: Marx, J. 2003.  Possible role for environmental copper in Alzheimer’s Disease.  Science, v. 301, p. 905

Setting the fossil record to rights

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Much has been made of ups and downs in the diversity of life from the global fossil record of the Phanerozoic, including the possibility of massive downturns in diversity related to a variety of cause for mass extinction.  However, there are many biases in what is an inevitably imperfect record of biodiversity.  There are anthropogenic influences, for a start.  Although they are becoming more adventurous, palaeontologists cut their teeth on sites close to home, and most of them live in the richer parts of the world.  Insatiable demand for fossils, but mainly of the spectacular and valuable kinds, has grown a world-wide industry of commercial fossil mining.  That may homogenise the geographic coverage of the fossil record, but it is very tempting to go for the richest troves and ignore meagre pickings.  Sedimentation is by no means guaranteed to have been constant through time, partly because of ups and downs of sea level and changes in the pace of erosion of earlier rocks.  Although Phanerozoic stratigraphy seems complete when sections from all over are pieced together, in any one place there are huge gaps of erosion or non-deposition.  It is very easy to come upon several  beds of sedimentary rock and conclude that the sequence represents a continuum in time.  Not so, as any examination of such beds forming today often reveals that intact preservation is the exception compared with erosion and reworking.  The global areas of exposed rocks that cover, say, 10 Ma chunks of Earth history is by no means constant either.  Another factor that conspires to cast doubt on the veracity of the existing fossil record is that the numbers of possible ecological niches that once existed in different tectonic environments are probably not the same.  Active oceanic arcs have few such niches, whereas tropical zones of shallow shelves have vastly more.  There are lots more uncertainties, and New Zealand palaeontologists have painstakingly tried to develop some means of allowing for them in the Tertiary record of their islands (Crampton, J.S. et al. 2003.  Estimating the rock volume bias in paleodiversity studies.  Science, v. 301, p. 358-360).  The simplest premise for estimating bias in the numbers of taxa preserved in rocks covering a particular time range is the available volume of rock from the period that can be sampled.  One approach is to see how geologists have divided up that period in terms of distinct rock formations, the other just uses estimates of the areas underlain by sedimentary rocks laid down during the period.  The first suggests that collecting should be systematically from formation to formation up a sequence, while the second implies that random grid sampling is the best approach.  The New Zealand data suggest that the area approach is most appropriate there, largely because the local rocks formed in a sedimentologically simple, active-margin environment.  Both methods seem to work in tectonically stable areas.  This is just a beginning, but is raises the issue of how much weight can be placed on existing fossil collections in pondering on both titanic and slow-but-sure episodes in the last 544 Ma.

On the same tack, attempts are underway to correct the entire fossil record from 30 thousand collections, using a similar approach to sampling bias.  John Alroy at the University of California, Santa Barbara has helped set up the Paleobiology Database (http://flatpebble.nceas.ucsb.edu/public/), following prompting by the most prolific fossil cataloguer, Jack Sepkoski, shortly before his untimely death in 1999.  The web site allows anyone to generate diversity curves, but the process is a little complicated and best tackled by experienced palaeontologists.  You can also enter information from your own collections.  Early results are conflicting.  Sepkoski’s original suggestion that diversity among marine faunas increased since the Triassic may be an artefact of the intensity of sampling which varies from age to age.  However, using just molluscs seems to confirm that at least they did indeed radiate tremendously as Sepkoski had concluded (Schiermeier, Q.  2003.  Setting the record straight.  Nature, v. 424, p. 482-483).

Origins of the vertebrates

Long before techniques were developed to investigate the genetic stuff of living organisms, and when the only known repository of primitive, soft-bodied animals was the Burgess Shale, basic anatomical analysis suggested that maybe the ancestors of vertebrates were worms, sea squirts and even echinoderms.  When the Burgess Shale fauna was re-evaluated and extended in the 1970’s by, among others, Simon Conway Morris of Cambridge University, it became clear that the fossil record was missing a great many delicate and sometimes very odd organisms.  Entirely unsuspected phylla numbered among the occupants of that famous lagerstätte (site of exceptional preservation), but little new about our own ultimate origins.

Vertebrates, echinoderms, sea squirts and a diverse collection of worm-like animals have one thing in common, though apparently very little else.  The first opening to emerge during embryonic development is the anus, whereas in the rest of the animals (protostomes) it becomes a mouth.  So, in the “supergroup” to which we belong, mouths appear at a later developmental stage; hence the sack-name “deuterostome”.  This oddly dichotomous embryonic unfolding points to a very early division among the animals, that might only be unveiled by discovery of even earlier lagerstätten than the Late Cambrian Burgess Shale.  So far, no such source of palaeontological richness has been discovered in late Precambrian sedimentary rocks – crude “molecular clock” approaches to genetic divergence suggest that a great deal went on before the Cambrian Explosion at 544 Ma.  However, the fossil-rich Cambrian of China does push back the record of delicate animals almost to that time.  The recently discovered lagerstätte of Chengjiang is about 530 Ma old, and, as Conway Morris and his Chinese colleagues have discovered, it is rich in fossil deuterostomes.  One group, the vetulicolians, bears a remarkable resemblance to what the pioneer vertebrate palaeontologist, Alfred Romer, suggested as a probable vertebrate ancestor – something with a front end bearing gill slits and a long, segmented tail.  The Chengjiang deposit also contains jawless fish, together with unique “almost fish” called yunnanozoans that may be intermediate links between vetulicolians and fish.  Similarly, there are intriguing hints that vetulicolians evolved towards the most primitive echinoderms, with bilateral symmetry rather than the fivefold form that emerged later.  Clearly, the Chengjiang fauna was extremely diverse and therefore had a long evolutionary history.  Since even more delicate, entirely soft-bodied Ediacaran animals were preserved as imprints in sandstones from the Late Neoproterozoic, it is maybe only a matter of time before low-energy lagerstätten are found from that time.  There are abundant undeformed mudstones from that period throughout the world, but only painstaking rock splitting will find such treasures, unlike the large, “trip-over” Ediacaran trace fossils.

Source:  Conway Morris, S.  2003.  Once we were worms.  New Scientist, 2 August 2003, p. 34-37.

Zircons that wander

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The crust beneath the British Isles is made up of several once widely separated terranes, parts of Laurentia, an arc segment called Avalonia that split from Gondwana around 500 Ma ago, and a similar terrane (Armorica) that followed Avalonia across the Iapetus Ocean to accrete to Laurentia at the end of the Palaeozoic Era.  Because of its maritime position, modern Britain is cloaked in vegetation so that rock occurrences are few and far between by comparison with less humid areas.  Conditions for geological investigations are made yet worse by a mantle of glacial sediments plastered on top of bedrock.  So, although having been studied for longer than almost every other piece of continental crust, the evolution of that beneath the British Isles is a subject of continual controversy and surprises.  Sitting at the interface between the Laurentian and Avalonian terranes, roughly where the Iapetus suture is thought to have consumed at least half of the eponymous ocean, sit the Lower Palaeozoic rocks of the Southern Uplands of Scotland.  They are widely thought to have formed as an accretionary prism on the edge of the plate underidden by subducted Iapetus oceanic lithosphere until Avalonia collided with the north-British terranes at the close of the Silurian.  Some of the Ordovician sediments in the pile contain clasts of volcanic rocks, which were long thought to be contemporary and giving evidence of the expected arc volcanism behind the prism.  However, they turn out to be much older, now that zircons from the sediments have been dated using high-preciiision methods (Phiilips, E.R. and 7 others 2003.  Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland.  Geology, v. 31, p. 625-628).  The zircons yielded Neoproterozoic ages (557 to 613 Ma), with evidence that some had been assimilated from older crust (1043 Ma) during volcanism.  Taken at face value, the Neoproterozoic ages are similar to those of volcanic rocks in England and Wales, which formed off Gondwana in an arc setting, when the terranes were widely separated.  The problem is one of getting the material across the subduction zone that separates the accreted terranes, but that is the issue proposed by the authors (all from the Natural Environment Research Council.  However, such a conclusion might stem from the authors’ narrow context; that of British geology.  Immediately to the north of the Southern Uplands terrane is another, poorly exposed crustal block that underlies the Scottish Midland Valley.  It was directly involved in the Ordovician Grampian orogeny that formed the highly deformed Precambrian rocks of the Scottish Highlands.  With a narrow view, that terrane is also a mystery, yet it has a counterpart in the Taconia terrane that is familiar to North American geologists, which was involved in orogenic events contemporary with the Grampian orogeny in Scotland.  Taconia has late Neoproterozoic to Ordovician arc volcanics.