Author: zooks777

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

Universal access to peer reviewed articles?

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Scientists without access to libraries that subscribe to scientific journals, or whose institutions are poorly funded, are cut off from the mainstream of research developments.  That is, unless they request offprints of papers from authors.  The growth of electronic versions of journals and increasing access to the Web, even in poor countries (Eritrea recently went “on-line”) seemed to promise wider availability of primary sources of research information.  That is an illusion.  Unless you are a subscriber to paper journals (for instance Nature and Science subscribers automatically get free access to on-line versions) or are registered with a library that has subscribed to all electronic journals made available by a publishing house, such as Elsevier’s Science Direct, then downloading more than an abstract is on a pay-per-view basis.  (Note:  even the Web of Science, that hosts the Science Citation Index database, requires a paid-for user id and password).  Being a university academic in a rich country, I have the luxury of free access to many electronic resources through the Open University Library’s subscriptions, and the same goes for any of our students.  However, the economic facts of academic life occasionally rear up.  A reference to an interesting paper in the Journal of Human Evolution came to my attention.  Using my id and password for the publisher’s web site, I was able to locate the entry for the paper.  However, we do not subscribe to that journal, and to download an Adobe Acrobat PDF file would have cost me about £35, charged to my credit card.  Instead I requested an offprint from the authors, and am still waiting for its arrival after 2 months.

Authors provide papers free of charge to publishers of journals, referees review submissions without payment, and many editors compile issues for little if any return, other than satisfaction and kudos.  Publishers of journals make enormous profits, and increase subscriptions at rates far above that of inflation (one veterinary science journal increased in price by 7 time between 1991 and this year).  The average total income received by publishers of the roughly 20 000 scientific journals for each one of the 2 million papers published each year is around US$2 000 – the trade has a US$4 billion annual income.  In the Earth sciences annual subscriptions are beyond the budgets of most 3rd World institutions (6 issues of Elsevier’s Journal of African Earth Sciences cost £1003), apart from a few (the University of Chicago’s Journal of Geology costs £79 per annum).

The Public Library of Science  – http://www.publiclibraryofscience.org -is campaigning for a way out of the increasing cost for freedom of access to scientific information.  One simple and foolproof strategy is for authors to “self-archive” their preprints and manuscripts of published papers in their institutions’ “e-print” archive in such a way that they can then all be harvested into a global virtual archive, its full contents freely searchable and accessible online by anyone.  Stevan Harnad of the University of Southampton is one of the driving forces for the self-archiving initiative, and provides full details of the possibilities at http://www.cogsci.soton.ac.uk/~harnad/Tp/nature4.htm

See also:  Harnad, S.  2001.  First Person: In the name of freedom.  New Scientist, 26 May 2001, p. 53.

Dinosaur update

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BBC-2’s Live from Dinosaur Island (4-16 June 2001) brought palaeontology into Britain’s living rooms.  Centred on a frantically excavated series of Jurassic sites on the Isle of Wight, and fronted by the irrepressible Bill “Birdman” Oddie and genuinely excited (and sometime irascible) professional palaeontologists, the series used the now familiar approach of Channel 4’s Time Team, with the added frisson of being unedited and live.  The BBC was in debt to its viewers after the truly dreadful, if visually astonishing, Walking with Dinosaurs, and has repaid them handsomely by showing the bone-people working in their natural habitat.  It should help repopularize geology after a century of our being the brightly coloured anoraks seen dimly in the drizzle.

Dinosaurs are perhaps the main link between the popular imagination and the Earth’s past.  However, leaving them at the level of awesome animals that a comet strike snuffed out 65 Ma ago may enthuse, but does not really educate.  Live from Dinosaur Island began to break the T rex – My Little Pony connection, by also showing how we can recreate the environments that long-dead creatures inhabited, and how they changed.  Climate and life (above), hints that dinosaur breath may even have affected climate during the Mesozoic.

Barely a month passes without dinosaur news.  The latest concerns the rediscovery of the Egyptian site, from which Ernst Stromer von Reichenbach gathered  a rich collection of animal fossils between 1911 and 1936.  Stromer’s collection, housed in the Bayerische Staatssammlung museum in Munich, was destroyed by wartime bombing.  Because Stromer left no clues regarding the precise location of his site, except that it was near the Baharyia Oasis in the Western Desert, it seemed unlikely ever to be found again.  A team from the University of Pennsylvania, let by Josh Smith, more or less tripped over the site by luck, when combing the area for coastal Upper Cretaceous sedimentary outcrops, after Smith’s inspiration by Stromer’s monographs (Smith, J.B. and 7 others 2001.  A giant sauropod from an Upper Cretaceous mangrove deposit in Egypt.  Science, v. 292, p. 1704-1706).  The highlight of their excavations is Paralititan stromeri, a sauropod reckoned to be the second most massive animal that lived, after South America’s Argentinosaurus.  The tidal sediments also yielded a diversity of lesser animals that matches and will certainly transcend Stromer’s destroyed collection.

See also:  Stokstad, E.  2001.  New dig at old trove yields giant sauropod.  Science, v. 292, p. 1623-1624.

Doubts cast on the increase in diversity with time

The late John Sepkoski of the University of Harvard painstakingly spent 20 years trawling the palaeontological literature to build an archive of the duration of every marine fossil known.  Others did similar work for terrestrial fossils, but Sepkoski’s database stands out, head and shoulders, for its comprehensiveness.  It is largely from his work that the record of extinction events took on semi-quantitative form.  Plotted against Phanerozoic time, his counts of genera also seem to show patterns that chart the fluctuations of biodiversity; rapid rise from the Cambrian Explosion to plateau in the mid-Palaeozoic, a decline in the late Palaeozoic and early Mesozoic, and then a post-Jurassic explosion in diversity.  Much speculation has hung on Sepkoski’s empirical data, such as the influence of “modern” evolutionary designs on the number of ecological niches that life can exploit.

Enormously important as Sepkoski’s work was, inevitably it rested on the selective nature of fossil collecting, itself partly determined by the variable quality and quantity of preservation, but also by the limited numbers of active palaeontologists, the manner in which they worked and their selection of sites.  There are gross biases in fossil collections, but how can archivists possibly allow for their influence?  Without a superhuman effort to re-collect more intensively, to plunder every conceivable stratum wherever it crops out and perhaps standardise what is meant by a genus, the only available means is through statistics.  Palaeontologists at the universities of California (Santa Barbara) and Harvard, led by John Alroy and Charles Marshall respectively, are compiling information along more comprehensive lines than did Sepkoski, including the dimension of geographic occurrence as well as duration, in the Palaeobiology Database.  Their first attempts to allow statistically for the welter of biases, published in the 25 May 2001 issue of Proceedings of the National Academy of Sciences, all point in the same direction.  The Cretaceous to Tertiary genera show patterns of change that are little different those for the Silurian to Carboniferous, compared with Sepkoski’s suggestion of explosive diversification in the first and a plateau in the second.  The main problem remains; vast as they are, fossil collections are not truly representative of life in the past.

Source:  Kerr, R.A. 2001.  Putting limits on the diversity of life.  Science, v. 292, p. 1481.

Earth System Processes conference

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Geoscientists from all over the world attended and spoke at this seminal meeting.  Its theme was moving from isolated studies to those linking contributions from many branches of science, thereby attempting to match the complex web of interactions on every scale and at every pace that is the essence of the way the world works and has evolved.  Topics covered a huge range, from the “Snowball Earth” hypothesis to issues linking life and inorganic processes, deep mantle processes to sea-floor hydrothermal systems, molecular palaeobiology to the fossil record.  Indeed, the scope is too broad for me adequately to summarise here.  While it remains active, readers can read all the abstracts of papers presented at: http://www.geosociety.org/meetings/edinburgh/prog.htm

Late Pleistocene mass extinction

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The recent fossil records of the Americas and Oceania are littered with species that became extinct in the last 100 000 years.  The majority of them are large animals whose body weights were greater than 45 kg – part of the megafauna.  While controversy rages about the date of entry of the first humans into both vast regions, for a long time archaeologists have suspected that the appearance of sophisticated hunters was somehow connected with the rapid decline in what would have been prey species.  One theory is that having never encountered weapon-bearing bipeds, large mammals were “naïve” and thus easily slaughtered.  Most visitors to the Americas and Australia soon notice how unafraid many animals are of humans, compared with their behaviour in Europe and Africa.  The suddenness of  the selective extinctions (around 15 to 11 000 and 47 000 years ago in North America and Australia respectively) is astonishing, if the cause was small bands of hunters, and other workers have suggested that human entry brought diseases that wiped out species susceptible to them, but with no immunity. The third main theory is that a sudden shift in climate wrought havoc among large herbivores and predators, by producing a change in vegetation.  The last is difficult to support for the Americas as the extinctions were in a period of increasing warmth and humidity following the termination of the last glacial period.  As always, new information from research directed at the problem has narrowed the choices, but revealed complexities.

Modelling the influence of changing predation on prey stems from the mathematical simplification of reality by Lotka and Volterra, in which “boom and bust” events pop out of the simulations.  John Alroy of the University of California applied an advanced version of the basic model to the likely effects of advanced hunters appearing suddenly in North America (Alroy, J.  2001.  A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction.  Science, v. 292, p. 1893-1896).  His model assumes slow human population growth, random hunting and the least possible effort – a conservative approach.  The results closely parallel the record, if human population expanded from 100 first entrants about 14 000 years ago to almost 1 million 750 years later, and suggest that a steady state population of around half that co-existed with the surviving fauna until the appearance of Europeans and their culture.  It is an entirely mechanistic model, but mimics what happened without recourse to any other influence, such as climate change.  So far, no human site in the Americas has been convincingly dated before 14 000 years ago.

Dating is even more of a problem in Australia, particularly for human arrival.  The earliest dated fossil is 60 000 years old (see Out of Africa hypothesis confounded? EP Feb 2001), but claims have been made for artefacts at least twice that age.  Alroy’s model applied to Australia would demand extinction (24 out of 25 genera Pleistocene megafaunal species) shortly after earliest arrival.  A large team of Australian scientists (Roberts, R.G. and 10 others 2001.  New ages for the last Australian megafauna: continent-wide extinction about 46 000 years ago.  Science, v. 292, p. 1888-1892) have systematically dated the age of burial of extinct faunas at 27 sites in coastal areas and the more humid SE of the continent (none from the vast, arid “red centre”), and one in Papua New Guinea.  The most likely interval for the extinctions, between 39 800 and 51 200 years ago, bears no relation to extreme aridity during the last glacial maximum, so the data weigh against that climatic cause.  However, the last 100 000 years have seen lesser, but still extreme shifts in climate, so climate change cannot be ruled out.  Though the authors also do not rule out humans eating their way through Australias bizarre megafauna, the lag between evidence for first entry and the extinction seems far longer than that in the Americas.  Closer inspection of their data, however,  does show precise 230Th/234U ages (+ 600 to 2 200) from 33 600 to 60 000 from 3 sites, and less well-constrained luminescence ages (+ 200-21 000) from 16 000 to 171 000 years from all the sites.  Applying simple statistics to samples from such a wide spread of localities does not seem justified to me – normal practice is for ages at individual sites to be accepted as dates within the errors of the method used.  Australia’s megafaunal extinction seems to have been protracted.  Using fuzzier dating of the extinction, earlier workers correlated it with evidence for an increase in bush fires marked by ash in offshore sediments.  Much of Australia’s flora is fire resistant, and the seeds of some species require light burning before they will germinate.  The most popular theory for the extinctions there is through deliberate fire setting by hunters – a culturally induced decline unique to Australia’s peculiar climate and terrain.

See also:  Dayton, L.  2001.  Mass extinction pinned on Ice Age hunters.  Science, v. 292, p. 1819.

Where do subducted slabs go?

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Geophysicists and geochemists are generally opposed on what happens to subducted lithosphere.  Seismic tomography of the deep mantle shows convincing evidence for slab-like cold bodies down to the core-mantle boundary, yet differences in trace-element and isotopic signatures of volcanic rocks formed at ridges from shallow mantle and ocean islands that relate to deep plumes persuades geochemists that restriction of convection within the upper mantle, at the 660 km deep discontinuity, best explains the differences.  There are other models that might account for geochemical differences, such as heterogeneities throughout a poorly stirred mantle or because material in slabs subducted to the bottom of the mantle rarely rises again, but displaces more pristine materials upwards.

The more earthquakes that seismographs detect and locate, the better geophysicists are able to map in 3-D the zones on which they take place.  One destructive margin long known to have aberrant seismicity is the northern part of the Tonga system in the Pacific Ocean.  This is where the fastest subduction anywhere consumes lithosphere that has little time to warm up while it descends – surely a site for slabs to fall steeply into the deep mantle.  Much of the Tonga system shows the expected zone of steeply plunging Earthquakes, yet west and north-west of Fiji there are earthquakes that do not fit the regional pattern.  They are far too shallow to result from motion on the main subduction zone.  By detailed analysis of seismic data Wang-Ping Chen and Michael Brudzinski have revealed a strong possibility that a piece of old subducted slab has slid to the 660 km discontinuity since it parted company with the now rapid and steep motion at the Tonga trench (Chen, W-P. and Brudzinski, M.R. 2001.  Evidence for a large-scale remnant of subducted lithosphere beneath Fiji.  Science, v. 292, p. 2475-2478).  If such behaviour turns out to be more widespread, large volumes of old lithosphere may indeed sit at the discontinuity, satisfying many geochemists as a means to maintain very old differences in composition of the mantle.  The problem is, increasingly good resolution in seismic tomography has so far failed to detect the tell-tale high seismic velocity signature of such cold slabs.  Chen and Brudzinski suggest that they may be “invisible” to this method, because of their mineralogy – perhaps the crustal lithosphere has not equilibrated to eclogitic materials, or is given neutral buoyancy by being heavily hydrated.

Between a rock and a hard place

Plate theory stems from the notion that the lithosphere is overwhelmingly rigid and deforms only at the boundaries between plates, particularly at destructive margins.  The Earth’s seismicity is overwhelmed by earthquakes at discrete boundaries, and the mapping of seismic events along narrow lines by the world-wide network of seismographs (set up as a means of pinpointing nuclear weapon tests) formed on of the main planks in developing the theory of plate tectonics.  The plate whose evolution drove India into Asia bucks this definition.  It has long been known to host seismicity well inside its boundaries.  Oceanographic work has slowly built up a means of relating Indian Ocean seismicity to plate structure, whereas analysis of earthquake first motions from seismographs reveals that the deformation differs between various block of the ocean floor.  The plate suffers folding and thrusting, and transcurrent motions along ancient transform faults, such as the Ninety East Ridge.  The most likely explanation for the Indian plate’s aberrance is that sea-floor spreading from the ridge separating the Indian Plate from that carrying Antarctica can no longer be accommodated by subduction of the subcontinent beneath Asia, whereas it can be taken up by subduction beneath the Java-Sumatra island arc.  The Central Indian basin is being compressed, and must deform in some way, perhaps eventually to become a new subduction zone.

Source:  Deplus, C.  2001.  Indian Ocean actively deforms.  Science, v. 292, p. 1850-1851.

Life on Earth even luckier than we thought?

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Continually improving resolution of telescopes is now beginning to reveal signs of planetary systems around other stars.  Because their gravitational effects on stellar motion are detectable, the 60 or so known planets in distant stellar systems are all gas-giants, similar to but bigger than Jupiter.  Surprisingly, calculations show that such massive planets are in very different orbits than those in the Solar System.  Their orbits are highly eccentric, and bring them remarkably close to the star, unlike the almost circular orbits in the Solar System.   Yet, if they are mainly gaseous, they must have formed far from the warming influence of their companion star, as did Jupiter, Saturn, Uranus and Neptune.  Somehow, they have been gravitationally perturbed over the billions of years of evolution of the stellar systems.

How, then, did such bodies move inwards?  One possibility is that they exchanged angular momentum with smaller, rocky planets, forcing both into eccentricity.  For the smaller bodies the effect would be more dramatic, potentially either flinging them into interstellar space or into collision with their star.  Spanish and Swiss astronomers using spectroscopes at an observatory on the Canary Islands have discovered a large lithium anomaly in the spectrum of one star with such an aberrant gas giant (Israelian, G.  et al. 2001.  Evidence for planet engulfment by the star HD82943.  Nature, v. 411, p. 163-166).  Because the anomaly is accompanied by greater than usual abundances of many elements heavier than helium, and because lithium is quickly consumed as stars “ignite”, Israelian and colleagues conclude that the star has engulfed an Earth-like planet.

If such processes are common, and theory suggests that it may be, our Solar System could be one of very few in which potentially life-building and sustaining planets had sufficient time to develop a biosphere.  It seems that the more small planets there are between a star and an outer gas-giant, the more likely it is for such perturbations to take place.  The Solar System has only four, and calculations using Jupiter’s mass and orbit point to a minute tendency for such eccentricities to evolve.  Looking on the bright side, at least for those committed to a view of life pervading the cosmos, current observational resolution is only able to detect giant planets in wildly eccentric orbits.  Many planetary could be more stable.

Se also:  Samuel, E.  2001.  Banished forever.  New Scientist, 12 May 2001, p. 15.

Late-Palaeocene red tides?

About 55 Ma ago, in the late-Palaeocene, the carbon-isotope record shows a sudden drop in 13C, signifying a sudden release of methane from ocean-floor gas hydrates or clathrates.  That period also reveals evidence of s brief global warming, against the general trend of cooling through the  Tertiary.  Since the discovery of this massive discharge of the “clathrate gun”, palaeontologists have looked for ecological effects in sea-floor sediments.  For them to be significant, it is important that climate-related ecological effects occurred at the same time in widely separated parts of the globe.

Geologists from the Netherlands, Denmark, New Zealand, Austria and Sweden have examined the microfossil record from two late-Palaeocene sequences in Austria and New Zealand, and show such synchronicity (Crouch, E.M. et al.  2001.  Global dinoflagellate event associated with the late Palaeocene thermal maximum.  Geology, v. 29, p. 315-318).  Exactly at the time of the d13C dip in both sections, the abundance of cysts of single-celled phytoplankton known as dinoflagellates rose dramatically, only falling when carbon isotopes recovered to usual levels.  The authors link this to exceptionally high surface-water temperature and photosynthetic productivity.  Over the same period, the fossil record shows a mass extinction of benthonic organisms, and noticeable turnover and diversification of plankton and mammals, though not as dramatic as other biological events.

Today, dinoflagellates explode in numbers, along with other phytoplankton, under similar conditions and when nutrients increase in surface waters, to create phenomena known as “red tides”.  Because some species of dinoflagellates produce potent neurotoxins, “red tides” often result in massive death of marine animal life.  The effects linger as such toxins build up in the cells of animals, such as bivalves, which survive the bloom.  The air above such blooms is filled with stinging, choking aerosols, not far different from nerve gas.  Rotting of dead organisms causes oxygen levels in local seawater to drop, further adding to the death tool at deeper levels.  Red tides that result from human input of nutrients in sheltered embayments often sterilize them for long periods.

Although it is impossible to tell if such neurotoxins built up during the late-Palaeocene thermal maximum, that is not an impossibility.  Such biological “warfare” (no-one knows why some dinoflagellates produce the toxins) might explain the biological crisis that accompanied methane release.

A broader view of the Permian-Triassic mass extinction

That the Palaeozoic Era ended in the greatest mass extinction is well know, although why it happened is still a topic of fierce debate.  Part of the problem is that its effects on land and in the oceans emerge from studies of widely separated P-Tr sections, and many of these are extremely thin.  Such condensed sequences are notoriously difficult to resolve in terms of relative and absolute timing, as well as to correlate from place to place.

As with much else, Greenland promises to throw light on the end-Palaeozoic events, thanks to a 700 metre sequence of siliciclastic sediments in East Greenland that spans the Permian-Triassic boundary without a break.  Its most exciting feature is the way in which marine and non-marine sediments interleave with one another.  Geologists from the USA, the Netherlands, Australia and Britain have pieced together the evidence of biological change from a small part of this little described occurrence (Twitchett, R.J. et al.  2001.  Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis.  Geology, v.  29, p. 351-354).

In marine sediments, the Permian biota collapse, together with evidence for disturbance of the sediment structure by burrowing , in a mere 50 cm of the almost 40 metre sequence that the authors analysed.  Over the same interval, pollens of Permian land plants also fall dramatically, but all the pollen types linger through the overlying 15 metres.  Only at a level 25 metres above the biotic collapse do  fully Triassic faunas and floras appear.  From estimates of the rate of sedimentation the marine and terrestrial collapse appears to have taken between 10 and 30 ka.  Oddly, the now well-known fall in 13C does not coincide with that in the biota.  The authors visualize two possibilites: that it resulted from the collapse itself, or reflects an external factor that played little or no role.  One interesting scenario that they suggest is that it may indicate a major release of methane by breakdown of gas hydrates (a now increasingly popular mechanism!).

Conferring strength to cratons

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Considering the continual processes that stress continental lithosphere from the time of its formation, it is a puzzle to find large areas that preserve its earliest parts in an almost pristine state.  Greater heat production in the past demands that the frequency and power involved in continental jostling were greater as we go back in geological time.  Zones that show little sign of having been tectonically reworked for more than a billion years are termed cratons, and most of them have at their core continental material that formed in the Archaean, more than 2.5 Ga ago.  Later orogens do show isotopic signs that deformed and partially melted Archaean crust was involved, but no so much as might be expected.  Somehow, having a nucleus of Archaean lithosphere confers strength to cratonic areas.  Geophysics reveals that  the lithosphere beneath cratons uniquely extends to depths of 200 km, forming a “keel” or tectosphere.

Most geochemists consider that deep mantle beneath cratons is so rigid because it is unable to come close to the beginning of melting, due to it having once been the source of massive amounts of basaltic magma.  Loss of the constituent elements of basalt and volatiles, including heat-producing isotopes of U, Th and K, renders it more inert than mantle that still has the potential to generate basalt under appropriate conditions.  Basalt magmas also remove significant amounts of iron, thereby adding buoyancy to tectosphere materials.

Occasionally, much younger magmas that do form at the depths of the tectosphere bring samples of it to the surface, in the form of xenoliths.  Their petrography and geochemistry reinforce the general idea of how cratonic “keels” form, but they have been difficult to date with confidence.  The relatively new rhenium-187/osmium-187 method makes dating more assured.  Cin-Ty Lee and colleagues from Harvard University (Lee, C et al.  2001.  Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.  Nature, v. 411, p. 69-73) used the method on xenoliths from two adjacent areas, the actively extending Basin and Range Province and the Colorado Plateau.  Both contain ancient rocks, Archaean in the former and Mesoproterozoic in the second, which behaves as a stable craton.  Xenoliths from mantle deep beneath them have similar ages to those in the oldest crustal rocks, helping confirm the geochemical connection between crust formation and lithospheric mantle.  However, those from beneath the Basin and Range have potentially “fertile” compositions, whereas the Colorado samples show signs of the depletion thought to confer strength and buoyancy.  Paradoxically, a younger craton sits next to Archaean lithosphere that is demonstrably weak. 

Lee and colleagues suggest that if part of Archaean crust formation did not create a tectosphere, it is quite possible that younger orogens might contain considerably more ancient crust than currently suspected.  On the other hand, the mismatch between the near certainty that continents formed more rapidly during the first third of recorded geological history and the disproportionately small volume of known Archaean crustal rock could signify that a lot of it became resorbed into the mantle.  That doesn’t appear to have been a significant process in later times.  However, the total lack of sialic rocks older than 4 Ga, yet the evidence from detrital zircons up to 4.4 Ga in much younger sediments that some did indeed form, suggests that crustal resorption was efficient during early tectonics.  Perhaps the Archaean marked the waning of such processes, in which an increasing proportion remained locked at the surface.

See also:  Nyblade, A.  2001.  Hard-cored continents.  Nature, v. 411, p. 39-39.

Partially melted zones beneath Tibet

Anomalously low seismic velocities, accompanied by a “muffling” of seismic energy, and high heat flow beneath the Tibetan Plateau have hinted at the possibility of active crustal melting, but such information cannot resolve whether that is the case or not.  Parts of the Plateau have been volcanically active in the near past, and that has been attributed by some workers  to the detachment and sinking into the mantle of a large chunk of sub-Tibetan lithosphere.  Freed of a substantial mass, the thick lithosphere beneath Tibet would then bob up, the rapid drop in pressure at depth inducing partial melting.  Being weak, a substantial partially melted zone would also help the Tibetan crust deform more easily.

One means of  adding support to the idea is looking for deep-crustal anomalies in electrical conductivity.  Because electric currents flow naturally in the Earth, the conventional means of resistivity survey can use them instead of an input current.  Such magnetotelluric surveys potentially give information down to depths of 100 km or more.  At these scales, zones of abnormally low conductivity are likely to be due either to pervasion of deep rock with watery fluids or with widespread partial melting.  A group of Chinese, Canadian and US geophysicisists (Wei, W. and 14 others 2001.  Detection of widespread fluids in the Tibetan crust by magnetotelluric studies.  Science, v. 292, p. 716-718) have shown that the middle to lower crust deeper than 15 to 20 km beneath most of the Tibetan Plateau is anomalous in this way.  The highest conductivity lies beneath the main Yarlung (Indus) – Tsangpo suture., and may be related to fluids released by subduction processes.  It is the anomaly beneath the Plateau itself that is most significant, for it extends for 4 degrees of latitude along the survey line.  Higher conductivity anomalies correlate closely with Plio-Pleistocene volcanically active areas, and much of the area is affected by hydrothermal fluids.  While adding detail to structure and rheological properties beneath Tibet, magnetotelluric studies still leave open the possibility that much of the electrical signature may be due to pervasive watery fluids, as well as to zones of melting.

Brazilian input to the growth of Gondwana

One of the most dramatic tectonic events known from the geological record is the break up of a supercontinent, dubbed Rodinia (from the Russian for motherland), in the Neoproterozoic.  From a unity of almost all earlier continental crust, this break up sent fragments scurrying across a plethora of new oceans.  Some of the fragments reassembled around 650 Ma ago to create what eventually became the southern part of the Carboniferous supercontinent of Pangaea; Gondwana.  The assembly of West Gondwana involved a vast network of orogenic belts in which juvenile arc materials were pinched between colliding continental fragments, as these oceans closed up.  Often called the Pan African event, because of its widespread signature in that continent, this assembly also affected eastern South America at the same time.

Fernando Alkmim, Stephen Marshak and Marco Fonseca (Alkmin, F.F.  2001.  Assembling West Gondwana in the Neoproterozoic: clues from the São Francisco craton region, Brazil.  Geology, v.  29, p. 319-322)  turn our attention from the much-described Pan African to its Braziliano counterpart in South America.  Their summary of current understanding suggests six stages in the rifting to collision, that involved major changes in palaeogeography.

Mapping with geophysical data

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In the same way that topographic contours can be transformed to models of continuous elevation change using surface fitting, measurements of gravitational and magnetic field potentials, at points on the ground or along aerial survey lines, are sources of imagery.  Expressed as contours joining points with the same value, spatial distributed data are notoriously difficult to interpret, however much information they contain.  Not only do contours simplify the data by dividing them into arbitrary steps, how we interpret contour maps depends on how we perceive them.  Our eyes evolved to extract information distributed as a continuum across our field of view, and our visual cortex developed many tricks to innately interpret clues to shape, perspective and distance, to extend the limits of stereoscopic vision (we see objects in true 3-D only if they are closer than about 400 metres).  Our innate abilities “interpret” contours in terms of the spacing between them; the closer they are together the darker we perceive the area of steep gradient.  In other words we have to convert an image that is the “negative” of the first derivative to an understanding of the actual shape represented by contours!  Unsurprisingly, we have to learn to “read” maps, and that is a great deal more difficult for those showing potential-field intensity than for topographic elevation.  Cartographers long ago latched onto our use of shadows as clues to shape, and designed maps with shading as if the Sun was shining from the top of the sheet.  They also use different colours as a second clue to what is high and low.  Combining the two aids helps transform images of geographic variables – basically bland shifts from high to low – into visually stunning, and therefore more easily interpreted pictures.  Surface modelling of elevation and geophysical data, with such graphic tricks, literally throws hidden, and often unsuspected features into sharp relief.

These techniques have revitalized desktop interpretation of the world, especially using results of geophysical surveys.  However, in the same way that detail of a terrain blurs and loses information as resolving power falls, low-resolution data of other kinds obscure buried features, or give ambiguous hints to what they are and where they go.  Reducing the spacing of aerial surveys, and the height from which they are acquired, increases the resolving power of the technique.  Stunning examples of the state of this particular art appear in recent work by the US Geological Survey (Grauch, V.J.S. 2001.  High-resolution aeromagnetic data, a new tool for mapping intrabasinal faults: example from the Albuquerque basin, New Mexico.  Geology, v. 29, p. 367-370.  See also http://rmmcweb.cr.usgs.gov/public/mrgb/airborne.html ). 

Grauch worked on an area in which superficial materials and rapid rounding of topography result in poor surface expression of all but the largest faults.  By using aeromagnetic images modelled from survey lines spaced at 100 to 150 metres, he picked out not only hidden faults, but also the magnetic signatures of pipelines, water tanks and buildings.

Far-Eastern control on African climate and hominid evolution

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The drying of East Africa’s climate since 5 Ma ago shifted the distribution of its ecosystems towards more widespread savannah.  In the most general sense that probably created conditions for ape speciation towards an upright gait and the potential for tool-using and growing consciousness that palaeoanthropologists visualize at the core of human evolution.  The apparently dominant influence of North Atlantic circulation changes on climate fluctuations since then has suggested to many climatologists that the shift to glacial-interglacial and dry-humid cycles, at high and low latitudes, stems from some trigger for a fundamental shift in that circulation.  The favoured process is the closure of open connection between Atlantic and Pacific Oceans when the Isthmus of Panama formed about 5 Ma ago.  That transformed Atlantic circulation, and probably set in motion the Gulf Stream.  However, there are several such gateways whose affects on ocean circulation link to plate movements.

One is the narrow passage between Indonesia and Australasia, which transfers Pacific water to the Indian Ocean.  Subduction permits Australasia to move gradually northwards, thereby narrowing the gateway and also shifting it relative to the major currents in the tropical Pacific.  Mark Cane and Peter Molnar of Columbia University and MIT have analysed the recent evolution of the Indonesian gateway (Cane, M.A. and Molnar, P.  2001.  Closing of the Indonesian seaway as a precursor to east African aridification around 3-4 million years ago.  Nature, v. 411, p. 157-162).  Their findings suggest that the main flow switched from warm, South Pacific surface waters to cooler waters that originate in the North Pacific at about 4 Ma.  Cooling of surface waters in the Indian Ocean would have reduced the amount of water vapour transferred to the air masses that are involved in the East African monsoons.  The reduction in seasonal rainfall would have dried that area substantially.  Though providing a plausible cause for regional climate change, the coincident transformations of two major ocean gateways adds greater complexity to the Plio-Pleistocene climate system.  In terms of modern climate, the Indonesian gateway provides a means of understanding the teleconnection that seems to exist from correlation between drought-flood cycles in East Africa and the El Niño – Southern Oscillation in the tropical Pacific.

See also: Wright, J,D. 2001.  The Indonesian valve.  Nature, v. 411, p. 142-143.

Multiregionalists nailed by Y chromosome?

One of the big problems in using genetic material from living people to chart relatedness, and perhaps evolutionary origins, is simply getting the material.  For the mitochondrial DNA studies that first hinted at a common African origin for all modern humans, the best material is placental tissue.  A focus on male lineage using Y chromosomes is not so difficult; it can be done using blood samples.  Nonetheless, a survey based on 12,127 samples from 163 population is a monumental achievement (Ke, Y. and 23 others  2001.  African origin of modern humans in East Asia: a tale of 12,000 Y chromosomes.  Science, v. 292, p. 1151-1153).

The significance of this study by a large team from China, the USA, Indonesia and Britain is that it focuses on the region most favoured by multiregionalists for the hypothetically separate descent of modern humans from ancient ancestors of Homo erectus stock in different parts of the Old World.  The male chromosomes all carry evidence of mutations to a Y-chromosome marker that originated in Africa, abetween 35 to 89 ka ago.  The huge mass of data from the whole of East Asia do not support even minimal contribution from any source other than one that originated in Africa around the time it is thought that fully modern humans began to leave in significant numbers.

Erosion on Mars

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Mars is the only planet in the Solar System that has landscapes that bear any resemblance to those we see on Earth.  The one factor common to both planets is that surfaces have been shaped by flowing water.  On Mars, that was a one-off event early in its history, and thereafter shaping the planet has been through continual movement of dust in its thin, but energetic atmosphere, the formation of impact craters and volcanism.  Evidence for fluvial processes occurs in the highland regions, which were built mainly by volcanic activity., and stems from careful examination of high-resolution photography from orbiting probes.  Whether the various kinds of valleys formed by catastrophic, short-lived floods of melt water released by impacts into deep frozen ground, through steady release of groundwater or actually by precipitation  are the ground for speculation and controversy.  A means of assessing the possibilities is using accurate data on topographic elevation.  Digital elevation models for the Earth, even at coarse resolution (GTOPO30 data at 1 km resolution), map out the intricacy of surface drainage of the continents.  A DEM produced by the laser altimeter aboard Mars Orbiter allows not only the various models to be assessed, but enables quantitative work on the amount and rate of water erosion and deposition of sediment when combined with evidence for the age and duration of Mars’ fluvial event (Hynek, B.M. and Phillips, R.J.  2001.  Evidence for extensive denudation of the Martian highlands.  Geology, v. 29, p. 407-410).

Hynek and Phillips show that the event was long lived, lasting 350 to 500 Ma around 4 billion years ago.  Their study was of an area the size of Europe.  Scaled up, their findings suggest that of the order of 5 million cubic kilometres of sediment was transported, equivalent to deposition of a 120 metre thick sediment layer in the flat plains of Mars’ northern hemisphere.  The average rate of erosion during the event compares closely with that typical of temperate maritime areas of mountains on Earth.  It is difficult to see how such prolonged erosion could have taken place without runoff fed by precipitation on the surface, and that implies a much warmer climate and thicker atmosphere than on modern Mars, albeit only for a very early episode in its evolution.