Author: zooks777

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

Satellite-based gravitational surveys

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

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

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

Prolonged Cretaceous hothouse

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

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

Continental growth and strike-slip tectonics

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

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

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

Homo erectus unification?

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

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

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

Phyllogeography and “Out of Africa”

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

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

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

Extinctions by impacts: smoking artillery

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

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

Dinosaur digest

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

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

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

Extremophiles and possibilities for extraterrestrial life

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

Taking stock of hominid evolution

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

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

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

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

New Japanese tectonics research centre

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

Collapsing islands

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

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

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

Credit where credit is due?

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

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

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

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

Review of thermohaline circulation

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

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