Month: January 2001

Out of Africa hypothesis confounded?

Published on by zooks777Leave a comment

Living humans are anatomically the most diverse animals of a single species on the planet.  The differences extend from limb bones to skull characteristics, including the bony underpinnings of our faces.  That shows up plainly in any crowded market, whether that be in Addis Ababa, Bombay or Birmingham. Yet our genetic make up is extremely narrow, and chimps from separate troupes in West African jungle show greater diversity than that of humans across the world.  When physical anthropologists’ only tool was empirical comparisons between the physiognomies of people from different populations, their findings helped serve a political agenda. Statistical groupings drawn from that diversity slaked racists’ thirst for “proof” of their ethnic group’s wished-for “superiority”.  Such furtive longings are as alive today as they ever were in the 1930s: a mischief based on rubbished pseudoscience and ignorance.  We are physically diverse, but genetically distinguishable only by the most exquisitely precise analyses of DNA and other heritable material.

The minute genetic differences between peoples, like those more obviously separating the languages that they speak, result from migrations across the planet that took place before about 10 thousand years ago.  The migrants lived as hunter-gatherers under the climatically adverse condition of the last ice age.  Before the invention in widely separate centres of animal husbandry and agriculture that allowed human populations to explode – no earlier than 10 thousand years ago – our forebears’ total numbers would have barely exceeded the attendance on a Saturday afternoon at English Premier League soccer matches.  Tiny population densities, coupled with groups living in isolation and the random effect of mutations, with time create genetic differences between these groups, and so too for language and culture.  The narrowness of modern peoples’ genetic diversity points strongly to their last common ancestor living not so long ago in geological terms.  Whereas the earliest anatomical evidence for modern humans – a skull from Ethiopia with the chin that sets us apart from other extinct human species – is 450 thousand years old, differences in DNA from mitochondria indicate that divergence of the female half of our make up was about 140 thousand years ago.  Evidence from living men’s Y chromosomes (see November 2000 Earth Pages Eve never met Adam) suggests an even more recent stem, about 70 thousand years ago.  Both analyses point strongly to Africa for the focus of later divergence, that no other lines of descent survived to the present, and that no DNA from different groups, such as Neanderthals or Homo erectus, was involved in living peoples’ ancestors since 140 thousand years ago.  These observations form the core of the “Out of Africa” hypothesis.

There are, however, physical anthropologists who still set great store by statistical analysis of anatomical features, specifically that of skulls from extant humans and fossil ones.  They hold a view that it is possible that modern human’s physical diversity arose by evolution from much older populations of earlier migrants to different regions from Africa – the “Multi-regional” hypothesis explored by Milford Wolpoff of the University of Michigan.  In the case of Asian and Australasians that might have been from H. erectus that arrived in China as long ago as 1.8 million years back – recent dating of sediments in which erects’ remains have been found in Indonesia shows that they survived until as recently as 20 thousand years ago.  Alternatively it could have been from more advanced humans who arrived in Asia less than half a million years ago; the Mapas whose remains resemble those of Neanderthals.  For Europe, the putative ancestors would be Neanderthals, who arrived there at least 350 thousand years ago.  Africans, say the multi-regionalists, evolved continuously from the earliest tool-using humans since 2.5 million years ago.

Wolpoff’s group has used the same statistical technique employed in DNA studies to analyse skull morphologies from 25 individual modern humans from the fossil records of Europe and Australia, and compared the results with those for well-accepted, earlier humans and modern ones from Africa.  They claim (Wolpoff, M.H. et al. 2001.  Modern human ancestry at the peripheries: a test of the replacement theory.  Science, v. 291, p. 293-297) a better statistical fit between data for pairings of modern-human and earlier inhabitants of Australia and Indonesia, and of Europe than between modern-human remains from different regions.  “Out of Africa” proponents question the validity of the method, particularly selection of parameters – facial characters are omitted – and actual fossils.  Statistics is always a problem in studying human fossils, because they are so rare and widely separated in time – the study by Wolpoff’s group used material ranging from 60- to 14 thousand years old, and a total of only 25 specimens.

Even rarer are data for genetic material separated from fossils.  Three years ago, palaeoanthropologists at the Max Planck Institute in Munich reported the first partial DNA sequence from Neanderthal remains, later confirmed by another extraction.  They showed how unlikely it is that conjugation of Neanderthals and contemporary modern humans resulted in any signature surviving in the genes of living people.  Likewise, the data seemed to rule out any relatedness between the two groups since possibly several hundred million years ago; bad news for the multi-regionalists.  Astonishingly, scientists at the Australian National University have recovered useful DNA from 10 fossil humans between that range from 2 to 60 thousand years old.  The oldest not only represents the earliest Australian yet found, but turned out to be very different from that of later inhabitants (Adcock, G.L. et al. 2001.  Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins.  Proceedings of the National Academy of Sciences, v. 98, p. 537-542).  One intriguing aspect is that a sequence in the mitochondrial DNA of “Mungo Man” exists as a remnant “insert” in modern DNA from chromosome 11, long suspected of being old mtDNA that has transferred to that in the cell nucleus.

Although no-one claims “Mungo Man” was an ancestor of living native Australians, there is many a spin that can be placed on the discovery.  The spanner in the works is that he is physically modern, beyond a shadow of doubt for comparative anatomists, but genetically archaic.  One possibility, espoused by the multi-regionalists, is that he evolved from pre-modern human migrants into Asia, either H. erectus or Mapas.  But that runs against the discovery of morphologically erect fossils from Indonesia that are much younger.  Perhaps he descended from interbreeding between early modern human migrants with earlier Asians, his DNA failing to be passed on to the present.  It is also possible that 60 thousand years ago, humans had a much greater range of genetic diversity, and that was filtered to today’s narrowness by a “bottleneck” due either to a disastrous fall in global population or to a cultural innovation that favoured only those who used it in the lottery of evolutionary fitness.  Though grist to the multi-regionalist mill, one DNA datum does not knock the “Out of Africa” hypothesis from its basis on thousands of results from living people.  Humans in one shape or other trekked from Africa to Asia at least three times since 1.8 million years ago, surviving in the case of the erects until quite recently.  It is what tool-equipped, socially conscious beings do, because they are sheltered from environmental pressures by what they do as much as by who they are.  That also surely means that all manner of changes in their genes and their morphology, which in mere beasts might snuff them out, can survive to confound the pure anatomist and the molecular biologist.  As the demise of the Neanderthals shows, when cultures are pitted in environments that offer limited resources, one gives way to another better suited.  Sadly, lifestyles and outlook, that we know to have driven human history for 6 000 years or so, leave little fossil record save stone tools and art, often inexplicable.  Accepting what makes humans unique has somehow to figure in all the empiricism around which centre current ideas on our origins.

(See also: Pennisi, E.  2001.  Skull study targets Africa-only origins.  Science, v. 291, p.231.  Dayton, L.  2001.  The man from down under.  New Scientist, 13 January 2001 issue, p. 6.  Holden, C.  2001.  Oldest human DNA reveals Aussie oddity.  Science, v. 291, p. 230-231)

Siberian role in climate change?

Published on by zooks777Leave a comment

Climate researchers at MIT in Cambridge, Massachusetts have analysed Northern Hemisphere climate data from 1972 to 1999, in the search for correlations that might help improve long-term weather forecasting.  The most striking match to emerge is that of winter climate with the extent of autumn snow cover in Siberia.  Snow reflects back to space a far greater proportion of incoming solar energy than any other kind of surface, with the exception of salt.  More snow results in less warming in the area.  Although Siberia is at the heart of the Asian continent, and therefore pretty dry, it has cold winters, so that when snow falls it covers large areas and tends to remain.  It is the focus for an enormous mid-continent high-pressure area in winter, appropriately named the Siberian High, which is one of three systems that dominate northern climate.

High-pressure areas do two things: air spills from them into surrounding areas; they isolate the area beneath them from warming, moist winds blowing from the oceans.  In winter the second creates cooling so intense that temperatures can steadily drop to -50°C or below , further building pressure because of the increase in air density.  Siberia sheds cold air westwards into Europe and over the North Pole into North America.  The MIT study bears out the obvious prediction based on this tendency.  However, it may also add the Siberian High to the range of large-scale terrestrial processes – shifts in air pressure over oceans, such as the El-Niño of the tropical Pacific and the North Atlantic Oscillation, and thermohaline controls over Atlantic surface currents – that make ice-age climate patterns so complex.

Cooling of northern Europe and the Canadian Shield does not have to be very extreme to lower the topographic elevation at which snow remains permanently, the glaciation limit – at present that level is only a couple of hundred metres above the tops of Britain’s highest mountains.  Should permanent snow cover return to the highest areas around the North Atlantic, that would amplify the present effect of Siberian autumnal snow and expand the high-pressure area.  That is a positive feedback driving climate towards increased frigidity, and larger winter highs would hold back maritime warming influences.

Computer modelling of the air-flow patterns over Asia shows that the primary influence is the Himalaya and Tibetan Plateau.  In particular, they dry out air passing over them during the South Asian Monsoon, and hinder its influence further into central Asia.  The two huge massifs seem to have risen rapidly and recently, beginning about 8 million years ago, despite the fact that India collided with Asia about 50 million years ago.  Together with other roughly E-W high mountain ranges in central Asia, they also channel Siberian cold air to spill westwards and eastwards, and over the pole.  Behaviour of the Siberian High almost certainly dates from the uplift of the Himalaya and Tibetan Plateau.

Adding another controlling factor to long-term northern climate has an intrinsic potential in refining academic studies of Pleistocene climate.  However, there is an immediacy to the observations.  For snow to cause cooling by reflecting away solar heat it does not have to be thick; a few centimetres will suffice.  The critical factor is the area covered by it.  Siberia is so cold in autumn and winter that it will snow there, provided moist air can enter.  Should more get in then more snow will cover a greater area, to feed the positive feedback to cooling.  Perversely, the more the climate warms globally, the more moisture evaporates from tropical and mid-latitude oceans to move polewards and towards continental interiors……

Mismatches from north to south proven

Whether or not climate changes, especially those of shorter duration than the full glacial-interglacial cycle, occur at the same time everywhere is something that vexes all climatologists.  It encapsulates all the problems of causation: orbital forcing, thermohaline circulation, shifts in the Polar Front and Intertropical Convergence Zone, etcetera.  The problem mainly stems from uncertainties in the correlation of  time series that show proxies for climate change.  This is particularly bad for ocean-floor sediment cores, which depend upon radiometric dates for calibration from depth to time sequences and an assumption of constant rates of sedimentation between dated samples.  Imprecision often means that correlations are not believable, except at a very general level.  Many analyses end up by correlating the patterns shown by the proxies, which defeats the object of assessing the degree of global synchronicity of climate changes.

Cores taken through ice sheets offer a way out, for annual layers of ice are there to be counted, but only in the upper parts.  For deeper parts, converting depth to time relies on models of how ice compacts and how it thins by glacial flow.  Another seeming advantage of ice-core records is that a great deal more ice accumulates than does ocean-floor sediment over a particular time.  That means that the resolution of ice core records can be finer – potentially at the level of decades compared with hundreds of years for sediment cores.  A seeming key to correlation between ice cores lies in the way that ice traps air.  Being rapidly mixed, the atmosphere should have the same composition everywhere.  This is particularly so for methane, partly because it soon becomes oxidised to carbon dioxide, and partly because its level is highly variable from emissions by rotting vegetation and unstable gas hydrate on the shallow ocean floor.  Thomas Blunier and Edward Brook of Princeton University and the University of Berne used the methane records of Greenland and Antarctic ice to correlate the other proxies therein over the last 90 thousand years (Blunier, T. and Brook, E.J. 2001.  Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period. Science, v. 291, p. 109-112).  They show a consistent mismatch between rapid warmings of the air over the two polar ice sheets, where Antarctic changes precede those over Greenland by 1500 to 3000 years.  Interestingly, when frigidity gave way to comparative warmth in a matter of a few decades over Greenland, the Antarctic was shifting from warm to cool conditions.

Commenting on the paper in Science’s Compass, Nicholas Shackleton of Cambridge University shows yet more emerging oddities (Shackleton, N. 2001.  Climate change across the hemispheres.  Science, v. 291, p. 58-59).  In the North Atlantic Ocean, surface water temperatures apparently changed according to Greenland’s pace, while those for deep water match that of the Antarctic.  To add to the complexity of climate change through the last glacial period – until a few years ago it was all supposed to link to the astronomical forcing of solar heating at high northern latitudes – the oxygen isotope changes in the same deep water of the North Atlantic match those of ice volume around the north pole.

Whereas Blunier and Brook have proved that air-temperature changes above ice sheets at high northern and southern latitudes are not synchronous, this still leaves problems in correlating between ice and sediment cores, and between the oceanic record at the many sites world-wide, especially those at low latitudes.  With a growing number of hypotheses for climate changes of the order of a few thousand years – driven by changes associated with northern ice sheets, Antarctica and the tropics – onlookers await with interest the development of a means of precise correlation among all the time series.

Hands-on planetology

Published on by zooks777Leave a comment

up with playing Solitaire or Hearts between those moments of productive inspiration?  NASA Ames Research Center has set up a cottage industry (unpaid) to help Mars specialists there build a catalogue of impact craters on the Martian surface.  As those flyers tucked under your windscreen wipers say, “No experience needed”.

Probably the most important scientific breakthrough from studies of the Moon since the 1960s has been the discovery that its pocked surface resulted from impacts by chunks of interplanetary debris.  The rate of impact and the size of the colliding bodies, and therefore the energy that they delivered, has varied since the Moon formed.  The lunar cratering record, backed up by accurate dates of its products, is a detailed chronology of how impacts influenced Earth’s evolution – vital, since signs of impacts rapidly become masked by our planet’s vitality.

Mars, on which NASA scientists and many more besides focus their undivided attention, is also cratered as a result of the same kind of process.  Counting craters, measuring their diameters (a proxy for the energy involved in their formation) and looking for their age relative to one another and other features of the Martian scene is an excellent means of assessing aspects of the Red Planet’s evolution.  But Mars is a great deal bigger than the Moon, and the sheer tedium of doing the work has become a burden.  Those geologists who compiled the lunar record have moved on, and few relish the task as a profession, hence Ames’ appeal for public participation.

The idea is that the basic information on crater occurrence, size and relative age – that’s based on relations between overlapping craters and degradation by Mars’ “weather” – can easily be gathered by interested, but untrained people.  The statistical work can then be done much more quickly.  If you fancy being a NASA “Clickworker”, then connect to http://clickworkers.arc.nasa.gov/top

Since inception on November 17, 2000, all clickworkers combined have contributed 340,070 crater-marking and 93,891 crater-classification entries.  It seems better by far than simply running the SETI distributed software to analyse radio frequencies for possible signs of intelligence out there.  You get to look at some magnificent high resolution images too.

China’s fossil treasure house

Published on by zooks777Leave a comment

For small, shelly faunas that just preceded the Cambrian Explosion, outcrops that span mass extinction events, the evolution of vertebrates and much else besides, the huge diversity of Chinese geology has become a hive of palaeontological activity.  Perhaps this is due to an astonishing run of good fortune through the Phanerozoic as regards excellence of preservation, or the patience, ingenuity and skill of Chinese fossil experts.  The embarras de richesse is probably a blend of both with the fact that for decades following the Cultural Revolution little work was possible for political reasons.  Pent-up enthusiasm and curiosity is a marvellous driving force in research when released.

Such is the degree of interest that the 12 January 2001 issue of Science devotes 10 pages (Stokstad, E., Normile, D. and Lei, X. 2001.  Paleontology in China.  Science v.  291, p. 232-241) to a summary of discoveries so far, how Chinese palaeontologists are organising and funding their work, the in-fighting that goes on (not so different from anywhere else!) and the dangers of unique material being looted in the manner of rare works of art.  One difference in fossil hunting between developed and poor countries that are geologically well-endowed, is that in the former most of it is by professionals or well-heeled amateurs seeking entertainment.  In China it is a potential source of extra income for rural people, in the same manner as artisanal gold working, widespread in Africa.  That is double edged: while leaving no stone left unturned where fossils crop up in soil, it is the source of semi-legal international trade in treasures like dinosaur eggs containing embryos, and untutored fossickers make no records of stratigraphy.

The most important issue discussed in the revue concerns how essential overseas resources focus on scientific potential in less well-heeled countries.  There is a tendency, which has tempted most scientists with access to funds to pay lip-service to transnational collaboration, merely to add names to proposals and publications of individuals who for various reasons have not played a full, or sometimes any role at all.  That is a device to attract funds with an air of philanthropy, and to get official access to material.  It has no benefit for transfer of knowledge, skills and technology.  Most Chinese palaeontologists now rightly demand to participate fully in order to boost and widen expertise in their community.

The Chinese experience offers plenty of lessons for Earth scientists in other poor countries.  For one thing, it has focussed the government’s attention on reversing the previous drain of excellence by earmarking affordable funds for research.  Another is that it shows how curiosity and plain hard work can open up entirely new knowledge from the previously overlooked.  There is no reason why their application in other poorly-known geological scenarios shouldn’t uncover crucial threads for many other problems of the Earth’s evolution – about 75% of the continental surface still remains to be mapped at scales better than 1:1 million.

Oh Dear, another weird dinosaur!

China isn’t the only new frontier for palaeontologists.  It looks as though Madagascar is on the fossil map, because of fine preservation in late-Cretaceous, terrestrial sediments there.  The latest find there is a somewhat diminutive (~1.8 m long), but nontheless strange abelisaurid theropod – the group best known for having T. rex as a member (Sampson, S.D. et al. 2001.  A bizarre predatory dinosaur from the late Cretaceous of Madagascar.  Nature, v. 409, p. 504-506).

Masiakasaurus knopfleri  (the expedition crew included the few surviving fans of Dire Straits) had nimble teeth; in fact a whole gob-full of them.  Not a beast on whose snout to place a little kiss, for lots of pointy and serrated fangs protrude in a most alarming manner.  “It shows there’s still more to theropod lifestyles than we thought”, observed Tom Holtz of the University of Maryland; something with which we can all agree.  But upon what victims did it prey?  There are similarly equipped fossil crocodiles, and M. knopfleri certainly seems well-equipped to snaffle the odd passing trout.  However, the late Cretaceous greenhouse world had an atmosphere with high oxygen levels due to much greater rates of photosynthesis than now.  It probably teemed with large flying insects, because oxygen levels determine the maximum size compatible with the high metabolism needed for flight.  The discoverers plump for an insectivorous lifestyle.

But just what constitutes “weirdness”, the adjective “bizarre”?  To me, they are appropriately applied to living beetles that boil formic acid and spit it on a predator, giant squid whose sexuality involves males injecting packets of sperm under high pressure into the tentacles of females, who, at their leisure, rip off the skin that heals the wounds to impregnate themselves, and, of course, the recently discovered phyllum that lives exclusively on the lips of lobsters.

Bacteria and dolomites

Published on by zooks777Leave a comment

from the atmosphere.  During the Phanerozoic times of “greenhouse” conditions both induced and were relieved by carbonate “factories” dominated by metazoans that secreted calcium carbonate in their hard parts.  An excellent example is the Chalk of the late Cretaceous.  Before the evolution of the metazoa some other means was needed.  Precambrian sequences contain abundant carbonate strata, but a great many of them contain lots of calcium-magnesium carbonate or dolomite.  The further back in time, the more dolomitic carbonates become.

Some of these dolomites contain mounds and cauliflower-like masses built of many thin laminae.  By analogy with similar structures forming nowadays in a few rare environments that are too saline to support metazoans, sedimentologists have ascribed these stromatolites to the expulsion of toxic calcium from their cells by blue-green bacteria or cyanobacteria.  Blue-greens are photosynthesisers that generate oxygen.  Evidence from carbon-isotope analyses of fossil organic material in old Precambrian sediments supports them having evolved very early.  Despite their antiquity and ability to break down water and release its oxygen, blue-greens were unable to build up oxygen in the Earth’s atmosphere until about 2 000 million years ago.  For half of life’s history conditions were lacking in oxygen, and bacteria that consumed dead things had to subsist with metabolisms that employed other chemical tricks than the oxidation of organic matter to carbon dioxide plus water, for which oxygen is essential.  One strategy is the reduction of sulphate (SO42-) to sulphide (S22-) ions (see Slime to the rescue, December 2000 issue).  That involves shifting of electrons so that the counterpart of a reduction is some form of oxidation, which such bacteria employ in their metabolism.

Modern environments devoid of oxygen encourage such organisms; hence their obnoxious odour from released hydrogen sulphide gas.  One such habitat is a very salty lagoon in Brazil, and that is also a place where dolomite precipitates in abundance.  A Swiss-French team of organic geochemists has shown experimentally that its sulphate-reducing micro-organisms actually encourage dolomite to leave solution (Warthmann, R. et al. 2000.  Bacterially induced dolomite precipitation in anoxic culture environments.  Geology v. 28, p. 1091-1094).  Not only does that add to the ways in which modern carbon dioxide leaves the atmosphere on a long-term basis, but suggests that such bacteria played the key role in climate balance in the earliest Precambrian.  The lack of oxygen before 2 000 million years ago would have made every niche available to them, for sulphate ions are continually added to sea water.  They showed in vitro how bacteria from the lagoon sediments cultured in sulphate-rich water did precipitate dolomite in curious dumbbell-shaped grains that aggregated to cauliflower-shaped masses in zones around the cultures.  By carefully isolating different species of bacteria, they found that sulphate reducers were the culprits.  As well as helping account for the preponderance of dolomite in ancient carbonates, it expands our vista of organic diversity represented by them, albeit of a very lowly kind.

Pushing back the “vestige of a beginning”

Published on by zooks777Leave a comment

About 4.5 billion years ago the Moon formed, probably as a result of a stupendous collision between the original Earth and a body about the size of Mars.  That would have left Earth with its outer parts molten in a global magma ocean, and without any atmosphere.  Such a dreadful condition formed the point of departure for all subsequent evolution of our home world; the beginning of geological history.  No matter how many terrestrial rocks geochronologists analyse, it seems pretty clear that they are never going to push back their erstwhile grail of the oldest one beyond 4 billion years.  Among the oldest rocks, those from Akilia in west Greenland contain sedimentary evidence for flowing water and the isotopic signature of established life.  The date 4 billion years before the present seems to be the maximum for every aspect of geological research that might support theory with concrete evidence, which is sad, because both continents and oceans existed, the planet was inhabited, some form of tectonics operated and water moved matter around.  Studying the emergence of such broadly familiar processes is a lost cause, at least on this planet, for a half billion years has simply vanished.

The enduring outer skin of the Earth, continental crust, is made mainly of two minerals, quartz and feldspar.  Feldspar can be dated, but it breaks down to clay and soluble compounds, so the weather removes it as a source of information,.  Quartz offers not a single clue to when it formed, even though its hardness and stable molecule mean that it is durable.  Its abundance of silicon demands several stages of evolution from the silicon-poor mantle.  Quartz is quintessentially continent stuff.  Probably among those quartz grains found on a beach or in a sandstone some date back to the emergence of the first crust, but you would never know.  Even more durable is zirconium silicate, or zircon, tiny amounts of which settle from many sands because it is denser than quartz.  Zircon’s structure is hospitable to several elements rarer still, including radioactive uranium and thorium. Build up of radiogenic lead isotopes inside zircon crystals means that grains carry their own history.  Zirconium finds no easy resting place in minerals that form the bulk of the mantle.  So it tends selectively to enter magma formed there.  Nor are the minerals of oceanic crust particularly accommodating.  Naturally, zirconium becomes concentrated in materials that end up as continental crust, so to form zircons.  A handful of zircons from beach sands continually sorted according to density on the Coromandel Coast contains the entire history of the formation of the Indian continent – they are sold in bottles by urchins at tourist resorts as one of Lord Krishna’s five varieties of “rice”.

The mount Narryer Quartzite of Western Australia is a similarly well sorted, though 3 billion-year old sedimentary repository.  Fourteen years ago, Bill Compston and Bob Pidgeon managed to extract 17 tiny zircons from it that extinguished at a stroke the ambitions of other geochronologists to date the oldest rock in the world.  Their ages, obtained by methods based on the build up of lead isotopes from decayed uranium and thorium reached back to 4.27 billion years.  They had discovered the oldest continent, but one sneeze and they would have lost the lot.  Mount Narryer made the front pages early in January by providing even older zircons that post-date “Year Zero” by a mere hundred million years.  Some continental material was around 4.4 billion years ago (Wilde, S.A. et al.  2001.  Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.  Nature, v. 409, p. 175-178).  Oxygen isotopes in these tiny, aged grains offer another insight.  They have contents of 18O that are too high to have formed other than in an environment that involved liquid water reacting with the source of the zircon-forming magma (Wilde et al., 2001; Mojzsis, S.J et al. 2001.  Oxygen-isotope evidence from ancient zircons for liquid water at the Earth’s surface 4,300 Myr ago.  Nature, v. 409, p. 178-181).

Evidence for such old liquid water drew attention from many planetary scientists.  Life is impossible without it.  The conclusion drawn is that it could have been around so close to “Year Zero” .  But evidence for early water is no surprise.  Earth’s high content of volatiles ensures that water in one phase or another must always play a role in its internal processes.  Hot as it must have been immediately following Moon formation, convection in its “magma ocean” and radiation from its surface (proportional to the fourth power of surface temperature) would have been so efficient that cooling to permit liquid water at the surface may have taken less than 100 million years.  The maximum temperature of the liquid water that interacted with the zircon-forming magma depended on the pressure of the environment where that happened.  That was not necessarily an ocean or even “some warm little pond”.  Water is liquid, if the pressure is high enough, at temperatures up to 274°C, which is too high for most of life’s molecules.

African roots

Published on by zooks777Leave a comment

Africa to a large degree exerts a control over modern plate tectonics, because it barely moves at all.  The base of its lithosphere connects in several places with the solid mantle, so that asthenosphere is not universally present beneath the continent.  These roots slow down Africa’s motion.  One name applied to them is “tectosphere”, and they are partly governed by the low heat production in the lithosphere and underlying mantle, as a result of U, Th and K having been extracted from depth by processes that led to separation of continental crust.  These processes reach completion beneath the most ancient segments of continental crust, and result in them eventually becoming geologically inert; they become cratons. 

Studies based on samples brought from deep below cratons by volcanism, particularly that which formed the kimberlite plugs of Africa, suggest that their roots date back almost as far as the age of continental material above them.  But that natural sampling is haphazard, and relationships cannot be found.  Where large extraterrestrial bodies have excavated material to great depths, tectosphere material might well have reached the surface en masse by rebound following impact.  Such a deep section formed around the Vredfort Dome in the Kaapvaal  Craton of southern Africa after a major impact about 2 billion years ago.  It exposes the crust-mantle boundary. 

A programme of dating the Vredfort materials (Moser, D.E. et al.  2001.  Birth of the Kaapvaal tectosphere 3.08 billion years ago.  Science, v. 291, p. 465-468) shows that welding of crust to mantle in Archaean times, and formation of both craton and tectosphere, took place about 3.1 billion years ago, more than a hundred million years after crustal material itself coalesced.  Tectospheres seem not to begin forming at the same time as large masses of continental crust.  Instead they accrete to the base of the crust through later processes that probably involve subduction.  Other workers have suggested that the Kaapvaal tectosphere accumulated from masses of oceanic lithosphere that failed to descend completely into the mantle.  Curiously, the fragments in kimberlite pipes from which those conclusions were drawn are very dense eclogites.  Such material should descend easily into the deep mantle because their density exceeds that of peridotite.  That poses the question of why they came to stay close to the surface so long ago.  Perhaps their eclogite mineralogy stabilized long after they accreted beneath Kaapvaal, and they are “stuck” in the inert tectosphere that they form, out of gravitational equilibrium.  Should such high-density roots eventually become detached from their overlying materials, then the surface would pop up to become eroded dow to great depths.  The fact that most of the worlds cratons (the continental “shields”) preserve great volumes of material that crystallized at quite shallow depths, suggests that such “delamination” does not commonly happen beneath them.