Author: zooks777

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

Sidelined by events?

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2001 was the year of the human genome, stem cells and carnage in the USA and Britain delivered by self-justifying fanatics and the agricultural wing of government respectively.  It ends with serious inroads into basic freedoms of expression and privacy in the wake of violence.  Both 11th September and the British foot and mouth epidemic feature prominently in the authoritative end-year reviews by both Science (v. 294, p. 2446-2447) and Nature (v.  414, p. 836-841).  It is hardly surprising that Earth science, bar comments on climate change, has no entry to rank with downturns in science budgets, development of neural circuitry, high-temperature superconductivity, bio-molecular chips, the unending hunt for the neutrino, adventures with the Large Hadron Collider and the farce of NASA’s Space Station .  Nonetheless, our progress has been marked by the sublime and the ridiculous, to a satisfying degree, as 2001 Earth Pages has tried to summarise.  No doubt its limitations have meant that some major advances have been missed, for which I apologise.  I leave it to readers to judge from the archive what were the highlights of the year.

In Britain, the last month has been one for either celebration or sober reflection by university academics, on announcement of the results of the latest round of the Research Assessment Exercise by HEFCE.  Science in general seems to have done rather too well over the last 5 years, for there is insufficient cash in the pot to suitably reward those departments whose rating has risen (Watson, A.  2001.  Universities raise their game, but the money doesn’t flow.  Science, v. 294, p. 2448-2449).  The £1.3 billion kitty to boost research infrastructure falls about £150 million short of the improved departments’ supposed expectations.  The board of HECFE is to tinker with the goalposts to eke out the dosh.  No doubt the well-endowed will benefit even further, the middle rankers getting less than their improvement ought to warrant, and then there are the also-rans.  In Earth sciences a financial squeeze ought not to have such an inequitable outcome as in more lab-dependent disciplines, but the whole exercise seems destined to result in marginalization of research topics into a dwindling bunch regarded as world-ranking.  That would be a recipe for a cut in diversity, that does not match the increasing need for breadth in getting to grips with processes in and history of the Earth System.

Returning, finally, to the events that have gripped the world for the last quarter of 2001, the central theme of most commentary is that the world changed on 11th September.  I do not believe that it did.  For two thirds of the world’s population it is “business as usual” – an ever widening gap between hope for the future and expectation of any relief from poverty, disease and the fear of falling victim to natural and anthropogenic calamities that scientific advance might bring.  Despicable as the perpetrators and those who motivated their actions were, the attacks on the USA arose from the growing powerlessness of hundreds of millions of dispossessed people to secure their livelihoods and lives.  Global communications ensure that they are confronted daily by what they lack set against what is possible, leading to a deep sense of unfairness and perpetual victimhood   Scientists, whose work is enmeshed with emergence of the possible, should dwell on how they might help close that growing human fault line, rather than raging at or cringing before the monstrosity that they have helped to nurture.  Assisting the dispossessed to secure safe, dependable water supplies, to improve their agricultural yields, to rid themselves of endemic disease, to gain access to cheap energy and transportation, and above all to acquire knowledge and the ability to solve their own problems is not a problem of cosmological or genomic proportions.  It is a simple, human duty.

Irish stalagmite reveals high-frequency climate changes

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Much of the information about glacial and interglacial climate change has come from cores drilled either from ocean-floor sediments or ice caps.  However, both suffer from limits to time resolution of the order of more than 100 years, although ice younger than about 5 thousand years clearly shows annual layers.  While groundwater is able to flow, speleothem (flowstone)  grows continuously in caves, under conditions of extremely stable temperature and humidity.  Depending on how they are analysed and how thick the deposits are, stalagmites and stalactites should give fine time resolution.  A half-metre long stalagmite from an Irish cave has grown since the start of the Holocene.  Using high-precision uranium-series dating, its length has been calibrated in calendar years before present.  A laser probe that releases oxygen from the speleothem calcite has provided oxygen isotope data whose resolution (between 7 and 18 years) is an order of magnitude better than sea-floor sediments and between 5 to 20 times better than from pre-5 ka ice cores (McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variabilty revealed by a high-resolution speleothem d18O record from SW Ireland.  Science, v. 294, p. 1328-1331).

Until recently, the best documented climate variations that are more rapid than can be explained by the Milankovich effect are the Dansgaard-Oeschger cycles in the Greenland ice cap.  They are of the order of 1 ka, but somewhat variable in their periodicity.  The Irish stalagmite shows that there were climate shifts throughout the once supposedly stable Holocene, with frequencies equivalent to periods of 625, 169 and 78 years, the latest of which coincide with warm and cool periods since Roman times.  One caution is that the oxygen isotope variations cannot be ascribed directly to variations in air temperature, because they would have been affected by differences in the surface seawater from which water vapour evaporated to fall as rain in SW Ireland.  Before about 4.5 ka 8 clear peaks and troughs occur at the same times in both the Irish stalagmite and the Greenland ice core; clear signs of regional changes.  These probably reflect releases of glacial meltwater to freshen surface waters of the North Atlantic.  Over Greenland they resulted in atmospheric cooling, in response to weakening of the effects of the Gulf Stream by reduced thermohaline circulation.  The correlation breaks down for the last 4 ka, and the fluctuations in the Irish data do not show features that coincide with ice-rafting events known from sea-floor sediment cores.  That suggests that ice-rafting was no longer able to cap the North Atlantic with fresher water.  Nonetheless, something was going on to impart isotopic changes to rain falling on Ireland, and that did coincide with the widespread climate changes of the recent past.  What the driving processes were is not known, but it seems inescapable that underlying the drive to global warming through industrial CO2 emissions is a more fundamental process.  Should anthropogenic warming reinforce it, as seems to be happening, their combined effects could flush fresh water into the North Atlantic’s surface layers, thereby slowing thermohaline circulation and the warming effect of the Gulf Stream.

Tiny tectonics and the hydrological cycle

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Anyone who has watched a watchmaker at work may well have felt a tinge of panic at the sheer tininess of the screws, sprockets and gears, and awe at the near-superhuman patience and concentration involved in such micro-engineering.  Developments in geodesy based on the Global Positioning System of navigational satellites  push towards such aching precision.  The fixed stations of the International GPS Service (IGS) measure geographic position and topographic elevation to within less than a millimetre.  Corrections for known plate motions and Earth tides reveal motions that must be due to other forces.

Ultimately, the forces shaping the Earth’s surface are gravitational, and thus reduce to shifts of mass within and upon our planet.  By far the most rapid movements of matter are those involving the atmosphere and the water vapour that it carries.  Through variations in atmospheric density and the mass of water residing in soil moisture and snow cover, which arises from varying precipitation, surface load changes on an annual cycle.  Meteorological and remote sensing estimates of these loads allow geophysicists to model the elastic response of the surface to the seasons.  Why they have done this is not abundantly clear to me, but starting position is essential to astronavigation, hence similar attention to the Chandler Wobble (see Atmosphere linked to Earth’s rotation, Earth Pages, September 2000).  Anyhow, the records suggest an annual mass transfer from hemisphere to hemisphere of around 1013 tonnes, which is sufficient to cause elastic deformation within the scope of GPS measurements. Geomaticians from the universities of Nevada and Newcastle upon Tyne (Blewitt, G. et al. 2001.  A new global mode of Earth deformation: seasonal cycle detected.  Science, v. 294, p. 2342-2345) have been able to chart the actual motions over the period from 1996 to mid-2001.

Performing the necessary computations on the weekly data from 66 IGS stations, and fitting curves to the results, Blewitt et al. present convincingly repetitive cycles in the motions towards the intersection of the Greenwich meridian and the Equator, towards the North Pole, and perpendicular to the surface.  These tie very well to the theoretical model.  Interestingly, they were able to model the shifts of displacement globally, and in series of maps show that the positions of maximum displacement shift along a path linking the continents.  That is not surprising in itself, for the oceans respond by changes in water level, and only exposed continental lithosphere is likely to flex.  The poles sink by around 3 mm each winter, and the Equator swings towards the winter pole by 1.5 mm.  Results tally extremely well with estimates of seasonal mass shifts and theory.  The surprises include an anomaly in vertical displacement in 1996-7 preceding the 1997-8 El Niño event, probably due to changes in Pacific sea level driven by winds and anomalous monsoon precipitation.

Yet more on tectonics of the Tibetan Plateau

In the previous issue of Earth Pages was a resumé of a paper in Science that discusses the lateral tectonic motions that result from India’s collision with Eurasia (Continental tectonics of eastern Eurasia December 2001 Earth Pages)).  The compliment to that appeared in the 22 November 2001 issue of Science (Tapponnier et al. 2001.  Oblique stepwise rise and growth of the Tibet Plateau.  Science, v. 294, p. 1671-1677).  How and when the India-Eurasia collision zone achieved its pattern of huge elevated masses is partly an issue of tectonics, but they bear on any climatic effect that changed elevations might have had on climate, both regionally in the case of the South Asian monsoon circulation, and globally (one view is that Tibet’s deflection of atmospheric circulation may have been an important trigger for the onset of northern hemisphere glacial conditions).

Many geologists have considered the whole lithosphere of the region to have behaved in a ductile manner during collision, so that shortening and thickening were distributed more or less evenly.  They ascribe the uniform height (>5 000 metres) to gravitational rebound when part of the thickened lithosphere detached and fell into the mantle, around the mid-Miocene.  Erosion being unable to keep pace with uplift, the Tibetan Plateau is then thought to have become unstable and started to collapse laterally.  That is seen by many as an explanation for clear evidence of E-W extension from both numerous N-S rift systems and extensional first motions on Tibetan earthquakes.  However, this vast area is clearly subdivided into several major blocks by large strike-slip systems.  The prevailing notion is that these faults are effects of “soft” collisional tectonics.  Tapponier et al. assemble detailed evidence in relation to these faults and the blocks that they bound.  They support tectonic evolution which has been controlled by coherent blocks of lithosphere, a process which was episodic rather than continuous, and accompanied by decoupling of crust and mantle lithosphere.

The linchpin of their model is the diachronous calc-alkaline magmatism of the region during the Tertiary, which becomes younger towards the north.  As well as the principal site of northward subduction of Indian lithosphere beneath the Zangbo Suture, they propose that this magmatism was related to southward subduction that migrated northwards, and is hidden by thickened crust.  The huge strike-slip systems are, to Tapponier et al., nothing less that oblique suture zones.  The crustal blocks that they separate are, according to their model, large thrust wedges founded on major crustal detachments that accomplished most of the shortening.  The process did not involve destruction of oceanic basins, but subduction of sub-continental mantle lithosphere, when crust and mantle became detached.  Each successive subduction-accretion episode added its own increment to surface uplift, there probably having been three major steps in creating the highest average topography on Earth.

Toffee found in meteorite?

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The origin of life on Earth would have been greatly accelerated had some of the compounds used in constructing complex bio-molecules simply rained onto the young planet from outer space.  Carbonaceous chondrite meteorites are known to contain a tremendous blend of many possible precursors, ranging from amino acids to the ampiphile molecules, whose curling-up in the presence of liquid water is seen by many cosmo-biologists as a route to the formation of cell walls.  The latest addition to possible ingredients are sugars and related compounds in the two most important such meteorites, Murchison and Murray (Cooper et al. 2001.  Carbonaceous meteorites as a source for sugar-related compounds for the early Earth.  Nature, v. 414, p. 879-883).  Detection of simpler carbon-based molecules in the spectra of interstellar molecular clouds, from which the Solar System probably accreted, suggests that a complex chain of photochemical reactions followed by thermochemistry as the pre-solar nebula became denser was the route to seeding the vicinity of the Earth with biological potential.  However, the next steps ending in chemical self-replication and its RNA/DNA control remain a great deal more mysterious than detection of suitable reagents.  For one thing, all life-molecules rotate polarized light in only one direction (anti-clockwise), whereas those of abiogenic origin, such as the compounds found in meteorites, rotate it both ways in roughly equal proportions.

See also:  Sephton, M.A. 2001.  Life’s sweet beginnngs.  Nature, v. 414, p. 857-858.

The “Big Five” become the “Big Three”?

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That mass extinctions mark several fundamental boundaries in the stratigraphic column (late-Ordovician, late-Devonian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Tertiary) seemed to have become a well established feature of geology, thanks to the vast compilation and analysis of marine and terrestrial  organisms by the late John Sepkoski and David Raup.  However, it is very much a numerological exercise matching extinctions, new arrivals and their precise timing.  Although not exactly “lies, damned lies and statistics”, analysing the fossil record depends on both data and algorithms.  A new crunching of Sepkoski and Raup’s data, by Richard Bambach and Andrew Knoll of Harvard University, casts doubt on two of the formerly outstanding extinctions.  They see a distinction between true mass extinctions – lots of genera popping their collective corks very quickly, and mass depletions, when a more general rate of extinction fails to be matched by newly evolved taxa.  According to Bambach and Kroll, the late-Devonian and end-Triassic events fall in the latter category, leaving only three “big ones”.

Palaeontologists seem quite relaxed about these demotions and an earlier degradation of the Cenomanian-Turonian extinction, but one wonders about those who have beavered away at possible causes.  Impactophiles have congregated lately on both boundaries, studying signs of correlation with large cratering events (Woodleigh in Western Australia and Manicouagan in Canada, respectively).  Because the fossil record has a great deal to do with where collectors have been (and that has usually been close to their home bases in Europe and North America), it is anthropogenically biased.  So far, new collections from further afield have failed to numerically overcome this skew, but the demise of the late-Devonian event stems largely from recent work in China.

Personally, I fail to see the distinction.  The failure of evolution to repopulate niches abandoned by extinct genera seems equally as odd as spikes in the rate of extinctions.  However, I have always been worried that the humble graptolite’s disappearance at the end of the Silurian hasn’t been recognised as a sign of dreadful times.  These meek and co-operative creatures spread far and wide as plankton throughout the Ordovician and Silurian, evolved with an unmatched enthusiasm, and yet failed to inherit the Earth as their meekness should have guaranteed.  Still, few now seem concerned with the vast panoply of graptolitic thecae and stipes.

Source:  Kerr, R.A. 2001.  Paring down the Big Five mass extinctions.  Science, v. 294, p. 2072-2073.  Report on November 2001 Annual Meeting in Boston of the Geological Society of America.

Length of childhood and the growth of teeth

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Unsurprisingly, palaeoanthropologists pay a great deal of attention to teeth and have friendly relations with dentists. The tendency of our ancestors’ remains to be gnawed and otherwise dismembered left more of them around than other skeletal bits and pieces.  Based on the old adage that we are what we eat, teeth reveal a great deal about hominin habits.  They also take up trace elements from the environment in which individuals lived at an early age, thereby giving hints to migration.  Astonishingly, tooth enamel grows day by day, and tooth development can be charted with great precision.  Together with the timing at which different teeth erupt in juveniles, fossil dental records potentially allow researchers to detect when in human evolution the unusually extended childhood of humans first appeared, and whether it developed gradually or suddenly.  The particular focus is on teeth from prematurely deceased hominins. 

Modern humans’ rates of enamel growth is much slower than that in apes.  Despite the many signs of a profound physiological differentiation between apes, australopithecines and early species of Homo, as far as tooth growth goes, they are all similar (Dean, C. et al. 2001.  Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins.  Nature, v. 414., p. 628-631).  The teeth of each grew faster than in modern humans.  In dentition at least, there is little sign of an advance in childhood development even in anatomically very modern-looking H. erectus.  That must have taken place in early modern humans, and needs to be checked in them and our co-descendants, the Neanderthals.

Teeth provide by no means the whole story.  The near-complete skeleton of the famous Turkana Boy provides lines to suggest that when he died, his growth was well within the range of modern human development (Moggi-Cecchi, J. 2001.  Questions of growth.  Nature, v. 414, p. 595-596).  It seems unwise to rely entirely on teeth.  One possibility is that several important features (brain size, growth of tooth enamel, and even bipedalism) may have undergone repeated evolution – two steps forward, one step back?

Strontium load of Himalayan rivers

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One process connected to long-term climate change is the way that weakly acid rainwater (containing dissolved CO2) weathers silicates in continental rocks, one product being carbonate in soils.  The process should draw CO2 from the atmosphere, thereby reducing its “greenhouse” effect.  The idea is by no means new, but received a boost in the mid 1990’s from Maureen Raymo’s suggestion that fluctuations in the strontium-isotope composition of the oceans through geological time should be a proxy for changes in the rate of continental weathering.  The 87Sr/86Sr of marine carbonates does show clear correlation with long-term climate shifts during the Phanerozoic..

Continental weathering should increase as topographic relief becomes greater through mountain building episodes.  The Himalaya’s rise through the late-Tertiary has been suggested as a major influence over climatic deterioration, partly by its effect on the Asian monsoon and partly as a huge site for the sequestration of atmospheric CO2 by chemical weathering.  Himalayan rivers have enormous flows and equally large sediment and dissolved element loads.  In particular they carry far more strontium than other rivers, and it has a highly radiogenic content of 87Sr.  There are three means of attaining these levels: from average continental crust which has a higher 87Sr/86Sr ratio than oceanic crust (the other main source of seawater strontium); from strontium rich limestones that acquired their isotopic signatures from the ocean when they were deposited; or from sources with unusually high 87Sr/86Sr ratios.  The Himalaya are well known for carbonate sediments, and for granites formed by melting of deeper, older continental material that gives them very high proportions of radiogenic strontium.  Recent work now shows that a significant contribution of highly radiogenic strontium to Himalayan rivers is hydrothermal activity (Evans, M.J. et al. 2001.  Hydrothermal source of radiogenic strontium to Himalayan rivers.  Geology, v. 29, p. 803-806).  Hot springs feeding a major tributary of the Ganges contribute up to 30% of its strontium load, and incidentally a great deal of CO2.  Both result from hydrothermal alteration of deeper rocks, and are unrelated to weathering if the water involved emanates from the deep crust.  It seems that these waters are recycled rainwater, so this is a case of a high-temperature chemical weathering.  Whatever, it further complicates the original notion of linkage between mountain building and climate.

Methane and Snowball Earth

The well-publicized “Snowball Earth “ model for Neoproterozoic glaciogenic rocks that occur at tropical palaeolatitudes has to involve an escape mechanism from global frigidity.  Without some means of warming, the high albedo of widespread ice would have locked the Earth into perpetual glaciation, which of course did not happen.

The main proponents of the model, Paul Hoffman and Dan Schragg of Harvard University suggested a gradual build up of volcanogenic CO2 during “Snowball” conditions, when a dry atmosphere would have retained the “greenhouse” gas instead of its being sequestered to the oceans and carbonate rocks by acid rain and continental weathering.  Gradually, atmospheric temperatures would have risen due to trapping of outgoing, long-wave radiation by CO2.  This simple aspect of the model leads to scenarios where warming overruns once ice sheets disappeared, to give extremely high-temperature conditions.  Using carbon-isotope data from marine carbonates is a means of supporting or refuting this escape mechanism, and also of detecting the influences of other components of the carbon cycle.  Carbonates take up carbon dissolved in seawater without fractionating its different isotopes, and provide measures of the degree to which organic processes did contributed to fractionation.  Cell processes preferentially take up 12C, and if large masses of undecayed organic matter ends up in seafloor sediments, the proportion of “heavier” 13C (indicated by the standardized ratio of the two main isotopes d13C) increases in seawater and the atmosphere.  Carbon of mantle origin, that emerges as volcanic CO2, has a constant d13C of about -5‰.  So these two processes contribute to an isotopic balance, which for most of the Mesozoic and Cenozoic Eras established a d13C of between 0 and +4 ‰ in sea water and limestones.  This is interpreted as a sign that the recent carbon cycle achieved a balance between volcanic additions and organic carbon burial weighted towards trapping of undecayed carbohydrate in sea-floor sediments.  Explanations for broad climate changes since 250 Ma therefore rely more on other mechanisms than on the carbon cycle

The most comprehensive study of Neoproterozoic carbon (Walter, M.R. et al. 2000.  Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulfur in seawater, and some interpretative models.  Precambrian Research, v. 100, p. 371-433) does indeed show dramatic see-sawing of d13C through supposed “Snowball” events, from highly positive values (<+10‰) before glaciogenic sedimentation to highly negative (>-10‰) in the immediate aftermath.  However, few data were available from within glaciogenic sediments, and resolution is insufficient to detect tell-tale trends.  The key approach needs detailed carbon isotopes through a single event, and such data appeared recently for the famous Neoproterozoic glaciogenic-cap carbonate sequence of Namibia (Kennedy, M.J. et al. 2001.  Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals.  Geology, v. 29, p. 443-446)

Kennedy et al. measure d13C in carbonate cements in the glaciogenic diamictites, in overlying cap carbonates and in cement to later clastic rocks.  Interestingly, there is little sign of a gradual decrease in 13C through the glaciogenic rocks.  Constant oceanic carbon composition would be expected if no volcanic CO2 entered seawater during frigid, dry conditions, and living processes were minimal.  In the cap carbonates d13C plummets from +3‰ to -4‰.  One simple explanation would be massive “rain-out” of volcanic CO2 (d13C of -5‰) that had built up in the air during the “Snowball” episode.

Whizz-bang at end of Permian

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Relating mass extinctions to the effects of impacts by comets or asteroids is now a major industry, and a great number of geologists who sneered at early suggestions of extraterrestrial influences over evolution are finding ever new ways to cook and eat their headgear.  Oddly, however, many of those who bore the brunt of such mean-spirited, and somewhat premature scorn still cling to the safe old K-T event.  Soon all the thin K-T boundary material will have been consumed by these cautious, if meticulous scientists.  Thankfully, some have ventured to seek evidence for other catastrophes that came out of the blue. In comparison with the end-Permian extinction, the K-T event is a mere bagatelle.  However, attaching it to an extraterrestrial cause has proved difficult.  It has attracted as many opponents of impact theories as “whizz-bang” aficionados, with much talk of the effects of sea-level changes, volcanism, ocean anoxia and climate shift.  They may be in for a big surprise.

The Permian-Triassic boundary in Meishan, China is at first sight a nondescript sequence of shallow marine strata, albeit complete.  The last occurrence of Permian marine genera there, with typical signs of mass extinction, coincides with a 20-fold increase in nickel concentrations.  Closer examination reveals other brusque geochemical and mineralogical anomalies, including magnetic grains of iron-silicon-nickel alloy, but no iridium anomaly (the popular target for detecting asteroidal impact horizons) or examples of shocked quartz and feldspar (Kaiho, K. et al. 2001.  End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle.  Geology, v. 29, p. 815-818).  Most significant is a sudden drop in 34S due to a large increase in the amount of isotopically light sulphur in the environment.  Kaiho et al. attribute this to vast emission of sulphur from the mantle.  A coincident fall in the 87Sr/86Sr ratio could also result from entry into the oceans of lots of mantle-derived strontium.

The P-Tr boundary also coincides with the time of eruption of the largest continental flood-basalt province, the Siberian Traps.  No doubt other scientists will seek to account for the chemical anomalies at Meishan as distant effects of the Siberian volcanism alone, as they have for the K-T boundary anomalies because of their coincidence with Deccan volcanism.  The authors prefer to suggest a causal link between impact and massive volcanism.

Surviving the Archaean with a UV jacket

Earth’s dominance, for at least the last half billion years or so, by oxygen-dependent and oxygen producing life forms stems from the evolution of photosynthetic organisms whose cell metabolism involves breaking the strong bonds in water molecules with solar energy.  Chemo-autotrophic life that exploits other energy sources has been consigned to niches that are very much narrower than they were at the biosphere’s outset.  The earliest primary producers using oxygenic photosynthesis were the cyanobacteria – arguably the predecessors of modern plants’ chloroplasts, in Lyn Margulis’ endosymbiotic model for the origin if the Eucarya.  Carbon isotopes from the early Archaean do suggest their presence close to the start of recordable geological history, and at around 3.5 Ga the first known stromatolites were almost certainly secreted by blue-green bacteria (See Carbonates and biofilms, Earth Pages August 2001).

To thrive and colonise ocean surface waters, the shallows and perhaps even the continental surface – their water-splitting, solar powered metabolism opened up those opportunities – cyanobacteria, more than any other prokaryotes, had to resist massive damage from ultraviolet radiation.  Lack of atmospheric oxygen, and therefore ozone, left Earth’s surface with no shield to the most biologically damaging, short-wave UV.  Despite the fact that modern “blue-greens” can survive climatic extremes from the frigidity of Antarctica’s Dry Valleys to superheated water in hot springs, as regards UV damage they are wimpish.  This is partly due to its bleaching effect on the light-harvesting pigment on which chlorophyll depends.  Cyanobacteria cells do have some biochemical protection against radiation damage, but it is of no avail when bathed in the “hardest” UV likely to have characterized Archaean surface environments.

A widely held view is that “blue-greens” survived and prospered because of another function common to many single-celled organisms; their tendency to promote nucleation of inorganic compounds outside their cell walls.  Stromatolites themselves are good examples of the production of biofilms, being made of minute laminae of carbonates, whose secretion helps cyanobacteria avoid calcium stress.  In modern hot springs that contain dissolved silica, these organisms often help generate sinters made of silica.  A team from the University of Leeds (Phoenix, V.R. et al.  2001.  Role of biomineralization as an ultraviolet shield: Implications for Archaean life.  Geology, v. 29, p. 823-826) has performed controlled experiments on living cyanobacteria from Icelandic hot springs to check their defences against short-wave UV.  With a biofilm screen (in the experiment they used wafers made from associated iron-silica sinter, as well as colonies with a biofilm) the organisms easily survived and continued to photosynthesize.  Exposed “naked” they succumbed after only a few days exposure.  It seems that traces of iron incorporated in the films dramatically enhance the UV-screening, without reducing photosynthesis.  Archaean iron-rich cherts are massively abundant in banded iron formations, and the first definite remains of cyanobacterial cells come from such silica-rich material.  However, the ubiquitous stromatolites in limestones of early Precambrian times are the main signs of life.  It remains for the UV-screening properties of carbonate biofilms to be assessed.

New phyllum from Chinese Cambrian

Incompleteness of the fossil record is partly a result of the bias towards organisms with hard parts and against soft tissue, during sedimentary processes.  For preservation of soft-bodied animals, together with that of intricate parts of the usual fossils, palaeontologists look to site where preservation is exceptionally good – lagersttätten.  An example is the Solenhöfen Limestone, famous for Archaeopterix.  Mudstones formed under highly reducing conditions, which excluded bacteria that complete oxidize flesh, provide similar opportunities.  Work through the last two decades by Simon Conway Morris of the University of Cambridge has resulted in working and interpretative methods that permit extremely detailed analysis of physiologies, beginning with the most famous lagersttätte, the Middle Cambrian Burgess Shale of British Columbia.  Conway Morris and others unearthed beasts so strange that they had little choice other than to erect new Linnaean Classes and Phylla to classify them.  Equally as important, such sites help fill in the details of early members of those which survive today, including the elusive penis worms.

Conway Morris has been part of a team based at the Northwest University in Xi’an China, which has discovered lagersttätten in the Lower Cambrian, closer in time to the explosive development and radiation of animals at the end of the Precambrian.  Once again, unsuspected novelty has turned up (Shu, D.-G. et al.  2001.  Primitive deuterostomes from the Chenjiang lagersttätte (Lower Cambrian, China).  Nature, v. 414, p. 419-424).  Along with excellent examples of agnathan fish and many familiar soft-bodied animals, the prize in this case are remains that warrant a new, extinct Phyllum, the Vetulicolia.  The organisms are small but complex, with two main body chambers that reveal mouth, innards and gill slits.  The last helps place them within the deuterostomes; an “umbrella” that groups chordates (sea squirts and vertebrates) and echinoderms (they have lost such slits, but are genetically closer to chordates than any other group).  Critical to the evolutionary significance of the vetulicolians is a groove that floors what is interpreted as the anterior part of their alimentary canals.  Such a groove characterizes the pharynx of chordates, where it serves as “gutter” for various glands – the endostyle, also involved with iodine in metabolism.  If the vetulicolian groove is an endostyle, then they are chordates.  However, lacking an axial stiffening rod (notochord of the chordates in general, and vertebral column in vertebrates) they must be primitive.  Occurring with true vertebrates, in the form of jawless fish, the vetulicolians are a relic of some earlier stage in vertebrate evolution.  Shu et al. take the cautious view that they are early deuterstomes from which echinoderms and chordates emerged – close to the fundamental division among animals into deuterostomes and protostomes.

(See also:  Gee, H.  2001.  On being vetulicolian.  Nature, v. 414, p. 407-408)

EarthScope

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North America, particularly its west coast, is the best studied natural laboratory for active tectonics.  Nonetheless, the downturn in Earth Science funding in the USA has threatened an ambitious project aimed at consolidating knowledge of plate interactions there.  Nature (15 November 2001, p. 241) reports that the EarthScope initiative now has strong backing from the US National Academy of Sciences.

EarthScope has 4 elements: a mobile grid of seismometers; an observatory to monitor movement of plates below the NW Pacific Ocean; a programme aimed at drilling into the San Andreas Fault System; an interferometric radar satellite that will accurately measure ground movements in relation to tectonic and volcanic features.  The total cost is around $400 million, shared equally between NASA and the National Science Foundation, if the funding proposal wins acceptance.

Information from:  http://www.earthscope.org

Continental tectonics of eastern Eurasia

Interferometric radar remote sensing provides high precision information on Earth motions associated with earthquakes (Radar analysis of Turkish earthquake, Earth Pages August 2001), but depends on “before and after” imaging.  Continental tectonics is not just the outcome of occasional large movements on major faults, but of strains that continually occur throughout the lithosphere.  Global positioning satellites provide means of precise location, particular when operated in differential mode, in which field-station signals are matched to those at fixed, geodetically precise base stations.  Precisions to within centimetres or better are now commonplace at low cost.  Structural geologists have been using GPS receivers for over a decade to check on the annual rates of plate motion across major structures such as the Alpine Fault of New Zealand and spreading centres such as that exposed on land in Iceland.  In the 19 October issue of Science, such geodetic analysis of tectonics leaped by an order of magnitude.

The jewel in the crown of continental tectonics is eastern Eurasia, where the active collision of the Indian sub-continent with Asia drives a huge array of very large faults that separate rigid blocks and others, such as the Tibetan Plateau, that are deforming en masse.  The spreading power of the Carlsberg and Central Indian Ridges is dissipated in motion of continental crust spanning 30° of latitude and 60° of longitude.  Chinese scientists and their collaborators from the US universities of Alaska and Colorado have measure GPS positions at 354 stations throughout China, every one or two years for the last decade.  Their analysis of the interim results (Wang, Q. et al.  2001.  Present-day crustal deformation in China constrained by global positioning system measurements.  Science, v.  294, p. 574-577) helps confirm or modify ideas about crustal motions that stemmed from seismic first-motion studies and regional field evidence.  More than a third of the tectonic power accounts for crustal shortening within the Tibetan Plateau.  While the western part of the huge system involves consistent motion towards the north-north-east, driving into Eurasia’s hinterland, the “free-edge” of eastern China  and Indo-China seems to encourage the escape tectonics first proposed by Molnar and Tapponier.  That involves a massive clockwise rotation around the East Himalayan Syntaxis, which takes up a great deal of motion.  Whereas Molnar and Tapponnier proposed the shoving of south-eastern China oceanwards by the “escape” of Tibet, Wang et al’s measurements reveal that its motion to the east is only between one third and a quarter that of the adjacent east Tibetan Plateau.  The lack of any sign that Tibetan crust is overriding that of south-east China, or that the latter is being shortened, may suggest that escape is funnelled around the East Himalayan Syntaxis into Burma and South-East Asia.

Fate of the Neanderthals

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Chris Stringer and William Davies report on two recent conferences about the Neanderthals in the 25 October issue of Nature (Stringer, C. and Davies, W 2001.  Those elusive Neanderthals.  Nature, v. 413, p. 791-792).  Debate continues on what happened to them, and why.  Assimilation by gene flow remains a possibility with a few researchers, despite the mismatch between fragmental Neanderthal DNA and that from modern people, and the inability to get Cro-Magnon genetic material is vexing.  Acculturation – the influence of the behaviours of groups on one another – is also an unresolved issue.  At the centre of that particular debate are tools associated with late-Neanderthal sites that bear close resemblance to those of early Cro-Magnons; the so-called Châtelperronian.  The problem is precision and accuracy of dating the material, which, of course, constitute the palaeoanthroplogist’s Sword of Damocles.  Dating using the decay of 14C has long been a right old mess, what with variations in the cosmogenic productivity of the isotope, and the tendency of common bone samples to pick up stratigraphically younger carbon from humic acids in soils.  Charcoal is the material of choice, but in the case of Châtelperronian artefacts only associated bone seems to be available.  Help might be on the way in resolving inaccuracy that stems from variable 14C productivity by using marine-core data to calibrate terrestrial 14C dates to calendar years (the “CalPal” curve).  It does, however, seem to be peeking over the horizon at present.

One of the alternative processes that might have snuffed out Neanderthals is climate change.  High-resolution marine records are not too useful in that regard, because they reflect global processes, and Neanderthal demise was a regional issue.  Pollen records from lake sediments in Italy now reveal the intricacies of European climate during the critical period around 30 ka.  It was time of rapid fluctuations in tree cover.  However, similar rapid vegetation shifts occurred long before modern human influx, and the Neanderthals survived them.  One possibility, allied to the competitive-disadvantage hypothesis, is that Cro-Magnons brought a steppe culture with them, which allowed them to occupy open country more successfully than Neanderthals with a woodland culture.

The topic is stymied by imprecise dating (it can be as bad as ± 4 ka), so that open-season for speculation is protracted.  There is a reluctance to consider extinction through epidemic diseases brought by newcomers, and against which Neanderthals had no immunity.  Disease has played such a huge role in population crashes throughout recorded history, that for it not to be at the forefront is curious.  It is a widely supported hypothesis for extinction of large mammals that coincided with first entry by modern humans into the Americas ( see Late Pleistocene mass extinction – July 2001 Earth Pages).  That would have had to involve jumps between species, rather than simple transmission of killers such as measles between genetically very similar populations of humans.

En route out of Africa

Finds of H. erectus and artefacts in China and Georgia date back as long ago as 1.8 Ma; the earliest signs of massive diffusion of early humans protected by their culture from entirely new climates and surroundings.  The great question is, “Which way did they go?”  To many palaeoanthropologists, obstacles presented by the Arabian Desert and Caucasus Mountains, favoured exit from Africa via the Straits of Bab el Mandab (closed at that time) and coastal diffusion.  It now seems that movements of early humans did reach the Levant at a very early date.  Ron Hagai and Shaul Levi have produced strong evidence for H. erectus’ presence in the Dead Sea rift at around the same time (Hagai, R. and Levi, S.  2001.  When did hominids first leave Africa?: New high-resolution magnetostratigraphy from the Erk-el-Ahmar Formation, Israel.  Geology, v. 29, p. 887-890).  They found that sediments enclosing primitive, Oldowan tools (but no skeletal remains) accumulated during the period between two magnetic polarity reversals.  With other evidence, these correlate with the Olduvai subchron from 1.96 to 1.78 Ma.  Definitely a “first” for the Middle East, but by no means proof that this lay on the route to wider colonization, even at Dmanisi, across the Caucasus in Georgia.  Little would prevent easy diffusion from East Africa along the proto-Nile or the Red Sea coast to reach the Dead Sea rift, but the obstacles to the north and east of Israel would have been far greater for poorly clad and equipped Erects.