US security clamp on vital data

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The geopolitical realities of remotely sensed data became plain in the aftermath of the 11th September attack by terrorists on the United States.  The US National Imagery and Mapping Agency (formerly the Defense Mapping Agency of the Department of Defense) placed a moratorium on release of digital elevation data derived from NASA’s February 2000 Shuttle Radar Topography Mission, “in the interests of national security”.  The SRTM, which used radar interferometry from dual antennae on a 60 metre long boom, was intended to satisfy the huge demand from Earth scientists for digital elevation models of the continents for a large range of applications, ranging from accurate hydrological mapping to sophisticated mathematical analyses of landforms.  An accurate, high-resolution DEM is central to rapid topographic mapping of those many parts of the world where published scales do not exceed 1:250 000.  NIMA also maintains the classified DTED Level-1C global elevation data set, derived from a variety of sources, including clandestine aerial and satellite photography, and which has a resolution as precise as 30 metres.  SRTM data are reported to be more revealing.  At the heart of cruise-missile guidance and the real-time imaging radar used for navigation in low-flying, all-weather military aircraft lies DTED Level-1C data.  Such facilities are not known to be in the possession of, or under development by any agencies other than the military of a small number of developed countries, for obvious economic reasons.  Oddly, elevation data as revealing as DTED Level-1C for the whole of the USA and its territories are still available freely from the US Geological Survey.  Anyone “targeting” installations, either for military or more innocent purposes, need look no further than the growing number of commercial image providers who sell satellite images with spatial resolutions as good as 1 metre.  Indeed, some such companies currently promote their wares through images of Manhattan Island in the aftermath of 9th September, and there is a thriving business in selling aerial photographs of real estate with resolutions up to the 10 centimetre level.

Browsing through the archive of data from the Terra satellite, particularly those from the ASTER instrument (visit http://edcimswww.cr.usgs.gov/pub/imswelcome/ ), reveals a disproportionate focus on Afghanistan compared with much of the rest of the world.  The majority of Afghan images were captured before 11th September, and the area is hardly a priority for scientific research.  ASTER produces stereographic images with a 15 metre resolution, suitable for producing high-quality digital elevation models that rival those of SRTM.  It would not be surprising to discover that US and British Special Forces engaged in Afghanistan not only carried large-scale topographic maps derived from ASTER images, but also commercial Ikonos 1-metre images, that are capable of pinpointing vehicles and concentrations of people.  Nor is it surprising that relief agencies, intent on delivering humanitarian supplies to emergencies of many different kinds in nearly unknown terrain, rarely if ever have such sophisticated navigational aids.

Interferometric radar and faults of the Mojave Desert

Though it requires considerable computing power and specialized software, the use of “before” and “after” radar data to detect small-scale subsidence or shifts in the horizontal plane, is a potentially powerful tool in neotectonics (see Radar analysis of Turkish earthquake, August 2001 Earth Pages).  Motion detection by such radar interferometry becomes even more useful as historic radar images accumulate.  The workhorse for radar interferometry is the European Space Agency ERS series of satellites, which produce synthetic aperture radar images about 150 km wide along the same track, orbit after orbit.  The system has operated since 1992, so there are rich possibilities for multitemporal use of the distance-measuring capacity inherent in radar imagery.  Means of assessing the regional build-up of strain in seismically active areas are important in earthquake prediction, and such synopses help understand the tectonics at work there.

In terms of seismicity and tectonics there is no better studied area than that extending from the Pacific coast of southern California across the San Andreas Fault and the Mojave Desert.  Radar interferometry provided by 25 pairs of ERS images from 1992 to 2000 produces a spectacular picture of the gradual development of ductile strain underlying this risky area (Peltzer, G. et al. 2001.  Transient strain accumulation and fault interaction in the Eastern California shear zone.  Geology, v. 29, p. 975-978).  Unsurprisingly, shear strain along the San Andreas fault system shows up well.  The Garlock Fault that marks the NW flank of the Mojave is apparently resting after 10 thousand years of motion that averaged 7 mm per year.  The authors focus on displacements associated with the diminutive, by Californian standards, Blackwater and Little Lake fault systems, which trend SE-NW to link the epicentres of the 1872 Owens Valley earthquake and that at Landers in 1992.  Within 10 km of these aligned faults are clear signs of a step in strain rate, that suggests that the lineament lies above a major, active ductile shear zone; perhaps the birth of a new fault system.  Should this system fail in a brittle fashion it is likely to result in an event with a magnitude greater than 7 on the Richter scale, and a surface break more than 100 km long.  Peltzer et al. have achieved a test of concept for interferometric radar’s use in seismic risk assessment, that can be deployed anywhere, given the computing resources.  Their work transcends after-the-event studies that do little to assist the victims of earthquakes.

New map resource for Earth scientists

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Researchers at Cornell University have been compiling digital maps of a wide range of data for the last 8 years.  The Digital Earth project’s web site is http://atlas.geo.ecornell.edu .  There, it is possible not only to download various data sets for use in a GIS, and to track down primary sources for data, but also to build your own maps.  Digital Earth comprises over 100 data sets, on global, regional and North American scales, that include geographical, geological and geophysical themes.  The mapping tool takes a while to get used to, and runs slowly with a 56k connection, but should behave well with broadband access.  I tested the tools by creating a geological map of NE Africa.  This was pleasingly up to date and showed moderate detail, but the lack of a legend is something of a drawback.  Understandably, the level of stratigraphic division is limited, so that all Precambrian areas appear in the same colour.  Similar detail is not yet available for Europe, only a coarser resolution world geology data set covering it.  Downloads are in either Postscript or jpeg form, the latter suffering from artefacts generated by compression.   This is a site well worth a visit.

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.

Experimental satellite to have extended mission

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The Earth Observing-1 (EO-1) satellite, launched by NASA in late 2000, carries two remote-sensing instruments that may become operational devices in the future, given a proven track record on EO-1 and, of course, sufficient funding.  One, the Advanced Land Imager (ALI) is a test bed for sensors earmarked for the follow-on to the current Landsat-7 Enhanced Thematic Mapper+ (ETM+).  As well as the existing ETM+ six bands, ALI covers three others close to existing bands.  Whether by design or good fortune, two of these help define the important VNIR broad absorption by ferric iron minerals, neglected in remote sensing since the early days of the Landsat Multispectral Scanner.  Like the ETM+, ALI also carries a panchromatic band that spans the visible range, and which is aimed at providing a means of sharpening detail in images.  On ALI, however, this band has an improved resolution of 10 metres as opposed to the current 15.

More innovatory is the Hyperion instrument, a hyperspectral device that spans the visible to short-wave infrared range with 242 bands that are 10 nanometre wide.  Hyperion is comparable with airborne hyperspectral devices, such as AVIRIS.  In the experiment it captures data swathes that 7.7 km wide, made up from 256 pixels with a resolution of 30 m.  After initial difficulties with allowing for atmospheric effects  on the data, newly calibrated Hyperion data closely mimic mineral spectra.

Early work on EO-1 data in many fields, including geology, has proved sufficiently promising that NASA has given the mission a year-long extension.  Although data are restricted to only a few target areas suggested by the investigators, the extension is good news.  It is a reassurance about continuity of the Landsat programme, and a tantalising indication that the ill-fated hyperspectral Lewis satellite may be resurrected.

Information from: http://eo1.gsfc.nasa.gov/

Handy guide to the significance of meteorites

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Although the press made a great fuss in 1999 about the supposed discovery of signs of life in a meteorite reckoned to have been blasted off Mars by a giant impact, meteorites in general are the only direct means of developing ideas about how the Earth and the rest of the planets formed.  The market in meteorites is beginning to resemble the London Metal Exchange in its frenzied bullishness, but being collectibles it is rare types that command the highest prices, rather than their significance.   An excellent review of current ideas among meteorite specialists appeared in the 6 July issue of Science (Alexander, C.M.O’D., Boss, A.P. and Carlson, R.W. 2001.  The early evolution of the Solar Syetem: a meteoritic perspective.  Science, v. 293, p. 64-68).

Since development of theoretical ideas about the generation of the elements in stellar processes, it has become almost a cliché to ponder about the ultimate dependence of every aspect of the natural world on supernovae and their “seeding” of the galaxy with the chemical mix that is so familiar.  Even the nuclear processes involved are easily grasped.  Not so the means whereby star stuff assembled into planetary systems and laid the potential for life, plate tectonics and virtually everything else.  Alexander and colleagues from the Carnegie Institute of Washington span the interactions between physical conditions around young and rapidly evolving stars, derived theoretically, and the kinds of compounds that they can generate.  Meteorite chemistry and mineralogy, which are very diverse, put flesh on the bones of these ideas.  The tangible properties of different meteorite classes, together with their radiometric ages, are analogous to fossils in piecing together both planetary evolution and the various kinds of environments in the early Solar System.

One conclusion in the review that surprised me concerns the oldest materials known to us – calcium-aluminium-rich inclusions found in some chondrites, such as the famous Allende meteorite that fell in Mexico.  The pale inclusions contain evidence for the former presence of short-lived isotopes, such as 26Al.  So short are their half-lives that the delay between their nucleosynthesis and the assembly of the pale inclusions can have been a few hundred thousand years at most.  There are two possibilities: either such isotopes were generated by energetic particles emitted by the growing early Sun, or they had their source in supernova events.  Theoretical work on local genesis has so far failed to match the relative abundance of all such short-lived isotopes, derived from the amounts of their decay products found in pale inclusions.  It seems highly likely that collapse of a pre-solar cloud of matter to form the nebula out of which Sun, planets and the parent bodies of meteorites emerged was set in motion by shock waves from a nearby supernova.  They would have taken the form of a high-speed interstellar “wind” of gas.  Observed differences in oxygen-isotope proportions in meteorites were once ascribed to heterogeneous mixing of this explosive introduction of exotic matter.  However, the oxygen heterogeneities do not show up in the isotopes of other elements.  That mismatch has led to ideas of chemical fractionation during Solar System evolution, akin to that so familiar from the different behaviours of “light” and “heavy” oxygen during evaporation of water and its uptake in skeletons of living things exposed to different climates.  Differences in oxygen isotopes now form a strand in assigning different meteorites to sources at different distances from the evolving Sun, and in deducing that some rare meteorites did indeed come from Mars.

Clearly behind the hype surrounding promotion of staffed and unstaffed missions to Mars and the increasingly shady world of the meteorite trade, exciting research is being done.

Ice and prebiotic chemistry

The problem with ice on Earth is that it will not support living chemistry.  The process of crystallization excludes impurities from its structure, so that reactions between organic compounds cannot go on.  Comets are mainly ice, and frozen water is a common occurrence in the infrared spectra of interstellar clouds, along with a host of complex CHON compounds (over 100 discovered to date).  How organic molecules form in cold molecular clouds is a difficult problem, or at least it was believed to be until recently. 

Researchers at the NASA Ames Research Center in California have probed the structure of solid water under all manner of physical conditions.  Below a temperature of 200 K (about that of liquid nitrogen)  the hexagonal symmetry of ice, familiar from snowflakes, changes to the simpler cubic form.  At yet cooler temperatures 10 to 125 K), ice has no crystalline structure.  Like flint, it is cryptocrystalline or amorphous.  Curiously, even only a few degrees above absolute zero it can flow like a viscous medium, in the manner of glass, when irradiated with ultraviolet radiation.  The breaking and reforming of hydrogen bonds, as in liquid water, but slower, creates the conditions for retaining impurities and their chemical combination.  This odd behaviour at precisely the temperatures of molecular clouds explains their richness in organic molecules.  Quite probably comets form by accretion of such interstellar icy material.  The experiments revealed that warming of amorphous ice above 125 K does not result in a complete transition to cubic ice, that would exclude impurities.  Instead, around two thirds retains its odd properties.  The discovery strongly hints that much of the basic work of producing precursors to life’s chemistry is not only feasible in interstellar space, but that they can be delivered to planets as they collide with comets giving a kick start to the origin of life.

Source:  Blake, D.F. and Jenniskens, P.  2001.  The ice of life.  Scientific American, August 2001, p. 36-41.

Dinosaur nose mystery resolved?

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Popular animations of dinosaurs in Jurassic Park and Walking with Dinosaurs are palaeontologically speaking “state of the art”.  That is, except for the beasts’ noses.  A close observer will have seen Tyrannosaurus and Triceratops with nostrils high on their snouts, and appealing brachiosaurs apparently breathing through the tops of their heads.  Such reconstructions rely on the position of the nasal passages where they enter the skull, and in dinosaurs such bony nostrils are large and complicated.  Traditionally, dinosaur reconstructors have gone for the rear of the cavity for the positions of the fleshy nostrils.

Despite their extinction at the end of the Cretaceous Period, dinosaurs have many living close relatives, such as birds, crocodiles and some primitive lizards.  All of them have fleshy nostrils situated at the front of the bony openings.  For that matter, so do mammals.  For several years Lawrence Witmer of the College of Osteopathic Medicine at Ohio University (Athens) has been pondering on this, even setting up the DinoNose project.  Not only did Witmer apply the principle of parsimony to this intriguing issue, but noted the marks left on skulls by the blood vessels that supply the muscles that enable land vertebrates to snuff the air in many interesting and useful ways.  Such marks appear on dinosaur skulls, towards the forward end of the nasal openings.  The outcome is a fundamental revision of  dinosaur physiognomy (Witmer, L.M. 2001.  Nostril position in dinosaurs and other vertebrates and its significance for nasal function.  Science, v. 293, p. 850-853).  The next logical step is to seek signs that carnivorous dinosaurs did indeed snarl.

Cambrian Explosion:  Shropshire hits the news

As if by magic, nearly all animal phyla suddenly appear in the fossil record around 545 Ma, at the base of the Cambrian period.  The most famous of these are trilobites, a group within the phylum Arthropoda, for enthusiasts of which the Cambrian of Shropshire has long been a happy hunting ground.  Temporary excavations into the Protolenus Limestone of Comley have revealed a somewhat diminutive, though nonetheless startling relative that helps resolve the long-running debate over the origins of animals (Siveter, D.J., Williams, M. and Wlaoszek, D.  2001.  A phosphatocopid crustacean with appendages from the Lower Cambrian.  Science, v. 293, p. 479-481).  Superbly preserved in calcium phosphate, the tiny beast reveals great detail of its body parts, peeping from between a two-valve, spherical carapace.  It is possibly an early ostracod, and certainly a crustacean.  That such an advanced arthropod occurs close to the base of the Cambrian lends support to the view that animal diversification into extant phylla, and some vanished ones too, may have gone on far back into the Neoproterozoic.  The other view is that this radiation was explosive, beginning only 10 Ma or so before the base of the Cambrian.

The “long-fuse” hypothesis for the emergence and diversification of the animals is also supported by differences in the molecular biology of distantly related modern animals.  Assuming that accumulation of genetic change is steady, and can be calibrated by the coexistence of such groups as far back as the Cambrian, the “molecular clock” for animals probably started between 700 to 1500 Ma ago.  The problem, of course, is that only animals with hard parts or which miraculously had soft tissue rendered preservable by mineralization can assist palaeobiologists resolve the issue.  That is unfortunate, as such fossils occur only after about 5 Ma before the start of the Cambrian, and the large ones are exclusively Cambrian or younger.  The “explosion” was the sudden appearance of skeletal material, using calcium compounds such as carbonates or complex phosphorus-bearing material.  Such is the fascination with the detail of phyllogeny, that the trigger for the explosive emergence of hard parts is often overlooked.

See also:  Fortey, R.  2001.  The Cambrian Explosion exploded?  Science, v. 293, p. 438-439.

True polar wander

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One of the powerful bits of evidence that support continental drift are the plots of magnetic pole positions determined from rocks of different ages exposed on a modern continent, relative to that of the present pole position. Comparing such plots from different continents sometimes reveals similarities in their shapes over long periods of time, so that the plots partly match when they are superimposed.  In such fits, other parts of the plots diverge considerably.  Such comparisons are best explained respectively by the former unity of the two modern continents and their movement together, and their separation to drift independently.  The plots are illusory, and are called apparent polar wander paths.  For most of the Phanerozoic Aeon such palaeomagnetic data tie in well with other evidence for the formation of composite continental masses, such as Pangaea, and plate movements since the Triassic.  That provides confirmation of the basic assumption in palaeomagnetic studies that the Earth’s magnetic poles remain close to those of its rotation, bar some circulation around the axis and magnetic reversals.  It is tempting to use the same assumption for earlier times, in the absence of  easy fitting of the margins of continental segments and the sea-floor magnetic stripes that are the key to plate tectonics since the early Mesozoic.  If magnetic poles did move well away from the poles of rotation at any time in the past, that would play havoc with continental reconstruction.  True polar wander is something that many tectonicians “Dinnae care to speak aboot”!  That is not surprising, for another reason.  Use of the term imply mean two things: a long-term shift in the Earth’s magnetic polarity relative to its axis of rotation (to me that warrants the adjective “true”, and ); a shift in the relative position of the whole crust and mantle relative to the core, whose dynamism determines the magnetic field.

A recent review (Irion, R. 2001.  Slip-sliding away.  New Scientist 18 August 2001, p. 34-37) concentrates on evidence for the second usage.  There is evidence that suggests a 20° shift of all continents over a period of 2 Ma, in the Cretaceous.  This is dwarfed by a suggestion of a 90° shift in 15 Ma that span the time of the Cambrian Explosion, so that a continent could have moved from the pole to the equator at a rate far faster then anything known from Mesozoic to Recent sea-floor spreading.  One explanation is destabilization of the Earth’s angular momentum by concentration of all crustal mass and the effect of a massive mantle plume beneath it at high latitudes.  That would distort the Earth’s shape.  A planet’s rotation is most stable when it its shape is fat around the equator.  The opposite, a prolate spheroid, is least stable, and a polar supercontinent could result in such instability, restoring steady state if the whole caboodle slipped to lower latitudes.  That is what is proposed to explain some odd palaeomagnetic pole positions newly and accurately gathered from early Cambrian rocks.  Such a notion takes on its own momentum, because of its association in time with the explosive diversification of animals with hard parts.

It is not a fundamentally new idea, for Alfred Wegener suggested that the mechanism for his hypothesis of continental drift was Pohlflucht (flight from the poles) of continental mass.

Sniff ethylene and become an oracle

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In a small temple on the south slopes of Mount Parnassus in Ancient Greece sits a somewhat befuddled lady, her inhibitions definitely down and she sometimes becomes delirious and thrashes around.  The great and the good seek her opinion on matters of state.  Perhaps for almost 2 millennia, successive pythia (pythia) of the Delphic Oracle had a steady passing trade.  Writers from the classic times of Greek and Rome leave little doubt that the pythia’s powers stemmed from three things: a fissure and a spring lying at the centre of what became revered as the Sanctuary of Apollo, and a vapour that emanated from her chamber.  The Oracle was seemingly a matter of geology and its mystique. 

Indeed there are two intersecting faults passing right beneath the Oracle.  Suitably encouraged, a team from Florida State and Weslyan Universities, and The University of Louisville, USA has been studying all aspects of the site since 1995 (de Boer, J.Z., Hale, J.R. and Chanton, J.  2001.  New evidence for the geological origins of the ancient Delphic oracle (Greece).  Geology, v. 29, p. 707-710).  Where others failed before them, they have discovered evidence for a spring and traces of hydrocarbon gas leaking from a bituminous limestone cut by the faults at depth.  One of the gases in the mixture is ethylene, once used as an anaesthetic, and known to cause just the symptoms in the pythia described in ancient accounts of her powers.

Seafaring Homo erectus?

The first Homo erectus fossils recorded by Eugene Dubois came from Java.  Dubois was not so good at recording the geological context of his finds, and most of the later Javan discoveries were by local farmers.  Consequently the dates of first arrival of the erects are a subject of continual debate, recent suggestions being that erects maintained a hold in Indonesia until as late as 20 thousand years ago.  The incompleteness of  records also led to few finds of artefacts, so much so that doubt has been cast on any significant H. erectus culture.  Later work throughout Indonesia did reveal something quite astonishing, however.  The erects crossed Wallace’s line to colonise one of the easternmost islands in the Indonesian arc, Flores, where undoubted stone artefacts occur in rich beds of fossil bones.

Alfred Russell Wallace noted that the flora and fauna of western Indonesia are to all intents the same as on continental Asia, whereas those of the islands east of Bali are very different.  This empirical division is now known to have arisen through the emergence of land bridges between the western islands and mainland Asia as sea level fell to expose shallow seafloor during Pleistocene glacial periods.  Wallace’s line coincides with straits that are very much deeper than could ever disappear during falling sea level.  Reaching islands such as Flores demands that H. erectus must have devised means of crossing wide stretches of open sea.  Fission-track dating of zircons gives ages for bone beds with tools that range from 840 to 700 thousand years (O’Sullivan, P.B. et al. 2001.  Archaeological implications of the geology and chronology of the Soa basin, Flores, Indonesia.  Geology, v. 29, p. 607-609).  Erects crossed at least two major seaways to reach Flores, predating the first seafaring modern humans, who reached Australia around 40 to 60 thousand years ago by an enormous span of time.

Unwholesome fare

Since Raymond Dart’s notoriously bloodcurdling views on the dietary habits of early hominids and “the mark of Cain” appeared in his 1950s essay “The Predatory Transition from Ape to Human”, palaeoanthropology has sometimes tried to brush under the carpet evidence for cannibalism among our ancestors.  Considering the many funerary traditions practised today, some of which involve dismemberment and defleshing of corpses, it is easy to pass off cut-marks on fossil bones as indicating last rites.  However, when evidence of cooking turns up (and the Anasazi people of 12th century Colorado left plenty of evidence for that, including making anthropic soup), the common notice in pub restaurants, “Children served”,  takes on grim undertones.

Tim White, co-director of the Laboratory for Human Evolutionary Studies at the University of California (Berkeley) is a palaeoanthropologist who commands attention.  It was he who discovered evidence for Anasazi cuisine, and has subsequently maintained an interest in assessing evidence for cannibalism.  It does go back a long way, to evidence for the first European’s (H. antecessor) gustatory relish of their fellows at the 800 ka site of Gran Dolina in northern Spain, and similar signs in Neanderthal sites spaced by hundreds of generations.  The questions of, “How often?”, and, “Under what circumstances?”, are difficult to answer.  However, it was a part of the cultures unearthed by excavation.

Source:  White, T.  2001.  Once were cannibals.  Scientific American, August 2001, p. 48-55.

Radar analysis of Turkish earthquake

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The destructive Kocaeli earthquake (magnitude 7.4) of August 17 1999 involved horizontal slip of up to 5 metres.  Although it is possible to measure strains precisely using GPS arrays, many stations are needed to fully grasp strain patterns.  Interferometric processing of before and after radar data (InSAR) presents an opportunity to examine seismic strains over very large areas.  Displacements associated with the Kocaeli earthquake on the North Anatolian Fault, recorded by InSAR, extended for up to 60 km either side of the fault (Mayer, L. and Lu, Z. 2001.  Elastic rebound following the Kocaeli earthquake, Turkey, recorded using synthetic aperture radar interferometry.  Geology, v. 29, p. 495-498). 

The fault runs parallel to the look direction of SAR beams from the ERS-2 satellite in its ascending orbits.  This fortuitous geometry charted relative motions in a horizontal sense on either flank of the major strike-slip fault system, with a precision of about 3 cm.  Interesting in its own right, the recorded strain helps understand how and where the elastic strain energy released by earthquakes was stored.  The key to energy storage is the rebound pattern associated with strain release during earthquakes, to which the InSAR results are an approximation.  This pattern depends theoretically on the displacement along the fault itself, the shear modulus of the rock involved and the depth to which faulting extends.  In the case of Kocaeli, faulting penetrated to between 6 and 15 km below the surface.  Because elastic strain builds up around active faults, it may be possible to use InSAR monitoring as a means of predicting the risk of future failures on dangerous faults, like the North Anatolian Fault.  Earthquake records show that successive failure migrates westwards along the Fault, getting ever closer to Istanbul.

Methane as the early “greenhouse” gas

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Various indicators, such as the presence of detrital uranium oxide and iron sulphide grains in sediments older than about 2.3 Ga and the appearance of terrestrial sediments stained red by the presence of ferric (Fe-3) oxides thereafter, have long been used to suggest that atmospheric oxygen was a mere trace before that time.  Generation of oxygen through photosynthesis by simple organisms, principally blue-green bacteria, could have led to an oxygenated atmosphere when their productivity exceeded the tendency for oxygen to be consumed by reaction with reducing agents, such as abundant ferrous (Fe-2) iron in sea water, and by burial of carbon-rich dead organic matter.  That method is a central plank in the Gaia hypothesis.  However, geochemical considerations suggest another scenario for oxygenation (Catling, D.C., Zahnla, K.J. and McKay, C.P. 2001.  Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth.  Science, v. 293, p, 839-843).  Unless carbon burial exceeded the rate at which reductants supplied to the outer Earth (including exposure of buried carbonaceous sediments) by geological processes consumed oxygen, the atmosphere would remain low in oxygen.

Lacking in oxygen, the early atmosphere would have been able to support build-up of methane from biogenic processes – today methane is soon oxidized to carbon dioxide and water.  Carbon isotope evidence suggests that early life was dominated by methanogens, and such organisms alive today are genetically very primitive.  Consequently, methane is a good candidate for keeping average surface temperature above the freezing point of water at a time when the Sun’s output of energy was considerably lower than it is now.  All hydrogen-bearing compounds become dissociated high in the atmosphere, to release hydrogen atoms, and they readily escape the Earth’s gravitational pull.  Fortunately, this does not happen now because the only significant H-compound, water, cannot rise above the tropopause.  The decline in temperature upwards acts as a cold trap for water.  Were this boundary not in place, and it is largely due to the presence of ozone in the stratosphere which absorbs radiation to give higher-level warming, Earth would long ago have lost most of its water, as did Mars and Venus.  In the early atmosphere, methane would not have been “cold trapped”, and nor is it today.  So, during that period, hydrogen would steadily have leaked from the Earth.

The chemical outcome of such a simple process would have been a steady decline in the reducing capacity of the Earth as a whole, for hydrogen is a powerful reductant.  Because most of our planet’s hydrogen was locked in water from the time of its accretion, its escape must have resulted in a net gain of oxygen somewhere in the Earth system.  Increased methane productivity by methanogen bacteria during the Archaean and early Proterozoic would have enhanced this tendency for the whole Earth to become more oxidizing.  Catling et al. argue that the continental crust became more oxidized, so that any gases released from it by metamorphism would become less reducing.  That would have reduced the tendency for immediate consumption of oxygen produced by photosynthetic organisms, culminating in its eventual ability to exist in the atmosphere in balance with biological processes at around 2.3 Ga.

Zircons’ window on the Hadean

The oldest tangible rocks that are not completely changed by deep-crustal metamorphism are those of Isua in West Greenland.  Interleaved with gneisses that originated probably from calc-alkaline intrusions are rocks formed at the Earth’s surface around 3.8 Ga ago.  The general scene represented by this Akilia Association is in many respects familiar – the operation of plate tectonics, rapid generation of what was to become continental crust, abundant evidence for the action of liquid water and even the isotopic traces of living organisms.  That 750 Ma after the Earth’s accretion the last two were present is no surprise.  The oddity is that, despite decades of effort, there is still no sign of continents older than 4 Ga.  That crustal rocks which had undergone considerable evolution from their mantle source did exist in the missing half-billion years emerged from the discovery of detrital zircons as old as 4.4 Ga in much younger Australian sedimentary rocks.  Some of the rare, tiny grains show isotopic evidence that the magmas in which they formed had contact with liquid water at the surface.

As well as containing sufficient uranium to allow the dating of single grains by the U-Pb method, zircons also contain hafnium, which is chemically very similar to zirconium.  Measurable quantities of 176Hf add to common 177Hf by the decay of 176Lu, giving a potential dating technique.  However, zircon contains only minute traces of lutetium, so that its 176Hf/177Hf ratio remains that of the ultimate source of its host rock.  Relative to hafnium, lutetium is more likely to remain in the residue left by partial melting of the mantle, or so theory suggests (geochemists can only deduce this from various lines of indirect evidence).  Consequently, mantle that has sourced continental crust builds up 176Hf from the time such crust formed., whereas continental crust has significantly lower levels.  Studying hafnium isotopes in very old zircons is therefore a means of seeking periods when significant amounts of continental crust separated from the mantle.  Because such tiny amounts of the radiogenic hafnium are involved, an accurate decay constant for 176Lu is vital (Scherer, E., Münker, C. and Mezger, K. 2001.  Calibration of the lutetium-hafnium clock.  Science, v. 293, p. 683-687).  Zircons from the oldest rocks in Greenland, Canada, Australia and South Africa fall into two, complementary groups; those with slight enrichment in 176Hf and those with slight depletion.  Simple geochemical theory seems to indicate that indeed magmas similar to those that contributed to formation of the bulk of continental crust did form as early as 4.4 Ga ago.  However, zircons with younger Archaean ages show little sign of deviant hafnium, which suggests that a large proportion of the mantle was not involved in early sial formation.  Hadean continental material no doubt formed, but not much.  That is no surprise, for involvement of surface-derived water in mantle melting above zones where earlier lithosphere returns to the mantle, whatever their form, seems inevitable in a planet noted for its high water content.  That is the basic “recipe” for the formation of silica-rich magmas.

Two things stem from this work: the probable futility of seeking Hadean continents; the unlikelihood that the chemical heterogeneity of the mantle stemmed from Hadean continet formation on a massive scale.

See also:  Kramers, J. 2001.  The smile of the Cheshire Cat.  Science, v. 293, p. 619-620

Yet more complexity

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The view that all manner of processes connected with climate – volumes of land ice, ocean temperature and flow, aspects of atmospheric composition and its motion, and the expansion and contraction of biological communities – are locked into the cycles of changing solar input steadily evolves into something less mechanical, as new data flows in.  The first serious doubts about Milutin Milankovic’s theory of astronomical forcing of the world’s climate, since oxygen isotope fluctuations in sea-floor sediments began to reveal the periodicities predicted by him, stemmed from a very different kind of deposit.  Devil’s Hole near Las Vegas, a fissure being slowly filled by calcite flow stone that precipitates from groundwater, presented a detailed record of oxygen-isotope variations over the last 600 thousand years.  Though showing the same patterns as ocean cores available at the time, Devil’s Hole revealed changes in continental climate that differed from those in land-ice volume by thousands of years.  Ice-core time series of Antarctic air temperatures also show that warming began up to 9000 years before the last four terminations of glaciation.  As more proxies for climate are devised, the more complex global climate shifts appear to have been.

The latest measure stems from fluctuations in the structure of compounds produced by marine algae as a result of shifts in sea-surface temperature.  Applied to sea-floor sediments deposited off California, an area influenced today by the southward, cold California Current, they reveal regional warming of the sea that began 10 to 15 thousand years earlier than the last five deglaciations of the northern hemisphere (Herbert, T.D. and 8 others  2001.  Collapse of the California Current during glacial maxima linked to climate change on land.  Science, v. 293, p. 71-76).  In cores south of the modern cold current, no such large discrepancies emerged.  In fact they accompanied the maximum extents of land ice.  It seems that, like the Gulf Stream, the California Current is prone to shutting down, but as a result of changed Pacific wind patterns in response to the North American ice sheets rather than to thermohaline deep circulation.  Here is an explanation for the vexing record from Devil’s Hole – regional climate shifts that do not “knock” Milankovic.

There is no doubt that changes in ice volume on the northern continents are the main characteristic of environmental change going back more than 2 Ma.  However, the mechanistic view that lots of ice means a cold, dry world and a great deal less points to warmth and more moist conditions is dead in the water as a useful paradigm.  Yet all models of climate are little more than Heath Robinson tangles of such reductionism, despite claims for their increasing incorporation of ideas that stem from measured realities.  As always, the devil lies in the detail, and Herbert et al.’s paper also shows from pollen records in the marine cores that dense warm-climate forests cloaked the Pacific seaboard during the last 5 glacial maxima.  For a vast area of western North America to be warm while ice sheets elsewhere were at their maximum should be a warning of unpredictable future climate shifts.

Growing concern about unpredictable and contrary change was amply expressed by a meeting of 1800 climate specialists in Amsterdam in early July.  They endorsed the distinct possibility of sudden shifts in regional climates that may stem from increased global warming, such as return of vegetation to the Sahara, aridity in the Amazon basin, and Europe’s plunging into a frigid climate as the Gulf Stream slows because of reduced thermohaline circulation (Pearce, F.  2001.  Violent future.  New Scientist, 13 July 2001, p. 4-5).

Magnetic stratigraphy works in the Devonian

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Using alternations of magnetic field intensity, and the patterns that they show over time, has been a standard method in stratigraphy for times back to about 200 Ma ago.  There is no sea floor older than that, and although reversals are known widely from earlier times, there is no continuity that allows its use.  Moreover, reversals are too widely spaced in time to allow for more than calibrating stratigraphic sequences.  Much finer stratigraphic resolution comes from direct and rapid measurement of the intensity of magnetization that can be induced in sediments from their content of various magnetic minerals.  The Ocean Drilling Programme and studies of loess sections in China have long established such magnetic susceptibility logging as a correlative tool.  Empirically, it works, and the loess studies suggested that variations relate to changes in global climate.  Its usefulness in marine sediments is now seen to relate to the production of massive amounts of very fine-grained magnetic minerals in tropical soil formation during warm-humid episodes.  Being so fine, the particles reach the most distant ocean basins after soil erosion.  Susceptibility seems to vary with global changes in the amount of continental erosion.

Detailed correlation between widely separated marine stratigraphic sequences of all ages is notoriously difficult.  Consequently, rapid methods based on magnetic susceptibility, which can produce near-continuous logs, have useful potential.  A team of Us, Spanish and Moroccan geologists has demonstrated its use in definitive correlation between Lower Devonian rocks found in Spain, Morocco and Bolivia (Ellwood, B.B. et al.  2001.  Global correlation using magnestic susceptibility data from Lower Devonian rocks.  Geology, v.  29, p. 583-586).

Climate and heavy breathing

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The kingdom of the eukaryotes rests on a very simple environmental economy.  Plants are producers of carbohydrate through photosynthesis, thereby generating excess oxygen from the photo- and molecular chemistry involved.  Animal consumers use up oxygen in their metabolism and return carbon dioxide, the ultimate source of carbohydrate, to the air.  A simple view is that animals contribute to global warming, whereas plants help cool the world.  Perhaps because of that “common sense” view, most environmental scientists take a very different line, linking it with volcanic exhalation of CO2, “capture of carbon through rock weathering and the burial of dead organic matter  in the global carbon cycle.  Greg Retallack of the University of Oregon is about to publish a reappraisal of the animal versus plant part of the C-cycle (in press, Journal of Geology) that is based on observed imbalances between the two opposed kinds of respiration.  Specialists in the C-cycle hold that there is a an overall balance, taking all components into account, whose inevitable result is the build up of oxygen in the atmosphere of an inhabited world.  Yet oxygen is extremely reactive and should quickly combine in mineral oxides and hydroxides – after all, the iron in an untended car reverts to its oxide ore in the space of a few decades at most.

Partly following James Lovelock’s Gaia hypothesis, Retallack focuses on the major fluctuations in atmospheric chemistry evidenced in the geochemical record, the most immediate being the see-saw fluctuation of modern levels of CO2 in the atmosphere – a 2% annual variation controlled by the waxing and waning of vegetation in the northern hemisphere (where plant cover is greatest) according to season.  One of the largest shifts in atmospheric CO2 concentration followed the evolution of land plants from about 450 Ma ago.  To thrive, they had to develop hard cellular material (lignin) that formed stems and trunks, which animals of the Palaeozoic were unable to oxidise efficiently.  Both living biomass and burial of undigested lignin drew down CO2 and boosted oxygen levels.  Animal evolution eventually exploited this “free lunch” through the humble termite and reptilian and then mammalian megafauns.  Retallack believes that heavy breathing that resulted from lignin digestion reversed the declining CO2 trend for the 200 Ma following the Carboniferous to Permian glacial epoch in Gondwana.  Though displaying some ups and downs, the Mesozoic saw a “greenhouse” world.  Removal of the mighty and extremely abundant herbivorous dinosaurs by the K-T mass extinction provided and opportunity for plant diversification.  Many Mesozoic plants evolved armour against browsing dinosaurs, exemplified by the surviving Andean “monkey puzzle” tree Araucaria.  Their demise removed the need, and the plant Kingdom’s evolutionary response was the appearance of grasses.  Reatallack points out that grass itself is not as good as lignin-rich plants in holding CO2, but grasslands encourage the development of thick carbon-rich soils that hold more than the soils of the forest floor.  It is this development that Retallack believes lay at the base of the decline in average global temperature through the Cainozoic, to culminate in the present Ice Age.  Unsurprisingly, proponents of the complexity and diversity of the C-cycle, particularly in the oceans, are disinclined to have truck with the hypothesis.

Source:  Pearce, F.  The Kingdoms of Gaia.  New Scientist, 16 June 2001, p. 30-33.

Carbonates and biofilms

Above the low level that is essential for their role in molecular “information” transfer, calcium ions pose a fatal threat to cell processes.  That is simply because excess calcium combines with carbonate ions to form minute calcium carbonate crystals within the cell when the solubility product of calcite is exceeded.  The solubility product is the concentration of calcium ions multiplied by that of carbonate ions, so that increase in one or the other can lead to supersaturation of calcium carbonate and imminent precipitation.  Because CO2 is an essential need for photosynthesis and a product of animal metabolism, this risk is always present.  In the most common photosynthesising bacteria, the cyanobacteria that have been around for at least 3.6 billion years, the drawing in of CO2 in the form of carbonate (CO32-) or bicarbonate (HCO3–) ions in water can result in supersaturation immediately around the cell.  When it occurs, the “blue-green” bacterial biofilms induce precipitation of calcium carbonate.  That is why such micro-organisms can act as reef builders, as they did to great effect during the early Precambrian (stromatolites), and also from Cambrian to Cretaceous times.

Calcite mineralization by biofilms is, however, a complicated process.  It is connected with highly reactive substances that cyanobacteria exude outside their cell walls.  Depending on their degree of ordering and the supply of calcium ions, these substances control the manner in which calcium carbonate precipitates.  The detailed biochemistry and the form of calcite biofilms obtained by study of modern cyanobacteria in different watery environments has allowed Gernot Arp and co-workers at the University of Göttingen to evaluate varying calcium and CO2 concentrations in ocean water since 540 Ma, and suggest differences in Precambrian oceans (Arp, G. et al. 2001.  Photosynthesis-induced biofilm calcification and calcium concentrations in Phanerozoic oceans.  Science, v. 292, p. 1701-1704).

Their studies suggest that up to the Cretaceous, the Phanerozoic oceans must have had higher calcium contents than they do today.  Microbial reefs formed in that period preserve details of the “blue-green’s” cell structure, suggesting that calcite was nucleated directly by the extracellular substances.   Vast burial of the calcite shells of planktonic metazoan organisms to form the Chalk deposits of Cretaceous age reduced very high levels to give the calcium-depleted oceans that prevailed during the Cainozoic.  Microbial carbonates of these younger ages show no structure.  The stromatolites that are so characteristic of Precambrian limestones are stuctureless too, although they show evidence of progressive build-up from myriads of thin layers.  Irrespective of the Precambrian oceans’ calcium content, this lack of structure can be explained by more dissolved CO2 that resulted from its higher concentration in the atmosphere.  About 700-750 Ma ago, stromatolites that contain calcified cyanobacterial cells appear, and that may signify the massive drawdown of CO2 from the atmosphere that is implicated in creating icehouse conditions on a global scale during the late Proterozoic Aeon.

Universal access to peer reviewed articles?

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

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

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

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

Dinosaur update

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

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

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

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

Doubts cast on the increase in diversity with time

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

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

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

Earth System Processes conference

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