Popper refuted

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In mid-Victorian times, Lord Kelvin peered down his nose at Charles Lyell’s estimation of sedimentation rate from the historic silting of the port of King’s Lynn, as a means to judge the vast time span represented by the stratigraphic column.  His words were not kind; “…when you cannot measure [what you are speaking about], when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind”.  Geologists cringed, particularly when Kelvin went on to reckon an age of 20 to 40 Ma for the Earth based on its cooling from a molten mass, using the physical laws of conduction and radiation.  He was fundamentally wrong on most counts, partly because he knew nothing of radioactive heat generation nor convective heat transfer.  Sadly his corpse could not be revived to eat his mean-spirited words.  Nonetheless, the gibe of Earth scientists’ being “unscientific” has stuck.  We rarely stick to the “scientific method”, reputedly stemming from the Elizabethan philosopher, Francis Bacon and his rationalization of the inductive method of reductionist experimentation.  There are few universal “truths” in Earth history, and the interweaving of limitless processes with a vast spectrum of rates, scales and magnitudes renders reductionism absurd.  Even more prone to reductio ad absurdem is the chemist Karl Popper’s supposedly logical insight that “proper” science rigorously subjects hypotheses to a “risky test”; an experiment that should yield evidence of refutation if the notion is unsound.  Popper’s method of falsification consigns to the dustbin of research any hypothesis which fails the test, with the corollary that in is not “best practice” to seek confirmation for a hypothesis.

Carol Cleland of the University of Colorado (Cleland, c.e. 2001.  Historical science, experimental science, and the scientific method.  Geology, v. 29, p. 987-990) demolishes the “recipe-book” approach to science, which has laid a dead hand on not only the Earth sciences, from the standpoint of philosophy and reality.  She starts from the position of Thomas Kuhn, by pointing out that, for Popper, the whole of Newtonian celestial mechanics should have bitten the dust when 19th century astronomers discovered that the orbit of Uranus deviated from Newtonian prediction.  A sustained search for reasons why concluded that there must be gravitational forces from planets beyond Uranus, and sure enough astronomers discovered Neptune.

There is an air of bullying about the “scientific method”, which has warped investigations and dulled imagination and curiosity for centuries.  It provides ammunition for those who carp and pontificate from the sidelines, and in many cases from positions of considerable power.  Cleland does us all a service by discussing philosophical matters of science in the context of the realities that confront us all, in an accessible way.  Her analogy is Holmesian detection (Sherlock was a deductionist, by the way, proceeding from the general to the particular), which discovers events and proceeds to trace their circumstances – the search, to my mind, for the artillery rather than a single “smoking gun” is far richer than the events themselves, because that deepens our sense of context for particular events, however dramatic they might seem to be.

 

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.