Month: May 2001

Life on Earth even luckier than we thought?

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Continually improving resolution of telescopes is now beginning to reveal signs of planetary systems around other stars.  Because their gravitational effects on stellar motion are detectable, the 60 or so known planets in distant stellar systems are all gas-giants, similar to but bigger than Jupiter.  Surprisingly, calculations show that such massive planets are in very different orbits than those in the Solar System.  Their orbits are highly eccentric, and bring them remarkably close to the star, unlike the almost circular orbits in the Solar System.   Yet, if they are mainly gaseous, they must have formed far from the warming influence of their companion star, as did Jupiter, Saturn, Uranus and Neptune.  Somehow, they have been gravitationally perturbed over the billions of years of evolution of the stellar systems.

How, then, did such bodies move inwards?  One possibility is that they exchanged angular momentum with smaller, rocky planets, forcing both into eccentricity.  For the smaller bodies the effect would be more dramatic, potentially either flinging them into interstellar space or into collision with their star.  Spanish and Swiss astronomers using spectroscopes at an observatory on the Canary Islands have discovered a large lithium anomaly in the spectrum of one star with such an aberrant gas giant (Israelian, G.  et al. 2001.  Evidence for planet engulfment by the star HD82943.  Nature, v. 411, p. 163-166).  Because the anomaly is accompanied by greater than usual abundances of many elements heavier than helium, and because lithium is quickly consumed as stars “ignite”, Israelian and colleagues conclude that the star has engulfed an Earth-like planet.

If such processes are common, and theory suggests that it may be, our Solar System could be one of very few in which potentially life-building and sustaining planets had sufficient time to develop a biosphere.  It seems that the more small planets there are between a star and an outer gas-giant, the more likely it is for such perturbations to take place.  The Solar System has only four, and calculations using Jupiter’s mass and orbit point to a minute tendency for such eccentricities to evolve.  Looking on the bright side, at least for those committed to a view of life pervading the cosmos, current observational resolution is only able to detect giant planets in wildly eccentric orbits.  Many planetary could be more stable.

Se also:  Samuel, E.  2001.  Banished forever.  New Scientist, 12 May 2001, p. 15.

Late-Palaeocene red tides?

About 55 Ma ago, in the late-Palaeocene, the carbon-isotope record shows a sudden drop in 13C, signifying a sudden release of methane from ocean-floor gas hydrates or clathrates.  That period also reveals evidence of s brief global warming, against the general trend of cooling through the  Tertiary.  Since the discovery of this massive discharge of the “clathrate gun”, palaeontologists have looked for ecological effects in sea-floor sediments.  For them to be significant, it is important that climate-related ecological effects occurred at the same time in widely separated parts of the globe.

Geologists from the Netherlands, Denmark, New Zealand, Austria and Sweden have examined the microfossil record from two late-Palaeocene sequences in Austria and New Zealand, and show such synchronicity (Crouch, E.M. et al.  2001.  Global dinoflagellate event associated with the late Palaeocene thermal maximum.  Geology, v. 29, p. 315-318).  Exactly at the time of the d13C dip in both sections, the abundance of cysts of single-celled phytoplankton known as dinoflagellates rose dramatically, only falling when carbon isotopes recovered to usual levels.  The authors link this to exceptionally high surface-water temperature and photosynthetic productivity.  Over the same period, the fossil record shows a mass extinction of benthonic organisms, and noticeable turnover and diversification of plankton and mammals, though not as dramatic as other biological events.

Today, dinoflagellates explode in numbers, along with other phytoplankton, under similar conditions and when nutrients increase in surface waters, to create phenomena known as “red tides”.  Because some species of dinoflagellates produce potent neurotoxins, “red tides” often result in massive death of marine animal life.  The effects linger as such toxins build up in the cells of animals, such as bivalves, which survive the bloom.  The air above such blooms is filled with stinging, choking aerosols, not far different from nerve gas.  Rotting of dead organisms causes oxygen levels in local seawater to drop, further adding to the death tool at deeper levels.  Red tides that result from human input of nutrients in sheltered embayments often sterilize them for long periods.

Although it is impossible to tell if such neurotoxins built up during the late-Palaeocene thermal maximum, that is not an impossibility.  Such biological “warfare” (no-one knows why some dinoflagellates produce the toxins) might explain the biological crisis that accompanied methane release.

A broader view of the Permian-Triassic mass extinction

That the Palaeozoic Era ended in the greatest mass extinction is well know, although why it happened is still a topic of fierce debate.  Part of the problem is that its effects on land and in the oceans emerge from studies of widely separated P-Tr sections, and many of these are extremely thin.  Such condensed sequences are notoriously difficult to resolve in terms of relative and absolute timing, as well as to correlate from place to place.

As with much else, Greenland promises to throw light on the end-Palaeozoic events, thanks to a 700 metre sequence of siliciclastic sediments in East Greenland that spans the Permian-Triassic boundary without a break.  Its most exciting feature is the way in which marine and non-marine sediments interleave with one another.  Geologists from the USA, the Netherlands, Australia and Britain have pieced together the evidence of biological change from a small part of this little described occurrence (Twitchett, R.J. et al.  2001.  Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis.  Geology, v.  29, p. 351-354).

In marine sediments, the Permian biota collapse, together with evidence for disturbance of the sediment structure by burrowing , in a mere 50 cm of the almost 40 metre sequence that the authors analysed.  Over the same interval, pollens of Permian land plants also fall dramatically, but all the pollen types linger through the overlying 15 metres.  Only at a level 25 metres above the biotic collapse do  fully Triassic faunas and floras appear.  From estimates of the rate of sedimentation the marine and terrestrial collapse appears to have taken between 10 and 30 ka.  Oddly, the now well-known fall in 13C does not coincide with that in the biota.  The authors visualize two possibilites: that it resulted from the collapse itself, or reflects an external factor that played little or no role.  One interesting scenario that they suggest is that it may indicate a major release of methane by breakdown of gas hydrates (a now increasingly popular mechanism!).

Conferring strength to cratons

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Considering the continual processes that stress continental lithosphere from the time of its formation, it is a puzzle to find large areas that preserve its earliest parts in an almost pristine state.  Greater heat production in the past demands that the frequency and power involved in continental jostling were greater as we go back in geological time.  Zones that show little sign of having been tectonically reworked for more than a billion years are termed cratons, and most of them have at their core continental material that formed in the Archaean, more than 2.5 Ga ago.  Later orogens do show isotopic signs that deformed and partially melted Archaean crust was involved, but no so much as might be expected.  Somehow, having a nucleus of Archaean lithosphere confers strength to cratonic areas.  Geophysics reveals that  the lithosphere beneath cratons uniquely extends to depths of 200 km, forming a “keel” or tectosphere.

Most geochemists consider that deep mantle beneath cratons is so rigid because it is unable to come close to the beginning of melting, due to it having once been the source of massive amounts of basaltic magma.  Loss of the constituent elements of basalt and volatiles, including heat-producing isotopes of U, Th and K, renders it more inert than mantle that still has the potential to generate basalt under appropriate conditions.  Basalt magmas also remove significant amounts of iron, thereby adding buoyancy to tectosphere materials.

Occasionally, much younger magmas that do form at the depths of the tectosphere bring samples of it to the surface, in the form of xenoliths.  Their petrography and geochemistry reinforce the general idea of how cratonic “keels” form, but they have been difficult to date with confidence.  The relatively new rhenium-187/osmium-187 method makes dating more assured.  Cin-Ty Lee and colleagues from Harvard University (Lee, C et al.  2001.  Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.  Nature, v. 411, p. 69-73) used the method on xenoliths from two adjacent areas, the actively extending Basin and Range Province and the Colorado Plateau.  Both contain ancient rocks, Archaean in the former and Mesoproterozoic in the second, which behaves as a stable craton.  Xenoliths from mantle deep beneath them have similar ages to those in the oldest crustal rocks, helping confirm the geochemical connection between crust formation and lithospheric mantle.  However, those from beneath the Basin and Range have potentially “fertile” compositions, whereas the Colorado samples show signs of the depletion thought to confer strength and buoyancy.  Paradoxically, a younger craton sits next to Archaean lithosphere that is demonstrably weak. 

Lee and colleagues suggest that if part of Archaean crust formation did not create a tectosphere, it is quite possible that younger orogens might contain considerably more ancient crust than currently suspected.  On the other hand, the mismatch between the near certainty that continents formed more rapidly during the first third of recorded geological history and the disproportionately small volume of known Archaean crustal rock could signify that a lot of it became resorbed into the mantle.  That doesn’t appear to have been a significant process in later times.  However, the total lack of sialic rocks older than 4 Ga, yet the evidence from detrital zircons up to 4.4 Ga in much younger sediments that some did indeed form, suggests that crustal resorption was efficient during early tectonics.  Perhaps the Archaean marked the waning of such processes, in which an increasing proportion remained locked at the surface.

See also:  Nyblade, A.  2001.  Hard-cored continents.  Nature, v. 411, p. 39-39.

Partially melted zones beneath Tibet

Anomalously low seismic velocities, accompanied by a “muffling” of seismic energy, and high heat flow beneath the Tibetan Plateau have hinted at the possibility of active crustal melting, but such information cannot resolve whether that is the case or not.  Parts of the Plateau have been volcanically active in the near past, and that has been attributed by some workers  to the detachment and sinking into the mantle of a large chunk of sub-Tibetan lithosphere.  Freed of a substantial mass, the thick lithosphere beneath Tibet would then bob up, the rapid drop in pressure at depth inducing partial melting.  Being weak, a substantial partially melted zone would also help the Tibetan crust deform more easily.

One means of  adding support to the idea is looking for deep-crustal anomalies in electrical conductivity.  Because electric currents flow naturally in the Earth, the conventional means of resistivity survey can use them instead of an input current.  Such magnetotelluric surveys potentially give information down to depths of 100 km or more.  At these scales, zones of abnormally low conductivity are likely to be due either to pervasion of deep rock with watery fluids or with widespread partial melting.  A group of Chinese, Canadian and US geophysicisists (Wei, W. and 14 others 2001.  Detection of widespread fluids in the Tibetan crust by magnetotelluric studies.  Science, v. 292, p. 716-718) have shown that the middle to lower crust deeper than 15 to 20 km beneath most of the Tibetan Plateau is anomalous in this way.  The highest conductivity lies beneath the main Yarlung (Indus) – Tsangpo suture., and may be related to fluids released by subduction processes.  It is the anomaly beneath the Plateau itself that is most significant, for it extends for 4 degrees of latitude along the survey line.  Higher conductivity anomalies correlate closely with Plio-Pleistocene volcanically active areas, and much of the area is affected by hydrothermal fluids.  While adding detail to structure and rheological properties beneath Tibet, magnetotelluric studies still leave open the possibility that much of the electrical signature may be due to pervasive watery fluids, as well as to zones of melting.

Brazilian input to the growth of Gondwana

One of the most dramatic tectonic events known from the geological record is the break up of a supercontinent, dubbed Rodinia (from the Russian for motherland), in the Neoproterozoic.  From a unity of almost all earlier continental crust, this break up sent fragments scurrying across a plethora of new oceans.  Some of the fragments reassembled around 650 Ma ago to create what eventually became the southern part of the Carboniferous supercontinent of Pangaea; Gondwana.  The assembly of West Gondwana involved a vast network of orogenic belts in which juvenile arc materials were pinched between colliding continental fragments, as these oceans closed up.  Often called the Pan African event, because of its widespread signature in that continent, this assembly also affected eastern South America at the same time.

Fernando Alkmim, Stephen Marshak and Marco Fonseca (Alkmin, F.F.  2001.  Assembling West Gondwana in the Neoproterozoic: clues from the São Francisco craton region, Brazil.  Geology, v.  29, p. 319-322)  turn our attention from the much-described Pan African to its Braziliano counterpart in South America.  Their summary of current understanding suggests six stages in the rifting to collision, that involved major changes in palaeogeography.

Mapping with geophysical data

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In the same way that topographic contours can be transformed to models of continuous elevation change using surface fitting, measurements of gravitational and magnetic field potentials, at points on the ground or along aerial survey lines, are sources of imagery.  Expressed as contours joining points with the same value, spatial distributed data are notoriously difficult to interpret, however much information they contain.  Not only do contours simplify the data by dividing them into arbitrary steps, how we interpret contour maps depends on how we perceive them.  Our eyes evolved to extract information distributed as a continuum across our field of view, and our visual cortex developed many tricks to innately interpret clues to shape, perspective and distance, to extend the limits of stereoscopic vision (we see objects in true 3-D only if they are closer than about 400 metres).  Our innate abilities “interpret” contours in terms of the spacing between them; the closer they are together the darker we perceive the area of steep gradient.  In other words we have to convert an image that is the “negative” of the first derivative to an understanding of the actual shape represented by contours!  Unsurprisingly, we have to learn to “read” maps, and that is a great deal more difficult for those showing potential-field intensity than for topographic elevation.  Cartographers long ago latched onto our use of shadows as clues to shape, and designed maps with shading as if the Sun was shining from the top of the sheet.  They also use different colours as a second clue to what is high and low.  Combining the two aids helps transform images of geographic variables – basically bland shifts from high to low – into visually stunning, and therefore more easily interpreted pictures.  Surface modelling of elevation and geophysical data, with such graphic tricks, literally throws hidden, and often unsuspected features into sharp relief.

These techniques have revitalized desktop interpretation of the world, especially using results of geophysical surveys.  However, in the same way that detail of a terrain blurs and loses information as resolving power falls, low-resolution data of other kinds obscure buried features, or give ambiguous hints to what they are and where they go.  Reducing the spacing of aerial surveys, and the height from which they are acquired, increases the resolving power of the technique.  Stunning examples of the state of this particular art appear in recent work by the US Geological Survey (Grauch, V.J.S. 2001.  High-resolution aeromagnetic data, a new tool for mapping intrabasinal faults: example from the Albuquerque basin, New Mexico.  Geology, v. 29, p. 367-370.  See also http://rmmcweb.cr.usgs.gov/public/mrgb/airborne.html ). 

Grauch worked on an area in which superficial materials and rapid rounding of topography result in poor surface expression of all but the largest faults.  By using aeromagnetic images modelled from survey lines spaced at 100 to 150 metres, he picked out not only hidden faults, but also the magnetic signatures of pipelines, water tanks and buildings.

Far-Eastern control on African climate and hominid evolution

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The drying of East Africa’s climate since 5 Ma ago shifted the distribution of its ecosystems towards more widespread savannah.  In the most general sense that probably created conditions for ape speciation towards an upright gait and the potential for tool-using and growing consciousness that palaeoanthropologists visualize at the core of human evolution.  The apparently dominant influence of North Atlantic circulation changes on climate fluctuations since then has suggested to many climatologists that the shift to glacial-interglacial and dry-humid cycles, at high and low latitudes, stems from some trigger for a fundamental shift in that circulation.  The favoured process is the closure of open connection between Atlantic and Pacific Oceans when the Isthmus of Panama formed about 5 Ma ago.  That transformed Atlantic circulation, and probably set in motion the Gulf Stream.  However, there are several such gateways whose affects on ocean circulation link to plate movements.

One is the narrow passage between Indonesia and Australasia, which transfers Pacific water to the Indian Ocean.  Subduction permits Australasia to move gradually northwards, thereby narrowing the gateway and also shifting it relative to the major currents in the tropical Pacific.  Mark Cane and Peter Molnar of Columbia University and MIT have analysed the recent evolution of the Indonesian gateway (Cane, M.A. and Molnar, P.  2001.  Closing of the Indonesian seaway as a precursor to east African aridification around 3-4 million years ago.  Nature, v. 411, p. 157-162).  Their findings suggest that the main flow switched from warm, South Pacific surface waters to cooler waters that originate in the North Pacific at about 4 Ma.  Cooling of surface waters in the Indian Ocean would have reduced the amount of water vapour transferred to the air masses that are involved in the East African monsoons.  The reduction in seasonal rainfall would have dried that area substantially.  Though providing a plausible cause for regional climate change, the coincident transformations of two major ocean gateways adds greater complexity to the Plio-Pleistocene climate system.  In terms of modern climate, the Indonesian gateway provides a means of understanding the teleconnection that seems to exist from correlation between drought-flood cycles in East Africa and the El Niño – Southern Oscillation in the tropical Pacific.

See also: Wright, J,D. 2001.  The Indonesian valve.  Nature, v. 411, p. 142-143.

Multiregionalists nailed by Y chromosome?

One of the big problems in using genetic material from living people to chart relatedness, and perhaps evolutionary origins, is simply getting the material.  For the mitochondrial DNA studies that first hinted at a common African origin for all modern humans, the best material is placental tissue.  A focus on male lineage using Y chromosomes is not so difficult; it can be done using blood samples.  Nonetheless, a survey based on 12,127 samples from 163 population is a monumental achievement (Ke, Y. and 23 others  2001.  African origin of modern humans in East Asia: a tale of 12,000 Y chromosomes.  Science, v. 292, p. 1151-1153).

The significance of this study by a large team from China, the USA, Indonesia and Britain is that it focuses on the region most favoured by multiregionalists for the hypothetically separate descent of modern humans from ancient ancestors of Homo erectus stock in different parts of the Old World.  The male chromosomes all carry evidence of mutations to a Y-chromosome marker that originated in Africa, abetween 35 to 89 ka ago.  The huge mass of data from the whole of East Asia do not support even minimal contribution from any source other than one that originated in Africa around the time it is thought that fully modern humans began to leave in significant numbers.

Erosion on Mars

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Mars is the only planet in the Solar System that has landscapes that bear any resemblance to those we see on Earth.  The one factor common to both planets is that surfaces have been shaped by flowing water.  On Mars, that was a one-off event early in its history, and thereafter shaping the planet has been through continual movement of dust in its thin, but energetic atmosphere, the formation of impact craters and volcanism.  Evidence for fluvial processes occurs in the highland regions, which were built mainly by volcanic activity., and stems from careful examination of high-resolution photography from orbiting probes.  Whether the various kinds of valleys formed by catastrophic, short-lived floods of melt water released by impacts into deep frozen ground, through steady release of groundwater or actually by precipitation  are the ground for speculation and controversy.  A means of assessing the possibilities is using accurate data on topographic elevation.  Digital elevation models for the Earth, even at coarse resolution (GTOPO30 data at 1 km resolution), map out the intricacy of surface drainage of the continents.  A DEM produced by the laser altimeter aboard Mars Orbiter allows not only the various models to be assessed, but enables quantitative work on the amount and rate of water erosion and deposition of sediment when combined with evidence for the age and duration of Mars’ fluvial event (Hynek, B.M. and Phillips, R.J.  2001.  Evidence for extensive denudation of the Martian highlands.  Geology, v. 29, p. 407-410).

Hynek and Phillips show that the event was long lived, lasting 350 to 500 Ma around 4 billion years ago.  Their study was of an area the size of Europe.  Scaled up, their findings suggest that of the order of 5 million cubic kilometres of sediment was transported, equivalent to deposition of a 120 metre thick sediment layer in the flat plains of Mars’ northern hemisphere.  The average rate of erosion during the event compares closely with that typical of temperate maritime areas of mountains on Earth.  It is difficult to see how such prolonged erosion could have taken place without runoff fed by precipitation on the surface, and that implies a much warmer climate and thicker atmosphere than on modern Mars, albeit only for a very early episode in its evolution.

Phanerozoic CO2 levels

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Because climate depends partly on the retention of solar heat by carbon dioxide in the atmosphere, a record of past CO2 fluctuations is important in linking evidence for shifting climate and environments to models.  Conversely, models that seek to mimic climates of the past depend heavily on the assumption that the “greenhouse” effect and the carbon cycle underpin global temperature and precipitation.  Current theorists consider that shifts in CO2 content of the atmosphere reflect a balance between its release through volcanism (itself a reflection of the rate of plate tectonics) and  its removal by weathering of silicate minerals and burial of dead biomass. 

The GEOCARB III model predicts rising atmospheric CO2 following the ice-house condition of the late-Precambrian, when rapid sea-floor spreading broke up and began to reassemble supercontinents during the Lower Palaeozoic.  In the early Cambrian CO2 levels come out at 25 times the modern amount.  Colonization of the land by plants through the Upper Palaeozoic, and the burial of a proportion of the increased amount of carbon fixed by them, allows the model to predict a massive fall in CO2.  That tallies very well with the long period of glaciation in southern Pangaea during the Carboniferous and Permian.  GEOCARB III suggests a recovery in levels through the Mesozoic, punctuated by extraordinary releases from plume activity, such as that implicated in the formation of ocean plateaux beneath the Pacific about 120 Ma ago.

From GEOCARB modelling stem predictions of the overall forcing of global temperatures.  However, only the last 100 Ma can be assessed as regards temperatures, by using accurate proxies provided by oxygen isotopes and the Ca:Mg ratio of marine carbonates.  Two of the leading climatic theorists, Thomas Crowley and Robert Berner of Texas A&M and Yale universities usefully summarise the range of other proxies that help validate their kind of modelling (Crowley, T.J. and Berner, R.A. 2001.  CO2 and climate change.  Science, v. 292, p. 870-872).  These include estimates from fossil soils, carbon isotopes in sediments, the pores in plant leaves (see Plant respiration and climate below) and how much boron is taken up in the shells of fossil animals.  There are considerable discrepancies with modelling, albeit encompassed by the high uncertainties in the calculations.  Crowley and Berner acknowledge the complexity of other factors that affect the global redistribution of heat, such as continental configurations in terms of area, geographic position, their effects on ocean circulation and even on the pace of the carbon cycle.  They see the need to expand climate models, taking other factors on board, in an attempt to quantify the discrepancies.

Methane and escape from Snowball Earth

Palaeomagnetic pole positions determined from areas characterized by thick glacigenic deposits around 750 Ma old leave little doubt that large volumes of ice covered the Earth to tropical latitudes.  Such evidence suggests an ice-bound world from which escape would have been very difficult because much of the Sun’s energy would have been reflected back to space.  Extreme and prolonged frigidity, from which Earth’s climate did escape is seen by a growing number of palaeobiologists as the most profound influence over later evolution and diversification of life.  The first fossil metazoans appear in the record shortly after a “Snowball Earth” event at 650 Ma, and the Cambrian explosion of animals with hard parts followed close on the heels of the last.  Carbon isotope studies from marine carbonates suggest that each global glaciation witnessed massive extinctions of single-celled organisms, and surviving life was presented with a virtual tabula rasa of niches to fill.  Such survivors, possessing characters that had ensured their survival – at which we can only guess – exploited them to the full.  It is reasonable to speculate that without such climatic upheavals life would not be as it is now, and that our eventual appearance depended on them.

That Earth’s climate broke out of runaway ice-house conditions is obvious, the question being how was that possible.  Volcanic emissions of carbon dioxide, which neither the Neoproterozoic biosphere nor silicate weathering were able to draw down into ocean water and sediments, would have accumulated in the atmosphere, to create “greenhouse” conditions.  That simple scenario, envisaging a spectacular shift from frigid to hot conditions, has its problems.  In order for climate to stabilize, without rushing into runaway heating along the path followed by Venus, demands implausibly high rates of silicate weathering to draw down CO2 in the period following the end of each “Snowball” event, and strontium isotopes that record the rate of continental weathering shwo no sign of anything so dramatic.  It also poses the question of how global ice cover could remain while CO2 slowly built up.  The key seems to lie in carbonates that everywhere cap the glacigenic deposits of this age.  The cap carbonates record rapid falls in the 13C proportion of the carbon in carbonate.  13C shows a rise in the glacial epochs that signifies massive burial of dead organic matter (enriched in lighter 12C), probably through mass extinction.  In a review of the geochemical basis for changes in oceanic carbon isotopes, and high-resolution data from cap carbonates, scientists from the University of California and the Lamont-Doherty Earth Observatory, suggest that the isotopic excursions could reflect massive release of methane from gas-hydrate layers in sediments that were frigid during the Snowball event (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).  Backing up this hypothesis are examples of structures in cap carbonates that are identical to those formed in modern sediments affected by break down of gas hydrates and release of methane from the sea floor.

Plant respiration and climate

Leaf surfaces are pockmarked by pores (stomata), through which cell metabolism draws in the carbon dioxide involved in photosynthesis and transpires its products, including oxygen.  When CO2 levels are low, more pores are needed, and vice versa.  Surprisingly, museum specimens of leaves collected since the start of the Industrial Revolution do show a decrease in the density of such pores that matches the documented rise in atmospheric CO2 levels.  Were it possible to find fossils of the same plant species, pore density would be an excellent proxy for the “greenhouse” effect.  That is not possible, because of evolution.  However, plants related to the Ginkgo have a pedigree that goes back about 300 Ma.  Morphologically, the four genera of Ginkgo-like leaves are very similar, so using them potentially gives an independent record of the “greenhouse” effect.

Gregory Retallack of the University of Oregon has measured the stomatal index of sufficient Ginkgo and related leaves to assess CO2 levels in a broad-brush sense for the period since the early Permian (Retallack, G.J. 2001.  A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles.  Nature, v. 411, p, 287-290).  His results tally broadly with oxygen-isotope and other proxies for palaeotemperature variations, and to some extent with CO2 modelling (see Phanerozoic CO2 levels above).  However, the stomatal record shows changes up to 10 Ma in advance of shifts in temperature.  That might be due to coarse resolution in Retallack’s data, but could signify other forces at work other than the “greenhouse” effect.  The most significant advance provided by leaf studies is that they help account for mismatches between evidence for cooling and predictions of highCO2 by modelling, for the Jurassic and Cretaceous, that have been a thorn in the side of the modellers.  Given fossil leaves more closely spaced in time, and using other plant groups, Retallack’s method potentially could revolutionize climate analyses and extend them back as far as 400 Ma ago.

See also:  Kürschner, W.M.  2001.  Leaf sensor for CO2 in deep time.  Nature, v. 411, p. 247-248.

Loss to geology

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Robert Shackleton FRS died aged 91 on 3 May 2001. Shackleton’s long career began as a survey geologist in Africa.  After a period at Liverpool University he took up a chair at Leeds, and became an Honorary Senior Research Fellow at the Open University.  He was not a retiring man, and was of the school of which it was said, “The best geologist is the one who sees the most rocks”.   His peregrinations were legendary.  Shackleton’s forte was structural geology and tectonics, and he was a central figure in driving forward our understanding of Africa’s evolution.  Sadly, he did not live to witness the publication of his Geology of Africa project.  His touch and his flair were felt by many throughout the world, and they will be missed.