Late Pleistocene mass extinction

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The recent fossil records of the Americas and Oceania are littered with species that became extinct in the last 100 000 years.  The majority of them are large animals whose body weights were greater than 45 kg – part of the megafauna.  While controversy rages about the date of entry of the first humans into both vast regions, for a long time archaeologists have suspected that the appearance of sophisticated hunters was somehow connected with the rapid decline in what would have been prey species.  One theory is that having never encountered weapon-bearing bipeds, large mammals were “naïve” and thus easily slaughtered.  Most visitors to the Americas and Australia soon notice how unafraid many animals are of humans, compared with their behaviour in Europe and Africa.  The suddenness of  the selective extinctions (around 15 to 11 000 and 47 000 years ago in North America and Australia respectively) is astonishing, if the cause was small bands of hunters, and other workers have suggested that human entry brought diseases that wiped out species susceptible to them, but with no immunity. The third main theory is that a sudden shift in climate wrought havoc among large herbivores and predators, by producing a change in vegetation.  The last is difficult to support for the Americas as the extinctions were in a period of increasing warmth and humidity following the termination of the last glacial period.  As always, new information from research directed at the problem has narrowed the choices, but revealed complexities.

Modelling the influence of changing predation on prey stems from the mathematical simplification of reality by Lotka and Volterra, in which “boom and bust” events pop out of the simulations.  John Alroy of the University of California applied an advanced version of the basic model to the likely effects of advanced hunters appearing suddenly in North America (Alroy, J.  2001.  A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction.  Science, v. 292, p. 1893-1896).  His model assumes slow human population growth, random hunting and the least possible effort – a conservative approach.  The results closely parallel the record, if human population expanded from 100 first entrants about 14 000 years ago to almost 1 million 750 years later, and suggest that a steady state population of around half that co-existed with the surviving fauna until the appearance of Europeans and their culture.  It is an entirely mechanistic model, but mimics what happened without recourse to any other influence, such as climate change.  So far, no human site in the Americas has been convincingly dated before 14 000 years ago.

Dating is even more of a problem in Australia, particularly for human arrival.  The earliest dated fossil is 60 000 years old (see Out of Africa hypothesis confounded? EP Feb 2001), but claims have been made for artefacts at least twice that age.  Alroy’s model applied to Australia would demand extinction (24 out of 25 genera Pleistocene megafaunal species) shortly after earliest arrival.  A large team of Australian scientists (Roberts, R.G. and 10 others 2001.  New ages for the last Australian megafauna: continent-wide extinction about 46 000 years ago.  Science, v. 292, p. 1888-1892) have systematically dated the age of burial of extinct faunas at 27 sites in coastal areas and the more humid SE of the continent (none from the vast, arid “red centre”), and one in Papua New Guinea.  The most likely interval for the extinctions, between 39 800 and 51 200 years ago, bears no relation to extreme aridity during the last glacial maximum, so the data weigh against that climatic cause.  However, the last 100 000 years have seen lesser, but still extreme shifts in climate, so climate change cannot be ruled out.  Though the authors also do not rule out humans eating their way through Australias bizarre megafauna, the lag between evidence for first entry and the extinction seems far longer than that in the Americas.  Closer inspection of their data, however,  does show precise 230Th/234U ages (+ 600 to 2 200) from 33 600 to 60 000 from 3 sites, and less well-constrained luminescence ages (+ 200-21 000) from 16 000 to 171 000 years from all the sites.  Applying simple statistics to samples from such a wide spread of localities does not seem justified to me – normal practice is for ages at individual sites to be accepted as dates within the errors of the method used.  Australia’s megafaunal extinction seems to have been protracted.  Using fuzzier dating of the extinction, earlier workers correlated it with evidence for an increase in bush fires marked by ash in offshore sediments.  Much of Australia’s flora is fire resistant, and the seeds of some species require light burning before they will germinate.  The most popular theory for the extinctions there is through deliberate fire setting by hunters – a culturally induced decline unique to Australia’s peculiar climate and terrain.

See also:  Dayton, L.  2001.  Mass extinction pinned on Ice Age hunters.  Science, v. 292, p. 1819.

Where do subducted slabs go?

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Geophysicists and geochemists are generally opposed on what happens to subducted lithosphere.  Seismic tomography of the deep mantle shows convincing evidence for slab-like cold bodies down to the core-mantle boundary, yet differences in trace-element and isotopic signatures of volcanic rocks formed at ridges from shallow mantle and ocean islands that relate to deep plumes persuades geochemists that restriction of convection within the upper mantle, at the 660 km deep discontinuity, best explains the differences.  There are other models that might account for geochemical differences, such as heterogeneities throughout a poorly stirred mantle or because material in slabs subducted to the bottom of the mantle rarely rises again, but displaces more pristine materials upwards.

The more earthquakes that seismographs detect and locate, the better geophysicists are able to map in 3-D the zones on which they take place.  One destructive margin long known to have aberrant seismicity is the northern part of the Tonga system in the Pacific Ocean.  This is where the fastest subduction anywhere consumes lithosphere that has little time to warm up while it descends – surely a site for slabs to fall steeply into the deep mantle.  Much of the Tonga system shows the expected zone of steeply plunging Earthquakes, yet west and north-west of Fiji there are earthquakes that do not fit the regional pattern.  They are far too shallow to result from motion on the main subduction zone.  By detailed analysis of seismic data Wang-Ping Chen and Michael Brudzinski have revealed a strong possibility that a piece of old subducted slab has slid to the 660 km discontinuity since it parted company with the now rapid and steep motion at the Tonga trench (Chen, W-P. and Brudzinski, M.R. 2001.  Evidence for a large-scale remnant of subducted lithosphere beneath Fiji.  Science, v. 292, p. 2475-2478).  If such behaviour turns out to be more widespread, large volumes of old lithosphere may indeed sit at the discontinuity, satisfying many geochemists as a means to maintain very old differences in composition of the mantle.  The problem is, increasingly good resolution in seismic tomography has so far failed to detect the tell-tale high seismic velocity signature of such cold slabs.  Chen and Brudzinski suggest that they may be “invisible” to this method, because of their mineralogy – perhaps the crustal lithosphere has not equilibrated to eclogitic materials, or is given neutral buoyancy by being heavily hydrated.

Between a rock and a hard place

Plate theory stems from the notion that the lithosphere is overwhelmingly rigid and deforms only at the boundaries between plates, particularly at destructive margins.  The Earth’s seismicity is overwhelmed by earthquakes at discrete boundaries, and the mapping of seismic events along narrow lines by the world-wide network of seismographs (set up as a means of pinpointing nuclear weapon tests) formed on of the main planks in developing the theory of plate tectonics.  The plate whose evolution drove India into Asia bucks this definition.  It has long been known to host seismicity well inside its boundaries.  Oceanographic work has slowly built up a means of relating Indian Ocean seismicity to plate structure, whereas analysis of earthquake first motions from seismographs reveals that the deformation differs between various block of the ocean floor.  The plate suffers folding and thrusting, and transcurrent motions along ancient transform faults, such as the Ninety East Ridge.  The most likely explanation for the Indian plate’s aberrance is that sea-floor spreading from the ridge separating the Indian Plate from that carrying Antarctica can no longer be accommodated by subduction of the subcontinent beneath Asia, whereas it can be taken up by subduction beneath the Java-Sumatra island arc.  The Central Indian basin is being compressed, and must deform in some way, perhaps eventually to become a new subduction zone.

Source:  Deplus, C.  2001.  Indian Ocean actively deforms.  Science, v. 292, p. 1850-1851.

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.

Java girl

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As if the jumble in cladistics of African hominins was not enough, the skull SM3, dubbed by some as “Java Girl”, adds to the bag of spanners that disrupts attempts to rationalise the human evolutionary bush  (See Earth Pages Apr 2001, Skulduggery, migration and confusion).  Java, of course is where the whole thing began, with Eugene Dubois’ (See Review of Pat Shipman’s biography of Dubois in Nature v. 410, p. 869) discovery of what seemed to him as Darwin’s “missing link”, in the form of Pithecanthropus (now Homo) erectus in 1892.  Miss palaeo-Java, is odd by comparison, largely because her brow ridges did not meet and her forehead was “nobly” high.  Morphologically, her skull shows features that could be transitional between H. erectus and H, sapiens.  New Scientist ran an article (Soares, C.. Talking heads.  New Sientist, 14 April 2001 issue, p. 26-29) that charts how her skull, found recently in a New York antique shop – she was smuggled out of Indonesia two decades ago, has been grist to the mill for the multiregionalists, already gleeful at the DNA sequence of Australia’s “Mungo Man” (See Earth Pages Feb 2001, Out of Africa hypothesis confounded?).  Thoughtfully, Christine Soares also mentions the growing doubts that shapes of skulls and even whole skeletal anatomies can contribute a great deal resolving the multiregional vs out-of-Africa debate.  This arises from Todd Disotell’s studies of  modern monkeys, where he found that genetically distant species had almost identical morphologies, whereas much more closely related species were the most different from each other cranially.

Impacts and human evolution

Few Earth scientists disagree with the notion that our planet’s evolution and that of its life has been repeatedly punctuated by catastrophic impacts with comets and asteroids.  The Moon’s surface is an excellent record of that bombardment in near-Earth space since about 4.45 Ga ago, when it formed in orbit around the Earth.  Both dating of impact glasses from the Apollo programme and assessment of the relative ages of lunar craters provide continually refined statistics of the distribution of impact events of different magnitudes through time.

Dr Benny Peiser, a social anthropologist at Liverpool John Moores University and Michael Paine, an impact researcher from the Planetary Society in Australia, applied these statistics to the roughly 5 Ma time span of human and hominin evolution.  Their suggestions were presented by Peiser at the Charterhouse Conference 2001 “Celebrating Britain’s Achievements in Space” in London (see the Cambridge Conference Network  archives at http://abob.libs.uga.edu/bobk/cccmenu.html .

They calculate that 552 impacts that formed craters between 5 and more than 20 km across occurred on land during human evolution, with an additional 6 ocean impacts that could be expected to produce moderate to severe global climate disruption. So far, 32 impact craters have been discovered that are younger than 5 million years.  Earth’s active erosion and sedimentation are like to have obscured more craters, even in such a brief period.

No-one would seriously dispute Peiser and Pain’s calculations, but where they proceed from them is a different matter.  They assign an impact origin to the genetic bottlenecks, which seem to be implicated in speciation and which show up in modern human gene sequences (see More molecular evidence for Cro-Magnon migration into Europe – Earth Pages Jan 2001 – and Eve never met Adam – Earth Pages Nov 2000).  No doubt the aftermath of sizeable impacts would place terrestrial life under considerable stress, but to jump from impact statistics to a hypothesis of external causes for hominin speciation is not likely to find much support.  It does not use evidence at all, but probabilities, as often quoted that each of us is as likely to perish from extraterrestrial impact as from a firework accident or murder.

The record of human evolution is blurred to a large degree by:

1.  the tiny number of fossils

2.  the dates assigned to those fossils

3.  the significance assigned to their morphology by different palaeoanthropologists – there are “lumpers” and “splitters”

4.  the total lack of knowledge about the interplay between physiology, culture and social interaction, as regards what constituted “fitness” in natural selection.

Aside from the bottlenecks implied by modern human genetic diversity, or rather lack of it, we do not have a clue when Orrorin, “Lucy” , H. erectus, “Bonzo” the chimp or fully modern humans appeared as species.  And there is another matter; the post-Miocene period has been punctuated by climatic shifts of dreadful magnitude that came thick and fast through Milankovich pacing.  To suggest any other trigger for speciation, without a “smoking crater” and a precise date coinciding with the first individual of a species, is neither sensible nor necessary – as if…  This is grandstanding, and the press have had a field day.

New human evolution web site

Science magazine’s NetWatch (10 April 2001) includes news of the Becoming Human web site developed by the Institute of Human Origins at Arizona State University.  http://www.becominghuman.org is multimedia, including a 30 minute “webcast” by Donald Johanson, the director, who found “Lucy” in 1974.  That can be skipped, and the meat found in various Exhibits, a glossary, references, links and news.  The site plans to launch a teachers’ resource centre in May.

The origin of microcontinental terranes

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Slivers of ancient crust make up part of the collages of accreted terranes found in many ancient orogens.  How they form is not well-known.   Clues might lie in modern microcontinents that still remain surrounded by oceanic lithosphere, such as Jan Mayen, the Seychelles and the East Tasman Plateau.  Geologists from the Universities of Sydney, and Aarhus and the Geological Survey of Canada believe that such fragments of continental crust form early in the evolution of passive margins, as a result of plume activity followed by asymmetric sea-floor spreading (Müller, D.M. 2001.  A recipe for microcontinent formation.  Geology, v.  29, p. 203-206).

One suspected microcontinent in the southern Indian Ocean is the Kerguelen Plateau – its shape is odd.  In the few places where it breaches surface in the Kerguelen Archipelago, there are rare occurrences of silicic plutonic rocks.  However, evidence from dredged samples seems to show that most of the Plateau formed by plume-related basaltic volcanism that began at the same time as the formation of the Rajmahal Traps in Bangladesh (about 117 Ma ago).  ODP drilling now reveals fluviatile sediment layers that contain high-grade gneisses, whose ages range back to the Proterozoic (Nicolaysen, K. and many others 2001.  Provenance of Proterozoic garnet-biotite gneiss recovered from Elan Bank, Kerguelen Plateau, southern Indian Ocean.  Geology, v.  29, p. 235-238).  The authors do not see this as directly supporting a Kerguelen microcontinent, but the formation of the plateau close to eastern India around 110 Ma ago, from where abundant Precambrian crustal debris would have been shed.  However, the presence of continental geochemical signatures in Kerguelen Plateau basalts, otherwise having plume affinities, might indicate a fragment of former Gondwanan lithosphere at the core of the Plateau, akin to the now exposed Danakil block in the nascent Red-Sea – Afar rift in NE Africa, that spalled off during the break-up of the Mesozoic supercontinent.

The start of North Atlantic Deep Water formation

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The most favoured means whereby the weak fluctuation in solar radiation due to the Milankovich-Croll Effect become amplified to affect climate’s ups and downs is the switching on and off of thermohaline circulation in the North Atlantic Ocean.  The key to such ocean circulation is formation today of dense, cold brine through sea-ice formation around Iceland.  To set circulation in motion, however, depends on these brines being able to move southwards, which they do now in a sea-floor channel between Shetland and the Faeroe Islands.  When the North Atlantic began to open, this route was blocked by a ridge between Greenland and Shetland, buoyed up by residual warmth in the lithosphere from volcanic activity at the Iceland plume.

It is important to assess when the Shetland-Faeroe “gateway” formed, so that the effects of thermohaline circulation on pre-glacial climate can be assessed.  Petroleum exploration using high-resolution seismic reflection profiles and drilling has resolved this particular issue.  Geologists and geophysicists from Exxon and Cardiff University have found signs that sediment drift dragged by such a deep flow began in the early Oligocene (about 35 Ma ago) (Davies, R.  et al. 2001.  Early Oligocene initiation of North Atlantic Deep Water formation.  Nature, v. 410, p. 917-920).  The evidence takes the form of multiple, moat-like erosion surfaces down to the base of sediment fill between the Faeroes and Shetland, shown superbly by the seismic data.  Drilling shows that these signs of deep-water flow stop abruptly in Early Oligocene sediments.

Astrology and ice

The early Oligocene marked the onset of serious ice cover on Antarctica, and it shows as a dramatic increase in d18O values in the ocean-floor record of benthic forms – lighter 16O had been trapped in land ice.  That may or may not be a coincidence with the finding about the start of  North Atlantic thermohaline flow in the previous item.  A lesser, but still dramatic increase marks the Oligocene-Miocene boundary, suggesting further growth of the Antarctic ice sheet, which is not so readily matched empirically.  Detailed study of the isotopic  “blip” at this time by a team from the Universities of California, Cambridge and South Florida (Zachos, J.C. et al. 2001.  Climate response to orbital forcing across the Oligocene-Miocene boundary.  Science, V.  292, p. 274-278) suggests that it related to a remarkable coincidence in the astronomical record of solar heating.

Round 23 Ma ago, the orbital eccentricity dropped almost to zero – Earth’s orbit would have been circular – at the same time as its axial tilt became very stable, the one reinforcing the climatic effect of the other.  The isotopic “blip” coincides exactly with the coincidence.  The detailed record also shows very clearly that minor fluctuations in climate at that time were in step with the 400 and 100 ka periods in the eccentricity variations, and with those of 41 ka that relate to changes in axial tilt.  If nothing else, these results confirm that it is unnecessary to turn to extraterrestrial influences over climate other than those which are predictable from Milankovich’s theory (see Impacts and human evolution, above).

Additional source:  Kerr, R.A. 2001.  An orbital confluence leaves its mark.  Science, v. 292, p. 191.

Start of Pleistocene environmental change in tropical Africa

Pollen records from an ODP core drilled off the Congo estuary provide a record of the fluctuation in the monsoon of western tropical Africa (Dupont, L.M. et al.  2001.  Mid-Pleistocene environmental change in tropical Africa began as early as 1.05 Ma.  Geology, v.  29, p. 1195-198).  Before 1.05 Ma there is little sign of a glacial-interglacial pulse in the fluctuation of vegetation in the Congo Basin.  Thereafter, ups and downs in pollen from various vegetation groups correlate well with the benthic foram oxygen-isotope time series.  However there are a few surprises.

Conventional wisdom is that Africa experienced drying during glacial epochs, rain forest expanding during interglacials.  In the Congo basin, grasses and savannah trees increased during interglacials while mountain trees fell in their influence, up to 600 ka.  This suggests the opposite trend of  warm, dry interglacials and cool, humid conditions during glacial periods, similar to the record for tropical South America.  In the later Pleistocene, the fluctuation switched to that indicated by fluctuating lake levels throughout the continent.  The pollen variations are backed up by variations in dinoflagellate cysts, which show that discharge from the Congo dropped during interglacials.  The other surprise is that the onset of astronomically paced environmental change in west Africa predated the change to a 100 ka domination of global climate, and the increase in amplitude of changes in land-ice volume at 900 ka by a hundred thousand years.  Dupont et al. suggest that the changes in albedo in tropical West Africa in response to vegetation changes could have had an influence on global climate when the fluctuations began.

As well as being interesting in terms of climate change, the new data throw doubt on the hypothesized link between climate in Africa and pulses of migration of early human species, such as H. ergaster and H. erectus.  There were fluctuations in humidity in the earlier Pleistocene, but they show no link to global climate change.  So, it seems unwise simply to look to the Milankovich forcing as a pacemaker in early human affairs.

A Late-Jurassic methane “gun”

Massive releases of methane from gas hydrate layers beneath the ocean floor, and its subsequent oxidation to carbon dioxide have been implicated in major climatic and oceanographic changes in the mid-Jurassic, Cretaceous and Palaeocene.  They can be detected by drops in the 13C content of marine carbonates, caused by the “light” carbon trapped in biogenic methane.  All those known also correlate with evidence for climatic warming.

The Swiss Jura mountains are a repository of great thicknesses of Jurassic carbonates, whose ammonite faunas allow fine stratigraphic division.  Between 157 and 156 Ma (late Middle Oxfordian) there is a major negative excursion in d13C whose duration was as short as 180 ka (Padden, M. et al. 2001.  Evidence for Late Jurassic release of methane from gas hydrate.  Geology, v.  29, p. 223-226).  The Swiss-French geochemists who discovered the anomaly believe that the release may have linked to opening of the ocean gateway that connected currents between Tethys and the easter Pacific oceans through what is now the Atlantic.

Surviving in salt?

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In the manner of Count Dracula’s dogged refusal to shed his mortal coil by hiding from sunlight, is it conceivable for primitive organisms to be immortal by being protected from UV radiation?  That it might be possible emerged from the revival of bacteria trapped in fluid inclusions in Permian rock salt by Russell Vreeland and William Rozenzweig of West Chester University in Pennsylvania (see Earth Pages, November 2000, The undead).  Despite taking stringent precautions to avoid any contamination of their samples by modern bacteria, Vreeland and Rozenzeig’s claim has been fiercely challenged.  It is possible that the dormant bacteria could have entered the salt in much younger solutions permeating the deposit (incidentally one of the most stable tectonically and hydrogeologically – it is the prospective site for burial of US radioactive wastes).

Vreeland’s team  found 4 bacterial strains – all salt-tolerant halobacteria – but have genetically fingerprinted only one so far.  It is related to a modern genus living in the Dead Sea that forms spores.  The minute fluid inclusions from which samples came have insufficient energy and nutrients to have sustained cell growth and division.  The inactivity involved in spore formation, combined with the slowing down of biological processes by dense brines in the inclusions, might just allow immensely long survival for 250 Ma without breakdown of the DNA essential for revivable dormancy.  Hydrogen diffusing into the salt and biological materials could have played a role in maintaining DNA’s integrity.  One snag is that the DNA sequence of the revived bacteria is 99% identical to that of its closest modern relative.  Using the theory of molecular clocks, they should have been different by 5 to 10%.  Yet, says Vreeland, salt deposits continually add to the surface environment, being soluble.  Any dormant bacteria within them would replenish fully living stocks in similar environments to those which formed the salt originally.  Such continual addition might preserve ancient genetics, that would otherwise evolve steadily.

Aside from giving comfort to proponents of life spreading throughout the universe as spores adrift on dust driven in the manner of a solar sail, the results encourage probing of older salt deposits, which go back in almost undisturbed form to the Mesoproterozoic.

(Source:  Knight, J.  The Immortals.  New Scientist, 28 April 2001, p. 36-39).

How the Earth works: “mega-blobs” in the mantle

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Seismic waves generated by large earthquakes arrive at different times at seismographs arranged in a world-wide network.  When they arrive depends on the relative positions of epicentres and receivers, but most importantly on variations in physical properties within the Earth that affect the speed at which they travel.  Given enough high-quality seismic records and powerful computing, such data allow geophysicists to map how wave speeds change with depth in the mantle and produce 3-D models.  In other words, seismic energy can produce geophysical homologues of medical CAT scans.  The second important means of visualizing the unseeable comes from the geochemistry of basaltic lavas formed by partial melting of the mantle in different tectonic settings.  Results from such studies reveal that the composition of the mantle is not homogeneous.  Combining information from both sources, in the light of motions of the lithosphere, provides a powerful input to modelling how the Earth behaves as a whole  (see Earth Pages, July 2000, Geodynamics).

Seismic tomography’s most important derivative stems from the manner in which wave speed depends on variations in the mechanical properties of the mantle.  For P-waves, speed varies with the mantle’s differing resistance to compression, and S-wave speed is directly proportional to the rigidity of the mantle.  Unusually high mantle temperatures cause decreases in compression resistance and rigidity, and therefore drops in the speeds of both kinds of body wave.  The cooler the temperature, the higher both speeds.  So, velocity variations in seismic tomographs are proxies for changing mantle temperature, and in turn for regions of different density – the hotter a material is, the lower is its density.  The implications are quite simple; high-speed anomalies signify cool, potentially sinking regions in the mantle, whereas low speeds suggest that matter is able to rise.  In practice, modelling the fundamental dynamics of the Earth’s mantle using seismic tomography is computationally difficult, often ambiguous and blurred because of the lack of suitable data.

Seismic tomography gave the first clues to the idea that subducted slabs penetrate all the way down to the core mantle boundary, and that at least some of the plumes suspected to underpin hot spots have their source at such depths.  Together, these findings support whole-mantle convection.  As well as improving the amount of high-quality seismic data and the software to analyse them, combining physical parameters with sketchy knowledge of variations in mantle chemistry and mineralogy is the next step in “sharpening” the focus of mantle models.  That seems to have been taken by Alessandro Forte and Jerry Mitrovica of the Universities of Western Ontario and Toronto (Forte, A.M. and Mitrovica, J.X. 2001.  Deep-mantle high-viscosity flow and thermochemical structure inferred from seismic and geodynamic data.  Nature, v. 410, p. 1049-1056).  Their work confirms the concept of whole-mantle convection resulting from thermal anomalies, but has an added bite.  They show evidence for vary large variations in deep-mantle composition – to megaplumes they have added “mega-blobs”.  Although the results of their analyses are limited by data availability and reliability, and by simplifying assumptions, they imply that such blobs can respond to temperature changes by rising and sinking periodically.  That is, the mantle may move as vast domes and downwellings as well as in the more tightly constrained plumes and sinking slabs.  One intriguing possibility is that such blobs may be primitive and retain high concentrations of elements that evolution of other parts of the mantle has transferred to the continental crust.  Such primitive signatures are passed on to the geochemistry of basalts forming from plumes beneath ocean islands.  However, there is a long way to go before a blob-plume-ocean island connection can be made.  If it proves to be plausible, then such ancient blobs would have to be very viscous to have resisted mixing over time with more evolved mantle.  Another possibility is that the blobs are themselves highly evolved, through the progressive accumulation of subducted slab material.

(See also:  Manga, M.  2001.  Shaken, not stirred.  Nature, v. 410, p. 1041-1042)

Atmospheric oxygen: yet more

Following last month’s Earth Pages briefing (Mantle overturn and oxygenation of the atmosphere)  Nature (19 April 2001) ran a news feature on the competing theories for when oxygen began to accumulate in Earth’s atmosphere (Copley, J.  2001.  The story of O.  Nature, v. 410, p. 862-864).  The paradox between evidence for oxygen production by photosynthetic cyanobacteria since 3.5 Ga and that supporting the first major influence of oxygen in redbeds at 2.2 Ga may be resolved by the ideas of Hiroshi Ohmoto of Pennsylvania State University.

Redbeds – terrestrial sediments containing abundant ferric hydroxides – form when iron enters its Fe-3 state, and are insoluble.  That results in weathering processes being unable to leach soils of their iron content, unless the waters involved have been rendered reducing by bacterial activity.  The most dramatic expression of this is laterite that blankets ancient erosion surfaces of most of the Gondwanan continents, much of which formed in Palaeocene times.  Palaeosols older than 2.2 Ga do not show the characteristic laterite ferricrete cap, implying that iron existed consistently in its soluble Fe-2 form and could be leached away.  Most geochemists regard that as evidence for a reducing atmosphere, lacking oxygen except as a trace.  Ohmoto suggests that organic acids formed by terrestrial cyanobacteria might also create the reducing conditions necessary for iron leaching..  He sees such “blue-greens” as having had a dual role, fixing iron in soils through oxidation and then releasing it to solution by formation of organic acids.  Ohmoto and Antonio Lasaga are developing a geochemical model for the iron, oxygen, carbon and sulphur cycles during the Archaean.  Early runs suggest that only 30 Ma after the appearance of cyanobacteria at 3.5 Ga their release of oxygen would have built up high levels in the early atmosphere.

That bucks the evidence for low oxygen provided by detrital sulphides and uranium oxide grains in Archaean high-energy sediments, such as the conglomerates of the Witwatersrand basin in South Africa – in the presence of oxygen, both should break down quickly in water.  Archaean banded iron formations, thought to form by reaction between Fe-2 ions in ocean water and oxygen produced locally by shallow-water cyanobacteria, have a dual significance – abundant oceanic Fe-2 suggests global lack of oxygen, and BIF deposition of ferric oxide would have formed a sink for any oxygen in the environment.  Ohmoto cites the re-appearance of BIFs at several times in the Proterozoic Eon as a sign that BIF formation was possible when atmospheric oxygen was abundant.

The debate seems destined to run, for two reasons.  Studies of sulphur isotopes – Ohmoto’s speciality – give evidence for fractionation through the influence of ultraviolet radiation.  Once oxygen rose in the air, its formation of ozone gas would have blocked UV and ended this kind of selective take-up of sulphur isotopes.  James Farquhar of the University of California in San Diego has found its effects common in Archaean rocks, but no sign in later rocks.  That favours an oxygen-poor early atmosphere.  Ohmoto counters with abundant evidence in the Archaean for the activity of bacteria that reduce sulphate ions to sulphide – in an oxygen-poor world, sulphate formation would have been suppressed.

Oxygen build-up demands complementary burial of organic matter formed by photosynthesis before it oxidized.  The influence of organic carbon burial  is to take with it 12C that biological processes favour over heavier 13C, so that carbon-rich rocks show higher 12C than carbonates precipitated from the seawater that was left.  Such enrichment in 12C shows up most clearly after 2.7 Ga ago, when carbon burial must have been stoked up somehow.  That points to a late build-up of oxygen in the air.  But why?  James Kasting, also of Pennsylvania State University, suggests a change in the Earth’s mantle from reducing to oxidizing conditions.  Before that time volcanic gases would have been dominated by reduced gases that could mop up any free oxygen.  Afterwards, oxidized volcanic gases could have co-existed with free oxygen.

Mantle overturn and oxygenation of the atmosphere

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The presence of abundant oxygen in Earth’s atmosphere defies Le Chatelier’s Principle – it should react rapidly with the rest of the environment through oxidation.  That it does not is sufficient evidence for an alien observer to conclude that our planet is dominated by photosynthetic life at its surface and the burial of carbohydrate by geological processes.  So, Le Chatelier is not defied on the long term, because the CO2 + H2O = carbohydrate + oxygen equilibrium does not reach a balance because of continual removal of organic material from the right-hand side!  That Mars has no atmospheric oxygen bears witness to its lifelessness in that respect, as concluded decades back by James Lovelock.

Before 2.5 Ga ago, in the Archaean, atmospheric oxygen was a trace gas.  Preservation of detrital grains of sulphides and uranium oxides in Archaean clastic sequences, that would have broken down in an oxidizing environment, is the main evidence for that.  The other side of the coin is that oxygen-producing photosynthesizers – the cyanobacteria – were abundant throughout the Archaean, leaving their trace as common stromatolitic carbonates and signs of the crucial enzyme rubisco in kerogens and the carbon-isotope record.

If cyanobacteria generated oxygen, then why did it not build up in the atmosphere throughout the Archaean, instead of from about 2.2 Ga ago?  The most likely explanation is that Archaean magmatism released vast amounts of Fe-II or ferrous iron to sea water, which then reacted with available oxygen to form the ferric oxide of banded iron formations (BIFs), with the biproduct of hydrogen gas that further drove Archaean environmental chemistry into a reducing condition.  Seawater circulating through Archaean ocean crust would also have enriched basalts in ferric iron by the same oxidizing reaction.  Such a chemical model still leaves unexplained the shift to an oxygenated atmosphere after the Archaean.

Norman Sleep of Stanford University, reviews an article by Kump et al. in  Geochemistry, Geophysics, Geosystems (2001) that deals with this dilemma (Sleep, N.H.  2001.  Oxygenating the atmosphere.  Nature, v. 410, p. 317-319).  Kump and his co-workers suggest that, rather than relating to a change in palaeoecology, the shift arose from subduction of dense ferric oxide-rich lithosphere to settle at the core-mantle boundary.  By the end of the Archaean oxidized material filled the lower mantle.  Heating reduced its density so that it became buoyant.  If that deep oxidized layer rose to displace more primitive, reducing mantle, later magmatism would have released less Fe-II, thereby allowing biologically generated oxygen to build up.  The converse effect would have been to bring down levels of reducing atmospheric gases, such as hydrogen, methane and carbon monoxide, to trace levels.

Except to its primitive producer – cyanobacteria – oxygen would have been anathema to the dominant anaerobic Bacteria and Archaea that constituted Archaean life.  An end-Archaean mantle overturn, implicated by the tectonic pandemonium from 2.7 Ga, could well have triggered accelerated extinction and evolution that encouraged the rise of the eukaryote cell that requires oxygen for its basic metabolism.  Nonetheless, such an upheaval would have been directly connected with earlier living processes.  That is something which will delight followers of the Gaia hypothesis.

Taming Lake Nyos, Cameroon

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On 21 August 1986 a huge cloud of carbon dioxide gas released from Lake Nyos in the Highlands of Yaounde District of Cameroon, killed 1,700 local people by suffocation

Lake Nyos is one of several maars produced by one-off explosive events in the recent past.  Isotopic analyses of gas remaining dissolved in the lake show that the CO2 is of volcanic origin.  The lakes are fed by springs on their beds, which is where the CO2 enters, so that CO2-rich water builds up at the bottom.  A thermal overturn of Lake Nyos may have caused dissolved gas to come out of solution as pressure decreased. 

Since 1986, gas levels have built up, so Lake Nyos once again threatens the local people and their livestock.  An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly.  This involves polyethylene pipes that descend to the lake bed.  Once primed by pumping, gas bubbles form as pressure drops.  Their rise up the pipe drags more water upwards, as in a soda siphon.  Fifteen years after the disaster, the first such siphon began operating with spectacular effects (Jones, N.  2001.  The monster in the lake.  New Scientist, 24 March 2001, p 36-40).  This only keeps pace with addition of CO2 and a full solution requires several siphons.

Some scientists worry that siphoning itself may disturb a precarious balance in the lake, so the French engineers who built it have included sensors and shut-off valves.  Not everyone agrees that the 1986 disaster resulted from processes within the deep lake itself.  That should have led to a regular succession of gas releases, for which there is little evidence.  Landslips or a gaseous eruption might have been the trigger.  Reducing dissolved CO2 levels in Lake Nyos and nearby Lake Monoun would seem to lessen risks of a future disaster, but could also lull locals into a false sense of security.

Ganymede’s water volcanism

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Jupiter’s giant moon Ganymede is an icy world, as are many satellites of the Outer Planets.  But is also one of the few showing signs of some kind of tectonics.  Its surface is made up of dark, cratered material, presumably an ancient mixture of rocky debris and ice, riven by swaths of lighter surface.  The latter, which covers two-thirds, has little cratering and is a later feature of the moon’s surface.  Somehow, Ganymede underwent a resurfacing, perhaps in a similar manner to neighbouring Europa – a simple ice ball – but not so all-consuming.

The event probably stemmed from the coming together of Jupiter’s largest moons into orbital resonance that generated sufficient gravitational energy to cause internal melting.  Precisely how this achieved the intricacies of Ganymede’s surface is something of a mystery.

Images from Voyager and Galileo missions form stereoscopic pairs from which the moon’s topography can be derived with useful precision (Schenk, P.M. et al.  2001.  Flooding of Ganymede’s’ bright terrains by low-viscosity water-ice lavas.  Nature, v. 410, p.57-60).  Using digital elevation data with high-resolution Galileo images, Schenk et al. have been able to subdivide the light swaths into three kinds of surface, reticulate, grooved and smooth at different elevations from highest to lowest.  Large elevation differences of the order of 2 km are involved.  That in itself is evidence that ice at the prevailing temperature behaves more like rock than glacial ice.

The greater surprise is that the lowest, smooth unit shows evidence of having formed by processes akin to volcanism, with calderas and features that engulf earlier structures.  However, even the fine resolution of the latest images does not reveal “lava” flows.  Some rifting mechanism seems to have encouraged emergence of water-ice “magma” to form the low smooth terrains.  All very counter-intuitive for terrestrial volcanologists, because water “magma” must be more dense than the solidified flows forming from it, unlike silicate liquids or those rich in sulphur on Io.  That makes the formation of high volcanoes impossible.

Presumably, the much higher grooved and reticulate terrains started in the same manner, as linear troughs, then to be deformed and thickened by “water tectonics”.

Bacterial sulphides from the Archaean

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Most of the sulphide mineralization involved in base-metal ore bodies formed by reaction between metal ions and those of sulphur released by bacteria that reduce sulphate ions in water.  They do that while oxidizing organic matter or hydrogen in their metabolism, under completely anaerobic conditions.  Like other biological processes, sulphide production at the cell level fractionates the isotopes of sulphur so that it becomes possible to chart sulphate-reducing bacteria through time.  Depletion of 34S in sedimentary sulphides relative to that in co-existing sulphates (such as baryte) was previously known with certainty back to 2.7 Ga.  Danish and Australian bio-geochemists have now pushed this particular bacterial metabolism back by 750 Ma (Shen, Y.  et al. 2001.  Isotopic evidence for microbial sulphate reduction in the early Archaean era.  Nature, v. 410, p. 77-81).

The data from the Pilbara Craton of Western Australia helps calibrate the evolutionary bush of the prokaryotes, which is based on comparisons between RNA in different living organisms.  The trouble is, sulphate-reducing species with very primitive genetics and similar lifestyles (hyperthermophilic) occur among both the Bacteria and Archaea.  Shen et al. go for the Bacteria Thermodesulfobacterium as the most likely organism responsible.  Their argument is that the mineralization replaces originally sedimentary gypsum, formed at low temperatures, and probably represents hydrothermal processes in which thermophilic organisms could have thrived.  Bacteria that reduce sulphate ions at low temperatures – gram-positive and purple bacteria – are genetically more advanced than their candidate.

See also:  Slime to the rescue Earth Pages December 2000

“Piltdown” bird

Fragmentary remains of vertebrates in particular are notoriously prone to misguided reconstruction – Gideon Mantell placed the Iguanodon’s thumb on its nose, thereby obscuring evidence for the first hitchhiking dinosaur for many decades.  The forger of fossils has two possible motives – spite in the case of Piltdown Man, or profit.  The skilled forgeries of Silurian trilobites by quarrymen from Dudley in Britain’s West Midlands are now more valuable because they were made for rapacious Victorian antiquaries, than bona fide Calymene specimens.  Missing links sought by professional palaeontologists and archaeo-biologists are in a field of their own.  It has long been suspected that birds evolved from small carnivorous dinosaurs, and the early Cretaceous of China has provided spectacular transitional fossils.  Archaeoraptor was announced as the final missing link in 1999.  Within a year it was denounced as a forgery that combines very skilfully the bones of a primitive bird with those of a non-flying dromeosaurid dinosaur.  How it was assembled has finally been revealed using X-ray tomography, which shows that as many as 5 different specimens were “cut and pasted” together (Rowe, T. et al. 2001.  The Archaeoraptor forgery.  Nature, v. 410, p. 539-40).

Cretaceous water lilies

Readers of Earth Pages will be delighted to learn that fossil flowers of Nymphaeales (water lilies) have been found in the Lower Cretaceous of Portugal.  (Friis, E.M. et al. 2001.  Fossil evidence of water lilies (Nympaeales) in the Early Cretaceous.  Nature, v. 410, p. 357-360).

When modern corals emerged

Fossil corals  fall into three taxonomic groups or Orders: tabulate, rugose and scleractinian.  Only the last group is alive today.  Scleractinian corals have been central to the “carbonate factories” that have drawn down CO2 from the atmosphere throughout the Mesozoic and Cainozoic Eras to form reef limestones.  They are major regulators of long-term climate fluctuation.  However, there is something very odd about their appearance in the fossil record, as discussed recently by George Stanley and Daphne Fautin (Stanley, G.D. and Fautin, D.G. 2001.  The origins of modern corals.  Science, v. 291, p. 1913-1914).

The rugose and tabulate corals were exclusively Palaeozoic colonial, carbonate-secreting organism.  Their record ends abruptly with the end-Permian mass extinction.  No examples of scleractinian corals have been found in rocks older than Triassic.  The oddity is a 14 Ma gap in known coral fossils in the earliest Triassic.  Scleractinians secrete calcium carbonate as aragonite, whereas rugose corals formed from calcite; an important difference in processes at the cellular level.  It is hard to avoid the conclusion that the ancestors of scleractinians did not secrete carbonate and were entirely soft-bodied taxa during the Palaeozoic Era.  If Permian Rugosa and Tabulata happily secreted carbonate, while proto-Scleractinia did not, there ought to be a biochemical or geochemical explanation for the last taking on a reef building role in Mesozoic times.

Molecular evidence suggests that scleractinian ancestry goes back to the Late Carboniferous, and that there is a complex “lawn” (as opposed to tree or bush) of genetic relationships between modern hard corals and soft-bodied organisms that are closely related.  The puzzle can potentially be resolved if modern corals and their ancestral lines lost and regained skeleton building several times in the Mesozoic and Cainozoic.  Exploring that requires more understanding of how carbonate is secreted at the cell level, and the geochemical conditions in seawater that underpin the need for secretion.

Following the greatest ever mass extinction at the end of the Permian, early Triassic oceans were almost sterile and anoxic.  Global CO2 levels were high, yet little carbonate was deposited in the marine environment.  That would have increased the amount of calcium and bicarbonate ions in sea water.  Many corals harbour algal symbionts that are involved in calcification.  As calcium carbonate saturation drops so too does carbonate secretion, and vice versa.  Calcium is a two-edged sword in cell metabolism.  On the one hand it is vital in “information” transfer, yet above a threshold it combines with CO2 to form crystalline carbonate within the cell wall, that spells cell death.  In Palaeozoic oceans rugose and tabulate corals, as well as a host of other carbonate secreting animals, would have buffered calcium concentrations below levels tolerable by other, soft-bodied animals.  Their sudden demise 251 Ma ago, along with most everything else, would have left calcium to build up in the early Triassic “Strangelove” ocean.  Survivors of the holocaust would have had a fierce task coping with potential calcium toxicity, and the scleractinians may well have adopted calcification as a survival mechanism.  Thereafter, oceans restocked with reef building organisms would have had tolerable calcium concentrations for most organisms, those now able to secrete carbonate having the benefit of armour against predation and a solid substrate for colony building.

Skulduggery, migration and confusion

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March was a fertile month for news concerning human origins and evolution.  The good news is that the palaeoanthropologists are at each other’s throats again!  I think it is good news because many of them have an air of smugness and triumph, and they get far more money than other Earth scientists (with the exception of those bent on finding a banth on Mars).  Tangling with hominins in Kenya is a sure route to trouble, as the finders of “Millenium Man” (Ororrin tugenensis) – Martin Pickford and Brigitte Senut now discover (Butler, D.  2001.  The battle of Tugen Hills.  Nature, v. 410, p. 508-509).  Not only is their claim that the 6 Ma old fossil is the oldest on the route to humanity hotly disputed (Aiello, L.C. and Collard, M.  2001.  Our newest oldest ancestor?  Nature, v. 410, p. 526-527), but has ended with their taking suit against Richard Leakey and the Kenyan National Museums for unlawful arrest, false imprisonment and malicious harassment over claims that they poached the site where Orrorin was found.  Never an easy atmosphere in which to work, human evolution is now one posing considerable dangers, so much so that some specialists will comment only anonymously.

Books in the field always sell like hot cakes, as much for the intrigue and the chutzpa as for the science that they convey.  Reviewers become drawn into the hype, despite their best intentions (White, T.D. 2001.  Adventures in the Bone Trade: the Race to Discover Human Ancestors in Ethiopia’s Afar Depression, by Jon Kalb.  Nature, v. 410, p. 517-518).  Areas in Afar and Danakil are physically dangerous because of current hostilities between Eritrea and Ethiopia, and dissatisfaction among the local people.  But they have enormous potential for hominin discoveries following those of “Lucy” and Ardepithecus.  On a recent visit to Eritrea I heard rumours of what might amount to claim jumping and attempts to acquire material clandestinely from new and potentially productive sites, hopefully without foundation.

Confusion is washing over hominin cladistics as ever more variants of accepted species, and fossils that seem to warrant new species and genera turn up.  This is particularly rife for early remains that predate the first stone tools (Lieberman, D.E. 2001.  Another face in our family tree.  Nature, v. 410, p. 419-20; Balter, M. 2001.  Fossil tangles the roots of human family tree.  Science, v. 291, p. 2289-2290; ).  Of course, much of the confusion stems from every new find seeming to bear different cranial and dental hallmarks, combined with dogged attempts to chart the path of our descent through the remains and a tendency to change genus and species names (Homo habilis is now sometimes assigned to Australopithecus, despite a probable association with primitive stone tools).  The latest bush figured by Lieberman is notable for every supposed cladistic link being marked by a query.  One gets the impression of rather too much shuffling around of anatomy, and too little consideration of the unseeable, but inevitably vital distinction between the human line and other fossils.  There are still very few hominin fossils!

Tools demand consciousness, and probably social links far stronger than those of other apes.  Only stone tools survive, from around 2.5 Ma ago, but must represent an advanced culture that arose from earlier beginnings.  Abstracting usefulness from surrounding nature and social organisation confer such advantages to its inventors that they set them apart from other animals in relation to natural selection.  Fitness no longer applies to the individual organism, but increasingly to its culture shared with others.  The formerly unfit becomes fit, and that can play havoc with physiological diversity and thereby the cladists’ shuffling.

Culture confers something equally powerful by enabling its carriers to diffuse beyond their geographic range.  The 2 March 2001 issue of Science devotes 33 pages to human migrations (Culotta, E, Sugden, A and Hanson, B. (eds)  2001.  Humans on the move.  Science, v. 291, p. 1721-1753).  For me, this is the most powerful and informative contribution to our self-knowledge in many years.  Eight articles cover the earliest Europeans, the relations between modern humans and Neanderthals, the first colonisers of the Americas, the roles of genetics in teasing out our origins and how tools track physiological change.  Appearing in the midst of tedious and self-regarding squabbles among the “bone people”, it surely marks a proper line of march in this abidingly gripping branch of Earth science.

Human genome “snips” and our evolution

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February 2001 saw the public release of the human genome, with entire issues of both Nature and Science substantially devoted to discussion of its implications, educational CDs and wall charts.  That is if the huge wadge of adverts capitalising on the genome’s release is discounted  Pundits have latched onto the fact that humans seem to possess not that many more genes (around 30 000) than grass, a worm or a fruit fly, making comments about how humbling that is.  Vastly outnumbering protein-coding genes are “snips” (single nucleotide polymorphisms – SNPs), and humans have around 1.4 million of these and possibly far more in the 3 billion sequences of four nucleotides.

The huge variability of “Snips” holds excellent prospects for deeper understanding of human origins and evolution, previously (and unsatisfactorily) addressed by using DNA in mitochondria and the Y chromosome.  Previous means of establishing molecular  “distance” to indicate relatedness and the times of divergence from last common ancestors rely on DNA that occurs only once in each cell, and does not undergo division and recombination during sexual reproduction, so that it is passed on in the female or male line of descent.  Whereas such haploid material is relatively easy to analyse and interpret, it behaves like a single gene.  Differences arise through natural selection or chance events that affect only one item.  That makes it possible only to address the history of one variable, rather than that of a whole species or a population – a single thread rather than the multitude that must constitute the signal of real events.

“Snips” potentially can help resolve the out-of-Africa and multiregional hypotheses for the origin and spread of fully modern humans, and even whether we do carry vestiges of other groups of the genus Homo, such as the Neanderthals or various groups of more archaic beings who began to leave Africa for the rest of the Old World around 1.8 Ma ago.  In a review of the possibilities, Mark Stoneking of the Max Planck Institute in Leipzig (Stoneking, M. 2001.  From the evolutionary past…  Nature, v. 409, p 821-822) cautions that much remains to be done before SNPs can really give believable information.

The earliest ecosystems

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Reconstructing an environment devoid of multicellular life requires some stretch of the imagination.  Before the appearance of the first metazoans in the Proterozoic ecology might seem to have been somewhat tedious.  However, discovery from molecular biology of the antiquity of living prokaryotes and detailed analysis of their highly diverse metabolism makes such a venture fascinating.  In a review of early life and habitats, Euan Nisbet and Norman Sleep (Nisbet, E.G. & Sleep, N.H. 2001.  The habitat and nature of early life.  Nature Insight, v.  409, p. 1083-1091) weave an intricate fabric of biology, geochemistry and tectonics that serves to enthuse undergraduates and professional Earth scientists alike.  The review is understandably speculative, dealing as it does with proxy evidence for Archaean life forms themselves and their possible precursors.  But it presents a useful logic for seeing the Archaean Aeon as having a highly diverse biosphere, albeit one that is probably as alien as any that humans are likely to find…. even if they get to Mars!

Buckyballs and the end-Palaeozoic extinction

The largest mass extinction in the 600 Ma history of multicellular life took place 251 Ma ago, at the close of the Palaeozoic Era.  The end-Permian event wiped out up to 90 percent of marine animals and 70 per cent of land vertebrates.  Spanning the Permian-Triassic boundary are the vast Siberian Traps (continental flood basalts), that have been the most widely suspected trigger for the extinction.  Their emissions of sulphur dioxide may have created acid rain and stratospheric aerosols that cooled conditions through the event.  The boundary shows up in ocean-floor sediments incorporated in a Japanese ophiolite, which suggest less than 100 000 years saw the massive die off.  The popular notion of an impact cause for mass extinctions seemed to be a non-starter for the P-Tr boundary until late February.  There had been sporadic reports of iridium anomalies from the boundary, but not so believable as that at the K-T boundary.

Another tell-tale sign of extraterrestrial causes for extinctions is the presence of peculiar molecules in which more than 60 carbon atoms are bonded in a structure similar to a geodesic dome, called fullerenes after the creator of  this architectural structure, Buckminster Fuller (they are nicknamed “buckyballs”).  Fullerenes are thought to be created in the aftermath of supernovae, and therefore likely to occur in comets from the outer Solar System, where the most primitive material resides.  Their structure allows them to act as immensely strong and impermeable “cages” for gases around at the time of their formation.

In 1996 US geochemists discovered fullerenes in rocks formed in a huge impact crater near Sudbury, Ontario that must have come from space nearly two billion years ago and arrived on Earth intact. Last year the same team showed that even more complex carbon molecules, with as many as 200 atoms, had survived an impact from space at the same time as an impact wiped out the dinosaurs at the K/T boundary.  Sensitive measurements of isotopes of helium ad argon locked within the carbon cages reveal that their proportions are uncharacteristic of more common Solar System materials and must have been formed by nucleosynthesis far off in space.

Samples from the Permian-Triassic boundary in China, Japan, and Hungary contain fullerenes with these unusual combinations of helium and argon isotopes (REF).  This is incontrovertible evidence for an impact influence.  As yet, no candidate crater has been found, though with 70 percent of the Earth being occupied by recyclable ocean floor, it may have vanished down a subduction zone (the oldest sea floor now is late Triassic).  However, the coincidence of impact, massive flood basalt eruptions and a mass extinction is familiar.  The long-running debate about the K-T event is fuelled by such a triple coincidence – the death of the dinosaurs and much else, the Deccan Traps and the Chicxulub structure in the Gulf of Mexico.

Some authorities believe that extinctions big enough to be adopted as the principal boundaries in the stratigraphic column may need a “double whammy” to occur.  But there is also evidence that links the timing of flood basalt events to other extinctions that have yet to reveal a correlation with impacts.  Undoubtedly an outcome of mantle upwelling in superplumes that might start from the core-mantle boundary, the seeming regularity of flood basalt events (around every 30 Ma) poses a conundrum.  Linking two major basalt floods with impacts raises the possibility that superplumes might be triggered by major impacts.  One idea is that seismic energy released by major impact travels to the core, to trigger dislodgement of core-mantle boundary material into a rising superplume at the opposite side of the planet.  The Decccan Traps are at almost the exact antipode of the Chicxulub structure.  Using this logic, the place to look for the P-Tr culprit would be at the antipode of Siberia, when it was part of Pangaea.  That conveniently places the possible site in the huge ocean that encompassed Pangaea at the end of the Permian – it would ultimately be subducted as Pangaea broke up and continents began their latest round of drifting.

BSE in reverse?

Some say that we are witnessing and are even the source of the latest mass extinction.  If so, it is not entirely a product of modern society.  The late-Pleistocene of Australia and the Americas saw massive losses among large marsupial and more advanced mammal species from around 70 thousand years ago, i.e. since the first arrival of humans.  A widely accepted view is that this selective extinction was because the vanished species were eaten because they were naïve and easy prey.  Certainly the disappearances were sudden.  A huge diversity of large American mammals, including several elephants, camels, giant sloths and sabre-toothed cats (about 30 species), was decimated from around 11 to 9 thousand years ago, as humans spread quickly southwards through two continents when climate emerged from the last Ice Age.  Gluttony on such a scale is difficult to comprehend.

In an article in the February issue of Scientific American, Ross McPhee of the American Museum of Natural History in New York introduces an alternative hypothesis, that the extinctions resulted from infectious diseases crossing species barriers.  His idea stems not from evidence for epidemics, but the lack of it for massive butchery in the form of cut marks on bones of these extinct beasts.  Isolated for millions of years from both humans and the diseases that evolved in the Old World, American and Australian mammal populations would have had no immunity to viruses or pathogenic bacteria brought in by the human colonisers.  The crash in population of native Americans following European colonization was mainly due to epidemics.  The fact that several human diseases originally evolved in other species – poxes among cattle, ‘flu in birds and AIDS in African apes, for example – points to mutations possibly occurring in the opposite direction.  Add to that the fact that human immigrants would have been accompanied by dogs and almost certainly rats and infesting insects, and the idea become plausible.

(Source:  McKie, R. Man’s germs wiped out mammoths.  The Observer, 28 January 2001)

Cretaceous owl?

Earth Pages has charted over the last 9 months a tendency for publication in the “journals of record” of fossil arcana.  February 2001 adds yet another.  Early Cretaceous sequences of Cuenca in Spain include lagerstätten (horizons of exquisite preservation).  One provided a near perfect example of a regurgitated pellet, similar to those coughed up by owls (Sanz, J.L. et al. 2001.  An early Cretaceous pellet.  Nature, v. 409, p. 998-999).  In it are the remains of four chicks, including evidence of feathers, of different bird species, whose bones show clear microscopic evidence of having been partially digested.  Being 23 cm2 in flattened form, the pellet is presumably from some predator approximating the dimensions of a modern owl.  That does not necessarily call for Cretaceous owls, for any small predator, such as a pterosaur or small theropod dinosaur may well have encountered difficulty passing bony debris to dung, and resorted to regurgitation.

Loss of Martian atmosphere

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Mars seem quite massive enough to have held a substantial atmosphere, as have Earth and Venus.  That it has barely any is a major puzzle.  One possible reason is that Mars has a tiny magnetic field.  A strong magnetic field on Earth serves to deflect the solar wind, a stream of charged particles emitted by the outer part of the Sun.  Undeflected in this way, the solar wind would gradually strip off an atmosphere.  Currently, Mars has so little atmosphere that photosynthetic life that combines water and carbon dioxide to build carbohydrate is impossible, despite the fact that most of what little air there is  comprises CO2.

In the great chattering about prospects for Martian life at some time in the planet’s past, a central issue is the timing of atmospheric loss.  It is inconceivable that Mars never had an atmosphere, because it possesses the largest volcanoes in the Solar System which must have vented mantle gases.  If its magnetic field slowly dwindled, that gives ample time for life to have emerged.

Unsurprisingly, one of the tasks of NASA’s Mars Global Surveyor Mission has, for the last two years, been a global survey of the Martian ionosphere.   That is a proxy for regional variation in magnetic field strength.  A recent meeting of the Mars Global Surveyor team revealed the maps and their implications to the public.  The oldest terrains – those showing the greatest density of impact structures, as in the Lunar Highlands – show evidence of remanent magnetism.  Those affected by the youngest major impacts – analogous to the 4 billion-year old lunar maria – do not.  This suggests that Mars lost its magnetic field some time in its first half billion years, and thereby any substantial atmosphere.  One possible reason for this loss is that Mars has long been a geologically sluggish planet.  It is turbulent motion in the Earth’s liquid outer core that generates a magnetic field.  That turbulence is probably kept in motion by convective heat transfer in the mantle – it is a companion of terrestrial plate tectonics or any kind of regular mantle overturn.  Mars’ mantle does not do that, either by tectonics or through plume activity (unlike Venus), so its core may well be devoid of motion.

Exactly when magnetism stopped, with the attendant effect of the solar wind on any atmosphere, is crucial for estimates of how long life might have had to appear and begin evolving.  The results certainly rule out evolution beyond the most primitive life forms.  However, establishing that date must await future Mars landers, either staffed or robotic, on which the most important experiments will aim at detecting signs of former of extant life.  The magnetic data are not encouraging for exobiologists.

(Source:  Samuel, E.  2001.  The day the dynamo died.  New Scientist, 10 February 2001 issue.)

And now, Martian glaciers

Readers will have seen scornful comments in Earth Pages, regarding the desperate search for evidence of liquid water on modern Mars.  That water once was there seemed cut and dried from the giant valleys scoured across the Red Planet’s surface.  It was said that vast volumes of deep-seated ice catastrophically melted to flood from large impact sites.  Like the supposed evidence for active watery emissions in recent time, that for past flooding which cut the large valley systems rested on interpretation of the landforms themselves.  Re-examination of the valleys shows that they almost exactly mimic features revealed by sonar sounding on the sea floor surrounding the Antarctic ice sheet.  The Antarctic features probably formed during increased flow regimes when sea level stood at its lowest during glacial maxima.  Such surges can flow uphill, and sure enough the valley systems on Mars do have uphill tracts.

Baerbel Luchita of the US Geological Survey applied work on structure beneath the Ross Ice Shelf to Mars, suggesting that impact-melted water froze on emergence at the surface to flow in a more or less glacial fashion.  Undoubtedly, ice flow is far more capable of large-scale excavation than an equal volume of water, but to form the 1000 km long systems on Mars implies a considerable head.  Also its branching nature forces the assumption of many coalescing glaciers over a very large area.  That meets problems in imagining a widely distributed source of energy that caused the melting.  Impacts are at points, so perhaps yet another mechanism, such as seismicity, will need to be invoked.

(Source:  Hecht, J.  Sliced by ice.  New Scientist, 27 January 2001 issue)

Out of Africa hypothesis confounded?

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

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

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

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

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

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

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

Siberian role in climate change?

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

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

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

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

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

Mismatches from north to south proven

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

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

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

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

Hands-on planetology

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

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

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

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

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