Author: zooks777

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

Hi-tech future may be saved by ocean floor sediments

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Global rare earth element production (1 kt=106...
China's growing REE market share. Image via Wikipedia

Since the now far-off founding of the Club of Rome and the re-emergence of Malthusian ideology, time and again there have been warnings about the imminent running out of resources that are essential for modern life. The latest concern one of the formerly haunted wings of the Periodic Table, central to petrogenetic geochemistry, but little else; the rare-earth elements. From early beginnings as the source for phosphors in the screens of colour televisions all 15 REEs now have a growing commercial role in applications ranging from precision guided weapons, night-vision goggles and stealth technology in the military sphere, through the satiation of artificial appetites for electronic gaming and mobile phones, to applications of super-efficient magnets in medial scanners and ‘green’ power generation. The crisis being discussed currently is not so much a shortage – REEs are not so rare – but the cornering of their mining by the Peoples’ Republic of China, which produces more than 95%  of RREs used at present (~120 thousand tons). Yet world reserves are estimated at almost 100 million t, of which China has 36 million. Mining is often in only a few known, high-grade deposits; for instance most of the US reserves of 13 million t are locked in the Mountain Pass Mine, California that is currently on a ‘care-and-maintenance’ regime, i.e. shut. This one-sided economy sends shudders through capital’s strategy forums, i.e. in the US, Silicon Valley and the Pentagon.

Not surprisingly, geochemists and oceanographers from Japan, the world’s most hi-tech country, have bent their collective will to finding alternative sources, and may have revealed one in an unexpected location (Kato, Y. et al. 2011. Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements. Nature Geoscience, v. 4, p. 535-539).  Their work stems from ‘mining’ existing geochemical data from deep-sea drilling projects on the floor of the Pacific Ocean, that reveal a wide range of REE concentrations in the ooze coating the seabed: from <250 to >2000 parts per million. The richest pickings seem to lie in a swath either side of the East Pacific Rise at around 15°S, where the group estimate that a 1 km2 plot could yield about one fifth of current world annual production, even though REE concentrations lie way below their on-shore economic cut-off grade. Apart from the need for dredging at depths around 3-5 km on the abyssal plains, and the inevitability of destroying a largely unknown ecosystem, the positive aspect of these metal-rich oozes is that the REE can be extracted simply by acid leaching of the goethite (FeOOH) in which the bulk of the elements reside. Goethite is something of a geochemical ‘mop’ with a capacity for adsorbing elements of all kinds on grain surfaces; so much so that it is being considered as a means of cleaning up heavy-metal pollution. Both the REEs and the iron probably arise from seabed hot springs where oxidising conditions result in dissolved ferrous iron combining in ferric form with oxygen to form goethite, which in turn scavenges other dissolved ions. Many of the on-shore REE deposits are carbonatites (intrusions of carbonate-rich magmas) that contain fluoro-carbonates and phosphates that host the REE, or beach sands in which wave swash concentrates the durable heavy phosphates in so-called black-sand deposits. Carbonatites are rare, most occurring in ancient ‘shields’, as in southern Africa, Canada and China, but being so unusual are not difficult to find.  One in the Canadian Shield known as the Big Spruce Lake deposit provides phosphorus- and potassium-rich soil that encourages the growth f conifers and so creates a geobotanical anomaly of large trees where local climate generally supports only stunted ones.

The rising demand and currently restricted supply of REEs is creating an exploration boom for carbonatites as the metal prices rise inexorably. Yet it may also produce a shift to what seems to be an alternative kind of source; iron-rich deep-sea sediments, though more likely those preserved on-shore in ophiolite complexes than at the huge depths of the abyssal plains. It is worth bearing in mind, however, that oceanographers and geochemists have pointed to untold metal riches before: manganese nodules that litter huge tracts of the seabed and contain sufficient copper, nickel and cobalt to maintain supplies for millennia. Despite a half-billion dollar investment in the 1960s and 70s, there is no nodule-dredging industry. There are however well-advanced plans for deep water mining of gold-rich hydrothermal sites, but miners will go just about anywhere to gloat over Marx’s ‘money commodity’

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Dust tied to climate

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This TOMS image shows a record-setting Asian d...
Dust moving in April 2001 from arid areas in Central Asia and North Africa to the oceans. From NASA's Nimbus-7 satellite. Image via Wikipedia

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant foot chain. Oddly, even when commonly known nutrients are brought to the ocean surface far from land by deep-sourced upwellings the effect on near-surface biomass is far from that expected. The key factor that is missing is dissolved divalent iron that acts as a minor nutrient for phytoplankton: even in deep ocean waters any such ferrous iron is quickly oxidised and precipitated as trivalent ferric compounds. One of the suggested means of geoengineering away any future climatic warming is to seed the far-off oceans reaches with soluble iron in the hope of triggering massive planktonic blooms, dead organisms sinking to be buried along with the their carbon content in the ocean-floor oozes. Retrospectively, it has been suggested that the slight mismatch between changes in atmospheric CO2 concentration and climate changes may be linked to fluctuating availability of iron dissolved from dust in ocean-surface waters, but so far that hypothesis has not been robustly tested. It is well known, however, that global cooling is accompanied by drying of continental climates and thereby an increase in the delivery of dust, even to polar ice caps where cores have shown dustiness to fluctuate with temperature.

Recently an ocean-floor sediment core from around 42° S has revealed a high-resolution record of the deposition of dust and iron at that location over the last 4 Ma (Martinez-Garcia, A. et al. 2011. Southern Ocean dust-climate coupling over the past 4 million years. Nature, v. 476, p. 312-315). In it one proxy for dust is the amount of organic compounds known as n-alkanes that are a major component of the waxes shed from plant leaves. Others are iron, titanium and thorium concentrations in the ooze. Dust proxies tally with land-ice volumes shown by the fluctuating d18O measured in bottom-dwelling foraminifera found as fossils in the core to form a convincing link between dust and climate over the Southern Ocean. Those proxies also match nicely the record of dust delivered to Antarctica that emerged from the 0.8 Ma Dome C ice core that was extracted and analysed by the EPICA consortium. The record shows boosts in iron and dust deposition at 2.7 Ma, when ice first took hold of northern high latitudes, and at 1.25 Ma when larger ice sheets began to develop and climate shifts switched to 100 ka cyclicity. Although the match between marine and glacial dust accumulation in the latter part of this mid-Pleistocene Transition is an important step forward in palaeoclimatology, it is a surprise that the new ocean-floor data is not plotted with the record of atmospheric CO2 in Antarctic ice bubbles: if there was a clear relationship that would have iced the cake.

A big hit in the Moon’s evolution

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South pole Aitken basin highlighted on the moo...
The South Pole - Aitken basin (blue-magenta) and part of the high lunar far side (yellow-red) on an elevation map. Image via Wikipedia

The most significant discovery from the Apollo lunar landings is that the Earth and Moon shared a fiery early history, when a planetary body around the size of Mars slammed into the Earth to fling off vaporised rock that condensed to create the Moon. Such a catastrophic event reset the geochemistry of the Earth, and both it and the Moon likely had an early phase dominated by a deep ocean of magma. The evidence for a magma ocean comes mainly from the lunar highlands which are dominated by almost pure calcium plagioclase feldspar (the rock anorthosite), suggesting that this high-temperature, low-density silicate mineral crystallised and then floated to the surface of the Moon. Yet there is a great deal of evidence about the Moon that did not depend on people setting foot on its surface. For instance, detailed photographic records of the surface and extremely precise measurements of the surface elevation stem from cheaper orbital missions, including coverage of the unvisited far side of the Moon.

The face of the Moon never seen from Earth has long been known to have one of the largest impact basins in the solar system, the South Pole – Aitken basin. Analysis of the far side’s surface elevation data from the Lunar Orbiter Laser Altimeter (LOLA) also shows that it is significantly higher than the near side. It is also far more heavily cratered than the near side. Now there is a plausible explanation for the dichotomy: the Moon received another stupendous blow (Jutzi, M & Asphaug, E. 2011. Forming the lunar farside highlands by accretion of a companion moon. Nature, v. 476, p. 69-72). But how come that didn’t blast the Moon apart or re-melt it and allow it to re-shape to a near perfect sphere? The modelling study suggests that if the culprit slowly collided – around 2-3 km s-1 – it would have wrapped around the early Moon to plaster the surface with debris, nicely shown by the paper’s graphics.  Such a ‘slow’ impact is only possible from a co-orbital companion moon, objects from outside the Earth-Moon system inevitably being accelerated by gravity to at least the equivalent of its escape velocity (about 11-12 km s-1). That exceeds the speed of sound through rock, leading at least to a very large hole, shock metamorphism and, with a massive body, to extensive melting (the energy would be ½ mv2) rather than the observed lunar far-side bulge. Jutzi and Asphaugs’s modelling comes up with a companion moon around 1200 km across, that may have formed from the same massive event that created the Moon itself. It could have accreted from the impact-induced vapour disc at a Trojan point in the lunar orbit, where gravitational forces balance to keep orbital objects apart. The gradual expansion of the lunar orbit in response to tidal forces – large in the early history of the Earth-Moon system – could have destabilised the balance so that the companion moon slowly drifted towards the Moon and eventual collision.

One such modelling becomes closer to known reality, i.e. the far-side bulge, it gets more tempting to look for secondary possibilities. One of these the effect of such a ‘slow’ impact on the remaining magma ocean on the Moon. It may have blurted that by then deep molten layer to the side opposite the impact. That, the authors suggest, may be responsible for the geochemical peculiarities of the flood basalts that filled the much later lunar maria on the near side. There are no signs of these KREEP basalt floors to large later craters on the far side, such as the Aitken basin, formed around 4.0 to 3.8 Ga ago at the same time as the near-side maria. A variety of new instruments orbit the Moon and more are planned, so this model presents a nice hypothesis for them to test: what is the betting that a robotic lander might eventually be sent to return samples from the enigmatic far side?

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Homes for hominin evolution

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African savannah exhibit at the National Zoolo...
Typical African savannah. Image via Wikipedia

Friedrich Engels’s notion in The Part Played by Labour in the Transition from Ape to Man (1876), encouraged by Darwin’s The Descent of Man (1871), that the road to modern humans began with walking on two legs, thereby freeing the hands for work and tool making has been central to discussion of human evolution for more than a century. The ‘descent from the trees’ that bipedalism signifies has long been supposed to stem from the replacement of tropical forests in East Africa by open woodland or savannah, but evidence to support that environmental change has been difficult to glean from the fossil record  since the Late Miocene. Even in terrestrial sediments plant remains are rare, so that much has rested on animal fossils in relation to the habitats of their living descendants: opinion is divided.

There is a round-about means of resolving this central issue: using the carbon-isotope record in fossil soils that depends on the fractionation effects of broadly different kinds of plants that once grew in the soils and the signature of that fractionation in carbonate nodules that formed in the soils. The d13C value (crudely the difference between the 13C/12C ratio of a sample and that of a carbon-rich standard) found in C4 plants (many grasses) is -16 to -10 ‰ whereas that in C3 plants (including almost all trees) it is much more depleted in the heavier 13C isotope (-33 to -24‰). Exchange of carbon between living and dead organic matter, and carbonates that are precipitated from soil waters through the intermediary of gases in the soil should leave a d13C signature in the carbonates that reflects the overall proportions of different photosynthetic plant groups living at the time the soil formed. The approach was developed in the early 1990s by Thure Cerling and Jay Quade of the US universities of Utah and Arizona respectively.

After a long gestation period, involving calibration using soils from different modern ecosystems, the soil C-isotope method has been applied painstakingly to palaeosols in which African hominin remains have turned-up (Cerling, T.E. and 9 others 2011. Woody cover and hominin environments in the past 6 million years. Nature, v. 476, p. 51-56). All the famous hominin sites from the Awash and Omo Valleys of Ethiopia and around Lake Turkana in Kenya, figure in this important study, in which the authors devise a proxy for ‘palaeo-shade’ based on their carbonate d13C data from 76 modern tropical soils: a good ‘straight-line’ plot of d13C against the fraction of woody cover at the different calibration sites. Applying the proxy to their 1300 samples of palaeosols they show convincingly that since about 6 Ma tree cover rarely rose above 40% in the homelands of all the East African hominins. From the times of Ardepithecus ramidus (~4 Ma) at Aramis in Ethiopia, through those of ‘Selam’ and ‘Lucy’, the 2.5 Ma first stone tools at Gona, the times when Africa was dominated by Homo erectus(1.8 to 1 Ma) to perhaps the first signs of modern human cranial remains (those with chins!) around 1 Ma, all hominins strode through open, grassy environments. One can imagine pleasured nods from the shades of Darwin and Engels now their prescience has finally been confirmed.

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Plate tectonics monitored by diamonds

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eclogite
Norwegian Eclogite. Image by kevinzim via Flickr

For more than 30 years a debate has raged about the antiquity of plate tectonics: some claim it has always operated since the Earth first acquired a rigid carapace not long after a molten state following formation of the Moon; others look to the earliest occurrences of island-arc volcanism, oceanic crust thrust onto continents as ophiolite complexes, and to high-pressure, low-temperature metamorphic rocks. The earliest evidence of this kind has been cited from as far apart in time as the oldest Archaean rocks of Greenland (3.9 Ga) and the Neoproterozoic (1 Ga to 542 Ma). A key feature produced by plate interactions that can be preserved are high-P, low-T rocks formed where old, cool oceanic lithosphere is pulled by its own increasing density into the mantle at subduction zones to form eclogites and blueschists. In the accessible crust, both rock types are unstable as well as rare and can be retrogressed to different metamorphic mineral assemblages by high-temperature events at lower pressures than those at which they formed. Relics dating back to the earliest subduction may be in the mantle, but that seems inaccessible. Yet, from time to time explosive magmatism from very deep sources brings mantle-depth materials to the surface in kimberlite pipes that are most commonly found in stabilised blocks of ancient continental crust or cratons. Again there is the problem of mineral stability when solids enter different physical conditions, but there is one mineral that preserves characteristics of its deep origins – diamond. Steven Shirer and Stephen Richardson of the Carnegie Institution of Washington and the University of Cape Town have shed light on early subduction by exploiting the relative ease of dating diamonds and their capacity for preserving other minerals captured within them (Shirey, S.B. & Richardson, S.H. 2011. Start of the Wilson cycle at 3 Ga shown by diamonds from the subcontinental mantle. Science, v. 333, p. 434-436). Their study used data from over four thousand silicate inclusions in previously dated large diamonds, made almost worthless as gemstones by their contaminants. It is these inclusions that are amenable to dating, principally by the Sm-Nd method. Adrift in the mantle high temperature would result in daughter isotopes diffusing from the minerals. Once locked within diamond that isotopic loss would be stopped by the strength of the diamond structure, so building up with time to yield an age of entrapment when sampled.  The collection spans five cratons in Australia, Africa, Asia and North America, and has an age spectrum from 1.0 to 3.5 Ga. Note that diamonds are not formed by subduction but grow as a result of reduction of carbonates or oxidation of methane in the mantle at depths between 125 to 175 km. In growing they may envelop fragments of their surroundings that formed by other processes.

A notable feature of the inclusions is that before 3.2 Ga only mantle peridotites (olivine and pyroxene) are trapped, whereas in diamonds younger than 3.0 Ga the inclusions are dominated by eclogite minerals (garnet and Na-, Al-rich omphacite pyroxenes). This dichotomy is paralleled by the rhenium and osmium isotope composition of sulfide mineral inclusions. To the authors these consistent features point to an absence of steep-angled subduction, characteristic of modern plate tectonics, from the Earth system before 3 Ga. But does that rule out plate tectonics in earlier times and cast doubt on structural and other evidence for it? Not entirely, because consumption of spreading oceanic lithosphere by the mantle can take place if basaltic rock is not converted to eclogite by high-P, low-T metamorphism when the consumed lithosphere is warmer than it generally is nowadays – this happens beneath a large stretch of the Central Andes where subduction is at a shallow angle. What Shirey and Richardson have conveyed is a sense that the dominant force of modern plate tectonics – slab-pull that is driven by increased density of eclogitised basalt – did not operate in the first 1.5 Ga of Earth history. Eclogite can also form, under the right physical conditions, when chunks of basaltic material (perhaps underplated magmatically to the base of continents) founder and fall into the mantle. The absence of eclogite inclusions seems also to rule out such delamination from the early Earth system. So whatever tectonic activity and mantle convection did take place upon and within the pre-3 Ga Earth it was probably simpler than modern geodynamics. The other matter is that the shift to dominant eclogite inclusions appears quite abrupt from the data, perhaps suggesting major upheavals around 3 Ga. The Archaean cratons do provide some evidence for a major transformation in the rate of growth of continental crust around 3 Ga; about 30-40 percent of modern continental material was generated in the following 500 Ma to reach a total of 60% of the current amount, the remaining 40% taking 2.5 Ga to form through modern plate tectonics

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Pristine mantle and basalt floods

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The Western Ghat hills at Matheran in Maharash...
Flood basalts of the Deccan Traps in Maharashtra State, India. Image via Wikipedia

Plot the ages of major extinctions against those of flood basalt events and you will get a straight line graph for six co-occurrences since 250 Ma, with very little error. Although the exact mechanism for mass death of species and families is argued over interminably, for those six, flood basalt events have to be deeply implicated. There again, every geologist and their aunties dispute the mechanisms behind monster basalt effusions that bury whole landscapes beneath flow after flow and create very distinctive landforms.  When they are eroded they form regularly stepped mountain sides, hence their formerly popular name trap basalts, after the Swedish word trappa meaning staircase.  There is a hint of cyclicity in their age distribution. But most important of all, no-one has witnessed these vast, pulsating events, the last having mantled the surroundings of the Columbia and Snake River catchments in the US states of Oregon and Washington between 14-17 Ma ago in the Middle Miocene. Some mark episodes of continental break-up, such as those flanking the Central Atlantic at the time of the end-Triassic (~200 Ma) mass extinction, while others are associated with hot spots, such as the Deccan Traps of western India erupted between 60-68 Ma as India drifted over the Reunion hot-spot and those of the Ethiopian highlands (30 Ma) associated with the Afar hot spot.

A common geochemical feature is beginning to emerge concerning the mantle from which the basalts were partially melted. Six sets of flood basalts exhibit the same trace-element and isotopic (Nd, Pb, Hf and He) characteristics, which suggest that their source had been little effected by previous extraction of crust-forming magmas; it is primitive and may be a relic of the original mantle formed at about 4500 Ma shortly after the catastrophic collision between the early Earth and a wandering Mars-sized planet that flung off the Moon (Jackson, M.G. & Carlson, R.W. 2011. An ancient recipe for flood basalt genesis. Nature, online (27 July 2011) doi:10.1038/nature10326). Although undepleted, the chemistry of the mantle source, worked out by back-calculation from that of the flood basalts, is not the same as the once-postulated original accretion of carbonaceous chondrite meteorites: conceivably a result of the chemical reworking when the Moon formed and the remaining Earth was probably molten from top to centre. The important feature is that the recast chemistry is rich in heat-producing elements compared with the source of ‘common-or-garden’ basalts that continually contribute to the ocean floors and island arcs. Wherever the relic mantle is, it is capable of heating itself, over and above the heating from the core and surrounding mantle, and thus likely to generate thermal and material plumes rising through the mantle.

Preceding the work of Jackson and Carlson, another group discovered that when flood basalt events since the Carboniferous are restored to their former geographic positions at the time they were erupted, they cluster above what are now two patches of more ductile mantle close to the cure-mantle boundary (Torsvik, T.H. et al. 2010. Diamonds sampled by plumes from the core–mantle boundary. Nature, v. 466, p. 352–355). If that is the source of basalt flood-forming plumes, then it is still there and, aside from giant impacts with extra-terrestrial projectiles, the most catastrophic upheavals of the Earth system inevitably will continue, perhaps in the next few million years.

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Feathers will fly: Archaeopteryx relegated

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Archaeopteryx
A not unimaginative reconstruction of Archaeopteryx. Image via Wikipedia

This year, 2011, is the 150th anniversary of the first Archaeopteryx specimen being unearthed from the famous Solnhofen  limestone lagerstätte. With its feathered, lizard-like tail; two-clawed, stubby wings; a bill-shaped muzzle with teeth but no keratin coating; feet capable of perching and unlike those of small dinosaurs; a ‘wishbone’ and lightweight bones, Archaeopteryx was just the half-and-half missing link in the fossil record so desperately needed to support Darwin’s Origin of Species, published two years beforehand.  It has remained controversial ever since, even having been claimed to be a forgery by such luminaries as cosmologist Fred Hoyle in 1985, despite its superbly preserved intricacies and the existence at the time of 6 slightly different specimens from the same source some discovered long after Hoyle’s supposed master craftsman must have died. Creationists soon after the first discovery claimed it was simply a bird created on a Friday together with fish (Genesis 1:20) and must have predated dinosaurs by a day, as they were created on the 6th Day along with all the ‘cattle and creeping thing and beast of the earth’ (Genesis 1:24-31). That scurrilous sect will certainly leap gleefully on the new discovery of a feathered dinosaur from the ever productive Late Jurassic Tiaojishan Formation in NE China (Xu, X. et al.2011. An Archaeopteryx-like theropod from China and the origin of the Avialae. Nature, v. 475, p. 465-470) because ironically, by itself, it could be said to be a missing link too.

Archaeopteryx lithographica, specimen displaye...
Cast of the first-described Archaeopteryx fossil. Image via Wikipedia

In fact, Xiaotingia zhengi possesses features very like those displayed by Archaeopteryx but convincingly close affinities to deinonychosaurian dinosaurs. The shared features show that neither is a bird (Avialae) and nor are they part of the clade that evolved to birds: they are part of the growing group of feathered dinosaurs that may well have glided or even flown. As Lawrence Witmer of Ohio University has observed (Witmer, L.M. 201. An icon knocked off its perch. Nature, v. 475, p. 458-459), ‘This finding is likely to be met with considerable controversy (if not outright horror)…’. However, Witmer still considers Archaeopteryx to have iconic status, indeed yet more, for its taxonomy and that of its related feathery dinosaurs provides compelling evidence that the origin and evolution of life was a ‘rather messy affair’. Undoubtedly, more feathered creatures hundreds of million years old will be unearthed; it is even possible that further finds will push the beast of Solnhofen back onto its avian perch. Let the celebrations begin!

Added 12 August 2011: Ironically, yesterday the German mint issued a €10 silver coin commemorating the 150th anniversary of the first discovery of Archaeopteryx, artwork of the skeleton with fully fledged arms on the reverse side of the coin compared with the stylised German eagle on the front. This event coincides with the greatest crisis facing the eurozone in its short history, though Germany still retains its ‘triple A’ financial status unlike France and the US. See: http://witmerlab.wordpress.com/2011/01/31/evolution-icon-archaeopteryx-turns-150-this-year-how-are-we-celebrating/

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A plume drive for tectonics?

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Himalaya Formation Source www.usgs.org US Gove...
India's tectonic travels. Image via Wikipedia

The theory of plate tectonics resolved Alfred Wegener’s search for a driving force for continental drift around half a century after his discovery faced near-universal rejection for not having one that was large enough or plausible. Plate theory recognises many forces, both driving and in opposition to tectonic movement. By far the largest is the gravitational pull exerted by subducting slabs of dense oceanic lithosphere, followed in distant second place by ridge-push, another gravity-driven force that arises from the slope on the ocean floors away from sea-floor spreading centres as the oceanic lithosphere cools and shrinks as it ages. Until very recently, no place was assigned in the theory to forces associated with the apparently non-tectonic plumes that rise through the mantle from well beneath the lithosphere from which plates are made, quite possibly because it seems logical to expect a vertically upwards force, if any, from hot plumes whereas plate tectonics is mainly concerned with horizontal movements. Looking around the present state of sea-floor spreading, the maximum pace at which plates move is just over 100 mm a-1 (100 km Ma-1) in the case of the Pacific Plate. Yet, during the Late Cretaceous and Early Palaeogene Periods after India had been wrenched away from the Gondwana supercontinent to move towards eventual collision with Eurasia the subcontinent experienced an extraordinary episode beginning around 68 Ma when its pace increased to as high as 180 km Ma-1. This accelerated motion continued over some 15 Ma and then equally abruptly slowed to less than 40 km Ma-1 around the start of the Eocene (Cande, S.C. & Stegman, D.R. 2011. Indian and African plate motions driven by the push force of the Réunion plume head. Nature, v. 475, p. 47-52; see also: Müller, R.D. 2011. Plate motion and mantle plumes. . Nature, v. 475, p. 40-41). The acceleration coincided with the start of continental flood-basalt volcanism that blanketed much of western India with the Deccan Traps across the K-P boundary when the subcontinent lay over the site of the Réunion hot spot. Coincidentally, the Réunion plume head formed at that time; i.e. the Indian continental lithosphere did not drift over an active plume, but was hit from below by one that happened to be rising to the surface. Curiously, while the Indian plate was accelerated, nearby Africa was slowed, explained by a push in the same direction of India’s travel towards a subduction zone beneath Asia and one applied to Africa that opposed its motion. Africa too resumed its usual tectonic progress at the start of the Eocene. But how did a mantle plume exert such a force: was it because it caused a local bulge from which the plates slid, or did mantle motion associated with the mushroom-like structure of the horizontally growing plume head exert viscous drag on the overlying plates? Such shifts in motion of major plates inevitably have an effect on the whole plate tectonic carapace, and the authors list a number of contemporary, distant consequences, speculating that the famous bend in the Hawaii-Emperor island and sea-mount chain in the Early Eocene resulted from the final waning of the Réunion plume head’s influence and major readjustment of tectonics.

Himalayan Horizon From Space
The result of India's final collision with Eurasia - the Himalaya. Image via Wikipedia
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From small beginnings

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Camarasaurus, Brachiosaurus, Giraffatitan, Euh...
Some really cool sauropods. Image via Wikipedia

The great vegetarian sauropod dinosaurs, such as Brachiosaurus, were the biggest animals to walk the Earth, weighing up to 100 tonnes, as long as 60 m from snout to the end of their tails and more than 10 m tall. So big, indeed, that even the largest contemporary predators would have been unable to get sufficient purchase with their jaws to do them much damage. This vast bulk, unlike even bigger modern whales, was unsupported by water and would have posed major problems had the sauropods not evolved very porous, low-density neck and tail bones and kept their heads small relative to the rest of their bodies. Such small heads needed to take in up to a tonne of vegetation each day to keep the monsters alive and  ambling. Their teeth are not those of a chewer, being peg- or spoon-like and pointed forwards; specialised for raking in leaves and twigs, swallowed unchewed in great gulps. Once that style of eating developed in their precursors, with no need for massive chewing muscles it became possible to evolve necks up to 15 m long with increasingly diminutive heads. Studies of large numbers of some species of sauropod precursors indicate that juveniles grew astonishingly quickly, essential if their initial vulnerability was to be outpaced; newly hatched they would have weighed little more than 10 kg. At the growth rates of modern reptiles, the largest sauropods would only have reached full size in about a century. The estimated growth rates suggest warm bloodedness, research suggesting that they maintained body temperatures up to 12°C higher than do alligators. Clearly, sauropod dinosaurs were highly specialised, and their evolution is now known to have been lengthy.

A major news feature in Nature (Heeren, F. 201. Rise of the titans. Nature, v. 475, p. 159-161) traces that evolution through several surprising stages. The earliest likely ancestors, which appear in the Late Triassic (~230 Ma), were about the size of a turkey and had teeth adapted for shredding fibrous plant material; other early dinosaurs show clear signs of a predatory lifestyle. There is a limit to the size of predators bound up with the energy balance between flesh consumption and the energy expended in casing down prey and killing them. The limits on the size of plant eaters are mechanical: how much they can stuff in and the strength of their bodies, especially legs. In a world dominated in numbers by predatory dinosaurs, the selection pressure for herbivores to outgrow them and become too big to bite would have been substantial.

Little Triassic Panphagia (‘eater of everything’) was also bipedal, but the fossil record of sauropod precursors clearly shows their growth to the order of 10 m by the Early Jurassic, but not yet a four-legged gait though they had evolved relatively short but sturdy legs, signs of mass-saving porous neck and tail bones, and jaws with a large gape suited to gulping rather than chewing. By the mid-Jurassic Period sauropods were big, strong and four-legged, and by the Cretaceous they reached unmatched dimensions with the titanosaurs. This evolutionary path was not the only one adopted for dinosaurian herbivory. The famous Iguanodon discovered in 1822 by Gideon Mantell in the Early Cretaceous of Sussex was a member of a bipedal group of herbivores, including the duck-billed dinosaurs, that spanned more or less the same time range as sauropods. Fredric Heeren’s article is accompanied by an on-line ‘tour’ of sauropod evolution (go.nature.com/c7zlct), while the American Museum of Natural History has a website for a major exhibition of sauropods (www.amnh.org/exhibitions/wld/ and http://www.youtube.com/AMNHorg ) that includes footage of  a full-scale animatronic Mamenchisaurus from China which breathes and moves, (Switek, B. 2011. Living it large: review of The World’s Largest Dinosaurs exhibition. Nature, v. 475, p. 172).

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Core’s comfort blanket and stable magnetic fields

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Pangea animation
Pangaea and its break-up. Image via Wikipedia

The record of the Earth’s magnetic field for the most part bears more than a passing resemblance to a bar-code mark, by convention black representing normal polarity, i.e. like that at the present, and white signifies reversed polarity. The bar-code resemblance stems from long periods when the geomagnetic poles flipped on a regular, short-term basis, by geological standards. The black and white divisions subdivide time as represented by geomagnetic into chrons of the order of a million-years and subchrons that are somewhat shorter intervals. Stemming from changes in the Earth’s core, magnetostratigraphic divisions potentially occur in any sequence of sedimentary or volcanic igneous rocks anywhere on the planet and so can be used as reliable time markers; that is, if they can be defined by measurements of the remanent magnetism preserved in rock, which is not universally achievable. Yet this method of chronometry is extremely useful, for most of the Phanerozoic. However, there were periods when the geomagnetic field became unusually stable for tens of million years so the method is not so good. These have become known as superchrons, of which three occur during Phanerozoic times: the Cretaceous Normal Superchron when the field remained as it is nowadays from 120 to 83 Ma; a 50 Ma long period of stable reversed polarity (Kiaman Reverse Superchron) from 312 to 262 Ma in the Late Carboniferous and Early Permian; the Ordovician Moyero Reverse Superchron from 485 to 463 Ma.

Because the geomagnetic field is almost certainly generated by a self-exciting dynamo in the convecting  liquid metallic outer core, polarity flips mark sudden changes in how heat is transferred through the outer core to pass into the lower mantle. It follows that if there are no magnetic reversals then the outer core continued in a stable form of convection; the likely condition during superchrons. But why the shifts from repeated instability to long periods of quiescence? That is one of geoscience’s ‘hard’ questions, since no-one really knows how the core works at any one time, let alone over hundreds of million years. There is however a crude correlation with events much closer to the surface. The Kiaman superchron spans a time when Alfred Wegener’s supercontinent Pangaea had finished assembling so that all continental material was in one vast chunk. The Cretaceous superchron was at a time when sea-floor spreading and the break-up of Pangaea reached a maximum. The Ordovician, Moyero superchron coincides with the unification of what are now the northern continents into Laurasia and the continued existence of the southern continents lumped in Gondwana, so that the Earth had two supercontinents. Those empirical observations may have been due to chance, but at least they provide a possible clue to linkage between lithosphere and core, despite their separation by 2800 km of convecting mantle that transfers the core heat as well as that produced by the mantle itself to dissipate at the surface. Enter the modellers.

How part of the Earth transfers heat is, not unexpectedly, very complex, depending not only on what is happening at that point but on heat-transfer processes and heat inputs both above and below it. The surface heat flow is complex in its own right ranging from less than 20 to as much as 350 mW m-2, the largest amount being through zones of sea-floor spreading and the least  through continental lithosphere. Wherever heat is released in the core and mantle, willy-nilly the bulk of it leaves the solid Earth along what is today a complex series of lines; active oceanic ridge and rift systems such as the mid-Atlantic Ridge.  These lines weave between six drifting continental masses and many more sites of additional heat loss – hot spots and mantle plumes. The many heat escape routes today complicate the deeper convective processes and there are many possibilities for the core to shed heat, yet they continually change pace and position. When, inevitably, all continental lithosphere unites in a supercontinent, almost by definition, the sites of heat loss simplify too, the supercontinent acting like an efficient insulating blanket. In a qualitative sense, this kind of evolving scenario is what modellers try to mimic by putting in reasonable parameters for all the dynamic aspects involved.  Two physicists at the University of Colorado in Boulder, USA, Nan Zhang and Shije Zhong, have formulated 3-D spherical models of mantle convection with plate tectonics as a basis for whole Earth thermal evolution over that last 350 Ma (Zhang, N & Zhong, S. 2011.  Heat fluxes at the Earth’s surface and core–mantle boundary since Pangea formation. Earth and Planetary Science Letters, v. 306, p. 205-216). The acid test is whether the model can end with a close approximation to modern variations in heat flow and distribution of ages on the sea floor; it does. A probable key to stability in the means of transfer of heat from core to lower mantle – itself a key to a constant outer-core dynamo and geomagnetic polarity – is reduced heat flow at equatorial latitudes; a sort of equatorial downflow of convection with upflows in both northern and southern hemispheres. Zhang and Zhong’s model produced minimal core-to-mantle heat flow at  the Equator at 270 and 100 Ma, both within geomagnetic-field superchrons. Well, that is a good start. Superchrons seem also to have occurred from time to time during the Precambrian, one being documented at the Mesoproterozoic-Neoperoterozoic boundary about 1000 Ma ago. At that time, all continental lithosphere was assembled in a supercontinent dubbed Rodinia (‘homeland’ or ‘birthplace’ in Russian).