Post-perovskite unveiled

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Kei Hirose, the discoverer in 2002 of a ultra high-pressure transformation of mantle mineralogy, has produced a highly readable review of the implications of his work for how the mantle functions (Hirose, K. 2010. The Earth’s missing ingredient. Scientific American, v. 302 (June 2010 issue), p. 58-65).

Seismology has long charted the occurrence of step-changes in mantle properties at a several more or less constant depths. Mantle above 410 km provide most of the samples available to geoscientists as inclusions in basalt lavas and is olivine-rich peridotite. From 410-660 km the elements forming olivine take on a different configuration more akin to the mineral spinel; also backed by some direct as well as theoretical/experimental evidence. At 660 km deep seismic properties change dramatically in a major transition zone. Experimental work in the 1970s with mantle chemical compositions at high pressures and temperatures showed that at greater depths the structure of magnesium silicates like olivine, pyroxene and spinel collapses to a denser form with very efficient packing of aoms that is the same as that of a broad group of minerals known as perovskites. That seemed to be the end of the matter. However, continued geophysical investigations and geochemical studies of basalts derived by partial melting of mantle rock teased out complexities in the once assumed simplicity of the mantle. In 1983 analysis of seismic records revealed a further step in physical properties of the deepest mantle (once designated the D layer) that forced a revision to recognise a transition at 2600 km deep, just 300 km above the core-mantle boundary. This now separates the 2000 km thick D’ layer from the lowest D” layer in the mantle. Subsequently, chemical heterogeneities in the deep mantle became a major puzzle.

Hirose and his team pushed experimental conditions to match the huge pressures below 2600 km and discovered a yet more efficient, hitherto unknown molecular configuration that arranges magnesium, silicon and oxygen into separate layers: dubbed ‘post-perovskite’ for want of a already known mineral structure. As well as a small (1.5%) increase in density, the mineralogical change unexpectedly releases rather than consumes heat energy. Such an exothermic process clearly had great implications for how the mantle works. If rock from higher levels finds its way down to and below the D’-D” transition, as might happen if subducted oceanic lithosphere slabs continue ever downwards, it gets an energy ‘kick’. Theoretical work revealed that the early Earth would have been too hot for post-perovskite to form. But once it had cooled below a threshold the phase change ‘snapped’ into existence: that must have significantly changed mantle dynamics. Convective motion in D” that brings material to the D’:D” boundary the post-perovskite to perovskite phase change produces a sharp decrease in density and an upward force. So, once D” formed plume formation and overall mantle convection would have increased. That impetus could not have been present before so that early Earth mantle dynamics were more sluggish. That would maintain a hotter core-mantle boundary, thereby slowing cooling of the liquid core and formation of the solid inner core. Moreover, the upper mantle would have been cooler than now, creating the paradox of less surface magmatism on the early Earth. Theoretically, development of D” should have been marked by a 20% increase in heat flow and a paroxysm of tectonics and crust formation. Was that linked with the formation of stable continental crust around 4 Ga, the spurt in continental growth in the late Archaean or some later event (Hirose suggests 2.3 Ga, but no major tectonic shift has that age)?

As well as tectonic implications, the affect of the D” layer on the pace of crystallisation of the solid inner core may have controlled increasing strength and stability of the geomagnetic field. Because only Earth’s strong magnetic field protects the surface from life-threatening cosmic rays and the solar wind, in a roundabout way post-perovskite possibly played a role in allowing the origin, evolution and survival of life on our home world. That possibility is pretty much the ultimate link between solid Earth and the biosphere: take note Gaians!
See also: Buffet, B.A. 2010. The enigmatic inner core. Science, v. 328, p. 982-983.

Why a glacial period ends

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The publicity and debate that sprang up in the 9 months after release of e-mails stolen (17 November 2009) from the British University of East Anglia’s Climatic Research Unit, and several debacles regarding pronouncements by the Intergovernmental Panel on Climate Change have in fact cleared the air on several purely scientific matters. , Contrary to what had become the broad public conception, thanks to massive and continuous propaganda about global warming that barely mentions anything else, greenhouse gas emissions are widely revealed to be not the ‘only game in town’ when it comes to past changes in climate. That is very much the lesson learned by decades of study of the greatest climate change that fully modern humans have experienced: the last glacial termination when the deepest frigidity about 20 ka ago gave way to very rapid warming. A review of that enormous world event carries important lessons about what really controls climate on our world and how complex that is (Denton, G.H. et al. 2010. The last glacial termination. Science, v. 328, p. 1652-1656).

Since the 1970s proxy data from deep-sea sediments that reveal the variation in the volume of glacial ice on land have showed how climate changes over the last 2.5 Ma are broadly correlated with the periods of astronomical effects on the amount of solar energy received by Earth or insolation, particularly that at high northern latitudes. This might suggest that glacial terminations occur when insolation reaches maxima. In fact over the last 800 ka terminations have also occurred at times of low insolation. The Milankovich signal is ubiquitous but it is not the primary driving factor for the end of glacial episodes. Nor do they tally exactly with increased CO2 in the atmosphere, as recorded in air bubble trapped in polar ice. In fact there is a lag between the record for greenhouse gases and those for warming and cooling. The clearest correlation is between terminations and the maximum volume of land ice in each glacial epoch, towards which Denton et al. direct most attention. Since Antarctic ice has barely changed volume since the Pliocene, pulsation in land-ice volume must stem mostly from Northern Hemisphere glaciation and deglaciation. That repeatedly occurred around the North Atlantic where the main sites for ocean-water downwelling occur. At their thickest the North American and European ice sheets also had their greatest isostatic effects, bowing down the crust, and increasing ice flow towards the ocean. Time after time in each glacial build-up such a configuration became unstable so that marginal ice collapsed to produce the iceberg ‘armadas’ known as Heinrich events. Freshening of the North Atlantic by iceberg melting shut down the downwelling, thereby thermally isolating high northern latitudes to give Dansgaard-Oeschger events comprising paired coolings, or stadials, followed by suddenly warming interstadials once deep circulation restarted.

What is also emerging is that, to maintain heat balance, as each stadial developed in the North Atlantic more heat was shifted to the Southern Hemisphere. Increased downwelling of cold saline water of the Southern Ocean drove this warming to higher southern latitudes. The net observed effect is a southern reversal of sea-surface and polar air temperatures compared with those of the Northern Hemisphere, especially clear in the late stages of the last termination, including the Younger Dryas. Each warming of the south encouraged the southern oceans to emit stored CO2 to the atmosphere, until finally sufficient to maintain global warm conditions when the arose during terminations.

Flatulence and the Younger Dryas
There is a widespread belief that the enlargement of domesticated ruminant herds, mainly cattle, goats and sheep, may have had some effect on recent climate: their enteric fermentation of grass cellulose generates methane, a powerful greenhouse gas. Livestock produce an estimated 80 million metric tons of methane annually, accounting for about 28% of anthropogenic methane emissions. Livestock aren’t the only methane emitting ruminants: giraffe; bison; yaks; water buffalo; deer; camels (including llamas and alpacas); and antelope. Elephants are not so efficient, but they do break wind a great deal. An adult elephant emits about half a ton of methane annually; enough to run a car 20 miles per day; on the school run for instance.

Livestock have become the dominant herbivores on the planet, but far more wild ruminants roamed the Earth during the last glacial epoch because of the much greater expanses of grasslands during cooler, more arid conditions. This was especially the case in North America, a much diminished impression being given by the vast herds of bison that were almost exterminated in the 19th century and those of caribou that still migrate across Alaska and northern Canada. The estimated ruminant population of late-Pleistocene prairies was so large that it too has been implicated in climate change during the last glacial termination (Smith, F.A. et al. 2010. Methane emissions from extinct megafauna. Nature Geoscience, v. 3, p. 374-375), with estimated annual emissions around 10 million tons. With atmospheric methane concentrations having reached around 650 parts per billion by volume (ppbv) by 15 ka – a third of those today – the farting animals of the prairies may have made a significant contribution to post-glacial global warming. Sometime around 13 ka immigrant humans from Asia entered the scene, armed with efficient hunting weapons. By 11.5 ka, the vast herds had more or less vanished through extinction, and the 10 megaton methane emission went with them. Felisa Smith and her colleagues from the University of New Mexico, Los Alamos National National Laboratory and the Smithsonian Institution, USA, note that over the same period atmospheric methane content fell from 650 to <500 ppbv. They speculate that part of this decline may have resulted from the extinction of the North American ‘megafauna’ and contributed to the Younger Dryas cooling between 12.8 to 11.5 ka. If that were the case, it would have been the earliest instance of a human effect on the Earth and, opine the authors, ought to be used to mark the start of what some geoscientists propose as a new geological Period: the ‘Anthropocene’. This parochial view surely ranks alongside that of a shower of nano-diamonds from an extraterrestrial explosion as the cause of the Younger Dryas, to the posthumous annoyance of William Seach of Occam.

Doubt cast on erosion and weathering theory of climate change
A seminal paper in the late 1980’s by Maureen Raymo, Flip Froelich and Bill Ruddiman proposed that the uplift of mountain ranges, their erosion and associated chemical weathering helped gradually shift global climate. Their main reasoning was that rotting of feldspars by carbonic acid formed when CO2 dissolves in rainwater locked the greenhouse gas in soil carbonates and supplied bicarbonate ions to sea water, where they would recombine with calcium and magnesium ions also released by weathering to form limestones. This process would draw down greenhouse gas levels in the atmosphere faster during episodes of major mountain building. Such carbonate burial has since been assumed to have helped the Earth’s climate cool during the Cenozoic era, after the Alps, Andes and especially the Himalaya began to form. There have been many publications about the processes involved and the geochemical signature of varying erosion, such as changes in the strontium isotope composition of limestones as a proxy for that of sea water. But the real test for whether or not there have been pulses in erosion controlled by orogeny would involve measuring changes over time in sediment deposition in all the world’s sedimentary basins. In a recent paper (Willenbring, J.K. & von Blanckenburg, F. 2010. Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature, v. 465, p. 211-214) published estimates of continent derived sedimentation plotted against atmospheric CO2 derived from various proxies show two features. First, there hasn’t been a truly significant decrease in CO2 since the end of the Oligocene (23 Ma). Secondly, although sedimentation over every 5 Ma rose from about 6 x 1015 to 1016 t between the end of the Oligocene and the start of the Pliocene. Repeated glaciation over the last 5 Ma helped increase global sedimentation to 3 x 1016 t, but even that tripling seems not to have had much effect on atmospheric CO2.

Willenbring and von Blanckenburg have attempted to improve the very uncertain evolution of the sedimentary record based on basin stratigraphy – despite seismic sections in many basins, costly and still rare 3-D cross sections are the only means of working out actual masses of sediment deposited through time. The authors re-examined the record of beryllium isotopes in sediments and manganese crusts from the deep-ocean floor, as a proxy for rates of weathering of continental debris. The principle behind this is the continuous production of radioactive 10Be in the atmosphere by cosmic rays, and its entry into the oceans. There it mixes with stable 9Be released to solution by weathering of rocks. Allowing for the decay of 10Be and assuming constant rates at which it is produced, the 10Be/9Be ratio in ocean water and sediments in contact with it is a proxy for global weathering. A decrease in the ratio implies an increase in continental weathering, while decreases signify periods of slowing rock breakdown. Over the last 10 Ma, the ratio has stayed more or less constant in the Pacific and Atlantic Oceans. The obvious conclusion is that the last 10 Ma showed no pulse in weathering and that period did not follow the Raymo-Froelich-Ruddiman model. There are several explanations for the ‘flat-lining’ Be isotopes (Goddéris, Y. 2010. Mountains without erosion. Nature, v. 465, p. 169-171), but a rethink of the significance of any link between orogeny and climate is clearly on the cards.

On the same topic, the start of Northern Hemisphere glaciations and its 30-40 Ma lead-in, Bill Ruddiman of the University of Virginia reviews a broader range of evidence (Ruddiman, W.F. 2010. A paleoclimatic enigma. Science, v. 328, p. 838-839) but not that presented by Willenbring and von Blanckenburg. He concludes that little has changed by way of explanation since the late 1990s, and decreased CO2¬ was the primary forcing factor. Yet his own plot of atmospheric CO2 estimated from marine-sediment alkenones (organic compounds produced by some phytoplankton) shows little fluctuation in the mean concentration since 20 Ma, which is around that for the Pliocene-Pleistocene Great Ice Age.

Arsenic update

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Partly because of natural processes and partly due to a shift to avoid pathogens in surface water used for domestic to a massive well-drilling programme much of rural Bangladesh and neighbouring West Bengal in India found itself the epicentre of ‘the largest mass poisoning of a population in history’, during the 1990s. The agent was soluble arsenic in various forms that reducing conditions in shallow aquifers had released by dissolving its host mineral, iron hydroxide coatings on sand grains. Geological and hydrological attributes of the two hard-hit areas helped develop a model for assessing the risks in other areas. More than a decade on from the world-wide recognition of the tragedy (local geoscientists had their suspicions much earlier) a review of arsenic hazard in both South and Southeast Asia (Fendorf, S. et al. 2010. Spatial and temporal variations of groundwater arsenic in south and south-east Asia. Science, v. 328, p. 1123-1127) is welcome but is not reassuring. The problem now extends to plains of the whole of the Ganges-Brahmaputra-Meghna system, the Red River of Vietnam and the Mekong of Vietnam, Cambodia, Laos and part of Thailand. Almost certainly the Indus and Irrawaddy plains are affected too, though few data are available. The review highlights a haphazard aspect of the distribution of affected wells, both in geographic location and the depth of the tapped aquifer. In the latter case, it was thought that deeper aquifers were less prone to contamination than those in the top 100 m of wells. It turns out that even at depth up to a third of wells exceed WHO recommended levels of arsenic. The positive feature is that many villagers are within walking distance of safe well water. But it is difficult to predict whether or not new wells will be risky, and little is know about safe well’s propensity to become contaminated by groundwater flow from elsewhere. Two clear messages are, first to refine methods of testing and assessing hydrogeological conditions, second to move from hand drawn water from individual wells to provision of piper water from high-yielding safe wells.

More wet minerals on Mars

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A remote-sensing geologist who focuses on terrestrial matters would likely grind their teeth on seeing papers that use far better data captured from the Martian or lunar surface than are ever likely to be available from the bulk of Earth’s land surface over the next decade at least. Mine are even closer to the gums after reading about hyperspectral data from Mars with high spatial resolution (~20 m), used to locate rocks altered by water on Mars (Carter, J. et al. 2010. Detection of hydrated silicates in crustal outcrops in the northern plains of Mars. Science, v. 328, p. 11682-1686). And, of course, there is no vegetation and not much of an atmosphere to cryptify spectral features of minerals: if there is enough of a mineral exposed to show up, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) carried by NASA’s Mars Reconnaissance Orbiter will spot it. If the mineral has unique features in its spectrum, and most of the hydrated silicates do, it can be classified nicely. Less spatially sharp hyperspectral data from the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) carried by ESA’s Mars Express is equally discriminating for larger patches.

The two instruments have shown up hundreds of small outcrops of minerals in the southern hemisphere that formed by reactions between the dominantly anhydrous minerals of Mars’s dominantly igneous crust and water. They record an early phase when liquid water was available at the surface. The question is, are they merely a thin veneer? As a check, John Carter (did bearing the same name as Edgar Rice Burroughs’ hero in his Mars novels encourage his fascination with the Red Planet?) of the University of Paris and colleagues used OMEGA and CRISM data to look at deep crust exhumed in several of Mars’s northern hemisphere craters. Clay minerals, chlorite and prehnite do show up clearly, and the hydration reactions must therefore have penetrated up to a kilometre into the crust. The same suite of minerals occur in the southern hemisphere, so during this early wet episode water was available far and wide across the Martian surface. Minerals like prehnite and chlorite are most familiar as products of low-grade metamorphism, which presents a puzzle. Maybe they formed as a result of the temperatures and pressure generated by the impacts themselves. But if that were the case they would be expected to pervade all the excavated rock, whereas they occur in distinct patches next to pristine, highly reactive olivine-rich rocks. One absentee mineral is serpentine that would definitely have formed by the reaction of water with olivine during impacts. So it looks like water pervaded the whole Martian crust down to maybe a kilometre, then this ‘weathered’ layer was blanketed much later by a thick volcanic layer which has been removed in some places by impact excavation.

• Tectonics
Underpinnings of Mediterranean tectonics
The region of the Mediterranean Sea, especially in the Aegean area, has among the most complex active tectonics on Earth. Both the African and Eurasian plates are now barely moving. The basic shaping of the region stems from Africa’s protracted collision with Europe since 40 Ma that resulted in the closure of the Mesozoic Tethys seaway and jumbled both its sedimentary fill and the continental lithosphere that lay on either side of the collision zone. But if surface motion has largely stopped, why is the Mediterranean region so tectonically active? It now seems as though it links to flow in the mantle beneath (Facenna, C. & Becker, T.W. 2010. Shaping mobile belts by small-scale convection. Nature, v. 465, p. 602-605). A mix of GPS tracking of surface motions, evaluation of surface uplift and subsidence, and analysis of seismic tomography of the mantle. Vertical motion of the mantle is most pronounced at shallow mantle depth (250 km), suggesting vigorous convection in quite small cells. The relations to tectonics are complex, but they are interlinked. For instance subducting slabs interfere with shallow mantle flow so that compensating upwellings result, and in turn help drive subduction and volcanism, as in Italy. Overall, the lithospheric motion, from GPS tracking, has a distinct vortex-like pattern in the eastern Mediterranean and Middle East, which can be modelled from the underlying mantle flow.

The ultimate iPhone app: a truly retro makeover

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Now that the Neanderthal genome has revealed that non-Africans have a bit of the old chap inside us (see Yes, it seems that they did… in EPN May 2010), why not seek your inner Neanderthal? The famous Smithsonian Institution in Washington DC has released an application for iPhones, its first ever venture into ‘apps’, that allows users to morph their faces to resemble how they might have looked as a male or female H. neanderthalensis, H. heidelbergensis or even tiny H . floresiensis. The ‘app’ is called Meanderthal, which is especially apt as that neologism is street slang for a sad individual who roams supermarket aisles with a mobile phone welded to his or her ear.

Male relative of ‘Lucy’
Many people know of the amazing skeleton of a possible ancestor to humans discovered in NE Ethiopia by Donald Johanson in the late 1970s, and they know why it was dubbed ‘Lucy’. That type specimen of a female Australopithecus afarensis still figures in the media, but little appears concerning males of the species. That is not surprising for they are represented by only fragmentary and ambiguous remains. So a report on a 40% complete fossil male A. afarensis that includes limb and pelvic bones, and those of the neck, shoulder and arm is sure to cause a stir (Haile-Selassie, W. and 8 others 2010. An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proceedings of the National Academy of Science USA, v. 107, p. 12121–12126. doi/10.1073/pnas.1004527107). For starters, he is very big indeed compared with ‘Lucy’, standing between 1.5 and 1.7 m tall, and fragments of other individuals suggest that some males were larger still and within the modern human range. The conclusion must be that A. afarensis was sexually dimorphic: big males and diminutive females, which is the norm for chimps, orang utans and gorillas. Legs longer than arms suggest an upright walking posture, but the shoulder assembly is more gorilla-like than human. Yet ribs that indicate a barrel chest show a more human form than would other great apes. The authors suggest that the lack of consistent resemblance to any one of the living hominids may indicate that the last common ancestor that we share with the others may not have closely resembled any of the living forms. The big problem with the find is its antiquity: at 3.6 Ma it is a lot older than ‘Lucy’. Without teeth or at least part of a skull, assigning it to the same species carries no certainty.

Neanderthal ‘bling’

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Led by João Zilhão of the University of Bristol, UK, a team of British, French, Italian and Spanish archaeologists and anthropologists have at a stroke rid our former companions in Europe, the Neanderthals, of the popular and academic stigma of being uncultured (Zilhao, J. and 16 others 2010. Symbolic use of marine shells and mineral pigments by Iberian Neandertals. Proceedings of the National Academy of Sciences, v. 107 p. 1023-1028). They wore jewellery in the form of necklaces and pendants of bivalve shells, remains of which have turned up in large numbers in caves and rock shelters in the interior of southeast Spain. Some of the perforated shells show clear signs of having been painted, and a few show grooves worn by string. They found even a paint container and painting tools made of small bones from a horse’s foot. The container and tools retain distinct traces of pigment made from the common iron colorants goethite, jarosite and hematite. One large, perforated scallop shell shows that its white interior was painted to match its reddish exterior.

It has often been commented that Neanderthal adornments ( a few possible finds precede this work) and intricate tools were simply copied from those of fully modern humans. The deposits containing this ornamentation are around 50 thousand years old: preceding modern human occupation of the Iberian Peninsula by at least 10 ka. Evidence for artistic work by early H. sapiens comes from South Africa as far back as 165 ka (see Technology, culture and migration in the Middle Palaeolithic of southern Africa in January 2009 EPN, and When and where ‘culture’ began in EPN of November 2007). Iron-based pigments are still widely used for body painting in many societies, but obviously that use will not feature directly in archaeological finds. Association of lumps of potential pigments with hominin tools go back even further in Africa, beyond the presence of fully modern humans, but to ascribe pieces of say hematite to cultural practice needs evidence for scraping or grinding. There seems no reason why Neanderthals and modern humans maintained an ancient cultural tradition.

The Younger Dryas flood

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In 2006 Wallace Broeker first suggested that the sudden interruption of emergence from the last glacial maximum by a frigid climate about 12.8 ka was due to a massive release of fresh water to the North Atlantic that shut down its thermohaline ‘conveyor’ (see The Younger Dryas and the Flood in June 2006 issue of EPN). He resurrected an earlier idea that a vast lake of glacial meltwater (Lake Agassiz) to the north-west of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. His hypothesis was that the resulting freshening of surface water in the North Atlantic and decreased density stopped the formation of cold dense brines that sink and drag warm water northwards. Setting aside the notion by some enthusiastic authors that a trigger for the Younger Dryas was an exploding comet and a kind of ‘nuclear winter’ (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008) Broeker’s hypothesis is widely accepted. However there are few signs, if any, of a catastrophic glacial-lake outburst through the Great Lakes region and down the St Lawrence. An alternative is that Lake Agassiz drained northwards towards the Arctic Ocean. (Since the North American ice sheet covered Hudson’s Bay that could not have been the destination.) At the end of the last last full glaciation there was a corridor with relatively little glacial cover between the main ice over the Canadian Shield and that mantling the Rocky Mountains, roughly along the course of the modern Mackenzie River. That route would serve the hypothesis well, and there is clear evidence that an outburst flood followed it (Murton, J.B. et al. 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature, v. 464, p. 740-743).

Sediments of the huge Mackenzie Delta of NW Canada contain a sharp erosion surface overlain by gravels that belie the low-energy of deposition today. Optically stimulated luminescence dating of sediment immediately below and above the erosion surface range from 13.4 (below) to 12.7 ka (above), the latter approximating the onset of frigid Younger Dryas conditions. The surface occurs all the way along the Mackenzie into its major tributary the Athabasca River. Near Fort MacMurray, 20 km north of what was the northern shore of Lake Agassiz, there is a terrace composed of massive boulders. Further evidence comes from the apex of the Mackenzie delta in the form of a 25 km long, 2 km wide spillway scoured of all loose sediment and with topographic features reminiscent of the famous Channelled Scablands of Washington State in the NW USA. Numerous beach lines record the drainage of Lake Agassiz, the highest being dated at the start of the Younger Dryas and giving a clue to the volume involved in the initial outburst flood: around 9500 km3. Dating of other features suggest that a second flooding into the Arctic Ocean occurred during the Younger Dryas around 11.5 ka, during its last stages, and a third at 9.3 ka. One effect of the Younger Dryas was a regrowth of the main ice sheet that allowed Lake Agassiz to refill periodically perhaps allowing quieter flooding events down the Mississippi and through the Great Lakes. There are no signs in the climate record of any major perturbation at 9.3 ka.

Broeker received the news graciously, commenting that a freshening of the Arctic Ocean would have been more effective at shutting down North Atlantic thermohaline circulation than a spillway down the St Lawrence, because the sites of modern day sinking of dense cold brine lie well to the north of its outlet. The only way additional water in the Arctic Ocean could escape would have been into the northernmost North Atlantic.

See also: Schiermeier, Q. & Monastersky, R. 2010. River reveals chilling tracks of ancient flood. Nature, v. 464, p. 657.

Archaeology and the Toba eruption

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Depending on when fully modern humans left Africa – and that itself depends on evidence that is at odds with any definite resolution – the forebears of the eventual colonisers of the rest of the world may, or may not, have had to survive the effects of the biggest volcanic eruption of the past 2 million years. Around 74 ka the huge, elliptical caldera lake at Toba in Sumatra was formed by a stupendous eruption that threw out 800 km3 of ash (see Ash Wednesday to put this in perspective with recent events). Toba deposited a 15-centimetre ash layer over the entire Indian subcontinent. Toba has taken on a near iconic status among some palaeoanthropologists as a possible means of reducing the entire human population to a mere few thousand: a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations that shaped such things as language and culture. Unsurprisingly major efforts are underway to get hard facts about the relationship of fully modern humans to the Toba event, a lot of the work-in-progress being outlined at toba.arch.ox.ac.uk/index.htm.

See also:  Balter, M. 2010. Of two minds about Toba’s impact. Science, v. 327, p. 1187-1188.

Moon rocks turn out to be wetter and stranger

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Since the original analyses of lunar rock samples brought back by the Apollo astronauts is has been widely accepted that they are almost totally anhydrous. Some even contain pristine metallic iron with not a trace of rust after more than 4 billion years. So, therefore, the entire Moon should be bone dry, except for possible rimes of ice preserved in deeply shadowed polar craters. This lack of water is one line of evidence used to support the Moon’s origin in a stupendous collision between the early Earth and a smaller companion planet shortly after their accretion. The event may have depleted volatile elements and compounds in the incandescent vaporised rock from which the Moon is believed to have condensed. There are traces of water in glass spherules from lunar dust, but that might have come from the impactors that blasted them from craters. But at this year’s Lunar and Planetary Science Conference – the fortieth since the first Apollo landing – evidence for water in lunar minerals was presented (Hand, E. 2010. Old rocks drown dry Moon theory. Nature, v. 464, p. 150-151). The water is in apatite grains that occur as crystals in lunar maria basalts, so must have come from the Moon’s mantle through partial melting. Modelling suggests tens of thousand time more water in the lunar interior than believed previously, albeit still much less than in the Earth. Equally surprising is the water’s isotopic composition: it has a much greater proportion of deuterium (2H) relative to hydrogen (1H) than does water in terrestrial igneous rocks. The giant impact hypothesis suggests that the proportions should be the same in both bodies. One possibility is that a fortuitous comet delivered water to a dried-out hot moon soon after it has coalesced from and orbiting incandescent cloud. Hopefully a full publication will appear soon.

 

Ash Wednesday

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On 14 March 2010 the Icelandic volcano Eyjafjallajoekull conspired with a major kink in the stratospheric jet stream, itself a possible outcome of ‘quiet Sun’ conditions, to load the lower atmosphere with its ash cloud. The cloud arrived over most of Europe the following day with outcomes that need no mention here.

Researchers collected samples from the plume over Britain, finding particles mainly of the order of 0.1 mm diameter ranging up to 3 mm. The larger particles account for much of the mass of suspended ash (Sanderson, K. Questions fly over ash-cloud models. Nature, v. 464, p. 1253), but that amounted to only 60 mg m-3 in the air over Britain compared with a ‘danger level’ of 2000 mg m-3 declared by the Civil Aviation Authority. That volcanic ash – and presumably dust from sand storms – is hazardous to aircraft is a truism, but little is known about the actual processes involved.

At the speed of modern jet aircraft, mineral or glass dust sandblasts flight deck windscreen, may damage or clog the tubes used to measure airspeed, build up electrostatic charge to interfere with communications and may melt to coat turbine blades (Wikipedia –“volcanic ash”). Two near-catastrophic encounters of Boeing 747 passenger aircraft with ash clouds in the 1980s formed the basis for precautionary halting of all air traffic over most of Europe in mid-April 2010. In both incidents all four engines overheated and cut out, as the ash melted onto turbine blades and prevented them cooling. Fortunately, descent below the ash cloud cooled and shattered the glass coating so that the engines could be restarted. However, unbalancing of the turbines potentially could have caused them to jam irreversibly. Jet engines run at around 1400º C so can potentially melt ash of any composition: at atmospheric pressure the melting temperature of both felsic and basaltic materials is 1000-1200º C. Both the 1980s incidents occurred suddenly in thick ash plumes close to volcanoes, in which ash particles would have been larger than those in the dispersed cloud over Europe in April 2010. Little is known about how melted ash might accumulate in and damage turbines during prolonged flight through very dispersed, ultra-fine-grained ash clouds.

Disruption of aviation schedules is just one continental-scale hazard from Icelandic volcanoes. In the summer of 1783 an eruption of Laki, a fissure volcano further inland, killed 80% of Iceland’s sheep, 50% of other livestock and by the end of the year 25% of its human population. The magma was enriched in fluorine and among the emitted gases was hydrogen fluoride that reacted with ash to form metal fluorides that coated vegetation across wide tracts of the island. Ingesting fluorides leads to fluorosis, a crippling disease to which sheep and cows are especially prone. Most of the human victims probably died of starvation. However, archaeologists who exhumed burials from the time of Laki’s last devastating eruption found skeletal signs of fluorosis: bony nodules and spiky fibres in joints (see Archaeology and fluorine poisoning in EPN for December 2004). It is a repeat of Laki’s toxic ash eruption that Icelanders most fear. During 1783 there were widespread reports from northern Europe of a bluish, acrid smelling haze, probably rich in sulfur dioxide. Contrary to the cooling effect of sulfuric acid aerosols in the upper atmosphere, this acrid fog seems to have warmed the regional summer to possibly the hottest in several centuries. Followed by a bitterly cold winter, Laki’s distant effect was devastation of crops, famine and deaths from starvation. It was not restricted to Europe, drought and famine affecting Egypt, India and Japan at the same time, with an estimated global death toll of more than 2 million. This suggests that some of the sulfur dioxide did become trapped in the stratosphere as climatically cooling sulfuric acid droplets that spread over the whole Northern Hemisphere. There are few records of wind patterns from the mid 1780s, yet the filling of Europe’s skies with Icelandic dust in 2010 suggests that a similar, wind system prevailed in 1783 – clockwise from Iceland around a large anticyclone centred on western Britain.

When the Eyjafjallajoekull volcano last erupted in 920, 1612, and 1821-1823, the much larger subglacial volcano Katla, 25 km to the east, followed suit. Around 10 600 years ago Katla emitted 6 to 7 km3 of ash, recognisable in Scotland, Norway and in North Atlantic sediment cores. Many Icelanders regard Katla as potentially their most dangerous volcano.

Yes, it seems that they did…

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Perhaps now the myth of brutish Neanderthals will finally be laid to rest. Thanks to the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, we have a nuclear genome of H. neanderthalensis; in fact a composite based on bones of three individuals from a Croatian cave. Carbon-14 dating shows that the bones are between  44 to 38 ka old: about the time of the first arrival of fully modern humans in Europe. Only ten years on from the publication of the first human genome, the team inspired by Svante Paabo (actually the last of 56 authors, but the founder of the lab and its peerless facilities) has engineered a scientific triumph that matches the achievement in 2000 led by James D. Watson at the U.S. National Institutes of Health and Craig Ventner of Celera Corporation (Green, R.E. and 55 others 2010. A draft sequence of the Neandertal genome. Science, v.  328, p. 710-722). Let’s be frank, to get to know another member of our genus nearly as well as ourselves, albeit in terms of A, C, T and G the nucleotide bases of DNA adenine, cytosine, thymine and guanine, puts the rest of science in somewhat distant perspective. It forms the basis for learning what, if anything, sets us apart from earlier humans, what we share with them and potentially how we came to be what we are.

Apart from a geologically brief period since 80 ka when fully modern humans and Neanderthals occupied the Mediterranean fringe of the Middle East, both had probably developed separately since forebears of the Neanderthals left Africa to arrive in Europe about 400 ka ago while ours seem to have stayed in Africa. Earlier genetic results show that both species shared a common ancestor, perhaps H. heidelburgensis. From the time when the main wave of African people ventured into Arabia, Asia and Europe, perhaps around 60 to 75 ka, chances are that encounters were inevitable, until the last Neanderthals met a lonely end on the Rock of Gibraltar around 25 ka. Variations in mtDNA data seem to show that the two species have little genetic overlap, but mitochondria hold only a small part of DNA. The 4 billion base pairs of nuclear DNA occur in thousands of segments that have evolved independently, and in us continue to do so: a source for very detailed comparisons indeed. The issue centres on how alike and how different such segments are, when compared with DNA from different modern human genomes. If similarities and contrasts are more or less the same in comparison with all modern human groups, then it is most likely that although Neanderthals and modern humans did meet they did not exchange genetic materials; i.e. they did not mate successfully. The new data show beyond much doubt that Neanderthals were more similar genetically to modern Europeans and Asians than they were to modern Africans. There was successful mating and the progeny entered the fully modern human population of Asia and Europe, to the extent that Asians and Europeans host 1 to 4% of Neanderthal ancestry.

The most famous human in genetics, simply because he arranged sequencing of his own DNA, which is the comparator used by the team, Craig Ventner can be highly confident that he contains segments of Neanderthal DNA. We must await his reaction in a mood of solemn gaiety, and react he most probably will: I did and I feel quite cheerfully proud. Interestingly, Neanderthals are as closely related to individuals from New Guinea and China as they are to a French person. Such uniformity among non-Africans suggests that the gene exchange (viz. sexual intercourse) took place shortly after fully modern humans migrated out of Africa. But who did what to whom under which circumstances will remain a mystery, although it appears that the gene flow was from Neanderthal to human and not vice versa. With a small colonising group of Africans, there need not have been a great deal of ‘sharing’ of bodily fluids for introduced genes to ‘surf’ throughout succeeding generations to reach us. So what is it that we lucky ones share with Neanderthals? This is a topic fraught with possible overtones, though they probably will not suit the outlook of those with a prejudiced racist tendency. The results suggest 15 genomic regions that include those involved in energy metabolism, possibly associated with type 2 diabetes; cranial shape and cognitive abilities, perhaps linked to Down’s syndrome, autism and schizophrenia; wound healing; skin, sweat glands, hair follicles and skin pigmentation; and barrel chests. Some may have been beneficial others not, but they have been retained through thousands of fully modern human generations.

Analyses of the genome are at a very early stage, but the sequencing technique and associated checks for contamination with modern DNA are sufficiently advanced that other Neanderthal remains and bones of ancient Europeans and Asians will surely add to the excitement. Just how far back analyses can be pushed remains to be seen, but it is now quite clear that human evolution was a great deal more complicated than the simple Out-of-Africa model that is currently almost universally accepted.

See also: Gibbons, A. 2010. Close encounters of the prehistoric kind. Science, p. 680-684.

Other rich hominin pickings

March and April 2010 were indeed exciting times for palaeoanthropology, with publication of evidence for two new species of hominin. Cave systems in the Archaean limestones of north-eastern South Africa have yielded so many fossil remains related to human evolution that the area liberally dotted with them has UN World Heritage status. The caves formed beneath a now-eroded plateau, and are so rich because creatures fell into surface sink holes, died and remained little disturbed by scavengers. The latest find has an unusual story behind it (Balter, M. 2010. Candidate human ancestor from South Africa sparks praise and debate. Science. v. 328, p. 154-155). The cave system was first explored by lime-kiln workers around the early 1900s, who brought out blocks which litter the ground around cave mouths. It was in one of these chunks that the 9-year old son of a South African palaeoanthropologist found bone that turned out to be a hominin lower jaw. Sadly, young Matthew Berger had to be excluded from the list of authors of the two important papers that ensued from his find, because of Science magazine’s rules for authorship (Berger, L.R. et al. 2010. Australopithecus sediba: a new species of Homo­-like australopith from South Africa. Science, v. 328, p.195-204. Dirks, P.H.G.M. and 11 others 2010. Geological setting and age of Australopithecus sediba from southern Africa. Science, v. 328, p.205-208). Nevertheless, he can be well satisfied as the full set of bones points to a new species, one that may arguably share more features with Homo species of about the same antiquity than any other australopithecine. Being coeval with H habilis, A. sediba cannot be ancestral but may have shared a common ancestor with the earliest known human species. Fitting the new find into the long and variously disputed cladistics of hominins will run and run, but at least it should re-emphasise one thing: there were several cohabiting hominin species in Africa around 2 Ma ago.

Such a multiplicity of co-existing hominins seemingly continued until quite recent times, as a remarkable piece of evidence from a Siberian cave has confirmed. Between about 30 to 48 ka, the cave was a popular venue for Neanderthal hunters who left tools and bones of their prey. Russian archaeologists combed the cave deposits for human remains but came up with only fragmentary finds of bone. One of these was the tip of someone’s little finger. The possibility of obtaining genetic material from relatively young finds in caves that have remained cold and untouched encouraged the excavators to handle their finds carefully. It’s just as well they did for the results from the Max Planck Institute for Evolutionary Anthropology in Leipzig Germany, famous for its work on Neanderthal DNA, held a surprise. The finger’s owner was neither a Neanderthal nor a fully modern human (Krause, J. et al. 2010. The complete mitochondrial DNA genome of an unknown hominin from Southern Siberia. Nature, v. 464, p. 894-897). The evidence for this is overwhelming. Fully modern human mtDNA ranges from 0 to about 100 differences in nucleotide positions, the difference between human and Nenaderthal mtDNA is just over 200, but the pinky bone revealed almost 400 differences from ourselves and almost as many from Neanderthals. Such differences suggest that ancestors of the unknown Siberian separated from the line of descent to Neanderthals and modern humans about a million years ago. Yet all three were in Asia a mere 40 ka ago. Add to that the diminutive H. floresiensis who survived to cohabit Flores with modern humans until about 9ka, and some evidence that H. erectus was also around in Java up to 25 ka, gives possibly 5 species of human in Asia who may have met and goodness knows what else.

See also: Dalton, R. 2010. Fossil finger points to new human species. Nature, v. 464, p. 472-473.

Ocean-floor topography-age correlation challenged

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One of the elements comprising the canon of plate tectonics is that as plates spread away from constructive margins the depth to the ocean floor increases in direct proportion to the square root of the underling lithosphere’s age. This is generally considered to reflect steady passive cooling and increasing density of initially hot lithosphere produced at ridge systems. The resulting slope of the ocean floor is said to result in one of the gravitational forces that sustain plate tectonics – ‘ridge slide’. The Pacific Ocean floor is a good test for the hypothesis, but unfortunately does not show a linear depth vs Öage relationship (Adam, C. & Vidal, V. 2010. Mantle flow drives the subsidence of oceanic plates. Science, v. 328, p. 83-85). Instead, the ocean floor flattens out beyond a threshold distance, which has been a source of puzzlement for decades. However, a plot of depth against the square root of distance from the ridge along estimated lines of mantle convective flow is consistently linear. The depth curve seems therefore to reflect past changes in the direction of sea-floor spreading and changes in the deeper mantle convection, thereby linking reality to the original model for continental drift that had mantle convection at its heart. That view was discarded by geophysicists on account of a widespread belief that the asthenosphere was too weak to transmit forces from below to the rigid lithospheric plates.

End-Cretaceous mass extinction moving towards ‘closure’?

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Apart from the change in name from the K-T (Cretaceous-Tertiary) to the K-Pg (Cretaceous-Palaeogene) Event, following the abolition by the International Commission on Stratigraphy of the name Tertiary – given by Giovanni Arduino to the penultimate geological Era, in favour of Cenozoic (Palaeogene + Neogene + Quaternary) the eponymous mass extinction has steadily become a less regular news item. Views had settled in to three camps: driven by an impact; by Deccan volcanism or by the two conspiring together. Yet a host of geoscientists, from institutions whose addresses take up 8 column inches in Science, have been beavering away to settle the issue one way or another (Schulte, P. and 40 others 2010. The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science, v. 327, p. 1214-1218).  The main biotic changes and geochemical signatures of the K-Pg Event all coincide at 65.5 Ma with the world-wide Chicxulub ejecta layer, after two thirds of the Deccan Traps had been erupted. In an extensive and readable summary of all the evidence the authors conclude that the Chicxulub impact did trigger the massive die-off. Despite global change associated with volcanism, life went on ‘down to the wire’ (a wire once marked the finish line in horseracing). The authors rule out the Deccan volcanism as a causative factor on account of little more than a 2º C warming effect while it lasted, set against the likely near-instantaneous release of at least 100-500 billion tons of SO2 by an impact into massive sulfate-rich sediments around the Chicxulub site (the release by Deccan volcanism has been estimated at 0.05 to 0.5 Gt per year throughout its million-year duration). Such a release along with dust and water vapour flung into the atmosphere are modelled to have reduced global temperatures by up to 10º C – a reduction greater than that reached by the last glacial maximum. The re-entry of such a mass in rainfall within a few years would have acidified large areas of surface ocean water: a 3-4 orders of magnitude larger effect than that of slow release by volcanism. The authors conclude that the most important remaining work is to delve deeper into the impact site itself to quantify likely chemical emissions, and then to develop models of the actual deadly processes that ensued.

‘Hard’ Snowball Earth softens

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The original hypothesis of Neoproterozoic global glacial conditions, proposed by Joe Kirschvink (California Institute of Technology) and Paul Hoffman (emeritus at Harvard) in the 1990s was that conditions became so severe that the Earth was encased in glacial- and sea ice from pole to pole. As EPN has charted since 2000, that ‘hard’ Snowball variant has become increasingly less favoured by most geoscientists (Kerr, R.A. 2010. Snowball Earth has melted back to a profound wintry mix. Science, v. 327, p. 1186). However, evidence supporting low latitude glaciations continues to emerge (, F.A. and 9 others 2010. Calibrating the Cryogenian. . Science, v. 327, p. 1241-1243). In the latest, diamictites of the so-called ‘Sturtian’ glaciation in north-western Canada are interbedded with volcanic rocks that give a very precise age of 716.5 Ma. That age happens to coincide with outpouring of the regionally massive Franklin flood basalts whose palaeomagnetism gives equatorial latitudes, the first recorded for the Sturtian glaciation: the later Marinoan glaciation (~635 Ma) provides most low-latitude evidence for Snowball conditions. The paper by Francis Macdonald and co-workers also gives detailed carbon isotope data for a continuous sedimentary record from >811 to 583 Ma.

A potential spanner in the works for the entire Snowball Earth hypothesis is the discovery of a strange anomaly concerning palaeomagnetic pole positions during latest Neoproterozoic times  (Abrajevitch, A. & van der Voo, R. 2010. Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis. Earth and Planetary Science Letters, v. 293, p. 164-170). Paleomagnetism from glaciogenic rocks is the lynchpin for the notion of Snowball Earth, some occurrences recording tropical latitudes. Alexandra Abrajevitch (Kochi University, Japan) and Rob van der Voo (University of Michigan) report palaeomagnetic results for igneous rocks between 600 and 550 Ma in what are now North America and Scandinavia. The data show original inclinations of the magnetic field that are both steep and shallow, indicating high and low latitudes respectively. Plotting inclination against radiometric age for what were separate continental masses in the Ediacaran Period reveals repeated rapid changes from high to low palaeolatitudes that simply cannot be accounted for by continental drift: plate tectonic rates would have to have been unaccountably fast (~45 cm yr-1). To account for the abrupt shifts the authors turn not to true polar wander – due to changes in the geometry of the geomagnetic dipole – but to rapid flips in the orientation of the dipole between a coaxial and an equatorial alignment, perhaps due to dramatic changes of circulation within the liquid outer core. Familiar geomagnetic reversals normally shift the magnetic poles between roughly the geographic pole positions. Yet there are data showing that for brief periods the reversing poles do pass through equatorial latitudes but at very low magnetic field strength. In the cases from the Ediacaran the geomagnetic poles dwelt at tropical latitudes for long periods and maintained a strong field. Were such strange behaviour demonstrated earlier in the Neoproterozoic, during the Cryogenian period of supposed Snowball events, that would undermine the whole basis for the hypothesis. It seems inevitable that geophysicists will scurry to check the earlier palaeomagnetic data, analysing more igneous rocks on all continents at the narrowest possible time intervals.

Crustal sagging during major volcanism

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Ice sheets during the last glaciation reached more than 2 km in thickness over vast high-latitude areas of the Northern Hemisphere. Even though ice has less than half the density of continental crust, their sheer mass forced the lithosphere down into the asthenosphere by up to several hundred metres. The displaced asthenosphere resulted in a corresponding bulge around the glacial fringe. Continental flood basalts are about three times as dense as ice and reach thicknesses up to 2-3 km, so they would have produced even more subsidence, although set against that is the uplifting effect of reduced density of the crust as a result of magmatic heating. The loading effects of individual volcanoes are well known. Yet surprisingly, there have been few accounts of subsidence caused by CFB loading, and the prevailing view is that plume-related large igneous provinces are preceded by doming and even erosion. Geophysicists at the University of Colorado modelled the effects of plumes and CFB eruption and reverse the general view decisively (Leng, W. & Zhong, S 2010. Surface subsidence caused by mantle plumes and volcanic loading in large igneous provinces. Earth and Planetary Science Letters, v. 291, p. 207-214). They found that phase changes in the rising mantle plume at the 660 km deep discontinuity cause subsidence themselves, so that even before volcanism begins the surface subsides. This is borne out by preservation of basinal sediments beneath some CFB provinces, such as the Siberian and Deccan Traps. Effectively, flood basalts may fill shallow basins that they recreate and maintain due to their loading effect on the crust during successive eruptions. The high elevations of many ancient CFB provinces are a product of later tectonic processes rather than being ‘built’ by volcanism.

‘Microdating’ sedimentary sequences

There are few minerals amenable to radiometric dating that are found in all sedimentary rock types. To give ages that are stratigraphically useful they would have had to form authigenically while the sediment itself was accumulating – glauconite in ‘greensands’ is an example. Calibrated stratigraphy largely depends on dateable igneous minerals found in volcanic rocks interlayered with sediments, the most common being zircon that can be dated precisely using U-Pb methods. The vast bulk of high quality ages of this kind depend on being able to collect sufficient volcanic ash or lava to yield zircon grains. So only volcanic layers thicker than a few centimetres have been used, and they are haphazard in their occurrence in sedimentary sequences. Much thinner ash layers do occur more commonly and uniformly in sequences from arc-related sedimentary basins, and being able to date those would permit much better control over rates of sedimentation and correlation between different sequences. The key is being able to date zircons in thin section (Rasmussen, B. & Fletcher, I.R. 2010. Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tiffs <1 mm thick. Geology, v. 38, p. 299-302). Rasmussen and Fletcher (Curtin University, Western Australia) applied ion-microprobe methods to polished this sections of diamond drill core through Archaean sediments of the Pilbara craton in Western Australia, specifically to date a thin sediment layer that contains spherules formed by a major asteroid impact. They were able to narrow its age down to that of a thin ash only 15 mm above the spherules, about 2632+7 Ma. Though with a specialised objective, they demonstrate that semi-continuous stable isotope data in sediments can be calibrated sufficiently precisely to allow global correlations

2010: already a terrible year for disaster.

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Early 2010 witnessed horrific scenes on Haiti following a magnitude 7.0 earthquake on the afternoon of 12 January to be followed early in the morning of 26 February by one of the largest ever recorded in Chile (magnitude 8.8). Haiti has suffered fatalities on a scale that match those of the Indian Ocean tsunamis of 26 December 2004, while a huge area of coastal Chile affected by seismic energies more than a hundred times greater had estimated fatalities of over 700, though rising at the time of writing. It is easy to ascribe the relative magnitudes of human tragedy, which are the opposite of the relative seismic magnitudes, entirely to the more advanced infrastructure of one of South America’s most advanced countries compared with that of one of the world’s poorest. But that is not the full story. Haiti suffered from a shallow event very close to major population centres whose energy easily reached the surface. The fault responsible involved transverse horizontal movements that sheared through thick soft coastal sediments, which liquefied beneath Port au Prince. That offshore of Chile was much deeper, on a subduction zone and involved vertical movements, so much of its energy was dissipated deep in the crust, yet the area of structural damage along Chile’s narrow coastal fringe is much larger than in Haiti.

Sure, Chile has long had stringent regulations for seismic safety of construction and a state of emergency preparedness commensurate with its history of devastating earthquakes, including the largest ever recorded on 26 May 1960 with magnitude 9.5 that released about ~32 times more energy than the recent one. It is a country well-endowed with income from its huge mining operations, well-developed wineries and much else, especially foreign investment. Haiti has nothing but the horrifying reputation of a string of governments. Until the recent tragedy the majority of its people were left to fend for themselves, close to the playgrounds of the super-rich and the offshore hidey holes of ‘non-doms’. Yet survivors in both countries face essentially the same physical privations of having to live rough and the lasting horror that no amount of wealth can remove. After experiencing the great Valdivia earthquake of 20 February 1835, also in Chile, Charles Darwin observed,

‘An earthquake like this at once destroys the oldest associations; the world, the very emblem of all that is solid, moves beneath our feet like a crust over fluid; one second of time conveys to the mind a strange idea of insecurity, which hours of reflection would never create.’

In both cases lessons may be learned, some socio-economic that are too obvious to repeat here. There is, though, one of that kind that transcends most of the others: the 21st century’s first decade has seen a seismic death toll of 640 thousand; a fourfold increase over the previous 20 years fatalities. That is a reflection of increasing drift of especially poor people to cities. If their dwellings are easily smashed they stand little chance. So far, the pledges of aid for reconstruction in Haiti amount to about US$5000 for each damaged structure.  For geoscientists, however, what is beginning to emerge from these and the various large earthquakes in Indonesia, Pakistan and China since 2004 is that past seismic history is a clue to future events.

Faults zones behave in a segmented fashion, each with its own crude cyclicity but each somewhat prone to being triggered by events from nearby sectors. Between 1750 to 1770 Haiti was repeatedly devastated when the culprit fault unleashed its pent up stresses. Since then it has been locked in the vicinity of Haiti, with tectonic motions of about 8 mm per year accumulating to the 2 m or so motion undergone by the fault on 12 January. Subduction zones accumulate strain in many sectors distributed along the plate boundary, sometimes locking as seamounts start to descend to ‘clog’ them. Statistical analysis of historical earthquakes and locating their probable epicentres in relation to fault segments, with estimates of their power that would now be measurable from seismograph data, can at least highlight future risk geographically even if timely predictions remain impossible. Yet will their be action that matches up to the potential hazard? 2000 years ago the destruction of Pompeii and Herculaneum in the Bay of Naples by Vesuvius was recorded in graphic detail of which the excavations presented a gruesome reminder. Yet Naples expands to urbanise the very slopes of Europe’s most dangerous natural threat.

See also: Bilham, R. 2010. Lessons from the Haiti earthquake. Nature, v. 463, p. 878-879.

Evolution of first land vertebrates in disarray

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The finding of Tiktaalik, a supposed ‘missing link’ between bony fishes and amphibians (see A fish-quadruped missing link in EPN issue for May 2006) seemed to resolve the descent of tetrapods nicely. As is common, if inconvenient, nature has thrown a spanner in the works through a remarkable find in Polish rocks much older than those containing Tiktaalik and more evolved tetrapods (Niedźwiedski, G. et al. 2010. Tetrapod trackways from the early Middle Devonian period of Poland. Nature, v. 463, p. 43-48). Quarrymen unearthed extensive tracks appeared during excavation of intertidal limestones of the Middle Devonian Eifelian Stage (392-398 Ma). The bedding surface also shows raindrop pits and desiccation cracks, so the tracks were made by creatures able to survive out of water. The prints (up to 26 cm wide) are three times bigger than the paws of later amphibians that left fossil remains, but like them they show signs of more than 5 toes. The maker of one trackway was a good walker, having left no trace of dragging its belly through the mud, and it either had no tail or carried it aloft since there is no trail left by a tail either. Another, smaller animal left a separate trackway showing a very different gait. There seems little doubt that these animals were well advanced towards completely terrestrial lifestyles. Tiktaalik from 380 Ma sediments in Arctic Canada obviously cannot have been ancestral to them, and nor are there any fossils from the Middle Devonian that look like candidates. The hunt is on for fossilised remains of whatever walked the walk, and may emerge in the not-too-distant future from subtidal sediments of the same formation.

See also: Janvier, P. & Clément, G. 2010. Muddy tetrapod origins. Nature, v. 463, p. 40-41.

‘Roger, I think that triffid just moved’

The nasty surprise awaiting the bulk of human population blinded by radiation from a meteor shower in John Wyndham’s Day of the Triffids was that the genetically engineered, oil-yielding triffid plants could not only deal out deadly stings but they walked and ate dead meat. So it is that palaeontologists have found with the flabby, quilted bag-like organisms of the late Neoproterozoic Ediacaran fauna. They were animals of some kind, but hitherto considered to be completely sessile, except in larval form. They seem not to have been able to bite or gnaw, but probably absorbed victuals through their skins. Imagine the shock when palaeontologists from Oxford and Memorial University of Newfoundland found trackways in the famous biome of Mistaken Point in Newfoundland (Liu, A.G. et al. 2010. First evidencee for locomotion in the Ediacaran biota from the 565 Ma Mistaken Point Formation, Newfoundland. Geology, v. 38, p. 123-126). This throws an entirely new light on the very first sizeable animals: some of them were muscular. But not very adventurous, for the trails are only up to 17.2 cm long. Several of the traces show curved ridges, much like though far smaller than those left in wet sand by a buttock-shuffling baby, but ascribed by the authors to use of an ‘inflatable pedal disk’ in the manner of some cnidarians today – they ‘blurted’ along no doubt. The darned things must have had a purpose in moving, and chasing down prey springs easily to mind, only to be swiftly rejected. Alarmingly, at least for their totally torpid companions, some of the trackways clearly end in a depression: did they lie in wait? Yet not a one shows the telltale three-fold pedestal symmetry of Wyndham’s triffids…

Believable Archaean fossils

Some years back a major spat broke out over the reality of microscopic features purported to be evidence for bacterial life in 3.5 Ga rocks from Western Australia (See Doubt cast on earliest bacterial fossils in April 2002 issue of EPN), which has rumbled on ever since among highly regarded groups of palaeontologists. Those who refuted those finds as merely mineralogical structures that just seem to look biogenic have more work pending. Much more convincing evidence has been found in 3.2 Ga cherty rocks from South Africa (Javaux, E.J. et al. 2010. Organic-walled microfossils in 3.2-billion-year-old shallow marine siliciclastic deposits. Nature, v. 463, p. 934-938).  They are big, by microfossil standards, 3-dimensional structures up to a third of a millimetre across, and clearly resemble cells. Some have even been separated from their matrices by dissolving away silica with hydrofluoric acid, so are not merely figments of the authors’ imagination. They are carbonaceous with very negative δ13C values typical of organically processed carbon and show abundant evidence of intricate structures found in living cells. Raman infrared spectroscopy also shows that they have been metamorphosed at the same grade as the rock that host them, so they cannot be later contaminants. In all these respects the little spherules are a cut above previously described structures reckoned to have been early Archaean life forms, convincingly taking concrete evidence for the existence of living things back a remarkable billion years: the previous oldest true fossils are about 2.2 billion years old.

In one respect the find may be truly breath taking. Spherules this size cannot be from the life-domain Archaea, and at the very least they are particularly large cells of Bacteria. Yet, bacterial cells contain little that could produce such robust little objects, which resemble single-celled eukaryotes known as acritarchs. The earliest definite acritarchs data back to 1.8 Ga. Geochemical evidence for eukaryotes was not sought in the spherules, but there has been speculation that some Archaean rocks have yielded chemical biomarkers that point to the presence of the ancestors of multicelled life at an astonishingly early date in Earth’s history. Clearly Javaux and colleagues work is a precursor of a lot more, now that we have hard-to-refute evidence for 3.2 Ga life.

A ginger dinosaur

The Early Cretaceous of SE China has become justifiably famous by providing a regular supply of superbly preserved small dinosaurs and early birds believed to have had a dinosaurian ancestry in the Jurassic. We have become accustomed to seeing computer generated graphics of brightly coloured dinosaurs since the BBC series Walking with Dinosaurs, first broadcast in 1999, but they owe more to imaginative assumptions based on strongly patterned living lizards than to fossil evidence. That is set to change, with the discovery of actual colouring agents in a Chinese find (Zhang et al. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature, v. 463, p. 1075-1078). The melanosomes are in exquisitely preserved feathers that adorned and probably warmed small dinosaurs as well as the famous bird fossils from the same sedimentary rocks. One specimen of Sinornithosaurus may have sported a coat patterned in black and russet, while Sinosauropteryx seems to have had a tail and back crest striped in shades of red-brown. Could this be for camouflage, display or some aspect of regulating heat? The big leap follows some 6 months on from the discovery of melanosomes in bird feathers from Eocene oil shales in Germany, that may have given them a starling- or hummingbird-like iridescent sheen (Vinther, J. et al. 2009. Structural coloration in a fossil feather. Biology Letters, v. 6, p. 128-131). The huge diversity of modern coloration among birds, from feathers and in the skins of lizards is widely believed to function primarily as a species-dependent means of display, with some influence from camouflage and thermal properties. Whichever, it must have been an integral aspect of speciation for a very long time indeed, yet even the best fossils cannot yield full ornament information, and reconstructions will rely on artistic licence, but now with a little more confidence that creatures didn’t just come in one colour, like Model-T Fords.

To spice up the stereotypical view that ginger = bad-tempered it seems that as well as being mottled with that hue Sinornithosaurus may have been venomous (Gong, E. et al. 2010. The birdlike raptor Sinornithosaurus was venomous. Proceedings of the National Academy of Science, v. 107 p. 766-768). Its skull shows grooved teeth, the grooves leading to a pocket at the base of the teeth. It may also have evolved to feed on birds…

Joining the Neoproterozoic dots

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Riven by the effects of at least two Wilson cycles of rifting drifting and collision, and then covered by a variety of later sediments, late-Precambrian rocks at high latitudes around today’s North Atlantic are nowhere near as coherent as their counterparts in, for instance, Africa. Also they have a long history of field investigation that began long before the unifying theory of plate tectonics, using a parochial rather than a ‘joined-up’ approach. Consequently there is a vast literature, as witness that of say the Moines or the Dalradian in Scotland, which has strangely acted as a hindrance rather than a boon to synthesisers: not that attempts haven’t been made in recent decades. Interestingly, a multi-hemisphere approach to unification, combining Australian and British geologists, seems to have made a great deal of ground (Cawood, P.A. et al. 2010. Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic. Geology, v. 38, p. 99-102).

The Rodinia (‘Motherland’) supercontinent united all continental lithosphere at the end of the Mesoproterozoic era, existed between 1100 and 750 Ma, then broke into eight drifting continents during the Neoproterozoic. Like the later Pangaea (‘all of mother Earth’) formed when all these wandering masses finally clanged together again, conditions deep in the interior of Rodinia were probably tectonically and geomorphologically almost static. All the action would have been around its rim, towards which much of global sea-floor spreading ultimately was directed. Far older continental material now juxtaposed across the high-latitude North Atlantic was in just such an exposed position at the edge of the supercontinent; Greenland abutting the present Baltic crystalline mass. Local sea-floor spreading twisted Baltica from this part of Rodinia in a clockwise manner, to leave a large triangular sea in its wake. This Asgard Sea (why not Toblerone?) received debris from uplifted masses of older crust, to fill a deep sedimentary basin ready for deformation should tectonics warrant that. Two such episodes (980-910, 830-710 Ma) created the older Neoproterozoic metamorphic belts which have long drawn geologists to study Greenland, Scotland and Scandinavia in great detail: for British geologists the attraction was the complexity of the Moine Schists in which John Ramsay famously laid the foundations of modern polyphase structural analysis in the late 1950s and 1960s. A noteworthy point is that by comparison with most mountain belts, the Valhalla orogen took an awfully long time to form: around 300 Ma.

An old theory resurrected

Before the wide acceptance of sea-floor spreading and continental drift geoscientists had to seek explanations for the common occurrence of very similar fossils on now widely separated land masses. On the other hand, Alfred Wegener used observations such as the presence of fossilised tongue-like Glossopteris leaves in the Permian sediments of all the southern continents, and similar distributions of reptiles to support his theory. His detractors tried to explain away the fossil evidence by suggesting now-vanished land bridges, ‘island hopping’, floating seeds, and natural Noah’s Arks carrying animals and so on. With the discovery of irrefutable evidence for sea-floor spreading Wegener was vindicated, albeit long after his death, and the views of his detractors became ridiculed and neglected in their turn. But one puzzle remained: the fauna of Madagascar. Beginning about 170 Ma ago, Madagascar along with India parted company with Africa, to the extent that Madagascar is now more than 430 km off the East African coast (India moved much further independently).

Madagascar, of course, is famous for its lemurs but its fauna includes other animals found nowhere else. Another oddity is that late-Mesozoic Malagasy sediments have yielded no evidence for ancestors to these animals, so the fauna could not have evolved from African stock set adrift with the microcontinent. The only explanation then seems to be that the little animal ancestors drifted on vegetation rafts from Africa – note this would be more unlikely for large animals. Yet today’s current patterns make any drift toward Madagascar highly unlikely. The puzzle may have been resolved, if one believes computer modelling, by the different surface flow patterns of the Indian Ocean during the Palaeocene (Ali, J.R. & Huber, M. 2010. Mammalian biodiversity on Madagascar controlled by ocean currents. Nature, v. 463, p. 653-656). At that time the drifting island was further south than it is now, and currents would intermittently have flowed from East Africa towards it. As it was driven northwards, so it entered the influence of the westward flowing, South Equatorial Current that now isolates it from its parent continent. The idea of rafting, first developed in 1940 by George Gaylord Simpson, an opponent of anything smacking of continental drift, also seems the only possibility if the arrival of New World monkeys in South America and other oddities are to be explained.

See also: Krause, D.W. 2010. Washed up in Madagascar. Nature, v. 463, p. 613-614.

Geochemical prospecting on Mars

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Since its atmosphere is so thin, there are things you can achieve from orbit around Mars that would be unthinkable for the Earth. One is imagery free of atmospheric shimmer or scattering, another is analysing gamma rays emitted by Martian rocks using a gamma-ray spectrometer (GRS), as carried by Mars Odyssey. Two processes produce the gamma rays: the decay of long-lived naturally-occurring radiogenic isotopes of potassium, uranium and thorium with their daughter isotopes, and by the interactions of high-energy cosmic-ray particles with other elements in surface materials. Again, with little atmosphere the Martian surface is heavily bombarded by cosmic rays. Using far larger gamma-ray detecting crystals carried on low-flying aircraft it is possible to remotely sense K, U and Th concentrations at the Earth’s surface. To get data on other terrestrial elements from far off would involve unsociable irradiation of the surface by artificial means.

Results from the Mars Odyssey GRS are somewhat blurred as the analysed radiation comes from 0.5º x 0.5º sampling ‘bins’ and is then filtered to a level of 5º x 5º (~ 25 x 25 km) (Taylor G.J. et al. 2010. Mapping Mars geochemically. Geology, v. 38, p. 183-186). So, the approach cannot match geological maps made by interpretation of high resolution images of reflected or thermal radiation. However, as well as K, U and Th estimates, the data cover Fe, Si, Ca, Cl and H2O: sufficient to crudely distinguish mafic and felsic igneous rocks and to detect any regional hydrothermal or groundwater alteration. The authors claim that the GRS separates  much of the Equatorial region of Mars into six kinds of geochemical province, all of roughly basaltic composition. With an estimated SiO2 range from 46.7 to 49.8% that doesn’t promise much by way of fractionation on the scale of terrestrial magmagenesis; i.e. there are no significant intermediate or felsic igneous rocks. A CaO range of 7.5 to 11.4 does indicate varying plagioclase feldspar content, but no anorthosites, unlike the Moon. The greatest variation is in K and Th content, but that does not match the much larger ranges in terrestrial basalts. The geochemical provinces do not match even a simplified photogeological map of the planet, and it seems quite likely that such variation as there is could have resulted from slight weathering and movement of dust and sand. Will a single returned sample of Mars basalt be all that is needed to characterise the Red Planet? More to the point, how does the estimated chemistry match that of purported Martian meteorites, or for that matter the analyses performed on the surface by the Martian rovers Spirit and Opportunity and by the earlier Mars Pathfinder? There is no comment…but Mars Pathfinder surface analyses revealed andesitic rocks at its landing site with up to 55% SiO2.

A challenge to sea-level calibration

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As well as revealing the Milankovich pacemaker for past climate change, studies of oxygen isotopes from deep-water of benthic foraminifera in marine sediment cores also give a guide to the height of former sea levels. That approach is based on several assumptions, of which two are central. One is that the isolation of deep-water organisms from temperature variations at the sea surface, which control the take up of 18O by near surface plankton: well supported by the measured constancy of cold deep ocean water. The other is that oxygen is rapidly and homogeneously mixed throughout the ocean water column. The reason why good mixing is critical stems from the very purpose of measuring benthic oxygen isotopes, itself based on a sound assumption. Ice masses on land lock up a proportion of evaporated ocean water. Evaporation favours the lighter 16O isotope in water molecules over the heavier, so that atmospheric water vapour has a lower 18O/16O ratio than seawater. When snow falls and turns into glacial ice that build up ice caps, surface water of the oceans becomes depleted in 16O so that its 18O/16O ratio (standardised as the δ18O value) increases. That makes oceanic δ18O values, measured from benthic foram shells, an indirect or proxy measure of both the amount of ice locked up on land and changing sea levels: the principal quantification of past global climate change whose record goes back to the oldest preserved ocean floor (Lower Jurassic, ~205 Ma). Modern humans eventually left Africa to colonise the rest of the world  sometime before 60 Ma ago, the first reliable age of evidence for colonisation outside Africa. Africa is surrounded by sea, except for the narrow strip of land into Palestine that ends up in a desert dead end to further migrations. So, it seems likely that the exodus was across the outlet of the Red Sea that would have become narrower and shallower as sea level fell when the Earth moved into the last glacial epoch after 117 thousand years ago, when sea-level was as high as it is today.

The assumption of rapid, efficient mixing of the oceans has not been thoroughly tested. In fact it is estimated that any complete turnover takes around a thousand years, so there is likely to be a significant time lag in the sea-floor record. New, independent evidence also suggests that the calibration of benthic δ18O needs revision (Dorale, J.A. et al. 2010. Sea-level highstand 81,000 years ago in Mallorca. Science, v. 327, p. 860-863). It comes from caves on the Mediterranean island of Mallorca that connect directly with the sea. Stalactites and stalagmites (collectively called speleothem) have formed in the caves, their growth being affected by flooding and drying as sea level rose and fell during the last 130 ka. At each flooding level encrustations formed around the speleothem to produce bulbous growths at different heights in the caves, which are clearly forming today at mean sea level. The researchers from the US, Mallorca, Italy and Romania dated the bulbs using the U/Th method appropriate for speleothems, and found three stages of formation: at 121, 116 and 80-82 ka. The two older encrustations are at ~2.6 m above modern sea level, bang on the oxygen isotope calibration for the end of the last interglacial. However, those formed between 80-82 ka ago – a period of warming during the overall trend to colder conditions as ice sheets grew – are about a metre above modern sea level: very different from the estimate of 10-20 m below­ based on the benthic δ18O calibration.

It is too early to tell in what quandary palaeo-oceanographers will be placed by this large discrepancy. There are four main possibilities for the aberrant results. First, the Mediterranean might have stood higher that global sea level for some reason, but that seems highly unlikely as the connection through the Straits of Gibraltar is deep enough to have maintained flow even at the last glacial maximum when global sea level was around 120 m below the present. Second is that the means of calibration using raised coral reefs on tectonically rising coastlines of New Guinea and Barbados  is seriously out for part of the last glacial period. Thirdly, somehow the Mallorcan crust was depressed during the last glacial period. The island is rising at about 0.2 mm yr-1, which would give an uplift of 16 m since 81 ka, but that conflicts with the good match with the last highest sea level at 121 and 116 ka. Finally, the authors suggest that at 81 ka the volume of the world’s ice caps was much the same as today, despite the higher-than-present δ18O values in contemporary sea-floor sediments.

Neanderthal ‘bling’

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Led by João Zilhão of the University of Bristol, UK, a team of British, French, Italian and Spanish archaeologists and anthropologists have at a stroke rid our former companions in Europe, the Neanderthals, of the popular and academic stigma of being uncultured (Zilhao, J. and 16 others 2010. Symbolic use of marine shells and mineral pigments by Iberian Neandertals. Proceedings of the National Academy of Sciences, v. 107 p. 1023-1028). They wore jewellery in the form of necklaces and pendants of bivalve shells, remains of which have turned up in large numbers in caves and rock shelters in the interior of southeast Spain. Some of the perforated shells show clear signs of having been painted, and a few show grooves worn by string. They found even a paint container and painting tools made of small bones from a horse’s foot. The container and tools retain distinct traces of pigment made from the common iron colorants goethite, jarosite and hematite. One large, perforated scallop shell shows that its white interior was painted to match its reddish exterior.

It has often been commented that Neanderthal adornments ( a few possible finds precede this work) and intricate tools were simply copied from those of fully modern humans. The deposits containing this ornamentation are around 50 thousand years old: preceding modern human occupation of the Iberian Peninsula by at least 10 ka. Evidence for artistic work by early H. sapiens comes from South Africa as far back as 165 ka (see Technology, culture and migration in the Middle Palaeolithic of southern Africa in January 2009 EPN, and When and where ‘culture’ began in EPN of November 2007). Iron-based pigments are still widely used for body painting in many societies, but obviously that use will not feature directly in archaeological finds. Association of lumps of potential pigments with hominin tools go back even further in Africa, beyond the presence of fully modern humans, but to ascribe pieces of say hematite to cultural practice needs evidence for scraping or grinding. There seems no reason why Neanderthals and modern humans maintained an ancient cultural tradition.

Evidence for early journeys from Africa to Asia

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A fragile consensus has developed concerning the date when fully modern humans left Africa then migrated to all habitable continents. It is based on genetic comparisons among living people, very sparse occurrences of H. sapiens remains that have been dated and on the environmental pressures in Africa to migrate during the highly erratic deterioration of climate since the last interglacial. The last included a series of abrupt cooling and drying episodes around 118, 110, 86, 75, 71 and 67 ka. That fully modern humans entered the Middle East from time to time between 130 and 75 ka is backed up by actual fossils, but most palaeoanthropologists believe that they moved no further, because of the growth of surrounding deserts, and probably did not return until around 45 ka. The consensus for the decisive move out of Africa to Eurasia is that it was via the Straits of Bab el Mandab at the entrance to the Red Sea, when sea level fell to a level that would have allowed a crossing by rafting over narrow seaways. The most likely was during the brief 67 ka cool/dry episode that coincided with an 80 m fall in global sea level: the largest since the previous glacial maximum. This would fit the earliest dates of fully modern human remains in Asia and Australasia. There had been falls of more than 50 m around 110, 86 and 75 ka, each followed by rising sea level. Each of them accompanied by cooling and drying in Africa conceivably could have allowed earlier migrations from Africa to southern Arabia. Emerging data seems set to complicate matters.

At a conference in Gibraltar during September 2009 (Balter, M. 2009. New work may complicate history of Neandertals and H. sapiens. Science, v. 326, p. 224-225) there were further reports of stone tools, which apparently resemble those of a similar age from Africa, beneath the 74 ka Toba ash in South India, and dated between 70 to 80 ka old in the Yemen and United Arab Emirates. Even more challenging are reports of archaic H. sapiens teeth and a jawbone with a chin – a sure sign of a fully modern human – from cave sediments in southern China that yield a date of about 110 ka (Stone, R. 2009. Signs of early Homo sapiens in China. Science, v. 326, p. 655). Given an opportunity and a need humans do tend to move in order to survive, a proclivity that would undoubtedly be boosted by our insatiable curiosity: after all H. erectus, antecessor and neanderthalensis all made tremendous migrations starting more than 1.6 Ma ago.

 

Fungal clue to fate of North American megafauna

More than 30 large mammal species, including elephants and giant sloths, that had roamed North America during the Pleistocene met their end between 13 and 11.5 ka. Whether or not predation by newly arrived humans caused these extinctions remains unresolved, as do the triggers for coinciding changes in plant communities and evidence for increased burning of biomass. While the ages of fossil bones are direct evidence for species being present, they are not found everywhere that a megafauna likely lived and occurrences are patchy in time. There is however a proxy for the presence or absence of large herbivores: spores of fungus that thrived on their dung (Gill, J.L. et al. 2009. Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America. Science, v. 326, p. 1100-1103). Sporormiella can only complete its life cycle after herbivores have digested plant matter. So its spores in sediment cores form an impressive link to the local presence of herds. In a lake core from New York State such fungal spores, having been much more abundant beforehand, fell to less than 2% of all spores and pollen about 13.7 thousand years ago. This suggests that large herbivores vanished from this area at that time. Interestingly, the timing is during a warm period (the Bølling-Allerød) rather than the stress of the Younger Dryas glacial re-advance. Moreover, the local disappearance predates the first signs of Clovis people, although there is evidence for earlier human colonisers back to 15 ka. It is possible that it was the disappearance of large herbivores that allowed the development of extensive mixed coniferous-deciduous woodland, broad-leaved trees having perhaps been browsed severely by earlier herbivores.

Climate-CO2 links since the Miocene

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The November 2009 issue of EPN (Boron isotopes and climate change) described how the 11B/10B ratios of planktonic forams correlate with the pH of seawater, and thus with the amount of dissolved CO2 that increases acidity. In fact the more easily analysed ratio between the boron and calcium contents of forams does the same, and for the last 800 ka correlates with the measured CO2 content of bubbles in Antarctic ice, which itself correlates very well with temperatures and sea levels (Tripati, A.K. et al. 2009. Coupling of CO2 and ice sheet stability over major climate transitions of the last 20 million years. Science, v. 326, p. 1394-1397). Extending this approach back to 20 Ma shows that in the Middle Miocene (~10 Ma) when glacial cover began to expand atmospheric CO2 fell from levels similar to those of the present day (387 ppm) to approximately those of the pre-industrial Holocene (~250 ppm). In the earlier Miocene from 14 to 20 Ma global mean surface temperatures were 3-6º C higher and sea level stood 40 m higher than at present. As well as this grim reminder of a possible future, the data support the general notion of a coupling between atmospheric CO2 and global climate.

Was the Archaean blazing hot or balmy?

Silica-rich sediments, notably cherts have been used to estimate ocean temperatures in the far off Archaean Eon. This is possible because SiO2 and water exchange oxygen atoms as the silica mud is forming, and in doing so its two main stable isotopes (18O and 16O) are preferentially treated depending on water temperature. The cooler it is the more 18O ends up in silica. Early Archaean cherts commonly show lower δ18O values than silica-rich ocean sediments forming now, so much lower that the temperature of Palaeoarchaean seas has been judged to have been between 55 to 85º C. Discomfortingly hot for bathers, and not very plausible considering that without a CO2­-rich atmosphere Archaean oceans would have been frozen solid because the Sun emitted much less energy than it does now. However, such estimates have to assume that the oxygen isotopic composition of seawater at 3.5 Ga was the same as now, when in fact it is known that environmental δ18O probably changes over long time periods. A way of avoiding an untestable assumption is to measure the isotopic composition of hydrogen (1H and 2H or D) in chert as well as that of oxygen. The cooler water is, the lower δD values are in silica that is precipitated from it.  Ordinary quartz contains no hydrogen except in unstable fluid inclusions, but the way chert forms as colloidal precipitates of opal-like material locks hydrogen in the form of OH– ions into its silica (Hren, M.T. et al. 2009. Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago. Nature, v. 462, p. 205-208). Combining the two measures for 3.42 Ga cherts from the famous Barberton Mountain Land Archaean complex results in a sea-surface temperature estimate of no more than 40º C.

Mid-continent earthquakes: warnings or memories?

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Perhaps the most infamously unexpected earthquake was that of 17 December 1811 that shook the historically quiescent middle Mississippi valley with an estimated magnitude of 7 on the Richter scale. The area centred on New Madrid has been resonating with seismic events of lesser magnitude ever since. So too has the area around Charleston, South Carolina on the passive Atlantic margin of the USA, which experienced a magnitude 7 earthquake in 1886. Geophysicists now know to expect major earthquakes at some time in some place along active plate margins, especially subduction zones and boundaries dominated by strike slip motion, although prediction is an art to be learned if indeed it will ever be possible. Yet even small tremors far from plate boundaries within continental parts of plates are a continual worry. The shock of totally unexpected devastation in New Madrid and Charleston makes seismic-risk assessors mark the card of any such events, especially if repeated. Ideally, plate interiors should be rigid and safe. The magnitude 7.9 Sichuan event in May 2008, which caused more than 80 thousand deaths along a fault with no history of activity, reinforced worry. All three examples were situated in areas with old faults, of which most areas of continental crust have plenty, though some are hidden. Somehow tectonic forces had built up and eventually they failed.

Protracted activity might seem to foretell more big ‘quakes. However, it now appears that faults in continental interiors behave very differently from those at plate boundaries: aftershocks, even some with magnitude 6, continue for centuries in the first case, but only for a few years or decades at tectonically active margins (Stein, S. & Liu, M. 2009. Long aftershock sequences within continents and implications for earthquake hazard assessment. Nature, v. 462, p. 87-89). The duration of aftershocks in inversely related to the tectonic load sustained by faults. A lesson suggested is that assigning high risk to continental areas with repeated seismicity overestimates the dangers. But does this mean those seismically stable areas in continental interiors pose underestimated risks? The answer is probably ‘Yes’, if they are near to old faults. That is not to say that the Caledonian and Variscan structures that divide Britain into many small blocks are about to ‘go off’ at any time. Some do generate small, noticeable tremors such as that beneath Market Weighton in east Yorkshire at 1 am on 27 February 2008 that woke people up to several hundred kilometres away (including me). Market Weighton was an area of reduced subsidence during Jurassic sedimentation, as a result of flanking Variscan faults in the crust beneath. However, if large structures – high-rise buildings, bridges, dams and power stations – are planned, it would be wise to look in detail at local faults. One approach is to map disturbance of superficial sediments that in Britain would show activity over the last 18 to 11 thousand years since ice sheets melted. Another is to check bedrock geology for the last major movements on faults. It may become possible to develop models of seismic cyclicity for all large structures to give realistic assessments of risk in the future.

See also: Parsons, T. 2009. Lasting earthquake legacy. Nature, v. 462, p. 42-43.

Late formation of Earth’s atmosphere

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Because the Earth’s mantle is rich in volatiles which escape from magmas that reach the surface, it has long been assumed that our planet’s atmosphere was self-produced by exhalation. But it turns out that noble gases in such exhalations do not match those in the atmosphere isotopically (Holland, G. et al. 2009. Meteorite Kr in Earth’s mantle suggests a late accretionary source for the atmosphere. Science, v. 326, p. 1522-1525). Greg Holland and colleagues from the Universities of Manchester and Houston measured krypton and xenon isotopes in volcanic CO2 emissions from New Mexico, and found that their proportions matched those in carbonaceous chondrites as does the Kr/Xe ratio. Those in the atmosphere are significantly different, resembling the values in the Sun. Comets may have delivered these gases after the original accretion of the Earth and the catastrophic formation of the Moon.