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.