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.

Geochemical clue to environmental effects of large igneous provinces

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Several flood volcanism events seem to link to mass extinctions, and they have been seen as the culprits for global environmental change. Since flood volcanism is outside human experience, geologists have little conception of what they do other than amass up to millions of cubic kilometres of lavas both mafic and silicic. They all probably emitted CO2 and contributed to global warming, but whether they are able to deliver sulfate and particulate aerosols to the stratosphere to trigger cooling is hard to judge. But it seems there is a proxy for their global influence (Peate, D. 2009. Global dispersal of Pb by large-volume silicic eruptions in the Paraná-Etendeka large igneous province. Geology, v. 37, p. 1071-1074). Lead is potentially a volatile element that would accompany large volcanic gas and dust emissions, and it also bears unique isotopic signatures. Lead isotope proportions in sediments in contemporaneous marine sediments could be matched with those of large igneous provinces (LIPs). Should their signature occur globally, then it would be a fair bet that the products of volcanism did reach cloud-free stratospheric altitudes, there to be mixed globally and to remain aloft for many years. Below the tropopause gas and dust would soon be rained out, so that signatures would remain local.

Dave Peate of the University of Iowa found that the 208Pb/204Pb and 206Pb/206Pb ratios of 132 Ma sediments from an Ocean Drilling Program core in the mid-Pacific fall in the same field as those of the Paraná-Etendeka large igneous province. The sediments occur just below and within a prominent δ13C anomaly that geochemists believe to signify a major change in the biosphere, and the site is almost at the antipode of the Paraná-Etendeka large igneous province. Sediments from below the shift in carbon isotopes show lead-isotope ratios that can be explained by derivation from the oceanic crust underlying them, whereas those that witness a profound change in the biosphere overlap with the field of the P-E LIP. Specifically, they match the lead ‘signature’ of silicic volcanics rather than basalts, and in particular those with low titanium contents. So it seems that in this case basalt floods may not have been implicated in global environmental change, but the much less voluminous but probably far more violent ignimbrite do seem likely culprits. There were more than 20 such events within an interval of less than 2 Ma that emitted >100 km3 of silicic magma, most exceeding 1000 km3.

The ‘real’ Flood

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At the end of the Miocene tectonic uplift in the region of the present Straits of Gibraltar cut the Mediterranean Sea off from the Atlantic. The only water able to flow into the isolated marine basin was that carried by the major rivers: the Rhône, Danube, Dneiper and Nile. Their volume was exceeded by evaporation, so the Mediterranean became more and more salty, eventually almost drying out completely to leave thick evaporite deposits that still underlie its deepest parts. 5.33 Ma ago, the tectonic barrier was breached so that Atlantic water flooded the whole Mediterranean basin. The Zanclean flood at the start of the Pliocene has been rated as the greatest catastrophic event in the Phanerozoic history of the oceans, but just how dramatic it was has previously only been guessed at. Seismic profiles across and along the line of flooding reveal channels several kilometres across, about 200 km long and up to 250 m deep, now filled with debris (Garcia-Castellanos, D. et al. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, v. 462, p. 778-781). Using a well-established model of river incision in mountain rivers, the authors have suggested how the flooding proceeded. From an initial trickle when the original barrier subsided below Atlantic sea level, flow grew exponentially over a few thousand years to about three times that of the modern Amazon discharge (~108 m s-1), at which rate incision reached more than 0.4 m per day. Around 90% of the Mediterranean basin’s entire volume was flooded in a matter of a few months to two years, sea level rising at up to 10 m per day.

Formation of BIFs halted by Sudbury impact

The peculiar story of banded iron formations (BIFs) is one that ‘runs and runs’, as journalists say. Most of the steel on which North American capital was built comes from gigantic BIF deposits around Lake Superior that formed during the Palaeoproterozoic. Apart from a brief return in the Neoproterozoic, associated with conditions peculiar to ‘Snowball Earth’ conditions, the Superior Province BIFs are the last of any consequence. Most geologists look to a gradual shift in the oxygen content of ocean water as photosynthetic life grew to dominate the Earth after about 2.4 Ga, but the BIFs around Lake Superior turn out to be capped by a blanket of ejecta from a massive extraterrestrial impact that formed the Sudbury Complex (Slack, J.F. & Cannon, W.F. 2009. Extraterrestrial demise of banded iron formations 1.85 billion years ago. Geology, v. 37, p. 1011-1014). But how could even a monstrous bolide have changed ocean chemistry so decisively? John Slack and William Cannon of the US Geological Survey believe that the impact was so violent that it resulted in wholesale mixing of oxygen-bearing surface waters with those of the deep ocean. The evidence they cite is a coincident change in the nature of deep-water hydrothermal deposits from sulfide-bearing to those dominated by iron-oxides.

The Sudbury impact produced a crater around 150 to 270 km across (one of the three largest known on Earth), and it is dominated by remelted basaltic rocks so almost certainly struck the Palaeoproterozoic ocean floor. Its ejected debris probably covered almost 2 million km2 and is found up to 800 km from Sudbury, Ontario. Yet, even with impact cavitation and massive tsunamis it seems barely feasible that an impact of a size dwarfed by those of the Lunar surface could completely remix the oceans. However, it is likely that in the Palaeoproterozoic continental crust was gathered together in a supercontinent so that tsunamis could scour much of the surrounding ocean. A plume of vaporised seawater may also have scavenged oxygen from the atmosphere. The evidence seems compelling, and another possibility is that Sudbury was not the only impact site…

And another oddity…

That a major climatic warming occurred at the end of the Palaeocene (55 Ma) is now undoubted, as is its probable cause by emission from the ocean floor of vast amounts of methane. Yet oddly the Palaeocene-Eocene Thermal Maximum (PETM) coincides with a brief geomagnetic reversal 53 ka long (Lee, Y.S. & Kodama, K. 2009. A possible link between the geomagnetic field and catastrophic climate at the Paleocene-Eocene thermal maximum. Geology, v. 37, p. 1047-1050). Both events were short, so a coincidence seems unlikely, in the authors’ opinion. They suggest a connection through the massive power imparted to climatic processes by the PETM (at least a terawatt and perhaps orders of magnitude more), including the deep thermohaline circulation of the oceans that did shift during the event. Had they exceeded a threshold power for circulation of the liquid outer core they may have triggered the brief reversal, which quickly reverted to its previous magnetic polarity. Ths association is not unique, detailed magnetic studies of the K-T boundary event at 65 Ma has revealed a similar short reversal spanning the duration of the iridium peak ascribed to the Chicxulub impact. However, Chicxulub delivered a power of the order a year’s solar radiation in about one second: vastly larger than the climate perturbation of the PETM. Are we seeing here a hidden signal of an extraterrestrial impact behind the methane release? Impacts are no longer as popular as they once were…

Dating subduction

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The most distinctive products of the high-pressure, low-temperature metamorphism along subduction zones are stunningly coloured blueschists formed from ocean-floor basalts, their colour deriving from the sodium-rich amphibole glaucophane. Yet the defining mineral for subduction-zone metamorphism is lawsonite, which takes up the calcium from plagioclase feldspar that becomes unstable. Having formed at depths of up to 100 km, blueschists found at the surface had to rise slowly from mantle depths after metamorphism. Consequently, it is nearly impossible to unravel the date of their formation from those of later events. Being basaltic, blueschists also lack the usual elements whose unstable isotopes are commonly used for radiometric dating: potassium, rubidium, uranium and thorium. However, they do contain rare-earth elements, an isotope of one (176Lu) being unstable. Applying the Lu-Hf dating method to lawsonite ties down precisely when basalts achieved the narrow P-T range at which lawsonite forms (Mulcahy, S.R. et al. 2009. Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes. Geology, v. 37, p. 987-990). Sean Mulcahy of the Unigversity of Nevada and colleagues from Washington State chose a sample from the type locality for lawsonite discovered in the late 19th century by Andrew Lawson: the Franciscan blueschists of the Tiburon Peninsula in California. The Franciscan Complex formed during subduction at 145.5 Ma.

Phew, there is a mantle plume under Hawaii after all

Along with constructive and destructive plate boundaries volcanic hotspots within plates and sometimes at plate boundaries epitomise modern Earth science. Assuming that they are fixed points of reference allows the absolute motions of tectonic plates to be worked out, although it seems that some do move around. The evidence for hotspots being fixed or at least moving much more slowly than do plates are the chains of extinct volcanic islands or seamounts that extend away from active volcanic centres in the direction of plate motion. The most debated aspect of hotspots is whether they stem from processes in the upper mantle just beneath the asthenosphere or are the heads of cylindrical plumes of hot mantle that rise from the region next to the outer core. Seismic tomography has been claimed capable of resolving between the two possibilities, but its spatial resolution depends very much on the spacing of seismometers that provide the data that tomography subjects to highly complex processing. Some have claimed that the resolution of early tomography lends itself to producing artefacts that look like sought-after mantle structures (see Geoscience consensus challenged in EPN of December 2003).

One hotspot that has all the characteristics of a plume head, but which seismic tomography has been unable to detect is the volcanically active Big Island of the Hawaiian chain. The response to that somewhat embarrassing failure has been to deploy 30-odd seismometers on the seabed immediately around Hawaii and then to shift them to a wider spacing further from the island between 2005 to 2007. Together with 10 stations on the islands themselves, the array recorded 2146 S-wave arrivals from 97 earthquakes (Wolfe, C.J. et al. 2009. Mantle shear-wave velocity structure beneath the Hawaiian hot spot. Science, v. 326, p. 1388-1390). The results are reassuring, for the show in detail that indeed there is a vertical zone of low S-wave speeds indicating hotter, less rigid mantle that extends down to at least 1200 km. It is several hundred kilometres across, and is indeed a plume surrounded by a ‘tube’ of colder more rigid mantle.

See also: Kerr, R.A. 2009. Sea-floor study gives plumes from the deep mantle a boost. Science, v. 326, p. 1330.

Hot tectonics in the Archaean

The first thing that strikes you when looking at a small-scale geological maps of many deformed Archaean terrains – most of them are deformed – is how different they seem compared with those of later aeons. Bulbous granitic plutons separate slim and irregular, sometimes cusp-adorned areas of volcanic and sedimentary rocks. This is classic granite-greenstone terrain. Many geologists who have worked on Archaean rocks find it hard to pin down signs of ‘modern’ plate tectonics and the typical orogens of continent-continent collision zones, yet non-uniformitarian ideas on Archaean tectonics have become passé in the last 25-30 years. That seems odd, considering that the Earth’s internal heat production by radioactive decay must have been higher as less radioactive U, Th and K isotopes would have decayed in the very distant past. Convective mantle flow would have been faster, lithosphere would not have been so thick as now, and plates would have moved more rapidly in order that radioactive heat and that left over from early accretion and the Moon-forming event could escape. Whichever way one looks at such a scenario – plates as big as modern ones or more small plates – there is no escaping that younger, warmer lithosphere would have re-entered the mantle. Geochemistry of Archaean granitic rocks is so different from those of later aeons that their formative processes must have differed too. Quite probably descending basaltic crust would not have dehydrated to produce eclogite under low-T, high-P conditions, and that would prevent steep subduction, so that slab-pull may not have been the driving force for Archaean tectonics.

Two recent papers refresh the idea that the present is not entirely a key to the Earth’s Archaean past. One suggests an entirely alien kind of orogenic activity: that of very hot deformation of weak lithosphere (Chardon, D. et al. 2009. Flow of ultra-hot orogens: A view from the Precambrian, clues for the Phanerozoic. Tectonophysics, v. 477, p. 105-108). Dominique Chardon of the Université de Toulouse and colleagues from the Université de Rennes, highlight the dominance in orogens of the Archaean and early Proterozoic of ductile deformation imposed on massive accretion of magma produced by mantle processes, compared with the dominantly brittle style that dominates modern, cold orogens. Accumulated radiometric dating of the main building material of the continents – diorites and grandiorites – indicates that the 1.5 Ga of the Archaean witnessed the formation of not only the earliest continental crust but most (65%) of the rest of it. A summary of an emerging explanation for explosive continent production appeared in the first 2010 issue of Scientific American (Simpson, S. 2009. Violent origins of continents. Scientific American v. 302(1), p. 46-53). This rests on rapidly growing evidence, much unearthed by Andrew Glikson of the Australian National University, for the influence of major impacts that flung debris far and wide and perturbed the mantle’s thermal structure on a massive scale (Glikson, A. 2008. Field evidence for Eros-scale asteroids and impact forcing of Precambrian geodynamic episodes, Kaapvaal (south Africa) and Pilbara (Western Australia) cratons. Earth and Planetary Science Letters, v. 267, p. 558-570). Beds of impact-related spherules are turning up throughout Archaean greenstone-belt sequences. There are also megabreccias that could be debris lifted by tsunamis vcaused by impacts in the Archaean oceans. Glikson has demonstrated that the timing of such evidence closely matches that of magmatic outbursts that created continental crust. He has proposed that the thermal effects of the large impacts set in motion or deflected a large number of convective mantle plumes that drove the necessary magmatism.

BIFs and bacteria

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Banded iron formations from the late Archaean, Palaeoproterozoic, and in a few short time intervals linked with Neoproterozoic tillites, have long fascinated geoscientists with their counterintuitive occurrence at times when the oceans contained little if any oxygen. Anoxic water allows iron to exist in its Fe2+ form, thereby able to dissolve readily. The vast thicknesses and masses of BIFs demands an abundance of mobile iron, but being made predominantly of hematite (Fe2O3) their formation requires a balancing superabundance of oxygen. Many geochemists believe photosynthesising blue-green bacteria to have excreted oxygen to oxidise soluble iron to Fe3+ and precipitate it as the oxide in shallow water. Yet plenty of BIFs show such delicate banding that deep water is implicated. All the BIF paradoxes would be resolved if another mechanism had caused the oxidation and precipitation of iron. A new clue to what that may have been is the discovery of iron-oxide stromatolites in the monster BIF deposits around Lake Superior (Planavsky, N. et al. 2009. Iron-oxidizing microbial ecosystems thrived in late Paleoproterozoic redox-stratified oceans. Earth and Planetary Science Letters, v. 286, p. 230-242). Iron isotopes and rare earth elements are good indicators of redox conditions, and those in the BIFs indicate anoxic waters, so free oxygen was not available. The stromatolites, however, strongly suggest biogenic precipitation of iron oxide, which is possible through the action of specialist Fe-oxidising bacteria. Indeed, filamentous microfossils occur in the stromatolites. That opens the possibility of BIFs having formed by direct bacterial precipitation in the oxygen-free world before the Great Oxidation Event around 2.2 Ga, in the absence of cyanobacteria.

Fast-moving rhyolite magma

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Highly fractionated, silica-rich magma poses the greatest danger of explosive volcanic eruption, characterised by glowing pyroclastic flows that produce the strange rock ignimbrite. For example, in the Andes, ignimbrites extend for large distances from the calderas that emitted them. Fortunately rhyolite eruptions are rare, but that poses a scientific problem – they have not been as well studied as more common magmatic phenomena. Until May 2008 the latest rhyolite eruption had been in Alaska during 1912. In 2008 the Chilean volcano Chaitén erupted for the first time in 9 thousand years. There was no warning. Andesitic and dacitic volcanoes are restless for months before an eruption, though that is not much comfort as exactly when they ‘go off’ is still unpredictable. But any warning helps prepare local populations for the worst. A volcanoes precursory rumblings and shakings reflect the slow upward movement of magma. In the case of Chaitén, magma rose at about 1 m s-1 that flabbergasted the volcanologists who rushed to study such a rare event (Castro, J.M. & Dingwell, D.B. 2009. Rapid ascent of rhyolitic magma at Chaitén volcano, Chile. Nature, v. 461, p. 780-783). The magma rose 5 km from its source in less than 4 hours. It is generally thought that the more silicic magma is, the more viscous and sluggish, which is certainly the case for rhyolite when it emerges: the melting of impurities in a coal fire produces a very silica-rich melt but such slag certainly does not dribble out of the fire box to pool on the hearth. High viscosity allows an erupting magma to retain gas escaping from solution as pressure drops, which is the source of the catastrophic blasts of massive ignimbrite events. Below the surface the Chaitén magma behaved in an extremely fluid manner, perhaps because it contained so much dissolved gas that it became a fluid froth at quite shallow depth. This unique observation is deeply disturbing for populations living in areas blanketed by ancient ignimbrites, as in the Andes. The very worst terrestrial events imaginable are ignimbrite eruptions that can blast out at such high velocities as to groove the ground and carry over thousands of km2 in matter of minutes. Without warning, there is no escape.

Wenchuan earthquake (May 2008) analysed

On 12 May 2008 a magnitude 7.90 earthquake killed more than 80 thousand people and left many more injured and homeless in the Wenchuan area of Sichuan province China. In the worst affected areas up to 60% of the population were killed. The catastrophe occurred at the densely populated western boundary of the Sichuan basin with the Tibetan Plateau, and involved surface displacement that propagated rapidly north-eastwards along a 235 km long zone. There was virtually no warning sign and although crossed by major faults, high-magnitude seismicity was a rarity in the area. Several satellites now repeatedly deploy synthetic aperture radar sensing along their ground swath, so that interferometric methods (InSAR) are able to assess ground motions between separate times of overpass, with sub-centimetre precision. Together with direct measurement of motions at GPS ground stations, InSAR allows an unprecedented ‘post-mortem’ of this dreadful event (Shen, Z-K et al. 2009. Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchauan earthquake. Nature Geoscience, v. 2, p. 718-724). The structural architecture of the surrounding area is of five fault-bounded blocks that jostled during the event, resulting in profound shifts in the geometry of motion along two parallel faults that ruptured. The event was so sudden and large because what would otherwise have been barriers to propagation of strain failed at the same time. All the strain cascaded through several fault segments. This is not a scenario that could have been easily predicted, the authors judging it to have been a once-in-4000 years concatenation of crustal failure.

Seismic unpredictability is something that seismologists now recognise (Chui, G. 2009. Shaking up earthquake theory. Nature, v. 461, p. 870-872). Active faults turn out not to be ‘creatures of habit’, and nor can we assume that long-quiet segments are the most likely to fail in future. Ominously, there is a growing body of evidence that great earthquakes are able somehow to trigger others, often far distant. An example is the giant Sumatra-Andaman event of 26 December 2004, tsunamis from which caused a toll of hundreds of thousand lives around the Indian Ocean. It was followed quickly by swarms of small tremors on the San Andreas Fault 8000 km away. Rapid successions of great earthquakes around the world, such as the October 2005 Pakistan earthquake 9 months after that in the Indonesian area, can no longer be regarded as ‘bad luck’. Seismic waves are able to weaken far-off segments of active faults.

‘Follow the water’

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Long, long ago an anonymous Roman wrote, ‘The first provision of any civilised society, after a code of law, is a reliable source of clean water’. Personally, I think the phrase ‘legalised bureaucracy’ in Latin was mistranslated to ‘code of law’. Whichever, planetary and life scientists might well like the adage for themselves: the sentiment applies nicely to active planetary tectonics and to the origin and survival of all conceivable life forms. The Earth has plenty of water at the surface and deep in the mantle. Without the second, the main mantle mineral olivine would be too stiff for the mantle to convect. Heat would build up within until magma formed in great abundance and emerged with a dreadful growl, as it did on Venus about 750 million years ago to repave the entire planet. It simply isn’t possible to think of answering the questions, ‘When did plate tectonics begin and life emerge?’ – let alone ‘How?’ – without first addressing where the Earth’s water came from and when our home world become so richly endowed.

In a very practical sense, these are the most important issues in geochemistry. Francis Albarède, of the École normale supérieure de Lyon, President of the European Association for Geochemistry and the first geochemist to deploy a multicollector, inductively coupled, plasma-source mass spectrometer, is a fitting person to review where the subdiscipline stands on them. (An MC-ICPMS is a tool for which many still yearn hopelessly.) His views appeared as a ‘Progress’ (a rare kind of Nature article) in the 29 October 2009 issue of Nature (Albarède, F. 2009. Volatile accretion history of the terrestrial planets and dynamic implications. Nature, v. 461, p. 1227-1233). The article casts doubt on the long-held views that when the Moon formed after a giant impact on the Earth, both bodies lost huge masses of volatiles, including water, and that Earth’s water-rich nature stemmed from repeated bombardment by volatile-rich comets up to about 3800 Ma.

Geochemical data are now available from a comet (Hyakutaki) and it contains twice the amount of deuterium relative to hydrogen that is in terrestrial seawater. The D/H ratio of carbonaceous chondrite meteorites is more Earth-like, and these primitive objects seem a more likely water source than comets. But did cataclysmic formation of the Earth-Moon system dehydrate both bodies and drive off other volatile matter? Planets and smaller bodies formed by gravitational accretion of solids that condensed from the initially hot gas or nebula that dominated the proto-solar system. Experiments show that condensation of the elements occurs in three discrete temperature ranges, separated by ranges in which few elements condense. Above around 1300 K the most refractory elements condensed, including oxides of some elements (Ca, Fe, Mg, Si) that now make silicate minerals, including the dominant mantle mineral olivine. Between 900-1200 K the alkali metals and some of the elements (chalcophile) that readily combine with sulfur emerged in solid form. In the third step from 500-800 K the more volatile chalcophile elements, including lead, and halogens condense, leaving four (Hg, O, N, C) that can take on solid form only below about 300 K. Interestingly, the proportions of volatile elements relative to refractory ones in the Earth, Moon and Martian meteorites are very low compared with those in carbonaceous chondrites. It is likely that volatile elements only accreted to the Inner Planets in small amounts before being swept to the outer reaches by an intense solar wind as the Sun was powering up, i.e. before nebular temperatures had fallen below about 1000 K. From that stems the inescapable conclusion that none of these planets were endowed with much water in their earliest forms.

Proportions of the lead isotopes 206Pb and 204Pb from terrestrial sulfide mineral deposits define a near-perfect linear relationship with the ages of mineralisation, from which an age can be estimated for the time the element lead appeared on Earth. That age is 4400 Ma; about 110 Ma younger than the actual age of the planet, and matches apparent ages derived from I-Xe and Pu-Xe decay schemes; iodine and xenon are volatile elements. This strongly supports the idea that 500-800 K condensates arrived late, and other evidence indicates that they and water ice were delivered by carbonaceous chondrite material falling towards the Sun from far beyond the orbits of the giant planets, once the early solar wind had lessened. That is, the Earth’s oceans formed very early in its history, and the mantle gained its water from them once hydrated lithosphere could founder deep into the evolving mantle by subduction. Albarède also summarises fascinating new ideas about the different course followed by Venus and Mars from essentially the same starting point. His ‘Progress’ is not difficult to read, and by marking the start of a new consensus in planetary evolution is of vital interest to all Earth scientists

Extraterrestrial water is also the subject of a Great Quest by NASA and other space agencies, though sadly an attempt on 9 October to prove that there is ice on the lunar surface, by hurling a US$79 million spacecraft at an obscure polar crater, produced no sensible results. Ironically, a couple of weeks later, three papers appeared in Science that document passive remote sensing evidence that the Moon contains a lot more water than long assumed (the most revealing is: Pieters, C.M. and 28 others 2009. Character and spatial distribution of OH/H2O on the surface of the Moon seen by M3 on Chandrayaan-1. Science, v. 326, p. 568-572). The Apollo samples  astonished geologists when they proved to be almost completely anhydrous, any signs of minor hydration being ascribed to contamination after collection. The Moon Mineralogy Mapper (M3) aboard India’s first lunar mission Chandrayaan is a hyperspectral imaging device that operates in the visible to SWIR range of EM wavelengths (0.4 – 3.0 mm). That range includes SWIR wavelengths beyond 2.4 mm where OH–, water and water ice have large absorption features that are masked in terrestrial remote sensing by the high moisture content of Earth’s atmosphere. Pieters et al. attempted to model hydroxyl and water content in the lunar surface, and discovered significant amounts (a few tenths of a percent) in the polar regions. That they got results when the Moon was fully illuminated by the Sun suggests that this is not due to ice hidden from heating in shadows, but to minerals that contain molecularly bound water and hydroxyl ions. That begs the question of how the water got there. One possibility is the late arrival of volatile condensates as above, another that it is due to hydrogen (protons) from the solar wind reducing iron in silicate minerals to metallic iron and combination with the oxygen released. Expect loud hurrahs from devotees of Star Trek and NASA because one prerequisite of civilised society seems to be there on the Moon. But judging from the bureaucracies involved in space, getting the funds to use it will not be easy.