Growing evidence for ‘hobbits’

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Various shenanigans within the Indonesian palaeoanthropology community have hindered evaluation of all the evidence surrounding the diminutive adult female skeleton found in Liang Bua cave on Flores in 2003.  Her skull was damaged after prolonged examination by a leading national figure in the science, and now further excavation in the cave has been blocked indefinitely. Whether she is indeed a member of new species of hominin, Homo floresiensis, or merely an individual modern human dwarfed by some genetic defect, as some claim, seems closer to resolution (Morwood, M.J. and 10 others 2005.  Further evidence for small-bodied hominins from the Late Pleistocene of Flores, Indonesia. Nature, v. 437, p. 1012-1017). During the 2004 field season at Liang Bua the Australian-Indonesian team unearthed remains of nine other individuals of similarly diminished stature. They included another jaw bone that is virtually identical to that of the first ‘hobbit’: neither have the chins that unify all fully modern humans.  Significantly, the new piece of lower jaw is dated at some 3 ka older than the original, so the chances of both being from physiologically unfortunate modern humans are remote.

The new finds also include stone tools, more advanced than any found in association with one of H. floresiensis’s possible ancestors, H. erectus.  Whoever they were, the ‘hobbits’ also butchered prey and cooked meat.  There is negative evidence in support of the new species hypothesis too: compared with human sites of the Late Pleistocene, Liang Bua is conspicuously lacking in evidence for any form of art. But the idea is not proven.  It would take a definite association between fossils and tools, as for instance in a burial, to show that the implements belonged to ‘hobbits’ rather than having been introduced by a fully human visitor. Moreover, should any evidence for moderns be found in Liang Bua or other caves of interest, the possibility of mixture of cultures and fossils would leave things up in the air.

It is worth noting that Indonesian scientists are not the only ones prone to obstructive tactics as regards hominin sites. They have long been a bone of contention throughout Africa, where both local and visiting scientists have tried to throw spanners in their colleagues’ research ambitions.

See also: Dalton, R. 2005. More evidence for hobbit unearthed as diggers are refused access to cave. Nature, v. 437, p. 934-935; Lieberman, D.E. 2005. Further fossil finds from Flores. Nature, v. 437, p. 957-958.

Congenital disease, human migration and population growth

The way in which genetic features are inherited has become a key feature in distinguishing human populations, the time and route of their migrations as separate groups, and when they merged with other groups.  The most familiar outcomes are those based on mitochondrial DNA and lines of female descent that show with little room for manoeuvre, that all of us descend from Africans alive around 150 to 200 ka.  Studies of the male Y chromosome help fine tune the record to show short periods when either populations fell so low that human survival passed through only a few small bands (e.g. around 70 ka) or Big Men corralled most women for their own purposes (the now famous case of Ghengis Khan’s genes still dominating the genetics of Central Asian people). Dennis Drayna of the US NIH outlines yet another revealing feature of genetics with historical connotation in the October 2005 issue of Scientific American (Drayna, D. 2005. Founder mutations. Scientific American, v. 293(4), p. 60-67).

Disabling congenital diseases, such as cystic fibrosis and sickle-cell anaemia, together with adverse reaction to alcohol and the ability of adults to tolerate the lactose in milk, are all passed down generations in different ways. Understanding the genetic processes involved obviously stems from medical research on genetic mutations so as to identify groups that are at risk.  From it has emerged details on the structure and location of the responsible genes in chromosomal DNA.  The feature that unites the four examples above is a special repetition of the same kind of mutant structure. Inherited conditions involve either different mutations in a single gene, or the identical change at a specific location.  Of the latter, it seems the most common is an innate tendency in DNA for the same mutation to affect a specific gene – so called ‘hot-spot’ mutation, which occurs in unrelated individuals.  More rare is a defect that is embedded in a length of DNA (a haplotype) whose structure is identical in all those who carry the mutation. That common identity suggests that the mutation arose once and has been passed down subsequently; a ‘founder’ mutation.

Since a ‘founder’ mutation arose at some time in the past it can potentially be used to trace population history, and so passes into the realm of palaeoanthropology. The fascinating and most useful feature is that the greater the separation in generations from the individual in whom the mutation occurred, the more restricted becomes the haplotype, in terms of its relative length in DNA.  That phenomenon is a consequence of sexual recombination among descendants.  In the founding individual, the whole chromosome is the haplotype, and the mutated part becomes increasingly ‘diluted’ with time.  Measuring its length today harks back to the time of foundation.  What has become clear is that not all founder mutations have any obvious consequence, and instead of being in as few as one millionth of a population, the general case for those causing disability and therefore conferring an adverse effect on natural selection, a few percent of people can carry them. Such abundance indicates either neutral effects or some subtle benefit to fitness.  Diseases ascribed to them appear when both parents contribute the mutation: most are recessive. 

A good example is a mutation of the HFE gene that confers above normal iron absorption, which is a decided advantage in protection against anaemia from iron-deficient diet.  An individual with two copies vastly overcompensates and iron accumulates to deadly levels in their cells.  Studies of its incidence in global populations indicate that it arose in Ireland, western Britain and Brittany and then spread south-eastwards. It appears to be a Celtic trait, although not from their original heartland in Central Europe but at the limit of their original migration more than 2000 years ago. Its haplotype is quite long and suggests a founder around 800 AD.  There are no records of significant late Celtic migrations, and quite possibly the spread was through wide-ranging Vikings who dominated parts of the western British Islands at that time. A more fascinating case is the founder mutation that prevents people who carry it from tasting bitterness.  Most people do experience bitter tastes, and that is very handy for avoiding toxic plants.  About 25% do not.  Maybe the mutation involved conferred some advantage, but the fact is that the haplotype is exceptionally short, representing a foundation at about 100 ka.  It occurs in Africa along with 6 variants of the bitter-taster gene, yet beyond that continent only one taster and the non-taster forms occur commonly.  That tallies with the hypothesis of the major movement out of Africa to populate the rest of the world with modern humans, around 75 ka ago. The surveys go intriguingly further: should descendants of those African migrants have bred successfully and regularly with earlier Eurasian hominins (Neanderthals and Erects), then non-African versions of the bitterness detecting gene ought to be present among non-African populations.  Not one ‘alien’ haplotype has been detected, and this novel approach seems to have lain to rest that particularly intriguing bit of sociology.

A dialogue concerning world-shattering events

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Scottish Gaelic mythology includes the ‘Dread Coruisk’, the largest of the each uisge, or water horses.  “ ‘Tis a thing of which we dinnae care tae speak”, say locals of the Isle of Skye, whose shores it nightly stalks. The same could be said of one of the most daring, and amusing, hypotheses of modern geosciences: that of the ‘Verneshot’ (see Mass extinctions and internal catastrophes in June 2004 issue of EPN).  Phipps Morgan, Reston and Ranero explored the possible consequences of a build-up of volatiles in plume-related magmas at the base of thick continental lithosphere beneath cratons, prior to the eruption of continental flood basalts. The suggested that pressure would eventually result in an explosive release at a lithospheric weak point, followed by collapse above the plume head that would propagate upwards, at hypersonic speeds. Modelling the forces involved, the authors of the novel idea considered that they would be sufficient to fling huge rock masses into orbit.  The notion neatly might explain the circumstances around mass extinctions: coincidence of CFB events; large impact structures, most likely at the antipode of the event; global debris layers containing shocked rock, melt spherules; unusual element suites and compounds (including fullerenes); and enough toxic gas to cause biological devastation.  As with the ‘Dread Coruisk’, little has been said, neither in support nor in dispute over the last year.  My comment at the time was, “As with all departures from “accepted wisdom”, the Geomar group’s ideas will come in for a lot of stick, quite possibly from the fans of giant impacts, who not so long ago were themselves dismissed as “whizz-bang kids” by many geoscientists.

It is good to be proved perceptive once in a while. One of the original butts of adverse opinion in the early days of impact hypotheses, Andrew Glikson of the Australian National University, has been the sole commentator (Glikson, A.Y. 2005. Asteroid/comet impact clusters, flood basalts and mass extinctions: Significance of isotopic age overlaps. Earth and Planetary Science Letters, v. 236, p. 933– 937).  He points out that Phipps Morgan et al. overlooked 6 overlaps of impact clusters and CFBs, three of which were associated with mass extinctions. Rather than adding grist to their mill, he goes on to say that it is the geochemical blend associated with impactite layers that points unerringly to an extraterrestrial source for the mass involved in creating large impact craters, rather than any known terrestrial rocks. Moreover, the extreme shock-metamorphism that is the hallmark of impactites has never been observed near any gas-rich volcanic structure formed by explosive venting.   He returns to the view that impacts of alien origin have sufficient energy to induce large-scale partial melting of the mantle, and thereby generate large igneous provinces.

Unsurprisingly, the original authors were onto Glikson’s comment, in leopard-like manner (Phipps Morgan, J., Reston, T.J & Ranero, C.R. 2005. Reply to A. Glikson’s comment on ‘Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: Are mantle plume-induced lithospheric gas explosions the causal link?’. Earth and Planetary Science Letters, v. 236, p. 938– 941).  First they emphasise that their concept of the tremendous power of a ‘Verneshot’ is not based on the explosive release of volatiles, but on the shock pressures associated with the collapse of ~80 km tall pipes due to gas venting, in a very short period of time. As regards the geochemical blend in impactite-related layers, dominated by iridium yet a dearth of other platinum-group metals, they cite evidence that very similar element proportions are released in the carbon- and sulfur-rich gas phases of plume-related volcanoes, as in Hawaii and Reunion. They are not crustal, but of mantle origin, carried by escaping volatiles, and fall in the field normally said to be meteoritic. Phipps Morgan et al. also dispute the likelihood of extraterrestrial-impact induced magmatism from its statistical unlikelihood – the chances of a one in 100 Ma bolide coinciding with 1 in 30 Ma CFB events is, on their count, 1 in 3000 Ma – and from the standpoint of the powers and work involved.  They agree that indeed there are extraterrestrial impact structures.

Surely, their well-argued idea is worth bearing in mind and considering as evidence continues to emerge – they do list a plausible set of characteristics that a ‘Verneshot’ would probably produce. There is some essential philosophy that has a good track record in the history of the geosciences, that of plate tectonics for one: the absence of evidence is not evidence of absense.

Irish mineralising fluids

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One of the most revealing field trips that I ever made was to the now-closed Pine Point lead-zinc deposit in Canada’s Northwest Territories. Being in the company of the late Doug Shearman (dcd. 2003) of Imperial College London helped a great deal, but the evidence exposed in and around the mine reawoke my interest in sedimentary processes that lead to economic mineralisation.  To cut a long story short, Pine Point developed by the passage of Devonian seawater from a vast evaporating basin through a barrier-reef complex, in which a variety of chemical and biological environments, and products of karst formation encouraged the fluid to deposit the metals that it contained on an awesome scale.  Limestone-hosted Pb-Zn ores occur widely in Britain and Ireland in rocks of Carboniferous age, the most familiar to me in the English Pennines being in narrow veins.  The biggest in Ireland, and they are world-class, are more pervasive of the carbonate host.  How they formed has been conjectural and based on geological relationships in what is a small area by comparison with the vast Late Palaeozoic sedimentary systems of the Canadian Shield.  Crucial large-scale evidence is meagre.  Studying the chemistry of the ore-bearing solution trapped in Irish fluid inclusions reveals a familiar picture (Wilkinson, J.J. et al. 2005.  Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield.  Geology, v. 33, p. 805-808).

Multi-element geochemistry plus strontium and sulfur isotope composition of the included fluids in Irish deposits reveals the signature of considerable concentration of the brines by evaporation, together with their having scavenged metals from crustal rocks as they circulated at depth.  Returning to the surface along fault-controlled conduits, the metal-rich brine seems to have mixed with another. As at Pine Point, the sulfur needed to precipitate metal ions as insoluble sulfide ore minerals was probably supplied as hydrogen sulfide excreted by anaerobic bacteria that reduce sulfate ions in seawater. Doug demonstrated this phenomenon in 1981 with a linen handkerchief soaked in lead acetate solution, which he dipped into a foetid swamp seething with such ‘bugs’ on the Pine Point muskeg. ‘Instant orebody’, he cried, as the hanky turned black from fine galena particles.

Although the Irish Zn-Pb ores are more related to faults than to limestone reefs, nonetheless local geology demonstrates considerable relief on the floor of the shallow Carboniferous sea.  Fully understanding the ‘plumbing’ and the geochemistry requires, as Wilkinson and colleagues suggest, a regional view of Carboniferous tectonics just before Africa collided with Laurussia. Just before that amalgamation, restricted, evaporation-prone basins would have formed.  On a continental scale the circulation of their concentrated brines would have followed active faults systems that reached the shallow sea bed: a great deal more complicated than what is plain to see at Pine Point, given the eye of one of post-WW2 Britain’s lions of geology.

State of the art seismic imaging

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For many decades the primary tool of petroleum exploration has been reflection seismic surveying. As oil has become harder to find, industry has hugely improved means of processing seismic waves that return to detectors and expanded data gathering as a means of showing subtle structures and sedimentological detail.  From individual seismic sections up to the 1970s, seismic surveys have moved towards multiple lines with ever-decreasing spacing as a means of producing 3-dimensional subsurface maps. Until recently the results of 3-D seismics have been glimpsed only rarely by the academic community, but once their commercial usefulness has been exploited they are increasingly becoming accessible.  Richard Davies of the 3DLab at the University of Cardiff, UK and Henry Posamentier of Anadarko Canada provide an exquisite overview of the possibilities for research in the October 2005 issue of GSA Today (Davies, R.J. & Posamentier, H.W. 2005.  Geologic processes in sedimentary basins inferred from three-dimensional seismic imaging. GSA Today, v. 15(10), p. 4-9).  They show examples of derivatives from 3-D seismics, produced by a variety of image-processing techniques as well as the basic seismic processing, which demonstrate the depth to which these data can be interrogated.  Featured are an example of meandering Pleistocene channels beneath the Gulf of Mexico, structures produced by sediment compaction between the Shetland and Faeroe islands in the North Atlantic, and the shapes taken by basaltic sills as they flowed into place.  The graphics are wonderful, and would certainly tempt an IT-literate researcher.  However, no funding agency could afford to commission such revealing surveys, and the geoscience community will always rely on the activities and generosity of the petroleum industry to enter this awesome world.  Some might think of midnight meetings at lonely crossroads or an armful of long-handled spoons.  Yet the potential results far transcend the kind of information one might extract from exposed geology.

Multimedia volcanoes

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Virtual field trips made possible by the considerable ingenuity of their authors are excellent means of taking school children and even undergraduates to places well off limits or resources. Most are available only on CD or DVD, but those on the web are especially valuable for all with sufficient connection speed to use them. A Swiss educational organisation hosts the work of Italian volcanologists Roberto Carniel and Marco Fulle with Swiss teacher Jürg Alean.  They make it possible to experience volcanological life vividly, by ‘visiting’ the famous Stromboli, Ethiopia’s Erta Ale lava lake, explosive Montserrat in the Caribean and others.

Visit http://www.swisseduc.ch/stromboli

Photosynthesis during a ‘Snowball’ epoch

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In Neoproterozoic sedimentary sequences evidence for low latitude glaciation crops up at two and probably several other times; so-called ‘Snowball Earth’ events.  Opinion is divided on several aspects of these events: whether or not they truly coated the Earth in glacial ice; their influence on biological evolution; the processes that started and terminated them.  From a biological standpoint, a completely ice-bound surface – both land and oceans – would have stressed organisms to the extreme.  Marine life (all that there was in those times) may only have survived in a few refuges from the ice, perhaps around submarine hydrothermal vents or in ephemeral sea-ice leads and polynya. If that were so, then these frigid episodes would have created important evolutionary ‘bottlenecks’, from which sprang several adaptive radiations: ‘Snowball’ epochs may have determined the forms and genetic diversity of all later life, especially among the Eucarya, of which we are a part. Probable deep-ocean anoxia would have been particularly stressful for organisms that depend on oxygen.

The key to establishing whether or not Neoproterozoic frigid episodes did bring eucaryan life to the verge of extinction lies in the diversity of life during those periods.  That is not an easy task as all life until just before the Cambrian Explosion was both soft-bodied and minute.  One means of assessing diversity is to study biochemical remnants of cell processes preserved in reduced ocean sediments (Olcott, A.N. et al. 2005. Biomarker evidence for photosynthesis during Neoproterozoic glaciation. Science, v. 310, p. 471-474). Olcott and colleagues studied black shales from Brazil whose age is within that of a frigid episode (740-700 Ma), and which contain textural evidence for abundant sea ice and low temperatures. Recovered biochemical compounds indicate considerable diversity, with a mixture of photosynthetic blue-green bacteria and eucaryan algae, with anaerobic bacteria of several types.  The results indicate open water to allow photosynthesis – although it is possible for light to penetrate several metres of sea ice – together with deeper anoxic waters.  Since the samples span a section almost 100 m thick, it seems this diversity persisted for a long period.  However, the most that it can establish with certainty is that thin sea ice or open water did persist at the low palaeolatitude of late-Precambrian Brazil.  The Neoproterozoic record has abundant, widespread black shales, and quite possibly there are others associated with evidence for glacial events.  The importance of the paper lies in showing that biomarkers can be used as effectively in the Precambrian as in the Phanerozoic, and an expansion of this approach can be expected.

Oxygen and mammalian evolution

So much in the geological history of surface processes depends on either the dearth or the superabundance of oxygen. That is no surprise for a host of reasons, one being that it is the most reactive common element when free of bonds, and another is that the most powerful means of releasing oxygen is the capture of energetic solar photons by the pigments residing at the heart of photosynthesis. To grossly paraphrase James Lovelock, the principal reason for not sending people to Mars to search for life is that the planet’s atmosphere tells us that even if was there, it wouldn’t be very exciting.  Oxygen gas is at vanishing low levels on the Red Planet, even if there is lots locked up in its iron-oxide rich surface.

The greatest event in the history of terrestrial life, apart from its emergence, was exploitation of the means of breaking hydrogen-oxygen bonding in water, which is what common photosynthesis is all about.  It opened the entire planet to life from the restricted, though diverse habitats of most Bacteria and Archaea in the earlier anoxic world.  First, oxygen-excreting cyanobacteria were able to colonise the entire ocean surface, depending on available nutrients. In doing so and generating free oxygen they threatened every other organism that used metabolisms based on other kinds of chemistry: oxygen is highly toxic because of it propensity to grab free electrons.  Balanced by its oxidation of iron in early oceans, severe oxygen stress did not emerge until halfway through Earth’s history.  Once it did become able to accumulate in air and water, all ecosystems faced havoc.  Dominant prokaryotes slunk to rare places of refuge, while others seem to have combined in resisting oxidation. Their creation of the Eucarya that depend completely on available oxygen led, through the emergence of algae and then plants, to an accelerated stoking up of oxygen generation.

Once vegetation began to cloak the land, an extra 30% of the planet’s surface opened new vistas for animals and increased oxygen production and complementary burial of carbon.  Indeed, explosive growth of atmospheric oxygen during the Carboniferous resulted in animal expansion to the air, through ominously huge insects.  The first clearly traced ancestors of mammals seem to have appeared in the Permian, though their descendants only got the chance to dominate once reptiles, especially dinosaurians, lost their grip as a result of the K-T extinction. At the time of a far greater loss of living diversity, at the end of the Permian, it is now clear that in a relatively short time oxygen levels had fallen from their highest to one of the lowest in the Phanerozoic record (see New twist for end-Permian extinctions in the May 2005 issue of EPN).

Anoxic oceans were a regular feature of the Mesozoic and early Cenozoic. It is their preservation of abundant buried carbon that holds a key to, in an anthropocentric sense, the greatest of evolutionary leaps; the rise of large mammals and ourselves.  A large team of US scientists has used the now abundant records of carbon isotopes in both buried organic matter and marine carbonates to reconstruct changes in atmospheric oxygen content (Falkowski, P.G. and 8 others 2005.  The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science, v. 309, p. 2202-2204). Their modelling suggests that at the start of the Jurassic, atmospheric oxygen stood at around only 10%.  Through that period it rose dramatically to 16%, fell equally abruptly and then rose again to about 18%, thereby creating the conditions for some of the largest sources of petroleum.  Cretaceous times saw a slow rise, until around the time of the global warming at the Palaeocene-Eocene boundary (55 Ma).  The middle of the Cenozoic was a further period of dramatic increase in oxygen levels, to their highest (~23% in the Oligocene) since the peak during the Carboniferous. Latterly atmospheric oxygen has waned to around 21% today.

Falkowski et al. compare their new atmospheric oxygen curve with evolutionary spurts among mammals, of which the simplest to understand is the parallel rise of mammalian average size.  The metabolism of all mammals, like birds, has 3 to 6 times the oxygen demand of reptiles.  Not only were Mesozoic mammals challenged in stature by the air they breathed, reptiles were easily able to grow to monstrous proportions because of their less demanding physiological processes.  The first signs of the placental nurturing of mammalian foetuses, which requires a high oxygen level, coincides roughly with the Mesozoic maximum (100-65 Ma).  The end-Cretaceous extinction of the dominant dinosaurian reptiles removed the main competition against the subtle advantages of placental mammals, and was followed by further increase in oxygen.  The Cenozoic permitted terrestrial mammals to reach sizes almost comparable with dinosaurs, and to go beyond them among whales.  Moreover, it saw explosive diversification, one branch of which, the primates, leads to ourselves.

Martian methane: a bit of a blow

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In Joseph Heller’s Catch 22, Hungry Joe is noted for ‘…snorting, stamping and pawing the air in salivating lust and grovelling need’. That is a close metaphor for reactions among some scientists (and astronauts) to observations that seem to support the notion that indeed, there is life on Mars. Remember the meteorite ALH84001? In 2004, a spectrometer carried by ESA’s Mars Express probe detected methane in the Martian atmosphere above areas that probably carry sub-surface water ice. Many exobiologists attributed this to exhalations by methanogen bacteria perhaps living in the ice, which seemed plausible. Sadly, it seems that hydrous alteration of the mineral olivine, which is widespread at the Martian surface, to serpentine is even more likely. The reaction can yield hydrogen, which generates methane by reducing carbon dioxide. Exobiologists are keeping their options open…. Meanwhile, it is not implausible that hydrogen from this simple reaction might be used to resolve global warming: olivine is the most abundant mineral in the rocky planets. Incidentally, it is serpentinisation of ultramafic rocks that best explains methane exhalation from the deep ocean floor and from crystalline basement, which Thomas Gold thought had a deep-mantle origin and was responsible for all hydrocarbon deposits.

Source: Schilling, G. Martian methane: rocky birth then gone with the wind? Science, v. 309, p. 1984.

Where do impactors come from?

All the rocky bodies in the Solar System (the Moon, Mars, Mercury, Venus, Earth and moons of the giant planets) preserve to some extent the signs of collisions with errant bodies. One period stands out dramatically: the Late Heavy Bombardment or LHB (4.0-3.8 Ga) that produced the lunar maria, and left its signature in Archaean rocks on Earth (see Tungsten and Archaean heavy bombardment, August 2002 EPN). The planet Venus was entirely resurfaced about 500 Ma ago, and its plains record the later flux of impactors in much smaller more widespread craters, as do the lunar maria, parts of Mars and to a very limited degree the Earth. The LHB stopped abruptly, having appeared equally out of the blue. The influence of astronomical collisions on planetary histories may be an established fact, but is still something of a mystery as regards its pace and intensity. High resolution images of large rocky bodies sustain a thriving cottage industry of measuring, counting and dating craters; the latter from stratigraphic evidence of relative age, such as craters that have been cratered, and ejecta mantles that bear signs of impact themselves.

Hidden inside such statistics are clues to the astronomical processes that lead to impacts (Strom, R.G. et al. 2005. The origin of planetary impactors in the Inner Solar System. Science, v. 309, p. 1847-1850). The crater-size distributions for the early events and those after 3.8 Ga are very different. Those of the later generation show features very like the size distribution of objects whose orbits intersect that of the Earth (near-Earth Objects or NEOs) and largely reflect the element of chance in a more or less stable late Solar System. The LHB pattern extends to craters more than an order of magnitude larger than the younger one, and resemble the size distribution of bodies that now orbit quite happily in the Main Belt of asteroids. It seems that during the period between 4.0 and 3.8 Ga, some main belt asteroids were flung out of their orbits to enter the Inner Solar System in large numbers. The analysis by Strom et al. suggests that the gravitational disturbance during that period might have been due to gradual migration of the giant Outer Planets before they took up their present stable orbits.

Climate change and human evolution

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One clear character of the record of investigations into human evolution is that, rather than becoming clearer as data increase, our origins become more of a puzzle. With every major fossil find the hominin clade or bush of descent acquires what appears to be another branch. With the recent publication of the genome of our closest living relative, the chimpanzee – and its earliest fossil remains – (Nature, v. 437, p. 47-108), it will hardly be surprising if the assumptions about a gene-based time of separation of the two clades (5-7 Ma) comes into question. Studies of the Y-chromosomes of living human males have suggested ‘bottlenecks’ in our recent evolutionary past, interpreted to indicate near-catastrophic declines in numbers to perhaps that of a few scattered bands. One such ‘near-extinction’ seems to have occurred about 70 thousand years ago, which has been linked to the huge explosion of the Toba ‘supervolcano’ in Indonesia in whose ash are poignantly preserved biface axes. Toba would have had a global climatic effect at a time when fully modern humans were migrating rapidly from Africa across Eurasia; thinly spread and easily isolated by disaster. What followed was an explosive development of both material and aesthetic culture, perhaps enabled by some serious selection amongst those who endured Toba’s global blast.

It is always tempting to restrict hypothesizing with the ‘Just gimme the facts’ outlook – as people of my generation will remember from the main detective in the Dragnet TV series. That is, ideas based on hominin remains alone. Yet all evolution takes place within a wider environmental context; for much of our history that of East Africa. Scanty knowledge of tropical climates there and a reliance on distant deep-sea records had led to the widespread belief that this centre of most hominin evolution gradually became drier since the late Miocene. Lake beds in the East African Rift system have held the key to a useful record, and now some of the detail is emerging (Trauth, M.H. et al. 2005. Late Cenozoic moisture history of East Africa. Science, v. 309, p. 2051-2053). Lakes in the Rift are handy for climate study because they span 8 degrees of latitude north and south of the equator, the spread helping to isolate more local effects of volcanism and tectonics on their sedimentary record from those of regional climate change. Many have little outflow and a local supply of water, so their levels depend mainly on the amount of local precipitation compared with evaporation. The actively subsiding basins in which they form have the opportunity to preserve unbroken, thick records of both lake and river sediments.

Trauth et al. compile environmental and chronological information from sediments in seven Rift basins, going back to about 3 Ma. Volcanic events provide plenty of dating opportunities to calibrate and correlate the sedimentary evidence. They show three rift-long episodes of deep lakes spanning broad periods from 2.7-2.5, 1.9-1.7 and 1.1-0.9 Ma. A few sections reveal lake-level fluctuations on Milankovich timescales. The longer episodes link in time to the intensification of Northern Hemisphere glaciation, to a shift in east-west air circulation over Africa and to the switch from the dominant glacial cyclicity of 41 ka to one of 100 ka, respectively. Wisely, they consider the climatic information to be crucial to studies of human evolution, but still too coarse to be used with confidence in relation to details of the fossil record. Long humid periods would have been ‘easy’, whereas the separating drier periods may have experienced ups and downs in humidity on Milankovich timescales. Fluctuating conditions would have been more stressful and likely to witness speciation. One very odd feature is that the 1.9-1.7 Ma period of deep rift lakes is the time when H. erectus became the first tooled-up being to migrate far beyond Africa. Many have regarded migration as a response to environmental stress, but just as likely is an expansion of opportunity.

Climate and the end-Permian extinction

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A time in Earth history (~251 Ma) when life was all but snuffed out and from which the creatures most familiar to us eventually emerged is understandably revisited quite often. Causes ranging from impacts (no convincing evidence as yet), through flood-basalt emissions, catastrophic methane release, low atmospheric oxygen to ocean anoxia have all been proposed. Hesitantly, opinion is converging on a climatic crisis of some kind, and indeed the coincidence of both terrestrial and marine faunal and flora extinctions points to climate being the global transmitter of some cause or a coincidence of causes. After the waning of Southern Hemisphere glaciations, the late Permian was warm, even at high latitudes. Until recently, attempts at modelling the end-Permian climate have not been entirely convincing because of limitations in the models themselves. Jeffrey Kiehl and Christine Shields of the US National Center for Atmospheric Research in Colorado have assembled a model that couples land, atmosphere, oceans, sea-ice and palaeogeography for the period (Kiehl, J.T. & Shields, C.A. 2005. Climate simulation of the latest Permian: Implications for mass extinction. Geology, v. 33, p. 757-760).

The critical test for the model is running it with parameters for the near-present, and it performs well. Several lines of evidence point to a much higher CO2 level in the Permian atmosphere, so this is the main input parameter. The outcome is a world with a mean surface temperature that is 8° C higher than now. Unlike today, there was no geographic hindrance to poleward heat transport, so the high mean temperature is reflected in the summer warmth and humidity of Permian high-latitude land. The sub–tropics on the other hand were scorching (around an average summer minimum of 51° C, 15° C higher than now); a clear contributor to minimising life there. Sea-surface temperatures at high latitudes are higher in the model outcomes, this warmth extending to depths of 3 km. Surprisingly, low-latitude sea temperature emerges as much the same as now. The model also suggests that seawater was saltier than now, and that results in greater uniformity of density with depth and location: a hindrance to bottomward circulation and mixing. There would probably have been no thermohaline circulation worth speaking of. The model helps confirm the likelihood of an oxygen-free lower ocean and little transfer of nutrients. The oceans too would have been inhospitable. A shutdown of biological productivity and therefore carbon burial would have accelerated warming. So, pushing the biosphere into a mass extinction would have been inevitable. The last straw may have been the additional stress of increasing acidity from sulphur dioxide emissions from the Siberian flood basalts.

Milankovich forcing and Early Jurassic methane

Periods of environmental crisis less severe than those leading to mass extinction appear throughout the fossil record. As well as minor extinction peaks they are often signified by departures of carbon-isotope records from long-lasting norms. Such a crisis appears in the d 13C record of the Early Jurassic, and is beautifully preserved in about 15 m of black shales on the North Yorkshire coast of England. Geoscientists from the Open University, UK and the University of Cologne, Germany have produced an extremely high-resolution time series of carbon-isotope data from the section (Kemp, D.B. et al. 2005. Astronomical pacing of methane release in the Early Jurassic period. Nature, v. 437, p. 396-399). The quality is sufficiently good to analyse the time series using Fourier analysis that yields the frequencies that contribute to the observed wave-like patterns in the data. Of course, the time in a stratigraphic time series is measured in metres, unless it is possible to calibrate the section by precise radiometric dating. The Yorkshire Jurassic contains only fossils and no dateable horizons, but the fine stratigraphic division based on ammonites is also widespread and calibration is possible from dates obtained elsewhere. The overwhelmingly dominant frequency in the carbon-isotope curve is 1.23 cycles m-1, which represents 21 ka after the calibration of depth to time. That is the signal of precession of the equinoxes, part of the astronomical forcing bound up in Miliutin Milankovich’s theory of astronomical forcing of climate.

Astronomical pacing turns up throughout the stratigraphic column, wherever sediments are suitable for time-series analysis (steady, unbroken sedimentation), so a precessional signal is no great surprise. The important feature is the profundity of the d 13C excursions; a total of –7‰, largely accomplished by three abrupt shifts of –2 to –3‰. The first two coincide with bursts in extinctions. The most likely phenomenon to have produced these shifts is massive release of methane by destabilization of submarine gas hydrates. Emissions seem to have been blurting out on a regular basis as the Earth’s rotational axis precessed like a gyroscope. So, the complete time period was one in which gas hydrate was unstable, probably due to overall warming. Yet something else must have triggered vast releases three times. The Lower Jurassic extinctions link in time with massive magmatism in Southern Africa and Antarctic (the Karoo-Ferrar large igneous province). Perhaps especially large volcanic events there set the stage for large precessional methane releases. An alternative view is that volcanic emissions of CO2 gradually produced enough widespread warming for the astronomical trigger to cause breakdown of gas hydrate simultaneously over very wide areas of the ocean floor. Other explanations have been suggested for the Lower Jurassic warming and carbon-isotope excursions, such as wildfires, impacts and connections with petroleum maturation and migration. The clear cyclicity rules them out.

A tsunami’s reach

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The Boxing Day 2004 Indian Ocean tsunamis were recorded by tidal gauges across the planet, both as amplitude and time of arrival. Armed with such calibrating data, detailed ocean-floor bathymetry and means of modelling wave propagation, oceanographers and geophysicists from the US, Canada and Russia have been able to estimate just how the terrible waves travelled the globe (Titov, V. et al. 2005. The global reach of the 26 December 2004 Sumatra tsunami. Science, v. 309, p. 2045-2048). Highlighting their article wonderfully is a colour-coded map that shows offshore amplitude and arrival time for the world’s oceans and shores. Its most fascinating feature is the manner in which the worst of the disturbance was guided by ocean-ridge systems, principally the Ninety-East and Southwest Indian Ridges, but also the mid-Atlantic Ridge. That is of no comfort to the survivors of the disasters around the Bay of Bengal, although the Irriwaddy delta in Myanmar was spared by the influence of the northern part of the Ninety East Ridge. That Madagascar and East Africa, except for northern Somalia, suffered far less than anticipated is thanks to the peculiar effect of the ridge systems.

The fluoride saga

Archaeological work on Icelandic burial grounds of the 18th century in the early 21st century exhumed victims of the Laki eruption of XXXX. Many skeletons bore the distinctive signs of bizarre bone growth that characterises massive ingestion of fluoride ions. The victims had endured prolonged and worsening suffering after exposure to hydrogen fluoride-rich gases that seem to characterise Laki’s effusions. It is a now well-documented geotragedy. Equally well recorded are the lives of Iceland’s early inhabitants from the 8th century onwards, but in the form of epic prose in Old Norse: the Sagas. Being prone to repeated volcanism, an obvious question is, “Did the Viking heroes experience the same problems?”

One of them was huge, both a righter of injustice and a tidy hand with the battleaxe. Egil Skallagrimsson was ‘a man who caught the eye’, reputedly being awesomely ugly and capable of jerking an eyebrow down to his chin line. Such attributes might seem to have been passed on to the legendary centre-half, ‘Skinner’ Normanton, who graced Barnsley football club in the 1950s. The traditions perhaps, but Egil’s visage was probably a result of chronic fluorosis rather than parentage (Weinstein, P. 2005. Palaeopathology by proxy: the case of Egil’s bones. Journal of Archaeological Science, v. 32, p. 1077-1082). His relatives Hallbjorn Half-troll and Grim Hairy-Cheeks seem from the saga to have been equally afflicted, yet successful. As befits a Viking battler, Egil had a thick skull; when exhumed by descendants in the 12th century, it was found to be ridged like a scallop shell – the attending priest hit it with the back of an axe, to no avail. Some have inferred abnormal bone growth and deformities due to Paget’s disease, but that tends to produce massive but weak growths, following repeated crumbling of bone. Weinstein’s theory may be verifiable, since Egil’s Saga reveals the final resting place of this enigmatic giant.

Source: Pain, S. 2005. Egil the enigmatic. New Scientist, 17 September 2005, p. 48-49

Earth’s biggest ‘bull’s eye’

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Since astronauts and satellite imaging devices first made pictures from orbit, top of the list for oddness is the Richat structure of Mauritania. Sitting out in the Sahara is series of perfectly concentric rings that are almost circular. The structure is at least 40 km across, and even today, many geoscientists use images of Richat as a superb example of a meteorite impact. It is not (Matton, G. et al. 2005. Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex. Geology, v. 33, p. 665-668). Spectacular from space, Richat is not easily accessible. Early field work reported a breccia on a kilometric scale at its high-relief core, which unsurprisingly added to its designation as an impact structure. There are other possibilities: a structural dome, perhaps due to interference between open folds of a couple of generation; the result of upward forces from magmatic activity, such as an underlying plutonic diapir.

The rocks involved are Neoproterozoic to Ordovician sediments of various kinds, which dip radially outwards from Richat’s core, so it is some kind of dome, rather than the sort of circular breach expected of an impact. Two large, basaltic ring dykes, whose centre coincides with that of the dome, cut the sediments. Other igneous materials are: carbonatites (formed from unusual carbonate-rich magmas) in dykes and sills; alkaline silicate-rich intrusions and flows occurring close to the central breccia; kimberlites in the form of plugs and sills. The central breccia is in fact a roughly horizontal lens, about 3 km across, that is made mainly of local sedimentary material, mainly once carbonates, set in a silica-rich matrix. The clasts range from highly angular to rounded, but show abundant evidence of some kind of corrosion and silicification. Matton et al. interpret the breccia as a zone of intense dissolution that caused the original sediments at the structure’s core to collapse as volume was reduced as magmatic gases (supercritical fluids) rushed to the surface. So the Richat structure has all the hallmarks of doming above an alkaline igneous pluton, followed by intense hydrothermal activity that was able to dissolve carbonates and produce features akin to those formed by weathering in areas of karst. Rather than being particularly ancient, the igneous activity dates to the Middle Cretaceous. Richat is still unique. Diatremes (vertical breccia tubes) formed by explosive release of fluids from alkaline magmas are quite common, especially in areas dotted with kimberlites, but nowhere else have they produced doming on such a grand scale and with such a spectacular shape.

Detecting the effects of slab to wedge fluid transfer in subduction zones

A fundamental hypothesis concerning the formation of magmas above subduction zones is that partial melting in the over-riding wedge of mantle is induced by upward transfer of water vapour produced by dehydration of the descending lithospheric slab. Many aspects of the chemistry of igneous rocks in supra-subduction zone settings are explained by such dehydration-hydration. However, such fluid transfer is difficult to demonstrate, other than by its ‘second-hand’ geochemical effects on crustal magmas. It should have another, physical effect: in the presence of water vapour, some of the dominant olivine in mantle rocks should break down to form hydrated minerals of the serpentine family. Since olivine is an iron-magnesium silicate, whereas serpentine contains only magnesium, the hydration reaction should release iron to crystallise in the form of iron oxide; specifically Fe3O4 or magnetite. Geophysicists at the US Geological Survey have been able to detect at first hand the effects of this process, thereby allowing zones of hydration in the mantle wedge to be mapped (Blakely, R.J. 2005. Subduction-zone magnetic anomalies and implications for hydrated forearc mantle. Geology, v. 33, p. 445-448). As well as finding substantial magnetic anomalies caused by the release of magnetite by olivine dehydration over the forearc of the Cascadia subduction zone in Oregon, they show gravity anomalies that reflect density variations in the underlying mantle. The other aspect of the olivine-serpentine transformation is a large decrease in density, which should result in a decrease in gravity anomaly should sufficient olivine have been transformed. The coincidence of gravity lows with magnetic highs allowed Blakely et al. to model the location of hydrated mantle wedge in the Cascadia subduction system: probably just above the zone where subducting oceanic crust is transformed to ecologite.

Serpentinite also has a marked effect on the rheology of mantle rocks, because of its ease of ductile deformation. It should allow subduction deformation to proceed in a continuous fashion within the part of the system where it occurs, yet may focus sudden strain in great earthquakes to shallow levels up-dip of its position.

Arsenic removal no cure

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It is now a decade since the enormity of natural arsenic contamination in groundwater below the great plains of northern India and Bangladesh came to light. In 1995 the World Health Organisation announced that this waterborne arsenic was causing the world’s largest case of mass poisoning. Since then other areas at risk have emerged in East and Central Asia and South America. The tragedy is that groundwater generally presents the safest option for drinking water because sediments filter water and encourage biogenic oxidation that remove common pathogens. That tens of million people in West Bengal and Bangladesh face stealthy poisoning results from channels cut in the low-lying plains during the last glacial maximum being filled rapidly with sediment as sea level rose during climatic recovery. Sedimentation buried large amounts of organic debris to form anoxic conditions in the shallower sediments. Reducing conditions encourage breakdown of the common colorant in sediments, iron hydroxide grain coatings that, having adsorbed most arsenic and other ions from water, releases them when it dissolves. That this should occur was unsuspected during a massive programme of well sinking to relieve endemic ill health from waterborne disease, yet early signs that arsenic had replaced pathogens as a hazard was widely ignored, despite a few warning voices who discovered the unmistakable signs of arsenicosis in the 1980s. They include disfiguring pigmented skin spots and horny growths on hands and feet.

By 1995, the rest of the world took notice, pouring in funds to document occurrences and causes, and to remediate a clearly catastrophic situation. There are three main strategies: to remove arsenic from well water using chemical filters; to return to water from surface sources, though with careful processing to remove pathogens; to sink wells below the level known to encourage arsenic release from iron hydroxide dissolution. For two decades affected populations had been bombarded with encouragement to turn to groundwater: against their better judgement – they termed it the Devil’s water. Once using wells they saw that infant mortality plummeted, so they developed a new enthusiasm for water deemed safe. Caught on the horns of a dilemma, when arsenicosis appeared they were reluctant to return to what appeared to be the greater of two evils. In only a few places were wells deepened to safe depths, and the externally sponsored drive for a solution centred on arsenic removal techniques. Even that was not widespread: of millions of risky wells some 2000 were equipped with arsenic extracting devices, at around US$ 1500 each. It now emerges that the technologies chosen are not doing their intended job (Hossain, M.A. (and 10 others) 2005. Ineffectiveness and Poor Reliability of Arsenic Removal Plants in West Bengal, India. Environmental Science & Technology, v. 39, p. 4300-4306). The team, led by Depankar Chakraborti, who first spoke out about arsenicosis in 1983, tested the efficacy of 18 different devices installed in West Bengal. Only two reduced arsenic levels to the maximum of 50 parts per billion accepted by the Indian government, which is itself five times more than that deemed safe by the WHO. The teams view, supported by the agency that did most to encourage the massive well-driving programme since the 1970s (UNICEF), is that the only realistic solution is a return to rainwater harvesting and purification.

See also: Ball, P. 2005. Arsenic-free water still a pipedream. Nature, v. 436, p. 313.

Legendary events at the Gibraltar Straits

Everyone has heard of Atlantis, but few would care either to point to its former position, or to accept its existence without a shed-full of salt. Nevertheless, no lesser an authority than Plato first described the legend of Atlantis in the 4th century BC, following verbal accounts that originated in pharaonic Egypt. In the last decade a number of legends, if not their religious connotations, have received scientific support. Foremost among these is that of the biblical Flood, which Ryan and Pitman pursued relentlessly, using the Epic of Gilgamesh as a geographic and chronological guide. They discovered that the Black Sea had catastrophically filled through the Bosphorus once global sea level topped the level of its floor, following glacial melting. Their evidence now includes numerous examples of habitations now inundated by the Black Sea.

As with Ryan and Pitman’s work, one key to resolving a real basis for a legend is carefully puzzling out clues in the most detailed accounts of it. In the case of Atlantis, the clues come from Plato himself (Gutscher, M-A. 2005. Destruction of Atlantis by a great earthquake and tsunami? A geological analysis of the Spartle Bank hypothesis. Geology, v. 33, p. 685-688). Marc-André Gutscher and previous workers focused on Plato’s geographic description of Atlantis, as well as its fate. Plato clearly specified an island in the Atlantic beyond the Straits of Gibraltar, and an earthquake and flood that put paid to the Atlanteans in a single day. Indeed, bathymetry does show well-defined shallows (less than 100 m depth) in such a location, but only about 5 km across. This is the Spartel palaeo-island, on which Gutscher turns his focus. Until the final, decisive rise in sea level after around 12 ka, Spartel would have been a low island. Plato’s account is supported by the existence of a proto subduction zone on the Atlantic sea floor off the Straits of Gibratlar, a major earthquake on which devastated Cadiz in 1755, partly because of a 10 m tsunami. Offshore sediments include turbidites that indicate 8 tsunamis since 12 ka, suggesting a 1500- to 2000-year periodicity of large earthquakes at the entrance to the Mediterranean. Plato’s version of the events includes a rough chronology that suggests a time around 11.6 ka before the present. The thickest of the tsunami-driven turbidites is of roughly that age. Unfortunately for the hypothesis that Spartel was Atlantis, at that time only two tiny islets would have stood above the waves. Seismic destruction of coastal regions by tsunamis is something that might easily become legendary, the more so in the distant past. There is one other possibility that might revive the Spartle hypothesis, demonstrated by the great Indian Ocean tsunami of 26 December 2004. Very powerful earthquakes can also result in massive displacement of the crust, or the order of tens of metres. Spartle might have sunk repeatedly since 11.6 ka, as a result of later events.

Documenting the Palaeogene transition from ‘hothouse’ to ‘icehouse’

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It is well-established that the first large ice sheets that presaged descent into the oscillating climate of the Neogene formed about 34 Ma ago (the Eocene-Oligocene boundary) on Antarctica. Some 21 Ma before, at the Palaeocene-Eocene boundary, global temperatures had leaped following what many believe was a massive blurt of methane previously held in cold storage in ocean-floor sediments as gas hydrate. A monstrous ‘greenhouse’ climatic system must sometime in the interim have reverted to the cooling trend begun at the outset of the Cenozoic. Defining that transformation relies on assembling and interpreting newly available, high-resolution records of climatic proxies through the Eocene and Early Oligocene (Tripati, A. et al. 2005. Eocene bipolar glaciation associated with global carbon cycle changes. Nature, v. 436, p. 341-346). Hitherto, the Eocene part of the ocean-floor sedimentary column had been poorly sampled, so that only broad trends showed.

As you might expect, the change was not a simple transition. At about 42 Ma the record of the Pacific Ocean calcite compensation depth (CCD – the depth at which carbonate remains are dissolved in the deep oceans) shows a remarkable perturbation long before the CCD dipped decisively from about 3.5 km to around 5 km at the start of the Oligocene. A close look at the oxygen isotope record of that age in a highly detailed marine sediment core shows an increase in d 18O that corresponds to either some 6° of cooling or a 120 m fall in sea-level due to build-up somewhere of ice on land. Coinciding with this perturbation are shifts in the carbon-isotope record in carbonates. The authors suggest that the mid-Eocene cooling and continental glaciation that produced falling sea level triggered the weathering of shallow-water carbonates, which together with river transport increased the oceans’ alkalinity. That would have increased deep-water carbonate formation enormously and accelerated the effective ‘burial’ of carbon from the atmosphere

Smithsonian geological timeline

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A measure of the quality of a science website, apart from its visual appeal, is a mixture of how much it teaches you and what you can snaffle to help teach others. As a point of departure for E-geology, it will be hard to beat the Smithsonian Institutions geotime site (www.nmnh.si.edu/paleo/geotime). That’s because it focuses first on the history, and if you care to you can discover how that was constructed from the geological record. Its central organiser is a slider that can be zoomed, which lays out the geological past – the literal time line divided into stratigraphic Eons, Eras, Periods and Epochs. Each division is clickable, although zooming in several times is needed to see the Cenozoic Epochs. But, hang on, there is no Ediacaran Period, the newest addition, nor the subdivision of the Proterozoic on the timeline. Whatever, clicking on a division opens a thumbnail sketch of each and links to pages that give more detail on the highlights, plus introductions to the founding concepts behind geological time and unravelling Earth and life processes. There is a glossary, which shows the influence of Encarta and Wikipedia. Here is a chance to learn for hours in a most convenient and engaging way, but graphics are few and far between in the various main panes. There are examples of important fossil organisms, but displayed at a size that lacks satisfying detail. What the site needs are maps and explanatory diagrams, which are available elesewhere. So the Smithsonian needs, I think, to liase a bit with other learning resources in the geosciences. It would be good to have a one-stop shop.

Has human evolution stopped?

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There can be no doubt that the way in which humans consciously build ‘shields’ of many kinds between themselves and their surroundings placed our species, and those leading up to it, in an increasingly different relationship to the environment than those of other organisms. Fire, habitations, tools, weapons and clothing emerged far back in our evolutionary ‘bush’, to be followed more recently by artificial means of feeding ourselves in a vast range of climatic conditions. In the last century these ‘shields’ have been added to by medical protection against pathogens.

Many of the physical traits of the modern human frame would not be ‘fit’ in a purely Darwinian sense for life unprotected by myriads of cultural devices: they arose from genetic potential largely because growing human culture allowed them to be fit for purposes other than survival at its simplest level. The range of basic physiognomies among modern humans does seem to reflect natural selection to suit various climatic regions, such as the differences between cold- and heat adapted peoples. That perhaps began during the great expansion out of Africa some 70 ka ago. But the much greater range of facial characteristics among all populations (a really human characteristic compared with other primates) is probably a result of genetic drift at random, rather than any kind of evolutionary selection. There are also differences that have arisen since the widespread adoption of agriculturally produced foods since about 10 ka ago, as in jaw shapes and those of the skull, probably linked to easier mastication. That can be explained most easily by the manner in which the use of muscle tends to sculpt the bone to which it is attached: it arises during the life of the individual.

With what appears to be the start of a global unification of cultures, and greater security for the more fortunate one third of humanity at least, it might be expected that natural selection is on the wane for humans. A mere 10 thousand years since the rise of agriculture and far less since modern cultures arose, it is perhaps too soon to conclude that we have cut loose from Darwinian processes. Indeed, recent genetic research has come up with several developments that must be recent results of natural selection. One is the split between adults who can metabolise cows’ milk and those who cannot. The first group, a minority, cluster around the Near East (most Europeans) and in a few parts of Africa where cattle domestication arose. A large block of the human genome, about a million base pairs of nucleotides, includes the gene that produces the necessary enzyme lactase, and its persistence in those adults able to digest milk. The large size of the whole haplotype is typical of recent genetic developments, and the researchers are certain that it resulted from selective pressure where dairy farming began at between 5-10 ka.

Genes that confer resistance to infectious diseases that can cut life short before successful reproduction are good candidates for showing the effects of natural selection, especially in those areas where medical care and drugs are not available. For a long while natural resistance among some west Africans to malaria parasites was linked with heritable sickle-cell anaemia, but recent research has shown a more complex reason that involves several genes. Interestingly, ‘dating’ of the associated genetic changes gives recent ages between 3 and 6 ka, perhaps linked to the rise of farming practices. Clearing land and ponding of water on fields would have encouraged the malaria-carrying Anopheles mosquitoes, which are not forest species: a cultural change presaged a genetic one. Similar results have emerged from studies of inherited protection against HIV/AIDS, yet that only appeared in pandemic form very recently (unless misidentified earlier). An explanation may centre on selective pressure on mutation to form the protecting gene as a result of the appearance of previous epidemics, such as plague and smallpox among early Europeans, who seem to have the highest resistance to HIV/AIDS.

So it is hard to say if selective pressures will work in future on the human genome, as culture convergence continues, and (hopefully) equitably shared living standards. Since the limit on human brain size is the skull, and that is limited by the near-maximum pathway through the human female pelvis, it is very difficult to imagine our evolution into big-heads.

Source: Balter, M. 2005. Are humans still evolving? Science, v. 309, p. 234-237.

Modelling the core

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Judging by the growing procession of research grant proposals aimed at studying the inner workings of the Earth’s core through computer modelling, it would be easy to assume that a major breakthrough was just over the horizon. What you need is some kind of supercomputer to handle the massive complexity of core fluid dynamics and then channel that through one of several concepts of a geodynamo, first towards simulating the present field and then to how the geomagnetic field swirls and occasionally flips. The fourth biggest there is belongs to the Japanese geophysical community; the Earth Simulator, which is certainly well ahead, in terms of power and speed, of facilities available to less endowed scientists. Recently, about 10% of its power was let loose for a 9 month modelling run that focussed on complex motion in the liquid outer core that theory should generate (Takahashi, F. et al. 2005. Simulations of a quasi-Taylor state geomagnetic field including polarity reversals on the Earth Simulator. Science, v. 309, p. 459-461). Hitherto, modelling had produced pictures of varying magnetic intensity that bore some resemblance to the real magnetic field at the Earth’s surface, and did indeed come up with reversals. Yet a variety of models all produced similarly plausible patterns in space and time. The snag was the limit to matching the viscosity of liquid iron with spin rate. Geomagnetists suspect that the Ekman number, which represents that relationship, is very low in the Earth’s core, i.e. there is very low drag in core circulation, and that adds to complexity. Until the Earth Simulator was built, no power on Earth could deal with the high spatial resolution needed to simulate properly motions at low Ekman numbers. Takahashi and colleagues were able to drop the Ekman number 10 times below any previous simulation.

Real-looking features did begin to emerge in the time sequence for the field at the core’s surface. The most interesting was the formation of zones of opposed polarity at high latitudes, soon (in about 1000 years of simulated time) to be followed by a reversal. The zones move progressively polewards to coalesce, when the overall magnetic polarity all but disappears, and then a reversed field becomes established. However, this is not real but a model dependant phenomenon, even though it is possible to see patterns akin to those observed today – many geophysicists believe the Earth is on a magnetic cusp before a reversal. Will it ever be real is an obvious question, in the same way that related climate simulations may flatter to deceive. The problem is not a lack of models, nor conceivably computing power, but a lack of real data. The ocean floor contains masses of information on past reversals, and cunning analyses of palaeomagnetism in lavas that cooled slowly through the Curie point at the time of a reversal show astonishing things that happened. Excellent maps of the modern field are available, but reality in a reversal is a time series of that mapped field. Without such data, and the time to collect it (the modelling simulates evolution over 5200 years) before the next order-of-magnitude jump in computing power (perhaps 10 years off), it is very difficult to see a justification for this kind of modelling, as opposed to that for climate, which does have a more rapid response time.

See also: Kerr, R.A. 2005. Threshold crossed on the way to a geodynamo in a computer. Science, v. 309, p. 364-365.

Did oil and gas fields form during the Precambrian?

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Since the origin of life it is certain that a proportion of biological materials would have been preserved in sediments after organisms died. As today, such material would have evolved or matured as the host sediments were buried and heated. There is plenty of evidence that such maturation did occur as far back as 3250 Ma ago, but signs that oil-fields formed by migration and trapping have proved elusive. Several lines of evidence, such as carbon-isotope anomalies in Precambrian limestones, point to periods when enormous amounts of organic material were buried, much as happens in the formation of Phanerozoic petroleum source rocks during periods of ocean anoxia. Before about 2400 Ma, when evidence for an oxidising surface environment first appears in the rock record, such conditions would have been pervasive. The first hints of large-scale petroleum formation and migration have been found in the low-grade Pilbara craton (3500-2850 Ma) of Western Australia and 2770-2450 Ma sediments that overlie the older Archaean complex (Rasmussen, B. 2005. Evidence for pervasive petroleum generation and migration in 3.2 and 2.63 Ga shales. Geology, v. 33, p. 497-500). Black shales in the Pilbara contain not only lots of fine-grained carbonaceous matter, but some in forms that clearly suggest that they had been thermally matured (‘cracked’) to low-viscosity fluids that could migrate. There are blobs of bitumen contained within iron sulfide layers that seem to have formed later, to engulf petroleum liquids. Molecules within the bitumens resemble those formed by photosynthesising blue-green bacteria, methanogen and sulfate-reducing bacteria and arguably perhaps primitive eukaryotes. It appears that the bitumens probably formed as residues as lighter and more fluid hydrocarbons migrated out of these substantial source rocks. What has yet to be demonstrated are Archaean and Palaeoproterozoic reservoir rocks where such migrating petroleum accumulated. Another question is whether or not the source rocks, which are extremely widespread and thick, might have retained some potential for sourcing petroleum much later in the geological history of Western Australia and similar cratons elsewhere.

Precise timing of petroleum migration

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In their slack moments, petroleum geologists ponder on when oil and gas got into a particular reservoir and became trapped.  One aspect of the conundrum is easy to answer: after the reservoir rock and trap formed.  But timing is not so trivial, for an important consideration in exploration for new oilfields concerns the actual rock that sourced hydrocarbons in known fields, almost always a highly reduced, black mudrock in which unoxidised dead organic matter accumulated and matured. Repeated anoxic events, both regional and global, provide several alternatives in many petroleum provinces.

Hydrocarbons, having formed under highly reducing conditions, contain several metals and other elements well above normal crustal concentrations.  Among these are rhenium and osmium, which allow radiometric dating through the decay of 187Re to 187Os.  In principle, therefore, it is possible to date oil and relate it to a particular source rock. Interestingly, it is easier to date the actual time at which oil has accumulated in a trap.  In an analogous way to the equilibration of parent and daughter isotopes in magmas, which is halted by crystallization so that the system evolves and dating can be done, once oil settles in a trap after migration the timing can be dated sing the Re-Os method.  David Selby and Robert Creaser of the University of Alberta, Canada applied this approach for the first time, using the vast reservoirs of oil sand in Alberta as a test (Selby, D. & Creaser, R.A. 2005.  Direct radiometric dating of hydrocarbon deposits using rhenium-osmium isotopes.  Science, v. 308, p. 1293-1295). The oil in the sands were emplaced around 112 ±5 Ma ago, during the Early Cretaceous, not long after the host sandstones had been deposited.  Previous work using ideas on oil maturation suggested that migration had taken place during the Early Palaeocene, around 60 Ma ago, when potential source rocks were heated by tectonic burial during the Laramide orogeny.  The Re-Os results point to migration from the west while the Cretaceous sedimentary basin was filling.  This may explain the high viscosity of the oils as a result of near-surface biodegradation.

Another product of isotopic dating is establishing the initial 187Os/188Os ratio of the petroleum system, which relates to that of the original source and its isotopic evolution.  In the case of the oil sands this value points to source rocks of earlier Mesozoic and even Palaeozoic age, rather than a Cretaceous source that had been suggested previously.

Britain above convecting mantle?

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Being able to picture Earth features far beneath the surface is what makes seismic tomography such an exciting tool, even though it is in its infancy. It shows variations in the velocity of P and S waves in 3-D. Regions of fast waves are likely be cooler than those in which wave speeds are relatively slow.  The detail depends on the spacing between seismic recorders and the distribution of natural seismic events, whose interactions produce tomographic data.  Despite being rarely affected by seismicity themselves, the British Isles have a remarkably dense network of seismic stations that was developed for research.  Given arrival times at the different stations by waves from earthquakes that occurred over a wide range of epicentral angles from the British Isles, it becomes possible to probe in detail what lies beneath.  Exploiting the potential to the full, a group of British and US geophysicists has shown that the ‘British’ mantle is far from boring (Arrowsmith, S.J. et al. 2005. Seismic imaging of a hot upwelling beneath the British Isles.  Geology, v. 33, p. 345-348).

Down to a depth of 600 km, Britain is underlain by a series of significantly slow and fast mantle ‘blobs’.  The seismically slow, probably warm mantle zones seem to follow large features last active during Early Palaeogene magmatism that affected the Hebrides and Northern Ireland, and roughly parallel the 60 Ma dyke swarms that radiate from these centres.  They also correlate with regions of anomalously high gravity.  It seems highly likely that both features are long-lived relics of a spur of the still active Iceland plume that is intimately associated with spreading on the Mid-Atlantic Ridge.  The warm zones also underlie those parts of the British Isles that were most affected by uplift and erosion during the Cenozoic: as much as 3 km in the case of the Irish Sea.  Such areas also focused extension at the time of the magmatism, and they are still most affected by minor seismicity.

Estimates of the magnitude of the temperature anomaly associated with the slowing of P-waves are as much as 200 °C above ambient mantle temperature; sufficient to be associated with partial melts.  That Britain might once more have active volcanoes is highly unlikely, and the anomalies are probable parts of the Iceland plume system that became trapped beneath zones of crustal thinning. Their loss of heat is sufficiently slow for them to have bolstered areas of uplift and erosion for tens of million years.  There is even a chance that some form of convection might yet be going on.

New data on starting point for Earth evolution

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Slowly, geochemists as well as planetary scientists have been taking up the implications of a likely infernal origin for the Earth-Moon system that resulted from a Mars-size planet colliding with the proto-Earth, shortly after planetary accretion.  The chemistries of both Earth and Moon have sufficient similarities for a common origin to be almost certain.  There is one difference: lunar rocks are more depleted in volatiles than those accessible on the Earth.  Terrestrial rocks were at some stage in their evolution purged of some volatile elements.  The Moon’s early history seems to be extraordinarily simple.  It is recorded in the pale rocks of the lunar highlands that are made dominantly of feldspars.  Their low density and abundance suggest that feldspars floated to the top of completely molten rock, in much the same way as similar anorthosites on Earth seem to have formed in large magma chambers. The difference is that lunar anorthosites probably once formed the entire crust of the early Moon, and formed by simple differentiation of a deep, all-encompassing magma ocean.  The late Dennis Shaw applied this simple notion to the Earth’s earliest evolution during the 1970s, but his vision was largely ignored by his geochemist peers.  A mantle-wide zone of complete melting was resurrected when William Hartmann’s giant impact theory appeared: the energy involved seems to make this an inevitable corollary of his idea.

Indirect analysis of the mantle from the geochemistry of its basaltic products has shown that the mantle is not homogeneous.  Some has been partially stripped of basalt-forming elements, and there are other chemical heterogeneities.  However, examined from the standpoint of isotopes of neodymium (142Nd and 144Nd) more or less every magmatic rock has been considered to have been ultimately derived from material with the same isotopic composition as chondritic meteorites, and by extension, that of the Galaxy in the vicinity of what became the Solar System.  That observation has been a major counter argument to the notion of an early terrestrial magma ocean. Differentiation of such a fundamentally molten Earth would have separated some of the samarium-146 (the source of 142‑Nd through radioactive decay) from 144Nd, thereby imparting different growth histories for 142Nd/144Nd ratios to different mantle ‘reservoirs’.  The half-life of 147Sm is about 100 million years, so that radiogenic 142Nd would accumulate most in Earth’s early history, thereafter tending towards a constant proportion of neodymium, unlike the 143Nd used in radiometric dating that accumulates much more slowly from decay of 147Sm (half life about 100 billion years).

There was a flaw in this counter argument.  The similarity of chondritic and terrestrial Nd isotope patterns might have stemmed from isotopic measurements that were insufficiently precise to detect significant differences. Mass spectrometry has undergone a near-quantum leap in precision.  Applied to the chondrite-Earth rock comparison, the neodymium data for chondrites remains as determined earlier, but the 142Nd/144Nd ratios of terrestrial rocks turn out to be 20 parts in a million higher than for chondrites (Boyet, M & Carlson, R.W. 2005. 142Nd Evidence for Early (>4.53 Ga) Global Differentiation of the Silicate Earth.  Science, Published online June 16 2005; 10.1126/science.1113634).  That doesn’t seem very much, but quite sufficient to suggest plausibly that indeed the Earth’s mantle did indeed evolve from a magma ocean.  Its upper part was enriched in samarium by its fractionation as a solid that probably crystallised downwards.  Whatever was left of the original liquid would be at the base of the protomantle, and in it many other elements that favoured melt over crystals – so-called ‘incompatible’ elements – would have been enriched.  Boyet and Carson suggest that such a deep, enriched layer may amount to between 5 to 30% of the current mass of the mantle. 

The implications, if the ideas are confirmed, are enormous, because geochemists up to now have taken the bulk of the mantle that supplies basalt magmas – and whose composition is quite well constrained – to represent the whole silicate Earth.  That may satisfy geochemical parameters, but worries geophysicists.  The ‘standard’ Earth has insufficient radioactive uranium, thorium and potassium to account for the heat that flows to the surface. In fact it generates about a half, leaving the rest to speculation. One school looks to supposed gravitational potential energy locked in the core when it formed by inward collapse of iron-nickel alloy and slowly released thereafter.  Another theorises about radioactive potassium-40 combined in sulphides of the core, which also ‘leaks’ out.  The possible existence of the last dregs of an early magma ocean, near the core-mantle boundary (CMB), would not only account for 43% of surface heat flow, but might also drive convection in the liquid outer core as a means of generating Earth’s magnetic field.  Even more important, it might fuel the rise of plumes from the CMB that are increasingly implicated in periodic repaving of the Earth’s surface by flood-basalt volcanism.  Since flood basalts are a popular source for mantle geochemists’ data, why are the signs of such a peculiar source region not clear in their analyses?  Either they are not looking with the requisite precision, or the source itself does not move with plumes, merely setting them in motion.  Eminent geochemists see a bit of a hectic time ahead….. 

See also: Kerr, R.A. 2005. New geochemical benchmark changes everything on Earth.  Science, v. 308, p. 1723-1724.

Here is the earthquake forecast

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Earth Pages News of June 2005 reported on the development by the US Geological Survey of the first daily seismic forecasting service, which covers California.  It has a web site at http://pasadena.wr.usgs.gov/step.   The forecast is for events, generally aftershocks of earlier earthquakes, with sufficient energy to throw objects off shelves (Modified Mercalli Index VI). On June 30 2005, Lake Tahoe had a chance around 1 in 100 of such a tremblor, with the length of the San Andreas and related fault systems highlighted at between 1 in 10 000 to 1000.  Of course, it will take some time before people link as quickly as they do to the weather forecast.

Stay of execution for Quaternary

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The last remaining division of geological time that Giovanni Arduino erected in the mid- to late 18th century, has been under threat for some time (see EPN of September 2004).  For over seven years, the ‘Time Lords’ of the International Commission of Stratigraphy have striven to resolve, at least for a while, al the fundamental divisions of stratigraphic nomenclature.  To the horror of researchers concerned with the last 2 million years or so, publication of the new time scale in 2004 seemed to have allowed the Neogene to swallow the Quaternary Period whole.  Muttering broke into a storm of angry e-mails demanding its restoration.

The reason behind the annoyance is simple.  The Quaternary is unique for two reasons: it includes the Great Ice Age, and it is the time of humanity – the first stone tools appear in the geological record between 2.4 and 2.6 Ma ago.  But those who demand the resurrection of the old name are not entirely in agreement among themselves, particularly about when it started.  The problem arose from the manner in which systematisation of both relative and radiometric time evolved.  Arduino recognised four divisions only, Primary, Secondary, Tertiary and Quaternary based on decreasing compactness and complexity of rocks that he had seen in Italy.  The Quaternary was defined as unconsolidated material that sat upon the other three.  As fossils became the main tools of establishing relative time and wide correlation, Primary and Secondary were soon dropped.  But Tertiary and Quaternary remained as broad divisions until the late 20th century.  Tertiary strata became divided into 5 lesser palaeontological divisions, and Quaternary into two: Pleistocene and Holocene.  Radiometric dating demonstrated the brevity of the Tertiary compared with major stratigraphic divisions further back in time, so it was designated as a Period, subdivided into 5 epochs.  Tertiary itself then became elevated to Era status as the Cenozoic, despite its short time span, and its first three and last two epochs were bracketed by two new periods: Palaeogene and Neogene.  Development of geosciences was clearly marginalizing the Quaternary Period to which many devotees cling tenaciously.

The furore burst at the 32nd International Geological Congress in Florence in August 2004, and the ICS was duly chastened and apologetic.  It set up a task force to reunite the warring forces, or at least to draw plans for a truce. The task force voted in early June 2005 to retain the name Quaternary and to set its beginning at 2.6 Ma, thereby defining it as both the Great Ice Age and that of humankind.  Ironically, 2.6 Ma also marks the start of the Late Pliocene, defined by a Global Boundary Stratotype Sections and Point (the midpoint of sapropelic Nicola Bed (“A5”), Monte San Nicola, Gela, Sicily, Italy). You see, there has to be somewhere that you can visit and ‘put your finger on the proper boundary’.  This particular GSSP is defined as a stage in the fluctuation of oxygen isotopes in deep-sea sediments, at the start of the Matuyama geomagnetic reversal, and just below the points of extinction of two echinoid species…..  Incidentally, the ICS is by far the largest of the bodies within the International Union of Geological Sciences, the ‘UN’ of the geoscience community.  Acquiring the prestige of a GSSP ranks with many countries’ geoscientists at least as high as hosting an Olympic Games. Italy hosts 9 of the 22 Cenozoic GSSPs (5 are not yet placed), so clearly Arduino’s influence has been long lasting in some respects.  Several features of the New Timescale as a whole may confuse far into the future (should it stand the test of time).  The Stage names, learned by generations of stratigraphers, often through cunning mnemonics, are mainly taken from places or regions.  Most of the GSSPs at their bases are somewhere else (browse http://www.stratigraphy.org/).

Source: Giles, J. 2005.  Geologists call time on dating dispute.  Nature, v. 435, p. 865.

Hydrogen sulfide and mass extinction

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Naughty school kids once used to hurl glass vials that launched the most pervading smell of rotten eggs when they smashed.  Stink bombs produce hydrogen sulfide.  Interestingly, if you can smell it you are more or less safe – though not from flying glass shards.  When H2S is more concentrated, it becomes an odourless and stealthy killer, as ‘sour gas’ emitted from oil drilling rigs.  A group of anaerobic bacteria generate the gas when there are abundant sulfate ions in oxygen-starved conditions.  They use these ions as electron acceptors in their metabolism, thereby reducing sulfate to sulfide ions; a common phenomenon in stagnant swamps, and especially prevalent at depth in the Black Sea.

Several times during the Phanerozoic global ocean depths became anoxic, when thermohaline circulation shut down.  The consequences show up in black mudrocks, rich in partially broken down hydrocarbons and iron sulfide.  Some of these are major source rocks for petroleum.  Unstirred by deep current flow, bottom waters pervaded by H2S are covered by oxygenated water, so it might seem that there is little threat to surface dwellers and air breathers, although any animal unwarily entering toxic bottom water would instantly die.  That is why black mudrocks are repositories of exquisite fossils.  Should H2S build up in deep water, however, there might be chemical instability that would result in large-scale emissions to the upper ocean and to the atmosphere.  Geochemists from the universities of Pennsylvania and Colorado have made some simple chemical calculations to see if such a potentially catastrophic leakage is within the bounds of possibility (Kump, L.R. et al. 2005.  Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia.  Geology, v. 33, p. 397-400).  Theoretically it is, once a threshold concentration of around 1 mmol kg-1 of H2S dissolved in deep water is exceeded.  There would be sulfidic upwellings involving emissions of the order of teratonnes of sulfide per year to the atmosphere; more than 2000 times that today from volcanoes, with the added risk that it would also permeate upper-ocean water.

As well as witnessing mass extinctions, the Late Devonian, end-Permian and Middle Cretaceous were characterized by widespread anoxia.  Leakage of H2S would not only have killed directly, but would have destroyed the ozone layer that protects from UV radiation.  Inevitably, methane produced by other anaerobic bacteria would also have been released in the same way to force global warming.  Rather than being the result of dramatic impacts or monstrous flood basalt effusions, mass extinctions at these times would have been quiet, but efficient nonetheless

Potted history of atmospheric oxygen

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Potted history of atmospheric oxygen

The most likely hallmark of an inhabited planet is an atmosphere that contains oxygen; a simple rule of thumb made popular by James Lovelock.  By assembling complex molecules based on carbon, life increases the degree of chemical reduction in its environment.  Effectively it draws in electrons, and the counterpart of that must be that some other component loses them through oxidation.  On Earth the source of electrons needed to make organic molecules through the action of photosynthesis is predominantly the oxygen atoms locked in molecules of water and carbon dioxide.  By losing 4 electrons, 2 oxygens bonded in those two simple compounds are oxidised to become the gas O2, which itself has become the commonest and most active acceptor of electrons from reduced ions and compounds.  Oxygen gives its name to oxidation, which is the inevitable fate of most organisms, thereby reversing the process of photosynthesis.   A planet whose surface topography is continually changing, because more radioactive energy is produced in its mantle than can be lost to space by simple conduction, generates physical conditions that continually bury and store some unoxidised carbon compounds.  Carbon burial together with continued living processes keeps the photosynthetic chemical equation weighted in favour of free oxygen.

Since the domain of living things to which we and all advanced organisms belong, the Eukarya, is almost wholly one to which oxygen is vital in metabolism, there can be few more important geoscientific topics than how and when oxygen emerged as a free element.  There have been major recent developments in addressing these questions, so it is useful and fascinating to find an up-to-date and easily read review (Kerr, R.A. 2005.  The story of O2.  Science, v. 308, p. 1730-1732).  Among its highlights is evidence that although cyanobacteria (the most primitive oxygenic photosynthesisers) were definitely around at 2.7 Ga, they may not have produced oxygen until about 300 Ma later, when the first signs of free environmental oxygen appear.  Photosynthetic release of oxygen during life’s early period was not the only reduction-oxidation regime adopted by organisms.  Another of huge importance was generation of methane, which can rise to the limits of the atmosphere unlike the other major hydrogen-bearing gas, water, which is condensed out at quite low altitudes.  Photochemical breakdown of methane at the limits of outer space would release hydrogen to leak away from the Earth, removing a reductant gas that would otherwise consume highly reactive oxygen: without this process, modelling suggests that Earth’s atmosphere would never have accumulated free oxygen, even had primitive life emerged.

Once free oxygen appeared, about 2.4 Ga ago, it took almost 2 billion years for enough to accumulate so that complicated, multicelled Eukarya could use its potential (see The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic in EPN of September 2002). What kept the levels down?  Quite probably it was oxidation of sulfide minerals on exposed land.  That supplied sulfate ions to a still reducing ocean, so that sulfide ions formed again to become metal sulfide precipitates, which drew from ocean water several essential nutrients for Eukarya.  Oxygen-producing Eukarya (algae) would not be able to bloom because of this ‘starvation’.  Nonetheless, about 600 Ma ago, surface oxidation potential soared to almost modern levels, sufficient for large organisms to appear and evolve, to lead to life as we know it. Another series of questions surrounds this tremendous event, but they remain to be answered convincingly.

Another view of causes for the Younger Dryas cooling event

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High latitudes in the North Atlantic, especially on its eastern side, are warmed today by the Gulf Stream.  That current, which defies the Coriolis effect, is pulled northwards by the sinking of cold dense sea water between Greenland, Iceland and Scandinavia to form North Atlantic Deep Water (NADW).  The thermohaline circulation here is driven by both cooling of salty surface water in the Gulf Stream and further salinisation as sea ice forms in this area each winter.  The Younger Dryas cold period between 13 and 11.5 ka is regarded by most oceanographers and climatologists to have resulted from sudden freshening of the North Atlantic at these critical high latitudes, so that surface water density became too low to sink.  Such a process had occurred several times during the last glacial period, each of which has been correlated with release of massive amounts of glacial ice as icebergs.  There melting caused the freshening. The Younger Dryas is a different kind of event, because it occurred well into the period of global warming that brought the Ice Age to an end.  A seemingly plausible explanation was suggested by Wallace Broecker in 1989, who looked to explosive release of meltwater trapped in glacial lakes roughly along the Canadian-US border along the present St Lawrence River Valley, effectively flooding the source of NADW with a surface layer of low-density, low-salinity water.

The problem with Broecker’s mechanism is that sea-level records through the Younger Dryas show no sudden rise, whereas at about 14 ka a meltwater pulse had resulted in a 20 m rise over about 500 years, with no sign of a climatic response to a shutdown of the Gulf Stream by the freshening that it caused.  A similar event occurred shortly after the waning of the Younger Dryas.  There is no doubt that throughout high northern latitudes the great ice sheets were melting since about 18 ka. A new approach to the Younger Dryas concentrates on where the meltwater formed in northern North America probably escaped to the sea (Tarasov, L & Peltier, W.R. 2005.  Arctic freshwater forcing of the Youner Dryas cold reversal.  Nature, v. 435, p. 62-665).  Through their analysis of the drainage chronology of the Canadian Shield Tarasov and Pelter conclude that at the time of the onset of the Younger Dryas most flow was roughly along the present MacKenzie River valley to the Arctic Ocean.  Freshening of the Arctic Ocean would escape through the narrow Fram Straits directly to the source region for NADW. It would not necessarily have been through currents, for escape of increased amounts of pack ice would have much the same effect.  Central to their hypothesis are new data that relate to extraordinarily thick continental ice in the Keewatin glacial dome, that formed just to the east of modern Great Slave Lake.

Acidification of the oceans

When gases such as CO2 and H2S permeate through ocean water they dissolve to form weak acids: carbonic and sulfurous acid respectively. So many organisms, plants as well as animals, incorporate carbonates into their hard parts that changes in acidity constitute an important kind of stress.  The acidity of water combines with increasing pressure as water deepens to create a zone (the lysocline) in which water is undersaturated in calcium carbonate.  Below the lysocline carbonate shells begin to dissolve.  Deeper still is a level (the carbonate compensation depth, or CCD) below which there is no free CaCO3 in the water column.  Falling shelly material dissolves completely, so that deep-ocean sediments contain few if any shells other than those of silica-secreting organisms.  At present the CCD is around 4 km deep.  Any shift in the pH of the oceans causes the CCD either to rise or fall.  The signatures of such shifts lie in the composition of ocean-floor sediments.  In the deepest parts, where silica and clays dominate, layers in which carbonate shells are preserved signify a decrease in acidity (increased pH) and descent of the CCD to below the elevation of the ocean floor.  On the other hand, the appearance of pure clay-silica oozes in otherwise shelly muds, where the sea floor has been well above the CCD for long periods, show that acidity increased (a drop in pH) over a period.  Such anomalous sediment layers are often easy to see in cores because their colour is different from the common sediments.

In cores from ocean depths between 2 and 4 km, the second kind of anomaly appears consistently at the level of the Palaeocene-Eocene boundary: it signifies a massive increase in acidity (Zachos, J.C. et al. 2005.  Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum.  Science, v. 308, p. 1611-1614). Carbon-isotope measurements from the same cores also show a marked shift.  The sediments are depleted in 13C, which has generally been taken to indicate a huge release of methane from storage as gas hydrate in sea-floor sediment at the time of the Palaeocene-Eocene boundary.  Most palaeoclimatologists consider the C-isotope “spike” to be a proxy for sudden, intense warming that resulted from methane – a more efficient ‘greenhouse’ gas than CO2 – and the carbon dioxide produced as it was oxidized.  The range of water depths where the carbonate-free layers occur enables marine geochemists to estimate the rate of acidification.  In around only 10 ka the CCD rose 1.3 to 2.0 km above its current level.  From the degree of acidification needed it seems that considerably more than 2 x 1012 t of carbon was released in the form of methane that eventually oxidized to CO2, and returned to the ocean.  The carbonate content of the ocean sediments rose gradually over the next 100 ka, by the end of which the former balance was restored. This information in turn gives a picture of the rate at which sudden ‘greenhouse’ events subside once their cause has stopped being produced, almost certainly by the drawdown of atmospheric CO2 by weathering of silicate minerals exposed on the continental surface.

At the end of the Palaeocene, the effect on organisms was mainly restricted to benthic foraminifera that live in moderately deep water, which show a selective extinction.  The eventual release by human activity of carbon contained in accessible fossil fuel reserves, will give a mass of carbon in ‘greenhouse’ gases of about twice that released at the Palaeocene-Eocene boundary over perhaps 300 years.  Such rapid release may result in acidity that is incompatible with carbonate-secreting organisms anywhere in the oceans: the CCD will effectively be at the sea surface