Banded iron formations (BIFs) are by far the largest repositories of economic iron ore on Earth, and mines in them dwarf all but the largest surface coal mines. They also present one of the most enduring paradoxes in geochemistry. BIFs consist of oxidised iron in the form of iron(III) oxide (mainly hematite, Fe2O3), yet formed before about 2 billion years ago, when the Earth’s atmosphere and oceans were devoid of free oxygen. In fact the very formation of BIFs presupposes that iron must have been freely available in seawater as dissolved ions of its reduced form, iron(II). Their formation has been linked to the excretion of oxygen by photosynthesising cyanobacteria in the photoc zone of Archaean and Palaeoproterozoic seas, which would immediately combine with iron(II), thereby buffering environmental oxygen at very low levels. The problem with that hypothesis is BIFs show every sign of having accumulated in extremely quiet conditions: they contain the most exquisitely fine banding that in some cases has been linked to a diurnal cycle. The photic zone would have been one of high wave energy. A more environmentally viable hypothesis has to take account of that and place the environment of BIF deposition in deeper water. Biogeochemists of the California Institute of Technology and the University of Alberta have perhaps helped to resolve all the paradoxes surrounding BIFs (Kappler, A. et al. 2005. Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology, v. 33, p. 865-868). The bacteria that they cite as agents for iron(III) precipitation use the photon energy of ultraviolet radiation to oxidise iron(II) to iron(III), and in doing so use the freed electrons to reduce CO2 and water to carbohydrate – this is not photosynthesis that uses light energy to increase the energy of electrons so that they perform the life-giving reduction. Solar ultraviolet radiation penetrates to much greater depths than the red light exploited by photosynthesisers, and could therefore fuel BIF formation below storm wave base at depths greater than 200m.
Growing evidence for ‘hobbits’
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.
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A dialogue concerning world-shattering events
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.
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Irish mineralising fluids
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.
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State of the art seismic imaging
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.
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Multimedia volcanoes
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
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Photosynthesis during a ‘Snowball’ epoch
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.
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Martian methane: a bit of a blow
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.
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Climate change and human evolution
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.
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Climate and the end-Permian extinction
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.
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