Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

The swaddled mantle

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A great deal of both theoretical petrology and tectonics hinges on how temperature changes with depth within the Earth. The geotherm, as this variation is termed, depends on how heat is conducted – by conduction, convection or radiation – and where it is produced – either as a relic of original heat of Earth’s accretion or through decay of radioactive isotopes. There are plenty of imponderables, and it would be safe to say that, below the depths at which we can measure temperature (a few km), geotherms are guesswork. Metamorphism, partial melting in crust and mantle, and the rigidity of rock depend on temperature and pressure. Rocks that are too cool to act in a plastic manner tend only to conduct heat, and they are poor conductors. This applies to most of the crust, especially the lower continental crust, which is also low in heat producing radioactive K, U and Th isotopes and rigid. The upshot of this is that the crust acts to insulate the mantle, and that implies build-up of heat and temperature just below the crust. A new means of measuring a rock’s thermal conductivity has revealed that thermal conductivity actually decreases as temperature rises (Whittington, A.G et al. 2009. Temperature dependent thermal diffusivity of the Earth’s crust and implications for magmatism. Nature, v. 458, p. 319-321). The range of crustal temperatures in both continental and oceanic crust roughly halves conduction in the lower crust from previously measured values. This further increases insulation of the mantle, boosting the chances of partial melting.

This tallies with a coincidentally published account of how seismic shear waves change speed with depth beneath the oceanic crust (Kawakatsu, H. et al. 2009. Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates. Science, v. 324, p. 499-502). As well as sharply showing up the lithosphere-asthenosphere boundary, thought to be a transition from brittle to ductile behaviour, it detects thin layers of partially melted peridotite, which facilitates plate tectonics. A further coincidence is publication of an analysis of 15 years of global earthquake records that focuses on the base of the lithosphere (Rychert, C.A. & Shearer, P.M 2009. A global view of the lithosphere-asthenosphere boundary. Science, v. 324, p. 495-498). As well as its thickness this effectively maps the top of the asthenosphere and therefore the thickness of tectonic plates across the planet, albeit crudely (previously both had been estimated from surface heat flow and theoretical models). Beneath cratons that have remained sluggish for more than a billion years, the asthenosphere is deep (~95 km) and thin, shallowing and thickening appreciably beneath more recently active continental belts. Despite being the uppermost Earth and the stuff of plates and the medium upon which they move, respectively, the lithosphere and asthenosphere are less-well known than the mantle and even the core in terms of the mechanical properties. That may sound odd, but there is a good reason why it is so: more deeply travelled seismic waves are a great deal easier to record by the global network of seismic stations than are shallow regions.

On the edge of chaos in the Younger Dryas

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Around 13 thousand years ago, the world was warming rapidly and the great northern ice sheets in retreat. Plants, animals and humans in Europe were able to and did migrate northwards. Sea level still being low, there was nothing to stop decolonisation of Britain by crossing the huge fluviatile plain of Doggerland where the southern North Sea now stands.(see Return to ‘Doggerland’ in September 2008 issue of EPN). At 12.9 ka there came the shock of a rapid temperature fall at the start of the Younger Dryas episode, when ice sheets began to re-establish themselves in the upland areas of Britain and Scandinavia. What happened to those intrepid migrants we may never know, but what they would have faced had they chosen to remain in the game-teeming NW Europe of that episode has become clearer with detailed investigations in sediments at the bottom of a Norwegian lake supplied by melt water from glaciers (Bakke, J. et al. 2009. Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience, v. 2, p. 202-205).

The research by Norwegian and German scientists used two interesting proxies for glacial advance and retreat: the amount of sedimentary titanium and the density of the sediment, both of which would have varied with the rate of glacial erosion. The data were calibrated to time by 96 14C dates, and the sampling frequency (every 0.06 mm for Ti and 5 mm for density through a 1.4m core that represents 1700 years) was sufficient potentially to resolve events to a few days and 6 months respectively. Allowing for background ‘noise’ effects, certainly monthly and annual changes should show up, and indeed they do. The pattern is one of rapidly changing conditions between warm and frigid, which the authors interpret as a result of repeated ‘boom and bust’. At 12.18 ka, further cooling occurred and the late Younger Dryas is the more chaotic part of the record. The hypothesis is that the fluctuations reflect growth and shrinkage of sea ice in the North Atlantic, matched by growth and melting of glaciers. Brief warming during periods of prevailing westerly winds melted glaciers, but fed vast amounts of fresh water to the North Atlantic that in turn encouraged surface waters to freeze. Sea-ice formation and the build-up of a polar high pressure area drove weather systems conducive to westerlies southwards, when glaciers grew. Something suddenly stopped this chaotic behaviour and high latitudes rapidly emerged from frigidity at 11.7 ka: the Holocene had begun and, soon, so would humanity in an equally chaotic manner.

 

Climate at the Eocene-Oligocene (E-O) boundary

Oxygen isotopes from benthic foraminifera in deep-sea sediment cores show an abrupt increase in δ18O at around 34 Ma, which spanned a mere 300 ka. This is taken to indicate withdrawal of ocean water to polar ice caps on land that diamictites from high southern latitudes link to the beginning of glaciation of Antarctic. Then as now, the south polar region was thermally isolated, probably as a result of its having become surrounded by seaways and development of the Antarctic Circumpolar Current from the Palaeocene onwards as a result of the final break-up of Gondwana when it became separated from Australia and South America. Other factors at the E-O boundary seem to have been decreasing atmospheric CO2 and low solar heating as a result of the Milankovich effect. Cooling due to such factors was disrupted and delayed by the spectacular global warming at the Palaeocene-Eocene boundary (55.8 Ma) as a result of massive methane release to the atmosphere. Detailed proxy records from both high- and low-latitude sea-floor sediment cores now resolve fine detail of the shifts in sea-surface temperature (SST) at the E-O boundary (Liu, Z.  et al. 2009. Global cooling during the Eocene-Oligocene climate transition. Science, v. 323, p. 1187-1190). The most profound shift in SST took place at high latitudes (in both Northern and Southern Hemispheres) with a drop of around 5 to 9ºC between 34 to 33.5 Ma. This was followed by slight rise to about 3ºC below pre-E-O conditions. Surprisingly, data from low latitudes ‘flat-lined’ at around 28ºC across the transition, suggesting steady evaporation of seawater, more of which would have precipitated as snow at high latitudes. The ‘hothouse’ conditions of the Cretaceous and early Cenozoic saw estimated high-latitude sea-surface temperatures rise from about 7ºC to 12ºC by the Early Eocene. The protracted global cooling that followed reached about 7ºC by about 42 Ma, which stabilised until 40 Ma when SST fell to about 4ºC just before the E-O boundary (see http://www.learner.org/courses/envsci/visual/visual.php?shortname=cenozoic).

The sudden start of Antarctic glaciation at 34 Ma looks increasing like an example of a chaos-like ‘flip’ in global climatic conditions brought on by a blend of factors that collectively reached a threshold, which once crossed permitted no escape, at least not over the last 30 Ma or so (Kump, L.R. 2009. Tipping pointedly colder. Science, v. 323, p. 1175-1176). That is a point that should not be lost at a time when anthropogenic global warming continues unabated, despite so much hype by the G20 leaders at their London meeting in early April 2009. Climatic ‘flips’ can go either way.

 See also: Documenting the Palaeogene transition from ‘hothouse’ to ‘icehouse’ in EPN for August 2005, and Magmatic link to the Palaeocene-Eocene warming in EPN for July 2007

Flirting with hand axes

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A biface, Acheulean hand axe is more than object of beauty produced by exquisite skill, this industrial genre was invented by African Homo ergaster around 1.6 Ma ago, became a central feature of Palaeolithic archaeology, and lasted until the last few hundred thousand years. Nobody doubts that production of these objects implies a brain that fashioned able to visualise a complex shape within a shapeless lump of rock and to devise a way of achieving it. Moreover, its longevity spanning several species of Homo to our own shows that skills were efficiently passed down through hundreds of thousand generations: possible evidence for linguistic skills in the makers and teachers. But what was it for? Experts have been at a loss to agree on a function: too heavy for hafting to a spear; more awkward for cutting than earlier Oldowan pebble fragments; produced with careful three-dimensional symmetry when a hand tool needs none; time consuming to make yet often found in great abundance and apparently hardly used. One idea is that they were in fact for throwing, in the manner of a discus, yet broken biface axes are rare. A more appealing hypothesis is that they were made for ‘show’ as an element in human sexual selection (Kohn, M. & Mithen, S. 1999. Hand axes: products of sexual selection? Antiquity, v. 73, p. 518-526). Kohn and Mithen argued that the primary function of hand axes was to advertise a maker’s “good genes”: an indicator of the knap­per’s geographic knowledge of suitable resources; his ability to execute a plan; his dexterity and patience; and his so­cial awareness. Those are all attractive qualities in a potential mate. They also suggested that the axes’ often near-pristine quality and occurrence in great numbers at some sites indicate that once their purpose was served, they were thrown away: ‘That man is so cool, he must be good at surviving’. Ten years after Kohn and Mithen first mooted the hypothesis it has come under criticism by April Nowell and Melanie Lee Chang, of the universities of Victoria, Canada and Oregon USA, respectively  (Nowell, A. & Chang M.L. 2009.The case against sexual selection as an explanation of handaxe morphology. Paleoanthropology, v. 2009, p. 77-88).

The critique begins by examining Kohn and Mithen’s interest in symmetry as an element in attractiveness, that Nowell and Chang concede, but consider to have arisen not in a sexual context but in development of vision, despite vision being an evolutionary ‘given’ vastly older than hominins. After a discussion of how fully modern human females base their sexual choices on non-physical attributes of potential mates, such as “niceness,” intelligence, sense of humour, compatibility, willingness to work hard and evidence that the partner in question is attracted to them, Nowell and Chang examine available archaeological evidence. Much of this concerns the ‘absence of evidence’. For instance, there is no evidence to suggest that females did not make hand axes and living females in gatherer-hunter societies do make tools. Other criticisms include: the absence of hand axes from Asia until migration there by H. sapiens [but the biface axe had not been invented when H. ergaster migrated there from Africa around 1.8 Ma]; not all biface axes are symmetrical [but they are nonetheless impressive]; and axes in large numbers generally occur where prey has been butchered, as at Boxgrove, and may have accumulated by hundreds of years of use and loss at such sites by seasonal hunting. The most serious criticism is that some hand axes do show minute patterns that indicate that they were used; although most axes have never been examined for wear patterns. My own conclusion is that the critique is based on absence of evidence for biface axes as ritual objects in sexual selection, but that is not evidence of absence, and I wonder if the 10 years taken to bring together contrary evidence has a bit to do with casting doubt on a not quite ‘PC’ idea. There are many intriguing facets of the fossil and archaeological records of hominins, none more so than those which may have a cultural connotation, like ochre caches (see Deeper roots of culture in EPN of March 2009) and the tear-shaped Acheulean axe. For most we may never know their true context, but can be sure that any curiosity and imagination we apply are reflections of imaginative and curious forebears.

Homo erectus in a cold climate

The famous Zhoukoudian Cave where Peking Man, now known to have been Homo erectus, was first found in 1929 is a lugubrious place. It seems the hominin fossil remains of at least 40 individuals were dragged there and eaten, hopefully by predators. They are by no means the oldest Asian hominins at less than 1 Ma, and their ancestors, probably African H. ergaster, migrated that far around 1.6 to 1.8 Ma ago. Until this year, decent ages from Zhoukoudian were a problem: the errors on estimates of around 500 ka were too large (the likely time lies in a ‘datability gap’ between the capabilities of Ar-Ar and 14C dating methods) to see if the hominins were living at such a high latitude (40ºN) in warm or cold conditions. The latter would be of great interest as it suggests both the use of fire and clothing, and probably adaptation to cooked tubers. In fact, even in the current interglacial episode Beijing gets mighty cold in winter. However, cosmic-ray bombardment can produce unstable isotopes that are suited to dating in that gap, provided materials have been exposed to them. The fossil-containing sediments in Zhoukoudian Cave contain quartz that was exposed at the surface and washed in at the same time as H. erectus individuals were dragged in. Decay of cosmogenic 26Al to 10Be and measurement of parent and daughter isotopes in quartz grains have yielded ages of 770±80 ka, somewhat older than earlier estimates (Shen, G. et al. 2009. Age of Zhoukoudian Homo erectus determined with 26Al/10Be dating. Nature, v. 458, p. 198-200). This age roughly correlates with layers in the western Chinese windblown loess deposits that were deposited during the dry conditions of a minor glacial episode.

See also: Ciochon, R.L. & Bettis, E.A. 2009. Asian Homo erectus converges in time. Nature, v. 458, p.153-154. Gibbo0ns, A. 2009. Ice age no barrier to ‘Peking Man’. Science, v.  323, p. 1419.

 

Walking with the ancestors

From time to time the most evocative hominin trace fossils come to light, such as the Australopithecus afarensis footprints fount by Mary Leakey at Laetoli in Tanzania. A recent one is of footprints of a probable H. ergaster dating back to 1.5 Ma near Lake Turkana in Kenya, not far from the site of the famous ‘Turkana Boy’ skeleton of the same species (Bennett, M.R. and 11 others 2009. Early hominin foot morphology based on 1.5-million-year old footprints from Ileret, Kenya. Science, v. 323, p. 1197-1201). Not only does the trackway reveal details of flesh, skin and bones of the feet, but careful analysis of 3-D scans of the prints, in the context of the mechanical properties of the material walked upon, allows the authors to show that the person who left them moved in essentially the same way as do we when walking through soft mud. They are distinctly different from the Laetoli prints, showing arches and very distinct big toes that are so necessary for ‘springiness’ and bipedal balance respectively.

See also: Crompton, R.W. & Pataky, T.C. 2009. Stepping out. Science, v. 323, p. 1174-1175.

‘Clean’ coal and soda pop

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An option much touted as a means of having our cake (power stations fired by fossil fuels, especially coal) and eating it (escaping runaway global warming while enjoying a high-energy lifestyle) is extracting carbon dioxide from flue gases, or even the atmosphere itself, and safely disposing of it in long-term storage. Carbon capture and storage (CCS) is not a well-tried technology. Yet some authorities claim it is at the least a means of ‘tiding-over’ an economy that depends to such a degree on fossil carbon burning as an energy source that it seems unlikely that alternative, carbon-neutral sources can be deployed in time to stave off increasingly awful and plausible climate and thereby social scenarios. There are others who are convinced that CCS is merely an excuse to continue with ‘business as usual’, and therefore fraught with dangers. Whichever, there are elements of CCS that do concern geoscientists, such as where should it be stored and in what form. Leaving aside some of the geological issues of storage, such as depleted natural petroleum fields or deep aquifers, what happens to CO2 at depth? There are five possibilities: it remains as a gas; under high pressure it may take on liquid form (CO2 can exist only as gas or ‘dry ice’ at atmospheric pressure); it reacts with the rock itself to form some kind of carbonate; under moderate pressure and low temperature it may combine with water to form a gas-hydrate ‘ice’, as does methane; or it may dissolve in water under high pressure.

The ideal form for long-term storage would be in the form of solid carbonate, but that demands bicarbonate ions combining with calcium, magnesium or perhaps sodium ions. One possibility is through dissolution in highly saline groundwater. The chemical reactions are not complex, but depend on the solubility of carbonates being exceeded because of massive increases in bicarbonate concentrations. However, experiments have had little success. Another means of solid storage is by the combination of atmospheric CO2 with calcium hydroxide to form calcium carbonate, which is what happens when lime plaster slowly ‘cures’. The downside is that the only means of making Ca(OH)2 is by kilning limestone: no free lunch there. To cut a long story short, a view is emerging that CO2 pumped, in whatever form, into wet rock will end up dissolving in groundwater, to form vast quantities of ‘sparkling’ water, or ‘soda pop’ (Gilfillan, S.M.V. and 10 others 2009. Solubility trapping in formation water as dominant CO2 sink in natural gas fields. Nature, v. 458, p. 614-618). The British, Canadian, US and Chinese team investigated nine natural gas fields in which CO2 is present as well as petroleum gas, using noble gases and carbon isotopes as tracers of the chemical fate of the natural CO2 as the reservoir rocks filled with oil and natural gas during maturation. They discovered that the bulk of CO2 ended up dissolving to form a weakly acidic water under pressure. This is a recipe for filling huge analogies of soda siphons. They did discover that some CO2 ended up as solid carbonate, but no more than 15%. As those who add Perrier or Volvic to their Scotch should know, carbonated springs are not unknown. Consequently, CCS that uses confined aquifers poses the danger of eventual leakage, whether CO2 is stored as gas, liquid or in solution. Petroleum geologists often claim that no trap is leak proof, and extensive areas of gas leakage are known over most oil fields; they are an important sign for explorationists, if they can be detected. The other issue is that fans of CCS set much store in re-use of depleted commercial oil and gas fields for sequestration. Such fields have already been depressurised, and nobody knows whether or not they were leaky to gas and water.

See also: Aeschbach-Hertig, W. 2009. Clean coal and sparkling water. Nature, v. 458, p. 583-4.

The Great Bend of the Pacific ocean floor

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Ocean island chains are trackways of moving lithospheric plates relative to the underlying mantle. Mantle hotspots act in a similar manner to a candle that would burn a line in a sheet of paper were one to be passed over it. The largest, most coherent and best studied ocean island chain is that of the Hawaiian Islands and the Emperor Seamounts  in the NW Pacific. The volcanoes that built the chain range in age continuously from Late Cretaceous (81 Ma) at the northern tip of the Emperor Seamounts where they touch the Kamchatka Peninsula to the present in the Big Island of Hawai’i itself. So far, so good for the hotspot-track hypothesis. But the chain is bent into a WNW segment (Hawaii) and one that trends NNW (Emperor). That might seem to be superb evidence that the direction of West Pacific sea-floor spreading underwent a sudden, 60º change around 47 Ma (the age of the Diakakuji seamount at the apex of the bend). However, measurements in 2001 of palaeomagnetic latitude in sea-floor cores along the chain revealed clear palaeomagnetic evidence that the Hawaiian hot spot has not always been fixed relative to moving lithospheric plates.  From Late Cretaceous to Late Eocene times the hotspot seems to have been was shifting southwards relative to the north magnetic pole at a rate comparable with that of sea-floor spreading, and then became stationary to explain the 60° bend in the chain (See American Geophysical Union 2001 Fall Meeting in EPN for January 2002).

Further work has been done since 2001, and a review of the huge oddity that bucks John Tuzo Wilson’s 1963 theory of hotspots fixed in space and time is timely (Tarduno, J. et al. 2009. The bent Hawaiian-Emperor hotspot track: inheriting the mantle wind. Science, v. 324, p. 50-53). Data have moved on to suggest that the hotspot is indeed the head of narrow mantle plume originating deep down, perhaps even near the core – mantle boundary (CMB). But could such a massive structure change it’s behaviour so that its head would move? Some have suggested the development of a propagating crack in the Pacific lithosphere and then its closure, but no evidence points unerringly that way. After considering a range of possible mechanisms, the authors suggest that the great bend records past changes in mantle flow beneath the West Pacific, so that the plume would itself have bent in the vertical dimension. Seismic tomography has revealed apparently low-angled zones of hot, low-velocity mantle, such as one that may (or may not) connect with the Afar plume beneath the triple junction of the East African Rift, the Red Sea and the Gulf of Aden after rising from the CMB south of Cape Town. They are tantalising results, because the resolution is simply not good enough to be sure. It needs an order of magnitude better tomographic resolution of mantle features to truly make more headway.

When the Mediterranean evaporated

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Much to geologists’ surprise seismic surveys and drilling of the Mediterranean basin revealed that it is floored by an immense thickness of evaporite salts, laid down during the Late Miocene about 6 Ma ago (Messinian Stage).  The event has been dubbed the Messinian salinity crisis, and ascribed to the cutting off from the Atlantic of the Mediterranean Sea causing lowering of sea level by evaporation. The formation of the evaporite sequence has been overshadowed by what happened to restore the Mediterranean: a humongous waterfall at the Straits of Gibraltar. New modelling of the salt-forming event has had a technically surprising outcome (Govers, R. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, v. 37, p. 167-170). It suggests that most of the salt body formed before sea level fell. Sea level lowering reduced the load on the sea floor and allowed isostatic uplift to develop a flow barrier at the Straits of Gibraltar, further cutting off resupply of Atlantic water. The other factor seems to have been the effect of sluggish eastward subduction of a lithospheric slab that eventually resulted in subsidence so that the Atlantic could re-flood the Mediterranean basin.

Wow! Columnar joints found in Martian lava flow…

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From time to time I wear my spleen on my sleeve over issues of scientific priority. Orbiting Mars are imaging devices whose data, if they were of the Earth’s surface, would cost geoscientists the proverbial arm and a leg.  ‘Astrogeologists’ get those from Mars for nothing. The latest result explains why I get annoyed; and I hope many others do as well (Milazzo, M.P. and a great many others 2009. Discovery of columnar jointing on Mars. Geology, v. 37, p. 171-174). The High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter, can resolve pixels 30 cm across (about the same as the best, classified military data of Earth from spy satellites). It has stereoscopic capacity capable of producing not only stunningly informative 3-D visualisations but also topographic elevation data sufficiently precise that they could be used ‘at home’ for large-scale civil engineering, for instance routing water pipelines. The US Department of Defence vetoes access by scientists to near-global SRTM DEMs with even a 30 m resolution, the degraded 90 m version being freely available. Sub-metre DEMs can be produced from aircraft for the Earth’s surface, but at very high cost.

The paper reports one of the most common features exhibited by thick lava flows and other tabular bodies of igneous rock that cooled slowly. Visit the Giant’s Causeway in Antrim to see columnar joints, and put your child on one for scale. In fact there are thousands of such sights on Earth, and any planet that has a volcanic history will have columnar joints. Similar quality data is awaited from the Moon, and you can bet your intimate garments that some bright spark will report much the same. Meanwhile, there are over a billion people drinking hazardous water when geologists armed with data this good – and the inclination – could find safe supplies in the rocks beneath them.

Comet slew large mammals of the Americas?

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Shortly before the start of the Younger Dryas cold period, around 12.9 ka, the Palaeoindian Clovis culture of North America seems to have come to an abrupt halt. The North American mammoths on which the Clovis people preyed also disappear from the fossil record. Some folk reckon that early immigrants from NE Asia devoured the last of the mammoths, as they ate their way through two continents en route to Tierra del Fuego. Equally imaginative scientists have been suggesting since 2007 that an extraterrestrial cataclysm was responsible for climate change and the demise of both mammoths and the Clovis people (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008). Evidence found just beneath a sediment layer that marks the outset of the Younger Dryas included: excess iridium; tiny spherules; fullerenes containing extraterrestrial helium; nanodiamonds and evidence for huge wildfires. Neither crater nor shocked mineral grains have been found, and the proponents of this controversial idea have opted for a cometary airburst as culprit – an impact would have produced shocked debris. The authors have had a ‘bad press’, but remain undeterred and have published photomicrographs of diamonds in minute spherules made of amorphous carbon (Kennett, D.J. and 8 others 2009. Nanodiamonds in the Younger Dryas boundary sediment layer. Science, v. 323, p. 94). There is a problem or two with the hypothesis: mammoths, albeit little ones, lived on Wrangel Island in the Arctic Ocean until 1650 BC; had some kind of cosmic encounter in North America set global cooling in motion at 12.9 ka, then the best place to look for evidence would be in the Greenland ice cores, in which diamonds have yet to be found. No-one doubts that diamonds do occur in the sediments formed just before the Younger Dryas, but experts don’t accept them as irrefutable evidence for impacts (Kerr, R.A. 2009. Did the mammoth slayer leave a diamond calling card? Science, v. 323, p. 26). But the plot thickens. A Belgian and German team has discovered that forest topsoils, grasslands and swamps, no more than a few thousand years old, from 70 sites across Europe also contain nanodiamonds. Although one member of that team reportedly has no idea where they came from, a website (http://www.chiemgau-impact.com/) hints that a very young (2500 years) impact site in Bavaria may be the source. While the end-Clovis diamonds may not have triggered global cooling and killed off mammoths, they could well set off a research line aimed at documenting hazardous extraterrestrial events of the recent past and puzzling occurrences in the archaeological record.

See also: Herd, C.D.K et al. 2009. Anatomy of a young impact event in central Alberta, Canada: Prospects for the missing Holocene impact record. Geology, v. 36, p. 955-958.

Chinese dam implicated in the 2008 Sichuan great earthquake

Four years after the completion of the Koyna Dam in India’s Maharashtra State in 1963, the surrounding area experienced a magnitude 6.5 earthquake. Because the region is free of active tectonics, the earthquake was a surprise. The possibility that it could be linked to filling of the reservoir behind the Koyna Dam became a proven fact when the region subsequently became plagued by minor seismicity. In the immediate aftermath of the magnitude 7.9 Wenchuan earthquake in Sichuan, China on 12 May 2008, which killed 80 thousand people, there were alarms about the possible failure of weakened dams and lakes blocked by landslides in the Longmen Shan mountains. But now suspicion has fallen on the earthquake having been caused by the load that filling a new reservoir created only 5 km from the epicentre and 500 m from the fault that failed during the disaster (Kerr, R.A. & Stone, R. 2009. A human trigger for the great quake of Sichuan? Science, v. 323, p. 322). Calculations of the stress from this loading suggest that it was 25 times that of the tectonic stresses in the region.

Rheic Ocean reviewed

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Since the late 1960s when John Dewey and a few other geologists began to apply plate-tectonic ideas to palaeogeography, most of us when asked to name an ancient ocean would have blurted out ‘Iapetus’. Yet, another Palaeozoic ocean, the Rheic Ocean, left a far more profound mark on the Palaeozoic world: its closure around the end of the Palaeozoic Era united all the continents in Wegener’s Pangaea supercontinent, and threw up a vast mountain belt at the suture. The earlier evolution of the Rheic Ocean  involved the spalling of a series of microcontinental slivers from the flank of the earlier Gondwana supercontinent. Damien Nance and Ulf Linneman review the fascinating story of the Rheic Ocean in a nicely succinct way (R.D. Nance & Linnemann, U. 2008. The Rheic Ocean: Origin, evolution and significance. GSA Today, v. 18 (December 2008m issue), p. 4-12).

 

Archaean ‘Waterworld’

Readers might remember with some pain the 1995 film Waterworld, starring Kevin Costner: an actor so wooden he could not sink. That was based on the unlikely scenario that if all the ice caps melted the continents would be drowned entirely. In fact that global melting would raise sea level by a mere 67 m. A far higher sea-level rise took place during the Cretaceous, arguably because fast sea-floor spreading and subduction created a larger volume of ‘warm’ and so less-dense ocean lithosphere than there is now. The volume of the ocean basins shrank as a result, displacing ocean water onto low-lying areas of the continents. Something more dramatic has been suggested for the Archaean Earth (Flament, N. et al. 2008. A case for late-Archaean continental emergence from thermal evolution models and hypsometry. Earth and Planetary Science Letters, v. 275, p. 326-336). The starting point for the discussion by Flament and his Australian and French colleagues from the universities of Sydney and Lyon is that the reason for the present hypsometric distribution of surface elevations between ocean floor and continents is cooling of the Earth that has changed the isostatic balance between oceanic and continental lithosphere. That progressively sharpens the topographic contrast thereby increasing continental freeboard. Archaean times involved a hotter mantle due mainly to greater radiogenic heat production. Flament ­et al. argue that would have lessened the rigidity of continental lithosphere, so reducing the ability of the crust to thicken, whereas ocean floor would have had a higher relative elevation, so reducing ocean basin volume. As in the Cretaceous oceans would have flooded continents, but to a far greater extent, so that as little as 3% of the Earth surface was land.

Nitrogen isotopes and a change in the Archaean biosphere

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All life forms require nitrogen fixation; pretty obvious since they are largely made of C, H, O, N and P. It happens through two main processes in the nitrogen cycle: anaerobic reduction of dinitrogen (N2) to ammonium ions (NH4+) and the degradation of that by oxidation to nitrite (NO2–) or nitrate (NO3–) ions (nitrification). Both kinds of process allow nitrogen to enter cells today, but before the Earth’s biota evolved oxygen production through photosynthesis only the first, anaerobic process was possible. As with many elements that have several stable isotopes – nitrogen has two: 14N and 15N – such chemical processes favour one isotope over the others leading to fractionation in the overall environment. A measure of the relative proportions of nitrogen isotopes is δ15N, and its mean value in modern seawater is +5‰ due mainly to the reduction of nitrite and nitrate ions by denitrification. In an oxygen-free ocean δ15N would be significantly lower. Nitrogen-isotope studies of the organic matter in ancient sediments should therefore be a test for the presence of free oxygen in the environment.

In Archaean shales that have not been much metamorphosed δ15N is generally low, as expected. However, there have been hints of higher values from the youngest Archaean strata that do indicate oxygen. The Hamersley Group of Western Australia, famous for its vast reserves of banded ironstone formations (BIFs), includes a 50 m thick carbonaceous shale deposited at the very end of the Archaean around 2.5 Ga (Garvin, J. et al. 2009. Isotopic evidence for an aerobic nitrogen cycle in the latest Archaean. Science, v. 323, p. 1045-1048). Detailed geochemical analyses through the shales and enveloping BIFs, including nitrogen isotopes, show considerable variations ascribed to environmental changes. Aerobic denitrification is marked by a shift from 1 to 7.5‰ in δ15N within the shales, which correlates with shifts in molybdenum and the proportions of sulfur isotope. The real significance of the paper is not that the study detected evidence of free oxygen in the Archaean – the BIFs formed by combination of iron-2 ions with oxygen. It shows that before 2.5 Ga prokaryote organisms had already to perform aerobic nitrification as well as denitrification, of which there are only three groups nowadays, two of Bacteria the other of Archaea.

 The Palaeocene Snake of Death and torrid times

As a reader of anything connected with exploration of the Amazon as a kid, I developed a perfectly rational fear of snakes, especially anacondas that ate pigs. To my horror I awoke one snowy February morning to an item on the BBC Radio 4 Today programme about the biggest snake that ever lived (Head, J.J. and 7 others 2009. Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures. Nature, v.  457, p. 715-717). At 13 m long and weighing in at over a ton, Titanoboa could have eaten an entire family at one sitting, and gone next door for seconds: and it would probably get in the house with the booid’s celebrated stealth. Becoming calmer, I saw how interesting this gigantic people crusher must have seemed to its discovers. Seemingly the maximum size of snakes is governed by ambient temperature. The anaconda that gave me bad dreams gets to a maximum length of around seven metres in present equatorial South America (mean annual temperature in the upper 20s). Modelling based on a range of snakes now living at different latitudes suggests that Titanoboa grew Topsy-like at hotter Palaeocene tropical latitudes (a mean around 33ºC at least). We can all be thankful that such tropical temperatures would require atmospheric CO2 levels around 2000 parts per million, but this century’s possible global warming will probably mean bigger anacondas and boas for the Amazonian explorer to grapple with.

Snowball Earth and the major division among animals

There are two basic kind of animals: those whose embryos show bilateral symmetry – bilaterians like ourselves, sea urchins and lobsters, for instance – and those that don’t, such as corals and sponges. Evidence from genetic differences among living animals suggests that the evolutionary separation of the two fundamental groups was probably during the Proterozoic Eon. Calibrating molecular clocks based on DNA sequences of living organisms is possible to some extent for animal groups and the ancestral kinds preserved as fossils, for instance humans and domesticated chickens share a common ancestor that lived during the Carboniferous Period. (A propos of very little, mammals have uvulas dangling in their throats that have no other function than to make one throw up if they are tickled, and we share the uvula with birds who still use them to sing: food for the imagination there.) However, the separation of bilaterians from the others, and a great many living phyla, must have taken place in Precambrian times among ancestors with no hard parts and therefore no palpable trace of their existence. Thus, any evidence of when one or another was around is highly useful in phylogenic studies. Most such evidence is likely to come from resistant kerogen and bitumen hydrocarbons found in reduced facies sediments that occur as far back as the Archaean.

Biomarkers include organic molecules that can sometimes be linked to specific phyla, and distinctive ones are associated with either side of the bilaterian-‘others’ split (Love, G.D. and 12 others 2009. Fossil steroids record the appearance of Demospongiae during the Cryogenian period. Nature, v.  457, p. 718-721). The US-UK-Australia team sampled kerogen and bitumen from reduced carbonate sediments in the now famous Omani sequence that almost continuously spans times from the Cryogenian Period of Snowball Earth episodes, through the trace-fossil rich Ediacaran and across the Cambrian boundary. Incidentally, strata like these are source rocks for petroleum reserves in many parts of the Arabian Peninsula. Among the various kinds of molecule identified by chromatography are 24-isopropylcholestanes, degraded remnants of steroids based on 30 carbon atoms per molecule. These are characteristic of one group of sponges, i.e. non-bilaterians, and occur in the oldest samples (around 700 Ma). This shows clearly that the big evolutionary divergence predated that time and may have happened during the climatically dramatic Cryogenian.