Earth-logs

South Asian arsenic update

Published on November 16, 2011 by zooks777Leave a comment

Skin lesions from arsenic poisoning in Bangladesh — The first signs of chronic arsenic poisoning: skin keratoses. Image by waterdotorg via Flickr

That groundwater in West Bengal, India was polluted with arsenic to such levels that symptoms of poisoning had become endemic was reported by Depankar Chakraborti in 1983, leading to his being branded a ‘panic monger’ by the Indian authorities. The news broke internationally in 1993 as the now infamous tragedy in neighbouring Bangladesh emerged. Means of mitigating the effects – lesions or keratoses and skin discoloration, and later increases in incidence of several forms of cancer – and ideas of how the pollution had occurred had to await proper geochemical analyses of well waters and logging of the mainly alluvial sediments from which water was being withdrawn; another 8 years went by. Reports of arsenicosis began to emerge from other areas of alluvial sediments in SE Asia, revealing by far the worst mass poisoning in history and the likelihood that the lives of millions would be blighted by what Bangladeshis dubbed ‘the Black Rain’ from the resemblance of the characteristic skin lesions to drops of black water.

Thanks principally to the work of water engineer Peter Ravenscroft with other geochemists, the source of arsenic in groundwater was narrowed down to the effect of reducing conditions in grey, carbonaceous sandstones and peats on the mineral goethite, an iron oxy-hydroxide that forms the main colorant in oxidised sediments and whose loose structure normally encourages the mopping-up by surface adsorption of a wide spectrum of dissolved ions, including those of arsenic. Goethite readily breaks down under reducing conditions, and when that happens all the adsorbed material is released into solution. The upper parts of the alluvial and deltaic sediments in the lower reaches of the Ganges and Brahmaputra rivers contain abundant organic remains picked up when vegetation burgeoned during the Holocene, which mixed with goethite-coated sand grains derived from erosion in the Himalayan stretches of the rivers. Purely natural sedimentary and hydrogeological processes created the dreadful plight of villagers. The terrible irony was that before the 1980s there were no signs of arsenicosis, yet mortality, especially of under-fives, was very high due to water-borne pathogens in surface water supplies. Indian and Bangladeshi authorities and UN agencies waged a campaign to sink shallow wells for drinking water rather than relying on river and pond supplies. At first rural people resisted the change since they regarded water from wells as the ‘Devil’s water’, but as infant mortality began to fall, the resistance turned to rapid construction nationwide of wells, both public and private. A few years later came the ‘Black Rain’.

In the attempts to mitigate the arsenicosis plague, filters containing adsorptive materials, including goethite, were installed on pumps. However, the geochemists showed that in the deeper wells there were consistently low concentrations of arsenic in sediments that were brown-coloured due to prevailing oxidising conditions and the presence of goethite. Although arsenic was present in the sediments it was safely locked in the goethite coatings of sand grains. Steadily major public supplies were transferred to deep, high-yield wells. Alluvial and deltaic deposits are generally highly permeable, so it was feared that as the deeper wells were pumped arsenic-rich water from the reduced shallow sediments would replace the safe groundwater. Thankfully, it seems that is not likely to be a problem (Radloff, K.A. and 12 others 2011. Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand. Nature Geoscience, v. 4, p. 793-798). The study injected As-bearing groundwater into a deep aquifer and monitored its arsenic concentration over time, once in place. Within a day, the concentration of dissolved arsenic fell by 70% and by 5 days had fallen below recommended maximum levels for drinking water; a dramatic demonstration of the clean-up power of even minute films of goethite in sediments, for that seems the only explanation for the fall. The US-Bangladeshi team verified this by testing samples of the deeper sediments from drill cuttings. They mixed highly contaminated groundwater with the cuttings, to find that arsenic sorption over about a week was extremely high (~40mg kg^-1).

Bangladesh well — Water well in Bangladesh. From http://www.flickr.com/photos/waterdotorg/3696304044

Rather than just publishing their reassuring findings, the team input them to hydrogeological models of the Bengal Basin, varying hypothetical pumping rates to assess the changes in deep-groundwater chemistry over time due to downward migration of the highly polluted near-surface waters. Sure enough, the As-rich waters would end up in the deep aquifer eventually to overwhelm the sorptive capacity of its goethite content; arsenic would once again enter well supplies. However, if deep extraction was limited to drinking water by limiting pumping for irrigation to intermediate depths, safe limits could be sustained theoretically for a thousand years or more, except in some areas especially prone downward intrusion of polluted shallow groundwater. (Use of highly contaminated shallow groundwater for irrigation would simply transfer the problem to crops.) Clearly, monitoring is obligatory, but one hopes this important study does resolve the horrifying plight faced by so many people in catchments fed by Himalayan waters.

Conflicting studies fuel arsenic debate (nature.com)
“Plastic Bottle” Filters Can Help Millions Poisoned by Arsenic-Contaminated Water (forbes.com)
Arsenic In Well Water: What To Do Besides Filtration To Undo Poison? (alternativendhealth.wordpress.com)

Fracking check list

Published on November 4, 2011 by zooks7772 Comments

Britain is on the cusp of a shale-gas boom (see Britain to be comprehensively fracked? : EPN 14 October 2011) and it is as well to be prepared for some potential consequences. In extensively fracked parts of the US – the states of New York, Pennsylvania, Texas and Colorado – there are reports of water taps emitting roaring flames after dissolved methane in groundwater ignites. This is largely due to common-place household water supplies from unprocessed groundwater, which are rare in Britain. But there are other hazards (Mooney, C. 2011. The truth about fracking. Scientific American, v. 305 (Nov 2011), p. 62-67) that have enraged Americans in affected areas, which are just as likely to occur in Britain. In fact the nature of shale-gas exploitation by horizontal drilling beneath large areas poses larger threats in densely populated area, as the people of Blackpool have witnessed in the form of small earthquakes that the local shale-gas entrepreneur Cuadrilla admit as side effects of their exploratory operations .

Chris Mooney succinctly explains the processes involved in fracking shale reservoirs; basically huge volumes of water laced with a cocktail of hazardous chemicals and sand being blasted into shales at high pressure to fracture the rock hydraulically and create pathways for natural gas to leak to the wells. One risk is that this water has to be recovered and stored in surface ponds for re-use. About 75% returns to the surface and also carries whatever has been dissolved from the shales, which can be extremely hazardous. By definition a shale containing hydrocarbons creates strongly reducing conditions, which in turn can induce several elements to enter solution as well as easily dissolved salts; for instance divalent iron (Fe²⁺) is highly soluble, whereas more oxidised Fe³⁺ is not, so waters having passed through gas-rich shales will be iron-rich. But that is by no means the worst possibility; one of the most common iron minerals in sedimentary rocks is goethite (FeOOH), which adsorbs many otherwise soluble elements and compounds. In reducing conditions goethite can break down to release its adsorbed elements, among which is commonly arsenic. The blazing faucet hazard results from hydrocarbon gases leaking through imperfectly sealed well casings to enter shallow groundwater, where the gases can also create reducing conditions and release toxic elements and compounds into otherwise pure groundwater by dissolution of ubiquitous goethite, as in the infamous arsenic crisis of Bangladesh and adjoining West Bengal in India where natural reducing conditions do the damage.

What is not mentioned in the Scientific American article is the common association of hydrogen sulfide gas with petroleum, produced from abundant sulfate ions in formation water by bacteria that reduce sulfate to sulfide in the metabolism. This ‘sour gas’, as it is known in the oil industry, is a stealthy killer: at high concentrations it loses its rotten-eggs smell and in the early days of the petroleum industry killed more oil workers than did any other occupational hazard. Visit the spa towns of Harrogate in Yorkshire and Strathpeffer in northern Scotland and sample their waters for examples of what Carboniferous and Devonian gas-rich shales produce quite naturally: noxious stuff of questionable efficacy. The environmental effects of such natural seepage from gas-rich rocks tell a cautionary tale as regards fracking. The highly reducing cocktail of hydrocarbon and sulfide gases in rising, mineral-rich formation water kills the microbiotic symbionts that are essential to plant root systems for nutrient uptake die and so too do trees. The onshore Solway Basin of Carboniferous age in NW England illustrates both points, having many chalybeate springs as the sulfide- and iron-rich waters are euphemistically known and also a strange phenomenon in many of the deep valleys cut by glacial melt waters as land rose following the last glacial maximum. Once trees reach a certain height – and correspondingly deep root systems – they die, to litter the valley woodland with large dead-heads. Also leaves on smaller trees turn to their autumnal colours earlier than on higher ground. Both seem to be due to minor gas seepages from thick sale sequences in the depths of the sedimentary basin. Indeed, both are botanical indicators to the hydrocarbon explorationist.

To recap, a common size of a fracking operation using several horizontal wells driven from a single wellhead is 4km in diameter entering gas-rich shales at up to 2 km depth. Each well can generate fractures of a hundred metres or more in the shales and surrounding rocks, as they have to for commercial production. In Britain, most of the sites underlain by shales with gas potential are low-lying agricultural- or urban land. The producing rock in the Blackpool area is the Middle Carboniferous Bowland Shale that lies beneath the Coal Measures of what was formerly the Lancashire coalfield, now a patchwork of expanding urban centres. On 23 May 1984 an explosion occurred in Abbystead, Lancashire at an installation designed to pump winter flood water between the rivers Lune and Wyre through a tunnel beneath the Lower to Middle Carboniferous Bowland Fells. The Abbystead Disaster coincided with an inaugural demonstration of the pumping station to visitors, of whom 16 were killed and 22 injured. Methane had escaped from Carboniferous shales to build up in the flood-balancing tunnel soon after its construction. Methane build-ups were by far the worst hazard throughout the history of British coal mining, thousands dying and being maimed as a result of explosions. One of the largest death tolls in British coal-mining history was 344 miners at Hulton Colliery in Westhoughton, Lancashire in 1910 after a methane explosion; the methane may well have escaped from the underlying Bowland Shales.

Fracking was ‘probable’ cause of Lancashire tremors (independent.co.uk)
Michael McCarthy: Can we really manage all the risks if we allow fracking in the hope of a gas bonanza? (independent.co.uk)

Snippets on human evolution

Published on October 28, 2011 by zooks777Leave a comment

Image copyright held by author, Chris Henshilw... — Artifacts from the Blombos Cave, South Africa, including deliberately etched block of hematite Image by Chris Henshilwood via Wikipedia

The news that most humans outside of Africa carry fragments of DNA that match with those of Neanderthals and the mysterious Denisovan archaic humans ( see Yes, it seems that they did… and Other rich hominin pickings in the May 2010 issue of EPN) has entered into popular culture; or soon will have! Similar dalliances with the ‘older folk’ seem also to have occurred among those humans who remained in Africa (Hammer, M.F. et al. 2011. Genetic evidence for archaic admixture in Africa. Proceedings of the National Academy of Sciences, v. 108, p. 15123-15128). The DNA of three groups in West Africa who maintain a hunter-gatherer lifestyles show regions that are not involved in coding for proteins that differ from the African norm. This suggests mating with an entirely separate and unknown group of hominins – probably archaic forms of humans – that produced fertile offspring, probably around 35 thousand years ago. The find spurred re-evaluation of bones with a mix of archaic and modern features that were discovered in a Nigerian cave in the 1960s (Harvati, K. et al. 2011. The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology. PLoS ONE, v. 6: e24024. doi:10.1371/journal.pone.0024024). The study confirms that the skulls are outside the fully modern human range, but display a close similarity with Neanderthal and H. erectus. The big surprise is that U-Th dating suggests they are quite recent, around 16 ka. The stage seems set for nor only a burst of exploration for human remains of less antiquity than early hominins but a ‘paradigm shift’ in our view of what constitutes a human species.

See also: Gibbons, A. 2011, African data bolster new view of modern human origins. Science, v. 334, p. 167.

Another interesting link with archaic humans who had the closest of relationships with some of our ancestors is that their union may have bolstered the resistance of migrants from Africa to Eurasian pathogens (Abi-Rached, L. and 22 others 2011. The shaping of modern human immune systems by multiregional admixture with archaic humans. Science, v. 334, p. 89-94). The focus was on the human leucocyte antigen (HLA) group that is a vital part of our immune system in the form of ‘killer cells’. Part of modern Eurasian DNA that codes for the group (HLA-B*73 allele) appears in the Neanderthal and Denisovan genomes; indeed more than half the HLA alleles of modern Eurasians may have originated in this way, and have also been introduced into Africans subsequently.

Also at the front line of genomic research into human origins, DNA sequenced from a lock of hair given to an Edwardian anthropologist by a native Australian turns out to have an extreme antiquity compared with that of other Eurasian people descended from African migrants (Rasmussen, M. and 57 others. An aboriginal Australian genome reveals separate human dispersals into Asia. Science, v. 334, p. 94-98). The unique aspects of the Australian genome signify separation of a group of individuals from the main African population around 62-75 thousand years ago; significantly earlier than and different from ‘run of the mill’ migrants from whom modern Asians arose at between 25 to 38 ka. There is little doubt that native Australians are descended from the pioneers who first diffused from Africa either by crossing the Straits of Bab el Mandab or taking another route and they moved more speedily across southern Asia than other waves made possible by climate change and sea-level falls following the Eemian interglacial of 133-115 ka.

Despite the lingering Eurocentrist view that somehow fully modern human consciousness sprang into being at the time the famous French and Spanish cave art was painted, around 30 ka, increasing evidence points to an African origin for a sense of aesthetics and the ability to express it. The latest is the discovery of a 100 ka ‘paint box’ in a South African coastal cave (Henshilwood, C.S. et al. 2011. A 100,000-year-old ochre-processing workshop at Blombos Cave, South Africa. Science, v. 334, p. 219-223). The material consists of two large abalone shells containing traces of red and orange ochre, together with a hammer stone and grinder with adhering ochre, and fat-rich bones which ground-up would have produced a binder for the ochre. No art occurs in the cave and it might be supposed that the pigments were intended for face- or body adornment.

The downside of sex with Neanderthals (guardian.co.uk)
Strong isolation between Neandertals and modern humans (dienekes.blogspot.com)

The ultra-deep carbon cycle

Published on October 28, 2011 by zooks777Leave a comment

A scattering of "brilliant" cut diam... — Image via Wikipedia

The presence of diamonds in the strange, potassium-rich, mafic to ultramafic igneous rocks known as kimberlites clearly demonstrates that there is carbon in the mantle, but it could have come from either biogenic carbon having moved down subduction zones or the original meteoritic matter that accreted to form the Earth. Both are distinct possibilities for which evidence can only be found within diamonds themselves as inclusions. There is a steady flow of publications focussed on diamond inclusions subsidised to some extent by companies that mine them (see Plate tectonics monitored by diamonds in EPN, 2 August 2011). The latest centres on the original source rocks of kimberlites and the depths that they reached (Walter, M.J. and 8 others 2011. Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions. Science, v. 334, p. 54-57). The British, Brazilian and US team analysed inclusions in diamonds from Brazil, finding assemblages that are consistent with original minerals having formed below the 660 km upper- to lower-mantle seismic boundary and then adjusting to decreasing pressure as the kimberlite’s precursor rose to melt at shallower levels. The minerals – various forms of perovskite stable at deep-mantle pressures – from which the intricate composites of several lower-pressure phases exsolved suggest the diamonds originated around 1000 km below the surface; far deeper than did more common diamonds. Moreover, their geochemistry suggests that the inclusions formed from deeply subducted basalts of former oceanic crust.

Previous work on the carbon isotopes in ‘super-deep’ diamonds seemed to rule out a biogenic origin for the carbon, suggesting that surface carbon does not survive subduction into the lower mantle. In this case, however, the diamonds are made of carbon strongly enriched in light ¹²C relative to ¹³C, with δ¹³C values of around -20 ‰ (per thousand), which is far lower than that found in mantle peridotite and may have been subducted organic carbon. If that proves to be the case it extends the global carbon cycle far deeper than had been imagined, even by the most enthusiastic supporters of the Gaia hypothesis.

‘Superdeep’ Diamonds Hint at Depth of Carbon Cycle (livescience.com)
+ Diamonds’ Flaws Give Clues to Earth’s Carbon Cycle (dyslexia.wordpress.com)

Low-lying Tibet before India-Asia collision

Published on October 19, 2011 by zooks777Leave a comment

The vast Tibetan Plateau at an average elevation of 4500 m is a major influence on the climate of Asia, being central to the annual monsoons, as well as one the world’s largest continental tectonic features. When it formed is crucial in palaeoclimatic modelling as well as to geomorphologists and structural geologists. Whether or not it was present before the Indian subcontinent collided with Asia at 50 Ma has been the subject of perennial debate; it could have formed during the more or less continual accretion of terranes to southern Eurasia since the Jurassic Period. A novel approach to timing uplift of Tibet is obviously needed to resolve the controversies, and that may have been achieved (Hetzel, R. et al. 2011. Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift. Geology, v.39, p.983-986). North of Lhasa is an area of coincident small plateaus at around 5200-5400 m into which are cut valleys a few hundred metres. It has the hallmarks of a peneplain stripped to the base level of erosion, and developed on Cretaceous granites. The German-Chinese-South African team applied a range of geochronological techniques to date the emplacement of the granites and their cooling history. U/Pb dating shows the granites to have crystallised between 120 to 110 Ma; U-Th/He dating of zircons in them indicate their cooling from 180° to 60°C between 90 and 70 Ma; apatite U-Th/He and fission-track dating show that the granites experienced surface temperatures by around 55 Ma during a period of erosion at a rate of 200-400 m Ma^-1. The clear inference is that an area >10 000 km² became a peneplain by the end of the Palaeocene, to be unconformably overlain by Eocene continental redbeds.

By the Eocene the northern Lhasa Block had become a low-elevation plain from which a vast amount of sediment had been removed to be deposited elsewhere – Palaeocene and Eocene sediments are not common throughout the whole Tibetan Plateau. This is strong evidence that uplift of the Plateau only began after the India-Asia collision during the Eocene. Despite that and the erosion that would have taken place, much of the peneplain remains; given resistant bedrock peneplains can be very long-lived.

Seafloor mud cores and the seismic record

Published on October 19, 2011 by zooks7771 Comment

The most important factors in attempting to assess risk from earthquakes are their frequency and the time-dependence of seismic magnitude. Historical records, although they go back more than a millennium, do not offer sufficient statistical rigor for which tens or hundreds of thousand years are needed. So the geological record is the only source of information and for most environments it is incomplete, because of erosion episodes, ambiguity of possible signs of earthquakes and difficulty in precise dating; indeed some sequences are extremely difficult to date at all with the resolution and consistency that analysis requires. One set of records that offer precise, continuous timing is that from ocean-floor sediment cores in which oxygen isotope variations related to the intricacies of climate change can be widely correlated with one another and with the records preserved in polar ice cores. For the past 50 ka they can be dated using radiocarbon methods on foraminifera shells The main difficulty lies in finding earthquake signatures in quite monotonous muds, but one kind of feature may prove crucial; evidence of sudden fracturing of otherwise gloopy ooze (Sakagusch, A. et al. 2011. Episodic seafloor mud brecciation due to great subduction zone earthquakes. Geology, v.39, p. 919-922).

The Japanese-US team scrutinised cores from the Integrated Ocean Drilling Program (IODP) that were drilled 5 years ago through the shallow sea floor above the subduction zone associated with the Nankai Trough to the SE of southern Japan. Young, upper sediments were targeted close to one of the long-lived faults associated with the formation of an accretionary wedge by the scraping action of subduction. Rather than examining the cores visually the team used X-ray tomography similar to that involved in CT scans, which produce precise 3-D images of internal structure. This showed up repeated examples of sediment disturbance in the form of angular pieces of clay set in a homogeneous mud matrix separated by undisturbed sections containing laminations. The repetitions are on a scale of centimetres to tens of centimetres and were dated using a combination of ¹⁴C and ²¹⁰Pb dating (²¹⁰Pb forms as a stage in the decay sequence of ²³⁸U and decays with a half-life of about 22 years, so is useful for recent events). The youngest mud breccia gave a ²¹⁰Pb age of AD 1950±20, and probably formed during the 1944 Tonankai event, a great earthquake with Magnitude 8.2. Two other near-surface breccias gave ¹⁴C ages of 3512±34 and 10626±45 years before present. These too probably represent earlier great earthquakes as it can be shown that mud fracturing and brecciation by ground shaking needs accelerations of around 1G, induced by earthquakes with magnitudes greater than about 7.0. So, not all earthquakes in a particular segment of crust would show up in seafloor cores, most inducing turbidity flow of surface sediment, but knowing the frequency of the most damaging events, both by onshore seismicity and tsunamis, could be useful in risk analysis. In its favour, the method requires cores that penetrate only about 10 m, so hundreds could be systematically collected using simple piston coring rigs where a weighted tube is dropped onto the sea floor from a small craft.

Earthquakes from the ocean: Danger zones (nature.com)

Mercury: anything new?

Published on October 15, 2011 by zooks777Leave a comment

The Sun’s nearest planet, Mercury, seems odd in some ways; for instance, it has a proportionately larger metallic core than any other planet. That feature has led some to suggest that somehow most of any original silicate mantle was lost. One possibility is that its proximity to the Sun resulted in Mercury’s surface being ablated. Another looks to a huge collision with another body that tore off much of the mantle; similar to the event that the chemical commonality of the Earth and Moon suggests early in Earth history. Both processes should have left a distinct geochemical signature on the surface of Mercury: some kind of residue of solar ablation or evidence of fractional crystallisation of a magma ocean, such as the feldspar-rich lunar highlands that are probably formed of crystals that floated as such a planetary silicate melt cooled and evolved. The seeming strangeness of Mercury helped underpin a well-equipped un-crewed mission, going by the acronym MESSENGER, that finally settled into Mercury orbit in March 2011 after a planned ‘yo-yoing’ path since launch in August 2004 that took it back and forth between Earth, Mercury and Venus in its early stages. Early analysis of results from the now permanent orbit appeared in the 30 September 2011 issue of Science.

MESSENGER carries several remote sensing instruments: a stereo imaging device to map landforms, and topography; a laser altimeter to back the stereo imager; a visible to short-wave infrared spectrometer to map variations in surface spectra and minerals; gamma-ray spectrometry to map distributions of naturally radioactive isotopes and emissions from other elements triggered by high-energy cosmic ray bombardment; using the Sun as a source of gamma- and X-rays to cause a variety of elements to emit lower energy X-rays – a variant of X-ray fluorescence spectrometry that is a workhorse of lab geochemistry.

The earlier Mercury fly-bys and previous missions clearly showed that its surface is heavily cratered but possesses areas resurfaced by lavas that obliterate older cratering. A little like the lunar maria in age and appearance, these smooth terrains show evidence of accumulations up to a kilometre thick formed by repeated lava flows (Head, J.W. and 25 others, 2011. Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER. Science, v. 333, p. 1853-1855). As regards the age of these major volcanic features, all that can be said is that they post-date the largest impacts, such as the huge Caloris Basin, and are more sparsely peppered with younger craters. Intriguingly, floors of some of the craters show clusters of small depressions and pits surrounded by light-coloured material of some kind, suggested to be solids condensed from gases that emerged from below (Blewett, D.T. and 17 others 2011. Hollows on Mercury: MESSENGER evidence for geologically recent volatile-related activity. Science, v. 333, p. 1856-1859). While it is only possible to assign youth of these features relative to the craters in which they occur, they indicate an underlying source of volatiles; a factor weighing against previous accounts of Mercury’s evolution by either solar ablation or giant impact.

Considerably more interesting – at least to me – are the results from the geochemically oriented instruments. Calcium, magnesium, aluminium and silicon estimates by the XRF-like instrument present not the slightest evidence for a feldspar-rich component of the early crust akin to the lunar highlands; another blow for the giant-impact and magma-ocean hypotheses. Mercury’s surface seems to be similar in composition to the most ancient terrestrial lavas: Mg-rich mafic to ultramafic komatiites, compared with the more iron-rich tholeiites of the lunar maria (Nittler, L.R. and 14 others. The major-element composition of Mercury’s surface from Messenger X-ray spectrometry. Science, v. 333, p. 1847-1850). They are ten-times more enriched in sulfur than surface rocks on the Earth or Moon, though iron content seems too low to accommodate it in minerals such as pyrite (FeS₂). High sulfur content could point to an origin for Mercury from accretion of highly reduced material in the solar nebula, the Earth-Moon system being broadly more oxidised. Gamma-ray spectrometry to analyse the abundances of potassium, uranium and thorium (Peplowski, P.N. and 16 others. Radioactive elements on Mercury’s surface from MESSENGER: implications for the planet’s formation and evolution. Science, v. 333, p. 1850-1852) doesn’t serve previous ideas about the planet’s history either. Potassium, which is moderately volatile, is too high relative to more refractory uranium and thorium to support any notion of solar ablation of the surface, but the U, Th and K proportions are roughly like those of the Earth’s oceanic crust. One of the plots shows K-Th relationships for supposed meteorites from Mars and the extensive gamma-ray data from Mars itself, in which few of the meteorites fall in the K-Th ‘cloud’ for the Martian surface: now there’s a thing….

It must be emphasised that the geochemical results are but a fraction of what should eventually emerge from these powerful instruments. However, these early data place Mercury in much the same envelope as the other rock worlds of the Inner Solar System (Kerr, R.A. 2011. Mercury looking less exotic, more a member of the family. Science, v. 333, p. 1812).

‘Hollows’ mark Mercury’s surface (bbc.co.uk)
Closer look at Mercury reveals mystery hollows on planets surface (theglobeandmail.com)

Britain to be comprehensively fracked?

Published on October 14, 2011 by zooks7771 Comment

In ‘Fracking’ shale and US ‘peak gas’ (EPN of 1 July 2010) I drew attention to the relief being offered to dwindling US self-sufficiency in natural gas by new drilling and subsurface rock-fracturing technologies that opens access to extremely ‘tight’ carbonaceous shale and the gas it contains. The item also hinted at the down-side of shale-gas. The ‘fracking’ industry has grown at an alarming rate in the USA, now supplying more than 20% of US demand for gas. This side of the Atlantic the once vast reserves of North Sea gas fields are approaching exhaustion. This is at a time when commitments to reducing carbon emissions dramatically depend to a large extent on hydrocarbon gas supplanting coal to generate electricity, releasing much lower CO₂ by burning hydrogen-rich gases such as methane (CH₄) than by using coal that contains mainly carbon. Without alternative, indigenous supplies declining gas reserves in Western Europe also seem likely to enforce dependency on piped gas from Russia or shipment of liquefied petroleum gas from those major oil fields that produce it. The scene has been set in Europe in general and Britain in particular for a massive round of exploration aimed at alternative gas sources beneath dry land. Unlike the US and Canada, the British are not accustomed to on-shore drilling rigs, seismic exploration and production platforms, and nor are most Europeans. Least welcome are the potential environmental and social hazards that have been associated with the US fracking industry, which seem a greater threat in more densely populated Europe.

The offshore oil and gas of the North Sea fields formed by a process of slow geothermal heating of solid hydrocarbons or kerogen in source rocks at a variety of stratigraphic levels, escape into surrounding rocks of the gases and liquids produced by this maturation, and their eventual migration and accumulation in geological traps. By no means all products of maturation leave shale source rocks because of their very low permeability. That residue may be much more voluminous than petroleum liquids and gases in conventional reservoir rocks; hence the attraction of fracking carbonaceous shales. British on-shore geology is bulging with them, particularly Devonian and Carboniferous lacustrine mudstones, Carboniferous and Jurassic coals, and the marine black shales of the Jurassic (see http://www.bgs.ac.uk/research/energy/shaleGas.html and https://www.og.decc.gov.uk/upstream/licensing/shalegas.pdf), to the extent that areas of potential fracking cover around a third of England, Wales and southern Scotland.

News is breaking of a major shale-gas discovery beneath Blackpool, the seaside resort ‘noted for fresh air and fun, where Mr and Mrs Ramsbottom went with Young Albert their son…’ (Albert poked a stick at Wallace the lion and was eaten), said by energy firm Cuadrilla to have gas reserves of 5.7 trillion m³. The announcement followed 6 months of exploratory drilling, and drew attention to the burgeoning interest by entrepreneurs in the upcoming 14^th Onshore Licensing Round for petroleum exploration in Britain. It isn’t just from major petroleum companies, but in some cases even what amount to family businesses finding sufficient venture capital to spud wells; similar in many respects to the US fracking boom that began a mere 10 years ago.

Deep under Lancashire, a huge gas find that could lead to 800 ‘fracking’ wells (independent.co.uk)
Fracking gas test drill discussed (bbc.co.uk)
Is fracking environmentally friendly? (environmenteng.wordpress.com)
The answer to cheaper gas bills? (bbc.co.uk)
Camp Frack mobilises against UK’s first shale gas well (guardian.co.uk)

The useful geoneutrino

Published on October 13, 2011 by zooks7771 Comment

While the wires were hot with news of neutrinos possibly having exceeded light speed as they were fired through the Alps by the Large Hadron Collider, steady research has been seeking answers rather than perhaps transmuting physicists’ hubris into a death wish. (Note: it has to be said that British theoretical physicist Jim Al-Khalili has sufficient confidence that the speeding ticket issued to the neutrinos will be rescinded that he promises to eat his underpants if it is upheld.) The more tangible work concerns antineutrinos that the Earth emits, dubbed ‘geoneutrinos’ to distinguish them from extremely exotic ones from deep space which, worryingly for some, pass from one side of the Earth to the other and through us as well. When unstable isotopes, such as those of uranium, thorium and potassium that help heat the Earth, decay they emit antineutrinos as well as electrons, helium nuclei and gamma-rays. Notoriously elusive, neutrinos and antineutrinos can now be detected with sufficient precision to make useful observations, as well as produce results that have many theoretical physicists quivering in cellars from which they emerge, from time to time, covered with chalk dust from their desperate exertions to explain a material speed faster than ‘little c’. To geoscientists, the results of an experiment using geoneutrinos at the Japanese Kamioka Liquid-Scintillator Antineutrino Detector (KamLAND), which involved 66 individuals from 15 Japanese, US and Dutch institutions, are much more interesting: they help resolve a long-standing puzzle about the source of geothermal heat that flows from the Earth’s surface at a rate of about 44 TW (The KamLAND Collaboration 2011. Partial radiogenic heat model for Earth revealed by geoneutrino measurements. Nature Geoscience, v. 4, p. 647-651).

A model of the Earth that assumes it accreted from chondritic meteorites with well-known abundances and proportions of heat-producing U, Th and K isotopes, supported by some measurements of peridotites from the mantle, suggests that less than half the geothermal flux is radiogenic, implying that a great deal is heat originally trapped in the Earth when it formed. This view depends on several assumptions: that the Earth’s mantle is indeed chondritic below the 200 km or so from which samples have been brought by volcanism; that the core doesn’t produce any heat by radioactive decay; and that a geophysical model of a well-mixed mantle is correct. Not surprisingly, geophysical and geochemical evidence is so flimsy that many different views have had their champions: that the core contains potassium; that there is a deep, barely tapped inner-mantle layer of high heat production formed from now-rare meteoritic material, and so on. Geoneutrinos, if distinguishable from those from elsewhere in the cosmos and indeed measurable, could help home-in on one or other hypothesis. Based on a spherical balloon containing 1000 t of hydrocarbon liquids in a deep mine shaft that floats in an 18 m metal sphere filled with buoyant oil, KamLAND relies on detecting the light emitted by very rare interactions of neutrinos with protons. That is hard enough, but the site is surrounded by Japan’s 53 neutrino-emitting nuclear reactors, so a great deal of cunning operating conditions and data processing is needed to sort the ‘wheat from the chaff’; at present errors are large, but now sufficiently constrained to throw light on the great heat-flux issue. The KamLAND Collaboration reports that between 16 and 68% of heat flow is due to decay of the most productive isotopes ²³²Th and ²³⁸U – there is insufficient ²³⁵U and ⁴⁰K in the Earth for geoneutrinos generated by their decay to be meaningfully estimated. Fuzzy as the results are, they are sufficient to support the view that Earth’s ‘primordial’ heat of formation is still a major source of geothermal energy, thus narrowing down the geochemical aspects open for disputation.

See also: Korenaga, J. 2011. Clairvoyant geoneutrinos. Nature Geoscience, v. 4, p. 581-582

Another candidate for earliest, direct human ancestor

Published on October 12, 2011 by zooks7771 Comment

In May 2010 EPN commented on a new find from the famous fossil-rich caves of north-eastern South Africa; a new hominin species called Australopithecus sediba. At least one of them fell into a deathtrap shaft, died and remained unchewed without bones being spread far and wide. Inevitably, near-complete skeletons of individual hominins are soon pored over by dozens of specialists in human evolution, as they were for the much older Ardepithecus ramidus found in sediments of Ethiopia’s Afar Depression (see Early hominin takes over Science magazine in the November 2009 issue of EPN). Now there are two near-complete, well-preserved skeletons of Au. sediba and the palaeoanthropological world is agog. Dating to about 1.98 Ma the specimens represent the same time as do far less impressive remains of H habilis from Tanzania that were found with associated rudimentary stone tools. The first hint (just a fragment of upper jaw) of any remains that might be tagged ‘Homo’ dates to 2.3 Ma and is from Ethiopia, as are the first undoubted stone tools going back as far as 2.5 Ma, though lacking association with a maker.

Five consecutive papers on Au. Sediba occupy 22 pages in the 9 September 2011 issue of Science and make for startling reading. The first concerns the shape of its brain case, and therefore crudely its brain, discerned by tomographic X-ray scanning (Carlson, K.J. et al. 2011. The endocast of MH1, Australopithecus sediba. Science, v. 333, p. 1402-1407). It isn’t any bigger than that of other members of the genus but shows ‘some foreshadowing of the human frontal lobes’ and other shifts from the basic ape model that the authors imply are en route to human features. The next considers the two pelvis regions (Kibii, J.M. et al. 2011. A partial pelvis of Australopithecus sediba. Science, v. 333, p. 1407-1411); again australopithecine-like in the small size of the birth canal but with a hint of the S-shape of humans. Most astonishingly well-preserved are the fragile bones of a complete hand (Kivell, T.L. et al. 2011. Australopithecus sediba hand demonstrates mosaic evolution of locomotor and manipulative abilities. Science, v. 333, p. 1411-1417), which convincingly shows the long thumb and short fingers (for a primate) that characterise Homo and are essential for a precision grip and making things. Actually, the thumb is longer relative to fingers (60%) than in humans (54%), but Lucy’s (Au. afarensis) was a closer match. No tools that such a hand might have created and wielded were found with the fossils. Then there is the foot (Zipfel, B. et al. 2011. The foot and ankle of Australopithecus sediba. Science, v. 333, p. 1417-1420), which, again, mixes human and australopithecine features, giving ‘a unique form of bipedality and some degree of arboreality’. The fifth paper (Pickering, R. et al. 2011. Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo. Science, v. 333, p. 1417-1420) is as remarkable for the precision of U-Pb dating of speleothem (cave carbonates), which at 1.977+0.002 Ma far exceeds the workhorse Ar-Ar method used for most other hominins, as it is for the absolute age that precedes that of undisputed remains of humans.

In short, for Australopithecus sediba there is an embarrassment of riches unmatched until those of the 1.5 Ma old H. erectus (‘Turkana Boy’) found at Nariokotome in NW Kenya. To some extent this throws a flock of peregrines in among the palaeoanthropology pigeons, as an account of a meeting earlier in 2011, at which the bones were grandstanded, shows (Gibbons, A. 2011. Skeletons present an exquisite paleo-puzzle. Science, v. 333, p. 1370). Naturally, the authors are making the most of their material especially, it seems, its finder Lee Berger of the University of Witwatersrand, South Africa, the last author in all the papers. Comparisons with more australopithecine remains were said to be needed. The soon-to-be-famous hand has been said to be essentially like others from the same genus. While the remains of the creature’s pelvis could imply that its evolution was more driven by a need for efficient upright walking than to birth big-headed babies, the ankle shows a primitive trait that would have forced Australopithecus sediba to walk strangely as the heel bone is small and angled unlike that in human feet, which is broad and flat. But all the species’s features are combined in two near-complete individuals, whereas for the rest of its contemporaries, predecessors and near successors in time speculation is based on fragments of several individuals, none more so than in the case of the earliest agreed human, near contemporaneous H. habilis, which barely stands up to taxonomic scrutiny (Gibbons, A. 2011. Who was Homo habilis – and was it really Homo? Science, v. 332, p. 1370-1371). Some would say that it was only the associated stone tools that assigned ‘Handy Man’ to more elevated status than slightly large-headed australopithecine. The fact is; stone tools were around since 2.5 Ma, at least in Ethiopia, and this newly found being could have handled them and even made them with its palpable dexterity. Finding tools and skeletons together is almost as rare as hens with teeth…

Pieces of the Human Evolutionary Puzzle: Who Was Australopithecus sediba? (blogs.scientificamerican.com)
Who were the Hobbit’s ancestors? (blogs.nature.com)
New evidence suggests that Au.sediba is the best candidate for the genus Homo (eurekalert.org)
First of Our Kind: Could Australopithecus sediba Be Our Long Lost Ancestor? (scientificamerican.com)

Asteroid dust said to resolve a conundrum

Published on October 12, 2011 by zooks777Leave a comment

In September 2005 a Japanese space probe, Hayabusa, twice landed lightly on the small (700 m long) asteroid Itokawa that habitually crosses the orbit of Mars. The plan was to scoop up a substantial amount of its rubbly surface and return it for lab analysis. In the event the main sampling device malfunctioned. The dismayed Hayabusa team were mollified to some extent by the second landing impact fortuitously directing dust particles up to 0.2 mm across into the sampler. After Hayabusa landed safely in Australia on 13 June 2010, the team thankfully recovered 1574 tiny grains. Most were made of single minerals: olivine, pyroxene, feldspar (including 14 alkali feldspar grains), sulfides, chromite, Ca phosphate and iron-nickel alloy. About 450 were silicate mixtures some containing K-bearing halite (NaCl) (Nakamura, T. and 21 others. Itokawa dust particles: a direct link between S-type asteroids and ordinary chondrites. Science, v. 333, p. 1113-1116 – followed by 5 other papers from the Hayabusa team in the same issue). The sample analyses clearly show that Itokawa chemically and mineralogically resembles ordinary LL chondrites that make up most meteorites found on Earth.

Hardly a surprise, then… Yet it was, for Itokawa is an S-type asteroid – the most common – whose spectra do not match those of ordinary chondrite meteorites despite the logic that commonly found meteorites ought to come from the break-up of commonly seen asteroids. S-type asteroids have annoyed astronomers for decades because of their cryptic appearance, and now they are broadly relieved. Any object floating around the inner Solar System for billions of years inevitably undergoes a process for which terrestrial weathering is a metaphor; it is affected by the stream of charged particles that constitutes the solar wind, by bumping other bodies and attracting debris from such collisions. The Itokawa dust particles turn out to have extremely thin veneers of sulfide and metallic blobs on the scale of a few nanometres that are thought to result from condensation of matter vaporised either by tiny impacts or the solar wind. This veneer gives Itokawa and probably other S-type meteorites their irritatingly uniform reddish colour. It strikes me that there is a problem here: all asteroids, no matter what their mineralogy and chemistry, would be subject to the same kind of process and end up with a similar veneer. Itakawa may well be an ordinary chondrite, but what about all the other S-type asteroids?

See also: Kerr, R.A. 2011. Hayabusa gets to the bottom of deceptive asteroid cloaking. Science, v. 333, p. 1081.

Extraterrestrial dust reveals asteroid’s past and future (newscientist.com)

First bi-face tools and Homo erectus

Published on September 12, 2011 by zooks777Leave a comment

ארקטוס — Reconstruction of H. erectus face. Image via Wikipedia

The elegant pear-shaped, double edged tool, known as the Acheulean ‘hand-axe’ is an icon for the distant past of humans. It appears in the record as a sharp contrast to the earlier crude cutting tools made of broken and sharpened pebbles, known generally as Oldowan, that around 2.5 Ma marked the appearance of some hominin species with the wit to exploit the inorganic world and begin manufacture. There can be little doubt that the visualisation of a useful shape within a formless block of stone and the dexterity to realise it as a tool marked a major change in human cognitive ability.

This article can be read in full at Earth-logs in the Human evolution and migrations archive for 2011

Life at the battery terminal

Published on August 23, 2011 by zooks777Leave a comment

Having an interior that is dominated by reducing conditions and oxidising surface environments since free oxygen gradually permeated from its initial build up in the atmosphere to the ocean depths, the Earth has been likened to a massive self-charging battery. Electrons flow continually as a consequence of the nature of the linked oxidation-reduction: in terms of electrons, oxidation involves loss while reduction involves gain (the OILRIG mnemonic). Although there are natural electrical currents, most of the electron flow is in the form of reduced compounds rich in electrons that make their way through the flow of fluids from the deep Earth – effectively an anode – towards the surface where the reduced compounds lose electrons to create the equivalent of a cathode. Reduction-oxidation (redox) is therefore a power source. Inorganic reactions, such as the precipitation on the sea floor of sulfides from hydrothermal fluids at ‘black smokers’ dissipate energy. Yet the power has considerable potential for organic life. Some bacteria oxidise hydrogen sulfide carried by hydrothermal fluids and others do the same to upwelling methane. In 1977 a teeming biome of worms, molluscs and higher animals was discovered in a totally dark environment around ocean-floor vents. It soon became clear that it could only subsist on chemical energy of this kind, rather than any form of photosynthesis. The key to some metazoans’ success had to be symbiosis with bacteria that could perform the chemical tricks possible in the cathode region of the Earth’s electron flow. There are several candidate compounds: H₂S, CH₄, NH₄, metal ions and even hydrogen gas.

As hydrothermal fluids cycle ocean water into the basaltic crust and underlying peridotite mantle, they not only hydrate the olivines and pyroxenes that dominate the oceanic lithosphere but trigger other reactions one of whose products is hydrogen. As well as a reaction being eyed by those keen on a cheap source of clean fuel, it generates more energy potential for biological metabolism in the guise of hydrogen than those which form other common compound in the returning fluids. Although the nature of hydrogen’s organic use has been elusive, it has now come to light in a surprising guise (Petersen, J.M. and 14 others 2011. Hydrogen is an energy source for hydrothermal vent symbioses. Nature, v. 476, p. 176-180).

One highly successful animal in ocean-floor hot spring systems is a mussel called Bathymodiolus. Genetic experiments by the German-French-US team revealed that a gene known as hupL is present in the mussels’ gill tissue; a gene found in bacteria that use either carbon monoxide or hydrogen as an electron donor. The hupL gene encodes for enzymes known as hydrogenases that are needed to set off the reaction H₂ = 2H⁺ + 2e^– that provides electrons needed in bacterial metabolism; a sort of living fuel cell. Hydrogen-using bacteria interact symbiotically with the mussels, which would otherwise be unable to live in the pitch black environment. Genomic sequencing of tube worms and shrimps that occur in the vent communities also contain the bacterial hupL gene. Hydrogenase enzymes are proteins with an iron-nickel core, and probably evolved far back in bacterial evolution around metal-rich hot springs. Interesting as the specific detail of hydrogen-based symbiosis is, the general concept of Earth’s redox systems’ having battery-like behaviour is very useful. On land groundwater sometimes comes into contact with sulfide ore bodies that are oxidised to yield hydrogen and sulfate ions ,while the groundwater is reduced: a battery comes into being with a cathode in the aerated groundwater and electrons flow from the unaltered orebody towards it. Such currents are useful in revealing hidden orebodies using the ‘self-potential’ or SP method. Indeed the downward change from oxidising to reducing groundwater, caused by the redox reactions involved in weathering and soil formation also result in weak negative and positive ‘electrodes’ with a sluggish flow of compounds that bacteria can exploit and thereby encourage metazoan life through symbiosis. In doing so, changes in redox conditions affect the inorganic load of the slowly moving groundwater so that reduced metal ions can be precipitated once they rise into the oxidising horizon. The general enrichment of the upper horizons of soils in iron oxides and hydroxides, and metal depletion in lower horizons probably stem from the ‘Earth battery’ produced by an interplay between inorganic and organic redox reactions. Be on the look-out for more on this topic as the quest for hydrogen fuels becomes more urgent. A former colleague, Gordon Stanger, investigating groundwater in the Semail ophiolite of the Oman for his PhD in the 1970s discovered to his surprise that in outcrops of the mantle sequence there were springs from which hydrogen bubbled freely: fortunately he was not a smoker…

Orphan, V.J. & Hoehler, T.M. 2011. Hydrogen for dinner. Nature, v. 476, p. 154-155.
Hydrogen highway in the deep sea (eurekalert.org)

Hi-tech future may be saved by ocean floor sediments

Published on August 22, 2011 by zooks777Leave a comment

Global rare earth element production (1 kt=106... — China's growing REE market share. Image via Wikipedia

Since the now far-off founding of the Club of Rome and the re-emergence of Malthusian ideology, time and again there have been warnings about the imminent running out of resources that are essential for modern life. The latest concern one of the formerly haunted wings of the Periodic Table, central to petrogenetic geochemistry, but little else; the rare-earth elements. From early beginnings as the source for phosphors in the screens of colour televisions all 15 REEs now have a growing commercial role in applications ranging from precision guided weapons, night-vision goggles and stealth technology in the military sphere, through the satiation of artificial appetites for electronic gaming and mobile phones, to applications of super-efficient magnets in medial scanners and ‘green’ power generation. The crisis being discussed currently is not so much a shortage – REEs are not so rare – but the cornering of their mining by the Peoples’ Republic of China, which produces more than 95% of RREs used at present (~120 thousand tons). Yet world reserves are estimated at almost 100 million t, of which China has 36 million. Mining is often in only a few known, high-grade deposits; for instance most of the US reserves of 13 million t are locked in the Mountain Pass Mine, California that is currently on a ‘care-and-maintenance’ regime, i.e. shut. This one-sided economy sends shudders through capital’s strategy forums, i.e. in the US, Silicon Valley and the Pentagon.

Not surprisingly, geochemists and oceanographers from Japan, the world’s most hi-tech country, have bent their collective will to finding alternative sources, and may have revealed one in an unexpected location (Kato, Y. et al. 2011. Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements. Nature Geoscience, v. 4, p. 535-539). Their work stems from ‘mining’ existing geochemical data from deep-sea drilling projects on the floor of the Pacific Ocean, that reveal a wide range of REE concentrations in the ooze coating the seabed: from <250 to >2000 parts per million. The richest pickings seem to lie in a swath either side of the East Pacific Rise at around 15°S, where the group estimate that a 1 km² plot could yield about one fifth of current world annual production, even though REE concentrations lie way below their on-shore economic cut-off grade. Apart from the need for dredging at depths around 3-5 km on the abyssal plains, and the inevitability of destroying a largely unknown ecosystem, the positive aspect of these metal-rich oozes is that the REE can be extracted simply by acid leaching of the goethite (FeOOH) in which the bulk of the elements reside. Goethite is something of a geochemical ‘mop’ with a capacity for adsorbing elements of all kinds on grain surfaces; so much so that it is being considered as a means of cleaning up heavy-metal pollution. Both the REEs and the iron probably arise from seabed hot springs where oxidising conditions result in dissolved ferrous iron combining in ferric form with oxygen to form goethite, which in turn scavenges other dissolved ions. Many of the on-shore REE deposits are carbonatites (intrusions of carbonate-rich magmas) that contain fluoro-carbonates and phosphates that host the REE, or beach sands in which wave swash concentrates the durable heavy phosphates in so-called black-sand deposits. Carbonatites are rare, most occurring in ancient ‘shields’, as in southern Africa, Canada and China, but being so unusual are not difficult to find. One in the Canadian Shield known as the Big Spruce Lake deposit provides phosphorus- and potassium-rich soil that encourages the growth f conifers and so creates a geobotanical anomaly of large trees where local climate generally supports only stunted ones.

The rising demand and currently restricted supply of REEs is creating an exploration boom for carbonatites as the metal prices rise inexorably. Yet it may also produce a shift to what seems to be an alternative kind of source; iron-rich deep-sea sediments, though more likely those preserved on-shore in ophiolite complexes than at the huge depths of the abyssal plains. It is worth bearing in mind, however, that oceanographers and geochemists have pointed to untold metal riches before: manganese nodules that litter huge tracts of the seabed and contain sufficient copper, nickel and cobalt to maintain supplies for millennia. Despite a half-billion dollar investment in the 1960s and 70s, there is no nodule-dredging industry. There are however well-advanced plans for deep water mining of gold-rich hydrothermal sites, but miners will go just about anywhere to gloat over Marx’s ‘money commodity’

Byron King: The Real REE Demand Opportunity (businessinsider.com)

Dust tied to climate

Published on August 20, 2011 by zooks777Leave a comment

This TOMS image shows a record-setting Asian d... — Dust moving in April 2001 from arid areas in Central Asia and North Africa to the oceans. From NASA's Nimbus-7 satellite. Image via Wikipedia

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant foot chain. Oddly, even when commonly known nutrients are brought to the ocean surface far from land by deep-sourced upwellings the effect on near-surface biomass is far from that expected. The key factor that is missing is dissolved divalent iron that acts as a minor nutrient for phytoplankton: even in deep ocean waters any such ferrous iron is quickly oxidised and precipitated as trivalent ferric compounds. One of the suggested means of geoengineering away any future climatic warming is to seed the far-off oceans reaches with soluble iron in the hope of triggering massive planktonic blooms, dead organisms sinking to be buried along with the their carbon content in the ocean-floor oozes. Retrospectively, it has been suggested that the slight mismatch between changes in atmospheric CO₂ concentration and climate changes may be linked to fluctuating availability of iron dissolved from dust in ocean-surface waters, but so far that hypothesis has not been robustly tested. It is well known, however, that global cooling is accompanied by drying of continental climates and thereby an increase in the delivery of dust, even to polar ice caps where cores have shown dustiness to fluctuate with temperature.

Recently an ocean-floor sediment core from around 42° S has revealed a high-resolution record of the deposition of dust and iron at that location over the last 4 Ma (Martinez-Garcia, A. et al. 2011. Southern Ocean dust-climate coupling over the past 4 million years. Nature, v. 476, p. 312-315). In it one proxy for dust is the amount of organic compounds known as n-alkanes that are a major component of the waxes shed from plant leaves. Others are iron, titanium and thorium concentrations in the ooze. Dust proxies tally with land-ice volumes shown by the fluctuating d¹⁸O measured in bottom-dwelling foraminifera found as fossils in the core to form a convincing link between dust and climate over the Southern Ocean. Those proxies also match nicely the record of dust delivered to Antarctica that emerged from the 0.8 Ma Dome C ice core that was extracted and analysed by the EPICA consortium. The record shows boosts in iron and dust deposition at 2.7 Ma, when ice first took hold of northern high latitudes, and at 1.25 Ma when larger ice sheets began to develop and climate shifts switched to 100 ka cyclicity. Although the match between marine and glacial dust accumulation in the latter part of this mid-Pleistocene Transition is an important step forward in palaeoclimatology, it is a surprise that the new ocean-floor data is not plotted with the record of atmospheric CO₂ in Antarctic ice bubbles: if there was a clear relationship that would have iced the cake.

A big hit in the Moon’s evolution

Published on August 6, 2011 by zooks777Leave a comment

The most significant discovery from the Apollo lunar landings is that the Earth and Moon shared a fiery early history, when a planetary body around the size of Mars slammed into the Earth to fling off vaporised rock that condensed to create the Moon. Such a catastrophic event reset the geochemistry of the Earth, and both it and the Moon likely had an early phase dominated by a deep ocean of magma. The evidence for a magma ocean comes mainly from the lunar highlands which are dominated by almost pure calcium plagioclase feldspar (the rock anorthosite), suggesting that this high-temperature, low-density silicate mineral crystallised and then floated to the surface of the Moon. Yet there is a great deal of evidence about the Moon that did not depend on people setting foot on its surface. For instance, detailed photographic records of the surface and extremely precise measurements of the surface elevation stem from cheaper orbital missions, including coverage of the unvisited far side of the Moon.

The face of the Moon never seen from Earth has long been known to have one of the largest impact basins in the solar system, the South Pole – Aitken basin. Analysis of the far side’s surface elevation data from the Lunar Orbiter Laser Altimeter (LOLA) also shows that it is significantly higher than the near side. It is also far more heavily cratered than the near side. Now there is a plausible explanation for the dichotomy: the Moon received another stupendous blow (Jutzi, M & Asphaug, E. 2011. Forming the lunar farside highlands by accretion of a companion moon. Nature, v. 476, p. 69-72). But how come that didn’t blast the Moon apart or re-melt it and allow it to re-shape to a near perfect sphere? The modelling study suggests that if the culprit slowly collided – around 2-3 km s^-1 – it would have wrapped around the early Moon to plaster the surface with debris, nicely shown by the paper’s graphics. Such a ‘slow’ impact is only possible from a co-orbital companion moon, objects from outside the Earth-Moon system inevitably being accelerated by gravity to at least the equivalent of its escape velocity (about 11-12 km s^-1). That exceeds the speed of sound through rock, leading at least to a very large hole, shock metamorphism and, with a massive body, to extensive melting (the energy would be ½ mv²) rather than the observed lunar far-side bulge. Jutzi and Asphaugs’s modelling comes up with a companion moon around 1200 km across, that may have formed from the same massive event that created the Moon itself. It could have accreted from the impact-induced vapour disc at a Trojan point in the lunar orbit, where gravitational forces balance to keep orbital objects apart. The gradual expansion of the lunar orbit in response to tidal forces – large in the early history of the Earth-Moon system – could have destabilised the balance so that the companion moon slowly drifted towards the Moon and eventual collision.

One such modelling becomes closer to known reality, i.e. the far-side bulge, it gets more tempting to look for secondary possibilities. One of these the effect of such a ‘slow’ impact on the remaining magma ocean on the Moon. It may have blurted that by then deep molten layer to the side opposite the impact. That, the authors suggest, may be responsible for the geochemical peculiarities of the flood basalts that filled the much later lunar maria on the near side. There are no signs of these KREEP basalt floors to large later craters on the far side, such as the Aitken basin, formed around 4.0 to 3.8 Ga ago at the same time as the near-side maria. A variety of new instruments orbit the Moon and more are planned, so this model presents a nice hypothesis for them to test: what is the betting that a robotic lander might eventually be sent to return samples from the enigmatic far side?

Did a Slo-Mo Crash Create the Two-Sided Moon? (news.sciencemag.org)
You: Second moon may have collided with our moon, say scientists (guardian.co.uk)

Homes for hominin evolution

Published on August 6, 2011 by zooks777Leave a comment

Friedrich Engels’s notion in The Part Played by Labour in the Transition from Ape to Man (1876), encouraged by Darwin’s The Descent of Man (1871), that the road to modern humans began with walking on two legs, thereby freeing the hands for work and tool making has been central to discussion of human evolution for more than a century. The ‘descent from the trees’ that bipedalism signifies has long been supposed to stem from the replacement of tropical forests in East Africa by open woodland or savannah, but evidence to support that environmental change has been difficult to glean from the fossil record since the Late Miocene. Even in terrestrial sediments plant remains are rare, so that much has rested on animal fossils in relation to the habitats of their living descendants: opinion is divided.

There is a round-about means of resolving this central issue: using the carbon-isotope record in fossil soils that depends on the fractionation effects of broadly different kinds of plants that once grew in the soils and the signature of that fractionation in carbonate nodules that formed in the soils. The d¹³C value (crudely the difference between the ¹³C/¹²C ratio of a sample and that of a carbon-rich standard) found in C4 plants (many grasses) is -16 to -10 ‰ whereas that in C3 plants (including almost all trees) it is much more depleted in the heavier ¹³C isotope (-33 to -24‰). Exchange of carbon between living and dead organic matter, and carbonates that are precipitated from soil waters through the intermediary of gases in the soil should leave a d¹³C signature in the carbonates that reflects the overall proportions of different photosynthetic plant groups living at the time the soil formed. The approach was developed in the early 1990s by Thure Cerling and Jay Quade of the US universities of Utah and Arizona respectively.

After a long gestation period, involving calibration using soils from different modern ecosystems, the soil C-isotope method has been applied painstakingly to palaeosols in which African hominin remains have turned-up (Cerling, T.E. and 9 others 2011. Woody cover and hominin environments in the past 6 million years. Nature, v. 476, p. 51-56). All the famous hominin sites from the Awash and Omo Valleys of Ethiopia and around Lake Turkana in Kenya, figure in this important study, in which the authors devise a proxy for ‘palaeo-shade’ based on their carbonate d¹³C data from 76 modern tropical soils: a good ‘straight-line’ plot of d¹³C against the fraction of woody cover at the different calibration sites. Applying the proxy to their 1300 samples of palaeosols they show convincingly that since about 6 Ma tree cover rarely rose above 40% in the homelands of all the East African hominins. From the times of Ardepithecus ramidus (~4 Ma) at Aramis in Ethiopia, through those of ‘Selam’ and ‘Lucy’, the 2.5 Ma first stone tools at Gona, the times when Africa was dominated by Homo erectus(1.8 to 1 Ma) to perhaps the first signs of modern human cranial remains (those with chins!) around 1 Ma, all hominins strode through open, grassy environments. One can imagine pleasured nods from the shades of Darwin and Engels now their prescience has finally been confirmed.

6 million years of African savanna (eurekalert.org)
Savanna, Not Forest, Was Human Ancestors’ Proving Ground (livescience.com)
Early hominin landscape use (eurekalert.org)

Plate tectonics monitored by diamonds

Published on August 2, 2011 by zooks777Leave a comment

eclogite — Norwegian Eclogite. Image by kevinzim via Flickr

For more than 30 years a debate has raged about the antiquity of plate tectonics: some claim it has always operated since the Earth first acquired a rigid carapace not long after a molten state following formation of the Moon; others look to the earliest occurrences of island-arc volcanism, oceanic crust thrust onto continents as ophiolite complexes, and to high-pressure, low-temperature metamorphic rocks. The earliest evidence of this kind has been cited from as far apart in time as the oldest Archaean rocks of Greenland (3.9 Ga) and the Neoproterozoic (1 Ga to 542 Ma). A key feature produced by plate interactions that can be preserved are high-P, low-T rocks formed where old, cool oceanic lithosphere is pulled by its own increasing density into the mantle at subduction zones to form eclogites and blueschists. In the accessible crust, both rock types are unstable as well as rare and can be retrogressed to different metamorphic mineral assemblages by high-temperature events at lower pressures than those at which they formed. Relics dating back to the earliest subduction may be in the mantle, but that seems inaccessible. Yet, from time to time explosive magmatism from very deep sources brings mantle-depth materials to the surface in kimberlite pipes that are most commonly found in stabilised blocks of ancient continental crust or cratons. Again there is the problem of mineral stability when solids enter different physical conditions, but there is one mineral that preserves characteristics of its deep origins – diamond. Steven Shirer and Stephen Richardson of the Carnegie Institution of Washington and the University of Cape Town have shed light on early subduction by exploiting the relative ease of dating diamonds and their capacity for preserving other minerals captured within them (Shirey, S.B. & Richardson, S.H. 2011. Start of the Wilson cycle at 3 Ga shown by diamonds from the subcontinental mantle. Science, v. 333, p. 434-436). Their study used data from over four thousand silicate inclusions in previously dated large diamonds, made almost worthless as gemstones by their contaminants. It is these inclusions that are amenable to dating, principally by the Sm-Nd method. Adrift in the mantle high temperature would result in daughter isotopes diffusing from the minerals. Once locked within diamond that isotopic loss would be stopped by the strength of the diamond structure, so building up with time to yield an age of entrapment when sampled. The collection spans five cratons in Australia, Africa, Asia and North America, and has an age spectrum from 1.0 to 3.5 Ga. Note that diamonds are not formed by subduction but grow as a result of reduction of carbonates or oxidation of methane in the mantle at depths between 125 to 175 km. In growing they may envelop fragments of their surroundings that formed by other processes.

A notable feature of the inclusions is that before 3.2 Ga only mantle peridotites (olivine and pyroxene) are trapped, whereas in diamonds younger than 3.0 Ga the inclusions are dominated by eclogite minerals (garnet and Na-, Al-rich omphacite pyroxenes). This dichotomy is paralleled by the rhenium and osmium isotope composition of sulfide mineral inclusions. To the authors these consistent features point to an absence of steep-angled subduction, characteristic of modern plate tectonics, from the Earth system before 3 Ga. But does that rule out plate tectonics in earlier times and cast doubt on structural and other evidence for it? Not entirely, because consumption of spreading oceanic lithosphere by the mantle can take place if basaltic rock is not converted to eclogite by high-P, low-T metamorphism when the consumed lithosphere is warmer than it generally is nowadays – this happens beneath a large stretch of the Central Andes where subduction is at a shallow angle. What Shirey and Richardson have conveyed is a sense that the dominant force of modern plate tectonics – slab-pull that is driven by increased density of eclogitised basalt – did not operate in the first 1.5 Ga of Earth history. Eclogite can also form, under the right physical conditions, when chunks of basaltic material (perhaps underplated magmatically to the base of continents) founder and fall into the mantle. The absence of eclogite inclusions seems also to rule out such delamination from the early Earth system. So whatever tectonic activity and mantle convection did take place upon and within the pre-3 Ga Earth it was probably simpler than modern geodynamics. The other matter is that the shift to dominant eclogite inclusions appears quite abrupt from the data, perhaps suggesting major upheavals around 3 Ga. The Archaean cratons do provide some evidence for a major transformation in the rate of growth of continental crust around 3 Ga; about 30-40 percent of modern continental material was generated in the following 500 Ma to reach a total of 60% of the current amount, the remaining 40% taking 2.5 Ga to form through modern plate tectonics

Deep Diamonds Shed Light on Ancient Continental Movements (news.sciencemag.org)
van Kranendonk, M.J. 2011. Onset of plate tectonics. Science, v. 333, p. 413-414.

Pristine mantle and basalt floods

Published on July 30, 2011 by zooks777Leave a comment

The Western Ghat hills at Matheran in Maharash... — Flood basalts of the Deccan Traps in Maharashtra State, India. Image via Wikipedia

Plot the ages of major extinctions against those of flood basalt events and you will get a straight line graph for six co-occurrences since 250 Ma, with very little error. Although the exact mechanism for mass death of species and families is argued over interminably, for those six, flood basalt events have to be deeply implicated. There again, every geologist and their aunties dispute the mechanisms behind monster basalt effusions that bury whole landscapes beneath flow after flow and create very distinctive landforms. When they are eroded they form regularly stepped mountain sides, hence their formerly popular name trap basalts, after the Swedish word trappa meaning staircase. There is a hint of cyclicity in their age distribution. But most important of all, no-one has witnessed these vast, pulsating events, the last having mantled the surroundings of the Columbia and Snake River catchments in the US states of Oregon and Washington between 14-17 Ma ago in the Middle Miocene. Some mark episodes of continental break-up, such as those flanking the Central Atlantic at the time of the end-Triassic (~200 Ma) mass extinction, while others are associated with hot spots, such as the Deccan Traps of western India erupted between 60-68 Ma as India drifted over the Reunion hot-spot and those of the Ethiopian highlands (30 Ma) associated with the Afar hot spot.

A common geochemical feature is beginning to emerge concerning the mantle from which the basalts were partially melted. Six sets of flood basalts exhibit the same trace-element and isotopic (Nd, Pb, Hf and He) characteristics, which suggest that their source had been little effected by previous extraction of crust-forming magmas; it is primitive and may be a relic of the original mantle formed at about 4500 Ma shortly after the catastrophic collision between the early Earth and a wandering Mars-sized planet that flung off the Moon (Jackson, M.G. & Carlson, R.W. 2011. An ancient recipe for flood basalt genesis. Nature, online (27 July 2011) doi:10.1038/nature10326). Although undepleted, the chemistry of the mantle source, worked out by back-calculation from that of the flood basalts, is not the same as the once-postulated original accretion of carbonaceous chondrite meteorites: conceivably a result of the chemical reworking when the Moon formed and the remaining Earth was probably molten from top to centre. The important feature is that the recast chemistry is rich in heat-producing elements compared with the source of ‘common-or-garden’ basalts that continually contribute to the ocean floors and island arcs. Wherever the relic mantle is, it is capable of heating itself, over and above the heating from the core and surrounding mantle, and thus likely to generate thermal and material plumes rising through the mantle.

Preceding the work of Jackson and Carlson, another group discovered that when flood basalt events since the Carboniferous are restored to their former geographic positions at the time they were erupted, they cluster above what are now two patches of more ductile mantle close to the cure-mantle boundary (Torsvik, T.H. et al. 2010. Diamonds sampled by plumes from the core–mantle boundary. Nature, v. 466, p. 352–355). If that is the source of basalt flood-forming plumes, then it is still there and, aside from giant impacts with extra-terrestrial projectiles, the most catastrophic upheavals of the Earth system inevitably will continue, perhaps in the next few million years.

Earth’s time bombs may have killed the dinosaurs (newscientist.com)
Ancient Source of Earth’s Biggest Eruptions Found (livescience.com)

Feathers will fly: Archaeopteryx relegated

Published on July 28, 2011 by zooks777Leave a comment

This year, 2011, is the 150^th anniversary of the first Archaeopteryx specimen being unearthed from the famous Solnhofen limestone lagerstätte. With its feathered, lizard-like tail; two-clawed, stubby wings; a bill-shaped muzzle with teeth but no keratin coating; feet capable of perching and unlike those of small dinosaurs; a ‘wishbone’ and lightweight bones, Archaeopteryx was just the half-and-half missing link in the fossil record so desperately needed to support Darwin’s Origin of Species, published two years beforehand. It has remained controversial ever since, even having been claimed to be a forgery by such luminaries as cosmologist Fred Hoyle in 1985, despite its superbly preserved intricacies and the existence at the time of 6 slightly different specimens from the same source some discovered long after Hoyle’s supposed master craftsman must have died. Creationists soon after the first discovery claimed it was simply a bird created on a Friday together with fish (Genesis 1:20) and must have predated dinosaurs by a day, as they were created on the 6^th Day along with all the ‘cattle and creeping thing and beast of the earth’ (Genesis 1:24-31). That scurrilous sect will certainly leap gleefully on the new discovery of a feathered dinosaur from the ever productive Late Jurassic Tiaojishan Formation in NE China (Xu, X. et al.2011. An Archaeopteryx-like theropod from China and the origin of the Avialae. Nature, v. 475, p. 465-470) because ironically, by itself, it could be said to be a missing link too.

In fact, Xiaotingia zhengi possesses features very like those displayed by Archaeopteryx but convincingly close affinities to deinonychosaurian dinosaurs. The shared features show that neither is a bird (Avialae) and nor are they part of the clade that evolved to birds: they are part of the growing group of feathered dinosaurs that may well have glided or even flown. As Lawrence Witmer of Ohio University has observed (Witmer, L.M. 201. An icon knocked off its perch. Nature, v. 475, p. 458-459), ‘This finding is likely to be met with considerable controversy (if not outright horror)…’. However, Witmer still considers Archaeopteryx to have iconic status, indeed yet more, for its taxonomy and that of its related feathery dinosaurs provides compelling evidence that the origin and evolution of life was a ‘rather messy affair’. Undoubtedly, more feathered creatures hundreds of million years old will be unearthed; it is even possible that further finds will push the beast of Solnhofen back onto its avian perch. Let the celebrations begin!

Added 12 August 2011: Ironically, yesterday the German mint issued a €10 silver coin commemorating the 150th anniversary of the first discovery of Archaeopteryx, artwork of the skeleton with fully fledged arms on the reverse side of the coin compared with the stylised German eagle on the front. This event coincides with the greatest crisis facing the eurozone in its short history, though Germany still retains its ‘triple A’ financial status unlike France and the US. See: http://witmerlab.wordpress.com/2011/01/31/evolution-icon-archaeopteryx-turns-150-this-year-how-are-we-celebrating/

no longer first bird (nature.com)
Archaeopteryx Raises Fears Over Creationists Response (thebibleistheotherside.wordpress.com) A fun read!
Some Bad Reporting about Archaeopteryx (theness.com)

A plume drive for tectonics?

Published on July 15, 2011 by zooks777Leave a comment

Himalaya Formation Source www.usgs.org US Gove... — India's tectonic travels. Image via Wikipedia

The theory of plate tectonics resolved Alfred Wegener’s search for a driving force for continental drift around half a century after his discovery faced near-universal rejection for not having one that was large enough or plausible. Plate theory recognises many forces, both driving and in opposition to tectonic movement. By far the largest is the gravitational pull exerted by subducting slabs of dense oceanic lithosphere, followed in distant second place by ridge-push, another gravity-driven force that arises from the slope on the ocean floors away from sea-floor spreading centres as the oceanic lithosphere cools and shrinks as it ages. Until very recently, no place was assigned in the theory to forces associated with the apparently non-tectonic plumes that rise through the mantle from well beneath the lithosphere from which plates are made, quite possibly because it seems logical to expect a vertically upwards force, if any, from hot plumes whereas plate tectonics is mainly concerned with horizontal movements. Looking around the present state of sea-floor spreading, the maximum pace at which plates move is just over 100 mm a^-1 (100 km Ma^-1) in the case of the Pacific Plate. Yet, during the Late Cretaceous and Early Palaeogene Periods after India had been wrenched away from the Gondwana supercontinent to move towards eventual collision with Eurasia the subcontinent experienced an extraordinary episode beginning around 68 Ma when its pace increased to as high as 180 km Ma^-1. This accelerated motion continued over some 15 Ma and then equally abruptly slowed to less than 40 km Ma^-1 around the start of the Eocene (Cande, S.C. & Stegman, D.R. 2011. Indian and African plate motions driven by the push force of the Réunion plume head. Nature, v. 475, p. 47-52; see also: Müller, R.D. 2011. Plate motion and mantle plumes. . Nature, v. 475, p. 40-41). The acceleration coincided with the start of continental flood-basalt volcanism that blanketed much of western India with the Deccan Traps across the K-P boundary when the subcontinent lay over the site of the Réunion hot spot. Coincidentally, the Réunion plume head formed at that time; i.e. the Indian continental lithosphere did not drift over an active plume, but was hit from below by one that happened to be rising to the surface. Curiously, while the Indian plate was accelerated, nearby Africa was slowed, explained by a push in the same direction of India’s travel towards a subduction zone beneath Asia and one applied to Africa that opposed its motion. Africa too resumed its usual tectonic progress at the start of the Eocene. But how did a mantle plume exert such a force: was it because it caused a local bulge from which the plates slid, or did mantle motion associated with the mushroom-like structure of the horizontally growing plume head exert viscous drag on the overlying plates? Such shifts in motion of major plates inevitably have an effect on the whole plate tectonic carapace, and the authors list a number of contemporary, distant consequences, speculating that the famous bend in the Hawaii-Emperor island and sea-mount chain in the Early Eocene resulted from the final waning of the Réunion plume head’s influence and major readjustment of tectonics.

Mantle plume propelled India towards Asia (nature.com)

From small beginnings

Published on July 15, 2011 by zooks777Leave a comment

The great vegetarian sauropod dinosaurs, such as Brachiosaurus, were the biggest animals to walk the Earth, weighing up to 100 tonnes, as long as 60 m from snout to the end of their tails and more than 10 m tall. So big, indeed, that even the largest contemporary predators would have been unable to get sufficient purchase with their jaws to do them much damage. This vast bulk, unlike even bigger modern whales, was unsupported by water and would have posed major problems had the sauropods not evolved very porous, low-density neck and tail bones and kept their heads small relative to the rest of their bodies. Such small heads needed to take in up to a tonne of vegetation each day to keep the monsters alive and ambling. Their teeth are not those of a chewer, being peg- or spoon-like and pointed forwards; specialised for raking in leaves and twigs, swallowed unchewed in great gulps. Once that style of eating developed in their precursors, with no need for massive chewing muscles it became possible to evolve necks up to 15 m long with increasingly diminutive heads. Studies of large numbers of some species of sauropod precursors indicate that juveniles grew astonishingly quickly, essential if their initial vulnerability was to be outpaced; newly hatched they would have weighed little more than 10 kg. At the growth rates of modern reptiles, the largest sauropods would only have reached full size in about a century. The estimated growth rates suggest warm bloodedness, research suggesting that they maintained body temperatures up to 12°C higher than do alligators. Clearly, sauropod dinosaurs were highly specialised, and their evolution is now known to have been lengthy.

A major news feature in Nature (Heeren, F. 201. Rise of the titans. Nature, v. 475, p. 159-161) traces that evolution through several surprising stages. The earliest likely ancestors, which appear in the Late Triassic (~230 Ma), were about the size of a turkey and had teeth adapted for shredding fibrous plant material; other early dinosaurs show clear signs of a predatory lifestyle. There is a limit to the size of predators bound up with the energy balance between flesh consumption and the energy expended in casing down prey and killing them. The limits on the size of plant eaters are mechanical: how much they can stuff in and the strength of their bodies, especially legs. In a world dominated in numbers by predatory dinosaurs, the selection pressure for herbivores to outgrow them and become too big to bite would have been substantial.

Little Triassic Panphagia (‘eater of everything’) was also bipedal, but the fossil record of sauropod precursors clearly shows their growth to the order of 10 m by the Early Jurassic, but not yet a four-legged gait though they had evolved relatively short but sturdy legs, signs of mass-saving porous neck and tail bones, and jaws with a large gape suited to gulping rather than chewing. By the mid-Jurassic Period sauropods were big, strong and four-legged, and by the Cretaceous they reached unmatched dimensions with the titanosaurs. This evolutionary path was not the only one adopted for dinosaurian herbivory. The famous Iguanodon discovered in 1822 by Gideon Mantell in the Early Cretaceous of Sussex was a member of a bipedal group of herbivores, including the duck-billed dinosaurs, that spanned more or less the same time range as sauropods. Fredric Heeren’s article is accompanied by an on-line ‘tour’ of sauropod evolution (go.nature.com/c7zlct), while the American Museum of Natural History has a website for a major exhibition of sauropods (www.amnh.org/exhibitions/wld/ and http://www.youtube.com/AMNHorg ) that includes footage of a full-scale animatronic Mamenchisaurus from China which breathes and moves, (Switek, B. 2011. Living it large: review of The World’s Largest Dinosaurs exhibition. Nature, v. 475, p. 172).

Necks for sex? No thank you, we’re sauropod dinosaurs [Tetrapod Zoology] (scienceblogs.com)
New Dino Finding: Warm-Blooded, Nimble Beasts (livescience.com)

Core’s comfort blanket and stable magnetic fields

Published on July 5, 2011 by zooks777Leave a comment

Pangea animation — Pangaea and its break-up. Image via Wikipedia

The record of the Earth’s magnetic field for the most part bears more than a passing resemblance to a bar-code mark, by convention black representing normal polarity, i.e. like that at the present, and white signifies reversed polarity. The bar-code resemblance stems from long periods when the geomagnetic poles flipped on a regular, short-term basis, by geological standards. The black and white divisions subdivide time as represented by geomagnetic into chrons of the order of a million-years and subchrons that are somewhat shorter intervals. Stemming from changes in the Earth’s core, magnetostratigraphic divisions potentially occur in any sequence of sedimentary or volcanic igneous rocks anywhere on the planet and so can be used as reliable time markers; that is, if they can be defined by measurements of the remanent magnetism preserved in rock, which is not universally achievable. Yet this method of chronometry is extremely useful, for most of the Phanerozoic. However, there were periods when the geomagnetic field became unusually stable for tens of million years so the method is not so good. These have become known as superchrons, of which three occur during Phanerozoic times: the Cretaceous Normal Superchron when the field remained as it is nowadays from 120 to 83 Ma; a 50 Ma long period of stable reversed polarity (Kiaman Reverse Superchron) from 312 to 262 Ma in the Late Carboniferous and Early Permian; the Ordovician Moyero Reverse Superchron from 485 to 463 Ma.

Because the geomagnetic field is almost certainly generated by a self-exciting dynamo in the convecting liquid metallic outer core, polarity flips mark sudden changes in how heat is transferred through the outer core to pass into the lower mantle. It follows that if there are no magnetic reversals then the outer core continued in a stable form of convection; the likely condition during superchrons. But why the shifts from repeated instability to long periods of quiescence? That is one of geoscience’s ‘hard’ questions, since no-one really knows how the core works at any one time, let alone over hundreds of million years. There is however a crude correlation with events much closer to the surface. The Kiaman superchron spans a time when Alfred Wegener’s supercontinent Pangaea had finished assembling so that all continental material was in one vast chunk. The Cretaceous superchron was at a time when sea-floor spreading and the break-up of Pangaea reached a maximum. The Ordovician, Moyero superchron coincides with the unification of what are now the northern continents into Laurasia and the continued existence of the southern continents lumped in Gondwana, so that the Earth had two supercontinents. Those empirical observations may have been due to chance, but at least they provide a possible clue to linkage between lithosphere and core, despite their separation by 2800 km of convecting mantle that transfers the core heat as well as that produced by the mantle itself to dissipate at the surface. Enter the modellers.

How part of the Earth transfers heat is, not unexpectedly, very complex, depending not only on what is happening at that point but on heat-transfer processes and heat inputs both above and below it. The surface heat flow is complex in its own right ranging from less than 20 to as much as 350 mW m^-2, the largest amount being through zones of sea-floor spreading and the least through continental lithosphere. Wherever heat is released in the core and mantle, willy-nilly the bulk of it leaves the solid Earth along what is today a complex series of lines; active oceanic ridge and rift systems such as the mid-Atlantic Ridge. These lines weave between six drifting continental masses and many more sites of additional heat loss – hot spots and mantle plumes. The many heat escape routes today complicate the deeper convective processes and there are many possibilities for the core to shed heat, yet they continually change pace and position. When, inevitably, all continental lithosphere unites in a supercontinent, almost by definition, the sites of heat loss simplify too, the supercontinent acting like an efficient insulating blanket. In a qualitative sense, this kind of evolving scenario is what modellers try to mimic by putting in reasonable parameters for all the dynamic aspects involved. Two physicists at the University of Colorado in Boulder, USA, Nan Zhang and Shije Zhong, have formulated 3-D spherical models of mantle convection with plate tectonics as a basis for whole Earth thermal evolution over that last 350 Ma (Zhang, N & Zhong, S. 2011. Heat fluxes at the Earth’s surface and core–mantle boundary since Pangea formation. Earth and Planetary Science Letters, v. 306, p. 205-216). The acid test is whether the model can end with a close approximation to modern variations in heat flow and distribution of ages on the sea floor; it does. A probable key to stability in the means of transfer of heat from core to lower mantle – itself a key to a constant outer-core dynamo and geomagnetic polarity – is reduced heat flow at equatorial latitudes; a sort of equatorial downflow of convection with upflows in both northern and southern hemispheres. Zhang and Zhong’s model produced minimal core-to-mantle heat flow at the Equator at 270 and 100 Ma, both within geomagnetic-field superchrons. Well, that is a good start. Superchrons seem also to have occurred from time to time during the Precambrian, one being documented at the Mesoproterozoic-Neoperoterozoic boundary about 1000 Ma ago. At that time, all continental lithosphere was assembled in a supercontinent dubbed Rodinia (‘homeland’ or ‘birthplace’ in Russian).

Search on for past tsunamis

Published on July 5, 2011 by zooks777Leave a comment

Spurred by the horrific scenes and death toll wrought by tsunamis following the 26 December 2004 Sumatran and 11 March 2011 Sendai giant earthquakes, environmental geologists are beginning to look for signs that can reveal past tsunamis in order to evaluate risk from region to region. Before the 11 March disaster Japanese scientists had in fact traced signs of a tsunami in 869 CE and showed that it had reached almost as far inland as that following the Sendai earthquake. There are a number of geological features that mark the wake of a tsunami: dislodgement of huge boulders on rocky shores; signs of powerful scouring of sallow marine sediments as water recedes from the land; chaotic sediments made up of a jumble of clasts; sediments associated with high-energy flow interleaved with those that mark long periods of low energy deposition; marine faunas unexpectedly found in otherwise terrestrial sediments.

Shortly after the 2004 Indian Ocean tsunamis Indian and Japanese scientists visited the Andaman Islands, which were at the northern end of the megathrust deformation, to seek onshore signs of previous catastrophes (Malik, J.N. et al. 2011. Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India. Geology, v. 39, p. 559-562). They discovered a layer of ripped-up lumps of mud set in a sandy matrix dumped on a low-energy black mud, the sandy unit showing inclined stratification that dips inland. All the evidence pointed to deposition by a tsunami. An earlier event reveals swamping of older non-marine sediments by the black mud unit that contains brackish-marine diatoms; a probable result of sudden subsidence linked to an earthquake affecting the Andamans in much the same was as did that of December 2004. The mud had also been intruded by a body of structureless sand , probably resulting from liquefaction as a result of the seismicity. Dating the events using radiocarbon methods proved difficult. Although dating of the earlier event suggested an event around 1670 CE, carbon from the later one gave much older ages, suggesting that the tsunami had ripped up older sediments and redeposited them. However it may be correlated with the major Arakan earthquake of 2 April 1762 close to the coast of Myanmar.

Evidence of this kind can easily be overlooked, and rather less research centres on recent coastal-zone sediments than on sedimentary rocks of the distant past. Areas where such signs of neotectonics have been sought assiduously are those surrounding coastal nuclear installations, but largely to check for evidence of recent faulting that may indicate potential seismic threat but not tsunamis. Clearly it was that kind of threat that decisively put the Japanese Fukushima Daiichi nuclear power station out of action and almost resulted in complete melt-down in March 2011, and severely set back construction of an advanced fast-breeder reactor on the eastern coat of India at Kalpakkam, near Chennai in 2004.

The Sendai great earthquake in close retrospect (earth-pages.co.uk)

Earliest animals from continental environments

Published on July 5, 2011 by zooks777Leave a comment

Skolithos trace fossil. Scale bar is 10 mm. — Skolithus burrows. Image via Wikipedia

Following closely on discovery in 1 Ga old sediments of the earliest evidence for eukaryote life in continental environments (see Eukaryote conquest of the continents posted June 11, 2011) it seems that metazoan animals colonised non-marine environments earlier than had previously been thought. Up to now most palaeontologists believed that there was a lag of at least 80 Ma between the emergence of marine bilaterian metazoans and their expansion into freshwater, due to a number of physiological hurdles that had to be overcome, such as regulation of trace element chemistry within their cells and bodily fluids. It has been know for more than a century that the first signs of sturdy animals in the marine realm are burrows in tidal sediments that formed more or less at the Cambrian-Precambrian boundary; the earlier sac-like Ediacaran fauna seemed ill-suited to a burrowing or infaunal habitat. A considerable thickness of clastic sediments occur in the Cambrian of eastern California, USA. The earliest are clearly shallow-marine and contain abundant evidence of burrowing. Succeeding them are intensively studied fluviatile sands and silts that have been used a model for sedimentation in the absence of the stabilising influence of land plants. What has been overlooked until recently is evidence for colonisation of the river-laid deposits by burrowing animals (Kennedy, M.J. & Droser, M.L. 2011. Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation. Geology, v. 39, p. 583-586).

The burrows include the vertical U-shaped forms given the name Arenicolites, which is the most common trace fossil, simple vertical tubes (Skolithus) and horizontal, meandering tubes with furrowed sides (Psammichnites). Anyone who has seen the Early Cambrian Pipe Rock of NW Scotland will also have seen these trace fossils, yet the Pipe Rock shows evidence of tidal deposition and is shallow marine. Their non-marine equivalents in California are coeval with the earliest known trilobites in the Cambrian marine sequence. It seems that whatever the burrowing animals were, they easily overcame any physiological or environmental barriers to adopting a life in freshwater, encouraged by the ready sustenance that terrestrially adapted acritarchs and cyanobacteria had provided for half a billion years previously.

Eukaryote conquest of the continents (earth-pages.co.uk)
Come On In, the Water’s Fresh (news.sciencemag.org)

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Related articles

Share this:

Share this:

Related articles

Share this:

Related articles

Share this: