Author: zooks777

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

Life originated as an oddity

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Coming up with a theory for the origin of something so complex and ancient as life on Earth might seem to be at the pinnacle of hubris, yet such ideas are not uncommon. A novel slant on the ‘Big Question’ centres on how cells get their energy, rather than on trying to put together all manner of chemical prerequisites (Lane, N. 2009. The cradle of life. New Scientist v. 204 (17 October 2009) p. 38-42). Mike Russell began his career as a geochemist looking at hydrothermal mineral deposits and the intricacies of their formation, while at the University of Strathclyde, Scotland. He now works at NASA-JPL in Pasadena, California inspired by the views of a self-funded eccentric Cornish farmer, Peter Mitchell. Cell energetics, according to Mitchell, are about pumping protons through cell membranes to effect the oxidation and reduction fundamentals of metabolism; in short electrochemical gradients. That is now recognised by every cell biologist, though once it was considered absurd. Russell’s take on that novel truism is that the environment of life’s origin must have involved similar processes taking place in the absence of living cells, which inherited proton pumping. His choice is mineralised pinnacles full of foam-like voids that can act as minute chemical factories: not the famous sulfidic black smokers of ocean ridge systems, but cooler features formed of carbonates precipitated from alkaline sea-floor hydrothermal vents. The carbonate foam in ancient examples, well-known to Russell from their mineralisation, contains bubbles lined with iron sulfides. Sulfides are known to have catalytic properties; proteins in living cells that convert CO2 to sugars have Fe-S bonds at the core of their structure; alkaline hydrothermal vents emit hydrogen released by alteration of olivine in ocean-floor basalt to serpentine minerals; bubbles in carbonate foam look very like potential precursors to cells. To produce the first living cells, these features together in one enclosed space need 10 steps of quite simple chemistry. Except, that is, for nucleic acid production…

End-Permian crisis not so bad for ammonites

The greatest known mass extinction at the end of the Permian Period snuffed out 85% of fossil marine species. It is widely understood to have taken at least five million years for ecosystems to begin recovering, and some animal groups remained depressed for longer still, especially those living at or near the sea floor. Yet one group of cephalopods, the ceratidid ammonites, almost immediately began to thrive, despite the ammonoid sub-Class having been among the hardest hit groups (Brayard, A. et al. 2009. Good genes and good luck: ammonoid diversity and the end-Permian mass extinction. Science, v. 325, p. 1118-1121). Only three genera of ceratidids survived the cataclysm, but within 1-2 Ma there were almost 100 representatives. A similar swift recovery is shown by the completely unrelated conodont animals (now-extinct eel-like vertebrates whose teeth are generally the only parts to be fossilised). For such a success story to emerge by pure chance seems intuitively unlikely: for cephalopod  equivalents of Lazarus to go forth and multiply so nicely requires genes well-suited to the conditions that followed the mass extinction.

Micro-gravity data chart shrinking ice caps

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The NASA and German Aerospace Centre Gravity Recovery and Climate Experiment (GRACE) launched in 2002 aims to measure variations over time in the Earth’s gravity field by gauging tiny changes in distance between two satellites using radar. Briefly, mass in the Earth tugs first on the leading satellite and then on the one trailing it, so if mass distribution stays constant so does the separation between the craft. If mass below a point on the Earth’s surface does change, GRACE detects this from a change in separation between the two craft. Between April 2002 and February 2009, monthly measurements over Greenland and Antarctica reveal losses in the amount of ice, and the rate at which the ice caps are shrinking is accelerating (Velicogna, I. 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, v. 36, L19503 doi:10.1029/2009GL040222). Isabella Velicogna of NASA/JPL shows that the Greenland ice cap (total mass~3 x 1015 t) lost 1.37 x 1011 t a-1 in 2002–3, rising to 2.86 x 1011 t a-1 in 2007–9 , the loss is accelerating at 3.0 ± 1.1 x 1010 t a-2). The ten times more massive Antarctic ice cap lost 1.04 x 1011 t a-1 in 2002–6 rising to 2.46 x 1011 t a-1 in 2006–9, giving an acceleration of 2.6 ± 1.4 x 1010 t a-2. Proportionate to size the Greenland ice cap is dwindling faster than Antarctica, but at these rates it still has 10 thousand years before it disappears.

Boron isotopes and climate change

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Boron has two stable isotopes, 10B and 11B. Like all isotopes of the same element, when boron is shifted from one host to another some fractionation between its isotopes is likely. In the case of boron being taken-up by planktonic foraminifera, their shells’ 11B/10B ratios correlate with the pH of seawater. Since the pH of the oceans is dominated by the effects of dissolved CO2, itself in equilibrium with the gas’s atmospheric concentration, boron isotope ratios in foram shells are a proxy for the greenhouse effect produced by carbon dioxide. This finding dates back to 1992, but has only recently been used. It is especially revealing for the period around the Eocene-Oligocene boundary (see Lead-in to icehouse conditions in July 2009 issue of EPN) when other evidence indicates that global cooling eventually allowed glaciers to grow on Antarctica and possibly at northern high latitudes (Pearson, P.N. et al. 2009. Atmospheric carbon dioxide through the Eocene-Oligocene climate transition. Nature, v. 461, p. 1110-1113). The boron data indicate a downward shift in atmospheric CO2 from around 1100 to 750 ppm by volume from 34.2-33.5 Ma, the lower value just preceding δ18O data for a rapid increase in polar glaciers. Oddly, δ11B then rises to levels suggesting a return to CO2 levels of >1000 ppm by volume at a time of constant high δ18O that show the survival of ice caps; perhaps a result of increased albedo forcing.

Impact cause for Younger Dryas panned again

In 2007 two dozen scientists presented evidence to suggest that onset of the Younger Dryas, extinction of many North American mammal species and the sudden end of the Clovis culture at 12.9 ka followed upper atmosphere explosions of cometary material (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN of July 2007 and May 2008). The Clovis culture of North America, signified by superbly crafted stone spear points, occupied a narrow time range between 13.3 and 12.8 ka, i.e. up to the start of the Younger Dryas interstadial. Some Clovis occupation sites are buried by organic-rich soils. Remarkably, the original proposers of a catastrophic event (Firestone, R.B. and 25 others 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences of the United States of America, v. 104, 16016-16021) claimed that the veneers contain magnetic microspherules, magnetic grains, iridium and nickel, charcoal, soot and polycyclic hydrocarbons, carbon spherules, fullerenes that trap helium with extraterrestrial isotopic proportions, glass-like carbon, and nanodiamonds. Missing from what looks like a supportive package are shocked minerals, which are the only materials formed uniquely by impact events.

Experts on extraterrestrial influences considered the team to be ‘over-enthusiastic’. In response Firestone and co-workers made replicate samples available for independent confirmation or refutation of their claims. This offer seems not to have been followed-up, but another large team recollected the black soil veneers from two of the same sites and 5 others of similar age (Surovell, T.A. and 8 others 2009. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences of the United States of America, v. 106, p. 18155–18158). They focussed on the claim for magnetic spherules, using the same techniques as Firestone et al. (2007), yet failed to find anomalous peaks at the time of the Clovis demise and opening of the Younger Dryas massive global cooling. Their conclusion was, ‘ In short, we find no support for the extraterrestrial impact hypothesis as proposed by Firestone et al.’. However, Surovell et al. did find magnetic spherules before, during and after the interstadial event. In fact, magnetic spherules are quite common in many sedimentary settings and have a history of controversy. In the late 1980s Robert S. Foote, an oil explorationist claimed that many oilfields were associated with geomagnetic anomalies with distinctive short wavelength ‘signatures’. He became widely regarded as a crank. But he persisted and discovered the first tangible evidence for lifeforms that thrive at high temperatures in deep oil wells – shiny, tiny magnetic spherules made of magnetite (Fe3O4). Magnetotactic bacteria living in highly reducing conditions produce them to form magnetosome chains. Magnetosomes are also present in the brains of far-migrating birds, with connections to their remarkable feats of navigation.

Just when you think it’s going to turn out alright…

The millennium of Younger Dryas global cooling from 12.8 to 11.5 ka ago caught forager-hunters on the hop as they followed herds in the wake of the general glacial retreat after 18 ka. The shut-down of the Gulf Stream when high-latitude North Atlantic surface waters freshened may have occurred in a decade or so. The end of the YD marked the start of more modern conditions in the Holocene Epoch, when northward recolonisation resumed in earnest. Climate records, such as the δ18O proxy for air temperature in the Greenland ice cores, suggest long-term but ‘noisy’ climatic constancy. That is, until one spreads out the Holocene records. At around 8200 years ago is a 200-year downward ‘blip’ in temperature to well below the Holocene average and then recovery. The perturbation also shows up in a Newfoundland mire (Daley, T.J. et al. 2009. Terrestrial climate signal of the “8200 yr B.P. cold event” in the Labrador Sea region. Geology, v. 37, p. 831-834) as a pronounced change in δ18O from moss cellulose. The event has been ascribed to slow-down in thermohaline circulation following a further freshening of North Atlantic surface water by drainage of a remaining ice-dammed lake (Lake Agassiz) on the Canadian Shield. By 8.2 Ka the northward spread of flora and fauna from refugia around the Mediterranean Sea was well underway, and included the arrival in southern Europe of Neolithic farming practices: the start of an agricultural revolution that was to reshape the entire sociocultural ethos of the ‘Old World’, from which today’s globalisation emerged. So it is interesting to learn that the ‘cold blip’ also left a signature at 41º N in northern Greece (Pross, J. et al. 2009. Massive perturbation in terrestrial ecosystems of the Eastern Mediterranean region associated with the 8.2 kyr B.P. climatic event. Geology, v. 37, p. 887-890). This study uses pollens collected from a lake-bed sediment core. The climatic event involved a rapid drop by 30 % in tree pollen abundances, matched by a 10% increase in pollen from shrubs, such as Artemisia (wormwood) normally associated with steppes further north. The end of the event involves a more sedate recolonisation by trees. From the pollen can be estimated the actual fall in winter temperature, which amounts to a devastating (for agriculture) decrease that was greater on average than 4º C. Interestingly, the German-French-Greek-Australian team ascribe some influence on the cooling to a spread of the Siberian High, a winter build-up of cold air on the steppes to the north of the Carpathians. The magnitude and extent of the Siberian High depends to a large extent on the albedo of the steppes in winter, which depends on snow cover and its persistence. This is a major influence today across much of Western Europe, as cold Siberian air spills from the continental anticyclone. At 8.2 ka it may have forced katabatic winds through Carpathian passes to cause winters that may have devastated the early farmers of northern Greece.

The march of the seismometers

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It used to be a joke in the Geological Surveys of the Soviet Union that they employed so many thousands of geologists that the entire USSR could be mapped in a few years if they all linked hands and walked from east to west. Geophysicists are trying for something similar to map the mantle underlying the USA in 3-D. The USArray involves 400 portable seismometers, currently spread out at 10 km intervals in the western States, is intended to act like a fly’s eye in monitoring arrival times of seismic waves from worldwide earthquakes. The plan is to steadily move the array eastwards until by 2013 it has reached the Atlantic coast. From that data the geophysicist hope vastly to improve the resolution of seismic tomographic images of the deep Earth (see Kerr, R.A. 2009. Scoping out unseen forces shaping North America. Science, v. 325, p. 1620-1621). Yep, they are definitely going for a high ‘Wow factor’ rating. Yet is seems that there are other expletives floating around as the strangely knobbly and discontinuous architecture that is emerging from early data processing refuses to fit many simple hypotheses being tested.

Groundwater depletion measured from orbit

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The NASA and German Aerospace Centre Gravity Recovery and Climate Experiment (GRACE) launched in 2002 aims to measure variations over time in the gravity field by gauging tiny changes in distance between two satellites using radar. The only significant changes in the short term are due to movements of water in one form or another. The best-known result from GRACE is its assessment of shrinking ice caps, and it can also detect shifting ocean currents and the drainage of lakes. The GRACE science team has noted a major change in gravity since launch over a nearly 3 million km2 area of NW India centred on Delhi. The only conceivable mechanism is gradual loss of groundwater though irrigation of the Gangetic plains (Rodell, M. et al. 2009. Satellite-based estimates of groundwater depletion in India. Nature, v. 460, p.999-1002). The authors estimate a decline of around 109 km3 of groundwater since 2002 – more than twice the storage capacity of India’s largest reservoir. In places local farmers are reportedly having to sink deeper and deeper wells as the water table sinks by over 6 m each year. The area is one of Asia’s largest producer of food grains and occupied by 600 million people. Most likely there has been a surge in withdrawal for irrigation during poor monsoons in the early 21st century, for pumping rates seem to be 70% greater than they were in the 1990s.

 

Gas hydrates soon to come on stream?

 The looming prospects of petroleum production outside of Arabia passing its peak and flexing of Russian economic power that stems from its control of the largest  untapped natural gas reserves are spurring evaluation of methane production from gas hydrates in onshore frozen peat mires and marine sediments. Gas hydrates are more equitably distributed than are much older petroleum reservoirs: even Japan, which is currently entirely dependent on foreign supplies, has what appear to be huge offshore reserves of gas hydrates. Estimates of the world‘s potential resources are enormous, at around 2 x 1016 m3 (annual US natural gas consumption is ~ 6 x 1011 m3) but in a variety of sands and muds at different concentrations (Boswell, R. 2009. Is gas hydrate energy within reach? Science, v. 325, p. 957-958). Experiments in northern Canada (see Onshore gas hydrate reserves close to recovery in March 2004 issue of EPN) indicates that drilling to induce lower pressure in gas hydrate bearing sediments induces dissociation of the hydrate crystals to release methane while retaining also present within their structure. Injection of CO2 into deposits should displace methane while CO2 enters the crystalline structure: killing two birds, including carbon sequestration, with one stone. The main technical stumbling block is that gas hydrates occur in unconsolidated sediments that may be destabilised during production, resulting in uncontrollable release of the powerful greenhouse gases as well as collapse of surface structures. From an environmental standpoint, all gas hydrates do is sustain reliance on carbon-based fossil fuels and continue emissions of greenhouse gases, though burning methane is a good deal ‘cleaner’ than coal or oil.

Did mantle chemistry change after the late heavy bombardment?

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During the Hadean the Inner Solar System was subject to a high flux of asteroidal debris, culminating in a dramatic increase in the rate of cratering on planetary surfaces between 4.0 and 3.8 Ga known as the late heavy bombardment. It left a subtle mark in tungsten isotopes of the Earth’s continental crust that formed during and shortly after the cataclysm (see Tungsten and Archaean heavy bombardment, August 2002 EPN). It has also been suggested that it enriched the mantle in elements, such as those of the platinum group, that have an affinity for metallic iron, a major constituent of many meteorites. The most likely rocks of the Archaean crust to show hints of such enrichment are ultramafic lavas known as komatiites, though to have formed by high degrees of partial melting of plumes rising from deep in the Archaean mantle. Komatiites from their type locality in South Africa and from the Pilbara area of Western Australia do indeed suggest that there was significant effects (Maier, W. D. et al. 2009. Progressive mixing of meteoritic veneer into the early Earth’s deep mantle. Nature, v. 460, p. 620-623). The Finnish-Australian-Canadian team found that the older komatiites (3.2-3.4 Ga) contain less platinum-group elements (PGE) than do those from the later Archaean and early Proterozoic (2.0-2.9 Ga). This they ascribe to a surface layer of meteoritic debris gradually being mixed into the mantle by convection. In their discussion they suggest that once the Earth’s core formed (almost certainly very soon after the Moon-forming event at 4.45 Ga) it effectively leached all PGE from the lower mantle, and could only have achieved higher concentrations by mixing of later meteoritic debris. Their results suggest that this went on through the Hadean, but reached its acme and then stabilised in the late Archaean once the earlier Archaean alien debris had been churned in.

Fire and tool making

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Native people in Australia have been spoiled for choice of materials from which to make superb stone tools, all kinds of silica rock being available in the bedrock and the widespread tropical soils, including multicoloured chalcedony and even opal. Their master craftsmen developed a form of heat treatment that subtly modifies silica’s internal structure so that gentle application of pressure to the edges of lumps removes small flakes to give intricate sharp edges, including barbs for fishing spears. This pyrotechnology leaves easily recognised signs in stone tools: colour changes and a pearly lustre.

A large team of archaeologists and geoscientists from South Africa, Australia, the UK and France have sifted through tools collected from the 35 to 280 ka African Middle Stone Age (defined differently from the European Mesolithic) in search of evidence for fire treatment (Brown, K.S. and 8 others 2009. Fire as an engineering tool of early modern humans. Science, v. 325, p. 859-862). Like signs of symbolic behaviour (see Technology, culture and migration in the Middle Palaeolithic of southern Africa and Deeper roots of culture in January and March 2009 issues of EPN) fire-worked silica tools appear as early as 164 ka ago. However, this is the first paper that reports a search for such technology, and since fire was definitely used by even earlier humans, such as Homo antecessor around 790 ka (see Early, microscopic evidence for human control of fire in November 2008 issue of EPN) expect earlier finds to be announced.

See also: Webb, J. and Domansski, M. 2009. Fire and stone. Science, v. 325, p. 820-821

Neanderthals few on the ground

Analysis of DNA from Neanderthal bones is gathering pace as cheaper and more reliable methods for sequencing emerge. The latest breakthrough is by a team working in Svante Pääbo’s lab at the Max-Planck Instuitute for Evolutionary Anthropology in Leipzig, Germany, which has defined full mitochondrial DNA sequences for five individuals (Briggs, A.W. and 17 others 2009. Targeted retrieval and analysis of five Neandertal mtDN genomes. Science, v. 325, p. 318-321). The samples are from almost the full geographic range known for Neanderthals, from Spain in the west to the eastern shore of the Black Sea in Russia, and are from 38 to 70 ka old; i.e. probably pre-dating the main influx of fully modern humans into Europe. The results show that the range of genetic diversity in the female line was only one third that found in humans today. That suggests that, compared with the modern human diaspora from Africa, total numbers of Neanderthals was low over the period analysed, and perhaps since their first colonisation of Europe and the Eurasian steppes around 400 ka.

See also: Wong, K. 2009. Twilight of the Neandertals. Scientific American, v. 301 (August 2009), p34-39.

Klondike gold rush pays dividends for Pleistocene

The 1896 discovery of gold in the Yukon Territory, Canada triggered the Klondike gold rush, which led to environmental wreckage that continues to this day. The placer deposits are in permanently frozen, but fragile alluvial sediments dating back as far as 700 ka. But as well as gold washed in by the Yukon’s rivers, the permafrost contains exceptionally well preserved records of the area’s late Pleistocene flora and fauna. The reason why that was possible at such high latitude (65ºN) through 6 or 7 glacial interglacial cycles is that it remained free of ice sheets for most of the Pleistocene. Fossils finds in the placer deposits therefore document the conditions on the western edge of the Bering Straits land bridge, or Beringia, which emerged each time that sea level fell during glacial maxima (Froese, D.G. et al. 2009. The Klondike goldfields and Pleistocene environments of Beringia. GSA Today, v. 19 (August 2009), p. 4-10). Beringia was the route presented to the earliest Asian human migrants into the Americas, possibly even before the Last Glacial Maximum 22 ka ago. Much of the evidence comes from wind-blown loess deposits that are prone to permafrost development. Also, being close to a number of active volcanoes the area was sporadically blanketed by ash deposits that are dateable by radiometric means, so a stratigraphy is possible even in the irregular and ice-disturbed sediments. During glacial episodes the area was steppe dominated by herds of bison, mammoths and horses; clearly a hunters paradise, despite the harsh conditions.

Nuclear test-ban monitoring promises a bonanza for seismic tomography

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The world-wide network of seismic recording stations was originally set up partly to improve detection of underground nuclear weapons tests. It is the source for the mapping of variations in seismic-wave speeds in the mantle by seismic tomography that is revolutionising ideas about the Earth’s internal dynamics. Nowadays nuclear explosions have been miniaturised so that detecting them and their locations and distinguishing them from small natural earthquakes has become difficult. The growing concerns about nuclear weapons proliferation have spurred an upgrade and expansion of seismic monitoring, and other means of verifying that seismic signals have indeed been produced by underground nuclear explosions, such as sensitive analysis of air sample for isotopes leaking from tests (Clery, D. 2009. Test ban monitoring: no place to hide. Science, v. 325, p. 382-385). If this enhanced source of seismic data is routinely made available to tomography researchers, it should boost resolution of seismic speed anomalies and sharpen up ideas about deep tectonics.

Supershear earthquakes

In an analogous fashion to the sonic booms made by aircraft travelling faster than sound, it seems possible that the rupture of a fault may travel faster than the seismic waves that it generates. Evidence is accumulating that such faults produce the equivalent of a sonic boom (Fisher, R. 2009. Seismic boom. New Scientist, v. 203 (1 August 2009), p. 32-35) despite mathematical suggestions that faults cannot propagate so fast. Experiments show that there is a seismic equivalent of the Mach fronts associated with sonic booms, and they amplify the shock of earthquakes that produce them. High amplitude at the Mach front causes it to travel further away from a fault line than normal seismic surface waves – those that cause most damage, and it also gives rise to ground motions different from those normally linked with earthquakes: more like a hammer blow than shaking. The net conclusion is that these ‘supershear’ earthquakes may pose hazards beyond those involved in risk assessment near active fault zones. Field evidence for supershear events are signs of disturbance by recent earthquakes that are further from an active fault zone than existing theory predicts. So far such evidence has only turned up along active strike-slip faults on continents, such as the Kun Lun Fault in Tibet and the North Anatolian Fault in Turkey. Yet, these form the longest seismically active zones, including the infamous San Andreas Fault in California.

And now; salt domes on Mars

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The front cover of the August 2009 issue of Geology could be mistaken for an exaggerated oblique aerial view of part of Iran’s Zagros Mountains, well known for their dissected salt domes. It is, however, a simulation of an aerial oblique using digital elevation data from the Valles Marineris area on Mars (Adams et al. 2009. Salt tectonics and collapse of Hebes Chasma, Valles Marineris, Mars. Geology, v.  37, p. 691-694). Hebes Chasma is a roughly oval, steep-sided depression the margins of which show clear signs of some kind of erosion. However, the depression has no outlet, so looks quite bizarre by terrestrial standards: and it is not the only such feature. At its core is a pericline of material that was formerly buried deeper than the flanks of the chasma, which are pretty much horizontal. Unlike the larger, nearby Valles Marineris, Hebes Chasma cannot have formed by erosion of the surface by a huge mass of flowing water, yet 100 thousand cubic kilometres of rock has simply disappeared. Explaining such a gigantic, weird feature taxes the imagination, but the authors do come up with a hypothesis. They reckon that the 105 km3 of material became some kind of thin, briny slurry during an early Martian heating event, which drained downwards into a vast aquifer. For that to happen demands a thick, subsurface layer of dirty ice that melted, and an extremely porous substrate able to channel away the escaping muddy brine. How the pericline formed is not explained, except that it appears in a lab model made of sand, glass beads and ductile silicone polymer, when the silicone drained out through slots in the model’s base. There is plenty of evidence that the surroundings of the chasma collapsed spectacularly, and if the pericline formed by the rising of low-density material dominated by ductile salts (or ice) then it is a likely story. But where did the 100 thousand km3 of gloopy brine go? My guess it followed a secret passage to emerge into the far larger Valles Marineris… Even if there is a crewed mission to Mars, to land anywhere near Valles Marineris would be suicidal, it is so precipitous. So, this is yet another Martian mystery that will linger in a febrile kind of way.

 

Detecting natural asbestos hazards

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All forms of asbestos (various serpentines and some amphiboles), but especially the blue variety, are carcinogenic because their dusts consist of minute fibres. Most publicity about the hazard that this mineral presents is from cases that stem from its use as an insulator in housing, shipbuilding and other constructions in developed countries. Areas where it has been mined or outcrops naturally are equally risky if wind can pick up asbestos dust under dry conditions. A large proportion of this now banned industrial mineral was mined in South Africa and many cases of asbestosis and mesothelioma in former mining areas have come to light there since the fall of apartheid. The locations of former asbestos mines are well known, and some attempts are being made to bury the waste. The most tragic cases are where the mining companies have either folded or been engulfed by larger transnational corporations; several legal actions for compensation have been dragging through the courts for a decade or more. However, asbestos minerals are common at what were non-commercial levels in many ultramafic rocks. Such rocks occur in ophiolite complexes and Archaean greenstone belts on every continent, and although ultramafics are in a minority as regards rock outcroppings, they are far from rare. In its natural state such land can shed asbestos-rich dust when dry, and urban and communications developments expose the material to wind action.

Asbestos minerals fortunately have distinctive infrared spectra in the short-wave infrared (SWIR), preferentially absorbing photons at around 2.3 micrometres because of their abundance of magnesium-oxygen bonds that such wavelengths cause to vibrate. Remote sensing is therefore a potentially useful means of screening areas of human habitation for asbestos risks (Swayze, G.A. et al. 2009. Mapping potentially asbestos-bearing rocks using imaging spectroscopy. Geology, v. 37, p. 763-766). The authors, from the US geological Survey and the California Department of Conservation, used a sophisticated and costly form of aerial remote sensing that covers the visible and infrared part of the EM spectrum with hundreds of narrow-wavelength bands: so-called hyperspectral imaging. It is possible to highlight areas containing asbestos minerals by matching the measured and mapped surface spectra with laboratory standard spectra of the pure minerals. In the case of the test area in northern California, where suburban expansion is likely to occur or has done already, the geology is known in some detail and the expensive airborne hyperspectral surveys could be focused. The approach gave results sufficiently accurate for preventive measure to be taken; not only for asbestos-rich bare soils, but also the specific kind of vegetation that ultramafic soils encourage.

There is another, far cheaper means of assessing asbestos risks that is not so accurate, but capable of covering very large areas of poorly known geology, especially in less well-off parts of the world. This uses the satellite remote sensing conducted by the US-Japanese ASTER instrument carried on NASA’s Terra satellite. ASTER data include 5 narrow wavebands that bracket the 2.3-micrometre part of SWIR, so that it is capable of assessing the distribution of ultramafic rock outcrops using software similar to that for hyperspectral data. The USGS/California DoC survey could have tested ASTER data to see how effective it would be if more costly airborne data was unaffordable. Sadly the team didn’t foresee how a local test of concept might benefit a great many areas elsewhere by using an ASTER scene that would cover their entire study area, be free to USGS scientists and cost only US$85 for anyone working in the Third World.

Nuclear waste: planning blight writ large

The artificial radioactive isotopes generated in nuclear fission reactors have half lives that range from days (131I) to a few million years (135Cs). They pose a thorny problem for disposal since the radiation that they emit collectively is likely to reach ‘safe’ levels only after tens to hundreds of thousand years, even if they were diluted by leakage into air or water or onto the land surface. They have to be contained, and that demands storage in rock. More over, underground disposal sites must ensure no leakage for geologically significant periods – a great many rare events, such as magnitude 9 earthquakes, large volcanic upheavals and rapid climate changes all become increasing likely the longer the delay time. Apart from Sweden and Finland, no country that uses nuclear energy has a deep disposal site. The focus has been on the temporary measure of reprocessing, and one major facility, that at Sellafield in the UK, is to close down.

In 1987 the US Congress designated only one potential site for investigation as a place for long term water storage in their vast, geologically diverse country: Yucca Mountain in Nevada. The reasoning was that the area is remote and arid, and not so far away from highly secure military sites, so it could be guarded unobtrusively. After 30 years of investigation, Yucca Mountain has been abandoned, with no equally-well researched fallback site (Ewing, R.C. & von Hippel, F.N. 2009. Nuclear waste management in the United States – starting over. Science, v. 325, 151-152). From a geological standpoint, that is not so surprising as Nevada is seismically active; there has been volcanism in the not-so-distant past, it does have groundwater, and that is present in the volcanic ash proposed for storage. Moreover, the water is oxidising and uranium in spent nuclear fuel easily dissolves under those conditions – storage was to be in titanium casks. Clay saturated in anoxic water is a better bet, while the Scandinavian approach seems safer still: galleries and boreholes in dry crystalline basement rock with canisters packed in clay.

Yucca Mountain has been wrangled over for 3 decades, and one component in its abandonment was a change in the proposed ‘regulatory period’ from 10 thousand to a million years. How compliance might be demonstrated for a period five time longer than our species has existed, and 500 time longer than the length of the Industrial Revolution is something of a problem for bureaucrats, as of course is judging the cost and time for decommissioning obsolescent nuclear plant. If nuclear energy is to play any role in cutting carbon emissions, the volume of nuclear waste is set to rise enormously, but this does not seem to concentrate the regulatory group mind wonderfully.

See also: Wald, M.L. 2009. What now for nuclear waste? Scientific American, v. 301 (August 2009), p. 40-47.

Methane: the dilemma of Lake Kivu

A massive discharge of carbon dioxide from the small but deep Lake Nyos in Cameroon in 1986 killed 1700 local people after a small earthquake and landslide disturbed the bottom water.  The lake is stagnant, and carbon dioxide released by exhalation from deep magma chambers beneath it had dissolved under pressure in in deepest levels. Once disturbed, the gas came out of solution to reduce bottom water density so a large volume rose to blurt out gas and deal silent death in the lake’s immediate surroundings.

Lake Kivu in the western branch of the East African Rift system borders the Democratic Republic of Congo (DRC) and Rwanda. With an area of 2700 km2 and a depth of over 400 m it is far larger than Lake Nyos, but similar in having stagnant water below a depth of about 75 m, in which gases are dissolved under pressure. Lake Kivu contains an estimated 256 km3 of carbon dioxide derived from magmas beneath the Rift and 65 km3 of methane that probably arises by anoxic bacterial reduction of the CO2. Cores into Lake Kivu’s sedimentary floor indicate massive biological die-offs at roughly millennial intervals, which probably result from magmatic destabilisation of the gas-rich lower waters. Experimental vent pipes have been installed in Lake Nyos and nearby Lake Monoun to remove gas from the deep water (see Taming Lake Nyos, Cameroon and Letting Cameroon’s soda-pop lakes go flat in EPN issues for April 2001 and March 2003, respectively), but such a solution for the much larger Lake Kivu would be far less predictable and extremely expensive (Nayar, A. 2009. A lakeful of trouble. Nature, v. 460, p. 321-323). Energy companies based in DRC and Rwanda are now starting to use the ‘soda siphon’ approach that relieved Cameroon’s deadly lakes to capture the methane potential in Lake Kivu. Perhaps that will dampen down the lake’s potential for explosive gas surges, but no one knows if it could instead destabilise its uneasy equilibrium. Furthermore, the deep cool water is nutrient rich and may set off planktonic blooms in Lake Kivu’s surface waters. DRC is notorious for bandit mining and politics and security even more unstable than the lake that it shares with its tiny neighbour Rwanda. Population density on the lake’s shore, always high because of the fisheries and agricultural potential, rose explosively in the aftermath of the Rwandan genocide of 1994.