‘Fracking’ shale and US ‘peak gas’

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Around 1970 the production of natural gas in the US reached its peak and has been slowly declining since then. The degree to which the US economy has grown to depend on natural gas and growing fears of becoming dependent on insecure supplies on the international LPG market has seen a stealthy growth in unconventional technologies to maintain indigenous supplies. The greatest growth has been in winning the useful fuel from ‘tight’ organic-rich shales that are usually regarded as source rocks for conventional petroleum rather than resources in their own right (Kerr, R.A.. 2010. Natural gas from shale bursts onto the scene. Science, 328, p. 1624-1626). The technology relies on drilling methods developed in the oil industry that allow several holes from a single platform to bend to pass at low angles through thin, gently dipping strata. That allows far larger volumes to be tapped than through a single, vertical well. Oil shales are not yet targeted for liquid petroleum because of the cost, but as Richard Kerr, a news writer for Science, reveals they are supplying an increasing proportion of US gas demand: from 1% to 20% since 2000. Being less of a source of carbon dioxide than coal or oil that might seem to be a ‘good thing’ all round, but there are worrying and little known problems with the technology.

To get the gas out demands that the permeability of shale is artificially increased by jacking open joints and fractures using very high-pressure fluids that carry sand to wedge them open when production begins open: this is ‘fracking’ in driller-speak. Not only gas starts to move, but also water locked into the shale for millions of years and often highly toxic. Drillers hope that all the fluids will follow the holes, but that is by no means guaranteed and some may make their way into aquifers and up to the surface. The fluids used in fracking are deliberately full of chemicals that help open up cracks and even biocides that keep them from being clogged by bacterial films: around 15 million litres used per well. Although aimed to be recycled these noxious fluids can escape, sometimes in massive blowouts. Uncontrolled gas and formation water escapes can cause explosions and kill of forested areas by disrupting tree-root biota.

Post-perovskite unveiled

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Kei Hirose, the discoverer in 2002 of a ultra high-pressure transformation of mantle mineralogy, has produced a highly readable review of the implications of his work for how the mantle functions (Hirose, K. 2010. The Earth’s missing ingredient. Scientific American, v. 302 (June 2010 issue), p. 58-65).

Seismology has long charted the occurrence of step-changes in mantle properties at a several more or less constant depths. Mantle above 410 km provide most of the samples available to geoscientists as inclusions in basalt lavas and is olivine-rich peridotite. From 410-660 km the elements forming olivine take on a different configuration more akin to the mineral spinel; also backed by some direct as well as theoretical/experimental evidence. At 660 km deep seismic properties change dramatically in a major transition zone. Experimental work in the 1970s with mantle chemical compositions at high pressures and temperatures showed that at greater depths the structure of magnesium silicates like olivine, pyroxene and spinel collapses to a denser form with very efficient packing of aoms that is the same as that of a broad group of minerals known as perovskites. That seemed to be the end of the matter. However, continued geophysical investigations and geochemical studies of basalts derived by partial melting of mantle rock teased out complexities in the once assumed simplicity of the mantle. In 1983 analysis of seismic records revealed a further step in physical properties of the deepest mantle (once designated the D layer) that forced a revision to recognise a transition at 2600 km deep, just 300 km above the core-mantle boundary. This now separates the 2000 km thick D’ layer from the lowest D” layer in the mantle. Subsequently, chemical heterogeneities in the deep mantle became a major puzzle.

Hirose and his team pushed experimental conditions to match the huge pressures below 2600 km and discovered a yet more efficient, hitherto unknown molecular configuration that arranges magnesium, silicon and oxygen into separate layers: dubbed ‘post-perovskite’ for want of a already known mineral structure. As well as a small (1.5%) increase in density, the mineralogical change unexpectedly releases rather than consumes heat energy. Such an exothermic process clearly had great implications for how the mantle works. If rock from higher levels finds its way down to and below the D’-D” transition, as might happen if subducted oceanic lithosphere slabs continue ever downwards, it gets an energy ‘kick’. Theoretical work revealed that the early Earth would have been too hot for post-perovskite to form. But once it had cooled below a threshold the phase change ‘snapped’ into existence: that must have significantly changed mantle dynamics. Convective motion in D” that brings material to the D’:D” boundary the post-perovskite to perovskite phase change produces a sharp decrease in density and an upward force. So, once D” formed plume formation and overall mantle convection would have increased. That impetus could not have been present before so that early Earth mantle dynamics were more sluggish. That would maintain a hotter core-mantle boundary, thereby slowing cooling of the liquid core and formation of the solid inner core. Moreover, the upper mantle would have been cooler than now, creating the paradox of less surface magmatism on the early Earth. Theoretically, development of D” should have been marked by a 20% increase in heat flow and a paroxysm of tectonics and crust formation. Was that linked with the formation of stable continental crust around 4 Ga, the spurt in continental growth in the late Archaean or some later event (Hirose suggests 2.3 Ga, but no major tectonic shift has that age)?

As well as tectonic implications, the affect of the D” layer on the pace of crystallisation of the solid inner core may have controlled increasing strength and stability of the geomagnetic field. Because only Earth’s strong magnetic field protects the surface from life-threatening cosmic rays and the solar wind, in a roundabout way post-perovskite possibly played a role in allowing the origin, evolution and survival of life on our home world. That possibility is pretty much the ultimate link between solid Earth and the biosphere: take note Gaians!
See also: Buffet, B.A. 2010. The enigmatic inner core. Science, v. 328, p. 982-983.

Why a glacial period ends

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The publicity and debate that sprang up in the 9 months after release of e-mails stolen (17 November 2009) from the British University of East Anglia’s Climatic Research Unit, and several debacles regarding pronouncements by the Intergovernmental Panel on Climate Change have in fact cleared the air on several purely scientific matters. , Contrary to what had become the broad public conception, thanks to massive and continuous propaganda about global warming that barely mentions anything else, greenhouse gas emissions are widely revealed to be not the ‘only game in town’ when it comes to past changes in climate. That is very much the lesson learned by decades of study of the greatest climate change that fully modern humans have experienced: the last glacial termination when the deepest frigidity about 20 ka ago gave way to very rapid warming. A review of that enormous world event carries important lessons about what really controls climate on our world and how complex that is (Denton, G.H. et al. 2010. The last glacial termination. Science, v. 328, p. 1652-1656).

Since the 1970s proxy data from deep-sea sediments that reveal the variation in the volume of glacial ice on land have showed how climate changes over the last 2.5 Ma are broadly correlated with the periods of astronomical effects on the amount of solar energy received by Earth or insolation, particularly that at high northern latitudes. This might suggest that glacial terminations occur when insolation reaches maxima. In fact over the last 800 ka terminations have also occurred at times of low insolation. The Milankovich signal is ubiquitous but it is not the primary driving factor for the end of glacial episodes. Nor do they tally exactly with increased CO2 in the atmosphere, as recorded in air bubble trapped in polar ice. In fact there is a lag between the record for greenhouse gases and those for warming and cooling. The clearest correlation is between terminations and the maximum volume of land ice in each glacial epoch, towards which Denton et al. direct most attention. Since Antarctic ice has barely changed volume since the Pliocene, pulsation in land-ice volume must stem mostly from Northern Hemisphere glaciation and deglaciation. That repeatedly occurred around the North Atlantic where the main sites for ocean-water downwelling occur. At their thickest the North American and European ice sheets also had their greatest isostatic effects, bowing down the crust, and increasing ice flow towards the ocean. Time after time in each glacial build-up such a configuration became unstable so that marginal ice collapsed to produce the iceberg ‘armadas’ known as Heinrich events. Freshening of the North Atlantic by iceberg melting shut down the downwelling, thereby thermally isolating high northern latitudes to give Dansgaard-Oeschger events comprising paired coolings, or stadials, followed by suddenly warming interstadials once deep circulation restarted.

What is also emerging is that, to maintain heat balance, as each stadial developed in the North Atlantic more heat was shifted to the Southern Hemisphere. Increased downwelling of cold saline water of the Southern Ocean drove this warming to higher southern latitudes. The net observed effect is a southern reversal of sea-surface and polar air temperatures compared with those of the Northern Hemisphere, especially clear in the late stages of the last termination, including the Younger Dryas. Each warming of the south encouraged the southern oceans to emit stored CO2 to the atmosphere, until finally sufficient to maintain global warm conditions when the arose during terminations.

Flatulence and the Younger Dryas
There is a widespread belief that the enlargement of domesticated ruminant herds, mainly cattle, goats and sheep, may have had some effect on recent climate: their enteric fermentation of grass cellulose generates methane, a powerful greenhouse gas. Livestock produce an estimated 80 million metric tons of methane annually, accounting for about 28% of anthropogenic methane emissions. Livestock aren’t the only methane emitting ruminants: giraffe; bison; yaks; water buffalo; deer; camels (including llamas and alpacas); and antelope. Elephants are not so efficient, but they do break wind a great deal. An adult elephant emits about half a ton of methane annually; enough to run a car 20 miles per day; on the school run for instance.

Livestock have become the dominant herbivores on the planet, but far more wild ruminants roamed the Earth during the last glacial epoch because of the much greater expanses of grasslands during cooler, more arid conditions. This was especially the case in North America, a much diminished impression being given by the vast herds of bison that were almost exterminated in the 19th century and those of caribou that still migrate across Alaska and northern Canada. The estimated ruminant population of late-Pleistocene prairies was so large that it too has been implicated in climate change during the last glacial termination (Smith, F.A. et al. 2010. Methane emissions from extinct megafauna. Nature Geoscience, v. 3, p. 374-375), with estimated annual emissions around 10 million tons. With atmospheric methane concentrations having reached around 650 parts per billion by volume (ppbv) by 15 ka – a third of those today – the farting animals of the prairies may have made a significant contribution to post-glacial global warming. Sometime around 13 ka immigrant humans from Asia entered the scene, armed with efficient hunting weapons. By 11.5 ka, the vast herds had more or less vanished through extinction, and the 10 megaton methane emission went with them. Felisa Smith and her colleagues from the University of New Mexico, Los Alamos National National Laboratory and the Smithsonian Institution, USA, note that over the same period atmospheric methane content fell from 650 to <500 ppbv. They speculate that part of this decline may have resulted from the extinction of the North American ‘megafauna’ and contributed to the Younger Dryas cooling between 12.8 to 11.5 ka. If that were the case, it would have been the earliest instance of a human effect on the Earth and, opine the authors, ought to be used to mark the start of what some geoscientists propose as a new geological Period: the ‘Anthropocene’. This parochial view surely ranks alongside that of a shower of nano-diamonds from an extraterrestrial explosion as the cause of the Younger Dryas, to the posthumous annoyance of William Seach of Occam.

Doubt cast on erosion and weathering theory of climate change
A seminal paper in the late 1980’s by Maureen Raymo, Flip Froelich and Bill Ruddiman proposed that the uplift of mountain ranges, their erosion and associated chemical weathering helped gradually shift global climate. Their main reasoning was that rotting of feldspars by carbonic acid formed when CO2 dissolves in rainwater locked the greenhouse gas in soil carbonates and supplied bicarbonate ions to sea water, where they would recombine with calcium and magnesium ions also released by weathering to form limestones. This process would draw down greenhouse gas levels in the atmosphere faster during episodes of major mountain building. Such carbonate burial has since been assumed to have helped the Earth’s climate cool during the Cenozoic era, after the Alps, Andes and especially the Himalaya began to form. There have been many publications about the processes involved and the geochemical signature of varying erosion, such as changes in the strontium isotope composition of limestones as a proxy for that of sea water. But the real test for whether or not there have been pulses in erosion controlled by orogeny would involve measuring changes over time in sediment deposition in all the world’s sedimentary basins. In a recent paper (Willenbring, J.K. & von Blanckenburg, F. 2010. Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature, v. 465, p. 211-214) published estimates of continent derived sedimentation plotted against atmospheric CO2 derived from various proxies show two features. First, there hasn’t been a truly significant decrease in CO2 since the end of the Oligocene (23 Ma). Secondly, although sedimentation over every 5 Ma rose from about 6 x 1015 to 1016 t between the end of the Oligocene and the start of the Pliocene. Repeated glaciation over the last 5 Ma helped increase global sedimentation to 3 x 1016 t, but even that tripling seems not to have had much effect on atmospheric CO2.

Willenbring and von Blanckenburg have attempted to improve the very uncertain evolution of the sedimentary record based on basin stratigraphy – despite seismic sections in many basins, costly and still rare 3-D cross sections are the only means of working out actual masses of sediment deposited through time. The authors re-examined the record of beryllium isotopes in sediments and manganese crusts from the deep-ocean floor, as a proxy for rates of weathering of continental debris. The principle behind this is the continuous production of radioactive 10Be in the atmosphere by cosmic rays, and its entry into the oceans. There it mixes with stable 9Be released to solution by weathering of rocks. Allowing for the decay of 10Be and assuming constant rates at which it is produced, the 10Be/9Be ratio in ocean water and sediments in contact with it is a proxy for global weathering. A decrease in the ratio implies an increase in continental weathering, while decreases signify periods of slowing rock breakdown. Over the last 10 Ma, the ratio has stayed more or less constant in the Pacific and Atlantic Oceans. The obvious conclusion is that the last 10 Ma showed no pulse in weathering and that period did not follow the Raymo-Froelich-Ruddiman model. There are several explanations for the ‘flat-lining’ Be isotopes (Goddéris, Y. 2010. Mountains without erosion. Nature, v. 465, p. 169-171), but a rethink of the significance of any link between orogeny and climate is clearly on the cards.

On the same topic, the start of Northern Hemisphere glaciations and its 30-40 Ma lead-in, Bill Ruddiman of the University of Virginia reviews a broader range of evidence (Ruddiman, W.F. 2010. A paleoclimatic enigma. Science, v. 328, p. 838-839) but not that presented by Willenbring and von Blanckenburg. He concludes that little has changed by way of explanation since the late 1990s, and decreased CO2¬ was the primary forcing factor. Yet his own plot of atmospheric CO2 estimated from marine-sediment alkenones (organic compounds produced by some phytoplankton) shows little fluctuation in the mean concentration since 20 Ma, which is around that for the Pliocene-Pleistocene Great Ice Age.

Arsenic update

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Partly because of natural processes and partly due to a shift to avoid pathogens in surface water used for domestic to a massive well-drilling programme much of rural Bangladesh and neighbouring West Bengal in India found itself the epicentre of ‘the largest mass poisoning of a population in history’, during the 1990s. The agent was soluble arsenic in various forms that reducing conditions in shallow aquifers had released by dissolving its host mineral, iron hydroxide coatings on sand grains. Geological and hydrological attributes of the two hard-hit areas helped develop a model for assessing the risks in other areas. More than a decade on from the world-wide recognition of the tragedy (local geoscientists had their suspicions much earlier) a review of arsenic hazard in both South and Southeast Asia (Fendorf, S. et al. 2010. Spatial and temporal variations of groundwater arsenic in south and south-east Asia. Science, v. 328, p. 1123-1127) is welcome but is not reassuring. The problem now extends to plains of the whole of the Ganges-Brahmaputra-Meghna system, the Red River of Vietnam and the Mekong of Vietnam, Cambodia, Laos and part of Thailand. Almost certainly the Indus and Irrawaddy plains are affected too, though few data are available. The review highlights a haphazard aspect of the distribution of affected wells, both in geographic location and the depth of the tapped aquifer. In the latter case, it was thought that deeper aquifers were less prone to contamination than those in the top 100 m of wells. It turns out that even at depth up to a third of wells exceed WHO recommended levels of arsenic. The positive feature is that many villagers are within walking distance of safe well water. But it is difficult to predict whether or not new wells will be risky, and little is know about safe well’s propensity to become contaminated by groundwater flow from elsewhere. Two clear messages are, first to refine methods of testing and assessing hydrogeological conditions, second to move from hand drawn water from individual wells to provision of piper water from high-yielding safe wells.

More wet minerals on Mars

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A remote-sensing geologist who focuses on terrestrial matters would likely grind their teeth on seeing papers that use far better data captured from the Martian or lunar surface than are ever likely to be available from the bulk of Earth’s land surface over the next decade at least. Mine are even closer to the gums after reading about hyperspectral data from Mars with high spatial resolution (~20 m), used to locate rocks altered by water on Mars (Carter, J. et al. 2010. Detection of hydrated silicates in crustal outcrops in the northern plains of Mars. Science, v. 328, p. 11682-1686). And, of course, there is no vegetation and not much of an atmosphere to cryptify spectral features of minerals: if there is enough of a mineral exposed to show up, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) carried by NASA’s Mars Reconnaissance Orbiter will spot it. If the mineral has unique features in its spectrum, and most of the hydrated silicates do, it can be classified nicely. Less spatially sharp hyperspectral data from the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) carried by ESA’s Mars Express is equally discriminating for larger patches.

The two instruments have shown up hundreds of small outcrops of minerals in the southern hemisphere that formed by reactions between the dominantly anhydrous minerals of Mars’s dominantly igneous crust and water. They record an early phase when liquid water was available at the surface. The question is, are they merely a thin veneer? As a check, John Carter (did bearing the same name as Edgar Rice Burroughs’ hero in his Mars novels encourage his fascination with the Red Planet?) of the University of Paris and colleagues used OMEGA and CRISM data to look at deep crust exhumed in several of Mars’s northern hemisphere craters. Clay minerals, chlorite and prehnite do show up clearly, and the hydration reactions must therefore have penetrated up to a kilometre into the crust. The same suite of minerals occur in the southern hemisphere, so during this early wet episode water was available far and wide across the Martian surface. Minerals like prehnite and chlorite are most familiar as products of low-grade metamorphism, which presents a puzzle. Maybe they formed as a result of the temperatures and pressure generated by the impacts themselves. But if that were the case they would be expected to pervade all the excavated rock, whereas they occur in distinct patches next to pristine, highly reactive olivine-rich rocks. One absentee mineral is serpentine that would definitely have formed by the reaction of water with olivine during impacts. So it looks like water pervaded the whole Martian crust down to maybe a kilometre, then this ‘weathered’ layer was blanketed much later by a thick volcanic layer which has been removed in some places by impact excavation.

• Tectonics
Underpinnings of Mediterranean tectonics
The region of the Mediterranean Sea, especially in the Aegean area, has among the most complex active tectonics on Earth. Both the African and Eurasian plates are now barely moving. The basic shaping of the region stems from Africa’s protracted collision with Europe since 40 Ma that resulted in the closure of the Mesozoic Tethys seaway and jumbled both its sedimentary fill and the continental lithosphere that lay on either side of the collision zone. But if surface motion has largely stopped, why is the Mediterranean region so tectonically active? It now seems as though it links to flow in the mantle beneath (Facenna, C. & Becker, T.W. 2010. Shaping mobile belts by small-scale convection. Nature, v. 465, p. 602-605). A mix of GPS tracking of surface motions, evaluation of surface uplift and subsidence, and analysis of seismic tomography of the mantle. Vertical motion of the mantle is most pronounced at shallow mantle depth (250 km), suggesting vigorous convection in quite small cells. The relations to tectonics are complex, but they are interlinked. For instance subducting slabs interfere with shallow mantle flow so that compensating upwellings result, and in turn help drive subduction and volcanism, as in Italy. Overall, the lithospheric motion, from GPS tracking, has a distinct vortex-like pattern in the eastern Mediterranean and Middle East, which can be modelled from the underlying mantle flow.

The ultimate iPhone app: a truly retro makeover

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Now that the Neanderthal genome has revealed that non-Africans have a bit of the old chap inside us (see Yes, it seems that they did… in EPN May 2010), why not seek your inner Neanderthal? The famous Smithsonian Institution in Washington DC has released an application for iPhones, its first ever venture into ‘apps’, that allows users to morph their faces to resemble how they might have looked as a male or female H. neanderthalensis, H. heidelbergensis or even tiny H . floresiensis. The ‘app’ is called Meanderthal, which is especially apt as that neologism is street slang for a sad individual who roams supermarket aisles with a mobile phone welded to his or her ear.

Male relative of ‘Lucy’
Many people know of the amazing skeleton of a possible ancestor to humans discovered in NE Ethiopia by Donald Johanson in the late 1970s, and they know why it was dubbed ‘Lucy’. That type specimen of a female Australopithecus afarensis still figures in the media, but little appears concerning males of the species. That is not surprising for they are represented by only fragmentary and ambiguous remains. So a report on a 40% complete fossil male A. afarensis that includes limb and pelvic bones, and those of the neck, shoulder and arm is sure to cause a stir (Haile-Selassie, W. and 8 others 2010. An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proceedings of the National Academy of Science USA, v. 107, p. 12121–12126. doi/10.1073/pnas.1004527107). For starters, he is very big indeed compared with ‘Lucy’, standing between 1.5 and 1.7 m tall, and fragments of other individuals suggest that some males were larger still and within the modern human range. The conclusion must be that A. afarensis was sexually dimorphic: big males and diminutive females, which is the norm for chimps, orang utans and gorillas. Legs longer than arms suggest an upright walking posture, but the shoulder assembly is more gorilla-like than human. Yet ribs that indicate a barrel chest show a more human form than would other great apes. The authors suggest that the lack of consistent resemblance to any one of the living hominids may indicate that the last common ancestor that we share with the others may not have closely resembled any of the living forms. The big problem with the find is its antiquity: at 3.6 Ma it is a lot older than ‘Lucy’. Without teeth or at least part of a skull, assigning it to the same species carries no certainty.

Neanderthal ‘bling’

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Led by João Zilhão of the University of Bristol, UK, a team of British, French, Italian and Spanish archaeologists and anthropologists have at a stroke rid our former companions in Europe, the Neanderthals, of the popular and academic stigma of being uncultured (Zilhao, J. and 16 others 2010. Symbolic use of marine shells and mineral pigments by Iberian Neandertals. Proceedings of the National Academy of Sciences, v. 107 p. 1023-1028). They wore jewellery in the form of necklaces and pendants of bivalve shells, remains of which have turned up in large numbers in caves and rock shelters in the interior of southeast Spain. Some of the perforated shells show clear signs of having been painted, and a few show grooves worn by string. They found even a paint container and painting tools made of small bones from a horse’s foot. The container and tools retain distinct traces of pigment made from the common iron colorants goethite, jarosite and hematite. One large, perforated scallop shell shows that its white interior was painted to match its reddish exterior.

It has often been commented that Neanderthal adornments ( a few possible finds precede this work) and intricate tools were simply copied from those of fully modern humans. The deposits containing this ornamentation are around 50 thousand years old: preceding modern human occupation of the Iberian Peninsula by at least 10 ka. Evidence for artistic work by early H. sapiens comes from South Africa as far back as 165 ka (see Technology, culture and migration in the Middle Palaeolithic of southern Africa in January 2009 EPN, and When and where ‘culture’ began in EPN of November 2007). Iron-based pigments are still widely used for body painting in many societies, but obviously that use will not feature directly in archaeological finds. Association of lumps of potential pigments with hominin tools go back even further in Africa, beyond the presence of fully modern humans, but to ascribe pieces of say hematite to cultural practice needs evidence for scraping or grinding. There seems no reason why Neanderthals and modern humans maintained an ancient cultural tradition.

The Younger Dryas flood

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In 2006 Wallace Broeker first suggested that the sudden interruption of emergence from the last glacial maximum by a frigid climate about 12.8 ka was due to a massive release of fresh water to the North Atlantic that shut down its thermohaline ‘conveyor’ (see The Younger Dryas and the Flood in June 2006 issue of EPN). He resurrected an earlier idea that a vast lake of glacial meltwater (Lake Agassiz) to the north-west of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. His hypothesis was that the resulting freshening of surface water in the North Atlantic and decreased density stopped the formation of cold dense brines that sink and drag warm water northwards. Setting aside the notion by some enthusiastic authors that a trigger for the Younger Dryas was an exploding comet and a kind of ‘nuclear winter’ (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008) Broeker’s hypothesis is widely accepted. However there are few signs, if any, of a catastrophic glacial-lake outburst through the Great Lakes region and down the St Lawrence. An alternative is that Lake Agassiz drained northwards towards the Arctic Ocean. (Since the North American ice sheet covered Hudson’s Bay that could not have been the destination.) At the end of the last last full glaciation there was a corridor with relatively little glacial cover between the main ice over the Canadian Shield and that mantling the Rocky Mountains, roughly along the course of the modern Mackenzie River. That route would serve the hypothesis well, and there is clear evidence that an outburst flood followed it (Murton, J.B. et al. 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature, v. 464, p. 740-743).

Sediments of the huge Mackenzie Delta of NW Canada contain a sharp erosion surface overlain by gravels that belie the low-energy of deposition today. Optically stimulated luminescence dating of sediment immediately below and above the erosion surface range from 13.4 (below) to 12.7 ka (above), the latter approximating the onset of frigid Younger Dryas conditions. The surface occurs all the way along the Mackenzie into its major tributary the Athabasca River. Near Fort MacMurray, 20 km north of what was the northern shore of Lake Agassiz, there is a terrace composed of massive boulders. Further evidence comes from the apex of the Mackenzie delta in the form of a 25 km long, 2 km wide spillway scoured of all loose sediment and with topographic features reminiscent of the famous Channelled Scablands of Washington State in the NW USA. Numerous beach lines record the drainage of Lake Agassiz, the highest being dated at the start of the Younger Dryas and giving a clue to the volume involved in the initial outburst flood: around 9500 km3. Dating of other features suggest that a second flooding into the Arctic Ocean occurred during the Younger Dryas around 11.5 ka, during its last stages, and a third at 9.3 ka. One effect of the Younger Dryas was a regrowth of the main ice sheet that allowed Lake Agassiz to refill periodically perhaps allowing quieter flooding events down the Mississippi and through the Great Lakes. There are no signs in the climate record of any major perturbation at 9.3 ka.

Broeker received the news graciously, commenting that a freshening of the Arctic Ocean would have been more effective at shutting down North Atlantic thermohaline circulation than a spillway down the St Lawrence, because the sites of modern day sinking of dense cold brine lie well to the north of its outlet. The only way additional water in the Arctic Ocean could escape would have been into the northernmost North Atlantic.

See also: Schiermeier, Q. & Monastersky, R. 2010. River reveals chilling tracks of ancient flood. Nature, v. 464, p. 657.

Archaeology and the Toba eruption

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Depending on when fully modern humans left Africa – and that itself depends on evidence that is at odds with any definite resolution – the forebears of the eventual colonisers of the rest of the world may, or may not, have had to survive the effects of the biggest volcanic eruption of the past 2 million years. Around 74 ka the huge, elliptical caldera lake at Toba in Sumatra was formed by a stupendous eruption that threw out 800 km3 of ash (see Ash Wednesday to put this in perspective with recent events). Toba deposited a 15-centimetre ash layer over the entire Indian subcontinent. Toba has taken on a near iconic status among some palaeoanthropologists as a possible means of reducing the entire human population to a mere few thousand: a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations that shaped such things as language and culture. Unsurprisingly major efforts are underway to get hard facts about the relationship of fully modern humans to the Toba event, a lot of the work-in-progress being outlined at toba.arch.ox.ac.uk/index.htm.

See also:  Balter, M. 2010. Of two minds about Toba’s impact. Science, v. 327, p. 1187-1188.

Moon rocks turn out to be wetter and stranger

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Since the original analyses of lunar rock samples brought back by the Apollo astronauts is has been widely accepted that they are almost totally anhydrous. Some even contain pristine metallic iron with not a trace of rust after more than 4 billion years. So, therefore, the entire Moon should be bone dry, except for possible rimes of ice preserved in deeply shadowed polar craters. This lack of water is one line of evidence used to support the Moon’s origin in a stupendous collision between the early Earth and a smaller companion planet shortly after their accretion. The event may have depleted volatile elements and compounds in the incandescent vaporised rock from which the Moon is believed to have condensed. There are traces of water in glass spherules from lunar dust, but that might have come from the impactors that blasted them from craters. But at this year’s Lunar and Planetary Science Conference – the fortieth since the first Apollo landing – evidence for water in lunar minerals was presented (Hand, E. 2010. Old rocks drown dry Moon theory. Nature, v. 464, p. 150-151). The water is in apatite grains that occur as crystals in lunar maria basalts, so must have come from the Moon’s mantle through partial melting. Modelling suggests tens of thousand time more water in the lunar interior than believed previously, albeit still much less than in the Earth. Equally surprising is the water’s isotopic composition: it has a much greater proportion of deuterium (2H) relative to hydrogen (1H) than does water in terrestrial igneous rocks. The giant impact hypothesis suggests that the proportions should be the same in both bodies. One possibility is that a fortuitous comet delivered water to a dried-out hot moon soon after it has coalesced from and orbiting incandescent cloud. Hopefully a full publication will appear soon.