Earth-logs

‘Follow the water’

Published on November 1, 2009 by zooks777Leave a comment

Long, long ago an anonymous Roman wrote, ‘The first provision of any civilised society, after a code of law, is a reliable source of clean water’. Personally, I think the phrase ‘legalised bureaucracy’ in Latin was mistranslated to ‘code of law’. Whichever, planetary and life scientists might well like the adage for themselves: the sentiment applies nicely to active planetary tectonics and to the origin and survival of all conceivable life forms. The Earth has plenty of water at the surface and deep in the mantle. Without the second, the main mantle mineral olivine would be too stiff for the mantle to convect. Heat would build up within until magma formed in great abundance and emerged with a dreadful growl, as it did on Venus about 750 million years ago to repave the entire planet. It simply isn’t possible to think of answering the questions, ‘When did plate tectonics begin and life emerge?’ – let alone ‘How?’ – without first addressing where the Earth’s water came from and when our home world become so richly endowed.

In a very practical sense, these are the most important issues in geochemistry. Francis Albarède, of the École normale supérieure de Lyon, President of the European Association for Geochemistry and the first geochemist to deploy a multicollector, inductively coupled, plasma-source mass spectrometer, is a fitting person to review where the subdiscipline stands on them. (An MC-ICPMS is a tool for which many still yearn hopelessly.) His views appeared as a ‘Progress’ (a rare kind of Nature article) in the 29 October 2009 issue of Nature (Albarède, F. 2009. Volatile accretion history of the terrestrial planets and dynamic implications. Nature, v. 461, p. 1227-1233). The article casts doubt on the long-held views that when the Moon formed after a giant impact on the Earth, both bodies lost huge masses of volatiles, including water, and that Earth’s water-rich nature stemmed from repeated bombardment by volatile-rich comets up to about 3800 Ma.

Geochemical data are now available from a comet (Hyakutaki) and it contains twice the amount of deuterium relative to hydrogen that is in terrestrial seawater. The D/H ratio of carbonaceous chondrite meteorites is more Earth-like, and these primitive objects seem a more likely water source than comets. But did cataclysmic formation of the Earth-Moon system dehydrate both bodies and drive off other volatile matter? Planets and smaller bodies formed by gravitational accretion of solids that condensed from the initially hot gas or nebula that dominated the proto-solar system. Experiments show that condensation of the elements occurs in three discrete temperature ranges, separated by ranges in which few elements condense. Above around 1300 K the most refractory elements condensed, including oxides of some elements (Ca, Fe, Mg, Si) that now make silicate minerals, including the dominant mantle mineral olivine. Between 900-1200 K the alkali metals and some of the elements (chalcophile) that readily combine with sulfur emerged in solid form. In the third step from 500-800 K the more volatile chalcophile elements, including lead, and halogens condense, leaving four (Hg, O, N, C) that can take on solid form only below about 300 K. Interestingly, the proportions of volatile elements relative to refractory ones in the Earth, Moon and Martian meteorites are very low compared with those in carbonaceous chondrites. It is likely that volatile elements only accreted to the Inner Planets in small amounts before being swept to the outer reaches by an intense solar wind as the Sun was powering up, i.e. before nebular temperatures had fallen below about 1000 K. From that stems the inescapable conclusion that none of these planets were endowed with much water in their earliest forms.

Proportions of the lead isotopes ²⁰⁶Pb and ²⁰⁴Pb from terrestrial sulfide mineral deposits define a near-perfect linear relationship with the ages of mineralisation, from which an age can be estimated for the time the element lead appeared on Earth. That age is 4400 Ma; about 110 Ma younger than the actual age of the planet, and matches apparent ages derived from I-Xe and Pu-Xe decay schemes; iodine and xenon are volatile elements. This strongly supports the idea that 500-800 K condensates arrived late, and other evidence indicates that they and water ice were delivered by carbonaceous chondrite material falling towards the Sun from far beyond the orbits of the giant planets, once the early solar wind had lessened. That is, the Earth’s oceans formed very early in its history, and the mantle gained its water from them once hydrated lithosphere could founder deep into the evolving mantle by subduction. Albarède also summarises fascinating new ideas about the different course followed by Venus and Mars from essentially the same starting point. His ‘Progress’ is not difficult to read, and by marking the start of a new consensus in planetary evolution is of vital interest to all Earth scientists

Extraterrestrial water is also the subject of a Great Quest by NASA and other space agencies, though sadly an attempt on 9 October to prove that there is ice on the lunar surface, by hurling a US$79 million spacecraft at an obscure polar crater, produced no sensible results. Ironically, a couple of weeks later, three papers appeared in Science that document passive remote sensing evidence that the Moon contains a lot more water than long assumed (the most revealing is: Pieters, C.M. and 28 others 2009. Character and spatial distribution of OH/H₂O on the surface of the Moon seen by M³ on Chandrayaan-1. Science, v. 326, p. 568-572). The Apollo samples astonished geologists when they proved to be almost completely anhydrous, any signs of minor hydration being ascribed to contamination after collection. The Moon Mineralogy Mapper (M³) aboard India’s first lunar mission Chandrayaan is a hyperspectral imaging device that operates in the visible to SWIR range of EM wavelengths (0.4 – 3.0 mm). That range includes SWIR wavelengths beyond 2.4 mm where OH^–, water and water ice have large absorption features that are masked in terrestrial remote sensing by the high moisture content of Earth’s atmosphere. Pieters et al. attempted to model hydroxyl and water content in the lunar surface, and discovered significant amounts (a few tenths of a percent) in the polar regions. That they got results when the Moon was fully illuminated by the Sun suggests that this is not due to ice hidden from heating in shadows, but to minerals that contain molecularly bound water and hydroxyl ions. That begs the question of how the water got there. One possibility is the late arrival of volatile condensates as above, another that it is due to hydrogen (protons) from the solar wind reducing iron in silicate minerals to metallic iron and combination with the oxygen released. Expect loud hurrahs from devotees of Star Trek and NASA because one prerequisite of civilised society seems to be there on the Moon. But judging from the bureaucracies involved in space, getting the funds to use it will not be easy.

Early hominin takes over Science magazine

Published on November 1, 2009 by zooks777Leave a comment

I first mentioned Ardipithecus ramidus in EPN for February 2002 (Taking stock of hominid evolution), and the remarkable first finds by Tim White and his team were in 1994. Fifteen years on, and having amassed fragments of at least 36 individuals (and thousands of vertebrate, invertebrate and plant fossils) – Owen Lovejoy of Kent State University remarked, ‘This team seems to suck fossils out of the ground’ – it’s pay day! A total of 54 pages of the 2 October 2009 issue of Science (v. 326, Issue 5949) are devoted to this diminutive and very old (4.4 Ma) hominin. Such mounds of data wrested from the cauldron of the Afar Depression needed a long incubation period, and what is presented in Science is a summary rather than being comprehensive: much more is available online, and yet to come. The now hugely experienced, 47-strong academic team built up by Tim White and his original colleagues deserve massive congratulations. But they depended on the eagle-eyed, mainly Ethiopian fossil finders, many of whom are Afar pastoralists who took to field palaeontology as ducks to water. Science in general owes a massive debt to all those who have wrested such a wealth of anatomical information from every aspect of the fossils and their environmental context. What they have achieved is more worthy of Nobel-status than the fumbling of gaggles of annual economist-laureates who still cannot grasp why the world economy continually does grave disservice to humanity. The Ar. ramidus team also have a lot more worth saying to us than those physicists who seek the grail of a theory of everything – racked by such hubris that they are both unintelligible and unrealistic in the most literal way.

I cannot do adequate justice to the work in that historic issue of Science, but there are some general points that will leave any interested person breathless. As regards previous assumptions about the environment under which hominins emerged, it was woodland not open savannah. Though upright and capable of walking, as revealed by pelvis remains, Ardipithecus had feet with opposable big toes: sort of foot-thumbs. So they would have been as comfortable on trees as on the ground. Yet, their foot-architecture shows signs of having evolved from monkey-like feet rather than any lin=ke those of modern gorillas and chimps. A degree of certainty accompanies anatomical discussions, for one individual female Ar. ramidus is represented by a large proportion of a full skeleton, rivalling the later remains of ‘Lucy’, an Australopithecus afarensis. Her skull, reconstructed from a badly crushed state using co0mputed tomography and digital piecing-together, gives a brain size around the same as bonobo chimpanzees, and less than that of australopithecines. The feet clearly show a walker able to clamber, rather than swing and knuckle walk. Hands, though primitive, are more human-like than those of living apes are. From that can be concluded that a common ancestor a million of so years earlier was not ape-like in manual terms: chimps have evolved in this respect perhaps a lot more than those on the human line. Teeth shape, wear and isotopic signatures suggest a broad diet, rather than specialisation, from which grasses and grass-eating prey seem absent. Moreover, there is no sign of large canines, that could indicate minimal social aggression. Males and females were of similar size, as are we, rather than showing the sexual dimorphism that characterised later australopithecines and both chimps and gorillas. This also seems to point backwards in time to the last common ancestor of ourselves and chimps being very different from both living genera. Yet in many respects chimps seem to have evolved more than hominins. Because of the work on Ar. Ramidus, a chimpanzee-centric view of our shared forebears and therefore of hominin evolution can now be rejected. Perhaps thankfully, speculation about aspects of our behaviour stemming from those of chimpanzees is probably worthless.

The mass of data concerning this small, Pliocene hominin holds out a promise of yet more to come, both further back in time, and to populate the gaps in time and morphology that currently plague palaeoanthropology. The terrestrial sediments in which White et al. found Ar. Ramidus are 300 m thick, cover 5.5 to 3.8 Ma and are exposed over a large area. The stratum from which most data were recovered represents at most about 10 thousand years. Elsewhere in the Afar-Danakil Depression are other sediments laid down in river and lake systems that go back as far the Miocene (the estimated time of the last common ancestor of other primates and humans), and are still being deposited today. If anything characterised this triumph of the human intellect, it combined patience, determination and an attention to detail that was shared by every participant.

Evidence for Hadean continental crust takes a knock

Published on November 1, 2009 by zooks777Leave a comment

The pre-4 Ga ages recorded by some of the detrital zircons from the 3 Ga Jack Hills sandstones have been used to suggest that continental crust formed from about 4.4 Ga onwards, which implies some kind of recycling process in the tectonics of the early earth to generate and fractionate the necessary silicic magmas. That assumes zircons only form in silicic magmas produced by fractionation in volcanic arcs. The plagiogranites found in small amounts in ophiolites also contain zircons, thereby countering the claim for Hadean continents. More revealing are zircons found in granite magmas that represent the last dregs of melts formed by giant impact (Darling, J. et al. 2009. Impact melt sheet zircons and their implications for the Hadean. Geology, v. 37, p. 927-930). The huge impact-induced mafic to ultramafic melt sheet at Sudbury, Ontario, formed around 1.85 Ga. Zircons extracted from late-stage granites in the body are similar to those with Hadean ages.

Life originated as an oddity

Published on November 1, 2009 by zooks777Leave a comment

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

Published on November 1, 2009 by zooks7771 Comment

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 10¹⁵t) 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

Published on November 1, 2009 by zooks777Leave a comment

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

Published on November 1, 2009 by zooks7771 Comment

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

Published on September 1, 2009 by zooks7772 Comments

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 21^st 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?

Published on September 1, 2009 by zooks777Leave a comment

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

Published on September 1, 2009 by zooks7771 Comment

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.

The Mother of all climate models and deglaciation hiccups

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In his latest book, The Vanishing Face of Gaia: A Final Warning (Allen Lane, London, 2009, ISBN 978186141850), James Lovelock more or less gives up on the ability of humanity in general, and science and engineering in particular, to fend off looming climatic catastrophe. He reserves his sharpest criticism for what he calls ‘American science’; a fundamentally reductionist approach that is fed into prediction of the future. For Lovelock, the assumption ‘that all we need to know about the climate can come from modelling the physics and chemistry of the air in ever more powerful computers’ has been a disastrous mistake. He is obviously not one for humble retrospection, as his early Gaia writings had at their centre a sort of reductio ad absurdum of that now prevailing genre in Earth system science. Daisyworld, reduced a planet’s life forms to white and black daisies, whose interplay with climatic change was governed by a formula known as a difference equation in the manner of Lotka and Volterra’s work on predator-prey interrelationships. The simplest difference equation is x_next = rx(1-x). Solving such non-linear relationships for minute increments in x led to the unmasking of chaos theory, the first instance being Edward Lorentz’s discovery that the simplest models of climatic turbulence go wonky if you tinker with them: the ‘Butterfly Effect’.

When his Gaia hypothesis drew together all manner of people from New Ageists mathematicians working on complex systems James Lovelock was exposed to friendly criticism and education about non-linearity and chaos. Clearly that revolutionised his world-view, which is fine, albeit a cause of some glumness for him. Far sadder is that he is probably right in criticising climate modelling – now that it has a stranglehold on the entire climate debate and indeed on the ears of the ‘Great and the Good’. A measure of where modelling has led is a simulation of what happened as the Northern Hemisphere emerged from the last glacial maximum, between 22 and 10 ka (Liu, Z. and 13 others 2009. Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming. Science, v. 325, p. 310-314). These ~10 millennia saw a return to a see-saw climate that lasted from 60 to 30 Ma as the Earth cooled towards the last glacial epoch, dominated by cooling-warming cycles with a similar pattern of slow cooling-sudden descent into frigidity-thousand year cold spells-sudden warming known as Dansgaard-Oeschger cycles.

The Chinese-US team developed and ran the first synchronously coupled atmosphere-ocean general circulation model to investigate a hiccup in warming of the sea surface one northern ice caps began to melt decisively. It is said to be ‘one of the most epic numerical modelling efforts of the climate community to date’ (Timmermann, A. & Menviel, L. 2009. What drives climate flip-flops? Science, v. 325, p. 273-274). Epic, well yes: one of the world’s largest operational supercomputer (Jaguar at the Oak Ridge National Laboratory, USA) was wrangling for 18 months. Lots of known empirical data for the period were fed in: insolation determined by astronomic effects; changes in greenhouse gases from ice cores; shifts in coastlines and ice-sheet volumes. Tinkering with the model involved varying freshwater influx to high-latitude North Atlantic seawater. The result was crude simulation of what actually happened to sea-surface temperatures at several locations around the North Atlantic, giving some insights into why changes occurred. But climate scientists have long suggested mechanisms for the Dansgaard-Oeschger cycles, Bølling-Allerød warming, and the final frigid paroxysm of the Younger Dryas in much the same framework, the only difference being they didn’t produce numerical models that mimicked reality.

It seems that another 2 to 3 million hours of time on Jaguar are needed to bring the project through to the present. The enormous funding needed to get this kind of number crunching done can only have been on the back of claims that it will help predict future anthropogenic climate shifts. Based on real data, it still didn’t get things right – millennium-long cooling and warmings are not trivial events. There are conflicting kinds of data for changes in the parameters since the start of the Industrial Revolution preceded by 10 ka of relatively stable Holocene conditions. The best that climate forecasting for the next 100 years has been able to do, also using pretty large amounts of CPU time, is a range of straight lines showing increases in global mean surface temperature. Yes, hindsight is wonderful…

What’s green and above sea level?

Published on September 1, 2009 by zooks777Leave a comment

Most geologists would answer, ‘The continents after the start of the Silurian Period’, and from now on they could be wrong. Evidence for an earlier ‘greening’ of the land comes from a detailed analysis of thousands of oxygen- and carbon-isotope measurements in Neoproterozoic carbonate rocks (Knauth, L.P. & Kennedy, M.J. 2009. The Neoproterozoic greening of the Earth. Nature, v. 460, p. 728-732). An important consideration in understanding the geochemistry of limestones is that however they originally formed as wet sediments at some later stage their constituents were largely transformed into crystalline aggregates by lithification through the intermediary of pore fluids. During lithification chemistry is equilibrated between crystals and the pore fluids, so if pore fluids are chemically (in this case isotopically) different from the sediment the resulting rock will have been changed isotopically. Studies of Cenozoic carbonates strongly suggest that the place where carbonate sediments are lithified most quickly is in coastal areas where terrestrial groundwater mixes with marine formation water in sediments. Since colonisation of the land by photosynthesising organisms groundwater C- and O-isotopes evolves in equilibrium with those organisms. The terrestrial biomass fixes ¹²C preferentially thereby depleting their proportion of ¹³C by up to 20‰. Groundwater, having originated as water vapour evaporated from the oceans that acts preferentially on ¹²O is also depleted in ¹⁸O. Consequently, low δ¹³C and δ¹⁸O signatures are passed on to groundwater and thence to carbonate rocks when groundwater participates in lithification.

Neoproterozoic carbonates plot in the same δ¹³C vs δ¹⁸O fields as those from the Phanerozoic. Earlier Precambrian carbonate data plotted in the same way show depletion in δ¹⁸O but not in δ¹³C, which signifies no terrestrial life, but normal preferential evaporation of ¹⁶O from the ocean surface to form rain and then groundwater. Knauth and Kennedy’s results suggest a strong likelihood that carbonates of the late Precambrian were lithified by groundwater from a land surface where photosynthetic organisms were well-established and abundant. There is likely to be a sceptical backlash to this remarkable conclusion, largely because it seems that the terrestrial biomass in the Neoproterozoic would have needed to be of the same order as that in later times. Yet molecular evidence from modern fungi, lichens, liverworts and mosses suggests that they evolved in the Neoproterozoic and Chinese scientists have found traces of what look remarkably like lichens in the 600 Ma Doushantuo lagerstätte – fungus-like hyphae and cells that resemble those of cyanobacteria (see The earliest lichens in May 2005 issue of EPN). In an earlier paper, Martin Kennedy had noted that around 700 Ma, the record of marine limestones show increasing ⁸⁷Sr/⁸⁶Sr ratios, suggesting an increase in the chemical weathering of ancient continental rocks. That may have coincided with biological agencies helping break down bare rock chemically to swelling clays that show a surge in Neoproterozoic sedimentary sequences (see Clays and the rise of an oxygenated atmosphere in March 2006 issue of EPN). The same paper pointed out that such clays increase the chances of preservation of buried organic matter, thereby boosting build-up of atmospheric and dissolved oxygen, as would terrestrial photosynthesisers. The feedback of increased oxygen to other eukaryotes that had evolved as heterotrophic animals would have enabled them to increase in size. Interestingly the earliest fossil animals occur in the same Chinese lagerstätte as the putative terrestrial photosynthesisers.

See also: Arthur, M.A. 2009. Carbonate rocks deconstructed. Nature, v. 460, p.698-699; Hand, E. 2009. When Earth greened over. Nature, v. 460, p.161.

‘Surf’s up’ from seismic noise

Published on July 1, 2009 by zooks777Leave a comment

Global warming is intensifying cyclonic storm systems, the energy retained by the greenhouse effect being redistributed to winds and in turn to ocean waves, which even have a small effect on local gravitational potential. The effects become coupled to the solid Earth and appear as the background ‘noise’ in seismograms. So historic seismograms, both digital and in paper form, potentially supply a proxy for climate change going back as far as the 1930s when seismographic stations first began to be set up. In some instances the records are continuous, and when digitised form a unique record that integrates, but one yet to be exploited fully (Bromirski, P.D. 2009. Earth vibrations. Science, v. 324, p. 1026-1027.

Is there a giant impact basin beneath the Antarctic ice?

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At present there are only two reliable means of surveying variations in the Earth’s gravitational field: at the surface using gravimeters and from space, by processing measurements the height of the ocean surface from radar measurements or by accurately measuring the variation in distance between two satellite travelling in tandem over the Earth’s surface. The last is used by the Gravity Recovery and Climate Experiment (GRACE) designed by NASA and the German Space Agency. It is the only realistic means of usefully precise gravity surveys over Antarctica. A truly multinational team (von Frese, R.R.B. et al. 2009. GRACE gravity evidence for an impact basin in Wilkes Land, Antarctica. Geochemistry,Geophysics, Geosystems, v. 10, Q02014, doi:10.1029/2008GC002149 – on-line journal) has discovered a prominent positive free-air gravity anomaly over a roughly 500-km diameter subglacial basin in Wilkes Land. A basin filled with low-density ice would normally give a negative gravitational ‘signature’, so the positive anomaly suggests either unusually dense crustal rocks beneath it, or that the mantle is unusually close to the surface; i.e. the crust is thin. The authors suggest that the central anomaly is surrounded by roughly concentric circular features, and that it is a hitherto unsuspected impact structure, three time larger than the Chicxulub structure (also mapped by gravity data off the Yucatan Peninsula of Mexico) that caused an upward bulge of the mantle. To my eye, the hypothesis only becomes convincing when concentric circles are drawn around the undoubted major anomaly, and the evidence for them is scant compared with the similarly detected structures of Mars and the Moon. What intrigues the authors is the position of the anomaly on a Permian continental reconstruction, It is at the antipode of the Siberian Traps flood basalt province, implicated strongly in the end-Permian mass extinction: the most devastating known. This harks back to speculation that the undoubted Chicxulub structure and caused the mantle to melt beneath its antipode to form the Deccan Traps…

Mantle link with biosphere

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It is pretty clear that events in the deep Earth, which give rise to surface changes, such as topographic uplift and increases or decreases in the pace of continental drift, feed into changes in the biosphere. A convincing example of that is the manner in which uplift of the flanks of the East African Rift System led to climate change that favoured bipedal apes. But is there a more direct link involving chemical influences?

It is likely that the earliest autotrophic organisms performed a variety of chemical tricks in order to create energy and chemical conditions that moved matter back and forth through their cell walls. As well as photoautotrophs of different kinds, including those that release oxygen as waste there would have been chemautotrophs, such as sulfate-sulfide reducers, methanogens and considerably more. Oxygenic photosynthesis apparently was functioning almost 3500 Ma ago, long before the Great Oxidation Event (see Early signs of oxygen…but in the wrong place in this issue) yet it was slow to make any impact on the atmosphere. In the Archaean oceans free oxygen would have been consumed by oxidation of soluble iron-II, probably creating banded iron formations. But photosynthesis has to take place in shallow sunlit water, so it would have been easy for oxygen to enter the atmosphere. Since carbon dioxide in the atmosphere is unable to react with oxygen, oxygen build up in the air might be expected to have built far faster than it did. That is, unless there was a reducing gas present in sufficient amounts to consume oxidation. The most likely buffering agent holding back an oxygen-bearing atmosphere is methane produced by methanogen autotrophs, and it has been suggested that falling methane levels towards the end of the Archaean and start of the Proterozoic Aeons eventually permitted atmospheric oxygen to remain unreacted. Since very little methane is produced by inorganic processes, that hypothesis has a corollary; that there was a decline in methanogen Bacteria and Archaea. So, how might that be tested?

A cunning piece of lateral thinking presents a test, and suggests a mechanism linked to processes in the Late Archaean – Palaeoproterozoic mantle (Konhhauser, K.O. and eight others 2009. Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event. Nature, v. 458, p. 750-753). The first cunning bit comes from the biochemistry of modern methanogens: Methyl-coenzyme M reductase (MCR) catalyses the formation of methane from methyl-coenzyme M and coenzyme B in methanogenic Archaea. This enzyme contains the nickel-centred porphinoid F430 tightly bound in its structure. Needless to say, the olivine-rich mantle contains abundant nickel, so the greater the percentage of mantle partial melting, the more nickel enters the surface environment. Archaean stratigraphy, especially its earlier parts, contains abundant ultramafic lavas known as komatiites, associated with some of the world’s big nickel mines. From the Late Archaean onwards, komatiites are rare rocks. The second master stroke by the authors is to find a means of charting the varying abundance in Archaean and Proterozoic seawater: they analysed the Ni content relative to that of Fe in banded iron formations. To as late as 2700 Ma the Ni/Fe ratio remains high in BIFs, but thereafter it falls sharply. That seems to support the hypothesis that a decline in the mass of methanogens did allow oxygen to build up in the atmosphere, and that decline reflected a fall in the supply of mantle nickel to the oceans. The next step would be to exploit the recently demonstrated ability of methanogen Archaea to fractionate nickel isotopes during their metabolism of dead organic matter. That would ideally be done using Ni-rich BIFs, as in this study.

Hadean not so hellish for life

Although the Earth’s history before 4 Ga is not the mystery that it was, following the discovery of 4.3 Ga-old metasedimentary rocks in NE Canada (see At last, 4.0 Ga barrier broken in November 2008 issue of EPN), the early history of the Moon suggests that it was hectic and plagued by very large asteroid and comet impacts. The mightiest events occurred around 3.9 Ga, forming the huge mare basins on the Moon. Scaling up for the Earth’s greater gravitational pull even larger catastrophes would have pounded our planet, although its turbulent tectonics has removed all tangible traces of them. From detailed studies of rocks and impact melts from the Moon – much of the lunar regolith comprises glass spherules produced by cratering over its entire history – the late heavy bombardment (LHB) was not prolonged in geological terms, lasting 20 to 200 Ma. Yet it involved the most extreme delivery of kinetic energy since the giant Moon-forming event around 2.45 Ga, which generated stupendous power – the rate of energy delivery by impactors moving at a minimum of 15 km s^-1 is about a second. This has encouraged speculation that the Earth was effectively sterilised for a second time in its history. The 500-600 Ma of Hadean history may have witnessed emerging life forms of the most basic kind, only to see them wiped out, perhaps more than once. It has been assumed, therefore, that the earliest living things which left descendants, including us, had a universal ancestor that appeared only after 3.9 Ga. Now it seems a serious rethink is needed (Abramov, O. & Mojzis, S.J. 2009. Microbial habitability of the Hadean Earth during the late heavy bombardment. Nature, v. 459, p. 419-422).

Feeding the impact data from the Moon and terrestrial planets into new modelling software run on a super-fast computer, Oleg Abramov and Stephen Mojzis of the University of Colorado have been able to model the degree of thermal metamorphism that the Earth’s crust may have undergone during the LHB. Interestingly, they reveal that less than 10% of the surface would have been heated above 500ºC, and only 37% would have been sterilised, even if all the huge impacts predicted for Earth landed at the same time. Assuming that any basic life forms that had arisen in the Hadean were randomly distributed at the surface and in the subsurface – a variety of extremophile bacteria still live at depths down to 4 km – populations would survive to leave descendants. If they could survive temperatures up to 110ºC, which modern hyperthermophiles do, then so much the better for life as a whole. Although based on modelling, the work by Abramov and Mozjis, gives palaeobiologists another half billion years in which inorganic processes could have assembled the immensely complex molecules the living processes demand. The earliest possible signs of life, based on carbon isotopes locked in stable minerals of a Greenland metasediment, date to 3.8 Ga. Previous assumptions about life’s slate being wiped clean by the LHB therefore left only a few tens of million years for that assembly by some kind of thermodynamic miracle. The new vista will please Mike Russell of the University of Strathclyde in Glasgow. Russell is an economic geochemist turned palaeo-biochemist set on testing the Oparin-Haldane hypothesis of the origin of life using apparatus and approaches that are much more sophisticated than those used by Miller and Urey who created amino acids in vitro during the early 50s. The 21 May 2009 issue of Nature includes an account of Russell’s plans and the views of those with a more cautious outlook (Whitfield, J. 2009. Nascence man. Nature, v. 459, p. 316-319).

See also: Rothschild, L.J. 2009. Life battered but unbowed. Nature, v. 459, p. 335-336.

Irresistible brevia

Surprisingly, the most abundant crustacean fossils are those of ostracodes, which have two carapace shells. They reach back as far as the Ordovician. Although modern ostracodes are an ecologically very diverse group, much used in assessing changing environmental conditions, they are not the most prepossessing creatures being small and externally smooth. Ostracode bodies and appendages are rarely found as fossils, but a German, Japanese, Czech, British and French team has set out to find soft parts using X-ray synchrotron tomography on a Brazilian ostracode of Cretaceous age (Matzke-Karasz, R et al. 2009. Sexual intercourse involving giant sperm in Cretaceous ostracode. Science, v. 324, p. 1535). A third of the ostracode’s body is devoted to reproduction, males having large Zenker organs or sperm pumps. This is unsurprising, when one is informed that the ostracode sperm are sometimes longer than an individual creature. Indeed, Matzke-Karasz et al. assign some significance to them; ‘persistence of reproduction with giant sperm through geological time may add a criterion to test for the pressure of sexual selection’…

Gas source for flood basalts

Although there are several coincidences between flood basalt eruptions from large igneous provinces and mass extinction, not all basalt flood events made an impact on the biosphere and not all mass extinctions link to a LIP. Where there is a connection, two mechanisms dominate discussion: dust and noxious gas such as SO₂, stratospheric aerosols from which can also induce global cooling, or global warming stemming from CO₂ emissions. The odd thing is that most flood eruptions in LIPs are of tholeiitic basalt magma, which is generally low in gas content. Of sizeable flood basalt provinces, the Ethiopian (30 Ma), Karoo (~180 Ma), Parana (130 Ma) and North Atlantic (55-60 Ma) had no truly significant impact on life. Those that certainly did were the Siberian Traps implicated in the end-Permian devastation, those of Emeishan in China at the time of35 % of all genera went extinct around 260 Ma, the Central Atlantic Province the main suspect for the end-Triassic extinctions and the Deccan Traps that coincided with the Chicxulub impact at the K-T boundary. Two of these massive tholeiitic magma events have been assessed in terms of how they might have emitted gases.

The Emeishan LIP emerged through crust that contains large volumes of carbonates of Proterozoic to Silurian age. Conceivably the magma might have released carbon dioxide by inducing thermal metamorphism (Ganino, C. & Arndt, N.T. 2009. Climate change caused by degassing of sediments during the emplacement of large igneous provinces. Geology, v. 37, p. 323-326). Clément Ganino and Nick Arndt of the University of Grenoble, France investigated a monstrous sill almost 2 km thick in the deeply eroded Emeishan province. It proved to have a 300 m contact aureole dominated by brucite (Mg(OH)₂) marble, evidence of melting of carbonates and calc-silicate marbles, production of which by metamorphism would have yielded huge amounts of CO₂. They go on to discuss other possibilities for gas generation by magmatism, involving thermal metamorphism of coals, oil shales and evaporites. The last is a distinct possibility in the case of the Siberian Traps (Li, C. et al. 2009. Magmatic anhydrite-sulfide assemblages in the plumbing system of the Siberian Traps. Geology, v. 37, p. 259-262). A large stratiform intrusion associated with the end-Permian flood basalts contains around 7% sulfides; truly huge for mafic magma and making it a major exploration target for platinum-group metals, yet unusual for a tholeiite. It also contains abundant anhydrite, calcium sulfate that is more usually found in sedimentary evaporites. The isotopic composition of sulfur in the intrusion is enriched in ³⁴S, suggesting that at least 50 % was derived from a sedimentary rather than a mantle source. The sedimentary sequence through which the Siberian flood basalt magmas passed contains evaporites around 5 km thick. That would be a suitable source for the sulfur in the intrusion, but would also yield stupendous amounts of SO₂if carried to the surface by erupting magma. An example of a LIP that had little if any effect on the biosphere is that which mantled both side of the North Atlantic with flood basalts in the Palaeocene. The magma that was involved moved through almost entirely crystalline ancient continental crust. The same set-up characterised the Ethiopian, Parana and Karoo provinces.

Social behaviour among giant trilobites

There’s something about a trilobite that causes outbreaks of hyperbole: as far as I know they are the only class of animals to warrant an expletive in serious literature (Fortey, R. 2001. Trilobite! Flamingo). The title conjures a vision of a three-lobed, segmented alien hurtling for one’s nether regions, hideous malice in its compound eye. Well, most trilobites were little, albeit with anorak-rending diversity in form and habit: they ranged from burrowing bottom feeders to inhabitants of the ocean meniscus, rather like early water boatmen. If you want to use an exclamation mark for an invertebrate, then it might be better to reserve it for the fearsome Eurypterids or sea scorpions. At up to 2 m, with mighty pincers and capable of galloping across a beach, they certainly would have best been avoided in the Ordovician to Permian. Yet, from time to time big trilobites do turn up, such as Paradoxides, Ogyginus and Hunioides that break the metre barrier. Rather a lot of them have been found in a Portuguese lagerstätte of Middle Ordovician age (Gutiérrez-Marco, J.C. et al. 2009. Giant trilobites and trilobite clusters from the Ordovician of Portugal. Geology, v. 37, p. 443-446). They were up to something, as the locality described by Gutiérrez-Marco et al. contains huge numbers that were apparently having been overwhelmed by a sudden turbidity flow once they had gathered together. Some of them are in single file… It could be some sexual frenzy; fearfulness when moulting synchronously or something at which we cannot even guess. Whatever, it seems likely that the gigantism in the deposit is something to do with these being high-latitude animals.

Lead-in to icehouse conditions

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At 33.5 Ma, around the time of the Eocene-Oligocene boundary, Earth’s climate took a sudden shift towards cooler conditions, coinciding with the onset of glaciation in the Northern Hemisphere and growth of Antarctic ice cover. Studies of a variety of proxies, including the density of pores or stomata on plant leaves, suggests that the transition resulted from a halving of atmospheric CO₂ content from more than 1000 ppm in the Early Eocene to ~560 ppm in the Oligocene. So, even at twice the pre-industrial level greenhouse warming was compatible with high-latitude frigidity. Ocean-floor sediments from a site close to the Arctic Circle in the Norwegian-Greenland Sea yield pollen and spore records that chart vegetation change from 50 to 30 Ma (Eldrett, J.S et al. 2009. Increased seasonality through the Eocene to Oligocene transition in high northern latitudes. Nature, v. 459, p. 969-973. The proxy data suggest that in the period preceding the decisive global climate change conditions became increasingly seasonal, with greater differences between winter and summer temperatures. This was largely due to increasingly cold winters, a more constant summer temperature suggesting that any land ice on Greenland was of the valley type rather than an all-covering ice sheet.

African genes

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Much of the interpretation of the growing database of human genetic variability has so far focused on migration out of Africa and across the habitable continents. To some extent the largest variability, of Africans themselves, has been undersampled, but a multinational team of Africans and non-Africans has now begun to redress the balance (Tishkoff and 24 others 2009. The genetic structure and history of Africans and African Americans. Science, v. 324, p. 1025-1043) partly to study genetically-linked epidemiology and partly anthropology. The study centres on African’s own ideas about their identity/ethnicity as well as documented cultural and linguistic division, and covers 3194 individuals from 121 populations in the continent, African-American populations in 4 US cities and 60 other populations from outside Africa. The team expands knowledge tremendously, as expressed by the many intricate diagrams. They use the statistical method of Bayesian clustering to tease out the ancestral bases for the genetic patterns preserved by Africans, which appear to be based on 14 major ancestral groups that mostly tally with cultural and linguistic divisions. Overall, the picture is one of repeated mixing of populations through migrations within the continent, many within historic times such as the shift of West Africans south-eastwards, but also much earlier movements such as the ancestors of the San people of southern Africa. These remaining gatherer-hunter people together with central African pygmies and the Hadza and Sandawe of Tanzania share ancestry and also, except for pygmies, language that involves click-sounds – the pygmies abandoned their original language in favour of that of the groups that now surround them in the Equatorial rain forests. Of the three groups, the Hadza most maintain the genetic structure of the earliest ancestors on the continent, but all three shared a common ancestor about 35 Ka ago. Interestingly, comparison with people outside Africa confirms earlier studies that indicated a source population for the out-of-Africa migration in East Africa close to the Red Sea. The paper is necessarily condensed and so difficult to follow, but clearly opens up great vistas in understanding intricacies at which anthropologists have previously only guessed. Like the physical landscape of Africa, that of its population reflects the range of factors that have shaped human evolution and hence a great deal of its destiny.

See also: Gibbons, A. 2009. African’s deep genetic roots reveal their evolutionary story. Science, v. 324, p. 575.

Very old human footprints in Mexico?

In 2006 palaeoanthropologists in the Americas, already at loggerheads about evidence for pre-Clovis (pre 13 ka) colonisation, were rocked to their boots. A team from Liverpool John Moores University, Bournemouth University and the Mexican Geophysics Institute claimed to have found human footprints more than 40 ka old in a volcanic ash deposit (Gonzalez, S. et al. 2006. Human footprints in Central Mexico older than 40,000 years. Quaternary Science Reviews, v. 25, p. 201-222). The extensive site exposed by quarrying carries many apparent footprints, both human and non-human. Moreover, some of the prints are in convincing-looking trackways. The very old date was obtained by optically stimulated luminescence dating of quartz-grains that measures the time since the grains were last exposed to sunlight or thermal baking. Were it not for that result probably little fuss would have been made. Now this remarkable find is under serious challenge (Feinberg, J.M. et al. 2009. Age constrains on alleged ‘footprints’ in the Xa;nene Tuff near Puebla, Mexico. Geology, v. 37, p. 267-270). This US-Mexican team applied Ar-Ar dating to the ash and found an age of about 1.3 Ma, confirmed by its association with reversed magnetic polarity in the deposit – at 40 ka the geomagnetic field was as it is today. On that basis, Feinberg and colleagues claim to have refuted the identification of human footprints, and claim that they are merely quarrying marks degraded by later weathering. The Xalnene Tuff in which the footprints were found was deposited in a lake that has been periodically filled and dried out. If the disputed features can be shown irrefutably to be footprints, then there are only two possibilities: either they date from a 40 ka lowstand when the tuff was rewetted and soft, or they are of Homo erectus who somehow found their way to the Americas after leaving Africa around 1.7 Ma ago and crossed the drying lake bed shortly after the tuff was ejected from a nearby volcano.

‘Hobbit’ news

Bones of at least 6 or 7 small people have turned up in the now famous Liang Bua cave on the island of Flores, Indonesia. Their stratigraphic positions span the period from 95 to 17 ka. There have been numerous claims that they do not represent a dwarfed human species – i.e. Homo floresiensis – but individuals who suffered from some form of pathological condition. The strongest evidence supporting that sceptical view is that the one near-complete skull does not fall on the well-established brain –body-size distribution that covers many species: it seems too small for either a normal pigmy modern human or a similarly diminutive H. erectus. Now crucial new anatomical evidence seems set to swing the balance. (Jungers, W.L. et al. 2009. The foot of Homo floresiensis. Nature, v. 459, p. 81-84; Weston, E.N. & Lister A.M. 2009. Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis. Nature, v. 459, p. 85-88). The foot bones of the most recent and most complete specimen are not like those of humans but more ape-like, although they show clear evidence of bipedalism. Interestingly, they seem to be more primitive than those of H. erectus, raising the possibility of an undocumented dispersal of perhaps from Africa into Eurasia as an ultimate ancestor. Curiously, the foot is disproportionately long compared with the rest of the skeleton; another bonus for ‘hobbit’ fans. Not having a snout, H. floresiensis certainly was no ape, indeed the skull is best expressed as a scaled-down version of either H. erectus or H. habilis. As to extremely small brain size in relation to the body size of H. floresiensis, insular dwarfism of fossil hippos in Madagascar provides a useful analogue, as Weston and Lister suggest. In adulthood they also have disproportionately small brains. As with many puzzles in human evolution, the stir caused by these new discoveries maintains H. floresiensis as a ‘hot topic’ and further excavations are inevitable – Flores has plenty of caves, as do many islands in the Indonesian chain.

See also: Lieberman, D.E. 2009. H. floresiensis from head to toe. Nature, v. 459, p. 41-42.

Quaternary snatched from jaws of extinction

Published on July 1, 2009 by zooks7771 Comment

At a stormy meeting in August 2004at the 32nd International Geological Congress in Florence, a rearguard action was mounted by a group of stalwart geologists to thwart an attempt to expunge the last remnant of the stratigraphic divisions inspired by Giovanni Arduino’s work in the 18th century from the minds of all future geologists (see December 2004 issue of EPN). The Quaternary was under siege. Despite the fact that the International Commission on Stratigraphy (ICS) of the IUGS had already prepared the ground for a coup de gras by stating that, “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”, its defenders seem to have won (Mascarelli, A.L. 2009. Quaternary geologists win timescale vote. Nature, v. 459, p. 624). The ICS voted on 21 May 2009 to formally define the base of the Quaternary at 2.6 Ma when the Earth began to cool, glaciation began in the Northern Hemisphere and stone tools first appeared in Africa (it was formerly set at 1.8 Ma, for no obvious reason) and to pass that to IUGS for ratification. Another minority group is enraged, with rumours of chewed carpets, as the Quaternary has annexed 800 ka of what previously was designated as Pliocene: ‘It’s kind of a land grab’, commented Philip Gibbard, a Quaternary expert from Cambridge University, possibly with a hint of glee. To me, it is a milestone decision that gives a proper place to tool making, bipedal apes – ourselves – which makes a great deal more sense that the absurd notion of the Anthropocene (see Epoch, Age, Zone or Nonsense? in March 2008 issue of EPN), whose base some deluded colleagues are trying to set at the beginning of the Industrial Revolution!

Early signs of oxygen…but in the wrong place
The so-called ‘Great Oxidation Event’ is marked by the first occurrence of iron-oxide bearing subaerial sediments or palaeosols, widely regarded as occurring at around 2400 Ma. That is probably around the time that photosynthesis overtook the rate of oxidation reactions that previously consumed the oxygen that it produced, so that oxygen could build-up continually in the air. But that date is far earlier than the origin of subaerial photosynthesis and oxygenic photosynthesis must have arisen among oceanic bacteria before then, but only those inhabiting shallow water where the sunlight is. Banded iron formations that go back into the Archaean are often cited as evidence for when such photosynthesis got underway. Their dominant mineral hematite probably formed by oxidation of soluble iron-II and combination of iron-III with free biogenic oxygen, presumed by most workers to be in shallow water. Among the oldest hematite-rich formations is the Marble Bar Chert of Western Australia, dated to 3460 Ma (Hoashi, M. et al. 2009. Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience, v. 2, p. 301-306). The hematite crystals in the chert seem to have formed at above 60ºC in ocean-floor hydrothermal springs that were discharging abundant dissolved iron-II. The authors estimate the basin in which the cherts formed to be between 200 to 1000 m deep. Since at such depths photosynthesis would not be possible, they claim that sufficient oxygen was produced by shallow-water photosynthesis to form oxygenated intermediate and deep ocean waters, reminiscent of far later times in Earth’s history. This is a minority view, and hinges on whether or not the hematite did form directly on the sea floor. One possibility is that it could have been precipitated colloidally from iron-II-rich ocean water in the photic zone where early photosynthesisers would be, to sink to the deeper sea floor. Eventually very fine iron oxide might recrystallise.
See also: Konhauser, K. 2009. Deepening the early oxygen debate. Nature Geoscience, v. 2, p. 241-242.

At last, a geoscientific April Fool joke?

Published on July 1, 2009 by zooks777Leave a comment

Maybe it was a coincidence, but the April issue of Geology contain a paper whose title looked suspiciously unreal (White, K. et al. 2009. Hydrologic evolution of the Edwards Aquifer recharge zone (Balcones fault zone) as recorded in the DNA of eyeless Cicurina cave spiders, south-central Texas. Geology, v. 37, p. 339-342). Seemingly, the Cretaceous Edwards Aquifer now flows through cavern systems at the base of a fault-controlled escarpment. At higher levels in the unit are air-filled caves, that are relics of previous karstic events. It is in these dark, dry caves that the arachnid troglobites dwell. Troglobitic animals (those that inhabit totally dark caves and have no eyes) originate as normal surface dwellers, which through successive generations lose functioning eyes and coloration. Conversely, they evolve improved senses of smell, taste and vibration detection. The species that emerge are among the rarest of creatures, for they often occur in only a single cave: a special case of allopatric speciation that may happen when small populations are cut off from one another. Technically, then, this study is no joke, for analysis of mtDNA from the spiders in different caves ought to show evidence of microcosmic evolution, and possible provide a molecular ‘clock’ to chart the times of cave colonisation. And this is what the authors from the University of Mississippi and the endangered invertebrate group of a Texan consulting company have tried to do. The spiders in the higher caves are more evolved than those at progressively lower levels. Moreover, since the karst evolution has developed in a structurally active setting, the spider data correlates with tectonic history…

The ancestral animal

Published on May 1, 2009 by zooks777Leave a comment

The Cambrian Explosion of shell-bearing animals and the preceding, diverse and very odd Ediacaran fauna that left imprints and moulds in the Late Neoproterozoic both posed two puzzles for early palaeontologists. What organisms evolved so that unmistakable traces of animal life were able to leave fossils after about 600 Ma, and what pace did evolution take to present us with virtually all the animal phyla, including some not around nowadays, ‘fully separated’? Molecular genetic studies of living animals are beginning to throw up some answers (Holmes, R. 2009. The mother of us all. New Scientist, v. 202 (2 May Issue), p. 38-41). It is a complex and growing field, so Bob Holmes’ review of current ideas on the last common ancestor of the animals is welcome for non-specialists. It does look as though the radiation was long before the Ediacaran, but may well have been very rapid. The genetically closest single-celled organism to metazoan animals are the rare choanoflagellates; filter feeders with a collar-like structure and a tail. They bear some resemblance to the feeding cells of sponges, but sponges in their current form seem highly unlikely as the Ur-creature, totally lacking any organs and really just a coexistence of clone-like cells. Gene sequencing from 42 animal groups puts sponges at the bottom of a relatedness tree, yet at the bottom of two of the main branches. So the sponges do indeed seem to have it as our ultimate ancestors. Yet the flurry of ever-more detailed sequencing, for more and more groups using increasingly sophisticated statistical analysis has fired up controversy. Jellyfish-like ctenophores now have a look-in too, as do mysterious placozoans, according to one or other researcher. This field is throwing up an object lesson for hubristic scientists used to counting their chickens… No, the votes are never all in, and surprises always lie ahead for both the unwary and the patient.

Luckily, Holmes closes by looking at a careful proposal for the ‘How’. Claus Nielson of the University of Copenhagen, a major ‘player’ in this field, has suggested how starting with a slab-like choanoflagellate, with all its function cells on the outside, might have evolved be curling to enclose a tube of inward facing cells; a precursor of a gut. One next step from there could be specialisation of some cells as nerves, then the development of a ‘mouth’ and ‘anus’ – the basis for the bilateral symmetry of all higher animals including ourselves. As for the ‘When’, there are sufficient leads from a molecular clock approach to settle on the oddest climatic events of the last 1.5 Ga of the Proterozoic, the near global glaciations or ‘Snowball Earth’ events that began around 750 Ma ago.

Photosynthesis from way back when: the hunt for RuBisCO

Charles Darwin had an abiding fascination with plants, though one that was essentially practical through observation and breeding. That is sufficient excuse in his bicentenary for reviews, but a good way to honour his legacy is again to push essays to the leading edge of present understanding (Leslie, M. 2009. On the origin of photosynthesis. Science, v. 323, p. 1286-1287). Being able to convert sunlight, water and carbon dioxide to the basis of their own life and that of the rest of the planet, plants and other photosynthesising organisms are the fundamental essence of the living world. Land plants are recent developments, emerging in the Silurian around 425 Ma ago with presumed terrestrial spores some 50 Ma earlier. Their forbears were almost certainly marine algae. Yet they are highly evolved, and it is not to separate precursors that palaeobotanists can look for origins, but to the internal chloroplasts that look remarkable like cells in their own right with separate DNA and RNA. They perform the astonishing trick of breaking the extremely strong OH-H bonds that form the water molecule otherwise achieved either by extremely high temperatures or by electrolysis. The trick is for an organism to grab an electron thereby releasing the bond and both hydrogen and oxygen. The hydrogen links to carbon and oxygen from CO₂, and the other oxygen is freed. Similar to a magician’s trick with smoke and mirrors, photosynthesis uses pigments. Colour in any object or material results from photons of one wavelength range in sunlight being absorbed so that those reflected make up the colour. The most familiar is chlorophyll which absorbs two wavelength ranges: the red and the blue regions to leave green to be reflected for us to see. It is actually a bit of quantum mechanics, as the absorbed photons carry the energy needed to stoke up that of electrons so that they can break free of the OH-H bond in water and split the molecule. The chain of organic chemistry which follows this trick is hugely complex, and it seems to have taken several forms reflected in specific genes in a growing array of photosynthesising bacteria of various genetic antiquities. There are green ones, blue ones, the reds, yellows and oranges.

Luckily the chemical remnants of photosynthesising bacteria are pretty robust, and also distinctive. The central one for most photosynthesising organisms is an enzyme that is complicated, called Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO for short. Euan Nisbet of Royal Holloway, University of London has been hunting RuBisCO for most of the latter part of his career as a Precambrian geologist. he and colleagues found relics of it in 2.7 Ga Archaean sediments from Zimbabwe and Canada (Nisbet, E.G. et al. 2007. The age of Rubisco: the evolution of oxygenic photosynthesis. Geobiology, v. 5, p. 311-335) and claim there are signs far older. Needless to say.

A fluffy grazing dinosaur

The Cretaceous of NE China is becoming a favoured destination for palaeobiologists interested in well-preserved vertebrates, little dinosaurs, especially. An increasing number turned up by fossil hunters have skin relics covered in feathers, although they are rarely if at all equipped for flight, are. Recently, something even more bizarre was unearthed (Zheng, X.-T. et al. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature, v. 458, p. 333-336). In plain-speak, Tianyulong confuciusi was fluffy. And as readers really ought to know, the heterodontosaurs were largely Jurassic herbivorous creatures, 70 Ma older than T. confuciusi; a good example of a ‘living fossil’ in its own time. They evolved to large Cretaceous herbivores, such as the famous duck-billed hadrosaurs, Triceratops and Stegosaurus, members of the Ornithischia as opposed to the more commonly carnivorous Saurischia. It was the latter that were widely believed to have been evolutionary branch from which birds sprang. There is a complex argument surrounding T. Confucius, based on which is a proposal that the ancestral dinosaurs were themselves fluffy. First, thoughts of brightly coloured ‘monsters’ and now the possibility that some may even have looked cuddly.

See also: Witmer, L.M. 2009. Fuzzy origins for feathers. Nature, v. 458, p. 293-295.

The swaddled mantle

Published on May 1, 2009 by zooks777Leave a comment

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

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

On the edge of chaos in the Younger Dryas

Published on May 1, 2009 by zooks777Leave a comment

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

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

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

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

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

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

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

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