Explosion of the exoplanets

Published on by zooks777Leave a comment
The size of Kepler's first five planet discoveries
First five confirmed planets discovered by Kepler mission Image via Wikipedia

There is little doubt that it can be done, but what is so compelling about the search for worlds that orbit other stars?

By the end of the 21st century’s first decade 500 such exoplanets had been discovered, ranging from super gas giants almost 10 thousand times the mass of the Earth to a few that are comparable in size to our home world. At present the records of size and orbital radius are biased by the relative ease

of detecting large bodies over that of Earth-sized objects. Another bias is the greater chance of observing the change in luminosity of a star as one of its planets passes between us and the star – a transit – if the planet’s orbital period is short, being close to the star. The majority of known exoplanets are less than about 8 times the Earth’s orbital radius (1 astronomical unit or AU) away from their star, although some truly huge bodies have been spotted that are up to a thousand times more remote from a star than ours is.

Labeled illustration of the Kepler spacecraft
Kepler spacecraft. Image via Wikipedia

The rate of discovery is set to burgeon now that data from NASA’s Kepler exoplanet-finding mission, launched in 2009, is producing data (Reich, E.S. 2011. Beyond the stars. Nature, v. 470, p. 24-26). The 0.95 m Kepler space telescope gazes continually at a patch of sky containing 150 thousand Milky Way stars, many of which are like the Sun. It uses the transit method, and because it is fixed on only one star field it can potentially pick up the variation of stars’ luminosity due to transiting planets that are about the size of the Earth and larger. The computations are, unsurprisingly, massive and any dips in the light curves for pixels that represent individual stars have to be confirmed by other methods or by Kepler detecting repeats of the fluctuation. One drawback is that the transit method only provides the radius of a planet and its orbital period. Mass is needed to work out an exoplanet’s density and that requires another method using the red-shift of a star due to the gravitational effect of a planet causing it to wobble; a technique fraught with difficulties and best applied to dwarf red stars. The density is important for discriminating silicate-rich exoplanets from gas-liquid bodies. The main aim of planet finders is to find those around the same size and mass as the Earth that orbit a star at a distance where they would be warm enough for liquid water to exist but not so warm that it existed only as a vapour: in the so-called ‘Goldilocks zone’.

There was an initial flurry of excitement in the press in 2010 when a scientist on the Kepler programme was misinterpreted while giving a conference presentation that resulted in headlines that hundreds of distant Earths had already been discovered in the experiment’s first year. So far Kepler has only 15 confirmed planets to its credit that range from 800 times to twice the Earth’s radius all with orbits less than that of the Earth around the Sun. Nonetheless, a couple orbit within their star’s Goldilocks zone. So there is a way to go before real excitement is justified, but Kepler data will undoubtedly be used to seek funds for other planet-dedicated programmes that can fill in the gaps and perhaps confirm the existence of distant worlds that bear some resemblance to ours. Out of Kepler’s 1235 candidate detections since launch, 68 would be Earth-sized if confirmed (Shiga, D. 2011. What’s an alien solar system like? New Scientist, v. 209 (26 February 2011 issue) p. 6-7). For such remote detection to suggest an exoplanet on which life has evolved demands that atmospheric composition can be deduced from spectra of electromagnetic radiation from the body itself: a far more difficult undertaking that finding and weighing. Free atmospheric oxygen, so far unique to the Earth, is an obvious target. However, its absence would not rule out life that did not use photosynthesis to split water molecules in making living matter, and there are plenty of life forms here that do that.

Related Articles

Comet water in lunar rocks

Published on by zooks777Leave a comment
Halebopp031197
Comet Hale-Bopp.Image via Wikipedia

There are two main hypotheses about the origin of Earth’s oceans: that they are filled with water that was locked in the meteoritic matter that initially accreted to form the Earth, or ocean water was delivered by massive comet bombardment in the first half billion years of the Earth’s history. It hasn’t yet been possible to decide whether one of these, or both were  involved, but the Moon might give a clue, even though until very recently it was regarded as being bone dry (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). The ratio between deuterium and hydrogen (D/H) gives a clue to the origin of water, in which both hydrogen isotopes occur (Greenwood, J.P. et al. 2011. Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, v. 4, p. 79-82). Using an ion microprobe to analyse the water in apatite, its dominant host in lunar rock samples, the authors were able to report two things. First, there is water in magmatic rocks of all ages found on the Moon: the earliest anorthosites of the lunar highlands and the younger basalts that fill the dark maria. Secondly, the water has D/H ratios significantly outside the terrestrial range. In detail, apatites with the greatest enrichment of deuterium relative to hydrogen are found in the maria basalts which fill enormous basins thought to have formed around 4 Ga ago as a result of cometary impacts. The D/H ratios are lower in apatites from the lunar highland anorthosites, which probably formed through flotation of low density calcium-rich feldspar as the Moon’s initially molten mantle crystallized not long after its formation through the impact of a small planet with the Earth. The highland D/H values are not wildly dissimilar from those found on Earth, yet those found in the mare basalts match the admittedly less well-constrained levels determined from comets hale-Bopp, Hyakutake and Halley. Because the Earth’s mass would ensure that it would corral 15 times more incoming extraterrestrial matter than would the Moon, the argument goes that if the Moon captured cometary water then Earth did so in trumps. The difference is that the Earths greater gravitational pull and thick atmosphere allowed it to retain gaseous and liquid water, while the Moon’s lower escape velocity let them leak away so that only mineralogically bound water could be retained.

Related Articles

Top story and most flawed hype of 2010

Published on by zooks777Leave a comment
Reconstruction of Neandertaler at Neanderthal ...
Reconstruction of a middle-aged Neanderthal man. Image via Wikipedia

EPN might seem to include a disproportionate number of items on hominin evolution, including several on genetic evidence. An outcome of the Earth System’s 4.5 billion-year evolution increasingly depending on physical resources, we lie at the focus of our own curiosity studying the past primarily for ourselves. That is why the discovery from the partial genome of Neanderthal remains that all humans outside those who live in Africa carry in our DNA the ‘fruits’ of intimate relations with Neanderthals is surely the most explosive development of the 21st century so far (see Yes, it seems that they did…in May 2010 issue of EPN). It is deepened by the publication in late 2010 (Reich, D and 27 others 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, v. 468, p. 1053-1060) of genetic findings from remains of a third distinct hominin group that inhabited central Siberia 30 to 50 ka ago (see Other rich hominin pickings in the May 2010 issue of EPN). [Thanks go to Dr Bill Deller, legal historian, for alerting me to this.] The DNA from a tooth and a finger bone show that the individual female was genetically neither a fully modern human nor a Neanderthal in a statistical sense, but parts of the sequence, as with the Neanderthal genome, pop up in the genomes of living people. The ‘Denisovan’ signature – the authors do not assign the female to a new species – contributes 4 to 6 % of the genomes of present-day inhabitants of Papua-New Guinea and other Melanesian people of the Pacific north of Australia, but appears in no others. Since Melanesians carry some Neanderthal genetic material the new finding can be interpreted as the result of similar interfertile mating between the ‘Denisovans’ and a limited group of early fully human travellers who crossed central Asia and eventually moved through Indonesia to cross the West Pacific to Papua-New Guinea and Melanesia about 45 ka ago. For up to a twentieth of the genetic outcome of such liaisons to survive to the present suggests no idle dalliance, but proportionately common relationships.

Solo tiene 400.000 años. Un chaval, vamos.

Reconstruction of Homo heidelbergensis, perhaps similar to a Denisovan. Image via Wikipedia

Denisovans shared a common ancestor with Neanderthals and ourselves, but seem to have followed a separate evolutionary path. Analysis of their DNA suggests that they diverged from Neanderthals around 640 ka  and from modern Africans around 800 ka. Although these ‘molecular clock’ dates show considerable uncertainty, they extend back to a period when fossil evidence suggests the presence in Europe and Africa of Homo heidelbergensis and H. erectus respectively. The molar tooth has a morphology similar to African H. erectus and to even earlier hominins, but distinct from the teeth of Neanderthals and fully modern humans. Could the ‘Denisovans’ represent a distinct wave of emigrants from Africa? Some hominin fossils from China are dissimilar to Neanderthals and Asian H. erectus and efforts will certainly be made to establish their genetic make-up. For the moment, these findings deny any simple linear explanation for the ‘Out-of-Africa’ movement of people. Equally important, and the reason why the researchers refuse to assign the ‘Denisovans’ to a new species, is that interfertility is generally accepted as the sign of mating between members of the same species. To some extent this harks back to the ideas of the ecologist Jonathan Kingdon (Kingdon, J. 1993, Self-made Man and His Undoing. Simon & Schuster: London) that humans are a line that did not speciate over the last couple of million years, but show morphological differences that arose within the growing protection from selection pressures conferred by the use and development of tools. Kingdon’s parsimonious approach to human evolution found little favour with palaeoanthropologists, perhaps because of the kudos associated with finding and naming new species.

See also: Callaway, E. 2010. Fossil genome reveals ancestral link. Nature, v. 468, p. 1012; Bustamante, C.D. & Henn, B.M. 2010. Shadows of early migrations. Nature, v. 468, p. 1044-1045.

Perhaps it is a generational thing, stemming from popular science fiction and scientists’ speculation in the 1970s and 80s, that has encouraged the growth of exo-, xeno- and astrobiology as subdisciplines. There is a certain sadness in that all practitioners can do at present is examine the organic diversity offered by our home world and speculate about alien life forms based on that terrestrial evidence. The Earth offers plenty of scope for studying the biologically odd and awesome, especially among prokaryotes, as there are extremophiles of all kinds: the hot, the cold and the deep biospheres. But all are based on the nucleic acids shared by all life on Earth; traces of familiar amino acids occur far and wide in the cosmos, but none whatsoever of anything more complicated that could source self-replication and evolution. So it was in a mood of solemn gaiety that EPN greeted the hint of truly alien life forms among us by NASA press officers in November. It turned out to presage a paper concerning bacteria peculiar to Mono Lake in California (Wolfe-Simon, F. And 11 others 2010. A bacterium that can grow by using arsenic instead of phosphorus. Science Express, DOI:10.1126/science.1197258). The paper hinted at arsenic being used to substitute for phosphorus in the structure of nucleic acids in the bacterium when it lived in low-phosphate environments. The paper’s substance was culturing the bacterium in vitro in increasingly P-deficient water that also contained arsenic. If replicable the notion of arsenic-DNA would seem to be pretty startling, but the paper faced a storm of adverse comment.

Mono Lake Tufa
Biomineralised columns at Mono Lake, California. Image via Wikipedia

A crucial feature of the DNA molecule is the bond between the sugar of one nucleotide and the phosphate group of another. As any geochemist knows, it is possible for elements to substitute for one another if they have similar atomic properties. Arsenic, being adjacent to phosphorus in the same group of the periodic table, is a potential substitute –  arsenate for phosphate, although the former is far less stable than phosphate. Wolfe-Simon’s team is not claiming the peculiar bacterium as a candidate for alien life forms, but that is the spin widely being put on their work. All they suggest is that some bacteria can survive high-As low-P conditions and may be found in chemically highly toxic environments elsewhere. Since the cosmic abundance of arsenic is about three orders of magnitude less than that of phosphorus it is unlikely that alien genetic material somehow uses arsenic in its architecture. Besides, why should DNA be the sole basis for self-replication, the essence of truly living beings?

Added 14 January 2011: Science gave Felisa Wolfe-Simon the opportunity to reply to critics (Pennisi, E. 2010. Discoverer asks for time, patience over arsenic bacteria controversy. Science, v. 330, p. 1734-1735). Personally, I sympathise with the beleaguered team: on the launch of Stepping Stones in 1999 I was pestered continuously by journalists from both low- and highbrow newspapers. In that case I had made a joke that perhaps the human coughing reflex had stemmed from ancestral reptiles that survived the Permian-Triassic mass extinction and the emissions of the Siberian Traps: the journalists actually believed it

See also: Pennisi, E. 2010. What Poison? Bacterium uses arsenic to build DNA and other molecules. Science, v. 330, p. 1302.

Related Articles

Surprise in store for coal burning

Published on by zooks7771 Comment
A coal mine in Wyoming, United States. The Uni...
Image via Wikipedia

Of the fossil fuels coal has long been assumed to be the most plentiful, even the most pessimistic forecasters having acknowledged a global lifetime of centuries for known reserves. The determination of the emerging giant economies of China and India and of the USA to fuel themselves through coal-burning seems inevitable if highly risky for the climate. But that depends on coal remaining the cheapest fuel, largely because of the sheer abundance of supplies. A recent commentary on coal (Heinberg, R. & Fridley, D. 2010. The end of cheap coal. Nature, v. 268, p. 367-369) suggests that there is a growing tendency for reserve estimates to decrease as geologists factor in practical restrictions – place, depth, seam thickness and quality – on feasibility under current mining conditions, instead of just looking at known masses of coal. Astonishingly, the end-19th century estimate of five thousand years of US coal supplies dropped to about 400 years by 1974 and is currently judged to be 240 years. China and India look likely to have less than 60 years-worth left. On top of that, the widely publicized turn to carbon capture and storage (CCS)for ‘clean-coal’ future supplies will inevitably drive-up prices of coal-fired energy. The two main factors in this remarkable transformation of ‘King Coal’ are fundamental economic forces in capitalism and the increasing refusal of miners to accept dangerous working conditions. The second is especially the case for China, where most coal is deep-mined; in the late 1990s it saw a drive to close down unsafe mines that caused production to fall, although it has greatly accelerated this century – further driving down coal’s lifetime there. It seems from this analysis that any realistic hope for a CCS-based coal economy, especially in China and India, depends on declining safety and environmental standards in their largely underground mines, which in turn depends on the highly unlikely willingness of their workforces to accept worse conditions.

Self plagiarism

Published on by zooks777Leave a comment

Some scientists have enormous publication records, a notorious case being one who claimed personal discovery of the HIV virus. During the 1980s, this person managed to figure as an author in up to 90 papers a year, despite mainly travelling back and forth to conferences. If the same name appears again and again in publications – it makes little difference where it figures in the list of authors – it is that name that is remembered as an “authority”. In some cases such an accolade is deserved, in others it is engineered by a variety of devices: the same data can be used over and over (most blatantly if those data are ‘engineered in the first place); a place in an authors’ list can result from being a ‘guest’, in the manner of a faded star, ‘down on their luck’, who pops up with a one-line cameo in a film (I rule out Alfred Hithcock’s appearance as a bystander in every film that he made); by nicking the ideas and words of others; through the device of self plagiarism. The last is an especially cunning ploy, as it also saves time crafting text. The italicized sentence above is an example self-plagiarised from the March 2002 issue of EPN (Credit where credit is due?), but as a blogger I can do that with a clear conscience; Earth Pages News is highly unlikely to get me into the ‘Professoriat’, especially my inability to resist occasional items such as this! Having provided the original source reference, I am safe from universal condemnation.

Potentially the game is up for plagiarists and pot-boilers in peer-reviewed journals through scanning software (e.g. Turnitin) that checks text against web-available journals. Self-plagiarism may well be an oxymoron, but it serves as CV fodder as well as creating academic redundancy. It hit the news (Reich, E.S. 2010. Self-plagiarism case prompts calls for agencies to tighten rules. Nature, v. 468, p. 745) because of a case in Canada where an author’s peer-reviewed portfolio was found to contain 20 instances. No academic censure ensued, but three of his papers were retracted. Using the Déjà Vu facility to check biomedical literature has resulted in 79 thousand cases of duplicated wording in abstracts and titles alone, and the eventual retraction of almost 100 articles. Seemingly, journal editors are allowing repeated use of text in the ‘methods’ sections of papers, so a geochemist minor co-author, who gets a ride for a small contribution based on use of a particular piece of equipment might be safe in that regard, there being safety in numbers. Yet as the use of anti-plagiarism software spreads into the wider on-line literature its original targets, undergraduate and graduate students, may decide that the biter ought to be bit and turn the cyber searchlight on their ‘betters’…

Related Articles

Linking oxygen levels to great animal radiations

Published on by zooks777Leave a comment
Dunkleosteus
Dunkleosteus (10 m long) of the Late Devonian. Image by Travis S. via Flickr

Probably the greatest ecological truism is that without oxygen there would be no life forms on Earth above the level of a restricted number of prokaryotes. Since around 2.4 Ga, when free atmospheric oxygen first appeared, levels have risen to the present 21% – it was probably as high as ~30% in the Carboniferous and Cretaceous Periods. Charting the rise has been difficult and the history of oxygen is written with a very broad brush. If there had been sudden increases in the availability of oxygen in the atmosphere and oceans there ought to have been a bursts of evolutionary radiation and diversity, but often oxygen-related causality for events such as the Cambrian Explosion have been speculative, as have cases for the inverse, declines due to downturns in oxygen levels (see Oxygen depletion before P-T extinction in the November 2003 issue of EPN). Recently a proxy for the redox chemistry of the global ocean, and therefore for relative changes in atmospheric oxygen, has been developed. It is based on the abundance and isotopic composition of the element molybdenum (Mo) in sedimentary rocks: higher 98Mo relative to 95Mo (the d98Mo value) signifies higher oxygen levels. Its recent use in relation to evolutionary radiations (Dahl, T.W. et al. 2010. Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish. Proceedings of the National Academy of the US, v. 107, p. 17911-17915) has produced interesting results. The US-Swedish-Danish-British team analysed the Mo in euxinic (reduced) marine black shales, which concentrate the element from seawater, in the Proterozoic and Phanerozoic Eons. Increases in δ98Mo occur at the time of the Cambrian Explosion, as expected, and also during the Devonian. The latter correlates with increasing diversification of large fishes and among early terrestrial plants, and may have been the greatest leap in the bioavailability of oxygen in Earth’s history, stemming from the ‘greening’ of the land. So far Mo-isotope data have not been obtained from Carboniferous, Permian or Cretaceous back shales, but the ratio of Mo to organic carbon content in black shales of those ages  – a less constrained proxy –  does confirm what has been suspected: highs (greater than present levels) in the Carboniferous and Cretaceous and lows during the Permian and Triassic. However, any hopes that the approach can be calibrated to actual oxygen levels seem likely to be optimistic as the controls over dissolved molybdenum supply to the oceans and its transfer to sediments are extremely complex.

Added 14 January 2011. Some of the team feature in a related article (Gill, B.C. et al. 2011. Geochemical evidence for widespread euxinia in the Later Cambrian ocean. Nature, v. 469, p. 80-83) that ticks all the geochemical boxes for the evolutionary effects of depleted oxygen; i.e. extinctions. They use new measurements of sulfur isotopes in conjunction with published carbon-isotope  and other geochemical data from a wide range of Late Cambrian sediment types and environments in six well-known sections of that age. Spikes in the relative abundance of 34S match those in 13C along with a decrease in Mo in one section (see above), suggesting temporary increases in carbon and sulfide burial during periods of oxygen deficiency in the Late Cambrian ocean. Massive sequestration of organic carbon may have led to the extremely cold Late Cambrian climate, as described in A chilly Late Cambrian (this issue). Combined with changes in redox conditions associated with ocean anoxia this would have especially stressed animals, even on continental shelves had oxygen depleted water risen from the depths where sulfur and carbon burial were going on.

See also: Shields-Zhou, G. 2011. Toxic Cambrian oceans. Nature, v. 469, p. 42-43.

Related Articles

Assorted developments in palaeoanthropology

Published on by zooks777Leave a comment

The notion that Neanderthals were dim and brutish compared with us continues to be undermined, but although their brain capacity was as large and in some cases distinctly larger than that of fully modern humans, its shape was significantly different; longer towards the rear than our more rounded brain. Studies of a Neanderthal baby and three children reveal that just after birth the Neanderthal brain was virtually identical to that of fully modern babies, i.e. elongate, but remains so in childhood through to maturity, whereas modern children’s brains develop towards the roundness of adults. Consequently, there must have been differences in the parts of the brain from which aspects of behaviour stem: Neanderthals almost certainly behaved differently from us both in childhood and as adults (Harvati, K. et al. 2010. Evolution of middle-late Pleistocene human cranio-facial form: a 3-D approach. Journal of Human Evolution, v. 59, p. 445-464. See also: Gibbons, A. 2010. Neandertal brain growth shows a head start for moderns. Science, v. 330, p. 900-901).

The now widely accepted hypothesis that modern humans did not begin to leave Africa to colonise Eurasia until about 60 ka may be under threat from reports of what seem to be fully modern human remains in China dated to ~105 ka (Liu, W. et al. 2010. Human remains from Zhirendong, South China, and modern human emergence in East Asia. Proceedings of the National Academy of the US, v. 107, p. 19201-19206). The dating appears to be sound, being based on the uranium-series (230Th) method applied to flowstone that rests on top of the sedimentary layer containing the remains in Zhirendong cave. The precipitated calcite layer completely sealed in the fossils as soon as it began to form about 105 ka ago, indicating that they are older still. Whether or not the remains are of fully modern humans is uncertain. Had they been found in Europe there would be little doubt about their affinities, the only other contemporary hominins being the Neanderthals. The problem in South China is that it was inhabited by H. erectus and the finds may be from ‘late’ members of that archaic species which arrived more than a million years earlier than fully modern humans. Judging by the DNA evidence for three interfertile hominin genetic groups cohabiting Eurasia, there is a host of possibilities for the Zhirendong fossils. One line of evidence that does not rule out that they are fully modern is the occurrence of stone tools more advanced than used by Asian H. erectus beneath the 74 ka Toba volcanic ash in India. It seems inevitable that these remains will be tried for DNA sequencing

See also: Dennell, R. 2010. Early Homo sapiens in China. Nature, v. 468, 512-513

It is well accepted that as with all forms of life the twists and turns in hominin evolution was surely tuned by changes in their environments. But that is not just linked to the immediate milieu of individuals: environments change on all scales up to that of the entire planet and reflect physical as well as biological processes. The largest scales are generally assumed to be the province of climate change, yet animals also occupy a landscape subject to geophysical forces such as tectonics and erosion. Geoffrey Bailey and Geoffrey King of the University of York, UK and the Institute de Physique du Globe in Paris, France have championed the view that water supplies and topography, for example, are just as influential over hominin evolution as interspecies competition and changing vegetation patterns for almost two decades. They have now put their ideas to rigorous tests (Bailey, G.N.& King, G.C., 2010 (in press) Dynamic landscapes and human dispersal patterns: Tectonics, coastlines, and the reconstruction of human habitats. Quaternary Science Reviews doi:10.1016/j.quascirev.2010.06.019). This fascinating and well illustrated paper correlates known hominin sites in Africa with variations in topography and its roughness, derived from global elevation data from the Shuttle Radar Topography Mission (SRTM), active seismicity, Neogene uplift and volcanicity.

Perspective view of the Afar depression and en...
Afar Depression: a cradle of human evolution

They concentrate on the rich palaeoanthropological pickings of the Afar Depression and the Sterkfontein area of South Africa, applying their ideas and findings to the eastern coast of the Red Sea at the recently discovered Palaeolithic site of Harat Al Birk south of Jeddah, and the Red Sea islands that would have been connected to either side of the Red Sea during the last glacial maximum because of a 130 m lower sea level. This application is vital for directing searches for new site that relate to the pathways out of Africa for early modern humans. Though a largely empirical study, it forms a link between human evolution and geological and landscape change that is not yet widely grasped and linked to climate studies.

See also: Marshall, M. 2010. Evolution by shake, rattle and roll. New Scientist, v. 208 (13 November 2010), p. 8-9.

Related Articles

Oxygen and the differentiation of magmas

Published on by zooks777Leave a comment
2006 eruption
Image via Wikipedia

The bulk of igneous rocks found within and upon the crust formed by one of two fundamental processes of magma differentiation: calc-alkaline and tholeiitic, responsible for island arcs and ultimately continents forming and for generating oceanic crust and flood basalts. The parental material for both is basaltic magma, but the first leads to a decrease in iron in more fractionated magmas, whereas an increase in iron characterised the second. In the first case conditions favour iron entering igneous minerals, whereas in the second they urge crystallising minerals to exclude iron. The most likely explanation is that the calc-alkaline magmas of volcanic arcs devour electrons so that iron exists in the oxidised ferric or Fe3+ state and readily forms dense iron oxide minerals whose progressive removal makes the remaining magma less and less rich in iron. More reducing conditions that lack an abundant electron acceptor, primarily oxygen, make the formation of iron oxides less likely, and iron can build up in residual magmas. But how greater oxidation occurs in arc magmas than in those of the oceanic crust has several possible explanations. The most-widely assumed is that it happens because volcanic arcs lie above subduction zones where hydrated and therefore oxidised ocean floor descends into the mantle conferring oxygen to the products of partial melting. Another candidate is the depth at which fractional crystallisation takes place and there are other possibilities. The oxidation state of fundamental magmatic processes can be proxied by determining in rocks produced by fractionation the relative proportions of elements that behave differently in conditions of increased or decreased oxygen. One such pair is insensitive zinc and sensitive iron (Lee, C.-T.A. et al. 2010. The redox state of arc mantle using Zn/Fe systematics. Nature, v. 468, p. 681-685). The surprise is that the parent magmas of both calc-alkaline and tholeiitic fractionation series have identical Zn/Fe ratios, suggesting that both partially melt from mantle with much the same availability of oxygen. The Zn/Fe ratios differ in more evolved igneous rocks from the two series, suggesting that it is in the fractionating magma chambers that the distinctively different oxygenation occurs, not in the zone of mantle melting.

Related Articles

A chilly Late Cambrian

Published on by zooks777Leave a comment

Application of the Uniformitarian Principle by geologists from Minnesota, USA (Runkel, A.C. et al. 2010. Tropical shoreline ice in the late Cambrian: implications for Earth’s climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event. GSA Today v.20 (November 2010), p. 4-10) may have shown that around the end of the Cambrian period (500 to 488 Ma) global climate was sufficiently cold for sea ice to have formed in the tropics of the time. The evidence comes from curious metre-scale clasts of cemented sands in Late Cambrian beach deposits of the northern USA, some of which show imbrication as if the bodies were shoved together. Others seem to have been extended into boudin-like plates without any sign of tectonic activity, so that isolated clasts occur in offshore deposits. Yet more have been bent to drape over irregularities in the surface beneath them. Somehow individual sand beds must have become cemented quickly so that water action could fracture them in a brittle fashion and then they became softer to experience ductile deformation and even boring by worm-like animals. Almost exact replicas of such structures form on the shores of the American Great Lakes in winter when water in shoreline sands freezes to cement the grains. Breaking waves and melting explain the peculiar structures in these intraclasts. Examples of ice-cemented sediments abound in glaciogenic deposits, but the Late Cambrian world is widely considered to have experienced greenhouse conditions.

Land distribution during late Cambrian.
Image via Wikipedia

Apparently not as the North American crust was definitely close to the Equator at that time. The intraclasts occur only in one stratigraphic Formation of the Minnesotan Cambrian, because it preserves littoral facies. There are no other reports from elsewhere, but that may well be because few geologists were able to combine the experience of modern frigid shore conditions with that of Cambrian stratigraphy as those from Minnesota surely do.

The Middle to Late Cambrian was a period of faunal hiccups, diversification after the Cambrian Explosion failing to get underway because of repeated minor extinctions spread across the known occurrences of rocks of that age (see Linking oxygen levels to great animal radiations, this issue) . The Minnesotan evidence could indicate that the global climate was extremely unstable at that time in the manner of Neoproterozoic ‘Snowball Earth’ conditions, but not so severe. The widespread occurrence of microbial carbonate facies of this age range has long been used as evidence of a warm Earth, but such carbonated form today over a wide range of latitudes: witness the huge coccolithophore blooms so common at high latitudes nowadays. Shoreline sandy sediments of Cambrian age are not uncommon, occurring throughout the English Midlands and in NW Scotland, for instance. So it might be interesting to re-examine easily-reached occurrences such as these to see if similar structures turn-up.

The timing of ups and downs of metamorphism

Published on by zooks777Leave a comment
An enlargeable topographic map of Sri Lanka
Complex topography of Sri Lanka via Wikipedia

As the temperature and pressure affecting crustal rocks go up and down, as for instance in the thickening of crust when two continents collide and then erosion strips off the cover so that the rocks slowly rise, the rocks undergo progressive changes in their mineral content; in both cases they are metamorphosed. Rising intensity of conditions gives rise to a prograde metamorphic sequence, and when they wane retrograde metamorphism takes place as the elements that combine in minerals react to adjust to new conditions. In some cases it is possible to use the mineral assemblages, specifically the proportions of different elements that are shared between two or more minerals, to chart the changes in temperature and pressure. That reveals the path taken by the rock through temperature- and pressure space, which is effectively a measure of the crustal processes involved and the geothermal conditions under which they acted: a P-T path. Adding the timing to give a sort of movie to all the changes has been hit-or-miss up to now, and based on radiometric ages from igneous rocks formed and emplaced during the metamorphic evolution. Thanks to the finely targeted mass spectrometry that an ion microprobe an achieve, adding the ‘t’ dimension is now possible from the metamorphic rocks themselves (Sajeev, K. et al. 2010. Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature  metamorphism exemplified by Sri Lankan granulites. Geology, v. 38, p. 971-974). Minerals based on the element zirconium (Zr), such as zircon and monazite are extremely resistant to the effects of temperature as regards the radioactive and radiogenic elements that they contain, specifically uranium (U) and thorium (Th) and the lead (Pb) isotopes that form when 235U, 238U and 232Th decay. Both these minerals become zoned as successive layers grow during metamorphism, and the ion microprobe can measure the isotopic composition on a later-by-layer and therefore event-by-event basis. The famous granulites (charnockites) of the island of Sri Lanka (Ceylon) reached the peak of their metamorphism (1050°C and 0.9 GPa) at ~570 Ma and began to retrogress about 20 Ma later around the start of the Cambrian. Previously it was not possible to separate metamorphic ages from those when the original rocks formed in the Archaean and early Neoproterozoic.  Such high temperatures are very difficult to attain in the crust under normal geothermal conditions unless extra heat is added by large volumes of basaltic magma ponding at the base of the crust during crustal thickening.

Degassing of sea-floor clathrates

Published on by zooks777Leave a comment
Locations of known and inferred gas hydrate (m...
Image via Wikipedia

Methane hydrates – natural gas held in clathrate solids that resemble water ice – that occur in sea-floor sediments are on the one hand a potential energy resource and on the other pose great risks. There are between 1015 to 1017 m3 buried beneath the ocean floors and an unknown amount in Arctic soils and lakes.  The temperature that confers stability on these peculiar solids depends on pressure. At pressures lower than those at a water depth of around 250m they are unstable. Clathrate crystals form from natural gas and water in sediments at 0°C at that depth and at progressively higher temperatures at deeper levels beneath the seafloor, until geothermal heat flow at a depth of around 2.5 km results in temperatures above about 20°C when they cannot form; there is a depth-temperature window in which gas hydrates may be found in seafloor sediments, which depends on the temperature of deep water. Little is known about the stability of gas hydrates. In some areas there is a steady release of methane that bubbles to the surface, whereas in others they can be detected by seismic surveys in huge volumes that appear to be stable with no release. One area rich in gas hydrates occurs at the continental edge off the Norwegian coast (the Storegga in Norwegian). Periodically sediments at the Storegga fail in massive sub-sea landslides which have resulted in tsunamis in the North Sea. The last such tsunami occurred around 6100 BCE after a slide displaced 3500 km3 of debris, devastating the east coast of Scotland. Either an earthquake triggered the slide or it was due to destabilizing of the clathrates. Either way huge amounts of methane would have been released. At the end of the Palaeocene Era (55 Ma) a global carbon-isotope anomaly coincides with evidence for very rapid climatic warming, which suggests that vast amounts of methane – a far more powerful greenhouse gas than CO2 – were released from submarine gas hydrates. In recent years the loss without trace of several large ships may have resulted from a lowering in the density of surface water by gas bubbles that caused the vessels to founder. One country that plans to exploit gas hydrates off its Pacific cast is Japan, and recent surveys indicate a large basin underlain by highly disturbed sediments which contain clathrates on the flank of the basin (Bangs, N.L. et al. 2010. Massive methane release triggered by seafloor erosion offshore southwestern Japan. Geology, v. 38, p. 1019-1022). It appears that bottom currents eroded the seafloor to destabilize the clathrates that then ‘erupted’ ripping through the sediments to release around 1.5 x 1011 m3 of methane. Clearly, drilling into gas hydrate deposits is going to be a risky business; drilling will reduce the pressure so that gas is released and it is not known whether or not this might trigger a form of chain reaction. In the longer term, warming of deep water as a result of climate change could place much larger areas of clathrate-rich seafloor in a knife edge.

Blood of the dinosaurs

Published on by zooks777Leave a comment
Epic battle in my backyard
Image by Cliff Beckwith via Flickr

Though it is highly likely that burial of fossils for millions of years destroys any trace of their DNA the massive bones of large creatures can preserve cell material. A near complete 67 Ma old Tyrannosaurus rex, fondly known as ‘Big Mike’ has revealed blood cells in thin sections of its bone (Schweitzer, M.H. 2010. Blood from stone. Scientific American, v. 303 (November 2010), p. 38-45). Her article also covers traces of blood vessels, and collagen of similar antiquity. The research involved positive reaction of antibodies against proteins, thereby proving the materials to be organic and not products of biomineralisation formed during the process of fossilisation. Potentially such forensic work can tease out relationships among animal groups whose fossils preserve organic materials, in a similar way to indications of the rise of prokaryote groups by biogeochemical marker molecules in carbonaceous shales. Indeed, sequences of fossil proteins from dinosaurs closely resemble that of modern birds. One of the great surprises of the late 20th century was the growing evidence that the stem-line for birds was dinosaurian, specifically the theropod group. This is nicely summarized by another review article (O’Donoghue, J. 2010. Flight of the living dead. New Scientist, v. 208 (11 December 2010), p. 36-40) that addresses the certainty of birds’ evolution from dinosaurs; which of the fossils is bird, which feathered dinosaur and when did they separate; and why did birds survive the end-Cretaceous mass extinction while dinosaurs famously succumbed – probably a matter of breeding; its pace, that is. The two articles together suggest a fruitful way forward for palaeobiologists.

Further material about biochemical relics in fossils and methods used to detect and analyse them can be found in Hecht, J. 2011. Waking the dead. New Scientist, v. 209 (22 January 2011 issue), p. 43-45.

Snatched from the Earth’s jaws

Published on by zooks777Leave a comment

Every geoscientist will salute the fortitude and bravery of the 33 Chilean miners rescued from a refuge 700 m below ground, that of the 5 volunteer rescuers who descended the 80 cm shaft, not knowing whether it was safe and the skills of voluntary engineers whose drill managed to find the small refuge, despite its depth. Many geologists have been in underground mines, though only a minority have worked in them, but all admire the mental and physical resilience of the 33. Trapped by the caved-in access tunnel on 5 August, the miners faced and survived 17 days with fading lamps and tiny supplies of food and liquids. The final rescue came with remarkable swiftness during 13-14 October. Apart from one with a chest infection all seemed little the worse for wear. The growing tension during the rescue was almost palpable, even at a distance of more than 11 000 km: would the narrow tunnel collapse; would the rescue shuttle jam? The likelihood of either grew with each rescue.

The rise in gold and copper price since the global crash of 2008 has seen the reopening of dozens of once uneconomic mines, kept for years on a ‘care and maintenance’ basis. Not knowing when the metal-price boom would collapse, mine owners have rushed to restart operations, paying locally premium wages to attract miners. The San José mine near Copiapo, was one such mine, whose fabric had deteriorated after years of neglect. It would be unsurprising if another disaster, with less happy outcomes, occurred during the current metal-mining boom.

Added 26/11/2010. So soon after such a victory over being buried alive for so long, it is especially tragic to learn that the methane explosion of 19 November in New Zealand’s largest coalmine at Pike River on the South Island killed 29 miners. They were declared dead after a second explosion on 24 November. Today a third blast ripped through the mine not long before a memorial service was to be held, vindicating the decision not to send in rescue parties as soon as the initial explosion took place. Inevitably, there will be a major inquiry into how such a build-up of explosive gas could possibly have gone unnoticed.

Record of rising Holocene sea-level in the tropics

Published on by zooks777Leave a comment

Areas beyond the zones of isostatic depression by ice-loading and recovery during glacial-interglacial cycles passively undergo sea-level fall and inundation. They best record the progress of Holocene ice-sheet melting and sea-level rise since 11.5 ka, especially if they are tectonically stable. The island state of Singapore, 1.5 º north of the Equator, is a near-ideal place for study (Bird, M.I. et al. 2010. Punctuated eustatic sea-level rise in the early mid-Holocene. Geology, v. 38, p. 803-806). The Australian and British geoscientists analysed a core through sediments in a mangrove swamp now just below sea level. The top 14 m penetrated a uniform though laminated sequence of marine muds, calibrated to time by radiocarbon dating of mollusc shells, mainly focused on the period from 9 to 6 ka period that the global oxygen-isotope record of ice volume suggests to have been the main period of final melting after the Younger Dryas.

Sedimentation was very rapid (~1 cm y-1) from  8.5 to 7.8 ka, probably as sea level rose too rapidly for the coast to be protected by mangrove growth.  Then for 400 years it slackened off to ~0.1 cm y-1 to rise again to 0.5 cm y-1 by 6.5 ka. The last date is the time of the mid-Holocene sea level highstand, after which sedimentation rate soon declined to 0.05 cm y-1, when mangroves became established at the site. Stable isotopes of carbon in the core (δ13C) show how the relative input of marine and terrestrial (mainly mangroves) organisms shifted over the period and are a proxy for the distance to the coastline and hence sea level. From 8.5 to 6.5 ka this was erratic from a starting point about 10 m lower than nowadays, showing rapid rises and falls that culminated in a sea level in Singapore about 3 m above present during the mid-Holocene sea level highstand that slowly declined to that of the present.

The team’s findings tally with evidence for the melting record of the North American ice sheet. An interesting aspect is that they also cover the period when rice cultivation in swampy areas of SE Asia got underway (~7.7 ka). Very rapid sedimentation would have encouraged development of the substrate for the highly fertile delta plains that now support the largest regional population densities on Earth. In turn they culminated in a series of early south and east Asian civilisations based on class societies.

Correction to marine biodiversity record and mass extinctions

Published on by zooks777Leave a comment

The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.
One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were especially noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos (Bennett, K. 2010. The chaos theory of evolution. New Scientist, v. 208 (16 October 2010), p. 28-31)?

Comet impacts’ candidature for origin of life

Published on by zooks777Leave a comment

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules. Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.

Phosphorus, Snowball Earth and origin of metazoans

Published on by zooks777Leave a comment

As any gardener knows, the element phosphorus is an essential plant nutrient or fertiliser, along with potassium and nitrogen plus a host of minor elements that are rarely mentioned as sufficient amounts are generally available in soils. The same necessities for life apply to oceans too, in which amounts vary a great deal from place to place and whose relative proportions have changed through geological time. For the oceans the main source of phosphorus is the continental crust, where the element resides mainly in the mineral apatite (Ca5(PO4)3(F,Cl,OH)). This is not an easily dissolved mineral, which is why for agricultural fertiliser it is generally made available in the soluble form of calcium superphosphate (Ca(H2PO4)2) that is produced by reaction between apatite and sulfuric acid. Since the land surface was colonised by plants about 450 Ma ago, biological processes made phosphorus more readily available to solution in river water by their break-down of apatite; supply by rivers to the ocean nowadays is of the order of 109 kg y-1. Ups and downs of P dissolved in ocean water though geological time would be expected to have influenced its overall biological productivity, controlled by photosynthetic phytoplankton and prokaryotes. Variations of carbon isotopes (δ13C) in organic and carbonate sediments are know to have occurred episodically since Archaean times, suggesting wide fluctuations in both bioproductivity and burial of dead organic matter. However, it has been hard to judge any geochemical reasons underpinning such variations. Since it is now clear that the common iron mineral goethite (FeOOH) ‘mops up’ many chemical species including phosphate ions by adsorption on its grain surfaces, measuring the P/Fe ratios in marine ironstones containing these minerals is a potential guide to the changing phosphorus concentration in the oceans (Planavsky, N.J. et al. 2010. The evolution of the marine phosphate reservoir. Nature, v. 467, p. 1088-1090).

The US-French-Canadian researchers charted P/Fe ratios in banded iron formations and ironstones precipitated around ocean-floor hydrothermal vents since the Archaean. What emerged were four episodes: from 2900 to 1700 Ma with generally low ratios; the Neoproterozoic from 750 to 635 Ma with much higher ratios; the Phanerozoic from Cambrian to Jurassic with low ratios and post-Cretaceous high ratios. There are several significant gaps in the record of ocean phosphate levels, notable one a billion years long from 750 to 1700 Ma. One factor that probably affected the variation is the way that dissolved silica (SiO2) drives down the proportion of phosphate adsorbing onto goethite. The rapid evolution and expansion since the Cretaceous of diatoms that secrete silica probably reduced SiO2 concentration in ocean water as their remains rained down to be buried on the ocean floor; that explains the high P/Fe ratios since about 100 Ma. Earlier Phanerozoic oceans are estimated to have had as much as seven times the present concentration of dissolved SiO2, thereby explaining the low values of P/Fe in ironstones deposited in the 100-540 Ma range. From 1700 to 3000 Ma the low P/Fe suggests oceanic phosphorus levels equivalent to those from the Jurassic to Cambrian (but perhaps up to 4 times that, depending on the poorly constrained SiO2 concentrations).

The Neoproterozoic phosphorus ‘spike’, at a time when dissolved SiO2 would have been no different from that in earlier times, suggests a massive influx of phosphate to the oceans at that time. It coincides with the two greatest glacial epochs the Earth has experienced: ‘Snowball’ Earth when glacial ice existed at tropic latitudes. In themselves the massive glaciations offer an explanation for high phosphorus delivery from the continents through glacial erosion and massive run-off during melting. More exciting is that the P/Fe ‘spike’ occurred at a time of massive perturbations in stable carbon isotopes ascribed to huge explosions of phytoplankton and organic carbon burial, which would have been permitted by high dissolved phosphate in the oceans. A large increase in primary biological productivity, i.e. photosynthesis, would have boosted oxygen levels; a necessity for the emergence of metazoan life forms soon after the end of ‘Snowball’ Earth conditions. But that begs the question of how glacially ground-up apatite, abundant as it would have been together with vast amounts of other rock debris, came to be dissolved. In today’s oceans crystalline apatite is barely soluble. It seems that apatite’s solubility decreases as temperature rises, and increases with pH – in alkaline conditions. As well as being cold, Neoproterozoic ocean water around the time of the ‘Snowball’ Earths was saturated with carbonate ions that helped thick, almost pure limestones to form globally after each glaciation. That spells alkaline conditions favouring phosphate solution. The authors speculate that global geochemical conditions during the Cryogenian Period (850-635 Ma) may have fostered the origin of the metazoans. Maybe, but their data have a billion-year gap immediately before that Period, and genomic molecular clocks suggest that the root metazoans emerged as much as half a billion years earlier.
See also: Filippelli, G.M. 2010. Phosphorus and the gust of fresh air. Nature, v. 467, p.1052-1053.

Threat to landscape from alien crayfish?

Published on by zooks777Leave a comment
Pacifastacus leniusculus 5
Image via Wikipedia

The stealthy invasion of rivers in Europe by the tasty American signal crayfish Pacifastacus leniusculus poses a threat not only to the indigenous European species Astacus astacus (P. leniusculus carries a fungal infection as well as being formidably armed), but conceivably to the very landscape itself (Johnson, M.F. et al. 2010. Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus). Geomorphology, v. 123, p. 269-278). Johnson and colleagues from the University of Loughborough, UK used captive alien crayfish to model the effects of their bioturbation under controlled laboratory conditions, assessing their activity by the use of millimetre-resolution gravel-surface elevation data generated by a laser altimeter. The sturdy little beasts behave like frenzied bulldozers creating mounds and pits in the gravel substrate, displacing on average about 1.7 kg of gravel every day over an area of 1 m2 thereby completely disrupting the perfectly flat experimental substrate onto which they were introduced in about 3 days. By this activity they render the surface more prone to erosion by flowing water so that greater grain transport ensues; they could effect bother erosion and deposition by increasing transportation of grains. To my knowledge, this is the first experimental study of bioturbation in the context of hydrology. We can expect more now that the technology is available: the burrowers as well as the diggers of the animal world. While the Phanerozoic is best know for having begun with the Cambrian Explosion of multicellular life, a sometimes overlooked attribute is that it coincided with the start of bioturbation. That may well have had a profound effect on sediment transport as the American invader suggests.
See also: Newton, A. 2010. Crayfish at work. Nature Geoscience, v. 3, p. 592

Whizz-bang hypothesis for the Younger Dryas bites the dust

Published on by zooks777Leave a comment

Such has been the urge to leap on the impact theory of Earth system change, that virtually every drastic event recorded in the geological timescale has been linked by someone or other to the effects of bombardment by extraterrestrial objects. The most recent concerns the Younger Dryas and the extinction of the mammoths (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008). The hypothesis stemmed from reports of an association of tiny magnetic spherules, soot and purported nanodiamonds and fullerenes (carbon molecules bonded into ‘geodesic’ spheres) with the onset of the Younger Dryas, the roughly coincident disappearance of Clovis tools and the demise of several large North American mammal species, including mammoths. Regular columnist for Science magazine, Richard Kerr, reports that independent searches for all the evidential materials at the sites where they were said to occur have drawn unrelieved blanks (Kerr, R.A. 2010. Mammoth-killer impact flunks out , Science, v. 329, p. 1140-1141). Nonetheless, the core supporters of the hypothesis are clinging to their guns.

Hard-core continental lithosphere

Published on by zooks777Leave a comment

The oldest and most stable parts of the continents are known as cratons, after the Greek word for strength κράτο (kratos). All the present continents have at least one craton (Africa and South America have 4 each, and Eurasia 6 or 7). Each has remained unaffected by major deformation for a billion years or more, even during continent-to-continent collisions in which they participated. Almost all cratons began to form during the Archaean Eon before 2500 Ma, but most became rigid long after. Several theories have been suggested to account for their durability, one commonly accepted being that somehow the crust ‘ripened’ so that most of the heat-producing radioactive isotopes of U, Th and K were moved by igneous and metamorphic processes to the uppermost crust, along with water; most cratons expose fragments of anhydrous granulites of tonalitic composition. These bear evidence of having formed at the base of the continental crust and have been heavily depleted in “granitophile” trace elements. As a result they cannot undergo partial melting under normal geothermal conditions and where they remain at great depth are much cooler than younger, deep crust. The other dominant feature of cratonic lithosphere is a mantle portion that is anomalously thick (sometimes down to 250 km); in some cases there is little if any sign of asthenosphere beneath such ‘keels’. Research on rocks brought up from the ‘roots’ of cratons by the kimberlite magmas famous for their diamonds points to that deep mantle itself having conferred great rigidity and thus longevity (Peslier, A.H. et al. 2010. Olivine water contents in the continental lithosphere and the longevity of cratons. Nature, v. 467, p. 78-81).

The presence of water in minerals that make up igneous and metamorphic rocks enables them to begin melting at lower temperatures than their dry equivalents, and also to behave in a more plastic fashion under stress. Anne Peslier of NASA in Houston and her US and German colleagues analysed the minerals in ultramafic mantle rocks dragged upwards by kimberlites that punched through the Kaapvaal craton in southern Africa long after it formed. The dominant mantle mineral is olivine (50-80%), generally thought of as anhydrous but typically containing a few hundred parts per million by weight. Olivines in the Kaapvaal mantle xenoliths become drier with increasing depth of their formation (determined from their mineralogy in which garnet is stable at the deepest levels). At depths around 150-250 km low water content in olivine makes it and the mantle itself 20 to 3000 times stronger than the asthenosphere, which protects it from the underlying flow associated with tectonic motions.

How such root zone of continents may have formed has been addressed by two papers on seismic structure beneath the best studied craton; that of the Canadian Shield (Yuan, H. & Romanowicz, B. 2010. Lithospheric layering in the North American craton. Nature, v. 466, p. 1063-1068; Miller, M.S. & Eaton, D.W. 2010. Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL044366). In the first, Yuan and Romanowicz of the Berkeley Seismological Laboratory, California use a form of seismic tomography to map anisotropy in the mantle along transects that cross the ancient core of the North American continent. Their results chart the depth of the base of the lithosphere and also define two layers in the lithospheric mantle. The upper layer (down to 150 km) only occurs beneath the Archaean craton, and the top of the asthenosphere ranges from 100-240 km down: at its deepest beneath the craton. The sub-craton mantle they ascribe to chemical depletion of its upper part during early lithospheric evolution, and later addition of the less chemically evolved deeper layer. Miller and Eaton of the Universities of California USA and Calgary Canada used S-wave data from eight seismic stations extending from WSW to ENE over the western cordillera and the Canadian Shield to the Arctic islands of Canada. Their results show a similar variation in dept of the base of the lithosphere and resolve several roughly eastward-dipping boundaries in the sub-craton lithospheric mantle, which they link to Precambrian volcanic arcs preserved in the upper crust above them; i.e. suggesting that the upper layer in the first paper stems from a major episode of arc accretion that built the Canadian Shield.

Record of rising Holocene sea-level in the tropics

Published on by zooks777Leave a comment

Areas beyond the zones of isostatic depression by ice-loading and recovery during glacial-interglacial cycles passively undergo sea-level fall and inundation. They best record the progress of Holocene ice-sheet melting and sea-level rise since 11.5 ka, especially if they are tectonically stable. The island state of Singapore, 1.5 º north of the Equator, is a near-ideal place for study (Bird, M.I.et al. 2010. Punctuated eustatic sea-level rise in the early mid-Holocene.Geology, v. 38, p. 803-806). The Australian and British geoscientists analysed a core through sediments in a mangrove swamp now just below sea level. The top 14 m penetrated a uniform though laminated sequence of marine muds, calibrated to time by radiocarbon dating of mollusc shells, mainly focused on the period from 9 to 6 ka period that the global oxygen-isotope record of ice volume suggests to have been the main period of final melting after the Younger Dryas.

Sedimentation was very rapid (~1 cm y-1) from  8.5 to 7.8 ka, probably as sea level rose too rapidly for the coast to be protected by mangrove growth.  Then for 400 years it slackened off to ~0.1 cm y-1 to rise again to 0.5 cm y-1 by 6.5 ka. The last date is the time of the mid-Holocene sea level highstand, after which sedimentation rate soon declined to 0.05 cm y-1, when mangroves became established at the site. Stable isotopes of carbon in the core (δ13C) show how the relative input of marine and terrestrial (mainly mangroves) organisms shifted over the period and are a proxy for the distance to the coastline and hence sea level. From 8.5 to 6.5 ka this was erratic from a starting point about 10 m lower than nowadays, showing rapid rises and falls that culminated in a sea level in Singapore about 3 m above present during the mid-Holocene sea level highstand that slowly declined to that of the present.

The team’s findings tally with evidence for the melting record of the North American ice sheet. An interesting aspect is that they also cover the period when rice cultivation in swampy areas of SE Asia got underway (~7.7 ka). Very rapid sedimentation would have encouraged development of the substrate for the highly fertile delta plains that now support the largest regional population densities on Earth. In turn they culminated in a series of early south and east Asian civilisations based on class societies.

 

Antipodean glaciers confirm complementary southern Younger Dryas warming

Published on by zooks777Leave a comment

Studies of air-temperature proxies in cores from the Antarctic ice cap show a roughly mirrored climate record to that found in the Greenland ice. While the Northern Hemisphere underwent a sudden climate collapse into almost full-glacial conditions around 12.9 ka and an equally dramatic warming around 11.7 ka, Antarctica steadily warmed over the same period to reach full interglacial conditions by 11.5. That this climatic inversion reached relatively low southern latitudes is confirmed by a record of the changing size of glaciers on mountains in New Zealand’s South Island (Kaplan, M.R. and 9 others 2010. Glacier retreat in New Zealand during the Younger Dryas stadial. Nature, v. 467, p. 194-197). The US-New Zealand-Norwegian-French partnership used detailed geomorphological mapping, and cosmogenic isotope studies of exposed rock fragments to show that after about 13 ka glaciers retreated by more than a kilometre in the succeeding 1500 years in contrast to the dramatic glacial advances in northern areas such as the Scottish Highlands.

Sabotage in Science

Published on by zooks7771 Comment

Scientists are supposedly objective but a recent case in Michigan USA sheds a worrying light on a dark reality of research. A former post-doctoral researcher at the Ann Arbor campus of the University of Michigan has been found guilty of changing the experimental results of a PhD student who worked in the same lab; the charge was malicious destruction of personal property, which in the USA usually means vandalism. The postdoc claims his otherwise inexplicable actions stemmed from internal pressures and that he intended to slow down the student’s work (Maher, B. 2010. Sabotage. Nature, v. 467, p. 516-518). At first the student believed that she was making mistakes herself, but then realised some unknown person had swapped labels on her samples. When she aired her suspicions she was told she was being paranoid and going through a bad patch in her studies. She persisted despite such resistance, until her supervisor alerted the university’s security officers. They launched an investigation into the student herself! After two interrogations and a lie-detector test, the university police installed cameras in the lab, which led to the culprit being caught red-handed.

Research misconduct is notoriously difficult to apprehend, institutional authorities often balk at clear evidence and end up in what amounts to a whitewash to protect the institution’s integrity. Daniele Fanelli of the University of Edinburgh UK has made a study of malpractice in science, ranging from this kind of willful derailing of a research project to withholding information and vindictive reviews that are rarely considered misconduct. She has found that up to 30% of scientists admit (anonymously) to lesser but still baleful issues, and a staggering 70% say they have witnessed deliberate damage to fellow researchers. This malice that dare not speak its name, even were it to be rarer than Famelli has discovered, is a blight that should be recognised by institutional authorities rather than ignored or actually turned against the complainants.

Crowd Science

Published on by zooks777Leave a comment

Malice and/or mendacity are not the sole ways to get on unfairly. A mild form is somehow to join a team, preferably with a role that involves little actual work. ‘Brownie-points’ in the promotion stakes are guaranteed nowadays by authorship in peer-reviewed journals: senior or sole author is best; next being in a small authors list in a journal that demands an account of the role of each; but even being an also-ran or last of a great many can go nicely on your CV. Does one have to have some je ne sais quoi to be accepted by a team? Well it depends on what the quois might be. Some might say seniority or prestige as that helps the paper to be accepted; others that having the only accessible scientific machine for the topic more or less guarantees a place; but is it possible merely to lurk in the corridor and still get on board?

The vast majority of author lists are surely completely honest, but there is a definite tendency for them to get longer as time goes by. During the days when analysis of lunar rocks from the Apollo Missions was booming a team of geochemists – the Lunatic Asylum – was formed at the California Institute of Technology (incidentally, in 1920 Caltech changed its name from Throop University – after Amos Gager Throop, former Mayor of Pasadena). Its founder and leader was and remains Gerry Wasserburg, and occasionally papers were published under the anonymity of the group, so it is hard to tell just how many of them were involved. The Atlas experiment at the CERN Large Hadron Collider has given rise to a paper authored by 230 individuals from 169 institutions (The ATLAS Collaboration et al. 2008. The ATLAS Experiment at the CERN Large Hadron Collider. Journal of Instrumentation, v. 3, doi: 10.1088/1748-0221/3/08/S08003), but that consortium does not hold the record. As far as I know, the biscuit is taken, for the moment, by Members of the Genetic Investigation of ANthropocentric Traits (GIANT) consortium (Allen, H.L et al. 2010. Hundreds of variants clustered in genomic loci and biological pathways affect human height. Nature, v. 467, p. 832-838) whose title is self-explanatory. Of its 7 pages, 3 are taken up by the names of its 287 authors, their 203 institutions and a not inconsiderable number of funding agencies. At just under 3000 words (not including the names and affiliations of the authors), each author on average has just over 10 words to their name. Interestingly, 10 of the authors (the first 6 and last 4 ) ‘contributed equally to this work’ – how is not specified, and 4 authors are each affiliated with 5 institutions. By comparison, geosciences is definitely little league as regards collaborative ventures, but opportunities there surely are.

 

Whizz-bang hypothesis for the Younger Dryas bites the dust

Published on by zooks777Leave a comment

Such has been the urge to leap on the impact theory of Earth system change, that virtually every drastic event recorded in the geological timescale has been linked by someone or other to the effects of bombardment by extraterrestrial objects. The most recent concerns the Younger Dryas and the extinction of the mammoths (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008). The hypothesis stemmed from reports of an association of tiny magnetic spherules, soot and purported nanodiamonds and fullerenes (carbon molecules bonded into ‘geodesic’ spheres) with the onset of the Younger Dryas, the roughly coincident disappearance of Clovis tools and the demise of several large North American mammal species, including mammoths. Regular columnist for Science magazine, Richard Kerr,  reports that independent searches for all the evidential materials at the sites where they were said to occur have drawn unrelieved blanks (Kerr, R.A. 2010. Mammoth-killer impact flunks out , Science, v. 329, p. 1140-1141). Nonetheless, the core supporters of the hypothesis are clinging to their guns.