Month: August 2006

Anonymous referees

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Anyone who submits their first paper to a journal soon becomes aware of the “peer review” process: probably the single greatest contributor to academic suspicion and anxiety.  Of course, these “peers” fall into two categories: the “esteemed colleague” (helpful); the “witless wonder” (negative, and prone to crushing your paper).  Write a book, a play or an operatic score, and your critics in the media have a name.  You could even find out where they keep their pet rabbit.  They are accountable. Yet, editors of journals claim to have a “duty of confidentiality” towards those referees who opt for anonymity: guess which category most often does. At one time or another, most academics asked for critiques by learned journals only to recommend rejection have succumbed to “taking the veil”.  Equally, there are few researchers who have not suffered a similar fate to one they may have meted out themselves.  Learning by experience is not necessarily a strong point among scientists.  A typical case came to my notice recently, but the identity of one faceless and repugnant referee eventually became clear.  I know him well.  He too had suffered acute stress from a grossly delayed manuscript and the vicious comment of an anonymous referee some years back, yet saw fit to indulge his own spleen when offered a place in the shade: goodness only knows why, but in this case I have my suspicions.

The whole scientific community grows increasingly uneasy about anonymous peer-review, and the abuse that it sometimes makes possible. Examples are deliberate delays by unnamed referees engaged in similar research or related commercial activity, plagiarism, incompetence and the self-indulgence of gratuitously destructive and belittling comment.  It is the near-universal policy of referee anonymity that allows these unwholesome practices to fester and grow.  Most journals give their referees the option of coming out of the closet, or remaining smugly behind its door.  Some assume anonymity, so that a referee has to ask explicitly for their name to be revealed.  Anonymous referees are simply moral cowards, along with editors of the journals that give them a cloak. What do they fear?  Are direct questions about their comments cause for timidity?

Referee malpractice can be removed completely by editors refusing to allow referees to skulk behind anonymity. Now, in the UK at least, it seems possible to challenge this unwholesome editorial prerogative, because of the Freedom of Information Act 2000 (2002 Scotland), which came into force on 1 Jan 2005, and the Data Protection Act 1998.  Resorting to the Acts ought not to be necessary as regards the activities of scholarly journals, yet editors continue to defend the more faceless of their referees.  No doubt there would be a temporary shortage of referees should compulsory “outing” become the norm, but it would remove those who do engage in malpractice.  The most important result would be an increase in objectively constructive comment, which softens the blow of a rejection slip by showing a way forward to authors.   Peer-review should work both ways, and should be seen to be honest.

 

Pliocene climate and a lesson for the near future?

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While most geoscientists use the products of processes that operate today to judge environments of the past, climatologists do the reverse: the past is the key to the present. While the climate record of the last 2.5 Ma is a key to understanding and perhaps even predicting rapid climate shifts during glacial-interglacial periods uncontaminated by human influences, such is the extent to which greenhouse emissions have affected the current climate that we have little idea what the outcomes may be. The possibility of greenhouse warming has become higher than in any previous interglacial epoch. To get even an inkling of what that might set in motion requires looking back to warmer times than the Late Pliocene and Pleistocene, at around 3 to 5 Ma. In the Early Pliocene it is very likely that CO2 in the atmosphere was no more than nowadays. Because the Earth’s geography was little different from the way it is now and the Milankovich forcing was the same too, modelling Early Pliocene climate might seem to result in similar patterns, but it doesn’t (Fedorov, A.V. et al. 2006. The Pliocene paradox (mechanisms for a permanent El Niño). Science, v. 312, p. 1485-1489). Sea level was some 25 metres higher than it is at present and mean global temperature was an extra 3°C, and sea-surface temperatures (from the oxygen isotopes in planktonic foraminifera) were high as well. Despite much the same forcing factors as today, the Pliocene lacked large high-latitude ice caps in Arctic regions. Milankovich-related fluctuations were damped down compared with those of the Pleistocene. Both modelling and geological evidence from the Early Pliocene suggests that Earth’s climate was dominated by a perpetual El Niño in the tropical oceans, because of an inability of cold water to upwell periodically along the western tropical margins of Africa and South America. Quite probably such conditions had persisted for the previous 50 Ma, despite gradual overall cooling.

Fedorov and colleagues point to very different Early Pliocene climates in several regions: Mild winters in central and north-eastern North America; droughts in Indonesia and torrential rains in western North and South America. Overall, it was a much more humid world, and since water vapour is a powerful greenhouse gas warmth and humidity were sustained despite no higher CO2 levels than now. At about 3 Ma, ocean surface waters began to cool, with signs that the alternations associated with El Niño and La Niña in the eastern Pacific began. An explanation for this is the gradual build up of very cold water deep in the ocean as a result of winds from continents cooling ocean surface water at high latitudes and causing it to sink. Without periodic upwellings, warm surface waters and cold deep waters could not mix, so inevitably the interface became shallower. At some critical depth, this thermocline could break surface, transforming both climate patterns and those of ocean currents, eventually to end up as the present tropical climate cyclicity which is connected with  other climate features of the Great Ice Age.

Fedorov et al  speculate that only a small descent of the ocean thermocline – a matter of a few tens of metres – could re-establish Pliocene conditions.  That might occur because of continued anthropogenic warming, and the ‘flip’ might be as quick as a few decades to centuries.

Accretion and core formation reviewed

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Painstaking work on meteorites and their re-evaluation has only a small, non-specialist readership, but now and again developments in the science and its bearing on how the Solar System and its planets formed need a review. The latest of these (Wood, B.J. et al. 2006. Accretion of the Earth and segregation of its core. Nature, v. 441, p. 825-833) doesn’t deviate much from generally accepted ideas, except in detail. For a long while it has seemed inescapable that gravitational potential energy accumulated from accretion of mass, together with energy released by decaying short-lived isotopes formed by a supernova near the dust cloud that gave birth to the Solar System would have led to hot protoplanets. So core formation by segregation of dense immiscible metal and sulphide melts was likely to have been sooner rather than later – such melts form at lower temperatures than do those made of silicates.

The daughter isotope (182W) of one short-lived isotope (182Hf) is especially revealing in both meteorites and the Earth. Hafnium favours entry into silicates while tungsten has an affinity for metallic iron; they are siderophile. So, when metallic melts form in a silicate body the Hf/W ratio increases in the silicates. If that segregation occurs before most 182Hf has decayed – within about 45 Ma – then the silicate part will express an excess of 182W while metals have a deficiency. In the case of metallic meteorites, 182W is so low as to indicate segregation of the metal from silicate within less than 5 Ma of the ultimate origin of the Solar System. Inevitably, Earth would have incorporated some of these early-formed metallic parts during its accretion. Tungsten isotopes from terrestrial rocks, however, suggest that core formation lasted about ten times longer, and imply that this early metal re-mixed with silicate in the mantle during accretion, and formation of the core was a secondary product of heating of the growing planet. The mantle has an excess of siderophile elements, which poses a problem. There are three possibilities: core formation was never completed, some of these elements remaining locked in silicate; it took place while overall chemical conditions changed from reducing to oxidizing, so that the most siderophile ended up in the core during the reduced phase then less siderophile elements progressively favoured silicate entry as conditions became oxidising; as the Earth grew the pressure under which segregation of core materials increased. The third scenario invokes a deep ‘ocean’ of magma through which droplets of metal fell, equilibrating with silicate melt and then forming a pond on the ‘ocean’ floor, ultimately to descend as large masses.

Wood et al. examine these three scenarios in the light of recent data and planetary modelling, suggesting that the second was the most likely by a process of ‘self-oxidation’ as its size increased, perhaps linked with the formation of perovskite in the deep mantle once a limiting radius had been achieved. Such a heterogeneous accretion and core segregation would explain the disparity between estimates of the timing of the core from tungsten and lead isotopes (~12 and ~28 Ma respectively)   They also revisit the oddly low density of the liquid outer core – about 8% less than expected of an iron-nickel alloy, ascribing it to a mixture of the low-atomic weight elements, silicon, sulphur, carbon and hydrogen, with an unknown proportion of oxygen.

Evidence for strain build up along faults in southern California

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In the centenary year of the 1906 San Francisco earthquake a lot of attention has been paid to the northern part of the infamous San Andreas Fault. That avoids the fact the its southerly extension to the south-east of Los Angeles has not ruptured in a devastating way for at least 250 years. Faults break after protracted build-up of elastic strain. Such strains are detectable using data from spaceborne radar systems. These have been available since 1992 from the European Space Agency ERS-1 and ERS-2 satellites. A sequence of data sets provides information about the annual rate of deformation (Fialko Y. 2006. Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system. Nature, v. 441, p. 968-971). Fialko shows that the parallel San Andreas and San Jacinto faults near the Salton Sea are building up strain at about 3 cm per year, so that about 7 to 10 metres will have accumulated since the last major earthquake in that part of the system. This exceeds the largest known seismic movement on the system, thereby suggesting that Los Angeles is likely to experience a ‘big one’ shortly.

Supervolcanoes

Outside of a major meteorite impact, the greatest danger posed by geological processes is a monster volcanic eruption. As well as the close-by effects of massive debris avalanches and ash falls, explosive eruptions blast sulphur gases into the stratosphere where they reside for a long time as sulphuric acid aerosols. Clouds of these tiny particles reflect a proportion of solar radiation back into space and so cause global cooling. The eruptions of Pinatubo and Krakatau in recent historic times did just that, as have several others with more devastating global effects such as famine. Yet these are tiny compared with eruptions known from the recent geological past that are marked by ash deposits over vast areas. About 71 ka ago, Toba in Indonesia  blasted out a 30 by 100 km caldera and its ash extends across much of south Asia and surrounding ocean floors. Genetic evidence from human Y-chromosomes suggests a massive decline in human numbers at the time, to create an evolutionary bottleneck. This near-extinction may have been connected in some way to eruption of the Toba supervolcano. Such events are a more likely risk than impacts, and a recent review of research into them highlights those that are well-known (Bindeman, I.N. 2006. The secrets of supervolcanoes. Scientific American, v. 294 (June 2006 issue), p. 26-33). The western USA has two potential threats: calderas in Yellowstone National Park and Long Valley. Between 760 and 640 ka both exploded to blanket the whole southern USA and northern Mexico with around 1000 cubic kilometres of ash. Bindeman’s own research sheds light on the details of magma evolution during such eruptions using isotopic signals in zircons contained within ash deposits..

Out of Africa and back again?

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Humans left Africa with a meagre tool kits at a remarkably early date, possibly around 1.9 Ma from finds of primitive stone tools in Pakistan and Central China, and certainly before 1.7 Ma in the case of the now celebrated human remains at Dmanisi in Georgia and in Java. Around 1.7 Ma sites with evidence for human occupation extend from southern to north-western Africa and over 2/3 of the width of southern Eurasia. Despite the increased chances of preservation in later times, such a wide-ranging expansion seems not to have recurred until the fully modern human diaspora from Africa that began around 70 to 100 ka. Fossil evidence suggests that descendants of these earliest known migrants thrived until as recently as 20 ka in south-east Asia, and perhaps longer, if tiny Homo floresiensis prove to be other than symptomatic of congenital dwarfism. They represent a puzzle, and absence of evidence has deterred palaeoanthropologists from sticking out their necks, until a recent review of possibilities (Dennell, R. & Roebroeks, W. 2005. An Asian perspective on early human dispersal from Africa. Nature, v. 438, p. 1099-1104).

For a long time all human remains dated between 1 and 1.9 Ma were ascribed to H. erectus, whose type specimen hails from Java, not Africa. Anatomical re-evaluation of specimens from Africa, notably the famous, 1.6 Ma old Turkana Boy from Kenya, shows that they are sufficiently different from Eugéne Dubois’s Javan H. erectus type specimen to warrant a different species name – ‘Action Man’ or H. ergaster. The Dmanisi humans have close affinities, but are older. Therein lies one puzzle: apart from the very much more primitive (and very rare) H. habilis of east Africa, there is no obvious African candidate as an ancestor for H. ergaster there. Dennell and Roebroeks speculate that they migrated back to Africa after evolving there from some unknown earlier species. Another puzzle centres on the tools carried by the early migrants from Africa.

Simple chopper and rough flake tools first appear in north-east Ethiopia at 2.6 Ma, but with no clear sign of who made them. The first discovery of the earliest known tool kit was at Olduvai Gorge in Tanzania – hence their name, Oldowan. They are associated with remains of the earliest known human species H. habilis, but date only to 1.8 Ma. Since Oldowan tool use is now known to have extended over a huge range of Africa and Eurasia at that time, the original emigrants must have carried the culture with them sometime after its first appearance in Ethiopia at 2.6 Ma. The emblematic artefact of ‘H. erectus’ is the beautiful pear-shaped biface axe, yet it first appeared at 1.5 Ma in Africa, and did not make an appearance outside the continent until about 700 ka and never made it to east Asia until carried their by fully modern humans: it was an African invention. Oddly, these highly crafted tools are often found with little sign of wear, and indeed opinion about what they were for is divided.

The great problem in palaeoanthropology is absence of fossils, which is hardly surprising. Dennell and Roebroeks comment that most Late Pliocene to Early Pleistocene terrestrial faunas are nearly always of large, robust animals, and even they are uncommon. The ravages of erosion and transportation also make it difficult to date finds of stone tools, as they may have been mixed with younger dateable materials. With confidence, they rely on the old adage (not well liked by the Popperian school of scientific methodology) that, ‘Absence of evidence is not evidence of absence’, and also that the earliest evidence for a new migrant is bound to be younger than its first presence. They look to the palaeoecological record of the period, which suggests a vast extent of open savannah covering much of Africa and southern Asia in the period when the climatic effects of glacial-interglacial cycles had not gripped low latitudes to create the desert barriers of later Pleistocene times. For species adapted to savannah life there was little to prevent their very wide migration, indeed simple diffusion would have moved them across the entire savannah range. Once thought to be confined to the East African Rift, australopithecines have turned up as far afield as modern Chad, 2500 km away, and as long ago as 3.5 Ma. If such diminutive creatures with no tools could diffuse so far, then what might have been the geographic limitation to the earliest tool users? Moreover, diffusion has no direction in the area that presents its possibility: movement could have been back and forth. An intriguing point made by Dennell and Roebroeks is that climatic instability first appeared around 2.6 Ma in Central China, so any emigrants moving north would have been subject to greater evolutionary-selective pressures for longer. Homo ergaster might have evolved in Asia and returned to Africa in the face of worsening conditions. This approach raises as many plausible hypotheses as a stick can be poked at, and should re-vitalise palaeoanthropological research outside Africa as a means of testing them.

Dee also: Kohn, M. 2006. Made in Savannahstan. New Scientist, v. 191 (1 July 2006 issue), p. 34-39.

Detection of rifting due to dyke emplacement

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The Afar Depression of NE Africa is a zone of complex continental rifting and nascent formation of new ocean floor that has been developing since the Late Oligocene, where the Red Sea, Gulf of Aden and Ethiopian rifts meet. Averaged out, the extension is at around 16 mm per year. In September and October 2005 small seismic events spread along about 60 km of a discrete segment of the Afar rifting system. Analysis of the vicinity of these earthquakes, using satellite-radar interferometry revealed an astonishing 8 m of extension in little more than a week (Wright, T.J. et al. 2006. Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature, v. 442, p. 291-294). This could not be accounted for by extensional faulting alone, indeed that would only add up to less than 10% of the motion. It seems likely that sideways injection of around 2.5 km3 of magma was responsible, forming a dyke extending from 2 to 9 km deep. Surface volcanism was barely noticeable, the event being represented by a small puff of felsic ash from a minor volcano while the dyke itself is twice the volume of the 1980 eruption of Mount St Helens

Near-pristine traces of life before Earth’s surface became oxidising

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Only around 2.2 Ga did the atmosphere contain sufficient oxygen to oxidise iron(II) to iron(III) and leave its trace in red soils and terrestrial sedimentary rocks. That opened the way for the emergence and evolution of the Eucaryan domain of organisms, most of which depend on oxygen. For their predecessors, the prokaryote Bacteria and Archaea, oxygen would have been intensely toxic, especially for those which used anoxygenic forms of metabolism. Almost certainly oxygen was released for more than a billion years before the Great Oxidation Event, by blue-green bacteria, only to be mopped up by oxidation of abundant iron(II) ions dissolved in sea water. Getting an idea of the diversity of pre-2.2 Ga life is possible by examining the organic chemicals produced when they decayed under anoxic conditions, i.e. from oil and kerogen. Unfortunately, the great age of their host rocks has resulted in many Precambrian sediments having been heated and metamorphosed, so that different biomarkers break down into less distinctive compounds. There are, however, sediments that may have remained more or less unaffected, and one sequence in the Canadian Shield has yielded astonishing results (Dutkiewicz, A. et al. 2006. Biomarkers from Huronian oil-bearing inclusions: An uncontaminated record of life before the Great Oxidation Event. Geology, v. 34, p. 437-440).

The sediments are conglomerates rich in uranium, having been deposited under reducing conditions that helps precipitate uranium from solution, and have been mined extensively in the Elliot Lake area of Ontario. Oil seems to have entered fluid inclusions in quartz that cemented the conglomerates, shortly after the conglomerates were deposited at about 2.45 Ga. The oil contains a host of complex organic compounds that have never been degraded by heating. Some can be linked to blue-green bacteria, which undoubtedly created of oxygen continuously. That they gave rise locally to favourable conditions for oxygen-using organisms is clear from other biomarkers. Those are steranes that are derived by breakdown of sterols, which in turn are only known to be created by the enzymes exclusive to Eucaryan metabolism. Steranes have been found in even older sediments, but they were back shales that could easily have been contaminated by much younger organic materials seeping through the host rock. Oil in fluid inclusion within diagenetic minerals is far less likely to have been contaminated, so the Elliot Lake samples define a minimum age for the emergence of the Eucarya far earlier than the appearance of actual microfossils that show the distinctive cell nucleus that defines the domain Eukarya.

Precambrian bonanza for palaeoembryologists

Signs of relatedness among groups of organisms often show up well during their early growth as embryos, so their fossils in very old rocks are of great use in establishing when different groups emerged (see Ancient baby penis worm hits the news in EPN February 2004 issue). A deposit containing possible embryos of deutorostomes (see Age range of early fossil treasure trove, in EPN March 2005 issue), in which the first orifice to emerge during embryonic development is the anus, is of considerable interest. Nowadays, the group contains animals with mirror symmetry (bilaterians), including the vertebrates. First reports of fossil embryos from the 580 Ma old Doushantuo Formation of southern China in 2004 drew fire from palaeontologists who preferred to believe that the smooth almost spherical objects, like the fictitious life forms in a supposedly Martian meteorite, were probably oolith-like mineral growths. Undeterred, their finders have extracted yet more from the exposures (Chen, J-Y. and 12 others 2006. Phosphatized polar lobe-forming embryos from the Precambrian of southwest China. Science, v. 312, p. 1644-1646). They demonstrate clearly that the objects do show lobes in an early stage of development that break the embryos initial symmetry so that different kinds of tissue can develop to form adults. The find matches well with evidence from the genes of modern bilaterians that the basic branching of the Animal Kingdom occurred well before the Cambrian Explosion of shelly fossils. Since more or less all modern phyla are represented by Cambrian fossils, that is not surprising.

Pocket sauropods

The largest animals to roam the land were vegetarian dinosaurs of the sauropod group. The biggest reached a length of more than 30 metres, and were commensurately tall. These giants permeate our perception of Mesozoic life on the continents, along with their monster predators. Now, children made nervous by such titanic creatures (and I was definitely one of them) can be reassured that there were ones that were not so crushingly big (Sander, P.M. et al. 2006. Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur. Nature, v. 441, p. 739-741). A near-complete skeleton of a sauropod that was only 6 metres long turned up in Lower Saxony in Germany, along with other remains suggesting individuals as small as 1.7 m. Europasaurus was first thought to be a juvenile of a much larger species, but Sander et al. developed means of microscopic bone analysis that clearly show fully mature bone growth. In the Late Jurassic central Germany was covered by sea, except for a number of large islands. The most likely explanation for such a tiny species is that it adapted to island life in much the same way as other, more recent mammals did, such as pigmy elephants and hippos on many islands in the Mediterranean and the Indonesian archipelago.

Hydrogen isotopes test uplift hypotheses

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When Cenozoic mountain belts and high plateaux began to rise and be eroded has become a disputed topic about most of them, such as the Himalaya and Tibetan Plateau, the North American Western Cordillera, the Andes and the Ethiopian Plateau. A host of techniques have been used, including plant-leaf stomatal indices, the bubbles in lavas, cosmogenic isotopes and various stratigraphic approaches. For the Tibetan Plateau (see When did Tibet rise? and Tibetan uplift: looking a gift horse in the mouth in the March and May 2006 issues of EPN) opinion is polarised: as soon as India collided with Asia, around 40-50 Ma ago, due to crustal thickening; as a result of a slab of lithosphere delaminating from the region as recently as the Late Miocene (8-10 Ma), when the crust rose gravitationally. There is support for both hypotheses. Much the same divergence of opinion applies to the Sierra Nevada of the Western USA, which may have been a major feature throughout the Cenozoic, or subject to delamination in the Pliocene (3 to 5 Ma ago). In its case, a new way of estimating topographic elevation in the past may resolve the disputation (Mulch, A. et al. 2006. Hydrogen isotopes in Eocene river gravels and paleoelevation of the Sierra Nevada. Science, v. 313, p. 87-89).

The Stanford University group led by Andreas Mulch have focused on the way in which the proportion of deuterium (2H) in rainwater changes as clouds rise over high areas. This is known quite precisely from modern hydrological studies. Of course, you cannot find ancient rainwater ponded in the place where it once fell, but that rain does find its way into clay minerals during weathering. The western flank of the Sierra Nevada is mantled by extensive Eocene graves deposited by rivers that drained the area now occupied by the mountains. Handily, those gravels were the main target for the 1849 California Gold Rush and subsequent alluvial gold mining, so they have been well exposed by the prospectors throughout the Sierra Nevada. At high elevations they are preserved in river terraces, so it is possible to trace roughly the ancient drainage courses and sample a range of modern elevations. The hydrogen isotope data from kaolinite in cobbles of weathered granitic rocks are extremely interesting. They show that the Eocene rainwater that entered kaolinite molecules at different modern heights had fallen at a very similar range some 50 Ma ago. There seems little doubt that the Sierra Nevada had risen more than 2 km before the Eocene