The use of radiometric dating based on the decaying away of radioactive 14C is the most useful technique for building sensible archaeological and climatic records over the last 50 thousand years. However, this radiocarbon is produced from 14N by cosmic rays in the upper atmosphere, and their flux varies with time. Consequently, the proportion of 14C in the environment varied in the past, and a radiocarbon age is not necessarily an age in calendar years “before present” (BP). Even BP is confusing, because it isn’t “before now” but before 1950 when the first hydrogen bombs produced 14C. The outcome is one of some confusion. If dates were recorded in calendar years, whether BP or AD/BC everything would be clear. But they aren’t. Many authors give their dating as either 14C ages (BP) or calendar years (BP), and the two can be very different. For instance, the date when the Younger Dryas glacial pulse began is 1000 calendar years older than its 14C age. One reason for the dichotomy is that no agreed conversion existed until about 1998, particularly for the time before which annual growth rings in trees can be built into an unambiguous record, using modern trees and those preserved in ancient timber. Bristlecone pines and other long-lived trees first gave an accepted conversion factor that went back around 6000 years. That has been extended to about 26 ka by dating annually layered corals, stalagmites (speleothem) and sediments. A way of going even further back is correlating large, world-wide events between their appearance in a record such as a marine sediment core, dated using 14C, and their appearance in a Greenland ice core, whose annual layering gives a calendar age. However, further back in time less radioactive 14C remains to be measured and contamination by later carbon introduced by percolating water blurs the dating. In September 2003 the 18th International Radiocarbon Conference tried to clear the air (Bard E. et al. 2004. A better radiocarbon clock. Science, v. 303, p. 178-179). The latest “official” calibration curve, (INTCAL04) goes back to 26 ka. But beyond that there are 3 quite different candidates for calibration, the sea-floor sediment-ice core curve, one based on annually layered lake sediments in Japan, and one from speleothem in a submerged cave in the Bahamas. For a vitally important archaeological find, such as the paintings in the Chauvet cave in France, the 14C date of 31ka could range from 33 to 38 ka in calendar years. Dates for fossil occurrences of Neanderthal and the first fully human Europeans could overlap or be so different that neither had an influence on the other. Everyone hopes that the sea-floor sediment-ice core curve can be validated by new results, thereby giving a common age framework to all dateable materials.
Ancient baby penis worm hits the news
China is proving to be the repository of a vast wealth of well-preserved ancient faunas, thanks to several lagerstãtten, the most famous being that which hosts early ancestral birds that show links with dinosaurs. But Chinese strata with exceptional preservation also occur in Cambrian sediments, close enough to the first appearance of preservable life forms to make any out-of-the-ordinary finds especially revealing. Ten years ago many palaeontologists scoffed at reports of trilobite embryos being unearthed in southern China, yet there has been a steady flow of material that opens up what might be called “palaeoembryology”. Being able to describe and analyse an entire life cycle of an organism is vital in studies of the inter-relatedness of living metazoans. The lack of data on fossil life histories to some extent thwarts attempts to place extinct animals accurately within an evolutionary scheme. Palaeontologists from the University of Bristol and Peking University have therefore put such studies on the map through finding exquisitely preserved Cambrian embryos of what is now a rare and bizarre animal group, but one thought to lie at the root of the explosive radiation of the arthropods, which includes insects (Dong, X. et al. 2004. Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. Nature, v. 427, p. 237-240). They are in eggs, and therefore had yet to hatch and develop further; true embryos, from their initial development to the last stage before emerging. They are Scalidophores, which include today the individual phylla of Priapulida, Kynorhyncha and Loricefera, all marine worm-like animals (the priapulids are the notorious, and fortunately rare, penis worms from their evocative contours). Interestingly. the embryonic stages clearly indicate direct development from egg to adult, rather than going through the intermediary larval stage that characterises most insects and other invertebrates. Such direct development seems to be a primitive evolutionary stage from which more complex life-histories developed later. Penis worms are well known to grow hugely once hatched, so the search is on for a fully grown adult from the Cambrian of southern China, as well as early developmental stages of other animal groups..
See also: Budd, G.E. 2004. Lost children of the Cambrian. Nature, v. 427, p. 205-206.
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National Geochemical Survey of the USA
The US Geological Survey has made publicly available a large repository of geochemical data (63 of the 91 naturally occurring elements) that it has acquired through a continuing nation-wide survey of stream sediments (available at http://tin.er.usgs.gov/geochem/doc/home.htm). The data coverage is incomplete and involves several generations of previous surveys. The most revealing stream sediment surveys involve collection of panned sediment samples in every small stream that has no upstream tributary, but that is a daunting task for such a vast area as the USA. That method allows the analyses to be treated as accurate representations of stream sediment composition in upstream catchments around 1 x 1 km in size. The USGS data are a mixed bunch, some dating from the National Uranium Resource Evaluation (NURE) of the 1970s when there was a scramble to find new uranium ore bodies. The NURE survey involved a sample density based on a 17 x 17 km grid, and made no distinction between stream order. The latest USGS survey is based on sample collection that uses 10 x 10 km grids drawn in the UTM co-ordinate system. Each 10 x 10 km cell is divided into four quadrants, and one is selected at random for sampling. In that one small stream selected at random is chosen for analysis. The data set is too coarse and too varied to create meaningful gridded interpolations that can be displayed as continuous tone images, unlike comparable geochemical atlases based on systematic, small-stream sampling, such as that developed for commercial leasing by the British Geological Survey. The NGS data will be a useful resource for scanning broad geochemical features of the country, such as for high levels of potentially toxic elements in water, bearing in mind that the analyses are of solid minerals not the water itself.
“Plumeology” site
The last issue of EPN showed that the debate over mantle plumes, their sources, and even their existence is hotting up (see Geoscience consensus challenged in EPN January 2004). However that pans out, vast areas of continental and submarine flood basalts compel geoscientists to ponder over them, the more so because they represent events never witnessed by humans and are therefore unimaginable. Now they have their own website (http://www.mantleplumes.org/) that has been compiled by Gillian Foulger of Durham University. It is an impressive and highly useful resource, the outstanding feature being pages on most aspects of large igneous provinces written by experts who are also excellent communicators. There is even a linked site at the Geological Society that hosts discussion on the Great Plume Debate, as well as a letters page, links and up to date news. For information, without unnecessary frills, this is the place to go, especially if you have to write an essay!
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Influence of continental weathering on climate boosted
Since the resurrection of Chamberlin’s idea that the rate of chemical weathering of continental crust helps regulate atmospheric CO2 by Maureen Raymo, the hypothesis has not yet been supported by convincing geochemical evidence. There is such a lag between changes in ocean chemistry and evidence for global climate change, that correlations are flimsy. The need is for a proxy for weathering of the land surface that resides in seawater for a geologically very short period. Such an element is osmium (Os), which passes from river water through the oceans to sea-floor sediments in about 25 thousand years, so changes in its abundance in sediments ought to match the pace of any climatic shifts. In principle, there are two main sources for elements in seawater, from sea-floor hydrothermal alteration of oceanic crust, and from continental weathering. The first can be considered to be more or less constant, except on time scales of tens of million years. Continental weathering is a response to climate change, and keeps pace with it. Researchers at the UK Open University and the University of Köln in Germany analysed samples for osmium and carbon isotopes through a sequence of Jurassic mudstones on the NE coast of England (Cohen, A.S. et al. 2004. Osmium isotope evidence for the regulation of atmospheric CO2 by continental weathering. Geology, v. 32, p. 157-160). The carbon isotopes show a sudden drop in d13C within a very hydrocarbon-rich unit famous for it contribution of jet (oil-rich lignite) to Victorian funereal jewellery. This negative excursion is recognisable world-wide at around 180 Ma. The most likely explanation is a monstrous blurt of methane from destabilised gas hydrate on the Jurassic sea floor (see Methane hydrate – more evidence for the ‘greenhouse’ time bomb, August 2000 issue of EPN). The Jet Rock of the Whitby coast therefore preserves a nice example of sudden climatic change, and by the end of its deposition carbon isotopes returned to Jurassic background values. Methane, a powerful “greenhouse” gas, is rapidly oxidised to CO2 in the atmosphere, so reducing its initial warming effect, but climate would have been hotter for some time afterwards until the excess CO2 was drawn down somehow. Interestingly, the Jet Rock also shows a sudden leap in the abundance of 187Os, reflected in the 187Os/186Os ratio of the samples, and an upward step in the value of the 87Sr/86Sr ratio – one of the fastest rises known. The latter is generally assigned to an increase in continental weathering, since continental crust contains more radiogenic 87Sr than does oceanic crust. The implication of the osmium-isotopic shift is odd; it requires an increase in the rate of continental weathering by 4 to 8 times that in the preceding period. That is a vast change, even if it only lasted for a short period, but it tallies with what is known about the temperature dependence of the dissolved loads of rivers in more recent times. If the osmium isotope excursion truly reflects massive continental weathering, then it is possible to calculate the drawdown of the excess CO2 in the atmosphere from a commensurate flux of calcium and magnesium ions from the continents, that would eventually form marine carbonates. The authors estimate a mere 37-123 ka to get rid of it. Yet continent-derived radiogenic 87Sr remained high for much longer, and the authors’ arguments become tricky. One interesting aside is that, unlike today, more groundwater found its way to the oceans than surface run-off during the Jurassic, perhaps 6 times more. It is easy to look on weathering as what happens at the interface between rocks and the weather; the land surface. Not so. A great deal of chemistry that releases soluble ions goes on in the subsurface, above and below the water table. It is by no means as simple as reactions between carbonic acid in rainwater and silicate minerals. Weathering is the product of hydrogen ions’ (whatever their source) effects on silicates. Bacteria are extremely important actors in modifying pH below the surface, for example the sulphate-sulphide reducers, and the oxidative dissolution of sulphides produces sulphuric acid. Even more interesting for the chemistry of groundwater is the curious role of iron hydroxide. Under oxidising conditions it adsorbs many elements from solution, including platinum-group elements, such as osmium. Should conditions become reducing, dissolution of goethite skins on sedimentary grains releases the accumulated elements. A warming trend almost inevitably results in increased precipitation, and rising water tables. It also should boost biological productivity on land and an increase in the amount of buried organic matter, which create reducing conditions in groundwater.
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Protecting your intellectual property
Long ago, most students entered research by thinking up their own project, albeit with advice from potential supervisors. That is rarely possible today, for many reasons. Instead, gifted students are recruited to research topics proposed to funding agencies by established scientists. More often than not, such projects slot into an overall strategy centred on an academic’s career or the ambitions of a research group. There are advantages in having the sometimes undivided attention of a “boss”, a structured approach to work within a broader framework, access to a group’s equipment and funding, and support from several co-thinkers. With the old style, there were risks in “ploughing a lone furrow”, such as abandonment by a disenchanted supervisor (the enchanted ones could be even more worrying). The single most important advantage of designing your own project, hard and risky as that might be, was one of possession from the outset. Such responsibility develops qualities that are otherwise not easy to get: independence of thought and action, time-management, resourcefulness, an ability to argue your case, and self-discipline – if you can really “hack” it. Except for the indolent and irrecoverably stupid, most people can, given some knowledge of where their subject is going and thesine qua non of curiosity. In those “old days”, the risks were more than offset by the advantage of ownership, and it was rare for postgraduates not to be successful, and the majority gained their doctorates within three years. Today, up to a third of enrolled graduate students withdraw or fail their degrees, and hardly any complete inside this reasonable period.
Funding agencies now demand guarantees that their outlay bears fruit. They increasingly direct lines of research, so that studentships follow previous funding. The funders are more accountable, and by the iron logic of the marketplace so too must be the recipients. The upshot is continual assessment of research performance by departments, the creation of “centres of excellence”, and the crushing of departments that do not measure up to an amoebic growth of criteria and guidelines. So, for anyone keen on testing their abilities to the limit and following their curiosity, the options are increasingly limited. Even if you have independent means, it is now a very rare department that encourages self-motivated research by students, or even by its established staff. In truth, most academics find it hard to be independent, because they no longer have the security that once guaranteed freedom of thought, action and expression. In Britain, if an academic began their career or earned promotion after 20 November 1987, they can be dismissed solely on grounds of redundancy, rather than “with good cause”, which was the rock on which tenure used to be based. “Gross moral turpitude” was, I believe, the operative and infinitely more expressive phrase in US institutions. So for your average supervisor the world has turned upside down. Now it’s a case of “publish or perish”, larded with citation and impact records, and bringing cash into your institution to boost its research assessment. There are very few academics with the energy, imagination, brass neck and wit to jump through all these hoops and remain sanely independent. So we see a growth of hidden but nonetheless unwholesome vices adopted by some to survive and prosper in this deranged environment. There are many victims, but the new researcher is most at risk. During the festive season it is customary to give and receive advice, as well as greetings. Here is some that concerns the vice that dare not speak its name –plagiarism – in the form of a bestiary to help you memorise potentially risky people.
- Chameleons Check out potential supervisors. The Science Citation Index will reveal their record of sole or senior authorship of papers (notreviews). If they are what they claim to be, that will dominate. Relative to that, how many times does their name appear within multi-author papers, of which they are not senior author? If the latter dominates, their reputation probably rests on offering technical facilities that they control, or the research talents of other people. You may find individuals who have a short publication list of either kind. They are either at the start of their career, or beyond all human help (except perhaps your own).
- Beavers Never let anyone else do any work for you, unless they are a kindly technician (who then deserves at least an acknowledgement). Where possible, keep your research materials under your personal control – in some institutions burial is a useful tactic.
- Curlews Be suspicious of a supervisor who shares your findings with the rest of a team; either you do that yourself or not at all.
- Moles Although communication with others is an essential aspect of research, until you are ready to submit a paper for peer review, do not reveal all in seminars and conferences. Pay particular attention to your posters. At every conference you will see people photographing them, whom you can safely assume are after your ideas.
- Hamsters Beware the friendly soul offering, without being asked, to read your first draft of a paper. Instead, plead with the most curmudgeonly academic around, the one who hammers your every utterance, for he or she will probably be honest.
- Tapeworms Do not allow your supervisor to routinely add their name or others in a research team to your papers. Authorship is not based on advice, basic training in research techniques or discussion of your work. That is your supervisor’s duty of care, and a good one should give far more than they take. Acknowledgements are the place to express gratitude for such assistance. Authors have to do real work, both analytical and intellectual, to deserve a place in the list.
- Squirrels Insist that your supervisor lets you read all drafts of their papers that bear on your own field, to check that your findings are not included, as well as to learn. If your work appears, you have a right to authorship.
- Weasels Be aware of the relationships among academics and post-docs in your department, and theirs with others in outside institutions. Keep an eye on “networking”, which often involves mutual sharing of information as well as gossip, particularly if joint bids for funding are in the offing.
- Ravens It is easy to be pressured overtly and subtly, particularly in a large research group. That may be beneficial, but can be to get you to toe the “party line”.
10. Wolverines Never tolerate anything that seems like plagiarism, manipulation, obstruction, exploitation, bullying or harassment. Best to confront politely yet firmly the person responsible, but that is not easy. Finding someone who can help is not easy either. Your institution may well have a policy of pastoral care based on designated individuals, who are deemed to be disinterested and trustworthy. In the real world there is a culture of protecting long-term colleagues, which extends throughout a university; you are transitory… In case of difficulty, ask to change your pastoral advisor. Other students of longer standing may know who is straight, or have similar experiences. Whatever, it is essential that you get honest support to resolve such problems. One useful tactic is to air your grievances as accurately as possible in writing, with a copy to someone that you can trust.
11. Diverse enchanted beasts The most difficult obstacle to ownership can be, oddly, the genuinely honest, kindly and enthusiastic supervisor. Because of their greater experience and breadth of knowledge, your work can easily become their obsession, usually because of their frustration with your progress. They will not steal your thunder consciously, but can easily end up driving you rather than the other way round. If you want to become their creature, fine. If not, then you have battles ahead, but they will serve both of you well!
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Recent snowfall on Mars
Evidence from the neutron detector on Mars Odyssey suggested the possible existence of subsurface water on Mars (Water on Mars, August 2002 Earth Pages News). I reluctantly succumbed to all the hype about what is implied by that, the more so when reports came in of dendritic drainages revealed by high-resolution elevation data (Case for Martian rainfall strengthens in October 2003 issue of EPN). In planetary exploration, including remote sensing of the Earth’s surface features, progressive improvement in resolution generally reveals novelty. The Mars Orbiter Camera, deployed by the Mars Global Surveyor mission has a resolution from 15 down to 2 metres. For the Earth, you can get 15 m images freely from the ASTER programme, but to match the 2 m images would be very costly. Given a broadband or better connection you can download the lot for Mars (http://pds-imaging.jpl.nasa.gov/atlas/). It is this resource that scientists from Brown and Boston Universities in the USA and the Kharkov National University of the Ukraine have used to reveal the latest paradigm buster from the Red Planet (Head, J.W. et al. 2003. Recent ice ages on Mars. Nature, v. 426, p. 797-802).
James Head and his colleagues focused on the smooth terrains, or mantles, which drape over older deposits above 30º latitude on both Martian hemispheres, especially where water had been indicated by the Mars Odyssey neutron detector. They were looking for signs of what on Earth would be regarded as periglacial features, formed by the growth and melting of subsurface ice. They found lots, including signs of flowing ice-bound debris, but they do not show them in the Article, which deals with the implications of their findings. An important conclusion is that at least some of the mantle may have formed by what could be described as very dirty snow – a mixture of ice and wind blown dust. Judging the age of the deposits directly depends on the standard stratigraphic method for all planets other than the Earth and Moon, their relationship to signs of impacts. There are very few fresh craters in the mantle, but many that have been “blurred” by it. Head et al. suggest that the mantle dates to at most 10 Ma. They resort to modelling climate shifts on Mars from its orbital and rotational history. Its rotational axis undergoes the greatest obliquity shifts of any planet, from about 15 to 35º over a 124,000-year cycle (unlike Earth’s tilt, which slowly rocks through a range of only 4 degrees thanks to the stabilising tuggings of our large Moon). At high obliquity, the polar caps probably evaporate. loading the atmosphere with water vapour, so unlike the Earth it is global warming that induces low-latitude ice accumulation. It is this modelling that encouraged the authors to suggest an ice age between 2 million and 400 thousand years ago.
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Deep-sea drilling project financed Liberian carnage
Despite the common knowledge of rapidly deteriorating conditions for civilians in Liberia for the last 10 years or so, the Joint Oceanographic Institutions’ drilling vessel Resolution and its predecessors continues to this day to be registered under a Liberian flag of convenience. Shipping registrations form a major part of Liberia’s foreign earnings, and have been used for purchase of arms that have been used on its population, and quite possibly on that of Sierra Leone. Flags of convenience allow ship owners to avoid taxation and internationally agreed regulations for the safety and working conditions of its crew. So, the International Ocean Drilling Program and NSF which funds it are in an awkward position. The whole venture is privatised, NSF funding JOI, which in turn co-owns the famous vessel with Transocean, the world’s largest offshore drilling company. ODP, which directs operations claims to have been too busy with that to consider the implications of ship registry….
Source: Dalton, R. 2003. Ship row flags up funding of war in Africa. Nature, v. 426, p. 485.
Wildfires and uplift chronology
The “next big thing” in geomorphological studies has been said to be precisely dating crustal exhumation during erosion and uplift. Fission tracks produced in some minerals by particles emitted by radioactive isotopes within them are preserved only when temperature is below that at which annealing can take place. That temperature varies from mineral to mineral. By counting the tracks it is possible to estimate the time since the containing mineral cooled below its annealing temperature during its rise to the surface. Analysing surface samples from different topographic elevations in an area can therefore build up a history of uplift, those lowest in the section being the last to pass through the temperature, and vice versa. Similarly, radiogenic gases only accumulate in a mineral once it cools below a temperature at which the molecular structure blocks diffusion of the gas from the mineral. One example is radiogenic argon produced by decay of 40K. Ages of potassium minerals, such as micas and feldspars, determined by the Ar-Ar technique relate to the time when the containing samples rose through the blocking temperature. There are numerous problems with fission track dating, although most users assume that the ages that they get are real. For Ar-Ar “thermochronology” the blocking temperatures are above 150ºC, which is also problematic, because for a normal continental geothermal gradient of 30ºC km-1 a sample would have to rise 5 km to reach the surface before yielding an age relevant to uplift and erosion history. Unless a study area has much higher geothermal heat flow, or has undergone enormous rapid uplift, most ages obtained by such studies are much older than the event of interest. In the case of helium, the blocking temperatures are lower, about 70ºC in the case of apatite. So dating the accumulation of helium produced by decay of uranium and thorium in apatite offers a tool that seems near-ideal for studying rapid exhumation of the order of a couple of kilometres, and that seems likely for many mountain belts and continental margins. It is the apatite U-Th/He dating method that has spurred a flurry of new studies, now that mass spectrometry is capable of precisely measuring the tiny amounts of helium in single apatite grains. But that has its drawbacks too. On that is pretty obvious is the effect of heating of the surface in recent times. Sara Mitchell and Peter Reiners of the universities of Washington and Yale studied the effects of biomass burning on the method (Mitchell, S.G & Reiners, P.W. 2003. Influence of wildfires on apatite and zircon (U-Th)/He ages. Geology, v. 31, p. 1025-1028) because modelling suggests that fires can reset apatite ages. They found that resetting and scrambling of ages does indeed occur, down to depths of 3 cm in surface samples. That casts doubt on this dating not only on detrital apatites found in soils and sediment, but also in rocks, unless the exposed surfaces are ground away before separating mineral grains. Fires are not the only means of heating rock surfaces, and high temperatures are experienced daily by many rocks due simply to solar heating at low latitudes. This affects depths down to as much as 30 cm, especially in rocks with a dark surface. It is possible to fry eggs on exposed rock in some parts of the world, though they are not very appetizing.
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Change of Geology Today’s editor
Those of you who subscribe to Geology Today will have noticed that its Editor is no longer Peter J. Smith. He was the founding editor of this, the leading digest of news, comment and articles for “lay” geoscientists. Geology Today arose out of Peter’s determination to develop a truly independent forum in the Earth sciences, which appeared as Open Earth back in the 1970s. One of the original five members of the Open University’s Department of Earth Sciences in 1969, Peter took early retirement in March 2003. His replacement at Geology Today is Peter Doyle.
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Geoscience consensus challenged
The history of science shows that what is widely agreed is generally wrong. Yet, there is more than the temptation of cosiness, and the ease of publication that goes with it, that induces even the most imaginative scientists rarely to stick their necks out. In their overthrow of the geocentric view of the cosmos, both Copernicus and Kepler felt ideological pressures that we can only guess at. Colleagues of Copernicus had been burnt at the stake, so he hid himself for the 40 years of his life and only dared publish his ideas so late that the galleys arrived at his deathbed. Kepler, a Protestant in the Holy Roman Empire, kept one step ahead of trouble by networking that would done many a modern scientist proud, and a sort of Bowdlerisation of his ideas so that they merged almost seamlessly with the prevailing ideology of both sides of European Christianity. Even the bravest, most honest and gifted scientists generally agree with their peers, simply because they rarely know any better. If they do, they either keep or are kept quiet. There is very little, if any objectivity in the science of any age… because it is scientists who do it! Kepler cuddled up to Tycho de Brahe, he of the gold and silver nose (fitted after student duelling), in order to gain access to Tycho’s observational data when the old feller died. He got them alright, and began to turn the universe back on its feet, thereby opening an avenue for Newton. Neither Kepler, an unstable hypochondiac who was good at geometry, but not much else, nor Tycho, an anal retentive maker of revolutionising instruments and the founder of empirical science, but devoid of ideas, would have been celebrated for four centuries if the one had not worked with the other. The evolution of science has been marked by the influence of non-conformists, but few worked in isolation against the mainstream.
One modern geoscientist who seems rarely to conform is Warren Hamilton of the Colorado School of Mines, and now he has gone for it big time (Hamilton, W.B. 2003. An alternative Earth. GSA Today, v. 13(11), p. 4-12). His starting point is to challenge the consensus among geophysicists and geochemists that the mantle has a still-unfractionated lower part beneath depleted upper mantle which has sourced oceanic and continental lithosphere progressively over time. Linked to that is the notion of easy circulation of material from top to bottom through descending, subducted slabs and plumes rising from the core-mantle boundary. Hamilton says that neither exists, and that upper and lower mantle are decoupled. His challenge stems from the certainty that the Earth accreted “hot, fast and violently”, and the strong likelihood that its Moon originated after a titanic collision of Earth with a Mars-sized planet less than 100 Ma after accretion. Chances are it became wholly molten and suffered massive loss of volatiles. Such a body would have fractionated rapidly, to produce a lower mantle very unlike that imagined by most geochemists and geophysicists. Moreover, it would have remained so, partly due to its likely perovskite mineralogy, highly fractionated nature and phase-change barriers to transfer of matter – the 630, 1000 and 2000 km discontinuities. Such an early scenario would have transferred most potassium, uranium and thorium into the outermost Earth, where the generation of radiogenic heat would have concentrated. This is very similar to models proposed in the 1960s and early 70’s by J.V. Smith and others, when lunar geochemistry, particularly that of the anorthositic highlands, set in motion ideas about a planet-wide magma ocean and global fractionation as it cooled. Like Smith and others, Hamilton considers continental crust to have formed rapidly, sequestering a large proportion of the elements that make mantle rocks “fertile”. But only traces remain in the form of a small pinch of pre-4 Ga zircons, that could easily be lost in a single sneeze. Much of this early sial returned swiftly to the upper mantle to make it increasingly heterogeneous – fertile parts and some not so petrogenetically prone.
The current consensus has its roots, according to Hamilton, in much older ideas about the early phases of Earth’s evolution. Harold Urey and others in the 1950s and early 60s considered the planet to have formed by slow, cold accretion of the most primitive meteoritic materials, chondrites, particularly those containing carbonaceous materials. They are petrogenetically highly fertile, and the radioactive heating of a chondritic Earth, plus that from core formation, would involve a continual, slow fractionation of the mantle that would probably still be going on today. That this fundamental set of assumptions still dominates, though is rarely mentioned, is down to the rapidly increasing number of mantle profiles based on seismic tomography, that are claimed to have imaged seismic-speed anomalies that could be explained by both slabs and plumes extending to the core-mantle boundary. Hamilton makes the reasonable point that the very irregular distribution of earthquakes in the top 600 km of the Earth leaves large volumes of the mantle in blind spots, and that the majority that are used are subduction related. That, he suggests, predestines tomograph images to create artifacts that just “look” like deep penetration of descending slabs. Moreover, stunning as they look in publications, there is much graphic sleight of hand that assigns primary colours to lower mantle anomalies that have an order of magnitude lower amplitude than those at shallower depths, as well as filling unimaged areas with average or interpolated values, placement of sections to look most plausible, and a great deal of data filtering. There is a “fudge factor” that hypes the hoped-for, and avoids alternative data analysis – you can’t do this kind of thing on a PC. The plume hypothesis is falsified exactly where it ought not to be – in the Emperor-Hawaiian seamount chain (see Wandering hot spots in the September issue of EPN). There the great bend dated at 45 Ma is not matched by any known change in the direction of Pacific sea-floor spreading. The magma source for the chain might well be a restricted volume of mantle, but it didn’t stay still as a plume must. Seismic tomography, at the time Hamilton’s essay went to press, had not verified a single plume sourced in the lower mantle – there are many cases of volcanic hotspots without any plume, and tomographically inferred hot mantle doesn’t always have a volcanic expression.
Hamilton’s essay is worth reading in its entirety, as it reviews the whole of Earth’s tectonic and magmatic evolution. I have just tried to pick out the critical aspects here.
More, or less plumes
In view of Warren Hamilton’s questioning the existence of mantle plumes (Geoscience consensus challenged), in the same month as his essay appeared a team of seismologists from the universities of Princeton, California, Colorado and the National Taiwan University used a new approach to seismic tomography to seek evidence for plumes (Montelli, R. et al, 2003. Finite-frequency tomography reveals a variety of plumes in the mantle. Science Express http://www.sciencexpress.org, 4 December 2003, p, 1-10). They present evidence for 32 suspected plumes. Some have a seismic expression at shallower depths than 650 km in the mantle, such as beneath Iceland and the Galapagos. Others seem to reach as deep as the core-mantle boundary, as beneath Hawaii and the Kerguelen Plateau. In fact most of the classic volcanic hotspots that have associated chains appear to have plumes beneath them, with the exception of Yellowstone. An apparent duality of shallow and deep plumes suggests to the authors a two-tier division in vertically moving mantle, above and below the 660 km discontinuity. The long-suspected major plumes beneath Africa and the Pacific also appear to spawn lesser plumes, that in turn sometimes split
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Fossil hamster’s food cache
It is uncommon to find fossilised nuts, so imagine the fervour that has greeted an actual cache of them, clearly secreted by some hoarding animal. The Garzweiler lignite pit near Cologne in Germany has long been a treasure house for Miocene terrestrial fossils, thanks largely to the keen eyes of miners who work there. In 1992 they came across 1800 nuts in one of the sand horizons that divides the lignite deposit. They were in a burrow through probable dune sands. Its dimensions give a clue to the hoarder, which was about 25 cm long and weighed in at 225 grams (Gee, C.T., Sander, P.M & Petzelberger, B.E.M. 2003. A Miocene rodent nut cache in coastal dunes of the Lower Rhine Embayment, Germany. Palaeontology, v. 46, p. 1133-1149). This is about the size of an extinct hamster, remains of which have been found at a similar level in the lignites. Evidently, hamsters have always worried about their future, especially when food is likely to be scarce, but are also dim-wittedly forgetful. The hazel-like nuts are the earliest-known example of a lost food cache (about 17 Ma), and have been suggested to represent the onset of seasonality in Europe during the late Early Miocene.
The selectivity of mass extinctions
Every mass extinction, whatever its magnitude, was selective; there always were surviving organisms, otherwise we wouldn’t be here. However, selectivity according to the lifestyles of animals that became extinct can give important clues to the causes of extinctions. Die-off across the ecological board strongly suggests a cause that was all encompassing, such as a major impact or geochemical stress that reached into every corner, as might occur with massive flood-basalt volcanism. At the end of the Pliensbachian Epoch of the Early Jurassic there was a significant mass extinction. Its victims were mainly marine organisms, especially molluscs. Study of the disappearances of bivalve species shows that those which lived in burrows suffered more than ones inhabiting open sea floor (Aberhan, M. & Baumiller, T.K. 2003. selective extinction among Early Jurassic bivalves: A consequence of anoxia. Geology, v. 31, p. 1077-1080). A likely cause is loss of oxygen from the upper layer of sea-floor sediments, but a less reducing environment immediately above the sediment surface
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Supergiant hydrocarbon field just leaked away
The largest producing hydrocarbon field, which is unlikely to be bettered, is the Gharwar oil field of Saudi Arabia. It extends for about 3500 km2 and still contains 80 billion barrels of oil. Anything comparable in size, or bigger, would have been tripped over decades ago, because of the sheer size of the geological trap structures. That is one of the reasons to believe that hydrocarbon resources are unlikely to last until the 22nd century, unless other kinds of accumulation can be exploited economically. There are vast onshore reserves of tar sands from which the more volatile hydrocarbons have leaked away, but for them to become generally useable requires very large rises in oil price. The same conditions will have to prevail before oil shales, the source rocks for conventional hydrocarbons, become viable.. Had tectonics not induced the Colorado Plateau to rise and be eroded, oil would be far cheaper and more secure, and the USA would have even more economic and political clout than it already has. The recognition of unroofed hydrocarbon fields in that region of western North America may therefore come as a relief to many people (Beitler, B., Chan, M.A. & Parry, W.T. 2003. Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs: Evidence of exhumed hydrocarbon supergiants. Geology, v. 31, p. 1041-1044).
The desert dune sandstones of the North American Jurassic form some of the world’s most spectacular scenery, because of their vast outcrops in Utah national parks, such as Monument Valley. Their attraction lies in the colours of the sandstones as well their deep incision. Discovery of what was once a series of supergiant hydrocarbon fields lies in variations of that coloration. When laid down, the sandstones were reddened by precipitation of ferric (Fe3+) oxides from water that seeped through them during diagenesis under oxidising conditions. However, large tracts now show signs of variable bleaching, which gives the variegation that tourists flock to see. Iron has been removed in places, and for that to happen, the insoluble Fe3+ has been reduced to the more soluble Fe2+, or ferrous form. That can occur when conditions in the rock change to highly reducing, as in the case of hydrocarbons migrating in along with water. Most wind-blown sands have good porosity and their uniform grain size induces excellent permeability as well, so they are near-ideal reservoirs. However, for them to become permeated by hydrocarbons that migrated from source rocks (usually shales) requires pathways and structures in which the hydrocarbons can be trapped. The Jurassic of the western USA has alternations of these sandstones with less permeable rocks, and was deformed into huge open anticlines during the Laramide orogeny, that originally might have created such traps on a regional scale. Brenda Beitler and her colleagues from the University of Utah have mapped the zones of bleaching using Landsat-7 Enhanced Thematic Mapper data. Sure enough, the most bleached areas coincide with the crests of the large upfolds, and with reverse faults that link them to basins with source rocks and may have acted as fluid migration pathways. The pore volume that could have been available for hydrocarbon trapping would have been 2200 km3, equivalent to 18.5 trillion barrels, about 6 times larger than estimates of the modern world’s recoverable oil. Since the Cretaceous, the Colorado Plateau has undergone more than 2 km of uplift and every single upfold has been breached and deeply incised. Sorry George, the oil leaked out long ago! The inevitable leakage of the gas fraction, perhaps as much as 2 billion tonnes, could have warmed the Tertiary climate, if a significant fraction were released quickly. The main incision of the Colorado Plateau was probably in the late Miocene (around 6 Ma), when ocean-floor data suggest global warming of the order of 0.5 to 1ºC.
Background to globalisation of water resources
“The second provision of any civilised society after a system of laws, is that of a safe water supply” is anonymously attributed in the repeated warnings about the parlous state of water provision for about two thirds of the world’s population. Many of the private companies that took over the public water authorities in Britain now stride the planet organising that provision. In South Africa, the resulting increases in water pricing are the main source of anger throughout the poorer sections of its population, especially in the townships. In Cochabamba, Bolivia there have been mass protests about similar price hikes that came years ahead of any improvement in supplies. A consortium of national and transnational companies needed the extra cash to finance a major dam project, instead of looking to global investors in the project. Science carried a lengthy article that provides a context for this new trend in globalisation (Gleick, P.H. 2003. Global freshwater resources: soft-path solutions for the 21st century. Science, v. 302, p. 1524-1528)
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Recalibrating the stratigraphic column
Managers of isotopic dating labs may be rubbing their hands with glee. The absolute dating on which proper correlation of events in Earth’s history depends, is “officially” a dog’s breakfast. This is partly due to the slowly improving precision and accuracy of radiometric dating applied to ever smaller samples, but also to the high cost of getting the age data. Many important geological boundaries were dated long ago by methods that would not pass muster today, yet those earlier dates are all that palaeontologists, sedimentologists and palaeoclimate specialists have to go on when estimating rates and correlating events. Many important stratigraphic and more complex igneous and metamorphic events remain undated, no matter how much their discoverers plead with the isotopic community. The trend has been to eschew mundane dating in favour of isotopic approaches to petrogenesis, now that really precise data can be had. Sponsored by a number of geochronological labs in the United States, a meeting in Washington, DC during October 2003 set in motion means to redress the balance. A proposal is being developed to obtain US$6 million to found three new labs devoted to dating in the USA (plus $2 to 3 million annual operating costs). The idea is to link similar labs internationally, which would use the same methodology and perform multiple analyses to set standardised dates for important events. Attendees from other countries will be busy formulating their own proposals at around the same levels, you can be sure.
Source: Clarke, T. 2003. Geologists seek to put an end to blind dates. Nature, v. 425, p. 550-551.
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Geochemical switch for Snowball conditions
Whether or not you believe that the Earth was totally encased in ice up to four times during the Neoproterozoic Era, there is convincing evidence that ice sheets did extend to the tropics during such “Snowball” episodes. How such extremely cold episodes came to prevail for several million years has been the subject of debate for 5 years, since Harland’s notion of global glaciations was resurrected by palaeomagnetic evidence for the low latitudes of Neoproterozoic glaciogenic rocks. Ice extending almost to the Equator, even if just on the continents, would have driven down global temperatures simply because it would have reflected away solar radiation. Increased albedo helps explain why frigid conditions lingered, but some other cooling mechanism must first have encouraged the widespread formation of ice sheets. Essentially, the supply of the “greenhouse” gas CO2 by volcanic activity must have been outstripped by burial or solution of carbon in some form. The two usually identified candidates are increased deposition of carbonate sediments and the accumulation of unoxidised organic carbon in sea-floor muds. It is the first of these that dominates climate control today, by the accumulation of carbonate shells of marine plankton, and that has probably prevailed since foraminifera and coccolithophores began to proliferate in the Mesozoic. No shelled organisms existed during the Precambrian, so a major factor in damping down climate fluctuations was missing before the start of the Phanerozoic. This crucial difference between the modern and Precambrian world focussed the attention of Andy Ridgwell, Martin Kennedy (University of California) and Ken Caldeira (Lawrence Livermore National Laboratory) in seeking an explanation for “Snowball” events (Ridgwell, A.J. et al. 2003. Carbonate deposition, climate stability and Neoproterozoic Ice Ages. Science, v. 302, p. 859-862).
Carbonate sediments are plentiful in the Precambrian record. Some formed as a result of organic action (stromatolitic limestones) and others show evidence for direct, inorganic precipitation of carbonates from sea water. The latter indicate sea water in which calcium and carbonate/bicarbonate ions exceeded the solubility of calcite and the ability of organic activity to remove calcite from solution. Evidence for such extreme oversaturation is rare, but the cap carbonates that overlie Neoproterozoic glaciogenic rocks are important examples. The key area of carbonate deposition has always been on shallow continental shelves, the main secreters of carbonates during the Precambrian having been blue-green bacteria that can photosynthesise only in shallow water. Falls in sea-level or a reduction in the area of shelves during the Phanerozoic reduced this sink for CO2 in the build-ups of coral and shelly limestones, but plankton of the open oceans continued to accumulate on the deep sea floor. Because calcite can be dissolved at depth, the deepest sea floor does not contain much carbonate. However, a fall in sea level, increases the area suitable for deep-water burial of shelly material, because the carbonate compensation depth or lysocline also falls. In the absence of shelly plankton, this modern balancing mechanism for ocean chemistry did not exist during the Precambrian. Superficially, it might seem that a reduction in the area of shelf deposition of carbonates, brought on by a sea-level fall, would allow CO2 to build up in the atmosphere, driving towards warmer conditions. However the way in which atmospheric carbon dioxide is related to dissolved carbonate (CO32-) and bicarbonate (HCO3–) ions tells a very different story. This is the equilibrium: CO2 + CO32- +H2O = 2HCO3–. Less carbonate accumulation on reduced continental shelves would drive up the carbonate-ion concentration of sea water, and also its pH. So, according to Le Chatelier’s Principle, the equilibrium proceeds to the right and adds to the more soluble bicarbonate ions in sea water. This consumes CO2, and drives down the “greenhouse” effect. Ridgwell and colleagues developed a model around this equilibrium, and applied it to conditions of falling sea level when carbonates were only deposited on continental shelves. Their results show that decreased shelf-carbonate burial during a period of sea-level fall would rapidly drive down the warming effect of atmospheric carbon dioxide. Combined with the lower solar energy output during the Neoproterozoic, that would be sufficient to create protracted periods of frigidity. Alkalinity of the oceans would increase through periods of glaciation, so that once sea-level rose, massive carbonate precipitation would form cap carbonates on the newly inundated shelves, thereby reducing the oceanic drawdown of CO2.
Ridgwell et al’s model is not easy to grasp, and relies on its initiation by falling sea-level. Either that resulted from build up of continental glaciers because of some other climatic mechanism, or internal processes increased the volume of the ocean basins. An example of the last is a decrease in sea-floor spreading, when cooling of the lithosphere increases it density so that it sags down. Periods of accelerated creation of oceanic lithosphere displace sea water upwards, and perhaps that might explain an increase in shelf areas, which would allow warming according to the new model. The model also needs special pleading to account for the 1 billion-year absence of glaciation before the period of Snowball events. The authors suggest that it could have been prevented by much wider shelves during earlier times, but without quoting evidence.
Continental erosion and climate
Maureen Raymo suggested in 1988 that long term climate change was modulated by the rise of mountain chains and their erosion and weathering. This is because chemical weathering of silicate minerals is a net consumer of atmospheric carbon dioxide. Raymo’s hypothesis, based on T.C. Chamberlin’s theory of glaciation, has set climatically concerned geochemists to analysing the trace element content of river water in many mountainous regions, because those such as strontium are proxies for the amount of weathering going on today. Others have looked at the flux of elements into seawater through the Phanerozoic in particular, by analysing marine carbonates, to see if the ups and down’s of water composition through time match the record of climate change. These time series do suggest some matching, but not precise enough for all to agree with the hypothesis. Measurements of river-water composition have also met set-backs. Much of the weathering flux from mountains seems to stem from dissolution of carbonate rocks, and that does not lead to long-term loss of CO2 from the atmosphere. In a bid to resolve the contributions of carbonates and silicates, Andrew Jacobson and Joel Blum of the University of Michigan have studied the flux from part of the Alps of New Zealand’s South Island (Jacobson, A.D. & Blum, J.D. 2003. relationship between mechanical erosion and atmospheric consumption in the New Zealand Southern Alps. Geology, v. 31, p. 865-868). Their area is a good choice because the New Zealand Alps are actively rising, precipitous and drenched with continual heavy rain and snowfall. Moreover, they offer something that the Andes and Himalaya do not; the rocks are pretty uniform. What they find will not please Raymo’s followers. As in many mountain ranges, mechanical erosion favours carbonate weathering over that of the CO2 sequestering alteration of silicates. With a low ratio of silicate:carbonate chemical weathering, mountain building in New Zealand does draw down carbon dioxide, but only by a factor of about 2. They conclude that more stable areas with lower relief are more likely to affect climate. Although chemical weathering in them is lower than in mountains, that of silicates is far higher than for carbonates. Moreover, active mountain ranges are minuscule compared with the extent of more subdued land. It seems likely from Jacobson and Blum’s findings that the major control of weathering over climate depends to a large degree on where continents are located relative to warm, humid climatic zones. For much of the early Cenozoic, the dominantly crystalline Precambrian shields of India, Africa, Australia and South America straddled the Equator, and witnessed intense weathering. Maybe that relationship helped draw down carbon dioxide, and gradually cooled the planet from the hot and humid climate of the late Mesozoic.
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Nemesis web site
If you like that frisson of fear that comes from contemplating the demise of the world as we know it, then the Near Earth Objects Dynamic Site (NEODyS) will give you hours of it (newton.dm.unipi.it/neodys). The more than 2500 NEOs that orbit within 45 million km of the Earth’s are fully catalogued there, along with impact risk assessment. The site also links to the on-line newletter Tumbling Stone, that has news on asteroidal matters, especially near misses…..and impending doom…..
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Insights into hydrocarbon reservoirs
Oil and natural gas are the dominant physical resources for modern society, having rapidly outstripped coal in the world’s economy. Yet using them poses the threat of global climatic changes. They are essentially a bank of solar energy, mediated by past photosynthesis into hydrocarbons; very long passed indeed. Their burial tens and hundreds of million years ago helped modulate solar warming and drove up the level of oxygen in the atmosphere. Using them reverses those aspects of the carbon cycle. As the wars in Sudan, Afghanistan and Iraq demonstrate, developed economies will go to any lengths to retain access to known reserves. Being so “hooked” on hydrocarbons, those economies have continually to find more. However, the days of “trip-over” oilfields, such as those of Persian Gulf, are gone forever. Exploration ventures into more and more difficult conditions, particularly offshore, where drilling is now going on in sea floor as deep as 2.5 km beneath the water surface. Every aspect of the hydrocarbon industry poses increasing challenges; it seems to be at a crux. For this reason, the 20 November 2003 issue of Nature includes a 56-page Insight supplement on a wide range of topics. It starts with a review of the place of the petroleum industry in human history (Hall, C. et al. 2003. Hydrocarbons and the evolution of human culture. Nature, v. 426, p. 318-322). Robert Berner of Yale University gives an up to date summary of the effects of fossil fuel use, in the context of the carbon cycle over geological time (Berner, R. 2003. The long-term carbon cycle, fossil fuels and atmospheric composition. Nature, v. 426, p. 322-326). The question, “How does petroleum form?” is addressed by Jeffrey Seewald of the Woods Hole Oceanographic Institute (Seewald, J.S. 2003. Organic-inorganic interactions in petroleum-producing sedimentary basins. Nature, v. 426, p. 327-333). The shift of exploration to ever deeper offshore areas brings it closer to the lines where continents split and drifted apart in the past. So it isn’t surprising that Nature Insight includes a review by Cambridge University and BP geoscientists of how those margins evolved (White, N., Thompson, M. & Barwise, T. 2003. Understanding the thermal evolution of deep-water continental margins. Nature, v. 426, p. 334-343). Organisms other than humans exploit the energy locked in oil, and geochemists from the University of Newcastle upon Tyne address their role in actually degrading petroleum, so that many of the largest onshore petroleum reserves (oil sands in particular) pose great difficulties for exploitation (Head, I.M., Jones, D.M. & Larter, S.L. 2003. Biological activity in the deep subsurface and the origin of heavy oil. Nature, v. 426, p. 344-352). Methane generated by anaerobic bacteria in sea-floor sediments and in bogs can combine with water in the form of an ice-like substance called methane hydrate, if the pressure is high enough and temperature is close to 0ºC. There is a lot of it about. On the one hand it has huge economic potential, but on the other it poses awesome threats to the climate. Several times in geological history vast amounts of methane have belched from the sea floor to drive up global temperature; it is a highly efficient “greenhouse” gas. Dendy Sloane of the Colorado School of Mines addresses issues related to methane hydrates (Sloane, E.D. 2003. Fundamental principles and applications of natural gas hydrates. Nature, v. 426, p. 353-359). All these articles are deeply informative and well written. They are “must-reads” for all geoscientists. The sequence ends with a word from “management” (Shell International), in the form of a look ahead to how oil companies might clean up their act and become “friends of the Earth” (Stankiewicz, B.A. 2003. Integration of geoscience and engineering in the oil industry – just a dream? Nature, v. 426, p. 360-363)
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First out of Africa?
In 1991 archaeologists working at the Georgian site of Dmanisi, which had been an important town on the Silk Road, found human remains, but they lay beneath the level at which several extinct mammals had been found. As work progressed in the deeper levels, head bones emerged. They were exceedingly primitive, and associated with equally archaic tools; not the elegant biface stone tools of Homo erectus and later, truly human people, but from the Oldowan culture found with the earliest Homo habilis in Tanzania. The first estimate of their age, based on the mammal remains, was 1.6 Ma. Apart from disputed finds in Indonesia and China, the Dmanisi hominids were the oldest found outside of Africa. Yet at that time, the larger, more brainy H. erectus was thriving in Africa, using the Acheulean biface axes. For the Georgian archaeologists, and the growing number of international collaborators, 9 years of painstaking work lay ahead before enough data had been gathered to draw conclusions confidently. A well illustrated summary of what Dmanisi has revealed appeared in the November issue of Scientific American ( Wong, K. 2003. Stranger in a new land. Scientific American, v. 289(5), p. 54-63). Lots fell into place, when eventually the stratigraphic position of the hominid remains was convincingly established using radiometric dating of basalts below and above it – 1.85 and 1.76 Ma respectively. With more cranial fossils, the Georgian team led by David Lordipanidze the late Leo Gabunia were able to show just how primitive the Dmanisi hominids were. Their brain capacity was half that of modern humans, and detailed skull features resembled the earliest known member of the human genus, H. habilis. They were small people too, and palaeoanthropologists really cannot decide whether they were australopithecines or part of our genus. Lordipanidze believes that they are transitional between habilines and erects. What is most surprising is that they migrated as far as Georgia. That would have involved either crossing the mountains of Turkey and Iran, or, had they taken the possible route out of Africa across the Straits of Bab el Mandab (possibly dry land at the time), an even more circuitous route following the coast of Arabia and perhaps up the Tigris-Euphrates rivers. Their journey began before H. erectus invented the biface axe, which up to now has been regarded as the first sign of both a leap in intellect and the beginning of some command over the rest of nature. The Dmanisi hominids made it and survived, despite their apparently puny frames, if the abundance of animal bones at the site marks long occupation.
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How mountains grow
In the Lake District of Cumbria, asking older local farmers how the fells grew will often get the response that they started out as pebbles. The justification of this seemingly implausible hypothesis is that once a field is cleared of boulders, about 20 to 30 years later new ones have appeared and the clearing has to start again. Geologists have their own ideas. Compressive deformation of continental crust will thicken it, and gravity acting on this low-density material will ensure that its surface rises. Counter-intuitively, the action of erosion can cause mountains to rise as well. Debris flushed from deep valleys lessens the load on the underlying crust, so that it continually rises to drive up the elevations of the remaining ridges and peaks. The compressional origin of the Himalaya is hard to dispute, yet they bounced up quite quickly, long after they began to form. Current ideas, backed up by a variety of evidence, suggests that a lump of the dense lithosphere beneath the India-Asia collision zone fell off (delaminated) and sank in the mantle. That reduced the mass of the lithosphere beneath and the gravitational field, so that the surface rose. The second highest mountains, the Andes, offer no such mechanism, for they are not products of compression associated with collision. Dense Pacific Ocean lithosphere subducts beneath them and the forces involved are insufficient to raise the Andes to even half their present elevation. Simon Lamb of the University of Oxford and Paul Davies of the University of California, Los Angeles have attempted an explanation for the anomalously high Central Andes (Lamb, S. & Davies P. 2003. Cenozoic climate change as a possible cause for the rise of the Andes. Nature, v. 425, p. 792-797). Their idea is that sediments that pour into subduction-related trenches from rising arcs, to form part of the accretionary prism where lithosphere starts to go down, lubricate subduction because of the pore water in them. If there is little sediment supply from the rising crust, then frictional forces build up along the line of the subduction zone. That focuses the plate boundary stresses over a narrow zone, thereby giving sufficient force to drive the crust higher and higher. Today the cold northward ocean current along western South America provides little rainfall to the Central Andes, so erosion is much slowed. Episodic global cooling since the Mid-Eocene probably reduced erosion there several times during the Cenozoic. So for long periods the worlds largest subduction zone would have been starved of lubricants, thereby driving up the Andes. The mountains themselves, by forcing maritime air upwards, would also starve the rising peaks and the great Altiplano plateau of rainfall, further influencing sediment supply to the trench system. Lamb and Davies reckon that the Andes are fortuitous results of a N-S subduction zone at a continental margin, combined with its development during a period of global cooling and tropical drying.
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Oxygen depletion before P-T extinction
The massive die-off at the end of the Palaeozoic Era (251.5 Ma) has focussed attention from a variety of geoscientists for over a decade. Theories for the cause abound, including the climatic influence of the huge Siberian continental flood basalt province, which formed around the same time, explosive release of sea-floor methane, oceanic anoxia, continental aridity and a massive belch of sulphur from the deep mantle. There is now another candidate, asphyxiation (Weidlich, O. et al. 2003. Permian-Triassic boundary interval as a model for forcing marine ecosystem collapse by long-term atmospheric oxygen drop. Geology, v. 31, p. 961-964). The explosion of land plants in the Carboniferous and early Permian that led to the world’s great coal deposits drove up atmospheric oxygen levels to their all-time peak. The occurrence at that time of giant insects, whose metabolism depends on direct diffusion of oxygen, suggests levels of as high as 35%. By the end of the Permian oxygen levels may have been as low as 15%. One line of support for such low concentrations is the growing abundance of fungal spores in the late Permian, which the authors suggest may have been related to a decline in insect populations which consume vast amounts of plant debris. Another is the widespread evidence of anoxic conditions in the Permian oceans, including isotopic features that support a “Strangelove” ocean at the P-T boundary. How oxygen was removed from the atmosphere in the Carboniferous to end-Permian is hard to assess. At levels above around 25% green vegetation catches fire easily, so large firestorms may have been characteristic of the coal-forming era. However, that would not drop levels much below those that prevail at present. Yet the Permian is famous for its continental red beds, the red coloration being due to iron oxide (hematite). Perhaps the missing oxygen became locked in Fe2O3 as the Earth took on a distinct reddishness as the Permian progressed.
“Archaean” ironstone pods prove to be very young
For a number of reasons, including evidence that the cell-chemistry of the most primitive bacteria includes heavy metals and sulphur, the most popular current theory for the place of life’s origin suggests ocean-floor hydrothermal vents. This has led to a search for remains of such “black smokers” in Archaean greenstone belts. One of the most celebrated sites is in the 3.5 Ga Barberton greenstone belt on the South Africa-Mozambique border. Within it are bodies rich in iron oxides, known as “ironstone pods” (not banded iron formations) that show many of the characteristic features of hydrothermal processes. As well as spurring many authors into concluding that the complex organic compounds in them indicate highly developed microbial ecosystems around early-Archaean seafloor vents, scientists have used fluids included in them to speculate on Archaean oceans, and the prevailing temperatures so long ago. They will be dismayed by a re-appraisal of the pods by Donald Lowe of Stanford University and Gary Byerly of Louisiana State University, which casts doubt on their antiquity (Lowe, D.R. & Byerly, G.R. 2003. Ironstone pods in the Archean Barberton greenstone belt, South Africa: Earth’s oldest hydrothermal vents reinterpreted as Quaternary hot springs. Geology, v. 31, p. 909-912). These pods are composed mainly of ferric hydroxide (goethite), which survives only at low temperatures, and are full of open pore spaces that include banded goethite indicating that it formed with the pores’ present orientation, The Barberton Archaean rocks are highly deformed and were metamorphosed at greenschist facies. The pods cut the foliation, and goethite is seen to partly replace Archaean cherts and serpentinised ultramafic lavas. As if these features were not sufficient to rule out the pods’ formation during Archaean times, Lowe and Byerly found one that is clearly related to a now inactive modern spring that formed terraces of botryoidal goethite. These show clear evidence of having formed as a result of modern bacterial action; they are biofilms. In places, modern landslide debris is cemented by goethite. Watch out for interesting correspondence in future issues of Geology from groups who stuck out their necks too far.
Artificial Archaean “fossils”
Debate on the existence of the world’s oldest microfossils from the 3.5 Ga Warrawoona cherts in Western Australia (see Doubt cast on earliest bacterial fossils, April 2002 EPN) has been stoked up by the creation of similar filamentous objects in vitro by geochemists from Spain and Australia (Garcia-Ruiz, J.M. et al. 2003. Self-assembled silica-carbonate structures and detection of ancient microfossils. Science, v. 302, p. 1194-1197). They did this by mixing soluble barium salts in an alkaline sodium silicate solution (pH 8.5-11) exposed to CO2 in the atmosphere. At high alkalinity CO2 dissolves to enrich solutions in carbonate and bicarbonate ions. Filaments made up of precipitated barium carbonate (witherite) and silica soon form. They take on shapes very similar to the tiny segmented worm-like structures that in 1996 were trumpeted as fossils in a now notorious Martian meteorite, as well as those from Warrawoona that are disputed by Schopf and Brazier. The experimenters went a step further, by immersing the filaments in a formaldehyde-phenol mixture and heating them to 125ºC. They then became coated in brownish, kerogen-like carbonaceous material, much as the Warrawoona structures are. Such organic coatings can also be produced by heating iron carbonate (siderite) to 300ºC in water vapour. These “test-tube” analogues of microfossils formed in plausible chemical compositions under not particularly special physical conditions. Interestingly, the Warrawoona chert contains both baryte and iron carbonate. Reaction to the paper was mixed!
Eucarya missing from Mesoproterozoic
Naively, I am always surprised to learn of Precambrian oilfields, even though petroleum in the vast fields of Saudi Arabia partly had its source in Neoproterozoic sediments and migrated into the overlying cover. Provided oil has not been degraded by later biological activity, it contains chemical traces of the organisms whose original decay produced the hydrocarbons, even a breakdown product of cholesterol (cholestane) that is characteristic of the former presence of Eucarya. In the Northern Territories of Australia, Mesoproterozoic sediments (~1430 Ma) that formed in a shallow marine basin are a target for oil exploration. Potential reservoir rocks contain bitumen in pore spaces, but there are fluid inclusion in fractures, which host liquid oil and brines. Organic geochemists at CSIRO, the University of Sydney and Macquarie University have analysed the oil’s molecular structure (Dutkiewicz, A. et al. 2003. Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia. Geology, v. 31, p. 981-984). Mass chromatography reveals a wealth of complex organic compounds, that are biomarkers for the kinds of organisms that were buried and then thermally matured to form the oil. These are exclusively those which point to prokaryotes, especially the cyanobacteria. Evidence for eukaryotic organism is completely absent. This is useful evidence in assigning a maximum age for the rise of the Eucarya that evolved into all modern complex organisms. The earliest likely eukaryote fossil is Grypania, a glossy carbonaceous spiral, found occasionally in sediments around 1400 Ma old, although dubious finds may indicate an origin as far back as 2100 Ma. The dominance of evidence for photosynthesising blue-green bacteria indicates that the oil-forming organisms thrived in an oxygenated, shallow environment. So there seems every reason to believe that Eukarya would have been capable of thriving as part of the trophic pyramid, had they arisen before 1430 Ma.
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Permian-Triassic boundary and an impact?
More than 20 years since the proposal that the end-Cretaceous mass extinction coincided with a major impact, confirmed by the discovery of Chicxulub, nobody has produced convincing evidence for an extraterrestrial culprit for others. Were geologists implanted with GPS tracking devices as soon as they graduated (no doubt on the cards in new health and safety regulations planned by the Blair government in Britain), then Big Brother would see strong clusters close to a number of boundaries on the geological map of the world. There would be many at P-T sites. Electronic tagging would have shown personnel from several US universities (Rochester, Harvard, California) in the Transantarctic Mountains, from time to time in the last few years. Allegedly, that near-pristine area exposes rocks at the juncture between Permian and Triassic strata over less than a metre. It is marked by the sudden disappearance of the famous Glossopteris flora, just below a clay breccia, from which this group of scientists have previously extracted evidence for shocked quartz and extraterrestrial fullerenes (football-shaped organic molecules) that contained odd noble-gas isotopes. Two members of the team have made other finds of fullerenes, at the P-T boundary in China and Japan, the K-T boundary and the ancient Sudbury impact in Canada, whereas other workers have not been so lucky. In fact, the duo are also the only people to have found fullerenes in meteorites, which is key evidence linking terrestrial finds to possible impact events. The team has hit the headlines again (Basu, A.R et al. 2003. Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica. Science, v. 302, p. 1388-1392). At first sight their discovery of pristine fragments of forsterite-enstatite rock with probable chondrules at the boundary suggests that indeed a major impact coincided with the biggest of all Phanerozoic mass extinctions. They even report tiny grains of metallic iron with an astonishing purity, perhaps formed by condensation from the plasma cloud associated with a really big meteorite impact. What is really odd, however, is that sedimentary rocks a quarter of billion years old should have preserved such highly unstable minerals. All other finds of fossil meteorite fragments have been highly altered relics, as any geologist would expect. There is a clamour for the Antarctic samples from other laboratories, so that the results can be confirmed or refuted.
See also: Kerr, R.A. 2003. Has an impact done it again? Science, v. 302, p. 1314-1316, and Oxygen depletion before P-T extinction (above)
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Low-cost disaster monitoring from satellites
With little hype, a British company (Surrey Satellite Technology Limited, linked to the University of Surrey) is beginning to develop a constellation of remote sensing satellites that aim at monitoring a variety of threatening phenomena across the whole planet. The Disaster Monitoring Constellation produces images at the same resolution (about 30 metres) as the US Landsat Thematic Mapper, but is unique in two aspects. The satellites and launching them are cheap, because they are tiny by comparison with the giants normally associated with remote sensing, weighing in at only a few hundred kilograms, and they also use off-the-shelf components including the imaging devices. Second, the four current DMC satellites fly in concert to cover the whole Earth with images 600 km across (Landsat images cover less than a tenth of the area) every day. No other system is capable of that degree of timeliness, the shortest “revist” time to now having been 16 days. SSTL does not own the satellites or the data, but builds them on contract for developing countries. The first to reach orbit, in November 2002, belongs to Algeria. It was joined on 27 September 2003 by three more, sponsored by Turkey, Nigeria and the UK, which were successfully launched by a Kosmos rocket from Plesetsk in northern Russia, at a total cost of around $85 million. These will be joined by similar platforms sponsored by China, Thailand and Vietnam in the next few years. The targets are wildfires, floods, windstorms, volcanic eruptions, erosion and potential landslides, with the added benefit of very detailed information about changes in agriculture and forestry, and baseline mapping of geological and hydrological features. Perhaps most important, it gives less affluent countries independent access to space imagery, which can only boost the confidence of natural scientists in the third world who are venturing into remote sensing after years of playing second fiddle to North American, Japanese and European specialists. Organisations, such as Reuters Foundation AlertNet and the International Charter, plus other international disaster relief organisations, can tap in for images at very short notice Astonishingly, SSTL has launched and is planning imaging satellites that weigh in as little as 7 kg. The low-key announcement of the launch of the 3 latest members of the DMC (www.sstl.co.uk) coincided with US and British hype-fests centred on the current missions to Mars. There is little doubt which will provide the most lasting benefits.
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Recognition of African contributions to palaeoanthropology
Science continues its occasional series on individuals who make an impact on the progress of science with a review of the growing number of Africans working at the forefront of human evolutionary studies (Gibbons, A. 2003. Africans begin to make their mark in human-origins research. Science, v. 301, p. 1178-1179). Ethiopians, Kenyans, Tanzanians and Eritreans have all made important finds and published their results over the last decade. Their hallmark is avid field work, backed up with growing interpretative skills. All credit the encouragement they have had from western colleagues, but now they are in a position to bring along a new generation of experts in their home countries.
The “Big Daddy” theory of human evolution!
One of the anthropological shocks of the 21st century was the discovery that the gene pool of central Asian men is dominated by such a limited range of Y-chromosome characteristics that the only conclusion is that one small group of closely related men dominated impregnation across the region about 800 years ago. They were probably all Mongols closely related to Genghis Khan (see, Darwinian evolution of humans challenged by Y-chromosome data? EPN March 2003). Studies by geneticists from Italy, Portugal and Spain recently suggested that sexual dominance by very few men may have been widespread before about 18 to 12 thousand years ago, around the beginning of the warming that closed the last glacial epoch (Dupanloup, I. et al. 2003. A recent shift from polygyny to monogamy in humans is suggested by the analysis of worldwide Y-chromosome diversity. Journal of Molecular Evolution, v. 57, p. 85-97). Mitochondrial (passed maternally) and Y-chromosome (paternal) DNA studies have been key tools in explaining the timing of migrations of humans over the last 100 thousand years, since their genetic patterns seem to cluster regionally. Molecular clock estimates that use the appearance of new genetic mutations indicate the timing of population separations. The study by Doupanloup and colleagues examined data from individuals who live on all continents. There is an odd and generally distributed difference in genetic diversity between mitochondrial and Y-chromosome DNA, which superficially suggests far more women than men during the last glacial epoch. In terms of births, that is clearly impossible. One explanation, favoured by Doupanloup et al., is widespread polygamy that dwarfs that which notoriously occurs within some religious sects today. Moreover, the “privilege” would have had to be passed on to successive generations of men directly related to the original “Big Daddies”. Rapid shifts in power would not have left such a clear imprint on global Y-chromosomes. How that was achieved without repression or slaughter of potentially competing men, is impossible to judge. However, probable changes in EuropeanY-chromosome patterns around 70, 40 and 20 thousand years ago, that have been ascribed to either evolutionary “bottlenecks” during periods of rapidly dwindling numbers or sudden migrations, might equally have been due to the rise of new patterns of a few males’ dominance over others. Dupanloup et al. show that the rise of agriculture around 10 thousand years ago seems to coincide with a breakdown of massive polygamy and more common monogamy. There are other possible interpretations of the data. In a largely monogamous society, if males stayed where they were born while women moved to live in their mates’ home area, men would be closely related to others in their area, eventually resulting in very similar Y-chromosomes being shared by many. Different migration patterns or early deaths for most men while hunting may also have led to the genetic bias that is causing great discussion among evolutionary geneticists.
Source: Bhattacharya, S. & Le Page, M. 2003. A few prehistoric men had all the children. New Scientist, 6 September 2003, p. 18.
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Case for Martian rainfall strengthens
“Everyone knows” about the huge valley systems on Mars, which through their relationships to other aspects of the planet’s features are thought to have formed catastrophically early in its history. The high-resolution Mars Global Surveyor images and altimetry bring a new perspective to fluvial features (Hynek, B.M. & Phillips, R.J. 2003. New data reveal mature, integrated drainage systems on Mars indicative of past precipitation. Geology, v. 31, p. 757-760). The authors, from Washington University in St Louis USA, show depressions extracted from the altimetry data by simulation of the paths likely to be taken by rain water falling on the surface. In some areas, the depressions link up in dendritic networks very like those that occur on the Earth’s surface. Previous data only picked up disconnected valleys. The newly outlined valleys are V-shaped, unlike the U-shaped systems that developed on Mars probably by sapping as groundwater emerged, either slowly or catastrophically. Such profiles are good evidence for surface run-off, and that can only indicate precipitation, either of rain, or as a result of melting snow. Only 11000 kilometres of valley segments can be identified, and are probably relics of a larger ancient system that later events have masked. Some however, reach to the rims of large craters and seem to post date them. Probably, the events that carved these systems occurred in Mars’ early history.
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High- and low-latitude climate changes almost match
Ten years ago the records of climate proxies from the Greenland ice sheet set new benchmarks for understanding how climate has varied over the last 100 thousand years – annual ice layers allowed division of that data to as fine as decades. Variations in the ice cores helped explain many of the variations found in more blurred data from sea-floor sediment cores in the Northern Hemisphere. Variations could be correlated with changes in the formation of North Atlantic deep water at high latitudes and the destabilisation of North American and Scandinavian glaciers. The whole hemisphere behaved in concert, through long-distance connections in climatic processes, but high-latitude processes seemed to dominate. Development of 234U/230Th dating extended high precision to carbonates that have been precipitated from groundwater to form stalagmites or speleothem. The latest results from speleothem, collected on the Indian Ocean island of Socotra, cover 14 thousand years between 56 and 42 ka, and resolve down to only 8 year intervals (Burns, S.J. et al. 2003. Indian Ocean climate and an absolute chronology over Dansgaard/Oeschger events 9 to 13. Science, v. 301, p. 1365-1367). They show variations in rainfall on the island, though the d18O proxy, and thus changes in the strength of the Indian Ocean monsoon. In terms of shape, the stalagmite record closely resembles d18O changes in the Greenland ice cores, although the two have opposite senses, because the Greenland proxy is for air temperature above the ice cap. During the frigid Heinrich events that saw massive southward waves of icebergs, rainfall over Socotra was low. It became higher as high-latitude conditions warmed in Dansgaard-Oeschger events. The fine speleothem resolution shows a dramatic change-over that took only 25 years or so. The explanation is that warmer conditions increased equatorial evaporation from the oceans. But water vapour is the dominant “greenhouse” gas, and a wetter atmosphere would become warmer. So the question of whether low- or high latitudes drove the changes is still an open one. If North Atlantic events were the driver, then the tropical processes would greatly amplify their effects. One big problem emerges from the joint research by US, Swiss and Yemeni scientists. The highly reliable U/Th dating gives ages for each event that are about 3000 years older than those interpreted from the ice cores. The authors are convinced that the ice-core ages need revision, yet there are discrepancies with the event-ages from other similarly dated speleothems. Commenting on the paper, Frank Sirocko of Johannes Gutenberg University of Mainz in Germany (Sirocko, F. 2003. What drove past teleconnections. Science, v. 301, p. 1336-1337) makes the point that maybe the quality and age of ice core records lie behind the widely accepted view that high-latitude process drive climate. He presents an excellent global image of modern sea-surface temperatures that show the main oceanic shifts of energy – the leakage of cold circum-Antarctic waters northwards, the westward movement of equatorial warm waters to which the El Niño – Southern Oscillation (ENSO) is due, and the unique movement of warm water to Arctic regions in the North Atlantic that is connected to deep water formation. To that he adds the major effect of continental winter snow cover in central Eurasia, that affects albedo and the size of the winter high-pressure zone there. Is there a teleconnection between that and events in the North Atlantic? Nobody knows, because there are no data to compare, yet. Another uncharted but likely linkage is between the ENSO and processes in the circum-Antarctic current. Using currently accepted dating of ice cores, records from those in the Antarctic show air temperature changes that precede those from Greenland by several thousand years. In that respect, the Socotra record possibly has a link with the South Polar climate. Until the issue of dating is sorted out, it will always be difficult to make concrete statements about global climate change.
Interestingly, in the same issue of Science, sea-floor data (between 9 and 16 ka) from the Cariaco Basin off Venezuela, at about the same latitude as Socotra, mimic the Greenland records to within 30 to 90 years (Lea, D.W. et al, 2003. Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination. Science, v. 301, p. 1361-1364).
“Greenhouse” controls challenged
There’s data gathering and there’s theorising. In palaeoclimate studies the two come into conflict. Theory suggests that CO2 is likely to be the principal driver for climatic ups and downs, probably on all time scales. Atmospheric CO2 estimates from the past are based on proxies of different kind, and the various models that they support do not tally vary well. Worst of all they do not fit climate records through the Phanerozoic at all well, except in the crudest possible way. Only the long-lived Carboniferous to Permian “icehouse” and Tertiary cooling tally, and then only in Berner’s GeocarbIII model. One of the best records of major climate shifts, aside from continental tillites, are marine sediments that contain ice-rafted debris, in particular the palaeolatitudes to which they extend. They record four major cooling episodes: Late Ordovician; Devonian to Late Permian; Late Jurassic to Mid Cretaceous; and those since about 35 Ma ago. The oxygen isotope record from Phanerozoic fossils, partly correlated with ocean temperatures also suggest 4 global coolings in the last 545 Ma. Either the CO2 modelling needs more detail, or the whole issue of the “greenhouse” effect is under question. That is the conclusion of a study by Nir Shaviv of the Hebrew University of Jerusalem, and Ján Veiser of the Ruhr University and The University of Ottawa (Shaviv, N.J. & Veiser, J. 2003. Celestial driver of Phanerozoic climate? GSA Today, Huly 2003, p. 4-10). Veiser has been analysing the chemistry of carbonates, especially their oxygen isotopes, for his 30 year career, and has amassed more data than any other geochemist on carbonate-related issues. The two have worked together because their interests fit together extremely well. Shaviv has reconstructed the variation of cosmic ray flux from studies of the exposure of iron meteorites to them, blended with analysis of how the Solar System moves through the spiral arms of our galaxy. Cosmic rays are known to affect the Earth’s cloudiness and therefore albedo. Greater cosmic ray flux should increase the amount of solar energy reflected away by the Earth, thereby causing global cooling. The degree of fit between the cosmic ray flux and palaeoclimatic records is so good that up to 2/3 of climate variation may be connected with the Earth’s celestial position. That is, as it passes through the star-rich spiral arms cosmic rays intensities go up. This happens every 140 Ma or so, which fits very well with the 4 icehouse periods during the Phanerozoic. They even suggest that the climate-CO2 relationship may be the opposite of that generally agreed; climate might drive carbon dioxide levels. A secondary role for “greenhouse” gases wreaks havoc on attempts at modelling climate change feared to result from increasing anthropogenic releases. Shaviv and Veiser’s work comes at a particularly awkward time for climate modellers, who have just initiated a programme for running huge simulations by corralling the combined computing power of millions of home PC users, similar to the approach pioneered by the SETI Institute (Allen, M.R. Possible or probable. Nature, v. 425, p. 242). Perhaps the view of Phillip Stott, that climate modelling is a complete waste of time (Stott, P. 2003. You can’t control the climate. New Scientist, 20 September 2003, p. 25) might sink in as a result of the possible link between cosmic ray flux and climates of the past. Stott believes that acting on the output of such models might perhaps even be dangerous, since we clearly do not understand short-term climate change well enough.
Precambrian CO2 levels
Whether or not fluctuations in the “greenhouse” effect drive climate change, the fact remains that CO2, methane and water vapour all act to retain solar heat in the Earth system. Were it nor for their presence in the atmosphere, the Earth would be about 33 degrees colder than it is. It would be covered by ice. Theoretical modelling of how stars evolve suggests that the Sun had progressive less energy output going back in Earth’s history. Only gaseous heat retention could have prevented a sterile, frigid planet. Yet periods of cooling sufficient to hold large amounts of water in surface ice have occurred only a few times, 4 in the Phanerozoic, a flurry of so-called “Snowball” epochs in the Neoproterozoic and the earliest known glaciation around 2200 Ma ago. The earliest coincided with the first evidence for free oxygen in the atmosphere, and may have been caused by that. Methane, a more powerful “greenhouse” gas than water or carbon dioxide and abundantly produced by anaerobic decay, is easily oxidised. In later time, it has been ephemeral in the atmosphere, unless continuously released, for instance by destabilisation of gas hydrate in sea-floor sediments. Warming by CO2 has undoubtedly kept total frigidity at bay since then. The problem is charting just how much was in the air, because most estimates have been based on studies of palaeosols that give odd and very imprecise results for the early Palaeozoic (see Shaviv and Veiser, 2003; previous item).
Photosynthetic organisms derived their carbon from CO2, either in the air or dissolved in water through equilibration with the atmosphere. The extraction favours lighter 12C, so biological activity results in their products being depleted in the heavier 13C by about 25 parts per thousand (‰) relative to carbon in air and water. If organic carbon becomes buried, the remaining carbon in the surface environment gets richer in 13C, and that signature becomes fixed in contemporaneous carbonates, both organic and inorganic. It is therefore possible to use the two carbon-isotope signatures to estimate the reservoir of CO2; its proportion in contemporary air. However, the degree of fractionation depends on the specific carbon metabolism of different organisms, yet most organic carbon in sediments is a mixed product of widely differing life styles. That severely blurs estimates of atmospheric carbon dioxide content. What is needed are data from a single source with known metabolism. Acritarchs are fossil remains of single-celled marine eukaryotes that were, and still are, marine photosynthesisers. They are made of degraded hydrocarbons. Advanced ion-microprobe resolution is now sufficient to produce carbon-isotope measurements of individual fossils (about 200 micrometres across). Sediments from northern China, roughly 1400 Ma old, contain abundant little-altered acritarchs and carbon isotope data from them give good estimates of atmospheric CO2 levels, that are independent of other methods (Kauffman, A.J. & Xiao, S. 2003. High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils. Nature, v. 425, p. 279-282). The estimates suggest between 10 to 200 times higher contents than today, but just about sufficient to keep the Earth above the limit of glacial temperatures when solar luminosity was about 88% of the present. Acritarchs are present throughout the Neoproterozoic, and it should prove possible to examine the critical periods of “Snowball” conditions using this method.
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Another K-T row
Since the discovery of the buried Chicxulub impact crater off the Yucatán Peninsula, Mexico, many geologists have regarded it as the “smoking gun” for the end-Cretaceous mass extinction. Such is the heft of K-T studies that money has been raised to drill into the crater and its overlying sediments. That began in late 2001 at an onshore site on the flank of the structure, and results are starting to emerge. However, research has been slow in getting underway on the crucial part of the core that goes through the boundary itself. That section was taken from the project’s headquarters in Mexico City to the Free University of Amsterdam, by Jan Smit, one of the pioneers of K-T boundary studies. Samples began to reach other researchers in December 2002, 6 months after the boundary section arrived in Amsterdam. For many, this was a little too slow and suspicions have been raised. Everyone wanted to get abstracts into the AGU/EGS/EUG bun fight in Nice in April 2003, where a conference session on Chicxulub had been scheduled. One report presented there seems set to stun the pro-impact school. Gerta Keller of Princeton University studied foraminifera in the samples immediately above the impact breccia – there were plenty. She claimed that they represented a period of about 300 thousand years of sedimentation that followed the impact. Moreover, they occurred below the level of a thin glauconite-rich horizon, which seems to represent the K-T extinction event itself. Not surprisingly, Keller concluded that the impact could not have caused the extinction. Smit dismisses the allegation of “hogging” the core samples, and also suggests that the foram-rich layers represent sediment that was washed back into the crater soon after it formed. It has always struck me as odd that whenever something startling emerges from scientific research, a sort of preciousness overwhelms supposed scientific “objectivity”. Counter claims and new variants of ideas rapidly evolve on the periphery of the discovery. There are reputations to be built, and defended, and of course “sexy” themes attract cash. The initial work that led to the recognition of a global layer of mass destruction, carried out by the Alvarez father and son team in the late 1970s, was a purer form of science – driven by curiosity and little else.
Sources: Dalton, R. 2003. Hot tempers, hard core. Nature, v. 425, p. 13-14. McKie, R. 2003. I’ve got a bone to pick with you, say feuding dinosaur experts. The Observer, 7 September 2003, p. 22.
Gamma-ray bursts and mass extinctions
There is a Gaelic saying, which roughly translated goes: There are more ways of killing a cat than drowning it in butter. It seems to apply to mass extinctions. A team of astrophysicists and palaeontologists from the University of Kansas and NASA, headed by Adrian Melott of the University of Kansas, has found peculiarities in the trilobite record after the Late Ordovician mass extinction (443 Ma) that are difficult to explain by the usual culprits. Planktonic trilobites were decimated, but those living in deeper water largely came through the extinction. Graptolites too incurred major changes, only the monograptids surviving until the Silurian. Many palaeontologists link the end-Ordovician extinctions to global cooling, evidenced by glacial rocks mainly in Africa. Melott and colleagues suggest that a realistic reason for a depth-related extinction pattern could be due to intense gamma rays emitted by the collapse of a nearby giant star into a black hole. Although most would be blocked by the Earth’s atmosphere, that would be at the expense of nitrogen oxides being created in large volumes from oxygen and nitrogen molecules. Nitrogen dioxide, the yellow colorant in photochemical smog would prevent solar radiation reaching the surface and trigger cooling. Also acid rain would lower the pH of surface water. Such a process could also explain the Late Ordovician glaciation of Africa.
Source Hecht, J. 2003. Did a gamma-ray burst devastate life on Earth? New Scientist, 27 September 2003, p. 17
Fossil oddities – a golfing trilobite and the ox-sized rodent
Gamblers and golfers do not like distractions, and many wear eye shades of some design or other. So it is intriguing to learn that a Devonian trilobite, Erbenochile, found in Morocco evolved a similar device. Richard Fortey and Brian Chatterton, of the British Museum of Natural History and the University of Alberta, respectively, analysed the peculiar eyes of this phacopid trilobite, and found that their tops had a sort of rim. Light shining down on the beast put the compound facets in shadow (Fortey, R. & Chatterton, B. 2003. A Devonian trilobite with an eyeshade. Science, v. 301, p. 1689). Not only would this arthropod have been undistracted from its activities by goings on above, but it could also see over its back.
Not since the discovery of the Late Miocene Bullockornis in Australia (see The Ducks of Death in EPN June 2000) have Neogene palaeontologists come up with a record beater. But now they have (Sanches-Villagra, M.R. et al. 2003. The anatomy of the world’s largest extinct rodent. Science, v. 301, p. 1708-1710). The Late Miocene of Venezuela has yielded a rodent (Phoberomys), whose bones suggest that it weighed in at about 0.7 tonnes. It is related to modern guinea pigs, and probably had much the same herbivorous habits. Its teeth suggest that it was grazer too, and like the modern capybara (one tenth the size of Phoberomys) it lived in swamps. Rodents now rank as the mammalian order with the greatest range of sizes. Because the digestive systems of mammals cannot efficiently break down the high cellulose content of grasses without the aid of internal bacteria, the bigger their gut, the more efficient they are as herbivores. So giant rodents make sense as regards their metabolism. However, they are not as well known for galloping as many other grazers, which is why smaller rodents prefer to escape predation by diving into burrows or among boulders. That would be difficult for a creature as big as an ox. Swamp dwellers, like the capybara and Phoberomys, can get away with not being fleet of foot, but would not do well on open grassland.
The compiler of EPN welcomes news of odd and awesome fossils, and hopes soon to learn of mighty hamsters and their adaptation to natural treadmills.
See also: Alexander, R.M. 2003. A rodent as big as a buffalo. Science, v. 301, p. 1678-1679).
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Wetting oceanic lithosphere
Loss of watery fluids from downgoing subduction zones and their rise into the over-riding mantle wedge is the main reason why arc magmas form there by partial melting under high pH2O conditions. It is usually assumed that all oceanic crust becomes thoroughly hydrated by circulation of seawater shortly after it forms at constructive plate margins. However, many oceanic basalts from ophiolites or dredged from the ocean floor are very fresh. It also seems that to explain the depth of fluid-influenced melting in some volcanic arcs, large amounts of water must be coming from the mantle part of the subducted slab. That is more difficult to hydrate by sea-floor hydrothermal processes. German and US geophysicists have found abundant evidence for faults oceanwards of where the Cocos Plate bends to descend below the Middle America Trench (Ranero, C.R. et al. 2003. Bending-related faulting and mantle serpentinization at the Middle America Trench. Nature, v. 425, p. 367-373). The faults show up clearly on detailed bathymetric images as wrinkles on the ocean floor off Nicaragua, and high-resolution seismic reflection profiles show that they penetrate deep into the mantle part of the Cocos Plate. Water can easily make its way down to form serpentinite from mantle peridotites just before the slab plunges down the subduction zone.
