Late Devonian: mass extinction or mass invasion?

Posted on January 25, 2012 by | 2 Comments
A hand made lookalike for User:Dragons flight'...

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The later part of the Devonian (the Frasnian and Famennian Stages) once marked the second largest marine mass extinction (~375 Ma) of the Phanerozoic Eon: it was one of the ‘Big Five’. For the last decade the drop in marine biodiversity in that interval has come under scrutiny: partly because it may have involved several  events;  no well-supported extinction mechanism has emerged; and extinctions seem have been concentrated on three animal groups – trilobites, brachiopods and reef corals. Something large did happen, as reef-building corals almost disappeared and coral reefs only returned with the rise of modern (scleractinian) corals in the Mesozoic. While the end of the Devonian still figures widely as having experienced a mass extinction event, more detailed palaeontological work at the genus and species level suggests another possibility.

‘Officially’ a mass extinction event must exceed the background extinction rate throughout the Phanerozoic and be above that in immediately preceding and following stages: statistically significant, that is. They always give rise to a marked reduction in biodiversity, but another mechanism can do that without extinctions suddenly increasing. The rate at which new species emerge can fall below that of species extinctions, when the overall number of living species falls. As far as ecosystems are concerned both processes are equally severe, but the causes may be very different.

Hederelloids encrusting a Spiriferida brachiop...

Brachiopod from the Devonian of Ohio, USA. Image via Wikipedia

Reviewing detailed records of Devonian species of two genera of brachiopods and one bivalve genus (50 species in all) in five North American stratigraphic sequences, Alycia Stigall of Ohio University, USA noted apparently significant variations in the local assemblages (Stigall, A. L. 2012. Speciation collapse and invasive species dynamics during the Late Devonian ‘Mass Extinction’. GSA Today, v. 22(1), p. 4-9). Speciation overall fell in the Frasnian and the preceding Givetian, while rate of extinction barely changed. For the three studied genera ,speciation reached low values in the Frasnian and Famennian, but that was accompanied by an equally large fall in extinctions. In this narrow sample we seem to be seeing not an extinction crisis but one of biodiversity. Why?

The Late Devonian saw repeated ups and downs in sea level, which repeatedly connected and disconnected shallow- to moderate-depth marine basins. The fossil record shows repeated cases of species from one basin colonising another, each invasion accompanying rapid marine transgression.. One means whereby species arise is through prolonged isolation of separate populations of the ancestral species through independent genetic drift and mutation. The episodic connection of basins may have prevented such allopatric speciation. Interestingly, the invading species  were dominantly animals with a broad tolerance for environmental conditions.

Whether this mechanism applied to all three main animal groups whose diversity plummeted in Late Devonian times remains to be seen, and it begs the question ‘why didn’t it happen among other animal groups that were less affected by whatever the events were?’ One of the problems associated with decreasing biodiversity in modern marine (and terrestrial) settings is growth in the numbers of invasive species, so the work on 375 Ma fossils might help understand and mitigate current ecological issues. The only difference is that for many of the hyper-successful invader species the means of invasion has been provided by human activities. brachiopod brachioopod

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Posted in Geobiology, palaeontology, and evolution

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Research misconduct

Posted on January 25, 2012 by | Leave a comment

whistleblowerIn 2011 there was a growing trickle of news about various kinds of research malpractice: data fabrication, falsification and obfuscation (not reporting adverse outcomes); plagiarism (http://earth-pages.co.uk/2008/01/01/watch-out-burglars-about/) ; repeated publication of data, text and diagrams (self-plagiarism); ‘guest’ contributors; plus other kinds of scientific fraud and chicanery (http://en.wikipedia.org/wiki/Scientific_misconduct). Motives are many, from malice to laziness, but more often than not are a mixture of ambition, greed, jealousy, desperation and paranoia that increasingly form the downside of academic life – not the least in science. Life is so hard in a career dominated by promotion, which in academe rests on: publication lists; peer citations; journal impact factors; institutional income generation and, let’s face it, by the kind of individual self-regard and hubris that drives people to seek fame and celebrity. The wider population has grown accustomed to this as bystanders watching Big Brother, the X-Factor and Fame Academy.

Fiddling research has reached such a level as to provoke the world’s most prestigious research outlet, Nature, to include an editorial on the topic (Editorial 19 January 2012. Face up to fraud. Nature, v. 481, p. 237-238), albeit after a first lead about the Antarctic Treaty at the centenary of the race to the South Pole, and followed by a puff for articles in the same issue on how to get research funding from the public or philanthropists.

As many scientists suspect, in fact what does emerge about research malpractice is the tip of the proverbial iceberg, some admitting to wandering from the path of righteousness themselves (but not saying how or where). One mild form is making unsubstantiated claims: a great many geologists (including me) have trodden very thin ice in this regard (unless they wisely included the ‘Get Out of Jail’ verb ‘to speculate’), but few innocent bystanders, if any, have met a horrid fate as a result of resultant health and safety ‘issues’. A great deal that should not does get through peer-review to enter the canon of whichever discipline. Academic fraud is a quasi-crime with few risks of detection, though punishment can be swingeing,  in the manner of being cast into the ‘dark place’.

According to Nature, what makes Britain seem to be a haven of academic honesty is the risk that both journals and ‘whistleblowers’ face from libel laws, should deeds and authors be named and linked. Moreover, certain kinds of gross malpractice never reach peer-reviewed publication. Examples are: malicious falsification of someone else’s data by a perpetrator with access to the data on, say, a lab server; swapping analytical sample labels; destroying lab records(http://earth-pages.co.uk/2010/11/04/sabotage-in-science-4/); petty theft of ideas (on which there is no formal copyright), for instance through copying poster presentations at conferences; misuse of peer-review privileges – generally anonymous (http://earth-pages.co.uk/2006/08/01/anonymous-referees-2/); menacing a presenter at a conference or disrupting their presentation. Victims of such actions rarely have any redress, unless the perpetrator is actually caught ‘in flagrante delicto’, so to speak (http://earth-pages.co.uk/2010/11/04/sabotage-in-science-4/).

‘Whistle blowers’ or complainants then face the defensive mechanisms of the academic world: not dissimilar to those of the musk ox. How far you get as regards redress depends to a large extent on the seniority of the perpetrator. An extremely brave friend cited, with abundant evidence, his vice-chancellor for gross cronyism: he was soon ‘on the cobbles’ with the VC (male) remaining ‘virgo intacto. Yet an Industrial Tribunal took a very dim view of the whole affair: my pal paid off his mortgage and lives well in retirement from the compensation awarded by the tribunal. It takes an exceptionally brave graduate student to take on their supervisor(s) for malfeasance (or even the lesser misfeasance and nonfeasance – http://en.wikipedia.org/wiki/Misfeasance). The likely best outcome (after long and harrowing procedures) is a kind of bribe – more time to complete – but most victims just disappear without completing. Unless the perpetrators are low on the academic scale (they might get a reprimand at worst), promotion to management or enhanced early retirement is a common response by senior management to mounds of incontrovertible evidence of guilt. The oddest fate for someone flying high in the institutional firmament was rumoured to be a posting to a far-flung outpost of the former British Empire: but I digress…

The geosciences seem immune to research malpractice, which may reflect at best the small numbers involved in the discipline or at worst because no-one notices, or cares for that matter. Unless, that is, dear reader, you know different…  Most important, for graduate students who are the most usual victims: protect yourselves (http://earth-pages.co.uk/2003/12/01/protecting-your-intellectual-property-2/).

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Dust: heating or cooling?

Posted on January 24, 2012 by | Leave a comment
In the left image, thin martian clouds are vis...

Mars: with and without dust storms in 2001. Image via Wikipedia

Once every 13 years on average dust blots out most of the surface of Mars turning it into an orange ball. The last such planet-encircling dust storm occurred in 2001, but lesser storms spring up on a seasonal basis. Yet Martian seasons have very different weather from terrestrial ones because of the greater eccentricity of Mars’s orbit, as well as the fact that its ‘weather’ doesn’t involve water. When Mars is closest to the Sun solar heating is 20% greater than the average, for both hemispheres. The approach to that perihelion marks the start of the dust season which last a half the Martian year. Unsurprisingly, the sedimentary process that dominates Mars nowadays is the whipping up and deposition of sand and dust, though in the distant past catastrophic floods – probably when subsurface ice melted – sculpted a volcanic landscape pockmarked with impact craters up to several thousand kilometres across. Waterlain sediments on early Mars filled, at least in part, many of the earlier craters and probably blanketed the bulk of its northern hemisphere that is the lowest part of the planet and now devoid of large craters. Erosion and sedimentation since that eventful first billion years has largely been aeolian. Some areas having spectacular dunes of many shapes and sizes, whereas more rugged surfaces show streamlined linear ridges, or yardangs (http://earth-pages.co.uk/2011/05/08/winds-of-change/), formed by sand blasting. Most of the dust on Mars is raised by high winds in the thin atmosphere sweeping the great plains and basins, and, by virtue of Stokes’s law, the grains are very much smaller than on Earth.

The dustiest times on Earth, which might have blotted out sizeable areas from alien astronomers, in the last million years have been glacial maxima, roughly every 100 ka with the latest 20 ka ago. Layering in the Antarctic ice core records such dust-dominated frigid periods very precisely. Less intricate records formed away from the maximum extent of ice sheets as layers of fine sediment known as loess, whose thickness variations match other proxy records of palaeoclimate nicely. Loess, either in place or redeposited in alluvium by rivers, forms the most fertile soil known – when the climate is warm and moist. The vast cereal production of lowland China and the prairies of North America coincides with loess: it may seem strange but a large proportion of 7 billion living humans survive partly because of dust storms during glacial periods of the past.

Being derived from rock-forming minerals dust carries with it a diverse range of chemical elements, including a critical nutrient common on land but in short supply in ocean water far offshore: iron in the form of oxide and hydroxide coatings on dust particles – the dust coating your car after rain often has a yellow or pinkish hue because of its iron content. Even when the well-known ‘fertilizer’ elements potassium, nitrogen and phosphorus are abundant in surface ocean water, they can not encourage algal phytoplankton to multiply without iron. Today the most remote parts of the oceans have little living in their surface layers because of this iron deficiency. Yet oceanographers and climatologists are pretty sure that this wasn’t always the case. They are confident simply because reducing the amount of atmospheric carbon dioxide and its greenhouse effect to levels that would encourage climate cooling and glacial epochs needed more carbon to be buried on the ocean floors than happens nowadays, and lifeless ocean centres would not help in that.

Dust plume off the Sahara desert over the nort...

Saharan dust carried over the Atlantic Ocean by a tropical cyclone. Image via Wikipedia

At present, the greatest source of atmospheric dust is the Sahara Desert (bartholoet, J. 2012. Swept from Africa to the Sahara. Scientific American, v. 306 (February 2012), p. 34-39). Largely derived from palaeolakes dating from a Holocene pluvial episode, Saharan dust accounts for more than half the two billion metric tonnes of particulate atmospheric aerosols dispersed over the Earth each year. Located in the SE trade-wind belt, the Sahara vents dust clouds across the Atlantic Ocean, most to fall there and contribute dissolved material to the mid-ocean near-surface biome but an estimated 40 million t reaches the Amazon basin, contributing to fertilising the otherwise highly leached tropical rain-forest soils. While over the ocean the high albedo of dust adds a cooling effect to the otherwise absorbent sea surface. Over land the fine particles help nucleate water droplets in clouds and hence encourages rainfall. The climatic functions of clouds and dusts are probably the least known factors in the climatic system, a mere 5% uncertainty in their climatic forcing may mean the difference between unremitting global warming ahead or sufficient cooling by reflection of solar radiation to compensate for the cumulative effects of industrial CO2 emissions.

Recording amounts of dust from marine sediments quantitatively is very difficult and impossible in terrestrial sediments, but superb records tied accurately to time at annual precision exist in ice sheets. Low dust levels in Greenland and Antarctic ice tally well with the so-called ‘Medieval Climate Anomaly’ (a warm period) whereas through the 13th to 19th centuries (the ‘Little Ice Age’) more dust than average circulated in the atmosphere. Crucially, for climate change in the industrial era, there has been a massive spike in dust reaching near-polar latitudes since the close of the 18th century during the period associated with signs of global warming: a counterintuitive relationship, but one that is difficult to interpret. The additional dust may well be a result of massive changes in land use across the planet following industrialised agricultural practices and growing population. There are several  questions: does the additional dust also reflect global warming with which it is correlated, i.e. evaporation of the huge former lakes in the Sahara (e.g. Lake Chad); is the dust preventing additional greenhouse warming that would have taken place had the atmosphere been clearer; is it even the ‘wrong kind of dust’, which may well reflect short-wave solar radiation away but also absorbs the longer wavelength thermal radiation emitted by the Earth’s surface, i.e. an aerosol form of greenhouse warming. Needless to say, neither clouds nor dust can be factored into climate prediction models with much confidence.

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Posted in Climate change and palaeoclimatology

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Within-plate earthquakes

Posted on January 17, 2012 by | Leave a comment

 

 

English: Earthquakes recorded in the New Madri...

Recent earthquakes in the US mid-west around New Madrid Missouri. Image via Wikipedia

 

Almost all devastating earthquakes within living memory and the tsunamis that ensued from some of them have occurred where tectonic plates meet and move past one another either horizontally through strike-slip motion or vertically as a result of subduction. This link between real events and the central theory of global dynamics gives an impression of inherent predictability about where damaging and deadly earthquakes might happen, if not the more useful matter of when the lithosphere might rupture. Such confidence is potentially highly dangerous: the most deadly earthquake in recorded history killed at least 800 thousand people in China’s Shanxi Province in 1556 when according to  a description written shortly afterwards, ‘… various misfortunes took place… In some places, the ground suddenly rose up and formed new hills, or it sank abruptly and became new valleys. In other areas, a stream burst out in an instant, or the ground broke and new gullies appeared…’. Shanxi is far from any plate boundary. A study of Chinese historic records covering the last two millennia (Liu, M. et al. 2011. 2000 years of migrating earthquakes in North China: How earthquakes in midcontinents differ from those at plate boundaries. Lithosphere, v. 3, p. 128-132) shows a pattern to the position of large intraplate events.  Rather than occurring along lines as do those at plate boundaries, earthquakes ‘hopped’ from place to place without affecting the same areas twice. Liu and colleagues consider this almost random pattern to result from reactivation of interlinked faults through broad-scale and gradual tectonic loading of the crust by far off plate movements. After a short period of reactivation one fault locks so that energy build-up is eventually released by another in the plexus of crustal weaknesses.

The best studied site of such intraplate seismicity lies midway along the Mississippi valley in the mid-US, between St Louis and Memphis. In 1811 and 1812 four Magnitude 7 to 8 earthquakes struck, the most affected place being the small township of New Madrid on the banks of the great river where mud and sand spouted from numerous sediment volcanoes. No-one died there but tremors were felt over a million square kilometers, bells ringing spontaneously as far away as Boston and Toronto. It is now known that this section of the Mississippi basin lies above a graben that affects the ancient basement beneath the alluvial sediments, one of whose faults was reactivated, perhaps in an analogous way to the hypothesis about Chinese seismicity. A coauthor in Liu et al. (2011), Seth Stein of Northwestern University, Illinois, believes stress redistribution through a Mid-western fault network was responsible and other events are likely at some uncertain time in the future on this and other areas underpinned by ancient fault complexes. Indeed sporadic ‘quakes up to Magnitude 7 have affected the eastern US and Canada and the Atlantic seaboard since European settlement. But since the largest of the New Madrid quartet of earthquakes, populations have grown across the likely areas of tenuous risk and future ones could have extremely serious consequences for which it is difficult to plan by virtue of unpredictability of both place and timing: in some respects a more worrying prospect than is the case where major events are inevitable – sometime – as along the San Andreas Fault. There are few, if any, major conurbations worldwide that could be considered seismically safe if the theory of networked stress redistribution through otherwise inert parts of continental crust is borne out.

In some respects the theory is a small-scale version of the suggested mechanical linkage through all major plate boundaries that has been suggested by some to account for the clustering in time of great earthquakes – around and above Magnitude 8 – around the globe. Since 2000 great earthquakes have occurred on subduction zones beneath Sumatra, the Himalaya, the Andes, Central America, Alaska, New Guinea, the mid-Pacific, Japan and the Kurile islands, on the strike-slip system that cuts New Zealand and in the intraplate setting of the 2008 Sichuan earthquake in China. Almost all plate boundaries link up globally, but although it seems likely that stress is redistributed along boundaries, especially between adjacent segments, as documented for the great Anatolian fault system of Turkey and the Indonesian subduction zone, a mechanism that transmits stress beyond individual plates seems unlikely.

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Posted in Environmental geology and geohazards

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Massive event in the Precambrian carbon cycle

Posted on January 10, 2012 by | 1 Comment
English: Cyanobacteria

Cyanobacteria: earliest producers of oxygen in the Precambrian. Image via Wikipedia

The entire eukaryote domain of life, from alga to trees and fungi to animals, would not exist had it not been for the emergence of free oxygen in the oceans and atmosphere about 2.4 billion years ago; thanks in large part to the very much simpler photosynthetic blue-green bacteria. The chemistry behind this boils down to organisms being able to transfer electrons from elements and compounds in the inorganic world to build organic molecules incorporated in living things. Having lost electrons the inorganic donors become oxidised, for instance ferrous iron (Fe2+ or Fe-2) becomes ferric iron (Fe3+ or Fe-3) and  sulfide ions (S2-) become sulfate (SO42-) and the organic products that receive electrons principally involve reduction of carbon, on the OilRig principal – Oxidation involves loss of electrons, Reduction involves gain. Since the Great Oxygenation Event (GOE), ferric iron and sulfate ions now account for 75% of oxidation of the lithosphere and hydrosphere while free oxygen (O2) is a mere 2-3 % (Hayes, J.M. 2011. Earth’s redox history. Science. V. 334, p. 1654-1655; an excellent introduction to the geochemistry involved in the GOE and the carbon cycle). Free oxygen is around today only because more of it is produced than is consumed by its acting to oxidize ferrous iron and sulfide ions supplied mainly by volcanism, and carbon-rich material exposed to surface processes by erosion and sediment transport.

Eukaryote life has never been snuffed out for the last two billion years or so, but it has certainly had its ups and downs. To geochemists taking the long view oxygen might well seem to have steadily risen, but that is hardly likely in the hugely varied chemical factory that constitutes Earth’s surface environments, involving major geochemical cycles for carbon, iron, sulfur, nitrogen, phosphorus and so on, that all inveigle oxygen into reactions. Tabs can be kept on one of these cycles – that involving carbon – through the way in which the proportions of its stable isotopes vary in natural systems. If all geochemistry was in balance all the time, all materials that contain carbon would show the same proportions of 13C and 12C as the whole  Earth, but that is never the case. Living processes that fix carbon in organic compounds favour the lighter isotope, so they show a deficit of 13C relative to 12C signified by negative values of δ13C. The source of the carbon, for instance CO2 dissolved in sea water, thereby becomes enriched in 13C to achieve a positive value of δ13C, which may then be preserved in the form of carbonates in, for instance, fossil shells that ended up in limestones formed at the same time as organic processes were favouring the lighter isotope of carbon. Any organic carbon compounds that ocean-floor mud buried before they decayed (became oxidised) conversely would add their negative δ13C to the sediment. Searching for δ13C anomalies in limestones and carbonaceous mudrocks has become a major means of charting life’s ups and downs, and also what has happened to buried organic carbon through geological time.

A most interesting time to examine C-isotopes and the carbon cycle is undoubtedly the period immediately following the GOE, in the Palaeoproterozoic Era (2500 to 1600 Ma). From around 2200 to 2060 Ma the general picture is roughly constant, high positive values of δ13C (~+10‰): more organic carbon was being buried than was being oxidised to CO2. However, in drill cores through the Palaeoproterozoic of NW Russia carbonate carbon undergoes a sharp decline in its heavy isotope to give a negative δ13C  (~-14‰) while carbon in organic-rich sediments falls too (to~-40‰): definitely against the general  trend (Kump, L.R. et al. 2011. Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event. Science. V. 334, p. 1694-1696). Oxygen isotopes in the carbonates affected by the depletion in ‘heavy’ carbon show barely a flicker of change: a clear sign that the 13C δ13C deficit is not due to later alteration by hydrothermal fluids, as can sometimes cause deviant δ13C in limestones. It is more likely that a vast amount of organic carbon, buried in sediments or dissolved in seawater was oxidised to CO2 faster than biological activity was supplying dead material to be buried or dissolved. In turn, the overproduction of carbon dioxide dissolved in seawater to affect C-isotopes in limestones. Such an event would have entailed a sharp increase in oxygen production to levels capable of causing the oxidation (~ 1% of present levels). Yet this was not the time of the GOE (2400 Ma) but 300-400 Ma later. A possible explanation is a burst in oxygen production by more photosynthetic activity, perhaps by the evolution of chloroplast-bearing eukaryotes much larger than cyanobacteria.

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Posted in Geochemistry, mineralogy, petrology and volcanology

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Excitement over early animals dampened

Posted on January 9, 2012 by | Leave a comment
Alga (Volvox sp.)

Volvox cyst. Image via Wikipedia

The Neoproterozoic lagerstätte in the Doushantuo Formation in the south of China was until recently thought to be a source of astonishing information about Earth’s earliest animals (See Ancestral animal? in EPN August 2004) that preceded the appearance of those with hard parts at the start of the Phanerozoic.  It contains well-preserved fossils that resemble embryos, algae, acritarchs, and small bilaterians. Dated at between 580 to 600 Ma(See Age range of early fossil treasure trove  in EPN February 2005), the Doushantuo directly overlies cap carbonates representing the emergence of Earth’s climate from a Snowball epoch represented by a tillite beneath the carbonate sequence. A detailed examination using synchrotron X-ray tomography of the putative animal embryos does show clear signs of cell doubling or palintomy (Huldtgren, T. et al. 2011. Fossilized nucluei and germination structures identify Ediacaran ‘animal embryos’ as encysting protists. Science. V. 334, p. 1696-1699) but also internal cell features most likely to be nuclei, but which have no counterparts in animal embryos. The organisms which the fossils most resemble are indeed eukaryotes, but of a kind separate from animals known as Holozoa. Yet there are striking resemblances with eukaryotes more distant from animals, such as the modern Volvox, a type of alga (Butterfield, N.J. 2011. Terminal developments in Ediacaran embryology. Science. V. 334, p. 1655-1656), that developed from an ancestor further back in time than the separation of metazoan animals from holozoans.

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Mistaken conclusions from Earth’s oldest materials

Posted on December 21, 2011 by | Leave a comment

Microscope projection close-upThe oldest materials on the planet are tiny zircon grains that were washed into conglomerate in Western  Australia about 2650 to 3050 Ma ago. It wasn’t the fact that the grains are zircons, which are among the most durable materials around, but the range of ages that they revealed when routinely analysed. U-Pb dating of detrital zircons is a well tested means of finding the provenance of sedimentary materials as an indicator of orogenic and igneous events that formed the crust from which they were eroded. In the original study of the Jack Hills zircons some showed ages that might reasonably have been expected from late sediments in an Archaean craton: around 3.5 billion years is about the maximum age for orogenic events there. What astonished all geoscientists was that a proportion of the grains gave ages of more than 4 billion years, some as old as 4.4 Ga: here was a window on the missing first half billion years of Earth history, the Hadean.

Subsequent work on yet more zircons confirmed the original age span but other kinds of analysis led to a variety of claims: that continental crust was around in abundance within 100 Ma of Earth having formed; geothermal heat =flow was not especially high;  liquid water was available for geological processes, including the origin of life; plate tectonics may have started early…. The topic has cropped up several times in EPN since the issue of 1 January 2001. Quite a lot of the claims emerged from studies of other minerals enclosed by the ancient zircons, such as quartz and micas, and now they have been checked again by geochemists from Western Australia (Rasmussen, B. et al. 2011. Metamorphic replacement of mineral inclusions in detrital zircons from Jack Hills, Australia: Implications for the Hadean Earth. Geology, v. 39, p. 1143-1146). It turns out that the inclusions formed at temperatures well below those of magmas, between 350 to 490°C: more like those of metamorphism. Indeed, uranium-bearing rare-earth phosphate minerals, xenotime and monazite, also locked in the zircons not only turn out to be metamorphic in origin too (both are also formed magmatically) but date to between 2700 and 800 Ma.

While the  Hadean zircon dates remain robust, a closer look at their inclusions shows that they did not remain geochemically closed systems thereafter. It was on the assumption of zircons being geological ‘time capsules’ that much of the excitement rested. Even using the presence of zircons from 4.4 Ga – they are most common in granites but do occur in mafic and intermediate igneous rocks – to suggest early ‘sialic’ continental crust is suspect. Despite having some tiny bits from Earth’s early days, it seems we are none the wiser.

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Galactic controls

Posted on December 15, 2011 by | 3 Comments
English: Artist's conception of the Milky Way ...

Artists impression of the Milky Way viewed along its axis. Image via Wikipedia

Palaeoclimatologists are quite content that an important element in controlling the vagaries of climate is due to gravitational forces that cyclically perturb Earth’s orbit, it axial tilt and the way the axis of rotation wobbles in a similar manner to that of a gyroscope. The predictions about this by James Croll in the late 19th century, which were quantified by Milutin Milankovich during his incarceration during World War I, triumphed when the predicted periods of change were found in deep-sea floor sediment records in 1972. Authors of ideas that link Earth system changes  to the progress of the Solar system through the Milky Way galaxy haven’t had the same accolades. One of the first to suggest a galactic link was Joe Steiner (Steiner, J. 1967. The sequence of geological events and the dynamics of the Milky Way Galaxy. Journal of the  Geological Society of  Australia, v.  14, p. 99–132.) but his work is rarely credited.

There has been an upsurge of interest in the last decade or so. In a recent issue of New Scientist Stephen Battersby reviews what galactic ‘forcings’ may have accomplished during the 4.5 billion-year history of our world (Battersby, S. 2011. Earth odyssey. New Scientist, v. 212 (3 December issue), p. 42-45). Having formed probably much closer to the galactic centre than its current position the Solar System has drifted, perhaps even ‘surfed’ gravitationally, outwards to reach its present ‘suburban’ position in one of the spiral arms. There are regularities to the now stabilised orbital movements: once every 200 million years the Solar System completes a full orbit; this orbit wobbles across the hypothetical plane of the galactic disc by as much as 200 light years, moving with and against the Milky Way’s cosmic motion. It has proved impossible so far to detect any sign of the orbital 200 Ma periodicity in events on the Earth, and most attention has centred on the wobble.

Steiner suggested that this motion may have crossed different polarities of the galactic magnetic field, perhaps triggering the periodicity of geomagnetic  changes in polarity, but this now seems unlikely. However, his suggestion that glacial epochs, such as those in the Palaeo- and Neoproterozoic, at the end of the Palaeozoic Era and at present, may have resulted from the Solar System’s passage through dust and gas banding in the Milky Way continues to have its attractions (e.g. Pavlov, A.A. et al. 2005. Passing through a giant molecular cloud: “Snowball” glaciations produced by interstellar dust, Geophysical Research Letters, v. 32, p. L03705). The direction of motion relative to the Milky Way’s cosmic drift governs the exposure to cosmic rays that result from a kind of ‘bow-shock’ ahead of the galaxy

Stellar motion through the Milky Way is semi-independent so that from time to time the Solar System may have been sufficiently close to regions of dense dust and gas that nurture the formation of super-massive stars. These huge objects quickly evolve to end in supernovae, proximity to which would have exposed life to ‘hard’ X- and  γ-rays and would be trigger for mass extinction, for instance by accompanying cosmic rays in destroying the ozone protection from UV radiation from the Sun.

The dynamism of the Earth and the resulting complexity of its surface processes makes it a poor place to look for physical signs of galactic influences. No so the Moon: for almost 4.5 billion years it has been a passive receptor for virtually anything that the cosmos could fling at it, and so geologically inert that its surface layers may well preserve a complete ‘stratigraphic’ record of all kinds of process. Should lunar landings with geological capabilities once more prove economically possible, or politically useful, that hidden history could be read.

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Posted in Planetary, extraterrestrial geology, and meteoritics

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Hominin updates

Posted on December 1, 2011 by | Leave a comment

A new approach to 14C dating at the Oxford Radiocarbon Accelerator Unit at the University of Oxford UK, combined with detailed analysis of human teeth to distinguish fully modern human remains from those of Neanderthals has pushed back the date and pace of migration into Europe by people whose tools define the Aurignacian and Italian Uluzzian technologies. These are the earliest modern-human cultures found in Europe, but some of the tools are similar to those produced by Neanderthals (Châtelperronian culture), raising the possibility of transfer of technologies between the two groups. So, without confirmation from human remains of the anatomical affinities the would be doubts about using tools of these kinds to signify the presence at a site of full modern humans. Teeth found decades ago at caves in SW England and southern Italy prove, on detailed comparative study, to be from ‘moderns’ (Higham, T. And 12 others 2011. The earliest evidence for anatomically modern humans in northwestern Europe. Nature, v. 479, p. 521-524; Benazzi, S. And 13others 2011. Early dispersal of modern humans in Europe and implications for Neanderthal behaviour. Nature, v. 479, p. 525-528).The new carbon-isotope method  efficiently eliminates chemical contamination of material by post-fossilisation processes and so tend to increase the measured age of samples. The two studies produced exciting results: dates of occupation between 42-43 and 43-45 ka from SW England and southern Italy respectively. Together with results from other sites throughout central and southern Europe, the discovery shows that widespread colonisation was accomplished in three to five thousand years by migrants probably from the Levant, who may have travelled along three routes fanning out from the Bosporus in modern Turkey: along the Danube; along the Adriatic coast; from southern Greece to the ‘heel’ of Italy.

In early 2011 a group of archaeologists led by Simon Armitage of the University of Birmingham, UK reported stone tools from a cave in the United Arab Emirates for which they derived possible ages of 125, 95 and 40 ka (see Human migration in EPN for January 2011). The older dates were coeval with anatomically modern humans in the Levant, but the tools themselves showed features that could not be matched decisively with those from any other sites, including those in the Leant, though they most resembled collections from East and NE Africa. Armitage and colleagues suggested that the people who occupied the UAE cave had crossed the Red Sea at the time of the glacial maximum around 130 ka, at a time of unprecedented low sea level. A recent paper adds considerable weight to this idea (Rose, J.I. and 9 others 2011. The Nubian Complex of Dhofar, Oman: An African Middle Stone Age Industry in Southern Arabia at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0028239). Jeffrey Rose, also of the University of Birmingham, and colleagues from Ukraine, US, UK, Germany, the Czech Republic and Australia excavated site in Dhofar southern Oman, much closer to the Straits of Bab el Mandab than the UAE. Chert tools found in the area are of the Levallois type, specifically resembling closely those found widely in the Nile Valley of southern Egypt and northern Sudan, and in the Afar Depression of Ethiopia, in deposits dated between 128 to 74 ka. The Omani tools yielded an optically stimulated luminescence age of about 106 ka. This nicely confirms that Africans had moved far beyond the confines of their home continent by the last interglacial episode, with the route to South Asia open to them along the shores of the Persian Gulf and Indian Ocean. However, the route that they had taken could equally have been around the head of the Red Sea as across the Bab el Mandab.

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Desert varnish: an outdoor canvas

Posted on November 30, 2011 by | Leave a comment
Petroglyphs carved in desert varnish at the Va...

Petroglyphs in desert varnish near Las Vegas, Nevada, USA. Image via Wikipedia

Early occupants of semi-arid areas found a cultural use for what is one of geology’s greatest annoyances: desert varnish. Annoying because once developed it leaves an extremely durable brownish to black, shiny coating over rock surfaces: be they dunite, marble or quartzite, sandstone or granite, desert outcrops all look very much the same. You have to bash them unmercifully to see the true texture and mineralogy, and, except on images of thermally emitted infrared, remote sensing doesn’t help as the varnish has the same reflectance whatever the wavelength of radiation. Yet to the former inhabitants of dry lands – and latter day ‘taggers’ – desert varnish has been irresistible for millennia. Lightly peck away with a sharp pebble – and some ability to depict your thoughts – and you can leave an almost indelible sign that you and your ideas were at that very rock face: a petroglyph, picked out for all time in the manner of chalk on a blackboard. Even more spectacular, given an oversight of a varnished cobbly plain and it is possible to magnify your tag, or whatever petroglyphs once signified, a hundredfold or more. That happened on the famous Nazca Plain of Peru  and continues to do so in especially dry places in the south-western US, as around Lake Havasu City in Arizona. Varnish forms only on the exposed face of cobbles, the downward side remaining more or less the original rock’s colour; generally lighter. Turn over the cobbles in an organised way, with a degree of persistence as well as talent and you too can make your mark on Google Earth! (Do not pass this on to Banksy – it doesn’t hurt the ecosystem, but will annoy the authorities immensely).

Français : Lignes de Nazca au Pérou. Le contra...

Ancient art depicting a hummingbird on the Nazca Plain, Peru. Image via Wikipedia

For all this period of artistic endeavour, stretching back in some places to the Palaeolithic, it now seems that desert varnish also records how environments have changed as well as the religiosity, humour or downright egotism of its inhabitants (Dickerson, R. 2011. Desert varnish – nature’s smallest sedimentary formation. Geology Today, v. 27 (November-December issue), p. 216-219). As well as reviewing how the varnish forms (see also Desert varnish in EPN May 2008, in Subjects: GIS and Remote Sensing)., Dickerson flags-up the little-known fact that the minute layers produced as varnish imperceptibly develops record changes in environmental conditions – wet, dry and middling – and, moreover they can be dated precisely despite being extremely thin (e.g. Liu, T. & Broeker, W.S. 2008. Rock varnish microlamination dating of late Quaternary geomorphic features in the dry lands of wester USA. Geomorphology, v. 93, p. 501-523). Liu and Broeker were able to match variations in the colour of varnish layers with important climatic episodes of the Northern Hemisphere, such as the Younger Dryas and other warming-cooling, dry-wet shifts as far back as the Last Glacial Maximum. Their approach offers a chance of dating petroglyphs and thereby cultural changes during critical stages in the history of modern human migrations, occupations and abandonments, even when no artefacts or bones remain. That is because once made, petroglyphs gradually become varnished themselves.

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