Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

Stress and the Cambrian Explosion

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The opening of the Phanerozoic Eon at the base of the Cambrian is, as everyone knows, characterised by the appearance of body fossils of organisms that were preserved because they had calcium-rich hard parts. Thereafter biological diversity grew and grew, despite episodic sets back. Why calcium carbonate and phosphate skeletal parts evolved is still a mystery, although it may have had something to do with an increase in the calcium-ion concentration of seawater. Earth had not long emerged from the last of several truly deep freezes, associated with evidence for which are carbon isotope signals that may indicate repeated mass extinctions of life forms that left few tangible traces. Whatever the truth, it must have lain in some major change in global environmental conditions. Evidence for one such widespread chemical stress has emerged from black shales at the Precambrian-Cambrian boundary in the Oman and China (Wille, M. et al. 2008. Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary. Nature, v. 453, p. 767-769).

Molybdenum, like many transition metals, has several valence states, some soluble in oxidising conditions, others when conditions are reducing. Solution or precipitation when redox conditions change may cause fractionation among stable isotopes, and isotopes of Mo are a case in point. The Swiss-German-US team found that close to the base of the Cambrian the 98Mo/95Mo ratio underwent rapid changes in black shales of Oman and China. They ascribe this to a major upwelling of hydrogen sulfide-rich deep seawater, indeed it would be difficult to suggest any other mechanism that could have caused the shift. Molybdenum is soluble in oxidising waters, and the increase in Mo concentrations in the shales at the time of the isotopic anomaly must mark a shift to reducing conditions in 542 Ma surface seas, hence the link to such an upwelling. Such rises in highly toxic ‘sour gas’-rich water have been suggested as a possible cause for the mass extinctions at the ends of the Permian and Triassic (see Global warming, sour gas and mass extinctions in the January 2007 issue of EPN).

The globally abundant Ediacaran fauna of soft, bag-like and quilted organisms that lived in the late Neoproterozoic has no counterpart in the Cambrian record, even in lagerstätten. Moreover, the Cambrian shelly fauna does not simply spring into place fully formed: it developed over a protracted period and did not simply succeed or evolve from the Ediacaran. It looks like there was the last of a succession of Neoproterozoic mass extinctions at the outset of the Phanerozoic. Indeed the Mo anomalies coincide with abnormally ‘light’ carbon isotopes in the black shales, due the accumulation of massive amounts of dead organisms, and formation of large phosphatic deposits globally.

Yet another blow for creationism

The Devonian transition from fish to four-legged animals is represent by one of the best time sequences showing the development of physical features from one use to another, in their case from fins to legs. Lobe-finned fishes and the earliest amphibians show this nicely, with the missing link of Tiktaalik found in 2006 (see A fish-quadruped missing link in EPN for June 2006) seeming to gild the lily. Now, yet another member of the sequence neatly connects the limb form and function of lobe-fins to the peculiar Tiktaalik (Ahlberg, P.E. et al. 2008. Ventastega curonica and the origin of tetrapod morphology. Nature, v. 453, p. 1199-1204). But perhaps the ID school will consider it a case of the designer continually changing his or her objective.

What, pray, is the platypus?

In a mood of solemn gaiety the world greeted the publication in May 2008 of the the duck-billed platypus or Ornithorhynchus anatinus genome (Warren, W.C. and a very large number of other authors 2008. Genome analysis of the platypus reveals unique signatures of evolution. Nature, v. 453, p. 175-183). My reaction to the title of the paper was, ‘So it blooming well should’. The eponymous platypus has few rivals for oddness: it has a beak for a start; detects its prey using electrosounding; has venom-injecting spurs; females lay eggs but suckle little platypuses, despite having no nipples (the milk is exuded by belly skin when sucked); has fur like an otter; no teeth and the male ejaculates sperm that hunt in packs. It lives in Australia and has kindly eyes. The vast authorship needed considerable space to fully document this strange package of characteristics, leaving little room to expand on the novelty of its genome. In a nutshell, the platypus combines features both reptilian and mammalian: no surprise there. But it is dissimilar from ducks.

Vivipary in armoured fishes

The extinct placoderms  were formidable predators of Silurian to Devonian seas and brackish waterways; in fact they were the vertebrates of those Periods. Being covered by articulated platy armour, their heads are well represented in the fossil record, but their aft parts are not, having been naked of protection. They were anatomically advanced creatures, but succumbed to the late-Devonian mass extinction, unlike other fishes, including those that figure in the ancestry of all terrestrial vertebrates. Placoderms provide the first example of the evolution of live birthing, not to recur until the evolution of the higher mammals in the last 100 Ma. Evidence for placoderm vivipary comes from an astonishing Australian fossil that contain embryos a few centimetres long (Long, J.A. et al. 2008, Live birth in the Devonian period. Nature, v. 453, p. 650-652).

A volcanic nursery for life

Aside from Darwin’s ‘warm, little pond’, all sorts of environments have been suggested for the origin and early nurturing of life. One possibility is in the nutrient-rich cavities between pillows in ocean-floor lavas. The evocative black smokers marking hydrothermal springs on the ocean floor have long been known to host abundant live, from the microbial to the large. Yet the entire volcanic pat of the oceanic lithosphere interacts with water to source hydrothermal vents. The hydration and oxidising reactions that take place in basalts are exothermic, and so yield plenty of energy, both thermal and chemical. This retrogression has offered potential for biological chemautotrophy since mantle-derived magmas first met liquid water; arguably since 4.5 Ga ago. A study of organic infestation of glassy pillowed basalts reveals that today there are up to ten thousand times more prokaryotic cells in exposed seafloor basalts than there are in the overlying seawater (Santelli, C. M. and 7 others 2008. Abundance and diversity of microbial life in ocean crust. Nature, v.  453, p. 653-656). The study relied on RNA sequencing of organic material in the glasses, rather than microscopic examination.

Using thin sections and high-powered microscopes shows up tell-tale signs of the effects of colonisation of surfaces on fractures in oceanic basalt, backed up evidence for the cells themselves. The effects are distinctive and potentially offer a means of judging microbial colonisation of ancient crust, especially that of early Archaean age.

A 0.8 Ma history of changing greenhouse gases

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Polar ice cores have presented us with the most exquisite records of how high-latitude climate has changed in the recent past from indirect clues presented by variations in stable isotopes of oxygen and deuterium (temperature change), dust and sulfate content (aridity and volcanicity respectively) in layers of ice. That proxy record extends back to 800 ka in the Dome C core from Antarctica, showing in great detail the course of the last nine glacial-interglacial cycles, both the astronomical effect of a changeover from a 40 ka pacing to one of around 100 ka and many intricacies on a millennial time scale. The most tangible archive of information resides in the air bubbles trapped by the original snow that eventually turned into ice. That reveals how the intricate pacing of climate change has been almost perfectly tracked by the global carbon cycle as shown by changes in the concentrations of carbon dioxide and methane. This was first demonstrated by cores through the Greenland ice cap, which penetrate just the last glacial episode and the warmth before and after.

After several years of painstaking bubble analyses at many collaborating labs, the full 800 ka greenhouse-gas records from Antarctica have now appeared (Luthi, D. and 10 others 2008. High resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, v.  453, p. 379-382. Lulergue, L. and 9 others 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature, v.  453, p. 383-386). These long records demonstrate the close connection between climate and greenhouse gases that must be maintained by complex (and not fully understood) feedback mechanisms. Different Earth processes affect the two principal gases, methane probably being controlled by effects of varying temperature and rainfall on peat-rich swamps in the tropics, whereas carbon dioxide’s main driver is capture and release of carbon by the oceans. The central feature remains that of astronomical forces, with perhaps some sign of a signal from the 413 ka component of orbital eccentricity from a shift in the range of temperatures and greenhouse gases in 100 ka cycles around 450 ka ago, and a broad change in methane concentrations. Yet, despite being a pole away from high northern latitudes where comparison of the Greenland ice record with North Atlantic sea-floor sediment data revealed a northern cause for dramatic short term shifts, much the same millennial cycles characterise the whole Antarctic record. It could be that these rapid changes are proxies for the course of northern climate vagaries – there are about 75 of them in the methane Antarctic record. So stunning are the new data that they are sure to spur attempts to go back even further by more drilling in Antarctica, probably in the eastern ice cap where current air temperature and snow fall are extremely low and a greater length of time may be preserved in a smaller thickness of ice. That is because the faster snow and ice accumulate the more rapidly flow removes the record: the reason why the thick Greenland ice, although capable of yielding time resolution of as little as individual years, cannot retain records much beyond 200 ka.

See also: Brook, E. 2008. Windows on the greenhouse. Nature, v.  453, p. 291-292.

 

The yellowing of the Sahara

As Earth emerged steadily from the last glacial maximum, around 14.8 ka when temperatures were close to those of the Holocene yet sea level still had a way to rise before reaching its current level, the Sahara became a land of wetlands, lakes and grassland. Many caves within its modern arid confines contain superb artwork depicting its fauna and the forager-hunters that preyed on it. Around the time of the earliest Pharaonic civilisation on the Nile floodplain (~3000 BCE) the humid episode ended, forcing inhabitants of the Sahara either to the Nile valley of the Mediterranean coast. Having spanned the millennium-long climatic upheaval of the Younger Dryas and the relative stability and warmth of the early Holocene, why it ended is something of a mystery. A small, amazingly beautiful lake in northern Chad seems to hold the key, as it has existed and gathered sediment for at least 6 thousand years (Kröpelin, S and 14 others 2008. Climate-driven ecosystem succession in the Sahara: the past 6000 years. Science, v. 320, p. 765-768), Lake Yoa is one of several permanent lakes fed by ancient groundwater from the vast Nubian Sandstone aquifer, yet receives negligible rainfall. The uppermost lake sediments are laminated in an annual fashion so that each layer and its contents of aquatic organisms, pollen and dust can be precisely dated.

Between 4200 and 3900 years ago the lake changed from a freshwater habitat to a salt lake when evaporation overcame recharge by rain. However, the environment as a whole did not change suddenly, but progressively. The sudden change in salinity resulted from Lake Yoa losing any outflow, which previously had removed salts accumulated by evaporation of the inflowing groundwater. The lake would then no longer have had any use for humans and their livestock, but conditions did not drive people out of the Sahara suddenly.

Complexities of the deep mantle

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The use of seismic signals from many receiving stations to probe physical properties of the Earth tomographically is producing increasingly sharp results from the deep mantle. In a fascinating review of the state of that art, combined with results of high-pressure experiments that throw light on deep mantle changes in mineralogy and density, Edward Garnero and Allen McNamara of Arizona State University present some stunning graphics (Garnero, E.J. & McNamara, A.K. 2008. Structure and dynamics of Earth’s lower mantle. Science, v.  320, p. 626-627). Their scope is global, and dominated by thermochemical upwelling plumes and superplumes, zones towards which whole-mantle convection has swept dense material, and some indication of a connection between the two huge phenomena. It seems there are also pockets of magma close to the core-mantle boundary, which are hinted at by abnormally low shear-wave velocities.

Global wildfires at the K-T boundary debunked

Among the minuscule treasures of the K-T boundary deposits across the world are abundant amounts of what researchers have generally called soot. Interpreted literally, these seem to point to massive combustion of living vegetation at the time of the Chicxulub impact. That presupposes two things: that oxygen levels in the late Cretaceous were sufficiently high (~30%) to support combustion of green vegetation and heating from the entry flash of the Chicxulub projectile. The first is possible, but not the second, for not all the planet would have been bathed in the flash caused by compressive heating of the atmosphere ahead of the inbound planetesimal. Nonetheless, global forest fires were the accepted wisdom. A closer look at the ‘soots’ from eight K-T boundary exposures reveals that they are not made of charcoal, which vegetation burning would produce (Harvey, M.C. et al. 2008. Combustion of fossil organic matter at the Cretaceous-Paleogene (K-P) boundary. Geology, v. 36, p. 355-358). Instead the resemble carbonaceous nanospheres that result from incomplete combustion of pulverised coal or oil aerosols in power stations. By chance, the Chicxulub impact was next to what is now one of the most productive oilfields on Earth; the Canterell field in Mexico.

Astonishing stratigraphy of the north pole of Mars

Since, so far as we know, not a single sentient being has set foot on the Martian surface the title of this item might seem strange; but it is true. One of the features of microwave radiation is that it is capable of penetrating through solid surfaces and imaging the subsurface, given the right conditions. This phenomenon is best exploited by ice, and ground-penetrating radar is routinely used for sounding Earths glaciers and ice caps. To a lesser extent sedimentary layers can be penetrated, provided they are very dry. Radar is also an extremely useful remote-sensing tool with which to examine surfaces, and no planetary mission would be complete without some kind of radar instrument. The US Mars Reconnaissance Orbiter carries a radar system targeted at just such penetration – the Shallow Radar or SHARAD.

SHARAD is operated along traverses and provides cross sections of the subsurface that look very like seismic sections, with structure picked out by reflecting surfaces. Crossing the north polar ice cap of Mars, SHARAD reveals a simple layered sequence (Phillips, R.J. and 26 others 2008. Mars north polar deposits: stratigraphy, age and geodynamical response. Science, v. 320, p. 1182-1185). Nonetheless the layering is interesting as it reveals what appear to be cyclical processes involved in the ice cap’s evolution; perhaps by ~million-year periodicity in Mars’s obliquity or orbital eccentricity. The radar transparency of the north polar region is probably down to almost pure ice, around 1 km thick. Therein lie clues to another Martian feature: its lithosphere is very strong and thick. That conclusion stems from the lack of any significant annular topographic bulge around  the ice cap. Kilometre thick ice on Earth would result in a measurable feature of that kind, due to displacement of the underlying asthenosphere. The post-glacial relaxation of such a bulge that once lay to the south of the British ice cap is responsible for the drowning of valleys in SW England especially, and measurable subsidence of southern Britain today.

See also: Kerr, R. 2008. Layers within layers hint at a wobbly Martian climate. Science, v. 320, p. 867.

Other Martian oddities

A wonderfully written and illustrated summary of some of the strange recent findings about Mars appeared in the 24 May 2008 issue of New Scientist (Clark, S. 2008. Fire & ice. New Scientist, v. 198 24 May 2008 issue, p. 35-39). It emphasises the role of water and the chaotic orbital and spin behaviour of the ‘Red Planet’ in shaping its surface. Clark draws a picture of mystery and weirdness that will surely appeal to all Mars buffs.

How to spot impact sites that others have missed

The Earth’s surface is not peppered with obvious impact craters, as are the surfaces of other planetary bodies, because our planet is active tectonically and in terms of weathering, erosion and sedimentary deposition. Craters here get ‘ironed-out’ or buried quickly. Yet there is no way that the Earth could have escaped the episodic rain of objects large and small that results from gravitational perturbation of asteroids and comets by the complex motions of the giant planets. Finding signs of past impacts adds to knowledge of their effects on life, for example, as well as on the processes that accompany ‘mountains that fall from the sky’: it is a damn sight cheaper than doing the field work on the Moon or Mars. Astonishingly, a large impact site straddling a major highway in New Mexico escaped detection until recently (Fackelman, S.P. et al. 2008. Shatter cone and microscopic shock-alteration evidence for a post-Paleoproterozoic terrestrial impact structure near Santa Fe, New Mexico, USA. Earth and Planetary Science Letters, v. 270, p. 290-299). The clue that something swift and terrible had occurred in New Mexico during the late Precambrian were strange structures in road cuttings that looked like cartoons of Christmas trees. They consist of multiple cone-shaped features nested together in masses up to 2 m long and 0.5 m across. Other processes can form these strange structures, but finds of shocked minerals and signs of melting in the rocks affected by the cones confirmed a suspicion of a nearby impact structure. Shatter cones can easily be overlooked by geologists who have never seen such features before. The fact that those in New Mexico occur in recent road cuttings helped the authors spot them. At known impact sites shatter cones occur exclusively within the zone of uplift at the centre of complex craters. Those in New Mexico occur over an area about 3 km across, suggesting a minimum size for the now vanished crater of 6-13 km across.

A drop off the old block?

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It is not so long ago that detachment and foundering of material from lithospheric blocks began to be visualised as a means to explain large areas of recent, rapid uplift of the continental surface. Chunks falling from the subducted slabs beneath Tibet and Kamchatka (see Evidence for slab break-off in subduction zones in EPN September 2002) may have generated unusual magmatism or stopped volcanism respectively. Massive Himalayan uplift and that of areas such as the Sierra Nevada in the western US seem to indicate foundering of large masses of mafic rocks from the base of thickened lower crust (see Mantle dripping off mountain roots in EPN October 2004). Even the end-Miocene Messinian salinity crisis in the Mediterranean has been ascribed to uplift resulting from delamination (see When the Mediterranean dried up in EPN May 2003). Yet convincing evidence from seismic data are conspicuous by their rarity. A necking, or monstrous boudinage of the subducting slab beneath the Hindu Kush region of the Himalayan chain is convincingly demonstrated by geophysicists from the Australian National University (Lister, G. et al, 2008. Boudinage of a stretching slablet implicated in earthquakes beneath the Hindu Kush. Nature Geoscience, v. 1, p. 196-201).

The setting for this remarkable ‘caught in the act’ phenomenon is where a minor ocean basin closed when the Kohistan arc was accreted to Asia during the closure of the Tethys ocean, and is in the process of vanishing. Wherever such minor basins have been caught up in major destructive-margin tectonics they seem to coincide with markedly arcuate orogens characterised by high-P metamorphism and repeated stacking of thrust slices. Once school of thought seeks a solution by some kind of ductile ‘dripping’ of mantle, which the authors sought to test by looking at seismicity beneath the most prominent of these arcuate mini-orogens. What they found was a zone of ‘necking’ defined by clustered earthquakes on either side. Detailed analysis suggests that a drop-shaped mass is in the process of detaching itself by a combination of brittle and ductile deformation –a boudin several orders of magnitude than any the have previously been described.

Desert varnish

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Just as vultures are annoyed by glass eyes, so geologists who use remote sensing detest vegetation cover. But the spectral blanket thrown over geology by grass and other plants is not the only irritation and one occurs where least expected. Arid terrain usually pays the best dividends in remote geological mapping, because the spectral properties of rocks and their constituent minerals emerge in reflected and emitted radiation  and bear close relationships to those determined in laboratories. Images captured from orbit that use carefully chosen wavebands are often stunningly informative in deserts. The bugbear is desert varnish, an often shiny black coating that completely masks what lies beneath, be it basalt, granite, sandstone or carbonate, even in the field. Generally it is no more than a millimetre thick, and often far thinner. Close examination often shows a minutely botryoidal texture and parallel laminae in cross section, very like a tiny stromatolite. Basically, desert varnish is such a biofilm deposit, and the responsible organisms are cyanobacteria, as in stromatolites, but exceptionally sturdy ones. However, the bulk of the material is inorganic, and it is spectrally featureless, hence the problem in remote sensing.

Widespread as it is in arid environments, desert varnish has not been deemed an appropriate subject of study, so any information is welcome (Garvie, L.A.J. et al. 2008. Nanometer-scale complexity, growth and diagenesis in desert varnish. Geology, v. 36, p. 215-218). Hailing from Arizona University, the authors are well placed. Their approach is no so much directed at organic aspects, which is a shame, but at the geochemistry of this annoying gunk. As previously known, they show the dominance of manganese phases, but mixed in with very fine-grained quartz, clays and iron oxy-hydroxides. The varnish seems to contain a wind-blown component, but the manganese and probably the iron is derived in some other way, having grain sizes less than 100 nanometres. Iron and manganese minerals dominate the fine laminae, and at very high electron microscope resolutions their grains show yet finer structure at 1 nm scale. The authors ascribe the cyclical structures and mineralogy to repeated wetting and drying, with leaching and oxidation of Fe and Mn. Both iron and manganese are multi-valent, Mn more so than Fe. For both to be leached, i.e. drawn into solution as Fe2+ and Mn2+ ions, requires strongly reducing conditions, and then oxidation to precipitate Fe3+ and Mn4+ or Mn7+ minerals. At this minute scale, whatever the source of the Fe and Mn, a biological influence seems crucial.

Renewed interest in desert varnish seems to be connected with Mars – the study was partly financed by NASA. Yet, none of the Martian remote sensing studies report annoyance with huge tracts blacked out by manganese minerals. Such surface alteration that has been analysed by the Mars Rovers proved to be iron-enriched with little significant manganese enrichment. If desert varnish is biogenically mediated, then its occurrence on Mars would be cause for excitement bordering on hysteria. The cyanobacteria in terrestrial varnishes are tough, and may date back into Precambrian times as the first colonisers of dry land. As yet, there have been no attempts to examine their genetic affinities.

New hope for very old molecular phylogeny …

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Although DNA has been obtained from a number of fossils, including Neanderthals, its complexity more or less rules out any being preserved in a useful state beyond a few hundred thousand years ago. However, information about molecular relatedness also emerges from protein sequences, albeit with less chance of detailed comparisons. Collagen from bone is a potential resource for palaeobiologists, and fossils as old as the Jurassic Period have provided useable sequences. Prime targets are large extinct animals, as the greater the mass of a bone, the better the chance that it preserves some. Two irresistible beasts are the American mastodon (Mammut americanum) and T. rex (Organ, C.L. et al. 2008. Molecular phylogenetics of mastodon and Tyrannosaurus rex. Science, v. 320, p. 499). Unsurprisingly, the research group from Harvard, Boston and North Carolina, found that a Pleistocene mastodon contains proteins closely similar to those of African elephants. The T. rex, however, has a passably close relationship to the ancestral chicken, the South Asian Red Junglefowl (Gallus gallus) and the ostrich (Struthio camelus).

In fact, both connections were expected by the team, for their research set out to show that it is possible to extract intact parts of protein sequences from fossil bones. The matches confirm their hopes, and seem set to launch attempts at resolving evolutionary relationships among vertebrates that hitherto have depended on morphology alone.

Impact cause for Younger Dryas draws flak

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Almost a year ago two dozen scientists presented evidence to suggest that onset of the Younger Dryas at 12.9 ka followed upper atmosphere explosions of cometary material (Firestone, R.B. and 25 others 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences of the United States of America, v. 104, 16016-16021; see Whizz-bang view of Younger Dryas in EPN July 2007). Evidence cited included: excess iridium; tiny spherules; fullerenes containing extraterrestrial helium; nanodiamonds and evidence for huge wildfires. Not quite the Full Monty, as neither crater nor shocked mineral grains were claimed, hence the teams’ opting for a cometary airburst. In North America such signs were said to overly the last known occurrences of Clovis tools at 7 archaeological sites (see Clovis First hypothesis dumped above). It was pretty clear that the suggestion for a hitherto unnoticed event with a widespread signature – 26 sites either side of the Atlantic were cited –  was going to be challenged, and so it has (see Kerr, R.A. Experts find no evidence for a mammoth-killer impact. Science, v. 319, p. 1331-1332), perhaps not unconnected with the blaze of publicity surrounding the paper’s appearance, including several TV documentaries.

Well, say experts, sooty layers do suggest large-scale fires, but forest fires occur every year, especially when humans are around. Fullerenes or ‘buckyballs’ equally can form terrestrially, except those containing ET helium. The last is regarded by many critics as ‘inventive’; they have never been isolated since such combinations were first reported in 2001 (see Extinctions by impacts: smoking artillery in EPN March 2002). The accepted methodology for detection of tiny diamonds seems to have been ignored, and that claimed to have found them misused. The iridium ‘spike’ – crucial in identifying the global nature of the K-T event – by itself is not enough for claims of impacts. Astonishingly, the authors cited such a Younger Dryas iridium spike in a Greenland ice core, yet the originator of those data says his paper does not report abnormal iridium at 12.9 ka or anywhere during the YD. Microspherules rain down all the time with interplanetary dust, and do not constitute sound evidence either.

So, what on Earth is going on? A collaboration between 26 authors, who willingly supply other workers with materials for checking surely cannot be conspiring at a hoax. Impact experts are hinting at ‘over-enthusiasm’ by a team outside the ‘impact community’. It all sounds oddly similar to the furore that in 1980 greeted  first suggestions by the Alvarezes for the K-T impact…

May geologists now synchronise their watches?

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Calibrating the stratigraphic column to absolute time depends, of course, on radiometrically dating geochemically suitable rocks or minerals. Yet there is a range of available methods based on decay of unstable isotopes, such as 14C, 40K, 87Rb, 147Sm, uranium and thorium. All depend on a variety of assumptions, of which that of a constant, well-established half-life is common to all. If all were perfect, several methods applied to the same materials should give the same results. The trouble is, each parent isotope favours different minerals and different compositions of igneous rocks, so that discrepancies in the dates assigned by different methods to the same stratigraphic unit may either be due to disturbance of one isotopic system relative to the other or to the half-life of one (or both) parent isotope being inaccurate. Currently, the two most widely used and best-regarded methods are U-Pb and Ar-Ar, the latter depending on 40K being converted to 40Ar by neutron bombardment. The first often uses zircons, the second various potassium minerals such as alkali feldspar. Both minerals are magmatic in origin and so the same igneous rock may sometimes be dated by either method or both. It is becoming increasingly clear that the two approaches do not give the same age, which is worrisome at the detailed level permitted by the high precision of each of the methods.

A means of checking the timing parameters for radiometric dating is to compare its results with absolute age determined by a non-radiometric method. The best-calibrated and most widely possible method that does not rely on radioactive decay is based on the astronomical pacing of climate, with its 100, 41, 23 and 19 ka cycles. Analysis of cyclicity in repetitive sedimentary sequences reveals patterns of frequencies that match the astronomical signals. So, within such a sequence it is possible to chart time differences to within a few thousand years. If there are igneous rocks interlayered with the cyclical sediments it should be possible to check their radiometric age differences against the difference determined independently. A Miocene sequence in Morocco has many intercalations of igneous tephras, and therefore provides a crucial test for radiometric approaches (Kuiper, K.F. et al. 2008. Synchronizing rock clocks of Earth history. Science, v. 320, p. 500-504). The team from the University of Utrecht, the Free University of Amsterdam in the Netherlands, and the University of California, dated sanidine (K-feldspar) from the tephras using the Ar-Ar method. This involved using a standard age determined for sanidines from a similar rock type at Fish Canyon in Colorado USA. By turning the approach on its head, i.e. by using astronomically calibrated ages for the samples, they recalculated the age of the Fish Canyon standard. It seems to be 0.65% older than previously thought (from rather dodgy U-Pb dating of  zircons in the Fish Canyon Tuff).

All Ar-Ar ages involve the Fish Canyon standard. So, an underestimate of its age would imply revision of quite a lot of geological events dated by Ar-Ar, especially those that happened abruptly, such as mass extinctions, impacts and magnetic reversals. Using the new standard age puts the K/T boundary event back to 66 Ma from 65.5 Ma. The formerly 251.0 Ma mass extinction at the end of the Permian becomes 252.5 Ma, which coincides better with the outpouring of the Siberian Traps. Similarly, the once 200 Ma end-Triassic extinction, but now possibly 201.6 Ma, links better to the Central Atlantic Magmatic Province outpourings. Quite a stir may be on the horizon, if Kuiper and colleagues’ recalibration is confirmed by similar independent measures.

That radiocarbon dates need to be used with caution is well known, as the amount of 14C produced by cosmic ray bombardment of atmospheric nitrogen varies markedly over time. Again, the ‘work-around’ involves using non-radiometric ages to calibrate the fluctuating relationship between radiocarbon ages and real time. The data of choice are those from tree-ring analysis, but ice cores also preserve ages with a 1-year precision from their annual layering. The Younger Dryas cold period that interrupted the global deglaciation began when atmospheric 14C production was high. It was also a tremendously important event in the progress of human migration and perhaps even genetics – population crashes in hard times can have a ‘bottleneck’ effect on evolution. A multinational team has addressed the interrelations between radiocarbon dating, ice-core climate proxy records and tree-ring analysis for this crucial episode (Muscheler, R. et al. 2008. Tree rings and ice cores reveal calibration uncertainties during the Younger Dryas. Nature Geoscience, v. 1, p. 263-267). They combined measures of varying 14C in tree rings and 10Be in ice cores, both of which are cosmogenic. Rather than resolving the issue, they discovered that the best marine record of the carbon-cycle during the YD, in the Cariaco basin off Venezuela, has a bias caused by anomalous concentration of 14C in shallow seawater as the YD began. Their study open the possibility of resolving such changes in the marine C-cycle.

See also: Kerr, R.A. 2008. Two geological clocks finally keeping the same time. Science, v. 320, p.434-435.

Great surprise: Deccan flood volcanism emitted gases

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The only documented volcanic eruption resembling those thought to characterise effusion of flood basalts was of the Icelandic Laki fissure in 1783. At 14 km3 its lava volume was minuscule compared with those of ancient flood-basalt flows, but it did have a remarkable effect on the atmosphere and climate of the Northern Hemisphere. A bluish, ground-hugging dry fog spread over much of Europe and North America. The fog caused severe chest ailments and was probably full of sulfuric acid aerosols. Such droplets also serve to increase the reflectivity of the atmosphere, thereby reducing solar heating. In fact, witnesses remarked on how dim the summer sun appeared that year, although it seems not to be particularly chilly. The climatic effects emerged the following winter with the average temperature in Paris falling by almost 5°C from the long-term average. On Iceland itself, crops failed during the eruption, but worse was to come. Both livestock and humans developed the awful bone lesions associated with fluorosis, for the basalt magma emitted hydrogen fluoride as well as SO2. Human and animal skeletons from the time show gross bone deformities, often like fibrous needles that would have grown through living flesh. Gas emissions from modern basalt flows chemically similar to those of Laki and far larger flood basalts are well documented, and the potential climate effects of continental flood basalt magmatism have been modelled repeatedly using those data.

Measuring actual gas contents of the magmas that fed ancient lava flows is difficult, simply because most magma degasses before it finally crystallises. Even vesicles are devoid of pristine gas that formed them, due to later percolation of fluids. In a few extremely fresh flows some of the original magma may have been preserved as glassy blobs trapped within phenocrysts such as olivine or Ca-plagioclase that formed in magma chambers before eruption. A group from the Open University, UK has analysed sulfur and chlorine content in four such minute samples by electron probe and XRF, finding levels up to 1400 and 900 ppm respectively (Self, S. et al. 2008. Sulfur and chlorine in late Cretaceous Deccan Magmas and eruptive gas release. Science, v. 319, p. 1654-1657).  The sulfur values are not unusual compared with modern basaltic glasses that have not lost their magmatic gases, though chlorine concentrations are somewhat high in the known range.

The climatic and environmental implications of both gases are noteworthy, mainly because each basalt flood would have emitted hundreds to thousands of teragrams of each annually – vastly more than modern emissions by both humanity and active volcanoes. In the lower atmosphere effects would have been like those of Laki – locally choking fogs acid rain, and cooling. Had chlorine reached the stratosphere it would have destroyed ozone to increase exposure of terrestrial life to UV radiation. So quite a few large-scale kill mechanisms may be ascribed to continental flood basalts such as the Deccan province.

This may well be the first direct evidence for actual gas-emission potential of ancient basalt magma samples. Sadly, however, the specimens containing glass were erupted some time before the K-T extinction event – the on-line data supplement reports ages of 66-68 Ma for the lower Deccan flows in which glass inclusions occur, between 0.5 to 2.5 Ma earlier than the end of the Cretaceous. That undermines, to some extent, the need to have analysed the glasses in the first place, when modern data serve well for modelling the effects of CFBs.  Still, even at the low end of S and Cl contents of modern undegassed basalt magmas, the stupendous volume of any flood basalt province – up to millions of km3 – would have repeatedly placed great stresses on the biosphere. The wonder is that not all CFBs are associated with mass extinctions, so maybe the environmentally less-destructive CFB provinces since 250 Ma ago (8 out of 11) involved magmas with extremely low S and Cl contents…

Clovis First hypothesis dumped

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For decades palaeoanthropology of the Americas has been dominated by a single idea; that nobody entered the continents before those people who used the elegant fluted spear blades first found near Clovis, New Mexico in the 1930s. These were eventually dated at a maximum age of around 13 ka before the present. One reason for accepting the Clovis people as the first Americans, apart from the lack of conclusive evidence for any earlier occupation, was the fact that glaciers blocked the route from the Bering land bridge of the last Ice age until about 13 ka. Increasing evidence has suggested earlier penetration by people who did not use Clovis tools from Asia, which reached Chile by around the same time and possibly as early as 33 ka. However, none of the evidence is definitive and the Clovis First hypothesis has been stoutly defended against this growing body of contrary evidence.

The ‘traditional’ idea of American occupation by humans after 13ka has taken a double whammy from an unusual set of fossils – of human excrement – discovered in a cave in Oregon. These have been dated at up to 15 ka and are unmistakably human, containing human mtDNA with genetic signatures typical of Native Americans (Waters, M.R. & Stafford, T.W., Jr. 2007. Redefining the Age of Clovis: Implications for the Peopling of the Americas. Science v. 315, p. 1122-1126; Gilbert, M.T.P et al. 2008. DNA from pre-Clovis human coprolites in Oregon, North America. Science, DOI:10.1126/science.1154116).

Ideas of how and when the Americas were colonised are changing rapidly after decades of ossification. A fascinating article in the 14 March 2008 issue of Science magazine reviews the issues and prospects (Goebel, E. et al. 2008. The late Pleistocene dispersal of modern humans in the Americas. Science, v. 319, p. 1497-1502). Genetic studies of living native Americans suggest their common ancestry in a Siberian population no earlier than 30 ka, and perhaps as late as 22 ka. The Beringia land bridge had repeatedly created a possible migration route during every major glaciation followed by many of the Pleistocene mammals that inhabited the Americas, but not by humans until the late stages of the last glaciation. Dating of archaeological sites and remains, including the human coprolites found by Waters and Stafford, is slowly pushing back the earliest evidence for a human presence to around 15 ka, several trhosand years before the Clovis culture appeared. Sometime before that, the first Americans had arrived and begun to spread. Ice barred their way through the interior of Alaska and NW Canada, and they must therefore have travelled along the coast, where the way was open from Beringia to Cape Horn; perhaps they used boats to move along the flat, but frigid shores of Beringia and the rugged western seaboard of North America. Early populations subsisting on shoreline resources would not have needed the heavy projectiles of the Clovis culture that are more attuned to ‘big-game’ hunting on plains. That may explain the sudden appearance of Clovis artefacts once access to plains was possible around 13.5 ka and its equally sudden disappearance at the start of the Younger Dryas around 12.8 ka when survival on icy plains would have become very difficult. Interestingly, the period of occupation of Siberia around 30 ka, would have presented the Beringia route to migration to North America when climate was similar to that following the last glacial maximum. So far, no tangible evidence

Homo floresiensis had big feet

Controversy has raged about her identity since the skull of a minute female hominin was unearthed from the Liang Bua cave on the Indonesian island of Flores. On the one hand are authorities who believe the fossil is that of a distinct human species, while on the other are sceptics convinced that the diminutive stature and chimp-like brain capacity reflect some pathological issue in a population of ordinary humans. The 12 April meeting of the American Association of Physical Anthropology in Columbus, Ohio (see Culotta, E. 2008. When hobbits (slowly) walked the Earth. Science, v. 320, p. 433-435) were treated to an anatomical exposition of the rest of the Liang Bua skeleton. A great deal more turns out to be different from human characteristics, including the legs and feet. Amusingly, for J.R.R. Tolkien’s Hobbit had them, the feet of H. floresiensis were disproportionately large. Also, her gait was quite different from ours – a kind of careful, high-stepping plod. Although not all agree, the post-cranial bones of H. floresiensis appear to bear close resemblance to those of early Homo species. Those favouring a separate species from our own suggest either that it arose through allopatric speciation from SE Asian H. erectus  after isolation of a population on Flores, or perhaps even that it is a relic of an early migration of H. habilis from Africa almost 2 Ma ago. Whatever, it is now going to be even more difficult not to speak of hobbits.

Orrorin walked the walk

Orrorin tugenensis is one of those fossils over which palaeontologists tend get heated. It is a hominin, old (~6 Ma) and fragmentary, so it just might be the daddy of us all. That possibility takes a significant step forward with statistical evidence that Orrorin walked upright in a similar manner to the much later australopithecines and paranthropoids (Richmond, B.G. & Junggers, W.L. 2008. Orrorin tugenensis femoral morphology and the evolution of hominin bipedalism. Science, v, 319, p. 1662-1665). The study was made independently of the original discoverers, who claim that the femur has especially human-like features. Whichever, one of the original suggestions that Orrorin  was on the ancestral line to gorillas has become improbable. The creature clearly displays the oldest known example of a bipedal gait (the older Sahelanthropus (~7 Ma) is known only from skull fragments and teeth, although its skull’s foramen magnum hints at bipedalism). In itself, Orrorin’s walking biomechanics is remarkable, as molecular evidence suggests that the branching that led to chimpanzees and to hominins is not much older than 6 Ma. It does seem as if that phylogenetic split may well have centred first on adaptation for traversing open ground from a forest common ancestor.

Colonisation of Europe pushed further back

Europe is so close to Africa that in recent years repeated waves of immigrants have crossed the Straits of Gibraltar, often on frighteningly flimsy craft. Their driving force is simply the search for a better life in the booming economies of Spain and Italy. Far more intense pressure from deteriorating climate and vanishing game drove Africans of many earlier times to escape their home continent, reaching back almost 2 million years. So how come the European hominin record is so short? At last count it went to H. antecessor around 750 ka, albeit a species that was sufficiently adventurous to reach British shores (see Earliest tourism in Northern Europe in EPN of January 2006). The famous Sierra de Atapuerca cave systems in northern Spain have now yielded clear evidence of much earlier occupants from around 1.1 to 1.2 Ma ago in the form of a lower jaw fragment in association with tools and bones showing signs of butchery (Carbonell, E. and 29 others 2008. The first hominin of Europe. Nature, v. 452, p. 465-469). Provisionally, the person has been assigned to H. antecessor, and there are two possible interpretations: either (s)he was a new immigrant from Africa, or represents a new speciation in northern Spain from an earlier population of African colonists. The paper’s title may prove to be premature.