Author: zooks777

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

Hydrogen isotopes test uplift hypotheses

Published on by zooks777Leave a comment

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

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

Has Dune been discovered?

Published on by zooks777Leave a comment

Titan, where Kurt Vonnegut’s Sirens sang, is, as we all know, a foggy world shrouded in hydrocarbons. The Huygens probe that sank to its surface revealed a tantalising glimpse of its strangeness, with possible erosion by liquid methane rivers and sediments of icy substances. But Huygens didn’t really tell us much, like the probe that lasted a few minutes on equally obscure Venus. To map a foggy world you need orbital radar. The Cassini mission, the mother ship for Huygens, carried a high-resolution radar imaging system, and the results are astonishing; Titan has monster sand dunes (Lorenz, R.D. and 39 others 2006. The sand seas of Titan: Cassini RADAR observations of longitudinal dunes. Science, v. 312, p. 724-727). They dwarf all but the largest terrestrial dunes in Namibia, rising to 200 m. They are linear dunes, spaced at around 4 km, and trend parallel to Titan’s Equator, where there must be a wind belt. So far only a few images have been returned, so the extent of the dune systems is unknown.  However, they correlate with optically dark material that is extensive in the equatorial region, so Titan may be dominated by dunes. For dunes to form presupposes an abundant supply of particles small enough to be picked up and transported by winds. The images from different latitudes suggest that transport is equatorwards. What those particles are made of is impossible to tell from radar returns, but most likely they are either organic solids or ice. Notions of Titan being bathed in hydrocarbon oceans now fall flat, as the areas that are not dunes seem to be topographic highs.

Folds in the mantle

Published on by zooks777Leave a comment

Seismic tomography – the processing of records of  seismic waves from many earthquakes that arrive at the world-wide network of receiving stations – continues to add detail to structures in the mantle. It is based on 3-dimensional mapping of variations in wave speeds that gives clues to variations in temperature and rheological properties at depth. One of its most fascinating outcomes has been the detection of thick, steeply dipping sheets of anomalous material well below the 660 km mantle discontinuity where earthquakes cease to occur, i.e. where the whole mantle behaves in a ductile manner. These show signs of linkage to near-surface destructive plate margins, and have been ascribed to lithospheric slabs that continue to be subducted as discrete entities to as deep as the core-mantle boundary (CMB). If that were the case, it follows that their accumulation in this D” region might displace other deep material laterally, perhaps to set mantle-wide convective plumes in motion.

One such sheet occurs deep beneath the Caribbean, and is attributed to the remnants of  a lithospheric plate, once forming the foundation of the eastern Pacific, which ceased to form once North America had overridden the East Pacific Rise. By analogy with the 160 Ma width of the West Pacific plate, this one would have been sufficiently extensive to reach the CMB once subducted. New tomography beneath the region no only suggests that it did, but that in doing so it accumulated as a heap of buckled material (Hutko, A.R. et al. 2006. Seismic detection of folded, subducted lithosphere at the core-mantle boundary. Nature, 441, p. 333-336). The reconstruction from tomographic results is highly reminiscent of the folding that occurs when honey or treacle is tipped into a tumbler of hot tea and falls to the bottom.. If the interpretation is correct, part of .the D” zone is made up of gigantic recumbent folds of former oceanic lithosphere.

Afar and the African superplume

Seismic tomography has also played a role in mapping zones in which hot, low-density mantle is likely to be rising – a contribution to understanding how plumes give rise to near-surface hot spots and major intra-plate volcanism. One of the largest active and long-lived zones of such thermal and magmatic activity is that of Ethiopia and Yemen, connected somehow with the opening of the Red Sea, Gulf of Aden and the East African Rift system; the Afar plume. This began about 45 Ma ago in Kenya and southern Ethiopia, reached its climax with the rapid extrusion of vast continental flood basalts of the Ethiopia-Yemen province around 30-26 Ma, and continues today in the Afar Depression. Thought by some to be a classic example of how a single upwelling of hot, low-density mantle generated a magmatic and tectonic hotspot, an alternative view is that the Afar plume is a mere near-surface part of a vast and complex system of anomalous mantle beneath the whole of southern and eastern Africa. Tomography based on the world-wide network of seismic observatories is unable to resolve the matter one way or the other. Geophysicists of the Pennsylvanian University and Carnegie Institution in the USA have analysed data from a more closely spaced network of temporary seismic stations around the famous RRR triple junction of Afar (Benoit, M.H. et al. 2006. Upper mantle P-wave speed variations beneath Ethiopia and the origin of the Afar hotspot. Geology, v. 34, p. 329-332).

The results outline a wide (>500 km), elongated region of low P-wave speeds below 400 km that trends south-west from Djibouti, roughly parallel to the Ethiopian Rift. This is far too large to represent a classic plume, whose tails are thought to be no more than 100-200 km diameter, and whose heads on reaching the base of the lithosphere are no more than 100-200 km thick, despite spreading laterally to a radius of up to 2000 km. The huge structure is more consistent with a broad mantle upwelling that penetrates down to the lower mantle. Lower-resolution tomography does show anomalous low-speed mantle in a broad zone, which is deep in the mantle below southern Africa then rises obliquely towards the vicinity of Afar. The more detailed results support the influence of this African ‘superplume’.

Crustal spreading from the Tibetan Plateau

In the mid 1970s Peter Molnar and Paul Tapponnier proposed that the active tectonics of eastern Asia were driven by gravitational collapse and lateral spreading of the huge mass of thickened crust that had accumulated beneath Tibet after India collided with Eurasia. The driving forces for such lateral spreading are variations in gravitational potential energy (GPE) due to regional differences in surface elevation. In the oceans, such GPE adds to plate driving forces as sliding from oceanic ridge systems that are elevated relative to abyssal plains because ridges are underlain by warmer, lower density oceanic lithosphere. Partly because the continental surface is not covered by water up to 4 km deep, the stresses resulting from GPE associated with Tibet’s high elevation are about twice as large as those connected with ridge slide. Computing the variations in GPE in eastern Asia and the adjoining oceans allows the magnitudes and directions of stresses due to gravitational spreading to be mapped (Ghosh, A. et al. 2006. Gravitational potential energy of the Tibetan Plateau and the forces driving the Indian plate. Geology, 34, p. 321-324).

One of the oddities discovered by Ghosh et al. is that the dominant stresses resulting from GPE differences in Tibet are oriented N-S and would tend to cause crustal spreading in those directions. Yet the surface of the Tibetan Plateau is riven with numerous N-S normal faults that indicate current spreading in E-W directions, as Molnar and Tapponier surmised. Somehow the N-S gravitational extension forces must be cancelled out, probably by traction between the lithosphere and motion of the underlying mantle driven by sea-floor spreading from the ridges in the Indian Ocean. One possibility is that the known buckling and thrusting within the oceanic part of the Indian Plate is a reflection of this balance. However, the stresses that emerge from the GPE calculations are simply not large enough to account for this intraplate deformation.

Implications of a mismatch between hominin genes and bones

Published on by zooks777Leave a comment

Finds in Kenya, Ethiopia and Chad during the first few years of the 21st century suggest that bipedal hominins, perhaps on the human clade, emerged as long ago as 7 Ma. Even using the previously accepted molecular-clock age for separation of chimpanzees and hominins, this is dangerously close to the time of the last common ancestor of both (5-10 Ma). Results from comparison of more detailed chimp and human genomics (Paterson, N. et al. 2006. Genetic evidence for complex speciation of humans and chimpanzees. Nature, doi:10.1038/nature04789, online) throw up a bewildering series of possibilities. On Patterson et al’s reckoning, our descent split from that of our nearest relatives no more than 6.3 Ma ago and perhaps as recently as 5.4 Ma, implying an overlap between tangible evidence and that based on DNA. Of even greater concern is the fact that human and chimp X-chromosomes are more similar than the rest, and seem to have diverged even later. One way in which this greater similarity could have arisen is if natural selection had been operating more strongly on X-chromosome genes, which studies of other related species show to have stemmed from hybridisation. Genes found on X-chromosomes that make hybrids less fertile can create strong selection pressures on this chromosome. An explanation that takes into account the young date of apparent splitting and strong selection operating on X-chromosomes is that the actual speciation(s) did take place before the time when the oldest hominin fossils were preserved, but that there was common interbreeding between the two closely related lines. 

Understandably, palaeoanthropologists and geneticists are arguing heatedly, but failing to recognise the great differences between fossils and extant genetic evidence: each is bound to tell a different part of the story. Yet another is the ecology connected to either lineage, the end point being a regional separation into creatures of forest and open savannah, separated by considerable distances in Africa – basically west and east of the East African Rift system. Before that climatic and vegetation-cover schism what would there have been to stop a great many branchings from either lineage of very closely related animals? The rarity of fossils from either may leave the true relationships early in the history of both clades completely impenetrable. One thing is for sure, although chimps and humans today do make close friendships, that is as far as it goes…

See also: Holmes, B. 2006. Did humans and chimps once merge?. New Scientist, v. 190 20 May 2006, p. 14. Pennisi, E. 2006. Genomes throw kinks in timing of chimp-human split. Science, v. 312, p. 985-986.

Hobbit matters

Debate about the significance of the tiny hominid fossils from the Indonesian island of Flores (H. floresiensis) continues to escalate. The remains are sufficiently complete for analysis of other things than size and morphology of skull and brain. It seems that the shoulder structure is different from that of modern humans, but more similar to that of full-sized H. erectus (see Culotta, E. 2006. How the hobbit shrugged: tiny hominid’s story take a new turn. Science, v. 312, p. 983-984). In ourselves, when standing straight, our inner elbows face slightly forwards so that we can work with both hands in front of the body. The necessary twist in the humerus is somewhat less in H. floresiensis, and by itself that would inhibit being able to make tools. However, the shoulder bones of the fossil articulate differently with the hobbit humerus so that a hunched posture would allow intricate work, but not an overarm throwing action. Much the same features characterise the well-preserved upper bodies of H. erectus fossils from Africa and Georgia. Incidentally, like J.R.R Tolkien’s fictional Hobbit, H. floresiensis also had disproportionately large feet.

It seems inescapable that H. floresiensis did make tools. As well as the 90-12 ka artefacts found in the Liang Bua cave with the hominid remains, which some have reckoned to be too complex for the small people to have made the, large numbers of similarly sophisticated stone tools have been found at other sites in Flores. These occur with similar prey species, but not hominid remains, from as long ago as 800 ka; a time at which only H. erectus was present in the Indonesian archipelago (Brumm, A. et al. 2006. Early stone technology on Flores and its implications for Homo floresiensis. Nature. V. 441, p. 624-628).

The minute size of H. floresiensis, with a brain capacity of a mere 400 cm3, continues to cause some researchers to doubts that the fossils – in fact 9 sets of remains from Luing Bua – were other than congenitally deformed modern humans: microcephalics. Anatomist Robert Martin of the Chicago Field Museum of Natural History (see www.sciencemag.org/cgi/content/full/312/5776/999b) used scaling factors of other dwarfed mammals from island faunas to model the body versus brain size to be expected for similarly dwarfed hominids that might arise from isolated H. erectus. He calculated that the 400 cm3 brain of H. floresiensis should be associated with a creature with around 11 kg body mass: about the size of small monkey. But that conflicts with the fact that the famous skull shows no signs of other deformities associated with microcephaly (See Culotta, E. 2006. How the hobbit shrugged: tiny hominid’s story take a new turn. Science, v. 312, p. 983-984).

The Younger Dryas and the Flood

Published on by zooks777Leave a comment

Between about 12.9 and 11.5 ka the progress of warming from the frigidity of the Last Glacial Maximum was rudely interrupted. For over a thousand years conditions returned to those of a mini ice age, with continental glaciers re-advancing on a large scale, an increase in aridity and a reversal of colonisation of high northern latitudes by both plants and humans. Pollen records become dominated by those of a diminutive alpine plant, the mountain avens (Dryas octopetala) from which the cold snap gets its name – the Younger Dryas. The pace at which cooling took place was dramatic, and glacial conditions swept in within a decade at most. The most likely scenario is failure of North Atlantic Deep Water to form, thereby shutting down the thermohaline circulation that draws the warming Gulf Stream into the Arctic Ocean off the northern cape of Norway. The reason for that was a massive and sudden freshening of surface water at high latitudes in the North Atlantic, but where the influx of fresh water came from is a puzzle. Wallace Broeker of the Lamont-Doherty Earth Observatory in New York State resurrected an earlier idea that a vast lake of meltwater in the region of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. Broeker has recently reviewed this hypothesis (Broeker, W.S. 2006. Was the Younger Dryas triggered by a flood? Science, v. 312, p. 1146-1148).

Oxygen isotope records from sediments in the Gulf of Mexico had been recording massive influx there of water depleted in 18O; a sure sign that the Mississippi was carrying much of the water produced by melting of the Laurentian ice sheet. That signature stops abruptly at the outset of the Younger Dryas. The meltwater must have found another outlet, but so far its oxygen isotope signature has not been conclusively discovered. As well as the St Lawrence escape route there are three other possibilities: north-westwards along the MacKenzie River valley; beneath the great ice sheet and through Hudson Bay; and by massive break-up of the ice sheet to launch an ‘armada’ of icebergs that quickly melted to freshen northern Atlantic waters. One of the clearest signs that vast proglacial lakes suddenly emptied is that they carve immense channels resembling canyons, in which there is abundant evidence for extreme scouring. Examples are the ‘channelled scablands’ of the state of Washington, and the Minnesota River valley. The volume escaping at the start of the Younger Dryas would have been so immense that such overflow channels would be dominant features of northern North America’s terrain; but there are few that fit the bill, and those that do exist are poorly constrained by radiocarbon dating. The lack of accurate dates for sediments and channels associated with the demise of the Laurentian ice sheet is the main obstacle, and surely evidence for exactly how the sudden plunge into glacial conditions was triggered will emerge sooner rather than later. One thing seems certain, the Younger Dryas was a freak event. The new ice core from Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) penetrates the previous six glacial maxima and shows no sign of a similar event at their terminations.

Sedimentary evolution of the Arctic Ocean: a start is made

For the Northern Hemisphere, especially around the North Atlantic, what happens in the Arctic exerts a strong influence over climate. On the one hand, ice-cover increases the proportion of solar energy that is reflected back to space, giving a cooling effect. On the other, cooling and increasing salinity of high-latitude water at the ocean surface results in its sinking to draw in warmer waters from further south, to extend warming further north. The two are linked intricately, for sea-ice formation adds to surface waters’ salinity. How and when the delicate balances arose remained poorly known while thick sea ice prevented ships penetrating to the highest possible latitudes in the Arctic Ocean, because the key to climate evolution depends on access to long core through ocean-floor sediments. Ironically, the decrease in Arctic ice cover with global warming has created greater access by icebreakers and drilling vessels. A consortium of countries around the Arctic funded a major effort to resolve the gap in knowledge through such a marine drilling programme in 2004. Results from the polar expedition have just begun to emerge (Moran, K and 36 other 2006. The Cenozoic palaeoenvironment of the Arctic Ocean. Nature, v. 441, p. 601-605). The cores were taken almost at the North geographic pole on the Lomonosov Ridge, a sliver of continental crust separated from its connection with the northern Russian continental shelf when North Atlantic sea-floor spreading nosed into the Arctic about 57 Ma ago.

The core is from sediments deposited on the Lomonosov ridge since it became detached from Russia, and is over 400 m long. Analyses are not yet complete, and the report by the IODP Arctic Coring Expedition covers the simplest parameters to determine: sediment bulk density and lithology, and micro-organisms. Nonetheless, these preliminary results provide a major surprise. Previously it was believed that frigid conditions in northern polar regions became established long after the Antarctic developed an ice cap 43 Ma ago, which matches the Cenozoic fall in atmospheric CO2 and other evidence for lower mean global temperatures. The first glaciation in the Arctic was thought to be at 2-3 Ma, when pebbles dropped by icebergs first appear in the cores from the North Atlantic floor. In the Arctic Ocean core, such pebbles appear at much the same time as those around the Antarctic. They become widespread by 14 Ma. At the time of the Palaeocene-Eocene global warming, in response to massive methane emissions at 55 Ma, the Arctic waters were as warm as 18°C. The record is one of transition from a greenhouse world to an ice house. Surprisingly, considering the later influence of thermohaline processes that draw in warm water from lower latitudes, the earliest period is marked by fresh or at most slightly brackish waters. That was probably a result of isolation from the Atlantic and an excess of precipitation over evaporation. The early sediments record abundant carbon, then at around 14 Ma, the percentage of buried organic carbon drops dramatically to mark the start of increasing frigidity, when icebergs dropped significantly more debris in the Arctic Ocean.

HOW THE AMAZON FORMED

The world’s largest drainage system in the Amazon basin is so huge that it might seem to be an eternal feature of South America, at least since that continent formed when opening of the South Atlantic wrenched it from Africa in the Triassic. The upper Amazon takes much of its flow from rainfall in the eastern slopes of the Andes, but that range is still in the process of formation by tectonic and volcanic forces. A review of the Amazon’s evolution in a recent issue of Scientific American (Hoorn, C. 2006. The birth of the mighty Amazon. Scientific American, v. 294 May 2006, p. 40-47) shows that the river system is much younger than you might have expected. Lots of evidence points to the major eastward flow only beginning in the late Miocene, after 15 Ma ago. Before that drainage was northwards into the Caribbean, the reason being that the north-eastern Andes of Columbia and western Venezuela had not formed. When they did begin to rise, they hindered flow to create a huge wetland in what is now eastern Columbia. Eventually a northward drainage route was definitively blocked, so that flow took the easiest remaining route to the ocean; eastwards, to create the Amazon basin.

San Francisco centenary

Published on by zooks777Leave a comment

That 18 April was 100 years since the Magnitude 7.9 earthquake that raised San Francisco to the ground and killed more than 3000 is no cause for celebration. Yet it focussed seismologists to commemorate the event, as if that was necessary following hard on the heels of two of the most shattering natural events of the last century. In fact San Francisco created the science of seismology, rocking as it did the most vibrant city in the world’s emerging superpower. It brought the San Andreas Fault into common parlance, and research on that huge and structurally odd fracture – one of the largest transcurrent systems on the continent – played a major role in the development of plate tectonics. In the US, a century of attention to seismic hazards has made it, along with Japan, the leader in attempts to forecast earthquakes and subdivide half a continent in terms of seismic risk (see Here is the earthquake forecast in the July 2005 issue of EPN).

The 1906 San Francisco earthquake is reviewed in issues of three generalist journals (Lubick, N. 2006. Breaking new ground. Nature, v. 440, p. 864-865. Holden, C. 2006. Reliving the ‘Frisco quake. Science, v. 312, p. 345. Marshall, J. 100 years on, you’d think San Francisco would be ready. New Scientist, v. 190 15 April 2006, p. 8-11). In each, different graphics show the estimated risk of earthquakes and the degree of seismic hazard in relation to the many large faults in California. Yet the Sumatra-Andaman earthquake that set the Indian Ocean tsunamis in motion on 26 December 2004, and that in Kashmir in October 2005, between 20 and 40 times more energetic than San Francisco, killed hundreds of times more people and devastated the lives of millions more. As well as more widely deploying well-known, sensible and moderate-cost measures to build and site habitations more safely as regards the shaking effects of seismic waves, a great deal is left to learn about the global nature of earthquake hazard. A first step is better understanding the actual processes to which great earthquakes are related, and lessons are beginning to stem from the research on the Sunda subduction zone, whose movement unleashed terror around the entire Indian Ocean (Briggs, R.W. and 13 others 2006. Deformation and slip along the Sunda megathrust in the great 2005 Nias-Simeulue earthquake. Science, v. 311, p. 1897-1901). The Nias earthquake involved failure of the Sunda subduction zone in a 400 km gap between that affected by the Sumtra-Andaman event of 2004 and a stretch further to the SE that had three great earthquakes between 1797 to 2000; i.e. a previously quieter sector had succumbed to tectonic forces. That emerged from seismic analysis at the University of Ulster (see Yet more Indian Ocean earthquakes? Sadly, yes in the April 2005 issue of EPN). Briggs et al. examined hundreds of patches of coral reef around the islands of Nias and Simeulue, using preciseGPS measurements of the elevation of coral heads that had been uplifted and killed by exposure to the air. Their results show that uplift was as high as 3 metres with some areas subsiding by around a metre, but the total movement by thrusting beneath the islands was of the order of 11 metres.

Tibetan uplift: looking a gift horse in the mouth

Published on by zooks777Leave a comment

The old saying stems from it being possible to tell the age of a horse, indeed that of a number of herbivores, from the number of dark and light bands that show on the worn surface of its teeth. Because grasses contain abrasive material, such animals’ teeth grow throughout their lives, different coloured material being laid down depending on the time of year. But there is a great deal more to this annual layering, from a chemical standpoint. By looking at various isotopes that are incorporated into enamel and dentine, it is possible to say where a horse – or a human for that matter – once lived (from variations in strontium-isotopes proportions for instance), and what it ate. The second forensic sign can be worked out from the carbon isotopes that a tooth has picked up during growth. Grasses have different proportions of carbon isotopes than those of other kinds of plans, such as shrubs and trees, the one depending on the so-called C3 type of photosynthesis and grasses on the C4 process. Each takes up carbon isotopes in measurably different proportion (d 13C in grasses is significantly lower than it is in C3 plants). Using carbon isotopes from teeth of fossil vegetarian animals is therefore a useful way of checking on the past proportions of grasses and other plants – often controlled in some way by climate. Neogene sediments of the Tibetan side of the High Himalaya contain abundant vertebrate faunas, and in view of the controversy over when the Tibetan Plateau began rapidly to rise (see When did Tibet Rise? in March 2006 issue of EPN) their dental geochemistry is a potentially useful approach to take. New results are somewhat at odds with those from other methods (Wang et al. 2006. Ancient diets indicate significant uplift of southern Tibet after ca. 7Ma. Geology, v. 34, p. 309-312).

Previous work using another approach (see When did Tibet Rise? in March 2006 issue of EPN) strongly suggests that southern Tibet was above 4 km elevation as far back as the Middle Eocene (40 Ma). Carbon isotopes in the teeth of Late Miocene Tibetan horses and rhinoceroses show that they ate a great deal of grass, unlike the modern yaks and wild herbivores that have to browse C3 plants. Wang and co-authors interpret this to signify that the southern Tibetan Plateau was considerably warmer than today, and also much lower: maybe around 2.5-3.5 km rather than the present 4 km or more. For elevation to change by 1-2 km in 7 million years suggests remarkably rapid uplift late in the evolution of the Plateau and adjoining Himalaya. Grasses, however, depend on both higher temperature and greater rainfall, but also on reduced CO2 in the atmosphere. They increased in their global cover only since about 8 Ma ago, when CO2 began to decline and climate cooled globally. Would it be possible for changes in the Asian monsoon to have had an effect on Tibetan vegetation, thereby explaining to dental evidence? Tibet is as dry as it is, because the monsoons now lose all their moisture in rising over the high Himalaya. If moist air and therefore cloud found its way into Tibet during the Miocene, maybe it would have been warmer too.

Mantle behaviour and the influence of minerals

Published on by zooks777Leave a comment

To most geologists minerals are a means to an end. Identifying them and working out their relative proportions in a rock provides a quick means of assessing its rough chemical composition. Textural relations between minerals help work out the sequence of processes that were involved in its evolution, and in the case of metamorphic minerals what pressure and temperatures were involved. In the case of the Earth’s mantle, however, mineralogy comprises only one or two abundant minerals – olivine and pyroxene at shallow depths, and the mineral perovskite (MgSiO3) at depths greater than about 670 km – and dominates the mantle’s physical properties and bulk behaviour. There are distinct, narrow zones or discontinuities that separate different seismic properties and these have long been considered to represent changes in mineralogy of the more or less uniform bulk composition of the mantle. The most likely phase transition is from olivine + pyroxene to perovskite, in response to increasing pressure, thought to occur at about 670 km down. That transition was confirmed by high-pressure experiments, but whether that simple mineralogy persists down to the outer core has remained a mystery. Using tiny diamond anvils in a laser-heated furnace to create the enormous pressures at depths up to 2700 km is fraught with technical difficulties, but Kei Hirose and Shigeaki Ono of the Japan Marine Science and Technology Centre have finally achieved them (see Cyranoski, D. 2006. Magical mantle tour. Nature, v. 440, p. 1108-1110).

Hirose and Ono discovered that perovskite itself collapses to produce another, more tightly-packed molecular structure – post-perovskite with a sheet-like structure. This phase transition occurred experimentally under conditions that characterise the thin D” layer just above the core-mantle boundary. Seismic tomography has suggested that a number of weird things happen there. For instance, seismic S waves near the CMB have different speeds according to their direction of travel, and even accelerate in some parts. The platy structure of post-perovskite, unlike the more regular perovskite, is likely to create such physical anisotropy, especially if grains are aligned. The mineral, when iron enters its structure, may also help to explain thin (5-40 mm) zones in the D” layer in which seismic wave speeds fall by 5 to 30% compared with expected values (Mao, W.L. et al. 2006. Iron-rich post-perovskite and the origin of ultralow-velocity zones. Science, v. 312, p. 564-565). When first detected by seismic tomography, these zones had been assumed to involve regions in which partial melting occurred. It also seems that the phase transition is temperature- as well as pressure-dependent, so that post-perovskite could form at shallower depths in cooler regions. Being denser than its parent, that could result in sinking: like slab-pull at shallow depths, such a gravitational force would contribute to whole mantle convection by displacing hotter D” material. That in turn would ‘flip’ through the phase transition in the reverse fashion to become less dense, perhaps encouraging the initiation of rising plumes.

Sure enough, what might seem to be a boring bit of exotic mineralogy promises to exert some control over speculation on what happens at the bottom of the mantle. But it is too early to say how seminal the discovery might be – the errors in the experiments correspond to a depth range of about 350 km. On top of that, other experiments need to be conducted under these extremely difficult conditions, such as finding out if post-perovskite can chemically interact with the iron-rich outer core, and if its electrical properties are in some way different from those of better-understood perovskite.

A fish-quadruped missing link

Published on by zooks777Leave a comment

Rich as the fossil record is, it is terribly incomplete, for the obvious reason that the chance of preservation over fragmentation and destruction of body parts is extremely small. That is especially the case for the high-energy and oxidising land and freshwater environments. Each fossil species can easily be assumed to be a one-off, appearing, thriving for a short while and then disappearing: ripe for the assumption of divine creation, as Linnaeus assumed. Very rarely indeed, specimens emerge that fill in the many gaps needed by evolutionary theory, the most celebrated being Archaeopterix that bridged the gap between dinosaurs and birds. That transition has been enriched by a whole series of older fossils from Chinese lagerstätten that show the transition in sublime detail.

The comparative anatomy of fish and land vertebrates suggests a common ancestry, and the Devonian to Early Carboniferous terrestrial record has yielded tantalising fish with lobed fins (e.g. Eusthenopteron and Panderichthys) and almost fish-like animals with four rudimentary limbs (e.g. Acanthostega and Ichthyostega). Yet a gap remained to be filled in the apparent transition from aquatic to land-dwelling vertebrates. US palaeobiologists engaged in seeking candidates from the Late Devonian of Arctic Canada have found one that reduces any uncertainty tremendously (Daeschler, E.B et al. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, v. 440, p. 757-763. Shubin, N.H. et al. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, v. 440, p. 764-771). The fossil, prepared with lengthy and painstaking care, shows such amazing anatomical detail as to demonstrate clearly that the fin and shoulder girdle are indeed intermediate between fish and tetrapods, whereas previous candidates supporting a transition are either definitely fish or tetrapods. Tiktaalik slots nicely into the time gap too, about 2 Ma younger than the most tetrapod-like fish Panderichthys and slightly older than fish-like quadrupeds. The outcome of a deliberate search for an animal to fit the gap, Tiktaalik above all demonstrates the predictive capacity of palaeontology, which counters a common epithet flung by those bent on divine intervention and/or intelligent design. Based on this outstanding success, fossil hunters will be encouraged to sift on a stratigraphically finer scale for yet more steps in vertebrate evolution, including our own.

See also: Ahlberg, P.E. & Clack, J.A. 2006. A firm step from water to land. Nature, v. 440, p. 747-749.

Hominid evolution: a line or a bush?

Published on by zooks777Leave a comment

From the late 19th century it has been clear that two species of our genus Homo inhabited Europe and the Middle East: modern humans and Neanderthals. Recent partial sequences of Neanderthal genetic material, compared with the human genome, confirm that the two did not interbreed; at least, no trace of Neanderthal genetics remains in that of modern humans. The discovery in Indonesia that fully modern immigrants occupied the same territory as Homo erectus from 70 to 20 thousand years ago adds more weight to the hypothesis of multiple occupancy of the world by different kinds of humans until recent times. The astonishing discovery in 2003 of the remains of tiny hominids (Homo floresiensis) on Flores whose occupancy lasted from at least 840 ka to as recent as 12 ka (see The little people of Flores, Indonesia, November 2004 issue of EPN) confirms mixed occupancy late in hominid evolution. That includes several different representatives of Homo – habilis, eragster and erectus – and also paranthropoids in Africa around 2 Ma years ago. As regards Homo, this cohabitation, especially that in Africa, supports two hypotheses: that our lineage was bush-like and involved separate extinctions and sudden appearances of new species (cladogenesis), or that the great variability in physiognomy (polymorphy) of modern humans extended back for a considerable time. The second is the view of Jonathan Kingdon, who believes insufficient hominid fossils have been collected to rule out polymorphism among tool-using and tool-creating beings. The idea of a single lineage since the first appearance of bipedal apes that led unerringly through gradual changes to modern humans (phyletic evolution) has been largely discarded. For at least part of the 6-7 Ma hominid record, that abandonment of phyletic evolution may have to be reconsidered, following a report of remarkably productive excavations in the Awash Valley of NE Ethiopia (White, T.D. and 21 others 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature, v. 440, p. 883-889).

The Middle Awash is the single most productive area for hominid remains and other fossils that help establish changes in their environment. That is so because of consistent collecting for more than two decades by a multinational team, co-led by Ethiopian and US palaeoanthropologists, from a sequence of flood plain sediments over 1 km thick, liberally interlayered with dateable volcanic horizons. Its middle parts record three species, Ardepithecus ramidus, Australopithecus anamensis and Australopithecus afarensis (of which ‘Lucy’ was a member), in an age range from 4.42 to 3.88 Ma. White and the other members of the team have unearthed 30 new fossils of all three species, but, so far, no examples of more than one in a particular thickness of sediments. Of course, ‘absence of evidence is not evidence of absence’, but this massive addition to the Pliocene hominid record is a challenge to the prevailing hypothesis of cladogenesis – Steven J. Gould’s idea of punctuated equilibrium, in which species arise by sudden appearance of new characteristics from earlier ancestors. Its test is whether or not ancestral species co-exist with new species for a time. In the Middle Awash, it seems that they do not, even though the critical 300 m of sediments represents only 200 thousand years.

The three species, and their predecessor Ardepithecus ramidus kadabba (5.5-5.8 Ma), show variations in their teeth, with Ar. r. kadabba and Ar. ramidus sharing some similarities, and Au. anamensis and Au. afarensis others. The shift between the two sets of common dentition can be explained by either gradual changes in a single lineage over about 2.5 to 3.0 Ma, or a sudden speciation event, perhaps around 4.5 Ma. The lack of overlap favours the first hypothesis. Complicating factors are rife, however, for there may have been migrations (Ar. Ramidus is known from far to the south in Kenya), and yet more evidence will undubtedly be found from the vast amount of sediment of this age in the Afar Depression.

See also: Dalton, R. 2006. Feel it in your bones. Nature, v. 440, p. 1100-1101.

Palaeodentistry

Those of a nervous disposition should not read this item.

A 7500 to 9000 year-old Neolithic graveyard in Pakistan has yielded remains of about 300 people who cultivated wheat, barley and cotton, and herded cattle. There is nothing remarkable in that, except that nine individuals have teeth that have clearly been drilled neatly (Coppa, A. et al. 2006. Nature, v. 440, p. 755). The holes are between 1-3 mm in diameter and up to 3.5 mm deep, and would have exposed sensitive parts of the tooth. In excavations of the nearby village of Merhgarh are found tiny flint drill heads associated with beads of various ornamental materials. The drills are of the same size as the tooth holes. Quite probably, miniature bow-drills tipped with flint would have been used by Neolithic dentists for at least 1500 years – there is no evidence for tooth drilling from younger cemeteries in the area, despite abundant evidence of dental decay. Experiments show that such drills would take less than a minute to produce the neat holes, probably wielded by jewellers rather than dentists.

Asian Homo erectus skilled in tool making

The 1.8 Ma emigrants from Africa who first populated the Far East have not been regarded as having been especially inventive. While their ‘cousins’ in Africa developed the aesthetically stunning bi-face axe about 1.6 to 1.4 Ma ago (the first instance of visualising a finished object within a rough piece of raw material), H. erectus in East Asia is associated with the most primitive stone tools made by simply breaking flinty stones. That seemed to have been the extent of their stone-using skills up to their final demise about 20 thousand years ago –not a lot of progress in 1.8 million years. A report in March at the Indo-Pacific Prehistory Association Congress (Manila) of yet to be published work by Harry Widianto of Indonesia’s National centre of Archaeology may force a revision of this less than charitable view of early Asians (Stone, R. 2006. Java Man’s first tools. Science, v. 312, p. 361). In the Solo district of Java, made famous by Renée Dubois who found the first fossils of H. erectus there, a wealth of finely worked flake tools has been discovered in sediments that are about 1.6 Ma old. Most are small and made from blood-red to beige, translucent chalcedony. It seems that necessity was the mother of invention in this case, because suitable materials for sharp tools are very scarce in Java.