Puffing up the Moon

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Since George Bush announced that US manned planetary missions are back on the agenda, albeit in an uncertain future for NASA, barely a month goes by without some kind of scientific justification for a return to the ‘good old days’. The latest as regards future lunar missions was in the 1 April 2006 of New Scientist, as a special report ‘It’s time to go back’.  It seems there are unique opportunities that the Moon presents for a range of scientific work (Chandler, D.L. 2006. The ultimate lab. New Scientist,1 April 2006 issue, p. 33-37). The lunar far side, being shielded from radio noise from Earth, is well suited to deploying an array of miniature radio telescopes. Half a dozen 1 m dishes spread over 20 km could simulate an enormous dish. The lack of an atmosphere suggests ideal stable conditions for optical telescopes, although being on a body with a large gravitational attraction would expose instruments to meteor flux. The lunar south pole is said to look good for science. For a start, there is a 5 km peak always lit by the Sun for continuous solar power, as well as data relay back to Earth. Nearby is the deep Shackleton crater that is never lit, and is immensely cold; ideal for an infrared telescope, and maybe harbouring water ice to support a manned lunar base.

The Apollo missions returned sufficient rock and soil samples to whet planetary scientists’ appetites.  They answered a lot of questions, and did revolutionise issues of planetary origins, evolution and bombardment history, yet they raised other interesting questions. Answering geological questions from the rocks of other worlds depends a great deal on luck, and the few small sites visited by the Apollo astronauts undoubtedly left out a great deal. What is needed, it seems is a ‘Serendipity Base’. The best one would be a deep crater with steep, rocky sides, and there is one that seems just right. The Aitken basin is 12 km deep and exposes a layered structure in its walls.

Perhaps the greatest attraction is the fact that anything that falls on the Moon remains in its pristine state for all time, provided it is not buried by accumulated meteoritic dust and impact ejecta. The Moon could be a really happy hunting ground for meteorite specialists, although finding interesting ones on the dull, grey surface might pose problems – you can tell a meteorite on Earth, if you search ice sheets, deserts and saline flats, by their contrast with the background.  There is a very odd notion, however, that well-preserved ejecta from impacts on the Earth and other planets that found their way to the lunar surface might hold the keys to the origin of life (Ward, P. 2006. House of flying fossils. New Scientist, 1 April 2006 issue, p. 38-41). The reasoning goes like this: like the Moon, all planets in the Solar System have for 4.55 Ga been whacked by impacts, which must have flung debris outside their gravitational attraction. Having a strong gravitational field itself, the Moon must have swept up a sizeable representative sample of all such debris hurtling around the Solar System.  Some of the biggest impacts – again as revealed by the lunar surface – were early in planetary evolution. Debris from them would therefore be samples of materials before they had been affected by later geological processes on their parent planets. Analyses of particles in the Apollo samples indicate that perhaps 3 kg of the third of a tonne of material is non-lunar, of which a few grams might be from Earth.

Terrestrial geology effectively stops once we go back to about 4 Ga, besides which very old rocks on Earth have been subject to all manner of chemical, erosive, tectonic and metamorphic influences. That is the reason why incontrovertible fossils and geochemical evidence for life have yet to be found before 3 Ga at the earliest. There are whiffs of earlier life, which people choose to believe or otherwise, but the potential for dispute fuels continual debate. But escaped ejecta from Hadean impacts on the Earth wouldn’t have been altered so much. They could be dated, and thereby tell geoscientists about the earliest crust, now vanished apart from a few minute grains of pre-4 Ga zircons. Most attractive is the possibility that they could harbour well-preserved organic materials that are traces of the very earliest life forms or their complex precursor chemicals. But would they survive the impacts that produced them? Although impacts from objects as small as 100 m could fling debris beyond the Earth’s pull without heating it too much, Hadean impacts would have had awesome energy because the colliders were huge, as witness the mare basins on the Moon that are over 100 km across. Much of the debris from those lunar big hits is in the form of once melted glasses, and the holes that they left filled with magma generated by the huge energies involved. Some meteorites do preserve their original magnetization, which suggests they never reached temperatures above the Curie points of the minerals responsible for it. Ward cites this evidence in support of once living materials being able to survive in ancient terrestrial ejecta that almost certainly will lie on the lunar surface. But he uses it to say that meteorite internal temperatures must have stayed below 100°C: the Curie point for common magnetic minerals is around 600°C. Given the date of publication, might we be reading of a pudding with too much egg? Whatever, the origin, if not the meaning of life exerts more pull on science purse strings than the prospect of gold nuggets hiding in shadowed craters…

Yet another weird world

Saturn is well-endowed with moons: 35 with names and a whole lot of moonlets.  The Saturnian System is astonishing in its diversity, and part of the Cassini probe’s mission is to examine in detail as many moons as possible– 20 flown by in the last year. Enceladus is by no means the largest (504 km in diameter), yet it is very odd indeed. One of its singular features is its ability to jet vast amounts of water from warm spots, and the fact that it seems to snow there.  The 10 March 2006 issue of Science magazine devotes 40 pages to articles on the oddities of Enceladus. To jet water ice and vapour to more than twice its diameter – in fact to drench much of the planetary system and replenish parts of the famed ring system – there must be a powerful heat source.  Just what that is has yet to be worked out: it could be bound up with internal radioactive decay or with vast tidal sources from Saturn itself, and maybe something else entirely. Its south pole is curiously its most active part, with sufficient heat energy beneath to create a major positive anomaly in long-wave infrared images. This is where much of Enceladus’s resurfacing by snow takes place. Saturn’s tidal forces have rucked up the surface to create hilly ridges, perhaps assisted by a kind of icy volcanism. Tidal or internal forces have also opened up great cracks in the surface, which false-colour images that use UV, green and short-wave infrared reveal to be compositionally different from the water-ice bulk of the surface. That may have resulted from hydrocarbon deposits leaking from deeper layers. It is the moon’s interior that causes most excitement.  In order for it to spray off watery jets, there must be a deep source of liquid water, either a liquid shell on which an ice ‘lithosphere’ floats or produced as internal plumes by melting at an interface with a rocky core.  That there are hydrocarbons suggests that some of the watery solids include gas-hydrates (ices that incorporate both water and gases).

Discoverer of arsenic in Bengal’s water supply speaks out

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Indian analytical chemist Dipankar Chakraborti of Jadvapur University, Kolkata was born and raised in one of West Bengal’s many small villages on the delta plains of the Ganges. Paying a visit to a friend’s village in 1988, he found people bearing visible symptoms of chronic arsenic poisoning, which had not been diagnosed before. Analysing samples of well water, Chakraborti found extremely high levels of the poisonous element. For years he was reviled by government agencies who paid no heed to his discovery, calling him a ‘panic monger’ – when more recently showing that Bihar and Assam had similar problems he received death threats. Almost single-handed he campaigned for attention to the undoubted problem, until in the mid 1990s it became clear that arsenic in drinking water from recently sunk wells was a plague of biblical proportions across low-lying West Bengal and neighbouring Bangladesh.

Massive funding, both for establishing the extent and distribution of the contamination and for installing means of removing arsenic from well water, flowed form a host of international donors and agencies. To the outside world it has seemed that the tragedy was being remedied by hugely qualified teams of international scientists, and would eventually be held in check. As revealed in a recent interview (Pearce, F & Chakraborti, D. 2006. Drinking at the west’s toxic well. New Scientist, 1 April 2006 issue, p. 48-49), Chakraborti believes that intervention at national and international levels is doing far less than claimed, even exacerbating the problem by pouring in remedial filtration units without teaching villagers to maintain them. Locals’ are encouraged to trust the remedies, yet continue to drink highly contaminated water once the units clog with silts.

Timely review of nuclear waste disposal

The grand old man of biogeochemistry and the Gaia hypothesis, James Lovelock, seems to have lost patience with life’s ability – and that of alternative energy resources – to keep the Earth system in balance. His view that global warming is past the point of no return as regards ‘green’ remedies has been widely publicised in recent months: he has come out in favour of an increase in the contribution of energy by nuclear reactors. He may have fallen out with many environmentalists, but may also have become an ally of politicians who are looking to nuclear power as a way of maintaining ‘business as usual’ yet putting their money where their mouths are, as regards reducing carbon emissions.  Nuclear power may yet have a resurgence, but that would pose again the thorny problem of secure disposal of radioactive wastes. Sweden supplies almost 50% of its electricity using eleven nuclear power stations: the highest number per capita anywhere, despite the country’s otherwise ‘green’ outlook. Should nuclear power rise rapidly elsewhere, then Sweden’s approach to waste disposal may well become a model to follow.  What that system is summarised in a recent issue of New Scientist (Nielsen, R.H 2006. Final resting place. New Scientist, 4 March 2006, p. 38-41). Sweden has discovered quite a challenge at its experimental nuclear-waste disposal facility, even though most of the country’s rocks are hard and crystalline, and therefore seemingly ideal for disposal sterilised from the outside world. Despite the common view that crystalline basement is totally impermeable, in reality it is not. Water will be present in any rocks used to cache waste, unless they are beneath almost totally arid deserts, of which only the USA among developed countries has one. It is also becoming increasingly clear that even at great depths, extremophile organisms infest the rock. Among the most common are those that use the reduction of sulfate to sulfide ions as a metabolic energy source: they produce sulphuric acid. That seems a considerable risk to the integrity of whatever form the waste is stored in. The response of the Swedish researchers has been to look for lateral solutions that either kill off the bacteria using clay packing, or exploit the potentially preservative effects of others.

Faster recovery after mass extinctions

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Mass extinctions have been the principal time markers in the Phanerozoic stratigraphic column since 19th century palaeontologists recognised sudden changeovers in the fossil record. Two close the Palaeozoic and Mesozoic Eras, two more end Periods (Ordovician and Triassic) and others mark Stage boundaries. Greatest focus has been on the magnitudes of each extinction, greatly assisted by the statistics compiled by the late Jack Sepkoski. The adaptive radiations that filled abandoned niches and restored and, in most cases, expanded diversity are equally interesting.  Such recoveries from depleted stocks of organisms have been of immense influence over biological evolution. Resulting from chance events, as far as the Earth’s biota are concerned, the families and species that arose would not otherwise have appeared: the most powerful blow to any notion that biological advances are in any way pre-ordained.

Until recently, it seemed that each recovery was an extremely protracted affair. Over 5 to 10 million years seemed to be the case for aftermaths of the largest extinctions. To a marked extent, analysing recoveries from the fossil record is not so easy as tying the great declines in diversity to a time. It is a matter of working out the rate at which new genera arose or originated through speciation, and that is affected by geographic biases in the fossil record.  They arise from less collecting in remote areas and variations in the volume of exposed strata in others.  Correcting the biases is possible to some extent, but that still leaves the challenge of statistical analysis. From an extraordinary expansion of analytical expertise, which extends to economists’ methods of understanding stock market trends and the flair of physicists, a very different story of restocking seems about to emerge. A technique called vector autoregression applied to faunal diversification corrected for biases suggests that recoveries were very much faster than previously thought, in fact almost immediate by comparison with the time-precision of the stratigraphic column (Lu, P.J. Motohiro Yogo, M and Marshall, C.R., 2006. Phanerozoic marine biodiversity dynamics in light of the incompleteness of the fossil record. Proceedings of the National Academy of Sciences, v. 103, p. 2736-2739).

See also: Kerr, R.A. 2006.  Revised numbers quicken the pace of rebound from mass extinctions. Science, v. 311, p. 931.

Is the Cambrian Explosion real evidence for an evolutionary burst?

About 543 Ma ago, remains of organisms that secreted hard parts suddenly appear in the fossil record.  Most palaeontology has focussed on such easily fossilised organisms from the Phanerozoic Eon that began at that time. Whether or not the Cambrian Explosion was a truly significant event, bar the appearance of hard parts – that is quite a mystery in itself – is highlighted by the presence of members of almost all modern animal phyla in the Early Cambrian record. Did they all suddenly explode onto the scene at its outset, or were they around well beforehand as almost completely soft-bodied creatures? Comparative molecular biology of living animals, and the concept of molecular ‘clocks’ has for a while suggested that the origination of modern phyla was considerably earlier than the start of the Phanerozoic. Increasing the database on which such ideas can be based helps improve their precision and scope, assisted by novel methods of mathematical analysis. The 23 December 2005 issue of Science contained an analysis of more than 12 thousand amino acids involved in the genomes of members of 9 or 26 extant animal phyla (Rokas, A.. et al. 2005. Animal evolution and the molecular signature of radiations compressed in time. Science, v. 310, p. 1933-1938). Preliminary study suggests that indeed the early history of the metazoans was remarkably compressed in time, probably in the 50 million years after the ~600 Ma Snowball Earth event, and possibly within a few million years of the base of the Cambrian. However, tests of hypotheses based on such indirectly related data are notoriously difficult, and Rokas et al. have taken a bit of stick (Jermiin, L.S. et al. 2005. Is the ‘Big Bang’ in animal evolution real? Science, v. 310, p. 1910-1911). It seems yet more work on molecular biology of the remaining 17 phyla and a great deal of mathematical wrangling is yet to come.

Zircons and early continents no longer to be sneezed at

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Dating of detrital zircon grains found in moderately old Archaean sediments from Western Australia first pushed known geological time beyond the previously impenetrable 4 Ga barrier. The record now goes back to around 4.4 Ga, within 95% of the date when the Earth and the Solar System came into being (4.55 Ga).  There has been much written about the oxygen isotopes in this tiny number of resistant minerals regarding whether or not they originated in a crust permeated by liquid water.  Because zircon is a mineral most usually associated with rocks of granitic composition, the very presence of extremely old ones seems to suggest that some degree of fractionation of primitive basaltic magmas must have taken place in the Hadean to form highly evolved magmas.  But did actual continental material arise so early? Processes in island arcs can generate evolved magmas in which zirconium is moderately enriched.  If such a host for the pre-4 Ga zircons was small in volume, it may have been easily recycled back to mantle depths, yet would enough zircons have been eroded from it to yield those preserved in sediments a billion years younger? It is possible to probe the processes involved in zircon formation by using the extremely sluggish radioactive decay of an isotope of the rare-earth element lutetium. The half-life of the 176Lu to 176Hf decay scheme (~37 Ga) is far longer than the time since the Big Bang, so detecting changes in the proportion of 176Hf to other hafnium isotopes is a tough nut to crack, the more so as 176Lu is very rare indeed.

A consortium of geochemists from Australia, the US, France and the UK have used the famous Jack Hills zircons to test the widely believed hypothesis that substantial continental crust has only emerged since 4 Ga ago (Harrison, T.M. et al. 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science, v. 310, p. 1947-1950). They found that deviations of 176Hf/177Hf from those assumed to characterise the bulk Earth (in fact the proxy of chondritic meteorites) show large variations in the zircons. Some of the deviations are negative, which is consistent with the very early formation of continental crust – perhaps from very soon after the Earth formed. On the other hand, some zircons show positive deviations, a sign that the mantle was depleted, also pointing to crust forming events. The authors boldly suggest that such anomalies refer to a very early geochemical upheaval in the Earth, that likely produced continental material. But the 4 Ga barrier for whole rocks seems clearly to suggest that none remains: either it was all subducted away, or was only a tiny fraction from which the Jack Hills zircons miraculously emerged on their long journey to a final resting place.

Commenting on the paper, Yuri Amelin of the Canadian Geological Survey, points out that no one agrees on the true composition of the bulk Earth (Amelin, Y. 2005. A tale of early Earth told in zircons. Science, v. 310, p. 1914-1915). Other isotopic evidence raises the spectre of our planet having accreted from a mixture of geochemically different meteorite types, and has never mixed thoroughly. Moreover, zircons are notorious for being compositionally zoned, as a result of being able to survive engulfment in later magmas from which new layers of zircon grow. The measurement of 176Hf/177Hf ratios is so difficult that only whole zircons give useful results, but those data hide the variations among the zones. Finally, he points out that studies of the 176Hf/177Hf in post 4 Ga basalts – and therefore the mantle from which they were derived – show that there is a clear divergence from chondritic meteorites that began around 4 Ga, the start of the record of existing continental rocks. In the kindest way, Amelin casts doubt on the sense in studies of such tiny relics of the Earth’s distant past.

When did Tibet rise?

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As plateaux go, that forming Tibet is by far the highest and the largest. Sitting at an average elevation above 5 km and spanning about 3500 x 1500 km, it dwarfs the next in the list, the Andean Altiplano (mean elevation 3.8 km). The position of the Tibetan Plateau, ahead of the Indian subcontinent’s northward collision with Eurasia marks it obviously as being of tectonic origin. Some plateaux are possibly buoyed up by underlying thermal anomalies in the mantle (the Colorado Plateau of North America, underpinned by a subducted spreading centre), while others, such as that of northern Ethiopia, result partly from vast outpourings of flood basalts and partly from thermal effects of active mantle plumes and rebound associated with massive crustal extension.

There are two basic models for Tibet. It may have formed as a result of a near doubling of crustal thickness as Indian crust was driven beneath that of Asia, low density of the thickened continental crust acting to buoy up its vast area.  If that is so, then as soon as India collided with Asia, around 40-50 Ma ago, Tibet would have steadily risen and its plateau would have grown in extent. There are however signs of sudden changes in thermal structure, marked by large-scale magmatism of roughly Late Miocene (8-10 Ma) age. That may have been induced by an extraordinary event, the detachment and foundering (delamination) of a large mass of underlying mantle, whose loss resulted in rapid uplift of the whole overlying region. Because Tibet is known to play a central role in the mechanism that drives the South Asian monsoon, assessing the timing of its formation is crucial to understanding the onset of the monsoon and the many phenomena of accelerated weathering and erosion associated with it. Cores from the floor of the Indian Ocean suggest that the monsoon suddenly increased in intensity at around 8 Ma. Both as a sink for carbon dioxide as a result of weathering of the continental crust, and as a means of obstructing and redirecting continental wind patterns, the growth of the Tibetan Plateau and the Himalaya in front of it have been assigned a major role in the decline of global mean temperatures that resulted in northern hemisphere glaciations. So establishing the timing of their formation makes or breaks two major geoscientific hypotheses of recent decades. The key is some form of proxy for past elevations in the area. One such proxy, the stomatal index of plant leaves found in Tibetan sediments of Miocene age, showed that 15 Ma ago the southern Plateau was just as high as today (see When did southern Tibet get so high? in March 2003 EPN). That cast doubt on a later cause of uplift, but remained unconfirmed.

Sediments deposited in lakes that periodically fill Tibet’s many basins form a record that goes back at least 35 Ma. Carbonates in such lacustrine sediments offer a geochemical means of charting changes in elevation (Rowley, D.B. & Currie, B.S. 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, Central Tibet. Nature, v. 439, p. 677-681). That depends on the proportion of 18O to the lighter 16O isotope of oxygen (δ18O) in carbonate, which is believed to be inherited from rainwater that originally drained into the basins. The higher the elevation at which water falls as rain or snow, the less of the heavier oxygen isotope it contains, so δ18O is a potential means of measuring the evolution of surface elevation. For central Tibet, this shows that the topography was at least 4 km high as early as 35 Ma ago. Results from other basins that span the Tibetan Plateau clearly suggest that 4 km elevation was achieved progressively later from south to north, anging from 40 to 10 Ma ago. So the delamination model for a sudden springing-up of the Plateau seems now to be a less plausible mechanism for the uplift than the simpler model of progressive crustal thickening following the collision of India. That does not entirely rule out an episode of delamination in the Miocene, for which geochemical evidence is fairly convincing. The implication of the new results is that if Tibet has been a major influence over climate, then it was one that developed progressively from the late Eocene.

See also: Mulch, A and Page Chamberlain, C. 2006.  The rise and growth of Tibet. Nature, v. 439, p. 670-671. Kerr, R.A. 2006. An early date for aising the roof of the world. Science, v. 311, p. 758.

Climate change and collapse of early civilisations

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About 4200 years ago early civilisations of the Old World underwent decline and collapse. Examples are the Akkadian civilisation in the upper Tigris and Euphrates basins, famed for Hammurabi’s Hanging Gardens of Babylon, the Harappan of the Indus Valley (Mohenjodaro), the phaoronic Old Kingdom and the Minoan of Crete. This period of the Bronze Age has been thought by some to have experienced either massive volcanism – the explosion of Santorini – or even a comet strike. Others have correlated collapses of city states with Biblical events. Whatever happened, its outcome spanned a vast area of western Asia and north-eastern Africa, so another candidate is climatic drying leading to drought and famine. That is perhaps not such a spectacular fate as near-instant environmental upheavals, but probably just as effective for societies dependant on regular agriculture production or, in the case of Crete, on wide-ranging trade.

Detecting climate change is now well established on proxy records of one kind or another, such as those based on isotopes and sedimentation changes from sea-floor sediments and flowstone (speleothem) in caves, and dust records in ice cores. Such time-series from the mid- to late Holocene are increasing in number, with particular interest growing in records from speleothem now that precise age sequences are possible using uranium-series dating. A flowstone record from a cave in northern Italy, has helped link other time series ranging from the North Atlantic floor, in the Middle East and East Africa (Drysdale, R. et al. 2006. Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone. Geology, v. 34, p. 101-104). A team of geochemists ad environmental scientists from Australia, Italy and the UK has shown a remarkable coincidence among these widely different records, centred on 3900-4200 b.p.. From the North Atlantic at high latitudes is an upsurge in fragments deposited by ice rafting, while mean sea-surface temperatures swung downwards. Kilimanjaro ice shows a marked peak in atmospheric dustiness. Carbonate deposition peaked in the Gulf of Oman. Finally, the Italian flowstone shows peaks in d18O, d13C and the magnesium:calcium ratio of its carbonates. The conclusion is a period of climatic cooling and drying that spanned 40 degrees of latitude over a period of several hundred years. This is not the signature likely to have been associated with instantaneous catastrophes. Yet nor is it typical of the episodic climate shifts of the order of a few thousand years, which were now well known features of the last glacial period and the current interglacial. It was certainly sufficiently prolonged and large enough to have wrought havoc on early civilisations, and throughout the Old World it clearly did.

The Digital Earth revolution

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Launched in July 2005, Google Earth (earth.google.com) has become familiar to many Earth scientists.  Some, like me, may have needed encouragement to try it out. Whatever, once up and running on a modern PC with Windows 2000 or XP and broadband connection, even the free version of the software that you need to access Google Earth is compelling, even addictive.  It takes no more than a few minutes to realise that it revolutionises teaching of many aspects of Earth science, and will be used too as a top-line research tool by anyone interested in spatial data.

Based primarily on natural-colour images that cover the entire Earth, much at Landsat TM 15-30 m resolution but for some areas using other images that resolve to the order of a couple of metres or better, Google Earth also uses global topographic elevation data. This is where it takes on its revolutionising role.  It is easy to view the surface of any part of the planet in oblique perspective, when all topographic and a great many geological features show up dramatically. It is the ultimate ‘Swiss Hammer’ – mapping the complex geology of the Alps was only possible by viewing exposures in one massif from the vantage point of another. Choosing appropriate zoom factors connects geological features that are on different scales. Design of the database – it is perfectly seamless, except where resolution changes in mostly urban areas – makes it possible  at broadband connection speeds to roam in real time at any scale. This allows you to simulate flight at any altitude and with any downward look angle: ‘grand tours’ to visit all the famous geological sites you have longed for on every continent become simple. The novelty of 3-D simulation also means that there is much to discover.

Sometimes, even in one’s homeland, it is possible to get lost, especially at large scale. By turning on GIS layers for rivers and roads (in many areas populated places, even street names and fast-food outlets show) navigation is made easier. It is the linking of images with other kinds of data that gives Google Earth its potential for research power. Designed as an easy-to-use geographic information system, by purchasing professional versions of some GIS software you can add layers interpreted, almost literally, ‘on the fly’ (Butler, D. 2006. The web-wide world. Nature, v. 439, p. 776-778).

An immediate attraction, both for globe-trotting geoscientists and, more importantly, people engaged in disaster relief, is the way Google Earth makes it easy to become familiar in moderate detail with the terrain that has to be faced. Solving problems of access, assessing where assistance may be most urgently needed is helped enormously by its highly realistic geographic visualisation. Of course, it cuts down the need for very expensive helicopter reconnaissance.  Google Earth has already proved invaluable for assessing the aftermath of the October 2005 earthquake in Kashmir. Google facilitates the mosaicing of new images of disaster areas, such as those struck by Hurricane Katrina, and their incorporation into the Google Earth database (Nourbakhsh, I. 2006. Mapping disaster zones. Nature, v. 439, p. 787-788).

A few people get frightened by some of the highest resolution images that are available – even the lines on tennis courts show up – as if their privacy was being invaded. More seriously, some governments worry about security implications of anyone being able to see intimate details of airfields and ports.  That is silly – at any time the Quickbird or Ikonos satellites can take a snap of any part of the planet at up to 65 cm resolution for anyone who has the cash to pay for its acquisition; most likely intelligence agencies and military strategists. Privacy, at least from several hundred kilometres above, is a thing of the past.  Every geologist would like to get one-metre resolution images of their research areas. If they see something intended to be hidden for one or another reason, they have an obligation to be discrete.

Exactly how does life shape landforms?

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The land’s present topography is not just the frontier between the lithosphere and the atmosphere and hydrosphere, but where plants of many different kinds grow. Whether in the form of cyanobacteria, lichens or luxuriant tropical rain forest, vegetation affects weathering, erosion and the deposition of sediments. Animals – leaving out humans – also have some influence, whether they be subterranean rabbits, moles and worms, or heavy-footed beasts that force soils to move downslope. Inevitably life-land interactions affect landforms, although rock-type and active geological processes tend to dominate. Nonetheless, a planet with life ought to show different styles of surface shapes from one that is organically dead. The central issues for geomorphologists is whether or not it is possible to define absolutely the differences, and then to use them as a means of detecting the likely former influence of life on other worlds.

Central to such a venture (Dietrich, W.E. & Perron, J.t. 2006. The search for a topographic signature of life. Nature, v. 439, p. 411-418) is the ability to map in detail the variation of topographic elevation. Digital topographic elevation data is now available for most of the Earth’s land surface at a resolution of between 90 and 30 m, the second only publicly available for the USA, from the groundbreaking Shuttle Radar Topography Mission of 2000. Aerial photography and high-resolution stereoscopic images from satellite such as Quickbird and Ikonos, allow resolution as sharp as a few metres.  Laser scanning from aircraft potentially can even improve that to the scale of a few tens of centimetres, but such high-resolution data are far from global. The planet Mars is now better endowed with elevation data than is our own planet, thanks to photogrammetric instruments carried by ESA’s Mars Express mission, and the shyness of various intelligence agencies to share publicly what they have gleaned from high-altitude aircraft and spy satellites. Nonetheless, it is now possible to analyse elevation data from the entire range of terrestrial biomes to see what signal vegetation has imposed on surface shape. An easy way to visualise that is simple – just use Google Earth (see The Digital Earth revolution above).

Dietrich and Perron review the mathematical approaches to modelling life’s topographic influences, beginning with an equation that relates elevation and time to rates of uplift, erosion and entry of sediment into storage, thereby expressing conservation of mass.  All the variables are themselves governed by a variety of processes, theoretically amenable to quantification, summarised in Dietrich and Perron’s review. In each there will be some potential biotic influence. On Earth there are sufficient landscapes devoid of all but a minute veneer of organisms to assess both end-members clearly. Mars and Venus ought to be good tests.  But, should such a rigorous quantification of lifeless and lively surfaces at a spectrum of scales be achieved, where would we deploy it?

Yet more on the end-Permian extinction

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Sequences that reveal the Permian-Triassic boundary continue to receive a great deal of attention, spurred by the seemingly cryptic nature of the conditions that caused up to 90% of all living things to die. Globally, the boundary is marked by a sudden and large fall in the proportion of 13C in carbonates and sedimentary organic matter.  Since the d13C anomaly follows the biotic decline, it is less likely to reflect any cause of the extinction, such as a massive methane release from destabilised gas hydrates and global warming, than an effect of whatever went on.  Joint research by UK, Dutch and US organic geochemists focused on the P/Tr boundary in northern Italy, where it is dominated by shallow-marine carbonates (Sephton, M.A. et al., 2005. Catastrophic soil erosion during the end-Permian biotic crisis. Geology, v. 33, p. 941-944). They analysed the organic compounds preserved in the section, and found that the extinction zone coincides with a major increase in total organic carbon, which is dominated by large amounts of compounds (polysaccharides) that typify soils and leaf litter.  They explain the anomaly as the result of a short period of rapid soil erosion from the terrestrial hinterland of the shallow Late Permian sea.  Since virtually all continental crust had stabilised in the Pangaea supercontinent, tens of millions of years beforehand, such erosion was unlikely have been a result of some sudden tectonic uplift. But it might have been triggered by sudden loss of the vegetation that retards soil erosion on the continental surface. The P/Tr extinction affected both marine and terrestrial organisms, and Sephton et al recognise that their discovery of evidence for soil stripping on a grand scale reflects that unified fate. Acid rain from the massive Siberian continental flood volcanism could well have been the trigger for ill thrift of land vegetation, or maybe removal of stratospheric ozone by release of halogen (chlorine and bromine) compounds let in destructive UV radiation.

Culture and human evolution

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Culture in the most general sense that encompasses tools, clothing, habitation and fire has increasingly set humans and their ancestors apart from the rest of the natural world. It might therefore seem that becoming more ‘human’ cushions our line from Darwinian natural selection since we have created our own ‘nature’ and carry it with us. Setting fully modern humans adrift in the environment, without that culture, would undoubtedly result in rapidly extinguishing the species. In that hypothetical context we are far from ‘fit’, in Darwin’s sense. However, the development of humanity’s cultural milieu has itself provided a continually changing, increasingly pervasive artificial set of conditions for natural selection. Culturally, the most dramatic step in human evolution, for which we have tangible evidence, emerged with the explosive appearance of graphic art and a complex ‘toolkit’ around 35 thousand years ago in Europe. That huge advance will undoubtedly be traced back maybe tens of millennia when archaeological finds in Africa and Australia, for instance, are more precisely dated.  Evidence from the DNA in male-carried Y chromosomes indicates that a profound genetic shift occurred around 70 ka, perhaps resulting from a decline in global human numbers to a very small population after the climatic disaster wrought by the explosive eruption of the Toba volcano in Indonesia. That too was a time when fully modern humanity distributed itself more thinly by a decisive exodus from Africa. Some specialists have speculated that the cultural explosion stemmed from that evolutionary ‘bottleneck’.  There are genetic signs of adaptation to cultural practices and selective pressures that accompanied them after the rise of agriculture and settlement (See Has human evolution stopped?, September 2005 issue of EPN). Recent work on the whole human genome gives an inkling that even more pervasive evolutionary changes took place in the last 50 thousand years (Wang, E.T. et al., 2005. Global landscape of recent inferred Darwinian selection for Homo sapiens. Proceedings of the National Academy of Science, www.pnas.org/cgi/doi/10.1073/pnas.0509691102).

Wang and colleagues from the University of California studied the occurrence of single-letter differences in the genetic code (single-nucleotide polymorphisms – SNPs). Scattered across all human chromosomes are about 1.6 million of these SNPs. They appear not to do anything, but can be linked to nearby genes. When natural selection favours a particular mutated variant of a gene, the associated SNPs can be selected as well. The approach used by Wang et al. is a statistical search for pairs of SNPs that occur together more often than could be possible by chance ‘reshuffling’ that occurs from generation to generation. Their analysis suggests that around 1800 genes, a remarkable 7% of the whole genome, have changed over the last 50 thousand years. Interestingly, that is similar to the degree of genetic change in maize since its domestication from its wild ancestor. As well as genes connected to protein metabolism that could have changed as new diets followed the rise of agriculture, some that are involved in brain function have been selected as well.

Although at an early stage, this kind of research confirms that we are indeed still evolving along Darwinian lines, perhaps unwittingly domesticating ourselves. It is easy to assume that ideas, skills and artistic sensibilities are passed on through language and learning and thereby grow and diversify, but in order for any of these to stimulate the deep feelings that they foster suggests that some aspects have become ‘hard-wired’ in all of us. Everyone unconsciously taps their feet to rhythm, can be moved to a vast range of emotions by music, words and visual stimuli, and can ‘sense’ an environment captured, even in abstraction, by a talented artist. They inspire further development. Until around 50 ka human culture, insofar as we can see evidence for it, remained fixed for more than a million years through several species and subspecies of the genus Homo. Appearing between 1.6 and 1.4 Ma ago the bi-face stone axe endured as humanity’s highest known achievement until those very recent times.

See also: Holmes, R. 2005. Civilisation left its mark on our genes. New Scientist, 24/31 December 2005 issue, p. 8.

Earliest tourism in northern Europe

Some years ago British palaeoanthropologists were in a state of high excitement about finds of stone tools, evidence of prolonged human habitation and fragmentary skeletal remains from a sandpit at Boxgrove on England’s southern coast.  They showed the earliest human presence at high latitudes around 400-500 ka. The date of early colonisation has now been pushed back more than half as long before that to 700 ka by finds in a shoreline exposure of riverine sediments on the coast of Suffolk on England’s east coat.  The Cromer Forest Bed of Middle Pleistocene age has been know since Victorian times as a rich source of the flora and fauna from one of the earliest interglacials of the current period of 100 ka climate cyclicity. At that time the North Sea had yet to establish a connection that would eventually separate the British Isles from Europe, and the site at Pakefield would have been the estuary of a now-vanished river system draining the Midlands and Wales.  So far no human bones have turned up in the excavations, which have to be conducted at low tide. But many flint tools pepper the organic-rich sediments (Parrfitt, S.A. et al., 2005. The earliest record of human activity in northern Europe. Nature, v. 438, p. 1008-1012). As with most terrestrial deposits, establishing the age of human occupation posed the greatest difficulty. A careful documentation of magnetic polarity combined with fossils – including distinct voles – and a new technique that relies on assessing the degree of protein degradation in bivalve shells helped tie-down the age precisely.

Around 800 ka human occupation had begun in Spain and the Pakefield site shows that migration northwards of flora and fauna following a glacial epoch was swift, to establish conditions considerable warmer than in the Holocene. It seems that this Mediterranean climate encouraged such northward penetration by humans, most likely during a short period of particular warmth. Long eyed by archaeologists as a potential source of human remains, patience has paid off in the Cromer Forest Beds.  Yet around the world there are many other, equally promising strata or Pleistocene age that have not had such undivided attention for so long, A glance at the distribution of keynote sites for palaeoanthropology shows how narrow the search for human origins and migratory destination has been up to now. Though it is understandable that once finds have been made, funds and scientists cluster where progress is best guaranteed. Very rarely, either a ‘shot in the dark’ pays off or something surprising turns up at a site being excavated for other purposes. Broadening the search may well have high financial and career risks, yet the more discoveries are made at well-trodden sites the greater the likelihood that the full story of human evolution and migration will be revealed by breaking new ground,

See also: Roebroeks, E. 2005. Life on the Costa del Cromer. Nature, v. 438, p.921-922.

Biogeochemical evidence for vegetation change when hominins evolved

A long-held theory that concerns the background to hominin evolution, is that the freeing of hands by bipedalism was triggered by a shift in the ecology of East Africa from forest to more open grassland.  That might well have happened as the Neogene uplift associated with development of the East African Rift transformed the regional wind and rainfall patterns to the way they are today, thereby creating the conditions for the modern savannahs and semi-deserts in the area long associated with human origins.  The lakes of East Africa are ephemeral in the context of Neogene climate change, and so their sediments are not much use in charting long-term shifts in flora.  However, the modern wind systems shift dust and organic particles consistently towards the Gulf of Aden, so sediment cores there potentially provide a continuous record of vegetation change.  That is, if they contain ‘biomarkers’ that distinguish the debris of trees from that of grasses. The first biomarker records from the Gulf of Aden seabed powerfully confirm the notion of vegetation change as a possible driver for hominin evolution (Feakins, S.J. et al., 2005. Biomarker records of late Neogene changes in northeast African vegetation. Geology, v. 33, p. 977-980).

Up to about 3.5 Ma the cores contain plant-derived waxes that are characteristic of trees that use C3 metabolic processes, but thereafter evidence for increasing C4 grasses predominates.  Coinciding with that broad trend is an increase in 13C in soil carbonates on land, which probably reflects increased grassland too.  Although records of hominin diversity before about 3 Ma are scanty, later times saw the rise of several bipedal species, grouped as the powerfully jawed parathropoids and the more daintily chewing members of the lineage that led to modern humans. Detail in those sections of marine core that were used – presumably costs prevented continuous measurements – shows that the carbon-isotopic signals in the waxes varied in harmony with evidence for climate change, so the proportions of savannah and woodland probably shifted quite rapidly.  However, because cold-dry periods have tended to be longer than those which were warm and more humid, savannah would have had more influence over faunas than ephemeral woodland. Fascinating as this empirical relationship between hominin evolution and vegetation change is, what Africa lacks – as indeed does most of the planet – is data that chart accurately how topography has changed with time. Cosmogenic and U-Th/ He apatite thermochronology, on which so much hope and funding have been invested, has proved spectacularly ineffectual compared with careful work on the likely effects of changing landforms.