Earth-logs

Setting the fossil record to rights

Published on July 1, 2003 by zooks777Leave a comment

Much has been made of ups and downs in the diversity of life from the global fossil record of the Phanerozoic, including the possibility of massive downturns in diversity related to a variety of cause for mass extinction. However, there are many biases in what is an inevitably imperfect record of biodiversity. There are anthropogenic influences, for a start. Although they are becoming more adventurous, palaeontologists cut their teeth on sites close to home, and most of them live in the richer parts of the world. Insatiable demand for fossils, but mainly of the spectacular and valuable kinds, has grown a world-wide industry of commercial fossil mining. That may homogenise the geographic coverage of the fossil record, but it is very tempting to go for the richest troves and ignore meagre pickings. Sedimentation is by no means guaranteed to have been constant through time, partly because of ups and downs of sea level and changes in the pace of erosion of earlier rocks. Although Phanerozoic stratigraphy seems complete when sections from all over are pieced together, in any one place there are huge gaps of erosion or non-deposition. It is very easy to come upon several beds of sedimentary rock and conclude that the sequence represents a continuum in time. Not so, as any examination of such beds forming today often reveals that intact preservation is the exception compared with erosion and reworking. The global areas of exposed rocks that cover, say, 10 Ma chunks of Earth history is by no means constant either. Another factor that conspires to cast doubt on the veracity of the existing fossil record is that the numbers of possible ecological niches that once existed in different tectonic environments are probably not the same. Active oceanic arcs have few such niches, whereas tropical zones of shallow shelves have vastly more. There are lots more uncertainties, and New Zealand palaeontologists have painstakingly tried to develop some means of allowing for them in the Tertiary record of their islands (Crampton, J.S. et al. 2003. Estimating the rock volume bias in paleodiversity studies. Science, v. 301, p. 358-360). The simplest premise for estimating bias in the numbers of taxa preserved in rocks covering a particular time range is the available volume of rock from the period that can be sampled. One approach is to see how geologists have divided up that period in terms of distinct rock formations, the other just uses estimates of the areas underlain by sedimentary rocks laid down during the period. The first suggests that collecting should be systematically from formation to formation up a sequence, while the second implies that random grid sampling is the best approach. The New Zealand data suggest that the area approach is most appropriate there, largely because the local rocks formed in a sedimentologically simple, active-margin environment. Both methods seem to work in tectonically stable areas. This is just a beginning, but is raises the issue of how much weight can be placed on existing fossil collections in pondering on both titanic and slow-but-sure episodes in the last 544 Ma.

On the same tack, attempts are underway to correct the entire fossil record from 30 thousand collections, using a similar approach to sampling bias. John Alroy at the University of California, Santa Barbara has helped set up the Paleobiology Database (http://flatpebble.nceas.ucsb.edu/public/), following prompting by the most prolific fossil cataloguer, Jack Sepkoski, shortly before his untimely death in 1999. The web site allows anyone to generate diversity curves, but the process is a little complicated and best tackled by experienced palaeontologists. You can also enter information from your own collections. Early results are conflicting. Sepkoski’s original suggestion that diversity among marine faunas increased since the Triassic may be an artefact of the intensity of sampling which varies from age to age. However, using just molluscs seems to confirm that at least they did indeed radiate tremendously as Sepkoski had concluded (Schiermeier, Q. 2003. Setting the record straight. Nature, v. 424, p. 482-483).

Origins of the vertebrates

Long before techniques were developed to investigate the genetic stuff of living organisms, and when the only known repository of primitive, soft-bodied animals was the Burgess Shale, basic anatomical analysis suggested that maybe the ancestors of vertebrates were worms, sea squirts and even echinoderms. When the Burgess Shale fauna was re-evaluated and extended in the 1970’s by, among others, Simon Conway Morris of Cambridge University, it became clear that the fossil record was missing a great many delicate and sometimes very odd organisms. Entirely unsuspected phylla numbered among the occupants of that famous lagerstätte (site of exceptional preservation), but little new about our own ultimate origins.

Vertebrates, echinoderms, sea squirts and a diverse collection of worm-like animals have one thing in common, though apparently very little else. The first opening to emerge during embryonic development is the anus, whereas in the rest of the animals (protostomes) it becomes a mouth. So, in the “supergroup” to which we belong, mouths appear at a later developmental stage; hence the sack-name “deuterostome”. This oddly dichotomous embryonic unfolding points to a very early division among the animals, that might only be unveiled by discovery of even earlier lagerstätten than the Late Cambrian Burgess Shale. So far, no such source of palaeontological richness has been discovered in late Precambrian sedimentary rocks – crude “molecular clock” approaches to genetic divergence suggest that a great deal went on before the Cambrian Explosion at 544 Ma. However, the fossil-rich Cambrian of China does push back the record of delicate animals almost to that time. The recently discovered lagerstätte of Chengjiang is about 530 Ma old, and, as Conway Morris and his Chinese colleagues have discovered, it is rich in fossil deuterostomes. One group, the vetulicolians, bears a remarkable resemblance to what the pioneer vertebrate palaeontologist, Alfred Romer, suggested as a probable vertebrate ancestor – something with a front end bearing gill slits and a long, segmented tail. The Chengjiang deposit also contains jawless fish, together with unique “almost fish” called yunnanozoans that may be intermediate links between vetulicolians and fish. Similarly, there are intriguing hints that vetulicolians evolved towards the most primitive echinoderms, with bilateral symmetry rather than the fivefold form that emerged later. Clearly, the Chengjiang fauna was extremely diverse and therefore had a long evolutionary history. Since even more delicate, entirely soft-bodied Ediacaran animals were preserved as imprints in sandstones from the Late Neoproterozoic, it is maybe only a matter of time before low-energy lagerstätten are found from that time. There are abundant undeformed mudstones from that period throughout the world, but only painstaking rock splitting will find such treasures, unlike the large, “trip-over” Ediacaran trace fossils.

Source: Conway Morris, S. 2003. Once we were worms. New Scientist, 2 August 2003, p. 34-37.

Zircons that wander

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The crust beneath the British Isles is made up of several once widely separated terranes, parts of Laurentia, an arc segment called Avalonia that split from Gondwana around 500 Ma ago, and a similar terrane (Armorica) that followed Avalonia across the Iapetus Ocean to accrete to Laurentia at the end of the Palaeozoic Era. Because of its maritime position, modern Britain is cloaked in vegetation so that rock occurrences are few and far between by comparison with less humid areas. Conditions for geological investigations are made yet worse by a mantle of glacial sediments plastered on top of bedrock. So, although having been studied for longer than almost every other piece of continental crust, the evolution of that beneath the British Isles is a subject of continual controversy and surprises. Sitting at the interface between the Laurentian and Avalonian terranes, roughly where the Iapetus suture is thought to have consumed at least half of the eponymous ocean, sit the Lower Palaeozoic rocks of the Southern Uplands of Scotland. They are widely thought to have formed as an accretionary prism on the edge of the plate underidden by subducted Iapetus oceanic lithosphere until Avalonia collided with the north-British terranes at the close of the Silurian. Some of the Ordovician sediments in the pile contain clasts of volcanic rocks, which were long thought to be contemporary and giving evidence of the expected arc volcanism behind the prism. However, they turn out to be much older, now that zircons from the sediments have been dated using high-preciiision methods (Phiilips, E.R. and 7 others 2003. Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland. Geology, v. 31, p. 625-628). The zircons yielded Neoproterozoic ages (557 to 613 Ma), with evidence that some had been assimilated from older crust (1043 Ma) during volcanism. Taken at face value, the Neoproterozoic ages are similar to those of volcanic rocks in England and Wales, which formed off Gondwana in an arc setting, when the terranes were widely separated. The problem is one of getting the material across the subduction zone that separates the accreted terranes, but that is the issue proposed by the authors (all from the Natural Environment Research Council. However, such a conclusion might stem from the authors’ narrow context; that of British geology. Immediately to the north of the Southern Uplands terrane is another, poorly exposed crustal block that underlies the Scottish Midland Valley. It was directly involved in the Ordovician Grampian orogeny that formed the highly deformed Precambrian rocks of the Scottish Highlands. With a narrow view, that terrane is also a mystery, yet it has a counterpart in the Taconia terrane that is familiar to North American geologists, which was involved in orogenic events contemporary with the Grampian orogeny in Scotland. Taconia has late Neoproterozoic to Ordovician arc volcanics.

Titanic solution to unpalatable water

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Currently around a billion people are at severe risk from drinking contaminated water, and whenever there is a major human crisis refugees are placed in the same plight. The main solution would seem to be drilling wells that tap groundwater that aerobic bacterial action cleanses of most pathogens. That is essentially true, but some groundwater is rejected even by people suffering the most extreme privations. It has the appearance of water from the radiator of an aged lorry, because it contains abundant dissolved iron that immediately precipitates as red-orange slime when exposed to the air, tainting food and staining clothes. A solution may arise from studies as far from drought-stricken areas as one could possibly get; concerning the way in which deep-sea wrecks decay away. The discovery of the wreck of the Titanic in 1985 and recovery of parts of it later by marine historian Robert Ballard, revealed that its ironworks were being consumed by bacteria that created stalactite-like masses of iron oxides, known as “rusticles”. Detailed microbiological studies found a highly complex harmony of different bacteria that created and inhabited the rusticles. Effectively, they were eating the mighty ship at a rate of about a tonne every ten days by exploiting the energy released by oxidation of iron. It may prove possible to harness the habits of these iron-loving bacteria to remove iron from groundwater and make it palatable

Source: Fry, C. 2003. Iron rations. New Scientist, 26 July 2003, p. 36-37.

Glaciers of Mars

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The world has been agog these last few years as evidence has mounted to suggest that Mars still has abundant water buried beneath its dusty surface, in the form of permafrost. Early in its history there are many signs of vast floods that carved huge meandering canyons and may have filled basins with moderately long-lived seas. Yet Mars has probably always been pretty cold, as it is now, and the most likely form that surface water would have taken is in glaciers; that is, if there was ever sufficient atmospheric water to precipitate snow. As on Earth, the likeliest places to look are in mountainous regions, and Mars is not lacking in very high places. By far the largest, and indeed they are the highest mountains in the Solar System, are the shield volcanoes of the Tharsis Rise, topping out around 18 km above the Martian version of the geoid. The volcanoes have gnarled surfaces, which until recently have been regarded by most as the result of volcano-related processes. Imaging of the Martian surface has stepped up several notches in resolution in recent years, and details of the small-scale features of the volcanoes are very clear. Above all else, they resemble aspects of the nearest analogue to Martian conditions on Earth – the Dry Valleys of Antarctica. Although the Dry Valleys are now largely free of ice sheets, they show many features of former glaciation, perhaps extending back 30 Ma to the Oligocene. Their frigidity has ensured that any glaciers there were frozen to the surface, rather than having zones of incipient melting at their bases. Such cold-based glaciers move sluggishly, and produce peculiar features. Among these are moraines produced by sublimation rather than melting of the ice – they evidence no reworking by melt water – and rock glaciers that are also products of sublimation and sometimes rest on relics of former glaciers. Probable examples of both occur on the flanks of the Tharsis volcanoes, together with weird track-like assemblies of concentric ridges, that are likely to have formed on the flanks of ablating glaciers as they reached a standstill and then retreated. (Head, J.W. & Marchant, D.R. 2003. Cold-based mountain glaciers on Mars: Western Arsia Mons. Geology, v. 31, p. 641-644). Interestingly, the relationship of the glacial features to impact craters suggests that glaciation took place during the period since about 1.8 billion years ago (the Amazonian phase of Mars’ history) when bombardment had slackened to almost terrestrial rates and liquid water was unable to form on the red planet. Of course, glaciers do not have to be made of water ice, and there is still a possibility that at such immense altitudes any glaciers might have been made of solid carbon dioxide. Head and Marchant speculate that some of the features might still sit upon relics of the glaciers. It could be a bit of a disappointment if future explorers of Mars landed there expecting a water supply.

Smithsonian Dynamic Earth site

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The Smithsonian Institute’s National Museum of Natural History has a new and evolving Earth science website at www.mnh.si.edu/earth (Flash 6 and printable versions). Currently only the Rocks and Mining section is up and running, but it is instructive at the introductory level. To come are sections on gemstones, plate tectonics and the Solar System. There are also downloads and a geogallery. It is somewhat slow in Flash using a normal dial-up connection., but the printable version has no images. With a fast connection, this is likely to become a favourite for elementary visualisation of Earth processes.

Cosmogenic nuclides and tropical erosion

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In the highlands of central Sri Lanka the sediment suspended in rivers suggest rates of soil loss from agricultural land of the order of up to 7000 tonnes per km² each year. However, it is difficult to judge how much would be eroded under natural conditions, compared with the probable loss as a result of deforestation and human activities, particularly from very rugged landscapes where seasonal rainfall is high.. Radionuclides produced by cosmic-ray bombardment of minerals exposed to them, such as ¹⁰Be and ²⁶Al, accumulate in soil that is being eroded at a rate that is inversely proportional to the rate of erosion. The nuclides form in the top 0.6 m of soil, which is the depth within which cosmic rays are normally absorbed. So erosion rates that can be calculated from the cosmogenic nuclides in minerals, such as quartz, in river sediments apply to the times taken to remove that depth of soil. Essentially, the rates that are measured represent the long-term erosion within a catchment basin. Swiss and Sri Lankan geoscientists have applied the technique to rivers in central Sri Lanka, whose catchments have different vegetation cover and land usage (Hewawasam, T. et al. 2003. Increase of human over natural erosion rates in tropical highlands constrained by cosmogenic nuclides. Geology, v. 31, p. 597-600), such as forest reserves, rice terraces, tea plantations, areas of slash and burn agriculture, and various levels of degraded land. The unmodified forest catchments give the lowest long-term erosion rates of 5-11 mm per 100 ka (13-30 tonnes per km² per year) as expected, but this is about a quarter of the rate of erosion measured by the same method throughout the highland region. That probably reflects the antiquity of erosion induced by agriculture, yet current rates measured from sediments being carried by rivers suggests that soil erosion is now between 10 and 100 times faster than would occur under natural conditions.

Remote signs of earthquakes

All manner of ground-based observations have been tried as means of timely predictors of pending earthquakes, ranging from strange behaviour of wildlife to emissions of radon from wells (see Radon emissions and earthquakes, July 2003 issue of EPN). So far, none of them have been universally useful, although there have been successful evacuations of threatened populations, principally in China, whose seismologists have focused on a wide range of signals. Ideally, what is needed is some kind of global monitoring, and as with attempts to predict volcanic eruptions the only realistic means is from satellite surveillance. Long ago, Doug Shearman of the Royal School of Mines at Imperial College, London introduced me to the peculiar properties of the mineral dolomite, as discovered by the man whose name it takes, Count Deodar de Dolomieu. If you rub two lumps of dolomite together in a darkened room, they emit a sinister glow, and so do other minerals, such as quartz and even sugar. Excellent for amusing the kids. But then I learnt of “earth lights”, which had been photographed by Japanese observers just before earthquakes, in the vicinity of active faults – previously they were supposed to be as mythological as the fire balls during thunder storms (also a proven fact now). At the time, the Landsat remote sensing satellite captured images during its night-time overpasses, on request. A nice, if a little “blue skies” research project. I submitted a brief proposal to my department’s research committee for ranking along with other studentship projects. Perhaps my wry attitude to what had become somewhat dominated by other disciplines than remote sensing coloured my efforts; it was rejected. So it was with some glee, a decade later, to find that NASA and the US Federal Emergency Management Agency had been testing the idea using weather satellites and the MODIS instrument carried by the Terra platform since 2000 (Enriquez, A. 2003. The shining. New Scientist, 5 July 2003, p. 26-29). Encouragingly, though not for their victims, the devastating 1999 Izmit and 2001 Gujarat earthquakes were preceded by increased infrared emissions, detected from space, 5 days before the event. Experiments show that when rock is stressed, emissions build up, and then vanish once the rocks fails, as in an earthquake, so the method looks very promising.

Another seismic phenomenon is changing magnetic fields around the site of failure. This was first noticed from magnetometer records on the ground before the 1989 Loma Prieta earthquake that damaged large tracts of northern California. Magnetic field variations too can be monitored from orbit. The privately funded QuakeSat, launched on 30 June 2003 aims to test this possibility, as will a more ambitious French satellite, due to reach orbit in April next year (Reichhardt, T. 2003. Satellites aim to shake up quake prediction. Nature, v. 424, p. 478).

Iron isotopes and ocean evolution

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The main driver for biological activity in the oceans far from land is the availability of iron, and this helps control the burial of organic carbon and hence aspects of global climate. At low Fe concentrations, as they have been since the oxygenation of the surface environment from 2 billion years ago, iron is cycled in the marine environment in a matter of a few hundred years. So, ocean water responds very quickly, in geological terms, to changes in the source of any dissolved iron. There are two main sources, discharge of hydrothermal fluids from the oceanic lithosphere and delivery of river water and dust derived from the continents. Of the last, riverine sources probably end up in near-shore sediments and only dust contributes significantly to deep ocean water. The slowly growing nodules and crusts, composed mainly of iron and manganese compounds, on the ocean floor can chart variations in the relative proportions of these sources, because their growth produces zonation. Measurements of d⁵⁶Fe in various materials show that the two sources are different in isotopic composition (Beard, B.L. et al. 2003. Iron isotope constrains on Fe cycling and mass balance in oxygenated Earth oceans. Geology, v. 31, p. 629-632). While continent derived materials exude iron that is essentially the same as that in terrestrial volcanic rocks (d⁵⁶Fe ~0.0‰), ocean-floor hydrothermal activity is significantly depleted in ⁵⁶Fe (‰⁵⁶Fe ~ -0.38‰). From 6 Ma to 1.7 Ma iron-manganese crusts record iron with a dominant hydrothermal origin, but during the glaciation-dominated period since 1.7 Ma the contribution of continent-derived dusts becomes overwhelming – cooling forces drying on a global scale. Because hydrothermal contributions probably stay much the same over very long periods, because of the sluggishness of plate tectonics, iron isotopes in deep marine sediments, such as Fe-Mn crusts, may be important tracers for glacial events in the distant past, such as the glaciations during the Neoproterozoic and Palaeozoic. Interestingly, the largest iron-rich deposits on the planet, the BIFs that peaked during Archaean and Palaeoproterozoic times, record far larger excursions in iron isotopes than any other. The very low d⁵⁶Fe values of some BIFs (down to – 2.4‰) probably signify the dominance of sea-floor sources, although a non-oxidising atmosphere would have mobilised dissolved iron from the continents too, which explains the range in BIFs up to +1.0‰.

Imaging radar and WMD

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A short article in New Scientist (Morris, H. 2003. Satellites hunt for buried treasure. New Scientist 12 July 2003, p. 12-13) reminded me of the puzzling failure of British and US forces in Iraq to discover any buried caches of weapons of mass destruction, either before the invasion of Iraq or in the aftermath of Saddam Hussein’s disappearance. Researchers at the Ben Gurion University of the Negev in Israel have tested the ground-penetrating capabilities of imaging radar that uses microwave pulses with various wavelengths. One of the principles of radar remote sensing is that microwaves can penetrate beneath the Earth’s surface, provided the materials contain little liquid water. The longer the wavelength the greater the depth from which information can be sensed. Ground-penetrating radar is a common tool in archaeological investigations and in glaciology (ice is “dry”), but is usually deployed along ground traverses. The Israeli experiments, which duplicated work done by remote sensing researchers at NASA’s Jet Propulsion Laboratory, used airborne imaging radar to detect buried metal target, which are highly reflective to microwaves. They used microwaves with moderately long wavelength, and showed that objects half a metre deep were easily detected.

Radar with a wavelength of around 70 cm is called P-band radar, and has the greatest potential for sub-surface mapping, with penetration up to 9 metres. In 1987, NASA’s Jet Propulsion Laboratory first flew an airborne radar imaging system (AIRSAR) that uses P-band, partly to exploit its ability to “see through” dense vegetation but also to produce ground-penetrating images in dry regions. AIRSAR has the potential to produce images with a resolution of 3.3 metres, and data produced by it have been available freely to civilian users. It would be no surprise, therefore, if there were imaging radar systems with P-band radar being used for intelligence gathering. The US National Imagery and Mapping Agency (NIMA), in conjunction with JPL and EarthData International, Inc., developed in 2000 the 2-metre resolution Geographic Synthetic Aperture Radar (GeoSAR) mapping system, that also includes a P-band imager. GeoSAR is funded by the US Defense Advanced Research Projects Agency (DARPA). NIMA, formerly the US Defense Mapping Agency, is a Department of Defense Combat Support and National Intelligence Community agency that provides imagery, image intelligence and geospatial information in support of US national security objectives. The French and Italian space agencies are also discussing the development such systems, perhaps to be deployed from orbit by the European Space Agency.

It was NASA/JPL’s Shuttle Imaging Radar missions in the 1980s and 90s that revealed dramatic evidence for former tributaries of the Nile River System that are buried beneath the sands of the arid eastern Sahara desert in Egypt and Libya. Although not so dry, the Tigris-Euphrates plain is a desert, and it would be very surprising if P-band radar imaging has not been used in the search for buried WMD. Since radar energy is barely affected by the atmosphere, and the microwaves used in radar imaging are effectively highly focussed laser beams, systems carried on satellites have the same spatial resolution as those carried on aircraft. Had a P-wave system been deployed on a military surveillance aircraft or satellite, then sizeable buried caches would have been difficult to miss. Even if the ground was damp, one of radar’s other features is that it responds to variations in the texture of the ground surface. Reworked soil over excavations would be easily spotted by any radar imaging system, either orbiting or on an aircraft. So it was somewhat odd when the US Secretary of State, Colin Powell, did not use any imaging radar evidence in his submission to the UN Security Council on 5 February 2003.

Impact database

Published on June 1, 2003 by zooks7771 Comment

The University of New Brunswick, Canada, maintains an illustrated archive of information on terrestrial impact craters. It lists 169, with exact co-ordinates for each and much other information besides. Many have satellite, aerial and/or ground images, plus full lists of references for each. The URL is http://www.unb.ca/passc/ImpactDatabase/

Rodinia muddles

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In the early 1990s, Ian Dalziel, Eldridge Moores and Paul Hoffman speculated on the former existence of a supercontinent comparable with Pangaea, between about 1100 and 750 Ma. The name Rodinia, from the Russian for Motherland, seemed appropriate. They based sketchy reconstructions on the way in which orogens formed almost globally between 1300 and 1000 Ma could be fitted together by shuffling older crustal fragments, along with evidence from sediments in North America, and Antarctica that the supercontinent began to disassemble around 800 Ma. A great conundrum of later Neoproterozoic times seemed to be partly resolved by what might have happened when Rodinia broke apart and its fragments drifted across the globe. This was the event that welded together the southern supercontinent of Gondwana between 800 to 500 Ma ago, forming the web of orogens known colloquially as the Pan African and Brazilide belts of Africa and South America. Palaeomagnetic pole positions for the 1200-750 Ma period, from the supposed components of Rodinia, were an obvious test of Rodinia’s former existence and its gross structure. As they appeared the palaeomagnetic data seemed to confirm the early ideas that were based on Wegener’s method of linking now far-separated orogens to reassemble his Carboniferous Pangaea supercontinent. A reasonable consensus existed by the early years of the 21^st century. One of the main contributors of palaeopole data for Rodinia reconstruction has been Trond Torsvik of the Geological Survey of Norway, so it is noteworthy that he has cast the first shadows of doubt on what seemed to be an elegant general solution to more than half a billion years of global tectonics (Torsvic, T.H. 2003. The Rodinia jigsaw puzzle. Science, v. 300, p. 1379-1381).

The problem that Torsvik recognises is that superficially convincing geological jigsaw fits are coming into increasing conflict with better evidence for the palaeolatitudes of different segments. This is compounded by a lack of palaeomagnetic data for some of the 13 major continental segments that had formed earlier in Precambrian times. The central element in the original Rodinia model was the way that India, Antarctica and Australia’s 1300-1000 Ma orogens fitted in what appeared to be a rational reconstruction of East Gondwana. The first fly in the ointment is that revision of Australia’s palaeolatitude seems to make its fit with India impossible. Likewise the position of the geologically fitted Congo and Kalahari cratons, that now make up West Africa, is less certain. Amazonia is also not “behaving” as expected, and Baltica may have been rotated by 180 degrees relative to its former orientation in the old Rodinia model. As well as varying quality of palaeomagnetic data, and its lack from crucial components such as Siberia and North China, their dates vary so much that it is impossible to allow for large-scale readjustments through the lifetime of the putative supercontinent. Torsvik figures a “worst case” scenario, in which the whole Rodinia concept becomes merely continents that were near one another and separated by a variety of active rifts; something of a dog’s breakfast that should spur more dating, palaeomagnetism and tectonic research on the orogens that first suggested a grand unification. That is, if the main proponents do not become so profoundly depressed that they simply give up!

Hydrological madness

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Regular readers of New Scientist know that Fred Pearce is the scourge of dam builders, especially those with near-megalomania about vast barriers and reservoirs. Back in the late 1960s Canadian environmentalists were horrified to learn of plans being developed to divert southwards water that naturally flows along the great rivers of the Canadian Shield to the Arctic Ocean and Hudson’s Bay. This was NAWAPA, the North American Water and Power Alliance. NAWAPA is still a live ambition for supplying the water-hungry west and mid-west states of the USA. The former Soviet Union put such grandiose plans into effect, one outcome being the dramatic shrinkage of the inland Aral Sea. Pearce returns to continental water transfer in an important review in the weekly for whom he has worked for many years (Pearce, F. 2003. Replumbing the planet. New Scientist, 7 June 2003, p. 30-34). His trigger is the filling of the giant Three Gorges reservoir on the Yangste, one of whose aims is to channel water northwards to augment supplies to the increasing parched plains of central eastern China. But this is only the start of an awesome venture, that will also shift the equivalent of 25% of the Nile’s flow from Tibet’s glacial meltwater that feeds the Yangste into the Yellow River, which now barely trickles into the Yellow Sea. India seems bent on snaffling much of the flow from the Ganges and Brahmaputra catchments into the drought-prone south of the subcontinent. As well as the huge disruption of people and environment that schemes such as these must entail, Pearce highlights the vast economic costs. India’s continental engineering will eat up the equivalent of 40% of its GNP.

Obviously, such huge ventures throw up equally large political and ethical questions, which are not easy to resolve. In many cases the perceived needs for regional water transfers stem from very wasteful water use, particularly in agriculture. Using drip or trickle irrigation, which needs large-scale application but relatively low-cost and simple technology can reduce water requirements dramatically, simply by reducing losses by evaporation from canals. In semi-arid areas as much as 70 % of channelled water never reaches the crops for which it is intended. Governments such as those of India and China depend so much on rural support that they might commit political suicide by pressing for changes to practices that date back millennia, so they opt for the spectacular, quick fixes. Yet there are other such schemes that might transform the livelihoods of some of the worlds most destitute people in the Sahel and Horn of Africa. One suggestion is to divert part of the largely unused river flow through humid tropical Central Africa across the Sahel to reach Lake Chad. Another, not mentioned by Pearce, is to dig a channel that will flood the Danakil Depression of Ethiopia and Eritrea, which lies about 100 m below sea level. Topographically, this would be relatively easy, because only about 30 km of low-lying coastal plain separates the Red Sea from the Depression. The flow could generate hydropower in a power-starved region, and evaporation from the resulting saline lake would boost rainfall in the world’s hottest place, and perhaps allow harvesting of the many salts that would be precipitated, including potash fertilisers. Solar energy could also be used for low-cost desalination. However, no-one can guess at the climatic and ecological consequences of changing humidity in both the Chad and Danakil basins. Yet, water is becoming the most strategically important physical resource so rapidly that the enormous economic implications for transnational contractors, and political prestige associated with regional transfer schemes will drive them ever onwards. There is one glimmer of hope, which Pearce mentions; ordinary people in Rajasthan, India’s driest state, have resurrected old practices of water harvesting, and find that they are more secure than those who rely on state-sponsored canal supplies. The root issue is that rainfall disappears either by run-off or evaporation in a matter of days, unless it is stored somehow. Any habitable place has rainfall, albeit irregular in drought-prone areas, and quite low-cost ingenuity can “bank” the transient spates where the water is needed.

Rasta man

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Ras Tefari Makkonnen (Haile Selassie) claimed direct descent from the illicit liaison between Solomon and the Queen of Sheba, several millennia before his reign over Ethiopia. Now, “everyone knows” that we are all descended from a single African woman who lived about 120 to 150 thousand years ago – only the line of descent from her proved continually fertile and survived until now. So, it is perhaps fitting that the earliest known remains of properly modern human beings have emerged from the soil of Ethiopia, in the highly fossiliferous sediments associated with the Awash river that drains into the Afar Depression. The cover of Nature (12 June 2003) shows a forensic reconstruction from a male skull found at Herto Bouri, and it bears an uncanny resemblance to the handsome fellows who roam with their herds in modern Afar. There the resemblance stops, for the Afar are not truly African but hale from Arabia, as do many other Ethiopians. These human fossils are 160 thousand years old, and may be contemporary with “African Eve”, or even earlier. The issue of “modernity”, as with others based on anatomical features in incomplete fossil remains, is a bone destined to be gnawed at continually. The discovering team was led by Tim White of the University of California, who regular readers of Earth Pages News will recall came up with the shocking suggestion that deformation of hominid remains could underlie a profligate splitting of human evolution since 4 Ma into many species, some of which might be spurious, even capricious (Ancestral lines squashed?, in EPN of May 2003). The central feature of the well-preserved and undeformed Herto fossils is that they look modern, yet pre-date the classic Neanderthals of Europe (White, T.D. and 6 others 2003. Pleistocene Homo sapiens from Middle Awash, Ethiopia. Nature, v. 423, p. 742-747). The paper shows nicely, by photographic comparison, how the 160 ka humans lie between the more heavily browed archaic H. sapiens from Ethiopia and Zimbabwe (ca 500 ka) and 100 ka humans from Israel. However, statistical plots show graphically the limited number of specimens that palaeoanthropologists have to grapple with, even for relatively recent hominids. Modern as they appear, the Herto fossils lie outside the spread of morphologies gleaned from anatomical studies of Holocene humans. But they do have an astonishingly human characteristic.

All three crania, two adult males and an infant, show clear signs of cut marks (Clark, J.D. and 12 others 2003. Stratigraphic, chronological and behavioural contexts of Pleistocene Homo sapiens from Middle Awash, Ethiopia. Nature, v. 423, p. 747-752). It appears as if the heads of the individuals were cleaned of any skin and flesh, probably by scraping with extremely sharp obsidian blades. The infant cranium is also polished, as if it had been carried around for a long period. Since the markings are very different from those produced by preparing carcasses for eating, and in any case only the brain is a substantial object for cannibalism of a human head, these marks must signify some post-mortem ritual.

Radon emissions and earthquakes

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Models abound for predicting earthquakes from past seismicity and detailed tectonic maps, analogous to those suggested for prediction of volcanic hazards. The Izmit earthquake of 17 August 1999 in Turkey was among the most savage in recent years and killed thousands. It was as powerful (magnitude 7.8) as the celebrated 1906 San Francisco earthquake, and like it stemmed from movement on a continental-scale strike-slip fault. The North Anatolian Fault is almost as well studied as the San Andreas line, and seismicity was known to be heading westwards well before the Izmit catastrophe. Indeed, the Izmit area was predicted to be next on the list, yet no preparation had been made, even by Turkish tectonicians who had been involved in seismic analysis. Chinese geoscientists take a different approach to seismic prediction than those in the west – over the last few centuries, hundreds of thousand Chinese people have perished in earthquakes. They are trying to organise local people to monitor possible precursors to earthquakes, such as rises in water levels in wells and strange behaviour of animals. They have had some notable successes, including preparation for one earthquake in recent years that saved an estimated 80 thousand people in one particularly hazard prone city. The Geological Survey of Israel has been testing a well known correlation between the times of anomalous radon emissions from the ground and earthquakes along the Aqaba Fault that controls the Dead Sea. Over a 7-year period, hourly scintillation-counter readings of radon emissions from springs, wells and especially gravels near known active faults allowed a rigorous test of a possible prediction system, because in that time there were almost 800 minor earthquakes (Steinitz, G. et al. 2003. Statistically significant relation between radon flux and weak earthquakes in the Dead Sea rift valley. Geology, v. 31, p, 505-508). For events beneath the Dead Sea rift, there is a good correlation between the start of radon emission increases and earthquakes, which suggests that about 3-days warning could be given, if the monitoring was widely deployed. The same cannot be said for small tremors with a source outside of the active fault zones. The success may possibly be because sufficient radon to be easily detected is generated by radioactive decay of uranium in a phosphorite bed that underlies the study area. Radon escape to the surface is possibly eased when microfractures begin to open as strains build before an earthquake.

Divine intervention?

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Christianity had a hard time in its first four centuries as a faith, especially at the centre of the Roman Empire. Persecution of Christians ended abruptly with the conversion of Emperor Constantine in 312 AD. Legend has it that, while faced with the double problem of northern barbarian hordes at the gates of Rome and dissident Christians within, Constantine saw a vision in the sky while preparing to take on the invaders. Immediately converting to Christianity, he saw off the hordes, albeit temporarily, and the rest, as they say, is history. One version of the legend, from the Sirente region of Central Italy, tells of a new star that came nearer and nearer to disappear behind the mountains, with a blaze of light from horizon to horizon and ground shaking. Unsurprisingly, impact theorists latched onto this because of its similarity to what probably happens when a substantial meteorite strikes the Earth. Geologists from Sweden have discovered a small crater field in the Sirente area, that consists of a 125 m wide, circular lake with a raised and deformed lip, and several lesser craters dotted around it. Preliminary dating gives an age of 412+ 40 years. Although this date is a century later than Constantine’s conversion, contamination with later material might have reduced the actual age. If the link does prove to be substantial, the Sirente impact will rank with other catastrophes that literally made history, such as the filling of the Black Sea which has been argued to be the inspiration for the Biblical Flood and the Epic of Gilgamesh, and the explosive volcanism of Santorini that wiped out Minoan civilisation on Crete and may well be recorded apocryphally in the Old Testament.

Source: Chandler, D.L. 2003. Crater find backs falling star legend. New Scientist, 21 June 2003, p. 13.

Middle Devonian extinction and impactite layer

Around 380 Ma there was a major extinction event (~40% of marine animals) that is recorded world-wide, along with negative shifts in ¹³C. As with other extinctions since the discovery that the Chixculub crater was exactly the same age as the famous K/T extinction, there has been a quest to link this Middle Devonian event to an extraterrestrial cause. Now there seems to be a positive result (Ellwood, B.B. and 4 others 2003. Impact eject layer from the mid-Devonian: possible connection to global mass extinctions. Science, v. 300, p. 1734-1737). A Devonian section in Morocco contains a thin layer rich in shocked quartz, microspherules of devitrified glass, and metals, that also has low d¹³C. The carbon-isotope shift could have resulted from either of two possible consequences: collapse of the marine ecosystem; or massive release of methane from gas hydrates destabilised by the impact. Only one crater coincides wit the date of the layer and the extinction, Kaluga in Russia, but it is only 15 km wide, so cannot have had any dramatic biological effect. However, the very presence of a moderate crater at exactly the right age might signify other impacts, because it is becoming increasing clear that impacts come in clusters, perhaps because large, approaching bodies break up before they hit the Earth.

The gas-hydrate “gun”

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The gas-hydrate “gun”

As fears of anthropogenic climate warming have risen, so more geoscientists have looked in detail at the stratigraphic record for signs of past warming, and funds have become more targeted towards palaeoclimatology. One of the most important discoveries was that the end of the Palaeocene, about 55 Ma ago, was a time of sudden global warming during the overall cooling that has characterised the Cenozoic. The first sign that something strange had happened then came from using the oxygen isotope geothermometer on plankton tests from marine drill core that passed through the boundary. There seemed to have been a 7º C jump in surface seawater temperature. An explanation for the thermal spike arose after carbon isotopes revealed a coincident spike in the lighter ¹²C. Periods of low primary biological production can impose such anomalies, because photosynthesis selectively binds light carbon in carbohydrate. However, some of that light carbon ends up buried in sea-floor sediments, so another explanation for a negative excursion in d¹³C is that organic carbon has somehow been released from sedimentary storage to the atmosphere. So, either there was a sterile ocean or a massive release of organic carbon at the Palaeocene/Eocene boundary. Some kind of erosion to achieve the second possibility could not have led to such a speedy shift in carbon isotopes. The accepted explanation, suggested in 1995, stemmed from organic carbon that had been metabolised by methanogen bacteria in anaerobic sea-floor sediments to form methane. Given low enough sea-bottom temperatures and sufficient pressure, methane can crystallise with water to form an icy substance, known as gas-hydrate or clathrate, in sea-floor sediments. Being an unstable compound, gas hydrate can break down rapidly if seafloor temperature rises or sea-level falls. And, of course, the methane can rush to the surface as bubbles. Being 4 times more efficient than carbon dioxide at trapping thermal radiation emitted by the Earth’s surface, methane releases are excellent explanations for sudden warmings in the stratigraphic record. And there is a great deal of methane locked as gas hydrate beneath the sea floor, about 2 teratonnes (2 x 10¹² t). Quirin Schiermeier reviews the basic concept (Scheiermeier, Q. 2003. Nature, v. 423, p. 681-682), but poses the question of how methane-induced warming is reversed. Methane is quickly oxidised to CO₂ in the atmosphere, so lessening its warming effect. So a “spike” that lasts thousands of years has to be fed by continual releases. Since warming drives gas hydrate breakdown, something must intervene to stop the releases before the warming becomes a “runaway greenhouse”. One view, and probably the correct one, is that warmth and more CO₂ drives up biological activity so that the increased atmospheric carbon is “pumped” down by living processes, back to sedimentary burial. If sufficient nutrients are available, there is no way of stopping this negative feedback until a balance is restored. Schiermeier reports that new ocean drilling plans to test the hypothesis that the Palaeocene/Eocene warming accelerated continental erosion, which was able to wash the crucial nutrients phosphorus and iron into the oceans. Experiments have shown that increased iron in ocean-surface water far from land – now pretty sterile because it is iron-deficient – sparks up photosynthetic plankton. That is one possible way of artificially drawing down anthropogenic CO₂. The problem is, if such a process was involved in cooling the Eocene Earth, it took about 100 thousand years.

Red Sea record links to northern hemisphere climate

In his forthcoming book, Out of Eden: the Peopling of the World (Constable and Robinson, July 2003), Stephen Oppenheimer offers the novel suggestion that fully modern humans left Africa by island hopping on log rafts across the Straits of Bab el Mandab, which connects the Red Sea to the Indian Ocean. The rationale to his suggestion is that sea-level falls during major glaciations would have partially exposed the shelf that lies beneath the Straits, presenting a route to SW Arabia across only 18 km of island-dotted sea. As today, it would have been impossible to trek across the deserts of the Middle East after a northward African migration along the Nile, without chains of wells. His thesis then sees humans migrating along coasts eventually to reach east Asia at about 70 ka. Precisely when the Straits of Bab el Mandab became shallow enough would have been determined by global climatic conditions, for only glacial maxima result in sufficient sea-level falls for such island hopping to be possible.

The shallowing of the shelf across the southern outlet of the Red Sea would have had a profound impact on seawater circulation. Already having restricted connection to the world’s oceans, Red Sea water has elevated ¹⁸O levels, because evaporation from it favours loss of lighter ¹⁶O. With more restricted circulation, evaporation would have driven this up further. Geoscientists from the Universities of Southampton, Tuebingen and Göttingen, and the Geological Survey of Israel have analysed the variation in oxygen isotopes of foraminifera from a Red Sea core to quantify ups and downs in sea level in more detail than possible from open-ocean cores, which have uncertainties of about ±30m) (Siddall, M. and 6 others 2003. Sea-level fluctuations during the last glacial cycle. Nature, v. 423, p. 853-858). The method that they used models the effects on Red Sea oxygen isotopes of evaporation and changed circulation to estimate how the depth of the Straits of Bab el Mandab changed. They claim a precision of ±12m. Through the period from 70 to 20 ka, leading up to the last glacial maximum, their sea-level record tallies nicely with climate records from both Antarctic and Greenland ice cores, including shifts linked to the short-lived Heinrich and Dansgaard-Oeschger cycles. During the last glacial maximum(18-20 ka), sea-level fell by almost 120 m, so that the Straits of Bab el Mandab were on average only 15 m deep. The first human Exodus out of Africa to populate Eurasia would have been between 120 to 130 ka, as suggested by Oppenheimer, when sea level probably fell a little further. However, at about 65 ka, sea level dropped to about 100 m below modern levels, perhaps presenting another window of opportunity.

Broecker reviews climate triggers

Wallace Broecker, of the Lamont-Doherty Earth Observatory at Columbia University, was the first to quantify in 1975 the 19^th century prediction of Svante Arrhenius that increasing atmospheric carbon dioxide would drive up global temperatures. Broecker’s early work lies at the centre of concern about global warming, and his subsequent contributions are enmeshed with the entire study of past climate change. A review by him of current ideas on palaeoclimates of the recent past is therefore compulsory reading, for all geoscientists (Broecker, W.S. 2003. Does the trigger for abrupt climate change reside in the ocean or in the atmosphere? Science, v. 300, p. 1519-1522. As well as the astronomically connected cyclicity that is apparent in all kinds of climate record through the Pleistocene, those records are punctuated by sudden, short-lived phenomena, whose magnitudes and pace are sufficiently dramatic to focus attention on processes that are probably entirely terrestrial. Foremost among these during the last glacial interglacial cycle are the astonishing coolings of Heinrich’s iceberg armada events and the possibly catastrophic (in a human as well as an ecological sense) Younger Dryas, which reversed warming from the last Glacial Maximum, and the equally sudden warmings associated with Dansgaard-Oeschger events. Broecker’s review focuses on the two mechanisms that have been suggested to underlie these overturns. One links such changes to shifts in whole-ocean water circulation, especially the ons and offs of deep-water circulation beneath the North Atlantic, the other to perturbations of the way in which atmosphere and ocean interact in the tropics.

An entirely plausible scenario for climate-driving changes in North Atlantic water circulation is flushes of freshwater from the surrounding continents, so that formation of sea ice leaves residual water that is not saline or dense enough to sink and drag in water from lower latitudes. The problem is that the complete thermohaline cycle, which impacts on global atmospheric circulation, has a period longer than the changes that might be induced by its perturbation in the North Atlantic. Tropical atmosphere-ocean dynamics are the largest elements in global climate, in terms of the energy and mass that are shifted, so they are a natural candidate for a driving mechanism. Tropical climate shifts abruptly today in well-known ways, most important being the El Niño-La Niña cycle. There is no ponderous underlying dynamic that would damp down connections between cause and global effect, and prevent sudden climate change. Yet, some kind of “flywheel” is essential to keep long-term cyclicity going and lock sudden changes into century to millennium-long climate “states”, which should rapidly decay if effect rebounded on cause, as it does in the case of El Niño-La Niña. Broecker covers all the critical evidence that has borne on both hypotheses up to now. His conclusion is interesting. Both hypotheses are very much model led, and in need of as much empirical support as can be had. Yet, and here is the nub, the crucial data are those bearing on correlating times of events that are recognised all over the place. Time resolution is of the greatest importance, since climate transitions are fast; faster in fact than we can presently resolve before historical times. It is entirely likely that suitable resolution of times past may be absolutely impossible. Both hypotheses have a lot of empirical and theoretical support. So, what is the problem of combining them in a cunning way? Partly, that may be because reductionism (controlling a few variables and looking for developments in another simple set) still plagues science. That is odd in climatology, where all motions and energy changes palpably relate to one another, with no control of a rational kind. Reductionism demands ever more staggering computing power and speed, to “keep all the eggs in the air”. There is always the feeling, as Jimmy “Shnozzle” Durante observed in his musical monologue, The Man Who Found The Lost Chord, that if you find a hitherto overlooked connection, then everything goes well; if you can remember it! Broecker suggests that the missing connection must “transmit” from deep ocean water to tropical atmosphere.

Geochemistry of the vanishingly tiny

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The British press has been awash with speculation that the Prince of Wales is worried about nanotechnology and the slim possibility that the next big threat after Osama and SARS might be minute, self-replicating robots that invade our bodily orifices. It stemmed from the Prince of Wales’ having asked experts for a briefing, and that may well have been just HRH’s curiosity about a changing world. There is rarely an issue of the weekly science journals without news of some discovery of phenomena that occur in nanotubes and minuscule cavities; the world at scales less than a micrometre is beginning to seem strange. Rocks are full of pore spaces and inter-grain boundaries with the dimensions on which new wings of the other sciences are emerging. So it is no surprise to learn that there will soon be “nanogeochemistry” (Wang, Y. et al. 2003. Nanogeochemistry: geochemical reactions and mass transfers in nanopores. Geology, v. 31, p. 387-390). The use of natural and artificial zeolites as ionic filters has been around for a long time, so this is a branch with a new name, rather than a fundamental breakthrough. But zeolites are profitable, and only now has “blue-skies” research turned up the magnification.

Typical nanopores and pathways are grain boundaries in crystalline rocks, cleavage planes in phyllosilicates and clay minerals, and pores in fine-grained sediments, such as diatomite and kaolin, and minerals that have been precipitated as amorphous masses rather than discrete crystals, a good example being the iron oxy-hydroxides in soils. To see these structures requires advanced transmission electron microscopy, and even with them the features are somewhat indistinct. Nanopores can make up to 40% of a material’s porosity, and having such minute radii they contribute as much as 90% of the internal surface area that is exposed to chemical reactions. Artificial materials that show nanoporosity have internal surface areas as high as hundreds of square metres per gram. Clearly, such materials in nature must play a major, but largely uncharted role in geochemical change. Among the oddities discovered by Wang and colleagues at the Sandia National Laboratories and the University of New Mexico, are inclusions of native copper in weathered clay minerals and equally small particles of gold along microfractures in mylonites. Their experiments with artificial simulants of natural fine-grained materials focussed on two simple phenomena: the electrical charge on small surfaces in relation to acidty; and their ability to absorb trace elements. The paper is highly technical, but the conclusions are surprising . Nanopores develop unusually high surface-charge densities that should affect their ability to adsorb ions, and also exert controls on reactions that might seem unlikely in macro-scale simulations of geological conditions. Indeed, finely porous materials enrich trace elements by an order of magnitude compared with isolated small particles, and encourage precipitation or solution of different compounds when that would be unexpected in more open systems. As well as bearing on burial of toxic and radioactive wastes, and on mineralising processes, nano-scale processes are probably central to the whole process of weathering. Interestingly, such small scales exclude even the tiniest bacteria, so that the geochemical processes seem unlikely to impinge on life. However, spaces in rocks comprise a nested series of dimensions, and changing conditions may well flush material from one scale to another. In particular, bacteria of various kinds can control pH at the micro-scale, thereby creating the ambient conditions for nano-scale geochemistry.

Potassium in the core

It might seem impossible for planetary cores dominated by iron-nickel alloys to contain any source of heat generation. The main three elements (uranium, thorium and potassium) with long-lived radioactive isotopes and sufficient abundance to produce substantial heat energy are all highly concentrated in the Earth’s crust. That is because they are incompatible with the minerals in mantle rocks, and so readily enter magmas that contribute to continental growth. However, the only natural materials that bear any resemblance to geoscientists’ notions of core materials, metallic meteorites, contain abundant sulphur. Theoretically, potassium can enter sulphide minerals. So, since as long ago as the 1970s there has been debate about whether motion in the core was driven entirely by residual heat from Earth’s accretion and the formation of the core, or that it contained its own heat source in the form of ⁴⁰K. If the first was true, then the self-exciting dynamo responsible for the Earth’s magnetic field has been running down over geological time, because heat is transferred across the core-mantle boundary, eventually to reach the surface by convection. The existence of a solid inner core might result from such cooling, though its formation would release latent heat of crystallization and prolong inner motion. However, some calculations suggest that core motion and so geomagnetism ought to have vanished long ago, through loss of core heat to the surface. Substantial potassium in the core would demand considerable revision of ideas about the bulk evolution of the Earth, and other rocky planets. Experiments to prove that iron-sulphur alloys can contain abundant potassium have had a chequered history. Research at the University of Minnesota and the Carnegie Institute of Washington has discovered why there were such ambiguous results (Murthy, V.R. et al, 2003. Experimental evidence that potassium is a substantial radioactive heat source in planetary cores. Nature, v. 423, p. 163-165). The problem was in the preparation of samples for analysis. Rama Murthy and colleagues found that the oils used in polishing samples for electron-microprobe analysis actually leach potassium from the sulphides in them, nearly all disappearing in a few days of contact. With great care, they repeated experiments on mixtures of metallic iron, iron sulphide and potassium bearing glass held at high temperature under pressures between 5 and 10 % of those experienced in the core. Their results show that potassium can indeed enter core materials with high sulphur contents. The higher the temperature the more gets in, and their most extreme run saw almost 4 % K in the quenched sulphide. Plan are afoot to discover if uranium and thorium might also be in core materials.

Incidentally, in the week that the film The Matrix: Reloaded was premiered in the USA, a proposal to send a probe to the core-mantle boundary also appeared (Stephenson, D.J. 2003. Mission to Earth’s core – a modest proposal. Nature, v. 423, p. 239). David Stephenson, of the California Institute of Technology, builds on the notion of the “China Syndrome”, in which meltdown of the core of a nuclear reactor would lead to superdense molten uranium melting its way through the mantle. In his proposal, ruggedised instruments in a capsule the size of a grapefruit would make the journey, along with about 10 million tons of molten iron, by propagating a large crack started by a 10 Mt nuclear explosion. Data is to be transmitted by modulated acoustic signals in the kHz range. The article helps to demonstrate the delays in publication, even in a prestigious weekly journal; it should have appeared 6 weeks earlier….

Long-term prediction of volcanic activity

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Unless it is possible to give people who live near dangerous volcanoes sufficient warning that they can escape disaster, eruption prediction might be looked on as a lugubrious topic. Up to now, there have been very few predictions that have been better than a few hours or days. Mexico’s Popocatapetl gave two days warning in late 2001, and that was sufficient for a completely successful evacuation of those threatened. In the case of the eruption of Nyirangongo in eastern Congo, a few months later, warning signs preceded eruption by 5 days, but the people of Goma were not told and 45 people died trying to rescue possessions from the quiet, but relentless movement of a lava stream (see EPN February 2002, Is volcanic eruption predictable?). In both cases it was abnormal seismicity that presaged the events. John Murray, of the British Open University, has analysed the statistics of seismic events and eruptions of possibly the world’s most monitored volcano, Etna on Sicily (Murray, J.B. 2003. Seismicity and time-lagged lava output at Mount Etna: A new method of long-term forecasting at a destructive volcano. Geology, v. 31, p. 443-446). Energy released during 19-year periods by earthquakes beneath the volcano since 1870 shows a inverse relationship with 9-year lava production, which suggests that seismicity and eruption are widely separated in time over long periods. However, by examining the correlation of seismic energy with eruption volume for time differences between the two from 0 to 50 years, Murray has been able to show that Etna increases its productivity roughly 25 years after major releases of seismic energy. Using this as an input to a model that might predict eruption intensity, he has been able to mimic the actual volcanism through the 20^th century with fair accuracy. In his opinion, the very high eruption rate since 1950, which reached a peak in the 1990s, is only likely to decline a quarter of a century after large earthquakes (> magnitude 6) return to Sicily. So, Sicilians have a difficult choice. Should they worry about lava flows or earthquake damage? Sadly, data suitable for broadening Murray’s method are available for very few volcanoes, all in quite prosperous countries.

Modelling the duration and extent of mining contaminants

Release of high concentrations of heavy metals and other pollutants to drainages is a natural consequence of geochemical anomalies associated with mineralization. However, these have come to balance with the rest of the environment over periods measured in thousands of years or even longer. The pose perpetual hazards, some of which are known, some not. Environmental disturbance by mining and associated activities scales up releases of pollutants many times over those of natural origin. Even with modern means of waste containment, escapes occur, sometimes of very large magnitude, such as the breaching of tailings dams or landslips in spoil heaps. Of course, these hit the news when they happen, but assessing how long the pollution dwells in downstream areas and how it moves is not easy. It requires some kind of model of the hydrology, erosion and sediment-transport characteristics of the affected drainage basins, that takes into account catchment topography and the size-distribution and density of escaped wastes. Such a modelling tool is now available, having been developed at the University of Wales in Aberystwyth (Coulthard, T.J. & Macklin, M.G. 2003. Modelling long-term contamination in river systems from historical metal mining. Geology, v. 31, p. 451-454). It is complex, because it combines the 3-D shape of basins with water discharge and depth, vegetation cover, depth to bedrock and the properties of released materials. In a simulation of hydrological dynamics. TRACER is able to take account not just of the fate of grains that enter drainages, but how they are deposited in alluvium and then reworked by later changes in hydrology. Coulthard and Macklin apply the model to the base-metal mining district of Swaledale in North Yorkshire, England, where production began in 1700 and ended 200 years later. Swaledale was a minor producer of lead and zinc in modern terms, and the miners paid scant attention to environmental protection. Results suggest that contamination spread downstream to the flat land of the Vale of York in only 10 years after mining started, but the pollution lingers, and seems likely to stay above safe limits until well after the start of the 22^nd century. When possible increases in rainfall through global warming are factored in, the simulation remains much the same for 10 to 25 % rises, and only moves towards clean-up with 50 to 100 % increases in precipitation, when clean sediments should dilute the pollutants. As well as predicting the general effects of contaminant releases, TRACER is able to highlight parts of a drainage basin that are particularly at risk due to trapping of sediments. Mining in Swaledale produced, at most, only about 600 thousand cubic metres of metal-rich waste, fine enough to be transported by water. Recent escapes from tailings dams and landslipped spoil heaps, as in Spain and OK Tedi in Papua New Guinea, were orders of magnitude larger.

Astronauts’ snaps

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When directing The Greatest Story Ever Told George Stevens was unimpressed by John Wayne’s delivery of his only line at the very end of the film. Stevens said, “You have to deliver with a little more awe, John”. And so we have one of the great conclusions in cinema history, “Aw, he truly was…..”. Astronauts have had a fair number of those “Aw” moments, and thankfully, most have them have carried cameras. They captured a great many views of odd and awesome phenomena and features of the Earth as they passed over. There are a great many (around 400 thousand), and NASA has compiled the best views of Earth’s surface on its new site at http://eol.jsc.nasa.gov/sseop/EFS/. I put in “faults” to see what came up; there were 184 images from every continent. Downloads are in two sizes, 300 kb and 13 Mb, so the images are of very high quality. The entire archive is searchable at http://eol.jsc.nasa.gov/sseop/.

Elderly South African Australopithecines

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The Sterkfontein Caves near Johannesburg in South Africa have provided some of the best preserved hominid remains, because they are enveloped in chemically precipitated cement. Fossils are also much more plentiful than at other sites, and the caves have yielded about 500 specimens. However, unlike sites in bedded sediments interleaved with volcanic horizons, cave deposits are difficult to date accurately. Up to now, correlation of other fossil animals in the breccias that encase Sterkfontein hominids with those at more amenable sites, together with dating based on palaeomagnetic reversals, have been hotly disputed. A new technique based on the radioactive decay of isotopes that cosmic-ray bombardment induces in quartz grains promises to resolve the paradox of wonderful fossils that cannot be dated. While quartz grains are at the surface, in alluvium or the debris on slopes, cosmic rays produce radioactive aluminium and beryllium isotopes in a fixed proportion. The longer the exposure time, the more radioactive isotopes are produced. But if such irradiated grains are buried, the isotopes decay away, because they are protected by overlying material. Detrital sediments enter cave systems very quickly, so they are near-ideal for the use of cosmogenic dating. Of the two most-used isotopes, ²⁶Al decays quicker than ¹⁰Be. So, the ²⁶Al/¹⁰Be ratio decreases with time and gives a measure of how long the sediment has been buried. Results from Sterkfontein (Partridge, T.C. et al. 2003. Lower Pliocene hominid remains from Sterkfontein. Science, v. 300, p. 607-612) show that the stratigraphically lowest fossils are much older than previously thought; around 4 Ma.. Previous age estimates suggested that the oldest Sterkfontein hominids lived around the same time as Australopithecus afarensis, of which the famous “Lucy” skeleton was an Ethiopian member. Four million years ago A. anamensis would have been a contemporary, yet the hominids at Sterkfontein seem quite different anatomically. Maybe there were two species in Pliocene Africa, one East African and the other a southern one. In fact, there are hints that perhaps two species of australopithecines, along with a more robust paranthropoid may have been washed into the caves. There are two problems though: cosmogenic dating is notoriously imprecise (the age reported is 4.2±0.3 Ma), and Sterkfontein has such excellent preservation that the number of specimens outweighs those from elsewhere – comparisons are not easy!

Tracking migrations with language

One of the first surprises that arose when genetic relatedness among living people and the estimated time of their separation began to encompass global populations was how well the genetic patterns matched with the distribution of the world’s languages. When populations move they not only carry their genetic heritage but their languages. Probably the greatest migrations in human evolution took place at the end of the last Ice Age, and so it might seem that plotting language distribution ought to chart the paths these wandering people took. Jared Diamond and Peter Bellwood (Diamond, J. & Bellwood, P. 2003. Farmers and their languages: the first expansions. Science, v. 300, p. 597-603) have reviewed just how complex such a task will be. Genes and language can tell only part of the story, because people carry skills and culture too. The two dominant cultures around 11 000 years ago were the age-old ways of the hunter-gatherer and the new agriculture and animal husbandry. There are at least five possibilities involved. Genes, language and lifestyle could mix between both groups when they came into contact. Hunters might take up farming but keep their identity. Hunters were as likely to shift as farmers when climate belts changed. Powerful incomers might impose their language but not their genes. When one group moved, another might take its place. Bearing in mind these caveats, Diamond and Bellwood review the main patterns of linguistic groups, using excellent graphics.

Plume debates

Published on May 1, 2003 by zooks777Leave a comment

Jason Morgan’s recognised in the early 1970-s that chains of volcanic islands and seamounts, such as the Hawaii-Emperor Chain, which cross sea-floor magnetic stripes, might have resulted from mantle “hot spots” that are fixed relative to motions of lithospheric plates. He went on to suggest that such magmatic anomalies might reflect narrow thermal upwellings within the deep mantle, and applied the term “plumes” to these notional convective zones. Geochemists have since flocked to active and extinct manifestations of within-plate magmatism, and developed a whole sub-culture of classification and hypotheses concerning their origin and inner workings. By the end of the 1990s over 5000 candidates for underlying plumes had been proposed, some still active and others inferred for past events, such as flood basalt provinces. Processing of seismic signals using supercomputers over the last few years has used them to map variations in P- and S-wave speeds at different depths in the mantle. Speeds below those expected are likely to reflect hot mantle relative to high-speed, colder regions. So seismic tomography potentially charts hot rising mantle and cool, descending parts; seemingly ideal for detecting mantle plumes and how deep they extend. Early results centred on proposed plumes were a mixed bag. Some seemed to have very deep origins, perhaps down to the core-mantle boundary, whereas others appeared to be above hardly anomalous mantle. Most exciting was a zone of hot, probably rising mantle with a source at the top of the core beneath the South Atlantic, yet whose upper parts sloped obliquely upwards towards the Red Sea. It seemed that the Afar plume, believed to have been responsible for continental flood volcanism in Kenya and the Ethiopian Plateau, and perhaps the East African Rift and opening of the Red Sea, still existed. Hot-spot activity is a minor aspect of global tectonics today, so it is not an ideal time to ponder on plumes. If they are real, then periods of massive flood volcanism would have been responses to superplumes, but the last in Ethiopia was 30 Ma ago.

Exciting as seismic tomography is, its resolution is currently too coarse to pick out the most revealing features of the plumes that potentially it could detect. To have sufficient gravitational potential energy to rise through the entire mantle, a very large volume is required, and that is assigned to the “plume head”. Some hotspots are over large volumes of hot mantle, but they lie just beneath the lithosphere, and could have their origin at any level in the mantle. The tracks that they followed, if any, and which might continue to be a conduit for uprising material would be much narrower. Such predicted “plume tails” are too small for resolution by current tomography. A compilation and re-classification of hot spots (Courtillot, V. et al. 2003. Three distinct types of hotspots in the Earth’s mantle. Earth and Planetary Science Letters, v. 205, p. 295-308) has whittled down candidates for mantle plumes to a mere 50 or so, with less than 10 likely to have risen from core depths. Two responses have arisen about this hugely popular topic: that Morgan’s ideas are still basically valid, but need more work (DePaulo, D.J. & Manga, M. 2003. Deep origin of hotspots – the mantle plume model. Sciene, v. 300, p. 920-921); that hotspots might be linked to plate tectonics, and that mantle plumes are nothing more than a “belief system” (Fouger, G.R. & Natland, J.H. 2003. Is “hotspot” volcanism a consequence of plate tectonics? Science, v. 300, p. 921-922). A sensible aim that might resolve matters is to seek materials from the largest magmatic events – flood basalts – that should contain unambiguous geochemical signs that their parent mantle was at some stage exchanging matter with the core, if they had formed after rise of a superplume. But, every line of approach to deep-mantle processes relies on proxy evidence, several steps removed from actual events and properties. That makes David Stephenson’s proposal for a mission to the core (above) so urgently in need of support!

Extinction at the Precambrian-Cambrian boundary

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The very beginning of the Cambrian is associated in every geologist’s mind with the explosive appearance and diversification of animals with hard parts. Why this dramatic introduction to the modern biological world occurred is one of the great questions in evolution. Some connection with the effects of “Snowball Earth” events in the late Neoproterozoic was thrown into doubt by evidence that it had little effect on micro-organisms (see Microbes showed no sign of change following a “Snowball Earth” in May 2003 EPN). Exactly at the boundary there is a marked fall in the abundance of carbon-13, and this negative d¹³C excursion is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age. One of the places that it occurs is in Oman, reported previously in EPN (A possible fuse for the Cambrian Explosion, January 2003). The paper describing the evidence from Oman that the carbon-isotope excursion relates to a mass extinction is now out (Amthor, J.E. and 6 others 2003. Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman. Geology, v. 31, p. 431-434) The disappearance of the distinctive eukaryote fossils coincides exactly with the carbon anomaly. Luckily, so too does a volcanic ash horizon from which zircons provide a very precise U-Pb age of 542±0.3 Ma. This matches less precise dates for the anomaly from Siberia and Namibia, and seems likely to become accepted as the definitive age for the start of the Phanerozoic.

“Snowball Earth” and evolutionary diversification: Australians speak out

By comparison with the vast amounts of Australian diamictites that span a range of Neoproterozoic ages, the sites elsewhere, from which evidence in support of the “Snowball Earth” hypothesis and possible effects on evolution have been drawn, are puny. Besides that, the Late Precambrian of Australia has the best record of biological change, including the type locality for the Ediacaran fauna that presaged the Cambrian Explosion. Although somewhat less hasty than the flurry of papers on the “Snowball” hypothesis, since 1998, the appearance of published data from the “Red Continent” is sure to push the debate decisively one way or another. Palaeontologists from the Geological Survey of Western Australia, Macquarie University and Mineral Resources Tasmania have just unveiled details of acritarchs from late-Neoproterozoic sediments that overlie the Marinoan (~600 Ma) glaciogenic rocks in South Australia (Grey, K. et al. 2003. Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? Geology, v. 31, p. 459-462). Acritarchs are spore-like fossils, that probably represent encysting algae. Their rapid diversification makes them useful biostratigraphic indicators from the Late Precambrian to the present. Grey et al. Found that the same assemblage of acritarchs occur before the Marinoan glaciogenic strata and after the succeeding “cap” carbonate. They are part of a group that can be traced back to the Mesoproterozoic However, higher in the sequence that they examined there is a distinctive layer of debris that contains evidence of impact-induced shock. This can be correlated with little doubt to the 90 km Acraman structure in South Australia, which formed at 580 Ma with an energy likely to have had a major influence on life. Sure enough, in the strata above this ejecta layer a completely new type of acritarch group appears and diversifies rapidly, while the pre-impact groups simply disappear. Clearly, the Acraman impact is implicated in this sudden biological change; an extinction followed by rapid diversification. Acritarchs are thought to represent the phytoplanktonic base of the Neoproterozoic food chain. Immediately above the strata in which the post-impact acritarchs diversified lie sandstones that contain the famous Ediacara fauna of the first large, soft bodied animals. The Marinoan “Snowball” event seems disconnected from this evolutionary leap.

Water resources and bullocks

Published on May 1, 2003 by zooks777Leave a comment

Desalination is often touted as a solution to shortages of clean drinking water, but the most common method, using reverse osmosis, is really a luxury. It relies on electric pumps driving salty water through a membrane, so that salt concentrates on the high-pressure side of the membrane, allowing nearly fresh water through it. This method is widespread among power-rich economies along desert coastlines, but has done nothing to help the less fortunate millions in countries where electricity is unaffordable. Indian scientists, unsurprisingly, have developed a means whereby fresh water might become accessible to most coastal people in the tropics. They have worked out how to gear bullock power to reverse-osmosis pumps, so that a pair can produce up to 3000 litres each day and supply entire villages. If a bullock can do it, then why not donkeys or camels in even more arid coastal areas?

Source: Coghlan, A 2003. All hooves to India’s pumps. New Scientist, 10 May 2003, p. 19.

Volcanic hazard assessment

Published on April 1, 2003 by zooks777Leave a comment

Unlike some natural catastrophes, there is no stopping a volcanic eruption. The best that can be done is to give people who live in the danger zones sufficient warning that they can escape disaster. Many volcanic areas are densely populated, largely because soils derived from lavas and ash are extremely fertile, and high volcanoes create decent rainfall because of their orographic effect. Naturally, nobody likes to up sticks, whatever the dangers, least of all if there are false alarms. As with seismic prediction, volcanologists do not have a good track record of foretelling big eruptions, even though a great many geologists cluster on and around volcanoes. Most of them flock to areas with active lavas, pyroclastic flows and other lugubrious after effects of major activity. However some do the painstaking work of trying to monitor the plumbing of volcanoes, to get a handle on which parameters are most likely to be authentic warnings of impending doom. It is no longer a matter of experienced volcano watchers and their instinctive feel for when one is about to blow its top, but one of ever more sophisticated instruments and software to analyse data and model volcanoes’ inner workings. The 28 March 2003 issue of Science (p. 2015-2030) devotes 16 pages to a review of volcano monitoring. While advances are being made, there is still a long way to go before they can pay dividends by reducing the loss of life. What is not going to go away, even in the best of all possible scientific worlds, is the economic devastation that follows any geohazard.

Ancestral lines squashed?

Published on April 1, 2003 by zooks777Leave a comment

Many of the famous finds of hominid crania, on which ideas of human descent hang, consist of small fragments that have to be glued together to reconstruct their form. The basic work of palaeoanthropology is very like doing a 1000-piece jigsaw puzzle, but in three dimensions. Tim White, one of the pioneers of modern studies of hominin fossils, is now worried that the fragmentation of bone is connected with distortion during burial (White, T. 2003. Early hominids – diversity or distortion. Science, v. 299, p. 1994-1997). His own studies of fossil pigs present a disturbing pattern of post-mortem distortion that spurred earlier workers to subdivide them “exuberantly”. There are even “flat-headed flat pigs” and “narrow pigs” (literally, from their given Linnean names), but they are now known to be mechanically distorted remains of a single early pig. Hominid crania viewed in this light, and there are nowhere near as many as those of pigs, are a mess. White gives one example, Kenyanthropus platyops (“flat face”), which may well be a distorted and quite ordinary Australopithecus afarensis. Combined with the shape variation within living species, notably humans but also among bonobo chimpanzees, distortion throws the bushy tree of human descent into considerable doubt, just as Jonathon Kingdon predicted 10 years ago in his book Self-Made Man and His Undoing. There are so few hominid remains, and most are a mess, that it seems impossible to decide whether many hominin species existed together at any one time in the Late Miocene to Early Pleistocene, or that just a few (even one?) spread to many different habitats across the face of Africa; something of a bombshell for those who make a tidy living from skull-hunting and hominin cladistics.

Walking with Slade

Imagine, if you will, the Pliocene savannah of East Africa and a band of upright apes (Australopithecus afarensis), each (even the females) with the trademark sideburns of Noddy Holder. Imagine too that peeping from the bush is a voyeuristic obstetrician who resembles Groucho Marx, drinking a hot beverage (Cuppasoup?) from a flask, and trying ever so hard to get one over on Whispering David (Attenborough). There is a story here, because one of the apes is Lucy, who gets clobbered in Pliocene Slade’s fracas with a rival band (Staus Quo?), her infant falling into the long grass. Her sister rescues the child, and all is well on the long road to humanity. That was the first episode of the BBC’s Walking With Cavemen, the third series aimed at popularizing palaeontology, which began with Walking with Dinosaurs. All three owe as much to Bambi and Dumbo as they do to computer animation and modern research, despite the best efforts of the numerous scientific advisors. I saw the trailer for the next episode, concerning Homo ergaster – quite apt, because that was “Action Man”, that was. Not only were they white with tangled grey locks, but despite the brow ridges it was hard to conceal the fact that they were Pan’s People and the Chippendales striding purposefully across a salt pan. Did even female H. ergasters have 6-packs? Physically arousing it may have been, again leaving out the brow ridges, the bad barnets and table manners, but I thought, “Tripe”, and watched the footy the following week. (Note: “barnet” – rhyming slang for hair, from Barnet Fair).

A genetic key to human evolution?

It will not be too long before the publication of the chimpanzee genome. Because chimps are our closest relatives, and we shared an ape ancestor about 5 to 7 Ma ago, there is bound to be a media hullabaloo (and agitation among creationists) on the day of the release. At first sight, a comparison of human and chimpanzee genomes might seem to offer plain clues about the genetic side of our co-evolution, but evolutionary biologists are not so optimistic about an imminent breakthrough (Carroll, S.B. 2003. Genetics and the making of Homo sapiens. Nature, v. 422, p. 849-857). Their hesitancy stems from a matter of arithmetic and the sheer volume of work that needs to be done, as well as because of gross uncertainties about how genes relate to the important traits of humans and their differences from closely related apes. The human genome consists of about 3 billion base pairs and the gross difference from that of chimpanzees is about 1.2% (incidentally, it is likely that all mammals, from mice to men, share around 80% of their genes). Assuming that this difference is split 50:50 between the results of evolution towards us and towards chimps over the last 5 to 7 Ma, the divergence from the genotype of our shared ancestor in the human genome should amount to about 16 million new base pairs. Some of them may be “chaff”, but the genetic side of human evolution is buried in this massive area of potential work. Maybe around 200 000 are tied to evolved changes in protein production, that could be the key candidates for research. Although there have been claims for genes that control this or that side of humanness, properly tying down traits to genes will be an awesome task.

The differences between chimpanzees and humans manifest themselves in anatomy and behaviour, and a huge body of knowledge on both has grown in the last two centuries. So biologists know pretty well what they are looking for in terms of interesting genotype-phenotype links. However, a chart of those parts of the genome that account for the differences, whenever that becomes a believable reality, really does not help with the hows and whens of the course taken by evolution over several million years. They rely on the fossil record. Astonishingly, chimpanzee fossils are almost totally unknown, especially in the early part of their phylogeny. Even by the most optimistic account, the record of our predecessors is patchy and only a handful of near-complete skeletons are known from before about 500 ka. Carroll uses the most “bushy” version of hominin cladistics claimed by palaeoanthropologists, with 19 species, to illustrate the current status of hominin descent. White’s view of the uncertainties (Ancestral lines squashed?, earlier in this issue) makes the crucial connections before about half a million years ago extremely flimsy. But, there will undoubtedly be a huge growth in human evolutionary studies, once the key chimpanzee data become available. Of course there will be a massive media hype as well, and all manner of outlandish claims. But maybe also more funds for palaeontology will stem from the potential to link the evidence from today’s graspable realities with the exciting though puzzling anatomical record since the late Miocene.

USGS photographic archive

Published on April 1, 2003 by zooks777Leave a comment

The US Geological Survey has placed 16 thousand of its archived field photographs on the web, at print-quality resolution. They can be accessed at http://libraryphotos.er.usgs.gov and carry no copyright, so anyone can use them for illustration of lectures or textbooks (it would be polite to acknowledge the USGSs generosity). The photos date back to the earliest days of geological research in the United States, and are in black and white, and colour. Although still under development, the site’s search engine works well and quickly. Putting in “unconformity” and “thrust” yielded 31 and 52 pictures, respectively. However, trying to find the highly photogenic thrust of grey Cambrian limestones over Permian redbeds west of Las Vegas in Nevada drew a surprising blank. Every geological survey holds enormous archives of photographs that never see the light, so the USGS initiative ought to encourage others to follow suit. In particular, it would be great news if the British Geological Survey, the world’s oldest, did likewise, instead of just generating a meagre flow of funds by selling a minute proportion of its collection as postcards.

Origins of the vertebrates

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Remote signs of earthquakes

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Middle Devonian extinction and impactite layer

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The gas-hydrate “gun”

Red Sea record links to northern hemisphere climate

Broecker reviews climate triggers

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Potassium in the core

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Modelling the duration and extent of mining contaminants

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Tracking migrations with language

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“Snowball Earth” and evolutionary diversification: Australians speak out

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Walking with Slade

A genetic key to human evolution?

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