Author: zooks777

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

Geochemical switch for Snowball conditions

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Whether or not you believe that the Earth was totally encased in ice up to four times during the Neoproterozoic Era, there is convincing evidence that ice sheets did extend to the tropics during such “Snowball” episodes.  How such extremely cold episodes came to prevail for several million years has been the subject of debate for 5 years, since Harland’s notion of global glaciations was resurrected by palaeomagnetic evidence for the low latitudes of Neoproterozoic glaciogenic rocks.  Ice extending almost to the Equator, even if just on the continents, would have driven down global temperatures simply because it would have reflected away solar radiation.  Increased albedo helps explain why frigid conditions lingered, but some other cooling mechanism must first have encouraged the widespread formation of ice sheets.  Essentially, the supply of the “greenhouse” gas CO2 by volcanic activity must have been outstripped by burial or solution of carbon in some form.  The two usually identified candidates are increased deposition of carbonate sediments and the accumulation of unoxidised organic carbon in sea-floor muds.  It is the first of these that dominates climate control today, by the accumulation of carbonate shells of marine plankton, and that has probably prevailed since foraminifera and coccolithophores began to proliferate in the Mesozoic.  No shelled organisms existed during the Precambrian, so a major factor in damping down climate fluctuations was missing before the start of the Phanerozoic.  This crucial difference between the modern and Precambrian world focussed the attention of Andy Ridgwell, Martin Kennedy (University of California) and Ken Caldeira (Lawrence Livermore National Laboratory) in seeking an explanation for “Snowball” events (Ridgwell, A.J. et al. 2003.  Carbonate deposition, climate stability and Neoproterozoic Ice Ages.  Science, v. 302, p. 859-862).

Carbonate sediments are plentiful in the Precambrian record.  Some formed as a result of organic action (stromatolitic limestones) and others show evidence for direct, inorganic precipitation of carbonates from sea water.  The latter indicate sea water in which calcium and carbonate/bicarbonate ions exceeded the solubility of calcite and the ability of organic activity to remove calcite from solution.  Evidence for such extreme oversaturation is rare, but the cap carbonates that overlie Neoproterozoic glaciogenic rocks are important examples.  The key area of carbonate deposition has always been on shallow continental shelves, the main secreters of carbonates during the Precambrian having been blue-green bacteria that can photosynthesise only in shallow water.  Falls in sea-level or a reduction in the area of shelves during the Phanerozoic reduced this sink for CO2 in the build-ups of coral and shelly limestones, but plankton of the open oceans continued to accumulate on the deep sea floor.  Because calcite can be dissolved at depth, the deepest sea floor does not contain much carbonate.  However, a fall in sea level,  increases the area suitable for deep-water burial of shelly material, because the carbonate compensation depth or lysocline also falls.  In the absence of shelly plankton, this modern balancing mechanism for ocean chemistry did not exist during the Precambrian.  Superficially, it might seem that a reduction in the area of shelf deposition of carbonates, brought on by a sea-level fall, would allow CO2 to build up in the atmosphere, driving towards warmer conditions.  However the way in which atmospheric carbon dioxide is related to dissolved carbonate (CO32-) and bicarbonate (HCO3–) ions tells a very different story.  This is the equilibrium: CO2 + CO32- +H2O = 2HCO3–.  Less carbonate accumulation on reduced continental shelves would drive up the carbonate-ion concentration of sea water, and also its pH.  So, according to Le Chatelier’s Principle, the equilibrium proceeds to the right and adds to the more soluble bicarbonate ions in sea water.  This consumes CO­2, and drives down the “greenhouse” effect.  Ridgwell and colleagues developed a model around this equilibrium, and applied it to conditions of falling sea level when carbonates were only deposited on continental shelves.  Their results show that decreased shelf-carbonate burial during a period of sea-level fall would rapidly drive down the warming effect of atmospheric carbon dioxide.  Combined with the lower solar energy output during the Neoproterozoic, that would be sufficient to create protracted periods of frigidity.  Alkalinity of the oceans would increase through periods of glaciation, so that once sea-level rose, massive carbonate precipitation would form cap carbonates on the newly inundated shelves, thereby reducing the oceanic drawdown of CO2.

Ridgwell et al’s model is not easy to grasp, and relies on its initiation by falling sea-level.  Either that resulted from build up of continental glaciers because of some other climatic mechanism, or internal processes increased the volume of the ocean basins.  An example of the last is a decrease in sea-floor spreading, when cooling of the lithosphere increases it density so that it sags down.  Periods of accelerated creation of oceanic lithosphere displace sea water upwards, and perhaps that might explain an increase in shelf areas, which would allow warming according to the new model.  The model also needs special pleading to account for the 1 billion-year absence of glaciation before the period of Snowball events.  The authors suggest that it could have been prevented by much wider shelves during earlier times, but without quoting evidence.

Continental erosion and climate

Maureen Raymo suggested in 1988 that long term climate change was modulated by the rise of mountain chains and their erosion and weathering.  This is because chemical weathering of silicate minerals is a net consumer of atmospheric carbon dioxide.  Raymo’s hypothesis, based on T.C. Chamberlin’s theory of glaciation, has set climatically concerned geochemists to analysing the trace element content of river water in many mountainous regions, because those such as strontium are proxies for the amount of weathering going on today.  Others have looked at the flux of elements into seawater through the Phanerozoic in particular, by analysing marine carbonates, to see if the ups and down’s of water composition through time match the record of climate change.  These time series do suggest some matching, but not precise enough for all to agree with the hypothesis.  Measurements of river-water composition have also met set-backs.  Much of the weathering flux from mountains seems to stem from dissolution of carbonate rocks, and that does not lead to long-term loss of CO2 from the atmosphere.  In a bid to resolve the contributions of carbonates and silicates, Andrew Jacobson and Joel Blum of the University of Michigan have studied the flux from part of the Alps of New Zealand’s South Island (Jacobson, A.D. & Blum, J.D. 2003.  relationship between mechanical erosion and atmospheric consumption in the New Zealand Southern Alps.  Geology, v. 31, p. 865-868).  Their area is a good choice because the New Zealand Alps are actively rising, precipitous and drenched with continual heavy rain and snowfall. Moreover, they offer something that the Andes and Himalaya do not; the rocks are pretty uniform.  What they find will not please Raymo’s followers.  As in many mountain ranges, mechanical erosion favours carbonate weathering over that of the CO2 sequestering alteration of silicates.  With a low ratio of  silicate:carbonate chemical weathering, mountain building in New Zealand does draw down carbon dioxide, but only by a factor of about 2.  They conclude that more stable areas with lower relief are more likely to affect climate.  Although chemical weathering in them is lower than in mountains, that of silicates is far higher than for carbonates.  Moreover, active mountain ranges are minuscule compared with the extent of more subdued land.  It seems likely from Jacobson and Blum’s findings that the major control of weathering over climate depends to a large degree on where continents are located relative to warm, humid climatic zones.  For much of the early Cenozoic, the dominantly crystalline Precambrian shields of India, Africa, Australia and South America straddled the Equator, and witnessed intense weathering.  Maybe that relationship helped draw down carbon dioxide, and gradually cooled the planet from the hot and humid climate of the late Mesozoic.

Nemesis web site

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If you like that frisson of fear that comes from contemplating the demise of the world as we know it, then the Near Earth Objects Dynamic Site (NEODyS) will give you hours of it (newton.dm.unipi.it/neodys).  The more than 2500 NEOs that orbit within 45 million km of the Earth’s are fully catalogued there, along with impact risk assessment.  The site also links to the on-line newletter Tumbling Stone, that has news on asteroidal matters, especially near misses…..and impending doom…..

Insights into hydrocarbon reservoirs

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Oil and natural gas are the dominant physical resources for modern society, having rapidly outstripped coal in the world’s economy.  Yet using them poses the threat of global climatic changes.  They are essentially a bank of solar energy, mediated by past photosynthesis into hydrocarbons; very long passed indeed.  Their burial tens and hundreds of million years ago helped modulate solar warming and drove up the level of oxygen in the atmosphere.  Using them reverses those aspects of the carbon cycle.  As the wars in Sudan, Afghanistan and Iraq demonstrate, developed economies will go to any lengths to retain access to known reserves.  Being so “hooked” on hydrocarbons, those economies have continually to find more.  However, the days of “trip-over” oilfields, such as those of Persian Gulf, are gone forever.  Exploration ventures into more and more difficult conditions, particularly offshore, where drilling is now going on in sea floor as deep as 2.5 km beneath the water surface.  Every aspect of the hydrocarbon industry poses increasing challenges; it seems to be at a crux.  For this reason, the 20 November 2003 issue of Nature includes a 56-page Insight supplement on a wide range of topics.  It starts with a review of the place of the petroleum industry in human history (Hall, C. et al. 2003.  Hydrocarbons and the evolution of human culture.  Nature, v. 426, p. 318-322).  Robert Berner of Yale University gives an up to date summary of the effects of fossil fuel use, in the context of the carbon cycle over geological time (Berner, R. 2003.  The long-term carbon cycle, fossil fuels and atmospheric composition. Nature, v. 426, p. 322-326).  The question, “How does petroleum form?” is addressed by Jeffrey Seewald of the Woods Hole Oceanographic Institute (Seewald, J.S. 2003.  Organic-inorganic interactions in petroleum-producing sedimentary basins. Nature, v. 426, p. 327-333).  The shift of exploration to ever deeper offshore areas brings it closer to the lines where continents split and drifted apart in the past.  So it isn’t surprising that Nature Insight includes a review by Cambridge University and BP geoscientists of how those margins evolved (White, N., Thompson, M. & Barwise, T. 2003.  Understanding the thermal evolution of deep-water continental margins. Nature, v. 426, p. 334-343).  Organisms other than humans exploit the energy locked in oil, and geochemists from the University of Newcastle upon Tyne address their role in actually degrading petroleum, so that many of the largest onshore petroleum reserves (oil sands in particular) pose great difficulties for exploitation (Head, I.M., Jones, D.M. & Larter, S.L. 2003.  Biological activity in the deep subsurface and the origin of heavy oil. Nature, v. 426, p. 344-352).  Methane generated by anaerobic bacteria in sea-floor sediments and in bogs can combine with water in the form of an ice-like substance called methane hydrate, if the pressure is high enough and temperature is close to 0ºC.  There is a lot of it about.  On the one hand it has huge economic potential, but on the other it poses awesome threats to the climate.  Several times in geological history vast amounts of methane have belched from the sea floor to drive up global temperature; it is a highly efficient “greenhouse” gas.  Dendy Sloane of the Colorado School of Mines addresses issues related to methane hydrates (Sloane, E.D. 2003.  Fundamental principles and applications of natural gas hydrates. Nature, v. 426, p. 353-359).  All these articles are deeply informative and well written.  They are “must-reads” for all geoscientists.  The sequence ends with a word from “management” (Shell International), in the form of a look ahead to how oil companies might clean up their act and become “friends of the Earth” (Stankiewicz, B.A. 2003.  Integration of geoscience and engineering in the oil industry – just a dream? Nature, v. 426, p. 360-363)

First out of Africa?

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In 1991 archaeologists working at the Georgian site of Dmanisi, which had been an important town on the Silk Road, found human remains, but they lay beneath the level at which several extinct mammals had been found.  As work progressed in the deeper levels, head bones emerged.  They were exceedingly primitive, and associated with equally archaic tools; not the elegant biface stone tools of Homo erectus and later, truly human people, but from the Oldowan culture found with the earliest Homo habilis in Tanzania.  The first estimate of their age, based on the mammal remains, was 1.6 Ma.  Apart from disputed finds in Indonesia and China, the Dmanisi hominids were the oldest found outside of Africa.  Yet at that time, the larger, more brainy H. erectus was thriving in Africa, using the Acheulean biface axes.  For the Georgian archaeologists, and the growing number of international collaborators, 9 years of painstaking work lay ahead before enough data had been gathered to draw conclusions confidently.  A well illustrated summary of what Dmanisi has revealed appeared in the November issue of Scientific American ( Wong, K. 2003.  Stranger in a new land.  Scientific American, v. 289(5), p. 54-63).  Lots fell into place, when eventually the stratigraphic position of the hominid remains was convincingly established using radiometric dating of basalts below and above it – 1.85 and 1.76 Ma respectively.  With more cranial fossils, the Georgian team led by David Lordipanidze the late Leo Gabunia were able to show just how primitive the Dmanisi hominids were.  Their brain capacity was half that of modern humans, and detailed skull features resembled the earliest known member of the human genus, H. habilis.  They were small people too, and palaeoanthropologists really cannot decide whether they were australopithecines or part of our genus.  Lordipanidze believes that they are transitional between habilines and erects.  What is most surprising is that they migrated as far as Georgia.  That would have involved either crossing the mountains of Turkey and Iran, or, had they taken the possible route out of Africa across the Straits of Bab el Mandab (possibly dry land at the time), an even more circuitous route following the coast of Arabia and perhaps up the Tigris-Euphrates rivers.  Their journey began before H. erectus invented the biface axe, which up to now has been regarded as the first sign of both a leap in intellect and the beginning of some command over the rest of nature.  The Dmanisi hominids made it and survived, despite their apparently puny frames, if the abundance of animal bones at the site marks long occupation.

Oxygen depletion before P-T extinction

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The massive die-off at the end of the Palaeozoic Era (251.5 Ma) has focussed attention from a variety of geoscientists for over a decade.  Theories for the cause abound, including the climatic influence of the huge Siberian continental flood basalt province, which formed around the same time, explosive release of sea-floor methane, oceanic anoxia, continental aridity and a massive belch of sulphur from the deep mantle.  There is now another candidate, asphyxiation (Weidlich, O. et al. 2003.  Permian-Triassic boundary interval as a model for forcing marine ecosystem collapse by long-term atmospheric oxygen drop.  Geology, v. 31, p. 961-964).  The explosion of land plants in the Carboniferous and early Permian that led to the world’s great coal deposits drove up atmospheric oxygen levels to their all-time peak.  The occurrence at that time of giant insects, whose metabolism depends on direct diffusion of oxygen, suggests levels of as high as 35%.  By the end of the Permian oxygen levels may have been as low as 15%.  One line of support for such low concentrations is the growing abundance of fungal spores in the late Permian, which the authors suggest may have been related to a decline in insect populations which consume vast amounts of plant debris.  Another is the widespread evidence of anoxic conditions in the Permian oceans, including isotopic features that support a “Strangelove” ocean at the P-T boundary.  How oxygen was removed from the atmosphere in the Carboniferous to end-Permian is hard to assess.  At levels above around 25% green vegetation catches fire easily, so large firestorms may have been characteristic of the coal-forming era.  However, that would not drop levels much below those that prevail at present.  Yet the Permian is famous for its continental red beds, the red coloration being due to iron oxide (hematite).  Perhaps the missing oxygen became locked in Fe­2O3 as the Earth took on a distinct reddishness as the Permian progressed.

“Archaean” ironstone pods prove to be very young

For a number of reasons, including evidence that the cell-chemistry of the most primitive bacteria includes heavy metals and sulphur, the most popular current theory for the place of life’s origin suggests ocean-floor hydrothermal vents.  This has led to a search for remains of such “black smokers” in Archaean greenstone belts.  One of the most celebrated sites is in the 3.5 Ga Barberton greenstone belt on the South Africa-Mozambique border.  Within it are bodies rich in iron oxides, known as “ironstone pods” (not banded iron formations) that show many of the characteristic features of hydrothermal processes.  As well as spurring many authors into concluding that the complex organic compounds in them indicate highly developed microbial ecosystems around early-Archaean seafloor vents, scientists have used fluids included in them to speculate on Archaean oceans, and the prevailing temperatures so long ago.  They will be dismayed by a re-appraisal of the pods by Donald Lowe of Stanford University and Gary Byerly of Louisiana State University, which casts doubt on their antiquity (Lowe, D.R. & Byerly, G.R. 2003.  Ironstone pods in the Archean Barberton greenstone belt, South Africa:  Earth’s oldest hydrothermal vents reinterpreted as Quaternary hot springs.  Geology, v. 31, p. 909-912).  These pods are composed mainly of ferric hydroxide (goethite), which survives only at low temperatures, and are full of open pore spaces that include banded goethite indicating that it formed with the pores’ present orientation,  The Barberton Archaean rocks are highly deformed and were metamorphosed at greenschist facies.  The pods cut the foliation, and goethite is seen to partly replace Archaean cherts and serpentinised ultramafic lavas.  As if these features were not sufficient to rule out the pods’ formation during Archaean times, Lowe and Byerly found one that is clearly related to a now inactive modern spring that formed terraces of botryoidal goethite.  These show clear evidence of having formed as a result of modern bacterial action; they are biofilms.  In places, modern landslide debris is cemented by goethite.  Watch out for interesting correspondence in future issues of Geology from groups who stuck out their necks too far.

Artificial Archaean “fossils”

Debate on the existence of the world’s oldest microfossils from the 3.5 Ga Warrawoona cherts in Western Australia (see Doubt cast on earliest bacterial fossils, April 2002 EPN) has been stoked up by the creation of similar filamentous objects in vitro by geochemists from Spain and Australia (Garcia-Ruiz, J.M. et al. 2003.  Self-assembled silica-carbonate structures and detection of ancient microfossils.  Science, v. 302, p. 1194-1197).  They did this by mixing soluble barium salts in an alkaline sodium silicate solution (pH 8.5-11) exposed to CO2 in the atmosphere.  At high alkalinity CO2 dissolves to enrich solutions in carbonate and bicarbonate ions.  Filaments made up of precipitated barium carbonate (witherite) and silica soon form.  They take on shapes very similar to the tiny segmented worm-like structures that in 1996 were trumpeted as fossils in a now notorious Martian meteorite, as well as those from Warrawoona that are disputed by Schopf and Brazier.  The experimenters went a step further, by immersing the filaments in a formaldehyde-phenol mixture and heating them to 125ºC.  They then became coated in brownish, kerogen-like carbonaceous material, much as the Warrawoona structures are.  Such organic coatings can also be produced by heating iron carbonate (siderite) to 300ºC in water vapour. These “test-tube” analogues of microfossils formed in plausible chemical compositions under not particularly special physical conditions.  Interestingly, the Warrawoona chert contains both baryte and iron carbonate.  Reaction to the paper was mixed!

Eucarya missing from Mesoproterozoic

Naively, I am always surprised to learn of Precambrian oilfields, even though petroleum in the vast fields of Saudi Arabia partly had its source in Neoproterozoic sediments and migrated into the overlying cover.  Provided oil has not been degraded by later biological activity, it contains chemical traces of the organisms whose original decay produced the hydrocarbons, even a breakdown product of cholesterol (cholestane) that is characteristic of the former presence of Eucarya.  In the Northern Territories of Australia, Mesoproterozoic sediments (~1430 Ma) that formed in a shallow marine basin are a target for oil exploration.  Potential reservoir rocks contain bitumen in pore spaces, but there are fluid inclusion in fractures, which host liquid oil and brines.  Organic geochemists at CSIRO, the University of Sydney and Macquarie University have analysed the oil’s molecular structure (Dutkiewicz, A. et al. 2003.  Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia.  Geology, v. 31, p. 981-984).  Mass chromatography reveals a wealth of complex organic compounds, that are biomarkers for the kinds of organisms that were buried and then thermally matured to form the oil.  These are exclusively those which point to prokaryotes, especially the cyanobacteria.  Evidence for eukaryotic organism is completely absent.  This is useful evidence in assigning a maximum age for the rise of the Eucarya that evolved into all modern complex organisms.  The earliest likely eukaryote fossil is Grypania, a glossy carbonaceous spiral, found occasionally in sediments around 1400 Ma old, although dubious finds may indicate an origin as far back as 2100 Ma.  The dominance of evidence for photosynthesising blue-green bacteria indicates that the oil-forming organisms thrived in an oxygenated, shallow environment.  So there seems every reason to believe that Eukarya would have been capable of thriving as part of the trophic pyramid, had they arisen before 1430 Ma.

How mountains grow

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In the Lake District of Cumbria, asking older local farmers how the fells grew will often get the response that they started out as pebbles.  The justification of this seemingly implausible hypothesis is that once a field is cleared of boulders, about 20 to 30 years later new ones have appeared and the clearing has to start again.  Geologists have their own ideas.  Compressive deformation of continental crust will thicken it, and gravity acting on this low-density material will ensure that its surface rises.  Counter-intuitively, the action of erosion can cause mountains to rise as well.  Debris flushed from deep valleys lessens the load on the underlying crust, so that it continually rises to drive up the elevations of the remaining ridges and peaks.  The compressional origin of the Himalaya is hard to dispute, yet they bounced up quite quickly, long after they began to form.  Current ideas, backed up by a variety of evidence, suggests that a lump of the dense lithosphere beneath the India-Asia collision zone fell off (delaminated) and sank in the mantle.  That reduced the mass of the lithosphere beneath and the gravitational field, so that the surface rose.  The second highest mountains, the Andes, offer no such mechanism, for they are not products of compression associated with collision.  Dense Pacific Ocean lithosphere subducts beneath them and the forces involved are insufficient to raise the Andes to even half their present elevation.  Simon Lamb of the University of Oxford and Paul Davies of the University of California, Los Angeles have attempted an explanation for the anomalously high Central Andes (Lamb, S. & Davies P. 2003.  Cenozoic climate change as a possible cause for the rise of the Andes.  Nature, v. 425, p. 792-797).  Their idea is that sediments that pour into subduction-related trenches from rising arcs, to form part of the accretionary prism where lithosphere starts to go down, lubricate subduction because of the pore water in them.  If there is little sediment supply from the rising crust, then frictional forces build up along the line of the subduction zone.  That focuses the plate boundary stresses over a narrow zone, thereby giving sufficient force to drive the crust higher and higher.  Today the cold northward ocean current along western South America provides little rainfall to the Central Andes, so erosion is much slowed.  Episodic global cooling since the Mid-Eocene probably reduced erosion there several times during the Cenozoic.  So for long periods the worlds largest subduction zone would have been starved of lubricants, thereby driving up the Andes.  The mountains themselves, by forcing maritime air upwards, would also starve the rising peaks and the great Altiplano plateau of rainfall, further influencing sediment supply to the trench system.  Lamb and Davies reckon that the Andes are fortuitous results of a N-S subduction zone at a continental margin, combined with its development during a period of global cooling and tropical drying.

Permian-Triassic boundary and an impact?

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More than 20 years since the proposal that the end-Cretaceous mass extinction coincided with a major impact, confirmed by the discovery of Chicxulub, nobody has produced convincing evidence for an extraterrestrial culprit for others.  Were geologists implanted with GPS tracking devices as soon as they graduated (no doubt on the cards in new health and safety regulations planned by the Blair government in Britain), then Big Brother would see strong clusters close to a number of boundaries on the geological map of the world.  There would be many at P-T sites.  Electronic tagging would have shown personnel from several US universities (Rochester, Harvard, California) in the Transantarctic Mountains, from time to time in the last few years.  Allegedly, that near-pristine area exposes rocks at the juncture between Permian and Triassic strata over less than a metre.  It is marked by the sudden disappearance of the famous Glossopteris flora, just below a clay breccia, from which this group of scientists have previously extracted evidence for shocked quartz and extraterrestrial fullerenes (football-shaped organic molecules) that contained odd noble-gas isotopes.  Two members of the team have made other finds of fullerenes, at the P-T boundary in China and Japan, the K-T boundary and the ancient Sudbury impact in Canada, whereas other workers have not been so lucky.  In fact, the duo are also the only people to have found fullerenes in meteorites, which is key evidence linking terrestrial finds to possible impact events.  The team has hit the headlines again (Basu, A.R et al. 2003.  Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica.  Science, v. 302, p. 1388-1392).  At first sight their discovery of pristine fragments of forsterite-enstatite rock with probable chondrules at the boundary suggests that indeed a major impact coincided with the biggest of all Phanerozoic mass extinctions.  They even report tiny grains of metallic iron with an astonishing purity, perhaps formed by condensation from the plasma cloud associated with a really big meteorite impact.  What is really odd, however, is that sedimentary rocks a quarter of billion years old should have preserved such highly unstable minerals.  All other finds of fossil meteorite fragments have been highly altered relics, as any geologist would expect.  There is a clamour for the Antarctic samples from other laboratories, so that the results can be confirmed or refuted. 

See also: Kerr, R.A. 2003.  Has an impact done it again?  Science, v. 302, p. 1314-1316, and Oxygen depletion before P-T extinction (above)

Low-cost disaster monitoring from satellites

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With little hype, a British company (Surrey Satellite Technology Limited, linked to the University of Surrey) is beginning to develop a constellation of remote sensing satellites that aim at monitoring a variety of threatening phenomena across the whole planet.  The Disaster Monitoring Constellation produces images at the same resolution (about 30 metres) as the US Landsat Thematic Mapper, but is unique in two aspects.  The satellites and launching them are cheap, because they are tiny by comparison with the giants normally associated with remote sensing, weighing in at only a few hundred kilograms, and they also use off-the-shelf components including the imaging devices.  Second, the four current DMC satellites fly in concert to cover the whole Earth with images 600 km across (Landsat images cover less than a tenth of the area) every day. No other system is capable of that degree of timeliness, the shortest “revist” time to now having been 16 days.  SSTL does not own the satellites or the data, but builds them on contract for developing countries.  The first to reach orbit, in November 2002, belongs to Algeria.  It was joined on 27 September 2003 by three more, sponsored by Turkey, Nigeria and the UK, which were successfully launched by a Kosmos rocket from Plesetsk in northern Russia, at a total cost of around $85 million.  These will be joined by similar platforms sponsored by China, Thailand and Vietnam in the next few years.  The targets are wildfires, floods, windstorms, volcanic eruptions, erosion and potential landslides, with the added benefit of very detailed information about changes in agriculture and forestry, and baseline mapping of geological and hydrological features.  Perhaps most important, it gives less affluent countries independent access to space imagery, which can only boost the confidence of natural scientists in the third world who are venturing into remote sensing after years of playing second fiddle to North American, Japanese and European specialists.  Organisations, such as Reuters Foundation AlertNet and the International Charter, plus other international disaster relief organisations, can tap in for images at very short notice  Astonishingly, SSTL has launched and is planning imaging satellites that weigh in as little as 7 kg.  The low-key announcement of the launch of the 3 latest members of the DMC (www.sstl.co.uk) coincided with US and British hype-fests centred on the current missions to Mars.  There is little doubt which will provide the most lasting benefits.

Recognition of African contributions to palaeoanthropology

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Science continues its occasional series on individuals who make an impact on the progress of science with a review of the growing number of Africans working at the forefront of human evolutionary studies (Gibbons, A. 2003.  Africans begin to make their mark in human-origins research.  Science, v.  301, p. 1178-1179). Ethiopians, Kenyans, Tanzanians and Eritreans have all made important finds and published their results over the last decade.  Their hallmark is avid field work, backed up with growing interpretative skills.  All credit the encouragement they have had from western colleagues, but now they are in a position to bring along a new generation of experts in their home countries.

The “Big Daddy” theory of human evolution!

One of the anthropological shocks of the 21st century was the discovery that the gene pool of central Asian men is dominated by such a limited range of Y-chromosome  characteristics that the only conclusion is that one small group of closely related men dominated impregnation across the region about 800 years ago.  They were probably all Mongols closely related to Genghis Khan (see, Darwinian evolution of humans challenged by Y-chromosome data? EPN March 2003).  Studies by geneticists from Italy, Portugal and Spain recently suggested that sexual dominance by very few men may have been widespread before about 18 to 12 thousand years ago, around the beginning of the warming that closed the last glacial epoch (Dupanloup, I. et al. 2003.  A recent shift from polygyny to monogamy in humans is suggested by the analysis of worldwide Y-chromosome diversity.  Journal of Molecular Evolution, v. 57, p. 85-97).  Mitochondrial (passed maternally) and Y-chromosome (paternal) DNA studies have been key tools in explaining the timing of migrations of humans over the last 100 thousand years, since their genetic patterns seem to cluster regionally.  Molecular clock estimates that use the appearance of new genetic mutations indicate the timing of population separations.  The study by Doupanloup and colleagues examined data from individuals who live on all continents.  There is an odd and generally distributed difference in genetic diversity between mitochondrial and Y-chromosome DNA, which superficially suggests far more women than men during the last glacial epoch.  In terms of births, that is clearly impossible.  One explanation, favoured by Doupanloup et al., is widespread polygamy that dwarfs that which notoriously occurs within some religious sects today.  Moreover, the “privilege” would have had to be passed on to successive generations of men directly related to the original “Big Daddies”.  Rapid shifts in power would not have left such a clear imprint on global Y-chromosomes.   How that was achieved without repression or slaughter of potentially competing men, is impossible to judge.  However, probable changes in EuropeanY-chromosome patterns around 70, 40 and 20 thousand years ago, that have been ascribed to either evolutionary “bottlenecks” during periods of rapidly dwindling numbers or sudden migrations, might equally have been due to the rise of new patterns of a few males’ dominance over others.   Dupanloup et al. show that the rise of agriculture around 10 thousand years ago seems to coincide with a breakdown of massive polygamy and more common monogamy.  There are other possible interpretations of the data.  In a largely monogamous society, if males stayed where they were born while women moved to live in their mates’ home area, men would be closely related to others in their area, eventually resulting in very similar Y-chromosomes being shared by many.  Different migration patterns or early deaths for most men while hunting may also have led to the genetic bias that is causing great discussion among evolutionary geneticists.

Source:  Bhattacharya, S. & Le Page, M. 2003.  A few prehistoric men had all the children.  New Scientist, 6 September 2003, p. 18.

Case for Martian rainfall strengthens

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“Everyone knows” about the huge valley systems on Mars, which through their relationships to other aspects of the planet’s features are thought to have formed catastrophically early in its history.  The high-resolution Mars Global Surveyor images and altimetry bring a new perspective to fluvial features (Hynek, B.M. & Phillips, R.J. 2003.  New data reveal mature, integrated drainage systems on Mars indicative of past precipitation.  Geology, v. 31, p. 757-760).  The authors, from Washington University in St Louis USA, show depressions extracted from the altimetry data by simulation of the paths likely to be taken by rain water falling on the surface.  In some areas, the depressions link up in dendritic networks very like those that occur on the Earth’s surface.  Previous data only picked up disconnected valleys.  The newly outlined valleys are V-shaped, unlike the U-shaped systems that developed on Mars probably by sapping as groundwater emerged, either slowly or catastrophically.  Such profiles are good evidence for surface run-off, and that can only indicate precipitation, either of rain, or as a result of melting snow.  Only 11000 kilometres of valley segments can be identified, and are probably relics of a larger ancient system that later events have masked.  Some however, reach to the rims of large craters and seem to post date them.  Probably, the events that carved these systems occurred in Mars’ early history.