Author: zooks777

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

The Early Cretaceous lagerstätten of NE China

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Barely a month passes without some weird fossil emerging from the widespread excavations in Early Cretaceous lacustrine sediments of north-east China.  It is probably the most productive palaeontological formation in the world, and has shed light on more than just the dinosaurian origin of birds, and rives ideas on the rise of angiosperm plants and early mammals.  As well as abundant fossils, the lagerstätten formed under low-oxygen conditions and preserves exquisite detail of soft tissue.  A review of the material and the environment in which it formed is welcomed by all palaeontologists (Zhou, Z, Barrett, P.M. & Hilton, J. 2003.  An exceptionally preserved Lower Cretaceous ecosystem.  Nature, v. 421, p. 807-814).  Zhou et al. Discuss the formation from two angles.  Scientifically their focus is on the potential for building a complete ecosystem for the area during the Early Cretaceous.  However, they also record the massive problems that result from haphazard collection by organised teams of locals and fossil dealers – incidentally the source of the infamous Archaeoraptor forgery (see “Piltdown” bird, in March 2001 issue of Earth Pages News).  Their review is also a plea for some kind of firm regulation of collection, although experience from many other lagerstätten suggests that is unlikely in the short-term.

Did terrestrial life emerge later than geochemists think?

A lot hangs on the notion that life can make it from abiogenic chemistry very quickly once a world has watery seas.  Evidence from oxygen isotopes in the oldest known terrestrial zircons suggests that liquid water was around on Earth by about 4400 Ma (see Pushing back the “vestige of a beginning” in Earth Pages News of February 2001, and The Hadean was cool June 2002).  It lies behind the search for signs of life on Mars and the fiasco surrounding the premature announcement of bacterial fossils in a meteorite reputedly from the Red Planet.  Right here, controversy has been raging over the once-living status of tiny patterns in 3500 Ma cherts from Western Australia (see Doubt cast on earliest bacterial fossils in Earth Pages News, April 2002), and on the true significance of isotopically light carbon trapped in apatite crystals in the 3800 ma Akilia metasediments of West Greenland.  Both have been claimed as signs of early, well-organised life, but the evidence is circumstantial.

Investigative journalism is very welcome in science, mainly because most scientists are either too polite, or grumble quietly in the coffee room.  Jon Copley, who teaches at Southampton University, has ventured into the field by interviewing some of the main antagonists in the “Is this a sign of life” debate (Copley, J. 2003.  Proof of life.  New Scientist, 22 February 2003, p. 28-31).  His article is most revealing, by getting down to brass tacks.  There is a lazy tendency in science to invoke William of Ockham’s “Razor”, i.e. that the simplest explanation of data is the best.  That is fine for the Old Bailey, in the manner of Roman legal argument of cui bono (who benefits?), but the natural world has a cussed tendency to pay no attention to human linear thought,  It is not a place for “elegance”, no matter how much scientists feel in awe of elegant mathematical proofs.  That it is wielded in favour of the most complex process in the universe to account for geochemical and other data is a bit odd.  Central to Copley’s sharp journalism lies something of which C-isotope specialists do not speak much.  At temperatures around 400ºC and a few hundred times atmospheric pressure can result in carbon monoxide and hydrogen combining to form hydrocarbons.  Fischer-Tropsch synthesis of hydrocarbons that fuelled Nazi Germany and South Africa under apartheid does occur in nature.  The ideal place is around deep-sea hydrothermal vents.  The reactions favour 12C over the heavier 13C and results in d13C just as negative as do living processes.  Isotopically light carbon in rocks that do not contain cast iron confirmation through tiny fossils, cannot be seen as proof that life existed.  Probably the oldest irrefutable fossils are of bacteria in the 1900 Ma Gunflint Chert of Ontario.  If we cannot be sure that C-isotopes help detect living processes on the early Earth, then results from missions, such as Beagle-2, to Mars could be exercises in futility.

Freezing the Antarctic

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Records of seawater oxygen isotopes and its Ca/Mg ratio shows that a substantial permanent ice sheet first formed in Antarctica in the Oligocene Epoch, about 34 Ma ago.  The favoured explanation, until this month, was that the South polar continent became thermally isolated from the rest of the planet when circumpolar currents were able to flow around it, once South America and Australia had separated from Antarctica and opened the “gateways” of the Drake and Tasmanian Passages.  But what if atmospheric CO2 played a role?  A drop in the “greenhouse” effect and global cooling could have driven polar temperatures low enough for ice formation without an oceanographic influence.  Once established, the albedo effect of a large ice sheet would seal Antarctica into permanent freeze-up.  Factoring all the likely components in a general circulation model leads to a surprise (DeConto, R.M. & Pollard, D. 2003.  Rapid Cenozoic glaciation of Antarctica by declining atmospheric CO2, Nature, v. 421, p. 245-249).  The opening of the Drake and Tasmanian Passages was not accompanied by a sufficient depth of water to support massive current reorganisation until several million years after the ice cap left its clear imprint on the marine record.  DeConto and Pollard’s model shows that even with closed Passages an ice cap would have formed, if CO2 levels had fallen below three times those that prevailed in the Holocene, before industrial emissions began.  Global cooling had begun somewhat earlier than Antarctic freeze-up, following the high around the Palaeocene/Eocene boundary (~55 Ma), falling to a plateau about 40 Ma ago.  Undoubtedly CO2 concentrations had fallen globally for this to have happened.  Of course, there is no Oligocene ice, from which glaciologists might extract trapped bubbles and samples of ancient air with which to refute or confirm the model.  However, a decrease in carbon dioxide would also cause the acidity of rainfall to decrease as well as the amount of rainfall globally, and that might show up in changed weathering processes, especially in the tropics of the time. 

How patterned ground forms

Visiting flat areas of permanently frozen ground brings you face to face with truly bizarre patterns at the ground surface.  Some are perfect hexagons of stones around finer soils, others doughnut-like circles and then a perplexing range of other features that look for all the world as though they were built by humans.  Undoubtedly, they result from the forces at work when the top soil layer freezes and thaws annually, together with soil creep down extremely shallow slopes, repeated over millennia.  However, exactly how the patterned ground develops has eluded geomorphologists for more than a century.  Rejecting the reductionist approach that any landform’s evolution can be deduced from basic principles of physics seems to be the key (Kessler, M.A. & Werner, B.T. 2003.  Self-organization of sorted patterned ground. Science, v. 299, p. 380-383).  Kessler and Werner of the University of California modelled the two likely processes of ice lensing that sorts stones and finer soil, and the transport of individual stones along the lines of accumulated stones as freezing fines expand, building in elements of spatial and time scales plus other parameters such as surface slope.  Their model is self-organising, and proceeds to mimic many of the intricacies of patterned ground, even the most labyrinthine.  It might seem a little heavy handed to crunch numbers to help explain what are really quite minor features.  But having demonstrated the power of non-linear modelling here, the authors open up a novel approach to landscape evolution of every scale and antiquity.

Carbon dioxide and Martian channels

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Despite the evidence from the neutron detector on Mars Odyssey for the possible existence of subsurface water on Mars (Water on Mars, August 2002 Earth Pages News) not everyone accepts that minor rills and channels on its surface are due to periodic melting of buried water ice (Water on Mars, July 2000Earth Pages News).  Two small pieces in New Scientist contest that view.  In a letter, Wytse Sikkema of Shell likens them to features carved by turbidity flows (suspensions of solid particles in a fluid, such as avalanches, ash flows and submarine turbidity currents) which they resemble more than stream channels (Sikkema, W. 2003.  Rivers of Dust.  New Scientist, 18 January 2003, p. 24).  Sikkema suggests that the supposed ocean-like basins on the Red Planet are filled with dusts carried by such flows.  Support for such a mechanism emerges from observations of gullying in progress during Mars’ late spring near the poles, when temperatures were too low for liquid water to exist.  Nick Hoffman of the University of Melbourne, suggests that the active gullying that he observed  on successive Mars Global Surveyor images involves rapid vaporisation of CO2 snow and ice to lubricate dust avalanches (Nowack, R. 2003.  Ravines hint at gas avalanches on Mars.  New Scientist, 18 January 2003, p. 14-15).  Hoffman also considers that massive release of gas by boiling of buried CO2 liquid could have carved the much larger valley systems on Mars by massive flows of dust-gas mixtures.  If he is correct, there is no reason to consider Mars either as a haven for early life or one for intrepid astronauts.  Britain’s Beagle 2 probe and two unnamed NASA Mars rovers, due for launch this year, should resolve the issue, but if water is not confirmed, there will be huge disappointment for both teams involved with those missions.

Frightened by impacts?

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If so, the site to visit is at NASA’s Jet Propulsion Laboratory ( www.jpl.nasa.gov/temlates/flash/neo/neo.htm ).  The Near Earth Objects team has designed the site for general education about the kinds of space chunks that might strike, the risks involved and what will probably happen when we get very unlucky.

The chemical conditions for life

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Robert Williams (Oxford University) and João Fraústo da Silva (Technical University of Lisbon) have an unconventional, but plausible take on the conditions for life’s origin and evolution (Williams, R.J.P & Fraústo da Silva J.J.R. 2003.  Evolution was Chemically Constrained.  Journal of Theoretical Biology, v. 220, p. 323-343).  However life began, presumably as cytoplasm containing DNA, RNA and proteins within a semi-permeable wall, it was surrounded by the chemistry of whatever environment it appeared in.  The proto-cell would have drawn hydrogen ions from water, to perform the proton pumping that is essential to all living organisms, and thereby created more oxidising conditions in its immediate vicinity.  Oxidation would have generated nitrogen from ammonia, released metals from their sulphides and converted other sulphides to sulphates.  Conversely, ions in its surroundings would have been able to “leak” into the cell itself.  By creating oxidised radicals, this inward leakage would have rebounded the cell’s activity on itself, with potentially toxic consequences.  Survival depended on two things: exploiting the opportunities, such as nitrogen fixation, using oxygen and even photosynthetic chemistry; and fending off potential toxic shock.  One of the most interesting aspects is the role assumed by calcium ions.  Their presence inside a cell would have precipitated DNA, by binding to it, with fatal consequences.  The upshot, according to Williams and Fraústo da Silva, is the special role of calcium as a messenger ion, perhaps having arisen through the necessity to pump it out again.  Today, the range of calcium concentrations in cells is extremely limited; too much or too little being fatal.  Perhaps a sudden change in the calcium-ion concentration in seawater in the late Neoproterozoic was responsible for the extreme excursions in carbon isotopes that are ascribed to mass extinction and equally massive adaptive radiations.  My own stab in the dark, is that a protective response to calcium stress by metazoans at that time may explain the sudden appearance of calcium-rich hard parts, which we know as the Cambrian Explosion.  They evolved means of excreting calcium from their many cells, so creating an outer “shell” that eventually developed into “armour” or “armament”.

The delightful aspect of Williams and Fraústo da Silva’s ideas is that they break from pure genetic determinism and the dominance of pure chance in addressing the central issue in the whole of science – the complete interconnectedness of real nature.

Archaean tectonics was different

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Higher mantle heat production in the past suggests that at some stage in the evolution of plate tectonics oceanic lithosphere would arrive at destructive margins too hot for oceanic basalt to dehydrate and form eclogite.  Without excess density over that of the mantle, conferred by subducted eclogite (3300 kg m-3), the lithospheric slab would descend at a shallow angle, oceanic crust would probably undergo wet partial melting, and maybe slab pull force would be so low that subduction was a hit or miss affair.  The thermal state of the Archaean Earth might not have had plate tectonics as we know it today.  However, studies of the oldest probable ocean floor (the >3800Ma Akilia Association of West Greenland) looks for all the world as if it formed as an accretionary prism as a result of normal-seeming plate forces.  Previous speculation about Archaean tectonics assumed basaltic oceanic crust, much like today’s.  High heat production also implies that Archaean constructive margins generated a great deal more magma by partial melting of mantle with higher potential temperature; probably more magnesian, picritic primary magma (Foley, S.F et al. 2003.  Evolution of the Archaean crust by delamination and shallow subduction.  Nature, v. 421, p. 249-252).  Instead of the lower oceanic crust being made from gabbroic cumulates, it was then probably dominated by ultramafic products of fractional crystallization.  Foley, and colleagues Stephan Buhre and Dorrit Jacob of the Universities of Greifswald and Franfurt in Germany, show from high-pressure experiments that such lower crust would form dense pyroxenites.  At destructive margins these might delaminate from the upper oceanic crust to subduct steeply, thereby conferring slab-pull force to drive tectonics.  Their eventual partial melting would source basaltic magmas to add to older oceanic crust that failed to subduct during the earliest Hadean times.  That would explain the lack of continental materials older than 4000 Ma.  .  The partial melting of garnet-bearing mafic materials (probably garnet amphibolite) that sourced Archaean continental crust would have had to await the end of such delamination, when the whole oceanic crust could descend, albeit with hot wet basalt in the upper part of the slab.  Interesting though the ideas in the paper are, apart from the authors suggestion of a connection with element depletion of the upper mantle progressively affecting an ever deeper zone, they hark back to thoughts on Archaean processes as early as the late 1970s.

Eskola’s mantled gneiss domes revisited

The Finnish geologist Pentti Eskola famously recognised in the 1940s that many basement terrains throughout the world, particularly in Scandinavia, have large tracts of gneiss in the form of domal structures separated by synforms (mantles to the domes) of supracrustal rocks.  These mantled domes give a curious “egg-box” appearance to the geology of many shield areas, usually picked out by the conventional pink colours used to signify granitic rocks and greens for supracrustal belts.  Once it was recognised that interference between upright folds of different ages and with different axial trends could produce “egg-box” structures on the outcrop scale, many structural geologists turned to this as an explanation for the huge features recognised by Eskola, even suggesting that the “mantles” were above profound unconformities.  Eskola’s view was that these regional features were due to differential uplift of low-density gneisses and more dense supracrustal rocks, and this view lingers with many other geologists.  Christain Teyssier and Donna Whitney, of the University of Minnesota, have reviewed the current state of knowledge for the phenomenon (Teyssier, C. & Whitney, D.L. 2002.  Gneiss domes and orogeny.  Geology, v. 30, p. 1139-1142), and conclude something more involved than either hypothesis.  Many of the gneiss domes show evidence for the involvement of crustal melting in response to decompression as orogens evolve, almost certainly resulting from removal of the upper crust, either by rapid erosion or extensional tectonics.  As well as forming bodies of melt or near-molten migmatites, such a process weakens he crust, allowing masses of low-density crust, including the partially melted bodies, to rise rapidly.  This feeds further decompression, the whole process becoming an effective means of advective heat transfer in large orogens.

Some old habits die hard

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Earth Pages News does not usually contain book reviews, but two that I read over the Christmas break deserve a comment.  The first, The Lunar Men: The Friends Who Made The Future (Jenny Uglow, Faber & Faber, 2002) shows how what is now becoming known as the geosciences was central to the wide-ranging discourses and research of that group of men who created the foundations of modern science in Britain.  The Lunar Society was a loose association of free-thinking individuals, which included Matthew Boulton, Erasmus Darwin, James Watt, James Priestley, Josiah Wedgwood and James Hutton, who became close friends and collaborators at the dawn of the Industrial Revolution.  All emerged from the religious nonconformism that lay outside the aristocratic establishment of the late 18th century, but each came from different backgrounds.  What united them was an all-consuming curiosity as well as a desire to make a living.  Each was driven in his own way to serve those less fortunate, as well as to take their own wealth and talents to whatever limits they had.  Not one was a specialist, and they shared all their interests, ideas and discoveries, as well as supporting one another intellectually, economically and socially.  These were not men in thrall to peer-review or the building of academic empires.  Collectively they challenged established views of all kinds, and took the greatest delight in doing so, even when subject to physical attack, as was Priestley.

The characters depicted in The Dinosaur Hunters (Deborah Cadbury, Fourth Estate, 2001) are from one or two generations later, and do not cut such a merry dash.  Central figures are Gideon Mantell, William Buckland and Richard Owen.  They worked in a period when the challenge of the Lunar Society had created a state defence of religious orthodoxy (almost a panic), and in which a new and partitioned scientific establishment had emerged.  The first dinosaur remains were discovered by the daughter of a carpenter, subsisting on Poor Relief, who supplemented her family’s subsistence by selling Jurassic fossils that she had become adept at finding on the beaches of Lyme Regis.  170 years before the advent of the Open University, Mary’s brilliant insights brought no academic benefits to her, but many to the Reverend geologists who plagiarised her in exchange for just enough cash to keep her and her family in bread and potatoes.  The central characters, however, are Gideon Mantell and Richard Owen.  Mantell, a rural doctor, became obsessed with ancient reptiles following his and his wife’s discoveries of fragments of the Iguanodon.  He felt driven to make his name in scientific circles from outside the establishment, and a tough time he had, despite his growing insight and assiduous collection.  Owen, who hardly collected a specimen in his entire career, relied on his anatomical skills to describe, classify and steal those of others, such as Mantell.  The founder of the Natural History Museum (with Prince Albert’s patronage), Owen clawed his way to the pinnacle of British science over the backs of those more honest and naïve than himself.  Although he was exposed as a plagiarist and scoundrel by Thomas Huxley, Charles Darwin’s “bulldog”, following the publication of On The Origin of Species, Owen’s main victim Mantell had already died a broken man.  William Buckland, by all accounts a genuinely nice man, ended his days in a lunatic asylum having tried to square the growth of material evidence for evolution with his own deep religious beliefs.

Having read both books in quick succession, it was difficult to avoid the conclusion that the best spirit of the “Lunar Men” seems largely to have departed from our science, while the meanest spirit of Victorian times lingers on among self-promoting empire builders with ever-narrower specialisations.

More pondering on new discoveries

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The recent publications that described extremely old primate remains (Orrorin and Sahelanthropus), which may be early beings on the tortuous road to the emergence of humans, has set the circle of palaeoanthropologists abuzz (see A considered view November 2002 Earth Pages News).  Sooner or later, Scientific American was bound to commission an article in plain words that expressed all the conflicting views and  illustrated them magnificently, and so it has (Wond, K. 2003.  An ancestor to call our own.  Scientific American, January 2003, p. 42-51).

BIFs and bacteria

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The steel in your car almost certainly contains iron mined from a banded iron formation or BIF.  These Precambrian sediments are the largest repository of high-grade iron ore on the planet, and nearly all of them formed before about 2 billion years ago, when Earth’s atmosphere and hydrosphere are reckoned by many to have contained very low amounts of free oxygen.  The enigma of BIFs is that, as well as vast amounts of iron, they contain equally large amounts of oxygen combined in hematite and magnetite.  However they formed, there must have been sufficient iron and oxygen in their environment to make these minerals in astounding quanties.  Iron is problematic, because in its Fe-3 form it is almost completely insoluble, and modern sea water contains very little because it is an oxidizing fluid now.  Nobody doubts that BIFs formed in a marine environment, and that would have had to contain plenty of soluble Fe-2.  So seawater before 2 Ga must have been a reducing fluid so that iron emanating from hydrothermal vents on the basaltic ocean floor could remain in solution and end up in near-surface water.  A popular explanation for the oxygen in BIFs is that it was released by the photosynthetic metabolism of blue-green bacteria, near to the basins where BIFs accumulated.  So BIFs mopped up any free oxygen that would otherwise have ended up in air or water and made both oxidising.  Eventually oxygen production outstripped that of soluble Fe-2 (perhaps by a gradual slowdown of sea-floor spreading) and thereby caused all hydrothermal iron to be precipitated near to ocean floor hydrothermal vents; the oceans became iron-poor after 2 Ga.

There is another plausible scenario for BIF formation, explored by a team from Canada, Britain, Australia and Denmark.  Some types of modern bacteria, chemolithoautotrophs and photosynthesisers that do not produce oxygen, are able to fix iron as Fe-3 hydroxides where there is very little oxygen or none at all.  The simple chemical equilibria that they exploit provide both energy and carbohydrate (Konhauser and 6 others 2002.  Could bacteria have formed the Precambrian banded iron formations?  Geology, v. 30, p. 1079-1082).  Evidence that such a process might have “grown” the massive BIFs comes from the famous Palaeoproterozoic Hamersley Group of Western Australia, the source of all the steel in cars produced in east Asia.  The Hamersley BIFs contain extraordinarily fine layers of iron oxides and silica, which may be annual or even daily records of biological cycles.  The key evidence lies in the relative concentrations of other elements in the deposit, phosphorus and trace metals (V, Mn, Co, Zn and Mo), which are close to the nutritional balance needed by the bacteria that Konhauser et al. suggest to have been involved.  Experiments with colonies modern bacteria of these kinds show that they are quite capable of depositing iron hydroxide at rates that would easily build vast thicknesses, given time.  Around 1022 individual cells could do the job at a rate that would have built the Hamersley BIFs – about 100 metres per million years.  That might seem to be an awful lot of bacteria, but it amounts to only about 40 thousand cells per cubic centimetre – far less than the number that build plaque on our teeth!

Phanerozoic marine strontium record throws spanners in the works

Jan Veizer of Ruhr University, Germany and the University of Ottawa is rightly known as “Dr Strontium”.  Almost single handedly he has created the record of strontium variation in seawater through geological time, by analysing carbonates that have extracted it along with calcium.  Input of strontium to the oceans is through continental weathering and hydrothermal solutions from the oceanic crust, and it has proved tempting to use variations in the Sr/Ca ratio of carbonates as a proxy for the rates of both processes, particularly using Sr isotopes.  It is not so simple however, as Thomas Steuber of Ruhr University and Veizer have shown (Steuber, T. & Veizer, J. 2002.  Phanerozoic record of plate tectonic control of seawater chemistry and carbonate sedimentation.  Geology, v. 30, p. 1123-1126).  As in many geochemical cycles, the other important process is burial of strontium in marine sediments, and that depends very much on the type of carbonate that carries it from solution.  Aragonite is between 8 and 4 times more efficient at mopping up dissolved strontium than the other common calcium carbonate, calcite.  So, if aragonite is the main carbonate that is buried, seawater strontium is likely to fall more rapidly than with calcite burial.  Which form dominates in sedimentation depends a great deal on the kind of animal that builds shells – most carbonate buried during the Phanerozoic has been of biogenic origin.  Corals and carbonate-secreting algae use aragonite, whereas molluscs, brachiopods, coccoliths and forams have calcite shells.

Other workers have suggested that there have been periods dominated by deposition of one or other form of calcium carbonate, mainly calcite until the mid-Carboniferous, then aragonite up to the mid-Jurassic, calcite through the Cretaceous and most of the Tertiary, and a current tendency for more aragonite.  Steuber and Veizer show how there is good correlation between changing ocean-crust formation and seawater Sr, and a negative correlation with the Mg/Ca ratio of seawater.  Clearly there are linkages between the three variables, as follows: hydrothermal alteration of new ocean crust exchanges Mg for Ca, so the rate of sea-floor spreading modulates the seawater Mg/Ca ratio; magnesium inhibits the formation of calcite, thereby encouraging aragonite formation; periods of slow spreading therefore favour a higher rate of strontium removal from seawater.  This has profound negative implications for the use of strontium isotopes in marine sediments to monitor the pace of continental weathering (the crux for some gross models of global climate change), and using the Mg/Ca ratio as a means of monitoring seawater temperature variations.

Volcano Webcams

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CCTV not only infests every street, trunk road and office block, but is beginning to be trained on volcanoes.  The US Geological Survey maintains a web site that links to more than 40 Webcams pointed at active volcanoes, including St Helen’s, Fuji, Ruapehu and Etna (vulcan.wr.usgs.gov/Photo/volcano_cams.html).  So, volcanologists, make sure your sensors, hard hats and reflective suits are packed, ready to go.  You can keep an eye out for your volcano starting to blow, even as you are eating your Rice Crispies!

United States geological database

As well as organising its geographic information, including topographic maps and digital elevation data, into a seamless browseable whole (Brown, K, 2002.  Mapping the future.  Science, v. 298, p. 1874-1875), the US Geological Survey has launched a national geological database from which anyone can download a vast amount of information in 100 categories (geode.usgs.gov).

Landsat images as art

A new web site at NASA’s Goddard Space Flight Center landsat.gsfc.nasa.gove/earthasart enables you to view, download and order some of the most dramatic and aesthetically pleasing images captured by the Landsat programme.