Earth-logs

Toffee found in meteorite?

Published on December 1, 2001 by zooks777Leave a comment

The origin of life on Earth would have been greatly accelerated had some of the compounds used in constructing complex bio-molecules simply rained onto the young planet from outer space. Carbonaceous chondrite meteorites are known to contain a tremendous blend of many possible precursors, ranging from amino acids to the ampiphile molecules, whose curling-up in the presence of liquid water is seen by many cosmo-biologists as a route to the formation of cell walls. The latest addition to possible ingredients are sugars and related compounds in the two most important such meteorites, Murchison and Murray (Cooper et al. 2001. Carbonaceous meteorites as a source for sugar-related compounds for the early Earth. Nature, v. 414, p. 879-883). Detection of simpler carbon-based molecules in the spectra of interstellar molecular clouds, from which the Solar System probably accreted, suggests that a complex chain of photochemical reactions followed by thermochemistry as the pre-solar nebula became denser was the route to seeding the vicinity of the Earth with biological potential. However, the next steps ending in chemical self-replication and its RNA/DNA control remain a great deal more mysterious than detection of suitable reagents. For one thing, all life-molecules rotate polarized light in only one direction (anti-clockwise), whereas those of abiogenic origin, such as the compounds found in meteorites, rotate it both ways in roughly equal proportions.

See also: Sephton, M.A. 2001. Life’s sweet beginnngs. Nature, v. 414, p. 857-858.

The “Big Five” become the “Big Three”?

Published on December 1, 2001 by zooks777Leave a comment

That mass extinctions mark several fundamental boundaries in the stratigraphic column (late-Ordovician, late-Devonian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Tertiary) seemed to have become a well established feature of geology, thanks to the vast compilation and analysis of marine and terrestrial organisms by the late John Sepkoski and David Raup. However, it is very much a numerological exercise matching extinctions, new arrivals and their precise timing. Although not exactly “lies, damned lies and statistics”, analysing the fossil record depends on both data and algorithms. A new crunching of Sepkoski and Raup’s data, by Richard Bambach and Andrew Knoll of Harvard University, casts doubt on two of the formerly outstanding extinctions. They see a distinction between true mass extinctions – lots of genera popping their collective corks very quickly, and mass depletions, when a more general rate of extinction fails to be matched by newly evolved taxa. According to Bambach and Kroll, the late-Devonian and end-Triassic events fall in the latter category, leaving only three “big ones”.

Palaeontologists seem quite relaxed about these demotions and an earlier degradation of the Cenomanian-Turonian extinction, but one wonders about those who have beavered away at possible causes. Impactophiles have congregated lately on both boundaries, studying signs of correlation with large cratering events (Woodleigh in Western Australia and Manicouagan in Canada, respectively). Because the fossil record has a great deal to do with where collectors have been (and that has usually been close to their home bases in Europe and North America), it is anthropogenically biased. So far, new collections from further afield have failed to numerically overcome this skew, but the demise of the late-Devonian event stems largely from recent work in China.

Personally, I fail to see the distinction. The failure of evolution to repopulate niches abandoned by extinct genera seems equally as odd as spikes in the rate of extinctions. However, I have always been worried that the humble graptolite’s disappearance at the end of the Silurian hasn’t been recognised as a sign of dreadful times. These meek and co-operative creatures spread far and wide as plankton throughout the Ordovician and Silurian, evolved with an unmatched enthusiasm, and yet failed to inherit the Earth as their meekness should have guaranteed. Still, few now seem concerned with the vast panoply of graptolitic thecae and stipes.

Source: Kerr, R.A. 2001. Paring down the Big Five mass extinctions. Science, v. 294, p. 2072-2073. Report on November 2001 Annual Meeting in Boston of the Geological Society of America.

Length of childhood and the growth of teeth

Published on December 1, 2001 by zooks777Leave a comment

Unsurprisingly, palaeoanthropologists pay a great deal of attention to teeth and have friendly relations with dentists. The tendency of our ancestors’ remains to be gnawed and otherwise dismembered left more of them around than other skeletal bits and pieces. Based on the old adage that we are what we eat, teeth reveal a great deal about hominin habits. They also take up trace elements from the environment in which individuals lived at an early age, thereby giving hints to migration. Astonishingly, tooth enamel grows day by day, and tooth development can be charted with great precision. Together with the timing at which different teeth erupt in juveniles, fossil dental records potentially allow researchers to detect when in human evolution the unusually extended childhood of humans first appeared, and whether it developed gradually or suddenly. The particular focus is on teeth from prematurely deceased hominins.

Modern humans’ rates of enamel growth is much slower than that in apes. Despite the many signs of a profound physiological differentiation between apes, australopithecines and early species of Homo, as far as tooth growth goes, they are all similar (Dean, C. et al. 2001. Growth processes in teeth distinguish modern humans from Homo erectus and earlier hominins. Nature, v. 414., p. 628-631). The teeth of each grew faster than in modern humans. In dentition at least, there is little sign of an advance in childhood development even in anatomically very modern-looking H. erectus. That must have taken place in early modern humans, and needs to be checked in them and our co-descendants, the Neanderthals.

Teeth provide by no means the whole story. The near-complete skeleton of the famous Turkana Boy provides lines to suggest that when he died, his growth was well within the range of modern human development (Moggi-Cecchi, J. 2001. Questions of growth. Nature, v. 414, p. 595-596). It seems unwise to rely entirely on teeth. One possibility is that several important features (brain size, growth of tooth enamel, and even bipedalism) may have undergone repeated evolution – two steps forward, one step back?

Strontium load of Himalayan rivers

Published on November 1, 2001 by zooks777Leave a comment

One process connected to long-term climate change is the way that weakly acid rainwater (containing dissolved CO₂) weathers silicates in continental rocks, one product being carbonate in soils. The process should draw CO₂ from the atmosphere, thereby reducing its “greenhouse” effect. The idea is by no means new, but received a boost in the mid 1990’s from Maureen Raymo’s suggestion that fluctuations in the strontium-isotope composition of the oceans through geological time should be a proxy for changes in the rate of continental weathering. The ⁸⁷Sr/⁸⁶Sr of marine carbonates does show clear correlation with long-term climate shifts during the Phanerozoic..

Continental weathering should increase as topographic relief becomes greater through mountain building episodes. The Himalaya’s rise through the late-Tertiary has been suggested as a major influence over climatic deterioration, partly by its effect on the Asian monsoon and partly as a huge site for the sequestration of atmospheric CO₂ by chemical weathering. Himalayan rivers have enormous flows and equally large sediment and dissolved element loads. In particular they carry far more strontium than other rivers, and it has a highly radiogenic content of ⁸⁷Sr. There are three means of attaining these levels: from average continental crust which has a higher ⁸⁷Sr/⁸⁶Sr ratio than oceanic crust (the other main source of seawater strontium); from strontium rich limestones that acquired their isotopic signatures from the ocean when they were deposited; or from sources with unusually high ⁸⁷Sr/⁸⁶Sr ratios. The Himalaya are well known for carbonate sediments, and for granites formed by melting of deeper, older continental material that gives them very high proportions of radiogenic strontium. Recent work now shows that a significant contribution of highly radiogenic strontium to Himalayan rivers is hydrothermal activity (Evans, M.J. et al. 2001. Hydrothermal source of radiogenic strontium to Himalayan rivers. Geology, v. 29, p. 803-806). Hot springs feeding a major tributary of the Ganges contribute up to 30% of its strontium load, and incidentally a great deal of CO₂. Both result from hydrothermal alteration of deeper rocks, and are unrelated to weathering if the water involved emanates from the deep crust. It seems that these waters are recycled rainwater, so this is a case of a high-temperature chemical weathering. Whatever, it further complicates the original notion of linkage between mountain building and climate.

Methane and Snowball Earth

The well-publicized “Snowball Earth “ model for Neoproterozoic glaciogenic rocks that occur at tropical palaeolatitudes has to involve an escape mechanism from global frigidity. Without some means of warming, the high albedo of widespread ice would have locked the Earth into perpetual glaciation, which of course did not happen.

The main proponents of the model, Paul Hoffman and Dan Schragg of Harvard University suggested a gradual build up of volcanogenic CO₂ during “Snowball” conditions, when a dry atmosphere would have retained the “greenhouse” gas instead of its being sequestered to the oceans and carbonate rocks by acid rain and continental weathering. Gradually, atmospheric temperatures would have risen due to trapping of outgoing, long-wave radiation by CO₂. This simple aspect of the model leads to scenarios where warming overruns once ice sheets disappeared, to give extremely high-temperature conditions. Using carbon-isotope data from marine carbonates is a means of supporting or refuting this escape mechanism, and also of detecting the influences of other components of the carbon cycle. Carbonates take up carbon dissolved in seawater without fractionating its different isotopes, and provide measures of the degree to which organic processes did contributed to fractionation. Cell processes preferentially take up ¹²C, and if large masses of undecayed organic matter ends up in seafloor sediments, the proportion of “heavier” ¹³C (indicated by the standardized ratio of the two main isotopes d¹³C) increases in seawater and the atmosphere. Carbon of mantle origin, that emerges as volcanic CO₂, has a constant d¹³C of about -5‰. So these two processes contribute to an isotopic balance, which for most of the Mesozoic and Cenozoic Eras established a d¹³C of between 0 and +4 ‰ in sea water and limestones. This is interpreted as a sign that the recent carbon cycle achieved a balance between volcanic additions and organic carbon burial weighted towards trapping of undecayed carbohydrate in sea-floor sediments. Explanations for broad climate changes since 250 Ma therefore rely more on other mechanisms than on the carbon cycle

The most comprehensive study of Neoproterozoic carbon (Walter, M.R. et al. 2000. Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulfur in seawater, and some interpretative models. Precambrian Research, v. 100, p. 371-433) does indeed show dramatic see-sawing of d¹³C through supposed “Snowball” events, from highly positive values (<+10‰) before glaciogenic sedimentation to highly negative (>-10‰) in the immediate aftermath. However, few data were available from within glaciogenic sediments, and resolution is insufficient to detect tell-tale trends. The key approach needs detailed carbon isotopes through a single event, and such data appeared recently for the famous Neoproterozoic glaciogenic-cap carbonate sequence of Namibia (Kennedy, M.J. et al. 2001. Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals. Geology, v. 29, p. 443-446)

Kennedy et al. measure d¹³C in carbonate cements in the glaciogenic diamictites, in overlying cap carbonates and in cement to later clastic rocks. Interestingly, there is little sign of a gradual decrease in ¹³C through the glaciogenic rocks. Constant oceanic carbon composition would be expected if no volcanic CO₂ entered seawater during frigid, dry conditions, and living processes were minimal. In the cap carbonates d¹³C plummets from +3‰ to -4‰. One simple explanation would be massive “rain-out” of volcanic CO₂ (d¹³C of -5‰) that had built up in the air during the “Snowball” episode.

Whizz-bang at end of Permian

Published on November 1, 2001 by zooks777Leave a comment

Relating mass extinctions to the effects of impacts by comets or asteroids is now a major industry, and a great number of geologists who sneered at early suggestions of extraterrestrial influences over evolution are finding ever new ways to cook and eat their headgear. Oddly, however, many of those who bore the brunt of such mean-spirited, and somewhat premature scorn still cling to the safe old K-T event. Soon all the thin K-T boundary material will have been consumed by these cautious, if meticulous scientists. Thankfully, some have ventured to seek evidence for other catastrophes that came out of the blue. In comparison with the end-Permian extinction, the K-T event is a mere bagatelle. However, attaching it to an extraterrestrial cause has proved difficult. It has attracted as many opponents of impact theories as “whizz-bang” aficionados, with much talk of the effects of sea-level changes, volcanism, ocean anoxia and climate shift. They may be in for a big surprise.

The Permian-Triassic boundary in Meishan, China is at first sight a nondescript sequence of shallow marine strata, albeit complete. The last occurrence of Permian marine genera there, with typical signs of mass extinction, coincides with a 20-fold increase in nickel concentrations. Closer examination reveals other brusque geochemical and mineralogical anomalies, including magnetic grains of iron-silicon-nickel alloy, but no iridium anomaly (the popular target for detecting asteroidal impact horizons) or examples of shocked quartz and feldspar (Kaiho, K. et al. 2001. End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle. Geology, v. 29, p. 815-818). Most significant is a sudden drop in ³⁴S due to a large increase in the amount of isotopically light sulphur in the environment. Kaiho et al. attribute this to vast emission of sulphur from the mantle. A coincident fall in the ⁸⁷Sr/⁸⁶Sr ratio could also result from entry into the oceans of lots of mantle-derived strontium.

The P-Tr boundary also coincides with the time of eruption of the largest continental flood-basalt province, the Siberian Traps. No doubt other scientists will seek to account for the chemical anomalies at Meishan as distant effects of the Siberian volcanism alone, as they have for the K-T boundary anomalies because of their coincidence with Deccan volcanism. The authors prefer to suggest a causal link between impact and massive volcanism.

Surviving the Archaean with a UV jacket

Earth’s dominance, for at least the last half billion years or so, by oxygen-dependent and oxygen producing life forms stems from the evolution of photosynthetic organisms whose cell metabolism involves breaking the strong bonds in water molecules with solar energy. Chemo-autotrophic life that exploits other energy sources has been consigned to niches that are very much narrower than they were at the biosphere’s outset. The earliest primary producers using oxygenic photosynthesis were the cyanobacteria – arguably the predecessors of modern plants’ chloroplasts, in Lyn Margulis’ endosymbiotic model for the origin if the Eucarya. Carbon isotopes from the early Archaean do suggest their presence close to the start of recordable geological history, and at around 3.5 Ga the first known stromatolites were almost certainly secreted by blue-green bacteria (See Carbonates and biofilms, Earth Pages August 2001).

To thrive and colonise ocean surface waters, the shallows and perhaps even the continental surface – their water-splitting, solar powered metabolism opened up those opportunities – cyanobacteria, more than any other prokaryotes, had to resist massive damage from ultraviolet radiation. Lack of atmospheric oxygen, and therefore ozone, left Earth’s surface with no shield to the most biologically damaging, short-wave UV. Despite the fact that modern “blue-greens” can survive climatic extremes from the frigidity of Antarctica’s Dry Valleys to superheated water in hot springs, as regards UV damage they are wimpish. This is partly due to its bleaching effect on the light-harvesting pigment on which chlorophyll depends. Cyanobacteria cells do have some biochemical protection against radiation damage, but it is of no avail when bathed in the “hardest” UV likely to have characterized Archaean surface environments.

A widely held view is that “blue-greens” survived and prospered because of another function common to many single-celled organisms; their tendency to promote nucleation of inorganic compounds outside their cell walls. Stromatolites themselves are good examples of the production of biofilms, being made of minute laminae of carbonates, whose secretion helps cyanobacteria avoid calcium stress. In modern hot springs that contain dissolved silica, these organisms often help generate sinters made of silica. A team from the University of Leeds (Phoenix, V.R. et al. 2001. Role of biomineralization as an ultraviolet shield: Implications for Archaean life. Geology, v. 29, p. 823-826) has performed controlled experiments on living cyanobacteria from Icelandic hot springs to check their defences against short-wave UV. With a biofilm screen (in the experiment they used wafers made from associated iron-silica sinter, as well as colonies with a biofilm) the organisms easily survived and continued to photosynthesize. Exposed “naked” they succumbed after only a few days exposure. It seems that traces of iron incorporated in the films dramatically enhance the UV-screening, without reducing photosynthesis. Archaean iron-rich cherts are massively abundant in banded iron formations, and the first definite remains of cyanobacterial cells come from such silica-rich material. However, the ubiquitous stromatolites in limestones of early Precambrian times are the main signs of life. It remains for the UV-screening properties of carbonate biofilms to be assessed.

New phyllum from Chinese Cambrian

Incompleteness of the fossil record is partly a result of the bias towards organisms with hard parts and against soft tissue, during sedimentary processes. For preservation of soft-bodied animals, together with that of intricate parts of the usual fossils, palaeontologists look to site where preservation is exceptionally good – lagersttätten. An example is the Solenhöfen Limestone, famous for Archaeopterix. Mudstones formed under highly reducing conditions, which excluded bacteria that complete oxidize flesh, provide similar opportunities. Work through the last two decades by Simon Conway Morris of the University of Cambridge has resulted in working and interpretative methods that permit extremely detailed analysis of physiologies, beginning with the most famous lagersttätte, the Middle Cambrian Burgess Shale of British Columbia. Conway Morris and others unearthed beasts so strange that they had little choice other than to erect new Linnaean Classes and Phylla to classify them. Equally as important, such sites help fill in the details of early members of those which survive today, including the elusive penis worms.

Conway Morris has been part of a team based at the Northwest University in Xi’an China, which has discovered lagersttätten in the Lower Cambrian, closer in time to the explosive development and radiation of animals at the end of the Precambrian. Once again, unsuspected novelty has turned up (Shu, D.-G. et al. 2001. Primitive deuterostomes from the Chenjiang lagersttätte (Lower Cambrian, China). Nature, v. 414, p. 419-424). Along with excellent examples of agnathan fish and many familiar soft-bodied animals, the prize in this case are remains that warrant a new, extinct Phyllum, the Vetulicolia. The organisms are small but complex, with two main body chambers that reveal mouth, innards and gill slits. The last helps place them within the deuterostomes; an “umbrella” that groups chordates (sea squirts and vertebrates) and echinoderms (they have lost such slits, but are genetically closer to chordates than any other group). Critical to the evolutionary significance of the vetulicolians is a groove that floors what is interpreted as the anterior part of their alimentary canals. Such a groove characterizes the pharynx of chordates, where it serves as “gutter” for various glands – the endostyle, also involved with iodine in metabolism. If the vetulicolian groove is an endostyle, then they are chordates. However, lacking an axial stiffening rod (notochord of the chordates in general, and vertebral column in vertebrates) they must be primitive. Occurring with true vertebrates, in the form of jawless fish, the vetulicolians are a relic of some earlier stage in vertebrate evolution. Shu et al. take the cautious view that they are early deuterstomes from which echinoderms and chordates emerged – close to the fundamental division among animals into deuterostomes and protostomes.

(See also: Gee, H. 2001. On being vetulicolian. Nature, v. 414, p. 407-408)

EarthScope

Published on November 1, 2001 by zooks777Leave a comment

North America, particularly its west coast, is the best studied natural laboratory for active tectonics. Nonetheless, the downturn in Earth Science funding in the USA has threatened an ambitious project aimed at consolidating knowledge of plate interactions there. Nature (15 November 2001, p. 241) reports that the EarthScope initiative now has strong backing from the US National Academy of Sciences.

EarthScope has 4 elements: a mobile grid of seismometers; an observatory to monitor movement of plates below the NW Pacific Ocean; a programme aimed at drilling into the San Andreas Fault System; an interferometric radar satellite that will accurately measure ground movements in relation to tectonic and volcanic features. The total cost is around $400 million, shared equally between NASA and the National Science Foundation, if the funding proposal wins acceptance.

Information from: http://www.earthscope.org

Continental tectonics of eastern Eurasia

Interferometric radar remote sensing provides high precision information on Earth motions associated with earthquakes (Radar analysis of Turkish earthquake, Earth Pages August 2001), but depends on “before and after” imaging. Continental tectonics is not just the outcome of occasional large movements on major faults, but of strains that continually occur throughout the lithosphere. Global positioning satellites provide means of precise location, particular when operated in differential mode, in which field-station signals are matched to those at fixed, geodetically precise base stations. Precisions to within centimetres or better are now commonplace at low cost. Structural geologists have been using GPS receivers for over a decade to check on the annual rates of plate motion across major structures such as the Alpine Fault of New Zealand and spreading centres such as that exposed on land in Iceland. In the 19 October issue of Science, such geodetic analysis of tectonics leaped by an order of magnitude.

The jewel in the crown of continental tectonics is eastern Eurasia, where the active collision of the Indian sub-continent with Asia drives a huge array of very large faults that separate rigid blocks and others, such as the Tibetan Plateau, that are deforming en masse. The spreading power of the Carlsberg and Central Indian Ridges is dissipated in motion of continental crust spanning 30° of latitude and 60° of longitude. Chinese scientists and their collaborators from the US universities of Alaska and Colorado have measure GPS positions at 354 stations throughout China, every one or two years for the last decade. Their analysis of the interim results (Wang, Q. et al. 2001. Present-day crustal deformation in China constrained by global positioning system measurements. Science, v. 294, p. 574-577) helps confirm or modify ideas about crustal motions that stemmed from seismic first-motion studies and regional field evidence. More than a third of the tectonic power accounts for crustal shortening within the Tibetan Plateau. While the western part of the huge system involves consistent motion towards the north-north-east, driving into Eurasia’s hinterland, the “free-edge” of eastern China and Indo-China seems to encourage the escape tectonics first proposed by Molnar and Tapponier. That involves a massive clockwise rotation around the East Himalayan Syntaxis, which takes up a great deal of motion. Whereas Molnar and Tapponnier proposed the shoving of south-eastern China oceanwards by the “escape” of Tibet, Wang et al’s measurements reveal that its motion to the east is only between one third and a quarter that of the adjacent east Tibetan Plateau. The lack of any sign that Tibetan crust is overriding that of south-east China, or that the latter is being shortened, may suggest that escape is funnelled around the East Himalayan Syntaxis into Burma and South-East Asia.

Fate of the Neanderthals

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Chris Stringer and William Davies report on two recent conferences about the Neanderthals in the 25 October issue of Nature (Stringer, C. and Davies, W 2001. Those elusive Neanderthals. Nature, v. 413, p. 791-792). Debate continues on what happened to them, and why. Assimilation by gene flow remains a possibility with a few researchers, despite the mismatch between fragmental Neanderthal DNA and that from modern people, and the inability to get Cro-Magnon genetic material is vexing. Acculturation – the influence of the behaviours of groups on one another – is also an unresolved issue. At the centre of that particular debate are tools associated with late-Neanderthal sites that bear close resemblance to those of early Cro-Magnons; the so-called Châtelperronian. The problem is precision and accuracy of dating the material, which, of course, constitute the palaeoanthroplogist’s Sword of Damocles. Dating using the decay of ¹⁴C has long been a right old mess, what with variations in the cosmogenic productivity of the isotope, and the tendency of common bone samples to pick up stratigraphically younger carbon from humic acids in soils. Charcoal is the material of choice, but in the case of Châtelperronian artefacts only associated bone seems to be available. Help might be on the way in resolving inaccuracy that stems from variable ¹⁴C productivity by using marine-core data to calibrate terrestrial ¹⁴C dates to calendar years (the “CalPal” curve). It does, however, seem to be peeking over the horizon at present.

One of the alternative processes that might have snuffed out Neanderthals is climate change. High-resolution marine records are not too useful in that regard, because they reflect global processes, and Neanderthal demise was a regional issue. Pollen records from lake sediments in Italy now reveal the intricacies of European climate during the critical period around 30 ka. It was time of rapid fluctuations in tree cover. However, similar rapid vegetation shifts occurred long before modern human influx, and the Neanderthals survived them. One possibility, allied to the competitive-disadvantage hypothesis, is that Cro-Magnons brought a steppe culture with them, which allowed them to occupy open country more successfully than Neanderthals with a woodland culture.

The topic is stymied by imprecise dating (it can be as bad as ± 4 ka), so that open-season for speculation is protracted. There is a reluctance to consider extinction through epidemic diseases brought by newcomers, and against which Neanderthals had no immunity. Disease has played such a huge role in population crashes throughout recorded history, that for it not to be at the forefront is curious. It is a widely supported hypothesis for extinction of large mammals that coincided with first entry by modern humans into the Americas ( see Late Pleistocene mass extinction – July 2001 Earth Pages). That would have had to involve jumps between species, rather than simple transmission of killers such as measles between genetically very similar populations of humans.

En route out of Africa

Finds of H. erectus and artefacts in China and Georgia date back as long ago as 1.8 Ma; the earliest signs of massive diffusion of early humans protected by their culture from entirely new climates and surroundings. The great question is, “Which way did they go?” To many palaeoanthropologists, obstacles presented by the Arabian Desert and Caucasus Mountains, favoured exit from Africa via the Straits of Bab el Mandab (closed at that time) and coastal diffusion. It now seems that movements of early humans did reach the Levant at a very early date. Ron Hagai and Shaul Levi have produced strong evidence for H. erectus’ presence in the Dead Sea rift at around the same time (Hagai, R. and Levi, S. 2001. When did hominids first leave Africa?: New high-resolution magnetostratigraphy from the Erk-el-Ahmar Formation, Israel. Geology, v. 29, p. 887-890). They found that sediments enclosing primitive, Oldowan tools (but no skeletal remains) accumulated during the period between two magnetic polarity reversals. With other evidence, these correlate with the Olduvai subchron from 1.96 to 1.78 Ma. Definitely a “first” for the Middle East, but by no means proof that this lay on the route to wider colonization, even at Dmanisi, across the Caucasus in Georgia. Little would prevent easy diffusion from East Africa along the proto-Nile or the Red Sea coast to reach the Dead Sea rift, but the obstacles to the north and east of Israel would have been far greater for poorly clad and equipped Erects.

Experimental satellite to have extended mission

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The Earth Observing-1 (EO-1) satellite, launched by NASA in late 2000, carries two remote-sensing instruments that may become operational devices in the future, given a proven track record on EO-1 and, of course, sufficient funding. One, the Advanced Land Imager (ALI) is a test bed for sensors earmarked for the follow-on to the current Landsat-7 Enhanced Thematic Mapper+ (ETM+). As well as the existing ETM+ six bands, ALI covers three others close to existing bands. Whether by design or good fortune, two of these help define the important VNIR broad absorption by ferric iron minerals, neglected in remote sensing since the early days of the Landsat Multispectral Scanner. Like the ETM+, ALI also carries a panchromatic band that spans the visible range, and which is aimed at providing a means of sharpening detail in images. On ALI, however, this band has an improved resolution of 10 metres as opposed to the current 15.

More innovatory is the Hyperion instrument, a hyperspectral device that spans the visible to short-wave infrared range with 242 bands that are 10 nanometre wide. Hyperion is comparable with airborne hyperspectral devices, such as AVIRIS. In the experiment it captures data swathes that 7.7 km wide, made up from 256 pixels with a resolution of 30 m. After initial difficulties with allowing for atmospheric effects on the data, newly calibrated Hyperion data closely mimic mineral spectra.

Early work on EO-1 data in many fields, including geology, has proved sufficiently promising that NASA has given the mission a year-long extension. Although data are restricted to only a few target areas suggested by the investigators, the extension is good news. It is a reassurance about continuity of the Landsat programme, and a tantalising indication that the ill-fated hyperspectral Lewis satellite may be resurrected.

Information from: http://eo1.gsfc.nasa.gov/

Handy guide to the significance of meteorites

Published on July 1, 2001 by zooks777Leave a comment

Although the press made a great fuss in 1999 about the supposed discovery of signs of life in a meteorite reckoned to have been blasted off Mars by a giant impact, meteorites in general are the only direct means of developing ideas about how the Earth and the rest of the planets formed. The market in meteorites is beginning to resemble the London Metal Exchange in its frenzied bullishness, but being collectibles it is rare types that command the highest prices, rather than their significance. An excellent review of current ideas among meteorite specialists appeared in the 6 July issue of Science (Alexander, C.M.O’D., Boss, A.P. and Carlson, R.W. 2001. The early evolution of the Solar Syetem: a meteoritic perspective. Science, v. 293, p. 64-68).

Since development of theoretical ideas about the generation of the elements in stellar processes, it has become almost a cliché to ponder about the ultimate dependence of every aspect of the natural world on supernovae and their “seeding” of the galaxy with the chemical mix that is so familiar. Even the nuclear processes involved are easily grasped. Not so the means whereby star stuff assembled into planetary systems and laid the potential for life, plate tectonics and virtually everything else. Alexander and colleagues from the Carnegie Institute of Washington span the interactions between physical conditions around young and rapidly evolving stars, derived theoretically, and the kinds of compounds that they can generate. Meteorite chemistry and mineralogy, which are very diverse, put flesh on the bones of these ideas. The tangible properties of different meteorite classes, together with their radiometric ages, are analogous to fossils in piecing together both planetary evolution and the various kinds of environments in the early Solar System.

One conclusion in the review that surprised me concerns the oldest materials known to us – calcium-aluminium-rich inclusions found in some chondrites, such as the famous Allende meteorite that fell in Mexico. The pale inclusions contain evidence for the former presence of short-lived isotopes, such as ²⁶Al. So short are their half-lives that the delay between their nucleosynthesis and the assembly of the pale inclusions can have been a few hundred thousand years at most. There are two possibilities: either such isotopes were generated by energetic particles emitted by the growing early Sun, or they had their source in supernova events. Theoretical work on local genesis has so far failed to match the relative abundance of all such short-lived isotopes, derived from the amounts of their decay products found in pale inclusions. It seems highly likely that collapse of a pre-solar cloud of matter to form the nebula out of which Sun, planets and the parent bodies of meteorites emerged was set in motion by shock waves from a nearby supernova. They would have taken the form of a high-speed interstellar “wind” of gas. Observed differences in oxygen-isotope proportions in meteorites were once ascribed to heterogeneous mixing of this explosive introduction of exotic matter. However, the oxygen heterogeneities do not show up in the isotopes of other elements. That mismatch has led to ideas of chemical fractionation during Solar System evolution, akin to that so familiar from the different behaviours of “light” and “heavy” oxygen during evaporation of water and its uptake in skeletons of living things exposed to different climates. Differences in oxygen isotopes now form a strand in assigning different meteorites to sources at different distances from the evolving Sun, and in deducing that some rare meteorites did indeed come from Mars.

Clearly behind the hype surrounding promotion of staffed and unstaffed missions to Mars and the increasingly shady world of the meteorite trade, exciting research is being done.

Ice and prebiotic chemistry

The problem with ice on Earth is that it will not support living chemistry. The process of crystallization excludes impurities from its structure, so that reactions between organic compounds cannot go on. Comets are mainly ice, and frozen water is a common occurrence in the infrared spectra of interstellar clouds, along with a host of complex CHON compounds (over 100 discovered to date). How organic molecules form in cold molecular clouds is a difficult problem, or at least it was believed to be until recently.

Researchers at the NASA Ames Research Center in California have probed the structure of solid water under all manner of physical conditions. Below a temperature of 200 K (about that of liquid nitrogen) the hexagonal symmetry of ice, familiar from snowflakes, changes to the simpler cubic form. At yet cooler temperatures 10 to 125 K), ice has no crystalline structure. Like flint, it is cryptocrystalline or amorphous. Curiously, even only a few degrees above absolute zero it can flow like a viscous medium, in the manner of glass, when irradiated with ultraviolet radiation. The breaking and reforming of hydrogen bonds, as in liquid water, but slower, creates the conditions for retaining impurities and their chemical combination. This odd behaviour at precisely the temperatures of molecular clouds explains their richness in organic molecules. Quite probably comets form by accretion of such interstellar icy material. The experiments revealed that warming of amorphous ice above 125 K does not result in a complete transition to cubic ice, that would exclude impurities. Instead, around two thirds retains its odd properties. The discovery strongly hints that much of the basic work of producing precursors to life’s chemistry is not only feasible in interstellar space, but that they can be delivered to planets as they collide with comets giving a kick start to the origin of life.

Source: Blake, D.F. and Jenniskens, P. 2001. The ice of life. Scientific American, August 2001, p. 36-41.

Dinosaur nose mystery resolved?

Published on July 1, 2001 by zooks777Leave a comment

Popular animations of dinosaurs in Jurassic Park and Walking with Dinosaurs are palaeontologically speaking “state of the art”. That is, except for the beasts’ noses. A close observer will have seen Tyrannosaurus and Triceratops with nostrils high on their snouts, and appealing brachiosaurs apparently breathing through the tops of their heads. Such reconstructions rely on the position of the nasal passages where they enter the skull, and in dinosaurs such bony nostrils are large and complicated. Traditionally, dinosaur reconstructors have gone for the rear of the cavity for the positions of the fleshy nostrils.

Despite their extinction at the end of the Cretaceous Period, dinosaurs have many living close relatives, such as birds, crocodiles and some primitive lizards. All of them have fleshy nostrils situated at the front of the bony openings. For that matter, so do mammals. For several years Lawrence Witmer of the College of Osteopathic Medicine at Ohio University (Athens) has been pondering on this, even setting up the DinoNose project. Not only did Witmer apply the principle of parsimony to this intriguing issue, but noted the marks left on skulls by the blood vessels that supply the muscles that enable land vertebrates to snuff the air in many interesting and useful ways. Such marks appear on dinosaur skulls, towards the forward end of the nasal openings. The outcome is a fundamental revision of dinosaur physiognomy (Witmer, L.M. 2001. Nostril position in dinosaurs and other vertebrates and its significance for nasal function. Science, v. 293, p. 850-853). The next logical step is to seek signs that carnivorous dinosaurs did indeed snarl.

Cambrian Explosion: Shropshire hits the news

As if by magic, nearly all animal phyla suddenly appear in the fossil record around 545 Ma, at the base of the Cambrian period. The most famous of these are trilobites, a group within the phylum Arthropoda, for enthusiasts of which the Cambrian of Shropshire has long been a happy hunting ground. Temporary excavations into the Protolenus Limestone of Comley have revealed a somewhat diminutive, though nonetheless startling relative that helps resolve the long-running debate over the origins of animals (Siveter, D.J., Williams, M. and Wlaoszek, D. 2001. A phosphatocopid crustacean with appendages from the Lower Cambrian. Science, v. 293, p. 479-481). Superbly preserved in calcium phosphate, the tiny beast reveals great detail of its body parts, peeping from between a two-valve, spherical carapace. It is possibly an early ostracod, and certainly a crustacean. That such an advanced arthropod occurs close to the base of the Cambrian lends support to the view that animal diversification into extant phylla, and some vanished ones too, may have gone on far back into the Neoproterozoic. The other view is that this radiation was explosive, beginning only 10 Ma or so before the base of the Cambrian.

The “long-fuse” hypothesis for the emergence and diversification of the animals is also supported by differences in the molecular biology of distantly related modern animals. Assuming that accumulation of genetic change is steady, and can be calibrated by the coexistence of such groups as far back as the Cambrian, the “molecular clock” for animals probably started between 700 to 1500 Ma ago. The problem, of course, is that only animals with hard parts or which miraculously had soft tissue rendered preservable by mineralization can assist palaeobiologists resolve the issue. That is unfortunate, as such fossils occur only after about 5 Ma before the start of the Cambrian, and the large ones are exclusively Cambrian or younger. The “explosion” was the sudden appearance of skeletal material, using calcium compounds such as carbonates or complex phosphorus-bearing material. Such is the fascination with the detail of phyllogeny, that the trigger for the explosive emergence of hard parts is often overlooked.

See also: Fortey, R. 2001. The Cambrian Explosion exploded? Science, v. 293, p. 438-439.

True polar wander

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One of the powerful bits of evidence that support continental drift are the plots of magnetic pole positions determined from rocks of different ages exposed on a modern continent, relative to that of the present pole position. Comparing such plots from different continents sometimes reveals similarities in their shapes over long periods of time, so that the plots partly match when they are superimposed. In such fits, other parts of the plots diverge considerably. Such comparisons are best explained respectively by the former unity of the two modern continents and their movement together, and their separation to drift independently. The plots are illusory, and are called apparent polar wander paths. For most of the Phanerozoic Aeon such palaeomagnetic data tie in well with other evidence for the formation of composite continental masses, such as Pangaea, and plate movements since the Triassic. That provides confirmation of the basic assumption in palaeomagnetic studies that the Earth’s magnetic poles remain close to those of its rotation, bar some circulation around the axis and magnetic reversals. It is tempting to use the same assumption for earlier times, in the absence of easy fitting of the margins of continental segments and the sea-floor magnetic stripes that are the key to plate tectonics since the early Mesozoic. If magnetic poles did move well away from the poles of rotation at any time in the past, that would play havoc with continental reconstruction. True polar wander is something that many tectonicians “Dinnae care to speak aboot”! That is not surprising, for another reason. Use of the term imply mean two things: a long-term shift in the Earth’s magnetic polarity relative to its axis of rotation (to me that warrants the adjective “true”, and ); a shift in the relative position of the whole crust and mantle relative to the core, whose dynamism determines the magnetic field.

A recent review (Irion, R. 2001. Slip-sliding away. New Scientist 18 August 2001, p. 34-37) concentrates on evidence for the second usage. There is evidence that suggests a 20° shift of all continents over a period of 2 Ma, in the Cretaceous. This is dwarfed by a suggestion of a 90° shift in 15 Ma that span the time of the Cambrian Explosion, so that a continent could have moved from the pole to the equator at a rate far faster then anything known from Mesozoic to Recent sea-floor spreading. One explanation is destabilization of the Earth’s angular momentum by concentration of all crustal mass and the effect of a massive mantle plume beneath it at high latitudes. That would distort the Earth’s shape. A planet’s rotation is most stable when it its shape is fat around the equator. The opposite, a prolate spheroid, is least stable, and a polar supercontinent could result in such instability, restoring steady state if the whole caboodle slipped to lower latitudes. That is what is proposed to explain some odd palaeomagnetic pole positions newly and accurately gathered from early Cambrian rocks. Such a notion takes on its own momentum, because of its association in time with the explosive diversification of animals with hard parts.

It is not a fundamentally new idea, for Alfred Wegener suggested that the mechanism for his hypothesis of continental drift was Pohlflucht (flight from the poles) of continental mass.

Sniff ethylene and become an oracle

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In a small temple on the south slopes of Mount Parnassus in Ancient Greece sits a somewhat befuddled lady, her inhibitions definitely down and she sometimes becomes delirious and thrashes around. The great and the good seek her opinion on matters of state. Perhaps for almost 2 millennia, successive pythia (pythia) of the Delphic Oracle had a steady passing trade. Writers from the classic times of Greek and Rome leave little doubt that the pythia’s powers stemmed from three things: a fissure and a spring lying at the centre of what became revered as the Sanctuary of Apollo, and a vapour that emanated from her chamber. The Oracle was seemingly a matter of geology and its mystique.

Indeed there are two intersecting faults passing right beneath the Oracle. Suitably encouraged, a team from Florida State and Weslyan Universities, and The University of Louisville, USA has been studying all aspects of the site since 1995 (de Boer, J.Z., Hale, J.R. and Chanton, J. 2001. New evidence for the geological origins of the ancient Delphic oracle (Greece). Geology, v. 29, p. 707-710). Where others failed before them, they have discovered evidence for a spring and traces of hydrocarbon gas leaking from a bituminous limestone cut by the faults at depth. One of the gases in the mixture is ethylene, once used as an anaesthetic, and known to cause just the symptoms in the pythia described in ancient accounts of her powers.

Seafaring Homo erectus?

The first Homo erectus fossils recorded by Eugene Dubois came from Java. Dubois was not so good at recording the geological context of his finds, and most of the later Javan discoveries were by local farmers. Consequently the dates of first arrival of the erects are a subject of continual debate, recent suggestions being that erects maintained a hold in Indonesia until as late as 20 thousand years ago. The incompleteness of records also led to few finds of artefacts, so much so that doubt has been cast on any significant H. erectus culture. Later work throughout Indonesia did reveal something quite astonishing, however. The erects crossed Wallace’s line to colonise one of the easternmost islands in the Indonesian arc, Flores, where undoubted stone artefacts occur in rich beds of fossil bones.

Alfred Russell Wallace noted that the flora and fauna of western Indonesia are to all intents the same as on continental Asia, whereas those of the islands east of Bali are very different. This empirical division is now known to have arisen through the emergence of land bridges between the western islands and mainland Asia as sea level fell to expose shallow seafloor during Pleistocene glacial periods. Wallace’s line coincides with straits that are very much deeper than could ever disappear during falling sea level. Reaching islands such as Flores demands that H. erectus must have devised means of crossing wide stretches of open sea. Fission-track dating of zircons gives ages for bone beds with tools that range from 840 to 700 thousand years (O’Sullivan, P.B. et al. 2001. Archaeological implications of the geology and chronology of the Soa basin, Flores, Indonesia. Geology, v. 29, p. 607-609). Erects crossed at least two major seaways to reach Flores, predating the first seafaring modern humans, who reached Australia around 40 to 60 thousand years ago by an enormous span of time.

Unwholesome fare

Since Raymond Dart’s notoriously bloodcurdling views on the dietary habits of early hominids and “the mark of Cain” appeared in his 1950s essay “The Predatory Transition from Ape to Human”, palaeoanthropology has sometimes tried to brush under the carpet evidence for cannibalism among our ancestors. Considering the many funerary traditions practised today, some of which involve dismemberment and defleshing of corpses, it is easy to pass off cut-marks on fossil bones as indicating last rites. However, when evidence of cooking turns up (and the Anasazi people of 12^th century Colorado left plenty of evidence for that, including making anthropic soup), the common notice in pub restaurants, “Children served”, takes on grim undertones.

Tim White, co-director of the Laboratory for Human Evolutionary Studies at the University of California (Berkeley) is a palaeoanthropologist who commands attention. It was he who discovered evidence for Anasazi cuisine, and has subsequently maintained an interest in assessing evidence for cannibalism. It does go back a long way, to evidence for the first European’s (H. antecessor) gustatory relish of their fellows at the 800 ka site of Gran Dolina in northern Spain, and similar signs in Neanderthal sites spaced by hundreds of generations. The questions of, “How often?”, and, “Under what circumstances?”, are difficult to answer. However, it was a part of the cultures unearthed by excavation.

Source: White, T. 2001. Once were cannibals. Scientific American, August 2001, p. 48-55.

Radar analysis of Turkish earthquake

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The destructive Kocaeli earthquake (magnitude 7.4) of August 17 1999 involved horizontal slip of up to 5 metres. Although it is possible to measure strains precisely using GPS arrays, many stations are needed to fully grasp strain patterns. Interferometric processing of before and after radar data (InSAR) presents an opportunity to examine seismic strains over very large areas. Displacements associated with the Kocaeli earthquake on the North Anatolian Fault, recorded by InSAR, extended for up to 60 km either side of the fault (Mayer, L. and Lu, Z. 2001. Elastic rebound following the Kocaeli earthquake, Turkey, recorded using synthetic aperture radar interferometry. Geology, v. 29, p. 495-498).

The fault runs parallel to the look direction of SAR beams from the ERS-2 satellite in its ascending orbits. This fortuitous geometry charted relative motions in a horizontal sense on either flank of the major strike-slip fault system, with a precision of about 3 cm. Interesting in its own right, the recorded strain helps understand how and where the elastic strain energy released by earthquakes was stored. The key to energy storage is the rebound pattern associated with strain release during earthquakes, to which the InSAR results are an approximation. This pattern depends theoretically on the displacement along the fault itself, the shear modulus of the rock involved and the depth to which faulting extends. In the case of Kocaeli, faulting penetrated to between 6 and 15 km below the surface. Because elastic strain builds up around active faults, it may be possible to use InSAR monitoring as a means of predicting the risk of future failures on dangerous faults, like the North Anatolian Fault. Earthquake records show that successive failure migrates westwards along the Fault, getting ever closer to Istanbul.

Methane as the early “greenhouse” gas

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Various indicators, such as the presence of detrital uranium oxide and iron sulphide grains in sediments older than about 2.3 Ga and the appearance of terrestrial sediments stained red by the presence of ferric (Fe-3) oxides thereafter, have long been used to suggest that atmospheric oxygen was a mere trace before that time. Generation of oxygen through photosynthesis by simple organisms, principally blue-green bacteria, could have led to an oxygenated atmosphere when their productivity exceeded the tendency for oxygen to be consumed by reaction with reducing agents, such as abundant ferrous (Fe-2) iron in sea water, and by burial of carbon-rich dead organic matter. That method is a central plank in the Gaia hypothesis. However, geochemical considerations suggest another scenario for oxygenation (Catling, D.C., Zahnla, K.J. and McKay, C.P. 2001. Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth. Science, v. 293, p, 839-843). Unless carbon burial exceeded the rate at which reductants supplied to the outer Earth (including exposure of buried carbonaceous sediments) by geological processes consumed oxygen, the atmosphere would remain low in oxygen.

Lacking in oxygen, the early atmosphere would have been able to support build-up of methane from biogenic processes – today methane is soon oxidized to carbon dioxide and water. Carbon isotope evidence suggests that early life was dominated by methanogens, and such organisms alive today are genetically very primitive. Consequently, methane is a good candidate for keeping average surface temperature above the freezing point of water at a time when the Sun’s output of energy was considerably lower than it is now. All hydrogen-bearing compounds become dissociated high in the atmosphere, to release hydrogen atoms, and they readily escape the Earth’s gravitational pull. Fortunately, this does not happen now because the only significant H-compound, water, cannot rise above the tropopause. The decline in temperature upwards acts as a cold trap for water. Were this boundary not in place, and it is largely due to the presence of ozone in the stratosphere which absorbs radiation to give higher-level warming, Earth would long ago have lost most of its water, as did Mars and Venus. In the early atmosphere, methane would not have been “cold trapped”, and nor is it today. So, during that period, hydrogen would steadily have leaked from the Earth.

The chemical outcome of such a simple process would have been a steady decline in the reducing capacity of the Earth as a whole, for hydrogen is a powerful reductant. Because most of our planet’s hydrogen was locked in water from the time of its accretion, its escape must have resulted in a net gain of oxygen somewhere in the Earth system. Increased methane productivity by methanogen bacteria during the Archaean and early Proterozoic would have enhanced this tendency for the whole Earth to become more oxidizing. Catling et al. argue that the continental crust became more oxidized, so that any gases released from it by metamorphism would become less reducing. That would have reduced the tendency for immediate consumption of oxygen produced by photosynthetic organisms, culminating in its eventual ability to exist in the atmosphere in balance with biological processes at around 2.3 Ga.

Zircons’ window on the Hadean

The oldest tangible rocks that are not completely changed by deep-crustal metamorphism are those of Isua in West Greenland. Interleaved with gneisses that originated probably from calc-alkaline intrusions are rocks formed at the Earth’s surface around 3.8 Ga ago. The general scene represented by this Akilia Association is in many respects familiar – the operation of plate tectonics, rapid generation of what was to become continental crust, abundant evidence for the action of liquid water and even the isotopic traces of living organisms. That 750 Ma after the Earth’s accretion the last two were present is no surprise. The oddity is that, despite decades of effort, there is still no sign of continents older than 4 Ga. That crustal rocks which had undergone considerable evolution from their mantle source did exist in the missing half-billion years emerged from the discovery of detrital zircons as old as 4.4 Ga in much younger Australian sedimentary rocks. Some of the rare, tiny grains show isotopic evidence that the magmas in which they formed had contact with liquid water at the surface.

As well as containing sufficient uranium to allow the dating of single grains by the U-Pb method, zircons also contain hafnium, which is chemically very similar to zirconium. Measurable quantities of ¹⁷⁶Hf add to common ¹⁷⁷Hf by the decay of ¹⁷⁶Lu, giving a potential dating technique. However, zircon contains only minute traces of lutetium, so that its ¹⁷⁶Hf/¹⁷⁷Hf ratio remains that of the ultimate source of its host rock. Relative to hafnium, lutetium is more likely to remain in the residue left by partial melting of the mantle, or so theory suggests (geochemists can only deduce this from various lines of indirect evidence). Consequently, mantle that has sourced continental crust builds up ¹⁷⁶Hf from the time such crust formed., whereas continental crust has significantly lower levels. Studying hafnium isotopes in very old zircons is therefore a means of seeking periods when significant amounts of continental crust separated from the mantle. Because such tiny amounts of the radiogenic hafnium are involved, an accurate decay constant for ¹⁷⁶Lu is vital (Scherer, E., Münker, C. and Mezger, K. 2001. Calibration of the lutetium-hafnium clock. Science, v. 293, p. 683-687). Zircons from the oldest rocks in Greenland, Canada, Australia and South Africa fall into two, complementary groups; those with slight enrichment in ¹⁷⁶Hf and those with slight depletion. Simple geochemical theory seems to indicate that indeed magmas similar to those that contributed to formation of the bulk of continental crust did form as early as 4.4 Ga ago. However, zircons with younger Archaean ages show little sign of deviant hafnium, which suggests that a large proportion of the mantle was not involved in early sial formation. Hadean continental material no doubt formed, but not much. That is no surprise, for involvement of surface-derived water in mantle melting above zones where earlier lithosphere returns to the mantle, whatever their form, seems inevitable in a planet noted for its high water content. That is the basic “recipe” for the formation of silica-rich magmas.

Two things stem from this work: the probable futility of seeking Hadean continents; the unlikelihood that the chemical heterogeneity of the mantle stemmed from Hadean continet formation on a massive scale.

See also: Kramers, J. 2001. The smile of the Cheshire Cat. Science, v. 293, p. 619-620

Yet more complexity

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The view that all manner of processes connected with climate – volumes of land ice, ocean temperature and flow, aspects of atmospheric composition and its motion, and the expansion and contraction of biological communities – are locked into the cycles of changing solar input steadily evolves into something less mechanical, as new data flows in. The first serious doubts about Milutin Milankovic’s theory of astronomical forcing of the world’s climate, since oxygen isotope fluctuations in sea-floor sediments began to reveal the periodicities predicted by him, stemmed from a very different kind of deposit. Devil’s Hole near Las Vegas, a fissure being slowly filled by calcite flow stone that precipitates from groundwater, presented a detailed record of oxygen-isotope variations over the last 600 thousand years. Though showing the same patterns as ocean cores available at the time, Devil’s Hole revealed changes in continental climate that differed from those in land-ice volume by thousands of years. Ice-core time series of Antarctic air temperatures also show that warming began up to 9000 years before the last four terminations of glaciation. As more proxies for climate are devised, the more complex global climate shifts appear to have been.

The latest measure stems from fluctuations in the structure of compounds produced by marine algae as a result of shifts in sea-surface temperature. Applied to sea-floor sediments deposited off California, an area influenced today by the southward, cold California Current, they reveal regional warming of the sea that began 10 to 15 thousand years earlier than the last five deglaciations of the northern hemisphere (Herbert, T.D. and 8 others 2001. Collapse of the California Current during glacial maxima linked to climate change on land. Science, v. 293, p. 71-76). In cores south of the modern cold current, no such large discrepancies emerged. In fact they accompanied the maximum extents of land ice. It seems that, like the Gulf Stream, the California Current is prone to shutting down, but as a result of changed Pacific wind patterns in response to the North American ice sheets rather than to thermohaline deep circulation. Here is an explanation for the vexing record from Devil’s Hole – regional climate shifts that do not “knock” Milankovic.

There is no doubt that changes in ice volume on the northern continents are the main characteristic of environmental change going back more than 2 Ma. However, the mechanistic view that lots of ice means a cold, dry world and a great deal less points to warmth and more moist conditions is dead in the water as a useful paradigm. Yet all models of climate are little more than Heath Robinson tangles of such reductionism, despite claims for their increasing incorporation of ideas that stem from measured realities. As always, the devil lies in the detail, and Herbert et al.’s paper also shows from pollen records in the marine cores that dense warm-climate forests cloaked the Pacific seaboard during the last 5 glacial maxima. For a vast area of western North America to be warm while ice sheets elsewhere were at their maximum should be a warning of unpredictable future climate shifts.

Growing concern about unpredictable and contrary change was amply expressed by a meeting of 1800 climate specialists in Amsterdam in early July. They endorsed the distinct possibility of sudden shifts in regional climates that may stem from increased global warming, such as return of vegetation to the Sahara, aridity in the Amazon basin, and Europe’s plunging into a frigid climate as the Gulf Stream slows because of reduced thermohaline circulation (Pearce, F. 2001. Violent future. New Scientist, 13 July 2001, p. 4-5).

Magnetic stratigraphy works in the Devonian

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Using alternations of magnetic field intensity, and the patterns that they show over time, has been a standard method in stratigraphy for times back to about 200 Ma ago. There is no sea floor older than that, and although reversals are known widely from earlier times, there is no continuity that allows its use. Moreover, reversals are too widely spaced in time to allow for more than calibrating stratigraphic sequences. Much finer stratigraphic resolution comes from direct and rapid measurement of the intensity of magnetization that can be induced in sediments from their content of various magnetic minerals. The Ocean Drilling Programme and studies of loess sections in China have long established such magnetic susceptibility logging as a correlative tool. Empirically, it works, and the loess studies suggested that variations relate to changes in global climate. Its usefulness in marine sediments is now seen to relate to the production of massive amounts of very fine-grained magnetic minerals in tropical soil formation during warm-humid episodes. Being so fine, the particles reach the most distant ocean basins after soil erosion. Susceptibility seems to vary with global changes in the amount of continental erosion.

Detailed correlation between widely separated marine stratigraphic sequences of all ages is notoriously difficult. Consequently, rapid methods based on magnetic susceptibility, which can produce near-continuous logs, have useful potential. A team of Us, Spanish and Moroccan geologists has demonstrated its use in definitive correlation between Lower Devonian rocks found in Spain, Morocco and Bolivia (Ellwood, B.B. et al. 2001. Global correlation using magnestic susceptibility data from Lower Devonian rocks. Geology, v. 29, p. 583-586).

Climate and heavy breathing

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The kingdom of the eukaryotes rests on a very simple environmental economy. Plants are producers of carbohydrate through photosynthesis, thereby generating excess oxygen from the photo- and molecular chemistry involved. Animal consumers use up oxygen in their metabolism and return carbon dioxide, the ultimate source of carbohydrate, to the air. A simple view is that animals contribute to global warming, whereas plants help cool the world. Perhaps because of that “common sense” view, most environmental scientists take a very different line, linking it with volcanic exhalation of CO₂, “capture of carbon through rock weathering and the burial of dead organic matter in the global carbon cycle. Greg Retallack of the University of Oregon is about to publish a reappraisal of the animal versus plant part of the C-cycle (in press, Journal of Geology) that is based on observed imbalances between the two opposed kinds of respiration. Specialists in the C-cycle hold that there is a an overall balance, taking all components into account, whose inevitable result is the build up of oxygen in the atmosphere of an inhabited world. Yet oxygen is extremely reactive and should quickly combine in mineral oxides and hydroxides – after all, the iron in an untended car reverts to its oxide ore in the space of a few decades at most.

Partly following James Lovelock’s Gaia hypothesis, Retallack focuses on the major fluctuations in atmospheric chemistry evidenced in the geochemical record, the most immediate being the see-saw fluctuation of modern levels of CO₂ in the atmosphere – a 2% annual variation controlled by the waxing and waning of vegetation in the northern hemisphere (where plant cover is greatest) according to season. One of the largest shifts in atmospheric CO₂ concentration followed the evolution of land plants from about 450 Ma ago. To thrive, they had to develop hard cellular material (lignin) that formed stems and trunks, which animals of the Palaeozoic were unable to oxidise efficiently. Both living biomass and burial of undigested lignin drew down CO₂ and boosted oxygen levels. Animal evolution eventually exploited this “free lunch” through the humble termite and reptilian and then mammalian megafauns. Retallack believes that heavy breathing that resulted from lignin digestion reversed the declining CO₂ trend for the 200 Ma following the Carboniferous to Permian glacial epoch in Gondwana. Though displaying some ups and downs, the Mesozoic saw a “greenhouse” world. Removal of the mighty and extremely abundant herbivorous dinosaurs by the K-T mass extinction provided and opportunity for plant diversification. Many Mesozoic plants evolved armour against browsing dinosaurs, exemplified by the surviving Andean “monkey puzzle” tree Araucaria. Their demise removed the need, and the plant Kingdom’s evolutionary response was the appearance of grasses. Reatallack points out that grass itself is not as good as lignin-rich plants in holding CO₂, but grasslands encourage the development of thick carbon-rich soils that hold more than the soils of the forest floor. It is this development that Retallack believes lay at the base of the decline in average global temperature through the Cainozoic, to culminate in the present Ice Age. Unsurprisingly, proponents of the complexity and diversity of the C-cycle, particularly in the oceans, are disinclined to have truck with the hypothesis.

Source: Pearce, F. The Kingdoms of Gaia. New Scientist, 16 June 2001, p. 30-33.

Carbonates and biofilms

Above the low level that is essential for their role in molecular “information” transfer, calcium ions pose a fatal threat to cell processes. That is simply because excess calcium combines with carbonate ions to form minute calcium carbonate crystals within the cell when the solubility product of calcite is exceeded. The solubility product is the concentration of calcium ions multiplied by that of carbonate ions, so that increase in one or the other can lead to supersaturation of calcium carbonate and imminent precipitation. Because CO₂ is an essential need for photosynthesis and a product of animal metabolism, this risk is always present. In the most common photosynthesising bacteria, the cyanobacteria that have been around for at least 3.6 billion years, the drawing in of CO₂ in the form of carbonate (CO₃^2-) or bicarbonate (HCO₃^–) ions in water can result in supersaturation immediately around the cell. When it occurs, the “blue-green” bacterial biofilms induce precipitation of calcium carbonate. That is why such micro-organisms can act as reef builders, as they did to great effect during the early Precambrian (stromatolites), and also from Cambrian to Cretaceous times.

Calcite mineralization by biofilms is, however, a complicated process. It is connected with highly reactive substances that cyanobacteria exude outside their cell walls. Depending on their degree of ordering and the supply of calcium ions, these substances control the manner in which calcium carbonate precipitates. The detailed biochemistry and the form of calcite biofilms obtained by study of modern cyanobacteria in different watery environments has allowed Gernot Arp and co-workers at the University of Göttingen to evaluate varying calcium and CO₂ concentrations in ocean water since 540 Ma, and suggest differences in Precambrian oceans (Arp, G. et al. 2001. Photosynthesis-induced biofilm calcification and calcium concentrations in Phanerozoic oceans. Science, v. 292, p. 1701-1704).

Their studies suggest that up to the Cretaceous, the Phanerozoic oceans must have had higher calcium contents than they do today. Microbial reefs formed in that period preserve details of the “blue-green’s” cell structure, suggesting that calcite was nucleated directly by the extracellular substances. Vast burial of the calcite shells of planktonic metazoan organisms to form the Chalk deposits of Cretaceous age reduced very high levels to give the calcium-depleted oceans that prevailed during the Cainozoic. Microbial carbonates of these younger ages show no structure. The stromatolites that are so characteristic of Precambrian limestones are stuctureless too, although they show evidence of progressive build-up from myriads of thin layers. Irrespective of the Precambrian oceans’ calcium content, this lack of structure can be explained by more dissolved CO₂ that resulted from its higher concentration in the atmosphere. About 700-750 Ma ago, stromatolites that contain calcified cyanobacterial cells appear, and that may signify the massive drawdown of CO₂ from the atmosphere that is implicated in creating icehouse conditions on a global scale during the late Proterozoic Aeon.

Universal access to peer reviewed articles?

Published on June 1, 2001 by zooks777Leave a comment

Scientists without access to libraries that subscribe to scientific journals, or whose institutions are poorly funded, are cut off from the mainstream of research developments. That is, unless they request offprints of papers from authors. The growth of electronic versions of journals and increasing access to the Web, even in poor countries (Eritrea recently went “on-line”) seemed to promise wider availability of primary sources of research information. That is an illusion. Unless you are a subscriber to paper journals (for instance Nature and Science subscribers automatically get free access to on-line versions) or are registered with a library that has subscribed to all electronic journals made available by a publishing house, such as Elsevier’s Science Direct, then downloading more than an abstract is on a pay-per-view basis. (Note: even the Web of Science, that hosts the Science Citation Index database, requires a paid-for user id and password). Being a university academic in a rich country, I have the luxury of free access to many electronic resources through the Open University Library’s subscriptions, and the same goes for any of our students. However, the economic facts of academic life occasionally rear up. A reference to an interesting paper in the Journal of Human Evolution came to my attention. Using my id and password for the publisher’s web site, I was able to locate the entry for the paper. However, we do not subscribe to that journal, and to download an Adobe Acrobat PDF file would have cost me about £35, charged to my credit card. Instead I requested an offprint from the authors, and am still waiting for its arrival after 2 months.

Authors provide papers free of charge to publishers of journals, referees review submissions without payment, and many editors compile issues for little if any return, other than satisfaction and kudos. Publishers of journals make enormous profits, and increase subscriptions at rates far above that of inflation (one veterinary science journal increased in price by 7 time between 1991 and this year). The average total income received by publishers of the roughly 20 000 scientific journals for each one of the 2 million papers published each year is around US$2 000 – the trade has a US$4 billion annual income. In the Earth sciences annual subscriptions are beyond the budgets of most 3^rd World institutions (6 issues of Elsevier’s Journal of African Earth Sciences cost £1003), apart from a few (the University of Chicago’s Journal of Geology costs £79 per annum).

The Public Library of Science – http://www.publiclibraryofscience.org -is campaigning for a way out of the increasing cost for freedom of access to scientific information. One simple and foolproof strategy is for authors to “self-archive” their preprints and manuscripts of published papers in their institutions’ “e-print” archive in such a way that they can then all be harvested into a global virtual archive, its full contents freely searchable and accessible online by anyone. Stevan Harnad of the University of Southampton is one of the driving forces for the self-archiving initiative, and provides full details of the possibilities at http://www.cogsci.soton.ac.uk/~harnad/Tp/nature4.htm

See also: Harnad, S. 2001. First Person: In the name of freedom. New Scientist, 26 May 2001, p. 53.

Dinosaur update

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BBC-2’s Live from Dinosaur Island (4-16 June 2001) brought palaeontology into Britain’s living rooms. Centred on a frantically excavated series of Jurassic sites on the Isle of Wight, and fronted by the irrepressible Bill “Birdman” Oddie and genuinely excited (and sometime irascible) professional palaeontologists, the series used the now familiar approach of Channel 4’s Time Team, with the added frisson of being unedited and live. The BBC was in debt to its viewers after the truly dreadful, if visually astonishing, Walking with Dinosaurs, and has repaid them handsomely by showing the bone-people working in their natural habitat. It should help repopularize geology after a century of our being the brightly coloured anoraks seen dimly in the drizzle.

Dinosaurs are perhaps the main link between the popular imagination and the Earth’s past. However, leaving them at the level of awesome animals that a comet strike snuffed out 65 Ma ago may enthuse, but does not really educate. Live from Dinosaur Island began to break the T rex – My Little Pony connection, by also showing how we can recreate the environments that long-dead creatures inhabited, and how they changed. Climate and life (above), hints that dinosaur breath may even have affected climate during the Mesozoic.

Barely a month passes without dinosaur news. The latest concerns the rediscovery of the Egyptian site, from which Ernst Stromer von Reichenbach gathered a rich collection of animal fossils between 1911 and 1936. Stromer’s collection, housed in the Bayerische Staatssammlung museum in Munich, was destroyed by wartime bombing. Because Stromer left no clues regarding the precise location of his site, except that it was near the Baharyia Oasis in the Western Desert, it seemed unlikely ever to be found again. A team from the University of Pennsylvania, let by Josh Smith, more or less tripped over the site by luck, when combing the area for coastal Upper Cretaceous sedimentary outcrops, after Smith’s inspiration by Stromer’s monographs (Smith, J.B. and 7 others 2001. A giant sauropod from an Upper Cretaceous mangrove deposit in Egypt. Science, v. 292, p. 1704-1706). The highlight of their excavations is Paralititan stromeri, a sauropod reckoned to be the second most massive animal that lived, after South America’s Argentinosaurus. The tidal sediments also yielded a diversity of lesser animals that matches and will certainly transcend Stromer’s destroyed collection.

See also: Stokstad, E. 2001. New dig at old trove yields giant sauropod. Science, v. 292, p. 1623-1624.

Doubts cast on the increase in diversity with time

The late John Sepkoski of the University of Harvard painstakingly spent 20 years trawling the palaeontological literature to build an archive of the duration of every marine fossil known. Others did similar work for terrestrial fossils, but Sepkoski’s database stands out, head and shoulders, for its comprehensiveness. It is largely from his work that the record of extinction events took on semi-quantitative form. Plotted against Phanerozoic time, his counts of genera also seem to show patterns that chart the fluctuations of biodiversity; rapid rise from the Cambrian Explosion to plateau in the mid-Palaeozoic, a decline in the late Palaeozoic and early Mesozoic, and then a post-Jurassic explosion in diversity. Much speculation has hung on Sepkoski’s empirical data, such as the influence of “modern” evolutionary designs on the number of ecological niches that life can exploit.

Enormously important as Sepkoski’s work was, inevitably it rested on the selective nature of fossil collecting, itself partly determined by the variable quality and quantity of preservation, but also by the limited numbers of active palaeontologists, the manner in which they worked and their selection of sites. There are gross biases in fossil collections, but how can archivists possibly allow for their influence? Without a superhuman effort to re-collect more intensively, to plunder every conceivable stratum wherever it crops out and perhaps standardise what is meant by a genus, the only available means is through statistics. Palaeontologists at the universities of California (Santa Barbara) and Harvard, led by John Alroy and Charles Marshall respectively, are compiling information along more comprehensive lines than did Sepkoski, including the dimension of geographic occurrence as well as duration, in the Palaeobiology Database. Their first attempts to allow statistically for the welter of biases, published in the 25 May 2001 issue of Proceedings of the National Academy of Sciences, all point in the same direction. The Cretaceous to Tertiary genera show patterns of change that are little different those for the Silurian to Carboniferous, compared with Sepkoski’s suggestion of explosive diversification in the first and a plateau in the second. The main problem remains; vast as they are, fossil collections are not truly representative of life in the past.

Source: Kerr, R.A. 2001. Putting limits on the diversity of life. Science, v. 292, p. 1481.

Earth System Processes conference

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Geoscientists from all over the world attended and spoke at this seminal meeting. Its theme was moving from isolated studies to those linking contributions from many branches of science, thereby attempting to match the complex web of interactions on every scale and at every pace that is the essence of the way the world works and has evolved. Topics covered a huge range, from the “Snowball Earth” hypothesis to issues linking life and inorganic processes, deep mantle processes to sea-floor hydrothermal systems, molecular palaeobiology to the fossil record. Indeed, the scope is too broad for me adequately to summarise here. While it remains active, readers can read all the abstracts of papers presented at: http://www.geosociety.org/meetings/edinburgh/prog.htm

Late Pleistocene mass extinction

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The recent fossil records of the Americas and Oceania are littered with species that became extinct in the last 100 000 years. The majority of them are large animals whose body weights were greater than 45 kg – part of the megafauna. While controversy rages about the date of entry of the first humans into both vast regions, for a long time archaeologists have suspected that the appearance of sophisticated hunters was somehow connected with the rapid decline in what would have been prey species. One theory is that having never encountered weapon-bearing bipeds, large mammals were “naïve” and thus easily slaughtered. Most visitors to the Americas and Australia soon notice how unafraid many animals are of humans, compared with their behaviour in Europe and Africa. The suddenness of the selective extinctions (around 15 to 11 000 and 47 000 years ago in North America and Australia respectively) is astonishing, if the cause was small bands of hunters, and other workers have suggested that human entry brought diseases that wiped out species susceptible to them, but with no immunity. The third main theory is that a sudden shift in climate wrought havoc among large herbivores and predators, by producing a change in vegetation. The last is difficult to support for the Americas as the extinctions were in a period of increasing warmth and humidity following the termination of the last glacial period. As always, new information from research directed at the problem has narrowed the choices, but revealed complexities.

Modelling the influence of changing predation on prey stems from the mathematical simplification of reality by Lotka and Volterra, in which “boom and bust” events pop out of the simulations. John Alroy of the University of California applied an advanced version of the basic model to the likely effects of advanced hunters appearing suddenly in North America (Alroy, J. 2001. A multispecies overkill simulation of the end-Pleistocene megafaunal mass extinction. Science, v. 292, p. 1893-1896). His model assumes slow human population growth, random hunting and the least possible effort – a conservative approach. The results closely parallel the record, if human population expanded from 100 first entrants about 14 000 years ago to almost 1 million 750 years later, and suggest that a steady state population of around half that co-existed with the surviving fauna until the appearance of Europeans and their culture. It is an entirely mechanistic model, but mimics what happened without recourse to any other influence, such as climate change. So far, no human site in the Americas has been convincingly dated before 14 000 years ago.

Dating is even more of a problem in Australia, particularly for human arrival. The earliest dated fossil is 60 000 years old (see Out of Africa hypothesis confounded? EP Feb 2001), but claims have been made for artefacts at least twice that age. Alroy’s model applied to Australia would demand extinction (24 out of 25 genera Pleistocene megafaunal species) shortly after earliest arrival. A large team of Australian scientists (Roberts, R.G. and 10 others 2001. New ages for the last Australian megafauna: continent-wide extinction about 46 000 years ago. Science, v. 292, p. 1888-1892) have systematically dated the age of burial of extinct faunas at 27 sites in coastal areas and the more humid SE of the continent (none from the vast, arid “red centre”), and one in Papua New Guinea. The most likely interval for the extinctions, between 39 800 and 51 200 years ago, bears no relation to extreme aridity during the last glacial maximum, so the data weigh against that climatic cause. However, the last 100 000 years have seen lesser, but still extreme shifts in climate, so climate change cannot be ruled out. Though the authors also do not rule out humans eating their way through Australias bizarre megafauna, the lag between evidence for first entry and the extinction seems far longer than that in the Americas. Closer inspection of their data, however, does show precise ²³⁰Th/²³⁴U ages (+ 600 to 2 200) from 33 600 to 60 000 from 3 sites, and less well-constrained luminescence ages (+ 200-21 000) from 16 000 to 171 000 years from all the sites. Applying simple statistics to samples from such a wide spread of localities does not seem justified to me – normal practice is for ages at individual sites to be accepted as dates within the errors of the method used. Australia’s megafaunal extinction seems to have been protracted. Using fuzzier dating of the extinction, earlier workers correlated it with evidence for an increase in bush fires marked by ash in offshore sediments. Much of Australia’s flora is fire resistant, and the seeds of some species require light burning before they will germinate. The most popular theory for the extinctions there is through deliberate fire setting by hunters – a culturally induced decline unique to Australia’s peculiar climate and terrain.

See also: Dayton, L. 2001. Mass extinction pinned on Ice Age hunters. Science, v. 292, p. 1819.

Where do subducted slabs go?

Published on June 1, 2001 by zooks777Leave a comment

Geophysicists and geochemists are generally opposed on what happens to subducted lithosphere. Seismic tomography of the deep mantle shows convincing evidence for slab-like cold bodies down to the core-mantle boundary, yet differences in trace-element and isotopic signatures of volcanic rocks formed at ridges from shallow mantle and ocean islands that relate to deep plumes persuades geochemists that restriction of convection within the upper mantle, at the 660 km deep discontinuity, best explains the differences. There are other models that might account for geochemical differences, such as heterogeneities throughout a poorly stirred mantle or because material in slabs subducted to the bottom of the mantle rarely rises again, but displaces more pristine materials upwards.

The more earthquakes that seismographs detect and locate, the better geophysicists are able to map in 3-D the zones on which they take place. One destructive margin long known to have aberrant seismicity is the northern part of the Tonga system in the Pacific Ocean. This is where the fastest subduction anywhere consumes lithosphere that has little time to warm up while it descends – surely a site for slabs to fall steeply into the deep mantle. Much of the Tonga system shows the expected zone of steeply plunging Earthquakes, yet west and north-west of Fiji there are earthquakes that do not fit the regional pattern. They are far too shallow to result from motion on the main subduction zone. By detailed analysis of seismic data Wang-Ping Chen and Michael Brudzinski have revealed a strong possibility that a piece of old subducted slab has slid to the 660 km discontinuity since it parted company with the now rapid and steep motion at the Tonga trench (Chen, W-P. and Brudzinski, M.R. 2001. Evidence for a large-scale remnant of subducted lithosphere beneath Fiji. Science, v. 292, p. 2475-2478). If such behaviour turns out to be more widespread, large volumes of old lithosphere may indeed sit at the discontinuity, satisfying many geochemists as a means to maintain very old differences in composition of the mantle. The problem is, increasingly good resolution in seismic tomography has so far failed to detect the tell-tale high seismic velocity signature of such cold slabs. Chen and Brudzinski suggest that they may be “invisible” to this method, because of their mineralogy – perhaps the crustal lithosphere has not equilibrated to eclogitic materials, or is given neutral buoyancy by being heavily hydrated.

Between a rock and a hard place

Plate theory stems from the notion that the lithosphere is overwhelmingly rigid and deforms only at the boundaries between plates, particularly at destructive margins. The Earth’s seismicity is overwhelmed by earthquakes at discrete boundaries, and the mapping of seismic events along narrow lines by the world-wide network of seismographs (set up as a means of pinpointing nuclear weapon tests) formed on of the main planks in developing the theory of plate tectonics. The plate whose evolution drove India into Asia bucks this definition. It has long been known to host seismicity well inside its boundaries. Oceanographic work has slowly built up a means of relating Indian Ocean seismicity to plate structure, whereas analysis of earthquake first motions from seismographs reveals that the deformation differs between various block of the ocean floor. The plate suffers folding and thrusting, and transcurrent motions along ancient transform faults, such as the Ninety East Ridge. The most likely explanation for the Indian plate’s aberrance is that sea-floor spreading from the ridge separating the Indian Plate from that carrying Antarctica can no longer be accommodated by subduction of the subcontinent beneath Asia, whereas it can be taken up by subduction beneath the Java-Sumatra island arc. The Central Indian basin is being compressed, and must deform in some way, perhaps eventually to become a new subduction zone.

Source: Deplus, C. 2001. Indian Ocean actively deforms. Science, v. 292, p. 1850-1851.

Life on Earth even luckier than we thought?

Published on May 1, 2001 by zooks777Leave a comment

Continually improving resolution of telescopes is now beginning to reveal signs of planetary systems around other stars. Because their gravitational effects on stellar motion are detectable, the 60 or so known planets in distant stellar systems are all gas-giants, similar to but bigger than Jupiter. Surprisingly, calculations show that such massive planets are in very different orbits than those in the Solar System. Their orbits are highly eccentric, and bring them remarkably close to the star, unlike the almost circular orbits in the Solar System. Yet, if they are mainly gaseous, they must have formed far from the warming influence of their companion star, as did Jupiter, Saturn, Uranus and Neptune. Somehow, they have been gravitationally perturbed over the billions of years of evolution of the stellar systems.

How, then, did such bodies move inwards? One possibility is that they exchanged angular momentum with smaller, rocky planets, forcing both into eccentricity. For the smaller bodies the effect would be more dramatic, potentially either flinging them into interstellar space or into collision with their star. Spanish and Swiss astronomers using spectroscopes at an observatory on the Canary Islands have discovered a large lithium anomaly in the spectrum of one star with such an aberrant gas giant (Israelian, G. et al. 2001. Evidence for planet engulfment by the star HD82943. Nature, v. 411, p. 163-166). Because the anomaly is accompanied by greater than usual abundances of many elements heavier than helium, and because lithium is quickly consumed as stars “ignite”, Israelian and colleagues conclude that the star has engulfed an Earth-like planet.

If such processes are common, and theory suggests that it may be, our Solar System could be one of very few in which potentially life-building and sustaining planets had sufficient time to develop a biosphere. It seems that the more small planets there are between a star and an outer gas-giant, the more likely it is for such perturbations to take place. The Solar System has only four, and calculations using Jupiter’s mass and orbit point to a minute tendency for such eccentricities to evolve. Looking on the bright side, at least for those committed to a view of life pervading the cosmos, current observational resolution is only able to detect giant planets in wildly eccentric orbits. Many planetary could be more stable.

Se also: Samuel, E. 2001. Banished forever. New Scientist, 12 May 2001, p. 15.

Late-Palaeocene red tides?

About 55 Ma ago, in the late-Palaeocene, the carbon-isotope record shows a sudden drop in ¹³C, signifying a sudden release of methane from ocean-floor gas hydrates or clathrates. That period also reveals evidence of s brief global warming, against the general trend of cooling through the Tertiary. Since the discovery of this massive discharge of the “clathrate gun”, palaeontologists have looked for ecological effects in sea-floor sediments. For them to be significant, it is important that climate-related ecological effects occurred at the same time in widely separated parts of the globe.

Geologists from the Netherlands, Denmark, New Zealand, Austria and Sweden have examined the microfossil record from two late-Palaeocene sequences in Austria and New Zealand, and show such synchronicity (Crouch, E.M. et al. 2001. Global dinoflagellate event associated with the late Palaeocene thermal maximum. Geology, v. 29, p. 315-318). Exactly at the time of the d¹³C dip in both sections, the abundance of cysts of single-celled phytoplankton known as dinoflagellates rose dramatically, only falling when carbon isotopes recovered to usual levels. The authors link this to exceptionally high surface-water temperature and photosynthetic productivity. Over the same period, the fossil record shows a mass extinction of benthonic organisms, and noticeable turnover and diversification of plankton and mammals, though not as dramatic as other biological events.

Today, dinoflagellates explode in numbers, along with other phytoplankton, under similar conditions and when nutrients increase in surface waters, to create phenomena known as “red tides”. Because some species of dinoflagellates produce potent neurotoxins, “red tides” often result in massive death of marine animal life. The effects linger as such toxins build up in the cells of animals, such as bivalves, which survive the bloom. The air above such blooms is filled with stinging, choking aerosols, not far different from nerve gas. Rotting of dead organisms causes oxygen levels in local seawater to drop, further adding to the death tool at deeper levels. Red tides that result from human input of nutrients in sheltered embayments often sterilize them for long periods.

Although it is impossible to tell if such neurotoxins built up during the late-Palaeocene thermal maximum, that is not an impossibility. Such biological “warfare” (no-one knows why some dinoflagellates produce the toxins) might explain the biological crisis that accompanied methane release.

A broader view of the Permian-Triassic mass extinction

That the Palaeozoic Era ended in the greatest mass extinction is well know, although why it happened is still a topic of fierce debate. Part of the problem is that its effects on land and in the oceans emerge from studies of widely separated P-Tr sections, and many of these are extremely thin. Such condensed sequences are notoriously difficult to resolve in terms of relative and absolute timing, as well as to correlate from place to place.

As with much else, Greenland promises to throw light on the end-Palaeozoic events, thanks to a 700 metre sequence of siliciclastic sediments in East Greenland that spans the Permian-Triassic boundary without a break. Its most exciting feature is the way in which marine and non-marine sediments interleave with one another. Geologists from the USA, the Netherlands, Australia and Britain have pieced together the evidence of biological change from a small part of this little described occurrence (Twitchett, R.J. et al. 2001. Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis. Geology, v. 29, p. 351-354).

In marine sediments, the Permian biota collapse, together with evidence for disturbance of the sediment structure by burrowing , in a mere 50 cm of the almost 40 metre sequence that the authors analysed. Over the same interval, pollens of Permian land plants also fall dramatically, but all the pollen types linger through the overlying 15 metres. Only at a level 25 metres above the biotic collapse do fully Triassic faunas and floras appear. From estimates of the rate of sedimentation the marine and terrestrial collapse appears to have taken between 10 and 30 ka. Oddly, the now well-known fall in ¹³C does not coincide with that in the biota. The authors visualize two possibilites: that it resulted from the collapse itself, or reflects an external factor that played little or no role. One interesting scenario that they suggest is that it may indicate a major release of methane by breakdown of gas hydrates (a now increasingly popular mechanism!).

Conferring strength to cratons

Published on May 1, 2001 by zooks777Leave a comment

Considering the continual processes that stress continental lithosphere from the time of its formation, it is a puzzle to find large areas that preserve its earliest parts in an almost pristine state. Greater heat production in the past demands that the frequency and power involved in continental jostling were greater as we go back in geological time. Zones that show little sign of having been tectonically reworked for more than a billion years are termed cratons, and most of them have at their core continental material that formed in the Archaean, more than 2.5 Ga ago. Later orogens do show isotopic signs that deformed and partially melted Archaean crust was involved, but no so much as might be expected. Somehow, having a nucleus of Archaean lithosphere confers strength to cratonic areas. Geophysics reveals that the lithosphere beneath cratons uniquely extends to depths of 200 km, forming a “keel” or tectosphere.

Most geochemists consider that deep mantle beneath cratons is so rigid because it is unable to come close to the beginning of melting, due to it having once been the source of massive amounts of basaltic magma. Loss of the constituent elements of basalt and volatiles, including heat-producing isotopes of U, Th and K, renders it more inert than mantle that still has the potential to generate basalt under appropriate conditions. Basalt magmas also remove significant amounts of iron, thereby adding buoyancy to tectosphere materials.

Occasionally, much younger magmas that do form at the depths of the tectosphere bring samples of it to the surface, in the form of xenoliths. Their petrography and geochemistry reinforce the general idea of how cratonic “keels” form, but they have been difficult to date with confidence. The relatively new rhenium-187/osmium-187 method makes dating more assured. Cin-Ty Lee and colleagues from Harvard University (Lee, C et al. 2001. Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States. Nature, v. 411, p. 69-73) used the method on xenoliths from two adjacent areas, the actively extending Basin and Range Province and the Colorado Plateau. Both contain ancient rocks, Archaean in the former and Mesoproterozoic in the second, which behaves as a stable craton. Xenoliths from mantle deep beneath them have similar ages to those in the oldest crustal rocks, helping confirm the geochemical connection between crust formation and lithospheric mantle. However, those from beneath the Basin and Range have potentially “fertile” compositions, whereas the Colorado samples show signs of the depletion thought to confer strength and buoyancy. Paradoxically, a younger craton sits next to Archaean lithosphere that is demonstrably weak.

Lee and colleagues suggest that if part of Archaean crust formation did not create a tectosphere, it is quite possible that younger orogens might contain considerably more ancient crust than currently suspected. On the other hand, the mismatch between the near certainty that continents formed more rapidly during the first third of recorded geological history and the disproportionately small volume of known Archaean crustal rock could signify that a lot of it became resorbed into the mantle. That doesn’t appear to have been a significant process in later times. However, the total lack of sialic rocks older than 4 Ga, yet the evidence from detrital zircons up to 4.4 Ga in much younger sediments that some did indeed form, suggests that crustal resorption was efficient during early tectonics. Perhaps the Archaean marked the waning of such processes, in which an increasing proportion remained locked at the surface.

See also: Nyblade, A. 2001. Hard-cored continents. Nature, v. 411, p. 39-39.

Partially melted zones beneath Tibet

Anomalously low seismic velocities, accompanied by a “muffling” of seismic energy, and high heat flow beneath the Tibetan Plateau have hinted at the possibility of active crustal melting, but such information cannot resolve whether that is the case or not. Parts of the Plateau have been volcanically active in the near past, and that has been attributed by some workers to the detachment and sinking into the mantle of a large chunk of sub-Tibetan lithosphere. Freed of a substantial mass, the thick lithosphere beneath Tibet would then bob up, the rapid drop in pressure at depth inducing partial melting. Being weak, a substantial partially melted zone would also help the Tibetan crust deform more easily.

One means of adding support to the idea is looking for deep-crustal anomalies in electrical conductivity. Because electric currents flow naturally in the Earth, the conventional means of resistivity survey can use them instead of an input current. Such magnetotelluric surveys potentially give information down to depths of 100 km or more. At these scales, zones of abnormally low conductivity are likely to be due either to pervasion of deep rock with watery fluids or with widespread partial melting. A group of Chinese, Canadian and US geophysicisists (Wei, W. and 14 others 2001. Detection of widespread fluids in the Tibetan crust by magnetotelluric studies. Science, v. 292, p. 716-718) have shown that the middle to lower crust deeper than 15 to 20 km beneath most of the Tibetan Plateau is anomalous in this way. The highest conductivity lies beneath the main Yarlung (Indus) – Tsangpo suture., and may be related to fluids released by subduction processes. It is the anomaly beneath the Plateau itself that is most significant, for it extends for 4 degrees of latitude along the survey line. Higher conductivity anomalies correlate closely with Plio-Pleistocene volcanically active areas, and much of the area is affected by hydrothermal fluids. While adding detail to structure and rheological properties beneath Tibet, magnetotelluric studies still leave open the possibility that much of the electrical signature may be due to pervasive watery fluids, as well as to zones of melting.

Brazilian input to the growth of Gondwana

One of the most dramatic tectonic events known from the geological record is the break up of a supercontinent, dubbed Rodinia (from the Russian for motherland), in the Neoproterozoic. From a unity of almost all earlier continental crust, this break up sent fragments scurrying across a plethora of new oceans. Some of the fragments reassembled around 650 Ma ago to create what eventually became the southern part of the Carboniferous supercontinent of Pangaea; Gondwana. The assembly of West Gondwana involved a vast network of orogenic belts in which juvenile arc materials were pinched between colliding continental fragments, as these oceans closed up. Often called the Pan African event, because of its widespread signature in that continent, this assembly also affected eastern South America at the same time.

Fernando Alkmim, Stephen Marshak and Marco Fonseca (Alkmin, F.F. 2001. Assembling West Gondwana in the Neoproterozoic: clues from the São Francisco craton region, Brazil. Geology, v. 29, p. 319-322) turn our attention from the much-described Pan African to its Braziliano counterpart in South America. Their summary of current understanding suggests six stages in the rifting to collision, that involved major changes in palaeogeography.

Mapping with geophysical data

Published on May 1, 2001 by zooks777Leave a comment

In the same way that topographic contours can be transformed to models of continuous elevation change using surface fitting, measurements of gravitational and magnetic field potentials, at points on the ground or along aerial survey lines, are sources of imagery. Expressed as contours joining points with the same value, spatial distributed data are notoriously difficult to interpret, however much information they contain. Not only do contours simplify the data by dividing them into arbitrary steps, how we interpret contour maps depends on how we perceive them. Our eyes evolved to extract information distributed as a continuum across our field of view, and our visual cortex developed many tricks to innately interpret clues to shape, perspective and distance, to extend the limits of stereoscopic vision (we see objects in true 3-D only if they are closer than about 400 metres). Our innate abilities “interpret” contours in terms of the spacing between them; the closer they are together the darker we perceive the area of steep gradient. In other words we have to convert an image that is the “negative” of the first derivative to an understanding of the actual shape represented by contours! Unsurprisingly, we have to learn to “read” maps, and that is a great deal more difficult for those showing potential-field intensity than for topographic elevation. Cartographers long ago latched onto our use of shadows as clues to shape, and designed maps with shading as if the Sun was shining from the top of the sheet. They also use different colours as a second clue to what is high and low. Combining the two aids helps transform images of geographic variables – basically bland shifts from high to low – into visually stunning, and therefore more easily interpreted pictures. Surface modelling of elevation and geophysical data, with such graphic tricks, literally throws hidden, and often unsuspected features into sharp relief.

These techniques have revitalized desktop interpretation of the world, especially using results of geophysical surveys. However, in the same way that detail of a terrain blurs and loses information as resolving power falls, low-resolution data of other kinds obscure buried features, or give ambiguous hints to what they are and where they go. Reducing the spacing of aerial surveys, and the height from which they are acquired, increases the resolving power of the technique. Stunning examples of the state of this particular art appear in recent work by the US Geological Survey (Grauch, V.J.S. 2001. High-resolution aeromagnetic data, a new tool for mapping intrabasinal faults: example from the Albuquerque basin, New Mexico. Geology, v. 29, p. 367-370. See also http://rmmcweb.cr.usgs.gov/public/mrgb/airborne.html ).

Grauch worked on an area in which superficial materials and rapid rounding of topography result in poor surface expression of all but the largest faults. By using aeromagnetic images modelled from survey lines spaced at 100 to 150 metres, he picked out not only hidden faults, but also the magnetic signatures of pipelines, water tanks and buildings.

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Methane and Snowball Earth

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Surviving the Archaean with a UV jacket

New phyllum from Chinese Cambrian

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Continental tectonics of eastern Eurasia

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En route out of Africa

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Ice and prebiotic chemistry

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Cambrian Explosion: Shropshire hits the news

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Seafaring Homo erectus?

Unwholesome fare

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Zircons’ window on the Hadean

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Carbonates and biofilms

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Doubts cast on the increase in diversity with time

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Between a rock and a hard place

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Late-Palaeocene red tides?

A broader view of the Permian-Triassic mass extinction

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Partially melted zones beneath Tibet

Brazilian input to the growth of Gondwana

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