Unlike some natural catastrophes, there is no stopping a volcanic eruption. The best that can be done is to give people who live in the danger zones sufficient warning that they can escape disaster. Many volcanic areas are densely populated, largely because soils derived from lavas and ash are extremely fertile, and high volcanoes create decent rainfall because of their orographic effect. Naturally, nobody likes to up sticks, whatever the dangers, least of all if there are false alarms. As with seismic prediction, volcanologists do not have a good track record of foretelling big eruptions, even though a great many geologists cluster on and around volcanoes. Most of them flock to areas with active lavas, pyroclastic flows and other lugubrious after effects of major activity. However some do the painstaking work of trying to monitor the plumbing of volcanoes, to get a handle on which parameters are most likely to be authentic warnings of impending doom. It is no longer a matter of experienced volcano watchers and their instinctive feel for when one is about to blow its top, but one of ever more sophisticated instruments and software to analyse data and model volcanoes’ inner workings. The 28 March 2003 issue of Science (p. 2015-2030) devotes 16 pages to a review of volcano monitoring. While advances are being made, there is still a long way to go before they can pay dividends by reducing the loss of life. What is not going to go away, even in the best of all possible scientific worlds, is the economic devastation that follows any geohazard.
Author: zooks777
Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry
Ancestral lines squashed?
Many of the famous finds of hominid crania, on which ideas of human descent hang, consist of small fragments that have to be glued together to reconstruct their form. The basic work of palaeoanthropology is very like doing a 1000-piece jigsaw puzzle, but in three dimensions. Tim White, one of the pioneers of modern studies of hominin fossils, is now worried that the fragmentation of bone is connected with distortion during burial (White, T. 2003. Early hominids – diversity or distortion. Science, v. 299, p. 1994-1997). His own studies of fossil pigs present a disturbing pattern of post-mortem distortion that spurred earlier workers to subdivide them “exuberantly”. There are even “flat-headed flat pigs” and “narrow pigs” (literally, from their given Linnean names), but they are now known to be mechanically distorted remains of a single early pig. Hominid crania viewed in this light, and there are nowhere near as many as those of pigs, are a mess. White gives one example, Kenyanthropus platyops (“flat face”), which may well be a distorted and quite ordinary Australopithecus afarensis. Combined with the shape variation within living species, notably humans but also among bonobo chimpanzees, distortion throws the bushy tree of human descent into considerable doubt, just as Jonathon Kingdon predicted 10 years ago in his book Self-Made Man and His Undoing. There are so few hominid remains, and most are a mess, that it seems impossible to decide whether many hominin species existed together at any one time in the Late Miocene to Early Pleistocene, or that just a few (even one?) spread to many different habitats across the face of Africa; something of a bombshell for those who make a tidy living from skull-hunting and hominin cladistics.
Walking with Slade
Imagine, if you will, the Pliocene savannah of East Africa and a band of upright apes (Australopithecus afarensis), each (even the females) with the trademark sideburns of Noddy Holder. Imagine too that peeping from the bush is a voyeuristic obstetrician who resembles Groucho Marx, drinking a hot beverage (Cuppasoup?) from a flask, and trying ever so hard to get one over on Whispering David (Attenborough). There is a story here, because one of the apes is Lucy, who gets clobbered in Pliocene Slade’s fracas with a rival band (Staus Quo?), her infant falling into the long grass. Her sister rescues the child, and all is well on the long road to humanity. That was the first episode of the BBC’s Walking With Cavemen, the third series aimed at popularizing palaeontology, which began with Walking with Dinosaurs. All three owe as much to Bambi and Dumbo as they do to computer animation and modern research, despite the best efforts of the numerous scientific advisors. I saw the trailer for the next episode, concerning Homo ergaster – quite apt, because that was “Action Man”, that was. Not only were they white with tangled grey locks, but despite the brow ridges it was hard to conceal the fact that they were Pan’s People and the Chippendales striding purposefully across a salt pan. Did even female H. ergasters have 6-packs? Physically arousing it may have been, again leaving out the brow ridges, the bad barnets and table manners, but I thought, “Tripe”, and watched the footy the following week. (Note: “barnet” – rhyming slang for hair, from Barnet Fair).
A genetic key to human evolution?
It will not be too long before the publication of the chimpanzee genome. Because chimps are our closest relatives, and we shared an ape ancestor about 5 to 7 Ma ago, there is bound to be a media hullabaloo (and agitation among creationists) on the day of the release. At first sight, a comparison of human and chimpanzee genomes might seem to offer plain clues about the genetic side of our co-evolution, but evolutionary biologists are not so optimistic about an imminent breakthrough (Carroll, S.B. 2003. Genetics and the making of Homo sapiens. Nature, v. 422, p. 849-857). Their hesitancy stems from a matter of arithmetic and the sheer volume of work that needs to be done, as well as because of gross uncertainties about how genes relate to the important traits of humans and their differences from closely related apes. The human genome consists of about 3 billion base pairs and the gross difference from that of chimpanzees is about 1.2% (incidentally, it is likely that all mammals, from mice to men, share around 80% of their genes). Assuming that this difference is split 50:50 between the results of evolution towards us and towards chimps over the last 5 to 7 Ma, the divergence from the genotype of our shared ancestor in the human genome should amount to about 16 million new base pairs. Some of them may be “chaff”, but the genetic side of human evolution is buried in this massive area of potential work. Maybe around 200 000 are tied to evolved changes in protein production, that could be the key candidates for research. Although there have been claims for genes that control this or that side of humanness, properly tying down traits to genes will be an awesome task.
The differences between chimpanzees and humans manifest themselves in anatomy and behaviour, and a huge body of knowledge on both has grown in the last two centuries. So biologists know pretty well what they are looking for in terms of interesting genotype-phenotype links. However, a chart of those parts of the genome that account for the differences, whenever that becomes a believable reality, really does not help with the hows and whens of the course taken by evolution over several million years. They rely on the fossil record. Astonishingly, chimpanzee fossils are almost totally unknown, especially in the early part of their phylogeny. Even by the most optimistic account, the record of our predecessors is patchy and only a handful of near-complete skeletons are known from before about 500 ka. Carroll uses the most “bushy” version of hominin cladistics claimed by palaeoanthropologists, with 19 species, to illustrate the current status of hominin descent. White’s view of the uncertainties (Ancestral lines squashed?, earlier in this issue) makes the crucial connections before about half a million years ago extremely flimsy. But, there will undoubtedly be a huge growth in human evolutionary studies, once the key chimpanzee data become available. Of course there will be a massive media hype as well, and all manner of outlandish claims. But maybe also more funds for palaeontology will stem from the potential to link the evidence from today’s graspable realities with the exciting though puzzling anatomical record since the late Miocene.
Share this:
USGS photographic archive
The US Geological Survey has placed 16 thousand of its archived field photographs on the web, at print-quality resolution. They can be accessed at http://libraryphotos.er.usgs.gov and carry no copyright, so anyone can use them for illustration of lectures or textbooks (it would be polite to acknowledge the USGSs generosity). The photos date back to the earliest days of geological research in the United States, and are in black and white, and colour. Although still under development, the site’s search engine works well and quickly. Putting in “unconformity” and “thrust” yielded 31 and 52 pictures, respectively. However, trying to find the highly photogenic thrust of grey Cambrian limestones over Permian redbeds west of Las Vegas in Nevada drew a surprising blank. Every geological survey holds enormous archives of photographs that never see the light, so the USGS initiative ought to encourage others to follow suit. In particular, it would be great news if the British Geological Survey, the world’s oldest, did likewise, instead of just generating a meagre flow of funds by selling a minute proportion of its collection as postcards.
Share this:
Homing in on the great end-Permian extinction
Discussing what actually killed off around 95% of all species 251 Ma ago has become the perennial mass-extinction topic, now that the K-T boundary event is more or less done and dusted, bar a little murmuring over the Deccan Trap. Michael Benton of the University of Bristol has summarised the current state of play for the Permian-Triassic (P-Tr) event (Benton, M. 2003. Wipeout. New Scientist, 26 April 2003, p. 38-41). Despite many attempts to link an impact to the annihilation – such evidence as there is (see Buckyballs and the end-Palaeozoic extinction, EPN March 2001) has not been reproduced by independent analysis of the material. Weighty evidence comes instead for an Earth-induced event, from the coincidence of the monstrous Siberian Traps with the 100 thousand years or less that the extinction occupied, and from complete sequences across the P-Tr boundary in a Japanese ophiolite and a shallow marine section at Meishan in South China. As well as an intricate series of faunal changes, the Meishan sequence has now provided a complete record of oxygen and carbon isotopes that span the boundary event. The oxygen data suggest a 6ºC rise in global temperature at exactly the stratigraphic level of the extinction and of a massive lurch towards light carbon. Such a high proportion of 12C occurs at the boundary that it cannot have been induced by sterilisation of the oceans, which may well have happened as a result of the extinctions. Nor can even the huge belch of mantle CO2 emitted by Siberian continental flood basalts. The two combined only account for 40% of the carbon-isotope excursion. Release of methane from long-term storage as gas hydrate on the Permian sea floor is the only conceivable candidate. So it looks as if a runaway “greenhouse”, plus toxic gas and maybe acid rain put paid to most living things. Such a wiping out left lifeless oxygen-poor oceans – originally dubbed “Strangelove oceans” by Ken Hsu after the eponymous insane doctor. Triassic times did not see explosive reoccupation of abandoned niches, recovery taking up to 50 Ma from a tiny population of not very diverse organism. Benton has written a book on the P-Tr event (When Life Nearly Died. Thames & Hudson), and that is likely to be a rattling read. New Scientist maintains it’s irritating habit of never referring to sources in its articles, so to go further, you will have to buy the book.
Microbes showed no sign of change following a “Snowball Earth”
The “Snowball Earth” hypothesis has suffered quite a lot since its original promotion (see: Meltdown for Snowball Earth?, February 2002 EPN; Snowball Earth hypothesis challenged, again, December 2002 EPN). Whatever the eventual fate of the notion that the entire Earth was iced over from pole to pole, the fact that glaciers reached sea level at low latitudes at least twice in the Neoproterozoic seems to be an established fact. Such climate swings must surely have had an effect on life, either by driving up the rates of extinction and adaptive radiation because of stress, or perhaps providing nutrients to the oceans in vast amounts that allowed the phytoplankton base of the food chain to explode (see: The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic, September 2002). One of the first discoveries of low-latitude glaciogenic deposits was around Death Valley, California by the late Preston Cloud, who worked there during the 1960s. So it is fitting that palaeobiologists associated with the Preston Cloud Research Laboratory at the University of California, Santa Barbara have dissected sediments within and immediately beneath the 750 Ma diamictites that Cloud interpreted as glacial in origin, to test for signs of evolutionary change (Corsetti, F.A., Awramik, S.M. & Pierce, D. 2003. A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA. Proceedings of the National Academy of Science, v. 100, p. 4399-4404). In cherts within carbonate units they found a surprisingly diverse range of undoubted microfossils, that are probably auto- and heterotrophic Eucarya, but no difference between pre-glacial and glacial levels, in terms of their biota. Although this single piece of work does not prove that there was no biological change associated with a major cooling during the Neoproterozoic, it does cast doubt on the severity of its effects on life. Most important, the study shows that well-preserved cellular material is available for study in sediments that occur with glaciogenic diamictites, and should open up a new line of research bearing on the rise of the metazoan (multi-celled) Eucarya, which appeared in large numbers shortly after the last (~600 Ma) glacial epoch. Most if not all Neoproterozoic carbonates, whose universal presence in close stratigraphic proximity to glaciogenic strata first hinted at low-latitude frigidity, contain abundant chert nodules that are the best preserving medium for delicate and tiny cell structures.
Share this:
No glacial refugia in the Amazon Basin?
Tropical rainforest in Africa and South America is the most diverse biome on the planet, both as regards plants and animals. One view of how such luxuriance arose is that the forests have blanketed the humid tropics for as long as 50 or 60 million years, and the fact that they encompass a huge variety of environments created by different levels in the dominant and diverse vegetation. Thousands of niches and the interactions between organisms that exploit them during lengthy stasis inevitably drives rapid evolution towards all kinds of specialisation. The other view is that rainforests are by no means static over millions of years, but climate shifts have caused them to retreat and advance, perhaps hundreds of times during the Cenozoic. Amazonia in particular shows surprising variation in diversity, some patches being far more biologically rich than others, and having regionally distinct assemblages of plants and animals. This theory suggests that climatic stress, probably drying associated with globally cool episodes, resulted in rainforest shrinking to “refugia”. In them, populations of plants and animals shrank, thereby reducing the gene pool and giving greater chance for evolution by natural selection; different in different refuge areas.
Tropical soils are continually reworked and their highly oxidising nature destroys organic remains. So no record of its development exists in rainforest. However, wind and rivers transport spores, pollen and other biomarkers to seafloor sediments, where a complete record of fluctuations in biomass and diversity becomes preserved. A test of the popular refugia hypothesis is therefore to analyse organic matter in continuous cores taken from offshore sediment. Known fluctuations in global climate, from the oxygen isotope record should be matched by changes in the record of terrestrial biomarkers carried to the sea. Cores from the deep-sea sediment fan off the mouth of the Amazon potentially provide such a test (Kastner, T.P. & Goñi, M.A. 2003. Constancy in the vegetation of the Amazon Basin during the late Pleistocene: Evidence from the organic matter composition of Amazon deep sea fan sediments. Geology, v. 31, p. 291-294). Kastner and Goñi, from the University of South Carolina, examined phenols and organic acids in the cores, which can discriminate between grassy plants and trees that would have dominated savannah and rainforest, whose relative cover of the Amazon basin should have changed, according to the refugia hypothesis, as climate shifted from globally cool-dry to warm-humid.. Although their record only spans the last glacial cycle since 70 ka, they detected no significant change in the proportion of grasses and trees in the Amazon catchment. Moreover, the biomarkers remained similar to those carried by the Amazon today, right through the last glacial maximum, when drying of the tropics would have been most likely to have driven a shrinkage of rainforest area. It seems unlikely that forest refugia developed during one of the most extreme climate shifts in the last 55 Ma. Global climate fluctuations were considerably less before 1 million years ago, when the current round of 100 ka cycles began. So there is little reason to doubt that the Amazon rainforest has had a more or less constant area for much of the Cenozoic. The same cannot be said for those in Africa and SE Asia, partly because there are no useful data from offshore sediments, but also because those regions have experienced changing topography due to major tectonic activity, whereas eastern South America has remained stable. To conclude, as the authors do, that the data signify no great fluctuation in rainfall is not so certain.
Share this:
Catastrophic floods and denudation
Working out the rate at which landscapes evolve depends on some means of dating surfaces formed at different stages in the cutting down of topography. Modern studies rely to a large extent on the build up of isotopes, such as 10Be, that form in minerals no more than a metre or so beneath the Earth’s surface when they are exposed to cosmic ray bombardment. If such transmuted nuclides stay in place, for instance on a relic surface or a series of alluvial terraces, cosmogenic isotope analysis dates the formation of that surface. No matter how precise such surface dating can be, and currently there is a slop of around 20% either side of an age, there is a limit to the number of suitable surfaces. So, the continual degradation of landscape can only be sampled at a few isolated times. At best, an average denudation rate over long periods is all that geomorphologists can hope for. The same goes for analysis of the range of times during which grains in a sediment were exposed to cosmic rays, before they were eroded, transported and finally protected from bombardment when they were buried in alluvium, that is another approach to timing erosion. Average rates of erosion are useful in assessing some aspects of landscape development, but they are not much good for judging how it took place. Standing in some awesome scenery easily gives the impression that it must have evolved by some continuous, steady process, and there is a long tradition dating back to James Hutton that views surface processes in that way. Changes in rates have been seen as responses to “rejuvenation”, either by falls in the base-level of erosion or tectonic uplift to add gravitational potential to a region that makes flowing water more energetic.
Another approach is to look at the actual transfer of mass, either carried by rivers during different seasons or in the volumes of sediments that were deposited by recognisable individual events, such as a flood. In large river basins that have a low average gradient it is well-accepted that occasional floods don’t have much effect on sediment movement in the long term, but most of the sediment moved in such basins is alluvium already supplied by earlier processes. In mountainous areas rivers carry material directly from bedrock and the regolith that lies on it. Anyone who has witnessed flash floods in a normally crystal clear mountain river knows their awesome power. They become mud torrents studded with boulders that even fly through the air; they are debris flows rather than streams in the normally accepted sense. Such flows are episodic, but frequently annual, and exert a major influence over denuding the landscape. Yet over millennia, they too should maintain a consistent down wearing. In the Appalachian mountains of the eastern USA denudation rates seem to average out between 2.5 to 5 centimetres per thousand years. However, four catastrophic Appalachian storms in the late 20th century, related to hurricanes, had an astonishing effect on erosion there (Eaton, L.S. et al. 2003. Role of debris flows in long-term landscape denudation in the central Appalachians of Virginia. Geology, v. 31, p. 339-342). Carbon-14 dating of ancient mass-flow deposits formed in Virginia by comparable storms indicates recurrences in particular drainage basins around every 2500 to 3500 years. During that time the average rate of erosion would have denuded the surface by a measurable amount (5 to 10 cm), yet the recent storms removed between 47 to 63% of that expected during periods measured in millennia. The Appalachians are well vegetated, and therefore well protected from the effects of extreme floods compared with the surfaces of really big mountains such as the Himalaya and rugged areas in arid regions. The obvious question is, “Are average denudation rates, no matter how precise, very relevant to the way landscapes actually develop?” It is an important one, because weathering of debris from mountains is regarded by many geochemists as a means of taking carbon dioxide from the atmosphere – silicate weathering that involves CO2 dissolved in rainwater locks atmospheric carbon in bicarbonate ions that carbonate-secreting creatures in the sea can sequester to deep storage when they die. If about half the erosion of mountains is in widely separated catastrophes, which shift and then dump debris in a matter of days, then it is possible that the sums based on equating rates of weathering with those of erosion are not entirely valid. Weathering of continental silicates is one means of forcing global cooling by reducing the greenhouse effect, and understandably mountain rivers teem with researchers sampling the water and sediment load, especially in the Himalaya. If the bulk of debris shed to plains, such as those of the Indus, Ganges and Brahmaputra, never had time to be weathered at high altitude because it moved in catastrophic pulses, then maybe the sampling should be done somewhere else. Processes in the vast alluvial tracts far below high mountains are slower and more constant wit time, so maybe looking at groundwater that moves through them might add to the current research..
Share this:
When the Mediterranean dried up
At the end of the Miocene (from 6 to 5.3 Ma) the connection between the Atlantic Ocean and the Mediterranean Sea was blocked somehow. Over 700 thousand years evaporation deposited a thick layer of salt that now lies beneath much of the Mediterranean basin. This is known as the Messinian salt crisis. Equally dramatic, the straits reopened suddenly to allow seawater to flood back in the early Pliocene, in a hydrological catastrophe. How the Mediterranean basin became cut off has been ascribed to a 60 m sea-level fall, crustal shortening associated with nappe formation in the Betic Cordillera of Spain and the Atlas mountains, or by some kind of tectonic uplift. Timing of the Messinian crisis rules out the first two options, but sedimentation in the former “gateway”, a shallow seaway through what is now southern Spain, shows evidence of rapid shallowing that would have resulted from regional uplift. The question is, what drove this regional upwarping? A team from the GEOMAR Research Centre for Marine Geosciences in Kiel, Germany has discovered evidence from the changing geochemistry of Miocene to Pliocene volcanic rocks in the western part of the Mediterranean (Duggen, S. et al, 2003. Deep roots of the Messinian salinity crisis. Nature, v. 422, p. 602-606). Send Duggan and his co-workers found that the lavas underwent a geochemical shift from affinities with subduction-zone processes to those typical of intra-plate magmatism around 6.3 Ma ago, volcanism largely ending about 4.8 Ma. The ending in the late Miocene of eastward subduction of Tethyan sea floor beneath the Mediterranean, which had initiated volcanism around 12 Ma, led to foundering of part of the lithosphere and uprise of asthenosphere. This is marked by a change from high-silica, early magmas to alkaline, more basaltic varieties during the period of the Messinian salinity crisis. Uplift resulting from this delamination would have pushed the formed connections between the Atlantic and the Mediterranean as much as 800 m above sea level. Duggan et al. Suggest that the axis of uplift gradually migrated westwards, so that by the end of the Messinian crisis the area now centred on the Straits of Gibraltar would have been bulged up. Massive gravitational sliding from this edge of the continental lithosphere into the Atlantic may then have opened the narrow passage through which Atlantic water once again flooded.
Share this:
Flying feathers
Steadily, the remarkable fossil record in Cretaceous terrestrial sediments in China is revolutionising ideas about vertebrate evolution, particularly among small dinosaurs and early birds (see The Early Cretaceous lagerstätten of NE China in EPN March 2003). The long-held view that birds simply emerged fully fledged and flying from the dinosaurs has had to be thoroughly amplified. The sheer diversity, combined with intricate preservation in the Chinese sediments reveals feathering on a host of animals that are not birds, but earlier, bipedal dinosaurs. Some may have flown, but others had feathers for some other reason. Feathers are not a prerequisite for flying, and are so odd and complex in morphology and growth, that it has always been probable that they emerged and evolved over a long period preceding the appearance of true birds. Now it is possible to begin dissecting that strange evolutionary divergence, and Richard Prum and Alan Brush of the Universities of Kansas and Connecticut combine information about feathers and discussion of new fossils in a superbly illustrated review in the March issue of Scientific American (Prum, R.O. & Brush, A.H. 2003. Which came first, the feather or the bird. Scientific American, March 2003, p. 60-69).
Squirrels and tectonics
The squirrel family (Sciuridae) is one of the most widespread groups of mammals, only Australia, the Pacific islands and Antarctica being squirrel-free. The main reason is that squirrels are basically a primitive group among the rodents, themselves accounting for almost 50% of all living mammals species. The earliest fossil squirrel (Douglassciurus jeffersoni) was found in Late Eocene sediments in western North America, and the family seems to have originated there. The present wide distribution of squirrels bears witness to the many opportunities for migration in the Palaeogene, when continental masses were much less dispersed than they are today, together with changing environmental conditions that would have acted to drive migration. In the same way as human migrations have been charted and timed using genetic sequencing and molecular clock hypotheses, this unique group has been studied in detail (Mercer, J.M. & Roth, L. 2003. The effects of Cenozoic global change on squirrel phylogeny. Science, v. 299, p. 1568-1572). The general picture outlined by Mercer and Roth is that the Sciuridae migrated first across Beringea to reach Asia, then Europe and eventually Africa. In terms of migration rates, this was fast, the earliest European squirrel (Palaeosciurus) occurring in Early Oligocene sediments – this is also the earliest representative of squirrels that bear signs of the distinctive chewing muscles whose use today delights us all. Near identical musculature is found in the Red Squirrel and many other tree squirrels (Sciurus sp.), and their “living fossil” anatomy is borne out genetically.
As well as giving a fascinating insight into how modern genetic techniques help organise the cladistics of animals, the paper is full of information about the sheer diversity that this lowly group has achieved in about 50 Ma. Ground squirrels, rock squirrels, marmots, and tree squirrels abound, but none are so fascinating as the flying squirrels. Their teeth are similar to those in early Oligocene fossils, and genetic analysis suggests a common ancestry relatively early in squirrel evolution and migration. However, fossils of flying squirrels, in the areas where they are found today (North America and Asia) appear quite late in the stratigraphic column. The authors suggest that perhaps flying ability arose several times independently, based on a labile trait in the genes of their clade. There is also evidence for population “bottlenecks” that preceded adaptive radiation in several area. For instance, the entire radiation of South American squirrels seems to have stemmed from a single lineage that crossed the Isthmus of Panama shortly after it formed in Pliocene times. African squirrels can be accounted for by just two colonisations in the Miocene, and those of Indonesian archipelago east of the Wallace Line by migration during the Late Miocene, when sea-level was at its lowest before the Pleistocene lowstands. Most astonishing of all, is the Giant Squirrel of Borneo (Rheithosciurus), which is genetically closest to the squirrels of North America rather than its more diminutive cousins in the Sunda Shelf islands – did its ancestors move with astonishing speed, or did all related squirrels along its migration route become extinct quite rapidly?
A possible answer to the origin of the Giant Squirrel of Borneo lies in a collection made recently from a unique lagerstätten in a clay-filled pocket within laterites of northern Karnataka in India. The discoverer, Dr P.U. Siffli of Sringeri Institute of Palaeontology, has posted provisional results on his web site (http://geocities.yahoo.com/pusiffli/squirrels.html). The range of fossil rodents from near Sringeri is astonishing. Among them are bones of an undoubtedly primitive squirrel of enormous dimensions – approximately the size of a large child. Its masticatory musculature is similar to that of the North American Douglassciurus jeffersoni of Eocene age, i.e. unlike that of modern tree-squirrels. The biggest surprise lies in the dentition of the Sringeri giant squirrel. The typical rodent second incisors are serrated and arranged in a similar way to the shearing canines of mammalian carnivores. Its back teeth bear close resemblance to carnivore carnassials. As if this was not sufficient, the body cavity of the best preserved fossil contains pellets made up exclusively of bones from primitive hamsters, which abound in the lagerstätten. In a personal communication, Pandit Unmer makes a convincing case that he has discovered the only known predatory squirrel (provisionally named Titanosciurus sringeriensis), and will soon submit his finding for peer review. His only regret is that establishing a stratigraphic age for the laterite-bound pocket is proving to be very difficult. Sitting atop Archaean gneisses, the laterite can be correlated with similar palaeosols that cover the 64 Ma Deccan flood basalts some 130 km to the north, yet they defy dating by palaeontological or radiometric means. Dr Siffli would welcome offers to date the Sringeri lagerstätten (pusiffli@yahoo.com).
Share this:
Chromium isotopes and Archaean impacts
As mentioned several times in Earth Pages News, geologists have been slow to accept that the Earth’s evolution has been substantially affected by impacts of extraterrestrial bodies. In hindsight, this stubborn scepticism seems perverse. The discovery of impact-induced melt spherules in the Late Triassic sediments of SW England (see Britain’s own impact in EPN, December 2002) went almost unnoticed. However, there is still an entrenched view that nothing really big has happened. When similar spherule beds were reported from the Early Archaean greenstone belts in Australia and South Africa in 1986, and deduced to have formed by an impact, the authors were pounced on by those who thought they could plausibly explain the very odd rocks by unremarkable, Earthly processes. How satisfied Donald Lowe and Gary Byerly, of Stanford and Louisiana State Universities must be to find their view now proven beyond doubt, and to share in publishing the evidence. The proof comes from isotopic studies of three spherule beds in the 3200 Ma-old Barberton greenstone belt in South Africa (Kyte, F.T. et al. 2003. Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type. Geology, v. 31, p. 283-286). Chromium isotopes in the rocks are so unearthly, that explaining them requires that they contain up to 60% of extraterrestrial material, probably from carbonaceous chondrite impactors. Compared with the global spherule-bearing and iridium-rich K/T boundary layer (3 mm thick on average), that is the ejecta from the Chicxulub impact, the Barberton beds are much thicker (10-20 cm). The authors estimate that, if the Barberton layers are globally representative, the impactor responsible for their formation could have been 50 to 300 times more massive than that which terminated the Mesozoic Era. Besides that, three such layers formed within 20 Ma, and that suggests bombardment flux more than ten times that late in Earth evolution.
Triggering core formation at the microscopic level
Since Birch’s discovery in the 1950’s that the Earth’s excessive density compared with exposed rocks could be explained by a metallic, iron rich core, whose presence was detected by studies of seismic waves, there have been many explanations for core formation. Some regarded the process as a slow accumulation of iron-rich melt as it sank from the mantle, others that it formed during Earth’s initial accretion from the iron-rich parents of metallic meteorites. Lead and tungsten isotope studies indicate clearly that the core formed very early in Earth’s evolution, taking as little as 30 Ma. However, for such a vast mass to have quickly segregated from the rest of the Earth poses awesome mechanical problems. Alloys of iron, nickel and sulphur do have much lower melting temperatures than silicate minerals, and planetary accretion releases gravitational potential energy. That serves to heat up a growing planet, but core-forming materials would melt long before the dominant silicates that envelop them, if indeed mantle materials did melt substantially. So, at the centimetre scale of rocks, a melt fraction, however dense, would have to migrate and accumulate in globules with sufficient gravitational potential to sink through the viscous early mantle. The boundaries of pores in which melts form are critical. If the angles between silicate facets and melt-filled pores are large, tiny amounts of molten metal cannot become interconnected and migrate, unless the silicates begin to melt too or are actively deformed. Since coexisting silicate and metal melts are not supported by geochemical evidence and deep planetary interiors are probably static, the fact that the interfacial angles of crystalline minerals are high poses quite a problem. Geochemists at the University of Yokohama in Japan have performed complex experiments at high pressure and temperatures to simulate likely conditions during planetary accretion (Yoshino, T. et al. 2003. Core formation in planetesimals triggered by permeable flow. Nature, v. 422, p. 154-157). They discovered that if metallic melts account for more than 5% by volume of the accreting body, then this melt can percolate through the solid rock, because the angles separating melt and solid fall below the critical value of 60º.
The implication is that even quite small planetesimals (>30 km radius) can quickly develop metallic cores, using energy released by the decay of short-lived isotopes that were plentiful early in Solar System history. This is borne out by studies of metallic meteorites Of course, the immense gravitational energy released by accretion of larger planetary bodies would result in the same differentiation, but if they formed by accumulation of smaller differentiated bodies there is no need to postulate within-planet processes on the microscopic scale. The core would be “pre-manufactured”, only requiring blending of many smaller cores of accreting planetesimals
See also: Minarik, B. 2003. The core of planet formation. Nature, v. 422, p. 126-127.
Share this:
“Greenhouse” gas website
Dave Reay of the School of Geosciences at the University of Edinburgh has developed an extremely useful website that covers all the breaking news about “greenhouse” gases and climate change at www.ghgonline.org, which is easy to navigate and regularly updated. It contains links to on-line publications and a comprehensive Links page.
Earth-logs 