Some years back, a near complete skeleton emerged from a terrace on the Columbia River, in the north-western USA, near to Kennewick. Preliminary examination suggested that the skull had distinct European features, and some thought that these were the remains of some early pioneer. Kennewick Man attracted considerable attention when the terrace was dated at 9300 years, because the individual would then have been among the earliest known colonizers of the Americas. Five local tribes of Native Americans laid claim to the bones under the Native American Graves Protection and Repatriation Act, considering him to be an ancestor. The bones were taken into custody, thereby halting further research. Several academics saw this in a malevolent light, since if it was proven that the skeleton was indeed of European origin instead of Asian that would undermine a major plank in Native Americans’ claims for primary occupation of land; the central issue in a vast raft of legislation over ownership of mineral reserves. Pressure for release of the bones for research has built over the last two years, finally to overcome concerted opposition that wished to re-bury the bones with due resepct. The magistrate who judged the case found the original decision for sequestration “arbitrary and capricious”, and so investigations can resume. Quite possibly DNA will be preserved, and that could set the cat among the pigeons in Native American circles. However, some experts who had a quick look at the skull suggested that it might well be of an Ainu, one of the earliest inhabitants of the Japanese islands, who bear passing resemblance to Caucasian people..
Sea level fluctuations and large igneous provinces
On a global scale, shifts in sea level recorded by stratigraphers and on seismic profiles stem from one of two main processes: changes in land-ice volume and the volume of the ocean basins. The latter most often results from changing rates of sea-floor spreading, so that when it is rapid a greater volume of the lithosphere near spreading centres retains sufficient buoyancy to displace the oceans onto continental margins. During slow spreading, cooling of the lithosphere and an increase in its density enlarges the deep abyssal plains, so that the oceans withdraw to low levels. The mid-Cretaceous saw vast outpourings of plume-related lavas onto the floor of the West Pacific. So large, that they reduced the volume of the Pacific basin enough to result in continental flooding that was unprecedented in the Phanerozoic Eon.
On a local scale, changes in sea level recorded by the stratigraphic record include those due to local processes, generally ascribed to tectonic events at continental margins, which involved rising continental lithosphere. However, one of the greatest forces for local change in the continental freeboard is changing density of the lithosphere due to thermal effects. Anywhere once affected by major igneous events should record relative falls in sea level during the acme of magmatism, and rises when activity waned. The British Tertiary Igneous Province, a precursor to the eventual rifting of the North Atlantic under the influence of the Iceland plume is a good candidate for charting magma-sea level connections. The central volcanic complexes of the Hebrides, and their enveloping flood basalt piles formed at the start of the Palaeocene (~60 Ma). Around that time, much of the British Isles underwent several kilometres of vertical uplift and exhumation, whose effects remain today. In the surrounding marine basins, this event is recorded by Palaeogene sandstone bodies, presumable derived by erosion of the uplifted crust. Yet local Palaeogene sediments also record episodes of rising sea level. John Maclennan and Brian Lovell of the French Institut de Physique du Globe and Cambridge University have modelled the likely effect on sea levels around the British Isles by crustal underplating of magmas formed during the BTIP magmatism (Maclennan, J. & Lovell, B. 2002. Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of the Earth’s crust. Geology, v. 30, p. 675-678).
Up to 8 km of mafic igneous rocks seem to have ponded at the base of the British Isles’ crust while the BTIP was active. This estimate stems from the fact that the lavas of the province evidence high-pressure fractional crystallization. Calculations of the percentage of cumulates needed to generate the bulk chemistry of the BTIP lavas suggest that their volume far outweighs that of the volcanic part of the province. Given estimates of the volume of underplated cumulates, modelling boils down to examining the consequences for lithospheric density of initial heating and its subsequent relaxation. The Palaeogene sedimentary record provides good support for the model, with massive uplift from 60-56 Ma (the period when the BTIP was forming). Sudden sea-level rise at the end of this period never reached the level prior to magmatism; in fact it amounts to one half the estimated uplift. That is precisely in line with the underplating model.
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Prediction of earthquake periodicity founders
In an number of well-studied areas of chronic seismicity it appears from historical records that earthquakes recur with regularity. If that was so, it might be possible at least to prepare to throw many methods of detecting imminent movements at such areas, when they are “due” to go off. The theory behind time-predictability is that earthquakes relieve tectonic stresses along faults, and that if the forces are maintained, stress builds up again, to be released after a roughly fixed time (the same might apply to volcanism where magma production stays constant). A corollary is that high-magnitude events have longer periodicities than those lower on the Richter scale. One of the best cases thought to support this view is a 25-km stretch of the San Andreas Fault near Parkfield in California. The area has had 5 or 6 earthquakes greater than magnitude 6 since 1857, roughly every 22 years, the last being in 1966. There ought to have been one in 1988, but the poor statistics give an uncertainty either way of 10 years. By now there should have been a magnitude-6 event in the area, but it hasn’t happened. Jessica Murray and Paul Segall of Stanford University have analyzed the physics of the last event, and of the period that followed it. (Murray, J. & Segall, P 2002. Testing time-predictable earthquake recurrence by direct measurement of strain accumulation and release. Nature, v. 419, p. 287-291).
Their work involved using precise geodetic measurements obtained over the last four decades to assess the 1966 Parkfield earthquake’s size, which combines the movement then along the San Andreas Fault, the area involved in the slip and how “stiff” the crust is locally. Comparing this with geodetic data since then suggests strongly that the strain released in 1966 must have recovered between 1973 and 1987. They have shown that another Parkfield earthquake is long overdue. Their method rigorously allows for the effects of movements along other nearby fault, and inherent unpredictability seems inescapable. While other tests of the time-predictability principle, theoretically the most plausible approach, will continue, most devastating earthquakes continue to occur without forewarning. That reflects the fact that there are only enough seismologists with fancy equipment to cover threatened areas in a few extremely rich countries. Most people who live along active fault zones know whether or not high-magnitude earthquakes occur in their vicinity, yet will not have the privilege of scientists and equipment to provide warnings of this kind for a very long time, for simple economic reasons. Perhaps some effort and funds should be diverted to providing warnings within days of a serious event, using less “robust” methods.
See also: Stein, R.S. 2002. Parkfield’s unfulfilled promise. Nature, v. 419, p. 257-258.
British Geological Survey sued over arsenic
The world’s largest ever class action has been launched in London against the British Geological Survey, over claims that it failed to spot arsenic contamination during a 1992water survey in Bangladesh. As many as 40 million Bengalis risk arsenic poisoning, following a major groundwater development programme in the 1970s and 80s. Arsenic poisoning at non-fatal doses often shows first as water blisters on hands and shins. Long-term exposure via drinking-water causes cancer of the skin, lungs, urinary bladder, and kidney.
Aid agencies, led by UNICEF sank four million wells deep into alluvium, in the hope that groundwater use would alleviate the chronic problem of heavily polluted surface water in Bangladesh. The arsenic is of natural origin, and stems from leaching of the toxic element from sulphide minerals by deep, reducing waters. The case hinges on BGS’ failing to test for arsenic, which is easily detected using low-cost semi-quantitative methods, only 3 years after they had completed a comprehensive evaluation of groundwater quality in Britain that did include arsenic measurements. Accusations of double standards have been flying. However, UNICEF also failed to test for arsenic during the original drilling, because they did not expect to find it in the water. World Health Organization guidelines are very clear that arsenic does pose a threat in groundwater, but most cases in the past have been associated with former mining areas.
Considerable work on measures to clean up well water has been conducted since the Bengal arsenic crisis surfaced. Under oxidizing conditions, arsenic is adsorbed by ferric hydroxide, and a simple remedy is passing the water through iron wool or over ground-up rust or natural ochres.
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Conodonts and late Devonian mass extinction
The Late Devonian saw sufficient extinctions (around 55 % of all genera) for it to rank among the Big Five, but most genera that disappeared were shallow-water marine, particularly rugose and tabulate corals. Although the Woodleigh impact structure, just north of Perth in Western Australia, has been suggested as a possible culprit, its age is not reliable. Another possible cause is climatic cooling at low-latitudes, because the extinction was followed by the spread to tropical localities of high-latitude faunas. The key to supporting a climatic influence is temperature data from areas most affected by the extinctions. Unusually, a recent study selected phosphatic conodonts (tooth-like microfossils) for oxygen-isotope investigations – carbonate-shelled creatures are the usual choice. Michael Joachimski and Werner Buggisch, of the University of Erlangen in Germany, found prominent oxygen isotope excursions close the Frasnian-famenian boundary (Joachimski, M.M & Buggisch, W. 2002. Conodont apatite d18O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction. Geology, v. 30, p. 711-714). Their data are well controlled stratigraphically, because the rapid evolution of conodonts in the Devonian allows fine biostratigraphic division.
The extinction event is bracketed by two espisodes of sea-surface cooling, estimated to involve a drop of 6°C from an otherwise constant ambient temperature of around 32°C. They coincide with significant positive shifts in d13C of seawater, interpreted by the authors as evidence of the burial of much organic carbon debris. Therein lies a possible cause for the cooling. Carbon burial would have drawn down atmospheric CO2 levels. The extinction does seem to have been a response to temperature stress, tallying with the colonization of low-latitude seas by high-latitude faunas. However, that still begs the question of why carbon burial underwent two spurts. Was there an increase in sediment supply to the oceans that might augment burial rates, or are the positive carbon-isotope excursions reflections of the extinctions themselves? The second still leaves open the possibility that the undoubted cooling events may have had other causes, such as an increase in stratospheric aerosols, resulting either from major explosive volcanism or perhaps impacts that are yet to be found.
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Landsat to be privatised, once more?
Remote sensing, once the domain of researchers seeking hitherto undiscovered potato fields, lost cities and the intricacies of drainage patterns, entered the commercial domain in a big way about a decade ago. As well as giving lugubrious views of factories reputed to be manufacturing weapons of mass destruction, the aftermath of their bombing and that of villages alleged to harbour agents of the “axis of evil”, remote sensing helps find physical resources, spots farmers who fraudulently claim subsidies for non-existent crops and is used to site cell-phone transmitter networks. There are now several orbiting systems launched by commercial outfits that offer pin sharp and spectrally revealing information, at a cost. The workhorse of remote sensing since 1972 has been the US Landsat series. Following the addition of the Thematic Mapper in 1984, pressure grew for Landsat’s privatization in 1988. Prices jumped tenfold, to the horror of researchers, and the venture became uneconomic because of insufficient private-sector interest. Landsat 7, which carries an Enhanced Thematic Mapper, made orbit in 1999, and is administered by the US Geological Survey. Landsat-7 ETM data sell at $600 per scene, which is a bargain. Such has been the demand for data that US authorities are once more trying to shed responsibility for data provision to private hands, by asking for bids to develop, launch and market the next Landsat. Prices will once again leap to profitable levels. The joint US-Japan ASTER system aboard the ostensibly research-oriented Terra satellite rivals Landsat ETM in quality, and many scientists have been trying out the data. Again, to their disquiet, pressure reputedly from the Japanese partners has resulted in once free data being assigned a price of $55 per scene
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Protocol wars
Finding a new species of fossil organism is not usually a big deal. There are lots out there, and palaeontological journals publish formal descriptions regularly. The finder moves on, and as often as not allows other scientists in the field free access to the original specimens. Free exchange of published data, allowing colleagues to add to knowledge of materials by direct study, and, in most branches of science, verification by inter-laboratory analysis of material is part and parcel of research. The priceless Apollo lunar samples and many meteorites move freely because of these informal protocols. Things are different when the materials are “hot news”, none more so than remains from the human bush of evolution (Gibbons, A. 2002. Glasnost for hominids: seeking access to fossils. Science, v. 297, p.1464-1468).
Protocols for hominid specimens often allow access only to the finders, their colleagues and trusted friends, until they have performed the most minute investigation and written detailed monographs. The rules are sometimes laid down legally at governmental level. This can extend even to casts and CT-scan facsimiles. There are often delays of a decade between first publication of a new species and basic information, and the fossils’ entering the public domain. Unsurprisingly, this frustrates palaeoanthropologists who do not have the luck to make a major discovery – useful hominid material is exceptionally rare, despite the fanfares which greet its first publication. Consequently, eager students of human origins try various ploys to get in on the act, such as detailed photography of specimens in museums, and furtive digs for new material at the original sites. Sometimes they are thwarted, sometimes not (See April Earth Pages News, Homo erectus unification?). Berhane Asfaw, of the Middle Awash Research Team that has done so much to advance knowledge of our early ancestors, commented, “You don’t know how we suffered in the field to get these fossils”, when putting a halt to such a disingenuous attempt to snaffle pictures.
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Long-range forecast: a prolonged interglacial
Provided the Milankovich theory of astronomical influences on insolation is indeed behind the pacing of glacial-interglacial episodes of the near past, it should be easier to forecast future change in overall climate than that of weather. It turns out that the fluctuation of Earth’s orbital eccentricity (behind the roughly 100 ka periodicity of climate change for the past 1 Ma) is entering an historic low, due to the 400 ka period of one of its two cycles. Modelling future insolation at high northern latitudes results in a damping of its fluctuations over the next 100 ka (Berger, A. and Loutre, M.F. 2002. An exceptionally long interglacial ahead? Science, v. 297, p. 1287-1288). Left to climates own devices, the small changes in insolation may prolong the Holocene interglacial for as much as another 50 ka, instead of being now on the cusp of a descent into more frigid conditions. Until recently, many climatologists looked to the last, Eemian interglacial as the model for the current one, and that lasted only 10 ka.
Of course, climate is no longer at the whim of astronomical forces and the Earth’s own circulation of energy, principally by the flow of energy in North Atlantic water, driven by deep water formed by sea-ice around Iceland. Atmospheric CO2 stands about 30% higher than during previous interglacials, because of anthropogenic emissions. Berger and Loutre factor in the “greenhouse” influence of the additional CO2, to find an ominous possibility that the Greenland ice sheet might well melt, with the climate entering an irreversible warming. The climate, however, is not a model, and there is really no inkling of what surprises are in store from counter-intuitive behaviour of the many forces at work in it, under conditions that have no analogue during the whole of human evolutionary history.
Analogue of Archaean carbon cycle in Black Sea reefs
The Archaean world almost certainly had an atmosphere and oceans that were more or less free of oxygen. Under such conditions the fate of dead organisms in the ocean, perhaps the remains of photosynthesizing cyanobacteria, would have been bacterial fermentation and the production of massive amounts of methane. Along with volcanic emissions of carbon dioxide, methane in the atmosphere would have helped warm the planet at a time when the Sun emitted considerably less energy than it does now. Methane is more strongly depleted in 13C than any organic or inorganic carbon compound. So large falls in the d13C composition of organic carbon in Archaean rocks, around 2700 Ma have been taken by some palaeobiologists to signify methane metabolism. Most methane-consuming bacteria today produce oxygen as a biproduct, so the negative excursions might indicate an early build up of more than a trace of oxygen in the Archaean atmosphere. Discovery of bacterial communities on the floor of the Black Sea, which consume methane without oxygen production (Michaelis, W. and 16 others 2002. Microbial reefs in the Black Sea fuelled by anaerobic oxidation of methane. Science, v. 297, p. 1013-1015), suggest strongly that there may be little reason to suppose that Archaean conditions did involve free oxygen.
Off the coast of Crimea there are numerous sea-bed methane seeps in shallow water. Surprisingly they are well-colonized by primitive bacteria, which produce thick mats held together by carbonate precipitates in completely anoxic conditions. Laboratory cultures of the communities reveal that the consist of archaea and bacteria that respectively consume methane and reduce sulphate ions to sulphide. The net result is that methane is oxidized by sulphate to produce calcium and magnesium carbonates, and lots of hydrogen sulphide (methane donates electrons for sulphate reduction, thereby becoming a source of carbon for cell metabolism). Since much of the methane’s carbon ends up in stable carbonate – perhaps ten times more than in organic matter, such a process in the Archaean would have helped stabilize the “greenhouse effect” then.
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Evidence for slab break-off in subduction zones
The detachment of lithospheric masses and their falling-off into the mantle, either by delamination of deep lithosphere beneath continents or the breaking of a subducted slab, have become popular means of explaining a variety of unusual phenomena in mountain belts. In the Himalaya and Tibetan Plateau, such models have been evoked for the formation of odd K-rich basalts in the Eocene and Miocene, and the crustal melting that generated leucogranites around 20 Ma ago along the entire length of the Greater Himalaya. Taking all the oddities of the Indo-Asian collision zone together does seem to support such a model (Kohn, M.J. & Parkinson, C.D. 2002. Petrologic case for Eocene slab breakoff during the Indo-Asian collision. Geology, v. 30, p. 591-594). However, there is still no tangible direct evidence beneath the region.
Using seismograms for deep-Earth tomography appears to be able to resolve a range of proposed variants of tectonics, as well as the gross behaviour of the deep mantle. The site where two plates are being subducted on the west side of the North Pacific, marked by the Kamchatka peninsula, is pretty odd as well. Although rates of subduction of both plates are high, the part of Kamchatka at one boundary no longer has active volcanoes, whereas the other does. In fact one of the volcanoes there holds the world record for magma output. Up to 5 Ma ago, the whole of Kamchatka was actively volcanic. An explanation for the sudden halt to volcanism is that the dehydrating slab which provides the essential watery fluid for partial melting of the overlying mantle wedge – the source of subduction-zone magmas – broke away from the subduction zone and “fell” into the mantle 5 Ma ago. That would have removed the source of hydrous fluid at a stroke. Seismic tomography now seems to be capable of resolving just such a foundered slab (Levin, V. et al. 2002. Seismic evidence for catastrophic slab loss beneath Kamchatka. Nature, v. 418, p. 763-767). There is no slab beneath the presently inactive volcanoes, whereas it is intact beneath the active ones. The authors also claim that the seismic structure reveals a more recently foundered piece of lithosphere, whose rapid loss of hydrous fluid helps explain the phenomenally high magma production of the Klyuchevskoy volcano. Such slab break-off is clearly a potential engine for enormous changes in magmatism, and the first seismic evidence for it is bound to spur a search for more examples.
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The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic
Proterozoic
Accepted biogeochemical wisdom suggests that about 2000 Ma ago, the terrestrial environment changed from one in which oxygen was a rare free element to an increasingly oxygenated world. One line of support for this involves the first appearances around that time of redbeds and lateritic palaeosols, that signify a surge in the O2 content of the atmosphere. The other pointer is the disappearance of banded iron formations (BIFs), suggesting that soluble iron-2 was no longer available in the oceans due to its oxidation near its main source at mid-ocean ridges. The first unambiguous microfossils of eukaryotes, which need oxygen for their metabolism, also appeared some two billion years ago.
There is, however, a different view; that there was a transition between the anoxic world of the Archaean and Early Palaeoproterozoic and that marked by pervasion of atmosphere and hydrosphere by oxygen. It stems from studies of sulphur isotopes in Proterozoic marine sediments by Donald Canfield of Odense University Denmark (Canfield, D.E., 1998. A new model for Proterozoic ocean chemistry. Nature, v. 396, p. 450-453). Canfield found evidence for steadily increasing sulphate ions in seawater from 2300 Ma, which he suggested would have led to increasing production of hydrogen sulphide in the deep oceans by sulphate-reducing bacteria. He proposed that it was combination with deep-ocean sulphide ions that shut off the supply of soluble iron-2, essential for the production of shallow-water BIFs. Today, sulphide precipitation is restricted to hydrothermal vents and most iron is removed by combination with oxygen in sediments on the main ocean floors. In short, Canfield proposed a transitional ocean akin to the Black Sea, with an oxic near-surface zone but anoxic at depth. Not only iron would have been removed in sedimentary sulphides, but many other metals, leading to their depletion in seawater. Ariel Anbar of the University of Rochester and Andrew Knoll of Harvard examine the biological repercussions of this transitional ocean (Anbar, A.D. & Knoll, A.H. 2002. Proterozoic ocean chemistry and evolution: a bioinorganic bridge? Science, v. 297, p. 1137-1142).
Iron and molybdenum are crucial elements for eukaryotes, albeit only in small quantities, because they are central to the enzymes that fix nitrogen. Insufficient quantities would put early eukaryotes at an evolutionary disadvantage to prokaryote life. Moreover it would reduce ocean productivity. This, they propose, can help explain the lack of evolution among eukaryotes until the late Proterozoic. The carbon isotope record of seawater (derived from limestones) shows a strange pattern that supports a period of biological stasis from 2000 to about 1200 Ma. From the end of the Archaean until 2 billion years ago, there are huge fluctuations (to highly positive and negative values) in the proportion of heavy 13C, and so too in the Neoproterozoic. The period in between shows no significant carbon-isotope fluctuation, d13C remaining at around zero, which Anbar and Knoll attribute to very low biological productivity. In their model, it was the release of massive amounts of metals by continental erosion during the “Snowball Earth” glacial periods of the Neoproterozoic that was able to kick start life, especially that of the eukaryotes. Emergence of the efficient, multicelled algal photosynthesizers drove up oxygen levels, eventually to oxygenate the deep oceans.
A cautionary note needs to be thrown in, however, especially when using analogies with the modern Black Sea (see Analogue of Archaean carbon cycle in Black Sea reefs). Biogenic carbonates on the Black Sea bed show huge negative excursions in their d13C, because organisms that formed them metabolized methane, thereby incorporating methane’s strong depletion in heavy carbon. As well as there being little direct evidence for Anwar and Knoll’s idea, the methane part of the carbon cycle needs to be factored into interpretations of the carbon-isotope record.
See: Kerr, R.A. 2002. Could poor nutrition have held life back? Science, v. 297, p. 1104-1105.
Isotopic evidence for early life may be from metamorphic processes
Controversy has surrounded reports of carbon-isotope evidence from the oldest recognisable sedimentary rocks that can be interpreted as signs of life 3800 Ma ago. The problem is that the data came from carbon trapped in resistant minerals, such as apatite, in the metamorphosed Isua supracrustal rocks of west Greenland. A detailed study of carbon in various forms in the Isua metasediments (van Zullen, M.A. et al. 2002. Reassessing the evidence for the earliest traces of life. Nature, v. 418, p. 627-630) strongly suggests that the isotopic evidence for life is flawed. It seems likely that both graphite and carbonates in the Isua rocks originated by chemical reactions that took place during metamorphism; they are probably metasomatic in origin. The wide range of d13C values found in both graphites and carbonates could have formed by isotopic exchange between graphite and carbonate during metamorphism. Graphite inclusions in apatite, the source of carbon isotopes claimed to reflect the earliest biological activity, are petrographically no different from inclusions in other minerals. Indeed, the sample originally used to suggest the isotopic influence of early life is of metasomatic origin.
All is not lost, however, for graphite that is highly depleted in heavy carbon-13 (a sign, albeit ambiguous, for organic processes) also occurs in turbidites that show graded bedding. These rocks show no petrographic signs of metasomatism, and may contain signs of life. Ominously, the US, Norwegian and Estonian co-workers, having looked in detail at carbon found in low concentration within BIFs and cherts from Isua, conclude that at least some is recent organic matter that groundwater flow has carried into the rocks.
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Bizarre impact structure beneath North Sea
The increasing use of finely-resolving 3-D seismic surveys in offshore exploration for hydrocarbons reveals exquisite detail of structure in strata beneath the sea floor. So it is no surprise that oil-company geophysicists are able to image features that would otherwise remain hidden to researchers in universities. If such discoveries are of little interest commercially, their finders are free to publish. During routine surveys in the southern North Sea, an array of seismic profiles gradually built up a picture of something more reminiscent of the surface of an icy moon of Jupiter than a sequence of basinal sediments (Stewart, S.A. & Allen, P.J. 2002. A 20-km-diameter multi-ringed impact structure in the North Sea. Nature, v. 418, p. 520-523). The circular feature found in strata at the top of the Cretaceous, might have been passed off as the product of deeper rise of salt diapirs from the widespread Permian evaporites of the North Sea basin, but for several features. The surveys revealed no signs of the low-density Permian salt having bulged upwards below the structure, and disruption stops at depth.
The feature consists of at least 10 concentric rings extending to 20 km diameter, and at its centre is a bowl-shaped depression around a clear peak. Not only is it an impact structure, but one of a particular class known as multi-ringed basins. Those known from the Moon, are vastly bigger and are thought to have formed by such immense energy that the lunar surface rippled to fail along large concentric faults. Lunar and terrestrial craters of the size of the North Sea structure usually have no concentric structure, being circular pits with rims and occasionally a central peak cause by rebound of the crust after impact. The only similar features known are from moons of the Giant Planets that are made mostly of ice. It is surprising that the North Sea example closely resembles them. Modelling of such craters on Callisto suggests that they form when surface materials are underlain at depth by weaker ones; possibly an ice-liquid slush on ice moons. The North Sea impact was into the Upper Cretaceous Chalk, whose upper strata are more homogeneous than those at deeper stratigraphic levels, which contain layers of mudstone. Had impact occurred while the strata were not completely lithified, then the clays would have allowed inward movement to fill the crater excavated by impact, the more rigid upper Chalk having fractured during this movement.
Whether or not the impact accompanied the Chicxulub crater, implicated in the end-Cretaceous mass extinction, is not certain, although it does seem to predate Tertiary sedimentation in the North Sea. There are probably many more impact structures on the sea floor, buried by marine sediments, but only in hydrocarbon-rich basins are they likely to be unmasked by seismic surveys.
Evidence builds for major impacts in Early Archaean
Following the discovery that anomalous tungsten isotope compositions of some Early Archaean rocks suggest a major component of extraterrestrial material in them (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment), geochemists from Louisiana State and Stanford universities report evidence of debris from very large impacts in the same period (Byerly, G.R. et al. 2002. An Archean impact layer from the Pilbara and Kaapvaal cratons. Science, v. 297, p. 1325-1327). Their case rests on the occurrence of layers of rock containing spherules of what formed as molten silicate droplets, in Early Archaean greenstone belts of the Barberton and Warrawoona areas of South Africa and Australia. Zircons from a single layer in both areas yield identical ages of 3470 Ma, suggesting that the layers formed during a single impact event. The authors speculate that a major unconformity in the Archaean of the Pilbara province in Australia, which is around the same age, may be the result of tsunamis induced by the impact. It seems as if the responsible impact had a global effect, and may have released 1 to 2 orders of magnitude more energy than that responsible for the K/T event. Judging by the lunar cratering record, this and previous finds help confirm expectations of similar bombardment on Earth during the Early Archaean.
Very early differentiation of planetary bodies
The radioactive decay of 182hafnium to 182tungsten seems likely to resolve the influence of impacts on the Earth ‘s evolution (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment). It is even more useful in refining ideas about the evolutionary pace of the parent bodies of meteorites. The half-life of 182Hf is only 9 million years (all of it has decayed away in the Solar System by now), so the amount of radiogenic 182W associated with hafnium in a meteorite is a guide to pervasive geochemical processes early in the history of their parent bodies. Hafnium has an affinity for silicates, whereas tungsten is siderophile and likely to enter planetary cores, should they form. Because 182Hf decays so quickly, it is not easy to work out its original abundance, relative to stable 180Hf, in the source material for the Solar System. That is a prerequisite for estimating when the hafnium-tungsten differentiation took place in a planetary body. Two papers in the final August 2002 issue of Nature agree on this initial ratio (Yin, Q. et al. 2002. A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites. Nature, v. 418, p. 949-952. Kleine, T. et al. 2002. Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry. Nature, v. 418, p. 952-955), which has important connotations; it is less than half the previously assumed value. They determined this initial ratio using Hf-W data from independently dated carbonaceous-chondrite meteorites, whose parent bodies were never fractionated.
The two research groups, from Harvard University and the French Laboratoire des Sciences de la Terre, and the universities of Münster and Köln, Germany, respectively, use the new initial ratio to estimate the age of core formation from a range of meteorites. Their estimates dramatically shorten the time between original accretion and core formation in a variety of bodies whose Hf-W isotopes have been studied previously. The parent of the eucrite class of meteorites, probably the asteroid Vesta, differentiated within only 3 to 4 Ma, whereas the cores of the Earth and Mars took a little longer – about 29 and 13 Ma respectively. In geological terms, accretion and core formation probably accompanied one another. Of course, such estimates based on isotopic decay systems assume that the initial ratios existed at the time of accretion. That may not be valid if the pre-Solar nebula took millions of years to evolve to the stage of self-collapse under gravity, which is the prerequisite for the formation of a planetary system. However, there is evidence from short-lived decay systems involving other radioactive isotopes, such as 26Al, in meteorites, that points to the influence of a nearby supernova that triggered the formation of our Solar System. Such an event is required to synthesize short-lived isotopes anyway. Moreover, the shock from a supernova could accelerate collapse to mere few tens of thousand years.
See: Cameron, A.G.W. 2002. Birth of a Solar System. Nature, v. 418, p. 924-925.
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Biofilms and BIFs
Biomineralization is a growing topic that ranges from life’s influence on the production of economic deposits of metal ores to even the suspicion that it might play a role in Alzheimer’s syndrome. The most common, and enduring evidence of the influence of micro-organisms in making rocks are stromatolites made of carbonates that blue-green bacteria have secreted, perhaps from as early as 3500 Ma ago. Something similar, though it involves eukaryotic algae, is the formation of tufa or travertine where springs emerge from limestones. Many a child, including my young self, consigned a cuddly toy to “petrifying” springs, such as Mother Shipton’s Well in Knaresborough, Yorkshire. Few retrieved them, which is why there aren’t many rock-like Teddies around.. Another childhood memory, that bears on biomineralization, is a spring surrounded by orange and brown slime that we supposed was so deadly that only bathing in helicopter fuel would ward off a dreadful end brought on by the faintest splash of the loathsome gunk. It is a great surprise to learn that such ochreous springs, common where coal mines drain to the surface, might hold a key to the formation of Precambrian banded iron formations (BIFs) (Brake, S.S. et al. 2002. Eukaryotic stromatolite builders in acid mine drainage: implications for Precambrian iron formations and oxygenation of the atmosphere. Geology, v. 30, p. 599-602).
Groundwater that has passed through iron-sulphide bearing rocks, becomes both acid and charged with iron-2 after oxidation of pyrite. It is high acidity and low Eh that dissolves toxic heavy metals and arsenic, rather than their iron content, that make springs of such waters so hazardous to small boys bent on careers as hydraulic engineers (check their shins and fingers for the lingering water blisters that are a sure sign of the onset of arsenic poisoning). It seems that Euglena, a common “animalcule” in such springs that is easily seen with a cheap microscope, is an ochre (iron-3 hydroxides and sulphates) forming agent. It is an acid-tolerant, oxygenic photosynthesizer that builds slimy mats. Given time and substantial supplies of dissolved iron, Euglena actually builds hard structures reminiscent of stromatolites. Brake and colleagues from Indiana State and Kansas universities, and the Colorado School of Mines, studied Euglena from coal-mine drainages under lab conditions, and provide details of their metabolism. The modern iron-stromatolites are so like some variants of BIFs from the Archaean and Palaeoproterozoic, when they were at their acme, that the authors suspect their origins in biofilms formed by prokaryotic organisms with similar metabolism to the more complex Euglena. Until their work, most geologists regarded BIFs as products of inorganic precipitation of iron-3 compounds and silica when iron-2 rich seawater met oxygen produced by photosynthesizing cyanobacteria. Indeed they speculate that the biofilm makers could have been early eukaryotes, despite the first unambiguous evidence for nucleus-bearing organisms being no older than 2100 Ma. If they are correct, then such communities would have needed free oxygen, and would themselves have contributed to oxygen build-up in the early atmosphere.
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Bonanza time for Bonzo
The big news of July was without doubt from the palaeoanthropologists; a report on finds at the 1.75 Ma Dmanisi site in Georgia (Vekua, A. and 11 others 2002. A new skull from Dmanisi, Georgia. Science, v. 297, p. 85-89.), and the unveiling of a hominid-like skull from Chad dated at 7 Ma (Brunet, M. and 37 others 2002. A new hominid from the Upper Miocene of Chad, central Africa. Nature, v. 4418, p. 145-151). Both threw the issues of human origins, evolution and migration back into the arena of debate.
Time and Newsweek, and once upon a time Life magazine often figure celebrities of the week or month on their covers. Nature entered the celebrity cult on 11 July with a front-page photo of the magnificent cranium of Sahelanthropus tchadensis’ holotype found and analysed by a vast team from France, Chad, USA, Switzerland and Spain. The skull is from Upper Miocene sediments around Lake Chad, dated from their varied fauna which is very like that of similar sediments in Kenya. Its hominid credentials stem from the skull’s face, jaw and teeth, but it is odd. From the back, it resembles a chimp, and so does the capacity of its brain case. From the front, it bears close resemblance to an advanced Australopithecine. Yet no limb bones have been recovered so far, and the attachment point of the skull to its backbone is not mentioned. Both features would be needed to prove upright gait. Undeterred, the authors and many commentators are convinced that it is the oldest human ancestor, from the very limit in time at which modern genetic analyses suggest that the human “bush” of descent parted from that which led to modern chimpanzees. Bernard Wood of George Washington University (Wood, B. 2002. Hominid revelations from Chad. Nature, v. 418, p. 133-135) discusses Sahelanthropus’ significance to human evolution, implying that it poses problems for both the linear model of descent from a single emergence of basic human anatomy and the “untidy” model, to which he subscribes – adaptive radiation to changed circumstances that occurred more than once. In the “untidy” model, even an excellent-looking candidate for the first in the line may not have been ancestral to us.
Palaeoanthropologists have never been as well-endowed with bones as they are with funds, and one detects hints of the protectiveness that has long plagued the discipline. The finders of the previous candidate for the first hominid – Brigite Senut and Michael Pickford of the Natural History Museum in Paris (Taking stock of hominid evolution, Earth Pages News, March 2002) who found Orrorin tugenensis, in 5.72 to 5.88 Ma sediments of the Tugen Hills in the Kenyan Rift – claim that Sahelanthropus is merely an ancestral gorilla, citing the creature’s large canines. Without a pelvis or footbones to back up the hominid claim, they could well be right. However, the good news is that East Africa has lost its primacy as the source of fossils bearing on human evolution. Being 1500 km west of the nearest previous site, and unrelated to the East African Rift system. The new sites in Chad open up a vast area for future searches of potentially fruitful Miocene sediments, that are neither abundant nor complete in the Rift (its formation is post-Miocene).
Georgia in the former Soviet Union has grown in significance since the first reports of very old human remains near Dmanisi, a decade ago. The site is well preserved, contains abundant mammalian remains, and the containing strata overlie a 1.85 Ma basalt. With supplementary palaeomagnetic stratigraphy, Abesalom Vekua and his colleagues from several Georgian institutions, the USA, Spain and Switzerland have narrowed the age of the site to 1.75 Ma. Their new find is a superbly preserved skull, together with a lower jaw, following earlier discoveries of two other cranial fossils. The site is well endowed with stone artefacts, similar to those of the Oldowan culture of East Africa.
The new skull has a smaller brain capacity than co-eval H. ergaster or H. erectus in Africa, and bears some resemblance to the earliest species of human, H. habilis, although the authors prefer not to muddy the waters with yet another species of Homo. However, had this skull been found first, they might well have gone for H. habilis, and in the paper suggest that it and the others may have descended from habilines that left Africa some time before they were preserved. As with Sahelanthropus, no limb bones have been found at Dmansi so far. The three fossils are not identical, and another important possibility is that these humans, like us, were polymorphic, though this needs to be tempered with the possibility of differences between males and females, or that the smallest may have been adolescent. Others have jumped on the differences to suggest that more than one species are represented. Here we see the problem of meagre evidence, so that anatomy alone permits either “lumping” or “splitting”. Jonathan Kingdon, in his book Self made man and his undoing (1993, Simon and Schuster) raised the issue of polymorphism, so characteristic of modern humans, to the consternation of most palaeoanthropologists, who remain largely silent on its implications for the whole issue of human classification.
There is no doubt that early humans with primitive tools were able to expand out of Africa as early as 1.75 Ma ago. They were not well-endowed with brain power, and they were little people – they did not stride purposefully into the wide, blue yonder. That they reached Georgia, of all places, is extremely odd, because a direct route from Africa is barred by the Caucasus mountain range, and the deserts of Syria and Iraq. They might have tramped around the coast of Asia Minor, following the Dardanelles to the Black Sea coast and then into the Georgian plains. A more extreme possibility is that first they crossed the Straits of Bab el Mandab (closed at the time) and, in Kingdon’s words, “standloped” to east Asia and the backtracked along the northern flanks of the great mountains of Asia to reach the steppes. Finds of Oldowan artefacts and meagre human remains in China also provide ages around 1.8 Ma.
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Water on Mars
From time to time Earth Pages News has tried to temper the flood of papers that seek every which way to support the notion that Mars is still well-endowed with water. That is what NASA seeks in order to fuel its bid for the vast funds needed to launch a staffed mission to the Red Planet. The evidence in each case was ambiguous. I have always thought that attention and money would be better directed towards the one sixth of the human population who have no access to safe and abundant water supplies. That remains my view, but the appearance of 10 pages of Science forces me to accept near proof of Martian water in abundance (Feldman, W.C. and 12 others 2002. Global distribution of neutrons from Mars:results from Mars Odyssey. Science, v. 297, p. 75-78. Mitrofanov, I and 11others 2002. Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey. Science, v. 297, p. 78-81. Boynton, W.V. and 24 others 2002. Distribution of hydrogen in the near surface of mars: evidence for subsurface ice deposits. Science, v. 297, p. 81-85).
The neutron and gamma-ray detectors aboard Mars Odyssey only needed to operate for a month to reveal the abundance of hydrogen across the surface of Mars. It varies a great deal, the highest levels showing up at high northern and southern latitudes. Preliminary modelling suggests that these regions have at least several metres of ice-rich debris, containing between 25-35 % water ice. Quite possibly the modelled ice-rich layer could reach a kilometre in thickness. High anomalies at lower latitudes are modelled as being due to hydrated minerals in the Martian soil.
More results at higher precision are to come from Mars Odyssey, and experts emphasize that the reported modelling of neutron fluxes and those of gamma rays emitted by neutron-capture reactions is complex and preliminary. However it does look like NASA scientists will soon by selecting sites for future landings on Mars. Even more certain, it will have sent a frisson of excitement through those intent on the glory of finding signs of life there.
Tungsten and Archaean heavy bombardment
One of the major revelations that arose from the Apollo missions to the Moon is that the vast maria basins, filled with basalt, formed when a series of huge impacts wracked the lunar interior. Surprisingly, they formed between 4 to 3.8 Ga ago, rather than in the earlier evolution of the Moon, and this “late heavy bombardment” (LHB) spans the period when the oldest rocks were forming on the Earth. Controversy has raged for 3 decades about whether the LHB had a major influence on early Archaean geology. The problem was that direct evidence has been hard to find, and difficult to get across to critics of such outlandish notions. A careful investigation by geochemists from the Universities of Queensland and Oxford seems likely to force some critics to eat hat (Schoenberg, R. et al. 2002. Tungsten isotope evidence from ~3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth. Nature, v. 418, p. 403-405).
Because stable 182W forms by the decay of 182Hf, with a short (9 Ma) half life, virtually none will have formed since the Earth accreted. The 182W/183W ratio of objects from different parts of the Solar System should show distinct differences, and so they do. Different classes of meteorites show tungsten isotopes that are significantly different from one another, and from products of mantle melting on Earth. Ronny Schoenberg and co-workers analysed tungsten from two early-Archaean sources: the dominant grey gneisses, which are probably calc-alkaline igneous rocks formed at mantle depths, and metasediments from the famous Isua area in Greenland and another around the same age (~3.8 Ga) in Labrador. The gneisses show no difference from later products of mantle processes, but the metasediments deviate significantly from the terrestrial isotopic composition of tungsten, towards that characteristic of meteorites. They conclude that the metasediments mix debris formed by weathering and erosion of normal early Archaean crustal rocks with that formed in major blankets of ejecta from meteorite-induced impacts.
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Exploration licence lepton by physicists
The search for hitherto undiscovered and totally hidden hydrocarbon reserves has attracted a bizarre range of patented techniques over the years. They range from using thermal images of the sea surface to pinpoint stationary cold spots that may mark deep water upwellings driven by rising natural gas bubbles, through helicopter borne hydrocarbon sniffers to fine-resolution aeromagnetic surveys to detect anomalies due to magnetite formed by bacteria that metabolise oil and reduce hematite to magnetite. Most have a rational scientific basis, but there are a few that defy reason. Most explorationists have been button-holed by dowsers, but the latest venture seems to have convinced Her Majesty’s Government, to the extent that the Department of Trade and Industry has granted three licences to explore parts of rural England, generally known for their fox-hunting aficionados.
A company, Technology Investment and Exploration Limited of Guernsey, has invented a device that they call a “microlepton generator”, supposedly based on the Nobel-winning work of Martin Perl of Stanford University, who discovered the subatomic tau lepton in the early 1990s ( http://physicsweb.org/article/news/6/7/1 ). They claim that their beam of microleptons, highlights areas underlain by hydrocarbon deposits, when used to illuminate satellite images. They contend that oil generates vast amounts of microleptons that produce subtle effects on such images, but they can only be detected by microlepton beams TIEL intends to deploy a hand-held microlepton detector from an aircraft overflying areas that they claim have given “tell-tale” signatures using their instrument. In this respect, they are one up on particle physicists, who have so-far failed to detect microleptons under laboratory conditions. The smallest known lepton is the electron that is 1000 times more massive than the microleptons claimed by TIEL at the base of their leading-edge technology. Despite that, it is hardly likely to have escaped discovery by the best-financed branch of science.
Robin Marshall, a particle physicist at Manchester University, discovered that microlepton technology is based on a paper published by a Russian physicist called Anatoly Okhatrin in the journal Doklady in 1989. “He was clearly either mad, drunk or deluded,” says Marshall. “He spun a cone of lead weighing several kilograms in front of a pin-hole camera and claimed to have photographed a ‘glow’ surrounding the cone that was due to microleptons.” Enough said? No. One of TIELs targets is in Charnwood Forest in Leicestershire, well-known to geologists for not being above an oil-prone basin. Indeed the area is underlain by Neoproterozoic volcanic rocks that bolster the Midland craton of central England, which thwarted extensional basin formation from the Silurian to modern times. Still, an onshore exploration licence is a handy item for a company’s CV.
TIEL is not the only outfit making these claims. Another, Alkor International, seems to have a Russian link, and its website (http://www.alkorinternational.com/ ) gives details of the method it uses; “special” photographic processes, computers and software, and is also claimed to locate water resources and gold deposits!
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Seismic tomography and the African superplume
Analysis of travel paths taken by many S waves that travelled beneath the African continent, largely by geophysicists at the California Institute of Technology, shows that beneath it is a large zone of anomalously low wave speeds. Part of the zone dips down obliquely from the rough location at shallow depths of the Afar plume beneath Ethiopia/Yemen to the core-mantle boundary between the surface locations of Africa and South America. The structure is well placed for seismic tomography, by virtue of its good match with useful earthquakes and the world-wide network of seismometers. More advanced analysis (Ni, S et al. 2002. Sharp sides to the African superplume. Science, v. 296, p. 1850-1862) shows up a strangely sharp-sided part of the plume that rises from the core-mantle boundary for about 1500 km below southern Africa. There its boundary with more normal mantle is little more than 50 km wide. Modelling suggests that the upward flow has caught up a dense layer with possibly different chemistry, which would result in a tilt towards the direction of movement so that instead of rising vertically, the plume would have an oblique trajectory. The tilt also fits with Africa’s north-eastwards drift (in an absolute frame of reference, relative to other hotspots) since 100 Ma ago.
Whatever its origin, a rising, hot mantle zone beneath Africa is consistent with the continent’s high overall topography, which has encouraged the lithosphere to rift. This extension has resulted in the East African Rift, which further encouraged partial melting in the underlying mantle and the resulting volcanism. By far the most important aspect of Africa’s recent volcanic activity has been the Eocene to Oligocene flood-basalt event of the Ethiopian Plateau and the current activity in the Afar part of the Rift.
Subduction metamorphism and earthquakes
The recently commissioned Hi-net array of 600 digital seismometers in Japan paid dividends in an unexpected way during 2001, by picking up long-lived vibrations rather than discrete seismic events Obara, K. 2002. Nonvolcanic seep tremor associated with subduction in southwest Japan. Science, v. 296, p. 1679-1681). The tremors occurred in a part of Japan where there are no active volcanoes, with which protracted vibrations are usually associated. Their epicentres define a clear zone, at about the depth of the Moho and on the Wadati-Benioff zone where the Philippine Plate is being subducted. This region is where dehydration reactions that convert cold, wet oceanic crust to dense eclogite, the driving force for plate tectonics through slab pull, are predicted to occur by thermodynamics. Kazushige Obara, of Japan’s National Research Institute for Earth Science and Disaster Prevention, suggests that this correlation might fit with the release and rise of hydrothermal fluids released by dehydration of the slab. Part of his evidence is that such tremors seem not to occur where the much older (and therefore cooler) Pacific Plate is being subducted beneath the northwest of Japan. It probably does not undergo such reactions until it has reached about 100 km depth, where temperature would be sufficient to enter the field of eclogite stability. Detecting fluid motion at 3 times the depth of that beneath southwest Japan might emerge with more specialized procesing.
See also: Julian, B. 2002. Seismological detection of slab metamorphism. Science, v. 296, p. 1625-1626.
Continental roots
Crustal shortening and thickening in collisional orogeny produces mountain belts with a root of crust beneath them. This truism is central to isostasy, where the mass of uplifted mountains is balanced by a compensating mass of low-density root material beneath that penetrates the mantle lithosphere. The classic story of the reduction of mountain belts to a peneplain involves continuous isostatic uplift as the topography is eroded away. Finally, no root remains and the exposed rocks reflect in their high-grade metamorphism a steady upward passage from the root. Later cover rests with profound unconformity upon this peneplain. Yet this essentially simple theory does not hold in many cases, especially for older collisional orogens. As Karen Fischer of Brown University, USA has shown (Fischer, K.M. 2002. Waning buoyancy in the crustal roots of old mountains. Nature, v. 417, p. 933-936), there is a crude correlation between the age of orogens and their ratio of elevation to root thickness. The ratio decreases from around 0.15 (root about 7 times thicker than surface elevation) in active orogens to zero before 1 Ga ago, when peneplained orogens still have a substantial root.
In order for this to happen, either the roots’ buoyancy must somehow decline with age or the mantle lithosphere which it penetrates becomes too rigid to allow isostatic uplift to occur. Resolving which has most effect depends on analysing the gravity anomalies above orogens. It is no easy task to model the two processes, and this is what Fischer has achieved. She finds that mantle viscosity is not responsible, and that the cause is variation in root density. This is probably a result of slow decline in heat flow, and the resulting mineralogical equilibria in the root. For mafic granulite roots, a change from heat flow values of 70mWm-2 to around 40 mWm-2 could increase their density by 100-150 kg m-3, by an increase in the proportion of garnet, perhaps to the extent of producing eclogites at the deepest levels. Eclogites would be seismically very similar to mantle lithosphere, so that even thicker, hidden roots may be present. Reduction in buoyancy by this means could take as little as 20 Ma, before which the elevation to root thickness ratio has declined below that in active orogens.
One implication of this process is that orogenic collapse by lateral extension of highly elevated crust, which might lead to rapid root thinning, is not the general process that many structural geologists believe. If it was, orogenic roots would be removed relatively quickly. Decrease in root buoyancy is also a plausible explanation for the creation of cratons, where quite low-grade metamorphic rocks, formed at shallow crustal levels occupy vast areas of low-lying shields.
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Flood basalts of Siberian Traps doubled at a stroke
Erupted at the time of the Palaeozoic-Mesozoic boundary, and coinciding with the largest mass extinction during the Phanerozoic, the Siberian Traps are by far the biggest example of flood-basalt volcanism known. They blanket a huge area of the Siberian Platform. To the east of their outcrops is a large extensional downwarp, known as the West Siberian Basin, where recent deep drilling has cut through up to 1 km of flood basalts. Dating samples from 15 boreholes proves that these too are members of the Siberian Trap suite (Reichow, M.K. et al. 2002. 40Ar/39Ar dates from the West Siberian Basin: Siberian flood basalt province doubled. Science, v. 296, p. 1846-1849). Combined, the two zones of Siberian Traps represent eruption of around 2.3 million km3 of plume-derived magma at around 250 Ma ago, possibly within 2 or 3 Ma. Gas release from such a stupendous event is implicated in the Permian-Triassic mass extinction, either through climate change associated with CO2 and SO2, or toxic effects of hydrofluoric acid. Unlike the end-Triassic and K-T extinctions, no clear evidence has emerged for coincident flood volcanism and major impact at the end of the Palaeozoic Era. However, the use of tungsten isotopes as “fingerprints” for extraterrestrial debris in boundary sediments may help resolve the issue of whether an impact accompanied the Siberian Traps (see Tungsten and Archaean heavy bombardment, this issue)
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Early Argentines did not witness a meteorite impact
Ten years ago, planetary scientist Peter Schultz and Argentine pilot Ruben Lianza observed several depressions shaped like tear drops while flying over the Pampa. Because they also found meteorites and tektite glass when they examined the structures on the ground, it seemed certain that the depressions had formed by the impact of bodies travelling almost parallel to the Earth’s surface. The structures were clearly no more than a few thousand years old, and the discovery encouraged lurid artistic impressions of terrified native South Americans cowering from an extraterrestrial firestorm. The Rio Cuarto structures were a godsend for those who fear social and economic disaster from Earth-bound NEOs (near-Earth objects), and have been lobbying for a sky watch for impending doom.
In reality, the Pampas of northern Argentina has hundreds of similar structures over an area of more than 50 thousand square kilometres, and their long axes parallel the prevailing wind direction (Bland, P.A. and 10 others 2002. A possible tektite strewn field in the Argentinian Pampa. Science, v. 296, p, 1109-1111). They are “blow-outs” developed in the fine loess soils of the Pampa, and much the same structures affect most loess plains. Being formed of wind-blown silica and clay dust, loess is not well known for its content of objects above a millimetre in size, so any larger objects found on wind-deposited plains stand a high chance of having arrived by some extraterrestrial process. Meteorites and tektites are rare, but ablation concentrates them in wind-blown depressions as they are too heavy to be blown away. That is the likely origin of the objects that Schultz and Lianza used in support of their hypothesis of impact devastation wrought on early South Americans. Phil Bland of the Open University, and his colleagues from Brazil, the USA, Australia, Russia, Argentine and Britain, were able to date organic matter in the Rio Cuarto structures using the C-14 method at 4000 years. Yet Ar-Ar ages of the meteorites range from 52 to 36 thousand years, so the two are unconnected. The glassy tektite fragments provided yet another age of 57 thousand years. Along with similar glasses at a couple of other sites in Argentina, these support melting of the homogeneous loess by an impact around that time, although no crater from which they might have been ejected is known. The search is on for the source of a hitherto unknown field of strewn tektites, although it seems strange that in the featureless plains of southern South America one hasn’t shown up long before now.
The mantle’s breath and Earth’s early evolution
Many lavas contain bubbles, which form when gases dissolved under pressure in magma froth out at low pressures. For the most part the gas is water vapour, carbon dioxide and sulphur dioxide. It comes from mantle peridotite, and represents the volatile fraction of the deep Earth. But there are traces of other gases, the most revealing of which are the noble gases helium, neon, argon, krypton and xenon, because some of their isotopes originate from radioactive decay of other elements (mainly potassium, uranium and thorium. Noble gases in basalts offer important insights into how the mantle has evolved since the origin of the Earth. Chris Ballentine of the University of Manchester, reviews how such trace-gas isotopes in basalts help resolve some otherwise intangible challenges (Ballentine, C.J. 2002. Tiny tracers tell tall tales. Science, v. 296, p. 1247-1248).
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Forensic geochemistry to foil “fencing” of conflict diamonds
The longest and most devastating wars in history have centred rather more on economic interests than nationalism or chivalrous defence of principles, and in some case a specific commodity created an issue that annexation served to resolve. For instance, the 1914-18 war was not unconnected with the vast iron ore reserves of Alsace-Lorraine. Similarly, the Nigerian civil war of the late 1960s was bound up with the oil reserves of the Niger delta, and that of Congo centred on base-metal resources of the Copper Belt, particularly the fact that vast strategic reserves of cobalt occur in its Congolese sector. The running sores of present conflicts in Africa – Angola, Congo, Sierra Leone, Liberia – are about and financed by gems that adorn the rich, the self-regarding and the lazy. These diamond wars are a direct concern of geologists, for who else finds the elusive kimberlites and traces the natural dispersion of the diamonds that they contain?
More than 30 years on from the start of gem-related carnage in Africa, in which dealers and giant mining corporations have been implicated up to their collective eyebrows, local people have been drawn into “illicit” diamond mining when their livelihoods have been destroyed by perpetual danger and insecurity. Preyed on by many so-called “rebel” groups, even kids as young as 8 or 9 have been armed and set upon one another and the inhabitants of regions blighted by the presence of what is no more than an allotrope of carbon. Eugenie Samuel writes on a possible means of defining the source of diamonds “fenced” by the gem trade from on-going conflict zones (Samuel, E. 2002. Diamond wars. New Scientist, 25 May 2002, p. 6-7). It seems that ultra-thin coatings on rough diamonds carry a geochemical signature from the chemically diverse kimberlites and other unusual mafic rocks that carry them from the mantle. Given research on rough stones from every kimberlite province it should be possible for this forensic approach to help stamp out what is the world’s largest blood trade.
The problems are many. For a start, trade in “conflict diamonds” is now illegal, so it is unlikely that rough stones used to calibrate the technique would be given a bona fide provenance by dealers. It would be a courageous geochemist who went sampling in interior Congo, Angola, Liberia or Sierra Leone. The method clearly requires funds, yet the obvious source, diamond mining and trading companies, are engaged in their own tagging schemes that use using ion beams to bar-code their products on a minute scale. In fact this tagging method was developed under great secrecy to distinguish from the “real” thing perfect artificial diamond gems synthesized by Russian geochemists. Finding diamonds requires considerable exploration, which involves systematic sampling of sediments along streams that drain likely kimberlite-bearing ground. Although high-quality rough found by geologists would be sold, there must be small diamonds archived from such sampling by mining companies and geological surveys. They could be supplied to forensic geochemists to calibrate the method. In Sierra Leone, for instance, the diamond fields were located in the early 1950s by geologists of the then Overseas Geological Survey – part of what became the modern British Geological Survey. Belgian and Portuguese equivalents may well have archival material from Congo and Angola.
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Nut-cracking chimps provide clues to the origin of tools
Ethoarchaeology attempts to use observation or experimental approaches to animal behaviour to shed light on features of fossil occurrences that relate to human origin. One example is examining the gnaw marks on bones in the dens of predators to check if they match similar signs on the bones of early hominids. Another is knapping flints to see if the flakes or debris produced match finds of broken fragments at sites with no clear sign of early-human involvement. Chimps use lumps of stone to break nuts on wooden anvils, and so provide natural subjects to probe what early hominids may have been up to. Anthropologists from George Washington University in the USA and the Max Planck Institute for Evolutionary Anthropology in Germany have painstakingly excavated the debris from a nut-cracking site beneath a large tree “traditionally” used as a source of nut protein by Ivory Coast chimps (Mercader, J. et al. 2002. Excavation of a chimpanzee stone tool site in the African rainforest. Science, v. 296, p. 1542-1455).
Broken fragments inadvertently created by the chimpanzee troupe do resemble the earliest Oldowan tools, which appear in the fossil record at around 2.5 Ma. The chimps can be shown to have brought hammer stones from several rock outcrops. However, any old rock serves their purpose and there is no sign of deliberate selection, unlike the makers of Oldowan tools, who clearly selected rocks that break to give sharp edges from outcrops up to several kilometres from the fossil sites. The first Oldowan tools demonstrate that they are the end product of what was probably a progression from accidental stone breakage. The way in which broken fragments from patterns around chimps favourite anvils for nut cracking should help identify earlier assemblages in the steps towards proper tool making. With luck, they may relate to fossils of the actual beings who were involved. The 2.5 Ma Oldowan tools from Ethiopia have yet to be linked to a hominid species. The earliest direct link between tools and their makers is the association of Oldowan artefacts with remains of Homo habilis about 2 Ma ago.
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Serpentine: the Vaseline of subduction
Although they are seismically precarious, the major coastal cities of the Americas and East Asia that lie close to destructive plate margins probably owe their survival to a greasy assemblage of hydrated ultramafic minerals – serpentine, talc and magnesium hydroxide (brucite). Detailed tomographic images using the records of natural earthquakes along the subduction zone beneath western North America show a zone of exceptionally reduced S-wave speeds at the “corner” formed by the subducted slab and the base of the crust (Bostock, M.G. et al. 2002. Inverted continental Moho and serpentinization of the forearc mantle. Nature, v. 417, p. 536-538). This low-speed zone coincides with the fore-arc region of the destructive margin, roughly along the coast. Normally the Moho marks a sudden increase in wave speed in the mantle underlying the crust, but here the situation is reversed (inverted). The best explanation is that S-wave speed slows because of an abundance of weak rock, between 35 and 60 km down. The likely candidate is mantle peridotite that has become hydrated by fluids seeping upwards from cold, wet oceanic lithosphere as it begins to be subducted. Low-temperature, high-pressure metamorphism of hydrothermally altered basaltic crust begins to transform it to anhydrous eclogite, so releasing masses of water vapour. It is this fluid release that is implicated in the generation of magmas beneath volcanic arcs, because it reduces the beginning-of-melting temperature in the overriding mantle wedge. However, such partial melting is possible only when temperature is high. In the cooler, shallow regions of the fore arc rising watery fluids serve to convert peridotite to hydrous minerals, especially serpentine. One outcome is the creation of anomalously low-density mantle, which bulges upwards to create fore-arc ridges at some destructive margins, even squirting serpentinite upwards in bizarre mud volcanoes. Yet all hydrated, ultramafic minerals are natural lubricants, and would act to ease sudden rupture along the subduction zone, thereby preventing extremely high-magnitude earthquakes whose surface effects would be devastating.
See also: Zandt, G. 2002. The slippery slope. Nature, v. 417, p. 497-498
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Rise of the dinosaurs after the Tr-J event
Whatever happened at the Triassic-Jurassic boundary (around 200 Ma ago), the palaeontological shifts then coincided with eruptions of flood basalts of the Central Atlantic Province and the start of Atlantic opening (see And now, the Tr-J boundary, Earth Pages May 2002). Although questioned as a mass extinction event, the boundary contains extremely high proportions of fern spores, that may signify the land being cloaked by rapidly spreading ferns after it had been wiped clean of other vegetation. New evidence suggesting the influence of an impact at the time emerges from a geochemical study of the fern-rich boundary layer (Olsen, P.E. and 9 others 2002. Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary. Science, v. 296, p. 1305-1307), which revealed anomalously high levels of iridium. High iridium is only one pointer to possible extraterrestrial influences, and the clinching factor of shocked mineral grains has yet to be shown convincingly.
The novel feature of the paper by Paul Olsen of the Lamont-Doherty Earth Observatory and colleagues from the USA, Canada, Italy and Austria is how they used trace fossils to reach a remarkable conclusion. They combed eastern US terrestrial sediments either side of the boundary for reptilian foot prints. They tracked time using evidence for climate change paced by Milankovich cycles. Their records of 10 thousand sets of tracks show a decline in non-dinosaur footprints, and a jump in the proportion left by dinosaurs from 20 to 50% of the total, as the boundary is crossed. Those of some Triassic reptiles that had survived for 20 Ma end abruptly at the boundary. It seems that, whatever the boundary event was, early dinosaurs were able to adapt to change better than evolutionarily more primitive reptiles, so that they could speciate rapidly when their Triassic companions bit the dust. Dinosaur evolution seems to have been similar to that of the mammalian adaptive radiation that followed the K-T extinction event.
Gigantic claims for “geogenomics”
Fossils and their stratigraphic ages no longer offer the only clues to biological evolution, now that is possible to judge the degree of relatedness between living organisms from sequences of genes and proteins that their cells contain. The molecularly inferred family trees of modern animals, plants and micro-organisms help scientists to visualize the relative antiquities of the sharing of a common ancestor by different pairs of a living group. By assuming constant rates for genetic mutation and protein evolution, some palaeobiologists have asserted that they are able to assign absolute ages to evolutionary divergences. If that were so, then it would be possible to correlate evolutionary milestones with transformations brought on by geological and climatic upheavals, and also with other past changes in the biosphere. Good examples would be linking fossil and genetic changes in ruminant mammals to the rise of grasses, or the rise and divergence of corals following the end-Permian mass extinction. The inter-linkage between palaeontology and genomics is in its infancy. That it promises a great deal by way of insights, as well as possible bloomers, is nicely brought out by a recent review (Benner, S.A. et al. 2002. Planetary biology – paleontological, geological and molecular histories of life. Science, v. 296, p. 864-868). Whether charting the “planetary proteome” will become “a civilization-wide enterprise”, as Steven Benner and his colleagues predict, is something that I would not care to comment on during the 2002 World Cup. As Bill Shankly once observed, some things are far more important than matters of life and death.
Too much iron, too little phosphorus delayed an oxygen-rich atmosphere
The age of the earliest blue-green bacteria hinges on the imagination of some palaeobiologists and how well they can focus a microscope (Doubt cast on earliest bacterial fossils, Earth Pages April 2002). Without doubt, it was blue-greens that first began breaking the chemical equilibrium of water to release free oxygen to the environment, yet it was some 2½ billion years after the Earth had formed that atmospheric oxygen had a tangible effect on the Earth’s bare surface. In rocks around 2.2 to 2.0 Ga old geologists find the first evidence for that in soils that are rich in oxidized Fe-3. For iron to lose an electron and change from soluble Fe-2 to Fe-3, whose oxides and hydroxides are highly insoluble, demands the abundant presence of an electron acceptor, or oxidizing agent. The most likely of these in the atmosphere and hydrosphere is oxygen. However, there are sedimentary rocks that form vast repositories of Fe-3 and oxygen that predate the first well-accepted oxygen-rich atmosphere. They are known as banded iron formations or BIFs, whose minuscule layering seems to signify that they formed as precipitates from water, when dissolved Fe-2 met a source of oxygen to produce hematite – Fe2O3 – and goethite – Fe(OH)3 BIFs signify deep ocean water devoid of oxygen, to enable soluble Fe-2 to circulate abundantly, yet a sizeable supply of oxygen where they were precipitated. Since only organic photosynthesis is capable of breaking the powerful bond in water, some kind of photosynthetic bacteria are implicated in the formation of BIFs. Whether or not palaeobiologists and geochemists can demonstrate evidence for the first appearance of such bacteria, BIFs more or less prove their existence, in the absence of any other plausible means of formation.
Until recently, the huge delay in the Earth’s surface environments becoming oxygenated has been ascribed to the mopping up of any biogenic oxygen by its reaction with a vast excess of dissolved Fe-2. However, once blue-green bacteria evolved photosynthesis, their chemical trick of splitting water molecules to provide hydrogen for processes at the cell level should have meant that they would have spread like wildfire across the ocean surface. In that respect they are unique among bacteria, most of which exploit very narrow ecological niches. Oxygen should have quickly come to dominate both oceans and atmosphere. That is, unless there was some check on the living ocean biomass. It turns out that BIFs may contain the answer, for they are rich in phosphates, adsorbed onto the surfaces of their iron minerals (Bjerrum, C.J. and Canfield, D.E. 2002. Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides. Nature, v. 417, p. 159-162). Phosphorus is vital in any organism, being an essential component of nucleic acids and phospoholipids. By working out the partition coefficient between water and iron oxide, and estimating the production rate of BIFs before 1.8 Ga when their production ceased, Bjerrum and Canfield conclude that phosphorus was an order of magnitude less abundant in sea water until then. Such a deficiency in a vital nutrient would have limited the scope of blue-greens, and the rate at which they produced oxygen.
Just why the Fe-P checks and balances on oxygen production collapsed around 2.2 to1.8 Ga is something of a mystery. One possibility is that the iron concentration in sea water fell, perhaps as sea-floor spreading waned from its high early rates; basalt magma provides the main input of iron through ocean-floor hydrothermal activity. Less production of BIFs would leave more phosphorus in solution, helping greater biological productivity, whose oxygen output would eventually remove soluble iron from sea water.
See also Hayes, J.M. 2002. A lowdown on oxygen. Nature, v. 417, p. 127-128.
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Glacial floods and climate change
One of the fundamental discoveries about climate change during the Plio-Pleistocene ice ages is how many climate fluctuations with periods too short to be ascribed to astronomical forcing link to shifts in deep-ocean circulation. In the case of the North Atlantic Ocean, if high-latitude seas become diluted by fresh water cold dense brines are less able to form. It is their sinking as a residue from the formation of sea ice that helps drive the “ocean conveyor” and draws warmer water into the Arctic from the tropics. If they do not form, then the conveyor shuts down and high-latitudes cool. The most spectacular of these ocean-driven events was the Younger Dryas cooling from about 12.9 to 11.6 ka, and it may well have occurred because of the sudden drainage of a giant lake of glacial meltwater down the St Lawrence Seaway to dilute the North Atlantic. The waning of every major ice sheet covering North America would have generated vast amounts of freshwater, and because repeated glaciation created basins by erosion and sagging of the low-relief surface, drainage of such lakes would have been characteristic of every transition to interglacial warmth. Steven Colman of the US Geological Survey reviews recent attempts to model how flooding may have escaped from the ice-sheet margins (Colman, S.M. 2002. A fresh look at glacial floods. Science, v. 296, p. 1251-1252).
The Hadean was cool
James Hutton’s observation that the geological history of Scotland had “no vestige of a beginning” applies everywhere, for no-one has dated rocks that are older than about 4.0 billion years (Ga) old, despite a great deal of effort. It seems that continental crust only became capable of remaining at the surface in large volumes almost 600 Ma after the Earth formed from the Solar nebula. Indirect isotopic evidence and dating of meteorites do indicate that the Earth accreted from dust and planetesimals about 4.56 Ga ago. There are terrestrial materials that break the 4 Ga barrier, but they are so few and so tiny that they could be lost with one powerful sneeze. These are crystals of the highly resistant mineral zircon, found as detrital grains in mid-Archaean sandstones in Western Australia. The oldest of these is a single grain dated at 4.404 Ga. All of them formed in igneous rocks produced by partial melting of the mantle, which concentrates zirconium in magma. Following their liberation to sedimentary processes by weathering, the zircons have probably been through several sedimentary cycles since the formed. So the pre-Archaean history of our world has left relics, but they are minuscule. Because of the absence of pre-4Ga crust, that period was probably turbulent, partly through rapid convective turnover of the mantle and higher degrees of melting because of higher heat production, and partly due to far more large impacts that the lunar surface shows during those times. Dating of lunar cratering and impact glasses suggests that bombardment reached a crescendo around 4.0 to 3.9 Ga. It is now fairly certain that the Moon formed from incandescent material ejected from the Earth when it collided with a Mars-sized planet around 4.45 Ga. Earth and its companion would, in that likely scenario, have begun their geological evolution completely molten in the case of the Moon and with a deep magma ocean on Earth. “Hellish” is a barely adequate adjective for such conditions, and the period before 4 Ga has been termed the Hadean. A vital question concerns when such extreme conditions waned to become potentially supportive of biochemistry and the origin of life.
Minute as they are, the pre-4.0 Ga zircons provide useful oxygen-isotope data, and their d18O is no different from that of more common zircons throughout the Archaean Aeon. The explanation for this is that the mantle and the magmas produced from it contained an H2O phase. Either the mantle has always had a water content – no surprise as it still does – or the magmas from which the zircons crystallized encountered near-surface water vapour, possibly as a result of hydrothermal exchange with a hydrosphere. Reviewing these data, John Valley and colleagues from the University of Wisconsin USA and Curtin University Australia pursue the second conjecture (Valley, J.W. et al. 2002. A cool early Earth. Geology, v. 30, p. 351-354), and argue for a surface temperature below the boiling point of water since 4.4 Ga, only 50 Ma years after geochemical “year zero”. The crux of their argument is that the high d18O values of four Hadean zircons indicate their equilibration with water vapour at temperatures below water’s critical point (374°C). If crystallization at depth was below that temperature, then the Earth would have had surface oceans. But is this such a surprising conclusion? Loss of heat by radiation being proportional to the fourth power of absolute temperature, an incandescent Earth’s surface at the time of Moon formation would have cooled below 100°C well within 50 Ma, unless it was blanketed by an opaque atmosphere. Impacts of the size of those which produced the lunar maria around 4.0-3.9 Ga could have boiled away any surface water from time to time, only for the surface to cool quickly once again. Conditions for bio-geochemistry could well have been present throughout the Hadean. The significance of that for the origin of life is hard to judge, because large impacts and ocean boiling would have extinguished any progress, so that the process may have had to restart again and again.
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The etymology of the Gaia hypothesis
Amid the desperate search for classical names to lend weight to the study of asteroids palaeotectonic features, and even theories of the Earth system, there has been one particularly unfortunate choice.
Gaia (Earth) emerged from Chaos, the great void of emptiness within the universe. She gave birth to Uranus (Sky), apparently by some form of parthenogenesis. Their incestuous coupling produced the 3 Cyclopes, 3 Hecatoncheires and the 12 Titans. Uranus was a bad father and husband. He particularly hated the Hecatoncheires (they had 100 arms and 50 heads each), and stuck them deep within Gaia’s womb causing her to plot against him. To rid herself of Uranus she begged her children to kill him. All refused apart from the youngest child, Chronos (Time and the father of Zeus). Gaia made Chronos a flint sickle, which he used to castrate his father and threw his testicles into the sea. From the spilt blood came the Giants, the Ash Tree Nymphs and the Erinnyes. When Uranus’ severed genitalia landed in the sea, foam bubbled around them. From this foam sprang Aphrodite (meaning foam-born), Goddess of Love.
This doesn’t quite tally with the eponymous hypothesis, but seems to have a more realistic ring for what we know about Earth history.
Source: http://www.csc.liv.ac.uk/~u9dam/myth/immortals/
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Mantle motions from seismic tomography
Variations in the density and rigidity of the mantle induce changes in the speed at which seismic waves move through it. Mapping mantle regions with slowed and faster waves in three-dimensions is the basis for assessing temperature anomalies within the deep Earth. It has been such tomography that has begun to test ideas about the depth from which mantle plumes rise and the fate of subducted slabs of oceanic lithosphere, and an increasingly certain model for mantle motions has evolved with improvements in the resolution of seismic analyses. However, the P and S waves used in tomography have other properties than simply speed. These include direction, polarization, signs of conversion of P to S waves, and even interference properties for which the birefringence observed in petrography is an analogue.
Analysing these properties reveals that there are deviations in the structure of the minerals that make up mantle rocks from random arrangement; there are anisotropies (Park, J and Levin, V. Seismic Anisotropy: tracing plate dynamics in the mantle. Science, v. 296, p. 485-489). Deformation lines up minerals in such a way that the bulk rock structure affects the propagation of seismic waves in different directions – again, the way in which crystallographic anisotropy of minerals affects light passing through them is a means of visualizing what happens on vastly larger scales. In their review, Park and Lewin describe how this novel approach is revealing aspects of convection in the upper mantle, how lithospheric plates have formed and features spatially related to accretionary boundaries in continents.
Field studies of ophiolites have shown that the dominant olivines of mantle peridotite are commonly aligned, probably as convection dragged it at right angles to the axes of lithospheric spreading. Indeed, seismic anisotropy confirms that view with trends normal to the mid-Atlantic, Pacific and Indian Ocean spreading centres. Destructive margins show two trends, those parallel to trenches and those in the direction of subduction, but there are complex variations depending on depth. Once resolved into indicators of past motions, that complexity may tell volcanologists a lot about large-scale variations in magmatism. The Hawaiian hot spot has associated vertical anisotropy, that is consistent with a disturbance of the overall flow of shallow mantle. Several ancient orogens in continents, dating back to the Precambrian, show anisotropy in the mantle beneath them, often parallel to the orogenic trends, but occasionally more complex. Clearly, this use of natural earthquake signals has a lot to contribute, but depends on much more complex computations than “conventional” tomography and awaits the wider distribution of software and powerful hardware.
The latest significant development from tomography based on detection of wave-speed anomalies relates to the Earth’s two major mantle plumes, beneath Africa and the Pacific Ocean (Romanowicz, B. and Gung, Y. 2002. Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux. Science, v. 296, p. 513-516). Both apparently persist through the transition zone of mantle wave speeds at 670 km below the surface, to become deflected laterally beneath the lithosphere. They may well be supplying heat to the asthenosphere that could find its way to spreading ridge systems. The lowering of viscosity in the asthenosphere as a result of this heat originally from the core-mantle boundary (some of it may be heat lost by the core) would act as a lubricant for plate motions. In particular, it could enhance the influence of slab-pull force at subduction zones, such as those around the Pacific, thereby speeding up tectonics. The mantle beneath the African lithosphere has probably been heated. The huge topographic and gravitational anomaly generated by massive flood basalt eruptions in Kenya and Ethiopia may more easily have been able to convert the resulting extensional stresses into extensional deformation, thereby driving the East African Rift system above a zone of thermal lubrication. Far more gravitationally unstable lithosphere beneath young orogens does undergo lateral collapse, but the lack of associated plumes makes it impossible for the entire lithosphere to fail through lack of such lubrication. And when superplumes eventually wane, as perhaps have those beneath Iceland and western North America, that too would influence both plate tectonics and that on more local scales by increasing viscous drag in the asthenosphere.
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A basaltic meteorite, but from where?
The vast majority of meteorites represent bodies in the Solar System that never became parts of planets; they are fragments of planetesimals. Of the 20,000 collected meteorites, only about 50 have been suggested from their geochemistry to hail from existing planetary bodies. They travelled to Earth as fragments that violent impacts on these bodies ejected from their surfaces. Since most meteorites have been recovered either from glacial ice or the surface of deserts, such suspected planetary fragments arrived recently in geological time, but had probably been travelling for immense periods of time since an impact dislodged them. Oddly, there are few if any meteorites with Earthly compositions, and only the Moon and Mars seem to be represented in collections. Suspected planetary meteorites have basaltic compositions, but so too do some likely to have originated from planetesimals. One of the keys to sorting them is analysis of their oxygen isotopes, as well as conventional element analyses and noble-gas composition. It was the resemblance of noble gases in the notorious Antarctic meteorite ALH84001, and others like it, to the very imprecise measurements made by the Viking lander in the 1970s that encouraged the view that it was from Mars. Their odd oxygen-isotope composition has also been said to indicate a Martian origin, mainly because they don’t fit with other specimens most likely to have originated from planetesimals.
In these uncertain times for manned and unmanned space missions, basaltic meteorites are probably as close as planetary scientists will ever get to the objects of their longing, perhaps for several generations. It is hardly surprising that collectors seize on petrogenetically evolved meteorites with glee. Such a desirable chunk from a desert surface in NW Africa has been analysed comprehensively by scientists from Japan and the USA (Yamaguchi, A. et al. 2002. A new source of basaltic meteorites inferred from Northwest Africa 011. Science, v. 296, p. 334-336). Its chemistry fits with no planetesimal or suspected planetary meteorite class, although for the most part it does resemble the eucrites, considered to originate from the large asteroid Vesta. Rare-earth elements, siderophile metals and oxygen isotopes put it in a class of its own. Although the authors are content to conclude that it probably evidences a range of planetesimals that underwent differentiation to produce basaltic magmas, some have been tempted to speculate on a planetary origin, perhaps on Mercury (Palme, H. 2002. A new Solar System basalt. Science, v, 296, p. 271-273). I am left wondering why the supposed Martian meteorite class, with all the kudos that such a suggested origin brings, has not been tempered by the likelihood of origin in a large planetesimal; but I am no specialist.
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