About 2.3 billion years ago, ancient soils begin to reveal that Earth, or more precisely life upon it had developed an atmosphere that contained oxygen, albeit at quite low levels. One of the most interesting events during the Proterozoic Aeon was the world-wide disappearance of vast deposits of iron oxides known as banded iron formations or BIFs, at about 1.8 billion years. Many authorities view that as the time when sufficient oxygen was dissolved in seawater to have removed soluble Fe-2 at its source, on the ocean floor near hydrothermal vents – BIFs formed in shallow water, and that requires Fe-2 to have permeated the entire oceans. There is another possibility. The presence of atmospheric oxygen would have ensured the oxidation of iron sulphide exposed at the land surface, thereby adding sulphate ions to river water, and eventually seawater. Another line of evidence for atmospheric oxygen is the disappearance of detrital sulphide grains from sedimentary rocks younger than 2.3 billion years, so a build-up of sulphate ions in later seawater is quite plausible. Should deep-ocean chemistry have been reducing, it is possible that sulphide ions would form there. The insolubility of iron sulphides would then remove Fe-2 from seawater equally as efficiently as would oxygen. Danish and Canadian geochemists have investigated this possibility using data from sediments in Canada that mark the last phase of major BIF deposition around 1.8 billion years (Poulton, S.W. et al. 2004. The transition to a sulphidic ocean ~1.84 billion years ago. Nature, v. 431, p. 173-177). They found that conditions changed from one in which seawater contained dissolved Fe-2 at the time of the last BIF deposition to one dominated by sulphide ions, similar to that found in modern anoxic waters such as those in the Black Sea. That would have sequestered any available Fe-2 to pyrite in sediments, a feature typical of many later Proterozoic sediments. Since seawater during the Phanerozoic was dominated by sulphate ions, except in periods of ocean anoxia, it looks likely that late Precambrian sulphidic oceans gave way to more modern sulphur chemistry following a rapid rise in atmospheric oxygen at the end of the Proterozoic. One consequence of highly-reducing deep ocean water would have been very efficient burial of dead organic matter while it lasted, because anaerobic bacteria do not fully convert organic molecules back to water and carbon dioxide. During the Neoproterozoic d13C in seawater underwent rapid swings from highly negative to highly positive, on which all kinds of connotations have been placed. Another explanation for the carbon hiccups might be that periodically there were short-lived increases in oxygenation of deep ocean water.
How often did it rain?
Geoscientists have become used to masses of climate data, often with better than 50 years resolution, from cores through ice sheets and sea-floor sediments. But all of it is from some kind of proxy; oxygen isotopes for air temperature and land-ice volume, methane for humidity, dust for windiness, and so forth. One aspect of both climate and the British obsession with weather is raininess, for which there is scant evidence. How many rainy days occur in a British summer is interesting, but for studies of past climate evidence for the onset or disappearance of seasonality, and the annual intensity and duration of rainfall would be invaluable, if it could be had. A piece of ingenious research shows that it is possible (Kano, A. et al. 2004. High-resolution records of rainfall events from clay bands in tufa. Geology, v. 32, p. 793-796). Akihiro Kano and Japanese colleagues studied the well-known layering of tufa – carbonate veneers laid down in freshwater that has high dissolved bicarbonate and calcium ions. In “hard-water” areas tufa can be deposited very quickly, at rates above a few millimetres per year, and it tends to be preserved, being quite tough. So tufas have the potential for preserving annual records of various fluctuations. Kano and colleagues saw that colour laminations represented clays deposited in the tufa when the water was turbid after prolonged rainfall. To record the variations they simply measured fluorescent X-rays emitted by silicon when slices of tufa were examined in an electron microprobe – silicon is present in clays and silt, but not in carbonate minerals. Because they used tufa deposited in recent times (1988-2002) they were able to correlate variations in clay content with detailed weather records from the site, thereby calibrating their method. The match was very good and followed rainfall closely at the level of a few days. Of 112 high rainfall days in the abnormally wet year of 1993, 100 showed up in the clay record. So, tufas are potentially more revealing than even the annual growth rings in wood, and some tufa deposits preserve long records.
Details of the last interglacial climate
Worries about how anthropogenic warming will affect the course of the Holocene interglacial in which we live might be tempered or exacerbated by knowing what went on during the previous, Eemian interglacial that ended about 120 ka ago. Data from cores through the Greenland and Antarctic ice sheets have been both ambiguous and plagued by resolution that does not show enough detail, but a core from a new position in Greenland seems to resolve both problems (North Greenland Ice Core Project members 2004. High-resolution record of Northern Hemisphere climate extending into the last interglacial period. Nature, v. 431, p. 147-151). Uniquely, the NGRIP ice still preserves the annual snow layering as far back as 123 ka. This is because the site shows little sign of the deformation at deep levels that characterised previous Greenland cores. That is probably because the site lies above a zone of high heat flow through the underlying crust, so that the base of the ice has melted. Melting helps prevent internal deformation, but that in itself is a surprise because the site was chosen because it is colder and drier at the surface than other sites. The drilling objective was to penetrate older ice than the Eemian to give a fuller record than from earlier cores, yet anticipated poor time resolution. The presence of resolvable annual records from depth was both a surprise and a bonus, although the melting had removed ice from the earliest part of the last interglacial. Despite that, preliminary oxygen-isotope results from the NGRIP core suggest that the Eemian had a remarkably stable climate and one that was warmer than that of the Holocene by about 5ºC; maybe it is an analogue for climate evolution during a future, artificially warmed world. That possibility stems from the observation that around 115 ka, North Atlantic climate suddenly warmed Thereafter, interglacial conditions did not suddenly change to glacial, as happened several times during the course of the last glacial epoch, but took around five millennia after the sudden warming. The authors make no claims that their preliminary data help resolve current fears of warming collapsing to glacial conditions in a matter of years to decades. That grim scenario has been widely trumpeted both by the media and some climate scientists. There is more to the Eemian than the period after 123 ka, and who knows what the eventual annual resolution will show up? The data presented in the paper are from a coarse sampling of 55 cm that represents about 40 year intervals.
See also: Kuffey, K.M. 2004. Into an ice age. Nature, v. 431, p. 133-134
For and against “Snowball Earth”
Reputedly glaciogenic sediments in the Neoproterozoic are reckoned to represent at least three separate cold episodes, the Sturtian (~720 Ma), Marinoan (~600 Ma) and Varangerian (~580 Ma). Sadly, the diamictites that characterise these episodes are not easily dated. Only two have well-defined radiometric ages, the Gubrah Member in the Oman (713 Ma), said to be Sturtian, and the Gaskiers Formation of Newfoundland (580 Ma), a possible example of the Varangerian that is better exposed in northern Norway. The truly whopping Sturtian and Marinoan diamictites of Australia are fitted to a global stratigraphy on the basis of carbon isotope variations, as are those of Namibia on which Paul Hoffman and colleagues stake their claims to “Snowball Earth” events. Another Hoffman, native to Namibia, and geochemists at MIT, have finally given a believable age to one of the Namibian diamictites (Hoffman, K.-H. et al 2004. U-Pb zircon dates from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation. Geology, v. 32, p. 817-820). Their zircons come from a thin volcanic ash within isolated Neoproterozoic diamictites in central Namibia, and yield an age of 636±1 Ma. Correlating the studied diamictites with the Namibian sequences elsewhere in the country relies on the presence of a supposed cap carbonate rather than lateral continuity. The authors link them with the younger of the two Namibian diamictites, the Ghaub Formation, rather than the Chuos Formation that lies at depth, despite the fact that both well-studied units are sometimes overlain by carbonate sediments. The conclusion is that the Ghaub is Marinoan, previously thought to be somewhere between 600 and 660 Ma. Interestingly, the new occurrence of diamictites is divided vertically by two thick sequences of volcanic lavas, neither of which have been dated by the authors.
One of the leading experts on what actually constitutes incontrovertible evidence for glacial sedimentation is Nicholas Eyles of the University of Toronto. He has become increasingly disenchanted with notions of Snowball conditions, on the basis of ambiguity in the very evidence said to signify them; diamictites with drop stones. He and Nicole Januszczac have assembled a monumental paper that counsels caution, and perhaps more (Eyles, N. & Januszczac, N. 2004. “Zipper-rift”: a tectonic model for Neoproterozoic glaciations during breakup of Rodinia after 750 Ma. Earth-Science Reviews, v. 65, p. 1-73). Part of their argument rests on the very lack of robust ages for Neoproterozoic diamictites that prevents believable correlations from continent to continent. It is the globally synchronous nature assumed for these glaciations that gave rise to the “Snowball Earth” notion. The palaeomagnetic latitudes are often used to support this, but they are error prone both palaeogeographically and geochronologically. Accepting evidence for glaciation at low latitudes is no guarantee of support for even cold extremes, let alone an icebound world. Solar heating in the Neoproterozoic was lower than now, and so, therefore, would be the elevations at which glaciers might form at different latitudes. But the main problem is reconciling the features of many supposed glaciogenic diamictites with modern ideas of what truly constitutes evidence for glacial transport and deposition. Few of the units on which the “Snowball Earth” hypothesis is based stand up to modern scrutiny. Most of the diamictite packages occur in tectonically controlled basins, that were subject to episodic rifting. Each can be considered to form the base of a “tectonostratigraphic” cycle, and many show abundant evidence of having formed as mass flows from a shelf into the basin. They include olistostromes with huge rafts of carbonates likely to represent failure of carbonate platforms and huge submarine landslides, similar to those being discovered off many large islands today. The 750 to 580 Ma period was one of the most dramatic episodes of continental break-up in Earth’s history as the Rodinia supercontinent was disassembled. Continental uplift, resulting either from mantle plume activity or rebound of rift shoulders, could have resulted in large areas rising above the ice limit, even at low latitudes in those cooler times. Those diamictites that are undoubtedly glaciogenic could easily have formed haphazardly in time. The carbon isotope record of immense shifts in d13C during the Neoproterozoic, linked by some to repeated collapses and resurrections of life, might just as easily have occurred through efficient organic burial in active extensional basins and repeated major volcanism from plumes. Only evidence of timing will tell, and three good dates for “Snowball Earth” events are simply not enough.
See also: Fanning, C.M & Link, P.K. 2004. U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho. Geology, v. 32, p. 881-884. Gives age of 709±5 Ma for tuff immediately beneath a supposed Sturtian diamictite. Also: Calver, C.R. et al. 2004. U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania. Geology, v. 32, p. 893-896. Gives 575±3 Ma age for sills intruding a “Marinoan” diamictite, and 582±4 Ma for a rhyodacite immediately beneath it, similar to Gaskiers age above – worth a read later
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The earliest granny factor
One of the unique features of humanity is the progress of women into infertility after the onset of the menopause. Females of all other animal species, including primates, remain potentially fertile until they die, even when kept alive in zoos well beyond their natural life spans. When the menopause arose is difficult, if not impossible to judge, but the advantage of surviving grandparents, especially grannies released from the burden of child-bearing and care, is huge. They carry knowledge from two generations or more before the lives of their descendants, and they have the time to confer it on children. Once grandparents became common members of families, effectively they would have doubled the potential for teaching and learning. That has immense importance for human survival and development. In 1990 I witnessed this in action in a remote and war-torn part of Eritrea. There was a drought worse than any since 1918, and villagers were frantically searching for drinking water for themselves and their livestock, to the extent that they were felling giant baobab trees, more than 300 years old, to get to their water-rich inner core. While we were attempting, with little success, to advise a group on where to dig a new well a young boy with a large camel arrived. On it was a couple well into their 80s. They directed attention to a particular spot, digging resumed, and after 2 hours water was struck. That place was where the couple remembered a well being dug in the great drought of 1918. It is possible to get some idea of when the possible influence of grandparents arose by finding evidence about age distribution in ancient populations. The further back in time, the more incomplete are human remains. However, teeth have the highest of all survival chances, and the do carry evidence of the age of the person who chewed with them, from the wear patterns and the presence or absence of late-erupting teeth (Caspari, R. & Lee, S.-H. 2004. Older age becomes common late in human evolution. Proceedings of the National Academy of Science, USA, v. 101, p. 10895-10900). Caspari and Lee’s work used more than 750 samples of human teeth, dating back to some of the earliest hominids. The measure that they adopted to assess onset of old age does not increase gradually into more recent times, but undergoes a remarkable jump around 30ka. Interestingly, this coincides with the explosion of art of the highest quality in Europe. Was it the oldsters who made that leap or was it their influence that opened up new horizons for their grandchildren. Other than this remarkable possibility, the opening of culture as we know it is hard to explain.
Black Sea flooding put to test
In the mid-1990s, William Ryan and Walter Pitman of the US Lamont-Doherty Earth Observatory captured a much wider audience than is the normally the case for geoscientists, when they announced evidence from the Black Sea that seemed to confirm legends of the Flood in the Old Testament and the Epic of Gilgamesh. They claimed that in early Holocene times, the Black Sea was a freshwater lake some 150 m below present sea level. At the time, global sea level was below the threshold of the floor of the Bosporus, thereby isolating the Black Sea from the world’s oceans. Yet sea level was rising inexorably as continental ice sheets melted back. Around 8000 years ago, sea water flooded through the Bosporus to fill the Black Sea to its present level. Evidence takes the form of submerged beaches and even possible townships (mounds similar to the tells in Turkey and Mesopotamia formed during long-term occupation by Neolithic to Bronze Age cultures). Other features on the floor of the Black Sea are zones of large sand waves and signs of incision, ascribed by Ryan and Pitman to massive currents when flow began through the Bosporus. The way in which such flooding might have take progressed is testable using hydraulic modelling, although the topographic parameters are complex (Siddall, M. et al. 2004. Testing the physical oceanographic implications of the suggested sudden Black Sea infill 8400 years ago. Paleoceanography, v. 19, PA1024, doi:10.1029/2003PA000903). The work of Siddall and colleagues suggests a flow rate of 60 thousand m3 s-1, about that of a river as powerful as the Brahmaputra (see Catastrophic erosion in Tibet, this issue of EPN). That would have taken around 30 years to fill the Black Sea to its present level; far longer than the Biblical 40 days and nights, but quick enough to force large-scale migration and to live on in legend. The model fits with the seabed sand waves and channelling, and being based only on known topography and post-glacial sea level rise, rather than the myths, it carries weight scientifically. However, little is known about the way in which young sediments in the Black Sea basin formed, and proper documentation awaits their coring..
See also: Schiermeier, Q. 2004. Noah’s flood. Nature, v. 430, p. 718-719.
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Mantle dripping off mountain roots
Continental arcs, such as the Andes, parts of the Himalaya and Tibetan Plateau and the Sierra Nevada of the western USA, are stuffed with granite intrusions. Large volumes coalesce to form classic batholiths. It is now well-accepted that very little of the granitic magma originated by melting of older continental crust, but by processes of fractionation from more mafic parent magmas. That presupposes a layer of dense, mafic to ultramafic cumulates below and complementing up to 30 km of batholithic crust. The overall density of the continental arc crust would be high relative to that of the granites themselves. So the fact that many batholithic cordilleras are topographically high suggests one of several processes: either the granitic part of the crust has become tectonically thickened relative to its denser root, or that root has separated from the continental lithosphere as a whole, and sunk into the mantle. Such decoupling, or delamination, would induce the remaining lithosphere to rise dramatically. Also, its descent could result in partial melting to produce peculiar potassium-rich basaltic magmas. The latter occur in Tibet and their presence there has been linked to foundering of deep lithosphere, that may have triggered the relatively recent surge in Himalayan uplift. Proving the existence of a descending lump of lithosphere is not easy, but developments in seismic processing can make a crucial contribution, if sufficient data are available for a suspected zone of delamination. The western USA is blessed with lots of seismic stations, so is a natural place to try out the new techniques as a test of the hypothesis. George Zandt of the University of Arizona, and other US colleagues have come up with interesting results (Zandt, G. et al. 2004. Active foundering of a continental arc root beneath the southern Sierra Nevada in California. Nature, v. 431, p. 41-46). Their analyses of seismic data shed light on a late stage in the development of the Sierra Nevada. During the Mesozoic Era, subduction beneath North America of the now disappeared Farallon plate of Pacific ocean lithosphere built up the Sierra Nevada batholith. About 10-16 Ma ago, subduction stopped and the plate margin became one of transpression, the most prominent feature of which is the San Andreas Fault. At that stage, a “drip” of dense cumulates began to form, and subsequently separated to descend into the mantle. Cruustal rebound was not simple but included zones of extension, as well as tell-tale high-K volcanism during the Pliocene.
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Tighter link of end-Permian extinction with Siberian Traps
The volcanism versus impact debate about the K-T boundary runs and runs, as newshounds tend to say. Things are not so evenly balanced for the biggest of all mass extinctions at the end of the Permian. Although signs have been reported, a link with an impacting extraterrestrial body has not convinced a decisive majority. On the other hand, there is a 1-2 Ma mismatch between the well-determined age (around 253 Ma) of the Siberian Traps and previous dates for the end of Permian stratigraphy in sections that have no depositional break with the Triassic. The extinction has all the hallmarks of a catastrophe, by definition a sudden event, so tying down its age and that of a plausible cause is essential. Not being able to do that for the K-T event and the Deccan Traps, and with uncertainties about the relationship of impact rocks to signs of extinction at the Chicxulub site, add fuel to that long-running debate. The accepted “golden spike” or GSSP for the Permian-Triassic boundary is at Meishan in eastern China, and there are other sites in China that run it close. The sections contain several volcanic ash layers, so zeroing in on a date for the extinction would seem straightforward, using U/Pb zircon dating. There is a problem. Some of the zircons in the ashes are xenocrysts rather than having formed during the various magmatic episodes, and they are microscopically indistinguishable from those that should give precise dates. All the zircons also show signs of having lost radiogenic lead during later alteration of the beds. The last could explain the mismatch with the Ar-Ar age of the Siberian Traps, the generally favoured culprits for the extinction. US and Australian geochemists have taken a new tack in dealing with these problems (Mundil, R et al. 2004. Age and timing of the Permian mass extinction: U/Pb dating of closed system zircons. Science, v. 305, p. 1760-1763). They have “aggressively” treated zircon grains to remove outer parts from which radiogenic lead has been lost, so leaving isotopically undisturbed cores of the grains. Their U/Pb data are mainly from a boundary section in central China (Shangsi), dating 8 separate ash layers, plus one from the boundary clay itself at the Meishan GSSP. The dates agree well with the stratigraphic sequence of the ashes, and hare high precision. Judging the actual age of the boundary at Shangsi relies on statistical analysis of the sequence of ages from the different ashes, and gives a date of 252.6±0.2 Ma. That is within error of the accepted Ar-Ar age of the Siberian Traps. As usual, this is not cut and dried, because there are other ages for the Siberian Traps, including one using the same U/Pb zircon method that suggests a 251.4 Ma age. Clearly the mismatches for the end-Permian events will be a meaty bone of contention, when all respected geochronologists turn up for a meeting early in 2005 to thrash out the conflicts that continually inflame their passions.
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Catastrophic erosion in Tibet
The world’s most awesome natural spectacle is probably the Brahmaputra River in full spate. Unlike most large rivers, it is constrained for most of its course within a deep, narrow gorge that has to take the snow melt from a huge catchment on the northern flank of the High Himalaya, brought partly by the Tibetan Tsangpo River. Each spate hurtles onto the plains of Bangladesh, loaded with debris, at a rate of around 70 thousand cubic metres per second. Although that is but a third of the flood discharge of the Amazon, for much of the Brahmaputra’s course it must pass through a gorge only a few hundred metres wide in places. This gives not inconsiderable erosive power, indeed probably the highest anywhere. Not surprisingly, little is known about the Tsangpo-Brahmaputra valley, because of its inhospitable character. With the recent release of ~90m resolution elevation data from the Shuttle Radar Topography Mission, it is now possible to analyse the whole catchment’s morphology in detail, without needing to follow the individual rivers. Parts of the lower Tsangpo have remarkably high gradients, including a 100 km stretch with a fall of more than 2 km, through a gorge with almost 7 km of relief on either flank that cuts N-S across the axis of the Eastern Syntaxis of the High Himalaya. The gorge lies downstream of a west to east stretch with lower gradients, falling around 1 km in 300 km, which suggests some dramatic incision begins at the junction of the two sections. US and Chinese geomorphologists visited the area and discovered that high on the flanks of the upper Tsangpo are terraces of lacustrine sediments, at about 3100 and 3500 m (200 and 600 m higher than the river) (Montgomery, D.R. et al. 2004. Evidence for Holocene megafloods down the Tsangpo River gorge, southeastern Tibet. Quaternary Research, 9 September 2004 issue). Charcoal in the sediments gives radiocarbon ages between 1200 to 1600 BP and 8800 to 9800 BP for the lower and higher terrace levels, so the lakes formed during the Holocene. The terraces stop at a zone of thick glacial moraine, cut by the Tsangpo, which suggests that both formed in lakes behind two ice dams. Using SRTM data allows the volume of water ponded in both ice-dammed lakes to be estimated. The older and higher level indicates about 830 km3, and the lower some 80 km3. Breaching of the dams would have caused the largest recorded erosive events in recent Earth history, and explains the gorge below. Each flood discharge would have been between 1 and 5 million cubic metres per second, equivalent 3 to 15 times the maximum flood discharge of the Amazon.
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Mars issue of Science
So, you are a geoscientist and you are interested in Mars. Excellent! Now read pages 793 to 845 of the 6 August 2004 issue of Science v. 305. There is much to learn from 11 papers about the less revealing of the two Mars Exploration Rovers, Spirit. Rover Opportunity has been getting the headlines, with its discoveries that relate to the influence of surface and subsurface water on superficial Martian minerals, such as the now well-publicised “blueberries” made of hematite, and the presence of sulphates. A more informative digest of the mineralogy of Mars appears in the same issues’ News Focus (Kerr, R.A. 2004, Rainbow of Martian minerals paints picture of degradation. Science, v. 305, p. 770-771). Kerr makes clear that the really revolutionising instrument is orbiting Mars; the Visible and Infrared Mineralogical Mapping Spectrometer or OMEGA. That is part of the payload of the ESA Mars Express, and measures radiant energy from the Martian surface with such spectral and spatial resolution, that the results can be compared with standard spectra of terrestrial minerals to see what the Martian surface is made of. Hopefully, OMEGA will produce a hyperspectral database for the entire planet. The on-surface readings from the various instruments on the NASA Rovers play much the same role as a field geologist would, by providing “ground truth” to validate the broader scope of the OMEGA instrument. The hematite that dominates the overall red colour of Mars, has been confirmed by the Rovers, but to nobody’s great surprise. The exciting find is just how much is owed to sulphate minerals, such as orange iron potassium sulphate, or jarosite. The sulphate-rich veneer could well point to the influence of sulphuric acid, let alone water in Mars’ early surface environment, probably emitted as sulphur dioxide during intense volcanic activity. Interestingly, the incompatibility of highly acid surface water with the preservation of carbonates could have thwarted drawdown of CO2 from the Martian atmosphere (Fairén, A.G. et al. 2004. Inhibition of carbonate synthesis in acidic oceasn on early Mars. Nature, v. 431, p. 423-426). Formation and preservation of soil carbonate minerals would have collapsed the “greenhouse” warming mechanism demanded by the now proven influence of flowing water early in Martian history. So long as sulphurous volcanic emissions overwhelmed carbonate formation, Mars might have stayed wet and warm. The key is the duration of massive volcanism, which could be tied down by seeing how lavas have been affected by impacts in the minute detail possible from another Mars Express imaging instrument, the High Resolution Stereo Camera. Planetary volcanic specialists reckon massive volcanism lasted for a considerable time
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Plastic deformation beneath Tibet
Plate tectonics’ basic tenet is that discrete segments of the lithosphere behave as rigid bodies, whose motion is accommodated by extensional, overriding and strike-slip faulting at equally discrete boundaries. That is true to a first approximation for the parts of plates made up from oceanic lithosphere, which is rheologically strong because of its mineralogical composition. Continental lithosphere is weakened by its quartz-rich crust, which tends to behave plastically at high temperatures deep within it. So it is no surprise that opposed motions of plates induce large-scale shortening and thickening of continental lithosphere that they carry, but there are no orogens in the ocean basins. The largest site of active continental shortening and thickening is, of course, the Alpine-Carpathian-Himalayan orogen. The Tibetan Plateau is underpinned by continental crust that is in the process of being thickened as India drives north-eastwards into Asia, at about 4 to 5 cm per year. Consequently it the largest area of high-elevations on the planet. In the 1973 John Dewey and Kevin Burke speculated that forces involved in continent-continent collisions with irregular margins might expel thickened continental lithosphere sideways, at right angles to the opposed plate motions. Peter Molnar of the University of Colorado in Boulder and Paul Tapponnier of the Institute of Global Physics in Paris applied this on a grand scale to the neotectonics of the Tibetan Plateau and East Asia in 1975. They considered that south-eastward expulsion was channelled by the many enormous strike-slip faults in the region. In a sense, this notion considers the continental tectonics to be akin to the rigid-body behaviour of oceanic parts of plates. If the overall motions involving Tibet and continental lithosphere to the east was dominated by plastic deformation in the deep crust and mantle, the motion would be taken up by a host of smaller faults in the brittle upper crust. Geodetic measurements using GPS over the last 17 years do conflict with the movement of discrete blocks of East Asian crust (see Quantifying motions inside continents, March 2004 EPN). Two papers published in July 2004 also lean towards plastic behaviour of the bulk continental lithosphere. One uses data from surface seismic waves to show about 30% ductile thinning in the middle and lower crust beneath Tibet (Shapiro, N.M. et al. 2004. Thinning and flow of Tibetan crust constrained by seismic anisotropy. Science, v. 305, p. 233-236). The other is based on interferometric analysis of radar data from satellites, which involves measuring signal differences between radar data captured on different dates, in this case between 1992 and 1999 (Wright, T.J. et al. 2004. InSAR observations of low slip rates on the major faults of western Tibet. Science, v. 305, p. 236-239). The technique has mainly been used to look for vertical displacements associated with earthquakes and volcanoes. By eliminating the effects on signals by terrain, using an accurate digital elevation model, InSAR results can estimate the motion of the surface along and opposite to the illumination direction of the radar pulses, thereby detecting horizontal ground movements over a period of several years with sub-centimetre precision. Rather than revealing large movements in the two opposed directions that are expected on either side of large strike-slip faults, such as the Karakorum and Altyn Tagh Faults, there was none. In a zone crossing western Tibet from NNE to SSW, much of the orogen appears to be moving slowly eastwards, irrespective of the large faults. Tapponnier still maintains the importance of the big faults, and perhaps the InSAR survey coincided with a period of tectonic quiescence.
Early Earth’s Nemesis
William Hartmann’s proposal that, shortly after it formed, the Earth suffered impact by a planet about as big as Mars has become a central feature on ideas about our planet’s evolution and the origin of the Moon. The problem with the theory is a conundrum that lies in quite esoteric geochemistry. Studies of meteorites show that the oxygen isotopes in them vary considerably, and that almost certainly resulted from their forming at varying distances from the Sun where fractionation among oxygen’s stable isotopes had different effects on their proportions. So it is possible to judge the original orbits in the solar system of meteorites’ parent bodies. Martian meteorites are identified on this basis. The difficulty with Hartmann’s idea is that rocks from the Earth and Moon have nearly identical oxygen isotope proportions. There seems no way that an errant planet that crashed into the Earth could not have left its mark in oxygen isotopes, particularly in those of the Moon, for debris flung off from the Earth would have mixed with that from the colliding body. It turns out that there is a possible explanation (Chown, M. 2004. The planet that stalked the Earth. New Scientist, 14 August 2004, p. 26-30). The Earth’s orbit could have involved the accretion of more than one planet from interstellar dust. This can happen once a planet has grown until it has sufficient gravitational potential to interact with solar gravity. The result is a series of points in the orbit (Lagrange points) where the two gravitational fields exactly balance. Matter that drifts into Lagrange points accumulates there rather than being swept up by the growing, larger planet. So considerable mass can build up, even enough to make a small companion planet. While all the main planets were growing, gravitational fields were continually changing, so the Lagrange points would not remain as stable as they are today. A small planet formed at one of them would begin to move erratically within the Earth’s orbit. Eventually it would be caught up by mutual attraction between the two, and then would collide with the Earth, but not at immense speed. So far, the hypothesis based on complex modelling of Lagrange accretion seems plausible. Geochemists will be pleased because it resolves their fundamental conundrum about the similar chemistries of the Earth and its Moon.
Uranium in the core?
The constant, but complex circulation in the Earth’s liquid outer core almost certainly results in the self-exciting dynamo believed to be responsible for the geomagnetic field and its periodic reversals in polarity. If an electrical conductor moves in a magnetic field a current is generated in it, which in turn creates a magnetic field, hence self-excitation. The outer core’s convective motion requires a heat supply of some kind. There are three general possibilities: the heat is left over from the Earth’s energy of accretion; it is generated from latent heat released as the solid inner core grows slowly by crystallization of iron-nickel alloy; or there is significant radioactive decay in the core. Compared with estimates for the Earth’s overall radioactive heat production, based on the composition of the primitive meteorites (ordinary chondrites) from which it is thought to have formed, there is excess heat flowing through the surface. This is believed to emanate from the core. Separating the three possible heat sources is not yet possible, but it is possible to rule the generation of enough to account for excess heat flow by one of the possible mechanisms. If the inner core has been crystallising out since the core formed around 4500 Ma ago, the latent heat being released is too small. Much attention has focussed on a possible radioactive source, for which the unstable natural isotope of potassium (40K) is a plausible candidate. The sulphide phases of metal and chondritic meteorites do contain potassium, so the element has affinities for sulphur as well as its dominant tendency to enter silicate melts and minerals. The core almost certainly contains a sizeable proportion of Fe-Ni sulphides. One geoscientist, Marvin Herndon based in San Diego, California, reckons there is another possibility (Battersby, S. 2004. Fire down below. New Scientist, 7 August 2004 issue, p. 26-29); uranium. To most geochemists, the idea is implausible, because uranium has such a strong affinity for silicates that it ought never to have entered the metallic and sulphide liquids that sank through the early Earth to form the core. Herndon bases his idea on an alternative type of meteorite from which the Earth could have formed by accretion, enstatite chondrites. They have lower oxygen contents than ordinary chondrites, and would have created strongly reducing conditions in the undifferentiated early Earth. Such planetary chemistry, claims Herndon, would induce uranium to enter dense sulphide liquids and the core. This view has not found much support, but experiments in detection of neutrinos and antineutrinos, when they are more efficient than at present, may resolve the issue of radioactivity in the core, because decay of unstable isotopes produces antineutrinos.
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Climate and mountain relief
The greater the rainfall, the more effective streams become as agents of erosion. So, “common sense” suggests that very wet mountain areas should be eroded more quickly and develop a more profound relief than those that are drier. With the advent of detailed digital elevation models that cover the world, it is very easy to calculate slope angles and relief over huge areas, and match them with rainfall records. Geomorphologists from the Universities of Montana and California have done this for the wettest and most rugged area in the world, the Annapurna area of the Himalaya (Gabet, E.J. et al. 2004. Climatic controls on hillslope angle and relief in the Himalayas. Geology, v. 32, p. 629-632). The main agent of erosion there as streams cut downwards is by landslides. The region also shows a profound gradient in annual rainfall from about 1000 mm in the High Himalaya to 4500 at the front of the range, where the monsoon rains hit hardest. “Common sense” is wrong, for the slopes decrease from an average of 35º to 25º as rainfall increases. The authors believe this is due to the influence of deeper weathering in more humid parts that reduces the strength of slope materials so that they must stabilise at lower angles than those in dry areas. Their other finding is that relief (elevation difference in small segments of an area) and slope angle have a strong positive correlation, so that relief itself is inversely related to rainfall. They are able to comment interestingly on various ideas about mountain evolution. Their main conclusion is that in any particular area, a transition from dry to wet conditions lowers mountain ridges faster than valley incision can shift the debris, whereas during drying, ridges are barely lowered, while streams cut unhindered into bedrock, thereby sharpening up the landscape.
Formation of gorges in tectonically quiet areas
The flanks of the North Atlantic probably became tectonically inactive in Mesozoic times, yet rivers large and small have cut large gorges, often through highly resistant bedrock. But they also have developed broad valleys over millions of years, and it is into them that the gorges are incised. Slow upward flexing caused by sediment loading on the continental shelves, a general lowering of sea level since Antarctica first formed a permanent ice cap, and isostatic response to gradual denudation help explain the full extent and shape of the rivers drainage basins. The gorges are young, and must have developed rapidly. Old ideas focussed on W.M. Davis’ theories of landscape evolution, particularly rejuvenation associated with changing base levels of erosion, but with no quantitative backing. The development of means of dating eroded surfaces using the decay of short-lived radioactive isotopes that cosmic-ray bombardment creates now offers an opportunity to test hypotheses rigorously and come up with others. Quite a few published works on cosmogenic dating applied to landform development seem to add little to geomorphological knowledge, so it is a relief to find one that does (Reusser, L.J. et al. 2004. Rapid late Pleistocene incision of Atlantic passive-margin river gorges. Science, v. 305, p. 499-502). The authors, from the Universities of Vermont and Maryland, the USGS and the Lawrence Livermore National Laboratory, focus on impressive gorges in the lower reaches of the Susquehanna and Potomac Rivers as they drain the eastern US into the Atlantic, and a series of higher surfaces which they cut into to leave as rocky straths. The oldest ages occur on the highest of these straths, as expected, and age decreases on successively lower ones to the rocky flood plain of the modern rivers just above their current channels. The highest levels are between 85 and 97 ka, the most prominent strath formed between 30 and 33 ka, succeeded by one at 19 ka and the lowest level seems to have formed between 13 and 14 ka. Interpreting the periods of intense erosion that cut each level must involve late Pleistocene climate change, sea-level shifts, and the bulging effect due to the North American ice sheet which reached its maximum extent in the northernmost part of the Susquehanna basin. It seems that during the early part of the last glacial episode, incision was slow, although probably faster than during the Holocene. But around 30 to 33 ka ago it accelerated rapidly to half a metre every thousand years, some 1 to 2 orders of magnitude greater than at present. This was at a time when ice loading was only half that at the glacial maximum around 20 ka, so it seems likely to have been initiated more by increased storminess and torrents, and indeed correlates with an abrupt increase in sea-salt content in the Greenland ice cap brought in by winds at that time. Lasting through the glacial maximum, increased frequency of flooding combined with more rapid sea-level fall, also beginning at around 32 ka, were probably the main driving forces for gorge incision. This still leaves a puzzle. Both drainage basins had been in existence since well before the cycles of glacial and interglacial periods began on the flanks of the North Atlantic around 2.5 Ma ago. Similar periods of accelerated incision must have been repeated, at least during the last 6 or 7 glaciations which were the most extensive. Did earlier topographic features exert any control over later ones, and do any relics of them remain?
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Antarctic climate back to 740 ka: cause for optimism?
Ice extracted from ice sheets by core drilling has provided the most detailed historical information on climate variation at high latitudes and about the varying gas and dust content of the atmosphere. It provides the best time-resolution currently available, sometimes of the order of 50 years. Cores from the Greenland ice sheet revolutionised ideas about the controls over short-term climate shifts in the northern hemisphere – the millennial-scale Heinrich and Dansgaard-Oeschger events. It is from those revelations that fears have arisen about the consequences of deep-ocean circulation shut-downs that might arise from current global warming. The Greenland ice goes back only to cover the last glaciation and part of the interglacial period the preceded it. Until recently, the Vostok ice core from Antarctica gave the greatest penetration into past climatic events, to around 430 ka that covers the last four glacial epochs. Again, Vostok revolutionised our understanding of past climate change, principally the differences between climate behaviour in interglacials, and those between the records from northern and southern hemispheres. North and south have not been in exact harmony, at least as far as high latitudes are concerned. Ocean-floor sediment cores and those from mid-latitude glaciers do give hints of a global harmonisation of events though. Since we live in an interglacial period, for the last three of which the previous ice-core records suggest a span around 10 ka, it has seemed likely that ours wouldn’t have lasted much longer than it already has under purely “natural” conditions. Modelling the possible effects of anthropogenic warming on climate that may be about to change anyway within this millennium, has left climatologists undecided about the future. That blurring is as much to do with the unknown direction that an unstable climate might take and the limitations of modelling, as with knowledge of past events. So, the more information on past interglacials, the better the chance of getting a “handle” on the climatic frying pan out of which humanity seems to be on the point of jumping. The European Project for Ice Coring in Antarctica (EPICA), which involves 57 scientists from 10 European countries, has dramatically expanded the scope for comparison with the past by a 3 kilometre core from one of the deepest parts of the Antarctic ice (EPICA, 2004. Eight glacial cycles from an Antarctic ice core. Nature, v. 429, p. 623-628). The potential information that eventually will flow from the core will dwarf that from any previous climatic research project. It covers the period when climate settled into a roughly 100 ka rhythm, probably linked with the weakest of the astronomical controls of solar heating, that of orbital eccentricity, and thereby a bit of a mystery even if it twangs the harmonics of purely terrestrial climatic processes.
The first focus, naturally enough, is on the fourth interglacial epoch before the present one, which ended about 400 ka ago. In terms of overall astronomical forcing, that is the time when insolation patterns were most similar to those during the Holocene. Vostok only covered the latter stages, but now its entire span is covered. All the preliminary time-series for it indicate that it was considerably longer than the last three interglacials, around 25 ka rather than 10. Its initiation following the waning of the preceding full glacial period follows a similar patter to the early Holocene; the warming was interrupted by a sudden, one-off cooling, somewhat like the Younger Dryas around 12 ka ago. Although the first EPICA report contains preliminary ideas on several important topics, the one that has caused a stir is that duration of the 5th interglacial. Maybe out own warm times will be naturally prolonged for several more millennia, in which case fears of instability and a plunge to full glaciation soon could be set aside with some relief. However, the abstract to the article, concludes ny saying, “…our results may imply that without human intervention, a climate similar to the present one would extend well into the future” [my italics]. But we do intervene, and nobody knows the outcome of that on a climatic pace of change that follows the almost infinitesimally small orbital-obliquity forcing of probable oceanic process that really call the tune.
Smoking gun for end-Palaeocene global warming: an igneous connection
The sudden warming of the Earth at the start of the Eocene 55 Ma ago has been a topic touched on several times in EPN. It is widely regarded as a consequence of rapid release of methane from sea-floor gas hydrate, a risk that modern anthropogenic warming presents if deep-water temperatures rise much above their present near-freezing temperatures. However, no evidence gives a direct connection to the “clathrate gun”. The disturbance in carbon isotopes of marine sediments at the P-E boundary is most easily linked to a massive methane release at the time, but precisely where it began has been unknown. Many shallow marine basins, such as the North Sea, have a pockmarked modern floor attributed to minor gas release in much more recent times. The phenomenon can destabilise the sea bed, so more recent releases have been carefully documented where oil-production platforms are situated. A clue to the much larger release at 55 Ma stems from detailed seismic exploration of western Norway that involved over 150 thousand kilometres of profiling (Svenson, H. et al. 2004. Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, v. 429, p. 542-545). The surveys revealed that beds immediately beneath the base of Eocene sediments are riddled with hydrothermal vents complexes, which take the form of mounds, craters and eye-shaped structures. Some are huge, extending to 5 km across. The profiles also show that beneath the vents are pipes of disrupted strata which extend to the depth of a complex of igneous sills of the North Atlantic large igneous complex, itself emplaced at about 55 Ma. The sills underlie about 80 thousand square kilometres and most of the vents occur within this area. Biostratigraphic dating of the youngest sediments disrupted by the vents gives ages between 55.0 and 55.8 Ma. Intrusion of magma into a deep sedimentary sequence unsurprisingly would set hydrothermal circulation going. If, as they did, the hot fluids reached the sea bed, they would pass through a zone of gas hydrate, destabilise it and release massive amounts of methane to the atmosphere. In the case of the Norwegian shelf, the intrusions were into deeply buried organic rich rocks, further encouraging methane formation; probably a great deal more than from gas hydrate. An estimate of 1012 tonnes of methane generated thermally off Norway is enough to result in a change in carbon isotopes as large as that known from the P-E boundary. In fact, similar sediments throughout the end-Palaeocene North Atlantic large igneous province are likely to have been “over matured” in this way, and no other explanation for the increase in “greenhouse” gases seems necessary. The clear connection with large scale magmatism in thick sedimentary basins may help focus ideas about similar methane-related episodes of global warming, such as the C-isotope excursions at the Permian-Triassic and Triassic-Jurassic boundaries, and within Jurassic and Cretaceous sequences.
Earth’s early climate and methane
At the time the Earth accreted, some 4.6 billion years ago, the Sun was less bright than it is now, so that its warming effect was 30% less. Without some means of retaining in the ancient atmosphere what heat was available, the Earth would have been frigid. This “faint, young Sun” problem would have persisted into the time when the geological record begins, around 4 billion years ago, slowly increasing in its energy output to its modern level. Even in the oldest rocks, there is abundant evidence for the dominance of liquid water at the surface in the form of oceans and river transport across continents. Low solar warming would have made that impossible, and pole-to-pole ice would have made the Earth a highly reflective planet that could never escape glacial condition. That is, unless the atmosphere contained sufficient “greenhouse” gases to retain far more solar energy than now. The favoured gas, until recently, has been the same one that dominates fears of global warming today – carbon dioxide – that volcanoes probably emitted throughout Earth’s history. However, estimates of how much would have been needed to keep the surface free of sea ice and land glaciers, for which there is no evidence until about 2.3 billion years, are extremely high (hundreds of time greater than now). Levels greater than 8 times present levels encourage the precipitation of iron carbonates in soils, yet soils from the late Archaean and Palaeoproterozoic contain none. At those times, CO2 concentrations less than 8 times present ones would not have prevented runaway “ice-house” conditions, so some other gas had to be involved in atmospheric warming. James Kasting of the University of Michigan, who has been involved in studies of ancient atmosphere and climate for 25 years, summarises the case for methane being the means of keeping Earth free of ice while the sun was fainter in a recent article (Kasting, J.F. 2004. When methane made climate. Scientific American, v. 291(1), p. 52-59). Only about 1000 parts per million of atmospheric methane would have been needed to keep the early Earth ice-free, because its “greenhouse” effect is extremely efficient. After oxygen rose to become a major atmospheric gas (since 2.2 billion years), heating induced by methane releases has been tempered by its rapid oxidation to CO2. At several times in the past, when there were massive methane releases from sea-floor sediments, such as the end of the Palaeocene, that oxidation prevented the opposite problem, a runaway “greenhouse”. That is “another story”, involving the rise of photosynthesising organisms. Kasting’s main theme is the role of methane-generating Archaea (once known as archaebacteria) soon after the origin of life. In the absence of oxygen, rising methane from thriving methanogen communities could itself have produced irreversible heating, were it not for methane’s ability to polymerise to heavier hydrocarbons through photochemical reactions. That would have produced a “smog” that not only would have acted as a reflector for solar radiation, but would have added chemical “feedstock” to early life. Kasting gives a fascinating, all-sided summary, but misses what seems to be an obvious point. Without atmospheric methane, any water on Earth would have frozen soon after it appeared, however that happened, perhaps by outgassing, perhaps delivered by comets. Without liquid water, life processes cannot develop. That opens the possibility for a much earlier origin of life, of the methane generating variety, than anyone has dared to speculate on. Many methanogens metabolise hydrogen and CO2. Volcanoes emit small amounts of hydrogen gas, but an even larger source is from sea-floor hydration of ultramafic lavas, common in early times. Almost certainly the very earliest times would have provided a suitable environment for methanogens to emerge.
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Carbon-isotope resonance of the end-Permian extinction
As with several major extinction events, the Permian-Triassic boundary is characterised by a major excursion in carbon isotopes of marine towards negative d13C. This is often taken to indicate a reduction in the burial of dead organic matter, perhaps because of low global biomass. US, Chinese and Canadian geoscientists have added great detail to the P-Tr carbon-isotope record from analysis of three continuous sections through carbonate-dominated sequences in an Early Triassic reef system in southern China (Payne, J.L. et al. 2004. Large perturbations of the carbon cycle during recovery from the end-Permian extinction. Science, v. 305, p. 506-509). This is no ordinary reef, for it was built by carbonate secretions by micro-organisms, either algae or bacteria. The tabulate coral reef builders of the Palaeozoic became extinct at the end of the Permian (251 Ma), and their successors, scleractinian corals, do not appear until about 10 Ma later. The Early Triassic was undoubtedly characterised by low animal diversity, before adaptive radiation could “re-stock” a devastated biosphere. The authors found a remarkable series of ups and downs in d13C within the reef carbonates, some of the negative excursions being even more severe than that just after the mass extinction. Some of the positive peaks go far beyond the d13C levels in preceding and following times, and could be due to periods of extremely high burial of organic matter. But the fossil record shows that such burial probably involved a restricted number of taxa, so perhaps there were huge “blooms” among a few groups that filled vacant ecological niches only to collapse. As suddenly as this see-sawing of the carbon cycle had begun, at about 246 Ma it settled to a more or less constant level, just after the start of the Middle Triassic. There are two reasonable explanations for the fluctuations. One is that biotic recovery from the mass extinction was set back three of four times by further environmental upheavals, thereby dashing diversification. The other is that the fluctuations reflect instability in the simple ecosystems of the Early Triassic and their control on carbon burial.
Calcium in the ocean and the Cambrian Explosion
If ever there was a geoscientific topic that would “run and run”, it would be explaining why creatures with hard parts just popped into being 542 Ma ago. Physiologically, members at the phylum level of the Cambrian fauna have little in common apart from hard parts made from calcium compounds, either carbonate or phosphate. Calcium carbonate was secreted as stromatolites by blue-green bacteria as far back as the Archaean, but not in an organised form linked to their function. In the very latest Precambrian, the Ediacaran, there are tiny shell-like bits and pieces in its very uppermost strata (the “small shelly fauna”) but they suggest no obvious function and no association with any of the various soft-bodied metazoans that define that Period. The Cambrian Explosion has no rudimentary precursor. Because calcium is an element with a very narrow tolerance in cells, from the level needed for viable function (it has a “messenger” function) to that at which it is fatally toxic, and it is a common element in all environments, adoption of calciferous hard parts seems very likely to have a risen as a means of avoiding toxicity, without any other role. Once established in large animals, hard parts provide a means of and a defence against predation, so losing the ability to secrete hard parts would be an evolutionary risky strategy; once established it cannot be lost except when substituted by other effective defences or mealtime tackle. There were times in the Precambrian record when calcium compounds exceeded their solubility, and they are marked by inorganically precipitated crystalline forms in sediments. The early Archaean was one such period, but if levels of Fe-2 are high in water those solubilities are enhanced. Therein lies a link between Archaean and Palaeoproterozoic stromatolites, banded iron formations and the oxidation potential of seawater. In fact precipitation of BIFs seems to link nicely with the abundance of stromatolites, because the production of oxygen by blue-green bacteria would locally have consumed electrons to oxidise soluble Fe-2 to Fe-3 that has insoluble oxides and hydroxides. This connection returned several times in the Neoproterozoic, oddly at the times of so-called “Snowball Earth” episodes, first noted by Preston Cloud. Could the last of these have triggered adoption of calcium secretion by the early metazoan animals? That is hard to judge, because it preceded the Cambrian by several tens of million years. Geochemists from the US Geological Survey, the State University of New York and the US Oak Ridge National Laboratory have taken a cunning route to shedding some light on the biggest of all palaeontological mysteries (Brennan, S.T. et al. 2004. Seawater chemistry and the advent of biocalcification. Geology, v. 32, p. 473-476). They sought crystals of evaporitic halite that spanned the Precambrian-Cambrian boundary, and which usually contain fluid inclusion containing samples of the brine from which they formed, hopefully seawater. So far, they have two sets of suitable halites that can be assigned to a marine environment, from Siberia and the Oman, and their measurements of calcium concentrations are very precise. The first is dated around 515 Ma the other set from 544 Ma. Two sample points are not enough to prove a role for elevated calcium levels in the ocean, but the results are encouraging. Calcium concentrations (with suitable corrections for changes during evaporation of restricted seas) jumped by a factor of 3 from the very end of Precambrian to Cambrian times. Over the same period, it is thought that global sea-floor spreading rates were much higher than at present, and there is also strontium-isotope evidence for an increase in ocean-floor hydrothermal activity that adds elements derived from oceanic basalts to seawater. That, however post-dates the start of the Cambrian by about 15 Ma. With a CO2-rich atmosphere and elevated continental weathering calcium is likely to have been supplied from the continents. Whatever, the results fit with models based on variation of continental and oceanic additions to seawater with changing spreading rates (Hardie, L.A. 2003. Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas. Geology, v. 31, p. 785-788). Hardie suggested that calcium in seawater fell to very low levels during the Neoproterozoic from an unprecedented high at its outset at 1000 Ma. That is a time when metazoans were probably not around, while the period when they appear in the later Neoproterozoic record was one of calcium-poor conditions. Large animals may have evolved when there was little danger of calcium shock, only to face it once they were well established. Then would have had to rid their cells of it very efficiently. Studies of fluid inclusions from marine precipitates seem likely to grow following Brennan et al.’s important discovery, though suitable samples are likely to be few and far between. One important role they need to play is verifying Hardie’s model for secular variation in seawater chemistry, which depends on difficult interpretations of rates of sea-floor spreading and continental erosion.
Ancestral animal?
The significant feature of the first appearance of widespread, large fossils during the Cambrian Explosion about 542 Ma ago was really the adoption of hard parts by most of the existing (and some now extinct) phylla of animals. The preceding Neoproterozoic Ediacaran Period witnessed lots of large life forms, but preserved them only as imprints; they were soft bodied. Superficially, the outset of the Cambrian appears to marked the simultaneous emergences of the rough blueprints of all subsequent animals. In reality, this was probably not a faunal explosion, but one of biochemical processes, wherein many phylla turned the fundamental cell process of excreting excess calcium as carbonate and phosphate to generating functional parts of their bodies. Why that happened explosively is still a mystery. Looking for the origin of animals requires going further back in geological time, and an element of luck as regards exceptional preservation of soft tissue. The other way is using a molecular clock approach to the genetic differences among modern phylla, but that is fraught with uncertainties and gives a very large time range (possibly 1500 to 600 Ma) in which to find tangible evidence. The maximum limit is around 2200 Ma, when oxygen became significant in the atmosphere and the upper ocean – the prime condition for eukaryote life. A rather dull carbonaceous fossil, with a spiral form and thought to be the first known multicelled eukaryote (Grypania) appears in the record about 1500 Ma ago, but what it was is unclear. The best place to look for ancestral animals is in known repositories of well preserved organisms. One such lagerstät is the Doushanto Formation in SW China. This goes back to the last “Snowball Earth” event at 600 Ma, and has been heavily mined for primitive life forms. Chinese palaeontologists, teamed up with others from the USA have indeed found something intriguing (Chan, J-Y. et al. 2004. Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian. Science, v. 305, p. 218-222). Only about 0.2 mm across, 10 specimens seems to show microscopic signs of all the basic elements of many members of the Animal Kingdom: bilateral symmetry, a mouth and gut, skin tissue and possible sensory organs. The layers from which they were extracted are between 580 to 600 Ma, well before the Cambrian Explosion. However, micropalaeontologists in general subscribe to the “once bitten, twice shy” outlook, especially following controversies over even earlier evidence for small organisms and those purported to occur in Martian meteorites, which are as likely to be results of inorganic mineralisation as fossils. Various mineral crusts and films, formed inorganically, can mimic organic structures. The one feature that persuades Chen and colleagues is that the same features show up in all the specimens, and they are all the same size. That is highly unlikely from some inorganic process.
Source: Stokstad, E. 2004. Controversial fossil could shed light on early animals’ blueprint. Science, v. 304, p. 1425.
The case of the stranded, tiny mammoths
It does seem likely that our ancestors ate all the mammoths (Mammuthus primigenius), a species that had wandered over the northern tundras bordering the Northern Hemisphere ice sheets through several glacial-interglacial periods. But some of them did escape to survive into the Holocene. They were stranded on high-latitude islands off NE Siberia and Alaska as sea levels rose. The last of them died on Wrangel island about 4 thousand years ago. A common tendency in small populations of large mammals that are restricted to islands is that they become smaller and smaller with each generation. This happened to the stranded mammoths of the Bering Straits islands, remains of which are often dwarfs. (Guthrie, R.D. 2004. Radiocarbon evidence of mid-Holocene mammoths stranded on an Alaskan Bering Sea island. Nature, v. 429, p. 746-749). St Paul Island is now only 91 km2 in area, too small to support even tiny, woolly elephants, but it was probably much larger when sea-level rise first isolated it from the vast Bering steppe across which mammoths roamed. It was that isolation about 13 thousand years ago that probably helped the stranded mammoth population avoid the hunters who colonised the Americas, until 7 900 when the last mammoth there died. The even later population on much larger Wrangel Island fell to human colonisation, but there are no signs of human intervention on St Paul. The earlier extinction there was probably a result of shrinking browse as sea level steadily rose., when St Paul would have been 5 to 10 times larger than it is now.
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Putting off the evil day
The US oil economist M.K. Hubbert issued a chilling warning in the late 1960s that foretold the eventual decline of the single most important physical resource of the global economy. His simple approach was to consider petroleum, and by implication a great many other commodities, as having a fixed abundance that was not added to naturally at a rate that could keep pace with its exploitation. Oil and natural gas are non-renewable, as far as human society is concerned; they are “wasting” resources built slowly and episodically over tens of million years. Hubbert matched the exponential rate at which petroleum is extracted with various notions of how much is in the ground and how the easiest to find and pump out inevitably will give way to more tenacious reserves. His model for the future of the petroleum economy centred on a theoretical bell-shaped curve relating production to time, and we are now entering his predicted period of increasing difficulties. Estimates of reserves have increased considerably in the last 35 years, and so has the efficiency of getting out the fluids. Recent news leaking from the Shell oil giant that there has been a certain fiddling of the books about how much remains in its licence areas (a 20% overestimate) is perhaps a sign of just how difficult it is to keep pace with growing demand. Oil companies hope for the best as regards how quickly new discoveries add to their assets, yet they can never voice their fears of the worst for the sake of investor confidence and the volatility of the oil futures market. The history of petroleum discovery is indeed a bell-shaped curve, and it has been on the slippery downward slope for about 30 years, with a few cheering but brief upswings. On average, annual discovery has decreased from about 50 to 10 billion barrels each year, noting that the size of the discoveries is always an estimate of what might eventually be extracted to be tempered by the fact that it rarely if ever is. A great deal of the petroleum products now being used emerge from massive discoveries in the late 30s and 40s and the mid 1960s. Nothing like the huge Arabian and Iraqi fields has been found since then. Many commentators, as usual, consider the present upsurge in oil prices to stem from political issues, but there are deeper economic and technical issues that suggest that it is an irreversible trend while ever demand is insatiable and supply more difficult to achieve. Standing above the generally quoted reserves that can reasonably be expected to flow using current methods, are several categories of petroleum in the ground that require new extraction methods and a higher price to implement them. They are considerably larger, though much more fuzzily defined, and range from the dregs that are not easily pumped, through viscous oils, tars sands to oil shales, the primary source rocks for conventional petroleum fields when geological processes free their organic content to move. So the future is likely to depend increasingly on new extraction technologies, that Jim Giles of Nature recently reviewed (Giles, J. 2004. Every last drop. Nature, v. 429, p. 694-695). There are several problems to solve in boosting production: decreasing the viscosity of oil, freeing oil that remains in sediment pore spaces, and driving the oil out under pressure. One interesting possibility is setting fire to oil in the reservoir rock, by pumping air into it. That would create gas pressure as well as lower viscosity, and has been tried before after Russian engineers accidentally set fire to a deposit by trying pressurised air to drive oil out. Following their surprise (and no doubt a ticking off by top political management), oil did flow more freely from nearby wells, but later experiments have had mixed success. Bacteria that metabolise oil are increasingly used to clean up spills. Since they break it down to lighter and less viscous molecules, and generate various gases, they have a role to play underground. However, all kinds of secondary recovery methods that are deployed today do not add a great deal to production – about 3 to 4% – and are unlikely to stave off eventual decline without further massive increases in price.
Structural control over hydrothermal gold mineralisation
One of the world’s richest gold provinces is centred on the town of Kalgoorlie in Western Australia, site of the “Golden Mile” whose production and reserves exceed 2500 tonnes of gold. The geological control is a 200 km long shear zone trending SSE that cuts Archaean greenstone associations of mafic-ultramafic and felsic lavas, and volcanoclastic rocks of the 2700 Ma Yilgarn Province. Exploration along the trend has revealed a number of other world-class gold deposits, and the Boulder-Lefroy Shear Zone has come to typify syn-tectonic hydrothermal mineralisation. Detailed work has long demonstrated that smaller shear zones slightly oblique to the main trend focus the mineralisation. That is because the main line of movement was probably in compression, having a strike-slip sense of motion. Depending on the local orientation of lesser shear zones, some have trends likely to have encouraged dilatation in transtensional environments. Fluids are more likely to favour such opening zones, thereby concentrating their flow and deposition of minerals from them. Much of the research in the area has focussed on detail, in an attempt to discover a means of predicting new deposits, and exploration is dominated by systematic drilling in what is not a particularly well-exposed terrain, and one where standard methods of stream sediment analysis are thwarted by low rainfall. Robert Weinberg of Monash University, Paul Hodkiewicz and David Groves of the University of Western Australia have taken a broader view of the structural setting (Weinberg, R.F. et al. 2004. What controls gold distribution in Archean terranes? Geology, v. 32, p. 545-548). So intensively explored is the gold province that it is unlikely that any large deposits remain to be discovered, but very similar shear zones affect most of the world’s Archaean granite-greenstone terranes, where exploration is at an earlier stage of progress. A model of regional controls over gold is therefore pretty valuable. Weinberg et al. divide the Boulder-Lefroy Shear Zone into boxes along its length, each centred on 8 gold “camps”. They plotted the deviation in trend of local segments of the shear zone in each box from its overall trend against the box’s known gold “endowment”. What emerged was a clear confirmation of the regional association of mineralisation with likely zones of regional transtension, trend deviation matching closely the estimated gold endowment. The abundance of structural data also enabled the authors to analyse the fractal dimension of all shears and fractures, thereby assessing the variation in overall geological complexity of the province. The results are odd. The least well-endowed parts of the gold province are more complex than those containing the most gold. The Golden Mile itself occurs where complexity changes from low to high. The ideas await testing on less mature shear zones cutting Archaean greenstones elsewhere in the world, such as in South India and East Africa.
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Sudbury impact turned the crust inside out
The 1800 Ma old Sudbury complex in eastern Canada is one of the largest repositories of nickel ores and contains commercial platinum deposits. It has also been ascribed to a major impact that produced a crater over 200 km across. The evidence is the common presence of shocked minerals and a sheet of very homogeneous, once molten rock, whose andesitic major-element composition suggests that it represents melting of the local upper crust. However, the trace elements, including platinum group metals, have all the hallmarks of the lower crust (Mungall, J.E. et al. 2004. Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts. Nature, v. 429, p. 546-548.). The melt sheet is mixed with upper crustal rocks, including sediments that formed in a shallow marine basin into which the meteorite plunged. This suggests that impact not only affected the whole crust, but excavated it as well, so that a 30 km deep crater formed at the instant of collision. The bulk of the homogenised crustal melt remained molten for long enough for complex fractional crystallisation to take place, thereby forming the classic layered Sudbury Igneous complex, in which the nickel ore bodies are located. They may well represent relics of the impactor itself, that mixed with molten crust.
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Seismic detection of zones of crustal melting
The Himalaya and Tibet are known for their huge granite batholiths that show geochemical signs of having formed by partial melting of the continental crust. They also show signs of ductile zones in the deep crust. Whether or not this ductility is associated with incipient melting cannot be judged easily, as there are no examples of active felsic volcanism. However, it is possible to predict theoretically where crustal temperatures exceed the solidus of the crust, whose paths in pressure-temperature space for various amounts of water content is well known. The problem is knowing the way in which temperature increases with depth. That is usually estimated from the surface heat flow and modelling the likely thermal conductivity of different crustal layers, but it isn’t suitably precise. German, US and Chinese geophysicists have tried a clever means of estimating crustal temperature using seismic data (Mechie, J. et al. 2004. Precise temperature estimation in the Tibetan crust from seismic detection of the a-b quartz transition. Geology, v. 32, p. 601-604). Experiments show that quartz in its low-temperature a form transforms to b quartz above 575ºC at atmospheric pressure, and at higher temperatures with increasing pressure. The P-T change in the transition is well known, so if b quartz can somehow be detected in the deep crust, its depth gives the crustal temperature. As luck would have it, the transition results in a significant change in the elastic properties of quartz that should effect the speed at which seismic waves travel through rocks rich in b quartz. More precisely, the P-wave speed should increase abruptly by a detectable amount. Mechie and colleagues have indeed found the depth of this transition below a seismic profile running across part of the Tibetan Plateau NNW of Lhasa. Its depth varies between about 20 to 15 km coinciding with the upper-middle crustal boundary. At its shallowest levels, the transition is directly above a large zone of high electrical conductivity, discovered by magnetotelluric surveys, which has been suggested to be due either to a high content of aqueous fluids or crustal melts. The geotherm (about 40ºC km-1) associated with the shallow a-b quartz transition crosses the wet granite solidus about 5 km beneath it, so the lower crust itself is likely to be generating granitic magmas. Although the deepest levels of the a-b quartz transition also predict likely conditions for wet melting in the lower and middle crust, below those zones there is no evidence that it is happening. One possibility is that water content varies considerably in the sub-Tibetan crust. Where melts or fluids are moving in the crust, heat transfer is not purely by conduction, and steep geothermal gradients can stem from heat being transported upwards with moving fluids.
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Arsenic tragedy in Bangladesh
Almost 35 million people in Bangladesh are probably drinking well water that contains arsenic well over the accepted safe limit. Why that is so is one of the greatest tragic ironies of our age. In an attempt to reduce the incidence of gastrointestinal disease from drinking polluted surface water, the government, with assistance from international agencies, sank millions of tube wells from the 1970s onward. The wells tapped abundant and seemingly clean groundwater from the alluvium beneath the Brahmaputra and Ganges plains. Health problems dropped dramatically, especially among children. But by 1983 a Calcutta dermatologist reported skin lesions on patients from neighbouring West Bengal in India that are a sure sign of arsenic poisoning. Even though the British Geological Survey conducted a pilot survey of water chemistry in some Bangladeshi well waters in 1991, the danger from arsenic remained unknown; BGS did not test for the element, despite routinely analysing it in British groundwater. Shortly after the report was published, typical symptoms of arsenic poisoning appeared from a wide tract of low-lying Bangladesh. Dermatological symptoms generally only start to appear about 10 years after individuals are exposed to low, but dangerous levels of arsenic in water. They are followed by a variety of cancers (of the skin, bladder, liver and kidneys) at around 20 years from the start of exposure. A number of affected Bangladeshi people have taken legal action against BGS for negligence (see British Geological Survey sued over arsenic in EPN of October 2002). However, on appeal against a legal decision to put their case to trial, Britain’s Natural Environment Research Council, of which BGS is a part, were judged to be too distant from the villagers to have had a duty of care. The issue will not go away, and informing as many people as possible about the arsenic tragedy, its causes and possible remedies is vital. This has been taken a step forward by a clear review article by a Bangladeshi health scientist, Mushtaque Chowdhury who co-chairs the UN Millennium Project’s task force on child and maternal health (Chowdhury, A.M.R. 2004. Arsenic crisis in Bangladesh. Scientific American, August 2004, p. 70-75).
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New benchmarks for geological time
In the December 2003 issue of EPN, I mentioned a programme aimed at sorting out the calibration of the stratigraphic column to an absolute or radiometric timescale (Recalibrating the stratigraphic column). The other side of this task is deciding on where to place the “golden spikes”, otherwise known as global standard stratotype-section and points (GSSPs). They are locations where the best exposures of world-wide events can be found. The first, defining the disappearance of graptolites at the Silurian-Devonian boundary (no-one knows why that happened), was placed in 1972 near the wonderfully named town of Klonk in the Czech Republic. GSSPs are essential in defining events, no matter if their ages change as dating methods and results advance. Until 1999 the problem was that only 15 of the 91 stage boundaries of the Phanerozoic had been defined agreeably by such “golden spikes”. That year the International Union of Geological Sciences (IUGS) spurred a crash programme of GSSP definition, but there have been political as well as geological disagreements. The most important “spike” is at the Permian-Triassic boundary – the end of the Palaeozoic Era, and the time of the largest ever mass extinction – and there have been heated discussions over whether to have it in Iran, Kashmir or China. Zhejiang Province in China won, and it now has a 6 metre high monument at the boundary! This and Klonk should be on every geologists’ future tourist itineraries. There are now 50 stage-boundary GSSPs, and together with a revision of currently accepted dates, the revised stratigraphic column can be downloaded as a (rather large) PDF from http://www.stratigraphy.org/. All is not so well with Precambrian time, for the obvious reason that it contains no tangible fossils, and it is still arbitrarily split by round-number dates. But there is some hope for a similar system of “golden spikes” that use probably global events such as glacial epochs, and perhaps shifts in the d13C of carbonate sediments that should record global changes in ocean composition.
Source: Whitfield, J. 2004. Time lords. Nature, v. 429, p. 124-125
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Abiotic formation of hydrocarbons by oceanic hydrothermal circulation
There has been speculation, particularly by Thomas Gold in his book The Deep Hot Biosphere, that methane can form without the intervention of organisms. In Gold’s case, he proposed an origin in the mantle that supported a thriving organic community at great depth in the crust, and that such abiogenic methane is the source of all hydrocarbon and coal deposits. Not many people believe Gold. However, there are chemically feasible means of generating simple hydrocarbons in the upper earth, notably the Fischer-Tropsch catalytic process that has been used to synthesise artificial fuels. The Fischer-Tropsch process hydrogenates a carbon-bearing gas, such as carbon dioxide, and commercially has used chromium oxide as a catalyst. In hydrothermal systems that permeate olivine- and orthopyroxene-rich ultramafic rocks, those minerals breakdown to serpentines, talc and magnetite, and the reactions generate hydrogen, which is often found dissolved in samples of oceanic hydrothermal fluids and occasionally in onshore springs, where mantle rocks in ophiolites are being weathered. So there is no shortage of hydrogen for potential reactions in sea-floor hydrothermal systems, and they contain lots of dissolved carbon dioxide. Ultramafic rocks are rich in chromium generally in the form of Fe-Cr oxide or chromite. Geochemists from the University of Minnesota simulated a hydrogen-carbon dioxide-chromite hydrothermal system to see if the Fischer-Tropsch process would work (Foustoukos, D.I. & Seyfried, W.E. 2004. Hydrocarbons in hydrothermal vent fluids: The role of chromium-bearing catalysts. Science, v. 304, p. 1002-1005). It did, producing methane, ethane and propane under simulated conditions of sea-floor vents. They conclude that these simple hydrocarbons help support thriving bacterial communities in “black smokers”. Their results also support the possibility of such vents having produced “feedstock” for processes that led to the origin of life, but also lend a cautionary note to claims for ancient organic matter (see Early biomarkers in South African pillow lavas in May 2004 EPN)
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Wildfires and oxygen
Ray Bradbury wrote a seminal political fiction in the 1950s, called Fahrenheit 451. It is about a repressive regime that tries to snuff out dissent by burning books, the title referring to the temperature (233ºC) at which paper spontaneously bursts into flame in the modern atmosphere. With no reference to book burning by some future oligarchy, geoscientists have speculated on the possibility of higher atmospheric oxygen contents being able to induce massive conflagration of green vegetation after lightning strikes or meteorite impacts. One often cited case is at the K/T boundary, where the thin layer that signifies the mass extinction event contains a high proportion of sooty particles. Late Cretaceous air probably had significantly higher oxygen content than now, generated by pole-to-pole luxuriant vegetation, and the idea of a global wildfire gained much support when first mooted. During the Carboniferous, there is very good evidence that oxygen levels were as high as 35% compared with 21% today. It was a time of giant flying insects, whose size is limited by the availability of oxygen. Carboniferous and Permian strata contain much charcoal, which suggests that indeed fires then were a great deal fiercer and more capable of spreading. They might have destroyed vegetation, despite evidence that the tree-sized plants of the period had developed fire-resistant structures. Experiments to simulate the effects up to now have used strips of paper in different oxygen levels, and showed a strong correlation between the minimum energy for ignition and oxygen concentration. US geologists, foresters and engineers have repeated the experiments using a range of natural plant materials as well as paper (Wildman, R.A. et al. 2004. Burning of forest materials under late Paleozoic high atmospheric oxygen levels. Geology, v. 32, p. 457-460). Their results approximately confirm Bradbury’s fictional paper-combustion temperature, but monkey-puzzle (Araucaria) leaves are more easily set alight. However, the temperature for ignition does not change as oxygen levels increase, although burning is faster. How natural materials burn depends on their relative proportions of cellulose and lignin, the higher the latter, the greater the temperature for complete combustion. They behave very differently from paper. Another finding was that the rate at which burning spreads did not rise as dramatically as expected for Carboniferous conditions. The limiting factor is moisture content, although that for no-burn does increase with oxygen levels. This is particularly important for the firing of dead vegetation lying on the surface, which is essential for catastrophic wildfires. Natural fires are started by lightning, and that occurs during heavy rainfall, when surface debris is thoroughly saturated. Fires in the canopy would have occurred at higher frequencies and with greater intensities, but the authors consider they would not have seriously threatened plant life.
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An enthusiastic view of deep-Earth processes
In EPN of January 2004, there appeared a summary of Warren Hamilton’s sceptical view of recent ideas about what happens beneath the 660 km mantle discontinuity (Geoscience consensus challenged). It is below that level that the dominant mantle mineral, olivine (MgSiO4), is thought to change to the more densely packed perovskite (MgSiO3). Encouraged by an experiment which suggests that at the pressure and temperature just above the core-mantle boundary (CMB) perovskite itself undergoes a phase change to define the D” seismic discontinuity (Murakami, M. et al. 2004. Post-perovskite phase transition in MgSiO3. Science, v. 304, p. 855-858), Edward Garnero of Arizona State University takes a very different view. In his Science Perspectives review of the CMB region (Garnero, E.J. 2004. A new paradigm for the Earth’s core-mantle boundary. Science, v. 304, p. 834-836) he builds into a comprehensive, illustrated model everything that Hamilton finds dubious: whole-mantle plumes and slab descent; zones of ultra-low velocity close to the CMB; undulations on it; and massive bulges of low-velocity mantle above D”, such as that suggested to underlie the South Atlantic and southern Africa from which constellations of plumes rise. He links this to a wealth of anisotropies which basalt-oriented geochemists have found and continue to relish. His enthusiastic account makes fascinating reading, but makes no mention of Hamilton’s and others’ doubts about gilding the lily of only a few short years of seismic tomography.
Mesoproterozoic large igneous province and Rodinia
Flood basalt events in the Phanerozoic seem generally to have preceded the break-up of supercontinents, and many geoscientists believe that their formation is implicated in the mechanism of continental disaggregation. So it comes as something of a surprise to learn that the assembly of most continental lithosphere to form the Rodinia supercontinent about 1100 Ma ago, which ranks in size with Pangaea, was probably accompanied by massive igneous activity (Hanson, R.E. et al. 2004. Coeval large-scale magmatism in the Kalahari and Laurentian cratons during Rodinia assembly. Science, v. 304, p. 1126-1129). The Proterozoic sediments of southern Africa and once-adjacent Antarctica are intruded, wherever they occur, by basaltic sills up to hundreds of metres thick. In a few places relics of flood basalts above the sedimentary groups have the same composition and age, around 1100 Ma. Like Phanerozoic large igneous provinces, most of the magmatism occupied only a few million years, perhaps less than 1Ma. The distribution of the probable feeder intrusions for the few relics of CFBs suggests that the province in the Kalahari craton formerly covered about 2 million km2, so it ranks in size with most Phanerozoic LIPs. In North America, cored by the craton of Laurentia, there occurs the Keeweenawan dyke swarm and other mainly mafic intrusions, that probably fed another veneer of CFBs. Dating them using the same single-crystal U-Pb method reveals ages that are within error of those from southern Africa. Combined, the two LIPs are much larger than the biggest know LIP from the Phanerozoic – the Ontong-Java Plateau that formed on the floor of the West Pacific Ocean during the Cretaceous. So, were there two massive, but short-lived igneous events while Rodinia was assembling, or one that unites both the Kalahari and Laurentian cratons? In many models of Rodinia, stitched together using orogenic belts that formed in the late Mesoproterozoic between1150 and 950 Ma, the Kalahari craton has been placed against Laurentia; both LIPs could be a single super-province. However, the same authors also measured palaeomagnetic pole positions from the southern African igneous rocks. They are different from those revealed by the Laurentian LIP, and imply considerable separation of the two continental masses at the time of igneous activity. That suggests either separate melting events in the mantle beneath both cratons at the same time, or that both are parts of an even larger magmatic upheaval that spanned about 1/5 of a hemisphere. Whichever turns out to be the case, this ancient large-scale mantle event bucks the Phanerozoic trend of LIPs’ presaging or accompanying continental break-up. Maybe the rare mantle upwellings thought to generate LIPs are really random in their positioning, and “just happened” to rise beneath Pangaea and its fragments from the Devonian onwards.
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Human origins site – the palaeoanthro weblog
This seems to be a blog well worth examining and mining – www.talkorigins.org/faqs/homs . The blogger, Jim Foley, maintains an excellent sense of humour as well as what appears to be considerable energy and knowledge. There is a link to a masterful April Fool’s Day joke at the expense of the Institute for Creation Research, which gulled their radio show, Science, Scripture and Salvation in 2000 into accepting at face value a spoof article in the April 1997 issue of Discover magazine. This was penned by the German palaeoanthropologist Oscar Todkopf (Deadheads are fans of the Grateful Dead) of Hindenburg University (Led Zeppelin and a well-known, flaming bag of gas), which documented a find of assorted musical instruments, (a 6 foot length of mammoth tusk turned into a tuba, a bagpipe-like instrument made from the bladder of a large animal, a triangle of thin bones, a collection of hollowed out bones of different lengths, which Todkopf suggested might be part of a xylophone (he called it a ‘xylobone’), the first known Neanderthal cave painting, showing marching musicians alongside some suspected musical notation, and a Neanderthal skull) in the famous Neander Valley, Germany. Even the fact that the eponymous author claimed that Neanderthal musicians played the bagpipes with their remarkably huge noses, did not deter the ICR’s Marvin Lubenow, author of the leading creationist book on human origins, Bones of Contention, from commenting, “There’s overwhelming evidence that Neanderthals were musically inclined.”, along with a further stream of howlers. For that alone, you must visit this site. However, it is probably the best source of human-origins information, illustrations and news that there is on the Web, and puts the EPN anthropology and geoarchaeology section to shame! There is a balance, for the site includes a great many items on creationist ideas, but this has to be tongue in cheek, despite the accuracy of the accounts there. I wonder who Jim Foley is….
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Crater linked to end-Permian extinction
In mid May news spread fast that a nearly circular feature that shows up in gravity data over the north-western continental margin of Australia could be a crater, about 220 km across, which formed at the end of the Permian (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia. Science Express 14 May 2004 – www.sciencexpress.org). Australian and US scientists have examined drill cuttings from exploratory oil wells that penetrate to the level of the hidden feature. They describe breccias and associated melt rock. A plagioclase separate from the exploration well has an Ar/Ar age of 250.1 ± 4.5 Ma, that is within error of the age (251 Ma) of the largest Phanerozoic mass extinction. Unfortunately, they have not discovered the easily recognised signs of shock damage to minerals – distinctive banded lamellae in quartz – nor any meteoritic chemical signature. Nevertheless, the structure is huge and looks very like the gravitational expression of the Chixculub crater off the Yucatan Peninsula of Mexico, drill core from which shows all the signs of having formed by an impact at the end of the Cretaceous. Evidence is accumulating from the Permian-Triassic boundary sequence that some event did produce all the signs usually attributed to a major impact in a global ejecta blanket (see Permian-Triassic boundary and an impact?, December 2003 EPN). Despite glass being included in the breccias, many experts on impact processes and products are sceptical that the Bedout structure was produced by an impact. But probably the only way in which such melts might have formed is by some kind of seismic shock, although that could have occurred during volcanism.. The structure is so huge that if it does have an origin by internal processes it ranks among the biggest to be found – could this ironically be a product of a Verneshot event (see Mass extinctions and internal catastrophes, above)?!
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Caves and snoticles
If ever there was “received wisdom” in the geosciences the most pervasive is the notion that the weak acid formed when carbon dioxide dissolves in rainwater is the cause of carbonate solution. Anyone hearing it in the spiel from a cave guide, while admiring caverns as big as cathedrals, is not surprisingly awe-struck by such an innocuous sculpting agent. Many speleologists have long wondered if there might be other mechanisms, and the discovery of bacterial films that generate strong sulphuric acid provides a good candidate. They can take the form of floppy, stalactite-like masses, that have become fondly known as “snoticles”. However, their role in cave formation had not been substantiated until April 2004. Microbial geochemists at the University of Texas carefully studied the geochemical balances in a cave system in Wyoming where such bacteria are abundant (Summers Engel, A. et al. 2004. Microbial contributions to Cave formation: New insights into sulfuric acid speleogenesis. Geology, v. 32, p. 369-372). The bacteria are members of two groups that live in aerated conditions and use the oxidation of sulphide ions (from hydrogen sulphide) as a source of metabolic energy. Oxidation results in sulphuric acid, which rapidly dissociates in water to generate abundant hydrogen ions (the source of acidity and low pH) and sulphate ions. So, to thrive the bacteria need a continuous source of hydrogen sulphide, of which more later. The study by Annette Summers Engels and two colleagues shows that hydrogen sulphide is efficiently consumed by the bacteria, so that little if any enters the cave’s atmosphere. Interestingly, water flowing through the cave isn’t particularly acid either, yet the bacteria generate a great deal of sulphuric acid. It is rapidly neutralised by reaction with calcium carbonate near the colonial mats, to increase the flux of calcium and sulphate ions into solution. The effect extends to limestone pebbles on the beds of the cave streams, so the bacteria encourage solution beneath water as well as near snoticles hanging from the roof. That suggests that they can live below the water table, where many caves are thought to have formed in the past, being left as open caverns as the water table fell as bulk permeability increased with solution. The studied cave does experience a constant flux of hydrogen sulphide, but where does that come from? There are other groups of bacteria that generate sulphide from dissolved sulphate ions, but under highly reducing conditions. They are the source of the “sour gas” that is a constant danger in oil production in some petroleum fields, consumed gleefully in dissolved form at a great many spas and generated in our own guts. These sulphate-sulphide reducing bacteria get their energy from dead organic matter, that many sediments deposited under reducing conditions contain in substantial volumes. Interestingly, connectivity between oxygen-rich and oxygen-starved groundwater might create a recycling of sulphur that involves both bacterial groups. Many limestones contain strata that are rich in organic remains and metal sulphides, in which conditions become reducing. Equally, interbedded, black shales might play a role.
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Mass extinctions and internal catastrophes
The four largest extinction events of the Phanerozoic (late Devonian, 370 Ma; end-Permian, 251 Ma; end-Triassic 201 Ma; end-Cretaceous , 65 Ma) each coincide with periods of rapid and voluminous continental flood-basalt volcanism. There is also evidence from the extinction horizons that each coincided with a major impact event as well, most widely accepted for the end-Cretaceous event. Geological time is so long that pure chance cannot be ruled out entirely to explain coeval impacts and CFB events, but is unlikely (a 1 in 8 chance for one coincidence, but 1 in 3500 for four). So there has been a long-running controversy over a volcanic or an extraterrestrial cause for extinctions, together with speculation that large impacts can somehow trigger CFB events. The last does not work for the end-Cretaceous extinction, because the Deccan volcanism began somewhat before the formation of the “smoking-gun” Chicxulub crater, and a linking mechanism is not clear. Taking into account lesser extinctions and CFB events, there is a rough periodicity of 30 Ma and similar ages for both. Geoscientists at the Geomar Institute of the University of Kiel in Germany have stoked up the controversy by taking a very different view of events (Phipps Morgan, J. et al. 2004. Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: are mantle plume-induced lithospheric gas explosions the causal link? Earth and Planetary Science Letters, v. 217, p. 263-284) albeit not a completely new one. They consider the processes at depth that presage CFB events, where rising mantle material impacts at the base of thick continental lithosphere. Each of the CFB provinces linked in time to the four large extinctions lies on an ancient craton, devoid of tectonic activity for over a billion years, and greatly depleted in heat-producing elements. Lithosphere beneath them is over 300 km thick and might have acted in the manner of the lid on a pressure cooker, building up gas pressure during the delay in breaking through overlying rock. Eventually pressure would be sufficient to breach the lithosphere, and gases (CO2 and SO2) would be explosively vented, perhaps creating globally toxic conditions. Release of the pressure would lead to collapse above the plume head that would propagate upwards, at hypersonic speeds according to the authors. Maybe that would fling enormous amounts of rock into the stratosphere. Some chunks might be large enough to cause big impact structures at the surface when they fell back, so explaining the coincidence. They account for the pre-extinction start of CFB outpourings, as in the case of the Deccan traps, by lateral and upwards migration of part of the plume to locally thinned lithosphere. The power involved in such an event extending through the entire lithosphere could account for the shocked grains, microspherules and fullerenes in known extinction horizons. Being sourced in mantle rock that may once have resided near the core-mantle boundary, such a process could also eject high iridium concentrations that were the signs that first led to the Alvarez’ hypothesis of impact-induced extinctions, but without an extraterrestrial culprit. Despite the attractions of the impact theory, no sign of meteoritic debris has been found in any of the ejecta horizons or the craters themselves. On Phipps Morgan and colleagues’ account that is not surprising, because the impacting objects would have been common Earth rock. The authors decided to dub these hypothetical events “Verneshots” after Jules Verne’s book From the Earth to the Moon, which involved a giant gun firing the space craft moonwards. If there is anything in the idea, then surely there would be spectacular evidence of the source of the blasts, but perhaps they are conveniently buried by later CFBs. Geophysical studies do show signs of circular features beneath both the Deccan and Siberian Traps. However, the associated seismic shock waves would pervade large volumes of crust outside the blast vent, and signs of that, such as shatter cones, are perhaps an easier target. As with all departures from “accepted wisdom”, the Geomar group’s ideas will come in for a lot of stick, quite possibly from the fans of giant impacts, who not so long ago were themselves dismissed as “whizz-bang kids” by many geoscientists.
That gas build-up might lead to catastrophic crustal collapse gets some support from a modelling study on the processes involved in volcanic collapse (Reid, M.E. 2004. Massive collapse of volcano edifices triggered by hydrothermal pressurization. Geology, v. 32, p. 373-376), albeit in miniature. Mark Reid of the USGS focuses on those volcano collapses that occur without any warning signs from eruptions and seismicity. His study examines the effects of deep intrusion of magma on the groundwater systems within stratovolcanoes. This could promote increases in gas pressures deep within the edifice. Their upward propagation would destabilise the entire volcanic structure, leading to its collapse in extreme situations. The modelling indicates increased likelihood of over-pressuring where permeability is low; a crude analogy to Phipps Morgan and colleagues’ pressure lid of inert cratonic lithosphere. Gas-rich magmas can emerge explosively in continental flood basalt provinces, normally regarded as forming by episodic, quiet outpourings from fissure systems. That is well demonstrated by the Ethiopian-Yemeni CFB province. The main basaltic trap sequence is followed by very widespread felsic ignimbrites on both sides of the Red Sea that formed by lateral blasts of incandescent debris and felsic lava shards. Only one example of an ignimbrite centre is known from the province. Lying about 60 km south of Sa’ana, near the small town of Mabar, it is a circular structure about 18 km across with clear concentric zoning. Interestingly the zones dip steeply towards the centre of the structure, in an inverted cone, that is possibly due to collapse even more dramatic than in the calderas that sourced the more familiar ignimbrites of the Andes.
See also: Ravilious, K. 2004. Four days that shook the world. New Scientist * may 2004, p. 32-35.
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Middle-eastern Prometheus
Several articles over the years in EPN have referred to the phenomenal movement of humans from Africa to much higher latitudes in Asia, from as early as 1.8 million years ago. Although that migration must have been a gradual diffusion rather than with any purpose, even in interglacial periods it took our ancestors into chilly winter climes. Many palaeoanthropologists have sought evidence for controlled use of fire that would have made survival more likely, but until recently little concrete signs have been found before the last glacial epoch. Israeli scientists, who have worked on an Acheulian site in the Jordan valley, found evidence of much earlier fire use (Goren-Inbar, N. et al. 2004. Evidence of hominin control of fire at Gesher Benot Ya’aqov, Israel. Science, v. 304, p. 725-727). A 34 m thick sequence of sediment on the shore of an ancient lake contains several tool-bearing horizons, in each of which they found flint artefacts that show signs of having been burned. There are also fragments of burnt wood. Were the burned remnants widely distributed they could be accounted for as the result of wildfires, but they occur in clusters. That strongly suggests hearths and a human origin. The age of the sequence is indicated by the layers that contain tools and evidence of controlled use of fire lying just above the Brunhes-Matuyama geomagnetic polarity reversal, whose end is dated at 790 thousand years ago, when the most likely inhabitants were Homo erectus. The thickness of sediment containing the layers with signs of human activity suggests several tens of thousand years occupation of the site. Some of the burnt vegetation is of edible species. However, despite finds of animal bones that show signs of having been processed for food, there are no burnt bones. So, fire may have been used for comfort, but there is no proof of cooking.
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And now….molybdenum isotopes! Ocean anoxia in the Proterozoic
“Everyone knows” that free atmospheric oxygen appeared about 2300 million years ago, thanks to the waste products of blue-green bacterial photosynthesis. At least the land surface became an oxidising environment and a progressively redder place, as Fe-2 was oxidised to Fe-3 which forms insoluble oxides and hydroxides. Paradoxically, the shallow sea floor of earlier times was redder than anything since, because of exactly the same oxygen-containing, ferric minerals. It hosted the largest build-up of any metal concentration in Earth’s history; the banded iron formations (BIFs) that have for a century or more been the source of industrial iron. A simple, and probably accurate explanation for BIFs is that iron dissolved in ocean water that lacked oxygen as Fe-2, and was supplied by sea-floor volcanism. Once blue-green bacteria began pumping out oxygen, an oxidising reaction dumped both elements as slimy red sediment where the two met. Dissolved iron consumed oxygen – just as well, because to most prokaryote life it is a poison – yet as oxygen productivity rose (and perhaps sea-floor spreading slowed) dissolved iron was increasingly removed by oxyidation from sea water. The tipping point, when air contained oxygen and sea water became starved of iron (a vital micronutrient for phytoplankton) is difficult to address since the two chemical environments are so different and interact in complicated ways. BIFs continued to form for about half a billion years after the first sign of atmospheric oxygen, then they disappear from the geological record at 1800 Ma ago. There were minor reappearences in the Neoproterozoic, at the time of “Snowball Earth” events, and that is a fascinating topic in its own right. Clearly, there was a long period of transition to what we can regard as a thoroughly modern world. Studies that use sulphur isotopes suggest that in the Mesoproterozoic the upper ocean was oxygenated while bottom waters were perpetually akin to those of the Black Sea today. Conditions in them may have been highly conducive to burial of dead organic matter – rapid drawdown of atmospheric CO2, but allowing the massive production of methane by anaerobic bacteria. Methane is a far more potent greenhouse gas than carbon dioxide, so controls over climate may have been very different from today’s. Molybdenum offers an independent and potentially useful means of testing hypotheses about ocean chemistry. It enters the sea in river water, which in post 2300 Ma times would have been oxygenated, allowing the formation of the soluble and very stable molybdate ion. In anoxic ocean floor conditions, bacteria that generate hydrogen sulphide remove molybdenum as the sulphide, which is why modern Mo concentrations remain stable – it ends up in a very small percentage of ocean floor sediments. The stable isotopes of molybdenum (97Mo and 95Mo) fractionate during precipitation of the element, the heavier one being preferentially removed during sulphide precipitation, to give high 97Mo/95Mo ratios in sediments. The opposite seems to occur if precipitation is in the oxide form, as in sea-floor manganese nodules. Geochemists from the Universities of Rochester and Missouri, USA have compared Mo isotopes from apparently anoxic Mesoproterozoic sediments with those in modern euxinic basins (Arnold, G.L. et al. 2004. Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans. Science v. 304, p. 87-90). The Precambrian results are isotopically much lighter than modern ones, suggesting that 97Mo did not become enriched in seawater as a result of oxide precipitation in the equivalent of modern manganese nodules. They estimate that 10 times more of the ocean floor was anoxic than today or since about 1300 Ma ago. So far no comparable work has been done of the extremely abundant black shales and schists of the Neoproterozoic, that link with “Snowball Earth” events. Whether or not “modern” redox conditions emerged 1300 Ma ago, with probably a big impact on climate controls, the oddest time climatically was between about 750 and 600 Ma ago. Not only were there several dramatic coolings and warmings, but the main indicator of organic carbon burial, d13C, went haywire. As did the BIFs, did ocean anoxic conditions once more get footholds. Molybdenum isotope data seem likely to shed some light on those strange times.
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