Epoch, Age, Zone or Nonsense

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The International Commission on Stratigraphy lists 37 Series/Epochs and 85 Stages/Ages in the latest version of the International Stratigraphic Chart for the 11 Systems/Periods of the Phanerozoic. A great battle against ICS’s attempt to extinguish the Quaternary, the only enduring Era originated by Giovanni Arduino (1714-1795) and Johann Gotlob Lehmann (1719-1767), now seems to have ended in a compromise (Kerr, R.A. 2008. A time war over the period we live in. Science, v. 319, p. 402-403). While that vigorous struggle has apparently petered out, the Stratigraphic Commission of the Geological Society of London has launched another by proposing a new Epoch – the Anthropocene. This follow a suggestion by Nobel laureate and chemist Paul Crutzen that the Holocene Epoch ended once humanity made a significant impact on the Earth system (Zalasiewicz, J. and 20 others 2008. Are we now living in the Anthropocene? GSA Today, v.18(ii), p. 4-8).

The device intended by the ICS to mark boundaries between Periods, Epochs and Ages in the Phanerozoic is a symbolic Global Standard Section and Point (GSSP), combining an absolute age definition and a type section. A growing number of boundaries are marked by a physical ‘golden’ spike (not necessarily made of gold) including a plaque engraved with the Period or Age names, welded into the agreed boundary itself. There is good reason for this seemingly odd behaviour; geologists need to have agreed nomenclature and locations so that their discourse can be internationally sensible. It is also a deeply exciting, even exalting moment when any geologist puts her/his finger on a boundary of global significance: and how supremely triumphant actually to wield the hammer that drives the spike home. So much so, that there have been monumental squabbles, some not far short of diplomatic ‘incidents’, about exactly where GSSPs should be placed.

But the whole bureaucratic process has its awkwardly humorous side. There is a proposal that the GSSP for the Pleistocene/Holocene boundary be located in a Greenland ice core. Is that to be in the hole left by the NGRIP core drill at the centre of Greenland, at the depth at which evidence for the warming at the end of the Younger Dryas (11.5 ka) occurs? Or should it be in the core itself – a GSSP in a fridge? Either way, it is going to be difficult to put a finger on that particular boundary Moreover, global warming and the attendant social disruption might remove both. The proposed Anthropocene might have an even stranger GSSP. For a start, when did it begin? An anthropogenic human signature appears clearly in the NGRIP core around 8 ka bp, and at a variety of levels in pollen records, but the GSL’s Stratigraphic Commission wants it to start at the beginning of the Industrial Revolution. Sadly, that is a profoundly diachronous, economic boundary. To make it Eurocentric, as Crutzen suggested, would be a bit non-PC.

Let’s face it, the Holocene is just an interglacial, similar to a great many since 2.4 Ma ago. It is noted only for the brief period in which humanity became separated into two groups: a very small one owning the means of production; the other, initially diverse, being forced to work for the first in order to survive. The Industrial Revolution marked a social simplification into two opposed classes, as clearly defined by Marx, and the increased dominance of human affairs by an inhuman entity called capital. The working through of the contradictions bound up in class society and in capital itself has been largely responsible for the huge environmental changes drawn on by Zalasiewicz et al. It seems our somewhat po-faced authors forget the great many more scholars of human affairs than there are geologists: historians and political economists. Already there are plenty of anthropocentric equivalents of GSSPs in London itself, in the form of its celebrated blue plaques. Historians and political economists might well agree that the rise to dominance of capital – and hence the emergence of rapid environmental change during the uniquely short-lived Anthropocene – began outside the Banqueting Hall on Whitehall at 2.04 pm on Tuesday 30 January 1649 with the separation of the head of the divinely righteous monarch, Charles I, from his body. Ladies and Gentlemen of the SC of the GSL, that is where you place your ‘golden’ spike. However, geology might yet have its say, any time now (and geologists cannot really foretell): a super-volcanic eruption; a comet strike or a cosmic gamma-ray burst. So you had better be quick, if your aim is posterity.

Watermills and meanders

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The classic notion of a floodplain is that the streams responsible for it meander to create point bars, overbank muds and all the other paraphernalia of the fluvial sedimentologist. River authorities seeking to restore floodplains see the meandering stream as the ideal to aim for, and increasingly as a means of natural flood amelioration. All this may turn out to be illusory following publication of a study on long-vanished human activities (Walter, R.C. & Merritts, D.J. 2008. Natural streams and the legacy of water-powered mills. Science, v. 319, p. 299-304). By mapping and dating alluvial deposits along 1st to 3rd order streams in the north-eastern USA, in relation to milldams recorded on 19th century maps, Walter and Merritts of Franklin and Marshall College, Pennsylvania found that up to 5 metres of sediment had accumulated behind the dams since the 17th century up to the abandonment of watermills.

The conclusion is that mill dams together with increased sediment load following deforestation for agriculture created valley flats on a vast scale – three counties in Pennsylvania had over a thousand mill dams. In places along the north-eastern Piedmont the density of water mills reaches as many as one per square kilometre, and the median density of around 1 per 10 km2 involved more than 22 000 mills out of a total in 1840 of >65 000. Once the mills were abandoned, either because their dams had silted up or milling turned to larger facilities powered other energy sources, streams developed meanders that gradually incised the artificial flood plains. The situation now is that the small floodplains rarely flood, spates being unable to spill over the current bank height. Consequently, many of the low-order streams in major river catchments discharge floods quickly to the larger streams and rivers, which themselves burst their banks to cause floods with major social and economic consequences.

Walter and Merritts’ findings are also based on their analysis of the kinds of sediment that accumulated before European colonisation. In most small valleys these indicate extensive forested wetlands with multiple small channels and drier islands. A major influence over this earlier state was the formation of logjams, and perhaps beaver lodges, that spread normal and spate flows. Slow steam flow carried less sediment than nowadays, and the older Holocene alluvial deposits are organic rich. In addition, stream flow, once directly connected to groundwater, has become disconnected thereby reducing both recharge and the flood balancing achieved by truly natural streams.

The whole of Europe had a history of milling around five times as long as that in the eastern USA, as well as higher population densities. In addition, urban mill dams for metal forging and textile manufacture were on a larger scale. The UK’s National River Authority, Environment Agency and Phil Woolas, the Minister of State (Environment) need to read this study with care, as another flood season is almost certain in the summer of 2008 or the winter of 2008-9. As far as I can judge, it demands a reassessment of flood prevention ‘best practice’ in any populated humid-temperate landscape. Whatever, Walter and Merritts’ study forces a new look at the European lowland and upland geomorphology used for teaching at all levels.

An old bat from Wyoming

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The Lower Eocene Green River Formation of Wyoming is dominated by fine-grained lake sediments, mainly made of laminated limy mudstones. Many layers constitute superb lagerstätten teeming with remains of delicate organisms. As well as much else, The Green River Formation is noted for its early bats, which suddenly appear in the fossil record with all the prerequisites for flight. The cover of the 14 February 2008 issue of Nature depicts a perfect specimen showing the four elongated ‘fingers’ that supported its wing membrane, and a long tail, which few modern bats have, except in atrophied form to support the rear part of the wing. In many respects it has a transitional structure between non-flying mammals and later bats, but would definitely have been a good flyer or rather flutterer-glider.

Not only is the fossil spectacularly well-preserved, detail of its head morphology helps resolve the issue of whether echolocation preceded flight (Simmons, .B. et al. 2008. Primitive Early Eocene at from Wyoming and the evolution of flight and echolocation. Nature, v. 451, p. 818-821). Other, slightly later fossil bats from the Green River Formation probably did echolocate, as evidenced by their stomach contents, and enlarged larynx and cochlea for transmitting and receiving the now typical high pitched squeaks of many bats. Onychonychteris doesn’t have such characteristics, so it seems as if echolocation did not evolve before flight, thereby resolving one of Darwin’s vexations about the universality of natural selection. Prior to the discovery by Simmons et al. many bat-oriented evolutionists speculated that echolocation evolved among small arboreal mammals so that they could detect passing insects. A habit of leaping to grab the prey in turn selected for an ability to glide from a strategic perch, for quite obvious reasons. Success further encouraged the evolution of powered flight. Yet no other living mammals have echolocation, probably because it is a highly energy-intensive habit. However, the muscles used by a flying mammal serve also to make squeaking a ‘cost-free’ bonus. So, the findings in Onychonychteris seem to resolve the matter nicely.

See also: Speakman, J. 2008. A first for bats. Nature, v. 451, p. 774-775.

Life perked up by repeated impacts

Following the blazes of publicity since the early 1980s about the demise of the dinosaurs at the K/T boundary it is easy to regard objects the size of mountains that fall out of the sky as bad news for life. That is despite the fact that, bar the Chicxulub impact structure that exactly matches the timing of the end-Cretaceous mass extinction, no other significant and rapid drop in the diversity of life has been found to be associated with an extraterrestrial impact. Whatever their cause, mass extinction events sometimes seem to be followed by bursts in biodiversity, presumably as the survivors eventually find lots of new opportunities and diversity to occupy them. One exception is the end-Ordovician mass extinction that was also preceded by a tripling in the number of families, which the extinction rudely interrupted. This has often been seen as a somewhat delayed exploitation of all the advantages and competitive opportunities conferred by the appearance of hard parts at the start of the Cambrian. But remarkable finds in the limestone-rich Ordovician of Scandinavia suggest an unexpected connection with meteorite bombardment (Schmitz, B. and 8 others 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience, v. 1, p. 49-53).

The most usual measure of diversity used by stratigraphic palaeontologists is the number of families at a particular time, and the overall tripling in the Middle to Upper Ordovician is notable. However, if specimens of individual groups, such as brachiopods, are collected from the Scandinavian limestones on a bed by bed basis, increased diversity at the species level is even more dramatic. There are sudden doublings or triplings over periods of what can be no more than a few hundreds of ka, especially around 470 Ma ago. In the 1960s potassium-argon dating of chondritic meteorite collections revealed a cluster of reheating ages between 500 and 450 Ma (Upper Cambrian to Upper Ordovician); about 20% of all meteorites fall into this age-cluster, and most show evidence of having been shocked as well as heated up. This seems to signify a major collision or series of collisions in the Asteroid Belt around the early Palaeozoic. More reliable and precise 40Ar-39Ar dating narrows this event to a period between 463 and 477 Ma in the Middle Ordovician. In 2001, Birger Schmitz of the University of Lund reported, with others, more than 50 sizeable chondritic meteorites in the Middle Ordovician limestones of Sweden. Schmitz and his Damnish, US and Chinese colleagues in the new paper give plots of brachiopod species and also the abundance of chromite grains of meteoritic origin in Middle Ordovician limestones from Sweden and China. Two sharp jumps in brachiopod species numbers are  preceded and accompanied by ‘spikes’ in the number of extraterrestrial chromite grains, so the link seems to be real. Yet what can have produced such a counter-intuitive result? One possibility is that the undoubted disturbance may have killed off species of one group, maybe trilobites, so that the resources used by them became available to more sturdy groups, whose speciation filled the newly available niches. Such a scenario would make sense, as mobile predators/scavengers (e.g. trilobites) may have been less able to survive disruption, thereby favouring the rise of less metabolically energetic filter feeders (e.g. brachiopods).

A Cretaceous Ice Age?

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Accepted geoscientific ‘wisdom’ is that the Cretaceous Period was so warm that forests reached polar latitudes and so too did cold-blooded reptiles. Planktonic foram oxygen isotopes indicate that the Cretaceous ‘hothouse’ in the Turonian (93.5-89.3 Ma) produced tropical sea-surface temperatures up to 37°C; warmer than human blood temperature. It also saw sea level reach an all time high. Both features have been attributed to the rate of ocean-floor volcanism being at its highest. It has, however, been difficult to model the warmth at high latitudes without fudging the input to general circulation models.

Measuring d18O in both planktonic and benthonic (ocean-floor) forams at centimetre spacings in Turonian ocean-floor sediments seems to have truly bamboozled specialists in the Cretaceous. They reveal a period of ~200 ka  at around 91.2 Ma where both show a sharp increase (Bornemann, A. and 8 others 2008. Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science, v. 319, p. 189-192). Respectively, the peaks reflect decreased sea-surface temperature (but only down to 32°C in the tropics) and an increase in the extraction of light 16O from the oceans; only likely when ice caps build up on land. The size of the benthonic d18O increase suggests ice caps about half the size of that now blanketing Antarctica. Other evidence includes rapid decreases in Turonian sea level in Europe, North America and Russia; only likely on such a scale as a result of glacio-eustasy. However, direct evidence in the form of tillites, striated pavements and glacio-marine sediments has yet to turn up

Until these convincing data emerged, it seemed that sufficient post-Permian frigidity for large-scale glaciation had not developed until Oligocene times. However, the paradox of high-latitude ice caps and low-latitude balmy seas is resolvable. Evaporation from the tropical sea surface would have been much greater than nowadays. Transport of moisture to cooler areas may have resulted in such immense winter snowfall at high latitudes that sufficient remained unmelted after winter darkness for its albedo to further cool the polar region. Almost certainly the site for the ice cap would have been Antarctica, which in the Cretaceous, as now, sat over the South Pole. Remove the present ice, and that continent would have had an average surface height of between 1 and 2 km that would have encouraged snow build up were sufficient to have fallen during the Turonian. Yet without the direct evidence for glaciation in sediments – much would be buried by the present Antarctic ice cap, if not eroded away – the scenario is difficult for some to believe.

Holocene cold spell and glacial lake burst

The most startling event during the gradual warming after the last glacial maximum was the millennium of icy conditions between 12.5 and 11.5 ka; the Younger Dryas. Long after Holocene warmth seemed well established and agriculture had been underway for two millennia, with perhaps increased human population, a smaller cold ‘snap’ took place, between 8.21 and 8.17 ka; i.e. for about 70 years. Its main effect was around the North Atlantic, but it was felt over the whole hemisphere. It must have been devastating for early farmers and new migrants into higher latitude lands. High-resolution records of many kinds are possible for such a young event, from both ice and marine cores, and also terrestrial pollen records. Norwegian, French and Dutch climate researchers have gleaned a great deal from a sea-floor core from between southern Greenland and Labrador (Kleiven, H.F. et al. 2008. Reduced North Atlantic deep water and the glacial Lake Agassiz outburst. Science, v. 319, p. 60-64). Their combined fossil, oxygen-isotope and mineralogical study shows anomalies from about 170 years before to 100 years after the drop in regional temperatures.  These include signs of decreased saltiness of the water in the Labrador Basin and a reduction in production of deep water in the North Atlantic. This is exactly the predicted signature for a shut-down of the Gulf Stream, similar to those implicated in Dansgaard-Oeschger events through the last Ice Age and the Younger Dryas itself.

The Younger Dryas has been linked to sudden drainage of huge glacially dammed lakes that once surrounded the ice cap of the Canadian Shield.  One scenario for that is a huge, protracted flood down the St Lawrence River into the North Atlantic, another being one down the MacKenzie River into the Arctic Ocean. Freshening of surface waters by such means would have reduced the formation of the dense cold brines that sink to form North Atlantic Deep Water today. In so doing these down-wellings drag surface waters northwards from low latitudes to form the Gulf Stream that makes the western side of the North Atlantic unusually warm. If they stop or slow significantly regional air temperatures fall, as they did again around 8.2 ka. In this case the likely cause was escape of water melted from the last dregs of the North American ice sheet that had been held in a glacial lake south of Hudson Bay: Lake Agassiz.

Pacific plate about to split?

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The world’s largest lithospheric plate lies to the west of the East Pacific Rise spreading axis, and extends from 60˚N to 60˚S. A string of volcanic islands connects Easter Island close to the East Pacific Rise to Samoa on the northern end of the Tonga Trench. Each lies above a small hot spot, which collectively define the most densely packed area of active within-plate volcanism on the Pacific Plate. Associated with it is an area of anomalously shallow ocean floor: the South Pacific Superswell. North of this zone the plate velocity has been faster than that of the southern part of the Plate for the last 7 Ma. One explanation for the hot-spot cluster is that it lies above a ‘tear’ that is starting to develop in the Pacific lithosphere (Clouard, V. & Gerbault, M. 2008. Break-up spots: Could the Pacific open as a consequence of plate kinematics? Earth and Planetary Science Letters, v. 265, p. 195-208). Others dispute this conjecture, but Clouard and Gerbault have modelled strain patterns across the Plate, using plate speeds derived from magnetic stripes and GPS measurements, to predict where volcanism might arise in relation to a focussed shear zone in the lithosphere. The model points directly at the linear cluster of hot spots. Maybe this is the site of a future division of the Pacific Plate into two, the current magmatism perhaps to generate a new, E-W spreading axis. That would be 5 to 20 Ma off, so there is plenty of time to discuss the processes going on.

See also: Reilly, M. 2008. I the Pacific splitting in two. New Scientist, v. 197 (26 January 2008 issue), p. 10.

Is plate tectonics a turn-on or a turn-off?

The dominant force that helps to drive plate motions is the pull exerted by dense cold lithosphere descending subduction zones. If the total length of subduction zones were to increase or decrease, or some other factor affecting the global rate of subduction changed then plate movements overall would be affected. Yet it is plate tectonics that actually removes the bulk of heat continuously generated in the deep Earth by radioactive decay, the amount of which changes very slowly over periods of tens to hundreds of million years. Should plate movements slow or stop that heat would either build up at depth or would emerge in a way unrelated to the motion of plates, perhaps as within-plate magmatism.

Should the Pacific Ocean close, then a large proportion of modern subduction would stop, and some kind of thermal and mechanical compensation would cut-in. There were times in the past when vast oceans did close as supercontinents formed: the formation of Rodinia in the late Mesoproterozoic; the Pan African orogeny of the late Neoproterozoic; the mid-Phanerozoic assembly of Pangaea. Each would have resulted in an order-of-magnitude fall in the rate of subduction. Paul Silver and Mark Behn of the Carnegie Institution of Washington and Woods Hole Oceanographic Institution have attempted to judge what kind of thermal and mechanical compensation may have taken place (Silver, P.G & Behn. M.D. 2008. Intermittent plate tectonics. Science, v. 319, p. 85-88). They look at geochemical parameters that ought to act as proxies for subduction processes – the way certain element and isotope proportions in the mantle (Nb/Th and 4He/3He) are affected by the productivity of arc magmatism. Another proxy for subduction intensity is the rate of production of continental crust, assuming reasonably that most is produced from magmas generated at volcanic arcs.

It has become increasingly clear, as the number of absolute ages from the crust has steadily increased, that the continents have formed in a stop-start fashion. Convincingly, Silver and Behn’s synthesis of Nb/Th and 4He/3He ratios in basalts also shows a marked fluctuation in the rate of the mantle’s chemical depletion. It peaked at the end of the Archaean, declined to a minimum around 1 Ga and rose again with the formation of Pangaea at about 300 Ma. The link with supercontinent formation is not simple, although a pattern emerges. Pangaea and the suspected Nuna supercontinent of the Palaeoproterozoic link to peaks in mantle depletion rate, whereas the supercontinents Rodinia and Pannotia (arising from the Pan African) formed while depletion rates were low. Silver and Behn ascribe the differences to two kinds of closure of Pacific-sized oceans following their origination by rifting and drifting of supercontinents. One scenario involves closure of the ocean that once surrounded the supercontinent, as seems to be on the cards for the modern Pacific; P-type closure. The other when the ocean formed passively by break-up is ‘outgunned’ by sea-floor spreading in the once surrounding ocean. That would be the case had the spreading on the East Pacific Rise not involved double subduction around the Pacific margins – the Atlantic would have opened only for both its margins to become subduction zones; A-type closure. Pangaea and possibly Nuna resulted from A-type closure. On the other hand Rodinia and Pannotia seem to have involved circumnavigation of drifting continents to collide at roughly the antipode of the split in a preceding supercontinent by P-type closure.

The conclusion is that plate tectonics was active in the early Earth, becoming intermittent in its middle life and resurrected since a billion years ago. From an examination of the deep thermal consequences of changes in plate motions in the outer Earth, it appears that mantle temperature has fluctuated markedly through time, albeit with a net decrease due to decayed radioactivity. This may have partially ‘switched off’ the conditions for mantle convection that favours plate formation and motion to a more sluggish form. By way of confirmation of their theoretical work, Silver and Behn point to the vast emplacement on most modern continents of granitic and anorthositic plutons under tectonically quiescent conditions that characterised the Grenvillian events preceding the formation of Rodinia between 1.6 to 1.3 Ga.

Deep geothermal processes

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Advances in seismic tomography of the mantle, greater knowledge of mineralogical phase changes right down to its base and modelling of processes within the core have revolutionised ideas on the physical aspects of deep mantle processes that contribute to convection and magmatism. The thermal features of the deep Earth are of crucial importance, so it is excellent to see a timely review of how heat moves at and around the core-mantle boundary (CMB) (Lay, T. et al. 2008. Core-mantle boundary heat flow. Nature Geoscience, v. 1, p. 25-32). The review gives a readable means of catching up with developments, using simple and not too speculative diagrams. You can find plenty about temperatures and physical properties at the CMB, the various contributions to heat flow and their magnitudes, and the significance of the newly discovered transformation of the deep mantle ‘catch-all’ mineral perovskite to another phase, post-perovskite. Heat that flows from the core into the lower mantle, as much as a third of the total current surface flux of about 45 terawatts, must make a profound contribution to convection in the core and thus to the geomagnetic dynamo. But there is a temperature contrast at the CMB of 500 to 1800 degrees that surely must affect physical processes in the deepest mantle, such as the initiation of mantle plumes. A puzzling new discovery is of ultra-low seismic velocities in the bottom few tens of kilometres of mantle, which  Thorne Lay, John Herlund and Bruce Buffett discuss. Finally, the whole of Earth history encapsulates the evolution of heat flow, which underpins the dynamics of our planet. The historically complex interplay between evolving sources of heat – inherited from Earth accretion and Moon formation; radiogenic sources, and physical and chemical phenomena that are played out as the core evolves – should be curricular issues for all Earth scientists.

What becomes of all the sediments?

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It used to be widely thought that sediment of the ocean floor and that at active continental margins or ahead of volcanic arcs were scraped off subducting lithosphere and simply added to continental growth. If that didn’t happen, then perhaps continents could be recycled by a combination of erosion and tectonics? Geochemists know better now, for a variety of compositional anomalies in volcanic rocks do suggest a measure of recycling of subducted lithosphere, and it is becoming clear that part of the oddity has a sedimentary source. “Which one?” is the question.

Hafnium and neodymium isotopes have become choice tracers of whether basaltic magmas formed from pristine mantle, that depleted by previously sourcing magma or some kind of mixture with recycled materials. . Catherine Chauvel and colleagues from the University of Grenoble have pondered on the sizeable amount of Hf and Nd isotopic data that has emerged from a couple of decades of fancy mass spectrometry of ocean-island and mid-ocean-ridge basalts, and a variety of sediments (Chauvel, C. et al. 2008. Role of recycled oceanic basalt and sediment in generating the Hf-Nd mantle array. Nature Geoscience, v. 1, p. 64-67). By modelling how various reasonable mixtures of isotopes of the two elements might fit the simple Hf-Nd relationship for the source mantle of all oceanic basalts they discovered that it couldn’t be derived from just the crystalline oceanic lithosphere, but must involve a substantial contribution from subducted sediments. Moreover, they seem to have demonstrated that much of the mantle involved in producing ocean-island, hot-spot basalts is a product of this recycling – both oceanic crust and its sedimentary cover get down to the levels where the mantle involved in hot-spot melting originates. Although there is a good probability of separation of sediment and crystalline components of subducted slabs according to density, it seems from the modelling that some sediment does get down to profound levels.

See also: Plank, T. & van Keken, P.E. 2008. The ups and downs of sediment. Nature Geoscience, v. 1, p. 17-18, especially their astonishing figure giving a graphic notion of the forms mantle convection might take (see Deep geothermal processes).

Hydrocarbons from the mantle: was Gold right?

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In 1999 the late Thomas Gold, cosmologist and quite a lot more, annoyed the geoscience community with publication of his book The Deep Hot Biosphere: The Myth of Fossil Fuels (Springer-Verlag: New York). In that book Gold reached the acme of his lone campaign for recognition that oil, gas and even coal formed from carbon and hydrogen feedstock that had been residing in the mantle since the Earth’s accretion. He suggested that it was mediated by a hidden yet teeming biosphere at much deeper levels than suspected at the time. I did my share of carpet gnawing, but was sorry to learn of the death in 2004 of such a supreme scientific provocateur. Although without mentioning Gold, a recent paper hints at a possibility that he may have been on to something (Proskurowski, G. et al. 2008. Abiogenic hydrocarbon production at Lost City hydrothermal field. Science, v. 319, p. 604-607).

Hydrocarbons are often found as blobs in fluid inclusions within gangue minerals of a variety of ore bodies. The US-Swiss research team examined hydrocarbons within the fluids that gush from a hydrothermal vent at 30˚N on the Mid-Atlantic Ridge; i.e. where there is no older sediment that might host biologically generated hydrocarbons, but where heat-loving microbial life could play a role. Molecular structure and carbon-isotope composition of the hydrocarbons point strongly to their formation by reduction of CO2 to methane and low molecular weight hydrocarbons by the catalytic action of mineral surfaces in the presence of a great deal of hydrogen. This is known as a Fischer-Tropsch reaction, the basis for making oil from coal, as in Nazi Germany and South Africa when under economic blockade.

The CO2 could have come from two possible sources: seawater or the mantle beneath the Lost City vents. Hydrogen can form abundantly when the olivine in peridotite beaks down to serpentinite as seawater is convected through the oceanic mantle. The vents have created towers made partly of carbonates, in whose pores there are microbes whose metabolism is based on use of hydrogen. However, the key finding is that the hydrocarbons contain no radioactive 14C, which forms by cosmic-ray interaction with nitrogen atoms in the atmosphere and is easily detectable in seawater. This rules out a seawater source for the CO2, but supports a mantle origin.

Watch out, watch out, there are burglars about

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f the major journals are anything to go by, the gravest crime that scientists can commit is to make up data and publish the results after peer review. The only thing worse in the eyes of us ‘academics’ is to publish the same makey-up data several times without being rumbled by referees. Once discovered, all the hammers of hell fall on the miscreant: they lose their jobs; their faces are splashed on the news pages of Nature and Science; they are blackballed internationally and can never work in academic circles again. Pretty harsh treatment for what, after all, is a good old-fashioned con (and often one of some ingenuity). In general, most of us love a rascally grifter, so long as they haven’t trousered our life savings. So why is the academic equivalent of the death penalty reserved for what is little different from getting a gullible public to believe that politicians act in the best interests of humanity? If any geologist looked deeply into his or her conscience most would find several cases where they had fudged a bit of data – marked a geological boundary on a map where there was barely a shred of evidence, for instance. We have all speculated well beyond the realms of reality, and often that has passed peer review easily. It is in the very nature of a dominantly observational science to do the odd bit of grifting and have it accepted.

What we detest in real life is the burglar, who desecrates our homes and work. Having anything stolen leaves a life-long trauma and a feeling of being somehow dirtied. In our academic world, theft is called plagiarism. It is most generally applied these days to the actions of students who snip bits and pieces from published sources to get a good mark from a term-paper or dissertation. Like the fabricator of data, they are generally hammered if caught at it. Yet there is a real theft that damages its victims rather than merely soiling the ‘clean image’ of education, and these victims are usually ‘junior partners’ in research. It is rife, and in one form is actually condoned and even encouraged. These days many research students are forced to more or less sign away their intellectual property to their supervisors, often a sizeable posse most of whom do very little, if anything at all. If a research student wants to publish the posse must be in the list of authors. Many commentators have noted that this riding on the backs of the inexperienced is how CVs are built up and fast-track promotion is achieved. It could be called the ‘pillion passenger’ route to greatness. But this kind of institutionalised pillaging is by no means the worst form that plagiarism can take. Far worse is to find out accidentally that one’s original ideas, data, or graphics are being published or uttered by someone else without any acknowledgement, especially if they have yet to be published.

The police rarely catch a burglar, and even less-often recover stolen goods. Similarly, victims of this worst form of academic plagiarism also know that having the record properly set straight is unlikely. The academic burglar excuses him/herself with the defence that, “there is no copyright over ideas”. To accuse such charlatans invites being actioned for libel, because of legal vagueness over intellectual property. Last month I witnessed an attempted burglary at a conference in London. In that case the burglar not only published purloined ideas previously but clearly fed his student those ideas. Unwittingly, she presented them, suitably tarted up, but with him as second author – i.e. trying to have his cake and eat it. All would have gone smoothly for the snaffler, but for one thing. The victim was there and gave the genuine presentation only 30 minutes before the blagged one hit the floor. Quite clearly, she knew what she was talking about whereas the coached presenter obviously did not. Thanks to two or three acute, and honest people in the audience, the game was up. The perpetrator of the burglary was, in the most polite (and legal) fashion, academically savaged with not inconsiderable relish. In a way, justice was done, but not entirely.

Anyone who attempts to build a career by theft needs to be stopped in their tracks, but in the ‘halls of academe’ only con-artists who are caught have the book thrown at them. So, keep your eyes and ears open in 2008, on behalf of others as well as yourself, for that is the only way metaphorically to give burglars a touch of the old Black and Decker about the knee caps.

Neoproterozoic climate modelling supports a ‘slushball’ Earth

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Following its first discovery, evidence for low-latitude glacial action at several times during the Neoproterozoic has fuelled one of the most publicised controversies in the geosciences. Was the Earth totally frozen over during these episodes, or was ice confined only to parts of the surface? Whatever, the last part of the Precambrian witnessed huge fluctuations of many kinds, and after the cold epochs the first large animals made a sudden appearance. The most dramatic geochemical ups and downs in Earth history took place, in the form of sudden extreme shifts in the relative proportions of the stable isotopes of carbon in seawater, as recorded by marine carbonate rocks. These fluctuations correlate closely with the evidence for low-latitude glaciations: large negative excursions of d13C with glacial epochs, and positive values developing between them. The first can be interpreted as the result of massive declines in photosynthetic fixation of organic carbon. The second suggests repeated recoveries of such biological productivity, which favours the extraction of 12C from seawater and an increase in the relative proportion of the heavier isotope as organic carbon becomes buried in seafloor sediments.

Since organic carbon is ultimately extracted photosynthetically from carbon dioxide in the atmosphere, a link between climate and living processes (and those that bury dead organisms) can be the basis for models attempting to explain the extraordinary events of Neoproterozoic times. If large amounts of organic carbon are buried or remain suspended in the oceans, the drawdown of atmospheric CO2 reduces the greenhouse effect and leads to cooling. Conceivably, the effect could be to so reduce global mean surface temperature that freezing conditions grip even the lowest latitudes. Once glacial and sea ice becomes established, its high reflectivity reduces the amount of incoming solar radiation that is absorbed to warm the Earth. The two processes combined would tend to lock frigid conditions in place until such time as gradual release of volcanic CO2 increased the atmospheric greenhouse effect. That is the theoretical essence of the Snowball Earth hypothesis in which complete ice cover sterilised surface biology for long periods. However, it leaves out two important factors: as water cools it is able to dissolve more gases from the atmosphere; organic carbon in ocean water can be transformed to dissolved CO2 if it is oxidised, thereby reducing the amount of carbon being buried. Modelling the carbon-climate link in the Neoproterozoic requires that both factors are accounted for (Peltier et al. 2007. Snowball Earth prevention by dissolved organic carbon remineralization. Nature, v. 450, p. 813-818).

The model devised by Richard Peltier and colleagues from the University of Toronto also incorporates the distribution of land at the time. Results from it show a looping behaviour, with recovery from frigidity as increases in dissolved oxygen convert organic carbon to dissolved carbon dioxide, whose increasing concentration in turn leads to more escape of the gas to the atmosphere. The model also suggests how glacial and sea ice might have developed during such a cycle, and with the late Precambrian configuration of drifting continents it allows for low-latitude continental glaciation, but not for all-enveloping sea ice. The implication is indeed glacial events vastly greater than those of the late Palaeozoic and during the present Ice Age, but less effect on marine photosynthesis than from Snowball conditions – a ‘Slushball’ Peltier et al. explain why the cyclical processes suggested by the model stopped before the start of the Phanerozoic, from carbon-isotope evidence for a massive oxidation of suspended marine organic carbon around 550 Ma. Thereafter, abundant oxygen and large animals ensured most dead organic carbon was oxidised in the oceans.

Unsurprisingly, one of the authors of the Snowball hypothesis finds flaws in the geochemical argument for its impossibility (Kaufman, A.J. 2007. Slush find. Nature, v. 450, p. 807-808). Not only was oxygen likely to have been at far lower atmospheric concentrations than it became in the Phanerozoic, the glacial epochs provide evidence that its concentration in seawater was very low. The marine diamictites associated with each contain both ironstones and iron-oxide cements. For them to have formed demands high concentrations of dissolved iron in sea water, in the form of reduced Fe2+ ions; incompatible with widespread oxidizing conditions that would favour Fe3+ whose compounds are insoluble.

Some good news about carbon burial

The second largest ‘sink’ for atmospheric CO2, after silicate weathering and formation of carbonate sediments, is the burial of organic carbon. Derived from photosynthesis of carbon dioxide in the air or dissolved in water, organic carbon descends from the photic zone of the oceans or is carried from the land by rivers. In the second case it is often believed that more than 70% of the carbon load of rivers is oxidised back to CO2 before having a chance of being buried in marine sediments. To estimate the proportion that does contribute to carbon sequestration is a complicated matter, involving measurement of the carbon budgeting for an entire river basin and its offshore sediments. This has been done by a team of French geochemists for the huge Ganges-Brahmaputra system that drains the northern Indian subcontinent and much of the Himalaya (Galy, V. et al. 2007. Efficient carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature, v. 450, p. 407-410). This system carries a stupendous load of sediment, especially during the monsoon season. At 1 to 2 billion t of sediment deposited from suspension in the Bay of Bengal each year, this is the largest single flux of sediment from land to the ocean floor. Even more is delivered as bed load (rolling and bouncing sand particles) to build up the Ganges-Brahmaputra delta of Bangladesh and West Bengal, India. The authors found that recently produced organic carbon is about 4 to 5 times more abundant in the suspended sediment load than is reworked fossil carbon derived by erosion of ancient sedimentary rocks, which itself is predominant in the bed load. Fossil carbon makes no difference to the modern carbon cycle, provided it does not get oxidised, which is less likely than for recent organic carbon in a form that can be metabolised.

By comparing the recent organic carbon load suspended in the rivers’ flow with that in the fine sediments of the Bengal Fan, Galy et al. have been able to show that most of that carbon is conserved without oxidation. As a result, the Bengal Fan accounts annually for about 15% of global carbon burial. There are two reasons for this remarkable efficiency: the low oxygen availability in deep waters of the Bay of Bengal; the very high sediment load from erosion of the Himalaya that buries carbon before oxidation is possible. Orogenic belts in humid areas are therefore key factors in exerting negative feedback on climate, whereas drainages of flat areas, such as the Amazon and especially its main tributary the Rio Negro, encourage oxidation in their lower reaches and offshore and are less important.

Moon formed from vapour cloud

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The Moon is generally believed to have formed from the debris ejected when a body (nicknamed Theia) about the size of Mars struck the partly formed Earth a glancing blow. That cataclysmic event can be considered to have marked the start of geochemical evolution of both Earth and Moon. From a purely mechanical standpoint, it seems almost inevitable that the Moon is made mainly from debris supplied by the offending small planet. Yet Earth and Moon have some profound geochemical similarities, the most remarkable being their now similar blend of oxygen isotopes. Meteorite studies suggest that oxygen isotopes varied widely in the early Solar System, probably differing according to distance from the Sun. That suggests that the Earth-Moon similarity is somewhat odd, unless the impacting planet formed in the same part of space as the Earth itself, i.e. in a very similar orbit. However, that is as mechanically unlikely as the Moon being a chunk of Earth flung off by the impact.

A new explanation for shared oxygen isotopes is based on a model for the collision that involves the vaporisation of most of the Earth and Theia (Pahlevan, K. & Stevenson, D.J. 2007. Equilibration in the aftermath of the lunar-forming giant impact. Earth and Planetary Science Letters. v. 262, p. 438–449). High temperature vapour would have involved sufficient turbulence for the geochemical signatures of both Earth and Theia to have been mixed efficiently.  The Moon would then have condensed from a disk of orbiting vapour of this mixed composition, most of the Earth re-accreting in a molten state too. Thus both bodies would have begun their evolution with deep magma oceans. The light-coloured, highland part of the Moon is thought to be a relic of the flotation of plagioclase crystals that floated to the top of its magma ocean as it began to cool; the lunar highlands are made of anorthosite and are at least 4.4Ga old. So far no tangible sign of such relics of early fractionation have appeared in the Earth’s geological record. Pahlevan and Stevenson’s model indicates that only between 100 to 1000 years would have elapsed from impact to appearance of the moon as a tangible body.

Another angle on the mysteries of the Hadean

Geochemists will be celebrating the end of 2007 after a steady growth in knowledge about times before formation of the first real rocks, albeit of a proxy nature. The latest addition stems from the isotopes of the rare-earth element neodymium. Its heaviest isotope 144Nd is a direct product of nucleosynthesis in supernova star explosions The middleweight isotope 143Nd is well-known as the daughter product of the decay of one unstable isotope of a sister element, samarium (147Sm, half-life 1.06 x 105 Ma). The Sm-Nd dating method, based on this decay, has been an important means of dating ancient mafic and ultramafic rocks and examining the geochemistry of their source rocks in the mantle for over 20 years. The lightest isotope is also a daughter of radioactive decay but would have formed from short-lived 146Sm (108 Ma half life). Potentially, 142Nd in old rocks can be used to judge processes in the Hadean mantle as 146Sm would have declined rapidly in the early Solar System – none is detectable nowadays. In meteorites it reveals complexities in the early differentiation of their parental planetesimals, and lunar studies show that too was subject to fractionation. That something odd happened in the early Earth became apparent when it was discovered that modern crust and mantle had more radiogenic 142Nd than the chondritic meteorites thought to have been the building blocks for the Earth. A study of neodymium isotopes in the two largest old chunks of continental crust – the  Archaean gneisses of SW Greenland and Western Australia – revealed yet more (Bennett, V.C. et al. 2007. Coupled 142Nd-143Nd   isotopic evidence for Hadean mantle dynamics. Science, v. 318, p. 1907-1910). The two blocks are different as regards their neodymium, and this suggests that a fundamental chemical division of the Earth’s mantle took place during the Hadean, which lasted for the next billion years at least. Yet another long-held idea about the Earth’s origin seems condemned to the status of myth. It had been assumed that the early Earth was well-mixed as a result of its accretion from countless planetesimals – it doesn’t really matter if they included different varieties because accretion would have been such a chaotic process. Discovering whether the now-established mantle fractionation resulted during accretion or after a cataclysmic collision with another world formed the Earth-Moon system is set to be the next challenge for students of the Hadean. It will probably be argued that this requires yet more samples to be brought from the Moon…

Last common ancestor of all the primates was a flying lemur

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Vertebrate palaeontologists sometimes become precious after a career peering at old bones, especially when they are as remarkably tiny as those of most Mesozoic mammals – and most of those fossils are teeth. Some defend to death the notion that primates descend from tree-shrews, while others foam at the mouth at the mere suggestion of the ur-shrew. ‘A key feature in primate evolution is reduction of the snout’, is axiomatic to yet others. Again, geneticists have provided extreme selection pressures that will either cause vertebrate palaeontologists rapidly to evolve or to become extinct.

Analysis of living primate genomes produces a phylogeny that links all primates with a group that has been said to be ‘the sort of animals that defy taxonomic categorization, confuse one’s sense of aesthetics, and seem to largely fall under the umbrella of “weird.” ‘ (Janecka, J.E and 7 others 2007. Molecular and genomic data identify the closest living relatives of primates. Science, v. 318, p. 792-794). These are the colugos, or flying lemurs that include the wonderfully named sugar glider.

Planet of the beetles

More than 20% of the known diversity of life on Earth is made up by the order Coleoptera, which includes several hundred thousand species. Although that huge number is largely thanks to beetle collectors, Charles Darwin having been a particularly voracious one, it is difficult believe that any other order or even class of multicelled organisms will prove to be as diverse. Yet there is only a sparse fossil record of these ubiquitous creepy-crawlies. The earliest known beetle fossils date back to the Lower Permian, and the Triassic saw their radiation into wood-eating, predatory and fungus-eating clades – from morphological similarities with living beetles. Their modern diversity depends on the vast range of ecological niches that beetles can fill, many of which are environmentally so subtle that only the beetles exploiting them show that the niches exist at all. Like all organisms the evolution of the beetles has been within the interconnectedness of the whole Earth system, and it through the linkages that such subtlety has emerged and evolved. One of the best known is the sensitivity of different beetle species to small climatic changes, which has allowed their growing use for charting climate change on land: they are far better proxies for temperature than are the foraminifera of the oceans.

Being only sparingly preserved in rocks, how beetles evolved has long been a mystery, considering their overwhelming presence on the planet. Yet again, the rapid rise of molecular phylogeny, including means of timing when mutations took place, is starting to supplant the skills of the traditional palaeontologist (Hunt, T. and 15 others 2007. A comprehensive phylogeny of beetles reveals the evolutionary origins of a superradiation. Science, v. 318, p. 1913-1916). Toby Hunt of London’s Natural History Museum and colleagues from the UK, Czech Republic, USA, Germany and Spain have combined their own RNA sequencing with existing databases of 1880 species from all the beetle suborders, series and superfamilies, 80% of families and 60% of subfamilies, to represent more than 95% of all described species. This establishes a phylogenetic tree for the lineages that they analysed, details of which will excite the coleopterist sororities and fraternities. The general picture, however, presents a more a broadly fascinating surprise. Because a vast number of beetles are associated with plants and fungi, it might seem inevitable that their evolution has parallels with that of plants, especially their explosive diversification once the angiosperms  (flowering plants) appeared. The molecular dating clearly shows that is not the case. While the angiosperms emerged in the Cretaceous Period, more than 100 living beetle lineages appeared earlier in the geological record. Unlike the Vertebrata, which diversified after mass extinctions (including the primates), the fundamental beetle lineages were clearly good survivors that were capable of their own diversification whenever opportunities arose. I think we might grow to worry about that…

Mammal evolution makeover

The Cenozoic has been the Era of mammals, and their diversification is the largest recorded adaptive radiation. However, the Linnean names of many mammal clades from the Mesozoic end in ..dont, i.e. they have been defined in terms of their teeth and not much else.  Most fossil mammals from the Mesozoic are small and fragile and only survive as teeth and jaw fragments. As a result most of the course of early mammal evolution has been a bit uncertain, to say the least. The view until recently has been that early mammalian evolution was a step-by-step affair in which key innovations accumulated in an orderly manner.  However, even on the basis of teeth, developing taxonomic approaches have proved able to reveal that considerably more complicated things happened (Luo, Z-X. 2007. Transformation and diversification in early mammal evolution. Nature, v. 450, p. 1011-1019). For a start, it turns out that mammals, despite their scanty remains, were almost as diverse during the Mesozoic as the dinosaurs that are often said to have driven early mammals underground or into the night (310 mammal to about 550 dinosaur genera). The potential for analysis stems from an explosive growth in fossil discoveries: from 116 genera in 1979 to the present 310, and a 200-fold increase in well-preserved specimens. Clearly, mammal-oriented palaeobiologists have been hard at work.

Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh crams most of the developments into a 6-page review, from which it is possible to learn a great deal, albeit needing quite a firm grasp of cladistic terminology. One of the highlights is how evolution of the mammals before 65 Ma involved repeated evolutionary convergence, i.e. the end products of evolutionary bursts often looked superficially similar. That tendency carried over into the Cenozoic on a grander scale. One example is that of adaptations for burrowing to produce mole-like end products, even some with semi-aquatic habits. Many of the rapid diversifications ended in extinction of the lineage, but all seem to indicate a great deal of ‘experimentation’ with a range of original forms that channelled towards similar functions. The outcome was a vigorous occupation of potential ecological niches in which mammals clearly had the advantage over reptiles, possibly because of their physiologically greater adaptability, partly stemming from warm-bloodedness.

Permian shark bites fish-biting amphibian

It is worth queuing to await the appearance of the 22 January 2008 issue of the Proceedings of the Royal Society B: Biological Sciences. It contains unique evidence of predator-prey relations and the food chain in the Lower Permian Zechstein Sea (Kriwet, J. et al. 2008. First direct evidence of a vertebrate three-level trophic chain in the fossil record. Proceedings of the Royal Society B: Biological Sciences, v. 275, p. 181-186). The object for your amazement is a shark whose gut contains two amphibians. The last meal of one of the amphibians was a small fish.

The paper promises to be reminiscent of the final part of the Monty Python Fish Slap Dance sketch, which can be viewed at http://www.youtube.com/watch?v=d1xfp6Xeu0c&feature=related

Neanderthals more ‘human’ than once thought

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Sébastien Chabal, the gigantic and hairy back-row forward in the 2007 French World Cup rugby team, was nicknamed ‘The Caveman’ by French fans. Indeed he is an awesome spectacle, at almost 2 m tall and weighing over a tenth of a tonne, with great black beard and locks. But is seems that Neanderthals were redheads and probably prone to sunburn (Lalueza-Fox, C. and 16 others. 2007. A melanocortin 1 receptor allele suggests varying pigmentation among Neanderthals. Science, v.  318, p, 1453-1455). The team analysed DNA extracted from Neanderthal bones from Spain and Italy, and identified the mc1r gene that regulates pigmentation in many mammals. In both specimens it turned out to be a variant that is associated with fair skin and red hair. An artist has rendered a French Neanderthal man’s physiognomy from his skull, by combining this information with modern facial reconstruction techniques (in Culotta, E. 2007. Ancient DNA reveals Neandertals with red hair, fair complexions. Science, v. 318, p. 546-547). He seems set to become a pin-up among those ladies who favour the larger gentleman, even having a nose far larger than that of Gerard Depardieu. Although proof of the growing power of genetic analysis of ancient tissue, that Neanderthals were probably pale-skinned is not really surprising. They inhabited high latitudes for at least 200 ka longer than modern Europeans have, and the pale variant of mc1r is advantageous where sunlight is at a premium for creating vitamin D. Like modern Europeans, their immediate ancestors who migrated northwards were almost certainly dark-skinned.

Yet by far the most scientifically exciting outcome of the team’s work is the extraction from the Spanish Neanderthal bones of the FOXP2 gene, which is implicated in the development of speech and language (Krause, J. and 12 others 2007. The derived FOXP2 variant of modern humans was shared with Neandertals. Current Biology, v. 17, p. 1908-1912). It shares two mutations with FOXP2 in modern humans, that had previously been suggested only to have developed in the last 100 ka, so must have been present in the last common ancestor of fully modern humans and Neanderthals, around 300 to 400 ka. Although this discovery cannot prove that Neanderthals spoke, taken along with emerging evidence that symbolic skills were used by even earlier hominins (see When and where ‘culture’ began in November 2007 issue of EPN) it does suggest they were capable of as much sophistication as the earliest fully modern humans.

Is human evolution speeding up?

Another outcome of the acceleration in genetic analysis is an ability to scan vast numbers of differences in DNA from many individuals.  Highly productive are single nucleotide polymorphisms or SNPs (‘snips’) that are available from the international HapMap project. From analysing almost 4 million SNPs from 270 individuals has emerged an intriguing parallel between human population explosion since about 40 ka and an increasing rate at which new genetic traits have been incorporated into the human genome (Hawks, J. et al. 2007. Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences, v. 104, p. 20753-20758). The link is not entirely surprising, for the exposure of more individuals to mutagenic factors will result in more mutations entering the total gene pool. Yet ‘weeding-out’ of unfavourable mutations also operates over time, so the fact that around 7 % of human genes seem to have changed over the last 40 ka, indicates the overall rate of human evolution must have speeded up remarkably. The analysis suggests that the rate rose to a peak between 5000 and 8000 years ago, for Europeans and West Africans respectively. ‘Received wisdom’ has for a long while been that fully modern humans went through a phenomenal spurt in evolution around 50 to 40 ka (but see When and where ‘culture’ began in November 2007 issue of EPN), and that somewhat Eurocentric view is overturned by the SNP evidence. Selection pressures must have risen to a peak around the time of the spread of agriculture and the rise of large social communities – big changes in diet and in exposure to communicable disease would be associated with those shifts.

In some respects the findings are cause for optimism. Global warming and rapid transformation of climate belts will expose billions of people to new experiences. Hundreds of millions, or more, may perish, yet our species’ evolution may speed up again. Let’s hope it leads to some improvement in avoiding self-induced misfortune.

See also: Holzman, D. 2007. How we adapted to a modern world. New Scientist, v. 196, 15 Dec 2007 issue, p. 8-9.

A shocking discovery

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Every introductory geology course hammers home the message that the finer the grains in a sedimentary rock, the lower the energy under which it was deposited. This ‘received wisdom’ links to the ways in which grains move in moving fluids: rolling; bouncing and in suspension. A reductionist view sees this as the influence of Stokes’ Law in the boundary conditions between turbulent and laminar flow, close to the bed of flow and higher up in the fluid respectively. Stokes’ Law is invoked as that explains how spheres falling through fluids reach a steady speed related to the fluid’s viscosity. The larger the radius of the sphere, the greater that settling speed is. For the smaller size ranges settling speed is proportional to the square of the radius (laminar flow conditions), whereas for large objects it is proportional to the square root of radius (turbulent flow).  This nicely explains the upward decreasing grain sizes in graded beds, formed when a mixture of grain sizes settles from moving fluids when their speed slow, as in turbidites and the on the lee sides of sand dunes. Since we often see silts and muds being deposited in low-energy lagoons and estuaries on the coast that too seems to verify the theory. However, muds that contain clay mineral particles are quite different from scaled-down spherical grains: they are platy; often have unbalanced electrical charges and are subject to Brownian motion that helps keep them in suspension.  When clays suspended in fresh river water meet the sea, ions in sea water encourage the plates to clump together as aggregates or floccules that are much larger than the clay particles themselves. Another oddity is that, once deposited, clays are not as easily eroded as uncemented sands, partly due to their hosting biofilms that hold the particles together.

Despite the accepted explanation of mudstones as indicators of past low-energy conditions based on reductionist notions, suspicion of awkward complexity dates back to Henry Clifton Sorby, one of the founders of geology, who suggested that the study of mudstones and shales was a great challenge for sedimentology. In reality there are probably more than 30 parameters that govern the shifting and deposition of muds, many bound up with flocculation. Confidently discussing the true environmental conditions of mudstone deposition is often thwarted by their ease of weathering and by small animals that munch their way through muds to exploit often high contents of organic debris. Even the fissility of shales is a mystery in the field. Now and again, muds do reveal surprises, such as ripples and cross lamination, that surely reflect current action. Only experimentation can throw light on Sorby’s great challenge (Schieber, J. et al. 2007. Accretion of mudstone beds from migrating floccule ripples. Science, v. 318, p. 1760-1763). Schieber and colleagues from MIT and Indiana University used experimental flumes to investigate what happens to clay floccules, seeding the materials with fine hematite grains to show up any bedforms clearly. The muds used were from 5 to 63 mm in size, which produced floccules between 0.1 to 1.0 mm.  Again and again the experiments produced migrating ripples, some like tiny barchan dunes made of clay floccules. The surprise lay in the flow speeds at which they began to form: between 10 to 30 cm s-1, much the same as those needed to produce sand ripples. Floccules were preserved in the experiments, but since they are made of clay minerals, compaction tends to destroy floccule outlines when mudstones form.

No doubt some fine-grained sedimentary rocks reflect low-energy environments, but without more careful examination of their small-scale features muds formed by energies as high as those involved in producing sandstones and many limestones will go unnoticed. Since mudstones are the most common sedimentary rocks in the geological record, some big surprises are in store.

See also: MacQuaker, J.H.S. & Bohacs, K.M. 2007. On the accumulation of mud. Science, v. 318, p. 1734-1735.

Detecting, mapping and understanding ancient soils

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A recent paper provides a clear guide and a new means of addressing one of geoscience’s great puzzles (Andrew Deller, M.E. 2006. Facies discrimination in laterites using Landsat Thematic Mapper, ASTER and ALI data — examples from Eritrea and Arabia. International Journal of Remote Sensing, v. 27, p. 2389–2409). During the early Cenozoic, and perhaps before that, huge areas of the exposed continental surface were subject to a hot, humid climate. Intense chemical weathering broke down every conceivable rock type to a few stable minerals. The resulting residual soils were preserved over vast areas of Africa, South America, India and Australia to form laterites, which M.E. Andrews Deller at the Open University UK points out are distinctly zoned [avoids repeat of layered] mineralogically and stunningly layered in colour. No one can fail to see laterites where they are exposed, if they know what to look for, but few geologists have set out to understand them properly. Andrews Deller documents in detail where these unique rocks occur, highlighting the importance of laterites as a resource; the frightening hazards that they pose to people throughout laterite-mantled Africa, and their relevance to the history of erosion and intraplate deformation.

The central theme of Andrews Deller’s paper is the essential first step of mapping laterites and discriminating their facies. This rests on their mineralogical simplicity, and the unique and distinct spectral properties of those constituent minerals. The author matches these to the spectral coverage of freely available remote sensing data — Landsat TM, ASTER and ALI — each of which offers nuances to be exploited in uniquely discriminating the different laterite horizons. Rather than setting out to ‘unveil’ sophisticated new methods of computer analysis (to which few people in laterite-encrusted areas would have access), Andrews Deller explores the simplest, most revealing approaches to a previously overlooked challenge: laterite facies have never been discriminated and mapped before using remote sensing. The results in this well-illustrated paper are stunning, and any geologist (and probably many lay people) can understand what the figures show and the importance of mapping laterites, thanks to careful discussion. The result is a paper that combines interest, novelty and usefulness.

Write about your favourite fossil

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The success of the online encyclopaedia called Wikipedia stems from millions of people being able to write about their own expertise, and also to add to, revise, correct and update any entry. Building up a knowledge base that way is a lot faster and more agreeable than individual efforts. The authors of a useful website on fossils (www.palaeos.com), begun in 2002, recently ran out of steam. Rather than allow it to become fallow, they have turned it into a wiki (wiki wiki means quickly in the Hawaiian language) at www.palaeos.org. Hopefully it will grow explosively, and I have suggested to  Prof P.U. Siffli, of Sringeri University in Karnataka, India, that he should contribute his hitherto private but astonishing knowledge on fossil hamsters.

Gold rush

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As they say, ‘Gold is where you find it’ – gold mineralisation has a great diversity of settings. One of the oddest gold mines is the Ladolam deposit on the island of Lihir off Papua New Guinea — it is also one of the largest, with reserves of around 1300 tons (~41 million troy ounces). There, gold is being extracted from an open pit, cooled by water injection, in the crater of a geothermally active volcano. Aside from that it is one of many different kinds of hydrothermal deposit in which metals are transported and deposited by a plumbing system that delivers hot watery fluids. The hydrothermal system on Lihir is obviously still active, and it is possible to sample the fluid itself by drilling to depths up to a kilometre. Deep sampling is needed to obtain pristine fluids, uncontaminated by mixing with groundwater. Their chemical composition trns out to be surprising (Simmons, S.F. & Brown, K.L. 2006. Gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit. Science, v. 314, p. 288-291).

The ground in which the deposit occurs is a breccia produced by explosive decompression when the volcano collapsed in its last magmatic throes, at about 400 ka. It is this brecciation that provided the intricate pathways in which gold was able to precipitate from the hydrothermal fluids. The samples have deuterium and oxygen isotopes that show that it is derived directly from magma. The fluid is extremely saline with very high chloride and sulfate ion concentrations. Around 50 kg of the fluid reaches the surface every second. Because it contains about 15 parts per billion of gold, it is possible to estimate how long it might have taken to produce the gold ore body: a surprisingly rapid 55 thousand years at the current rate of 24 kg of gold per year. Even more surprising is that the Lihir hydrothermal fluid is not particularly rich in gold compared with the fluids emerging from some active volcanoes. For instance Mount Etna is estimated to be delivering up to a tone of gold every year. However, before setting off on a gold rush to extinct volcanoes in the last hydrothermal phase, it is worth bearing in mind that forming a super-rich giant gold deposit requires that both gold transport and deposition are closely synchronised in a small volume of rock, otherwise the gold merely ends up in such a vast volume of rock that its extraction is not economic.

Long-term stability of the magnetic poles

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Back to about 200 Ma ago, charting the motions of plates is relatively simple using the striped patterns of magnetic field strength above the ocean floor, which reflect periodic reversals of polarity of the geomagnetic field. Post-Triassic plate motions can also be assessed in an absolute reference frame with the use of hot spot tracks. Since no ocean floor is older than 200 Ma, the method cannot be used before then. Instead, the inclination and direction of remanent magnetism in continental rocks, suitably corrected for any tilting by deformation, take on the role of tracking motions. The direction is taken as being towards the magnetic poles at the time a rock formed, whereas the inclination supposedly varies in a simple fashion with latitude as it does today; vertical at the poles and horizontal at the ancient Equator. The post-Triassic break-up of Pangaea allows the palaeomagnetic method to be tested, and for that period it holds up extremely well. The models that chart how continental masses separated from a late-Precambrian supercontinent, drifted and then clanged together in the Devonian to early Permian to form Pangaea use the assumption of a consistently dipolar magnetic field that was lined up with the Earth’s axis of rotation: about as uniformitarian as one can get. They are models that delight tectonicians and students alike. There is however, a period in Earth’s history, from about 750 to 600 Ma, when palaeomagnetic positioning gives worrying results. Evidence of glaciation occurs at nearly equatorial palaeolatitudes at least three times.

Taken at face value, these results form the basis for the ‘Snowball Earth’ hypothesis, and the 750 to 600 Ma period has been dubbed the Cryogenian. But there are two other ways of explaining what is about as far from uniformitarian as can be. Maybe there were long periods when the geomagnetic field was neither dipolar nor lined-up with the rotational axis, in which case palaeolatitudes for those periods would be totally meaningless. The other possibility, which is alarmingly odd, is that before about 600 Ma the angle between the Earth’s axis of rotation and the plane in which it orbits the Sun was not about 23.5°, but more than 58°. At a high obliquity, Earth’s rotation would then ensure that high latitudes were warmer than low ones, which would neatly explain away much of the evidence for ‘Snowball Earth’ conditions. It is a worrying idea, simply because some considerable force, i.e. a stupendous impact, would be needed to change the axial tilt from >58° to what it is now and probably has been throughout the Phanerozoic. Settling the matter once and for all seems now to have been achieved by David Evans of Yale University, using a simple yet ingenious approach (Evans, D.A.D 2006. Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes. Nature, v. 444, p. 51-55).

Evans based his study on the uniformitarian assumption that conditions are just right for strong evaporation of shallow, enclosed seas between 15 to 35° of latitude either side of the Equator, which is where evaporite deposits are forming now. If true, and if the geomagnetic field has been much the same as it is now, except during reversals, then all evaporites should give palaeolatitude results with this narrow range. There are lots of them, going back to 2.3 Ga ago, and being quite soft it is easy to drill cores from them. Furthermore they contain wind-blown dust, the magnetic component of which would line up nicely with the geomagnetic field while salts crystallised. The results from 54 world-wide sample are quite a triumph, for no evaporite palaeolatitudes are further than 40° from the Equator, and their means fall within the modern latitude range of an excess of evaporation over precipitation. There are differences between different time periods – before Pangaea existed evaporites formed slightly closer to the Equator than in later times. The fact that they cluster also shows that the dominant component of the geomagnetic field has been consistently been a dipole. However, even though the fundamental assumptions on which palaeomagnetic measurements are based seem sound, there are still problems for the Snowball hypothesis. Are the magnetic measurements up to scratch and do the stratigraphic and radiometric ages of samples refer to the evidence for glaciation?

Bad news for lunar base

Whether or not the Moon becomes once again a target for exploration by astronauts, and for use as a launch pad for Mars, depend on whether there is any water there. There has been considerable optimism that perpetual shadows in some of the deep craters close to the south lunar pole might contain ice that has not been exposed to solar heating. There is a way of telling using radar imaging, and reconnaissance results from orbiting probes had suggested that ice was indeed there, hence the excited men in suits of various kinds. A check using far more revealing radar data produced using the Areceibo radio telescope – it has also produced images of Venus at far greater distances – show that both sunlit and shadowed areas on the Moon can give a signal that is theoretically that from ice (Campbell, D.B. et al. 2006. No evidence for thick deposits of ice at the lunar south pole. Nature, v. 443, p. 835-837). Since ice could never survive in full sunlight, the similar results cast great doubt on ice being anywhere else on the Moon. There also seems to be a correlation in degree of belief with degree of involvement with future lunar exploration preparation.

Microbial alteration of oceanic crust

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The transformation of ocean-floor lavas from pristine assemblages of anhydrous minerals to cold, wet masses of hydrated silicates is of central importance to subduction processes that pull oceanic lithosphere apart and generate the hydrous arc magmas that can eventually become parts of the continents. This geochemical heat engine is usually ascribed to hydrothermal circulation of seawater through hot new oceanic crust. When these fluids emerge as hydrothermal vents they sustain seething colonies of prokaryote and eukaryote life from the most minute Archaea to substantial metazoans. That this long-hidden part of the biosphere might play a role in plate-tectonic systems is beginning to seem possible. Evidence is emerging from the study of altered basaltic glass that the biosphere does extend deep into the ocean floor (Staudigel, H. et al. 2006. Microbes and volcanoes: A tale from the oceans, ophiolites and greenstone belts. GSA Today, v. 16, October 2006 issue, p. 4-10). The US, Canadian and Norwegian team reviews observations of modern unicellular organisms in the cracks that permeate volcanic glass when it forms by rapid cooling of lava erupted into seawater. They seem rapidly to colonise tiny cracks and to act as a medium through which water is more easily able to transform the sterile glass into complex clay assemblages known as palagonite. The bugs are everywhere, down to at least 300 m in modern ocean floor. High-powered microscopy of ancient ophiolites, such as those of the Cretaceous Troodos Complex on Cyprus, reveals structures that appear exactly the same, including convincing evidence of the organisms themselves. Similar structures, but no irrefutable cell-like structures, occur in Archaean greenstone belt lavas too, as far back as 3.4 Ga: possibly the oldest tangible signs of living processes.

From a cell-biology standpoint, hydration reactions in mafic to ultramafic lavas are potentially highly fertile, the formation of serpentine minerals by hydration being a well-known generator of hydrogen. Modern methanogens use the reaction of hydrogen with carbon dioxide as an energy source, with methane as a by-product. Other organisms exploit the oxidation of sulfides or the reduction of sulfates in a similar way. All these processes can go on inorganically, and the possibility that tiny cracks in volcanic glasses may have harboured the origin of life, as well as thriving ‘ecosystems’, is a possibility worth further exploration. If there is one process that has undoubtedly occurred since the Earth cooled sufficiently for liquid water to exist, it is the alteration of mantle-derived lavas.

Oxygen in the atmosphere: why the delay?

Several lines of evidence suggest that the Earth’s atmosphere only accumulated sufficient oxygen for it to be significantly oxidising around 2.4 Ga ago. Yet the much earlier emergence of blue-green bacteria, assumed to be the organisms that secreted the intricate biofilms that make up stromatolites, suggest that it was being generated by photosynthesis as a much as a billion years beforehand. Many geochemists now suggest that oxygen was readily mopped up in the oceans by the conversion of soluble iron(II) ions derived from sea-floor lavas to insoluble compounds of iron(III), through oxidation reactions. As the rate of production of oceanic lithosphere gradually slowed, there would come a point when all available iron(II) was precipitated leaving excess photosynthetic oxygen to accumulate and enter the atmosphere. But other factors would have been at work: burial of organic carbon produced by photosynthesisers also works to increase the rate at which oxygen remains uncombined (otherwise it combines with oxygen to reproduce carbon dioxide). Complicating the geochemistry of atmospheric oxygen is the way in which it may combine with biogenic methane by reactions catalysed by ultraviolet radiation. Since UV penetration also falls as oxygen levels rise, because of the formation of ozone. That makes possible extremely complex systems of positive and negative feedback. Assessing such mechanisms, three British environmental scientists suggest a kind of potential ‘flipping’ from two possible states for the Archaean to Palaeoproterozoic atmosphere; one rich in oxygen the other forced to have low levels (Goldblatt, C. et al. 2006. Bistability of atmospheric oxygen and the Great Oxidation. Nature, v. 443, p. 683-686). Various permutations of the rates of carbon burial, methane and oxygen production might have locked the pre-2.4 Ga atmosphere in a low-oxygen state. The authors estimate that just a 3% increase in organic carbon burial could have flipped the dynamics towards a state of rapid oxygen accumulation that by generating ozone would be destined to persist. Their model helps resolve a number of awkward geochemical observations that an iron buffering model cannot explain.

See also: Kasting, J.F. 2006. Ups and downs of ancient oxygen. Nature, v. 443, p. 643-645.

Is weathering due to the weather?

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The name ‘weathering’ has always been taken to indicate a direct relationship between the atmosphere and the breakdown of rocks, i.e. at or very close to the surface. This is so easy to test that it comes as a surprise to find that nobody has really tried until recently (Yokoyama, T. & Matsukura, Y. 2006. Field and laboratory experiments on weather rate of granodiorite: Separation of chemical and physical processes. Geology, v. 34, p. 809-812). Tadashi Yokoyama and Yukinori Matsukura of universities of Osaka and Tsukuba, Japan, placed small cut tablets of identical fresh granodiorite in three position: at the surface, buried above the water table and buried beneath the water table in one small catchment. These samples stayed there for 10 years. The only sample to show much sign of chemical breakdown of minerals was that buried below the water table. Does anyone claim that there is weather in groundwater? Just exposing fresh granodiorite in the laboratory to a constant flow of water chemically similar to the groundwater doesn’t accomplish the weathering (it is 50 times slower than when samples are buried). Chemical weathering needs to involve soaking, when grain boundaries break down so that individual grains can become detached and allow yet more penetration.

Most geoscientists who work on topics that involve chemical weathering, such as the changing release of tracer isotopes of strontium to estimate rates of weathering in the past, assume that it is all done by atmospheric carbon dioxide dissolved in rainwater or released by organisms in soil. It is accomplished by hydrogen ions that can be released by a great deal more processes than the formation of vary weak carbonic acid (e.g. organic acids and breakdown of sulfides). It now seems very clear that chemical weathering is a product of groundwater and burial, so should we call it weathering at all?

Calibrating the deepest ice core

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Although the ice that makes up the upper parts of the Greenland and Antarctic ice sheets is annually layered, for times before about 70 ka the layering disappears because of plastic deformation. Earlier ages have to estimated from models of the deformation, and a second check is to match the data records from ice cores against those from sea floor sediments. Different processes contribute to those records: for instance, the marine record of oxygen isotopes in benthonic forams tracks the changing volume of ice locked on land, while the same record from ice cores depends on the air temperature above the ice cap. The correlation does seem to work, however. But not, it seems, for the very deepest ice recovered from beneath Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) which extends to around 800 ka.

French scientists involved in the EPICA Dome C ice-core project have cunningly discovered a means of checking on the otherwise undateable deep Antarctic ice (Raisbeck, G.M. et al. 2006. 10Be evidence for the Matuyama-Brunhes geomagnetic reversal in the EPICA Dome C ice core. Nature, v. 444, p. 82-84). The core penetrated to an estimated time that should include the most recent magnetic reversal, dated very precisely to 778±2 ka. Although the exact details of how the magnetic field behaved during this reversal, it is known that when its polarity flips the intensity of the field becomes very small. While the field is stable it is sufficiently strong to deflect charged particles, both in the Solar wind and in cosmic rays, so that less pass through the atmosphere. Cosmic rays are so energetic that they can perform isotopic transformations, one product being 10Be. So if the magnetic field decreased so the proportion of 10Be in the atmosphere would go up. Raisbeck and colleagues have examined the 10Be record in the EPICA core in great detail. In a 10 m thick section from a depth of almost 3.2 km the isotope rises to a peak, which they interpret as the signature of the reversal. If correct, this gives a ‘golden spike’ against which the depth to age conversion can be refined.

Balmy shores of the Precambrian

Before the appearance of fossil organisms that could give clues to past climates the only sources of information take the form of proxies. One of the best examples might seem to be the oxygen isotope composition of carbonate rocks that relate to sea-surface temperature. In fact it isn’t useful for the Precambrian because estimates of SST depend on being able to identify the shells of planktonic animals and use their d18O as a proxy. That is a pity, because limestones are common throughout the geological record and various aspects of their geochemistry have been used extensively as proxies for other crucial information, such as the relationship between their strontium isotope composition and the pace of continental weathering. Another palaeothermometer relies on the same temperature dependent fractionation of oxygen isotopes between seawater and the precipitation of dissolved silica to form cherts, whose d18O decreases with temperature. The trouble is that silica is notoriously prone to being remobilised and reprecipitated as pH changes in the fluids within sedimentary rocks. Some results from Precambrian cherts gave such low d18O that seawater temperature would have been tens of degrees higher than they were during the Phanerozoic, but they have been wisely suspected of having been affected by much later alteration by warmer fluids passing through cherty sequences. Now the approach has been given a boost by geochemists at the French National History Museum (Robert, F. & Chaussidon, M. 2006. A paleaotemperature curve for the Precambrian ocean based on silicon isotopes in cherts. Nature, v. 443, p. 969-972).

François Robert and Marc Chaussidon analysed the silicon isotopes in cherts for which oxygen isotope data are available. Since the two isotopic systems would both change, yet would behave differently during hydrothermal or metamorphic alteration, if the results correlate well both should be undisturbed. Except in samples that show the lowest d18O values (i.e. highest temperatures) there is a good correlation. That finding validates many of the O-isotope seawater temperatures, but Si isotopes fractionate during precipitation too, again in relation to temperature. So Robert and Chaussidon take Precambrian ocean temperature data to a new level with estimates based on two methods. Their results are fascinating: as well as confirming a decline from around 70°C 3.4 Ga ago to between 10 to 40°C in the Phanerozoic, the d30Si data show sharp downward ‘spikes’ at about 2.5 Ga and 1.8 Ga. Between about 1.5 Ga to 600 Ma ocean temperature was steady at around 20°C, so there is no sign of continually cold oceans through the period of ‘Snowball Earth’ events – the number of samples cannot yet resolve the individual events, but the ‘Cryogenian’ is an obvious target for more work. The data are also important as they hint at all kinds of possible biological outcomes for such global warmth, and explanations are definitely needed.  Does the record suggest greater geothermal heating, or was it an outcome of the greenhouse effect? Will more details show periods of changing burial of organic carbon? Whatever, the Precambrian has become a stranger world to contemplate.

See also: de la Rocha, C.L. 2006. In hot water. Nature, v. 443, p. 920-921.

Fossil bee: the right place and the right time

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Amber from a mine in Myanmar generates a steady income from sales to palaeoentomologists, each bead of the lithified resin being a possible lagerstätte in its own right. Two scientists at Oregon State University and Cornell were fortunate enough to find a small, Early Cretaceous bee that is so well-preserved as to show even the leg hairs on which bees carry pollen (Poinar, G.O. & Danforth, B.N. 2006. A fossil been from Early Cretaceous Burmese amber. Science, v. 314, p. 614). Indeed, the hairs carry several grains of pollen. This is the oldest known bee by more than 35 Ma, and it coincides with the start of the explosive radiation of flowering plants.

So, farewell planet Pluto…

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One theological mode of discourse is casuistry, best known for disputing the number of angels who can sit on a pinhead. Amongst astronomers, at least those who meet every three years at the General Assembly of the International Astronomical Union (IAU), this form of sophism crops up from time to time.  It does too among geologists, and probably more often, as they have a many things to argue about. At 13.32 GMT on the 24th of August the 26th GA of the IAU in Prague upset a great many people by casting Pluto, formerly known as Planet Pluto, into the indignity of dwarf-planet status. NASA may be well-miffed, as their New Horizon probe has been on its way there since mid-January 2006.

The issue of Pluto’s status popped up after a larger Sun-orbiting object was announced in 2005 (2003 UB313), which, like Pluto is beyond the orbit of Neptune. That new body is the largest known in the dim and distant Kuiper Belt, and Pluto may well be a stray from that region, having a very odd orbit. IAU decided, somewhat late in its existence, to define ‘planet’. Committees were appointed. The primary criterion decided by the final committee to report to IAU was that planets need to orbit the Sun, not another bigger planet. Second, they have to have sufficient mass for their gravitational force to make them nice and round. Sadly, it seems that the committee made quite a gaffe. In order to distinguish trans-Neptunian planets that take more than 200 years to orbit, they suggested the term ‘pluton’ (oh dear). Whatever, that would give the Solar System 12 planets: trans-Neptunian Pluto, Charon (in binary orbit with Pluto) and 2003 UB313; and Ceres, formerly just the largest asteroid known. But the Kuiper Belt might easily have lots of other massive and round objects in it, awaiting discovery. So, has the old Jesuitical mind-expanding exercise been ‘larged-up’? Probably not, in a strictly scientific sense, because additional criterion for planetary status, added by the 26th GA of the IAU, is that one should be massive enough either to have ‘swept’ its orbit clear of minor bodies early on, or to have flung them far away. Since Pluto and Ceres have done neither, they are officially to be considered ‘of diminished stature’. Some worry that traumatised children, fond of Pluto, will be driven from an interest in science. Who knows? But if IAU persists in the name ‘pluton’ as a sop to public opinion, there will be trouble…

Climate moves mountains

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Several times in Earth Pages News the topic of how erosion contributes to uplift has cropped up. That is more than just the iceberg-like bobbing up of the crust as the load on the underlying asthenosphere is eased by surface rock removal. One oddity is that as large valleys are carved the ridges and peaks that they separate can rise higher than the original lands surface from which they developed (see Erosion and plate tectonics in May 2005 issue of EPN). Now it is becoming clear that sideways movement of the crust beneath mountain ranges can also be a response to erosion; thrusts and nappes can respond to erosion as well as to plate tectonic forces. The most likely place where this might be happening is in the Himalaya, which produce a huge contrast in climate and erosion rate between their southern and northern sides by creating the world’s largest rain shadow. The evidence for this possibility is nicely reviewed by Kip Hodges of Arizona Sate University (Hodges, K. 2006. Climate and the evolution of mountains. Scientific American, v. 295 (August 2006 issue), p. 54-61).

The highest erosion rates take place where rainfall during the Indian monsoon is greatest, on the SSW face of the Himalaya, especially in the foothills between about 1000 and 3500 m. The Tibetan Plateau lies in the rain shadow of the Himalaya, and erosion is far less intense. Yet the Tibetan plateau is buoyed up by crust that is double the normal thickness, to an average elevation of around 5 km. In a crude way Tibet can be regarded as having a pressure head ‘dammed’ to the north of the Himalaya. Intense erosion at the foot of the mountain ‘dam’ is likewise akin to one cause of landslides: erosion of the toe of a slope. The gravitational potential of Tibet, combined with continual undermining of the Himalayan front must create a lateral force. Where the crust is able to behave in a plastic fashion, i.e. at depth, and if there are surfaces on which movement is possible — the north-dipping frontal thrusts of the Himalaya — then deep crust should be extruded sideways. In fact there are faults systems just to the north of the Himalaya that have the same dip as the thrusts, but an opposite sense of movement, directed northwards to create extensional detachments. The crustal zone in-between is the most likely to undergo extrusion. GPS measurements there and cosmogenic dating of the surface reveal that indeed this zone is experiencing  anomalously high rates of uplift. It is producing extremely high gradients on both hillsides and valley floors.

Threatening Earth

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The US Geological Survey has recently launched its Natural Hazards Gateway at www.usgs.gov/hazards to give access to data and educational material on volcanoes, landslides, hurricanes, floods, earthquakes, tsunamis and wildfires. The coverage is global, naturally with a great deal on the US. The links within USGS and to other agencies are comprehensive. When USGS sets out its stall, it groans with produce.