Phosphorus, Snowball Earth and origin of metazoans

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As any gardener knows, the element phosphorus is an essential plant nutrient or fertiliser, along with potassium and nitrogen plus a host of minor elements that are rarely mentioned as sufficient amounts are generally available in soils. The same necessities for life apply to oceans too, in which amounts vary a great deal from place to place and whose relative proportions have changed through geological time. For the oceans the main source of phosphorus is the continental crust, where the element resides mainly in the mineral apatite (Ca5(PO4)3(F,Cl,OH)). This is not an easily dissolved mineral, which is why for agricultural fertiliser it is generally made available in the soluble form of calcium superphosphate (Ca(H2PO4)2) that is produced by reaction between apatite and sulfuric acid. Since the land surface was colonised by plants about 450 Ma ago, biological processes made phosphorus more readily available to solution in river water by their break-down of apatite; supply by rivers to the ocean nowadays is of the order of 109 kg y-1. Ups and downs of P dissolved in ocean water though geological time would be expected to have influenced its overall biological productivity, controlled by photosynthetic phytoplankton and prokaryotes. Variations of carbon isotopes (δ13C) in organic and carbonate sediments are know to have occurred episodically since Archaean times, suggesting wide fluctuations in both bioproductivity and burial of dead organic matter. However, it has been hard to judge any geochemical reasons underpinning such variations. Since it is now clear that the common iron mineral goethite (FeOOH) ‘mops up’ many chemical species including phosphate ions by adsorption on its grain surfaces, measuring the P/Fe ratios in marine ironstones containing these minerals is a potential guide to the changing phosphorus concentration in the oceans (Planavsky, N.J. et al. 2010. The evolution of the marine phosphate reservoir. Nature, v. 467, p. 1088-1090).

The US-French-Canadian researchers charted P/Fe ratios in banded iron formations and ironstones precipitated around ocean-floor hydrothermal vents since the Archaean. What emerged were four episodes: from 2900 to 1700 Ma with generally low ratios; the Neoproterozoic from 750 to 635 Ma with much higher ratios; the Phanerozoic from Cambrian to Jurassic with low ratios and post-Cretaceous high ratios. There are several significant gaps in the record of ocean phosphate levels, notable one a billion years long from 750 to 1700 Ma. One factor that probably affected the variation is the way that dissolved silica (SiO2) drives down the proportion of phosphate adsorbing onto goethite. The rapid evolution and expansion since the Cretaceous of diatoms that secrete silica probably reduced SiO2 concentration in ocean water as their remains rained down to be buried on the ocean floor; that explains the high P/Fe ratios since about 100 Ma. Earlier Phanerozoic oceans are estimated to have had as much as seven times the present concentration of dissolved SiO2, thereby explaining the low values of P/Fe in ironstones deposited in the 100-540 Ma range. From 1700 to 3000 Ma the low P/Fe suggests oceanic phosphorus levels equivalent to those from the Jurassic to Cambrian (but perhaps up to 4 times that, depending on the poorly constrained SiO2 concentrations).

The Neoproterozoic phosphorus ‘spike’, at a time when dissolved SiO2 would have been no different from that in earlier times, suggests a massive influx of phosphate to the oceans at that time. It coincides with the two greatest glacial epochs the Earth has experienced: ‘Snowball’ Earth when glacial ice existed at tropic latitudes. In themselves the massive glaciations offer an explanation for high phosphorus delivery from the continents through glacial erosion and massive run-off during melting. More exciting is that the P/Fe ‘spike’ occurred at a time of massive perturbations in stable carbon isotopes ascribed to huge explosions of phytoplankton and organic carbon burial, which would have been permitted by high dissolved phosphate in the oceans. A large increase in primary biological productivity, i.e. photosynthesis, would have boosted oxygen levels; a necessity for the emergence of metazoan life forms soon after the end of ‘Snowball’ Earth conditions. But that begs the question of how glacially ground-up apatite, abundant as it would have been together with vast amounts of other rock debris, came to be dissolved. In today’s oceans crystalline apatite is barely soluble. It seems that apatite’s solubility decreases as temperature rises, and increases with pH – in alkaline conditions. As well as being cold, Neoproterozoic ocean water around the time of the ‘Snowball’ Earths was saturated with carbonate ions that helped thick, almost pure limestones to form globally after each glaciation. That spells alkaline conditions favouring phosphate solution. The authors speculate that global geochemical conditions during the Cryogenian Period (850-635 Ma) may have fostered the origin of the metazoans. Maybe, but their data have a billion-year gap immediately before that Period, and genomic molecular clocks suggest that the root metazoans emerged as much as half a billion years earlier.

See also: Filippelli, G.M. 2010. Phosphorus and the gust of fresh air. Nature, v. 467, p.1052-1053.

Hard-core continental lithosphere

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The oldest and most stable parts of the continents are known as cratons, after the Greek word for strength κράτο (kratos). All the present continents have at least one craton (Africa and South America have 4 each, and Eurasia 6 or 7). Each has remained unaffected by major deformation for a billion years or more, even during continent-to-continent collisions in which they participated. Almost all cratons began to form during the Archaean Eon before 2500 Ma, but most became rigid long after. Several theories have been suggested to account for their durability, one commonly accepted being that somehow the crust ‘ripened’ so that most of the heat-producing radioactive isotopes of U, Th and K were moved by igneous and metamorphic processes to the uppermost crust, along with water; most cratons expose fragments of anhydrous granulites of tonalitic composition. These bear evidence of having formed at the base of the continental crust and have been heavily depleted in “granitophile” trace elements. As a result they cannot undergo partial melting under normal geothermal conditions and where they remain at great depth are much cooler than younger, deep crust. The other dominant feature of cratonic lithosphere is a mantle portion that is anomalously thick (sometimes down to 250 km); in some cases there is little if any sign of asthenosphere beneath such ‘keels’. Research on rocks brought up from the ‘roots’ of cratons by the kimberlite magmas famous for their diamonds points to that deep mantle itself having conferred great rigidity and thus longevity (Peslier, A.H. et al. 2010. Olivine water contents in the continental lithosphere and the longevity of cratons. Nature, v. 467, p. 78-81).

The presence of water in minerals that make up igneous and metamorphic rocks enables them to begin melting at lower temperatures than their dry equivalents, and also to behave in a more plastic fashion under stress. Anne Peslier of NASA in Houston and her US and German colleagues analysed the minerals in ultramafic mantle rocks dragged upwards by kimberlites that punched through the Kaapvaal craton in southern Africa long after it formed. The dominant mantle mineral is olivine (50-80%), generally thought of as anhydrous but typically containing a few hundred parts per million by weight. Olivines in the Kaapvaal mantle xenoliths become drier with increasing depth of their formation (determined from their mineralogy in which garnet is stable at the deepest levels). At depths around 150-250 km low water content in olivine makes it and the mantle itself 20 to 3000 times stronger than the asthenosphere, which protects it from the underlying flow associated with tectonic motions.

How such root zone of continents may have formed has been addressed by two papers on seismic structure beneath the best studied craton; that of the Canadian Shield (Yuan, H. & Romanowicz, B. 2010. Lithospheric layering in the North American craton. Nature, v. 466, p. 1063-1068; Miller, M.S. & Eaton, D.W. 2010. Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL044366).In the first, Yuan and Romanowicz of the Berkeley Seismological Laboratory, California use a form of seismic tomography to map anisotropy in the mantle along transects that cross the ancient core of the North American continent. Their results chart the depth of the base of the lithosphere and also define two layers in the lithospheric mantle. The upper layer (down to 150 km) only occurs beneath the Archaean craton, and the top of the asthenosphere ranges from 100-240 km down: at its deepest beneath the craton. The sub-craton mantle they ascribe to chemical depletion of its upper part during early lithospheric evolution, and later addition of the less chemically evolved deeper layer. Miller and Eaton of the Universities of California USA and Calgary Canada used S-wave data from eight seismic stations extending from WSW to ENE over the western cordillera and the Canadian Shield to the Arctic islands of Canada. Their results show a similar variation in dept of the base of the lithosphere and resolve several roughly eastward-dipping boundaries in the sub-craton lithospheric mantle, which they link to Precambrian volcanic arcs preserved in the upper crust above them; i.e. suggesting that the upper layer in the first paper stems from a major episode of arc accretion that built the Canadian Shield.

Threat to landscape from alien crayfish?

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The stealthy invasion of rivers in Europe by the tasty American signal crayfish Pacifastacus leniusculus poses a threat not only to the indigenous European species Astacus astacus (P. leniusculus carries a fungal infection as well as being formidably armed), but conceivably to the very landscape itself (Johnson, M.F. et al. 2010. Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus). Geomorphology, v. 123, p. 269-278). Johnsson and colleagues from the University of Loughborough, UK used captive alien crayfish to model the effects of their bioturbation under controlled laboratory conditions, assessing their activity by the use of millimetre-resolution gravel-surface elevation data generated by a laser altimeter. The sturdy little beasts behave like frenzied bulldozers creating mounds and pits in the gravel substrate, displacing on average about 1.7 kg of gravel every day over an area of 1 m2 thereby completely disrupting the perfectly flat substrate onto which they were introduced in about 3 days. By this activity they render the surface more prone to erosion by flowing water so that greater grain transport ensues; they could effect bother erosion and deposition by increasing transportation of grains. To my knowledge, this is the first experimental study of bioturbation in the context of hydrology. We can expect more now that the technology is available: the burrowers as well as the diggers of the animal world. While the Phanerozoic is best know for having begun with the Cambrian Explosion of multicellular life, a sometimes overlooked attribute is that it coincided with the start of bioturbation. That may well have had a profound effect on sediment transport as the American invader suggests.

See also: Newton, A. 2010. Crayfish at work. Nature Geoscience, v. 3, p. 592

Antipodean glaciers confirm complementary southern warming during the Younger Dryas

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Studies of air-temperature proxies in cores from the Antarctic ice cap show a roughly mirrored climate record to that found in the Greenland ice. While the Northern Hemisphere underwent a sudden climate collapse into almost full-glacial conditions around 12.9 ka and an equally dramatic warming around 11.7 ka, Antarctica steadily warmed over the same period to reach full interglacial conditions by 11.5. That this climatic inversion reached relatively low southern latitudes is confirmed by a record of the changing size of glaciers on mountains in New Zealand’s South Island (Kaplan, M.R. and 9 others 2010. Glacier retreat in New Zealand during the Younger Dryas stadial. Nature, v. 467, p. 194-197). The US-New Zealand-Norwegian-French partnerships used detailed geomorphological mapping, and cosmogenic isotope studies of exposed rock fragments to show that after about 13 ka glaciers retreated by more than a kilometre in the succeeding 1500 years in contrast to the dramatic glacial advances in northern areas such as the Scottish Highlands.

Record of rising sea-level in the tropics

Areas beyond the zones of isostatic depression by ice-loading and recovery during glacial-interglacial cycles passively undergo sea-level fall and inundation. They best record the progress of Holocene ice-sheet melting and sea-level rise since 11.5 ka, especially if they are tectonically stable. The island state of Singapore, 1.5 º north of the Equator, is a near-ideal place for study (Bird, M.I. et al. 2010. Punctuated eustatic sea-level rise in the early mid-Holocene. Geology, v. 38, p. 803-806). The Australian and British geoscientists analysed a core through sediments in a mangrove swamp now just below sea level. The top 14 m penetrated a uniform though laminated sequence of marine muds, calibrated to time by radiocarbon dating of mollusc shells, mainly focused on the period from 9 to 6ka period that the global oxygen-isotope record of ice volume suggests to have been the main period of final melting after the Younger Dryas.

Sedimentation was very rapid (~1 cm y-1) from  8.5 to 7.8 ka, probably as sea level rose too rapidly for the coast to be protected by mangrove growth.  Then for 400 years it slackened off to ~0.1 cm y-1 to rise again to 0.5 cm y-1 by 6.5 ka. The last date is the time of the mid-Holocene sea level highstand, after which sedimentation rate soon declined to 0.05 cm y-1, when mangroves became established at the site. Stable isotopes of carbon in the core (δ13C) show how the relative input of marine and terrestrial (mainly mangroves) organisms shifted over the period and are a proxy for the distance to the coastline and hence sea level. From 8.5 to 6.5 ka this was erratic from a starting point about 10 m lower than nowadays, showing rapid rises and falls that culminated in a sea level in Singapore about 3 m above present during the mid-Holocene sea level highstand that slowly declined to that of the present.

The team’s findings tally with evidence for the melting record of the North American ice sheet. An interesting aspect is that they also cover the period when rice cultivation in swampy areas of SE Asia got underway (~7.7 ka). Very rapid sedimentation would have encouraged development of the substrate for the highly fertile delta plains that now support the largest regional population densities on Earth. In turn they culminated in a series of early south and east Asian civilisations based on class societies.

Correction to marine biodiversity record and mass extinctions

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The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.

One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos?

Comet impacts’ candidature for origin of life

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules.  Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.

Low-angle extensional detachments at ocean ridges

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The discovery in the 1970s that some low-angled faults have an extensional or normal sense of displacement stemmed from extensional systems in the continental crust, exemplified by the Basin and Range Province of western North America. Yet the largest extensional systems on Earth are those associated with mid-ocean ridges, and in the 1980s some of those were shown to involve low-angled detachments too. Michael Cheadle and Craig Grimes (University of Wyoming and Mississippi State University, USA) review the latest word on oceanic extensional complexes revealed at the AGO Chapman Conference in May 2010 (Cheadle, M. & Grimes, C. 2010. To fault or not to fault. Nature Geoscience, v. 3, p.454-456). As in continental extension, this kind of deformation at divergent margins may produce core complexes uplifted as a result of tectonic unroofing by low-angled detachments, thereby revealing oceanic mantle lithosphere on the ocean floor. Such peculiarities seem to be absent from fast spreading ridges such as the East Pacific Rise and occur where spreading is slow. They are best developed where spreading is starved of magma injection to produce the classic sheeted-dyke complexes of the middle oceanic crust, and with unusually thick oceanic lithosphere. Yet the ocean floor must spread at these localities, and that is achieved by extensional tectonics that accommodates up to 125 km of spreading with next to no magmatism: 4 Ma-worth of spreading.

For extensional faults to develop into low-angled detachments rocks must be weak, otherwise simple steep, domino-style faults would form. Penetration of seawater down faults weakens oceanic lithosphere through hydration reactions that produce clays and serpentines, which encourage the formation of ductile shear zones. Interestingly, some of the largest hydrothermal systems on the mid-Atlantic Ridge coincide with core complexes, and exude hydrogen – a product of serpentinisation – as well as methane and metal-rich brines.

Plate theory moves on

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The last 40 to 50 years have seen the theory of plate tectonics supported by more and more empirical evidence from sea-floor magnetism, seismicity, bathymetry and a growing number of other features that relate to Earth’s dynamism. Yet the original concepts of rigid plates and their dislocation from one another and the underlying mantle have been undermined to a degree by the wealth of data now available. Increasing resolution of seismic tomography is revealing what is happening in the depths of the mantle on which growing confidence can be placed. Matching these increasingly revealing sources of data has been the computing power to try to blend them all with rheological theory and thereby model the way the world works. The latest of these modelling ventures does seem to move plate theory onto a significantly higher plane (Stadler, G. et al. 2010. The dynamics of plate tectonics and mantle flow: from local to global scales. Science, v. 329, p. 1033-1038). The keys to this step are: increasingly sophisticated software that encompasses the contributory factors, akin to models used by mechanical and hydraulic engineers; faster computing that allows a decrease in the size of the 3-D cells used in assessing all the interactions as realistically as possible, and a great deal of graphic creativity so that we can visualise the results. At its centre is varying rock strength, the principal ‘engineering’ input derived from seismic tomography, blended with the gravitational and thermal forces that drive Earth’s ‘engine’.

Stadler et al.’s development divides up the planet into a 3-D mesh whose resolution varies according to the likely complexity of motions within and upon the Earth. For instance there is not much call for detail for what lies below abyssal plains of the ocean floor, so available computing power can be focused on the more intricate parts of the tectonic set-up, especially subduction zones that are both the most spectacular features of the Earth’s behaviour and the source of the main force that drives its surface parts – slab pull. Already the approach is producing more questions than answers. For instance, building in the data that show something of convection in the deep mantle makes the model’s output for the more shallow-seated and better known processes deviate more than expected from what is observed – less comprehensive and more coarse approaches previously seemed to be match deep and shallow processes quite well. This is a difficult topic to express merely in words, but fortunately the paper has been made freely available at http://users.ices.utexas.edu/~carsten/papers/StadlerGurnisBursteddeEtAl10.pdf

See also: Becker, T. 2010. Fine-scale modelling of global plate tectonics. Science, v. 329, p. 1020-1021.

Carbonates on Mars

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Ancient valley systems, huge water-carved gorges and sedimentary deposits signify with little room for doubt that early in its history Mars was wet; it must therefore have been warm. A thick CO2-rich atmosphere seems obligatory to give the kind of greenhouse warming that prevented Earth from freezing over when the young Sun was weaker than now. The question is, where did the CO2 go so that the planet became chilled? Gravity on Mars is sufficient to have retained the gas, unlike water vapour that dissociates to hydrogen and oxygen, of which hydrogen easily escapes even a much stronger gravitational field. A consensus is developing that it resides in carbonate minerals. The other likely greenhouse gas is sulfur dioxide, for whose drawdown there is ample evidence in the form of sulfates detected from orbit and by surface rovers. Carbonates have a relatively simple, and unique spectrum of reflected solar radiation, with an absorption feature at a wavelength around 2.3 micrometres. Carbonates have been detected on Mars using orbital hyperspectral imaging, but only in patches. The NASA rovers rely on serendipity for any discovery, yet Spirit did stumble on a large carbonate-rich outcrop identified by its on-board Mössbauer spectrometer (Morris, R.V. and 12 others 2010. Identification of carbonate-rich outcrops on Mars by the Spirit rover. Science, v. 329, p. 421-424). It appears to be a Fe-Mg variety in association with olivines, and carbonate makes up to 34 % of part of the outcrop. The texture is granular, yet the area abounds with evidence for hydrothermal activity in the form of sulfates and silica-rich materials, implying that some kind of circulation system deposited the carbonates. The associated olivine is odd, as that mineral is prone to rapid breakdown to serpentines in the presence of water.

The discovery of carbonate rock does help the CO2 early greenhouse theory and the fate of the warming gas, but aside from the fact the identification has been done at vast distance does it rank with geoscience that can be accomplished on Earth? It is a small piece in the jigsaw of Mars’s climatic evolution, but cannot resolve the issue of drawdown of greenhouse gas. The real drama there lay in the finding of abundant signs of water erosion on many scales set against today’s surface hyperaridity; evidence for glaciation and subsurface water ice in apparently large volumes. Earth had to have had a thick CO2-rich atmosphere at the same time as that of Mars, but we are still not sure where all that carbon ended up in the early Precambrian, despite limestones and carbon-rich mudstones dating back to 3.4 Ga: as we cannot quantify that aspect of Earth’s history, neither can we expect an early answer for Mars. Indeed, what is the benefit set against the cost?

See also: Harvey, P. 2010. Carbonates and Martian climate. Science, v. 329, p. 400-401.

Wet Moon, dry Moon

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Regular readers will remember my remarkable though very reluctant conversion to the notion that there may be water on Mars. My stubborn reaction had been against the background that shrouded the hypothesis with a certain desperation; the need of any future crewed mission to Mars for a water supply and thereby one of hydrogen fuel, plus the determination of the whole Mars-oriented community to justify such a mission by hyping ‘xenobiology’ on the ‘Red Planet’. A similar desperation claoked the search for surface water on the Moon, although one more dominated by the ‘Everest’ syndrome: since the boot prints and flags appeared, everyone wants to go. The Moons internal water is an entirely different kettle of fish. The hypothesis of the Moon’s formation by condensation from an incandescent mass flung into orbit after a planet – planet collision involving the Earth has the corollary that the lunar mantle ought to be bone dry: and so it seemed to be from bulk analyses of rocks brought back by the Apollo missions. In fact, there are a number of possibilities to explain vanishingly small amounts of internal water: the Moon is made of impactor that happened to be dry rather than terrestrial material; Earth and Moon are a mix of both and both Earth and impactor started out dry, but the Earth later received its water from comets; low pressure condensation of the Moon ruled out water entering itss silicate minerals and so on. Then water was found in apatite grains from lunar maria basalts (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). Within a couple of months we are back to the dry-as-an-alco’s-throat view (Sharp, Z.D. et al. 2010. The chlorine isotope composition of the Moon and implications for an anhydrous mantle. Science, v. 329, p. 1050-1053). Both terrestrial and meteoritic chlorine isotopes are in remarkably consistent proportions, but lunar rocks show an 25 times greater spread by comparison. To cut a long and complicated discussion short, such a range could only have formed if chlorides of a variety of metals were vaporised from lunar magmas each having its own effect on fractionation of Cl isotopes. In turn, combination of chlorine with metal ions requires virtually no hydrogen ions and therefore vanishingly little water in the moon, otherwise chlorine would have been combined in HCl and not subject to any fractionation when that volatilised on eruption. So that seems settled, then…

Catastrophic canyon formation

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Huge canyons, such as the Grand Canyon and the Gorge of the Blue Nile, have generally been supposed to have resulted from steady-state erosion through resistant rocks, accelerating during annual floods. There are exceptions that produced spectacular gorges during emptying of proglacial lakes in North America and on a lesser scale in northern Britain. Just how efficient at erosion individual floods may be was demonstrated by release of reservoir water through a spillway in Texas for about 3 days in 2002 (Lamb M.P. & Fonstad, M.A. 2010. Rapid formation of a modern bedrock canyon by a single flood event. Nature Geoscience, v. 3, p. 477-481). The peak discharge was ~1500 m3s-1, which is not especially huge, yet up to 12 m of erosion occurred through bedrock to produce a sizeable canyon in what was previously a typical small stream valley. Although some erosion was by plucking of joint blocks a considerable amount occurred by potholes scoured by boulders swirling in the rapid currents. Small islands, resembling those preserved in glacial lake outburst floods, were sculpted mainly by suspended sediment rather than by boulder impacts. Another feature that forces a rethink of erosional processes is that waterfalls show no sign of headward retreat by undercutting, but seem to have formed as slabs were plucked by the hydraulic force and slid down stream to form tabular boulders. The implication is that canyons may form episodically during flood events, when the shear stress of the flow on its bed is sufficient to lift and slide joint-bounded slabs.

The vestige of a beginning

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Geoscientists take it for granted that the Earth has a certain age (currently estimated at 4.54 Ga), but it is one divined from indirect evidence, lead isotopes in meteorites and ancient ores of lead derived from uranium. If ever geoscientists are to grasp the nature of the early planet the evidence would be geochemical, yet also second-hand because relics must lie somewhere in the mantle as the crust is constantly being changed. For decades it has been known that the mantle shows geochemical heterogeneity as a result of episodes of partial melting from which the oceanic and continental crust emerged. Even with such an ancient origin it seems intuitively likely that there should be some mantle that has not been interfered with. Now a group of geochemists from the US and Britain have presented evidence for just such ur-mantle (Jackson, M.G et al. 2010. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, v. 466, p. 853-856). Their data come from Cenozoic lavas collected on Baffin Island and in West Greenland, which gave an earlier clue for having melted from a truly antique source: they contain the highest ratio of helium produced in the Big Bang (3He) to that released by radioactive decay (4He). Repeated melting of the mantle gradually drives off, yet radioactive decay continually replenishes its complement of 4He, so the more reworked a mantle source for lavas is the lower its 3He/ 4He ratio. This notion is backed up by the lead and neodymium isotopes in the Baffin Island and West Greenland lavas and they suggest an age of formation of the mantle source between 4.45-4.55 Ga.

Convection over billions of years ensures a degree of mixing in the mantle, but such is the viscosity of the Earth that there is a good chance that some areas have remained unchanged, the more so if the bulk of magmatism involving deep mantle has been linked to narrow rising plumes. But what emerges from the rest of the geochemistry of these lavas? Provided they have not been contaminated by continental crust through which they have passed, it should be possible using models for the way different elements are contributed to or withheld from magma by mantle minerals to estimate the source mantle’s overall composition. The team did this, bearing in mind the uncertainties. Plotted relative to a ‘guestimate’ of the original bulk Earth based on the geochemistry of chondritic meteorites they sshoww a very good fit for those elements that are likely to be retained by mantle minerals during partial melting: the so-called ‘compatible’ elements. But the estimated source for the lavas seems to have been depleted in the ‘incompatible’ elements that are highly likely to enter magma as soon as partial melting starts. This pattern would be expected if the early mantle had undergone some kind of differentiation as a whole, and that would be a consequence of the entire mantle having been molten and then crystallising: some low-density minerals could preferentially have taken in incompatible elements and floated upwards to deplete those elements in the deep mantle. That is compatible with the idea of Moon formation as a result of a collision between the proto-Earth and a Mars-sized planet, which could have released sufficient energy in the form of heat tp completely melt the outermost Earth.

So the data reveal a great deal, especially that this ancient mantle may well have been the parent for all later mantle compositions as the Earth evolved by dominantly igneous processes. But they do not resolve the perennial debate as to whether the Earth accreted from a uniform mix of nebular material of which meteorites are relics, roughly the composition of chondrites, or heterogeneously from different materials that had condensed from incandescent vapour at different nebular temperatures at different times. Moon formation would have mixed up the latter efficiently in a mantle-wide magma ocean, so we may never know. However some of the oldest meteorites contain fragments of condensates that did form at different temperatures.

The anatomy of a small landslip

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At the centre of the Peak District National Park in England is a small mountain called Mam Tor, at the summit of which is a large Iron Age fort complete with defensive ramparts and ditches. Complete, that is, except for its southern parts, which are chopped through by a large arcuate cliff. Below that is hummocky ground typical of landslips, but such disturbed ground is common over large tracts in the Peak District that lie below hills, especially those underlain by Lower Namurian shales of the region. Mam Tor is the only one of these that has an active landslip. Since my early childhood the local authority has tried to keep trafficable a once major road linking the cities of Sheffield and Manchester, but to no avail; most winters it was buckled and cracked by continued motion. The road was abandoned in 1979 and is now a magnificent laboratory for judging the kind of motion involved in the Mam Tor slip. The Iron Age people had much the same problem, as the slip began around 1500 BC long before the fort was built. Clearly, they were not engineering geologists, though the unclimbable scar was maybe a defensive bonus, provided the old, the bewildered and the very young were kept well away from it, as they are today.

Records of the movement have been kept since the road was constructed in 1820, and one milestone has moved 50 m in 190 years at a constant annual rate, but just how it moves has only become clear since Manchester University geologists installed tilt and creep meters, and 50 survey stations in 2004-5. Their preliminary results are just in (Green, S. et al. 2010. The effects of groundwater level and vegetaion on creep of the Mam Tor landslip. Geology Today, v. 26, p. 134-139). The creep rate is clearly governed by groundwater level beneath the slip, and has risen as high as 19.5 mm per day. From the logarithmic plot between the two variables it is possible to estimate the creep rate with completely saturated ground, which would be an ominous 0.6 m per day. Thankfully, drainage through the slip is good, as beneath lie highly unstable mudstones; but things could change. The team has also monitored local rainfall, and precipitation underwent a marked increase from 2000 onward (1.64 m per year) compared with the average since 1930 of 1.3 m per year. Fortunately, spring and summer rains are quickly returned to the atmosphere by vigorous evapotranspiration by the lush grasses and ferns on the slipped mass. The greatest creep takes place in the winter when vegetation has died back.  Mam Tor is indeed highly instructive, but at present poses no great hazard, yet it might become less predictable should annual rainfall increase. It is unlikely to attain the awesome pace of that in Calabria, southern Itaaly on 15 February 2010 at Maierato near Vibo Valentia (view www.stumbleupon.com/su/9LP6H7/sorisomail.com/email/42722/ja-viram-desmoronar-uma-montanha.html).

New clues to origin of porphyry-type ore deposits

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The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

Earlier colonisers of northern Europe

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The Pleistocene of East Anglia in England is a rich source of the high-latitude flora and fauna from early interglacials of the 1 Ma long series of 100 ka climate cycles. Eyed by archaeologists for decades as a potential source of human remains, a coastal site at Pakefield in Suffolk finally yielded stone tools in 2005 (see Earliest tourism in northern Europe in EPN January 2006). The enclosing sediments, to widespread excitement, turned out to be around 700 ka old, establishing the earliest known human colonisation at that latitude (52ºN). At that time East Anglia was connected to Europe during both glacial and interglacial periods, and was crossed by a now-vanished river system draining the Midlands and Wales into the proto-North Sea. Stone artifacts have now emerged from similar interglacial terrestrial sediments on the shore below the village of Happisburgh (pronounced ‘Haze-burra’) further north still, in Norfolk (Parfitt, S.A and 115 others 2010. Early Pleistocene human occupation at the edge of the boreal zone in northwest Europe. Nature, v. 466, p. 229-233). Magnetostratigraphy pushes back the human influence here to more than 800 ka, maybe as far back as 950 ka. As yet no human remains have been turned up, and the site is below high-tide level and liable to be destroyed by winter storms so work proceeds as fast as possible. Yet cliff erosion will inevitably reveal new material each spring.

Fauna and flora from Happisburgh indicate a slow flowing river flanked by coniferous forest with grassed clearings. Beetle fossils suggest summer temperatures slightly warmer than those in modern southern Britain, but with winters some 3ºC colder than now. The climate was analogous to that in southern Norway today, at the transition from temperate to boreal vegetation zones; certainly tough in winter for people without shelter. Yet the permanent connection with continental Europe would have permitted easy seasonal migration across great plains that extended to warmer southern climes. The tool-using people were not the earliest Europeans, for several archaeological sites in Spain, southern France and Italy extend back to 1.3 Ma. Who or rather what hominin species they were needs bones, preferably those of the head. The discovery that there were at least 4 hominin species cohabiting Eurasia during the last glacial epoch encourages caution in any speculation.

See also: Roberts, A.P. & Grűn, R. 2010. Early human northerners. Nature, v. 466, p. 189-190.

Survival by the seaside

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Increasingly, hominins have survived swings of climate by their wits and by chance. Neither underpin the instinct to migrate when times are hard, but where one ends up depended, until the Holocene, more on chance than design. Early migrations must have been more by diffusion than purposeful, especially in the vastness of the African continent. Yet groups of hominins found their way into Eurasia several times and thrived there. Far more of them would have met the coast far from a continental exit route, such as the Levant or the Straits of Bab el Mandab. However, in stressful glacial episodes reaching the coast was a key to survival as its food resources are almost limitless (see Human migration and sea food May 2000 issue of EPN). Our own species found refuge by the sea not long after we originated (Marean, C.W. 2010. When the sea saved humanity. Scientific American, v. 303 (August 2010), p. 40-47). Around 195 ka climate began to cool and dry to reach a glacial maximum at roughly 123 ka. Curtis Marean (Arizona State University, USA) was one of the first scientists to look for signs of coastal refuges in Africa during the early 1990s, particularly at its southern tip. With co-workers he found several caves on the coast of South Africa that yielded the evidence on which he has based a review of littoral survival opportunities and the skills that we developed. This particular coastal stretch has a huge diversity of plant life, most unique to it, and many of which store carbohydrate in tubers, bulbs and corms. They are adapted to dry conditions and need only the simplest technology – digging sticks and fires for cooking – to exploit starchy, easily digested energy resources, along with the more obvious animal protein sources present on all shorelines. Marean’s review puts in plain language all the discoveries made by his group over the last 20 years, including evidence of the use of fire treatment to improve flaked stone tools and the development of art based on iron-oxide pigments, plus his own take on their anthropological significance.

Another big surprise

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The discovery from the Neanderthal genome that people outside Africa have such a muscular bloke in their distant ancestry (see Yes, it seems that they did…in May 2010 issue of EPN) ought to be quite enough of a shock for one year, but hard on its heels comes another. Animal bones from Ethiopia in sediments dated at more than 3.4 Ma show clear signs of having flesh cut from them with a sharp blade (McPherron, S.P. et al. 2010. Evidence for stone-tool assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature, v. 466, p. 857-860). The oldest known stone tools date back only 2.4 Ma (none were found at Dikika), and those associated with a known hominin (H. habilis) to half a million years later than that. No species of the genus Homo is known to have been living 3.4 Ma ago, so a likely candidate for making and wielding stone tools then would be Australopithecus afarensis: Lucy’s genus. In fact the infant A. afarensis named Selam (see ‘Peace’ (Selam) disturbed in October 2006 issue of EPN) was found a mere 300 m away from the cut-marked bones.

There are several problems that arise from these butchered bones, as regards their implications. Do hominin specialists reserve the genus Homo exclusively for tool makers? If so, do Lucy and Selam become H. afarensis? But without actual tools associated with the bones, it is impossible to decide whether they were specifically made to deflesh prey or carrion, or were just sharp, naturally occurring bits of stone that some creature with insubstantial teeth happened to use to snaffle a quick snack from competing carnivores. Even more intriguing, in the light of the immense rarity of hominin remains, was there some creature more advanced than A. afarensis roaming the stifling plains of Ethiopia’s Awash valley 1.4 Ma before the first known tool maker? The various Awash projects will run and run after this new and startling discovery.

Catastrophic canyon formation

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Huge canyons, such as the Grand Canyon and the Gorge of the Blue Nile, have generally been supposed to have resulted from steady-state erosion through resistant rocks, accelerating during annual floods. There are exceptions that produced spectacular gorges during emptying of proglacial lakes in North America and on a lesser scale in northern Britain. Just how efficient at erosion individual floods may be was demonstrated by release of reservoir water through a spillway in Texas for about 3 days in 2002 (Lamb M.P. & Fonstad, M.A. 2010. Rapid formation of a modern bedrock canyon by a single flood event. Nature Geoscience, v. 3, p. 477-481). The peak discharge was ~1500 m3s-1, which is not especially huge, yet up to 12 m of erosion occurred through bedrock to produce a sizeable canyon in what was previously a typical small stream valley. Although some erosion was by plucking of joint blocks a considerable amount occurred by potholes scoured by boulders swirling in the rapid currents. Small islands, resembling those preserved in glacial lake outburst floods, were sculpted mainly by suspended sediment rather than by boulder impacts. Another feature that forces a rethink of erosional processes is that waterfalls show no sign of headward retreat by undercutting, but seem to have formed as slabs were plucked by the hydraulic force and slid down stream to form tabular boulders. The implication is that canyons may form episodically during flood events, when the shear stress of the flow on its bed is sufficient to lift and slide joint-bounded slabs.

The anatomy of a small landslip

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At the centre of the Peak District National Park in England is a small mountain called Mam Tor, at the summit of which is a large Iron Age fort complete with defensive ramparts and ditches. Complete, that is, except for its southern parts, which are chopped through by a large arcuate cliff. Below that is hummocky ground typical of landslips, but such disturbed ground is common over large tracts in the Peak District that lie below hills, especially those underlain by Lower Namurian shales of the region. Mam Tor is the only one of these that has an active landslip. Since my early childhood the local authority has tried to keep trafficable a once major road linking the cities of Sheffield and Manchester, but to no avail; most winters it was buckled and cracked by continued motion. The road was abandoned in 1979 and is now a magnificent laboratory for judging the kind of motion involved in the Mam Tor slip. The Iron Age people had much the same problem, as the slip began around 1500 BC long before the fort was built. Clearly, they were not engineering geologists, though the unclimbable scar was maybe a defensive bonus, provided the old, the bewildered and the very young were kept well away from it, as they are today.

Records of the movement have been kept since the road was constructed in 1820, and one milestone has moved 50 m in 190 years at a constant annual rate, but just how it moves has only become clear since Manchester University geologists installed tilt and creep meters, and 50 survey stations in 2004-5. Their preliminary results are just in (Green, S. et al. 2010. The effects of groundwater level and vegetaion on creep of the Mam Tor landslip. Geology Today, v. 26, p. 134-139). The creep rate is clearly governed by groundwater level beneath the slip, and has risen as high as 19.5 mm per day. From the logarithmic plot between the two variables it is possible to estimate the creep rate with completely saturated ground, which would be an ominous 0.6 m per day. Thankfully, drainage through the slip is good, as beneath lie highly unstable mudstones; but things could change. The team has also monitored local rainfall, and precipitation underwent a marked increase from 2000 onward (1.64 m per year) compared with the average since 1930 of 1.3 m per year. Fortunately, spring and summer rains are quickly returned to the atmosphere by vigorous evapotranspiration by the lush grasses and ferns on the slipped mass. The greatest creep takes place in the winter when vegetation has died back.  Mam Tor is indeed highly instructive, but at present poses no great hazard, yet it might become less predictable should annual rainfall increase. It is unlikely to attain the awesome pace of that in Calabria, southern Itaaly on 15 February 2010 at Maierato near Vibo Valentia (view http://www.stumbleupon.com/su/9LP6H7/sorisomail.com/email/42722/ja-viram-desmoronar-uma-montanha.html).

Wet Moon, dry Moon

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Regular readers will remember my remarkable though very reluctant conversion to the notion that there may be water on Mars. My stubborn reaction had been against the background that shrouded the hypothesis with a certain desperation; the need of any future crewed mission to Mars for a water supply and thereby one of hydrogen fuel, plus the determination of the whole Mars-oriented community to justify such a mission by hyping ‘xenobiology’ on the ‘Red Planet’. A similar desperation claoked the search for surface water on the Moon, although one more dominated by the ‘Everest’ syndrome: since the boot prints and flags appeared, everyone wants to go. The Moons internal water is an entirely different kettle of fish. The hypothesis of the Moon’s formation by condensation from an incandescent mass flung into orbit after a planet – planet collision involving the Earth has the corollary that the lunar mantle ought to be bone dry: and so it seemed to be from bulk analyses of rocks brought back by the Apollo missions. In fact, there are a number of possibilities to explain vanishingly small amounts of internal water: the Moon is made of impactor that happened to be dry rather than terrestrial material; Earth and Moon are a mix of both and both Earth and impactor started out dry, but the Earth later received its water from comets; low pressure condensation of the Moon ruled out water entering itss silicate minerals and so on. Then water was found in apatite grains from lunar maria basalts (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). Within a couple of months we are back to the dry-as-an-alco’s-throat view (Sharp, Z.D. et al. 2010. The chlorine isotope composition of the Moon and implications for an anhydrous mantle. Science, v. 329, p. 1050-1053). Both terrestrial and meteoritic chlorine isotopes are in remarkably consistent proportions, but lunar rocks show an 25 times greater spread by comparison. To cut a long and complicated discussion short, such a range could only have formed if chlorides of a variety of metals were vaporised from lunar magmas each having its own effect on fractionation of Cl isotopes. In turn, combination of chlorine with metal ions requires virtually no hydrogen ions and therefore vanishingly little water in the moon, otherwise chlorine would have been combined in HCl and not subject to any fractionation when that volatilised on eruption. So that seems settled, then…

Carbonates on Mars

Ancient valley systems, huge water-carved gorges and sedimentary deposits signify with little room for doubt that early in its history Mars was wet; it must therefore have been warm. A thick CO2-rich atmosphere seems obligatory to give the kind of greenhouse warming that prevented Earth from freezing over when the young Sun was weaker than now. The question is, where did the CO2 go so that the planet became chilled? Gravity on Mars is sufficient to have retained the gas, unlike water vapour that dissociates to hydrogen and oxygen, of which hydrogen easily escapes even a much stronger gravitational field. A consensus is developing that it resides in carbonate minerals. The other likely greenhouse gas is sulfur dioxide, for whose drawdown there is ample evidence in the form of sulfates detected from orbit and by surface rovers. Carbonates have a relatively simple, and unique spectrum of reflected solar radiation, with an absorption feature at a wavelength around 2.3 micrometres. Carbonates have been detected on Mars using orbital hyperspectral imaging, but only in patches. The NASA rovers rely on serendipity for any discovery, yet Spirit did stumble on a large carbonate-rich outcrop identified by its on-board Mössbauer spectrometer (Morris, R.V. and 12 others 2010. Identification of carbonate-rich outcrops on Mars by the Spirit rover. Science, v. 329, p. 421-424). It appears to be a Fe-Mg variety in association with olivines, and carbonate makes up to 34 % of part of the outcrop. The texture is granular, yet the area abounds with evidence for hydrothermal activity in the form of sulfates and silica-rich materials, implying that some kind of circulation system deposited the carbonates. The associated olivine is odd, as that mineral is prone to rapid breakdown to serpentines in the presence of water.

The discovery of carbonate rock does help the CO2 early greenhouse theory and the fate of the warming gas, but aside from the fact the identification has been done at vast distance does it rank with geoscience that can be accomplished on Earth? It is a small piece in the jigsaw of Mars’s climatic evolution, but cannot resolve the issue of drawdown of greenhouse gas. The real drama there lay in the finding of abundant signs of water erosion on many scales set against today’s surface hyperaridity; evidence for glaciation and subsurface water ice in apparently large volumes. Earth had to have had a thick CO2-rich atmosphere at the same time as that of Mars, but we are still not sure where all that carbon ended up in the early Precambrian, despite limestones and carbon-rich mudstones dating back to 3.4 Ga: as we cannot quantify that aspect of Earth’s history, neither can we expect an early answer for Mars. Indeed, what is the benefit set against the cost?

See also: Harvey, P. 2010. Carbonates and Martian climate. Science, v. 329, p. 400-401.

The vestige of a beginning

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Geoscientists take it for granted that the Earth has a certain age (currently estimated at 4.54 Ga), but it is one divined from indirect evidence, lead isotopes in meteorites and ancient ores of lead derived from uranium. If ever geoscientists are to grasp the nature of the early planet the evidence would be geochemical, yet also second-hand because relics must lie somewhere in the mantle as the crust is constantly being changed. For decades it has been known that the mantle shows geochemical heterogeneity as a result of episodes of partial melting from which the oceanic and continental crust emerged. Even with such an ancient origin it seems intuitively likely that there should be some mantle that has not been interfered with. Now a group of geochemists from the US and Britain have presented evidence for just such ur-mantle (Jackson, M.G et al. 2010. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, v. 466, p. 853-856). Their data come from Cenozoic lavas collected on Baffin Island and in West Greenland, which gave an earlier clue for having melted from a truly antique source: they contain the highest ratio of helium produced in the Big Bang (3He) to that released by radioactive decay (4He). Repeated melting of the mantle gradually drives off, yet radioactive decay continually replenishes its complement of 4He, so the more reworked a mantle source for lavas is the lower its 3He/ 4He ratio. This notion is backed up by the lead and neodymium isotopes in the Baffin Island and West Greenland lavas and they suggest an age of formation of the mantle source between 4.45-4.55 Ga.

Convection over billions of years ensures a degree of mixing in the mantle, but such is the viscosity of the Earth that there is a good chance that some areas have remained unchanged, the more so if the bulk of magmatism involving deep mantle has been linked to narrow rising plumes. But what emerges from the rest of the geochemistry of these lavas? Provided they have not been contaminated by continental crust through which they have passed, it should be possible using models for the way different elements are contributed to or withheld from magma by mantle minerals to estimate the source mantle’s overall composition. The team did this, bearing in mind the uncertainties. Plotted relative to a ‘guestimate’ of the original bulk Earth based on the geochemistry of chondritic meteorites they sshoww a very good fit for those elements that are likely to be retained by mantle minerals during partial melting: the so-called ‘compatible’ elements. But the estimated source for the lavas seems to have been depleted in the ‘incompatible’ elements that are highly likely to enter magma as soon as partial melting starts. This pattern would be expected if the early mantle had undergone some kind of differentiation as a whole, and that would be a consequence of the entire mantle having been molten and then crystallising: some low-density minerals could preferentially have taken in incompatible elements and floated upwards to deplete those elements in the deep mantle. That is compatible with the idea of Moon formation as a result of a collision between the proto-Earth and a Mars-sized planet, which could have released sufficient energy in the form of heat tp completely melt the outermost Earth.

So the data reveal a great deal, especially that this ancient mantle may well have been the parent for all later mantle compositions as the Earth evolved by dominantly igneous processes. But they do not resolve the perennial debate as to whether the Earth accreted from a uniform mix of nebular material of which meteorites are relics, roughly the composition of chondrites, or heterogeneously from different materials that had condensed from incandescent vapour at different nebular temperatures at different times. Moon formation would have mixed up the latter efficiently in a mantle-wide magma ocean, so we may never know. However some of the oldest meteorites contain fragments of condensates that did form at different temperatures.

Another big surprise

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The discovery from the Neanderthal genome that people outside Africa have such a muscular bloke in their distant ancestry (see Yes, it seems that they did…in May 2010 issue of EPN) ought to be quite enough of a shock for one year, but hard on its heels comes another. Animal bones from Ethiopia in sediments dated at more than 3.4 Ma show clear signs of having flesh cut from them with a sharp blade (McPherron, S.P. et al. 2010. Evidence for stone-tool assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature, v. 466, p. 857-860). The oldest known stone tools date back only 2.4 Ma (none were found at Dikika), and those associated with a known hominin (H. habilis) to half a million years later than that. No species of the genus Homo is known to have been living 3.4 Ma ago, so a likely candidate for making and wielding stone tools then would be Australopithecus afarensis: Lucy’s genus. In fact the infant A. afarensis named Selam (see ‘Peace’ (Selam) disturbed in October 2006 issue of EPN) was found a mere 300 m away from the cut-marked bones.

There are several problems that arise from these butchered bones, as regards their implications. Do hominin specialists reserve the genus Homo exclusively for tool makers? If so, do Lucy and Selam become H. afarensis? But without actual tools associated with the bones, it is impossible to decide whether they were specifically made to deflesh prey or carrion, or were just sharp, naturally occurring bits of stone that some creature with insubstantial teeth happened to use to snaffle a quick snack from competing carnivores. Even more intriguing, in the light of the immense rarity of hominin remains, was there some creature more advanced than A. afarensis roaming the stifling plains of Ethiopia’s Awash valley 1.4 Ma before the first known tool maker? The various Awash projects will run and run after this new and startling discovery.

Survival by the seaside

Increasingly, hominins have survived swings of climate by their wits and by chance. Neither underpin the instinct to migrate when times are hard, but where one ends up depended, until the Holocene, more on chance than design. Early migrations must have been more by diffusion than purposeful, especially in the vastness of the African continent. Yet groups of hominins found their way into Eurasia several times and thrived there. Far more of them would have met the coast far from a continental exit route, such as the Levant or the Straits of Bab el Mandab. However, in stressful glacial episodes reaching the coast was a key to survival as its food resources are almost limitless (see Human migration and sea food May 2000 issue of EPN). Our own species found refuge by the sea not long after we originated (Marean, C.W. 2010. When the sea saved humanity. Scientific American, v. 303 (August 2010), p. 40-47). Around 195 ka climate began to cool and dry to reach a glacial maximum at roughly 123 ka. Curtis Marean (Arizona State University, USA) was one of the first scientists to look for signs of coastal refuges in Africa during the early 1990s, particularly at its southern tip. With co-workers he found several caves on the coast of South Africa that yielded the evidence on which he has based a review of littoral survival opportunities and the skills that we developed. This particular coastal stretch has a huge diversity of plant life, most unique to it, and many of which store carbohydrate in tubers, bulbs and corms. They are adapted to dry conditions and need only the simplest technology – digging sticks and fires for cooking – to exploit starchy, easily digested energy resources, along with the more obvious animal protein sources present on all shorelines. Marean’s review puts in plain language all the discoveries made by his group over the last 20 years, including evidence of the use of fire treatment to improve flaked stone tools and the development of art based on iron-oxide pigments, plus his own take on their anthropological significance.

Earlier colonisers of northern Europe

The Pleistocene of East Anglia in England is a rich source of the high-latitude flora and fauna from early interglacials of the 1 Ma long series of 100 ka climate cycles. Eyed by archaeologists for decades as a potential source of human remains, a coastal site at Pakefield in Suffolk finally yielded stone tools in 2005 (see Earliest tourism in northern Europe in EPN January 2006). The enclosing sediments, to widespread excitement, turned out to be around 700 ka old, establishing the earliest known human colonisation at that latitude (52ºN). At that time East Anglia was connected to Europe during both glacial and interglacial periods, and was crossed by a now-vanished river system draining the Midlands and Wales into the proto-North Sea. Stone artifacts have now emerged from similar interglacial terrestrial sediments on the shore below the village of Happisburgh (pronounced ‘Haze-burra’) further north still, in Norfolk (Parfitt, S.A and 115 others 2010. Early Pleistocene human occupation at the edge of the boreal zone in northwest Europe. Nature, v. 466, p. 229-233). Magnetostratigraphy pushes back the human influence here to more than 800 ka, maybe as far back as 950 ka. As yet no human remains have been turned up, and the site is below high-tide level and liable to be destroyed by winter storms so work proceeds as fast as possible. Yet cliff erosion will inevitably reveal new material each spring.

Fauna and flora from Happisburgh indicate a slow flowing river flanked by coniferous forest with grassed clearings. Beetle fossils suggest summer temperatures slightly warmer than those in modern southern Britain, but with winters some 3ºC colder than now. The climate was analogous to that in southern Norway today, at the transition from temperate to boreal vegetation zones; certainly tough in winter for people without shelter. Yet the permanent connection with continental Europe would have permitted easy seasonal migration across great plains that extended to warmer southern climes. The tool-using people were not the earliest Europeans, for several archaeological sites in Spain, southern France and Italy extend back to 1.3 Ma. Who or rather what hominin species they were needs bones, preferably those of the head. The discovery that there were at least 4 hominin species cohabiting Eurasia during the last glacial epoch encourages caution in any speculation.

See also: Roberts, A.P. & Grűn, R. 2010. Early human northerners. Nature, v. 466, p. 189-190.

Plate theory moves on

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more empirical evidence from sea-floor magnetism, seismicity, bathymetry and a growing number of other features that relate to Earth’s dynamism. Yet the original concepts of rigid plates and their dislocation from one another and the underlying mantle have been undermined to a degree by the wealth of data now available. Increasing resolution of seismic tomography is revealing what is happening in the depths of the mantle on which growing confidence can be placed. Matching these increasingly revealing sources of data has been the computing power to try to blend them all with rheological theory and thereby model the way the world works. The latest of these modelling ventures does seem to move plate theory onto a significantly higher plane (Stadler, G. et al. 2010. The dynamics of plate tectonics and mantle flow: from local to global scales. Science, v. 329, p. 1033-1038). The keys to this step are: increasingly sophisticated software that encompasses the contributory factors, akin to models used by mechanical and hydraulic engineers; faster computing that allows a decrease in the size of the 3-D cells used in assessing all the interactions as realistically as possible, and a great deal of graphic creativity so that we can visualise the results. At its centre is varying rock strength, the principal ‘engineering’ input derived from seismic tomography, blended with the gravitational and thermal forces that drive Earth’s ‘engine’

Stadler et al.’s development divides up the planet into a 3-D mesh whose resolution varies according to the likely complexity of motions within and upon the Earth. For instance there is not much call for detail for what lies below abyssal plains of the ocean floor, so available computing power can be focused on the more intricate parts of the tectonic set-up, especially subduction zones that are both the most spectacular features of the Earth’s behaviour and the source of the main force that drives its surface parts – slab pull. Already the approach is producing more questions than answers. For instance, building in the data that show something of convection in the deep mantle makes the model’s output for the more shallow-seated and better known processes deviate more than expected from what is observed – less comprehensive and more coarse approaches previously seemed to be match deep and shallow processes quite well. This is a difficult topic to express merely in words, but fortunately the paper has been made freely available at http://users.ices.utexas.edu/~carsten/papers/StadlerGurnisBursteddeEtAl10.pdf

See also: Becker, T. 2010. Fine-scale modelling of global plate tectonics. Science, v. 329, p. 1020-1021.

Low-angle extensional detachments at ocean ridges

The discovery in the 1970s that some low-angled faults have an extensional or normal sense of displacement stemmed from extensional systems in the continental crust, exemplified by the Basin and Range Province of western North America. Yet the largest extensional systems on Earth are those associated with mid-ocean ridges, and in the 1980s some of those were shown to involve low-angled detachments too. Michael Cheadle and Craig Grimes (University of Wyoming and Mississippi State University, USA) review the latest word on oceanic extensional complexes revealed at the AGO Chapman Conference in May 2010 (Cheadle, M. & Grimes, C. 2010. To fault or not to fault. Nature Geoscience, v. 3, p.454-456). As in continental extension, this kind of deformation at divergent margins may produce core complexes uplifted as a result of tectonic unroofing by low-angled detachments, thereby revealing oceanic mantle lithosphere on the ocean floor. Such peculiarities seem to be absent from fast spreading ridges such as the East Pacific Rise and occur where spreading is slow. They are best developed where spreading is starved of magma injection to produce the classic sheeted-dyke complexes of the middle oceanic crust, and with unusually thick oceanic lithosphere. Yet the ocean floor must spread at these localities, and that is achieved by extensional tectonics that accommodates up to 125 km of spreading with next to no magmatism: 4 Ma-worth of spreading.

For extensional faults to develop into low-angled detachments rocks must be weak, otherwise simple steep, domino-style faults would form. Penetration of seawater down faults weakens oceanic lithosphere through hydration reactions that produce clays and serpentines, which encourage the formation of ductile shear zones. Interestingly, some of the largest hydrothermal systems on the mid-Atlantic Ridge coincide with core complexes, and exude hydrogen – a product of serpentinisation – as well as methane and metal-rich brines

New clues to origin of porphyry-type ore deposits

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The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

The earliest multicelled life

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Being multicellular does not necessarily qualify a fossilised organism as being a member of the eukaryote domain: such a classification is assured when there is strong evidence for many cells constituting a functional whole with specialised parts. That eukaryotes also have cells with nuclei and a variety of organelles is a prerequisite for living members, though such evidence is extremely rare and disputed for fossils, and the earliest convincing examples are from 1700 Ma sedimentary rocks. Using a molecular clock approach to the differences in genetic make-up between modern eukaryotes might seem one means of estimating when the last common ancestor of all of them lived, but the Catch-22 is having incontrovertible examples from the distant past as means of calibrating that approach. A fourth possible ‘fingerprint’ is the presence of biomarker chemicals in sedimentary rocks that are exclusive to living Eucarya, steranes derived from sterols being an example.

Since the 1970s the oldest candidate for eukaryote status has been a coiled form a few centimetres across made from a strap-like carbon film, known as Grypania that some regard as a primitive alga. Yet it could equally be a colonial bacterium. Grypania are know as far back as those found in the 1900 Ma ironstones of Michigan, USA. Thin black shales from a mixed marine and terrestrial sequence of 2100 Ma siltstones and sandstone in Gabon, West Africa now provide something far more spectacular (El Albani, A. and 21 others 2010. Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature, v. 466, p. 100-104). They are complex and look a little like an irregular discus 1-2 cm across. Being replaced by fine grained iron sulfide they preserve odd internal structures discernible using X-ray tomography – folds of their central node – signs of flexibility in the original material – and scalloped flanges with radial slits. To the authors this suggests coordinated growth rather than the amorphous characteristics of bacterial biofilms such as stromatolites. They are completely unlike any living colonial bacteria. Their host rocks have yielded steranes characteristic of eukaryote biochemistry, but contamination from groundwater cannot yet be ruled out.

Only the one and a half billion year younger Ediacara fauna comes close in terms of complexity of form to the Gabon fossils. Yet whether they are the earliest-known eukaryotes or bacterial colonies whose growth was coordinated between the cells of which they were composed by some unknown exchange of information cannot be pinned down. However, their age is appropriate for the rise of the oxygen-demanding Eucarya, a few hundred million years after the start of planetary oxygenation. Perhaps more important, the surprising find will give palaeobiologists the impetus and confidence that large body fossils can indeed be found in the Palaeoproterozoic Era.
See also: Donoghue, P.C.J & Antcliffe, J.B. 2010. Origins of multicellularity. Nature, v. 466, p. 41-42.

Ordovician lagerstätte in Morocco
It was during the Ordovician Period that multicelled life really took off (see The Great Ordovician Diversification in the September 2008 issue of EPN), but the fossil record seems to suggest that the wonderfully diverse soft-bodies fauna of the Cambrian, exemplified by that from the Burgess Shale, didn’t survive to take part. It turns out that this may be an artefact of imperfect preservation, for a Lower Ordovician equivalent of the Burgess Shale has been unearthed in Morocco (Van Roy, P. et al. 2010. Ordovician faunas of Burgess Shale type. Nature, v. 465, p 215-218). It is just as rich and even shows more organic detail, highlighted in reds, oranges and yellows because iron sulfide that mineralised soft parts has since been gently oxidised. A fascinating link with the Burgess Shale is that the fossil taxa from the Moroccan lagerstätte are related to those in that most famous Middle Cambrian rock unit.

On the subject of exceptionally preserved fossil material, one of the Burgess Shale oddities, new specimens of Nectocaris pteryx allow a detailed reconstruction. What a stunning beast! This 5 cm stem-group cephalopod had two tentacles, enormous stalked eyes and a funnel shaped device that may have been for squid-like jet propulsion. Reconstruction of its back-end suggests a cuttlefish-like means of propulsion by a flap of tissue around the main body, but with no sign of any stiffening ‘bone’.

Possible abiotic mechanisms for DNA splitting and cell membranes
The central feature of the DNA helix is its ability to ‘unzip’ and recombine as part of the replication that is essential for all known living things. In doing this, DNA copies itself. It is one thing to deduce this from DNA’s structure and the meiotic aspect of reproduction, but quite another to figure out how it might have arisen. An experiment that mimics conditions in porous sea-floor lava – a temperature gradient and small-scale convection inside a capillary tube – shows that this lengthways splitting does occur on the hot side of the gradient. On the cool arm of the convection the halved ribbons of DNA reassemble (Mast, C.B. & Braun, D. 2010. Thermal trap for DNA replication. Physical Review Letters, v. 104, p. 188102-188105). This is a long way from life’s origin and even that of DNA as an isolated entity without a cell, but perhaps one step towards a better understanding of both. It seems pretty certain from a range of evidence – e.g. heavy metal centred proteins and heat-shock proteins – that life sprang from physical and chemical processes around hot vents on the ocean floor. What’s next on the experimental agenda: membranes to bag-up genetic material such as DNA as a precursor to the cell? It’s been done (Budin, I. et al. 2009. Formation of protocell-like vesicles in a thermal diffusion column) using fatty acids that are relatively easy to generate abiotically. Some can transform into flexible membranes that curve in on themselves – amphiphiles – and vesicles of these formed in Budin et al.’s capillary tubes.
See also: McAlpine, K. 2010. Life cooked up in undersea cauldrons. New Scientist, v. 206 (29 May 2010 issue), p. 14.

‘Fracking’ shale and US ‘peak gas’

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Around 1970 the production of natural gas in the US reached its peak and has been slowly declining since then. The degree to which the US economy has grown to depend on natural gas and growing fears of becoming dependent on insecure supplies on the international LPG market has seen a stealthy growth in unconventional technologies to maintain indigenous supplies. The greatest growth has been in winning the useful fuel from ‘tight’ organic-rich shales that are usually regarded as source rocks for conventional petroleum rather than resources in their own right (Kerr, R.A.. 2010. Natural gas from shale bursts onto the scene. Science, 328, p. 1624-1626). The technology relies on drilling methods developed in the oil industry that allow several holes from a single platform to bend to pass at low angles through thin, gently dipping strata. That allows far larger volumes to be tapped than through a single, vertical well. Oil shales are not yet targeted for liquid petroleum because of the cost, but as Richard Kerr, a news writer for Science, reveals they are supplying an increasing proportion of US gas demand: from 1% to 20% since 2000. Being less of a source of carbon dioxide than coal or oil that might seem to be a ‘good thing’ all round, but there are worrying and little known problems with the technology.

To get the gas out demands that the permeability of shale is artificially increased by jacking open joints and fractures using very high-pressure fluids that carry sand to wedge them open when production begins open: this is ‘fracking’ in driller-speak. Not only gas starts to move, but also water locked into the shale for millions of years and often highly toxic. Drillers hope that all the fluids will follow the holes, but that is by no means guaranteed and some may make their way into aquifers and up to the surface. The fluids used in fracking are deliberately full of chemicals that help open up cracks and even biocides that keep them from being clogged by bacterial films: around 15 million litres used per well. Although aimed to be recycled these noxious fluids can escape, sometimes in massive blowouts. Uncontrolled gas and formation water escapes can cause explosions and kill of forested areas by disrupting tree-root biota.