Earth-logs

Improved dating sheds light on Neanderthals’ demise

Published on August 26, 2014 by zooks7772 Comments

As Earth Pages reported in December 2011 a refined method of radiocarbon dating that removes contamination by younger carbon has pushed back the oldest accessible ¹⁴C dates. Indeed, materials previously dated using less sophisticated methods are found to be significantly older. This has led archaeologists to rethink several hypotheses , none more so than those concerned with the relationship in Europe between anatomically modern humans (AMH) and Neanderthals, especially the extinction of the latter.

The team of geochronologists at Oxford University who pioneered accelerator mass spectrometry (AMS) of carbon isotopes, together with the many European archaeologists whose research has benefitted from it, have now published results from 40 sites across Europe that have yielded either Neanderthal remains or the tools they are thought to have fashioned (Higham, T. and 47 others. The timing and spatiotemporal patterning of Neanderthal disappearance. Nature, v. 512, p. 306-309) . One such site is Gorham’s Cave in the Rock of Gibraltar where earlier dating suggested that Neanderthals clung on in southern Iberia until about 25 ka. Another hypothesis concerns the so called Châtelperronian tool industry which previous dating at the upper age limit of earlier radiocarbon methodology could not resolve whether or not it preceded AMH colonisation of Europe; i.e. it could either have been a Neanderthal invention or copied from the new entrants. Most important is establishing when AMH first did set foot in previously Neanderthal’s exclusive territory and for how long the two kinds of human cohabited Europe before the elder group met its end.

Deutsch: Rekonstruierter Neandertaler im Neand... — Reconstruction of Neanderthal life from the Neandertahl Museum(credit: Wikipedia)

The new data do not quash the idea of Neanderthals eking out survival almost until the last glacial maximum in the southernmost Iberian Peninsula, since material from Gorham’s Cave could not be dated. However, occupation levels at another site in southern Spain in which Neanderthal fossils occur and that had been dated at 33 ka turned out to be much older (46 ka). So it is now less likely that Neanderthals survived here any longer than they did elsewhere.

Neanderthal remains are generally associated with a tool kit known as the Mousterian that is not as sophisticated as that carried by AMH at the same time. Of the Mousterian sites that yielded AMS ages, the oldest (the Hyaena Cave in Devon, Britain) dates to almost 50 ka. The youngest has a 95% probability of being about 41 ka old. Of course, Neanderthals may have survived until later, but there is no age data to support that conjecture. The earliest known AMH remains in Europe are those associated with the so-called Uluzzian tool industry of the Italian peninsula. In southern Italy Mousterian tools are replaced by Uluzzian between about 44.8 and 44.0 ka, while Mousterian culture was sustained in northern Italy until between 41.7 to 40.5 ka.

Châtelperronian tools associated with Neanderthal remains occur in south-western France and the Pyrenees. The new AMS dating shows that the culture arose at about the same time (~45 ka) as the Uluzzian tool industry began in Italy and ended in those areas where it was used at about the same time (~41 ka) as did the more widespread Mousterian culture. So the question of whether Neanderthals copied stone shaping techniques from the earliest Uluzzian-making AMH more than 500 km to the east, or invented the methods themselves remains an open question. But does it matter as regards the cognitive abilities of Neanderthals? Copying methodology is part and parcel of the success and survival of succeeding AMH, but o too is the capacity to invent useful novelties from scratch. So, yes it does matter, for Neanderthals had sustained the Mousterian culture for tens to hundreds of thousand years with little change.

The upshot of these better data on timing is that AMH and Neanderthals co-existed in Europe for between 2.6 to 5.4 ka; as long as the time back from now to the Neolithic and early Bronze Age. Even allowing for low population density to make contacts only occasional, this is surely too long for systematic slaughter of Neanderthals by AMH. Yet it gives plenty of time for two-way transmission of cultural and symbolic activities, and even for genetic exchanges: assimilation as well as out-competition.

Incidentally, Scientific American’s September 2014 issue is partly devoted to broader issues of human evolution (Wong, K. (editor) The Human Saga. Scientific American, v. 311(No 3), p. 20-75) with a focus on new developments. These cover: a revised time line; the emerging complexity of hominin evolution by veteran palaeoanthropologist Bernard Wood.; the influence of climate change; by Peter de Menocal; cultural evolution in the broad hominin context by Ian Tattersall; a discussion of hominin mating arrangements by Blake Edgar; two contributions on cooperation versus competition among hominins by Frans de Wall and GGry Stix; two articles on recent biological and future cultural evolution by John Hawks and Sherry Turkle (interview).

Did Out of Africa begin earlier?

Published on August 15, 2014 by zooks777Leave a comment

It is widely thought that anatomically modern humans (AMH) began to diffuse out of Africa during the climatic cooling that followed the last interglacial episode. Periods of build-up of ice sheets, or stadials, also saw falls in sea level, which would have left shallow seas dry and easily crossed. The weight of evidence seems to point towards the narrowing of the Red Sea at the Straits of Bab el Mandab between modern Eritrea and the Yemen. Because the Red Sea spreading axis goes onshore through the Afar region of Ethiopia further north, the Straits today are shallow. Between about 70 and 60 ka, during a major stadial, much of the Bab el Mandab would have been dry. Dating of the earliest AMH remains in Asia and Australasia seems to suggest that the move out of Africa probably began around that time. But, of course, that presupposes the AMH fossils being the oldest in existence, although some would claim that genetic evidence also supports a 70-60 ka migration. Yet, AMH human remains dated at around 100 ka have been found in the Middle East on a route that would also lead out of Africa, but for the major problem of crossing deserts of modern Syria and Iraq. The supposed desert barrier has led many to suggest that the earlier venture into the Levant met a dead end. Should AMH fossils older than 70 ka turn up in Eurasia or Australasia then a single migration becomes open to doubt.

Mitochondrial DNA-based chart of large human m... — Chart of large human migrations based on variations in mitochondrial DNA in living humans(Numbers are millennia before present.) (credit: Wikipedia)

It appears that challenge to what has become palaeoanthropological orthodoxy has emerged (Bae, C.J. et al. 2014. Modern human teeth from Late Pleistocene Luna Cave (Guangxi, China). Quaternary International, In Press). Scientists from the US, China and Australia found two molar teeth within calcite flowstone in Lunadong (‘dong’ means ‘cave’). That speleothem is amenable to uranium-series dating, and has yielded ages between 70 and 127 ka. That antiquity does open up the possibility of earlier migration, perhaps during the interglacial that ended at about 115 ka when sea levels would have stood about as high as it does nowadays (in fact it was only after about 80 ka that it stood low enough to make a move across the Bab el Mandab plausible). If that were the case, the migration route would have more likely been through the Middle East, perhaps along the Jordan valley and thence to the east. Had there been greater rainfall over what is now desert then there would have been no insurmountable barrier to colonisation of Asia.

These teeth are not the only evidence for earlier entry of AMH into east Asia; a date of 66 ka for a modern human toe bone was recently reported from the Philippines. Yet many experts remain unconvinced by teeth alone, especially from east Asia where earlier humans had evolved since first colonisation as early as 1.8 Ma ago. There are other pre-70 ka east Asian bones with more convincing AMH provenance, however.

There is another approach to the issue of earlier Out of Africa migration; one resting on theoretical modelling of the observed genetic and morphological variation among living Eurasians, especially the decreasing diversity proceeding eastwards (Reyes-Centeno, H. et al. 2014. Genomic and cranial phenotype data support multiple modern human dispersals from Africa and a southern route into Asia. Proceedings of the National Academy of Sciences, v. 111, p. 7248-7253. doi: 10.1073/pnas.1323666111). The authors, from Germany, Italy and France, challenge the single-exit hypothesis based on genetic data, suggesting that those data are also commensurate with several Out of Africa dispersals beginning as early as 130 ka. They favour the Bab el Mandab exit point and migration around Eurasia at that time when sea-level was extremely low during a glacial maximum. They hint at the ancestors of living native Australians and Melanesians being among those first to leave Africa, other Asian and European populations having dispersed from a later wave.

“Out-of-Africa” is morphing into “Out-of Africarabia” as genetic and archaeological time-lines converge

Serious groundwater depletion in western US

Published on August 15, 2014 by zooks777Leave a comment

The 2300 km long Colorado River whose catchment covers most of Arizona and parts of the states of Colorado, California, Nevada, Utah, New Mexico and Wyoming is one of the world’s most harvested surface water resources. So much so that barely a trickle now ends up in Baja California where the huge river once flowed into the sea. The lower reaches of the river system cross arid lands and it is the water source for several major cities and areas of intensive agriculture, serving as many as 40 million people and 16 thousand km² of irrigated fields. It has been nicknamed the US Nile because of its economic importance, but Egypt’s Nile has far less pressure put on it, although its exit flow to the Mediterranean is also hugely reduced from its former peak volume. The water crisis affecting the Colorado River and the areas that it serves has peaked during the 14-year drought over its lower reaches. To ease conditions in the former wet lands of Mexico near the river’s outlet 2014 saw deliberate major releases from giant reservoirs higher in the Colorado’s course.

English: New map of the Colorado River watersh... — The Colorado River Basin (credit: Wikipedia)

Surface abstraction is not the only drain on water resources of the Colorado River basin: groundwater pumping from the sediments beneath has grown enormously for both irrigation and urban use. That it is possible to play golf at many courses in the desert and to see monstrous musical fountains in Las Vegas is down largely to groundwater exploitation. There have been concerns about depletion of underground reserves once abstraction outpaced natural recharge by infiltration of rainfall and snow melt, but highlighting the magnitude of the problem required a rather dramatic discovery: so much water has been lost from aquifers that the missing mass has reduced the Earth’s gravitational field over the south-west US (Castle, S.L. et al. 2014. Groundwater depletion during drought threatens future water security of the Colorado River Basin. Geophysical Research Letters, doi: 10.1002/2014GL061055).

The evidence comes from the Gravity Recovery and Climate Experiment (GRACE), jointly funded by NASA and Germany’s DLR and launched in March 2002. GRACE uses two satellites that follow the same orbit with a spacing of 220 km between them. Range finders on each measure their separation distance, and so their ups and downs as gravity varies, with far greater accuracy than any other method. Measuring the Earth’s entire gravitational field at their orbital height takes about a month. Groundwater depletion beneath the Gangetic Plains of northern India, to the tune of 109 km³, was detected in 2009 and the same approach has been applied to the Colorado Basin for nine years between 2004 and 2013. It shows that during this part of one of the longest droughts in the history of the south-west US 50 km³ have been lost from beneath, as a rate of about 5.5 km³ per year. Though the total is half the loss from beneath northern India, it should be remembered that more than ten times as many people depend on the Ganges Basin. Moreover, there is no monsoon recharge in the south-western states.

Any excuse to return to the Moon

Published on August 8, 2014 by zooks777Leave a comment

Humans first set foot on the Moon 45 years ago, yet by 42 years ago the last lunar astronaut left: by human standards staffed lunar exploration has been ephemeral. Yet for several reasons – romantic and political – once again getting living beings onto other worlds has become an obsession to some, in much the same manner that increasing numbers of countries seem hell-bent in increasing the redundancy of equipment in orbit; redundant because many of the satellites being launched all do much the same thing, especially in the remote sensing field. It’s all a bit like the choice between buying a Ferrari or hiring a perfectly serviceable vehicle when needed – prestige is high on the list of motivators. A new obsession is extraterrestrial mining and some very rich kids on the block are dabbling in that possibility: James Cameron of Aliens and Avatar fame (both films with space mining in the plot); a bunch of Google top dogs; billionaire entrepreneurs and oligarchs with cash to burn. Resource exploitation has also motivated Indian, Russian and Chinese interest in a return to the Moon, at least at an exploratory level.

NASA's proposed Moon colony concept from early... — NASA’s proposed Moon colony concept from early 2001 (image: NASA)

The main prospective targets have been water, as a source of hydrogen and oxygen through electrolysis to make portable rocket fuel, and helium, especially its rare isotope He-3, for use in fusion reactors. Helium is more abundant on the Moon than it is on Earth: only 300 grams of He-3 per year leaks out of the Earth’s depths. On the Moon there may be as much as 50 parts per billion in its dusty regolith cover where it remains supercooled in areas of permanent shadow. But to get a ton of it would require shifting 150 million tons of regolith. A decade ago geologists suggesting that metals might be mined on the Moon – noble metals and rare-earth elements have been mooted (the latter’s export being embargoed by Earth’s main producer China) – would have been laughing stocks, but now they get air time. Yet none of these materials occur on the Moon in the type of ore deposit found on Earth; if they did the anomalous nature of such enrichments on a body devoid of vegetation would have ensured their detection already. Even if there were lunar ore bodies, anyone with a passing familiarity with resource extraction knows just how much waste has to be shifted to make even a super-rich deposit economic on Earth, and that vast amounts of water are deployed in enriching the ‘paying’ metal to levels fit for smelting. For instance, while the rise in gold price since it was detached from a fixed link with paper money in 1971 has enabled very low concentrations to be mined, the methods involve grinding ore in water and then dissolving the gold in sodium cyanide solution, re-precipitating it on carbon made from coconut husks, redissolving and then precipitating the gold again by mixing the ‘liquor’ with zinc dust. Dry ore processing methods – based on density, magnetic and electrical properties – are hardly used in major mining operations nowadays.

The other, and perhaps most important issue with lunar or asteroid mining is that the undoubtedly high costs of whatever beneficiation process is deemed possible must be offset against income from the product; i.e. determined by market price on the home world which would have to be far higher than now. Such a rise in price would work to make currently uneconomic resources here worth mining, and anyone who believes that mining on the Moon would ever be competitive in that capitalist scenario risks being en route to the chuckle farm. Unless, of course, their motive is an exclusivist hobby par excellence and the bragging rights that accompany it – a bit like big game hunting, but the buzz coming from risking their billions rather than their lives.

But it turns out that a refocus on bringing stuff back from the Moon is not confined to floating stock on the financial markets. There are academic efforts to rationalise the Dan Dare spirit. There aren’t many scientific journals with a level of kudos to match the Philosophical Transactions of the Royal Society, the first journal in the world exclusively devoted to science and probably the longest running since it was established in 1665 at the same time as the Royal Society itself. Recently one of its thematic issues dubbed ‘‘Shock and blast: celebrating the centenary of Bertram Hopkinson’s seminal paper of 1914’ (Hopkinson, B. 1914. A method of measuring the pressure produced in the detonation of high explosives or by the impact of bullets. Philosophical Transactions of the Royal Society A v. 213, p. 437-456) a paper appeared that examines the likelihood of fossils surviving the shocks of a major impact (Burchell, M.J. et al. 2014. Survival of fossils under extreme shocks induced by hypervelocity impacts. Philosophical Transactions of the Royal Society A v. 372, 20130190 Open Access).

The authors, based at the University of Kent, UK, used a high-velocity air gun to fire quite fragile fossils of diatoms frozen in ice into water at speeds up to 5.34 km s^-1. They then looked at solids left in the target to see if any recognisable sign of the fossils remained. Even at the highest energies of impact some diatomaceous material did indeed remain. Their conclusion was that meteorites derived by large impacts into planetary bodies, such as those supposedly from Mars or the Moon, could reasonably be expected to carry remnants of fossils from the bodies, had the impact been into sedimentary rock and that the bodies had supported living organisms that secreted hard parts. My first thought was that the paper was going to resurrect the aged notion of panspermia and a re-examination of the ALH84001 meteorite found in Antarctica claimed in 1996 to contain a Martian fossil (and believed by then US President Bill Clinton). Likewise it might be cited in support of the similar claim, made by panspermia buff Chandra Wickramasinghe, regarding fossils reputedly in a meteorite that fell in Sri Lanka on 29 December 2012: widely regarded as being mistaken. Yet Wickramasinghe’s team reported diatoms in the meteorite!

However, Burchell has suggested that their results open up the possibility of meteorites on the Moon that had been blasted there from Earth might preserve terrestrial fossils. Moreover, such meteorites might preserve fossils from early stages in the evolution of life on Earth, since when both rocks and whatever they once contained have been removed by erosion or obliterated by deformation and metamorphism on our active planet. ‘Another reason we should hurry back to the Moon’ says Kieren Torres Howard of New York’s City University…

Breathing spaces or toxic traps in the Archaean ocean

Published on July 31, 2014 by zooks777Leave a comment

The relationship between Earth’s complement of free oxygen and life seems to have begun in the Archaean, but it presented a series of paradoxes: produced by photosynthetic organisms oxygen would have been toxic to most other Archaean life forms; its presence drew an important micronutrient, dissolved iron-2, from sea water by precipitation of iron-3 oxides; though produced in seawater there is no evidence until about 2.4 Ga for its presence in the air. It has long been thought that the paradoxes may have been resolved by oxygen being produced in isolated patches, or ‘oases’ on the Archaean sea floor, where early blue-green bacteria evolved and thrived.

A stratigraphic clue to the former presence of such oxygen factories is itself quite convoluted. The precipitation of calcium carbonates and therefore the presence of limestones in sedimentary sequences are suppressed by dissolved iron-2: the presence of Fe²⁺ ions would favour the removal of bicarbonate ions from seawater by formation of ferrous carbonate that is less soluble than calcium carbonate. Canadian and US geochemists studied one of the thickest Archaean limestone sequences, dated at around 2.8 Ga, in the wonderfully named Wabigoon Subprovince of the Canadian Shield which is full of stromatolites, bulbous laminated masses probably formed from bacterial biofilms in shallow water (Riding, R. et al. 2014. Identification of an Archean marine oxygen oasis. Precambrian Research, v. 251, p. 232-237).

Limestones from the sequence that stable isotope analyses show to remain unaltered all have abnormally low cerium concentrations relative to the other rare-earth elements. Unaltered limestones from stromatolite-free, deep water limestones show no such negative Ce anomaly. Cerium is the only rare-earth element that has a possible 4+ valence state as well one with lower positive charge. So in the presence of oxygen cerium can form an insoluble oxide and thus be removed from solution. So cerium independently shows that the shallow water limestones formed in seawater that contained free oxygen. Nor was it an ephemeral condition, for the anomalies persist through half a kilometer of limestone.

The study shows that anomalous oxygenated patches existed on the Archaean sea floor, probably shallow-water basins or shelves isolated by the build up of stromatolite reef barriers. For most prokaryote cells they would have harboured toxic conditions, presenting them with severe chemical stress. Possibly these were the first places where oxygen defence measures evolved, that eventually led to more complex eukaryote cells that not only survive oxygen stress but thrive on its presence. That conjecture is unlikely to be fully proved, since the first undoubted fossils of eukaryote cells, known as acritarchs, occur in rocks that are more than 800 Ma years younger.

Oxygen oasis for early life found in ancient rock

Planet Mercury and giant collisions

Published on July 30, 2014 by zooks777Leave a comment

Mercury is quite different from the other three Terrestrial Planets, having a significantly higher density. So it must have a considerably larger metallic core than the others – estimated to make up about 70% of Mercury’s mass – and therefore has a far thinner silicate mantle. The other large body in the Inner Solar System, our Moon, is the opposite, having the greatest proportion of silicate mantle and a small core.

The presently favoured explanation for the Moon’s anomalous mass distribution is that it resulted from a giant collision between the proto-Earth and a Mars-sized planetary body. Moreover, planetary theorists have been postulating around 20 planetary ‘embryos’ in the most of which accreted to form Venus and Earth, the final terrestrial event being the Moon-forming collision, with smaller Mars and Mercury having been derived from the two remaining such bodies. For Mercury to have such an anomalously large metallic core has invited mega-collision as a possible cause, but with such a high energy that much of its original complement of silicate mantle failed to fall back after the event. Two planetary scientists from the Universities of Arizona, USA, and Berne, Switzerland, have modelled various scenarios for such an origin of the Sun’s closest companion (Asphaug, E. & Reuffer, A. 2014. Mercury and other iron-rich planetary bodies as relics of inefficient accretion. Nature Geoscience, published online, doi: 10.1038/NGEO2189).

Their favoured mechanism is what they term ‘hit-and-run’ collisions in the early Inner Solar System. In the case of Mercury, that may have been with a larger target planet that survived intact while proto-Mercury was blasted apart to lose much of it mantle on re-accretion. To survive eventual accretion into a larger planet the left-overs had to have ended up in an orbit that avoided further collisions. Maybe Mars had the same kind of lucky escape but one that left it with a greater proportion of silicates.

One possible scenario is that proto-Mercury was indeed the body that started the clock of the Earth-Moon system through a giant impact. Yet no-one will be satisfied with a simulation and some statistics. Only detailed geochemistry of returned samples can take us any further. The supposed Martian meteorites seem not to be compatible with such a model; at least one would expect there to have been a considerable stir in planetary-science circles if they were. For Mercury, it will be a long wait for a resolution by geochemists, probably yet to be conceived.

Trapping Martian life forms

Published on July 21, 2014 by zooks777Leave a comment

No matter how optimistic exobiologists might be, the current approaches to discovering whether or not Mars once hosted life or, the longest shot of all, still does are almost literally hit or miss. First the various teams involved try to select a target area using remotely sensed data to see if rocks or regolith have interacted with water; generally from the presence or absence of clay minerals and /or sulfates that hydrous alteration produces on Earth. Since funding is limited the sites with such ingredients are narrowed down to the ‘best’ – in the case of NASA’s Curiosity rover to Gale Crater where a thick sequence of sediments shows occasional signs of clays and sulfates. But a potential site must also be logistically feasible with the least risk of loss to the lander. Even then, all that can be achieved in existing and planned mission is geochemical analysis of drilled and powdered samples. Curiosity’s ambition is limited to assessing whether the conditions for life were present. Isotopic analysis of any carbon content to check for mass fractionation that may have arisen from living processes is something for a future ESA mission.

Neither approach is likely to prove the existence now or in far-off times of Martian life, though scientists hope to whet the appetite of those holding the purse strings. Only return of rock samples stands any realistic chance of giving substance to the dreams of exobiologists. But what to collect? A random soil grab or drill core is highly unlikely to provide satisfaction one way or the other. Indeed only incontrovertible remains of some kind of cellular material can slake the yearning. Terrestrial materials might provide a guide to (probably) robotic collectors. Kathleen Benison and Francis Karmanocky of West Virginia University have followed this up by examining sulfates from one of the least hospitable places on Earth; the salt flats of the high Andes of Chile (Benison, K.C. & Karmanocky, F.J. 2014. Could microorganisms be preserved in Mars gypsum? Insights from terrestrial examples. Geology, v. 42, p. 615-618).

Evaporite minerals from Andean salars precipitated from extremely acidic and highly saline lake water originating from weathering of surrounding volcanoes. Oddly few researchers have sought cellular life trapped in crystals of salt or gypsum, the two most common minerals in the high-elevation salt pans. Fluid inclusions in sedimentary halite (NaCl) crystals from as far back as the Triassic are known to contain single-celled extremophile prokaryotes and eukaryotes, but gypsum is more likely to be found on Mars. Benison and Karmanocky document a variety of cellular material from Chilean gypsum that has been trapped in the solid mineral itself or in fluid inclusions. This is the most likely means of fossilisation of Martian life forms, if they ever existed. The salar gypsum contain cells that can be cultured and thereby revived since several species can remain dormant for long periods. The authors suggest that transparent cleavage fragments of Martian gypsum could be examined at up to 2000x magnification on future Mars landers. Finding convincing cells would see dancing in exobiology labs, and what if they should move…

A Most Earthly Mineral on Mars

Fieldwork and geological education

Published on July 16, 2014 by zooks7775 Comments

In March 2013 EPN carried an item connected with the abandonment of field training at week-long summer schools by the UK’s Open University. After 40 years of geoscientific summer schools connected with courses at Levels-1, -2 and -3 anonymous performance statistics were available for thousands of students who had studied those OU Earth Science courses that offered summer-school experiences in the field, first as compulsory modules (1971-2001) then as an optional element (2002-2011) and finally with no such provision. The March 2013 item compared statistics for the three kinds of provision. It should be noted that the OU once had possibly the world’s largest throughput of degree-level geoscience students for a single higher educational institution.

After 2001, pass rates feel abruptly and significantly; in the Science Foundation Course the rate fell from an annual average of 69 to 54%, and in level-2 Geology from 65 to 55%. This was accompanied by a significant decrease in enrolment in equally and more popular geoscience courses that had never had a summer school element. The second statistical drop was of the order of 30 to 40%. It seemed that residential schools played a vital role in boosting confidence and reinforcing home studies, as well as transferring practical field skills. After further falls in enrolment since summer schools were removed from the curriculum in 2012, the OU is in the process of completely revising its geoscientific courses and attempting to substitute virtual, on-line field and lab ‘experiences’. Time will tell if it ever manages to reach its former level of success and acceptance

So, discovering that The Geological Society of America had surveyed attendees at its Annual Meetings (Petcovic, H.L. et al. 2014. Geoscientists’ perceptions of the value of undergraduate field education. GSA Today, v. 24 (July 2014), p. 4-10) piqued my interest. Almost 90% of those polled agreed that field studies should be a fundamental requirement of undergraduate programmes; very few agreed that becoming an expert geoscientist is possible without field experience. Field courses develop the skills and knowledge specific to ‘doing’ geoscience; teach integration of fundamental concepts and broaden general understanding of them; inculcate cooperation, time management and independent thinking that have broader applications. Fieldwork also has personal and emotional impacts: reinforcing positive attitudes to the subject; creating a geoscientific esprit de corps; helping students recognise their personal strengths and limitations. Then there is the aspect of enhanced employability, highlighted by all categories of respondents.

Set against these somewhat predictable sentiments among geoscientists are the increasing strains posed by cost, time commitment, and liability, as well as the fact that some potential students do not relish outdoor pursuits. Yet overall the broad opinion was that degree programmes should involve at least one field methods course as a requirement, with other non-compulsory opportunities for more advanced field training

Mass extinctions’ connections with volcanism: more support

Published on July 4, 2014 by zooks7771 Comment

Plot the times of peaks in the rates of extinction during the Mesozoic against those of flood basalt outpourings closest in time to the die-offs and a straight line can be plotted through the data. There is sufficiently low deviation between it and the points that any statistician would agree that the degree of fit is very good. Many geoscientists have used this empirical relationship to claim that all Mesozoic mass extinctions, including the three largest (end-Permian, end-Triassic and end-Cretaceous) were caused in some way by massive basaltic volcanism. The fact that the points are almost evenly spaced – roughly every 30 Ma, except for a few gaps – has suggested to some that there is some kind of rhythm connecting the two very different kinds of event.

Major extinctions and flood basalt events during the Mesozoic (credit: S Drury) — Major Mesozoic extinctions and flood basalt events (credit: S Drury)

Leaving aside that beguiling periodicity, the hypothesis of a flood-basalt – extinction link has a major weakness. The only likely intermediary is atmospheric, through its composition and/or climate; flood volcanism was probably not violent. Both probably settle down quickly in geological terms. Moreover, flood basalt volcanism is generally short-lived (a few Ma at most) and seems not to be continuous, unlike that at plate margins which is always going on at one or other place. The great basalt piles of Siberia, around the Central Atlantic margins and in Western India are made up of individual thick and extensive flows separated by fossil soils or boles. This suggests that magma blurted out only occasionally, and was separated by long periods of normality; say between 10 and 100 thousand years. Evidence for the duration of major accelerations, either from stratigraphy and palaeontology or from proxies such as peaks and troughs in the isotopic composition of carbon (e.g. EPN Ni life and mass extinction) is that they too occurred swiftly; in a matter of tens of thousand years. Most of the points on the flood-basalt – extinction plot are too imprecise in the time dimension to satisfy a definite relationship. Opinion has swung behind an instantaneous impact hypothesis for the K-P boundary event rather than one involving the Deccan Traps in India, simply because the best dating of the Deccan suggests extinction seems to have occurred when no flows were being erupted, while the thin impact-related layer in sediments the world over is exactly at the point dividing Cretaceous flora and fauna from those of the succeeding Palaeogene.

Yet no such link to an extraterrestrial factor is known to exist for any other major extinctions, so volcanism seems to be ‘the only game in town’ for the rest. Until basalt dating is universally more precise than it has been up to the present the case is ‘not proven’; but, in the manner of the Scottish criminal law, each is a ‘cold case’ which can be reopened. The previous article hardens the evidence for a volcanic driver behind the greatest known extinction at the end of the Permian Period. And in short-order, another of the Big Five seems to have been resolved in the same way. A flood basalt province covering a large area of west and north-west Australia (known as the Kalkarindji large igneous province)has long been known to be of roughly Cambrian age but does it tie in with the earliest Phanerozoic mass extinction at the Lower to Middle Cambrian boundary? New age data suggests that it does at the level of a few hundred thousand years (Jourdan, F. et al. 2014. High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early-Middle Cambrian (Stage 4-5) extinction. Geology, v. 42, p. 543-546). The Kalkarindji basalts have high sulfur contents and are also associated with widespread breccias that suggest that some of the volcanism was sufficiently explosive to have blasted sulfur-oxygen gases into the stratosphere; a known means of causing rapid and massive climatic cooling as well as increasing oceanic acidity. The magma also passed through late Precambrian sedimentary basins which contain abundant organic-rich shales that later sourced extensive petroleum fields. Their thermal metamorphism could have vented massive amounts of CO₂ and methane to result in climatic warming. It may have been volcanically-driven climatic chaos that resulted in the demise of much of the earliest tangible marine fauna on Earth to create also a sudden fall in the oxygen content of the Cambrian ocean basins.

Nickel, life and the end-Permian extinction

Published on June 23, 2014 by zooks777Leave a comment

The greatest mass extinction of the Phanerozoic closed the Palaeozoic Era at the end of the Permian, with the loss of perhaps as much as 90% of eukaryote diversity on land and at sea. It was also over very quickly by geological standards, taking a mere 20 thousand years from about 252.18 Ma ago. There is no plausible evidence for an extraterrestrial cause, unlike that for the mass extinction that closed the Mesozoic Era and the age of dinosaurs. Almost all researchers blame one of the largest-ever magmatic events that spilled out the Siberian Traps either through direct means, such as climate change related to CO₂, sulfur oxides or atmospheric ash clouds produced by the flood volcanism or indirectly through combustion of coal in strata beneath the thick basalt pile. So far, no proposal has received universal acclaim. The latest proposal relies on two vital and apparently related geochemical observations in rocks around the age of the extinctions (Rothman, D.H. et al. 2014. Methanogenic burst in the end-Permian carbon cycle. Proceedings of the National Academy of the United States, v. 111, p. 5462-5467).

Siberian flood-basalt flows in Putorana, Taymyr Peninsula. (Credit: Paul Wignall; Nature http://www.nature.com/nature/journal/v477/n7364/fig_tab/477285a_F1.html)

In the run-up to the extinction carbon isotopes in marine Permian sediments from Meishan, China suggest a runaway growth in the amount of inorganic carbon (in carbonate) in the oceans. The C-isotope record from Meishan shows episodes of sudden major change (over ~20 ka) in both the inorganic and organic carbon parts of the oceanic carbon cycle. The timing of both ‘excursions’ from the long-term trend immediately follows a ‘spike’ in the concentration of the element nickel in the Meishan sediments. The Ni almost certainly was contributed by the massive outflow of basalt lavas in Siberia. So, what is the connection?

Some modern members of the prokaryote Archaea that decompose organic matter to produce methane have a metabolism that depends on Ni, one genus being Methanosarcina that converts acetate to methane by a process known as acetoclastic methanogenesis. Methanosarcina acquired this highly efficient metabolic pathway probably though a sideways gene transfer from Bacteria of the class Clostridia; a process now acknowledged as playing a major role in the evolution of many aspects of prokaryote biology, including resistance to drugs among pathogens. Molecular-clock studies of the Methanosarcina genome are consistent with this Archaea appearing at about the time of the Late Permian. A burst of nickel ‘fertilisation’ of the oceans may have resulted in huge production of atmospheric methane. Being a greenhouse gas much more powerful than CO₂, methane in such volumes would very rapidly have led to global warming. Before the Siberian Traps began to be erupted nickel would only have been sufficiently abundant to support this kind of methanogen around ocean-floor hydrothermal springs. Spread globally by eruption plumes, nickel throughout the oceans would have allowed Methanosarcina or its like to thrive everywhere with disastrous consequences. Other geochemical processes, such as the oxidation of methane in seawater, would have spread the influence of the biosphere-lithosphere ‘conspiracy’. Methane oxidation would have removed oxygen from the oceans to create anoxia that, in turn, would have encouraged other microorganisms that reduce sulfate ions to sulfide and thereby produce toxic hydrogen sulfide. That gas once in the atmosphere would have parlayed an oceanic ‘kill mechanism’’ into one fatal for land animals.

There is one aspect that puzzles me: the Siberian Traps probably involved many huge lava outpourings every 10 to 100 ka while the magma lasted, as did all other flood basalt events. Why then is the nickel from only such eruption preserved in the Meishan sediments, and if others are known from marine sediments is there evidence for other such methanogen ‘blooms’ in the oceans?

What’s happening at the core-mantle boundary?

Published on June 16, 2014 by zooks7772 Comments

The lithosphere that falls into the mantle at subduction zones must end up somewhere in the deep Earth; the question is, where and what happens to it. There are hints from seismic tomography of the mantle that such slabs penetrate as deep as the boundary between the lowermost mantle and the molten outer core. The lithosphere’s two components, depleted mantle and oceanic crust, are compositionally quite different, being peridotitic and basaltic, so each is likely to be involved different petrological processes. As regards the physics, since seismic activity ceases below a depth of about 700 km neither entity behaves in a brittle fashion in the lower mantle. Such ductile materials might even pile up in the manner of intestines on the lithological equivalent of the abattoir floor; Bowels of the Earth as John Elder had it in his book of the same name.

Sketch of the lower 1000 km of the Earth’s mantle (credit: Williams, Q. 2014. Deep mantle matters. Science, v. 344, p. 800-801)

Pressure would make these recycled components mineralogically different, as indeed a relative light squeeze does in the upper mantle, where cold wet basalts become dry and denser eclogites thereby pulling more lithosphere down Wadati and Benioff’s eponymous zones to drive plate tectonics. Decades old experiments at lower-mantle pressures suggest that mantle minerals recompose from olivine with a dash of pyroxene to a mixture of more pressure-resistant iron-magnesium oxide and perovskite ((Mg,Fe)SiO₃). Experiments in the early 21^st century, under conditions at depths below 2600 km, revealed that perovskite transforms at the very bottom of the mantle (the D” zone) into layers of magnesium plus iron, silicon and oxygen. This is provisionally known as ‘post-perovskite’. The experiments showed that the transition releases heat. So, should oceanic lithosphere descend to the D” zone, it would receive an energy ‘kick’ and its temperature would increase. Conversely, if D”-zone materials rose to the depth of the perovskite to post-perovskite transition they would become less dense: a possible driver for deep-mantle plumes.

Now a new iron-rich phase stable in the bottom 1000 km of the mantle has emerged from experiments, seeming to result from perovskite undergoing a disproportionation reaction (Zhang, L. And 11 others 2014. Disproportionation of (Mg,Fe)SiO₃ perovskite in Earth’s deep mantle. Science, v. 344, p. 877-882). In the same issue of Science other workers using laser-heated diamond anvils have revealed that, despite the huge pressures, basaltic rock may melt at temperatures considerably below the solid mantle’s ambient temperature (Andrault, D. et al. 2014. Melting of subducted basalt at the core-mantle boundary. Science, v. 344, p. 892-895). Both studies help better understand the peculiarities of the deepest mantle that emerge from seismic tomography (Williams, Q. 2014. Deep mantle matters. Science, v. 344, p. 800-801).

Huge blocks with reduced S-wave velocities that rise above the D” zone sit beneath Africa and the Pacific Ocean. There are also smaller zones at the core-mantle boundary (CMB) with shear-wave velocities up to 45% lower than expected. These ultralow-velocity zones (ULVZs) probably coincide with melting of subducted oceanic basalts, but the magma cannot escape by rising as it would soon revert to perovskite. Yet, since ultramafic compositions cannot melt under such high pressures the ULVSs indirectly show that subduction does descend to the CMB. Seismically defined horizontal layering in the D” zone thus may result from basaltic slabs whose ductility has enabled them to fold like sheets of lasagne as the reach the base of the mantle. Development of variants of the laser-heated diamond anvil set-up seem likely to offer insights into our own world’s ‘digestive’ system at a far lower cost and with vastly more relevance than the growing fad for speculating on Earth-like planets that the current ‘laws’ of physics show can never be visited and on ‘exobiology’ that cannot proceed further than the extremes of the Earth’s near-surface environment and the DNA double helix.

Year Zero: the giant-impact hypothesis

Published on June 16, 2014 by zooks7771 Comment

On close examination, the light-coloured Highlands of the Moon look remarkably like an old sign by a North American road through hunting country: they are pocked by impact craters of every size. More than that, a lengthy period of bombardment is signified by signs that the craters themselves are cratered to form a chaotic landscape dominated by interlocking and overlapping circular feature. In contrast the dark basaltic plains, called maria (seas), are pretty smooth albeit with some craters. They are clearly much younger than the Highlands. The discovery by Apollo astronauts that the older lunar Highlands are made almost exclusively of calcic plagioclase feldspar was a major surprise, requiring an astonishing event to explain them. Such anorthosites may form by flotation of low-density feldspar from a cooling and crystallising basaltic magma. Yet to form the bulk of the Moon’s early crust from such materials requires not simply a deep magma chamber, but literally an ocean of molten material at least 200 km deep. The anorthosites also turned out to be far older than the oldest rocks on Earth, close to 4.5 billion years. The most likely explanation seemed to be that the melting resulted from a gargantuan collision between two protoplanets, the Earth’s forebear and another now vanished. This would have melted and partially vaporised both bodies. After this discovery the Moon was widely believed to have formed from liquid and vaporised rock flung into orbit around what became the Earth.

Artist’s depiction of a collision between two planetary bodies likely to have formed the Moon (Credit: Wikipedia)

Such a catastrophic model for joint formation of the Earth and Moon shortly after planets of the Solar System had formed is hard to escape, but it carries two major puzzles. First, Earth and Moon seem to have very similar, indeed almost the same chemistry: So what happened to the colliding planet? If it had been identical in composition to the proto Earth there is no problem, but a different composition would surely have left some detectable trace in a Moon-Earth geochemical comparison. Initial models of the collision suggested that the other planet (dubbed Theia) was about the size of Mars and should have contributed 70 to 90% of the lunar mass: the Moon-Earth geochemical difference should have been substantial The second issue raised in the early days of the hypothesis was that since the Moon seemed to be almost totally dry (at least, the first rock analyses suggested that), then how come the Earth had retained so much water?

For decades, after an initial flurry of analyses, the Apollo samples remained in storage. Only in the last 10 years or so, when the need to gee-up space exploration required some prospect of astronauts one more to be sent beyond Earth orbit, have the samples been re-examined. With better analytical tools, the first puzzle was resolved: lunar rocks do contain measurable amounts of water, so the impact had not entirely driven off volatiles from the Moon. The bulk geochemical similarity was especially puzzling for the isotopes of oxygen. Meteorites of different types are significantly ear-marked by their relative proportions of different oxygen isotopes, signifying to planetary scientists that each type formed in different parts of the early Solar System; a suggestion confirmed by the difference between those in meteorites supposedly flung from Mars and terrestrial oxygen isotope proportions. A clear target for more precise re-examination of the lunar samples, plus meteorites reckoned to have come from the Moon, is therefore using vastly improved mass spectrometry to seek significant isotopic differences (Harwartz, D. et al. 2014. Identification of the giant impactor Theia in lunar rocks. Science, v. 344, p. 1146-1150). It turns out that there is a 12 ppm difference in the proportion of ¹⁷Oin lunar oxygen, sufficient to liken Theia’s geochemistry to that of enstatite chondrites. However, that difference may have arisen by the Earth, once the Moon had formed, having attracted a greater proportion of carbonaceous-chrondrite material during the later stages of planetary accretion by virtue of its much greater gravitational attraction. That would also account for the much higher volatile content of the Earth.

The new data do help to support the giant-impact hypothesis, but still leave a great deal of slack in the big questions: Did Theia form in a similar orbit around the Sun to that of Earth; was the impact head-on or glancing; how fast was the closure speed; how big was Theia and more besides? If Theia had roughly the same mass as the proto-Earth then modelling suggests that about half the mass of both Moon and Earth would be made of Theia stuff, giving the Moon and post-impact Earth much the same chemistry, irrespective of where Theia came from. Were William of Ockham’s ideas still major arbiters in science, then his Razor would suggest that we stop fretting about such details. But continuing the intellectual quest would constitute powerful support for a return to the Moon and more samples…

Fracking in the UK; will it happen?

Published on June 3, 2014 by zooks777Leave a comment

Whether or not one has read the Tractatus Logico-Philosophicus of Ludwig Wittgenstein, there can be little doubt that one of his most famous quotations can be applied to much of the furore over hydraulic fracturing (fracking) of hydrocarbon-rich shale in south-eastern Britain: ‘Whereof one cannot speak, one must remain silent’ (more pithily expressed by Mark Twain as ‘Better to remain silent and be thought a fool than to speak and remove all doubt’). A press release by the British Geological Survey in late May 2014 caused egg to appear on the shirts of both erstwhile ‘frackmeister’ David Cameron (British Prime Minister) and anti-fracking protestors in Sussex. While there are oil shales beneath the Weald, these Jurassic rocks have never reached temperatures sufficient to generate any significant gas reserves (see: Upfront, New Scientist, 31 May 2014 issue, p. 6). Yet BGS estimate the oil shales to contain a total of 4.4 billion barrels of oil. That might sound a lot, but the experience of shale fracking companies in the US is that, at best, only about 5% can be recovered and, in cases that are geologically similar to the Weald, as little as 1% might be expected. Between 44 and 220 million barrels is between two and six months’ worth of British oil consumption; and that is only if the entire Wealden shales are fracked.

Areas where petroleum-rich shales occur at the surface in Britain. (credit: British Geological Survey)

Why would any commercial exploration company, such as Cuadrilla, go to the trouble of drilling wells, even of an ‘exploratory nature’, for such meager potential returns? Well, when there is sufficient hype, and the British Government has gushed in this context for a few years, bigger fish tend to bite and cash flows improve. For instance, Centrica the owner of British Gas forked out $160 million to Cuadrilla in June 2013 for a quarter share in the well-publicised licence area near Blackpool in Lancashire; the grub stake to allow Cuadrilla to continue exploration in exchange for 25% of any profit should commercial quantities of shale-gas be produced.

Sedimentary rock sequences further north in Britain whose geological evolution buried oil shales more deeply are potential gas producers through fracking; an example is the Carboniferous Bowland Shale beneath the Elswick gasfield in west Lancashire, targeted by Cuadrilla. Far greater potential may be present in a large tract of the Pennine hills and lowlands that flank them where the Bowland Shale occurs at depth.

Few people realize just how much detail is known about what lies beneath their homes apart from maps of surface geology. That is partly thanks to BGS being the world’s oldest geological survey (founded in 1835) and partly the sheer number of non-survey geologists who have prowled over Britain for 200 years or more and published their findings. Legally, any excavation, be it an underground mine, a borehole or even the footings for a building, must be reported to BGS along with whatever geological information came to light as a result. The sheer rarity of outcropping rock in Britain is obvious to everyone: a legacy of repeated glaciation that left a veneer of jumbled debris over much of the land below 500m that lies north of the northern outskirts of the London megalopolis. Only highland areas where glacial erosion shifted mullock to lower terrains have more than about 5% of the surface occupied by bare rock. Of all the data lodged with BGS by far the most important for rock type and structure at depth are surveys that used seismic waves generated by vibrating plates deployed on specialized trucks. These and the cables that connected hundreds of detectors were seen along major and minor roads in many parts of Britain during the 1980s during several rounds of licenced onshore exploration for conventional petroleum resources. That the strange vehicles carried signs saying Highway Maintenance lulled most people apart from professional geologists as regards their actual purpose. Over 75 thousand kilometers of seismic sections that penetrated thousands of metres into the Earth now reside in the UK Onshore Geophysical Library (an Interactive Map at UKOGL allows you to see details of these surveys, current areas licenced for exploration and the locations of various petroleum wells).

Seismic survey lines in northern England (green lines) from the interactive map at the UK Onshore Geophysical Library

Such is the detail of geological knowledge that estimates of any oil and gas, conventional or otherwise, residing beneath many areas of Britain are a lot more reliable than in other parts of the world which do not already have or once had a vibrant petroleum industry. So you can take it that when the BGS says there is such and such a potential for oil or gas beneath this or that stretch of rural Britain they are pretty close to the truth. Yet it is their raw estimates that are most often publicized; that is, the total possible volumes. Any caveats are often ignored in the publicity and hype that follows such an announcement. BGS recently announced that as much as 38 trillion cubic metres of gas may reside in British shales, much in the north of England. There followed a frenzy of optimism from Government sources that this 40 years’ worth of shale gas would remove at a stroke Britain’s exposure to the world market of natural gas, currently dominated by Russia, and herald a rosy economic future to follow the present austerity similar to the successes of shale-gas in North America. Equally, there has been fear of all kinds of catastrophe from fracking on our ‘tight little island’ especially amongst those lucky enough not to live in urban wastelands. What was ignored by both tendencies was reality. In the US, fracking experience shows that only 10% at most of the gas in a fractured shale can be got out; even the mighty Marcellus Shale of the NE US underlying an area as big as Britain can only supply 6 years of total US gas demand. Britain’s entire shale-gas endowment would serve only 4 years of British gas demand.

To tap just the gas in the upper part of the Bowland basin would require 33 thousand fracking wells in northern Britain. Between 1902 and 2013 only 19 onshore petroleum wells were drilled here in an average year. To make any significant contribution to British energy markets would require 100 per annum at a minimum. Yet, in the US, the flow rate from fracked wells drops to a mere zephyr within 3 years. Fracking on a large scale may well never happen in Britain, such are the largely unstated caveats. But the current hype is fruitful for speculation that it will, and that can make a lot of cash sucked in by the prospect – without any production whatsoever.

Carbon Briefing: UK shale gas resource (carbonbrief.org)
Carbon Briefing: the what, where, and how much of UK shale oil (carbonbrief.org)
The cost of fracking Britain (bbc.co.uk)
Fracking company wants UK drilling laws relaxed (itv.com)

Impact melts and their destination

Published on May 28, 2014 by zooks7771 Comment

The work done by an asteroid or a comet that hits the Earth is most obviously demonstrated by the size of the crater that it creates on impact, should it have survived erosion and/or burial by sediments. Since some is done in flinging material away from the impact, the furthest point at which ejecta land is also a rough measure of the power of the hit. All this and much more derived from the kinetic energy of the object, which from Newton’s laws of motion amounts to half the product of the body’s mass and the square of its speed (mv²/2). It’s the speed that confers most energy; doubling the speed quadruples the energy. At a minimum, the speed of an object from far-off in space is that due to acceleration by the Earth’s gravitational field; the same as Earth’s escape velocity (about 11.2 km s^-1). In March 1989 Earth had a close encounter with Newton’s laws writ large; an asteroid about 500 m across passed us with just half a million kilometres to spare. Moving at 20 km s^-1 it carried kinetic energy of around 4 x 10¹⁹J. Had it hit, all of this immense amount would have been delivered in about a second giving a power of 4 x 10¹⁹ W. That is more than two hundred times greater than the power of solar heating of the day-side of the Earth. A small part of that power would melt quite a lot of rock.

As well as the glass spherules that are one of the hallmarks of impact ejecta on Earth and more so on the Moon’s surface, some of the larger known impact craters are associated with various kinds of glassy rock produced by instantaneous melting. Some of this melt-rock occurs in thin dykes, but sometimes there is an entire layer of once molten ‘country’ rock at the impact site. The most spectacular is in the Manicougan crater in Quebec, Canada. In fact a 1 km thick impact-melt sheet dominates most of the 90 km wide structure and it is reputed to be the most homogeneous large rock mass known, being a chemical average of every rock type involved in the Triassic asteroid strike. Not all craters are so well endowed with an actual sheet of melt-rock. This has puzzled some geologists, especially those who studied the much larger (160 km) Vredfort Dome in South Africa, which formed around 2 billion years ago. As the name suggests this is now a positive circular topographic anomaly, probably due to rebound and erosional unloading, the structure extending down 20 km into the ancient continental lithosphere of the Kaapvaal craton. Vredfort has some cracking dykes of pseudotachylite but apparently no impact melt sheet. It has vanished, probably through erosion, but a relic has been found (Cupelli, C.L. et al. 2014. Discovery of mafic impact melt in the centre of the Vredfort dome: Archetype for continental residua of early Earth cratering? Geology, v. 42, p. 403-406). One reason for it having gone undiscovered until now is that it is mafic in composition, and resembles an igneous gabbro intrusion. Isotope geochemistry refutes that mundane origin. It is far younger than the rocks that were zapped, and may well have formed as huge energy penetrated to the lower crust and even the upper mantle to melt a sizeable percentage of 2.7 to 3.0 Ga old mafic and ultramafic rock.

Oddly, the same issue of Geology contains an article that also bears on the Vredfort Dome structure (Huber, M.S. et al. 2014. Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredfort impact event. Geology, v. 42, p. 375-378). Drill core from a Palaeoproterozoic limestone revealed millimetre-sized glass droplets containing excess iridium – an element at high concentration in a variety of meteorites. The link to Vredfort is the age of the sediments, which are between 1.98 and 2.05 Ga, neatly bracketing the timing of the large South African impact. Using reasonably well-constrained palaeogeographic positions at that time for Karelia and the Kaapvaal craton suggests that the glassy ejecta, if indeed they are from Vredfort, must have been flung over 2500 km.

How the first metazoan mass extinction happened

Published on May 12, 2014 by zooks777Leave a comment

The end-Ordovician mass extinction was the first of five during the Phanerozoic, andthe first that involved multicelled organisms. It happened in two distinct phases that roughly coincided with an intense but short-lived glaciation at the South Pole, then situated within what is now the African continent. Unlike the other four, this biotic catastrophe seems unlinked to either a major impact structure or to an episode of flood volcanism.

seadiorama ordovician — Artist’s impression of an Ordovician shallow-sea community (credit: drtel)

In 2009 Earth Pages reported the curious occurrence in 470 Ma (Darriwilian Stage) Swedish limestones of a large number of altered chondritic meteorites, possible evidence that there may have been an extraterrestrial influence on extinction rates around that time. In support is evidence that the meteorite swarm coincided with megabreccias or olistostromes at what were then Southern Hemisphere continental margins: possible signs of a series of huge tsunamis. But in fact this odd coincidence occurred at a time when metazoan diversity was truly booming: the only known case of impacts possibly favouring life.

Number One of the Big Five mass extinctions occurred during the late-Ordovician Hirnantian stage (443-445 Ma) and has received much less attention than the later ones. So it is good see the balance being redressed by a review of evidence for it and for possible mechanisms (Harper, D.A.T et al. 2014. End Ordovician extinctions: A coincidence of causes. Gondwana Research, v. 25, p. 1294-1307). The first event of a double-whammy mainly affected free-swimming and planktonic organisms and those of shallow seas; near-surface dwellers such as graptolites and trilobites. The second, about a million years later, hit animals living at all depths in the sea. Between them, the two events removed about 85% of marines species – there were few if any terrestrial animals so this is close to the extinction level that closed the Palaeozoic at around 250 Ma.

No single process can be regarded as the ‘culprit’. However the two events are bracketed by an 80-100 m fall in sea level due to the southern hemisphere glaciation. That may have given rise to changes in ocean oxygen content and in the reduction of sulfur to hydrogen sulfide. Also climate-related may have been changes in the vertical, thermohaline circulation of the oceans, falling temperatures encouraging sinking of surface water to abyssal depths providing more oxygen to support life deep in the water column. Sea-level fall would have reduced the extent of shallow seas too. Those consequences would explain the early demise of shallow water, free swimming animals. Reversal of these trends as glaciation waned may have returned stagnancy and anoxia to deep water, thereby affecting life at all depths. The authors suggest generalized ‘tipping points’ towards which several global processes contributed.

What Triggers a Mass Extinction?

Age calibration of Mesozoic sedimentary sequences: can it be improved?

Published on May 12, 2014 by zooks777Leave a comment

Relative age sequences in sequences of fossiliferous sediments are frequently intricate, thanks to animal groups that evolved quickly to leave easily identifiable fossil species. Yet converting that one-after-the-other dating to absolute values of past time has been difficult and generally debateable. Up to now it has relied on fossil-based correlation with localities where parts of the sequence of interest interleave with volcanic ashes or lavas that can be dated radiometrically. Igneous rocks can provide reference points in time, so that age estimates of intervening sedimentary layers emerge by assuming constant rates of sedimentation and of faunal speciation. However, neither rate can safely be assumed constant, and those of evolutionary processes are of great biological interest.

Setting Sun at Whitby Abbey — Sunset at St Hilda’s Abbey, Whitby NE England; fabled haunt of Count Dracula (credit: epicnom)

If only we could date the fossils a wealth of information would be accessible. In the case of organisms that apparently evolved quickly, such as the ammonites of the Mesozoic, time resolution might be extremely fine. Isotopic analysis methods have become sufficiently precise to exploit the radioactive decay of uranium isotopes, for instance, at the very low concentrations found in sedimentary minerals such as calcium carbonate. So this prospect of direct calibration might seem imminent. Geochemists and palaeontologists at Royal Holloway University of London, Leicester University and the British Geological Survey have used the U-Pb method to date Jurassic ammonites (Li, Q. et al. 2014. U–Pb dating of cements in Mesozoic ammonites. Chemical Geology, v. 376, p. 76-83). The species they chose are members of the genus Hildoceras, familiar to junior collectors on the foreshore below the ruined Abbey of St Hilda at the small port of Whitby, in NE England. The abundance and coiled shape of Hildoceras was once cited as evidence for the eponymous founder of the Abbey ridding this choice locality of a plague of venomous serpents using the simple expedient of divine lithification.

The target uranium-containing mineral is the calcite formed on the walls of the ammonites’ flotation chambers either while they were alive or shortly after death. This early cement is found in all well-preserved ammonites. The Hildoceras genus is found in one of the many faunal Zones of the Toarcian Age of the Lower Jurassic; the bifrons Zone (after Hildoceras bifrons). After careful selection of bifrons Zone specimens, the earliest calcite cement to have formed in the chambers was found to yield dates of around 165 Ma with precisions as low as ±3.3 Ma. Another species from the Middle Jurassic Bajocian Age came out at 158.8±4.3 Ma. Unfortunately, these precise ages were between 10-20 Ma younger than the accepted ranges of 174-183 and 168-170 Ma for the Toarcian and Bajocian. The authors ascribe this disappointing discrepancy to the breakdown of the calcium carbonate (aragonite) forming the animals’ shells from which uranium migrated to contaminate the after-death calcite cement.

Impacts in the early Archaean

Published on April 28, 2014 by zooks777Leave a comment

From the days when advocates of impacts by extraterrestrial objects as explanations of geological features were widely regarded as ‘whizz-bang artistes’ a great many hats have probably been eaten, albeit in closely guarded privacy. In 1986, when beds of glassy spherules similar to those found in lunar soil and in the K-T boundary sequence were reported from early Archaean greenstone belts in Australia and South Africa, and deduced to have formed by an impact, the authors, Donald Lowe of Stanford University, USA and colleagues, were pounced on by those who thought they could plausibly explain the very odd rocks by unremarkable, Earthly processes. Subsequent work on their geochemistry overwhelmingly supported their formation by an impact of a large carbonaceous chondrite asteroid. And at one site, the Barberton Mountain Land greenstone belt in northeastern South Africa, there was evidence for at least three such impacts formed in a 20 Ma period. In hindsight, given the lunar bombardment history that peaked between 4 and 3.8 Ga, early Archaean rocks were a great deal more likely to contain materials formed by giant impacts than less antiquated ones.

Barberton greenstone belt, South Africa (credit: Barberton World Heritage Site)

Lowe has been steadily working on his original idea since then, his enthusiasm drawing in others. The latest focus is on evidence for other likely consequences in the Archaean record of the vast power unleashed by incoming asteroids travelling at speeds around 15 km s^-1 (Sleep, N.H. & Lowe, D.R. 2014. Physics of crustal fracturing and chertdike formation triggered by asteroid impact, ~3.26 Ga, Barbertongreenstone belt, South Africa. Geochemistry, Geophysics, Geosystems, v. 15, doi:10.1002/2014GC005229). The damage at Barberton not only produced spherule beds but opened fractures on the shallow sea bed into which liquefied sediments, including some spherules, were injected. These swarms of up to 10 m wide cherty dykes extend up to 100 m below what was then the sea floor strewn with impact spherules, and contain evidence of successive pulses of sediment injection.

Sleep and Lowe explain these dyke swarms as fractures caused by seismicity associated with a major impact. Their complexity suggests extreme shaking for upwards of 100 seconds; far longer than that from large, tectonic earthquakes. The fact that cracks opened to accommodate the sedimentary dykes indicates extension of the affected crust, which the authors suggest resulted from gravitational sliding of the shocked surface sediments down a gentle slope. Possibly the sediments, including the direct products of impact, the spherules, were swept into the cracks by currents associated with tsunamis induced by the impact.

Interestingly, the spherules and dykes formed upon crust largely formed of mafic to ultramafic lavas, yet volcanism following close on the heels of the impact event was of felsic composition. Did the impact trigger a shift locally from oceanic magmatism to that characteristic of island arcs; that is, did it start a new subduction zone?

Oxygen, magnetic reversals and mass extinctions

Published on April 17, 2014 by zooks777Leave a comment

In April 2005 EPN reported evidence for a late Permian fall in atmospheric oxygen concentration to about 16% from its all-time high of 30% in the Carboniferous and earlier Permian.. This would have reduced the highest elevation on land where animals could live to about 2.7 km above sea level, compared with 4 to 5 today. Such an event would have placed a great deal of stress on terrestrial animal families. Moreover, it implies anoxic conditions in the oceans that would stress marine animals too. At the time, it seemed unlikely that declining oxygen was the main trigger for the end-Permian mass extinction as the decline would probably have been gradual; for instance by oxygen being locked into iron-3 compounds that give Permian and Triassic terrestrial sediments their unrelenting red coloration. By most accounts the greatest mass extinction of the Phanerozoic was extremely swift.

The possibility of extinctions being brought on by loss of oxygen from the air and ocean water has reappeared, though with suggestion of a very different means of achieving it (Wei, Y. and 10 others 2014. Oxygen escape from the Earth during geomagnetic reversals: Implications to mass extinction. Earth and Planetary Science Letters, v. 394, p. 94-98). The nub of the issue proposed by the Chinese-German authors is the dissociation and ionization by solar radiation of O₂ molecules into O⁺ ions. If exposed to the solar wind, such ions could literally be ‘blown away’ into interplanetary space; an explanation for the lack of much in the way of any atmosphere on Mars today. Mars is prone to such ionic ablation because it now has a very weak magnetic field and may have been in that state for 3 billion years or more. Earth’s much larger magnetic field diverts the solar wind by acting as an electromagnetic buffer against much loss of gases, except free hydrogen and to a certain extent helium. But the geomagnetic field undergoes reversals, and while they are in progress, the field drops to very low levels exposing Earth to loss of oxygen as well as to dangerous levels of ionising radiation through unprotected exposure of the surface to the solar wind.

Field reversals and, presumably, short periods of very low geomagnetic field associated with them, varied in their frequency through time. For the past 80 Ma the reversal rate has been between 1 and 5 per million years. For much of the Cretaceous Period there were hardly any during a magnetic quiet episode or superchron. Earlier Mesozoic times were magnetically hectic, when reversals rose to rates as high as 7 per million years in the early Jurassic. This was preceded by another superchron that spanned the Permian and Late Carboniferous. Earlier geomagnetic data are haphazardly distributed through the stratigraphic column, so little can be said in the context of reversal-oxygen-extinction connections.

Wei et al. focus on the end-Triassic mass extinction which does indeed coincide, albeit roughly, with low geochemically modelled atmospheric oxygen levels (~15%). This anoxic episode extended almost to the end of the Jurassic, although that was a period of rapid faunal diversification following the extinction event. Yet it does fall in the longest period of rapid reversals of the Mesozoic. However, this is the only clear reversal-oxygen-extinction correlation, the Cenozoic bucking the prediction. In order to present a seemingly persuasive case for their idea, the authors assign mass extinctions not to very rapid events – of the order of hundreds of thousand years at most – which is well supported by both fossils and stratigraphy, but to ‘blocks’ of time of the order of tens of million years.

My own view is that quite possibly magnetic reversals can have adverse consequences for life, but as a once widely considered causal mechanism for mass extinction they have faded from the scene; unlikely to be resurrected by this study. There are plenty of more plausible and better supported mechanisms, such as impacts and flood-basalt outpourings. Yet several large igneous provinces do coincide with the end of geomagnetic superchrons, although that correlation may well be due to the associated mantle plumes marking drastic changes around the core-mantle boundary. According to Wei et al., the supposed 6^th mass extinction of the Neogene has a link to the general speeding up of geomagnetic reversals through the Cenozoic: not much has happened to either oxygen levels or biodiversity during the Neogene, and the predicted 6^th mass extinction has more to do with human activity than the solar wind.

Remote sensing for fossils

Published on April 15, 2014 by zooks7772 Comments

With the growing diversity of data from those parts of the electromagnetic spectrum that pass freely though Earth’s atmosphere, mainly acquired from orbit, an increasing number of attributes of the surface can be mapped remotely. The initial impetus to launch remote sensing satellites in the 1960’s and early 70’s had two strands: to monitor weather conditions and assess vegetation cover with the early metsats, such as TIROS-1, and the first Landsat platform that exploited green plants’ propensity for absorbing visible and largely reflecting near-infrared (NIR) radiation. With the incorporation in the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instruments of wavelength regions in which minerals show spectral diversity, in the reflected short-wave infrared (SWIR) and emitted thermal infrared (TIR), remote sensing became a viable and useful tool for geologists. It figures strongly in lithological mapping and also in the detection of minerals related to various kinds of alteration associated with metal mineralisation and the migration of hydrocarbon-related fluids. The more wavebands with narrower coverage of radiation wavelengths, the more likely are the subtle differences in mineral spectra able to be detected and mapped. Yet, apart from one experimental system (Hyperion aboard NASA’s EO-1 orbital platform) our home planet is not as well served by such hyperspectral systems as is Mars, blessed by two which have fuelled the on-going search for past habitable zones on the Red Planet.

The May 2014 issue of Scientific American includes an article on remote sensing that follows what to many might seem an odd direction: how to increase the chance of finding rich fossil deposits (Anemone, R.L. & Emerson, C.W. 2014. Fossil GPS. Scientific American, v. 310(5), p. 34-39). Apart from targeting a particular stratigraphic unit on a geological map, palaeontological collection has generally been a hit-or-miss affair depending on persistence and a keen eye, with quite a lot of luck. Once a productive locality turns up, such as the Cambrian Burgess shale, the dinosaur-rich Cretaceous sandstone of the Red Deer River badlands of southern Alberta in Canada and the hominin sites of Ethiopia’s Afar Depression, palaeontologists often look no further until its potential is exhausted. Robert Anemone and Charles Emerson felt, as may palaeobiologists do, that one fossil ‘hotspot’ is simply not enough, yet balked at the physical effort, time and frustration needed to find more by trekking through their area of interest, the vast Tertiary sedimentary basins of Wyoming, USA. They decided to try an easier tack: using the few known fossil localities as digital ‘training areas’ for a software interrogation of Landsat Enhanced Thematic Mapper data in the hope that fossiliferous spots might be subtly different in their optical properties from those that were barren.

The teeth and bones of early Eocene mammals that had drawn them to Wyoming turn up in sandstone beds of the basins. They are pretty distinctive elements of landscape, forming ridges of outcrop because of their relative resistance to erosion, yet for that very reason present a huge selection of possibilities. Being simple mineralogically they also presented a seemingly daunting uniformity. Anemone and Emerson decided on a purely statistical approach using the six visible, NIR and SWIR bands sensed by Landsat ETM, rather than a spectrally oriented strategy using more sophisticated ASTER data with 14 spectral bands. Their chosen algorithm was that based on an artificial neural network that the fossil rich sandstones would train to recognise patterns present in ETM data recorded over them. This purely empirical approach seems to have worked. Of 31 sites suggested by the algorithm 25 yielded abundant vertebrate fossils. Applied to another of Wyoming’s Tertiary basins it also ‘found’ the three most productive known mammal sites there. So, what is it about the fossil-rich sandstones that sets them apart from those that are more likely to be barren? The authors do not offer an explanation. Perhaps it has something to do with reducing conditions that would help preserve organic material better than would sandstones deposited in an oxidising environment. Iron minerals and thereby colour might be a key factor, oxidised sandstones are generally stained red to orange by Fe-3 oxides and hydroxides, whereas reduced sandstone facies may be grey because of iron in the form of sulfides

Human, artificial intelligence join forces to pinpoint fossil locations

Update on giant fossil squirrel

Published on March 31, 2014 by zooks777Leave a comment

Eleven years on from his announcement in March 2003 of a giant member of the Family Sciuridae (squirrels) found in a lateritic lagerstätte in the Western Ghats of Karnataka State in India (see http://geocities.yahoo.com/pusiffli/squirrels.html – note: this site may no longer be extant) Professor Pandit U. Siffli of the emeritus faculty at the Sringeri Institute of Palaeontology has sent me further news of his investigations. The clay-filled pocket within the mottled zone has proved astonishingly fruitful now that Pandit Unmer has more free time following his retirement. He and his recently graduated colleague, Dr G.B. Harm, have unearthed several more exquisite specimens of Titanosciurus sringeriensis – long-standing readers will recall that the body cavity of the child-sized type specimen of T. sringeriensis contained bones of primitive hamsters, that no doubt the squirrel had consumed, confirming Siffli’s speculation that the creature was the only known member of the Sciuridae that was an obligate carnivore. This view stemmed originally from its formidable dentition.

Confirmation of this astounding revelation comes from two new lines of evidence discovered by Harm – the principle excavator since Siffli became encumbered by what he has described to me as his ‘blessed game leg’. In his letter he says, ‘young Grivas Bodili has informed me in a mood of solemn gaiety that there are burrows in the lagerstätte which contain complete skeletons of hamsters in a cowering posture. There are also abundant coprolites associated with one of the more corpulent specimens of T. sringeriensis that are a rich source of tiny hamster bones and one example of a partly digested avian flight feather’. The pair now have a paper in press (Harm, G.B. & Siffli, P.U. in press 2014. A large predatory sciurid from the Kudremukh laterites, Karnataka, India: evidence from a well-preserved rodent warren. Earth and Sanitary Appliance Letters, doi:11.3319/esal55164).
It seems likely that the early squirrels and hamsters borrowed into the laterite soon after intense tropical weathering has ceased due to climatic cooling associated with the onset of glaciation in Antarctica, probably in late-Eocene times. At that stage the upper laterite must have been soft enough for early mammals to dig into it. Subsequently the palaeosol became indurated as a result of regional desiccation, allowing exquisite preservation. Exact dating by the Ar-Ar method may soon be possible, given samples containing potassium-rich authigenic minerals. The search is now surely on for similar subterranean lagerstätten in the lateritic veneers covering vast tracts of the southern continents, whose formation probably came to a close at roughly the same time as did those of South India.

Artist's impression of the Sringeri carnivorous squirrel (credit: network54.com) — Artist’s impression of T. sringeriensis (credit: network54.com)

Prof Siffli tells me he would welcome communications from other sciurid and laterite researchers at pusiffli@gmail.com

The Higgs, gravity waves and now: dark matter and the dinosaurs

Published on March 27, 2014 by zooks777Leave a comment

The discovery around 50 years ago that in orbiting the centre of the Milky Way galaxy the solar system regularly wobbles to either side of its path. If the galaxy’s physical properties varied in a direction at right angles to the plane of the Milky Way then the Sun and its planets would experience that variation in a regular and predictable way (see Galactic controls http://earth-pages.co.uk/2011/12/15/galactic-controls/). Such oscillations might therefore show up as periodic changes in the geological record. There are loads of such cycles some not so regular, such as the accretion and disaggregation of supercontinents, and some involved in climatic change that have almost the predictability of a metronome.
One of these periodicities has thrilled geoscientists ever since it first began to emerge from improved dating of events in the geological record and more extensive knowledge of what it contained. Massive floods of basaltic magma blurt from the mantle every so often; more specifically approximately every 35 Ma. Intriguingly, there is a rough tally between the timing of such large igneous provinces and pulses in biological extinction. The wobbles in the solar system’s galactic passage are – wait for it – about every 35 Ma. A supposed link between LIPs, extinctions and galactic motions simply will not go away as a topic for speculation. Add to that some evidence that terrestrial impact cratering might have a 35 Ma period and you have ‘a story that will run and run’. The apparent periodicity of impacts, besides encouraging links with life and death and magmas, now seems to have spurred links with the dark side of cosmology.

It does indeed seem that the galactic magnetic field and dust concentrations vary across the plane of the Milky Way, but their affects during solar peregrinations have been raised long before now (Steiner, J. 1967. The sequence of geological events and the dynamics of the Milky Way Galaxy. Journal of the Geological Society of Australia, v. 14, p. 99–132.). The latest novelty concerns the possibility that galaxies might somehow collect the fabled but as yet undiscovered ‘dark matter’ in a flat disc within the galactic plane. Well, matter, ‘dark’ or not, should have mass, and mass must have a gravitational effect (thanks of course to the Higgs boson), even if it is hidden. Instead of some Nemesis or Death Star, as once was proposed to nudge comets from the outer reaches of the solar system, a gigantic dish of dark matter through which the Sun might pass on a regular basis might serve more plausibly (Randall, L. & Reece, MM. 2014. Dark matter as a trigger for periodic comet impacts. Physical Review Letters. arXiv:1403.0576 [astro-ph.GA]). Interestingly, Comments on the paper at the arXiv site read “Accepted by Physical Review Letters. 4 figures, no dinosaurs”

Solar System, in Perspective (credit: NASA Goddard SFC)

Did dark matter help kill off the dinosaurs?

Damp Earth: hydrous minerals in deep mantle rock

Published on March 14, 2014 by zooks777Leave a comment

A large number of water-oriented tropes have been applied to Earth for ‘artistic effect’, ranging from Waterworld to the Blue Planet, but from a geoscientific perspective H₂O in its many forms – liquid, solid, gas, supercritical fluid and chemically bound – has as much influence over the way the world works as do its internal heat production and transfer. Leaving aside surface processes, the presence of water has dramatic effects on the temperature at which rocks – felsic, mafic and ultramafic – begin to melt and deform and on the rates of important chemical reactions bound up with internal processes.

For a long while many geologists believed that the oceans were the product of water being transferred from the mantle by degassing through volcanoes so that the deep Earth has steadily been desiccated. But now it has become clear that such is the rate at which subduction can shift water back to the mantle that the entire volume of modern ocean water may have been cycled back and forth more than 3 times in Earth history (see Subduction and the water cycle). Besides, it is conceivable that accretion of cometary material up to about 3.8 Ga may have delivered the bulk of it.

An important aspect of the deep part of the water cycle concerns just how far into the mantle subduction can transport this most dominant volatile component of our planet. Ultra high-pressure experimental petrology has reached the stage when conditions at depths more than halfway to the core-mantle boundary (pressures up to 50 GPa) can be sustained using diamond anvils surrounding chemical mixtures that approximate mantle ultramafic materials. Previously, it was thought that serpentinite, the hydrous mineral most likely to be subducted, broke down into magnesium-rich, anhydrous silicates at around 1250 km down. This would prevent the deepest mantle from gaining any subducted water and retaining any that it had since the Earth formed. A team of Japanese geochemists has discovered a hint that hydrous silicates can, through a series of phase changes, achieve stability under the conditions of the deepest mantle (Nishi, M. 2014. Stability of hydrous silicate at high pressures and water transport to the deep lower mantle. Nature Geoscience, v. 7, p. 224-227). Their experiments yielded a yet unnamed mineral (phase H or MgSiH₂O₄) from approximate mantle composition that could remain stable in subducted slabs down to the core-mantle boundary. This development may help explain why the lowermost mantle is able to participate in plume activity through reduction in viscosity at those depths.

A parallel discovery concerns conditions at the base of the upper mantle; the 410 to 660 km mantle seismic transition zone. It comes from close study of a rare class of Brazilian diamonds that have been swiftly transported to the Earth’s surface from such depths, probably in kimberlite magma pipes, though their actual source rock has yet to be discovered. These ultra-deep diamonds prove to contain inclusions of mantle materials from the transition zone (Pearson, D.G. and 11 others 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, v. 507, p. 221-224). Australian geochemist Ted Ringwood pioneered the idea in the 1950s and 60s that the mantle transition zone might be due to the main mantle mineral olivine ((Mg,Fe)₂SiO₄) being transformed to structures commensurate with extremely high pressures, including one akin to that of spinel. Such a mineral was first observed in stony meteorites that had undergone shock metamorphism, and was dubbed ringwoodite in honour of its eponymous predictor. Yet ringwoodite had never been found in terrestrial rocks, until it turned up in the Brazilian diamonds thanks to Pearson and colleagues.

Partial cross-section of the Earth showing the location of ringwoodite in the mantle. Credit: Kathy Mather — Partial cross-section of the Earth showing the location of ringwoodite in the mantle Credit: Kathy Mather

Earlier experimental work to synthesise ultra-deep minerals discovered that ringwoodite may contain up to 2% water (actually OH groups) in its molecular lattice: an astonishing thing for material formed under such extreme conditions. The ringwoodite inclusions in diamond show infrared spectra that closely resemble its hydrous form. From this it may be inferred that the 401-660 km transition zone contains a vast amount of water; roughly the same as in all the oceans combined, though the find is yet to be confirmed in a wider selection of diamonds. One of the puzzles about diamondiferous kimberlites is that the magma must have been rich in water and carbon dioxide. That can now be explained by volatile-rich materials at the depths where diamonds form, But that does not necessarily implicate the whole transition zone: there may be pockets ripe for kimberlitic magma formation in a more widely water-poor mantle.

Keppler, H. 2014. Earth’s deep water reservoir. Nature , v. 507, p. 174-175

Evidence for North Atlantic current shut-down ~120 ka ago

Published on March 7, 2014 by zooks7771 Comment

Gulf stream map — Warming surface currents of the North Atlantic (credit: Wikipedia)

A stupendous amount of heat is shifted by ocean-surface currents, so they have a major influence over regional climates. But they are just part of ocean circulation systems, the other being the movement of water in the deep ocean basins. One driver of this world-encompassing system is water density; a function of its temperature and salinity. Cold saline water forming at the surface tends to sink, the volume that does being replaced by surface flow towards the site of sinking: effectively, cold downwellings ‘drag’ major surface currents along. This is especially striking in the North Atlantic where sinking cold brines are focused in narrow zones between Canada and Greenland and between Greenland and Iceland. From there the cold water flows southwards towards the South Atlantic at depths between 1 and 5 km. The northward compensating surface flow, largely from tropical seas of the Caribbean, is the Gulf Stream/North Atlantic Current whose warming influence on climate of western and north-western Europe extends into the Arctic Ocean.

Circulation in the Atlantic Ocean. the orange and red water masses are those of the Gulf stream and North Atlantic Deep Water (credit: Science, Figure 1 in Galaasen et al. 2014)

Since the discovery of this top-to-bottom ‘conveyor system’ of ocean circulation oceanographers and climatologists have suspected that sudden climate shifts around the North Atlantic, such as the millennial Dansgaard-Oeschger events recorded in the Greenland ice cores, may have been forced by circulation changes. The return to almost full glacial conditions during the Younger Dryas, while global climate was warming towards the interglacial conditions of the Holocene and present day, has been attributed to huge volumes of meltwater from the North American ice sheet entering the North Atlantic. By reducing surface salinity and density the deluge slowed or shut down the ‘conveyor’ for over a thousand years, thereby drastically cooling regional climate. Such drastic and potentially devastating events for humans in the region seem not to have occurred during the 11.5 thousand years since the end of the Younger Dryas. Yet their suspected cause, increased freshwater influx into the North Atlantic, continues with melting of the Greenland ice cap and reduction of the permanent sea-ice cover of the Arctic Ocean, particularly accelerated by global warming.

The Holocene interglacial has not yet come to completion, so checking what could happen in the North Atlantic region depends on studying previous interglacials, especially the previous one – the Eemian – from 130 to 114 ka. Unfortunately the high-resolution climate records from Greenland ice cores do not extend that far back. On top of that, more lengthy sea-floor sediment cores rarely have the time resolution to show detailed records, unless, that is, sediment accumulated quickly on the deep sea bed. One place that seems to have happened is just south of Greenland. Cores from there have been re-examined with an eye to charting the change in deep water temperature from unusually thick sediment sequences spanning the Eemian interglacial (Galaasen, E.V. and 7 others 2014. Rapid reductions in North Atlantic Deep Water during the peak of the last interglacial period. Science, v. 343, 1129-1132).

The approach taken by the consortium of scientiosts from Norway, the US, France and Britain was to analyse the carbon-isotope composition of the shells of foraminifers that lived in the very cold water of the ocean floor during the Eemian. The ratio of ¹³C to ¹²C, expressed as δ¹³C, fluctuates according to the isotopic composition of the water in which the forams lived. What show up in the 130-114 ka period are several major but short-lived falls in δ¹³C from the general level of what would then have been North Atlantic Deep Water (NADW). It seems that five times during the Eemian the flow of NADW slowed and perhaps stopped for periods of the order of a few hundred years. If so, then the warming influence of the Gulf Stream and North Atlantic Current would inevitably have waned through the same intervals. Confirmation of that comes from records of surface dwelling forams. This revelation should come as a warning: if purely natural shifts in currents and climate were able to perturb what had been assumed previously to be stable conditions during the last interglacial, what might anthropogenic warming do in the next century?

The launch of modern life on Earth

Published on March 1, 2014 by zooks777Leave a comment

To set against five brief episodes of mass extinction – some would count the present as being the beginning of a sixth – is one short period when animals with hard parts appeared for the first time roughly simultaneously across the Earth. Not only was the Cambrian Explosion sudden and pervasive but almost all phyla, the basic morphological divisions of multicellular life, adopted inner or outer skeletons that could survive as fossils. Such an all-pervading evolutionary step has never been repeated, although there have been many bursts in living diversity. Apart from the origin of life and the emergence of its sexual model, the eukaryotes, nothing could be more important in palaeobiology than the events across the Cambrian-Precambrian boundary.

This eminent event has been marked by most of the latest issue of the journal Gondwana Research (volume 25, Issue 3 for April 2014)in a 20-paper series called Beyond the Cambrian Explosion: from galaxy to genome (summarized by Isozaki, Y., Degan, S.., aruyama,, S.. & Santosh, M. 2014. Beyond the Cambrian Explosion: from galaxy to genome. Gondwana Research, v. 25, p. 881-883). Of course, these phenomenal events have been at issue since the 19^th century when the division of geological time began to be based on the appearance and vanishing of well preserved and easily distinguished fossils in the stratigraphic column. On this basis roughly the last ninth of the Earth’s history was split on palaeontological grounds into the 3 Eras, 11 Periods, and a great many of the briefer Epochs and Ages that constitute the Phanerozoic. Time that preceded the Cambrian explosion was for a long while somewhat murky mainly because of a lack of means of subdivision and the greater structural and metamorphic damage that had been done to the rocks that had accumulated over 4 billion years since the planet accreted. Detail emerged slowly by increasingly concerted study of the Precambrian, helped since the 1930s by the ability to assign numerical ages to rocks. Signs of life in sediments that had originally been termed the Azoic (Greek for ‘without life’) gradually turned up as far back as 3.5 Ga, but much attention focused on the 400 Ma immediately preceding the start of the Cambrian period once abundant trace fossils had been found in the Ediacaran Hills of South Australia that had been preceded by repeated worldwide glacial epochs. The Ediacaran and Cryogenian Periods (635-541 and 850-635 Ma respectively) of the Neoproterozoic figure prominently in 9 of the papers to investigate or review the ‘back story’ from which the crucial event in the history of life emerged. Six have a mainly Cambrian focus on newly discovered fossils, especially from a sedimentary sequence in southern China that preserves delicate fossils in great detail: the Chengjian Lagerstätte. Others cover geochemical evidence for changes in marine conditions from the Cryogenian to Cambrian and reviews of theories for what triggered the great faunal change.

Since the hard parts that allow fossils to linger are based on calcium-rich compounds, mainly carbonates and phosphates that bind the organic materials in bones and shells, it is important to check for some change in the Ca content of ocean water over the time covered by the discourse. In fact there are signs from Ca-isotopes in carbonates that this did change. A team of Japanese and Chinese geochemists drilled through an almost unbroken sequence of Ediacaran to Lower Cambrian sediments near the Three Gorges Dam across the Yangtse River and analysed for ⁴⁴Ca and ⁴²Ca (Sawaki, Y. et al. 2014. The anomalous Ca cycle in the Ediacaran ocean: Evidence from Ca isotopes preserved in carbonates in the Three Gorges area, South China. Gondwana Research, v. 25, p. 1070-1089) calibrated to time by U-Pb dating of volcanic ash layers in the sequence (Okada, Y. et al. 2014. New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng’an and Chengjiang areas, South China. Gondwana Research, v. 25, p. 1027-1044). They found that there were significant changes in the ratio between the two isotopes. The isotopic ratio underwent a rapid decrease, an equally abrupt increase then a decrease around the start of the Cambrian, which coincided with a major upward ‘spike’ and then a broad increase in the ⁸⁷Sr/⁸⁶Sr isotope ratio in the Lower Cambrian. The authors ascribe this to an increasing Ca ion concentration in sea water through the Ediacaran and a major perturbation just before the Cambrian Explosion, which happens to coincide with Sr-isotope evidence for a major influx of isotopically old material derived from erosion of the continental crust. As discussed in Origin of the arms race (May 2012) perhaps the appearance of animals’ hard parts did indeed result from initial secretions of calcium compounds outside cells to protect them from excess calcium’s toxic effects and were then commandeered for protective armour or offensive tools of predation.

"SNOWBALL EARTH" - 640 million years ago — Artists impression of a Snowball Earth event 640 Ma ago (credit: guano via Flickr)

Is there is a link between the Cambrian Explosion and the preceding Snowball Earth episodes of the Cryogenian with their associated roller coaster excursions in carbon isotopes? Xingliang Zhang and colleagues at Northwest University in Xian, China (Zhang, X. et al. 2014. Triggers for the Cambrian explosion: Hypotheses and problems. Gondwana Research, v. 25, p. 896-909) propose that fluctuating Cryogenian environmental conditions conspiring with massive nutrient influxes to the oceans and boosts in oxygenation of sea water through the Ediacaran set the scene for early Cambrian biological events. The nutrient boost may have been through increased transfer o f water from mantle to the surface linked to the start of subduction of wet lithosphere and expulsion of fluids from it as a result of the geotherm cooling through a threshold around 600 Ma (Maruyama, S. et al. 2014. Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion. Gondwana Research, v. 25, p. 910-944). Alternatively the nutrient flux may have arisen by increased erosion as a result of plume-driven uplift (Santosh, M. et al. 2014. The Cambrian Explosion: Plume-driven birth of the second ecosystem on Earth. Gondwana Research, v. 25, p. 945-965).

A bolder approach, reflected in the title of the Special Issue, seeks an interstellar trigger (Kataoka, R. et al. 2014. The Nebula Winter: The united view of the snowball Earth, mass extinctions, and explosive evolution in the late Neoproterozoic and Cambrian periods. Gondwana Research, v. 25, p. 1153-1163). This looks to encounters between the Solar System and dust clouds or supernova remnants as it orbited the galactic centre: a view that surfaces occasionally in several other contexts. Such chance events may have been climatically and biologically catastrophic: a sort of nebular winter, far more pervasive than the once postulated nuclear winter of a 3^rd World War. That is perhaps going a little too far beyond the constraints of evidence, for there should be isotopic and other geochemical signs that such an event took place. It also raises yet the issue that life on Earth is and always has been unique in the galaxy and perhaps the known universe due to a concatenation of diverse chance events, without structure in time or order, which pushed living processes to outcomes whose probabilities of repetition are infinitesimally small.

Massive trove of Canadian fossils gives near-unprecedented glimpse of Cambrian explosion

Traces of the most ancient Britons

Published on February 16, 2014 by zooks7772 Comments

Perhaps the most evocative traces of our ancestors are their footprints preserved in once soft muds or silts, none more so than the 3.6 Ma old hominin trackway at Laetoli in Tanzania, discovered by Mary Leakey and colleagues in 1978. Such records of living beings’ activities are by no means vanishingly rare. In 2003 footprints of Neanderthal children emerged in volcanic ash that had formed on the slopes of an Italian volcano. The fact that the tracks zig-zagged and included handprints seemed to suggest that the children were playing on a tempting slope of soft sediment, much as they do today (see The first volcanologists? and Walking with the ancestors). The muddy sediments of the Severn and Mersey estuaries in England yield younger footprints of anatomically modern humans of all sizes every time tidal flows rip up the sedimentary layers. Now similar examples have been unearthed from 1.0 to 0.78 Ma old Pleistocene interglacial sediments at a coastal site in Norfolk, England, in which stone tools had been found in 2010 .

Coastal exposure of Pleistocene laminated sediments at Happisburgh (credit: Ashton et a. 2014 PLOS1) — Coastal exposure of Pleistocene laminated sediments at Happisburgh; the top surface exposes the hominin trackway (credit: Ashton et al. 2014 PLOS1)

A team funded by the Pathways to Ancient Britain Project, involving scientists from a consortium of British museums and universities, rapidly conserved a 12 m² surface of laminated sediments fortuitously exposed on the foreshore at Happisburgh (pronounced ‘Haze-burra’) by winter storms. It was covered in footprints (Ashton, N. and 11 others 2014. Hominin Footprints from Early Pleistocene Deposits at Happisburgh, UK. PLoS ONE v. 9: e88329. doi:10.1371/journal.pone.0088329). Analysis of the prints suggested a band of individuals who had tramped southwards across mudflats at the edge of an estuary. They were possibly members of an early human species, known as Homo antecessor, skeletal remains of whom are known from northern Spain. The Happisburgh individuals were of mixed size, probably including adults and juveniles: three footprint sets suggested 1.6 to 1.73 m stature; nine less than 1.4 m.

View from above of the well-trodden trackway at Happisburgh, with an enlarged example of one of the foot prints (credit: Ashton et al 2014 PLoS1) — View from above of the well-trodden trackway at Happisburgh, with an enlarged example of one of the foot prints (credit: Ashton et al. 2014 PLoS1)

From pollen samples, East Anglia during the interglacial had a cool climate with pine, spruce, birch and alder tree cover with patches of heath and grassland. That it had attracted early humans to travel so far north from the Mediterranean climate where skeletal remains are found, suggests that food resources were at least adequate. It is hard to imagine the band having been seasonal visitors from warmer climes further south. They must have been hardy, and from the stone tools we know they were well equipped and capable of killing sizeable prey animals, bones of which marked by clear cut marks being good evidence for their hunting skills.

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