Improved dating sheds light on Neanderthals’ demise

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 stone tools

Châtelperronian stone tools (credit: Wikipedia)

Mousterian tool from France

Mousterian blade tool from France (credit: Wikipedia)

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?

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.

Serious groundwater depletion in western US

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).

Global Gravity Anomaly Animation over land fro...

Global Gravity Anomaly Animation over land from GRACE (credit: Wikipedia)

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

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!

The Martian meteorite ALH84001 shows microscop...

The Martian meteorite ALH84001 shows microscopic features once suggested to have been created by life. (credit: Wikipedia)

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

 

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).

English: Stromatolites in the Hoyt Limestone (...

Limestone formed from blue-green bacteria biofilms or stromatolites (credit: Wikipedia)

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.

 

 

 

Planet Mercury and giant collisions

Full-color image of from first MESSENGER flyby

Mercury’s sun-lit side from first MESSENGER flyby (credit: Wikipedia)

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

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…

Fieldwork and geological education

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

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

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)

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?