The post-glacial North Atlantic

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Microfossils from marine sediments - planktoni...
Neogloboquadrina pachyderma. Image via Wikipedia

One of the main controls over Earth’s climate is the way that water in the North Atlantic convects. At present it is behaving like a liquid conveyor belt that links the tropics and well to the north of the Arctic Circle. Warm salty water that reaches boreal latitudes cools and also becomes saltier as sea ice freezes out fresh water. It therefore gets denser and sinks to the ocean floor in the Denmark Strait between Iceland and Greenland, and between Iceland and the Faeroe Isles. This downwelling drags surface water polewards from the tropics to replenish the system, thereby creating the Gulf Stream and North Atlantic Drift that warms coastal north-western Europe as far as the northern tip of Scandinavia. It was not always this way; evidence has accumulated to indicate that the North Atlantic ‘conveyor’ shut down periodically during the run-up to the last glacial period and in the climatic hiccup of the Younger Dryas (12.6-11.5 ka). The best supported hypothesis as to why it may do that is through massive influx of freshwater to lower the density of surface water in the northernmost North Atlantic. The progressive summer retreat of sea-ice in the Arctic Ocean and the likelihood of ice-free summers there in the near future raises fears that such a shut-down may occur once again, because of freshening of surface water by ice meltwater, with devastating climatic results for Europe at least. The circulation also transports carbon dioxide dissolved in cold descending surface water to abyssal depths helping buffer its atmospheric concentration: a shut-down would allow greenhouse gas emitted by society to build up in the air.

One means of investigating the mechanisms that underlie ‘on’ and ‘off’ switching in ocean convection is to use sea-floor sediment data from the18 ka long period since the last glacial maximum (Thornalley. D.J.R. et al. 2011. The deglacial evolution of North Atlantic convection. Science, v. 331, p. 202-205). The British-US consortium used oxygen isotope data from the planktonic (near-surface) foraminifera Neogloboquadrina pachyderma preserved in sea-floor sediment cores from south of Iceland, close to where surface water descends today, to assess sea-surface temperature variations. Because of the continual exchange of CO2 between surface water and the atmosphere, the ocean surface contains the same radioactive 14C content in carbon as does the atmosphere, at whose top the isotope is produced. When water descends this connection is cut and the proportion of 14C in it decays so that it is theoretically possible to work out the time at which deep water began to descend – its ‘ventilation age’. In practice this is done by measuring the ‘age’ of  carbon preserved in planktonic and benthonic (deep- and bottom-water) foram shells, the planktonic age being the actual age used to assess the age difference between deep and surface waters. In the case of a complete shut-down of the convection the ventilation age should be high and constant; exactly the case during the last glacial maximum (19-22 ka) and most of Heinrich Stadial 1 (16.5-19 ka). When the ‘conveyor’ is functioning the ventilation age should be low, in fact from about 16-11.5 ka the ventilation age fluctuates to show 3 major and 2 lesser low to high episodes during the Bølling-Allerød and Younger Dryas, suggesting that indeed there was repeated turning-on and turning-off of the conveyor, probably triggered by pulses of fresh water into the northern North Atlantic from glacial melting. The resolution of these data is of the order of 350 years, so there may be finer detail of great interest as regards future climate.

See also: Sarnthein, M. Northern meltwater pulses, CO2, and changes in Atlantic convection. Science, v. 331, p. 156-158.

Explosion of the exoplanets

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The size of Kepler's first five planet discoveries
First five confirmed planets discovered by Kepler mission Image via Wikipedia

There is little doubt that it can be done, but what is so compelling about the search for worlds that orbit other stars?

By the end of the 21st century’s first decade 500 such exoplanets had been discovered, ranging from super gas giants almost 10 thousand times the mass of the Earth to a few that are comparable in size to our home world. At present the records of size and orbital radius are biased by the relative ease

of detecting large bodies over that of Earth-sized objects. Another bias is the greater chance of observing the change in luminosity of a star as one of its planets passes between us and the star – a transit – if the planet’s orbital period is short, being close to the star. The majority of known exoplanets are less than about 8 times the Earth’s orbital radius (1 astronomical unit or AU) away from their star, although some truly huge bodies have been spotted that are up to a thousand times more remote from a star than ours is.

Labeled illustration of the Kepler spacecraft
Kepler spacecraft. Image via Wikipedia

The rate of discovery is set to burgeon now that data from NASA’s Kepler exoplanet-finding mission, launched in 2009, is producing data (Reich, E.S. 2011. Beyond the stars. Nature, v. 470, p. 24-26). The 0.95 m Kepler space telescope gazes continually at a patch of sky containing 150 thousand Milky Way stars, many of which are like the Sun. It uses the transit method, and because it is fixed on only one star field it can potentially pick up the variation of stars’ luminosity due to transiting planets that are about the size of the Earth and larger. The computations are, unsurprisingly, massive and any dips in the light curves for pixels that represent individual stars have to be confirmed by other methods or by Kepler detecting repeats of the fluctuation. One drawback is that the transit method only provides the radius of a planet and its orbital period. Mass is needed to work out an exoplanet’s density and that requires another method using the red-shift of a star due to the gravitational effect of a planet causing it to wobble; a technique fraught with difficulties and best applied to dwarf red stars. The density is important for discriminating silicate-rich exoplanets from gas-liquid bodies. The main aim of planet finders is to find those around the same size and mass as the Earth that orbit a star at a distance where they would be warm enough for liquid water to exist but not so warm that it existed only as a vapour: in the so-called ‘Goldilocks zone’.

There was an initial flurry of excitement in the press in 2010 when a scientist on the Kepler programme was misinterpreted while giving a conference presentation that resulted in headlines that hundreds of distant Earths had already been discovered in the experiment’s first year. So far Kepler has only 15 confirmed planets to its credit that range from 800 times to twice the Earth’s radius all with orbits less than that of the Earth around the Sun. Nonetheless, a couple orbit within their star’s Goldilocks zone. So there is a way to go before real excitement is justified, but Kepler data will undoubtedly be used to seek funds for other planet-dedicated programmes that can fill in the gaps and perhaps confirm the existence of distant worlds that bear some resemblance to ours. Out of Kepler’s 1235 candidate detections since launch, 68 would be Earth-sized if confirmed (Shiga, D. 2011. What’s an alien solar system like? New Scientist, v. 209 (26 February 2011 issue) p. 6-7). For such remote detection to suggest an exoplanet on which life has evolved demands that atmospheric composition can be deduced from spectra of electromagnetic radiation from the body itself: a far more difficult undertaking that finding and weighing. Free atmospheric oxygen, so far unique to the Earth, is an obvious target. However, its absence would not rule out life that did not use photosynthesis to split water molecules in making living matter, and there are plenty of life forms here that do that.

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Comet water in lunar rocks

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Halebopp031197
Comet Hale-Bopp.Image via Wikipedia

There are two main hypotheses about the origin of Earth’s oceans: that they are filled with water that was locked in the meteoritic matter that initially accreted to form the Earth, or ocean water was delivered by massive comet bombardment in the first half billion years of the Earth’s history. It hasn’t yet been possible to decide whether one of these, or both were  involved, but the Moon might give a clue, even though until very recently it was regarded as being bone dry (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). The ratio between deuterium and hydrogen (D/H) gives a clue to the origin of water, in which both hydrogen isotopes occur (Greenwood, J.P. et al. 2011. Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, v. 4, p. 79-82). Using an ion microprobe to analyse the water in apatite, its dominant host in lunar rock samples, the authors were able to report two things. First, there is water in magmatic rocks of all ages found on the Moon: the earliest anorthosites of the lunar highlands and the younger basalts that fill the dark maria. Secondly, the water has D/H ratios significantly outside the terrestrial range. In detail, apatites with the greatest enrichment of deuterium relative to hydrogen are found in the maria basalts which fill enormous basins thought to have formed around 4 Ga ago as a result of cometary impacts. The D/H ratios are lower in apatites from the lunar highland anorthosites, which probably formed through flotation of low density calcium-rich feldspar as the Moon’s initially molten mantle crystallized not long after its formation through the impact of a small planet with the Earth. The highland D/H values are not wildly dissimilar from those found on Earth, yet those found in the mare basalts match the admittedly less well-constrained levels determined from comets hale-Bopp, Hyakutake and Halley. Because the Earth’s mass would ensure that it would corral 15 times more incoming extraterrestrial matter than would the Moon, the argument goes that if the Moon captured cometary water then Earth did so in trumps. The difference is that the Earths greater gravitational pull and thick atmosphere allowed it to retain gaseous and liquid water, while the Moon’s lower escape velocity let them leak away so that only mineralogically bound water could be retained.

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Top story and most flawed hype of 2010

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Reconstruction of Neandertaler at Neanderthal ...
Reconstruction of a middle-aged Neanderthal man. Image via Wikipedia

EPN might seem to include a disproportionate number of items on hominin evolution, including several on genetic evidence. An outcome of the Earth System’s 4.5 billion-year evolution increasingly depending on physical resources, we lie at the focus of our own curiosity studying the past primarily for ourselves. That is why the discovery from the partial genome of Neanderthal remains that all humans outside those who live in Africa carry in our DNA the ‘fruits’ of intimate relations with Neanderthals is surely the most explosive development of the 21st century so far (see Yes, it seems that they did…in May 2010 issue of EPN). It is deepened by the publication in late 2010 (Reich, D and 27 others 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, v. 468, p. 1053-1060) of genetic findings from remains of a third distinct hominin group that inhabited central Siberia 30 to 50 ka ago (see Other rich hominin pickings in the May 2010 issue of EPN). [Thanks go to Dr Bill Deller, legal historian, for alerting me to this.] The DNA from a tooth and a finger bone show that the individual female was genetically neither a fully modern human nor a Neanderthal in a statistical sense, but parts of the sequence, as with the Neanderthal genome, pop up in the genomes of living people. The ‘Denisovan’ signature – the authors do not assign the female to a new species – contributes 4 to 6 % of the genomes of present-day inhabitants of Papua-New Guinea and other Melanesian people of the Pacific north of Australia, but appears in no others. Since Melanesians carry some Neanderthal genetic material the new finding can be interpreted as the result of similar interfertile mating between the ‘Denisovans’ and a limited group of early fully human travellers who crossed central Asia and eventually moved through Indonesia to cross the West Pacific to Papua-New Guinea and Melanesia about 45 ka ago. For up to a twentieth of the genetic outcome of such liaisons to survive to the present suggests no idle dalliance, but proportionately common relationships.

Solo tiene 400.000 años. Un chaval, vamos.

Reconstruction of Homo heidelbergensis, perhaps similar to a Denisovan. Image via Wikipedia

Denisovans shared a common ancestor with Neanderthals and ourselves, but seem to have followed a separate evolutionary path. Analysis of their DNA suggests that they diverged from Neanderthals around 640 ka  and from modern Africans around 800 ka. Although these ‘molecular clock’ dates show considerable uncertainty, they extend back to a period when fossil evidence suggests the presence in Europe and Africa of Homo heidelbergensis and H. erectus respectively. The molar tooth has a morphology similar to African H. erectus and to even earlier hominins, but distinct from the teeth of Neanderthals and fully modern humans. Could the ‘Denisovans’ represent a distinct wave of emigrants from Africa? Some hominin fossils from China are dissimilar to Neanderthals and Asian H. erectus and efforts will certainly be made to establish their genetic make-up. For the moment, these findings deny any simple linear explanation for the ‘Out-of-Africa’ movement of people. Equally important, and the reason why the researchers refuse to assign the ‘Denisovans’ to a new species, is that interfertility is generally accepted as the sign of mating between members of the same species. To some extent this harks back to the ideas of the ecologist Jonathan Kingdon (Kingdon, J. 1993, Self-made Man and His Undoing. Simon & Schuster: London) that humans are a line that did not speciate over the last couple of million years, but show morphological differences that arose within the growing protection from selection pressures conferred by the use and development of tools. Kingdon’s parsimonious approach to human evolution found little favour with palaeoanthropologists, perhaps because of the kudos associated with finding and naming new species.

See also: Callaway, E. 2010. Fossil genome reveals ancestral link. Nature, v. 468, p. 1012; Bustamante, C.D. & Henn, B.M. 2010. Shadows of early migrations. Nature, v. 468, p. 1044-1045.

Perhaps it is a generational thing, stemming from popular science fiction and scientists’ speculation in the 1970s and 80s, that has encouraged the growth of exo-, xeno- and astrobiology as subdisciplines. There is a certain sadness in that all practitioners can do at present is examine the organic diversity offered by our home world and speculate about alien life forms based on that terrestrial evidence. The Earth offers plenty of scope for studying the biologically odd and awesome, especially among prokaryotes, as there are extremophiles of all kinds: the hot, the cold and the deep biospheres. But all are based on the nucleic acids shared by all life on Earth; traces of familiar amino acids occur far and wide in the cosmos, but none whatsoever of anything more complicated that could source self-replication and evolution. So it was in a mood of solemn gaiety that EPN greeted the hint of truly alien life forms among us by NASA press officers in November. It turned out to presage a paper concerning bacteria peculiar to Mono Lake in California (Wolfe-Simon, F. And 11 others 2010. A bacterium that can grow by using arsenic instead of phosphorus. Science Express, DOI:10.1126/science.1197258). The paper hinted at arsenic being used to substitute for phosphorus in the structure of nucleic acids in the bacterium when it lived in low-phosphate environments. The paper’s substance was culturing the bacterium in vitro in increasingly P-deficient water that also contained arsenic. If replicable the notion of arsenic-DNA would seem to be pretty startling, but the paper faced a storm of adverse comment.

Mono Lake Tufa
Biomineralised columns at Mono Lake, California. Image via Wikipedia

A crucial feature of the DNA molecule is the bond between the sugar of one nucleotide and the phosphate group of another. As any geochemist knows, it is possible for elements to substitute for one another if they have similar atomic properties. Arsenic, being adjacent to phosphorus in the same group of the periodic table, is a potential substitute –  arsenate for phosphate, although the former is far less stable than phosphate. Wolfe-Simon’s team is not claiming the peculiar bacterium as a candidate for alien life forms, but that is the spin widely being put on their work. All they suggest is that some bacteria can survive high-As low-P conditions and may be found in chemically highly toxic environments elsewhere. Since the cosmic abundance of arsenic is about three orders of magnitude less than that of phosphorus it is unlikely that alien genetic material somehow uses arsenic in its architecture. Besides, why should DNA be the sole basis for self-replication, the essence of truly living beings?

Added 14 January 2011: Science gave Felisa Wolfe-Simon the opportunity to reply to critics (Pennisi, E. 2010. Discoverer asks for time, patience over arsenic bacteria controversy. Science, v. 330, p. 1734-1735). Personally, I sympathise with the beleaguered team: on the launch of Stepping Stones in 1999 I was pestered continuously by journalists from both low- and highbrow newspapers. In that case I had made a joke that perhaps the human coughing reflex had stemmed from ancestral reptiles that survived the Permian-Triassic mass extinction and the emissions of the Siberian Traps: the journalists actually believed it

See also: Pennisi, E. 2010. What Poison? Bacterium uses arsenic to build DNA and other molecules. Science, v. 330, p. 1302.

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Surprise in store for coal burning

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A coal mine in Wyoming, United States. The Uni...
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Of the fossil fuels coal has long been assumed to be the most plentiful, even the most pessimistic forecasters having acknowledged a global lifetime of centuries for known reserves. The determination of the emerging giant economies of China and India and of the USA to fuel themselves through coal-burning seems inevitable if highly risky for the climate. But that depends on coal remaining the cheapest fuel, largely because of the sheer abundance of supplies. A recent commentary on coal (Heinberg, R. & Fridley, D. 2010. The end of cheap coal. Nature, v. 268, p. 367-369) suggests that there is a growing tendency for reserve estimates to decrease as geologists factor in practical restrictions – place, depth, seam thickness and quality – on feasibility under current mining conditions, instead of just looking at known masses of coal. Astonishingly, the end-19th century estimate of five thousand years of US coal supplies dropped to about 400 years by 1974 and is currently judged to be 240 years. China and India look likely to have less than 60 years-worth left. On top of that, the widely publicized turn to carbon capture and storage (CCS)for ‘clean-coal’ future supplies will inevitably drive-up prices of coal-fired energy. The two main factors in this remarkable transformation of ‘King Coal’ are fundamental economic forces in capitalism and the increasing refusal of miners to accept dangerous working conditions. The second is especially the case for China, where most coal is deep-mined; in the late 1990s it saw a drive to close down unsafe mines that caused production to fall, although it has greatly accelerated this century – further driving down coal’s lifetime there. It seems from this analysis that any realistic hope for a CCS-based coal economy, especially in China and India, depends on declining safety and environmental standards in their largely underground mines, which in turn depends on the highly unlikely willingness of their workforces to accept worse conditions.

Self plagiarism

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Some scientists have enormous publication records, a notorious case being one who claimed personal discovery of the HIV virus. During the 1980s, this person managed to figure as an author in up to 90 papers a year, despite mainly travelling back and forth to conferences. If the same name appears again and again in publications – it makes little difference where it figures in the list of authors – it is that name that is remembered as an “authority”. In some cases such an accolade is deserved, in others it is engineered by a variety of devices: the same data can be used over and over (most blatantly if those data are ‘engineered in the first place); a place in an authors’ list can result from being a ‘guest’, in the manner of a faded star, ‘down on their luck’, who pops up with a one-line cameo in a film (I rule out Alfred Hithcock’s appearance as a bystander in every film that he made); by nicking the ideas and words of others; through the device of self plagiarism. The last is an especially cunning ploy, as it also saves time crafting text. The italicized sentence above is an example self-plagiarised from the March 2002 issue of EPN (Credit where credit is due?), but as a blogger I can do that with a clear conscience; Earth Pages News is highly unlikely to get me into the ‘Professoriat’, especially my inability to resist occasional items such as this! Having provided the original source reference, I am safe from universal condemnation.

Potentially the game is up for plagiarists and pot-boilers in peer-reviewed journals through scanning software (e.g. Turnitin) that checks text against web-available journals. Self-plagiarism may well be an oxymoron, but it serves as CV fodder as well as creating academic redundancy. It hit the news (Reich, E.S. 2010. Self-plagiarism case prompts calls for agencies to tighten rules. Nature, v. 468, p. 745) because of a case in Canada where an author’s peer-reviewed portfolio was found to contain 20 instances. No academic censure ensued, but three of his papers were retracted. Using the Déjà Vu facility to check biomedical literature has resulted in 79 thousand cases of duplicated wording in abstracts and titles alone, and the eventual retraction of almost 100 articles. Seemingly, journal editors are allowing repeated use of text in the ‘methods’ sections of papers, so a geochemist minor co-author, who gets a ride for a small contribution based on use of a particular piece of equipment might be safe in that regard, there being safety in numbers. Yet as the use of anti-plagiarism software spreads into the wider on-line literature its original targets, undergraduate and graduate students, may decide that the biter ought to be bit and turn the cyber searchlight on their ‘betters’…

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Linking oxygen levels to great animal radiations

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Dunkleosteus
Dunkleosteus (10 m long) of the Late Devonian. Image by Travis S. via Flickr

Probably the greatest ecological truism is that without oxygen there would be no life forms on Earth above the level of a restricted number of prokaryotes. Since around 2.4 Ga, when free atmospheric oxygen first appeared, levels have risen to the present 21% – it was probably as high as ~30% in the Carboniferous and Cretaceous Periods. Charting the rise has been difficult and the history of oxygen is written with a very broad brush. If there had been sudden increases in the availability of oxygen in the atmosphere and oceans there ought to have been a bursts of evolutionary radiation and diversity, but often oxygen-related causality for events such as the Cambrian Explosion have been speculative, as have cases for the inverse, declines due to downturns in oxygen levels (see Oxygen depletion before P-T extinction in the November 2003 issue of EPN). Recently a proxy for the redox chemistry of the global ocean, and therefore for relative changes in atmospheric oxygen, has been developed. It is based on the abundance and isotopic composition of the element molybdenum (Mo) in sedimentary rocks: higher 98Mo relative to 95Mo (the d98Mo value) signifies higher oxygen levels. Its recent use in relation to evolutionary radiations (Dahl, T.W. et al. 2010. Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish. Proceedings of the National Academy of the US, v. 107, p. 17911-17915) has produced interesting results. The US-Swedish-Danish-British team analysed the Mo in euxinic (reduced) marine black shales, which concentrate the element from seawater, in the Proterozoic and Phanerozoic Eons. Increases in δ98Mo occur at the time of the Cambrian Explosion, as expected, and also during the Devonian. The latter correlates with increasing diversification of large fishes and among early terrestrial plants, and may have been the greatest leap in the bioavailability of oxygen in Earth’s history, stemming from the ‘greening’ of the land. So far Mo-isotope data have not been obtained from Carboniferous, Permian or Cretaceous back shales, but the ratio of Mo to organic carbon content in black shales of those ages  – a less constrained proxy –  does confirm what has been suspected: highs (greater than present levels) in the Carboniferous and Cretaceous and lows during the Permian and Triassic. However, any hopes that the approach can be calibrated to actual oxygen levels seem likely to be optimistic as the controls over dissolved molybdenum supply to the oceans and its transfer to sediments are extremely complex.

Added 14 January 2011. Some of the team feature in a related article (Gill, B.C. et al. 2011. Geochemical evidence for widespread euxinia in the Later Cambrian ocean. Nature, v. 469, p. 80-83) that ticks all the geochemical boxes for the evolutionary effects of depleted oxygen; i.e. extinctions. They use new measurements of sulfur isotopes in conjunction with published carbon-isotope  and other geochemical data from a wide range of Late Cambrian sediment types and environments in six well-known sections of that age. Spikes in the relative abundance of 34S match those in 13C along with a decrease in Mo in one section (see above), suggesting temporary increases in carbon and sulfide burial during periods of oxygen deficiency in the Late Cambrian ocean. Massive sequestration of organic carbon may have led to the extremely cold Late Cambrian climate, as described in A chilly Late Cambrian (this issue). Combined with changes in redox conditions associated with ocean anoxia this would have especially stressed animals, even on continental shelves had oxygen depleted water risen from the depths where sulfur and carbon burial were going on.

See also: Shields-Zhou, G. 2011. Toxic Cambrian oceans. Nature, v. 469, p. 42-43.

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Assorted developments in palaeoanthropology

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The notion that Neanderthals were dim and brutish compared with us continues to be undermined, but although their brain capacity was as large and in some cases distinctly larger than that of fully modern humans, its shape was significantly different; longer towards the rear than our more rounded brain. Studies of a Neanderthal baby and three children reveal that just after birth the Neanderthal brain was virtually identical to that of fully modern babies, i.e. elongate, but remains so in childhood through to maturity, whereas modern children’s brains develop towards the roundness of adults. Consequently, there must have been differences in the parts of the brain from which aspects of behaviour stem: Neanderthals almost certainly behaved differently from us both in childhood and as adults (Harvati, K. et al. 2010. Evolution of middle-late Pleistocene human cranio-facial form: a 3-D approach. Journal of Human Evolution, v. 59, p. 445-464. See also: Gibbons, A. 2010. Neandertal brain growth shows a head start for moderns. Science, v. 330, p. 900-901).

The now widely accepted hypothesis that modern humans did not begin to leave Africa to colonise Eurasia until about 60 ka may be under threat from reports of what seem to be fully modern human remains in China dated to ~105 ka (Liu, W. et al. 2010. Human remains from Zhirendong, South China, and modern human emergence in East Asia. Proceedings of the National Academy of the US, v. 107, p. 19201-19206). The dating appears to be sound, being based on the uranium-series (230Th) method applied to flowstone that rests on top of the sedimentary layer containing the remains in Zhirendong cave. The precipitated calcite layer completely sealed in the fossils as soon as it began to form about 105 ka ago, indicating that they are older still. Whether or not the remains are of fully modern humans is uncertain. Had they been found in Europe there would be little doubt about their affinities, the only other contemporary hominins being the Neanderthals. The problem in South China is that it was inhabited by H. erectus and the finds may be from ‘late’ members of that archaic species which arrived more than a million years earlier than fully modern humans. Judging by the DNA evidence for three interfertile hominin genetic groups cohabiting Eurasia, there is a host of possibilities for the Zhirendong fossils. One line of evidence that does not rule out that they are fully modern is the occurrence of stone tools more advanced than used by Asian H. erectus beneath the 74 ka Toba volcanic ash in India. It seems inevitable that these remains will be tried for DNA sequencing

See also: Dennell, R. 2010. Early Homo sapiens in China. Nature, v. 468, 512-513

It is well accepted that as with all forms of life the twists and turns in hominin evolution was surely tuned by changes in their environments. But that is not just linked to the immediate milieu of individuals: environments change on all scales up to that of the entire planet and reflect physical as well as biological processes. The largest scales are generally assumed to be the province of climate change, yet animals also occupy a landscape subject to geophysical forces such as tectonics and erosion. Geoffrey Bailey and Geoffrey King of the University of York, UK and the Institute de Physique du Globe in Paris, France have championed the view that water supplies and topography, for example, are just as influential over hominin evolution as interspecies competition and changing vegetation patterns for almost two decades. They have now put their ideas to rigorous tests (Bailey, G.N.& King, G.C., 2010 (in press) Dynamic landscapes and human dispersal patterns: Tectonics, coastlines, and the reconstruction of human habitats. Quaternary Science Reviews doi:10.1016/j.quascirev.2010.06.019). This fascinating and well illustrated paper correlates known hominin sites in Africa with variations in topography and its roughness, derived from global elevation data from the Shuttle Radar Topography Mission (SRTM), active seismicity, Neogene uplift and volcanicity.

Perspective view of the Afar depression and en...
Afar Depression: a cradle of human evolution

They concentrate on the rich palaeoanthropological pickings of the Afar Depression and the Sterkfontein area of South Africa, applying their ideas and findings to the eastern coast of the Red Sea at the recently discovered Palaeolithic site of Harat Al Birk south of Jeddah, and the Red Sea islands that would have been connected to either side of the Red Sea during the last glacial maximum because of a 130 m lower sea level. This application is vital for directing searches for new site that relate to the pathways out of Africa for early modern humans. Though a largely empirical study, it forms a link between human evolution and geological and landscape change that is not yet widely grasped and linked to climate studies.

See also: Marshall, M. 2010. Evolution by shake, rattle and roll. New Scientist, v. 208 (13 November 2010), p. 8-9.

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Oxygen and the differentiation of magmas

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2006 eruption
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The bulk of igneous rocks found within and upon the crust formed by one of two fundamental processes of magma differentiation: calc-alkaline and tholeiitic, responsible for island arcs and ultimately continents forming and for generating oceanic crust and flood basalts. The parental material for both is basaltic magma, but the first leads to a decrease in iron in more fractionated magmas, whereas an increase in iron characterised the second. In the first case conditions favour iron entering igneous minerals, whereas in the second they urge crystallising minerals to exclude iron. The most likely explanation is that the calc-alkaline magmas of volcanic arcs devour electrons so that iron exists in the oxidised ferric or Fe3+ state and readily forms dense iron oxide minerals whose progressive removal makes the remaining magma less and less rich in iron. More reducing conditions that lack an abundant electron acceptor, primarily oxygen, make the formation of iron oxides less likely, and iron can build up in residual magmas. But how greater oxidation occurs in arc magmas than in those of the oceanic crust has several possible explanations. The most-widely assumed is that it happens because volcanic arcs lie above subduction zones where hydrated and therefore oxidised ocean floor descends into the mantle conferring oxygen to the products of partial melting. Another candidate is the depth at which fractional crystallisation takes place and there are other possibilities. The oxidation state of fundamental magmatic processes can be proxied by determining in rocks produced by fractionation the relative proportions of elements that behave differently in conditions of increased or decreased oxygen. One such pair is insensitive zinc and sensitive iron (Lee, C.-T.A. et al. 2010. The redox state of arc mantle using Zn/Fe systematics. Nature, v. 468, p. 681-685). The surprise is that the parent magmas of both calc-alkaline and tholeiitic fractionation series have identical Zn/Fe ratios, suggesting that both partially melt from mantle with much the same availability of oxygen. The Zn/Fe ratios differ in more evolved igneous rocks from the two series, suggesting that it is in the fractionating magma chambers that the distinctively different oxygenation occurs, not in the zone of mantle melting.

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A chilly Late Cambrian

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Application of the Uniformitarian Principle by geologists from Minnesota, USA (Runkel, A.C. et al. 2010. Tropical shoreline ice in the late Cambrian: implications for Earth’s climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event. GSA Today v.20 (November 2010), p. 4-10) may have shown that around the end of the Cambrian period (500 to 488 Ma) global climate was sufficiently cold for sea ice to have formed in the tropics of the time. The evidence comes from curious metre-scale clasts of cemented sands in Late Cambrian beach deposits of the northern USA, some of which show imbrication as if the bodies were shoved together. Others seem to have been extended into boudin-like plates without any sign of tectonic activity, so that isolated clasts occur in offshore deposits. Yet more have been bent to drape over irregularities in the surface beneath them. Somehow individual sand beds must have become cemented quickly so that water action could fracture them in a brittle fashion and then they became softer to experience ductile deformation and even boring by worm-like animals. Almost exact replicas of such structures form on the shores of the American Great Lakes in winter when water in shoreline sands freezes to cement the grains. Breaking waves and melting explain the peculiar structures in these intraclasts. Examples of ice-cemented sediments abound in glaciogenic deposits, but the Late Cambrian world is widely considered to have experienced greenhouse conditions.

Land distribution during late Cambrian.
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Apparently not as the North American crust was definitely close to the Equator at that time. The intraclasts occur only in one stratigraphic Formation of the Minnesotan Cambrian, because it preserves littoral facies. There are no other reports from elsewhere, but that may well be because few geologists were able to combine the experience of modern frigid shore conditions with that of Cambrian stratigraphy as those from Minnesota surely do.

The Middle to Late Cambrian was a period of faunal hiccups, diversification after the Cambrian Explosion failing to get underway because of repeated minor extinctions spread across the known occurrences of rocks of that age (see Linking oxygen levels to great animal radiations, this issue) . The Minnesotan evidence could indicate that the global climate was extremely unstable at that time in the manner of Neoproterozoic ‘Snowball Earth’ conditions, but not so severe. The widespread occurrence of microbial carbonate facies of this age range has long been used as evidence of a warm Earth, but such carbonated form today over a wide range of latitudes: witness the huge coccolithophore blooms so common at high latitudes nowadays. Shoreline sandy sediments of Cambrian age are not uncommon, occurring throughout the English Midlands and in NW Scotland, for instance. So it might be interesting to re-examine easily-reached occurrences such as these to see if similar structures turn-up.