More early art from South Africa?

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Silcrete flake from Blombos with crosshatching drawn in red ochre. (Credit: C. Foster)

The Blombos Cave 300 km east of Cape Town is where the earliest signs of art produced by anatomically modern humans were found (see Snippets on human evolution October 2011). The most publicized was a shaped piece of ochre etched with a hashed pattern of lines (Henshilwood, C.S. et al. 2018. An abstract drawing from the 73,000-year-old levels at Blombos Cave, South Africa. Nature v. 561, online; DOI: 10.1038/s41586-018-0514-3). This and the ochre-processing workshop where it was found gave a date of about 100 ka, Now another item has hit the newsrooms; a  ground piece of flinty silcrete that shows signs of being the product of knapping, on which has been drawn a similar pattern, which resembles the now ubiquitous ‘hashtag’ associated with Twitter. The level in the excavation from which it was removed gives an age of about 75 ka. Like the earlier artifact, it involved the use of ochre but in a way that has been said to be an example of drawing or painting, rather than etching. It is likely to have been produced by a sharpened piece of solid ochre, perhaps a kind of crayon

For some reason the object has been hyped as the earliest example of art and of advanced cognitive abilities. But the pattern is not as complex as that on the original etched ochre block from Blombos, or even those on a freshwater mussel from Trinil in Java that could have =been made by associated Homo erectus between 430 and 500 ka ago. This does not take the context at Blombos into account. There is ample evidence that ochre, along with charcoal and burnt seal bone, was being ground there and made into paint found in an abalone shell. It can be surmised that such paint was used for some kind of decoration that has not yet been discovered. That is quite possibly because it was used for body paint as similar materials are still widely used.  Now anyone – male or female – who uses cosmetics today, be it foundation, lipstick, eye-liner and -shadow or the truly fabulous make-up used by the Kathakali performers of Kerala, takes an age to try and to decide on which of an almost imperceptible range of shades to apply. Ochres are like that, as any native Australian artist will tell you.

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Lord Rama face paint in Kathakali

To me, the most likely origins of both kinds of Palaeolithic hashtag are: in the case of the ‘drawing’, checking the colour and ‘grindability’ of a sharpened piece of red ochre before use; and for the etched block, using a sharp tool to grind off small amounts from what may have been a well-used block of an especially valued hue.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Neanderthal Mum meets Denisovan Dad

Two bone fragments from the Denisova Cave – the former abode of an 18th century Russian hermit called Denis – in the Altai region of Siberia yielded ancient  DNA. One matches that from previously analysed Neanderthal remains and the other a genome that could only be ascribed to a hitherto unknown ancient-human population, now known as the Denisovans. Since their discovery further analysis of both modern and ancient DNA has shown that modern humans living outside of Africa contain a few percent of DNA from both ancient-human groups. Soon after leaving Africa some of their ancestors interbred with both; indeed a 40 ka-old modern-human jaw from Romania revealed genetic evidence that the individual had a Neanderthal great-great grandparent. Their descendants spread far and wide to populate Eurasia, Australasia and the Americas. Using the ancient DNA to peer back in time suggests that Neanderthals and Denisovans diverged from a common ancestor between 470 and 380 ka, itself having split from modern-human ancestry between 770 to 550 ka. Denisovan DNA also contains evidence that its ancestry included segments that could only have come from a totally unknown hominin species. Interestingly, DNA from the Neanderthal bone fragment found at Denisova contains fragments from an anatomically modern-human.

Tourists at the entrance to Denisova Cave, Rus...

Tourists at the entrance to Denisova Cave, Russia (credit: Wikipedia)

With such riches from tiny fragments of human bones unearthed from the Denisova Cave, it is no surprise that the team led by Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has subsequently analysed others that showed signs of human proteins. The latest ‘takes the biscuit’. A fragment of limb bone from someone who was at least 13 years old yielded DNA commensurate with their having been the child of a Neanderthal mother and a Denisovan father (Slon, V. and 18 others 2018. The genome of the offspring of a Neanderthal mother and a Denisovan father. Nature, v. 560, published on-line; doi: 10.1038/s41586-018-0455-x). Their child was a girl, who has been nicknamed ‘Denny’ by the team, though ‘Denise’ might seem more appropriate. The only clues to what her father, or any Denisovan, might have looked like stem from a few teeth and a skull fragment from the cave that have yielded Denisovan DNA. The teeth are much larger and the skull fragment is thicker than those of Neanderthals, suggesting that Denisovans were distinctly bigger and more robust than even the sturdy Neanderthals.

The father came from a population related to a later Denisovan found in the cave – the first to be sequenced. This suggests long-term occupancy of the area by Denisovans. But his genome also carries traces of Neanderthal ancestry. Surprisingly, the mother is more closely related to Croatian Neanderthals, rather than to an earlier Neanderthal found in the cave. Neanderthals were clearly capable of migrating between Europe and eastern Eurasia; more than 5000 km in this case. Even though very few archaic humans have been genetically sequenced it is beginning to look as if genetic mixing between diverse hominin groups in the last half million years was common, when they actually met. A custom of marrying outside a closely related group (exogamy) has been popular throughout recorded history; indeed it makes sound genetic sense. With the tiny human population density during the Late Pleistocene, it may then have been cause for mutual celebration.  As documented in Chapters 2 and 3 of David Reich’s Who We Are and How We Got Here (Oxford University Press, 2018) human origins since about 470 ka until the present chart a history of episodic migrations and genetic mixing that certainly makes nonsense of earlier ideas of ‘racial purity’ and casts doubt even on the term ‘species’ as regards members of the genus Homo.

If we are ever to discover who the Denisovans were and what they looked like, the evidence is likely to come from East Asia at latitudes where climate favours preservation of DNA. Advanced sequencing equipment and techniques are now operational in China, where suspected Denisovan remains have been found

See also: Warren, M. 2018. First ancient-human hybrid. Nature, v. 560, p. 417-418; doi: 10.1038/d41586-018-06004-0); Sample, I. 2018. Offspring of Neanderthal and Denisovan identified for first time. The Guardian (22 August 2918).

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Multiregional human evolution in Africa

Africa is not only a large continent, but is subdivided into many different climatic zones and ecosystems and these have changed drastically over the last 2 Ma. It is further subdivided by terrain features, such as the courses of major rivers, large plateaus, tectonic rift systems and the mountains that frequently define their flanks. Getting around Africa is not easy today, was more difficult before modern transport, and many geomorphic provinces may have been mutually inaccessible in the distant past. For instance, the Sahara Desert forms a major barrier to travellers on foot because access to surface water is non-existent except at widely spaced oases. Without boats or rafts the Nile and Congo cannot be crossed for a thousand miles or more. Migration was perhaps a very rare event outside of periods of widespread humid climates or when great environmental stress forced people either to move or perish. Despite these physical and ecological divisions and barriers palaeoanthropologists have, until recently, tended to regard the evolution of Homo sapiens and earlier human and hominin species as having occurred within single populations: a linear view forced on them by scanty fossil remains and limited methodologies. Logically, when human numbers were small Africa probably had several isolated population Physical isolation would have engendered genetic isolation in which our ancestors evolved for tens of thousand years.

Anatomically modern human (AMH) remains found at Jebel Irhoud in Morocco turned out to be 315 ka old, displacing those from Ethiopia (190 ka) as the earliest known examples of AMH. Several more archaic H. sapiens fossils have turned up in southern Africa and as far afield as the Middle East, suggesting that the early evolution of AMH was in an Africa-wide context rather than in one area – the rift system of Ethiopia and Kenya – from which a new species radiated outwards. This breadth of finds has encouraged Eleanor Scerri of Oxford University and her many international colleagues to resurrect what was once a widely discarded hypothesis; a multiregional model of modern human origins, originally proposed to have arisen from pre-sapiens groups in Eurasia by Milford Wolpoff but which was sunk once genetic connections among living humans turned out to be rooted in Africa. (Scerri, E.M.L. and 22 others 2018. Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology & Evolution, v. 33, p. 582-594; (PDF) doi: 10.1016/j.tree.2018.05.005). Scerri et al’s model is sited in Africa and the paper’s authors include several leading palaeoanthropologists who once opposed multiregionalism and established the Recent African Origin hypothesis on the back of the early genetic data.

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Different early AMH cranium shapes: left Jebel Irhoud, Morocco (315 ka), right Qafzeh, the Levant (85 ka) (credit: Scerri et al, 2018; Figure 1)

From region to region in Africa, the oldest AMH crania show significant differences from each other, but within a distinct combination of features that clearly distinguish us from our fossil relatives and ancestors, such as Homo heidelbergensis from Zimbabwe and the primitive-looking H. naledi found in a South African cave in 2015. Improved dating now shows that the Zimbabwean H. heidelbergensis and H.naledi remains are roughly the same age as the Jebel Irhoud AMH specimens. The first has long been held as the progenitor of AMH and descended from H. antecessor, perhaps the common ancestor for AMH, Neanderthals and Denisovans about 700 ka ago. The three human species cohabited Africa early in the evolutionary history of AMH. It is now abundantly clear from ancient and modern genomes that AMH, Neanderthals and Denisovans interbred in Eurasia. The proximity in time and space of earlier African AMH to two more ancient human species opens up a similar possibility earlier in the emergence of all living humans. There is evidence for that too: Yoruba people living in West Africa, whose genomes have been analysed, carry up to 8% of genetic ancestry that originated in an unidentified ancient population that was non-sapiens. At present, DNA analysis with the same high precision and information content from other living Africans has not been performed, and deterioration of ancient DNA in African climates has so far thwarted genomic studies of ancient African fossils.

The new view of our origins points to repeated hybridisation involving other coexisting human species, as well as evolution in isolation, from the outset. It continued through later times while Neanderthals and Denisovans survived. Even recent human genetic history is peppered with intermingling of a great variety of migrants passing through all the habitable continents. Another issue: In the earliest times, were cultures exchanged as well as genes? The first appearance of AMH coincides with that of a new stone technology (Levallois technique), moving away from the earlier dominance by handaxes towards more delicate, leaf-shaped points, that characterise the African Middle Stone Age. Similar techniques reached Europe with the Neanderthals. Was this an invention of the earliest AMH or a joint venture?

You can find an excellent review of these issues in the September 2018 issue of Scientific American (Wong, K. 2018. Last hominin standing.  Scientific American, v. 319(3), p. 56-61) along with several other articles on human evolution.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

How does subduction start?

Robert Stern of the University of Texas at Dallas, USA, and Taras Gerya of ETH, Zurich, have produced a masterly review of how subduction gets started from place to place, and from time to time in geological history (Stern, R.J. & Gerya, T. 2018. Subduction initiation in nature and models: A review. Tectonophysics, v. 744 (in press); (PDF). It is the foundering of oceanic lithosphere into the mantle and gravity that give modern plate tectonics the bulk of energy that drives it along by slab pull. Yet the mantle’s consumption of a lithospheric slab somehow has to be set in motion from the symmetrical spreading of ocean floor as occurs either side of a constructive margin. It could not happen were the lithosphere to retain its low bulk density relative to mantle peridotite for all time. Moreover, it wouldn’t last for long were the lithosphere not to retain its strength through hundreds of kilometres depth as it sinks into the mantle. Active subduction zones have consumed vast amounts of oceanic lithosphere, for more than 65 million years, especially in fast-spreading ocean basins such as the western and eastern Pacific. The record is held by the destructive margin on the west flank of South America where more than 150 million years-worth of eastern Pacific lithosphere has been swallowed. Yet in order for oceanic lithosphere, which is stronger than that beneath the continents, somehow to fail and begin to sink a linear weak zone must develop at the interface between two incipient new plates. On top of that, all subduction on Earth is one-sided. A simple mechanism involving just thermal convection predicts that both plates either side of a break would have similar density so both should sink, more or less symmetrically.

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Various ways in which subduction may start. (Credit: Stern and Gerya 2018 – in press – Figure 4)

Geophysical observations reveal that terrestrial subduction can be divided into that which is induced by plate motions and changes in force balance within spreading plates, or spontaneously due to unique conditions developing along the line of initiation. In the first class are cases where a microcontinent is driven into another continental margin and extinguishes the subduction responsible, while spreading continues behind the accreted microcontinent drive older lithosphere beneath the suture (this may have happened in the past but is not seen today). Another, similar, induced case occurs where an oceanic island arc accretes by subduction beneath it so that subduction flips in polarity to consume the driving sea-floor spreading. The loading of oceanic lithosphere by sediments piled onto it by erosion of a continental margin may spontaneously collapse to result in subduction beneath the sedimentary wedge and the continent (again, not happening today, but inferred from examples inferred by earlier geological history). Spontaneous failure may also occur where old, cold lithosphere is juxtaposed with younger by transform faulting, or where a mantle plume heats up lithosphere to create a thermally weakened zone.

Stern and Gerya do not leave the issue at simple mechanics but discuss how plates may develop weak zones or inherit them from earlier tectonic events. The role of water released by metamorphism of descending materials may encourage the observed one-sidedness of subduction by reducing frictional resistance and plate strength and make the process self-sustaining. The paper also discusses the various permutations and combinations that affect the style of induced destructive margins in compressional and extensional environments and a whole variety of nuanced cases of spontaneous initiation. Numerical modelling of the subduction process plays an important, though somewhat bewildering role in discussion, as do considerations of the forces likely to be at play. Applying theoretical considerations to actual examples from the geological record are sublimely enlivening, as are speculations about the future evolution of the passive margins of the Atlantic. Clearly, there is a healthy future for field and mathematical study on the processes at destructive plate margins, such as building in the aspects of magmagenesis. Since Stern has built his career on study of long dead collusions zones, products of arc accretion etcetera, development of their understanding is undoubtedly the main thrust of his and Gerya’s tour de force. Stern provides a full PDF at his University of Texas website for the benefit of anyone who wants to delve deeper than space at Earth-pages and my limited intellect permit!

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Snowball Earth: A result of global tectonic change?

The Snowball Earth hypothesis first arose when Antarctic explorer Douglas Mawson (1882-1958)speculated towards the end of his career on an episode of global glaciations, based on his recognition in South Australia of thick Neoproterozoic glacial sediments. Further discoveries on every continent, together with precise dating and palaeomagnetic indications of the latitude at which they were laid down, have steadily concretised Mawson’s musings. It is now generally accepted that frigid conditions enveloped the globe at least twice – the Sturtian (~715 to 660 Ma) and Marinoan (650 to 635 Ma) glacial episodes – and perhaps more often during the Neoproterozoic Era. Such an astonishing idea has spurred intensive studies of geochemistry associated with the events, which showed rapid variations in carbon isotopes in ancient seawater, linked to the terrestrial carbon cycle that involves both life- and Earth processes. Strontium isotopes suggest that the Neoproterozoic launched erratic variation of continental erosion and weathering and related carbon sequestration that underpinned major climate changes in the succeeding Phanerozoic Eon. Increased marine phosphorus deposition and a change in sulfur isotopes indicate substantial change in the role of oxygen in seawater. The preceding part of the Proterozoic Eon is relatively featureless in most respects and is known to some geoscientists as the ‘Boring Billion’.

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Artist’s impression of the glacial maximum of a Snowball Earth event (Source: NASA)

Noted tectonician Robert Stern and his colleague Nathan Miller, both of the University of Texas, USA, have produced a well- argued and -documented case (and probably cause for controversy) that suggests a fundamental change in the way the Precambrian Earth worked at the outset of the Neoproterozoic (Stern, R.J. & Miller, N.R. 2018. Did the transition to plate tectonics cause Neoproterozoic Snowball Earth. Terra Nova, v. 30, p. 87-94). To the geochemical and climatic changes they have added evidence from a host of upheavals in tectonics. Ophiolites and high-pressure, low-temperature metamorphic rocks, including those produced deep in the mantle, are direct indicators of plate tectonics and subduction. Both make their first, uncontested appearance in the Neoproterozoic. Stern and Miller ask the obvious question; Was this the start of plate tectonics? Most geologists would put this back to at least the end of the Archaean Eon (2,500 Ma) and some much earlier, hence the likelihood of some dispute with their views.

They consider the quiescent billion years (1,800 to 800 Ma) before all this upheaval to be evidence of a period of stagnant ‘lid tectonics’, despite the Rodinia supercontinent having been assembled in the latter part of the ‘Boring Billion’, although little convincing evidence has emerged to suggest it was an entity formed by plate tectonics driven by subduction. But how could the onset of subduction-driven tectonics have triggered Snowball Earth? An early explanation was that the Earth’s spin axis was much more tilted in the Neoproterozoic than it is at present (~23°). High obliquity could lead to extreme variability of seasons, particularly in the tropics. A major shift in axial tilt requires a redistribution of mass within a planetary body, leading to true polar wander, as opposed to the apparent polar wander that results from continental drift. There is evidence for such an episode around the time of Rodinia break-up at 800 Ma that others have suggested stemmed from the formation of a mantle superplume beneath the supercontinent.

Considering seventeen possible geodynamic, oceanographic and biotic causes that have been plausibly suggested for global glaciation Stern and Miller link all but one to a Neoproterozoic transition from lid- to plate tectonics. Readers may wish to examine the authors’ reasoning to make up their own minds –  their paper is available for free download as a PDF from the publishers.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Evolution of the River Nile

The longest river in the world, the Nile has all sorts of riveting connotations in terms of archaeology, Africa’s colonial history, the romance of early exploration and is currently the focus of disputes about rights to its waters. The last stems from its vast potential for irrigation and for hydropower. It is probably the most complex of all the major rivers of our planet because it stretches across so many climatic zones, topographic systems geological and tectonic provinces. Mohamed Abdelsalam of Oklahoma State University, who was born in the Sudan and began his career at the confluence of the White and Blue Nile in its capital Khartoum, is an ideal person to produce a modern scientific summary of how the Nile has evolved. That is because he has studied some of the key elements of the geology through which the river and its major tributaries travel, but most of all because he is a leading geological and geomorphological interpreter of remotely sensed data. Only space imagery can let us grasp the immense span and complexity of the Nile system. His recent review of its entirety (Abdelsalam, M.G. 2018. The Nile’s journey through space and time: A geological perspective. Earth Science Reviews, v. 177, p. 742-773; doi: 10.1016/j.earscirev.2018.01.010) is a tour de force, many years in the compilation, and it makes fittingly compulsive reading.

Abdelsalam lays out the geomorphology, underlying geology and regional tectonics of the Nile drainage basin, synthesized from publications over the last century, including his own work on the evolution of the Blue Nile in Ethiopia. On the regional scale elements of its complexity can be ascribed to the upwelling of mantle plumes beneath the Ethiopian Highlands and Red Sea, and under the Lake Plateau centred on Kenya, Tanzania, Rwanda and Burundi. These plumes are part of a much larger mantle mass rising from the core-mantle boundary beneath the African continent. Their influence on the lithosphere of north-east Africa began over 30 million years ago, producing vast outpourings of flood basalts followed by regional doming, the formation of large shield volcanoes and rifting to transform a once muted surface to one with a topographic range of up to 5 kilometres in the Nile’s two main source regions in Ethiopia and the Lakes Plateau.

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The geological underpinnings of the Nile system (Credit: Abdelsalam 2018; Fig. 5)

The basin can be divided into six distinct provinces, from south to north the Lakes, Sudd, Central Sudan, Ethiopia – East Sudan, Cataract and Egyptian Niles. Each of them has had a different history; in fact, the making of the Nile system as we know it has taken at least 6 million years and probably longer. For instance, the Lakes Nile basin, founded mostly on Precambrian crystalline basement, seems original to have drained westward through the Congo system to the Atlantic Ocean. Sometime between 20 and 12 Ma the western branch of the East African Rift System began to form along with slow, broad uplift, hindering westward flow to create the forerunners of the Great Lakes. The flow was reversed around 2.5 Ma ago by the rise of the Rwenzori and Virunga massifs on the western rift flank and eventually forced northwards into the low-lying Sudd, breaching a major divide in Northern Uganda. The vast swamps there have acted as a buffer for sediment supply, other than the finest silts and clays, into the northern stretches of the White Nile. The Blue Nile’s tortuous trajectory evolved as the Ethiopian flood basalt province rose after 30 Ma, rifted to form the Lake Tana Basin and drained to initiate erosion into the rising plateau with the interference of huge shield volcanoes that formed as uplift proceeded.

Other events are recorded along the Nile’ general trajectory by huge, abandoned alluvial fans, relics of now vanished lakes and evidence from satellite radar of palaeo-drainages with reversed flow beneath the surface of the eastern Sahara. The system evolved episodically, in five or more steps, at the whim of broad tectonic processes that affected flow direction and erosive capacity. The Cataract Nile that cuts through hard basement rocks perhaps records the increase in energy added by the Blue Nile which, which in turn may have encouraged the drainage of the huge Sudd swamps that established the White Nile’s course. Even the Mediterranean Sea played a role: the Egyptian Nile may have formed when the sea vanished to expose a deep saline basin during the Messinian Salinity Crisis 5.5 Ma ago. This reduction in the regional base level of erosion possibly directed drainage into the present course of the Nile. The various provinces only became a unified drainage system during the last half million years, and that emerged in its present form as recently as 15 thousand years ago.  But as Abdelsalam points out, there is a great deal to learn about the fabled river system. Hopefully his review will encourage others to take investigations forward and into previously unstudied regions.

The hobbits of Flores: An update

Homo floresiensis (the "Hobbit")

Homo floresiensis from Liang Bua Cave, Flores, Indonesia. (Credit: Wikipedia)

In October 2004 the world’s news media headlined the discovery of fossil remains of a tiny adult human on the Indonesian island of Flores, dated at around 18 ka. At only 1 m tall, with a brain cavity around a third the size of ours, yet having used stone tools and fire she was a sensational find. Someone so tiny and with such a small brain seemed highly unlikely to some palaeoanthropologists. Others claimed she was of a different species altogether. Homo floresiensis was also challenged as a new species and attributed to some congenital cause of small stature in a modern human – H. sapiens had first colonised Flores between 50 and 35 ka. But the subsequent discovery of remains of nine more individuals revealed skeletal details that were definitely un-human, with a suggestion of greater affinity to H. erectus. Her stature even suggested to a few anthropologists that she may have descended from migrant H. habilis, previously known only from 2 Ma ago in East Africa. The issue of relatedness was partly resolved by further dating of the cave strata that entombed the ‘hobbit’ which pushed her back to between 190 to 50 ka, beyond the earliest date of modern human colonisation. Further fragmentary fossil finds in more easily dated sediments on Flores showed the earliest known H. floresiensis lived around 700 ka ago. Stone tools and butchered prey remains on the island go back to 1 Ma, when the hominin trail goes cold.

English: Cave where the remainings of ' where ...

Liang Bua cave where the remains of Homo floresiensis were discovered in 2003. (credit: Wikipedia)

A plausible theory for these human’s ‘hobbit’-like size is an evolutionary process known as island dwarfism, akin to that which produced the tiny elephants (Stegodon) on which they preyed. Such dramatic size reduction may arise through the influence of stringently limited food resources on the evolution of descendants from a restricted founder population, genetically cut-off from larger, more widespread populations. It now appears that such dwarfism has also affected a modern human population living on Flores (Tucci, S and 14 others 2018. Evolutionary history and adaptation of a human pygmy population of Flores Island, Indonesia. Science,  v. 361, p. 511-516; doi: 10.1126/science.aar8486). A group of people of diminished stature live within shouting distance of the Liang Bua cave in which Homo floresiensis was first discovered. On average adults in the village are about 1.45 m tall. They certainly are not relict H. floresiensis, just significantly smaller than other Indonesian people living on Flores.

Serena Tucci and colleagues analysed the DNA of 32  adult pygmies from the village of Rampasasa. They show no sign of DNA from any other archaic human population than the Neanderthal and Denisovan traces that every living person outside of Africa carries – the pygmies are not descendants of H. floresiensis and are little different from other Indonesians and the rest of us. The analysis does show, however, that their ancestors carried a mixture of DNA from East Asia and New Guinea; perhaps a result of several waves of migration between 50 and 5 ka. They also carry significantly more DNA segments that are linked to short stature than do other East Asians. This suggests natural selection favored existing genes for shortness while the pygmies’ ancestors were on Flores; in other words they display an example of island dwarfism akin to that probably explaining the ‘hobbits’. Moreover, the people of Rampasasa show signs of an evolutionary adaptation to an almost exclusively meat and seafood diet, possibly arising after they migrated to Flores and had to depend on the available fauna but little in the way of plant foods.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Hot-spot track beneath the Greenland ice cap

Around 63 Ma ago, during the Palaeocene Epoch, major igneous activity broke out in what are now both sides of the North Atlantic Ocean. After initial sputtering it culminated massively between 57 and 53 Ma. Relics are to be seen in Baffin Island, West and East Greenland, the Faeroes and north-western parts of the British Islands, in the form of flood basalts, dyke swarms and scattered remnants of central volcanoes. Offshore drilling on the North Atlantic’s continental shelves suggests that the volcanism extended over 1.3 million km2 and blurted out around 6.6 million km3 of magma. Not for nothing have the products of this event been categorised as a Large Igneous Province. Its formation took place before the North Atlantic existed. It began to form as this precursor magmatic paroxysm waned.  Continued basaltic magma production created the ocean floor each side of the mid-Atlantic Ridge system to divide North America and Greenland from northern Europe. Sea floor spreading continues, rising above sea level in Iceland, which is underlain by a large mantle plume.

The plume beneath Iceland may have been present at a fixed position in the mantle for tens of million years. A hot spot over which plate movements have shifted lithosphere to be heated in a similar way to a sheet of paper dragged slowly over a candle flame. The Iceland plume may have left a hot-spot track similar to that involved in the Hawaiian island chain. The ocean floor to the east and west of Iceland is shallower and forms broad rides at right angles to the trend of the Mid-Atlantic Ridge system, judged to be such tracks that are still warm and buoyant after formation over the plume. But are there traces of earlier passage of drifting lithosphere over the plume. A way to detect older hot-spot tracks is through variations in geothermal heat flow through the continental surface, a linear pattern raising suspicions of such trace of passage. There is no sign to the east beneath Europe, so what about to the west. Greenland, being mainly blanketed in ice, is not a good place to conduct such a search as it would involve deep drilling through the ice at huge cost for each hole. But there is a roundabout way of obtaining geothermal information without even setting foot on Greenland’s icy wastes.

The geomagnetic field measured at the surface records anomalies in rock magnetisation in the solid Earth beneath. Near-surface variations due to large variations in rock types that comprise the continental crust appear as sharp, high frequency signals. Aeromagnetic surveys over Greenland are characterised by such noisy patterns because the subsurface geology is extremely complicated. However, the underlying upper mantle beneath all continents is geologically quite bland, but being uniformly rich in iron it contains a high proportion of magnetic minerals such as magnetite (Fe3O4). The upper mantle should therefore leave a signal in the surface geomagnetic field, albeit a commensurately bland one. Like radio signals that span a large range of wavelengths, Earth properties that vary spatially, such as the geomagnetic field, may be analysed using filters. Once the high-frequency geomagnetic features of the crust are filtered out what should remain is a signal that reflects the magnetic structure of the upper mantle. It should be more or less featureless, yet beneath Greenland it isn’t.

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Estimated Curie depth variation below Greenland (left) converted to geothermal heat flow variation (right). (Credit: Martos et al. 2018; Figures 1b and 1c)

Magnetic anomalies are created by magnetisation induced in magnetic minerals in rocks by the Earth’s magnetic field. Yet minerals lose their ability to be magnetised at temperatures above a threshold known as the Curie point, which is 580 °C for magnetite, the most abundant magnetic mineral. Depending on the geothermal heat flow the Curie point is exceeded at some depth in the lithosphere. So magnetic anomalies can safely be assumed to be produced only by rocks above the so-called Curie depth. Yasmina Martos of the British Antarctic Survey (now at the University of Maryland) and scientists from Britain, the US and Spain used a complex procedure, including gravity data and a few direct measurements of heat flow below Greenland as well as filtered aeromagnetic data, to estimate the variation in Curie depth beneath the ice cap. (Martos, Y.M. et al. 2018. Geothermal heat flux reveals the Iceland hotspot track underneath Greenland. Geophysical Research Letters, v. 45, online publication; doi: 10.1029/2018GL078289). Using that as an inverse proxy for heat flow they were able to map the likely geothermal variation beneath the island. Rather than a random and narrow variation in depth, as would be expected for roughly uniform heat flow, the Curie depth varied in a non-random way by over 20 km, equivalent to roughly 20 mW m-2.

The shallowest Curie depth and highest estimated heat flow occurs in East Greenland around Scoresby Sund where the largest sequence of Palaeocene flood basalts occur. It is also on a line perpendicular to the mid-Atlantic Rift system that meets the active Iceland plume. Running north-west from Scoresby Sund is a zone of locally high estimated heat flow. Martos et al. suggest that this is the track of Greenland’s motion over the Iceland hot spot from about 80 Ma to the period of maximum on-shore volcanism and the start of sea-floor spreading at around 50 Ma.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

The Great Barrier Reef and the Last Glacial Maximum (LGM)

The 2,300 km stretch of coral reefs and islands in the Coral Sea off the coast of Queensland, Australia is the largest single structure on Earth built by living organisms. The dominant reef builders are four hundred species of coral, most of which are a symbiosis that conjoins marine invertebrates in the class Anthozoa – part of the phylum Cnidaria – and photosynthesising single-celled eukaryotes known as dinoflagellates. These algae are mainly free-living marine plankton, some species of which evolved to be co-opted by corals. Their role in the symbiosis is complex; on the one hand providing energy in the form of sugars, glycerol and amino acids; on the other consuming the coral polyps’ carbon dioxide output. The latter is fixed, in the case of hard corals, by the secretion of calcium carbonate: the key to reef formation.

Marine photosynthesisers demand clear water in the upper few tens of metres of the sea, together with sunlight least affected by the atmosphere, as in the tropics where the sun rises to the zenith year round. The coral animal-algae connection limits reef growth to shallow seas, the top of the reef being close to mean sea level, sometimes rising above it at low tide. Hence the formation of fringing and barrier reefs. In the case of atoll reefs, a connection with sea-floor volcanoes that rose from hotspots on the oceanic abyssal plains to form active volcanic islands that began to sink once they became extinct. The pace at which reefs can grow is generally able to match that of crustal subsidence so that atolls remain throughout the Western Pacific. Reef growth is also capable of coping with global sea-level changes, so that the present top level of the Great Barrier Reef has been in balance with the generally static sea level of the Holocene since the ice caps of the last glaciation melted back to roughly their present extent about seven thousand years ago.

There are many cases of different reef levels on and around islands that match the sea-level fluctuations during the last Ice Age.  High-resolution bathymetry produced by multi-beam sonar across the eastern edge of parts of the Great Barrier Reef reveals a series of submerged terraces down to almost 120 m below modern sea-level (Yokoyama, Y. and 17 others 2018. Rapid glaciation and a two-step sea level plunge in the Last Glacial Maximum. Nature, v. 559, p. 603-607; doi:10.1038/s41586-018-0335-4). Globally, the LGM began at around 31 ka when sea level fell by about 40 metres, thanks to massive accumulation of glacial ice at high latitudes. Previous studies to chart the changes in global mean sea level during the LGM suggested a steady fall until about 20 ka, followed by rapid rise as ice caps melted back. The multinational team led by Yusuke Yokoyama of the University of Tokyo, obtained precise ages of coral samples from different depths in drill cores through the coral terraces. These data revealed a more complex pattern of sea-level change, in particular a hitherto unsuspected plunge between 21.9 and 20.5 ka of 20 m to reach -118 m. This immediately preceded the warming-related rise that continued to Holocene levels.

GBR Bathymetry

High-resolution sonar images of the sea floor at two sites on the eastern edge of Australia’s Great Barrier Reef. They show terraces associated with, the lowest of which corresponds to the Last Glacial Maximum. (Credit: Yokoyama et al. 2018, Figure 1)

Curiously, this massive phenomenon is not shown by sea-level estimates derived from the records of changing oxygen isotopes in ocean-floor sediments and ice cores. The team’s complex modelling incorporated global changes in land and sea-bed levels, and thus changes in the volume of the ocean basins, due to the changing isostatic effects of both ice-cap and ocean masses. From these it is possible to reach an interesting conclusion (Whitehouse, P. 2018. Ancient ice sheet had a growth spurt. Nature, v. 603, p. 487-488; doi:10.1038/d41586-018-05760-3). Rather than an increase in snowfall onto ice-caps, their retreat may have been hindered by thickening of marginal floating ice shelves that created buttresses around Antarctica and the northern ice sheets. Slowed glacial flow to the oceans could have promoted ice sheet growth for a time as melting of calved icebergs was hindered, especially in the case of the ice sheet over northern North America. Certainly, this crucial climatic turning point was a lot more complex than previously believed.

Technical problem with Earth-pages News

Followers of Earth-pages News have been unable to access the site for more than three weeks due to a technical problem that disabled the earth-pages.co.uk link. The fault has been found and remedied. Apologies for the loss of service.

Steve Drury