Within-plate earthquakes

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English: Earthquakes recorded in the New Madri...
Recent earthquakes in the US mid-west around New Madrid Missouri. Image via Wikipedia

 

Almost all devastating earthquakes within living memory and the tsunamis that ensued from some of them have occurred where tectonic plates meet and move past one another either horizontally through strike-slip motion or vertically as a result of subduction. This link between real events and the central theory of global dynamics gives an impression of inherent predictability about where damaging and deadly earthquakes might happen, if not the more useful matter of when the lithosphere might rupture. Such confidence is potentially highly dangerous: the most deadly earthquake in recorded history killed at least 800 thousand people in China’s Shanxi Province in 1556 when according to  a description written shortly afterwards, ‘… various misfortunes took place… In some places, the ground suddenly rose up and formed new hills, or it sank abruptly and became new valleys. In other areas, a stream burst out in an instant, or the ground broke and new gullies appeared…’. Shanxi is far from any plate boundary. A study of Chinese historic records covering the last two millennia (Liu, M. et al. 2011. 2000 years of migrating earthquakes in North China: How earthquakes in midcontinents differ from those at plate boundaries. Lithosphere, v. 3, p. 128-132) shows a pattern to the position of large intraplate events.  Rather than occurring along lines as do those at plate boundaries, earthquakes ‘hopped’ from place to place without affecting the same areas twice. Liu and colleagues consider this almost random pattern to result from reactivation of interlinked faults through broad-scale and gradual tectonic loading of the crust by far off plate movements. After a short period of reactivation one fault locks so that energy build-up is eventually released by another in the plexus of crustal weaknesses.

The best studied site of such intraplate seismicity lies midway along the Mississippi valley in the mid-US, between St Louis and Memphis. In 1811 and 1812 four Magnitude 7 to 8 earthquakes struck, the most affected place being the small township of New Madrid on the banks of the great river where mud and sand spouted from numerous sediment volcanoes. No-one died there but tremors were felt over a million square kilometers, bells ringing spontaneously as far away as Boston and Toronto. It is now known that this section of the Mississippi basin lies above a graben that affects the ancient basement beneath the alluvial sediments, one of whose faults was reactivated, perhaps in an analogous way to the hypothesis about Chinese seismicity. A coauthor in Liu et al. (2011), Seth Stein of Northwestern University, Illinois, believes stress redistribution through a Mid-western fault network was responsible and other events are likely at some uncertain time in the future on this and other areas underpinned by ancient fault complexes. Indeed sporadic ‘quakes up to Magnitude 7 have affected the eastern US and Canada and the Atlantic seaboard since European settlement. But since the largest of the New Madrid quartet of earthquakes, populations have grown across the likely areas of tenuous risk and future ones could have extremely serious consequences for which it is difficult to plan by virtue of unpredictability of both place and timing: in some respects a more worrying prospect than is the case where major events are inevitable – sometime – as along the San Andreas Fault. There are few, if any, major conurbations worldwide that could be considered seismically safe if the theory of networked stress redistribution through otherwise inert parts of continental crust is borne out.

In some respects the theory is a small-scale version of the suggested mechanical linkage through all major plate boundaries that has been suggested by some to account for the clustering in time of great earthquakes – around and above Magnitude 8 – around the globe. Since 2000 great earthquakes have occurred on subduction zones beneath Sumatra, the Himalaya, the Andes, Central America, Alaska, New Guinea, the mid-Pacific, Japan and the Kurile islands, on the strike-slip system that cuts New Zealand and in the intraplate setting of the 2008 Sichuan earthquake in China. Almost all plate boundaries link up globally, but although it seems likely that stress is redistributed along boundaries, especially between adjacent segments, as documented for the great Anatolian fault system of Turkey and the Indonesian subduction zone, a mechanism that transmits stress beyond individual plates seems unlikely.

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Massive event in the Precambrian carbon cycle

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English: Cyanobacteria
Cyanobacteria: earliest producers of oxygen in the Precambrian. Image via Wikipedia

The entire eukaryote domain of life, from alga to trees and fungi to animals, would not exist had it not been for the emergence of free oxygen in the oceans and atmosphere about 2.4 billion years ago; thanks in large part to the very much simpler photosynthetic blue-green bacteria. The chemistry behind this boils down to organisms being able to transfer electrons from elements and compounds in the inorganic world to build organic molecules incorporated in living things. Having lost electrons the inorganic donors become oxidised, for instance ferrous iron (Fe2+ or Fe-2) becomes ferric iron (Fe3+ or Fe-3) and  sulfide ions (S2-) become sulfate (SO42-) and the organic products that receive electrons principally involve reduction of carbon, on the OilRig principal – Oxidation involves loss of electrons, Reduction involves gain. Since the Great Oxygenation Event (GOE), ferric iron and sulfate ions now account for 75% of oxidation of the lithosphere and hydrosphere while free oxygen (O2) is a mere 2-3 % (Hayes, J.M. 2011. Earth’s redox history. Science. V. 334, p. 1654-1655; an excellent introduction to the geochemistry involved in the GOE and the carbon cycle). Free oxygen is around today only because more of it is produced than is consumed by its acting to oxidize ferrous iron and sulfide ions supplied mainly by volcanism, and carbon-rich material exposed to surface processes by erosion and sediment transport.

Eukaryote life has never been snuffed out for the last two billion years or so, but it has certainly had its ups and downs. To geochemists taking the long view oxygen might well seem to have steadily risen, but that is hardly likely in the hugely varied chemical factory that constitutes Earth’s surface environments, involving major geochemical cycles for carbon, iron, sulfur, nitrogen, phosphorus and so on, that all inveigle oxygen into reactions. Tabs can be kept on one of these cycles – that involving carbon – through the way in which the proportions of its stable isotopes vary in natural systems. If all geochemistry was in balance all the time, all materials that contain carbon would show the same proportions of 13C and 12C as the whole  Earth, but that is never the case. Living processes that fix carbon in organic compounds favour the lighter isotope, so they show a deficit of 13C relative to 12C signified by negative values of δ13C. The source of the carbon, for instance CO2 dissolved in sea water, thereby becomes enriched in 13C to achieve a positive value of δ13C, which may then be preserved in the form of carbonates in, for instance, fossil shells that ended up in limestones formed at the same time as organic processes were favouring the lighter isotope of carbon. Any organic carbon compounds that ocean-floor mud buried before they decayed (became oxidised) conversely would add their negative δ13C to the sediment. Searching for δ13C anomalies in limestones and carbonaceous mudrocks has become a major means of charting life’s ups and downs, and also what has happened to buried organic carbon through geological time.

A most interesting time to examine C-isotopes and the carbon cycle is undoubtedly the period immediately following the GOE, in the Palaeoproterozoic Era (2500 to 1600 Ma). From around 2200 to 2060 Ma the general picture is roughly constant, high positive values of δ13C (~+10‰): more organic carbon was being buried than was being oxidised to CO2. However, in drill cores through the Palaeoproterozoic of NW Russia carbonate carbon undergoes a sharp decline in its heavy isotope to give a negative δ13C  (~-14‰) while carbon in organic-rich sediments falls too (to~-40‰): definitely against the general  trend (Kump, L.R. et al. 2011. Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event. Science. V. 334, p. 1694-1696). Oxygen isotopes in the carbonates affected by the depletion in ‘heavy’ carbon show barely a flicker of change: a clear sign that the 13C δ13C deficit is not due to later alteration by hydrothermal fluids, as can sometimes cause deviant δ13C in limestones. It is more likely that a vast amount of organic carbon, buried in sediments or dissolved in seawater was oxidised to CO2 faster than biological activity was supplying dead material to be buried or dissolved. In turn, the overproduction of carbon dioxide dissolved in seawater to affect C-isotopes in limestones. Such an event would have entailed a sharp increase in oxygen production to levels capable of causing the oxidation (~ 1% of present levels). Yet this was not the time of the GOE (2400 Ma) but 300-400 Ma later. A possible explanation is a burst in oxygen production by more photosynthetic activity, perhaps by the evolution of chloroplast-bearing eukaryotes much larger than cyanobacteria.

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Excitement over early animals dampened

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Alga (Volvox sp.)
Volvox cyst. Image via Wikipedia

The Neoproterozoic lagerstätte in the Doushantuo Formation in the south of China was until recently thought to be a source of astonishing information about Earth’s earliest animals (See Ancestral animal? in EPN August 2004) that preceded the appearance of those with hard parts at the start of the Phanerozoic.  It contains well-preserved fossils that resemble embryos, algae, acritarchs, and small bilaterians. Dated at between 580 to 600 Ma(See Age range of early fossil treasure trove  in EPN February 2005), the Doushantuo directly overlies cap carbonates representing the emergence of Earth’s climate from a Snowball epoch represented by a tillite beneath the carbonate sequence. A detailed examination using synchrotron X-ray tomography of the putative animal embryos does show clear signs of cell doubling or palintomy (Huldtgren, T. et al. 2011. Fossilized nucluei and germination structures identify Ediacaran ‘animal embryos’ as encysting protists. Science. V. 334, p. 1696-1699) but also internal cell features most likely to be nuclei, but which have no counterparts in animal embryos. The organisms which the fossils most resemble are indeed eukaryotes, but of a kind separate from animals known as Holozoa. Yet there are striking resemblances with eukaryotes more distant from animals, such as the modern Volvox, a type of alga (Butterfield, N.J. 2011. Terminal developments in Ediacaran embryology. Science. V. 334, p. 1655-1656), that developed from an ancestor further back in time than the separation of metazoan animals from holozoans.

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Mistaken conclusions from Earth’s oldest materials

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Microscope projection close-upThe oldest materials on the planet are tiny zircon grains that were washed into conglomerate in Western  Australia about 2650 to 3050 Ma ago. It wasn’t the fact that the grains are zircons, which are among the most durable materials around, but the range of ages that they revealed when routinely analysed. U-Pb dating of detrital zircons is a well tested means of finding the provenance of sedimentary materials as an indicator of orogenic and igneous events that formed the crust from which they were eroded. In the original study of the Jack Hills zircons some showed ages that might reasonably have been expected from late sediments in an Archaean craton: around 3.5 billion years is about the maximum age for orogenic events there. What astonished all geoscientists was that a proportion of the grains gave ages of more than 4 billion years, some as old as 4.4 Ga: here was a window on the missing first half billion years of Earth history, the Hadean.

Subsequent work on yet more zircons confirmed the original age span but other kinds of analysis led to a variety of claims: that continental crust was around in abundance within 100 Ma of Earth having formed; geothermal heat =flow was not especially high;  liquid water was available for geological processes, including the origin of life; plate tectonics may have started early…. The topic has cropped up several times in EPN since the issue of 1 January 2001. Quite a lot of the claims emerged from studies of other minerals enclosed by the ancient zircons, such as quartz and micas, and now they have been checked again by geochemists from Western Australia (Rasmussen, B. et al. 2011. Metamorphic replacement of mineral inclusions in detrital zircons from Jack Hills, Australia: Implications for the Hadean Earth. Geology, v. 39, p. 1143-1146). It turns out that the inclusions formed at temperatures well below those of magmas, between 350 to 490°C: more like those of metamorphism. Indeed, uranium-bearing rare-earth phosphate minerals, xenotime and monazite, also locked in the zircons not only turn out to be metamorphic in origin too (both are also formed magmatically) but date to between 2700 and 800 Ma.

While the  Hadean zircon dates remain robust, a closer look at their inclusions shows that they did not remain geochemically closed systems thereafter. It was on the assumption of zircons being geological ‘time capsules’ that much of the excitement rested. Even using the presence of zircons from 4.4 Ga – they are most common in granites but do occur in mafic and intermediate igneous rocks – to suggest early ‘sialic’ continental crust is suspect. Despite having some tiny bits from Earth’s early days, it seems we are none the wiser.

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Galactic controls

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English: Artist's conception of the Milky Way ...
Artists impression of the Milky Way viewed along its axis. Image via Wikipedia

Palaeoclimatologists are quite content that an important element in controlling the vagaries of climate is due to gravitational forces that cyclically perturb Earth’s orbit, it axial tilt and the way the axis of rotation wobbles in a similar manner to that of a gyroscope. The predictions about this by James Croll in the late 19th century, which were quantified by Milutin Milankovich during his incarceration during World War I, triumphed when the predicted periods of change were found in deep-sea floor sediment records in 1972. Authors of ideas that link Earth system changes  to the progress of the Solar system through the Milky Way galaxy haven’t had the same accolades. One of the first to suggest a galactic link was Joe Steiner (Steiner, J. 1967. The sequence of geological events and the dynamics of the Milky Way Galaxy. Journal of the  Geological Society of  Australia, v.  14, p. 99–132.) but his work is rarely credited.

There has been an upsurge of interest in the last decade or so. In a recent issue of New Scientist Stephen Battersby reviews what galactic ‘forcings’ may have accomplished during the 4.5 billion-year history of our world (Battersby, S. 2011. Earth odyssey. New Scientist, v. 212 (3 December issue), p. 42-45). Having formed probably much closer to the galactic centre than its current position the Solar System has drifted, perhaps even ‘surfed’ gravitationally, outwards to reach its present ‘suburban’ position in one of the spiral arms. There are regularities to the now stabilised orbital movements: once every 200 million years the Solar System completes a full orbit; this orbit wobbles across the hypothetical plane of the galactic disc by as much as 200 light years, moving with and against the Milky Way’s cosmic motion. It has proved impossible so far to detect any sign of the orbital 200 Ma periodicity in events on the Earth, and most attention has centred on the wobble.

Steiner suggested that this motion may have crossed different polarities of the galactic magnetic field, perhaps triggering the periodicity of geomagnetic  changes in polarity, but this now seems unlikely. However, his suggestion that glacial epochs, such as those in the Palaeo- and Neoproterozoic, at the end of the Palaeozoic Era and at present, may have resulted from the Solar System’s passage through dust and gas banding in the Milky Way continues to have its attractions (e.g. Pavlov, A.A. et al. 2005. Passing through a giant molecular cloud: “Snowball” glaciations produced by interstellar dust, Geophysical Research Letters, v. 32, p. L03705). The direction of motion relative to the Milky Way’s cosmic drift governs the exposure to cosmic rays that result from a kind of ‘bow-shock’ ahead of the galaxy

Stellar motion through the Milky Way is semi-independent so that from time to time the Solar System may have been sufficiently close to regions of dense dust and gas that nurture the formation of super-massive stars. These huge objects quickly evolve to end in supernovae, proximity to which would have exposed life to ‘hard’ X- and  γ-rays and would be trigger for mass extinction, for instance by accompanying cosmic rays in destroying the ozone protection from UV radiation from the Sun.

The dynamism of the Earth and the resulting complexity of its surface processes makes it a poor place to look for physical signs of galactic influences. No so the Moon: for almost 4.5 billion years it has been a passive receptor for virtually anything that the cosmos could fling at it, and so geologically inert that its surface layers may well preserve a complete ‘stratigraphic’ record of all kinds of process. Should lunar landings with geological capabilities once more prove economically possible, or politically useful, that hidden history could be read.

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Hominin updates

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A new approach to 14C dating at the Oxford Radiocarbon Accelerator Unit at the University of Oxford UK, combined with detailed analysis of human teeth to distinguish fully modern human remains from those of Neanderthals has pushed back the date and pace of migration into Europe by people whose tools define the Aurignacian and Italian Uluzzian technologies. These are the earliest modern-human cultures found in Europe, but some of the tools are similar to those produced by Neanderthals (Châtelperronian culture), raising the possibility of transfer of technologies between the two groups. So, without confirmation from human remains of the anatomical affinities the would be doubts about using tools of these kinds to signify the presence at a site of full modern humans. Teeth found decades ago at caves in SW England and southern Italy prove, on detailed comparative study, to be from ‘moderns’ (Higham, T. And 12 others 2011. The earliest evidence for anatomically modern humans in northwestern Europe. Nature, v. 479, p. 521-524; Benazzi, S. And 13others 2011. Early dispersal of modern humans in Europe and implications for Neanderthal behaviour. Nature, v. 479, p. 525-528).The new carbon-isotope method  efficiently eliminates chemical contamination of material by post-fossilisation processes and so tend to increase the measured age of samples. The two studies produced exciting results: dates of occupation between 42-43 and 43-45 ka from SW England and southern Italy respectively. Together with results from other sites throughout central and southern Europe, the discovery shows that widespread colonisation was accomplished in three to five thousand years by migrants probably from the Levant, who may have travelled along three routes fanning out from the Bosporus in modern Turkey: along the Danube; along the Adriatic coast; from southern Greece to the ‘heel’ of Italy.

In early 2011 a group of archaeologists led by Simon Armitage of the University of Birmingham, UK reported stone tools from a cave in the United Arab Emirates for which they derived possible ages of 125, 95 and 40 ka (see Human migration in EPN for January 2011). The older dates were coeval with anatomically modern humans in the Levant, but the tools themselves showed features that could not be matched decisively with those from any other sites, including those in the Leant, though they most resembled collections from East and NE Africa. Armitage and colleagues suggested that the people who occupied the UAE cave had crossed the Red Sea at the time of the glacial maximum around 130 ka, at a time of unprecedented low sea level. A recent paper adds considerable weight to this idea (Rose, J.I. and 9 others 2011. The Nubian Complex of Dhofar, Oman: An African Middle Stone Age Industry in Southern Arabia at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0028239). Jeffrey Rose, also of the University of Birmingham, and colleagues from Ukraine, US, UK, Germany, the Czech Republic and Australia excavated site in Dhofar southern Oman, much closer to the Straits of Bab el Mandab than the UAE. Chert tools found in the area are of the Levallois type, specifically resembling closely those found widely in the Nile Valley of southern Egypt and northern Sudan, and in the Afar Depression of Ethiopia, in deposits dated between 128 to 74 ka. The Omani tools yielded an optically stimulated luminescence age of about 106 ka. This nicely confirms that Africans had moved far beyond the confines of their home continent by the last interglacial episode, with the route to South Asia open to them along the shores of the Persian Gulf and Indian Ocean. However, the route that they had taken could equally have been around the head of the Red Sea as across the Bab el Mandab.

Desert varnish: an outdoor canvas

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Petroglyphs carved in desert varnish at the Va...
Petroglyphs in desert varnish near Las Vegas, Nevada, USA. Image via Wikipedia

Early occupants of semi-arid areas found a cultural use for what is one of geology’s greatest annoyances: desert varnish. Annoying because once developed it leaves an extremely durable brownish to black, shiny coating over rock surfaces: be they dunite, marble or quartzite, sandstone or granite, desert outcrops all look very much the same. You have to bash them unmercifully to see the true texture and mineralogy, and, except on images of thermally emitted infrared, remote sensing doesn’t help as the varnish has the same reflectance whatever the wavelength of radiation. Yet to the former inhabitants of dry lands – and latter day ‘taggers’ – desert varnish has been irresistible for millennia. Lightly peck away with a sharp pebble – and some ability to depict your thoughts – and you can leave an almost indelible sign that you and your ideas were at that very rock face: a petroglyph, picked out for all time in the manner of chalk on a blackboard. Even more spectacular, given an oversight of a varnished cobbly plain and it is possible to magnify your tag, or whatever petroglyphs once signified, a hundredfold or more. That happened on the famous Nazca Plain of Peru  and continues to do so in especially dry places in the south-western US, as around Lake Havasu City in Arizona. Varnish forms only on the exposed face of cobbles, the downward side remaining more or less the original rock’s colour; generally lighter. Turn over the cobbles in an organised way, with a degree of persistence as well as talent and you too can make your mark on Google Earth! (Do not pass this on to Banksy – it doesn’t hurt the ecosystem, but will annoy the authorities immensely).

Français : Lignes de Nazca au Pérou. Le contra...
Ancient art depicting a hummingbird on the Nazca Plain, Peru. Image via Wikipedia

For all this period of artistic endeavour, stretching back in some places to the Palaeolithic, it now seems that desert varnish also records how environments have changed as well as the religiosity, humour or downright egotism of its inhabitants (Dickerson, R. 2011. Desert varnish – nature’s smallest sedimentary formation. Geology Today, v. 27 (November-December issue), p. 216-219). As well as reviewing how the varnish forms (see also Desert varnish in EPN May 2008, in Subjects: GIS and Remote Sensing)., Dickerson flags-up the little-known fact that the minute layers produced as varnish imperceptibly develops record changes in environmental conditions – wet, dry and middling – and, moreover they can be dated precisely despite being extremely thin (e.g. Liu, T. & Broeker, W.S. 2008. Rock varnish microlamination dating of late Quaternary geomorphic features in the dry lands of wester USA. Geomorphology, v. 93, p. 501-523). Liu and Broeker were able to match variations in the colour of varnish layers with important climatic episodes of the Northern Hemisphere, such as the Younger Dryas and other warming-cooling, dry-wet shifts as far back as the Last Glacial Maximum. Their approach offers a chance of dating petroglyphs and thereby cultural changes during critical stages in the history of modern human migrations, occupations and abandonments, even when no artefacts or bones remain. That is because once made, petroglyphs gradually become varnished themselves.

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Pan African Review

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A terrane boundary close to the Nile in the Sudan, detected by radar from the Space Shuttle: the Keraf Suture. From NASA

Undoubtedly the best exposed and one of the biggest examples, the accretionary orogen of the Arabian-Nubian Shield (ANS) is a witness to the creation of a supercontinent from the remnants of an earlier one. At about 1 Ga, most of the Earth’s continental material was clumped together in the Rodinia supercontinent that existed for a quarter of a billion years. At a time of massive mantle upheaval that left most crust of that age affected by basaltic magmatism, in the form of lava flows and dyke swarms, Rodinia began to break up at 800 Ma to scatter continental fragments. Subduction zone accommodated this continental drift to form many ocean and continental-margin volcanic arcs. The ANS is a repository for many of these arcs which episodically accreted between earlier cratons to the west in Africa and those comprising Somalia and the present Indian subcontinent. Primarily the terranes are oceanic in origin and formed in the aftermath of the dismemberment of Rodinia, although a few slivers of older, reworked crust occur in Saudi Arabia and Yemen. Among the various components are ophiolites marking sutures and other major tectonic features of the orogen. The shape of the Shield is unlike that of any other major orogen of later times, for it shrinks from a width estimated at ~2000 km in Arabia to the north to vanish just south of the Equator in southern Kenya. This ‘pinched’ structure has suggested to some that the bulk of the new crust was forced laterally northwards when the African and Indian cratons collided, in the manner of toothpaste from a trodden-on tube.

Today the ANS is a harsh place, some off-limits to geologists either for political reasons or the sheer hostility and remoteness of the environment. Yet a picture has emerged, bit by bit, over the last 30 years. So a detailed review of the most extensive and varied part from 7° to 32°N and 26° to 50°E – in Egypt, Saudia Arabia, eastern Sudan, Eritrea, Yemen and northern Ethiopia is especially welcome (Johnson, P.R. et al. 2011. Late Cryogenian–Ediacaran history of the Arabian–Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the closing stages of the northern East African Orogen. Journal of African Earth Sciences, v. 61, p. 167-232). Peter Johnson himself compiled a vast amount of information during his career with the US Geological Survey Mission in Saudi Arabia and has blended the inevitably diverse ideas of his 7 co-workers – but by no means all the ideas that are in the literature. The result is a readable and well illustrated account of how the ANS assembled tectonically during times when a near-global glaciation took place, and the first macroscopic animals appear in the fossil record. Tillites and other glaciogenic rocks from the Marinoan ‘SnowBall’ occur from place to place in the ANS, as do banded iron formations that made a surprise return after a billion-year or longer absence in the Cryogenian Period . Coincidentally, glacial conditions returned to the region twice in Ordovician and Carboniferous to Permian times, forming distinctive, tectonically undisturbed sediments in the Phanerozoic cover that unconformably overlies the Neoproterozoic orogen.

Except in a few areas only recently explored, geologists have assiduously dated events in the ANS, showing nicely that all the basement rock formed after 800 Ma, and that orogenic events culminated before the start of the Cambrian period, although one or two unusual granites intruded as late as the Ordovician. The deformation is immense in places, with huge nappes, often strike-slip shear zones and exposure ranging from the lowest metamorphic grade to that in which water and granitic magma was driven from the lower Pan African crust. The range of exposed crustal levels stems partly from the tectonics, but owes a lot to the 2-3 km of modern topographic relief, unique to NE Africa and Arabia. Yet it is not uncommon to come upon delicate features such as pillowed lavas, conglomerates and finely laminated volcanoclastic tuffs. Following tectonic welding, more brittle deformation opened subsiding basins that contain exclusively sedimentary rocks derived from the newly uplifted crust, both marine and terrestrial in formation (basins of this type, in Eritrea and Ethiopia, unfortunately do not figure in the regional maps). Much of the ANS is currently the object of a gold rush, encouraged by a rising world price for the ‘inflation-proof’ comfort blanket provided by the yellow metal. Consequently, newcomers to the stampede will be well advised to mug-up on the regional picture of occurrences and gold-favourable geology provided in the review, and may be interested by other exploration possibilities for rare-earth metals and other rising stars on the London Metal Exchange, such as tin, which are often hosted in evolved granites, that stud the whole region.

Water sources and early migration from Africa

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SeaWiFS collected this view of the Arabian Pen...
The Arabian Peninsula today. Image via Wikipedia

In March 2011 EPN reported in Human migration a puzzle relating to evidence for modern human occupation of Arabia on the southern shore of the Persian Gulf during the last Eemian interglacial at 125 and 95 ka. At that time sea level would have been as it is now, discouraging any attempt to cross the Red Sea via the Straits of Bab el Mandab; a widely suggested short-cut from East Africa to the rest of the world. Around 125 ka modern humans were making a living from coastal resources in Eritrea, leaving abundant stone tools in shoreline deposits at the head of the Gulf of Zula, and in the Sodmein Cave on Egypt’s Red Sea coast. They had also reached the famous Qafzeh and Skhul caves of Mount Carmel in today’s Israel around 100 thousand years ago. A route out of Africa through the Levant has not been widely favoured and the humans of Qafzeh and Skhul have been suggested to have reached a geographic cul-de-sac with no eastward exit because of the aridity of the Arabian Peninsula. Yet once in the Levant they could have skirted the desert interior by following the east coast of the Red Sea, and ‘strandloped’, as Jonathan Kingdon has dubbed following the coastline. But continuous access to fresh water would still have been essential.

The shores of the Red Sea preserve many examples of uplifted coral reefs, indeed signs of human presence in Eritrea occur in such a terrace. Being extremely porous, reef terraces are potential aquifers and a sign that they may have sourced freshwater springs is the conversion of the intricate coral skeletons from one form of calcium carbonate to another; original aragonite changes to calcite in the presence of fresh water, a complete replacement being estimated to take a thousand years of continual contact with fresh water. This change allowed Boaz Lazar and Mordechai Stein of the Hebrew University of Jerusalem and the Geological Survey of Israel to check for the presence of freshwater coastal springs in the past (Lazar, B. & Stein, M. 2011. Freshwater on the route of hominins out of Africa revealed by U-Th in Red Sea corals. Geology, v. 39, p. 1067-1070). Their test site was a series of uplifted reefs near Aqaba on the Red Sea coast of Jordan. The authors determined variations in the 230Th/238U ratio in the reefs relative to that of 234U/238U and showed open-system addition of 230Th and 234U during the aragonite to calcite recrystallization, that results in an isotopic compositional trend charting the timing of any alteration. Thus, the original age of reef terraces can be backtracked, revealing at Aqaba successively higher terraces formed recently and at 120, 142 and  190 ka. The oldest of the terraces seems to have been flooded with fresh water at the start of the Eemian interglacial (~140 ka), and may have been a source of springs that would have served the earliest human travellers well. It remains to use Lazar and Stein’s approach at other reef terraces along the postulated northern exit route for the earliest modern human emigrants from Africa and, more important, to find traces of their passage.

Added 21 December 2011. The likely route for leaving Africa got a push towards the Bab el Mandab with publication of evidence for a greener south Arabia at several times in the late Pleistocene (Rosenberg, T.M. and 8 others 2011. Humid periods in southern Arabia: Windows of opportunity for modern human dispersal. Geology, v. 39, p. 1115-1118). On the eastern edge of the now hyper-arid Rub al Khali are a series of former lakes with thin sediments. When first discovered they yielded radiocarbon ages of fossil molluscs of around 40 to 20 and 10.5 to 6 ka. However recent dating using optically stimulated luminescence (OSL) of the dune sands between which occur lacustrine muds and silts suggest that the lakes were water-filled  for lengthy periods  before those ages – radiocarbon dating can be reset to younger ages by precipitation of carbonates on older  fossils.  The OSL results show wet periods around 80, 100 and 125 ka, suggesting that around these times the Intertropical Convergence Zone was pulled northwards so taking seasonal monsoon rains well into the Arabian Peninsula. They tie in nicely with a variety of other parameters, including the timing of lowstands of the Red Sea. This created episodes a few thousand years long that would have been conducive to humans living there and passing through en route to Asia around eastern Arabia and perhaps to the Levant up the west side of the sub-continent. Potential occupancy was shut off by long arid periods, which might have allowed only pulses of migration. Had such episodic diffusion occurred it might have left a record in human DNA that ongoing and planned population genetic research may reveal.

South Asian arsenic update

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Skin lesions from arsenic poisoning in Bangladesh
The first signs of chronic arsenic poisoning: skin keratoses. Image by waterdotorg via Flickr

That groundwater in West Bengal, India was polluted with arsenic to such levels that symptoms of poisoning had become endemic was reported by Depankar Chakraborti in 1983, leading to his being branded a ‘panic monger’ by the Indian authorities. The news broke internationally in 1993 as the now infamous tragedy in neighbouring Bangladesh emerged. Means of mitigating the effects – lesions or keratoses and skin discoloration, and later increases in incidence of several forms of cancer – and ideas of how the pollution had occurred had to await proper geochemical analyses of well waters and logging of the mainly alluvial sediments from which water was being withdrawn; another 8 years went by. Reports of arsenicosis began to emerge from other areas of alluvial sediments in SE Asia, revealing by far the worst mass poisoning in history and the likelihood that the lives of millions would be blighted by what Bangladeshis dubbed ‘the Black  Rain’ from the resemblance of the characteristic skin lesions to drops of black water.

Thanks principally to the work of water engineer Peter Ravenscroft with other geochemists, the source of arsenic in groundwater was narrowed down to the effect of reducing conditions in grey, carbonaceous sandstones and peats on the mineral goethite, an iron oxy-hydroxide that forms the main colorant in oxidised sediments and whose loose structure normally encourages the mopping-up by surface adsorption of a wide spectrum of dissolved ions, including those of arsenic. Goethite readily breaks down under reducing conditions, and when that happens all the adsorbed material is released into solution. The upper parts of the alluvial and deltaic sediments in the lower reaches of the Ganges and Brahmaputra rivers contain abundant organic remains picked up when vegetation burgeoned during the Holocene, which mixed with goethite-coated sand grains derived from erosion in the Himalayan stretches of the rivers. Purely natural sedimentary and hydrogeological processes created the dreadful plight of villagers. The terrible irony was that before the 1980s there were no signs of arsenicosis, yet mortality, especially of under-fives, was very high due to water-borne pathogens in surface water supplies. Indian and Bangladeshi authorities and UN agencies waged a campaign to sink shallow wells for drinking water rather than relying on river and pond supplies. At first rural people resisted the change since they regarded water from wells as the ‘Devil’s water’, but as infant mortality began to fall, the resistance turned to rapid construction nationwide of wells, both public and private. A few years later came the ‘Black Rain’.

In the attempts to mitigate the arsenicosis plague, filters containing adsorptive materials, including goethite, were installed on pumps. However, the geochemists showed that in the deeper wells there were consistently low concentrations of arsenic in sediments that were brown-coloured due to prevailing oxidising conditions and the presence of goethite. Although arsenic was present in the sediments it was safely locked in the goethite coatings of sand grains. Steadily major public supplies were transferred to deep, high-yield wells. Alluvial and deltaic deposits are generally highly permeable, so it was feared that as the deeper wells were pumped arsenic-rich water from the reduced shallow sediments would replace the safe groundwater. Thankfully, it seems that is not likely to be a problem (Radloff, K.A. and 12 others 2011. Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand. Nature Geoscience, v. 4, p. 793-798). The study injected As-bearing groundwater into a deep aquifer and monitored its arsenic concentration over time, once in place. Within a day, the concentration of dissolved arsenic fell by 70% and by 5 days had fallen below recommended maximum levels for drinking water; a dramatic demonstration of the clean-up power of even minute films of goethite in sediments, for that seems the only explanation for the fall. The US-Bangladeshi team verified this by testing samples of the deeper sediments from drill cuttings. They mixed highly contaminated groundwater with the cuttings, to find that arsenic sorption over  about a week was extremely high (~40mg kg-1).

Water well in Bangladesh. From http://www.flickr.com/photos/waterdotorg/3696304044

Rather than just publishing their reassuring findings, the team input them to hydrogeological models of the Bengal Basin, varying hypothetical pumping rates to assess the changes in deep-groundwater chemistry over time due to downward migration of the highly polluted near-surface waters. Sure enough, the As-rich waters would end up in the deep aquifer eventually to overwhelm the sorptive capacity of its goethite content; arsenic would once again enter well supplies. However, if deep extraction was limited to drinking water by limiting pumping for irrigation to intermediate depths, safe limits could be sustained theoretically for a thousand years or more, except in some areas especially prone downward intrusion of polluted shallow groundwater. (Use of highly contaminated shallow groundwater for irrigation would simply transfer the problem to crops.) Clearly, monitoring is obligatory, but one hopes this important study does resolve the horrifying plight faced by so many people in catchments fed by Himalayan waters.

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