Earth-Pages

We hope that you enjoy this section of Earth Pages. It gives one geologist's personal views about some of the major stories that appeared in leading journals, such as Geology, Science and Nature, in the last month. The items have been written and compiled by Steve Drury author of a number of Earth science books, including Stepping Stones: The Making of our Home World (Oxford University Press, 1999) and Image Interpretation in Geology (3rd edition) (Taylor & Francis, and Blackwell Science (USA), 2001).

If you have comments on this month's items you can contact the author at s.a.drury@open.ac.uk

News for January 2010

Anthropology and geoarchaeology

Evidence for early journeys from Africa to Asia

A fragile consensus has developed concerning the date when fully modern humans left Africa then migrated to all habitable continents. It is based on genetic comparisons among living people, very sparse occurrences of H. sapiens remains that have been dated and on the environmental pressures in Africa to migrate during the highly erratic deterioration of climate since the last interglacial. The last included a series of abrupt cooling and drying episodes around 118, 110, 86, 75, 71 and 67 ka. That fully modern humans entered the Middle East from time to time between 130 and 75 ka is backed up by actual fossils, but most palaeoanthropologists believe that they moved no further, because of the growth of surrounding deserts, and probably did not return until around 45 ka. The consensus for the decisive move out of Africa to Eurasia is that it was via the Straits of Bab el Mandab at the entrance to the Red Sea, when sea level fell to a level that would have allowed a crossing by rafting over narrow seaways. The most likely was during the brief 67 ka cool/dry episode that coincided with an 80 m fall in global sea level: the largest since the previous glacial maximum. This would fit the earliest dates of fully modern human remains in Asia and Australasia. There had been falls of more than 50 m around 110, 86 and 75 ka, each followed by rising sea level. Each of them accompanied by cooling and drying in Africa conceivably could have allowed earlier migrations from Africa to southern Arabia. Emerging data seems set to complicate matters.

At a conference in Gibraltar during September 2009 (Balter, M. 2009. New work may complicate history of Neandertals and H. sapiens. Science, v. 326, p. 224-225) there were further reports of stone tools, which apparently resemble those of a similar age from Africa, beneath the 74 ka Toba ash in South India, and dated between 70 to 80 ka old in the Yemen and United Arab Emirates. Even more challenging are reports of archaic H. sapiens teeth and a jawbone with a chin – a sure sign of a fully modern human – from cave sediments in southern China that yield a date of about 110 ka (Stone, R. 2009. Signs of early Homo sapiens in China. Science, v. 326, p. 655). Given an opportunity and a need humans do tend to move in order to survive, a proclivity that would undoubtedly be boosted by our insatiable curiosity: after all H. erectus, antecessor and neanderthalensis all made tremendous migrations starting more than 1.6 Ma ago.

Fungal clue to fate of North American megafauna

More than 30 large mammal species, including elephants and giant sloths, that had roamed North America during the Pleistocene met their end between 13 and 11.5 ka. Whether or not predation by newly arrived humans caused these extinctions remains unresolved, as do the triggers for coinciding changes in plant communities and evidence for increased burning of biomass. While the ages of fossil bones are direct evidence for species being present, they are not found everywhere that a megafauna likely lived and occurrences are patchy in time. There is however a proxy for the presence or absence of large herbivores: spores of fungus that thrived on their dung (Gill, J.L. et al. 2009. Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America. Science, v. 326, p. 1100-1103). Sporormiella can only complete its life cycle after herbivores have digested plant matter. So its spores in sediment cores form an impressive link to the local presence of herds. In a lake core from New York State such fungal spores, having been much more abundant beforehand, fell to less than 2% of all spores and pollen about 13.7 thousand years ago. This suggests that large herbivores vanished from this area at that time.

Interestingly, the timing is during a warm period (the Bølling-Allerød) rather than the stress of the Younger Dryas glacial re-advance. Moreover, the local disappearance predates the first signs of Clovis people, although there is evidence for earlier human colonisers back to 15 ka. It is possible that it was the disappearance of large herbivores that allowed the development of extensive mixed coniferous-deciduous woodland, broad-leaved trees having perhaps been browsed severely by earlier herbivores.

Climate change and palaeoclimatology

Climate-CO2 links since the Miocene

The November 2009 issue of EPN (Boron isotopes and climate change) described how the ¹¹B/¹⁰B ratios of planktonic forams correlate with the pH of seawater, and thus with the amount of dissolved CO₂ that increases acidity. In fact the more easily analysed ratio between the boron and calcium contents of forams does the same, and for the last 800 ka correlates with the measured CO₂ content of bubbles in Antarctic ice, which itself correlates very well with temperatures and sea levels (Tripati, A.K. et al. 2009. Coupling of CO₂ and ice sheet stability over major climate transitions of the last 20 million years. Science, v. 326, p. 1394-1397). Extending this approach back to 20 Ma shows that in the Middle Miocene (~10 Ma) when glacial cover began to expand atmospheric CO₂ fell from levels similar to those of the present day (387 ppm) to approximately those of the pre-industrial Holocene (~250 ppm). In the earlier Miocene from 14 to 20 Ma global mean surface temperatures were 3-6º C higher and sea level stood 40 m higher than at present. As well as this grim reminder of a possible future, the data support the general notion of a coupling between atmospheric CO₂ and global climate.

Was the Archaean blazing hot or balmy?

Silica-rich sediments, notably cherts have been used to estimate ocean temperatures in the far off Archaean Eon. This is possible because SiO2 and water exchange oxygen atoms as the silica mud is forming, and in doing so its two main stable isotopes (¹⁸O and ¹⁶O) are preferentially treated depending on water temperature. The cooler it is the more ¹⁸O ends up in silica. Early Archaean cherts commonly show lower δ¹⁸O values than silica-rich ocean sediments forming now, so much lower that the temperature of Palaeoarchaean seas has been judged to have been between 55 to 85º C. Discomfortingly hot for bathers, and not very plausible considering that without a CO₂-rich atmosphere Archaean oceans would have been frozen solid because the Sun emitted much less energy than it does now. However, such estimates have to assume that the oxygen isotopic composition of seawater at 3.5 Ga was the same as now, when in fact it is known that environmental δ¹⁸O probably changes over long time periods. A way of avoiding an untestable assumption is to measure the isotopic composition of hydrogen (¹H and ²H or D) in chert as well as that of oxygen. The cooler water is, the lower δD values are in silica that is precipitated from it. Ordinary quartz contains no hydrogen except in unstable fluid inclusions, but the way chert forms as colloidal precipitates of opal-like material locks hydrogen in the form of OH- ions into its silica (Hren, M.T. et al. 2009. Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago. Nature, v. 462, p. 205-208). Combining the two measures for 3.42 Ga cherts from the famous Barberton Mountain Land Archaean complex results in a sea-surface temperature estimate of no more than 40ºC.

Environmental geology and geohazards

Mid-continent earthquakes: warnings or memories?

Perhaps the most infamously unexpected earthquake was that of 17 December 1811 that shook the historically quiescent middle Mississippi valley with an estimated magnitude of 7 on the Richter scale. The area centred on New Madrid has been resonating with seismic events of lesser magnitude ever since. So too has the area around Charleston, South Carolina on the passive Atlantic margin of the USA, which experienced a magnitude 7 earthquake in 1886. Geophysicists now know to expect major earthquakes at some time in some place along active plate margins, especially subduction zones and boundaries dominated by strike slip motion, although prediction is an art to be learned if indeed it will ever be possible. Yet even small tremors far from plate boundaries within continental parts of plates are a continual worry. The shock of totally unexpected devastation in New Madrid and Charleston makes seismic-risk assessors mark the card of any such events, especially if repeated. Ideally, plate interiors should be rigid and safe. The magnitude 7.9 Sichuan event in May 2008, which caused more than 80 thousand deaths along a fault with no history of activity, reinforced worry. All three examples were situated in areas with old faults, of which most areas of continental crust have plenty, though some are hidden. Somehow tectonic forces had built up and eventually they failed.

Protracted activity might seem to foretell more big ‘quakes. However, it now appears that faults in continental interiors behave very differently from those at plate boundaries: aftershocks, even some with magnitude 6, continue for centuries in the first case, but only for a few years or decades at tectonically active margins (Stein, S. & Liu, M. 2009. Long aftershock sequences within continents and implications for earthquake hazard assessment. Nature, v. 462, p. 87-89). The duration of aftershocks in inversely related to the tectonic load sustained by faults. A lesson suggested is that assigning high risk to continental areas with repeated seismicity overestimates the dangers. But does this mean those seismically stable areas in continental interiors pose underestimated risks? The answer is probably ‘Yes’, if they are near to old faults. That is not to say that the Caledonian and Variscan structures that divide Britain into many small blocks are about to ‘go off’ at any time. Some do generate small, noticeable tremors such as that beneath Market Weighton in east Yorkshire at 1 am on 27 February 2008 that woke people up to several hundred kilometres away (including me). Market Weighton was an area of reduced subsidence during Jurassic sedimentation, as a result of flanking Variscan faults in the crust beneath. However, if large structures – high-rise buildings, bridges, dams and power stations – are planned, it would be wise to look in detail at local faults. One approach is to map disturbance of superficial sediments that in Britain would show activity over the last 18 to 11 thousand years since ice sheets melted. Another is to check bedrock geology for the last major movements on faults. It may become possible to develop models of seismic cyclicity for all large structures to give realistic assessments of risk in the future.

See also: Parsons, T. 2009. Lasting earthquake legacy. Nature, v. 462, p. 42-43.

Geochemistry, mineralogy, petrology and volcanology

Geochemical clue to environmental effects of large igneous provinces

Several flood volcanism events seem to link to mass extinctions, and they have been seen as the culprits for global environmental change. Since flood volcanism is outside human experience, geologists have little conception of what they do other than amass up to millions of cubic kilometres of lavas both mafic and silicic. They all probably emitted CO₂ and contributed to global warming, but whether they are able to deliver sulfate and particulate aerosols to the stratosphere to trigger cooling is hard to judge. But it seems there is a proxy for their global influence (Peate, D. 2009. Global dispersal of Pb by large-volume silicic eruptions in the Paraná-Etendeka large igneous province. Geology, v. 37, p. 1071-1074). Lead is potentially a volatile element that would accompany large volcanic gas and dust emissions, and it also bears unique isotopic signatures. Lead isotope proportions in sediments in contemporaneous marine sediments could be matched with those of large igneous provinces (LIPs). Should their signature occur globally, then it would be a fair bet that the products of volcanism did reach cloud-free stratospheric altitudes, there to be mixed globally and to remain aloft for many years. Below the tropopause gas and dust would soon be rained out, so that signatures would remain local.

Dave Peate of the University of Iowa found that the ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁶Pb ratios of 132 Ma sediments from an Ocean Drilling Program core in the mid-Pacific fall in the same field as those of the Paraná-Etendeka large igneous province. The sediments occur just below and within a prominent d¹³C anomaly that geochemists believe to signify a major change in the biosphere, and the site is almost at the antipode of the Paraná-Etendeka large igneous province. Sediments from below the shift in carbon isotopes show lead-isotope ratios that can be explained by derivation from the oceanic crust underlying them, whereas those that witness a profound change in the biosphere overlap with the field of the P-E LIP. Specifically, they match the lead ‘signature’ of silicic volcanics rather than basalts, and in particular those with low titanium contents. So it seems that in this case basalt floods may not have been implicated in global environmental change, but the much less voluminous but probably far more violent ignimbrite do seem likely culprits. There were more than 20 such events within an interval of less than 2 Ma that emitted >100 km³ of silicic magma, most exceeding 1000 km³.

Planetary, extraterrestrial geology, and meteoritics

Late formation of Earth’s atmosphere

Because the Earth’s mantle is rich in volatiles which escape from magmas that reach the surface, it has long been assumed that our planet’s atmosphere was self-produced by exhalation. But it turns out that noble gases in such exhalations do not match those in the atmosphere isotopically (Holland, G. et al. 2009. Meteorite Kr in Earth’s mantle suggests a late accretionary source for the atmosphere. Science, v. 326, p. 1522-1525). Greg Holland and colleagues from the Universities of Manchester and Houston measured krypton and xenon isotopes in volcanic CO₂ emissions from New Mexico, and found that their proportions matched those in carbonaceous chondrites as does the Kr/Xe ratio. Those in the atmosphere are significantly different, resembling the values in the Sun. Comets may have delivered these gases after the original accretion of the Earth and the catastrophic formation of the Moon.

Sedimentology and stratigraphy

The ‘real’ Flood

At the end of the Miocene tectonic uplift in the region of the present Straits of Gibraltar cut the Mediterranean Sea off from the Atlantic. The only water able to flow into the isolated marine basin was that carried by the major rivers: the Rhône, Danube, Dneiper and Nile. Their volume was exceeded by evaporation, so the Mediterranean became more and more salty, eventually almost drying out completely to leave thick evaporite deposits that still underlie its deepest parts. 5.33 Ma ago, the tectonic barrier was breached so that Atlantic water flooded the whole Mediterranean basin. The Zanclean flood at the start of the Pliocene has been rated as the greatest catastrophic event in the Phanerozoic history of the oceans, but just how dramatic it was has previously only been guessed at. Seismic profiles across and along the line of flooding reveal channels several kilometres across, about 200 km long and up to 250 m deep, now filled with debris (Garcia-Castellanos, D. et al. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, v. 462, p. 778-781). Using a well-established model of river incision in mountain rivers, the authors have suggested how the flooding proceeded. From an initial trickle when the original barrier subsided below Atlantic sea level, flow grew exponentially over a few thousand years to about three times that of the modern Amazon discharge (~10⁸ m ^s-1), at which rate incision reached more than 0.4 m per day. Around 90% of the Mediterranean basin’s entire volume was flooded in a matter of a few months to two years, sea level rising at up to 10 m per day.

Formation of BIFs halted by Sudbury impact

The peculiar story of banded iron formations (BIFs) is one that ‘runs and runs’, as journalists say. Most of the steel on which North American capital was built comes from gigantic BIF deposits around Lake Superior that formed during the Palaeoproterozoic. Apart from a brief return in the Neoproterozoic, associated with conditions peculiar to ‘Snowball Earth’ conditions, the Superior Province BIFs are the last of any consequence. Most geologists look to a gradual shift in the oxygen content of ocean water as photosynthetic life grew to dominate the Earth after about 2.4 Ga, but the BIFs around Lake Superior turn out to be capped by a blanket of ejecta from a massive extraterrestrial impact that formed the Sudbury Complex (Slack, J.F. & Cannon, W.F. 2009. Extraterrestrial demise of banded iron formations 1.85 billion years ago. Geology, v. 37, p. 1011-1014). But how could even a monstrous bolide have changed ocean chemistry so decisively? John Slack and William Cannon of the US Geological Survey believe that the impact was so violent that it resulted in wholesale mixing of oxygen-bearing surface waters with those of the deep ocean. The evidence they cite is a coincident change in the nature of deep-water hydrothermal deposits from sulfide-bearing to those dominated by iron-oxides.

The Sudbury impact produced a crater around 150 to 270 km across (one of the three largest known on Earth), and it is dominated by remelted basaltic rocks so almost certainly struck the Palaeoproterozoic ocean floor. Its ejected debris probably covered almost 2 million km2 and is found up to 800 km from Sudbury, Ontario. Yet, even with impact cavitation and massive tsunamis it seems barely feasible that an impact of a size dwarfed by those of the Lunar surface could completely remix the oceans. However, it is likely that in the Palaeoproterozoic continental crust was gathered together in a supercontinent so that tsunamis could scour much of the surrounding ocean. A plume of vaporised seawater may also have scavenged oxygen from the atmosphere. The evidence seems compelling, and another possibility is that Sudbury was not the only impact site...

And another oddity...

That a major climatic warming occurred at the end of the Palaeocene (55 Ma) is now undoubted, as is its probable cause by emission from the ocean floor of vast amounts of methane. Yet oddly the Palaeocene-Eocene Thermal Maximum (PETM) coincides with a brief geomagnetic reversal 53 ka long (Lee, Y.S. & Kodama, K. 2009. A possible link between the geomagnetic field and catastrophic climate at the Paleocene-Eocene thermal maximum. Geology, v. 37, p. 1047-1050). Both events were short, so a coincidence seems unlikely, in the authors’ opinion. They suggest a connection through the massive power imparted to climatic processes by the PETM (at least a terawatt and perhaps orders of magnitude more), including the deep thermohaline circulation of the oceans that did shift during the event. Had they exceeded a threshold power for circulation of the liquid outer core they may have triggered the brief reversal, which quickly reverted to its previous magnetic polarity. Ths association is not unique, detailed magnetic studies of the K-T boundary event at 65 Ma has revealed a similar short reversal spanning the duration of the iridium peak ascribed to the Chicxulub impact. However, Chicxulub delivered a power of the order a year’s solar radiation in about one second: vastly larger than the climate perturbation of the PETM. Are we seeing here a hidden signal of an extraterrestrial impact behind the methane release? Impacts are no longer as popular as they once were...

Tectonics

Dating subduction

The most distinctive products of the high-pressure, low-temperature metamorphism along subduction zones are stunningly coloured blueschists formed from ocean-floor basalts, their colour deriving from the sodium-rich amphibole glaucophane. Yet the defining mineral for subduction-zone metamorphism is lawsonite, which takes up the calcium from plagioclase feldspar that becomes unstable. Having formed at depths of up to 100 km, blueschists found at the surface had to rise slowly from mantle depths after metamorphism. Consequently, it is nearly impossible to unravel the date of their formation from those of later events. Being basaltic, blueschists also lack the usual elements whose unstable isotopes are commonly used for radiometric dating: potassium, rubidium, uranium and thorium. However, they do contain rare-earth elements, an isotope of one (¹⁷⁶Lu) being unstable. Applying the Lu-Hf dating method to lawsonite ties down precisely when basalts achieved the narrow P-T range at which lawsonite forms (Mulcahy, S.R. et al. 2009. Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes. Geology, v. 37, p. 987-990). Sean Mulcahy of the Unigversity of Nevada and colleagues from Washington State chose a sample from the type locality for lawsonite discovered in the late 19th century by Andrew Lawson: the Franciscan blueschists of the Tiburon Peninsula in California. The Franciscan Complex formed during subduction at 145.5 Ma.

Phew, there is a mantle plume under Hawaii after all

Along with constructive and destructive plate boundaries volcanic hotspots within plates and sometimes at plate boundaries epitomise modern Earth science. Assuming that they are fixed points of reference allows the absolute motions of tectonic plates to be worked out, although it seems that some do move around. The evidence for hotspots being fixed or at least moving much more slowly than do plates are the chains of extinct volcanic islands or seamounts that extend away from active volcanic centres in the direction of plate motion. The most debated aspect of hotspots is whether they stem from processes in the upper mantle just beneath the asthenosphere or are the heads of cylindrical plumes of hot mantle that rise from the region next to the outer core. Seismic tomography has been claimed capable of resolving between the two possibilities, but its spatial resolution depends very much on the spacing of seismometers that provide the data that tomography subjects to highly complex processing. Some have claimed that the resolution of early tomography lends itself to producing artefacts that look like sought-after mantle structures (see Geoscience consensus challenged in EPN of December 2003).

One hotspot that has all the characteristics of a plume head, but which seismic tomography has been unable to detect is the volcanically active Big Island of the Hawaiian chain. The response to that somewhat embarrassing failure has been to deploy 30-odd seismometers on the seabed immediately around Hawaii and then to shift them to a wider spacing further from the island between 2005 to 2007. Together with 10 stations on the islands themselves, the array recorded 2146 S-wave arrivals from 97 earthquakes (Wolfe, C.J. et al. 2009. Mantle shear-wave velocity structure beneath the Hawaiian hot spot. Science, v. 326, p. 1388-1390). The results are reassuring, for the show in detail that indeed there is a vertical zone of low S-wave speeds indicating hotter, less rigid mantle that extends down to at least 1200 km. It is several hundred kilometres across, and is indeed a plume surrounded by a ‘tube’ of colder more rigid mantle.

See also: Kerr, R.A. 2009. Sea-floor study gives plumes from the deep mantle a boost. Science, v. 326, p. 1330.

Hot tectonics in the Archaean

The first thing that strikes you when looking at a small-scale geological maps of many deformed Archaean terrains – most of them are deformed – is how different they seem compared with those of later aeons. Bulbous granitic plutons separate slim and irregular, sometimes cusp-adorned areas of volcanic and sedimentary rocks. This is classic granite-greenstone terrain. Many geologists who have worked on Archaean rocks find it hard to pin down signs of ‘modern’ plate tectonics and the typical orogens of continent-continent collision zones, yet non-uniformitarian ideas on Archaean tectonics have become passé in the last 25-30 years. That seems odd, considering that the Earth’s internal heat production by radioactive decay must have been higher as less radioactive U, Th and K isotopes would have decayed in the very distant past. Convective mantle flow would have been faster, lithosphere would not have been so thick as now, and plates would have moved more rapidly in order that radioactive heat and that left over from early accretion and the Moon-forming event could escape. Whichever way one looks at such a scenario – plates as big as modern ones or more small plates – there is no escaping that younger, warmer lithosphere would have re-entered the mantle. Geochemistry of Archaean granitic rocks is so different from those of later aeons that their formative processes must have differed too. Quite probably descending basaltic crust would not have dehydrated to produce eclogite under low-T, high-P conditions, and that would prevent steep subduction, so that slab-pull may not have been the driving force for Archaean tectonics.

Two recent papers refresh the idea that the present is not entirely a key to the Earth’s Archaean past. One suggests an entirely alien kind of orogenic activity: that of very hot deformation of weak lithosphere (Chardon, D. et al. 2009. Flow of ultra-hot orogens: A view from the Precambrian, clues for the Phanerozoic. Tectonophysics, v. 477, p. 105-108). Dominique Chardon of the Université de Toulouse and colleagues from the Université de Rennes, highlight the dominance in orogens of the Archaean and early Proterozoic of ductile deformation imposed on massive accretion of magma produced by mantle processes, compared with the dominantly brittle style that dominates modern, cold orogens. Accumulated radiometric dating of the main building material of the continents – diorites and grandiorites – indicates that the 1.5 Ga of the Archaean witnessed the formation of not only the earliest continental crust but most (65%) of the rest of it. A summary of an emerging explanation for explosive continent production appeared in the first 2010 issue of Scientific American (Simpson, S. 2009. Violent origins of continents. Scientific American v. 302(1), p. 46-53). This rests on rapidly growing evidence, much unearthed by Andrew Glikson of the Australian National University, for the influence of major impacts that flung debris far and wide and perturbed the mantle’s thermal structure on a massive scale (Glikson, A. 2008. Field evidence for Eros-scale asteroids and impact forcing of Precambrian geodynamic episodes, Kaapvaal (south Africa) and Pilbara (Western Australia) cratons. Earth and Planetary Science Letters, v. 267, p. 558-570). Beds of impact-related spherules are turning up throughout Archaean greenstone-belt sequences. There are also megabreccias that could be debris lifted by tsunamis vcaused by impacts in the Archaean oceans. Glikson has demonstrated that the timing of such evidence closely matches that of magmatic outbursts that created continental crust. He has proposed that the thermal effects of the large impacts set in motion or deflected a large number of convective mantle plumes that drove the necessary magmatism.