Earth Pages News of June 2005 reported on the development by the US Geological Survey of the first daily seismic forecasting service, which covers California. It has a web site at http://pasadena.wr.usgs.gov/step. The forecast is for events, generally aftershocks of earlier earthquakes, with sufficient energy to throw objects off shelves (Modified Mercalli Index VI). On June 30 2005, Lake Tahoe had a chance around 1 in 100 of such a tremblor, with the length of the San Andreas and related fault systems highlighted at between 1 in 10 000 to 1000. Of course, it will take some time before people link as quickly as they do to the weather forecast.
Stay of execution for Quaternary
The last remaining division of geological time that Giovanni Arduino erected in the mid- to late 18th century, has been under threat for some time (see EPN of September 2004). For over seven years, the ‘Time Lords’ of the International Commission of Stratigraphy have striven to resolve, at least for a while, al the fundamental divisions of stratigraphic nomenclature. To the horror of researchers concerned with the last 2 million years or so, publication of the new time scale in 2004 seemed to have allowed the Neogene to swallow the Quaternary Period whole. Muttering broke into a storm of angry e-mails demanding its restoration.
The reason behind the annoyance is simple. The Quaternary is unique for two reasons: it includes the Great Ice Age, and it is the time of humanity – the first stone tools appear in the geological record between 2.4 and 2.6 Ma ago. But those who demand the resurrection of the old name are not entirely in agreement among themselves, particularly about when it started. The problem arose from the manner in which systematisation of both relative and radiometric time evolved. Arduino recognised four divisions only, Primary, Secondary, Tertiary and Quaternary based on decreasing compactness and complexity of rocks that he had seen in Italy. The Quaternary was defined as unconsolidated material that sat upon the other three. As fossils became the main tools of establishing relative time and wide correlation, Primary and Secondary were soon dropped. But Tertiary and Quaternary remained as broad divisions until the late 20th century. Tertiary strata became divided into 5 lesser palaeontological divisions, and Quaternary into two: Pleistocene and Holocene. Radiometric dating demonstrated the brevity of the Tertiary compared with major stratigraphic divisions further back in time, so it was designated as a Period, subdivided into 5 epochs. Tertiary itself then became elevated to Era status as the Cenozoic, despite its short time span, and its first three and last two epochs were bracketed by two new periods: Palaeogene and Neogene. Development of geosciences was clearly marginalizing the Quaternary Period to which many devotees cling tenaciously.
The furore burst at the 32nd International Geological Congress in Florence in August 2004, and the ICS was duly chastened and apologetic. It set up a task force to reunite the warring forces, or at least to draw plans for a truce. The task force voted in early June 2005 to retain the name Quaternary and to set its beginning at 2.6 Ma, thereby defining it as both the Great Ice Age and that of humankind. Ironically, 2.6 Ma also marks the start of the Late Pliocene, defined by a Global Boundary Stratotype Sections and Point (the midpoint of sapropelic Nicola Bed (“A5”), Monte San Nicola, Gela, Sicily, Italy). You see, there has to be somewhere that you can visit and ‘put your finger on the proper boundary’. This particular GSSP is defined as a stage in the fluctuation of oxygen isotopes in deep-sea sediments, at the start of the Matuyama geomagnetic reversal, and just below the points of extinction of two echinoid species….. Incidentally, the ICS is by far the largest of the bodies within the International Union of Geological Sciences, the ‘UN’ of the geoscience community. Acquiring the prestige of a GSSP ranks with many countries’ geoscientists at least as high as hosting an Olympic Games. Italy hosts 9 of the 22 Cenozoic GSSPs (5 are not yet placed), so clearly Arduino’s influence has been long lasting in some respects. Several features of the New Timescale as a whole may confuse far into the future (should it stand the test of time). The Stage names, learned by generations of stratigraphers, often through cunning mnemonics, are mainly taken from places or regions. Most of the GSSPs at their bases are somewhere else (browse http://www.stratigraphy.org/).
Source: Giles, J. 2005. Geologists call time on dating dispute. Nature, v. 435, p. 865.
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Hydrogen sulfide and mass extinction
Naughty school kids once used to hurl glass vials that launched the most pervading smell of rotten eggs when they smashed. Stink bombs produce hydrogen sulfide. Interestingly, if you can smell it you are more or less safe – though not from flying glass shards. When H2S is more concentrated, it becomes an odourless and stealthy killer, as ‘sour gas’ emitted from oil drilling rigs. A group of anaerobic bacteria generate the gas when there are abundant sulfate ions in oxygen-starved conditions. They use these ions as electron acceptors in their metabolism, thereby reducing sulfate to sulfide ions; a common phenomenon in stagnant swamps, and especially prevalent at depth in the Black Sea.
Several times during the Phanerozoic global ocean depths became anoxic, when thermohaline circulation shut down. The consequences show up in black mudrocks, rich in partially broken down hydrocarbons and iron sulfide. Some of these are major source rocks for petroleum. Unstirred by deep current flow, bottom waters pervaded by H2S are covered by oxygenated water, so it might seem that there is little threat to surface dwellers and air breathers, although any animal unwarily entering toxic bottom water would instantly die. That is why black mudrocks are repositories of exquisite fossils. Should H2S build up in deep water, however, there might be chemical instability that would result in large-scale emissions to the upper ocean and to the atmosphere. Geochemists from the universities of Pennsylvania and Colorado have made some simple chemical calculations to see if such a potentially catastrophic leakage is within the bounds of possibility (Kump, L.R. et al. 2005. Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia. Geology, v. 33, p. 397-400). Theoretically it is, once a threshold concentration of around 1 mmol kg-1 of H2S dissolved in deep water is exceeded. There would be sulfidic upwellings involving emissions of the order of teratonnes of sulfide per year to the atmosphere; more than 2000 times that today from volcanoes, with the added risk that it would also permeate upper-ocean water.
As well as witnessing mass extinctions, the Late Devonian, end-Permian and Middle Cretaceous were characterized by widespread anoxia. Leakage of H2S would not only have killed directly, but would have destroyed the ozone layer that protects from UV radiation. Inevitably, methane produced by other anaerobic bacteria would also have been released in the same way to force global warming. Rather than being the result of dramatic impacts or monstrous flood basalt effusions, mass extinctions at these times would have been quiet, but efficient nonetheless
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Potted history of atmospheric oxygen
Potted history of atmospheric oxygen
The most likely hallmark of an inhabited planet is an atmosphere that contains oxygen; a simple rule of thumb made popular by James Lovelock. By assembling complex molecules based on carbon, life increases the degree of chemical reduction in its environment. Effectively it draws in electrons, and the counterpart of that must be that some other component loses them through oxidation. On Earth the source of electrons needed to make organic molecules through the action of photosynthesis is predominantly the oxygen atoms locked in molecules of water and carbon dioxide. By losing 4 electrons, 2 oxygens bonded in those two simple compounds are oxidised to become the gas O2, which itself has become the commonest and most active acceptor of electrons from reduced ions and compounds. Oxygen gives its name to oxidation, which is the inevitable fate of most organisms, thereby reversing the process of photosynthesis. A planet whose surface topography is continually changing, because more radioactive energy is produced in its mantle than can be lost to space by simple conduction, generates physical conditions that continually bury and store some unoxidised carbon compounds. Carbon burial together with continued living processes keeps the photosynthetic chemical equation weighted in favour of free oxygen.
Since the domain of living things to which we and all advanced organisms belong, the Eukarya, is almost wholly one to which oxygen is vital in metabolism, there can be few more important geoscientific topics than how and when oxygen emerged as a free element. There have been major recent developments in addressing these questions, so it is useful and fascinating to find an up-to-date and easily read review (Kerr, R.A. 2005. The story of O2. Science, v. 308, p. 1730-1732). Among its highlights is evidence that although cyanobacteria (the most primitive oxygenic photosynthesisers) were definitely around at 2.7 Ga, they may not have produced oxygen until about 300 Ma later, when the first signs of free environmental oxygen appear. Photosynthetic release of oxygen during life’s early period was not the only reduction-oxidation regime adopted by organisms. Another of huge importance was generation of methane, which can rise to the limits of the atmosphere unlike the other major hydrogen-bearing gas, water, which is condensed out at quite low altitudes. Photochemical breakdown of methane at the limits of outer space would release hydrogen to leak away from the Earth, removing a reductant gas that would otherwise consume highly reactive oxygen: without this process, modelling suggests that Earth’s atmosphere would never have accumulated free oxygen, even had primitive life emerged.
Once free oxygen appeared, about 2.4 Ga ago, it took almost 2 billion years for enough to accumulate so that complicated, multicelled Eukarya could use its potential (see The Malnourished Earth hypothesis – evolutionary stasis in the mid-Proterozoic in EPN of September 2002). What kept the levels down? Quite probably it was oxidation of sulfide minerals on exposed land. That supplied sulfate ions to a still reducing ocean, so that sulfide ions formed again to become metal sulfide precipitates, which drew from ocean water several essential nutrients for Eukarya. Oxygen-producing Eukarya (algae) would not be able to bloom because of this ‘starvation’. Nonetheless, about 600 Ma ago, surface oxidation potential soared to almost modern levels, sufficient for large organisms to appear and evolve, to lead to life as we know it. Another series of questions surrounds this tremendous event, but they remain to be answered convincingly.
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Another view of causes for the Younger Dryas cooling event
High latitudes in the North Atlantic, especially on its eastern side, are warmed today by the Gulf Stream. That current, which defies the Coriolis effect, is pulled northwards by the sinking of cold dense sea water between Greenland, Iceland and Scandinavia to form North Atlantic Deep Water (NADW). The thermohaline circulation here is driven by both cooling of salty surface water in the Gulf Stream and further salinisation as sea ice forms in this area each winter. The Younger Dryas cold period between 13 and 11.5 ka is regarded by most oceanographers and climatologists to have resulted from sudden freshening of the North Atlantic at these critical high latitudes, so that surface water density became too low to sink. Such a process had occurred several times during the last glacial period, each of which has been correlated with release of massive amounts of glacial ice as icebergs. There melting caused the freshening. The Younger Dryas is a different kind of event, because it occurred well into the period of global warming that brought the Ice Age to an end. A seemingly plausible explanation was suggested by Wallace Broecker in 1989, who looked to explosive release of meltwater trapped in glacial lakes roughly along the Canadian-US border along the present St Lawrence River Valley, effectively flooding the source of NADW with a surface layer of low-density, low-salinity water.
The problem with Broecker’s mechanism is that sea-level records through the Younger Dryas show no sudden rise, whereas at about 14 ka a meltwater pulse had resulted in a 20 m rise over about 500 years, with no sign of a climatic response to a shutdown of the Gulf Stream by the freshening that it caused. A similar event occurred shortly after the waning of the Younger Dryas. There is no doubt that throughout high northern latitudes the great ice sheets were melting since about 18 ka. A new approach to the Younger Dryas concentrates on where the meltwater formed in northern North America probably escaped to the sea (Tarasov, L & Peltier, W.R. 2005. Arctic freshwater forcing of the Youner Dryas cold reversal. Nature, v. 435, p. 62-665). Through their analysis of the drainage chronology of the Canadian Shield Tarasov and Pelter conclude that at the time of the onset of the Younger Dryas most flow was roughly along the present MacKenzie River valley to the Arctic Ocean. Freshening of the Arctic Ocean would escape through the narrow Fram Straits directly to the source region for NADW. It would not necessarily have been through currents, for escape of increased amounts of pack ice would have much the same effect. Central to their hypothesis are new data that relate to extraordinarily thick continental ice in the Keewatin glacial dome, that formed just to the east of modern Great Slave Lake.
Acidification of the oceans
When gases such as CO2 and H2S permeate through ocean water they dissolve to form weak acids: carbonic and sulfurous acid respectively. So many organisms, plants as well as animals, incorporate carbonates into their hard parts that changes in acidity constitute an important kind of stress. The acidity of water combines with increasing pressure as water deepens to create a zone (the lysocline) in which water is undersaturated in calcium carbonate. Below the lysocline carbonate shells begin to dissolve. Deeper still is a level (the carbonate compensation depth, or CCD) below which there is no free CaCO3 in the water column. Falling shelly material dissolves completely, so that deep-ocean sediments contain few if any shells other than those of silica-secreting organisms. At present the CCD is around 4 km deep. Any shift in the pH of the oceans causes the CCD either to rise or fall. The signatures of such shifts lie in the composition of ocean-floor sediments. In the deepest parts, where silica and clays dominate, layers in which carbonate shells are preserved signify a decrease in acidity (increased pH) and descent of the CCD to below the elevation of the ocean floor. On the other hand, the appearance of pure clay-silica oozes in otherwise shelly muds, where the sea floor has been well above the CCD for long periods, show that acidity increased (a drop in pH) over a period. Such anomalous sediment layers are often easy to see in cores because their colour is different from the common sediments.
In cores from ocean depths between 2 and 4 km, the second kind of anomaly appears consistently at the level of the Palaeocene-Eocene boundary: it signifies a massive increase in acidity (Zachos, J.C. et al. 2005. Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum. Science, v. 308, p. 1611-1614). Carbon-isotope measurements from the same cores also show a marked shift. The sediments are depleted in 13C, which has generally been taken to indicate a huge release of methane from storage as gas hydrate in sea-floor sediment at the time of the Palaeocene-Eocene boundary. Most palaeoclimatologists consider the C-isotope “spike” to be a proxy for sudden, intense warming that resulted from methane – a more efficient ‘greenhouse’ gas than CO2 – and the carbon dioxide produced as it was oxidized. The range of water depths where the carbonate-free layers occur enables marine geochemists to estimate the rate of acidification. In around only 10 ka the CCD rose 1.3 to 2.0 km above its current level. From the degree of acidification needed it seems that considerably more than 2 x 1012 t of carbon was released in the form of methane that eventually oxidized to CO2, and returned to the ocean. The carbonate content of the ocean sediments rose gradually over the next 100 ka, by the end of which the former balance was restored. This information in turn gives a picture of the rate at which sudden ‘greenhouse’ events subside once their cause has stopped being produced, almost certainly by the drawdown of atmospheric CO2 by weathering of silicate minerals exposed on the continental surface.
At the end of the Palaeocene, the effect on organisms was mainly restricted to benthic foraminifera that live in moderately deep water, which show a selective extinction. The eventual release by human activity of carbon contained in accessible fossil fuel reserves, will give a mass of carbon in ‘greenhouse’ gases of about twice that released at the Palaeocene-Eocene boundary over perhaps 300 years. Such rapid release may result in acidity that is incompatible with carbonate-secreting organisms anywhere in the oceans: the CCD will effectively be at the sea surface
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The earliest lichens
Lichens are not individual species, although they are given Linnaean names, but symbiotic associations of two or more species. In the lichens the mutual relationship is between entirely different organisms: fungi with either algae or blue-green bacteria. Although lichen form one of the plagues set to try geologists, their fossil record is extremely sparse. Once again, Chinese lagerstätten in the Doushantuo Formation establish a first, in this case preserved in phosphorites (Yuan, X. et al. 2005. Lichen-like symbiosis 600 million years ago. Science, v. 308, p. 1017-1020). The fossils show exquisite detail, sufficient to reveal both fungus-like hyphae and cells that resemble those of cyanobacteria. They are from the late Neoproterozoic, Ediacaran period, when all manner of evolutionary developments were taking place. One question that is unanswered is whether or not these fossils were marine or subaerial. Modern lichens are intolerant of salt water.
Methuselah
Since the 1960s claims have been made for the oldest living organism being found in brine inclusions from salt deposits, and most have been dismissed as modern contaminants. In 2000 that easy avoidance was ruled out by super-sterile culturing of the contents of a fluid inclusion in a Permian halite crystal from New Mexico (Vreeland, R.H. et al. 2000. Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal. Nature, v. 407, p. 897-900). The research produced a culture of a salt-tolerant bacterium that was dubbed Virgilbacillus. However, the odd nature of the crystal could have formed much later than the deposition of the salt beds. Confirming a Permian age for a fluid inclusion is not easy. One approach is by comparing the composition and formation temperature of the bacterium-hosting fluid with that from other, more usual inclusions in the same deposit and from fluids that form when salt deposits are exposed to air (“weeps”), as might be included when salt deposits recrystallise long after their formation (Satterfield, C.L. et al. 2005. New evidence for 250 Ma age of halotolerant bacterium from a Permian salt crystal. Geology, v. 33, p. 265-268). The study found that the inclusion fluids along with others from halite at the same level in the salt deposit have significantly different compositions from “weeps”. The latter reflect the composition of the salts in the deposit which formed by precipitation of the less soluble components of seawater. The inclusions have compositions more like sea water that has been concentrated by evaporation, albeit different from that of modern halite inclusions. So it does indeed seem as if Virgilbacillus is a Permian creature. Yet to emerge are DNA analyses that can be compared with modern salt-tolerant bacteria.
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Mars: the best may yet be to come
The US and ESA satellites orbiting Mars have so far deployed remote sensing instruments that detect visible to thermal infrared radiation from the planet’s surface. Ultimately the energy involved is from the Sun: these are passive instruments. Engrossing as they are, images from these sensors reveal only details of surface mineralogy and the Martian topography. So far, virtually nothing is known about what lies buried beneath it, apart from inferences about ground ice. The ESA Mars Express has one last imaging trick up its sleeve, which uses energy generated on board and beamed obliquely down to the surface. This is the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). Radar remote sensing on Earth generally uses high-frequency microwaves in the wavelength range from 0.01 to 0.1 metres, and the images produced show how much energy is scattered by surfaces of varying roughness, to be received by antennae deployed from an aircraft or satellite. The longer the wavelength the greater the height of small-scale surface irregularities that cause scattering and therefore a received signal. Smooth perfectly surfaces reflect all the energy away from the antennae, like a mirror, so no energy returns to be sensed. How microwaves interact with the Earth’s surface depends on the electrical properties of the materials. Good electrical conductors, such as metals and liquid water are extremely efficient reflectors, whereas minerals are poor conductors and tend to absorb microwaves to some extent. If soils are extremely dry, with less than 1% moisture content, as in some deserts, some of the absorbed energy is scattered by materials below the surface and images show subsurface features. This lies behind the principle of ground penetrating radar, but since many soils are damp, only radar waves generated at the surface give good signals in most areas, to be exploited by civil engineers and archaeologists. Ice is very different from liquid water, being so poorly conductive that it is almost transparent to microwaves. Consequently it has proved possible to sound the depth of glaciers and ice sheets using ground penetrating radar deployed from aircraft. The depth of penetration, and of course that involves energy returning to the surface in order to get a signal, is governed by the radar wavelength. For instance, unknown former courses of the River Nile’s tributaries have been detected by 0.25 m radar waves beneath the hyperarid eastern Sahara through about 3 metres of dry sand.
MARSIS can transmit microwaves with 4 wavelengths 170 , 100 , 80 and 60 m. Given rocks and soils free of liquid water, which comprise most of Mars’s surface, or ice, it can penetrate as deep as almost 5 km. The multi-wavelength arrangement can also potentially discriminate water ice from rock and soil. A great deal of speculation and some evidence suggest that parts of Mars may be underlain by permafrost, that is melted only under unusual conditions, such as after meteorite impacts. There are also suggestions that glaciogenic-like landforms may still be underlain by ice, and bizarrely that there are frozen seas (see The triumph of the old on Mars in April 2005 EPN). MARSIS may well throw Mars investigations into a turmoil, but maybe not. The delay in sparking it up has been caused by fears that deploying its antennae might damage the whole spacecraft, and the first attempt seems to have got stuck. It’s other drawback is limited power so that horizontal resolution will be between 5 to 10 km and vertically only 100 m, so results may be so blurred as to be inconclusive. NASA plans a similar device aboard its Mars Reconnaissance Orbiter (launch date August 2005). The Shallow Subsurface Radar (SHARAD) will use microwaves with 12 to 20 m wavelengths that give penetration to 1 km, but horizontal and vertical resolutions of 300 and 15 metres.
See: Reichhardt, T. 2005. Going underground. Nature, v. 435, p. 266-267.
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Water and the G8
On May 24 the government of Tanzania cancelled a contract with the commercial water giant Biwater, which was supposed to bring clean water to the country’s largest city Dar es Salaam, and establish a privatised water supply. The UK-based company had won a £76.5 million contract from the World Bank, with the support of the British government’s Department for International Development (DfID). DiFID had paid the free-market thinktank £0.5 million in fees to advise the Tanzanian government and promote privatisation, out of a total expenditure of more that £36 million since 1998 for similar consultancies. In two years Biwater has failed to install a single pipe (Vidal, J. 2005. Flagship water privatisation fails in Tanzania. The Guardian 25 May 2005, p. 4).
In her statement to the International Conference on Water and Sustainable Development in Paris (March 1998) Clare Short (British minister then heading DfID) outlined the New Labour government’s “vision” on water resources in the Third World, “Partnerships among governments, the private sector and civil society are critical to sustainable development [of water resources]”. Policy of the International Monetary Fund is to enforce “structural adjustment programmes” on poorer countries as a condition for rescheduling debt repayments. Into these are written the privatisation of formerly public assets, such as water utilities. The first targets for this in Africa were the townships of South Africa, following the removal of apartheid. Although very poor by western standards, and with unemployment running at up to 50%, people in South African townships are better off than the majority of sub-Saharan Africans. Potential profits from water metering seemed attractive. However, a great many people found themselves cut off from this most basic necessity in 2000, being unable to pay the increased water rates. This led to nationwide protests, the most violent being in the arid Transvaal. The company involved in that region was also Biwater, with bids for contracts worth 12 billion rand. The company has an interesting history, having been an early beneficiary of the Conservative government’s “aid for trade” programme in the 1980s, including dam and water distribution contracts in Malaysia and Thailand that were linked to British arms supplies to the governments involved.
Water privatisation is a target outside Africa, perhaps the most notorious case being in South America. Bolivian trades unionists demonstrated on 6 April 2000 against a 35% rise in water prices imposed on the city of Cochabamba. Military forces opened fire, killing 6 demonstrators, and a state of siege was declared by the authorities. The price hike stemmed from the new owner of the region’s water system – International Waters Ltd (IWL) of London, a subsidiary of Bechtel, based in San Francisco. IWL’s Bolivian operation centres on the Misicuni dam project. Water from the dam will cost 6 times more than it would from alternative sources. The increased water charges were to recover the cost of the dam, with one problem: the dam had not been built, and IWL/Bechtel had put no funds into the construction project. Subsequently, public pressure forced the ending of the contract. Similar upheavals have been seen in Ghana, Trinidad, Argentina and the Phillipines.
News of Tanzania’s decision to end the ill-fated contract with Biwater followed announcements in the same week that the EU would effectively double its Third World aid. In early July, Britain will host the 2005 G8 summit, which will be dominated by discussion of ways to increase the flow of finance into Africa in particular. This follows the publication in early 2005 of the Commission for Africa Report sponsored by the New Labour government. Two thirds of the world’s population lacks sanitation that is adequate for healthy living. Of them, one billion people, including the majority of Africans, have no access to safe drinking water. Poor water supplies form the main contributor to the death of children under five years old. For hundreds of millions of people, getting water for domestic use consumes much of their daily labour, which involves mainly women and children trudging to distant water sources and carrying it home, on average twice each day. The failure of private enterprise to deliver water to the needy suggests that the small print of any declaration from the G8 summit needs the most careful scrutiny.
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The route and the pace out of Africa
Tool making hominid species left their African homeland several times in the past, the earliest being shortly after the appearance of Homo erectus, about 1.8 Ma ago. Those early migrants ended up in eastern Asia, where they thrived until as recently as 12 thousand years ago (if indeed H. floresiensis does prove to be a miniature erect). Europe was reached by at least three waves: possibly advanced H. erectus around 0.5 Ma; Neanderthals as early as 0.25 Ma; modern humans around 40 thousand years ago, at the earliest. The fully modern human record in Asia begins at 67 thousand years ago, suggesting an exodus from Africa at between 80 and 70 thousand years. There is an oddity here: simple geography suggests that Europe should have been colonised first in each wave out of Africa, because it is closer. But the Nile to Middle East to Europe route was not successfully used by our immediate forebears until long after they moved eastwards, although there is evidence of H. sapiens temporary occupation of parts of Palestine between 100 to 80 thousand years. Several reasons for this have been suggested, including the possibility of direct competition with Neanderthals who occupied the same 100 ka sites in the Middle East, and the relative difficulty of passage along the Nile compared with a coastal route in NE Africa.
Eritrean and US archaeologists have shown that around 100 ka the Eritrean coast was occupied by humans who subsisted on seafood: always available whatever the climate, whereas terrestrial game potential fluctuates. That has led to the suggestion that Africans who colonised Asia and Australasia left by island hopping across the narrow Straits of Bab el Mandab when sea-level began to fall around 70 ka. A coastal route, well stocked with food items would have allowed rapid movement eastwards. That seems intuitively likely, because an eastward route through the Middle East is barred by deserts, which would have been even more arid as glacial conditions developed. Moreover, a Middle Eastern route would have led more directly to Asia Minor and ultimately Europe. The conundrum deepens, since the Straits of Bab el Mandab would have been even easier to cross at the time of the last glacial maximum, around 20 ka, yet there are no archaeological signs of populations of that age in Yemen and Oman; research has hardly begun there. Unravelling routes is possible, just, by analysing modern population genetics (Macaulay, V. et al. 2005. Single, rapid coastal settlement of Asia by analysis of complete mitochondrial genomes. Science, v. 308, p. 1034-1036). People living in the Andaman islands and the Malaysian Peninsula include groups who differ substantially from their neighbours and may be descendants of the original colonisers. Mitochondrial DNA from these groups indicates a branching from an original type around 65 ka, remarkably suggesting a single founding woman. That cannot be taken exactly at face value, but does suggest that only a small band migrated to these two areas, perhaps no larger than a few hundred. The fact that they reached the Andaman islands may indicate that theirs was a boat-using culture. Whatever, movement was rapid, possibly as high as 4 km per year, thereby allowing the early colonisation of Australia.
Analyses of mtDNA in Africa suggest that about 85 ka ago there was a major expansion of people, whose descendants make up more than two thirds of modern Africans. Could it be that this expansion reflected climate and ecological change, so that migration from elsewhere drove inhabitants of the Red Sea coast to cross the daunting Straits of Bab el Mandab because of severe competition? Perhaps it was the driving force as late as 40 ka, when modern humans reached Europe itself, undoubtedly along the Middle East route.
See also: Forster, P. & Matsumura, S. 2005. Did early humans go north or south? Science, v. 3308, p. 965-966.
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Scientific lessons from the Boxing Day 2004 earthquake
Fortunately, the most devastating earthquakes with magnitudes greater than 9 on the Richter Scale occur less than once in a human generation. Records show that when such strain is released there may be two or more as major faults adjust to the release by the first. That was the case for the Sumatra-Andaman earthquake (magnitude 9.1 to 9.3) of 24 December 2004 that created the Indian Ocean tsunamis. On 28 March 2005 it was followed by the magnitude-8.7 Nias earthquake to the south of the movement zone of the earlier event. Both occurred on the subduction zone that consumes the Indo-Australian plate obliquely, from SW of the Indonesian archipelago through the ocean floor west of the Nicobar and Andaman islands to link with the Himalayan subduction system. The last seismic event of such magnitude was beneath Alaska in 1964, before modern seismograph development. How such events propagate could only be guessed at by analogy with lesser earthquakes, so scientific interest in the seismograph records of these two and their analysis has been very high. The 20 May 2005 issue of Science devotes 22 pages to full accounts of the findings (Hanson, B. 2005. Learning from natural disasters; and 5 other papers. Science, v. 308, p. 1125-1146).
The Sumatran-Andaman earthquake involved movements of up to 20 m vertically that lasted about an hour, and thrusting “unzipped” the subduction zone over a length of around 1300 km, proceeding from south to north. The energy released was equivalent to that of 100 thousand one megaton nuclear explosions, or the energy used in the US in 6 months. It set up resonances in the entire Earth that are still reverberating, and changed the shape of the crust across a hemisphere by an amount measurable using high-precision GPS monitoring, which has raised global sea level by about 0.1 mm. Half a globe away, the surface waves from the earthquake triggered several minor shocks in Alaska in exact harmony with their passage. In social terms, the loss of 300 thousand lives resulted from the displacement of around 30 km3 of sea water by the movement of the faults. The prolonged event was complex, and one sobering feature is that in the northern part of its propagation it moved slowly, thereby failing to unleash yet more tsunamis: they would have devastated most of the coast of eastern India and the west of Myanmar and Thailand. Much of what occurred was unpredictable, and quite possibly the lessons learned here may not be directly applicable to future earthquakes of this magnitude, except for one: hazard assessment based on scaling up from lesser events underestimates enormously what actually happens. What the seismograph data will not do is help warn when similar events will occur elsewhere, with sufficient leeway to take measure that will mitigate effects.
Promising developments for forecasting lesser earthquakes
Although there are many places that are riskier, California is widely regarded as the earthquake capital of the world, mainly because so many people live there with such an economically huge infrastructure. At any rate, it is indeed the centre for the most advanced seismic forecasting based on far more data that are available for analysis than anywhere else. Until recently, forecasting was limited to the likely aftershocks following unpredictable large earthquakes. Seismologists of the US Geological Survey and at ETH in Zurich have developed an advanced modelling system based on the wealth of data (Gerstenberger, M.C. et al. 2005. Real-time forecasts of tomorrow’s earthquakes in California. Nature, v. 435, p. 328-331). Their model allows day-by-day calculation of probabilities for strong shaking (> Mercali Intensity VI), using the way in which seismic events cluster along different faults and monitored lesser movements that might presage a major fault break. These take the form of extremely graphic maps of hazard across the whole state. The system has been tested using historic data that preceded historic earthquakes.
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Zircon and the quest for life’s origin
At a rough estimate the material that has pushed back the oldest direct dating of supposedly continental material is about the size of a pinch of salt. It consists of detrital zircon grains contained in Archaean sedimentary quartzites from Western Australia, the oldest of which give U-Pb ages of 4.4 Ga, 400 Ma older than the earliest rocks of the continents. Arguably, the zircons are products of repeatedly recycled debris from the earliest silica-rich magmas formed in the Hadean: zircon is hard and not affected by sedimentary processes. Any subduction processes in the early Earth might well have produced silicic magmas by a variety of petrogenetic processes: modern ocean crust contains tiny amounts of plagiogranites. Minute inclusions of quartz, mica and feldspar in the zircons suggest that such igneous rocks may have formed by partial melting of the clay-rich sedimentary veneer on Hadean oceanic crust when it descended. So, the only surprise in a chronological sense is that a few grains have been found among those formed in the 1.4 Ga until the deposition of the 3 Ga old Jack Hills quartzite in which they found a resting place. The zircons are controversial for another reason. They contain high concentrations of 18O that indicate a role for water in their formation.
Bruce Watson and Mark Harrison of the Rensselaer Polytechnic Institute, New York and the Australian National University have devised a way of establishing the temperatures at which the zircon formed, from their content of titanium (Watson, E.B & Harrison, T.M. 2005. Zircon thermometer reveals minimum melting conditions on earliest Earth. Science, v. 308, p. 841-844). Their results from 54 zircons aged from 4.0 to 4.35 Ga cluster around 700°C, which is what would be expected had their parent magmas formed at the minimum temperature for partial melting of sediments to form granite-like magmas in the presence of a water-rich fluid (the “wet-granite minimum”): they look very similar to modern zircons. This confirms the results from earlier oxygen-isotope studies. Because the oldest of the Jack Hills zircons are only 75 Ma younger than the mighty thermal effect of the Earth’s collision with a smaller planetary body that excavated matter that formed the Moon, the influence of water in the zircons’ formation has been interpreted as having monumental significance for the effectively vanished 400 Ma-long Hadean Eon. It has been taken as support for oceans at the Earth’s surface, as well as “normal” plate tectonic processes that can generate continental crust, but also that conditions amenable to pre-biotic chemistry and even the origin of life existed.
The Earth could not have escaped the massive Hadean bombardment of the lunar surface by planetesimals that climaxed between 4.0 and 3.8 Ga. Rocks from the lunar highlands preserve ages back to 4.45 Ga, close to the time of its origin, and at that time the Moon must have had a solid crust below about 400°C for radiogenic isotopes to accumulate in minerals. The Earth equally must have had at least a surface veneer of relative cool rock at that time. So, since the Apollo samples yielded these dates in the 1970’s, the popular image of a long-lived magma ocean has been insupportable, even though it probably existed shortly after the cataclysm of the formation of the Earth-Moon system. In that sense, evidence in ancient zircons for plate-like processes is not a surprise, although an interesting confirmation of long-held beliefs. Nor does their showing the influence of water come as a shock. The Earth is tectonically active partly through it not having been thoroughly dried by Moon formation; lunar rocks are a great deal drier and the Moon is as dead as a doorknob. At 700°C water cannot exist as a liquid, so its influence in partial melting is not evidence for surface water. However, the most efficient means of heat loss from any heated body is by radiation to space, and simple calculations show that it would be highly unlikely for Earth not to have had liquid surface water about 100 Ma after Moon formation. That in itself indicates that there would have been a water-rich atmosphere too. No matter how much “shock and awe” might colour our view of repeated bombardment during the Hadean, no sane impact theorist has suggested that sufficient energy was delivered to recreate a global magma ocean. Water may have been boiled off to the atmosphere by the biggest, but only to fall again as rain between major impacts. Given favourable chemical conditions and liquid water, the route to life might well have opened up in the Hadean itself: some have suggested that it happened again and again only to be snuffed out by high powered impacts, until the Inner Solar System became a safer place after 3.8Ga. The real mystery of the aged zircons concerns the rocks in which they crystallised: where on Earth are they? Four decades of radiometric dating of actual rocks has failed to break the 4.0 Ga barrier, so if relics do remain they are either buried or have been reduced to sediments, as the Jack Hills quartzite so nicely demonstrates.
See also: Reich, E.S. 2005. What the hell…? New Scientist 14 May 2005, p. 41-43.
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Thermal metamorphism and ocean anoxia
Now and again in the geological record, evidence turns up that suggests that the deep oceans were devoid of oxygen. Ocean anoxia encourages burial of dead organic remains that gives rise to carbon-isotope “excursions”: signals of the anoxia itself. A likely mechanism that starves the deep oceans of oxygen is the shut down of that part of the ocean “conveyor” driven by sinking of cold, dense brines, as happens today in the North Atlantic and around Antarctica. Gases dissolve more efficiently in cold water than in warm. Quite probably most oceanic anoxia events are related to global warming and increases in the “greenhouse” effect due to CO2 rises in the atmosphere. A group of US and British geoscientists have examined one such anoxia event in the Lower Jurassic (~183 Ma) of Denmark using both carbon isotopes and the density of pores (stomata) on fossil leaves (McElwain, J.C. et al. 2005. Changes in carbon dioxide during an oceanic anoxia event linked to intrusion into Gondwana coals. Nature, v. 435, p. 479-482). Stomatal density is inversely related to the amount of CO2 in the atmosphere, so is very useful in seeking evidence for an anoxia-climate link.
This particular anoxia event has been linked either to release of methane, which quickly causes warming and then oxidises to CO2, from gas hydrate or to massive release of carbon dioxide itself. McElwain et al. neatly show that the event first experienced drawdown of ”greenhouse” gas and cooling of around 2.5 °C, then sudden quadrupling of CO2 and warming of around 6.5°C. Such an odd pattern cannot be ascribed to methane release, but coincides with the formation of the Karroo-Ferrar continental flood-basalt igneous activity in southern Africa and Antarctica. That involved massive intrusion into coal-bearing strata, whose thermal metamorphism would have released huge amounts of “greenhouse” gases. Calculations of the amount of carbon mobilised to cause the shifts in CO2 suggest between 2.5 and 4.4 trillion metric tons, vastly more than the probable amount of methane hydrate beneath the Jurassic sea floor.
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How the core controls Earth’s magnetic field
While most geoscientists are well aware that past changes in the geomagnetic field are useful as a means of timing sea-floor spreading and stratigraphic correlation, and that records of the direction of palaeomagnetism are keys to ancient plate movements. Most, however, understand only vaguely why Earth has a magnetic field that flips polarity from time to time: there is some kind of self-sustaining dynamo due to motion in the liquid-metal outer core. That aspect of geomagnetism involves tough theory and maths. So for Scientific American to present an up-to-date review of how that dynamo might work is both surprising and welcome (Glatzmaier, G.A. & Olson, P. 2005. Probing the geodynamo. Scientific American April 2005, p. 33-39). The review covers what is currently known about convective motion in the outer core, both laminar and turbulent, and how the simpler laminar convection has been used in computer modelling that simulates how the geodynamo works. It is complex even at that level of simplification, because thermal convection is affected by the Coriolis effect: much like that in the atmosphere. Even though the idea of a dynamo inducing magnetic flux is a basic principle of physics, one based on fluid circulation is in constant motion and change. Surface monitoring of shifts in the magnetic field help chart that aspect. The issue of reversal is, literally, the knottiest problem for geomagnetists, and they have to resort to the old idea of lines of flux and the effect of contortions by motion at the core-mantle boundary to grapple with how polarity flips might occur. Computer simulations show the development of what can only be described as chaos in the geomagnetic field at the core-mantle boundary, and much smoothed, but nonetheless odd variability at the surface, as the poles prepare to reverse. For a period of around 6 000 years the field wobbles like a massive jelly as it lurches across the planet, sometimes splitting into several “blobs” of different polarity. Eventually it settles down into its new configuration. To some extent this strange behaviour is matched by what little is known in detail about the progress of reversals from the geological record (see Magnetic polarity reversals in May 2004 issue of EPN).
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Two sides to reducing carbon emissions
Scientists in developed countries are more or less unanimous that climate is warming because of rising CO2 levels from the burning of fossil fuels. That spurs calls for less reliance on fossil fuels and more use of renewable energy resources, including biomass. The situation for the other two-thirds of humanity is much different. The majority depends on biomass fuels (wood products, agricultural waste or animal dung). Unprotected burning of biofuels releases such levels of carcinogens that 1.6 million people including 400 thousand in sub-Saharan Africa, mainly women and infants, meet an early death each year. By 2030 this may rise to over 9 million, if current fuel use continues. Biofuels also devastate woodland cover, and burning animal dung reduces natural fertiliser used on fields: two contributors to the inexorable decline in conditions of life in the “Two-Thirds World”.
Energy researchers at Harvard and the University of California have examined the options for household fuels in the light of these “counter-environmentalism” facts (Bailis, R. et al. 2005. Mortality and greenhouse impacts of biomass and petroleum energy futures in Africa. Science, v. 308, p. 98-103). A safer alternative to wood and dung burning is the use of charcoal, yet that would increase CO2 emissions by around 50%, as well as increasing loss of woodland. The higher energy content of non-coal fossil fuels would actually decrease the “greenhouse” burden, while improving health dramatically. They estimate that a shift to petroleum-based household fuels would delay between 1.3 to 3.7 million deaths per annum, by 2030
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Changing the world
Because humanity and its activities have transformed the vegetated face of our home planet, caused its climate to warm and pushed an increasing number of other species over the edge of extinction, some circles have coined the name “Anthropocene” for the last half of the Holocene Epoch. Human induced change almost certainly began as soon as settled agriculture arose to dominate most societies (see Did the earliest agriculture kick-start global warming?, in EPN of April 2005). In terms of atmospheric emissions and mobilizing metals we now push natural rates close: facts that emerge from annual reviews of mining and energy use. But are we truly significant geological agents as well as influences on the atmosphere and biosphere? Two articles in April 2005 suggest that we are.
Quarries, mines and other excavations are obvious signs of human erosive power, but our farming activities produce insidious results by inducing soil erosion. Although its effects are well known from such areas as the Ethiopian Highlands and the 1930’s “Dust Bowl” of the US mid-west, a global measure of the rates involved requires a careful compilation of quantitative data. Bruce Wilkinson of the University of Michigan has made the first attempt (Wilkinson, B.H. 2005. Humans as geological agents: A deep-time perspective. Geology, v. 33, p. 161-164). Throughout the Phanerozoic, the volume of sedimentary rocks suggests that enough erosion has taken place to have stripped a uniform blanket 3 km deep from the continental surface. That gives an average erosion rate for the last half-billion years of Earth history of the order of tens of metres per million years. Assembling information about current rates of human-induced stripping, roughly divided 30:70 between excavation and soil erosion, Wilkinson arrives at a staggering figure for anthropogenic denudation: hundreds of metres per million years. Our activities in the outer part of the rock cycle are an order of magnitude greater than purely natural rates of weathering, erosion and transportation. He suggests that humanity began to outpace sedimentology sometime around the time of the Norman Conquest.
This awesome picture might seem to indicate that rates of sediment deposition on continental margins are also tremendously elevated by our actions. That aspect has been studied by geoscientists from the US and Holland (Syvitski, J.P.M. 2005. Impact of humans on the flux of terrestrial sediment to the global coastal ocean. Science, v. 308, p. 376-380). The opposite is now happening. Syvitski et al.’s analysis of historical sediment loads in the catchments and lower reaches of the worlds major rivers shows that while overall sediment transport has increased by 2.3 billion t per year, since human effects became noticeable in the sedimentary record, the amount delivered to the sea has fallen. Some 1.4 billion t no longer add to marine sedimentation each year. Instead, that mass ends up behind dams of one kind or another. In the last 50 years, more than 100 billion t, containing 1 to 3 billion t of carbon is in silted up reservoirs, or redistributed to farmland by irrigation diversions. One of the outcomes is that natural coastal protection by spits and sand bars is growing less effective. Another is that less nutrients are getting to the near-shore marine biosphere, with possible effects on fish stocks, coral reefs and other habitats.
Caring among the Erects
Dmanisi in Georgia provided one great surprise in human evolution by yielding abundant remains of 1.7 Ma old Homo erectus where they might be least expected: north of the Caucasus mountains that would have formed a tremendous barrier to any migration from further south. The archaeological sites have provided another surprise in the form of a well-preserved skull of a completely toothless individual. It is clear from the regrowth of bone into the sockets that this “masticatorily impaired” individual survived for years after losing all their teeth (Lordkipanidze, D. et al. 2005. The earliest toothless hominin skull. Nature, v. 434, p. 717-718). It is impossible to believe that the individual could have survived on a tough meat and vegetable diet without special preparation of soft victuals. Although the person’s survival cannot prove that other Erects helped out, that is a distinct possibility. Losing teeth through dental disease or trauma would have been immensely painful and debilitating, yet the individual did survive. We have to move forward to around 40 thousand years ago for compelling evidence that Neanderthal society cared for disadvantaged people, when several near-complete skeletons show evidence of long-term, crippling damage.
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Erosion and plate tectonics
This item can be read at Earth-logs in the Geomorphology archive for 2005
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New twist for end-Permian extinctions
There is a Gaelic proverb, which loosely translated goes: “There are more ways of killing a cat than drowning it in butter”. That seems apt for mass extinctions, particularly the most severe, at the end of the Palaeozoic. A new hypothesis points the finger towards breathing problems, but not those likely from massive, ground-hugging emissions of sulphur dioxide from the Siberian flood basalts that coincide with the P-Tr extinction: “everyone knows” that they resulted in the universal coughing reflex in all surviving land vertebrates….. Raymond Huey and Peter Ward of the University of Washington reckon a major contributing factor for terrestrial extinctions was a fall in atmospheric oxygen (Huey, R.B. & Ward, P.D. 2005. Hypoxia, global warming and terrestrial Late Permian extinctions. Science, v. 308, p. 398-401).
For most of the Carboniferous and Early Permian Earth flipped in and out of glacial conditions that dominated the southern supercontinent of Gondwana. Tropical latitudes were cloaked in dense vegetation for the first time. Rapid sedimentation buried vast amounts of carbon in the form now taken by the world’s largest and most extensive coal deposits. Net carbon burial for 90 to 100 Ma resulted in extraordinary oxygen concentrations in the atmosphere. One line of evidence for that is the huge size of Carboniferous and Early Permian insect fossils, such as those of dragonflies. Insects do not breathe, but take in oxygen by a diffusive process through spiracles on the underside of their bodies. The more oxygen the larger they can grow. Carbon burial also links in with the global cooling that made the Carbonierous and Early Permian susceptible to astronomic forcing of glacial-interglacial cyclicity: CO2 fell.
The present-day oxygen concentration in the air is about 22%, whereas estimates for the Carboniferous Permian peak are around 30%. Most land animals today, including ourselves, have an altitude limit to permanent life of around 4 to 5 km, though the vast majority live much lower. In the Early to Middle Permian, the availability of oxygen for respiration corresponding to that at sea level today would have been around 6 km altitude, and at the top of a mountain the height of Everest breathing would be easy. The limit to altitude range of animals would have been temperature rather than oxygen availability. So, given sufficient warmth, the area available for animal life would have been very high. Estimates of the oxygen level at the end of the Permian are as low as about 16%. Even living at sea level would have demanded an ability to survive at about 2.7 km today, and at 6 km during the oxygen-rich Early and Middle Permian. Evolution of land animals during the 100 Ma long “global winter” would have adjusted to elevated oxygen availability, which Huey and Ward believe would have led to at least a limited altitude stratification of available ecosystems, governed by temperature. Their hypothesis is that declining oxygen forced extinctions by reducing the habitable range severely, and increased competition among those taxa able to live in the reduced, low-altitude land area: probably patches of “refugia”.
The decline in oxygen was accompanied by global warming. Permian and Triassic sedimentary records show a dramatic increase in red terrestrial sediments, coloured by iron oxide. Iron had been released and oxidised to insoluble iron(III), possibly by increased continental weathering, which would have sequestered oxygen by the formation of iron oxide coatings to sedimentary grains. Increased oxidation would also have encouraged biodegradation by aerobic bacteria, which may have run-away to help boost atmospheric CO2 levels. One testable outcome of such events is the rate of extinction during the Late Permian, which should have risen slowly, rather than plummeting at the P-Tr event. Another is that survivors might show signs of adaptation to low oxygen levels, and indeed some Triassic reptiles do. All in all, those times were stressful on land. Yet the extinctions were just as severe in marine ecosystems, where the fossil record is more complete. Less oxygen and warmer seas would have resulted in similar hypoxia for aquatic animals.
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Ejecta from the Sudbury impact
Sudbury in Ontario, Canada hosts one of the largest nickel and platinum-group metal deposits, and it in turn is associated with the world’s second largest impact structure (260 km diameter), dated at 1850 Ma. About 650 km to the WNW is another of Canada’s Precambrian treasures, the Gunflint Chert beds that contain the earliest incontrovertible fossil cells. Those cherts are also roughly the same age as the Sudbury impact structure, so what better place to seek material excavated and ejected by the offending meteorite? No need either to thrash around the bush to collect rocks; the succession has been penetrated by 5 drill cores near Thunder Bay and in northern Minnesota. Sure enough, all the cores show signs of impact ejecta (Addison, W.D. et al. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, v. 33, p. 193-196). The proof takes the form of shocked quartz and feldspar grains and melt spherules, but in a sequence of silicified carbonates above the level of the Gunflint Chert. Ejecta material is about 0.6 m thick. Because the carbonates contain no volcanic horizons, establishing the age of the ejecta depends on a thin volcanic ash 5 m above it, which yielded zircon U-Pb ages between 1827 to 1832 Ma. There are no other known impacts around this time, so Sudbury is the most likely source of the ejecta. Apart from being the oldest impactite layer known that can be tied to a source, there are a couple of intriguing features. The ejecta layer occurs almost at the top of the Gunflint Formation famous for its cellular remains, yet the overlying strata contain no sign of fossils. The authors wonder if this might represent mass extinction, but these slightly younger sediments are clastic rocks in which cell microfossils are unlikely to have been preserved. However, they do show signs of anoxia, including high organic carbon content and sulfide minerals. Hopefully carbon isotope data from the section might throw light on how impacts in a world exclusively that of single-celled organisms affected the biota: an interesting comparison with the K-T boundary. The other puzzle is that the ejecta are in shallow-marine sediments. Being only a few hundred km from the linked impact structure, some sign of disturbance by tsunamis or water-release by huge seismic shocks might be expected within the sediments. No signs of such disturbances have been reported.
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Snowball Earth gets a boost
enveloping glaciations during the Neoproterozoic Eon, that notion of “Snowball” conditions has received many severe knocks, charted by numerous items in EPN. Geochemists and geologists from the Universities of Vienna and Witwatersrand realised that a good test of the hypothesis would be to concentrate on a rather obvious property of an ice-bound planet (Bodiselitsch, B. et al. 2005. Estimating duration and intensity of Neoproterozoic Snowball glaciations from Ir anomalies. Science, v. 308. P. 239-242). Whatever falls on an ice sheet, whether it is cosmic dust from outside the Earth or ash from volcanoes, becomes trapped in the annual layers of ice. When the ice melts, that accumulated content is transferred to the oceans very quickly. With weathering in suspended animation during the glacial epoch, transport of many elements would have slowed to very low levels. So, marine sediments deposited immediately after the diamictites that are allegedly glaciogenic ought to contain anomalously high levels of several elements. The most important of these would be those which show very different abundance patterns in meteorites form those in terrestrial rocks.
Bodiselitsch et al. hit what seems to be “paydirt” in carbonates above a prominent diamictite in central Africa. Their samples are impeccable, being from diamond-drill cores produced during evaluation of sediment-hosted mineralization in the famous Neoproterozoic Copper Belt of Zambia and Congo. The core contains a prominent iridium anomaly at the very base of the carbonates, with a “signature” relative to other anomalous elements that points to a cosmic origin. Normally such an anomaly would be ascribed to a meteorite impact, but in this case the coincidence would be too good to be true. Instead, the authors use the magnitude of the anomaly to estimate how long cosmic dust had to accumulate to build up such a high level if it was released by rapid deglaciation. Deep-ocean sediments from the last 80 Ma are a guide to the long-term accumulation rate of cosmic material. If that rate is applied to the cap-carbonate anomaly, it gives a total time for accumulation in the hypothesised global ice cover of around 12 Ma. Presumably this would have been from ice immediately overlying the area being studied. An ice age that long defies any idea of more “normal”, astronomically forced glaciation, which would be expected to have cyclically formed and receded many times, thereby releasing the dust particles much more gradually. Any anomalies would be expected in the diamictites themselves, yet there are none. Although sample spacing is rather patchy through the entire succession, they are most dense around the anomaly itself. Moreover, another suspected glaciogenic “package” higher in the sequence shows exactly the same iridium “spike”.
Arguing against such support for the “Snowball Earth” hypothesis will be difficult, but other sequences require similar tests, most importantly those of Namibia, where Hoffman and colleagues developed their ideas, and the much more extensive deposits of Australia. This diamictite sequence is reckoned to represent both postulated deep-freeze events of the Neoproterozoic, around 710 Ma (Sturtian) and 635 Ma (Marinoan). There is one nagging problem. Data from one area are likely to record ice-retained cosmic dust only from ice in its immediate vicinity, and therefore do not represent the entire planet. Much of the controversy is between supporters of a whole-Earth ice cover, and those who favour patchy glaciation (the “Slushball” model). Unfortunately, Neoproterozoic stratigraphic correlation and radiometric age calibration is not sufficiently good to detect the same intervals elsewhere and look for anomalies there. In fact, the stratigraphy is generally correlated from place to place by matching the diamictites themselves. There is plenty of evidence that they may all coincide in time.
Tracking ocean circulation during the last glacial period
The use of various ocean-floor sediment proxies for climate change, such as the ups and downs of heavy 18O that chart waxing and waning continental ice cover, has progressively revealed the complexity of shifts during glacial and interglacial periods. Yet more emerged from finer-resolution time-series contained with Greenland and Antarctic ice cores. The diversity of information that proxy for many different, climate-related processes has in the last decade enabled palaeoclimatologists to begin piecing together possible causative mechanisms, beyond the initial discovery of an astronomical signal in early oxygen-isotope records. One of enormous significance is the possibility that sudden millennial-scale cooling and warming link to changes in ocean circulation, especially that performed by the Gulf Stream driven by thermohaline processes at high northern latitudes. Shutting down that poleward transfer of heat, probably because freshwater made high-latitude surface water less dense, has been implicated in sudden cooling or “stadials”, and its restart linked to warming or “or interstadials”. The last such sudden climate event, the Younger Dryas between about 12 and 11 thousand years ago, is widely believed to have resulted from a collapse of the Gulf Stream. That has raised fears that current anthropogenic warming might achieve the same thing, thereby plunging Western Europe into a counterintuitive frigid period through loss of its maritime warming.
Ocean circulation has lacked a proxy that might help resolve such worrying scenarios, but it seems that one has arrived, because of improvements in mass spectrometry (Piotrowski, A.M. et al. 2005. Temporal relationships of carbon cycling and ocean circulation at glacial boundaries. Science, v. 307, p. 1933-1938). Different bodies of ocean-surface water have subtly different chemical compositions, due to the varied geochemistry of surrounding landmasses. Weathering of exposed rocks results in some elements entering solution in river water, and that mixes with surface water in the nearby ocean. Among the most useful elements are those with an isotope to which radioactive decay of unstable isotopes of another element contributes. A good example is 87Sr that is formed when 87Rb decays. Where continents expose large expanses of very ancient rocks they contribute more 87Sr to seawater than do continents veneered with younger rocks. Strontium isotopes have been used successfully for charting very-long term changes in the overall erosion of continental crust, in relation to climate shifts, but being related to calcium are taken up quickly by carbonate secreting organisms, such as foraminifera, at many different levels in the ocean as it circulates. So they are not very useful for short-term studies. A more useful isotopic system involving an daughter of slow radioactive decay is that of neodymium, because it does not get taken up in this way. It does however enter the manganese minerals that slowly precipitate on the deep ocean floor. Moreover, its isotopic composition varies greatly in different ocean-water masses. Piotrowski et al. used neodymium isotopes from deep ocean cores to see if changes in this circulation proxy coincided with known climate proxies. For interstadial, warming events there is a match, so a Gulf-stream control over millennial-scale climate shifts is indeed supported. But for the start and end of the full glacial period control by ocean circulation did not happen. Instead, changes in the neodymium record lag behind the climate proxies, suggesting climatic control of circulation, which then “kicked in” to boost changes that were well underway.
See also: Kerr, R.A. 2005. Ocean flow amplified, not triggered, climate change. Science, v. 307, p. 1854.
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Tree-ring heaven
Growth rings in tree trunks are among the best records of local climate variation that there are: they provide an annual “stratigraphy”. So intricate are the records that it has proved possible to match ring sequences in ancient but still growing trees to those found in logs of even greater antiquity, thereby building up a “dendrochronology” that extends back into history. Tree rings help historians link human affairs to a background of changing conditions for life. Henri Grissino-Mayer of the University of Tennessee has brought together a wealth of dendrochronological information in his Ultimate Tree Ring Pages at web.utk.edu/%7Egrissino/default.html.
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Yet more Indian Ocean earthquakes? Sadly, yes
The shores of the Indian Ocean and the people who live near them will take years and maybe decades to recover from the awful events of 26 December 2004. While relief and reconstruction efforts are underway, so too is the scientific analysis of what happened. Throwing a malevolent shadow is the uncertainty of whether there may yet be more tsunamis so soon after the first in the region for 150 years. The Sunda trench where the massive earthquake took place had remained stable for a long time. Stresses built up, eventually to cause the subduction zone to fail catastrophically. However stress relief in one place redistributes that which remains along other fault lines, and can create space in which new breaks might occur. Geophysicists from the University of Ulster have analysed the likely disruption of stress in the eastern Indian Ocean (McCloskey, et al. 2005. Earthquake risk from co-seismic stress. Nature, v. 434, p. 291) following the distribution of about 20 m displacement on the Sunda subduction zone over a N-S length of around 500 km. They feared that such a huge perturbation may activate other large faults. A changed stress field seems to have been the cause of the Izmit earthquake that devastated central Turkey and also set in motion repeated seismicity along the subduction system off Japan in the past. McCloskey and colleagues foresaw two worrying possibilities for the Sunda subduction system: stress localised just to the south of the Boxing Day event could migrate southwards to trigger release again on the subduction zone; a large strike-slip fault that runs down the centre of Sumatra, itself linked to subduction, may fail soon. fear that the second is the more likely. Since modern seismology emerged, so few earthquakes have occurred in the area compared with other large subduction settings that prediction is difficult. The Ulster scientists were correct, very soon after their prediction was published. On 28 March 2005, a magnitude 8.7 earthquake occurred on the subduction zone about 150 km south-west of that on Boxing Day 2004. Its motion involved vertical displacement, so it was feared to trigger yet more tsunamis and sirens sounded throughout the previously devastated areas. The warnings were heeded. Apart from some panic that cause two deaths in Sri Lanka, people moved quickly to safe ground. Thankfully, perhaps miraculously considering an energy release not far short of that at the end of 2004, there were no tsunamis of any consequence. Yet the places on the nearby Indonesian island of Nias were devastated by the shock waves, killing upwards of a thousand people. This is a grim warning that McCloskey and colleagues’ interpretation of stresses moving southwards along the main ocean floor fault system is happening. The risk of further devastation soon is by no means over.
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Mineral maps of Mars
Lots of space has been devoted in science journals to results from NASA’s robot rovers on Mars. Well, haven’t they been exciting? Iron-oxide “blueberries, a cliff with bedded sediments and some iron-aluminium sulphate in a combined traverse of a kilometre at most: imagine a geologist coming back from a terrestrial field trip costing a year’s GDP of a small poor country and writing a report for the funding agency! That is a bit cruel, for in planetary exploration the themes are context, context and context, but we did know that Mars is red and orange, which is enough for most of us to feel happy with a lot of iron coloration. At the same time as the rovers were deployed, the European Space Agency’s Mars Express was going into orbit (so named because it was assembled in something of a hurry). That bristles with the geoscientist’s other modern tools: those aimed at sensing materials from their electromagnetic spectra. There is the High-Resolution Stereo Camera that produces images to rival high-altitude aerial photos of the Earth, and with stereoscopic overlap from which accurate models of Mars’ topographic elevation can be calculated, of which more in the next item. The principal mineral and rock mapping tool is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité (OMEGA), that builds on the spectral mapping by NASA’s Thermal Emission Spectrometer deployed by the earlier Mars Global Surveyor and a similar instrument aboard Mars Odyssey. OMEGA is every remote sensing geologist’s dream machine, because its coverage of the short-wave end of electromagnetic radiation by 350 narrow bands can match spectra reflected from rocks and soils with those measured under laboratory conditions for several hundred important minerals. Research geologists don’t get much of that quality of data from Earth, mainly because it is commercially successful in mineral exploration, and very expensive (for much of the Earth, such hyperspectral data is not very useful, because vegetation masks most mineral signatuires). But data are free from Mars Express (or will be when the main investigators have had a reasonable time to satisfy their curiosity) and has a terrestrially useful resolution down to 100m. They also cover an awful lot of the planet’s surface and should eventually give 100% coverage.. The 11 March 2005 issue of Science devotes 24 pages (p. 1574-1597) to summarising OMEGA results. Various papers reveal variations in the composition of pyroxenes in the predominantly mafic Martian surface rocks, those minerals, such as the sulphates gypsum and jarosite, which contain water and signs of weathering by water, and an awful lot about water and CO2 ices around the poles. But this is not the geology in full of course, but driven by the search for potential habitability. Common rocks are not made of sulphates and ice, but silicates, which can be assessed by multispectral thermal emission data that prove very useful on Earth. The lack of information about such fundamental divisions of Martian igneous rocks as ultramafic, mafic, intermediate and felsic is a great disappointment, but perhaps the thermal instrument aboard Mars Odyssey will eventually come up with those more mundane goodies. Oddly, the planetary treasures of Mars are not being revealed by such sophisticated instruments, but by what is still the work horse for a great deal of geological image interpretation, black and white stereo images.
The triumph of the old on Mars
Except perhaps for some of the current generation of geologists, who are immersed in their remote sensing training by false colour images of spectrally revealing multispectral image data, a great many professionals who engage in mapping cut their teeth on what is known simply as photogeology. And it is simple. Provided images are taken of an area from different angles, with the simplest of instruments most people’s innate stereoscopic vision enables them to see startling illusions in three dimensions. Stereoscopy has been to geologists of the mid to late 20th and early 21st centuries what the binoculars were to those earlier scientist who discovered the great nappes of the Alps and thrust belts of the Rockies. A stereoscope of some kind is the latter-day analogue of that “Swiss Hammer”. Two stereo images reveal a great deal more than twice the information of one flat image, no matter how detailed. Using complex software, which converts the parallax differences that enable us to see 3-D to the differences in topographic elevation that cause relative shifts in the position of features on overlapping images creates accurate models of the elevation itself. That enables quantitative measure of many features related to topography, and allows the images to be viewed in perspective, as if they were indeed captured by binoculars from a high view point. Results from the Mars Express High-Resolution Stereo Camera (HRSC) have proved able to revolutionise our understanding of the Martian surface. The 17 March 2005 issue of Nature reports three important new results that stem from HRSC data. For several years the possibility of glaciers having carved some features on Mars have been suspected from lower resolution elevation data. Now it is certain from exquisite perspective views of debris aprons that record the flow of smashed rock from large mountains, almost certainly because the debris was once extremely dirty glacial ice (Head, J.W. et al. 2005. Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars. Nature, v. 434, p. 346-351). The flows are reminiscent of rock-rich glaciers in the hyper-arid Dry Valleys of Antarctica. These authors present evidence that suggests that the flows are as young as 130 Ma, and may yet contain water ice. A second paper also reveals the influence of near-surface ice on Mars (Hauber, E. et al. 2005. Discovery of a flank caldera and very young glacial activity at Hhecates Tholus, Mars. Nature, v. 434, p. 356-361). In its case it seems to have been mobilised by an explosive volcanic eruption, possibly as young as 20 Ma, to produce debris flows and also very well preserved drainage channels at a much smaller scale than those known from Mars’ earliest history. The drainages might have resulted from subsurface ice melting by high heat flow and emergence of the “groundwater” to carve the meandering channels. There is an important caution: any dating on Mars depends on assuming a timescale based on counting impact craters and noting their relations to each other and different kinds of surface. The third paper observes something very different (Murray, J.B. et al. 2005. Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator. Nature, v. 434, p. 352-356). HRSC images reveal an area about the same size as the North Sea that is not only completely flat, but shows features very like those associated with pack ice in the Arctic and around Antarctica. They are plates whose edges can be fitted together, and in some cases islands have resulted in pressure ridges very like those seen where terrestrial pack ice meets land. There are even examples of impact craters that have been flooded. Murray and colleagues attribute all this to a large volume of subsurface water released by very recent volcanism along fissures close to the Martian equator. Basalt floods had been identified in the region before, but not evidence for a possible sea-sized, frozen lake. Similar, but not so revealing features elsewhere on Mars have been interpreted as lava rafts that once floated on flood basalts. Naturally, Mars scientists are very excited about the possibility of a large ice sheet at the equatorial surface, which may be as much as 45 metres deep. Unfortunately, the observations are from an area not yet covered by spectral data that would resolve whether the surface is ice-rich or more mundane lavas.
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Evolutionary rhythms
The late Jack Sepkoski did a lasting service for those who study life’s record by combing the literature to compile the first and last appearance of each marine fossil genus. It is from this archive that we have been able to visualise mass extinctions and those less in magnitude numerically. As well as the “Big Five” there are other die-offs, particularly through the Mesozoic and Cenozoic record. To some extent the extinction patterns also appear among terrestrial taxa that have been less well documented, partly because few have had Sepkoski’s determination and partly because land organisms leave fewer traces. It quickly became apparent to him and other palaeontologists that extinction occurred sharply, which is why the biologically-determined division of Phanerozoic time since 542 Ma is so well defined world-wide. What also emerged from inspection of the time series of genus and family numbers was a pulse in the timing of significant extinctions, which appears to have been between 25 and 30 Ma. That struck a chord with specialists in volcanic activity, and there is a good correlation between the occurrence of flood-basalt outpourings and extinctions. But at least one of the five largest extinctions, at the K-T boundary, coincides with abundant evidence for a major impact by an extraterrestrial body. Planetary scientists then began looking for a pulsed variation in the intensity of bombardment of the Inner Solar System. There is no tangible evidence of that, although there are theoretical arguments that suggest that the Sun in its ~250 Ma orbit around the galactic centre wobbles through dust arranged in bands close to the galactic plane every 30 Ma.
Extinctions are not, of course, the only features of the fossil record. Primarily it charts variations in diversity, of which suddenly lowered numbers are one aspect in broader fluctuations. Each extinction eventually precedes an increase in diversity as adaptive radiation from surviving taxa fills ecological niches left vacant or under-populated. That part of the record has its fascinations, as complexity seems to have emerged in three great pulses, through the Palaeozoic, Mesozoic and Cenozoic Eras, each producing more diverse forms than its predecessor. There are also slackenings in the pace and periods of apparent stasis. Getting to numerical grips with the full record requires analysis that uses similar mathematical techniques to that which unlocked proof of Milankovich’s theory of astronomical pacing of climate from finely calibrated oceanic-sediment records. It is possible to analyse time series in terms of discrete frequencies from which the curves can be reconstructed. Physicists Robert Rohde and Richard Muller of the University of California have used this Fourier analysis on the 36 thousand strong catalogue published after Sepkoski’s death, with some recalibration of the time scale and some pruning of data – they removed genera with only a single record or whose age is poorly known (Rohde, R.A. & Muller, R.A. 2005. Cycles in fossil diversity. Nature, v. 434, p. 208-210). There are definitely distinct frequencies that dominate the record, and they cannot be present by chance, although that is a purely statistical view. But to their surprise, and everyone else’s, they are completely unexpected ones at 62 and 140 Ma. It is proving exceedingly difficult to come up with plausible Earthly or extra-terrestrial explanations. There are two interesting features: the 62 Ma periodicity dominates the record of relatively short-lived genera; and the “Big Five” seem to fit neatly into the patterns of diversity, albeit at unequally spaced intervals, when the effects of background fluctuations have been removed. That filtering may allow for increasing preservation towards recent times. One major control over diversity is, logically, a mixture of the number of potential niches and their geographic isolation, and both are probably related to plate tectonic activity. Unfortunately, fluctuations in 2 and even 3 geographic dimensions have only the broadest calibration to time. Added to that is the complex way in which global sea level has changed with time. So we can expect a great deal of head scratching, and it may come as a relief that the crowing of some volcanologists and impact theorists may have been silenced at a single stroke!
See also: Kirchner, J.W. & Weil, A. 2005. Fossils make waves. Nature, v. 434, p. 147-8.
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Making sense of glacial-interglacial cycles?
The competing periodicities of the three astronomical “drivers” of climate – orbital eccentricity (~100 ka), axial obliquity (~40 ka) and axial precession (~20 ka) – lie behind several models for the climate changes of the last 0.7 Ma. Taking in the theories that sway towards the influence of variables in the Earth system itself, around 30 models have some currency at present. Since climate forecasters have to take account of which factors drive climate in the absence of human emissions, as well as piece together their own particular models, it is easy to see how critics of global warming get a wide hearing: compared with creationists, they have it easy! Is there any way of resolving what is quite bluntly a theoretical mess? It is a mess simply because the available data are so complex, and in the case of both main sources, ocean-floor sediments and ice cores, not only are their devils in the detail, but there are whopping contradictions, such as the mismatches in timing between the Greenland and Antarctic ice cores. Add all the other sources, such as stalactites, tree rings etcetera, together with caveats like the difficulty in time calibration using 14C dating, and the volume of diverse records become bewildering. It is tempting that a reversion to a statistical approach, that includes more bells and whistles than hitherto (see Evolutionary rhythms below), can resolve matters. Peter Huybers and Carl Wunch, of Woods Hole Oceanographic Institution and MIT, have tried that for pacing of the last 0.7 Ma of climate cycles (Huybers, P. * Wunsche, C. 2005. Obliquity pacing of the late Pleistocene glacial terminations. Nature, v. 434, p. 491-494). Generally accepted “wisdom” holds that the last 7 glacial-interglacial cycles are paced by ~100 ka eccentricity forcing, even though it has the weakest effect on solar heating, by a very long way. But there are smidgens of evidence for some interaction between that and the much stronger influence of changes in the Earth’s axial tilt or obliquity. Huybers and Wunsch go for the Popperian rigor of first defining a null hypothesis, that obliquity has no effect, and then designing a test. It isn’t easy to decide how the contrary hypothesis that it does can be evaluated though. The clearest features in all climate records are the ends of glacial epochs or termination: they are sudden, sharp and generally look the same. Most other features have some kind of pattern, but little consistent comparability. Using the most advanced statistical techniques, which employ many iterations to test for stability in statistical models, they can show that the null hypothesis fails. The positive result is that the time between terminations that are repeatedly modelled falls into two envelopes, around 120 and 80 ka, which simple arithmetic shows are divisible by 40 ka. But how can axial obliquity only have an effect every two of three of its cycles, while a single cycle does not appear in the time-series; is it nature skipping beats somehow. One means that the authors suggest is that the underlying pace of eccentricity can effect the temperature at the base of ice sheets, depending on their thickness. If they are thin, then the heating is insufficient to trigger ice-sheet collapse because the base is very cold, whereas if ice is thick the effects of thermal conductivity and heat flow makes the ice base warmer and more subject to perturbation beyond its failure limit. It was at this point that I gave up, but wish the authors good luck in promoting their possibly unifying hypothesis for what finishes off glacial epochs…..
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Curiously low-velocity material at the core-mantle boundary (CMB)
One of the oddities of the deep Earth is the presence of zones of the order of 1 to 10 km thick close to the boundary between the lower mantle and the outer core that have seismic wave speeds well below those expected at such depths. Because wave speed is inversely proportional to density, the chances are that they are “ponds” of extremely dense solid materials. Denser in fact than basalt might become in the form of eclogite, even compressed appropriately to these extreme depths. The zones have been a puzzle, but that seems to have been resolved by work from University College, London (Dobson, D.P. & Brodholt, J.P. 2005. Subducted banded iron formations as a source of ultralow-velocity zones at the core-mantle boundary. Nature, v. 434, p. 371-374). The densest materials found commonly at crustal levels are iron oxides and hydroxides, but today they are disseminated through much larger volumes or quartz-rich sediments. Up to about 1.8 billion years ago, they were produced in huge abundance in sedimentary rocks, along with interbedded cherts, to form banded iron formations (BIFs). That is widely agreed to have been a phenomenon only possible when the ocean was oxygen free so that iron could be dissolved in the oceans, and that they were precipitated when that Fe(II) came into contact with oxygen being produced by photosynthesising blue-green bacteria in shallow water. Without any shadow of doubt, BIFs are the densest sediment that the Earth has ever produced, with a 50:50 mix of iron oxide and chert having a density of 3900 kg m-3 at near-surface pressures, compared with 3100 for the upper mantle. Long ago, Bob Newton of the University of Chicago reckoned that they “didn’t oughta be around still”: Precambrian BIFs are so vast and so dense that they are even more likely to be subducted than oceanic basalt converted to eclogite. And they would not even need to be metamorphosed to do that. So, it has taken a long time for someone to cotton on to Newton’s typical prescience. Quite possibly, BIFs were a tectonic driving force at a time when the basalt-eclogite transformation was thermodynamically unlikely. Dobson and Brodholt observe that BIF density can only get larger (much larger; 6600 kgm-3 at CMB pressure) if they sink This is a nice hypothesis, for BIFs fit the bill exactly for the ultra-low velocity zones, and carries some interesting corollaries. BIFs contain a great deal of oxygen, in fact probably the entire productivity of the early Precambrian biosphere: that would have a biogenic isotope signature. Could that be added to any plume material emanating from the CMB? Equally, BIFs contain unusually high concentrations of transition metals, and there is another possibility for deep-mantle geochemists to juggle with. The authors also observe that iron-oxides have high electrical conductivity compared with silicates, and ponder on the electromagnetic consequences of that so close to the core. One thing seems certain; iron oxides probably would not melt, but, depending on the amount of oxygen in the core, they might dissolved in the molten outer core.
