Scientific lessons from the Boxing Day 2004 earthquake

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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.

Zircon and the quest for life’s origin

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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.

Thermal metamorphism and ocean anoxia

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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.

How the core controls Earth’s magnetic field

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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).

Two sides to reducing carbon emissions

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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

Changing the world

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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.

New twist for end-Permian extinctions

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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.

Ejecta from the Sudbury impact

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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.

Snowball Earth gets a boost

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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.