Category: Climate change and palaeoclimatology

As well as revealing the Milankovich pacemaker for past climate change, studies of oxygen isotopes from deep-water of benthic foraminifera in marine sediment cores also give a guide to the height of former sea levels. That approach is based on several assumptions, of which two are central. One is that the isolation of deep-water organisms from temperature variations at the sea surface, which control the take up of ¹⁸O by near surface plankton: well supported by the measured constancy of cold deep ocean water. The other is that oxygen is rapidly and homogeneously mixed throughout the ocean water column. The reason why good mixing is critical stems from the very purpose of measuring benthic oxygen isotopes, itself based on a sound assumption. Ice masses on land lock up a proportion of evaporated ocean water. Evaporation favours the lighter ¹⁶O isotope in water molecules over the heavier, so that atmospheric water vapour has a lower ¹⁸O/¹⁶O ratio than seawater. When snow falls and turns into glacial ice that build up ice caps, surface water of the oceans becomes depleted in ¹⁶O so that its ¹⁸O/¹⁶O ratio (standardised as the δ¹⁸O value) increases. That makes oceanic δ¹⁸O values, measured from benthic foram shells, an indirect or proxy measure of both the amount of ice locked up on land and changing sea levels: the principal quantification of past global climate change whose record goes back to the oldest preserved ocean floor (Lower Jurassic, ~205 Ma). Modern humans eventually left Africa to colonise the rest of the world  sometime before 60 Ma ago, the first reliable age of evidence for colonisation outside Africa. Africa is surrounded by sea, except for the narrow strip of land into Palestine that ends up in a desert dead end to further migrations. So, it seems likely that the exodus was across the outlet of the Red Sea that would have become narrower and shallower as sea level fell when the Earth moved into the last glacial epoch after 117 thousand years ago, when sea-level was as high as it is today.

The assumption of rapid, efficient mixing of the oceans has not been thoroughly tested. In fact it is estimated that any complete turnover takes around a thousand years, so there is likely to be a significant time lag in the sea-floor record. New, independent evidence also suggests that the calibration of benthic δ¹⁸O needs revision (Dorale, J.A. et al. 2010. Sea-level highstand 81,000 years ago in Mallorca. Science, v. 327, p. 860-863). It comes from caves on the Mediterranean island of Mallorca that connect directly with the sea. Stalactites and stalagmites (collectively called speleothem) have formed in the caves, their growth being affected by flooding and drying as sea level rose and fell during the last 130 ka. At each flooding level encrustations formed around the speleothem to produce bulbous growths at different heights in the caves, which are clearly forming today at mean sea level. The researchers from the US, Mallorca, Italy and Romania dated the bulbs using the U/Th method appropriate for speleothems, and found three stages of formation: at 121, 116 and 80-82 ka. The two older encrustations are at ~2.6 m above modern sea level, bang on the oxygen isotope calibration for the end of the last interglacial. However, those formed between 80-82 ka ago – a period of warming during the overall trend to colder conditions as ice sheets grew – are about a metre above modern sea level: very different from the estimate of 10-20 m below­ based on the benthic δ¹⁸O calibration.

It is too early to tell in what quandary palaeo-oceanographers will be placed by this large discrepancy. There are four main possibilities for the aberrant results. First, the Mediterranean might have stood higher that global sea level for some reason, but that seems highly unlikely as the connection through the Straits of Gibraltar is deep enough to have maintained flow even at the last glacial maximum when global sea level was around 120 m below the present. Second is that the means of calibration using raised coral reefs on tectonically rising coastlines of New Guinea and Barbados  is seriously out for part of the last glacial period. Thirdly, somehow the Mallorcan crust was depressed during the last glacial period. The island is rising at about 0.2 mm yr^-1, which would give an uplift of 16 m since 81 ka, but that conflicts with the good match with the last highest sea level at 121 and 116 ka. Finally, the authors suggest that at 81 ka the volume of the world’s ice caps was much the same as today, despite the higher-than-present δ¹⁸O values in contemporary sea-floor sediments.

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

Was the Archaean blazing hot or balmy?

Silica-rich sediments, notably cherts have been used to estimate ocean temperatures in the far off Archaean Eon. This is possible because SiO₂ and water exchange oxygen atoms as the silica mud is forming, and in doing so its two main stable isotopes (¹⁸O and ¹⁶O) are preferentially treated depending on water temperature. The cooler it is the more ¹⁸O ends up in silica. Early Archaean cherts commonly show lower δ¹⁸O values than silica-rich ocean sediments forming now, so much lower that the temperature of Palaeoarchaean seas has been judged to have been between 55 to 85º C. Discomfortingly hot for bathers, and not very plausible considering that without a CO_2­-rich atmosphere Archaean oceans would have been frozen solid because the Sun emitted much less energy than it does now. However, such estimates have to assume that the oxygen isotopic composition of seawater at 3.5 Ga was the same as now, when in fact it is known that environmental δ¹⁸O probably changes over long time periods. A way of avoiding an untestable assumption is to measure the isotopic composition of hydrogen (¹H and ²H or D) in chert as well as that of oxygen. The cooler water is, the lower δD values are in silica that is precipitated from it.  Ordinary quartz contains no hydrogen except in unstable fluid inclusions, but the way chert forms as colloidal precipitates of opal-like material locks hydrogen in the form of OH^– ions into its silica (Hren, M.T. et al. 2009. Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago. Nature, v. 462, p. 205-208). Combining the two measures for 3.42 Ga cherts from the famous Barberton Mountain Land Archaean complex results in a sea-surface temperature estimate of no more than 40º C.

Boron has two stable isotopes, ¹⁰B and ¹¹B. Like all isotopes of the same element, when boron is shifted from one host to another some fractionation between its isotopes is likely. In the case of boron being taken-up by planktonic foraminifera, their shells’ ¹¹B/¹⁰B ratios correlate with the pH of seawater. Since the pH of the oceans is dominated by the effects of dissolved CO₂, itself in equilibrium with the gas’s atmospheric concentration, boron isotope ratios in foram shells are a proxy for the greenhouse effect produced by carbon dioxide. This finding dates back to 1992, but has only recently been used. It is especially revealing for the period around the Eocene-Oligocene boundary (see Lead-in to icehouse conditions in July 2009 issue of EPN) when other evidence indicates that global cooling eventually allowed glaciers to grow on Antarctica and possibly at northern high latitudes (Pearson, P.N. et al. 2009. Atmospheric carbon dioxide through the Eocene-Oligocene climate transition. Nature, v. 461, p. 1110-1113). The boron data indicate a downward shift in atmospheric CO₂ from around 1100 to 750 ppm by volume from 34.2-33.5 Ma, the lower value just preceding δ¹⁸O data for a rapid increase in polar glaciers. Oddly, δ¹¹B then rises to levels suggesting a return to CO₂ levels of >1000 ppm by volume at a time of constant high δ¹⁸O that show the survival of ice caps; perhaps a result of increased albedo forcing.

Impact cause for Younger Dryas panned again

In 2007 two dozen scientists presented evidence to suggest that onset of the Younger Dryas, extinction of many North American mammal species and the sudden end of the Clovis culture at 12.9 ka followed upper atmosphere explosions of cometary material (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN of July 2007 and May 2008). The Clovis culture of North America, signified by superbly crafted stone spear points, occupied a narrow time range between 13.3 and 12.8 ka, i.e. up to the start of the Younger Dryas interstadial. Some Clovis occupation sites are buried by organic-rich soils. Remarkably, the original proposers of a catastrophic event (Firestone, R.B. and 25 others 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences of the United States of America, v. 104, 16016-16021) claimed that the veneers contain magnetic microspherules, magnetic grains, iridium and nickel, charcoal, soot and polycyclic hydrocarbons, carbon spherules, fullerenes that trap helium with extraterrestrial isotopic proportions, glass-like carbon, and nanodiamonds. Missing from what looks like a supportive package are shocked minerals, which are the only materials formed uniquely by impact events.

Experts on extraterrestrial influences considered the team to be ‘over-enthusiastic’. In response Firestone and co-workers made replicate samples available for independent confirmation or refutation of their claims. This offer seems not to have been followed-up, but another large team recollected the black soil veneers from two of the same sites and 5 others of similar age (Surovell, T.A. and 8 others 2009. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences of the United States of America, v. 106, p. 18155–18158). They focussed on the claim for magnetic spherules, using the same techniques as Firestone et al. (2007), yet failed to find anomalous peaks at the time of the Clovis demise and opening of the Younger Dryas massive global cooling. Their conclusion was, ‘ In short, we find no support for the extraterrestrial impact hypothesis as proposed by Firestone et al.’. However, Surovell et al. did find magnetic spherules before, during and after the interstadial event. In fact, magnetic spherules are quite common in many sedimentary settings and have a history of controversy. In the late 1980s Robert S. Foote, an oil explorationist claimed that many oilfields were associated with geomagnetic anomalies with distinctive short wavelength ‘signatures’. He became widely regarded as a crank. But he persisted and discovered the first tangible evidence for lifeforms that thrive at high temperatures in deep oil wells – shiny, tiny magnetic spherules made of magnetite (Fe₃O₄). Magnetotactic bacteria living in highly reducing conditions produce them to form magnetosome chains. Magnetosomes are also present in the brains of far-migrating birds, with connections to their remarkable feats of navigation.

Just when you think it’s going to turn out alright…

The millennium of Younger Dryas global cooling from 12.8 to 11.5 ka ago caught forager-hunters on the hop as they followed herds in the wake of the general glacial retreat after 18 ka. The shut-down of the Gulf Stream when high-latitude North Atlantic surface waters freshened may have occurred in a decade or so. The end of the YD marked the start of more modern conditions in the Holocene Epoch, when northward recolonisation resumed in earnest. Climate records, such as the δ¹⁸O proxy for air temperature in the Greenland ice cores, suggest long-term but ‘noisy’ climatic constancy. That is, until one spreads out the Holocene records. At around 8200 years ago is a 200-year downward ‘blip’ in temperature to well below the Holocene average and then recovery. The perturbation also shows up in a Newfoundland mire (Daley, T.J. et al. 2009. Terrestrial climate signal of the “8200 yr B.P. cold event” in the Labrador Sea region. Geology, v. 37, p. 831-834) as a pronounced change in δ¹⁸O from moss cellulose. The event has been ascribed to slow-down in thermohaline circulation following a further freshening of North Atlantic surface water by drainage of a remaining ice-dammed lake (Lake Agassiz) on the Canadian Shield. By 8.2 Ka the northward spread of flora and fauna from refugia around the Mediterranean Sea was well underway, and included the arrival in southern Europe of Neolithic farming practices: the start of an agricultural revolution that was to reshape the entire sociocultural ethos of the ‘Old World’, from which today’s globalisation emerged. So it is interesting to learn that the ‘cold blip’ also left a signature at 41º N in northern Greece (Pross, J. et al. 2009. Massive perturbation in terrestrial ecosystems of the Eastern Mediterranean region associated with the 8.2 kyr B.P. climatic event. Geology, v. 37, p. 887-890). This study uses pollens collected from a lake-bed sediment core. The climatic event involved a rapid drop by 30 % in tree pollen abundances, matched by a 10% increase in pollen from shrubs, such as Artemisia (wormwood) normally associated with steppes further north. The end of the event involves a more sedate recolonisation by trees. From the pollen can be estimated the actual fall in winter temperature, which amounts to a devastating (for agriculture) decrease that was greater on average than 4º C. Interestingly, the German-French-Greek-Australian team ascribe some influence on the cooling to a spread of the Siberian High, a winter build-up of cold air on the steppes to the north of the Carpathians. The magnitude and extent of the Siberian High depends to a large extent on the albedo of the steppes in winter, which depends on snow cover and its persistence. This is a major influence today across much of Western Europe, as cold Siberian air spills from the continental anticyclone. At 8.2 ka it may have forced katabatic winds through Carpathian passes to cause winters that may have devastated the early farmers of northern Greece.

In his latest book, The Vanishing Face of Gaia: A Final Warning (Allen Lane, London, 2009, ISBN 978186141850), James Lovelock more or less gives up on the ability of humanity in general, and science and engineering in particular, to fend off looming climatic catastrophe. He reserves his sharpest criticism for what he calls ‘American science’; a fundamentally reductionist approach that is fed into prediction of the future. For Lovelock, the assumption ‘that all we need to know about the climate can come from modelling the physics and chemistry of the air in ever more powerful computers’ has been a disastrous mistake. He is obviously not one for humble retrospection, as his early Gaia writings had at their centre a sort of reductio ad absurdum of that now prevailing genre in Earth system science. Daisyworld, reduced a planet’s life forms to white and black daisies, whose interplay with climatic change was governed by a formula known as a difference equation in the manner of Lotka and Volterra’s work on predator-prey interrelationships. The simplest difference equation is x_next = rx(1-x). Solving such non-linear relationships for minute increments in x led to the unmasking of chaos theory, the first instance being Edward Lorentz’s discovery that the simplest models of climatic turbulence go wonky if you tinker with them: the ‘Butterfly Effect’.

When his Gaia hypothesis drew together all manner of people from New Ageists mathematicians working on complex systems James Lovelock was exposed to friendly criticism and education about non-linearity and chaos. Clearly that revolutionised his world-view, which is fine, albeit a cause of some glumness for him. Far sadder is that he is probably right in criticising climate modelling – now that it has a stranglehold on the entire climate debate and indeed on the ears of the ‘Great and the Good’. A measure of where modelling has led is a simulation of what happened as the Northern Hemisphere emerged from the last glacial maximum, between 22 and 10 ka (Liu, Z. and 13 others 2009. Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming. Science, v. 325, p. 310-314). These ~10 millennia saw a return to a see-saw climate that lasted from 60 to 30 Ma as the Earth cooled towards the last glacial epoch, dominated by cooling-warming cycles with a similar pattern of slow cooling-sudden descent into frigidity-thousand year cold spells-sudden warming known as Dansgaard-Oeschger cycles.

The Chinese-US team developed and ran the first synchronously coupled atmosphere-ocean general circulation model to investigate a hiccup in warming of the sea surface one northern ice caps began to melt decisively. It is said to be ‘one of the most epic numerical modelling efforts of the climate community to date’ (Timmermann, A. & Menviel, L. 2009. What drives climate flip-flops? Science, v. 325, p. 273-274). Epic, well yes: one of the world’s largest operational supercomputer (Jaguar at the Oak Ridge National Laboratory, USA) was wrangling for 18 months. Lots of known empirical data for the period were fed in: insolation determined by astronomic effects; changes in greenhouse gases from ice cores; shifts in coastlines and ice-sheet volumes. Tinkering with the model involved varying freshwater influx to high-latitude North Atlantic seawater. The result was crude simulation of what actually happened to sea-surface temperatures at several locations around the North Atlantic, giving some insights into why changes occurred. But climate scientists have long suggested mechanisms for the Dansgaard-Oeschger cycles, Bølling-Allerød warming, and the final frigid paroxysm of the Younger Dryas in much the same framework, the only difference being they didn’t produce numerical models that mimicked reality.

It seems that another 2 to 3 million hours of time on Jaguar are needed to bring the project through to the present. The enormous funding needed to get this kind of number crunching done can only have been on the back of claims that it will help predict future anthropogenic climate shifts. Based on real data, it still didn’t get things right – millennium-long cooling and warmings are not trivial events. There are conflicting kinds of data for changes in the parameters since the start of the Industrial Revolution preceded by 10 ka of relatively stable Holocene conditions. The best that climate forecasting for the next 100 years has been able to do, also using pretty large amounts of CPU time, is a range of straight lines showing increases in global mean surface temperature. Yes, hindsight is wonderful…

At 33.5 Ma, around the time of the Eocene-Oligocene boundary, Earth’s climate took a sudden shift towards cooler conditions, coinciding with the onset of glaciation in the Northern Hemisphere and growth of Antarctic ice cover. Studies of a variety of proxies, including the density of pores or stomata on plant leaves, suggests that the transition resulted from a halving of atmospheric CO₂ content from more than 1000 ppm in the Early Eocene to ~560 ppm in the Oligocene. So, even at twice the pre-industrial level greenhouse warming was compatible with high-latitude frigidity. Ocean-floor sediments from a site close to the Arctic Circle in the Norwegian-Greenland Sea yield pollen and spore records that chart vegetation change from 50 to 30 Ma (Eldrett, J.S et al. 2009. Increased seasonality through the Eocene to Oligocene transition in high northern latitudes. Nature, v. 459, p. 969-973. The proxy data suggest that in the period preceding the decisive global climate change conditions became increasingly seasonal, with greater differences between winter and summer temperatures. This was largely due to increasingly cold winters, a more constant summer temperature suggesting that any land ice on Greenland was of the valley type rather than an all-covering ice sheet.

Around 13 thousand years ago, the world was warming rapidly and the great northern ice sheets in retreat. Plants, animals and humans in Europe were able to and did migrate northwards. Sea level still being low, there was nothing to stop decolonisation of Britain by crossing the huge fluviatile plain of Doggerland where the southern North Sea now stands.(see Return to ‘Doggerland’ in September 2008 issue of EPN). At 12.9 ka there came the shock of a rapid temperature fall at the start of the Younger Dryas episode, when ice sheets began to re-establish themselves in the upland areas of Britain and Scandinavia. What happened to those intrepid migrants we may never know, but what they would have faced had they chosen to remain in the game-teeming NW Europe of that episode has become clearer with detailed investigations in sediments at the bottom of a Norwegian lake supplied by melt water from glaciers (Bakke, J. et al. 2009. Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience, v. 2, p. 202-205).

The research by Norwegian and German scientists used two interesting proxies for glacial advance and retreat: the amount of sedimentary titanium and the density of the sediment, both of which would have varied with the rate of glacial erosion. The data were calibrated to time by 96 ¹⁴C dates, and the sampling frequency (every 0.06 mm for Ti and 5 mm for density through a 1.4m core that represents 1700 years) was sufficient potentially to resolve events to a few days and 6 months respectively. Allowing for background ‘noise’ effects, certainly monthly and annual changes should show up, and indeed they do. The pattern is one of rapidly changing conditions between warm and frigid, which the authors interpret as a result of repeated ‘boom and bust’. At 12.18 ka, further cooling occurred and the late Younger Dryas is the more chaotic part of the record. The hypothesis is that the fluctuations reflect growth and shrinkage of sea ice in the North Atlantic, matched by growth and melting of glaciers. Brief warming during periods of prevailing westerly winds melted glaciers, but fed vast amounts of fresh water to the North Atlantic that in turn encouraged surface waters to freeze. Sea-ice formation and the build-up of a polar high pressure area drove weather systems conducive to westerlies southwards, when glaciers grew. Something suddenly stopped this chaotic behaviour and high latitudes rapidly emerged from frigidity at 11.7 ka: the Holocene had begun and, soon, so would humanity in an equally chaotic manner.

 

Climate at the Eocene-Oligocene (E-O) boundary

Oxygen isotopes from benthic foraminifera in deep-sea sediment cores show an abrupt increase in δ¹⁸O at around 34 Ma, which spanned a mere 300 ka. This is taken to indicate withdrawal of ocean water to polar ice caps on land that diamictites from high southern latitudes link to the beginning of glaciation of Antarctic. Then as now, the south polar region was thermally isolated, probably as a result of its having become surrounded by seaways and development of the Antarctic Circumpolar Current from the Palaeocene onwards as a result of the final break-up of Gondwana when it became separated from Australia and South America. Other factors at the E-O boundary seem to have been decreasing atmospheric CO₂ and low solar heating as a result of the Milankovich effect. Cooling due to such factors was disrupted and delayed by the spectacular global warming at the Palaeocene-Eocene boundary (55.8 Ma) as a result of massive methane release to the atmosphere. Detailed proxy records from both high- and low-latitude sea-floor sediment cores now resolve fine detail of the shifts in sea-surface temperature (SST) at the E-O boundary (Liu, Z.  et al. 2009. Global cooling during the Eocene-Oligocene climate transition. Science, v. 323, p. 1187-1190). The most profound shift in SST took place at high latitudes (in both Northern and Southern Hemispheres) with a drop of around 5 to 9ºC between 34 to 33.5 Ma. This was followed by slight rise to about 3ºC below pre-E-O conditions. Surprisingly, data from low latitudes ‘flat-lined’ at around 28ºC across the transition, suggesting steady evaporation of seawater, more of which would have precipitated as snow at high latitudes. The ‘hothouse’ conditions of the Cretaceous and early Cenozoic saw estimated high-latitude sea-surface temperatures rise from about 7ºC to 12ºC by the Early Eocene. The protracted global cooling that followed reached about 7ºC by about 42 Ma, which stabilised until 40 Ma when SST fell to about 4ºC just before the E-O boundary (see http://www.learner.org/courses/envsci/visual/visual.php?shortname=cenozoic).

The sudden start of Antarctic glaciation at 34 Ma looks increasing like an example of a chaos-like ‘flip’ in global climatic conditions brought on by a blend of factors that collectively reached a threshold, which once crossed permitted no escape, at least not over the last 30 Ma or so (Kump, L.R. 2009. Tipping pointedly colder. Science, v. 323, p. 1175-1176). That is a point that should not be lost at a time when anthropogenic global warming continues unabated, despite so much hype by the G20 leaders at their London meeting in early April 2009. Climatic ‘flips’ can go either way.

 See also: Documenting the Palaeogene transition from ‘hothouse’ to ‘icehouse’ in EPN for August 2005, and Magmatic link to the Palaeocene-Eocene warming in EPN for July 2007

One of the fuelling factors in the debate about short-term climate change during the Holocene is the suggestion that variations in cosmic-ray bombardment might affect climate through these extra-solar particles’ possibly nucleating low-altitude clouds. It is a complicated idea, because changes in the Sun’s activity – the solar wind – modulate the cosmic ray flux, and short-term changes in solar irradiance at the Earth’s surface have also been suggested as a climate driver. To further obscure matters, any changes in the geomagnetic field would also affect cosmic-ray flux, yet geomagnetism is in turn a measurable proxy for cosmic ray intensities on Earth (Knudsen, M.F. & Riisager, P. 2009. Is there a link between Earth’s magnetic field and low latitude precipitation? Geology, v. 37, p. 71-74). The two Danish scientists have compiled the Holocene record of the geomagnetic dipole moment: effectively a measure of the strength of the magnetic field. In the paper they compare that record with δ¹⁸O changes in stalactites from China and the Oman, which are a proxy for changing low-latitude precipitation – in part the signal of the Indian Ocean monsoons. A correlation did emerge from the study, supporting the cosmic ray-climate theory. This further complicates the Earth’s climate system and therefore the models used by climatologists.

The peridotite mantle sequence of ophiolites often shows signs of having been altered by processes that form calcite and magnesite (CaCO₃ and MgCO₃) veins. It is a mundane feature and few geologists have paid it any heed, other than to note the veining. Such theories as there are generally suggest that the veining took place at the time of obduction of the ophiolitic masses onto continental margins, which was generally accompanied by some metamorphism. Nonetheless, the veins must have taken up carbon dioxide from some reservoir, either hydrothermal fluids derived from seawater or groundwater, but ultimately from the atmosphere: there are no primary carbonates in ophiolites. Dating the veins was deemed impossible, but someone had a go at veins in the Oman ophiolite using the ¹⁴C method (Keleman, P.B. & Matter, J. 2008. In situ carbonation of peridotite for CO₂ storage. Proceedings of The National Academy of Sciences of the USA, v. 105, p. 17295-17300), discovering a great surprise; the veins are very much younger than the Eocene age of ophiolite emplacement. Their ages span 1.6 to 43 ka, about the same as the period over which a surface tufa deposit formed. Calcite and magnesite form by the breakdown of olivine and clinopyroxene in the presence of slightly acid water in which CO₂ is dissolved, their young ages suggesting the veins formed during weathering by rainwater, the tufa deposits probably forming through related processes. Keleman and Matter estimated the volume of veins in peridotites exposed in new road cuttings at about 1%. The 15 m thick weathering horizon in the exposed Oman peridotite therefore corresponds to about 10¹² kg of CO₂, which accumulated at an average rate of around 4 x10⁷ kg of CO₂ per year. If this could be increased by 100 thousand times, the Oman peridotite could sequester about 10% of anthropogenic emissions. Is that possible?

Higher temperatures could speed up the carbonation reactions. The reactions are exothermic and sustaining a temperature around 185ºC is feasible by stimulating the reactions through shallow drilling and pumping carbon dioxide and water into shattered rock. Interestingly, the reactions might be capable of limited geothermal power generation. The potential absorption by such a plant in the Oman ophiolite could be up to 1 billion tonnes of CO₂, and there are many other ophiolites rich in olivine. But that is not the end of the story: other olivine breakdown reactions involving water generate hydrogen, as discovered by Australian hydrogeologist Gordon Stanger. While conducting his PhD field work in Oman as part of the Open University Oman Ophiolite Project, Stanger discovered natural springs from which hydrogen gas was bubbling (Stanger, G. 1986. The hydrogeology of the Oman mountains. Unpublished PhD thesis, The Open University, Milton Keynes, UK).

The most interesting times in human prehistory were those when African beings set off from their home continent for new habitats. The earliest seems to have been the migration of Homo ergaster some 1.8 Ma ago, and the most riveting, of course, was that of modern humans who set out to colonise the entire habitable planet sometime around 80 to 60 ka ago. It is pretty certain that the population movements were driven by environmental changes that provided a driving pressure to seek survivable conditions beyond Africa, such as episodes of drying in East Africa, and passable exits from the continent, such as sea-level falls to produce land bridges like that of the Straits of Bab el Mandab. One of the glaring gaps in knowledge about those circumstances is evidence for climate change from Africa itself. The problem has been that many of the Great Lakes did not fill until the last 12 ka or so, so provide no sediment cores and proxy climate records for the crucial period in human history. Lake Tanganyika is an exception, being so enormously deep that it survived much of the last glacial episode when Africa was probably a lot drier than now. Cores of Lake Tanganyika sediment reach back at least to 60 ka (more might be had if coring was done using drilling rather than piston coring) and a surprising record has emerged from that time (Tierney, J.E. et al. 2008. Northern Hemisphere controls on tropical southeast African climate during the last 60,000 years. Science, v. 322, p. 252-255).

Deuterium and organic geochemical data from the cores are proxies for water temperature and precipitation in the lake’s catchment, and show fluctuations that clearly match the familiar patterns of climate change from Greenland ice cores, and the intensity of the Asian monsoon recorded in Chinese cave deposits. This match shows clearly that the East African climate followed closely the orbitally-induced changes in solar input at high northern latitudes. But the cause of the linkage is not clear. One candidate is the varying position of the Intertropical Convergence Zone (ITCZ). Yet it seems that known shifts in the ITCZ are not linked to East African fluctuations. So the connection with the Asian monsoon hints at controls by the changes in Indian Ocean sea-surface temperature. The ‘teleconnection’ is characterised by very abrupt shifts from humidity to aridity, and profound aridity around 57, 47.5 and 38 ka. These may have resulted in extreme ecological shifts in the southern East African Rift System, resulting in considerable stresses on human groups. Sadly, data from the most probable first period of migration out of Africa by modern humans (70-80 ka) have not been reached by the piston coring method – maybe they will eventually be accessed by rotary drilling. However, the close linkage with the Greenland record does suggest that cool/arid conditions occurred in the modern human heartland around 70 and 74 ka, when sea level was beginning to fall to 80 m below that at present.

Younger Dryas and the Bat Cave

It seems that bats have a remarkable loyalty to their chosen cave, whatever the weather. Thick guano deposits coat the floors of most caves that are now popular with bats. While the deposits are bioturbated by a narrow range of unwholesome insects, sufficient stratigraphy remains intact for more intrepid scientists to chance their hand at proxy records of climate in the caves’ vicinity; but data are, unsurprisingly, rather scanty. Arid conditions enhance preservation of such cave-floor deposits, and Bat Cave in the Grand Canyon of Arizona has attracted attention (Wurtster, C.M. et al. 2008. Stable carbon and hydrogen isotopes from bat guano in the Grand Canyon, USA, reveal Younger Dryas and 8.2 ka events. Geology, v. 36, p. 683-688). The team from Scotland, Canada, the USA and New Zealand show that both the Younger Dryas and a lesser global cold spell at 8.2 ka are discernible in the guano core from Bat Cave, but the signals arise from a rather circuitous cause. Bat guano is largely made up of the chitinous remains of the insects eaten by the bats, and it is the isotopic variation in the insects’ diet that the chitin preserves. That in turn stems from local vegetation, in some cases pollen or nectar consumed by the bugs, or even the blood of mammals or birds taken by bloodsucking insects – itself several metabolic steps from the local vegetation. These complexities may account for the rather ‘noisy’ guano data, yet it seems likely that other caves will be probed in arid areas where speleothem (from stalactites) has not developed continually through the caves’ lifetimes.

Whereas Antarctica began to develop significant ice caps in the early Oligocene (maybe in late Eocene times) those of the Northern Hemisphere, principally on Greenland, did not arise until about 3 Ma ago. There are several hypotheses for that onset of the Great Ice Age: closure of the Panama seaway and increased poleward heat transport in the North Atlantic; perhaps related development of the El Niño cycle in the East Pacific; uplift of the Himalaya and Rocky Mountains changing atmospheric circulation; lowered atmospheric CO₂, and a combination of all four that allowed the Milankovich astronomical forcing to get a grip on Earth’s climate ‘machine’. Testing the hypotheses is somewhat more difficult than find empirical support for them; i.e. coincidences in timing. Climate scientists from Bristol, Cambridge and Leeds universities in the UK have attempted such a test, using a complex climate model involving coupled atmosphere-ocean circulation and ice-sheet models (Lunt, D.J. et al. 2008. Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO₂ levels. Nature, v. 454, p. 1102-1105). Only a decrease in the greenhouse effect could have transformed climate over Greenland sufficiently to equip it with a large ice sheet, the other three main hypotheses falling a long way short, although each could have led to small ice volumes. Significantly, the study failed to find support for any of the terrestrial processes having been capable of ‘priming’ orbital and rotational forcing to such an extent that they triggered glaciation. Despite the claims by the authors, as computing power goes up and the resolution of feasible climate modelling comes down it is quite likely that within a few years there will be another view ‘supported’ by models.

Climate shock of the Younger Dryas

Between 12,900-11,500 years before the present, high northern latitudes returned to almost full glacial conditions, after about 6000 years of warming since the last glacial maximum. Just prior to the Younger Dryas cooling event, conditions had warmed sufficiently that European people had migrated northwards, some to occupy what are now the British Isles. Temperate grasslands teeming with game were the probable attraction, and still-low sea levels permitted crossing of what became the North Sea. Although it is possible that some people remained in Britain through the thousand-year mini glaciation, conditions would have been at the extremes of winter cold and year-long windiness, judging from the Greenland ice-core records of air temperature and dust. Those records have shown for some time that the transition from warmth to frigidity was rapid, but not how rapid. The cold spell had much in common with sudden, millennial-scale coolings repeated several times during the run-up to the last glacial maximum. Each such event has been linked with interruptions in the shallow and deep circulation of North Atlantic ocean waters, a likely trigger having been reduction in the salinity of surface waters as a result of floods of fresh water, either through collapses of ice caps and melting of icebergs or, in the case of the Younger Dryas, release of massive amounts of fresh water from glacially-blocked lakes in North America. One result would have been failure of cold surface water to sink at high latitudes, thereby shutting down the suction effect that drags warm water northwards to raise temperatures, especially in NW Europe.

There are concerns that unsuspected climate shifts that stem from the Earth System rather than astronomical influences – the Milankovich effect – may characterise the period of global warming caused by human activities. Increased precipitation at high northern latitudes or melting of ice on Greenland could result in falling ocean salinity and slowing or shutdown of the North Atlantic heat conveyor. Two sets of data published in August 2008 highlight potential climate shifts that may arise with virtually no warning. Both rely on the potentially high resolution of cores through ice caps and stagnant lakes that are annually layered, which has hitherto not been fully exploited by climate scientists. European and North American researchers have focussed on the upper part of the latest core through the Greenland ice cap, using two or three samples from each annual layer (Steffensen, J.P. and 19 others 2008. High-resolution Greenland ice core data show abrupt climate change happens in few years. Science, v. 321, p. 680-684). Deuterium and oxygen isotopes during the onset of the Younger Dryas show a marked cooling at the source of moisture precipitated as snow within 1 to 3 years, which the authors ascribe to the Intertropical Convergence Zone migrating northwards through a major change in atmospheric circulation. Temperature over the Greenland ice cap also changed, but over about 50 years [note however, that the sharp warming of the Bolling episode took less than a decade].

The second study uses annually varved lake sediments that accumulated in an isolated lake in central Germany that filled a circular depression formed by explosive volcanism (Brauer, A. et al. 2008. An abrupt wind shift in western Europe at the onset of the Younger Dryas cold period. Nature Geoscience, v. 1, 520-523). The seasonal sediment layers change in thickness, colour and mineralogy as warmth gave way to the frigidity of the Younger Dryas. One of the proxies, the iron content of the sediments deposited under anoxic conditions during winters fell significantly within a year at 12679 BP, along with a 4-5 fold increase in the rate of sediment deposition. Together with shifts in the lake biota, these features suggest to the authors that within a year wind strength increased greatly, probably due to a greater incidence of storm-force westerlies brought on by a change in the position of the jet stream. Today, westerly winds add to warming in northern Europe, around 12.7 ka they added to cooling, which can only be explained by global cooling or a southward excursion of sea ice in the North Atlantic.

Neither abrupt climate shift can be produced by validation of today’s climate models using actual data from the time just before they took place. It follows therefore that similar shifts in the near future could make themselves felt with no warning.

Opinion has drifted back and forth regarding the global effects of the Younger Dryas, evidence for its effects in the Southern Hemisphere being scanty. The best place to look for direct evidence would be in mid-latitude glaciers, especially where they are abundant in South America and New Zealand. A study of the largest of these, the Southern Patagonian Icefield (Ackert, R.P et al. 2008. Patagonian glacier response during the late glacial-Holocene transition. Science, v. 321, p. 392-395) indicates that the ice there advanced around the time of the YD. However, its dating indicates that the advance lay outside the 1300 year span of the cold period in the Northern Hemisphere. It was more likely due to a local response to increased precipitation from air moving from the east.

See also: Flückiger, J. 2008. Did you say “fast”? Science, v. 321, p. 650-651.

Polar ice cores have presented us with the most exquisite records of how high-latitude climate has changed in the recent past from indirect clues presented by variations in stable isotopes of oxygen and deuterium (temperature change), dust and sulfate content (aridity and volcanicity respectively) in layers of ice. That proxy record extends back to 800 ka in the Dome C core from Antarctica, showing in great detail the course of the last nine glacial-interglacial cycles, both the astronomical effect of a changeover from a 40 ka pacing to one of around 100 ka and many intricacies on a millennial time scale. The most tangible archive of information resides in the air bubbles trapped by the original snow that eventually turned into ice. That reveals how the intricate pacing of climate change has been almost perfectly tracked by the global carbon cycle as shown by changes in the concentrations of carbon dioxide and methane. This was first demonstrated by cores through the Greenland ice cap, which penetrate just the last glacial episode and the warmth before and after.

After several years of painstaking bubble analyses at many collaborating labs, the full 800 ka greenhouse-gas records from Antarctica have now appeared (Luthi, D. and 10 others 2008. High resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, v.  453, p. 379-382. Lulergue, L. and 9 others 2008. Orbital and millennial-scale features of atmospheric CH₄ over the past 800,000 years. Nature, v.  453, p. 383-386). These long records demonstrate the close connection between climate and greenhouse gases that must be maintained by complex (and not fully understood) feedback mechanisms. Different Earth processes affect the two principal gases, methane probably being controlled by effects of varying temperature and rainfall on peat-rich swamps in the tropics, whereas carbon dioxide’s main driver is capture and release of carbon by the oceans. The central feature remains that of astronomical forces, with perhaps some sign of a signal from the 413 ka component of orbital eccentricity from a shift in the range of temperatures and greenhouse gases in 100 ka cycles around 450 ka ago, and a broad change in methane concentrations. Yet, despite being a pole away from high northern latitudes where comparison of the Greenland ice record with North Atlantic sea-floor sediment data revealed a northern cause for dramatic short term shifts, much the same millennial cycles characterise the whole Antarctic record. It could be that these rapid changes are proxies for the course of northern climate vagaries – there are about 75 of them in the methane Antarctic record. So stunning are the new data that they are sure to spur attempts to go back even further by more drilling in Antarctica, probably in the eastern ice cap where current air temperature and snow fall are extremely low and a greater length of time may be preserved in a smaller thickness of ice. That is because the faster snow and ice accumulate the more rapidly flow removes the record: the reason why the thick Greenland ice, although capable of yielding time resolution of as little as individual years, cannot retain records much beyond 200 ka.

See also: Brook, E. 2008. Windows on the greenhouse. Nature, v.  453, p. 291-292.

 

The yellowing of the Sahara

As Earth emerged steadily from the last glacial maximum, around 14.8 ka when temperatures were close to those of the Holocene yet sea level still had a way to rise before reaching its current level, the Sahara became a land of wetlands, lakes and grassland. Many caves within its modern arid confines contain superb artwork depicting its fauna and the forager-hunters that preyed on it. Around the time of the earliest Pharaonic civilisation on the Nile floodplain (~3000 BCE) the humid episode ended, forcing inhabitants of the Sahara either to the Nile valley of the Mediterranean coast. Having spanned the millennium-long climatic upheaval of the Younger Dryas and the relative stability and warmth of the early Holocene, why it ended is something of a mystery. A small, amazingly beautiful lake in northern Chad seems to hold the key, as it has existed and gathered sediment for at least 6 thousand years (Kröpelin, S and 14 others 2008. Climate-driven ecosystem succession in the Sahara: the past 6000 years. Science, v. 320, p. 765-768), Lake Yoa is one of several permanent lakes fed by ancient groundwater from the vast Nubian Sandstone aquifer, yet receives negligible rainfall. The uppermost lake sediments are laminated in an annual fashion so that each layer and its contents of aquatic organisms, pollen and dust can be precisely dated.

Between 4200 and 3900 years ago the lake changed from a freshwater habitat to a salt lake when evaporation overcame recharge by rain. However, the environment as a whole did not change suddenly, but progressively. The sudden change in salinity resulted from Lake Yoa losing any outflow, which previously had removed salts accumulated by evaporation of the inflowing groundwater. The lake would then no longer have had any use for humans and their livestock, but conditions did not drive people out of the Sahara suddenly.

Almost a year ago two dozen scientists presented evidence to suggest that onset of the Younger Dryas at 12.9 ka followed upper atmosphere explosions of cometary material (Firestone, R.B. and 25 others 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences of the United States of America, v. 104, 16016-16021; see Whizz-bang view of Younger Dryas in EPN July 2007). Evidence cited included: excess iridium; tiny spherules; fullerenes containing extraterrestrial helium; nanodiamonds and evidence for huge wildfires. Not quite the Full Monty, as neither crater nor shocked mineral grains were claimed, hence the teams’ opting for a cometary airburst. In North America such signs were said to overly the last known occurrences of Clovis tools at 7 archaeological sites (see Clovis First hypothesis dumped above). It was pretty clear that the suggestion for a hitherto unnoticed event with a widespread signature – 26 sites either side of the Atlantic were cited –  was going to be challenged, and so it has (see Kerr, R.A. Experts find no evidence for a mammoth-killer impact. Science, v. 319, p. 1331-1332), perhaps not unconnected with the blaze of publicity surrounding the paper’s appearance, including several TV documentaries.

Well, say experts, sooty layers do suggest large-scale fires, but forest fires occur every year, especially when humans are around. Fullerenes or ‘buckyballs’ equally can form terrestrially, except those containing ET helium. The last is regarded by many critics as ‘inventive’; they have never been isolated since such combinations were first reported in 2001 (see Extinctions by impacts: smoking artillery in EPN March 2002). The accepted methodology for detection of tiny diamonds seems to have been ignored, and that claimed to have found them misused. The iridium ‘spike’ – crucial in identifying the global nature of the K-T event – by itself is not enough for claims of impacts. Astonishingly, the authors cited such a Younger Dryas iridium spike in a Greenland ice core, yet the originator of those data says his paper does not report abnormal iridium at 12.9 ka or anywhere during the YD. Microspherules rain down all the time with interplanetary dust, and do not constitute sound evidence either.

So, what on Earth is going on? A collaboration between 26 authors, who willingly supply other workers with materials for checking surely cannot be conspiring at a hoax. Impact experts are hinting at ‘over-enthusiasm’ by a team outside the ‘impact community’. It all sounds oddly similar to the furore that in 1980 greeted  first suggestions by the Alvarezes for the K-T impact…

Accepted geoscientific ‘wisdom’ is that the Cretaceous Period was so warm that forests reached polar latitudes and so too did cold-blooded reptiles. Planktonic foram oxygen isotopes indicate that the Cretaceous ‘hothouse’ in the Turonian (93.5-89.3 Ma) produced tropical sea-surface temperatures up to 37°C; warmer than human blood temperature. It also saw sea level reach an all time high. Both features have been attributed to the rate of ocean-floor volcanism being at its highest. It has, however, been difficult to model the warmth at high latitudes without fudging the input to general circulation models.

Measuring d¹⁸O in both planktonic and benthonic (ocean-floor) forams at centimetre spacings in Turonian ocean-floor sediments seems to have truly bamboozled specialists in the Cretaceous. They reveal a period of ~200 ka  at around 91.2 Ma where both show a sharp increase (Bornemann, A. and 8 others 2008. Isotopic evidence for glaciation during the Cretaceous supergreenhouse. Science, v. 319, p. 189-192). Respectively, the peaks reflect decreased sea-surface temperature (but only down to 32°C in the tropics) and an increase in the extraction of light ¹⁶O from the oceans; only likely when ice caps build up on land. The size of the benthonic d¹⁸O increase suggests ice caps about half the size of that now blanketing Antarctica. Other evidence includes rapid decreases in Turonian sea level in Europe, North America and Russia; only likely on such a scale as a result of glacio-eustasy. However, direct evidence in the form of tillites, striated pavements and glacio-marine sediments has yet to turn up

Until these convincing data emerged, it seemed that sufficient post-Permian frigidity for large-scale glaciation had not developed until Oligocene times. However, the paradox of high-latitude ice caps and low-latitude balmy seas is resolvable. Evaporation from the tropical sea surface would have been much greater than nowadays. Transport of moisture to cooler areas may have resulted in such immense winter snowfall at high latitudes that sufficient remained unmelted after winter darkness for its albedo to further cool the polar region. Almost certainly the site for the ice cap would have been Antarctica, which in the Cretaceous, as now, sat over the South Pole. Remove the present ice, and that continent would have had an average surface height of between 1 and 2 km that would have encouraged snow build up were sufficient to have fallen during the Turonian. Yet without the direct evidence for glaciation in sediments – much would be buried by the present Antarctic ice cap, if not eroded away – the scenario is difficult for some to believe.

Holocene cold spell and glacial lake burst

The most startling event during the gradual warming after the last glacial maximum was the millennium of icy conditions between 12.5 and 11.5 ka; the Younger Dryas. Long after Holocene warmth seemed well established and agriculture had been underway for two millennia, with perhaps increased human population, a smaller cold ‘snap’ took place, between 8.21 and 8.17 ka; i.e. for about 70 years. Its main effect was around the North Atlantic, but it was felt over the whole hemisphere. It must have been devastating for early farmers and new migrants into higher latitude lands. High-resolution records of many kinds are possible for such a young event, from both ice and marine cores, and also terrestrial pollen records. Norwegian, French and Dutch climate researchers have gleaned a great deal from a sea-floor core from between southern Greenland and Labrador (Kleiven, H.F. et al. 2008. Reduced North Atlantic deep water and the glacial Lake Agassiz outburst. Science, v. 319, p. 60-64). Their combined fossil, oxygen-isotope and mineralogical study shows anomalies from about 170 years before to 100 years after the drop in regional temperatures.  These include signs of decreased saltiness of the water in the Labrador Basin and a reduction in production of deep water in the North Atlantic. This is exactly the predicted signature for a shut-down of the Gulf Stream, similar to those implicated in Dansgaard-Oeschger events through the last Ice Age and the Younger Dryas itself.

The Younger Dryas has been linked to sudden drainage of huge glacially dammed lakes that once surrounded the ice cap of the Canadian Shield.  One scenario for that is a huge, protracted flood down the St Lawrence River into the North Atlantic, another being one down the MacKenzie River into the Arctic Ocean. Freshening of surface waters by such means would have reduced the formation of the dense cold brines that sink to form North Atlantic Deep Water today. In so doing these down-wellings drag surface waters northwards from low latitudes to form the Gulf Stream that makes the western side of the North Atlantic unusually warm. If they stop or slow significantly regional air temperatures fall, as they did again around 8.2 ka. In this case the likely cause was escape of water melted from the last dregs of the North American ice sheet that had been held in a glacial lake south of Hudson Bay: Lake Agassiz.

Following its first discovery, evidence for low-latitude glacial action at several times during the Neoproterozoic has fuelled one of the most publicised controversies in the geosciences. Was the Earth totally frozen over during these episodes, or was ice confined only to parts of the surface? Whatever, the last part of the Precambrian witnessed huge fluctuations of many kinds, and after the cold epochs the first large animals made a sudden appearance. The most dramatic geochemical ups and downs in Earth history took place, in the form of sudden extreme shifts in the relative proportions of the stable isotopes of carbon in seawater, as recorded by marine carbonate rocks. These fluctuations correlate closely with the evidence for low-latitude glaciations: large negative excursions of d¹³C with glacial epochs, and positive values developing between them. The first can be interpreted as the result of massive declines in photosynthetic fixation of organic carbon. The second suggests repeated recoveries of such biological productivity, which favours the extraction of ¹²C from seawater and an increase in the relative proportion of the heavier isotope as organic carbon becomes buried in seafloor sediments.

Since organic carbon is ultimately extracted photosynthetically from carbon dioxide in the atmosphere, a link between climate and living processes (and those that bury dead organisms) can be the basis for models attempting to explain the extraordinary events of Neoproterozoic times. If large amounts of organic carbon are buried or remain suspended in the oceans, the drawdown of atmospheric CO₂ reduces the greenhouse effect and leads to cooling. Conceivably, the effect could be to so reduce global mean surface temperature that freezing conditions grip even the lowest latitudes. Once glacial and sea ice becomes established, its high reflectivity reduces the amount of incoming solar radiation that is absorbed to warm the Earth. The two processes combined would tend to lock frigid conditions in place until such time as gradual release of volcanic CO₂ increased the atmospheric greenhouse effect. That is the theoretical essence of the Snowball Earth hypothesis in which complete ice cover sterilised surface biology for long periods. However, it leaves out two important factors: as water cools it is able to dissolve more gases from the atmosphere; organic carbon in ocean water can be transformed to dissolved CO₂ if it is oxidised, thereby reducing the amount of carbon being buried. Modelling the carbon-climate link in the Neoproterozoic requires that both factors are accounted for (Peltier et al. 2007. Snowball Earth prevention by dissolved organic carbon remineralization. Nature, v. 450, p. 813-818).

The model devised by Richard Peltier and colleagues from the University of Toronto also incorporates the distribution of land at the time. Results from it show a looping behaviour, with recovery from frigidity as increases in dissolved oxygen convert organic carbon to dissolved carbon dioxide, whose increasing concentration in turn leads to more escape of the gas to the atmosphere. The model also suggests how glacial and sea ice might have developed during such a cycle, and with the late Precambrian configuration of drifting continents it allows for low-latitude continental glaciation, but not for all-enveloping sea ice. The implication is indeed glacial events vastly greater than those of the late Palaeozoic and during the present Ice Age, but less effect on marine photosynthesis than from Snowball conditions – a ‘Slushball’ Peltier et al. explain why the cyclical processes suggested by the model stopped before the start of the Phanerozoic, from carbon-isotope evidence for a massive oxidation of suspended marine organic carbon around 550 Ma. Thereafter, abundant oxygen and large animals ensured most dead organic carbon was oxidised in the oceans.

Unsurprisingly, one of the authors of the Snowball hypothesis finds flaws in the geochemical argument for its impossibility (Kaufman, A.J. 2007. Slush find. Nature, v. 450, p. 807-808). Not only was oxygen likely to have been at far lower atmospheric concentrations than it became in the Phanerozoic, the glacial epochs provide evidence that its concentration in seawater was very low. The marine diamictites associated with each contain both ironstones and iron-oxide cements. For them to have formed demands high concentrations of dissolved iron in sea water, in the form of reduced Fe²⁺ ions; incompatible with widespread oxidizing conditions that would favour Fe³⁺ whose compounds are insoluble.

Some good news about carbon burial

The second largest ‘sink’ for atmospheric CO₂, after silicate weathering and formation of carbonate sediments, is the burial of organic carbon. Derived from photosynthesis of carbon dioxide in the air or dissolved in water, organic carbon descends from the photic zone of the oceans or is carried from the land by rivers. In the second case it is often believed that more than 70% of the carbon load of rivers is oxidised back to CO₂ before having a chance of being buried in marine sediments. To estimate the proportion that does contribute to carbon sequestration is a complicated matter, involving measurement of the carbon budgeting for an entire river basin and its offshore sediments. This has been done by a team of French geochemists for the huge Ganges-Brahmaputra system that drains the northern Indian subcontinent and much of the Himalaya (Galy, V. et al. 2007. Efficient carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature, v. 450, p. 407-410). This system carries a stupendous load of sediment, especially during the monsoon season. At 1 to 2 billion t of sediment deposited from suspension in the Bay of Bengal each year, this is the largest single flux of sediment from land to the ocean floor. Even more is delivered as bed load (rolling and bouncing sand particles) to build up the Ganges-Brahmaputra delta of Bangladesh and West Bengal, India. The authors found that recently produced organic carbon is about 4 to 5 times more abundant in the suspended sediment load than is reworked fossil carbon derived by erosion of ancient sedimentary rocks, which itself is predominant in the bed load. Fossil carbon makes no difference to the modern carbon cycle, provided it does not get oxidised, which is less likely than for recent organic carbon in a form that can be metabolised.

By comparing the recent organic carbon load suspended in the rivers’ flow with that in the fine sediments of the Bengal Fan, Galy et al. have been able to show that most of that carbon is conserved without oxidation. As a result, the Bengal Fan accounts annually for about 15% of global carbon burial. There are two reasons for this remarkable efficiency: the low oxygen availability in deep waters of the Bay of Bengal; the very high sediment load from erosion of the Himalaya that buries carbon before oxidation is possible. Orogenic belts in humid areas are therefore key factors in exerting negative feedback on climate, whereas drainages of flat areas, such as the Amazon and especially its main tributary the Rio Negro, encourage oxidation in their lower reaches and offshore and are less important.

Although the ice that makes up the upper parts of the Greenland and Antarctic ice sheets is annually layered, for times before about 70 ka the layering disappears because of plastic deformation. Earlier ages have to estimated from models of the deformation, and a second check is to match the data records from ice cores against those from sea floor sediments. Different processes contribute to those records: for instance, the marine record of oxygen isotopes in benthonic forams tracks the changing volume of ice locked on land, while the same record from ice cores depends on the air temperature above the ice cap. The correlation does seem to work, however. But not, it seems, for the very deepest ice recovered from beneath Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) which extends to around 800 ka.

French scientists involved in the EPICA Dome C ice-core project have cunningly discovered a means of checking on the otherwise undateable deep Antarctic ice (Raisbeck, G.M. et al. 2006. ¹⁰Be evidence for the Matuyama-Brunhes geomagnetic reversal in the EPICA Dome C ice core. Nature, v. 444, p. 82-84). The core penetrated to an estimated time that should include the most recent magnetic reversal, dated very precisely to 778±2 ka. Although the exact details of how the magnetic field behaved during this reversal, it is known that when its polarity flips the intensity of the field becomes very small. While the field is stable it is sufficiently strong to deflect charged particles, both in the Solar wind and in cosmic rays, so that less pass through the atmosphere. Cosmic rays are so energetic that they can perform isotopic transformations, one product being ¹⁰Be. So if the magnetic field decreased so the proportion of ¹⁰Be in the atmosphere would go up. Raisbeck and colleagues have examined the ¹⁰Be record in the EPICA core in great detail. In a 10 m thick section from a depth of almost 3.2 km the isotope rises to a peak, which they interpret as the signature of the reversal. If correct, this gives a ‘golden spike’ against which the depth to age conversion can be refined.

Balmy shores of the Precambrian

Before the appearance of fossil organisms that could give clues to past climates the only sources of information take the form of proxies. One of the best examples might seem to be the oxygen isotope composition of carbonate rocks that relate to sea-surface temperature. In fact it isn’t useful for the Precambrian because estimates of SST depend on being able to identify the shells of planktonic animals and use their d¹⁸O as a proxy. That is a pity, because limestones are common throughout the geological record and various aspects of their geochemistry have been used extensively as proxies for other crucial information, such as the relationship between their strontium isotope composition and the pace of continental weathering. Another palaeothermometer relies on the same temperature dependent fractionation of oxygen isotopes between seawater and the precipitation of dissolved silica to form cherts, whose d¹⁸O decreases with temperature. The trouble is that silica is notoriously prone to being remobilised and reprecipitated as pH changes in the fluids within sedimentary rocks. Some results from Precambrian cherts gave such low d¹⁸O that seawater temperature would have been tens of degrees higher than they were during the Phanerozoic, but they have been wisely suspected of having been affected by much later alteration by warmer fluids passing through cherty sequences. Now the approach has been given a boost by geochemists at the French National History Museum (Robert, F. & Chaussidon, M. 2006. A paleaotemperature curve for the Precambrian ocean based on silicon isotopes in cherts. Nature, v. 443, p. 969-972).

François Robert and Marc Chaussidon analysed the silicon isotopes in cherts for which oxygen isotope data are available. Since the two isotopic systems would both change, yet would behave differently during hydrothermal or metamorphic alteration, if the results correlate well both should be undisturbed. Except in samples that show the lowest d¹⁸O values (i.e. highest temperatures) there is a good correlation. That finding validates many of the O-isotope seawater temperatures, but Si isotopes fractionate during precipitation too, again in relation to temperature. So Robert and Chaussidon take Precambrian ocean temperature data to a new level with estimates based on two methods. Their results are fascinating: as well as confirming a decline from around 70°C 3.4 Ga ago to between 10 to 40°C in the Phanerozoic, the d³⁰Si data show sharp downward ‘spikes’ at about 2.5 Ga and 1.8 Ga. Between about 1.5 Ga to 600 Ma ocean temperature was steady at around 20°C, so there is no sign of continually cold oceans through the period of ‘Snowball Earth’ events – the number of samples cannot yet resolve the individual events, but the ‘Cryogenian’ is an obvious target for more work. The data are also important as they hint at all kinds of possible biological outcomes for such global warmth, and explanations are definitely needed.  Does the record suggest greater geothermal heating, or was it an outcome of the greenhouse effect? Will more details show periods of changing burial of organic carbon? Whatever, the Precambrian has become a stranger world to contemplate.

See also: de la Rocha, C.L. 2006. In hot water. Nature, v. 443, p. 920-921.

The so-called Cryogenian Period of the Neoproterozoic rests on evidence for coincident glaciation at all latitudes. It has been supposed to include at least two, maybe three and perhaps more frigid ‘snowball’ events, each with a pattern of lower diamictites and an upper carbonate cap rock. The most widely supposed glacial epochs are the Sturtian at 712 Ma, the Marinoan at 635 Ma and the Gaskiers at 580 Ma, but Precambrian sedimentary sequences are notoriously difficult to tie down in time. Only if dateable igneous events bracket evidence for glaciation is an age truly valid. Yet the global 3-fold division depends largely on correlation of stratigraphic and carbon-isotope sequences with the odd few that are dated in an absolute time-frame. The developing field of rhenium-osmium (Re-Os) radiometric dating offers a more universal check, since it provides a means of dating highly reduced black shales, that are abundant in the Neoproterozoic. The first reported results come as a blow to the ‘Snowball Earth’ community (Kendall, B. et al. 2006. Re-Os geochronology of postglacial black shales in Australia: constraints on the timing of the ‘Sturtian’ glaciation. Geology, v. 34, p. 729-732).

Bruce Kendall and colleagues from the Universities of Alberta, Canada and the Durham, UK have constrained some of the principal occurrences of the Sturtian event in Australia to between 643 and 657 Ma, by dating the shales which envelop the diamictites and cap carbonates. They are younger than even the widest range previously suggested for the Sturtian: either the glaciation was grossly diachronous, or this is yet another glaciation of ‘Sturtian’type. The best that can be concluded is that the ‘Cryogenian’ was cold but glaciation shifted from place to place – a ‘slushball’ model?

While most geoscientists use the products of processes that operate today to judge environments of the past, climatologists do the reverse: the past is the key to the present. While the climate record of the last 2.5 Ma is a key to understanding and perhaps even predicting rapid climate shifts during glacial-interglacial periods uncontaminated by human influences, such is the extent to which greenhouse emissions have affected the current climate that we have little idea what the outcomes may be. The possibility of greenhouse warming has become higher than in any previous interglacial epoch. To get even an inkling of what that might set in motion requires looking back to warmer times than the Late Pliocene and Pleistocene, at around 3 to 5 Ma. In the Early Pliocene it is very likely that CO₂ in the atmosphere was no more than nowadays. Because the Earth’s geography was little different from the way it is now and the Milankovich forcing was the same too, modelling Early Pliocene climate might seem to result in similar patterns, but it doesn’t (Fedorov, A.V. et al. 2006. The Pliocene paradox (mechanisms for a permanent El Niño). Science, v. 312, p. 1485-1489). Sea level was some 25 metres higher than it is at present and mean global temperature was an extra 3°C, and sea-surface temperatures (from the oxygen isotopes in planktonic foraminifera) were high as well. Despite much the same forcing factors as today, the Pliocene lacked large high-latitude ice caps in Arctic regions. Milankovich-related fluctuations were damped down compared with those of the Pleistocene. Both modelling and geological evidence from the Early Pliocene suggests that Earth’s climate was dominated by a perpetual El Niño in the tropical oceans, because of an inability of cold water to upwell periodically along the western tropical margins of Africa and South America. Quite probably such conditions had persisted for the previous 50 Ma, despite gradual overall cooling.

Fedorov and colleagues point to very different Early Pliocene climates in several regions: Mild winters in central and north-eastern North America; droughts in Indonesia and torrential rains in western North and South America. Overall, it was a much more humid world, and since water vapour is a powerful greenhouse gas warmth and humidity were sustained despite no higher CO₂ levels than now. At about 3 Ma, ocean surface waters began to cool, with signs that the alternations associated with El Niño and La Niña in the eastern Pacific began. An explanation for this is the gradual build up of very cold water deep in the ocean as a result of winds from continents cooling ocean surface water at high latitudes and causing it to sink. Without periodic upwellings, warm surface waters and cold deep waters could not mix, so inevitably the interface became shallower. At some critical depth, this thermocline could break surface, transforming both climate patterns and those of ocean currents, eventually to end up as the present tropical climate cyclicity which is connected with  other climate features of the Great Ice Age.

Fedorov et al  speculate that only a small descent of the ocean thermocline – a matter of a few tens of metres – could re-establish Pliocene conditions.  That might occur because of continued anthropogenic warming, and the ‘flip’ might be as quick as a few decades to centuries.

Between about 12.9 and 11.5 ka the progress of warming from the frigidity of the Last Glacial Maximum was rudely interrupted. For over a thousand years conditions returned to those of a mini ice age, with continental glaciers re-advancing on a large scale, an increase in aridity and a reversal of colonisation of high northern latitudes by both plants and humans. Pollen records become dominated by those of a diminutive alpine plant, the mountain avens (Dryas octopetala) from which the cold snap gets its name – the Younger Dryas. The pace at which cooling took place was dramatic, and glacial conditions swept in within a decade at most. The most likely scenario is failure of North Atlantic Deep Water to form, thereby shutting down the thermohaline circulation that draws the warming Gulf Stream into the Arctic Ocean off the northern cape of Norway. The reason for that was a massive and sudden freshening of surface water at high latitudes in the North Atlantic, but where the influx of fresh water came from is a puzzle. Wallace Broeker of the Lamont-Doherty Earth Observatory in New York State resurrected an earlier idea that a vast lake of meltwater in the region of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. Broeker has recently reviewed this hypothesis (Broeker, W.S. 2006. Was the Younger Dryas triggered by a flood? Science, v. 312, p. 1146-1148).

Oxygen isotope records from sediments in the Gulf of Mexico had been recording massive influx there of water depleted in ¹⁸O; a sure sign that the Mississippi was carrying much of the water produced by melting of the Laurentian ice sheet. That signature stops abruptly at the outset of the Younger Dryas. The meltwater must have found another outlet, but so far its oxygen isotope signature has not been conclusively discovered. As well as the St Lawrence escape route there are three other possibilities: north-westwards along the MacKenzie River valley; beneath the great ice sheet and through Hudson Bay; and by massive break-up of the ice sheet to launch an ‘armada’ of icebergs that quickly melted to freshen northern Atlantic waters. One of the clearest signs that vast proglacial lakes suddenly emptied is that they carve immense channels resembling canyons, in which there is abundant evidence for extreme scouring. Examples are the ‘channelled scablands’ of the state of Washington, and the Minnesota River valley. The volume escaping at the start of the Younger Dryas would have been so immense that such overflow channels would be dominant features of northern North America’s terrain; but there are few that fit the bill, and those that do exist are poorly constrained by radiocarbon dating. The lack of accurate dates for sediments and channels associated with the demise of the Laurentian ice sheet is the main obstacle, and surely evidence for exactly how the sudden plunge into glacial conditions was triggered will emerge sooner rather than later. One thing seems certain, the Younger Dryas was a freak event. The new ice core from Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) penetrates the previous six glacial maxima and shows no sign of a similar event at their terminations.

Sedimentary evolution of the Arctic Ocean: a start is made

For the Northern Hemisphere, especially around the North Atlantic, what happens in the Arctic exerts a strong influence over climate. On the one hand, ice-cover increases the proportion of solar energy that is reflected back to space, giving a cooling effect. On the other, cooling and increasing salinity of high-latitude water at the ocean surface results in its sinking to draw in warmer waters from further south, to extend warming further north. The two are linked intricately, for sea-ice formation adds to surface waters’ salinity. How and when the delicate balances arose remained poorly known while thick sea ice prevented ships penetrating to the highest possible latitudes in the Arctic Ocean, because the key to climate evolution depends on access to long core through ocean-floor sediments. Ironically, the decrease in Arctic ice cover with global warming has created greater access by icebreakers and drilling vessels. A consortium of countries around the Arctic funded a major effort to resolve the gap in knowledge through such a marine drilling programme in 2004. Results from the polar expedition have just begun to emerge (Moran, K and 36 other 2006. The Cenozoic palaeoenvironment of the Arctic Ocean. Nature, v. 441, p. 601-605). The cores were taken almost at the North geographic pole on the Lomonosov Ridge, a sliver of continental crust separated from its connection with the northern Russian continental shelf when North Atlantic sea-floor spreading nosed into the Arctic about 57 Ma ago.

The core is from sediments deposited on the Lomonosov ridge since it became detached from Russia, and is over 400 m long. Analyses are not yet complete, and the report by the IODP Arctic Coring Expedition covers the simplest parameters to determine: sediment bulk density and lithology, and micro-organisms. Nonetheless, these preliminary results provide a major surprise. Previously it was believed that frigid conditions in northern polar regions became established long after the Antarctic developed an ice cap 43 Ma ago, which matches the Cenozoic fall in atmospheric CO₂ and other evidence for lower mean global temperatures. The first glaciation in the Arctic was thought to be at 2-3 Ma, when pebbles dropped by icebergs first appear in the cores from the North Atlantic floor. In the Arctic Ocean core, such pebbles appear at much the same time as those around the Antarctic. They become widespread by 14 Ma. At the time of the Palaeocene-Eocene global warming, in response to massive methane emissions at 55 Ma, the Arctic waters were as warm as 18°C. The record is one of transition from a greenhouse world to an ice house. Surprisingly, considering the later influence of thermohaline processes that draw in warm water from lower latitudes, the earliest period is marked by fresh or at most slightly brackish waters. That was probably a result of isolation from the Atlantic and an excess of precipitation over evaporation. The early sediments record abundant carbon, then at around 14 Ma, the percentage of buried organic carbon drops dramatically to mark the start of increasing frigidity, when icebergs dropped significantly more debris in the Arctic Ocean.

HOW THE AMAZON FORMED

The world’s largest drainage system in the Amazon basin is so huge that it might seem to be an eternal feature of South America, at least since that continent formed when opening of the South Atlantic wrenched it from Africa in the Triassic. The upper Amazon takes much of its flow from rainfall in the eastern slopes of the Andes, but that range is still in the process of formation by tectonic and volcanic forces. A review of the Amazon’s evolution in a recent issue of Scientific American (Hoorn, C. 2006. The birth of the mighty Amazon. Scientific American, v. 294 May 2006, p. 40-47) shows that the river system is much younger than you might have expected. Lots of evidence points to the major eastward flow only beginning in the late Miocene, after 15 Ma ago. Before that drainage was northwards into the Caribbean, the reason being that the north-eastern Andes of Columbia and western Venezuela had not formed. When they did begin to rise, they hindered flow to create a huge wetland in what is now eastern Columbia. Eventually a northward drainage route was definitively blocked, so that flow took the easiest remaining route to the ocean; eastwards, to create the Amazon basin.

The groundbreaking Vostok ice core from Antarctica is the deepest ever to have been drilled. It recorded 440 ka of climate and atmospheric history, but unfortunately the very depth of the ice beneath the drilling station made that the limit in time terms. Thick ice begins to deform and flow, and the lowest parts of the Vostok core were clearly scrambled by that. The European Project for Ice Coring in Antarctica (EPICA) focussed its effort on a region of the East Antarctic ice sheet (Dome Concordia) whose location may always have ensured low accumulation of snow. Hopefully that would ensure that ice thickness was not so much as to result in complex flow at depth and that a fuller record would be preserved. The idea paid off, and the Dome C core penetrates back as far as 740 ka, giving an additional 3 glacial-interglacial cycles during the early part of the 100 ka periodicity; but falling just short of the first of those major cycles that are reflected in the marine oxygen-isotope record.

Results are now starting to emerge from Dome C (Siegenthaler, U and 10 others 2005.  Stable carbon cycle-climate relationship during the Late Pleistocene. Science, v. 310, p. 1313-1317. Spahni, R. and 10 others 2005. Atmospheric methane and nitrous oxide of the Late Pleistocene from Antarctic ice cores. Science, v. 310, p. 1317-1321). The results are high-quality, and reveal some new features. The first three cycles conform to the 100 ka signal of the very weak variation in orbital eccentricity, as expected, but show lower amplitude shifts in CO₂ and methane in air trapped in bubbles than do the later four cycles.  The two ‘greenhouse’ gases vary in concert, and their earlier low levels match with less extreme shifts in temperature as shown by the changes in deuterium content of the ice itself. This is probably due to the transition from the previous dominance by the 40 ka pace of changing axial tilt. Nitrous oxide values, although patchy down the core, seem to have fluctuated but at much the same amplitude throughout the last 720 00 ka. Dome C has yet to be ‘bottomed out’ so there is a chance that the record may yet reach the 40-100 ka boundary around 900 ka ago.  What is striking – and should ring alarm bells – from the results so far is that in each of the previous 7 interglacials atmospheric neither CO₂ nor methane levels came close to those of the last century. Whatever its eventual effects, anthropogenic addition to the ‘greenhouse effect’ is an incontrovertible fact.

See also: Brook, E.J. 2005. Tiny bubbles tell all. Science, v. 210, p. 1285-7

 

A time in Earth history (~251 Ma) when life was all but snuffed out and from which the creatures most familiar to us eventually emerged is understandably revisited quite often. Causes ranging from impacts (no convincing evidence as yet), through flood-basalt emissions, catastrophic methane release, low atmospheric oxygen to ocean anoxia have all been proposed. Hesitantly, opinion is converging on a climatic crisis of some kind, and indeed the coincidence of both terrestrial and marine faunal and flora extinctions points to climate being the global transmitter of some cause or a coincidence of causes. After the waning of Southern Hemisphere glaciations, the late Permian was warm, even at high latitudes. Until recently, attempts at modelling the end-Permian climate have not been entirely convincing because of limitations in the models themselves. Jeffrey Kiehl and Christine Shields of the US National Center for Atmospheric Research in Colorado have assembled a model that couples land, atmosphere, oceans, sea-ice and palaeogeography for the period (Kiehl, J.T. & Shields, C.A. 2005. Climate simulation of the latest Permian: Implications for mass extinction. Geology, v. 33, p. 757-760).

The critical test for the model is running it with parameters for the near-present, and it performs well. Several lines of evidence point to a much higher CO₂ level in the Permian atmosphere, so this is the main input parameter. The outcome is a world with a mean surface temperature that is 8° C higher than now. Unlike today, there was no geographic hindrance to poleward heat transport, so the high mean temperature is reflected in the summer warmth and humidity of Permian high-latitude land. The sub–tropics on the other hand were scorching (around an average summer minimum of 51° C, 15° C higher than now); a clear contributor to minimising life there. Sea-surface temperatures at high latitudes are higher in the model outcomes, this warmth extending to depths of 3 km. Surprisingly, low-latitude sea temperature emerges as much the same as now. The model also suggests that seawater was saltier than now, and that results in greater uniformity of density with depth and location: a hindrance to bottomward circulation and mixing. There would probably have been no thermohaline circulation worth speaking of. The model helps confirm the likelihood of an oxygen-free lower ocean and little transfer of nutrients. The oceans too would have been inhospitable. A shutdown of biological productivity and therefore carbon burial would have accelerated warming. So, pushing the biosphere into a mass extinction would have been inevitable. The last straw may have been the additional stress of increasing acidity from sulphur dioxide emissions from the Siberian flood basalts.

Milankovich forcing and Early Jurassic methane

Periods of environmental crisis less severe than those leading to mass extinction appear throughout the fossil record. As well as minor extinction peaks they are often signified by departures of carbon-isotope records from long-lasting norms. Such a crisis appears in the d ¹³C record of the Early Jurassic, and is beautifully preserved in about 15 m of black shales on the North Yorkshire coast of England. Geoscientists from the Open University, UK and the University of Cologne, Germany have produced an extremely high-resolution time series of carbon-isotope data from the section (Kemp, D.B. et al. 2005. Astronomical pacing of methane release in the Early Jurassic period. Nature, v. 437, p. 396-399). The quality is sufficiently good to analyse the time series using Fourier analysis that yields the frequencies that contribute to the observed wave-like patterns in the data. Of course, the time in a stratigraphic time series is measured in metres, unless it is possible to calibrate the section by precise radiometric dating. The Yorkshire Jurassic contains only fossils and no dateable horizons, but the fine stratigraphic division based on ammonites is also widespread and calibration is possible from dates obtained elsewhere. The overwhelmingly dominant frequency in the carbon-isotope curve is 1.23 cycles m^-1, which represents 21 ka after the calibration of depth to time. That is the signal of precession of the equinoxes, part of the astronomical forcing bound up in Miliutin Milankovich’s theory of astronomical forcing of climate.

Astronomical pacing turns up throughout the stratigraphic column, wherever sediments are suitable for time-series analysis (steady, unbroken sedimentation), so a precessional signal is no great surprise. The important feature is the profundity of the d ¹³C excursions; a total of –7‰, largely accomplished by three abrupt shifts of –2 to –3‰. The first two coincide with bursts in extinctions. The most likely phenomenon to have produced these shifts is massive release of methane by destabilization of submarine gas hydrates. Emissions seem to have been blurting out on a regular basis as the Earth’s rotational axis precessed like a gyroscope. So, the complete time period was one in which gas hydrate was unstable, probably due to overall warming. Yet something else must have triggered vast releases three times. The Lower Jurassic extinctions link in time with massive magmatism in Southern Africa and Antarctic (the Karoo-Ferrar large igneous province). Perhaps especially large volcanic events there set the stage for large precessional methane releases. An alternative view is that volcanic emissions of CO₂ gradually produced enough widespread warming for the astronomical trigger to cause breakdown of gas hydrate simultaneously over very wide areas of the ocean floor. Other explanations have been suggested for the Lower Jurassic warming and carbon-isotope excursions, such as wildfires, impacts and connections with petroleum maturation and migration. The clear cyclicity rules them out.

It is well-established that the first large ice sheets that presaged descent into the oscillating climate of the Neogene formed about 34 Ma ago (the Eocene-Oligocene boundary) on Antarctica. Some 21 Ma before, at the Palaeocene-Eocene boundary, global temperatures had leaped following what many believe was a massive blurt of methane previously held in cold storage in ocean-floor sediments as gas hydrate. A monstrous ‘greenhouse’ climatic system must sometime in the interim have reverted to the cooling trend begun at the outset of the Cenozoic. Defining that transformation relies on assembling and interpreting newly available, high-resolution records of climatic proxies through the Eocene and Early Oligocene (Tripati, A. et al. 2005. Eocene bipolar glaciation associated with global carbon cycle changes. Nature, v. 436, p. 341-346). Hitherto, the Eocene part of the ocean-floor sedimentary column had been poorly sampled, so that only broad trends showed.

As you might expect, the change was not a simple transition. At about 42 Ma the record of the Pacific Ocean calcite compensation depth (CCD – the depth at which carbonate remains are dissolved in the deep oceans) shows a remarkable perturbation long before the CCD dipped decisively from about 3.5 km to around 5 km at the start of the Oligocene. A close look at the oxygen isotope record of that age in a highly detailed marine sediment core shows an increase in d ¹⁸O that corresponds to either some 6° of cooling or a 120 m fall in sea-level due to build-up somewhere of ice on land. Coinciding with this perturbation are shifts in the carbon-isotope record in carbonates. The authors suggest that the mid-Eocene cooling and continental glaciation that produced falling sea level triggered the weathering of shallow-water carbonates, which together with river transport increased the oceans’ alkalinity. That would have increased deep-water carbonate formation enormously and accelerated the effective ‘burial’ of carbon from the atmosphere

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

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.

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.

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…..