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.