Quaternary Glaciations: from Observations to Theories

Quaternary Science Reviews 107 (2015) 11e24

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Quaternary Science Reviews

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Invited review Quaternary glaciations: from observations to theories

Didier Paillard

Laboratoire des Sciences du Climat et de l'Environnement, IPSL, CEA-CNRS-UVSQ, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette, France article info abstract

Article history: Ice ages are known since the mid-nineteenth century. From the beginning, they have been at the center Received 2 July 2014 of theories of climate and climate change. Still, the mechanisms behind these large amplitude oscillations Received in revised form remain poorly understood. In order to position our current knowledge of glacialeinterglacial cycles, it is 3 October 2014 useful to present how the notion of climate change appeared in the XIXth century with the discovery of Accepted 4 October 2014 glacial periods, and how the two main theories, the astronomical one and the geochemical one, emerged Available online 4 November 2014 progressively both from sound physical principles but also from extravagant ideas. Major progresses in geochemistry in the XXth century led first to the firm evidence of an astronomical pacemaker of these Keywords: Quaternary cycles thanks to the accumulation of paleoceanographic data. Still, the Milankovitch's theory predicts an Paleoclimatology ice age cyclicity of about 41,000 years, while the major periodicity found in the records is 100,000 yr. Ice ages Besides, ice cores from Antarctica proved unambiguously that the atmospheric carbon dioxide was lower Models during glacial periods. Even more importantly, during the last termination, the atmospheric pCO2 in- Carbon cycle creases significantly by about 50 ppm, several millenia before any important change in continental ice volume. This fact, together with many other pieces of information, strongly suggests an active role of greenhouse gases in the ice age problem, at least during deglaciations. Since terminations are precisely at the heart of the 100-ka problem, we need to formulate a new synthesis of the astronomical and geochemical theories in order to unravel this almost two-century-old question of ice ages. The foundations of such a theory have already been put forward, and its predictions appear in surprisingly good agreement with many recent observations. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction therefore try to provide a short historical account of the development of ideas on ice ages and how they relate to the building of climate It is too often stated in textbooks but also in many scientific pa- sciences in general. In particular, I will present the birth and the pers, that Quaternary glaciations are “caused” by the astronomical development of the two major theories of ice ages, the astronomical forcing, according to the Milankovitch theory. Consequently, there is one and the geochemical one, as well as the development of the sometimes a diffuse feeling, even within scientific communities major paleoclimatic reconstruction techniques, and how these new working on climatic questions, that this problem was fully settled data have shifted the balance of evidence on one or the other side. several decades ago. But if indeed ice ages are “paced” by the as- Then, I will show why we currently need a synthesis of these two tronomy, the key physical mechanisms are far from being under- theories and finally I will provide a track towards this goal. stood. On the other extreme, quite too often, some recent papers are discussing aspects of glacial cycles, like the role of the different orbital parameters on Earth's climate, without providing any clue 2. Historical background towards the 150-year long historical context of theories and dis- fi coveries of Quaternary Sciences. But explaining that we are “standing Though it is probably dif cult to pinpoint the emergence of the on the shoulders of giants” is not only a question of acknowledg- notion of Earth's climatic change in the history of Sciences, it is ment: it is also a necessity to articulate our understanding of a sci- interesting to note that the evidence of a changing environment was entific question. As a result, there is often some confusion among described since the Antiquity. For instance, in the context of the as- students and non-specialists, on what is the current knowledge on tronomical theory of ice ages, Aristotle's writings (Meteorologica, glacial cycles and what are the key questions. In the following, I will Book 1, Chapter 14, translation by Lee, 1951) might seem quite prophetical: “The same parts of the earth are not always moist or dry, but E-mail address: [email protected]. change their character according to the appearance or failure of http://dx.doi.org/10.1016/j.quascirev.2014.10.002 0277-3791/© 2014 Elsevier Ltd. All rights reserved. 12 D. Paillard / Quaternary Science Reviews 107 (2015) 11e24

rivers. ...sea replaces what was once dry land, and where there is regions as evidenced in the coal mines, to a warm Eocene and then now sea, is at another time land. This process must however be to the present-day temperate climate. This idea was also somewhat supposed to take place in an orderly cycle.” in accordance with the “plutonist” view that rocks formed in fire and that the Earth was initially molten: For instance, in Les epoques de la nature (1778) Buffon computed an age of the Earth based on Then he speculates on the causes of such changes: the cooling rate of iron, which led to an age of 75,000 years (though “we should suppose that the cause of all these changes is that, when accounting for the slow diffusion of heat, Lord Kelvin in 1897 just as there is a winter among the yearly seasons, so at fixed obtained an age between 20 and 40 millions of years). intervals in some great period of time, there is a great winter In this general context, the discovery of ice ages was a crucial and excess rain”. step forward. Indeed, for the first time the evidences of climatic change were not based on paleontological interpretations, but on much less ambiguous, physically based observations, like moraines, Quite clearly Aristotle was not talking of ice ages, which were not erratic boulders, or glacial striations. It furthermore strongly sug- known at this time. He describes hydrological changes that were gested that climate evolution was not a simple long term cooling either observed historically, or inferred more indirectly, in Egypt or in trend, as usually believed. Greece, and insists that the changes are mostly local or regional ones, with different regions experiencing sometimes opposite changes. 3. Discovery of ice ages and early theories of climate This would be now called evidences of local or regional climatic changes. Still, Aristotle's writings were certainly very influencial in 3.1. The evidence of past glaciations the construction of modern science in general, including geology. For instance, the above short sentence is cited by C. Lyell (1830) as a The erratic blocks found in lots of places in the Alps and in “theory of periodical revolutions of the inorganic world”. The same northern Europe had been subject to many speculations for a long word “revolution” is used by Cuvier in his book “Discours sur les time, often involving giants or trolls, while the usual scientific revolutions de la surface du globe” (1825) (A Discourse on the Rev- explanation involved again catastrophic floods or diluvium. But a olutions of the Surface of the Globe). The use of the word “revolution” new suggestion slowly emerged in the first half of the XIXth cen- is quite meaningful, and the same word is also used by Adhemar tury: glaciers should be the cause. The morphology and geology of (1842) for the title of his book on the first astronomical theory of the Alps was indeed investigated by swiss scientists, in particular by ice ages (“Revolutions de la mer, deluges periodiques ”). It probably Venetz (1833), de Charpentier (1836) and Agassiz (1840), who have aims at reminding, in some way, Copernicus' book “De revolutionibus carefully described and analyzed many morphological features in orbium coelestium” which is often presented as the foundation stone, the Swiss Alps, with the unambiguous conclusion that the only valid on which Kepler and Newton developed modern physics. There was explanation for moraines, erratic boulders, or glacial striations, obviously among natural scientists a strong desire to build Geology involved episodes of significant glacial advances in the past. Similar on a similarly footing, ie. the occurrence of cyclic changes, whose observations and suggestions were also made in Scandinavia by unknown ultimate causes might possibly be related to some “cos- Esmark (1827) (Andersen, 1992) or in Scotland by Jameson, who mic” phenomena, either astronomical or divine depending on au- unfortunately did not publish his findings. A very nice and detailed thors. Cuvier's book presents his theory of cataclysmic changes that account of the history of this discovery is given in Berger (1988, paced the succession of fossils, which together with the work of 2012) or in Bard (2004) or more recently in Krüger (2013) or William Smith, led to the foundation of stratigraphy. Since the An- Woodward (2014). tiquity, geologists had found marine fossils over lands, even on top of As noted by Lyell, geological theories were still too often associ- mountains. This led to centuries of discussions on how these might ated with cosmogonies and philosophical preconceptions. In this have formed, and the dominant view at the beginning of the XIXth respect, in contrast to the plutonists, the neptunists favored the idea century was that recurrent floods or catastrophes occurred in the that all rocks are sedimentary in origin and were formed in water. past, the last one being the “Great Flood” from the Bible. Looking for They were also more inclined to follow the idea of a cold origin for the an explanation of these “revolutions” was a scientific challenge, as Earth, like the famous poet, but also minister of mining, J. W von noted by the astronomer John Herschel (1830): Goethe, a proponent of ice ages, who presents his doctor Faust in “Impressed with the magnificence of that view of geological favor of neptunism, while Mephistopheles was, of course, a plutonist. revolutions which regards them rather as regular and necessary Though necessarily very speculative, a possible scientific explanation effects of great and general causes, than as resulting from a for such glacial periods was put forward by J. Esmark. He notes that series of convulsions and catastrophes regulated by no laws and according to William Whiston's theory, Isaac Newton's successor at reducible to no fixed principles, the mind naturally turns to Cambridge University, the Earth was initially a “comet” on a very those immense periods with whose existence in the planetary eccentric orbit. It was therefore in a frozen state when far away from system the astronomer is familiar”. the Sun. Accordingly, ice ages would occur when the Earth was very young, something which soon will be proved wrong. This was nevertheless probably the first suggestion of a possible link between Still, the notion that climate might change through time, in the eccentricity of the Earth and ice ages. But it stoods somewhat in particular on a rather large or even a global scale, was controversial contradiction with astronomical knowledge at this time. J. Herschel at the beginning of the XIXth century. Many fossils from former (1830) states that “Geometers have demonstrated the absolute invari- tropical environments were found in high northern latitude, but ability” of the Earth's major axis, and therefore the length of the year: their interpretation was not necessarily straightforward. For Increasing the eccentricity would only warm up the mean climate, instance, if mammoths, ie. “siberian elephants”, were often cited as since “the total quantity of heat received by the earth from the sun in a proof of a former warm “african type” climate in arctic regions, one revolution is inversely proportional to the minor axis of the such analogies were also strongly (and rightly) criticized by other orbit”. He further adds that for any significant mean annual change, scientists (eg. J. Fleming, 1829). The dominant view was, never- the seasons would be so extreme that they would “produce a climate theless, that the Earth probably experienced a gradual cooling, from perfectly intolerable”. He also considered that neither obliquity nor a hot Paleozoic time period, with giant ferns growing even in arctic precessional changes (see below) could account for significant D. Paillard / Quaternary Science Reviews 107 (2015) 11e24 13 climatic changes (see eg. Paillard, 2010; for a brief presentation of the James Geikie (1874) in Scotland, that a succession of colder and climatically relevant astronomical parameters). warmer periods occurred. As explained by Croll (1864): The present-day distribution of temperatures was also not well “The recurrence of colder and warmer periods evidently points explained. At the beginning of the XIXth century, a big difficulty to some great, fixed, and continuously operating cosmical law. was to understand why the Southern hemisphere appears colder (… ) The true cosmical cause must be sought for in the relation than the Northern hemisphere, and in particular to account for the of our earth to the sun”. recently discovered Antarctica, a new continent that was still un- explored, but appeared covered by a huge amount of ice. A first idea was to look for asymmetries in the astronomical forcing. C. Lyell Croll's theory of ice ages is a more elaborate version of writes in his book (Princ of Geol Vol 1 p174): Adhemar's theory. He particularly insists on the role of eccentricity, as a modulation of the precessional forcing, and intense “Before the amount of difference between the temperature of the glaciations would therefore occur only when eccentricity is large two hemispheres was ascertained, it was referred by many as- enough. Still, the mechanism by which precession could act was tronomers to the precession of the equinoxes, or the acceleration of not well understood, though Croll suggests that “the great accu- the earth's motion in its perihelium; in consequence of which the mulation of ice and snow in the northern regions, arising from the spring and summer of the southern hemisphere are now shorter, severity of the seasons, would tend to keep the air in the northern by nearly eight days, than those seasons north of the equator”. regions of the globe much colder”. Croll (1867) also recognized that the obliquity could play a major role. Though “the increase in But immediately continues, citing Herschel (1830): obliquity would not sensibly affect the polar winter”, it would help “But Sir J. Herschel reminds us that … it is demonstrable that melt the ice in the summer hemisphere. This latter idea appears whatever be the ellipticity of the earth's orbit, the two hemi- extremely close to Milankovitch's theory. Interestingly, Joseph J. spheres must receive equal absolute quantities of light and heat Murphy (1869, 1876) suggested that cool summers, not cold per annum, the proximity of the sun in perigee exactly winters, were more important to grow glaciers, as evidenced by compensating the effect of its swifter motion”. numerous field data. He therefore suggested that Croll's theory should be reversed, with glaciations associated with summer at In other words, the astronomical forcing, or at least the pre- the aphelion, in accordance with Milankovitch's theory and also cessional one, cannot easily explain the current southern hemi- with our current understanding of ice sheet mass balance. Murphy sphere climate, and Lyell was convinced that the present-day (1876) still notes that: distribution of temperatures was mostly explained in terms of the “There is one fact of physical geography, which seems at first geographical distribution of lands and seas, with less land area in sight to support Mr. Croll's theory (… ). In the Antarctic regions the southern hemisphere. Consequently, he speculates that climate there is a glacial climate now” may have changed through time, due to the vertical movements of the Earth crust, as evidenced by marine fossils at high elevations: but concludes correctly that this was mostly the result of the repartition of land and sea, not the consequence of the astronom- “ let the Himalaya mountains, with the whole Hindostan, sink ical forcing. In other words, the theoretical foundations of the as- down, and their place be occupied by the Indian ocean, while an tronomical theory of climate were readily available in the late XIXth equal extent of territory and mountains, of the same vast height, century. Still, these ideas were not properly quantified. The great rise up between North Greenland and the Orkney islands. It achievement of Milankovitch was to provide a rigorous computa- fi seems dif cult to exaggerate the amount to which the climate of tion of all the necessary elements, from the astronomical planetary ” the northern hemisphere would then be cooled . parameters, down to regional seasonal insolation values. For the first time, it was therefore possible to discuss the respective role of e 3.2. Astronomical theories each astronomical parameter eccentricity, obliquity and precession e on the incoming radiation. This paved the road for con- fi Though Adhemar (1842) was aware of Herschel's objection to structing a chronology of ice ages, and nally to the success of his theory. He writes (1941): the role of precession, he suggested that, if temperature depends on the input of solar radiation, it might also depend on other factors. “I have dealt with Croll's theory (… ) and I have found that its He therefore built the first ice age theory, based on precession as inadequacy lies in the fact that the influence of the variability of exposed above by Lyell, in which longer winters are associated with the obliquity upon insolation is not sufficiently taken into glacial periods, as “demonstrated” by the Southern hemisphere account”. today. But he went as far as suggesting that the southern ocean was currently much deeper than the arctic ocean, due to the gravita- tional attraction of Antarctic ice that he assumed to be almost Indeed, the obliquity forcing is dominant at high latitudes, thus its influence on the timing of glaciations. This is even more true 100 km thick, therefore the small amount of emerged lands in the south since they are currently flooded or in a “diluvium” period. when integrating the forcing over a fraction of the year, as done by “ ” Croll (1867) also discussed the link between glaciations and sea Milankovitch in his caloric seasons since, as explained above, the level, but with more realistic assumptions concerning Antarctica: precessional forcing fully disappears in annual mean. Interestingly, these questions are still subject to discussions now (eg. Huybers “A mile of ice removed from off the antarctic continent (… ) and Wunsch, 2005; Raymo et al., 2006). would raise the general level of the sea 206 feet. And to this must be added the rise resulting from the displacement of the earth's centre of gravity, which in the latitude of Scotland mould 3.3. Geochemical theories amount to about 74 feet”. A fundamental advantage of the astronomical theories of ice Though the periodicity of ice ages was not proven, it became ages was their ability to predict the timing of glaciations. This was clear with the work of Archibald Geikie (1863) and his brother also their weakness, as noted by Arrhenius (1896): 14 D. Paillard / Quaternary Science Reviews 107 (2015) 11e24

“It seems that the great advantage which Croll's hypothesis warming of 5 Ce10 C. Svante Arrhenius pointed out that, the promised to geologists, viz. of giving them a natural chronology, atmosphere being a small carbon reservoir in contrast to the ocean predisposed them in favour of its acceptance. But this circum- or rocks, it is rather easy to change its concentration in the long stance, which at first appeared advantageous, seems with the term by accumulating small imbalances, for instance between advance of investigation rather to militate against the theory” volcanic sources, and chemical weathering, the major carbon sink on geological time scales. It is worth emphasizing that, on all these points, Arrhenius was basically correct, even if his climate model Croll's theory was indeed rejected mostly on the grounds that was erroneous (eg. Dufresne, 2009). geological data on the last Glacial period indicated an age of about The american geologist Thomas Chamberlin also explained that 10e30 ka BP (before present), thus invalidating the expectations there is no reason for the geological sources and sinks of carbon to from Croll's theory that the Last Glacial time was coïncident of the compensate, and even described (1897) what might be the first eccentricity maxima between 80 and 140 ka BP. Consequently, the coupled carbon e ice-sheet self-sustained oscillatory model, with role of past changes in atmospheric CO was considered an inter- 2 colder oceans favoring organic preservation and thus pCO esting and credible alternative theory by geologists. 2 lowering in a positive feedback, while in a later stage the ice-sheet Indeed, if astronomers or mathematicians were inclined to- advance reduces silicate weathering, thus providing a mechanism wards astronomical theories of climatic change, chemists often had to increase pCO again. A simple mathematical implementation of a different opinion. Since the work of J. Fourier (1824), it was sus- 2 this idea is shown on Fig. 1 for illustration. pected that greenhouse gases have a significant impact on Earth climate. The first suggestion that the Earth carbon cycle might not 3.4. Some remarks be constant through time and could possibly affect climate, was probably made by J. Ebelmen. In 1845, he described the major The available paleoclimatic data was very sparse in the XIXth geochemical processes controlling the atmospheric composition of and early XXth century. It was mostly qualitative and sometimes carbon dioxide and oxygen on geologic time scales. He then con- difficult to interpret. Besides, the timing of the geological past was cludes that: largely an open question, continental drift was unknown, and the “many circumstances nonetheless tend to prove that in ancient physical mechanisms governing meteorology or oceanography fi geologic epochs the atmosphere was denser and richer in car- were not fully understood. It was therefore very dif cult for sci- bonic acid and perhaps oxygen, than at present. To a greater entists to sort out the different phenomena and to build theories fi weight of the gaseous envelope should correspond a stronger focused speci cally towards only one of them. In retrospect, while condensation of solar heat and some atmospheric phenomena many of these ideas are to some extent currently still valid ones, the of a greater intensity. ” theories exposed by the scientists were too ambitious and therefore, in the end, incorrect. For instance, Adhemar was right in building an astronomical theory of climate, but wrong to apply it to Though his results were later mostly forgotten (see Berner and “periodic diluvia”, which led him to an extravagant hypothesis. Maasch, 1996), this conclusion was further reinforced by the work Croll was right to include the effect of eccentricity in this astro- of J. Tyndall. Tyndall was the first to measure the infrared absorp- nomical theory, but wrong to explain the present day Antarctic tion and emission spectra of atmospheric gases and to discover climate with it, which probably led him to reject Murphy's sug- that, while the major atmospheric constituents, nitrogen and oxy- gestion on the role of summer temperature on the ice sheets. gen, were mostly transparent to infrared radiations, the greenhouse Similarly, proponents of the role of CO2 where eager to explain effect was due mostly to water vapor and carbon dioxide. Tyndall Eocene climates as well as glacial climates in the same general was also an accomplished mountaineer: he was the first to climb framework of small imbalances in the carbon fluxes. Opinions were the Weisshorn swiss peak and wrote a book on the dynamics of often rather clear cut, probably to compensate for the sparsity of glaciers (1860). He was well aware of the climatic and geologic geological information. Still, a few scientists tried to accommodate implications of his spectrometric measurements (1861): in some ways for what each theory had best to offer on each point. “Now if, as the above experiments indicate, the chief influence For instance, Ekholm (1901) rejected Croll's theory based on pre- be exercised by the aqueous vapor, every variation of this con- cession, being convinced that CO2 was responsible for ice ages, as stituent must produce a change of climate. Similar remarks explained by Arrhenius. Still, he explained that smaller climatic would apply to the carbonic acid diffused through the air (… ) variations, such as the cooling observed in Scandinavia over the Such changes in fact may have produced all the mutations of Holocene, was best accounted for by the decline in obliquity, a climate which the researches of geologists reveal. ” conclusion that certainly holds today (eg. Marchal et al., 2002). He concluded that: “ … This led the Swedish chemist Svante Arrhenius to perform the the present burning of pit-coal is so great that it will un- first computation of the role of CO on climate. As explained by S. doubtedly cause a very obvious rise of the mean temperature of 2 … Arrhenius (1896), his motivation was not theoretical, but was the Earth By such means also the deterioration of the climate aiming at answering the ice age problem: of the northern and Arctic regions, depending on the decrease of the obliquity, may be counteracted. ” “I should certainly not have undertaken these tedious calculations if an extraordinary interest had not been connected with them. In the Physical Society of Stockholm there have been occasionally very lively discussions on the probable causes of Ice Age … ”. 4. From paleoceanographic discoveries to modern astronomical theories

He computed that a one-third reduction in atmospheric pCO2 4.1. Marine geology and isotope geochemistry would result in a global 3 C cooling, something which was consistent with the available data. He also explained the warm During the XXth century, major advances on the ice age problem Eocene climates by multiplying pCO2 levels by two or three, thus a concerned the accumulation of many different paleoclimatological D. Paillard / Quaternary Science Reviews 107 (2015) 11e24 15

Fig. 1. A “Chamberlin carbon-Ice sheet oscillator”. Solution of the coupled system { V' ¼ (C0eC)/tV; C' ¼ aCe (V0e V)/tc }, ie. the ice volume V(t) decreases when it is warm, i.e. when atmospheric carbon dioxide concentration C(t) is high, while C(t) increases when the ice volume is large due to reduced silicate weathering (here C(0) ¼ 300 ppm; V(0) ¼ 120 m sea lev equi., C0 ¼ 300, V0 ¼ 100, tV ¼ 10, tc ¼ 5, a ¼ 0). The organic matter feedback a is destabilizing: if a > 0, the model would spiral up in an unbounded fashion (if a < 0, the oscillations are damped). A more realistic model should include non-linear terms. records, together with their more and more detailed interpretations. Today it is clear that stage 2 and 4 are only parts of the same Key ingredients were the development of marine geology, isotopic glacial period within the last climatic cycle. Even if Emiliani (1955) geochemistry, geochronology and also plate tectonics, which notes: together provided the necessary general informations on ice ages. In “some inconsistencies. For example the lower value of the this respect, the Swedish expedition on the Albatross (1947e1948) insolation peak at 45,000 years corresponds to the low- was a landmark event. Indeed, the slow accumulation of sediments temperature maximum of stage 3” in the bottom of the ocean was long suspected to provide the time- continuous records needed for a better understanding of past cli- it was extremely tempting to associate the climatic cycles to the mates. The techniques used before, on the Challenger expedition insolation cycles from the Milankovitch theory. These are largely (1872e1876), or on the german ship Meteor (1925e1927), could not dominated by obliquity, therefore the 40 ka cyclicity of Emiliani's recover more than about 1 m of sediment. B. Kullenberg devised a ice ages in 1955. A decade later, in a compilation of isotopic data, new piston-corer that could easily provide sediment cores up to ten Emiliani (1966) suggested significantly older ages for the earlier or 20 m long (eg. Pettersson, 1953). Equiped with this new device, stages, but the correlation with insolation, or more precisely the Albatross was able to recover in her world-round expedition over “caloric seasons”, was still holding quite well, with ice age cycles 200 deep ocean sediment cores, and some of them were used as corresponding to either one or two obliquity cycle. rosetta stones for the ice age problem. They were studied by Gustaf The association of light carbonate d18O isotopic values with Arrhenius, Svante Arrhenius' grandson, who was on board. In warmer temperatures was largely confirmed by many comple- particular, he observed the evidence of very clear cycles in carbonate mentary studies, in particular on micropaleontological faunal as- deposition (Arrhenius, 1952). semblages (eg. Ericson and Wollin, 1956; Imbrie and Kipp, 1971). At about the same time, Harold Urey, who received the nobel But its precise quantitative interpretation became more clear, with price for his discovery of deuterium, discussed the thermody- the work of Shackleton (1967) who showed that, in contrast to namics of isotopic exchanges. He writes (1947): Emiliani's assumptions, the isotopic content of the ocean was a significant part of the signal. In addition to being paleother- “These calculations suggest investigations of particular interest mometers, marine isotopic curves were also rather direct records of to geology … Accurate determinations of the 18O content of the ice-sheets volume, ie. a global signal that could, in particular, be carbonate rocks could be used to determine the temperature at used for stratigraphy. This was further reinforced by the dating of which they were formed”. fossil coral reefs (Chappell, 1974; Chappell and Shackleton, 1986) which clearly confirmed the link between the marine isotopic re- Consequently, using some cores from the Swedish expedition, cords and sea level changes. Emiliani (1955) performed the isotopic analysis of fossil foraminifera, thus providing for the first time an unequivocal and quan- 4.2. Absolute chronology and the 100 ka problem tified record of the succession of ice ages over the last few hundred thousands of years. Dating has always been a central difficulty in geology. If many It is also at this time that the radiocarbon method was devel- radiometric techniques were developed in the 1940's and 1950's, oped, as well as methods based on thorium isotopes. This gave reliable dates could only be obtained with technological advances Emiliani some chronological constraints, but only in the youngest in mass-spectrometers and better controls on the geochemical part of the records, with the Last Glacial maximum (LGM) at about context of the samples. Consequently, a reliable chronology of the 20 ka BP, and a previous cold period at about 65 ka BP, which he ice ages became available only in the 1970's. In contrast to expec- named stage 4. The ages of the older parts were based on simple tations, it became progressively clear that ice ages were not simply extrapolation, which finally led to results shown on Fig. 2. paced by obliquity. Broecker and van Donk (1970) identified a clear 16 D. Paillard / Quaternary Science Reviews 107 (2015) 11e24

Fig. 2. a/summer insolation according to Milankovitch theory and b/the definition of marine isotopic stages with odd numbers for warm periods (redrawn from Emiliani, 1955); c/a decade later, though earlier stages are significantly older, they still correlate with the obliquity-driven insolation forcing (redrawn from Emiliani, 1966); d/this is no more the case with a chronology comparable to present-day knowledge (redrawn from Emiliani and Shackleton, 1974) with two alternative time scales for termination II. asymmetric “sawtoothed” characteristic shape, with long periods astronomers in the XIXth and early XXth century were eager to of glacial growth, followed by rapid deglaciations, or “terminations”. discuss theories on climate, but progresses in meteorology and, In addition to these typical cycles, there are secondary oscillations later, the appearance of numerical models of the atmosphere, superimposed that closely parallel precessional changes. They also introduced new severe constraints that were difficult to ignore. note that: This might be the reason why, in the 1970's, no scientist would dare building a numerical model of ice ages, even though the newly “The two preceding terminations appear to correlate with times available paleoceanographic data had provided some revolutionary of maximum eccentricity necessary to the generation of pre- insights into the question. The task was therefore taken up by a cession maxima. ” journalist, or science writer, Nigel Calder, who was then working on a BBC film on climate. He cautions that (1974): In other words, the link between ice ages and the insolation “Meteorological processes are so notoriously nonlinear that my forcing appeared at the same time more robust, but also more assumptions are almost frivolous” complex than initially thought. This is finally synthesized in the landmark paper of Hays et al. (1976) which is often presented as the “proof” of the astronomical theory, since it is demonstrated that But in retrospect, though the shape of the predicted cycles is not astronomical periodicities are indeed present in the geological re- very good, Calder's model (1974) actually did correctly predict the cords. As explained in the title, the astronomical forcing is, without timing of almost all terminations (see Fig. 3), something that many doubt, the “pacemaker of the ice ages”. The main 100-ka cycle ap- much more recent models are unable to reproduce. This timing pears clearly associated to eccentricity, and more precisely the seemed initially quite wrong, but the discrepancy was linked to an paper demonstrates the “association of glacial times with intervals of incorrect estimation in the data of the absolute age of the Brun- low eccentricity”. But this left entirely open the question of the heseMatuyama magnetic reversal. With the revised ages obtained mechanisms involved, and the authors conclude that: about 20 years later (Bassinot et al., 1994), the timing predicted by the Calder model appears now very good. This probably stands as “the 100,000-year climate cycle is driven in some way by the best proof, that the 100 ka cycles of the ice ages are strongly changes in orbital eccentricity. As before, we avoid the obliga- linked to the eccentricity forcing, as noted in the Hays et al. (1976) tion of identifying the physical mechanism of this response, and paper, as shown on Fig. 3. instead characterize the behavior of the system only in general Calder was using “caloric seasons” as defined by Milankovitch as terms. Specifically, we abandon the assumption of linearity”. a forcing. Later on, it became more usual to use daily insolation at some specific orbital positions, like the summer solstice or mid- This is the precise statement of the so-called “100-ka problem”: july, since it can be much more easily computed and it better there is an obvious link between eccentricity and ice ages, but no “fits” the last termination (eg. Broecker and van Donk, 1970; Berger, obvious mechanism. In particular, once again, the expectations 1978), and since the precessional imprint, and therefore the ec- from Croll are entirely reversed: if ice ages are indeed linked to centricity modulation, is much more pronounced in the daily eccentricity changes, they are associated with low values, not high insolation than in the « caloric seasons », as shown on Fig. 4. Still, ones. with a simple “rectifier model”, ie a model that extract power from the envelop of the precessional forcing, like in Imbrie and Imbrie 4.3. Ice-only theories (1980), we are faced with the “400-ka problem”: the model ex- tracts not only the 100-ka periodicity, but also (and inevitably) the Climate is often presented as a “complex system” with many much larger 400-ka one that characterizes eccentricity variations. different interacting components. Physicists, chemists or The lack of this 400-ka signature in the ice volume record is a D. Paillard / Quaternary Science Reviews 107 (2015) 11e24 17

Fig. 3. a/The data used as a reference in order to evaluate b/Calder's model (redrawn from Calder, 1974); c/comparison with present-day knowledge of the chronology of ice ages (stack LR04, Lisiecki and Raymo, 2005). The timing of Calder's model appears a posteriori extremely good. The reason can be found in the link, in this model, between glacial maxima and eccentricity minima. This synchronization is also noted by Hays et al., 1976. strong indication that the inﬂuence of eccentricity on ice ages is MacDonald, 1997). Clearly, in order to account for observed large probably not straightforward. glacial cycles that are phase-locked to eccentricity, some non-linear In spite of the Hays et al. (1976) paper, or of the predictions of mechanism is needed. Suggested concepts are stochastic resonance the Calder model, the conclusion that ice ages are predictable and (Benzi et al., 1982), internal oscillations (eg. Kall€ en et al., 1979; paced “in some way” by eccentricity, is still today challenged (eg. Saltzman and Moritz, 1980; Saltzman et al., 1981; Gildor and Cruciﬁx, 2013) and many authors insist on introducing other forc- Tziperman, 2000), combination tones (eg. Ghil and Le Treut, ings or other mechanisms to account for the main (about 100-ka) 1981; Le Treut and Ghil, 1983) or chaotic systems (eg. Saltzman periodicity, like explaining it as multiples of obliquity (eg. Huybers and Maasch, 1990; for some parameter settings, see; Mitsui and and Wunsch, 2005), or induced by orbital inclination (Muller and Aihara, 2014). A key question for all these suggestions remains the synchronization of glacial maxima with eccentricity minima, as observed in the records. It is worth mentioning that for many of these models, the phase-locking to the astronomical forcing has interesting non-trivial properties that are currently under investigation (eg. De Saedeleer et al., 2013; Mitsui and Aihara, 2014).

4.4. Relaxation oscillations

Among the many possibilities that have been suggested, probably the simplest and most efficient mechanisms are based on a relaxation oscillation. Indeed, this concept explains quite well the most important qualitative features of the glacial cycles over the last million years: the fast deglaciations and slower glaciations, and the more or less constant amplitude of cycles, ie. the general saw- tooth shape of the records. Furthermore, this generic mechanism can easily be locked to the precessional forcing. A number of models can be classified into this category, based on simple climatic box models (eg. Gildor and Tziperman, 2000), on simplified ice- sheet physics (eg. Pollard, 1983; Saltzman and Sutera, 1984), on empirical rules (Paillard, 1998; Imbrie et al., 2011) or on simpler formulations like the van der Pol oscillator (Saltzman and Moritz, 1980; Crucifix, 2012). Though generally not required in all models, it is useful to have some insolation forcing threshold below which the slow glaciation phase may start. This glaciation threshold will determine rather directly the duration of interglacials. The inevitable conclusion is that, in case of small insolation changes

when the eccentricity is small, like today, the interglacials will be Fig. 4. Caloric seasons in red (ie. here the mean power in W.m 2 received during the half-year centered around the june solstice) vs. mean insolation in blue (the mean exceptionally long (eg. Paillard, 2001; Berger and Loutre, 2002)as power in W.m 2 received between the march equinox and the september equinox), shown on Fig. 5. But the most critical aspect of these relaxation both in the time domain (top) and frequency domain (bottom). The difference results oscillators is the triggering of terminations: indeed, a common from the varying length of seasons (from 169 to 196 days between equinoxes). Both feature of all these models is a slow glaciation inﬂuenced by the series have the same 41-ka content, but the precessional imprint on caloric seasons is precessional and obliquity forcings, up to a point where the ice- signiﬁcantly smaller. The eccentricity modulation appears through the doubling of precession lines. The usual daily summer solstice insolation is very similar to the blue sheet is so large, that it triggers some new phenomenon, like a curve. lack of moisture due to sea ice extent (Gildor and Tziperman, 2000), 18 D. Paillard / Quaternary Science Reviews 107 (2015) 11e24

meteorological and glaciological states of present day ice-sheets are rather sparse due to the logistic difficulties involved in obtaining in situ measurements, and covers mostly only the last one or two decades. Building a realistic coupled ice-sheet e climate model is therefore still a difficult challenge. This may have some serious implications on the estimations of the future evolution of ice- sheets. Interestingly, the simulation of their past evolution might be a useful benchmark in this context. To address this question, it is necessary to integrate the climatic evolution over hundreds of thousands of years, something fully out reach of general circulation models. This is mostly why “intermediate complexity” models were developed (Claussen et al., 2002). If the first coupled ice-sheet climate models were two-dimensional (Deblonde and Peltier, 1991; Gallee et al., 1991; Loutre and Berger, 2000; Crucifix et al., 2001), more recent models have a coarse but realistic geographic representation (Calov and Ganopolski, 2005; Ganopolski and Calov, 2011). More recently, Abe-Ouchi et al. (2013) have used many snapshot simulations from an atmospheric general circulation model to cover various astronomical, CO2 and ice sheet extent. With a three dimensional ice-sheet model, they have computed a coupled climate e ice sheet evolution by interpolating between the climatic results. In these different experiments, it is generally more difficult to obtain a strong 100-ka oscillation, when the atmospheric 100-ka CO2 forcing is absent.

5. From glaciological discoveries to integrated theories of glacial cycles

Fig. 5. Results from simple “ice-only” conceptual models. From top to bottom: blue curve is the insolation forcing (at 65N, at the summer solstice, Laskar et al., 2004); 5.1. A more complete picture of ice ages: continents, atmospheric purple curve (Calder, 1974); red curve (Imbrie and Imbrie, 1980), thick black curve CO2 and ocean circulation (Paillard, 1998) and thin black curve (idem, with a lower threshold for the triggering of glaciations). If paleoceanography did provide some critical advances on the question of ice ages, contributions from continental records were also very important. In particular, it was possible to correlate quite enhanced catastrophic calving (Pollard, 1983), isostatic rebound precisely many observations from loess records (eg. Kukla, 1977)or (Oerlemans, 1983; Birchfield and Grumbine, 1985) or some non- from pollen records (eg. Woillard, 1978). Soon, it became clear that specified mechanism in rule-based or mathematical models. This the marine isotopic stratigraphy should be used as the reference, general picture appears also to hold when using a more complex because of its continuity and its global significance as a record of three-dimensional ice-sheet model coupled asynchronously to an global ice volume, as explained by Kukla (1977): atmospheric general circulation model (Abe-Ouchi et al., 2013). Interestingly, a closer inspection of the deglacial triggering in rule- “In summary, the isotopic record of deep-sea cores provides the based models suggests that terminations should be triggered not most accurate hitherto known information on Pleistocene cli- only by a large ice-sheet as explained above, but also by a favorable mates, unique in its continuity and global extent. Validity of any astronomical forcing, in which case it is possible to account rather other climatostratigraphic system of Pleistocene must be tested accurately for the varying phase and durations of individual tran- by comparison with the deep-sea record. ” sitions (Parrenin and Paillard, 2003, 2012; Imbrie et al., 2011). This further suggests that other climatic mechanisms may have a critical It became progressively accepted to refer to regional climatic role in the triggering of terminations. As we will see below, a very changes, like temperature records, monsoons, or any environ- likely candidate is the observed rise in atmospheric CO2 mental or biogeochemical changes, in a unique common strati- concentration. graphic framework linked to the Earth global ice volume: the marine isotopic stages as defined by Emiliani (1955). This led to the 4.5. Physically based models famous reference stacked isotopic record SPECMAP, with an astronomically tuned timescale (Imbrie et al., 1984), that as been Since the first attempts to provide simple physically based ice used for about 20 years as the main stratigraphic framework of the sheets models (Weertman, 1976; Pollard, 1978) for the ice age Quaternary. A better and longer stack record has been build more problem, it is clear that a precise estimation of accumulation and recently (Lisiecki and Raymo, 2005). ablation over ice sheets is a critical and difficult question. For Glaciologists also used the isotopic composition of the ice from instance, a rather small 1 cm/year error in snow accumulation will Greenland and Antarctica to provide estimations of temperature rapidly grow into several hundred meters of ice sheet height error changes over current ice sheets, Greenland and Antarctica. But a within a glacial cycle. Besides, the ablation zone on the margins of new critical piece information appeared in the 1980's. Based on air the ice sheet has a geographically very restricted area of typically a bubbles recovered from Antarctic ice cores, it was possible to few tens of kilometers. The dynamics of ice sheets is also largely measure rather directly the air composition of the Last Glacial controlled by small-scale processes involving kilometer-sized gla- maximum (Delmas et al., 1980; Neftel et al., 1982). As assumed by S. ciers and ice-streams, that are usually not captured by ice-sheet Arrhenius almost a century before, pCO2 levels appeared indeed models. Last but not least, the data sets available on the significantly lower during glacial periods by almost 100 ppm, and D. Paillard / Quaternary Science Reviews 107 (2015) 11e24 19 the evolution of atmospheric CO2 levels was shown to match quite simulations with state-of-the-art models, since the ice sheets are closely the evolution of temperature over Antarctica for each glacial responsible for a half of the LGM cooling. But the Milankovitch cycles (Barnola et al.,1987, Petit et al., 1999; Lüthi et al., 2008). Such theory does not account for other possible climatic features, like CO2 changes certainly had some role on the Earth climate, and abrupt changes, CO2 variations or any other mechanisms that are current climate modeling results suggest that it would account for now recognized climatically relevant. It is therefore not entirely about half the cooling during glacial times. Still a critical question surprising that its predictions of a quasi-linear, mainly obliquity concerned the relative timing between ice ages and CO2 variations, driven oscillation, is not corresponding exactly to observations. in order to untangle the causes and consequences. Indeed, if carbon Indeed, climatic changes might, in turn, also have an impact on ice- entered the scene as a potential important player in the ice age sheets. For instance, it is now a classical result from general circu- problem, there is no doubt that the periodicity is controlled by the lation models that CO2 accounts for almost the other half of the astronomical forcing. In some ways, both Arrhenius and Milanko- LGM cooling (cf Fig. 6). vitch were right after all, and the astronomical theory certainly But the most important piece of information is the relative needs a geochemical component to account for observed paleocli- timing between the last deglaciation, ie. the melting of the ice matic changes. But this adds complexity to the problem, since a sheets, and the rise in CO2 as shown on Fig. 7. theory of ice ages now needs to account not only for ice sheet As already noted for many years (eg. Pichon et al., 1992), tem- changes, but also for atmospheric CO2 variations. Geochemists peratures in the Southern ocean and Antarctica increased several were surprised by the size of the glacialeinterglacial change in millenia before the melting of northern hemisphere ice sheets and pCO2, and even now it is still quite difficult to find an explanation the warming in the North. This observation was since confirmed for such a large difference. If many theories have been suggested, many times by more systematic studies (eg. Shakun et al., 2012). most scientists agree that the carbon was stored more efficiently in Obviously, this cannot be explained within the Milankovitch the- the ocean during glacial periods. But the precise physical or ory. Therefore, if Milankovitch theory is clearly necessary to explain biogeochemical mechanisms responsible for this oceanic glacial the rhythm of ice ages, it is probably not sufficient to account for ice carbon storage are still currently under investigation. ages dynamics. It now seems indeed very unreasonable to believe Progresses in paleoceanography did also provide some impor- that the evolution of Quaternary climate, and a fortiori of even older tant informations on past ocean circulation. By using the carbon time periods, will only be explained by the astronomy. In contrast isotopic composition of benthic foraminifera, it was shown that the to scientists in the early XXth century that were eager to find a Atlantic circulation underwent significant changes between the simple unique mechanism for ice ages, we now have many paleo- LGM and today (Duplessy et al., 1988), with a significant reduction climatic records to build up more complex scenarios, that are during the LGM of the depth of the North Atlantic Deep Waters furthermore consistent with results obtained with current com- (NADW) that remained above about 2000 m deep, while they reach puter simulations. The most natural and promising direction is to today down to 4000 m. In contrast the bottom of the Atlantic was try to account for CO2 changes by including the geochemical part of filled with Antarctic bottom waters. More generally, a deep strati- the story. This was quite well understood already in the 1980's with fication at about 2000 m appears to be present not only in the the first attempts to build a coupled ice sheetecarbon cycle model Atlantic, but also in the Indian ocean (Kallel et al., 1988). More (Saltzman and Sutera, 1987; Saltzman and Maasch, 1988). These recently, it has been suggested that very salty and dense waters models were build on a set of three non-linear (polynomial) ordi- were filling the deep ocean from the Southern Ocean (Adkins et al., nary differential equations, in the spirit of dynamical system the- 2002). As explained below, it is very likely that such a different ory, representing ice volume, some deep ocean property ocean state had a strong impact on the carbon cycle. (temperature or overturning) and atmospheric CO2. Through The discovery of rapid climatic variability in ice cores or Dans- providing many interesting results, a weak point of these models is gaardeOeschger events (Dansgaard et al., 1982; Johnsen et al., the difficulty to identify and quantify the key physical processes 1992) as well as the evidence of abrupt iceberg discharges in the involved, in particular in the carbon cycle part of the equations. North Atlantic, the so-called Heinrich events (Heinrich, 1988; Bond et al., 1992) also demonstrated that Quaternary climates could be 5.3. Explaining lower CO2 during glacial times significantly influenced by internal variability on millenial scales, probably even without any external forcing. This was a considerable The lack of a well accepted theory of the glacial carbon cycle is surprise and prompted the awareness that glacial cycles, paced by currently the main missing piece in the puzzle of Quaternary cli- the astronomy, were obviously not the “final story” of Quaternary mates. Many possible physical and biogeochemical processes have climates. This paradigm shift suggested that ice ages might be been suggested (eg. Archer et al., 2000; Sigman and Boyle, 2000) but largely influenced by other changes, in the carbon cycle or in the there is currently no consensus on the most important ones. Most deep ocean, and that looking for an ice-sheet-only theory might not likely, the deep ocean contained more carbon during glacial times be the best strategy for explaining Quaternary climates. since this is the only large carbon reservoir involved within the time scales of interest. Most likely also, the Southern ocean is the key area, 5.2. The return of geochemical theories since the evolution of atmospheric CO2 closely parallels the evolution of Southern records, as examplified by the strong correlation Concerning Milankovitch's theory, it is worth insisting on the between pCO2 and temperature reconstructions over Antarctica point that the central object of interest is not “climate”, ie. the (Petit et al., 1999; Lüthi et al., 2008). Most studies are trying to temperature over different regions of the Earth, but the ice-sheets, simulate the carbon cycle during the Last Glacial maximum with the and more precisely ice-sheets in high northern latitudes for the hope of obtaining atmospheric levels about 100 ppm lower than Quaternary period. It can be said that Milankovitch's theory is during pre-industrial times, as recorded in ice cores. In other words, clearly not a theory of “climate”, but a theory of ice-sheets. In most studies are trying to explain lower pCO2 as a consequence of contrast to the geochemical theory which states that CO2 and global cold, ice-sheet induced, climate. But as outlined above, we know climate are driving ice sheet changes, the hypothesis of Milanko- quite well the succession of events, at least during the deglaciation, vitch is just the opposite: ice-sheets are driving global climatic and the causal link between ice-sheets and carbon appears to be just changes over the Quaternary, in particular through their large al- the opposite. Furthermore, we also need to identify a “switch bedo effect. This mechanism is largely confirmed by LGM mechanism” in order to explain terminations and the 100-ka cycles 20 D. Paillard / Quaternary Science Reviews 107 (2015) 11e24

Fig. 6. (left) Last Glacial maximum regional cooling in gray shades compared simulations in PMIP-2 (circles): current ocean-atmosphere general circulation models simulate reasonably well temperatures during the Last Glacial maximum, the major forcings (right) being the prescribed ice-sheet and greenhouse gases concentrations, each one accounting for about half of the cooling (redrawn from IPCC Fourth Assessment Report, 2007).

as explained in the preceding sections, and Fig. 7 suggests that pCO2 is a very plausible candidate. In other words, the key question might not be “why is pCO2 lower during glacial times”, but “why does pCO2 rise just after glacial maxima”. This dynamical perspective provides a very stringent constraint on the possible carbon cycle mechanisms involved. Indeed, most of the suggested processes for lowering CO2 are most efﬁcient during glacial maxima. According to the data shown on Fig. 7, a mechanism for lowering pCO2 should break down

Fig. 7. From top to bottom: insolation at 65N at the june solstice (Laskar et al., 2004); Fig. 8. Scheme of the mechanism (Paillard and Parrenin, 2004). Top: current situation, sea level records (Fairbanks, 1989; Bard et al., 1996) Greenland d18O record as a proxy where salty waters from brine rejection are mostly mixed with open ocean waters. of Northern hemisphere climate (Dansgaard et al., 1993); atmospheric CO2 (Monnin Middle: with lower sea-level, more intense sea ice formation, less ice shelf melting, it et al., 2001) and antarctic dD as a proxy of Southern hemisphere climate (Stenni is likely that the salty waters will overspill down to the bottom of the Ocean. Bottom: et al., 2001). At the onset of the BollingeAllerod, about 14.6 ka BP, the sea level is shortly after the glacial maximum in the north, Antarctica will reach its maximum area within glacial values, while atmospheric CO2 has increased by about 50 ppm. More and will cover a substantial part of the continental shelves (schemes from Bouttes generally, the southern hemisphere is warming several thousands of years before the et al., 2012). The critical parameter is the fraction (“frac”) of salty “brine” waters northern hemisphere one. reaching the bottom of the Ocean, which relates to the green curve of Fig. 10. D. Paillard / Quaternary Science Reviews 107 (2015) 11e24 21

constructed (Paillard and Parrenin, 2004), in which the key process for storing carbon in the deep ocean is linked to the formation of very dense and salty bottom waters on the continental margins of Antarctica during cold climates, due to sea ice formation and brine rejection. In particular, these dense waters can reach the bottom of the Ocean when they overspill the continental shelf and flow as gravity currents down the topography (eg. Foldvik et al., 2004). This mechanism stops naturally when the Antarctic ice sheet covers most of the continental shelves, since there is almost no margin left for bottom water formation, as shown on Fig. 8. This expansion of Antarctica results largely because of the reduced sea level at the glacial maximum (Ritz et al., 2001; Anderson et al., 2002). This conceptual idea has since been tested in an intermediate complexity model of the climate and carbon cycle (Bouttes et al., 2010). Interestingly, the corresponding simulations of the carbon isotopic composition in the deep ocean is in good agreement with the geographic patterns observed in paleoceanographic data (Bouttes et al., 2011), thus explaining quite well the mystery of the strong deep ocean stratification during glacial times (Kallel et al., 1988; Curry and Oppo, 2005), as well as the observed bottom salty waters (Adkins et al., 2002). The corresponding simulation of radiocarbon (Mariotti et al., 2013) is also in good agreement with D14C measurements in the atmosphere and ocean. Furthermore, the time evolution during the last deglaciation of pCO2 and carbon isotopes at different ocean depths is quite well represented (Bouttes et al., 2012) with a simultaneous increase in pCO2 and southern temperatures (see Fig. 9), both being linked to the de-stratification of the southern ocean. This synchronicity was recently confirmed through a careful analysis of the ice core chronology (Parrenin et al., 2013). Recent observations of dense salty water formation on the continental shelves of Antarctica associated with sea ice brine rejection (Ohshima et al., 2013) gives undoubtedly some support to Fig. 9. Top: pCO2 and antarctic temperatures according to ice cores (Monnin et al., the hypothesis outlined above, since this processes 2001) to be compared with two model scenarios of salty bottom water formation, contribute today to some non-negligible part of AABW (Meredith, either an abrupt stop at 18 ka BP, or a more progressive one (Bouttes et al., 2012). 2013) and is likely to be much more important in a colder climate. As mentioned above, at the glacial maximum the Antarctic within a few millenia of the glacial maximum. According to the ice-sheet covers a significant part of the continental shelf and this relaxation oscillator hypothesis for explaining the 100-ka cycles, this mechanism breaks down. A key open question is the dynamics of mechanism should in fact break down mainly as a direct conse- the gravity currents that transport these salty shelf waters down to quence of the glacial maximum. Following these lines, a conceptual the bottom of the Southern ocean (Foldvik et al., 2004; Ohshima model for the coupled ice sheetecarbon cycle system has been et al., 2013). Unfortunately, these processes are not well

Fig. 10. Conceptual model of coupled ice sheet e CO2 evolution over the last 5 million years. From top to bottom, black: simulated pCO2; blue and red, simulated northern hemisphere ice volume and Antarctica extent; black and red: isotopic records of ice ages; green: the critical parameter that controls the formation of salty deep waters and the oceanic storage of carbon (from Paillard and Parrenin, 2004). 22 D. Paillard / Quaternary Science Reviews 107 (2015) 11e24 understood, and not well represented by current state-of-the-art References climate models. Another important mechanism is the basal melting under the ice shelves, that provides today a significant Abe-Ouchi, A., Saito, F., Kawamura, K., Raymo, M.E., Okuno, J., Takahashi, K., Blatter, H., 2013. Insolation-driven 100,000-year glacial cycles and hysteresis of amount of fresh water to the continental shelf waters, thus ice-sheet volume. 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