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3.4 Does the Dansgaard–Oeschger cycle have a periodicity?
evidence for the existence of a 100-kyr cycle in Pleistocene climate is overwhelming, starting with Hays et al. (1976) frequency analysis that provoked the reinstatement of Milankovitch theory. Since that data was measuring changes in 18O isotope, it is clear that the nature of the 100-kyr cycle corresponds to an ice cycle. Extensive continental ice sheets expand and wane simultaneously at both hemispheres according to a 100-kyr periodicity. This periodicity obviously results from eccentricity, as it shows phase coherence with it. It can be concluded from the evidence that the 100-kyr cycle determines global ice buildup, while the 41-kyr cycle determines interglacials. They are two related but independently paced cycles and this is the source of most confusion in the field. In brief, low eccentricity promotes global ice build up, while high eccentricity promotes global ice melting, and obliquity is responsible for opening up windows during which an interglacial is possible, and closing them making an interglacial impossible. The decision whether there is an interglacial or not when obliquity opens the window of opportunity is taken by a combination of insolation forcing and the status of the 100-kyr ice cycle at the time (see next section). Figure 2.12 shows how the two cycles interplay constituting two sides of the same coin. The top side determines the frequency of interglacials (focus on Fig. 2.12a, that depends on obliquity), while the bottom side reflects the 100-kyr cycle determined by the ice proxy (focus on Fig. 2.12b, that depends on eccentricity). Frequency analysis does not differentiate between both halves of the data, producing the 100-kyr, 80-kyr and 41-kyr bands observed in Fig. 2.4c, right panel, without informing us that the bands come from different values in the data, the obliquity band from the low !18O values, and the 100kyr band from the high !18O values.
With this information the 100-kyr periodicity stops being a mystery. It is linked to eccentricity but in the opposite way most authors consider. Eccentricity-derived precession-linked insolation forcing does its main role fighting the intrinsic accumulation of ice during glacial periods and has only a supporting role at terminations. At the MPT the world became so cold and the intrinsic ice build-up so extensive that the change in insolation caused by the increase in obliquity was insufficient to melt all the ice that accumulated during the previous low obliquity period. Some interglacials became colder, with more ice at high latitudes. When eccentricity is low, 23-kyr precession cycles produce low summer insolation at high latitudes, resulting in cool summers, with the ice growing through the precession cycle. Low eccentricity thus leads to ice accumulation due to low summer energy precession cycles, while high eccentricity leads to ice melting through high summer energy precession cycles. When an obliquity increase coincides with high eccentricity and high insolation from precession at the right time, it produces an interglacial, melting all the extrapolar ice and resetting the ice clock. Then, as eccentricity and obliquity decline, ice accumulates. 41-kyr after the interglacial, eccentricity has become low, and precession cycles have low insolation during the summer. Under those conditions high obliquity is unable to produce an interglacial on its own. Therefore more ice accumulates. By the time conditions are right again, and eccentricity is sufficiently high, four to five precession cycles had taken place (90–115 kyr) until insolation is again high enough to melt the ice and contribute to an obliquity paced interglacial. The larger the icevolume accumulated over the 100-kyr period the more unstable the ice sheets and the stronger the feedbacks to drive fast melting dynamics (albedo, rising sea levels,
Fig. 2.12 Cycles of ice – cycles of warmth
LR04 temperature and ice proxy (black line) shows two different periodicities during periods of high and low global ice-volume. a) Periods of low ice-volume follow the obliquity cycle (orange curve). Time between interglacials (upper numbers) fits multiples of 41 kyr. Obliquity oscillations that do not produce an interglacial often coincide with periods of warming (arrowheads). Obliquity curve has been shifted to the right by 6,500 years to account for the lag in the response. Data from Lisiecky & Raymo 2005. b) Periods of high icevolume follow the eccentricity cycle (blue curve). The amount of ice extends as an inverse of the eccentricity, and in the graph tends to reach the eccentricity curve as drawn. The average of the periods between ice maxima is close to 95 kyr (bottom numbers), with the latest 6 periods averaging 102 kyr. The 405-kyr cycle modulation is clearly observable in the last four periods (dotted line).
dust, CO2, volcanic activity). The large ice-sheet instability hypothesis introduced by Paillard (1998) has received support from studies that show it could be mediated at least in part by the delayed effect of slow glacial isostasis adjustment maintaining ice-sheets at low altitude through the rapid melting process (Abe–Ouchi et al. 2013). Once the ice is melted in a new interglacial, the ice clock is reset. This explains why the 405-kyr periodicity is absent, as the ice cycle is re-initialized after every successful interglacial. However, the amplitude modulation by the 405kyr eccentricity cycle is clearly visible in the high icevolume during the Last Glacial Maximum (LGM), and the glacial maximum 415 kyr earlier, compared to the low icevolume during the glacial maxima c. 200 ka ago (Fig. 2.12b, dotted line). It follows that since we are in a very low eccentricity situation and moving towards even lower, almost zero eccentricity, there will be considerable ice build up over the next three precession oscillations, preventing a new interglacial in the next obliquity cycle, and leading to high volume of continental ice (though not as high as at the LGM) in 70 kyr (see Chap. 14). So low eccentricity does not promote long interglacials. It does exactly the opposite. It promotes long harsh glacial periods.
Obliquity determines when an interglacial might take place but, for the past million years, if the interglacial finally takes place or not does not depend on obliquity. If it did there would be an interglacial in every obliquity oscillation, as it happened in the Pliocene–Early Pleistocene. The 100-kyr ice cycle appears at the MPT, when the cooling of the planet stimulates ice growth and allows the survival of large continental ice sheets outside Greenland and Antarctica during certain obliquity oscillations. These ice sheets can grow for two or even three periods of low obliquity reaching sizes not seen in the planet for the last 250 million years.
The growth of these extrapolar ice sheets has two opposing effects. On one side they make the planet colder, making it more difficult to warm and leading to a decrease in the frequency of interglacials. On the other side when they become very large they reach lower latitudes and extend on continental platforms freed by lowering sea levels, and become increasingly unstable, prone to catastrophic break ups, releasing iceberg armadas, as during Heinrich
Fig. 2.13 Elements participating in interglacial determination
a) Interglacials are allowed when obliquity (black curve) is above 23° (black horizontal line), defining windows of opportunity marked with a colored bar, orange for interglacial positive and light blue for interglacial negative, and numbered on top from O–1 to O–24. b) 65°N summer insolation (black curve) favors interglacials when above 521 W/m2 (orange horizontal line and circles) and hinders them when below that value. Insolation values above 549 W/m2 (dark red horizontal line and circles) directly produce an interglacial. Astronomical data after Laskar et al. (2004). c) Global ice-volume (LR04 proxy, black curve; after Lisiecki & Raymo 2005) promotes interglacials when above (lower in the graph) a threshold value (4.56‰ !18O dark blue horizontal line and circles) and hinders them when below that level (light blue circles). The graph reveals that after obliquity, global ice-volume is the most important determinant for interglacials (see O–11, for example), and most high obliquity periods without an interglacial are due to insufficient ice (small number 1 at bottom), while some are due to insufficient insolation (small number 2 at bottom). The odd MIS 13a interglacial (question mark), cannot be explained in these terms, but it is quite similar except for a temperature spike to MIS5c (O–3) that is not considered an interglacial. Observe that the numbering of obliquity cycles almost exactly coincides with Marine Isotope Stages numbering derived from data. It is another indication of the great importance of obliquity changes for climate.
