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Figure 2.14 Fluctuating oxygen-isotope (180/16<0) ratios recorded through the recent'GRIP Summit' ice core from Greenland. The various ' inter stadials' recognized in the sequence are indicated by numbers (in the central column), and correlated tentatively with those recognized in pollen and other stratigraphic sequences in northwestern Europe. After Dansgaard et al. 1993. (N.B. More recent work on the core has cast doubt on the reality of the apparent climatic fluctuations during the period of the Eemian interglacial: see Grootes et al. 1993.)

less than 50 years or so (Dansgaard et al. 1993; Grootes et al 1993). Most of these warmer oscillations were short lived, of the order of ca 1,000 years. At four points in the sequence, however, there is evidence for periods of more prolonged warming of 2,000-4,000 years and with temperature peaks (usually at the start of each interval) slightly higher than those of the shorter intervals. The clearest illustration of these patterns which has been published so far is shown in Fig. 2.14.

It is now apparent that many of the shorter interstadials within stage 3 were either too brief, or not sufficiently marked, to show up in many of the other records of climatic change preserved, for example, in most of the deep-sea core oxygen-isotope sequences (but see Bond et al. 1993 for a possible exception) or in most pollen sequences from sites in northern and western Europe. The shorter intervals may have been simply too brief for trees to migrate into northern Europe from their refuge areas further south. The longer and more major interstadials, however, are clearly represented in many of these sequences, including for example the pollen records from La Grande Pile and Les Echets in eastern France (Woillard 1978; de Beaulieu & Reille 1984; Guiot et al. 1989), in some shorter pollen sequences from northern Europe (Behre 1990; Zagwijn 1990; Behre & Plicht 1992), in oxygen-isotope records from the Mediterranean and north Atlantic (Labeyrie 1984; Paterne et al 1984), and in some of the more detailed and high resolution records of sea-surface temperatures (Sancetta et al. 1973; Bond et al. 1993) (see Figs 2.6-2.11, 2.15). Unfortunately the close spacing of several of these intervals combined with the poor resolution of most absolute dating methods within this time range make an exact correlation of the individual climatic oscillations within the different sequences extremely difficult (see Fig. 2.16).

Perhaps the main point to emphasize is that the ecological effects of these various interstadial episodes within isotope stage 3

Isotope Age B P stages x1000 4

Isotope Age B P stages x1000 4

Oxygen Isotope Stage Neanderthal Map

108,000 (U-Th) 115,000 (U-Th) 120,000 (U-Th) 131,000 (U-Th)

Figure 2.15 Changing oxygen-isotope ratios over the past 140,000 years recorded in core KET-8004, from the north Mediterranean. In this area, variations in the oxygen-isotope ratios are likely to he much more directly controlled hy temperature variations than in the major ocean basins. Absolute dates for the major stadial and interstadial phases obtained by Potassium-Argon and Uranium-Thorium methods are indicated on the right. After Labeyrie 1984 (see also Paterne et al. 1986).

108,000 (U-Th) 115,000 (U-Th) 120,000 (U-Th) 131,000 (U-Th)

Figure 2.15 Changing oxygen-isotope ratios over the past 140,000 years recorded in core KET-8004, from the north Mediterranean. In this area, variations in the oxygen-isotope ratios are likely to he much more directly controlled hy temperature variations than in the major ocean basins. Absolute dates for the major stadial and interstadial phases obtained by Potassium-Argon and Uranium-Thorium methods are indicated on the right. After Labeyrie 1984 (see also Paterne et al. 1986).

were much less marked than those which characterized the earlier, more pronounced interstadials of isotope 5a and 5c. In the pollen records from northern Europe, for example, the Denekamp and Hengelo interstadials were marked simply by a shift from open tundra to shrub tundra communities, whereas the earlier, much longer, Amers-foort/Brorup and Odderade interstadials had been characterized in the same areas by full coniferous forest (Zagwijn 1990; Behre 1990; Behre & Plicht 1992; Van der Hammen et al 1967: see Figs 2.9, 2.10). Similarly in the pollen records from Grande Pile and Les Echets, the various interstadial episodes of stage 3 were characterized by arboreal pollen percentages of only ca 30-50 percent (consisting almost entirely of birch and pine) while the earlier St Germain I and II interstadials had been marked by tree frequencies of around 90 percent, and consisting of full deciduous forest not very different from that of the preceding interglacial (Figs. 2.6, 2.7). Most of these ecological and climatic records from Europe converge on figures of around 5-7°C for the overall scale of the temperature rises during the major interstadial episodes of isotope stage 3 (see Figs 2.2, 2.8, 2.9, 2.11). Even if climatic conditions during stage 3 were highly oscillatory, therefore, they never seem to have attained the degree of climatic amelioration which characterized the more major interstadials during the earlier stages of the last-glaciation.

The exact causes of these rapid and sudden climatic oscillations during isotope stage 3 have recently generated lively controversy among Quaternary climatologists (e.g. Dans-gaard et al 1993; Bond et al 1993; Kerr 1993). The most significant discovery is that the most pronounced periods of rapid cooling seem to have coincided with periods when large numbers of icebergs broke away from the Laurentide ice sheet in the North Atlantic region (leaving clearly defined layers of det-rital material in the contemporaneous deep-sea sediments) which in turn would have reduced the salinity of the surface waters in the North Atlantic, and triggered off movements in ocean currents which would have caused an influx of much warmer waters - a pattern which has recently been described as 'Bond cycles' and related 'Heinrich events' (Bond et al 1993; Kerr 1993). Other climatologists, however, are inclined to see most of these climatic oscillations as a possible direct result of various astronomical orbital-forcing

20 30 40 _50_60 kyrs BP

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