CENTRAL POLAND 10° 20° C July
CENTRAL POLAND 10° 20° C July
Figure 2.9 Principal stadial and interstadial periods recognized in different areas of northern and western Europe during the earlier part of the last glaciation, with associated summer temperature estimates. After Zagwijn 1990.
The essentially periglacial character of the climate during these two periods is indicated equally clearly by the composition of the pollen records from many parts of northern, western and central Europe (Figs 2.6, 2.7, 2.10). In pollen diagrams ranging from southern Denmark to the Rhone valley the vegetation during both stages 5d and 5b seems to have been open in character, characterized predominantly either by grasses, sedges or other tundra-like plants, or, further to the south and east, by high frequencies of more steppic species such as Artemisia, Thalictrum and Chenopodiaceae (Zagwijn 1990; Behre 1990). Nevertheless, in all these areas there is evidence that some localized patches of more hardy trees such as pine, birch and willow were able to survive during these periods, suggesting that climatic conditions never became quite as severe as those experienced during the later full glacial periods of isotope stages 2 and 4. The vegetation over the different zones of northern and western Europe during both stages 5d and 5b has been char-
acterized by Zagwijn (1990: 62) as essentially 'open with patches of birch-pine forest7, though with an increasing component of woodland further to the south and east across the continent.
The precise implications of these vegetational patterns in climatic terms are more difficult to reconstruct. Zagwijn (1990) has suggested that over a zone of northern Europe extending from the Netherlands through northern Germany to central Poland, mid-summer temperatures during both stages 5d and 5b can hardly have risen above ca 8°C - implying an overall depression in temperatures (compared with those of the present day in the same regions) of at least 8-10°C (Fig. 2.9). Further to the south, the scale of temperature reduction seems to have been much less. From various multivariate analyses of vegetational patterns at La Grande Pile and Les Echets, Guiot et al. (1989) have estimated an average depression in year-round temperatures during these two periods of around 5-6°C (Fig. 2.8). As noted earlier, these and other lines of evidence indicate a much sharper pattern of climatic gradients from north to south and from east to west across Europe than those recorded during the preceding interglacial period (Sejrup & Larsen 1991).
Exactly what caused these episodes of sharp climatic deterioration during the earlier stages of the last glacial period is still a matter for debate. Almost certainly, a major part of this climatic cooling was due to a sharp reduction in the intensity of solar radiation around 118,000 BP as the Milanko-vitch hypothesis would predict (Martinson et al. 1987; Imbrie et al. 1989; LIGA 1991a,b). But in the more northern zones of Europe there was probably a major feedback factor in the accumulation of major ice masses in the Scandinavian region, combined with a major southward extension in the flow of polar waters from the north Atlantic region, into the central Atlantic zone (Zagwijn 1990). According to the reconstructions by Ruddi-man and Maclntyre (1976; Maclntyre et al. 1972, 1975), this polar front extended southwards to the latitude of ca 52°N (i.e. approximately to the latitude of southern England) during both stages 5d and 5b (Fig. 2.12), deflecting the warmer waters of the Gulf Stream far to the south, and providing a further source of extremely cold air masses to much of the northern and western zones of Europe (Lamb 1977; LIGA 1991b).
In some ways the most striking feature of the early glacial period is not the severity of the colder periods but the relative warmth of the intervening 'interstadial' periods represented by stages 5c and 5a of the oxygen-isotope records. The implications of the ocean core isotope ratios (Fig. 2.1) are that the total volume of ice masses during these periods was reduced to approximately half the volume attained during the colder episodes of stages 5d and 5b - although still much greater than those which existed during fully interglacial periods (Shackleton 1977, 1987). In the same way there is clear evidence that the position of the polar front in the North Atlantic region retreated several hundred miles to the north of the earlier glacial episodes, from around the latitude of 50°N to above 60°N (Fig. 2.12) (Maclntyre et al 1972, 1975; Ruddiman & Maclntyre 1976).
The most detailed reflection of these major interstadial periods is provided once again in the long pollen sequences recorded at La Grande Pile, Les Echets and elsewhere (Figs 2.6=2.8). As noted earlier, all these diagrams show two major periods of climatic warming (generally referred to as St Germain phases I and II) which can be correlated unambiguously with stages 5c and 5a respectively of the deep-sea core sequences (Zagwijn 1990; Woil-lard & Mook 1982; de Beaulieu & Reille 1984; Behre 1990) and with only slightly more reservations with the well known sequence of Amersfoort/Brorup and Odderade intersta-dials within the more fragmentary pollen records of northern Europe (Fig. 2.9) (LIGA 1991a; Sejrup & Larsen 1991; Behre & Plicht 1992). As both Zagwijn (1990) and Behre (1990) have recently emphasized, it is now clear that the exact ways in which these two interstadials were reflected in the local vege-tational and climatic records varied sharply in
Estimated summer sea-surface temperature (core V23-82) (0°C)
5 10 15
5 10 15
Figure 2.11 Estimated mid-summer sea-surface temperatures in the north Atlantic region over the past 130,000 years, based on analyses of the varying frequencies of temperature-sensitive surface-living organisms in deep-sea core V23-82. After Sancetta et al. 1973.
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