The Middle Glacial period ca BP

The end of the early glacial period was marked by the onset of much more rigorous climatic conditions which define stage 4 of the oxygen-isotope sequence. This period between ca 75,000 and 60,000 BP (Martinson et al. 1987), clearly represents the main glacial maximum of the first half of the last glaciation. In the oxygen-isotope records this period is marked by a rapid decrease in the l80/160 ratios (Fig. 2.1) indicating a major expansion of the continental ice sheets far beyond the limits reached during the earlier cold periods of stages 5d and 5b (Shackleton 1977,1987; Chappell & Shackleton 1986). The same period is equally apparent in the various faunal indicators of sea-surface temperatures in the North Atlantic region (Fig. 2.11) which suggest that surface temperatures in this region fell to at least 3-4°C lower than those attained during any of the preceding stages of the glaciation (Sancetta et al. 1973; Imbrie et al. 1989). According to the reconstructions of Maclntyre et al. (1975) it would seem that the position of the extremely cold

Figure 2.13 Estimated mid-summer and mid-winter sea-surface temperatures in the Atlantic region at the time of the overall maximum of the last glaciation, ca 18,000 BP. The approximate positions of the continental ice sheets and coastlines of Europe at this time are also shown. After CLIMAP 1981.

-ifvprfrfr ice sheets —10 sea surface temperatures (°C)

-ifvprfrfr ice sheets —10 sea surface temperatures (°C)

Figure 2.13 Estimated mid-summer and mid-winter sea-surface temperatures in the Atlantic region at the time of the overall maximum of the last glaciation, ca 18,000 BP. The approximate positions of the continental ice sheets and coastlines of Europe at this time are also shown. After CLIMAP 1981.

polar front in the North Atlantic area extended to the latitude of ca 45°N, that is, at least 500 miles to the south of the positions attained during isotope stages 5d and 5b and almost as far south as that reached during the overall maximum of the last glaciation at around 18,000 BP (Fig. 2.12; see also Ruddi-man & Maclntyre 1976).

In Europe there are clear indications of the character of ecological and climatic conditions during this period in most of the long vegetational sequences discussed earlier. In Holland, Denmark and northern Germany it seems that vegetation during this period consisted entirely of open tundra communities with perhaps some areas of almost barren 'polar desert' during the coldest climatic phases (Figs 2.9,2.10) (Behre 1990; Zagwijn 1990). Mid-summer temperatures in this region have been estimated by Zagwijn (1990) as around 5°C, with average temperatures which must have remained well below freezing point for large parts of the year. A similar pattern is reflected in the pollen records from La Grande Pile and Les Echets. Here it seems that conditions were more steppic than tundra-like in character (reflected in high frequencies of species such as Artemisia and Chenopodiaceae, implying generally drier and more continental climates in these more southerly areas) and included a slightly higher component of some of the hardier tree species such as birch and pine (Woillard 1978; Woillard & Mook 1982;" de Beaulieu & Reille 1984). Nevertheless, the overall temperatures in these regions are thought to have fallen, once again, to at least 3-4°C lower than those experienced at any earlier stage in the last glaciation (Fig. 2.8) (Guiot et al. 1989). There is now no question that the period of isotope stage 4 witnessed some of the most severe climatic conditions of the last glaciation - in most respects probably only slightly less severe than those which characterized the period of the overall maximum of the last glaciation at around 18,000 BP (Figs 2.13,2.31).

Exactly how far the ice sheets expanded in northern Europe during isotope stage 4 is still uncertain. It is clear from the isotope records of deep-sea cores that the ice sheets were generally much less extensive during stage 4 than during the period of the glacial maximum at ca 18,000 BP (Shackleton 1987; Chappell & Shackleton 1986) so that most of the potential geological traces of the precise limits of the earlier glacial advances have been largely obliterated from the geological record. In Britain, Bowen (1990) has suggested that the ice sheets during stage 4 probably extended to around the latitude of the Isle of Man; and in northern Germany it has been suggested that the partially overridden Stetten end moraine may have formed during this period (Butzer 1972). For the present, however, most of these suggestions remain highly speculative. Overall, there is no doubt that stage 4 must have been characterized by fairly massive and extensive glaciation over the northern parts of Europe, but the exact limits of this glaciation remain unclear.

Isotope stage 3

The ensuing period of isotope stage 3, from ca 60,000 to 25,000 BP, is one of the most enigmatic parts of the last glaciation. In broad terms this was a period of predominantly mild climate, in which the extent of global glaciation was substantially reduced (see Fig. 2.1 Shackleton 1977, 1987; Martinson et al. 1987). The difficulties of reconstructing climatic and environmental patterns during this interval stem from the sharply oscillatory nature of climatic events, which can be seen clearly in all the associated climatic and pal-aeoenvironmental records.

The best records of these rapid climatic oscillations can be seen in the recent, high-resolution studies of two long ice cores (the so-called 'GRIP7 and /GISP2/ cores) from central Greenland (GRIP 1993; Dansgaard et al. 1993; Bond et al. 1993; Grootes et al. 1993; Boulton 1993; Kerr 1993). Through detailed studies of the oxygen-isotope ratios in the cores it has been possible to identify at least a dozen significant climatic oscillations between ca 25,000 and 60,000 BP, in which temperatures over the area of the ice sheet itself seem to have risen by between 5 and 8°C, often within remarkably short periods of

Denekamp

Denekamp

14 Glinde

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