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Last Interglacial Period

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Figure 2.3 Typical vegetational succession recorded in pollen sequences spanning the last inter glacial period ('Eemian' interglacial) in northern Europe. Similar patterns of vegetational development are recorded over large areas of Europe during this period. The Eemian interglacial is generally agreed to correlate with isotope stage 5e, from ca 126-118,000 BP. After Zagwijn 1990.

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Figure 2.3 Typical vegetational succession recorded in pollen sequences spanning the last inter glacial period ('Eemian' interglacial) in northern Europe. Similar patterns of vegetational development are recorded over large areas of Europe during this period. The Eemian interglacial is generally agreed to correlate with isotope stage 5e, from ca 126-118,000 BP. After Zagwijn 1990.

whole of the last interglacial throughout most regions of Europe.

Exactly how climatic conditions varied during the interglacial is still open to some dispute. Most authors are agreed that the initial rise in temperatures at the start of the interglacial must have been very rapid and that the warmest temperatures were probably attained during the earlier stages of the interglacial, around 120-125,000 BP (Fig. 2.4) (Watts 1988). At that time, world-wide sea levels seem to have risen to at least 5-6 metres above present-day levels (implying a significant reduction in the total volume of the continental ice sheets) and the temperature records in the Arctic and Antarctic ice cores suggest that average year-round temperatures were at least 2-3°C higher than those in the same regions today (Figs 2.2, 2.14, 2.17) (Shackleton 1987; Jouzel et al 1987; Lorius et al 1985; Dansgaard et al 1993; LIGA 1991a; Sejrup & Larsen 1991). There is equally strong evidence for a clear climatic optimum at this time in the vegetational records of northern Europe (Fig. 2.3). As Watts (1988) points out, several species such as Stratiotes (water aloe) and Hydrocharis (frog bit) extended further to the north during the peak of the Eemian interglacial than at any time during the present postglacial period and there is evidence for the presence of such semi-tropical species as hippopotamus and the European pond tortoise in the faunal records of northern Europe (see also West 1977; Stuart 1982; Bowen 1978; Zagwijn 1990; LIGA 1991a). Watts argues that the earlier stages of the interglacial were probably characterized by a relatively continental pattern of climate, (i.e. with substantially warmer summers than at present and rather cooler winters) which became more oceanic, with less marked seasonal contrast, during the middle and later stages of the interglacial.

Most of the debate in the literature has

Figure 2.4 Estimated mid-summer temperatures across Europe at the peak of the last (Eemian) inter glacial, reconstructed from vegetational data by Zagwijn 1990.
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Figure 2.5 Estimated mid-summer temperatures in Europe at the time of the Amersfoort interstadial (= isotope stage 5c, ca 100,000 BP) as reconstructed from vegetational data by Zagwijn 1990. Note the much sharper temperature gradients from southwest to northeast than those during the Eemian inter glacial (Fig. 2.4), presumably due to the presence of an ice sheet over northern Scandinavia.

Figure 2.5 Estimated mid-summer temperatures in Europe at the time of the Amersfoort interstadial (= isotope stage 5c, ca 100,000 BP) as reconstructed from vegetational data by Zagwijn 1990. Note the much sharper temperature gradients from southwest to northeast than those during the Eemian inter glacial (Fig. 2.4), presumably due to the presence of an ice sheet over northern Scandinavia.

centred on the precise causes of the changes in vegetational patterns during the closing stages of the interglacial - the so-called 'post-temperate' or 'telocratic' phases (Watts 1988). As noted above, this period is characterized by an increase in the frequencies of various coniferous species of pine, spruce and silver fir and by a general expansion in more open vegetation at the expense of closed woodland (see Fig. 2.3). One early suggestion put forward by Iversen (1958) and others was that these changes could have been caused purely by progressive deterioration in soil conditions during the course of the interglacial rather than by any significant change in climate. It now seems more likely, however, that there was indeed a significant deterioration in climatic conditions (at least in summer temperatures) during the later part of the interglacial (Watts 1988), as reflected, for example, in the temperature records of ice cores, in the gradual increase in global ice volumes reflected in oceanic oxygen-isotope ratios and in the various faunal indications of sea-surface temperatures in the North Atlantic (Figs 2.1, 2.2, 2.11, 2.14) (Jouzel et al 1987; Imbrie et al 1989; Sancetta et al 1973). A similar deterioration in climate during the later stages of the interglacial also seems predicted from theoretical models of solar radiation implied by the Milankovitch hypothesis (Watts 1988). Thus, all evidence suggests that the last interglacial period came to an end in a much more gradual way than it began and was characterized by a progressive shift towards increasingly glacial conditions rather than by any sudden climatic event.

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