Lake Windermere Interstadial

FIGURE 8.7 Reconstructed July paleotemperatures based on insect remains in areas of the southern and central British Isles since the last (Ipswichian) interglacial. Annual temperature ranges are also shown (Coope, l977b).The period before 50,000 yr B.R (dashed line) is very uncertain and there may have been a more gradual, monotonic decline in temperature from 120,000 yr B.R to 60,000 yr B.P.

any climatic amelioration (Coope, 1975b). Evidently the Coleoptera were sufficiently mobile that they could rapidly move northward as the climate improved, whereas certain plants could not migrate northward fast enough to become established in Great Britain before the climate once again deteriorated. A similar situation occurred at the end of the Devensian (Weichselian) cold phase when temperatures again rose abruptly, but for only a relatively short period (the Lake Windermere Interstadial; Coope and Pennington, 1977). At this time, an abrupt change from arctic to thermophilous beetle assemblages took place, with maximum warmth occurring around 12,500-12,000 yr B.P. Shortly thereafter (when from pollen data it is apparent that birch began to colonize the north of England), the interstadial peak of warmth had passed and a significantly cooler episode was already beginning. The newly established birch forest declined and the thermophilous beetle assemblage was replaced by a northern assemblage typical of tundra regions today. By 9500 yr B.P. this sequence had been entirely reversed and thermophilous species again rapidly replaced the arctic stenotherms that had been abundant only

500 yr earlier (Osborne, 1974, 1980). Again, the more mobile insects were in advance of the vegetation and provide a more accurate assessment of paleoclimatic conditions than could be obtained simply from palynological data.

It is perhaps appropriate to note that coleopteran and palynological data do not always appear to be out of phase; such situations are probably the exception rather than the rule. The Chelford interstadial (radiocarbon dated at -60,000 yr B.P.), for example, must have lasted long enough for trees to migrate northward into Great Britain following the cold, early Devensian period. Coleopteran assemblages are in complete accord with palynological evidence for a cool but quite continental climate at this time (see Fig. 8.7); conditions in central England were similar to those in southern Finland today (Simpson and West, 1958; Coope, 1959, 1977b).

A more rigorous, quantitative approach to paleoclimatic reconstruction has been applied to coleopteran fauna from a set of 14C-dated sites in Great Britain, spanning the last 22,000 yr. The "Mutual Climatic Range" method is based on climatic conditions found across the modern range of particular species, which defines their tolerance limits (Grichuk, 1969). The climate of locations where several fossil species once coexisted is defined by the overlapping range of climatic conditions that is compatible with their occurrence together in one place (Fig. 8.8). For beetles, this "mutual climatic range" is defined in terms of the mean temperature of the warmest and coldest months (Atkinson et al., 1986b, 1987). Figure 8.9 shows the temperatures reconstructed in this way for Great Britain (50-55° N) for the last

Chelford Interstadial

Annual range of monthly temperature °C

FIGURE 8.8 Schematic diagram illustrating the mutual climatic range of two species, defined by the overlapping region of "climate space" (Atkinson et al., 1987).

Annual range of monthly temperature °C

FIGURE 8.8 Schematic diagram illustrating the mutual climatic range of two species, defined by the overlapping region of "climate space" (Atkinson et al., 1987).

Interstadial

FIGURE 8.9 Mean temperature of the warmest and coldest months of the year (TMAX and TMIN) for intervals over the last 22,000 yr, according to beetle remains in sediments from various sites across the British Isles, calibrated using the mutual climatic range method.The dark center lines give the best estimate of temperature, and the upper and lower lines, the extreme ranges of estimates (based on the average of samples of similar ages).The inner horizontal lines give the range of decadal mean temperatures, in the warmest and coldest months, recorded in central England over the interval A.D. 1659-1980; the outer horizontal lines give the range of warmest and coldest individual years over the same interval (Atkinson et o/„ 1987).

22,000 yr, compared to the observed range of temperatures in central England from

A.D. 1659-1980. Of particular note is the extremely low winter temperatures during the last Glacial Maximum (22-18 ka B.P.) and also from -14.5 to 13 ka B.P., when the mean temperature of the coldest month was -16 °C and <-20 °C, respectively. Such low temperatures must certainly have been associated with extensive sea ice in the Atlantic, west of Great Britain at those times, or the oceanic influence would have produced a much more moderate climatic regime. From 13.3-12.5 ka

B.P., rapid warming took place (by +25 °C in winter and +7-8 °C in summer), indicating northward migration of the sea-ice front at that time. For a short interval around 12,000 yr B.P., temperatures were similar to those of today, but in the ensuing Younger Dryas cold event the region was plunged back into glacial-like conditions, followed by abrupt warming to a more equable maritime climate by the early Holocene. One note of caution is required regarding the apparent abrupt changes: radiocarbon "plateau x" (see Section 3.2.1.5) make the definition of a precise chronology difficult at certain times (especially around 10,000 14C yr B.P.). This may tend to exaggerate the rapidity of environmental changes at these times.

The longest insect-based paleotemperature reconstructions using the mutual climatic range approach are those of Ponel (1995), who examined insects in cores from the Grande Pile (France). This record extends back to before the last interglacial and provides insect-based paleotemperature estimates from -135 ka to -25 ka B.P. (see Figs. 9.16 and 9.20). The analysis indicates that in the coldest part of the last glaciation, mean temperatures in the warmest month were -10 °C lower than in the Eemian optimum, and in the coldest month of the year they may have been >20 °C colder in glacial times than in the last interglacial.

The mutual climatic range approach has not yet been widely employed beyond Europe, though applications to North American data have begun (Elias, 1996; Elias et al., 1996a). A number of sites that have been studied in North America are summarized in Elias (1994). However, it has not yet been possible to reconstruct long-term temperature variations in any detail, as Coope has done for Great Britain. This is due primarily to two factors: (a) The insect fauna of North America is significantly larger than that of Europe and systematic relationships between the modern faunal elements are not as well known, (b) The distribution and ecology of modern insects are also not well known in North America and many areas remain entomo-logically unexplored (Ashworth, 1980; Morgan and Morgan, 1981). Consequently, it is more difficult in North America to identify paleoenvironmental conditions precisely by fossil insect faunal assemblages. As more studies of both modern and fossil assemblages are carried out, this situation should improve significantly. Similar database problems exist elsewhere, but important results are nevertheless possible in some areas. For example, studies of late glacial sites in Chile suggest that there was no Younger Dryas episode in that area, shedding new light on an on-going controversy (Hoganson and Ashworth, 1992).

8.3.2 Paleodimatic Reconstruction Based on Aquatic Insects

In studies of lake sediments, certain aquatic insects have proven useful in paleodimatic reconstruction. Midge flies (Order: Diptera; Family: Chironomidae) can be identified by the characteristic chitinous head capsules that are often preserved in sediments (Hofmann, 1986; Walker, 1987). Walker et al. (1991a) showed that assemblages of chironomids in a suite of lakes from Labrador are related to the surface water temperature of the lakes in summer. Although the relationship may involve other factors (Hann et al., 1992) down-core analysis of chironomid remains provides an estimate of former lake surface temperatures (Walker et al., 1991b). Applying this relationship to chironomid remains in lake sediments from Maine, Cwynar and Levesque (1995) found strong evidence of a pronounced climatic reversal, which they correlated with the Younger Dryas episode (Fig. 8.10). Changes in chironomid assemblages indicate an abrupt drop in temperature of -10 °C at around 11,000 yr B.P., with temperatures then remaining low for 1-1.5 ka, followed by a similarly abrupt warming phase. This change has not been recognized in pollen data from the region (though there was an increase in Alnus and Picea at that time) so in this case the chironomid record appears to be a more sensitive climatic indicator and adds

Cold Water-

-Temperate-

Cold Water-

-Temperate-

Chelford Sediments

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16 20 25

20 40 HI 60

Chironofoid Porcenlago Diagram

Total sum or squares

FIGURE 8.10 Chironomld percentage diagram for the late Pleistocene/early Holocene section of a sediment core from Trout Pond, Maine. Radiocarbon dates are indicated on the left.The down-core changes in chironomid taxa have been calibrated in terms of summer lake surface temperature, providing the paleotemperature estimates in the second column.The pronounced drop in temperature from — I I to 9.8 kyr B.R is correlative with the Younger Dryas episode, widely seen in European sediments (Cwynar and Levesque, 1995).

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fioio tois-io^o

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