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Chironofoid Porcenlago Diagram

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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).

new insight into conditions prevailing in the region at the late Pleistocene/Holocene transition. Brooks et al. (1997) also found that the chironomid record from southeastern Scotland provided considerably more insight into rapidly changing late glacial conditions than could be obtained from pollen data alone.

In certain situations, chironomid remains may be indicative of salinity conditions in a lake, so that some insight into the precipitation-evaporation balance (P-E) over time may be possible, provided an appropriate set of lakes is selected. Studies of lakes with a wide range of salinity in a small area of British Columbia demonstrate a strong statistical relationship between water salinity and chironomid assemblages (Walker et al., 1995). Hence, in areas where pronounced changes in salinity are likely to have occurred, chironomids may be useful paleo-ecological indicators. However, in most situations they will be of secondary importance to diatoms, which are more diagnostic of salinity conditions (Fritz et al., 1991).

Apart from chironomids, the use of aquatic insects in paleo-environmental reconstruction has been somewhat exploratory. Williams and Eyles (1995) suggested that the occurrence of different caddisflies (Trichoptera) in deposits of the last interglacial and early Wisconsin age from near Toronto, Canada indicate a change in temperature of 4-5 °C from 80 to 55 ka B.P. This estimate is based on modern climatic conditions in the present day distribution of the different species identified. Such studies provide complementary information to support more comprehensive paleo-environmental reconstructions (N. E. Williams et al., 1981).

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