CALIBRATION OF LACUSTRINE ISOTOPIC AND FAUNAL PALAEOTEMPERATURE RECORDS

An understanding of the impact of past climate change on terrestrial environments requires reliable and quantitative palaeotemperature measurements at high stratigraphic resolution. The oxygen stable isotopic composition of lacustrine carbonates is commonly used as a climatic indicator, but, the effects of changing temperature and hydrology are difficult to resolve. Recently developed indices based on chironomid (midge larvae) populations in relatively acid lakes potentially offer both high resolution and high precision - but it is questionable whether the the calibration extends to carbonate lakes.

An investigation of lacustrine sediments from Hawes Water, a small carbonate lake in NW England, has been carried out in conjunction with a study of carbonate precipitation in the modern lake. This has enabled a site-specific relationship between temperature and isotopic composition to be determined.

A high resolution sediment record from the late glacial and early Holocene from the Hawes Water reveals 6 abrupt negative excursions in the oxygen isotopic composition in addition to the Younger Dryas stadial. These can be correlated with events recognised in the GRIP ice core and marine and terrestrial records from NW Europe and are interpreted as cold events: all are accompanied by local floral and/or faunal changes, including changes in chironomid populations, that indicate deteriorating climate.

The pattern of events at Hawes water are almost identical to those recognised in a published study of chironomid SST's from southern Scotland suggesting that the isotopic record faithfully records local temperature. Direct calibration of the isotopic record has proved problematic: the magnitude of temperature change implied by the isotopic record is comparable to that from studies of beetles and marine diatoms from the same region but is significantly greater than the changes inferred from the chironomid data.