A HIGH RESOLUTION HOLOCENE PALEOCLIMATE RECORD FROM WESTERN IRELAND: EVIDENCE FROM POPULATION, BIOMETRIC AND STABLE ISOTOPE VALUES OF FRESHWATER MOLLUSKS

Freshwater lakes situated in the limestone bedrock of western Ireland precipitate copious carbonate sediment that is often capped by several meters of peat. This sediment includes abundant bivalve and gastropod shells. A 4.5 m core was recovered from beneath the Headford blanket bog, County Galway, to assess trends in bivalve and gastropod populations. Bivalve and gastropod abundances were determined at 5 cm intervals to evaluate ecophenotypic variability through the carbonate-rich sequence from 1.5 to 4.5 m below the bog surface. Gastropod and bivalve population data exhibit frequent, high amplitude variation that suggests large-scale changes in lake levels, depositional conditions and/or water chemistry. The pulmonate gastropod species Lymnaea peregra displays nine peaks in abundance throughout the core at 5 cm intervals, and disappears at 2.75 m. Some specimens of L. peregra also display a change in coiling direction from dextral to sinistral beginning above 1.60 m. This sinistrality is coupled with a smaller, more involute, flexible shell. The operculate gastropod species Bithynia tentaculata displays seven major peaks, which tend to coincide with low levels of L. peregra. Other species of mollusks, including the gastropods Lymnaea stagnalis and Planorbis contortus, as well as the bivalve Sphaerium corneum appear as the carbonate sequence grades upward into peat. To provide additional evidence for fauna-based interpretations, specimens of mollusks were then chosen from selected 5 cm intervals up the core for computer-assisted micromilling to obtain high-resolution d¹³C and d¹⁸O values. Thus, isotope values provide supporting evidence for variation in seasonality of precipitation and temperature.