HOLOCENE MOLLUSCAN FAUNAL HISTORY AND ENVIRONMENTAL CHANGE AT CHINE DRIVE FAN, SCARBOROUGH, SOUTHERN ONTARIO

Quaternary non-marine molluscs occur in a wide range of sediments, including oxidized deposits that rarely preserve other fossil groups. In addition to conveying a regional palaeoclimatic overview, fossil land snail assemblages provide detailed information relating to local palaeoenvironments. This important archive is generally overlooked in eastern North America and consequently, detailed, well-dated terrestrial mollusc successions are extremely rare. About 50 m above Lake Ontario at Scarborough Bluffs, 12,000-year old glacial Lake Iroquois eroded a terrace and shorebluff in older glacial sediments. Gully erosion in the Iroquois shorebluff spread alluvial fans onto the terrace; Lake Ontario shore erosion has provided sporadic exposures in the fan sediments. Although no pollen has endured in the calcareous, oxidized sediments, the palaeoenvironmental history of Chine Drive Fan has nevertheless been reconstructed from the preserved land snail succession. The earliest fauna is extremely impoverished and characterised by open and marsh taxa, such as Vallonia pulchella, Deroceras/Limax and Succineidae. The immediately overlying sediments are dominated by the aquatic Fossaria modicella, indicating some standing water. The concurrent terrestrial fauna remains that of an open marsh, although some catholic (Euconulus fulvus, Nesovitrea electrina) and woodland species (Discus whitneyi) begin to appear. A barren, leached palaeosol horizon (charcoal 14C: 8710 ± 120 yr BP; 8980 ± 120 yr BP) then temporarily interrupts the faunal history. The upper half of the sequence is predominantly comprised of shade-demanding elements, such as D. whitneyi, Strobilops labyrinthica and especially Carychium exile, although marsh and aquatic taxa remain important. Damp woodland, again with some standing water, is indicated. This work clearly demonstrates the validity of molluscan faunal analyses in reconstructing climatic and environmental change in eastern North American settings and provides a basis for extending the technique to other Quaternary sites, mirroring the success of well-established European palaeoenvironmental work. We hope to extend this work further, not only at Chine Drive Fan, but also to encompass a range of similar sites across North America.