Three detailed, radiocarbon-dated, Holocene tufa and alluvial fan mollusc successions from southern Ontario: The first in northeastern North America

Fossil mollusc remains, particularly those of land snails, provide a wealth of detailed information concerning local Quaternary paleoenvironments and regional paleoclimatic conditions. However, this important resource is generally overlooked in North America and consequently quantitative, well-dated terrestrial mollusc successions are extremely rare here. In an attempt to redress this imbalance, a project examining the Holocene molluscan faunal history of northeastern North America has been initiated. Thus far, detailed mollusc successions and associated radiocarbon age determinations have been obtained from three sites in southern Ontario; two alluvial fans (Chine Drive Fan and Cudia Park Fan, both from the Scarborough Bluffs east of Toronto) and a tufa (West River Road, near Cambridge). The observed land snail successions to provide an initial view of the molluscan faunal history of this region during the period from the late glacial to the mid Holocene, between approximately 10,500 and 5000 radiocarbon years BP (yr BP). The molluscan data have also been used to reconstruct the paleoenvironmental history of southern Ontario during this time, documenting the progression from exposed, open habitats characterised by species such as Vertigo modesta, Vallonia albula, Columella columella and Vitrina angelicae limpida, to closed woodland biotopes indicated by the presence of taxa including Carychium nannodes, Nesovitrea binneyana, Paravitrea multidenta and Helicodiscus parallelus. This study has already demonstrated the potential of this technique in the study of Quaternary climatic and environmental change in eastern North America and represents a strong foundation for extending the analysis of fossil land snail successions to other sites throughout this region. It has also provided new and important insights into the establishment of the modern molluscan fauna, which are essential to the understanding of present day distributional patterns and environmental associations.